Polarization Mode Dispersion

Introduction

      There are three fundamentally different dispersive phenomena in optical fiber, of which polarization mode dispersion (PMD) is the most complex. In digital multimode fiber systems, a light pulse separates into multiple spatial paths or modes. Each component reaches the receiver at a slightly different time as shown in the figure broadening the received pulse. Single-mode fiber solves the differential mode delay problem, allowing data rates to be increased until chromatic dispersion — the variation of propagation speed with wavelength — produces unacceptable pulse spreading. The amount of chromatic dispersion that a system can tolerate is inversely proportional to the square of the bit rate because an increased data rate means not only a wider spectrum and increased spreading, but also narrower bit slots that are more sensitive to the spreading of neighboring pulses. 

Three Different types of dispersion in fiber

     When chromatic dispersion is compensated — typically to a small but nonzero value in dense wavelength division multiplexed (DWDM) systems — the bit rate can be increased until it is limited by the third dispersive effect, PMD. Every network exhibits two slightly different propagation delays that correspond to different input polarization's. Some of the pulse energy experiences the longer delay and the rest of the energy experiences the shorter delay. As with the other dispersive effects, the result is a broadening of the received pulse. 

      PMD is considerably more subtle and interesting than this, however, and the topic accounts for a rapidly growing body of technical literature. This article will explore the origins, statistical character, measurement and mitigation of first-order polarization mode dispersion. 

Properties of polarized light

     The electric and magnetic fields of a lightwave fluctuate at right angles to one another in the plane perpendicular to the direction of propagation like in the figure below. PMD in single-mode optical fiber originates with noncircularity of the core  Fiber birefringence has two components. Form birefringence is a basic characteristic of any oval waveguide. Stress birefringence — generally dominant — is induced by the mechanical stress field that is set up when the fiber is drawn to other than a perfectly circular shape. Over short lengths, fiber birefringence splits the input pulse into linear slow and fast polarization modes, behaving like a linearly birefringent crystal. The corresponding difference in propagation time is called the differential group delay (DGD), expressed in picoseconds (1 ps = 10-12 s). Together, the differential group delay and the orthogonal polarization modes are the fundamental manifestations of first-order PMD. 

Lightwave Fluctuating between electric and magnetic field

   Given the extremely weak birefringence of telecom fiber, mode coupling is easily induced in the fiber by the mechanical forces arising from spooling, cabling or installation. 

 Core noncircularity is the root of PMD in single-mode fiber.

     The differential group delay at a given wavelength and time is called the instantaneous differential group delay. The average value of the DGD over wavelength is called the PMD delay. The average DGD divided by the square root of fiber length is called the PMD coefficient. 

How much can be tolerated?

    Digital transmission systems are designed to tolerate 10 to 15 percent of a bit period of average differential group delay, or 10 to 15 ps for a 10 Gb/s system. The average differential group delay of long routes of legacy fiber is often greater than this limit and in particularly severe cases can exceed 100 ps. 

     New optical fiber generally exhibits an average differential group delay in the range of 0.05 to 0.10 ps/km1/2. 

PMD mitigation

      The development of PMD mitigation techniques is driven by the upgrade of legacy fiber links to 10+ Gb/s. Any mitigation scheme must account for random changes in the differential group delay and principal states of polarization. One approach is to eliminate pulse spreading by coupling the transmitter output to a single input principal state of polarization of the link. Drawbacks are the need for specialized hardware at both transmitter and receiver and the delay of the feedback loop, which is twice the length of the link. 

    PMD can also be electrically mitigated by means of an equalizer circuit installed following the receiver photodetector. The detected signal is split into several paths to be differentially delayed and scaled, then recombined to squeeze the pulse back to a narrower shape. This technique has a history in microwave communications. PMD mitigation is receiving wide research attention and field trials have been run on several methods. 

Summary 

   When chromatic dispersion is compensated, PMD becomes a bit-rate limiting factor in digital fiber optic communications systems. The high PMD of many legacy fibers calls for measurement of the installed fibers and motivates the development of PMD mitigation. Specifications for components and fibers are tightening, and an understanding of polarized light and its interaction with hardware has become a key success factor for component manufacturers