- Email: [email protected]
Laser-diode injection seeding of a flashlamp-pumped Q-switched Ti:Al2O3 laser oscillator
I December 1996
C•3
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 132 (1996) 263-268
Laser-diode injection seeding of a flashlamp-pumped Q-switched Ti: A1203 laser oscillator J. Yu l, M. Douard, P. Rambaldi, B. Vezin, J.P. Wolf Laboratoire de Spectrora~trielonique et Mol~culaire, UMR n°5579 CNRS, Universit~Claude Bernard-Lyon L Bat. 205, Campus de la Doua, 69622 Villeurbanne Cedex, France
Received 1 March 1996; revised version received 24 May 1996; accepted 5 June 1996
Abstract Narrow bandwidth operation within 250 MHz of a flashlamp-pumped Ti: Al20 3 laser is successfully demonstrated with injection seeding from a CW laser diode. Seeding is shown sensitive even to very small injected power. Typically, 70% energy of a 100 mJ and 50 ns pulse is locked into the narrow bandwidth. This seeding efficiency is essentially limited in our experimentsrby.the spatial hole burning in a laser rod with linearly polarized laser beam. it is also limited by mode mismatching betwe~en slave laser and seeding laser due to thermal effect on the laser rod induced by short and intense pumping flash.
Owing to its broad coverage of the near-ir~frared spectral region (0.7-1.0 p.m), titanium-doped sapphire (Ti:Al203) has emerged as an important laser medium, and today the Ti:AI203 laser is the most widely used tunable solid-state laser. In spite of the short fluorescence lifetime of Ti : A1203 (~'= 3.2 I~s), efforts have been made to have successful flashlamp-pumped laser operation [1]. Combined with the Q-switching technique, a compact flashlamp-pumped laser delivers short ( ~ 50 ns), high energy ( ~ 200 mJ) and tunable pulses at a repetition rate of about 20 Hz. Without any wavelength selective element, laser output is in a broad spectral band of several nm centrered at 790 nm. Narrow spectral emission, and
i E-mail: [email protected].
even single-longitudinal mode operation is required for applications such as high resolution spectroscopy, non-linear optics and remote sensing with the DIAL technique (Differential Absorption Lidar). In the last application, for which our laser system is developed, the laser is alternatively operated at two different wavelengths. One of them corresponds to an absorption line of the atmosphere constituent to be measured. The other is detuned outside the resonance and is used to determine atmospheric backscattering and extinction [2]. For such a system to be efficient, laser linewidth should be smaller than or comparable to the linewidth of the atmosphere constituent to be measured. In the near-infrared spectrum region, this implies a linewidth on the order of 1.0 pm (or 0.5 GHz in frequency) [3]. Extensive works have already been dedicated to the development of high energy
0030-4018/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pli S0030-401 8(96)00383-5
264
J. Yu et al. / Optics Communications 132 (1996) 263-268
and narrow bandwidth lasers for water vapor DIAL measurement in 720 nm and 935 nm bands [4]. Achievement of narrow bandwidth operation in a pulsed laser typically requires a series of wavelength selective elements in the laser cavity and may require some type of cavity Q or gain variation control to allow sufficient time for the laser pulse to be narrowed in the spectrum [5]. As pointed out by several authors, this standard method suffers from some disadvantages [6]. Laser output energy is decreased because of loss introduced by wavelength selective elements. And optical damage susceptibility of these elements limits also laser output energy. This puts the requirement of narrow spectral bandwidth in contradiction with that of high output energy. In addition, wavelength narrowing is limited by the relatively short pulse evolution time in the cavity, especially for laser media with short fluorescence lifetime like Ti:AI203. In fact, for an efficient extraction of energy from a laser cavity, the laser pulse should evolve in a time too short to allow its spectral narrowing. An alternative method sequentially uses two different resonators in a self-injection seeding arrangement [7,8]. Wavelength selective elements are inserted in a first cavity, which is a low Q and low energy cavity. While a second cavity is free of optical selective elements and provides a large optical gain. A timing control on the Q-switch allows a nascent laser pulse to travel through the wavelength selective elements in the first cavity and get further narrowed on each pass. Before the pulse becomes too large, it is switched into the second cavity and undergoes further amplification before the extraction. Self-injection seeding obviates problems related to optical loss and damage. But the linewidth narrowing potential is limited by the finite time interval spent by the laser pulse in the first cavity. Laser operation in a narrow bandwidth of the order of 0.05 nm has been reported for a self-injection seeded flashlamppumped Q-switched Ti:AI203 laser [8]. A smooth continuous tuning is however, difficult to obtain with a self-injection seeded laser, because of resonant coupling between the low energy and the high energy cavities. The method that we used to achieve narrow bandwidth operation was injection seeding. For a pulsed laser, the injection seeding is accomplished by intro-
ducing radiation from a low power single-mode master oscillator into the cavity of a high energy pulsed laser during the pulse buildup period. The high energy laser is referred as slave laser or power oscillator. Both injected radiation and spontaneous emission in the slave laser cavity will be regeneratively amplified. If the seeding beam has enough power on one of the slave cavity modes, this mode will first saturate the homogeneously broadened gain medium, suppressing further growth of adjacent longitudinal modes [9]. When the seeding mode matches exactly one of the slave cavity longitudinal modes, injection is more efficient and single-longitudinal mode oscillation can be reached. When the seeding mode is detuned from that of the slave cavity, the injection efficiency decreases, but seeding still occurs if the detuning is within the injection seeding bandwidth [6,10]. The main advantage of injection seeding over other methods is that the spectral property of the output high energy pulses is determined by an independent low power (or low energy) laser. Efficient techniques are available to obtain single mode oscillation in CW or pulsed low power lasers. Tuning on the master oscillator can provide tuning of high energy pulses. Use of a laser diode or diodepumped solid-state laser, makes master oscillators reliable and compact. Broad tunability can be obtained with an external-cavity laser diode [11] or diode-pumped tunable solid-state laser, Cr: LiSAF for example [12]. Experimental demonstration of injection seeding has been carried out for a varity of lasers. Extensive study has been done for the Nd:YAG laser [13]. Recently, interest has been turned to a tunable solid-state laser. An Alexandrite laser has been injection-seeded by a CW Ti:AI203 laser [10] and a CW laser diode [14]. A frequencydoubled Nd: YAG laser-pumped Ti: AI203 laser has been injection-seeded by a pulsed single-mode dye laser [15] and a CW laser diode [16]. In this paper, we present the injection seeding of a flashlamp pumped Q-switched Ti: A1203 oscillator by a CW laser diode. This is to our knowledge the first demonstration of a compact injection-seeded Ti: Ai203 laser oscillator, which combines flashlamp pumping and laser-diode seeding. Our experimental setup is schematically presented in Fig. 1. The slave laser used is a commercial flashlamppumped Ti: AI203 laser (ELIGHT Ti: Flash series).
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J. Yu et aL / Optics Communications 132 (1996) 263-268
HR Mirror
Brewster Window
PockelsCell
[
50% Output Coupler
x
x,, // Diaphragms or k/4 Plates
From Diode Fig. I. Schematic of the experimental setup. The diaphragms ~ d the quaxter-wave plates are only used in some specific experimental configurations.
The laser rod, 6 mm diameter, 100 mm length and 0.15% by weight doped, is pumped by four lamps supplied by discharge pulses of about 5 p,s duration. Two flat mirrors surround the laser rod forming a cavity of about 50 cm length. One of the mirrors is high reflection coated and the other, the output coupler, has a transmission of 50%. Q-switching is achieved by a Pockels cell. One or several dispersion prisms can be inserted in the cavity for a rough bandwidth narrowing (,-, 0.2 nm). When the prisms are not used, a Brewster window is inserted in the cavity to provide the polarization selection. The seeding beam comes from a CW laser diode (SDL5311-Gl), which emits a diffraction-limited beam and has a maximum power of 100 mW at 800 nm. An objective of 8 mm focal length and of 0.5 NA collimates the diode beam, and a cylindrical telescope corrects its astigmatism to get a circular-section beam of 4 mm diameter. Two optical isolators axe used to get an isolation of 60 dB to prevent the diode from optical damage. A spherical telescope is used to adapt the diode beam to the 6 mm-diameter Ti: AI203 beam. The seeding beam was coupled into the Ti: A1203 cavity through an intra-cavity reflecting surface. When the dispersion prisms were used, one of the Brewster-angle-set prism facets provided the coupling surface. Otherwise, the Brewster window was used for coupling. For an s-polarized seeding beam, about 15% (15% for a BK7 surface, 13% for a synthetic fused silica surface) power of the laser diode beam was coupled into the cavity and directed toward the high reflection mirror through the Pockels cell. In this arrangement, the seeding beam is sliced by the Q-switch [10]. When the Q-switch is closed, the injected beam is converted to p-polarization when
it is reflected back from the HR mirror, and can pass through Brewster facets without loss. After a roundtrip in the cavity, the injected beam is converted back to s-polarization and is rejected out of the cavity by Brewster facets. While the Q-switch opens and the cavity Q increases, a slice of the injection beam is confined in the Ti:AI203 cavity with ppolarization (Ti: A1203 lasing polarization). The duration and the form of the confined slice depend on the Pockels cell high voltage decreasing time and the Ti: A1203 cavity length. The spectrum of the sliced injection beam is in general broadened, this makes injectio~ seeding less sensitive to the mismatching between seeding mode and Ti : A1203 cavity modes [10]. The seeding process was monitored by observing the buildup time of the laser pulse. In Fig. 2, the laser output is recorded with a digital oscilloscope triggered by the Q-switch opening pulse. The signal is not completely time-resolved because of limited pass bandwidth of our oscilloscope. When the seeding beam is blocked, laser pulses build up from spontaneous emission with a buildup time of 320 ns (Fig. 2a). When the injection seeding occurs, laser pulses build up from stimulated emission induced by the seeding beam. The pulse buildup time, 240 ns, is shorter. To know how the Ti:A1203 laser is sensitive to injection seeding, we decreased the diode power. Effective seeding occurred even for 500 p,W coupled into the slave cavity.
.1 I
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Time (ns) Fig. 2. Temporal pulse shape without seeding (a) and with seeding (b).
J. Yu et al. / Optics Communications 132 (1996) 263-268
266
0
0 Fabry-Perot Scanning Ramp Fig. 3. Fabry-Perot transmission signal of injection-seeded Ti:Al20 a output (a), and laser diode beam (b). The Fabry-Petot has a FSR of 750 MHz and a f'messe of 20, it is scanned by a high voltage ramp of 20 seconds duration.
For a more quantitative study, we analysed the output pulse spectrum using a 10 cm-length confocal Fabry.Perot with a FSR of 750 MHz and a finesse of 20. The seeding power coupled into the slave cavity was fixed at about lO roW. Fine adjustment of diode current allowed us to tune the seeding mode into resonance with one of the slave cavity modes. While one of the mirrors of the Fabry-Perot was scanned by a PZT, the transmission from the Fabry-Perot was detected by a pulse detector and recorded by a digital oscilloscope. The scanning ramp duration was 20 s. In Fig, 3 we present typical recorded signals: (a) Transmission of injection-seeded Ti:Al203 beam, each output pulse gives a vertical line in the recorded trace; (b) Transmission of laser diode beam. For the second trace, the pulse detector was replaced by a photodiode. We remark that injection-seeded pulses have narrow transmission peaks with a FWHM of 250 MHz. For our 50 cm-length slave lase:" cavity, this implies that the laser operation is in one or two longitudinal modes. Another remark is that there is a transmission signal between two transmission peaks. This implies that a part of the output energy is not locked into the narrow spectral band. We define then an experimentally measurable injection seeding effi-
ciency from the Fabry-Perot transmission trace (Fig. 3a). We can consider that the energy of each laser pulse is spectrally distributed in a narrow peak, injection-seeded part, and broad lateral wings, freeruning part. When the Fabry-Perot transmission of such a pulse is detected, the free-tuning part gives a constant level versus the scanning high voltage, while the injection-seeded part provives a signal modulated by the Fabry-Perot transmission function. Our experimentally measurable seeding efficiency is then defined as the ratio between the intensity of the narrow peak, measured from the free-runing background to the top of the peak, and the total transmission intensity, measured from zero to the top of the peak. Several variant configurations of the slave laser cavity were used in our experiments. For each configuration, the seeding efficiency was measured as a function of Ti: Al~O 3 rod flash pumping level. Fig. 4 presents measured seeding efficiencies. The seeding beam was first coupled into the slave cavity through a Brewster window (Fig. 4a), then it was coupled through a prism (Fig. 4b). We studied then the effects of two quarter-wave plates and two diaphragms of 5 mm diameter surrounding the laser rod (Fig. 4c and Fig. 4d). We see that in general, the seeding efficiency decreases as pumping level increases, it runs from about 80% to about 40%.
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Pumping Discharge High Voltage (kV) Fig. 4. Seeding efficiency as a function of the pumping discharge high voltage for several different configurations of the slave laser cavity. (a) Injection through a Brewster window. (b) Injection through a prism facet. (c) With two quarter-wave plates in the cavity. (d) With two diaphragms in the cavity.
J. Yu et d/Optics
Communications
Typically, 70% energy of a 100 mJ pulse is injection-seeded into tbe 250 MHz narrow spectral band. Comparing the cavity configurations used, we get the following information: A better efficiency is obtained in curve c for high pumping level, implying that the spatial hole burning in the laser rod limits the seeding efficiency. The use of two quarter-wave plates converts the laser beam to circular polarization inside the Ti : Al,O, rod, which avoids the spatial hole burning. This solution is used in an injectionseeded Nd : YAG laser. For uniaxial laser crystals like Ti : Al,O,, unfortunately, non-negligible birefringence draws laser output energy down. An output energy of 65 mJ is obtained at a pumping level of 13.5 kV instead of 200 mJ without quarter-wave plates. The diaphragms seem beneficial for a higher seeding efficiency, because they limit flee-tuning lasing in the peripheral part of the laser rod at high pumping level. The loss introduced by the diaphragms is small, an output energy of 180 mJ is obtained at a pumping level of 13.5 kV. Injections through a prism facet (Fig. 4b) and through a Brewster window (Fig. 4a) give comparable efficiencies for middle and high pumping levels. For a low pumping level however, the injection through a prism provides a poor efficiency. This is due to the competition between the wavelength selections by the prism and by the injection seeding. In our experiments, when a prism was inserted in the slave cavity, a spectrometer was used to tune the slave laser to around the injection wavelength. The Brewster window coupling presents a simpler configuration, because the seeding mode provides the only wavelength selection. The effect of mode matching between seeding mode and slave cavity modes was studied in otr experiments. In practice, mode mismatching is due to drift of both modes, especially, that of slave modes induced by fluctuation of its cavity length. Smooth and slow fluctuation is due to ambient temperature change, and rapid variation is induced by heat dispersion in the laser rod at each flashlamp pumping. The last type of cavity length variation is particularly important for Ti : Al,O,, because short discharge pulses of several IJS are applied to the flashlamps to deposit sufficient energy within the fluorescence lifetime. In our experiments the seeding efficiency was measured for different pumping repe-
132 11996) 263-268
267
tition rates. The obtained results show that we got a better efficiency for a larger repetition rate at a given pumping level. At a pumping level of 12 kV, for a repetition rate of 6 Hz, an efficiency of 0.60 was obtained, this efficiency was increased to 0.75 for a repetition rate of 18 Hz. This increase of efficiency can be interpreted by the thermal lensing in the laser rod. A higher average temperature for 18 Hz repetition rate induces a thermal lens with shorter focal length, which provides more Easingtransverse modes. The effective slave cavity mode is thus broadened, this compensates for mode drift during flashlamp pumping, and allows a higher probability for a resonance between cavity modes and seeding laser mode. The transverse mode mixing could also be provided by the use of a concave mirror in the slave laser cavity, leading to a less critical seeding mode matching condition. The standard active mode matching techniques used in an injection-seeded Nd: YAG laser [17] should also be efficient for a Ti : Al,O, laser to improve mode matching stability. But efforts should be made on rapid electronics because of the large difference in fluorescence lifetimes between Nd : YAG and Ti : Al,O,. The observation on the far-field of injection-seeded pulses by a CCD array gave a diffraction pattern, specific to a single-longitudinal mode and multi-transverse mode laser beam. In conclusion, narrow bandwidth operation within 250 MHz has been successfully demonstrated for a commercial flashlamp pumped Q-switched Ti : Al,O, laser with injection seeding from a CW laser diode. Injection seeding is shown to be sensitive even to very small seeding power. The seeding efficiency depends on the specific slave laser cavity configuration and its pumping level. Typically, 70% energy of a 100 mJ and 50 ns pulse is !ccked into the narrow bandwidth of 250 MHz, which corresponds to one or two longitudinal modes of the slave laser cavity. For P single-longitudinal mode operation with a higher spectral purity, a ring-cavity slave laser free of spatial hole burning should be used. The use of a concave mirror in the slave laser cavity, allowing for more lasing transverse modes, should lead to a less critical seeding mode matching condition. An electronic servo system should improve mode matching stability between seeding and slave lasers, leading to a better seeding efficiency. The use of an extemalcavity laser diode or a diode-pumped Cr: LiSAF
268
J, Yu et al./ Optics Communications 132 (1996) 263-268
laser will provide a compact tunable ma~ter oscillator. A diode-pumped Cr:LiSAF laser is in development in our laboratory. The transverse .,~ ~e control is also in our future program. This necessitates the use of an unstable cavity for a higher transverse mode discrimination.
The authors would like to acknowledge M. N~ri and M. Barbaire for their much appreciated technical supports.
References [1] H. Kneipp, J. Kolcnda, P. Rairoux, B. Stein. D. Weidauer, J.P. Wolf and L. W~ste, Optical Methods in Atmospheric Chemistry, SPlE (1993) 1715. [2] M. Beniston, M. Beniston-Rebetez, H.J. K~lsch, P. Rairoux, J.P. Wolf and L. W~stc, J. Geophys. Rcs. 95 (D7) (1990) 13. [3] J.C. Barnes, N.P. Barnes, L.G. Wang and W. Edwards, IEEE J. Quantum Electron. 29 (1993) 2684. [4] C. Cahen, G. M~gie, P. Flamant, J. Appl. Meteorology 21 (1982) 1506; N.S. Higdon, E.V. Browell, P. Ponsardin, B.E, Grossmann, C.F. Butler, T.H. Chyba, M.N. Mayo, RJ. Allen, A.W.
Heuser, W.B. Grant, S. Ismail, S.D. Mayor and A.F. Caner, Appl. Optics 33 (1994) 6422. [5] W. Koeclmet, Solid-state laser engineering (Spingcr, Berlin, 1992). [6] N.P. Barnes and J.C. Barnes, IEEE J. Quantum Electron. 29 (1993) 2670. [7] N.P. Barnes, J.A. Williams, J.C. Barnes and G.E. Lockard, IEEE J. Quantum Electron. 24 (1988) 1021. [8] J.A. Russel and R.A. Siena, preprint (1995). [9] Y.K. Park, G. Giuliani and R.L. Bycr, Optics LetL 5 (I 98O)96. [10] D. Bruneau, T. Amand des Lions, P. Quaglia and J. Pelon, Appl. Optics 33 (1994) 3941. [11] M. De Labachclerie and P. Cerez, Optics Commun. 55 (1985) 174. [12] H.H. Zenizie, A. Finch and P.F. Moulton, Optics Lett. 20 (1995) 2207. [13] See for example, R.L. Schmitt and L.A. Ralm. Appl. Optics
25 (1986) 629. [14] K. Choi, S.H. Lin, E. Korevaar and C.S. Lin, Optics Comm. 88 (1992) 385. [15] P. Brockman, C.H. Bait, J.C. Berries, R.V. Hess and E.V. Browell, Optics Lett. I I (1986) 712. [16] C.H. Bait, P. Brockman, R.V. Hess and E.A. Moldin, IEEE J. Quantum Electron. 24 (1988) 1045. [17] L.A. Rahn, Appl. Optics 24(1985) 940; E.S. Fry, Qiquan Hu and Xingfu Li, AppL Optics 30 (1991) 1015.
C•3
OPTICS COMMUNICATIONS ELSEVIER
Optics Communications 132 (1996) 263-268
Laser-diode injection seeding of a flashlamp-pumped Q-switched Ti: A1203 laser oscillator J. Yu l, M. Douard, P. Rambaldi, B. Vezin, J.P. Wolf Laboratoire de Spectrora~trielonique et Mol~culaire, UMR n°5579 CNRS, Universit~Claude Bernard-Lyon L Bat. 205, Campus de la Doua, 69622 Villeurbanne Cedex, France
Received 1 March 1996; revised version received 24 May 1996; accepted 5 June 1996
Abstract Narrow bandwidth operation within 250 MHz of a flashlamp-pumped Ti: Al20 3 laser is successfully demonstrated with injection seeding from a CW laser diode. Seeding is shown sensitive even to very small injected power. Typically, 70% energy of a 100 mJ and 50 ns pulse is locked into the narrow bandwidth. This seeding efficiency is essentially limited in our experimentsrby.the spatial hole burning in a laser rod with linearly polarized laser beam. it is also limited by mode mismatching betwe~en slave laser and seeding laser due to thermal effect on the laser rod induced by short and intense pumping flash.
Owing to its broad coverage of the near-ir~frared spectral region (0.7-1.0 p.m), titanium-doped sapphire (Ti:Al203) has emerged as an important laser medium, and today the Ti:AI203 laser is the most widely used tunable solid-state laser. In spite of the short fluorescence lifetime of Ti : A1203 (~'= 3.2 I~s), efforts have been made to have successful flashlamp-pumped laser operation [1]. Combined with the Q-switching technique, a compact flashlamp-pumped laser delivers short ( ~ 50 ns), high energy ( ~ 200 mJ) and tunable pulses at a repetition rate of about 20 Hz. Without any wavelength selective element, laser output is in a broad spectral band of several nm centrered at 790 nm. Narrow spectral emission, and
i E-mail: [email protected].
even single-longitudinal mode operation is required for applications such as high resolution spectroscopy, non-linear optics and remote sensing with the DIAL technique (Differential Absorption Lidar). In the last application, for which our laser system is developed, the laser is alternatively operated at two different wavelengths. One of them corresponds to an absorption line of the atmosphere constituent to be measured. The other is detuned outside the resonance and is used to determine atmospheric backscattering and extinction [2]. For such a system to be efficient, laser linewidth should be smaller than or comparable to the linewidth of the atmosphere constituent to be measured. In the near-infrared spectrum region, this implies a linewidth on the order of 1.0 pm (or 0.5 GHz in frequency) [3]. Extensive works have already been dedicated to the development of high energy
0030-4018/96/$12.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. Pli S0030-401 8(96)00383-5
264
J. Yu et al. / Optics Communications 132 (1996) 263-268
and narrow bandwidth lasers for water vapor DIAL measurement in 720 nm and 935 nm bands [4]. Achievement of narrow bandwidth operation in a pulsed laser typically requires a series of wavelength selective elements in the laser cavity and may require some type of cavity Q or gain variation control to allow sufficient time for the laser pulse to be narrowed in the spectrum [5]. As pointed out by several authors, this standard method suffers from some disadvantages [6]. Laser output energy is decreased because of loss introduced by wavelength selective elements. And optical damage susceptibility of these elements limits also laser output energy. This puts the requirement of narrow spectral bandwidth in contradiction with that of high output energy. In addition, wavelength narrowing is limited by the relatively short pulse evolution time in the cavity, especially for laser media with short fluorescence lifetime like Ti:AI203. In fact, for an efficient extraction of energy from a laser cavity, the laser pulse should evolve in a time too short to allow its spectral narrowing. An alternative method sequentially uses two different resonators in a self-injection seeding arrangement [7,8]. Wavelength selective elements are inserted in a first cavity, which is a low Q and low energy cavity. While a second cavity is free of optical selective elements and provides a large optical gain. A timing control on the Q-switch allows a nascent laser pulse to travel through the wavelength selective elements in the first cavity and get further narrowed on each pass. Before the pulse becomes too large, it is switched into the second cavity and undergoes further amplification before the extraction. Self-injection seeding obviates problems related to optical loss and damage. But the linewidth narrowing potential is limited by the finite time interval spent by the laser pulse in the first cavity. Laser operation in a narrow bandwidth of the order of 0.05 nm has been reported for a self-injection seeded flashlamppumped Q-switched Ti:AI203 laser [8]. A smooth continuous tuning is however, difficult to obtain with a self-injection seeded laser, because of resonant coupling between the low energy and the high energy cavities. The method that we used to achieve narrow bandwidth operation was injection seeding. For a pulsed laser, the injection seeding is accomplished by intro-
ducing radiation from a low power single-mode master oscillator into the cavity of a high energy pulsed laser during the pulse buildup period. The high energy laser is referred as slave laser or power oscillator. Both injected radiation and spontaneous emission in the slave laser cavity will be regeneratively amplified. If the seeding beam has enough power on one of the slave cavity modes, this mode will first saturate the homogeneously broadened gain medium, suppressing further growth of adjacent longitudinal modes [9]. When the seeding mode matches exactly one of the slave cavity longitudinal modes, injection is more efficient and single-longitudinal mode oscillation can be reached. When the seeding mode is detuned from that of the slave cavity, the injection efficiency decreases, but seeding still occurs if the detuning is within the injection seeding bandwidth [6,10]. The main advantage of injection seeding over other methods is that the spectral property of the output high energy pulses is determined by an independent low power (or low energy) laser. Efficient techniques are available to obtain single mode oscillation in CW or pulsed low power lasers. Tuning on the master oscillator can provide tuning of high energy pulses. Use of a laser diode or diodepumped solid-state laser, makes master oscillators reliable and compact. Broad tunability can be obtained with an external-cavity laser diode [11] or diode-pumped tunable solid-state laser, Cr: LiSAF for example [12]. Experimental demonstration of injection seeding has been carried out for a varity of lasers. Extensive study has been done for the Nd:YAG laser [13]. Recently, interest has been turned to a tunable solid-state laser. An Alexandrite laser has been injection-seeded by a CW Ti:AI203 laser [10] and a CW laser diode [14]. A frequencydoubled Nd: YAG laser-pumped Ti: AI203 laser has been injection-seeded by a pulsed single-mode dye laser [15] and a CW laser diode [16]. In this paper, we present the injection seeding of a flashlamp pumped Q-switched Ti: A1203 oscillator by a CW laser diode. This is to our knowledge the first demonstration of a compact injection-seeded Ti: Ai203 laser oscillator, which combines flashlamp pumping and laser-diode seeding. Our experimental setup is schematically presented in Fig. 1. The slave laser used is a commercial flashlamppumped Ti: AI203 laser (ELIGHT Ti: Flash series).
265
J. Yu et aL / Optics Communications 132 (1996) 263-268
HR Mirror
Brewster Window
PockelsCell
[
50% Output Coupler
x
x,, // Diaphragms or k/4 Plates
From Diode Fig. I. Schematic of the experimental setup. The diaphragms ~ d the quaxter-wave plates are only used in some specific experimental configurations.
The laser rod, 6 mm diameter, 100 mm length and 0.15% by weight doped, is pumped by four lamps supplied by discharge pulses of about 5 p,s duration. Two flat mirrors surround the laser rod forming a cavity of about 50 cm length. One of the mirrors is high reflection coated and the other, the output coupler, has a transmission of 50%. Q-switching is achieved by a Pockels cell. One or several dispersion prisms can be inserted in the cavity for a rough bandwidth narrowing (,-, 0.2 nm). When the prisms are not used, a Brewster window is inserted in the cavity to provide the polarization selection. The seeding beam comes from a CW laser diode (SDL5311-Gl), which emits a diffraction-limited beam and has a maximum power of 100 mW at 800 nm. An objective of 8 mm focal length and of 0.5 NA collimates the diode beam, and a cylindrical telescope corrects its astigmatism to get a circular-section beam of 4 mm diameter. Two optical isolators axe used to get an isolation of 60 dB to prevent the diode from optical damage. A spherical telescope is used to adapt the diode beam to the 6 mm-diameter Ti: AI203 beam. The seeding beam was coupled into the Ti: A1203 cavity through an intra-cavity reflecting surface. When the dispersion prisms were used, one of the Brewster-angle-set prism facets provided the coupling surface. Otherwise, the Brewster window was used for coupling. For an s-polarized seeding beam, about 15% (15% for a BK7 surface, 13% for a synthetic fused silica surface) power of the laser diode beam was coupled into the cavity and directed toward the high reflection mirror through the Pockels cell. In this arrangement, the seeding beam is sliced by the Q-switch [10]. When the Q-switch is closed, the injected beam is converted to p-polarization when
it is reflected back from the HR mirror, and can pass through Brewster facets without loss. After a roundtrip in the cavity, the injected beam is converted back to s-polarization and is rejected out of the cavity by Brewster facets. While the Q-switch opens and the cavity Q increases, a slice of the injection beam is confined in the Ti:AI203 cavity with ppolarization (Ti: A1203 lasing polarization). The duration and the form of the confined slice depend on the Pockels cell high voltage decreasing time and the Ti: A1203 cavity length. The spectrum of the sliced injection beam is in general broadened, this makes injectio~ seeding less sensitive to the mismatching between seeding mode and Ti : A1203 cavity modes [10]. The seeding process was monitored by observing the buildup time of the laser pulse. In Fig. 2, the laser output is recorded with a digital oscilloscope triggered by the Q-switch opening pulse. The signal is not completely time-resolved because of limited pass bandwidth of our oscilloscope. When the seeding beam is blocked, laser pulses build up from spontaneous emission with a buildup time of 320 ns (Fig. 2a). When the injection seeding occurs, laser pulses build up from stimulated emission induced by the seeding beam. The pulse buildup time, 240 ns, is shorter. To know how the Ti:A1203 laser is sensitive to injection seeding, we decreased the diode power. Effective seeding occurred even for 500 p,W coupled into the slave cavity.
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Time (ns) Fig. 2. Temporal pulse shape without seeding (a) and with seeding (b).
J. Yu et al. / Optics Communications 132 (1996) 263-268
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0
0 Fabry-Perot Scanning Ramp Fig. 3. Fabry-Perot transmission signal of injection-seeded Ti:Al20 a output (a), and laser diode beam (b). The Fabry-Petot has a FSR of 750 MHz and a f'messe of 20, it is scanned by a high voltage ramp of 20 seconds duration.
For a more quantitative study, we analysed the output pulse spectrum using a 10 cm-length confocal Fabry.Perot with a FSR of 750 MHz and a finesse of 20. The seeding power coupled into the slave cavity was fixed at about lO roW. Fine adjustment of diode current allowed us to tune the seeding mode into resonance with one of the slave cavity modes. While one of the mirrors of the Fabry-Perot was scanned by a PZT, the transmission from the Fabry-Perot was detected by a pulse detector and recorded by a digital oscilloscope. The scanning ramp duration was 20 s. In Fig, 3 we present typical recorded signals: (a) Transmission of injection-seeded Ti:Al203 beam, each output pulse gives a vertical line in the recorded trace; (b) Transmission of laser diode beam. For the second trace, the pulse detector was replaced by a photodiode. We remark that injection-seeded pulses have narrow transmission peaks with a FWHM of 250 MHz. For our 50 cm-length slave lase:" cavity, this implies that the laser operation is in one or two longitudinal modes. Another remark is that there is a transmission signal between two transmission peaks. This implies that a part of the output energy is not locked into the narrow spectral band. We define then an experimentally measurable injection seeding effi-
ciency from the Fabry-Perot transmission trace (Fig. 3a). We can consider that the energy of each laser pulse is spectrally distributed in a narrow peak, injection-seeded part, and broad lateral wings, freeruning part. When the Fabry-Perot transmission of such a pulse is detected, the free-tuning part gives a constant level versus the scanning high voltage, while the injection-seeded part provives a signal modulated by the Fabry-Perot transmission function. Our experimentally measurable seeding efficiency is then defined as the ratio between the intensity of the narrow peak, measured from the free-runing background to the top of the peak, and the total transmission intensity, measured from zero to the top of the peak. Several variant configurations of the slave laser cavity were used in our experiments. For each configuration, the seeding efficiency was measured as a function of Ti: Al~O 3 rod flash pumping level. Fig. 4 presents measured seeding efficiencies. The seeding beam was first coupled into the slave cavity through a Brewster window (Fig. 4a), then it was coupled through a prism (Fig. 4b). We studied then the effects of two quarter-wave plates and two diaphragms of 5 mm diameter surrounding the laser rod (Fig. 4c and Fig. 4d). We see that in general, the seeding efficiency decreases as pumping level increases, it runs from about 80% to about 40%.
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Pumping Discharge High Voltage (kV) Fig. 4. Seeding efficiency as a function of the pumping discharge high voltage for several different configurations of the slave laser cavity. (a) Injection through a Brewster window. (b) Injection through a prism facet. (c) With two quarter-wave plates in the cavity. (d) With two diaphragms in the cavity.
J. Yu et d/Optics
Communications
Typically, 70% energy of a 100 mJ pulse is injection-seeded into tbe 250 MHz narrow spectral band. Comparing the cavity configurations used, we get the following information: A better efficiency is obtained in curve c for high pumping level, implying that the spatial hole burning in the laser rod limits the seeding efficiency. The use of two quarter-wave plates converts the laser beam to circular polarization inside the Ti : Al,O, rod, which avoids the spatial hole burning. This solution is used in an injectionseeded Nd : YAG laser. For uniaxial laser crystals like Ti : Al,O,, unfortunately, non-negligible birefringence draws laser output energy down. An output energy of 65 mJ is obtained at a pumping level of 13.5 kV instead of 200 mJ without quarter-wave plates. The diaphragms seem beneficial for a higher seeding efficiency, because they limit flee-tuning lasing in the peripheral part of the laser rod at high pumping level. The loss introduced by the diaphragms is small, an output energy of 180 mJ is obtained at a pumping level of 13.5 kV. Injections through a prism facet (Fig. 4b) and through a Brewster window (Fig. 4a) give comparable efficiencies for middle and high pumping levels. For a low pumping level however, the injection through a prism provides a poor efficiency. This is due to the competition between the wavelength selections by the prism and by the injection seeding. In our experiments, when a prism was inserted in the slave cavity, a spectrometer was used to tune the slave laser to around the injection wavelength. The Brewster window coupling presents a simpler configuration, because the seeding mode provides the only wavelength selection. The effect of mode matching between seeding mode and slave cavity modes was studied in otr experiments. In practice, mode mismatching is due to drift of both modes, especially, that of slave modes induced by fluctuation of its cavity length. Smooth and slow fluctuation is due to ambient temperature change, and rapid variation is induced by heat dispersion in the laser rod at each flashlamp pumping. The last type of cavity length variation is particularly important for Ti : Al,O,, because short discharge pulses of several IJS are applied to the flashlamps to deposit sufficient energy within the fluorescence lifetime. In our experiments the seeding efficiency was measured for different pumping repe-
132 11996) 263-268
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tition rates. The obtained results show that we got a better efficiency for a larger repetition rate at a given pumping level. At a pumping level of 12 kV, for a repetition rate of 6 Hz, an efficiency of 0.60 was obtained, this efficiency was increased to 0.75 for a repetition rate of 18 Hz. This increase of efficiency can be interpreted by the thermal lensing in the laser rod. A higher average temperature for 18 Hz repetition rate induces a thermal lens with shorter focal length, which provides more Easingtransverse modes. The effective slave cavity mode is thus broadened, this compensates for mode drift during flashlamp pumping, and allows a higher probability for a resonance between cavity modes and seeding laser mode. The transverse mode mixing could also be provided by the use of a concave mirror in the slave laser cavity, leading to a less critical seeding mode matching condition. The standard active mode matching techniques used in an injection-seeded Nd: YAG laser [17] should also be efficient for a Ti : Al,O, laser to improve mode matching stability. But efforts should be made on rapid electronics because of the large difference in fluorescence lifetimes between Nd : YAG and Ti : Al,O,. The observation on the far-field of injection-seeded pulses by a CCD array gave a diffraction pattern, specific to a single-longitudinal mode and multi-transverse mode laser beam. In conclusion, narrow bandwidth operation within 250 MHz has been successfully demonstrated for a commercial flashlamp pumped Q-switched Ti : Al,O, laser with injection seeding from a CW laser diode. Injection seeding is shown to be sensitive even to very small seeding power. The seeding efficiency depends on the specific slave laser cavity configuration and its pumping level. Typically, 70% energy of a 100 mJ and 50 ns pulse is !ccked into the narrow bandwidth of 250 MHz, which corresponds to one or two longitudinal modes of the slave laser cavity. For P single-longitudinal mode operation with a higher spectral purity, a ring-cavity slave laser free of spatial hole burning should be used. The use of a concave mirror in the slave laser cavity, allowing for more lasing transverse modes, should lead to a less critical seeding mode matching condition. An electronic servo system should improve mode matching stability between seeding and slave lasers, leading to a better seeding efficiency. The use of an extemalcavity laser diode or a diode-pumped Cr: LiSAF
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J, Yu et al./ Optics Communications 132 (1996) 263-268
laser will provide a compact tunable ma~ter oscillator. A diode-pumped Cr:LiSAF laser is in development in our laboratory. The transverse .,~ ~e control is also in our future program. This necessitates the use of an unstable cavity for a higher transverse mode discrimination.
The authors would like to acknowledge M. N~ri and M. Barbaire for their much appreciated technical supports.
References [1] H. Kneipp, J. Kolcnda, P. Rairoux, B. Stein. D. Weidauer, J.P. Wolf and L. W~ste, Optical Methods in Atmospheric Chemistry, SPlE (1993) 1715. [2] M. Beniston, M. Beniston-Rebetez, H.J. K~lsch, P. Rairoux, J.P. Wolf and L. W~stc, J. Geophys. Rcs. 95 (D7) (1990) 13. [3] J.C. Barnes, N.P. Barnes, L.G. Wang and W. Edwards, IEEE J. Quantum Electron. 29 (1993) 2684. [4] C. Cahen, G. M~gie, P. Flamant, J. Appl. Meteorology 21 (1982) 1506; N.S. Higdon, E.V. Browell, P. Ponsardin, B.E, Grossmann, C.F. Butler, T.H. Chyba, M.N. Mayo, RJ. Allen, A.W.
Heuser, W.B. Grant, S. Ismail, S.D. Mayor and A.F. Caner, Appl. Optics 33 (1994) 6422. [5] W. Koeclmet, Solid-state laser engineering (Spingcr, Berlin, 1992). [6] N.P. Barnes and J.C. Barnes, IEEE J. Quantum Electron. 29 (1993) 2670. [7] N.P. Barnes, J.A. Williams, J.C. Barnes and G.E. Lockard, IEEE J. Quantum Electron. 24 (1988) 1021. [8] J.A. Russel and R.A. Siena, preprint (1995). [9] Y.K. Park, G. Giuliani and R.L. Bycr, Optics LetL 5 (I 98O)96. [10] D. Bruneau, T. Amand des Lions, P. Quaglia and J. Pelon, Appl. Optics 33 (1994) 3941. [11] M. De Labachclerie and P. Cerez, Optics Commun. 55 (1985) 174. [12] H.H. Zenizie, A. Finch and P.F. Moulton, Optics Lett. 20 (1995) 2207. [13] See for example, R.L. Schmitt and L.A. Ralm. Appl. Optics
25 (1986) 629. [14] K. Choi, S.H. Lin, E. Korevaar and C.S. Lin, Optics Comm. 88 (1992) 385. [15] P. Brockman, C.H. Bait, J.C. Berries, R.V. Hess and E.V. Browell, Optics Lett. I I (1986) 712. [16] C.H. Bait, P. Brockman, R.V. Hess and E.A. Moldin, IEEE J. Quantum Electron. 24 (1988) 1045. [17] L.A. Rahn, Appl. Optics 24(1985) 940; E.S. Fry, Qiquan Hu and Xingfu Li, AppL Optics 30 (1991) 1015.