A QCL model with integrated thermal and stark rollover mechanisms

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

A model for a pulsed terahertz quantum cascade laser under optical feedback

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

Terahertz sensing and imaging using a quantum cascade laser

We demonstrate terahertz (THz) frequency imaging and sensing using a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilising the effect of self-mixing in the THz QCL, and specifically by monitoring perturbations to the voltage across the QCL induced by light reflected from an external object back into the laser cavity. Self-mixing offers high sensitivity, a potentially fast response, and a simple, compact optical design. We show that it can be used to obtain high-resolution reflection images of exemplar structures, as well as for the measurement of the displacement of a remote target, both with and without opaque (in the visible spectrum) materials in the beam path. We also demonstrate displacement sensing over a stand-off distance of 7m through air

Terahertz imaging through self-mixing in a quantum cascade laser

We demonstrate terahertz (THz) frequency imaging through use of a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilising the effect of self-mixing in the QCL, and specifically by monitoring perturbations to the voltage across the QCL induced by light reflected from an external object back into the laser cavity. Self-mixing imaging offers high sensitivity, a potentially fast response, and a simple, compact optical design, and we show that it can be used to obtain high-resolution reflection images of exemplar structures

Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis

The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of high-power radiation with a narrow intrinsic linewidth. As such, THz QCLs are extremely promising sources for applications including high-resolution spectroscopy, heterodyne detection, and coherent imaging. We exploit the remarkable phase-stability of THz QCLs to create a coherent swept-frequency delayed self-homodyning method for both imaging and materials analysis, using laser feedback interferometry. Using our scheme we obtain amplitude-like and phase-like images with minimal signal processing. We determine the physical relationship between the operating parameters of the laser under feedback and the complex refractive index of the target and demonstrate that this coherent detection method enables extraction of complex refractive indices with high accuracy. This establishes an ultimately compact and easy-to-implement THz imaging and materials analysis system, in which the local oscillator, mixer, and detector are all combined into a single laser

Self-mixing interferometry with a Terahertz Quantum Cascade Laser: feedback induced voltage signal

We report here a compact and efficient sensing technique that utilises the self-mixing effect in terahertz Quantum Cascade Lasers (QCLs). A terahertz QCL is used both for the emission and the detection of light. Sensors based on this technique promise high sensitivity, simplicity of optical design and the potential for implementation in monolithic arrays. © 2010 IEEE

Self-mixing sensor based on a terahertz quantum cascade laser

In recent years, the quantum cascade laser (QCL) [1] has attracted extensive interest as a narrowband terahertz (THz) source. To date, THz QCLs have been demonstrated with operating temperatures as high as 186 K in pulsed mode [2], and output powers exceeding 100 mW in continuous-wave (cw) operation [3]. The compactness and high power of such sources makes THz QCLs potentially well-suited to a range of sensing applications including biomedicine [4], pharmaceuticals [5], and biological and chemical sensing [6,7]. However, existing sensing implementations inevitably result in bulky and complex systems, typically requiring a vibration isolation platform

Coherent imaging and sensing using the self-mixing effect in THz quantum cascade lasers

We present recent advancements in the development of coherent THz imaging and sensing systems that exploit the self-mixing (SM) effect in quantum cascade lasers (QCLs). SM occurs when radiation from a laser is partially reflected from an external object and injected back into the laser cavity. The reflected radiation interferes („mixes‟) with the inter-cavity field, producing variations in the emitted power and terminal voltage. Thus, by combining the local oscillator, mixer, and the detector all in a single laser, this technique allows the development of simple, self-aligned systems that can sense both the phase and amplitude of the THz field reflected from samples. We demonstrate the coherent nature of this sensing technique for depth-resolved reflection imaging, whereby the phase-shift induced upon reflection is interpreted in terms of surface morphology of the sample. We will also present an alternative, novel sensing modality based on this self-mixing approach

Self-mixing effect in THz quantum cascade lasers: applications in sensing and imaging

The paper introduces self-mixing interferometry in semiconductor lasers in general, and then discusses recent advancements in the coherent THz imaging and sensing systems based on the self-mixing effect in terahertz quantum cascade lasers. Two different imaging modalities are used to illustrate the coherent nature of this sensing technique and its applications to three-dimensional surface profiling and material identification