Abstract
A 448-Gb/s four-level pulse amplitude modulation (PAM4) free-space optical (FSO) communication through 600 m free-space link was constructed, utilizing polarization-multiplexing injection-locked vertical-cavity surface-emitting lasers (VCSELs) for presentation. When uniting four-wavelength polarization-multiplexing and PAM4 modulation schemes, the transmission capacity of FSO communications is substantially multiplied, with an aggregate transmission rate of 448 Gb/s [56 Gb/s PAM4/wavelength × 4 wavelengths × 2 polarizations (xand y-polarizations)]. The results show that four 1.55 μm VCSEL transmitters with injection locking technique are sufficiently powerful for 448 Gb/s PAM4 signal transmission. Adopting a tunable optical band-pass filter and a polarization beam splitter for wavelength filtering and polarization de-multiplexing, the polarized wavelengths are effectually filtered and de-multiplexed in each polarized state. As the polarization state is well preserved in the scenario over 600 m free-space transmission, a sophisticated polarization tracker is not required in this proposed FSO communication. High bit error rate performance and accepted PAM4 eye diagrams are attained through 600 m free-space transmission. The four-wavelength polarization-multiplexing PAM4 FSO communication established here reveals the prominent benefits of high aggregate transmission capacity and long-distance free-space link.
Highlights
Free-space optical (FSO) communications are essentially signal transmission between two locations through the freespace
The channel capacity of FSO communications is noticeably multiplied by combining polarization-multiplexing injection-locked verticalcavity surface-emitting lasers (VCSELs) with
Four optical PAM4 signals with parallel and orthogonally polarized dual wavelengths are combined using a 4×1 optical combiner [insert (d)], amplified by an erbium-doped fiber amplifier (EDFA), optimized by a variable optical attenuator (VOA), and communicated through 600 m free-space link using a couple of doublet lenses
Summary
Free-space optical (FSO) communications are essentially signal transmission between two locations through the freespace. Line of sight is required between the transmitting site and the receiving end for FSO communications. License-free propagation, large available bandwidth, high directivity, high reliability, and strong confidentiality characteristics have made FSO communications very popular [1]–[6]. In the state of heavy transmission loading, a high-speed data transmission configuration should be built to alleviate the critical concern of heavy transmission loading. A 448-Gb/s four-level pulse amplitude modulation (PAM4) FSO communication with polarization-multiplexing injection-locked verticalcavity surface-emitting lasers (VCSELs) through 600 m free-space link is thereby proposed and feasibly demonstrated. Four optical wavelengths are polarized in x and y orthogonal polarizations. The channel capacity of FSO communications is noticeably multiplied by combining polarization-multiplexing injection-locked VCSELs with
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