In order to achieve high-speed transmission over optical transport networks (OTNs) and maximize its throughput, we propose using a rateadaptive polarization-multiplexed coded multilevel modulation with coherent detection based on component non-binary quasi-cyclic (QC) LDPC codes. Compared to prior-art bit-interleaved LDPC-coded modulation (BI-LDPC-CM) scheme, the proposed non-binary LDPC-coded modulation (NB-LDPC-CM) scheme not only reduces latency due to symbol-instead of bit-level processing but also provides either impressive reduction in computational complexity or striking improvements in coding gain depending on the constellation size. As the paper presents, compared to its prior-art binary counterpart, the proposed NB-LDPC-CM scheme addresses the needs of future OTNs, which are achieving the target BER performance and providing maximum possible throughput both over the entire lifetime of the OTN, better.

the value corresponding to the selected reference point. This was realized by detecting the averaged intensity of the pulse sequence through the etalon, and by applying this signal in a feedback loop to control the temperature of the silicon chip. The control method relies on the average power staying constant during the measurement. The feedback signal was generated by modulating the pulse width of an external frequency generator by the detected average intensity, and by using a microcontroller to generate a current proportional to the width of the pulse. The microcontroller was programmed to perform all of the operations needed in the measurements. Ž . First, the transmission spectrum of the etalon see Fig. 1 is scanned by applying a high heating current. During the scan, the average power of the laser should stay constant to avoid errors when the reference points are determined. The value for the transmission at the maximum and minimum points T and T are stored, and the desired reference point T max min 0 Ž . is calculated. We chose the reference point T see Fig. 1 The performance of the device was tested by measuring the frequency chirp of a DFB laser that was modulated using a pseudorandom-bit sequence at the rate 2.5 Gbitsrs. The time trace of the detected signal intensity for a 12 bit sequence ''010110011101'' and the corresponding time-resolved frequency chirp are shown in Figure 3. The average power of the modulated laser output was y0.8 dBm. The adiabatic part of the chirp varies linearly with the signal power, but there are also larger frequency transients present at the fast changes of the optical power. The detected time trace of the signal is an average of 64 values at each measurement point. CONCLUSIONWe have developed and demonstrated a simple and inexpensive device for measurements of the time-resolved frequency chirp in narrowband light sources used for telecommunication purposes. The device makes use of a solid silicon wafer as a frequency discriminator to convert fluctuations in the laser frequency into variations in the transmitted signal intensity. The transmission of the etalon is tuned by controlling the refractive index of silicon by changing the temperature of Figure 3 Time traces of the modulated laser output power and the measured frequency chirp for a DFB laser operating at 1.55 m the chip. The FSR of 105 GHz allows frequency chirp up to "25 GHz to be measured with a time resolution of about 20 ps. The attractive features of the chirp analyzer include a broad wavelength range, insensitivity to mechanical vibrations, and to the polarization state of the light.

A simple and non-blocking polarisation-mode dispersion (PMD) monitoring technique using coherent detection is demonstrated and applied to a 40 Gbit=s live fibre-optic system. The results demonstrated a clear correlation between the instantaneous differential group delay and the receiver Q-margin. It was also demonstrated that PMDinduced outage can be eliminated by an adaptive PMD compensator.Introduction: In high-speed fibre-optic communication systems, polarisation-mode dispersion (PMD) is one of the most important factors of performance degradation. In-situ monitoring of PMD in live, multichannel WDM systems will be a key requirement for future dynamic optical networks to ensure quality of operation. Especially, for optical links with exceptionally high PMD values, an in-situ monitoring of PMD during operation will help network engineers to determine the impact of PMD and to select wavelength bands where PMD appears less damaging [1]. Many PMD measurement techniques have been proposed and demonstrated during the last two decades; the most popular include the fixed analyser method, the Jones matrix method, the Poincare arc method and the pulse delay method [2]. All these methods were developed for PMD characterisation instrumentation, which usually require access to both ends of an optical fibre cable to be measured. In live optical networks with installed optical fibres, the source and receiver are at distance and not accessible at the same time. We have recently introduced a novel and simpler method to evaluate differential group delay (DGD) using coherent detection and RF signal processing. This method utilises the spectral characteristics of the digital signal carried by each wavelength channel and measures the PMD-induced polarisation walk-off between the carrier and the clock components [3]. By tuning the wavelength of the local oscillator, the measurement can be performed across various channels in a WDM optical network.In [3], the PMD monitoring technique was briefly introduced and tested with a fixed PMD emulator. In this Letter, we provide a more detailed signal processing algorithm and report the results of our field measurement applying this technique to a 200 km OC768 SONET optical link. Most importantly, we show that this PMD monitoring technique does not rely on the existence of clock components of the signal and therefore it is independent of the optical modulation format. The results demonstrate clear correlation between the instantaneous DGD and the Q-margin at the receiver. We also demonstrate that PMDinduced outage can be eliminated by an adaptive PMD compensator.

Abstract-The development of 100 Gb/s transponder technology is progressing rapidly to meet the needs of next-generation optical/IP carrier networks. In this paper, we describe the upgrade of an installed 10 Gb/s field system to 100 Gb/s using a real-time single-carrier, coherent 100 Gb/s polarization-multiplexed quadrature-phase-shift keyed channel. Performance sufficient for error-free operation after forward-error-correction was achieved over installed 900 and 1800 km links, proving the viability of 100 Gb/s upgrades to most installed systems. Excellent tolerance to fiber polarization mode dispersion and narrowband optical filtering demonstrates the applicability of this technology over the majority of installed fiber plant and through existing 50 GHz reconfigurable optical add/drop multiplexers.Index Terms-Communication system performance, optical fiber communication, polarization multiplexing, quadrature phase-shift keying, signal processing.

Abstract-The parity-check matrix of a nonbinary (NB) low-density parity-check (LDPC) code over Galois field GF( ) is constructed by assigning nonzero elements from GF( ) to the 1s in corresponding binary LDPC code. In this paper, we state and prove a theorem that establishes a necessary and sufficient condition that an NB matrix over GF( ) constructed by assigning nonzero elements from GF( ) to the 1s in the parity-check matrix of a binary quasi-cyclic (QC) LDPC code, must satisfy in order for its null-space to define a nonbinary QC-LDPC (NB-QC-LDPC) code. We also provide a general scheme for constructing NB-QC-LDPC codes along with some other code construction schemes targeting different goals, e.g., a scheme that can be used to construct codes for which the fast-Fourier-transform-based decoding algorithm does not contain any intermediary permutation blocks between bit node processing and check node processing steps. Via Monte Carlo simulations, we demonstrate that NB-QC-LDPC codes can achieve a net effective coding gain of 10.8 dB at an output bit error rate of 10 12 . Due to their structural properties that can be exploited during encoding/decoding and impressive error rate performance, NB-QC-LDPC codes are strong candidates for application in optical communications.