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Terahertz-frequency quantum cascade lasers (THz QCLs) [1] have been used as compact, yet powerful sources of THz radiation in a range of gas spectroscopy techniques [2], including both in situ active sensing [3] and heterodyne radiometry [4]. A novel approach has recently been demonstrated, based on self-mixing interferometry (SMI) in a QCL [5]. This effect occurs when radiation is fed back into the QCL from an external reflector [6]. The resulting interference within the QCL perturbs the terminal voltage, and the absorption spectrum of a gas within the external cavity may be inferred from the amplitude of these perturbations. This eliminates the need for an external THz detector, doubles the interaction-length for absorption spectroscopy, and the scanning speed can potentially be raised to the time-scale of the QCL lasing dynamics (~10 GHz). A limitation reported in the previously published work is that the QCL emission frequency was inferred from prior FTIR measurements of the unperturbed laser. However, the actual system QCL frequency is perturbed by SMI feedback effects and is therefore dependent on the gas absorption crosssection, leading to apparent frequency shifts in the measured spectral lines. In this work, we demonstrate a technique to measure the frequency directly by extending the external cavity length modulation to 200-mm using a motorised linear translation stage [Fig. 1(a)]. The QCL in this system can be tuned by adjusting the drive current, over a 1.5 GHz bandwidth, around a centre frequency of 3.394 THz. Fig. 1(b) shows the transmitted radiation intensity through a 73-cm gas cell with TPX windows, filled with methanol vapour at a pressure of 2 Torr, as a function of drive current, measured using a pyroelectric detector. Two absorption lines are clearly resolved. By replacing the detector with a planar mirror, and recording the QCL voltage modulation as a function of stage position, a full interferogram can be acquired, and a Fourier transform can then be used to determine the laser frequency and the amplitude of the transmitted signal [Fig. 1(c)]. In this paper, we will demonstrate the reconstruction of the methanol absorption spectrum, with direct measurement of the laser frequency using this technique.

Terahertz-frequency quantum cascade lasers (THz QCLs) have been used as compact, yet powerful THz radiation sources in a range of gas spectroscopy techniques, including both in situ active sensing and heterodyne radiometry. However, all such approaches require external THz instrumentation (detectors or mixers) in addition to the QCL, thus raising the system complexity and cost. A partial solution has recently been demonstrated, based on self-mixing interferometry (SMI) in a QCL, which occurs when radiation is fed back into the QCL from an external reflector. The resulting interference within the QCL perturbs the terminal voltage, and the absorption spectrum of a gas within the external cavity may be inferred from the amplitude of these perturbations. This both eliminates the need for an external THz detector or mixer, doubles the interaction-length for absorption spectroscopy, and the scanning speed can potentially be raised to the time-scale of the QCL lasing dynamics (~10 GHz). A limitation reported in the previous work is that the QCL emission frequency must be inferred from prior spectral measurements of the unperturbed laser, which introduces two principal problems: (1) additional THz instrumentation is still required, and (2) the system QCL frequency is itself perturbed by feedback effects, leading to apparent frequency shifts in the measured spectral lines. In this work, we demonstrate a technique to measure the QCL frequency directly by extending the external cavity length modulation to 400-mm using a motorised linear translation stage. By recording the QCL voltage modulation as a function of stage position, a full interferogram can be acquired, and a Fourier transform can then be used to determine the laser frequency and the amplitude of the transmitted signal. The QCL was shown to be tunable by adjusting the drive current over a 1.5-GHz bandwidth, around a centre frequency of 3.4052 THz. To demonstrate gas spectroscopy, a 1-m gas cell with TPX windows was filled with methanol vapour, and the transmitted QCL power was measured as a function of drive current through SMI analysis. Two absorption lines are clearly resolved. The technique was found to be accurate to partial methanol pressures of < 10 mTorr. In conclusion, we have demonstrated an accurate and low-cost THz gas spectroscopy technique based on self-mixing in a THz QCL, without the need for any external THz mixer or detector, or a priori calibration of the QCL emission frequency.

We present an electromagnetic model of a THz QCL, integrated with a micro-machined waveguide and dual diagonal feedhorns, enabling simultaneous access to both facets of the QCL. A hybrid finite-element/Fourier transform approach enables analysis of both the near and far-fields in agreement with experimental observations. The far-field pattern shows enhancement of the beam profile when compared with an unmounted QCL, in terms of beam divergence and side-lobe suppression ratio.

An “elegant breadboard” system has been developed, which demonstrates the integration of terahertz-frequency (THz) sources, optics and compact cryocooler technology for the LOCUS satellite (Linking Observations of Climate, the Upper Atmosphere and Space Weather). This proposed satellite instrument has the aim of providing the first global mapping of key molecular species within the mesosphere and lower thermosphere (MLT) from low-earth orbit (LEO), using compact radiometers operating in the 0.8–4.7 THz band and a set of infrared detectors. The LOCUS THz radiometers will incorporate planar-Schottky-diode (SD) mixers, driven using waveguide-integrated local-oscillators (LOs). The LOs will be based on SD multipliers operating at 0.8 and 1.1 THz, and THz quantum-cascade lasers (QCLs) operating at 3.5 and 4.7 THz. A key technological challenge, addressed by the LOCUS elegant breadboard, is the integration of these components into a compact and robust satellite payload, including space-qualified cryocooler technology, and suitable fore-optics. In this paper, we discuss recent progress in QCL integration within the 3.5-THz channel.

We present a THz-QCL spectroscopy technique, for analyzing deuteration reactions in gases. This is an important analytical chemistry technique, underpinning mechanistic studies. We use a self-mixing interferometry approach, in which radiation from a multimode QCL is fed-back into the device, via a gas cell, causing perturbations to the QCL voltage. This enables “detector-free” sensing over a 17-GHz range, allowing speciated measurements of reactions involving H2O, D2O, HDO, CH3OH and CH2DOD.

The use of quantum cascade lasers (QCLs) for laser feedback interferometry (LFI) has received significant attention since it enables a wide range of sensing applications without requiring a separate detector, and hence simplifies experimental apparatus [1]. LFA (based on the self-mixing effect) refers to the partial reinjection of the radiation emitted from a laser after reflection from a target; the injected radiation field then interacts with the intra-cavity field causing measurable variations of the QCL terminal voltage. The theory of LFI with conventional laser sources is well studied and explained by the Lang–Kobayashi model [2, 3]. However, while this enables the dynamic state populations and light interaction to be modelled, a linear relationship between the change in cavity light power, ∆P, and terminal voltage variation is commonly assumed, i.e. VSM ∝ ∆P [4, 5]. This is not strictly applicable to QCL structures since carrier transport is dominated by the mechanisms of electron subband alignment, intersubband scattering and photon driven transport between subbands with energy separations that change with applied bias (terminal voltage). We present experimental results of a QCL which departs significantly from this assumed linear behavior. We observe strong enhancement of the self-mixing signal in regions where the local gradient of the current-voltage (I–V) curve increases. We explain the origin of this signal using an extended density matrix (DM) approach [6] which accounts for coherent transport and interaction of the optical light field with the active region. The model is used to calculate the I–V characteristics of a bound-to-continuum (BTC) terahertz (THz) QCL and predict the effect of light variation on terminal voltage at a fixed drive current. This approach is shown to predict the experimental signal with good agreement.

Practical supra-terahertz (1–5 THz) heterodyne radiometry systems require compact local oscillator and mixer components, which are suitable for robust waveguide integration. We demonstrate detection of radiation emitted by quantum cascade lasers at frequencies between 2.2 and 3.5 THz using room-temperature quasi-optically mounted planar Schottky diodes. These diodes are more readily integrated into waveguides than point-contact diodes, do not require the cryogenic environment of hot-electron bolometers and offer potentially better noise performance than superlattice devices.

Terahertz-frequency quantum cascade lasers (THz QCLs) are compact sources of 1–5 THz radiation, which show great promise for use as local oscillators in satellite-borne heterodyne radiometers. We present a waveguide-integration scheme, in which a THz QCL is mounted in a copper heat-sink block, with radiation outcoupled into a precision micromachined rectangular waveguide. Electrical bias is provided by an integrated SMA connector and mounting points are provided for attachment to a cryocooler and a temperature sensor. The integration scheme is mechanically robust and is shown to have negligible impact on the thermal performance or threshold current of the device. The emitted beam quality is significantly improved, compared with that of a conventional device, with single-lobed profile with divergence

An “elegant breadboard” system has been developed, which demonstrates the integration of terahertz-frequency (THz) sources, optics and compact cryocooler technology for the LOCUS satellite (Linking Observations of Climate, the Upper Atmosphere and Space Weather). This proposed satellite instrument has the aim of providing the first global mapping of key molecular species within the mesosphere and lower thermosphere (MLT) from low-earth orbit (LEO), using compact radiometers operating in the 0.8–4.7 THz band and a set of infrared detectors. The LOCUS THz radiometers will incorporate planar-Schottky-diode (SD) mixers, driven using waveguide-integrated local-oscillators (LOs). The LOs will be based on SD multipliers operating at 0.8 and 1.1 THz, and THz quantum-cascade lasers (QCLs) operating at 3.5 and 4.7 THz. A key technological challenge, addressed by the LOCUS elegant breadboard, is the integration of these components into a compact and robust satellite payload, including space-qualified cryocooler technology, and suitable fore-optics. In this paper, we discuss recent progress in QCL integration within the 3.5-THz channel.

The LOCUS satellite (Linking Observations of Climate, the Upper Atmosphere and Space weather) is a proposed mission to explore and observe key gas species within the upper atmosphere using a novel terahertz-frequency (THz) heterodyne spectrometer, in which the emission spectra are simultaneously recorded in four channels in the 0.8–4.7 THz band [1]. THz quantum-cascade lasers (QCLs) will be exploited as local oscillators for the first time in space because they are powerful, yet compact THz sources, providing > 1 mW continuous-wave output at key LOCUS frequencies (3.5 and 4.7 THz). Furthermore, they have been integrated successfully into precision-micro-machined waveguide blocks and operated in space-qualified Stirling-cycle cryo-coolers (~60 K) [2]. A key development challenge will be stabilising the emission frequency of the QCL by locking it to a stable reference oscillator. \ud \ud In this paper, we present a new QCL dual-feedhorn integration technique, which advances this goal by enabling access to radiation emitted from both facets of the QCL simultaneously. In this configuration, the THz emission may be coupled simultaneously to a mixer and to a stabilisation subsystem. Figure 1(a) shows the integration of a 3.5 THz QCL into a precision micro-machined channel in a copper block. A second, symmetrical copper block is placed on top of this to form a rectangular waveguide around the QCL, with a diagonal feedhorn at either end. Figure 1(b) shows the simulated far-field beam pattern for a single feedhorn at 3.5 THz, in which the maximum collected power is represented in the bright central area. Figure 1(c) shows the experimental results for the dual-feedhorn waveguide block, in which the radiation from each horn is reflected onto the same plane, and measured using a raster-scanned Golay-cell detector. Good agreement with the simulated single-feedhorn emission has been observed, with similar beam width and side-lobe content.\ud \ud In conclusion, the integration of a 3.5-THz QCL into a dual-feedhorn waveguide block has been demonstrated. This opens the way towards simultaneous integration of the QCL with a frequency stabilisation system and a supra-THz mixer.