Self-mixing Interferometry System Research Articles

A Novel Method for Quadrature Signal Construction in a Semiconductor Self-Mixing Interferometry System Using a Liquid Crystal Phase Shifter

We have established a novel method for quadrature signal construction in a semiconductor laser diode self-mixing interferometry system using two photodiodes and a beam splitter with a liquid crystal phase shifter (LCPS). This method entails placing an LCPS between the photodiode and the beam splitter so that another phase shift self-mixing signal can be obtained. Then, an arctangent phase algorithm can be used to demodulate the pair of quadrature signals to reconstruct the vibration information of the target object. This method simplifies the self-mixing signal demodulation process and the reconstruction of vibration information. Our experimental results demonstrate the feasibility of using self-mixing phase shifter detection for self-mixing optical measurements. The work illustrates a sort of efficient and referable novel design guidance model which supports the quadrature signals construction in a self-mixing interferometer based on a semiconductor laser diode.

Robust Signal Extraction Based on Time-Frequency Joint Analysis and GRNN for a Laser SMI System

In pursuit of vibration recovery, with subwavelength precision based on laser self-mixing interferometry (SMI), the estimation of two key parameters, i.e., the optical feedback parameter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$C$</tex-math></inline-formula> and linewidth enhancement factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha$</tex-math></inline-formula> , is of vital importance. However, the above process is not ideal in terms of accuracy and real-time performance. Aiming to overcome these deficiencies, a time-domain-based frequency-domain processing method (TFPM) has been proposed for the SMI system in this paper, which realizes robust and fast extraction of the vibration signals without the estimation of the parameters and does not require fringe detection. The method achieves the extraction of motion information via a mathematical transformation, namely, short-time Fourier transform (STFT), in which the limitations caused by STFT are alleviated by using generalized regression neural network (GRNN). In processing the SMI signal of the target with non-stationary motion, the TFPM finally achieves an expected root-mean-square (RMS) error of 0.24 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\%$</tex-math></inline-formula> , which demonstrates the high performance of the proposed method.

Determining System Parameters and Target Movement Directions in a Laser Self-Mixing Interferometry Sensor

Self-mixing interferometry (SMI) is a promising sensing technology. As well as its compact structure, self-alignment and low implementation cost, it has an important advantage that conventional two-beam interferometry does not have, i.e., SMI signal fringe evolves into asymmetrical shape with increasing optical feedback level, which leads to discrimination of target movement directions for unambiguous displacement measurement possible by a single-channel interferometric signal. It is usually achieved by using SMI signals in moderate feedback regime, where the signals exhibit hysteresis and discontinuity. However, in some applications, e.g., in biomedical sensing where the target has a low reflectivity, it is hard for the SMI system to operate in a moderate feedback regime. In this work, we present comprehensive analyses on SMI signal waveforms for determining system parameters and movement directions by a single-channel weak feedback SMI signal. We first investigated the influence of two system parameters, i.e., linewidth enhancement factor and optical feedback factor, on the symmetry of SMI signals. Based on the analyses on signal waveform, we then proposed a method of estimating the system parameters and displacement directions. The method was finally verified by experiments. The results are helpful for developing sensing applications based on weak feedback SMI systems.

Suppression of undamped relaxation oscillation in a laser self-mixing interferometry sensing system.

Undamped relaxation oscillation (RO) in a laser self-mixing interferometry (SMI) system may occur in some common application conditions, which may impact the stable operation of the system and degrade its sensing performance. In this work, we proposed to suppress the undamped RO by controlling the system operation parameters in a laser SMI sensing system. By numerically solving the famous Lang Kobayashi equations, the stability of a laser SMI system in a 3-parameter space of external cavity length, injection current and optical feedback factor were investigated. Based on the stability analyses, we determined the system operation conditions required for suppressing the undamped RO and derived an analytical expression for describing the relationship between the operation parameters. An experimental SMI system based on a laser diode (Sanyo, DL4140-001s) was implemented and verified the suppressing method. The experimental results showed that the SMI system in a moderate feedback regime can operate in steady state without undamped RO by setting proper operation parameters. This work provides useful guidance to design a stable SMI sensing system for practical applications.

Laser Self-Mixing Sensor for Simultaneous Measurement of Young’s Modulus and Internal Friction

The Young’s modulus and internal friction are two important parameters of materials. Self-mixing interferometry (SMI) is an emerging non-destructive sensing method that has been employed for various applications because of its advantages of simple structure, ease of alignment and high resolution. Some recent works have proposed the use of SMI technology to measure the Young’s moduli and/or internal frictions by measuring the resonance frequencies and damping factors of specimen vibrations induced by impulse excitation. However, the measurement results may be affected by frequencies of SMI fringes, and the implementation requires extra signal processing on SMI fringes. In this work, we developed an all-fiber SMI system without SMI fringes to measure the Young’s modulus and internal friction simultaneously. Simulations and experiments were carried out to verify the feasibility of the proposed method. Two specimens of brass and aluminum were tested. The experimental results show that the standard deviations of Young’s moduli for brass and aluminum are 0.20 GPa and 0.14 GPa, and the standard deviations of internal frictions are 4.0×10−5 and 5.4×10−5, respectively. This method eliminates the influences of the SMI fringe frequency on the resonant frequency and requires no signal processing on SMI fringes, contributing to its simplicity as a method for the measurement of the Young’s modulus and internal friction.

All-Fiber Laser-Self-Mixing Sensor for Acoustic Emission Measurement

Self-mixing interferometry (SMI) is a promising non-destructive sensing technology with advantages of simplicity in system structure, low cost in implementation, ease in optical alignment, and high resolution in measurement. It consists of a laser diode, a photodiode packaged at the rear of the laser diode, a lens, and a target to be measured. Typically, SMI-based sensing is to measure the phase change in an SMI signal with a fringe pattern as sinusoidal or saw-tooth like. The phase is called optical phase in external cavity that is linked to the quantity to be measured, e.g., displacement, velocity, and vibration. Each SMI fringe maps to a half-laser-wavelength displacement of the target. With increasing feedback level in an SMI system, some fringes may lost or even totally disappear. In this case, the laser output from an SMI system can trace the optical phase to achieve a linear sensing relationship. In this article, we design an all-fiber SMI linear sensor for real-time and fast tracking acoustic emission (AE) signals. Firstly, a system design in terms of selection of operating parameters is presented. Then, a proof of concept experimental system was built for detecting AE signals. The results show that the proposed SMI sensor has comparable AE detection capability to piezoelectric sensor, providing a compact and cost-effective optical AE sensing solution without need of extra signal processing, which can be considered as a viable alternative to the well-established piezoelectric AE sensor.

Experimental observation of differential self-mixing interference signals using a randomly polarized laser: a differential self-mixing interferometry.

Differential measurement has strong anti-interference ability. We report the first, to the best of our knowledge, experimental demonstration of differential self-mixing interference signals using a randomly polarized laser for differential self-mixing interferometry (SMI). In the differential SMI system, the detection light can be divided into interference signals of $p$p-polarized and $s$s-polarized light with identical intensity and pi-shift phase difference. By exploiting such a new experimental phenomenon, we propose a noise-robust and low-cost self-mixing interferometer. Experiments show that the proposed approach can effectively suppress both periodic and aperiodic noise. Thus, the reported phenomenon and approach has a good application prospect in self-mixing interferometers.

A New Algorithm for Displacement Measurement Using Self-Mixing Interferometry With Modulated Injection Current

Using self-mixing interferometry (SMI) with a periodical modulated injection current, high resolution displacement sensing can be achieved by retrieving the initial phase of an SMI signal at each modulation period. However, the existing initial-phase-based detection methods can only obtain a single point measurement of displacement within each single modulation period. Thus, they are only effective when the target is subject to slow movement, or the injection current is modulated by a signal of very high frequency, which are not practical in many applications. In this work, a new method is proposed to tackle the problem. Firstly, a reference signal is obtained by setting the target still. Then Fast Fourier Transform and its inverse (FFT/IFFT) are applied to the reference signal and the SMI signal, leading to a formulation to obtain the SMI signal phase, which enables the SMI system to retrieve the time varying displacement in each modulation period. As the proposed method is able to measure displacement at multiple discrete time instances (dependent on the number of samples for FFT), the measurement resolution is significantly improved over existing method. Hence, the measurement capability of the SMI system is enhanced greatly. Both simulation and experiments are conducted and the results are presented to verify the proposed algorithm.

Phase modulation self-mixing interferometry by using rotated optical flat

A self-mixing interferometry based on a simple and effective phase modulation is proposed. The core of the technique is the use of an optical flat for phase modulation. The optical flat is placed in the laser external cavity. By rotating the optical flat, the phase is modulated because the rotation of the optical flat changes the optical path length of the laser beam in the external cavity. With the phase modulation, accurate micro-displacement and vibration measurement can be done. We experimentally demonstrate that when the measurement range is less than 2000 nm, the maximum measurement error of the proposed technique is less than 9 nm. The vibration displacement of a loudspeaker is approximately linear with the driving voltage in the range of 0˜80 mV. The proposed technique has the advantages of large modulation range, high precision, simple operation and low cost. It can potentially be used in self-mixing interferometry system widely.

Effect of dual-channel optical feedback on self-mixing interferometry system

Laser feedback interference is widely used for displacement measurement of non-cooperative targets due to its ultra-high sensitivity. However, by the same token, laser feedback interferometry (LFI) suffers from multi-channel optical feedback, which compromises measurement stability and accuracy. We perform both simulations and experiments to study the effect of dual-channel optical feedback on LFI systems based displacement measurement. Dual-channel optical feedback interference is the simplest and most representative form of multi-channel feedback interference. We evaluate system performance by varying which is the ratio of feedback level in the primary channel to the one in the secondary channel. The results indicate that the measured displacement curve is a linear line superimposed with nonlinear periodic errors when the target in the primary channel moves linearly and the target in the secondary channel stays stationary. Whereas, the curve is a periodic curve instead of a linear one, for the case when the target in the secondary channel moves linearly while the one in the primary channel stays stationary. Furthermore, the amplitude and shape of the periodic curves also depend on the parameter When is much less than 1, the displacement curve is sinusoidal, whereas the curve has a saw-tooth shape with larger amplitude for close to 1. In view of this situation, a new displacement measurement method, counting the cycles of the periodic displacement curves, is proposed. The current work can provide theoretical and experimental suggestions for improving the performance of existing self-mixing interferometry systems, and implementing ones with multi-channel optical feedback channels.

Enhanced self-mixing Doppler velocimetry by fiber Bragg grating

An enhanced laser self-mixing Doppler velocimeter by fiber Bragg grating (FBG) is proposed. In fiber sensing, FBG is often used as an optical filter to select the wavelength of the light using the narrow reflection spectrum. The Doppler frequency shift from the rotating target can be parsed in the self-mixing interferometry (SMI) signal. The frequency-fluctuant self-mixing signal is transformed into intensity variations by a FBG when the laser wavelength is set to the edge of the FBG steep transmission profile. An experimental comparison between the enhanced and the traditional SMI approaches is made. The magnitude of obtained FBG-enhanced SMI signal is about 40 times stronger than that from traditional SMI. The result of the experiments indicates that enhanced self-mixing Doppler velocimetry by FBG can be applied to velocity sensing. This method has potential application in the engineering industry because the FBG is easy to fabricate and connect to a SMI system and the wavelength of FBG can be changed to match the different lasers for an enhanced SMI signal.

Common-path heterodyne self-mixing interferometry with polarization and frequency multiplexing.

A heterodyne Nd:YVO4 microchip laser self-mixing interferometry based on frequency and polarization multiplexing has been demonstrated. By using two orthogonally polarized lights to measure the measurement and reference target, the effect of the acousto-optic crystal thermal creep and air disturbance in interference light path is eliminated. In addition, the measurement error caused by the difference between two identical microchip lasers is compensated for by shifted frequency multiplexing technique. A rate equations model with multi-channel frequency-shifted feedback is established for interpreting the principle of the interferometry. Due to the ultrahigh sensitivity of the microchip laser, the target used in the experiment is a non-cooperative object which is different from the targets in conventional Michelson interferometers. Under typical room conditions, the short-term resolution is better than 2.5 nm, and the long-term zero drift is less than 60 nm within 7 h. The result shows that this self-mixing interferometry system is feasible and robust in the field of displacement measurement.

Removing the impulsive noise contained in a self-mixing interferometry system using outlier detection

Impulsive noise is a major problem that seriously degrades the performance of self-mixing interferometry (SMI). A new method to rectify this issue is proposed. First, an outlier detection approach is employed to detect the data samples corrupted by the impulsive noise, and then the SMI signal waveform is rectified by means of least square (LS) curve fitting. The results show that the proposed method can effectively remove the impulsive noise without introducing distortion to the original waveform and thus lead to improvement in the performance of an SMI system.

Dynamic stability analysis for a self-mixing interferometry system.

A self-mixing interferometry (SMI) system is a laser diode (LD) with an external cavity formed by a moving external target. The behavior of an SMI system is governed by the injection current J to the LD and the parameters associated with the external cavity mainly including optical feedback factor C, the initial external cavity length (L₀) and the light phase (ϕ₀) which is mapped to the movement of the target. In this paper, we investigate the dynamic behavior of an SMI system by using the Lang-Kobayashi model. The stability boundary of such system is presented in the plane of (C, ϕ₀), from which a critical C (denoted as C(critical)) is derived. Both simulations and experiments show that the stability can be enhanced by increasing either L₀ or J. Furthermore, three regions on the plane of (C, ϕ₀) are proposed to characterize the behavior of an SMI system, including stable, semi-stable and unstable regions. We found that the existing SMI model is only valid for the stable region, and the semi-stable region has potential applications on sensing and measurement but needs re-modeling the system by considering the bandwidth of the detection components.