Optical fibers


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Optical fibers

Flexible transparent fiber devices, sometimes called lightguides, used for either image or information transmission, in which light is propagated by total internal reflection. In simplest form, the optical fiber or lightguide consists of a core of material with a refractive index higher than the surrounding cladding. The optical fiber properties and requirements for image transfer, in which information is continuously transmitted over relatively short distances, are quite different than those for information transmission, where typically digital encoding of information into on-off pulses of light (on = 1, off = 0) is used to transmit audio, video, or data over much longer distances at high bit rates. Another application for optical fibers is in sensors, where a change in light transmission properties is used to sense or detect a change in some property, such as temperature, pressure, or magnetic field. See Reflection of electromagnetic radiation, Refraction of waves

There are three basic types of optical fibers. Propagation in these lightguides is most easily understood by ray optics, although the wave or modal description must be used for an exact description. In a multimode, stepped-refractive-index-profile fiber (illus. a), the number of rays or modes of light which are guided, and thus the amount of light power coupled into the lightguide, is determined by the core size and the core-cladding refractive index difference. Such fibers, used for conventional image transfer, are limited to short distances for information transmission due to pulse broadening. An initially sharp pulse made up of many modes broadens as it travels long distances in the fiber, since high-angle modes have a longer distance to travel relative to the low-angle modes. This limits the bit rate and distance because it determines how closely input pulses can be spaced without overlap at the output end.

Types of optical fiber designsenlarge picture
Types of optical fiber designs

A graded-index multimode fiber (illus. b), where the core refractive index varies across the core diameter, is used to minimize pulse broadening due to intermodal dispersion. Since light travels more slowly in the high-index region of the fiber relative to the low-index region, significant equalization of the transit time for the various modes can be achieved to reduce pulse broadening. This type of fiber is suitable for intermediate-distance, intermediate-bit-rate transmission systems. For both fiber types, light from a laser or light-emitting diode can be effectively coupled into the fiber. See Laser, Light-emitting diode

A single-mode fiber (illus. c) is designed with a core diameter and refractive index distribution such that only one fundamental mode is guided, thus eliminating intermodal pulse-broadening effects. Material and waveguide dispersion effects cause some pulse broadening, which increases with the spectral width of the light source. These fibers are best suited for use with a laser source in order to efficiently couple light into the small core of the lightguide and to enable information transmission over long distances at very high bit rates.

A special class of single-mode fibers comprises polarization-preserving fibers. In an ideal, perfectly circular single-mode fiber core, the polarization state of the propagating light is preserved, but in a real fiber various imperfections can cause birefringence; that is, the two orthogonally polarized modes of the fundamental mode travel at different speeds. For applications such as sensors, where controlling the polarization is important, polarization-maintaining fibers can be designed that deliberately introduce a polarization. This is typically accomplished by using noncircular cores (shape birefringence) or by introducing asymmetric stresses (stress-induced birefringence) on the core. See Birefringence, Polarized light

The attenuation or loss of light intensity is an important property of the lightguide since it limits the achievable transmission distance, and is caused by light absorption and scattering. Optical fibers based on silica glass have an intrinsic transmission window at near-infrared wavelengths with extremely low losses. Glass fibers, intrinsically brittle, are coated with a protective plastic to preserve their strength. See Optical materials

References in periodicals archive ?
In a relevant development last year, Iranian scientists at the University of Wisconsin-Milwaukee (UWM) found a new mechanism to transmit light through optical fibers.
Optical fiber: originally an underestimated invention which later completely changed the world of telecommunications technologies.
Safety Technology Holdings (STH) has announced the acquisition of OpTek Systems, a manufacturer of laser processing equipment and provider of micro-machining and optical fiber processing services, the company said.
Arregui, "High sensitive refractometers based on lossy mode resonances (LMRs) supported by ITO coated D-shaped optical fibers," Optics Express, vol.
An experiment was performed to compare the performance between two 20 mm optical fibers with and without an UV filter.
Compared with conventional monitoring techniques, optical fibers which are used in BOTDR-based monitoring system are both sensors and propagation medium of signals.
To survey the deformation of each optical fiber under loading conditions of 20 kg, 50 kg, 70 kg, 100 kg, and 140 kg, the amount of deformation of optical fibers was first surveyed before surcharging to obtain a background load.
TOKYO, Aug 8, 2017 - (JCN Newswire) - Nippon Telegraph and Telephone Corporation, and six partners, KDDI Research, Inc., Sumitomo Electric Industries, Ltd., Fujikura Ltd., Furukawa Electric Co., Ltd., NEC Corporation, and Chiba Institute of Technology have demonstrated the world's largest transmission capacity of 118.5 Tera-bit/s(1) using a multi-core fiber with four optical paths (cores) in the same diameter of currently used optical fiber.
Optical Fiber Displacement Sensor Based on Reflective Light Intensity-Modulated Principle.
Dorosz, "The xanthene dyes doped PMMA microspheres for optical sensor applications," in Proceedings of the 16th Conference on Optical Fibers and Their Applications, Poland, September 2015.
Fujitsu Laboratories and Furukawa Electric have developed a connector that can accommodate different lengths of optical fiber with a spring mechanism that obviates the need for this polishing process, slashing by more than half the cost required to connect optical fibers.
Seepage Monitoring System with Optical Fibers. In order to model seepage by DTS technology, the experiment system is designed, which consists of heating system, seepage system, DTS system, data processing, and analysis system.

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