PMD impairments in optical fiber transmission at 10 Gbps and 40 Gbps

Abstract

The continuous need for greater bandwidth and capacity to support existing and emerging technologies, such as fiber-to-the-home (FTTH) and Internet Protocol Television(IPTV), drive optical-communication systems to higher and higher data rates per wavelength channel, from 10 to 40 Gbps and above. Degrading effects that tended to cause non catastrophic events at lower bit rates have become critical concerns for high-performance networks. Among them, polarization-mode dispersion (PMD) is perhaps the largest concern and, therefore, has garnered a great amount of attention. The PMD arises in an optical fiber from asymmetries in the fiber core that induce a small amount of birefringence that randomly varies along the length of the fiber. This birefringence causes the power in each optical pulse to split between the two polarization modes of the fiber and travel at different speeds, creating a differential group delay (DGD) between the two modes that can result in pulse spreading and intersymbol interference. PMD becomes a unique and challenging hurdle for high-performance systems mainly due to its dynamic and random nature. The polarization state is generally unknown and wanders with time. In general, PMD effects are wavelength (channel) dependent and can vary over a time scale of milliseconds. As a random variable, the DGD follows a Maxwellian distribution for which high-DGD points in the tail of the distribution can lead to network outages. Typically, system designers require the outage probability for high-performance networks to be 10−5 or less (penalty > 1dB for <30 min/yr). Clearly, the most straightforward approach to overcoming the effects of PMD is to employ newly manufactured low-PMD optical fibers, which have PMD values < 0.1 . However, much of the previously embedded fiber has high PMD values between 0.5 and 1 or even higher. The reality of deploying new systems over the embedded fiber means that the PMD monitoring and compensation are important for PMD mitigation. Unlike other degrading effects such as chromatic dispersion, the PMD is a time-varying random process making compensation difficult.The aim of this work is to study the trend of PMD effects over two different system, at 10 and 40 Gbps with two kind of fiber with high (0.5 ) and low (0.1 ) PMD coefficient. The first one corresponds to an old fiber’s type, that is used in the majority in the current transmission system; while the second one corresponds to a new fiber’s type, designed to have a lower response to the PMD phenomenon, making possible the transmission over long distances at high bit-rates. This work is structured as follows: After a short introduction, the second chapter is a review of PMD theory; where the PMD is faced from a theoretical point of view. It’s reported how the PMD arises in a fiber, how the DGD has a Maxwellian probability distribution, and the outage limits to design a system under the influences of PMD. In the third chapter there is a literary review over the PMD mitigation. Over the years, research groups from around the globe have proposed and/or demonstrated different strategies for PMD compensation. In this chapter an overview of these strategies shall be given, mentioning their relative merits and demerits. Following that, methods to increase the tolerance of a fiber-optic communication system to PMD, will also be discussed. After this theoretical introduction the central part’s of this study starts. In the fourth chapter the limitations imposed by the PMD are investigated. We starts probing the theoretical distance limits imposed by the only PMD, setting all other fiber’s impairments and attenuation to be negligible. Sequentially two single span optical transmission systems are compared on the basis of fiber PMD coefficient and bit-rates, to find the maximum distance that can be reached with a bit error rate of 10-10, taking in account or not the PMD and setting only the attenuation of the fiber. After this first investigation, the real impact of PMD was reported, performing a simulation of a multi span system, where the fiber’s attenuation of each section is compensated by an amplifier, so to find the maximum reachable distances over long-haul transmission and clearly see how is the PMD impact.In the last chapter a first-order polarization compensator is tested. Firstly in order to show how the compensator could works, the monitor signal’s simulation (based on the analysis of the Power Spectral Densities at selected frequency) is made, to show how the PMD level is related to the PSD. After that, the compensator is tested, performing two simulations at 10 and 40 Gbps with different value of DGD reached at the end of the fiber, to demonstrate the real capability of the compensator. The last study done is over the compensation applied to the previous multi-span system, to study how the performance of a system get increasing with a PMD compensation, and what is the system tolerance to PMD with or without compensation. All the simulation of this work are made with the use of a software package (1) used in the optical laboratory of Universitat Politècnica de Catalunya.