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 paper, 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.

Fiber Bragg grating (FBG) is considered a good candidate for acoustic emission (AE) measurement. The sensing and measurement in traditional FBG-based AE systems are based on the variation in laser intensity induced by the Bragg wavelength shift. This paper presents a sensing system by combining self-mixing interference (SMI) in a laser diode and FBG for AE measurement, aiming to form a new compact and cost-effective sensing system. The measurement model of the overall system was derived. The performance of the presented system was investigated from both aspects of theory and experiment. The results show that the proposed system is able to measure AE events with high resolution and over a wide dynamic frequency range.

Laser dynamics have great potential for various applications, ranging from cryptography to microwave photonics and instrumentation. This letter presents a design for achieving high sensitive sensing and measurement using the dynamics of a laser diode (LD) with a dual external cavity. In the design, one cavity is used to control the dynamics, making the LD operate at the period-one (P1) oscillation state, and the other one is associated with the quantities to be measured. The Lang-Kobayashi equations are modified and solved to develop a bifurcation diagram for the design, from which we determine the P1 state and investigate the sensing performance within this state. It is shown that, when operating in P1, the laser intensity exhibits an oscillation with its amplitude modulated by a traditional optical feedback interferometric (OFI) signal (generated with a single cavity and LD operating at the steady state). It is also observed that the modulation depth is remarkably larger than the magnitude of a traditional OFI signal. This leads to a significant increase in the sensitivity of sensing and measurement and hence provides an attractive solution for the detection of very small or weak physical quantities. An experimental system is designed, and the experimental results verify the high sensitive sensing performance of the proposed design.

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.

This paper presents a new method for measuring the linewidth enhancement factor (alpha factor) by the relaxation oscillation (RO) frequency of a laser with external optical feedback (EOF). A measurement formula for alpha is derived which shows the alpha can be determined by only using the RO frequencies and no need to know any other parameters related to the internal or external parameters associated to the laser. Unlike the existing EOF based alpha measurement methods which require an external target has a symmetric reciprocate movement. The proposed method only needs to move the target to be in a few different positions along the light beam. Furthermore, this method also suits for the case with alpha less than 1. Both simulation and experiment are performed to verify the proposed method.

Semiconductor lasers (SLs) show relaxation oscillation (RO) due to the cavity damping rate being higher than the carrier damping rate. The presence of RO in SLs contributes to abundant complex dynamics when the laser is perturbed by external optical feedback (EOF). In this work, the influence of feedback optical phase on the relaxation oscillation frequency (ROF) in an SL is investigated both theoretically and experimentally. By numerically solving the well-known Lang Kobayashi equations, the relationship between the ROF and feedback optical phase was obtained, which shows ROF is in a sinusoidal manner with respect to the feedback optical phase under weak feedback strength. A simplified mathematic expression for ROF was derived to describe such a sinusoidal relationship. Potential sensing applications can be developed based on the relationship. As an example, a new method of measuring linewidth enhancement factor of an SL was presented. Finally, an experimental setup was built and experiments were carried out to verify the relationship and the measurement method for linewidth enhancement factor.