Effects of optical feedback with dispersive loss on the amplitude noise and relaxation resonance of semiconductor lasers

Optical feedback from an external cavity containing an element of dispersive loss was used to reduce the amplitude noise of a semiconductor laser. At feedback levels of P_(fb)/P_(out) ≈ 10^(-2)), a maximum amplitude noise reduction of 16 decibel was measured close to threshold but the potiential for reduction was reduced considerably at higher injection currents as the laser noise approached the shot noise limit. In addition, the threshold current decreased and the linewidth was reduced to 10 kHz. The relaxation oscillation peak in the amplitude noise spectrum was also found to be dramatically suppressed and we find evidence that the relaxation resonance can be moved to much higher frequencies using optical feedback techniques