Multiple wavelength operation of a unidirectional Er-doped fiber ring laser with optical feedback

Multiple wavelength laser operation in the 1.55 p m lowest-loss window has attracted a great attention recently because of its potential application in wavelength division multiplexed communication.’-’ We report here a equal power output, narrow bandwidth, and parasitic oscillation-free Er-doped fiber unidirectional ring laser using optical feedback. Figure 1 shows the configuration of this new ring laser. Fiber gratings are used as wavelength-selective elements. Both the polarization-sensitive isolator and the polarization controller PC1 keep the ring laser operating in unidirection preventing from two directional spatial mode competition. Meanwhile, through altering the polarization states of the multiwavelength lasing oscillation via adjusting the PCl, roughly equal power oscillation may be obtained. The closed lasing circulation path is A+B+C+D+E-+A, while the feedback path is B+F-+G+A. Assuming that the laser power at X, is P, at point B under stable state oscillation, and ignoring the insertion loss of the devices such as CP1 and CP2 as well as fiber gratings, the selected power by the fiber grating with center frequency Ai is 0.25RR,P,, optical feedback is 0.5( l-R)P,, and the laser output is [0.5R2 + 0.5(1-R)’ + 0.25R( 1-R)R,]P,, where R is the coupling ratio of the CP 1 and R, is the reflectivity of the fiber grating. 10 ni Er-doped fiber with doping concentration of4.4 X 10” cm-’, pumped at 980 nm, provides a gain to balance the ring cavity loss. The polarization controller PC2 is added into the feedback path so as to match the polarization state of the feedback with that in the ring cavity. We have compared the cases of R = 1 (without feedback), 0.9 (weak feedback), and 0.7 (large feedback). The experimental results show that optical feedback provides the following three advantages: (1) Stable laser operation with equal power outputs can be realized by optical feedback that functions through adjusting the dynamic inhomoge-