Gated viewing at 2.09 µm laser wavelength: experimental system assessment and comparison to 1.57 µm

We have set up a Gated Viewing (GV) system operating at a laser wavelength of 2.09 μm in the short-wave infrared (SWIR) spectral range to experimentally assess the potential of such a system for security and military applications like long-range target identification and intelligence, surveillance and reconnaissance (ISR) in low visibility conditions. In particular, we compare this system with GV systems operating at the widely used SWIR wavelength of 1.57 μm. Our focus is on examining physical effects such as laser reflection and speckles at the target surface as well as atmospheric impacts like transmission and turbulence. Finally, estimates of system ranges are made. The gated viewing camera is based on an array of 640 × 512 mercury cadmium telluride (MCT) avalanche photodiodes (APD) with a pitch of 15 μm. The cut-on and cut-off wavelengths are 0.9 μm and 2.55 μm, respectively, providing sensitivity in the extended SWIR (eSWIR) spectral range. This allows to capture both laser wavelengths 1.57 μm and 2.09 μm with the same GV camera. The camera is equipped with an aspherical F/3 lens with a focal length of 600 mm, resulting in a field-of-view (FOV) of 0.92° × 0.73°. The 1.57 μm laser is based on a commercial flashlamp-pumped Nd:YAG laser combined with an optical parametric oscillator (OPO) with a maximal pulse energy of 65 mJ at 20 Hz pulse repetition frequency (PRF) and a pulse width of τ = 11 ns. The 2.09 μm laser is an in-house developed solution with approximately 20 mJ at 20 Hz PRF and τ = 12 ns.