High-efficiency, broadly tunable, tandem optical parametric oscillators

Yelena Isyanova, Dicky Lee, Bhabana Pati, and Peter F. Moulton

Q-Peak, Inc.,135 South Road, Bedford, MA 01730

Phone: 781-275-9535

FAX: 781-275-9726

email: isyanova@qpeak.com

Abstract: We report recent advances in KTA - CdSe tandem OPO systems. Our results include: 1) difference – frequency generation between the KTA signal and the idler 2) injection seeding of the KTA signal and 3) operation of the system at a 4-kHz pulse rate.

Ó 1999 Optical Society of America

OCIS codes: (190.4970) Parametric oscillators; (140.0140) Laser and laser optics

1. Introduction

We previously reported a high-conversion-efficiency, tandem KTA-CdSe OPO source capable of covering four bands in the spectral range from 1.5 to 10 mm [1]. The design uses the angle-tuned, idler output of a KTA OPO to pump a non-critically phase-matched (NCPM) CdSe OPO. In this work, we use two techniques to increase KTA OPO efficiency 1) difference-frequency generation (DFG) in a second KTA crystal placed in the KTA OPO cavity, and 2) injection-seeding of single-frequency radiation at the signal wavelength into the KTA OPO cavity. We also demonstrate a tandem OPO pumped by a cw-diode-pumped, Q-switched, 4-kHz-repetition-rate, Nd:YLF laser.

2. System description and experimental results

Figure 1 shows a schematic layout of one of our experimental arrangements. The pump laser was a 20-Hz, Q-switched, flashlamp-pumped, Nd:YLF ring-resonator oscillator with a 5x100-mm rod. As a seeding source we used a 20-mW cw, single-frequency, diode-pumped 1053-nm Nd:YLF laser. A 6x100-mm rod, single-pass amplifier followed the oscillator, and generated 200 mJ of energy.

It has recently been shown [2] that mixing the signal and idler waves in a nonlinear crystal placed into the OPO resonator can produce long-wavelength radiation at the difference frequency and enhance the generation of idler photons. Our calculations showed that, by angle-tuning both a KTA DFG and a KTA OPO crystal, we would expect to cover the spectral range 2.7-4.7 m m for DFG and 3.5–2.7 m m for the idler. For experimental verification, we used a two-crystal, ring resonator. From our calculations, we found that there exists a degenerate angle, »  600, where KTA operates as an OPO and DFG simultaneously. Fine angle-tuning of both two crystals allowed us to find an angular position at which the idler energy increased by 20% and the DF pulse emerged. The signal, idler and DFG wavelengths were measured to be 1.688, 2.80, and 4.255 m m, respectively. The idler and DFG output energies were 15 mJ and 1.25 mJ, respectively.

For injection-seeding experiments, we used a singly-resonant (for the signal) standing-wave, angle-tuned, two-mirror cavity for the KTA OPO. In Figure 2 we plot experimental data on signal and idler output pulse energies versus tuning angles when neither the pump laser nor the OPO were seeded. Also shown are the corresponding signal and idler wavelengths. We observed an increase of the OPO energy by ~10% because of injection seeding of the pump laser. Once seeding of the OPO occurred, the spectral width of the signal decreased from a few lines to a single line (Fig. 3).

We also found that there exist two different modes of seeded OPO operation, which are illustrated by the oscilloscope traces in Fig. 4. First, the OPO is seeded and the pump wavelength is resonant for the OPO cavity. The idler pulse energy is maximal 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 for the seeded, pump-resonant case. Thus, the pump enhancement reported here and earlier [3] is found to provide a substantial improvement in OPO efficiency.

With the KTA idler as a pump, we were able to operate the CdSe OPO over signal and idler wavelength ranges of 3.66-5.12 and 7.99-10.57 m m, respectively.

 

 

Fig. 1. Tandem OPO system setup.

Fig. 2. Unseeded KTA OPO output pulse energies and corresponding wavelengths versus tuning angle.

Finally, we report efforts to increase the average output power and pulse rate of the tandem OPO. The pump source was a diode-pumped, 1.047- m m Nd:YLF laser, which delivered 30 W of average output power or 7.5 mJ/pulse at a 4-kHz rate. We used two x-cut KTA NCPM crystals in a singly-resonant two-mirror cavity, in which the pump was double-passed. The output data for the KTA OPO are shown in Fig. 5, with signal and idler wavelengths of 1.52 and 3.45 m m . We were able to use the KTA idler output to pump a CdSe OPO just over threshold, generating signal and idler wavelengths of 5.12 and 10.57  m m . This represents, to our knowledge, the highest-pulse-rate CdSe OPO ever operated.

3. References
  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.
  2. M. E. Dearborn, K. Koch, G. T. Moore, and J. C. Diels, "Greater than 100% photon-conversion efficiency from an optical parametric oscillator with intracavity difference-frequency mixing," Opt. Letters 23, 759-761, 1998.
  3. K .Schneider, P. Kramper, S. Schiller, and J. Mlynek, "Toward an optical synthesizer: a single-frequency parametric oscillator using periodically poled LiNbO3," Opt. Letters 22, 1293-1295, 1997.

  Fig. 3. Signal spectral output in seeded (left) and unseeded (right) operation.

Fig. 4. Oscilloscope traces of the idler output pulses.

Fig. 5.  KTA OPO signal and idler outputs as a function of 1.047-m m pump power at 4-kHz repetition rate.

WB01624_.gif (281 bytes) If you got here from a search engine, click the icon to go to Recent Technical Papers at Q-Peak