We show that a glass micro-sphere resonator can be used as a wavelength-selective mirror in
fiber lasers. Due to their high quality-factor (Q ~108), microsphere resonators possess a narrow
reflection bandwidth. This feature enables construction of single-frequency fiber lasers even
when the cavity is long. We also propose and demonstrate an active Q-switched fiber laser using
a high-Q micro-sphere resonator as the Q-switching element. The laser cavity consists of an Er-doped
fiber as the gain medium, a glass micro-sphere reflector (coupled through a fiber taper) at
one end of the cavity, and a fiber Bragg grating reflector at the other end. The reflectivity of the
micro-sphere is modulated by changing the gap between the micro-sphere and the fiber taper.
Active Q-switching is realized by oscillating the micro-sphere in and out of contact with the
taper. Nonlinear effects (such as stimulated Raman lasing) were also observed in our setup at
relatively low pump powers.