IEC 60793-1-44 pdf download

IEC 60793-1-44 pdf download.Optical fibres – Part 1-44: Measurement methods and test procedures – Cut-off wavelength
1 Scope
This part of IEC 60793 establishes uniform requirements for measuring the cut-off wavelength of single-mode optical fibre, thereby assisting in the inspection of fibres and cables for commercial purposes. This standard gives the methods for measuring the cut-off wavelength of fibre and cable There are two methods for measuring cable cut-off wavelength, λ cc : • Method A: using uncabled fibre; • Method B: using cabled fibre. There is only one method (Method C) for measuring fibre cut-off wavelength, λ c . The test method in this standard describes procedures for determining the cut-off wavelength of a sample fibre in either an uncabled condition ( λ c ) or in a cable ( λ cc ). Three default configurations are given here: any different configuration will be given in a detail specification. These procedures apply to all category B and C fibre types (see Normative references). All methods require a reference measurement. There are two reference-scan techniques, either or both of which may be used with all methods: • bend-reference technique; • multimode-reference technique using category A1 multimode fibre.
3 Background
Theoretical cut-off wavelength is the shortest wavelength at which only the fundamental mode can propagate in a single-mode fibre, as 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. For purposes of determining fibre performance in a telecommunications network, theoretical cut-off wavelength is less useful than the lower value actually measured when the fibre is deployed. Measured cut-off wavelength is defined as the wavelength greater than which the ratio between the total power, including launched higher-order modes, and the fundamental mode power has decreased to less than 0,1 dB. According to this definition, the second-order (LP 1 1 ) mode undergoes 1 9,3 dB more attenuation than the fundamental (LP 01 ) mode at the cut-off wavelength. Because measured cut-off wavelength depends on the length and bends of the fibre, the resulting value of cut-off wavelength depends on whether the measured fibre is configured in a deployed, cabled condition, or it is short and uncabled. Consequently, there are two overall types of cut-off wavelength: • Cable cut-off wavelength, measured in an uncabled fibre deployment condition (method A), or in a cabled condition (method B); • Fibre cut-off wavelength, measured on a short length of uncabled, primary-coated fibre. Cable cut-off wavelength is the preferred attribute to be specified and measured.
4 Overview of methods
All of the methods shall use the transmitted-power technique, which measures the variation with wavelength of the transmitted power of a fibre under test compared to a reference transmitted-power wavelength scan. The reference scan normalizes wavelength-dependent fluctuations in the measurement equipment so that the attenuation of the LP 1 1 mode in the specimen can be properly characterized and the cut-off wavelength precisely determined. The reference scan uses one of the following two techniques: • the specimen with an additional, smaller-radius fibre bend; • a (separate) category A1 multimode fibre. This procedure can determine the cut-off wavelength of a fibre specimen in either a cabled or uncabled condition. Each method has its own default configurations; the detail specification will give any different configuration required. The fibre cut-off wavelength, ( λ c ), measured under the standard length and bend conditions described in this standard, will generally exhibit a value larger than λ cc . For normal installed cable spans, it is common for the measured λ c value to exceed the system transmission wavelength. Thus cable cut-off wavelength is the more useful description of system performance and capability. For short cables, e.g. a pigtail with a length shorter (and possibly a bending radius larger) than described in this method, the cable may introduce modal noise near the cut-off wavelength when lossy splices are present (>0,5 dB).