To accurately measure the insertion loss of an optical device, we use the following setup and procedure during production and final test.
To measure a device having optical connectors, make a fiber optical jumper cable of the same fiber and connectors.
Use a stabilized laser having the type agreed on the PO or datasheet. The laser must have a feedback controller. An optical isolator is preferred to prevent back refection instability. Different types of lasers will have different measurement results. For a single-mode laser, a sharp peak laser may generate special effects inside the device. For broadband single-mode laser, different type affects the measurements, especially the return loss. Different multimode lasers may have orders of magnitude and huge differences in test results. Most lasers have ER drift issues for polarization maintenance device measurements, although the power is stabilized. A fiber laser-based PM light source having both power and ER stabilized is preferred (we make this type).
Use a good benchtop optical power meter. We use only Agilent Optical Power Meters with large area detect heads that have shortwave silicon heads and IR germanium heads.
If the measuring device has optical connectors, connect the fiber jump cable in place of the fiber optical device to be tested and zero the meter. The meter should read 0.00 dB. If the device has bare fibers, put the bare fiber end of the laser into the large area detector head using an Agilent bare fiber adaptor. Zero the meter. The meter should read 0.00 dB. One can use a jumper to connect the laser and leave the other end of bare fiber exposed.
For devices with connectors, replace the fiber jump cable with the device to be measured. The insertion loss will now show on display. This method still has errors from the connectors; although the jumper calibrates to some extent, the connector-to-connector is varied due to its eccentric nature.
For a device with bare fibers, splice one end to the laser and put the other end into the large area detector head using an Agilent bare fiber adaptor. The insertion loss will now show on display. This is the most accurate way known.