A schematic depiction of the organization of an archetypical optical system is given in Figure 12.1. The model used here, as we will see in the subsequent discussion, is quite general and applies to practically all-optical systems that exist, including communications as well as sensing systems. For example, the earliest types of optical communication systems were probably ones in which a lamp was sequentially hidden from sight for fixed periods between periods of being in plain view. Such a system was employed by Paul Revere in 1776, but it is very likely that such systems had been in operation since the times of the early Greeks or Egyptians . In such a system, the information source is the lamp holder, the encoder is his arms together with the occluding sheet, the receiver optics are the eyes of the beholder, the detector is the beholder's retinae, the receiver is the brain of the beholder, and the information out is the response of the beholder. One could say that such a system employs digitally encoded on-off keyd intensity modulation with direction detection (a DEOOKIMDD or DIM system, depending on the receiver design ) if one were to classify this system. A similar block diagram description could apply to an Indian on a hill sending smoke signals back to his tribe a fiber optic gas detection system, a telescope, or, as we will see as we go on, practically any optical system we can devise.
Perhaps a major point in having a model such as in Figure 12.1 is that such a model is so general that we can use it as a basis for comparing widely differing systems that at first may appear incomparable. Certainly, however, any tow systems can be compared by comparing the data launched into the system with that exit the system (bit error rate (BER) counting in digital systems or noise figure measurement in analog systems). Other figures of merit can, indeed, also allow comparison of the specific blocks of the model. Such comparison will be a major occupation of this course - to compare performance of the various schemes for performing the various functions of the blocks of Figure 12.1. The following four subsections will discuss first sources, then fibers, and then detectors. The last section will give some systems architecture perspective, as the other types of components which make up the blocks of Figure 12.1 in the archetypical "modern-day" optical communication systems are very architecture-dependent. The discussion that follows will be given from a somewhat historical perspective.
Optical Paths
The second major development to occur during 1970 was the development of the low loss optical fiber. The idea of communicating through free space is well and good, but as was previously stated, a major advantage of optical transmitters was their directivity. For broadcasting, it is hard to beat radio waves.For point to point, it is hard to beat optics. But there are problems with using free space as a transmission medium.