power into both single mode and multimode optical fibers; however. LDs usually are used only in single mode fiber systems because they require more complex driver circuitry and cost more than LEDs.

Optical power produced by optical sources can range from microwatts (m W) for LEDs to tens of milliwatts (m W) for semiconductor LDs; however, it is not possible to couple all the available optical power effectively into the optical fiber for transmission.

The amount of optical power coupled into the fiber is the relevant optical power. It depends on the following factors:

The angles over which the light is emitted

The size of the light-emitting area of the source relative to the fiber core size

The alignment of the source and fiber

The coupling characteristics of the fiber

Typically, semiconductor lasers emit light spread out over an angle of 10 to 15 degrees. Semiconductor LEDs emit light spread out at even larger angles. Coupling losses of several decibels (dB) can easily occur when coupling light from an optical source to a fiber, especially with LEDs.

SEMICONDUCTOR MATERIAL

Understanding optical emission in semiconductor lasers and LEDs requires knowledge of semiconductor material and device properties. Providing a complete description of semiconductor properties is beyond the scope of this text. In this chapter we will only discuss the general properties of semiconductor LEDs and LDs.

Semiconductor sources are diodes, with all of the characteristics typical of diodes; however, their construction includes a special layer, called the active layer, that emits photons (light particles) when a current passes through the layer. The particular properties of the semiconductor are determined by the materials used and the layering of the materials within the semiconductor. Silicon (Si) and gallium arsenide (GaAs) are the two most common semiconductor materials used in electronic and electro-optic devices. In some cases, other elements, such as aluminum (Al), indium (In), and phosphorus (P), are added to the base semiconductor material to modify the semiconductor properties. These elements are called dopants. Current flowing through a semiconductor optical source causes it to produce light.

LEDs generally produce light through spontaneous emission when a current is passed through them. Spontaneous emission is the random generation of photons within the active layer of the LED. The emitted photons move in random directions. Only a certain percentage of the photons exit the semiconductor and are coupled into the fiber. Many of the photons are absorbed by the LED materials and the energy is dissipated as heat. This process causes the light output from a LED to be incoherent, have a broad spectral width. and have a wide output pattern.

Laser diodes are much more complex than LEDs. Laser is an acronym for Light Amplification by the Stimulated Emission of Radiation. Laser diodes pro- duce light through stimulated emission when a current is passed through them. Stimulated emission describes how light is produced in any type of laser. In the laser diode, photons, initially produced by, spon- taneous emission, interact with the laser material to produce additional photons. This process occurs with- in the active area of the diode called the laser cavity.

As with the LED, not all of the photons produced are emitted from the laser diode. Some of the photons are absorbed and the energy dissipated as heat. The emission process and the physical characteristics of the diode cause the light output to be coherent, have a narrow spectral width. and have a narrow output pattern.

It is important to note that in both LED and laser diodes all of the electrical energy is not converted into optical energy. A substantial portion is converted to heat. Different LED and laser diode structures convert different amounts of electrical energy into optical energy.

FIBER-OPTIC TRANSMITTERS

As stated previously, a fiber-optic transmitter is a hybrid electro-optic device. It converts electrical signals into optical signals and launches the optical signals into an optical fiber. A fiber-optic transmitter consists of an interface circuit, a source drive circuit, and an optical source. The interface circuit accepts the incoming electrical signal and processes it to make it compatible with the source drive circuit. The source drive circuit intensity modulates the optical source by varying the current through it. The optical signal is coupled into an optical fiber through the transmitter output interface.

Although semiconductor LEDs and LDs have many similarities. unique transmitter designs result