Loss in hollow-core optical fibers: mechanisms, scaling rules, and limits

Abstract

Over the past few years, progress in hollow-core optical fiber technology has reduced the attenuation of these fibers to levels comparable to those of all-solid silica-core single-mode fibers. The sustained pace of progress in the field has sparked renewed interest in the technology and created the expectation that it will one day enable realization of the most transparent light-propagating waveguides ever produced, across all spectral regions of interest. In this work we review and analyze the various physical mechanisms that drive attenuation in hollow-core optical fibers. We consider both the somewhat legacy hollow-core photonic bandgap technology as well as the more recent antiresonant hollow-core fibers. As both fiber types exploit different guidance mechanisms from that of conventional solid-core fibers to confine light to the central core, their attenuation is also dominated by a different set of physical processes, which we analyze here in detail. First, we discuss intrinsic loss mechanisms in perfect and idealized fibers. These include leakage loss, absorption, and scattering within the gas filling the core or from the glass microstructure surrounding it, and roughness scattering from the air–glass interfaces within the fibers. The latter contribution is analyzed rigorously, clarifying inaccuracies in the literature that often led to the use of inadequate scaling rules. We then explore the extrinsic contributions to loss and discuss the effect of random microbends as well as that of other perturbations and non-uniformities that may result from imperfections in the fabrication process. These effects impact the loss of the fiber predominantly by scattering light from the fundamental mode into lossier higher-order modes and cladding modes. Although these contributions have often been neglected, their role becomes increasingly important in the context of producing, one day, hollow-core fibers with sub-0.1-dB/km loss and a pure single-mode guidance. Finally, we present general scaling rules for all the loss mechanisms mentioned previously and combine them to examine the performance of recently reported fibers. We lay some general guidelines for the design of low-loss hollow-core fibers operating at different spectral regions and conclude the paper with a brief outlook on the future of this potentially transformative technology.

Hollow‐Core Fiber: Breaking the Nonlinearity Limits of Silica Fiber in Long‐Distance Green Laser Pulse Delivery

“…The seven-element cladding design offers an excellent compromise between confinement loss, higher-order mode suppression, and bend-loss sensitivity, allowing efficient single-mode delivery in practical fiber deployments. [19,46] The fiber was drawn to a total length of ≈500 m, but >1 km lengths are possible with our current preform design. The inner micro-structure was consistent over the fiber length, as confirmed by imaging both ends of the entire fiber (not shown).…”

“…The development of HCFs for this wavelength range represents a higher fabrication challenge due to the smaller structural features. [17][18][19] Furthermore, nonlinearity in HCFs in the visible relative to the infrared region is significantly higher, which is due not only to the smaller fiber core size but also to the shorter operation wavelength. In particular, Raman scattering strength, which is the primary nonlinearity in this context, scales inversely with approximately the fourth power of the laser wavelength.…”

4.8-μm CO-filled hollow-core silica fibre laser

“…A numerical simulation employing nite element analysis was used to extrapolate the loss of the HCF in additional spectral bands 43 . In the MIR spectral band, especially above 3.7 μm, the signi cant mechanisms that cause attenuation are leakage loss and material absorption of silica 34,37 . Hence, the simulation solely took into account these two variables.…”

“…However, the CO-lled bre laser has not yet been reported, primarily due to the overly extended wavelength band. While it is true that advanced HCFs have the capability to minimize the overlap between material and mode domains to approximately 10 − 5 , the transmission loss of HCFs is still affected by the signi cant loss of silica materials in the MIR band 34 . According to the curve presented in Fig.…”

Anti-resonant acoustic waveguides enabled tailorable Brillouin scattering on chip

“…Emerging anti-resonant reflection offers an alternative approach to achieve field confinement 32 – 36 . This approach has been successfully applied in hollow-core fibers 32 – 34 , effectively confining optical modes within the lower-refractive-index air core.…”