Wavelength tuning of fiber lasers using multimode interference effects.

We report on a novel scheme to fabricate a simple, cheap, and compact tunable fiber laser. The tuning is realized by splicing a piece of single-mode fiber to one end of an active double-clad fiber, while the other end of the single-mode fiber is spliced to a 15 mm long section of 105/125 multimode fiber. The fluorescence signal entering into the multimode fiber will be reproduced as single images at periodic intervals along the propagation direction of the fiber. The length of the multimode fiber is chosen to be slightly shorter than the first re-imaging point, such that the signal coming out from the single mode fiber is obtained in free space, where a broadband mirror retroreflects the fluorescence signal. Since the position of the re-imaging point is wavelength dependent, different wavelengths will be imaged at different positions. Therefore, wavelength tuning is easily obtained by adjusting the distance between the broadband mirror and the multimode fiber facet end. Using this principle, the tunable fiber laser revealed a tunability of 8 nm, ranging from 1088-1097 nm, and an output power of 500 mW. The simplicity of the setup makes this a very cost-effective tunable fiber laser.