Methods for measuring reflections
Two methods are in general use. The Optical Continuous-Wave Reflectometer (OCWR) utilizes a
continuous or modulated stable light source with a high sensitivity time-averaging optical power
meter. It is suitable for measuring the optical return loss of the cable plant at point S or the
reflectance of the receiver at point R. The Optical Time-Domain Reflectometer (OTDR) utilizes a
pulsed source having a low-duty cycle along with a sensitive time-resolving optical receiver. It is
suitable for measuring discrete reflectances between S and R or the receiver reflectance at R.
Both instruments utilize 2
×
1 optical couplers, and both are available commercially. Instructions
contained with the instrument may supersede those given below. Moreover, test procedures are under
development.
For calibration purposes, a jumper with a known end reflector may be used. The value of reflectance
may be near zero (as obtained with careful index matching and/or a tight bend in the fibre), or about
−
14.5 dB (as with a good cleave), or some other known reflectance R
0
(as with an imperfect cleave or
an applied thin film coating). The connection between the jumper and the instrument must have a
low reflectance.
I.1
Optical continuous-wave reflectometer
The coupler nomenclature is shown in Figure I.1, and the following calibration measurement needs
to be performed only once. Power P
s
is measured by connecting the optical source directly to the
power meter. The source is then connected to output port 3 of the coupler, while the power meter
measures P
32
at the input port 2. The source is now connected to input port 1, while the meter
measures power P
13
at port 3. Finally, the non-reflecting jumper is connected to port 3, while power
P
0
is measured at port 2.
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