Tuning of fiber lasers by use of a single-mode biconic fiber taper.

We report a novel scheme to build a compact, tunable fiber laser. The tuning mechanism is based on the transmission property of a single-mode biconic fiber taper. While pulling the taper, we observe oscillations in the transmitted optical power that are due primarily to interference between a pair of excited modes within the tapered region, which are eventually coupled into the unstretched single-mode fiber at the end of the taper. A similar mechanism causes the modulation of the transmitted optical spectrum after the taper has been pulled and stabilized. It is this spectral modulation by the taper that is exploited here to control the wavelength of a fiber laser. The modulation can be adjusted by stretching the taper, thus enabling the tuning of the laser wavelength. We have built a 32 mW Er-doped tunable ring fiber laser with a continuous tuning range of over 20 nm and a signal-to-noise ratio of better than 45 dB over the entire tuning range; our output power is limited only by the available pump power.