Birefringent crystal Mach-Zehnder interferometer

A birefringent Mach-Zehnder interferometer (MZI) is provided for optically sensing a small fluctuation from an un-polarized light beam. The birefringent MZI includes first and second birefringent crystals arranged coaxially, the first crystal to receive the beam; and first and second 45° polarizers positioned behind respective the first and second crystals. The first crystal divides the beam into first ordinary and extraordinary rays. The first polarizer converts the first rays into first 45° rays. The second crystal divides the first 45° rays into second ordinary, extraordinary and recombination rays. The second polarizer converts the second rays into second 45° rays.