| Laser is extensively used in the field of precision engineering and measurement with the advantages of fine monochromaticity,good coherence,and high directivity.Laser ranging technology provides a variety of means for length and distance measurement with the widest dynamic range and the highest accuracy,which plays a vital role in fields of industrial production,equipment manufacturing,defense technology and aerospace,etc.With the development of modern industry,the increasing demand in large-scale precision measurement and positioning becomes serious;however,traditional laser ranging based on incoherent and coherent theory has been difficult to meet the needs of large-scale and high-precision distance measurement.As one of the greatest contributions on physics in this century,optical frequency comb of femtosecond laser not only realizes the direct connection between optical frequency and microwave frequency,but also facilitates the technological innovation in distance measurement and length metrology.It introduces optical coherence into the traditional laser pulse ranging,making the incoherent and coherent measurements perfectly integrated,and hence solves the prominent contradiction between long distance and high precision while many innovative methods and technology for large-scale and precise laser ranging are arouse.In order to achieve large scale,high precision and real-time absolute distance measurement,researches on absolute distance measurement based on time-frequency characteristics of optical comb,including related theories and methods,are carried out.Especially,large-scale and real-time absolute distance measurement by optical-comb-referenced multi-wavelength interferometry is emphasized and has been systematically studied.The main work of this dissertation is summarized as follows:1.An innovative method of measuring absolute distance based on multi-heterodyning of dual optical combs is proposed for large-scale and high-precision measurement.A radio-frequency synthetic chain is built on the combination of multi-heterodyning of dual-comb cross-correlation with inter-mode beats of optical comb,and thus the ideal absolute ranging accuracy of micrometer or sub-micrometer within kilometer range is achieved without scanning repetition-rate of optical comb.A measuring model of the proposed method has been built,and the practicability is revealed from the corresponding simulation results.A model of space absolute ranging by cross-correlation with repetition-rate scan has been built on the theory of intensity cross-correlation between femtosecond pulses,which improves the cross-correlation method in time domain.The feasibility of the proposed improvement is validated through simulation as well.2.The method of real-time absolute distance measurement by optical-comb-referenced synchronous-locking multi-wavelength interferometry is put forward.As the traditional laser source for multi-wavelength interferometry and methods with wavelength scanning interferometry have difficulty in meeting the needs of real-time measurement and large-scale absolute distance measurement,the method of real-time absolute distance measurement by optical-comb-referenced synchronous-locking multi-wavelength interferometry is proposed from behaviors of optical comb as a ultra-precision frequency rule.With optical comb treats as the optical frequency reference source,multiple different wavelengths,comprised of several single-wavelength lasers simultaneously optical phase locked to the comb with high frequency stability,are generated for optical interferometry.Non-ambiguity range of absolute ranging by interferometry is extended through synchronous detection of multi-wavelength interferometric phases and excess fraction algorithm.The proposed method is capable of retaining measurement resolution and accuracy of single-wavelength laser interferometer,and also makes the measurement result traceable to the microwave frequency standard,which is of the vital significance to the definition of the meter and length metrology.3.The method of secondary equivalent synthetic wavelength for extending non-ambiguity range is presented.To realize the wavelength demodulation which is difficult in large-scale absolute distance measurement obtained only by one-time equivalent synthetic wavelength,the concept of secondary equivalent synthetic wavelength is proposed,which overcomes the limitation of the minimum wavelength gap by secondary synthesis of two one-time equivalent synthetic wavelengths.On the basis of this proposal,a further study on multi-wavelength demodulation algorithm,wavelength selection,and non-ambiguity range extension,is presented.Simulation results show that optical-comb-referenced four-wavelength interferometry can achieve a non-ambiguity range of 36 mm and five-wavelength interferometry can reach the magnitude of meter.4.To generate multi-wavelength laser light source with high frequency stability,optical comb needs to be constructed firstly.Based on fiber-based femtosecond pulse laser light source,the repetition rate and carrier-envelope-offset frequency are locked by technologies of second harmonic generation,f-2f self-referencing method and phase locked loop,with the frequency stability of 1.1×10-12 and 5.1×10-15 in Allan deviation respectively,and thus the preparation of optical comb is achieved.Based on the construction of optical comb as a frequency reference source,multiple tunable continuous wavelength lasers are optical phase locked to different modes of optical comb,resulting in a relative frequency stability of 1.9×10-15 in Allan deviation,so multi-wavelength optical frequency generator with high frequency stability is realized.Due to the need for real-time measurement by multi-wavelength interferometry,multi-wavelength heterodyne interferometer and multi-channel synchronous phase demodulation module have been developed,which facilitates multi-wavelength ranging system with no nonlinear phase caused by mixed interference,wavelength demodulation and fractional phase accuracy is better than 0.001 to achieve.The software for real-time absolute distance measurement algorithm is developed to improve the efficiency of excess-fraction-method calculation for integer phase,so that the update time for continuous measurement is about 10 ms,well meeting the needs of real-time absolute distance measurement.5.Absolute distance measurement system based on optical-comb-referenced synchronous-locked multi-wavelength interferometry and its experimental platform are built up,and then the coarse value of initial absolute distance is acquired by the frequency scanning method with millimeter accuracy.Experiments for verification of non-ambiguity range and linear displacement tests have been carried out.Through comparison with a heterodyne laser interferometer,it is proved that non-ambiguity range of four-wavelength interferometry in the constructed system reaches 44.6 mm.Besides,the PV value of residual errors during 1m linear comparison is 61.9 nm,corresponding to a relative accuracy of a magnitude of 10-8,which proves that the constructed system based on optical-comb-referenced synchronous-locked multi-wavelength interferometry is feasible to achieve nanometer precision in large-scale absolute distance measurement.6.The method of integrating the radio-frequency synthetic wavelength with multi-wavelength interferometry is proposed for further expanding non-ambiguity range.Due to the limitation and low efficiency in extending non-ambiguity range of secondary equivalent synthetic wavelength in five-wavelength interferometry,the idea of radio-frequency synthetic wavelength interferometry is proposed based on four-wavelength interferometry,which greatly improves the efficiency of extending the non-ambiguity range in the synthetic wavelength chain.With the proposed method,the linear displacement comparison of 3m range has been performed,and the P.V value of the residual errors fitted with the reference displacement is 35.3 nm.Moreover,experiments of anti-path-interruption capability in real-time absolute distance measurement,short-term test and 24 h long-term test are carried out,testing results of anti-path-interruption ability,measurement resolution,short-term accuracy,long-term stability and anti-dead-path error show good performance of absolute distance measurement system based on optical-comb-referenced multi-wavelength interferometry and its potential for future application.Finally,measurement uncertainty of the measured absolute distance by optical-comb-referenced synchronous-locked multi-wavelength interferometry is analyzed and summarized,and influences of air refractive index variations are discussed,which is related to the accurate determination of integer phase and the measurable maximum range in air by multi-wavelength interferometry. |
PDF Full Text Request