Self-mixing Research Articles

Self-mixing dual-frequency laser Doppler velocimeter

A self-mixing (SM) dual-frequency (DF) laser Doppler velocimeter (LDV) (SM DF-LDV) is proposed and studied, which integrates the advantages of both the SM-LDV and the DF-LDV. An optically injected semiconductor laser operated in a dual-frequency period-one (P1) dynamical state is used as the light source. By probing the target with the light-carried microwave generated from the beat of the two optical frequency components, the spectral broadening in the Doppler signal due to the speckle noise can be significantly reduced. Together with an SM configuration, the SM DF-LDV has the advantages of direction discriminability, self-alignment, high sensitivity, and compact setup. In this study, speckle noise reduction and direction discriminability with an SM DF-LDV are demonstrated. The signal-to-noise ratios (SNRs) at different feedback powers are investigated. Benefiting from the high sensitivity of the SM configuration, an SNR of 23 dB is achieved without employing an avalanched photodetector or photomultiplier tube. The velocity resolution and the SNR under different speckle noise conditions are studied. Average velocity resolution of 0.42 mm/s and SNR of 22.1 dB are achieved when a piece of paper is rotating at a transverse velocity of 5 m/s. Compared with a conventional single-frequency LDV (SF-LDV), the SM DF-LDV shows improvements of 20-fold in the velocity resolution and 8 dB in the SNR.

Classification of laser self-mixing interferometric signal under moderate feedback

This paper presents a different approach to classify self-mixing (SM) signals operating in the moderate feedback regime. A total of six distinct classes of SM signals can be defined based on the SM inherent shapes, which depend on both the feedback factor C and the linewidth enhancement factor α. This classification allows clear identification of SM signals for which normalization issues can arise and thus for which displacement precision is inherently reduced due to the very nature of the signal itself. Finally, it is shown that phase unwrapping approaches can theoretically retrieve displacement with subnanometer precision for usual laser diodes in the moderate feedback regime, in the absence of noise, only for α values greater than approximately 4.3.

Self-mixing sensing system based on uncooled vertical-cavity surface-emitting laser array: linking multichannel operation and enhanced performance

We compare the performance of a self-mixing (SM) sensing system based on an uncooled monolithic array of 24×1 vertical-cavity surface-emitting lasers (VCSELs) in two modes of operation: single active channel and the concurrent multichannel operation. We find that the signal-to-noise ratio of individual SM sensors in a VCSEL array is markedly improved by multichannel operation, as a consequence of the increased operational temperature of the sensors. The performance improvement can be further increased by manufacturing VCSEL arrays with smaller pitch. This has the potential to produce an imaging system with high spatial and temporal resolutions that can be operated without temperature stabilization.

Real time self-mixing interferometric laser sensor for embedded applications

We present a Self-Mixing (SM) interferometric laser displacement sensor that is capable of providing correct target measurements in real time, even when it is subject to extraneous parasitic movements. The sensor achieves such robustness by using an embedded MEMS Solid -State Accelerometer (SSA) that has been coupled with the laser sensor. The SSA thus measures the extraneous movement acting on the laser s ensor and this information is used to provide correct sensing. The proposed SSA-SM sensing system uses Consecutive-Samples based Unwrapping (CSU) to process the SM interferometric signal while a Digital Signal Processor (DSP) takes care of band-pass filtering, double integration as well as phase and gain corrections needed for the acceleration signal. Hence, a compact, real-time, precise and self-aligned SSA-SM sensor has been designed that has a displacement measurement precision of approximately 100 nm with a parasitic movement elimination of 31dB for a laser diode emitting at 785 nm.

Study of Laser Feedback Phase Under Self-Mixing Leading to Improved Phase Unwrapping for Vibration Sensing

In this paper, the inherent error as well as the robustness of a previously published displacement retrieval technique called the phase unwrapping method (PUM) is analyzed. This analysis, based on a detailed study of laser feedback phase behavior, results in a new algorithm that removes the PUM inherent error while maintaining its robustness. The said algorithm has been successfully tested on simulated and experimental self-mixing (SM) interferometric signals. Simulations in weak and moderate feedback regimes demonstrate that the said algorithm can reach a subnanometric precision compared with approximately 25 nm for PUM. For experimental SM signals affected by noise, the measured rms displacement error and the maximum absolute error are 14 and 37 nm, respectively, for the proposed algorithm and 34 and 123 nm for the PUM, which indicates a threefold displacement precision improvement over the PUM. Finally, it is explained that the precision can be further improved by a reduction of the noise level of experimental SM signals.

Design and Analysis of an Embedded Accelerometer Coupled Self-Mixing Laser Displacement Sensor

This paper presents the operating principle and signal processing needed for the design of a reliable solid-state accelerometer (SSA) coupled self-mixing (SM) interferometric laser displacement sensor for embedded applications. The influence of signal processing methods and accelerometer characteristics on the complete sensing system performance is studied, and four different SSA-SM sensing systems are examined and characterized. Through comparing their performance, the sensing system precision is limited by the noise density of the employed accelerometer as well as the used SM displacement retrieval technique, whereas the system bandwidth is mainly limited by the choice of a given accelerometer. Furthermore, this paper analyzes the phase and gain-matching properties that the SSA-SM should reach to guarantee proper extraneous vibrations correction. Finally, the proof of concept of a real-time SSA-SM sensing system indicating 30-dB correction is presented. This prototype demonstrates the possibility of using such a real-time sensing system for embedded and industrial applications in which the presence of extraneous movements would hinder traditional sensors use.

Self-Mixing Interferometry With Terahertz Quantum Cascade Lasers

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.

Infrasound detection by laser diode self-mixing interferometry

An alternative technique for infrasound detection based on the self-mixing (SM) interference of a laser diode is described. The principle involved is the dependence of the power emitted by the laser diode on infrasound-induced membrane vibration. The Fourier transform and fringe-counting methods are used to analyze the self-mixing signal. Infrasound signals are experimentally measured from 2 to 20 Hz with a resolution of 0.25, and the results well agree with the theoretical ones.

Self-Mixing Laser Sensor for Large Displacements: Signal Recovery in the Presence of Speckle

Laser self-mixing (SM) sensors are successfully used to measure displacement in the absence of speckle. However, speckle deforms the SM signal rendering it unusable for standard displacement extraction techniques. This article proposes a new signal-processing technique, based on tracking the signal envelope, to remedy this problem. Algorithm was successfully employed to measure long-range displacements (25 mm), in the presence of speckle and the lateral movement of the target, both causing severe corruption of the SM signal. It therefore enabled the use of the sensor on noncooperative targets without the need for sensor positioning and/or alignment. The results have been obtained for SM signals in which the envelope amplitude is varied by a factor of 28, without the loss of interferometric fringes. The use of this technique effectively removes the need for opto/electro-mechanical components traditionally used to measure long-range displacement in the presence of speckle.

Responsivity and Noise of Self-Mixing Photodetection Schemes

Responsivity and noise properties of the self-mixing (SM) detection process are analyzed. Starting with the photodiode and SM laser scheme as a detector of weak optical echoes from remote targets, we find equivalence to a coherent homodyne detection of the returning signal. In particular, in the VIS-NIR wavelength range, echoes can be detected down to about -90 dB of relative amplitude, typically. Then we consider the laser-diode voltage self-mixing (LV-SM) detection and find it is also a coherent homodyne scheme, though noise performance is limited by shot-noise of the bias current and by the relatively small signal supplied as an output. Theoretical results are finally compared to recent experimental data, finding a substantial agreement and confirming that the LV-SM is an attractive alternative to conventional photo-detection, especially for THz waves and other spectral ranges where good low-noise detectors may be difficult to employ.

MEMS accelerometer embedded in a self-mixing displacement sensor for parasitic vibration compensation

A self-mixing (SM) laser displacement sensor coupled with a microelectromechanical system (MEMS) accelerometer is presented that enables reliable displacement measurements even in the case of a nonstationary laser head. The proposed technique allows the use of SM-based sensors for embedded applications. The system resolution is currently limited to approximately 300 nm due to the noise characteristics of the currently used accelerometer. It is shown that this resolution can be greatly improved by the use of a low noise accelerometer.

GaN laser self-mixing velocimeter for measuring slow flows

The self-mixing (SM) laser sensing technique allows for a simple, self-aligned, and robust system for measuring velocity. Low-cost blue emitting GaN laser diodes have recently become available owing to the high volume requirements for Blu-ray Disc devices such as high-definition video players and gaming consoles. These GaN lasers have a significantly shorter wavelength (around 405 nm) compared with other semiconductor lasers (generally around 800 nm for SM sensors). Therefore, if used in SM flow sensors, they allow measuring of flow rates that would otherwise be too slow to measure. In this Letter we report what we believe to be the world's first SM flow measurement system based on a blue emitting semiconductor laser, demonstrating the ability to measure flow rates down to 26 microm/s.

Adaptive Transition Detection Algorithm for a Self-Mixing Displacement Sensor

A new algorithm for self-mixing (SM) sensors has been developed to perform displacement measurements. It is able to differentiate between the different SM regimes (very weak, weak, moderate, and strong) and thus converges automatically to the optimum threshold level required to detect all the SM fringes, independently of the shape of the signal. Displacement reconstructions based on this algorithm have been validated with counter measuring commercial sensors for both weak and moderate regime acquisitions which are most frequently encountered under experimental conditions.

A Self-Mixing Laser Sensor Design With an Extended Kalman Filter for Optimal Online Structural Analysis and Damping Evaluation

We have developed a new algorithm based on the extended Kalman filter in order to improve the resolution of a self-mixing (SM) optical displacement sensor. This noncontact sensor, which provides vibration measurement with a very high accuracy, can be used for online quality control, for example, measuring the damping of excited mechanical structures. This SM sensor subject to weak feedback has been tested in comparison with a commercial vibrometer in order to measure the frequency response function (FRF) of a plate with a passive damping to be characterized, and to show the efficiency of a damping treatment.