Annex B

                   (to Recommendation G.652)

              Test methods for single-mode fibres


        The  mode field diameter may be determined in the far-field domain from
the  far  field intensity distribution, F2(q), from the knife-edge transmission
function, K(’); or from the complementary aperture power transmission function,
(’);  in  the  offset  joint  domain from the  square  of  the  autocorrelation
function,  T();  in  the  near-field  domain  from  the  near  field  intensity
distribution, f2(r); according to the equivalent definitions shown in   A.1  in
Annex A to Recommendation G.652.

B.1.2  Test apparatus



















        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  with  the
source  modulation  frequency. The detecting system should  have  substantially
linear sensitivity characteristics.

B.1.2.4  Launching conditions

        The  launching  conditions  used  must  be  sufficient  to  excite  the
fundamental (LP01) mode. For example, suitable launching techniques could be:







        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.









        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 the fibre axes. It is recommended that the end  faces  be
flat  and  perpendicular to the fibre axes to within 1o. For the  offset  joint
technique, the fibre will be cut into two approximately equal lengths.

B.1.2.7  Offset or scan apparatus

       a)   Far-field scan system

             A  mechanism to scan the far-field intensity distribution shall be
     used  (for example, a scanning photodetector with pinhole aperature  or  a
     scanning  pig-tailed  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 Wb/O, where 2 W is the  expected  mode  field
     diameter of the fibre to be measured, and b is the diameter of the  active
     area of the detector.

II.    Offset joint domain

Traversing joint

        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 m  is
recommended.  The  optical power transmitted through the  traversing  joint  is
measured by a detector.

III.   Near-field domain

Note  - The NA of the collecting optics in I b) and I c) must be large enough  not
to affect the measurement results.

I.     Far-field domain

        A  nominal 1300 mn 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.

Note - The validity of the image shearing technique is under study and needs to be
confirmed.