Frequency locking of an erbium-doped fiber ring laser to an external fiber Fabry - Perot resonator.

There have been several demonstrations of frequency stabilized lasers operating in the important 1.5-,ttm telecommunications window. Techniques include absolute frequency stabilization using atomic lines' and frequency locking to the resonance peak of a reference cavity using either the transmitted laser field2 or the reflected laser field. 3'4 For application to wavelength-division-multiplexed systems, the Pound-Drever approach (i.e., the reflected field method) is attractive since it provides evenly spaced multiple resonance peaks that can be shared by several lasers and since the reflected wave approach provides response to laser phase fluctuations that is not limited by the reference resonator lifetime. 3'4 We have recently demonstrated an all-fiber erbiumdoped ring laser whose characteristics include quantum-limited intensity noise operations narrow linewidth (<4 kHz),6 large sidemode suppression (>60 dB), and large tuning range.' This device is based on a novel tandem fiber Fabry-Perot (FFP) concept in which a first, narrow-bandwidth Fabry-Perot (free spectral range 6 GHz, bandwidth 50 MHz) provides mode selection while a second, broadband device (free spectral range 4 THz, bandwidth 26 GHz) provides tuning (see Ref. 8 for a discussion of FFP technology). Although it is quite stable, this device does exhibit periodic mode hops on a time scale of minutes and experiences slow frequency shifts because of thermal drift of the resonator. In this Letter, we demonstrate an all-fiber stabilization scheme based on the Pound-Drever approach, which eliminates mode hopping altogether and locks the lasing frequency to an external FFP resonator. The resulting device is stable for periods of several hours and opens up the possibility of locking multiple fiber ring lasers to a single reference. Such a system is inherently compatible with optical fiber and could be of potential use in a fiber link employing wavelengthdivision multiplexing, in a fiber sensing network, or as a spectroscopic source. The laser setup and the electrical schematic for the error signal generation are shown in Fig. 1. The laser includes broadband and narrow-band FFP's for tuning and mode selection, a Corning FiberGain module with a 980-nm pump diode, polarizers and a phase modulator with a polarization controller, metal-clad fiber (MCF) for laser cavity-length control, and isolators for unidirectional operation and isolation from external reflections. All the components were fiber pigtailed and assembled by using either a mechanical splicer or a fusion splicer. Lasing emission was coupled out by a 3-dB fiber coupler. The laser cavity length was 65 m (free spectral range 3 MHz). The laser was housed in a styrofoam box to provide shielding from room microphonics, which cause mode hopping. The stabilization system consists of two independent control circuits. The first causes the internal mode-selection FFP filter to track the lasing frequency as it drifts or shifts owing to tracking provided by the second control circuit. This first circuit eliminates mode hopping. The second circuit causes the lasing frequency to track a particular resonance of the external FFP reference. Both circuits utilize the Pound-Drever method. For the first circuit, we obtained the frequency deviation error signal between the lasing mode and