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Hollow Antiresonant Fibers with Reduced Attenuation
Tools Used:
Hollow core optical fibers (HCF), in which light is guided in an air or vacuum core, have attracted scientists for decades due to their low non-linear response, low latency and lower dispersion as compared to solid fibers. More recently a new design for HCFs was proposed, where the hollow core is surrounded by a microstructure cladding composed of tubes attached to the jacket glass. An antiresonant hollow core fiber (HAF) is one that confines light using the anti-resonance principle. When the thickness of all the tubes forming the cladding of the fiber is chosen to be in anti-resonance with the desired operational wavelength, this structure gives a smooth and wide transmission window. The confinement loss of HAF is a function of the geometry and the key parameters are: the size of the core, the size of gaps between primary capillaries, and the size of the nested element. Anti-resonant fibers can confine light in the core over broadband spectral regions aside from where resonances with the membranes occur. A modified form of HAF (structure 1AE in Figure 1) was introduced by adding additional antiresonant elements to further reduce the coupling from the core into cladding voids (Ref.1). Our finite element mode solver FemSIM is a great tool to calculate the mode for HAF structures, find the attenuation loss, bending loss, and design a desired structure at needed band area.
Figure 1. Schematic picture of HAF and 1AE.
Figure 1 is the schematic picture of HAF and 1AE structure. The core radius Rc is 47um, silica wall thickness t is 2.66um. The 8 adjacent cladding holes are 58.27um in diameter. In the modified design 1AE, extra silica rings (with inner diameter d1) are nested within the original ones. The silica thickness of these inclusions is still the same as in HAF (t), so that it is also antiresonant, the diameter d1 is half of d.
We first calculated fundamental modes for different structures in the 2.8um to 3.8um wavelength range. The waveguide leakage of loss was obtained with metrics function directly at the end of the scan. The results are shown in Figure 2. We also calculated hollow core tube fiber for comparison. Overall, the leakage loss is reduced by several orders of magnitude from the standard HAF design to the 1AE structure compared to HCF. The leakage losses in the case of 1AE are reduced about to 0.1 dB∕km range.
Figure 2. The loss attenuation for three structures.
The index and their fundamental mode profiles at 3.05um wavelength are shown in Figure 3. The mode profiles are clearly showing how the field is suppressed inside the core because of the anti-resonant effect.
Figure 3. The index profile and their corresponding modes at 3.05um for HAF, 1AE and HOF.
The bending losses are also investigated to explore the effects of the structures on fiber performance. The results at 3.05um wavelength are shown in Figure 4.
Figure 4. Bending loss at wavelength 3.05um for three structures.
Simulation Settings
The following settings were used for the above simulations.
- Since the hollow core fiber mode is leaky mode, the leaky mode calculation needs to be turned on in the Advanced dialog.
- Since these hollow core type of fiber structure is big, it supports too many modes. In order to find fundamental mode, a large number of mode calculations (e.g. 200) is set and lowest loss is used to sort the mode. In most cases the fundamental mode will be the first mode calculated. It is also good to check the mode profile to assure that.
- Seed Neff should be turned on for the MOST scan of wavelength from short to long or bending radius from large to small.
- To choose the initial guess for the MOST scan, first find the wanted mode at the strongest guiding case, which is at the shortest wavelength or the largest bending radius within the scan range, then use its Neff as the starting Neff.
- The scan step should be small enough to avoid mode hopping.
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References
- Water Belardi, Jonathan C. Knight “Hollow antiresonant fibers with reduced attenuation” April 1, 2014 / Vol. 39, No. 7 / OPTICS LETTERS