A charge-coupled-device-based heterodyne technique for parallel photodisplacement imaging

Abstract

A heterodyne interferometry technique for parallel photodisplacement imaging is presented. In the parallel photodisplacement technique, a linear region of photothermal displacement is excited using a line-focusing intensity-modulated laser beam and is detected with a parallel heterodyne interferometer in which a charge-coupled device linear image sensor is used as a detector. The integration and sampling effects of the sensor provide spatiotemporally multiplexing of the interference light. To extract the spatially resolved photodisplacement component from the multiplexed sensor signal for heterodyne interferometry, a scheme of phase-shifting light integration under an undersampling condition is developed. The frequency relation for the heterodyne beat signal, modulation signal, and sensor gate signal is optimized so as to eliminate undesirable components, allowing only the displacement component to be extracted. Preliminary experimental results using a point-focused laser beam demonstrate that the technique is effective, making it possible to accurately extract photodisplacement components from the multiplexed interferogram. Subsurface structures and defects in silicon wafers are clearly imaged with a detection time of 5.3μs∕pixel. In combination with a line-focusing laser beam, this technique is very promising for high-throughput subsurface imaging with detection speeds more than 10 000 times faster than conventional photoacoustic microscopy.