Optical transmitters generate the light signals carried by a fiber optic communication system. a transmitter includes a light source but also includes signal generation circuit and components such as an electro-optic modulator.

The transmitter contains electronics that modulate the light source. At low speeds, the transmitter directly modulates light emission by varying the drive current to the light source. But the transmitter needs a high performance external modulator such as a electro-optic modulator for high speed applications. It maintains a constant drive current to the light source, producing a steady output beam, which passes through the external modulator that produces the 1 and 0 signals.

Transmitters can have more than one light source. Each light source operates at different wavelengths and are modulated separately with their corresponding signals. These light sources are then coupled to a single fiber with WDM (wavelength division multiplexing) technology. DWDM (dense wavelength division multiplexing) technology essentially expand a single fiber's information carrying capability by hundreds of times.

:: What are fiber optic light sources?

A fiber optic light source emits continuous light without any modulation or signal processing. It is one component of a fiber optic transmitter.

In fiber optic communication, the standard light sources are LEDs(light-emitting diodes) and semiconductor lasers. Other types of lasers such as gas lasers may be used in some special cases.

Lasers emit much higher powers than LEDs and can transmit much faster signals. But lasers are also much more expensive than LEDs.

:: LED light sources

All plastic fibers transmit visible light better than infrared light, although they can only carry low speed signals at short distances. So visible red LEDs are widely used in all-plastic fiber data links. One sample application is a data link on same floor of an office building.

Light sources emitting at 750nm to 900nm (central wavelength is 850nm) are used for higher speed and longer distance applications which utilize glass optical fibers. This type of light sources include 850nm LEDs and 850nm semiconductor lasers which are made from gallium arsenide (GaAs) and gallium-aluminum arsenide (GaAlAs).

A typical application for this type of light source is between buildings on a campus. LEDs do not offer any cost advantage over inexpensive GaAs lasers so they are less common.

:: Semiconductor laser sources

Semiconductor lasers emitting at 1310nm, 1550nm and up to 1610nm are used in much longer distance glass optical fiber networks. The applications range from inter-building links to transcontinental cables.

These lasers are made from indium gallium arsenide phosphate (InGaAsP). They produce much higher power and speed. Their major applications are in telecommunication systems.

:: Considerations for light source

Many factors determine the choice of light sources for fiber optic systems. Here are a few listed.

1. The wavelength must fall within a transmission window of the optical fiber being used

2. The power must be high enough to span the distance between optical amplifiers

3. The power must not be too high so that it causes nonlinear effects in the fiber or overloads the receiver

4. The range of wavelengths emitted by the source should not be so broad that dispersion limits transmission speed

5. The light source must be able to couple light effectively into the optical fiber