Study On Absolute Distance Measurement Technology Based On Femtosecond Optical Frequency Comb

Length is one of the seven basic physical quantities. The accurate measurement oflength has extremely important significance in the forefront of basic science and advancedtechnology applications. With the advent of femtosecond optical frequency comb, thefemtosecond pulse laser is used in the absolute distance measurement. Compared to lightsources used in traditional ranging method such as continuous wave laser, pulsed laser andwhite light, femtosecond pulse laser has very unique advantages. It can be considered as aseries of equally spaced pulse sequence in the time domain which can provide a series ofequally spaced frequency distribution from microwave frequencies to optical frequencies inthe frequency domain. The research in this dissertation was supported by National NaturalScience Foundation and Beijing Natural Science Foundation. The research and explorationof the absolute distance measurement based on femtosecond optical frequency comb waspresented in the paper. The main research contents are as the followings:(1) The existing methods of absolute distance measurement are analyzed. The domesticand international research present situation and the development trend of the absolutedistance measurement techniques based on femtosecond optical frequency comb areintroduced.(2) The absolute distance measurement principles based on femtosecond opticalfrequency comb are summarized. The two basic principles of femtosecond laser and itsfrequency locking and traceability are concluded.(3) Based on the dispersion pulse propagation theory, the discrete model of thecross-correlation signal the femtosecond optical frequency comb propagating in anunbalanced Michelson interferometer is proposed. Considering phase velocity, groupvelocity and the group delay dispersion the model of the cross-correlation functions of theultrashort pulse sequence propagating in air is built with different pulse center wavelength,different spectral distribution and with the change of environmental parameters. Thesimulation results show that the cross-correlation pattern has stable chirp and linearbroadening with the increase of propagation distance, but the minor changes of theenvironmental parameters, such as air pressure, temperature, humidity, and CO2content, give rise to the shift of the correlations patterns without any chirp or linear broadening.(4) Based on the discrete model of the cross-correlation patterns of the femtosecondabsolute distance measurement, the continuous model of the cross-correlation functions offemtosecond optical frequency comb propagating in an unbalanced Michelson interferometeris developed. By introducing the continuous model and quadratic approximation, theposition of the brightest fringe in the correlation pattern is found out. The simulation resultsshow that the position of the brightest fringe in the correlation pattern varies non-linearly forshort delay distances, and when a specific distance is reached, the brightest fringe varieslinearly. The concept of the non-linear dispersion depth is proposed. It is showed that theshape of the cross-correlation patterns at the distances greater than the non-linear dispersiondepth is determined by the source spectral profile. It is also observed that each intensity pointof the correlation pattern is formed by the contribution of one dominant stationary frequency.(5) Femtosecond absolute distance measurement of spectrally resolved interferometryon the basis of white light dispersion interference is advanced. The performance of spectrallyresolved interferometry method in the case of3648and1824filtered optical modes ofoptical frequency comb, with reference to the case of ideal sampling is summarized.Simulation results show that the minimum measurable distance of this method is6.3μm, thenon-ambiguity range is5.75mm and the measurement accuracy is±4μm. The maximumdistance can be extended to arbitrary length. The Fabry-Perot etalon, which is used to filterout the optical frequency comb mode, is developed. The setup of spectrally-resolvedinterferometry for absolute distance measurement is designed and realized. The experimentresult shows that the distance measurement range is±1mm, and the measurement differencereaches±5μm.