Michal Mlejnek

Fiber design considerations for 40 Gb/s systems

In this paper, we review the fundamental advantages and drawbacks of 40-Gb/s systems from a fiber manufacturers perspective. Based on modeling, experimental results, and fundamental understanding, we correlate the fiber design parameters with the expected performance of long-haul systems operating at 40 Gb/s. Nonlinear penalties, dispersion tolerances, modulation formats, polarization-mode dispersion, and Raman amplification are covered. We also present the fiber features required for both metro and submarine networks at this specific data rate.

Coherent frequency-selective polarimeter for polarization-mode dispersion monitoring

Frequency-selective polarimeters measure the state of polarization of the individual spectral components of a modulated optical signal. They can be used either as stand-alone measuring devices or as parts of adaptive polarization-mode dispersion (PMD) compensators. This paper presents a novel frequency-selective polarimeter based on coherent detection, which has superior accuracy compared to previously proposed direct detection-based counterparts. This is due to the high-frequency resolution and power sensitivity of coherent detection, features that minimize the systematic and random error, respectively, in the measurement of the state of polarization of the individual spectral components of the received optical signal. The accuracy of the measurement is independent of the received signal bit rate and modulation format. The proposed frequency-selective polarimeter is studied both theoretically and experimentally. The primary theoretical contribution of this paper is a unified formalism, which allows the modeling of both direct and coherent detection-based frequency-selective polarimeters. Analytical expressions for the output signal of both types of frequency-selective polarimeters are derived. Based on these expressions, a common algorithm is proposed for the evaluation of the Stokes parameters. In addition, an example error signal is used as a metric in order to test the agreement of the theoretical model with the experimental measurements. The successful operation of the coherent frequency-selective polarimeter is demonstrated experimentally for a 10-Gb/s intensity-modulated nonreturn-to-zero (NRZ) optical signal in the presence of first-order polarization-mode dispersion. There is an excellent agreement between theory and experiment.

Laser-induced birefringence in fused silica from polarized lasers

Fused silica, when exposed to excimer laser light, exhibits permanent anisotropic birefringence and wavefront changes. These laser-induced changes depend on the silica composition and processing conditions. The optical anisotropy is most clearly observed in samples that are exposed with linear polarization. This polarization-induced effect has been known for several years, but has become much more important with the advent of immersion lithography and its associated very high numerical apertures. High numerical aperture optics require controlled polarization, notably linear polarization, in order to maintain phase contrast at the image. When birefringence and wavefront changes are induced by laser exposure, the image contrast at the wafer deteriorates. We interpret the changes in optical properties in terms of permanent anisotropic strain induced by laser damage, and the associated strain-induced optical effects. This is accomplished using the mathematics of tensors to account for anisotropic strain and optical anisotropy, and using finite element analysis to calculate the strain fields taking the sample and exposure geometries into account. We report the relations between underlying density and strain anisotropy changes and the induced birefringence and wavefront for a given experimental sample geometry. We also report some examples of the different degree of laser damage from silica with different compositions and processing conditions.

Coupled-Mode Theory of Multipath Interference in Quasi-Single Mode Fibers

We use the power coupled-mode theory to study the interplay between multipath interference (MPI) and differential mode attenuation (DMA) in quasi-single mode (QSM) fibers. The analytical expressions derived assuming two mode propagation in QSM fibers show that MPI scales differently as a function of the span length for low and high DMA. Furthermore, we derive analytical expressions for the performance improvement of long-haul coherent optical communication systems using QSM fibers, taking into account the impact of excess loss and MPI on system performance. From these expressions, we calculate the maximum allowable coupling coefficient for different values of the DMA. We show, for example, that a QSM fiber with an effective area of 250 μm2, a coupling coefficient κ ≤ 6 × 10-4 km-1, and DMA equal to 4 dB/km offers a 1-dB performance advantage over a reference pure silica core single-mode fiber for spans of 100 km.

Time-resolved performance analysis of a second-order PMD compensator

Design, test, and performance requirement and analysis for a polarization-mode-dispersion compensator (PMDC) with four degrees of freedom is presented. The performance is analyzed on the basis of time-integrated and time-resolved bit-error ratio (BER) measurements. Signal impairments are generated by both, first- and higher-order emulators. The probability distributions of bit errors measured over many one second intervals exhibit very long tails. Therefore even a PMDC with a good average BER performance may result in a significant total outage time for a given system.

Coherent heterodyne frequency-selective polarimeter for error signal generation in higher-order PMD compensators

We proposed and demonstrated a coherent heterodyne receiver architecture that can frequency-resolve the Stokes parameters of a modulated signal. We also studied the feasibility of generation of an error signal based on the Stokes parameters variance. We presented experimental results for a 10 Gb/s NRZ signal with first-order polarisation mode dispersion (PMD) and theoretical results for a 40 Gb/s RZ signal with higher-order PMD.

Quasi-Single-Mode Fiber Transmission for Optical Communications

The transmission of a single fundamental mode in a fiber with cutoff wavelength above the transmission band is studied as a means of allowing a larger fiber effective area and reducing fiber nonlinearity. The reduction of nonlinear impairments is achieved at the expense of a potential new linear impairment in the form of multipath interference (MPI). We use a power-coupled-mode formalism to analyze the growth of MPI, and the effects of fiber and cable attributes on its magnitude and the required complexity of digital signal processing to combat the MPI. Hybrid fiber spans comprised partially of a quasi-single-mode fiber are also analyzed using a modification of the Gaussian noise model of coherent systems to predict optimal configurations, and results from transmission experiments are presented that demonstrate very high spectral efficiencies and performance surpassing that of a purely single-mode fiber system.

A Geometrical Perspective on the Coherent Multimode Optical Field and Mode Coupling Equations

The generalization of the Poincaré sphere to N ≥ 2 modes is the (N - 1)-dimensional complex projective space CP(N - 1). There is a minimal set of 2N - 2 Stokes vector components that determine the coherent multimode optical field. These are obtained from the inverse stereographic projection of coordinate hyperplanes in CP(N -1) into a 2N -2 sphere, just as in the N = 2 case. We derive N-mode analogs of Pooles optical fiber polarization-mode dispersion (PMD) equations that involve only 2N - 2 independent variables. This is achieved by means of an explicit generalized coherent state representation of the optical field, which enables the components of the PMD vector to be expressed in terms of the optical state and its frequency derivatives. Pooles equations describe mode coupling as a flow on CP(N -1). We give general constraints on the mode-coupling matrix and Stokes vector components. The group delay operator is shown to be a rank-2 perturbation of a diagonal matrix.

Viewing Angle and Voltage Dependence of the Visibility of Cell Gap Defects in LCD Panels

Perception of visible defects induced by local cell gap variations in liquid crystal display (LCD) panels is studied as a function of the viewing direction and applied voltage. Our results are reported in terms of the LCD sensitivity parameter defined in our previous publication and can be readily used in conjunction with the psycho-physical models of Mura perception. We apply our methods to a particular example of a normally white TN LCD and show that the Mura visibility depends strongly on the viewing angle as well as the applied voltage. We conclude that a complete study of the -parameter as a function of the viewing angle and applied voltage is crucial in the correct assessment of the sensitivity of an LCD panel to cell gap variation.

A study of visible defects caused by local cell gap variations in LCD panels

We relate local cell gap variations to visible defects in LCD panels using psychophysical methods of human eye perception of intensity variations. Our analysis is applicable to general shape of cell gap variation for any LCD mode. We use our method in an explicit example to determine the visible cell gap variation threshold for TN LCD panels.