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The fabrication process and characterization of composite step-index fibers with a large refractive index difference (Δn = 0.336 at 1.54 μm) between the tellurite glass-made core and the germanate-tellurite glass-made cladding are presented. In order to fabricate these composite fibers, the composition of the cladding glass was selected because of its thermal and optical properties corresponding to those of the core glass. This work demonstrates that even if these two glasses have relatively different chemical compositions, their association results in a good quality fiber. This fiber design combines strong confinement of the optical modes inside its core and good environmental stability for nonlinear applications in the mid-infrared.
© 2014 Optical Society of America
1. Introduction
Third order nonlinear effects can be generated in optical fibers for different applications such as optical switching [1, 2] and supercontinuum generation [3–6]. To generate these phenomena with better efficiencies, it is desirable to employ waveguides based on materials having a high nonlinear refractive index (n2) and designed to have a small effective mode area (Aeff). These conditions are met, among others, by microstructured optical fibers (MOFs) made from high n2 glasses, for example, chalcogenide or heavy metal oxide glasses [3–6]. In addition, the high confinement of the optical modes in these fibers also allows management of their chromatic dispersion, which is a relevant feature for optimizing the generation of nonlinear effects [5]. However, MOFs made from non-silica materials and having a cross-section with a large air fraction suffer frequently from poor mechanical properties and low environmental stability compared to step-index fibers which hinders their inclusion in systems requiring high reliability.
Another approach for fabricating an optical fiber showing strong confinement without compromising its environmental stability is to exploit a composite fiber design [7–10]. In this design, two glasses based on different glassy network formers and having a large difference between their respective refractive index are combined into a fiber to obtain strong optical guiding. Its main advantages over MOFs resides in the opportunity to have an all-solid cross-section in the fiber which prevents the exposition of the optical modes to the ambient air. However, to draw a prototype of this type of fiber with good mechanical properties, several physical parameters between the cladding and the core glasses must be matched.
This paper presents a fabrication technique for making a composite step-index fiber having a high refractive index difference between its core and its cladding. The proposed design associates the 47.5GeO2 – 17.5TeO2 – 20ZnO – 15Na2O (GTZN) glass for the cladding with the 69TeO2 – 23WO3 – 8La2O3 (TWL) glass [11] for the core. These two glasses are based on different glassy network formers; the TWL glass is mainly constituted of tellurium oxide (TeO2) and the GTZN glass, of germanium oxide (GeO2). In order to draw these two glasses together into a fiber, the chemical composition of the GTZN cladding glass has been optimized to match its thermal properties with those of the TWL core glass. This work demonstrates that these two glasses, even if they possess relatively dissimilar compositions, can be drawn into a composite fiber without introducing any optical or structural defects at the core-cladding interface.
There are two main incentives for using the GTZN glass for the cladding of the fiber. First, this glass has a significantly lower refractive index than that of the core glass which is beneficial for having high optical confinement resulting in a small Aeff. Secondly, its transmission spectrum has the potential to extend up to around 5.5 μm in the mid-infrared which is the same spectral range as the core glass. This last condition is essential for exploiting nonlinear effects on the full transmission range of the core glass without being limited by excessive propagation losses. With all these assets, this fiber is an interesting option for reaching high nonlinearity and environmental stability for mid-infrared applications.
2. Experimental methods
Glasses were prepared by mixing oxides and carbonates (with purity ≥99.9%) using the previously mentioned molar ratios and by melting these powders at temperatures between 800 °C and 1000 °C under ambient air. The TWL and the GTZN glasses were respectively melted in 100% gold and 95%platinum/5%gold crucibles. The preform was fabricated by using the modified built-in casting technique [12]. To increase the ratio between the diameters of the cladding and of the core, the preform was stretched into a thin rod that was inserted into an assembly of jacketing tubes made from cladding glass. This assembly was then drawn into a fiber at ∼ 550 °C by using the rod-in-tube technique. This last process was done under a dry oxygen atmosphere to prevent surface crystallization [13] and it produced an approximate length of 100 m of continuous fiber. Two acrylate polymers were successively coated on the fiber to increase its tensile strength and to protect its surface against the environment. The resulting fiber had enough mechanical strength to be manipulated without any special care and to be cleaved with a standard fiber cleaver.
Differential scanning calorimetry (DSC) measurements were acquired with a Netzsch DSC 404 F3 Pegasus calorimeter at a heating rate of 10 °C/min. The thermomechanical analyses (TMA) were performed on samples shaped into cylindrical rods having a diameter of 10 mm and a thickness around 5 mm to ensure good measurement accuracy. These measurements were done with a Netzsch TMA F1 Hyperion analyzer at a heating rate of 5 °C/min and a load of 0.02 N. The thermal expansion coefficient of each glass was then calculated from the slope of the section of their TMA curve located between 100 °C and 200 °C. The softening points were defined as the temperature at which the thermal expansion on the TMA curve of each glass stops and begins to drop [14]. For optical characterization, glass samples were shaped into windows having two parallel polished surfaces (sample thickness, GTZN: 4.27 mm, TWL: 2.27 mm). UV/Vis/Nir (Varian, Cary 500) and FT-IR (Perkin Elmer, Frontier) spectrometers were used for measuring the transmission spectrum of each glass in the 300 nm to 6 μm spectral range. The refractive indices were measured at five different wavelengths (532 nm, 632.8 nm, 972.4 nm, 1308.2 nm and 1537.7 nm) with a prism coupler refractometer (Metricon, 2010/M). From these last measurements, the dispersion curve of each glass was calculated by fitting a two-pole Sellmeier curve through these points [15]. The two-pole Sellmeier equation is given by :
where λ is the wavelength in micrometers and A, B1, B2, C1 and C2 are optical constants determined by the least squares method.The nonlinear refractive indices of the cladding and the core glasses were measured by the Z-scan technique [16]. The laser source employed was a mode-locked titanium:sapphire oscillator (Coherent RegA 9000) centered at 788 nm and emitting pulses of 90 fs at a repetition rate of 100 kHz. However, this laser source produces an elliptic Gaussian beam which is incompatible with the original Z-scan technique in which the normalized transmittance of a sample is measured through a circular aperture. In consequence, the Z-scan set-up has been adapted by using the technique described by Tsigaridas et al. [17]. Instead of measuring the normalized transmittance, this technique uses a beam profiler camera for measuring the normalized long and short semi-axes of the elliptic Gaussian beam in the far-field after it has been focused through the sample with an achromatic lens. This technique also has the advantage of reducing the measurement noise coming from the pointing instabilities and from the energy fluctuations of the incident beam. To attenuate the temporal noise, each experimental point is an averaged value over 90 measurements. The quality factors (M2) of the focused laser beam (focal length: 300 mm) have been characterized along both the short and long semi-axes with the beam pro-filer, and these parameters have been incorporated into the theoretical fitting procedure. The Fresnel reflections have also been included in the calculations and a second scan at low power for each sample has been made to reduce inaccuracies coming from beam deformations caused by refractions on surface defects. To avoid contributions from higher-order nonlinearities, the intensity of the incident light was adjusted to produce a nonlinear phase-shift smaller than 1.0 rad. To calculate the nonlinear refractive indices, the analyses have been done only on the Z-scan traces corresponding to the short semi-axis of the laser beam because the traces associated with the long semi-axis were too weak to be analyzed accurately. The glass samples had a thickness around 1.5 mm which is smaller than the Rayleigh length of the experimental laser beam (2.6 mm) in order to prevent contributions from linear diffraction.
The propagation losses of the fiber were measured by the cut-back method. For accurate measurements, the cladding modes were removed by applying a metallic solution of indium-gallium on the bare fiber cladding over a length of about 30 cm.
3. Results and analysis
Thermal and optical properties of the core (TWL) and the cladding (GTZN) glasses are listed in Table 1. First, the DSC measurements show that both glasses are sufficiently thermally stable against crystallization and able to be drawn into a fiber. To draw a heavy metal oxide glass, its temperature difference (ΔT) between the onset of crystallization on heating (Tx) and the glass transition temperature (Tg) should be as high as possible [18, 19]. In the case of the TWL and the GTZN glasses, these temperature differences are equal to 235 °C and 164 °C respectively, which is significantly above 100 °C, a value considered as an approximate threshold for stability [20]. The thermal expansion coefficient of the core glass is 1 ppm/°C higher than that of the cladding glass in the range of 100 °C to 200 °C (TWL: 12.8 ppm/°C, GTZN: 11.8 ppm/°C). This situation is desirable because it ensures having a moderate compressive stress on the cladding which is favorable for having a good mechanical strength in the final fiber [21]. The softening point (Ts) of the cladding glass is 30 °C higher than that of the core glass. Even if it is assumed that relatively near softening temperatures are needed in order to get viscosities sufficiently similar for successful fiber drawing, this last mismatch was not high enough to cause any problem during the drawing process.
Table 1. Thermal and optical parameters of the 69TeO2 – 23WO3 – 8La2O3 (TWL) and the 47.5GeO2 – 17.5TeO2 – 20ZnO – 15Na2O (GTZN) glasses.
The complete transmission spectra of the GTZN and TWL glasses are shown in Fig. 1(a). Both glasses have a good optical transmission up to 2.5 μm in the mid-infrared. Beyond this wavelength, absorption bands due to the presence of hydroxyl (OH) groups in the matrix of the glasses strongly attenuate the electromagnetic radiation. However, these OH bands can be removed by using different techniques like, melting the glasses under a dry oxygen atmosphere or by introducing fluorinated reagents in the glass composition [11, 22]. If these last techniques are performed under sufficiently controlled conditions, the OH content of these glasses can be low enough to extend their transmission spectrum in the mid-infrared up to their phonon absorption edge located around 5.5 μm.
Fig. 1 (a) Optical transmission spectra (sample thickness, GTZN: 4.27 mm, TWL: 2.27 mm) and (b) dispersion curves of the linear refractive index for the TWL and the GTZN glasses.
The dispersion curves of the refractive index of the core and the cladding glasses are shown in Fig. 1(b). The Sellmeier coefficients associated with these curves are listed in Table 2. The refractive index difference between these glasses is 0.336 at 1.54 μm, which corresponds to a numerical aperture of 1.13. This index profile is sufficiently contrasted to have a strong optical confinement inside the core of the fiber.
Table 2. Sellmeier coefficients for the TWL and the GTZN glasses.
The inspection of the cross-section of the fabricated fiber was made with an optical microscope in reflection mode. It shows that the fiber has a total diameter of 125 μm and that its core has a diameter of 5.4 μm [Fig. 2(a)]. Images of the side view of the fiber taken in transmission mode also show that the interface between the core and the cladding is of optical quality [Fig. 2(b)]. The particularity here is that even if these glasses are made from relatively dissimilar chemical compositions, there is no apparent sign of any crystallization or delamination at their interface, which is a crucial aspect for having low propagation losses.
Fig. 2 (a) Cross-sectional and (b) side views of the TWL-GTZN composite fiber observed with an optical microscope. (c) Calculated dispersion curve of the composite fiber. Z-scan traces (circles) with theoretical fit (solid line) for the (d) TWL and (e) GTZN glasses.
The dispersion curve and the effective mode area (Aeff) of the fundamental mode of the composite fiber were calculated by numerical simulations using a commercial software (COM-SOL Multiphysics) [Fig. 2(c)]. The zero dispersion wavelength (ZDW) is located at 1.88 μm and the fundamental mode at this wavelength has a relatively small Aeff of 13 μm because of the highly contrasted step-index. This strong optical confinement also offers the opportunity to blue-shift the ZDW by decreasing the size of the core. This feature may be interesting for nonlinear applications requiring pumping in the anomalous dispersion regime. It also enables a multi-mode propagation over the entire transmission spectrum of the fiber. However, although the fiber supports several transverse spatial modes, single-mode propagation may be possible. The small dimension of the core and the high step-index provide a considerable wave-vector mismatch between the lower-order modes that prevents their mutual coupling [23]. With an optimized injection inside the core and by avoiding perturbations, it is possible to excite and to propagate mainly in the fundamental mode of the composite fiber.
By doing a theoretical fit over the measured Z-scan traces [Fig. 2(d) and 2(e)], it was possible to calculate the nonlinear refractive indices of the glasses constituting the composite fiber. First, the accuracy of the experimental set-up was verified by analyzing a fused silica glass window. A nonlinear refractive index of (2.6 ± 0.3) × 10−20 m2/W was obtained, which is consistent with the reference values within the uncertainty [24]. In using the experimental set-up with the same conditions, values of nonlinear refractive indices of (6.9 ± 0.7) × 10−19 m2/W and of (2.1 ± 0.2) × 10−19 m2/W for the TWL and the GTZN glasses were respectively measured. By combining the value of the n2 of the TWL glass with the Aeff obtained previously by numerical simulations, we calculated a nonlinear parameter (γ = [2πn2] / [λ Aeff]) of (220 ± 20) W−1 km−1 at 1.55 μm for the composite fiber. This nonlinear parameter is of the same order of magnitude as that of single-mode As2S3 chalcogenide fibers [25].
The propagation losses were measured at 1.55 μm by the cut-back method and they are equal to (13.4 ± 0.4) dB/m. It is assumed that they are mainly due to the presence of impurities such as OH groups and transition metals inside the glasses since the interface between the core and the cladding of the fiber is of relatively good quality.
4. Conclusion
A composite step-index fiber possessing a large refractive index difference of 0.336 at 1.54 μm between its core and its cladding were fabricated. The core and the cladding are composed of the TWL and the GTZN glasses respectively. The GTZN glass was developed to correspond its thermal and optical properties with those of the TWL glass in order to avoid the introduction of structural and optical defects during the fabrication process. The fiber has a core diameter of 5.4 μm and its nonlinear parameter is evaluated as being equal to (220 ± 20) W−1 km−1. Its optical transmission in the mid-infrared is limited by the OH absorption bands. However, if adequate purification methods were employed during glass synthesis for removing these absorption bands, the fiber obtained would have the potential of having an optical transmission up to ∼ 5.5 μm. This composite fiber design gives the opportunity of associating relatively high nonlinearity, optical transmittance in the mid-infrared and good environmental stability.
Acknowledgments
We acknowledge financial support from the Canada Excellence Research Chairs (CERC), the Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI).
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