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.
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.
Abstract-Imperfect antireflection coatings in external-cavity mode-locked semiconductor lasers can cause multiple output pulse generation. The incorporation of an intrawaveguide saturable absorber segment into the laser suppresses this problem. Single pulse outputs of less than 2.8 ps and 0.7 pJ of energy are obtained using such devices with both quantum well and bulk active regions.ODE-LOCKED semiconductor lasers are important M sources of short optical pulses for optical communications, optoelectronic measurement systems, and physics experiments. Active and hybrid activelpassive mode-locking are the most important modulation techniques for these applications because of their amplitude and temporal stability [l] and the presence of a system timing reference. However, actively mode-locked external cavity semiconductor lasers with a single gain segment are very susceptible to multiple pulse formation [2]. The undesired secondary pulses are initiated by reflections from imperfect antireflection coatings on the laser-diode facet. The reflected pulse is then amplified because the main pulse does not fully deplete the gain and current drive to the segment may still be creating new carriers. Schell et al. [3] have calculated that even a very good antireflection coating, will cause multiple pulse generation at the round-trip time of the diode laser. In this letter we show that incorporation of reverse-biased intrawaveguide saturable absorber segments [4] into the cavity suppresses the multiple pulse generation problem. The technique is applicable to both bulk and quantum-well active region lasers. Single pulse outputs of less than 2.8 ps with 0.7 pJ of energy are obtained using these techniques.The device and configuration used to suppress the multiple pulse output problem is shown in Fig. 1. The top contacts of the laser diode are separated to allow sections of the laser to be biased independently. The long segment provides the overall gain for the device. The reverse biased segment acts as an intrawaveguide saturable absorber and p-i-n photodetector. To illustrate how the saturable absorber can suppress multiple pulse formation, the propagation of a pulse around the mode-locked cavity of Fig. 1 is outlined in Fig. 2. The modeled device is a two section laser with a 500 pm overall length. The saturation energy, Esat, of a gain or an absorbing segment is defined as [5] where hv is the photon energy, dgldn is the differential gain, and A is the mode cross sectional area. The saturation energy is a measure of the energy required to saturate the gain of a gain section or the absorption of an absorber segment. The E,,, of the gain section is 2.1 pJ and the E,, of the absorber is 0.7 pJ in this simulation. The single pass unsaturated energy gain of an amplifier segment is 17 and the unsaturated transmission through the absorber is 0.05. Fig. 2(a) shows rate equation simulations of the pulse energy versus distance for the main pulse as the pulse enters from the external cavity and propagates through the gain and absor...
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