[Paper Review] Hollow-core fibers with reduced surface roughness and ultralow loss in the short-wavelength range

Jonas H. Osório, Foued Amrani+10 more|arXiv (Cornell University)|Jul 22, 2022

Photonic Crystal and Fiber Optics31 references84 citations

TL;DR

This paper presents a novel fabrication technique for hollow-core photonic crystal fibers (HCPCFs) that reduces core surface roughness by applying counter-directional gas flows during drawing, significantly lowering surface scattering loss. The method achieves a root-mean-square roughness reduction from 0.40 nm to 0.15 nm, enabling record ultralow loss of 50 dB/km at 290 nm, 9.7 dB/km at 369 nm, and sub-2 dB/km at visible wavelengths, surpassing silica-core fiber limits in the UV and visible range.

ABSTRACT

While optical fibers display excellent performances in the infrared, visible and ultraviolet ranges remain poorly addressed by them. Obtaining better fibers for the short-wavelength range has been restricted, in all fiber optics, by scattering processes. In hollow-core fibers, the scattering loss arises from the core roughness and represents the limiting factor in reducing their loss regardless of the fiber cladding confinement power. To attain fibers performing at short wavelengths, it is paramount developing means to minimize the height variations on the fiber microstructure boundaries. Here, we report on the reduction of the core surface roughness of hollow-core fibers by modifying their fabrication technique. In the novel process proposed herein, counter directional gas fluxes are applied within the fiber holes during fabrication to attain an increased shear rate on its microstructure. The effect of the process on the surface roughness has been quantified by optical profilometry and the results showed that the root-mean-square surface roughness has been reduced from 0.40 nm to 0.15 nm. The improvement in the fiber core surface quality entailed fibers with ultralow loss in the short-wavelength range. We report on fibers with record loss values as low as 50 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, and 1.8 dB/km at 719 nm. The results reveal this new approach as a promising path for the development of hollow-core fibers guiding at short wavelengths with loss that can potentially be orders of magnitude lower than the ones achievable with their silica-core counterparts.

Motivation & Objective

• To address the fundamental limitation of surface roughness-induced scattering loss in hollow-core fibers for short-wavelength operation.
• To develop a fabrication method that reduces core surface roughness below 0.40 nm, the current benchmark in HCPCFs.
• To enable ultralow loss in the ultraviolet and visible spectral ranges by minimizing surface scattering loss.
• To demonstrate that simple SR-TL HCPCF structures can achieve record-low loss without complex cladding designs.
• To establish a scalable, process-based approach to surface smoothing applicable to future high-performance HCPCFs.

Proposed method

• The novel fabrication process applies counter-directional gas fluxes within the fiber holes during drawing to increase shear rate on silica membranes.
• Shear flow is used to suppress thermal capillary waves that otherwise freeze into surface roughness at the glass transition temperature.
• Optical profilometry is used to measure surface height profiles and calculate root-mean-square (rms) roughness.
• Atomic force microscopy (AFM) validates high-frequency surface texture at nanoscale resolution.
• Loss measurements are performed using supercontinuum and plasma lamps for wavelengths above and below 400 nm, respectively.
• Index-matching liquid is used in profilometry to eliminate unwanted reflections from cladding tubes.

Experimental results

Research questions

• RQ1 Can counter-directional gas flows during fiber drawing reduce surface roughness in hollow-core fibers?
• RQ2 To what extent does reduced surface roughness lower scattering loss in the UV and visible range?
• RQ3 Can SR-TL HCPCFs achieve ultralow loss below the silica Rayleigh scattering limit without complex cladding designs?
• RQ4 What is the quantitative improvement in surface roughness and loss performance using the new fabrication method compared to standard techniques?
• RQ5 Is the reduction in surface roughness sufficient to enable loss values below 10 dB/km in the 250–400 nm range?

Key findings

• The root-mean-square surface roughness of the hollow-core fiber was reduced from 0.40 nm to 0.15 nm using the counter-directional gas flow technique.
• Fiber A achieved a record loss of 50 dB/km at 290 nm, the lowest reported for any fiber in the deep UV range.
• Fiber B achieved 1.8 dB/km at 719 nm and 0.9 dB/km at 558 nm, demonstrating sub-2 dB/km performance in the visible spectrum.
• The measured loss values for both fibers lie below the silica Rayleigh scattering limit (SRSL), indicating a fundamental performance breakthrough.
• The improvement in loss is directly attributed to reduced surface scattering loss (SSL), which scales quadratically with surface roughness.
• The results confirm that surface roughness is the dominant loss mechanism in short-wavelength HCPCFs and can be effectively mitigated by process engineering.