Ethernet-over-OWC Using VCSELs: Transparent Gigabit Links with Low Latency and Robust Alignment Tolerance

We demonstrate a fully bidirectional 1 Gbs Ethernet over OWC link over a 1m free space path using a VCSEL-PIN pair and only commercially available components. The unamplified, transparent system achieves error-free operation, with a latency of less than 25 ns, and a centimetre-scale alignment tolerance.

💡 Research Summary

The paper presents a fully bidirectional, real‑time Ethernet‑over‑optical‑wireless (OWC) link that operates at 1 Gb/s using only off‑the‑shelf components. A custom 980 nm vertical‑cavity surface‑emitting laser (VCSEL) and a high‑bandwidth silicon PIN photodiode form an unamplified transmitter‑receiver pair. Ethernet traffic generated by a server is converted to optical SFP format via a standard RJ45‑to‑SFP media converter (TP‑Link MC220L). The SFP evaluation board (Finisar FDB‑1032‑SFP) drives the VCSEL directly by adding a DC bias to the high‑speed electrical signal, eliminating the need for external line drivers or pre‑emphasis circuitry. The emitted beam is collimated with a single aspheric lens and propagates across a 1‑meter free‑space channel. At the receiver, the PIN photodiode (FEMTO HSPRX‑I‑1G4‑SI‑FS, 1.4 GHz bandwidth) converts the optical signal back to electricity, which is fed into a second SFP evaluation board, re‑encoded into optical SFP format, and finally reconverted to copper Ethernet by a second media converter for delivery to a client laptop. The reverse path mirrors this architecture, providing true full‑duplex operation without any protocol conversion or software modification.

Performance was evaluated in three key areas: signal quality, throughput, alignment tolerance, and latency. Eye‑diagram measurements yielded a Q‑factor of 7.44 at optimal alignment, corresponding to an estimated BER of ~10⁻¹³ (well below the Ethernet requirement of 10⁻¹²). Real‑world speed tests using speedtest.net recorded download and upload rates of 813.95 Mbps and 929.53 Mbps respectively, approaching the theoretical limit of Gigabit Ethernet after accounting for protocol overhead. Alignment tolerance was assessed by laterally shifting the receiver; the Q‑factor remained above 6 (BER < 10⁻⁹) for displacements up to ±5.5 mm, giving a total tolerance of approximately 1.15 cm. This centimetre‑scale tolerance is an order of magnitude more relaxed than typical fiber‑coupled free‑space links that demand sub‑millimetre precision, making the system robust to vibration and mis‑alignment in practical deployments.

Latency was measured with a Keysight P9370A vector network analyzer by extracting group delay between a back‑to‑back reference and the transmitted signal. The total end‑to‑end delay introduced by the OWC link was less than 25 ns, including roughly 3.3 ns of pure propagation time over the 1‑meter path. Such low latency satisfies the stringent timing requirements of industrial control, autonomous vehicles, and edge‑computing scenarios.

The authors emphasize that the link is “transparent”: Ethernet frames traverse the wireless segment unchanged, allowing seamless integration into existing network infrastructures without any additional protocol stack or firmware changes. The use of only commercially available VCSELs, PIN photodiodes, and standard SFP modules keeps component costs low and eliminates the need for expensive optical amplifiers or wavelength‑division components. The demonstrated combination of unamplified operation, full‑duplex transparency, sub‑25 ns latency, >900 Mbps effective throughput, and centimetre‑scale alignment tolerance establishes a new benchmark for practical OWC deployments as a direct wireless extension of wired Ethernet.

In conclusion, the work showcases a practical, low‑cost, and high‑performance solution for short‑range, high‑throughput wireless back‑haul or “last‑meter” connectivity. It fulfills five critical criteria—amplifier‑free, bidirectional, transparent, low‑latency, and robust alignment—making it suitable for smart factories, data‑center interconnects, and automotive networks. Future research directions suggested include extending the free‑space range to tens of meters, implementing multi‑user MIMO schemes, adding automatic beam‑steering for dynamic alignment, and scaling the architecture to higher Ethernet standards such as 25 GbE or 100 GbE.