Parameter settings for OTDR fiber testing

The optical time domain reflectometer (OTDR) uses the optical time domain measurement method to emit a certain pulse width of light into the optical fiber under test, and draws the OTDR curve by detecting the distribution of the Rayleigh scattering and Fresnel reflection optical signal power along the time axis to measure various optical cable and connector parameters to locate the optical fiber fault point and understand the distribution of optical cable loss. The following is an analysis and explanation using the OTDR tester of Shenzhen Xiaguang Communication Measurement Technology Co., Ltd. (referred to as "Xiaguang") as an example.

Five parameter settings to ensure OTDR accuracy

Xiaguang OTDR parameter settings:

Wavelength: that is, the test wavelength. The average loss of 1310nm in optical fiber is greater than that of 1550nm.

Range: that is, the measuring range. The recommended measuring range is 1.5 times the length of the optical fiber.

Pulse: pulse width, select according to actual situation.

Duration: Recommended value: 60 seconds.

IOR: Index of Refraction, provided by the cable or optical fiber manufacturer.

1. Test wavelength selection

Since OTDR is used for optical fiber communication, the test wavelength should be selected before conducting optical fiber testing. For single-mode optical fiber, only 1310 nm or 1550 nm is selected. Since the influence of 1550 nm wavelength on optical fiber bending loss is much more sensitive than 1310 nm wavelength, whether it is optical cable line construction or optical cable line maintenance or conducting experiments and teaching, when using OTDR to test the full-length optical fiber backscattering signal curve of a certain optical cable or optical fiber transmission link, 1550 nm wavelength is generally selected.

The test curves at 1310nm and 1550nm are the same in shape, and the measured fiber connector loss values ​​are basically the same. If no problems are found in the 1550nm wavelength test, then there will definitely be no problems in the 1310nm wavelength test.

By choosing the 1550 nm wavelength test, it is easy to find out whether there is excessive bending in the entire fiber. If a large loss step is found somewhere on the curve, retest with a 1310 nm wavelength. If the loss step disappears at 1310 nm, it means that there is indeed excessive bending at that location, which needs to be further found and eliminated. If the loss step is also large at 1310 nm, there may be other problems in the fiber at that location, which also need to be found and eliminated. In single-mode fiber line testing, the 1550 nm wavelength should be used as much as possible, so that the test effect will be better.

2. Test range selection

The range of OTDR refers to the maximum distance that the horizontal axis of OTDR can reach. When testing, the range should be selected according to the length of the fiber being tested. It is better to select the range that is 1.5 times the length of the fiber being tested. If the range is too small, the display screen of the optical time domain reflectometer will not show the whole picture; if the range is too large, the horizontal axis on the display screen of the optical time domain reflectometer will be compressed and unclear.

According to the actual experience of engineering and technical personnel, when the test range is selected so that the backscatter curve occupies approximately 70% of the OTDR display screen, both length testing and loss testing can obtain relatively good direct viewing effects and accurate test results.

In the test of optical fiber communication system, the link length is from hundreds to thousands of kilometers, the relay section length is 40 to 60 km, and the single-reel optical cable length is 2 to 4 km. Reasonable selection of OTDR range can obtain good test results.

3. Test pulse width selection

If the optical pulse width is set too large, a strong Fresnel reflection will be generated, which will increase the blind area. Although a narrower test optical pulse has a smaller blind area, the optical power will definitely be too weak when the test optical pulse is too narrow, and the corresponding backscattering signal will also be weak. The backscattering signal curve will be uneven and the test error will be large. The optical pulse width should be set to ensure that there is no excessive blind area effect, and to ensure that the backscattering signal curve has sufficient resolution to see the situation at every point along the optical fiber.

Generally, an appropriate test pulse width is selected based on the length of the tested optical fiber. After one or two pre-tests, an optimal value is determined. When the distance of the tested optical fiber is short (less than 5,000 m), the blind area can be less than 10 m; when the distance of the tested optical fiber is long (less than 50,000 m), the blind area can be less than 200 m; when the distance of the tested optical fiber is very long (less than 2,500,000 m), the blind area can be as high as 2,000 m or more.

When testing a single disk, properly selecting the optical pulse width (50 nm) can keep the blind area below 10 m. By testing in both directions or taking the average of multiple tests, the impact of the blind area will be even smaller.

4. Fiber refractive index selection

The refractive index of single-mode optical fiber currently used is basically in the range of 1.460 0 to 1.480 0, and should be accurately selected according to the actual value provided by the optical cable or optical fiber manufacturer. For G.652 single-mode optical fiber, if a 1310 nm wavelength is used in actual testing, the refractive index is generally selected at 1.468 0; if a 1550 nm wavelength is used, the refractive index is generally selected at 1.468 5. Inaccurate refractive index selection affects the test length.

Even the smallest mistake in optical cable maintenance and troubleshooting can lead to significant errors, so sufficient attention must be paid during testing.

5. Averaging time selection

Since the backscattered light signal is extremely weak, multiple statistical averaging methods are generally used to improve the signal-to-noise ratio. The OTDR test curve samples the reflected signal after each output pulse and averages the multiple samples to eliminate random events. The longer the averaging time, the closer the noise level is to the minimum value, and the larger the dynamic range. The dynamic range obtained with an averaging time of 3 minutes is 0.8 dB higher than the dynamic range obtained with an averaging time of 1 minute.

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

In the fiber optic communication splicing test, selecting 1.5 min (90 s) can achieve satisfactory results.

If using Xiaguang OTDR for testing, the recommended duration is 1 minute (60 seconds).

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