Hi Pelin,
Well, Polarization is a basic property associated with light just like its Amplitude, Phase, Frequency. There could be many applications in which polarization of light can be used e.g. in Polrization modulation where you modulate the polarization of your optical carrier to transmit data. Another application where polarization can be made use of is called Polarization Division Multiplexing where you transmit data in both the orthogonal polarization of light. In this case, you trasmit twice the data using the same Bandwidth.
Attached you will find the quite opened Eye Diagram of a 40 Gbps Polarization Division Multiplexed signal. I got this result from my simulation in OptiSystem.
I hope it helps.
Cheers,
Ahmad