CW mid-IR OPO based on OP-GaAs

Abstract

Summary form only given. Orientation-patterned GaAs (OP-GaAs) has grown in importance over the last decade as a quasi-phasematched (QPM) nonlinear optical (NLO) material for frequency conversion applications in the mid-infrared. OP-GaAs is attractive due to its high nonlinearity (d 14 = 94 pm/V), high thermal conductivity (46 W/mK), low absorption loss, and wide transparency range (~0.9-18 μm). Since the first reports of pulsed (ns) SHG and OPO operation in OP-GaAs, ns OPOs have improved in output power and efficiency and device demonstrations have expanded to broader spectral ranges (including THz generation) and temporal regimes (fs and ps). Continuous wave (cw) frequency conversion in OP-GaAs has been limited to DFG, and more recently SHG demonstrations at relatively low power and efficiency. Despite attempts by several groups, cw OPO operation in OP-GaAs has not been previously achieved. OP-GaAs must be pumped at λ > 1.73 μm to avoid two-photon absorption and free-carrier absorption, but to date cw OPO has yet to be demonstrated in any NLO material pumped beyond 1.55 μm since the OPO threshold scales as the cube of the pump wavelength. A 3-inch OP-GaAs template was produced by MBE, and a 1.7-mm-thick QPM grating structure was grown using low-pressure hydride vapor phase epitaxy. A 39.7 x 8.3 x 2.3 mm3 OPO sample with a grating period of 63.5 μm was cut from this wafer, polished, and AR-coated (R<; 0.5% per surface at 2.09 and 3.4-4.8 μm). Absorption losses of 0.004cm-1 at 2.4 μm were measured using photo-thermal common path interferometry. The Ho:YAG 2.09-μm MOPA (60 W, M2~1.3) was pumped by thulium fiber lasers. The oscillator had an AO Q-switch (enabling pulsed operation for initial alignment) and an etalon to narrow the spectral width to ~0.3 nm. The pump was focused to 126 μm diameter centered in the OP-GaAs crystal and polarized in the plane of the wafer. The OPO resonator was configured as a bow-tie ring where M1, M3, and M4 were coated for high transmission at the pump and idler and high reflection at the signal. M2 was coated for 2%T at the signal. All mirrors were used at 5 degree incidence: mirrors M3 and M4 had curvatures of 10 cm while M1 and M2 were flat. The physical cavity length was ~87 cm and the arms were adjusted to optimally overlap the signal cavity mode with the pump over the crystal. A long pass beam splitter was used to strip the residual pump away from the OPO beam exiting M4. The output power from M1, M2, and M4 were measured through various filters to separate the content in each spectral range. The OPO output power was measured at a crystal temperature of 41°C where the 3.8 μm signal (7nm FWHM) and 4.7 μm idler (9.5nm FWHM) were best suited for the mirror reflectivity. The maximum total output power (signal plus idler) was 5.3W from M1, M2, and M4. The majority of the OPO power exited from output mirror M4 where the content was 4 W of idler. The 2% coupling M2 allowed an additional 1 W of signal output. M1 leaked a small amount of both waves owing to imperfect mirrors. The OPO threshold was 11.5W, and maximum conversion efficiency of 23.6% occurred at 1.8 times threshold. The idler beam was TEM00 and with an M2 of approximately 1.4.