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Frequency-doubled microchip lasers
January 1999
Optical Materials 11 (1999) 217±233
Frequency-doubled microchip lasers Bruce D. Sinclair
1
J.F. Allen Research Laboratories, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, Scotland KY16 9SS, UK
Abstract Microchip lasers with internal second harmonic generation allow the ecient generation of visible light from particularly compact and readily manufactured devices. This paper reviews the underlying physics of these devices, and points to some of the reasons for their success. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Solid state laser; Diode-pumped laser; Nonlinear optics; Second harmonic generation; Laser resonator
1. Introduction Microchip lasers are miniature, diode-laserpowered, solid state lasers, of particularly simple design. The microchip laser consists of a slice of gain material polished to be plane parallel, with coatings applied to the crystal faces to de®ne the laser resonator. Thermal and gain-related eects bring the plane±plane cavity into geometrical stability. The simple nature of these lasers make them particularly attractive for mass production. The concept was proposed at about the same time by Dixon [1] and by Zayhowski [2]. The latter authors a paper in this issue [3] that gives an excellent overview of the potential of these devices operating at their fundamental frequency. Many applications require not the fundamental (IR) frequency of a solid state laser, but a frequency in the visible range. Such applications include reprographics, displays, alignment, and biomedical instrumentation. Several of these applications require a reliable, compact, ecient, and 1
E-mail: [email protected].
inexpensive cw laser generating a few tens of milliwatts in the visible. Our group has pioneered the ecient intra-cavity frequency doubling of these lasers to produce ultracompact sources of blue [4], green [5], and red [6] radiation. Fig. 1 shows one embodiment of the concept of the frequency-doubled microchip laser. We form a ``sandwich'' cavity consisting of the two elements needed in the laser. These are a slice of gain material (Nd:YVO4 ) and a slice of frequency doubling material (KTP). Both are polished to be planeparallel, and the two slices are stuck together. In experiments at St. Andrews we have used a halocarbon oil to provide a non-permanent bond. The outer surfaces of the sandwich are coated to be highly re¯ecting at the 1064 nm fundamental wavelength. When the laser is pumped by a half watt laser diode, a very strong circulating ®eld is built up at 1064 nm. The cavity mode diameter is small (tens of micrometres), which results in a high circulating intensity. As the fundamental ®eld passes through the KTP, a signi®cant amount is converted to the second harmonic at 532 nm. In this particular example, up to 50 mW of cw green light was generated in each direction. This diagram
0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 4 5 - 7
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Fig. 1. The green microchip laser. The a-cut Nd:YVO4 is aligned to the pump to maximise the absorption coecient. The axes of the KTP crystal are at 45° to those of the Nd:YVO4 .
shows the light from the diode laser being transferred merely by proximity coupling (i.e. the diode is brought close to the input surface of the solid state laser), though we often use collimating and focussing optics between the pump and the solid state laser. This review attempts to put these devices into context, and describe the physics of their operation. In Section 2 we put our results into context with other developments around the world. In Section 3 we describe some of the issues associated with the stability of the microchip laser cavity, and in Section 4 we concentrate on the details of the physics of the intra-cavity frequency-doubled device. This review is intended to be of a tutorial nature, reviewing the subject, and giving references to where more detailed information can be found. This review will concentrate on the work done at the University of St. Andrews. 2. Miniature visible cw solid state lasers There have been great strides in the development of green and blue solid state lasers over the last decade. For example, there are now commercially available multi-watt green lasers [7] that are set to eclipse the water-cooled argon-ion laser
(even higher powers and greater frequency stability have been produced in research laboratories). These lasers are still relatively complicated to manufacture, and have a signi®cant price tag. Many applications require visible radiation, but with far less power, and at much less cost [8]. The air-cooled argon-ion laser has dominated this market, producing tens of milliwatts of stable TEM00 laser radiation at 488 and 514 nm. This technology is well developed, and sold, for example, into the reprographics and semiconductor industries. Although diode-lasers are increasing in power and lifetime, and gradually pushing down to shorter wavelengths, it seems we are still some years away from obtaining inexpensive and longlived blue diode lasers. Direct diode-pumping of a solid state laser to produce visible radiation is possible only with some form of upconversion process. There has been recent progress in incoherent upconversion processes in ®bres in particular [9], but the alignment tolerances and the powers involved perhaps make this a less attractive option. Coherent upconversion appears to be much more attractive. Coherent upconversion through intra-cavity doubling of diode-laser pumped lasers was ®rst achieved by Baer [10]. He was also the ®rst to describe the ``green problem''; this is the name now commonly given to the intensity instability associated with the frequency doubling of a laser that is oscillating on a few longitudinal modes of the cavity. Baer used a 6 cm long cavity containing a Nd:YAG gain crystal and a KTP doubling crystal. A conventional output coupler was used. With a 200 mW diode laser as a pump source, up to 10 mW of green light was obtained. Baer describes how the dierent cavity modes are coupled through the nonlinear eect of sum-frequency generation. This, when combined with the cross saturation associated with gain saturation and spatial-hole burning, leads to mode competition, and a strong periodic or chaotic ¯uctuation in the powers of the individual modes and the generated green power. Thus while this paper represented the start of a new era for compact green lasers, it also heralded the start of a quest to overcome this problem of intensity noise. Oka and Kubota [11]
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reported how the intensity of a two-mode Nd:YAG laser could be stabilised if a quarterwave plate was used within the cavity. The two modes were polarised along the directions of the axes of the KTP, and if these were aligned to be at 45° to the axes of a quarter-wave plate then stable operation could be achieved. This was due to this geometry not producing a sum-frequency wave. Since then there has been much work published on the subject [12], including some comments that suggest that the only reliable way to avoid the green problem is to enforce single-frequency operation of the cw laser by using a ring cavity [7]. The miniaturisation of green lasers continued with impetus from the optical data storage markets. Masuda et al. [13], for example, published the design of their ``migreen'' laser, which eliminated the intensity noise using a principle similar to that proposed by Oka and Kubota. They created an entirely solid cavity, with a quarter-wave plate, Nd:YAG slice, KTP doubling crystal, and a spacer all mounted together. This device was only a few millimetres long, and produced 10 mW of green for a 250 mW pump. Dixon and Grubb [14] created a shorter laser using the microchip concept, and they attempted to increase the green output power by using resonant enhancement within the KTP. They achieved an output of 1.2 mW in the green. Work at St. Andrews also concentrated on the microchip approach, as outlined above, but with the minimum possible loss within the cavity. This has generated up to 158 mW of TEM00 green using a 1.2 W pump. The short length of the cavity promoted single-frequency operation, and even when more than one frequency was oscillating, regions of low intensity noise were readily obtained. The reasons for this will be described in detail in Section 4. This sandwich concept was subsequently successfully used by Huber and coworkers, who used Nd:LSB as the gain material [15]. They have scaled the power levels of these lasers up to a spectacular 1.2 watts of green output, although this was not in a single transverse mode [16]. Small green lasers are now marketed by a number of manufacturers, keen to attract the users of air-cooled argon-ion lasers. Some may not avoid the green noise problem at all. Some use a
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lengthy cavity to ensure very many modes are oscillating, and some use a ring cavity to ensure single-frequency operation. The microchip concept is the basis of both the blue and green lasers manufactured at the Uniphase Corporation, which has taken out a license on the patented technology developed in our group. 3. Cavity stability The plane±plane microchip cavity is on the limit of cavity stability. Despite this apparently ill-de®ned resonator, diraction limited, well-con®ned laser modes are produced. This can be due to one of several reasons, all of which are associated with the tight localisation of the pumping. The population inversion itself will cause ``guiding'' due to the preferential gain in the forward direction, and the population inversion can result in changes in the frequency-dependent refractive-index across the beam that give rise to refractive lensing eects. The inverse of this gain guiding can occur if the unpumped laser material is partially absorbing; this eect is particularly strong in quasi-three-level lasers, and has been termed aperture guiding. Despite the high eciency of many microchip lasers, there will always be some ``waste'' heat associated with the pumping process. This will result in a temperature distribution across the microchip, and an associated distributed thermal lens through dn/dT eects. The temperature distribution can also give rise to thermal expansion, and the production of eectively a curved mirror on the input surface. A positive result of these eects is that they are automatically associated with the gain region. These lasers are almost self-aligning. In the remainder of this section we shall explore the relative importance of these eects. Zayhowski [17] was the ®rst to show that in Nd:YAG microchip lasers the predominant guiding mechanism was a distributed thermal lens. In these systems, which have a relatively low absorption coecient for the pump light, the heat is deposited fairly uniformly along the microchip laser cavity. The chip is cooled at its edges, and this induces a temperature distribution from the
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centre of the pump beam to the edge of the chip. Cousins [18] has shown that if the pump pro®le can be modelled as a ``top-hat'', the temperature in the pumped region is parabolic, and outside the pumped region it falls o logarithmically. Zayhowski postulates that the resultant refractive index distribution can act as a weak waveguide. In Nd:YAG the thermo-optic coecient (dn/dT) is positive, so the refractive index distribution mirrors that of the temperature. Even if the microchip laser beam falls outside the pumped region, the gradual change in refractive index there can still result in index-guided waveguide operation. Zayhowski's experimental studies of beam waist against pump power, and of beam waist against pump spot radius, matched what he stated would be expected from this theory. In more strongly absorbing materials, such as lithium neodymium tetraphosphate (LNP), the thermal load at the pumped surface can be much greater. In this particular material dn/dT is negative, so any distributed lens of the type described above will act against cavity stability. In this case, only gain guiding and surface deformation are able to act to provide cavity stability. MacKinnon and Sinclair [19] used Fizeau interferometry to explore the change in optical thickness of pumped LNP microchip lasers, and found that these changes matched well with what was expected from the beam divergence of the microchip laser. The pumped surface bulged, and this bulge was su-
ciently close to a spherical surface over the dimensions of the strongly localised microchip laser beam that a diraction limited beam could be produced. In this case, the surface deformation was the predominant mechanism for cavity stability. More recently we have explored the operating characteristics of Nd:YVO4 . This material has a positive thermo-optic coecient and a high absorption coecient. In this case we would expect positive contributions from both mechanisms. A Fizeau interferometer was set up as shown in Fig. 2. The light re¯ected from the two chip surfaces interfered in the image plane, to produce fringes of equal optical thickness, which were examined using a CCD camera, frame grabber, and computer. Full details of this study will be published elsewhere, but an example of the results from a 0.25 mm thick 3% doped Nd:YVO4 chip pumped with 450 mW of diode-laser radiation are shown in Fig. 3. In order to ®nd the eective size of the pumped region, the 1.06 lm ¯uorescence of a non-lasing chipset was measured looking into the pump-beam direction. Integrated in this way along the line of sight, the smallest ¯uorescent spot sizes in the two directions were about 30 and 55 lm full width at half maximum (FWHM) intensity. However, if the chip was taken only a little (0.1 mm) away from the position of maximum pump intensity, a roughly circular ¯uorescent spot size of about 55±60 lm FWHM was observed.
Fig. 2. Fizeau interferometer used to analyse the optical thickness of a Nd:YVO4 microchip under lasing conditions. Helium±Neon (633 nm) or green microchip (532 nm) lasers were separately used to illuminate the interferometer. The laser light was usually arranged to have its polarisation parallel to a crystal axis in the microchip.
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Fig. 3. Results of the Fizeau interferometry for a 0.25 mm thick 3% doped Nd:YVO4 microchip laser pumped by 450 mW of diode laser radiation. Part (a) shows the calculated change in optical thickness of the chip as a function of position across the chip, but as this part of the analysis has not taken account of the direction of change, the central dip should be considered as being re¯ected into the continuation of a wider central peak. Part (b) shows this change in optical thickness as a function of the square of the distance from the centre of the pump-induced deformation.
The fringe intensity across the centre part of the chip is monitored, and from this the change in optical thickness of the chip is calculated, as shown in part (a) of the ®gure. If the chip was acting as a spherical lens, one would expect a change in optical thickness that varied linearly with the square of the distance R from the centre of the bulge. Part (b) of the ®gure shows such a plot, and additionally has marked on the values for R2 at which R 20 and 30 lm. Over this range, it can be seen that the relationship is close to being linear, indicating a change in optical thickness consistent with a low-aberration spherical lens. Taking least-squares ®ts to this data indicates an expected microchip beam waist of 24.7 or 23.7 lm depending on whether the gradient is taken across 30 or 20 lm. The far ®eld pro®le of the beam was measured using a photodiode array or a CCD camera, and from this the beam waist was calculated to be 22 lm. The good agreement between this ®gure and that expected from the interferometry suggests that the thermal lens and deformation model for this system is a good one, at least at this pump power and laser-frequency. Gain-related eects have been explored extensively theoretically due to an interest in spatiotemporal pattern formation, amongst other things [20]. As mentioned above, the gain-related eects can be classed into two groups, those associated with guiding due to the integrated gain being higher in the forward direction, and those associ-
ated with the change in refractive index due to the presence of the population inversion [21]. The ®rst group will often give a positive contribution to eective cavity stability [22,23]. A variant of this is in aperture guiding, in which quasi-three level lasers show signi®cant loss in unpumped gain material. Fan [24] demonstrated that this was the predominant guiding mechanism in Yb lasers, for example. His calculations showed that the guiding eects in his laser were very much stronger than those that would be expected from an extrapolation of Zayhowski's results for thermal guiding in Nd:YAG. The change in refractive index due to the presence of a population inversion is shown schematically in Fig. 4. The refractive index perturbation depends on the frequency of the mode, and the magnitude of the population inversion [22]. For lasers near threshold one would expect the population inversion to be higher in the centre of the pumped region than at the edges, and thus to see a variation in refractive index across the pumped region. Given the variation of refractive index shown in Fig. 4, one can see that this will give rise to guiding eects for frequencies higher than that of the gain peak, and antiguiding for frequencies less than the gain peak. Longhi et al. [22] have shown experimentally this frequency dependence in a 700 lm long Nd:YVO4 microchip laser pumped by a Ti:sapphire laser. They operated the microchip laser near threshold, and examined the
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Fig. 4. The variation in refractive index associated with gain.
variation in cavity mode size with mode frequency and pump spot size. The transverse mode size associated with the dierent frequency modes agreed with the analysis based on frequency and gain dependent refractive index variations. Both the St. Andrews [19] and Lincoln Lab [17] groups considered how important gain-related effects might be in their Nd lasers when pumped far above threshold. Both determined that at the relatively high pump powers that they were normally using, the thermal eects dominated. This was checked experimentally using a chopped pump beam. As gain-related eects should depend on the instantaneous pump power, but thermal eects on a time-averaged pump power, both sets of workers made a comparison between true cw operation and pulsed operation with the same average power. At high pump powers no dierence was seen, suggesting that the thermal eects dominated. However, the St. Andrews group did see evidence of gain-related eects becoming more important at lower average powers [25]. The pump beam was chopped with a variable duty cycle. The pump power that was needed during the ``on'' part of the cycle to ensure TEM00 operation was measured as the average incident power was varied (by changing the duty cycle of the chopping). If the cavity stability mechanisms were dependent solely on gain-related eects, this graph would be a hori-
zontal line. If, however, the eects were purely thermal, the required instantaneous pump power would vary with the duty cycle, and in particular would become very large as the average incident pump power went to zero. Neither of these is seen to happen in Fig. 5. This means that gain-related eects are becoming important at the low average powers, but that longer-term, thermal eects are important at the higher average powers. We have also seen evidence of what we believe are gain-related self-Q-switching eects in Nd:YVO4 lasers [26]. Relaxation oscillations and spiking are well known in solid state lasers, and can occur when a second mode starts to oscillate in the cavity. For some lasers, we saw not a conventional train of relaxation oscillations when a second mode came above threshold, but a very large spike in power, followed by a train of more conventional relaxation oscillations. We believe that this is associated with the second mode, which is at a lower frequency than the ®rst mode, being antiguided by the gain in the cavity. This antiguiding will be particularly strong at the nodes of the ®eld of mode one. This results in the width of mode 2 being rather greater than in a cavity with no gain-related changes in refractive index, thus pushing up the threshold of this mode further than expected. However, once mode two does reach threshold, and starts what would otherwise be a spiking train, the gain is saturated, and the antiguiding rapidly becomes much less. This means that mode two is now much further above
Fig. 5. A modulated diode pump source was used to power the solid state laser. The amount of power required during the ``on'' part of the cycle to ensure TEM00 operation of the Nd laser was recorded as a function of the average diode power.
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threshold than would otherwise be expected, and a form of Q-switched pulse is produced. This has a peak power of up to some 28 W, compared to the cw level of 11 mW, and a pulse width down to 1.85 ns. Our analysis, which takes into account the size of the pump beam and laser modes, the eects of gain saturation, and the gain-related changes in refractive index gives good agreement with theory. The eects of gain guiding obviously depend strongly on the frequency of operation, and the nature of any lensing eect will depend on the exact shape of the saturated gain distribution. These are areas of continued investigation. This section has given some indication of the involved process of cavity stability in microchip lasers operating at their fundamental frequency. The mode size will depend on the pump geometry, the cavity losses, the frequency of the laser mode, and the polarisation of the laser mode. Equally, the frequency of the laser mode will depend on temperature, which in turn depends on the pump power. The mode size is important for the operation of the fundamental microchip laser, but becomes even more so for the frequency-doubled microchip laser, as will be described in the following section.
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4. Intra-cavity-doubled microchip lasers Our group has shown that intra-cavity-doubled microchip lasers work well. We believe that we understand many of the processes that allow these lasers to operate in a stable and almost selfaligning manner. However, the physics of their operation is complex, as can be seen from a diagram showing some of the interdependence of various parameters that eect the second harmonic generation process, reproduced here as Fig. 6. The second harmonic generation process relies on an intense fundamental ®eld of appropriate polarisation passing through the nonlinear crystal. The greater the circulating intra-cavity power, the greater is the conversion eciency. Thus the linear losses in the cavity are vitally important in their role in determining the circulating power in the laser. Equally, the cavity stability mechanisms will de®ne the size of the circulating mode, and thus the intensity of the circulating beam. Some aspects of the physics of this process can be studied by using a very simple plane wave model of intra-cavity second harmonic generation [27]. While we should not expect this to model accurately every aspect of the laser operation, this model should at least guide us in our under-
Fig. 6. Some of the factors aecting the operation of the intra-cavity-doubled microchip laser, and some of their interdependencies. The complicated inter-relationship is readily seen.
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standing and in our optimisation processes. As in any other laser operating in steady state conditions, the round trip saturated gain of the intracavity-doubled laser must equal the round trip losses. These losses will, however, consist of both linear and nonlinear terms. For a standing wave cavity, in which the eects of spatial-hole burning are ignored, we may write 2g0 lg L ÿ ln R1 R2 2KI 1 2I=Is in which we are equating the saturated gain to the sum of parasitic losses L, the losses due to output coupling of the fundamental ®eld through mirror coatings of re¯ectivity R1 and R2 , and nonlinear losses due to the generation of the second harmonic. In this equation I is the intensity of the ®eld propagating in one direction, Is the saturation intensity of the material, g0 the small signal gain coecient, and lg the length of the gain medium. The amount of power converted to the second harmonic is dependent on the ®eld intensity I and a constant K that depends on the parameters of the nonlinear crystal. In particular, K is proportional to the square of both the length of the nonlinear crystal and the eective nonlinear coecient. This very simple model allows us to explore how sensitive the laser is to losses, second harmonic generation crystal length, the nonlinear coecient, and other parameters. One example of the comparison of the model and experiment is
shown in Fig. 7. Here we model the diode as providing a top-hat pump pro®le of 55 lm radius, and the laser mode is assumed to have an equivalent top-hat pro®le of the same size. These are obviously relatively crude approximations. In this case, with a 0.2% linear loss, a 0.1% transmission of the output coupler to the fundamental, and a 2 mm long KTP crystal, the model predicts the IR and green output powers shown in the part (a) of the ®gure. The form and magnitude of these curves are similar to those obtained experimentally, and displayed in part (b) of the ®gure. Having veri®ed that the model works reasonably, we then show how it can be used to investigate the role of losses and nonlinear coupling. Fig. 8 shows the calculated IR and green powers for a 1 W diode pump as a function of intracavity losses. The two graphs show the results for a 99.9% re¯ecting output coupler and a 99.5% output coupler. Not surprisingly, both graphs show the powers dropping o rapidly with increasing linear losses. However, in the 99.9% re¯ecting case in particular, it can be seen that the green output power drops o much more rapidly than the IR power, due to the squared dependence on the circulating intensity. At the lowest loss levels, the green output is considerably higher than in the IR, which is generally what would be desired. If the output coupling of the IR is increased, we see the IR output power rise, but the green power fall catastrophically. These graphs indicate what one
Fig. 7. Theoretical and experimental curves showing the IR and green output from an idealised microchip laser consisting of 0.5 mm of Nd gain material and 2 mm of KTP. The plane wave approximation was used for the calculations, and the assumptions of a 0.2% linear loss and a 0.1% transmission of the output coupler were made.
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Fig. 8. Calculated IR and Green output from the idealised microchip laser described in Fig. 7 and in the text. The two graphs show the result of changing the parasitic (linear) losses in the cavity, with the two graphs calculated for dierent output coupler re¯ectivities in the IR.
might reasonably expect, that it is important to keep intra-cavity losses low, but perhaps more importantly they also indicate the magnitudes of loss that may be tolerated. The length of the frequency-doubled microchip laser is an important parameter. The mode size of the laser depends on its length (for a given thermal load), and the intra-cavity mode spacing will depend on the laser length. Generally both these characteristics are improved if the laser is made short. However, as the laser is made up entirely of the gain material and the nonlinear crystal, any shortening of the laser cavity requires shortening of one or other of these crystals. The gain crystal requires to be long enough to absorb a reasonable fraction of the incident pump power, and also to be long enough to be readily handled. Quarter millimetre thick slices are the shortest that we have found practical. The longer is the nonlinear crystal, the greater the conversion will be from the IR to the green. However, it can be shown that there is an optimum length of crystal, depending on the laser parameters [27]. This is generally longer than we have used, and given the other deleterious effects of lengthening the cavity, it is an important question as to how short a nonlinear crystal can be used without seriously compromising the conversion eciency. In an ideal laser, with no linear
losses, perfect re¯ectors of the fundamental, and a ®nite nonlinearity in the nonlinear crystal, the crystal can be made arbitrarily small. Fig. 9 shows the eects of changing crystal length with a real laser (note that these calculations do not take into account the changes of mode size with laser length, or any length dependent attenuation within the doubling crystal). Part (a) shows that for a relatively lossy laser (0.4%), there is indeed a strong dependence on nonlinear crystal length. However, part (b) shows that if the losses are kept small (here 0.1%), as long as the nonlinear crystal is at least 2 mm long, there is little to be gained from going to a greater length. This shows one of the many advantages of the microchip laser geometry. The naturally short laser has the potential to be built as a low-loss device. If this potential is realised with good coatings forming the resonator, then even with relatively short (and therefore less expensive) nonlinear crystals good second harmonic generation eciencies can be achieved. This simple model shows that in the selection of gain and nonlinear materials one should be looking for materials that have low attenuation, and high gain and nonlinear coecients as appropriate. It is useful to take the ideas above and apply them to a choice of materials for frequency-dou-
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Fig. 9. Calculations of the expected IR and green output powers from the idealised microchip laser described in Fig. 7, as a function of the length of the nonlinear crystal. These calculations were made for a 1 W pump, and for coatings of the two mirrors that are respectively 100% and 99.9% re¯ecting at 1064 nm. The two graphs are for dierent values of parasitic (linear) loss in the cavity.
bled microchip lasers. In addition to the parameters above, one wishes to have a gain material which has · a suciently strong absorption coecient to allow strong absorption of the pump in a short length; · thermal properties appropriate for forming a tightly con®ned single transverse mode at pump power levels of interest;
· a polarisation-dependent gain in order to attempt to drive the laser into a polarisation suitable for frequency doubling; · a narrow emission bandwidth for reducing the number of oscillating modes, ideally to one. Table 1 gives parameters for a few of the more promising materials for green microchip lasers. The loss parameters for the materials listed are all small, and will all give rise to losses of less than
Table 1 Comparison of some possible hosts for the Nd 1.06 lm laser Nd:YAG
1% Nd:YVO4 E||z
3% Nd:YVO4 E||z
10% LSB E||x
Eective stimulated emission cross section r (10ÿ19 cm2 )
3.3
16
16
1.3
Upper state lifetime s (ls)
230
97
50
118
rs product (10ÿ23 cm2 s)
7.6
9.5
8
1.5
Emission line width (nm)
0.6
0.8
0.8
4
Absorption coecient (cmÿ1 )
12
40
110
36
Absorption line width (nm)
0.8
8
8
3
0.001
0.04 0.011
0.04
Losses (cmÿ1 )
0.0003
Data from Zayhowski and Harrison [28], Meyn and Huber [15], Koechner [29], Huber [30], and ITI datasheets [31]. There is some variation between sources; in particular, the loss ®gures of 0.04 are quoted by the ITI datasheets, with the additional (lower) ®gures from Huber.
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0.06% for a 0.25 mm thick slice. At these levels it is likely that other loss mechanisms in the cavity will dominate the overall cavity loss. Vanadate and LSB have the advantage over YAG of a high absorption coecient and a polarised gain spectrum, and vanadate has the advantage over LSB of a greater rs product. For doubling of the 1064 nm transition KTP is currently the material of choice. It has a high nonlinear coecient, it has low losses, and has high angular and temperature acceptance tolerances. The sandwich of Nd:YVO4 and KTP form a short cavity, which keeps linear losses low, and aids single-frequency operation. The short length of KTP that is needed ensures high temperature bandwidths for the doubling process, and may contribute to the ease of operation without evidence of the green problem. Fig. 10 shows the results of calculations based on the above simple model, where the green output power is plotted as a function of pump power and linear loss in the cavity. Also shown are the results for similar calculations for our blue and red lasers. These lasers are based on similar principles, but use in the red laser the 1342 nm transition in Nd:YVO4 and a non-critically phase matched LBO crystal, and in the blue laser the 946 nm transition in Nd:YAG and a critically phase matched crystal of KNbO3 . The relatively low gain
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and low nonlinear coecient in the red laser give rise to a much greater sensitivity to linear losses than in the green laser. The blue laser seems to be predicted to work very well, as the KNbO3 has a high nonlinear coecient. However, the 946 nm transition suers from reabsorption loss, and there are other technical problems which reduce its effectiveness, as will be outlined below. In the above discussion no account has been taken of the issues of phase matching and the polarisation states of the modes within the cavity. In the green laser, in which type II phase matching is used in KTP, both the gain and nonlinear materials are birefringent, and the gain in Nd:YVO4 is strongly polarisation dependent. Our studies of the polarisation states of the dierent longitudinal modes have shown good agreement between experiment and a theory based on Jones matrices. At certain temperatures the birefringence can be such that the circulating IR ®eld for at least one longitudinal mode can be optimally polarised for the gain material and optimally polarised for the frequency doubling process. Under other conditions this may not be the case, and the eciency of the device can be reduced. The polarisation of the dierent modes is also important in regard to their sum-frequency generation and the impact this has on ``green noise''. These issues are currently still
Fig. 10. Comparison of the expected idealised performance of the blue, green, and red lasers, as described in the text. The contours show lines of equal power output. The 25 mW output power contour is labelled as such, and the 100 mW contour is distinguished by the thick line. The contour intervals are 25 mW. The wave-like nature of the curves for the red laser are due to the interpolation routines used.
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under investigation and will be covered in forthcoming publications. However, the sensitivity of the laser to temperature is not great, and in a laboratory situation the lasers can be aligned to be ``quiet'' at set pump-power levels with no temperature stabilisation at all. The green laser has provided up to 158 mW of green output when pumped by a 1.2 W diode, as shown in Fig. 11(a). In this case the diode-laser radiation was collimated and refocussed using 8 mm focal length lenses. This created a tightly con®ned population inversion. We monitored the 1064 nm ¯uorescence on a non-lasing chip using a CCD camera, and found, under the conditions that had given maximum 1064 nm laser output, a ¯uorescent spot size (integrated through the thickness of the laser crystal) of 50 ´ 110 lm (full width). Fig. 11(a) gives the appearance of a smooth power in/out curve, but in practice this is not always the case. Fig. 11(b) shows a similar relationship, but with continuous monitoring of power levels. Large jumps in the IR and green power can be seen, which are associated with major changes in the operating characteristics of the laser. Fig. 6 gave an indication of the coupled processes that go to determining the operation of the laser. It does not seem unreasonable to expect that these coupled and sometimes nonlinear relationships will lead to bistable and similar eects in the laser operation. This complicated form of the power out/in curve means that this type of laser is best suited for a particular power level, and is likely to be less suited to applications where a varying green power level is required through direct modulation of the laser diode. The green noise of these lasers can be remarkably small. We routinely see the intensity noise as low as 0.5%, even when more than one longitudinal mode is oscillating. We postulate that the low levels of noise are due to the relatively small coupling between the longitudinal modes, which is in turn due to two reasons. The ®rst reason is that the short length of the cavity often leads to one longitudinal mode being of much greater intensity than a second subsidiary mode. The second reason is that the KTP is short, and thus provides relatively little nonlinear coupling between the modes.
In conditions where there are two modes that do generate a strong sum-frequency wave, we can see green noise producing an intensity modulation of up to 150% at frequencies from 150 kHz upwards. Fortunately, it is not dicult with most chipsets to ®nd broad regions of intensity stability. Another major potential source of intensity noise in solid state lasers is relaxation oscillations. These can be stimulated by ¯uctuations in the cavity loss or gain. The monolithic and stable nature of the frequency-doubled microchip laser means that the former mechanism is suppressed, and the latter can be minimised by using a stable diode laser pump source. The inclusion of a frequency doubling crystal within the cavity also acts to reduce the magnitude of relaxation oscillations [32]. Our red microchip lasers are based on the 1342 nm transition in Nd:YVO4 . The 1.3 lm transition in vanadate is considerably stronger than in YAG, and also has the signi®cant advantage of being strongly polarisation dependent. The rs product is, however, still much less than at 1.06 lm. A summary of some of the relevant properties of these two materials at the two wavelengths is given as Table 2. A 1.3 lm device was constructed in the form of a monolithic resonator made of a 0.25 mm long 3% doped Nd:YVO4 slice, with 5% output coupling, and pumped with a 1.2 W laser. This generated up to 105 mW of single-frequency radiation at this technologically interesting wavelength [33]. We then incorporated a 2-mm long type II noncritically phase matched crystal of LBO into a cavity consisting of a 0.5 mm thick slice of 1% doped Nd:YVO4 and a discrete output coupler to allow frequency doubling of the laser into the red. We were not in a position to create the usual allsolid cavity, but instead had to rely on antire¯ection coatings within the cavity. These, combined with a relatively poor coating on the output coupler, led to a relatively low circulating 1342 nm ®eld. When this was compounded with the relatively low conversion eciency of LBO, only 10 mW of red was coupled out of the laser, which was pumped with a 1.2 W diode. However, as our modelling shows, with improvements to the intracavity losses, signi®cantly higher red powers could be generated. Over much of the operating range
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229
Fig. 11. Power output from green microchip lasers as a function of incident pump power. Part (a) shows the results from a chipset pumped with 1.2 W diode laser. A maximum green output power of 158 mW was obtained. Part (b) shows for a dierent chipset the output powers in the green and IR as the pump power is varied. This shows the jumpy nature of the output power obtained with some chipsets. This is not surprising, given the coupled nonlinearities that are partially described by Fig. 6.
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Table 2 Comparison of properties of the 1.06 and 1.3 lm transitions in Nd:YAG and Nd:YVO4 Nd:YAG
Nd:YVO4
1064 nm
1321 nm
1064 nm E||z
1342 nm
Eective stimulated emission cross section r (10ÿ19 cm2 )
3.3
0.7
16
7.6
Fluorescence lifetime s (ls)
230
230
100 (1%) 47 (2%)
100 (1%) 47 (2%)
2
cm s)
7.6
1.6
16 (1%) 7.5 (2%)
7.6 (1%) 3.6 (2%)
Emission line width (nm)
0.6
0.6
0.8
1.6 [8]
ÿ23
rs (10
Refractive index
1.82
1.96, 2.17
Pump absorption coecient (cmÿ1 )
12
41 (1%) 72 (2%)
Absorption bandwidth (nm)
1
8
Scatter losses (cmÿ1 )
0.003
0.04
Thermal conductivity (W cmÿ1 Kÿ1 )
0.14
0.05
Data from Zayhowski and Harrison [28], and/or ITI Datasheets [31] except where otherwise stated.
the laser gave an intensity stability of better than 1%, and reliably operated in a TEM00 mode. The 2 mm long slice of non-critically phase matched LBO had an acceptably wide temperature acceptance bandwidth of 30°. However, over this temperature range considerable changes take place in the polarisation states of the cavity modes. These gradual changes with LBO temperature of polarisation state of the fundamental resulted in a variation in the red output power. The second harmonic power was modulated with a period of 7°C due to these polarisation eects. A single-frequency red output was frequently obtained, even with the relatively extended cavity (20 mm) used. While many applications can be addressed by the green laser, a number of important processes have been developed for the blue line of the argonion laser. This wavelength also has the advantage of a smaller minimum spot size than in the green. For these reasons we investigated various options for the generation of blue light in the microchip laser geometry. Neodymium has a tolerably strong transition from the 4F3=2 to 4I9=2 state, which, when frequency doubled will be in the blue. The best known host for this laser is Nd:YAG, which has been used for a number of blue laser demonstrations. This laser is a quasi-three level laser, as the
lower laser level is only a few kT above the ground state, and is thus thermally populated. The advent of ecient diode laser pumping kindled interest in this transition in the 1980s [34], though we are not aware of the widespread use of these lasers. What advantages would come from microchip laser operation? A major one is that the low losses associated with this geometry would allow a shorter nonlinear crystal to be used, and thus would provide a wider temperature acceptance bandwidth than the larger crystals used in larger lasers. There will also be the usual microchip laser advantages of ease of production, small size, and so on. We performed a survey of some Nd host materials to evaluate the best options for this laser. While vanadate was attractive with its polarised gain spectrum, the lower laser level was particularly close to the ground state. We also had diculties in the procurement of coatings on the vanadate that were good transmitters at 809 nm and very good re¯ectors at the Nd laser wavelength. Instead, we chose to work with Nd:YAG, and used critically phase matched KNbO3 as the nonlinear medium, in the laser geometry sketched in Fig. 12. With a 1.2 W diode pump we achieved up to 33 mW of TEM00 blue output, as shown in Fig. 13. The short length of potassium niobate
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Fig. 12. Schematic of the blue microchip laser. The 1.5 mm thick KNbO3 crystal was cut for type I critical phase matching at a temperature of 45°C. The orientation of doubling crystal and the polarisations of the light produced are indicated. The 1.1% doped Nd:YAG crystal was 1 mm thick.
The polarisation state varied across the crystal set, and was optimised near particular lines across the crystal. We postulate that the joining process of the two materials has led to residual stress, and it is this that provides the necessary polarisation discrimination. In the regions of best polarisation discrimination we achieved in the region of 15±35 mW of blue generation, in other regions it dropped to 5±10 mW. These power levels are appropriate for many applications, though the presence of some intensity noise must be taken into account. 5. Conclusions Fig. 13. Performance of the blue microchip laser.
that was used allowed a tolerably broad temperature acceptance bandwidth of 2.2°C. It was a little surprising that this laser generated any blue light at all, as there would not appear to be any reason for the laser not to operate on a polarisation that would be incorrectly aligned for phase matched frequency doubling. However, the laser always chose to operate with at least some of the circulating 946 nm ®eld aligned in the correct direction.
Frequency-doubled cw microchip lasers are ef®cient, reliable, ultracompact, and readily manufacturable sources of quality laser radiation. We have reviewed the major aspects behind their physics of operation, and attempted to show how an understanding of this physics can lead to optimisation of these lasers. At a time when diode-laser pumped solid state lasers are starting to break into the high-power high-cost market, we see the microchip lasers established in the low-power, lowcost arena.
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Acknowledgements The research programme at the University of St. Andrews has been undertaken by a team that has included at dierent times Richard Conroy, Graham Friel, Alan Kemp, David Matthews, Neil MacKinnon, and the author. Their work has been supported by the UK SERC and EPSRC through grant numbers GR/F83341 and GR/K14766. The work has also bene®tted from the input of IE Optomech and Uniphase. References [1] G.J. Dixon, L.S. Lingvay, R.H. Jarman, Properties of close-coupled, monolithic, lithium neodymium tetraphosphate lasers, Proc. SPIE 1104 (1989) 107. [2] J.J. Zayhowski, A. Mooradian, Single frequency microchip lasers, Opt. Lett. 14 (1989) 24. [3] J.J. Zayhowski, Microchip lasers, Opt. Mater. 11 (1999) 255. [4] D.G. Matthews, R.S. Conroy, B.D. Sinclair, N. MacKinnon, Blue microchip laser fabricated from Nd:YAG and KNbO3 , Opt. Lett. 21 (1996) 198±200. [5] N. MacKinnon, B.D. Sinclair, A laser diode array pumped, Nd:YVO4 /KTP composite material microchip laser, Opt. Commun. 105 (1994) 183±187. [6] R.S. Conroy, A. Kemp, B.D. Sinclair, N. MacKinnon, Comparison of 671/1342 nm generation with 532/1064 nm in Nd:YVO4 microchip lasers, Paper CFO6, in: Conference on Lasers and Electo-Optics, Baltimore, May 1997. [7] M. Gitin, C. Seaton, Laser Focus World, November 1996, p. 153. [8] G. Mitchard, Laser Focus World, January 1997, p. 82. [9] For example: T. Sandrock, H. Scheife, E. Heumann, G. Huber, High-power continuous-wave upconversion ®ber laser at room temperature, Opt. Lett. 22 (1997) 808, and references therein. [10] T. Baer, Large amplitude ¯uctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers, J. Opt. Soc. Am. B 3 (1986) 1175. [11] M. Oka, S. Kubota, Stable intracavity doubling of orthogonal linearly polarised modes in diode-pumped Nd:YAG lasers, Opt. Lett. 13 (1988) 905. [12] See, for example, S. Falter, K.M. Du, Y. Liao, M. Quade, J. Zhang, P. Loosen, R. Poprawe, Dynamics and stability of a laser system with second order nonlinearity, Opt. Lett. 22 (1997) 609; G.J. Dixon, Laser Focus World, April 1997, p. 85, and references therein. [13] H. Masuda, F. Maeda, M. Oka, Y. Kaneda, S. Kubota, Compact Blue-Green Laser Technical Digest, Santa Fe, 1992, 6FA-3-1, as reported in E.N. Eguchi, Y. Akiyama, Jpn. J. Appl. Phys. 32 (1993) 5307.
[14] G.J. Dixon, S.G. Grubb, Paper CPDP37 at the Conference on Lasers and Electro-Optics, 1990. [15] J.P. Meyn, G. Huber, Intracavity frequency doubling of a continuous wave, diode laser-pumped neodymium lanthanum scandium borate laser, Opt. Lett. 19 (1994) 1436. [16] V.G. Ostroumov, F. Heine, S. Kuck, G. Huber, V.A. Mikhailov, I.A. Shcherbakov, Intracavity frequency-doubled diode-pumped Nd:LaSc3 (BO3 )4 lasers, Appl. Phys. B 64 (1997) 301. [17] J.J. Zayhowski, Thermal guiding in microchip lasers, in: H.P. Jenssen, G. Dube (Eds.), OSA Proceedings on Advanced Solid State Lasers, March 1990, p. 9. [18] A.K. Cousins, Temperature and thermal stress scaling in ®nite-length end-pumped laser rods, IEEE J. Quantum Electron. 28 (1992) 1057. [19] N. MacKinnon, B.D. Sinclair, Pump power induced cavity stability mechanisms in lithium neodymium tetraphosphate (LNP) lasers, Opt. Commun. 94 (1992) 281. [20] F.G. Laeri, N. Deutsch, G. Angelow, M. Muller, H. Sakowski, Spatio-temporal coupling of laser ¯uctuations ± observations on a laser with internal frequency conversion, Appl. Phys. B 63 (1996) 339. [21] L.W. Casperson, A. Yariv, Gain and dispersion focussing in a high gain laser, Appl. Opt. 11 (1972) 462. [22] S. Longhi, G. Cerullo, S. Taccheo, V. Magni, P. Laporta, Experimental observation of transverse eects in microchip solid state lasers, Appl Phys. Lett. 65 (1994) 3042. [23] S. Longhi, Theory of transverse modes in microchip lasers, J. Opt. Soc. Am. B 11 (1994) 1098; S. Longhi, P. Laporta, Longitudinal-transverse mode interplay and conical emission in microchip lasers, J. Opt. Soc. Am. B 12 (1995) 1511. [24] T.Y. Fan, Aperture guiding in quasi-three-level lasers, Opt. Lett. 19 (1994) 554. [25] R.S. Conroy, A. Kemp, D.G. Matthews, B.D. Sinclair, Gain guiding and thermal distortion in diode-pumped Nd:YVO4 microchip lasers, Paper CTuP4, in: Conference on Lasers and Electro-Optics, Anaheim, June 1996. [26] R.S. Conroy, T. Lake, G.J. Friel, A.J. Kemp, B.D. Sinclair, Self Q-switched Nd:YVO4 microchip lasers, Opt. Lett. 23 (1998) 457±459. [27] R.G. Smith, Theory of intracavity optical second harmonic generation, IEEE J. Quantum Electron. 6 (1970) 215. [28] J.J. Zayhowski and J. Harrison, Miniature solid state lasers, in: M.C. Gupta (Ed.), The Handbook of Photonics, Ch. 8, CRC Press, Boca Raton, 1997. [29] W. Koechner, Solid State Laser Engineering, Springer, Berlin. [30] G. Huber, Compact diode pumped solid state lasers in the visible spectral region, in: Presentation at the Summer School on Miniature Coherent Light Sources in Dielectric Media, Les Houches, June 1997. [31] Data sheets from ITI Electro-Optics Corporation, 1993. [32] J. Chandler, J.D. Barry, Stability of an intracavity frequency-doulbed Nd:YAG laser, IEEE J. Quantum Electron. 10 (1974) 596.
B.D. Sinclair / Optical Materials 11 (1999) 217±233 [33] R.S. Conroy, A.J. Kemp, G.J. Friel, B.D. Sinclair, Microchip Nd:Vanadate lasers at 1342 nm and 671 nm, Opt. Lett. 23 (1997) 1781. [34] T.Y. Fan and R.L. Byer, Continuous-wave room-temperature Nd-YAG laser at 946 nm, J. Opt. Soc. Am. B 3
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(1986) 109; W.P. Risk, W. Lenth, Room-temperature, continuous-wave, 946-nm Nd-YAG laser pumped by laserdiode arrays and intracavity frequency doubling to 473nm, Opt. Lett. 12 (1987) 993.
Optical Materials 11 (1999) 217±233
Frequency-doubled microchip lasers Bruce D. Sinclair
1
J.F. Allen Research Laboratories, School of Physics and Astronomy, University of St. Andrews, North Haugh, St. Andrews, Fife, Scotland KY16 9SS, UK
Abstract Microchip lasers with internal second harmonic generation allow the ecient generation of visible light from particularly compact and readily manufactured devices. This paper reviews the underlying physics of these devices, and points to some of the reasons for their success. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Solid state laser; Diode-pumped laser; Nonlinear optics; Second harmonic generation; Laser resonator
1. Introduction Microchip lasers are miniature, diode-laserpowered, solid state lasers, of particularly simple design. The microchip laser consists of a slice of gain material polished to be plane parallel, with coatings applied to the crystal faces to de®ne the laser resonator. Thermal and gain-related eects bring the plane±plane cavity into geometrical stability. The simple nature of these lasers make them particularly attractive for mass production. The concept was proposed at about the same time by Dixon [1] and by Zayhowski [2]. The latter authors a paper in this issue [3] that gives an excellent overview of the potential of these devices operating at their fundamental frequency. Many applications require not the fundamental (IR) frequency of a solid state laser, but a frequency in the visible range. Such applications include reprographics, displays, alignment, and biomedical instrumentation. Several of these applications require a reliable, compact, ecient, and 1
E-mail: [email protected].
inexpensive cw laser generating a few tens of milliwatts in the visible. Our group has pioneered the ecient intra-cavity frequency doubling of these lasers to produce ultracompact sources of blue [4], green [5], and red [6] radiation. Fig. 1 shows one embodiment of the concept of the frequency-doubled microchip laser. We form a ``sandwich'' cavity consisting of the two elements needed in the laser. These are a slice of gain material (Nd:YVO4 ) and a slice of frequency doubling material (KTP). Both are polished to be planeparallel, and the two slices are stuck together. In experiments at St. Andrews we have used a halocarbon oil to provide a non-permanent bond. The outer surfaces of the sandwich are coated to be highly re¯ecting at the 1064 nm fundamental wavelength. When the laser is pumped by a half watt laser diode, a very strong circulating ®eld is built up at 1064 nm. The cavity mode diameter is small (tens of micrometres), which results in a high circulating intensity. As the fundamental ®eld passes through the KTP, a signi®cant amount is converted to the second harmonic at 532 nm. In this particular example, up to 50 mW of cw green light was generated in each direction. This diagram
0925-3467/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 8 ) 0 0 0 4 5 - 7
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B.D. Sinclair / Optical Materials 11 (1999) 217±233
Fig. 1. The green microchip laser. The a-cut Nd:YVO4 is aligned to the pump to maximise the absorption coecient. The axes of the KTP crystal are at 45° to those of the Nd:YVO4 .
shows the light from the diode laser being transferred merely by proximity coupling (i.e. the diode is brought close to the input surface of the solid state laser), though we often use collimating and focussing optics between the pump and the solid state laser. This review attempts to put these devices into context, and describe the physics of their operation. In Section 2 we put our results into context with other developments around the world. In Section 3 we describe some of the issues associated with the stability of the microchip laser cavity, and in Section 4 we concentrate on the details of the physics of the intra-cavity frequency-doubled device. This review is intended to be of a tutorial nature, reviewing the subject, and giving references to where more detailed information can be found. This review will concentrate on the work done at the University of St. Andrews. 2. Miniature visible cw solid state lasers There have been great strides in the development of green and blue solid state lasers over the last decade. For example, there are now commercially available multi-watt green lasers [7] that are set to eclipse the water-cooled argon-ion laser
(even higher powers and greater frequency stability have been produced in research laboratories). These lasers are still relatively complicated to manufacture, and have a signi®cant price tag. Many applications require visible radiation, but with far less power, and at much less cost [8]. The air-cooled argon-ion laser has dominated this market, producing tens of milliwatts of stable TEM00 laser radiation at 488 and 514 nm. This technology is well developed, and sold, for example, into the reprographics and semiconductor industries. Although diode-lasers are increasing in power and lifetime, and gradually pushing down to shorter wavelengths, it seems we are still some years away from obtaining inexpensive and longlived blue diode lasers. Direct diode-pumping of a solid state laser to produce visible radiation is possible only with some form of upconversion process. There has been recent progress in incoherent upconversion processes in ®bres in particular [9], but the alignment tolerances and the powers involved perhaps make this a less attractive option. Coherent upconversion appears to be much more attractive. Coherent upconversion through intra-cavity doubling of diode-laser pumped lasers was ®rst achieved by Baer [10]. He was also the ®rst to describe the ``green problem''; this is the name now commonly given to the intensity instability associated with the frequency doubling of a laser that is oscillating on a few longitudinal modes of the cavity. Baer used a 6 cm long cavity containing a Nd:YAG gain crystal and a KTP doubling crystal. A conventional output coupler was used. With a 200 mW diode laser as a pump source, up to 10 mW of green light was obtained. Baer describes how the dierent cavity modes are coupled through the nonlinear eect of sum-frequency generation. This, when combined with the cross saturation associated with gain saturation and spatial-hole burning, leads to mode competition, and a strong periodic or chaotic ¯uctuation in the powers of the individual modes and the generated green power. Thus while this paper represented the start of a new era for compact green lasers, it also heralded the start of a quest to overcome this problem of intensity noise. Oka and Kubota [11]
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reported how the intensity of a two-mode Nd:YAG laser could be stabilised if a quarterwave plate was used within the cavity. The two modes were polarised along the directions of the axes of the KTP, and if these were aligned to be at 45° to the axes of a quarter-wave plate then stable operation could be achieved. This was due to this geometry not producing a sum-frequency wave. Since then there has been much work published on the subject [12], including some comments that suggest that the only reliable way to avoid the green problem is to enforce single-frequency operation of the cw laser by using a ring cavity [7]. The miniaturisation of green lasers continued with impetus from the optical data storage markets. Masuda et al. [13], for example, published the design of their ``migreen'' laser, which eliminated the intensity noise using a principle similar to that proposed by Oka and Kubota. They created an entirely solid cavity, with a quarter-wave plate, Nd:YAG slice, KTP doubling crystal, and a spacer all mounted together. This device was only a few millimetres long, and produced 10 mW of green for a 250 mW pump. Dixon and Grubb [14] created a shorter laser using the microchip concept, and they attempted to increase the green output power by using resonant enhancement within the KTP. They achieved an output of 1.2 mW in the green. Work at St. Andrews also concentrated on the microchip approach, as outlined above, but with the minimum possible loss within the cavity. This has generated up to 158 mW of TEM00 green using a 1.2 W pump. The short length of the cavity promoted single-frequency operation, and even when more than one frequency was oscillating, regions of low intensity noise were readily obtained. The reasons for this will be described in detail in Section 4. This sandwich concept was subsequently successfully used by Huber and coworkers, who used Nd:LSB as the gain material [15]. They have scaled the power levels of these lasers up to a spectacular 1.2 watts of green output, although this was not in a single transverse mode [16]. Small green lasers are now marketed by a number of manufacturers, keen to attract the users of air-cooled argon-ion lasers. Some may not avoid the green noise problem at all. Some use a
219
lengthy cavity to ensure very many modes are oscillating, and some use a ring cavity to ensure single-frequency operation. The microchip concept is the basis of both the blue and green lasers manufactured at the Uniphase Corporation, which has taken out a license on the patented technology developed in our group. 3. Cavity stability The plane±plane microchip cavity is on the limit of cavity stability. Despite this apparently ill-de®ned resonator, diraction limited, well-con®ned laser modes are produced. This can be due to one of several reasons, all of which are associated with the tight localisation of the pumping. The population inversion itself will cause ``guiding'' due to the preferential gain in the forward direction, and the population inversion can result in changes in the frequency-dependent refractive-index across the beam that give rise to refractive lensing eects. The inverse of this gain guiding can occur if the unpumped laser material is partially absorbing; this eect is particularly strong in quasi-three-level lasers, and has been termed aperture guiding. Despite the high eciency of many microchip lasers, there will always be some ``waste'' heat associated with the pumping process. This will result in a temperature distribution across the microchip, and an associated distributed thermal lens through dn/dT eects. The temperature distribution can also give rise to thermal expansion, and the production of eectively a curved mirror on the input surface. A positive result of these eects is that they are automatically associated with the gain region. These lasers are almost self-aligning. In the remainder of this section we shall explore the relative importance of these eects. Zayhowski [17] was the ®rst to show that in Nd:YAG microchip lasers the predominant guiding mechanism was a distributed thermal lens. In these systems, which have a relatively low absorption coecient for the pump light, the heat is deposited fairly uniformly along the microchip laser cavity. The chip is cooled at its edges, and this induces a temperature distribution from the
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centre of the pump beam to the edge of the chip. Cousins [18] has shown that if the pump pro®le can be modelled as a ``top-hat'', the temperature in the pumped region is parabolic, and outside the pumped region it falls o logarithmically. Zayhowski postulates that the resultant refractive index distribution can act as a weak waveguide. In Nd:YAG the thermo-optic coecient (dn/dT) is positive, so the refractive index distribution mirrors that of the temperature. Even if the microchip laser beam falls outside the pumped region, the gradual change in refractive index there can still result in index-guided waveguide operation. Zayhowski's experimental studies of beam waist against pump power, and of beam waist against pump spot radius, matched what he stated would be expected from this theory. In more strongly absorbing materials, such as lithium neodymium tetraphosphate (LNP), the thermal load at the pumped surface can be much greater. In this particular material dn/dT is negative, so any distributed lens of the type described above will act against cavity stability. In this case, only gain guiding and surface deformation are able to act to provide cavity stability. MacKinnon and Sinclair [19] used Fizeau interferometry to explore the change in optical thickness of pumped LNP microchip lasers, and found that these changes matched well with what was expected from the beam divergence of the microchip laser. The pumped surface bulged, and this bulge was su-
ciently close to a spherical surface over the dimensions of the strongly localised microchip laser beam that a diraction limited beam could be produced. In this case, the surface deformation was the predominant mechanism for cavity stability. More recently we have explored the operating characteristics of Nd:YVO4 . This material has a positive thermo-optic coecient and a high absorption coecient. In this case we would expect positive contributions from both mechanisms. A Fizeau interferometer was set up as shown in Fig. 2. The light re¯ected from the two chip surfaces interfered in the image plane, to produce fringes of equal optical thickness, which were examined using a CCD camera, frame grabber, and computer. Full details of this study will be published elsewhere, but an example of the results from a 0.25 mm thick 3% doped Nd:YVO4 chip pumped with 450 mW of diode-laser radiation are shown in Fig. 3. In order to ®nd the eective size of the pumped region, the 1.06 lm ¯uorescence of a non-lasing chipset was measured looking into the pump-beam direction. Integrated in this way along the line of sight, the smallest ¯uorescent spot sizes in the two directions were about 30 and 55 lm full width at half maximum (FWHM) intensity. However, if the chip was taken only a little (0.1 mm) away from the position of maximum pump intensity, a roughly circular ¯uorescent spot size of about 55±60 lm FWHM was observed.
Fig. 2. Fizeau interferometer used to analyse the optical thickness of a Nd:YVO4 microchip under lasing conditions. Helium±Neon (633 nm) or green microchip (532 nm) lasers were separately used to illuminate the interferometer. The laser light was usually arranged to have its polarisation parallel to a crystal axis in the microchip.
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221
Fig. 3. Results of the Fizeau interferometry for a 0.25 mm thick 3% doped Nd:YVO4 microchip laser pumped by 450 mW of diode laser radiation. Part (a) shows the calculated change in optical thickness of the chip as a function of position across the chip, but as this part of the analysis has not taken account of the direction of change, the central dip should be considered as being re¯ected into the continuation of a wider central peak. Part (b) shows this change in optical thickness as a function of the square of the distance from the centre of the pump-induced deformation.
The fringe intensity across the centre part of the chip is monitored, and from this the change in optical thickness of the chip is calculated, as shown in part (a) of the ®gure. If the chip was acting as a spherical lens, one would expect a change in optical thickness that varied linearly with the square of the distance R from the centre of the bulge. Part (b) of the ®gure shows such a plot, and additionally has marked on the values for R2 at which R 20 and 30 lm. Over this range, it can be seen that the relationship is close to being linear, indicating a change in optical thickness consistent with a low-aberration spherical lens. Taking least-squares ®ts to this data indicates an expected microchip beam waist of 24.7 or 23.7 lm depending on whether the gradient is taken across 30 or 20 lm. The far ®eld pro®le of the beam was measured using a photodiode array or a CCD camera, and from this the beam waist was calculated to be 22 lm. The good agreement between this ®gure and that expected from the interferometry suggests that the thermal lens and deformation model for this system is a good one, at least at this pump power and laser-frequency. Gain-related eects have been explored extensively theoretically due to an interest in spatiotemporal pattern formation, amongst other things [20]. As mentioned above, the gain-related eects can be classed into two groups, those associated with guiding due to the integrated gain being higher in the forward direction, and those associ-
ated with the change in refractive index due to the presence of the population inversion [21]. The ®rst group will often give a positive contribution to eective cavity stability [22,23]. A variant of this is in aperture guiding, in which quasi-three level lasers show signi®cant loss in unpumped gain material. Fan [24] demonstrated that this was the predominant guiding mechanism in Yb lasers, for example. His calculations showed that the guiding eects in his laser were very much stronger than those that would be expected from an extrapolation of Zayhowski's results for thermal guiding in Nd:YAG. The change in refractive index due to the presence of a population inversion is shown schematically in Fig. 4. The refractive index perturbation depends on the frequency of the mode, and the magnitude of the population inversion [22]. For lasers near threshold one would expect the population inversion to be higher in the centre of the pumped region than at the edges, and thus to see a variation in refractive index across the pumped region. Given the variation of refractive index shown in Fig. 4, one can see that this will give rise to guiding eects for frequencies higher than that of the gain peak, and antiguiding for frequencies less than the gain peak. Longhi et al. [22] have shown experimentally this frequency dependence in a 700 lm long Nd:YVO4 microchip laser pumped by a Ti:sapphire laser. They operated the microchip laser near threshold, and examined the
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Fig. 4. The variation in refractive index associated with gain.
variation in cavity mode size with mode frequency and pump spot size. The transverse mode size associated with the dierent frequency modes agreed with the analysis based on frequency and gain dependent refractive index variations. Both the St. Andrews [19] and Lincoln Lab [17] groups considered how important gain-related effects might be in their Nd lasers when pumped far above threshold. Both determined that at the relatively high pump powers that they were normally using, the thermal eects dominated. This was checked experimentally using a chopped pump beam. As gain-related eects should depend on the instantaneous pump power, but thermal eects on a time-averaged pump power, both sets of workers made a comparison between true cw operation and pulsed operation with the same average power. At high pump powers no dierence was seen, suggesting that the thermal eects dominated. However, the St. Andrews group did see evidence of gain-related eects becoming more important at lower average powers [25]. The pump beam was chopped with a variable duty cycle. The pump power that was needed during the ``on'' part of the cycle to ensure TEM00 operation was measured as the average incident power was varied (by changing the duty cycle of the chopping). If the cavity stability mechanisms were dependent solely on gain-related eects, this graph would be a hori-
zontal line. If, however, the eects were purely thermal, the required instantaneous pump power would vary with the duty cycle, and in particular would become very large as the average incident pump power went to zero. Neither of these is seen to happen in Fig. 5. This means that gain-related eects are becoming important at the low average powers, but that longer-term, thermal eects are important at the higher average powers. We have also seen evidence of what we believe are gain-related self-Q-switching eects in Nd:YVO4 lasers [26]. Relaxation oscillations and spiking are well known in solid state lasers, and can occur when a second mode starts to oscillate in the cavity. For some lasers, we saw not a conventional train of relaxation oscillations when a second mode came above threshold, but a very large spike in power, followed by a train of more conventional relaxation oscillations. We believe that this is associated with the second mode, which is at a lower frequency than the ®rst mode, being antiguided by the gain in the cavity. This antiguiding will be particularly strong at the nodes of the ®eld of mode one. This results in the width of mode 2 being rather greater than in a cavity with no gain-related changes in refractive index, thus pushing up the threshold of this mode further than expected. However, once mode two does reach threshold, and starts what would otherwise be a spiking train, the gain is saturated, and the antiguiding rapidly becomes much less. This means that mode two is now much further above
Fig. 5. A modulated diode pump source was used to power the solid state laser. The amount of power required during the ``on'' part of the cycle to ensure TEM00 operation of the Nd laser was recorded as a function of the average diode power.
B.D. Sinclair / Optical Materials 11 (1999) 217±233
threshold than would otherwise be expected, and a form of Q-switched pulse is produced. This has a peak power of up to some 28 W, compared to the cw level of 11 mW, and a pulse width down to 1.85 ns. Our analysis, which takes into account the size of the pump beam and laser modes, the eects of gain saturation, and the gain-related changes in refractive index gives good agreement with theory. The eects of gain guiding obviously depend strongly on the frequency of operation, and the nature of any lensing eect will depend on the exact shape of the saturated gain distribution. These are areas of continued investigation. This section has given some indication of the involved process of cavity stability in microchip lasers operating at their fundamental frequency. The mode size will depend on the pump geometry, the cavity losses, the frequency of the laser mode, and the polarisation of the laser mode. Equally, the frequency of the laser mode will depend on temperature, which in turn depends on the pump power. The mode size is important for the operation of the fundamental microchip laser, but becomes even more so for the frequency-doubled microchip laser, as will be described in the following section.
223
4. Intra-cavity-doubled microchip lasers Our group has shown that intra-cavity-doubled microchip lasers work well. We believe that we understand many of the processes that allow these lasers to operate in a stable and almost selfaligning manner. However, the physics of their operation is complex, as can be seen from a diagram showing some of the interdependence of various parameters that eect the second harmonic generation process, reproduced here as Fig. 6. The second harmonic generation process relies on an intense fundamental ®eld of appropriate polarisation passing through the nonlinear crystal. The greater the circulating intra-cavity power, the greater is the conversion eciency. Thus the linear losses in the cavity are vitally important in their role in determining the circulating power in the laser. Equally, the cavity stability mechanisms will de®ne the size of the circulating mode, and thus the intensity of the circulating beam. Some aspects of the physics of this process can be studied by using a very simple plane wave model of intra-cavity second harmonic generation [27]. While we should not expect this to model accurately every aspect of the laser operation, this model should at least guide us in our under-
Fig. 6. Some of the factors aecting the operation of the intra-cavity-doubled microchip laser, and some of their interdependencies. The complicated inter-relationship is readily seen.
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standing and in our optimisation processes. As in any other laser operating in steady state conditions, the round trip saturated gain of the intracavity-doubled laser must equal the round trip losses. These losses will, however, consist of both linear and nonlinear terms. For a standing wave cavity, in which the eects of spatial-hole burning are ignored, we may write 2g0 lg L ÿ ln R1 R2 2KI 1 2I=Is in which we are equating the saturated gain to the sum of parasitic losses L, the losses due to output coupling of the fundamental ®eld through mirror coatings of re¯ectivity R1 and R2 , and nonlinear losses due to the generation of the second harmonic. In this equation I is the intensity of the ®eld propagating in one direction, Is the saturation intensity of the material, g0 the small signal gain coecient, and lg the length of the gain medium. The amount of power converted to the second harmonic is dependent on the ®eld intensity I and a constant K that depends on the parameters of the nonlinear crystal. In particular, K is proportional to the square of both the length of the nonlinear crystal and the eective nonlinear coecient. This very simple model allows us to explore how sensitive the laser is to losses, second harmonic generation crystal length, the nonlinear coecient, and other parameters. One example of the comparison of the model and experiment is
shown in Fig. 7. Here we model the diode as providing a top-hat pump pro®le of 55 lm radius, and the laser mode is assumed to have an equivalent top-hat pro®le of the same size. These are obviously relatively crude approximations. In this case, with a 0.2% linear loss, a 0.1% transmission of the output coupler to the fundamental, and a 2 mm long KTP crystal, the model predicts the IR and green output powers shown in the part (a) of the ®gure. The form and magnitude of these curves are similar to those obtained experimentally, and displayed in part (b) of the ®gure. Having veri®ed that the model works reasonably, we then show how it can be used to investigate the role of losses and nonlinear coupling. Fig. 8 shows the calculated IR and green powers for a 1 W diode pump as a function of intracavity losses. The two graphs show the results for a 99.9% re¯ecting output coupler and a 99.5% output coupler. Not surprisingly, both graphs show the powers dropping o rapidly with increasing linear losses. However, in the 99.9% re¯ecting case in particular, it can be seen that the green output power drops o much more rapidly than the IR power, due to the squared dependence on the circulating intensity. At the lowest loss levels, the green output is considerably higher than in the IR, which is generally what would be desired. If the output coupling of the IR is increased, we see the IR output power rise, but the green power fall catastrophically. These graphs indicate what one
Fig. 7. Theoretical and experimental curves showing the IR and green output from an idealised microchip laser consisting of 0.5 mm of Nd gain material and 2 mm of KTP. The plane wave approximation was used for the calculations, and the assumptions of a 0.2% linear loss and a 0.1% transmission of the output coupler were made.
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225
Fig. 8. Calculated IR and Green output from the idealised microchip laser described in Fig. 7 and in the text. The two graphs show the result of changing the parasitic (linear) losses in the cavity, with the two graphs calculated for dierent output coupler re¯ectivities in the IR.
might reasonably expect, that it is important to keep intra-cavity losses low, but perhaps more importantly they also indicate the magnitudes of loss that may be tolerated. The length of the frequency-doubled microchip laser is an important parameter. The mode size of the laser depends on its length (for a given thermal load), and the intra-cavity mode spacing will depend on the laser length. Generally both these characteristics are improved if the laser is made short. However, as the laser is made up entirely of the gain material and the nonlinear crystal, any shortening of the laser cavity requires shortening of one or other of these crystals. The gain crystal requires to be long enough to absorb a reasonable fraction of the incident pump power, and also to be long enough to be readily handled. Quarter millimetre thick slices are the shortest that we have found practical. The longer is the nonlinear crystal, the greater the conversion will be from the IR to the green. However, it can be shown that there is an optimum length of crystal, depending on the laser parameters [27]. This is generally longer than we have used, and given the other deleterious effects of lengthening the cavity, it is an important question as to how short a nonlinear crystal can be used without seriously compromising the conversion eciency. In an ideal laser, with no linear
losses, perfect re¯ectors of the fundamental, and a ®nite nonlinearity in the nonlinear crystal, the crystal can be made arbitrarily small. Fig. 9 shows the eects of changing crystal length with a real laser (note that these calculations do not take into account the changes of mode size with laser length, or any length dependent attenuation within the doubling crystal). Part (a) shows that for a relatively lossy laser (0.4%), there is indeed a strong dependence on nonlinear crystal length. However, part (b) shows that if the losses are kept small (here 0.1%), as long as the nonlinear crystal is at least 2 mm long, there is little to be gained from going to a greater length. This shows one of the many advantages of the microchip laser geometry. The naturally short laser has the potential to be built as a low-loss device. If this potential is realised with good coatings forming the resonator, then even with relatively short (and therefore less expensive) nonlinear crystals good second harmonic generation eciencies can be achieved. This simple model shows that in the selection of gain and nonlinear materials one should be looking for materials that have low attenuation, and high gain and nonlinear coecients as appropriate. It is useful to take the ideas above and apply them to a choice of materials for frequency-dou-
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Fig. 9. Calculations of the expected IR and green output powers from the idealised microchip laser described in Fig. 7, as a function of the length of the nonlinear crystal. These calculations were made for a 1 W pump, and for coatings of the two mirrors that are respectively 100% and 99.9% re¯ecting at 1064 nm. The two graphs are for dierent values of parasitic (linear) loss in the cavity.
bled microchip lasers. In addition to the parameters above, one wishes to have a gain material which has · a suciently strong absorption coecient to allow strong absorption of the pump in a short length; · thermal properties appropriate for forming a tightly con®ned single transverse mode at pump power levels of interest;
· a polarisation-dependent gain in order to attempt to drive the laser into a polarisation suitable for frequency doubling; · a narrow emission bandwidth for reducing the number of oscillating modes, ideally to one. Table 1 gives parameters for a few of the more promising materials for green microchip lasers. The loss parameters for the materials listed are all small, and will all give rise to losses of less than
Table 1 Comparison of some possible hosts for the Nd 1.06 lm laser Nd:YAG
1% Nd:YVO4 E||z
3% Nd:YVO4 E||z
10% LSB E||x
Eective stimulated emission cross section r (10ÿ19 cm2 )
3.3
16
16
1.3
Upper state lifetime s (ls)
230
97
50
118
rs product (10ÿ23 cm2 s)
7.6
9.5
8
1.5
Emission line width (nm)
0.6
0.8
0.8
4
Absorption coecient (cmÿ1 )
12
40
110
36
Absorption line width (nm)
0.8
8
8
3
0.001
0.04 0.011
0.04
Losses (cmÿ1 )
0.0003
Data from Zayhowski and Harrison [28], Meyn and Huber [15], Koechner [29], Huber [30], and ITI datasheets [31]. There is some variation between sources; in particular, the loss ®gures of 0.04 are quoted by the ITI datasheets, with the additional (lower) ®gures from Huber.
B.D. Sinclair / Optical Materials 11 (1999) 217±233
0.06% for a 0.25 mm thick slice. At these levels it is likely that other loss mechanisms in the cavity will dominate the overall cavity loss. Vanadate and LSB have the advantage over YAG of a high absorption coecient and a polarised gain spectrum, and vanadate has the advantage over LSB of a greater rs product. For doubling of the 1064 nm transition KTP is currently the material of choice. It has a high nonlinear coecient, it has low losses, and has high angular and temperature acceptance tolerances. The sandwich of Nd:YVO4 and KTP form a short cavity, which keeps linear losses low, and aids single-frequency operation. The short length of KTP that is needed ensures high temperature bandwidths for the doubling process, and may contribute to the ease of operation without evidence of the green problem. Fig. 10 shows the results of calculations based on the above simple model, where the green output power is plotted as a function of pump power and linear loss in the cavity. Also shown are the results for similar calculations for our blue and red lasers. These lasers are based on similar principles, but use in the red laser the 1342 nm transition in Nd:YVO4 and a non-critically phase matched LBO crystal, and in the blue laser the 946 nm transition in Nd:YAG and a critically phase matched crystal of KNbO3 . The relatively low gain
227
and low nonlinear coecient in the red laser give rise to a much greater sensitivity to linear losses than in the green laser. The blue laser seems to be predicted to work very well, as the KNbO3 has a high nonlinear coecient. However, the 946 nm transition suers from reabsorption loss, and there are other technical problems which reduce its effectiveness, as will be outlined below. In the above discussion no account has been taken of the issues of phase matching and the polarisation states of the modes within the cavity. In the green laser, in which type II phase matching is used in KTP, both the gain and nonlinear materials are birefringent, and the gain in Nd:YVO4 is strongly polarisation dependent. Our studies of the polarisation states of the dierent longitudinal modes have shown good agreement between experiment and a theory based on Jones matrices. At certain temperatures the birefringence can be such that the circulating IR ®eld for at least one longitudinal mode can be optimally polarised for the gain material and optimally polarised for the frequency doubling process. Under other conditions this may not be the case, and the eciency of the device can be reduced. The polarisation of the dierent modes is also important in regard to their sum-frequency generation and the impact this has on ``green noise''. These issues are currently still
Fig. 10. Comparison of the expected idealised performance of the blue, green, and red lasers, as described in the text. The contours show lines of equal power output. The 25 mW output power contour is labelled as such, and the 100 mW contour is distinguished by the thick line. The contour intervals are 25 mW. The wave-like nature of the curves for the red laser are due to the interpolation routines used.
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under investigation and will be covered in forthcoming publications. However, the sensitivity of the laser to temperature is not great, and in a laboratory situation the lasers can be aligned to be ``quiet'' at set pump-power levels with no temperature stabilisation at all. The green laser has provided up to 158 mW of green output when pumped by a 1.2 W diode, as shown in Fig. 11(a). In this case the diode-laser radiation was collimated and refocussed using 8 mm focal length lenses. This created a tightly con®ned population inversion. We monitored the 1064 nm ¯uorescence on a non-lasing chip using a CCD camera, and found, under the conditions that had given maximum 1064 nm laser output, a ¯uorescent spot size (integrated through the thickness of the laser crystal) of 50 ´ 110 lm (full width). Fig. 11(a) gives the appearance of a smooth power in/out curve, but in practice this is not always the case. Fig. 11(b) shows a similar relationship, but with continuous monitoring of power levels. Large jumps in the IR and green power can be seen, which are associated with major changes in the operating characteristics of the laser. Fig. 6 gave an indication of the coupled processes that go to determining the operation of the laser. It does not seem unreasonable to expect that these coupled and sometimes nonlinear relationships will lead to bistable and similar eects in the laser operation. This complicated form of the power out/in curve means that this type of laser is best suited for a particular power level, and is likely to be less suited to applications where a varying green power level is required through direct modulation of the laser diode. The green noise of these lasers can be remarkably small. We routinely see the intensity noise as low as 0.5%, even when more than one longitudinal mode is oscillating. We postulate that the low levels of noise are due to the relatively small coupling between the longitudinal modes, which is in turn due to two reasons. The ®rst reason is that the short length of the cavity often leads to one longitudinal mode being of much greater intensity than a second subsidiary mode. The second reason is that the KTP is short, and thus provides relatively little nonlinear coupling between the modes.
In conditions where there are two modes that do generate a strong sum-frequency wave, we can see green noise producing an intensity modulation of up to 150% at frequencies from 150 kHz upwards. Fortunately, it is not dicult with most chipsets to ®nd broad regions of intensity stability. Another major potential source of intensity noise in solid state lasers is relaxation oscillations. These can be stimulated by ¯uctuations in the cavity loss or gain. The monolithic and stable nature of the frequency-doubled microchip laser means that the former mechanism is suppressed, and the latter can be minimised by using a stable diode laser pump source. The inclusion of a frequency doubling crystal within the cavity also acts to reduce the magnitude of relaxation oscillations [32]. Our red microchip lasers are based on the 1342 nm transition in Nd:YVO4 . The 1.3 lm transition in vanadate is considerably stronger than in YAG, and also has the signi®cant advantage of being strongly polarisation dependent. The rs product is, however, still much less than at 1.06 lm. A summary of some of the relevant properties of these two materials at the two wavelengths is given as Table 2. A 1.3 lm device was constructed in the form of a monolithic resonator made of a 0.25 mm long 3% doped Nd:YVO4 slice, with 5% output coupling, and pumped with a 1.2 W laser. This generated up to 105 mW of single-frequency radiation at this technologically interesting wavelength [33]. We then incorporated a 2-mm long type II noncritically phase matched crystal of LBO into a cavity consisting of a 0.5 mm thick slice of 1% doped Nd:YVO4 and a discrete output coupler to allow frequency doubling of the laser into the red. We were not in a position to create the usual allsolid cavity, but instead had to rely on antire¯ection coatings within the cavity. These, combined with a relatively poor coating on the output coupler, led to a relatively low circulating 1342 nm ®eld. When this was compounded with the relatively low conversion eciency of LBO, only 10 mW of red was coupled out of the laser, which was pumped with a 1.2 W diode. However, as our modelling shows, with improvements to the intracavity losses, signi®cantly higher red powers could be generated. Over much of the operating range
B.D. Sinclair / Optical Materials 11 (1999) 217±233
229
Fig. 11. Power output from green microchip lasers as a function of incident pump power. Part (a) shows the results from a chipset pumped with 1.2 W diode laser. A maximum green output power of 158 mW was obtained. Part (b) shows for a dierent chipset the output powers in the green and IR as the pump power is varied. This shows the jumpy nature of the output power obtained with some chipsets. This is not surprising, given the coupled nonlinearities that are partially described by Fig. 6.
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Table 2 Comparison of properties of the 1.06 and 1.3 lm transitions in Nd:YAG and Nd:YVO4 Nd:YAG
Nd:YVO4
1064 nm
1321 nm
1064 nm E||z
1342 nm
Eective stimulated emission cross section r (10ÿ19 cm2 )
3.3
0.7
16
7.6
Fluorescence lifetime s (ls)
230
230
100 (1%) 47 (2%)
100 (1%) 47 (2%)
2
cm s)
7.6
1.6
16 (1%) 7.5 (2%)
7.6 (1%) 3.6 (2%)
Emission line width (nm)
0.6
0.6
0.8
1.6 [8]
ÿ23
rs (10
Refractive index
1.82
1.96, 2.17
Pump absorption coecient (cmÿ1 )
12
41 (1%) 72 (2%)
Absorption bandwidth (nm)
1
8
Scatter losses (cmÿ1 )
0.003
0.04
Thermal conductivity (W cmÿ1 Kÿ1 )
0.14
0.05
Data from Zayhowski and Harrison [28], and/or ITI Datasheets [31] except where otherwise stated.
the laser gave an intensity stability of better than 1%, and reliably operated in a TEM00 mode. The 2 mm long slice of non-critically phase matched LBO had an acceptably wide temperature acceptance bandwidth of 30°. However, over this temperature range considerable changes take place in the polarisation states of the cavity modes. These gradual changes with LBO temperature of polarisation state of the fundamental resulted in a variation in the red output power. The second harmonic power was modulated with a period of 7°C due to these polarisation eects. A single-frequency red output was frequently obtained, even with the relatively extended cavity (20 mm) used. While many applications can be addressed by the green laser, a number of important processes have been developed for the blue line of the argonion laser. This wavelength also has the advantage of a smaller minimum spot size than in the green. For these reasons we investigated various options for the generation of blue light in the microchip laser geometry. Neodymium has a tolerably strong transition from the 4F3=2 to 4I9=2 state, which, when frequency doubled will be in the blue. The best known host for this laser is Nd:YAG, which has been used for a number of blue laser demonstrations. This laser is a quasi-three level laser, as the
lower laser level is only a few kT above the ground state, and is thus thermally populated. The advent of ecient diode laser pumping kindled interest in this transition in the 1980s [34], though we are not aware of the widespread use of these lasers. What advantages would come from microchip laser operation? A major one is that the low losses associated with this geometry would allow a shorter nonlinear crystal to be used, and thus would provide a wider temperature acceptance bandwidth than the larger crystals used in larger lasers. There will also be the usual microchip laser advantages of ease of production, small size, and so on. We performed a survey of some Nd host materials to evaluate the best options for this laser. While vanadate was attractive with its polarised gain spectrum, the lower laser level was particularly close to the ground state. We also had diculties in the procurement of coatings on the vanadate that were good transmitters at 809 nm and very good re¯ectors at the Nd laser wavelength. Instead, we chose to work with Nd:YAG, and used critically phase matched KNbO3 as the nonlinear medium, in the laser geometry sketched in Fig. 12. With a 1.2 W diode pump we achieved up to 33 mW of TEM00 blue output, as shown in Fig. 13. The short length of potassium niobate
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231
Fig. 12. Schematic of the blue microchip laser. The 1.5 mm thick KNbO3 crystal was cut for type I critical phase matching at a temperature of 45°C. The orientation of doubling crystal and the polarisations of the light produced are indicated. The 1.1% doped Nd:YAG crystal was 1 mm thick.
The polarisation state varied across the crystal set, and was optimised near particular lines across the crystal. We postulate that the joining process of the two materials has led to residual stress, and it is this that provides the necessary polarisation discrimination. In the regions of best polarisation discrimination we achieved in the region of 15±35 mW of blue generation, in other regions it dropped to 5±10 mW. These power levels are appropriate for many applications, though the presence of some intensity noise must be taken into account. 5. Conclusions Fig. 13. Performance of the blue microchip laser.
that was used allowed a tolerably broad temperature acceptance bandwidth of 2.2°C. It was a little surprising that this laser generated any blue light at all, as there would not appear to be any reason for the laser not to operate on a polarisation that would be incorrectly aligned for phase matched frequency doubling. However, the laser always chose to operate with at least some of the circulating 946 nm ®eld aligned in the correct direction.
Frequency-doubled cw microchip lasers are ef®cient, reliable, ultracompact, and readily manufacturable sources of quality laser radiation. We have reviewed the major aspects behind their physics of operation, and attempted to show how an understanding of this physics can lead to optimisation of these lasers. At a time when diode-laser pumped solid state lasers are starting to break into the high-power high-cost market, we see the microchip lasers established in the low-power, lowcost arena.
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Acknowledgements The research programme at the University of St. Andrews has been undertaken by a team that has included at dierent times Richard Conroy, Graham Friel, Alan Kemp, David Matthews, Neil MacKinnon, and the author. Their work has been supported by the UK SERC and EPSRC through grant numbers GR/F83341 and GR/K14766. The work has also bene®tted from the input of IE Optomech and Uniphase. References [1] G.J. Dixon, L.S. Lingvay, R.H. Jarman, Properties of close-coupled, monolithic, lithium neodymium tetraphosphate lasers, Proc. SPIE 1104 (1989) 107. [2] J.J. Zayhowski, A. Mooradian, Single frequency microchip lasers, Opt. Lett. 14 (1989) 24. [3] J.J. Zayhowski, Microchip lasers, Opt. Mater. 11 (1999) 255. [4] D.G. Matthews, R.S. Conroy, B.D. Sinclair, N. MacKinnon, Blue microchip laser fabricated from Nd:YAG and KNbO3 , Opt. Lett. 21 (1996) 198±200. [5] N. MacKinnon, B.D. Sinclair, A laser diode array pumped, Nd:YVO4 /KTP composite material microchip laser, Opt. Commun. 105 (1994) 183±187. [6] R.S. Conroy, A. Kemp, B.D. Sinclair, N. MacKinnon, Comparison of 671/1342 nm generation with 532/1064 nm in Nd:YVO4 microchip lasers, Paper CFO6, in: Conference on Lasers and Electo-Optics, Baltimore, May 1997. [7] M. Gitin, C. Seaton, Laser Focus World, November 1996, p. 153. [8] G. Mitchard, Laser Focus World, January 1997, p. 82. [9] For example: T. Sandrock, H. Scheife, E. Heumann, G. Huber, High-power continuous-wave upconversion ®ber laser at room temperature, Opt. Lett. 22 (1997) 808, and references therein. [10] T. Baer, Large amplitude ¯uctuations due to longitudinal mode coupling in diode-pumped intracavity-doubled Nd:YAG lasers, J. Opt. Soc. Am. B 3 (1986) 1175. [11] M. Oka, S. Kubota, Stable intracavity doubling of orthogonal linearly polarised modes in diode-pumped Nd:YAG lasers, Opt. Lett. 13 (1988) 905. [12] See, for example, S. Falter, K.M. Du, Y. Liao, M. Quade, J. Zhang, P. Loosen, R. Poprawe, Dynamics and stability of a laser system with second order nonlinearity, Opt. Lett. 22 (1997) 609; G.J. Dixon, Laser Focus World, April 1997, p. 85, and references therein. [13] H. Masuda, F. Maeda, M. Oka, Y. Kaneda, S. Kubota, Compact Blue-Green Laser Technical Digest, Santa Fe, 1992, 6FA-3-1, as reported in E.N. Eguchi, Y. Akiyama, Jpn. J. Appl. Phys. 32 (1993) 5307.
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