Experimental Investigations Of The Orthogonally Demodulated Pound-Drever-Hall Method For Two-Cavity Dual-Frequency Solid-State Laser Frequency Stabilization

The tunable two-cavity dual-frequency solid-state laser with large frequency-difference can be used as an ideal light source for synthetic wavelength absolute distance interferometric measurement system because of its large tuning range.The accuracy of absolute distance interferometric measurement depends on the magnitude of synthetic wavelength and its stability.The magnitude of synthetic wavelength depends on the frequency difference of dual-frequency laser,and its stability is determined by the stability of the frequency difference of dual-frequency laser.Therefore,research of the frequency difference stabilization technology of two-cavity dual-frequency solid-state laser will has important implications for its application in synthetic wavelength absolute distance interferometric measurement.This dissertation makes a further study of the frequency stabilization method of two-cavity dual-frequency solid-state laser.The Pound-Drever-Hall(PDH)laser frequency stabilization technology based on orthogonal demodulation is adopted to design the laser frequency stabilization scheme for dual-frequency laser.The feasibility experimental study on the frequency difference stabilization scheme is carried out.Firstly,the characteristics of PDH laser frequency stabilization technology and noise sources that might limit the accuracy of the laser frequency stabilization are analyzed.A noise model of PDH laser frequency stabilization system is established and the closed loop noise contributions of the different noise sources to the output are analyzed.The transfer function of every element in PDH laser frequency stabilization system is obtained by analyzing their input-output responses in frequency fast pull-in area.A complex frequency domain model is established and the steady-state and dynamic performances are researched.The theoretical basis for the parameters selection of loop filter is given.Secondly,on the basis of analyzing the principle of PDH laser frequency stabilization method and the detecting error caused by analog phase sensitive detection in typical PDH laser frequency stabilization system,the orthogonally demodulated PDH method without phase shifter based on narrow-band signal orthogonal decomposition and orthogonal baseband transformation theory is proposed.A PDH laser frequency stabilization system based on orthogonal demodulation is designed.Three sine and cosine signals with the same frequency and amplitude are generated simultaneously by use of a direct digital synthesizer(DDS).One is used to drive an electro-optic modulator to produce the phase sidebands and the others with orthogonal phase act as the reference signals for the mixer.Analog demodulation is used to obtain the in-phase component and orthogonal component of the frequency drift error signal.By the AD conversations,the two orthogonal components are digitally detected to obtain the frequency drift.Main error sources of detection unit in PDH laser frequency stabilization system based on orthogonal demodulation are analyzed and a digital correcting filter to optimize system structure is also designed.Thirdly,laser frequency discrimination and frequency tracking experimental investigation of PDH laser frequency stabilization based on orthogonal demodulation for one-cavity single-frequency solid-state laser at 1064nm are carried out.The results show that the PDH laser frequency stabilization scheme based on orthogonal demodulation is feasible.A dispersion error curve is obtained.The linear dynamic range and sensitivity are 8.39MHz and 24.4mV/MHz.The frequency drift is adopted to carry out feedback adjustment of the length of F-P reference cavity to track a certain center frequency of F-P reference cavity towards that of single-frequency laser automatically.The locked time is about 1h.Finally,on the basis of analyzing the principles of various laser frequency stabilization methods,a frequency difference stabilization scheme based on orthogonally demodulated PDH method is designed according to the structural features of two-cavity dual-frequency laser.A F-P reference cavity with high finesse and high stability is used as shared laser frequency standard and two frequencies of two-cavity are locked on two different center frequencies of it using PDH laser frequency stabilization technology based on orthogonal demodulation to achieve the frequency difference stabilization.Two frequency stabilization units share the same local oscillator,A/D and software besides of F-P reference cavity.The above researches not only explore orthogonally demodulated Pound-Drever-Hall laser frequency stabilization method,but also provide a new scheme for design of laser frequency difference stabilization system for two-cavity dual-frequency solid-state laser,and provide important foundation for future research on laser frequency stabilization and application technologies for two-cavity dual-frequency solid-state laser as well.