A. Klekamp

Electronic Dispersion Compensation

The performance of different electronic equalization and processing schemes for 40- and 10-Gb/s optical transmission over single-mode fiber (SMF) are discussed, from the point of their ability to compensate chromatic dispersion (CD) and polarization mode dispersion (PMD). In addition, the impact of fiber nonlinearity and modulation format on equalization is also investigated. The main objective of this paper is to present an overview and a comparison of the performances rather than a detailed explanation of the principles of the different equalization schemes. The equalizers which will be covered are analog equalizer (feedforward and decision feedback type), maximum likelihood sequence estimator (MLSE), electronic precompensation, coherent/intradyne detection with digital signal processing (DSP) equalization, DSP-based optical orthogonal frequency division multiplexing (OFDM), and turbo equalization.

An improved single-mode Y-branch design for cascaded 1:2 splitters

The power splitting ratio of Y-branches based on step index waveguides has been calculated using three-dimensional (3-D) finite-difference beam propagation method (FD-BPM). Due to interference of the guided mode with quasiguided modes generated at distortions of the waveguide the splitting ratio depends on the wavelength and design parameters of the Y-branch. The simulations show how variations of the splitting ratio can be minimized using a mode-filter in the input section without significant increase of losses or component size.

Nonlinear limitations of electronic dispersion pre-compensation by intrachannel effects

The nonlinear threshold (NLT) characteristics of 10 Gbit/s transmission using electronic pre-compensation is studied by numerical simulations over 1 span and 12 spans of 80 km SMF for NRZ and DPSK, respectively. We found a reduction of NLT of up to 9 dB for ASK.

Advantages of frequency shift keying in 10 Gb/s systems

10.7 Gb/s frequency shift keying (FSK) modulation format applying directly modulated DFB lasers is reported. This low cost FSK approach has been compared with OOK and DPSK format by simulations. The proposed FSK solution has been proven by transmission experiments.

Calculation of imaging errors of AWG

This paper reports on the modeling of arrayed-waveguide gratings (AWGs) by means of Fourier optics where the coupling between the grating waveguides and defocus of the star couplers is taken into account as well. Special interest is drawn to the geometrical modeling of AWGs. The paraxial approximation for the free-space propagation within the star couplers, the exact calculation of the coupling coefficients for the star couplers by an orthonormal basis (ONB) expansion, and the drawbacks due to the nonunitary transfer matrix provided by the Greens functions approach for the coupling between the grating waveguides, as well as possible improvements thereof, are identified as crucial points. An efficient orthonormalization procedure by means of an eigenvector decomposition is presented. A unitary approximation of the transfer matrix, which is in itself approximate but in general not unitary, introduced within the impulse response method, is obtained. The presented simulation technique allows for ab initio simulations, which do not fall back on any experimental input, for the spectral response of AWGs, which are found in good agreement with experimental results.

Experimental Comparison of PMD Tolerances of Binary 43Gb/s Modulation Formats without and with PMD Compensation

We compared experimentally PMD tolerance on 43Gb/s ASK/DPSK NRZ/RZ formats taking into account first and second order PMD. At 10-5 outage NRZ and RZ33 exhibit 2.8 and 4ps tolerance, respectively, showing negligible differences between ASK and DPSK.

DWDM and single channel fibre nonlinear thresholds for 43 Gb/s ASK and DPSK formats over various fibre types

We experimentally compared nonlinear-thresholds for single channel and DWDM (0.4 bit/s/Hz) over various fibre types. NZDSFs show generally slightly reduced NLT vs. SMF and only ASK modulated DWDM systems over true wave are penalized versus single channel transmission.

Impairments of bit-to-bit alternate-polarization on non-linear threshold, CD and DGD tolerance of 43 Gb/s ASK and DPSK formats [fibre optic communication]

Applying bit-to-bit alternate-polarization modulation at 43 Gb/s in ASK/DPSK formats, we found experimentally non-linear threshold benefits up to 4 dB with RZ and 2 dB with NRZ, reduction of DGD tolerance up to 3 ps with RZ formats and a dispersion-tolerance of 20% with DPSK-NRZ.

Electronic Mitigation of Impairments by Signal Processing

In a comprehensive overview different aspects of 40 Gbit/s electronic distortion equalization (EDE), of electronic pre-compensation, and receiver circuit requirements for fast polarization scrambling based PMD mitigation are discussed.

Phase error distribution and spectral response in silica-based AWG multiplexers

PHASARs or Arrayed Waveguide Grating (AWG) multiplexers for application in Dense Wavelength Division Multiplexing optical networks have been realized using silica-waveguide technology. Using waveguide tapers, a flat-top passband characteristic is obtained. Using Fourier transform spectroscopy employing a low coherence interferometer, the power-distribution coefficients and the phase error distribution in such components is evaluated. Because of the coherence length of the used light source being smaller than the optical path length difference in the PHASAR, it is possible to resolve individual waveguides of the array. Thus, the transfer functions of the individual optical paths are accessible to the measurement, enabling the full simulation of the PHASAR transmission spectrum. It is demonstrated that the measured phase error distribution in tapered PHASAR devices consists of contributions from both tapered waveguides and arrayed waveguide grating regions. An evaluation method suitable for investigating both contributions separately is developed. Phase shift introduced by the waveguide tapers is in good agreement with simulation calculations based on the Beam Propagation Method. An analytic transmission calculation based on measured phase and power-distribution coefficients enables a full simulation of PHASARs including insertion loss, bandwidth and crosstalk performance. Excellent agreement with transmission measurements is performed.