Ultra-Low Noise Transfer of Spectral Purity between Two Lasers of Different Wavelength using a Heterodyne Fiber Interferometer

Many research fields using lasers, such as atomic physics and quantum technologies, require low frequency noise at specific or tunable wavelength or low frequency noise between two lasers with different wavelengths. Transferring the low frequency noise of a laser to another laser with a different wavelength is a task which can be achieved with a frequency comb, but this remains a costly solution. In 2020 a new technique has been presented [1] using a Mach-Zehnder interferometer with a fiber delay line and an acousto-optic modulator (AOM). Two interferometric phase signals are digitally extracted from both heterodyne signals generated from each laser. The two phases are digitally subtracted to obtain an error signal proportional to the frequency fluctuations difference of the two lasers. One of the lasers (slave) can be frequency locked to the other (master) using a servo system. The performances published, with a transfer over 665 nm, were limited by a 1 Hz/√Hz noise floor at 100 Hz.