Lab tests confirm excellent imaging properties of hexabundles

We are now moving into an era where multi-object wide-field galaxy surveys, which traditionally use single fibres to observe many targets simultaneously, can exploit compact integral field units in place of single fibres. Current multi-object integral field instruments, such as SAMI, have driven the development of new imaging fibre bundles (hexabundles) for multi-object spectrographs. The technology underpinning hexabundles was developed at the University of Sydney, by fusing optical fibres using a complex glass fibre processing facility.

However, optical fibres can be the source of loss in an instrument if not handled correctly. Focal ratio degradation (FRD) is such a loss in all fibres because it causes the light to come out of the fibre with a larger light cone than it went in. If this cone is larger than the acceptance cone of the spectrograph, then the throughput of the system will be reduced. A unique aspect of the hexabundle development was that the fibres could be fused in such a way as to remove the FRD seen in previous devices.

In a recent paper, CAASTRO member Dr Julia Bryant and co-workers have characterised the performance of hexabundles to assess the FRD and throughput for a range of input light cones (f-ratios) typically used in astronomy. This work proves that at low f-ratios, typical of fibre instruments, the FRD in hexabundles is as good as that in a single fibre of the same fibre type.

Another unique feature of the hexabundle design is that the cladding on the fibres is etched from 10 microns thickness down to between 1 to 8 microns. If the cladding on a fibre is too thin, then it will no longer sufficiently guide light, leading to losses into adjacent fibres, or ‘cross-talk’. The researchers found that cladding thicknesses as thin as 2 microns still gave a level of cross-talk that is negligible compared to the effects of atmospheric seeing in an observation. This means that by reducing the cladding thickness, hexabundles can be made with a higher fraction of the face being fibre cores that accept light, and less being cladding that does not accept light. This higher fill-fraction is unique to these devices and further increases the throughput of the system.

The performance results they have presented can be used to set a limit on the f-ratio of a system based on the maximum loss allowable for a planned instrument. These results confirm that hexabundles are a successful alternative for fibre imaging devices for multi-object spectroscopy on wide-field telescopes and have prompted further development of hexabundle designs with hexagonal packing and square cores.

by J. Bryant

Figure caption: Front face of a hexabundle with 61 fused optical fibres. Each of the fibre cores is 105 microns in diameter, with a very thin 5 micron cladding around each fibre, showing up as a thin dark circle. Special glue fills the interstitial holes between the fibres. The total actual width of the hexabundle face as shown in this image is ~1mm across. This hexabundle is now installed in the SAMI instrument.

Publication details:
J. Bryant, J. Bland-Hawthorn, L. Fogarty, J. Lawrence, S. Croom in MNRAS (2014) “Focal ratio degradation in lightly-fused hexabundles”