Abstract-Coherent imaging using laser feedback interferometry with pulsed-mode terahertz quantum cascade lasers

Yah Leng Lim, Karl Bertling, Thomas Taimre, Tim Gillespie, Chris Glenn, Ashley Robinson, Dragan Indjin, Yingjun Han, Lianhe Li, Edmund H. Linfield, A. Giles Davies, Paul Dean, and Aleksandar D. Rakić

Fig. 1 (a) Schematic diagram of the experimental setup. (b) Expanded view of the QCL cold finger module. (c) Photo of the THz QCL mounted on the cold finger of the Stirling cooler.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-7-10221

We report a coherent terahertz (THz) imaging system that utilises a quantum cascade laser (QCL) operating in pulsed-mode as both the source and detector. The realisation of a short-pulsed THz QCL feedback interferometer permits both high peak powers and improved thermal efficiency, which enables the cryogen-free operation of the system. In this work, we demonstrated pulsed-mode swept-frequency laser feedback interferometry experimentally. Our interferometric detection scheme not only permits the simultaneous creation of both amplitude and phase images, but inherently suppresses unwanted background radiation. We demonstrate that the proposed system utilising microsecond pulses has the potential to achieve 0.25 mega-pixel per second acquisition rates, paving the pathway to video frame rate THz imaging.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Abstract-Continuous frequency tuning with near constant output power in coupled Y-branched terahertz quantum cascade lasers with photonic lattice

Iman Kundu, Paul Dean, Alex Valavanis, Joshua R. Freeman, Mark C. Rosamond, Lianhe H. Li, Yingjun Han, Edmund H. Linfield, and Alexander Giles Davies

https://pubs.acs.org/doi/abs/10.1021/acsphotonics.8b00251?journalCode=apchd5

We demonstrate continuous frequency tuning in terahertz quantum cascade lasers with double metal waveguides using a Y-branched coupler. Two THz QCLs placed side-by-side couple by evanescent fields across the air gap between them. Each QCL waveguide comprises a 48-μm-wide coupler and S-bend section, which are connected to an 88-μm-wide Y-branch through an impedance matching tapered section. Photonic lattices are patterned on top of the coupler section in each QCL using focused ion-beam milling to control the spectral characteristics. The waveguide design used for individual QCL sections is optimized using finite element modelling and the spectral characteristics are modelled using a transfer matrix model. Continuous frequency tuning of ~19 GHz is demonstrated while maintaining an output power of ~4.2–4.8 mW and a heat sink temperature of 50 K. The tuning is controlled electrically through Stark shift and cavity pulling effects by driving both QCLs simultaneously and represents the widest electrically-controlled continuous tuning performance from a THz QCL without significant change in output power

Abstract-Gas spectroscopy with integrated frequency monitoring through self-mixing in a terahertz quantum-cascade laser

Rabi Chhantyal-Pun, Alexander Valavanis, James T. Keeley, Pierluigi Rubino, Iman Kundu, Yingjun Han, Paul Dean, Lianhe Li, A. Giles Davies, and Edmund H. Linfield

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-10-2225

We demonstrate a gas spectroscopy technique, using self-mixing in a 3.4 terahertz quantum-cascade laser (QCL). All previous QCL spectroscopy techniques have required additional terahertz instrumentation (detectors, mixers, or spectrometers) for system pre-calibration or spectral analysis. By contrast, our system self-calibrates the laser frequency (i.e., with no external instrumentation) to a precision of 630 MHz (0.02%) by analyzing QCL voltage perturbations in response to optical feedback within a 0–800 mm round-trip delay line. We demonstrate methanol spectroscopy by introducing a gas cell into the feedback path and show that a limiting absorption coefficient of ∼1×10−4  cm−1$\sim 1\times {10}^{-4}{\mathrm{cm}}^{-1}$ is resolvable.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.