Research On Transmission Effect In Optical Fiber Based Stable Radio Frequency Transmission System

Stable radio frequency transmission plays an important role in navigation,deep space network,radio astronomy and comparison of atomic clocks.However,for the traditional radio frequency transmission methods based on wireless,such as Global Position System and Two-Way Satellite Time-Frequency Transmission,the transmission signal is severely interfered by the open atmospheric channel atmosphere.And the transmission accuracy is limited to the order of 10-15,which cannot meet the requirements of high-performance applications.Optical fiber has shown its superiority as a transmission medium with low loss and high security.The existing underground optical fiber network has been relatively mature,which brings convenience to the research of fiber based RF transmission.In the actual environment,temperature changes and mechanical vibrations will cause optical path fluctuations of optical light transmitted in optical fibers,and introduce random jitter of transmission delay.Therefore,in order to obtain a stable signal at the remote end,it is necessary to compensate the random phase changes caused by the fiber link in real time.In addition to the above environmental factors,some transmission effects peculiar to optical fibers,such as Rayleigh scattering,dispersion,laser relative intensity noise,etc.,will affect the symmetry of the fiber link in bidirectional transmission and the detection of effective signals in the compensation system,thus reducing the stability of the transmission system.In stable radio frequency transmission,studying the transmission effect in optical fiber is helpful to further improve the transmission stability of compensation scheme.Rayleigh scattering and dispersion are two factors which have great influence on the stability of radio frequency phase stabilization.Rayleigh backscattering leads to the decrease of signal-to-noise ratio during the signal detection.In order to suppress the influence of Rayleigh backscattering,researchers at home and abroad used wavelength division multiplexing to isolate the round-trip transmission signals,but its dispersion effect would affect the long-term stability of the system due to the slow change of temperature.Therefore,in recent years,a passive compensation scheme of mixed frequency modulation is proposed to suppress Rayleigh backscattering by means of electrical filtering,and at the same time,optical carriers with the same central optical wavelength are applied to suppress dispersion.However,the passive compensation scheme has the problems of low compensation accuracy and low compensation bandwidth.Aiming at this problem,this paper proposes an active compensation scheme which can suppress Rayleigh scattering and dispersion effects simultaneously and achieve high compensation accuracy.The main work is as follows:1.An active compensation scheme for round-trip transmission using different modulation signals is proposed,and Rayleigh backscattering can be effectively suppressed.In this paper,phase-locked loop is used as an active compensation device to drive phase-locked loop to pre-compensate the signals sent from the local end through the error signals obtained by the fluctuation feedback of the optical fiber link during the round-trip transmission.Moreover,the radio frequency signals transmitted in the forward and backward directions are different by the way of frequency division at the remote end,so the Rayleigh backscattering can be suppressed by the way of electrical filtering.2.In order to improve the symmetry of two-way transmission,optical carriers with the same central wavelength are applied in both directions.Compared with multi-wavelength compensation system,the influence of temperature-induced group velocity dispersion is reduced effectively.3.In this paper,the experimental verification of the proposed scheme is performed on an 80 km optical fiber link,which proves the feasibility of the scheme.In the process of building the transmission system,a set of measurement system with wide frequency input range and simple structure is designed.Finally,the feasibility of the proposed scheme is verified by experiments,and the stable frequency distribution of 2.4 GHz RF signals on 80 km optical fiber is realized,with the short-term stability of 5.6E-14 at 1 s and the long-term stability of 3.0E-17 at 10000s.