A phase-modulated feedback method for testing optical isolators assembled into the laser diode package

We present an easy-to-implement method for measuring the isolation factor of an optical isolator mounted in front of its laser diode chip. The method is based on phase-modulated feedback that is induced by a moveable mirror reflecting the emitted power back into the laser, and it consists of measuring the intensity modulation of power that is found to be well correlated to the round-trip attenuation. Measurement sensitivities up to -80 dB in terms of isolation have been obtained. The method has been tested successfully for several kinds of laser diodes, either FP or DFB with emission in first and third windows.