.rs .\" Troff code generated by TPS Convert from ITU Original Files .\" Not Copyright ( c) 1991 .\" .\" Assumes tbl, eqn, MS macros, and lots of luck. .TA 1c 2c 3c 4c 5c 6c 7c 8c .ds CH .ds CF .EQ delim @@ .EN .nr LL 40.5P .nr ll 40.5P .nr HM 3P .nr FM 6P .nr PO 4P .nr PD 9p .po 4P .rs \v | 5i' .sp 2P .LP \fBRecommendation\ G.652\fR .RT .sp 2P .sp 1P .ce 1000 \fBCHARACTERISTICS\ OF\ A\ SINGLE\(hyMODE\fR \fBOPTICAL\ FIBRE\ CABLE\fR .EF '% Fascicle\ III.3\ \(em\ Rec.\ G.652'' .OF '''Fascicle\ III.3\ \(em\ Rec.\ G.652 %' .ce 0 .sp 1P .ce 1000 \fI(Malaga\(hyTorremolinos, 1984; amended at Melbourne, 1988)\fR .sp 9p .RT .ce 0 .sp 1P .LP The\ CCITT, .sp 1P .RT .sp 1P .LP \fIconsidering\fR .sp 9p .RT .PP (a) that single\(hymode optical fibre cables are widely used in telecommunication networks; .PP (b) that the foreseen potential applications may require several kinds of single\(hymode fibres differing in: .LP \(em geometrical characteristics; .LP \(em operating wavelength; .LP \(em attenuation dispersion, cut\(hyoff wavelength, and other optical characteristics; .LP \(em mechanical and environmental aspects; .PP (c) that recommendations on different kinds of single\(hymode fibres can be prepared when practical use studies have sufficiently progressed, .sp 1P .LP \fIrecommends\fR .sp 9p .RT .PP a single\(hymode fibre which has the zero\(hydispersion 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). .PP This fibre can be used for analogue and for digital transmission. .PP The geometrical, optical and transmission characteristics of this fibre are described below, together with applicable test methods. .PP 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\(hymode fibre Recommendations are agreed upon. .RT .sp 2P .LP \fB1\fR \fBFibre characteristics\fR .sp 1P .RT .PP Only those characteristics of the fibre providing a minimum essential design framework for fibre manufacture are recommended in \(sc\ 1. Of these, the cable fibre cut\(hyoff 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. .PP This Recommendation applies to fibres having a nominally circular mode field. .RT .sp 1P .LP 1.1 \fIMode field diameter\fR .sp 9p .RT .PP The nominal value of the mode field diameter at 1300\ nm shall lie within the range 9\ to 10\ \(*mm. The mode field diameter deviation should not exceed the limits of \(+- | 0% of the nominal value. .PP \fINote\ 1\fR \ \(em\ A value of 10 \(*mm is commonly employed for matched cladding designs, and a value of 9\ \(*mm is commonly employed for depressed cladding designs. However, the choice of a specific value within the above range is not necessarily associated with a specific fibre design. .PP \fINote\ 2\fR \ \(em\ It should be noted that the fibre performance required for any given application is a function of essential fibre and systems parameters, i.e., mode field diameters, cut\(hyoff wavelength, total dispersion, systems operating wavelength, and bit rate/frequency of operation, and not primarily of the fibre design. .PP \fINote\ 3\fR \ \(em\ The mean value of the mode field diameter, in fact, may differ from the above nominal values provided that all fibres fall within \(+- | 0% of the specified nominal value. .bp .RT .sp 1P .LP 1.2 \fICladding diameter\fR .sp 9p .RT .PP The recommended nominal value of the cladding diameter is 125 \(*mm. The cladding deviation should not exceed the limits of \(+- | .4%. .PP For some particular jointing techniques and joint loss requirements, other tolerances may be appropriate. .RT .sp 1P .LP 1.3 \fIMode field concentricity error\fR .sp 9p .RT .PP The recommended mode field concentricity error at 1300\ nm should not exceed 1\ \(*mm. .PP \fINote\ 1\fR \ \(em\ For some particular jointing techniques and joint loss requirements, tolerances up to 3\ \(*mm may be appropriate. .PP \fINote\ 2\fR \ \(em\ The mode field concentricity error and the concentricity error of the core represented by the transmitted illumination using wavelengths different from 1300\ nm (including white light) are equivalent. In general, the deviation of the centre of the refractive index profile and the cladding axis also represents the mode field concentricity error but, if any inconsistency appears between the mode field concentricity error, measured according to the reference test method (RTM), and the core concentricity error, the former will constitute the reference. .RT .sp 2P .LP 1.4 \fINon\(hycircularity\fR .sp 1P .RT .sp 1P .LP 1.4.1 \fIMode field non\(hycircularity\fR .sp 9p .RT .PP In practice, the mode field non\(hycircularity of fibres having nominally circular mode fields is found to be sufficiently low that propagation and jointing are not affected. It is therefore not considered necessary to recommend a particular value for the mode field non\(hycircularity. It is not normally necessary to measure the mode field non\(hycircularity for acceptance purposes. .RT .sp 1P .LP 1.4.2 \fICladding non\(hycircularity\fR .sp 9p .RT .PP The cladding non\(hycircularity should be less than 2%. For some particular jointing techniques and joint loss requirements, other tolerances may be appropriate. .RT .sp 1P .LP 1.5 \fICut\(hyoff wavelength\fR .sp 9p .RT .PP Two useful types of cut\(hyoff wavelengths can be distinguished: .RT .LP a) the cut\(hyoff wavelength \(*l\fI\fI\d\fIc\fR\u | of a primary coated fibre according to the relevant fibre RTM; .LP b) the cut\(hyoff wavelength \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u | of a cabled fiber in a deployment condition according to the relevant cable RTM. .PP The correlation of the measured values of \(*l\fI\fI\d\fIc\fR\u | and \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u | epends on the specific fibre and cable design and the test conditions. While in general \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u< \(*l\fI\fI\d\fIc\fR\u, a quantitative relationship cannot easily be established. The importance of ensuring single\(hymode transmission in the minimum cable length between joints at the minimum system operating wavelength is paramount. This can be approached in two alternate ways: .LP 1) recommending \(*l\fI\fI\d\fIc\fR\u | to be less than 1280\ nm; when a lower limit is appropriate, \(*l\fI\fI\d\fIc\fR\ushould be greater than 1100\ nm; .LP 2) recommending \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u | to be less than 1270\ nm. .PP \fINote\fR \ \(em\ A sufficient wavelength margin should be assured between the lowest\(hypermissible system operating wavelength \(*l\fI\fI\d\fIs\fR\uof 1270\ nm, and the highest\(hypermissible cable cut\(hyoff wavelength\ \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u. Several Administrations favour a maximum\ \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\uof 1260\ nm to allow for fibre sampling variations and source wavelength variations due to tolerance, temperature, and ageing effects. .PP \fR .PP \fR These two specifications need not both be invoked; users may choose to specify\ \(*l\fI\fI\d\fIc\fR\uor\ \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\uaccording to their specific needs and the particular envisaged applications. In the latter case, it should be understood that \(*l\fI\fI\d\fIc\fR\umay exceed 1280\ nm. .bp .PP In the case where the user chooses to specify\ \(*l\fI\fI\d\fIc\fR\uas in\ 1), then\ \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\uneed not be measured. .PP \fI\fR In the case where the user chooses to specify\ \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u, it may be permitted that\ \(*l\fI\fI\d\fIc\fR\ube higher than the minimum system operating wavelength, relying on the effects of cable fabrication and installation to yield\ \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\uvalues below the minimum system operating wavelength for the shortest length of cable between two joints. .PP In the case where the user chooses to specify \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u, a qualification test may be sufficient to verify that the \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\urequirement is being met. .RT .sp 1P .LP 1.6 \fI1550 nm loss performance\fR .sp 9p .RT .PP In order to ensure low\(hyloss operation of deployed 1300\ nm\(hyoptimized fibres in the 1550\ nm wavelength region, the loss increase of 100\ turns of fibre loosely\(hywound with a 37.5\ mm radius, and measured at 1550\ nm, shall be less than 1.0\ dB. .PP \fINote\ 1\fR \ \(em\ A qualification test may be sufficient to ensure that this requirement is being met. .PP \fINote\ 2\fR \ \(em\ The above value of 100 turns corresponds to the approximate number of turns deployed in all splice cases of a typical repeater span. The radius of 37.5\ mm is equivalent to the minimum bend\(hyradius widely accepted for long\(hyterm deployment of fibres in practical systems installations to avoid static\(hyfatigue failure. .PP \fINote\ 3\fR \ \(em\ If for practical reasons fewer than 100 turns are chosen to implement this test, it is suggested that not less than 40\ turns, and a proportionately smaller loss increase be used. .PP \fINote\ 4\fR \ \(em\ If bending radii smaller than 37.5 mm are planned to be used in splice cases or elsewhere in the system (for example, R\ =\ 30\ mm), it is suggested that the same loss value of 1.0\ dB shall apply to 100\ turns of fibre deployed with this smaller radius. .PP \fINote\ 5\fR \ \(em\ The 1550 nm bend\(hyloss recommendation relates to the deployment of fibres in practical single\(hymode fibre installations. The influence of the stranding\(hyrelated bending radii of cabled single\(hymode fibres on the loss performance is included in the loss specification of the cabled fibre. .PP \fINote\ 6\fR \ \(em\ In the event that routine tests are required a small diameter loop with one or several turns can be used instead of the 100\(hyturn test, for accuracy and measurement ease of the 1550\ nm bend sensitivity. In this case, the loop diameter, number of turns, and the maximum permissible bend loss for the several\(hyturn test, should be chosen, so as to correlate with the 1.0\ dB loss recommendation of the 37.5\ mm radius 100\(hyturn functional test. .RT .sp 2P .LP 1.7 \fIMaterial properties of the fibre\fR .sp 1P .RT .sp 1P .LP 1.7.1 \fIFibre materials\fR .sp 9p .RT .PP The substances of which the fibres are made should be indicated. .PP \fINote\fR \ \(em\ Care may be needed in fusion splicing fibres of different substances. Provisional results indicate that adequate splice loss and strength can be achieved when splicing different high\(hysilica fibres. .RT .sp 1P .LP 1.7.2 \fIProtective materials\fR .sp 9p .RT .PP The physical and chemical properties of the material used for the fibre primary coating, and the best way of removing it (if necessary) should be indicated. In the case of a single jacketed fibre similar indications shall be given. .RT .sp 1P .LP 1.8 \fIRefractive index profile\fR .sp 9p .RT .PP The refractive index profile of the fibre does not generally need to be known; if one wishes to measure it, the reference test method in Recommendation\ G.651 may be used. .bp .RT .sp 1P .LP 1.9 \fIExamples of fibre design guidelines\fR .sp 9p .RT .PP Supplement No. 33 gives an example of fibre design guidelines for matched\(hycladding fibres used by two organizations. .RT .sp 2P .LP \fB2\fR \fBFactory length specifications\fR .sp 1P .RT .PP Since the geometrical and optical characteristics of fibres given in \(sc\ 1 are barely affected by the cabling process, \(sc\ 2 will give recommendations mainly relevant to transmission characteristics of cabled factory lengths. .PP Environmental and test conditions are paramount and are described in the guidelines for test methods. .RT .sp 1P .LP 2.1 \fIAttenuation coefficient\fR .sp 9p .RT .PP Optical fibre cables covered by this Recommendation generally have attenuation coefficients in the below 1.0\ dB/km in the 1300\ nm wavelength region, and below 0.5\ dB/km in the 1550\ nm wavelength region. .PP \fINote\fR \ \(em\ The lowest values depend on the fabrication process, fibre composition and design, and cable design. Values in the range 0.3\(hy0.4\ dB/km in the 1300\ nm region and 0.15\(hy0.25\ dB/km in the 1550\ nm region have been achieved. .RT .sp 1P .LP 2.2 \fIChromatic dispersion coefficient\fR .sp 9p .RT .PP \fI\fR The maximum chromatic dispersion coefficient shall be specified by: .RT .LP \(em the allowed range of the zero\(hydispersion wavelength between \(*l \d\fIomin\fR \u = 1295 nm and \(*l \d\fIomax\fR \u = 1322 nm; .LP \(em the maximum value \fIS \domax \u\fR = 0.095 ps/(nm\u2\d | (mu | m) of the zero\(hydispersion slope. .PP The chromatic dispersion coefficient limits for any wavelength\ \(*l within the range 1270\(hy1340\ nm shall be calculated as \v'6p' .sp 1P .ce 1000 \fID\fR\d1\u(\(*l) = [Formula Deleted] @ left [ \(*l~\(em { (*l~$$Ei:4:\fIomin\fR~_ } over { (*l\u3\d } right ] @ .ce 0 .sp 1P .ce 1000 .sp 1 \fID\fR\d2\u(\(*l) = fIS~\domax~\u\fR @ left [ \(*l~\(em { (*l~$$Ei:4:\fIomax\fR~_ } over { (*l\u3\d } right ] @ .ce 0 .sp 1P .LP .sp 1 .PP \fINote\ 1\fR \ \(em\ The values of \(*l \d\fIomin\fR \u, \(*l \d\fIomax\fR \u, and \fIS \domax \u\fR yield chromatic dispersion coefficient magnitudes | | fID\fR\d1\u | and | | fID\fR\d2\u | equal to or smaller than the maximum chromatic dispersion coefficients in the table: .ce \fBH.T. [T1.652]\fR .ps 9 .vs 11 .nr VS 11 .nr PS 9 .TS center box; cw(60p) | cw(72p) . Wavelength (nm) { Maximum chromatic dispersion coefficient [ps/(nm\(mukm)] } _ .T& cw(60p) | cw(72p) . 1285 | (hy | 330 \ 3.5 .T& cw(60p) | cw(72p) . 1270 | (hy | 340 \ 6 | .T& cw(60p) | cw(72p) . 1550 20 | _ .TE .nr PS 9 .RT .ad r \fBTable [T1.652], p.\fR .sp 1P .RT .ad b .RT .PP (An exception occurs at 1285 nm, where the value of | | fID\fR\d2\u | is 3.67\ ps/(nm | (mu | m). A smaller value would be achieved by reducing \fIS \domax \u\fR or \(*l\fI \domax \u\fR ; this item requires further study.) .PP \fINote\ 2\fR \ \(em\ Use of these equations in the 1550 nm region should be approached with caution. .bp .PP \fINote\ 3\fR \ \(em\ For high capacity (for example, 4 \(mu 140\ Mb/s or above) or long length systems, a narrower range of \(*l \d\fIomin\fR \u, \(*l \d\fIomax\fR \u may need to be specified, or if possible, a smaller value of \fIS \domax \u\fR be chosen. .PP \fINote\ 4\fR \ \(em\ It is not necessary to measure chromatic dispersion coefficient of single mode fibre on a routine basis. .RT .sp 2P .LP \fB3\fR \fBElementary cable sections\fR .sp 1P .RT .PP An elementary cable section usually includes a number of spliced factory lengths. The requirements for factory lengths are given in \(sc\ 2 of this Recommendation. The transmission parameters for elementary cable sections must take into account not only the performance of the individual cable lengths but also amongst other factors, such things as splice losses and connector losses (if applicable). .RT .sp 1P .LP 3.1 \fIAttenuation\fR .sp 9p .RT .PP The attenuation \fIA\fR of an elementary cable section is given by: \v'6p' .RT .sp 1P .ce 1000 \fIA\fR = @ pile { fIm\fR above sum above \fIn\fR~=1 } @ \(*a\fI\fI\d\fIn\fR\u | (mu | fIL\fR\d\fIn\fR\u+ \fIa\fR\d\fIs\fR\u | (mu | \fIx\fR + \fIa\fR\d\fIc\fR\u | (mu | fIy\fR .ce 0 .sp 1P .LP .sp 1 where .LP \fI\(*a\fI\d\fIn\fR\u = attenuation coefficient of \fIn\fR th fibre in elementary cable section, .LP \fIL\fR\d\fIn\fR\u = length of \fIn\fR th fibre, .LP \fIm\fR = total number of concatenated fibres in elementary cable section, .LP \fIa\fR\d\fIs\fR\u = mean splice loss, .LP \fIx\fR = number of splices in elementary cable section, .LP \fIa\fR\d\fIc\fR\u = mean loss of line connectors, .LP \fIy\fR = number of line connectors in elementary cable section (if provided). .PP A suitable allowance should be allocated for a suitable cable margin for future modifications of cable configurations (additional splices, extra cable lengths, ageing effects, temperature variations,\ etc.). .PP The above expression does not include the loss of equipment connectors. .PP The mean loss is used for the loss of splices and connectors. The attenuation budget used in designing an actual system should account for the statistical variations in these parameters. .RT .sp 1P .LP 3.2 \fIChromatic dispersion\fR .sp 9p .RT .PP 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 \(sc\ 2.2). .RT .ce 1000 ANNEX\ A .ce 0 .ce 1000 (to Recommendation G.652) .sp 9p .RT .ce 0 .ce 1000 \fBMeaning of the terms used in the Recommendation\fR .sp 1P .RT .ce 0 .PP The terms listed in this Annex are specific for single\(hymode fibres. Other terms used in this Recommendation have the same meaning as given in Annex\ A to Recommendation\ G.651. .sp 1P .RT .sp 1P .LP A.1 \fBmode field diameter\fR .sp 9p .RT .PP The mode field diameter 2\fIw\fR is found by applying one of the following definitions. The integration limits are shown to be\ 0 to\ \(if, 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 \fIq\fR is [Formula Deleted] the integrands rapidly approach zero before this value is reached. .bp .RT .LP i) FAR\(hyFIELD DOMAIN: In this domain theree different measurement implementations are possible: .LP a) FAR\(hyFIELD SCAN: The far\(hyfield intensity distribution \fIF\fR \u2\d(\fIq\fR ) is measured as a function of the far\(hyfield angle\ \(*h, and the mode field diameter (MDF) at the wavelength\ \(*l is \v'6p' .ce 1000 2\fIw\fR = [Formula Deleted] @ left [ 2~$$4o pile { (if above int above 0 } fIq\fR~\u3\d\fIF\fR~\u2\d(\fIq\fR )\fIdq\fR~$$4u pile { (if above int above 0 } fIqF\fR~\u2\d(\fIq\fR )\fIdq\fR~$$4e right ] @ \u\(em1/2\d, where \fIq\fR = [Formula Deleted] .ce 0 .ad r (1) \v'2P' \v'3p' .ad b .RT .LP .sp 1 .LP b) KNIFE\(hyEDGE SCAN: The knife\(hyedge power transmission function \fIK\fR (\fIx\fR ) is measured as a function of knife\(hyedge lateral offset\ \fIx\fR with the plane of the knife\(hyedge separated by a distance\ \fID\fR from the fibre, and the MFD is \v'6p' .ce 1000 2\fIw\fR = [Formula Deleted] @ left [ 4~$$4o pile { (if above int above 0 } fIK\fR~` (\fIx\fR )\fIq\fR~\u2\d\fIdq\fR~$$4u pile { (if above int above 0 } fIK\fR~` (\fIx\fR )\fIdq\fR~$$4e right ] @ \u\(em1/2\d, where \fIx\fR = \fID\fR tan \(*h, \fIK\fR ` (\fIx\fR ) = @ { fIdK\fR (\fIx\fR ) } over { fIdx\fR } @ and \fIq\fR = [Formula Deleted] .ce 0 .ad r (2) \v'2P' \v'3p' .ad b .RT .LP .sp 1 .LP c) VARIABLE APERTURE TECHNIQUE: The complementary aperture power transmission function \(*a(\fIx\fR ) is measured as a function of aperture radius\ \fIx\fR with the plane of the aperture separated by a distance\ \fID\fR from the fibre, and the MFD is \v'6p' .ce 1000 2\fIw\fR = [Formula Deleted] @ left [ 4 pile { (if above int above 0 } fIa\fR (\fIx\fR )\fIqdq\fR right ] @ \u\(em1/2\d, where \fIx\fR = \fID\fR tan \(*h and \fIq\fR = [Formula Deleted] .ce 0 .ad r (3) \v'10p' .ad b .RT .LP .sp 1 .LP ii) OFFSET JOINT DOMAIN: The power transmission coefficient \fIT\fR (\(*d) is measured as a function of the transverse offset\ \(*d and \v'6p' .ce 1000 2\fIw\fR = 2 @ left [ \(em2~$$1o\fIT\fR (0) $$3u left [ { fId\fR~\u2\d\fIT\fR } over { fId\fR~\(*d\u2\d } right ] \d\\u(*d\d=\\d0\u$$3e right ] @ \u1/2\d .ce 0 .LP (4) \v'1P' \v'10p' .LP .sp 1 iii) NEAR\(hyFIELD DOMAIN: The near field intensity distribution \fIf\fR \u2\d(\fIr\fR ) is measured as a function of the radial coordinate\ \fIr\fR \ and \v'6p' .ce 1000 2\fIw\fR = 2 @ left [ 2~$$4o pile { (if above int above 0 } fIrf\fR~\u2\d(\fIr\fR )\fIdr\fR~~$$4u pile { (if above int above 0 } fIr\fR left [ { fIdf\fR (\fIr\fR~ ) } over { fIdr\fR } right ] $$2x2~\fIdr\fR~$$4e right ] @ \u1/2\d .ce 0 .ad r (5) \v'2P' \v'3p' .ad b .RT .PP .sp 1 \fINote\fR \ \(em\ The mathematical equivalence of these definitions results from transform relations between measurement results obtained by different implementation. These are summarized in Figure\ A\(hy1/G.652. .bp .LP .rs .sp 26P .ad r \fBFigure A\(hy1/G.652, p.\fR .sp 1P .RT .ad b .RT .sp 1P .LP A.2 \fBcladding surface\fR .sp 9p .RT .PP The outer surface of the glass that comprises the optical fibre. .RT .sp 1P .LP A.3 \fBcladding surface centre\fR .sp 9p .RT .PP For a cross\(hysection of an optical fibre, it is the position of the centre of the circle which best fits the locus of the cladding surface in the given cross\(hysection. .PP \fINote\fR \ \(em\ The best fit method has to be specified, and is currently under study. .RT .sp 1P .LP A.4 \fBcladding surface diameter\fR .sp 9p .RT .PP The diameter of the circle defining the cladding centre. .PP \fINote\fR \ \(em\ For a nominally circular fibre, the cladding surface diameter in any orientation of the cross\(hysection is the largest distance across the cladding. .RT .sp 1P .LP A.5\fR \fBnon\(hycircularity of the cladding surface\fR .sp 9p .RT .PP The difference between the maximum cladding surface diameter\ \fID \dmax \u\fR and minimum cladding surface diameter\ \fID \dmin \u\fR (with respect to the common cladding surface centre) divided by the nominal cladding diameter, \fID\fR , i.e., \v'6p' .RT .sp 1P .ce 1000 \fINon\(hycircularity\fR = (\fID \dmax \u\fR \(em \fID \dmin \u\fR ) / \fID\fR .ce 0 .sp 1P .PP .sp 1 \fINote\fR \ \(em\ The maximum and minimum cladding surface diameters are respectively the largest and smallest distances between the two intersections of a line through the cladding centre with the cladding surface. .bp .sp 1P .LP A.6 \fBmode field\fR .sp 9p .RT .PP The mode field is the single\(hymode field distribution giving rise to a spatial intensity distribution in the fibre. .RT .sp 1P .LP A.7 \fBmode field centre\fR .sp 9p .RT .PP The mode field centre is the position of the centroid of the spatial intensity distribution in the fibre. .PP \fINote\ 1\fR \ \(em\ The centroid is located at \fIr\fR \fI\fI\d\fIc\fR\u, and is the normalized intensity\(hyweighted integral of the position vector\ $$1\(rad \fIr\fR $$1\(raf. \v'6p' .RT .ce 1000 \fIr\fR \fI\fI\d\fIc\fR\u= @ int @ @ int @ \dAREA \u $$1\(rad \fIr\fR $$1\(raf \fII\fR ( $$1\(rad \fIr\fR $$1\(raf) dA \ \ $$2/ @ int @ @ int @ \dAREA \u \fII\fR ( $$1\(rad \fIr\fR $$1\(raf) dA .ce 0 .sp 1P .ce 1000 \v'9p' .ce 0 .sp 1P .LP .sp 1 .PP \fINote\ 2\fR \ \(em\ 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. .sp 1P .LP A.8 \fBmode field concentricity error\fR .sp 9p .RT .PP The distance between the mode field centre and the cladding surface centre. .RT .sp 1P .LP A.9 \fBmode field non\(hycircularity\fR .sp 9p .RT .PP Since it is not normally necessary to measure mode field non\(hycircularity for acceptance purposes (as stated in \(sc\ 1.4.1) a definition of mode field non\(hycircularity is not necessary in this context. .RT .sp 1P .LP A.10 \fBcut\(hyoff wavelength\fR .sp 9p .RT .PP The cut\(hyoff wavelength is the wavelength greater than which the ratio between the total power, including launched higher order modes, and the fundamental mode power has decreased to less than a specified value, the modes being substantially uniformly excited. .PP \fINote\ 1\fR \ \(em\ By definition, the specified value is chosen as 0.1\ dB for a substantially straight 2\ metre length of fibre including one single loop of radius 140\ mm. .PP \fINote\ 2\fR \ \(em\ The cut\(hyoff wavelength defined in this Recommendation is generally different from the theore tical cut\(hyoff wavelength that can be computed from the refractive index profile of the fibre. The theoretical cut\(hyoff wavelength is a less useful parameter for determining fibre performance in the telecommunication network. .PP \fINote\ 3\fR \ \(em\ In \(sc 1.5, two types of cut\(hyoff wavelength are described: .RT .LP i) a cut\(hyoff wavelength \(*l\fI\fI\d\fIc\fR\u | measured in a short length of uncabled primary\(hycoated fibre; .LP ii) a cut\(hyoff wavelength \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u | measured in a cabled fibre in a deployment condition. .PP To avoid modal noise and dispersion penalties, the cut\(hyoff wavelength \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u | of the shortest cable length (including repair lengths when present) should be less than the lowest anticipated system wavelength, \(*l\fI\fI\d\fIs\fR\u: \v'6p' .ce 1000 \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u< \(*l\fI\fI\d\fIs\fR\u .ce 0 .ad r (1) .ad b .RT .LP .sp 1 .PP This ensures that each individual cable section is sufficiently single mode. Any joint that is not perfect will create some higher order (\fILP\fR\d1\\d1\u) 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 \fILP\fR\d1\\d1\umode before it reaches the next joint. If inequality\ (1) is satisfied in the shortest cable section, it will be satisfied \fIa fortiori\fR in all longer cable sections, and single mode system operation will occur regardless of the elementary cable section length. .bp .PP Specifying \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u< \(*l\fI\fI\d\fIs\fR\ufor the shortest cable length (including loops in the splice enclosure) ensures single mode operation. It is frequently more convenient, however, to measure \(*l\fI\fI\d\fIc\fR\u, which requires only a two\(hymetre length of uncabled fibre. \(*l\fI\fI\d\fIc\fR\udepends on the fibre type, length, and bend radius, and \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u, in addition, depends on the structure of a particular cable. The relationship between \(*l\fI\fI\d\fIc\fR\uand \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u, therefore, is dependent on both the fibre and cable designs. In general, \(*l\fI\fI\d\fIc\fR\uis several tens of\ nm larger than \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u; \(*l\fI\fI\d\fIc\fR\ucan even be larger than the system wavelength, without violating inequality\ (1). Higher values of \(*l\fI\fI\d\fIc\fR\uproduce tighter confinement of the \fILP\fR\d0\\d1\umode and, therefore, help to reduce potential bending losses in the 1550\ nm wavelength region. .PP Short fibre lenghts (<20m) are frequently attached to sources and detectors, and are also used as jumpers for interconnections. The cut\(hyoff wavelength of these fibres, as deployed, should also be less than\ \(*l\fI\fI\d\fIs\fR\u. Among the means of avoiding modal noise in this case are: .RT .LP a) selecting only fibres with sufficiently low \(*l\fI\fI\d\fIc\fR\ufor such uses; .LP b) deployment of such fibres with small radius bends. .sp 1P .LP A.11 \fBchromatic dispersion\fR .sp 9p .RT .PP 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. .PP \fINote\fR \ \(em\ 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\(hysymmetric fibres. .RT .sp 1P .LP A.12 \fBchromatic dispersion coefficient\fR .sp 9p .RT .PP The chromatic dispersion per unit source spectrum width and unit length of fibre. It is usually expressed in ps/(nm\ \(mu\ km). .RT .sp 1P .LP A.13 \fBzero\(hydispersion slope\fR .sp 9p .RT .PP The slope of the chromatic dispersion coefficient versus wavelength curve at the zero\(hydispersion wavelength. .RT .sp 1P .LP A.14 \fBzero\(hydispersion wavelength\fR .sp 9p .RT .PP That wavelength at which the chromatic dispersion vanishes. .RT .ce 1000 ANNEX\ B .ce 0 .ce 1000 (to Recommendation G.652) .sp 9p .RT .ce 0 .ce 1000 \fBTest methods for single\(hymode fibres\fR .sp 1P .RT .ce 0 .PP Both reference and alternative test methods are usually given in this Annex for each parameter and it is the intention that both the RTM and the ATM(s) may be suitable for normal product acceptance purposes. However, when using an ATM, should any discrepancy arise it is recommended that the RTM be employed as the technique for providing the definitive measurement results. .sp 1P .RT .sp 2P .LP \fBB.1\ \(em\ Section\ I\ \(em\fR \fITest methods for the mode field diameter of\fR \fIsingle\(hymode fibres\fR .sp 1P .RT .sp 1P .LP B.1.1\ \ \fIReference test method for the mode field diameter of single\(hymode\fR \fIfibres\fR .sp 9p .RT .sp 1P .LP B.1.1\ \ \fIObjective\fR .sp 9p .RT .PP The mode field diameter may be determined in the far\(hyfield domain from the far field intensity distribution, \fIF\fR \u2\d(\fIq\fR ), from the knife\(hyedge transmission function, \fIK\fR (\fIx\fR ), or from the complementary aperture power transmission function, \(*a\ (\fIx\fR ); in the offset join domain from the square of the autocorrelation function, \fIT\fR (\(*d); in the near\(hyfield domain from the near\(hyfield intensity distribution, \fIf\fR \u2\d(\fIr\fR ); according to the equivalent definitions shown in \(sc\ A.1 in Annex\ A to Recommendation\ G.652. .bp .RT .sp 2P .LP B.1.1.2\ \ \fITest apparatus\fR .sp 1P .RT .sp 1P .LP B.1.1.2.1\ \ \fIGeneral\fR .sp 9p .RT .PP For\(hynear field measurements, the magnifying optics are required to create an image of the output end of the fibre in the plane of the detector. For offset joint measurements a means of traversing one fibre end face across another is required. For the three far\(hyfield measurements, appropriate scanning devices are required. .RT .sp 1P .LP B.1.1.2.2\ \ \fILight source\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.1.1.2.3\ \ \fIModulation\fR .sp 9p .RT .PP 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 to the source modulation frequency. The detecting system should have substantially linear sensitivity characteristics. .RT .sp 1P .LP B.1.1.2.4\ \ \fILaunching conditions\fR .sp 9p .RT .PP The launching conditions used must be sufficient to excite the fundamental (\fILP\fR\d0\\d1\u) mode. For example, suitable launching techniques could be: .RT .LP a) jointing with a fibre, .LP b) launching with a suitable system of optics. .PP 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. .sp 1P .LP B.1.1.2.5\ \ \fICladding mode strippers\fR .sp 9p .RT .PP Precautions shall be taken to prevent the propagation and detection of cladding modes. .RT .sp 1P .LP B.1.1.2.6\ \ \fISpecimen\fR .sp 9p .RT .PP 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 fibre axes. It is recommended that the end faces be flat and perpendicular to the fibre axes to within\ 1\(de. For the offset joint technique, the fibre will be cut into two approximately equal lengths. .RT .sp 1P .LP B.1.1.2.7\ \ \fIOffset or scan apparatus\fR .sp 9p .RT .PP Due to the characteristically narrower near\(hyfield intensity distributions and wider far\(hyfield intensity distributions of G.653\ fibres compared with G.652\ fibres, additional precautions must be taken as detailed below. .PP One of the following shall be used: .RT .LP I \fIFar\(hyfield domain\fR .LP a) \fIFar field scan system\fR .LP A mechanism to scan the far\(hyfield intensity distribution shall be used (for example, a scanning photodetector with pinhole aperture or a scanning pig\(hytailed photodetector). The scan may be either angular or linear. The detector should be at least 20\ 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 20\fIwb\fR /\(*l, where 2\fIw\fR is the expected mode field diameter of the fibre to be measured, and\ \fIb\fR is the diameter of the active area of the detector. The scan half\(hyangle should be 25\(de or greater. Alternatively, the scan should extend to at least \(em50\ dB of the zero\(hyangle intensity. .bp .LP b) \fIKnife\(hyedge assembly\fR .LP A mechanism to scan a knife\(hyedge linearly in a direction orthogonal to the fibre axis and to the edge of the blade is required. Light transmitted by the knife\(hyedge is collected and focused onto the detector. The collection optics should have a NA of\ 0.4 or greater. .LP c) \fIAperture assembly\fR .LP A mechanism containing at least twelve apertures spanning the half\(hyangle range of numerical apertures from\ 0.02 to\ 0.4 should be used. Light transmitted by the aperture is collected and focused onto the detector. .LP II \fIOffset joint domain\fR .LP \fITraversing joint\fR .LP The joint shall be constructed such that the relative offset of the fibre axes can be adjusted. A means of measuring the offset to within 0.1\ \(*mm is recommended. The optical power transmitted through the traversing joint is measured by a detector. Particular care should be taken with regard to the precision and accuracy of the offset apparatus. .LP III \fINear\(hyfield domain\fR .LP \fINear\(hyfield imaging optics\fR .LP 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\(hytailed 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 indepently known with sufficient accuracy. .LP \fINote\fR \ \(em\ The NA of the collecting optics in I\ b) and I\ c) must be large enough not to affect the measurement results. .sp 1P .LP B.1.1.2.8\ \ \fIDetector\fR .sp 9p .RT .PP A suitable detector shall be used. The detector must have linear characteristics. .RT .sp 1P .LP B.1.1.2.9\ \ \fIAmplifier\fR .sp 9p .RT .PP An amplifier should be employed in order to increase the signal level. .RT .sp 1P .LP B.1.1.2.10\ \ \fIData acquisition\fR .sp 9p .RT .PP The measured signal level shall be recorded and processed according to the technique used. .RT .sp 1P .LP B.1.1.2.11\ \ \fIMeasurement procedure\fR .sp 9p .RT .PP 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 appropriate output device. .PP One of the following procedures should be followed. .RT .LP I \fIFar\(hyfield domain\fR .LP a) By scanning the detector in fixed steps, the far\(hyfield intensity distribution \fIF\fR \u2\d(\fIq\fR ) is measured, and the mode field diameter is calculated from \(sc\ A.1, Equation\ (1) in Annex\ A. .LP b) The power transmitted by the knife\(hyedge is measured as a function of knife\(hyedge position. This function, \fIK\fR (\fIx\fR ), is differentiated and the mode field diameter is found from \(sc\ A.1, Equation\ (2) in Annex\ A. .LP c) The power transmitted by each aperture, \fIP\fR (\fIx\fR ), is measured, and the complementary aperture transmission function, \fIa\fR (\fIx\fR ), is found as: \v'6p' .sp 1P .ce 1000 \fIa\fR (\fIx\fR ) = 1 \(em @ { fIP\fR (\fIx\fR ) } over { fIP~\dmax~\u\fR } @ .ce 0 .sp 1P .LP .sp 1 where \fIP\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\u | is the power transmitted by the largest aperture and\ \fIx\fR is the aperture radius. The mode field diameter is computed from \(sc\ A.1, Equation\ (3) in Annex\ A. .bp .LP II \fIOffset joint domain\fR .LP By offsetting the joint transversely in discrete steps, the power transmission coefficient \fIT\fR (\(*d), is measured, and the mode field diameter is calculated from \(sc\ A.1, Equation\ (4) in Annex\ A. .LP III \fINear\(hyfield domain\fR .LP The near field of the fibre is enlarged by the magnifying optics and focused onto the plane of the detector. The focusing shall be performed with maximum accuracy, in order to reduce dimensional errors due to the scanning of a defocused image. The near field intensity distribution, \fIf\fR \u2\d(\fIr\fR ), is scanned and the mode field diameter is calculated from \(sc\ A.1, Equation\ (5) in Annex\ A. Alternatively, the near field intensity distribution \fIf\fR \u2\d(\fIr\fR ) may be transformed into the far field domain using a Hankel transform and the resulting transformed far field \fIF\fR \u2\d(\fIq\fR ) may be used to compute the mode field diameter from \(sc\ A.1, Equation\ (1) in Annex\ A. .sp 1P .LP B.1.1.2.12\ \ \fIPresentation of the results\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) Measurement technique used, including test set\(hyup arrangement, dynamic range of the measurement system, processing algorithms, and a description of the imaging, offsetting, or scanning devices used. .LP b) If the offset joint technique is used, the employed fitting method should be indicated (including the scan angle or NA, if applicable). .LP c) Launching conditions. .LP d) Wavelength and spectral linewidth FWHM of the source. .LP e) Fibre identification and length. .LP f ) Type of cladding mode stripper and filter (if applicable). .LP g) Magnification of the apparatus (if applicable). .LP h) Type and dimensions of the detector. .LP i) Temperature of the sample and environmental conditions (when necessary). .LP j) Indication of the accuracy and repeatability. .LP k) Mode field diameter. .PP \fINote\fR \ \(em\ As with other test methods, the apparatus and procedure given above cover only the essential basic features of the reference test method. It is assumed that the detailed instrumentation will incorporate all necessary measures to ensure stability, noise elimination, signal\(hyto\(hynoise ratio,\ etc. .sp 2P .LP \fBB.2\ \(em\ Section\ II\ \(em\fR \fITest methods for the geometrical characteristics\fR \fIexcluding the mode field diameter\fR .sp 1P .RT .sp 1P .LP B.2.1\ \ \fIReference test method: The\fR \fItransmitted near\(hyfield\fR \fItechnique\fR .sp 9p .RT .sp 1P .LP B.2.1.1\ \ \fIGeneral\fR .sp 9p .RT .PP The transmitted near\(hyfield technique shall be used for the measurement of the geometrical characteristics of single\(hymode optical fibres. Such measurements are performed in a manner consistent with the relevant definitions. .PP The measurement is based on the scanning of the magnified image(s) of the output end of the fibre under test over the cross\(hysection(s) where the detector is placed. .RT .sp 1P .LP B.2.1.2\ \ \fITest apparatus\fR .sp 9p .RT .PP A schematic diagram of the test apparatus is shown in Figure\ B\(hy1/G.652. .RT .sp 1P .LP B.2.1.2.1\ \ \fILight source\fR .sp 9p .RT .PP A nominal 1550 nm 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 defocussing of the image. .bp .RT .sp 1P .LP B.2.1.2.2\ \ \fILaunching conditions\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.1.2.3\ \ \fIMode filter\fR .sp 9p .RT .PP In the measurement, it is necessary to assure single\(hymode operation at the measurement wavelength. In these cases, it may be necessary to introduce a bend in order to remove the \fILP\fR\d1\\d1\umode. .RT .sp 1P .LP B.2.1.2.4\ \ \fICladding mode stripper\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.1.2.5\ \ \fISpecimen\fR .sp 9p .RT .PP The specimen shall be a short length of the optical fibre to be measured. The fibre ends shall be clean, smooth and perpendicular to fibre axis. .RT .sp 1P .LP B.2.1.2.6\ \ \fIMagnifying optics\fR .sp 9p .RT .PP The magnifying optics shall consist of an optical system (e.g., a microscope objective) which magnifies the specimen output near\(hyfield, focussing it onto the plane of the scanning 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 spatial resolution, and shall be recorded. .PP Image shearing techniques could be used in the magnifying optics to facilitate accurate measurements. .PP \fINote\fR \ \(em\ The validity of the image shearing technique is under study, and needs to be confirmed. .RT .sp 1P .LP B.2.1.2.7\ \ \fIDetector\fR .sp 9p .RT .PP A suitable detector shall be employed which provides the point\(hyto\(hypoint intensity of the transmitted near\(hyfield pattern(s). For example, any of the following techniques can be used: .RT .LP a) scanning photodetector with pinhole aperture; .LP b) scanning mirror with fixed pinhole aperture and photodetector; .LP c) scanning vidicon, charge coupled devices or other pattern/intensity recognition devices. .PP The detector shall be linear (or shall be linearized) in behaviour over the range intensities encountered. .sp 1P .LP B.2.1.2.8\ \ \fIAmplifier\fR .sp 9p .RT .PP 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. If such a procedure is adopted, the amplifier should be linked to the source modulation frequency. .RT .sp 1P .LP B.2.1.2.9\ \ \fIData acquisition\fR .sp 9p .RT .PP The measured intensity distribution can be recorded, processed and presented in a suitable form, according to the scanning technique and to the specification requirements. .RT .sp 2P .LP B.2.1.3\ \ \fIProcedure\fR .sp 1P .RT .sp 1P .LP B.2.1.3.1\ \ \fIEquipment calibration\fR .sp 9p .RT .PP For 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. .bp .RT .sp 1P .LP B.2.1.3.2\ \ \fIMeasurement\fR .sp 9p .RT .PP 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 focussed image(s) of the output end of the fibre shall be scanned by the detector, according to the specification requirements. The focussing shall be performed with maximum accuracy, in order to reduce dimensional errors due to the scanning of a defocussed image. The desired geometrical parameters are then calculated according to the definitions. .RT .sp 1P .LP B.2.1.4\ \ \fIPresentation of the results\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) test set\(hyup arrangement, with indication of the scanning technique used; .LP b) launching conditions; .LP c) spectral characteristics of the source(s); .LP d) fibre identification and length; .LP e) type of mode filter (if applicable); .LP f ) magnification of the magnifying optics; .LP g) type and dimensions of the scanning detector; .LP h ) temperature of the sample and environmental conditions (when necessary); .LP i) indication of the accuracy and repeatability; .LP j) resulting dimensional parameters, such as cladding diameters, cladding non\(hycircularities, mode field concentricity error,\ etc. .LP .rs .sp 13P .ad r \fBFIGURE B\(hy1/G.652, p.\fR .sp 1P .RT .ad b .RT .sp 1P .LP B.2.2\ \ \fIAlternative test method: the\fR \fIrefracted near\(hyfield\fR \fItechnique\fR .sp 9p .RT .PP This technique is described in Recommendation G.651. The decision levels on the various refractive index difference interfaces are defined as: .RT .LP Core/cladding 50% .LP Cladding/index matching fluid 50% .PP Geometry analyses consistent with the terms in Annex A, G.652, can be achieved by raster scanning of the input light spot. .sp 1P .LP B.2.3\ \ \fIAlternative test method: the\fR \fIside\(hyview method\fR .sp 9p .RT .PP The validity of the side\(hyview method for Recommendation G.653 fibres needs to be confirmed. .RT .sp 1P .LP B.2.3.1\ \ \fIObjective\fR .sp 9p .RT .PP The side\(hyview method is applied to single\(hymode fibres to determine geometrical parameters (mode field concentricity error (MFCE)), cladding diameter and cladding non\(hycircularity) by measuring the intensity distribution of light that is refracted inside the fibre. .bp .RT .sp 1P .LP B.2.3.2\ \ \fITest apparatus\fR .sp 9p .RT .PP A schematic diagram of the test apparatus is shown in Figure B\(hy2/G.652. .RT .sp 1P .LP B.2.3.2.1\ \ \fILight source\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.3.2.2\ \ \fISpecimen\fR .sp 9p .RT .PP The specimen to be measured shall be a short length of single\(hymode 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. .RT .sp 1P .LP B.2.3.2.3\ \ \fIMagnifying optics\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.3.2.4\ \ \fIDetector\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.3.2.5\ \ \fIData processing\fR .sp 9p .RT .PP A computer with appropriate software shall be used for the analysis of the intensity distributions. .RT .sp 2P .LP B.2.3.3\ \ \fIProcedure\fR .sp 1P .RT .sp 1P .LP B.2.3.3.1\ \ \fIEquipment calibration\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.3.3.2\ \ \fIMeasurement\fR .sp 9p .RT .PP 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. .PP Intensity distributions in the observation plane along the line perpendicular to the fibre axis (a\ \(em\ a\ `\ in\ A\ , in Figure\ B\(hy2/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\ ). 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. .PP As shown in Figure B\(hy3/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 .PP rotation angle, resulting in values for maximum and minimum diameters to determine the value of cladding non\(hycircularity according to the definition. .bp .RT .LP .rs .sp 27P .ad r \fBFigure B\(hy2/G.652, p.4\fR .sp 1P .RT .ad b .RT .LP .rs .sp 21P .ad r \fBFigure B\(hy3/G.652, p.5\fR .sp 1P .RT .ad b .RT .LP .bp .sp 1P .LP B.2.3.3.3\ \ \fIPresentation of the results\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) test arrangement; .LP b) fibre identification; .LP c) spectral characteristics of the source; .LP d) indication of repeatability and accuracy; .LP e) plot of nominal MFCE versus rotation angle; .LP f ) MFCE, cladding diameter and cladding non\(hycircularity; .LP g) temperature of the sample and environmental conditions (if necessary). .sp 1P .LP B.2.4\ \ \fIAlternative test method: the\fR \fItransmitted near\(hyfield image\fR \fItechnique\fR .sp 9p .RT .sp 1P .LP B.2.4.1\ \ \fIGeneral\fR .sp 9p .RT .PP The transmitted near\(hyfield image technique shall be used for the measurement of the geometrical characteristics of single\(hymode optical fibres. Such measurements are performed in a manner compatible with the relevant definitions. .PP The measurement is based on analysis of the magnified image(s) of the output end of the fibre under test. .RT .sp 1P .LP B.2.4.2\ \ \fITest apparatus\fR .sp 9p .RT .PP A schematic diagram of the test apparatus is shown in Figure\ B\(hy4/G.652. .RT .sp 1P .LP B.2.4.2.1\ \ \fILight source\fR .sp 9p .RT .PP The 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. .RT .sp 1P .LP B.2.4.2.2\ \ \fILaunching conditions\fR .sp 9p .RT .PP The 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. .RT .sp 1P .LP B.2.4.2.3\ \ \fICladding mode stripper\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.4.2.4\ \ \fISpecimen\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.2.4.2.5\ \ \fIMagnifying optics\fR .sp 9p .RT .PP The 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. .PP Image shearing techniques could be used in the magnifying optics to facilitate accurate measurements. .bp .RT .sp 1P .LP B.2.4.2.6\ \ \fIDetection\fR .sp 9p .RT .PP The fibre image shall be examined and/or analyzed. For example, either of following techniques can be used: .RT .LP a) image shearing .FS The validity of the image shearing technique is under study and needs to be confirmed. .FE ; .LP b) grey\(hyscale analysis of an electronically recorded image. .sp 1P .LP B.2.4.2.7\ \ \fIData acquisition\fR .sp 9p .RT .PP The data can be recorded, processeed and presented in a suitable form, according to the technique and to the specification requirements. .RT .sp 2P .LP B.2.4.3\ \ \fIProcedure\fR .sp 1P .RT .sp 1P .LP B.2.4.3.1\ \ \fIEquipment calibration\fR .sp 9p .RT .PP For 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. .RT .sp 1P .LP B.2.4.3.2\ \ \fIMeasurement\fR .sp 9p .RT .PP 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\(hyfield measurement, the focussed image(s) of the ouput 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. .RT .sp 1P .LP B.2.4.4\ \ \fIPresentation of the results\fR \v'3p' .sp 9p .RT .LP a) test set\(hyup arrangement, with indication of the technique used; .LP b) launching conditions; .LP c) spectral characteristics of the source; .LP d) fibre identification and length; .LP e) magnification of the magnifying optics; .LP f ) temperature of the sample and environmental conditions (when necessary); .LP g) indication of the accuracy and repeatibility; .LP h) resulting dimensional parameters, such as cladding diameters, cladding non\(hycircularities, mode field concentricity error,\ etc. .LP .rs .sp 12P .ad r \fBFigure B\(hy4/G.652, p.\fR .sp 1P .RT .ad b .RT .LP .bp .sp 2P .LP \fBB.3\ \(em\ Section\ III\ \(em\fR \fITest methods for the\fR \fIcut\(hyoff wavelength\fR .sp 1P .RT .sp 1P .LP B.3.1 \fIReference test method for the cut\(hyoff wavelength (\(*l\fI\d\fIc\fR\u\fI)\fR \fIof the primary coated fibre: the\fR \fItransmitted power technique\fR .sp 9p .RT .sp 1P .LP \fR B.3.1.1\ \ \fIObjective\fR .sp 9p .RT .PP This cut\(hyoff wavelength measurement of single\(hymode fibres is intended to assure effective single\(hymode operation above a specified wavelength. .RT .sp 1P .LP B.3.1.2\ \ \fIThe transmitted power technique\fR .sp 9p .RT .PP This method uses the variation with wavelength of the transmitted power of a short length of the fibre under test, under defined conditions, compared to a reference transmitted power. There are two possible ways to obtain this reference power: .RT .LP a) the test fibre with a loop of smaller radius, or .LP b) a short (1\(hy2\ m) length of multimode fibre. .sp 2P .LP B.3.1.2.1\ \ \fITest apparatus\fR .sp 1P .RT .sp 1P .LP B.3.1.2.1.1\ \ \fILight source\fR .sp 9p .RT .PP A light source with linewidth not exceeding 10 nm (FWHM), stable in position, intensity and wavelength over a time period sufficient to complete the measurement procedure, and capable of operating over a sufficient wavelength range, shall be used. .RT .sp 1P .LP B.3.1.2.1.2\ \ \fIModulation\fR .sp 9p .RT .PP 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 to the source modulation frequency. The detecting system should be substantially linear. .RT .sp 1P .LP B.3.1.2.1.3\ \ \fILaunching conditions\fR .sp 9p .RT .PP The launching conditions must be used in such a way to excite substantially uniformly both \fILP\fR\d0\\d1\uand\ \fILP\fR\d1\\d1\umodes. For example, suitable launching techniques could be: .RT .LP a) jointing with a multimode fibre, or .LP b) launching with a suitable large spot \(em large NA optics. .sp 1P .LP B.3.1.2.1.4\ \ \fICladding mode stripper\fR .sp 9p .RT .PP 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 \fILP\fR\d1\\d1\u\ mode. .RT .sp 1P .LP B.3.1.2.1.5\ \ \fIOptical detector\fR .sp 9p .RT .PP 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. .RT .sp 2P .LP B.3.1.2.2\ \ \fIProcedure\fR .sp 1P .RT .sp 1P .LP B.3.1.2.2.1\ \ \fIStandard test sample\fR .sp 9p .RT .PP 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 radius. 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 ouput power\ \fIP\fR\d1\u\ (\(*l) shall be recorded versus\ \(*l in a sufficiently wide range around the expected cut\(hyoff wavelength. .PP \fINote\fR \ \(em\ The presence of a primary coating on the fibre usually does not affect the cut\(hyoff wavelength. However, the presence of a secondary coating may result in a cut\(hyoff wavelength that may be significantly shorter than that of the primary coated fibre. .bp .RT .sp 1P .LP B.3.1.2.2.2\ \ \fITransmission through the reference sample\fR .sp 9p .RT .PP Either method a) or b) may be used. .RT .LP a) Using the test sample, and keeping the launch conditions fixed, an output power \fIP\fR\d2\u(\(*l) is measured over the same wavelength range with at least one loop of sufficiently small radius in the test sample to filter the \fILP\fR\d1\\d1\umode. A typical value for the radius of this loop is 30\ mm. .LP b) With a short (1\(hy2 m) length of multimode fibre, an output power\ \fIP\fR\d3\u\ (\(*l) over the same wavelength range. .PP \fINote\fR \ \(em\ The presence of leaky modes may cause ripple in the transmission spectrum of the multimode reference fibre, affecting the result. To reduce this problem, light\(hylaunching conditions may be restricted to fill only 70% of the multimode fibre's core diameter and NA or a suitable mode filter may be used. .sp 1P .LP B.3.1.2.2.3\ \ \fICalculations\fR .sp 9p .RT .PP The logarithmic ratio between transmitted powers \fIP\fR\d1\u(\(*l) and \fIP\fR\fI\d\fIi\fR\u\ (\(*l) is calculated as: \v'6p' .RT .sp 1P .ce 1000 \fIR\fR (\(*l) = 10 log [\fIP\fR\d1\u(\(*l)/\fIP\fR\d\fIi\fR\u(\(*l)] .ce 0 .sp 1P .LP .sp 1 where .PP \fIi\fR \ =\ 2 or 3, methods a) or b) respectively. .PP \fINote\fR \ \(em\ In method a) the small mode filter fibre loop eliminates all modes except the fundamental for wavelengths greater than a few tens of nm below the cut\(hyoff wavelength\ \(*l\fI\fI\d\fIc\fR\u. For wavelengths more than several hundred nm above\ \(*l\fI\fI\d\fIc\fR\u, even the fundamental mode may be strongly attenuated by the loop. \fIR\fR (\(*l) is equal to the logarithmic ratio between the total power emerging from the sample, including the\ \fILP\fR\d1\\d1\umode power, and the fundamental mode power. When the modes are uniformly excited in accordance with\ B.1.2.1.3, \fIR\fR (\(*l) then also yields the \fILP\fR\d1\\d1\umode attenuation\ \fIA\fR (\(*l) in dB in the test sample: \v'6p' .RT .sp 1P .ce 1000 \fIA\fR (\(*l) = 10 log [(\fIP\fR\d1\u(\(*l)/\fIP\fR\d2\u(\(*l) \(em 1)/2] .ce 0 .sp 1P .LP .sp 1 B.3.1.2.2.4\ \ \fIDetermination of cut\(hyoff wavelength\fR .sp 9p .RT .PP If method a) is used, \(*l\fI\fI\d\fIc\fR\u | is determined as the largest wavelength at which \fIR\fR (\(*l) is equal to 0.1\ dB (see Figure\ B\(hy5/G.652). .PP If method\ b) is used, \(*l\fI\fI\d\fIc\fR\uis determined by the intersection of a plot of \fIR\fR (\(*l) and a straight line\ (2) displaced 0.1\ dB and parallel to the straight line\ (1) fitted to the long wavelength portion of \fIR\fR (\(*l) (see Figure\ B\(hy6/G.652). .PP \fINote\fR \ \(em\ According to the definition, the \fILP\fR\d1\\d1\umode attenuation in the test sample is 19.3\ dB at the cut\(hyoff wavelength. .RT .sp 1P .LP B.3.2.1.2.2.5\ \ \fIPresentation of results\fR \v'3p' .sp 9p .RT .LP a) test set\(hyup arrangement; .LP b) launching condition; .LP c) type of reference sample; .LP d) temperature of the sample and environmental conditions (if necessary); .LP e) fibre identification; .LP f ) wavelength range of measurement; .LP g) cut\(hyoff wavelength; .LP h) plot of \fIR\fR (\(*l) (if required). .sp 2P .LP B.3.2\ \ \fIAlternative test method for \(*l\fI \fIsplit\(hymandrel\fR \fItechnique\fR .sp 1P .RT .sp 1P .LP B.3.2.1\ \ \fIObjective\fR through B.3.2.2.1.5 \fIOptical detector\fR (as in B.3.1.1\fR through B.3.1.2.1.5) .bp .sp 9p .RT .sp 1P .LP B.3.2.2.2\ \ \fIProcedure\fR .sp 9p .RT .sp 1P .LP B.3.2.2.2.1\ \ \fIStandard test sample\fR .sp 9p .RT .LP .rs .sp 15P .ad r \fBFigure B\(hy5/G.652, p.\fR .sp 1P .RT .ad b .RT .LP .rs .sp 15P .ad r \fBFigure B\(hy6/G.652, p.\fR .sp 1P .RT .ad b .RT .PP 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 contain a full turn (360\ degrees) consisting of two arcs (180\ degrees each) of 140\ mm radius connected by tangents. The remaining part of the fibre shall be substantially free of external stresses. .PP While some incidental bends of larger radii are permissible, they must not introduce a significant change in the measurement result. The output power\fR \fIP\fR\d1\u(\(*l) shall be recorded versus\ \(*l in a sufficiently wide range around the expected cut\(hyoff wavelength. .PP As shown in Figure B\(hy7/G.652, 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. .RT .sp 1P .LP B.3.2.2.2.2 through B.3.2.2.2.5 (as in B.3.1.2.2.2 through B.3.1.2.2.5) .bp .sp 9p .RT .LP .rs .sp 22P .ad r \fBFigure B\(hy7/G.652, p.\fR .sp 1P .RT .ad b .RT .sp 2P .LP \fB B.3.3 \fIReference test method for the cut\(hyoff wavelength\fR \fI(\(*l\fI\d\fIc\fR\\d\fIc\fR\u\fI) of the cable fibre: the\fR \fItransmitted power\fR \fItechnique\fR .sp 1P .RT .sp 1P .LP B.3.3.1 \fIObjective\fR .sp 9p .RT .PP This cut\(hyoff wavelength measurement which is performed on cabled single\(hymode fibres in a deployment condition which stimulates outside plant minimum cable lengths, is intended to assure effective single\(hymode operation above a specified wavelength. .RT .sp 1P .LP B.3.3.2\ \ \fIThe transmitted power technique\fR .sp 9p .RT .PP This method uses the variation with wavelength of the transmitted power of the fibre cable under test, under defined conditions, compared to a reference transmitted power. There are two possible ways to obtain this reference power. .RT .LP a) the cabled test fibre with a loop of smaller radius; .LP b) a short (1\(hy2\ m) length of multimode fibre. .sp 2P .LP B.3.3.2.1\ \ \fITest apparatus\fR .sp 1P .RT .sp 1P .LP B.3.3.2.1.1\ \ \fILight source\fR (as in B.3.1.2.1.1) .sp 9p .RT .sp 1P .LP B.3.3.2.1.2\ \ \fIModulation\fR (as in B.3.1.2.1.2) .sp 9p .RT .sp 1P .LP B.3.3.2.1.3\ \ \fILaunching conditions\fR (as in B.3.1.2.1.3) .sp 9p .RT .sp 1P .LP B.3.3.2.1.4\ \ \fICladding mode stripper\fR (as in B.3.1.2.1.4) .sp 9p .RT .sp 1P .LP B.3.3.2.1.5\ \ \fIOptical detector\fR (as in B.3.1.2.1.5) .bp .sp 9p .RT .sp 1P .LP B.3.3.2.2\ \ \fIProcedure\fR .sp 9p .RT .sp 1P .LP B.3.3.2.2.1\ \ \fIStandard test sample\fR .sp 9p .RT .PP The measurement shall be performed on a length of single\(hymode 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 XX\ mm radius shall be applied to each uncabled fibre length (see Figure\ B\(hy8/G.652). 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\ \fIP\fR\d1\u(\(*l) shall be recorded versus\ \(*l in a sufficiently wide range around the expected cut\(hyoff wavelength. .PP \fINote\fR \ \(em\ 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. .RT .sp 2P .LP B.3.3.2.2.2\ \fITransmission through the reference sample\fR (as in B.1.2.2.2) .sp 1P .RT .sp 1P .LP B.3.3.2.2.3\ \ \fICalculations\fR .sp 9p .RT .PP The logaritmic ratio between the transmitted powers \fIP\fR\d1\u(\(*l) and \fIP\fR\d1\u(\(*l) is calculated as \v'6p' .RT .ce 1000 \fIR\fR (\(*l) = 10 log [\fIP\fR\d1\u(\(*l)/\fIP\fR\d\fIi\fR\u(\(*l)] \ \ \ \ (dB) .ce 0 .ad r (1) .ad b .RT .LP .sp 1 where \fIi\fR \ =\ 2 or 3 for methods a) or b), respectively. .sp 1P .LP B.3.3.2.2.4\ \ \fIDetermination of cabled fibre cut\(hyoff wavelength\fR .sp 9p .RT .PP If method a) is used, \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\u | is determined as the largest wavelength at which \fIR\fR (\(*l) is equal to 0.1\ dB (see Figure\ B\(hy5). If method\ b) is used, \(*l\fI\fI\d\fIc\fR\\d\fIc\fR\uis determined by the intersection of a plot of \fIR\fR (\(*l) and a straight line\ (2) displaced 0.1\ dB and parallel to the straight line\ (1) fitted to the long wavelength portion of\ \fIR\fR (\(*l)\ see Figure\ B\(hy6). .RT .sp 1P .LP B.3.3.2.2.5\ \ \fIPresentation of results\fR \v'3p' .sp 9p .RT .LP a) test set\(hyup arrangment (including the radius XX of the loops); .LP b) launching condition; .LP c) type of reference sample; .LP d) temperature of the sample and environmental conditions (if necessary); .LP e) fibre and cable identification; .LP f ) wavelength range of measurement; .LP g) cabled fibre cut\(hyoff wavelength, and plot of \fIR\fR (\(*l) (if required); .LP h) plot of \fIR\fR (\(*l) (if required). .LP .rs .sp 11P .ad r \fBFigure B\(hy8/G.652, p.\fR .sp 1P .RT .ad b .RT .LP .bp .LP \fBB.4\ \(em\ Section\ IV\ \(em\fR \fITest methods for attenuation measurements\fR .sp 1P .RT .sp 1P .LP B.4.1\ \ \fIIntroduction\fR \v'3p' .sp 9p .RT .LP B.4.1.1\ \ \fIObjectives\fR .PP The 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. .RT .sp 1P .LP B.4.1.2\ \ \fIDefinition\fR .sp 9p .RT .PP The attenuation \fIA\fR (\(*l) at wavelength\ \(*l between two cross\(hysections and separated by distance \fIL\fR of a fibre\fR is defined, as \v'6p' .RT .ce 1000 \fIA\fR (\(*l) = 10 log [\fIP\fR\d1\u(\(*l)/\fIP\fR\d2\u(\(*l)]\ \ \ \ (dB) .ce 0 .ad r (1) .ad b .RT .LP .sp 1 where \fIP\fR\d1\u(\(*l) is the optical power traversing the cross\(hysection 1 and \fIP\fR\d2\u(\(*l) is the optical power traversing the cross\(hysection\ 2 at the wavelength\ \(*l. .PP For a uniform fibre, it is possible to define an attenuation per unit length, or an attenuation coefficient which is dependent of the length of the fibre: \v'6p' .ce 1000 \(*a(\(*l) = \fIA\fR (\(*l)/\fIL\fR \ \ \ \ (dB/unit of length) .ce 0 .ad r (2) .ad b .RT .PP .sp 1 \fINote\fR \ \(em\ Attenuation values specified for factory lengths should be measured at room temperature (i.e., a single value in the range 10\ to 35 | (deC). .sp 1P .LP B.4.2\ \ \fIThe reference test method: the\fR \fIcut\(hyback technique\fR .sp 9p .RT .PP The cut\(hyback technique is a direct application of the definition in which the power levels\ \fIP\fR\d1\uand\ \fIP\fR\d2\uare measured at two points of the fibre without change of input conditions. \fIP\fR\d2\uis the power emerging from the far end of the fibre and\ \fIP\fR\d1\uis the power emerging from a point near the input after cutting the fibre. .RT .sp 1P .LP B.4.2.1\ \ \fITest apparatus\fR .sp 9p .RT .PP Measurements 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\(hy9/G.652. .RT .sp 1P .LP B.4.2.1.1\ \ \fIOptical source\fR .sp 9p .RT .PP A suitable radiation source shall be used, such 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. .RT .sp 1P .LP B.4.2.1.2\ \ \fIModulation\fR .sp 9p .RT .PP 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 to the source modulation frequency. The detecting system should be substantially linear. .RT .sp 1P .LP B.4.2.1.3\ \ \fILaunching conditions\fR .sp 9p .RT .PP The launching conditions used must be sufficient to excite the fundamental mode. For example, suitable launching techniques could be: .RT .LP a) jointing with a fibre, .LP b) launching with a suitable system of optics. .sp 1P .LP B.4.2.1.4\ \ \fIMode filter\fR .sp 9p .RT .PP Care must be taken that higher order modes do not propagate through the cut\(hyback length. In these cases, it may be necessary to introduce a bend in order to remove the higher modes. .bp .RT .sp 1P .LP B.4.2.1.5\ \ \fR \fICladding mode stripper\fR .sp 9p .RT .PP A cladding mode stripper encourages the conversion of cladding modes to radiation modes; as a result, cladding modes are stripped from the fibre. .RT .sp 1P .LP B.4.2.1.6\ \ \fIOptical detector\fR .sp 9p .RT .PP 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 characteristics. .RT .sp 1P .LP B.4.2.2\ \ \fIMeasurement procedure\fR \v'3p' .sp 9p .RT .LP B.4.2.2.1\ \ \fIPreparation of fibre under test\fR .PP 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. .RT .sp 1P .LP B.4.2.2.2\ \ \fIProcedure\fR \v'3p' .sp 9p .RT .LP 1) The fibre under test is placed in the measurements set\(hyup. The output power\ \fIP\fR\d2\uis recorded. .LP 2) Keeping the launching conditions fixed, the fibre is cut to the cut\(hyback length (for example, 2\ m from the launching point). The cladding mode stripper, when needed, is refitted and the output power\ \fIP\fR\d1\ufrom the cut\(hyback length is recorded. .LP 3) The attenuation of the fibre, between the points where\ \fIP\fR\d1\uand\ \fIP\fR\d2\uhave been measured, can be calculated from the definition using\ \fIP\fR\d1\uand\ \fIP\fR\d2\u. .sp 1P .LP B.4.2.2.3\ \ \fIPresentation of results\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) test set\(hyup arrangement, including source type, source wavelength, and linewidth (FWHM); .LP b) fibre identification; .LP c) length of sample; .LP d) attenuation of the sample quoted in dB; .LP e) attenuation coefficient quoted in dB/km; .LP f ) indication of accuracy and repeatability; .LP g) temperature of the sample and environmental conditions (if necessary). .sp 1P .LP B.4.3\ \ \fIFirst alternative test method; the\fR \fIbackscattering\fR \fItechnique\fR .sp 9p .RT .PP \fINote\fR \ \(em\ This test method describes a procedure to measure the attenuation of a homogenous sample of single\(hymode 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. .RT .sp 1P .LP B.4.3.1\ \ \fILaunching conditions\fR .sp 9p .RT .PP The 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. .RT .sp 1P .LP B.4.3.2\ \ \fIApparatus and procedure\fR \v'3p' .sp 9p .RT .LP B.4.3.2.1\ \ \fIGeneral considerations\fR .PP The signal level of the backscattered optical signal will normally be small and close to the noise level. In order to improve the signal\(hyto\(hynoise 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 pulse width in order to obtain a compromise between resolution and pulse energy. Special care should be taken to minimize the Fresnel reflections. .PP Care must be taken that higher order modes do not propagate. .PP An example of apparatus is shown in Figure\ B\(hy10a/G.652. .bp .RT .sp 1P .LP B.4.3.2.2\ \ \fIOptical source\fR .sp 9p .RT .PP A stable high power optical source of an appropriate wavelength should be used. The wavelength of the source should be registered. The pulse width and repetition rate should be consistent with the desired resolution and the length of the fibre. Optical non\(hylinear effects should not be present in the part of the fibre under test. .RT .sp 1P .LP B.4.3.2.3\ \ \fICoupling device\fR .sp 9p .RT .PP The coupling device is needed to couple the source radiation to the fibre and the backscattered radiation to the detector, while avoiding a direct source\(hydetector coupling. Several devices can be used, but devices based on polarization effects should be avoided. .RT .sp 1P .LP B.4.3.2.4\ \ \fIOptical detection\fR .sp 9p .RT .PP A 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. .PP Signal processing is required to improve the signal to noise ratio, and it is desirable to have a logarithmic response in the detection system. .PP A 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\(hyoff between time resolution and noise reduction. .RT .LP .rs .sp 33P .ad r \fBfigure\ B\(hy9/G.652, p.\fR .sp 1P .RT .ad b .RT .LP .bp .sp 1P .LP B.4.3.2.5\ \ \fICladding mode stripper\fR .sp 9p .RT .PP See \(sc\ B.2.1.5. .RT .sp 1P .LP B.4.3.2.6\ \ \fIProcedure\fR \v'3p' .sp 9p .RT .LP 1) The fibre under test is aligned to the coupling device. .LP 2) Backscattered power is analyzed by a signal processor and recorded on a logarithmic scale. Figure B\(hy10b/G.652 shows such a typical curve. .LP 3) The attenuation between two points A and B of the curve corresponding to two cross\(hysections of the fibre is \v'6p' .sp 1P .ce 1000 @ pile { { t\fIA\fR (\(*l) } above { ~\fIA\fR~\s6\fIA\fR~\(ra\fIB\fR~\s } } @ = [Formula Deleted] \dA\u\fR \(em \fIV \dB\u\fR )\ \ \ \ (dB) .RT .ce 0 .sp 1P .LP .sp 1 where \fIV\fR\d\fIA\fR\uand \fIV\fR\d\fIB\fR\uare the corresponding power levels given on a logarithmic scale. .LP \fINote\fR \ \(em\ Attention must be given to the scattering conditions at points\ A and\ B when calculating the attenuation in this way. .LP 4) If so required, bi\(hydirectional 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. .sp 1P .LP B.4.3.2.7\ \ \fIResults\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) measurement types and characteristics; .LP b) launching techniques; .LP c) test set\(hyup arrangement; .LP d) relative humidity and temperature of the sample (when necessary); .LP e) fibre identification; .LP f ) length of sample; .LP g) rise time, width and repetition rate of the pulse; .LP h) kind of signal processing used; .LP i) The recorded curve on a logarithmic scale, with the attenuation of the sample, and under certain conditions the attenuation coefficient in dB/km. .PP \fINote\fR \ \(em\ The complete analysis of the recorded curve (Figure\ B\(hy10b/G.652) shows that, independently from the attenuation measurement, many phenomena can be monitored using the backscattering technique: .LP a) reflection originated by the coupling device at the input end of the fibre; .LP b) zone of constant slope; .LP c) discontinuity due to local defect, splice or coupling; .LP d) reflection due to dielectric defect; .LP e) reflection at the end of the fibre. .sp 1P .LP B.4.4\ \ \fISecond alternative test method: the\fR \fIinsertion loss\fR \fItechnique\fR .sp 9p .RT .PP Under consideration. .RT .sp 2P .LP \fBB.5\ \(em\ Section\ V\ \(em\fR \fITest methods for chromatic dispersion\fR \fIcoefficient measurement\fR .sp 1P .RT .sp 1P .LP B.5.1\ \ \fIReference test method for\fR \fIchromatic dispersion\fR \fIcoefficient measurement\fR \v'3p' .sp 9p .RT .LP B.5.1.1\ \ \fIObjective\fR .PP The 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. .bp .RT .LP .rs .sp 39P .ad r \fBfigure\ B\(hy10/G.652, p.12\fR .sp 1P .RT .ad b .RT .PP The group delay can be measured either in the time domain or in the frequency domain, according to the type of modulation of the source. .PP In 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. .PP The chromatic dispersion may be measured at a fixed wavelength or over a wavelength range. .RT .sp 1P .LP B.5.1.2\ \ \fITest apparatus\fR .sp 9p .RT .PP A schematic diagram of the test apparatus is shown in Figure\ B\(hy11/G.652. .bp .RT .sp 1P .LP B.5.1.2.1\ \ \fISource\fR .sp 9p .RT .PP The source shall be stable in position, intensity and wavelength over a time period sufficiently long to complete the measurement procedure. Laser diodes, 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. .PP In any case, the modulating signal shall be such as to guarantee a sufficient time resolution in the group delay measurement. .RT .sp 1P .LP B.5.1.2.2\ \ \fIWavelength selection\fR .sp 9p .RT .PP A wavelength selector is used to select the wavelength at which the group delay is to be measured. Optical switch, monochromator, dispersive devices, optical filters, optical coupler, connectors,\ etc., may be used, depending on the type of light sources and measurement set\(hyup. The selection may be carried out by switching electrical driving signals for different wavelength light sources. The wavelength selector may be used either at the input or at the output end of the fibre under test. .RT .sp 1P .LP B.5.1.2.3\ \ \fIDetector\fR .sp 9p .RT .PP The light emerging from the fibre under test, the reference fibre or the optical divider\ etc., is coupled to a photo detector whose signal\(hyto\(hynoise ratio and time resolution are adequate for the measurement. The detector is followed by a low noise amplifier if needed. .RT .sp 1P .LP B.5.1.2.4\ \ \fIReference channel\fR .sp 9p .RT .PP The reference channel may consist of 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. .RT .sp 1P .LP B.5.1.2.5\ \ \fIDelay detector\fR .sp 9p .RT .PP The delay detector shall measure the delay time or the phase shift between the reference signal and the channel signal. In the case of sinusoidal modulation, a vector voltmeter could be used. In the case of pulse modulation, a high speed oscilloscope or a sampling oscilloscope could be used. .RT .sp 1P .LP B.5.1.2.6\ \ \fISignal processor\fR .sp 9p .RT .PP 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. .RT .sp 1P .LP B.5.1.3\ \ \fIProcedure\fR .sp 9p .RT .PP The fibre under test is suitably coupled to the source and to the detector through the wavelength selector or the optical divider,\ etc. If needed, a calibration of the chromatic delay of the source may be performed. A suitable compromise between wavelength resolution and signal level must be achieved. Unless the fibre under test is also used as the reference channel line, the temperature of the fibre must be sufficiently stable during the measurement. .PP The time delay or phase shift between the reference signal and the channel signal at the operating wavelength are to be measured by the delay detector. Data processing appropriate to the type of modulation is used in order to obtain the chromatic dispersion coefficient at the operating wavelength. When needed, a spectral scan of the group delay versus wavelength can be performed; from the measured values a fitting curve can be completed. .bp .PP The measured group delay per unit fibre length versus wavelength shall be fitted by the quadratic expression: \v'6p' .RT .sp 1P .ce 1000 \(*t(\(*l) = \(*t\d0\u+ [Formula Deleted] (\(*l \(em \(*l\d0\u)\u2\d .ce 0 .sp 1P .LP .sp 1 where \(*t\d0\uis the relative delay minimum at the zero\(hydispersion wavelength\ \(*l\d0\u. The chromatic dispersion coefficient \ \fID\fR (\(*l) = \fId\fR \(*t/\fId\fR \(*l can be determined from the differentiated quadratic expression: \v'6p' .sp 1P .ce 1000 \fID\fR (\(*l) = (\(*l \(em \(*l\d0\u)\fIS\fR\d0\u .ce 0 .sp 1P .LP .sp 1 .LP where \fIS\fR\d0\uis the (uniform) zero\(hydispersion slope, i.e., the value of the dispersion slope \fIS\fR (\(*l) = \fIdD\fR /\fId\fR \(*l at \(*l\d0\u. .PP \fINote\ 1\fR \ \(em\ These equations for \(*t(\(*l) and \fID\fR (\(*l) are sufficiently accurate over the 1500\(hy1600\ nm range. They are not meant to be used in the 1300\ nm region. .PP \fINote\ 2\fR \ \(em\ Alternatively, the chromatic dispersion coefficient can be measured directly, for example by the differential phase shift method. In this case, a straight line shall be fitted directly to the dispersion coefficient for determining\ \(*l\d0\uand\ \fIS\fR\d0\u. .RT .sp 1P .LP B.5.1.4\ \ \fIPresentation of results\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) test set\(hyup arrangement; .LP b) type of modulation used; .LP c) source characteristics; .LP d) fibre identification and length; .LP e) characteristics of the wavelength selector (if present); .LP f ) type of photodetector; .LP g) characteristics of the delay detector; .LP h) values of the zero\(hydispersion wavelength and the zero\(hydispersion slope. .LP If the frequency domain technique is used, the time group delay\ \(*t will be deduced from the corresponding phase shift\ \(*f through the relation\ \(*t\ =\ \(*f/(2\(*p\fIf\fR ), \fIf\fR \ being the modulation frequency; .LP i) fitting procedures of relative delay data with the used fitting wavelength range; .LP j) temperature for the sample and environment conditions (if necessary). .LP .rs .sp 16P .ad r \fBFigure B\(hy11/G.652, p.\fR .sp 1P .RT .ad b .RT .LP .bp .sp 2P .LP B.5.2\ \ \fIAlternative test method for chromatic dispersion coefficient\fR \fImeasurement: the\fR \fIinterferometric test method\fR .sp 1P .RT .sp 1P .LP B.5.2.1\ \ \fIObjective\fR .sp 9p .RT .PP The interferometric test method allows the dispersion to be measured, using a short piece of fibre (several metres). This offers the possibility of measuring the longitudinal chromatic dispersion homogeneity of optical fibres. Moreover, it is possible to test the effect of overall or local influences, such as temperature changes and macrobending losses, on the chromatic dispersion. .PP According to the interferometric measuring principle, the wavelength\(hydependent time delay between the test sample and the reference path is measured by a Mach\(hyZehnder interferometer. The reference path can be an air path or as a single\(hymode fibre with known spectral group delay. .PP It should be noted that the extrapolation of the chromatic dispersion values derived from the interferometric test on fibres of a few metres length, to long fibre sections assumes longitudinal homogeneity of the fibre. This assumption may not be applicable in every case. .RT .sp 1P .LP B.5.2.2\ \ \fITest apparatus\fR .sp 9p .RT .PP Schematic diagrams of the test apparatus using a reference fibre and an air path reference are shown in Figures\ B\(hy12/G.652 and\ B\(hy13/G.652 respectively. .RT .sp 1P .LP B.5.2.2.1\ \ \fIOptical source\fR .sp 9p .RT .PP The source should be stable in position, intensity and wavelength for a time period sufficiently long to complete the measurement procedure. The source must be suitable,\ e.g. a YAG laser with a Raman fibre or a lamp and LED optical sources\ etc. For the application of lock\(hyin amplification techniques, a light source for low\(hyfrequency modulation (50\ to 500\ Hz) is sufficient. .RT .sp 1P .LP B.5.2.2.2\ \ \fIWavelength selector\fR .sp 9p .RT .PP A wavelength selector is used to select the wavelength at which the group delay is measured. A monochromator, optical interference filter, or other wavelength selector may be used depending on the type of optical sources and measurement systems. The wavelength selector may be used either at the input or the output end of the fibre under test. .PP The spectral width of the optical sources is to be restricted by the dispersion measuring accuracy, and it is about\ 2 to 10\ nm. .RT .sp 1P .LP B.5.2.2.3\ \ \fIOptical detector\fR .sp 9p .RT .PP The optical detector must have a sufficient sensitivity in that wavelength range in which the chromatic dispersion has to be determined. If necessary, the received signal has to be upgraded, with for example a transimpedance circuit. .RT .sp 1P .LP B.5.2.2.4\ \ \fITest equipment\fR .sp 9p .RT .PP For the recording of the interference patterns, a lock\(hyin amplifier may be used. Balancing of the optical length of the two ways of the interferometer is performed with one linear positioning device in the reference path. Concerning the positioning device, attention should be paid to the accuracy, uniformity and stability of linear motion. The variation of the length should cover the range from 20\ to 100\ mm with an accuracy of about 2\ \(*mm. .RT .sp 1P .LP B.5.2.2.5\ \ \fISpecimen\fR .sp 9p .RT .PP The specimen for the test can be uncabled and cabled single\(hymode fibres. The length of the specimen should be in the range 1\ m to 10\ m. The accuracy of the length should be about \(+- | \ mm. The preparation of the fibre endfaces should be carried out with reasonable care. .RT .sp 1P .LP B.5.2.2.6\ \ \fIData processing\fR .sp 9p .RT .PP For the analysis of the interference patterns, a computer with suitable software should be used. .bp .RT .sp 1P .LP B.5.2.3\ \ \fITest procedure\fR \v'3p' .sp 9p .RT .LP 1) The fibre under test is placed in the measurement set\(hyup (Figures\ B\(hy12/G.652,\ B\(hy13/G.652). The positioning of the endfaces is carried out with 3\(hydimensional micro\(hypositioning devices by optimizing the optical power received by the detector. Errors arising from cladding modes are not possible. .LP 2) The determination of the group delay is performed by balancing the optical lengths of the two interferometer paths with one linear positioning device in the reference path for different wavelengths. The difference between position\ \fIx\fR\d\fIi\fR\uof the maximum of the interference pattern for wavelength\ \(*l\fI\fI\d\fIi\fR\uand position\ \fIx\fR\d0\u(Figure\ B\(hy14/G.652) determines the group delay difference ?63\fIt\fR\d\fIg\fR\u\ (\(*l\fI\fI\d\fIi\fR\u) between the reference path and the test path as follows: \v'6p' .sp 1P .ce 1000 \fIt\fR\d\fIg\fR\u(\(*l\fI\fI\d\fIi\fR\u) = @ { fIx\fR\d0\u\(em~\fIx\fR\d\fIi\fR\ } over { fIc\fR\d0\ } @ .ce 0 .sp 1P .LP .sp 1 where \fIc\fR\d0\u | is the velocity of light in the vacuum. The group delay of the test sample is calculated by adding the value ?63\fIt\fR\d\fIg\fR\u\ (\(*l\fI\fI\d\fIi\fR\u) and the spectral group delay of the reference path. Dividing this sum by the test fibre length then gives the measured group delay per unit length\ \(*t(\(*l) of the test fibre. .LP .rs .sp 27P .ad r \fBFigure B\(hy12/G.652, p.14\fR .sp 1P .RT .ad b .RT .LP .bp .LP .rs .sp 23P .ad r \fBFigure B\(hy13/G.652, p.15\fR .sp 1P .RT .ad b .RT .LP .rs .sp 24P .ad r \fBFigure B\(hy14/G.652, p.16\fR .sp 1P .RT .ad b .RT .LP .bp .PP The measured group delay per unit fibre length versus wavelength shall be fitted by the quadratic expression \v'6p' .sp 1P .ce 1000 \(*t(\(*l) = \(*t\d0\u+ [Formula Deleted] (\(*l \(em \(*l\d0\u)\u2\d .ce 0 .sp 1P .LP .sp 1 where \(*t\d0\u | is the relative delay minimum at the zero\(hydispersion wavelength\ \(*l\d0\u. The chromatic dispersion coefficient \fID\fR (\(*l)\ =\ \fId\fR \(*t/\fId\fR \(*l can be determined from the differentiated quadratic expression: \v'6p' .sp 1P .ce 1000 D\fR (\(*l) = (\(*l \(em \(*l\d0\u)\fIS\fR\d0\u .ce 0 .sp 1P .LP .sp 1 where \fIS\fR\d0\uis the (uniform) zero\(hydispersion slope, i.e., the value of the dispersion slope \fIS\fR (\(*l) = \fIdD\fR /\fId\fR \(*l at \(*l\d0\u. .PP \fINote\fR \ \(em\ These equations for \(*t(\(*l) and \fID\fR (\(*l) are sufficiently accurate over the 1500\(hy1600\ nm range. They are not meant to be used in the 1300\ nm region. .sp 1P .LP B.5.2.4\ \ \fIPresentation of results\fR .sp 9p .RT .PP The following details shall be presented: .RT .LP a) test set\(hyup arrangement; .LP b) source characteristics; .LP c) fibre identification and length; .LP d) characteristics of the wavelength selector (if present); .LP e) type of the photodetector; .LP f ) values of the zero\(hydispersion wavelength and the zero\(hydispersion slope; .LP g) fitting procedures of relative delay date with the used fitting wavelength range; .LP h ) temperature of the sample and environmental conditions (if necessary). .LP .rs .sp 26P .LP \fBMONTAGE:\ \fR REC.\ G.653 SUR LE RESTE DE CETTE PAGE .sp 1P .RT .LP .bp