The present invention relates generally to optical communications and, more particularly, to multiple symbol polarization switching differential-detection modulation formats.
As Internet traffic grows exponentially because of a variety of user terminals and internet services, it has prompted strong research interests on high-speed optical networks, which are the backbone infrastructure of current “Globe Village”. The data rate for optical fiber communications has moved from 10 Gbits/s to 40 Gb/s and 100 Gbits/s or even 1 Tbits/s per channel. However, one of the major challenges facing the ultra-high-data-rate dense wavelength division multiplexing (DWDM) optical fiber transmissions is the fiber nonlinearity, which causes optical signal distortions due to the various nonlinear effects in optical fiber and sets the limit of the maximal reach. DQPSK modulation is an important format for high-speed optical communications by transmitting 2 bits per symbol. At 40 Gb/s, DQPSK systems employing direct detection are attractive by having low complexity and being generally available.
In a digital coherent optical communication system, different types of digital signal processing (DSP) functions can be applied, to mitigate the fiber nonlinearity, such as digital back-propagation algorithms. However, the existing DSP-based fiber nonlinearity mitigation algorithms are demanding on the hardware resources, which are relatively limited and sophisticated due to the requirements of very high electronic processing bandwidth. Meanwhile, most of the existing nonlinearity mitigation algorithms show very limited system performance improvements in real experimental testing.
In another approach, the phase conjugation scheme has been proposed to improve the systems' nonlinearity tolerance. However, the deployment of this scheme requires at the exact middle point of the entire transmission link, thus imposing a strict and thus unpractical restrictions on the system deployment. Its spectrum efficiency would be halved because of the fiber four-wave mixing effects. In another prior effort, the polarization states for adjacent symbols are arranged in orthogonal states for improving fiber nonlinearity tolerance.
Accordingly, there is a need for a low-cost solution to increase the nonlinearity tolerance of a direct-detection optical DQPSK system