In many cases, it is not sufficient to eliminate the PDL influence from the insertion-loss result. Instead, the PDL is considered as a separate characteristic and must be measured in addition to the insertion loss. The two most important PDL-measurement techniques are polarization scanning and the Mueller method. Both methods are explained in detail below.
>> Polarization Scanning Method
The figure below shows the principle measurement setup for polarization scanning. This measurement is usually based on the fact that laser sources produce nearly 100% polarized light. The polarization controller has to convert the fixed input polarization state to all possible output states, including all linear, elliptical, and circular states.
The typical polarization controller used for this measurement consists of several motorized fiber loops. To generate polarization scanning, the motors run at different speeds, so that completely random polarization states are generated. The following figure shows a Poincare plot of the polarization states generated with a fiber-loop type polarization controller within 30 seconds.
The traces were measured with a real-time polarization analyzer. The points leading to minimum and maximum transmission are always opposite to each other, for example, the North Pole and the South Pole or the west and the east.
The PDL value is finally calculated using:
where Tmax, Tmin are the transmission coefficients and Pmax, Pmin are the output powers.
To obtain good accuracy in this measurement, the following is important:
1. Constant input power to the device under test (DUT). Changing the polarization state should not change the input power to the DUT. A well-designed polarization controller is necessary to accomplish this. Quite often, it is overlooked that laser sources react with unstable power when light of varying polarization state is back-reflected like in this measurement. Therefore, an attenuator or isolator should be inserted between the laser source and the polarization controller.
2. Generation of all polarization states. The evolution of the polarization state is most easily understood using a real-time polarization analyzer: After some time, the Poincare sphere is fully covered with traces with only small gaps in between. The question is: How much time is needed to make an accurate measurement? It can be answered by asking for the maximum allowable error angle, δmax, on the Poincare sphere. The relative PDL error can be expressed by:
3. Polarization-independent power meter. Many optical power meters have unacceptable polarization dependence for this type of measurement. Therefore, the power meter should be characterized or specified for polarization-dependent responsivity (PDR).
As always, the total uncertainty is essentially the root-sum-square of the individual uncertainty contributions.
It can be said that this is a robust, accurate technique that is easy to implement, provided that the appropriate measurement equipment is selected. No programming is needed if the polarization controller features automatic scanning and the power meter is sufficiently fast and capable of recording minimum and maximum values. The relative long measurement time of 10 seconds is acceptable for PDL measurements at only one wavelength.
>> Mueller Method
The Mueller method is based on applying four well-known polarization states to the test device. The optical power transmission is measured at these four states only. The PDL is calculated from the four transmission results.
A setup for PDL measurements with the Mueller method is shown in the following figure. It is based on a polarization controller using waveplates. The particular design shown synthesizes the different polarization states with a quarter-waveplate (Q) and a half waveplate (H).
A polarizer (P) is added in front of the waveplates to ensure a polarized signal of fixed orientation. The first step in using this setup is to rotate the polarizer (αp) until a best match with the incoming signal occurs and maximum transmission is achieved. The Q and H plates are always to be aligned with respect to the reference angle αp (see the table below).
The measurement procedure starts without the DUT by measuring the optical power at the four well-defined polarization states. Then the DUT is inserted and the power is measured again at the same polarization states. The following table illustrates this process (power levels with capital subscripts indicate measurements with the DUT, and power levels with lower-case subscripts are without the DUT).
The Mueller matrix is a 4×4 matrix which describes the transmission and polarization characteristics of the DUT, similar to the Jones matrix. It can shown that only the elements of the first row, m11 to m14, are needed to calculate the PDL. The following equation shows how these elements can be calculated from the transmission coefficients T:
The maximum and minimum transmissions are given by:
Finally, the PDL can be calculated using:
The critical points in this measurement are similar to those of the scanning method, for example, constant power from the laser source and polarization-independent power meter. The requirement for constant power from the polarization controller is relaxed because the polarization dependence of the power level is mathematically corrected. This is necessary because polarization controllers with waveplates often produce larger power variations than polarization controllers with fiber loops.
In comparison with the scanning method, this method is mathematically more complex, and it consists of two measurement steps, one without and one with the DUT inserted. However, calibration step is only needed once in a series of measurements. Altogether, the Mueller method features at least the same accuracy as the polarization scanning method: For example, accuracies in the order of +/- 0.003 to +/- 0.005 dB are attainable for PDL values of 0.1 dB (this includes the repeatability of the waveplate positions).
The big advantage of the Mueller method is its speed: less than 2 seconds per PDL result can be achieved. This makes the Mueller method attractive for wavelength-dependent PDL measurements.