Lecture4 - 1 Characterization of Optical Fibres; Optical...

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Unformatted text preview: 1 Characterization of Optical Fibres; Optical Time Domain Reflectometry (OTDR); Optical Fibre Materials; Fabrication of Optical Fibres; Optical Fibre Cables EE4035 Optical Communications Semester A 2010-11 Lecture 4 2 Intended Learning Outcomes (ILOs) • Explain the operation principles of various methods for the measurement of the transmission characteristics of multimode and single-mode fibres. • Explain the operation principle and the applications of optical time domain reflectometry (OTDR). • Explain the requirements for optical fibre material and the advantages and applications of silica fibres and plastic fibres. • Describe the processes commonly used for the fabrication of optical fibres. • Explain the requirements for optical fibre cables and describe typical fibre cable structures. 3 Characterization of Multimode Fibres Measurement of Numerical Aperture Far-field technique Measurement of Attenuation Cut-back method Measurement of Dispersion and Bandwidth Time-domain technique Frequency-domain technique 4 Measurement of Numerical Aperture Overfill Multimode fibre ~2m Far-field pattern 0.05 1.0 2 θ a The numerical aperture of a multimode fibre is obtained as the sine of the half-angle where the far-field intensity decreases to 5 percent of its maximum value. Various experimental approaches have been employed for measuring the numerical aperture. These include: rotating a (small-area) detector in the far field, placing a detector array in the far field, rotating the fibre (with a fixed detector), and calculating the far-field angle on the basis of visual assessment of the far-field pattern. D A A D if D 2 A D 4 A A sin 2 2 >> ≈ + = = a NA θ 5 Measurement of Fibre Attenuation The most accurate method for determining the fibre attenuation is the cut-back method . The method involves taking a set of optical output power measurements over the required range of wavelengths using a long length of fibre (typical > 1 km for multimode fibre; a longer length for single-mode fibre). The fibre is then cut back to a point roughly two metres from the input end, and maintaining the same launch condition, another set of power output measurements is taken. This allows the optical attenuation per unit length of fibre to be calculated from the two sets of measurements. 01 02 10 2 1 log 10 P P L L − = α (in dB/km) where L 1 and L 2 are the original and cut-back lengths (in km), respectively, and P 01 and P 02 are the corresponding output powers at a specific wavelength. The cut-back method has the major drawback of being a destructive technique. Example Given L 1 = 2km, L 2 = 2m, λ = 0.85 μ m, the output voltages from the photo-receiver are 2.1V for L 1 and 10.7V for L 2 ....
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Lecture4 - 1 Characterization of Optical Fibres; Optical...

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