General techniques for improving PDL measurement accuracy

Measurement accuracy can be maximized for a given PDL measurement system by taking the following precautions.

1. Minimize the optical insertion loss variations of the polarization controller by using the Agilent 118966A  olarization Controller with Option 025 which provides fiber pigtail interfaces that can be cut and spliced to the test system and to the DUT for minimal reflections.

2. Maintain constant source output power for each wavelength being measured.

3. Use an optical source which approaches 100% degree of polarization (DOP). Unpolarized light will not be attenuated by the PDL of a test device; therefore, the lower the DOP is, the more unpolarized light is available to adversely affect measurement accuracy.

4. Be sure that the DUT’s maximum polarization mode dispersion (PMD, also known as differential group delay) is less than one-half of the coherent length of the test signal. PMD values greater than this can depolarize (cause DOP degradation) the test signal and cause PDL measurement errors. PMD measurements can be performed with the Agilent 8509 Lightwave Polarization Analyzer.

5. Be sure the test device and test equipment are thermally stable before beginning a measurement. One-hour warm-up is usually sufficient time for most equipment and components to thermally stabilize.

6. Ensure that the test device and test system’s optical fibers do not move during a measurement. The polarization transfer function of a fiber changes with movement because the stress induced birefringence, which affect polarization transfer function, changes with movement. It requires one minute or more for fiber stress patterns to relax and stabilize after a fiber is moved.

7. Connector pairs cause reflections and PDL within the connector interface. Minimize optical reflections and etalons by using proper connector care and fusion spliced connections when possible. For this reason the Agilent 11896A Option 025 is recommended.