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4035 results in Optics, Optoelectronics and Photonics

Page 51 of 202

  • 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.

  • 6 - Dispersion Management Techniques

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 279-318

  • Summary

    When all the spectral components are separated from an optical signal, it is termed dispersion. It usually occurs when optical signals travel along optical fiber from transmitter to receiver in an optic–fiber communication link. Dispersion causes distortion in the transmitted optical signal (analog or digital transmission) along the optical fiber. As the optical pulses travel along the optical fiber channel, when digital modulation is used in transmitting optical signals, the dispersion phenomenon causes the broadening of optical pulses. There are different types of dispersion effects such as modal dispersion (in multimode fiber, transmitted optical pulse tends to spread due to time delay between lower- and higher-order propagation modes, causing bandwidth limitation), chromatic dispersion (combination of material dispersion as well as waveguide dispersion that results in spreading of transmitted optical pulse as they travel through the optical fiber), and polarization mode dispersion. Material dispersion happens because of variations in the fiber core refractive index with respect to operating wavelength, and waveguide dispersion happens due to nature of the physical structure of the optical fiber. Due to variations in the fiber core refractive index, different wavelengths of the light beam would travel at somewhat different velocities of light. As a result, an optical pulse gets broadened, causing dispersion. With the introduction of optical amplifiers (as discussed in the previous chapter) as in-line amplifiers in an optic–fiber link, the signal attenuation due to fiber is no more the major concern for achieving desired performance for optical fiber communication systems. However, they aggravate the dispersion problems. In order to achieve the lowest attenuation, there is a need of implementing efficient dispersion management techniques. There are different varieties of optical fibers available including dispersion compensating fibers. With an objective of controlling the spread of transmitted optical pulse in optical fiber communications systems, the dispersion management (also known as dispersion compensation) techniques must be applied.

    This chapter focuses on dispersion management in optical fiber communications. The discussion begins with the need of dispersion management because dispersion-induced pulse broadening imposes the severe limitations on the performance of the system. This is followed by detailed discussions on different techniques of dispersion management as pre-compensation as well as post-compensation. The discussion is carried forward by describing various types of dispersion-compensating fibers including fiber Bragg gratings. Finally, different methods of fabrication of chirped fiber gratings are covered.

  • 1 - Introduction

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 1-36

  • Summary

    Light wave at higher frequency range of electromagnetic spectrum (3 × 1011–3 × 1016 Hz) is used for transmission of information through fibers as transmitting medium in optical fiber communications. It can offer a large bandwidth (more than 50 THz) for data transmission. The emergence of fiber optics as a dominant technology for long-distance broadband services is discussed in this chapter. The basic configuration of optical fiber communication system comprises of an information source, a voltage-to-current converter, an optical source, a channel coupler, an optical fiber channel, an optical repeater, an optoelectronic detector, an electronic receiver, and the output device. The need, advantages, disadvantages, and wide range of applications of optical fiber communication are covered so as to generate interest to know more about the subject in subsequent chapters.

    Historical Development

    Optical fiber communication has been developed over the last two centuries. The first optical communication system, known as the ‘optical telegraph’, was invented in the 1790s by French engineer Claude Chappe. In 1880, Alexander Graham Bell patented the photophone—an optical telephone device for transmission of speech using a beam of light. During the 1920s, an experiment was conducted by J. L. Baird of England and C. W. Hansell of the USA, for transmission of images for TV/Facsimile systems using arrays of uncoated fiber cables. In the 1950s, A. V. Heel and H. Hopkins protected a bare glass fiber by covering it with a transparent cladding having lower index of refraction. As a result, crosstalk between fibers was greatly reduced in addition to providing protection from contamination. This led to the development of the flexible fiberscope, which is widely used in the medical field.

    Note: In 1956, N. S. Kapany of England coined the term ‘fiber optics’. The initial applications of optical fiber were not in communications at all, because the early fibers were too lossy. Bundles of fibers were used for medical imaging to view inaccessible parts of the human body.

    By 1960, attenuation of the order of 1 dB/m was achieved with glass-clad fibers. This was acceptable for medical imaging applications but not for voice/data transmissions. The invention of the lasers in the 1960s marked the beginning of a new era in modern optics, called Photonics.

  • 4 - Optical Receivers

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 177-230

  • Summary

    The purpose of a receiver in an electronic communication system is to extract the information sent by the corresponding transmitter with as minimum a carrier power level as possible. The primary function of an optical receiver in an optical fiber communication link is to convert the received optical signal into an equivalent electrical signal and recover the data. One of the main components of an optical receiver is a photodetector that converts incident optical signals into electric signals using photoelectric effects. High Sensitivity, dynamic range, fast response (i.e., acquisition time), high reliability, low noise, compatible size with that of fiber, and low cost are some of the important requirements of a photodetector. These requirements are best met by semiconductor photodetectors that convert an optical signal transmitted via optical fiber cables to equivalent electrical signals for further processing to achieve the desired output. The type of photodetectors suitable for three optical spectrum ranges of 800–900 nm, 900–1100 nm, and 1100–1600 nm vary in the material used for their fabrication as well as assembly techniques. A p–i–n photodiode is an ideal semiconductor photodetector device, because it can provide high quantum efficiency, fast response and capability to operate at higher modulation frequencies. The minimum received optical power that can be detected by a photodetector is limited by noise. A fully integrated single beam optical receiver comprises of a semiconductor photodiode, preamplifier in the electric domain, digital logic circuits, and an off-chip electronic driver circuit. This chapter discusses all the important aspects of photodetectors and optical receivers. The discussion begins with basic concepts behind the photo detection process, followed by description of different types of photodetectors usually used by optical receivers. Next, the components used in an optical receiver unit are explained. Finally, different types of noise sources in optical receivers that limit the signal-to-noise ratio, the receiver sensitivity parameter and its degradation are covered in sufficient detail.

    Requirements for a Photodetector

    In an optical fiber communication system, a semiconductor photodetector is the basic component in an optical receiver, as shown in Fig. 4.1.

  • 7 - WDM Concepts and Components

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 319-391

  • Summary

    Wavelength division multiplexing (WDM) is the second major fiber–optic revolution in the field of telecommunications. WDM is a technology which combines many different segments of wavelength range, called different independent optical channels, into the same optical fiber. The best feature of an optical fiber is that it has a wide spectral region which ranges from 1260 nm to 1675 nm. The light source used in high-capacity optical fiber communication systems emits a narrow wavelength band of less than 1 nm, thus enabling simultaneous transmission of many optical channels using the same optical fiber. WDM allows a huge increase in capacity of an optical fiber as compared to point-to-point link that carries only a single optical channel. Another big advantage of WDM is that different transmission formats can be supported by various optical channels. It means that without the need of common signal format, any data rate can be transmitted simultaneously and independently using the common optical fiber.

    This chapter focuses on WDM concepts and components used in high-capacity optic–fiber communication networks. The discussion begins with the principle of wavelength division multiplexing which contains an orthogonal set of optical carriers with a suitable guard band, which a single-mode fiber can propagate. This is followed by a brief discussion on WDM system configuration involving a number of optical devices. An account of applications of WDM systems is presented next. The discussion is carried forward by describing various types of WDM components, including transmitters and receivers. Finally, an analysis of system performance issues and WDM soliton systems are covered.

    Principle of Wavelength Division Multiplexing

    Wavelength division multiplexing (WDM) is based on the fundamental physical principle which states that many optical rays having different wavelengths can be propagated together over a common optical channel with no interference. The concept of WDM is analogous to the basic concept of frequency division multiplexing (FDM) in which the available bandwidth of a communications channel in its frequency domain is divided into multiple sub-bands (called user channels). It implies that each user channel occupies only a part of the wide frequency spectrum. In WDM, each user channel is recognized by an optical wavelength. Remember the relationship between the wavelength and frequency as, which implies that shorter the wavelength of the signal, higher will be its frequency, and vice-versa.

  • 8 - Optical Measurements

  • T. L. Singal
  • Book:
    Optical Fiber Communications
    Published online:
    06 August 2018
    Print publication:
    06 May 2016, pp 392-420

  • Summary

    Optical measurements are necessary to verify the operational characteristics of the optical fiber communication link. Various measurement techniques and special-purpose test equipments are employed for determining key performance parameters of the constituent components and devices including the optical fiber. It is quite obvious that optical measurements are needed at different levels of research and design, manufacturing and production of optical components and devices, installation and commissioning of optical fiber communication systems in the field. There is wide variety of optical measurement and test equipments used. These include optical power meter, optical oscilloscope and spectrum analyzer, optical time-domain reflectometer (OTDR), optical waveform analyzer, connector inspection microscope, dispersion analyzer, live fiber detector, talk-set, optical test set (combined source and power meter), etc. All these measurements are wavelength specific. Fiber attenuation and occurrence of faults in the optical fiber link is the main concern in ensuring the desired performance. There are several challenges involved with optical measurements like multiple wavelengths/channels, high optical power levels, need to carry tests remotely along with a high degree of automation.

    Optical power and insertion loss measurements are among the easiest yet the most important optical measurements in optical fiber communications. An OTDR has several uses such as loss measurements as well as fault detection. Live fiber detectors and talk-sets are useful portable test equipment for the purpose of installation, maintenance and repair. Software prediction of an OTDR trace is a recent development in optical measurements. This chapter focuses on optical measurements of transmission properties of major constituents of optical fiber communication system such as optical source power output, optical amplifier noise characteristics, modulation response, insertion loss, fiber attenuation, dispersion parameters, and link fault detection.

    Requirements of Optical Fiber Measurements

    Optical fiber communication systems are evolving with innovations and numerous applications. Existing copper cables are being replaced with optical fibers everywhere in all accessible areas.

Page 51 of 202