Loss characteristics of Optical fiber: 1. Atte Attenu nuat atio ion n 2. Absorpti ption 3. Scattering 4. Banding lo loss 5. Disp Disper ersi sion on Loss Loss 6. Coup Coupli ling ng loss losses es 1. Atte Attenu nuat atio ion: n: Attenuation is the loss of optical energy as it travels through the fiber; this • loss is measured in dB/km. Attenuation is a transmission loss that can be measured as a difference • between the output signal power and the input signal power. It can be expressed in dB as – Attenuation loss α = 10 log10 (Pinput / Poutput ) dB The attenuation loss of fiber in dB/km is then expressed as α = 10 log10 (Pinput / Poutput ) / L dB/km Attenuation is a measure of the loss of signal strength or light • power that occurs as light pulses propagate through a run of multimode or single-mode fiber. 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. Causes of Attenuation: Empirical research has shown that attenuation in optical fiber is caused primarily by both scattering and absorption.
Significance of measuring attenuation: 1. Attenuatio Attenuation n is an important important considera consideration tion in the the design design of optical optical transmissi transmission on links since it determines the maximum repeaterless transmission distance between Tx and Rx. Attenuation depends on – a) Attenuation depends on wavelength used (i.e. frequency used). The most common peak wavelengths are 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm. b) Attenuation depends on light intensity i.e input light power
c) Attenuation depends on diameter of optical fiber (diameter of core mainly). For single/mono mode attenuation is minimum since lesser the traversed distance lesser the power loss. d) Attenuation definitely depends on distance. Distance between optical source and repeater/detector. •
Glass fiber (which has a low attenuation) is used for long-distance fiber optic cables; plastic fiber has a higher a ttenuation and hence shorter range.
How Attenuation can be minimized?
2. Abs Absorpt orptio ion: n: Absorption is the way by which the energy of o f a photon is taken up u p by matter, typically the electrons of an atom. Thus, Thu s, the light energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation propag ation is often called attenuation. Types or Classification: Absorption of light in optical fibers may be intrinsic or extrinsic. Intrinsic Absorption: Intrinsic absorption is caused by basic fiber-material properties. If an optical fiber were absolutely pure, with no imperfections or impurities, then all absorption would be intrinsic. •
In fiber optics, silica (pure glass) fibers are used. Silica fibers are used because of their low intrinsic material absorption at the wavelengths of operation.
In silica glass, the wavelengths of operation range from 700 nanometers (nm) to 1600 nm. Cause of Intrinsic Absorption:
Intrinsic absorption is due to material and electron absorption. Material absorption is a loss mechanism which results in the dissipation of some of the transmitted optical power into heat in the optical fiber. An absolutely pure silicate glass has little intrinsic absorption due • to its basic material structure. Electron Absorption: Intrinsic absorption in the ultraviolet region is caused by electronic absorption.
Basically, absorption occurs when a light particle (photon) interacts with an electron and excites it to a higher energy level. ((Light is absorbed as photons that excite electrons of the core atom to a higher energy level.))
Figure shows the level of attenuation at the wavelengths of operation. This wavelength of operation is between two intrinsic absorption regions. The first region is the ultraviolet region (below 400-nm wavelength). The second region is the infrared region (above 2000-nm wavelength).
How Intrinsic Absorption can be minimized? The effect of intrinsic absorption can be minimized by suitable composition of core and cladding materials. Moreover, fibers made of fluoride glasses, for example ZF 4 have low losses at higher wavelength.
Extrinsic Extri nsic Absorpt Ab sorption ion. – Extrinsic absorption is caused by impurities introduced into the fiber material. Metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. •
impurities Extrinsic absorption is caused by the presence of transition metal impurities known as impurity absorption.
Causes of Extrinsic Absorption: •
Metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication cause extrinsic absorption.
•
Extrinsic absorption also occurs when hydroxyl ions (OH-) (due to presence of water vapor) are introduced into the fiber.
•
Chromium and copper can cause attenuation in excess of 1 dB/km in the near infra-red region (~400 GHz).
How Extrinsic Absorption can be minimized??? Extrinsic absorption can be minimized by glass refining techniques such as vapor-phase oxidation which largely eliminates the effects of these metallic impurities. A new kind of glass fiber, known as dry fiber , the OH ion concentration is reduced to
such low levels that the 1.39um 1. 39um peak almost disappears. So, by using Dry fiber Extrinsic absorption can be minimized.
Scattering Loss: •
•
•
•
Scattering occurs when light strikes a substance which emits light of its own at the same wavelength as the incident light. Scattering is a general physical process where some forms of radiation, such as light are forced to deviate dev iate from a straight line by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections that undergo scattering are often called diffuse reflections. Scattering losses are caused by the interaction of light with density fluctuations within a fiber. Density changes are produced when optical fibers are manufactured. The propagation of light through the core of an optical fiber is based on total internal reflection of the lightwave. Rough and irregular surfaces, even at the molecular 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.
Causes of Scattering: During manufacturing, regions of higher and lower molecular density areas, relative to the average density of the fiber, are created. Light traveling through the fiber interacts with the density areas as shown in figure 2-22. Light is then partially scattered in all directions. •
Microscopic variations in the material density, compositional fluctuations, structural inhomogeneities and structural defects occurring during fiber
fabrication causes scattering. These gives rise to refractive-index variations within the glass, these index variations cause scattering of light. Figure 2-22. - Light scattering.
Classification/Types: a) Linear Scatte Scattering ring Losses Losses -1) Raylei Rayleigh gh Scattering Scattering and and 2) Mie Scatter Scattering ing b) Non-lin Non-linear ear Scatte Scatteri ring ng Losse Lossess a) Line Linear ar Scat Scatte teri ring ng Losse Losses: s: In this scattering, optical power transferred from one mode to another mode linearly. Since, it is linear so there is no change chang e in frequency. i.e. the parent/incident pa rent/incident signal’s frequency and scattered/children signal’s frequencies are same. Linear scattering are of two types – a) Rayl Raylei eigh gh Scat Scatte teri ring ng and and b) b) Mie Mie Scatt Scatter erin ing. g.
The degree of scattering varies as a function of the ratio of the particle diameter to the wavelength of the radiation, along with many other factors including polarization, angle, and coherence. Rayleigh scattering: •
Rayleigh scattering is a scattering of light by particles much smaller than the wavelength of the light, which may be individual atoms or molecules.
•
•
Rayleigh scattering is a process in which light is scattered by a small spherical volume of variant refractive index, such as a particle, bubble, droplet, or even a density den sity fluctuation. As light travels in the core, it interacts with the silica molecules in the core. Rayleigh scattering is the result of these elastic collisions between the light wave and the silica molecules in the fiber. Rayleigh scattering accounts for about 96 percent of attenuation in optical fiber.
Something Important to Read: LIGHT IN THE AIR (Taken From: http://www.sciencemadesimple.com/sky_blue.html ) Light travels through space in a straight line as long as nothing disturbs it. As light moves through the atmosphere, it continues to go straight until it bumps into a bit of dust or a gas molecule. Then what happens to the light depends on its wave length and the size of the thing it hits. Dust particles and water droplets are much larger than the wavelength of visible light. When light hits these large particles, it gets reflected, or b ounced off, in different directions. The different colors of light are all reflected by the particle in the same way. The reflected light appears white because it still contains all of the same colors. Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies frequenc ies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering.
Causes of Rayleigh Scattering: • •
It results from non-ideal physical properties of the manufactured fiber. It results from inhomogeneities in the core and cladding.
Because of these inhomogeneities problems occur like – a) Fluctu Fluctuati ation on in refrac refracti tive ve index index b) density density and compos compositi itional onal variat variation ions. s.
How to minimize Rayleigh Scattering? •
Rayleigh scattering is caused due to compositional variations which can be reduced by improved fabrication.
(Fluctuation of refractive index is caused by the freezing in of density inhomogeneities can not be avoided. ) Equation of Rayleigh Scattering: Light scattering can be divided into three domains based on a dimensionless size parameter, α which is defined as Α = πD p / / λ
where π D p is the circumference(The boundary line of a circle) of a particle and λ is the wavelength of incident radiation. Based on the value of α, these domains are: α<<1: Rayleigh scattering (small particle compared to wavelength of light) α≈1: Mie scattering (particle about the same size as wavelength of light) Mie Scattering: •
Mie scattering is a scattering of light by particles approximately equal to the wavelength of the light, which may be individual atoms or molecules.
Causes of Mie Scattering: •
• • •
Occurred due to inhomogeneities in the composition of silica. (i.e. inhomogeneities in the density of SiO2 ) Irregularities in the core-cladding interface, Difference in core cladding refractive index, Diameter fluctuations
•
Due to presence of strains and bubbles.
The scattering caused by such inhomogeneities is mainly in the forward direction depending upon the fiber material, design and manufacture.
How Mie scattering can be minimized? Mie scattering is mainly caused by inhomogeneities which can be minimized by – • •
Removing imperfection due to glass manufacturing p rocess Carefully controlled extrusion(To push or thrust out) and coating of the fiber
Both Mie and Rayleigh scattering are considered elastic scattering (elastic scattering is also called Linear scattering) processes, in which the energy (and thus wavelength and frequency) of the light is not substantially changed. Non-linear Scattering losses: If the output optical power does not increase directly proportional to the input optical power then the system is called non-linear. Non-linear scattering is also called Non-elastic scattering since wavelength and frequency may change. •
The non-linear scattering causes transfer of optical power from one mode to the same or other mode in the either the forward or the reverse direction at different frequency (i.e. in different wavelength).
Classification/types of Non-linear scattering: a) Stimulated Stimulated Brillou Brillouin in Scattering Scattering and b) b) Stimulated Stimulated Raman Raman Scatteri Scattering ng stim·u·late Means -
To rouse to activity or heightened action, as by spurring or goading; excite. 2. To increase temporarily the activity of (a body organ or part). 3. To excite or invigorate (a person, for example) with a stimulant. 1.
a) Stimu Stimulat lated ed Brill Brilloui ouin n Scatter Scattering ing:: Brillouin scattering occurs when light in a medium (for example in Optical fiber) interacts with optical density variations and changes its energy (frequency) and path.
((The density variations may be due to acoustic modes (acoustic wave – the wave that carries sound), such as phonons (***Phonon - a quantum/packet of sound or vibratory {{the thing that vibrates}} elastic energy, being the analogue of a photon of electromagnetic energy), magnetic modes, such as magnons {{A magnon is a collective
excitation of the electrons' spin structure in a crystal lattice}}, or temperature gradients. As described in classical physics, when the medium is compressed its index of refraction changes and the light's path necessarily bends. }}
