Mid-infrared supercontinuum generation in soft-glass suspended core photonic crystal fiber

Increasing number of applications in spectroscopy and life-sciences, as well as telecommunications, can benefit from broadband and compact sources of coherent radiation (Unterhuber et al. 2004; Kaminski et al. 2008; Swiderski and Maciejewska 2012). Supercontinuum sources based on silica photonic crystal fibers (PCF), operating in spectral range from the visible to the near-infrared, have been already commercialized. Interest is now focused on supercontinua covering near-infrared/mid-infrared part of spectrum, where vibration bands of a number of important organic compounds are located (Waynant et al. 2001; Werle et al. 2002; Guo et al. 2004). Effective supercontinuum generation at wavelengths longer than 2,000 nm requires optical fibers with transmission in the mid-infrared. Supercontinuum generation in excess of 4,000 nm has been reported in step index fibers or suspended core photonic crystal fibers made of various multicomponent glasses, e.g. ZBLAN (ZrF\(_{4}\)–BaF\(_{2}\)–LaF\(_{3}\)–AlF\(_{3}\)–NaF) (Agger et al. 2012), tellurite (eg. 75TeO\(_{2}\)–12ZnO–5PbO–3PbF\(_{2}\)–5Nb\(_{2}\)O\(_{5}\)) (Domachuk et al. 2008) and chalcogenide (As\(_{2}\)S\(_{3})\) (Marandi et al. 2012). Usually, these glasses exhibit increased scattering losses (with exception of ZBLAN), which necessitates use of only short lengths of fiber for supercontinuum generation. In this situation fiber nonlinearity plays crucial role. Suspended core photonic crystal fibers (SC-PCF) are a relatively new class of fibers, in which ultra-confinement is achieved through specially designed microstructure (Dong et al. 2008). Resulting small effective mode area, typically in the order of several \(\upmu \mathrm{m}^{2}\), promotes relatively large values of nonlinear coefficient. This comes at a cost of lower coupling efficiency with fiber, due to very small core diameter. Literature reports on supercontinuum generation in suspended core fibers are less available, than for other types of photonic crystal fibers. Libin Fu et al. demonstrated octave spanning supercontinuum in silica photonic crystal fibers pumped at around 800 and 1,000 nm (Fu et al. 2008). Also in silica suspended core PCFs, Mergo et al. obtained nearly octave spanning supercontinua using as pump various wavelengths between 740 and 880 nm (Mergo et al. 2008). More recent reports were focused on suspended core PCFs drawn from different non-silica glasses, with transmission windows enabling mid-infrared generation (Domachuk et al. 2008; El-Amraoui et al. 2010; Savelii et al. 2011, 2012). It is to be noted, that for photonic crystal fibers drawn from soft glasses, suspended core geometry enables reduction of thermal processing steps to the minimum, thus helping to avoid recrystallization of glass.

In this work, we report on supercontinuum generation in suspended core photonic crystal fiber, drawn from lead-bismuth-galate oxide glass, which was pumped with 70 fs pulses at a wavelength of 1,500 nm. Among mostly tellurite and chalcogenide compositions of glasses used for suspended core PCF drawing, our glass is a new selection for such a fiber. Yet it offers advantages, like higher mechanical durability and recrystallization resistance against tellurites, chalcogenides and fluorides, as well as relatively high nonlinear refractive index, which falls between the typical values reported for tellurite oxide glasses and chalcogenide glasses (Lorenc et al. 2008).

Demonstrated SC-PCF has been fabricated using stack and draw method at Institute of Electronic Materials Technology. The structure of fiber is shown in SEM images in Fig. 1. It consists of three large air channels, separated with thin struts supporting fiber core. Diameter of photonic microstructure of fiber is 54.3 \(\upmu \)m, diameter of the core area is roughly 1.22 \(\upmu \)m, and the struts are just 290 nm thick. Commercial mode solver (Lumerical Mode Solutions) has been used to determine modal properties of fiber, which does support several higher order modes, apart from the fundamental mode. Obtained thin and relatively long struts supporting the core ensure a high degree of isolation of the core area, which results in a negligible modal confinement loss for the fundamental mode. This allows to assume fiber loss for the fundamental mode as roughly equal to loss resulting from glass attenuation. Its measured characteristic is presented in Fig. 2, for a 2 mm thick sample.

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The glass oxide composition (weight %), enabling fiber drawing without recrystallization, in this case is following: PbO: 39.17, Bi\(_{2}\)O\(_{3}\): 27.26, Ga\(_{2}\)O\(_{3}\): 14.26, SiO2: 14.06 and CdO: 5.26. This glass has been successfully used for photonic crystal fiber development previously (Ertman et al. 2012; Stepien et al. 2012). The glass has one of the highest values of nonlinear refractive index among oxide based soft glasses, \(\hbox {n}_{2}=4.3\times 10^{-19}\,\hbox {m}^{2}\)/W, measured with Z-scan method at 1,240 nm (Lorenc et al. 2008). We are not aware of any wavelength-dependent measurements of n\(_{2}\) for this type of glass, therefore we took a flat n\(_{2}\) value for calculation of nonlinear coefficient in our fiber. Together with calculated effective mode area of the fiber at around 4 \(\upmu \mathrm{m}^{2}\), nonlinear coefficient at the used pump wavelength of 1,500 nm was relatively high for this fiber, \(\upgamma =(\upomega _{0}\cdot \mathrm{n}_{2})/(\mathrm{c}\cdot \mathrm{A_{eff}}) = 411 \mathrm{W}^{-1}\cdot \mathrm{km}^{-1}\). Notably, the transmission window of the PBG-08 glass extends from about 500 nm up to over 4,500 nm, with a characteristic dip at about 3,000 nm, which is related to OH\(^{-}\) content in glass. OH\(^{-}\) contamination does not decrease transmission to below 65 % in the investigated sample, however, this translates to tens of dB/m of attenuation in the drawn fiber. We believe this is the main reason for which obtained supercontinuum spectrum in this fiber does not extend further into mid-infrared, where absorption related to OH\(^{-}\) ion impurities increases. Further improvements to glass synthesis are necessary to overcome this issue. OH\(^{-}\) concentrations as low as 1 ppm (0.1 dB/m loss) were recently reported in oxide tellurite glass fibers, where glass synthesis incorporated in-house purification of commercial raw materials, as well as replacement of OH\(^{-}\) ions with fluorine ions (Savelii et al. 2013). Fiber sample used in our experiment was not end-capped. The reason for this was to avoid introducing additional coupling losses of pump pulses. We also believe, that ambient moisture has very little detrimental effect on this particular glass, once the fiber is drawn.

Dispersion characteristic of the developed fiber has been calculated based on previously predicted modal properties of fiber. The resulting profile is shown in Fig. 3 along with calculated group velocity dispersion characteristic (GVD) and its Taylor series expansion fit, used in numerical modeling. Dispersion of the developed fiber has two zero dispersion wavelengths (ZDW), the first one is at around 1,600 nm and at around 2,600 nm. Taylor series coefficients for GVD fitting, taken around used pump wavelength of 1,500 nm, are given in Table 1.

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Table 1

Taylor series expansion coefficients of GVD characteristic \(\upbeta _{2} =-\lambda ~\cdot D/(2\cdot \pi \cdot \)c), around pump wavelength of 1,500 nm, used in supercontinuum numerical modeling

\({\upbeta }_2[\mathrm{ps}^{2}/\mathrm{m}]\)	\({\upbeta }_3[\mathrm{ps^3}\mathrm{/m}]\)	\({\upbeta }_4[\mathrm{ps^4}\mathrm{/m}]\)	\({\upbeta }_5[\mathrm{ps^5}\mathrm{/m}]\)	\({\upbeta }_6[\mathrm{ps^6}\mathrm{/m}]\)
70.63\(\times \)10\(^{-3}\)	\(-\)32.89\(\times \)10\(^{-6}\)	\(-\)21.40\(\times \)10\(^{-7}\)	28.37\(\times \)10\(^{-9}\)	694.45\(\times \)10\(^{-12}\)

\({\upbeta }_{7}[\mathrm{ps^7}\mathrm{/m}]\)	\({{\upbeta }}_{8}[\mathrm{ps^8}\mathrm{/m}]\)	\({\upbeta }_{9}[\mathrm{ps^9}\mathrm{/m}]\)	\({\upbeta }_{10}[\mathrm{ps^{10}}\mathrm{/m}]\)	\({\upbeta }_{11}[\mathrm{ps^{11}}\mathrm{/m}]\)
\(-\)87.73\(\times \)10\(^{-13}\)	\(-\)243.51\(\times \)10\(^{-15}\)	28.60\(\times \)10\(^{-16}\)	79.35\(\times \)10\(^{-18}\)	\(-\)849.57\(\times \)10\(^{-22}\)

We designed and fabricated new photonic crystal fiber with a suspended core, using in-house synthesized lead-bismuth-galate oxide glass. We verified experimentally the fiber’s suitability for supercontinuum generation. Physical dispersion profile of drawn fiber had first ZDW at around 1,600 nm, which was longer, that designed, and limited range of generated spectrum, since disposed pumping at 1,500 nm fell into normal dispersion range. Despite this, generated bandwidth of 800–2,600 nm, obtained under 70 fs pulses, is encouraging and makes the fiber an attractive platform for further development. This would specifically include improvement of dispersion control and purification of OH\(^{-}\) contamination during glass synthesis and fiber drawing, which combined gives potential for extending spectrum generated in this fiber towards the mid-infrared.