A simple and efficient optical interconnection consisting of only a laser, two lenses, and a detector has been used to connect two high-speed digital circuits separated by over 24 cm. No external laser driver, detector preamplifier, or other interface circuitry was required. The interconnection operates at data rates of 400 Mb/s.
Dean Z. Tsang,
"Free-Space Optical Interconnects", Proc. SPIE 0994, Optoelectronic Materials, Devices, Packaging, and Interconnects II, (9 February 1989); doi: 10.1117/12.960113; https://doi.org/10.1117/12.960113