Fibre Optics PDF - A Textbook of Applied Physics

## A Textbook of Applied Physics Volume I-Second Edition ### 6. Fibre Optics #### 6.1 Introduction - In the middle of the last century, experiments were carried out to find the viability of propagating information carrying light beam through the open atmosphere. - It was realized that besides a hu...

## A Textbook of Applied Physics Volume I-Second Edition ### 6. Fibre Optics #### 6.1 Introduction - In the middle of the last century, experiments were carried out to find the viability of propagating information carrying light beam through the open atmosphere. - It was realized that besides a huge loss of intensity, variable factors like rain, fog, dust, etc., are hindrances and a guiding medium is a must through which light can propagate. - After intensive investigations, optical fibre was found to be the most viable option. - Using optical fibre, information carrying light can be transmitted over a long distance without any significant loss. - Optical fibre also enables us to peep into otherwise inaccessible locations like the interior of human body or the jet engines. #### 6.2 Optical Fibre - An optical fibre is a very thin, flexible thread of transparent glass or plastic in which light is transmitted through multiple total internal reflections. - It consists of a central cylinder, called core, surrounded by a layer of material called cladding, which in turn is covered by a protective jacket. - It is the core through which the light actually propagates and is usually made up of glass or plastic of refractive index $µ_1$. - Cladding keeps the light waves within the core because the refractive index $µ_2$ of the cladding material is less than that of the core ($µ_1 < µ_2$). - It is also made up of glass or plastic and provides some strength to the core. - The jacket protects the fibre from abrasion, crushing, moisture, etc. - A schematic representation of an optical fibre is shown in Fig. 6.1. - Typically, the core diameter varies from 5 µm to 100 µm while the cladding diameter is usually about 125 µm. #### 6.3 Numerical Aperture - It is not always practical to talk about acceptance angle and refractive indices of core and cladding using the ray theory of light. - A parameter, called the numerical aperture (NA) of the fibre, is more useful and is introduced below. #### 6.4 Types of Optical Fibres There are two types of optical fibres: - **(i) Step-Index Optical Fibre (SI Fibre):** In this type of the core is homogeneous with constant refractive index $µ_1$ and the cladding has also a constant refractive index $µ_2$ ($µ_1 > µ_2$). - **(ii) Graded-Index Optical Fibre (GRIN Fibre):** In this type of fibre, the core has non-uniform refractive index that gradually decreases from the centre towards the core-cladding interface, while the cladding has a constant refractive index. #### 6.6 Attenuation in Optical Fibre - Many types of losses occur during the propagation of light in an optical fibre, i.e., an optical signal gets progressively weaker as it propagates through the fibre. - This reduction of signal strength is called attenuation of signal. - The attenuation of signal is represented by a parameter, a, called fibre attenuation and is expressed as: - $α = \frac{10 log \frac{P_i}{P_0}}{L}$, where, $P_i$ is the input optical power, $P_0$ the output optical power and $L$ the length of the optical fibre in km. #### 6.7 Dispersion in Optical Fibres - Broadening (or spreading) of pulse during its propagation through an optical fibre is referred to as dispersion. - Due to dispersion, a pulse is broader at the output of an optical fibre compared to that at the input. - As a result, the pulse gets distorted. #### 6.8 Optical Communications - Optical communication refers to the transfer of information carrying light from one place to another, using light wave as a carrier. - The information carrying capacity of the optical system makes optical communication unique. #### 6.9 Applications of Optical Fibres Optical fibre is being extensively used in almost all fields of human activity. Some of the important applications are mentioned below: - **(i) Optical Communication:** - **(ii) Medical Applications:** - **(iii) Millitary Applications:** - **(iv) Entertainment/Television Applications:** - **(v) Industrial Applications:** - **(vi) Computer Networking:** ## 7. Holography - We are familiar with photograph and photography. - In conventional photography, a photograph is a two-dimensional recording of a three-dimensional scene or object. - What is recorded is merely the intensity distribution in the original scene on a light sensitive medium which is sensitive only to the intensity variations of light. - Consequently, the information about the relative phases of the light waves from different parts of the scene is lost. - In other words, information about the phase or the relative optical paths from different parts of the scene is lost. - Therefore, a photograph gives a two-dimensional view of a three-dimensional object. - Thus, the three-dimensional character (e.g. parallax) of the object is lost. ### 7.1 Basic Principle of Holography - This limitation of photography is overcome in holography. - In holography the complete wave field, both the intensity and the phase of the light scattered by the object is recorded. - The holography was developed by Dennis Gabor (awarded Nobel Prize in 1971) in which he conceived the idea of recording the phase and amplitude of the light scattered by the object. - Since all recording media respond only to the intensity, it is necessary to convert the phase information of the scattered light into intensity variation. - This is done by coherent illumination discussed in the next section. - A hologram is a two-dimensional recording but produces a three-dimensional image ('Holos' in Greek means, 'whole'). - The wide availability of laser light has made the use of holography more widespread. ### 7.2 Construction and Reconstruction of Image on Hologram - The process of recording the image on a hologram or the construction of hologram is shown in Fig. 7.1. - In this process, also called coherent illumination, the scattered wave (emanating from the object) is superimposed with another coherent wave, called the reference wave (usually a plane wave) and the photographic plate is made to record the resulting interference pattern. - Thus, what is recorded on the photographic plate is the interference pattern produced by the two interfering waves. - The intensity at any point in this pattern depends on the phase as well as the amplitude of the wave scattered by the object. - The processed photographic plate, called a hologram, contains information on both the phase and the amplitude of the object wave. - Unlike a photograph, a hologram bears no resemblance to the object and the information in the hologram exists in coded form.