Introduction
An optical fiber is made up of the core (Carries the light pulses), the cladding (reflects the light pulses back into core) and the buffer coating (protects the core and cladding from moisture, damage, etc). Together, all of this creates a fiber optic which can carry up to million messages at any time using light pulses.
Fiber optics is the overlap of applied science and engineering concerned with the design and application of optical fibers. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communication.
Fibers are used instead of metal wires because signals travel along them with less loss and are also immune to electromagnetic interference. Fibers are also used for illumination, and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.
Joining lengths of optical fiber is more complex than joining electrical wire or cable. The ends of the fibers must be carefully cleaved, and then spliced together either mechanically or by fusing them together with an electric arc. Special connectors are used to make removable connections.
Pee Pantsy
Fiber optics, through used extensively in the modern world, is a fairly simple and old technology. Guiding of light by refraction, the principle that makes fiber optics possible, was first demonstrated by Daniel colladon and Jacque Babinet in Paris in the early 1840s.
The use of optic fibers for communication purposes were first carried out in western Europe in the late 19th and early 20th century, such as they were used to diagnose a patient’s stomach by a doctor, and those communications within short ranges. Especially, transfer of images by optical fiber was largely popularized at the beginning of 21st century, Due to the growing medical and television demands.
In 1991, the emerging field of photonic crystals led to the development of photonic- crystal fiber which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers become commercially available in 2000. Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.
Applications
1. Optical fiber communication
Optical fiber can be used as a medium for telecommunication and network because it is flexible and can be bundled as cables. It is especially advantageous for long- distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the per- channel light signals propagating in the fiber have been modulated at rates as high as 111 gigabits second by NTT, although 10 or 40 Gb/s is typical in deployed systems. Each fiber can carry many independent channels, each using a different wavelength of light. The net data rate per fiber is the per channel data rate reduced by the FEC overhead, Multiplied by the number of channels.
For short distance applications, such as creating a network within an office building, fiber –optic cabling can be used to save space in cable ducts. This is because a single fiber can often carry much more data than many electrical cables, such as 4 pair of cat- 5 Ethernet cabling. Fiber is also immune to electrical interference; there is no cross- talk between signals in different cables and no pick up of environmental noise. Non-armored fiber cables do not conduct electricity, which makes fiber a good solution for protecting communications equipment located in high voltage environments such as power generating facilities, or mental communication structures prone to lightning strikes.
2. Fiber optic sensors
Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non- fiber optic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor.
Optical fibers can be used as sensors to measure strain, temperature, pressure and other quantities by modifying a fiber so that the quantity to be measured modulates the intensity, phase, polarization, and wavelength or transit time of light in the fiber. Sensors that vary the intensity of light are simplest, since only a simple a simple sources and detector are required. A particularly useful feature of such fiber optic sensors is that they can, if required, provide distributed sensing over distances of up to one meter.
A major benefit of extrinsic sensors is their ability to reach places which are otherwise inaccessible. An example is the measurement of temperature inside aircraft jet engines by using a fiber to transmit radiation into a radiation pyrometer located outside the engine. Extrinsic sensors can also be used in the same way to measure the internal temperature of electrical transformers, where the extreme electromagnetic field present makes other measurement techniques impossible. Extrinsic sensors are used to measure vibration, rotation, displacement, velocity, acceleration, torque, and twisting.
3. other uses of optical fibers
Fibers are widely used in illumination applications. They are used as light guides in medical and other applications where bright light needs to be shone a target without a clear line-of-sight path. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see non-imaging optics). Optical fiber illumination is also used for decorative applications, including signs, art and artificial Christmas trees. Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angels while only employing one light source. Optical fiber is an intrinsic part of the light-transmitting concrete building product, LiTraCon.
In spectroscopy optical fiber bundles are used to transmit light from a spectrometer to a substance which cannot be placed inside the spectrometer itself, in order to analyze it composition.
An optical fiber doped with certain rare earth elements such as erbium can be used as the gain medium of a laser or optical amplifier. Rare-earth doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular optical fiber line both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave.
The process that causes the amplification is stimulated emission.
Optical fiber can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment.
Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.
Principle of operation
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer, both of which are made of dielectric materials. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.
Total internal reflection
When light travelling in a dense medium hits a boundary at a steep angle (larger than the “critical angle” for the boundary), the light will be completely reflected. This effect is used in optical fibers to confine light in the core. Light travel along the fiber bouncing back and forth off the boundary. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles can travel down the fiber without leaking out. This range of angels is called the acceptance cone of the fiber.
In simpler terms, there is a maximum angle from the fiber axis at which light may enter the fiber so that it will propagate, or travel, in the core of the fiber. The sine of this maximum angle is the numerical aperture (NA) of the fiber. Fiber with a larger NA requires less precision to splice and work with than fiber with a smaller NA. Single-mode fiber has a small NA.
Total internal reflection
Multi-mode fiber
The propagation of light through a multi-mode optical fiber.
Fiber with large core diameter (greater than 10 micrometers) may be analyzed by geometrical optics. Such fiber is called multi-mode fiber, from the electromagnetic analysis (see below). In a step-index multi-mode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meets the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected.
The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle and refracted from the core into the cladding, and do not convey and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture.
A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber.
However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber.
Single- mode fiber
Single-mode fiber
The structure of a typical single-mode fiber.
1. Core: 8 um diameter
2. Cladding: 125 um dia.
3. Buffer: 250 um dia.
4. Jacket: 400 um dia.
Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must analyze as an electromagnetic structure, by solution of Maxwell’s equations as reduced to the electromagnetic. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-fiber. The behavior of larger-core multi-mode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core id large enough to support more than a few modes.
The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.
Special-purpose fiber
Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery made propagation.
Photonic-crystal fiber is made with a regular pattern of index variation (often in the form of cylindrical holes that run along the length of the fiber). Such fiber uses diffraction effects instead of or in addition to total internal reflection, to confine light to the fiber’s core. The properties of the fiber can be tailored to a wide variety of applications.
Mechanisms of attenuation
Light attenuation by ZBLAN and silica fibers
Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the relatively high quality of transparency of modern optical transmission media.
Light scattering and specular reflection at the back
Defuse reflection
The propagation of light through the core of an optical fiber is based on total internal reflection of the light wave. Rough and irregular surfaces, even at the modular level, can cause light rays to be reflected in random directions. This is called diffuse reflection or scattering, and it is typically characterized by wide variety of reflection angles.
Light scattering depends on the wavelength of the light being scattered. Thus, limits to spatial scales of visibility arise, depending on the frequency of the incident light –wave and the physical dimension (or spatial scale) of scattering center, which is typically in the form of some specific micro-structure feature. Since visible light has a wavelength of the order of one micron (one millionth of a meter) scattering centers will have dimensions on a similar spatial scale.
Similarly, the scattering of light in optical quality glass fiber is caused by molecular level irregularities (compositional fluctuations) in the glass structure. In deed, one emerging school of thought is that a glass is simply the limiting case of polycrystalline solid. Within this framework, “domains” exhibiting various degrees of short – range order become the building blocks of metals and alloys, as well as glasses and ceramics.
UV- Vis – IR absorption
In addition to light scattering, attenuation or signal loss also occur due to selective absorption of specific wavelengths, in a manner similar to that responsible for the appearance of color. Primary material considerations include both electrons and molecules as follows.
1) At the electronic level, it depends on whether the electron orbitals are spaced (or “quantized”) such that they can absorb a quantum of light ( or photon) of a specific wavelength or frequency in the ultraviolet (UV) or visible ranges. This is what gives rise to color.
2) At the atomic or molecular level, it depends on the frequencies of atomic or molecular vibrations or chemical bonds, how close- packed its atoms or molecules are, and weather or not the atoms or molecules exhibit long- range order. These factors will determine the capacity of the material transmitting longer wavelengths in the infrared (IR), far IR, radio and microwave ranges.
Normal modes of vibration in a crystalline solid.
Thus, multi- phonon absorption occurs when two or more phonons simultaneously interact to produce electric dipole moments with which the incident radiation may couple. These dipoles can absorb energy from the incident radiation, reaching a maximum coupling with the radiation when the frequency is equal to the fundamental vibrational mode of the molecular dipole (e.g Si-O bond) in the far – infrared, or one of its harmonics.
The selective absorption of infrared (IR) light by a particular material occurs because the selected frequency of the light wave matches the frequency (or an intergral multiple of the frequency) at which the particles of that material vibrate. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared (IR) light.
Reflection and transmission of light waves occurs because the frequencies of the light wave do not match the natural resonant frequencies of vibration of the objects. When IR light of these frequencies strikes an object, the energy is either reflected or transmitted.
To be Continue in next session.............
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