Subpicosecond monolithic colliding-pulse mode-locked multiple quantum well lasers

Generation of short optical pulses with semiconductor laser diodes is important for high bit rate time-division multiplexed communication systems, ultrafast data processing, and picosecond optoelectronic applications. Because the pulse shaping mechanisms are determined by the saturation and recovery time of the gain and absorber sections in mode-locked lasers,’ it is possible to generate short optical pulses with a repetition rate beyond the relaxation oscillation frequency of the semiconductor laser. The unique optical properties of quantum well semiconductor epitaxial layers such as low dispersion, broad gain spectrum, and the fast saturation and recovery times can well be utilized as the gain and absorber media to generate short mode-locked pulses.2-4 The use of an integrated waveguide cavity in semiconductor lasers further eliminates the uncontrollable multiple pulse bursts produced by nonperfectly antireflection-coated surfaces in mode-locked lasers with external cavities.5-9 The colliding-pulse mode-locking (CPM) scheme has been widely used to generate short optical pulses in dye lasers, which utilizes the interaction of counterpropagating pulses in the absorber to synchronize, stabilize, and shorten the pulses.‘c-‘2 We reported the generation of transform-limited 1.4 ps pulses from a monolithic active mode-locked CPM multiple quantum well (MQW) semiconductor laser with an external microwave oscillator.‘3 However, a very stable low-noise highfrequency electronic oscillator as well as a fast modulation section are required for the active mode-locking scheme. In this letter, we have implemented a monolithic passive colliding pulse mode-locked laser with a strained multiple quantum well laser structure to generate continuous wave (cw ) subpicosecond optical pulses without external microwave sources or any pulse sources. The pulse repetition rate of this passive CPM scheme is only limited by the cavity length and its saturation and recovery characteristics. The monolithic CPM lasers were fabricated with a 1.5 pm buried heterostructure (BH) GaInAsP graded index separate confinement (GRIN-SCH) lattice-strained MQW structure prepared by a two-step organo-metallic vapor phase epitaxy (OMVPE) growth technique. In the first growth, the lower part of the graded index confining InGaAsP layers were deposited on top of a 2-pm-thick n-InP cladding layer with step-like decreasing band-gap layers of 1.08 pm (25 nm thick), 1.16 pm (25 nm thick), and 1.25 pm (25 nm thick), and followed by five InGaAs quantum wells (5 nm thick) and 1.25 pm (22.5 nm thick) InGaAsP barriers. The upper graded index InGaAsP confining layers, similar to the lower part, were then grown with increasing band gap, and followed by a 2 pm p-InP cladding layer and a 120 nm p ’ -1nGaAsP contact layer (Zn doped to 5 x 1018 cm 3). After the l-pm-wide continuous waveguide stripes were formed by etching down to the lower n cladding layer with a Si02 mask, an iron-doped semi-insulating InP layer was selectively grown around the waveguide strips to provide electrical isolation and optical confinement. Detailed growth conditions and device performance of the MQW lasers were reported in Ref. 14. Standard lithography and wet chemical etching were used to construct the final structure. The schematic diagram of the monolithic passive CPM laser is depicted in Fig. 1. As shown in Fig. 1, the continuous optical waveguide is divided into three sections by segmented p-contact metal stripes. The electrical isolation between contact metals is achieved by removing the heavily doped top p-type contact epitaxial layer with wet chemical etching. Typical resistance across the 10 pm gap is 1 K fI. The saturable absorber is located in the symmetry center of the linear cavity between two cleaved Fabry-Perot facets. The remaining active cavity sections are connected together and forward biased as the gain sections for the integrated CPM laser. As in the passive CPM dye lasers, the initial transients proceed to form two counterpropagating pulses at steady state. These two pulses time themselves to collide in the center saturable absorber because minimum energy is lost. Compared to a single pulse traveling in the cavity, the pulse shaping is more effective with the CPM configuration because there are two pulses added coherently to saturate the absorber and only one pulse to saturate the gain section. The transient grating generated by the colliding pulses further reduces the saturation energy of the absorber and limits the necessary spectral bandwidth.“*‘* Devices of cavity length 2.1, 1.0, 0.534, and 0.25 mm were fabricated to generate optical pulses with repetition rates of 39.2, 80.4, 156, and 350 GHz, respectively. The repetition rate corresponds to one half of the round-trip time between two facets because of the CPM configuration. The length of the saturable absorber is 15 pm for lasers with 0.25 mm cavity length, and is 50 ,um for lasers with other cavity lengths. Typical continuous-wave light-current (L-I) charac-