3.9-μm Optical parametric oscillator based on MgO:PPLN pumped at 1966 nm using a high-repetition-rate Tm3+:Lu2O3 ceramic laser

摘要

Mid-infrared coherent sources at 3-4 μm have a wide variety of applications that include, but are not limited to gas sensing, material processing and production control. The MgO-doped periodically poled lithium niobate (MgO:PPLN) crystal is a well-known nonlinear element for quasi-phase-matched optical parametric oscillators (OPOs) typically pumped by 1-μm laser radiation [1]. However, strong multi-phonon absorption losses at wavelengths above 4 μm limit the use of MgO:PPLN for mid-IR OPO applications [2-4].The OPO used in our experiments was based on a MgO:PPLN crystal with 20×3×10 mm spatial dimensions and a 29.7-μm domain period (Labfer Ent., Russia). It was pumped by a Tm3+:Lu2O3 ceramic laser at 1966 nm, which in turn was pumped using a Raman-shifted Erbium fiber laser at 1670 nm. The ceramic laser was actively Q-switched by an acousto-optical modulator [5]. The high-quality laser beam at 1966 nm had the pulse duration of 40-100 ns at the 10-30 kHz repetition rate, and average output power of 10-12 W. An intracavity quartz etalon (with 200-μm thickness) tilted at a Brewster angle was used to decrease laser oscillation bandwidth and to stabilize the beam polarization. The PPLN (with the surface AR coated for 2 μm and 4 μm) was placed into a termostabilized cell with the temperature control, that allowed variation from 200C to 1600C. The ceramic laser beam was focused into the PPLN cell with a focal beam diameter of ~250 μm (at e-2 intensity). The double resonant OPO cavity was formed by two plane mirrors: the pump mirror with high transmission at 2 μm and high reflection at 3.7-4.1 μm, and the output coupler with ~50% reflection at 3.7-4.1 μm. The 24 mm cavity length was found to be optimal. The OPO oscillation at mid-IR wavelengths demonstrated strong temperature dependence (Fig. 1a). The optimal MgO:PPLN temperature was measured to be ~1470C , which corresponded to the degeneracy point of the OPO. The average output power reached ~530 mW (in mid-IR signal + idler) at the repetition rate of ~15 kHz (Fig. 1b). The broadband oscillations including signal and idler waves were measured at ~3.7-4.1 μm. The additional visible red wave arose simultaneously with the mid-IR OPO oscillations (the red wave at ~655 nm can be identified as a sum frequency generation in PPLN of the pump at 1966 nm and its second harmonic at 984 nm).