Permanent refractive-index modification in germanium-doped optical fibers by use of red light

We observed photoinduced birefringence in elliptical-core optical fibers by using a continuous-wave krypton-ion laser. We induced the birefringence by injecting 20 mW of 647-nm or 50 mW of 676-nm light into the fiber at 45 degrees to the slow axis. The rate of change of the refractive index was found to be proportional to the square of the average power. Polarization mode couplers written into the fibers have been stable for more than 2 years and can be erased by use of light polarized perpendicularly to the original writing beam.