Infrared Detection by Optical Mixing

A theoretical discussion is given of infrared detection systems employing an optically nonlinear crystal, a laser in the visible, and photomultiplier to detect the light produced at the sum or difference frequency. Three optical mixing systems are considered in detail and compared with direct detection: (a) cinnabar (HgS) in a single-pass optical system with the He–Ne 0.6328 μm cw laser, (b) the same crystal and laser with a ring resonator and narrow-band output filter, and (c) an ideal resonant system with a crystal as nonlinear as HgS but without absorption or double refraction. The noise output consisting of up-converted thermal noise and (in the case of the difference frequency) optical parametric noise is computed quantitatively. These systems have too small a quantum efficiency to compete with a heterodyne system employing an ir laser and a detector of high quantum efficiency such as a Ge:Cu-cooled photoconductor. The Ge:Cu detector has however a large dark noise compared to a good photomultiplier, and consequently optical mixing can surpass nonheterodyne direct detection if the required value of B≡(S/N)Δf is sufficiently small. The range of superiority of optical mixing over nonheterodyne direct detection extends high enough in B for Morse code for system (a) (if sum frequency is used), nearly high enough for a telephone channel for (b) and up to the television level for (c).