Fiber Optics Study Material PDF - Lovely Professional University

Lovely Professional University Punjab Study Material for Engineering Physics Course code: 206 Topic: Fib...

Lovely Professional University Punjab Study Material for Engineering Physics Course code: 206 Topic: Fiber Optics Prepared by: 21703: G. Joshva Raj April 2, 2020 Contents 1 Introduction to Fiber Optics 1 1.1 Refractive index........................... 1 1.2 Refraction of Light at an Interface................. 2 2 V-number 4 3 Classification of fibers based on refractive index profile 4 3.1 Step index fiber............................ 4 3.2 Graded index fiber.......................... 4 4 Losses in Optical Fibers 5 4.1 Dispersion loss............................ 5 4.2 Absorption loss............................ 5 4.2.1 Intrinsic absorption..................... 6 4.2.2 Extrinsic absorption..................... 6 4.3 Bending loss.............................. 6 4.3.1 Macrobending loss...................... 6 4.3.2 Microbending loss...................... 8 4.4 Scattering loss............................ 8 4.4.1 Rayleigh scattering...................... 8 4.4.2 Mie scattering........................ 8 5 Applications of optical fibers 8 1 Introduction to Fiber Optics 1.1 Refractive index Refractive index (n) of a given medium is defined as the ratio between the velocity of light (c) in vacuum and the velocity of light in the medium (v). c n= (1) v 1 Such parameter can also be termed as absolute refractive index. Refractive index of vacuum is a standard one and is taken as 1. Since light travel slower in any medium than vacuum, refractive index of a given medium cannot be less than 1. Note: Light travel faster in rarer medium than denser medium. Note: Refractive index cannot be less than 1 but can be negative. Relative refractive index is a parameter which is defined as the ratio of refractive index of a given medium (n1 ) with respect to the refractive index of another medium (n2 ) (not air). The relative refractive index is given by n1 n12 =. (2) n2 Relative refractive index difference is given by n1 − n2 (3) n1 1.2 Refraction of Light at an Interface Refraction of light at an interface between two media of different refractive index is governed by Snell’s law: sin i nr = , (4) sin r ni where i is the angle of incidence and r is the angle of refraction. ni is the refractive index of the medium from which angle of incidence is taking place and nr is the refractive index of the medium in which the light is refracted. When light travel from a lower refractive index medium (n1 ) to a higher refractive index medium (n2 ), it bends towards the normal at the interface between the two medium, as shown in the figure. Similarly, if light travel from a higher refractive index medium (n2 ) to a lower refractive index medium (n1 ), then the light ray bends away from the normal, as shown in the Fig. 1. If the angle of incidence φ1 is increased, the refracted light bends further away from the normal. Keep increasing on the angle of incidence, at a particular angle φc the refracted ray travel parallel to the interface. The angle φc is called critical angle and the same is shown in Fig. 2. The expression for critical angle is given by the following expression: n2 φc = sin−1 (5) n1 If the angle of incidence is further increased beyond φc , the refracted angle reflected back into the same medium. This reflection of light fully back into the same medium is called total internal reflection. The phenomenon of total internal reflection is shown in Fig. 3. The angle of incidence above which the light gets totally reflected into the same medium is called acceptance angle. 2 Figure 1: Light passes through an interface between a higher refractive index material and a lower refractive index material Figure 2: Critical angle The light gathering capacity of an optical medium is given by the following expression called numerical aperture: q N A = n21 − n22. (6) The relationship between acceptance angle and numerical aperture is given by θa = sin−1 N A (7) 3  Figure 3: Total internal reflection 2 V-number V-number is used to distinguish whether a fiber is single mode or multi-mode. The V-parameter is given by 2πa q V = n21 − n22 , (8) λ where a is the radius of the fiber and λ is the wavelength of the propagating light in the fiber. If V-number is less than 2.405, then the fiber exhibits single mode. If V-number is more than 2.405, then the fiber is a multimode fiber. 3 Classification of fibers based on refractive in- dex profile Based on the refractive index profile, optical fibers can be classified into two different categories: 3.1 Step index fiber In step index fiber, the refractive index of the core is uniform through out its cross section and make a change in step at core-cladding interface. The cladding have a constant refractive index value but less than that of the step index fiber. In step index fiber, the fiber make a zig-zag path and pass through the axis of the fiber. The refractive index profile of a step index fiber is given as follows: (9) 3.2 Graded index fiber In graded index fiber, the refractive index is not constant in the core. It is maximum at the core and decreases gradually towards the edge of the core. In 4  Figure 4: Phenomenon of dispersion graded index fiber, the propagating light do not cross the axis and makes a helical path. 4 Losses in Optical Fibers The attenuation in optical fiber is given by the following expression: Pi 10log10 , (10) Po where Pi is the power of input and Po is the power of output. Generally, Pi > Po There are different type of losses occur when light propagate through an optical fiber. 4.1 Dispersion loss When a light pulse propagate through an optical medium, different waves in the pulse may get deviated from each other. As a result, a time delay is developed for the arrival of information at the end of the fiber. Such a phenomenon is called dispersion. The phenomenon of dispersion is shown in Fig. 4. It is seen that a pulse which consists of three waves with different wavelengths propagate together before entering into the fiber. When leaving the fiber (in fact, during the propagation through the fiber), the waves deviate from each other. This process is called dispersion. This happen due to the wavelength dependent nature of refractive index n. ie. dn dλ 4.2 Absorption loss Absorption loss happens due to the absorption of light energy by the material. The absorbed energy will be dissipated as heat. As a result of absorption of light energy, there will be a power loss in signal. Absorption of light by a material depends on the wavelength of the propagating light. A given material may absorb light of particular wavelength(s) and transparent to other wavelengths. There are two types of absorption: 5 Figure 5: Absorption spectrum of silica (Courtesy: Applied optoelectronic cen- ter) 4.2.1 Intrinsic absorption In this type of absorption, the light energy is absorbed by the glass material itself by which the fiber is made up of. Usually, an optical fiber is made up of silica glass. The absorption spectra of pure silica glass is given in Fig. 5. From the figure, it is seen that silica glass is transparent between 800 nm 1600 nm absorbs light in other regions. 4.2.2 Extrinsic absorption Absorption of light due to the unwanted foreign molecules (impurities) present in the fiber medium is called extrinsic absorption. Impurities might have been added to the fiber medium during the manufacturing process. Different impu- rities and their corresponding absorption wavelength is shown in Fig. 6. 4.3 Bending loss Due to a bend or defect at the core-cladding interface may lead into a power loss of propagating optical signal. This type of loss is called bending loss. There are two types of bending loss. 4.3.1 Macrobending loss Loss occurs due to the bending of the fiber as shown in Fig. 7. This type of bending may happen during the installation of fiber cabled and could be avoided easily. 6 Figure 6: Names of impurities and their corresponding absorption wavelength (Courtesy: Applied optoelectronic center) Figure 7: Macrobending of fiber (Courtesy: http://community.fs.com/blog/understanding-loss-in-fiber-optic.html) 7  Figure 8: scattering of light 4.3.2 Microbending loss Microbending loss occurs due to small defects at core-cladding interface which develop during the manufacturing process. Fig. ?? shows such type of bending. This type of bending could be avoided by taking care of manufacturing process. 4.4 Scattering loss Scattering is a process which happens when light interacts with the molecules of a medium. Scattering is different from reflection, in the sense that, reflection occurs when light travels from one medium to another medium while scattering occurs when light directly interact with a medium. Due to scattering, the signal will be scattered in different direction so that there will be a loss in the information. The phenomenon of scattering is shown in Fig. 8. In general, there are two types of scattering. 4.4.1 Rayleigh scattering The scattering process that occurs due to the interaction of light waves with the particles which are small in size with the interacting waves is called Rayleigh scattering. Such type of scattering occurs due to the inhomogeneities in the arrangement of atoms or molecules in the medium. 4.4.2 Mie scattering Mie scattering is a type of scattering that occurs when a light wave interacting with a bulk entity which is comparable with the wavelength of incident wave. Mie scattering occurs due to the non-uniform structure of fiber and irregularities in the fiber structure. 5 Applications of optical fibers 1. Communication 8 2. Endoscopy 3. Laparoscopy 4. Different types of sensors 5. Photonic components 6. Fiber optics lasers 7. Refer more applications from internet and other sources. 9