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Improvement of the wavelength tunability of etalon-type laser diodes and mode recognition and stabilization in diode laser spectrometers
SPECTROCHIMICA ACTA PART B
ELSEVIER
Spectrochimica Acta Part B 52 (1997) 1215-1221
Technical Note
Improvement of the wavelength tunability of etalon-type laser diodes and mode recognition and stabilization in diode laser spectrometers A. Zybin 1, K. Niemax* Institute of Physics, University of Hohenheim, Garbenstrasse 30, D-70599 Stuttgart. Germany
Received 17 November 1996; accepted 27 January 1997
Abstract
A simple arrangement is presented for the improved wavelength tunability, mode recognition and stabilization of commercial single mode laser diodes. A glass plate with or without a partially reflective coating is mounted in front of the laser diode chip. The plate acts as the low-reflectivity mirror of an extended resonator and helps to select particular longitudinal modes within the gain profile even where laser action is not possible without the feedback. Mode selection is performed by piezoelectric tuning of the external cavity. Mode recognition is achieved by the measurement of the reflected intensity from a second very thin glass plate in the laser beam acting as a low-finesse etalon. The intensity pattern from the etalon is used for mode stabilization of the arrangement. © 1997 Elsevier Science B.V. Keywords: Diode laser spectrometry; Mode recognition; Wavelength stabilization; Wavelength tuning of laser diode
1. Introduction
A significant restriction in the spectroscopic application of commercial etalon-type laser diodes is caused by the existence of gaps in the wavelength tuning range (see, e.g., [1-3]). In particular, commercial InGaAsP laser diodes, which operate in the range 6 3 5 - 6 9 0 nm, typically cover only 2 0 - 3 0 % of the potential tuning ranges given by their gain curves. In principle, this problem can be overcome by optical feedback techniques. The most popular techniques involve wavelength dispersive components in extended cavities or feedback from external resonators [1,2]. However, the advantages of laser diodes, compactness, stability and simplicity of operation, are often lost if these techniques are applied. * Corresponding author. On leave from the Institute of Spectroscopy,Russian Academy of Science, 142092 Troitzk, Moscow Region, Russia.
There is a simple and inexpensive way to solve the problem of limited tunability within the gain profile of the laser diode: the application of a weak optical feedback provided by a thin glass plate mounted very close to the laser diode chip [4-7]. The surfaces of the glass plate form coupled resonators of low finesse with the facets of the laser diode. The resonator length is tuned by applying a voltage to a piezoelement (PZE) attached to the laser diode mount. By this arrangement, the dominating longitudinal mode at a fixed operating temperature and diode current can be suppressed, and the laser diode can operate in a selected neighbouring longitudinal mode. Furthermore, the wavelength range of continuous tuning without a mode hop can be significantly extended by simultaneous tuning of the PZE and the laser diode wavelength by current or by temperature. Unfortunately, this simple arrangement does not have high stability against mode hops. Even if the
0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0584-8547(97)00013-X
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A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
operating temperature and the current of the diode are stabilized, and the voltage applied to the PZE is kept constant, there are always small temperature drifts in the arrangement that change the length of the external cavity. As a result, mode hops are observed after some minutes up to 3 h in the best case. We will discuss two different constructions of the external cavity and the influence of the cavity parameters on mode behaviour, and we will present a simple way to identify and to stabilize the modes by variation of the PZE parameters.
2. Theoretical considerations Let us consider a single mode etalon-type laser diode at constant temperature and current. The gain profile of the laser diode (LD) is shown schematically in Fig. 1. It has an asymmetrical shape which reflects the Fermi-Dirac distributions of the electrons and holes in the active region of a semiconductor laser diode [8]. As it is a single mode laser diode it prefers the longitudinal mode with the largest gain, which is mode A in Fig. 1. The gain profile is modified if we extend the cavity by mounting a weakly reflecting glass plate near to the laser chip. In our simple treatment we will use a glass plate with one reflecting surface, the other side bearing a perfect antireflection coating. Now the gain is modulated by the Airy function given by the low-finesse extended cavity (the full curve in Fig. 1). The free spectral range of the
gain profile of LD
II
B A
longitudinal modes of LD ~
C
wavelength Fig. 1. Schematic gain profiles of the extended cavity for two slightly different cavity lengths (full and short-dashed curves) which favour the longitudinal modes A and B, respectively, of the laser diode. The gain profile of the laser diode without the extended cavity is also shown.
extended resonantor is smaller than that of the laser diode itself, because the period of modulation is given by AX = X2(2A)-l, where A = nlaserLlaser + nairLai r is the optical length of the extended cavity; nlaser and nair are the indices of refraction, and Llaser and Lai r a r e the lengths of the laser chip and the air gap between the laser and the glass plate, respectively. In Fig. 1, the free spectral range of the extended resonator AX=0.75AXLo. The laser will operate in mode A if a maximum of the Airy function coincides with the position of this mode. However, mode A can be suppressed and the laser can operate in other longitudinal modes if the length of the extended cavity is varied and the total gain for mode A becomes smaller than for other modes. In Fig. 1 we show the condition for a detuned cavity (short-dashed curve) which prefers the neigbouring mode B. By fine tuning of the cavity length with a PZE we can force the laser diode to operate in different modes (wavelengths) within the gain profile. In the example given in Fig. 1, there is only one additional mode which can be activated, mode C. However, the number of possible modes increases if AX approaches AXED. In this case, the distance Lair between the diode laser chip and the glass plate has to be reduced. An example will be shown below. In particular, we will give an example for the sequence of modes in dependence on the PZE voltage if AX = 0.85AXED. In reality, the conditions are more complex. We have to take into account that the glass plate has a finite thickness. This means that the reflection from the glass plate is modulated, because the plate acts as a low-finesse etalon. Furthermore, the front facet of the laser chip and the glass plate form another external resonator of low finesse which has an impact on the mode selection. However, both coupled external resonators have smaller effects on the mode selection than the total extended cavity discussed above. A theoretical description of coupled resonators in semiconductor lasers can be found, for example, in [8].
3. Experimental details Commercial laser diodes come in hermetically sealed housings. The windows of the housings have an antireflective coating. In order to place the glass plate very near to the laser chip, the housing-can with
1217
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
(c) te
(a) ,late
~ode
(b) late
~e Fig. 2. Experimental arrangements of the extended cavity diode laser spectrometer with mode recognition for stabilization: (a) open version with piezoceramic plate; (b) closed version with piezo-stack; (c) mode recognition with a glass plate etalon.
the window has to be removed. There are special "can-openers" for laser diodes on the market. We have tested two different arrangements with feedback plates as shown in Fig. 2. In arrangement (a), the glass plate was glued to a bimorph PZE (Philips PXE 5 Series, code 4322020-0824); in arrangement (b) it was affixed to a PZE stack (PI CERAMIC, PL055.20) bored through by a diamond drill. As shown in Fig. 2(b), the PZE stack was put on the mount with the laser diode chip. Compared with type (a), the advantages of arrangement (b) are the higher thermal stability and the protection of the laser chip against dust; the disadvantage is the higher price of the bored PZE stack. Typical voltages of 0 - 3 0 V were applied in both arrangements. Two types of glass plate were used, (i) a simple 0.15 mm thick glass plate, as common in optical microscopy, and (ii) the small windows of the laser housings which had previously been removed. In the latter case, the antireflection-coated windows were coated on one side with a gold layer. The reflectivities of the layers were typically beween 0.1 and 0.3. Different commercially available AIGaAs and InGaAsP laser diodes from Sharp, Hitachi, Mitsubishi, Philips and SDL were used. As the spectroscopic properties of the InGaAsP laser diodes are much poorer than
those of the near infrared A1GaAs laser diodes [3], more detailed investigations were performed with two types of InGaAsP lasers which operate in the visible red spectral range, namely the HL 6714 from Hitachi and the ML 1412R from Mitsubishi.
4. Experimental results The free spectral range of the extended cavity decreases with increasing distance Lai r between the glass plate and the laser chip. It becomes more and more difficult to suppress the dominating longitudinal mode of the laser diode by piezoelectric tuning. The influence of the cavity length on the generated modes of laser diode HL 6714 is shown in Fig. 3. At constant temperature and current, and at a distance of 0.7-0.8 mm, we could observe lasing only on two modes, whereas seven modes could be generated at 0.2 mm. However, we wish to point out that there were modes which only existed in longer cavities. On the other hand, the longitudinal laser mode expected at ~, = 666.69 nm could not be generated at the selected constant temperature and laser diode current. A possible reason for this behaviour will be discussed in more detail below.
A. Zybin, K. Niemax/59ectrochimica Acta Part B 52 (1997) 1215-1221
1218
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wavelength, nm Fig. 3. Modes for which laser action was observed at constant temperature and diode current by piezoelectric cavity tuning in dependence on the distance between the laser chip (HL 6714) and the glass plate.
Fig. 4 we show the single mode tuning curves by injection current for the laser diode ML 1412R at 20.24°C. The thick lines indicate the tuning range of the free running laser diode. Outside these ranges, the laser diode is operating on two or even more modes simultaneously [3]. The thinner lines represent the
Another very important feature of the extended cavity laser diode is the considerable improvement in the mode hop-free wavelength tunability of the laser diodes. In particular, the wavelength ranges of single mode tunability of InGaAsP laser diodes by current at constant temperature are very small [3]. In
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c u r r e n t of l a s e r d i o d e , m A Fig. 4. Wavelength characteristic of the laser diode ML 1412G with tuning by current at constant temperature without (thick lines) and with (thin lines) an extended cavity.
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
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single mode tuning curves of the extended cavity laser (Lair ~ 0.25 mm) by additional tuning of the PZE. It should be noted that the inner side of the glass plate had a gold coating (refiectivity 25%). The higher finesse of this extended cavity allowed the enforcement of laser operation at all wavelengths within the gain profile when, in addition to the current, the temperature of the laser diode was tuned. Such perfect tuning behaviour could not be achieved without the coating. Furthermore, the extended cavity laser could also be scanned free from mode hop over greater wavelength ranges than without the coating. For example, it was impossible to tune our selected laser diode ML 1412R with coatings on the glass plate of less than 25% reflectivity to the wavelength of the uranium line at 682.691 nm.
5. M o d e identification and control
As mentioned above, the long-term stability of the extended cavity arrangement at constant temperature, diode current and PZE voltage was not very good. Mode hops were observed after, typically, 10 min to 1 h and after 30 min to 3 h using arrangements (a) and (b), respectively, of Fig. 2. The instabilities are due to thermal drift of the cavity length. In principle, the stability can be improved by very precise temperature
control of the whole arrangement. However, if such a measure is taken, the arrangement becomes very complex. Therefore, we chose a different and less expensive way, a simple mode recognition by measurement of the reflected intensity from a tilted, uncoated 0.15 mm thick glass plate which acts as a low-finesse etalon with about 1 nm free spectral range. This second glass plate was placed in the laser beam a few cm from the laser diode arrangement and the reflected intensity was measured by a photodiode (see Fig. 2(c)). The dependence of photodiode signal on the PZE voltage between 3 and 30 V is shown in Fig. 5. Each intensity signal corresponds to one longitudinal mode of our extended cavity diode laser. The intensity differences can be well discriminated and the intensity pattern repeats periodically after 12 V. In Fig. 6 we have plotted the intensities of Fig. 5 versus the laser wavelength measured by a Burleigh WA 20 wavemeter. The measured values fit to the Airy function of the glass plate etalon. However, they can be also fitted with good precision to U (~ki) = U0[1 + Asin(27rz~X/~, i + ~o)]
(1)
where A is the amplitude of the modulation, AX is the free spectral range and ~o is the phase. As long as the temperature and the diode current are constant within certain error bars, and the reflection angle and the thickness of the glass plate do not vary, the modes
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
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Fig. 6. Reflected intensities from the thin etalon versus wavelength. The data are taken from Fig. 5. The curve represents a fit to the measured data.
can be well discriminated. This information can be used for piezoelectic retuning of the cavity to the desired mode if there are mode hops due to thermal drift. It is interesting that the sequence of active modes can be understood if the free spectral range of the extended cavity is 0.85AXLD (AXEDis the free spectral range of the laser diode). Taking into account n t.... and L l a s e r of the laser diode and nair ~ - 1, the distance between the front facet of the laser chip and the feedback glass plate w a s L a i r = 265 /xm. There are two irregularities in the mode sequence shown in Figs 5 and 6. There are no direct mode hops between modes 5 and 4, and between modes 9 and 8 (see Fig. 5). Normally, the adjacent mode with shorter wavelength than mode 1 ("mode 0") should follow mode 5, but the arrangement favours mode 4. The missing mode between 3 and 4 (see Fig. 6) should follow mode 9, but the extended cavity laser diode jumps to mode 8. The suppression of the mode between 3 and 4 is most likely due to the modulation of the reflectivity of the glass plate (etalon effect). It can be avoided by tilting of the feedback plate. Small changes in the reflection angle of the feedback plate were also the reason for the suppression of the modes at 666.82 and 667.22 nm at different distances between the laser chip and the glass plate (see Fig. 3). Most probably, "mode 0 "
does not oscillate because of the low gain at long wavelengths. There are no significant uncertainties in the sequence and assignment of the modes due to temperature changes of the thickness of the plate if the temperature is kept stable within _+10°C. Furthermore, the stability of the reflection angle should be better than 0.01 °. In principle, the optical surface quality and the parallelism of the etalon plate should be of interferometric grade. As mentioned above, we used glass plates from optical microscopy, which have poor optical properties. Even small changes of the laser beam position on the glass plate caused changes of phase ~oin Eq. (1) and modified the reflected intensity. Nevertheless, it is possible to identify the modes without using the wavemeter again. The basis of this procedure is the unchanged sequence of modes at constant temperature and diode current if the glass plate of the extended cavity is tuned piezoelectrically. The unchanged mode sequences were observed over several weeks. For example, as shown in Fig. 5, with increasing PZE voltage mode 3 always follows mode 8, mode 7 always follows mode 3, etc., if the temperature and current of ML 1412G laser diode have the same values. If the phase ~ is changed, for example, by a slight variation of the reflection angle, the
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
sinusoidal reflection pattern, as displayed in Fig. 6, shifts in wavelength. It is an easy task to find the shifted sinusoidal function from the changed reflection intensities, taking into account the given sequence of modes and a constant free spectral range of the etalon plate, i.e. we have to minimize [ U i - U ( X i ) ] 2 by variation of the phase ,p, where Ui are the measured intensity data and U(Xi) is given by Eq. (1).
1221
wavemeter, it can be used for recognition of modes even when there are slight changes in the optical arrangement. We believe that the arrangement presented is not only a satisfactory, simple and inexpensive solution for laser spectroscopic laboratories, but also an interesting and stable arrangement for commercial diode laser spectrometers.
Acknowledgements 6. Conclusion The wavelength tunability and stability of laser diodes with feedback from a glass plate have been studied in order to improve the applicability of diode laser spectrometers. It was found that such a simple arrangement allows the laser diode to be tuned by current and by temperature to all wavelength within its gain profile and extends mode hop-free tuning to much greater wavelength ranges. However, the long-term stability of the extended cavities against mode hops was found to be poor, even in very compact arrangements. The unwanted mode hops can be recognized by the measurement of the changed reflected intensity of an uncoated, low finesse etalon in the laser beam. The error signal can be used for tuning the extended cavity back to the mode of interest. Once the wavelength dependence of the reflected intensity has been measured with a laser
Financial support by LaserSpec Analytik GmbH, Munich is gratefully acknowledged.
References [1] C. Wieman, L. Hollberg, Rev. Sci. Instrum. 62 (1991) 1. [2] R.W. Fox, C.S. Weimer, L. Hollberg, G.C. Turk, Spectrochim. Acta Rev. 15 (1993) 291. [3] J. Franzke, A. Schnell, K. Niemax, Spectrochim. Acta Rev. 15 (1993) 379. [4] A.P. Bogatov,P.G. Eliseev, L.P. Ivanov,A.S. Logginov,M.A. Manko, K.Y. Senatorov, IEEE J. Quantum Electron. QE-9 (1973) 329. [5] L. Goldberg, H.F. Taylor, A. Dandridge, J.F. Weller, R.O. Miles, IEEE J. Quantum Electron. QE-18 (1982) 555. [6] A. Hemmerich, D.H. Mclntyre, C. Zimmermann, T.W. H~insch, Opt. Lett. 15 (1990) 372. [7] G.P. Barwood,P. Gill, W.R.C. Rowley,Meas. Sci. Technol. 2 (1992) 406. [8] W.W. Chow, S.W. Koch,M. SargentIII, Semiconductor-Laser Physics, Springer Verlag, New York, 1994.
ELSEVIER
Spectrochimica Acta Part B 52 (1997) 1215-1221
Technical Note
Improvement of the wavelength tunability of etalon-type laser diodes and mode recognition and stabilization in diode laser spectrometers A. Zybin 1, K. Niemax* Institute of Physics, University of Hohenheim, Garbenstrasse 30, D-70599 Stuttgart. Germany
Received 17 November 1996; accepted 27 January 1997
Abstract
A simple arrangement is presented for the improved wavelength tunability, mode recognition and stabilization of commercial single mode laser diodes. A glass plate with or without a partially reflective coating is mounted in front of the laser diode chip. The plate acts as the low-reflectivity mirror of an extended resonator and helps to select particular longitudinal modes within the gain profile even where laser action is not possible without the feedback. Mode selection is performed by piezoelectric tuning of the external cavity. Mode recognition is achieved by the measurement of the reflected intensity from a second very thin glass plate in the laser beam acting as a low-finesse etalon. The intensity pattern from the etalon is used for mode stabilization of the arrangement. © 1997 Elsevier Science B.V. Keywords: Diode laser spectrometry; Mode recognition; Wavelength stabilization; Wavelength tuning of laser diode
1. Introduction
A significant restriction in the spectroscopic application of commercial etalon-type laser diodes is caused by the existence of gaps in the wavelength tuning range (see, e.g., [1-3]). In particular, commercial InGaAsP laser diodes, which operate in the range 6 3 5 - 6 9 0 nm, typically cover only 2 0 - 3 0 % of the potential tuning ranges given by their gain curves. In principle, this problem can be overcome by optical feedback techniques. The most popular techniques involve wavelength dispersive components in extended cavities or feedback from external resonators [1,2]. However, the advantages of laser diodes, compactness, stability and simplicity of operation, are often lost if these techniques are applied. * Corresponding author. On leave from the Institute of Spectroscopy,Russian Academy of Science, 142092 Troitzk, Moscow Region, Russia.
There is a simple and inexpensive way to solve the problem of limited tunability within the gain profile of the laser diode: the application of a weak optical feedback provided by a thin glass plate mounted very close to the laser diode chip [4-7]. The surfaces of the glass plate form coupled resonators of low finesse with the facets of the laser diode. The resonator length is tuned by applying a voltage to a piezoelement (PZE) attached to the laser diode mount. By this arrangement, the dominating longitudinal mode at a fixed operating temperature and diode current can be suppressed, and the laser diode can operate in a selected neighbouring longitudinal mode. Furthermore, the wavelength range of continuous tuning without a mode hop can be significantly extended by simultaneous tuning of the PZE and the laser diode wavelength by current or by temperature. Unfortunately, this simple arrangement does not have high stability against mode hops. Even if the
0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0584-8547(97)00013-X
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A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
operating temperature and the current of the diode are stabilized, and the voltage applied to the PZE is kept constant, there are always small temperature drifts in the arrangement that change the length of the external cavity. As a result, mode hops are observed after some minutes up to 3 h in the best case. We will discuss two different constructions of the external cavity and the influence of the cavity parameters on mode behaviour, and we will present a simple way to identify and to stabilize the modes by variation of the PZE parameters.
2. Theoretical considerations Let us consider a single mode etalon-type laser diode at constant temperature and current. The gain profile of the laser diode (LD) is shown schematically in Fig. 1. It has an asymmetrical shape which reflects the Fermi-Dirac distributions of the electrons and holes in the active region of a semiconductor laser diode [8]. As it is a single mode laser diode it prefers the longitudinal mode with the largest gain, which is mode A in Fig. 1. The gain profile is modified if we extend the cavity by mounting a weakly reflecting glass plate near to the laser chip. In our simple treatment we will use a glass plate with one reflecting surface, the other side bearing a perfect antireflection coating. Now the gain is modulated by the Airy function given by the low-finesse extended cavity (the full curve in Fig. 1). The free spectral range of the
gain profile of LD
II
B A
longitudinal modes of LD ~
C
wavelength Fig. 1. Schematic gain profiles of the extended cavity for two slightly different cavity lengths (full and short-dashed curves) which favour the longitudinal modes A and B, respectively, of the laser diode. The gain profile of the laser diode without the extended cavity is also shown.
extended resonantor is smaller than that of the laser diode itself, because the period of modulation is given by AX = X2(2A)-l, where A = nlaserLlaser + nairLai r is the optical length of the extended cavity; nlaser and nair are the indices of refraction, and Llaser and Lai r a r e the lengths of the laser chip and the air gap between the laser and the glass plate, respectively. In Fig. 1, the free spectral range of the extended resonator AX=0.75AXLo. The laser will operate in mode A if a maximum of the Airy function coincides with the position of this mode. However, mode A can be suppressed and the laser can operate in other longitudinal modes if the length of the extended cavity is varied and the total gain for mode A becomes smaller than for other modes. In Fig. 1 we show the condition for a detuned cavity (short-dashed curve) which prefers the neigbouring mode B. By fine tuning of the cavity length with a PZE we can force the laser diode to operate in different modes (wavelengths) within the gain profile. In the example given in Fig. 1, there is only one additional mode which can be activated, mode C. However, the number of possible modes increases if AX approaches AXED. In this case, the distance Lair between the diode laser chip and the glass plate has to be reduced. An example will be shown below. In particular, we will give an example for the sequence of modes in dependence on the PZE voltage if AX = 0.85AXED. In reality, the conditions are more complex. We have to take into account that the glass plate has a finite thickness. This means that the reflection from the glass plate is modulated, because the plate acts as a low-finesse etalon. Furthermore, the front facet of the laser chip and the glass plate form another external resonator of low finesse which has an impact on the mode selection. However, both coupled external resonators have smaller effects on the mode selection than the total extended cavity discussed above. A theoretical description of coupled resonators in semiconductor lasers can be found, for example, in [8].
3. Experimental details Commercial laser diodes come in hermetically sealed housings. The windows of the housings have an antireflective coating. In order to place the glass plate very near to the laser chip, the housing-can with
1217
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
(c) te
(a) ,late
~ode
(b) late
~e Fig. 2. Experimental arrangements of the extended cavity diode laser spectrometer with mode recognition for stabilization: (a) open version with piezoceramic plate; (b) closed version with piezo-stack; (c) mode recognition with a glass plate etalon.
the window has to be removed. There are special "can-openers" for laser diodes on the market. We have tested two different arrangements with feedback plates as shown in Fig. 2. In arrangement (a), the glass plate was glued to a bimorph PZE (Philips PXE 5 Series, code 4322020-0824); in arrangement (b) it was affixed to a PZE stack (PI CERAMIC, PL055.20) bored through by a diamond drill. As shown in Fig. 2(b), the PZE stack was put on the mount with the laser diode chip. Compared with type (a), the advantages of arrangement (b) are the higher thermal stability and the protection of the laser chip against dust; the disadvantage is the higher price of the bored PZE stack. Typical voltages of 0 - 3 0 V were applied in both arrangements. Two types of glass plate were used, (i) a simple 0.15 mm thick glass plate, as common in optical microscopy, and (ii) the small windows of the laser housings which had previously been removed. In the latter case, the antireflection-coated windows were coated on one side with a gold layer. The reflectivities of the layers were typically beween 0.1 and 0.3. Different commercially available AIGaAs and InGaAsP laser diodes from Sharp, Hitachi, Mitsubishi, Philips and SDL were used. As the spectroscopic properties of the InGaAsP laser diodes are much poorer than
those of the near infrared A1GaAs laser diodes [3], more detailed investigations were performed with two types of InGaAsP lasers which operate in the visible red spectral range, namely the HL 6714 from Hitachi and the ML 1412R from Mitsubishi.
4. Experimental results The free spectral range of the extended cavity decreases with increasing distance Lai r between the glass plate and the laser chip. It becomes more and more difficult to suppress the dominating longitudinal mode of the laser diode by piezoelectric tuning. The influence of the cavity length on the generated modes of laser diode HL 6714 is shown in Fig. 3. At constant temperature and current, and at a distance of 0.7-0.8 mm, we could observe lasing only on two modes, whereas seven modes could be generated at 0.2 mm. However, we wish to point out that there were modes which only existed in longer cavities. On the other hand, the longitudinal laser mode expected at ~, = 666.69 nm could not be generated at the selected constant temperature and laser diode current. A possible reason for this behaviour will be discussed in more detail below.
A. Zybin, K. Niemax/59ectrochimica Acta Part B 52 (1997) 1215-1221
1218
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wavelength, nm Fig. 3. Modes for which laser action was observed at constant temperature and diode current by piezoelectric cavity tuning in dependence on the distance between the laser chip (HL 6714) and the glass plate.
Fig. 4 we show the single mode tuning curves by injection current for the laser diode ML 1412R at 20.24°C. The thick lines indicate the tuning range of the free running laser diode. Outside these ranges, the laser diode is operating on two or even more modes simultaneously [3]. The thinner lines represent the
Another very important feature of the extended cavity laser diode is the considerable improvement in the mode hop-free wavelength tunability of the laser diodes. In particular, the wavelength ranges of single mode tunability of InGaAsP laser diodes by current at constant temperature are very small [3]. In
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c u r r e n t of l a s e r d i o d e , m A Fig. 4. Wavelength characteristic of the laser diode ML 1412G with tuning by current at constant temperature without (thick lines) and with (thin lines) an extended cavity.
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
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30
voltage on piezoelement,V Fig. 5. Reflected intensity of the thin etalon in dependence on the voltage applied to the piezoelement.
single mode tuning curves of the extended cavity laser (Lair ~ 0.25 mm) by additional tuning of the PZE. It should be noted that the inner side of the glass plate had a gold coating (refiectivity 25%). The higher finesse of this extended cavity allowed the enforcement of laser operation at all wavelengths within the gain profile when, in addition to the current, the temperature of the laser diode was tuned. Such perfect tuning behaviour could not be achieved without the coating. Furthermore, the extended cavity laser could also be scanned free from mode hop over greater wavelength ranges than without the coating. For example, it was impossible to tune our selected laser diode ML 1412R with coatings on the glass plate of less than 25% reflectivity to the wavelength of the uranium line at 682.691 nm.
5. M o d e identification and control
As mentioned above, the long-term stability of the extended cavity arrangement at constant temperature, diode current and PZE voltage was not very good. Mode hops were observed after, typically, 10 min to 1 h and after 30 min to 3 h using arrangements (a) and (b), respectively, of Fig. 2. The instabilities are due to thermal drift of the cavity length. In principle, the stability can be improved by very precise temperature
control of the whole arrangement. However, if such a measure is taken, the arrangement becomes very complex. Therefore, we chose a different and less expensive way, a simple mode recognition by measurement of the reflected intensity from a tilted, uncoated 0.15 mm thick glass plate which acts as a low-finesse etalon with about 1 nm free spectral range. This second glass plate was placed in the laser beam a few cm from the laser diode arrangement and the reflected intensity was measured by a photodiode (see Fig. 2(c)). The dependence of photodiode signal on the PZE voltage between 3 and 30 V is shown in Fig. 5. Each intensity signal corresponds to one longitudinal mode of our extended cavity diode laser. The intensity differences can be well discriminated and the intensity pattern repeats periodically after 12 V. In Fig. 6 we have plotted the intensities of Fig. 5 versus the laser wavelength measured by a Burleigh WA 20 wavemeter. The measured values fit to the Airy function of the glass plate etalon. However, they can be also fitted with good precision to U (~ki) = U0[1 + Asin(27rz~X/~, i + ~o)]
(1)
where A is the amplitude of the modulation, AX is the free spectral range and ~o is the phase. As long as the temperature and the diode current are constant within certain error bars, and the reflection angle and the thickness of the glass plate do not vary, the modes
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
1220
8,5 8,0 7,5 >
o" 7,0 "13
.g
6,5
6,0 0 t-c~. 5,5 4,--
O
.{:~ ol
5,0 4,5
4,0
4•6
3,5 686,4
,
I
686,6
i
I
686,8
n
I
687,0
,
I
687,2
,
687,4
wavelength, nm
Fig. 6. Reflected intensities from the thin etalon versus wavelength. The data are taken from Fig. 5. The curve represents a fit to the measured data.
can be well discriminated. This information can be used for piezoelectic retuning of the cavity to the desired mode if there are mode hops due to thermal drift. It is interesting that the sequence of active modes can be understood if the free spectral range of the extended cavity is 0.85AXLD (AXEDis the free spectral range of the laser diode). Taking into account n t.... and L l a s e r of the laser diode and nair ~ - 1, the distance between the front facet of the laser chip and the feedback glass plate w a s L a i r = 265 /xm. There are two irregularities in the mode sequence shown in Figs 5 and 6. There are no direct mode hops between modes 5 and 4, and between modes 9 and 8 (see Fig. 5). Normally, the adjacent mode with shorter wavelength than mode 1 ("mode 0") should follow mode 5, but the arrangement favours mode 4. The missing mode between 3 and 4 (see Fig. 6) should follow mode 9, but the extended cavity laser diode jumps to mode 8. The suppression of the mode between 3 and 4 is most likely due to the modulation of the reflectivity of the glass plate (etalon effect). It can be avoided by tilting of the feedback plate. Small changes in the reflection angle of the feedback plate were also the reason for the suppression of the modes at 666.82 and 667.22 nm at different distances between the laser chip and the glass plate (see Fig. 3). Most probably, "mode 0 "
does not oscillate because of the low gain at long wavelengths. There are no significant uncertainties in the sequence and assignment of the modes due to temperature changes of the thickness of the plate if the temperature is kept stable within _+10°C. Furthermore, the stability of the reflection angle should be better than 0.01 °. In principle, the optical surface quality and the parallelism of the etalon plate should be of interferometric grade. As mentioned above, we used glass plates from optical microscopy, which have poor optical properties. Even small changes of the laser beam position on the glass plate caused changes of phase ~oin Eq. (1) and modified the reflected intensity. Nevertheless, it is possible to identify the modes without using the wavemeter again. The basis of this procedure is the unchanged sequence of modes at constant temperature and diode current if the glass plate of the extended cavity is tuned piezoelectrically. The unchanged mode sequences were observed over several weeks. For example, as shown in Fig. 5, with increasing PZE voltage mode 3 always follows mode 8, mode 7 always follows mode 3, etc., if the temperature and current of ML 1412G laser diode have the same values. If the phase ~ is changed, for example, by a slight variation of the reflection angle, the
A. Zybin, K. Niemax/Spectrochimica Acta Part B 52 (1997) 1215-1221
sinusoidal reflection pattern, as displayed in Fig. 6, shifts in wavelength. It is an easy task to find the shifted sinusoidal function from the changed reflection intensities, taking into account the given sequence of modes and a constant free spectral range of the etalon plate, i.e. we have to minimize [ U i - U ( X i ) ] 2 by variation of the phase ,p, where Ui are the measured intensity data and U(Xi) is given by Eq. (1).
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wavemeter, it can be used for recognition of modes even when there are slight changes in the optical arrangement. We believe that the arrangement presented is not only a satisfactory, simple and inexpensive solution for laser spectroscopic laboratories, but also an interesting and stable arrangement for commercial diode laser spectrometers.
Acknowledgements 6. Conclusion The wavelength tunability and stability of laser diodes with feedback from a glass plate have been studied in order to improve the applicability of diode laser spectrometers. It was found that such a simple arrangement allows the laser diode to be tuned by current and by temperature to all wavelength within its gain profile and extends mode hop-free tuning to much greater wavelength ranges. However, the long-term stability of the extended cavities against mode hops was found to be poor, even in very compact arrangements. The unwanted mode hops can be recognized by the measurement of the changed reflected intensity of an uncoated, low finesse etalon in the laser beam. The error signal can be used for tuning the extended cavity back to the mode of interest. Once the wavelength dependence of the reflected intensity has been measured with a laser
Financial support by LaserSpec Analytik GmbH, Munich is gratefully acknowledged.
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