Long-haul Optical Communication Systems Research Articles

Irregular Low-Density Parity-Check Codes for Long-Haul Optical Communications

A novel forward error correction (FEC) scheme for long-haul optical communication systems based on irregular low-density parity check codes and iterative decoding is proposed. The proposed FEC scheme for long-haul optical transmission significantly outperforms all previously reported FEC schemes, and it has a potential for achieving theoretical limits on reliable transmission through an optical communication channel.

Projective geometry LDPC codes for ultralong-haul WDM high-speed transmission

In this letter, we investigate the performance of low-density parity-check (LDPC) codes in long-haul optical communication systems for carrier-suppressed return-to-zero signal format at 40 Gb/s per channel in a wavelength-division-multiplexing environment. We are particularly concerned with high-rate codes based on projective geometries. These codes have large minimum distance and simple iterative decoding algorithms, which makes them good candidates for such high-speed application. We consider iterative decoding based on min-sum algorithm since it requires only simple addition and minimum operations and, as such, is suitable for high-speed optical transmission. Contrary to the common practice of considering the performance of error controlling schemes using the additive white Gaussian noise channel assumption, we consider the performance of the proposed LDPC schemes, taking into account in a natural way all major impairments in long-haul optical transmission such as amplifier spontaneous emission noise, pulse distortion due to fiber nonlinearities, chromatic dispersion or polarization dispersion, crosstalk effects, intersymbol interference, etc.

Low-density parity check codes and iterative decoding for long-haul optical communication systems

A forward-error correction (FEC) scheme based on low-density parity check (LDPC) codes and iterative decoding using belief propagation in code graphs is presented in this paper. We show that LDPC codes provide a significant system performance improvement with respect to the state-of-the-art FEC schemes employed in optical communications systems. We present a class of structured codes based on mutually orthogonal Latin rectangles. Such codes have high rates and can lend themselves to very low-complexity encoder/decoder implementations. The system performance is further improved by a code design that eliminates short cycles in a graph employed in iterative decoding.

40 Gb/s postamplifier and PIN/preamplifier modules for next generation optical front-end systems

Postamplifier and p-i-n/preamplifier modules suitable for 40-Gb/s short-haul and long-haul optical communication systems are presented. The postamplifier module consists of two stages of pseudomorphic high-electron mobility transistor (PHEMT) GaAs amplifiers mounted in a housing. A test fixture is used to provide bias supplies that control the output signal. The postamplifier has a 20-dB gain and 4.5-dB noise figure. The input voltage swing of the postamplifier can be varied from 10 to 148 mV p-p while providing an open output eye-pattern response over the complete range. The p-i-n/preamplifier module consists of a waveguide p-i-n/photodetector, with a polarization insensitive spot-size converter and a PHEMT-based GaAs transimpedance amplifier. The components are integrated inside a hermetically sealed compact package using hybrid techniques. The p-i-n/preamplifier module having 150-/spl Omega/ transimpedance exhibits a linear output voltage swing over 800 mV p-p and a responsivity of 0.8 A/W. The receiver module has a 3-dB bandwidth as high as 45 GHz, and the input equivalent noise current density is less than 15 pA//spl radic/(Hz). The electrical sensitivity of the postamplifier is found to be 7.8 mV p-p for a 40 Gb/s, 2/sup 7/-1 pseudorandom binary sequence nonreturn-to-zero signal. When used in combination with a 150-/spl Omega/ 40-Gb/s p-i-n/preamplifier module, a sensitivity of less than -11.2 dBm is achieved. These results indicate that the postamplifier module, in combination with the p-i-n/preamplifier module, can be used in 40-Gb/s short-haul systems, while the p-i-n/preamplifier module itself can be used for long-haul front-end receiver systems.

Low-density parity check codes for long-haul optical communication systems

Forward error correction (FEC) scheme based on low density parity check codes (LDPC) codes is presented in this paper. We show that LDPC codes provide a significant system performance enhancement with respect to the state-of-the-art FEC schemes employed in optical communication systems.

A Novel PSK Homodyne Optical Receiver for Long Haul Communications

In this paper a novel balanced optical phase locked loop has been proposed for the reception of PSK signals in long-haul optical communication systems. The effect of loop propagation delay, which cause serious restriction on the requirement of laser linewidth, have been considerably reduced in this new loop. Moreover, the data-to-phase-lock cross-talk effect has been eliminated. The performance of the new loop has been studied when the loop is preceded by optical amplifiers.

Recirculating loop for experimental evaluation of EDFA saturated regime effects on optical communication systems

We demonstrate an optical-fiber recirculating loop for experimental simulation of long-haul optical communication systems using cascaded erbium-doped fiber amplifiers (EDFA's) operating in the gain saturation regime. The loop contains sections of dispersion shifted fibers (DSF's), standard fiber, and a set of in-line devices, such as tuning filters, optical amplifiers, polarization controllers, and a variable attenuator. The main results presented here are related to the observation of the effects due to the slow dynamics of the EDFA. We also discuss the validity of using an optical attenuator to simulate an extra length of fiber.

Suppression of soliton jitter by a copropagating support structure

A new way to suppress the soliton’s jitter in long-haul optical communication systems is proposed that does not use in-line filters. Parallel to the information-carrier soliton stream, a periodic stiff array (consisting of bright or dark solitons) is launched in another mode, which may be either an orthogonal polarization or, more realistically, another wavelength. This support array induces, through the cross-phase modulation, an effective potential pinning of the signal solitons. Most promising is the scheme in which the support structure is an array of dark solitons (while the information is carried by the bright ones). In the analytical part of the work we derive a Langevin equation for the information soliton in the presence of the induced potential and of a random force representing the jitter, for both cases when the support channel has anomalous or normal dispersion. Next, we derive the corresponding Fokker–Planck equation. A solution to this equation explicitly demonstrates a strong suppression of the jitter in comparison with the Gordon–Haus limit. To check this mechanism directly we perform numerical simulations of the coupled nonlinear Schrodinger equations for the information and support channels, including the jitter-generating random perturbations acting in both of them. The simulations demonstrate that, while in the absence of the support structure the soliton’s jitter is growing in accord with the Gordon–Haus law, the support structure strongly suppresses the jitter, as is predicted by the analysis.

Design of multiwavelength optical communication systems based on an integrated waveguide amplifier compensator

We theoretically analyze the performance of longhaul multichannel optical communication systems based on an integrated WDM power level compensator. By placing an array of Er/sup 3+/-Yb/sup 3+/ co-doped silica waveguides in parallel between a demultiplexer/multiplexer pair and preceeding it by an EDFA, we demonstrate the possibility of transmitting eight unequally spaced WDM channels over long distances, without electronic regenerators and using span lengths as high as 50 km. This technique allows substantial reduction of both four-wave mixing crosstalk and gain peaking.

WDM systems with unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A method is discussed to find non-uniform channel separations for which no four-wave mixing product is superimposed on any of the transmitted channels, therefore suppressing FWM crosstalk. The residual crosstalk, due to channel power depletion only, is analytically evaluated for intensity-modulated repeaterless wavelength-division-multiplexed (WDM) systems and compared to experimental results. The theory includes the effect of the channel depletion on the amplitude of each phase-matched FWM wave. The probability of error is evaluated including the statistics of the pattern dependent channel depletion. The BER curve computed for an 8-channel WDM system is found to be in good agreement with experimental results. In the experiment, repeaterless transmission of eight 10 Gb/s WDM channels over 137 km (11 Tb/s-km) of dispersion-shifted fiber was demonstrated and error-free operation was achieved over a wide range of input powers using unequally spaced channels. The same system with equally spaced channels could not achieve a probability of error lower than 10/sup -6/. The use of unequal channel spacing allowed fiber input power to be increased by as much as 7 dB, which could be translated into a fivefold increase of the bit rate per channel (and therefore of the system capacity), or to an increase in the system length of about 30 km. >

Reduction of four-wave mixing crosstalk in WDM systems using unequally spaced channels

Crosstalk due to four-wave mixing (FWM) is the dominant nonlinear effect in long-haul multichannel optical communication systems employing dispersion-shifted fiber. A technique to design the channel frequency allocation in order to minimize the crosstalk due to FWM is presented. It is shown that suitable unequal channel separations can be found for which no four-wave mixing product term is superimposed on any of the transmitted channels. This is obtained at the expense of some expansion of the system bandwidth. Simulations are presented to show the effectiveness of this technique in a 10-channel, 10-Gb/s per channel, system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Simulation of Dispersion Tolerant Long-Haul Optical Fiber Communication Systems with Distributed In-Line Group-Delay Equalizers

Article Simulation of Dispersion Tolerant Long-Haul Optical Fiber Communication Systems with Distributed In-Line Group-Delay Equalizers was published on December 1, 1994 in the journal Journal of Optical Communications (volume 15, issue 6).

High-speed optical receiver with soft decision ISI cancellation

Due to the stimulating success in developing optical amplifiers, intersymbol interference (ISI) will become the domi- nant limit on both bit rate and transmission distance in long-haul optical fiber communication systems. Here we present a receiver with Soft Decision IS1 Cancellation (SDIC) to deal with IS1 in high-speed lightwave systems. The analytical and simulation results show that the transmission distance can be doubled with SDIC. In addition, the receiver with SDIC allows the IS1 cancellation circuit operating with processing and propagation delay much longer than 1-b period. Therefore, it is very promising in very high bit rate optical systems.

Erratum: Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Erratum: Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Signal linewidth broadening due to nonlinear Kerr effect in long-haul coherent systems using cascaded optical amplifiers

Signal spectral linewidth broadening due to the interaction between nonlinear Kerr effect and spontaneous emission from optical amplifiers in long-haul coherent optical fiber communication systems using optical amplifiers is examined theoretically and experimentally. By theoretical consideration and recirculating-loop experiments, it is shown that this effect becomes an essential limitation on maximum transmission length in transoceanic coherent optical fiber submarine cable systems. >

Change of field spectrum of signal light due to fibre nonlinearities and chromatic dispersion in long-haul coherent systems using in-line optical amplifiers

The change of a field spectrum of signal light in long-haul coherent optical fibre communication systems using in-line amplifiers is measured in the normal and anomalous dispersion regions of a fibre. Results show that the interplay of nonlinear Kerr effect and fibre chromatic dispersion cause a fatal change in the field spectrum in the anomalous dispersion region.

Observation of low-chirp modulation in InGaAs-InAlAs multiple-quantum-well optical modulators under 30 GHz

Modulated light spectra were measured in long-wavelength InGaAs-InAlAs multiple-quantum-well intensity modulators under 30-GHz large-signal modulations. The linewidth broadening factor alpha is determined from the relation between the intensity modulation index and the sideband strength relative to the carrier. The minimum alpha value is estimated to be 0.70 at 1.54 mu m, which is almost the same as the lowest value so far reported in a bulk Franz-Keldysh modulator. This is significantly lower than what is obtained from direct-intensity modulation of injection lasers, making this a useful device for application to high-bit-rate long-haul optical communication systems.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Signal linewidth broadening due to fibre nonlinearities in long-haul coherent optical fibre communication systems

Signal spectral linewidth broadening due to the Kerr effect in long-haul coherent optical fibre communication systems is examined theoretically and experimentally. By recirculating loop experiments, it is shown that linewidth broadening becomes serious in a system of more than a few thousand kilometres.

Long-haul coherent optical fiber communication systems using optical amplifiers

Studies on long-haul coherent optical fiber communication systems with in-line optical amplifier repeaters are made theoretically and experimentally. By theoretical calculation it was found that coherent systems can achieve wider dynamic range for an amplifier input power as compared with the intensity-modulation direct-detection (IM-DD) systems. The feasibility of such systems using traveling-wave semiconductor laser amplifiers (TWSLAs) and erbium-doped fiber amplifiers (EDFAs) was investigated, and 546 km, 140 Mb/s CPFSK transmission using TWSLAs and 1028-km, 560-Mb/s CPFSK transmission using EDFAs were successfully demonstrated. >

Optics and communications products

Mitsubishi announce a new high-efficiency distributed feedback laser diode module for use in high-capacity long-haul analogue optical communications systems. The FU65SDF-3 has its peak emission wavelength in the 155 μm band and offers a high speed response of 2 GHz (min.). The module couples the output into a monomode fibre 'pigtail' for direct connection into optical systems. Optical output from the fibre end is 4 mW. Also included in the module are photodiodes for the monitoring of optical output. These have a forward current rating of 2 mA, and a reverse voltage rating of 20 V. Features include an integral thermal electric cooler, for efficient temperature control. T1