EL CONCEPTO DE REFUGIO EN EL CONTROL SOSTENIBLE DE PARÁSITOS Y EL DESARROLLO DE LA RESISTENCIA

This section will discuss aspects pertinent to the fluorescence detection system used in the fibre traps. Points raised here are in addition to those previously considered in chapter 3 and 4. Further description of each specific probe arrangement is included in the later, trap specific sections.

A fluorescence-probe detection system was used in all fibre-guiding experiments. Fluorescence detection was used instead of absorption probing to ensure the highest detection sensitivity; a photomultiplier tube (PMT) and phase sensitive detector (PSD) were combined to provide as much sensitivity as possible.

Fluorescence-Probe Beam Arrangement

An extended cavity diode laser (ECDL), described in chapter 3, provided a probe beam of intensity I≈5Isat (Isat=1.6mW/cm2). This was tuned to the 5S1/2F=3→5P3/2F’=4 cooling transition of Rb85 for use in the majority of fibre- guiding experiments. It was collimated, directed under the fibre (through the flux

path) and retro-reflected through a quarter-wave plate (QWP). Care was taken to keep the probe normal to the surfaces of the detection cube so as to prevent stray reflections interacting with falling atoms before the main probe beam.

Probe centre was positioned 2.2mm beneath the fibre exit aperture. This allowed it to fit under the fibre without clipping the fibre end; probe light striking the fibre may enter and drive atoms into the fibre walls prior to fibre exit. Following a lack of data the probe was raised closer to the fibre in attempts to improve atom interception.

The spatial profile of the probe beam was varied between experiments. The desire to bring the centre of the beam closer to the fibre exit-aperture motivated an elliptical probe ‘sheet’ profile, oriented flat on the horizontal plane with dimensions 4mm×1mm. Fears of unbalanced angular momentum components between the input and retro-reflected arms of this probe returned the probe to a circular collimated Gaussian profile of ~4.3mm diameter.

Photomultiplier Tube Arrangement

The same detection system as described in chapter 3.6.1 (and shown in figure 3.21) was used for fluorescence detection in the fibre-guiding experiments. In brief, fluorescence was detected through use of a, Hamamatsu, H7710-03, Photomultiplier tube (PMT) and phase sensitive detector (PSD). As shown in figure 3.21, an iris and 2f optic system provided a 1:1 imaging ratio of the probe region onto the PMT. The optics system consisted of a cage plate assembly with an f=38.1mm lens mounted at one end, 2f away from the atom flux. An iris was mounted on the other side of the lens and positioned 2f from it. The PMT was mounted just far enough from the iris to allow signal light to efficiently fill the PMT’s sensor surface. Thus the iris diameter defined the volume of probe region visible in the PMT’s field of view (FoV) (shown in Figure 7.3). The PMT and its optics assembly were shrouded in an opaque cover and mounted on a Newport XYZ micro-translator. Probe laser light was tuned to the 5S1/2F=3→5P3/2F’=4 cooling transition of Rb85.

The detector position was fine tuned through use of a CCD camera temporarily mounted in place of the PMT on its optics assembly. When the CCD chip was positioned exactly where the PMT’s sensor would sit, we were able to see what the PMT ‘sees’ beneath the fibre. Once aligned perfectly with the fibre it was lowered vertically to view the centre of the probe beam immediately below the fibre. The aperture in the 2f system was essential for removing a large amount of background noise light. Figure 7.3 a), b) and c) give an impression of the amount of light that required filtering out. In particular guide-light leaked from the fibre walls and scattered from the end facet causing spurious noise.

Figure 7.3 : a) The view available to the PMT with the noise-reducing aperture closed to its smallest diameter of 1mm, b) with the aperture opened to ~1.5mm. The PMT is lowered to remove the fibre from view before experiments commence. c) a photograph taken through an IR viewer, showing detection-chamber glass cube with a fibre hanging within, (this was an early trap design; later traps had supports to prevent the fibre swaying). The fibre is glowing as there is guide light coupled into it. The analysing microscope objective can be seen beneath the glass cube.

An aperture, and thus FoV, of 1mm diameter (minimum aperture diameter) was used throughout the majority of detection attempts. This was increased only in later experiments within the Cross Trap where an increase in noise was tolerated in order to increase the visible probe region area; a maximum of 3mm diameter was reached before complete saturation of detection equipment occurred.

An extra f=38.1mm curved mirror was installed coaxially with the collecting lens on the opposite side of the detection cube. It faced towards the PMT and was precisely 2f away from the probe region. Hence it functioned as a signal capture lens, doubling the detected signal level by refocusing its collected fluorescence back through the detection cube, beneath the fibre and into the PMT optics stack.

Noise

The primary noise source in the detection chamber was scattered guide light. All other light could be suppressed by shielding the trap, by tuning the PSD system or by tweaking the probe alignment. Despite the chopped probe, the detected signal increased to saturation when the guide was incident on the fibre. This was attributed to, 1. the large amplitude of the guide light and 2. vibration of the suspended fibre end. The guide amplitude forced adjustment of both PMT and PSD gain settings away from those appropriate to fluorescing atoms in a probe field, thus perceived background noise level was increased. Despite mounting the entire trap on Sorbithane pads, the hanging fibre suffered pendulum like oscillations induced by vibrations transmitted through the superstructure of the UHV system. The oscillations were weakly damped by the fibre’s tensility and, in later trap designs, by surrounding surfaces. They caused guide-scatter to fluctuate at matching frequencies and created signal amplitudes up to 10 times the regular signal level. Figure 7.4 demonstrates typical noise levels seen in raw captured signals.

-0.6 -0.4 -0.2 0 0.2 0.4 0.6 -0.05 0 0.05 0.1 0.15 0.2 Time (s) F lu o rescen ce ( a rb . u n

Figure 7.4 : Two sets of raw fluorescence-probe data captured using a 10mW guide beam detuned 10GHz red from resonance. Oscillations building in magnitude from left to right are seen in both, these are fairly calm examples indicating no serious vibrations at this time (readings obscured by vibration were typically not recorded). The large spike just after the origin, marks the appearance of guide light. Note how flat the signal is prior to guide appearance. The spike saturated the PSD in arbitrary polarity. However setting a fast time constant on the PSD (10μs) allowed rapid suppression of the initial noise spike. Noise from fibre vibration was in addition to this.

The narrow, convoluted path between MOT chamber and detection chamber is expected to have prevented almost all Rb diffusion into the detection chamber. Hence noise fluorescence from background Rb vapour was mostly discounted in the detection cube. Furthermore, the narrow-linewidth of the extended cavity diode laser used for the probe, implied a significant fluorescence contrast between thermal vapour and guided cold atoms (due to the order of magnitude greater Doppler broadening of vapour atoms).

Flux Divergence

It is acknowledged that an atom flux will diverge on exit from a fibre, intuitively the divergence rate will increase with narrowing core diameter. The guide beam will also diverge on exit, following the fibre’s characteristic NA. Similarly, it was feared that slight intensity disparity between the input and retro-reflected probe beams (due to Fresnel reflections and mirror inefficiency) may push falling atoms out of the PMT field of view thus reducing available fluorescence.

Throughout the course of experiments, probe positions beneath the fibre were positioned progressively closer until their was no possibility of atom divergence outwith the probe region, similarly the PMT field of view was widened sufficiently to encompass all possible flux paths.

Estimates of flux divergence based on transverse velocities of atoms on the limit of guidance, and estimates of the atom push from probe disparity were made. However these were both deemed to contribute very little to our experimental work, and so will not be discussed here.

7.3 Atom Fibre-Guiding Experiments

In the following sections multiple trap designs are discussed for their ability to support an atomic source in close proximity to a hollow-fibre. Techniques used to develop cold atomic sources and position them in close proximity to the fibre are outlined. The methods used in attempts to optically guide atoms into and through the