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2.5.12 Properties of optical fibres
An optical fibre consists of at least two distinct regions known as the core and cladding. When the refractive index of the core, n1, is made higher than the refractive index of the cladding, n2, a light beam may be confined to the core region of the fibre. The most commonly used material for the fabrication of optical fibres is silicon dioxide in which the refractive index can be modified by the addition of dopants such as GeO2, P2O5, F and B2O3 (Waynant, 1993). There are two categories of optical fibre which differ in both propagation properties and in their construction. Multi-mode fibre has a 40 to 300 µm core diameter and a cladding diameter in the range 125 µm to over 300 µm; light can travel via many paths or modes. Single-mode fibre has a core diameter of the order 3 to 10 µm and a cladding diameter of 125 µm. The fibre parameter (Snyder, 1983), V, provides information on the number of modes in a fibre and is defined as
where ρ is the core radius, λ is the wavelength of radiation and Δ is the profile height parameter given by
A step profile fibre having a uniform core and uniform cladding is single-moded when V < 2.405 and the cut-off wavelength, λc, above which only one mode can propagate is given by
The mode field is the single-mode field distribution giving rise to a radial intensity distribution in the fibre. The mode field diameter (MFD) 2W is defined (Petermann, 1983) from the far field intensity distribution F2(q), where q = (1/λ) sin θ, θ being the far field angle,
Attenuation of a signal within a fibre occurs as a result of absorption, material scattering and perturbation of the optical path by bending the fibre. The attenuation A(λ) at wavelength λ is given by
where P1 and P2 are
the optical powers traversing cross-sections 1 and 2 separated by distance
L; for a uniform fibre the attenuation coefficient
Several effects contribute to distort the shape of light pulses propagating down a fibre. In multi-mode fibres, different modes have different optical paths and so a pulse comprising several modes will spread out as it propagates (intermodal dispersion). In a circularly symmetric, single mode fibre the intermodal dispersion is zero and the remaining intramodal dispersion arises from the finite spectral width of the light source, since the group delay of the mode varies with wavelength. This results from the wavelength dependence of refractive index (material dispersion) and from the propagation characteristics of the guiding structure (waveguide dispersion).
The generic characteristics of optical fibres widely used in telecommunication networks are detailed in Recommendations of the Telecommunication Standardization Sector (formerly the Consultative Committee for International Telephony and Telegraphy, CCITT) of the International Telegraph Union.
CCITT Recommendation G651–Characteristics of a 50/125 μm
multi-mode graded index optical fibre cable.
Typical parameters of telecommunication fibres
a Values in the range 2–2.5
dB/km are achieved.
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