A Fiber Optics Transmitter

A transmitter is a device that converts an electrical analog or digital signal into a corresponding optical signal, so it stands to reason that a fiber optics transmitter is a device that transmits electrical (analog) signals into optical signals and back again. The most common known devices used as the fiber optics transmitter for this purpose are the light emitting diode (LED) or the laser diode (LD).The most popular wavelengths of operation for optical transmitters are 850, 1310, or 1550 nanometers.
Both, of course, are probably known more for their practical uses.

LED

LED or “light emitting diode” is a semiconductor device that emits narrow-spectrum light in a forward direction. A semiconductor is any material with an electrical conductivity level that is between that of an insulator or a conductor. The effect of LED comes out in the form of the light that is emitted, which is dependent on the form of chemical composition within the light itself. LED light can either be ultraviolet, near invisible, or infrared.

The effect of LED light is called “electroluminescence,” which is an optical phenomenon in which a material emits light in response to an electrical current or a strong electrical field. This is, of course, different from the light emitted from a heat source or another light source, including a chemical source, in that the origin of the light is from an electrical source.

LED lights are often used as information lights to denote system information on embedded systems such as airport scanning systems, destination displays for transportation outlets, light bars on emergency vehicles, and some model railroading applications.

LD

LD, sometimes erroneously thought of as the now-defunct laserdisc technology, stands for laser diode, and is the situation in which the active medium is a semiconductor, much the same as it is with LED. The most common type of laser diode is formed from what is known as a “p-n junction” or a combination of semiconductors of a certain type. The p-n junction is then powered by an electrical current and is often referred to as an “injected laser diode.”

Laser diodes, as a type of fiber optics transmitter, serve as the most common type of laser especially for industry. They are used most often in telecommunications as a fiber optics transmitter because of their ease of modification and their reliability as coupled light sources. Infrared laser diodes are also used in DVD and CD players as well as CD-ROM devices whereas the blue-violet laser will soon find wide commercial use (according to experts) in “Blu-Ray” technology involving the DVD and CD markets.

LDs have advantages over LED's in the following ways.

- They can be modulated at very high speeds.
- They produce greater optical power.
- They have higher coupling efficiency to the fiber optic cable.

LED's have advantages over LD's because they have

- higher reliability
- better linearity
- lower cost

Both the LED and LD generate an optical beam with such dimensions that it can be coupled into a fiber optic cable. However, the LD produces an output beam with much less spatial width than an LED. This gives it greater coupling efficiency. Each can be modulated with a digital electrical signal. For very high-speed data rates the link architect is generally driven to a Transmitter having a LD. When cost is a major issue the link architect is generally driven to a Transmitter having an LED.