Doctor of Philosophy
School of Electrical, Computer and Telecommunications Engineering
Gao, Yan, Simultaneous measurement of the system parameters for a self-mixing interferometry, Doctor of Philosophy thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4357
In recent decades, self-mixing interferometry (SMI) has been an active and promising technique for non-contact sensing and instrumentation. The basic structure of an SMI consists of a Semiconductor laser (SL), or called laser diode (LD), a focusing lens and a moving target which forms the external cavity of the LD. When a portion of light emitted by the LD is backscattered or reflected by the vibrating external target and reenters the laser cavity, a modulated lasing field will be generated. The modulated laser power, detected by a photodiode (PD) packaged at the rear facet of LD, is called an SMI signal. It carries the information of the external target vibration and the parameters related to the LD. Generally, there are two classes of applications for SMI based sensing: (1) estimation of parameters associated with the LD and (2) measurement of the metrological quantities of the external target of the LD. The existing methods show that each class requires the knowledge from the other, e.g. the estimation of the parameters associated with the LD requires the knowledge of the target movements while the accurate measurement of the target movements needs to know the knowledge on LD parameters. Hence, it is very important to investigate how to simultaneously measure the parameters associated to both the target and the LD using SMI technology.
In this dissertation, an SMI experimental system is implemented for investigating the features of SMI signals. The system behaviour can be described by the widely accepted SMI model which is derived from Long-Kobayashi (L-K) equations.
Firstly, the method for simultaneously retrieving the vibration of the target and the LD parameters are proposed under the condition of that the target is in periodical vibration. According to the L-K equations, the waveform of an SMI signal is determined by both internal and external cavity parameters of the LD. The parameters include linewidth enhancement factor (also referred as a ), optical feedback level factor (denoted as C ) and the movement status of the external target. a is an important physical parameter for an LD as it characterizes the linewidth, the chirp, the injection lock range in an LD. However, its value can be modified by the LD operation conditions such as the injection current, the feedback level and so on. The parameter C tells the operational modes of an LD application system. An LD is considered to operate in weak optical feedback regime when 0 less than C less than 1; it runs in moderate feedback regime for 1 less than C less than 4.6 and in strong feedback regime when C more than 4.6.
Starting from the existing SMI model, a set of linear equations is derived. By careful selection of data samples, the linear equations can be made independent and then used to determine a and C , as well as the movement of the target. Simulations and experiments are both conducted to verify the proposed method, and the results reveal that the proposed approach is capable of working at weak, moderate and strong optical feedback regimes.
Secondly, an improved method is presented for lifting the limitation existing in above proposed method. As the above method only works when the target is subject to a periodic movement, the purpose of the improved method is to present a new approach which is able to simultaneously retrieve LD related parameters and the target movement in arbitrary form, thus lifting all the restrictions in existing methods. I n this method, the arbitrary movement of the target is expressed by a polynomial. Also based on the existing SMI model, a new matrix equation is derived for the measurement of those parameters. The measurement matrix is built by employing all the available data samples obtained from an SMI signal. The Total Least Square estimation approach is used to estimate the parameters. The effectiveness of the improved method is verified by both simulations and experiments.
To facilitate the proposed methods and improve the measurement accuracy, the reduction of the noises contained in experimental SMI signal, especially for the impulsive noise, is discussed in the dissertation. Two approaches named outlier detection and sequential least square estimation are proposed to enhance the signalnoise ratio (SNR).
In addition, the error analysis and data processing method for frequency-domain based alpha measurement are also conducted. The presented results are helpful for investigating the influence factors on the alpha factor.
All the results on parameters C and alpha presented in the dissertation are compared with the existing methods and the movement of the target obtained by the proposed methods are verified by the commercial displacement sensor LTC-025-04-SA from MTI instruments.