[Show abstract][Hide abstract] ABSTRACT: We demonstrate coherent terahertz (THz) frequency imaging using the self-mixing effect in a quantum cascade laser (QCL). Self-mixing voltage waveforms are acquired at each pixel of a two-dimensional image of etched GaAs structures and fitted to a three-mirror laser model, enabling extraction of the amplitude and phase parameters of the reflected field. From the phase, we reconstruct the depth of the sample surface, and we show that the amplitude can be related to the sample reflectance. Our approach is experimentally simple and compact, and does not require frequency stabilization of the THz QCL.
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: Based on the nature of self-mixing signals, we propose the use of the multiple signal classification (MUSIC) algorithm in place of the fast Fourier transform (FFT) for processing signals obtained from self-mixing interferometry (SMI). We apply this algorithm to two representative SMI measurement techniques: range finding and velocimetry. Applying MUSIC to SMI range finding, we find its signal-to-noise ratio performance to be significantly better than that of the FFT, allowing for more robust, longer-range measurement systems. We further demonstrate that MUSIC enables a fundamental change in how SMI Doppler velocity measurement is approached, letting one discard the complex fitting procedure and allowing for a real-time frequency estimation process.
[Show abstract][Hide abstract] ABSTRACT: Optical sensing offers an attractive option for detection of surface biopotentials in human subjects where electromagnetically noisy environments exist or safety requirements dictate a high degree of galvanic isolation. Such circumstances may be found in modern magnetic resonance imaging systems for example. The low signal amplitude and high source impedance of typical biopotentials have made optical transduction an uncommon sensing approach. We propose a solution consisting of an electro-optic phase modulator as a transducer, coupled to a vertical-cavity surface-emitting laser and the self-mixing signal detected via a photodiode. This configuration is physically evaluated with respect to synthesized surface electrocardiographic (EKG) signals of varying amplitudes and using differing optical feedback regimes. Optically detected EKG signals using strong optical feedback show the feasibility of this approach and indicate directions for optimization of the electro-optic transducer for improved signal-to-noise ratios. This may provide a new means of biopotential detection suited for environments characterized by harsh electromagnetic interference.
[Show abstract][Hide abstract] ABSTRACT: The need to accurately measure flow profiles in microfluidic channels is well recognised. In this work, we present a new optical feedback interferometry (OFI) flow sensor that accurately measures local velocity in fluids and enables reconstruction of a velocity profile inside a microchannel. OFI is a self-aligned interferometric technique that uses the laser as both the transmitter and the receiver thus offering high sensitivity, fast response, and a simple and compact optical design. The system described here is based on a commercial semiconductor laser and has been designed to achieve a micrometer-range spatial resolution. The sensor performance was validated by reconstructing the velocity profile inside a circular cross-section flow-channel with 320 lm internal diameter, with a relative error smaller than 1.8 %. The local flow velocity is directly measured, thus avoiding the need for model based profile calculation and uncertainties inherent to this approach. The system was validated by successfully extracting the flow profiles in both Newtonian and shear- thinning liquids.
Microfluidics and Nanofluidics 01/2013; 14:113-119. DOI:10.1007/s10404-012-1029-0 · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Terahertz frequency quantum cascade lasers (THz QCLs) are compact sources of coherent THz radiation with potential applications that include astronomy, trace-gas sensing, and security imaging. However, the reliance on slow and incoherent thermal detectors has limited their practical use in THz systems. We demonstrate THz sensing using self-mixing (SM) interferometry, in which radiation is reflected from an object back into the QCL cavity, causing changes in the laser properties; the THz QCL thus acts simultaneously as both a source and detector. Well-established SM theory predicts a much weaker coupling in THz QCLs than in diode lasers, yielding a near-linear relationship between the phase of SM signals and the external cavity length. We demonstrate velocimetry of an oscillating reflector by monitoring SM-induced changes in the QCL drive voltage. We show that this yields data equivalent to that obtained by sensing the emitted THz power, thus allowing phase-sensitive THz-SM sensing without any external detector. We also demonstrate high-resolution SM-imaging at a round-trip distance of 21 m in air-the longest-range interferometric sensing with a THz QCL to date.
[Show abstract][Hide abstract] ABSTRACT: This paper describes the proposed optical electrocardiograph (ECG) using self-mixing interferometer technique with a customized electro-optic phase modulator as the transducer. The surface ECG signals were recorded using commercially available electrodes demonstrating the feasibility of measuring the ECG signal using the SMI technique with its attendant benefits of intrinsic electrical safety and high level of EMI immunity.
[Show abstract][Hide abstract] ABSTRACT: Self-mixing laser sensors require few components and can be used to measure velocity. The self-mixing laser sensor consists of a laser emitting a beam focused onto a rough target that scatters the beam with some of the emission re-entering the laser cavity. This 'self-mixing' causes measurable interferometric modulation of the laser output power that leads to a periodic Doppler signal spectrum with a peak at a frequency proportional to the velocity of the target. Scattering of the laser emission from a rough surface also leads to a speckle effect that modulates the Doppler signal causing broadening of the signal spectrum adding uncertainty to the velocity measurement. This article analyzes the speckle effect to provide an analytic equation to predict the spectral broadening of an acquired self-mixing signal and compares the predicted broadening to experimental results. To the best of our knowledge, the model proposed in this article is the first model that has successfully predicted speckle broadening in a self-mixing velocimetry sensor in a quantitative manner. It was found that the beam spot size on the target and the target speed affect the resulting spectral broadening caused by speckle. It was also found that the broadening is only weakly dependent on target angle. The experimental broadening was consistently greater than the theoretical speckle broadening due to other effects that also contribute to the total broadening.
[Show abstract][Hide abstract] ABSTRACT: A novel integrated optofluidic flow sensing platform based on the self-mixing effect is presented. The system measures flow of a blood phantom through a 250 μm channel and exhibits good linearity, repeatability and accuracy.
[Show abstract][Hide abstract] ABSTRACT: This paper investigates the resolution and contrast in a near-infrared VCSEL based interferometric confocal microscope and proposes an architecture that can be readily applied in a range of bipolar and unipolar semiconductor lasers. Homodyning nature of the interferometric imaging technique helps overcome the problems related to weak scattered field while imaging nano-particles and low index-contrast objects.
[Show abstract][Hide abstract] ABSTRACT: Measurement of small-signal equivalent circuit parameters was carried out to estimate the RF performance of conventional inversion channel and depletion channel SOS MOSFET's. It was shown that the latter has significantly higher cutoff frequency fT attributed to electron mobility of the depletion channel.
[Show abstract][Hide abstract] ABSTRACT: The self-mixing laser sensor makes compact, low-cost velocimetry sensing possible. In this work we describe a process for modelling the velocimetry signal that includes the dynamic speckle effect. We give results showing a good match between experimentally acquired signals and the model.
[Show abstract][Hide abstract] ABSTRACT: In this paper we present a novel method for measuring change in real refractive index using the self-mixing effect in a semiconductor laser. The polished flat tip of a graded index multimode fiber was imaged and the changes observed attributed to the change in refractive index.
[Show abstract][Hide abstract] ABSTRACT: Recently demonstrated self-mixing effect in terahertz quantum cascade lasers (QCL) opens new possibilities for detectorless sensing in this range of electromagnetic spectrum. In this paper we compare self-mixing signals obtained from variations in QCL terminal voltage against the signals obtained from variations in QCL radiated power monitored using a bolometer. We show that these two signals are equivalent allowing for the disposal of a conventional bulky and slow thermal detector.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we critically compare two techniques for the parametrization of silicon-on-sapphire MOSFETs' high-frequency small-signal equivalent circuit and discuss the scalability of high-frequency equivalent circuit parameters. We demonstrate that the same values of the high-frequency circuit elements are obtained from both the vector network analyzer and the low-frequency $LCR$ measurements. We show that this holds even when majority carriers in the isolated body of the transistor are not in the equilibrium state, implying that the equivalence does not depend on quasi-static response of the carriers.
IEEE Transactions on Electron Devices 01/2012; 59(1):20-25. DOI:10.1109/TED.2011.2170426 · 2.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The off-state source-to-drain leakage current and punchthrough voltage are the quantities that frequently limit the performance of short-channel floating-body silicon-on-sapphire (SOS) n-channel MOSFETs. In this paper, we demonstrate that the high-temperature hydrogen annealing of the SOS film prior to the device fabrication leads to marked improvement in these two parameters. The effect is attributed to the impact of hydrogen on the out-diffused thin alumina layer formed at the silicon-sapphire interface during the anneal. The thin alumina layer acting as a p-type dopant source at the back interface eliminates the back surface depletion of SOS n-MOSFETs. It also acts as a recombination center to eliminate the floating-body effect of floating-body n-MOSFETs. This technique provides a practical and reliable process to build short-channel floating-body SOS n-MOSFETs with off-state leakage as low as the junction leakage and punchthrough voltage as high as 6 V or higher at the gate length of 0.5 μm without any degradation on the inversion layer carrier mobility or increase in the junction leakage current.
IEEE Transactions on Electron Devices 12/2011; 58(11-58):3787 - 3792. DOI:10.1109/TED.2011.2168401 · 2.47 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a method for maintaining the maximum signal-to-noise ratio (SNR) of the signal obtained from the self-mixing sensor based on a vertical-cavity surface-emitting laser (VCSEL). It was found that the locus of the maximum SNR in the current-temperature space can be well approximated by a simple analytical model related to the temperature behavior of the VCSEL threshold current. The optimum sensor performance is achieved by tuning the laser current according to the proposed model, thus enabling the sensor to operate without temperature stabilization in a wide temperature range between -20 °C and +80 °C.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate terahertz (THz) frequency imaging using a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilizing 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 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.
[Show abstract][Hide abstract] ABSTRACT: We present an ultra-compact sensor based on optical feedback in a Vertical-Cavity Surface-Emitting Laser (VCSEL) that does not require any optical element. The setup proposed measures displacement and velocity with an operating range of several centimeters.
Optoelectronic and Microelectronic Materials and Devices (COMMAD), 2010 Conference on; 01/2011