- [Show abstract] [Hide abstract] ABSTRACT: For the first time to our knowledge, we apply the multiple signal classification (MUSIC) algorithm to signals obtained from a self-mixing flow sensor. We find that MUSIC accurately extracts the fluid velocity and exhibits a markedly better signal-to-noise ratio (SNR) than the commonly used fast Fourier transform (FFT) method. We compare the performance of the MUSIC and FFT methods for three decades of scatterer concentration and fluid velocities from 0.5 to 50 mm/s. MUSIC provided better linearity than the FFT and was able to accurately function over a wider range of algorithm parameters. MUSIC exhibited excellent linearity and SNR even at low scatterer concentration, at which the FFT’s SNR decreased to impractical levels. This makes MUSIC a particularly attractive method for flow measurement systems with a low density of scatterers such as microfluidic and nanofluidic systems and blood flow in capillaries.
- [Show abstract] [Hide abstract] ABSTRACT: We demonstrate an active phase-nulling scheme for terahertz (THz) frequency quantum cascade lasers (QCLs) under optical feedback, by active electronic feedback control of the emission frequency. Using this scheme, the frequency tuning rate of a THz QCL is characterized, with significantly reduced experimental complexity compared to alternative approaches. Furthermore, we demonstrate real-time displacement sensing of targets, overcoming the resolution limits imposed by quantization in previously implemented fringe-counting methods. Our approach is readily applicable to high-frequency vibrometry and surface profiling of targets, as well as frequency-stabilization schemes for THz QCLs.
- [Show abstract] [Hide abstract] ABSTRACT: We demonstrate coherent three-dimensional terahertz imaging by frequency modulation of a quantum cascade laser in a compact and experimentally simple self-mixing scheme. Through this approach, we can realize significantly faster acquisition rates compared to previous schemes employing longitudinal mechanical scanning of a sample. We achieve a depth resolution of better than 0.1 mu m with a power noise spectral density below -50 dB/Hz, for a sampling time of 10 ms/pixel. (C) 2015 Optical Society of America
- [Show abstract] [Hide abstract] ABSTRACT: 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.
- [Show abstract] [Hide abstract] ABSTRACT: We present a simple analytical model that describes the injection current and temperature dependence of optical feedback interferometry signal strength for a single-mode laser diode. The model is derived from the Lang and Kobayashi rate equations, and is developed both for signals acquired from the monitoring photodiode (proportional to the variations in optical power) and for those obtained by amplification of the corresponding variations in laser voltage. The model shows that both the photodiode and the voltage signal strengths are dependent on the laser slope efficiency, which itself is a function of the injection current and the temperature. Moreover, the model predicts that the photodiode and voltage signal strengths depend differently on injection current and temperature. This important model prediction was proven experimentally for a near-infrared distributed feedback laser by measuring both types of signals over a wide range of injection currents and temperatures. Therefore, this simple model provides important insight into the radically different biasing strategies required to achieve optimal sensor sensitivity for both interferometric signal acquisition schemes.
- [Show abstract] [Hide abstract] ABSTRACT: In this paper, we demonstrate a simple square-wave electrical modulation scheme for imaging with laser feedback interferometry (LFI). Distinct advantages of this scheme include: 1) the straightforward creation of the modulating signal, even for high-current lasers and 2) its natural suitability for lock-in detection. We compare this simple scheme against two established imaging modalities for LFI: 1) mechanical modulation using an optical chopper and 2) 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.
Conference Paper: A QCL model with integrated thermal and stark rollover mechanisms[Show abstract] [Hide abstract] ABSTRACT: 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.
- [Show abstract] [Hide abstract] ABSTRACT: Characterisation of the standard deviation of a time-series signal has uncommon, yet widespread applications. The usual requirement for a representation of signal standard deviation in real-time implies a high computation speed. A method based on a field programmable gate array (FPGA) implementation is presented. The technique is benchmarked against conventional computational approaches and shows a single windowed standard deviation update calculation of a 16 bit sample can be achieved in 11 ns on a modern CPU. The FPGA implementation is found to be superior to all other approaches examined with an operation time of below 10 ns, and thus provides a useful tool for the real-time measurement of the standard deviation of signals above 100 MHz.
- [Show abstract] [Hide abstract] ABSTRACT: We have investigated the effect of the optical system design on the characteristics of the Doppler spectrum of a laser Doppler velocimeter, by means of a Monte Carlo simulation model. We show that numerical aperture is one of the parameters that strongly affect the full width at half maximum and signal to noise ratio of the Doppler spectrum. The profile and intensity of the enhanced backscattering of the laser beam from a diffusive rough aluminum disk surface is measured using a range of lenses with different focal lengths and the results are incorporated into the Monte Carlo model.
- [Show abstract] [Hide abstract] ABSTRACT: This study introduces optical feedback interferometry as a simple and effective technique for the two-dimensional visualisation of acoustic fields. We present imaging results for several pressure distributions including those for progressive waves, standing waves, as well as the diffraction and interference patterns of the acoustic waves. The proposed solution has the distinct advantage of extreme optical simplicity and robustness thus opening the way to a low cost acoustic field imaging system based on mass produced laser diodes.
- [Show abstract] [Hide abstract] ABSTRACT: There is considerable interest in the interrogation of biological tissue at terahertz (THz) frequencies, largely due to the contrast in the optical properties of different biological tissues which occur in this electro-magnetic radiation band. Of particular interest are THz biomedical images, which have the potential to highlight different information than those acquired in other frequency bands, thereby providing an augmented picture of biological structures. In this work, we demonstrate the feasibility of an interferometric biological imaging technique using a THz quantum cascade laser (QCL) operating at 2.59 THz to perform coherent imaging of porcine tissue samples. We show the potential of this new THz biomedical imaging technique for in vivo studies, by virtue of its reflection geometry and useful tissue penetration depth enabled by the large THz powers emitted by the quantum cascade laser used in this work. The observed clustering of interferometric tissue signatures opens a pathway towards automatic techniques for the discrimination of healthy tissue types for the study of normal physiology and possible therapeutic approaches.
- [Show abstract] [Hide abstract] ABSTRACT: The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of THz radiation offering high power, high spectral purity and moderate tunability. As such, these sources are particularly suited to the application of THz frequency imaging across a range of disciplines, and have motivated significant research interest in this area over the past decade. In this paper we review the technological approaches to THz QCL-based imaging and the key advancements within this field. We discuss in detail a number of imaging approaches targeted to application areas including multiple-frequency transmission and diffuse reflection imaging for the spectral mapping of targets; as well as coherent approaches based on the self-mixing phenomenon in THz QCLs for long-range imaging, three-dimensional imaging, materials analysis, and high-resolution inverse synthetic aperture radar imaging.
- [Show abstract] [Hide abstract] ABSTRACT: The rate-equation model for a semiconductor laser under optical feedback is a useful tool for studying complex laser dynamics. Optical feedback from scatterers suspended in fluid flow and rough bodies in motion consists of many, spectrally close, Doppler-shifted frequency components with random phase. How this type of feedback is included in the well-established Lang and Kobayashi model is clarified, resulting in simulated results that agree well with the experiment in both time and frequency domains. Equipped with this model, the dynamic effects of laser feedback systems measuring flow or rough targets in motion may be studied numerically.
- [Show abstract] [Hide abstract] ABSTRACT: Recently, we demonstrated an interferometric materials analysis scheme at terahertz frequencies based on the self-mixing effect in terahertz quantum cascade lasers. Here, we examine the impact of variations in laser operating parameters, target characteristics, laser–target system properties, and the quality calibration standards on our scheme. We show that our coherent scheme is intrinsically most sensitive to fluctuations in interferometric phase, arising primarily from variations in external cavity length. Moreover we demonstrate that the smallest experimental uncertainties in the determination of extinction coefficients are expected for lossy materials.
- [Show abstract] [Hide abstract] ABSTRACT: Terahertz frequency quantum cascade lasers (THz QCLs) are compact electrically-driven semiconductor sources of radiation in the 1–5 THz (λ = 300–60 μm) band. Through a systematic optimisation of the device design and processing, we have recently demonstrated a world-record peak output power in excess of 1 W from a single facet at 10 K (more than double the previous record) and \textgreater 400 mW at 77 K. We review the development and performance of this laser, and consider a range of novel imaging techniques that could exploit high-powered THz emission. The first such technique senses the relatively weak scattered radiation from a powdered material, thus obtaining sensitivity to both the particle geometry and the bulk spectral absorption properties of the material, in good agreement with conventional THz time-domain spectroscopy. An alternative self-mixing technique is discussed, in which feedback of radiation into the laser cavity from an external object causes perturbations to the laser voltage, allowing ‘detector-free’, highly sensitive, confocal and phase-sensitive imaging of concealed three-dimensional surface morphologies.
- [Show abstract] [Hide abstract] ABSTRACT: Self-mixing laser sensors show promise for a wide range of sensing applications, including displacement, velocimetry, and fluid flow measurements. Several techniques have been developed to simulate self-mixing signals; however, a complete and succinct process for synthesizing self-mixing signals has so far been absent in the open literature. This article provides a systematic numerical approach for the analysis of self-mixing sensors using the steady-state solution to the Lang and Kobayashi model. Examples are given to show how this method can be used to synthesize self-mixing signals for arbitrary feedback levels and for displacement, distance, and velocity measurement. We examine these applications with a deterministic stimulus and discuss the velocity measurement of a rough surface, which necessitates the inclusion of a random stimulus.
- [Show abstract] [Hide abstract] ABSTRACT: We propose a terahertz (THz)-frequency synthetic aperture radar imaging technique based on self-mixing (SM) interferometry, using a quantum cascade laser. A signal processing method is employed which extracts and exploits the radar-related information contained in the SM signals, enabling the creation of THz images with improved spatial resolution. We demonstrate this by imaging a standard resolution test target, achieving resolution beyond the diffraction limit. (C) 2014 Optical Society of America
- [Show abstract] [Hide abstract] ABSTRACT: Quasi-static interferometric signals in lasers under feedback arise from slowly varying perturbations of the intracavity electric field resulting from the reinjection of a portion of the emitted field into the cavity. Such interferometric signals are well described by the steady-state solution to the Lang-Kobayashi rate equation model. We give an exact series expansion for this steady-state solution that shows precisely how Acket's characteristic parameter C and Henry's linewidth enhancement factor α influence such signals. We show how the series coefficients can be extracted easily and explain how to determine C and α directly from them. Moreover, we draw a precise analogy between self-mixing and FM signals, showing that C plays exactly the same role in self-mixing as the modulation index does in FM.
University of Queensland
Brisbane, Queensland, Australia
- School of Information Technology and Electrical Engineering
The University of Hong Kong
Hong Kong, Hong Kong
- Department of Electrical and Electronic Engineering
University of Belgrade
Beograd, Central Serbia, Serbia
- School of Electrical Engineering