Signal processing scheme for broadband heterodyne gigahertz interferometry with a broadband and a second low-noise photodetector with limited bandwidth

Abstract

There is a need for highly accurate vibration measurements in the gigahertz range. To measure these vibrations with heterodyne interferometers, methods in the state of the art require both high photodetector bandwidths and high carrier frequencies. However, conventional methods such as acousto-optic modulators rarely achieve frequency shifts above 500 MHz and are inefficient at higher frequencies. Additionally, detector bandwidths are limited, or the noise level of high bandwidth detectors is insufficient. In this paper, we propose a solution to these limitations by using a setup with two phase-locked lasers to create a beat frequency in combination with a signal processing scheme that utilizes a broadband and a second low-noise photodetector with a much smaller bandwidth and low noise. Our method could enable gigahertz heterodyne vibration measurements with high resolution. The novelty of our concept is that we only detect the lower sidebands and are still insensitive to AM. This is achieved by two consecutive measurements with frequency shifting of the lasers, effectively swapping the upper and lower sidebands.