Research On A New Detection Method Of Laser Self-mixing Interference

The application of laser self-mixing interference in the field of precision measurement has been extensively studied since its discovery in the 1960s.Due to advantages of the inherent compact optical setup,high accuracy,self-alignment and non-contact,it displays a broad application prospects.In recent decades,the research and development of many high-tech manufacturing industries and frontiers of science has a higher request for laser self-mixing interference technology.The development of modern photon detecting technology provides new methods for laser self-mixing interference detection.A self-mixing birefringent dual-frequency laser Doppler velocimeter(SBD-LDV)for high-resolution velocity measurements is presented.The velocity information of the object can be accurately extracted from the self-mixing Doppler frequency shift of the birefringent light-carried microwave signal.We generate a stable light-carried microwave by using a birefringent dual-frequency He-Ne laser which further simplifies the structure of the light source.Moreover,the optical configuration based on the laser self-mixing interference brings benefits of compact optical setup,self-alignment,and direction discriminability.Experimentally,we extracted the Doppler beat frequency signal by the low-frequency(millihertz)phase lock-in amplifier,measured the beat frequency precisely in time-domain and calculated the magnitude of velocity.Compared with the previous self-mixing LDV,the average velocity resolution of SBD-LDV is improved to 0.030 mm/s for a target with longitudinal velocity,benefiting from the high stability of light-carried microwave.A new detection scheme is proposed for an enhanced self-mixing laser Doppler velocimeter(SLDV)using two-photon absorption(TPA),which is based on the TPA-induced quadratic photoconductivity dependence of a photodiode on the incident radiance at 1550nm with self-mixing interference(SMI).In an FBG-based enhanced SLDV,the harmonics of the SMI signal in the frequency spectrum is enhanced simultaneously,while the quadratic response induced by TPA can suppress harmonics,making their amplitude into the same level as the noise.From the experimental results presented in this letter,TPA effect in a commercial Si photodiode implements a sort of ultra-wide bandwidth filter without passively adjusting the cut-off frequencies for an unknown Doppler frequency,which simplifies the Doppler signal processing.The SNR is also improved to a considerable level of 15dB for a Doppler frequency of 4kHz due to the amplification of a EDFA.A simple synthetic-wavelength self-mixing interferometer is proposed for precision displacement measurement.Choosing the frequency difference of the orthogonally polarized dual frequency He-Ne laser appropriately,we introduce synthetic wavelength theory into self-mixing interference principle and demonstrate a feasible optical configuration by simply adjusting the optical design of self-mixing interferometer.The phase difference between the two orthogonally polarized feedback fringes is observed,and the tiny displacement of the object can be measured through the phase change of the synthetic signal.Since the virtual synthetic wavelength is 106 times larger than the operating wavelength,sub-nanometer displacement of the object can be obtained in millimeter criterion measurement without modulation,demodulation and complicated electrical circuits.Experimental results verifies the.synthetic wavelength self-mixing interferometer's ability of measuring nanoscale displacement,which provides a potential approach for contactless precision displacement measurement in a number of scientific and industrial applications.We develop an autocorrelator for measuring the temporal profile of pulses at 1550 nm from an erbium-doped fiber laser based on the two-photon photovoltaic(TPP).effect in a GaAs PIN photodiode.The temporal envelope of the autocorrelation function contains two symmetrical temporal side lobes due to the third order dispersion of the laser pulses.Moreover,the joint time-frequency distribution of the dispersive pulses and the dissimilar two-photon response spectrum of GaAs and Si result in different delay for the appearance of the temporal side lobes.Compared with Si,GaAs displays a greater sensitivity for pulse shape reconstruction at 1550nm,benefiting from the higher signal-to-noise ratio of the side lobes and the more centralized waveform of the autocorrelation trace.We also measure the pulse width using the GaAs PIN photodiode,and the resolution of the measured full width at half maximum of the TPP autocorrelation trace is 0.89 fs,which is consistent with a conventional SHG crystal autocorrelator.