Optical CDMA with Polarization Modulations
Polarization shift keying (PolSK) is the only modulation scheme that makes use of the vector nature of the lightwave. Like frequency shift keying (FSK), it is a multiple-array (M-ary) signalling transmission scheme, and unlike FSK the spectrum of a PolSK signal corresponds to an equivalent amplitude shift keying (ASK) signal at the same bit rate .
When a polarized lightwave is transmitted through a single-mode fibre-optic (SMF), its state of polarization (SOP) changes due to the presence of waveguide birefringence. However the fibre birefringence only causes a rigid rotation of the lightwave's polarization constellation over the Poincaré sphere . In other words, each signal point is displaced although the Stokes parameters illustrating the spatial relations among signal points in the Poincaré sphere are preserved. Thus, the information is not degraded. To compensate for the constellation rotation, some form of processing is required either optically by a polarization controlling and tracking or electronically by a digital signal processing (DSP) at the decision processor . To perform PolSK detection avoiding optical birefringence compensation, it is necessary to use a receiver that extracts the Stokes parameters of the incoming lightwaves . The receiver which extracts the Stokes parameters can generally be divided into two sections: (i) the optical front-end that probes the input electrical field ...