Quantum cascade laser combs: effects of modulation and dispersion.

Area of Science:

• Quantum optics
• Semiconductor lasers

Background:

• Quantum cascade lasers (QCLs) are semiconductor devices capable of emitting light in the mid-infrared and terahertz regions.
• Frequency combs are sources of coherent laser light with a spectrum composed of regularly spaced frequency lines, analogous to the teeth of a comb.

Purpose of the Study:

• To theoretically investigate the formation of frequency combs in quantum cascade lasers.
• To analyze the influence of cavity dispersion and active modulation on comb formation.
• To identify optimal conditions for achieving broadband quantum cascade laser frequency combs.

Main Methods:

• Utilizing a Maxwell-Bloch formalism.
• Employing a modal decomposition approach to account for dispersion.
• Simulating frequency comb formation in mid-infrared and terahertz QCLs.

Main Results:

• Frequency comb formation is sustained in mid-infrared QCLs with weak cavity dispersion (500 fs² mm⁻¹).
• Comb formation is suppressed at higher dispersion values (30,000 fs² mm⁻¹).
• Active modulation at the round-trip frequency induces mode-locking in terahertz QCLs with long upper-state lifetimes (tens of picoseconds).

Conclusions:

• Mode-locking via four-wave mixing in broadband gain, low dispersion cavities is the most effective strategy for generating broadband QCL frequency combs.
• Cavity dispersion plays a critical role in the feasibility of frequency comb generation in QCLs.
• Active modulation can be a viable method for achieving mode-locking in specific QCL designs.