Harshad P. Sardesai

Code-division multiple-access encoding and decoding of femtosecond optical pulses over a 2.5-km fiber link

We report for the first time proof-of-concept transmission experiments for femtosecond pulse code division multiple access (CDMA) operating over kilometer lengths of optical fiber in the 1.5-/spl mu/m communication band. Our CDMA link consists of a femtosecond mode-locked Er-fiber laser and two chirped pulse fiber amplifiers, a pair of low-loss fiber-pigtailed pulse shapers for encoding and decoding, a 2.5-km dispersion-compensated transmission fiber, and an ultrafast nonlinear optical receiver. Our results demonstrate high fidelity transmission of spectrally encoded femtosecond pulses over 2.5 km of fiber, followed by the use of the nonlinear CDMA receiver to discriminate between correctly and incorrectly decoded signals with 20-dB contrast.

Effects of self-phase modulation on sub-500 fs pulse transmission over dispersion compensated fiber links

The effects of nonlinearity on sub-500 fs pulse transmission over dispersion compensated fiber links using dispersion compensating fiber technique are investigated numerically and experimentally. The pulse broadening and recompression ratio of the 2.5-km transmission link is over 300. The postcompensated and precompensated links are compared when the input pulse energy ranges from 15 to 159 pJ. At high powers, self-phase modulation (SPM) degrades the pulse recompression process and provides an upper bound on the transmitted pulse energy. The SPM effect is stronger in the postcompensated link than in the precompensated link. A dramatic spectral narrowing effect was observed in the postcompensated link. Pulse energies up to tens of pJ, consistent with high quality communication, should be possible for a sub-500 fs pulse in such dispersion compensated links.

Measurement and system impact of multipath interference from dispersion compensating fiber modules

Dispersion compensating modules (DCM) are key components in long distance and high-speed fiber-optic transmission systems. However, many types of DCMs exhibit multipath interference (MPI) that acts as noise in transmission and degrades the bit error rate of the system. Further, MPI generated in DCMs is mostly due to forward scattering and mostly of the coherent type. This type of MPI cannot be correctly measured using conventional measurement techniques that use electrical spectrum analysis to measure post-detection noise. We demonstrate an accurate and simple method of MPI measurement using a swept wavelength system apparatus. We discuss the strengths and limitations of this measurement method and show system results of Q penalty due to MPI from concatenated DCMs.

Impairment-Aware Lightpath Routing and Regenerator Placement in Optical Transport Networks With Physical-Layer Heterogeneity

We develop a framework that supports impairment-aware lightpath routing and wavelength assignments in optical transport networks. Different from most existing studies, we consider a more generic optical transport network with physical-layer heterogeneity, including different fiber types, variable amplification span distances and attenuation coefficients. In addition, rather than a single amplifier type as in most of the existing studies, we consider multiple amplifier types for different amplification situations. Owing to the high cost of OEO regeneration, the total number of required regenerators is considered as the major objective for optimization. A signal-quality-aware routing algorithm is developed to find routes that are expected to require the fewest regenerators. The first-fit wavelength assignment algorithm is extended to assign wavelength(s) for lightpaths after placement of some regenerators which can freely function as wavelength converters. Simulation studies indicate that the proposed algorithm can significantly reduce the required number of regenerators compared to the simple shortest-path routing algorithm. Moreover, it is found that the signal-quality-aware algorithm shows stronger benefits when a network demonstrates higher physical-layer heterogeneity such as different fiber types and non-uniform span losses. The signal-quality-aware algorithm also demonstrates better performance when a network has a higher average nodal degree. Finally, the results indicate that multiple amplifier options are important for cost-effective optical transport network design. For a network with high physical-layer heterogeneity, multiple amplifier options can significantly reduce the required number of regenerators (up to 50%) over a single amplifier option.

Design of Polarization De-Multiplexer and PMD Compensator for 112 Gb/s Direct-Detect PDM RZ-DQPSK Systems

The design of a polarization de-multiplexer and a polarization mode dispersion compensator (PMDC) for direct-detect polarization division multiplexed (PDM) return-to-zero differential quadrature phase shift keying (RZ-DQPSK) systems are studied in detail. The impact of polarization dependent loss is studied in polarization de-multiplexers with different error detection configurations for both bit-aligned and bit-interleaved PDM systems. The level of the clock frequency of the combined pulse train of the two polarizations is proposed as the error signal for the PMDC. It enables the PMDC to work in the cancellation mode. Two separate control loops are proposed for the polarization de-multiplexer and the PMDC to allow them to work independently. The DGD tolerances for the one-stage and two-stage PMDC are measured and discussed. Finally the glitch problem in the polarization tuning algorithm is studied. An advanced dithering algorithm and the corresponding architecture of the polarization controller are proposed to solve the glitch problem.

Optimal Node Hardware Module Planning for Layer-One Optical Transport Networks

Most of the studies on traffic grooming focus on minimizing network link capacity and providing serving-relationship between client services and link capacity. Subsequent to this step, it is important to plan for adding/dropping client services over client service ports and setting up end-to-end lightpaths over network ports, which is however seldom investigated. We call such effort node hardware module planning. This is an industrially practical problem aiming to minimize node hardware cost since hardware modules are the most expensive components in a network. Based on a link-based traffic grooming result that provides information on end-to-end capacity units incident to nodes and aggregation relationship between client services and capacity units, we develop an Integer Linear Programming (ILP) model to optimally plan hardware modules. To overcome the computation difficulty of the ILP model under large-sized planning scenarios, we also develop a fast sub-optimal heuristic for hardware module planning. Simulation studies indicate that the heuristic is efficient to achieve a design close to an optimal solution obtained by the ILP model. Also, the evaluation of the impact of switch backplane size shows that given a certain set of network modules, an optimal switch backplane size exists, which achieves the lowest hardware cost.

Optimal WDM layer partitioning and transmission reach in optical networks

We investigate WDM layer network cost in typical North American networks and show that optical transparency at degree 2 and system reach between 1000-2000 km is sufficient for traffic loads between 1 -13 Tb/s.

Higher-order fiber dispersion compensation using a pulse shaper with a programmable phase-modulator array

Summary form only given. We present the use of a pulse shaper with a programmable liquid crystal phase modulator, which can provide any order of phase variation independently to completely remove the residual dispersion of a 2.5-km dispersion-optimized single mode fiber-dispersion compensating fiber link. As a result, we demonstrate transmission of sub-500-fs pulses over 2.5-km of fiber with no observable distortion.

Third order characterization of dispersion compensating gratings for 40Gbps DQPSK transmission systems

We present a third (and higher) order local dispersion characterization method for quality control of dispersion compensating gratings in 40 Gbps RZ-DQPSK systems with small (< 0.5dB) system penalty margin. Simulation results show that this approach is a simpler and more accurate indicator of system penalty due to group delay ripple, when compared to conventional methods based on phase ripple.

Influence of filter shape and bandwidth on 44 Gb/s DQPSK systems

Filter shape and bandwidth are studied for multiplexers/demultiplexers in DQPSK systems. Simulation shows both factors have strong impact on signal performance. In 50GHz spaced systems with 44Gb/s RZ-DQPSK signals, 2nd-order super-Gaussian filters outperform higher-order filters.