The impact of insufficient fiber bending radius on ODN link attenuation

ODN link downlink attenuation is generally abnormal

The GPON network uses 1490nm/1310nm wavelengths for both downstream and upstream, while the ODN (Optical Distribution Network) mainly uses G.652D and G.657A2 fibers, which have attenuation of 0.23dB/km and 0.36dB/km at 1490nm/1310nm wavelengths, respectively, 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.

After analyzing the downlink/uplink attenuation of an ODN link in a certain 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 increase of about 0.53dB/km for each link. That is, the downlink loss exceeded the normal value by about 0.53+0.13=0.66 (dB/km). The attenuation of some links is shown in Table 1 below.

ODN link attenuation is more sensitive to long wavelengths

Using OTDR with wavelengths of 1550nm/1310nm (as the ODTR used does not have a wavelength of 1490nm, 1550nm wavelength was used instead) to test the fiber optic link, it was found that the attenuation of 1550nm wavelength at the fiber optic cable joint was significantly longer than that of 1310nm wavelength, as shown in Figure 2.

In fiber optic cable sections containing multiple joints, almost all joints exhibit the same phenomenon of attenuation, as shown in Figure 3.

The attenuation of existing ODN links exhibits significant wavelength sensitivity, with larger attenuation at longer wavelengths.

The fiber bending radius of the attenuation anomaly point is significantly insufficient

What factors at the cable joint can cause the attenuation to be affected by long wavelengths?

We know that the macro bending loss of optical fibers is sensitive to long wavelengths. Could it be caused by insufficient bending radius of the fiber. Let’s first take a lok at the winding of the optical fiber inside the cable joint, as shown in Figure 4.

According to the construction and acceptance specifications for communication line engineering, the reserved radius of G.652 fiber should not be less than 30mm. However, the estimated reserved fiber radius at the joint is only about 15mm (indicated by the arrow in the figure).

Let’s take a look at the bending radius of the optical fibers at other fiber winding points. The termination of the cable splitter box is shown in Figure 5.

The bending situation of the optical fibers in the fiber optic cable junction box’s direct melting tray and fusion integrated module is shown in Figure 6.

In the entire ODN link, there is a common situation where the fiber bending radius does not meet the specification requirements in all areas involving fiber optic cables. Mainly because it is difficult to control the remaining length of the optical fiber during the construction process. After winding several standard loops in the fiber box, the remaining length of the optical fiber can only be wound in small loops.

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

What is the difference in additional loss at different wavelengths when the bending radius of the optical fiber is below the standard value?

We wound G.652D optical fiber onto cylinders of different radii, and conducted specific tests on the testing site as shown in Figure 7.

Select cylindrical shapes with diameters of 40mm, 30mm, and 20mm, and wind the optical fiber 10, 20, and 40 times respectively. The additional loss test results are shown in Table 2.

From the above table, it can be seen that when the curvature radius of G.652D fiber is less than 15mm, the additional loss at long wavelengths is more pronounced. The smaller the bending radius, the more coils there are, and the greater the additional loss.

Conclusion and Suggestions

Due to the sensitivity of fiber macro bending loss to long wavelengths, when the ODN downlink attenuation significantly exceeds the normal value, it is generally caused by the fiber’s bending radius not meeting the specification requirements. With the application of xG-PON, the downstream of ODN will use a longer wavelength of 1577nm, and the macro bending loss of the fiber will be greater. The insufficient bending radius of the fiber may limit the application of xG-PON.

Due to the difficulty of winding the excess length of the optical fiber to meet regulatory requirements at the fiber optic splice, it is difficult to eliminate the problem of insufficient fiber bending radius in low-cost and low-quality access network projects. Therefore, Yingda suggests the following:

• (1) At the completion of the access network project, it is necessary to conduct attenuation tests on the ODN link using corresponding wavelengths for both downlink and uplink. The uplink attenuation should be at least 0.1dB/km higher than the downlink attenuation;
• (2) Currently, the prices of G.657A2 fiber and G.652D fiber are almost the same, and all wired access network cable lines should use G.657A2 fiber.