Once a technician has spliced a fiber optic cable, he or she must test the splice to verify it is strong and has low loss. The technician must add the test data to the documentation for future reference and present it to the cable plant owner to verify the installation has been done correctly.
The testing process begins in the fusion-splicing machine. After fusing the fibers, the machine pulls on the splice to ensure it is mechanically strong. Most splicing machines also provide the operator a display of splice loss, but it is only an estimate based on the fiber alignment method the splicer uses.
Most splicers today use “profile alignment,” which is based on the video images of the fibers being spliced. Splicers use video images from two axes angled 90 degrees apart to analyze the cores’ alignment (or sometimes the cladding) of the fibers being spliced. The analysis evaluates the geometry of both fibers and the alignment to estimate the loss. If the fibers are within specifications and of the same type, the estimate should be reasonable. Splicing mismatched fibers may make the estimate less reliable.
Some older splicers use a local injection and detection (LID) system to align fibers and estimate loss. This system bends the fibers to inject some light on one side of the splice and detect it on the other side. Fibers are aligned when the maximum amount of light is transmitted through the joint between the fibers. Since the coupling into and out of the fibers is not always the same, absolute loss measurements are impossible, so the splicer can only estimate the splice loss.
Some types of fibers can fool both of these methods, especially when splicing different fiber types together. The new bend-insensitive fibers may not be a problem for some profile alignment splicers but are a big problem with LID alignment versions.
The only legitimate method to check splice loss is using an optical time-domain reflectometer (OTDR). If you know how to use one, you know you need to check from both directions and average to get trustworthy data. The way OTDRs work—measuring fiber backscatter over distance to imply fiber attenuation—causes that quirk of OTDR measurements.
If you break a fiber and splice it back together, the fibers will have the same backscatter coefficient on both sides of the splice, and the OTDR’s splice loss measurement will be correct. If you are splicing two fibers together from different batches and have different backscatter coefficients, that difference will cause directional variation in the measured loss, depending on the direction.
If you splice a high backscatter fiber to a low backscatter fiber, there will be directional differences. OTDR testing from high to low backscatter fibers will show a higher loss than the real loss. Testing from low to high backscatter fiber will show lower loss or even a “gainer” where the OTDR sees gain instead of loss, an impossible situation in the real world, making it obvious that the data is erroneous.
The solution is to test in both directions and average the results, which removes the errors caused by backscatter differences. In most cases, this is impractical while the tech is making the splices and will be performed only after all of the cable plant has been spliced together and final testing is done.
Knowing all of the problems, is there a more practical solution? The best bet is to use care in the splicing process and to trust your splicing machine’s estimates, then test and document consistently.
If you correctly maintain your splicer and cleaver, they should operate properly. You can check the fiber cleaves on the splicer display, and the machine should automatically check and reject bad cleaves. If your equipment starts encountering problems, have it serviced by an authorized dealer immediately.
Remember that the fusion splicer has preprogrammed operational cycles for each type of fiber that sets the proper prefuse cycle, correct arc heat and fusion time for the actual splice. You always need to ensure the splicer is properly set for the types of fibers being spliced.
If you set up the machine properly and follow the correct procedures, you can be confident that the machine will do its job and provide reliable results. If you compare the machine’s splice estimates with some bidirectional OTDR test results, you can confirm whether the machine’s estimates are trustworthy. That’s a good experiment for a dreary day in the office when you are tired of doing paperwork.