LEDs used for Free Space Optical Communications

LED devices have some advantages of low cost and lower safety classification than lasers.  However, LEDs have disadvantages which mean they are not normally used for long range FSO links. These include:

• Hard to modulate fast: high drive currents are required, therefore making high speed FSO links problematic.
• Hard to focus into collimated beam: source aperture is larger than a laser.
• Temperature/lifetime Stability: devices age with time.
• No monitoring diode included.

LED technology

Light Emitting Diodes (LEDs) are solid-state p-n junction devices hat emit light when forward biased. Unlike broad-spectrum incandescent lamps, LEDs emit narrow-band light, often perceived as a single colour. Infrared Emitting Diodes (IREDs) operate similarly in the infrared spectrum. LEDs have many advantages including small size, long life, low power consumption, low cost, and compatibility with solid state drive circuitry.

LEDs are made from a wide range of semiconductor materials, which, with the processing, determine the emitted peak wavelength, in the visible or near infrared part of the spectrum.

One region of the material is doped with donor atoms, and the adjacent region is doped with acceptor atoms to form the p-n junction. Under forward biased conditions, carriers are given enough energy to overcome the potential barrier at the junction, so recombine: some radiatively, emitting photons (light), and others non-radiatively, producing heat. Output power increases approximately linearly with forward current.

LED chips are typically cubic, with metal contacts on the top and bottom. Light is emitted from all sides, but some is reabsorbed or internally reflected, reducing efficiency. Packaging improves durability, increases the proportion of photons escaping, and focuses the light using lenses and reflectors.

In FSO, LEDs are only used in very low-cost systems. For FSO applications requiring high power and high bandwidth, lasers are preferred.

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