Passive Mode-Locking and Millimeter-Wave Modulation of Quantum Well Lasers
Quantum well lasers with their optical cavities electrically divided into regions of saturable absorption and gain are passively mode-locked to generate picosecond pulse trains. Current is injected into the gain region, and the absorber is biased below the p-n junction turn -on voltage to control the absorption level and the rate of extraction of photogenerated carriers. Using only DC or low frequency electrical bias on monolithic devices, pulse trains at repetition rates above 100 GHz are generated for the first time. Theoretical considerations show that quantum well laser properties including the reduction of differential gain with increasing carrier density and the fast recovery time of the absorber allow passive mode-locking to occur at repetition rates far beyond the direct modulation bandwidth of a semiconductor laser, at frequencies above 100 GHz. The saturable absorption in the cavity, however, also induces self-sustained pulsations at a few gigahertz that can interfere with the higher frequency mode-locking process. However, it is shown that increasing the photon lifetime in the cavity can inhibit self-sustained pulsations while extending the range of conditions for high-frequency passive mode -locking. Multisection stripe lasers are fabricated from quantum well materials by etching through the highly conductive cap layer to electrically isolate regions along the stripes. A two-section monolithic triple quantum well GaAs/AlGaAs stripe laser is passively mode-locked at a 108 GHz repetition rate, with pulsewidths averaging 2.4 ps. More stable mode -locking is observed in a three-section passively mode-locked InGaAs/AlGaAs double quantum well laser emitting 5.9 ps pulses at a 42 GHz repetition rate. By coupling a two-section quantum well buried heterostructure laser to an external cavity, the pulse train period becomes comparable to the gain and absorber recovery times and compatible with conventional optical detectors and electronics. It is shown that the laser can operate at six different harmonics of the 1.17 GHz repetition rate, by adjustment of the gain section current only. Both small and large-signal saturation models for passive mode-locking are described and applied to determine the conditions where the laser should operate at the different harmonics and are in reasonable agreement with the experimental results. From power spectrum measurements of a laser mode -locked at 546 MHz, the timing jitter is determined to be 5.5 ps above 50 Hz, and the pulse energy fluctuations less than 0.52% above 200 Hz. While it is expected that stable pulse trains can be generated at these lower repetition rates, as is demonstrated, the stability of the millimeter -wave mode-locked lasers remains a critical problem for future research.
- Pub Date:
- January 1992
- Physics: Optics; Engineering: Electronics and Electrical