SUB-PICOSECOND OPTICAL PULSE GENERATION AT 350 GHz IN MONOLITHIC PASSIVE CPM MQW LASERS

Sub-picosecond optical pulses have been successfully generated using a novel monolithic passive colliding pulse mode-locked (CPM)l semiconductor multiple quantum-well (MQW) laser. Transform-limited pulse width of 830 femto-seconds is achieved at a wavelength of 1.5pm and a repetition rate of 80 GHz. The measured timebandwidth product is 0.31. A record high repetition rate of 350 GHz is also demonstrated with 900 femto-seconds pulse width. INTRODUCTION Generation of short optical pulses using monolithic mode-locked semiconductor lasers has attracted a great deal of attention recently.2-8 In active mode-locking case, the phases of the longitudinal modes are locked together by an external microwave source. Passive mode-locking requires the saturable absorber cross-section to be larger than the gain cross-section9 Multiple quantum well (MQW) material is ideal to realize monolithic passive mode-locked lasers because this criterion can be satisfied by operating the same quantum well materials under different bias conditions." Other properties of MQW such as low dispersion, broad gain spectrum, and fast saturation and recovery time further improve the performance of the monolithic mode-locked Previousely, we have incorporated the colliding pulse mode-locking (CPM) technique into a monolithic active mode-locked quantum-well laser?78 An RF source was used to synchronize two counter-propagating optical pulses to collide in a center saturable absorber. In this paper, we report a passive monolithic CPM laser that generates continuous and stable sub-picosecond optical pulses without using any synchronization sources. The two counter-propagating pulses time themselves to collide in the center saturable absorber because of minimum energy loss. Shorter pulse can be obtained with the enhanced effectiveness of the saturable absorber.' The transient grating formed by the two pulses at the saturable absorber helps to generate pulses with transform-limited width. Optical puls.es with transform-limited widths of 830 femto-seconds are achieved. This is the first time that tranform-limited pulses with sub-picosecond widths are generated with any monolithic modelocked semiconductor lasers.