Fibre-Optic Feed-through Design

In many vacuum applications light must be coupled through a vacuum envelope for purposes of viewing sample positioning, introducing laser light or analysing optical events occurring within a vacuum chamber etc. This is not always a straightforward affair as there are no commercially available feed-throughs capable of sealing a fibre-optic bundle that passes continuously through a vacuum chamber wall. The most common method employs a pure silica core that is welded into a specific flange with the fibre optic elements being coupled to either end. This is therefore not a true feed-through as the fibre bundle is not continuous through the vacuum envelope. These feed-throughs are difficult to

obtain, are generally expensive, suffer transmission losses (typically of the order 2 dB) and also limit the user to a specific wavelength range, as dictated by the optical properties of the construction materials.

Spectral analysis of the extremely low light levels produced by laser ablation events demands that the maximum possible flux is obtained with minimal transmission losses. In order to investigate LIBS under vacuum conditions, a fibre-optic feed-through must be used that: enables flexible positioning of the fibre collection head to maximise collection efficiency, is continuous ensuring no coupling interface transmission losses, and is also vacuum tight. After an extensive survey of commercially available feed-throughs it was apparent that there were none available that met the above criteria.

Abraham and Cornell describe a Teflon feed-through using a 3.2 mm Swagelok tube fitting connector suitable for UHV applications, but this design is limited to single fibres with diameters of 120 - 160 m. When they tried to scale this design to a 12.7 mm (½ inch) tube fitting connector they were less successful (Abraham and Cornell 1998). Miller and Moshegov report the design and construction of an all-metal UHV optical fibre feed-through, found to be leak tight to 10-9cm3/min helium when repeatedly baked to 250°C (Miller and Moshegov 2001). This feed-through design uses unmodified Swagelok tube fitting connectors and is again only suitable for single fibres (400 µm diameter plus aluminium jacket). A machined aluminium plug replacing the forward ferrule of the compression fitting provides the vacuum seal.

Weiss and Stoever (Weiss and Stoever 1985) describe an o-ring and epoxy sealed feed-through for optical fibre bundles. Their design preserves fibre continuity, is deemed to be rugged enough for field use and is easy to construct, but is fairly destructive in that it requires a length of the fibre bundle cladding to be cut away. Testing of this design on a 10 mm diameter fibre bundle yielded a base pressure ~ 10-4mbar.

As there were no suitable feed-throughs in existence, such a device was designed, constructed and tested. The feed-through design was based on standard easily available 'tried and tested' components keeping in mind the design criteria as outlined above. The use of stock parts enabled the feed-through design to be easily tailored for a specific application. The feed-through was constructed using Swagelok Ultra-Torr fittings as shown in Figure 5-4. This simple and effective design used two 12.7 mm diameter unions, one 12.7 mm diameter convoluted tube of length 150 mm and a specially designed threaded aluminium plug (based on a standard fluid tube feed-through) which seals on the outer wall of the chamber with an elastomer o-ring and terminates in a 12.7 mm diameter nipple. All components were ultrasonically degreased in solvent at room temperature prior to assembly.

An Ultra-Torr union connects the aluminium plug to a 150 mm length of 12.7 mm diameter stainless steel convoluted tubing, and a second Ultra-Torr union terminates the convoluted tubing and seals around the end of the fibre-optic cable, shown expanded in Figure 5-5.

Figure 5-4 Schematic diagram of the fibre-optic feed-through, drawn to scale

The ends of the 12.7 mm convoluted tubing are reinforced with Ultra-Torr 12.7 mm XOA adapter cuffs. The fibres comprising the bundle are sealed in epoxy at the collection head, and the surface of the collection head potted in a protective epoxy coating; as such there is no gas permeation along the fibres or fibre cladding into the vacuum envelope. The Ultra-Torr unions are designed to accept 12.7 mm diameter tubing, whereas the outside diameter of the fibre-optic bundle head measures 10mm. In order to create a vacuum tight seal around the fibre-optic bundle head an o-ring of larger diameter than the stock Ultra-Torr o- ring was inserted; shown in Figure 5-5.

Use of convoluted tubing allows ease of manipulation of the fibre-optic and increases flexibility in accurately positioning the fibre-optic bundle to collect the maximum flux from laser-induced plasma plumes. Replacing the sealing elastomer o-rings with Viton o-rings would enable a gentle baking out of the chamber (maximum 200ºC) to attain base pressures ~10-8mbar.

Figure 5-5 Interior view showing the collection end of the feed-through, drawn to scale

Figure 5-6 shows the constructed fibre-optic feed-through in position. The feed- through is secured through the base of the vacuum chamber and clamped in position; the collection end of the fibre-optic can be seen protruding through the clamp.

With the feed-through in place the pump-down time and ultimate base pressure of the vacuum chamber are unaltered. A residual gas analyser connected to the chamber was used to helium leak check the feed-through; no leak was found. An up-leak check of the vacuum chamber revealed no discernible change in leak rate with the introduction of the feed-through. The design of the feed-through proved to be extremely successful, enabling LIBS investigations at low pressures.