Abstract
In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal - well known in interband diode lasers - was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback (C ≤ 1) to fringes modulated by SPs under moderate feedback (1 < C ≤ 4.6), and then [under strong feedback (C > 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.
Highlights
Terahertz (THz) quantum cascade lasers (QCLs) are high-power sources of coherent radiation [1,2,3,4] in the THz band, a spectral region which has many unique properties, such as low photon energy, penetration of conventionally opaque materials, and provide spectral fingerprints for many biomolecules [5]
We investigated the influences of the intrinsic SP dynamics from a THz QCL under optical feedback (OF) on the conventional SM signal created by driving current sweeping and target movement
We observed fringe loss in THz QCLs under strong feedback conditions, which is the first report of fringe loss in THz QCLs, to the best of our knowledge
Summary
Terahertz (THz) quantum cascade lasers (QCLs) are high-power sources of coherent radiation [1,2,3,4] in the THz band, a spectral region which has many unique properties, such as low photon energy, penetration of conventionally opaque materials, and provide spectral fingerprints for many biomolecules [5]. We demonstrated that the SPs observed originate from the beating between the lasing mode of the internal cavity and external cavity modes This interaction results in periodic oscillations in emission power and terminal voltage of the THz QCL. These SPs are further found superposed on conventional SM interferometric fringes when the reinjected optical field has a varying phase perturbation (resulting from driving current sweeping or target movement).
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