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Fiber Optics
Published in Lazo M. Manojlović, Fiber-Optic-Based Sensing Systems, 2022
The dependence of the fiber core and cladding indices of refraction on the wavelength can affect the light propagation along the fiber. In the previous chapter, we have dealt with the phenomenon of modal dispersion where it was noticed that different modes travel with different velocities thus giving a rise to a modal dispersion phenomenon. The modal dispersion can severely affect the data transport at higher rates thus limiting the maximal allowable data rates where the signal deciphering is still achievable. Similar situation is if a multimode optical fiber is deployed in some kind of a measurement instrument that is based on a phase-sensitive detection, that is, the interferometer base measurement system. Typically, if the higher modes are present in the fiber-optic interferometer, the coherence length of the fiber propagating light is thus reduced that further can affect the measurement if the interferometer branches lengths are misbalanced.
Link Bandwidth
Published in Lynne D. Green, Fiber Optic COMMUNICATIONS, 2019
Modal dispersion is due to different rays (modes) having different arrival times at the end of the fiber. For a step-index fiber, the ray that travels in a straight line takes time L/v to arrive at the end of the fiber. The ray that travels at the critical angle (θc) takes a time L/(v sin θc) to arrive. The modal pulse spreading width is the difference of these two times: twmod=L/[v(1−1/sinθc)]=n1ΔL/c=(NA2)(L)/(2n1c)
High Manufacturing Technology
Published in Abdul Al-Azzawi, Advanced Manufacturing for Optical Fibers and Integrated Photonic Devices, 2017
This fiber type is sometimes called graded-index fiber cables and shortened to GRIN fiber cable, as shown in Figure 5.4(c). Graded-index and multimode fiber cables have similar diameters. Common graded-index fibers have core diameters of 50, 62.5, or 85 μm and a cladding diameter of 125 μm. The core consists of numerous concentric layers of glass, somewhat like the annular rings of a tree or a piece of onion. Each successive layer outward from the central axis of the core has a lower index of refraction until reaching the inner diameter of the cladding. Light travels faster in an optical material with a lower index of refraction. So the further the light is from the center axis, the greater is its speed. Each layer of the core refracts the light according to Snell’s law. Instead of being sharply reflected as it is in a step-index fiber, the light is now bent or continually refracted in an almost sinusoidal pattern. Those light rays that follow the longest path by traveling near the outside of the core have a faster average velocity. The light ray traveling near the center of the core has the slowest average velocity. As a result, all rays tend to reach the end of the fiber at the same time. Thus, one way to reduce modal dispersion is to use graded-index fibers. This type of fiber optic cable is popular in applications requiring a wide range of wavelengths, especially in telecommunication systems, scanning, imaging, and data processing.
Advances in Surface Plasmon Resonance-Based Plastic Optical Fiber Sensors
Published in IETE Technical Review, 2022
Riadh A. Kadhim, Abdul Kareem K. Abdul, Liming Yuan
POFs, which have intrinsic features of any fiber optics, are flexible, easy to handle, and have large diameters ( mm). POF allows the utilization of low precision connectors, which decrease the total cost associated with a complete system. The value of Young modulus for the bulk poly(methyl methacrylate) is 3.2 GPa and that of silica fibers is 72 GPa. The polymer also has advantages to resist impacts and vibrations with a low density (1.195 kg m). The NA for the POF is 0.50, thus indicating the conjugation of light in the fiber and reduction in the loss related to macrobending. The disadvantage is the large modal dispersion, resulting in low bandwidth [54]. The schematic of the dimensions of POF is illustrated in Figure 2.