Laser feedback interferometry in multi-mode terahertz quantum cascade lasers

The typical modal characteristics arising during laser feedback interferometry (LFI) in multi-mode terahertz (THz) quantum cascade lasers (QCLs) are investigated in this work. To this end, a set of multi-mode reduced rate equations with gain saturation for a general Fabry-Pérot multi-mode THz QCL under optical feedback is developed. Depending on gain bandwidth of the laser and optical feedback level, three different operating regimes are identified, namely a single-mode regime, a multi-mode regime, and a tuneable-mode regime. When the laser operates in the single-mode and multi-mode regimes, the self-mixing signal amplitude (peak to peak value of the self-mixing fringes) is proportional to the feedback coupling rate at each mode frequency. However, this rule no longer holds when the laser enters into the tuneable-mode regime, in which the feedback level becomes sufficiently strong (the boundary value of the feedback level depends on the gain bandwidth). The mapping of the identified feedback regimes of the multi-mode THz QCL in the space of the gain bandwidth and feedback level is investigated. In addition, the dependence of the aforementioned mapping of these three regimes on the linewidth enhancement factor of the laser is also explored, which provides a systematic picture of the potential of LFI in multi-mode THz QCLs for spectroscopic sensing applications

Correction to “Temperature-Dependent High-Speed Dynamics of Terahertz Quantum Cascade Lasers”

Corrections to author affiliation information is presented in the above named paper

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

The effects of optical feedback (OF) in lasers have been observed since the early days of laser development. While OF can result in undesirable and unpredictable operation in laser systems, it can also cause measurable perturbations to the operating parameters, which can be harnessed for metrological purposes. In this work we exploit this ‘self-mixing’ effect to infer the emission spectrum of a semiconductor laser using a laser-feedback interferometer, in which the terminal voltage of the laser is used to coherently sample the reinjected field. We demonstrate this approach using a terahertz frequency quantum cascade laser operating in both single- and multiple-longitudinal mode regimes, and are able to resolve spectral features not reliably resolved using traditional Fourier transform spectroscopy. We also investigate quantitatively the frequency perturbation of individual laser modes under OF, and find excellent agreement with predictions of the excess phase equation central to the theory of lasers under OF

Sensing and Imaging using Laser Feedback Interferometry with Quantum Cascade Lasers

Quantum cascade lasers (QCLs) are high-power sources of coherent radiation in the midinfrared and terahertz (THz) bands. Laser feedback interferometry (LFI) is one of the simplest coherent techniques, for which the emission source can also play the role of a highly-sensitive detector. The combination of QCLs and LFI is particularly attractive for sensing applications, notably in the THz band where it provides a high-speed high-sensitivity detection mechanism which inherently suppresses unwanted background radiation. LFI with QCLs has been demonstrated for a wide range of applications, including the measurement of internal laser characteristics, trace gas detection, materials analysis, biomedical imaging, and near-field imaging. This article provides an overview of the QCLs and the LFI technique, and reviews the state of the art in LFI sensing using QCLs

Detection sensitivity of laser feedback interferometry using a terahertz quantum cascade laser

We report on the high detection sensitivity of a laser feedback interferometry scheme based on a terahertz frequency quantum cascade laser. We show that variations on the laser voltage induced by optical feedback to the laser can be resolved with reinjection of powers as low as ~−125 dB of the emitted power. Our measurements demonstrate a noise equivalent power of ~1.4 pW/√Hz, although after accounting for reinjection losses we estimate this corresponds to only ~1 fW/√Hz being coupled to the quantum cascade laser active region

Laser feedback interferometry with THz QCLs: A new technology for imaging and materials analysis

Considerable interest exists for sensing and imaging technologies in the terahertz (THz) spectral range, in particular for the interrogation of materials of an organic or biological nature. Development in THz quantum cascade lasers is seeing higher operating temperatures and peak output powers in pulsed mode, accentuating their place as the preferred source of coherent THz frequency radiation. Technological development of interferometric sensing schemes continues to take advantage of practical improvements in THz quantum cascade lasers. In this Summary, we give a brief overview of some recent developments in this regard

Terahertz Radar Cross Section Characterization using Laser Feedback Interferometry with a Quantum Cascade Laser

Radar cross section (RCS) measurements of complex, large objects are usually performed on scale models so that the measurement is carried out in a well-controlled environment. This letter explores the feasibility of RCS measurement using a terahertz quantum cascade laser via laser feedback interferometry. Numerical simulations show that the RCS information embedded in the non-linear interferometric signals obtained from simple targets can be retrieved through numerical fitting of the well-known excess phase equation. The method is validated experimentally using a terahertz quantum cascade laser and the results are well matched with those obtained from numerical simulations

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