TAILIEUCHUNG - Optical Networks: A Practical Perspective - Part 16

Optical Networks: A Practical Perspective - Part 16. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 120 Components Consider the grating shown in Figure a . Multiple narrow slits are spaced equally apart on a plane called the grating plane. The spacing between two adjacent slits is called the pitch of the grating. Light incident from a source on one side of the grating is transmitted through these slits. Since each slit is narrow by the phenomenon known as diffraction the light transmitted through each slit spreads out in all directions. Thus each slit acts as a secondary source of light. Consider some other plane parallel to the grating plane at which the transmitted light from all the slits interferes. We will call this plane the imaging plane. Consider any point on this imaging plane. For wavelengths for which the individual interfering waves at this point are in phase we have constructive interference and an enhancement of the light intensity at these wavelengths. For a large number of slits which is the case usually encountered in practice the interference is not constructive at other wavelengths and there is little light intensity at this point from these wavelengths. Since different wavelengths interfere constructively at different points on the imaging plane the grating effectively separates a WDM signal spatially into its constituent wavelengths. In a fiber optic system optical fibers could be placed at different imaging points to collect light at the different wavelengths. Note that if there were no diffraction we would simply have light transmitted or reflected along the directed dotted lines in Figure a and b . Thus the phenomenon of diffraction is key to the operation of these devices and for this reason they are called diffraction gratings. Since multiple transmissions occur in the grating of Figure a this grating is called a transmission grating. If the transmission slits are replaced by narrow reflecting surfaces with the rest of the grating surface being nonreflecting we get the reflection grating of Figure b . The principle of .

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