Parallel detection of low modulation depth signals: Application to picosecond ultrasonics

We demonstrate parallel detection of laser ultrasonic signals above 50 GHz, where the signals are encoded as small modulations on a large dc background signal. The measurement problem addressed here is generic for many situations, particularly pump/probe experiments, and this paper discusses the problems of moving from single point to parallel detection and practical solutions. This is achieved with a commercial detector array, custom interface electronics and a carefully selected phase stepping algorithm. The parallel detection of Brillouin oscillations illustrates the very low modulation depths that can be measured with this technique. Noise performance and projected improvements for future custom detectors are also considered.