A study on optic fibers

2.1 INTRODUCTION TO FIBERS:

Optical fiber is a type of optical fiber mostly used for communication over shorter distances, such as within a building or on a campus.Optic fibers support applications from 10 Mbit/s to 10 Gbit/s over link lengths of up to 550 meters—more than sufficient for the majority of premises applications.

Multi-mode fibers are described by their core and cladding diameters. Thus, 62.5/125 µm multimode fiber has a core size of 62.5 micrometers (µm) and a cladding diameter of 125 µm. In addition, multi-mode fibers are described using a system of classification determined by the ISO 11801 standard — OM1, OM2, and OM3 — which is based on the bandwidth of the multi-mode fiber.

Optic fiber has higher “light-gathering” capacity than single-mode optical fiber. Because of its high capacity and reliability, multi-mode optical fiber generally is used for backbone applications in buildings.

In practical terms, the larger core size simplifies connections and also allows the use of lower-cost electronics such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs).

2.2 WHAT IS FIBER OPTIC CABLE?

This image-transmitting device, which used the first practical all-glass fiber, was concurrently devised by Brian O’Brien at the American Optical Company and Narinder Kapany (who first coined the term “fiber optics” in 1956) at the Imperial College of Science and Technology in London.

Early all-glass fibers experienced excessive optical loss, the loss of the light signal as it traveled the fiber, limiting transmission distances. So they were not suitable for telecommunication. Light could not be transferred due to total internal reflection.

Due to the impurities present in the core material in the fiber there was huge data loss. Thus making scientists to develop glass fibers. This motivated scientists to develop glass fibers that included a separate glass coating.

The innermost region of the fiber, or core, was used to transmit the light, while the glass coating, or cladding, prevented the light from leaking out of the core by reflecting the light within the boundaries of the core.

Thus it helped in minimum loss of light which in turn helped in minimum loss of data when there is transmission going on in the optic fibers.

2.3 WHAT IS A SINGLE MODE OPTIC FIBER?

A single-mode optical fiber (SMF) is an optical fiber designed to carry only a single ray of light (mode).This ray of light often contains a variety of different wavelengths.

Although the ray travels parallel to the length of the fiber, it is often called the transverse mode since its electromagnetic vibrations occur perpendicular (transverse) to the length of the fiber.

Single-mode optical fibers are also called monomode optical fibers, single-mode optical waveguides, or unimode fibers. Single mode fibers are also better at retaining the fidelity of each light pulse over long distances than are multi-mode fibers. For these reasons, single-mode fibers can have a higher bandwidth than multi- mode fibers.

2.4 WHAT IS A MULTIMODE OPTIC FIBER?

Multi-Mode cable has a little bit bigger diameter, with a common diameter in the 50-to-100 micron range for the light carry component .Most an application in which Multi-mode fiber is used, 2 fibers are used.

POF is a newer plastic-based cable which promises performance similar to glass cable on very short runs, but at a lower cost.

However, in long cable runs (greater than 3000 feet [914.4 meters), multiple paths of light can cause signal distortion at the receiving end, resulting in an unclear and incomplete data transmission.

Multimode Fiber is an optical fiber that can support many modes of a given wavelength of light. In general, multiple modes are undesirable because they are susceptible to dispersion effects. However, multimode fiber is less expensive to manufacture and less fragile than single mode fiber, so it is often used in less demanding applications.

2.5 PROBLEMS OF OPTIC DETECTION

These disadvantages can be largely avoided by relocation of the primary light source relative to the optical detector which is less important when using optical fiber. A single primary light source can illuminate multiple optical fibers simultaneously, a single source of light can be directed to a number of optical detectors simultaneously via multiple optical fibers.

If the alignment varies in time, even slightly, or there is a power fluctuation causing a change in the intensity of light, the variation will be different from fiber to fiber and from fibers to reference.

The amount of transmitted light is influenced by many factors. Typical variables are the types of glass used, the purity of each glass, the quality of the fused interface between the core and the clad, the quality of the end finishing and assembly of the part.

2.7 RECTIFICATIONS:

These disadvantages can be largely avoided by relocation of the primary light source relative to the optical detector which is less important when using optical fiber.

A single primary light source can illuminate multiple optical fibers simultaneously, a single source of light can be directed to a number of optical detectors simultaneously via multiple optical fibers.

If the alignment varies in time, even slightly, or there is a power fluctuation causing a change in the intensity of light, the variation will be different from fiber to fiber and from fibers to reference. This means that even slight changes in the relative position of the source and the multi-fiber bundle, or diffraction of the light beam by fluctuating air currents, will cause variations.

This means that even slight changes in the relative position of the source and themulti-fiber bundle, or diffraction of the light beam by fluctuating air currents,will cause variations.

2.8 TYPES OF DISPERSIONS AND LOSS:

• Material dispersion
• Waveguide dispersion
• Multimode dispersion

2.9 MATERIAL DISPERSION:

This problem arise due to the difference in refractive index in core material with the wavelength or frequency of light. It is directly proportional to the frequency bandwidth of the transmitted pulse. Material dispersion can be a desirable or undesirable effect in optical applications.

The dispersion of light by glass prisms is used to construct spectrometers an spectroradiometers. Holographic gratings are also used, as they allow more accurate discrimination of wavelengths. However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade images in microscopes, telescopes and photographic objectives.

The most commonly seen consequence of dispersion in optics is the separation of white light into a color spectrum by a prism. From Snell’s law it can be seen that the angle of refraction of light in a prism depends on the refractive index of the prism material.

Since that refractive index varies with wavelength, it follows that the angle that the light is refracted will also vary with wavelength. By using a optical source with narrow width the material dispersion can be reduced.

2.10 WAVEGUIDE DISPERSION:

This problem arises due to the finite frequency bandwidth or velocity in the frequency of light. Optical fibers, which are used in telecommunications, are among the most abundant types of waveguides.

The higher the frequency bandwidth of the transmitted pulse, higher will be the waveguide dispersion. Dispersion in these fibers is one of the limiting factors that determine how much data can be transported on a single fiber.

The transverse modes for waves confined laterally within a waveguide generally have different speeds (and field patterns) depending upon their frequency (that is, on the relative size of the wave, the wavelength) compared to the size of the waveguide.The fiber design must be in such a way that waveguide dispersion can be avoided.

2.11 MULTIMODE DISPERSION:

This arises due to variation of group velocity for each mode at a single frequency. Pulse broadening in large core step-index optical fibers is dominated by multimode dispersion.

Different modes arrive at the end of each fiber at different times. The bandwidth of PMMA core polymer optical fiber depends on launch conditions, being substantially greater for collimated input than for mode-filled launching. The dispersion behavior is significantly affected by both mode dependent attenuation and by power coupling between modes.

The results of time-domain dispersion measurements performed at 650 nm are presented and examined. Mode conversion is studied by examining far-field output patterns under various launch conditions.

Disturbances caused by sharp bends, splices, and couplers can modify the distribution of energy among modes and thus effect the dispersion. Because only the propagation after the disturbance ips effected, a given fiber optic link can have different effective bandwidths in the two counter propagating directions.yThis can be avoided by maintaining the velocity of each mode but with different frequency.

2.12 TRANSMISSION LOSS IN FIBERS:

The main cause of transmission loss in optic fibers is due to absorption and scattering.

Transmission loss can be classified into two types.

(a)INTRINSIC LOSS (abortions loss)

(b)EXTRINSIC LOSS (scattering loss)

2.13 INTRINSIC LOSS:
A)

This loss can be least avoided because this is not caused due to human errors so there will be loss mainly due to natural causes and which are there in its characteristics. There will only be only minimal loss due to this intrinsic loss.

This loss is partly caused due to human errors. In this there will be loss in terms that when impurities are there in this core cable there will be internal absorption that will cause in interior loss.

So it is mainly advised in case of metal cables pure silica must be use so that there will be reduction in a large ratio so that there will be minimal loss due to the introduction of silica cables.

These losses are caused due to various natural factors and errors during the manufacturing of the cables. This leading to the only rectification in this type of loss.

2.13 EXTRINSIC LOSS:
B)

When there is non uniformity in core cladding boundary. There will be a loss of data and light thus mainly leading to extrinsic loss.

Difference in alignment between fibers.If there are also imperfect connections in between fibers there will be loss in data and light. If there are any leaky modes through which there can be light loss.

There will be loss even due to microbending. This is mainly caused due to the bend which is in more bend then there will be loss in transmission data due to this. There will also be loss due to the radiation of leaky modes leading to many loss. These losses can be largely avoided if proper care is taken during manufacturing and installation.

2.14 FRESNEL LOSS:

Light striking the input surface of the fiberwill be reflected, rather than continuing through the fiber, even though the incidence angle is within the acceptance angle of the fiber.

This phenomena is caused by a difference in refractive indices (light traveling in air with one refractive index, meets the core glass surface, with a different refractive index).

Loss is minimized when the two fiber cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fiber’s core will propagate.

This is caused due to various refractive indexes that is caused due to the core that has cladding that will be a small disadvantage in this type of loss. So there must be proper rectifications that will. Enable less loss.

In addition, because light is typically focused on the surface area of the entire fiber, we should also account for cladding losses

2.15 BUNDLE LOSS:

As glass fibers are actually cylinders, when they are grouped together to form a bundle, a space is created between cylinders.

Known as interstitial spacing, this wasted area accounts for 9-11% of the total bundle area. When we add in manufacturing process, the maximum practical transmission efficiency is about 60%.

To use this calculator, we have the freedom to type in a new efficiency value if you desire. Therefore, a perfectly constructed fiber bundle starts with a transmission efficiency of about 64-68%cladding loss, 9-11% interstitial spacing loss.

Very few industrial fiber optic applications use a single fiber. Most customers require a component made from a “Bundle” of fibers in a specific configuration. This bundle also has characteristics which contribute to losses.The best way to determine efficiency is to test the actual part.

2.16 COLOR SHIFTS IN FIBERS:

As visible light travels through fiber, shorter wavelengths attenuate, turning the output greenish yellow.Typical loss in borosilicate glass is .002% per inch. The color shift becomes noticeable in products with fiber lengths longer than 10 feet.

There will be color shifts when there is light transmission in optic fibers this will give various colors and it is used in many showpiece items this the color shifts in optic fibers

2.17 FACTS OF OPTIC FIBERS:

Fiber cables made of glass does not break even if a fully loaded truck runs over it.Water does not corrode optic fibers which has been thought for long this is a myth.When cables get cut there will not be no cracks in the fibers even if high pressure is given in the cables.

Fibers of different manufacturers can be combined together of same type. Eg: single mode 2 single mode ,In multimode fibers different core sized cables can also be joined.Fiber optic cables do not have holes in the center to pass light through it ,it has cladding glass having a reflective light trap.

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