Single Polarization-Mode-Beating Frequency Fiber Laser

We demonstrate a single-frequency fiber laser cavity that allows for measuring changes in the relative phase of two independent orthogonal polarization modes. Longitudinal-mode selection is carried out by a matched-pair of fiber Bragg gratings in a Fabry-Perot configuration and an intracavity polarizing beam splitter allows for adjustments on each polarization independently. The polarization-mode-beating signals generated with this laser are stable over long periods of time, owing to the noise cancellation effect achieved by the common cavity shared by both polarizations. This heterodyne detection scheme allows for measuring changes in frequency as small as 5 kHz, or equivalently, wavelength changes of 40 am (attometers) which are practically impossible to resolve in the wavelength domain. The proposed configuration has possible significance as an ultrasensitive and stable polarimetric optical fiber sensor.