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Losses and Dispersion in Waveguides

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Losses and Dispersion in Waveguides Losses and Dispersion in Waveguides Wei-Chih WangInstitute of Nanoengineeirng and Microsystems National Tsing Hua University w.wang 1 • dB is a ratio of the power received verses the power transmitted Loss (dB) = 10log (power transmitted / power received) w.wang 2 Intrisic Loss Mechanism • Scattering Loss Predominated in glass or dielectric (such as oxide) waveguides. • Absorption Loss Most important in semiconductor and other crystalline materials. • Radiation Loss w.wang 3 Higher order mode Lower order mode w.wang 4 (loss coefficient) is the propagation constant in x direction = in substrate and cover tg is glass thickness w.wang 5 Scattering The scattering from molecules and very tiny particles (< 1 /10 wavelength) is predominantly Rayleigh scattering. For particle sizes larger than a wavelength, Mie scattering predominates. This scattering produces a pattern like an antenna lobe, with a sharper and more intense forward lobe for larger particles. HyperPhysics Mie scattering is not strongly wavelength dependent and produces the almost white glare around the sun when a lot of particulate material is present in the air. It also gives us the the white light from mist and fog. w.wang 6 Blue Sky HyperPhysics w.wang 7 Rayleigh Scattering (small particle scattering) The random localized variations of the molecular positions in glass create random inhomogeneities of the refractive index that acts as tiny scattering centers. The amplitude of the scattered field is proportional to 2. The intensity is therefore proportional to 4 or 1/4 Joseph C. Palais Rayleigh scattering occurs when the size of the density fluctuation (waveguide defect) is less than one-tenth of the w.wangoperating wavelength of light, fucntion of wavelength. See blue sky because shorter wavelength 8 Rayleigh Scattering HyperPhysics It is Rayleigh scattering off the molecules of the air which gives us the blue sky. w.wang 9 Absorption Coefficient Due to Rayleigh Scattering w.wang 10 Absorption Coefficient Due to Rayleigh Scattering Shorter the wavelength the more w.wang Rayleigh scattering 11 Mie scattering (larger particle scattering) If the size of the defect is greater than one-tenth of the wavelength of light, the scattering mechanism is called Mie scattering. Mie scattering, caused by these large defects in the fiber core, scatters light out of the fiber core. However, in commercial fibers, the effects of Mie scattering are insignificant. Optical fibers are manufactured with very few large defects. w.wang 12 • Light striking the Ge molecules in the core can be scattered into new pathways out of the fiber • Rayleigh Scattering accounts for 95% of fiber attenuation • Optical Time Domain Reflectometers (OTDR) use this property to measurew.wang loss in a fiber 13 Absorption Joseph C. Palais w.wang 14 Absorption Absorption is a major cause of signal loss in an optical fiber. Absorption is defined as the portion of attenuation resulting from the conversion of optical power into another energy form, such as heat. Absorption in optical fibers is explained by three factors: •Imperfections in the atomic structure of the fiber material •The intrinsic or basic fiber-material properties •The extrinsic (presence of impurities) fiber-material properties Imperfections in the atomic structure induce absorption by the presence of missing molecules or oxygen defects. Absorption is also induced by the diffusion of hydrogen molecules into the glass fiber. Since intrinsic and extrinsic material properties are the main cause of absorption w.wang w.wang 16 = w.wang 17 w.wang 18 Absorption in Silica based Optical Fiber Silica glass (SiO2) is strongly dependent on wavelength. The material has strong absorption around middle-infrared (due to vibrational transitions) and UV (due to electronic and molecular transitions) Absorption in optical fibers is explained by three factors: • Imperfections in the atomic structure of the fiber material - induce absorption by the presence of missing molecules or oxygen defects. • The intrinsic or basic fiber-material properties - UV (due to electronic and molecular transitions) - middle-infrared (due to vibrational transitions) • The extrinsic (presence of impurities) fiber-material properties - OH radicals - transition metal ions w.wang 19 Dependence of the attenuation coefficient of silica glass on wavelengths Scientific and Technological Education in Photonics Silica glass (SiO2) is strongly dependent on wavelength. w.wang 20 UV- electric transitions associated with bandgap in SiO2 IR – molecules vibrations (phonons) Fundamental Molecular vibration B-O 7.3 m P-O 8.0 m Si-O 9.0 m Ge-O 11.0 m w.wang 21 UV and IR Absorption Urbach Tail: GeO2 shifts to longer wavelength: 4.6/ uv = (1.59/(47g+60)) e g= mole function =0.02 (GeO2) small @1.3m wavelength in mm uv => dB/km .IR/ IR= A e Where A = 8x1011 dB/km .IR/= 48.5m w.wang 22 Assignment Calculation of losses using the dB scale become easy. For example, if we have a 40-km fiber link (with a loss of 0.4 dB/km) having 3 connectors in its path and if each connector has a loss of 1.8 dB, the total loss will be the sum of all the losses in dB; or 0.4 dB/km × 40 km + 3 × 1.8 dB = 21.4 dB. Let us assume that the input power of a 5-mW laser decreases to 30 W after traversing through 40 km of an optical fiber. Attenuation of the fiber in dB/km is therefore [10 log (166.7)]/40 0.56 dB/km. (dB) = 10 log10 (P1/P2) w.wang 23 The main cause of intrinsic absorption in the infrared region is the characteristic vibration frequency of atomic bonds. In silica glass, absorption is caused by the vibration of silicon-oxygen (Si-O) bonds. The interaction between the vibrating bond and the electromagnetic field of the optical signal causes intrinsic absorption. Light energy is transferred from the electromagnetic field to the bond. The tail of the infrared absorption band is shown in figure. w.wang 24 • Intrinsic attenuation is controlled by the fiber manufacturer • Absorption caused by water molecules and other impurities • Light strikes a molecule at the right angle and light energy is converted into heat • Absorption accounts for 3-5% of fiber attenuation these is near w.wangthe theoretical limit 25 Extrinsic Absorption Extrinsic Absorption. - Extrinsic absorption is caused by impurities introduced into the fiber material. Trace metal impurities, such as iron, nickel, and chromium, are introduced into the fiber during fabrication. Extrinsic absorption is caused by the electronic transition of these metal ions from one energy level to another. Extrinsic absorption also occurs when hydroxyl ions (OH-) are introduced into the fiber. Water in silica glass forms a silicon-hydroxyl (Si-OH) bond. This bond has a fundamental absorption at 2700 nm. However, the harmonics or overtones of the fundamental absorption occur in the region of operation. These harmonics increase extrinsic absorption at 1383 nm, 1250 nm, and 950 nm. Last figure shows the presence of the three OH- harmonics. The level of the OH- harmonic absorption is also indicated. These absorption peaks define three regions or windows of preferred operation. The first window is centered at 850 nm. The second window is centered at 1300 nm. The third window is centered at 1550 nm. Fiber optic systems operate at wavelengths defined by one of these windows. The amount of water (OH-) impurities present in a fiber should be less than a few parts per billion. Fiber attenuation caused by extrinsic absorption is affected by the level of impurities (OH-) present in the fiber. If the amount of impurities in a fiber is reduced, then fiber attenuation is reduced. w.wang 26 w.wang 27 w.wang 28 w.wang 29 w.wang 30 w.wang 31 w.wang 32 w.wang 33 w.wang 34 w.wang 35 Radiation loss due to bending straight Small bend Large bend w.wang 36 w.wang 37 w.wang 38 Extrinsic attenuation can be controlled by the cable installer w.wang 39 Macrobend loss Radiation attenuation coefficient is t= c1exp (-c2R) Where R is radius of curvature, c1 and c2 are constants independent of R Large bending loss tends to occurs at critical radius Rc 2 2 2 3/2 Rc~ 3n1 / (4(n1 -n2 ) ) Loss maybe reduced by -large relative refractive index difference -Operating at shortest wavelength possible w.wang 40 Fiber bend losses • Bend losses can be approximated using a combination of raytracing results (H. Lambrecht, et.al.) • Validation through optical path measurements 90° bend X axis: NA fiber Y axis: bending radius Blue color = high losses (source: H. Lambrecht) w.wang 41 • Microbends may not be visible with the naked eye • Microbends may be: o bend related o temperature related o tensile related w.wang o crush related 42 Microbend Loss Microbends are caused by small discontinuities or imperfections in the fiber. Uneven coating applications and improper cabling procedures increase microbend loss. External forces are also a source of microbends. An external force deforms the cabled jacket surrounding the fiber but causes only a small bend in the fiber. Microbends change the path that propagating modes take, as shown in figure. Microbend loss increases attenuation because low-order modes become coupled with high-order modes that are naturally lossy. w.wang 43 Total loss due to Scattering Absorption and Radiation w.wang 44 w.wang 45 Coupling Loss w.wang 46 w.wang Cleaving the waveguide 47 Measure only overall light loss w.wang 48 Dispersion w.wang 49 Dispersion w.wang 50 w.wang 51 DISPERSION There are two different types of dispersion in optical fibers. The types are intramodal and intermodal dispersion. Intramodal, or chromatic dispersion occurs in all types of fibers. Intermodal, or modal, dispersion occurs only in multimode fibers.
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