Measurement of the thermal-noise-limited frequency stability of a fiber-optic Bragg-grating laser

Abstract

Tunable single-frequency fiber lasers that can be integrated into optical-fiber networks are of great interest in the fiber-communication area for wavelength-division-multiplexing work.1 In particular, fiber Bragg gratings (FBGs) with a spectral bandwidth of ≈0.2 nm are attractive as laser-cavity reflectors for the construction of compact single-frequency fiber lasers. Laser-cavity length of the order of a few centimeters (2-5 cm) can be constructed by using a length of Er/Yb-doped fiber (fiber-laser high-gain medium) connected between two matched fiber-Bragg-grating reflectors.2 The Er/Yb-doped fiber is pumped optically to achieve lasing. The central lasing wavelength of this type of single-frequency fiber-Bragg-grating laser is determined by Bragg-resonance condition and thus provides a high degree of wavelength selectivity by writing gratings at different Bragg wavelengths within the gain bandwidth of the active fiber. In using this type of FBG laser in high-data- rate communication systems, both the long-term and the shortterm frequency stability of the laser output are important. For example, long-term stability, which is usually affected by the laser-cavity drift (slow length changes) and can be controlled by a laser-cavity stabilization scheme, is essential to the communication-channel assignments when the WDM approach is used. Likewise, short-term stability, which is more difficult to control, is essential for signal integrity in high-data-rate optical heterodyne detection systems, for example. This paper reports the measurement of a thermal-noise-limited short-term (<1 ms) frequency stability from an FBG single-frequency laser by using an interferometric technique.