In this paper, we propose a linear polarization coding scheme (LPC) combined with the phase conjugated twin signals (PCTS) technique, referred to as LPC-PCTS, for fiber nonlinearity mitigation in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The LPC linearly combines the data symbols on the adjacent subcarriers of the OFDM symbol, one at full amplitude and the other at half amplitude. The linearly coded data is then transmitted as phase conjugate pairs on the same subcarriers of the two OFDM symbols on the two orthogonal polarizations. The nonlinear distortions added to these subcarriers are essentially anti-correlated, since they carry phase conjugate pairs of data. At the receiver, the coherent superposition of the information symbols received on these pairs of subcarriers eventually leads to the cancellation of the nonlinear distortions. We conducted numerical simulation of a single channel 200 Gb/s CO-OFDM system employing the LPC-PCTS technique. The results show that a Q-factor improvement of 2.3 dB and 1.7 dB with and without the dispersion symmetry, respectively, when compared to the recently proposed phase conjugated subcarrier coding (PCSC) technique, at an average launch power of 3 dBm. In addition, our proposed LPCPCTS technique shows a significant performance improvement when compared to the 16-quadrature amplitude modulation (QAM) with phase conjugated twin waves (PCTW) scheme, at the same spectral efficiency, for an uncompensated transmission distance of 2800 km.
KEYWORDS: Forward error correction, Digital signal processing, Transmittance, Optical networks, Telecommunications, Optical communications, Transponders, Remote sensing, Fluctuations and noise, Head
This paper will at first explain the requirement of high speed optical transport network
on forward error correction (FEC) codes in terms of code length, code rate, coding gain, burst
error correction capability, error floor, latency, coding/decoding complexity. Then, a few code
schemes used in current optical transport systems such as Reed-Solomon codes recommended by
ITU-T G.709 and enhanced FECs listed in ITU-T, G.975.1 are introduced. Advanced codes
recently developed by vendors used for 100Gbps systems and their performances are summarized.
Features and special requirements on soft decoding FEC (SDFEC) especially inter-working
between SDFEC and equalizer, with and without deferential coding etc. are analyzed. Some
perspectives of future FEC for optical transport are also given.
KEYWORDS: Forward error correction, Digital signal processing, Receivers, Signal detection, Polarization, Signal processing, Monte Carlo methods, Transmittance, Transmitters, Modulators
The OSNR margin improvements due to FEC limit elevation in 112Gb/s PDM-DQPSK systems under two different
receiver schemes, direct detection and coherent detection with digital signal processing (DSP), are analyzed. FEC limit
elevation provides more benefit to the direct detection scheme than to the coherent detection scheme in 112Gb/s
PDMRZ-DQPSK system. For the cases of non-ideal receivers employment and long haul fiber link transmission, FEC
performance improvement is more profitable than that for the ideal BtB case.
The design criteria of the Panda-type erbium-doped polarization-maintaining fiber (EDPMF) are presented, which take into account the cutoff wavelength, mode field diameter, modal birefringence and background loss. The structural parameters are optimized in terms of the design criteria. A Panda-type EDPMF has been manufactured. The fabrication process and the parameter control of the Panda-type EDPMF are in detail described. Its refractive index profile, birefringence and absorption spectra are experimentally investigated.
The finite element method with 2nd-order transparent boundary conditions is proposed to analysis the modes of photonic crystal fibers. These boundary conditions preserve the sparse matrix and offering accuracy of O(r-9/2), here r denoting the position of the computational boundary with respect to a chosen origin of the structure. The effective mode-index, the mode distribution, the confinement loss and the dispersion properties of the total internal reflection photonic crystal fibers (TIR-PCF) with triangle lattices and rectangle lattices are calculated in this method respectively. The results agree with the published results. At the same time the finite element method in the paper lessens the computation domain remarkably. It offers an easy and credibility approach in the study on the PCF and other fibers with irregular cross-section index distribution, even fibers with anisotropy materials.
Erbium-doped photonic crystal fiber (EDPCF) is not in the endless single-mode as the refractive index of the core in EDPCF is higher than that of silica cladding. There is a variation between the EDPCF and the conventional PCF. The modified average population inversion iteration method is proposed for simulating the gains and noises of EDPCF amplifiers. The effect of the structural parameters of EDPCF on the cutoff wavelengths, splice loss and the amplification properties is studied in detail by means of the improved average population iterative method combined with the finite element method. According to the design criteria of erbium-doped fiber, the four structural parameters of EDPCF-core radius, the refractive index difference between the core and silica cladding, the relative size of the core and the relative size of air holes are optimized.
Kanakidis et al presented several kinds of all-optical chaotic communication systems using two encoding techniques and various dispersion compensation maps [1]. It shows that the permitted transmission distances are different for various dispersion compensation maps and various encoding techniques. In order to explore the upper limits of the transmission distance, the parameters of the all-optical chaotic communication system introduced by D. Kanakidis et al. [1] is optimized using genetic algorithm. Some useful results are presented.
Hole-assisted lightguide fiber (HALF) is a microstructured fiber composed of a high index core, a low index cladding and a small number of air holes surrounding the core. The characteristics of HALF are studied by using the full-vector finite element method. The contour lines of power flow intensity and transverse electric distributions are plotted for the fundamental mode and the first four higher order modes. The effect of the structural parameters, such as hole-to-core spacing and relative size of air holes to the cutoff wavelengths of fundamental mode and higher order mode, core power confinement factor, and mode field diameter is analyzed. Due to these holes, there are variations between HALF and conventional step-index fiber. It's found that when the distance between the air holes and core is shorter or the relative size of air holes becomes larger, the effect of air hole on the HALF turns greater: core power confinement factor becomes larger; mode diameter becomes smaller; the cutoff wavelengths of fundamental mode and higher order mode move to shorter wavelength. In terms of waveguide design, the structural parameters of erbium-doped HALF are optimized to obtain high-efficiency operation of a fiber laser or amplifier.
Bragg fibres have many special characteristics. Therefore this kind of fibre attracts more and more attention. In this paper, genetic algorithm is applied to design Bragg fibres to realise desired dipersion and attenuation characteristics.
The asymmetry index profile in the transverse plane, which could be induced during the writing of fiber Bragg gratings with UV side-exposure techniques, may cause the photo-induced birefringence, as was numerically evaluated by Kokou Dossou et al. But the impact of this photo-induced birefringence on the performance of fiber grating components remains to be determined. In this paper, we numerically analyzed this kind of impact by using a finite-element method with a full vectorial formulation and the coupled-mode equations. It shows how the birefringence affects the reflection spectrum, the group delay.
This numerical method will be useful in designing and analyzing fiber gratings.
In this paper, we demonstrate a chirped grating designed to compensate for both second- and third-order fiber dispersion. The fabrication technique of chirped gratings is the phase-mask beam scanning method by using a standard unchirped phase mask and by tapering a fiber in the region of the grating. We have made theoretical analysis on the chirp induced by linear and nonlinear taper profiles. The reflectivity and time delay curve of such fiber Bragg grating have been calculated.
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