The sensitivity of optical interferometry is always limited by noise. A classical fundamental limiting factor is shot noise, as was the case in gravitational wave detectors such as LIGO before the transition to non-classical squeezed light sources. Another source of noise that can impose a limitation is laser relative intensity noise (RIN).

In a previous work [1] we presented a new technique for high-sensitivity interferometry: phase-shift-amplified interferometry (PAI).  We have shown that this technique is useful for improving the sensitivity in the presence of RIN, ADC quantization error and the non-linearity of the detector. In an experiment, we verified the theoretical prediction of sensitivity amplification, and demonstrated a sensitivity amplification factor G of 11.  

This work builds upon these results and improves them. We utilize heterodyne detection and demonstrate a sensitivity amplification factor of 35, giving 7.9⋅ 10^-4 rad, or 40 pm displacement resolution. This was achieved due to the improved immunity of PAI to the total RIN of the system. On a fundamental level, we predict that by combining PAI with coherent detection principles, the shot-noise-limited phase-shift sensitivity is improved by a factor of √2 as compared to a regular Mach-Zehnder interferometer (MZI).

Reference:

1. M. B. Ayun, E. Liokumovitch and S. Sternklar, “Phase-shift-amplified interferometry”, Optic Letters vol. 43(11) (2018).