Spectral line narrowing of a photonically generated microwave frequency comb with vertical-cavity surface-emitting...

A technique is presented for narrowing the spectral linewidth of microwave signals generated photonically by heterodyning a pair of vertical-cavity surface-emitting lasers forming an extended optically coupled cavity. The experimentally demonstrated linewidth reduction, by as much as a factor of 10(4)--to less than 10 kHz in microwave frequencies up to a couple of gigahertz--is approximately ten times that expected with conventional line-narrowing techniques such as optical feedback. An interpretation is given in terms of mode locking in pairs of optically coupled lasers as a first demonstration in the frequency domain of lag synchronization of coupled oscillators. The results of theoretical modeling agree well with the experimental results.