Insertion Loss Troubleshooting Tips

A helpful tip for troubleshooting any singlemode insertion loss testing problem with your product is to keep the following in mind:

(1) 1310nm is more sensitive to alignment issues

(2) 1550nm is more sensitive to fiber bending

(3) 1310 and 1550 are similar

Fiber optic components will test about the same on a 1310 or 1550 if made properly. The 1550 insertion loss results are usually a few hundredths of a dB better, partly due to its lower fiber attenuation. Typically, the insertion loss values for a connector are ~0.01-0.05 dB better at 1550 than at 1310.

(4) 1310 is higher than 1550

A connector or the overall product design, the insertion loss at 1310 is significantly higher than the insertion loss at 1550, indicating that there may be a problem with the core-to-core alignment between the two mated ferrules. The difference may be small and actually acceptable. The greater the misalignment, the greater the insertion loss at 1310 compared to 1550. The cause of misalignment can be due to many factors, the most common reasons may be contamination of the product and test components, or poor concentricity of the fiber core to the ferrule.

It is hoped that contamination can be eliminated, and that previous manufacturing processes can be improved to remove contamination before testing. However, poor concentricity is usually caused by using an oversized ferrule, so insertion loss cannot be improved without replacing the connector. "Relatively oversized" is relative: the diameter of the ferrule hole is larger than the outer diameter of the fiber, then the fiber can at most sit on the center side of the ferrule, so the expected insertion loss (1310) is greater.

(5) 1550 is higher than 1310

A connector, or the entire product design, with significantly higher insertion loss at 1550 than at 1310 indicates that there may be a stress point on the fiber somewhere in the fiber optic assembly - most likely the fiber is bent beyond the operating bend radius, or within the product The fiber is "pinched" or slightly bent somewhere. The higher the stress (more bending), the higher the insertion loss at 1550 compared to 1310. However, while the aforementioned core shift issues are often a normal consequence of raw material selection, any excessive stress placed directly on the fiber represents a serious problem. Product reliability is at risk, so IL value @ 1550 is especially important for monitoring and troubleshooting.

It is good practice to test all products at 1550, even with "reduced bend radius" fibers. Doing so can reveal serious product defects, especially in products where fiber is routed within a distribution closet (such as a cassette or within a fiber distribution closet, or a functional zone fan-out transition). If your product has a significantly higher insertion loss of 1550 than 1310, your product is likely under stress from the fiber and you need to understand why.