Internet Standards/CD2.mdf/ccitt/1992/g/g651.asc

Text File | 1993-06-26 | 12KB | 740 lines

                              -  -
                          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] -½


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 ¼-O
                   Bmodal total =  º Bmodal  ÄÄ ½
                                   »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)]                       =
ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ
                                          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)½

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.