Range-gated active short-wave infrared imaging for rain penetration

Abstract

A fixed wavelength optical parametric amplifier laser was constructed to create nano-second pulsed light at 1527 nm for eye-safe range gated active shortwave infrared (SWIR) imaging. The laser pulse is generated using a Nd:YAG operating at 1064 nm to pump a potassium titanyle arsenate crystal in a single-pass geometry, resulting in ≈85 mJ at up to 28 Hz at 1527 nm with a pulse duration of ∼8 ns. The SWIR imager can be gated in as short as a 70-ns window (active time) with time steps as small as 5 ns (e.g., 0 to 70 ns and 5 to 75 ns). This allows for significant weather penetration by reducing the contribution from back-scattered photons reaching the receiver from scattering along the path prior to the imaging target. In addition, by converting to 1527 from 1064 nm, the maximum permissible exposure limit for eye-safety is ≈4 orders of magnitude larger, allowing for higher fluences without risk of injury. NEXRAD weather data along the line-of-sight were utilized to facilitate atmospheric propagation modeling via MODTRAN to retrieve an estimate of the path transmittance during testing. The integrated system herein is shown to extend imaging ranges with improvement factors of ≈2.5-3.1× during periods of rain. An image of a water tower at a 10-km range was generated with visibilities between 3 and 4 km during testing.