Internet Standards/CD2.mdf/ccitt/1992/g/g652_b3.asc

Text File | 1993-06-26 | 12KB | 287 lines

B.1.2.2  Wavelength selection

        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-up. 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.

B.1.2.3     Detector

        The light emerging from the fibre under test, the reference fibre or
the  optical  divider etc., is coupled to a photo detector whose  signal-to-
noise  ratio  and  time  resolution are adequate for  the  measurement.  The
detector is followed by a flow noise amplifier if needed.

B.1.2.4     Reference channel

        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.

B.1.2.5     Delay detector

        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.

B.1.2.6     Signal processor

        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.

B.1.3  Procedure

        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.

        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.

        The  measured group delay per unit fibre length versus wavelength shall
be fitted by the three-term Sellmeier expression:

                                   So 1    Oo2 ¼2
                       (O) = o + ÄÄ 3O - ÄÄÄ 3
                                    8 »     O  _
        Here  o is the relative delay minimum at the zero-dispersion wavelength
Oo. The chromatic dispersion coefficient D(O) = d/dO can be determined from the
differentiated Sellmeier expression:

                              So 1    Oo4 ¼
                       D(O) = ÄÄ 3O - ÄÄÄ 3
                               4 »     O3 _

        Here So is the zero-dispersion slope i.e., the value of the dispersion-
dD
slope   S(O) = ÄÄ  at Oo.
               dO

Note  1  - These equations for (O) and D(O) are sufficiently accurate over  the
1270-1340  nm range, but are less accurate in the 1550 nm region.  Because  the
dispersion  in  the  latter  region  is large,  the  reduced  accuracy  may  be
acceptable;  if  not, it can be improved by including data  from  the  1550  nm
region  when  performing the fit. However, it should be  noted  that  this  may
reduce the accuracy in the 1300 nm region.

Note  2  -  Alternatively the chromatic dispersion coefficient can be  measured
directly,  for example by a differential phase shift method. In this case,  the
differentiated  Sellmeier equation shall be fitted directly to  the  dispersion
coefficient for determining Oo and So.

B.1.4  Presentation of results

       The following details shall be presented:

       a)   Test set-up arrangement

       b)   Type of modulation used

       c)   Source characteristics

       d)   Fibre identification and length

       e)   Characteristics of the wavelength selector (if present)

       f)   Type of photodetector

       g)   Characteristics of the delay detector.

        h)    Values of the zero-dispersion wavelength and the zero- dispersion
   slope

            If  the frequency domain technique is used, the time group delay  t
            will  be  deduced from the corresponding phase shift   through  the
            relation t = /(2f), f being the modulation frequency

        i)    Fitting procedures of relative delay data with the used fitting
   wavelength range

        j)    Temperature  of  the  sample  and  environment  conditions  (if
   necessary)

















                             FIGURE B-11/G.652

                 Typical arrangement of the test apparatus

B.2      Alternative   test  method  for  chromatic  dispersion   coefficient
    measurement: the interferometric test method

B.2.1  Objective

        The interferometric test method allows the chromatic 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.

        According to the interferometric measuring principle, the wavelength-
dependent  time  delay  between the test sample and  the  reference  path  is
measured by a Mach-Zehnder interferometer. The reference path can be  an  air
path or a single-mode fibre with known spectral group delay.

        It  should  be  noted that 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.

B.2.2  Test apparatus

        Schematic diagrams of the test apparatus using a reference fibre and  an
air path reference are shown in Figures B-12/G.652 and B-13/G.652 respectively.

B.2.2.1     Optical source

        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-in amplification techniques,  a  light
source with low-frequency modulation (50 to 500 Hz) is sufficient.

B.2.2.2     Wavelength selector

       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.

        The  spectral  width of the optical sources is to be restricted  by  the
dispersion measuring accuracy, and it is about 2 to 10 nm.

B.2.2.3     Optical detector

        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  could  be  upgraded,  with  for   example   a
transimpedance circuit.

B.2.2.4     Test equipment

        For the recording of the interference patterns, a lock-in amplifier  may
be  used. Balancing of the optical length of the two paths 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 m.

B.2.2.5     Specimen

        The specimen for the test can be uncabled and cabled single-mode fibres.
The  length of the specimen should be in the range 1 m to 10 m. The accuracy  of
the  length should be about ± 1 mm. The preparation of the fibre endfaces should
be carried out with reasonable care.

B.2.2.6     Data processing

        For  the analysis of the interference patterns, a computer with suitable
software should be used.

B.2.3  Test procedure

        1)   The fibre under test is placed in the measurement set-up (Figure B-
   12,  B-13/G.652).  The positioning of the endfaces is  carried  out  with  3-
   dimensional  micro-positioning  devices  by  optimizing  the  optical   power
   received  by  the  detector.  Errors arising  from  cladding  modes  are  not
   possible.

        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 xi of the maximum of the interference pattern for wavelength
   Oi  and position xo (Figure B-14/G.652) determines the group delay difference
   _tg(Oi)  between the reference path and the test path as follows:

                                     xo - xi
                           _tg(Oi) = ÄÄÄÄÄÄÄ
                                        co

where  co  is the velocity of light in the vacuum. The group delay of  the  test
sample is calculated by adding the value _tg(Oi) 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 (O) of the test fibre.





        From  the  individual  group delay values of the  fibre  under  test  an
interpolation  curve  can be derived. The measured group delay  per  unit  fibre
length versus wavelength shall be fitted by the three-term Sellmeier expression:
                           So 1    Oo2 ¼2         (O) = o + ÄÄ 3O - ÄÄÄ 3
                            8 »     O  _

        Here  o  is the relative delay minimum at the zero-dispersion wavelength
Oo.  The chromatic dispersion coefficient D(O) = d/dO can be determined from the
differentiated Sellmeier expression:

                      So 1    Oo4 ¼
            D(O) = ÄÄ 3O - ÄÄÄ 3
                       4 »     O3 _
        Here So is the zero-dispersion slope, i.e., the value of the dispersion-
slope
               S(O) = dD
                         ÄÄ  at Oo.                           dO

Note - These equations for (O) and D(O) are sufficiently accurate over the 1270-
1340  nm  range,  but  are  less accurate in the 1550  nm  region.  Because  the
dispersion  in  the  latter  region  is  large,  the  reduced  accuracy  may  be
acceptable, if not, it can be improved by including data from the 1550 nm region
when  performing the fit. However, it should be noted that this may  reduce  the
accuracy in the 1300 nm region.

B.2.4  Presentation of results

       The following details shall be presented:

       a)   Test set-up arrangement

       b)   Source characteristics

       c)   Fibre identification and length

       d)   Characteristics of the wavelength selector (if present)

       e)   Type of the photodetector

        f)    Values  of  the zero-dispersion wavelength and the zero-dispersion
   slope

        g)    Fitting  procedures of relative delay data with the  used  fitting
   wavelength range

         h)    Temperature  of  the  sample  and  environmental  conditions  (if
   necessary).