The core of an optical fibre has a refractive index that varies with polarisation (birefringence). This causes different polarisation states to travel through the fibre core at different speeds. The delay between the fastest and slowest polarisation states results in pulse broadening. This limits the maximum data rate and distance that a signal can propagate before degradation is so great as to lose signal integrity. Polarisation mode dispersion limits the transmission capacity of an optical network.
Test and measurement
Optical fibres can be tested for polarisation mode dispersion by measuring the time delay between the arrival of two discrete polarisation modes at the receiving end of the fibre (“differential group delay”). This is achieved by launching polarised broadband light into the fibre with an optical spectrum analyser (OSA) at the receiving end. The OSA measures the spectrum in the frequency domain, and then Fourier transforms the spectrum. The differential group delay (DGD) can then be calculated from the resulting time domain spectrum.
The differential group delay resulting from polarisation mode dispersion can have adverse effects on optical data transmission in optical fibre, including an increased bit error rate (BER), and a degraded optical signal.
There are a number of ways to reduce the effect of polarisation mode dispersion. The simplest ways are to reduce the bit rate, or shorten the length of fibre.
A more advanced method, known as coherent modulation, is used to encode more bits into each optical pulse (“symbol”). As fewer symbols are required to transmit the same amount of data, coherently modulated signals are less sensitive to polarisation mode dispersion.