Open Access
Add to BibSonomy
Abstract
The hollow core anti-resonant fibers (HC-ARFs) based on soft glass are in high demand for 3-6 µm laser delivery. A HC-ARF based on tellurite glass with 6 touching capillaries as cladding was designed and fabricated for the first time, to the best of our knowledge. A relatively low loss of 3.75 dB/m at 4.45 µm was realized in it. The effects of capillary number, core diameter, wall thickness of capillary, and material absorption loss on the loss of the HC-ARF were analyzed by the numerically simulation. The output beam quality was measured and the influence of bending on the fiber loss was discussed. The results of numerical simulation suggested that the theoretical loss of the prepared fiber can be reduced to 0.1 dB/m, indicating that tellurite HC-ARFs have great potential for mid-infrared laser applications.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The 3-6 µm mid-infrared (MIR) lasers have attracted huge attentions owing to its diverse applications, such as medical surgery, military application, environment monitor, and etc. [1]. Many of these applications require flexible laser transmission media, therefore the glass fibers that can transmit the MIR lasers are followed with interest. In solid core fibers, transmittance window of silica fibers cannot cover MIR band. The flexible fibers based on soft glasses, including fluoride, tellurite and chalcogenide (ChG) ones, have been used to transmit MIR laser, however, the delivering laser power is limited because of the relatively low intrinsic laser-damage-threshold (LDT) of them [2–4]. The researchers therefore proposed large mode field optical fibers, in which the hollow core anti-resonance fibers (HC-ARFs) are more attractive. They confine 99.99% of the light propagated in the hollow fiber core by the anti-resonance effect, thereby achieving high laser damage thresholds.
HC-ARFs based on silica glass has been rapidly developed in recent years [5–11]. Due to the propagation of laser in the air core, silica HC-ARFs have the possibility of transmitting MIR laser. In 2019, Yu at al. reported a silica HC-ARF with losses of 18 dB/km and 40 dB/km at 3.1 and 4 µm, respectively [12]. In the same year, Belardi et al. achieved the 3.8-5.2 µm transmission through a borosilicate HC-ARF and measured a loss of 4 dB/m between 5 and 5.2 µm [13]. In 2022, Fu et al. realized the delivery of 3.12-3.58 µm MIR pulses through a 108 m long silica HC-ARF [14]. Next year, the same research team reported laser transmission at more than 6 µm in silica HC-ARF with a loss of 6.95 dB/m at 6.045 µm [15]. Although the energy proportion in the cladding material can be reduced by reducing the wall thickness of the cladding tube, it is difficult to obtain low loss in silica HC-ARFs in the band of 4.4 µm due to the huge material absorption. Correspondingly, the low loss transmission band of soft glass HC-ARF can be extended to 6 µm.
Among soft glasses, the thermal stability of fluoride glass is poor, which makes it a higher tendency to crystallization than other two glasses at higher temperatures than Tg and cannot meet the preparation requirements of HC-ARF, considering the multiple heating and softening processes from the preparation of fiber preforms to fiber drawing. At present, the developed MIR HC-ARFs are mainly based on ChG and tellurite glasses, which have good thermal stability. In 2011, Kosolapov et al. reported the first soft-glass HC-ARF based on the Te20As30Se50 ChG. Due to the obvious deviation of the actual fiber structure from the ideal one, the fiber loss with 11 dB/m at 10.6 um was relatively high [16]. In 2023, we prepared a ChG HC-ARF with 7 touching capillaries by using the “stack-and-draw” method. By controlling the gas pressure difference between the capillary tubes and the air core during the fiber drawing process, the structural deviation between the prepared and the designed fiber was very small. The fiber loss was as low as 1.29 dB/m at 4.79 µm [17].
Compared to ChGs, tellurite glass is essentially an oxide glass with better chemical and thermal stability. The transmission band of tellurite bulk glass can extend up to ∼6.5 µm, and its refractive index is ∼1.8 at 3-5 µm. The research findings presented by Wu et al. demonstrate that the refractive index around 1.8 is preferred to achieve a flattened profile of group dispersion in the transmitting range [18]. In 2019, Tong et al. fabricated the first tellurite HC-ARF with 6 non-touching capillaries by using the “stack-and-draw” method. The fiber exhibited obvious alternating low and high transmittance bands caused by the resonance and anti-resonance effects in the range of 0.4-2.4 µm [19]. In the same year, they tested the transmission spectrum of another 17 cm tellurite HC-ARF in the range of 2-3.9 µm and suggested that the low-loss transmission band in tellurite HC-ARF could be extended to 6 µm [20]. In 2020, Ventura et al. also developed a tellurite HC-ARF with 6 non-touching capillaries through the “extrude-and-draw” approach. The fiber losses were ∼8.2, 4.8 and 6.4 dB/m at 5 µm, 5.6 µm and 5.8 µm, respectively, by using an OPO as the laser source and the output beam quality was good (M2 = 1.2) [21]. The theoretical loss of this tellurite HC-ARF can be reduced to 0.3 dB/m in the range of 5.4-6.1 µm. At present, many optical applications, such as molecular spectroscopy [22], pulse compression [23], Raman conversion [24], and high-power laser transmission [25] have been achieved in silica HC-ARFs. With the advancement of tellurite HC-ARFs preparation technology, the working wavelengths of above applications can be further extended to 6 µm. However, few tellurite HC-ARFs have been prepared, and there is a lack of understanding of their optical properties.
In this work, we designed and fabricated a tellurite HC-ARF with touching capillaries to realize low loss transmission in the range of 3-6 µm. The effects of capillary number, air core diameter, material absorption loss, and the prepared tolerant on the confinement loss were studied. A measured fiber loss of 3.75 dB/m at 4.45 µm was realized in it. The output beam quality was measured and the bend loss of the tellurite HC-ARF was discussed.
2. Experiments
2.1 Glass preparation and fiber fabricated processes
The composition of 70TeO2-15BaF2-10La2O3-5LaF3 (TBLL) was used and its preparation process was introduced in detail in our previous work [26]. As shown in Fig. 1(a), the tellurite HC-ARF with 6 touching capillaries was fabricated by the “stack-and-draw” method. First, the TBLL tellurite glass tubes with an outer diameter of 18 mm were synthetized by the rotary casting method and polished the outer face to meet the preparation requirements of tellurite HC-ARFs. This process could reduce the scattering loss introduced by surface mass. Then, the glass tubes were drawn into capillary tubes with an outer diameter of 4.7 mm. The HC-ARF preform with an inner core diameter of 4.7 mm was prepared by assembling the outer jacket tube and six capillary tubes according to the designed fiber structure. Meanwhile, two 10 mm-long supporting tubes were placed into the air core at both ends of the stacked preform to prevent the relative movement between the capillary and jacket tubes. Subsequently, the stacked preform was heated to 420 °C to soften the TBLL glass and bonded the capillary and jacket tubes in a dry and protective atmosphere glove box.
The infrared fiber drawing tower was used to draw the tellurite HC-ARFs. Due to the thin thickness of the capillary glass wall in the fiber preform, which is easy to deformation, it is difficult to maintain an ideal fiber structure during the fiber drawing processes. Therefore, a precise dual pressure controlling system was used to control the gas pressure in the air core (Pcore) and capillaries (Pcapillary) during the fiber drawing processes. Among them, as shown in Fig. 1(b), Pcore was higher than the atmospheric pressure, and Pcapillary was higher than the Pcore. The pressure difference ΔP = Pcapillary – Pcore was kept as a positive value. On this basis, by collaboratively controlling of fiber drawing parameters such as fiber drawing temperature, fiber drawing speed, and ΔP, the tellurite HC-ARF with an expected structure can be achieved.
2.2 Characterization
The thermal properties of prepared tellurite glasses were characterized by the differential scanning calorimetry (DSC) (Netzsch 404F1, JPN, argon gas atmosphere, heating rate of 10 K/min). The transmission spectrum of 0.4-1.7 µm and 1.7-7 µm was measured by FT-IR spectrometer (Nicolet 6700, USA) and UV/VIS/NIR spectrophotometer (Jasco V-570, JPN), respectively. The infrared variable angle spectroscopic ellipsometer (J.A. Woollam, IR VASE Mark II) was used to measure the refractive indices of the tellurite glass. The loss of prepared tellurite HC-ARF fiber was measured by the cut-back method. A 4.25-4.47 µm tunable quantum cascade laser (QCL) (DAYLIGHT, TLS-SK-41043-MHF) was used as the laser source with 1 nm bandwidth of the emitted light and <1 rms (over 5 minutes) of power stability. The laser was coupled into the HC-ARF by a ZnSe lens with a focal length of 22 mm.
3. Results and discussion
3.1 Thermal properties and transmission spectrum
The DSC curve of TBLL glass is shown in Fig. 2(a) and the characteristic temperature is marked. The Tg is about 430 °C, which is much higher than those of the tellurite glasses in the reports [27,28]. Higher Tg usually indicates the glass can resist a higher power laser irradiation without damage. In addition, the ΔT is usually to value the thermal stability of glasses, the ΔT of TBLL glass is greater than 112 °C, meeting the needs of fiber drawing.
Fig. 1. (a) The photos of the jacket tube, capillary tubes, and fiber preform. (b) The schematic diagram of the TBLL HC-ARF fabrication process.
The transmission spectrum of TBLL glass is shown in Fig. 2(b), the transmission range of the bulk TBLL glass can be expanded to 6.5 µm. The absorption coefficient of OH- groups at ∼3 µm was calculated by Formula (1):
Among them, the α, T, and l are the absorption coefficient, the transmittance measured by the instrument, and the thickness of glass sample, respectively. The α is calculated to be 0.012 cm-1, which is much lower to those in the previous reports of tellurite HC-ARFs [20,21]. The guidance mechanism of HC-ARFs is different from that of solid core fiber, and most of the light is confined in the air core. Thus, the total fiber loss includes confinement loss and material absorption loss, and is calculated by the following formula:
where αtotal is the total loss of HC-ARF, αCL is the confinement loss obtained by simulation, η is the modal overlap factor between core mode and cladding material, and αabs is the material absorption loss. In this work, η can be as low as 10−5 in the anti-resonant band. Generally, when the glass absorption loss is less than 100 dB/m, the influence of glass absorption loss on the total loss of HC-ARFs can be neglected [17].3.2 Fiber design
The results of refractive index dependent on light wavelength are shown in Fig. 3. In the range of 3-5 µm, the refractive index of TBLL glass is 1.77-1.8. The measured refractive index curve can be well fitted with the Sellmeier formula:
Fig. 3. The measured refractive indices and the fitting curve of TBLL glass. (This figure has been revised.)
Table 1. The Sellmeier coefficients of TBLL glass
The HC-ARF based on TBLL glass with touching capillary tubes as cladding were designed to realize a low-loss transmission in the range of 3-6 µm by using Finite Element Method, and their confinement loss curves were simulated. The core diameter of HC-ARF is fixed to be 150 µm and the number of touching capillaries is set to 5, 6, 7, and 8, respectively. The structure schematic diagrams and the fundamental mode of the designed HC-ARF are shown in Fig. 4(a), which can effectively confine the fundamental mode in the hollow air core. In addition, the structural parameters of HC-ARF are marked in Fig. 4(a). N is the number of capillary tubes. Dcore is the diameter of air core. dx is the inner diameter of capillary tube. T and t are the wall thickness of the capillaries and jacket tube, respectively. The calculated confinement loss curves of HC-ARFs are shown in Fig. 4(b). There are several low-loss transmission bands in the range of 3-6 µm. As the number of capillary tubes increases from 5 to 8, the confinement loss of HC-ARF in the 3-6 µm band gradually decreases and the corresponding reduction amplitude in confinement loss gradually slows down. In general, as the number of capillaries in HC-ARF increases, the curvature of the capillaries increases and the fiber confinement loss decreases [29]. However, the increase in the number of capillaries will also lead to the enhancements in the number of nodes between the capillaries, which will raise the confinement loss [30]. The competition between these two effects will gradually reduce the decrease extent of fiber confinement loss with the increase in the number of capillaries. It is observed that the confinement loss of HC-ARF composed of 6 capillaries is only half an order of magnitude higher than that of the fiber with 7 and 8 capillaries. Taking into account the optical performance and preparation difficulty, the HC-ARF with the 6 touching capillaries was chosen as the target fiber in this paper.
Fig. 4. (a) The structure schematic diagrams and the amplitude of the normalized electric field for the fundamental mode of TBLL tellurite HC-ARFs with the touching capillaries vary from 5 to 8. (b) The confinement loss curves of the TBLL tellurite HC-ARFs dependent on the number of capillaries.
The effects of fiber air core diameter and capillary wall thickness on the confinement loss of the HC-ARF at the anti-resonant wavelength around 4.0 µm were numerically simulated, as shown in Fig. 5(a) and (b). The results show that as the core diameter increases from 90 to 150 µm, the confinement loss of HC-ARF decreases monotonically. As the wall thickness of the capillary increases from 5.52 to 9.75 µm, the fiber loss exhibits a periodic alternating resonant and anti-resonant effects. When the fiber loss is 0.1 dB/m, the wall thickness preparation tolerance of the capillary can reach to 0.75 µm. This will be benefit for reducing the preparing difficulty for the low-loss tellurite HC-ARFs. As shown in Fig. 5(b), the difference between the minimum loss in the anti-resonance regions is small, we choose a small wall thickness of ∼6 µm to obtain a large transmission bandwidth.
Fig. 5. The confinement loss dependent on (a) the air core diameter and (b) the cladding capillary thickness.
In addition, the effect of material absorption loss on the total loss of HC-ARF was estimated. As shown in Fig. 6, in the anti-resonant band, the ratio of time-averaged power flow of the z-direction in the wall glass of capillary is about 1 × 10−5 of that for the total transmission power, indicating that the contribution of material absorption loss to the total fiber loss is very small. Meanwhile, almost all the energy in the air hole of the fiber is concentrated in the fiber air core, indicating that the fiber can effectively confine the transmitting energy within the air core in the anti-resonant band. On the contrary, in the resonant band, most of the energy will enter the air hole of the capillaries. And some of the transmission power will enter the wall glass of capillary, which will further increasing the total loss of the HC-ARF.
Fig. 6. The proportion of power transmitted in the air core, wall glass of cladding capillaries, and air to the total transmitted power.
Based on the results of above numerical simulation, the structure of target tellurite HC-ARF is shown in the Fig. 7(a), and the corresponding structural parameters are marked in it. The structure diagram of the prepared TBLL tellurite HC-ARF is shown in Fig, 7(b), and the structural parameters of the fiber are marked. The fabricated fiber was not coated in this work, so its bending radius cannot be less than 20 cm at present. There is a structural deviation between the prepared tellurite HC-ARF and the theoretical designed one. The structural deviation is mainly caused by the mismatch among fiber drawing temperature, fiber drawing speed, and ΔP. The wall thickness of the 6 capillaries is ∼5.49, 5.89, 6.11, 5.44, 5.89, and 5.83 µm, respectively. The difference of capillary glass wall thickness between the actual fiber and the designed one is less to 0.51 µm, which is much smaller than the preparation tolerance (0.75 µm).
Fig. 7. (a) The structure diagram of the target fiber. (b) The cross section of the prepared TBLL tellurite HC-ARF.
3.3 Fiber characterization
The output power was measured 4 times (3 cuts) from 75 cm to 45 cm, and the corresponding data is shown in Fig. 8. In the test, the fiber is placed as straight as possible to reduce the impact of bending. The loss of the fiber is measured as well as 3.75 ± 0.15 dB/m at 4.45 µm, which is lower than that (4.8 ± 0.4 dB/m@5.6 µm) in the tellurite HC-ARF with a core diameter of 139 µm prepared by the “extrude-and-draw” method [21]. The effective mode field area is calculated to be larger than 10600 µm2 in simulation, which will be benefit for reducing energy density and improving laser transmission capacity in MIR laser applications.
As shown in Fig. 9, the theoretical confinement loss curve (black line) of the prepared tellurite HC-ARF were simulated based on the actual structural parameters of the fiber and the measured loss (red dots) values are slightly higher than the simulated confinement loss curve but agree well. Low losses as 4.37 ± 0.25 dB/m, 7.5 ± 0.15 dB/m and 3.75 ± 0.15 dB/m were achieved at 4.28 µm, 4.34 µm and 4.45 µm, respectively. Compared to the simulated confinement loss curves in Fig. 4(b), several new resonance peaks appear, located at 3.24µm, 3.48µm, 4.04µm and 4.32µm, which is due to the non-uniformity of the wall thickness. In addition, the theoretical confinement loss of the prepared tellurite HC-ARF can reach to 0.1 dB/m at multiple wavelengths, such as 3.4, 3.9, and 4.7 µm. Especially, the confinement loss at 3.15 µm can even be as low as 0.01 dB/m, but it cannot be tested due to the limitation of the laser sources. Therefore, improving the uniformity of capillaries in fiber is the next optimization direction, which will be benefit for reducing the loss and expand the bandwidth of anti-resonant bands of the fibers.
Fig. 9. Re-simulated confinement loss curve according to the actual fiber structure, and the measured loss (red dots) in 3-5 µm range.
The output beam profiles of a 25 cm long fiber were measured at 4.36 µm and 4.45 µm by a beam analyzer (DataRay, WinCamD-IR-BB). Figure 10(a) depicts the output beam profile at the resonant wavelength of 4.36 µm. The results show propagation mode transmits in both the hollow air core and the cladding tube, which is mainly attributed to the strong coupling between the core mode and cladding mode due to the similar size of cladding tubes and fiber core. It is observed that the core mode is elliptical in the resonant band, which may be caused by resonance effect of part of the wall thickness. Figure 10(b) depicts the output beam profile at the anti-resonant wavelength of 4.45 µm. The results show that the core mode appears circular when deviating from the resonance band, but the coupling between core mode and cladding mode still exists. Although the tube mode can also conduct part of the light, its loss is higher than that of the core mode, which is why the measured loss is slightly higher than the simulation result in Fig. 9. The core output beams in both bands are near-Gaussian distribution. No higher-order modes were observed in fiber core, which may be due to the low power of the laser coupled to the fiber core. Therefore, effect of higher-order modes on the total loss were not specifically discussed in this work. As shown in Fig. 11, the beam quality was measured from a 20-cm-long tellurite HC-ARF at 4.45 µm. The M2 is calculated to be 1.666 and 1.933 in the x and y directions, respectively, attributed to the asymmetry in fiber structure. And the large M2 is caused by the coupling between the core and tube modes.
Since the fabricated fiber was not coated, the measurement of fiber bending loss is very difficult. Therefore, the bending loss based on the actual fiber structure was simulated at by changing the refractive index distribution in the x and y directions according to the formulas:
Fig. 12. Left: Bending loss simulation in x (black line) and y (red line) direction. Right: The fundamental mode at 3.9 µm for a fiber with a curvature radius of 34 cm bending in x direction and for a fiber with a curvature radius of 51 cm bending in y direction.
4. Conclusion
A tellurite HC-ARF with 6 touching capillaries as cladding was fabricated by the “stack-and-draw” method. The results of numerical simulations and experiments showed that several low confinement loss band can be realized in the range of 3-6 µm with a large effective mode field area of 10600 µm2 in simulation. A low loss of 3.75 ± 0.15 dB/m was measured at 4.45 µm, and a lower loss of 0.1 dB/m can be achieved at 3.4, 3.9, and 4.7 µm in theory. The measurement of output beam from a 25 cm long tellurite HC-ARF shows the coupling between the core mode and cladding mode due to the similar size of cladding tubes and fiber core. The beam quality was measured and the bend loss was discussed. The above results indicate that tellurite HC-ARF is a promising medium for the MIR laser applications.
Funding
National Natural Science Foundation of China (61935006, 62005312, Grants Nos. 62090065); Natural Science Foundation of Shaanxi Province (2023-JC-JQ-31, J23-016- III); Special Research Assistant Program of CAS (J22-068-III).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. J. S. Sanghera, C. M. Florea, L. B. Shaw, et al., “Non-linear properties of chalcogenide glasses and fibers,” J. Non-Cryst. Solids 354(2-9), 462–467 (2008). [CrossRef]
2. W. C. Wang, B. Zhou, S. H. Xu, et al., “Recent advances in soft optical glass fiber and fiber lasers,” Prog. Mater. Sci. 101, 90–171 (2019). [CrossRef]
3. S. Wuthrich, W. Luthy, and H. P. Weber, “Optical damage thresholds at 2.94, ur in fluoride glass fibers,” (n.d.).
4. Y. Yao, F. Yang, S. Dai, et al., “Mid-infrared femtosecond laser-induced damage in TeO2 -BaF2 -Y2O3 fluorotellurite glass,” Opt. Mater. Express 12(4), 1670 (2022). [CrossRef]
5. W. Belardi and J. C. Knight, “Hollow antiresonant fibers with low bending loss,” Opt. Express 22(8), 10091 (2014). [CrossRef]
6. P. Uebel, M. C. Günendi, M. H. Frosz, et al., “Broadband robustly single-mode hollow-core PCF by resonant filtering of higher-order modes,” Opt. Lett. 41(9), 1961 (2016). [CrossRef]
7. A. D. Pryamikov, A. S. Biriukov, A. F. Kosolapov, et al., “Demonstration of a waveguide regime for a silica hollow - core microstructured optical fiber with a negative curvature of the core boundary in the spectral region > 35 µm,” Opt. Express 19(2), 1441 (2011). [CrossRef]
8. F. Yu, W. J. Wadsworth, and J. C. Knight, “Low loss silica hollow core fibers for 3–4 µm spectral region,” Opt. Express 20(10), 11153 (2012). [CrossRef]
9. G. K. Alagashev, A. D. Pryamikov, A. F. Kosolapov, et al., “Impact of geometrical parameters on the optical properties of negative curvature hollow-core fibers,” Laser Phys. 25(5), 055101 (2015). [CrossRef]
10. A. V. Newkirk, J. E. Antonio-Lopez, R. A. Correa, et al., “Extending the transmission of a silica hollow core fiber to 4.6 µm,” Opt. Continuum 1(9), 2062 (2022). [CrossRef]
11. F. Yu and J. C. Knight, “Spectral attenuation limits of silica hollow core negative curvature fiber,” Opt. Express 21(18), 21466 (2013). [CrossRef]
12. F. Yu, P. Song, D. Wu, et al., “Attenuation limit of silica-based hollow-core fiber at mid-IR wavelengths,” APL Photonics 4(8), 080803 (2019). [CrossRef]
13. W. Belardi and P. J. Sazio, “Borosilicate based hollow-core optical fibers,” Fibers 7(8), 73 (2019). [CrossRef]
14. Q. Fu, Y. Wu, I. A. Davidson, et al., “Hundred-meter-scale, kilowatt peak-power, near-diffraction-limited, mid-infrared pulse delivery via the low-loss hollow-core fiber,” Opt. Lett. 47(20), 5301 (2022). [CrossRef]
15. Q. Fu, I. A. Davidson, G. T. Jasion, et al., “Low-loss fused silica hollow-core fiber delivery of mid-infrared light at 6-µm,” in 2023 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) (IEEE, 2023), pp. 1.
16. A. F. Kosolapov, A. D. Pryamikov, A. S. Biriukov, et al., “Demonstration of CO2-laser power delivery through chalcogenide-glass fiber with negative-curvature hollow core,” Opt. Express 19(25), 25723 (2011). [CrossRef]
17. H. Zhang, Y. Chang, Y. Xu, et al., “Design and fabrication of a chalcogenide hollow-core anti-resonant fiber for mid-infrared applications,” Opt. Express 31(5), 7659 (2023). [CrossRef]
18. D. Wu, F. Yu, Y. Liu, et al., “Dependence of waveguide properties of anti-resonant hollow-core fiber on refractive index of cladding material,” J. Lightwave Technol. 37(21), 5593–5699 (2019). [CrossRef]
19. H. T. Tong, N. Nishiharaguchi, T. Suzuki, et al., “Fabrication of a tellurite hollow core optical fiber with six non-touching cladding air holes,” J. Ceram. Soc. Japan 127(12), 918–923 (2019). [CrossRef]
20. H. T. Tong, N. Nishiharaguchi, T. Suzuki, et al., “Mid-infrared transmission by a tellurite hollow core optical fiber,” Opt. Express 27(21), 30576 (2019). [CrossRef]
21. A. Ventura, J. G. Hayashi, J. Cimek, et al., “Extruded tellurite antiresonant hollow core fiber for Mid-IR operation,” Opt. Express 28(11), 16542 (2020). [CrossRef]
22. K. Johnson, P. Castro-Marin, C. Farrell, et al., “Hollow-core fiber delivery of broadband mid-infrared light for remote spectroscopy,” Opt. Express 30(5), 7044 (2022). [CrossRef]
23. S. Loranger, P. St. J. Russell, and D. Novoa, “Sub-40 fs pulses at 1.8 µm and MHz repetition rates by chirp-assisted Raman scattering in hydrogen-filled hollow-core fiber,” J. Opt. Soc. Am. B 37(12), 3550 (2020). [CrossRef]
24. A. Benoit, B. Beaudou, B. Debord, et al., eds. (2017), p. 100880 H.
25. A. Urich, R. R. J. Maier, B. J. Mangan, et al., “Delivery of high energy Er:YAG pulsed laser light at 294 µm through a silica hollow core photonic crystal fibre,” Opt. Express 20(6), 6677 (2012). [CrossRef]
26. C. Liu, S. Feng, X. Xiao, et al., “Intense 2.85 µm mid-infrared emissions in Yb3+/Ho3+ codoped and Yb3+/Er3+/Ho3+ tridoped TBLL fluorotellurite glasses and their energy transfer mechanism,” Ceram. Int. 48(4), 5267–5273 (2022). [CrossRef]
27. E. A. Anashkina, A. V. Andrianov, V. V. Dorofeev, et al., “Development of infrared fiber lasers at 1555 nm and at 2800 nm based on Er-doped zinc-tellurite glass fiber,” J. Non-Cryst. Solids 525, 119667 (2019). [CrossRef]
28. W. C. Wang, J. Yuan, L. X. Li, et al., “Broadband 27 µm amplified spontaneous emission of Er3+ doped tellurite fibers for mid-infrared laser applications,” Opt. Mater. Express 5(12), 2964 (2015). [CrossRef]
29. W. Belardi and J. C. Knight, “Effect of core boundary curvature on the confinement losses of hollow antiresonant fibers,” Opt. Express 21(19), 21912 (2013). [CrossRef]
30. A. N. Kolyadin, A. F. Kosolapov, A. D. Pryamikov, et al., “Light transmission in negative curvature hollow core fiber in extremely high material loss region,” Opt. Express 21(8), 9514 (2013). [CrossRef]
