Low frequency intensity noise characterization of an all fiber single frequency erbium ring laser

Abstract

Erbium doped fiber ring lasers are drawing increasing interest because of their narrow linewidth, broad tunability, and inherent compatibility with optical fiber. We have recently developed such a laser that is broadly tunable (>30 nm) and stable from mode hopping for a period of time on the order of minutes in spite of having a cavity length on the order of 30 m. This is achieved through the use of tandem fiber Fabry-Perot filters. One broadband filter (30 GHz bandwidth, 30 nm free spectral range) provides tuning. The other narrowband filter (125 MHz bandwidth, 10 GHz free spectral range) provides sufficient loss difference between cavity modes to achieve a sidemode suppression greater than 50 dB. We report here on the low frequency intensity noise characteristics of this device. The relaxation oscillation frequency has been characterized as a function of laser power. It is found to vary as the square root of laser power, as expected. The relaxation oscillation frequency varies from 10 kHz to 30 kHz when the laser power is varied from 0.088 mW to 0.840 mW. Mode hopping amplitude instabilities were also investigated. When the broadband tuning element was tuned so as to give approximately equal loss to two competing modes one narrowband filter free spectral range apart, a sustained, large amplitude oscillation was observed. The oscillation has a power dependence similar to that of the relaxation oscillation frequency.