Optical feedback stabilization of the intensity oscillations in ultrahigh-frequency passively modelocked monolithic quantum-well lasers

The effect of optical feedback on the stability of the modelocking frequency of monolithic, passively modelocked semiconductor lasers at approximately 50 GHz is investigated. Optical feedback leads to linewidth narrowing, frequency pulling, and multiple modes of the intensity modulations in modelocked lasers, reminiscent of similar effects on the optical frequency in conventional continuous wave (CW) lasers. The narrowest observed line has a full-width-at-half-maximum (FWHM) of 56 kHz, which represents close to two orders of magnitude reduction of the linewidth compared to the situation without feedback. The effects of the optical feedback vary periodically as the length of the feedback delay is varied, with a periodicity that roughly matches the mode-locking wavelength. This wavelength is on the order of millimeters, which makes the mechanical stability of the optical feedback loop relatively uncritical.<>