Research On Fiber Laser Self-mixing Grating Interference Micro-displacement Sensing Technology

In recent years,laser self-mixing interferometry which emits from the external optical feedback effect has been used in many fields.Self-mixing interference occurs when a portion of light emitted from a laser source is reflected or scattered back into the laser cavity by a remote target,the reflected light mixes with the light inside the cavity,causing a modulation of both the power and the frequency of the lasing field.Self-mixing interference has proved to be a powerful tool in the precision sensing of velocity,displacement,vibration and absolute distance.However,when the external target is in a narrow and harsh environment,there is not enough space to place interferometer.Besides,the laser self-mixing interferometer cannot achieve satisfied measurement accuracy due to the disturbance of external environmental.By contrast,the optical fiber laser self-mixing interferometer can adapt to various harsh environments.The optical fiber is slight and highly flexible,it can travel in harsh and limited working areas,which has huge advantages in practical applications.However,as with laser self-mixing interferometry,the measurement accuracy of fiber laser self-mixing interferometry strongly depends on the wavelength stability of the light source.But the wavelength will be affected by the variations in temperature,humidity,air pressure,and air flow in the environment.As a result,the environmental conditions have to be strictly controlled to achieve high measurement accuracy,which greatly increases the measurement cost.In contrast,the grating interference technique uses the grating pitch as the measurement standard,has lower requirements on environmental factors,and has strong capacity of resisting disturbance.First,an all-fiber laser self-mixing grating interference system for dynamic displacement measurement is proposed by combining fiber laser self-mixing interferometry and grating interferometry.When the light emitted from a fiber laser is incident on a reflection grating at Littrow angle,the diffracted light will return into the laser along the original path,thus generating self-mixing grating interference effect.Phase modulation technique is introduced into the system to improve the measurement accuracy.Experimental results show that the proposed system has several tens of nanometers resolution.It provides a potential displacement sensor with high flexibility,simple mechanical structure,and good reliability.Second,the paper proposed an accelerometer testing technique based on fiber laser self-mixing grating interferometer.The external diffraction grating of the system and the accelerometer to be measured are fixed back-to-back on a vibration exciter.Fringe counting method or Bessel function zero method are used to extract acceleration from the interference signal according to the different vibration frequency of the vibration exciter.The results of multiple test groups show that the minimum standard deviation of the accelerometer sensitivity obtained by the test system can reach 0.014 m V/(m/s~2),which is 1.84 m V/(m/s~2),showing a good testing performance.