Test and Analysis of Optical Fiber Connection loss

The optical fiber connection loss is an important parameter in the performance index of the optical fiber communication system. The loss value directly affects the overall transmission quality of the optical transmission system. During the optical fiber cable construction and maintenance testing, the use of scientific testing and analysis methods is extremely important for improving the quality and maintenance of the entire optical fiber connection construction, in particular, it is of practical significance to further study the communication performance, transmission attenuation, measurement accuracy, inspection and maintenance of long-wavelength Single-Mode Optical Fiber in optical communication.

I. Optical Fiber Connection Loss Analysis

1. Causes of Optical Fiber Connection loss

1.1 Intrinsic loss

The intrinsic loss is a kind of inherent loss of the optical fiber material. It is determined after the premade rod is drawn into a fiber. This loss cannot be avoided, and the main cause of the intrinsic loss of the optical fiber is scattering and absorption, scattering is produced by the uneven density of materials. absorption is mainly caused by the absorption of impurity particles in the optical fiber material to some wavelengths of light.

1.2 Additional optical fiber loss

Additional losses are produced after fiber formation. They are mainly caused by bending and microbending of the optical fiber. Bending is inevitable during the construction of cables and optical cables, therefore, additional losses will occur. For single-mode optical fiber, the two fiber cables connected to each other, due to the differences in the die Field Diameter, The concentricity of the core and the package layer, and the non-roundness parameters of the core, this may cause Optical Fiber Connection loss. When the two optical fibers are fully aligned and the face gap is neglected, the connection loss mainly depends on the difference in the diameter of the optical fiber mode field. The connection loss is calculated as follows: B = 20lg [1/2 (d1/d2 + d2/d1)], and d1 and d2 respectively represent the die Field Diameter of the two optical fibers. The calculation formula shows that, when the diameter of the die field of the two optical fiber cables is equal to that of d1 = d2, the connection loss B = 0.

2. Causes of impact on Optical Fiber Connection loss

The causes of Optical Fiber Connection loss are mainly the structural parameters of the optical fiber and the welding quality of the melting machine. There are also some human and mechanical factors, such as the bending loss of the optical fiber receiving tray, cross Section quality of Optical Fiber cutting, horizontal mismatch, vertical separation, axial skew, etc.

Ii. Test and Analysis of Optical Fiber Connection loss

1. estimation principle of connection loss for melting splices

Melting splices are connected by adjusting the misalignment of the X axis and Y axis of the optical fiber, and weld at the most point of the Axis dislocation. This method of adjusting the axis is called the fiber-core direct-viewing method, this method is different from the power detection method, and the exact value of connection loss cannot be known on site. During the whole adjustment of the axis and connection process, the information about the state of the core is detected through the camera, the analysis program sent to the melting machine, and then the melting machine calculates the connection loss value. In fact, to be accurate, this only shows the degree of fiber axis alignment, the loss is not affected by the inherent characteristics of the optical fiber. The OTDR test method is backward scattering, which contains different forms of reflection loss with the optical fiber parameters, therefore, the data displayed by the melting machine is used together with the observation of the optical fiber connection section to roughly estimate the loss of the optical fiber connection point, and cannot be used as the actual value of the optical fiber connection loss.

2. How OTDR works

The back-to-back scattering method injects High-Power Narrow pulse light into the optical fiber to be tested, and then detects the scattered light power returned along the axial direction of the optical fiber at the same end. Due to the uneven density of the optical fiber, its own defects and doping composition are uneven. When the pulse is transmitted through the optical fiber, each point along the length of the optical fiber will cause the secondary scattering, and some of them will enter the numerical aperture angle of the optical fiber, reversely transmits data along the fiber optic axis to the input end. The wavelength of the scattering light is the same as the wavelength of the incident light. The optical power is proportional to the incident light power of the scattering point. The signal of the Back-to-Secondary Scattering Light returned along the axial direction of the optical fiber can be collected to the transmission loss information along the optical fiber, the attenuation of the optical fiber is measured.

The Optical Time Domain Reflector enters the input optical fiber by sending a pulse through the light. At the same time, it receives the freell reflection light and the ripple back scattering light at the input end, and then becomes an electrical signal, which is displayed on the oscilloscope over time.

When OTDR is used to test the optical fiber connection loss, the 1550nm wavelength is more sensitive to the bending loss of the optical fiber than 1310nm. Therefore, the 1550nm wavelength should be selected for the optical fiber connection loss test, in order to observe whether the loss will increase due to excessive bending of the Optical Fiber During the laying and connection of the optical fiber, but the 1310nm and 1550nm optical power meters should be used for the whole transmission loss test.

OTDR is used to monitor the optical fiber connection. There are two common methods. The first method is the forward one-way test method. OTDR is used to test the optical fiber connection point, the test point and connection point are always separated by only one optical fiber cable length. the attenuation of the test joint is more accurate and the test speed is faster. In most cases, the loss value of the optical fiber connection can be obtained more accurately, however, the disadvantage is that all the measured loss values are unidirectional test data, which cannot fully and accurately reflect the true loss values of optical fiber connection. :

The second method is the forward two-way test method. The positions of the OTDR test point and the connection point are still the same as those of the forward one-way monitoring, but the loop should be performed at the beginning of the connection direction, that is to say, at the beginning of the connection direction, the optical fiber cables in each group of beam tubes are uplinked to form a loop. As the cycle point is added, the OTDR test can measure the bidirectional value of connection loss, the results of the two-way optical fiber test with the OTDR are sometimes different, the main reason is that the optical fiber core diameter and the relative refractive index are not the same, that is, different brands or different batches of Optical Fiber fusion), not only will the welding loss increase, it will also be made into OTDR two direction A side to B side or B side to A side) the measured value is very different, when the two fused optical fiber mold field diameter is not the same, because the small-Mode Field Diameter optical fiber has better ability to transmit the ripple scattering light than the large-Mode Field Diameter optical fiber, when the two diameter optical fibers are fused, if the small-Mode Field Diameter optical fiber is tested in the direction of the large-Mode Field Diameter optical fiber, the welding loss may be negative, that is, false gain), and vice versa, there will be a high loss value, which is a surface phenomenon, it is because the diameter of Different Modulus field is not the actual loss of the weld point because of the difference in the transmission capacity of the secondary scattering light, so after testing in two different directions and taking the average value, the resulting loss is the actual loss of the weld point. For example, the loss measured from A to B is 0.18dB, And the loss measured from B to A is-0.12dB, in fact, the loss of this header is [0.18 +-0.12)]/2 = 0.03dB. However, if it is determined from the unidirectional value of 0.18dB, it may be mistaken that the connection is unqualified and the connection is disconnected and reconnected, therefore, bidirectional testing can avoid such misjudgment. The specific testing principles and methods are as follows:

1. After completing the connection end face of the 1st fiber at the connection point, the test point uses OTDR to find the position of the connection point based on the strongest freell reflection peak, and records the n1

2. After the first fiber is welded to the connection point, the test point will monitor the positive loss value of the to B of the 1st fiber from A to B, and record the positive loss value of the m1 fiber.

3. After the end face of the 2nd fiber connection is completed at the connection point, the test point uses the OTDR to locate the position of the 2nd fiber B to the position of the connection point in the direction after the loop of the 1 and 2 fibers, and records the n2

4. After the connection point is fused with the 2nd fiber, the test point changes from A to B and changes to the reverse loss value of B to A of the 2nd fiber in the B to A direction after the loop is returned, and the m2 reverse loss value is recorded.

5. Connect the 2nd fiber to the OTDR test port and use the n1 position recorded above to quickly locate the positive connection loss value between the and B directions of the 2nd fiber, and record the positive value of m2, next, we use the n2 position recorded above to quickly locate the reverse connection loss value from B to A of 1st fiber and record the m1 reverse connection loss value.

6. Calculate the connection loss of 1st fiber and 2nd fiber: Loss1 = 1/2 (m1 positive + m1 reverse)

Loss2 = 1/2 (m2 positive + m2 reverse)

The advantage of this method is that it can accurately assess the optical fiber connection loss. The disadvantage is that bidirectional testing increases the workload and slows down the testing speed, such as long lines in the relay segment, connect at 20km from the starting end. For example, if the optical fiber is 2 km long, test at 2km from the starting end. The positive value of the test is the attenuation value at 2km, the reverse test is the attenuation value at 42km points. As the distance is getting longer and longer, the signal is weak and the reverse value cannot be accurately measured. To continue two-way monitoring, the connection and test must be performed on the other end. In this way, a header cannot be monitored in two directions.

In short, the above two optical fiber connection loss testing methods are currently available testing methods. Different testing methods can be used according to different site conditions and actual requirements, with the advancement and development of optical communication technology, we believe that more advanced and accurate optical fiber attenuation testing equipment and testing methods will soon be applied to the analysis of optical fiber construction and optical line transmission quality.

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