Theoretical analysis of backscattering in hollow-core antiresonant fibers

Abstract

We present a theoretical analysis into the fundamental physical mechanisms contributing to backscattering in hollow-core antiresonant fibers. We consider Rayleigh scattering originating from the hollow regions of the fiber, which may be filled with gases, Rayleigh scattering from within the glass membranes, as well as the contribution from scattering at the glass surfaces. We derive expressions for the backscattering coefficient into any specified guided mode for an arbitrary excitation. These lead to general scaling relations with the core size and wavelength, which are found to be the key parameters for backscattering, regardless of the exact antiresonant geometry. For a nested antiresonant hollow-core fiber with a core diameter of 35 µm, the only antiresonant fiber geometry for which the experimental data are available in the literature, we find that the surface-scattering limited backscattering coefficient of the fundamental mode into itself is nearly 40 dB below that of a single mode fiber, in good agreement with recently published measurements.

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

“…6. At wavelengths away from the edges of the transmission windows, the field near the core boundary scales approximately as 𝜆/𝑎 [54,115,155]. When normalized according to Eq.…”

“…One approach to calculating the contribution from Rayleigh scattering is to consider that the inhomogeneities resulting from thermodynamic density fluctuations are effectively equivalent to a small, real perturbation to the dielectric permittivity Δ𝜀(𝑥, 𝑦, 𝑧). A perturbation treatment can then be used to estimate the total power scattered as a result of such perturbation, see for example [155]. A more intuitive estimate of the contribution from Rayleigh scattering can be made by realising that the Rayleigh loss coefficients for most constituent materials of the hollow-core fiber are well-known.…”

“…We first compute the current density induced by the presence of the surface inhomogeneities within this section of the waveguide, and from it the far field distribution of the radiated field and thus loss. Describing the geometric perturbations caused by roughness on the air-glass interfaces with a small, zero-mean, deviation ℎ(𝑠, 𝑧) (𝑠 is a curvilinear coordinate along the relevant interface in the cross-section) assumed not to change the local surface normal nor the refractive index, the induced current density at a point (𝑠, 𝑧) can be expressed as [155,166,179]:…”

“…Although the scattering loss at each wavelength ultimately depends on the precise mode field distributions within the fibers and near the air-glass interfaces in particular, useful simplifications can be made. This is because to first order approximation, the field strength near the air-glass interfaces at the core boundary scales with 𝜆/𝑎 [54,115,155]. If for simplicity we approximate this core boundary with a circle of radius 𝑎 and assume the mode field is constant on the core boundary , then to first order, the integral in Eq.…”

Stable Optical Frequency Comb Distribution Enabled by Hollow‐Core Fibers

“…Back‐scattering from the antiresonant HCF microstructure has been shown to be −118 dB m −1 at 1550 nm, more than 45 dB lower than SMF. [ 47 ] When filled with atmospheric air at atmospheric pressure, back‐scattering was predicted to increase to −100 dB m −1 , [ 52 ] which is, however, still ≈30 dB less than for the SMF. The property, which has not previously been discussed in literature and which we show in this article, is the thermal sensitivity of the chromatic dispersion, which is also significantly lower in antiresonant HCFs.…”