The Influence Of Insufficient Fiber Bending Radius On ODN Link Attenuation

ODN link downlink attenuation is generally abnormal

The downlink/uplink of the GPON network uses 1490nm/1310nm wavelengths respectively. The ODN (Optical Distribution Network) mainly uses G.652D and G.657A2 fibers. The attenuation of these two fibers at 1490nm/1310nm wavelengths is 0.23dB/ km and 0.36dB/km, as shown in Figure 1. Therefore, under normal circumstances, the downlink attenuation of the ODN link should be about 0.13dB/km lower than the uplink attenuation.

Figure 1 Optical fiber attenuation index in the access optical cable

However, when FCST analyzed the down/uplink attenuation of the ODN link of a metropolitan area network, it was found that the downlink attenuation of the ODN link in use exceeded the uplink attenuation by 100%, with an average of about 0.53dB/higher per link. km, that is, the downlink loss exceeds the normal value by approximately: 0.53 + 0.13 = 0.66 (dB/km). The attenuation of some links is shown in the table below.

ODN link attenuation is more sensitive to long wavelengths

Test the fiber link with the 1550nm/1310nm wavelength of OTDR (because the ODTR used does not have the 1490nm wavelength, so use 1550nm wavelength instead) respectively to test the optical fiber link, and found that the attenuation of the 1550nm wavelength at the optical cable joint is significantly larger than that of the 1310nm wavelength, as shown in Figure 2. Shown.

Figure 2 Example of the backscatter curve of an access optical cable 1

 In the optical cable segment containing multiple connectors, almost all connector attenuation exhibits the same phenomenon, as shown in Figure 3.

Figure 3 Example of the backscatter curve of an access optical cable 2

The attenuation of the existing ODN link shows obvious wavelength sensitivity, that is, the attenuation of the long wavelength is relatively large.

The bending radius of the fiber at the abnormal attenuation point is obviously insufficient

What factors at the optical cable joint will cause the attenuation of the optical cable joint to be affected by the long wavelength? We know that the macro-bending loss of an optical fiber is sensitive to long wavelengths. Could it be caused by the insufficient bending radius of the optical fiber. Let's take a look at the coiling of the optical fiber in the optical cable connector, as shown in Figure 4.

Figure 4 The coiling of the fiber core at the optical cable connector

According to the requirements of the construction and acceptance specifications of the communication line project, the reel radius of the G.652 optical fiber should not be less than 30mm, but the reel radius of the optical fiber at the joint is estimated to be only about 15mm (the arrow in the figure).

Look at the bending radii of the optical fibers at other coils. Figure 5 shows the end of the optical fiber splitter box.

Figure 5 The bending of the fiber at the end of the FAT

The bending situation of the optical fiber in the direct fusion tray and the fusion-distribution integrated module of the optical cable transfer box is shown in Figure 6.

Figure 6. The bending of the optical fiber in the Fiber Interconnect Cabinet

In the entire ODN link, where the coil fiber is involved, there are obvious cases where the fiber bend radius does not meet the requirements of the specification. The main reason is that it is difficult to control the excess length of the optical fiber during the construction process. The excess length of the optical fiber must be coiled only in small coils after coiling several coils that meet the standard in the fiber reel box.

Additional loss test of G.652D fiber at small bending radius

When the bend radius of the fiber is less than the specified value, how big is the difference in the additional loss at different wavelengths? We coiled G.652D optical fiber onto cylinders with different radii. Under specific tests, the test site is shown in Figure 7.

Figure 7. Attachment loss test of G.652D fiber at different bending radii

The additional loss test results of 40mm, 30mm, and 20mm cylinders with diameters of 40mm, 30mm, and 20mm, respectively, with the optical fiber coiled for 10, 20, and 40 turns are shown in the following table.

It can be seen from the above table that when the radius of curvature of the G.652D fiber is less than 15mm, the additional loss at long wavelengths is more obvious. The smaller the bending radius, the more the number of coils, and the greater the additional loss.

Conclusions and suggestions

Since the optical fiber macrobend loss is more sensitive to long wavelengths, when the ODN downlink loss significantly exceeds the normal value, it is generally caused by the bending radius of the optical fiber that does not meet the specification requirements. With the application of XG-PON, the downstream of ODN will use the longer wavelength of 1577nm, the macro-bending loss of the optical fiber will be greater, and the insufficient bending radius of the optical fiber may restrict the application of XG-PON.

Since it is not easy to coil the excess length of the optical fiber to meet the specification requirements at the optical fiber connection, it is difficult to eliminate the problem of insufficient optical fiber bending radius in low-cost and low-quality access network projects. Therefore, FCST recommends the following:

(1) When the access network project is completed, the down/uplink of the ODN link must be tested for attenuation with corresponding wavelengths, and the test results should meet the design requirements;

(2) At present, the price of G.657A2 optical fiber and G.652D optical fiber is almost the same, and all the optical cable lines of the wired access network should use G.657A2 optical fiber.