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 are 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
LEDs are solid state p-n junction devices which emit light when forward biased. An LED is a Light Emitting Diode – a generic term. An IRED is an Infrared Emitting Diode.. Unlike incandescent lamps which emit light over a very broad range of wavelengths, LEDs emit light over such a narrow bandwidth that they appear to be emitting a single “color”. Their small size, long operating lifetimes, low power consumption, compatibility with solid state drive circuitry and relatively low cost, make LEDs the preferred light source in many applications.
LEDs are made from a wide range of semiconductor materials. The emitted peak wavelength depends on the semiconductor material chosen and how it is processed. LEDs can be made which emit in the visible or near infrared part of the spectrum.
The P-N junction is formed by doping one region of the material with donor atoms and the adjacent region with acceptor atoms. Like all P-N junction devices, LEDs exhibit the familiar diode current-voltage characteristics. LEDs emit light only when they are biased in the forward direction. Under forward biased conditions carriers are given enough energy to overcome the potential barrier existing at the junction. After crossing the junction these carriers will recombine. A percentage of the carriers will recombine by a radiative process in which the hole-electron recombination energy is released as a photon of light. The remaining carriers recombine by a nonradiative process and give up their energy in the form of heat. The amount of light generated, or power output of the LED, varies almost linearly with forward current. Doubling the forward current approximately doubles the power output.
Physically, most LED chips resemble a cube with a metallized bottom surface and a top metal contact. Some visible LED devices are planar processed with buried junctions. The majority of high efficiency IRED chips have P-N junctions which extend out to the four sides of the chip. Since injected carrier recombination takes place within a few diffusion lengths of the junction, the light produced by the IRED is generated in this region. Once generated, the light travels out in all directions. Thus, light is not only emitted from the top surface of the chip but also from the sides. As the light travels through the chip some is reabsorbed. Light that strikes the LED chip surface at an angle greater than the critical angle of the dielectric interface is internally reflected. Only that light that exits the LED chip is useful. The packaging used to house the LED chip serves three functions; to protect the chip and its lead wire(s) from hostile environments, to increase the percentage of photons that can escape from the chip to the outside world and to “focus” the light through the use of incorporated lenses and reflectors.
In summary, LEDs are very low-cost devices, with modest output power and modulation bandwidths . For Free Space Optics, LEDs are only used on extremely low-cost systems. For reliable high power and high bandwidth operation, lasers are invariably used.
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