- -
AP IX-60-E
(3201)
6. Recommendation G.651
CHARACTERISTICS OF A 50/125 m MULTIMODE GRADED INDEX
OPTICAL FIBRE CABLE
The CCITT,
considering that
(a) multimode optical fibre cables are used widely in
telecommunication networks;
(b) the foreseen potential applications may require multimode fibres
differing in:
- nature of material
- geometrical characteristics
- operating wavelength region(s)
- transmission and optical characteristics
- mechanical and environmental aspects;
(c) Recommendations on different kinds of multimode fibres can be
prepared when practical use studies have sufficiently progressed;
recommends
a graded index, multimode fibre, which may be used in the region of
850 nm or in the region of 1300 nm or alternatively may be used in both
wavelength regions simultaneously.
This fibre can be used for analogue and for digital transmission.
Its geometrical, optical, and transmission characteristics are
described below.
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
multimode fibre Recommendations are agreed upon.
1. Fibre characteristics
The fibre characteristics dealt with in 1 are those which ensure the
interconnection of fibres with acceptable low losses.
Only the intrinsic fibre characteristics (not depending on the cable
manufacture) are recommended in 1. They will apply equally to individual
fibres, fibres incorporated into a cable wound on a drum, and fibres in
installed cables.
1.1 Geometrical characteristics of the fibre
1.1.1 Core diameter
The recommended nominal value of the core diameter is 50 m.
The core diameter deviation should not exceed the limits of
+ 6% (+ 3 m).
1.1.2 Cladding diameter
The recommended nominal value of the cladding diameter is 125 m.
The cladding diameter deviation should not exceed the limits of
+ 2.4% (+ 3 m).
2.2.1 Modal distortion bandwidth: amplitude response
The modal bandwidth amplitude response is specified in the form of -
3 dB optical (-6 dB electrical) points of the bandwidth of the total
amplitude/frequency curve corrected for chromatic dispersion. A more complete
curve of the total bandwidth response should also be given.
2.2.3 Chromatic dispersion
When required the manufacturer of the optical fibres should indicate
the chromatic dispersion coefficient values of the fibre type in the operating
wavelength region(s). The test method is contained in Annex B, section V, to
Recommendation G.652.
Note 1 - For multimode fibres the dominant chromatic dispersion mechanism is
material dispersion.
Note 2 - Typical values of the chromatic dispersion coefficient for high grade
silica optical fibres are the following:
3.2 Baseband response (overall -3dB optical bandwidth)
The baseband response is given in the frequency domain and includes the
effects of both modal distortion and chromatic dispersion and can be
represented by the expression:
BT = [Bmodal-2 + Bchromatic-2] -«
where
BT = overall bandwidth (including modal distortion and chromatic
dispersion)
Bmodal = modal distortion bandwidth
Bchromatic = chromatic dispersion bandwidth (see Note 3)
Note 1 - Both the fibre modal distortion baseband response and the source
spectrum are assumed to be Gaussian.
Note 2 - For certain applications the effect of chromatic dispersion is
negligible, in which case chromatic dispersion can be ignored.
Note 3 - Bchromatic, the chromatic bandwidth, is inversely proportional to the
section length and, if the source spectrum is assumed to be Gaussian, can be
expressed as:
Bchromatic (MHz) = (_O D(O) 10-6 L/0.44)-1
where
_O = FWHM source line width (nm)
D(O) = chromatic dispersion coefficient (ps/(nm.km))
L = section length (km)
3.2.1 Modal distortion bandwidth
The modal distortion bandwidth values for individual cable lengths in
an elementary cable section are obtained from the relevant fibre
specification. However, the overall modal distortion bandwidth of the
elementary cable section may not be a linear addition of the individual
responses due to mode coupling and other effects at splices and, sometimes,
along the length of the fibre.
The modal distortion bandwidth for an elementary cable section is
therefore given by:
1x -1 ¬-O
Bmodal total = § Bmodal ŽŽ «
¯1 n O _
where
Bmodal total = overall modal distortion bandwidth of an elementary
cable section
A.1 alternative test method (ATM)
A test method in which a given characteristic of a specified class of
optical fibres or optical fibre cables is measured in a manner consistent with
the definition of this characteristic and gives results which are reproducible
and relatable to the reference test method and to practical use.
A.2 attenuation coefficient
In an optical fibre it is the attenuation per unit length.
Note - The attenuation is the rate of decrease of average optical power with
respect to distance along the fbire and is defined by the equation:
P(z) = P(0) 10-(z/10)
where
P(z) = power at distance z along the fibre
P(0) = power at z = 0
= attenuation coefficient in dB/km if z is in km.
From this equation the attenuation coefficient is:
- 10 log10 [P(z)/P(0)] =
ŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽŽ
Z
This assumes that is independent of z.
A.3 bandwidth (of an optical fibre)
That value numerically equal to the lowest frequency at which magnitude
of the baseband transfer function of an optical fibre decreases to a specified
fraction, generally to -3dB optical (-6dB electrical), of the zero frequency
value.
Note - The bandwidth is limited by several mechanisms: mainly modal distortion
and chromatic dispersion in multimode fibres.
A.4 chromatic dispersion
The spreading of a light pulse in an optical fibre caused by the
different group velocities of the different wavelengths composing the source
spectrum.
Note - The chromatic dispersion may be due to one or more of the following:
material dispersion, waveguide dispersion, profile dispersion. Polarization
dispersion does not give appreciable effects in circularly-symmetric fibres.
A.5 chromatic dispersion coefficient
The chromatic dispersion per unit source spectrum width and unit length
of fibre. It is usually expressed in ps/(nm.km).
A.6 cladding
That dielectric material of an optical fibre surrounding the core.
A.7 cladding mode stripper
A device that encourages the conversion of cladding modes to radiation
modes.
A.8 core
The central region of an optical fibre through which most of the
optical power is transmitted.
A.9 core area
For a cross section of an optical fibre the area within which the
refractive index everywhere (excluding any index dip) exceeds that of the
innermost homogeneous cladding by a given fraction of the difference between
the maximum of the refractive index of the core and the refractive index of
the innermost homogeneous cladding.
Note - The core area is the smallest cross-sectional area of a fibre excluding
any index dip, which is contained within the locus of points where the
refractive index n3 is given by
n3 = n2 + k(n1 - n2) (see Figure A-1/G.651)
where
n1 = maximum refractive index of the core
n2 = refractive index of the innermost homogeneous cladding
k = a constant
Note - Unless otherwise specified a k value of 0.05 is assumed.
A.10 core (cladding) centre
For a cross-section of an optical fibre it is the centre of that circle
which best fits the outer limit of the core area (cladding).
Note 1 - These centres may not be the same.
Note 2 - The method of best fitting has to be specified.
A.11 core (cladding) diameter
The diameter of the circle defining the core (cladding) centre.
A.12 core (cladding) diameter deviation
The difference between the actual and the nominal values of the core
(cladding) diameter.
A.13 core/cladding concentricity error
The distance between the core centre and the cladding centre divided by
the core diameter.
A.14 core (cladding) tolerance field
For a cross-section of an optical fibre it is the region between the
circle circumscribing the core (cladding) area and the largest circle,
concentric with the first one, that fits into the core (cladding) area. Both
circles shall have the same centre as the core (cladding).
NAt max = (n12 - n22)«
where
n1 = maximum refractive index of the core
n2 = refractive index of the innermost homogeneous cladding
Note - The relationship between NA (see A.21) and NAt max is given in section I
of Annex B, B.2.2.
A.18 mode filter
A device designed to accept or reject a certain mode or modes.
A.19 mode scrambler; mode mixer
A device for inducing transfer of power between modes in an optical fibre,
effectively scrambling the modes.
Note - Frequently used to provide a mode distribution that is independent of
source characteristics.
A.20 non-circularity of core (cladding)
The difference between the diameters of the two circles defined by the
core (cladding) tolerance field divided by the core (cladding) diameter.
A.21 numerical aperture
The numerical aperture NA is the sine of the vertex half-angle of the
largest cone of rays that can enter or leave the core of an optical fibre,
multiplied by the refractive index of the medium in which the vertex of the cone
is located.
A.22 reference surface
The cylindrical surface of an optical fibre to which reference is made for
jointing purposes.
Note - The reference surface is typically the cladding or primary coating
surface. In rare circumstances it could be the core surface.
A.23 reference test method (RTM)
A test method in which a given characteristic of a specified class of
optical fibres or optical fibre cables is measured strictly according to the
definition of this characteristic and which gives results which are accurate,
reproducible and relatable to practical use.