Abstract

Recently, we demonstrated extremely low threshold optical parametric generation (OPG) pumping a CdSiP₂ (CSP) crystal with 500-ps-long pulses from a Nd-laser system [1]. Diode laser seeding enabled narrow-band non-critical OPG operation at 1-10 kHz repetition rate with the idler at ~6100 nm. In comparison to [2], with a simpler Q-switching based pump system, a 4-fold improvement of the time-bandwidth product of the mid-IR pulses was obtained. Further reduction of this product, which was still ~5 times above the Fourier limit, is reported here. The pump laser system is a Q-switched Master Oscillator Power Amplifier (MOPA) laser. The master oscillator is a micro-chip PQS laser providing a train of single longitudinal mode, 120-ps long pulses, at 230 kHz repetition rate, with 4-mW average power. The average power is increased in a two-stage Nd:YVO₄ amplifier up to 5.4 W, corresponding to a single pulse energy of ~ 24 μJ and a peak-power of about 0.2 MW at 1064 nm, with a close to diffraction limit beam quality [3]. The OPG is based on a 21.4-mm long CSP crystal, cut for non-critical (90°), type-I (oo-e) phase matching. Both 4.1 × 6.1 mm² crystal facets are AR-coated with a single layer of Al₂O₃, optimized only for the pump and signal wavelengths (20% per surface residual reflectivity at 6100 nm). The experimental setup is shown in Fig. 1(a). The pump beam is focussed in the CSP crystal with a spherical lens to a spot of 0.62 × 0.53 mm² (diameters) along the horizontal and vertical (parallel to crystal c-axis) directions respectively. In these conditions the unseeded OPG threshold was as low as ~ 6.5 μJ incident pump pulse energy, corresponding to an on-axis peak fluence of ~ 5 mJ/cm², lower compared to the results reported in [1]. We obtained 120 mW maximum idler output power, equivalent to a pulse energy of 0.52 μJ at ~6100 nm. The maximum signal output power was >1 W, with a total conversion efficiency >20%.

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