Finesse Limits in Hollow Core Fiber based Fabry-Perot interferometers

Due to their low sensitivity to changes to the external environment, low optical nonlinearity, low chromatic dispersion, and compatibility with fiber systems, hollow-core optical fibers (HCFs) represent an ideal medium for fiber Fabry-Perot interferometers (FPs). Many applications can benefit from the availability of FPs with high finesse or high finesse-length product. However, the mechanisms that limit the performance of HCF-based FPs are yet to be fully elucidated to the best of our knowledge. In this paper, we present a comprehensive analysis of several factors that impact HCF-FP performance and limit their finesse, e. g., mirror tilt, the distance between the HCF end-face and mirror, HCF cleave angle and HCF attenuation. In a sequence of experiments, we built and characterized five HCF-FPs with lengths ranging from 0.65 m to 9.25 m. By fitting the experimental data with derived analytical expressions, we found the mirror-assisted coupling loss to be below -0.0028 dB (corresponding to a coupling efficiency of 99.94%), which should allow finesse values greater than 5000 to be achieved. Experimentally, we demonstrate here a value of 2400, limited by the parameters of the mirrors available to us presently. We then show that the low coupling loss and high repeatability of mirror alignment and HCF cleave quality allows the effective use of such high-finesse FP for reliable measurements of HCF attenuation, even with short fiber length samples (10 m in our demonstration).