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We provide an experimental evidence of transient instabilities (TIs) in a terahertz (THz) quantum cascade laser (QCL) under optical feedback, in contrast to the widely accepted claim that THz QCLs are ultrastable against feedback. The TIs appear as periodic oscillations in emitted power or terminal voltage of the laser with an increasing oscillation frequency as feedback increases. The absence of relaxation oscillations and low linewidth enhancement factor in THz QCLs makes them a platform uniquely suitable for exploring external-cavity-related dynamics in semiconductor lasers. This work opens a pathway to a THz sensing and imaging modality based on these TIs, which has much reduced complexity compared to existing approaches using laser feedback interferometry.

Terahertz quantum cascade-lasers (QCLs) are able to produce higher optical output power at higher temperatures when operated in pulsed mode. Predicting a laser's behavior under pulsed operation in order to achieve performance requirements is, however, a nontrivial exercise: the complex and nonlinear interplay between current, electric field, and thermal transients gives rise to complex responses in both optical output power and emission frequency. In applications where it is important to predict and control these behaviors, establishing the link between current drive, emission frequency, and optical output power is necessary. In this paper, we demonstrate, via a realistic laser-specific model, that by appropriate manipulation of the drive pulse we can not only obtain a higher optical output at increased operating temperature but also both extend and linearize a QCL's frequency sweep. We suggest that consideration of laser behavior through realistic and comprehensive modeling is not only useful but is also required in any pulsed application in which emission frequency change is likely to affect performance.

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.

We report measurements of switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. We observe hopping between lasing modes on picosecond-timescales and temporal evolution of transient multi-mode emission into steady-state single mode emission.

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.

We propose a laser feedback interferometer operating at multiple terahertz (THz) frequency bands by using a pulsed coupled-cavity THz quantum cascade laser (QCL) under optical feedback. A theoretical model that contains multi-mode reduced rate equations and thermal equations is presented, which captures the interplay between electro-optical, thermal, and feedback effects. By using the self-heating effect in both active and passive cavities, self-mixing signal responses at three different THz frequency bands are predicted. A multi-spectral laser feedback interferometry system based on such a coupled-cavity THz QCL will permit ultra-high-speed sensing and spectroscopic applications including material identification.

In [1], the affiliation for Andrew Grier was incorrect. The correct affiliation where his contribution was made is as follows: A. T. Grier was with the School of Electronic and Electrical Engineering, University of Leeds, LS2 9JT Leeds, U.K. (e-mail: atgrier4@gmail.com).

Terahertz (THz) quantum cascade lasers (QCLs) are compact sources of radiation in the 1–5 THz range with significant potential for applications in sensing and imaging. Laser feedback interferometry (LFI) with THz QCLs is a technique utilizing the sensitivity of the QCL to the radiation reflected back into the laser cavity from an external target. We will discuss modelling techniques and explore the applications of LFI in biological tissue imaging and will show that the confocal nature of the QCL in LFI systems, with their innate capacity for depth sectioning, makes them suitable for skin diagnostics with the well-known advantages of more conventional confocal microscopes. A demonstration of discrimination of neoplasia from healthy tissue using a THz, LFI-based system in the context of melanoma is presented using a transgenic mouse model. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Optical feedback effects in lasers may be useful or problematic, depending on the type of application. When semiconductor lasers are operated using pulsed-mode excitation, their behavior under optical feedback depends on the electronic and thermal characteristics of the laser, as well as the nature of the external cavity. Predicting the behavior of a laser under both optical feedback and pulsed operation therefore requires a detailed model that includes laser-specific thermal and electronic characteristics. In this paper we introduce such a model for an exemplar bound-to-continuum terahertz frequency quantum cascade laser (QCL), illustrating its use in a selection of pulsed operation scenarios. Our results demonstrate significant interplay between electro-optical, thermal, and feedback phenomena, and that this interplay is key to understanding QCL behavior in pulsed applications. Further, our results suggest that for many types of QCL in interferometric applications, thermal modulation via low duty cycle pulsed operation would be an alternative to commonly used adiabatic modulation.

The authors acknowledge support the European Cooperation in Science and Technology (COST) Action BM1205. This research was supported under Australian Research Council’s Discovery Projects funding scheme (DP 120 103703) and the State of Bavaria. Y.L.L. acknowledges support under the Queensland Government’s Smart Futures Fellowships programme. P.D. acknowledges support from the EPSRC (UK). S. H. gratefully acknowledges support by the Royal Society and the Wolfson Foundation. In this work, we demonstrate a simple square-wave electrical modulation scheme for imaging with laser feedback interferometry (LFI). Distinct advantages of this scheme include (a) the straightforward creation of the modulating signal, even for high-current lasers, and (b) its natural suitability for lock-in detection. We compare this simple scheme against two established imaging modalities for LFI: mechanical modulation using an optical chopper and the swept-frequency feedback interferometry approach. The proposed scheme lends itself to high frequency modulation which paves the way for high frame-rate LFI imaging with no motion artefacts using off-the-shelf equipment. Publisher PDF

Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity., Single-mode, tuneable monolithic semiconductor lasers are important light sources for integrated photonics. Here, Kundu et al. observe the switch-on dynamics and mode competition of a terahertz quantum cascade laser and explain the behaviour with a carrier and photon transport model.

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.

Recent research shows that terahertz quantum cascade lasers are well-suited to high speed free space communication. The results of both theoretical and laboratory work indicate the devices are able to deliver bandwidths in the gigahertz to tens of gigahertz range without the burden of relaxation oscillations found in diode lasers. Using a novel rate equation model we explore the frequency response characteristics of a real device and report on the finding of a strongly peaked bias current-dependent response.

Terahertz-frequency quantum cascade lasers (THz QCLs) based on bound-to-continuum active regions are difficult to model owing to their large number of quantum states. We present a computationally efficient reduced rate equation (RE) model that reproduces the experimentally observed variation of THz power with respect to drive current and heat-sink temperature. We also present dynamic (time-domain) simulations under a range of drive currents and predict an increase in modulation bandwidth as the current approaches the peak of the light-current curve, as observed experimentally in mid-infrared QCLs. We account for temperature and bias dependence of the carrier lifetimes, gain, and injection efficiency, calculated from a full rate equation model. The temperature dependence of the simulated threshold current, emitted power, and cut-off current are thus all reproduced accurately with only one fitting parameter, the interface roughness, in the full REs. We propose that the model could therefore be used for rapid dynamical simulation of QCL designs. (C) 2015 AIP Publishing LLC

We explain the origin of voltage variations due to self-mixing in a terahertz (THz) frequency quantum cascade laser (QCL) using an extended density matrix (DM) approach. Our DM model allows calculation of both the current–voltage (I–V) and optical power characteristics of the QCL under optical feedback by changing the cavity loss, to which the gain of the active region is clamped. The variation of intra-cavity field strength necessary to achieve gain clamping, and the corresponding change in bias required to maintain a constant current density through the heterostructure is then calculated. Strong enhancement of the self-mixing voltage signal due to non-linearity of the (I–V) characteristics is predicted and confirmed experimentally in an exemplar 2.6 THz bound-to-continuum QCL.

A vacuum balance that compares the weights of 10-kg stainless-steel masses suspended in evacuated tubes at different levels in a hydroelectric reservoir is being used to measure the gravitational attractions of layers of lake water up to 10 m in depth. The mean effective distance between interacting masses in this experiment is 22 m, making it the largest-scale measurement of G using precisely controlled moving masses. The experiment extends laboratory-type measurements into the range previously explored only by geophysical methods. Assuming purely Newtonian physics the value of the gravitational constant determined from data obtained so far is G=6.689(57)×10-11 m3 kg-1s-2, which agrees with laboratory estimates. The data admit at a 0.6 standard deviation level the parameters of non-Newtonian gravity inferred from geophysical measurements in mines and a tower. These measurements push the estimated ranges of non-Newtonian forces down to a scale accessible to our reservoir experiment, so that experimental improvements now at hand may provide a critical test of non-Newtonian effects.

Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity.

Remarkable progress in terahertz (THz) technology over the past decade has been driven by the potential applications of THz waves in areas such as biomedical imaging, long-range screening, and organic materials identification [1]. This growth is in no small measure related to the success of the quantum cascade laser (QCL) which has established itself as one of the most promising radiation sources at terahertz frequencies [2]. The appeal of these novel semiconductor lasers stems from their compact size, broad spectral coverage (∼ 1–5 THz), and high output powers [3]. The ability of THz QCLs to generate coherent emission with quantum noise-limited linewidths, make them particularly suited to the development of interferometric THz sensing and imaging systems.

There is a need for a model that accurately describes dynamics of a bound-to-continuum terahertz quantum cascade laser over its full range of operating temperatures and bias conditions. In this paper we propose a compact model which, through the inclusion of thermal and Stark effects, accurately reproduces the light-current characteristics of an exemplar bound-to-continuum terahertz quantum cascade laser. Through this model, we investigate the dynamics of this laser with a view to applications in high-speed free space communications.