Abstract
Passively mode-locked pulse generations at a 40 GHz repetition rate were demonstrated without using any saturable absorbers. In a conventional dispersive Fabry-Perot cavity with the length corresponding to a mode-locking frequency of lower tens GHz, the phase differences between longitudinal modes become large and cause the out-of-phase mode-locking. In the present work, the gain section was restricted to a half-length of the cavity, then the phase differences were suppressed, and mode-locked pulse generations were achieved. A distributed Bragg reflector (DBR) was implemented in the cavity as an oscillating mode selector. The obtained pulses were almost transform-limited. A typical time-bandwidth product of 0.45 was obtained for a DBR length of 291 µm. By varying the DBR current, the center wavelength of mode-locked pulses was changed over 5 nm. Time-bandwidth products for pulses generated from lasers with various coupling lengths of DBR are also investigated experimentally.