Low-noise optical heterodyne spectrum analyzer

A prototype setup for a low-noise, high-resolution, multi-Gigahertz-span optical heterodyne spectrum analyzer is presented. The frequencies of the unknown optical spectrum are converted to zero intermediate frequency by heterodyning with the output from a tunable single-mode local oscillator laser in an optical mixer. The low-frequency beats are rectified, and their strength is recorded while the spectrum is scanned by tuning the local oscillator. Low-noise heterodyning requires double balance (or at least single balance) of the optical mixer. Further optimization of the noise performance can be achieved by adjusting the power levels of the light beams. Successful operation of the analyzer has been demonstrated by recording the output spectrum of a mode-locked dye laser.