Electro-Optic Control of Intra-Cavity Image Up-Conversion for Fast Range-Gated Systems

Abstract

Nonlinear image up-conversion based on intra-cavity nonlinear sum-frequency mixing (SFM) [1] is a useful tool for extending laser illuminated range-gated systems to the eye-safe [2] and other infrared (IR) spectral regions. In general, a range-gated system benefits from the high luminance gain and fast gating of image intensifier tubes or ICCDs [3]. In order to take advantage of the high gain of EMCCD cameras as an alternative to ICCDs in range-gated applications, a fast image gating mechanism has to be added to the system. Presently, EMCCD sensors can not be electronically shut below the μs regime [4]. This is too long a gating time for imaging through some backscattering media like dense fog [3]. For single-pass external SFM up-conversion of an IR image with a pump laser beam, it suffices electro-optically shutting the pump beam (usually collimated). Electro-optic gating of the IR image is not useful, as the different image rays undergo different optical path lengths in the electro-optic shutter, creating obscure rings in the up-converted image. In CW intra-cavity image up-conversion, electro-optic extinction of the pump laser beam prevents image up-conversion. However, it leads to Q-Switched laser operation, with an up-converted image gating time width set by the Q-Switched laser pulse duration which can not be controlled arbitrarily. We present a technique to allow high gain EMCCDs to be used for range-gated imaging with fast and arbitrary time-width in intra-cavity continuous-wave (CW) image up-conversion systems. It is based on electrooptic control of the laser beam polarization travelling through an intra-cavity biaxial nonlinear crystal, so that the Type II phase matching condition is spoiled transiently, whilst maintaining the CW operation of the laser to avoid the generation of a definite-time Q-Switch pulse, impeding arbitrary control of the up-conversion time width.