OBR 4600 & 4613 - Checking Calibration

How can I tell if the calibration I just performed on my OBR 4600 (or 4613) is valid? In the OBR User Guide, pages 38-39, a method is provided that guides the user through measuring the Return Loss (RL) value of the gold reflector (calibration artifact).

In particular, Step 14 lays out checking a curve for two parameters: flatness and a mean value of 0.0 to -1.0 dB. This range does not always accurately reflect what is sometimes observed. However, the range given does not actually impact calibration quality. A good calibration can occur using a reflective device with an RL value as low as -20.0 dB. The gold reflector artifacts are not chosen for their reflectivity accuracy, but for their polarization insensitivity (low Polarization Dependent Loss). The calibration reflector needs to have the same reflectivity at all polarization states, over the entire wavelength range of the OBR. The reflector is used to calibrate the polarization diverse detector to be equally responsive to all polarization states, and the reflector RL values are not used to calibrate the OBR RL levels. The OBR instead uses the Rayleigh scatter level of the fiber inside the front panel to calibrate the RL levels. That section of fiber has been continuously sourced for all 4600/4613 instruments from the same spool of Corning SMF28 E+ fiber. This approach guarantees uniformity between instruments.

Using an external reflector for a RL calibration artifact is challenging because the RL value any instrument records is affected by the Insertion Loss (IL) in the path to the reflector, so one would also need to measure and calibrate out the IL of the connector at the front panel of the instrument to get a consistent, accurate calibration. Using the fiber Rayleigh scatter of the fiber just before the front panel connector avoids this pitfall.

A better method exists for checking the calibration, which matches checks that Luna performs when testing each of our OBR instruments.

Checking the Calibration by checking Rayleigh Scatter level:

1. Calibrate as in the manual, following the steps on pages 37-38 ("To calibrate the system").
2. Click Scan (nothing should be attached to the front panel).
3. From the parameters menu or Title Bar in the upper graph, select
   Amplitude. This may already be displayed, as it is the default setting.
4. Turn on the vertical cursors in the upper graph.
5. In the Data Processing area, set Integration Width to 0.5 m.
6. Select the cross-hair tool, then click and drag the yellow cursor to about -0.5 m, so that the right side of the cursor does not capture any of the front panel peak.
7. The Ret Loss (dB) value for the yellow cursor will be displayed. This should be -76.5 dB (+/- 0.2 dB) for C&L-band (4600) or -74.5 dB (+/- 0.2 dB) for O-band (4613) instruments.
8. If the value is incorrect, make sure the OBR is aligned and then attempt to calibrate again. If the value is still incorrect, please contact Luna Support.