The field calculation in the stepindex fiber as well as the chromatic dispersion are explained. Optical fibers are classified into two types based on the refractive index profile of core and cladding. For pure silica core fibers, a step index profile can instead be achieved by downdoping the. Graded index or step index multimode fiber publie par emily twain categories. For a step index fiber with a constant refractive index core, the wave equation is bessel differential equation and solutions are cylindrical functions, therefore if cladding is removed the step index fiber. This work provides a numerical study of the scattering of lowcoherent light by an infinite right circular cylinder and various types of optical fiber with step and gradedindex profiles in the. It is customary in the theory of optical fibers to make the weakly guiding approximation n. The light that enters at less than the critical angle is guided along the fiber. Stepindex optical fiber sizing with a rainbow technique. For singlemode fibers, that fraction is low for low v values e. Stepindex fibers are used for the transmission of laser power in sensor technology, for industrial applications, and in medical technology. Step index fibers are used for the transmission of laser power in sensor technology, for industrial applications, and in medical technology. To help reduce modal dispersion, fiber manufacturers created graded index fiber.
The light in the fiber propagates by bouncing back and forth from corecladding interface. Graded index or step index multimode fiber fiber optic. Details of optical fibers here, i have explained two types of optical fiber 1. There are two methods to classify fiber optic cables. Their task is to slightly increase and decrease the refractive index of pure silica.
It compares both step index and graded index fibers. For an optical fiber, a step index profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the corecladding interface so that the cladding is of a lower refractive index. Some popular fiber sizes the transition of the optical parameters from the core to the cladding can be discontinuous stepindex fiber or smooth gradedindex fiber. Air serves as the cladding in the simple glass tube example. For simpler cases, the rp fiber calculator software is sufficient definition. The image to the right is a photograph of a step index fiber, shown next to the tip of a sewing needle. A multimode or singlemode optical fiber with a uniform refractive index throughout the core.
The refractive index of the core of step index fiber is constant throughout the core. Rp photonics encyclopedia v number, normalized frequency. In step index fibers the refractive index of the core medium is uniform and undergoes an abrupt change at the interface of core and cladding as shown in figure. In a step index fiber, the index of refraction changes abruptly from the core to the cladding. A bonded hardclad silica enhances the fibers strength and fatigue resistance. The step is the shift between the core and the cladding, which has a lower refractive index. They get their name from the stepshaped refractiveindex profile between the fiber core and fiber cladding. The question then arises as on how to compute the other.
Derive the expression of numerical aperture of step index fiber. The core refractive index is made to vary as a function of the radial distance from the center of the fiber. Unlike a longitudinal acoustic mode with negligible transverse displacement, a sam has both considerable transverse and longitudinal displacements. The stepindex profile corresponds to a powerlaw index profile with the profile parameter approaching infinity. Flexible biodegradable citratebased polymeric stepindex. During the lightsound coupling process, the fundamental and highorder sams can be guided and excited, ultimately generating a. Step index fiber is a fiber in which the core is of a uniform refractive index and there is a sharp decrease in the index of refraction at the cladding. Graded index fiber is of only of one type that is multi mode fiber. In the second part, a step index fiber bend with a 3 mm radius of curvature is analyzed with respect to propagating modes and radiation loss. The refractive index of the core material is constant throughout the length and diameter and an abrupt change occurs in step at the core cladding boundary. Optical fibers are therefore classified as stepindex or gradedindex, and multimode or singlemode, as illustrated in fig. Rp photonics encyclopedia stepindex fibers, optical fiber.
Because of an abrupt index change at the core cladding interface such fibers are called step index fibers. Our standard glassclad fiber has a silica core with fluorinedoped cladding and acrylate coating. Derive the expression of numerical aperture of step index. Step index optical fiber the refractive index of core and cladding are constant. The light rays,propagate in the form of skew rays or helical rays. The disparity between arrival times of the different light rays is known as dispersion. Jul 16, 2016 step index multimode fiber was the first fiber design but is too slow for most uses, due to the dispersion caused by the different path lengths of the various modes. The high signal bandwidth of optical fibers provides. Guide modes of stepindex and other fibers can be calculated and optimized with the rp fiber power software. The first part of the application computes the modes for a straight step index fiber made of silica glass.
Kim a, zhiwen liu b, jian yang a, a department of biomedical engineering, materials research institute, the huck institutes of the life sciences, the pennsylvania state university, university park, pa, 16802, usa. They work on the principle of total internal reflection. Jan 25, 2020 optical fibers are thread like structure made of glass and plastic in which light travel by total internal reflection. To help reduce modal dispersion, fiber manufacturers created gradedindex fiber. Stepindex fibers have a uniform core with one index of refraction, and a uniform cladding with a smaller index of refraction. The core of a step index fiber has a uniform index of refraction right up to the cladding interface where the index changes in a step like fashion. For an optical fiber, a stepindex profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the corecladding interface so that the cladding is of a lower refractive index. The principle of total internal reflection applies to multimode step index fiber.
An accurate numerical method is described for solving the helmholtz equation for a. Graded index fiber has an index of refraction which gradually increases as it progresses to the center of the core. Polymicro technologies optical fibers are available in a variety of standard and specialty options. In that way, one may accurately simulate a fiber with nearly the same mode structure is a true stepindex fiber. The light ray propagate through it in the form of meridiognal rays which cross the fiber axis during every. The refractive indices in the core and cladding are constant. What is the difference between step index and graded index. A typical value is an 0,2 leading to a maximum angle of incidence. Based on above, there are three types of fiber optic cable.
Three basic types of fiber optic cable are used in communication systems. Brillouin scattering induced by shear acoustic mode in a step. The basic material used in the manufacture of optical. Types of optical fiber learning outcome epg pathshala. Step index optical fiber multi mode and single mode step. Mode analysis of different step index optical fibers at. Anyway, real fibers usually also exhibit some smoothing of the corecladding interface, caused by diffusion in the fiber drawing process before the fiber is cooled down. In the case of singlemode sm fibers and a proportion of multimode mm fibers, the fiber has a step index profile. This hcs fiber has a silica glass core and an optical cladding of strong polymer. A new compact expression for the powers of hybrid modes of. Fiber laser, effective refractive index, evanescent field. Yun harvard medical school and wellman center for photomedicine massachusetts general hospital. Within the multimode fiber choices, the degree of refraction change between the core and the cladding varies between graded index and step index. The v number determines the fraction of the optical power in a certain mode which is confined to the fiber core.
Graded index or step index multimode fiber fiber optic solution. Light travels slower as the index of refraction increases. Structure, basics and types of optical fibers step index. Principles of optical fiber communications tutorialspoint. The principle of total internal reflection applies to multimode stepindex fiber. For example, the number of guided modes in a step index multimode fiber is given by v 2 2, and a step index fiber becomes singlemode for a given wavelength when v density function pdf for a fiber of arbitrary refractiveindex profile in the limiting cases in which the. The diameter of core is about 10micrometers in case of single mode fiber and 50 to 200 micrometers in multi mode fiber. In a graded index fiber the velocity increases with distance from the core axis since the refractive index decreases. The core of a stepindex fiber has a uniform index of refraction right up to the cladding interface where the index changes in a. It consists of a core surrounded by the cladding which has a single uniform index of refraction. Numerical aperture is the ability of fiber to collect the light from the source and save the light inside it by maintaining the condition of total internal reflection.
The optical fiber with a core of constant refractive index n1, and a cladding of a slightly lower refractive index n2 is known as step index. Some popular fiber sizes the transition of the optical parameters from the core to the cladding can be discontinuous step index fiber or smooth graded index fiber. Structure, basics and types of optical fibers step index and. In order to achieve a step index profile, the core is typically doped with a uniform level of germanium. Dec 31, 2016 on the basis of refractive index there are two type of optical fiber. Index optical fiber made of biocompatible hydrogels.
Custom fibers, sizes, jackets and coatings are available, including dualcladding with numerical apertures ranging from 0. We report on the first stepindex biodegradable polymer optical fiber biopof fabricated using commercially available polyesters, with a core made from polyd,llacticcoglycolic acid and a. The classification based on the refractive index is as follows. A multimode stepindex fiber with a core diameter of 80. Sep 21, 2017 therefore this type of step index fiber is known as single mode step index fiber. You can see the image shown above, how a single mode of light is propagating through the core of the optical fiber. For extreme temperature or ultrahighvacuum environments, we also offer aluminumcoated fiber and polyimidecoated fiber. A multimode stepindex fiber with a core diameter of 80 m and a relative index difference of 1. Osa brillouin scattering induced by shear acoustic mode. The expression is found with the help of the pauli matrices. The total number of guided modes m for a stepindex fiber is approximately related to the v number for v 20 as follows, 2 m. There are also lowerquality fibers available which. Gradedindex fiber has an index of refraction which gradually increases as it progresses to the center of the core.
This work provides a numerical study of the scattering of lowcoherent light by an infinite right circular cylinder and various types of optical fiber with step and graded index profiles in the. The refractive index of the optical fiber decreases as the radial distance from the fiber axis increases. The simplest form of an optical waveguide consists of a light. What will happen to numerical aperture if cladding is removed. Index of refraction is maximum at the center of the core.
The index of refraction in the core is higher than the index of refraction of the cladding. Our solarizationresistant fiber does not degrade with uv. The image to the right is a photograph of a stepindex fiber, shown next to the tip of a sewing needle. The refractive index of the core is uniform throughout and undergoes an abrupt change or step at the cladding boundary.
This change in material creates a strong mismatch in core and cladding index of refraction creating the step index profile. When plotted on a graph as a function of distance from the center of the fiber, the index of refraction resembles a stepfunction. In a gradedindex fiber the velocity increases with distance from the core axis since the refractive index decreases. We present the mechanism of backward brillouin scattering induced by shear acoustic mode sam in a stepindex fiber. This chapter emphasizes the nature of optical modes and their group velocities in stepindex and gradedindex fibers. Aug 25, 2017 therefore this type of step index fiber is known as single mode step index fiber. This particular fiber is fairly large by modern standards, but it provides a nice example of the different layers in an optical fiber.
It leads to the concept of linearly polarized modes. Pdf the pluta polarizing interference microscope is used to measure the refractive index profile and material dispersion of step index optical. Graded index fiber is of only one type,that is multi mode fiber. The expression is reducible to widely used approximate forms for hybrid mode powers of weaklyguided fibers. Step index fiber in a step index fiber, the refractive index changes in a step fashion, from the centre of the fiber, the core, to the outer shell, the cladding. We report on the first step index biodegradable polymer optical fiber biopof fabricated using commercially available polyesters, with a core made from polyd,llacticcoglycolic acid and a. It simplifies the characteristic equation for the modes. Many fiber parameters can be expressed in terms of v, such as. Rp photonics encyclopedia stepindex fibers, optical. They get their name from the step shaped refractive index profile between the fiber core and fiber cladding. Index of refraction is constant for cladding part of the fiber.
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