CHAPTER 6 FIBER OPTICS AND LIGHTING SYSTEMS

INTRODUCTION This chapter will expand on the information you learned in the CE Basic TRAMAN on fiber optics and area lighting and also introduce you to airfield lighting systems.

FIBER OPTICS

The CE Basic TRAMAN taught you that a fiber- optic data link had three basic functions:

To convert an electrical input signal to an optical signal

To send the optical signal over an optical fiber

To convert the optical signal back to an electrical signal

The fiber-optic data link converts an electrical signal into an optical signal, permitting the transfer of data along an optical fiber. The fiber-optic device responsible for that signal conversion is a fiber-optic transmitter.A fiber-optic transmitter is a hybrid device. The transmitter 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 the source. An optical source converts electrical energy (current) into optical energy (light). Light emitted by an optical source is launched, or coupled, into an optical fiber for transmission.

Fiber-optic data link performance depends on the amount of optical power (light) launched into the optical fiber. This chapter provides an overview of optical sources and fiber optic transmitters.

OPTICAL SOURCE PROPERTIES

The development of efficient semiconductor optical sources, along with low-loss optical fibers, has led to substantial improvements in fiber-optic communications. Semiconductor optical sources have the physical characteristics and performance properties necessary for successful implementations of fiber-optic systems. Optical sources should do the following:

Be compatible in size to low-loss optical fibers by having a small light-emitting area capable of launching light into fiber.

Launch sufficient optical power into the optical fiber to overcome fiber attenuation and connection losses, allowing for signal detection at the receiver.

Emit light at wavelengths that minimize optical fiber loss and dispersion. Optical sources should have a narrow spectral width to minimize dispersion.

Allow for direct modulation of optical output power. Maintain stable operation in changing environmental conditions (such as temperature).

Cost less and be more reliable than electrical devices, thereby permitting fiber-optic communication systems to compete with conventional systems.

Semiconductor optical sources suitable for fiber- optic systems range from inexpensive light-emitting diodes (LEDs) to more expensive semiconductor lasers. Semiconductor LEDs and laser diodes (LDs) are the principal light sources used in fiber optics.

SEMICONDUCTOR LIGHT-EMITTING DIODES AND LASER DIODES

Semiconductor LEDs emit incoherent light. Spontaneous emission of light in semiconductor LEDs produces light waves that lack a fixed-phase relationship. Light waves that lack a fixed-phase relationship are referred to as incoherent light. LEDs are the preferred optical source for multimode systems because they can launch sufficient power at a lower cost than semiconductor laser diodes (LDs).

Semiconductor LDs emit coherent light. Light waves having a fixed-phase relationship are referred to as coherent light. Since semiconductor LDs emit more focused light than LEDs, they are able to launch optical