We previously reported a high-conversion-efficiency, tandem KTA-CdSe OPO source capable of covering the spectral range from 1.5 to 10 mm [1]. In that work, the first OPO, using KTA, was pumped by a 1.053-mm, Q-switched Nd:YLF laser, and the idler output was used to pump the second, CdSe-based OPO. Angle tuning of x-cut KTA crystals provided signal and idler wavelengths in the regions 1.5-2.1 and 2.1-3.5 mm, respectively. The broadly tunable source was developed for use in an OPO-based DIAL system. In this work, we use single-frequency, injection-seeding techniques to control spectral linewidths of both the pump laser and the KTA OPO in order to obtain narrow linewidths required in high-resolution gas spectroscopy. Injection seeding of the pump laser alone allowed us to increase the total OPO output by 10%.

Figure 1 shows a schematic layout of our experimental arrangement. A Nd:YLF MOPA system was used as a pump source and included a Q-switched, flashlamp-pumped, Nd:YLF ring oscillator followed by a single-pass amplifier. The singly-resonant (for the signal) OPO cavity was a simple two-mirror, 4-cm-long, standing-wave resonator. The input mirror transmitted ~ 90% of the pump and was highly reflective at the signal wavelength. The output mirror provided 30% output coupling for the signal, was highly-reflective for the pump and transmitted approximately 90% of the idler. For this work, we used a 10 x 8(y) x 20-mm size KTA crystal cut at q =60 and j =0 degrees. The crystal was placed in the center of the resonator on a rotation stage to allow angle tuning.

For injection seeding at the signal wavelengths we used a single-frequency, external-cavity diode laser tunable within 1530-1560 nm (EOSI Model 2010). We seeded the OPO through the output coupler. Since the TDL output beam is highly divergent in the fast plane, we used a cylindrical telescope to collimate the beam. A Faraday isolator was used to protect the diode laser from high-energy signal pulses. A beamsplitter transmitted the signal and reflected the idler for pumping CdSe or, as we will describe below, to conduct experiments on methane gas spectroscopy.

A small portion (~ 2%) of the signal beam reflected from the beamsplitter was used for signal beam diagnostics. This beam and the idler beam propagated at different angles due to idler walkoff in the KTA crystal. Diagnostics for seeding included observation of (1) temporal behavior of the idler output using a fast HgCdTe detector, and (2) signal spectral properties as measured by a Fabry-Perot interferometer with a resolution of 0.006 nm (740 MHz). The interferograms were visualized by using an Electro-Physics MicronViewer.

A layout of a Q-switched, Nd:YLF ring oscillator is shown in Fig. 2. We used a triangular ring cavity formed by three mirrors and a Dove prism [2]. The design provided a 90-degree rotation of the beam cross-section after a full trip through the resonator. The Q-switch consisted of a l /2 Pockels cell, a half-wave plate, and a thin-film polarizer. The laser rods of both the oscillator and amplifier were 5-mm in diameter, and 100-mm long.

As a seed source, we used a cw, single-frequency, fiber-coupled-diode-pumped, 1053-nm Nd:YLF laser with a single-frequency output power of 10 mW. We injected the seed laser beam through the output coupler of the ring resonator, thus providing single-frequency, unidirectional operation and alignment. An optical isolator was used to protect the seed laser in case of bi-directional operation of the ring laser. Injection of the seed laser light caused a reduction of the Q-switched pulse buildup time. The effect of pulse-build-up-time-reduction was used for automatic adjustment of the resonator length through a use of a locking loop and PZT-mounted cavity mirror. This technique allowed us to frequency lock a longitudinal mode of the ring laser to the frequency of the seed laser.

The output-pulse energy of the single-frequency oscillator was 75 mJ at 10-30 Hz repetition rates. The amplifier output had essentially the same beam quality (1.2 times the diffraction limit) and pulse width (25 ns) as oscillator. Seeding led to a slight decrease of the output pulse energy, as shown in Fig. 3.

With one KTA crystal we were able to tune the OPO over a spectral range from 2066 to 1573 nm for the signal, and 2167 to 3184 nm for the idler. While angle tuning the KTA crystal from 50⁰ to 70⁰, we took data on the OPO output pulse energies at 200-mJ of pump-pulse energy. In Fig. 4 we plot the experimental data on signal and idler output pulse energy versus tuning angles when the pump laser, as well as the OPO, was not seeded. Also shown are the corresponding signal and idler wavelengths.

When we seeded the pump oscillator the efficiency of the OPO increased by approximately 10%, as illustrated by Fig. 5.

We were particularly interested in generating a 3250.237-nm idler output, which corresponds to one of the absorption lines of methane gas. The associated KTA OPO signal is 1557.68 nm. We tuned the IR-seed laser to this wavelength, as measured by a cw wavemeter having a resolution of 0.001 nm. Then, by angle-tuning the KTA crystal, we matched the signal wavelength to the seed wavelength. A piezo-translator attached to the input mirror was used for adjusting and locking the cavity length on resonance with the seed-laser frequency. Once seeding occurred, the spectral width of the signal decreased from a few lines to a single line (Fig. 6). Since the spacing between adjacent longitudinal modes of the resonator at 1557.68 nm is ~ five times the interferometer resolution, our measurements indicate single-frequency operation of the KTA OPO. Seeding was stable for several minutes with no adjustments, down to 0.4 mW of injected power.

We also found that there exist two different modes of the seeded OPO operation, which are illustrated by the oscilloscope traces in Fig. 7. First, the OPO is seeded and the pump wavelength is resonant for the OPO cavity. The idler pulse energy is maximum and exceeds that of the unseeded output by ~10% (upper and middle oscilloscope traces, respectively). Second, the OPO is seeded but the pump is not resonant (lower oscilloscope trace). The idler output is ~50% lower than at the seeded, pump-resonant situation.

Thus, the pump enhancement reported here and earlier [3] provides a substantial improvement in the OPO efficiency. However, it precludes fine-tuning between adjacent longitudinal cavity modes since pump-enhanced seeding occurs only when both the signal wave and the pump wave are resonant for the cavity. When tuning the OPO by tuning the seed laser, we observed wavelength jumps by 0.343 nm which, according to our calculations, corresponds to eleven longitudinal mode spacings for the signal and five spacings for the pump.

In conclusion, we have demonstrated a single-frequency, broadly tunable OPO source that can be used for high-resolution measurements, such as gas spectroscopy. When used as a pump for another OPO based on CdSe, the source forms the basis for narrow-linewidth generation at longer wavelengths. We have shown that injection seeding of the laser source provides a 10% increase of the OPO efficiency. Another 10%-increase is obtained when the KTA OPO was seeded. Even more substantial, 50%-enhancement can be achieved when the pump wavelength is resonant for the OPO cavity.

This work was supported by the Air Force Small Business Innovative Research Program, Contract No. F29601-97-C-0120.

1. Y. Isyanova,. A. Dergachev, D. Welford, and P. F. Moulton, "Multi-wavelength, 1.5-10 m m tunable, tandem OPO," OSA TOPS, 26, 548-553, 1999.

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