6.1.1.2.1.  Light source

The light source shall be stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. The spectral characteristics of the source should be chosen to preclude multimode operation. The FWHM spectral width shall be no greater than 10 nm.

6.1.1.2.2.  Modulation

It 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 have substantially linear sensitivity characteristics.

6.1.1.2.3.  Launching conditions

The launching conditions used must be sufficient to excite the fundamental (LP₀₁) mode. For example, suitable launching techniques could be:

1. jointing with a fibre;

2. launching with a suitable system of optics.

Care should be taken that higher order modes do not propagate. For this purpose, it may be necessary to introduce a loop of suitable radius or another mode filter in order to remove higher order modes. For example, a one-turn bend with a radius of 30 mm on the fibre is generally sufficient for most ITU-T G.65x fibres. For some [ITU-T G.657] fibres, smaller radius, multiple bends or longer sample lengths can be applied to remove high-order propagating modes.

6.1.1.2.4.  Cladding mode stripper

Precautions shall be taken to prevent the propagation and detection of cladding modes.

6.1.1.2.5.  Specimen

The specimen shall be a short length of the optical fibre to be measured. Primary fibre coating shall be removed from the section of the fibre inserted in the mode stripper, if used. The fibre ends shall be clean, smooth and perpendicular to the fibre axes. It is recommended that the end faces be flat and perpendicular to the fibre axes to within 1°.

6.1.1.2.6.  Scan apparatus

A mechanism to scan the far-field intensity distribution shall be used (for example, a scanning photodetector with pinhole aperture or a scanning pig-tailed photodetector). The detector should be at least 10 mm from the fibre end, and the detector's active area should not subtend too large an angle in the far field. This can be assured by placing the detector at a distance from the fibre end greater than 40 wb/λ, where $2w$ is the expected mode field diameter of the fibre to be measured and b is the diameter of the active area of the detector.

The minimum dynamic range of the measurement should be 50 dB. This corresponds to a maximum scan half-angle of 20° and 25°, or greater, for fibres covered by [ITU-T G.652] and [ITU-T G.653], respectively.

6.1.1.2.7.  Detector

A suitable detector shall be used. The detector must have linear sensitivity characteristics.

6.1.1.2.8.  Amplifier

An amplifier should be employed in order to increase the signal level.

6.1.1.2.9.  Data acquisition

The measured signal level shall be recorded and suitably processed.

6.1.2.2.1.  Light source (as in clause 6.1.1.2.1)

6.1.2.2.2.  Modulation (as in clause 6.1.1.2.2)

6.1.2.2.3.  Launching conditions (as in clause 6.1.1.2.3)

6.1.2.2.4.  Cladding mode stripper (as in clause 6.1.1.2.4)

6.1.2.2.5.  Specimen (as in clause 6.1.1.2.5)

6.1.2.2.6.  Aperture apparatus

A mechanism containing at least twelve apertures spanning the half-angle range of numerical apertures from 0.02 to 0.25 (0.4 for fibres covered by [ITU-T G.653]) should be used. Light transmitted by the aperture is collected and focused onto the detector.

6.1.2.2.7.  Detector (as in clause 6.1.1.2.7)

6.1.2.2.8.  Amplifier (as in clause 6.1.1.2.8)

6.1.2.2.9.  Data acquisition (as in clause 6.1.1.2.9)

6.1.3.2.1.  Light source (as in clause 6.1.1.2.1)

6.1.3.2.2.  Modulation (as in clause 6.1.1.2.2)

6.1.3.2.3.  Launching conditions (as in clause 6.1.1.2.3)

6.1.3.2.4.  Cladding mode stripper (as in clause 6.1.1.2.4)

6.1.3.2.5.  Specimen (as in clause 6.1.1.2.5)

6.1.3.2.6.  Scan apparatus

Magnifying optics (e.g., a microscope objective) shall be employed to enlarge and focus an image of the fibre near field onto the plane of a scanning detector (for example, a scanning photodetector with a pinhole aperture or a scanning pig-tailed photodetector). The numerical aperture and magnification shall be selected to be compatible with the desired spatial resolution. For calibration, the magnification of the optics should have been measured by scanning the length of a specimen whose dimensions are independently known with sufficient accuracy.

6.1.3.2.7.  Detector (as in clause 6.1.1.2.7)

6.1.3.2.8.  Amplifier (as in clause 6.1.1.2.8)

6.1.3.2.9.  Data acquisition (as in clause 6.1.1.2.9)

where:

w_d

is the mode field diameter of the dead-zone fibre

w_s

is the mode field diameter of the specimen fibre

L_d

is the change in backscatter (dB) across the splice when measuring from the dead-zone fibre

L_s

is the change in backscatter (dB) across the splice when measuring from the specimen fibre

g

is a wavelength and fibre design dependent adjustment factor

f

is a wavelength and fibre design dependent adjustment factor

6.1.4.3.1.  Measurement of a fibre at a given wavelength

1. Align the fibre so that light is launched from the dead-zone fibre A into the specimen fibre. (From the OTDR across splice A into the specimen fibre as shown in Figure 2).

2. Measure the change in backscatter across the splice (splice A as shown in Figure 2), avoiding any reflection, and record the value as L_d.

3. Align the fibre so that light is launched from the specimen fibre into the dead-zone fibre A. (From the OTDR, across splice B into the specimen fibre, then across splice A in Figure 2).

4. Measure the change in backscatter across the splice (splice A as shown in Figure 2), avoiding any reflection, and record the value as L_s.

5. Calculate the mode field diameter according to Equation (6-4).

6.1.4.3.2.  Qualification for a fibre type, design and wavelength

1. Select a sample of fibres of the type and design to be measured, for which the mode field diameter, w_s, has been measured at the desired wavelength using either the reference test method or the first or second alternative methods, so that the range of mode field diameter values for the fibre type and design are represented in the sample.

2. Complete procedures a) to d) of clause 6.1.4.3.1 to determine the changes in backscatter across the splice L_d and L_s.

3. Compute $20{\log }_{10}\left(w_s/w_d\right)$ for each fibre and perform a linear regression of it versus $\left(L_d-L_s\right)$ to determine g (slope) and f (intercept).

4. Select a second sample of fibres, independent from the first set to determine g and f, for which the mode field diameter has also been measured at the desired wavelength using either the reference test method or the first or second alternative methods.

5. Complete the procedure in clause 6.1.4.3.1, using the values of g and f determined in c) to determine the mode field diameter, w_s. Find the difference to the value measured with the reference test method or the first or second alternative methods.

6. Calculate the average difference (bias), and the standard deviation of differences (σ_d) to determine if equivalence has been demonstrated.

7. An acceptable measure of equivalence may be obtained by calculating the equivalence level, B, where B = |bias| + 2 σ_d/sqrt(n), with n being the sample size. A typical upper limit on B is 0.1 μm.

8. If B exceeds the upper limit, adjustments to the procedure such as improving the splice or butt-joint are recommended.

6.2.1.2.1.  Light sources

A light source, L1, for illuminating the core shall be chosen, adjustable in intensity and stable in position, over a period of time which is sufficiently long to complete the measurement procedure. Wavelengths above and/or below the fibre's cut-off wavelength may be used. A second light source, L2, with similar characteristics, shall be used to illuminate the cladding.

6.2.1.2.2.  Launching conditions

The launch optics shall be arranged so that the light source uniformly overfills the fibre angularly and spatially. At the output end, the cladding shall be illuminated uniformly.

6.2.1.2.3.  Cladding mode stripper

Cladding mode light shall be stripped from the specimen near the input end. When the fibre under test has a primary coating with a refractive index higher than that of the glass, this coating acts as a cladding mode stripper.

6.2.1.2.4.  Specimen

The 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.

6.2.1.2.5.  Magnifying optics

The magnifying optics shall consist of an optical system (e.g., a microscope objective) which magnifies the specimen output near field, focusing it onto the plane of the detector. 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 resolution, and shall be recorded.

6.2.1.2.6.  Detector

CCD video cameras, scanning vidicons, or other pattern/intensity recognition devices shall be used to detect the magnified output near-field image and transmit it to a video monitor. The video digitizer performs the digitization of the image for further computer analysis. The video system shall be sufficiently linear so that, after calibration, the measurement uncertainty is not greater than required.

6.2.1.2.7.  Video image monitor

A video image monitor shall be used to display the detected image. The screen on the monitor typically shows a pattern, such as cross-hairs, to assist the operator in centring the image of the specimen. Computer-controlled alignment and/or focusing may be used.

6.2.1.2.8.  Data system

The measurement, data acquisitions and calculations are performed using a computer. A printer provides a hard copy of the information and measurement results.

6.2.1.3.1.  Calibration

Calibration shall be according to the procedures given in [IEC 61745].

6.2.1.3.2.  Measurement

The prepared specimen shall be aligned at the input end to achieve the launch condition specified. The near-field image of the output end shall be focused and centred in the monitor. The intensity of the core image illumination at the input end and the intensity of the cladding image illumination at the output end shall be adjusted according to an established, internal standard for the particular test equipment.

The digitized video image of the output face shall be recorded and the points representing both the cladding image edge and the core image edge shall be determined and recorded in edge tables. The decision levels of the boundaries in the near-field image are the following:

Core image boundary: This level shall be chosen from 5% to 50% of the maximum near-field intensity.

Cladding image boundary: Different methods for determining the cladding boundary can be used, depending on the illumination method. The method used in practice shall be the same as that used in calibration.

6.2.1.3.3.  Calculations

The raw data of the core and cladding edge are fitted to smooth, mathematically closed forms to determine best estimates of the actual edges. These smooth mathematically closed forms are then fitted to a circle in order to determine the geometrical characteristics, including the first order deviations from the ideal circular shape of each respective edge boundary. These values and the mathematical edge representation are used to determine the parameters as follows:

• X_co, Y_co (μm) fitted core centre;

• R_cl (μm) fitted cladding radius;

• X_cl, Y_cl (μm) fitted cladding centre;

• R_mincl (μm) minimum distance of the cladding edge to centre;

• R_maxcl (μm) maximum distance of the cladding edge to centre;

• cladding diameter (μm) = 2R_cl;

• cladding non-circularity (%) = 100(R_maxcl – R_mincl)/R_cl;

• core concentricity error (μm) = [(X_cl – X_co) ² + (Y_cl – Y_co) ²] ^1/2.

The smooth mathematically closed forms used to represent the edges are required to allow a variation of curvature that is greater than or equal to that found in an ellipse. For non-elliptical forms, the data can be converted to polar coordinates about a roughly estimated centre before fitting the radius versus angular position.

Active filtering, or removal of raw data points that represent cleave damage from those that are fitted to the mathematical form, is allowed. The choice of the curve, the equipment, the cleave method and the filtration algorithm are interactive in their contribution to the quality of the cladding measurement results.

6.2.2.2.1.  Source

A stable laser giving about 1 mW of power in the TEM₀₀ mode is required, such as a HeNe laser.

A quarter-wave plate is introduced to change the beam from linear to circular polarization because the reflectivity of light at an air-glass interface is strongly angle and polarization-dependent.

6.2.2.2.2.  Launching conditions

The launch optics, which are arranged to overfill the numerical aperture of the fibre, bring a beam of light to a focus on the flat end of the fibre. The optical axis of the beam of light should be within 1° of the axis of the fibre. The resolution of the equipment is determined by the size of the focused spot, which should be as small as possible in order to maximize the resolution, e.g., less than 1.0 μm. The equipment enables the focused spot to be scanned across the fibre cross section.

6.2.2.2.3.  Cell

The cell will contain a fluid with a refractive index slightly higher than that of the fibre cladding. The position of the cell will be controlled by X-Y motors driven by the computer and detected by X-Y micrometres.

6.2.2.2.4.  Detection

The refracted light is collected by suitable collecting optics and brought to the detector in any convenient manner provided that all the refracted light is collected. By calculation, the required size of disc and its position along the central axis can be determined.

6.2.2.2.5.  Data acquisition

The measured intensity distribution can be recorded, processed and presented in a suitable form, according to the scanning technique and to the specification requirements. A computer will be used to drive the X-Y motors, to record the X-Y position of the cell and the corresponding power levels, and to process the measured data.

6.2.2.3.1.  Preparation of fibre under test

A length of fibre less than 2 m is required.

The primary fibre coating shall be removed from the section of fibre immersed in the fluid cell.

The fibre ends shall be clean, smooth and perpendicular to the fibre axis.

6.2.2.3.2.  Equipment calibration

The equipment is calibrated with the fibre removed from the fluid cell. During the measurement, the angle of the cone of light varies according to the refractive index seen at the entry point to the fibre (hence the change of power passing the disc). With the fibre removed and the fluid index and cell thickness known, this change in angle can be simulated by translating the disc along the optic axis. By moving the disc to a number of predetermined positions, one can scale the profile in terms of relative index. Absolute index can only be found if the cladding or fluid index is known accurately at the measurement wavelength and temperature.

More convenient calibration procedures can be performed by means of a thin rod of known constant refractive index or by means of a multimode-multistep fibre, where the various refractive index values are known with great accuracy. This latter technique can also be useful in checking the linearity of the apparatus. In this respect, it may also be useful to control the fluid temperature in the fluid cell.

6.2.2.3.3.  Raster scan

The launch end of the fibre to be measured is immersed in the fluid cell and the laser beam is simultaneously centred and focused on the fibre end face.

The disc is centred on the output cone. Refracted modes passing the disc are collected and focused onto the detector.

The focused laser spot is scanned across the fibre end cross section and a two-dimensional distribution of fibre refractive index is directly obtained. From this distribution, the geometrical characteristics will be calculated.

6.2.2.3.4.  Geometrical characteristics

Once the raster scan of refractive index is performed, the core contour is obtained taking the points at the core-cladding interface of refractive index coinciding with the mean value between the averaged refractive indices of core and cladding respectively. The cladding contour is determined in a similar way but at the cladding-index matching fluid interface. Geometry analyses consistent with the terms in clause 3 will be performed starting from the core and cladding contours data. An index profile measurement actually yields the core concentricity error.

6.2.3.2.1.  Light source

The 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.

6.2.3.2.2.  Specimen

The specimen to be measured shall be a short length of single-mode fibre. The primary fibre coating shall be removed from the observed section of the fibre. The surface of the fibre shall be kept clean during the measurement.

6.2.3.2.3.  Magnifying optics

The 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.

6.2.3.2.4.  Detector

A 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.

6.2.3.2.5.  Data processing

A computer with appropriate software shall be used for the analysis of the intensity distributions.

6.2.3.3.1.  Equipment calibration

For 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.

6.2.3.3.2.  Measurement

The 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.

Intensity distributions in the observation plane along the line perpendicular to the fibre axis (a-a' in A, in Figure 5) 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. The cladding diameter and the central position of the fibre are determined by analysing the symmetry of the radial intensity distribution in the magnified image (shown as b in B). The central position of the core is determined by analysing 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 core concentricity error.

As shown in Figure 6, fitting the sinusoidal function to the experimentally obtained values of the mode field concentricity error (see clause 3.3.7, Note 2) plotted as a function of the rotation angle, the actual core concentricity error 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.

Figure 6 — Measured value of the mode field concentricity error as a function of rotation angle

6.2.4.2.1.  Light source

A nominal 1310 nm (for fibres covered by [ITU-T G.652]) or 1550 nm (for fibres covered by [ITU-T G.653] and [ITU-T G.654]) light source for illuminating the core shall be used. The light source shall be adjustable in intensity and stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. The spectral characteristics of this source should be chosen to preclude multimode operation. 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 defocusing of the image.

6.2.4.2.2.  Launching conditions

The launch optics, which will be arranged to overfill the fibre, will bring a beam of light to a focus on the flat input end of the fibre.

6.2.4.2.3.  Mode filter

In the measurement, it is necessary to assure single-mode operation at the measurement wavelength. In these cases, it may be necessary to introduce a bend in order to remove the LP₁₁ mode.

6.2.4.2.4.  Cladding mode stripper

A suitable cladding mode stripper shall be used to remove the optical power propagating in the cladding. Whenmeasuring the geometrical characteristics of the cladding only, the cladding mode stripper shall not be present.

6.2.4.2.5.  Specimen

The 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.

6.2.4.2.6.  Magnifying optics

The magnifying optics shall consist of an optical system (e.g., a microscope objective) which magnifies the specimen output near field, focusing it onto the plane of the scanning detector. The numerical aperture, and hence the resolving power ofthe 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.

6.2.4.2.7.  Detector

A suitable detector shall be employed which provides the point-to-point intensity of the transmitted near-field pattern(s). For example, any of the following techniques can be used:

1. scanning photodetector with pinhole aperture;

2. scanning mirror with fixed pinhole aperture and photodetector;

3. scanning vidicon, charge coupled devices or other pattern/intensity recognition devices.

The detector shall be linear (or shall be linearized) in behaviour over the range of intensities encountered.

6.2.4.2.8.  Amplifier

An amplifier may be employed in order to increase the signal level. The bandwidth of the amplifier shall be chosen according to the type of scanning used. When scanning the output end of the fibre with mechanical or optical systems, it is customary to modulate the optical source. Ifsuch a procedure is adopted, the amplifier should be linked to the source modulation frequency.

6.2.4.2.9.  Data acquisition

The measured intensity distribution can be recorded, processed and presented in a suitable form, according to the scanning technique and to the specification requirements.

6.2.4.3.1.  Equipment calibration

For 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.

6.2.4.3.2.  Measurement

The 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 focused image(s) of the output end of the fibre shall be scanned by the detector, according to the specification requirements. The focusing shall be performed with maximum accuracy, in order to reduce dimensional errors due to the scanning of a defocused image. The desired geometrical parameters are then calculated according to the definitions.

Algorithms for defining edges and calculating the geometrical parameters are under study.

6.3.1.2.1.  Light source

A light source with a line width not exceeding 10 nm (FWHM), stable in position, intensity and wavelength over a time period which is sufficient to complete the measurement procedure, and capable of operating over a sufficient wavelength range, shall be used.

6.3.1.2.2.  Modulation

It 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.

6.3.1.2.3.  Launching conditions

The launching conditions must be used in such a way as to excite substantially and uniformly both LP₀₁ and LP₁₁ modes. For example, suitable launching techniques could be:

1. jointing with a multimode fibre; or

2. launching with a suitable large spot-large NA optics.

6.3.1.2.4.  Cladding mode stripper

The cladding mode stripper is a device that encourages the conversion of cladding modes to radiation modes; as a result, cladding modes are stripped from the fibre. Care should be taken to avoid affecting the propagation of the LP₁₁ mode.

6.3.1.2.5.  Optical detector

A suitable detector shall be used so that all of the radiation emerging from the fibre is intercepted. The spectral response should be compatible with the spectral characteristics of the source. The detector must be uniform and have linear sensitivity.

6.3.1.3.1.  Standard test sample

The measurement shall be performed on a 2 m length of fibre. The fibre is inserted into the test apparatus and bent to form a loosely constrained loop. The loop shall complete one full turn of a circle of 140 mm radiusor a full turn (360 degrees) consisting of two arcs (180 degrees each) of 140 mm radius, connected by tangents (split mandrel configuration). The remaining part of the fibre shall be substantially free of external stresses. While some incidental bends of larger radii are permissible, they must not introduce a significant change in the measurement result. The output power P₁(λ) shall be recorded versus λ in a sufficiently wide range around the expected cut-off wavelength.

In split mandrel configuration as shown in Figure 8, the lower semicircular mandrel moves to take any slack from the fibre loop without requiring movement of the launch or receive optics or placing the fibre sample under any significant tension.

Figure 8 — Fibre deployment: Cut-off wavelength by the split-mandrel technique

6.3.1.3.2.  Transmission through the reference sample

Either method a) or b) may be used.

1. Using the test sample, and keeping the launch conditions fixed, an output power P₂(λ) is measured over the same wavelength range with at least one loop of sufficiently small radius in the test sample to filter the LP₁₁ mode. The exact value of the smaller radius may be determined prior to measurement. It should be small enough to attenuate the second-order mode but not the primary mode, but not too small in order to avoid macrobending effects at higher wavelengths. Aradius between 10 and 30 mm is typical for most ITU-TG.65x fibres. For some [ITU-T G.657] fibres, the radius shall be much smaller, and this measurement procedure may not be adequate for these fibres. See Note 2.

2. With a short (1-2 m) length of multimode fibre, an output power P₃(λ) is measured over the same wavelength range.

6.3.1.3.3.  Calculations

The spectral attenuation of the test specimen, relative to the reference power is:

where i = 2 or 3 for methods a) or b), respectively, in clause 6.3.1.3.2.

Assuming a straight-line representation of the upper wavelength region, the deviation of higher-order modes from the fundamental mode is:

A_u and B_u are determined so that (A_u + B_uλ) represents the portion of the spectral attenuation curve at wavelengths above the region where the attenuation of higher order modes is accelerated (transition region). For method a), both A_u and B_u may be set to zero. See Figure 9-a and Figure 10-a.

6.3.1.3.4.  Determination of cut-off wavelength

In the transition region, higher-order mode power is reduced with increasing wavelength. Fibre cut-off wavelength, λ_c, is defined as the wavelength at which the higher-order mode power relative to the fundamental mode power, Δa(λ), has been reduced to 0.1 dB.

Figure 9-b and Figure 10-b illustrate "humps" that sometimes appear near the cut-off wavelength. In the absence of humps (see Figure 9-a and Figure 10-a), accurate determination of λ_c can be achieved without algorithms. Optionally, for precision improvement, fitting algorithms based on the following equations can be used when humps are present. Appendix I contains examples of such algorithms.

Unless otherwise specified,

When the coefficients of:

are determined for wavelengths in the transition region, then:

Figure 9-a — Single-mode reference cut-off plot

Figure 9-b — Single-mode reference cut-off plot with humps

Figure 10-a — Multimode reference cut-off plot

Figure 10-b — Multimode reference cut-off plot with hump

6.3.2.2.1.  Light source (as in clause 6.3.1.2.1)

6.3.2.2.2.  Modulation (as in clause 6.3.1.2.2)

6.3.2.2.3.  Launching conditions (as in clause 6.3.1.2.3)

6.3.2.2.4.  Cladding mode stripper (as in clause 6.3.1.2.4)

6.3.2.2.5.  Optical detector (as in clause 6.3.1.2.5)

6.3.2.3.1.  Standard test sample

The measurement shall be performed on a length of single-mode fibre in a cable. A cable length of 22 m shall be prepared by exposing 1 m uncabled fibre length at each end, and the resulting 20 m cabled portion shall be laid without any small bends which could affect the measurement value. To simulate the effects of a splice organizer, one loop of X = 40 mm radius shall be applied to each uncabled fibre length (see Figure 11). The uncabled fibre is deployed with secondary coating (if present) intact. While some incidental bends of larger radii are permissible in the fibre or cable, they must not introduce a significant change in the measurements. The output power P₁(λ) shall be recorded versus λ in a sufficiently wide range around the expected cut-off wavelength.

Alternatively, uncabledsingle-mode fibre in a deployment condition can be used as a test sample which assures that the results for λ_cc are in agreement with those achieved in measurements conducted on cabled fibres.

A fibre length of 22 m is inserted into the test apparatus; the inner 20 m are bent to form loosely constrained loops of a radius r ≥ 140 mm. One loop of X = 40 mm radius shall be applied to each fibre end (see Figure 12).

Though the relationship between λ_cc values measured with cabled and uncabled fibre samples depends on the specific cable design, λ_cc measured with an uncabled fibre sample is generally higher than that measured with a cabled fibre sample.

Figure 11 — Deployment condition for measurement of λ_cc on cabled fibre

Figure 12 — Deployment condition for measurement of λ_cc on uncabled fibres

6.3.2.3.2.  Transmission through the reference sample (as in clause 6.3.1.3.2)

6.3.2.3.3.  Calculations

The logarithmic ratio between the transmitted powers P₁(λ) and P_i(λ) is calculated as:

where i = 2 or 3 for methods a) or b) in clause 6.3.1.3.2, respectively.

6.3.2.3.4.  Determination of cabled fibre cut-off wavelength

The calculations and method of determining cable cut-off wavelength, λ_cc, are the same as for fibre cut-off wavelength. See clauses clause 6.3.1.3.3 and clause 6.3.1.3.4.

6.4.1.2.1.  Optical source

A 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 line width (FWHM) shall be specified so that the line width is narrow compared with any features of the fibre spectral attenuation.

6.4.1.2.2.  Modulation

It 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.

6.4.1.2.3.  Launching conditions

The launching conditions used must be sufficient to excite the fundamental mode. For example, suitable launching techniques could be:

1. jointing with a fibre;

2. launching with a suitable system of optics.

6.4.1.2.4.  Mode filter

Care must be taken that higher order modes do not propagate through the cut-back length. In these cases, it may be necessary to introduce a bend in order to remove the higher modes. For example, a one-turn bend with a radius of 30 mm on the fibre is generally sufficient for most ITU-T G.65x fibres. For some [ITU-T G.657] fibres, smaller radius, multiple bends or longer sample length can be applied to remove high-order propagating modes.

6.4.1.2.5.  Cladding mode stripper

A cladding mode stripper encourages the conversion of cladding modes to radiation modes; as a result, cladding modes are stripped from the fibre.

6.4.1.2.6.  Optical detector

A 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.

6.4.1.3.1.  Preparation of fibre under test

Fibre 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.

6.4.1.3.2.  Procedure

1. The fibre under test is placed in the measurement set-up. The output power P₂ is recorded.

2. Keeping 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 P₁ from the cut-back length is recorded.

3. The attenuation of the fibre, between the points where P₁ and P₂ have been measured, can be calculated, using P₁ and P₂, from the definition Equation (3-4) and Equation (3-5).

6.4.2.2.1.  General considerations

The 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. In addition, adjustment of the pulse width may be required to obtain a compromise between resolution and dynamic range.

Care must be taken that higher order modes do not propagate.

An example of apparatus is shown in Figure 14-a.

6.4.2.2.2.  Optical source

Use a stable, high power optical source of appropriate wavelengths and record them. The pulse width and repetition rate should be consistent with the desired resolution and the length of the fibre.

6.4.2.2.3.  Optical coupling system

An optical system for an efficient coupling of the beam into the fibre under test, the branching device or the optical detector shall be used. Various devices, such as index matching materials, can be added to reduce Fresnel reflections.

6.4.2.2.4.  Branching device

A branching device is needed to couple the source radiation into the fibre and the backscattered radiation onto the detector, while avoiding a direct source-detector coupling. Avoid using devices with polarization-dependent properties.

Figure 14-a — Schematic of apparatus

Figure 14-b — Example of a undirectional backscattering loss curve

Figure 14 — The backscattering technique

6.4.2.2.5.  Optical detector

A detector shall be used so that the maximum possible backscattered power is intercepted. The detector response shall be compatible with the levels and wavelengths of the detected signal. A substantially linear detector response is required for attenuation measurements.

6.4.2.2.6.  Amplifier

A suitable amplifier shall follow the optical detector, so that the signal level becomes adequate for signal processing. The amplifier bandwidth should involve a trade-off between time resolution and noise reduction.

6.4.2.2.7.  Signal processor

A signal processor able to improve the signal-to-noise ratio, to calculate the attenuation curve from the two unidirectional backscattering loss curves, and to provide a logarithmic response in the detection system is required. An oscilloscope for a direct view of the backscattering trace and a data acquisition system to store the measurement results can be connected to the signal processor.

6.4.2.2.8.  Cladding mode stripper

See clause 6.4.1.2.5

6.4.2.2.9.  Fibre sample configuration

The measurement may be made with the fibre in a number of fibre configurations (e.g., as cabled fibre, on a suitable shipping spool or as required for the reference text method).

6.4.3.2.1.  Optical source

A suitable intensity stable radiation source shall be used, such as a lamp, a laser or a light emitting diode. If a broad spectrum source is used, it should be followed by a wavelength selection device (alternatively this device can be inserted before the detector). In every case, the nominal wavelength of the source (possibly taking into account the wavelength selection device) shall be known.

The spectral width (FWHM) should be narrow compared with any features of the fibre spectral attenuation.

6.4.3.2.2.  Modulation

See clause 6.4.1.2.2.

6.4.3.2.3.  Launching conditions

For option a)

The source is coupled to a short length of single-mode fibre having the same nominal characteristics of the fibre under test and equipped with a mode filter and a cladding mode stripper (see below).

The above single-mode fibre is coupled to the fibre under test with a very precise coupling device to minimize coupling losses and ensure meaningful results. If the fibre under test is equipped with a semi-connector, a compatible high quality semi-connector shall be fixed to the launching fibre.

For option b)

The source is coupled through a suitable optic to the fibre under test in such a way that the launched spot at the fibre input-end face has a near-field and a far-field intensity almost uniform within the mode field diameter and the far-field intensity of the fibre under test.

The system can employ lenses and a fibre positioner; alternatively, the light can be launched in a step-index multimode fibre to be connected to the fibre under test.

This is accomplished with any coupling device or a semi-connector compatible with those terminating the fibre under test.

6.4.3.2.4.  Reference system (option b) only)

This system is composed of a short length of single-mode fibre having the same nominal characteristics of the fibre under test. The fibre is equipped with a mode filter and a cladding mode stripper; both devices shall not introduce any loss on the fundamental mode.

6.4.3.2.5.  Mode filter

The mode filter shall allow the propagation along the fibre of the fundamental mode only. It can be implemented by a suitable bending condition on the fibre. For example, a one-turn bend with a radius of 30 mm on the fibre is generally sufficient for most ITU-T G.65x fibres. For some [ITU-T G.657] fibres, a smaller radius, multiple bends or longer sample lengths can be applied to remove high-order propagating modes.

6.4.3.2.6.  Cladding mode stripper

A cladding mode stripper encouraging the conversion of cladding modes to radiation modes should be employed. This device is not necessary if the fibre itself does not allow the propagation cladding modes.

6.4.3.2.7.  Optical detection

The spectral response of the optical detector shall be compatible with the spectral characteristics of the source. It must have linear sensitivity characteristics.

For option a)

The detector is connected to a single-mode fibre having the same nominal characteristics of the fibre under test. The fibre should be equipped with a mode filter and a cladding mode stripper.

For the coupling with the fibre under test, the same indication as in clause 6.4.3.2.3, option a), is used.

For option b)

The end of the fibre under test is positioned in front of the detector.

A suitable detector shall be used so that all the radiation emerging from the fibre is intercepted. The detector should be spatially uniform.

Alternatively, the detector is connected to a step-index multimode fibre. This fibre is coupled to the fibre under test by any coupling device or a semi-connector compatible with those terminating the fibre under test.

6.4.3.3.1.  Preparation of fibre under test

See clause 6.4.1.3.1.

If the fibre is fitted with connectors, an appropriate cleaning procedure is required.

6.4.3.3.2.  Procedure

1. Once a measurement wavelength is selected, the power P₁ is firstly measured in the following way:

   For option a)

   The fibre of the launching system is connected to the fibre of the receiving system. The received power P₁ is then recorded.

   For option b)

   The reference system is connected between the launching and the receiving systems. The received power P₁ is then recorded.

2. Successively, the fibre under test is connected between the launching and the receiving systems. The received power P₂ is then recorded.

3. Finally, the attenuation A of the fibre section is calculated in the following way:

   For option a)

   $A=10\log \begin{array}{c}\frac{P_1\left(\lambda \right)}{P_2\left(\lambda \right)}\end{array}+C_r-C_1-C_2\left(dB\right)$  (6-16a)

   where C_r, C₁, and C₂ are the nominal average losses (in dB) of the connections respectively in the reference conditions at the input of the fibre under test and at its output.

   For option b)

   $A=10\log \begin{array}{c}\frac{P_1\left(\lambda \right)}{P_2\left(\lambda \right)}\end{array}\left(dB\right)$  (6-16b)

   NOTE 1 – The use of option b) assumes that the fibre under test does not allow the propagation to the receiving end of modes other than the fundamental one.

   NOTE 2 – Fibre attenuation measurements are also possible with a hybrid test set-up, using a launching system as in option a) and a receiving system as in option b), or vice versa.

   The measurement procedure for P₁ is in both cases similar to the one listed above for option a); no reference system is required and the launching system is connected directly to the receiving system.

   In both cases the fibre section attenuation can be calculated as:

   $A=10\log \begin{array}{c}\frac{P_1\left(\lambda \right)}{P_2\left(\lambda \right)}\end{array}-C_a\left(dB\right)$  (6-17)

   where C_a is the nominal average loss (in dB) of the connection between the fibre under test and that part of the test set-up (launch or receive) belonging to option a).

   NOTE 3 – The intrinsic capability of option a) to evaluate the semi-connectors behaviour does not imply its use whenever this evaluation is required.

   Alternative possibilities are in using, even at an end where the semi-connector evaluation is required, an option b) set-up, previously connecting a single-mode cord to the fibre under test. The nominal loss of the fibre-to-cord connector is to be subtracted from the measured loss.

The test apparatus to be used in practice should be chosen to minimize the error sources, taking into account the available instrumentation and connecting devices. The use of a hybrid set-up (a-launch, b-receive) plus a cord at the receiving end is usually the best solution when both the semi-connectors have to be evaluated.

where:

W

is the modified vector

w

comes from $w=M\cdot v$

e

is the correction-factor vector

6.5.1.2.1.  Optical source

The optical source shall be stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. Laser diodes, (laser diode array (LD-array)), wavelength tunable laser diodes (WTL) (e.g., an external cavity laser (ECL)), LEDs or broadband sources, (e.g., an Nd:YAG laser with a Raman fibre) may be used, depending on the wavelength range of the measurement.

In any case, the modulating signal shall be such as to guarantee a sufficient time resolution in the group delay measurement.

6.5.1.2.2.  Wavelength selection

The wavelength selector and monitoring are used to select and monitor the wavelength at which the group delay is to be measured. As a wavelength selector, an optical switch, a monochromator, dispersive devices, optical filters, optical couplers, connectors may be used, depending on the type of light sources and measurement set-up. The selection may be carried out by switching electrical driving signals for different wavelength light sources.

The wavelength monitoring may be carried out by an optical fibre coupler and a wavelength meter. The wavelength selector and monitor may be used either at the input or at the output end of the fibre under test.

If a mathematical fit is made to the data, at least one data point must be within 100 nm of λ₀.

6.5.1.2.3.  Detector

The light emerging from the fibre under test, the reference fibre or the optical divider, etc., is coupled to a photodetector whose signal-to-noise ratio and time resolution are adequate for the measurement. The detector is followed by a low noise amplifier if needed.

6.5.1.2.4.  Reference channel

The reference channel may consist of an electrical signal line or optical signal line. A suitable time delay generator may be interposed in this channel. In certain cases, the fibre under test itself can be used as the reference channel line.

6.5.1.2.5.  Delay detector

The delay detector shall measure the phase shift between the reference signal and the channel signal. A vector voltmeter could be used.

6.5.1.2.6.  Signal processor

A signal processor can be added in order to reduce the noise and/or the jitter in the measured waveform. If needed, a digital computer can be used for purposes of equipment control, data acquisition and numerical evaluation of the data.

6.5.2.2.1.  Optical source

The optical source shall be stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. Laser diodes, (laser diode array (LD-array)), wavelength tunable laser diodes (WTL) (e.g.,an external cavity laser (ECL)), broadband sources (e.g., an Nd:YAG laser with a Raman fibre) may be used, depending on the wavelength range of the measurement.

In any case, the modulating signal shall be such as to guarantee a sufficient time resolution in the group delay measurement.

6.5.2.2.2.  Wavelength selection

A wavelength selector and monitoring are used to select and monitor the wavelength at which the group delay is to be measured. As a wavelength selector, an optical switch, a monochromator, dispersive devices, optical filters, optical couplers, connectors may be used, depending on the type of light sources and measurement set-up. The selection may be carried out by switching electrical driving signals for different wavelength light sources.

The wavelength monitoring may be carried out by an optical fibre coupler and a wavelength meter. The wavelength selector and monitor may be used either at the input or at the output end of the fibre under test.

If a mathematical fit is made to the data, at least one data point must be within 100 nm of the zero-dispersion wavelength λ₀.

6.5.2.2.3.  Detector

The light emerging from the fibre under test, the reference fibre or the optical divider, etc., is coupled to a photodetector whose signal-to-noise ratio and time resolution are adequate for the measurement. The detector is followed by a low noise amplifier if needed.

6.5.2.2.4.  Reference channel

The reference channel may consist of an electrical signal line or optical signal line. A suitable time delay generator may be interposed in this channel. In certain cases, the fibre under test itself can be used as the reference channel line.

6.5.2.2.5.  Delay detector

The delay detector shall measure the delay time between the reference signal and the channel signal. A high-speed oscilloscope or a sampling oscilloscope could be used.

6.5.2.2.6.  Signal processor

A signal processor can be added in order to reduce the noise and/or the jitter in the measured waveform. If needed, a digital computer can be used for the purposes of equipment control, data acquisition and numerical evaluation of the data.

6.6.1.2.1.  Sample length

The specimen shall be a known length of fibre, as specified in the detailed specification. In particular, the length of the sample tested for loss is determined by the measurement set-up, i.e., curvature radius $\left(R\right)$ and number of turns (N); any further fibre length does not affect the measurement results, provided that the signal-to-noise (S/N) ratio is optimized.

6.6.1.2.2.  Number of coils

The number of coils should be in accordance with the values stated in the relevant [ITU-T G.652], [ITU-T G.653], [ITU-T G.654], [ITU-T G.655], [ITU-T G.656] and [ITU-T G.657].

For single-mode fibres, the attenuation increases in a linear fashion with the number of coils.

For each radius, the number of coils shall be chosen in such a way that:

1. the induced loss is significantly higher than the detection limit of the set-up; when necessary, e.g., for low-bend loss fibres, tests may be carried out with more coils than the specification requires, followed by linear normalization to the specified number;

2. the induced loss is significantly lower than the onset of the non-linear region in the set-up; for bending radii in the range 5 to 10 mm this may imply that not more than 5 to 10 coils should be used.

6.6.1.2.3.  Bend radius

The value of bend radius shall be in accordance with the values stated in the relevant [ITU-T G.652], [ITU-T G.653], [ITU-T G.654], [ITU-T G.655], [ITU-T G.656] and [ITU-T G.657]. It should be considered that the macrobending losses increase exponentially as radius decreases.

6.6.1.2.4.  Measurement wavelength

The measurement wavelength shall be 1550 nm or 1625 nm, in accordance with the relevant [ITU-T G.652], [ITU-T G.653], [ITU-T G.654], [ITU-T G.655], [ITU-T G.656] and [ITU-T G.657]. It should be considered that macrobending losses increase exponentially with the wavelength.

6.6.1.3.1.  Bending tool

The bending tool is used to hold the sample bent with a radius as stated in the specification. A mandrel or a guiding groove on a flat surface is applicable.

Since the actual curvature radius is critical, a maximum tolerance of ± 0.1 mm (for radii lower than or equal to 15 mm) or ± 0.5 to 1.0 mm (for larger radii) is accepted (a tighter tolerance on small radii is required for the higher measurement sensibility).

The test can be carried out on samples either making complete (360°) turn(s) in open air or around a suitable support (mandrel), or making u-turn(s) (180°) in open air or around suitable supports; the length under test is different in the two configurations, the length of a complete turn being twice the length of a u-turn. In the following, the term "coil" refers to one complete turn: one "coil" is made by two consecutive "u-turns" 3. This should be taken into account when normalizing the results to the length of the sample (number of coils).

6.6.1.3.2.  Loss measurement instrument

The loss-measurement instrument uses either the transmitted power monitoring technique (methodA of [IEC 60793-1-46]) or the cut-back technique (as in clause 6.4.1), taking care of the appropriate launch condition for the specific fibre type.

6.6.1.4.1.  Calculation

The results are reported in dB as:

where P_str is the power measured without the bend and P_bend is the power measured with the bend present.

6.7.1.1.1.  Proof test parameters

1. The proof stress, σ_p, is specified to control the length of the surviving sections of the fibre. The stress applied during the proof test, σ_a, is illustrated in Figure 17. The load and unload times, t_l and t_u, and the dwell-time, t_d, are also shown. The tensile load shall be applied for as short a time as possible, yet sufficiently long to ensure the glass experiences the proof stress, typically much less than one second.

2. The applied stress shall exceed the specified proof stress at all times. The unload time shall be controlled to be less than some maximum values to be agreed between user and manufacturer to control unloading damage.

Figure 17 — Stress σ versus time t during proof testing

6.7.1.2.1.  Operating procedure requirements

1. In the pay-out and take-up regions, the fibre is maintained with a low value of stress typically not exceeding 10% of the proof stress (see Figure 17).

2. In the loading region, fibre stress ramps up from a low stress level to the full proof stress. The load time is t_l.

3. In the proof test region, the applied proof stress, σ_a, is maintained at values greater than the specified proof stress, σ_p.

4. In the unloading region, fibre stress ramps down from the applied stress to a low value of stress. The time to unload the fibre is t_u.

5. The unloading time is controlled to be less than a maximum value to be agreed between user and manufacturer. It can be varied by changing the processing speed or by the gripping capstan design.

6. The capstans and other support pulleys shall be designed and operated to ensure that they do not induce excessive damage. The gripping capstans shall be capable of maintaining the applied stress without inducing additional damage from slipping.

6.7.1.2.2.  Proof test machines

There are several possible machine designs, all of which perform the basic functions required for measuring fibre proof with the indicated general operating requirements. Care should be used in the design so as to prevent coating damage.

Two machine types are used:

• braked-capstan machine;

• dead-weight machine.

Either machine may be used during the fibre-drawing process (online for coated fibre only), or as a separate process step (offline).

1. Braked-capstan machine (Figure 18)

   The fibre is paid out with constant, low tension. Also the rewinding after the proof test is done with constant tension. The levels of the pay-off and take-up tensions are adjustable.

   The proof test load is applied to the fibre between the brake and drive capstans by creating a speed difference between the capstans. Two belts are used to prevent slippage at the capstans. One design can be that the high precision tension gauge measures the load on the fibre and controls the speed difference to achieve the required proof test load. The load level and operating speed of the equipment can be independently set. Another design can be that the difference in speeds between the two capstans is set and controlled directly according to the desired fibre elongation (strain), without tension measurements.

   

   Figure 18 — Typical arrangement of a braked-capstan proof test machine

2. Dead-weight machine (Figure 19)

   The pay-out dancer and the take-up dancer pulleys are light enough to guide the fibre with minimal tension. The pay-out capstan and the take-up capstan are synchronized with each other. The capstan pinch belts prevent slippage at the capstans, but without additional stress to the fibre or damage to the fibre coatings. A load-arm and a dead-weight on a plate are attached to the shaft of a dead-weight dancer pulley to provide the proof stress to the fibre. An optional idler pulley provides an increased fibre gauge length if needed.

   

   Figure 19 — Dead-weight proof test machine

6.7.1.3.1.  Sample

1. The test sample shall consist of the entire length of optical fibre, minus short sections at the ends in which all requirements, i.e., maximum unloading time, may not be met. This end allowance length, typically 25 m to 50 m, shall be presented.

2. Fibre failure after proof testing shall be evidenced by complete breakage. Examination methods include visual inspection and OTDR measurements. After broken areas are removed, the surviving fibre lengths are considered to have passed the proof test procedure.

6.7.1.3.2.  Calculation

If the machine is calibrated in tension, the stress is calculated from Equation (6-19). Strain may be obtained from Equation (6-18).