Use of polarization freedom beyond polarization-division multiplexing to support high-speed and spectral-efficient data transmission

Increasing the system capacity and spectral efficiency (SE) per unit bandwidth is one of the ultimate goals for data network designers, especially when using technologies compatible with current embedded fiber infrastructures. Among these, the polarization-division-multiplexing (PDM) scheme, which supports two independent data channels on a single wavelength with orthogonal polarization states, has become a standard one in most state-of-art telecommunication systems. Currently, however, only two polarization states (that is, PDM) can be used, setting a barrier for further SE improvement. Assisted by coherent detection and digital signal processing, we propose and experimentally demonstrate a scheme for pseudo-PDM of four states (PPDM-4) by manipulation of four linearly polarized data channels with the same wavelength. Without any modification of the fiber link, we successfully transmit a 100-Gb s−1 PPDM-4 differential-phase-shift-keying signal over a 150-km single-mode fiber link. Such a method is expected to open new possibilities to fully explore the use of polarization freedom for capacity and SE improvement over existing fiber systems. A scheme for manipulating four linearly polarized data channels down existing fibre optic cables has been developed by researchers in China. Polarization-division multiplexing uses the polarization of light waves to differentiate multiple data streams travelling through a single-mode fibre. Lian-Shan Yan and co-workers from Southwest Jiaotong University have now found a way to accommodate more polarization states at a single wavelength, boosting the potential capacity of data networks. The team implemented a setup that multiplexes four signals having separate 0°, 30°, 90° and 120° polarizations using an optical controller. After transmission, the received signal is analyzed by coherent detectors aligned to the original polarization states, and demultiplexing algorithms. This strategy enabled data to be transferred at 100 Gb s-1 through a 150-km single-mode fibre.