Multi-level signaling in the Stokes space and its application to large-capacity optical communications

The Stokes vector of an optical signal does not depend on its absolute phase; therefore, we can construct the phase-insensitive optical communication system, using the Stokes vector as a modulation parameter. In such a system, multi-level optical signals can effectively be designed in the three-dimensional Stokes space and demodulated either by direct detection or by coherent detection, where low-complexity digital-signal processing (DSP) is employed. Although this system has the disadvantage that adaptive equalizers can hardly be implemented in the digital domain, it is still an attractive solution to large-capacity (≥ 100 Gbit/s) and medium-or short-reach (≤ 100 km) transmission. In this paper, we discuss the receiver configuration for the multi-level signal in the Stokes space and the efficient DSP algorithm for demodulating such a signal. Simulation results demonstrate that 2-, 4-, 8-, 16-, and 32-ary signals in the Stokes space have good bit-error rate (BER) characteristics. Especially, the 16-ary signal at the moderate symbol rate of 25 Gsymbol/s can reach the bit rate of 100 Gbit/s even by using direct detection.