IMPORTANCIA DE LA PROLONGACIÓN DE LA VIDA ÚTIL DE LOS ANTIHELMÍNTICOS

wavelength allowed its path to be traced and thus its rate of expansion to be measured on exit from the fibre. Despite the disparity between HeNe and guide light wavelengths, an approximate input NA for the guide light could be calculated, and thus, a desirable input beam diameter to the launch lens set accordingly. During atom guiding experiments the NA would give an indication of guide dipole potential persisting between fibre and probe region. This would have a confining effect on flux divergence, albeit a rapidly diminishing one.

The retro-fired HeNe laser proved to be the superior method for guide alignment; two apertures positioned sufficiently far upstream (in the guide beam path) permitted accurate matching of input guide to output HeNe.

After many fibres had been tested it was found that most NA and spot size requirements could be met with a standard AR coated, convex, f=38.1mm launch lens with a few standby lenses for the beam telescope, (rare exceptions benefited from a longer f=50.2mm or f=62.9mm launch lens). The launch lens was mounted on a Newport XYZ translation stage to aid coupling adjustment. Typically, beams input to this were collimated at ~3.5mm diameter, with a ±1mm variation for better mode matching.

6.3 Fibre Transmission Results

6.3.1 Transmission Efficiency

The purpose of all fibre analysis reported herein was to identify fibres suitable for atom guiding experiments. Capillaries, when identified, were dropped in favour of structured fibres hence they are not included here. Many fibres with poor transmission efficiencies were also dropped after their optimum throughput was attained, these too, are not included here. An arbitrary naming regime was applied to many of the fibres received; this was forced by a lack of data available on each. The table below contains

information on the best-transmitting fibres analysed throughout this thesis, all figures are quoted at optimum coupling condition specific to each fibre;

Fibre ID _Diameter Core (μm)

Length (cm) Best Transmission

“BG19-800” (PCF) 12μm 15cm 66.9% / -1.7dB “Blaze 800” (PCF) 9.2μm (short axis), 9.5μm (long axis) 15cm 39% / -4dB “SMF28-FC-5” (Q-PCF) 15μm 6cm 21% “Δ” (Q-PCF) 9μm 2.4cm, 4cm, 15cm 59.8%, 30.1%, 8.7% “Blue 4” (Q-PCF) 28μm 4.8cm – 15cm 34% - 4.8% “Pos6” (Q-PCF) 11μm 2.4cm - 15cm 54% - 8% “8μm” (Q-PCF) 8μm 15cm 5.6% “22μm” (Q-PCF) 22μm 15cm 5.2% “ ” (Q-PCF) 11μm 2.3cm – 14.5cm 65.5% -3.6% “ ” (Q-PCF) 10μm 3.9cm 46.6% “ ” (Q-PCF) 10μm 4.2cm 32.9% “Pos5” (Q-PCF) 12μm 2.3cm 46.9% Figure 6.3 : Transmission figures, coupling optics and fibre statistics for fibres deemed

most suitable for guiding atoms within. Ranked from top to bottom, best to worst. Transmission percentages are given under optimum coupling conditions. Note, throughput is also given in dB for the length independent PCF. Insufficient data was captured with the Q-PCF to allow separation of coupling loss from other length dependant mechanisms, hence there are no figures in dBm.

NA and Core Diameter Relationship

In general a counter intuitive relationship between NA and core diameter existed, that is, as the core expanded, a greater dependence on input light collinearity was observed. A slightly oblique launch angle or large solid-angle (of input light to the core), reduced transmission through broad cores more so than in narrower cores. However the less stringent NA requirements of narrow core fibre was easily overridden by moderate mode mismatch between the achievable input beam waist and the supported core mode. An input spot with ±3μm mismatch between it and the core- mode diameter resulted in a ~30% transmission drop, dropping exponentially after the

±3μm mark. For input spots smaller than the core-mode this loss came from an NA restriction, for spots larger than the core mode the loss was due to sharp break-up of the input wave-front between the core and crystalline structure.

PCF Loss Mechanism

As expected the primary source of optical loss in fibres demonstrating a PBG was the coupling process. It is believed the imposition of a minimum 38mm separation between fibre entrance facet and launch lens prevented near 100% transmission through the “BG19-800” and “Blaze-800” fibres. Successive shortening of these fibres showed ~constant transmission efficiencies, thus indicating photonic bandgap properties. Figure 6.4 illustrates transmission efficiency through various lengths of BG19-800 PCF.

Q-PCF also confirmed its predicted decrease in throughput with increasing length; this is shown, in the table-summary above, through comparison between the ~2.4cm, ~4cm and 15cm fibre lengths, (cut at these lengths for expected upcoming trap dimensions). A graph of this data is not included due to the sporadic fibre lengths used; as upcoming traps were designed newly arrived fibres were tested with lengths dictated by these designs and thus little consistency existed between successive fibre tests. This is regrettable, nevertheless the fibres of choice, the PCF, were fully tested.

0 10 20 30 40 50 60 70 80 0 5 10 15 Fibre Length (cm) Transmission % BG19-800

Figure 6.4 : Transmission efficiency plotted against fibre length for PCF entitled “BG19- 800”. Note the near constant transmission irrespective of fibre length. The dip in the centre is attributed to varying coupling quality, (ie. launch collinearity and cleave quality) ; it is not believed to be due to any PBG effects.

Polarisation Maintenance

A fibre’s ability to maintain a linear polarisation state was measured. A linearly polarised laser source was coupled into the hollow core. A Polaroid, linear polariser sheet, was inserted between the microscope and power meter in the detection arrangement shown in Figure 6.1 (polariser not shown). This was rotated while the power output from the fibre core (only) was measured.

It was found that a linearly polarised Gaussian coupled into a PCF segment was partially rotated such that, after 15cm transit, 6% ±2% was retarded 90° into the orthogonal axis, producing a slightly elliptical beam. However a 15cm Q-PCF segment maintained polarisation state to a lesser degree; 26% ±2% retarded into the orthogonal axis producing quite an elliptical polarisation.

Mode Profiles

As a possible alternative to Gaussian guiding, a Laguerre Gaussian (LG) beam was launched into the hollow cores of the “BG19-800” and “Blaze-800” fibres in the

hope that the LG intensity profile would be maintained during transit. If so, it would offer the possibility of even tighter atom confinement in a blue-detuned light-pipe, itself confined by the allowable modal area of the hollow-core. This mode compression could help bring fibre atom transport a step closer to single-atomic-mode operation2,8. In this state the atom flux path is compressed such that adjacent cold atoms are held closer to each other, thus it is easier for their de Broglie wavelengths to overlap and so begin coherent interaction.

However, the LG mode was not supported within their cores. A large drop in optical transmission was seen and the input LG profile was converted into a messy approximation of a Gaussian profile. As discussed in chapter 1, input profiles are transported in LP fibre modes and generally reform into Gaussians on exit; it is believed LG modes do not convert well into LP modes and LG profiles are not natural progressions on output.