Study On The Multi-wavelength Absolute Distance Measurement Based On The Heterodyne Dual Optical Frequency Combs

As one of the most cutting-edge scientific research, the gravitational wave detection puts forward an ultimate challenge of million thousands of meter range and picometer level accuracy for absolute distance measurement technique. The requirements of satellite formation flying reach nanometer accuracy in thousand meters range as well. The existing classical laser absolute distance measurement methods can no longer meet these needs. The born of optical frequency comb has greatly improved the development of laser absolute distance measurement technique. The requirements above can be met with this technique. However, there are still three main problems that prevent the achievement of ultra large range, rapid and high accuracy. Firstly, it is hard to achieve a distance measurement with large range, high speed and accuracy at the same time. Secondly, the existing model and stabilization method of optical frequency comb limit the measurement accuracy. Thirdly, the distance information of various comb modes can hardly be separated and extracted from the interference signal both rapidly and precisely.Aiming the problem above, this subject of “Study on the multi-wavelength absolute distance measurement based on the heterodyne dual optical frequency combs” is meant to provide a large range, rapid, high accuracy and easily traceable absolute distance measurement method with an optical frequency comb based multi-wavelength heterodyne interferometry. The measurement principle and laboratory environment testment in 20 meters are fouced in this thesis. The results of study can be further used in the fields of gravitational wave detection and satellite formation flying control. The main research works are as follows:A heterodyne dual optical frequency combs based multi-wavelength absolute interferometry is proposed to achieve distance measurement with large range, rapid and high accuracy. The heterodyne dual optical frequency combs are characterised by the offset-locked center comb modes and slightly different repetition rates. The different scales synthetic wavelengths for coarse and fine distance measurements are synchronously generated with the many comb modes. According to the heterodyne interference signals of each comb modes, t he unkown distance is finally derived from a fusion process of the distance information of many comb modes. The complete model of the heterodyne dual optical frequency combs based multi-wavelength absolute interferometry is established. The results from analysis and experiments demonstrated that the mathod is capable of large range, rapid and high accuracy distance measurement. For a continuous measurement of 20 m distance static target mirror in 30 min, t he distance measurement uncertainty derived from the phase measurement error is decreased from 21.3?m to 8?m by using the information confusion of the 8th order synthetic wavelengths from the centre 15 optical frequency comb modes.For the limited distance measurement accuracy brought by inaccurate power spectrum model and imperfect stabilisation method of the cavity enhanced phase modulation optical frequency comb, an accuate power spectrum model is first ly established by the superposition calculation of the laser electric-field intensity. The effections of parameters are simulated and analyzed in detail. According to the analysis of frequency comb electric-field, a Pound-Drever-Hall principle based comb stabilisation method is proposed. By detecting the reflection beam from the front cavity mirror, the interference signal among comb modes is extracted. Demodulated with the phase modulation signal, t he error signal for feedback control is hence generated. The accurate model of the error signal for this stabilisation method is established elaborately. Simulation and experimental results show that the model accuracy of the accuate power spectrum model proposed in this paper is one order of magnitude better than the existing approximate model. And an optical frequency comb of about 33 comb modes is generated with the stabilisation method above. The correponding optical spectrum is about 294.4GHz.To overcome the problems of only extract specific spectrum information and easily influenced by noise spectrum with the existing phase detection schemes, a heterodyne dual optical frequency comb and digital lock-in detection based phase separation and extraction method for many comb modes is proposed. The heterodyne dual optical frequency comb is generated with dual acousto-optic frequency shifting and synchronous different frequencies driving techniques. Hence, the requirements of offset-locked center comb mode and slightly different repetition rates are fully met for the multi-wavelength heterodyne interferometry. Moreover, the atomic time standard referenced synchronous different frequencies driving signals ensure the direct traceability to the SI definition of meter. According to the characteristic of interference signal spectrum, the distance measuring information of many comb modes is clearly separated and extracted by digital lock-in detection. The simulated results show that the phase measurement error is less than ±0.01° for the center 15 comb modes. And the phase resolution is better than 0.001°.An experimental multi-wavelength heterodyne absolute interferometer based on the heterodyne dual optical frequency combs has been established. The performances of constitution units and the system have been tested, including the generation and stabilisation of optical frequency combs, the interference signal spectrum of heterodyne dual optical frequency combs, the phase separation and extraction of many comb modes, the measurement stability of system and displacement comparison measurement with a reference interferometer. The experimental results show that the 30 min stability of the heterodyne dual optical frequency comb based multi-wavelength heterodyne absolute interferometer can achieve up to 4.1×10-7, and the distance measurement uncertainty in 20 m range is less than 10.6?m, the relative measurement uncertainty at 20 m distance can reach 5.3×10-7.