C-Band tunable Brillouin fiber-laser with sub-Hz intrinsic linewidth

Abstract

The increase of spectral purity for a laser (fibers, microsphere, integrated optics, etc.) is of major interest for various optical functions (sources, sensors, filters, optical clocks, etc.) and for different fields of application (optical communication, spectroscopy, defense, metrology, environment, health, etc.) as the system’s margin could be considerably improved. One of the most promising approach to generate compact and narrow linewidth lasers is based on the stimulated Brillouin scattering (SBS) optical nonlinearity. The establishment of SBS in an optical cavity gives rise to the emission of a Stokes wave, which is more coherent than the optical pump used in the linear process. Impressive noise performances have been reported in such Brillouin lasers. They are measured in terms of frequency noise and reduction factor higher than 40 dB may be reached when the noise level of Stokes wave is compared to that of the pump. Frequency noise analysis consists in studying the fluctuations of a laser optical frequency. (A derivative links phase noise and frequency noise). Measurements of the laser power spectral density in the electrical domain (DC-MHz) using an electrical spectrum analyzer give all the properties of the laser line (centered on its optical frequency). The wavelength-tunability could be a very attractive property for many applications. We realize such a tunable C-Band laser with excellent performances for the whole wavelength range. Its intrinsic laser linewidth is a few ten’s of mHz. Its integrated linewidth is in the kHz (or sub-kHz) range and it could be potentially improved by locking the laser signal to a better stable reference. The frequency noise reduction is above 60 dB. . Its integrated linewidth is in the kHz (or sub-kHz) range and it could be potentially improved by locking the laser signal to a better stable reference. The nice point of our approach is that different commercial tunable C-band sources may be used, for which we simply improved the spectral properties. The method can be generalized to other wavelength ranges.