Polarisation multiplexing is commonly used in 100Gb/s line side transmission as a method to increase the bit rate over the line without increasing the transmitted symbol rate.
Using polarisation controllers it is possible to generate two perpendicular polarised optical signals on the same wavelength for transmission through a single fibre. As two separate signals are now in use this doubles the bitrate whilst maintaining the same symbol rate.
In the case of 100Gb/s transmission each of the signal will typically be using QPSK modulation (see here for more information on phased based modulations).
If we now consider the case of carrying an OTU4 at 112Gb/s, we know that using QPSK in isolation we can transmit a symbol rate of only 56Gb/s using the modulations 2 bit encoding to achieve the 112Gb/s bitrate. By using polarisation multiplexing we can again reduce the symbol transmission rate to 28Gb/s.
In this case the modulation is referred to as dual polarised quadrature phase shift keying or DP-QPSK.
Dual polarised signal are subject to effects within the fibre and we will discuss these in the future.