Abstract
We consider here a time domain model representing the dynamics of quantum cascade lasers (QCLs) generating frequency combs (FCs) in both THz and long wave infrared (LWIR λ = 8-12µm) spectral ranges. Using common specifications for these QCLs we confirm that the free running laser enters a regime of operation yielding a pseudo-randomly frequency modulated (FM) radiation in the time domain corresponding to FCs with stable phase relations in the frequency domain. We provide an explanation for this unusual behavior as a consequence of competition for the most efficient regime of operation. Expanding the model previously developed in [Opt. Eng. 57(1), 011009 (2017)] we analyze the performance of realistic THz and LWIR QCLs and show, despite the vastly different scale of many parameters, that both types of lasers offer very similar characteristics, namely FM operation with an FM period commensurate with the gain recovery time and an FM amplitude comparable with the gain bandwidth. We also identify the true culprit behind pseudo-random dynamics of the FM comb to be spatial hole burning, rather than the more pervasive spectral hole burning.
Highlights
Frequency combs [1, 2], initially developed in the near-infrared and visible domains, have changed the world of metrology and spectroscopy as we know it
This higher gain is required for the THz laser as the waveguide free carrier absorption is much higher in this domain. It may not be perfectly discernible in the graph, the gain remaining after passing light through the gain medium is greater when no frequency modulated (FM) is present, amounting to an inefficient use of the spectral gain
As the laser begins to oscillate, the inhomogeneously broadened gain of the Quantum Cascade lasers (QCLs) naturally tends toward multimode operation, subsequently four wave mixing (FWM) creates and locks these modes
Summary
Frequency combs [1, 2], initially developed in the near-infrared and visible domains, have changed the world of metrology and spectroscopy as we know it. The observed QCL FC’s were quite different from all other FC’s in the sense that rather than producing trains of ultra-short pulses in time domain their output power was almost constant in time while the frequency of oscillation was modulated, a stable phase relation between the oscillating modes existed This relation is maintained by four wave mixing (FWM) in the fast saturable, intersubband gain medium of QCL. The time domain model has been recently developed in [16] using Optical Bloch Equations and the derivation there have shown that as long as the period of FM is substantially longer than coherence time of the gain medium (0.2ps for LWIR and 0.7ps for THz) one can use a rate equation approximation and greatly reduce complexity of calculations. By mitigating hole burning the QCL is able to lower its threshold, this regime of operation prevails
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