Measuring Polarization States in the OBR vs. OVA

Both the OBR and OVA can show polarization data collected from a device under test.  However, the instruments operate very differently, and it is important to understand these differences.  

OBR: 

The grey and white traces in the Polarization States plot are the responses at the two detectors in the OBR’s polarization diverse receiver.  Because the OBR is an interferometric device, and light between a reference path and a measurement path only interferes if it is in the same polarization state, our instrument uses a Polarizing Beam Splitter in the path to S and P detectors, and we control the polarization state of the light in the reference path to equally illuminate the S and P channels, so that the instrument will be uniformly sensitive to any arbitrary returned state.  

When measuring polarization states in an OBR, you'll see three values for each trace.  The white and grey traces represent the S and P detector signals.  These are combined to produce the total RL blue trace.  Since the fiber used in the OBR and OVA is Single-Mode and not Polarization-Maintaining, minor levels of birefringence can cause the polarization state to rotate, so we generally don’t expect the OBR’s S and P state to align to the test device TE and TM states.

The Polarization State plot in the time domain, or changes in the spectral Polarization State plot for successive reflection events, can tell you if there are changes in the evolution of the polarization state with distance down the test path.  For example, if you examined the Polarization State plot in the time domain for a tightly coiled SM jumper, you would expect to see a beat pattern in the S and P amplitudes, and the period of the beat pattern would tell you the beat length of the bend induced birefringence.

Many customers use a polarization controller between the OBR and the PIC to align the instrument polarization state to the PIC TE or TM state if there is an expected polarization dependence.  For example, if the PIC is coupled to PM fiber, the user can observe the reflection for the end of the PM lead and use the polarization controller to align the OBR’s light to either the fast or the slow axis.  Or if the PIC is expected to have high loss in the TM state, the user can observe the strength of the reflection of an event deep into the PIC and use the polarization controller to minimize or maximize the reflection amplitude of the event to align either to the TE or TM state.

OVA:

The OVA (using the OVA or Polarization Analysis software, not the OFDR software) produces two orthogonal polarization states and distinguishes between two orthogonal polarization states at the receiver.  Through careful calibration techniques, we can use the 4 received states to calculate the Jones Matrix for the test device, which gives us the full polarization response of the test device.  

The OVA can distinguish between the high and low loss states in a device with PDL, and between the fast and slow states in a device with high PMD.  If your PIC is coupled to a PM fiber and you know if it is slow or fast axis coupled, or if the PIC's TE and TM modes have different propagation losses and you know which is the low loss state, you can use this information to separate out the TE and TM response.

OVA with OFDR Software:

When using the OFDR software package on the OVA, only a single output state is launched, so you will not get the full polarization response of the test device.  This behaves similar to the OBR.