Polarization Effects on Thermally Stable Latency in Hollow-Core Photonic Bandgap Fibers

Conveyance of light in air endows hollow-core optical fibers with remarkably low sensitivity of the propagation delay to temperature changes. This sensitivity was demonstrated to be further reduced and even made negative (crossing zero) in photonic bandgap type of hollow core fibers. When operating long lengths of this fiber close to the zero sensitivity wavelength, it was observed experimentally that there is a small residual variation in propagation delay which had no apparent correlation to imposed temperature changes. In this article, we analyze the polarization effects that give rise to this variation, showing that the highest level of practically achievable thermal stability of the latency is limited by polarization mode dispersion. We show measurements of differential group delay between polarization modes in long lengths of photonic bandgap fiber at various temperatures and focus on spectral regions where thermally stable latency is predicted and measured. Our experimental observations, corroborated by numerical simulations, indicate the presence of strong polarization mode coupling in the fibers in addition to birefringence. The detailed understanding gained through this study allows us to propose practically achievable (i.e., manufacturable) fiber designs with up to three orders of magnitude lower polarization mode dispersion at wavelengths where the latency is insensitive to thermal fluctuations. This paves the way to fibers with polarization independent and thermally stable latency to serve a multitude of applications.