Semiconductor mode-locked lasers : modeling, characterization and applications

This thesis describes the modeling and characterization of mode-locked semiconductor lasers. An enhanced dynamic model is developed to describe the startup and steadystate behavior of mode-locked lasers. Two new applications for mode-locked lasers are given and their potential is discussed. A new technique to characterize the optical pulses emitted from a mode-locked laser is analyzed and demonstrated. A combined time and frequency-domain dynamic model is introduced for semiconductor mode-locked lasers. The model includes both linear mode-coupling effects through carrier density modulation at harmonics of the mode-spacing as well as nonlinear effects like gain saturation and additional mode-coupling through four wave mixing. The model is used to study the behavior of a 2 mm long mode-locked semiconductor laser with a gain section of 1900 µm and an absorber section of 100 µm. Without the inclusion of spontaneous emission, steady state mode-locking is achieved after a few tens of nanoseconds producing chirped picosecond pulses. The inclusion of spontaneous emission disturbs the steady state mode-locking solution into a quasi-steady state which causes timing and amplitude jitter of the pulse train. The potential of a semiconductor mode-locked laser with a dense mode spacing (~25 GHz) as an optical source for wavelength division multiplexing is studied. One of the locked modes is filtered out by a narrow band fiber Bragg grating and its use as a single wavelength source is examined. The bit error rate (BER) performance of the source is measured but no "error free" transmission is achieved due to mode competition noise. The laser is next used in an external feedback configuration where the feedback is provided by a fiber Bragg grating. Lasing only occurs when the fiber Bragg grating is tuned to one of the monolithic cavity modes leading to a discretely tunable single wavelength source whose channel spacing is determined by the mode spacing of the semiconductor laser. Single mode operation of the laser with more than 40 dB side mode suppression is obtained. The BER performance of several channels is examined by stretching the fiber Bragg grating. "Error free" performance is obtained for all channels. As another new application, the use of semiconductor mode-locked lasers in a photonic analog to digital (A/D) converter is proposed. The method uses wavelength multiplicity to increase the sampling rate of A/D converters. The optical output of a number of semiconductor lasers each mode-locked at a different center wavelength is spectrally stitched and time-interleaved into a high repetition rate multi-wavelength sampling pulse train (MW-SPT) which can be used in a photonic A/D converter to sample a high-end microwave signal. The amplitude modulated high repetition rate MW-SPT is next wavelength demultiplexed into parallel pulse streams with a lower sampling rate which can be processed by conventional electronic state-of-the-art A/D converters in a parallel fashion. Finally, a new method for the characterization of ultrashort pulses called time resolved optical gating based on dispersive propagation (DP-TROG) is introduced and demonstrated. The DP-TROG technique is a new non-interferometric method for characterizing ultra-short optical pulses in amplitude and phase without the need for a short optical gating pulse. An algorithm is developed for the reconstruction of the pulse amplitude and phase from the measurements. The pulse train emitted from a mode-locked semiconductor laser at 1.5µm is characterized using this new technique and excellent pulse retrieval is achieved