Doctor of Philosophy
School of Electrical, Computer and Telecommunications Engineering
Self-mixing interferometry (SMI), or also called optical feedback interferometry (OFI) is a promising non-contact sensing technology. It is based on the self-mixing effect which occurs when a fraction of light back-reflected or back-scattered by an external target re-enters the laser internal cavity. A sensing system by using SMI technique consists of a laser diode (LD), a photodiode (PD) packaged at the rear of the LD, a lens and a target. The LD is called self-mixing laser diode (SMLD). This configuration reflects a minimum part-count scheme, which is useful for engineering implementation. Compared to traditional interferometry, e.g., Michelson or Mach-Zehnder interferometry, SMI has the advantages of simplicity in system structure, low cost in implementation, and ease in optical alignment. Using these merits, SMI technology has been developed for various applications, such as measurement of displacement, vibration, velocity, imaging, material related parameters, laser related parameters, etc.
Most of the SMI-based applications and behavior study on SMLD system are based on the analytical SMI model, which is derived from the steady-state solution of the Lang and Kobayashi (L-K) equations, or from the classical three-mirror model, by assuming the system operates in stable mode, i.e. both the electric field and carrier density in an LD with a stationary external cavity can reach constant state after transient period. However, undamped relaxation oscillation (RO) may occur under some operation conditions, e.g. it is found an SMLD in moderate feedback regime exhibits undamped RO. The moderate feedback regime is quite commonly employed by researchers. Based on our in-depth study, the behavior of an SMLD system with undamped RO cannot be described by the existing analytical SMI model. The laser intensity (called as sensing signal) from such SMLD system shows some new characteristics. In order to differentiate the conventional SMI signals, we name the SMI signals with undamped RO as RO-SMI signals. Usually, the PD packaged inside the LD is used for detecting SMI signals. Such PD has limited bandwidth usually less than 1GHz. However, RO frequency of an LD can be higher than several GHz. Hence, using the existing SMI configuration for detection of RO-SMI signal, many frequency components cannot be observed. Therefore, it is of great interest and significance to reveal these phenomena and find their potential applications.
In this thesis, in Chapter 2, the features and waveforms of an SMLD with undamped RO are theoretically investigated. Firstly, an improved stability boundary of an SMLD system is obtained. The influence of injection current and initial external cavity length on the stability boundary is analyzed. Based on the stability boundary analysis, intensive simulations by numerically solving L-K equations are performed to conclude the features of an SMLD system under different conditions. The influence of the photodetector bandwidth on a RO-SMI signal is discussed. Furthermore, an analytical expression for describing RO-SMI signals when the SMLD system operates in period-one oscillation is derived, which clearly describes the features of RO-SMI signals and indicates that such signal has the potential for displacement measurement.
In Chapter 3, an experimental system is implemented for experimentally investigation on the behavior of an SMLD system with undamped RO. The details of each part in the experimental system are presented firstly. Intensive experiments are conducted for verifying the theory presented in Chapter 2. In addition, the influence of photodetector bandwidth on the captured RO-SMI signals is experimentally studied. The experimental results are consistent with the theoretical analysis in Chapter 2.
Two new displacement sensing methods by using an SMLD with undamped RO are proposed in Chapter 4. Firstly, a micro-displacement sensing method with very high resolution is presented. The influence of the injection current and the initial external cavity on the sensing performance is investigated. Secondly, displacement sensing by employing both the time-domain RO-SMI signal and its RO frequency is developed, showing this method has large measurement range, high sensitivity and resolution.
In Chapter 5, an all-fiber SMLD system is built to demonstrate that an SMLD in moderate feedback regime with a high injection current and long external cavity can be always stable. Then, this system is used to measure the acoustic emission events successfully, contributing to a novel compact system in structure health monitoring.
All the results presented in this thesis are confirmed by both simulations and experiments, which unveil the behavior of an SMLD system in moderate or strong feedback regime with undamped relaxation oscillation. Several sensing applications are proposed and verified by both theory and experiment. The results in this thesis provide useful guidance for developing an SMLD sensing system.
Liu, Bin, Behavior Study for a Self-mixing Laser Diode with Undamped Relaxation Oscillation and Its Sensing Applications, Doctor of Philosophy thesis, School of Electrical, Computer and Telecommunications Engineering, University of Wollongong, 2018. https://ro.uow.edu.au/theses1/477