Performance of AFC for phase noise reduction of optical sources

The letter evaluates the use of an automatic frequency control (AFC) circuit for reducing the phase noise of an optical source. The results show that this type of circuit reduces efficiently only the low-frequency variations of the phase noise. They also show that the high-frequency components of the phase noise can be significantly enhanced relative to those of the free-running source if the product of the loop gain by the loop bandwidth is made too large. These effects result from the time delay presented by the error signal and by the large loop gain needed to reduce sufficiently the phase noise of the optical source.