Research On Absolute Distance Measurement Based On Self-mixing Interferometer Using Laser Diode

Self-mixing interferometer(SMI)using laser diode(LD)is a new way of interferometry.In SMI optical configuration,a modulation of the laser frequency and amplitude is induced by coupling a fraction of the emitting light into laser cavity.The coupling factor induces the change of gain,carrier density and current threshold,therefore induces the change of the power,phase and the laser voltage.In summary,the LD cavity plays an important roles as an optical mixer with filtering and preamplifier function for SMI signals.With respect to conventional interferometers,SMI typically shows lower performances,due to its small amplitude modulation,but it also shows some advantages,mainly due to the its simplicity and portability.Indeed,in the last years,SMI technology was proposed for various applications in displacement,vibrations,velocity,absolute distance measurements,other laser-cavity-related measurement and the biomedical sensors.However,the strong electromagnetic disturbances and background noise in industrial environment affect both the laser current supply and the trans-impedance stage.We propose a new method using balanced detection for SMI to improve the signal-to-noise ratio(SNR),by using two photodiodes for implementing a differential acquisition.Besides,we develop a laser instrument for measuring very-short distances with a minimum sensor size.The sensor size is limited to the laser chip and the contacts;therefore,it is applicable also in very demanding applications,such as the diameter measurement of a hole.The dissertation includes four main part:Firstly,we analyzes the phase opposition of the front and rear laser outputs,for selfmixing interferometry.This effect is evident for semiconductor laser diodes,when cavity damping rate is much higher than carrier damping rate.A simple model based on three-mirror cavities is proposed to theoretically explain the phase opposition,mainly related to pump current,photon lifetime and light round trip time.Experimentally,the self-mixing signal of the two facets outputs,measured by two photodiodes,shows good consistency with the theory.We analyzes the contribution of LD parameters in SMI deeply,such as amplitude reflection,pump current.Thanks to this technique,it is possible to improve the performances of selfmixing based instruments.Secondly,we propose a new detection scheme for self-mixing interferometry,using two photodiodes for implementing a differential acquisition.The method is based on the phase opposition of the self-mixing signal,measured between the two laser-diode facets outputs.The subtraction of the two outputs implements a sort of balanced detection that improves the signal quality,and allows canceling of unwanted signals due to laser modulation and disturbances on laser supply and trans-impedance amplifier.Empirical measurements demonstrate the benefits of differential acquisition,in a system for both absolute distance and displacement-vibration measurement.Also we discuss the main noise source in SMI and the small loss induced by beam-splitter.The new technique may help SMI to reach the shot-noise limited.Thirdly,we develop a small volume and cheap prototype based on balanced detection for SMI,using ARM contribute into hardware with purpose of control and data acquisition.The data is processed by PC to elaborate the frequency of the SMI fringes.The precision of the prototype is 150 ?m at 12 k Hz of acquisition rate in dynamics of about 15—260 cm for real-time measurement,and shows a huge potential in industrial application.Finally,we develop a laser sensor for measuring very-short distances with a minimum size.The absolute distance measurement is obtained through a modulated self-mixing interferometer,realized with a lens-less red vertical-cavity surface-emitting laser(VCSEL).With current reshape,the linearity of SMI is obtained,also the modulation residual is deceased by three different amplitude modulation.In a range between 3 mm and 8 mm,the sensor shows a resolution of about 40 ?m at 12 k Hz of acquisition rate.