Tandem heterodyne light sources with dual acousto-optic modulators as an example of the optical rotation measurement of water as a function of temperature

We propose a new, to the best of our knowledge, heterodyne light source based on a series of dual acousto-optic modulators (AOMs). This structure differs from the Mach–Zehnder architecture. The two acoustic waves with a little frequency difference are in opposite directions to each other, and there are two mirrors and a half-wave plate between them. The zeroth-order beam from the first AOM passing through the wave plate is normally incident into the second AOM. Adjusting the two mirrors lets the +1st-order beams of the two AOMs coincide, as do the two −1st order beams. Consequently, two sets of common-path heterodyne beams with orthogonal polarizations are formed. The phase resolution of a heterodyne interferometer that we implemented with this design is about 0.04 deg. We use it to measure the optical rotation (OR) of water as a function of temperature. When the temperature rises, the OR increases and goes from negative to positive.