Lecture3 - 1 Loss Mechanisms in Fibres; Dispersion Effects...

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Unformatted text preview: 1 Loss Mechanisms in Fibres; Dispersion Effects in Fibres; Nonlinear Effects in Fibres; Polarization Effects in Fibres; Special Fibres EE4035 Optical Communications Semester A 2010-11 Lecture 3 2 Intended Learning Outcomes (ILOs) • Explain various loss mechanisms in silica glass. • Sketch the attenuation spectrum of a typical low-loss single-mode fibre. • Explain and evaluate various dispersion effects in optical fibres. • Describe various types of single-mode fibres. • Explain various dispersion compensation schemes and the concept of dispersion management. • Describe various nonlinear effects in optical fibres. • Explain and evaluate the effects of birefringence and polarization mode dispersion in a single-mode fibre. • Describe the properties of some special optical fibres. 3 Loss Mechanisms in Optical Fibres Material Absorption Losses • Intrinsic Absorption • Extrinsic Absorption Linear Scattering Losses • Rayleigh Scattering • Mie Scattering 4 Intrinsic Absorption Pure GeO 2-SiO 2 glass Ultraviolet Absorption due to the stimulation of electron transitions within the glass Infrared Absorption due to the interaction of photons with molecular vibrations within the glass. Strong absorption bands arise from oscillations of structural units, such as Si-O (9.2 μ m), P-O (8.1 μ m), B-O (7.2 μ m), and Ge-O (11.0 μ m) within the glass. 5 Extrinsic Absorption Transition metal element impurities Absorption losses caused by some of the more common metallic ion impurities in glasses, together with the absorption peak wavelengths Peak wavelength (nm) One part in 10 9 (dB km − 1 ) Cr 3+ 625 1.6 C 2+ 685 0.1 Cu 2+ 850 1.1 Fe 2+ 1100 0.68 Fe 3+ 400 0.15 Ni 2+ 650 0.1 Mn 3+ 460 0.2 V 4+ 725 2.7 Transition element contamination may be reduced to acceptable levels (i.e., one part in 10 10 ) by glass refining techniques, such as vapour-phase oxidation, which largely eliminates the effects of these metallic impurities. 6 Extrinsic Absorption Water as OH ion The hydroxyl groups (OH) are bonded into the glass structure and have fundamental stretching vibrations, which occur at wavelengths between 2.7 and 4.2 μ m depending on group position in the glass network. The fundamental vibrations give rise to overtones appearing almost harmonically at 1.38, 0.95, and 7.2 μ m. Furthermore, combinations between overtones and the fundamental SiO 2 vibration occur at 1.24, 1.13, and 0.88 μ m. 1 ppm OH → 1 dBkm − 1 at 0.95 μ m 2 dBkm − 1 at 1.24 μ m 4 dBkm − 1 at 1.38 μ m 7 Attenuation Spectrum of an Ultra-Low-Loss Single-Mode Fibre OH ions reduced to one part in 10 7 The lowest attenuation occurs at 1.55 μ m and is ~ 0.2 dBkm − 1 , which approaches the minimum possible attenuation of around 0.18 dBkm − 1 at this wavelength....
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This note was uploaded on 04/17/2011 for the course EE 4035 taught by Professor Prof.chiang during the Spring '10 term at City University of Hong Kong.

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Lecture3 - 1 Loss Mechanisms in Fibres; Dispersion Effects...

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