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Abstract
We experimentally demonstrate for the first time a successful fabrication of a tellurite hollow core optical fiber which has a mid-infrared transmission range. The wall thickness of each cladding air-hole is about 2.8 µm and the outer diameter of the full air-hole structure D is approximately 110 µm. The results show that the measured transmission spectrum can expand up to 3.9 µm. In addition, it is expected that the transmission can extend to around 6 µm. When the input light is linearly polarized, it can be maintained after propagating through a 17-cm-long fiber.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
From the late 1990s, hollow core photonic crystal fibers (HC-PCFs) which confine the electromagnetic field inside a central air-core surrounded by a microstructured air-hole cladding [1] have drawn great attention of researchers around the globe. The key advantage of HC-PCFs is that more than 99.99% of the travelling light can be concentrated in air rather than in glass [2]. Due to this air-guiding characteristic, HC-PCFs offer remarkable transmission properties such as high optical power delivery [3], high damage threshold [3], low loss levels [4,5] and broad bandwidth from vacuum ultraviolet to the NIR [6] up to the mid-IR region [7] or even terahertz range [8] where conventional solid silica fibers cannot reach.
Light can be guided in the air-core by photonic bandgap effect [9] due to the presence of the periodic air-hole structure in the cladding or by antiresonant effect [10] which enables light confinement in a medium with a lower refractive index than that of its surroundings [11,12]. This antiresonant mechanism allows a much broader spectral transmission than that achieved in photonic bandgap fibers [13,14]. Therefore, hollow core antiresonant fibers [2,12,15,16] have been widely studied. Since no bandgap is used, it is not necessary to have a complicated air-hole structure in the cladding. As mentioned in [17], the first ring of air holes mainly determines the guiding properties and low loss transmission band, even in the mid-IR wavelength region (λ > 4µm), is possible to be obtained by using one ring of air hole despite of very high material losses of silica. This new type of hollow core fiber is also called as negative curvature fibers (NCFs) [11,18] whose core boundary has a convex shape when it is seen from the center of the fiber [12] and they possess just one ring of cladding air holes. It has been proved that the“negative”curvature of the core boundary is essential for reducing confinement losses [12]. Due to their superior properties, NCFs have many potential applications including data communications [19,20], optical data transmission [21], terahertz propagation [8,22,23], power beam delivery for industrial applications such as cutting, welding, and engraving [3], medical applications [24,25] and chemical sensing [26–28].
Another important feature of NCFs is that their excellent guidance properties are preserved even when both core and cladding channels are filled with gas or liquid. The ability to quickly adjust the gas species, gas pressure or liquid filling procedure inside large air holes and air core of NCFs cannot be attained by using conventional solid fibers. However, most of NCFs are based on fused silica glass which limits the choice of liquids or gases because their refractive indices must be less than that of silica (nG = 1.45 at 600 nm) to preserve the benefits of single-mode guidance. To overcome this issue, glasses with higher refractive index are highly required. Recently, non-silica glasses, such as chalcogenide, have been used for NCFs due to their simple structure but the development of chalcogenide hollow core fibers has encountered several fabrication difficulties [29–31]. With wide transmission regions up to the IR region (from 0.35 to 6 µm), high thermal stability and good corrosion resistance [32], tellurite glasses can also be potential candidates for the development of non-silica negative curvature fibers. However, the performance of tellurite NCFs has not been demonstrated yet. In our previous work [33], a tellurite HC-PCF with a large hexagonal hollow core in the center and touching air-hole microstructure in the cladding was demonstrated. In this work, a tellurite NCF with 6 non-touching air holes in the cladding was successfully fabricated. In addition, its transmission and polarization properties were studied numerically and experimentally for the first time. It is shown that the transmission range of the fiber can expand up to 3.9 µm and the linear polarization can be maintained after a 17-cm-long fiber.
2. Fiber development
2.1 Glass material developments
The tellurite glass composed of 76.5TeO2-6ZnO-11.5Li2O-6Bi2O3 (TZLB) was developed because of its good optical and thermal properties. Glass sample was prepared by the dry melting process [33]. The transmission spectrum measured by an UV-VIS spectrometer (Perkin Elmer, Lambda 900) in the wavelength range from 0.2 to 2.5 µm and by an FT-IR spectrometer (Perkin Elmer, Spectrum 100) in the wavelength range from 2.5 to 10 µm was shown in Fig. 1. Compared to the glass in our previous work [33], the transmission range has improved and expanded further to long wavelength side.
Fig. 1. Measured transmission spectra of the TZLB 76.5TeO2-6ZnO-11.5Li2O-6Bi2O3 glass (bolded line).
The minimum deviation method [34] was employed to measure the refractive index of the developed TZLB glass at different wavelengths from 0.5 to 4.1 µm by using a triangular glass prism. The measured refractive indices were fitted to the Sellmeier equation [35,36] as given by Eq. (1) and its coefficients were shown in Table. 1. In Fig. 2, the measured refractive indices and fitted wavelength-dependent curve are plotted.
Table 1. Sellmeier coefficients of the TZLB glass (76.5TeO2-6ZnO-11.5Li2O-6Bi2O3).
2.2 Fiber development
In this section, fiber geometry and confinement loss are investigated numerically by using the commercial software Comsol Multiphysics, the finite element method and the perfectly-matched boundary condition. The mesh resolution was automatically optimized to maintain the calculation accuracy and reduce the calculation time. However, the largest mesh size was equal to 0.25 µm which is equal to 1/8 [37] of the shortest wavelength of the wavelength range from 2 to 6 µm. A schematic image of tellurite NCF with 6 non-touching cladding air-hole structure was plotted in Fig. 5. This structure was considered as a possible way to reduce the overall fiber attenuation [1]. The geometrical parameter D1 is the air-core diameter, D2 is the cladding air-hole diameter, D is the outer diameter of the full air-hole structure, N is the number of the cladding air hole, t is the wall thickness of the cladding air-hole and R is the ratio between D2 and D1.
Based on the current conditions of our facilities, when D is larger than 110 µm and t is smaller than 2.8 µm, the walls of the cladding jacket tube and capillary tubes which are necessary for fiber fabrication are very thin and difficult to obtain. In addition, the fibers were broken easily during the drawing process. Therefore, numerical calculations in this work were carried out when N and R were changed but D and t were kept constant at 110 µm and 2.8 µm, respectively. Figure 6 shows the N dependence of the confinement loss calculated for the fundamental mode in the air-core of the fiber by the solid line (N = 5), the dashed line (N = 6) and the dotted line (N = 7). In the inset, schematic cross-sectional images of the fibers with N = 5, 6 and 7 were plotted. As can be seen, the confinement loss was less than 10 dB/m in four wavelength bands from 2 to 6 µm. It becomes much lower when N is larger than 5, but it slightly increases when N is larger than 6.
Fig. 6. Calculated confinement loss spectra of the fundamental mode with the different value of N (5, 6 and 7).
Figure 7 shows the images of the intensity distribution profiles calculated for the fundamental mode and the 1st order mode which can propagate in the air core of the fiber when N = 6 and R = 0.5. In Fig. 8, the images of the intensity distribution profiles calculated for the fundamental mode are plotted when R are 0.1, 0.5 and 0.86, respectively. As can be seen, when R is as small as 0.1, the air-core diameter is very large and the gap between two adjacent cladding air holes is also large. When R increases, the gap becomes smaller and it disappears when R is 0.86. At this value of R, the walls of two adjacent cladding air holes touch each others.
Fig. 7. Calculated intensity distribution of the fundamental mode and the 1st order mode in the air-core when N = 6 and R = 0.5.
Fig. 8. Calculated intensity distribution of the fundamental mode in the air-core when N = 6 and R = 0.1, 0.5 and 0.86, respectively.
The confinement losses of the fundamental mode and the 1st order mode when R changed from 0.1 to 0.86 were calculated and plotted in Fig. 9. Firstly, it can be noticed in Fig. 9 that the confinement loss of the 1st order mode is larger than that of the fundamental mode regardless of R value. As an example, it is obvious in Fig. 7 (R = 0.5) that the 1st order mode is not confined in the air core of the fiber as tightly as compared to the fundamental mode. The leakage of electric field to cladding air-holes causes high confinement loss of the 1st order mode.
Fig. 9. Calculated confinement loss spectra of the fundamental mode and 1st order mode with different value of R (from 0.1 to 0.86).
Secondly, the confinement loss reduces when R increases from 0.1 to 0.5, but when R becomes larger than 0.5, the confinement loss increases rapidly, especially for the 1st order mode as shown in Fig. 9. Similar results were obtained in [11]. When the gap between two cladding air-hole is too large (R <0.5), the confinement loss is high due to the leakage of the electric field through the air gap [11,18]. On the other hand, when the gap is smaller (R > 0.5), the diameter of the cladding air-holes becomes larger than the diameter of the air-core and a weak coupling between the fundamental mode and the cladding air-hole mode occurs. Because of this weak coupling, the confinement loss in the air core increases. When the gap disappears, the walls of two cladding air-holes connect to each other forming nodes where the mode field can reside [1,18,38] and it causes a rapid increase in confinement loss. This feature can be confirmed in Fig. 10 which shows the glass mode profile when the walls of two nearby cladding air-holes touch each other.
Fig. 10. Calculated mode profile in the glass wall when N = 6 and R = 0.86 (the walls of two nearby cladding air-hole touch each other).
2.3 Fiber fabrication
Following the finding of the above numerical analysis, we demonstrated the fabrication of a tellurite NCF with 6 non-touching air holes in the cladding by using the stack-and-draw technique. A schematic diagram which illustrates the fiber fabrication process was shown in Fig. 11. A cylindrical TZLB tube was first prepared by using the rotational casting method [32]. Its outer and inner diameters were 15 and 10 mm, respectively. The outer surface of the tube was polished so that its wall thickness was controlled to be as thin as 1 mm. An elongation process was carried out to obtain TZLB capillary tubes which have 0.25-mm wall thickness. Each tube was about 15-cm long. After that, a set of 6 capillary tubes was used to form a preform which has a hexagonal cladding air-hole structure by stacking and soldering them inside a cylindrical TZLB jacket tube. Figure 12 shows images of experimental cladding tube and capillary tubes. Finally, the preform was drawn into fiber whose diameter was about 150 µm. During this fiber drawing process, an inflation pressure was kept inside of 6 capillary tubes to prevent their shape-deformation. At the same time, a negative pressure was applied in the center of the TZLB jacket tube. The cross-sectional image of the fiber was taken by a scanning electron microscope and shown in Fig. 13. The air-hole structure is depicted by the black area. The wall thickness of each cladding air-hole was about 2.8 µm. The outer diameter of the full air-hole structure D was 110 µm and the parameter R was about 0.4.
Fig. 11. Schematic diagram depicts the fiber fabrication of tellurite NCF with a 6 non-touching cladding air-hole structure.
Fig. 13. Cross-sectional image of the fabricated tellurite NCF taken by a scanning electron microscope.
3. Results and discussions
3.1 Transmission properties
To clarify the transmission properties of the fabricated tellurite NCF, a strong light beam from a tunable femtosecond laser (Coherent Chameleon) was coupled into its air-core by using a focus lens (Thorlabs DCX 25.4X). A variable density filter was placed after the laser output to control the light beam power launched into the fiber. The output light after a 17-cm-long fiber was coupled into a ZBLAN fiber (ZMF-60/20-N0.29) which was connected to an optical spectrum analyzer (OSA) (Thorlabs, OSA 205). The objective lens (Edmund optics, 2-12 UM AR) was used as a collimator. The recorded data was shown by red dots in Fig. 14(b). The measured spectrum includes high transmission bands in the wavelength ranges from 2.0 to 2.3 µm, 2.5 to 2.9 µm and 3.2 to 3.9 µm. To confirm the measured result, the transmission spectrum was calculated and shown by the solid line in Fig. 14(b). A good correspondence between them can be observed. In addition, the calculated spectrum had a high transmission band from about 5.0 to 6.0 µm, but it was not obtained experimentally due to the limit of the operation range of the current laser source.
Fig. 14. (a) Experimental setup for the transmission measurement of the tellurite NCF; (b) measured and calculated transmission spectra.
The OSA in Fig. 15(a) was then replaced by a CCD camera in order to observe the mode field intensity distribution at 3.1 and 3.4 µm as shown in Fig. 15(a). The result in Fig. 15(b) shows that a fundamental-like mode was propagated in the fiber core at the peak of the high transmission band at 3.4 µm. In contrast, the mode field profile was not observed in the fiber core at 3.1 µm. Consequently, the transmission at that wavelength was drastically reduced.
Fig. 15. Experimental setup to investigate mode field intensity distribution of a 17-cm-long fabricated tellurite NCF at 3.1 and 3.4 µm.
3.2 Polarization properties
Finally, the polarization properties of the tellurite NCF were measured. The experimental setup similar to Fig. 14(a) was employed. In addition, two identical linear polarizers (LPMIR050-MP2) were inserted before and after a 17-cm-long fiber section as illustrated by (LP1) and (LP2) in Fig. 16(a). The linear polarizer LP1 was fixed. Only the linear polarizer LP2 was rotated and the corresponding intensity variation was normalized and plotted in Fig. 16(b). The wavelength of the input light was 2.1 µm. The intensity of the transmitted light has the peaks at 0, 180 and 360 degree. At these angles, a fundamental-like mode field was observed. When LP2 was at perpendicular angles (90 and 270 degree) as compared to LP1, the intensity reduced to a minimum value and the mode field was not able to be observed as shown in Fig. 16(b). The image of the mode field at 90 and 270 degree was captured and shown in Fig. 17. Compared to the calculated result in Fig. 17, this mode field can be considered as the 1st order mode.
Fig. 16. (a) Experimental setup to study polarization properties of the fabricated tellurite NCF at 2.1 µm. (b) The dependence of LP2 rotation angle on the output intensity after a 17-cm-long fiber.
Fig. 17. Measured mode field intensity distribution when LP2 in Fig. 16 is at 90 and 270 degree and the calculated mode field of the 1st order mode.
4. Conclusions
The tellurite NCF which has 6 non-touching air holes in the cladding was studied in this work. By numerical analysis, it is realized that the confinement loss in the core will be high when the gap between two nearby cladding air-holes is large or when they connect to each other. When the diameter ratio between the central air-core and cladding air-hole diameter is about 0.5, low confinement loss can be obtained. A successful fiber fabrication was carried out and a tellurite NCF with 6 non-touching cladding air holes was demonstrated experimentally for the first time. Although its transmission range expanded up to 3.9 µm due to the current operating range of the laser source, it is expected to extend to the mid-IR range around 6 µm as shown in our calculated results. When the input light at 2.1 µm was linearly polarized, the measured result shows that it can be maintained in the fiber because the fundamental mode is dominant and the 1st order mode was found with very weak coupled power.
Funding
The JSPS-CNRS joint research program; Japan Society for the Promotion of Science (Kakenhi 15H02250, Kakenhi 17K18891, Kakenhi 18H01504).
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