Polarization Related Tests for Coherent Detection Systems

Coherent detection with polarization division multiplexing (PDM) has emerged as the key technology enabler for 40 Gbps and 100 Gbps networks because it significantly increases the spectral efficiency of each channel and allows each channel to transmit at a high bit rate with relatively small optical bandwidth. Consequently, 40 Gbps, 100 Gbps, or even 160 Gbps channels can be transmitted over existing 10 Gbps WDM infrastructures with 50 GHz channel spacing [1]. PDM doubles the spectral efficiency by combining two polarization channels of the same bit rate at the same wavelength [2-8]. Also, coherent detection allows multiple levels of phase and amplitude modulation at lower rates to be multiplexed on the same wavelength channel [916]. For example, PDM, together with quadrature-phase-shiftkeying (QPSK) modulation, enables 40 Gbps transmission within the bandwidth of a 10 Gbps direct channel [13]. At such a small bandwidth, impairments due to polarization mode dispersion (PMD) and chromatic dispersion (CD) are greatly reduced and have less impact on the bit error rate (BER) of the transmission. In addition, because both the amplitude and phase information of the two orthogonal polarizations of a signal are preserved in a coherent detection system, PMD, PDL (polarization dependent loss) and CD compensations can be performed by digital signal processing (DSP) in the electrical domain to further extend system tolerance of the effects of PMD, PDL and CD.