BOOK_CONTENT
From Optical Communications Essentials

When you go to a large hardware store, you will see dozens of different light-bulbs ranging in size and power from flashlight to floodlight applications. A similar situation holds for light sources used in optical communications. In this case the sources are much smaller, but they also range from simple, inexpensive light-emitting diodes (LEDs) to costly, high-power laser diodes with complex semiconductor structures.

Normally a user does not obtain an isolated light source, but purchases it as part of an optical transmitter. However, it is important to know the characteristics of the source in order to choose the transmitter properly. Thus this chapter first presents the characteristics of LED and laser diode light sources. Following this we will see how the sources are incorporated into transmitters. The transmitter can range from a simple inexpensive, LED-based package that fits on a circuit board for short-distance links, to expensive, laser diode-based modules that also contain sophisticated electronics for controlling the temperature, wavelength stability, and optical output power level of the laser diode.

6.1 General Source Characteristics

To understand the differences between LEDs and laser diodes, let us first look at some general characteristics. These include the operating wavelengths of various device materials, the source output spectral width (the range of wavelengths that are contained in an optical output pulse), and the device modulation capability (how fast a device can be turned on and off).

6.1.1 Materials

Semiconductor-based light sources are about the size of a grain of salt. This size allows efficient...

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Products & Services
Fiber Optic Transmitters
Fiber optic transmitters are devices that include an LED or laser source, and signal conditioning electronics, to inject a signal into fiber.
Light Emitting Diodes (LED)
Light-emitting diodes (LED) are PN junction devices that give off light radiation when biased in the forward direction. LEDs are used in a wide variety of indication applications.
Fiber Optic Test Sources
Fiber optic test sources review the performance of a system by injecting light through the fibers.
Fiber Optic Transceivers
Fiber optic transceivers include both a transmitter and a receiver in the same component.
Fiber Lasers
Fiber lasers use optical fibers doped with low levels of rare-earth halides as the lasing medium to amplify light.

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The basic optical transmitter converts electrical input signals into modulated light for transmission over an optical fiber. Depending on the nature of this signal, the resulting modulated light may...

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