Chapter 3. Optical Transmitters
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 be turned on and off or may be linearly varied in intensity between two predetermined levels. Figure 3.1 shows a graphical representation of these two basic schemes .
The most common devices used as the light source in optical transmitters are the light emitting diode (LED) and the laser diode (LD). In a fiber-optic system, these devices are mounted in a package that enables an optical fiber to be placed in very close proximity to the light-emitting region to couple as much light as possible into the fiber. In some cases, the emitter is even fitted with a tiny spherical lens to collect and focus "every last drop" of light onto the fiber and, in other cases, a fiber is "pigtailed" directly onto the actual surface of the emitter .
LEDs have relatively large emitting areas and as a result are not as good light sources as LDs. However, they are widely used for short to moderate transmission distances because they are much more economical, quite linear in terms of light output versus electrical current input, and stable in terms of light output versus ambient operating temperature. In contrast, LDs have very small light-emitting surfaces and can couple many times more power to the fiber than LEDs. LDs are also linear in terms of light output versus ...