Germanate-tellurite composite fibers with a high-contrast step-index design for nonlinear...

Germanate-tellurite composite fiberswith a high-contrast step-index design

for nonlinear applications

Mathieu Boivin,* Mohammed El-Amraoui, Yannick Ledemi, SteeveMorency, Real Vallee, and Younes Messaddeq

Center for Optics, Photonics and Lasers (COPL), Universite Laval, Quebec, G1V 0A6,Canada

*[email protected]

Abstract: 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 thermaland optical properties corresponding to those of the core glass. Thiswork demonstrates that even if these two glasses have relatively differentchemical compositions, their association results in a good quality fiber.This fiber design combines strong confinement of the optical modes insideits core and good environmental stability for nonlinear applications in themid-infrared.

© 2014 Optical Society of America

OCIS codes: (190.4400) Nonlinear optics, materials; (190.4370) Nonlinear optics, fibers;(060.2280) Fiber design and fabrication; (060.2290) Fiber materials; (060.4370) Nonlinear op-tics, fibers; (060.2390) Fiber optics, infrared.

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#212087 - $15.00 USD Received 16 May 2014; revised 20 Jun 2014; accepted 20 Jun 2014; published 31 Jul 2014(C) 2014 OSA 1 August 2014 | Vol. 4, No. 8 | DOI:10.1364/OME.4.001740 | OPTICAL MATERIALS EXPRESS 1740

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1. Introduction

Third order nonlinear effects can be generated in optical fibers for different applications suchas optical switching [1, 2] and supercontinuum generation [3–6]. To generate these phenomenawith better efficiencies, it is desirable to employ waveguides based on materials having a highnonlinear refractive index (n2) and designed to have a small effective mode area (Ae f f ). Theseconditions are met, among others, by microstructured optical fibers (MOFs) made from highn2 glasses, for example, chalcogenide or heavy metal oxide glasses [3–6]. In addition, the highconfinement of the optical modes in these fibers also allows management of their chromatic dis-persion, which is a relevant feature for optimizing the generation of nonlinear effects [5]. How-ever, MOFs made from non-silica materials and having a cross-section with a large air fractionsuffer frequently from poor mechanical properties and low environmental stability compared tostep-index fibers which hinders their inclusion in systems requiring high reliability.

Another approach for fabricating an optical fiber showing strong confinement without com-promising its environmental stability is to exploit a composite fiber design [7–10]. In this de-sign, two glasses based on different glassy network formers and having a large difference be-tween 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 inthe fiber which prevents the exposition of the optical modes to the ambient air. However, to drawa prototype of this type of fiber with good mechanical properties, several physical parametersbetween the cladding and the core glasses must be matched.


This paper presents a fabrication technique for making a composite step-index fiber havinga high refractive index difference between its core and its cladding. The proposed design asso-ciates the 47.5GeO2 − 17.5TeO2 − 20ZnO− 15Na2O (GTZN) glass for the cladding with the69TeO2−23WO3−8La2O3 (TWL) glass [11] for the core. These two glasses are based on dif-ferent 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 togetherinto a fiber, the chemical composition of the GTZN cladding glass has been optimized to matchits thermal properties with those of the TWL core glass. This work demonstrates that these twoglasses, even if they possess relatively dissimilar compositions, can be drawn into a compositefiber 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 bene-ficial for having high optical confinement resulting in a small Ae f f . Secondly, its transmissionspectrum has the potential to extend up to around 5.5 µm in the mid-infrared which is the samespectral range as the core glass. This last condition is essential for exploiting nonlinear effectson the full transmission range of the core glass without being limited by excessive propagationlosses. With all these assets, this fiber is an interesting option for reaching high nonlinearityand environmental stability for mid-infrared applications.

2. Experimental methods

Glasses were prepared by mixing oxides and carbonates (with purity ≥99.9%) using the pre-viously mentioned molar ratios and by melting these powders at temperatures between 800 ◦Cand 1000 ◦C under ambient air. The TWL and the GTZN glasses were respectively melted in100% gold and 95%platinum/5%gold crucibles. The preform was fabricated by using the mod-ified built-in casting technique [12]. To increase the ratio between the diameters of the claddingand of the core, the preform was stretched into a thin rod that was inserted into an assemblyof 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 oxygenatmosphere to prevent surface crystallization [13] and it produced an approximate length of100 m of continuous fiber. Two acrylate polymers were successively coated on the fiber to in-crease its tensile strength and to protect its surface against the environment. The resulting fiberhad enough mechanical strength to be manipulated without any special care and to be cleavedwith a standard fiber cleaver.

Differential scanning calorimetry (DSC) measurements were acquired with a NetzschDSC 404 F3 Pegasus calorimeter at a heating rate of 10 ◦C/min. The thermomechanical analy-ses (TMA) were performed on samples shaped into cylindrical rods having a diameter of 10 mmand a thickness around 5 mm to ensure good measurement accuracy. These measurements weredone with a Netzsch TMA F1 Hyperion analyzer at a heating rate of 5 ◦C/min and a load of0.02 N. The thermal expansion coefficient of each glass was then calculated from the slope ofthe section of their TMA curve located between 100 ◦C and 200 ◦C. The softening points weredefined as the temperature at which the thermal expansion on the TMA curve of each glass stopsand begins to drop [14]. For optical characterization, glass samples were shaped into windowshaving 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 formeasuring the transmission spectrum of each glass in the 300 nm to 6 µm spectral range. Therefractive 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 theselast measurements, the dispersion curve of each glass was calculated by fitting a two-pole Sell-meier curve through these points [15]. The two-pole Sellmeier equation is given by :


n2 = A+B1

1−C1/λ 2 +B2

1−C2/λ 2 (1)

where λ is the wavelength in micrometers and A, B1, B2, C1 and C2 are optical constants deter-mined by the least squares method.

The nonlinear refractive indices of the cladding and the core glasses were measured by theZ-scan technique [16]. The laser source employed was a mode-locked titanium:sapphire oscil-lator (Coherent RegA 9000) centered at 788 nm and emitting pulses of 90 fs at a repetition rateof 100 kHz. However, this laser source produces an elliptic Gaussian beam which is incom-patible with the original Z-scan technique in which the normalized transmittance of a sampleis measured through a circular aperture. In consequence, the Z-scan set-up has been adaptedby using the technique described by Tsigaridas et al. [17]. Instead of measuring the normal-ized transmittance, this technique uses a beam profiler camera for measuring the normalizedlong and short semi-axes of the elliptic Gaussian beam in the far-field after it has been focusedthrough the sample with an achromatic lens. This technique also has the advantage of reducingthe measurement noise coming from the pointing instabilities and from the energy fluctuationsof the incident beam. To attenuate the temporal noise, each experimental point is an averagedvalue 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. TheFresnel reflections have also been included in the calculations and a second scan at low powerfor each sample has been made to reduce inaccuracies coming from beam deformations causedby refractions on surface defects. To avoid contributions from higher-order nonlinearities, theintensity of the incident light was adjusted to produce a nonlinear phase-shift smaller than1.0 rad. To calculate the nonlinear refractive indices, the analyses have been done only on theZ-scan traces corresponding to the short semi-axis of the laser beam because the traces associ-ated with the long semi-axis were too weak to be analyzed accurately. The glass samples hada thickness around 1.5 mm which is smaller than the Rayleigh length of the experimental laserbeam (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 accuratemeasurements, 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 inTable 1. First, the DSC measurements show that both glasses are sufficiently thermally stableagainst 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 theglass transition temperature (Tg) should be as high as possible [18, 19]. In the case of the TWLand the GTZN glasses, these temperature differences are equal to 235 ◦C and 164 ◦C respec-tively, which is significantly above 100 ◦C, a value considered as an approximate thresholdfor stability [20]. The thermal expansion coefficient of the core glass is 1 ppm/◦C higherthan 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 compressivestress on the cladding which is favorable for having a good mechanical strength in the finalfiber [21]. The softening point (Ts) of the cladding glass is 30 ◦C higher than that of the coreglass. Even if it is assumed that relatively near softening temperatures are needed in order toget viscosities sufficiently similar for successful fiber drawing, this last mismatch was not highenough to cause any problem during the drawing process.


Table 1. Thermal and optical parameters of the 69TeO2 − 23WO3 − 8La2O3 (TWL) andthe 47.5GeO2 −17.5TeO2 −20ZnO−15Na2O (GTZN) glasses.

Parameters TWL GTZNRefractive index at 1.54 µm (−) 2.062 1.726Glass transition Tg (◦C) 425 401Onset of crystallization Tx (◦C) 660 565ΔT = Tx −Tg (◦C) 235 164Thermal expansion coefficient (ppm/◦C ) 12.8 11.8Softening point Ts (◦C ) 412 442

1 2 3 4 5 6

20

40

60

80

100

Wavelength (µm)

Tran

smis

sion

(%) GTZN

TWL

0.6 0.8 1 1.2 1.41.7

1.8

1.9

2

2.1

2.2

Wavelength (µm)R

efra

ctiv

e in

dex GTZN

TWL

OH bands

(a) (b)

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 andthe 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 thiswavelength, absorption bands due to the presence of hydroxyl (OH) groups in the matrix ofthe glasses strongly attenuate the electromagnetic radiation. However, these OH bands can beremoved by using different techniques like, melting the glasses under a dry oxygen atmosphereor by introducing fluorinated reagents in the glass composition [11, 22]. If these last techniquesare performed under sufficiently controlled conditions, the OH content of these glasses canbe low enough to extend their transmission spectrum in the mid-infrared up to their phononabsorption edge located around 5.5 µm.

The dispersion curves of the refractive index of the core and the cladding glasses are shownin Fig. 1(b). The Sellmeier coefficients associated with these curves are listed in Table 2. Therefractive index difference between these glasses is 0.336 at 1.54 µm, which corresponds to anumerical aperture of 1.13. This index profile is sufficiently contrasted to have a strong opticalconfinement inside the core of the fiber.

Table 2. Sellmeier coefficients for the TWL and the GTZN glasses.

Glass compositions A B1 B2 C1 (µm2) C2 (µm2)TWL 2.225 2.009 1.475 0.04767 149.2GTZN 1.679 1.292 0.500 0.03045 149.5

The inspection of the cross-section of the fabricated fiber was made with an optical mi-croscope in reflection mode. It shows that the fiber has a total diameter of 125 µm and thatits core has a diameter of 5.4 µm [Fig. 2(a)]. Images of the side view of the fiber taken intransmission mode also show that the interface between the core and the cladding is of optical


ZDW= 1.88 µm

(c)(a)

(e)(d)

(b)

Fig. 2. (a) Cross-sectional and (b) side views of the TWL-GTZN composite fiber observedwith an optical microscope. (c) Calculated dispersion curve of the composite fiber. Z-scantraces (circles) with theoretical fit (solid line) for the (d) TWL and (e) GTZN glasses.

quality [Fig. 2(b)]. The particularity here is that even if these glasses are made from relativelydissimilar chemical compositions, there is no apparent sign of any crystallization or delamina-tion at their interface, which is a crucial aspect for having low propagation losses.

The dispersion curve and the effective mode area (Ae f f ) of the fundamental mode of thecomposite 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 µmand the fundamental mode at this wavelength has a relatively small Ae f f of 13 µm because ofthe highly contrasted step-index. This strong optical confinement also offers the opportunityto blue-shift the ZDW by decreasing the size of the core. This feature may be interesting fornonlinear applications requiring pumping in the anomalous dispersion regime. It also enables amulti-mode propagation over the entire transmission spectrum of the fiber. However, althoughthe 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-vectormismatch between the lower-order modes that prevents their mutual coupling [23]. With anoptimized injection inside the core and by avoiding perturbations, it is possible to excite and topropagate 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 possibleto 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 consistentwith the reference values within the uncertainty [24]. In using the experimental set-up with thesame 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 Ae f f obtained previously bynumerical simulations, we calculated a nonlinear parameter (γ = [2πn2]/

[λAe f f

]) of (220±

20) W−1 km−1 at 1.55 µm for the composite fiber. This nonlinear parameter is of the sameorder 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 equalto (13.4±0.4) dB/m. It is assumed that they are mainly due to the presence of impurities suchas OH groups and transition metals inside the glasses since the interface between the core andthe 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 µmbetween its core and its cladding were fabricated. The core and the cladding are composed ofthe TWL and the GTZN glasses respectively. The GTZN glass was developed to correspond itsthermal and optical properties with those of the TWL glass in order to avoid the introductionof structural and optical defects during the fabrication process. The fiber has a core diameterof 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 ab-sorption bands, the fiber obtained would have the potential of having an optical transmissionup to ∼ 5.5 µm. This composite fiber design gives the opportunity of associating relatively highnonlinearity, optical transmittance in the mid-infrared and good environmental stability.

Acknowledgments

We acknowledge financial support from the Canada Excellence Research Chairs (CERC), theEngineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation(CFI).


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