Quantum cascade lasers (QCLs) are the most prominent and compact coherent light sources in the wavelength range from 3.5 to 20 μm. Remarkable design degrees of freedom make QCLs a unique candidate to serve as a semiconductor source of ultrashort pulses in the mid-infrared (MIR) and terahertz (THz) regions [123,185,186]. Ultrashort pulses which are generated in QCL media have been employed for various applications, such as non-linear frequency conversion [187,188], high-speed free space communication [189], and trace gas detection [190].
A common technique for generation of ultrashort pulses is mode locking which is realized either by an internal mechanism (passive mode locking) or an external one (active mode locking) [191]. Active modelocking does not lead to ultrashort pulses, because the frequency of modulation cannot be raised arbitrarily while a passive saturable absorber is much more effective in ultrashort pulse generation [128]. In lasers with a relatively long gain recovery time compared to the cavity round-trip time, the instability caused by a saturable absorber (SA) can often lead to passive mode locking [128]. Because of ultra-fast tunneling and inter-subband transitions, QCLs, unlike conventional semiconductor lasers, have a gain recovery (in the order of picoseconds) faster than the cavity round trip time. Therefore, SA dominated self-mode locking seems impossible in typical QCLs. However, as reported in Ref. [125], under these conditions the elusive Risken-Nummedal-Graham-Haken (RNGH)-like instability can be observed in QCLs. It is demonstrated that fast gain recovery of QCLs exhibits two kinds of instabilities in the multi-mode regime: the RNGH-like instability and one associated with spatial hole burning (SHB) [126].
The active mode locking in QCLs is strongly limited by SHB which leads to a proliferation of modes with random phases and destroys coherence [191].
In this chapter we investigate performance optimization and passive mode locking caused by a saturable absorber in a ring cavity QCL in which the SHB instability does not occur because of the absence of standing waves. Performance optimization of QCLs has achieved significant progress in recent years [104,192,193]. The performance of QCLs can be improved by optimizing the epitaxial growth, fabrication process, and active region design [194–196]. The latter is important in order to insure efficient carrier injection in the upper lasing state and carrier extraction out of the lower lasing state [104].
So far, optimization techniques proposed to design QCL structures are based on genetic algorithms [104,192,197,198]. A multi-variable multi-objective optimization algorithm for terahertz QCLs is presented in [193]. In [104,192], a technique to design quantum cascade structures in the mid-infrared is proposed. Particle swarm optimization (PSO) [199] is as an alternative to traditional evolutionary algorithms. It attempts to mimic the goal-seeking behavior of biological swarms. In PSO, a possible solution of the optimization problem is represented as a particle, and the algorithm operates in an iterative manner. Unlike traditional evolutionary algorithms, particles in PSO do not perform the operation of genetic recombination between particles, but they work individually with social behavior in swarms. PSO has some attractive characteristics. In particular, it has memories, so that knowledge of good solutions can be retained by all particles (solutions). This method has been successfully used to solve many discontinuous and complex problems with good results [200,201].
We perform an optimization study of ring cavity QCLs considering the laser instability condition. Laser design parameters, including the barrier and well thicknesses and applied electric field, are modified for maximizing laser gain under a desired instability condition. Passive mode locking with saturable absorber is investigated for the optimized QCLs. The effects of saturable absorber and pumping strength on the instability characteristics are studied. A large optical gain below the instability threshold is achieved for optimized QCL designs. A numerical calculation based on the Maxwell-Bloch equations is performed to analyze the optimized structure.