5.     Recommendation G.654

     CHARACTERISTICS OF A 1500 nm WAVELENGTH LOSS-MINIMIZED
                SINGLE-MODE OPTICAL FIBRE CABLE


       The CCITT,

considering

        (a)   that very low loss fibres are required in some telecommunication
network applications;

       (b)  that the foreseen potential applications may require several kinds
of single-mode fibres differing in:

       -    geometrical characteristics;

       -    operation wavelength;

       -    attenuation dispersion and other optical characteristics;

        (c)  that Recommendations on different kinds of single-mode fibres can
be prepared when practical use studies have sufficiently progressed,

recommends

        a  single-mode fibre which has the zero dispersion wavelength  in  the
1300  nm wavelength region which is loss minimized at a wavelength around 1550
nm and which is designed for use in this region.

       The geometrical, optical and transmission characteristics of this fibre
are described below.

        The  meaning  of the terms used in this Recommendation  are  given  in
Annex  A  and the guidelines to be followed in the measurements to verify  the
various characteristics are indicated in Annex B.

Note  -  The  characteristics of this fibre and the relevant  values  will  be
verified as studies and experience progress.

1.     Fibre characteristics

1.1    Mode field diameter

        The  nominal value of the mode field diameter at 1550 nm shall be  xx  m.
The  mode field diameter deviation should not exceed the limits of ñ 10%  of  the
nominal value.

Note  -  The  value for xx has to be specified. A value of 10.5  for  xx  is  one
possibility.

1.2    Cladding diameter

        The  recommended nominal value of the cladding diameter  is  125  m.  The
cladding deviation should not exceed the limits of ñ 2.4%* (ñ 3 m).

1.3    Mode field concentricity error

        The  recommended  mode field concentricity error at 1550  nm  should  not
exceed 1 m*.

1.4    Non-circularity

1.4.1  Mode field non-circulatory

        In  practice,  the mode field non-circularity 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-circularity.  It  is  not  normally
necessary to measure the mode field non-circularity for acceptance purposes.

1.4.2  Cladding non-circularity

        The  cladding non-circularity should be less than 2%. For some particular
jointing  techniques  and  joint  loss  requirements,  other  tolerances  may  be
appropriate.

1.5    Cut-off wavelength

       The cut-off wavelength values shall be between xxxx nm and yyyy nm for Oc,
and smaller than zzzz nm for Occ.

Note  -  The values for xxxx, yyyy and zzzz have to be specified; values of  1350
for xxxx, 1600 for yyyy and 1530 for zzzz are one possibility.

1.6    1550 nm bend loss performance

       Under study.

Note  - The performance of this fibre should not be worse than fibre designed  to
meet Recommendation G.653.

1.7    Material properties of the fibre

       This is given in section 1.7 of Recommendation G.652.

1.8    Example of fibre design guidelines

        Supplement  33  gives an example of fibre design guidelines  for  matched
cladding fibres used by one organization.




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*  Under study.

2.     Factory length specifications

2.1    Attenuation coefficient

       Optical fibre cables covered by this Recommendation shall have attenuation
coefficients in the 1550 nm region* below 0.25 dB/km*.

Note  -  The  lowest values depend on fabrication process, fibre composition  and
design, and cable design. Values of 0.15 to 0.20 dB/km in the 1550 nm region have
been achieved.

2.2    Chromatic dispersion coefficient

        The  maximum  chromatic dispersion coefficient in the 1550 nm  wavelength
region of single-mode fibres covered in this Recommendation shall be           20
ps/(nm.km).

3.     Elementary cable sections

       As given in  3 of Recommendation G.652.




                                    ANNEX A

                           (to Recommendation G.654)

                MEANING OF THE TERMS USED IN THE RECOMMENDATION


        Most of the definitions contained in Annex A to Recommendation G.652  are
in  principle  applicable  also  to  loss-minimized  fibre.  Because  of  limited
experience  with  this type of fibre, further study of the  suitability  of  some
definitions is needed.




                                    ANNEX B

                           (to Recommendation G.653)

               TEST METHODS FOR LOSS-MINIMIZED SINGLE-MODE FIBRES


        The  present  experience on loss-minimized single-mode fibres  is  rather
limited; therefore further study is needed on some reference and alternative test
methods  for this type of fibre. Nevertheless, most of the test methods described
in  Annex  B  to Recommendation G.652 are in principle applicable also  to  loss-
minimized  fibres.  Therefore,  for  this  annex,  reference  is  made   to   the
corresponding test methods of Annex B in Recommendation G.652; the  specifics  of
each  test  procedure  need further study. It should be noted  that  the  working
wavelength for G.654 fibres is in the 1550 nm region.



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*  Under study.