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Large telescopes use adaptive optics to correct aberrations in wavefronts over large areas. Such aberrations can be measured by a Shack-Hartmann wavefront sensor, an array of individual sensor elements each comprising a micro-lens with a detector at its focal plane. Any phase variation across the wavefront incident upon a sensor element causes the focal spot to move on the detector. The location of the spot therefore provides information about the magnitude and direction of the local tilt in the wavefront at that element.
We demonstrate a novel wavefront sensor based on coupling in a multi-core optical fibre. The fibre contains three identical single-mode cores symmetrically positioned at the corners of an equilateral triangle. The fibre is designed to have negligible coupling between the cores, so that their power distribution is faithfully transmitted along the fibre despite any time-varying perturbations. However, the input end of the fibre is locally narrowed down so that the cores couple over 1/4 of a coupling beatlength. This section acts like the lens in a Shack-Hartmann element, in that it converts phase variations at the input (due to wavefront tilt) into intensity variations at the output. Thus sensing of wavefront tilt can be physically remote from detection, in a compact lens-less all-fibre structure.
We fabricated a three-core fibre with cores of numerical aperture 0.11, diameter 10 μm and core-to-core centre separation of 40 μm. It was designed so that, for 1550 nm light, less than 0.1% of the light coupled from one core to the others over a length of 150 m. The input section was narrowed over about 40 mm by tapering (ie, heating and stretching) the fibre with an oxy-butane flame. To obtain the required 1/4 beatlength of coupling, we launched 1550 nm light into one core and monitored the outputs in all three as the fibre was elongated. Tapering was stopped when coupled power was maximised, corresponding to 1/2 beatlength of coupling [1]. The narrowed section was then cleaved in the middle, yielding 1/4 beatlength of coupling.
To investigate wavefront tilt, the narrowed end of the fibre was mounted in a reflective puck on a mirror mount 7 cm from the end of a single-mode fibre carrying 1550 nm light. This projects a plane wave onto the sensor. The angle and direction of tilt were varied by adjusting the mirror mount, mimicking a perturbation of the plane wavefront. The position of a 635 nm laser beam reflected by the puck onto a screen recorded the angle of tilt. In our preliminary experiments, a range of tilt angles of 2.5 degrees could be measured, along with the direction of the tilt.
In due course, we envisage a full wavefront sensor array constructed from a multi-core fibre containing many such groups of three cores, tapered at one end and connected to a remotely-placed sensor array at the other.
References
[1] R. J., Black et al., Electron. Lett. 22, 1311-1313 (1986)
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