6.5.1
Aim
The first experiment was to try to guide thermal rubidium atoms. All reports to date of red-detuned guiding in hollow core capillaries have used a thermal source of atoms.
Aside from the being easier to setup than experiments involving cold atoms, the high longitudinal velocity of the thermal atoms through the fibre minimises the time that the atoms spend within the guide. As discussed in section 7.6.3, the optical potential within the guide may be distorted, especially near the coupling
region. Consequently, the shorter period spent in the guide will reduce the atoms’ exposure to inhomogeneities in the optical potential.
This experiment was designed to essentially reproduce the red-detuned capillary guiding experiments of Renn and co-workers [17, 51], but for HC-PBG fibres.
6.5.2
Experimental set-up
The vacuum chamber used for this experiment was the single ion pump system as described in detail in chapter 5. In brief, the system consisted of two sub- chambers. The top chamber contained rubidium getters, excellent optical access and the intra-cavity mirrors needed for the cold atom experiment as described in the next section. The intra-cavity mirrors were not used in this experiment as I did not want the atoms to be cooled. The bottom chamber contained the hot wire detector (consisting of a hot wire and CEM) and an ion pump. A small
3 mmdiameter pipe linked the two chambers so that the two chambers could be maintained at UHV using a single ion pump. A fibre holder allowed for two fibres to run between the two chambers.
For this experiment, I loaded the fibre holder with two of the11.7µmcore diam- eter hollow core photonic band gap fibres (model:FB-BG-19c-800). Both fibres were∼ 8 cmlong with the output of the fibre∼11 mmaway from the hot wire. The two fibres gave the experiment a redundancy in case one fibre was contami- nated or became damaged during the experiment.
6.5.3
Experimental procedure
The guide beam was supplied by the Ti:Sapphire laser described in section 3.2.8. The beam passed through an AOM (Isle Optics, model: LM080), which was used to turn the beam on and off. The frequency shift of the AOM does not need to be taken into account due to the large detunings, with respect to the rubidium transitions, used. The beam was steered towards the top of the trap and then coarsely coupled into the fibre using the beams steering mirrors and a convex lens
with focal lengthf = 38.1 mm which was mounted on an xyz linear positioning stage (Newport, model: 562 series). The diameter of the beam before entering the lens was3.5 mm. The resultant spot size was therefore10.8µm.
A basic laser pointer was then pointed at the output end of the fibre. Despite the absence of a coupling lens, a small amount of light was coupled into the fibre and exited from the input end. The coupling lens, was adjusted so that pointer beam passed through the middle of the lens. The guide beam’s beam steering mirrors were then adjusted so that the guide beam and the laser pointer light emitted from the fibre overlapped over as long a beam path as was observable. The laser pointer was then removed. A power meter was then used monitor the output power from the fibre. A glass slide placed before the power meter directed a small percent- age towards a lens which imaged the end of the fibre on a CCD camera. This method allowed the output mode to be observed simultaneously while optimising the transmission efficiency. As the fibre was multimode, it was important to not only optimise the power, but to optimise the shape of the mode so that it looked Gaussian.
The hot wire detector was then manoeuvred to a position directly under the fibre. This was done by observing the influence of diffraction, in the fibre’s output, from the hot wire. The hot wire was set to a temperature of 1300 K and allowed to stabilise for∼10minutes. The CEM was then turned on.
By heating up the rubidium getters, the source chamber filled with thermal rubid- ium, building up to a pressure of∼10−7mbar. As the getters were situated along a small diameter pipe away from the entrance of fibre, rubidium in this chamber was considered to be at room temperature.
A LabVIEW VI was then used to control the AOM and turn the guide on and off, with an adjustable period. The LabVIEW program then recorded the number of pulses being being generated by the CEM control box.
In principle, the introduction of a red-detuned guide beam, with sufficient power to guide the atoms and sufficient detuning to minimise heating effects, should result in the atoms being guided through the fibre. The guided atoms would then increase the signal being detected by the hot wire detector.
6.5.4
Results and discussion
This attempt at vapour guiding produced only a partial result. As shown in fig- ure 6.4, a clear increase in the detected flux was observed when the red-detuned (from both the rubidium-85 and rubidium-87 D2 transitions) guide beam was turned on. Initial optimism was quickly subdued when the same effect was seen for a blue-detuned (from both the rubidium-85 and rubidium-87 D2transitions) guide beam; indicating that the dipole force was not the cause of this observation. I am still not entirely sure how to explain this effect. As the influence of the dipole force had been discounted, the only other optical force was due to the scattering. As seen in the LG guide beam experiment in chapter 4, increased scattering force can mean an increase in flux due to the reduction of time spent in the guide. Such an explanation would not apply in this situation as the increase in velocity would only have been very small compared to the high velocity of the thermal atoms. One possibility was that the laser, by heating effects or otherwise, had increased the desorption of rubidium deposited on the walls of the fibre core, thus increasing the quantity of rubidium leaving the fibre. This seems unlikely however, as a similar effect was not seen when the getters were turned off.
Also apparent from the data, as can be seen from figure 6.4, is that the flux rate built up slowly when the guide beam was turned on. This build up correlated with a curious effect seen when viewing the output of the fibre. If coupling was maximised, and subsequently the laser turned off for more than∼10seconds, the coupling efficiency would be greatly reduced on the re-introduction of the laser. Over the next few seconds, the coupling would improve back to the previous level. The time scale of this build up in coupling matched the build up in atom flux. The nature of this reduction in coupling suggested that heating effects in the fibre were causing some form of elastic mechanical deformation.
Despite trying numerous guide parameters and different pressures of rubidium in the source chamber, I was unable to produce a conclusive result. When the pressure of the source chamber was high enough to observe any effect, it was always independent on detuning. Consequently, it was decided to switch to the cold atom guiding method, as this should have increased the number of atoms
0 2 4 6 8 140 160 180 200 C o u n t s ( s^ ( - 1 ) ) Time (mins)
(a)4.5 mWguide power at16 GHzred detuning resulting in a potential
well depth of1.01 K. On-off period of2 mins.
1 2 3 4 5 6 7 200 210 220 230 240 250 260 C o u n t s ( s^ ( - 1 ) ) Time (mins)
(b) 4.9 mWguide power at11 GHzblue detuning resulting in a potential wall height of1.6 K. On-off period of3 mins.
Figure 6.4: Data taken for thermal guiding for blue- and red-detuned guide. An increase in flux is observed when both detunings are present, indicating that mech- anism is not due to the dipole force. Note the gradual increase in flux rate as the laser is turned on. This coincided with an observed increase in coupling.
with sufficiently low enough transverse velocity to be guided, without needing high vapour pressures in the source chamber.