B. Gerard

All-epitaxial fabrication of thick, orientation-patterned GaAs films for nonlinear optical frequency conversion

Orientation-patterned GaAs (OPGaAs) films of 200 μm thickness have been grown by hydride vapor phase epitaxy (HVPE) on an orientation-patterned template fabricated by molecular beam epitaxy (MBE). Fabrication of the templates utilized only MBE and chemical etching, taking advantage of GaAs/Ge/GaAs heteroepitaxy to control the crystal orientation of the top GaAs film relative to the substrate. Antiphase domain boundaries were observed to propagate vertically under HVPE growth conditions so that the domain duty cycle was preserved through the thick GaAs for all domain periods attempted. Quasiphase-matched frequency doubling of a CO2 laser was demonstrated with the beam confocally focused through a 4.6 mm long OPGaAs film.

Optical parametric oscillation in quasi-phase-matched GaAs

We demonstrate a GaAs based OPO. The singly resonant, pulsed OPO utilized an all-epitaxially-grown orientation-patterned GaAs (OP-GaAs) crystal. By tuning either near-IR pump wavelength or OP-GaAs temperature, we achieved broad tunability between 2 and 10 microns.

Improved dispersion relations for GaAs and applications to nonlinear optics

The refractive index of GaAs has been measured in the wavelength range from 0.97 to 17 μm, which covers nearly the entire transmission range of the material. Linear and quadratic temperature coefficients of the refractive index have been fitted to data measured between room temperature and 95 °C. In the midinfrared, the refractive index and temperature dependence are obtained from analysis of etalon fringes measured by Fourier-transform spectroscopy in undoped GaAs wafers. In the near infrared, the refractive index is deduced from the quasiphasematching (QPM) wavelengths of second-harmonic generation in orientation-patterned GaAs crystals. Two alternative empirical expressions are fitted to the data to give the refractive index as a function of wavelength and temperature. These dispersion relations agree with observed QPM conditions for midinfrared difference-frequency generation and second-harmonic generation. Predictions for various nonlinear optical interactions are presented, including tuning curves f...

Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation

Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mum grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO(3) were directly compared, and a ratio of nonlinear coefficients d(14)(GaAs)/d(33) (LiNbO(3))=5.01+/-0.3 was found at 4.1-mum fundamental wavelength. For input pulse energies as low as 50muJ and approximately 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs.

Difference frequency generation of 8-µm radiation in orientation-patterned GaAs

First-order quasi-phase-matched difference frequency generation of narrowband tunable mid-infrared light is demonstrated in orientation-patterned GaAs. The all-epitaxial orientation-patterned crystal is fabricated by a combination of molecular beam epitaxy and hydride vapor phase epitaxy. Lasers at 1.3 and 1.55 microm were mixed to give an idler output at 8 microm, with power and wavelength tuning consistent with theoretical estimates, indicating excellent material uniformity over the 19-mm-long and 500-microm-thick device.

Quasi-phase-matched gallium arsenide for versatile mid-infrared frequency conversion

Progress in processing low-loss quasi-phase-matched gallium arsenide crystals makes it possible to benefit from their excellent nonlinear properties in practical mid-infrared sources. This paper addresses both crystal growth aspects and the most recent device demonstrations.

Longwave infrared, single-frequency, tunable, pulsed optical parametric oscillator based on orientation-patterned GaAs for gas sensing.

We demonstrate a nanosecond single-frequency nested cavity optical parametric oscillator (NesCOPO) based on orientation-patterned GaAs (OP-GaAs). Its low threshold energy of 10 μJ enables to pump it with a pulsed single-frequency Tm:YAP microlaser. Stable single-longitudinal-mode emission is obtained owing to Vernier spectral filtering provided by the dual-cavity doubly-resonant NesCOPO scheme. Crystal temperature tuning covers the 10.3-10.9 μm range with a quasi-phase-matching period of 72.6 μm. A first step toward the implementation of this device in a differential absorption lidar is demonstrated by carrying out short-range standoff detection of ammonia vapor around 10.4 μm. Owing to the single-frequency emission, interferences due to absorption by atmospheric water vapor can be discriminated from the analyte signal.

Optical parametric oscillator based on microstructured GaAs

We demonstrate an optical parametric oscillator (OPO) based on GaAs. The OPO utilized an all-epitaxially-grown orientation-patterned GaAs (OP-GaAs) crystal, 0.5-mm-thick, 5-mm-wide, and 11-mm-long, with a domain reversal period of 61.2 μm. By tuning either the near-IR pump wavelength between 1.75 and 2 μm, or the temperature of the GaAs crystal, the mid-IR output tuned between 2 and 11 μm, limited only by the spectral range of the OPO mirrors. The pump threshold of the singly-resonant OPO was 16 μJ for the 6-ns pump pulses, and the photon conversion slope efficiency reached 54%. Also, we show experimentally the possibility of pump-polarization-independent frequency conversion in GaAs.

OP-GaAs OPO pumped by 2μm Q-switched lasers: Tm;Ho:silica fiber laser and Ho:YAG laser

Improvement in hybrid vapour phase epitaxy growing techniques of quasi-phase-matched orientation-patterned GaAs (OP-GaAs) allows larger sample thickness and permits efficient operation as a mid-infrared optical parametric oscillator at Watt-level average output powers [1-3]. Especially its low absorption loss (- 0.01 cm-1), its laser damage threshold comparable to ZGP (- 2 J/cm2) combined with a large nonlinear coefficient, a good thermal conductivity, excellent mechanical properties, and a wide transparency range (0.9-17 μm) are suitable properties for efficient non-critical phase matched OPOs. As there is no natural birefringence in GaAs, phase matching is independent of polarization and propagation direction, offering the ability to pump OP-GaAs with a variety of polarization states. Thus, even unpolarized or poorly polarized sources like simple fiber lasers have been efficiently used as pump sources [4-5]. The paper discuss the best OP-GaAs OPO results achieved, to our knowledge, using a Q-switched 2.09 μm Ho:YAG laser as pump source as well as results obtained with an OP-GaAs OPO directly pumped by a 2.09 μm Q-switched Tm,Ho:silica fiber laser. With a 2.09 μm Q-switched Ho:YAG fiber laser pump source up to 2.9 W of average output power was achieved at 20 kHz repetition rate, 3.9 W at 40 kHz and 4.9 W at 50 kHz. With a 2.09 μm Q-switched Tm3+,Ho3+:silica fiber laser pump source, up to 2.2 W of average output power was achieved at 40 kHz repetition rate, 1.9 W at 60 kHz and 1.3 W at 75 kHz in the mid-infrared range.

Compact efficient mid-infrared laser source: OP-GaAs OPO pumped by Ho3+:YAG laser

Due to a wide transparency range (0.9-17 μm), a low absorption loss (~ 0.01 cm-1), and a laser damage threshold comparable to ZGP crystals (~ 2 J/cm2), combined with excellent nonlinear, thermal and mechanical properties, quasi-phase-matched orientation-patterned gallium arsenide (OP-GaAs) crystals are well adapted for efficient mid-infrared optical parametric oscillators (OPOs). The paper discusses the best results obtained, to our knowledge, with an OP-GaAs OPO pumped by a Qswitched 2.09 μm Ho3+:YAG laser. The compact (33 × 48 cm) high-repetition rate source developed allows to achieve 4.0 W of average output power in the 3-5 μm range at 40 kHz repetition rate with a 45 % slope efficiency and a very good beam quality (M2 < 1.8). 6.4 W were obtained at 70 kHz with a 51 % slope efficiency, and 7.7 W at 100 kHz with a 46 % slope efficiency. At 40 kHz and 70 kHz, an optical damage occurred at a fluence of 1.9 J/cm2 and 1.5 J/cm2 respectively. The power is limited by the OP-GaAs crystal thickness and is expected to be scaled in thicker samples recently fabricated.