Opt. Lett. 39, 6549–6552 (2014)

An optical voice recorder based on the principle of off-axis digital holography is the latest innovation from Osamu Matoba and co-workers at Kobe University and the Kyoto Institute of Technology in Japan. The system relies on the fact that the pressure of sound waves modulates the refractive index of a medium, which in turn modulates the phase distribution of a light wave. An interferometer equipped with an image sensor can thus detect and store these phase distributions in the form of digital holograms, which contain the information required to reproduce the original sounds. The Japanese team performed tests with coherent 532 nm light from an Nd:YVO4 laser and a Mach–Zehnder interferometer equipped with a 512 × 512 pixel image sensor with a frame rate of 2,000 fps. Using this set-up they were able to record and reconstruct sounds from a tuning fork vibrating at 440 Hz and a human voice at frequencies of up to 1 kHz. The highest frequency sound wave that can be reconstructed is half of the frame rate of the camera. As state-of-the-art image sensors are now available with frame rates of ∼107 fps, in principle ultrasonic waves with frequencies as high as 5 MHz could be recorded using this technique.