Conventional low earth orbit (LEO) satellite communications over long distances may involve multiple hops between satellites and ground-based antennae with a subsequent increase in latency. Providing direct communications between LEO satellites using lasers allows a communications session to be forwarded between satellites minimising the number of satellite to ground station links involved in the session.

The Amazon LEO initiative Project Kuiper has recently completed a test of an optical inter-satellite link (OISL) operating at 100 Gbit/s over a distance of over 600 km. Project Kuiper has the aim of providing a mesh of inter-satellite links providing global low latency communications.

Reducing the number of hops reduces propagation distances with a subsequent reduction in delay. Since optical signals travel faster in space than through glass fibre, the latency of the OISL system should be lower than that of an equivalent terrestrial optical fibre system.

But terrestrial optical links offer even faster transmission speeds. Cienna has recently rolled out a nation-wide optical fibre network in South Korea spanning 1,000 km and capable of transmitting at 600 Gbit/s per wavelength.
The Cienna system achieves these high speeds using coherent detection. Optical add-drop multiplexers allow individual wavelengths to be switched to serve different locations.

PTT offers online courses introducing LEO satellite communications and optical coherent detection.