For the Insertion Loss Measurement (ILM) on single-mode fiber, we use equipment from EXFO, type FOT 932-4. These testers can perform bidirectional attenuation measurements at three wavelengths (1310, 1550 and 1625 nm) through a single fiber. They show also the fiber length (real cable length is a few ‰ shorter). The measurement of Optical Return Loss (ORL) is another option and requires special test leads.
With these power meters you can easily test very long links bidirectional, even 100 km is not a problem. The lasers are tolerated very closely, especially at 1625 nm, where the slope of attenuation depending on wavelength is very high. This is the reason for very close readings at bidirectional test.
This power meters store the data internally and increment the fiber counter automatically. For safety reasons, the results can be stored on both devices. In the Fast Test mode (automatic measurement) EXFO gives a loss range of 56 dB at 1550 nm. Even distances over 200 km in length should not be a problem.
For transportation of the equipment we use special cases and the test leads (Patch Cords) must not be separated from the device after the zero adjustment (taking reference) has been done