IMPACTO DE LOS RESIDUOS QUÍMICOS

The characteristics which make fibre-guiding traps distinct from free-space traps are;

1. The inclusion of a fibre; with associated mounts, heat shields and clearance for optical access at both ends of the fibre.

2. The inclusion of a MOT mirror and quarter wave-plate inside the UHV system.

3. Approximate vacuum isolation between MOT and detection chambers

These are elaborated below.

1. The inclusion of a fibre; with associated mounts, heat shields and clearance for optical access at both ends of the fibre.

The majority of fibre trap design constrictions derive from a necessity for optical access. Both entrance and exit fibre facets must be correctly positioned for exposure to input guide light and for output monitoring, respectively. As will be detailed in chapter 7, the guide beam in our experiments is launched from an f = 38.1mm lens and had to have a direct collinear path to the fibre hollow core. The volume in the MOT chamber also had to have sufficient exposure to all cooling beams (hence the long ~35mm separation between fibre facet and trap cube roof). The exit

facet required similar exposure, though with a reduced surrounding volume. This volume needed only to allow a probe beam to pass under the fibre. A long working distance (LWD) microscope objective lens, focused on the fibre exit, provided confirmation of guide coupling into the fibre core.

Fibre and optics mounts inside each trap were designed & manufactured in- house. They were cut from 0.5mm thick steel sheeting and bolted to an internal flange surface. These served dual purposes; vertical fibre suspension at correct height and rigid support of the internal MOT mirror. These had an adverse effect of conducting heat from the nearby Rubidium oven to the mirror/QWP and fibre.

Radiative heating of fibre and optics was reduced through inclusion of heat shields. In early traps a thin sheet of polished steel was the sole heat shield, later designs sported the same shield plus a Berillium sheet which blocked line of site to every area of the fibre, the mirror and the QWP, yet still left a large area for Rb vapour to disperse into the trapping volume.

To hold the fibre in position something was required to grip it while not crushing it. The first 2 traps employed a magnetic clamp; a small magnetic bar, similar to that found on a fridge-magnet was built onto the mount structure and combined with a 15mm×5mm iron plate. The plate had a narrow groove etched vertically (along the 5mm axis) which encouraged fibres to settle therein, sandwiched between the plate and the magnetic bar. The associated magnetic field was deemed to have no significant effect on either fibre-guided atoms transiting nearby or the MOT above.

However manipulation of the metal-plate fibre-grip was awkward and often led to non-vertical fibre positioning. Its magnetic nature subjected the fibre to a slight crushing force. This was not quantified, however attention was drawn when the 1st incarnation of the QWP trap was deconstructed to reveal a fibre stuck to the magnet; it is believed heat conductance through the mount induced melting of the fibre cladding during trap operation. Subsequent transmission checks indicated the fibre inner structure was unaffected by this heating, however fear of distorting the supported optical mode prompted creation of a more gentle mount.

This lead to the mount structure employed in the latter QWP trap and Cross trap. Here the fibre is held by a relatively weak, strained steel foil strip, tack welded to one side of the mount only. The strip was 15mm×3mm, its elasticity allowed sufficient fibre grip and its reduced width aided fibre adjustment. Heat transfer was unavoidable with metal mounts, but this design did not crush the fibre to the same extent, thus reduced fears of mode distortion.

Neither fibre-grip method permitted multi-fibre mounting. The groove width in the steel plate and the curvature of the strained foil prohibited grip of extra fibres. Re-shaping of the grips was forsaken in favour of quick trap assembly. Fortunately a dust-blocked fibre was never encountered throughout fibre-guiding experiments.

2. The inclusion of a MOT mirror and quarter wave-plate inside the UHV system.

A significant divergence from free-space trap design is made with the inclusion of a MOT mirror within the vacuum chamber. As will be described in chapter 7, the internal mirror was never a standard mirror, its design and construction underwent many revisions. It had to be mounted internally as the input cooling light could not pass the fibre mount and flange assembly to reach an external retro- reflecting mirror. This imposed a loss of beam steerage, however it brought the locus of all essential cooling beams into one compact volume above the fibre facet.

A half-wave rotation of input cooling beam polarisation is also required. Thus a quarter-wave plate was fitted between the MOT region and mirror. This brought the mirror and QWP into problematic close proximity with the atom oven; heat transfer to, and subsequent expansion of, the QWP is blamed for a shortening of trap lifetime. Cloud lifetime was limited to <3 minutes prior to QWP warp deteriorating cooling ability. This was a strong motivator for the inclusion of radiative-heat shields, however these could not prevent conductive heating through the flange and mount surfaces; ultimately limiting trap lifetime to <10mins before a ~5 minute cool down period was needed.

The fibre protruded through a 1mm diameter aperture in the mirror/QWP ensemble. Such apertures, and in fact, all mirror cutting and drilling was performed by Mr. Fritz Akerboom. Metallic coating of mirror/QWP surfaces was performed by the author using an Edwards 306, electron beam evaporator, provided courtesy of Professor T. Krauss. 200nm of Gold deposited on top of 20nm of Nickel. Provided reflectivities not below 85% for any mirror created, (Nickel was necessary for Gold adhesion to glass). Deposition of Silver for higher reflectivity was not permitted due to its prolific contamination of all metal sources within the evaporator.

3. Approximate vacuum isolation between MOT and detection chambers

The volumes surrounding each end of the fibre were used for fibre loading and flux detection. The detection volume required suppression of any unguided atom flux in order to reduce background fluorescence noise levels. Hence, either two separate vacuum systems (each with associated pumps) were required, or, as herein, the fibre could be passed through multiple narrow apertures; through flanges and thick mirrors in order to reduce unguided atom diffusion. The length of these narrow passages provided sizable surface areas for atoms to adsorb onto and eliminated any direct line of sight path. In this way background levels in the detection chamber were held a few orders of magnitude below those of the loading chamber, (calculated by fluorescence of passing probe beams)

Better isolation was always desirable for “cleaner” detection signals. This could be obtained through greater separation of the two chambers, however separation is limited by the choice of fibre length, in turn dictated by the level of optical guidance exhibited by the fibre. Traps utilising quasi-PBG fibre require loading and detection chambers in close proximity because of the rapid depletion of light intensity within the fibre, (recall Equation 1.38). The advent of PBG fibres diminished this length dependence however the series of fibre-trap systems described herein began with non-PBG fibre. Hence traps were constructed around short fibre lengths; ranging from 15cm down to 6.6cm chronologically. These short lengths still permitted competition with published work since their cores were narrow enough to demonstrate superior transmission properties relative to capillary fibres. Equation 1.38 denotes a cubed dependence on core diameter for transmitted intensity in a

capillary fibre, PBG fibre is not bound by this constriction thus selection of narrow core PBG fibre was encouraged and limited only by the minimum laser spot size achievable for guide coupling into the core

Additionally, the fibre exit volume was often designed to accommodate a 2nd MOT region beneath the fibre in order to provide an auxiliary detection method.

Thus the optical access for atom cooling, probing and monitoring ultimately dictated the volume left available for an Rb oven, fibre mount, mirror, QWP, electrical feed-through cables and the fibre.