The Research On Key Technologies For Optical Communication System Based On Direct Detection

With the popularization of high bandwidth applications such as multimedia,video conferencing and virtual reality in the internet and the development of big data technologies such as internet of things and cloud computing,the demand for bandwidth in the market is increasing with each passing day.The transmission technology of Intensity Modulation and Direct Detection?IM/DD?which can provide high bandwidth and low cost in optical fiber communication has been widely concerned and developed rapidly.Pulse amplitude modulation?PAM?technology based on IM/DD has become a hot research in industry and academia in recent years because of its simple structure,low cost and easy implementation.Single carrier PAM modulation system based on direct detection is usually used in cost-sensitive environment,so it faces different technical challenges such as bandwidth limitation of low-cost devices and dispersion damage in long-distance transmission.In this paper,the theory and implementation technology of modulation and coding technology,digital equalization technology and multi-core optical fiber transmission technology in optical fiber communication are studied in depth by using PAM modulation technology based on IM/DD as the basic innovative means.Aiming at the scientific problems in direct detection optical communication system,the following experiments are studied in this paper.With the help of KK field reconstruction,DCR and Volterra equalization,single channel 56-Gb/s and 64-Gb/s PAM-4 signal transmission over 720-km and 1440-km SSMF can be successfully achieved.By employing transmitter-side digital duobinary encoding of PAM-4 signal,digital precompensation and receiver-side 7-level training sequence aided least-mean square?TS-LMS?algorithm,we experimentally demonstrate single channel 112-Gb/s transmission with PAM-4 signal over 1-km SSMF with commercial 18-Gbps DML.With the aid of a linear DPD equalizer,we experimentally transmitted a single channel single-polarization IM/DD based 224-Gb/s PAM-16 signal over 100-m SSMF.By using linear DPD,Nyquist filtering and MLSE,a transmission of 1.176-Tb/s?168-Gb/s×7?PAM-8signal over 7-core single-mode fiber based on IM/DD has been successfully achieved.The main innovations are as follows:?1?Aiming at the problem of direct detection of frequency selective fading damage caused by cumulative dispersion in signal transmission in optical communication,which limits its transmission distance,this paper proposes a pilot-assisted scheme base on MZM and direct detection to extend the fiber transmission distance.The bias point of MZM is set near the null point for power efficient transmission with reduced SSBN.DCR is proposed to mitigate the effect of laser phase noise based on the proposed scheme.The experimental results show that single channel 64-Gb/s PAM-4 signal is successfully transmitted over 1440-km SSMF at the 20%soft-decision forward error correction?SD-FEC?threshold of 2×10^-2,and single channel 56-Gb/s PAM-4 signal can achieve transmission distance of 720 km at the 7%hard-decision forward error correction?HD-FEC?threshold of 3.8×10^-3.?2?Aiming at the problem of bandwidth limitation due to the use of low-speed optical/electrical devices in the short-distance optical communication,which makes the system unable to carry out high baud rate and large capacity signal transmission,duobinary decoding and frequency domain equalization are proposed to reduce the bandwidth of PAM signal to half of the original signal,and to equalize the bandwidth-constrained channel effectively.The experimental results show that single channel 112-Gb/s PAM-4 transmission based on IM/DD by using commercial 18-Gbps DML is experimentally demonstrated over 1-km standard single-mode fiber.?3?Aiming at the problem of the complexity of the receiver algorithm and noise enhancement caused by post-equalization algorithms in the direct detection system with high order modulation,a linear digital compensation technique is proposed to compensate system transmission damage by using only low-order linear filters at both the transmitter and receiver in this paper.Based on the above methods,the transmission of a single channel 224-Gb/s PAM-16 signal is successfully achieved over 100-m SSMF,with the bit error rate?BER?below the 7%overhead hard-decision forward error correction code limit of 3.8×10^-3.?4?Aiming at the problem of low spectral efficiency for PAM modulation and low cost and large capacity expansion in short distance transmission environment,a linear digital pre-distortion equalizer,Nyquist filtering and MLSE are applied to compensate all the distortions from the system components.The transmission of a 1.176-Tb/s Nyquist-PAM-8 signal is successfully achieved over 10-m 7-core fiber with residual dispersion up to 64-ps/nm?CD of 4-km SSMF?,and the bit error rate is below the forward error correction limit of 1×10^-3.To the best of our knowledge,it is the first demonstration of IM/DD based PAM-8 signal at a line rate over 1T-Gb/s in MCFs.The experimental results indicate that MCFs are well-suited in the IM/DD system,and it offers an upgrade path to higher data rate for the short reach optical communication.?5?Direct detection technology is widely used in different communication fields.It can also be used in the field of visible light transmission.A few applications of modulation and coding technology based on visible light transmission system are also studied in this paper.Aiming at the problem of non-linear damage caused by high peak-to-average power ratio,this paper proposes a modulation coding scheme based on DCTs-OFDM,which reduces the PAPR of the system under the condition of low algorithm complexity.The experimental results show that the proposed scheme can effectively reduce the PAPR by 3.2dB.Aiming at the problem of frequency selective fading caused by the multipath effect of the received signal due to the multi-source illumination in the indoor visible light communication system,SSD technology is proposed to reduce the influence of frequency selective fading.The experimental results show that the Q value of 16QAM-OFDM signal can be effectively increased 1.3dB under the multipath condition with 8ns delay.