Abstract
A polarisation-division-multiplexing (PDM)-based four-level pulse amplitude modulation (PAM4) fibre-free-space optical (FSO) convergent system with optical band-pass filters (OBPFs) for polarisation de-multiplexing is feasibly demonstrated for the first time. In a PDM scenario with PAM4 modulation, the transmission capacity of fibre-FSO convergent systems is enhanced four times with an aggregate channel capacity of 128 Gb/s (64 Gb/s PAM4/polarisation × 2 polarisations). With an OBPF, polarisation-tracking free de-multiplexing is attained by eliminating other optical carrier with orthogonal polarisation. An OBPF is a simple polarisation de-multiplexing scheme in which the polarisation-orthogonal carrier can be effectively de-multiplexed and the cross-polarisation interference can be nearly eliminated. Compared with traditional PDM-based fibre-FSO convergent systems with sophisticated polarisation-tracking mechanism and elaborate digital signal processing (DSP) approach, it reveals a noteworthy one with the advantage of simplicity. Through 25 km single-mode fibre transport and 500 m FSO link, sufficiently low bit error rate and qualified PAM4 eye diagrams are attained. This proposed polarisation-tracking free PDM-based fibre-FSO convergent system is notable because it not only incorporates the fibre backbone and optical wireless feeder, but it also simplifies the framework since complicated polarisation-tracking mechanism and DSP approach are not involved.
Highlights
In the past few years, fibre-free-space optical (FSO) convergent systems have attracted considerable attention to connect fibre backbone and optical wireless feeder networks
For traditional PDM-based lightwave transmission systems, sophisticated polarisation-tracking mechanism and elaborate digital signal processing (DSP) approach are needed for polarisation de-multiplexing[14,15,16,17,18,19,20,21]
The distributed feedback (DFB) laser diode (LD) is modulated with 12.5 GHz sinusoidal RF signal
Summary
In the past few years, fibre-free-space optical (FSO) convergent systems have attracted considerable attention to connect fibre backbone and optical wireless feeder networks. As for the traditional PDM-based fibre-FSO convergent systems, the polarisation-orthogonal carriers can’t be de-multiplexed by utilizing OBPFs because they are modulated at the same wavelength. A polarisation-tracking free PDM-based fibre-FSO convergent system is a promising convergence into which optical fibre backbone and optical wireless feeder networks are incorporated, but the traditional and complicated polarisation-tracking mechanism and DSP approach are not needed. This illustrated polarisation-tracking free PDM-based fibre-FSO convergence provides high transmission capacity and directs an access to accelerate the widespread applications through SMF transport with FSO transmission
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