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4948 results in Electronic, optoelectronic devices, and nanotechnology

Page 84 of 248

  • Preface

  • Sudhir Ranjan Jain
  • Book:
    Mechanics, Waves and Thermodynamics
    Published online:
    05 May 2016
    Print publication:
    09 May 2016, pp xvii-xviii

  • Summary

    A scientific appreciation of the world around us requires classical physics. The familiar, however, looks somehow more complex than what is learned at school. That a distilled set of equations will allow us an understanding seems somewhat doubtful. During 2011–2014, in my lectures on “Classical physics” a four-month long semester course to first-year undergraduates, I tried to bring some of these observations into the classroom. The encouragement received from the students provided me enthusiasm to write these notes. I have used a large number of sources - several fascinating books and articles published in journals. Hence, this work does not claim originality. Here a compact account is presented, which is understandable in one semester. This has been experimentally verified, so to say!

    The goal is to dig deeper into the mechanics of single particle to that of many particles to kinetic theory, and, to proceed from oscillations to waves to sound, eventually we discuss the elementary concepts of thermodynamics. Problem-solving is an integral part of the course; this was done by student-teams, each consisting of five to six students. Cooperation is a lesson that is learnt best, early in life.

    What is classical physics?

    Is it physics where the Planck's constant is zero? That does not sound very meaningful as we are given a world or a universe which is labelled by a value of the Planck's constant. It would be more meaningful to say that “Classical Physics” describes phenomena where we might do away with the Planck's constant. For instance, planetary motion may be understood without worrying about quantum mechanics. However, it is useful and important that we pursue this study with an awareness of quantum theory. Many aspects of quantum mechanics are intricately linked and even dependent on the structure of classical mechanics. The meaning of momentum operator in quantum mechanics is understood with the help of the usual notion of momentum in classical mechanics. On the other hand, there are phenomena described classically where the concept of a quantum is employed. For instance, in our dealing with turbulence of fluids or plasma the interaction between a particle or a wave with another wave is described using the quanta called plasmons - a quantum of a collective excitation.

  • 2 - Basics of Optical Fibers

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 37-130

  • Summary

    An optical fiber is the core component of an optical fiber communication link. Popularly known as optical fiber cables, they are the most promising type of guided transmission medium for virtually all forms of digital and data communications applications. With optical fibers, electromagnetic light waves propagate through the media composed of a transparent material without using electrical current flow. Optical fibers are mostly made of glass or plastic material having properties such that the phenomena of total internal reflection takes place that enables light waves to propagate within it in a properly guided manner similar to that of electromagnetic waves through a metallic transmission medium. This chapter begins with an easy-to-understand ray model of the propagation of light through optical fibers. It is followed by a discussion on the concept of modes and the modal analysis of step-index as well as the graded-index type of fibers. Finally, the type of losses and dispersions are explained to assess the limitation of optical fibers.

    Review of Optical Ray Theory

    In essence, an optical fiber communications system is one that uses light (optical signal) as the carrier of analog or digital information signal. Propagating light waves, carrying information, through the earth's atmosphere is difficult and often impractical. The optical energy in a light wave follows narrow paths, called light rays or beams. For most practical applications, the light rays are used to describe a number of optical phenomena geometrically. In fact, ray theory is known as geometric optics. It is these rays (geometrical paths traversed by light) which actually carry the optical energy.

    Velocity of Propagation

    Electromagnetic energy, such as light waves, travels at a velocity of c = 3 × 108 m/sec approximately in free space (a vacuum). Moreover, the velocity of propagation is the same for all light frequencies in free space. However, it has been demonstrated that

  • • All light frequencies are not propagated with the same velocity.

  • • Since materials are denser (possess higher refractive index) than free space, electromagnetic waves travel slower in materials than in free space.

  • • When the velocity of an electromagnetic wave is reduced as it travels from one medium to another medium of denser material, the light ray refracts (i.e., bends or changes direction) toward the normal.

  • • Likewise, when an electromagnetic wave travels from a denser material into a lighter one, it gets refracted away from the normal.

  • Appendix C - Wireless Optics

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 427-429

  • Summary

    The wireless optics holds great potential for fixed wireless communications as well as for other wireless applications. In spite of all other technology developments, wireless optics has promised an economical alternative to last mile connection solution in the wireless domain. The concept of wireless optics was originally developed over few decades ago by the military. An optical wireless technology, also known as free space optics (FSO) technology, provides broadband data communication links between line-of-sight (point-to-point) locations.

    Wireless optics (sometimes called fibreless optics) is an optical, wireless, line-of-sight, point-to-point, broadband technology. It basically uses infrared transmission instead of RF in the unlicensed higher frequency spectrum above 300 GHz. FSO signals are transmitted using low-power infrared beams (invisible) through free space, thereby limiting the coverage range. Another constraint on its usage is that the radiated power under any circumstances must not cross the specified limits in order to avoid any visual impairment to the human eye.

    Wireless optics transceivers are generally installed in the middle or upper floors, or on an open roof of the building so that a clear line-of-sight transmission path between two stations is available. However, in some applications, these transceivers can be mounted behind a window in an existing office/home building.

    In short, we can say that wireless optics is a matured technology based on line-of-sight propagation which uses optical light for transmission of user information such as voice, data, image, or video in open space. It allows optical connectivity without the use of optical fiber cables. Wireless optics system requires a light source such as LED or a LASER at the transmitter end which is capable of emitting a highly-focused light beam. The LASER beams are preferred in wireless optics because of its advantages as offered in optical fiber communications. Obviously, the only difference is the transmission medium – free space (unguided) as compared to optical fiber (guided). We know that the light travels all the way through air only at a much faster speed than through the optical fiber cable. Wireless optics, also referred to as open-air photonics or optical wireless or free-space photonics (FSP), deals with propagation of modulated optical beams in visible infrared (IR) range over air to provide broadband wireless communications.

  • Appendix A - Fiber Optic Sensors

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 421-424

  • Summary

    Recent advances in fiber optic technology and optoelectronic devices led to emergence of fiber optic sensors. With the availability of highly sensitive detectors and material loss almost approaching to negligible, it is possible to sense even slight variations in intensity level, phase shift and wavelength from external distresses on the optical fiber itself. This is the fundamental concept of fiber optic sensors. The primary function of a fiber optic sensor is to measure or monitor a physical quantity such as temperature, pressure, corrosion, humidity, and similar environmental factors. The basis of measurement is the net effect on its intensity modulation, operating wavelength, phase angle of the incident optical ray, or polarization of the light propagating through the optical fiber.

    Advantages of Fiber Optic Sensors

  • • Multifunctional sensing capabilities including current, electric field, position, vibration, strain, viscosity, chemicals, and acoustic signals.

  • • Multiplexing capability to form sensing networks.

  • • Immune to electromagnetic and radio frequency interference.

  • • High sensitivity.

  • • Robust under worst operating conditions.

  • • Remote sensing capability.

  • • Inability to conduct electric current.

  • • Generally cylindrical geometry.

  • • Small size.

  • • Easy integration into composite materials and different types of natural or man-made structures.

  • • Lightweight.

    • General Structure of Fiber Optic Sensor

    In its simplest form, a fiber optic sensor system comprises of an appropriate sensing element (a transducer having capability of converting the measurand quantity into an equivalent optical signal), an optical source such as an LED or an Injection Laser Diode), an optical fiber for transmission of optical signal from source to destination, an optical detector such as photodiode, and measuring instruments such as an optical spectrum analyzer (OSA) or an analog/digital oscilloscope.

    The main principle of working of an optical fiber sensor is that the output of the physical transducer is used to modulate light intensity, phase angle, operating wavelength, or polarization of the optical signal. This results into a corresponding change in the operational characteristics of the optical signal received at the optical detector after traveling through optical fiber.

    Classification of Fiber Optic Sensors

    There are different ways of classifying fiber optic sensors. For example, these can be categorized based on their principle of operation including the process of modulation and demodulation.

Page 84 of 248