A new approach for self-mixing interference in fiber ring laser and its application to vibration measurement is presented. Fourier transform method is proposed to improve the measurement accuracy of such an active sensing, in which the basic frequency of self-mixing interference signal is used to determine the frequency of vibration, and the stop order of the harmonic components is proportional to the amplitude of vibration. We have demonstrated this system to measure the microscopic vibration of a remote target. Some errors due to this method are discussed. The maximum error of the amplitude is about lambda/10 and the maximum error of the frequency is about 10%, showing a good agreement with the simulative results.

A new method based on fractal theory is proposed to analyze velocity sensing. The waveform of a self-mixing speckle signal is processed as a pattern of a fractal. Fractal boxes are defined as a set of grids used to divide the fractal pattern, and box-counting (BC) is introduced to characterize the statistical property of a speckle signal. A group of simulated speckle signals are analyzed by calculating the BCs corresponding to different velocities of the object. A linear dependence between the BCs of speckle signals and velocities is obtained, the result of which is validated by the analysis of a group of signals obtained from experiments. The performance of the fractal analysis is compared with those of the previous analysis methods. Better linearity and higher measurement sensitivity of the fractal analysis are indicated. The experimental result shows that the fractal method can be used as a valid analysis tool for the self-mixing speckle signal in velocity sensing.

The experiment observation of self-mixing interference in distributed feedback (DFB) laser has been illuminated in this paper. The influences on self-mixing interference have been discussed in both simulation and experiment through changing the conditions of external cavity. The experiment results show a good agreement with the simulation results, and validate the feasibility of DFB lasers for self-mixing interference application. Combining the self-mixing interference technique and DFB laser, we can obtain the compact structure and high-accuracy self-mixing interference sensors.

The effects of self-mixing interference on gain-coupled (GC) distributed-feedback (DFB) lasers are analyzed. From coupled-wave theory, the oscillation frequency and threshold gain variations are theoretically deduced. The influences on self-mixing from the coupling length coefficient, the linewidth enhancement factor of the GC DFB laser, and the reflection coefficient of the external reflector are discussed along with numerical analysis and are compared with the effects of self-mixing interference of lambda/4 phase-shifted DFB lasers and Fabry-Perot (F-P) lasers. Our results show that high-accuracy self-mixing sensors can be obtained with GC DFB lasers.

Theoretical analysis and experimental results of self-mixing speckle interference in a distributed feedback (DFB) laser are presented in this paper. Self-mixing speckle interference occurs when external optical feedback comes from a moving rough surface. Dynamic output variations in the DFB laser as well as their probability density functions (PDFs) are analyzed on the basis of speckle theory and self-mixing interference in the DFB laser. Numeric simulations and experiments are in agreement with each other. The both results show that self-mixing speckle interference in DFB laser can be used to measure velocity of target.