ÿWPCL ûÿ2BJ|xÐ ` ÐÐÌÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿH øÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÿÌÐÐ °°°è ÐÑ Âx„|ü@Ž ÑÐ Å°6Ø'°6Ø'Å ÐÕNÏ Ð ` Ð Áà@Á©  ©ƒ Áà<ÁAP IX©60©E ƒNÕÕNÏ Ð ` Ð Áà@Á©  ©ƒ Áà<ÁAP IX©60©E ƒNÕÕI (3201) ÕÕI (3201) ÕÐ °èX Ð6.ÁHÁÓÓÃÃRecommendation G.651ÄÄ ÁàÁCHARACTERISTICS OF A 50/125 ÀÀm MULTIMODE GRADED INDEX ƒ OPTICAL FIBRE CABLE ÁHÁThe CCITT, ÃÃconsidering thatÄÄ ÁHÁ(a)Á   Ámultimode optical fibre cables are used widely in telecommunication networks; Ð H ÐÁHÁ(b)Á   Áthe foreseen potential applications may require multimode fibres differing in: ÁHÁ©Á  Ánature of material ÁHÁ©Á  Ágeometrical characteristics ÁHÁ©Á  Áoperating wavelength region(s) ÁHÁ©Á  Átransmission and optical characteristics ÁHÁ©Á  Ámechanical and environmental aspects; ÁHÁ(c)Á   ÁRecommendations on different kinds of multimode fibres can be prepared when practical use studies have sufficiently progressed; ÃÃrecommendsÄÄ ÁHÁa graded index, multimode fibre, which may be used in the region of 850 nm or in the region of 1300 nm or alternatively may be used in both wavelength regions simultaneously. ÁHÁThis fibre can be used for analogue and for digital transmission. ÁHÁIts geometrical, optical, and transmission characteristics are described below. ÁHÁThe 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. ÁHÁAnnexes A and B may become separate Recommendations as additional multimode fibre Recommendations are agreed upon. 1.ÁHÁÃÃFibre characteristicsÄÄ ÁHÁThe fibre characteristics dealt with in ÀÀ 1 are those which ensure the interconnection of fibres with acceptable low losses. ÁHÁOnly the intrinsic fibre characteristics (not depending on the cable manufacture) are recommended in ÀÀ 1. They will apply equally to individual fibres, fibres incorporated into a cable wound on a drum, and fibres in installed cables. 1.1ÁHÁGeometrical characteristics of the fibre 1.1.1ÁHÁCore diameter ÁHÁThe recommended nominal value of the core diameter is 50 ÀÀm. ÁHÁThe core diameter deviation should not exceed the limits of ÃÃ+ÄÄ 6% (ÃÃ+ÄÄ 3 ÀÀm). 1.1.2ÁHÁCladding diameter ÁHÁThe recommended nominal value of the cladding diameter is 125 ÀÀm. ÁHÁThe cladding diameter deviation should not exceed the limits of ÃÃ+ÄÄ 2.4% (ÃÃ+ÄÄ 3 ÀÀm). Ô _(ÔŒ 2.2.1ÁHÁModal distortion bandwidth: amplitude response ÁHÁThe modal bandwidth amplitude response is specified in the form of ©3 dB optical (©6 dB electrical) points of the bandwidth of the total amplitude/frequency curve corrected for chromatic dispersion. A more complete curve of the total bandwidth response should also be given. 2.2.3ÁHÁChromatic dispersion ÁHÁWhen required the manufacturer of the optical fibres should indicate the chromatic dispersion coefficient values of the fibre type in the operating wavelength region(s). The test method is contained in Annex B, section V, to Recommendation G.652. ÃÃNote 1ÄÄ © For multimode fibres the dominant chromatic dispersion mechanism is material dispersion. ÃÃNote 2ÄÄ © Typical values of the chromatic dispersion coefficient for high grade silica optical fibres are the following: 3.2 ÁHÁBaseband response (overall ©3dB optical bandwidth) ÁHÁThe baseband response is given in the frequency domain and includes the effects of both modal distortion and chromatic dispersion and can be represented by the expression: ÁàH>ÁBÃÃTÄÄ = [BÃÃmodalÄÄÃé2ÄÄ + BÃÃchromaticÄÄÃé2ÄÄ] ©ÀÀƒ where ÁHÁBÃÃTÄÄ = overall bandwidth (including modal distortion and chromatic dispersion) ÁHÁBÃÃmodalÄÄ = modal distortion bandwidth ÁHÁBÃÃchromaticÄÄ = chromatic dispersion bandwidth (see Note 3) ÃÃNote 1ÄÄ © Both the fibre modal distortion baseband response and the source spectrum are assumed to be Gaussian. ÃÃNote 2ÄÄ © For certain applications the effect of chromatic dispersion is negligible, in which case chromatic dispersion can be ignored. ÃÃNote 3ÄÄ © BÃÃchromaticÄÄ, the chromatic bandwidth, is inversely proportional to the section length and, if the source spectrum is assumed to be Gaussian, can be expressed as: ÁàH6ÁBÃÃchromaticÄÄ (MHz) = (ÀˆÀÀ/À ÀÀ D(À/À) ÀÀ 10Ãé6ÄÄ ÀÀ L/0.44)Ãé1ÄÄ ƒ where ÁHÁÀˆÀÀ/À = FWHM source line width (nm) ÁHÁD(À/À) = chromatic dispersion coefficient (ps/(nm.km)) ÁHÁL = section length (km) 3.2.1ÁHÁModal distortion bandwidth ÁHÁThe modal distortion bandwidth values for individual cable lengths in an elementary cable section are obtained from the relevant fibre specification. However, the overall modal distortion bandwidth of the elementary cable section may not be a linear addition of the individual responses due to mode coupling and other effects at splices and, sometimes, along the length of the fibre. ÁHÁThe modal distortion bandwidth for an elementary cable section is therefore given by: ÀÀx Ãé1ÄÄ ÀÀ©À'À BÃÃmodal totalÄÄ = ÀÀÀ$À BÃÃmodalÄÄ ÀÀÀÀ À$À ÀÀ1 ÃÃnÄÄ ÃÃÀ'ÀÄÄ À À where ÁHÁBÃÃmodal totalÄÄ = overall modal distortion bandwidth of an elementary cable section ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ ÁàHNÁƒ A.1ÁHÁà Ãalternative test method (ATM)Ä Ä ÁHÁA test method in which a given characteristic of a specified class of optical fibres or optical fibre cables is measured in a manner consistent with the definition of this characteristic and gives results which are reproducible and relatable to the reference test method and to practical use. A.2ÁHÁà Ãattenuation coefficientÄ Ä ÁHÁIn an optical fibre it is the attenuation per unit length. ÃÃNoteÄÄ © The attenuation is the rate of decrease of average optical power with respect to distance along the fbire and is defined by the equation: ÁàHCÁP(z) = P(0) 10Ãé(ÀÀz/10)ÄÄ ƒ where ÁHÁ ÁHÁP(z) = power at distance z along the fibre ÁHÁP(0) = power at z = 0 ÁHÁÀÀ = attenuation coefficient in dB/km if z is in km. ÁHÁFrom this equation the attenuation coefficient is: ÁàHNÁƒ © 10 logÃÃ10ÄÄ [P(z)/P(0)] ÀÀ = ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ Z ÁHÁThis assumes that ÀÀ is independent of z. A.3ÁHÁà Ãbandwidth (of an optical fibre)Ä Ä ÁHÁThat value numerically equal to the lowest frequency at which magnitude of the baseband transfer function of an optical fibre decreases to a specified fraction, generally to ©3dB optical (©6dB electrical), of the zero frequency value. ÃÃNoteÄÄ © The bandwidth is limited by several mechanisms: mainly modal distortion and chromatic dispersion in multimode fibres. A.4ÁHÁà Ãchromatic dispersionÄ Ä ÁHÁThe spreading of a light pulse in an optical fibre caused by the different group velocities of the different wavelengths composing the source spectrum. ÃÃNoteÄÄ © The chromatic dispersion may be due to one or more of the following: material dispersion, waveguide dispersion, profile dispersion. Polarization dispersion does not give appreciable effects in circularly©symmetric fibres. A.5ÁHÁà Ãchromatic dispersion coefficientÄ Ä ÁHÁThe chromatic dispersion per unit source spectrum width and unit length of fibre. It is usually expressed in ps/(nm.km). A.6ÁHÁà ÃcladdingÄ Ä ÁøÁ ÁHÁThat dielectric material of an optical fibre surrounding the core. A.7ÁHÁà Ãcladding mode stripperÄ Ä ÁHÁA device that encourages the conversion of cladding modes to radiation modes. A.8ÁHÁà ÃcoreÄ Ä ÁHÁThe central region of an optical fibre through which most of the optical power is transmitted. A.9ÁHÁà Ãcore area Ä Ä ÁHÁFor a cross section of an optical fibre the area within which the refractive index everywhere (excluding any index dip) exceeds that of the innermost homogeneous cladding by a given fraction of the difference between the maximum of the refractive index of the core and the refractive index of the innermost homogeneous cladding. ÃÃNoteÄÄ © The core area is the smallest cross©sectional area of a fibre excluding any index dip, which is contained within the locus of points where the refractive index nÃÃ3ÄÄ is given by ÁHÁnÃÃ3ÄÄ = nÃÃ2ÄÄ + k(nÃÃ1ÄÄ © nÃÃ2ÄÄ) (see Figure A©1/G.651) where ÁHÁnÃÃ1ÄÄ = maximum refractive index of the core ÁHÁnÃÃ2ÄÄ = refractive index of the innermost homogeneous cladding ÁHÁk = a constant ÃÃNoteÄÄ © Unless otherwise specified a k value of 0.05 is assumed. A.10ÁHÁà Ãcore (cladding) centreÄ Ä ÁHÁFor a cross©section of an optical fibre it is the centre of that circle which best fits the outer limit of the core area (cladding). ÃÃNote 1ÄÄ © These centres may not be the same. ÃÃNote 2ÄÄ © The method of best fitting has to be specified. A.11ÁHÁà Ãcore (cladding) diameterÄ Ä ÁHÁThe diameter of the circle defining the core (cladding) centre. A.12ÁHÁà Ãcore (cladding) diameter deviationÄ Ä ÁHÁThe difference between the actual and the nominal values of the core (cladding) diameter. A.13 ÁHÁà Ãcore/cladding concentricity error Ä Ä ÁHÁThe distance between the core centre and the cladding centre divided by the core diameter. A.14ÁHÁà Ãcore (cladding) tolerance fieldÄ Ä ÁHÁFor a cross©section of an optical fibre it is the region between the circle circumscribing the core (cladding) area and the largest circle, concentric with the first one, that fits into the core (cladding) area. Both circles shall have the same centre as the core (cladding). ÁàHCÁNAÃÃtÄÄ ÃÃmaxÄÄ = (nÃÃ1ÄÄÃÃ2ÄÄ © nÃÃ2ÄÄÃÃ2ÄÄ)ÃÃÀÀÄă where ÁHÁnÃÃ1ÄÄ = maximum refractive index of the core ÁHÁnÃÃ2ÄÄ = refractive index of the innermost homogeneous cladding Ð à ÐÃÃNoteÄÄ © The relationship between NA (see A.21) and NAÃÃtÄÄ ÃÃmaxÄÄ is given in section I of Annex B, ÀÀ B.2.2. A.18ÁHÁà Ãmode filterÄ Ä ÁHÁA device designed to accept or reject a certain mode or modes. A.19ÁHÁà Ãmode scrambler; mode mixerÄ Ä ÁHÁA device for inducing transfer of power between modes in an optical fibre, effectively scrambling the modes. ÃÃNoteÄÄ © Frequently used to provide a mode distribution that is independent of source characteristics. A.20ÁHÁà Ãnon©circularity of core (cladding)Ä Ä ÁHÁThe difference between the diameters of the two circles defined by the core (cladding) tolerance field divided by the core (cladding) diameter. A.21ÁHÁà Ãnumerical apertureÄ Ä ÁHÁThe numerical aperture NA is the sine of the vertex half©angle of the largest cone of rays that can enter or leave the core of an optical fibre, multiplied by the refractive index of the medium in which the vertex of the cone is located. A.22ÁHÁà Ãreference surfaceÄ Ä ÁHÁThe cylindrical surface of an optical fibre to which reference is made for jointing purposes. ÃÃNoteÄÄ © The reference surface is typically the cladding or primary coating surface. In rare circumstances it could be the core surface. A.23ÁHÁà Ãreference test method (RTM)Ä Ä ÁHÁA test method in which a given characteristic of a specified class of optical fibres or optical fibre cables is measured strictly according to the definition of this characteristic and which gives results which are accurate, reproducible and relatable to practical use. Ô k+ÔŒ Ô k+ÔŒ Õ3I CCITT\AP©IX\DOC\060E2.TXS 3ÕÕ2I CCITT\AP©IX\DOC\060E2.TXS 2ÕÐ XÈ Ð