All single mode fibers work very similarly at any wavelength, and if your fiber optic components are properly constructed using quality materials and good technique, then the insertion loss value for any given fiber optic connector when tested on a 1310 or 1550 Should be very similar.
This has led some manufacturers to use only one wavelength to test their products, and only test two wavelengths when customers specifically request it. Testing at both wavelengths requires additional equipment, which in some cases appears to be nothing more than a "necessary evil". However, it is beneficial to make it standard practice to test all fiber optic cable assemblies at 1310 and 1550: the variation in insertion loss between the 1310nm and 1550nm test wavelengths can be very helpful in identifying serious problems with the product and/or process.
A helpful tip for troubleshooting any singlemode insertion loss test of a product is to keep the following in mind:
(1) 1310nm is sensitive to alignment issues
(2) 1550nm is sensitive to fiber bending
(3) 1310 and 1550 are similar
Fiber optic components will perform approximately the same tests on a 1310 or 1550 if manufactured properly. Insertion loss results for the 1550 are typically a few percent better, partly due to its lower fiber attenuation. Typically, the insertion loss value of the connector is ~0.01-0.05 dB better at 1550 than at 1310.
(4) 1310 is higher than 1550
For the Type A connector or the overall product design, the insertion loss at 1310 is significantly higher than the insertion loss at 1550, indicating a possible core-to-core alignment issue between the two mating ferrules. The difference may be small and actually acceptable. The greater the misalignment, the greater the insertion loss of the 1310 compared to the 1550. Misalignment can be caused by a variety of factors, the most common being 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 prior to testing. However, poor concentricity is often 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, and the fiber can only sit up to the center side of the ferrule, so the expected insertion loss (1310) is greater.
(5) 1550 is higher than 1310
The insertion loss of the connector or the entire product design at 1550 is significantly higher than at 1310, indicating 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 inside the product The fiber is "pinched" or slightly bent somewhere. The higher the stress (more bending), the greater the insertion loss at 1550 compared to 1310. However, while the aforementioned core shift issues are often a normal result of raw material selection, any excessive stress applied directly to the fiber is a serious concern. Product reliability is at risk, so IL values @ 1550 are essential for monitoring and troubleshooting Particularly important.
It is good practice to test all products at 1550, even if "reduced bend radius" fibers are present. Doing so can reveal serious product defects, especially in products that are wired in wiring closets (such as boxes or fiber closets) or in functional area fan-out transitions. If your product has a significantly higher insertion loss for the 1550 than the 1310, your product is likely being stressed by the fiber and you need to understand why.
