The Research On Decreasing Relative Intensity Noise Using Balanced-detection Method In ROF Link

Radio over Fiber(ROF)technology combines wireless communication technology with wired optical network technology,which has the characteristics of high bandwidth and low loss of fiber.So it has great advantages in the long-distance transmission.However,the relative intensity noise of the laser in the ROF transmission system is one of the important factors that restrict the performance of the system.In digital coherent optical communication system,balanced detection is an effective means to reduce the relative intensity of noise,which has important reference value in ROF analog optical communication system.In this paper,we studied the method of balanced detection in ROF transmission system,including the feasibility of reducing the relative intensity noise and its influencing factors.the content are studied as follows:1.The method of reducing the relative intensity noise in ROF system is analyzed theoretically.It is proved that the relative intensity noise can be reduced by reducing the direct current term,but the additional phase difference noise,the optical power of local oscillator and relative intensity noise will affect the effect of noise decreasing.On this basis,a balanced detection model is established on the VPI platform,and the simulation results show that the balanced detection in the ROF link reduces the relative intensity noise by 8.6dB / Hz compared with the direct detection.2.the article studied several influencing factors which affect the relative intensity noise decreasing,including RIN values of the signal light laser and the local oscillator,the optical power of the signal light and the local oscillator,and the linewidth of the signal light laser and the local oscillator.VPI simulation shows that when the RIN value of the signal light lasers increases and that of the local oscillator laser decreases,the RIN of the ROF system will be reducing.And it will also decrease when optical power of the signal light and the local oscillator increase.The increase in the linewidth value of the signal light and the local oscillator laser will degrade the effect of reducing the relative intensity noise,but it is mainly limited by a laser with larger linewidth.According to these characteristics,the simulation analysis of the detection signal is carried out,and we found that the performance of the balanced detection system is improved by comparing the constellation and the calculated RIN value.3.The effect of two-link imbalance in balanced detection is studied.The influence of three kinds of unbalance factors(transmission delay,coupler splitting ratio and detector responsibility)on the reduction of RIN is analyzed and simulated.The results show that the smaller the relative transmission delay is,the more uniform the coupler is,and when the responsibility of the detector is close to the same,the balanced detection can reduce the relative intensity noise more effectively than the direct detection.In addition,the influence of the propagation delay on the relative intensity noise is periodic.When the two links are out of an imbalance range,the balance detection method will not achieve the effect of decreasing the relative intensity noise.Considering about the influence of linewidth of lasers,the tolerance range of the two imbalances is further studied,and the maximum delay range and the ratio of the detector responsitivity and the coupling coefficient are simulated and analyzed.The simulation results show that with the increase of the linewidth of the laser,the tolerance of two-link imbalance range obviously decreased.