o a location or spot that matches the properties of the optical fiber, which is the core of the fiber. This process can be considered as collimating in reverse and requires the spot diameter from the focused beam to be less than or equal to the mode field diameter (MFD) of the optical fiber. Here the mode field diameter is a measure of the intensity of light traveling within a fiber where the intensity drops to 13.5 percent of the peak intensity. For a single-mode fiber the MFD is approximately 15 percent greater than the actual size of the core. Chapter Four 3. Precise alignment of optics is necessary to couple light from one medium to another. Figure 4.13 illustrates the relationship between a laser mounted in housing, a collimating lens, a focusing lens, and an output fiber. As illus- trated in this figure, the entire apparatus can be considered representative of one type of fiber coupler. Actually, there are several types of couplers developed by different manufacturers, and the selection of a particular coupler depends on several factors. These factors include the diameter of the laser diode housing, the connector method, the coupling efficiency, and the diameter of the fiber to be coupled. Concerning the connector method, various products on the market when this book was written included support for screw locking, pigtail coupling, and threading. Con- cerning coupler efficiency, this is normally closely coupled to the laser diode or fiber and converts it into a beam of parallel light. Although a laser's light is less diverse than that of an LED, a collimator is required for both light sources. If we focus on the collimator lens, we can begin to understand how the collimating process operates, SO let's do SO. The Collimating Process Figure 4.14 illustrates the operation of a fiber collimator. By extrapola- tion from optical relationships, it is possible to predict the behavior of the collimated beam. For example, if the focal length of the lens is f, Figure 4.13 screw There are many types of laser diode to f