Optical fiber - Eter - Biometric Tecnologies

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The optical fibers are strands of glassy materials or polymeric, constructed so as to be able to lead to their internal light, and which find important applications in telecommunications, medical diagnostics, and security lighting.

Available in the form of cables, are flexible, immune to electrical noise and the most extreme weather conditions, and not very sensitive to temperature variations. They have a diameter of cladding (outer covering) of usually 125 micrometers (about the size of a human hair) and weigh very little: a single fiber weighs about 60 g / km, including the sheath that covers it.

Optical fibers are classified as dielectric waveguides: simply the optical fiber can be thought of as a pipeline in which flows the light, comparable to a tube in which the water flows. They, in other words, allow to convey and guide inside them an electromagnetic field of frequency sufficiently high (usually near infrared) with losses extremely limited. Are commonly used in telecommunications as a means of transmission of optical signals over long distances or even on the transport network and the provision of access to wired broadband network (from 10 Mbit / s to Tbit / s using the most sophisticated technologies and WDM).

A rigorous study of the physics of optical fibers requires concepts of quantum optics.

Using a comparison of classical optics, in optical fibers occurs a phenomenon of total internal reflection, for which the discontinuity of the refractive index between the materials of the core and the mantle traps the light radiation until this maintains an angle oblique enough, in practice until the fiber does not make too sharp curves.

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The figure illustrates how two light rays, that is, two trains of electromagnetic radiation, affecting the interface between the core and mantle within the optical fiber.

The optical fibers are divided into multimode and single mode.

Multimode fibers allow the use of cheaper devices, but undergo the phenomenon of intermodal dispersion, to which the different modes are propagated at slightly different speeds, and this limits the maximum distance at which the signal can be received correctly.

Single mode fiber in contrast have a much higher price than multimode, but fail to cover distances and speeds much higher.

Multimode fibers can be further divided into step-index fibers and graded index:

  • in step-index fibers, the refractive index is constant along the whole section of the core and suddenly changes when it meets the cladding
  • in graded index fiber the refractive index gradually changes from the core to the cladding, allowing the use of multi-chromatic light

Constitution

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Diagram of optical fiber single mode (SM)
1.- Core 8 µm
2.- Cladding 125 µm
3.- Buffer 250 µm
4.- Jacket 400 µm

Every single optical fiber is composed of two concentric layers of transparent material extremely pure: a central cylindrical core, or core, and a cladding or cladding around it. The core has a very small diameter of about 10 uM for single mode and 50 um for Multimodal, while the cladding has a diameter of about 125 microns. The two layers are made of materials with slightly different refractive index, the cladding must have a lower refractive index (typically applies 1.475) compared to the core (that is about 1.5). As a further feature of the mantle (buffer) should have a thickness greater than the length of damping evanescent wave, characteristic of the transmitted light so as to capture the light that is not reflected in the core.

The optical fiber operates as a kind of mirror tubular. The light that enters the core at a certain angle (angle limit) propagates through a series of reflections at the surface of separation between the two materials of the core and the cladding.

Outside of the fiber there is a protective sheath polymer that said jacket serves to give resistance to physical stress and corrosion and avoid contact between the fiber and the external environment.

Different types of fibers are distinguished by the diameter of the core, refractive indices, the material characteristics, transition profile of the refractive index and doping (addition of small amounts of other materials to modify the optical characteristics).

The core and the cladding of the optical fiber can be made of silica (glass) or in plastic polymers (plastic fiber).

Silica

The optical fiber is a single fiber of glass.

The fibers are made from ultrapure silica, which is obtained from the reaction between silicon tetrachloride and oxygen. In silicon intended for the production of the core of germanium is added (in the form of germanium tetrachloride) in order to increase the refractive index without changing the attenuation. In destined to silica cladding instead of boron is added in order to reduce the refractive index.

Polymers

The fiber is made from a plastic material.

In these polymer optical fibers the size of the core is much larger (1 mm) compared to the silica fibers, then it has a numerical aperture higher and the possibility to realize multimode fibers. However this type of optical fiber has an attenuation quite high and a low thermal resistance.

Dispersion

Working with physical phenomena at very high frequency (light waves), with the optical fibers would ideally possible transmission speed very high. In practice, however, involved the physical factors which cause distortion and hence intersymbol interference, limiting the transmission speed as possible in an optical fiber.

Loss mechanisms in optical fiber

Ideally, the optical fibers are a transmission means perfect. In fact, in addition to not be affected in any way of electromagnetic interference or crosstalk, if properly structured to ensure the total reflection of the input signal, theoretically allow to completely transfer the input power in the output:

  • the intrinsic properties of the medium;
  • the presence of impurities in the material;
  • specification of open dielectric waveguides.

Specific losses of dielectric waveguides

In addition to losses due to the medium, there are other types of loss in a dielectric waveguide. These are not due to the type of light used but are related to the deformations and discontinuities present in the guide; to have a significant effect it is necessary that the periodicity of the perturbations is such as to generate a constructive interference.

Curvature of the dielectric waveguide

The curvature of the dielectric waveguide has a dual effect on the optical signal:

  • deformation of the distribution of the electromagnetic field;
  • excitation spectrum components side.

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The curvature generates a coupling between the guided mode and ways radiative spectrum. In the case where the radius of curvature is large enough, it can be assumed that the field distribution of the guided mode undergoes a slight deformation and irrelevant. So the loss is linked corner of curvature of the guide.

Corrugations of the dielectric guide

During machining of the guide can be created of the corrugations along the external walls. They may be periodic and thus give rise to a loss of power that is radiated externally. Also in this case there is a coupling between the guided mode and radiative modes.

Imperfections in the manufacture

The signal loss can also be caused by imperfections in the manufacture of fiber: dust particles, micro-voids and cracks. These imperfections, if they have size comparable to the wavelength interfere producing phenomena of diffraction, absorption, etc.

According to this physical principle, a decrease of fiber optic cable generates an attenuation of the light signal transmitted.

Our technology allows the use of fiber optic systems for security applications. Our system VPX is constituted by an electronic analyzer, equipped with a transmitter and a receiver for light signal to which is connected an optical fiber cable of multimode glass, able to measure the variation of the luminous radiation in the fiber optic cable.

Download the PDF of the VPX system

The operation is extremely simple: the system is self-calibrating, the installation is finished and activated, the device generates and analyzes the light that is sent inside the ring of optical fiber for the entire length, detects the variation of the intensity and frequency of the train of light pulses sent from the transmitter to the receiver, due to cutting, tearing or bending of the cable and immediately signals the alarm.

The device carries out the analysis of the bending of the fiber optic cable and detecting the effect on the light signal transmitted in it: the bend of the cable generates an intensity variation caused by the curvature of the dielectric medium (fiber) and the frequency of the pulses transmitted from the cable.

The applications of the system as a burglar alarm are manifold, both for the protection of photovoltaic panels, both the protection of materials and equipment of value located in open places and subjects at high risk of theft, type construction sites, warehouses, airports or other, as well as protection intrusion of fences of any type (mesh, welded, deleted, etc ...).

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