A simple method for metal re-coating of optical fibre Bragg gratings
Introduction
After a Bragg-grating has been written into a bare optical fibre the fibre needs to be coated to protect it from mechanical and chemical damage. A method for applying an approximately 20 μm thick nickel coating to an optical fibre will be described in detail in Section 2. The intention of this study is to support further applications involving metal embedding of fibre optical Bragg-gratings as sensors. These metal-embedded Bragg-gratings are intended for condition monitoring of high-temperature machine components up to 600 °C. The coating method should be simple and inexpensive. In existing literature, for applying metal coatings to optical fibres, sputtering and dip-coating methods are mentioned. Li et al. [1] first sputter coated a thin layer (1 μm) of titanium to the fibre to enhance the adhesion. Then a nickel layer (about 1 μm) was sputter coated on the titanium layer. Seo et al. [2] describe a method for melt coating of optical fibres. In this technique the fibre coating is applied by drawing the fibre through molten tin. Similar techniques are also used for coating fibres with aluminium, gold and copper. The authors have previously worked with similar Bragg grating nickel coatings as those manufactured in this work. In that instance, the nickel coating service was provided by a British company. This company does not exist anymore and they never reported how the coating was applied. We therefore had to develop a coating method of our own in order to be able to continue research on metal embedded Bragg gratings.
In this work the method of electroplating was chosen for application of the metal coating, because of its simplicity. Methods not applying electro-based techniques can be used to deposit, e.g., nickel or copper on the fibre. However, these are suitable for thin layers only (maximum of only a few micrometers, cf. Section 5). Nickel was chosen as the coating material, because most of the high-temperature alloys we prefer to embed the coated fibre in are nickel based. However, as a bare fibre is electrically non-conductive, the fibre surface therefore needs to be made electrically conductive before the electroplating process can be applied. This was achieved by depositing a thin (about 1 μm) silver interlayer onto the fibre using a method similar to that used in “mirror making”. This method as was selected also due to its simplicity. Once the silver interlayer is applied a nickel layer of desired thickness (about 20–25 μm in this case) can be applied by electroplating. Although both methods are simple, utmost care should be taken in order to avoid holes, pores and cracks in the coating. Furthermore, the coating should have a high degree of cylindrical symmetry to avoid light attenuation caused by micro-bending. The methods for applying the silver interlayer and the technique for the electroplating of nickel are naturally both individually well-documented. However, the application of these techniques in combination to acquiring high-quality nickel-coatings of desired thickness on optical fibres with in-fibre Bragg gratings required research; hence, we deemed it necessary to publish the results at this stage.
Section snippets
The metal coating process
This section gives a detailed description of the two methods needed for applying a nickel coating to an optical fibre; application of the silver interlayer (Section 2.1) and the electroplating process (Section 2.2). A notification of invention has been filed1 (Sandlin et al. [3]).
Microscopic examination of the applied coating
To verify the quality of the coating, a piece of a nickel coated fibre was embedded in epoxy and polished so that the fibre could be examined from both ends (cf. Fig. 2) displaying a fibre cross-section as seen through a microscope. Fig. 2 shows a relatively high-quality nickel coating around the fibre, with only a few small pores or inclusions. The diameter of the fibre without coating is 125 μm. Before fine-tuning of the process, voids such as the one shown in Fig. 3 were typical. The coating
Response of the Bragg peak to metal embedding
In this section the results of metal embedding of nickel coated optical fibres are presented. The nickel coated fibres were embedded by a vacuum brazing technique using a silver-based brazing alloy (Cusin-1 ABA), manufactured by Wesgo Metals. The alloy was in the form of a paste and had the following composition: 62.0% Ag, 34.25% Cu, 1.75% Ti, 1.0% Sn. The fibre was brazed to a substrate of Inconel (a nickel-based high-temperature resistant alloy) by placing it in a groove of the dimensions 0.3 ×
Discussion
This report gives a detailed description of a two-step method for metal coating of optical fibres. What has been demonstrated is that a high-quality coating can be produced using the presented method. No special equipment is needed for the application of the method. The chemicals required are inexpensive and commonly available and the instruments needed belong to the cadre of ordinary laboratory equipment. The application of the method certainly requires a fair amount of skill, but after some
Acknowledgements
The authors are indebted to Ms Päivi Varis for SEM-photography, Ms Sanna Lavola for cross-section preparation and photography and to MSc. Ilari Sohlo and for improving the English. We also gratefully acknowledge the financial support from the Academy of Finland (decision 72327) and the Technical Research Centre of Finland, VTT.
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