Polarization-maintaining, double-clad fiber amplifier employing externallyapplied stress-induced birefringence

We report a new approach to obtaining linear-polarization operation of a ra re-earth-doped fiber amplifier in which the gain fiber is coiled under tens ion to induce birefringence. We demonstrated this method by constructing an Er/Yb-doped, double-clad, single-mode fiber amplifier with an output power of 530 mW and a polarization extinction ratio of >17 dB (when seeded with linearly polarized light) at a wavelength of similar to 1.5 mu m. The techn ique is achromatic, permits single- or multiple-pass operation of the ampli fier, requires no additional components in the optical path, leaves the fib er ends unobstructed, and is inexpensive to implement. (C) 2000 Optical Soc iety of America OCIS codes: 060.2420, 060.2320, 260.1440, 140.4480, 140.351 0.