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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Study on optical heterodyne detection with photorefractive adaptive crystal

Changqing C. Cao, Xiaodong Zeng, Yili Zheng, Zhejun Feng, Wenrui Zhang

Published Nov 29, 2007• 0 citations

Abstract

A photorefractive adaptive optical heterodyne detection system is proposed. Its principle is introduced and typical numerical calculation is illustrated, which includes the reference beam output power and minimum detectable signal power versus system global power ratio, the effect of crystal coupling strength on reference beam output power and minimum detectable signal power, and the minimum detectable signal power versus nonlinear coupling strength. Several key factors which affect the system characteristics strongly are pointed out. In the proposed system, only one photorefractive crystal is used to attain optical heterodyne detection. The system features simple structure, low noise and high responsibility. It also answers two key questions of heterodyne detection: 1) phase distortion; 2) coherent length of light source.

DOI: 10.1117/12.746570

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Two-wave mixing in photorefractive crystals has been shown to combine coherently a modulated optical signal beam with a strong pump optical field.’ For a phase-modulated signal beam, the induced index of refraction phase grating inside the crystal yields a coupling of the optical fields of the pump and signal so that subsequent intensity detection results in apparent intensity modulation of the transmitted signal or pump beams. This provides an easy way to detect phase differences between two optical beams. Some of the advantages the above scheme offers over commonly used coherent optical phase detection schemes are: no loss of 3 dB in the signal beam intensity at the 50/50 beam splitter (used to combine signal and local oscillator fields); automatic alignment of the index of refraction grating with the two incident optical wavefronts; insensitivity to slow random changes in the phases of both beams. Experimental results are given, and a new definition for the resulting mixing gain for the phase-modulated beam is introduced, through which optimal conditions for input beam intensity ratios can be found. Applications to coherent homodyne detection are discussed.

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