Compact picosecond mid-IR PPLN OPO in burst-mode operation

Abstract

Short-pulsed mid-infrared (MIR) lasers have many important applications including spectroscopy, material processing and free space communication, where pulse parameters such as a high repetition rate and high peak power can play a critical role in enhancing features e.g. processing speed and signal-to-noise ratio [1]. Optical parametric oscillators (OPOs) are commonly used as MIR laser sources but they require temporal synchronisation between the pump and signal when used to produce short pulses [2]. Hence, picosecond (ps) OPO systems normally operate with a short cavity length for high repetition rate pulses with low peak power, or with a relatively long cavity length for pulses at low repetition rate and high peak power. Here we report a compact short-cavitylength ps OPO that can still provide high-peak-power pulses through burst-mode operation. MIR pulses at a high repetition rate of 1.5 GHz and controllable peak powers up to a maximum of 1.2 kW are demonstrated. The pump source for the OPO is a 1040-nm gain-switched laser-diode (GSLD) seeded Ytterbium fibre Master Oscillator Power Amplifier (MOPA) system, similar to the MOPA described in reference [3]. The GSLD technique offers highly controllable pulses at a choice of repetition rates and with flexible pulse duration. In this work, a pulse width of 37 ps and repetition rate of 1.5 GHz are selected for the experiment. In the MOPA, an electro-optic modulator (EOM) is employed as a time gate to suppress a variable number of pulses every 1 µs to enable burstmode operation with a 1-MHz burst repetition rate (Fig. 1a). The MOPA generates 14-W of maximum average output power for all numbers of pulses-per-burst. The OPO is of a signal-resonant linear cavity design consisting of two concave mirrors with a 40-mm-long periodically poled lithium niobate (PPLN) nonlinear gain medium. Both mirrors are of high reflectivity at a wavelength of 1.5 µm (signal) and ~81% transmission at both 1 µm (pump) and 3 µm (idler). The cavity length was set at just 15 cm to allow synchronous pumping every two signal roundtrips at a 1.5 GHz repetition rate. The PPLN crystal, having 5 channels of grating periods ranging from 29.52-31.59 µm , is mounted in an oven that allows temperature tuning from 20 to 200 °C, from which an idler wavelength tunability of 2260-3573 nm was realized (Fig. 1b). Time (ns) 900 ns Window Voltage