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InfoMagic Standards 1994 January
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1988
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.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
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.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
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.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
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B.5.2.2.5\ \ \fISpecimen\fR
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.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
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B.5.2.2.6\ \ \fIData processing\fR
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.RT
.PP
For the analysis of the interference patterns, a computer with
suitable software should be used.
.bp
.RT
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B.5.2.3\ \ \fITest procedure\fR \v'3p'
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.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
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.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
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.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 23P
.ad r
\fBFigure B\(hy13/G.652, p.15\fR
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.RT
.ad b
.RT
.LP
.rs
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.ad r
\fBFigure B\(hy14/G.652, p.16\fR
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.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.
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B.5.2.4\ \ \fIPresentation of results\fR
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.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
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.RT
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