Suppression Of Rayleigh Backscattering Noise In Wdm-pon

With the fast development of information technology,various broadband internet services have been gradually applied,such as high definition video on demand,network cloud storage or computation,enterprise VPN and so on.The demand of end user's bandwidth has grown rapidly.Through utilizing wavelength division multiplexing technology,wavelength division multiplexing passive optical network(WDM-PON)has been developed to upgrade the whole capacity of network.It has multiple advantages:(1)Abundant bandwidth resources.Every subscriber use typical wavelength in WDM-PON and can occupy whole bandwidth of that wavelength.With the increasing number of subscribers,only new wavelength resources need to be added in WDM-PON which can avoid bandwidth reduction of per subscriber,due to bandwidth sharing between different subscribers.(2)Easy upgrade.Compared with ethernet passive optical network(EPON)and gigabit passive optical network(GPON)which use complex synchronization and timing protocols,WDM-PON is transparent to different services and compatible with different transmission speed.So there is no need to replace all the equipments in upgrade.(3)High power budget.Wavelength multiplexer/de-multiplexer has been used in WDM-PON which has low insertion loss.The transmission loss won't changed with the increasing number of ONU and it's also useful to increasing network coverage.(4)High security.Compared with downstream broadcast scheme,each ONU operates on different wavelength in WDM-PON and cannot receive other ONU's message which improve the network security.In order to reduce the deployment cost of WDM-PON,the already existing single fiber bidirectional transmission and colorless ONU structures are both used to simplify the complexity of WDM-PON.But serious Rayleigh backscattering(RB)noise exists in single fiber bidirectional transmission WDM-PON,which may limit the transmission performance and network coverage.This thesis focus on RB noise mitigation techniques in WDM-PON system.Several novel RB noise mitigation schemes have been proposed to improve the performance of WDM-PON.The detailed works are devided into following parts.1.RB noise mitigation techniques based on DPSK demodulation in WDM-PONDifferential phase shift keying(DPSK)signal can enhance the dispersion and nonlinear effects tolerance in fiber optical communication systems.However,there are still some challenges to apply DPSK signal in WDM-PON.(1)The existing DPSK demodulation schemes lack of the abilities to demodulate bit rate flexible signal,so they are difficult to be ultilized in WDM-PON.(2)In single fiber bidirectional transmission WDM-PON,the performance of upstream DPSK signal may degraded by RB noise.Focusing on these two problems,the following researches have been carried out.(1)Bit rate transparent DPSK signal demodulation scheme based on injection locking Fabry-Perot laser diode(FP-LD)has been proposed.The polarization of free-running and injection-locking states of FP-LD has been studied.We ultilize the frequency deviation of DPSK signal generated by phase modulator(PM),and unstable injection locking state of FP-LD is realized in every phase shift.So the polarization state change corresponds to every phase shift is achieved.In the end,through extracting the typical polarization state,DPSK signal demodulation is accomplished.Bit rate transparent DPSK signal demodulation from 1.25 Gb/s to 10 Gb/s has been verified and this scheme is suitable for bit rate variable WDM-PON(2)The wavelength conversion and RB noise mitigation scheme based on self injection locking FP-LD has been proposed.Based on the unstable injection locking state of FP-LD,red shift of longitude modes of FP-LD is generated by PM modulated DPSK signal.So original stable self injection locking state is destroyed.In every phase shift,the self injection locking state is recovered which may induce power difference at original longitude mode of self injection locking.DPSK signal demodulation is accomplished by detecting this kind of power difference.Meanwhile,by using longitude mode of self injection locking as the upstream re-modulation carrier,the RB noise generated by downstream DPSK signal is eliminated and error floor of 25 km bidirectional transmission has been removed.2.RB noise mitigation techiques based on colorless ONU in WDM-PONThe cost and complexity of subscribers in WDM-PON are able to be reduced by colorless ONU with simplified management and maintenance functions.The RB noise influence of upstream and downstream receiving signals is reduced by using colorless ONU techniques to mitigate RB noise in WDM-PON.Existing RB noise mitigation schemes based on colorless ONU are still complicated which need to re-design the whole network structures or use high cost optoelectronic devices.In this paper,we propose two colorless ONU structures to suppress RB noise in WDM-PON.(1)Carrier RB noise suppression scheme based on cascaded semiconductor optical amplifier(SOA).We study the quasi-high pass filtering effect generated by high gain saturation effect of cascaded SOA.This effect has been used in colorless ONU to suppress the central carrier of downstream multi-subcarrier signal.The bias free central carrier suppression scheme is achieved which reduce the carrier RB noise with reflected signal of downstream seeding light at the central carrier.By using the microwave photonic filter(MPF)in optical line terminal(OLT),carrier RB noise at high frequency is also eliminated to realize 10 Gb/s 45 km bidirectional upstream transmission.(2)Signal RB noise suppression scheme based on cascaded reflective semiconductor optical amplifier(RSOA).In remodulation WDM-PON,gain saturation effect of single stage RSOA is able to be improved by the cascaded structure with the enhanced data erasing performance of downstream high exitinction ratio signal,which increase downstream exitinction ratio to 11 dB and downstream noise tolerance performance has been enhanced.In upstream direction,signal RB noise reflected back to ONU is also suppressed.We use experiment to validate 30 km error free bidirectional transmission.3,RB noise mitigation techniques based on upstream real-time heterodyne receiving in WDM-PONIn WDM-PON,by using loop-back structure based on external light seeding,light source at subscriber side is simplified and upstream high speed transmission is realized with modulator in ONU.But the seeding light transmits in both upstream and downstream direction,high insertion loss is achieved.In addition,upstream performance of loop-back WDM-PON is also limited by RB noise.So special designed upstream receiving schemes have to be used at receiver side in OLT.In this paper,we propose interferometric noise suppression scheme based on electro-absorption modulator(EAM)and RB noise suppression scheme in WDM-PON based on real-time heterodyne EAM receiver.(1)Interferometric noise suppression scheme based on EAM.We study the zero-slope regioin and linear modulation region of nonlinear modulation curve of EAM.The zero-slope region and linear modulation region are used to suppress noise amplitude and amplify signal amplitude,respectively.The noise suppression performance of EAM under different bias voltage is also analyzed.And bit rate transparent interferometric noise suppression from 2.5 Gb/s to 10 Gb/s is achieved by using the EAM receiver.(2)RB noise mitigation scheme based on real-time heterodyne receiving of EAM in WDM-PON.The EAM has been used in heterodyne receiver to realize 5 Gb/s upstream heterodyne detection and upstream receiving sensitivity of WDM-PON is also improved.We use the nonlinear modulation curve of EAM to suppress carrier RB noise in WDM-PON and 45 km bidirectional transmission is achieved.