Research On Simplified Coherent Optical System For Short-distance Optical Transmission

In modern era,the development of information technology is increasing fast.People place greater demands on the various performances of the optical communication system as the basic facility of the high speed communication.Short distance optical fiber communication occasions such as data centers are playing an important role of the modern global network.The relative researchment of opical communication used under such background is turning into a hot topic followed by domestic and overseas researchers.Traditional intensity modulation direct detection technology is facing challenges when supporting high capacity communications due to the lack of sensitivity and the frequency selective fading caused by fiber dispersion.With the successful application in the backbone network communications,the coherent optical technology begins to be used in short distance scenarios such as data centers and access networks,and simplified coherent optical system is being focused by reducing complexity and cost.On the one hand,simplified coherent technology makes the simplify operations on the coherent structure,reducing the complex hardware structure and cost.On the other hand,it operates on the digital coherent demodulation,reducing the complexity of the digital signal processing.It has broad prospect to study the simplified coherent schemes to achieve the reduction of the targets such as complexity and cost.This thesis mainly discusses the simplified coherent optical system for short distance optical transmission,and makes the simplify operations on the traditional coherent constructions from structure and algorithm.At the level of the coherent optical communication structure,this thesis studyies a simplified coherent system based on a 3×3 coupler.By building the simulation system,this thesis verifies and realizes a simplified coherent system based on a 3×3 coupler.It transmits On-Off Keying(OOK)signal without complex digital signal processing at the receiver.This system is polarization insensitive but it is sensitivity to the fiber dispersion.Meanwhile,this thesis also studies the low complexity reception of the self-coherent optical communication system.The receiver needs only two photodiode detectors and two analog-to-digital converters to demodulate.By adjusting the local oscillator(LO)to signal power ratio,the system can succeed in demodulating the signal without compensating the signal-signal beat interference,thus can reduce the complexity of the receiver.From the perspective of the digital coherent demodulation algorithms,this thesis discusses from the aspect of the application of inside the data center and that of interconnections between the data centers.On the occasion of short distance optical connection,this thesis proposed an improved joint equalizer,which utilizes 1-tap butterfly equalizer to depolarize the signal meanwhile combining the phase noise compensation.When the distance of the optical communication is 2 km,this equalizer can raise the tolerance of the laser linewidth with only 0.07 d B loss of the sensitivity compared to the traditional butterfly filter.When the distance of the optical communication is 25 km,the loss is 0.5 d B while the complexity decreases about 40%.On the occasion of medium distance distance optical connection,the performance of the proposed single-tap depolarizing joint equalizer is getting worse with the increasing of the transmission distance.Therefore,the multi-tap joint equalizer combining the depolarization and phase noise compensation is utilized.Both equalizers cascade the phase noise compensation module based on the training sequences into the depolarization algorithm,which can raise the tolerance of the phase noise caused by the large laser linewidth.Therefore it is possible for the system to utilize lasers with large linewidth to reduce the cost.Thus it can achieve the goal of simplifying the coherent optical communication.