NRZ versus RZ in 10-40-Gb/s dispersion-managed WDM transmissionsystems

We compare nonreturn-to-zero (NRZ) with return-to-zero (RZ) modulation format for wavelength-division-multiplexed systems operating at data rates up to 40 Gb/s. We find that in 10-40-Gb/s dispersion-managed systems (single-mode fiber alternating with dispersion compensating fiber), NRZ is more adversely affected by nonlinearities, whereas RZ is more affected by dispersion. In this dispersion map, 10- and 20-Gb/s systems operate better using RZ modulation format because nonlinearity dominates. However, 40-Gb/s systems favor the usage of NRZ because dispersion becomes the key limiting factor at 40 Gb/s     

An electroabsorption modulator module for digital and analogapplications

This paper presents an electroabsorption modulator (EAM) module for digital and analog (D/A) applications. Optically broad-band operation of the EAM module is studied for such digital application as wavelength division multiplexing (WDM) systems. Utilizing anisotropic electroabsorption of a multiple quantum-well (MQW) EAM, 40- and 100-nm bandwidth operations in 2.5-Gb/s digital signal transmission over 200-km standard fiber are confirmed by the experiments and the simulations, respectively. For analog applications, low distortion and high link gain characteristics of the EAM module are investigated at the wavelength of 1535 nm. High spurious-free dynamic range of 123 dB·Hz^4/5 and high link gains of -10.3 dB with matching circuit and -20.6 dB without matching circuit are obtained using the EAM module     

Wide Temperature Range Operation of a 1.55-$mu$m 40-Gb/s Electroabsorption Modulator Integrated DFB Laser for Very Short-Reach Applications

We present the uncooled operation of a 1.55- mum 40-Gb/s InGaAlAs electroabsorption modulator (EAM) integrated distributed-feedback (DFB) laser within a temperature range of 95degC (-15degC to 80degC ). To the best of our knowledge, this is the largest temperature range reported so far for such a 40-Gb/s EAM integrated DFB laser. We designed the EAM to operate at high speed by reducing the electrical parasitics, and we achieved a 3-dB frequency bandwidth of over 39 GHz for an EAM length of less than 150 mum. We demonstrated a 2-km single-mode fiber (SMF) transmission at 40-Gb/s over a wide temperature range of -15degC to 80degC by adjusting only the bias voltage to the EAM while keeping the modulation voltage swing constant at 2.0 V when the temperature changed. We achieved a dynamic extinction ratio of over 8.2 dB and a 2-km SMF transmission with a power penalty of less than 2 dB over a wide temperature range.     

EAM-integrated DFB laser modules with more than 40-GHz bandwidth

Electroabsorption modulator (EAM)-distributed feedback (DFB) modules with record-high bandwidth of 41 GHz have been developed. 40-Gb/s nonreturn to zero (NRZ) operation with 12-dB extinction ratio and -1.6-dBm average output power have been successfully achieved by optimizing the EAM length and detuning. A clearly opened eye diagram was maintained even after 80-km nonzero dispersion shifted fiber (NZ-DSF) transmission with dispersion compensation, 40-GHz short pulse trains with 6-ps pulsewidth have been also generated by sinusoidal electrical modulation     

Tunable dispersion compensation at 40-Gb/s using a multicavity etalon all-pass filter with NRZ, RZ, and CS-RZ modulation

We present a multichannel tunable dispersion compensator (TDC) based on multicavity all-pass etalons that is capable of operation at 40 Gb/s. The device has a tuning range of +200/-220 ps/nm with a group delay ripple < /spl plusmn/5 ps over a channel bandwidth of 80 GHz, an overall loss of < 5.2 dB, very low insertion loss ripple, and can operate on any channel on a 200-GHz grid over the C-band. In addition, we present system performance results at 40 Gb/s using NRZ, RZ, and CS-RZ modulation, compensating up to 45 km of nonzero dispersion shifted fiber (NZDSF). Our results show that this device introduces very little excess system penalty with signal frequency drifts of up to 20 GHz when operated near the center of its tuning range. For single channel experiments with fiber, the system penalty increase versus signal detuning is more significant, but can be reduced by dynamically optimizing the device dispersion during detuning. Finally, we demonstrate simultaneous compensation of 4 channels across the C-band over 25 km of NZDSF.     

Dispersion tolerance and transmission distance of a 40-Gb/sdispersion management soliton transmission system

We investigate the tolerance of the variation of average dispersion in a 40-Gb/s dispersion-managed soliton (DMS) transmission system. It is theoretically shown that dispersion tolerance is governed by pulse broadening and soliton interaction, and that the largest dispersion tolerance can be achieved by optimizing the pulse energy depending on the transmission distance. We construct a 40-Gb/s recirculating loop transmission system and show that the dispersion tolerance of over 180 ps/nm, which is much larger than that of a linear nonreturn-to-zero (NRZ) format system, can be realized by the optimization of the pulse energy at a transmission distance of more than 1000 km     

Application of a Prechirp Technique to 10-Gb/s Transmission at 1064 nm Through 24 km of Photonic Crystal Fiber

The prechirp effect on 10-Gb/s transmission in the 1000-nm band over photonic crystal fiber (PCF) was investigated both experimentally and theoretically. We performed transmission experiments using a 24-km-long PCF whose optical loss and chromatic dispersion were 0.94 dB/km and$-$20 ps/nm/km at 1064 nm, respectively. We confirmed an improvement in the bit-error-rate performance after the transmission, namely a “negative power penalty” of about$-$0.5 dB. Our experimental result and theoretical estimation revealed that the signal degradation induced by the chromatic dispersion can be effectively suppressed by employing the prechirp technique with a conventional$Z$-cut lithium niobate modulator.     

A Four‐Channel Laser Array with Four 10 Gbps Monolithic EAMs Each Integrated with a DBR Laser

A distributed Bragg reflector (DBR) laser and a high speed electroabsorption modulator (EAM) are integrated on the basis of the selective area growth technique. The typical threshold current is 4 to 6 mA, and the side mode suppression ratio is over 40 dB with single mode operation at 1550 nm. The DBR laser exhibits 2.5 to 3.3 mW fiber output power at a laser gain current of 100 mA, and a modulator bias voltage of 0 V. The 3 dB bandwidth is 13 GHz. A 10 Gbps non‐return to zero operation with 12 dB extinction ratio is obtained. A four‐channel laser array with 100 GHz wavelength spacing was fabricated and its operation at the designed wavelength was confirmed.     

Performance degradations of 2.4 Gbit/s NRZ/RZ lightwave systems dueto reflection-induced phase-to-intensity-noise conversion

Performance degradations in 2.4-Gb/s NRZ (nonreturn to zero) and RZ lightwave systems due to phase-to-intensity-noise conversion between two connectors have been evaluated using computer simulation techniques. Both NRZ and RZ systems have approximately the same penalty if the roundtrip time delay between the two connectors is an exact integer number of bits. If the roundtrip time delay is slightly offset, however, the RZ system penalty is significantly reduced. For example, the RZ system penalty is reduced from 3 dB (roundtrip delay between the two connectors=40 b) to 1.5 dB (roundtrip delay=40.5 b) for two connectors with 8-dB return loss each     

2500 km-10 Gbps RZ transmission system based on dispersion compensation CFBGs without electric regenerator

The characteristics of chirped fiber Bragg gratings (CFBGs) are optimized so that the ripple coefficient of the power reflectivity spectrum and group time delay are less than 1 dB and |± 15| ps, group delay is about 2600 ps/nm, polarization module dispersion is very small, PMD<2 ps, -3 dB bandwidth is about 0.35 nm, and insertion loss is about 4-5 dBm. Using dispersion compensation CFBG, a 2500 km-10 Gbps RZ optical signal transmission system on G.652 fiber was successfully demonstrated without an electric regenerator by optimizing dispersion management and loss management. The RZ optical signal was generated through a two-stage modulation method. At 2081 km, the power penalty of transmission is about 3 dB (conditions: RZ signal, BER = 10-12, PRBS = 1023 - 1); At 2560 km, the power penalty is about 5 dB. It is superior to the system using NRZ under the same conditions.     

40-Gb/s Low Chirp Electroabsorption Modulator Integrated With DFB Laser

A 40-Gb/s monolithically integrated transmitter containing an InGaAsP multiple-quantum-well electroabsorption modulator (EAM) with lumped electrode and a distributed-feedback semiconductor laser is demonstrated. Superior characteristics are exhibited for the device, such as low threshold current of 20 mA, over 40-dB sidemode suppression ratio at 1550 nm, and more than 30-dB dc extinction ratio when coupled into a single-mode fiber. By adopting a deep ridge waveguide and planar electrode structures combined with buried benzocyclobutene, the capacitance of the EAM is reduced to 0.18 pF and the small-signal modulation bandwidth exceeds 33 GHz. Negative chirp operation is also realized when the bias voltage is beyond 1.6 V.      

Fully electrical 40-Gb/s TDM system prototype based on InP HEMTdigital IC technologies

This paper presents a fully electrical 40-Gb/s time-division-multiplexing (TDM) system prototype transmitter and receiver. The input and output interface of the prototype are four-channel 10-Gb/s signals. The prototype can be mounted on a 300-mm-height rack and offers stable 40-Gb/s operation with a single power supply voltage. InP high-electron mobility transistor (HEMT) digital IC's perform 40-Gb/s multiplexing/demultiplexing and regeneration. In the receiver prototype, unitraveling-carrier photodiode (UTC-PD) generates 1 V_pp output and directly drives the InP HEMT decision circuit (DEC) without any need for an electronic amplifier. A clock recovery circuit recovers a 40-GHz clock with jitter of 220 fs_pp from a 40-Gb/s nonreturn-to-zero (NRZ) optical input. The tolerable dispersion range of the prototype within a 1-dB penalty from the receiver sensitivity at zero-dispersion is as wide as 95 ps/nm, and the clock phase margin is wider than 70° over almost all the tolerable dispersion range. A 100-km-long transmission experiment was performed using the prototype. A high receiver sensitivity [-25.1 dBm for NRZ (2⁷-1) pseudorandom binary sequence (PRBS)] was obtained after the transmission. The 40-Gb/s regeneration of the InP DEC suppressed the deviation in sensitivity among output channels to only 0.3 dB. In addition, four-channel 40-Gb/s wavelength-division-multiplexing (WDM) transmission was successfully performed     

On-the-Fly All-Optical Contention Resolution for NRZ and RZ Data Formats Using Packet Envelope Detection and Integrated Optical Switches

We present a packet-by-packet contention resolution scheme that combines packet detection, optical space switching, and wavelength conversion performed in the optical domain by integrated optical switches. The packet detection circuit provides the control signals required to deflect and wavelength-convert the contending packets so that all the packets are forwarded to the same output without any collision or packet droppings. We demonstrate the compatibility of the scheme with both nonreturn-to-zero (NRZ) and return-to-zero (RZ) modulation formats by recording error-free operation for 10-Gb/s NRZ and 40-Gb/s RZ packet-mode traffic     

Optical Signal-to-Noise Ratio Estimation Using Reference Asynchronous Histograms

This paper presents and experimentally demonstrates a novel method for the estimation of optical signal-to-noise ratio (OSNR) based on the comparison of an asynchronous histogram of the signal under analysis with a reference asynchronous histogram. The latter is acquired from the signal under analysis at a calibration stage. The proposed method allows the use of optical amplification to increase the sensitivity of the optical monitoring system (OMS) by a factor 20 dB, when using an erbium doped fiber preamplifier. In addition, the use of a semiconductor optical preamplifier, initially designed for nonlinear operation at 2.5 Gb/s is used in the OMS to preamplify 40-Gb/s signals, achieving a sensitivity gain of 10 dB. It will be experimentally demonstrated that the proposed method is applicable to 40-Gb/s nonreturn to zero (NRZ) signals arbitrarily degraded by group velocity dispersion (GVD). Furthermore, accurate monitoring of the OSNR of return-to-zero (RZ) signals will also be possible using a simple RZ-to-NRZ converter based on narrow-band optical filtering within the OMS. The proposed method also allows estimating of the GVD-induced OSNR penalty between the signal under analysis and the signal at the calibration stage.     

Simultaneous Compensation of Polarization Mode Dispersion and Chromatic Dispersion Using Electronic Signal Processing

In this paper, we study the performance of 10.7-Gb/s nonreturn-to-zero (NRZ) and return-to-zero (RZ) on-off keying signals in the joint presence of first-order polarization mode dispersion (PMD) and chromatic dispersion (CD) based on optical signal-to-noise ratio penalty measurements. Our investigations show that the tolerance of RZ to first-order PMD is severely reduced by the presence of typical values of residual CD. Three different receiver strategies are studied: 1. unequalized threshold detection; 2. combined feed-forward and decision-feedback equalization; and 3. maximum-likelihood sequence estimation (MLSE). In all three cases, the presence of CD eliminates the advantage of RZ over NRZ in terms of PMD tolerance. For NRZ, we find that the MLSE improves the tolerance to first-order PMD by 60%-70%, even in the presence of residual CD.     

Fabrication and Packaging of 40-Gb/s AlGaInAs Multiple-Quantum-Well Electroabsorption Modulated Lasers Based on Identical Epitaxial Layer Scheme

High-speed AlGaInAs multiple-quantum-well (MQW) electroabsorption modulated lasers (EMLs) based on an identical epitaxial layer (IEL) integration scheme are developed for 40-Gb/s optical fiber communication systems. A self-aligned planarization technique has been adopted to reduce the capacitance of the electroabsorption modulator (EAM). The IEL structure EML chips exhibit a small signal modulation bandwidth around 40 GHz. The influence of residual reflection at the modulator facet on the small signal modulation response is investigated. Submount containing a grounded coplanar waveguide (GCPW) transmission line is used for packaging the EML chips into transmitter modules. The optimization of the GCPW structure to suppress resonances in frequency response due to parallel-plate modes is presented. Clear eye opening under 40-Gb/s nonreturn-to-zero (NRZ) modulation has been demonstrated for the packaged EML module.     

All-Raman ultralong-haul single-wideband DWDM transmission systems with OADM capability

In this paper, we present a comprehensive experimental investigation of an all-Raman ultrawide single-band transmission system for both 10 and 40 Gb/s line rates. Enabling technologies include forward-Raman pumping of the transmission fiber, counter-Raman pumping of the fiber spans and dispersion compensation modules, wideband dispersion, and dispersion-slope compensation, and modulation formats resistant to both linear and nonlinear impairments. Ultralong-haul (ULH) 128/spl times/10 Gb/s return-to-zero (RZ) and ultrahigh-capacity (UHC) 64/spl times/40 Gb/s carrier-suppressed (CS) RZ transmission are demonstrated for commercially deployed fiber types, including both standard single-mode fiber (SSMF) and nonzero dispersion shifted fibers (NZDSF). The span losses of 23 dB (NZDSF) and 20 dB (SSMF) are consistent with those encountered in terrestrial networks. The optical reaches for 10 Gb/s rate are 4000 km (NZDSF) and 3200 km (SSMF). Using the same distributed Raman amplification (DRA) scheme, UHC over 2.5 Tb/s at a 40-Gb/s per channel rate is also demonstrated for all of the tested fiber types and for optical reaches exceeding 1300 km. We then study the impact of including optical add/drop modules (OADMs) in the transmission system for both 10 and 40 Gb/s channel rates. System performance is characterized by the system margin and the transmission penalty. For all of the experiments shown in this paper, industrial margins and small transmission penalties consistent with operation in commercially deployable networks are demonstrated, showing the feasibility of practical implementation of all-Raman amplified systems for ULH and UHC optical backbones. Attractive features of single-wideband transmission enabled by DRA include simplicity of design, flexible gain and gain-ripple control, good noise performance, and a small system footprint.     

Optically preamplified receiver performance due to VSB filtering for 40-Gb/s optical signals modulated with various formats

Optically preamplified receiver performance according to the vestigial sideband (VSB) filtering has been numerically investigated for 40-Gb/s optical signals modulated with nonreturn-to-zero, duobinary nonreturn-to-zero (NRZ), return-to-zero (RZ), carrier-suppressed RZ, and duobinary carrier-suppressed RZ formats. The VSB filtering enables the spectral widths of NRZ, duobinary NRZ, and RZ signals to be reduced without severe power penalties at the receiver. On the other hand, carrier-suppressed RZ and duobinary carrier-suppressed RZ signals have no large advantages over VSB filtering because of the characteristics of their signals. Our results suggest that RZ signals are the most suitable modulation format for VSB filtering, without considering the filter loss, because of the tolerance of the intersymbol interference and a large spectral width. However, duobinary NRZ signals are the most suitable modulation format for VSB filtering, considering the filter loss, because of their narrow spectral width.     

Dispersion penalty reduction using an optical modulator with adjustable chirp

Using a unique Ti:LiNbO/sub 3/ modulator, the value of the modulation chirp parameter that minimizes the transmission power penalty caused by fiber chromatic dispersion was experimentally identified. System experiments at 5 Gb/s using nonreturn-to-zero (NRZ) amplitude-shift-keyed (ASK) transmission with direct detection reception are discussed, and the optimum values of the modulation chirp parameter versus distance for transmission at 1.5 mu m wavelength over fibre having zero dispersion at 1.3 mu m are identified. 5 Gb/s NRZ transmission was achieved through distances of 128, 192, and 256 km of conventional fiber while incurring dispersion penalties of -0.5, 0.1, and 1.1 dB respectively, by operating at the quantum chirp value.<>     

Numerical comparison between distributed and discrete amplificationin a point-to-point 40-Gb/s 40-WDM-Based transmission system with threedifferent modulation formats

We describe a detailed numerical investigation on the relative merits of gain flattened distributed Raman amplification (DRA) and discrete gain flattened amplifiers. We simulate a system with forty 40-Gb/s channels spaced at 100 GHz and compare the performance of three different modulation formats nonreturn-to-zero (NRZ), return-to-zero (RZ) and carrier-suppressed RZ (CS-RZ). Three types of amplifiers, multifrequency backward- and forward-pumped DRAs, and an idealized discrete gain flattened amplifier are examined for various signal powers and transmission distances. For the backward-pumped DRA, we also describe calculated tolerance limits imposed by incomplete dispersion slope compensation and polarization mode dispersion (PMD) level