Parameter setting of OTDR optical fiber test method

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OTDR
is the main instrument in the field of optical fiber testing technology. It is
widely used in the maintenance and construction of optical cable lines. It can
measure optical fiber length, optical fiber transmission attenuation, joint
attenuation, and fault location. OTDR has the advantages of short test time,
fast test speed, and high test accuracy.

 

Optical
Time Domain Reflectometer (OTDR), using the method of optical time domain
measurement, emits a certain pulse width of light into the tested fiber, and
detects the Rayleigh scattering (Rayleigh scattering) and Fresnel reflection
(Fresnel reflection) optical signal power along the time Axis distribution,
draw OTDR curve, to measure various optical cable and joint parameters to
locate optical fiber fault points, and understand the distribution of optical
cable loss. The following takes fiber-mart.com OTDR
tester as an example to analyze and explain.

 

1.
Test wavelength selection

 

Since
OTDR is for optical fiber communication, select the test wavelength before
performing fiber test. Only 1310 nm or 1550 nm is selected for single-mode
fiber. Since the 1550nm wavelength has a much more sensitive influence on the
bending loss of the fiber than the 1310nm wavelength, it does not matter.
Whether it is fiber optic cable line construction or fiber optic cable line
maintenance or experimentation and teaching, use OTDR to test the entire fiber
backscatter signal curve of a certain optical cable or a certain optical fiber
transmission link. Generally, the wavelength of 1 550 nm is used.

 

The
shape of the test curves at 1310nm and 1550nm wavelengths is the same, and the
measured optical fiber connector loss values ​​are basically the same. If no
problems are found in the 1550 nm wavelength test, then the 1310 nm wavelength
test must be no problem.

 

Choosing
the 1550 nm wavelength test, you can easily find whether there is excessive
bending of the fiber throughout the entire process. If a large loss step is
found somewhere on the curve, repeat the measurement with a wavelength of 1310
nm. If the loss step disappears at a wavelength of 1310 nm, it means that there
is indeed an excessive bending situation, which needs to be further searched
and eliminated. If the loss step is the same at the 1310 nm wavelength, there
may be other problems in the fiber, which need to be found and eliminated. In
the single-mode optical fiber line test, the 1550 nm wavelength should be
selected as much as possible, so that the test effect will be better.

 

2.
Test range selection

 

The
range of the OTDR refers to the maximum distance that the abscissa of the OTDR
can reach. The measurement range should be selected according to the length of
the optical fiber under test. It is better for the measurement range to be 1.5
times the length of the optical fiber under test. When the range is selected
too small, the display of the optical time domain reflectometer is not
comprehensive; when the range is selected too large, the abscissa on the
display of the optical time domain reflectometer is compressed and can not be
seen clearly.

 

According
to the actual experience of engineers and technicians, when the test range is
selected so that the backscatter curve accounts for about 70% of the OTDR
display screen, both the length test and the loss test can get a better
direct-view effect and accurate test results.

 

In
the optical fiber communication system test, the link length is several hundred
to several thousand kilometers, the relay section length is 40-60 km, and the
single-reel optical cable length is 2 to 4 km. A reasonable selection of the
OTDR range can get good test results.

 

3.
Test pulse width selection

 

Setting
the light pulse width too large will produce strong Fresnel reflection, which
will increase the blind area. Although the narrow test light pulse has a small
blind area, the light power of the test light pulse is too narrow, and the
corresponding backscatter signal is also weak. The backscatter signal curve
will be undulating and the test error is large. The set optical pulse width
must not only ensure that there is no excessive blind zone effect, but also
ensure that the backscattered signal curve has sufficient resolution and can
see every point along the optical fiber.

 

Generally,
according to the length of the fiber under test, an appropriate test pulse
width is selected first, and an optimal value is determined from the pre-test
once or twice. When the distance of the tested fiber is short (less than 5 000
m), the blind area can be less than 10 m; when the distance of the tested fiber
is longer (less than 50,000 m), the blind area can be less than 200 m; the
distance of the tested fiber is very long (Less than 2 500 000 m), the blind
area can be as high as more than 2 000 m.

 

In
single-disk testing, proper selection of the optical pulse width (50 nm) can
make the blind zone less than 10 m. By taking the average of two-way test or
multiple tests, the impact of the blind zone will be smaller.

 

Generally
speaking, the longer the averaging time, the higher the test accuracy. In order
to increase the test speed and shorten the overall test time, the test time can
be selected within 0.5 to 3 min.

 

In
the optical fiber communication connection test, a satisfactory result can be
obtained by choosing 1.5 min (90 s).

 

If
Fiber-mart OTDR is used for testing, the recommended duration is 1min (60s).

 

Only
by accurately setting the basic parameters of the test can conditions be
created for accurate testing.