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Cable Transmission Mediums
Published in Mário Marques da Silva, Cable and Wireless Networks, 2018
The characteristics of optical fibers were exposed, including the advantages of optical fibers as compared to metallic transmission mediums. It was viewed that optical fibers allow a bandwidth much higher than twisted pairs and coaxial cables. Moreover, the immunity of optical fibers to electromagnetic interference was also described. This results from the fact that optical fibers are typically made of silica glass, which is not a metallic material, and therefore, it does not induct interferences. There are two main types of optical fibers: multimode and single mode. It was described that multimode step index presents higher modal dispersion than multimode graded index. It was also viewed that the single-mode optical fiber presents a lower amount of modal dispersion, which translates in the ability to support a higher bandwidth.
Single- and Few-Mode Structures and Guiding Properties
Published in Le Nguyen Binh, Wireless And Guided Wave Electromagnetics, 2017
An optical fiber consists of two concentric dielectric cylinders. The inner cylinder, or core, has a refractive index of n(r) and radius a. The outer cylinder, or cladding, has an index n2 with n(r) > n2 and a larger outer radius. A core of about 4–9 μm and a cladding diameter of 125 μm are the typical values for a silica-based single-mode optical fiber. A schematic diagram of the structure of a circular optical fiber is shown in Figure 6.1. Figure 6.1(a) shows the core and cladding region of the circular fiber, while Figure 6.1(b) and (c) show the figure of the etched cross sections of a multimode and single mode, respectively. The silica fibers are etched in a hydroperoxide solution so that the core region doped with impurity would be etched faster than that of pure silica, and thus the exposure of the core region, as observed. Figure 6.2 shows the index profile and the structure of circular fibers. The refractive index profile can be step or graded.
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Published in Toru Yoshizawa, Handbook of Optical Metrology, 2015
Figure 4.4 shows different conditions of light propagation depending on types of optical fibers. With a step-index-type optical fiber, light is propagated by repeating total reflection at the interface of high-refraction factor core part and low-refraction factor clad part. With graded-index-type optical fiber, light is propagated with smooth meandering according to the distribution of the refraction factor. Although an arbitrary number of optical paths can be set from geometric optics viewpoint, only one with the same phase as the first light is present in one cycle of reflection or meandering in the optical fiber, due to the relationship with the interference with light, and is transmitted. This is referred to as optical fiber mode. With a multimode optical fiber, since the propagation rate is different in every mode, light being incident to the input end becomes a timewise expanded pulse at the output end due to the time difference for every mode. In the meantime, with single-mode-type optical fiber, transmission of a large amount of signals is possible because there is no modal dispersion. Further, optical fibers with a hole inside, referred to as holey fibers, are available to be used for large-capacity and long-distance transmission. Typical dimensions of the optical fiber are core diameter 50 μm/clad and outside diameter 125 μm for multimode and core diameter 10 μm/clad and outside diameter 125 μm for single mode. When the optical fiber is used for optical path of the interferometer, single-mode optical fiber is used because light with single phase is handled.
Optical fiber sensing technology for full-scale condition monitoring of pavement layers
Published in Road Materials and Pavement Design, 2020
Hua-Ping Wang, Ping Xiang, Li-Zhong Jiang
The physical principle of Brillouin optical time domain analysis (BOTDA) is a technique for long-span sensing with relatively high strain/temperature resolution and precision. The interrogatory system is based on the interaction between a laser light and the glass material in an optical fiber. When a light travels through a transparent media, a large portion of the light is transmitted straightforwardly and a small portion of the light is backscattered, as displayed in Figure 2 (Lan, Zhou, & Ou, 2012; Wang et al., 2014). The simulated Brillouin scattering caused by acoustical phonons results in a frequency shift (Wang et al., 2014). Two laser sources, one a pump (pulse) laser source and the other a probe laser source, are introduced into the sensing fiber from two ends. When the frequency difference between the two lasers is equal to the Brillouin frequency shift, the back Brillouin scattering is simulated. It has been found that the Brillouin shift of optical fiber is linearly related to applied strain and temperature (Bao & Chen, 2011). BOTDA is one of the demodulating systems used to obtain distributed strain or temperature measurements along the fiber by using the good linear relationship between the Brillouin frequency shift and strain and temperature. The function can be expressed by where, νB, νB0, Cε and CT indicate the Brillouin frequency shift, original Brillouin frequency shift, strain and temperature coefficients, respectively. This technique uses standard low-loss single-mode optical fiber offering the longest distance range with unrivalled performances. Brillouin frequency-based technique is opposed to intensity based techniques (i.e. Raman scattering) (Culshaw, 2004) and is inherently more accurate and stable in the long term.