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High power single-mode cw dye laser with Michelson mode selector
OPTICS COMMUNICATIONS
Volume 37. number 2
15 April 1981
HIGH POWER SINGLE-MODE CW DYE LASER WITH MICHELSON MODE SELECTOR C.G. AMINOFF and M. KAIVOLA Helsinki Universityof Technology, Department of Technical Physics, SF-02150 Espoo 15, Finland Received 16 December 1980
The application of a Michelson mode selector for high power single-mode operation of a cw dye laser is reported. By employing the selector for compensation of spatial hole burning effects in a standing-wave cavity, a tunable single-mode output power of 1 W has been obtained at 590 nm. A double Michelson selector has also been applied with the spatial hole burning taken into account.
1. Introduction
Single-mode cw dye lasers are capable of providing high power in a very narrow line with a tunable frequency. To establish single-mode operation, several selective elements are, in general, inserted into the cavity. In linear laser configurations the mode selection is critical due to the standing-wave field pattern creating a non-uniform spatial distribution of the population inversion in the active medium [ 11. This effect, termed spatial hole burning, is known to favour simultaneous oscillation in several modes [2]. In conventional mode selection techniques, Fabry-Perot (FP) etalons of high finesse tend to reduce the conversion efficiency by significant insertion losses due to walkoff phenomena and energy storage effects [3,4]. On the other hand, insufficient selectivity will eventually result in a second mode reaching threshold when the pump power is increased. In travelling-wave ring lasers the absence of spatial hole burning allows homogeneous saturation, strong mode coupling and increased conversion efficiency [S] . Only low finesse methods are required for singlemode selection, allowing lower loss, high efficiency and very high output powers exceeding one watt [6]. The Michelson mode selector, although of low finesse, has been shown to be capable of single-mode selection in linear dye lasers [7]. Recently, the advantages of this method have been further exploited in the successful application of double and triple 0 030-40
18/8 1/OOOO-0000/$02.50
0 North-Holland
Michelson selectors (DM and TM, respectively) with increased selectivity for high power single-mode performance [8,9]. The insertion losses of these devices can be kept low, because i) the normal incidence operation eliminates walk-off loss; ii) the number of optical surfaces is reduced; iii) the energy inside the element is divided instead of being accumulated. Due to the enhanced selectivity of the TM, single-mode output powers in excess of 1 W have been obtained with a standing-wave cavity 191. This communication reports on the application of a Michelson mode selector for single-mode operation taking into account the spatial hole burning effects in a jet stream dye laser. Spatial hole burning has previously been exploited for mode selection based on FP type methods [lo] and thin absorbing films [ 111. It is our purpose to demonstrate that a single Michelson selector (SM), in combination with a birefringent filter, provides sufficient selectivity for standing-wave single-mode oscillation even at high pump powers. Due to the low insertion losses, the Michelson system is capable of supplying high single-mode power; output levels of 1 W are reported in this work. The application of a DM for compensation of spatial hole burning effects and for improved tuning properties is also demonstrated. 2. Spatial hole burning and Michelson mode klectors We consider a linear dye laser cavity with the jet Publishing Company
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Fig. 1. Schematic of the dye laser cavity with a Michelson mode selector. Mr is the outcoupling mirror; Mz, MS are high reflectors; BSr , BS? are beamsplitters; P is the tuning plate
and B is the three-plate birefringent filter. The dashed contours represent the double Michelson selector configuration.
stream at the distance lo from the cavity end mirror (see fig. 1.). The outcoupling end is formed by a Michelson interferometer operating as a selective reflector. The selector consists of a beam-splitter BS, and two mirrors Ml, M2, one of which is the output mirror (Ml) of the laser. In addition, the cavity contains a broadband filter B (e.g. a birefringent filter) for a coarse selection of the wavelength. In conventional jet stream cavities, the thickness of the active medium is much smaller than lo. The saturating effect of a single longitudinal standing-wave mode with frequency v1 will produce a spatial modulation of the population inversion in the jet. A second mode with frequency v2 may then interact with the pumped molecules in the nodal regions of the first mode. The field pattern of the second mode will be exactly “out of phase” in the pumped region at the frequency separation Av=lv1-v21=(~+~)~c/210,
n=0,1,2,...
(1)
where c is the velocity of light. Consequently, mode competition wilI be weak, favouring oscillation in these modes. On the other hand, the field pattern of a second mode at the frequency separation
Av=n*c/210,
n=0,1,2
,...
(2)
will be spatially coincident with the mode VI in the pumped region, resulting in strong mode coupling. In a single-mode situation, the excess gain available for a second mode will then have a modulation, as a function of Au, with the period c/21,. As soon as one of the maxima at (1) reaches threshold, a two-mode situation will result. The Michelson mode selector is convenient for in134
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traducing selective loss for compensation of the excess gain. The path difference formed by the two arms of the selector is designated e = IS + I, - I, 1(see fig. 1). We assume the SM selector to be adjusted to maximum reflectivity for the mode VI. Then the reflectivity of a second mode v2 will have a sinusoidal dependence on Au with the period c/2e. The compensation of excess gain will be efficient if all modes at the reflectivity maxima of the SM have a minimum of effective gain according to (2). This occurs if the path difference is chosen to be e=Eolm,
m = 1,2,3, ...
(3)
At these positions the selector allows oscillation only in modes strongly competing with each other. A slight spatial “dephasing” through the thickness of the pumped layer occurs for large Au, resulting in decreased mode coupling, but this effect is assumed to be compensated by the broadband selectivity. The broadband filter finally favours the selected mode vl, with single-mode oscillation as a result. The most efficient mode selection is achieved at the position m = 1 of (3) where the periods of the gain and loss curves are equal. The threshold for a second mode will then in practice be determined by the selectivity of the broadband filter and the compensation of the dephasing effect. In principle, the SM system will yield sufficient selectivity for single-mode operation even at high pump powers. The low insertion losses then allow high efficiency and a high output power. On account of the reduced FSR of the selector at large values of e, a somewhat limited frequency tuning range is however to be expected, without simultaneous tuning of the broadband filter. The double Michelson selector is capable of further increasing the selectivity and, in some cases, of raising the threshold for a second mode. A DM may be constructed by inserting a second beam-splitter BS2 with a mirror M, into the arm of the interferometer (the dashed contours in fig. 1). If the mirror M3 is at a distance I, from the plate BS2, then we can define two path differences for the DM: el=
Ist~(12+13)-11i,
e2 = I4 - 131 . When e2 is small compared with el , the two upper
(44 Cab)
arms in fig. 1 will form a selector yielding a reflectivity “envelope” curve of coarse or intermediate’selectivity . Considering the correct compensation of spatial hole burning effects in the gain, and taking into account the phase inversion occuring in the reflectivity function of the DM, we find that single-mode selection will take place for el=
[Zo*(n+$)-e2]/m,
m=1,3,...
(10fn*e2)/m,
m = 2,4, ...
(
15 April 1981
OPTICS COMMUNICATIONS
Volume 37. number 2
(5)
withn=O, 1,2,... More details will be found in [ 121. Thus the positions (3) will acquire a split structure for e2 having values of typically a few millimeters, one finds that the most efficient selectivity, in general, is achieved for a small e2, typically e2 < 1 mm for the standard dye laser design, and with el = Zo [ 121. With servocoupling of the selector mirrors to the cavity mode frequency [8], the DM provides a tunable broadband selection curve which extends the continuous tuning range.
3. Experimental results 3. I. Single Michelson selector
A Michelson selector was designed for a commercial jet stream dye laser cavity (SP 375) of the type shown in fig. 1. An Ar+ laser (SP 17 l-06) provided the pump beam. As a broadband filter we used three birefringent plates of thickness ratios 1: 4 : 16, with the thickest plate measuring 6.4 mm. The birefringent filter alone provided a laser linewidth of 25 GHz. The SM contained a 50% reflecting beam-splitter at 45’ incidence with an AR coated rear surface. The selector was mounted on the output plate of the cavity. An output mirror transmission of 14% was used (no optimization of this parameter was attempted). The high reflectivity mirror of the SM was servocoupled to the laser mode [8] for tuning and stabilization of thermal drift. The dye used was rhodamine 6G dissolved in ethylene glycol. Three scanning FP spectrum analyzers of different FSR’s were applied for monitoring the laser spectrum. A calibrated calorimeter was used for power measurements. The thickness of the jet at the orifice of the nozzle was 0.2 mm and somewhat smaller at the beam axis. The effective thickness of the pumped region in the
dye depends, however, on the crossing angle between the non-collinear pump and dye laser beams as well as on the beam waists and pump power. The distance Z,, between the jet and the cavity end mirror was close to 50 mm in this cavity. The output mirror MI was mounted on a micrometer translator for distance measurement. By varying the path difference e of the SM, we found stable single-mode operation at distinct values of e approximately at 50 mm and 25 mm, corresponding to m = 1,2 in (3). Because of geometrical inconvenience, other positions of (3) were not systematically explored. The most efficient selectivity was found at e = 50 mm. Pumping with the line 5 14 nm yielded a single-mode threshold of 0.9 W at the dye laser wavelength 590 nm. The maximum one-line pump power available was 5.5 W, providing a singlemode output of 620 mW at 590 run. The single-mode power ate = 50 mm for one-line pumping is plotted in fig. 2, curve (a). The average slope efficiency is about 13.5%. Pumping with all lines raised the threshold to 1.2 W but provided a maximum of more than 12 W in available pump power. The single-mode output with all-lines pumping is plotted as curve (b) in fig. 2. A power of 1 W in the main output beam was measured for 11 W of pump power. The slope efficiency stayed below the one-line value and dropped significantly at I
I
1
I
1
,
I
0
2
L
6
8 POWER
IO (WI
12
1
1.2-
’
PUMP
Fig. 2. The single-mode dye laser power measured in the main output beam at 590 nm as a function of pump power. (a) One-line pumping (5 14 nm), single Michelson mode selector with path difference e = 50 mm; (b) all-lines pumping, single Michelson mode selector, same e; (c) all-lines pumping, double Michelson mode selector with path differences el = 48.5 mm and e2 = 3 mm.
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high powers, mainly due to serious thermal lensing phenomena in the jet at these pump levels [ 131. The experimental limit was thus set by the available pump power and thermal effects rather than by the secondmode threshold. No reoptimization of the pump beam focusing at each power level was attempted. Particular care had to be taken to suppress higher order transverse mode appearing with increasing pump power. These modes were partly due to the mediocre quality of the jet surface (the pump beam remained in the TEM,, mode). Higher order modes were eliminated either by beam readjustment or by using an aperture in front of the end mirror. Thereby the fundamental mode was also affected by additional loss, which contributed to the efficiency drop. The power leakage in the secondary beam through the beam-splitter increased from about 10% of the output power near threshold to about 30% at high power (300 mW at the output of 1 W). This was probably a consequence of thermal deformations of coatings and mode shape. The fairly large path difference in the selector did not appear to introduce any significant additional loss caused by mode matching problems (cf. [9]). Appropriate mirror curvatures for mode matching in the SM could, however, reduce diffraction losses [8]. Note that the one-line slope efficiency increases to about 16% and the maximum output to 1.3 W with the inclusion of the leakage beam, which is available for experiments. The effective linewidth of the free-running mode, determined by jitter, was about 10 MHz at intermediate powers. For improved efficiency at high powers, the dye jet velocity was increased, which resulted in frequent air bubbles in the jet. As no efficient air bubble filtering was applied, the effective linewidth and frequency stability deteriorated. At the position e = 25 mm, a maximum all-lines power of 4 W could be applied in a preliminary experiment on single-mode operation, providing 300 mW of output power. We performed tuning of the frequency by tilting a galvanometer controlled intra-cavity glass plate inclined at Brewster’s angle (cf. fig. 1). With the SM at e = 50 mm, one would expect the tuning range to be limited to the FSR of the selector, viz. 3 GHz, without simultaneous tuning of the birefringent filter. By taking account of the cavity mode spacing (346 MHz) and the “vernier” effect, this range can however be 136
15 April 1981
further extended. Thus, the single-mode frequency was continuously tunable over 12 GHz. This range was approximately the excursion limit of the piezoelectric mirror translator in the SM. A measurement of the single-mode power as a function of wavelength for a constant pump level reproduced the gain profile of the dye. 3.2. Double Michelson selector A DM was applied to the cavity by inserting another 50% reflecting beam-splitter and a mirror in the selector, according to fig. 1. Appropriate servo-control of mirrors was used [8]. Testing the mode selection at moderate power for e2 = 3 mm with el close to 50 mm, we found single-mode operation at the distinct points el = 50 f (n + 0.5)*3 mm for n = 0, 1,2, and closeto25mmatel=25~n~1.5mmforn=0,1, 2,3, in close agreement with (5). The most selective positions were those for n = 0. We did not observe any significant improvement in selectivity compared with the SM result at available pump powers. For er close to 50 mm the limit was again set by the applicable pump power. The single-mode power results for the DM at er = 48.5 mm and e2 = 3 mm are represented by the curve (c) of fig. 2. Some additional loss is introduced by the second beam-splitter and leakage beam. This raised the threshold to 1.3 W (all lines) and reduced the efficiency slightly (about 8% lower output), but the 1 W output level was reached for about 12 W of all-lines power in the pump beam. In preliminary attempts at el = 25 mm, no observable increase in the second-mode threshold was found, suggesting insufficient narrowband selectivity at that path difference. The tuning stability and range did, however, improve with the DM. To overcome the tuning limit of the mirror translators, the selector can be adjusted for single-mode operation at some smaller el according to (5) at the expense of selectivity and power. For approximately el = lo/4 and at low pump power, the DM system allowed continuous tuning over 50 GHz.
4. Discussion When the path difference e = 50 mm is employed in the Michelson selector, a continuous scan of the
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OPTICS COMMUNICATIONS
cavity mode frequency over 10 GHz requires an excursion of 1 pm for the mirror M2. Thus, rather long piezoceramics are needed for extended scans, but a thin galvanometer plate in the selector arm could also be applied for this purpose. The Michelson systems are also well adapted to pressure tuning [8], amethod that eliminates the need for large mirror excursions when a compensating glass plate is used in one selector arm. As the selectivity of the Michelson device does not appear to determine the practical pumping limit, improvements of the single-mode efficiency and power seem possible. Defocusing of the pump beam may reduce heating effects in the jet (cf. [9]). The thermal properties of water-based solvents and high-velocity jets are known to be well adapted to high power pumping [ 141. An optimized cavity configuration and a betterjet stream quality could improve on transverse mode stability. DM selectors with beam-splitters at Brewster’s angle for reduced reflection loss have also been designed [8]. In previous applications of DM and TM selectors [8,9] the path difference el = Zo/2 has been exploited. As the TM provided enhanced selectivity, the secondmode threshold for that value of el was not encountered for the powers applied. The results of the present work were not obtained in the optimal conditions for high power efficiency, but they demonstrate that the selectivity of the Michelson selector is considerably enhanced by the adequate exploitation of spatial hole burning effects, Sufficient selectivity for single-mode operation even at high pump powers is obtained already with a single Michelson selector in conjunction with a birefrmgent filter. The low losses of this system allow the extraction of very high tunable frequency single-mode powers from linear dye lasers. The application of the DM further improves the tuning range and mode stability. Due to simple design, low cost, and easy applicability to conventional dye laser cavities, the Michelson systems may provide an interesting alternative to sophisticated ring lasers for high power singlemode requirements.
15 April 1981
Acknowledgements The authors are indebted to Prof. E. Byckling for his generous and continuing interest. This work has been supported by the Academy of Finland.
References [l] C.L. Tang, H. Statz and G. dehlars, J. Appl. Phys. 34 (1963) 2289. [2] K.KaufmannandW.Weidlich,Z.Phys. 217 (1968) 113; J.M. Green, J.P. Hohimer and F.K. Tittel, Optics Comm. 7 (1973) 349; C.T. Pike, Optics Comm. 10 (1974) 14. [ 31 W.R. Leeb, Appl. Phys. 6 (1975) 267. [4] D. Frllhlich, L. Stem, H.W. Schrtlder and H. Welling, Appl. Phys. 11 (1976) 97. [S] G. Marowsky and K. Kaufmann, IEEE J. Quantum Electron. QE-12 (1976) 207. [6] H.W. Schr8der, L. Stein, D. Frtlhlich, B. Fugger and H. Wellii, Appl. Phys. 14 (1977) 377; SM. Jarrett and J.F. Young, Optics Lett. 4 (1979) 176; T.F. Johnston, Jr. and W.P. Proffitt, in: Laser spectroscopy IV, Proc. Fourth Intern. Conf. on Laser spectroscopy, eds. H. Walter and K.W. Rothe (Sprhxger Verlag, Berlin, Heidelberg, New York, 1979) p. 649. [7] S. Liberman and J. Phrard, Appl. Phys. Lett. 24 (1974) 142. [8] M. Pinard, C.G. Aminoff and F. Lalog, Appl. Phys. 15 (1978) 371. [9] M. Pinard, M. Leduc, G. Tr&xc, C.G. Aminoff and F. Lalog, Appl. Phys. 19 (1979) 399. [lo] H.W. Schrbder, H. Dux and H. Welling, Appl. Phys. 7 (1975) 21. [ll] U.N. Beterov, Yu.M. Kirin and B.Ya. Yurshhr, Optics Comm. 13 (1975) 238. [ 121 C.G. Aminoff and M. Kaivola, report TKK-F-A430 (1980), Helsinki University of Technology (to be published). [ 131 B. WelIegehausen, L. Laepple and H. Welling, Appl. Phys. 6 (1975) 335. [ 141 S. Leutwyler, E. Schumacher and L. W8ste, Optics Comm. 19 (1976) 197.
137
Volume 37. number 2
15 April 1981
HIGH POWER SINGLE-MODE CW DYE LASER WITH MICHELSON MODE SELECTOR C.G. AMINOFF and M. KAIVOLA Helsinki Universityof Technology, Department of Technical Physics, SF-02150 Espoo 15, Finland Received 16 December 1980
The application of a Michelson mode selector for high power single-mode operation of a cw dye laser is reported. By employing the selector for compensation of spatial hole burning effects in a standing-wave cavity, a tunable single-mode output power of 1 W has been obtained at 590 nm. A double Michelson selector has also been applied with the spatial hole burning taken into account.
1. Introduction
Single-mode cw dye lasers are capable of providing high power in a very narrow line with a tunable frequency. To establish single-mode operation, several selective elements are, in general, inserted into the cavity. In linear laser configurations the mode selection is critical due to the standing-wave field pattern creating a non-uniform spatial distribution of the population inversion in the active medium [ 11. This effect, termed spatial hole burning, is known to favour simultaneous oscillation in several modes [2]. In conventional mode selection techniques, Fabry-Perot (FP) etalons of high finesse tend to reduce the conversion efficiency by significant insertion losses due to walkoff phenomena and energy storage effects [3,4]. On the other hand, insufficient selectivity will eventually result in a second mode reaching threshold when the pump power is increased. In travelling-wave ring lasers the absence of spatial hole burning allows homogeneous saturation, strong mode coupling and increased conversion efficiency [S] . Only low finesse methods are required for singlemode selection, allowing lower loss, high efficiency and very high output powers exceeding one watt [6]. The Michelson mode selector, although of low finesse, has been shown to be capable of single-mode selection in linear dye lasers [7]. Recently, the advantages of this method have been further exploited in the successful application of double and triple 0 030-40
18/8 1/OOOO-0000/$02.50
0 North-Holland
Michelson selectors (DM and TM, respectively) with increased selectivity for high power single-mode performance [8,9]. The insertion losses of these devices can be kept low, because i) the normal incidence operation eliminates walk-off loss; ii) the number of optical surfaces is reduced; iii) the energy inside the element is divided instead of being accumulated. Due to the enhanced selectivity of the TM, single-mode output powers in excess of 1 W have been obtained with a standing-wave cavity 191. This communication reports on the application of a Michelson mode selector for single-mode operation taking into account the spatial hole burning effects in a jet stream dye laser. Spatial hole burning has previously been exploited for mode selection based on FP type methods [lo] and thin absorbing films [ 111. It is our purpose to demonstrate that a single Michelson selector (SM), in combination with a birefringent filter, provides sufficient selectivity for standing-wave single-mode oscillation even at high pump powers. Due to the low insertion losses, the Michelson system is capable of supplying high single-mode power; output levels of 1 W are reported in this work. The application of a DM for compensation of spatial hole burning effects and for improved tuning properties is also demonstrated. 2. Spatial hole burning and Michelson mode klectors We consider a linear dye laser cavity with the jet Publishing Company
133
outout
Fig. 1. Schematic of the dye laser cavity with a Michelson mode selector. Mr is the outcoupling mirror; Mz, MS are high reflectors; BSr , BS? are beamsplitters; P is the tuning plate
and B is the three-plate birefringent filter. The dashed contours represent the double Michelson selector configuration.
stream at the distance lo from the cavity end mirror (see fig. 1.). The outcoupling end is formed by a Michelson interferometer operating as a selective reflector. The selector consists of a beam-splitter BS, and two mirrors Ml, M2, one of which is the output mirror (Ml) of the laser. In addition, the cavity contains a broadband filter B (e.g. a birefringent filter) for a coarse selection of the wavelength. In conventional jet stream cavities, the thickness of the active medium is much smaller than lo. The saturating effect of a single longitudinal standing-wave mode with frequency v1 will produce a spatial modulation of the population inversion in the jet. A second mode with frequency v2 may then interact with the pumped molecules in the nodal regions of the first mode. The field pattern of the second mode will be exactly “out of phase” in the pumped region at the frequency separation Av=lv1-v21=(~+~)~c/210,
n=0,1,2,...
(1)
where c is the velocity of light. Consequently, mode competition wilI be weak, favouring oscillation in these modes. On the other hand, the field pattern of a second mode at the frequency separation
Av=n*c/210,
n=0,1,2
,...
(2)
will be spatially coincident with the mode VI in the pumped region, resulting in strong mode coupling. In a single-mode situation, the excess gain available for a second mode will then have a modulation, as a function of Au, with the period c/21,. As soon as one of the maxima at (1) reaches threshold, a two-mode situation will result. The Michelson mode selector is convenient for in134
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traducing selective loss for compensation of the excess gain. The path difference formed by the two arms of the selector is designated e = IS + I, - I, 1(see fig. 1). We assume the SM selector to be adjusted to maximum reflectivity for the mode VI. Then the reflectivity of a second mode v2 will have a sinusoidal dependence on Au with the period c/2e. The compensation of excess gain will be efficient if all modes at the reflectivity maxima of the SM have a minimum of effective gain according to (2). This occurs if the path difference is chosen to be e=Eolm,
m = 1,2,3, ...
(3)
At these positions the selector allows oscillation only in modes strongly competing with each other. A slight spatial “dephasing” through the thickness of the pumped layer occurs for large Au, resulting in decreased mode coupling, but this effect is assumed to be compensated by the broadband selectivity. The broadband filter finally favours the selected mode vl, with single-mode oscillation as a result. The most efficient mode selection is achieved at the position m = 1 of (3) where the periods of the gain and loss curves are equal. The threshold for a second mode will then in practice be determined by the selectivity of the broadband filter and the compensation of the dephasing effect. In principle, the SM system will yield sufficient selectivity for single-mode operation even at high pump powers. The low insertion losses then allow high efficiency and a high output power. On account of the reduced FSR of the selector at large values of e, a somewhat limited frequency tuning range is however to be expected, without simultaneous tuning of the broadband filter. The double Michelson selector is capable of further increasing the selectivity and, in some cases, of raising the threshold for a second mode. A DM may be constructed by inserting a second beam-splitter BS2 with a mirror M, into the arm of the interferometer (the dashed contours in fig. 1). If the mirror M3 is at a distance I, from the plate BS2, then we can define two path differences for the DM: el=
Ist~(12+13)-11i,
e2 = I4 - 131 . When e2 is small compared with el , the two upper
(44 Cab)
arms in fig. 1 will form a selector yielding a reflectivity “envelope” curve of coarse or intermediate’selectivity . Considering the correct compensation of spatial hole burning effects in the gain, and taking into account the phase inversion occuring in the reflectivity function of the DM, we find that single-mode selection will take place for el=
[Zo*(n+$)-e2]/m,
m=1,3,...
(10fn*e2)/m,
m = 2,4, ...
(
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(5)
withn=O, 1,2,... More details will be found in [ 121. Thus the positions (3) will acquire a split structure for e2 having values of typically a few millimeters, one finds that the most efficient selectivity, in general, is achieved for a small e2, typically e2 < 1 mm for the standard dye laser design, and with el = Zo [ 121. With servocoupling of the selector mirrors to the cavity mode frequency [8], the DM provides a tunable broadband selection curve which extends the continuous tuning range.
3. Experimental results 3. I. Single Michelson selector
A Michelson selector was designed for a commercial jet stream dye laser cavity (SP 375) of the type shown in fig. 1. An Ar+ laser (SP 17 l-06) provided the pump beam. As a broadband filter we used three birefringent plates of thickness ratios 1: 4 : 16, with the thickest plate measuring 6.4 mm. The birefringent filter alone provided a laser linewidth of 25 GHz. The SM contained a 50% reflecting beam-splitter at 45’ incidence with an AR coated rear surface. The selector was mounted on the output plate of the cavity. An output mirror transmission of 14% was used (no optimization of this parameter was attempted). The high reflectivity mirror of the SM was servocoupled to the laser mode [8] for tuning and stabilization of thermal drift. The dye used was rhodamine 6G dissolved in ethylene glycol. Three scanning FP spectrum analyzers of different FSR’s were applied for monitoring the laser spectrum. A calibrated calorimeter was used for power measurements. The thickness of the jet at the orifice of the nozzle was 0.2 mm and somewhat smaller at the beam axis. The effective thickness of the pumped region in the
dye depends, however, on the crossing angle between the non-collinear pump and dye laser beams as well as on the beam waists and pump power. The distance Z,, between the jet and the cavity end mirror was close to 50 mm in this cavity. The output mirror MI was mounted on a micrometer translator for distance measurement. By varying the path difference e of the SM, we found stable single-mode operation at distinct values of e approximately at 50 mm and 25 mm, corresponding to m = 1,2 in (3). Because of geometrical inconvenience, other positions of (3) were not systematically explored. The most efficient selectivity was found at e = 50 mm. Pumping with the line 5 14 nm yielded a single-mode threshold of 0.9 W at the dye laser wavelength 590 nm. The maximum one-line pump power available was 5.5 W, providing a singlemode output of 620 mW at 590 run. The single-mode power ate = 50 mm for one-line pumping is plotted in fig. 2, curve (a). The average slope efficiency is about 13.5%. Pumping with all lines raised the threshold to 1.2 W but provided a maximum of more than 12 W in available pump power. The single-mode output with all-lines pumping is plotted as curve (b) in fig. 2. A power of 1 W in the main output beam was measured for 11 W of pump power. The slope efficiency stayed below the one-line value and dropped significantly at I
I
1
I
1
,
I
0
2
L
6
8 POWER
IO (WI
12
1
1.2-
’
PUMP
Fig. 2. The single-mode dye laser power measured in the main output beam at 590 nm as a function of pump power. (a) One-line pumping (5 14 nm), single Michelson mode selector with path difference e = 50 mm; (b) all-lines pumping, single Michelson mode selector, same e; (c) all-lines pumping, double Michelson mode selector with path differences el = 48.5 mm and e2 = 3 mm.
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high powers, mainly due to serious thermal lensing phenomena in the jet at these pump levels [ 131. The experimental limit was thus set by the available pump power and thermal effects rather than by the secondmode threshold. No reoptimization of the pump beam focusing at each power level was attempted. Particular care had to be taken to suppress higher order transverse mode appearing with increasing pump power. These modes were partly due to the mediocre quality of the jet surface (the pump beam remained in the TEM,, mode). Higher order modes were eliminated either by beam readjustment or by using an aperture in front of the end mirror. Thereby the fundamental mode was also affected by additional loss, which contributed to the efficiency drop. The power leakage in the secondary beam through the beam-splitter increased from about 10% of the output power near threshold to about 30% at high power (300 mW at the output of 1 W). This was probably a consequence of thermal deformations of coatings and mode shape. The fairly large path difference in the selector did not appear to introduce any significant additional loss caused by mode matching problems (cf. [9]). Appropriate mirror curvatures for mode matching in the SM could, however, reduce diffraction losses [8]. Note that the one-line slope efficiency increases to about 16% and the maximum output to 1.3 W with the inclusion of the leakage beam, which is available for experiments. The effective linewidth of the free-running mode, determined by jitter, was about 10 MHz at intermediate powers. For improved efficiency at high powers, the dye jet velocity was increased, which resulted in frequent air bubbles in the jet. As no efficient air bubble filtering was applied, the effective linewidth and frequency stability deteriorated. At the position e = 25 mm, a maximum all-lines power of 4 W could be applied in a preliminary experiment on single-mode operation, providing 300 mW of output power. We performed tuning of the frequency by tilting a galvanometer controlled intra-cavity glass plate inclined at Brewster’s angle (cf. fig. 1). With the SM at e = 50 mm, one would expect the tuning range to be limited to the FSR of the selector, viz. 3 GHz, without simultaneous tuning of the birefringent filter. By taking account of the cavity mode spacing (346 MHz) and the “vernier” effect, this range can however be 136
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further extended. Thus, the single-mode frequency was continuously tunable over 12 GHz. This range was approximately the excursion limit of the piezoelectric mirror translator in the SM. A measurement of the single-mode power as a function of wavelength for a constant pump level reproduced the gain profile of the dye. 3.2. Double Michelson selector A DM was applied to the cavity by inserting another 50% reflecting beam-splitter and a mirror in the selector, according to fig. 1. Appropriate servo-control of mirrors was used [8]. Testing the mode selection at moderate power for e2 = 3 mm with el close to 50 mm, we found single-mode operation at the distinct points el = 50 f (n + 0.5)*3 mm for n = 0, 1,2, and closeto25mmatel=25~n~1.5mmforn=0,1, 2,3, in close agreement with (5). The most selective positions were those for n = 0. We did not observe any significant improvement in selectivity compared with the SM result at available pump powers. For er close to 50 mm the limit was again set by the applicable pump power. The single-mode power results for the DM at er = 48.5 mm and e2 = 3 mm are represented by the curve (c) of fig. 2. Some additional loss is introduced by the second beam-splitter and leakage beam. This raised the threshold to 1.3 W (all lines) and reduced the efficiency slightly (about 8% lower output), but the 1 W output level was reached for about 12 W of all-lines power in the pump beam. In preliminary attempts at el = 25 mm, no observable increase in the second-mode threshold was found, suggesting insufficient narrowband selectivity at that path difference. The tuning stability and range did, however, improve with the DM. To overcome the tuning limit of the mirror translators, the selector can be adjusted for single-mode operation at some smaller el according to (5) at the expense of selectivity and power. For approximately el = lo/4 and at low pump power, the DM system allowed continuous tuning over 50 GHz.
4. Discussion When the path difference e = 50 mm is employed in the Michelson selector, a continuous scan of the
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cavity mode frequency over 10 GHz requires an excursion of 1 pm for the mirror M2. Thus, rather long piezoceramics are needed for extended scans, but a thin galvanometer plate in the selector arm could also be applied for this purpose. The Michelson systems are also well adapted to pressure tuning [8], amethod that eliminates the need for large mirror excursions when a compensating glass plate is used in one selector arm. As the selectivity of the Michelson device does not appear to determine the practical pumping limit, improvements of the single-mode efficiency and power seem possible. Defocusing of the pump beam may reduce heating effects in the jet (cf. [9]). The thermal properties of water-based solvents and high-velocity jets are known to be well adapted to high power pumping [ 141. An optimized cavity configuration and a betterjet stream quality could improve on transverse mode stability. DM selectors with beam-splitters at Brewster’s angle for reduced reflection loss have also been designed [8]. In previous applications of DM and TM selectors [8,9] the path difference el = Zo/2 has been exploited. As the TM provided enhanced selectivity, the secondmode threshold for that value of el was not encountered for the powers applied. The results of the present work were not obtained in the optimal conditions for high power efficiency, but they demonstrate that the selectivity of the Michelson selector is considerably enhanced by the adequate exploitation of spatial hole burning effects, Sufficient selectivity for single-mode operation even at high pump powers is obtained already with a single Michelson selector in conjunction with a birefrmgent filter. The low losses of this system allow the extraction of very high tunable frequency single-mode powers from linear dye lasers. The application of the DM further improves the tuning range and mode stability. Due to simple design, low cost, and easy applicability to conventional dye laser cavities, the Michelson systems may provide an interesting alternative to sophisticated ring lasers for high power singlemode requirements.
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Acknowledgements The authors are indebted to Prof. E. Byckling for his generous and continuing interest. This work has been supported by the Academy of Finland.
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