Studying focal ratio degradation of optical fibres with a core size of 50 μm for astronomy

Abstract

Along with the spectral attenuation properties, the focal ratio degradation (FRD) properties of optical fibres are the most important for instrumental applications in astronomy. We present a special study about the FRD of optical fibres with a core size of 50 μm to evaluate the effects of stress when mounting the fibre. Optical fibres like this were used to construct the Eucalyptus integral field unit. This fibre is very susceptible to the FRD effects, especially after the removal of the acrylate buffer. This operation is sometimes necessary to allow close packing of the fibres at the input to the spectrograph. Without the acrylate buffer, the protection of the cladding and core of the fibre may be easily damaged. In the near future, fibres of this size will be used to build the Southern Observatory for Astronomical Research (SOAR) integral field unit spectrograph (SIFS) and other instruments. It is important to understand the correct procedure which minimizes any increase in FRD during the construction of the instrument.

The Sydney-AAO Multi-object Integral field spectrograph

“…Mechanical stress on the fibres when mounted in the slit block can increase the FRD (see e.g. Oliveira, de Oliveira & dos Santos 2005), and hence decrease the throughput within f/3.15.…”

Design and development of the fibre cable and fore optics of the HERMES Spectrograph for the Anglo-Australian Telescope (AAT)

“…For this reason, a combination of aluminium oxide lapping films are used with particle sizes ranging from 12µm to 0.3µm on the final polish. A high quality polish is critical to maximise fibre performance as focal ratio degradation (FRD) is strongly affected by surface defects like scratches and pits [4], [5] and the resulting induced stresses all contribute to effectively changing the effective Fnumber of the indicencent light entering the fibre.…”

Focal ratio degradation: a new perspective

“…For ease of handling and polishing the optical fiber ends were mounted in rigid stainless steel ferules with polyimide strain relief tubes using the low shrinkage adhesive Araldite Super strength [8], [10] . The adhesive was allowed to cure for 30 hours before polishing.…”

“…Two FRD measurement techniques commonly used are the cone [5], [6], [7], [8], [9], [10] and the parallel laser beam [2], [11], [12] . The cone technique gives a good estimate of the total light loss that might be expected in a fiber based system, however it can be highly sensitive to alignment errors and does not provide information about the possible sources that may be contributing to any FRD.…”