When the optical fiber patch cord is tested with a red light VFL, the light leakage of its fiber is often encountered, but the insertion loss and reflection loss of the patch cord are qualified after tested again. This problem is especially obvious in single mode fiber.
 

When the single mode fiber is tested by red light VFL, it could happen to leak partial light at the end near to VFL (Fig. 1). The leakage is more obvious in darker environment (Fig. 2).

(Fig. 1)

(Fig. 2)

The light leakage would happen seriously at the over-bent part of fiber too. Part of red light will be lost at this position, but finally it can reach the further end too (Fig. 3, Fig. 4).

(Fig. 3)

(Fig. 4)

Once there is a breakage at the middle of the fiber, the light leakage will be very serious, and the red light won’t pass through the break to reach the further end of the fiber (fig. 5 and 6).

(Fig. 5)

(Fig. 6)

When the multi-mode fiber is tested with red light VFL, no obvious light leakage is found under normal lighting conditions (Fig. 7). Only in a dark environment, slight light leakage can be seen (Fig. 8).

(Fig. 7)

(Fig. 8)

Based on the official statement from YOFC, JFOPT makes the following summary on the causes of light leakage in the test of FO fiber with red light VFL:

1. Red light is visible light, whose wavelength is 650nm, much smaller than the single mode G652 patch cord’s cutoff wavelength 1260nm. Therefore, it transmits in multimode instead of single mode in G652 fiber.

2. The light source of red light VFL is surface light, which diameter is bigger than the fiber core. When the red light passes through the optical fiber in patch cord, a large number of light will be transmitted in fiber cladding, especially the higher-order modes of red light can be easily penetrated out of the fiber.

3. If the optical fiber is over-bent, too, its cross-section will be distorted, and the conditions for total reflection may be damaged, so the refracted light will be transmitted out of the fiber. But the remaining reflected light will continue to go to the end of the fiber after passing through this bend.

Therefore, in the process of testing the optical fiber patch cord with a red light VFL, it is hard to determine there is a quality problem of the patch cord itself only by the phenomenon of red light leakage. In order to accurately judge the quality of optical fiber patch cord, it is necessary to measure the insertion loss and return loss.