The increasing demand for high bandwidth due to the dramatic upsurge of traffic in long-haul backbone networks is a catalyst for the emergence of the 40G transmission system.  Under the same physical conditions as the 10G system, a 40G system’s optical signal-to-noise ratio (OSNR) is required to be four times higher (6dB); chromatic dispersion tolerance and polarization mode dispersion (PMD) tolerance are respectively sixteen and four times lower; and nonlinear effects are worsened.  The limitations must be addressed if 40G transmission is to be successfully commercialized.

Numerous technologies can be employed to this end, including FEC, Raman amplification, dispersion management, modulation coding, polarization management, and inverse multiplexing. Among these, we believe modulation coding technology is the most critical. Many modulation coding technologies are currently available, for example, phase modulation-based DQPSK.  DQPSK modulation offers advantages in terms of high spectral efficiency and good dispersion tolerance, so it is utilized for transmission at 40G system. 
 
DP-QPSK is a phase-modulation system that transmits 2-bit signals represented by four phases on two orthogonally polarized light beams respectively. The DP-QPSK signaling scheme reduces baud rate to one quarter of the system bit rate. This lowered baud rate permits the use of electrical and optical components with lower operating bandwidths and improves the system immunity to factors that significantly limit transmission, e.g. chromatic dispersion, polarization mode dispersion. DP-QPSK systems are widely recognized as being a leading-edge technology on the market today. DP-QPSK transmission technology using a digital coherent receiving system is now commonly used for 100G transmissions.