Eye safe large field of view homodyne detection using a photorefractive CdTe:V crystal

Abstract : We demonstrate a self-adapting homodyne detection system working at a wavelength of 1.55 μm. The system uses a photorefractive CdTe:V crystal as the element that combines local oscillator and signal beams. The device spontaneously adapts to slow phase and direction changes in the incoming light. Using a pump beam illumination of 66 mW*cm-2 and a signal power of 25 μW, we detect phase modulations corresponding to an optical path variation of a few tenths of nanometers at modulation frequencies higher than a cut-off frequency of 15 Hz. The performance of the system is not affected when the signal light consists of scattered light with an "étendue" of up to 0.1 mm2*sr, limited only by the lenses we are currently using. We obtain a detection limit of (6.2±0.4)10-7 nm* W/Hz . This is only about 20 times above the theoretical limit of an ideal interferometric system. We show how to scale these results to the optimized crystals and higher laser intensities necessary to obtain higher sensitivity (< 10-7 nm* W/Hz ) and higher cut-off frequencies (> 1 kHz)