The current status of lithium-niobate external-modulator technology is reviewed with emphasis on design, fabrication, system requirements, performance, and reliability. The technology meets the performance and reliability requirements of current 2.5-, 10-, and 40-Gb/s digital communication systems, as well as CATV analog systems. The current trend in device topology is toward higher data rates and increased levels of integration. In particular, multiple high-speed modulation functions, such as 10-Gb/s return-to-zero pulse generation plus data modulation, have been achieved in a single device.

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A review of lithium niobate modulators for fiber-optic communications systems

This tutorial describes the basic principles and performance analysis of optical intensity modulators using electrooptic and electroabsorption effects, for use in analog and digital communication systems. These include lithium niobate modulators, semiconductor electroabsorption modulators, semiconductor Mach-Zehnder modulators, and polymer modulators.

Optical intensity modulators for digital and analog applications

Digital backpropagation (BP) is a universal method for jointly compensating dispersion and nonlinear impairments in optical fiber and is applicable to signals of any modulation format. In this paper, we compare single carrier (SC) versus orthogonal frequency-division multiplexing (OFDM) for polarization-multiplexed transmission. We show that at realistic data rates and transmission distances, polarization mode dispersion does not significantly degrade the performance of BP and can be mitigated by a post-BP linear equalizer. Dispersion unmanaged transmission can significantly reduce nonlinearity, and in this transmission regime, SC and OFDM have similar nonlinear tolerance. Multichannel BP is effective at mitigating interchannel nonlinearity for point-to-point links.

Nonlinear Compensation Using Backpropagation for Polarization-Multiplexed Transmission

https://www.dtc.umn.edu/s/resources/gg-se.pdf

A bibliography on nonlinear system identification

Next-generation optical fiber systems will employ coherent detection to improve power and spectral efficiency, and to facilitate flexible impairment compensation using digital signal processors (DSPs). In a fully digital coherent system, the electric fields at the input and the output of the channel are available to DSPs at the transmitter and the receiver, enabling the use of arbitrary impairment precompensation and postcompensation algorithms. Linear time-invariant (LTI) impairments such as chromatic dispersion and polarization-mode dispersion can be compensated by adaptive linear equalizers. Non-LTI impairments, such as laser phase noise and Kerr nonlinearity, can be compensated by channel inversion. All existing impairment compensation techniques ultimately approximate channel inversion for a subset of the channel effects. We provide a unified multiblock nonlinear model for the joint compensation of the impairments in fiber transmission. We show that commonly used techniques for overcoming different impairments, despite their different appearance, are often based on the same principles such as feedback and feedforward control, and time-versus-frequency-domain representations. We highlight equivalences between techniques, and show that the choice of algorithm depends on making tradeoffs.

Fiber Impairment Compensation Using Coherent Detection and Digital Signal Processing

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.

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

We experimentally demonstrate dynamic dispersion compensation using a novel nonlinearly chirped fiber Bragg grating in a 10-Gb/s system. A single piezoelectric transducer continuously tunes the induced dispersion from 300 to 1000 ps/nm. The system achieves a bit-error rate=10/sup -9/ after both 50 and 104 km of single-mode fiber by dynamically tuning the dispersion of the grating between 500 and 1000 ps/nm, respectively. The power penalty after 104 km is reduced from 3.5 to <1 dB.

Dynamic dispersion compensation in a 10-Gb/s optical system using a novel voltage tuned nonlinearly chirped fiber Bragg grating

We propose and experimentally implement a method of demultiplexing two data channels, which were multiplexed using orthogonal polarizations on the same wavelength in a power ratio of 1:2 in order to double the bandwidth utilization in the fiber. Our technique facilitates the straightforward decoding of the two data channels and does not require the demultiplexing of the two orthogonal polarizations at the receiver. The two polarization channels each at 2.2 Gb/s are recovered after 100 km of fiber transmission with <1.0-dB power penalty.

Doubling of bandwidth utilization using two orthogonal polarizations and power unbalancing in a polarization-division-multiplexing scheme

Analytical expressions involving both system parameters and step-size are proposed to represent the local simulation error for the symmetrized split-step Fourier (SSSF) simulation method. This analytical expression can be used for a step-size selection rule to achieve comparable local simulation accuracy for SSSF simulations. This can lead to computational savings since there is no waste of computation in each simulation step. Furthermore, based on the local error expression, scaling rules are derived to achieve comparable global simulation accuracy for wide ranges of key system parameter values. This is significant in enhancing the computational efficiency in optical fiber communication system design and optimization. Extensive validation tests were performed to explore the application range of the proposed step-size selection and scaling rules. The desired global accuracy can be achieved with the use of our local error expression and scaling rules by only a couple of test trial simulation runs for a variety of practical applications.

Symmetrized Split-Step Fourier Scheme to Control Global Simulation Accuracy in Fiber-Optic Communication Systems

We analyze 10-Gb/s nondispersion-managed and dispersion-managed wavelength-division multiplexed (WDM) systems that use pre-compensation, post-compensation, or dual-compensation of each channel to minimize dispersion and nonlinear effects. We find that dual-compensation gives the minimal penalty for each channel in dispersion-managed WDM systems. Furthermore, we find that the optimal amount of pre- or post-compensation depends upon the specific dispersion map used in the WDM system.

M.I. Hayee

Papers

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

Dynamic dispersion compensation in a 10-Gb/s optical system using a novel voltage tuned nonlinearly chirped fiber Bragg grating

Doubling of bandwidth utilization using two orthogonal polarizations and power unbalancing in a polarization-division-multiplexing scheme

Symmetrized Split-Step Fourier Scheme to Control Global Simulation Accuracy in Fiber-Optic Communication Systems

Pre- and post-compensation of dispersion and nonlinearities in 10-Gb/s WDM systems