Novel WDM-PON architecture for simultaneous transmission of unicast data and multicast services

We propose a novel wavelength-division multiplexed passive optical network （WDM-PON） to simultaneously transmit unicast data and multicast services with upstream data re-modulation in optical network units （ONUs）. For each wavelength channel in the optical line terminal （OLT）, the downstream unicast data are applied to one arm of a dual-parallel Mach-Zehnder modulator （DPMZM） to generate baseband unicast non-return-to-zero （NRZ） signal. A radio frequency （RF） control signal is applied to the other arm to present two un-modulated sidebands for multicast data modulation in a differential phase-shift keying （DPSK） format. The flexible and dynamic multicast services are realized by simply switching the RF control signal on or off. The proposed scheme is experimentally demonstrated with 1.25-Gb/s downstream unicast, multicast, and upstream data.     

Influence of modulation formats performance of the combined modulation in WDM-PON system

Different modulation modes of DPSK and OOK are separately employed in the upstream and downstream link, after the comparison with different modulation formats in the downstream including the codes of non return-to-zero (NRZ), return-to-zero (RZ) and inverse return-to-zero (IRZ), the symmetric rate of 10 Gbps with 20 km transmission is realized without the dispersion compensation. It can be shown that in the combined modulation mode with the downstream of IRZ code, higher extinction ratio in the downstream and better performance of dispersion tolerance in the upstream could be achieved, enhancing the overall property of the transmission system.     

Characterization of the performance of optical amplitude-shift keying-differential phase-shift keying orthogonally modulated signals

We analytically investigate the signal performance of orthogonal modulation with slow amplitude-shift keying (ASK) data superimposed upon a high-speed differential phase-shift keying (DPSK) signal. The receiver sensitivities for both the ASK and the demodulated DPSK signals are formulated based on a probability model, and their dependence on the bit rate and the extinction ratio of the ASK data is theoretically investigated. A simple result of analysis of DPSK data performance has been derived. We verified the analytical results by experimental measurements.     

All-optical amplitude-shift keying and differential phase-shift keying to differential phase amplitude-shift keying format combination in highly nonlinear fiber

We propose an all-optical modulation formats combination scheme that merges an amplitude-shift keying （ASK） signal and a differential phase-shift keying （DPSK） signal into a single differential phase amplitude- shift keying （DPASK） signal based on parametric amplification in a highly nonlinear fiber. By optimizing the power of the ASK channel, formats combination of ASK and DPSK to DPASK signal is successfully demonstrated by computational simulation. The demodulation process of the generated DPASK pulses is investigated and the relationship between optical signal-to-noise ratio （OSNR） penalty and the input ASK power is presented. The proposed scheme may be used for increasing spectral efficiency and all-optical logic device.     

Novel orthogonal modulation format DRZ-FSK/DPSK for high-speed long-haul optical communication

We propose a novel advanced orthogonal modulation format dark return-to-zero frequency shift keying/differential phase shift keying （DRZ-FSK/DPSK） and its realization scheme. The DRZ-FSK/DPSK is generated by the combination of a 40-Gb/s return-to-zero （RZ） signal and a DRZ signal which is converted from the RZ using a semiconductor optical amplifier （SOA） based on nonlinear cross polarization rotation （XPR） and then re-modulated by high-bit-rate DPSK at 40 Gb/s. The feasibility of the scheme is exper-imentally demonstrated. Bit error rate （BER） results of the total 80-Gb/s DRZ-FSK/DPSK orthogonal modulation signal with a subsequent 100-km single-mode fiber （SMF） transmission link show its potential for future high-speed long-haul optical communication.     

A novel wavelength shift keying transmitter using a pair of Mach-Zehnder modulators

A novel and simple method to achieve wavelength shift keying (WSK) modulation is proposed and experimentally demonstrated. The proposed WSK transmitter consists of a pair of Mach-Zehnder modulators (MZMs) that are biased appropriately to achieve complementary operation. When a non-return-to-zero data stream is applied to the two MZMs, thanks to the complementary operation of the MZMs, the WSK-modulated signal can be obtained by combining the two intensity-modulated optical signals. A tunable optical delay line (ODL) is applied to achieve synchronization between the two optical signals. The ODL can also be used to compensate for the dispersion-induced walk-off effect between the two optical carriers. The application of the proposed WSK transmitter in a passive optical network is demonstrated, in which the downstream WSK signal is reused as optical carrier for upstream intensity-modulated data transmission. The proposed approach is experimentally realized; experimental results verify the feasibility of the approach.     

A cost-effective structure of a centralized-light-source WDM-PON utilizing inverse-duobinary-RZ downstream and DPSK upstream

In this paper, we propose a new structure of a centralized-light-source wavelength division multiplexed passive optical network (WDM-PON) utilizing inverse-duobinary-return-to-zero (inverse-duobinary-RZ) downstream and DPSK upstream. It reuses downstream light for the upstream modulation, which retrenches lasers assembled at each optical network unit (ONU), and ultimately cuts down the cost of ONUs a great deal. Meanwhile, a 50-km-reach WDM-PON experiment with 10-Gb/s inverse-duobinary-RZ downstream and 6-Gb/s DPSK upstream is demonstrated here. It is revealed to be a novel cost-effective alternative for the next generation access network.     

Performance analysis of FSO links under strong atmospheric turbulence conditions using various modulation schemes

Strong atmospheric turbulence is a major hindrance in wireless optical communication systems. In this paper, the performance of a wireless optical communication system is analyzed using different modulation formats such as, binary phase shift keying-subcarrier intensity modulation (BPSK-SIM), differential phase shift keying (DPSK), differential phase shift keying-subcarrier intensity modulation (DPSK-SIM), M-ary pulse position modulation (M-PPM) and polarization shift keying (PolSK). The atmospheric channel is modeled for strong atmospheric turbulences with combined effect of turbulence and pointing errors. Novel closed-form analytical expressions for average bit error rate (BER), channel capacity and outage probability for the various modulation techniques, viz. BPSK-SIM, DPSK, DPSK-SIM, PolSK and M-PPM are derived. The simulated results for BER, channel capacity and outage probability of various modulation techniques are plotted and analyzed.     

Label swapping and packet transmission of DPSK-labeled PPM signal in optical label switching

A label swapping scheme of an optical labeled signal with differential phase shift keying(DPSK) for label at 2.5 Gb/s and pulse position modulation(PPM) for payload at 40 Gb/s is demonstrated by simulation.Power penalties of ～1.8 and ～0.8 dB are achieved for both the payload and label over 80-km single mode fiber(SMF) transmission.This labeling scheme allows the use of four-wave mixing(FWM) in semiconductor optical amplifier(SOA) to perform label erasure,with advantages of transparence for bit rate,high processing rate,simple architecture,and low cost.Label swapping is demonstrated with appropriate penalties of-3.5 and 0.8 dB for PPM payload and new DPSK label,respectively.To further prove the effectiveness of the proposed scheme,label swapping in the case of using 10-Gb/s DPSK label is also investigated with the power penalties of 6 and 2 dB for PPM payload and new DPSK label.     

Improvement of switching properties and cascadability of an ultra-fast reconfigurable optical crosspoint switch matrix using DPSK payloads

In this paper, we demonstrate mitigation of pattern-induced degradation in an optical crosspoint switch (OXS) matrix by utilizing differential phase shift keying (DPSK) modulation format. We experimentally demonstrate 4 × 4 unicast optical packet switching and dynamic reconfiguration for 4-channel, 200 GHz spacing of RZ-DPSK payloads. Reconfigurable time as fast as 2 ns is achieved owing to the optimized control circuit and device fabrication. The power and wavelength dependence are obtained for the RZ-DPSK payload. We also investigate the cascadability of the OXS based on re-circulating loops. Due to the great suppression of the pattern effect in OXS, DPSK has shown dramatical improvement of switching properties compared to conventional ON-OFF keying (OOK) signal. The DPSK payload can outperform OOK for 3.2 dB after 9 hops optical switching.     

Comparison and analysis of codes for remodulation WDM-PON with adaptive code selection

In a remodulation PON, the upstream signal quality can be improved when the downstream signal is coded. But low code efficiency may result in network congestion in downlink. Based on the downlink traffic, a self-adapting PON can select the proper downstream modulation codes to achieve the optimal network performance. With adaptive code selection, network congestion can be avoided and the remodulated upstream signal suffers minimal performance degradation. Some codes of various coding efficiency are required to be selected in this self-adapting PON. These codes should induce as little crosstalk to the upstream signal as possible. Several candidate codes with coding efficiencies from 50% to 80%, such as Manchester code, 3b5b code, 4b5b code, 4b6b code and 6b8b code are tested through simulation and experiment in this paper, and their performances are compared. The results show that the optimum code for downstream modulation depends on the downlink traffic and the upstream bit rate. The results will help the self-adapting remodulation WDM-PON to select the proper downstream modulation codes in different traffic situations.     

Orthogonal labelling scheme using inverted RZ as payload

In this paper, we present an orthogonal labelling scheme using inverted return-to-zero (IRZ) as payload and differential-phase-shift-keying (DPSK) as label. Transmission performances of orthogonal labelling with 40 Gb/s IRZ payload and 10 Gb/s label are evaluated. Simulation results show that orthogonal labelling using IRZ as payload exhibits a much larger extinction ratio margin than that of using return-to-zero (RZ) as payload. The asynchronous effect between payload and label on the transmission performances of orthogonal labelling using IRZ and RZ as payload is evaluated. Simulation results show that the degradation of asynchronous orthogonal labelling using IRZ as payload is relatively worse than that of using RZ as payload. The impact of optical filter bandwidth before receivers is also investigated. Results show that the receiver sensitivities of label for IRZ/DPSK and payloads for both IRZ/DPSK and RZ/DPSK are degraded with the reduction of the filter bandwidth, while the label of RZ/DPSK exhibits an opposite trend.     

Optical homodyne receiver utilizing image rejection in FSK/ASK re-modulation WDM-PON

Optical frequency shift keying （FSK） homodyne detection utilizing image rejection in FSK/amplitude shift keying （ASK） re-modulation wavelength-division-multiplexed passive optical network （WDM-PON） is proposed and demonstrated. Image rejection is adopted to achieve better performance. We show the result in back-to-back and 40-km distance respectively and apparent improvement is obtained in the latter situation by using image rejection. Several factors which affect the image rejection receiver are analyzed in order to find out the optimum parameters. Result shows that the presented scheme is suitable for super long reach access network, especially for future metro access network.     

Demultiplexing of OTDM-DPSK signals based on a single semiconductor optical amplifier and optical filtering

We propose and demonstrate the use of a single semiconductor optical amplifier (SOA) and optical filtering to time demultiplex tributaries from an optical time division multiplexing-differential phase shift keying (OTDM-DPSK) signal. The scheme takes advantage of the fact that phase variations added to the target channel by cross-phase modulation from the control signal are effectively subtracted in the differential demodulation scheme employed for DPSK signals. Demultiplexing from 80 to 40 Gbit/s is demonstrated with moderate power penalty using an SOA with recovery time twice as long as the bit period at 80 Gbit/s. Large dynamic ranges for the input power and SOA current are experimentally demonstrated. The scheme is expected to be scalable toward higher bit rates.     

A long-reach WDM passive optical network enabling broadcasting service with centralized light source

We propose a long-reach wavelength-division-multiplexed (WDM) passive optical network (PON) to provide conventional point-to-point (P2P) data and downstream broadcasting service simultaneously by superimposing, for each WDM channel, the differential-phase-shift-keying (DPSK) broadcasting signal with the subcarrier multiplexing (SCM) modulated downstream P2P signal, at the optical line terminal (OLT). In the optical network units (ONUs), by re-modulating part of the downstream signal with a reflective semiconductor optical amplifier (RSOA), we realize color-less ONUs for upstream data transmission. The proposed scheme is numerically verified with a 5 Gb/s downstream P2P signal and broadcasting services, as well as 2.5 Gb/s upstream data through a 60 km bidirectional fiber link. In particular, the influence of the downstream lightwave's optical carrier–subcarrier ratio (OCSR) on the system performance is also investigated.     

Radio over fiber access network architecture employing RSOA with downstream OQPSK and upstream re-modulated OOK data

We have demonstrated a bidirectional reflective semiconductor optical amplifier (RSOA) based on wavelength division multiplexing ROF network utilizing an offset quadrature differential phase shift keying (OQPSK) signal for down-link and an on-off keying (OOK) signal re-modulated for up-link. A 50 km range colorless WDM-ROF without dispersion compensation was demonstrated for both 1 Gbit/s downstream and upstream signals. The BER performance of our scheme shows that our scheme is a practical solution to meet the data rate and cost-efficient of the optical links simultaneously in tomorrow's ROF access networks.     

Three-chip differential phase-shift keying maximum likelihood sequence estimation for chromatic-dispersion and polarization-mode-dispersion compensation

We propose a novel three-chip differential phase-shift keying (DPSK) maximum likelihood sequence estimation (MLSE) for chromatic-dispersion (CD) and first-order polarization-mode-dispersion (PMD) compensation to extend the transmission reach of the DPSK signal. Such a technique searches the most probable path through the trellis for DPSK data sequence estimation by exploiting the phase difference between not only the adjacent optical bits but also the bits that are one bit slot apart. The proposed scheme significantly outperforms conventional two-chip DPSK MLSE in CD and PMD compensation. We show that the proposed three-chip DPSK MLSE can enhance the CD tolerance of 10 Gbit/s DPSK signal to 2.5 times of that by using two-chip DPSK MLSE and can bound the penalty for 100 ps differential group delay by 1.4 dB.     

Improving channel capacity by nonlinearity compensation and noise suppression in high-speed multi-span optical transmission systems

In this paper, the channel capacity of 40 Gb/s multi-span nonlinear optical transmission systems with nonlinearity compensation and self-adapting Wiener filtering is studied by use of Finite State Machine (FSM) approach. The comparison of channel capacity of the system with differential phase shift keying (DPSK) and on–off keying (OOK) modulation is also investigated. The channel capacity increases monotonically with the input power for transmission systems with simultaneous compensation of dispersion and nonlinearity, which performs as well as linear system. DPSK shows a much better performance and the channel capacity has an improvement of at least 48 percent over that OOK modulation. A further reduction of the amplified spontaneous emission (ASE) noise accumulation is obtained by Wiener filtering, which improves the channel capacity considerably when the input peak power is less than 3 mW.     

基于半导体光放大器四波混频效应的多种调制格式的波长转换实验

实验报道了利用半导体光放大器(SOA)的四波混频(FWM)效应实现多种码型的波长转换.其中对于非归零(NRZ)信号实现了从单信道到三信道的多波长转换.调制速率从10 Gb/s到40 Gb/s均实现多波长转换.对于归零(RZ)信号分别实现了20 Gb/s和40 Gb/s的RZ格式的波长转换和40 Gb/s的载波抑制归零(CSRZ)格式的波长转换,利用光纤布拉格光栅(FBG)作为带陷滤波器消除共轭光和抽运光之间的串扰.对于非归零差分相移键控(NRZ_DPSK)信号分别实现了20 Gb/s和40 Gb/s的波长转换,利用实验室自制的光纤延时干涉仪进行NRZ-DPSK信号的解调.基于FWM效应的转换光的输出消光比大于7 dB,转换后消光比退化约为3 dB.     

Investigation of a high-speed optical FSK scheme for WDM-PON applications with centralized lightwave source

This paper proposes and numerically investigates a novel high-speed wavelength-division-multiplexed passive optical network (WDM-PON) architecture with colorless user terminals based on the use of orthogonal modulation scheme for downstream and upstream transmission. The 40 Gb/s optical frequency shift keyed (FSK) downstream data is generated based on carrier-suppressed modulation. At optical network unit, the downstream signal is directly re-modulated by the 2.5 Gb/s up-stream data and sent back with the same fiber. Error free transmission over 20 km single mode fiber can be observed for both downstream and upstream signals in our simulation. Power budget, tolerance of channel spacing, FSK tone spacing and dispersion are all investigated. Factors that might influence the stability of the system are analyzed and an extended hybrid wired/wireless version of the scheme is also given.