Abstract
Optical feedback effects in lasers may be useful or problematic, depending on the type of application. When semiconductor lasers are operated using pulsed-mode excitation, their behavior under optical feedback depends on the electronic and thermal characteristics of the laser, as well as the nature of the external cavity. Predicting the behavior of a laser under both optical feedback and pulsed operation therefore requires a detailed model that includes laser-specific thermal and electronic characteristics. In this paper we introduce such a model for an exemplar bound-to-continuum terahertz frequency quantum cascade laser (QCL), illustrating its use in a selection of pulsed operation scenarios. Our results demonstrate significant interplay between electro-optical, thermal, and feedback phenomena, and that this interplay is key to understanding QCL behavior in pulsed applications. Further, our results suggest that for many types of QCL in interferometric applications, thermal modulation via low duty cycle pulsed operation would be an alternative to commonly used adiabatic modulation.
Highlights
Terahertz quantum cascade lasers (THz QCLs) are compact, electrically driven sources of radiation in the ∼ 1–5 THz band [1] that hold enormous potential for sensing [2,3] and communication applications [4,5,6,7]
The optical feedback model we introduce here draws on and parallels that of an early seminal paper for diode lasers [35], in which terms representing re-injection of photons into the internal cavity are included in a reduced set of rate equations
Optical feedback effects in lasers can be observed using a photo-detector or the laser’s terminal voltage, which is known to be proportional to optical output power under small signal conditions [13]
Summary
Terahertz quantum cascade lasers (THz QCLs) are compact, electrically driven sources of radiation in the ∼ 1–5 THz band [1] that hold enormous potential for sensing [2,3] and communication applications [4,5,6,7]. Laser feedback interferometry (LFI) with THz QCLs is a recently-developed coherent sensing technique [8,9,10], ideally-suited to the development of compact sensing systems, in which radiation is reflected back into the internal laser cavity from an external target of interest This optical feedback gives rise to measurable changes in the electronic and optical behavior of the laser, in a phenomenon referred to as “self-mixing” [11,12,13]. A challenge remains, though, in the interpretation of LFI signals when a pulsed source is used, since the lasing dynamics are significantly more complex than in cw operation This is caused by the interplay between the electro-optic response to the retro-injected THz field and to the thermal transients occurring in a pulsed THz QCL.
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