Measuring Hollow Core Fiber (HCF) on a Lightwave Analyzer

There are a few differences between standard telecom grade fused silica fibers and hollow core fibers.  First, hollow core fibers have a lower group index of refraction (1.0003 instead of 1.4682) than silica fibers.  The OBR or LWA can only apply one group index to the entire measurement, so it is important to understand which group index you have set into the instrument to know if the length measurements are going to be accurate for the section you are interested in.  

The second difference is that HCF will have a lower Rayleigh scatter levels than silica fibers.  This Rayleigh scatter level may be below the noise floor of the instrument.  If the instrument cannot observe a consistent Rayleigh scatter level above the noise floor, it cannot measure distributed IL events based on drops in the Rayleigh scatter level. 

Finally, it can be difficult to couple standard telecom fibers to hollow core fibers without a large Return Loss event because of the large difference in the group index.  These bright reflections, coupled with the finite Return Loss Dynamic Range of the instrument, may increase the noise floor to the point where making meaningful RL and IL measurements is difficult. 

However, keeping in mind these challenges, Luna's instruments (Lightwave Analyzer and Optical Backscatter Reflectometer) can still measure time-of-flight delays, distances (based on the appropriate group index), and the RL of connectors with components.  Luna expects that bend loss events and cracks/breaks will still show up as RL events, even if the instrument can’t see corresponding drops in the Rayleigh scatter level.  

Below is a screenshot of a patch cord that has SMF tails on either side of a section of HCF, measured on an LWA 7601-CL.  Note that because of the presence of SMF on either side of the HCF segment, it is possible to measure the insertion loss through the entire assembly, including the losses associated with the SMF-HCF transition, the HCF itself, and the HCF-SMF transition.  In this screenshot, the group index is set to 1.4682, that of glass fiber, making the 2.2m long section of HCF appear shorter than it really is.