ARTICLE IN PRESS
Optik 119 (2008) 500–502 www.elsevier.de/ijleo
Structured electrodes on optical ﬁbers A. Straußa,, S. Bru¨cknera, R. Po¨hlmanna, V. Reichela, H. Bartelta,b a
Optical Fibers & Fiber Applications, Institute of Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena, Germany Faculty of Physics and Astronomy, University of Jena, Max-Wien-Platz 1, 07743 Jena, Germany
Received 13 November 2006; accepted 10 February 2007
Abstract We have investigated three different techniques for electrode production directly on surfaces of D-shaped optical ﬁbers. All three techniques were capable of producing suitable structural shapes and structural sizes for different metallic electrodes. The produced electrodes differ, however, in their electrical insulation properties and in the reproducibility of the electrode structures. Best results were achieved by a photolithographic structuring process. r 2007 Elsevier GmbH. All rights reserved. Keywords: Microstructuring; Photolithography; D-shaped optical ﬁbers
1. Introduction The integration of electrical or electronic functionality into optical ﬁbers offers great potential for the active manipulation of light in optical ﬁber systems. Recent examples for such developments are solid-state optoelectronic elements within ﬁbers, and methods for the poling of ﬁbers in order to achieve nonlinear properties in silica [1–3]. The poling of optical ﬁbers requires electrically conductive electrodes on the ﬁbers as electric components. We have investigated techniques to achieve such structured electrodes directly bonded to an optical ﬁber. Such electrodes for poling are required to meet a number of demands, which would also be of relevance for other types of optoelectronic applications with optical ﬁbers. The ﬁrst requirement is a strong adhesion between electrode material and silica surface, even with the latter slightly curved. Other requirements are the stability of the electrodes at high temperatures of around 300 1C, and stability with applied high voltage Corresponding author. Fax: +3641 206298.
E-mail address: [email protected]
(A. Strauß). 0030-4026/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2007.02.007
in the kV range while avoiding surface currents. As additional speciﬁc requirements, e.g., for quasi-phasematched second harmonic generation, structured comb electrodes with precise structural width and period in the mm range are needed. Therefore, we have investigated three different fabrication techniques for electrodes in order to compare their applicability. As a basic optical structure, D-shaped optical ﬁbers have been used. These ﬁbers have one relatively ﬂat lateral surface on which electrodes can be placed by different technologies.
2. Electrode structuring by laser ablation Laser ablation is a fast and ﬂexible technique, allowing an in situ monitoring of the structures produced and correction of the process parameters during laser ablation. To prepare the ﬁber, its surface is thoroughly cleaned. This procedure is carried out in a clean-room environment with a standard cleaning procedure (acetone, isopropanol and distilled water). Then the surface is coated with a 150 nm chromium layer. For optical structuring we used an ArF laser emitting at 193 nm. Its maximum pulse energy is 13 mJ, and the average power is
ARTICLE IN PRESS A. Strauß et al. / Optik 119 (2008) 500–502
Fig. 1. Comb electrode on the ﬂat of a D-shaped optical ﬁber structured by laser ablation with a structure period of approximately 40 mm.
6.5 W at pulse lengths of 20 ns. The beam dimensions used were 3 mm 6 mm. Using an aperture and a lens system, we obtained structural beam sizes of 5 mm. The ﬁber itself was positioned on a movable, computercontrolled XY table. For laser ablation, the chromium layer was irradiated with 20 ns pulses. The ablation rate depends on the layer material used, the optical ﬂuence and the scanning velocity of the sample. Fig. 1 depicts a chromium electrode on the surface of a D-shaped optical ﬁber produced by the laser ablation method. Special care is necessary to ﬁnd the optimum energy for complete ablation of the chromium layer and for not inducing defects on the surface of the clean silica surface. As a major limitation of applicability, a high residual electric conductivity was identiﬁed, probably due to condensation of the evaporated chromium. The resulting low electrical insulation between the electrodes is not satisfactory for high-voltage applications.
After cleaning and drying of the ﬁber, the remaining gold oxide structure was reduced to achieve the ﬁnal gold electrode as described in step three. Simple heating of the ﬁber for reducing the gold oxide would give only a low-grade gold electrode. By this method of local laser reduction, electrode structure sizes down to 3 mm were produced. The surface quality of the electrodes made by laser reduction is very good. In our process, the mechanical stability of the scanning setup and ﬂuctuation of the laser power turned out to be limiting factors for the electrode quality.
4. Electrode structuring by photolithographic technique The fabrication of surface structures on planar devices by photolithography is a standard technology today. Photolithography at the mm scale requires a photomask technique and a clean-room environment. Due to the fact that D-shaped optical ﬁbers do not have a perfectly planar surface, the implementation of such
3. Electrode structuring by local laser reduction Similar to laser ablation, local laser reduction is an in situ procedure, which works without a mask and allows ﬂexible adjustment during the process. Local laser reduction is a ﬁve-step process. First, the D-shaped ﬁber was mounted on a substrate for better handling. Then a standard cleaning procedure as described above was carried out. The next step was coating of the ﬁber, in this case with gold oxide by a sputter technique. By this technique, a layer thickness of 300–500 nm can be produced. In step three, the structuring by local laser reduction was performed. For this purpose, a cw Ar+-ion laser emitting at 514 nm was used. The beam was shaped by a microscope objective down to a spot size diameter of approximately 8 mm, allowing feature sizes down to 3 mm. Typical power densities used were between 10 and 40 kW/ cm2 for reduction from gold oxide AuO2 to gold. The ﬁber was positioned on a piezo-controlled, movable XY table. After the laser reduction, we etched the ﬁber in a speciﬁc potassium iodine solution for 1 min. Because gold and gold oxide have different etching rates, the reduced gold is dissolved while the gold oxide remains intact.
Fig. 2. Etching of a gold/gold oxide-coated D-shaped ﬁber in potassium iodine solution.
Fig. 3. Electrode structures on the surface of a D-shaped ﬁber with periods of 44 mm produced by a photolithographic process.
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reproducible structures by photolithography is a demanding task . The accurate positioning of D-shaped ﬁbers, imbedded in a substrate, is difﬁcult since the ﬁber may twist or shift due to stress induced during positioning. For exact positioning, we used a stereomicroscope and a 3601 ﬁber rotator. After exact adjustment, the ﬁber is glued on the substrate. The risk of ﬁber defects during spin coating increases with ﬁber length. Therefore, the ﬁbers were positioned in turns as shown in Fig. 2. By this method, electrode structures of 50 nm thickness with periods of 44 mm were produced (Fig. 3). The quality and purity of these structures were very good and are well suitable for applications.
5. Conclusion We compared three different fabrication techniques for electrode production on the surfaces of optical Dshape ﬁbers. All three techniques are capable of producing suitable structural sizes for electrodes. The application of the laser ablation method was limited due to low-grade insulation between the electrodes. The method of local laser reduction suffered from defects in the reduction process. The electrodes fabricated by a photolithographic process showed the best electrical
properties and have been used successfully to pole fused silica .
Acknowledgment This work was supported by the Deutsche Forschungsgemeinschaft (DFG) (BA 1724/7-1).
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