1. Introduction

At present, CO₂-lasers remain the most required laser type for applications in the technology, medicine and certain special fields. In these applications, the CO₂-laser radiation is, as a rule, transmitted through the air using a complicated system of mirrors. In recent years, however, more expensive high-power near-IR fiber and disk lasers began to expel CO₂-lasers, because, among other things, they allow the use of optical fibers for the delivery and manipulation of the laser beam. Therefore, the development of fibers capable of transmitting powerful mid-IR radiation would regenerate interest in CO₂-lasers.

By now, the optical loss in step-index chalcogenide glass fibers has been reduced to 1.5-2 dB/m at λ=10.6 µm owing to a reduction of the impurity As-O and Se-O bonds absorbing in this spectral region [1–3]. The maximum power transmitted through a 1-m length of such a fiber amounted to 10.7 W, when an anti-reflection coating and water cooling were used [1].

Better results have been achieved with hollow-core fibers, in which radiation power is transmitted through the air. A hollow-core polymer fiber with a cladding in the form of a multi-layer Bragg mirror demonstrated a loss below 1 dB/m at λ=10.6 µm [4]. It is known that near and mid - IR hollow-core microstructured optical fibers (HC MOF) may feature a much lower loss than the material from which the fiber is made [4, 5]. In addition, the advantages of the hollow core fibers as compared to step index fibers are low nonlinearity and flat dispersion. For example, the fiber created in [6] featured a loss of less than 1.2 dB/km at λ = 1.62 µm. However, although calculations [7] predict a loss of less than 1 dB/m at λ = 9.3 µm in microstructured fibers of the TAS glass and although the feasibity of microstructured chalcogenide fibers has been demonstrated many times [7–9], successful fabrication of a hollow-core microstructured optical fiber capable of transmitting the CO₂-laser radiation, as far as we know, has not yet been reported.

In this paper, we propose a technologically simple HC MOF design which is based on our previous work [5]. The HC MOF is formed by eight contiguous capillaries, the core boundary being of negative curvature. The term ‘negative curvature’ means that the surface normal to the core boundary is oppositely directed with a radial unit vector in a cylindrical coordinate system. We succeeded in fabricating such a fiber from a chalcogenide glass of the Te-As-Se system. The transmission of the CO₂ laser radiation in such a fiber was obtained and the minimal fiber loss at λ~10.6 µm proved to be about 11 dB/m.

2. Fiber modeling and fabrication

The HC MOF with a negative curvature of the core boundary considered in this paper (Fig. 1 ) belongs to the type of HC MOFs that do not support photonic band gaps [5]. Such HC MOFs have a relatively high transmission loss in comparison with photonic band gap HC MOFs, but possess a larger bandwidth. The latter is due to the fact that the air-core localized modes are only weakly coupled with the cladding modes in the low loss wavelength regions. The high loss wavelengths in the transmission spectrum correspond to the avoided crossing of the air-core modes and the cladding modes. These wavelength regions are described by the ARROW model [10]. Authors of [7] computed the core diameters for the chalcogenide HC MOF meeting the anti-resonant condition. In the case of the structure under consideration (Fig. 1) the ARROW model is also applicable because the capillary radius is much larger than the operating wavelength [11]. Moreover, there exists an irregular spectral behavior of the leakage loss in the low loss regions. The latter effect is due to the weak coupling of the core modes with the dielectric modes having a high azimuthal dependence (modes with a high azimuthal index), the cut-off wavelengths of which fall in these spectral regions [11]. The same HC MOF type with a positive curvature of the core boundary and a simplified cladding structure has been considered in [12,13].

 

Fig. 1 A negative-core-curvature HC MOF with a cladding consisting of eight capillaries.

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The chalcogenide glass refractive index at λ = 10.6 µm was taken to be 2.9064 [14] and the material loss, 50 dB/m (the latter is a typical and well-reproducible value). The geometry parameters d_ins and d_out (Fig. 1) are much larger as compared to the source wavelength λ = 10.62 μm. This means that an individual capillary of the cladding possesses a high density of states of the leaky modes with a high azimuthal dependence and the loss spectrum must demonstrate an irregular behavior even in very narrow ranges of the low-loss spectral regions. To model the optimal design of the HC MOF, we used the FemLab 3.1 software. By means of the finite element method, we calculated the loss level of the fundamental HE₁₁ air-core mode for the fiber with d_ins/d_out = 0.8 and 0.85 for two values of D_core = 260 and 380 μm (Fig. 2 ).

 

Fig. 2 (a) The computed loss dependence of the HE₁₁ air-core mode on the wavelength for a HC MOF with an air core diameter D_core = 260 μm and with the value of ratio d_ins/d_out = 0.8 (solid) and d_ins/d_out = 0.85 (dashed) (b) the same dependencies for an HC MOF with an air core diameter D_core = 380 μm.

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The calculations were made in a narrow spectral region in the vicinity of the CO₂-laser wavelength used in our experiments. As was expected, in the case of D_core = 260 µm, the calculated loss spectrum is strongly inhomogeneous even in a spectral interval of ~8 nm and varies from 0.2 to ~1 dB/m (Fig. 2(a)). In the case of D_core = 380 µm, the loss level is lower and the wavelength dependence is much smoother (Fig. 2(b)). In our opinion, in the case of a bigger value of D_core one obtains a weaker coupling with the cladding modes with a high azimuthal dependence. Based on the results obtained, we produced several fibers with D_core ~380 μm and d_ins/d_out ~0.85.

The fiber preform was manufactured by the «stack and draw» technique from a substrate tube and eight capillaries. First, high-purity chalcogenide glass of As₃₀Se₅₀Te₂₀ composition was produced by chemical-distillation melt purification [15, 16]. The glass obtained had a low content of the limiting impurities: hydrogen - <2·10⁻⁶ mass%, oxygen - 2·10⁻⁵ mass%, carbon - 1·10⁻⁴ mass%, silicon - 5·10⁻⁵ mass% (as follows from laser mass spectroscopy and IR spectroscopy).

Next, we fabricated the substrate tube and the capillaries. The substrate tube with the outer diameter of 16 mm, the inner diameter of 11 mm, and the length of 120 mm was fabricated by centrifugal casting of the chalcogenide glass inside a vacuumized silica tube. The capillaries are usually drawn from a tube which is also fabricated by centrifugation. However, in this work, capillaries with the outer diameter of 3 mm and the inner diameter of 2.4 mm were prepared by the double crucible method from the chalcogenide glass melt [15]. For this, the TAS glass was placed in the cladding crucible, while the core crucible remained empty. The capillary wall thickness was determined by the geometrical dimensions of the double crucible die, the excess inert gas pressure over the melt, and the pressure inside the capillary. The elimination of centrifugation from the fabrication process of the capillaries allowed us to lower the probability of glass recrystallization and, thereby, to lower the fiber optical loss. The final preform was an assembly of the substrate tube and 8 capillaries forming a complete azimuthally symmetric layer of holes on the inner surface of the tube. The fiber was drawn at a conventional drawing tower at a temperature of 240°C. Some overpressure was maintained in the capillaries during fiber drawing to prevent their collapsing. The outer diameter of the obtained fiber was 750 µm, the core diameter of ~380 µm, and the capillary wall thickness of 13 µm (Fig. 3 ).

 

Fig. 3 Fiber cross-section photograph obtained with the help of an electron microscope.

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3. Results and discussion

The fiber loss spectrum was measured by the cut-back method on a Bruker IFS-113v Fourier Transform Infrared Spectrometer with a resolution of 0.1 cm⁻¹ (Fig. 4 (left)). In this experiment, light propagated not only through the air core but also through the substrate tube and the cladding capillaries. Therefore, the measured loss was the average of the loss in the air core and in the glass elements.

 

Fig. 4 Measured optical loss spectra of the chalcogenide microstructured hollow-core fiber in the region of the CO₂-laser radiation (left) (the square dot was measured with help of the CO₂ laser) and the CO₂-laser radiation intensity (in a.u.) distribution over the fiber core (right).

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Fiber pieces, up to ~1 m in length, were found to be sufficiently transparent in the spectral range 2-10 µm, the loss at λ = 10.6 µm being ~13.5 dB/m, which makes the СО₂-laser radiation transmission possible. Note that the TAS-glass microstructured hollow-core fibers first described in [7] were nontransparent.

We also investigated the distribution of the CO₂-laser radiation intensity over the fiber cross-section. In this experiment, a ZnSe lens was used to launch the radiation into the fiber, while the intensity distribution over the fiber output endface was observed with the help of an Electrophysics PV320 thermal imaging camera. We varied the fiber excitation conditions so as to excite preferentially either the hollow core, or the capillaries and the substrate tube. It was found that a much greater power can be transmitted through the core, which means that the light propagating through the glass owing to the total internal reflection experiences much stronger attenuation. The distribution in Fig. 4 (right) obtained under proper excitation conditions shows that the CO₂-laser radiation is well confined in the hollow core. The additional maxima around the major maximum are due to light propagation through the glass on the places of contact of the capillaries. It was also established that the propagation regime in the core was multimode. The loss of the air-core modes measured with help of the CO₂ laser was ~11 dB/m (Fig. 4 (left), square dot). The measured and calculated loss (Fig. 2(b)) in the vicinity of λ = 10.6 μm differ markedly. In our opinion, this difference is due to size and shape variations of the fiber cross section (Fig. 3) and also due to an increase in material loss after the drawing process [17].

Next, we investigated the influence of bending on the loss level of the fiber with D_core = 380 μm. The bending loss was calculated by the method described in [18]. The dependence of the fundamental air-core mode on bending radius is shown in Fig. 5 for two values of d_ins/d_out = 0.8 and 0.85. It is seen that the bending loss level is rather low, but there are several maxima in the loss dependence related to the resonance coupling of the air-core mode with the leaky modes propagating in the air holes of individual capillaries [19]. This assumption is confirmed by Fig. 6 . It is seen that at a bend radius R_bend = 40 cm for the fiber with d_ins/d_out = 0.8 some tunneling into the air region of an individual capillary occurs (Fig. 6 (left)). At R_bend = 37 cm, the tunneling becomes much stronger and a leaky air mode of an individual capillary is excited (Fig. 6 (right)). In addition, a leaky mode of the capillary wall is excited as well. This parasitic effect and methods to suppress it will be considered in a subsequent publication.

 

Fig. 5 Calculated dependence of the bending loss of the fundamental air-core mode on bending radius at λ = 10.6 μm for d_ins/d_out = 0.8 (solid line) and d_ins/d_out = 0.85 (dashed line). Inset shows the measured near-field pattern at a bending radius of 30 cm (the bending center is at the top).

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Fig. 6 The fundamental air-core mode distribution at λ = 10.6 μm for two values of the bend radius (left) R_bend = 40 cm and (right) R_bend = 37 cm at d_ins/d_out = 0.8.

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Lastly, we measured the effect of fiber bending to a radius as small as 30 cm. We observed only a slight perturbation of the output near-field pattern under such bending (Fig. 5 inset), which means that the fiber continued to propagate light.

4. Conclusion

For the first time, guidance of the 10.6 µm СО₂-laser radiation through a chalcogenide-glass microstructured hollow-core fiber has been demonstrated, the glass composition being As₃₀Se₅₀Te_20. A further improvement of the technology and design of such fibers is expected to yield a reduction of the optical loss to below 1 dB/m. Because light proved to be well localized in the core, such fibers hold much promise for the delivery of the СО₂-laser radiation.