Picosecond, high power pulse generation by actively mode-locked two section semiconductor lasers

Abstract

The generation of optical pulses with short durations below 4 ps and high peak powers of more than 900 mW at repetition rates of 500 MHdsec is demonstrated by means of actively mode-locked two section semiconductor lasers. Time-bandwidth product widening is explained as due to self-phase modulation. Actively mode-locked semiconductor lasers often exhibit instabilities and multiple peaks in their outputs, especially for pulses of short duration. Pulses that are stable and free of bursts need carefully aligned cavities, excellent antireflection coatings, and well controlled modulation. The laser structures used to generate short, high power pulses at 0.9 pm wavelengths are of the two section AlGaAs/GaAs transverse junction stripe type [ 13. To achieve high output powers, the gain section of 210 prn in length is continuously pumped. For active mode-locking the short, 30 pm long, modulator section is driven into transparency by means of electrical pulses of 210 mA and 130 ps FWHM at 500 MHz. Stable pulse operation is achieved by coupling the optimally antireflection coated laser through a microscope objective to an external cavity with a grating of 600 grooves/ that can be rotated for wavelength tuning. To find the optimum conditions for short pulses with high peak powers, frequency, current and wavelength tuning experiments have been performed. For shortest optical pulses, that are still stable, the 'modulator is driven at a frequency that is 1.6 MHz lower than the resonance of the combined cavity at 495.8 MHz. The modulator acts as an actively controlled saturable absorber that performs pulse compression. The wavelength is tuned to the short wavelength side of the gain spectrum, to 3.5 nm below the wavelength of the gain maximum at 893.5 nm. Stable output pulses, which are almost free of multiple pulsing are observed up to drive currents of 110 mA that corresponds to twice the threshold of the gain section. Under optimum conditions, the shortest, highest peak power pulses, as shown in Fig. 1, exhibit FWHM of 3.8 ps and 920 mW. The pulse spectrum, top of Fig. 2, has a FWHM of 180 GHz. The corresponding time-bandwidth products of 0.68 are 6.2 times larger than the transform limit assuming one sided exponentials. Depletion of carriers from the gain region by these high power pulses leads to self phase modulation (SPM) and time-bandwidth product widening. With growing pulse energies, the spectra, shown in Fig. 2 become wider with growing spectral energy content at the short wavelength sides, thus corroborating the SPM model of [2]. The combined effect of SPM induced spectral broadening and bandwidth limitation of the grating cavity is found to set a lower limit on the achievable high energy pulse duration of the present experimental configuration., These semiconductor lasers are amongst the best reported in terms of the combination of large pulse power and short pulse durations. The almost background free pulses appear well suited for nonlinear optics, optical communication and measurement techniques. [l] J. Werner, H. Melchior and G. Guekos, El. Lett., vo1.24 (3), p. 140 ff (1988) [2] G.P. Agrawal and N.A. Olsson, IEEE J. Quantum Electron., QE-25 (ll), p. 2297 ff (1989)