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Abstract
Tellurium (Te)-based suspended-core fiber (SCF) consisting of dual chalcogenide glasses has been fabricated for the first time using a combined extrusion method. The SCF transmitting ability was evaluated by the simulation of the beam spots, energy distribution and ratio of core/fiber. The fiber was tapered into a small core size of 5 µm and the corresponding zero-dispersion-wavelength (ZDW) was 4.9 µm. The nonlinear coefficient γ was 3.983 m−1W−1 at 5 μm. Mid-infrared (MIR) supercontinuum (SC) spanning from 1.7 µm to 11.3 µm was achieved in a 20 cm Te-based complex SCF, pumped at 5 µm with an average power of 25 mW. The strong SC generated in the long wavelength was ascribed to the excellent transmitting ability in the SCF.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
During the last decade, plenty studies have been devoted to supercontinuum (SC) generation in the mid-infrared (MIR) because of its potential applications, such as chemical and biomedical sensing, military applications and materials processing [1]. To achieve broad SC spectra, it is essential to limit the light in a restricted geometry. For example, MIR SC generations have been reported in chalcogenide waveguides [2, 3] as well as in chalcogenide fibers [4–8]. Xin et al. have reported an SC spectrum in As2S3 waveguide [9] and Yi et al. demonstrated linearly polarized 2-10 μm SC generation in Ge-As-Se waveguide [10]. SC generations covering 1.5-7 μm [4] and 1.4-13.3 μm [6] have been demonstrated in As-S and As-Se step-index fibers. 1.5-14 µm SC generation in a Ge20As20Se15Te45 step-index fiber and 2.0-16 μm SC generation in a (Ge10Te40)90-(AgI)10 step-index fiber [11, 12] have also been reported. Compared with S- or Se-based glasses, Te-based chalcogenide glasses are the ideal candidates for MIR SC generation since they have the highest optical nonlinearity and broadest transmission window due to the high atomic weight of Te [13, 14]. However, because of the material dispersion of Te-based glasses is generally over 10.5 µm [11], it is highly desired to shift the zero-dispersion-wavelength (ZDW) into a shorter wavelength, enabling SC spectra generated by cheap and commercial available fiber lasers near the ZDW.
Compared with the normal step-index fiber, suspended-core fiber (SCF, a special microstructure fiber which supports a small core surrounded by several large air holes) provides another way to tune chromatic dispersion and enhance nonlinearity of the fibers [15]. For example, SC generation has been demonstrated in As-S-based SCF [16–18], but the broadening of the SC spectrum is limited by strong multi-phonon absorption in long-wave infrared. Recently, Møller et al. [19] reported MIR SC generations in a low-loss As-Se SCF with a zero-dispersion wavelength (ZDW) of 3.5 μm. Cheng et al. [20] investigated MIR SC generations in a novel AsSe/As-S hybride SCF with a ZDW of 3.38 μm. However, until now, no Te-based SCFs have been shown yet, since it is challenging to prepare low-loss Te-based chalcogenide SCFs due to the tendency for crystallization. Nevertheless, taking the high nonlinear optical properties of Te-based glass and the ability of tunable chromatic dispersion of SCF structure into account, Te-based SCF should be one of the ideal candidates for mid-infrared SC generation.
In this paper, we employed innovation technique to prepare Te-based SCF for the first time. The complex SCF, fabricated by extrusion, had a well-defined four-hole structure. The core was Ge20As20Se15Te45 glass and the supporting frame was Ge20Sb5Se75 glass. The ZDW of the Te-based SCF could be blue-shifted to 4.9 μm by tapering. The SCF showed higher nonlinearity in a wide wavelength range. A MIR SC covering 1.7- 11.3 μm was achieved in a 20 cm-long SCF pumped by 5 μm femtosecond laser.
2. Experiments
Previous reports have shown that glasses with similar transition temperatures (Tg) can be easily co-extruded [21]. Generally, the extrusion process can be performed at lower temperature, which is in contrast with fiber drawing process that is usually done at higher temperature. Therefore, the tendency of crystallization, the appearance of the bubbles and the possible contaminations that could appear in the process of fiber drawing can be suppressed in the extrusion process [22]. Ge20Sb5Se75 glass had a Tg of 187 °C that is almost identical to Ge20As20Se15Te45 with a Tg of 195 °C [23]. Thus, the Ge20As20Se15Te45 glass was chosen as the core of the SCF and the nontoxic Ge20Sb5Se75 glass as the supported ring and outer material. Chalcogenide glasses were prepared from purified raw materials by the melt-quenching method. Especially, the core glasses were purified using Mg to remove oxide impurities.
The preform consists of a 46 mm diameter cladding and a 9 mm diameter core glass rod with a cladding-core ratio of near 5:1. The thin core was extruded out with four thin walls growing to touch the outer solid cladding ring. Finally, a SCF with a core diameter of 40 μm and a cladding diameter of 280 μm was obtained in a home-made fiber drawing tower under the protection of nitrogen atmosphere. The schematic of the preform drawing process and the cross-section image of the preform were shown in Fig. 1. It can be seen clearly that, the core glass is different from the supporting ring. No crystallization or chemical reaction was found during the fiber-drawing process. The as-prepared SCF was used for optical measurement and SC generation. The pump laser was the femtosecond OPA system described in reference [11].
Fig. 1 The schematic diagram of the extrusion process and the extruded preform (the black ring is GeSbSe and the gray core is GeAsSeTe).
3. Results and discussions
The refractive indices of the core glass were measured by an IR ellipsometer (IR-VASE MARK II, J. A. Woollan Co.). The Ge20As20Se15Te45 glass shows high values of refractive indices, around 3.14 in IR region, as shown in Fig. 2(a).
The fiber was cut to examine whether the structure was destroyed or not during the fiber drawing process. The cross-section image of the fiber was recorded by an optical microscope with a hyper focal depth of view (Keyence, VHX-1000), as shown in Fig. 3(a) where the slight damage of the surface was caused by the fiber cut. Such a damaged surface cannot be improved by polishing since the fiber is too fragile, and any particles dropping into the hole of the fiber could lead to contamination of the fiber. Visual inspection showed that the suspended-core fiber structure was almost same as the original design containing 4 holes around a solid core as shown in Fig. 3(a), and such a structure generally kept almost identical over more than 10 m of fiber. The diameter of the black holes and the whole fibers in Fig. 3(a) was ~80 µm and ~280 µm, respectively. The core within the red rectangle in Fig. 3(a) was suspended by four ~80 µm long supporting bridges that were nearly 2 µm in the thinnest parts. The red part in Fig. 3(a) was further examined by a scanning electron microscope (SEM, Tescan VEGA 3 SBH) and the corresponding image was shown in Fig. 3(b). The diameter of the core diameter was defined as the yellow circle in Fig. 3(b), being ∼40 μm. The widths of these bridges gradually decreased to less than 2 µm, and the averaged width was 6 µm. Those supporting bridge allowed the solid core region to guide light and isolate the core from the outer solid regions of ring in the fiber.
Fig. 3 Cross section of the four-hole Te-based complex SCF: (a) Optical microscope image (500X), (b) SEM.
As it was hard to couple the FTIR light into the tiny core with IR lens and measure the fiber loss, here the optical loss was evaluated from a capillary which was drawn from the structured preform, with a core size of ~100 μm, as shown in Fig. 2(b). Obviously, this suspended core capillary shows a bit higher loss than the step-index fiber reported in reference [11]. Then, we employed the full vectorial finite element method (FEM) to simulate energy distribution of laser beam in SCF with a core of 40 µm and an ideal bridge width of 1 µm, 6 µm, 9 µm, 12 µm and 18 µm, respectively. In order to figure out the light-transmitting ability of the SCF, we defined a ratio R of light energy as the ratio of energy distribution in the core to that in the whole fiber.
Here, the energy intensity can be figured out from the grey values of each pixel in the beam profile images, and all the light intensity in the fiber were collected pixel by pixel. is the light intensity of each point and is the mean light intensity as confined in the fiber core, is the light intensity of each point and is the mean light intensity as laser energy distribution in the whole fiber cross-section, Acore is the area of the fiber core, Afiber is the area of the whole fiber cross-section. ∆ is the number of pixels per unit imaging area.Given an average bridge width of 6 μm, as shown in Fig. 3, the energy distribution of the fiber was numerical simulated by the COMSOL software. These results are presented in Fig. 4(a), where the curve line means the tendency of R and the insets are the 3-D profiles of the transmission light beam at wavelengths of 1.5, 3.5, 6.5, 7.5, 8.5 and 11.5 µm, respectively. When the light wavelength is longer than 7.5 µm (larger than the bridge width of SCF), the value of R is stable above 0.9, indicating that the light energy is well confined in the SCF core and this fiber is more suitable for working at long wavelengths (larger than 7.5 µm). We define the knee point wavelength of the curve as the cutoff wavelength (λc). Figure 4(b) shows the relation between λc and the bridge width. It was found that, λc redshifts with the increase of bridge width. So the operating wavelength of this SCF can be changed by adjusting the bridge width. However, in the practical point of view, it is hard to prepare SCF with a bridge width less than 6 μm, as the thin bridge would be broken easily during the extrusion process. Therefore, in this paper, SCF with a bridge width of 6 μm was prepared and its SC generation was recorded too.
Fig. 4 (a) Calculated energy ratio R in the SCF with wavelength increasing, inset: The 3-D profiles of the numerical simulated beam spots in the Ge20As20Se15Te45 SCF at typical wavelengths, (b) The cut-off wavelength of the SCF along with the width of bridges.
It is well known that, microstructured fiber can offer more flexibility to engineer the dispersion profile in fiber [24]. In this paper, flatter dispersion profile of the SCF was achieved via increasing waveguide dispersion to compensate material dispersion. Moreover, the SCF was further tapered to the core diameter of 5 µm and 15 µm. The dispersion of the SCF with different core diameter was analyzed by commercial software (Lumerical MODE Solution) with full-vectorial mode-solver. Figure 5(a) shows the chromatic dispersion curve of the fundamental mode in those fibers. The ZDW in the SCF with a core diameter of 40 µm shifts significantly towards shorter wavelength of 7.7 µm compared with the bulk Ge20As20Se15Te45 glass with a ZDW at 10.5 µm, and the ZDW of the tapered SCF shifts to 7.2 µm and 4.9 µm for the core size of 15 µm and 5 µm, respectively. This indicates that either suspended-core structure or smaller core diameter can shift the ZDW towards shorter wavelength effectively.
Fig. 5 (a) Calculated dispersions for the fundamental mode of the Te-based complex SCF with different diameter and normal step-index fiber. Inset: surface of tapered SCF (Outer cladding diameter is 13 µm and 100 µm); (b) Calculated Aeff and γ of the fundamental mode.
High nonlinear coefficients (γ) and low mode areas (Aeff) of a fiber are beneficial directly for potential SC generation. We simulated the Aeff and γ of a tapered SCF (core diameter of 5 µm) by employing the FEM under different laser wavelengths from 1 µm to 14 µm based on Ge20As20Se15Te45 glass, and the results were shown in Fig. 5(b). It was found that effective mode area increased but the nonlinear coefficient decreased with increasing wavelength. This SCF shows high nonlinearity in the whole wavelength range. The typical value of γ is 3.983 m−1W−1 at 5 μm.
In order to maximize the pump power via difference frequency generation (DFG), shorter wavelengths (4.5 µm, 5 µm) close to ZDW were chosen to pump the fiber for SC generation. The MIR pump source started from a Ti: Sapphire mode-locked seed laser (Coherent Mira 900), the seed pulses possessed spectrum bandwidth of 12 nm at 800 nm, then were delivered to a Coherent Legend pulse picker regenerative amplifier for boosting the pulse energy to about 1 mJ at a low repetition rate of 1 kHz. The amplified signals were collinearly combined together and passed through a DFG unit to generate a MIR pulse tunable from 1 µm to 12 μm and with a pulse width of ∼150 fs. The signal OPA laser beam were free-space-coupled into a 20 cm-long Te-SCF via a ZnSe lens, the SC signals were detected by a grating monochromator and a HgCdTe (MCT) detector.
The spectral behavior of the SC was investigated by changing the parameters of pump wavelength and pump power. Figure 6(a)(i) shows the obtained SC pumped by a beam with a wavelength of 4.5 μm and a laser power of 15 mW. The SC covers a range of 9 μm (from 1.7 μm to 10.7 μm) at a flatness of 30 dB. As shown in Fig. 6(a)(ii), when the pump wavelength red shifts to 5 μm which is located at the anomalous but close to ZDW dispersion region, the SC spans up to over 9.4 μm (from 1.7 μm to 11.1 μm). It is amazing to find out that the SC is stronger in the red-shifting part, although it is far from the pumping laser center (4.5 µm or 5 µm). This could be due to the fact that that the light could be well confined in the core and transmitted effectively in the complex SCF when the wavelength is longer than 8 µm as claimed in Fig. 4. Although the efficiency of SC generation is reduced with increasing wavelength, the excellent transmission of the SCFs makes SC stronger at the longer wavelength.
Fig. 6 Experimental SC results in the SCFs pumping: (a) at different wavelengths; (b) with increased power; (c) numerical simulated result with 5 μm pump at 5 kW (peak power), γ = 3.983 m−1W−1, 20 cm (fiber length).
Then we pumped the complex SCF at a wavelength of 5 μm with different pump powers. Figure 6(b) shows the variation of SC when the pump power is changed from 5 mW (33 kW peak power) to 25 mW (165 kW peak power), the broadest of the SC with a range of 9.6 μm (from 1.7 μm to 11.3 μm) is obtained. When the fiber is pumped at a relatively low power of 5 mW, the SC spanning is very weak. As the pump power increases, the spectrum becomes broad. We can see that there is no any minimum power band around 4 µm, 4.5 µm and 5 µm, because all these wavelengths are near the pump wavelength, and the SC spectra broaden cross those absorption bands easily. Further, SC generation was simulated in a 20 cm-long fiber pumped at 5 μm with a peak power of 5 kW and γ = 3.983 m−1W−1, as shown in Fig. 6(c). Due to its high nonlinearity, a broadband SC generation covering 1.5-11.5 μm can be easily excited by only 5 kW peak power in the SCF. So we can estimate that the couple efficiency in this case is about 3%. Comparing with the previous SC results in the reference [11], we found that the SC differences between Ge20As20Se15Te45 step-index fiber and the SCF are just decided by the cutoff wavelength which in turn came from the structure of fiber cross-section, and thus the bridge width shows much effect on the SC generation, shape and flatness.
4. Conclusions
In conclusion, a complex Te-based chalcogenide SCF with a perfect four-hole structure has been fabricated successfully via the extrusion method for the first time. No crystallization has been observed during fiber production. A ZDW of 7.7 μm can be obtained in the SCF with a 40 μm-core diameter, and this can decrease to 4.9 μm in the SCF with a 5 μm-core diameter. The nonlinear coefficient γ is as high as to 3.983 m−1W−1 at 5 μm. We simulated the laser pulse transmitting modes in the core at different wavelengths and found the details of beam spot shape and energy distributions between fiber core and supporting frame, which can be used to determine the bridge width dependence of the cutoff wavelength in SCFs. For a SCF with a bridge width of 6 μm, the fiber is suitable for laser transmission when its wavelength is longer than λc of 7.5 µm. The SC spectral behavior was investigated by changing the pump wavelength and power. The broadest SC spectrum covering 1.7 µm to 11.3 µm was generated from a 20 cm Te-based complex SCF pumped by a 5 μm OPA laser with an average power of 25 mW.
Funding
Natural Science Foundation of China (Grant Nos. (Grant Nos. 61705091, 61775109, 61627815 and 61377099); Zhejiang Provincial Natural Science Foundation of China (Grant No. LR18F050002); 3315 Innovation Team in Ningbo City, Zhejiang Province, China; the Opening Project of Key Laboratory of Optoelectronic Detection Materials and Devices of Zhejiang Province, China (Grant No. 2017004); Program for Science and Technology of Jiaxing, China (Grant No. 2017AY13010); and the K. C. Wong Magna Fund in Ningbo University, China.
References and links
1. S. D. Jackson, “Towards high-power mid-infrared emission from a fibre laser,” Nat. Photonics 6(7), 423–431 (2012). [CrossRef]
2. M. R. E. Lamont, B. Luther-Davies, D.-Y. Choi, S. Madden, and B. J. Eggleton, “Supercontinuum generation in dispersion engineered highly nonlinear (γ = 10 /W/m) As2S3) chalcogenide planar waveguide,” Opt. Express 16(19), 14938–14944 (2008). [CrossRef] [PubMed]
3. X. Gai, R. P. Wang, C. Xiong, M. J. Steel, B. J. Eggleton, and B. Luther-Davies, “Near-zero anomalous dispersion Ge11.5As24Se64.5 glass nanowires for correlated photon pair generation: design and analysis,” Opt. Express 20(2), 776–786 (2012). [CrossRef] [PubMed]
4. F. Théberge, N. Thiré, J. F. Daigle, P. Mathieu, B. E. Schmidt, Y. Messaddeq, R. Vallée, and F. Légaré, “Multioctave infrared supercontinuum generation in large-core As2S3 fibers,” Opt. Lett. 39(22), 6474–6477 (2014). [CrossRef] [PubMed]
5. T. Cheng, K. Nagasaka, T. H. Tuan, X. Xue, M. Matsumoto, H. Tezuka, T. Suzuki, and Y. Ohishi, “Mid-infrared supercontinuum generation spanning 2.0 to 15.1 μm in a chalcogenide step-index fiber,” Opt. Lett. 41(9), 2117–2120 (2016). [CrossRef] [PubMed]
6. C. R. Petersen, U. Moller, I. Kubat, B. Zhou, S. Dupont, J. Ramsay, T. Benson, S. Sujecki, N. Abdel-Moneim, Z. Tang, D. Furniss, A. Seddon, and O. Bang, “Mid-infrared supercontinuum covering the 1.4-13.3 μm molecular fingerprint region using ultra-high NA chalcogenide step-index fibre,” Nat. Photonics 8(11), 830–834 (2014). [CrossRef]
7. B. Zhang, W. Guo, Y. Yu, C. C. Zhai, S. S. Qi, A. P. Yang, L. Li, Z. Y. Yang, R. P. Wang, D. Y. Tang, G. M. Tao, and B. Luther-Davies, “Low Loss, High NA Chalcogenide Glass Fibers for Broadband Mid-Infrared Supercontinuum Generation,” J. Am. Ceram. Soc. 98(5), 1389–1392 (2015). [CrossRef]
8. R. R. Gattass, L. B. Shaw, V. Q. Nguyen, P. C. Pureza, I. D. Aggarwal, and J. S. Sanghera, “All-fiber chalcogenide-based mid-infrared supercontinuum source,” Opt. Fiber Technol. 18(5), 345–348 (2012). [CrossRef]
9. X. Gai, D. Y. Choi, S. Madden, Z. Yang, R. Wang, and B. Luther-Davies, “Supercontinuum generation in the mid-infrared from a dispersion-engineered As2S3 glass rib waveguide,” Opt. Lett. 37(18), 3870–3872 (2012). [CrossRef] [PubMed]
10. Y. Yu, X. Gai, P. Ma, K. Vu, Z. Yang, R. Wang, D. Y. Choi, S. Madden, and B. Luther-Davies, “Experimental demonstration of linearly polarized 2-10 μm supercontinuum generation in a chalcogenide rib waveguide,” Opt. Lett. 41(5), 958–961 (2016). [CrossRef] [PubMed]
11. Z. Zhao, X. Wang, S. Dai, Z. Pan, S. Liu, L. Sun, P. Zhang, Z. Liu, Q. Nie, X. Shen, and R. Wang, “1.5-14 μm midinfrared supercontinuum generation in a low-loss Te-based chalcogenide step-index fiber,” Opt. Lett. 41(22), 5222–5225 (2016). [CrossRef] [PubMed]
12. Z. Zhao, B. Wu, X. Wang, Z. Pan, Z. Liu, P. Zhang, X. Shen, Q. Nie, S. Dai, and R. Wang, “Mid-infrared supercontinuum covering 2.0-16 μm in a low-loss telluride single-mode fiber,” Laser Photonics Rev. 11(2), 1700005 (2017). [CrossRef]
13. T. Wang, X. Gai, W. H. Wei, R. P. Wang, Z. Y. Yang, X. Shen, S. Madden, and B. Luther-Davies, “Systematic z-scan measurements of the third order nonlinearity of chalcogenide glasses,” Opt. Mater. Express 4(5), 1011–1022 (2014). [CrossRef]
14. A. A. Wilhelm, C. Boussard-Pledel, Q. Coulombier, J. Lucas, B. Bureau, and P. Lucas, “Development of far-infrared-transmitting te based glasses suitable for carbon dioxide detection and space optics,” Adv. Mater. 19(22), 3796–3800 (2007). [CrossRef]
15. J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation in photonic crystal fiber,” Rev. Mod. Phys. 78(4), 1135–1184 (2006). [CrossRef]
16. M. El-Amraoui, G. Gadret, J. C. Jules, J. Fatome, C. Fortier, F. Désévédavy, I. Skripatchev, Y. Messaddeq, J. Troles, L. Brilland, W. Gao, T. Suzuki, Y. Ohishi, and F. Smektala, “Microstructured chalcogenide optical fibers from As2S3 glass: towards new IR broadband sources,” Opt. Express 18(25), 26655–26665 (2010). [CrossRef] [PubMed]
17. W. Gao, M. El Amraoui, M. Liao, H. Kawashima, Z. Duan, D. Deng, T. Cheng, T. Suzuki, Y. Messaddeq, and Y. Ohishi, “Mid-infrared supercontinuum generation in a suspended-core As2S3 chalcogenide microstructured optical fiber,” Opt. Express 21(8), 9573–9583 (2013). [CrossRef] [PubMed]
18. O. Mouawad, J. Picot-Clémente, F. Amrani, C. Strutynski, J. Fatome, B. Kibler, F. Désévédavy, G. Gadret, J. C. Jules, D. Deng, Y. Ohishi, and F. Smektala, “Multioctave midinfrared supercontinuum generation in suspended-core chalcogenide fibers,” Opt. Lett. 39(9), 2684–2687 (2014). [CrossRef] [PubMed]
19. U. Møller, Y. Yu, I. Kubat, C. R. Petersen, X. Gai, L. Brilland, D. Méchin, C. Caillaud, J. Troles, B. Luther-Davies, and O. Bang, “Multi-milliwatt mid-infrared supercontinuum generation in a suspended core chalcogenide fiber,” Opt. Express 23(3), 3282–3291 (2015). [CrossRef] [PubMed]
20. T. Cheng, Y. Kanou, X. Xue, D. Deng, M. Matsumoto, T. Misumi, T. Suzuki, and Y. Ohishi, “Mid-infrared supercontinuum generation in a novel AsSe2-As2S5 hybrid microstructured optical fiber,” Opt. Express 22(19), 23019–23025 (2014). [CrossRef] [PubMed]
21. S. D. Savage, C. A. Miller, D. Furniss, and A. B. Seddon, “Extrusion of chalcogenide glass preforms and drawing to multimode optical fibers,” J. Non-Cryst. Solids 354(29), 3418–3427 (2008). [CrossRef]
22. C. Jiang, X. Wang, M. Zhu, H. Xu, Q. Nie, S. Dai, G. Tao, X. Shen, C. Cheng, Q. Zhu, F. Liao, P. Zhang, P. Zhang, Z. Liu, and X. Zhang, “Preparation of chalcogenide glass fiber using an improved extrusion method,” Opt. Eng. 55(5), 056114 (2016). [CrossRef]
23. Z. Yang, T. Luo, S. Jiang, J. Geng, and P. Lucas, “Single-mode low-loss optical fibers for long-wave infrared transmission,” Opt. Lett. 35(20), 3360–3362 (2010). [CrossRef] [PubMed]
24. J. M. Dudley and J. R. Taylor, “Ten years of nonlinear optics in photonic crystal fibre,” Nat. Photonics 3(2), 85–90 (2009). [CrossRef]
