Modification of borosilicate glass composition for joint thermal processing with lead oxide glasses for development of photonic crystal fibers

Multi-component silicon oxide based glasses have become of considerable interest due to their various properties and possibility to modify them by adjustment of composition. Large number of possible compositions enabling fiber drawing, make this type of glasses attractive to investigate in context of fiber optics technology and other optical applications.

Adjusting of glass properties by composition change enables possibility to match two different glasses for joint thermal processing. All-solid photonic crystal fibers, in which glass rods take place of air holes in the photonic structure, first shown by (Feng et al. 2003) find applications in supercontinuum generation (Pniewski et al. 2013), sensing (Gu et al. 2011), and lasing (Olausson et al. 2008). Notably, chalcogenide glasses have also been used in all-solid glass photonic crystal fiber drawing (Toupin et al. 2013).

Photonic crystal fibers (PCF) drawn from a single type of glass and comprising a lattice of air-holes offer a degree of freedom in fiber designing, which is limited by properties of the single used glass. Fabricating PCF fibers with lattice composed of a different type of glass instead of air, broadens PCF design freedom, regarding to possibility to modify the refractive index contrast between two glasses. (Buczynski et al. 2012). In case of photonic crystal fibers made of one glass and air holes, the difference in refractive index is determined only by chromatic dispersion of glass. In all-solid photonic crystal fibers this contrast is determined by difference between the dispersion of refractive indices of the two used glasses. This results in possibility of tuning the modal and dispersion properties, which constitutes the additional degree of freedom in fiber designing. Additionally, joining two types of glasses enables other advantages, like matching glasses with different spectral transmission ranges or easier formation of nanostructured fibers, which is impossible with just an air-hole based photonic lattice. Choosing fused silica as the main material for all-solid PCF fiber enables to obtain attenuation below 20 dB/km around 1,550nm (Bouwmans et al. 2005). Also long period gratings in this type of fibers has been reported (Jin et al. 2008). One of the highest degrees of miniaturization in a fiber-optics based optical component fabricated from jointly synthesized and drawn glasses is the nanostructured gradient lens, which includes sub-wavelength structures to shape an incident beam, and can serve e.g. as an output collimator glued directly to an optical fiber’s facet (Nowosielski et al. 2012). Additionally the need of low index glass family for fabricating dual glass structures like birefringent materials (Waddie et al. 2011), with high index glasses (Stępień et al. 2013), offer wide possibility to fabricate structures with high index contrast between them.

High-alkaline borosilicate glasses are relatively easy to synthesize—their melting, homogenizing and clarifying temperatures rarely exceed 1 300 \(^{\circ }\hbox {C}\). Homogenous material with very good optical quality can be obtained by use of conventional melt-quenching technique, without applying special melting conditions like protective atmosphere.

The glasses of this type are optical materials which thermal and optical properties, such as viscosity, thermal expansion, refractive index, can be easily modified in a broadband range through the concentration changes of the whole glass components. This way the properties of the newly developed borosilicate glass can be matched to the properties of the commercially available lead-silicate glasses, such as Schott’s F2 and SF6, to enable joint thermal processing into photonic fibers and microstructured optical elements.

When considering matching of glass properties for joint thermal processing (i.e. fabrication of fiber optic components at a fiber drawing tower), critical parameters adjusted in the first place are those describing thermo-mechanical and crystallization properties while optical properties, like transmission and refractive index, are usually considered in the second place.

In this work we investigate modification of borosilicate glass properties by variation of composition, in order to allow joint thermal processing with commercial Schott’s F2 or SF6 lead-silicate glass for photonic crystal fiber fabrication. The goal of the study was to experimentally validate optimization of glass composition, which would enable fabrication of all-solid PCF and such a fiber, drawn as a proof- of -concept is presented.

Final result confirming the possibility to join two glasses into stable material is fabrication of microstructured optical fiber. From the investigated glasses we choose glass designated NC-21A, which has the most favorable viscosity, thermal expansion and crystallization resistance properties for joint thermal fiber drawing. The fibers were made by the stack and draw method.

The photonic structure of the fiber labelled NL21, made of F2 and NC-21A glasses, was designed using numerical methods. The initial parameters of the fiber were: lattice constant \((\Lambda )\,=1.8\,\upmu \hbox {m},\) and linear filling factor \(\ge \)0.9. The structure had a hexagonal lattice with 15 elements on diagonal, with a solid core made of F2 glass in the center. The fluctuations of the air pressure inside the air holes were eliminated by replacing them with glass rods.

Second pair of glasses, SF6 and NC-21A, were used to fabricate photonic crystal fiber (NL22) with lattice composed of 7 rings of air holes with solid core made of NC-21A, with a nano-inclusion of SF6 glass. This defect in the core was added to investigate influence of the nanostructure in the core on the dispersion of the fiber. It allows shifting dispersion characteristic of fundamental mode without changing mode area and mode properties (Pniewski et al. 2012). The initial parameters were: lattice constant \((\Lambda ) =2.8\upmu \hbox {m},\) and linear filling factor \(\ge \)0.9. The outer diameters of both fibers were \(125\pm \,2\,\upmu \hbox {m}\). SEM images of fabricated fibers are shown in Fig. 5 and in Fig. 6. The brighter colors correspond to the Schott lead-silicate glasses.

This work demonstrates the possibility of joint thermal processing of commercially available lead-silicate glass with high-alkaline borosilicate glass after optimization of its composition. Presented pairs of glasses F2—NC-21A and SF6—NC-21A were successfully synthesized for stacking and drawing of all-solid photonic crystal fibers. Difference in thermal expansion coefficients of the two glasses at a level of \(4.2 \times 10^{-7}\, \hbox {K}^{-1}\) enabled fabrication of mechanically stable, all-solid photonic crystal fibers. The refractive index contrasts between our NC-21A borosilicate glass and the F2 or SF6 glass were 0.094 and 0.279 respectively. Further work in this area will include optimization of borosilicate glass for thermal processing with our compositions of highly nonlinear glasses (Lorenc et al. 2008) and maintaining a refractive index difference larger than 0.4.

The all-solid photonic crystal fibers composed of two glasses enable e.g. engineering of dispersion profiles flattened over several hundred nanometers, which can find application in the pulse compression and supercontinuum generation. Further works on this type of fibers shall enable achieving refractive index contrast comparable to that in classic photonic crystal fibers made of fused silica with air holes. Additionally, application of two glasses allows creating subwavelength structures with unique properties (Nowosielski et al. 2012). An additional advantage will be experimental investigation of joining two glasses with different spectral transmission ranges.