Insertion Loss Through Materials With Dissimilar Rayleigh Scatter Levels

The OBR measures IL through a connector by comparing the relative levels of the Rayliegh Scatter on either side of the connector.  This works well when the fiber on both sides of the connector is the same.  However, there are times when there is a need to measure the IL through a connection with dissimilar materials.  In this case, the different Rayleigh Scatter levels make computing the IL through the connection difficult. 

However, if an OBR measurement can be taken from both directions, that is a forward measurement looking East-to-West and a reverse measurement looking West-to-East, then it is possible to do some post-processing of the OBR data and compute the IL through the input and output facet of the device.

As a simple example, Luna measured a length of Single-Mode Low-Bend-Loss fiber spliced in between two pieces of Single-Mode Fiber.  The Singe-Mode Fiber is Corning SMF28E+, and the LBL fiber is OFS BF-06160.  While this example is relatively simple, the same theory applies to Photonic Integrated Circuit (PIC) development, where the substrate of the PIC has a much higher Rayleigh Scatter level than the fiber leads connecting the PIC to the measurement equipment. 

Below are the measurements taken on a Luna Innovations OBR 4600.  The blue trace is the forward direction, and the red trace is the reverse direction. 

From the OBR’s perspective, we can see a noticeable difference between the Rayleigh Scatter levels of the Single-Mode Fiber before and after the region of the Low-Bend-Loss Fiber.  This difference is caused by the IL going through the splices and LBL segment, but the question then becomes how much IL is from the front splice and how much is the from the second splice?  

For post-processing and time-alignment, the OBR data for the two scans were loaded into LabVIEW.  The x-axis of the second scan was flipped, and the position adjusted until the traces lined up.  The connector reflections from either side of the jumper were used to align the two measurements in the x-axis.  A 100 um boxcar filter was applied on the linear amplitude values to smooth out some of the scatter variation. 

Having flipped and time-aligned the edges of the two traces, the scatter level average (in dB scale) was calculated to get the scatter level change between the SMF and LBL segments.  An additional 10 mm bandpass smoothing filter was used to make it easier to see the scatter level change over the spatial variation:

As expected, the SMF scatter level before and after the LBL section is consistent.  The scatter level change from SMF to LBL was + 6.31 dB. 

The scatter difference (in dB) between the two measurements was calculated and then divided by 4 to show the IL drops.  Again, a 10 mm smoothing filter was applied to make it easier to see the IL drops at the splices:

By placing horizontal cursors on each region of differential scatter levels, we can see the scatter levels of the three regions.  By taking the differences again, we can calculate the insertion loss through the multiple transitions:

The IL of the first splice is indicated by the first drop:  1.10014 – 0.10531 = - 0.99dB.

The IL of the second splice is indicated by the second drop:  0.10531 – -0.9067 = -1.01dB.

This procedure should work equally well for a PIC with fiber pigtails, for a simple loopback path, or a delay line path.  By using this method, it is possible to compute the scatter level change, the facet IL, and the distributed IL in the PIC.