An unbalanced interferometer insensitive to wavelength drift

Frequency-modulated interferometry offers a unique way to create phase modulation in 2-arm interferometers with a small footprint and cost. However, this approach is not widely used in common interferometric devices due to its sensitivity to small wavelength fluctuations and the difficulty to handle intricated phase and amplitude modulation. In this paper, we tackle these issues by performing unbalanced interferometry immune to laser wavelength fluctuations via a slightly modified setup comprising an unstabilized laser source and an additional reference arm. High performances are obtained with minimal equipment by using a power modulated VCSEL and the “generalized lock-in amplifier” method: the slow phase drift due to temperature change is efficiently canceled out while the high frequency noise is divided by a factor of 2.4. The measurement of slow displacement (0.1-10 Hz) with a sub-nanometric resolution is demonstrated. The proposed approach can be used in a number of interferometric setups including slightly unbalanced interferometers for lasers having small coherence lengths.