Research And Application Of DFB Laser-Doppler Velocimetry Based On The Self-mixing Effect

"laser self-mixing" is that a portion of light emitted or scattered from a laser source by an external target is reflected or scattered back into the laser cavity.The reinjected laser mixes with the light from the laser cavity and changes of the power and frequency,then forms the optical feedback effect.The optical feedback effect of the lasers with the defect of changing model,frequency instability or chaos, noise, widening spectra,is regard as the disturbing original which affects the spectra of the laser.With the further research,the optical feedback phenomena is changed to be used to metrology from the affect of eliminating light feedback,thus generates the self-mixing interference technology.Self-mixing interference in a LD occurs when a portion of light emitted from the LD is reflected by an object and coupled into the LD cavity.The reentered light mixes with the original light in the LD cavity and changes the output power and spectra of the laser.The resultant Doppler shift was accepted by PD.The Self-mixing interference effect can be used in metrology and sensing technology,such as velocity, displacement,ranging and vibration measurement.Research self-mixing interference in He-Ne lasers and DFB lasers,the general expression of the threshold gain and frequency in the DFB lasers was deduced by using the coupled wave function.Numerical simulations indicate that,for specific coupling coefficient and cavity length value,the DFB lasers are more sensitive to optical feedback than the Fabry-Perot semiconductor laser.These results show that high-accuracy self-mixing sensors can be obtained by using the DFB lasers.The high-accuracy self-mixing sensors can be used to measure the liquid velocity.When a small fraction of the light back-reflected or back-scattered by the liquid re-enters into the laser cavity,the output gain change in the cavity will be caused.And,the high-accuracy self-mixing sensors can be used to measure the target temperature.The target temperature change the air refractive index.The frequency shift is changed by the air refractive index.The temperature value is extracted by the signal processing.