Fibre alignment

Two methods of alignment were used during the investigation. Initially a v-groove slide was used but this was later abandoned in favour of a free-space method.

V-groove alignment

In order to accurately align the capillary in a repeatable way and to ensure the fibres were co-linear, I used a 20_× 20 mm slide that had been etched with a

∼ 100 µm deep v-groove (Adamant Kogyo Co., Ltd.). A slot was cut in this slide so that the height of square capillary could be freely adjusted between the fibres without any hindrance from the v-groove slide. The v-groove slide was then glued to a normal slide to increase stability and two small sections of slide were positioned over each of the grooves. The fibres were then placed facing each other in the grooves. By adding pressure to the small sections of slides, using electrical tape, a weak clamp was formed which held the fibres in the v-groove. The v-groove slide set-up is shown in figure 8.8.

Figure 8.9 shows the apparatus used in addition to the v-groove. External to the v-groove, each fibre was also attached to an xyz linear positioning stage (Newport, model: 562 Series). The stages allowed the fibres to be manoeuvred into place on the v-grooved slide. Once held in the v-groove, the separation of the fibres could be controlled by adjusting the appropriate axis of the xyz stages; the v-groove slide ensured that the fibres remained approximately co-axial as the separation was adjusted.

The capillary was then manoeuvred into position using the xyz linear positioning stage and the rotation stage. The height and position was adjusted to optimise trapping and locate an optimal area of the sample.

Although in principle, this arrangement should have ensured exact alignment, in reality this method proved to have two problems. Firstly, the fixed alignment was not as accurate as was later achieved using manually adjusted linear positioning and rotation stages. This was possibly due to mechanical torque being exerted on

the slide, thus destroying the alignment; or due to the centre of the fibres not be- ing concentric with the cladding. Although adequate and repeatable results were achieved for single particles, the lack of any freedom in the lateral or rotational alignment of the fibres was detrimental when studying multiple particle chains. Secondly, it was observed during the setting up of the supercontinuum experiment, that the output of the fibre could be severely affected by applying pressure or an abrupt bend near the output end of the fibre. This was presumably as a result of the bend or pressure induced deformity introducing a mode-conversion loss, also known as transition loss, where an abrupt change of curvature results in modal distortion [101] therefore coupling light into higher order modes. I observed that in such situations, a speckle pattern would appear not only in the the mode output but also in the crystal structure of the fibre, suggesting that the fibre was operating in many modes.

Although I could not confirm whether the v-groove set-up induced such mode- conversion problems, as it was impossible to measure the output of the fibre while situated in the v-groove, I assumed that there was a high likelihood that it would, due to the pressure on the fibre from the slide clamps. Subsequent to this discov- ery, the v-groove method was abandoned.

Free-space alignment

Figure 8.9 shows the experimental set-up, identical to that used with the v-groove slide, however it requires more careful adjustment to compensate for the absence of the slide. Although alignment in free-space was generally more labour intensive than when using the v-groove, having more degrees of freedom usually resulted in superior alignment. Using the xyz positioning stages and rotation stages, the fibres and hollow square capillary were coarsely positioned by eye and then using the microscope. Subsequent to this coarse alignment, a number of fine alignment techniques were used. By blocking one of the coupling beams, and then placing a power meter at the back of the coupling objective facing the input end of the ‘off’ fibre, it was possible to measure the power being coupling between the output ends of the left fibre and right fibre. The coupling could then be optimised by

Glass slide base

V-groove slide

Slide clamps

ESM-PCF

Hollow square capillary

Figure 8.8: Diagram of the fibre alignment when using the v-groove slide. The two ESM-PCF fibres were held in the v-groove by the two clamps that were made from small sections of microscope slide. The separation of the fibres was con- trollable using one axis of the xyz linear translators; the v-groove insured that the fibres remained relatively co-axial. The square capillary was attached to a micro- scope slide (not shown), which was then attached to a rotation stage and an xyz linear translator, allowing the sample to be aligned correctly and moved.

xyz linear translator

rotation stage ESM-PCF

Microscope slide Capillary

Figure 8.9: Schematic of the fibre trap assembly. The fibres were mounted on glass slides which were then attached to two xyz linear positioning stages. One of the slides was also mounted to a rotation stage; this allowed the fibres to be adjusted so that they were co-linear. The capillary was mounted on a slide which was attached to a rotation stage and an xyz linear translator stage. This allowed the capillary to be aligned correctly and the sample to be moved. When used, the v-groove slide (not shown) was positioned between the two xyz linear translators.

adjusting the xyz linear translators and the rotation stages holding the right hand fibre and the capillary.

It was also possible to optimise the alignment by observing the behaviour of the trapped particles. Misalignment could be manifested in a number of ways, for example, by blocking one of the beams the particle is seen to move in an unex- pected direction. Furthermore, arrays of particles were particularly sensitive to misalignment.

As with the v-groove set-up, the capillary’s height and position was adjusted to optimise trapping and locate an optimal area of the sample.

The free-space alignment method was not immune to the pressure/bend induced mode conversion problems that may have affected the v-groove alignment method, as the fibres still had to be securely mounted. However it was relatively easy to ob- serve the mode output of the fibres after they had been secured. If any mode con- version effects were observed, the fibre was simply re-positioned and re-secured in a way that resulted in a clean mode.