GENERATION OF SINGLE FEMTOSECOND PULSES BY HYBRID MODE-LOCKING OF A SEMICONDUCTOR-LASER

Generation of sub-ps pulses at a repetition frequency of f almost-equal-to 1 GHz by a wavelength detuned hybrid mode-locked semiconductor laser is presented. This method is capable of producing single optical pulses, while trailing pulses spaced by the laser chip round-trip time are suppressed. The laser is antireflection (AR) coated at one facet and ion implanted through the other facet to create a saturable absorber inside the laser. The laser is placed in an external cavity, where a reflection grating allows the selection of the emission wavelength, and is synchronously excited by a sinusoidal injection current. The dependence of the transient optical emission on implantation dose, excitation frequency, and wavelength detuning is investigated. It is found that the actual parameters of the ion implantation in conjunction with wavelength detuning is crucial for single pulse generation. Using a numerical analysis it is shown that the saturable absorber suppresses the perturbations created by spontaneous emission or spurious reflections at the AR-coated facet, which for purely active mode locking and our low injection current modulation inhibit the generation of pulses shorter than about 30 ps. The carrier lifetime in the absorber region is found to mainly influence the pulse turn-on delay time jitter (jitter), while detuning of the excitation frequency leads to a periodically varying time delay of the emitted pulse with respect to the excitation. The differential gain in the absorber must be above a certain level to enable pulse shaping, while a further increase reduces the pulse-width only slightly.