Optical Cable Construction Site and acceptance testing methods and standards (1)

The on-site test of optical cable construction is very important. It is used to prepare for the overall commissioning of the optical terminal. The optical fiber test items in the optical fiber have the measurement of transmission attenuation. for Multimode Optical Fiber, test the baseband response when necessary.

The purpose of the single-disk optical cable test is to ensure the quality of the factory products. the test after the construction deployment is to check whether there is any damage during the deployment process and use it as a pre-Connection check; the connection test is to check whether the connection achieves low loss. After the connection, the unit optical cable segment is tested to check whether the connection meets the design requirements for total transmission attenuation and total baseband response, as the preparation for the overall commissioning of the optical terminal.

A single-mode optical fiber is characterized by dispersion coefficient. The dispersion coefficient of a single-mode optical fiber is very low. For the 140 Mbit/s system, the limit is 300 ps/nm. Therefore, when the length of the relay field is less than 50km, this limit has a large margin, the construction process does not need to be measured; the quota of 565 Mbit/s five groups is 120 ps/nm, so it is necessary to consider in the design and verify the measurement after construction.

1. Measurement of on-site transmission attenuation

Attenuation of 1.1 Optical Fiber

When optical signals are transmitted along the optical fiber, the loss of optical power is the attenuation of the optical fiber. The unit of attenuation A is dB,

A = 10lgP1/P2dB)

P1 and P2 are the optical power at the injection end and output end respectively.

1.2 add injection system between optical cables

In order to obtain accurate measurement results, it is necessary to ensure that the power distribution is a steady-state mode. Therefore, an injection system is added between the light source and the tested optical cable. The injection system is a simulated device composed of a disturbing die, a filter die, And a package layer die stripping device. for Multimode optical fibers, an area of more than 1km can be used, and an optical fiber can be circled with a certain curvature radius.

1.3 comparison of three test methods

CCITT recommends G.651 to recommend three testing methods. That is, the shear, and backward scattering methods. The shear method has high precision but is destructive. The interventional loss method is non-destructive, and the accuracy is not as good as the shear method. The backward scattering method, that is, the OTDR) is used to measure the method with full functionality, high accuracy, and no destructive effect, the measurement data can be printed directly.

1.4 advantages of OTDR

OTDR) only needs to be tested at one end of the optical fiber, as shown in 1 and 2. This type of meter can not only measure the attenuation coefficient of the optical fiber, it also provides detailed information about the attenuation characteristics along the length of the optical fiber, detects the locations of physical defects or defects of the optical fiber, determines the attenuation and position of the joint, and the length of the optical fiber to be tested, this kind of instrument has a printer, which can print the curve of the ing.

Optical Time Domain Reflector

Typical Curve of backward scattering power

The on-site optical fiber connection is monitored by OTDR. After a core is fused, the fusion splitters will provide the estimated attenuation value of the fusion point. The estimation is generally based on the direct monitoring of the local fiber core, that is, the attenuation value is estimated by observing the performance of the core connection. Whether the connection work is in good condition, and the monitor will notify the connection worker after measurement. Advantages of this method: first, OTDR remains unchanged. Vehicle and human and material resources required for Instrument Transfer are omitted. Second, the test site is selected where there is mains and no generator is required. Third, the test point is fixed, which reduces the optical fiber stripping.

1.5 OTDR Measurement Parameter Selection

1) select an appropriate range: OTDR has different ranges. The operator should select an appropriate range based on the length of the optical cable to display the test curve in the middle of the screen as much as possible so that the reading can be accurate, the error is small.

2) Select the appropriate pulse width: OTDR can select the optical pulse width parameter injected into the tested optical fiber. In the case of the same amplitude, the energy of the wide pulse must be greater than that of the narrow pulse, it can be tested for a long distance, but the error is large. Therefore, the operator should select an appropriate pulse width based on the length of the optical fiber to be tested, so that it can test the distance as long as possible while ensuring accuracy.

3) Select the appropriate refractive index: Because different fiber manufacturers use different materials, resulting in different optical fiber transmission speeds, that is, different optical fibers have different refractive indexes, therefore, the appropriate refractive index should be selected during the test so that the measurement of the length of the optical fiber can be accurate.

4) Reasonable Selection of test points: at present, most OTDR test joints use the 5-point method. During the test, the cursor should be positioned at the joint point as one point, the remaining four points should correspond to the optical fiber characteristics on both sides of the joint points. In this way, the joint test can be accurate.

1.6 cable connector unidirectional Test Method

In this way, an OTDR is placed at the beginning of the connection direction to perform one-way test on all the connection points.

When the length of the relay segment is short and there are not many optical fiber connections, for example, the local relay optical fiber cable, the attenuation requirement of the joint is not very accurate, you can use the time domain reflector to monitor from one end, the conductor can adjust the connection set to reach a relatively optimal value. From observation to c in the figure, a small "Step" occurs in the waveform of the point ", the attenuation size can be estimated by the size of the step.

This method is not as accurate as the comparison method, but simple. As long as two points of monitoring are used together, it is suitable for the construction of Optical Cable segments with a large attenuation margin of the relay section, and the progress can be increased quickly.

1.7 Bidirectional Ring Test of optical cable connector

This method is to short the two optical fiber cables at the beginning of the connection direction to form a loop. OTDR performs a bidirectional test on all the connection points before the connection start point. As the cycle point is added, the bidirectional value of connection attenuation can be measured on the OTDR. The advantage of this method is that the joint quality can be accurately evaluated.

Due to the testing principle and the optical fiber structure, one-way OTDR monitoring may lead to false gains, and correspondingly, a false high attenuation may also occur. For a joint, the actual attenuation value can be accurately reflected by the mathematical average of the attenuation value in two directions. The optical fiber attenuation constant standard is: at 1310mm wavelength, the average attenuation value should be less than or equal to 0.36dB/km, and the maximum attenuation value should be less than or equal to 0.4dB/km. At 1550mm wavelength, the average attenuation value should be less than or equal to 0.22dB/km, and the maximum attenuation value should be less than or equal to 0.25dB/km. When the optical fiber connection is established, the bidirectional average joint loss should not be greater than 0.08dB.

After completion, the light source and optical power meter are used for two-way testing. The attenuation value must meet the design requirements. The OTDR bidirectional method is used to check whether the backward scattering curve meets the requirements.

2. baseband response tests of on-site Optical Fibers

Multimode Optical Fiber indirectly represents the dispersion of the optical fiber based on the baseband response. The frequency or time zone method can be used to test the baseband response of the optical fiber in a single disk. The frequency domain method is described as follows.