Abstract-Mode-locked semiconductor lasers which incorporate multiple contacting segments are found to give improved performance over single-segment designs. The functions of gain, saturable absorption, gain modulation, repetition rate tuning, wavelength tuning, and electrical pulse generation can be integrated on a single semiconductor chip. The optimization of the performance of multisegment mode-locked lasers in terms of material parameters, waveguiding parameters, electrical parasitics, and segment length is discussed experimentally and theoretically.
A comprehensive timing jitter comparison is made for mode-locked semiconductor lasers using active, passive, and hybrid mode-locking techniques in both external and monolithic cavity configurations. Active mode locking gives the lowest residual rms timing jitter of 65 fs ( 150 Hz-50 MHz), followed by the hybrid and passive mode-locking techniques. It is found that monolithic cavity devices with all active waveguides have higher timing jitter leveis than the comparable external cavity case.Mode-locked semiconductor lasers have produced ultrashort optical pulses in monolithi?and external cavity configurations.5 Monolithic cavities offer the advantage of mechanical stability, small size, and ease of use as compared to external cavity devices. In this letter, monolithic and external cavity multiquantum well lasers are compared using active, passive, and hybrid mode-locking techniques. Quantum well rather than bulk active region lasers are chosen because of the larger ratio in the differential gain between the reverse biased saturable absorber segments and the forward biased gain segments.6 In addition to pulse-width and spectral-width measurements, this paper concentrates on a comparison of the timing jitter properties of the lasers. Pulse to pulse timing jitter is an important noise parameter that contributes directly to the time resolution in most applications of mode-locked lasers. Figure 1 shows a diagram of the monolithic and external cavity devices used in the active, passive, and hybrid mode-locking experiments. The active region for all of the devices is comprised of four GaAs quantum wells with the lateral index guide formed by impurity induced disordering.' The monolithic structure is 6. l-mm long with the top electrode divided into two short end segments and a long center section. With all sections connected together, the device has a threshold of 115 mA and a single facet differential quantum efficiency of 4%. The 80-pm end segment is reverse biased for use as a saturable absorber, terminated in 50 s1, and high-reflection coated. The impulse response of the saturable absorber was measured using a sampling oscilloscope as the termination. The loss recovers quickly (50 ps full width at half-maximum). The middle section is dc biased to act as an active waveguide. The 4.00~pm end segment is modulated with a 24.5-dBm, 5.5-GHz sinusoid. The external cavity device of Fig. 1 (b) is a two segment device cleaved from one of the monolithic devices. The external cavity laser has a threshold current of 13 mA with an output facet differential quantum efficiency of 10%. The absorber facet has a 70% reflection coating and the opposite facet is antireflection coated for external cavity operation at a 5.5-GHz repetition rate. Table I gives a summary of the bias conditions and the performance results for active, passive, and hybrid mode locking of the monolithic and external cavity configurations. The average output power is held at 1 mW in all cases and the mode-locking frequency is nominally 5.5
Absorption recovery dynamics of GaAs/AlGaAs field-enhanced waveguide saturable absorbers are studied by pump-probe differential transmission measurements. We compare the response of bulk and single quantum well absorbers at different reverse bias levels and pump powers, and find an ultrafast transient in the response, followed by a slower rise before the final recovery. The absorption fully recovers after a few picoseconds, which is an important result for mode-locked lasers.
Mode-locked vertical-cavity lasers have a large cross-sectional area and consequently a large saturation energy and large peak powers. We analyze excess optical bandwidth generation in these lasers and find that self-phase modulation due to optical pumping and gain saturation is the dominant factor in inducing laser pulse chirping. The large magnitude of the chirp makes intracavity prism-pair compensation difficult. Adjustment of the cavity length has a major impact on the pulse chirping, as observed experimentally. Proper adjustment can result in a large linear frequency chirp which can be compensated using external pulse compression techniques.
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