Permanent photoinduced birefringence in a Ge‐doped fiber

We report the observation of permanent photoinduced birefringence in a Ge‐doped fiber exposed to cw mode locked at 532 nm. This effect is enhanced by increasing the Ge concentration in the core of the fiber, or by submitting it to a thermal hydrogen treatment. The photoinduced birefringence is however very weak, or nonexistent, with irradiation by 514 or 488 nm cw argon laser light.