Three common methods for OTDR fiber testing
OTDR tests optical cables and optical fibers.
The test occasions include factory testing of optical cables and optical
fibers, construction testing of optical cables and optical fibers, maintenance
testing of optical cables and optical fibers, and regular tests. The test
connection of OTDR is shown as in Fig. 1.
The method of testing the connection is:
OTDR-Optical fiber connector-The first reel of optical cable-The second reel of
optical cable-The nth reel of optical cable, and the terminal is not connected
to any equipment. According to the actual test work, there are mainly the
following three methods:
1．OTDR backward test method
This method is
mainly used to monitor the optical cable splicing, and the optical cable
splicing must be equipped with a dedicated optical fiber fusion splicer and an
optical time domain reflectometer (OTDR). Fusion machine will generally give
the estimated decay value of this contact after the fusion of a fiber core.This
method of testing has three advantages:
(1) OTDR is
fixed, omitting the vehicles and a lot of manpower and material resources
required for instrument transfer;
(2) The test point is
selected where there is city power and no gasoline generator is needed;
(3) The test point is fixed,
which reduces the cable stripping.
At the same time,
this method also has two disadvantages:
(1) Due to
distance and terrain restrictions, sometimes it is impossible to guarantee
smooth communication;
(2) As the connection
distance continues to increase, the test range and accuracy of the OTDR are
restricted.
There are generally three ways to solve these problems:
①Using
mobile phones in the city and suburbs enables testers and connection personnel
to keep in touch at any time, facilitates organization and coordination, and
helps improve work efficiency.
②Use the phone to
communicate. Make sure to use an optical fiber (such as the red optical fiber
of the blue optical fiber unit) to connect to the optical phone as a tie line.
Of course, the last fiber used for communication cannot be monitored because it
cannot be connected during splicing and coiling. Even so, the possibility of
problems will still be greatly reduced (if it is a 24-core optical cable, the
probability of problems will be reduced to less than 1/24 of the original).
③When the optical
cable connection reaches a relay distance, the OTDR moves forward.
Test practice has
proved that these monitoring methods are effective in ensuring quality and
reducing rework.
2. OTDR
forward one-way test method
The OTDR is
tested at a splice point before the fiber connection direction, and the test
instrument and test personnel are always transferred ahead of time with a
construction vehicle. Using this method for monitoring, the test point and the
connection point are always only one reel of optical cable length, the test
connector has high attenuation accuracy, and it is convenient for
communication. At present, the length of a reel of fiber optic cable is about 2
to 3 km, and communication can be ensured by using walkie-talkies under general
terrain. If the optical cable has a corrugated steel belt protective layer, you
can also use a magnet phone to contact.
The shortcomings of this
test method are also obvious. Moving the OTDR to each test point is
time-consuming and laborious, and it is not conducive to the protection of the
instrument. The test point is also limited by terrain, especially when the line
is far away from the highway and the terrain is more troublesome. Use portable
OTDR for monitoring. Short-distance testing does not require high instrument
dynamic range, and the small OTDR is small in size, light in weight and
convenient to move, which can greatly reduce the workload of testers and
improve test speed and work efficiency.
3. OTDR forward two-way test method
The OTDR position is still the same as the "forward one-way"
monitoring, but the two optical fibers are respectively short-circuited at the
beginning of the connection direction to form a loop. This method can satisfy
the fiber test of the relay section, and can also monitor the fiber connection.
For the fiber test of the relay section, the incident light pulse, reflected
light pulse, joint point, break point, fault point and attenuation distribution
curve can be clearly seen on the display of the optical time domain reflectometer.
The OTDR test event type and display are shown in Figure 2. It can provide
convenience for fiber optic cable maintenance.
When the fiber connection is
monitored, the loopback point is added, so the two-way value of the connection
attenuation can be measured on the OTDR. The advantage of this method is that
it can accurately assess the quality of the joint.
Due to the testing principle and fiber structure, the use of OTDR
unidirectional monitoring will cause false gains, and correspondingly false
large losses will occur. For an optical fiber connector, the mathematical
average of the attenuation values in the two directions can accurately reflect
its true attenuation value. For example, the attenuation of a connector
measured from A to B is 0.16 dB, and from B to A is -0.12 dB. In fact, the
attenuation of this head is [0.16+(-0.12)]/2=0.02 dB.