Study On The Arbitrary And Absolute Length Measurement System Based On Multiple Pulse Interference

Length is one of the seven fundamental physical quantities with importantsenses. In length measurement, expanding the range, improving the precision,and shortening the testing time have become the pursuing goals. However, itshould be often made out a balance between the large range and high precision byusing the traditional methods. With the advent and development of the opticalfrequency comb, it is considered to be a promising means of achieving abreakthrough for the long distance measurement with high precision, whichbecomes the strategic demand in the fields of basic physics, space and massproduction. On the basis of that, the optical frequency comb in length measurement isstudied in this thesis. By combing with the technique of atmospheric effects’ systemerror correction, we are aiming at exploring the new principles of thefemtosecond optical frequency comb for length measurement.This thesis makes a systemized research and discussion on the arbitrary andabsolute length measurement by the femtosecond optical frequency comb. In orderto solve the problems of low resolution of length measurement by theconventional time-of-flight (TOF) method and the difficulties in obtainingconsecutive values by the pulse interferometry method, a method based on themodified Michelson interferometer of multi-pulses interferometry for thearbitrary and absolute length measurement with high precision is proposed. Ingeneral, the main parts of this dissertation are as follows:1) We expound the main principle of the arbitrary and absolute lengthmeasurement that on the basis of multi-pulses interferometry and then construct thetemporal correlation model of multiple pulse train interference fringes. After that, thefeasibility of this method is verified in theory. Furthermore, the impacts of the systemthat could not be ignored during the measurement are analyzed.2) We provide the time and frequency domain characteristics of femtosecondpulses, and analysis the propagation characteristics of those in dispersive media.Based on the pulse propagation principles in the dispersive media, a time-correlationmodel of the traditional unbalanced Michelson interferometer is established. Atechnique of multi-pulses interference theory is employed to build the first-andsecond-order correlation models. And then, the effect of dispersion on the correlation fringes is also analyzed. In addition, the effects of the atmosphereenvironmental variations including temperature, humidity, air pressure, and CO2content on the group refractive index, which will affect the measuring results, aresimulated.3) We carry out the theoretic analysis on measuring precision of themulti-pulses distance measurement system. The uncertainty of the fraction part inthe formula is determined according to the white light interferometer, while thatof the integral part is determined by the excess fraction method. Then thecombination is carried out. From these analysis, two main conclusions areobtained: The minimum absolute distance measurement can achieve nanometerorder accuracy in vacuum, which is limited to the scanning precision of thereference mirror; Whereas in air, the precision of air reflective index becomes alimiting factor for improving the measuring precision, which leads to the fact thatthe measurement with a minimum relative precision of10-8is realized.4) We set up an arbitrary and absolute length measurement system based onthe multiple pulse train interference. The timing difference is calculated by thefirst-order and second-order optical cross-correlation signals separately; therefore theabsolute distance is obtained. The utility of the proposed method is demonstrated witha0.6m distance measurement. Results show a good measuring linearity with theresolution of less than1μm, measurement precision in the range of±0.5μm, andrepeatability error in the range of±108nm.