from differences between LED and LD sources. Transmitter designs compensate for differences in optical output power, response time, linearity, and thermal behavior between LEDs and LDs to ensure proper system operation. Fiber-optic transmitters using LDs require more complex circuitry than transmitters using LEDs.

Transmitter output interfaces generally fall into two categories: optical connectors and optical fiber pigtails (fig. 6-1). Optical pigtails are attached to the transmitter optical source. This pigtail is generally routed out of the transmitter package as a coated fiber in a loose buffer tube or a single fiber cable. The pigtail is either soldered or epoxied to the transmitter package to provide fiber strain relief. The buffer tube or single fiber cable also is attached to the transmitter package to provide additional strain relief.

The transmitter output interface may consist of a fiber-optical connector. The optical source may couple to the output optical connector through an intermediate optical fiber. One end of the optical fiber is attached to the source. The other end terminates in

Figure 6-1. - Pigtailed and connectorized fiber-optic devices.

the transmitter optical output connector. The optical source also may couple to the output optical connector without an intermediate optical fiber. The optical source is placed within the transmitter package to launch power directly into the fiber of the mating optical connector. In some cases, lenses are used to more efficiently couple light from the source into the mating optical connector.

OPTICAL DETECTORS AND FIBER- OPTIC RECEIVERS

A fiber-optic transmitter is an electro-optic device capable of accepting electrical signals, converting them into optical signals, and launching the optical signals into an optical fiber. The optical signals propagating in the fiber become weakened and distorted because of scattering, absorption, and dispersion. The fiber-optic device responsible for converting the weakened and distorted optical signal back to an electrical signal is a fiber-optic receiver.

A fiber-optic receiver is an electro-optic device that accepts optical signals from an optical fiber and converts them into electrical signals. A typical fiber- optic receiver consists of an optical detector, a low- noise amplifier, and other circuitry used to produce the output electrical signal (fig. 6-2). The optical detector converts the incoming optical signal into an electrical signal. The amplifier then amplifies the electrical signal to a level suitable for further signal processing. The type of other circuitry contained within the receiver depends on what type of modulation is used and the receiver's electrical output requirements.

A transducer is a device that converts input energy of one form into output energy of another. An optical detector is a transducer that converts an optical signal into an electrical signal. It does this by generating an electrical current proportional to the

Figure 6-2. - Block diagram of a typical fiber-optic receiver.