Nonlinear image up-conversion based on intra-cavity nonlinear sum-frequency mixing (SFM)  is a useful tool for extending laser illuminated range-gated systems to the eye-safe  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 . 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 . This is too long a gating time for imaging through some backscattering media like dense fog . 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.
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