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Demonstration of CO2-laser power delivery through chalcogenide-glass fiber with negativecurvature hollow core Alexey F. Kosolapov,1 Andrey D. Pryamikov,1 Alexander S. Biriukov,1 Vladimir S. Shiryaev,2 Maxim S. Astapovich,1 Gennady E. Snopatin,2 Victor G. Plotnichenko,1 Mikhail F. Churbanov,2 and Evgeny M. Dianov1 2

1 Fiber Optics Research Center RAS, 38 Vavilov Str., 119333 Moscow, Russia Institute of Chemistry of High-Purity Substances of RAS, 49 Tropinin Str., 603950 Nizhny Novgorod, Russia [email protected]

Abstract: A technologically simple optical fiber cross-section structure with a negative-curvature hollow-core has been proposed for the delivery of the CO2 laser radiation. The structure was optimized numerically and then realized using Te20As30Se50 (TAS) chalcogenide glass. Guidance of the 10.6 µm СО2-laser radiation through this TAS-glass hollow-core fiber has been demonstrated. The loss at λ=10.6 µm was amounted ~11 dB/m. A resonance behavior of the fiber bend loss as a function of the bend radius has been revealed. ©2011 Optical Society of America OCIS Codes: (060.2280) Fiber design and fabrication; (060.4005) Microstructured fibers; (060.2400) Fiber properties; (060.2390) Fiber optics, infrared.

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Received 30 Sep 2011; revised 17 Nov 2011; accepted 21 Nov 2011; published 1 Dec 2011

5 December 2011 / Vol. 19, No. 25 / OPTICS EXPRESS 25723

14. L. G. Aio, A. M. Efimov, and V. F. Kokorina, “Refractive index of chalcogenide glasses over a wide range of compositions,” J. of Non.-Crys Solids 27, 299–307 (1978). 15. G. E. Snopatin, V. S. Shiryaev, V. G. Plotnichenko, E. M. Dianov, and M. F. Churbanov, “High-purity chalcogenide glasses for fiber optics,” Inorg. Mater. 45(13), 1439–1460 (2009). 16. V. S. Shiryaev, J.-L. Adam, X. H. Zhang, C. Boussard-Plédel, J. Lucas, and M. F. Churbanov, “Infrared fibers based on Te-As-Se glass system with low optical losses,” J. Non-Cryst. Solids 336(2), 113–119 (2004). 17. L. Brilland, J. Troles, P. Houizot, F. Désévédavy, Q. Coulombier, G. Renversez, T. Chartier, T. N. Nguyen, J.-L. Adam, and N. Traynor, “Interface impact on the transmission of chalcogenide photonic crystal fibres,” J. Ceram. Soc. Jpn. 116(1358), 1024–1027 (2008). 18. W. A. Gambling, H. Matsumura, and C. M. Ragdale, “Curvature and microbending losses in single - mode optical fibres,” Opt. Quantum Electron. 11(1), 43–59 (1979). 19. G. Renversez, P. Boyer, and A. Sagrini, “Antiresonant reflecting optical waveguide microstructured fibers revisited: a new analysis based on leaky mode coupling,” Opt. Express 14(12), 5682–5687 (2006).

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 aircore modes and the cladding modes. These wavelength regions are described by the ARROW

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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.

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 dins and dout (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 dins/dout = 0.8 and 0.85 for two values of Dcore = 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 Dcore = 260 µm and with the value of ratio dins/dout = 0.8 (solid) and dins/dout = 0.85 (dashed) (b) the same dependencies for an HC MOF with an air core diameter Dcore = 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 Dcore = 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 Dcore = 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 Dcore one obtains a weaker coupling with the cladding modes with a high azimuthal dependence. Based on the results obtained, we produced several fibers with Dcore ~380 µm and dins/dout ~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 -