A detailed analysis of cross-phase modulation effects on OOK and dpsk optical WDM transmission systems in the presence of GVD,SPM, and ASE noise

A performance analysis is carried out to evaluate the effect of cross-phase modulation (XPM) on a dispersion-managed 20 Gb/s optical wavelength-division multiplexing (WDM) transmission system using either the on-off keying (OOK) or the different-phase-shifting keying (DPSK) modulation, in the presence of the group-velocity dispersion (GVD), self-phase modulation (SPM), and amplified spontaneous emission (ASE). It is found that to achieve a bit error rate (BER) of 10⁻⁹ at a distance of 160 km, a 1.0 dB XPM power penalty is incurred for input channel power of 3 dBm in the OOK transmission and 7 dBm in the DPSK transmission. The power penalty increases with input channel powers and is inversely proportional and exhibits oscillations with respect to the channel separation. The oscillation is evenly spaced for the DPSK but not for the OOK and suggests the presence of optimum separation values. The XPM penalty decreases when a high dispersion fiber is used and increases linearly with increasing dispersion slope. Small residual dispersion can reduce the penalty of nonlinear effects.     

高速光纤通信系统中抑制偏振模色散的新机制

重点研究了偏振模色散、群速度色散、自相位调制三者之间在高速光纤通信系统中的相互作用，从时域角度分析脉冲的演变，从频域角度分析频谱的变化，提出一定条件下，啁啾、色散、自相位调制可以部分补偿偏振模色散的思想。通过对40Gbit／s系统进行偏振模色散、群速度色散和自相位调制共同作用的仿真，从统计意义上验证了它们之间的相互影响，并找到最佳传输方案，对系统设计提供了参考。     

Performance enhancement of the power penalty in DWDM FSO communication using DPPM and OOK modulation

This paper presents the design and performance enhancement of the power penalty (PP) in a dense wavelength division multiplexing based on free space optical communication (FSOC) link using digital pulse position modulation (DPPM) and on–off keying (OOK) modulation. Such a system has a high performance, low cost, robust and power efficient, reliable, excessive flexibility, and higher data rate for access networks. The system performance is evaluated for an 8-channel wavelength-division-multiplexing for hybrid fiber FSOC system at 2.5 Gbps on widely accepted modulation schemes under various atmospheric turbulence (AT) regimes conditions. The performance of system is introduced in terms of PP, bit-error rate (BER), transmission distance and the average received optical power. The numerical results shows that the improvement of the PP using DPPM modulation of 0.2–3.0 dB for weak turbulence (WT) regimes for BER of 10^?6 and above 20, 25 dB for strong turbulence (ST) regimes are reported for BER of 10^?6 and 10^?9, as respectively (depending on the AT level). Further, we develop of improvement the PP caused by multiple-access interference about 6.686 dB which is predicted for target BER of 10^?9 in WT and 1 dB at target BER of 10^?6 in ST when the 8 user are active on the system of optical network units. Additionally, the optical power budget and margin losses of a system are calculated with different link length. The proposed approach of DPPM merges superiority with higher enhancement of PP about 0.8 dB for BER equal 10^?9 at FSO link length l_fso?=?2000 m compared to OOK at 1 dB for WT. An improvement of 2 dB is observed using the DPPM scheme over an OOK due to capability of detect pulses under background noise conditions with increased receiver sensitivity.     

光传输中RZ-ODPASK调制格式的SPM效应及其补偿研究

研究了单信道40 Gbps 归零八进制差分相位幅度调制格式（Return-to-zero Octal Phase-amplitude-shift Keying,RZ-ODPASK）光传输系统中,自相位调制(Self-phase Modulation,SPM）效应对于相位支路差分解调性能的影响.理论推导了相邻两个不同幅度的码元由于SPM效应,产生的非线性相移差表达式.提出了一种非线性相移差预补偿方法.仿真结果表明,该方法能够有效降低相位支路解调信号的眼张开代价,提高RZ-ODPASK长距离通信系统的传输性能.     

单模光纤中蓝移啁啾和红移啁啾斜率的最佳匹配

从理论上分析了自相位调制(SPM)引起的频率啁啾和群速度色散(GVD)引起的频率啁啾的传输特性,讨论了SPM和GVD频率啁啾的斜率匹配和导致两者啁啾斜率匹配的最佳输入功率。     

All-optical RZ-to-NRZ data format conversion using spectral broadening effect in a dispersion-shifted fiber

We propose a simple and efficient approach to obtain all-optical RZ-to-NRZ data format conversion using self-phase modulation (SPM) in a dispersion-shifted fiber. By exploiting SPM induced spectral broadening together with group velocity dispersion in the normal regime, a 10 Gb/s return-to-zero data signal has been converted to the non-return-to-zero format. The proposed scheme can work with different signal bit rates and does not require any external pulse duplicator. The 10 Gb/s bit-error-rate (BER) measurement shows a power penalty of less than 1.5 dB at 10⁻⁹ BER level.     

200 nm Supercontinuum Generated by Subpicosecond Optical Pulses and its Evolution Process Observation

200 nm superconticnumm (SC) with high flatness (<3 dB) is demonstrated with subpicosecond optical pulses generated by a passively mode-locked figure-eight-laser. The evolution process of SC under different optical pumping power is observed experimentally. Three sequential evolution stages are found: (1) SC spectrum is asymmetrically broadened due to self-phase modulation (SPM) and group velocity dispersion (GVD). (2) When the broadened spectrum exceeds the zero dispersion point, cross phase modulation (XPM) and four wave mixing (FWM) will cause symmetrical spectrum broadening. (3) stimulated Raman scattering (SRS) causes the spectrum to broaden toward long wavelength quickly when pumping power is intense enough.     

200 nm Supercontinuum Generated by Subpicosecond Optical Pulses and its Evolution Process Observation

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光传输系统中SPM效应的研究

从理论上分析了单信道光传输系统中SPM效应对传输性能的影响,并通过仿真得出了不同入纤功率情况下SPM效应对单信道光传输系统影响的定量数据,当入纤功率小于0 dBm时,SPM效应不明显;入纤功率在0～12 dBm时,SPM效应减弱了色散效应的作用,延长了系统的受限距离;入纤功率大于15 dBm时,SPM效应起主导作用,单信道光传输系统极限受限功率为18 dBm.     

Minimum Channel Spacing in an OFDM CPFSK Optical Coherent System

Channel spacing in optical frequency-division multiplexed (OFDM) coherent optical systems with continuous-phase frequency-shift-keying (CPFSK) is obtained both theoretically and experimentally. The bit error ratio is derived taking into account the non-Gaussian interference statistics, expressed in terms of the Marcum Q-function, and the sensitivity penalty is reported as a function of the signal-to-interference ratio and the channel spacing for different IF filters. Laboratory demonstration of a state-of-the-art fully engineered two-channel 2.5 Gb s^-1 CPFSK coherent system at 1550 nm has been performed, showing an inter-channel crosstalk measurement in agreement with the theoretical analysis. A 0.1 nm channel spacing is experimentally confirmed.     

利用光纤-光栅对1.06μm皮秒光脉冲压缩

本文报道了利用光脉冲在10m单模光纤中传播所产生的自相位调制(SPM)、互相位调制(XPM)、群速色散(GVD)和受激喇曼散射(SRS)的共同作用,实现光脉冲光谱和时间加宽,经光栅对线性啁啾补偿后,将脉宽15ps的1.06μm光脉冲压缩至小于6ps,能量大于100nJ,压缩比>2.5。     

Dual-Output Mach–Zehnder Modulator for Optical Access Networks

This paper employs dual-output Mach–Zehnder Modulator (MZM) for optical access networks without optical filters. Light waves generated from multiple laser sources are multiplexed and fed into dual-output MZM. Biasing the dual-output MZM at null point generates central carriers in one output port and first-order sidebands in another output port. Reflective semiconductor optical amplifier modulates both the central carriers and sidebands with wired and wireless data, respectively. The modulated optical signals are combined by polarization beam splitter and transmitted through 25-km single-mode fiber. The performance of the proposed scheme is proved by clear eye-diagrams and great bit error rate (BER) curves. Moreover, the power penalty at the BER of 10^-9 is less than 1 dB for both wired and wireless signals. Therefore, the proposed system simultaneously transmits wired and wireless signals.     

A novel encoder/decoder design scheme utilizing differential detection in OCDMA systems

In this paper, we report the use of differential detection in optical code-division multiple access (OCDMA) systems which use encoders and decoders based on unbalanced codes and utilize bipolar modulation/differential detection to demonstrate a significant improvement in system performance. Differential detection has been reported earlier also requiring a large number of one-chips in the code for (balanced codes) or with unbalanced codes with very long temporal dimensions requiring a large number of delay elements (either the delay lines in the ladder-type structures or the gratings in the multiple fiber gratings) making the encoder/decoder structures. We use single pulse per row codes to design our system and carry out the simulations for showing the results. The improved BER performance is achieved that can be traded for increasing the number of codes. Using optical simulation tool, the OCDMA system has been simulated for four users. The results of this system utilizing differential detection and another system that uses the same coding scheme but employing direct detection, for the same number of interfering users, are shown for comparison. The BER and the power penalty plots are shown for the two systems. Also, it is shown that performance degradation occurs due to the linear and non-linear effects of the fiber medium. The BER worsens more rapidly with link length for direct detection case, whereas for differential detection with an initial improvement of 8 dB, the increase in BER is insignificantly small for a link length of 500 km.     

Performance analysis of NRZ, RZ, CRZ and CSRZ data formats in 10 Gb/s optical soliton transmission link under the impact of chirp and TOD

This paper presents the performance analysis of non-return-to-zero (NRZ), return-to-zero (RZ), chirped return-to-zero (CRZ) and carrier suppressed return-to-zero (CSRZ) data formats in optical soliton transmission link under the impact of chirp and third-order dispersion (TOD). The performance of these data formats has been analyzed on the basis of certain performance metrics, viz, bit error rate (BER), Q² (dB), OSNR, eye opening, etc. It has been reported here that the performance of CRZ and CSRZ modulation format is better as compared to NRZ and RZ in a soliton transmission link. Further, CSRZ modulation format has been found to deliver optimum performance on the basis of performance evaluation metrics reported in this paper. In case of NRZ and CSRZ, comparatively narrow power spectrum has been observed. Best eye opening, highest value of Q² (dB) of 18 dB and lowest value of BER of the order of 10⁻¹⁶ has been reported in case of CSRZ among the considered data formats. The results have been obtained by varying noise figure from 3.0 to 9.0. No considerable effect of noise was observed. It was observed that at very narrow and ultra short pulse width, OSNR value suffers heavily and reduced to even negative values in dB, thus inducing a high degree of OSNR power penalty. The results were obtained by varying chirp factor from −0.6 to +0.6. Negative chirp resulted in improved OSNR as compared to positive chirp. RZ data format yielded a broader optical spectrum, comparatively low spectral efficiency and poor OSNR thus it was found that RZ format is not suitable for optical soliton transmission under the impact of chirp and TOD.     

Performance of directly modulated OFDM data over multimode fiber link

The performance of orthogonal-frequency-division-multiplexing (OFDM) technology in the multimode fiber (MMF) optical communication system is investigated numerically. The mode dispersion in MMF is analyzed by utilizing the optical angular spectrum analysis method. Simulation results in terms of bit-error-rate (BER) and signal-to-noise ratio (SNR) show that the transmission distance can reach up to 2.5-km at 10^− 3 BER for 40-Gb/s data. Results of different modulation formats further indicate that better performance can be achieved by adopting multi-level modulation format.     

Performance degradation due to cross-phase modulation in optical fdm ask coherent detection systems

The performance degradation due to cross-phase modulation in optical FDM ASK coherent detection systems has been investigated in this paper. The effect of modulation statistics and phase tracking of the receivers is taken into account. Based on the theoretical treatment, power penalty and allowable launched power per channel have been estimated.     

An optical continuous phase FSK modulation scheme with an arbitrary modulation index over long-haul transmission fiber link

This paper presents an optical continuous phase frequency shift keying (CPFSK) modulation scheme with an arbitrary modulation index. The detailed principle on the optical CPFSK generation is derived and analyzed, which includes the special case of the minimum-shift keying (MSK) with a modulation index h = 1/2. The differential detection and the coherent detection of CPFSK are also depicted. The performances of the four kinds of the optical CPFSK modulated system with a 40 Gb/s modulation rate whose modulation index are h = 1/2, h = 2/3, h = 3/4 and h = 1 are simulated via the spectral efficiency and the receiver sensitivity over fiber link respectively. In addition, comparison with the differential phase shift keying (DPSK) is taken. Through the calculation of the spectral efficiency of each modulation formats, CPFSK has higher spectral efficiency than DPSK with the same optical devices. The transmission performances of our CPFSK over the fiber link change better as the modulation index increases under the condition of the first order dispersion of the fiber link is completely compensated. Through simulations, a 1200 km transmission distance can be achieved with a modulation index h = 1.     

Using Fabry-Perot laser diode and reflective semiconductor optical amplifier for long reach WDM-PON system

In this investigation, we propose and investigate the simple self-injection locked Fabry-Perot laser diodes (FP-LDs) in optical line terminal (OLT); and wavelength-tunable optical network unit (ONU) using reflective optical semiconductor amplifier (RSOA) and FP-LD laser for downstream and upstream traffic in long reach (LR) wavelength division multiplexed-passive optical network (WDM-PON) respectively. The output performance of the proposed two laser sources in terms of power and side-mode suppression ratio (SMSR) has been discussed. Here, for the downstream traffic, the proposed optical transmitter can be directly modulated at 2.5 Gb/s on-off keying (OOK) format with nearly 0.4 dB power penalty at bit error rate (BER) of 10⁻⁹ through 75 km single-mode fiber (SMF) transmission. Moreover, the proposed upstream transmitter can be directly modulated at 1.25 and 2.5 Gb/s with nearly 0.5 and 1.1 dB power penalty, respectively, at the BER of 10⁻⁹.     

Analysis of a DPSK soliton transmission system

We present a closed form expression to analyse the error performance of a differential phase-shift keying (DPSK) digital modulation format in an optical soliton communication system. In the analysis we include the effect of Gordon-Haus jitter, which is mainly due to the amplified spontaneous emission noise of the amplifiers. Gordon-Haus jitter is found to be less significant when the bit period is 15 times greater than the jitter variance and becomes more significant when the signal-to-noise ratio is greater than 20 dB. Due to a good bit error rate (BER) performance (BER ≈ 10⁻²⁰at SNR= 21 dB), the DPSK modulation format has a potential application for a high-bit-rate and low-noise soliton communication system.     

Temperature independent 80 Gb/s transmission system using spectral phase modulation-based TDC

A temperature independent 80-Gb/s 100-km transmission system is demonstrated with the use of spectral phase modulation-based tunable dispersion compensator (TDC). The principle of dispersion compensation based on spectral phase modulation as well as the relationship between spectral phase modulation function and group velocity dispersion (GVD) are theoretically studied. TDC based on spectral phase modulation is implemented. The performance of 80-Gb/s transmission system is experimentally evaluated. The nonlinear relationship between temperature and temperature-induced dispersion fluctuations is demonstrated through the asymmetric temperature-induced power penalty without dispersion compensation. With respect to the low temperature area, the temperature-induced dispersion fluctuations are smaller than those in the high temperature area. By using the proposed TDC, temperature independent 80-Gb/s transmission is successfully demonstrated under a temperature range of -20 - 60 ?C with a power penalty of less than 0.8 dB.