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 °è° ÐB.3ÁHÁÓÓÃÃAlternative test methodÄÄ: ÃÃThe side©view methodÄÄ B.3.1ÁHÁÃÃObjectiveÄÄ Ð ° ÐÁHÁThe side©view method is applied to single©mode fibres to determine geometrical parameters (mode field concentricity error (MFCE), cladding diameter and cladding non©circularity) by measuring the intensity distribution of light that is refracted inside the fibre. B.3.2ÁHÁÃÃTest apparatusÄÄ ÁHÁA schematic diagram of the test apparatus is shown in Figure B©2. B.3.2.1Á  ÁLight source ÁHÁThe emitted light shall be collimated, adjustable in intensity and stable in position, intensity and wavelength over a time period sufficiently long to complete the measuring procedure. A stable and high intensity light source such as a light emitting diode (LED) may be used. B.3.2.2Á  ÁSpecimen ÁHÁThe specimen to be measured shall be a short length of single©mode fibre. The primary fibre coating shall be removed from the observed section Ð à Ðof the fibre. The surface of the fibre shall be kept clean during the measurement. B.3.2.3Á  ÁMagnifying optics ÁHÁThe magnifying optics shall consist of an optical system (e.g., a microscope objective) which magnifies the intensity distribution of refracted light inside the fibre onto the plane of the scanning detector. The observation plane shall be set at a fixed distance forward from the fibre axis. The magnification shall be selected to be compatible with the desired spatial resolution and shall be recorded. B.3.2.4Á  ÁDetector ÁHÁA suitable detector shall be employed to determine the magnified intensity distribution in the observation plane along the line perpendicular to the fibre axis. A vidicon or charge coupled device can be used. The detector must have linear characteristics in the required measuring range. The detector's resolution shall be compatible with the desired spatial resolution. B.3.2.5Á  ÁData processing ÁHÁA computer with appropriate software shall be used for the analysis of the intensity distributions. B.3.3ÁHÁÃÃProcedureÄÄ B.3.3.1Á  ÁEquipment calibration ÁHÁFor equipment calibration the magnification of the magnifying optics shall be measured by scanning the length of a specimen whose dimensions are already known with suitable accuracy. This magnification shall be recorded. B.3.3.2Á  ÁMeasurement ÁHÁThe test fibre is fixed in the sample holder and set in the measuring system. The fibre is adjusted so that its axis is perpendicular to the optical axis of the measuring system. ÁHÁIntensity distributions in the observation plane along the line perpendicular to the fibre axis ( a © a ' in A , in Figure B©2/G.652) are recorded (shown as B ) for different viewing directions, by rotating the fibre around its axis, keeping the distance between the fibre axis and the observation plane constant. Cladding diameter and the central position of the fibre are determined by analyzing the symmetry of the diffraction pattern (shown as b in Figure B ). The central position of the core is determined by analyzing the intensity distribution of converged light (shown as C ). The distance between the central position of the fibre and that of the core corresponds to the nominal observed value of MFCE. ÁHÁAs shown in Figure B©3/G.652, fitting the sinusoidal function to the experimentally obtained values of the MFCE plotted as a function of the rotation angle, the actual MFCE is calculated as the product of the maximum amplitude of the sinusoidal function and magnification factor with respect to the lens effect due to the cylindrical©structure of the fibre. The cladding diameter is evaluated as an averaged value of measured fibre diameters at each rotation angle, resulting in values for maximum and minimum diameters to determine the value of cladding non©circularity according to the definition. B.3.3.3Á  ÁPresentation of the results ÁHÁThe following details shall be presented. ÁHÁ ÁHÁa)Á   ÁTest arrangement ÁHÁb)Á   ÁFibre identification ÁHÁc)Á   ÁSpectral characteristics of the source ÁHÁd)Á   ÁIndication of repeatability and accuracy ÁHÁe)Á   ÁPlot of nominal MFCE vs. rotation angle ÁHÁf)Á   ÁMFCE, cladding diameter and cladding non©circularity ÁHÁg)Á   ÁTemperature of the sample and environmental conditions (if necessary) ÁàÀIÁFIGURE B©2/G.652ƒ ÁàÀQÁƒ ÁàÀ>ÁÃÃSchematic diagram of measurement systemÄă ÁàÀQÁƒ ÁàÀGÁrotation angle (deg)ƒ ÁàÀQÁƒ ÁàÀIÁFIGURE B©3/G.652ƒ ÁàÀQÁƒ ÁàÀ=ÁÃÃMeasured value of the MFCE as a functionÄă ÁàÀIÁÃÃof rotation angleÄă B.4ÁHÁÃÃAlternative test methodÄÄ: ÃÃThe transmitted near field image techniqueÄÄ B.4.1ÁHÁÃÃGeneralÄÄ ÁHÁThe transmitted near field image technique shall be used for the measurement of the geometrical characteristics of single©mode optical fibres. Such measurements are performed in a manner compatible with the relevant definitions. ÁHÁThe measurement is based on analysis of the magnified image(s) of the output end of the fibre under test. B.4.2ÁHÁÃÃTest appartatusÄÄ ÁHÁA schematic diagram of the test apparatus is shown in Figure B©4/G.652. B.4.2.1Á  ÁLight Source ÁHÁThe light source for illuminating the core shall be adjustable in intensity and stable in position and intensity over a time period sufficiently long to complete the measurement procedure. A second light source with similar characteristics can be used, if necessary, for illuminating the cladding. The spectral characteristics of the second light source must not cause defocussing of the image. B.4.2.2Á  ÁLaunching conditions ÁHÁThe launch optics, which will be arranged to overfill the fibre, will bring the beam of light to a focus on the flat input end of the fibre. B.4.2.3.Á  ÁCladding mode stripper ÁHÁA 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. B.4.2.4Á  ÁSpecimen ÁHÁThe specimen shall be a short length of the optical fibre to be measured. The fibre ends shall be clean, smooth and perpendicular to the fibre axis. B.4.2.5Á  ÁMagnifying optics ÁHÁThe magnifying optics shall consist of an optical system (e.g., a microscope objective) which magnifies the specimen output near field. The numerical aperture and hence the resolving power of the optics shall be compatible with the measuring accuracy required, and not lower than 0.3. The magnification shall be selected to be compatible with the desired spatial resolution, and shall be recorded. ÁHÁImage shearing techniques could be used in the magnifying optics to facilitate accurate measurements. B.4.2.6Á  ÁDetection ÁHÁThe fibre image shall be examined and/or analyzed. For example, either of the following techniques can be used: ÁHÁa)Á   Áimage shearing*; ÁHÁb)Á   Ágrey©scale analysis of an electronically recorded image. B.4.2.7Á  ÁData acquisition ÁHÁThe data can be recorded, processed and presented in a suitable form, according to the technique and to the specification requirements. B.4.3ÁHÁÃÃProcedureÄÄ B.4.3.1Á  ÁÃÃEquipment calibrationÄÄ ÁHÁFor the equipment calibration the magnification of the magnifying optics shall be measured by scanning the image of a specimen whose dimensions are already known with suitable accuracy. This magnification shall be recorded. B.4.3.2Á  ÁMeasurement ÁHÁThe launch end of the fibre shall be aligned with the launch beam, and the output end of the fibre shall be aligned to the optical axis of the magnifying optics. For transmitted near field measurement, the focussed image(s) of the output end of the fibre shall be examined according to the specification requirements. Defocussing errors should be minimized to reduce dimensional errors in the measurement. The desired geometrical parameters are then calculated. B.4.4ÁHÁÃÃPresentation of the resultsÄÄ ÁHÁa)Âh   ÂTest set©up arrangement, with indication of the technique usedÆÆ ÁHÁb)Âh   ÂLaunching conditionsÆÆ ÁHÁc)Âh   ÂSpectral characteristics of the sourceÆÆ ÁHÁd)Âh   ÂFibre identification and lengthÆÆ ÁHÁe)Âh   ÂMagnification of the magnifying opticsÆÆ ÁHÁf)Âh   ÂTemperature of the sample and environmental conditions (when necessary)ÆÆ ÁHÁg)Âh   ÂIndication of the accuracy and repeatabilityÆÆ ÁHÁh)Âh   ÂResulting dimensional parameters, such as cladding diameters, cladding non©circularities, mode field concentricity error, etc.ÆÆ ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ * The validity of the image shearing technique is under study and needs to be confirmed. ÁàÀIÁFIGURE B©4/G.652ƒÔ k+ÔŒ ÃÃSection IIIÄÄ: ÃÃTest methods for the cut©off wavelengthÄÄ B.1ÁHÁÃÃReference test method for the cut©off wavelenth (À/ÀÃÃcÄÄ) of the primary ÄÄ ÃÃcoated fibreÄÄ: ÃÃThe transmitted power techniqueÄÄ ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ * Including image shearing optics, where appropriate ** When appropriate ÃÃNoteÄÄ © The value of XX is under study. Several administrations indicated that a value of 45 mm is appropriate. The loops are intended to simulate deployment conditions, and should be chosen according to the practice of a particular administration. One option to be considered is deleting the loops, if that is the administration's practice. B.3.2.2.2 Transmission through the reference sample (as in B.1.2.2.2) B.3.2.2.3 Calculations B.3.2.2.4 Determination of cabled fibre cut©off wavelength ÁHÁIf method a) is used, À/ÀÃÃcÄÄ is determined as the largest wavelength at which R(À/À) is equal to 0.1 dB (see Figure B©5). If method b) is used, À/À is determined by the intersection of a plot of R(À/À) and a straight line (2) displaced 0.1 dB and parallel to the straight line (1) fitted to the long wavelength portion of R(À/À) (see Figure B©6). B.3.2.2.5 Presentation of results ÁHÁa)Á   ÁTest set©up arrangement (including the radius XX of the loops) ÁHÁb)Á   ÁLaunching condition ÁHÁc)Á   ÁType of reference sample ÁHÁd)Á   ÁTemperature of the sample and environmental conditions (if necessary) ÁHÁe)Á   ÁFibre and cable identification ÁHÁf)Á   ÁWavelength range of measurement ÁHÁg)Âh   ÂCabled fibre cut©off wavelength, and plot of R(À/À) (if required)ÆÆ ÁHÁh)Á   ÁPlot of R(À/À) (if required). ÁàÀQÁƒ ÁàÀOÁcableƒ ÁàÀIÁFIGURE B©8/G.652ƒ ÁàÀQÁƒ ÁàÀ<ÁÃÃDeployment condition for measurement of theÄă ÁàÀBÁÃÃcabled fibre cut©off wavelengthÄă ÃÃSection IVÄÄ: ÃÃTest methods for attenuation measurementsÄÄ B.1ÁHÁÃÃIntroductionÄÄ B.1.1ÁHÁÃÃObjectivesÄÄ ÁHÁThe attenuation tests are intended to provide a means whereby a certain attenuation value may be assigned to a fibre length such that individual attenuation values may be added together to determine the total attenuation of a concatenated length. B.1.2ÁHÁÃÃDefinitionÄÄ ÁHÁThe attenuation A(À/À) at wavelength À/À between two cross sections and separated by distance L of a fibre is defined, as A(À/À) = 10 log [PÃÃ1ÄÄ(À/À)/PÃÃ2ÄÄ(À/À)] (dB) (1) ÁàÀ(Áƒwhere PÃÃ1ÄÄ(À/À) is the optical power traversing the cross section 1 and PÃÃ2ÄÄ(À/À) is the optical power traversing the cross section 2 at the wavelength À/À. ÁHÁFor a uniform fibre, it is possible to define an attenuation per unit length, or an attenuation coefficient which is independent of the length of the fibre: ÀÀ(À/À) = A(À/À)/L (dB/unit length) (2) ÃÃNoteÄÄ © Attenuation values specified for factory lengths should be measured at room temperature (i.e., a single value in the range 10À-À to 35À-ÀC). B.2ÁHÁÃÃThe reference test methodÄÄ: ÃÃthe cut©back techniqueÄÄ ÁHÁThe cut©back technique is a direct application of the definition in which the power levels PÃÃ1ÄÄ and PÃÃ2ÄÄ are measured at two points of the fibre without change of input conditions. PÃÃ2ÄÄ is the power emerging from the far end of the fibre and PÃÃ1ÄÄ is the power emerging from a point near the input after cutting the fibre. B.2.1ÁHÁÃÃTest apparatusÄÄ ÁHÁMeasurements may be made at one or more spot wavelengths, or alternatively, a spectral response may be required over a range of wavelengths. Diagrams of suitable test equipments are shown as examples in Figure B©9/G.652. B.2.1.1Á  ÁOptical source ÁHÁA suitable radiation source shall be used as a lamp, laser or light emitting diode. The choice of source depends upon the type of measurement. The source must be stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. The spectral linewidth (FWHM) shall be specified such that the linewidth is narrow compared with any features of the fibre spectral attenuation. B.2.1.2Á  ÁModulation ÁHÁIt is customary to modulate the light source in order to improve the signal/noise ratio at the receiver. If such a procedure is adopted, the detector should be linked to a signal processing system synchronous with the source modulation frequency. The detecting system should be substantially linear in sensitivity. B.2.1.3Á  ÁLaunching conditions ÁHÁThe launching conditions used must be sufficient to excite the fundamental mode. For example, suitable launching techniques could be: ÁHÁa)Á   Ájointing with a fibre; ÁHÁb)Á   Álaunching with a suitable system of optics. B.2.1.4Á  ÁMode filter ÁHÁCare must be taken that higher order modes do not propagate through the cut©back length. In these cases it may be necessary to intoduce a bend in order to remove the higher modes. B.2.1.5Á  ÁCladding mode stripper ÁHÁA cladding mode stripper encourages the conversion of cladding modes to radiation modes; as a result, cladding modes are stripped from the fibre. B.2.1.6Á  ÁOptical detector ÁHÁA suitable detector shall be used so that all of the radiation emerging from the fibre is intercepted. The spectral response should be compatible with spectral characteristics of the source. The detector must be uniform and have linear sensitivity characteristics. B.2.2ÁHÁÃÃMeasurement procedureÄÄ B.2.2.1Á  ÁPreparation of fibre under test ÁHÁFibre ends shall be substantially clean, smooth, and perpendicular to the fibre axis. Measurements on uncabled fibres shall be carried out with the fibre loose on the drum, i.e., microbending effects shall not be introduced by the drum surface. B.2.2.2Á  ÁProcedure ÁHÁ1)Âh   ÂThe fibre under test is placed in the measurement set©up. The output power PÃÃ2ÄÄ is recorded.ÆÆ ÁHÁ2)Âh   ÂKeeping the launching conditions fixed, the fibre is cut to the cut©back length (for example, 2 m from the launching point). The cladding mode stripper, when needed, is refitted and the output power PÃÃ1ÄÄ from the cut©back length is recorded.ÆÆ ÁHÁ3)Âh   ÂThe attenuation of the fibre, between the points where PÃÃ1ÄÄ and PÃÃ2ÄÄ have been measured, can be calculated from the definition using PÃÃ1ÄÄ and PÃÃ2ÄÄ. ÆÆ B.2.2.3Á  ÁPresentation of results ÁHÁThe following details shall be presented: ÁHÁa)Âh   ÂTest set©up arrangement, including source type, source wavelength, and linewidth (FWHM)ÆÆ ÁHÁb)Âh   ÂFibre identificationÆÆ ÁHÁc)Âh   ÂLength of sampleÆÆ ÁHÁd)Á   ÁAttenuation of the sample quoted in dB ÁHÁe)Âh   ÂAttenuation coefficient quoted in dB/kmÆÆ ÁHÁf)Âh   ÂIndication of accuracy and repeatabilityÆÆ ÁHÁg)Âh   ÂTemperature of the sample and environmental conditions (if necessary).ÆÆ B.3ÁHÁÃÃFirst alternative test methodÄÄ: ÃÃThe backscattering techniqueÄÄ ÃÃNoteÄÄ © This test method describes a procedure to measure the attenuation of a homogeneous sample of single©mode optical fibre cable. The technique can be applied to check the optical continuity, physical defects, splices, backscattered light of optical fibre cables and the length of the fibre. B.3.1ÁHÁÃÃLaunching conditionsÄÄ ÁHÁThe launch beam shall be coaxially incident on the launch end of the fibre; various devices such as index matching materials can be used to reduce Fresnel reflections. The coupling loss shall be minimized. B.3.2ÁHÁÃÃApparatus and procedureÄÄ B.3.2.1 General considerations ÁHÁThe signal level of the backscattered optical signal will normally be small and close to the noise level. In order to improve the signal©to©noise ratio and the dynamic measuring range it is therefore customary to use a high power light source in connection with signal processing of the detected signal. Further, accurate spatial resolution may require adjustment of the pulse width in order to obtain a compromise between resolution and pulse energy. Special care should be taken to minimize the Fresnel reflections. ÁHÁCare must be taken that higher order modes do not propagate. ÁHÁAn example of apparatus is shown in Figure B©10a/G.652. B.3.2.2Á  ÁOptical source ÁHÁA stable high power optical source of an appropriate wavelength should be used. The wavelength of the source should be recorded. The pulse width and repetition rate should be consistent with the desired resolution and the length of the fibre. Optical non©linear effects should not be present in the part of the fibre under test. B.3.2.3Á  ÁCoupling device ÁHÁThe coupling device is needed to couple the source radiation to the fibre and the backscattered radiation to the detector, while avoiding a direct source©detector coupling. Several devices can be used, but devices based on polarization effects should be avoided. B.3.2.4Á  ÁOptical detection . ÁHÁA detector shall be used so that the maximum possible backscattered power should be intercepted. The detector response shall be compatible with the levels and wavelengths of the detected signal. For attenuation measurements the detector response shall be substantially linear. ÁHÁSignal processing is required to improve the signal to noise ratio, and it is desirable to have a logarithmic response in the detection system. ÁHÁA suitable amplifier shall follow the optical detector, so that the signal level becomes adequate for the signal processing. The bandwidth of the amplifier will be chosen as a trade©off between time resolution and noise reduction. ÁàÀIÁFIGURE B©9/G.652ƒ ÁàÀQÁƒ ÁàÀFÁÃÃThe cutback techniqueÄÄ ƒ B.3.2.5Á  ÁCladding mode stripper ÁHÁSee ÀÀ B.2.1.5. B.3.2.6Á  ÁProcedure ÁHÁ1)Á   ÁThe fibre under test is aligned to the coupling device. ÁHÁ2)Âh   ÂBackscattered power is analyzed by a signal processor and recorded on a logarithmic scale. Figure B©10b/G.652 shows such a typical curve.ÆÆ ÁHÁ3)Âh   ÂThe attentuation between two points A and B of the curve corresponding to two cross©sections of the fibre is ÆÆ ÁàÀCÁA(À/À) = ÃÃ1ÄÄ (VÃÃAÄÄ © VÃÃBÄÄ) dBƒ A©B 2 ÁHÁÂX  Âwhere VÃÃAÄÄ and VÃÃBÄÄ are the corresponding power levels given in the logarithmic scale.ÆÆ ÃÃNoteÄÄ © Attention must be given to the scattering conditions at points A and B when calculating the attenuation in this way. ÁHÁ4)Âh   ÂIf so required, bi©directional measurements can be made, together with numerical computation to improve the quality of the result and possibly to allow the separation of attenuation from backscattering factor.ÆÆ B.3.2.7Á  ÁResults ÁHÁThe following details shall be presented: ÁHÁa)Âh   ÂMeasurement types and characteristics ÆÆ ÁHÁb)Âh   ÂLaunching techniques ÆÆ ÁHÁc)Âh   ÂTest set©up arrangement ÆÆ ÁHÁd)Âh   ÂRelative humidity and temperature of the sample (when necessary) ÆÆ ÁHÁe)Âh   ÂFibre identification ÆÆ ÁHÁf)Á   ÁLength of sample ÁHÁg)Âh   ÂRise time, width and repetition rate of the pulse ÆÆ ÁHÁh)Âh   ÂKind of signal processing used ÆÆ ÁHÁi)Âh   ÂThe recorded curve on a logarithmic scale, with the attenuation of the sample, and under certain conditions the attenuation coefficient in dB/km.ÆÆ ÃÃNoteÄÄ © The complete analysis of the recorded curve (Figure B©10b/G.652) shows that, independently from the attenuation measurement, many phenomena can be monitored using the backscattering technique: ÁHÁa)Âh   ÂReflection originated by the coupling device at the input end of the fibre ÆÆ ÁHÁb)Âh   ÂZone of constant slope ÆÆ ÁHÁc)Âh   ÂDiscontinuity due to local defect, splice or coupling ÆÆ ÁHÁd)Á   ÁReflection due to dielectric defect ÁHÁe)Âh   ÂReflection at the end of the fibre.ÆÆ B.4ÁHÁÃÃSecond alternative test methodÄÄ: ÃÃThe insertion loss techniqueÄÄ ÁHÁUnder consideration. ÃÃSection VÄÄ: ÃÃTest methods for chromatic dispersion coefficient measurementÄÄ B.1ÁHÁÃÃReference test method for chromatic dispersion coefficient measurementÄÄ B.1.1ÁHÁÃÃObjectiveÄÄ ÁHÁThe fibre chromatic dispersion coefficient is derived from the measurement of the relative group delay experienced by the various wavelengths during propagation through a known length of fibre. ÁHÁThe group delay can be measured either in the time domain or in the frequency domain, according to the type of modulation of the source. ÁHÁIn the former case the delay experienced by pulses at various wavelengths is measured; in the latter the phase shift of a sinusoidal modulating signal is recorded and processed to obtain the time delay. ÁHÁThe chromatic dispersion may be measured at a fixed wavelength or over a wavelength range. B.1.2ÁHÁÃÃTest apparatusÄÄ ÁHÁA schematic diagram of the test apparatus is shown in Figure B©11/G.652. B.1.2.1Á  ÁSource ÁHÁThe source shall be stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. Laser diodes, LED's or broadband sources, (e.g., an Nd:YAG laser with a Raman fibre) may be used, depending on the wavelength range of the measurement. ÁHÁIn any case, the modulating signal shall be such as to guarantee a sufficient time resolution in the group delay measurement. Õ7I(3201) CCITT\AP©IX\DOC\060E4.TXS 7ÕÕ7I(3201) CCITT\AP©IX\DOC\060E4.TXS 7ÕÐ °è Ð