EYE-SAFE LARGE FIELD-OF-VIEW HOMODYNE DETECTION USING A PHOTOREFRACTIVE CDTE-V CRYSTAL

We demonstrate a self-adapting homodyne detection system working at a wavelength of 1.55 mu m. The system uses a photorefractive CdTe:V crys tal as the element that combines local oscillator and signal beams. Th e device spontaneously adapts to slow phase and direction changes in t he incoming light, Using a pump beam illumination of 66 mW cm(-2) and a signal power of 25 mu W, we detect phase modulations corresponding t o an optical path variation of a few tenths of nanometers at modulatio n frequencies higher than a cut-off frequency of 15 Hz. The performanc e of the system is not affected when the signal light consists of scat tered light with an ''etendue'' of up to 0.1 mm(2) sr, limited only by the lenses we are currently using, We obtain a detection limit of (6. 2 +/- 0.4) x 10(-7) nm root W/Hz. This is only about 20 times above th e theoretical limit of an ideal interferometric system. We show how to scale these results to the optimized crystals and higher laser intens ities necessary to obtain higher sensitivity (< 10(-7) nm root W/Hz) a nd higher cut-off frequencies (> 1 kHz).