Mid-infrared laser source for field tests

Abstract

Mid-infrared (mid-IR or 3.5 – 5.0 μm) laser sources are widely employed in applications such as laser projectors, remote sensing of the atmosphere and countermeasures against heat seeking missiles. In this paper, we describe the development of a mid-IR laser source which can be used as a ground-based system for remote sensing applications and testing countermeasure effectiveness in the field. For this purpose, we developed an optical parametric oscillator (OPO) based on a ZnGeP₂ (ZGP) crystal for converting a 2.1-μm pump laser into mid-IR. The pump laser is an acousto-optically Q-switched Ho:YAG laser pumped by a continuous-wave (cw) Tm:fiber laser. The maximum possible average mid-IR power obtained is 6.5 W at a pump power of 20 W with a 33% power conversion efficiency. The pump pulses resulting from the Q-switched operation of the Ho:YAG laser have a pulse width of 30 ns (FWHM) at a pulse repetition rate (PRR) of 50 kHz. Between the pump laser and the OPO, we use a Faraday isolator for protecting the pump laser from back reflections of the output beam coupling into the pump laser cavity, we also use a polarization rotator and a lens which focuses the pump beam down to a spot of 275 μm (1/e2 , diameter) at the center of the 15-mm long and 55°-cut ZGP crystal located in the middle of a 19-mm long cavity formed by two flat mirrors. The OPO cavity is doubly resonant, the input mirror is a high transmitter at 2.1 μm and a high reflector in mid-IR whereas the output mirror is a high transmitter at 2.1 μm but a partial reflector with a reflectance of 50% in mid-IR. The mid-IR and residual pump beams are separated from each other using a dichroic mirror. We characterized the performance of the OPO in terms of the mid-IR power, power conversion efficiency and pump depletion as functions of the input pump power. We recorded the wavelength spectrum of the midIR beam.