| With the rapid development of atomic clock and optical fiber communication technology,and relying on the advantages of low loss,wide bandwidth and anti-electromagnetic interference of optical fiber,the stable time and frequency signal transmission over the optical fiber has become a hot research topic,and gradually plays an increasingly important role in the science research,deep space exploration,national defense construction and other fields.The key problem of the time-frequency transmission over the optical fiber is that the changes in environmental factors such as temperature and mechanical stress on the fiber links may cause transmission delay variations,which degrade the phase and time stability at the remote end.This thesis focuses on this problem.Firstly,an innovative approachs to key issues such as phase detection and phase control of frequency signals is proposed,and the stable phase transmission of microwave and millimeter wave signals via optical fiber is realized.Then an innovative method for time-delay measurement of time signals is proposed,and finally the stable transmission of time-frequency signals over fiber is completed.The main work and key innovations in this thesis are listed as follows:?1?A high-stable millimeter wave signal transfer method over optical fiber based on optical frequency tuning.Facing the requirements for stable distribution of millimeter-wave local oscillator signals in the deep-space detection interference antenna array,this thesis adopts the extraction of optical frequency comb to generate the millimeter wave signal.According to the mapping between lightwaves and millimeter wave signals in optical millimeter wave,a millimeter-wave signal phase detection method with dual-heterodyne phase error transfer?DHPT?is proposed.The optical fiber path and the carrier separation path induced phase fluctuation is identically transferred to an intermediate frequency?IF?by using the DHPT.This method has made a breakthrough in the frequency,accuracy and sensitivity of phase error detection of millimeter wave signals.Then the phase correction mechanism of optical frequency tuning is proposed.It drives acousto-optic modulator through a controlled low-frequency voltage-controlled oscillator?VCO?to frequency-modulate one carrier of the optical millimeter-wave signal to complete the phase control of the millimeter-wave signal.Compared with the phase correction mechanism using the variable delay line in the optical domain,it has the advantages of fast response and unlimited compensation range.Based on the above two innovations,the transmission system is then modularized and designed to complete the prototype of a 40 km fiber optic transmission system with 300 GHz millimeter wave signals.The long-term stability of the transmission system reaches 6.5×10-17 at 2000s averaging time,which meets the transmission requirements of commercial hydrogen clocks.?2?A high-stable microwave dissemination method via optical fiber based on single sideband modulation.To meet the requirements of the stable phase distribution of microwave local oscillator signals in the distributed coherent aperture radar system,this thesis uses optical carrier suppression modulation method to generate microwave signal and completes the phase detection of microwave signal according to the improved DHPT.Then a phase correction mechanism based on single-sideband modulated is proposed.Compared with the traditional direct phase correction scheme for high-frequency VCO,the scheme uses a small tuning sensitivity low-frequency VCO instead of a high-frequency VCO to controls the phase of the microwave signal.This method greatly improves the accuracy of microwave signal phase control.Based on the above phase correction method,we develop a 20 GHz microwave signal dissemination system over 100 km fiber with the frequency stability of8×10-17 at 10,000 s averaging time.This thesis further studies the effect of the frequency division or frequency doubling processing on the stability of transmission system during phase discrimination.Therefore,this thesis optimizes the design of the transmission system by using two electric single sideband modulators to perform offset and tuning functions on the frequency signals respectively.It ensures that the IF signal obtained by the DHPT can perform the same frequency phase discrimination with the atomic clock frequency reference signal without frequency division and frequency doubling processing.The experimental data shows that the optimized system improves an order of magnitude longer-term stability compared to the frequency division transmission system.?3?An fiber delay measurement method based on frequency division and phase measurement is proposed,and the stable transmission of time-frequency signals over fiber is realized.At present,the accuracy of fiber delay measurement is limited by the time interval timer?TIC?resolution can only reach the ps level,which becomes the main factor limiting for the high stability transmission of time signals.This thesis proposes a high-precision fiber delay measurement method for frequency division phase measurement.It achieves high-precision delay measurement by phase measurement of the frequency signal that sense the fiber delay.At the same time,frequency division is used to avoid the problem of phase ambiguity to extend the range of delay measurement.Finally,the digital signal processing is used to synthesize the delay information measured under different frequency division ratios,which overcomes the problem of large-scale and high-precision delay measurement.For 40 km fiber delay measurement,this method improves the measurement accuracy by two orders of magnitude to 25 fs compared to the pulse interval measurement method.In order to realize the coherent synthesis of signals in distributed antenna arrays,according to the aforementioned phase compensation method and the fiber delay measurement method,the stable time-frequency signals over optical fiber is studied.When the loop is locked,the phase jitter of the frequency signal due to the fiber delay variation is compensated by the phase of the VCO.At the same time,the fiber delay information can be obtained by monitoring the phase of the VCO.It avoids the additional increase of the delay measurement of the fiber and simplifies the structure of the system.The segmented delay line is controlled according to the obtained delay information to pre-compensate the time signal to realize stable transmission.Finally,this thesis completes the stable transmission of20 GHz frequency signal and 1 PPS time signal over 80 km fiber.The frequency stability of the transmission system achieves 1.5×10-17 at 10000s averaging time,and the time stability reaches 0.97 ps at 4000s averaging time. |
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