| The traditional two-way satellite time transmission(TWSTT)and satellite common view have been unable to meet the high-precise time synchronization requirements.The time synchronization system which based on optical fiber has become a hotspot issue in the construction of ground clock-benchmark-networks due to its low cost,strong anti-interference and high stability.However,in optical fiber communication systems,the noise and nonlinearity of laser,pulse generator,electro-optic modulator,optical receiver as well as optical fiber links have become the primary factors restricting the improvement of system performance.While,the optical receiving and relaying modules in the fiber-optic time synchronization system shoulder the important mission of receiving and transmitting optical signals normally,which undoubtedly has a significant impact on system performance.Thus,ensuring that the receiver adapts to the input-signal varying in wide dynamic range and achieving "distortion-free" amplification of the time synchronization signal with less noise introduced while maintaining high stability,will be the key of optical receiving and relaying module designing.Due to the phase ambiguity of periodic single-frequency sinusoidal signals and the time synchronizing technology based on frequency difference by high-precision frequency locking is still immature.Pulse is usually taken as carrier of synchronization signal in fiber-optic time synchronization systems,by means of intensity modulation or phase modulation,etc,the electrical pulse will be converted into optical signal and then been sent into optical fiber link;Relying on the pulse rising edge,the received signal with round-trip and the original transmitted signal will be compared at the system transmitting end to realize verification and feedback for adjusting time synchronization characteristics in real time so as to achieve coarse time synchronization.Obviously,optimization of the pulse edge and receiving performance affect the system synchronization level greatly,which is the focus of attention.Based on the background above,the main research contents of thispaper are as follows:(1)Low noise broadband balance photoelectric detectorThe time synchronization signal often uses 2?5 V pulse with periodof one second,of which the rising time near nanosecond,covering wideband frequency components from DC to radio frequency.So it is a big challenge to make low-noise design ensuring small interference been introduced under wideband detection when receiving the pulse;in addition,sharp attenuation of optical signal after long-distance transmission requires the detector to provide higher sensitivity;what is more,trigger level of the relaying module must be matched when signal been re-transmitted after reception;finally,to be compatible with coherent demodulation system,balance detection needs to be taken into consideration.To this end,this paper has discussed and self-developed a variety ofphotodetectors.Among them,based on the high gain,large dynamic and high-speed characteristics of transimpedance amplification(TIA),special low-current-noise photoelectric detection chips are selected and low-noise designs are used to limit the broadband high-frequency noise;chips with big slew rate are chosen to achieve large amplitude and fast rising edge response;and Bias-Tee was introduced to separate low-frequency and high-frequency signal to avoid the influence of DC coupling on OP AMP linear operation with low-frequency precision amplification been performed individually;finally,a dual-channel low noise(balance)photoelectric detector was designed and realized.The detector has a high-frequency passband of 10 kHz ? 360 MHzwith transimpedance gain of 10.8 k? and background noise of 13 mVpp;the low-frequency channel provides the electro-optic modulator with a DC ? 4 kHz low-noise feedback control signal of which the background noise is less than 6 mVpp.It is on par with commercial photodetectors.The common-mode-rejection-ratio(CMRR)between two arms of the balance detector reaches more than 11 dB in high-frequency band.After experimental test and verification,the time synchronization jitter STD was measured equals to 29.77 ps@27 hour in a 1400 km fiber-optic time bidirectional transfer system in laboratory using the self-developed detector;and STD equals to 13.18 ps@45 hour in back-to-back condition;meeting the basic requirements of laboratory system.(2)Broadband power dividerWhen performing time comparison,a broadband power divider is needed to ensure the high-consistency(high-homology)of the comparison signals from two path,thereby improving the accuracy and stability of time synchronization.Therefore,it is necessary to design a two split or even N split equalized power divider,of which the second and third port must have as high isolation as possible to weaken the interaction of signals between the ports.On the basis of Wilkinson power divider theory,based on ?/4 multi-section stepped impedance transformation and impedance transformation low-pass prototype filter,the DC ? 400 MHz broadband two-split equalized power divider have been realized with both microstrip and discrete devices like capacitance and inductance.The distribution loss of the splitter is less than 3.7 dB,and the isolation between the second and third port is more than 13 dB;by time comparing,the measured time jitter STD less than 1.4 ps when the two signals come from the same port of square-wave generator,which is significantly better than the STD of about 7 ps measured at two different ports of the square-wave generator.(3)Sub-nanosecond pulse generatorAt the same time,in order to improve the pulse quality in the optical fiber link,a pulse generator can be introduced at the transmitting end or the receiving-relaying module,which can be triggered by the original pulse to regenerate a higher quality pulse signal.Based on the avalanche effect of radio frequency transistor(RF BJT),the sub-nanosecond pulse generator is designed and implemented.It is measured that the falling edge of the output pulse is within 600 ps,and the edge rise time of the sync pulse is significantly shortened when been connected to square-wave generator system;the edge falling time jitter STD is less than 8 ps,which does not significantly reduce the stability of the source pulse. |
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