A "dual", feed-forward + feed-back frequency control for efficient and convenient diode laser line narrowing

DFB diode lasers have linewidths in the MHz range. Narrowing the linewidth can be obtained by locking the diode to a frequency discriminator (Michelson interferometer,...). But the lock bandwidth has to reach 10 MHz or higher. The current-to-frequency transfer function of DFB lasers is complex: thermal effects at low frequency, carrier physics at high frequency, and a [1MHz-20MHz] transition zone in which the phase and amplitude show variations that prevent from locking in the MHz range. Adding a phase modulator in feed-forward (FF) configuration allows convenient control of the laser frequency, in excess of 15MHz, and convenient+efficient line narrowing with commercial, off the shelf components of the telecom industry.