Project Report

by Klitos Papaefstathiou

Project aims

This project is a low-energy laser-powered microphone that captures sound waves by measuring the fluctuations they induce on a reflective surface. The vibrating surface will introduce tiny changes in the frequency of the reflected laser beam due to the Doppler effect. By measuring these frequency variations, the resulting waveform can be demodulated to produce a sound signal. Other than supporting a wider bandwidth, this approach to sound capturing has the potential to offer higher sound quality over a conventional membrane microphone, due to greater sensitivity to vibrations. The concept is not novel, and several attempts of similar systems can be found online. However, the sound output of such projects tends to be muffled and of low quality. A notable example of a well operating device demonstrating the potential of the concept is the industrial laser microphone by Xarion. The team is aiming to build the laser system and attempt to improve the sound quality of the output by applying advanced audio processing techniques in both analogue hardware and digital signal processing form, to create a clear sound stream.

Results

Acoustic microphones have become an integral part of everyday life. Nonetheless, they present limitations that are not found in alternative technologies such as optical microphones, whose potential is being exploited in research. This project used a modified version of a Mach-Zehnder interferometer to design an optical microphone.

Speech was successfully captured by the optical microphone, then reconstructed and finally tuned using a variety of software tools. Additionally, the electronics of the device were developed and tested.

The original sound, sent to the microphone was this…
… and this was the processed sound received from the microphone.

The electronics board comprises a photodetector that captures the output of the interferometer, a transimpedance amplifier that amplifies the photodetector signal and a microcontroller that stores the recorded data such that it can then be processed by a computer.