Difference between step index fiber and graded index fiber. As explained in the section titled the structure and physics of an optical. Owing to this, multimode fibers can be categorized into step index fiber and graded index fiber seen in figure 1. The input light pulse is made up of a group of modes. Depending upon number of modes propagated by the fiber we have single mode and multimode step index fibers. Difference between single mode and multi mode fiber.
There are three principal possibilities which are multi mode step index, single mode step index and multi mode graded index. Fibers that carry more than one mode are called multimode fibers. In a multimode fiber with a step profile of the refraction index all rays. To meet this condition, a single mode fiber has a much smaller core size and na compared to a multimode fiber at the same wavelength. Compared with single mode fiber patch cables, multimode fiber has a larger core diameters, allowing for the larger numbers of modes. The classification based on the refractive index is as follows. In step index fiber, the refractive index of core is uniform throughout and undergoes an abrupt change step change at the core cladding interface. 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 disparity between arrival times of the different light rays is known as dispersion. In fiberoptic communication, a singlemode optical fiber smf is an optical fiber designed to. The numerical aperture na of a fiber is defined as the sine of the largest. You can see the image shown above, how a single mode of light is propagating through the core of the optical fiber. Therefore this type of step index fiber is known as single mode step index fiber. Single mode fiber optic cable has a small diametral core that allows only one mode of light to propagate.
For a given fiber optic cable, the number of modes that exist depends on the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section. A fiber made of a core and cladding with two reflective indices near and clad is called step index. The advantage of the multi mode graded index compared to the multi mode step index is the considerable decrease in modal dispersion. The single mode fiber has an exceedingly small core diameter of only 5. Modes are the possible solutions of the helmholtz equation for waves, which is obtained by combining maxwells equations and the boundary conditions. Therefore, the concept of a ray and a mode are equivalent, and in the rest of this chapter the term mode will be used. The stepindex profile is used in most single mode fibers and some multimode fibers. A typical singlemode optical fiber, showing diameters of the component layers. Discuss in brief single mode step index fiber and multi. Propagation modes fiberoptic technologies cisco press. Compare dispersion shifted and dispersion flattened fiber. The core is surrounded by a optical material called the cladding that traps the light. The refractive indices in the core and cladding are constant.
Consider an optical fibre having a core of refractive index n 1 and cladding of refractive index n 2. Having a basic understanding of the guiding properties of optical fibers. The index of refraction in the core is higher than the index of refraction of the cladding. The single mode fiber has an exceedingly small core diameter of only 5 to 10 m. One type is step index multimode fiber and the other type is graded index multimode fiber. A singlemode fiber is a single glass fiber strand used to transmit a single mode or ray of light. The term mode is described as the method by which light is propagated within the fibre,i. There are two methods to classify fiber optic cables. In step index the density of the core remains constant from center of edges and the refractive index changes in a step fashion from center of fiber. This indicates that in step index, core and cladding have their own constant index of refractions n1 and n2 respectively. Single mode fibers propagate only the fundamental mode.
These wavelengths were chosen because they best match the transmission properties of. Singlemode fiber is a common type of optical fiber that is used to transmit over longer distances. The refractive index of the optical fiber decreases as the radial distance from the fiber axis increases. Single mode means the fiber enables one type of light mode to be propagated at a time. For step index fibers, the condition for singlemode guidance can be formulated using the v number normalized frequency, which can be calculated from the wavelength, the core radius, and the numerical aperture na. Single mode fibers are defined with a vnumber cutoff of v fiber s fundamental mode.
To help reduce modal dispersion, fiber manufacturers created graded index fiber. We will see the differences taking into consideration various comparison factors of transmission system such as data rate, distance, attenuation, time to travel and so on. Multi mode fiber has a fairly large core diameter that enables multiple light modes to be propagated and limits the maximum length of a transmission link because of modal dispersion. Single mode fibers smf are characterized by a small core diameter from 5 to 10 microns fig. In fiber optic communication, a single mode optical fiber smf is an optical fiber designed to carry only a single mode of light the transverse mode. Rp photonics encyclopedia stepindex fibers, optical. It is customary in the theory of optical fibers to make the weakly guiding approximation n. Multimode fiber and multimode fiber optic cable tutorial. Multimode fibers will propagate many number of modes say hundreds of modes. Rp photonics encyclopedia singlemode fibers, launching. Graded index fiber has an index of refraction which gradually increases as it progresses to the center of the core.
Dispersion phenomena in optical fibers halina abramczyk. It simplifies the characteristic equation for the modes. They get their name from the step shaped refractive index profile between the fiber core and fiber cladding. In fiber optic communicationan intramodal dispersionis a category of dispersion that occurs within a single mode optical fiber. The number of modes in a fiber optic cable depends upon the dimensions of the cable and the variation of the indices of refraction of both core and cladding across the cross section. It is one of the two optical fiber types, the other being multi mode fiber. Multi mode fiber optic cable fibre optics is sending signals down hairthin strands of glass or plastic fibre.
The stepindex profile corresponds to a powerlaw index profile with the profile parameter approaching infinity. Guide modes of step index and other fibers can be calculated and optimized with the rp fiber power software. This type of fiber is normalized by the international telecommunications. 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. Singlemode stepindex fiber allows for only one path, or mode, for light to. This page on single mode vs multimode fiber describes difference between single mode and multi mode fiber systems.
V step index fiber to exhibit single mode operation when the core refractive index is 1. The difference between single mode and multimode fiber mainly lies in fiber core diameter, wavelength, light source and bandwidth. Step index fibers are used for the transmission of laser power in sensor technology, for industrial applications, and in medical technology. In single mode light takes single path through the fiber core. Single mode operation occurs when the value of the fiber s normalized frequency is between 0 and 2. Single mode fiber core diameter is much smaller than multimode. The optical properties of the fiber material which is usually fused silica. Graded index or step index multimode fiber fiber optic. What you need to know about step index fiber 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.
It is also a step index fibre, but due to its small diameter of the core 210 micrometre, it can support only single mode of light. Step index means sharp step in the index of refraction between core and cladding interface. A ray which enters the fiber with an angle greater than the acceptance angle will be lost in the cladding. Single mode fiber optic cable single mode fiber optic cable has a small diametral core that allows only one mode of light to propagate. Multi mode links can be used for data rates up to 100 gbits. Derive the expression of numerical aperture of step index. In a fiber, if only one mode is transmitted through it then it is said to be a single mode fiber. It leads to the concept of linearly polarized modes.
Modal dispersion can be further decreased by selecting a smaller core size less than 510. This article will describe the main characteristics of optical fiber in general, and the comparison. Dimensions of the multimode fiber a and the single mode fiber b one can say intuitively that because of small core diameter, in the single. Step index optical fiber multi mode and single mode step. As a result, graded index fibers have bandwidths which are significantly greater than step index fibers, but still much lower than single mode fibers. As explained earlier, the modes of a multimode fiber travel at different. Multi mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Basically optical fibers can be classified according to the number of modes they can propagate. It consists of a core surrounded by the cladding which has a single uniform index of refraction. This requires that the core radius is small, particularly for fibers with high na. In a graded index fiber the velocity increases with distance from the core axis since the refractive index decreases. A step index fiber is characterized by the core and cladding refractive indices n 1 and n 2 and the core and cladding radii a and b. Because of this, the number of light reflections created as the light passes through the core decreases, lowering attenuation and creating the.
For example, the number of guided modes in a step index multimode fiber is given by. Single mode fibers will propagate only the fundamental mode. V22, and a step index fiber becomes single mode for a given wavelength when v fiber core radius n core is the index. In a multi mode fiber the radius of the core is much larger 5062. Typical bandwidths for multimode mm fibers are between 200 and. A multimode fiber where the core is all the same index of refraction. The other difference between terms are also provided. The simplest form of an optical waveguide consists of a lightconducting. Another way to reduce modal dispersion is to reduce the cores diameter, until the fiber only propagates one mode efficiently. The light is guided down the center of the fibre called the core. The various modes include multimode step index, single mode step index, single mode dual step index, and multimode graded index.
In a step index fiber, the index of refraction changes abruptly from the core to the cladding. While multimode means the fiber can propagate multiple modes. The bandwidth is about 50 mhz km for multimode step index fiber whereas it is more than 100 mhz km in case of single mode step index fiber. Light travels slower as the index of refraction increases. The principle of total internal reflection applies to multimode stepindex fiber. For simpler cases, the rp fiber calculator software is sufficient definition. The fibre optic is also covered by a plastic jacket for protection. These modes define the way the wave travels through space, i.
Differentiate between multimode and singlemode fibers. The following illustration shows the differences between these two types of multimode fibers on refractive index. The light that enters at less than the critical angle is guided along the fiber. Singlemode fiber features only one transmission mode.
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