The internal optical feedback effect in fiber ring lasers is similar to self-mixing interference, and the former has same
phase sensitivity with the later: one fringe shifting corresponds to a target displacement of half optical wavelength λ. The
optical feedback effect presented in this paper is achieved inside the laser cavity. So the feedback part becomes part of
the fiber ring laser, and makes the laser not only a light source, but also a sensitive element. The similarities and
differences between optical feedback in fiber ring lasers and self-mixing interference in semiconductor lasers are
analyzed, which offers the theoretical foundation and experiment technology for fiber active sensing applications. The
internal optical feedback system based on fiber ring lasers satisfies the need of microminiaturized sensing device and the
interrogation demand of optical fiber. This system can easily perform noncontact and remote measurement of velocity,
vibration, displacement, etc.
In recent years, active sensing, which emits radiation directed toward the target to be investigated, has attracted more and
more attention. Laser is not only as a light source, but also as a sensitive element viz. sensing source presented in the
active sensing system which based on the theory of self-mixing interference in this paper. This application can make the
sensing system more simple, compact, and less cost. In this paper, we propose the active sensing by the combination of
DBR lasers and the self-mixing interference technique. The self-mixing interference in DBR lasers based on the study of
F-P cavity and coupling wave equations has been analyzed. Three self-mixing interference models in DBR lasers are
presented, and one of them has been taken for instance to simulate the output variations of self-mixing interference. The
influence from different internal or external parameters on the output variations and the inclination degree of waveform
of the active sensing were discussed in simulation also. And thereby good signal of self-mixing interference in DBR
lasers can be obtained. This application can satisfies the request of microminiaturized sensing device and the demand of
interrogation of optical fiber, and predigests the structure of sensing system.
A new approach for velocity detecting based on laser speckle feedback injected erbium-doped fiber (EDF) ring laser is
presented in this paper. 1550nm wavelength light is outgone through an optical fiber and shoots onto a moving object by
a lens. Portion of the light scattered from the object produces dynamic speckle, when it is back-coupled into the same
fiber and injects the EDF ring laser, random intensity feedback causes changes of both intensity and frequency of the
laser. We theoretically analyzed the laser output when speckle feedback injects an EDF ring laser by using model of the
injection-seeded EDF ring laser, and experimentally obtained the dynamic speckle-modulated laser output signal by a
photodiode (PD). The signal is analyzed by way of FFT analysis, and the mean speckle frequency (MSF), which is
defined as the ratio of number of fluctuations in the detected signal to the measurement time, is obtained. We studied the
relationship between the velocities and MSFs by changing the velocity of object, thus a linear dependent relationship
between them is obtained, which indicates that speckle feedback injected EDF ring laser is an effective approach for
velocity detecting, it is promising to develop a new generation of optical fiber active sensor.
The statistical analysis and experimental results of self-mixing speckle interference in the distributed feedback (DFB) laser are presented in this paper. Dynamics solution of output gain variation in the DFB cavity is deduced on the basis of speckle theory and self-mixing interference in the DFB laser, when external optical feedback comes from a moving rough surface. By numeric simulations and experiments, the dynamic output variations of the DFB laser as well as their probability density functions (PDFs) are analyzed. Both results of simulations and experiments are in agreement with each other. The experimental results show that this speckle signal processing can be used to measure velocity of target.
Self-mixing interference in DFB-LD for fiber sensing application has been analyzed in this paper. Due to the characteristics of good model and narrow spectrum, the DFB-LD has the application potentiality in the filed of self-mixing interference, and optical communication technique has rapidly driven the development of optical fiber sensing technique. The combination of self-mixing interference technique and optical fiber sensing technique can satisfy the request of microminiaturized sensing device and the demand of interrogation of optical fiber. It is possible to form a novel optical fiber sensing measurement network, and the combination is helpful to the application at the aspects of avigation, industrial automation, medical examination, etc. In this work, based on the theory of coupled wave, the variation of laser output caused by self-mixing interference has been presented. For optical fiber sensing application, the self-mixing interference under the condition of transmitting the external optical signal by fiber has been analyzed. The influence from the variation of attenuation caused by the increment of fiber length and the reflectivity of the remote target to the output signal of self-mixing interference has been discussed in numerical simulation.
The laser feedback interference in gain-coupled DFB laser diodes (Distribute Feedback LD) is analyzed. The threshold gain and frequency variation in the DFB laser diode are theoretically deduced. Considering the experiment in practice, we analyze the laser feedback interference at the condition of multiple external cavities. Some parameters have influences on the feedback interference, e.g., the coupling coefficient, the linewidth enhancement factor of gain-coupled DFB laser, the reflection coefficient of external reflector. We discussed these different parameters value in numerical analysis, including the simulation at the condition of multiple external cavities. Compared with the effect of laser feedback interference on F-P and λ/4 phase-shifted DFB lasers, the gain-couple DFB lasers are more sensitive to optical feedback for a specific κL value (coupling coefficient and L cavity length). These results show that the gain-couple DFB lasers can be used as laser feedback sensors with a high accuracy.
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