5 mJ, sub-nanosecond PPSLT OPA at 0.5 kHz, tunable in the water absorption band at 3 microns

Abstract

Summary form only given. Coherent mid-IR sources (2.5-4 μm) combining high average power and high pulse energy are essential for a great number of industrial, scientific and commercial applications. Furthermore, they have broad medical implications, due to the high water absorption around 3μm. Due to the limited choice of laser materials emitting directly in this spectral range, covering it can be efficiently achieved through nonlinear frequency conversion devices, pumped by Q-switched Nd-laser systems, operating at kHz repetition rates. A fundamental efficiency limitation in short-pulsed OPOs is the available resonant wave build-up time. In order to keep this time period small compared to the pump pulse duration in short cavity OPOs, one should make use of highly nonlinear optical media. Furthermore, pulsed OPOs reaching mJ level output are prone to optical component damage due to the high amount of energy circulating inside the OPO cavity [1]. Mid-IR OPO output can be scaled significantly beyond this threshold by employing an optical parametric amplification (OPA) stage, determining the output energy and efficiency of the whole system [2]. Naturally, given the potential benefits, the search for nonlinear materials for frequency conversion in mid IR have recently intensified [1, 2]. However for really high power (energy) frequency down-conversion devices in the spectral region between 2.5 and 4 μm, periodically poled stoichiometric lithium tantalate (PPSLT) with its low coercive field (0.8 kV/mm), high refraction damage threshold and transparency up to 5 μm, is amongst the very few suitable candidate.Here we demonstrate a highly efficient OPA system with high pulse energy at kilohertz repetition rate, seeded by a singly resonant OPO tunable between 3 and 3.5 μm. The OPO employs a 20 mm long, 10 mm wide, and 3.2 mm (along z axis) thick PPSLT crystal with three polled zones with different domain inversion periods (30.2, 30.3 and 30.4 μm). The OPA stage employs a similar 37 mm long crystal, which together with the OPO crystal is antireflection coated for the pump, signal and idler waves. The OPO cavity is 27 mm long with plane parallel mirrors, highly reflective for the signal wave. The output idler wave is collimated and then focused in the OPA by two CaF2 lens with 100 mm focal lengths. The pump source is a diode-pumped MOPA system providing 35 mJ pulses with high beam quality (Mx2 x My2=1.3 x 1.1), short pulse duration (1.2 ns), at 0.5 kHz repetition rate.The maximum output idler energy is 5.1 mJ, when seeded with 0.52 mJ at 3.04 μm, while the OPA pump energy is 27 mJ, corresponding to idler conversion efficiency of 18.3 % and total conversion efficiency of 48.4 % (Fig1.a). By changing the temperature of the two PPSLT crystals from 40°C up to 265°C and employing the three domain inversion periods, we were able to achieve continuous tunability from 3 to 3.5 μm. In order to measure the idler pulse duration we frequency doubled the idler wave in 3 mm thick KTP crystal. After deconvolution with the response function of the detection system we obtain 580 ps for the SH of the idler, corresponding to an idler pulse duration of 820 ps, expected shorter than the pump pulse duration (1.2 ns) - Fig.1b. The beam quality of the idler wave was Mx2 = 50 and My2 = 44 (Fig.1c). To the best of our knowledge, this is the first sub-nanosecond coherent source, that incorporates high energy pulses (up to 5.1 mJ) with high repetition rate (0.5 kHz) and tunability in this highly relevant for biomedical applications spectral region.