Nested multibar cladding elements in negative curvature fibers for CO laser guidance

A numerical study on the multi-bar nested cladding design of chalcogenide glass-based negative curvature hollow-core fiber was carried out to achieve a low-loss light guidance in the mid-infrared spectrum centered at 5.4 μm. Fiber design parameters were systematically optimized, and the effect of the nested bars on the confinement and total loss performance of a five-tubular cladding structure was investigated. An ultra-low transmission loss of 0.112 dB km−1 at 5.4 μm was achieved with As2Se3 triple-bar negative curvature fiber while maintaining low bending sensitivity. The design is also suitable for high transmission performance with alternative infrared glasses and can be potentially used for low-loss light guidance in a wide mid-infrared spectrum.


Introduction
The low-loss light guidance of solid-core silica optical fibers (SC-SOFs) has brought significant transformation in communication infrastructure and also found applications in other fields such as biomedicine, imaging, and sensing [1][2][3][4].Although the performance and applications of SC-SOFs have progressed over time, the basic design and the guidance mechanism of SC-SOFs have mostly remained the same.In contrast, negative curvature hollow-core fibers (NCFs) introduced a new design approach that allows light guidance through the hollow-core with the help of microstructured cladding.NCFs offer several advantages over SC-SOFs such as they can guide light with ultra-low-loss and low non-linearity through a wide transmission band in air which has significantly lower Rayleigh scattering and nonlinear coefficients compared to silica glass [5][6][7].NCFs offer reduced latency and higher damage thresholds and can be used for gas/liquid-light interaction in a long fiber length for various applications [5,8,9].Moreover, the hollow-core design offers low-loss transmission in the regions where solidcore fibers absorb electromagnetic radiation and such fibers are more robust against environmental perturbations compared to SC-SOFs [5][6][7].The simple cladding structure of NCFs as these fibers do not require a periodic cladding structure makes the fabrication process further simplified and enables a broader range of applications [5,6].Application in communication systems, medical imaging, high-power light delivery, nonlinear optics, and sensing across a wide spectral range are some of the proposed usage areas for NCFs [8][9][10][11][12][13].The microstructured elements in the fiber cladding guide light through the hollow core by preventing the coupling between the modes guided inside the core and the modes guided inside the fiber's cladding.Low phase mismatch and spatial overlap between the core-guided modes and the cladding modes are necessary for maximum light confinement inside the hollow air-core region of the fiber [6,14].
Infrared (IR) transparent soft glasses have attractive material properties and are usually preferred to design NCFs for different applications in the mid-IR spectral region [15][16][17].Over the years, various chalcogenide glass-based NCFs have been proposed in the mid-IR spectrum due to the favorable optical properties of chalcogenides [12,16,[18][19][20][21][22][23][24].Chalcogenide glasses such as As 2 S 3 and As 2 Se 3 have good material transparency in the mid-IR spectrum, and they are suitable for the thermal fiber drawing process, that is a robust fabrication method amenable to kilometer-long fiber drawing.Although silica and borosilicate glass-based NCFs have been reported to guide mid-IR radiation with attractive mechanical robustness and chemical durability, the increased material absorption in the longer spectral region of the mid-IR limit them to shorter wavelengths in this region [16,[25][26][27][28][29][30].Thus, alternative materials with good transparency in the mid-IR region are needed to guide the electromagnetic radiations [15, 16, 21] [28, 31-33].The spectral region centered at 5.4 μm is important for the CO laser, an alternative light source in the mid-IR region for material processing and spectroscopy [17].Various NCF structures such as tubular [11], elliptical [18,19,34], lotus-shaped [35], conjoined-tube [36], double-ring [37], adjacent nested [38], nested compounded [39], double nested [40,41], and bar supported NCFs [41][42][43][44] have been reported to improve the transmission losses of hollow-core structures.Although several studies on bar-supported structures have been reported, they are mainly focused on light guidance in either the near-IR or terahertz (THz) regions.To the best of our knowledge, a detailed analysis of bar-supported soft-glasses-based NCFs has not been conducted in the mid-IR region.
In this work, a detailed numerical study on the optimal positioning of multi-bar structures in tubular NCFs is presented to achieve low transmission losses in the mid-IR region with a centered wavelength of 5.4 μm.The fiber loss reaches a minimum when the triple supporting bars are parallel positioned toward the fiber core.Triple bar design decreases the transmission loss by more than two orders compared to a simple tubular NCF.Further analysis on the bending sensitivity of the proposed fiber structure shows that multi-bars further help in decreasing the bending loss.The transmission performance of the proposed bar-supported structure with silica and chalcogenide-based glasses (As 2 S 3 and As 2 Se 3 ) is also included to show the advantage of using chalcogenide NCFs in the mid-IR spectrum.The reported fiber can be used for different mid-IR applications, such as low-loss delivery of CO laser, which has applications in laser cutting, material processing, and medical procedures.

Simulation method and fiber design
Light leakage in NCFs is governed by the antiresonance effect inside the glass capillaries and by achieving low spatial overlap and phase mismatch through the inhibited coupling mechanism between the core-guided modes and the cladding modes [5,6,14].The low-loss transmission window between alternative resonant wavelengths (λ res ) can be expressed as follows [5]: Here, t is the thickness of the high-index fiber cladding elements, m is a positive integer, i.e., 1,2,3K denoting the resonance order, n 0 is the refractive index of air and n 1 is the refractive index of cladding material.For any wavelength other than the λ res , light is confined and guided in the hollow air-core region of the fiber with minimum leakage to the outer cladding structure.Modal analysis was performed using a commercial finite element method (FEM) based software (COMSOL Multiphysics).The full vectorial FEM method is used in various scientific fields to numerically investigate complicated geometries for which the analytical solution is challenging to obtain.Through this technique, the model body is divided into small finite bodies (elements), and each element is then solved with a set of defined algebraic equations within the model.The accumulated results are later combined to obtain a final solution.The thickness of the circular perfectly matched layer (PML) and mesh size at different regions of the fiber were optimized through convergence studies.Wavelength-dependent real and imaginary parts of the refractive indices were incorporated into the numerical models to account for both the confinement and material loss in all fiber geometries.The real part of the refractive index used for the cladding in our numerical models for As 2 S 3 , As 2 Se 3 , and silica glass at the centered wavelength of 5.4 μm was around 2.40, 2.79, and 1.31 respectively.More details regarding the simulation method used in this paper can be found in our previous work [18,42].
The cross-sectional diagrams of the proposed single-bar NCF (SB-NCF), double-bar NCF (DB-NCF), and triple-bar NCF (TB-NCF) fiber designs are shown in figure 1. NCFs with five tubular elements in their cladding are numerically studied and focused due to their strong single-mode light guidance performance compared to NCFs with a greater or smaller number of cladding elements while maintaining low-loss guidance [19,45].The cross-sections are annotated with the fiber's design parameters.Here, Ø core represents the core diameter, Ø tube represents the cladding tube diameter, t denotes the thickness of the cladding tubes, and d 1 , d 2, and d 3 show the bar distances of SB-NCF, DB-NCF, and DB-NCF structures to the cladding tubes, respectively.It is important to mention that the fiber core is always placed on the right side of the bar nested cladding tube shown as schematics/insets throughout this work to prevent confusion.Although increasing the Ø core decreases the confinement losses in hollow-core structures, a constant Ø core of 150 μm was used for all fiber designs to limit the overall fiber diameters for any future realization of the fibers [46].

Results and discussions
The design parameters were systematically optimized for all proposed fiber structures by iteratively sweeping them at 5.4 μm.The parametric optimization started by determining the gap between the cladding tubes to achieve optimal light confinement within the fiber core.Figure 2(a) shows the contour plot depicting the change in the confinement loss for As 2 Se 3 simple tubular NCF (ST-NCF) design without any bar placed inside of the cladding tubes as the Ø tube and the t were changed from 95 μm to 135 μm and 1.0 μm to 2.5 μm, respectively.Ø tube of 115 μm and t of 1.5 μm were found as the low-loss design parameters for the ST-NCF at 5.4 μm (indicated with the black star).A similar optimization study was carried out for As 2 Se 3 SB-NCF design, and the results are shown in figure 2(b).Here, it is worth noting that, for this study, the nested bar was fixed at the optimized d 1 value of 75 μm.A Ø tube value of 155 μm and t of 1.5 μm were found as the low-loss design parameters.A small tube diameter means an excess spacing between the cladding elements, resulting in electric field leakage to the outer cladding.In the case of large tube diameters, elevated coupling between the core and cladding modes occurs, especially when the tubes are large enough to overlap, leading to the formation of intersectional nodes.Similar numerical optimization studies were carried out for DB-NCF and TB-NCF for Ø tube and t, and the results indicate that keeping the t and Ø tube constant as 1.5 μm, and 155 μm, respectively, for SB-NCF, DB-NCF, and TB-NCF provides better comparison with low transmission losses.
Once the values of Ø tube and t were found, the position of the nested bar was changed to find the optimum bar position inside the cladding tubes.A secondary nested bar was placed inside the cladding tube to optimize the confinement losses of the As 2 Se 3 DB-NCF.Figure 4(a) shows the change in the confinement losses with respect to the distance (d 2 ) between the second nested bar and the cladding tube.Here it is important to mention that the first nested bar was fixed at the previously found optimum d 1 value of 75 μm while calculating the optimal low-loss position for the second bar.A d 2 value of 90 μm, indicated by the blue dot in figure 4(a), was found as the low-loss position for the second nested bar.Once the optimal low-loss position for the second nested bar was calculated, both bars were moved together within the cladding tube while maintaining a constant gap between them.A distance of zero (s = 0) in the upper horizontal axis of figure 4(a) means the initial positions of the bars, as found previously.A positive value of s means that bars move away from the core, whereas a negative value means they are placed closer to the core as illustrated in the inset of figure 4(a).The simulation confirms that the initial positioning of the bar provides the optimum losses.Furthermore, figure 4(b) shows the change in confinement loss in the proposed spectrum for three different positions of the second bar in the DB-NCF design, as identified with red, blue, and green dots in figure 4(a).As observed in the case of the SB-NCF, placement of the second nested bar in the DB-NCF either close (d 2 = 15 μm) or away (d 2 = 135 μm) from the core lacks in minimizing the leakage loss, resulting in high losses.The optimum transmission performance of DB-NCF was achieved when d 2 = 90 μm, and the confinement loss at 5.4 μm was found to be as low as 0.104 dB km −1 , which is significantly lower than the SB-NCF.
The effect of a third nested bar and its position inside the cladding tube on the confinement loss was also studied to design the TB-NCF.A similar optimization study was performed and the design parameters (Ø tube , Ø nest , d 1 , d 2 , and d 3 ) for all proposed NCF designs are summarized in table 1. Figure 5(a) shows the effect on the confinement loss as the distance (d 3 ) of the third nested bar is changed inside the cladding tube.An optimized distance of 120 μm was found as the optimum low-loss position for the third nested bar which offers the lowest confinement loss within the three designed fibers as 0.055 dB km −1 at 5.4 μm.spectrum between 4.0 μm and 7.0 μm.The confinement loss is maximum when the nested bar is placed either very close or away from the fiber core, as discussed above.
The relationship between the confinement losses and the angular position of nested bar(s) was investigated to further improve the optimum design of the proposed fiber structures.The change in confinement loss for the SB-NCF, DB-NCF, and TB-NCF with the counterclockwise angular orientation of the nested bar(s) from the initial vertical (0°) position to the horizontal (90°) and then back to the initial vertical position (180°) is shown in figure 6(a).The confinement loss increased for all the NCF designs as the angular position of the nested bar (s) was altered, and reached a maximum value when the bar (s) were positioned 90°to the initial position (s), and the nested elements can not effectively help in improving the light confinement.Furthermore, a study on the effect of the change in the angular orientation of individual nested bars for DB-NCF and TB-NCF structure on the confinement loss was also included, and a loss comparison for four possible orientations of the nested bar structure is shown in figure 6(b).Here, it is worth noting that for all four scenarios considered, the confinement loss increases as the nested bar structure is placed in an angular position compared to the straight (0°) placement.The incomplete curves correspond to the stopped rotations of the bars to prevent any overlapping of the nested bars.
To understand the effect of the bar shape on the confinement loss, positive and negative curvatures were introduced to the nested bars with fixed curvature radii.The curvature radii for both cases, i.e., positive and  negative curvatures, were defined from the right edge and the left edge of the cladding tube, respectively, as shown in the insets in figure 7. Here, it is worth mentioning that the curvatures were introduced into the nested bars for both SB-NCF and DB-NCF structures when they were fixed at their optimum location calculated in figures 3 and 4, and maximum possible curvatures were introduced by preventing any overlap between the curved bar and the cladding tube.[42].Although the positively curved structure for DB-NCF has high confinement loss, as expected, the calculated loss for negatively curved DB-NCF design was slightly better than the straight fiber structure over the studied spectrum, which can be attributed to the improved localization of the field distribution when two alternative nested negatively curved structure is used.A similar numerical study was also carried out for TB-NCF design, and the straight nested bar NCF structure was chosen as the low-loss optimized fiber geometry, which not only offers low confinement loss at the centered wavelength but also has comparatively better fabrication feasibility than curved bar nested geometries.
A comparative analysis on the confinement loss of the proposed designs is essential to understand the effect of the fiber geometry on light guidance.On the other hand, comparing the total loss, which includes both the confinement loss and the loss resulting from the interaction of the fiber's material and the guided light, is necessary to predict the overall loss of the fibers and assess their fabrication feasibilities.Confinement and total losses of the ST-NCF, tube nested NCF (TN-NCF), SB-NCF, DB-NCF, and TB-NCF designs in the proposed mid-IR spectrum between 4 μm to 7 μm are shown in figure 8. Here, it is worth mentioning that an optimized Ø tube of 115 μm was used for the ST-NCF, whereas, for a clear fiber loss comparison, a Ø tube of 155 μm, the same as the cladding tube diameter of bar-nested structures, was used for the TN-NCF geometry with an optimized nested tube diameter (Ø nest ) of 90 μm.Confinement losses generally dominate the total losses for all designs, and the material absorption does not have a significant effect on the fiber loss performance in the proposed spectrum, thus showing the significance of fiber geometry in achieving low-loss transmission.ST-NCF and TN-NCF designs have higher losses than the bar-nested structures, and the total loss performance of the TB-NCF is much more favorable than the other designs in the proposed spectrum by providing more than two orders of magnitude improvement in the fiber loss.Approximately an order of magnitude improvement was seen by changing the numbers of nested bars from one to three, as indicated by the vertical cyan dashed line and colored dots in figure 8. Total losses were calculated as ST-NCF = 20.1 dB km −1 , T −1 N −1 -NCF = 1.90 dB km −1 , S −1 B −1 -NCF = 0.670 dB km −1 , D −1 B −1 -NCF = 0.177 dB km −1 , and TB-NCF = 0.112 dB km −1 at 5.4 μm.
Bending loss of all the proposed NCF designs was investigated to analyze and understand the bending sensitivity of the fibers.The bending loss performance of the TB-NCF structure was compared with DB-NCF and SB-NCF design in figure 9(a) for bend radii between 2 cm to 65 cm at the target wavelength of 5.4 μm.The bending loss of the SB-NCF design reaches the saturation point at a very low bending radius and decreases monotonically after a bending radius of 10 cm.In the case of DB-NCF and TB-NCF, the bending losses experience characteristic peaks before reaching a region from where the loss decreases constantly.This is due to the relatively complex and compound structure compared to ST-NCF and SB-NCF design [39].The critical bending radii of SB-NCF, DB-NCF and TB-NCF were calculated as 3 cm, 15 cm, and 18 cm, respectively.Figure 9(b) shows the change in the loss performance of TB-NCF design with 0 cm (straight fiber), 10 cm, and 18 cm (critical bending radius) between 4.0 μm and 7.0 μm.Dispersion is another key parameter to consider when designing NCFs for communication applications and propagation of ultrashort pulses to avoid pulse and signal mixing.An in-depth study of this parameter is not carried out in this work where the main focus is to design a low-loss NCF for laser delivery in the mid-IR spectrum [19,47,48].
A comparative study on the effect of materials to the total losses of the optimized TB-NCF was conducted with silica glass and chalcogenide glasses (As 2 S 3 and As 2 Se 3 ) in the proposed mid-IR spectrum [15,16,28,49,50].It is important to mention that although the same optimized design parameters of TB-NCF were used for each fiber material, strut thickness, t, was optimized separately for each design to achieve low fiber losses at 5.4 μm in a wide transmission window.Therefore, t of 1.5 μm, 1.5 μm, and 2.0 μm were selected for silica, As 2 Se 3 and As 2 S 3 glasses, respectively.As seen in figure 10, the overall transmission performance of chalcogenide-based TB-NCFs is more favorable in our proposed spectrum, and it is worth noting that although   As 2 Se 3 TB-NCF can have a slightly better loss performance than As 2 S 3 NCF at 5.4 μm, both glasses can offer attractive transmission performance to guide light in the mid-IR region.Moreover, As 2 Se 3 offers a wider lowloss transmission window in the mid-IR region [21,22].Silica NCF has high transmission loss due to the drastic increase in the material absorption of silica in the mid-IR region; thus, the general loss trend of silica TB-NCF is higher than the others.Thermal drawing of mid-IR soft-glass fibers sometimes poses a significant challenge due to their viscositytemperature characteristics, and a comprehensive understanding of a material's thermomechanical properties is essential when drawing such IR fibers [15].Over the years, different fabrication techniques such as stack-anddraw, extrusion, and the recently proposed 3D printing technique have been utilized to fabricate mid-IR NCFs [12,17,22].Although stack-and-draw is a commonly used technique for fabricating silica glass-based NCFs and is also employed in fabricating some chalcogenide glass-based fibers with simple geometries, fabrication issues such as capillary dislocation and fusing especially with complex NCF geometry, which has a significant effect on the final drawn fiber optical performance, is a drawback when fabricating soft glass-based NCFs using this method [12,17,21,24,51].The proposed TB-NCF design has several crucial fabrication aspects.Although the fabrication of nested bars within the tubular cladding structure without any significant deformation can be challenging, the extrude-and-draw approach, as discussed and reported in different works on soft glass fibers, can be used to achieve the desired NCF structure [17,21].Nested vertical bars inside the cladding elements in a five-tube TB-NCF preform can be extruded using a modified metallic extrusion die.Later, the fibers can be drawn using the extruded preform by a fiber draw tower removing the stacking and thermal steps which are needed in the conventional stack-and-draw fabrication technique.Alternatively, flat vertical nested bars and tubes can be extruded separately and later firmly stacked together using necessary supporting rods and then drawn into the final fiber.Moreover, fabrication of a chalcogenide glass-based NCF using the 3D printing technique has already been demonstrated, and a careful and improvised use of this technique using stereolithography (SLA) and fused modeling method (FDM) can be an alternative method to potentially fabricate the TB-NCF soft-glass preform to fabricate fibers [22].
Detailed characteristics of some of the proposed NCFs for light guidance in the mid-IR spectral region are presented in table 2. The table includes details about the fiber's material, design specifications, reported numerical and experimental loss, and applications in different fields.Although silica and borosilicate glassbased NCFs are also proposed in the mid-IR region, soft glass-based NCFs are usually preferred for light guidance in longer wavelengths due to their attractive material transparency.The proposed TB-NCF predicts very low confinement and total loss in the targeted wavelength and can be an attractive NCF design for light transmission in the mid-IR region which has important applications as discussed in table 2.

Conclusion
In summary, multi-bar nested chalcogenide glass-based hollow-core NCF was proposed for low-loss light guidance in the mid-IR spectrum with a centered wavelength of 5.4 μm.The design parameters of the proposed fiber geometries were systematically optimized, and the effect of the nested bar on the confinement and total loss performance of a five-tubular cladding NCF was studied.The addition of nested bars significantly improved the transmission losses compared to simple tubular or tube nested tubular structures, and a total loss of 0.112 dB km −1 at 5.4 μm was achieved with As 2 Se 3 TB-NCF.Furthermore, bending sensitivity of TB-NCF design was studied and the proposed fiber features low bending sensitivity with a critical bending radius of 18 cm.The models also predict that fibers have good transmission performance if fabricated with alternative materials in the proposed spectrum.The proposed NCF structure can potentially guide light with low transmission loss in a wide mid-IR spectrum and can be used for different applications in this region, such as for the delivery of CO lasers operating around 5.4 μm.
Figure 3(a) shows the effect of the nested bar distance (d 1 ) of the SB-NCF design on the confinement loss at 5.4 μm.The confinement loss is maximum when the bar is placed close to either the cladding tube or the core, and it decreases as the distance d 1 is increased, reaching a minimum when the position of the nested bar is close to the center of the cladding tube.An optimum low-loss d 1 value of 75 μm was found as the bar placement position, as shown with the red dot in figure 3(a).
Figure 3(b) further clarifies the change in the confinement loss in the proposed mid-IR spectrum by showing the simulation results of three different bar positions between 4.0 μm and 7.0 μm.Here, a d 1 value of 15 μm (blue dotted line) means the bar is

Figure 1 .
Figure 1.An illustration of the cross-sectional diagrams showing the proposed SB-NCF, DB-NCF, and TB-NCFs annotated with design parameters.Each fiber design has a consistent core diameter (Ø core ) of 150 μm.

Figure 2 .
Figure 2. Contour plots showing the change in confinement loss with different Ø tube and t of the cladding tubes for (a) As 2 Se 3 ST-NCF design and (b) As 2 Se 3 SB-NCF design at the centered wavelength of 5.4 μm.

Figure 3
Figure 3 (a) Confinement loss of SB-NCF with respect to the positioning of the nested bar (d 1 ) at 5.4 μm.(b) Simulation results of SB-NCF at three different bar positions within the cladding tube in the proposed spectrum.A schematic illustration of the fiber for each d 1 value is shown on the top.

Figure 4 .
Figure 4. (a) Effect of d 2 on the confinement losses for DB-NCF (black solid line and lower horizontal axis), and effect of s on the losses of the proposed design (red dashed line and upper horizontal axis).(b) Confinement loss performance of DB-NCF design with three different placements of the second nested bar.A schematic illustration of the fiber for each d 2 value is shown on the top.
Figure 5(b) shows the confinement loss performance of TB-NCF for three different positions of the third nested bar in the proposed

Table 1 .Figure 5 .
Figure 5. (a) Change in the confinement loss of TB-NCF with different position (d 3 ) of the third nested bar at 5.4 μm.(b) Confinement loss of TB-NCF in the proposed mid-IR spectrum for three different positions of the third nested bar.A schematic illustration of the fiber for each d 3 value is shown on the top.

Figure 6 .
Figure 6.(a) Change in the confinement loss of SB-NCF, DB-NCF, and TB-NCF by the angular orientation of the nested bar(s).Illustrations for the selected angular positions of bars are shown in the inset.(b) Effect of the independent angular rotation of individual nested bar for DB-NCF and TB-NCF designs on the confinement loss.The minimum loss is achieved when the nested bars are placed vertically in the tubes and parallel to each other.

Figure 7 .
Figure 7. Confinement loss comparison of (a) SB-NCF and (b) DB-NCF with straight, negatively, and positively curved nested bar structures.

Figure 7 (
a) compares the confinement loss of SB-NCF with three different types of nested bar structures: straight (no curvature), negatively curved (a curvature radius of −110 μm), and positively curved (a curvature radius of 110 μm).

Figure 7 (
b) compares the confinement loss of straight DB-NCF with negatively and positively curved structures.A curvature radius of 97 μm was used for the second nested bar while fixing the first bar at 110 μm in DB-NCF design.The different radius of curvatures is due to the overlap of the secondary bar to the cladding tube.The negative and positive curvatures in SB-NCF prevented the localization of the field distribution within the cladding tubes and led to additional leakage of the guided light.Therefore, the straight bar structure is the ideal low-loss nested design for SB-NCF

Figure 8 .
Figure 8.(a) Confinement loss comparison of ST-NCF, TN-NCF, SB-NCF, DB-NCF, and TB-NCF designs in the proposed mid-IR region.A schematic of the fiber design is shown on the top.(b) Total loss comparison of all the proposed NCF designs between 4.0 μm to 7.0 μm.The loss value for each design is indicated with cyan dashed lines and colored dots at the centered wavelength of 5.4 μm.

Figure 9 .
Figure 9. (a) Bending loss comparison of SB-NCF, DB-NCF, and TB-NCF for different bend radii between 2 cm to 65 cm at 5.4 μm.The critical bending radius for TB-NCF design was found as 18 cm.(b) Bending loss trend of the TB-NCF for straight fiber, 10 cm, and 18 cm bending radii in the proposed mid-IR wavelength range of 4.0 μm and 7.0 μm.

Figure 10 .
Figure 10.Total loss performance of silica and chalcogenide-based TB-NCFs in the mid-IR spectrum between 4.0 μm to 7.0 μm.Chalcogenide glass-based (As 2 S 3 & As 2 Se 3 ) TB-NCF design offers significantly better overall transmission performance than silica TB-NCF in the proposed spectrum.

Table 2 .
A comparison of recently reported NCFs for light guidance in the mid-IR region.