Challenges in characterization of photonic crystal fibers

We present experience with photonic crystal fiber (PCF) characterization gathered during COST Action 299 "FIDES", focusing on phenomena causing errors and ways to mitigate them. PCFs developed at IPHT Jena (Germany) and UMCS Lublin (Poland), designed for single mode operation were coupled to test instruments by fusion splicing to intermediate lengths of telecom single mode fibers (SMF). Fiber samples were short (0.5-100 m) and lossy (20-70 dB/km). Optical Time Domain Reflectometer (OTDR) was best for measuring loss as most PCFs produced strong backscattering. On the other hand, relatively large and variable splice losses as well as difficulties with PCF cleaving for optical power measurements made cutback and insertion loss measurements inaccurate. Short review of experience with PCF handling and cleaving is included. Splice quality was critical. Excitation of holey structure modes produced errors and strong “noise” during measurements of polarization parameters like PMD or PDL. Multimode propagation and vibration-induced interference precluded testing of fine dependence of PMD on temperature or strain, causing random variations of test data comparable to true changes of PMD. OTDR measurements were not affected, but testing of short fiber sections with very different backscattering intensities puts special demands on instrument performance. Temperature testing of liquid-infiltrated PCF was time-consuming, as settling of fiber parameters after temperature change could take up to 40 minutes. Photonic crystal fibers were fragile, breaking at strain below 2%, sometimes in unusual way when twisted.