Publisher Summary Polarization mode dispersion (PMD) is a linear effect that can be compensated in principle. In an ideal circularly symmetric fiber, the two orthogonally polarized modes have the same group delay. However, in reality, fibers exhibit a certain amount of birefringence because of imperfections in the manufacturing process or mechanical stress on the fiber after manufacture. It is noted that fluctuations in the polarization mode and fiber birefringence produced by the environment lead to dispersion that varies statistically with time and frequency. PMD causes different delays for different polarizations and when the difference in the delays approaches a significant fraction of the bit period, it leads to pulse distortion and system penalties. Environmental changes— including temperature and stress—cause the fiber PMD to vary stochastically in time. PMD, illustrating the basic concepts, the measurement techniques, the PMD measurement, the PMD statistics for first- and higher orders, the PMD simulation and emulation, the system impairments, and the mitigation methods has been summarized in the chapter. Both the optical and the electrical PMD compensations are considered.

Polarization-Mode Dispersion

Chapter 6 – Polarization Effects in Lightwave Systems

Dynamic Bragg gratings can be recorded in rare-earth (e.g. Er, Yb) doped optical fibres by two counter-propagating mutually coherent laser waves via local saturation of the fibre optical absorption or gain (in optically pumped fibres). Typical recording cw light power needed for efficient grating formation is of sub-mW–mW scale which results in characteristic recording/erasure times of 10–0.1 ms. This review paper discusses fundamental aspects of the population grating formation, their basic properties, relating wave-mixing processes and also considers different applications of these dynamic gratings in single-frequency fibre lasers, tunable filters, optical fibre sensors and adaptive interferometry.

https://iopscience.iop.org/article/10.1088/0022-3727/41/22/224002/pdf

Dynamic population gratings in rare-earth-doped optical fibres

Polarization-related impairments have become a critical issue for high-data-rate optical systems, particularly when considering polarization-mode dispersion (PMD) and polarization-dependent loss (PDL). In general, polarization effects are stochastic processes and can occur on short or long time scales. Therefore, control and monitoring of these effects may be crucial in any systems-level mitigation. This tutorial will highlight the following key issues: PMD and PDL, monitoring schemes, emulation of proper statistics, interaction, and applications of polarization scrambling.

Monitoring and control of polarization-related impairments in optical fiber systems

Single- and dual-wavelength tunable lasing (around 1552 nm) has been obtained in an Er-doped low-Bi fiber laser with a Bragg grating mirror in a Hi-Bi fiber. Equal output powers for both laser lines and spectral spacing of up to 0.26 mm, larger than the original spacing between the resonance peaks of the grating (0.2 nm), can be easily achieved by employing a polarization controller inside of the laser cavity.

Fiber laser polarization tuning using a Bragg grating in a Hi-Bi fiber

A computer model for polarization dependent gain (PDG) in Er-doped fiber amplifiers (EDFA) is presented. The model assumes that each erbium ion possesses an ellipsoidal gain surface and that all ion orientations are equally likely. By dividing the ions into subsets based upon orientation and computing the inversion of each subset in the presence of polarized pump and signal waves, the model predicts the dependence of the PDG induced by this polarization hole-burning (PHB) on the design of the EDFA, the signal degree and state of polarization (SOP), and the pump SOP. For moderate gain amplifiers (made from the same fiber) with the same gain peak wavelength and the same compression level, the magnitude of the PDG is nearly independent of the EDFA gain. Internal and random fiber birefringence are included to model real fibers. In fibers which cause the signal SOP to walk rapidly around the Poincare sphere, the PDG is reduced by a factor of 2/3 when compared with a linear polarization-maintained signal. Scrambled signals and partially-polarized saturating tones are also considered. Simple rules are derived for predicting the PDG of a given EDFA.

V. Mazurczyk

Papers

Polarization dependent gain in erbium-doped fiber amplifiers: computer model and approximate formulas