Performance comparison between 160 Gb/s WDM and TDM systems

The performances of 160 Gb/s time-division multiplexing (TDM) and 4 × 40 Gb/s wavelength-division multiplexing (WDM) signals are comparatively studied in the nonzero-dispersion shifted fibers (NZDSFs). TDM format is superior to WDM, and with the increase of distance, this advantage is enhanced. In the case of adopting the dispersion managed soliton transmission and dispersion flattened fibers (DFFs) technique, the Q values of both formats change little when the channel space varies. So, TDM technique is applicable to the dense and very long haul transmissions. Only by utilizing the conventional loss and dispersion compensation schemes (NZDSFs + DCFs + EDFAs), the available transmission distance of dense WDM signals reaches 1000 km, and for TDM format, it even extends to 2000 km. Both systems have the analogue characteristics: a higher pulse power benefits system's working; ASE noise is the dominant impact factor of system performance; both format's system performances are improved for the case of less channel number; the channel space and duty cycle of return zero pulse have little effect on Q; the impact of duty cycle relates to the filter bandwidths, XPM induced sidebands and pulse broadening effect; the influence of channel space is determined by the walk-off effect.     

利用非线性光纤环镜160Gb/s到10Gb/s解时分复用

研究了160 Gb/s光时分复用(OTDM)系统的解复用技术.针对160 Gb/s速率的特点,对高非线性光纤(HNLF)的光纤环镜(NOLM)特性及解复用进行了数值仿真.计算了低信号光时间抖动下解复用误码特性对时钟与信号的走离及时钟功率的依赖关系.计算了三种走离值消光比随时钟功率增加的变化趋势并给出:存在一个能获得最大的解复用窗口消光比、并能降低相邻信道串扰的合适的时钟功率范围.利用自制的基于电吸收调制器和压缩技术的超短光脉冲源建立了160 Gb/s OTDM实验系统,测量了不同信号光功率下NOLM的消光比,它基本不随信号增大而变化,在信号功率为7.3 dBm时仍大于23 dB.利用上述装置实现了无误码的160 Gb/s到10 Gb/s全光解复用.     

Performance evaluation for 160-Gb/s optical phase conjugation systems considering dispersion mapping and third-order dispersion

The impact of third-order dispersion (TOD) is investigated by numerical simulations in 160-Gb/s singlechannel systems incorporated with dispersion mapping and optical phase conjugation (OPC). System performances using retrun-to-zero (RZ) or carrier-suppressed RZ (CSRZ) modulation format are evaluated on the optimized dispersion map. The results indicate that even though TOD has been fully compensated,the intra-channel nonlinearity induced by local TOD would degrade the system performance in nonlinear regime. The scheme with an optimized dispersion map provides a much higher performance and offers a larger tolerance on a variation of pre-compensation. CSRZ modulation format is more robust due to its tradeoff between tolerances on intra-channel nonlinearity and dispersion.     

The characters of dense dispersion managed soliton in optical fiber transmission systems

The properties of ultra-short dense dispersion-managed soliton (DBMS) in optical fiber links are investigated. They show some excellent characters, such as, reducing pulse's breathing extent greatly, facing fewer mutual interactions and tolerating larger local dispersion. In general, DBMS is more stable than a conventional dispersion-managed soliton in high-capacity systems. Excessively dense dispersion compensation is more suitable for systems with weak nonlinear effect.     

Impact of PMD induced penalties on 40 Gb/s duobinary optical communication system for fixed and variable scattering section dispersion

In this paper, we evaluate the performance of 40 Gb/s duobianry optical transmitter for different scattering section dispersion and polarization-mode dispersion coefficient of single mode fiber. We observe that the variable scattering section dispersion considerably improves the performance of duobinary optical system as compared to fixed scattering section dispersion where the Q value fluctuates over wide frequency range. Also, duobinary system observes pulse delay of 7 ps for the fixed scattering section dispersion while there is no delay in the received output signal if variable scattering section dispersion is considered.     

Optimizing amplifier spacing to improve performance in RZ-rectangular pulse based 10 Gb/s single channel dispersion managed optical communication system

In this paper, we optimize the inter-amplifier spacing in combination with duty cycle of RZ data format and EDFAs power so that link length of system can be maximized. The results for EDFA amplifier placement in 10 Gbps single channel dispersion managed optical communication system have been presented. By increasing the length of standard single mode fiber of dispersion 16 ps/nm/km in proportion to the increase in length of compensating fiber of dispersion −80 ps/nm/km, the pre-, post- and symmetrical-dispersion compensation schemes of the system have been compared. Further, schemes are observed at 8, 10 and 12 dBm values of EDFA power in the link with different duty cycle values of RZ optical pulse in the range of 0.2-0.8 with step size of 0.2 in relation to amplifier spacing to get lower value of bit error rate and timing jitter. The graphical results obtained show strong relationship among duty cycle of RZ optical pulse, EDFA power and, dispersion compensation scheme.     

All-optical 40 Gb/s wavelength conversion using hybrid SOI-based MZI and SOAs configuration

In this paper, we present a compact combination of silicon-based Mach–Zehnder interferometer (MZI) and semiconductor optical amplifiers (SOAs) to perform all-optical return to zero (RZ)-to-RZ and non-return to zero (NRZ) wavelength conversions at a bit rate of 40 Gb/s. Using the proposed theoretical project, the attractive issue is that converted signals with inverted and noninverted formats can be simultaneously achieved at the same target wavelength. Moreover, both extinction ratio and eye-opening ratio of the converted RZ and NRZ signals can be significantly optimized by judiciously adjusting the system parameters and initial conditions.     

Turbo-switched Mach-Zehnder interferometer performance as all-optical signal processing element at 160 Gb/s

Two variations of the active Mach-Zehnder interferometer (MZI) that incorporate the recently proposed turbo-switch effect are introduced to carry out three key functionalities in forthcoming high-speed optical telecommunication networks, namely, all-optical wavelength conversion, photonic XOR gating and optical time-division demultiplexing. Their performance is numerically investigated at 160 Gb/s using a sophisticated semiconductor optical amplifier (SOA) model. The more practical of the two proposed geometries shows error-free operation as XOR Boolean gate, low patterning as wavelength converter, and poor performance as demultiplexer. For comparison, results derived from well-accepted (or typical) schemes are also presented, and the role of the required extra SOAs as distinguishing elements of the new architectures is investigated.     

Using RLS-TEQ to reduce CP length for 20.48 Gb/s direct-detection optical OFDM transmission over 2400 km of SSMF

This paper uses a Recursive Least Squares-Time Domain Equalizer (RLS-TEQ) to reduce the Cyclic Prefix (CP) length in Direct-Detection Optical Orthogonal Frequency Division Multiplexing (O-OFDM) transmission over 2400 km of Standard Single Mode Fiber (SSMF). The RLS-TEQ can cancel the residual Inter Symbol Interference (ISI) caused by both the Group Velocity Dispersion (GVD) and the CP length being shorter than the Channel Impulse Response (CIR). Using RLS-TEQ reduces size of the CP, and consequently leading to system performance improvement.     

10 Gb/s optical carrier distributed network with W-band (0.1 THz) short-reach wireless communication system

Millimeter-wave (mm-wave) operated in W-band (75 GHz–0.11 THz) is of particular interests, since this frequency band can carry signals at much higher data rates. We demonstrate a 10 Gb/s optical carrier-distributed network with the wireless communication system. The mm-wave signal at carrier frequency of 0.1 THz is generated by a high speed near-ballistic uni-traveling carrier photodiode (NBUTC-PD) based transmitter (Tx), which is optically excited by optical short pulses. The optical pulse source is produced from a self-developed photonic mm-wave waveform generator (PMWG), which allows spectral line-by-line pulse shaping. Hence these optical pulses have high tolerance to fiber chromatic dispersion. The W-band 10 Gb/s wireless data is transmitted and received via a pair of horn antennas. The received 10 Gb/s data is envelope-detected and then used to drive an optical modulator at the remote antenna unit (RAU) to produce the upstream signal sending back to the central office (CO). 20 km single mode fiber (SMF) error free transmission is achieved. Analysis about the optimum repetition rate of the optical pulse source and the transmission performance of the upstream signal are also performed and discussed.     

Performance enhancement of 10 Gb/s direct modulation optical OFDM by external optical injection

We show experimentally and by simulation a performance enhancement of a directly modulated 10 Gb/s optical Orthogonal Frequency Division Multiplexing (OFDM) system due to external optical injection. The experiment is performed back to back and over 12 km of single mode fiber. The injection extends the range of linear operation of the laser and therefore extends the usable bandwidth for direct modulation formats which are susceptible to nonlinearity, such as OFDM. Nonlinearity in the system and its reduction due to injection are estimated by means of a two tone test. Additionally the performance enhancement on OFDM systems was verified in both simulation and experimentally by the comparisons of the average Bit Error Rate (BER) and Error Vector Magnitude (EVM).     

Amplification of 12 × 10 Gb/s WDM signals with negligible FWM crosstalk in a double-pumped fiber optical parametric amplifier

We report on the amplification of 12 × 10 Gb/s wavelength-division-multiplexed signals by a double-pumped fiber optic parametric amplifier (2P-FOPA). A gain of 10 dB is obtained using a 4.3 km-long conventional dispersion shifted fiber (DSF) as nonlinear medium. Our spectra show negligible generation of spurious FWM products, and we attribute this to the small variations of the zero-dispersion wavelength of the DSF. The 2P-FOPA performance is assessed through Q-factor measurements, and we show that for output powers per channel ranging from −15 to 3 dBm the power penalty is less than 0.5 dB.     

Investigations on order and width of RZ super Gaussian pulse in different WDM systems at 40 Gb/s using dispersion compensating fibers

In optical wavelength division multiplexed (WDM) systems the dispersion management is a key issue. In optical systems a lot of research is going on to reduce dispersion by selecting proper dispersion compensating techniques and proper modulation format for input data. One way to reduce dispersion is by using dispersion compensating fibers in the WDM systems. This paper analyzes the use of RZ super Gaussian pulse inputs for different WDM systems i.e. for conventional, dense and ultra dense WDM systems employing dispersion compensating fibers. The pulse width and the order of the RZ super Gaussian pulse was varied to evaluate the performance at 40 Gb/s. The experiment showed that to get minimum BER, pulse width of 7.5 ps and 10 ps along with third-order RZ super Gaussian pulse were found suitable and recommended to be used.     

40 Gb/s optically transparent heterogeneous networks: A non-distributed dispersion map optimisation methodology

We describe a dispersion management methodology suitable for optimising lumped dispersion compensating schemes in heterogeneous terrestrial optical networks. The technique affords for a significant reduction in time expensive numerical simulations and is demonstrated to be effective for the transmission of 40 Gb/s DPSK in representative example networks made up of standard single-mode fibre.     

Phase encoded spread in time signals for suppression of propagation non-linearities in 40 Gb/s CDM/WDM communications systems using optical phase conjugation for dispersion compensation

The non-linear degradation along a RZ-WDM 40 Gb/s transmission link is shown to be reduced when a spectral direct sequence bipolar phase shift keying (DS/BPSK) encoding format is used. Standard single mode fibre transmission links are studied which use optical phase conjugation for dispersion compensation in the encoded system. For a WDM system with 100 GHz channel spacing, an improvement of 6 dB is found in the link output power margin at a distance of 300 km after five amplification stages.     

Effectiveness of electronic equalizer for filter concatenation effect of low-cost 2.5 Gb/s rated DML sources operated at 10 Gb/s

The improvement on the impact of filter concatenation effect on optical signal quality is investigated and discussed for applications in metropolitan optical networks utilizing cost-effective 10-Gb/s transmitters. The sources are low-cost conventional directly modulated lasers (DMLs), fabricated for operation at 2.5 Gb/s but modulated at 10 Gb/s. Performance improvement is achieved by using decision-feedback equalization (DFE) at the receiver end. Experimental studies consider both transient and adiabatic chirp dominated DMLs sources with different chirp characteristics. Measurements have been obtained using a recirculating loop set-up and the performance improvement is evaluated in terms of bit-error-rate (BER) versus number of loops.     

Analysis of 160 Gb/s all-optical NRZ-to-RZ data format conversion using quantum-dot semiconductor optical amplifiers assisted Mach-Zehnder interferometer

A scheme of all-optical data format conversion from nonreturn-to-zero to return-to-zero is proposed using quantum-dot semiconductor optical amplifiers (QD SOAs) assisted Mach-Zehnder interferometer (MZI). The proposed scheme has the potential to operate at much larger bit rate ∼160 Gb/s, and the converted signal has a lower frequency chirp. 160 Gb/s all-optical format conversion is verified through numerical simulations, and the output contrast ratio and Q-factor are analyzed to evaluate the system performance. With properly selected parameters, the converted signal with a contrast ratio over 8 dB and a Q-factor over 8 can be achieved.     

The system of L-band 2 × 10 Gb/s WDM transmission over conventional single mode fibre with 600 km by chirped fibre Bragg gratings dispersion compensation

A chirped fibre Bragg grating according to ITU-T suggested L-band （2nd channel λ1 = 1570.83 nm; 80th channel λ2 = 1603.57 nm） with more than 1800 ps/nm single channel dispersion compensation is presented in this paper, of which the cladding mode loss, the delay curve ripple and the power fluctuation of the reflected spectrum are less than 0.5 dB, 50 ps and 0.25 dB, respectively. With this new FBG as dispersion compensation device, a 2 × 10 Gb/s wavelength division multiplexing （WDM） L-band transmission of 600 km based on conventional single mode fibre （G.652 fibre） is performed without forward error correction. The bit error rate （BER） is less than 10-12 and the power penalties of the 2nd and 80th channel of L-band are 1.8 dB and 2.0 dB, respectively.     

A Clock Enhanced Loop for Simultaneous Error-Free Demultiplexing and Clock Recovery of 160Gb/s OTDM Signal Single-Channel Transmission over 100km

A simple clock enhanced loop of cascaded electro-absorption modulators （EAMs） and 10 GHz clock recovery modules is presented. The intensity of harmonic of clock-frequency component is analyzed theoretically and verified experimentally in a 160 Gb/s OTDM 100 km transmission system. The 10 GHz clock component is enhanced obviously before launching into the clock recovery module and the recovered clock signal exhibits low rms jitter of 〈400 fs. Moreover, completely error^-1free （10-12） transmission is observed for more than two hours without using forward error correction technology. The power penalty is about 3.6 dB. The proposed loop has merits of enhancing base clock component, simultaneously de-multiplexing and clock recovery, which make the performance of this loop more stable and high suppression of non-target channels.