Calibrating the OBR when used with other optical instruments

Many customers use Luna's OBR 4600 or 4613 in a test setup with other optical devices, such as polarization controllers, that are located in between the OBR and the DUT.  This leads to the question - during the OBR's Calibration routine, where should the Gold Tip Reflector calibration artifact be connected - directly to the OBR, or at the end of the optical path, where the DUT would be connected?  

As mentioned in a separate article, the Calibration routine adjusts the sensitivity of the detectors to optimize the detector network to be able to best detect reflections from the DUT.  The OBR does this in two steps.

First, The OBR measures the RL of a section of fiber inside the front panel connector.  This is done during the period of time when the OBR asks for nothing to be connected to the front panel. 

The second step is that the OBR makes fine adjustments to the spectral gain of the S and P polarization detectors.  This is done during the period of time when the OBR asks for the reflector to be attached.  In order to do this, the OBR requires a very bright, polarization-independent reflection.  This is provided by the gold tip reflector.  The reflectors used are not perfect reflectors and cannot be considered a 0dB return loss.  In fact, you may measure the RL of the gold reflector to be -1 or -2dB, and that is perfectly acceptable for the way the OBR uses the reflector in the Calibration routine. 

While you can leave other devices connected during the first step of the Calibration routine, such as a patch cord or sacrificial jumper, a bright reflection may raise the noise floor above the Rayleigh Scatter level of the fiber inside the front panel, which will cause the RL values to be inaccurate. By disconnecting everything from the front panel connector, including the dust cap, any potential sources of bright reflections are removed.  

During the second step of the Calibration routine, because of the ‘polarization-independent’ reflection requirement, it is best if you connect the gold reflector directly to the OBR.  This will ensure that any downstream optical devices, such as polarization controllers, do not adversely affect the setting of the spectral gain of the polarization diverse detectors. 

Luna recommends that the calibration routine be completed at least once every 24 hours, or if the ambient temperature changes by +/- 5C (which is unlikely in a temperature-controlled engineering lab setting).  As a best practice, performing a Calibration routine first thing in the morning when you start using the instrument will ensure that the instrument is good for the rest of the day.  Doing so will provide the most accurate RL values possible.