A. Finch, P.F. Moulton and G.A. Rines
Schwartz Electro-Optics, Inc.
Research Division
Abstract
We report on the operation of a Q-switched, cw-pumped Ti:sapphire laser, which generated 190-W peak-power, 22-ns-duration pulses at a 100-kHz rate.
Summary
The enhancement in output power provided by Q-switched operation of a cw-pumped laser is proportional to the ratio of the laser upper-level lifetime to the laser-cavity lifetime. For many systems the latter is on the order of tens of ns and thus solid state lasers such as Nd:YAG, with a 240 µs upper-level lifetime, can produce peak powers 103-104 times greater than the cw levels. On the other hand, there is no benefit to Q-switching conventional dye lasers, which have ns-upper-level lifetimes. The Ti:sapphire laser is an intermediate case, as the laser transition has a room-temperature lifetime of 3.2 µs, and some enhancement would be expected for sufficiently short cavity lifetimes.
Q-switching in Ti:sapphire has been briefly reported on by Scheps et al [1] in which the Q-switching device was a chopper placed inside the laser cavity. In this case, output pulses consisted of narrow spikes, followed by relaxation oscillations damping down to quasi-cw behaviour. In the experiments reported below, we present the first quantitative data on a cw-pumped, repetitively Q-switched Ti:sapphire laser. Output pulses were extremely clean and stable, with no inter-pulse ringing. We observed operation over a wide range of repetition rates, with peak powers of 140 W in <40-ns-duration pulses at pulse rates up to 100 kHz and peak powers of 20 W in 100 ns pulses at a 900 kHz rate, all at a 800 nm wavelength with a 24% output coupling. The laser system studied was capable of 2.3 W of cw output and thus we obtained a peak-power enhancement of over 60. In a separate Ti:sapphire laser, with a larger (40%) output coupling, we obtained 22 ns pulses at a 100 kHz rate. In this case peak powers were greater than 190 W for 11 W pump levels. In one application of the Q-switched laser we generated 1.7 W and 4.9 mW of peak and average second-harmonic power at 400 nm through the use of a BBO external doubling crystal. In all cases the Ti:sapphire laser operated on the TEM00 mode.
The Ti:sapphire laser consisted of a 0.75-cm-long Brewster-angled Ti:sapphire crystal placed in an 4-mirror, astigmatically compensated, X-configuration cavity. Also in the cavity was a 3-plate birefringent crystal and a Brewster-angle acousto-optic Q-switch driven at 24 MHz. The total cavity length was 72 cm. A variety of output couplers were used to vary the effective cavity lifetime. The data presented below were taken with an output coupler transmission of 24%. The pump source was an all-lines argon-ion laser and the threshold pump power was 1.6 W; data were taken at a pump power of 8.8 W. The level of power enhancement is, to some extent, due to the use of a heavily doped crystal and a small pump-beam spot size, which leads to high gain and the ability of the laser to operate well over threshold with a relatively high level of output-mirror transmission.
We measured the peak power, pulsewidth and average power as a function of pulse repetition rate. At the highest rate, the average power was 1.8 W. As a check on our results we compared the data with the Q-switching theory of Chesler et al. [2]. The latter is a simplification that omits the effects of a finite pulse buildup time. In fact, we were limited to a maximum repetition rate of 900 kHz by the buildup time, which increased to over 650 ns at the highest pulse rate.Figure 1 shows the measured and calculated pulsewidths of the laser, while Figure 2 presents the measured and calculated peak powers, respectively, also as a function of pulse rate. The agreement with theory is reasonably good, considering that the theory also assumes the inversion density and cavity modes to be spatially uniform in the gain medium, as distinguished from the actual case of three-dimensionally varying pump and laser-mode intensities.
Figure 1. Pulsewidth vs. the pulse repetition rate.
Figure 2. Peak output power vs. pulse repetition rate.
In harmonic-generation experiments, we focused the laser output, run at a 100-kHz pulse rate, into a pair of BBO crystals, each 6 mm long and cut for Type I SHG phasematching at an angle of 27.2 degrees (normal incidence phasematching for 860 nm). The resultant 1.7 W and 4.9 mW peak and average powers represented a conversion efficiency of 0.9%. In contrast, with the system run at a 2.3-W cw level, we were only able to observe 170 µW of 400-nm output. As the cavity lifetime in our system was determined almost entirely by the cavity length and output coupling, we expect that even higher power enhancements and shorter pulses would result from the use of shorter cavities and/or higher levels of output-mirror transmission.
Our primary goal during this experimentation was to aid the setup of a high resolution absorption spectroscopic study of atomic mercury vapor at 253.7 nm [3]. A single frequency ring, continuously scanning, Ti:sapphire laser was used in this experiment, operating at 761 nm. The UV radiation was generated by second harmonic generation (SHG) and subsequent sum-frequency generation (SFG), both in BBO. With typically available, Watt-level, Ti:sapphire cw output powers, simple external single pass SHG and SFG processes can generate tens of nW of 253.7 nm radiation. Although such power levels are perfectly adequate for detection purposes, the alignment of optics, absorption cells, etc., and apertures (to discriminate between fundamental and SHG radiation) is tedious. Obviously any enhancement in the SFG signal is advantageous. One method is to use external resonant frequency doubling and tripling to improve conversion efficiencies. Another is to use intracavity harmonic generation. Unfortunately, such methods were incompatible with the existing continuously scanning laser or could not be employed in a timely manner. Consequently we decided to initially enhance the SHG and SFG processes by increasing the peak power of the fundamental via the relatively simple means of Q-switching. With the subsequent peak-power enhancement, the setup of the SHG and SFG system was greatly facilitated.
In conclusion we have demonstrated repetitive Q-switching of a cw-pumped Ti:sapphire laser. The output pulse train is extremely stable and provides peak powers as high as 190 W, an enhancement by a factor of up to 60 over normal cw levels. This comparatively simple technique should find applications in non-linear optics and time resolved diagnostics.
Acknowledgements
We are grateful to G. Scoles, W. Lempert, and coworkers of Princeton University for their collaboration in some of these experiments. Funding for this work was provided in part by a NASA SBIR contract (contract number given in [3]).
References
[1] R. Scheps and J.F. Myers, "Dual-wavelength coupled-cavity Ti:sapphire with active mirror for enhanced red operation and efficient intracavity sum frequency generation at 459 nm", J. Quantum Electron. (to be published).
[2] R.B. Chesler, M.A. Karr and J.E. Geusic, "An experimental and theoretical study of high repetition rate Q-switched Nd:YAG Lasers", Proc. IEEE 58, 1899 (1970).
[3] NASA Contract: NAS2-13799, "Laser-based instrumentation for nonintrusive diagnostics of hypersonic reactive flows".
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