- - AP IX-60-E (3201) 7. Recommendation G.652 CHARACTERISTICS OF A SINGLE-MODE OPTICAL FIBRE CABLE The CCITT, considering that (a) single-mode optical fibre cables are widely used in telecommunication networks; (b) the foreseen potential applications may require several kinds of single-mode fibres differing in: - geometrical characteristics, - operating wavelengths, - attenuation dispersion, cut-off wavelength, and other optical characteristics, - mechanical and environmental aspects; (c) Recommendations on different kinds of single-mode fibres can be prepared when practical use studies have sufficiently progressed; recommends A single-mode fibre which has the zero-dispersion wavelength around 1300 nm and which is optimized for use in the 1300 nm wavelength region, and which can also be used in the 1550 nm wavelength region (where this fibre is not optimized). This fibre can be used for analogue and for digital transmission. The geometrical, optical, and transmission characteristics of this fibre are described below, together with applicable Test Methods. The meaning of the terms used in this Recommendation is given in Annex A, and the guidelines to be followed in the measurements to verify the various characteristics are indicated in Annex B. Annexes A and B may become separate Recommendations as additional single-mode fibre Recommendations are agreed upon. 1. Fibre characteristics Only those characteristics of the fibre providing a minimum essential design framework for fibre manufacture are recommended in 1. Of these, the cabled fibre cut-off wavelength may be significantly affected by cable manufacture or installation. Otherwise, the recommended characteristics will apply equally to individual fibres, fibres incorporated into a cable wound on a drum, and fibres in installed cable. This Recommendation applies to fibres having a nominally circular mode field. Note - A sufficient wavelength margin should be assured between the lowest-permissible system operating wavelength Os of 1270 nm, and the highest-permissible cable cut-off wavelength Occ. Several Administrations favour a maximum Occ of 1260 nm to allow for fibre sampling variations and source wavelength variations due to tolerance, temperature, and ageing effects. These two specifications need not both be invoked; users may choose to specify Oc or Occ according to their specific needs and the particular envisaged applications. In the latter case, it should be understood that Oc may exceed 1280 nm. In the case where the user chooses to specify Oc as in I, then Occ need not be measured. In the case where the user chooses to specify Occ, it may be permitted that Oc be higher than the minimum system operating wavelength, relying on the effects of cable fabrication and installation to yield Occ values below the minimum system operating wavelength for the shortest length of cable between two joints. In the case where the user chooses to specify Occ, a qualification test may be sufficient to verify that the Occ requirement is being met. 1.9 Examples of fibre design guidelines Supplement No. 33 gives an example of fibre design guidelines for matched- cladding fibres used by two organizations. 2.1 Attenuation coefficient Optical fibre cables covered by this Recommendation generally have attenuation coefficients below 1.0 dB/km in the 1300 nm wavelength region, and below 0.5 dB/km in the 1500 nm wavelength region. 2.2 Chromatic dispersion coefficient The maximum chromatic dispersion coefficient shall be specified by: - the allowed range of the zero-dispersion wavelength between Oomin = 1295 nm and Oomax = 1322 nm; - the maximum value Somax - 0.095 ps/(nm2km) of the zero-dispersion slope. 3.2 Chromatic dispersion The chromatic dispersion in ps can be calculated from the chromatic dispersion coefficients of the factory lengths, assuming a linear dependence on length, and with due regard for the signs of the coefficients and system source characteristics (see 2.2). ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ The mode field diameter 2w is found by applying one of the following definitions. The integration limits are shown to be 0 to , but it is understood that this notation implies that the integrals be truncated in the limit of increasing argument. While the maximum physical value of the argument q is 1/O, the integrands rapidly approach zero before this value is reached. i) FAR-FIELD DOMAIN: In this domain three different measurement implementations are possible: a) FAR FIELD SCAN: The far field intensity distribution F2(q) is measured as a function of the far-field angle , and the mode field diameter (MFD) at the wavelength O is iii) NEAR-FIELD DOMAIN: The near field intensity distribution f2(r) is measured as a function of the radial coordinate r and Note - The mathematical equivalence of these definitions results from transform relations between measurement results obtained by different implementations. These are summarized in Figure A-1/G.652. FIGURE A-1/G.652 Mathematical relations between measurement implementations The difference between the maximum cladding surface diameter Dmax and minimum cladding surface diameter Dmin (with respect to the common cladding surface centre) divided by the nominal cladding diameter, D, i.e., A.6 Mode field The mode field is the single-mode field distribution giving rise to a spatial intensity distribution in the fibre. A.7 Mode field centre The mode field centre is the position of the centroid of the spatial intensity distribution in the fibre. Note 1 - The centroid is located at rc and is the normalized intensity-weighted integral of the position vector r: rc = r I(r) dA / I(r) dA AREA AREA Note 2 - For fibres considered in this Recommendation, the correspondence between the position of the centroid as defined and the position of the maximum of the spatial intensity distribution requires further study. This ensures that each individual cable section is sufficiently single mode. Any joint that is not perfect will create some higher order (LP11) mode power and single mode fibres typically support this mode for a short distance (of the order of metres, depending on the deployment conditions). A minimum distance must therefore be specified between joints in order to give the fibre sufficient distance to attenuate the LP11 mode before it reaches the next joint. If inequality (1) is satisfied in the shortest cable section, it will be satisfied a fortiori in all longer cable sections, and single mode system operation will occur regardless of the elementary cable section length. Specifying Occ < Os for the shortest cable length (including loops in the splice enclosure) ensures single mode operation. It is frequently more convenient, however, to measure Oc, which requires only a two metre length of uncabled fibre. Oc depends on the fibre type, length, and bend radius, and Occ, in addition, depends on the structure of a particular cable. The relationship between Oc and Occ, therefore, is dependent on both the fibre and cable designs. In general Oc is several tens of nm larger than Occ: Oc can even be larger than the system wavelength, without violating inequality (1). Higher values of Oc produce tighter confinement of the LP01 mode and, therefore, help to reduce potential bending losses in the 1550 nm wavelength region. Short fibre lengths (<20m) are frequently attached to sources and detectors, and are also used as jumpers for interconnections. The cut-off wavelength of these fibres, as deployed, should also be less than Os. Among the means of avoiding modal noise in this case are: a) selecting only fibres with sufficiently low Oc for such uses; b) deployment of such fibres with small radius bends. A.11 Chromatic dispersion The spreading of a light pulse per unit source spectrum width in an optical fibre caused by the different group velocities of the different wavelengths composing the source spectrum. Note - The chromatic dispersion may be due to the following contributions: material dispersion, waveguide dispersion, profile dispersion. Polarization dispersion does not give appreciable effects in circularly-symmetric fibres. A.12 Chromatic dispersion coefficient The chromatic dispersion per unit source spectrum width and unit length of fibre. It is usually expressed in ps/(nm . km). A.13 Zero-dispersion slope The slope of the chromatic dispersion coefficient versus wavelength curve at the zero-dispersion wavelength. A.14 Zero-dispersion wavelength That wavelength at which the chromatic dispersion vanishes. A suitable cladding mode stripper shall be used to remove the optical power propagating in the cladding. When measuring the geometrical characteristics of the cladding only, the cladding mode stripper shall not be present.