Fibre Attributes
Fibre attributes: Fibre attributes are those characteristics that are retained throughout cabling and installation processes. The values specified for each type of fibre can be found in the appropriate ITU-T Recommendation for multimode fibre (Recommendation ITU-T G.651.1) or single-mode fibre Recommendations ITU-T G.652, …, G.657.
Core characteristics: A value for the core diameter and for core non-circularity is specified for multimode fibres. The core centre is the centre of a circle which best fits the points at a constant level in the near-field intensity pattern emitted from the central region of the fibre, using wavelengths above and/or below the fibre’s cut-off wavelength. Usually the core centre represents a good approximation of the mode field centre. The cladding centre is the centre of a circle which best fits the cladding boundary. The core concentricity error is the distance between the core centre and the cladding centre.
The tolerances on the physical dimensions of an optical fibre (core, mode field, cladding) are the primary contributors to splice loss and splice yield in the field. The maximum value for these tolerances (concentricity errors, non-circularities, etc.) specified in ITU-T Recommendations help to reduce systems
costs and support a low maximum splice-loss requirement typically around 0.1 dB. Fibres with tightly controlled geometry tolerances will not only be easier and faster to splice, but will also reduce the need for testing in order to ensure high-quality splice performance. This is particularly true when fibres are spliced by passive, mechanical or fusion techniques for both single fibres and fibre ribbons
Mode field characteristics: The mode field is the single-mode field distribution of the fundamental mode (LP01) giving rise to a spatial intensity distribution in the fibre. For step-index fibres, the mode field can be well represented as a twodimensional Gaussian form. The mode field diameter (MFD) represents a measure of the transverse extent of the electromagnetic field intensity of the mode in a single-mode fibre cross-section. Mode field diameter describes the size of the light carrying portion of the fibre. For single-mode fibre this portion includes the fibre core as well as a small portion of the surrounding cladding glass. MFD is an important parameter for determining the fibre resistance to bend induced loss and can affect splice loss as well. MFD, rather than the
core diameter, is the functional parameter that determines optical performance when a fibre is coupled to a light source. It is a function of wavelength, core diameter and the refractive index difference between the core and the cladding
Effective area (Aeff): Effective area (Aeff ) is a parameter that is closely related to optical fibre non-linearities that will affect the transmission quality of the optical fibre systems, especially in long-haul, optically amplified systems. For Gaussian-shaped fields, Aeff is very close to πMFD2/4.
Cladding characteristics: The cladding is the outermost region of glass in the fibre cross-section. The cladding centre is the centre of an circle which best fits the cladding boundary. The cladding diameter is the diameter of the circle defining the vcladding centre. The cladding non-circularity is the difference between the maximum and minimum radii divided by the radius of the best fit circle, expressed as a percent. As above said, the nominal value of the cladding diameter is 125 μm for all the types of optical fibres specified in ITU-T. A tolerance is also specified and the cladding deviation from nominal shall not exceed the specified tolerance.
Cut-off wavelength: Theoretical cut-off wavelength is the shortest wavelength at which only the fundamental (LP01) mode can propagate in a single-mode fibre. At wavelengths below the theoretical cut-off wavelength, several modes propagate and the fibre is no longer single-mode, but multimode. This parameter can be computed from the refractive index profile of the fibre. In optical fibres, the change from multimode to single-mode behaviour does not occur at an isolated wavelength, but rather smoothly over a range of wavelengths. Consequently, for determining fibre performance in a telecommunication network, theoretical cut-off wavelength is less useful than the actual threshold wavelength for single-mode performance when the fibre is in operation. Thus, a more effective parameter, called cut-off wavelength, has been introduced for single-mode fibre specifications.