SHOT-NOISE-LIMITED RADIOFREQUENCY LOCK-IN PHOTODETECTION WITH A CONTINUOUS-WAVE MODE-LOCKED LASER

We have developed quite a simple and original lock-in photodetection s ystem that works at a fixed radio frequency. The system includes a pho todetector, a frequency down converter and a standard low-frequency lo ck-in amplifier. The resonant implementation of the radio-frequency at tachment provides high gain and high selectivity, which have enabled u s to attain the shot-noise-limited sensitivity to relative optical pow er change delta P/P similar or equal to 4 X 10(-8) Hz(-1/2) at a detec tion frequency of 6.2 MHz in measurements with a cw mode-locked Nd:ytt rium aluminum garnet laser. We have realized the precise position-sens itive photodetection for laser pointing noise measurements and picosec ond photoacoustic spectroscopy of solids. (C) 1997 American Institute of Physics. [S0034-6748(97)00410-3].