Single-mode hollow-core photonic crystal fiber made ... - OSA Publishing

Single-mode hollow-core photonic crystal fiber made from soft glass X. Jiang,1,* T. G. Euser,1 A. Abdolvand,1 F. Babic,1 F. Tani,1 N. Y. Joly,1,2 J. C. Travers,1 and P. St.J. Russell1,2 1

Max-Planck-Institute for the Science of Light, Guenther-Scharowsky-Strasse 1/Bau 24, Erlangen 91058, Germany 2 Department of Physics, University of Erlangen-Nuremberg, Erlangen 91058, Germany *[email protected]

Abstract: We demonstrate the first soft-glass hollow core photonic crystal fiber. The fiber is made from a high-index lead-silicate glass (Schott SF6, refractive index 1.82 at 500 nm). Fabricated by the stack-and-draw technique, the fiber incorporates a 7-cell hollow core embedded in a highly uniform 6-layer cladding structure that resembles a kagomé-like lattice. Effective single mode guidance of light is observed from 750 to 1050 nm in a large mode area (core diameter ~30 µm) with a low loss of 0.74 dB/m. The underlying guidance mechanism of the fiber is investigated using finite element modeling. The fiber is promising for applications requiring single mode guidance in a large mode area, such as particle guidance, fluid and gas filled devices. ©2011 Optical Society of America OCIS codes: (060.2280) Fiber design and fabrication; (060.5295) Photonic crystal fibers.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

P. St. J. Russell, “Photonic crystal fibers,” Science 299(5605), 358–362 (2003). J. C. Knight, “Photonic crystal fibres,” Nature 424(6950), 847–851 (2003). P. St. J. Russell, “Photonic-crystal fibers,” J. Lightwave Technol. 24(12), 4729–4749 (2006). F. Benabid, “Hollow-core photonic bandgap fiber: new light guidance of new science and technology,” Philos. Trans. R. Soc. London Ser. A 364(1849), 3439–3462 (2006). R. F. Cregan, B. J. Mangan, J. C. Knight, T. A. Birks, P. St. J. Russell, P. J. Roberts, and D. C. Allan, “Singlemode photonic band gap guidance of light in air,” Science 285(5433), 1537–1539 (1999). F. Benabid, J. C. Knight, G. Antonopoulos, and P. St. J. Russell, “Stimulated Raman scattering in hydrogen-filled hollow-core photonic crystal fiber,” Science 298(5592), 399–402 (2002). F. Couny, F. Benabid, and P. S. Light, “Large-pitch kagome-structured hollow-core photonic crystal fiber,” Opt. Lett. 31(24), 3574–3576 (2006). D. G. Ouzounov, F. R. Ahmad, D. Müller, N. Venkataraman, M. T. Gallagher, M. G. Thomas, J. Silcox, K. W. Koch, and A. L. Gaeta, “Generation of megawatt optical solitons in hollow-core photonic band-gap fibers,” Science 301(5640), 1702–1704 (2003). F. Benabid, J. C. Knight, and P. St. J. Russell, “Particle levitation and guidance in hollow-core photonic crystal fiber,” Opt. Express 10(21), 1195–1203 (2002). T. G. Euser, M. K. Garbos, J. S. Y. Chen, and P. St. J. Russell, “Precise balancing of viscous and radiation forces on a particle in liquid-filled photonic bandgap fiber,” Opt. Lett. 34(23), 3674–3676 (2009). F. Benabid, F. Couny, J. C. Knight, T. A. Birks, and P. St. J. Russell, “Compact, stable and efficient all-fibre gas cells using hollow-core photonic crystal fibres,” Nature 434(7032), 488–491 (2005). A. Abdolvand, A. Nazarkin, A. V. Chugreev, C. F. Kaminski, and P. St. J. Russell, “Solitary pulse generation by backward Raman scattering in H2-filled photonic crystal fibers,” Phys. Rev. Lett. 103(18), 183902 (2009). J. Nold, P. Hölzer, N. Y. Joly, G. K. L. Wong, A. Nazarkin, A. Podlipensky, M. Scharrer, and P. St. J. Russell, “Pressure-controlled phase matching to third harmonic in Ar-filled hollow-core photonic crystal fiber,” Opt. Lett. 35(17), 2922–2924 (2010). N. Y. Joly, J. Nold, W. Chang, P. Hölzer, A. Nazarkin, G. K. L. Wong, F. Biancalana, and P. St. J. Russell, “Bright spatially coherent wavelength-tunable deep-UV laser source using an Ar-filled photonic crystal fiber,” Phys. Rev. Lett. 106(20), 203901 (2011). J. A. Savage, “Materials for infrared fibre optics,” Mater. Sci. Eng. Rep. 2(3), 99–137 (1987). M. C. J. Large, A. Argyros, F. Cox, M. A. van Eijkelenborg, S. Ponrathnam, N. S. Pujari, I. M. Bassett, R. Lwin, and G. W. Barton, “Microstructured polymer optical fibres: new opportunities and challenges,” Mol. Cryst. Liq. Cryst. 446(1), 219–231 (2006). H. Ebendorff-Heidepriem, T. M. Monro, M. A. van Eijkelenborg, and M. J. C. Large, “Extruded high-NA microstructured polymer optical fiber,” Opt. Commun. 273(1), 133–137 (2007).

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Received 3 Jun 2011; revised 13 Jul 2011; accepted 13 Jul 2011; published 27 Jul 2011

1 August 2011 / Vol. 19, No. 16 / OPTICS EXPRESS 15438

18. T. M. Monro and H. Ebendorff-Heidepriem, “Progress in microstructured optical fibers,” Annu. Rev. Mater. Res. 36(1), 467–495 (2006). 19. V. V. Kumar, A. K. George, W. H. Reeves, J. C. Knight, P. Russell, F. Omenetto, and A. Taylor, “Extruded soft glass photonic crystal fiber for ultrabroad supercontinuum generation,” Opt. Express 10(25), 1520–1525 (2002). 20. J. Y. Y. Leong, P. Petropoulos, J. H. V. Price, H. Ebendorff-Heidepriem, S. A. Asimakis, R. C. Moore, K. E. Frampton, V. Finazzi, X. Feng, T. M. Monro, and D. J. Richardson, “High-nonlinearity dispersion-shifted leadsilicate holey fibers for efficient 1-µm pumped supercontinuum generation,” J. Lightwave Technol. 24(1), 183– 190 (2006). 21. H. Hundertmark, S. Rammler, T. Wilken, R. Holzwarth, T. W. Hänsch, and P. St. J. Russell, “Octave-spanning supercontinuum generated in SF6-glass PCF by a 1060 nm mode-locked fibre laser delivering 20 pJ per pulse,” Opt. Express 17(3), 1919–1924 (2009). 22. J. H. V. Price, T. M. Monro, H. Ebendorff-Heidepriem, F. Poletti, P. Horak, V. Finazzi, J. Y. Y. Leong, P. Petropoulos, J. C. Flanagan, G. Brambilla, X. Feng, and D. J. Richardson, “Mid-IR supercontinuum generation from nonsilica microstructured optical fibers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 738–749 (2007). 23. H. Ebendorff-Heidepriem, R. C. Moore, and T. M. Monro, “Progress in the fabrication of the next-generation soft glass microstructured optical fibers,” presented at the 1st Workshop on Specialty Optical Fibers and Their Applications, Sao Pedro, Brazil, 20–22 Aug. 2008. 24. H. Ebendorff-Heidepriem, S. C. Warren-Smith, and T. M. Monro, “Suspended nanowires: fabrication, design and characterization of fibers with nanoscale cores,” Opt. Express 17(4), 2646–2657 (2009). 25. J. S. Wang, E. M. Vogel, and E. Snitzer, “Tellurite glass: a new candidate for fiber devices,” Opt. Mater. 3(3), 187–203 (1994). 26. J. A. Harrington, Infrared Fibers and Their Applications (SPIE-The International Society for Optical Engineering, 2004). 27. X. Jiang, J. Lousteau, B. Richards, and A. Jha, “Investigation on germanium oxide-based glasses for infrared optical fibre development,” Opt. Mater. 31(11), 1701–1706 (2009). 28. R. H. Doremus, “Viscosity of silica,” J. Appl. Phys. 92(12), 7619–7629 (2002). 29. F. Désévédavy, G. Renversez, J. Troles, P. Houizot, L. Brilland, I. Vasilief, Q. Coulombier, N. Traynor, F. Smektala, and J.-L. Adam, “Chalcogenide glass hollow core photonic crystal fibers,” Opt. Mater. 32(11), 1532– 1539 (2010). 30. A. Argyros and J. Pla, “Hollow-core polymer fibres with a kagome lattice: potential for transmission in the infrared,” Opt. Express 15(12), 7713–7719 (2007). 31. T. G. Euser, G. Whyte, M. Scharrer, J. S. Y. Chen, A. Abdolvand, J. Nold, C. F. Kaminski, and P. St. J. Russell, “Dynamic control of higher-order modes in hollow-core photonic crystal fibers,” Opt. Express 16(22), 17972– 17981 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-22-17972. 32. JCMwave V.2.3.4.beta, JCMwave GmbH, Germany http://www.jcmwave.com/ 33. E. A. J. Marcatili and R. A. Schmeltzer, “Hollow metallic and dielectric waveguides for long distance optical transmission and laser,” Bell Syst. Tech. J. 43, 1783–1809 (1964).

1. Introduction Hollow-core photonic crystal fiber (HC-PCF) comes in two different varieties [1–4]. In the first, the periodically microstructured cladding supports a full two-dimensional photonic bandgap, permitting very low loss (1 dB/km at 1550 nm in the best case) over restricted bands of wavelength [5]. In the second, the periodic cladding is in the form of a kagomé lattice, and while it does not support a photonic bandgap it nevertheless provides relatively low loss guidance (a few tenths of dB/m in the best cases) over a very broad wavelength range [6,7]. The first we denote PBG-PCF and the second kagomé-PCF. Due to tight confinement of light in the hollow core, these fibers are ideal for applications such as high power laser pulse delivery [8], particle guidance [9,10] and studies of light-matter interactions [11–14]. Most HC-PCFs reported to date are made up from fused silica, a material that is well known for its good thermal stability and mechanical properties [15]. In particular, the gentle dependence of the viscosity on temperature has allowed the use of the highly flexible, and well-controlled stack-and draw technique [3]. Despite the advantageous properties of silica glass, there has been a growing interest in the fabrication of HC-PCFs using other materials such as polymers [16,17] and soft glasses [18– 24]. Soft glasses, or compound glasses, are defined as glasses which have lower melting and processing temperatures than those of silica [15,18]. Soft glasses can be classified into three categories: oxides, fluorides and chalcogenides [15,25,26]. There are four important advantages of soft glasses over silica: first, they allow transmission of light in the IR [15,18–26], a wavelength range that is of great interest for applications in mid-IR spectroscopy, LIDAR and material processing. For example, a typical AsGeSeTe-based chalcogenide glass has a transmission window between 4 and 11 µm. In

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contrast, transmission at wavelengths longer than 2.3 µm is inhibited in silica fibers by strong material absorption [25,26]. Second, their Kerr nonlinearity coefficient can be three orders of magnitude higher than that of silica glass (n2 ~1020 m2/W). For example, a typical chalcogenide glass has a nonlinear refractive index n2 ~1017 m2/W [25,26]. The combination of high values of n2 with control of group velocity dispersion has led to low-power supercontinuum (SC) generation in tellurite [19] and lead-silicate glass solid core PCFs [20,21]. Third, their rare-earth ion solubility is orders of magnitude higher than that of silica glass, which is of interest for high gain fiber amplifiers [15,26]. Finally, their typical melting temperature is in the 200-1000°C range (depending on glass composition), much lower than that of silica (1800-2300°C), potentially facilitating the fabrication process. Despite these advantages, fabrication of soft glass PCFs has not proved straightforward, for two main reasons. First, the viscosity changes rapidly with temperature, limiting the fiber drawing range to less than 50°C [27] – for fused silica it is a few hundred degrees [28]. As a consequence, small temperature fluctuations can result in dramatic viscosity changes that cause large structural distortions in soft glass PCFs [18–24]. Second, some soft glasses such as heavy metal oxides, fluorites and chalcogenides show thermal instability and crystallization during heating. To avoid crystallization, the entire fabrication process must take place in an inert gas environment. As a result of these difficulties, successful fabrication of soft glass PCFs has been limited to a few different glass compositions such as TeO2-ZnO-Na2O based tellurite [19] and SiO2-PbO based lead-silicate glasses [20,21]. Furthermore, light-guidance has only been demonstrated in solid-core soft glass PCFs, where the behavior during the drawing process is intrinsically more stable than that of HC-PCF. Recently a chalcogenide soft glass HC-PCF was reported by Desevedavy et al [29]. However, despite the relatively uniform cladding structure, no optical guidance was observed. In this paper, we report the first guiding soft-glass HC-PCF. The fiber has a 7-cell hollow core surrounded by a highly uniform cladding consisting of six kagomé-like cladding layers. The glass used is a high-index lead silicate glass (Schott SF6, refractive index 1.82 at 500 nm wavelength). The fiber demonstrates broadband single mode guidance in the visible and NIR spectral regions with a loss of less than 1 dB/m. Although higher order modes are guided, they have very much higher loss (much higher than in silica-based kagomé PCFs), rendering the fiber effectively single-mode. 2. Fabrication of SF6 HC-PCFs The SF6 glass HC-PCFs were fabricated using the well-known stack-and-draw technique. It has generally been believed that this technique is not suitable for fabrication of soft glass PCFs, mainly due to the difficulty of controlling the microstructure during drawing [20]. By careful optimization of the fabrication procedure, however, we have been able to minimize temperature fluctuations in the furnace and prevent devitrification of the glass. As a result, structural distortions in the fibers are almost completely eliminated. In our fabrication procedure, uniform heating of the stack is extremely important. Therefore a custom-designed spiral-shaped heating element was used to provide uniform heat distribution inside the furnace. Other improvements include relatively fast feeding (~6 mm/min) and slow drawing (~1.9 m/min) rates, which reduce the residence time of the fiber preform in the hot zone of the furnace to less than one minute. Within this time, the glass preform reaches a low enough viscosity to be drawn to fiber, while staying below the crystallization temperature. The pitch, core diameter and air-filling fraction (AFF) of the fiber are controlled by separately pressurizing the core and cladding holes during drawing. The drawing temperature is 610 ± 10°C, corresponding to a viscosity range of 10 5.5 to 105.9 Pa.s for the SF6 glass. By carefully tuning the drawing parameters, three SF6 HC-PCFs with different AFFs and pitch sizes were fabricated. Figure 1 shows scanning electron micrographs (SEMs) of the three fibers. Their slightly different structural parameters and transmission properties are summarized in Table 1. All of the fibers have a 7-cell defect core and an AFF over 85%. Interestingly, after using the improved drawing process, the structural uniformity of these SF6 glass HC-PCFs is #148672 - $15.00 USD

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comparable to that of fused-silica HC-PCFs. The small interstitial triangular gaps originate from the gaps between circular capillaries during stacking. The effect of these gaps on the guidance mechanism of the fiber will be discussed later in the paper. In going from the first fiber (Fig. 1(a)) to the third fiber (Fig. 1(c)), the pitch increases from 7.2 to 8.8 µm, and the core diameter from 29.0 to 33.3 µm. The AFF ranges from 85.3% to 88.0%. Table 1. Structural Parameters of the SF6 HC-PCFs

Fiber 1 Fiber 2 Fiber 3

OD (µm)

Core (µm)

Pitch (µm)

AFF

195.5 200.5 207.0

29.2 29.0 33.3

7.2 7.6 8.8

85.3% 86.0% 88.0%

Peak positions (nm) 828 795 775

959 945 931

Fig. 1. SEMs of three SF6-glass HC-PCFs with pitch (a) Fiber 1: 7.2 µm; (b) Fiber 2: 7.6 µm and (c) Fiber 3: 8.8 µm. The air-filling-fraction ranges from 85.3% to 88.0%. (d)-(f) Enlarged SEM images of the core structure. Core diameters are (d) Fiber 1: 29.2 ± 0.7 µm; (e) Fiber 2: 29.0 ± 0.5 µm and (f) Fiber 3: 33.3 ± 1.1 µm.

3. Transmission spectra and loss measurement The fiber loss was measured using the cut-back technique. The light source was a broadband PCF-based supercontinuum (SC) source (500-2200 nm). The light from the SC source was coupled into the SF6 HC-PCF through a 4x objective mounted on a fiber launch stage. The light emerging from the output end was split into two, one beam going to a CCD camera and the other to an optical spectrum analyzer (OSA). A spatial filter is used to select light originating from the core only. Figures 2(a)–2(c) presents the experimental near-field intensity profiles of the core mode in Fibers 1, 2 and 3. The corresponding transmission and loss spectra are presented in Figs. 2(d)–2(f). By measuring the intensity profiles before and after the cleave, it was verified that only the fundamental core mode was excited. Two distinct loss peaks at 828, 959 nm for Fiber 1, 795, 945 nm for Fiber 2 and 775, 931 nm for Fiber 3, are observed. The variations in peak position can be attributed to changes in core diameter, pitch and AFF, which were controlled by applying different pressures during fiber drawing. The observed minimum losses in the Fibers 1, 2 and 3 are 0.84, 0.74 and 0.98 dB/m respectively. We note that the minimum loss in each fiber remains below 1 dB/m, well below the bulk absorption value for SF6 glass (~2 dB/m), as expected for a hollow-core PCF. An unexpected experimental observation was the absence of any higher-order modes despite the relatively large core diameter (~30 µm). Similar effects have been reported in

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polymer kagomè fibers [30]. The fundamental mode loss in these structures is, however, rather high (10 dB/m). This effective single-mode guidance is in sharp contrast to a silica kagomé HC-PCF with similar structural parameters, which supports many higher order modes [31]. It seemed likely that higher order modes experience much higher losses in the SF6 HCPCFs. In order to test this we re-examined a shorter length of the fiber (23 cm). By careful adjustment of the coupling angle, we were indeed able to excite a weakly guiding LP 11 mode. Other higher order modes could not however be observed, even in this short length of fiber.

Fig. 2. (a)–(c) Experimental near-field profiles of guided mode and normalized intensity profiles taken through the mode centre. The lengths of fiber used were 1.59, 1.52 and 2.27 m, respectively. The small bubble-like artefacts originate from flaws in the CCD camera. (d)–(f) Transmission and loss spectra of core mode. The transmission window and minimum loss in each fiber are (d) 800-1050 nm, 0.84 dB/m; (e) 750-1025 nm, 0.74 dB/m and (f) 725-1000 nm, 0.98 dB/m.

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4. Finite-element calculations To explore the guidance behavior theoretically, finite-element (FE) calculations were performed using JCMwave software [32]. The calculations were based on high-resolution SEM images of Fiber 2 (Fig. 1(b)). To minimize the computational load, only the first two rings of cladding holes were included in the calculation mesh (see Fig. 3(a)). The resulting mode profiles and confinement losses for the LP 01, LP11, LP02 and LP31 guided modes at 830 nm wavelength are presented in Figs. 3(b)–3(e) and Table 2, respectively. The calculated loss for the fundamental LP01 mode is 0.34 dB/m, comparable to the experimentally observed value. For the higher order modes losses increase to 7.92 dB/m for the LP 11 mode, 56.92 dB/m for the LP02 mode, and 57.64 dB/m for the LP 31 mode. The increase of the losses with mode number is much stronger than previously observed in silica HC-PCF [29], and agrees with our experimental observations. Further numerical studies show that two factors are responsible for the unique single-mode guidance: the higher refractive index of the SF6 glass and the presence of triangular interstitial gaps in the fiber geometry. Calculations on a geometrical identical structure made from fusedsilica glass (Fig. 3(a)) show that its lowest-loss mode is the LP11 mode. A similar dependence of loss on mode-order and glass index was predicted by Marcatili et al., who investigated guidance in circular-cylindrical hollow capillaries [33]. Next we performed similar calculations on a SF6-based HC-PCF, however this time we filled all the triangular gaps with SF6 glass, creating a new fiber structure that is otherwise similar to the one shown in Fig. 3(a). In this case the numerical simulations do not yield an LP 01 mode; instead, the LP11 is the lowest order mode. These calculations show that effective single-mode guidance in the SF6 HC-PCFs relies on fulfillment of the two conditions mentioned above.

Fig. 3. Results of finite-element calculations of the modes guided in Fiber 2. The structure was extracted from the SEM image (Fig. 1(b)). (a) The meshed two-row air-hole structure of the Fiber 2. At 830 nm wavelength, (b) the LP01 mode (c) the LP11 mode: (d) the LP02 mode (e) the LP31 mode. Table 2. Calculated Confinement Losses And Effective Indices of the LP 01, LP11, LP02 and LP31 Modes in a SF6 HC-PCF with Two Rings of Hollow Channels Around the Core

Loss (dB/m) Re(ñ) Im(ñ)

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LP01

LP11

LP02

LP31

0.34 0.99976241 0.00000001

7.92 0.99938478 0.00000012

56.92 0.99875058 0.00000087

57.64 0.99838297 0.00000088



5. Conclusions In conclusion, it is possible to fabricate hollow core PCFs with a high degree of structural perfection using soft glass, despite the high temperature dependence of the glass viscosity. Particular care must be taken to control the temperature distribution in the drawing furnace and to avoid recrystallization. In the fibers realised, guidance bands 200-300 nm wide were measured, with minimum losses below 1 dB/m (less than that of bulk SF6 glass). Despite having a relatively large core diameter (30 μm), the fibers remained single-mode over the whole guidance band (~750 to ~1050 nm). This property we attribute to the higher index of the SF6 glass and the presence of interstitial gaps in the cladding. Experimental observations and finite element simulations confirm that although higher order modes are supported, they have very much higher loss than the fundamental mode, creating effectively single mode guidance. The fibers may be useful in applications which require a large mode area as well as good beam quality. Acknowledgments We would like to thank Dr. J. Nold, Dr. M. Scharrer, Dr. R. Keding and S. Rammler at the Max-Planck-Institute for the Science of Light and Dr. H. Hundermark at the Menlo Systems GmbH for helpful discussions and technical support.

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