How to measure the phase noise of a subhertz-linewidth laser
The trick is to cross-correlate between three lasers of the same type.
Ultranarrow-linewidth lasers are used for more than just atomic clocks: certain types of spectroscopy, coherent optical communication, and interferometric sensors benefit as well. In addition to wavelength stability, phase noise (or preferably, lack of it) is important for these lasers. Phase noise is the departure of the laser beam's time-varying electrical field from a pure sine wave. The presence of phase noise limits the resolution of systems theses lasers are used in.
Phase noise for such ultraprecise lasers is also hard to measure. Now, researchers from CNRS and the CEDRIC Laboratory (both in Paris, France) and the National Institute of Standards and Technology (NIST; Boulder, CO) have come up with a cross-correlation method in which three subhertz-linewidth lasers are compared with each other, allowing the phase noise of each to be derived.1
The lasers are ultrastable laser diodes emitting at 1542 nm and are each locked to its own ultrastable optical cavity, with frequency differences between the lasers of less than 600 MHz. The measurement of each laser takes place with the other two lasers acting as references.
The researchers measured phase noise power spectral density over a 0.5 Hz to 0.8 MHz Fourier frequency range. They determined that the noise floor of the cross-correlator itself was very low -- far lower than that of any ultranarrow-linewidth laser. The three lasers themselves had integrated phase noise of less than 2/𝜋 down to a Fourier frequency of 0.5 Hz.
1. Xiopeng Xie et al., Optics Letters (2017); https://doi.org/10.1364/OL.42.001217