Research And Application On Vibration Measurement Based On Laser Diode Self-mixing Interference

With the rapid development of measurement instrumentation, the field of metrological sensing continues to rely on optics based solutions to provide a precise and contact-less measurement technology. Within this context, vibration measurement based on laser diode(LD) self-mixing interference is probably the most attractive one due to its good precision and the merit which the conventional interferometer systems can not comparable, such as, simple optical arrangements, signal containing the direction information of the target, self-aligned, compact and cost-effective.Vibration measurement based on laser diode self-mixing interference is an approach based on weak coupling phenomena in laser diodes. The principle of this sensor is based on the optical feedback occurring when the optical beam reflected or diffused by a vibrating target is coupled back into the LD active cavity. The reinjected laser beam is then mixed with the light inside the LD cavity, causing variations in the optical output power when the optical path length varies by the movement of a target. This phenomenon is called as self-mixing (SM) effect. In order to extract the target motion information from the SM signal, various signal processing methods have been proposed. Therefore, the development of precise algorithms to improve such technique has long been a significant subject of research effort. The objective of this subject is to investigate the signal processing methods for vibration measurement based on laser diode self-mixing interference. Moreover, the main further intention is to try our best to improve the noise immunity, to minimize the cumulative error of the reconstructed displacement and to overcome the problem that the existing reconstruction algorithms can not be universal between different optical feedback regimes. Consequently, the laser diode self-mixing interference can be made more suitable for vibration measurement. And we also try to expand the application fields of vibration measurement based on laser diode self-mixing interference. The main research contributions of this dissertation are as follows:Firstly, the study is focus on correctly SM transitions detection and minimization the cumulative error while using phase unwrapping method to reconstruct the target vibrating displacement from SM signals with poor signal-to-noise ratio (SNR) in the moderate feedback regime. According to the mechanism of self-mixing effect and the principle of phase unwrapping method, the phase unwrapping method based on piece-wise transition detection algorithm is presented. The piece-wise transition detection algorithm is used to detect the SM fringes at a low SNR and in the presence of disturbances without filtering operation. And the optimization of a criterion depending on the minimum drift of the whole reconstructed displacement waveform is used to estimate the optimal number of segments and to reduce the overall reconstruction error caused by the cumulative error. While the SNR of SM signal is as low as16dB, the cumulative error is less than7.15%for the target vibration amplitude less than3m. The proposed algorithm is not very sensitive to noise, and provides a reliable signal processing method for low-power and economical red laser diodes applied in self-mixing interference system.Secondly, existing SM processing techniques are only suitable for their specific interference signal. However, a varying level of optical feedback then results in variation of the SM signal, not only in the shape but also in the amplitude of the SM fringes. And to maintain the SM signal waveform under practical conditions is quite difficult because of the variation of optical feedback level and the difference of target surface roughness. Aiming to overcome the problem that the existing reconstruction algorithms can not be universal between different optical feedback regimes, the spectral characteristics of SM signals at different optical feedback regimes are analyzed. The dominant harmonic order determination algorithm based on fast Fourier transform is proposed to extract target motion information over wide SM regimes (very weak, weak, and moderate regimes). Linear relationship between the target vibration amplitude and the dominant harmonic order is deduced, utilizing the feature that the order of the Bessel function of the first kind reaching a maximum is the order of the dominant harmonic, which reduces the complexity of the vibration information extraction process. Because only the first harmonic and the dominant harmonic are extracted, the proposed algorithm demonstrates good performance with very low SNRs and in the presence of disturbances, even when the SM fringes are almost drowned in the noise with a SNR of5dB.At last, taking advantage of vibration measurement based on laser diode self-mixing interference, such as simple optical arrangements, self-aligned and compact, this study is focus on its application in infrasound measurement. Considering the shortcoming of existing infrasound sensors, such as the un-flat frequency response, an infrasound sensor is developed based on laser diode self-mixing interference. According to infrasound propagation theory and the circular membrane forced vibration theory, the maximum center displacement of the membrane impinged by the infrasound wave is deduced, the material of the membrane is selected and the structure of the sensor is designed. Experimental results indicated that the proposed infrasound sensor has a flat frequency response, with a fluctuation of±2.0dB from1Hz to20Hz. It can be an alternative sensor for cost-effective and portable infrasound measurement.