Recommendation G.957 (06/99)
23
To measure the reflectance of the detector, the connector at point R is connected to port 3; to
measure the ORL of the cable plant, the connector at point S is connected to port 3. In either case,
power P
R
is measured by the meter at port 2. The reflectance of the detector is:
(
)
R
P P
P
P P
s
R
=
−
10
10
0
13 32
log
The ORL of the cable plant is:
ORL
R
= −
I.2
Optical time-domain reflectometer
Here the coupler is usually internal to the instrument. A variable optical attenuator, and a pigtail of
length beyond the dead-zone of the instrument are both supplied, if they are not already internal to
the instrument. The following calibration measurement needs to be performed only once. A jumper
with known reflectance R
0
is attached, giving an OTDR trace schematically shown in Figure I.2. The
optical attenuator is adjusted until the reflection peak falls just below the instrumental saturation
level, and the peak height H
0
is noted. The calibration factor:
F
R
H
=
−
−
0
10
10
10
1
0
5
log
is calculated. (If the temporal duration D of the pulse is measured, the backscatter coefficient of the
fibre is B
=
F
−
10 log
10
D. If D is in ns, B is about
−
80 dB.)
T1509050-92
P
S
1
2
3
S/R
P
R
P
13
Source
Detector
Coupler
Cable
plant
Chia sẻ với bạn bè của bạn: |
