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

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

Detector-free gas spectroscopy, with integrated frequency monitoring, through self-mixing in a terahertz quantum-cascade laser

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

Origin of terminal voltage variations due to self-mixing in a terahertz frequency quantum cascade laser

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

Publisher Correction: Measurement of the emission spectrum of a semiconductor laser using laser-feedback interferometry

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper

Self-Mixing as a means of Spectral Characterisation of a Terahertz QCL

Fast, sensitive and compact coherent systems for imaging and interferometry can be developed through the use of a single terahertz (THz) quantum cascade laser (QCL) device as both emitter and detector in a self-mixing (SM) scheme [1]. Here, radiation re-injected to the laser cavity interferes (‘mixes’) with the intra-cavity electric field, causing small variations in the fundamental laser parameters [2]. Of particular importance is the voltage perturbation induced by optical feedback. This can be used as a method of measuring the self-mixing effect through monitoring the terminal voltage of the device, and is dependent on the external cavity length and lasing frequency. As such, interferometric fringes can be acquired in a SM system by simply changing the external cavity length. In this work we demonstrate the use of SM interferometry for performing spectral characterisation of a multi-mode THz QCL in a scheme that offers much reduced experimental complexity, compared with typical Fourier Transform infrared spectroscopy (FTIR) systems

Spectral Characterisation of a Terahertz QCL through Self-Mixing

The use of a single terahertz (THz) quantum cascade laser (QCL) device as both emitter and detector in a self-mixing (SM) scheme allows for the development of fast, sensitive and compact coherent systems for imaging and interferometry [1]. In this scheme, radiation re-injected to the laser cavity interferes (‘mixes’) with the intra-cavity electric field, causing small variations in the fundamental laser parameters, as described in the seminal paper by Lang and Kobashi (L–K) [2]. In particular, the voltage perturbation induced by optical feedback is described by a sinusoidal variation dependent on both the external cavity length LExt and the emission frequency under feedback ν, and is given byΔVsm ∝ cos(2 v/c). As such, interferometric fringes can be acquired in a SM system by simply changing the external cavity length and concurrently monitoring the terminal voltage of the device. In this work we demonstrate the use of SM interferometry for performing spectral characterisation of a multi-mode THz QCL in a scheme that offers much reduced experimental complexity when compared with typical Fourier Transform infrared spectroscopy (FTIR) systems. In addition, we report the first direct measurements of the perturbation of the lasing frequency under feedback, and compare the results with predictions from the L–K model