Research On Relay And Amplification For Optical Fiber Time And Frequency Synchronization Systems

The stabilities improvement of precision time and frequency(T/F)signals promotes the development of basic scientific research,such as precise experimental measurement and basic physical law verification.And the development of T/F synchronization technology owns great significance in many national defense and industrial production,e.g.deep space exploration and satellite navigation and positioning.Optical fiber link has the unique advantages of anti-electromagnetic interference,stable and symmetrical two-way propagation path and propagation delay,so the T/F synchronization system which based on optical fiber can achieve higher signal transmission stability than the traditional system based on satellite network.As an indispensable part of optical fiber T/F synchronization system,optical amplifiers are used to compensate the power attenuation in the process of T/F signals transmission,which are the key to realize the long-distance fiber-optic T/F synchronization.Therefore,according to the bidirectional transmission characteristics of the T/F synchronization system,it is necessary to design a relay and amplification system with high gain and low noise characteristics to extend the transmission distance of T/F signals and reduce the influence of amplifier noise on the stabilities of T/F signals.In addition,it is also necessary to classify all kinds of noises that affect the stabilities of the system which based on a specific amplifier structure to point out the direction of improving the stabilities of T/F signals.In this paper,Erbium-doped fiber amplifier(EDFA),fiber Raman amplifier(FRA),and fiber Brillouin amplifier(FBA)are studied.This paper focuses on how to improve the amplifier gain and reduce the amplifier noise,as well as the the influence of the noise introduced by amplifiers on the transmission signals stabilities of the optical fiber T/F synchronization system.The main innovations are summarized as follows:1.Research on EDFA for optical fiber T/F synchronizationThe stabilities of the transmitted T/F signals will be deteriorated by the amplified spontaneous emission(ASE)noise of the EDFA.This paper studies the noise in optical fiber T/F system based on high isolation bidirectional Erbium-doped fiber amplifier(Bi-EDFA)from the physical mechanism.It is found that the Rayleigh scattering noise can be suppressed due to the high isolation design,while the ASE noise generated by the high isolation Bi-EDFA and the bidirectional asymmetry of the transmission link caused by the high isolation Bi-EDFA will reduce the stability of the system.The intensity noises which originate from the laser and detector,the transfer delay fluctuations caused by the variation in ambient temperature and the jitter in laser output wavelength are also studied.The research results show that the short-term stability of the T/F synchronization system which composed of high isolation Bi-EDFAs is limited by the accumulation of ASE noise and the laser frequency drift,while the long-term stability is limited by periodic variation in ambient temperature and the asymmetry of the amplifiers.Presented methods of analysis are useful for understanding the influences of each independent devices on the instabilities of the transmitted T/F signals from the physical mechanism,comparing the noises introduced by the devices,pointing out the direction for improving the stability of the system.In order to reduce the influence of ASE noise produced by EDFA on the stability of T/F synchronization system,an EDFA with modified dual-stage single-pass configuration is proposed.Two wavelength division multiplexers are used to build a transmission channel for unused pump between two Erbium-doped fibers in the original dual-stage single-pass configuration to reduce the noise figure(NF)of the amplifier.Based on this modified configuration,a high isolation Bi-EDFA relay system is designed,which is suitable for optical fiber T/F synchronization system.The measured NF of the modified EDFA is reduced to about 3.2 dB from 4.3 dB of the originate EDFA.2.Research on FRA/EDFA hybrid bidirectional amplifier for optical fiber T/F synchronizationIn order to further reduce the influence of amplifier noise on the stability of the T/F synchronization system,this paper designs a FRA/EDFA hybrid two-way relay system which is suitable for optical fiber T/F synchronization system and the equivalent NF of the hybrid amplifier can be less than 0 dB.The hybrid amplifier combines the low noise performance of FRA with the high gain performance of EDFA.The experimental results show that the T/F synchronization system composed of FRA/EDFA Hybrid amplifier can achieve higher transmission stability than that using EDFA alone.The transfer stabilities of free-running frequency synchronization system reach 1.9678×10-13/1s and 2.0248 ×10-1/1 s when FRA+EDFA and EDFA+FRA hybrid amplifiers are used,respectively,both exhibiting better performance than the stability of 3.0905 ×10-13/1 s obtain by EDFA.3.Research on FBA for optical fiber T/F synchronizationThe power of amplified spontaneous Brillouin scattering(ABS)noise introduced by the FBA is low for the narrow gain bandwidth.However,the gain efficiency of FBA depends on the relative state of polarization(SOP)of pump and signal,and the random birefringence effect in optical fiber will reduces the signal gain and causes power fluctuation of the amplified signal.In order to ensure that FBA can obtain maximum signal power and minimum ABS noise power.Based on the principle of signal amplification and ABS noise generation in FBA,a modified vector propagation equations for calculating the signal and ABS noise power are given in Stokes space.The results show that the maximum signal power and minimum ABS noise power can be obtained when the SOPs of the pump and signal are parallel to each other.And when the relative SOPs of pump and signal are changed,the ABS noise power increases gradually with the decreases of the signal power,but the total power of them is basically unchanged.And the closer the relative SOP of the pump and signal is,the more obvious ABS noise suppression effect will be brought by increasing the input signal power.In order to suppress the the signal gain reduction and power fluctuation caused by random birefringence effect in the optical fiber,a FBA based on orthogonal double pumps is designed.The theoretical and experimental results prove that the orthogonal double pumps can overcome the fiber random birefringence effect.And the signal with arbitrary SOPs can obtain stable high gain without adjusting its incident SOPs.By using the proposed scheme,compared with the traditional FBA which based on just single pump,the gain gap of the signal with different SOPs reduces to 2.7 dB from 9 dB,and the maximum output power of the signal is increased by 3.74 dB.Theoretically,the gain difference and fluctuation for signal with different SOPs can be further diminished to zero by the proposed double orthogonal pumps mechanism.In summary,in order to compensate the transmission attenuation of the signals in the optical fiber T/F synchronization system,this paper systematically studies the signal amplification and noise generation process in EDFA,FRA and FBA.The schemes for improving amplifier gain and reducing amplifier noise are designed,and the influences of the noise introduced by the amplifiers and other devices in the system on the stability of the optical fiber T/F synchronization system are studied.The research results of this paper are helpful to realize long-distance and high stability optical fiber T/F signals transmission.