| Stable frequency propagation is widely used,such as navigation,deep space network,geodesy,gravity wave detection and remote distributed radio telescopes.Traditional frequency transmission uses satellite link,which has low stability.It introduces a lot of noise to the frequency source in the transmission process and reduce the stability of the frequency signal.Compared with traditional satellite link,optical fiber has the advantages of wide bandwidth,low attenuation and anti-electromagnetic interference,and is considered to be an ideal fr-equency transmission medium.However,due to the change of temperature and mechanical disturbance,the optical fiber link changed,which affect the stability of frequency signal transmission.Additional real-time phase noise is introduced into the stable transmission of radio frequency signal.Therefore,the compensation of phase fluctuation is one of the key problems in the stable transmission of optical fiber frequency.In order to improve the stability of the optical fiber transmission signal,the round-trip link transmission is used to transmit the radio frequency signal back and forth to introduce the jitter of the optical fiber link.The existing schemes are mainly based on active compensation and passive compensation.Active compensation devices exist in active compensation system.It is necessary to use phase discriminator to extract phase error and compensate them by active compensation devices.Passive compensation uses mixing to realize automatic compensation,does not need to use complex PID circuit and so on.The compensation is fast and the compensation range is wide.Considering the advantages of passive compensation,this paper adopts passive compensation technology.Frequency signals transmitted in existing passive frequency stabilization systems are often frequency doubled and frequency doubled.In the frequency mixing process of passive compensation schemes,frequency doubling signals are usually generated to achieve phase conjugation with the detected signals transmitted back and forth,thus carrying the phase drift of the round trip link.As we all know,in the process of electro-optic modulation and electric mixer,due to the non-linear effect,it produce higher harmonics,and the second harmonic is the largest.When the harmonic firequency is the same as the transmission signal frequency,the second harmonic will introduce additional phase jitter.In order to reduce the interference of the second harmonic,the previous passive compensation schemes often used to reduce the power of the signal source.This method will reduce the modulation depth,and then worsen the signal-to-noise ratio,affect the transmission distance.This paper mainly studies the passive frequency stabilization transmission scheme to avoid the second harmonic interference.The innovation and main work of this paper are as follows:1?A stable frequency transmission scheme to avoid second harmonic interference is proposed.Firstly,the existence of the second harmonic and its influence on the stability of the system are verified by theoretical proof and simulation.Then,a stable frequency transmission scheme to avoid the interference of the second harmonic is proposed.The scheme uses passive mixing to automatically correct the jitter of the system.In order to avoid the interference of second harmonic,the system uses the transmission of higher frequency signals:frequency doubling and frequency quadrupling signals.2?The improvement of system stability is proved by comparative simulation.In order to better measure the influence of the second harmonic,and to simulate the system scheme of transmitting the first and second harmonic signals,the conclusion proves that our scheme of avoiding the second harmonic interference can improve the stability of the system and effectively compensate the phase jitter caused by the second harmonic.In the simulation experiment,the frequency source transmits 10 GHz and 40 GHz signals in a 50 Km optical fiber.After passive compensation,the instability of the system reaches 7.1 ×10-14/s and 2.0×10-18/104s. |
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