Design and fabrication of a silicon based micro fiber holder for DOTIFS Integral field unit and associated IFU tests

Abstract

Devasthal Optical Telescope Integral Field Spectrograph (DOTIFS) is a new multi-object integral field spectrograph being built by the Inter-University Centre for Astronomy and Astrophysics (IUCAA), Pune, India for the 3.6m Devasthal Optical Telescope, (DOT). Spectrographs, which disperse the light to study various aspects of the source, traditionally follow long slit to enter the light into to the instrument. However, varying slit width, atmospheric dispersion, crowded object field etc, causes this approach to lose its efficiency. A 2D field access using optical fibres solves most of these issues and introduces additional modularity. However, the circular shape of the fibre tip causes light loss as the fill factor < 80 %. There are mainly two ways to solve these issues, one is with 2D field splicers which are usually mirrors which break the field and reflect the beams in different directions, the other a more convenient alternative is using micro lens arrays (MLA) coupled with optical fibres, increasing the fill factor < 90 %. Even with this advantage, the miss alignment of the optical fibre with the MLA can cause increase in optical entropy and hence loss of effective transmission. Therefore, making a precise fiber to micro-lenslet array holder is a necessity. In DOTIFS which uses a combination of MLA and fibers to transmit the light, the IFU unit of the spectrograph consist of fiber array held on a mask and glued to a micro lenslet array. The plano-convex MLA (PCMLA) forms a hexagonal honeycomb structure of 12×12 spaxels. The on-sky footprint of an IFU is designed to be 8.7”×7.4” for a spatial sampling of 0.8”/300µm. The PCMLA has a thickness of 2.32 mm to create the pupil at its flat surface where fibers would be butted to sample the pupil. Each microlens curved side will receive a f/21.486 beam from the magnifier optics assembly sitting between IFU assembly and the Cassegrain side port selection mirror of the telescope. The microlens converts the incoming beam from the magnifier to f/4.5 beam and creates an ideal pupil of 76µm diameter at its at back surface. In this paper we present deep reactive ion etching technique based fibre holder manufacturing for holding the fibres of IFU of DOTIFS spectrograph. We present the design details, fiber routing scheme, manufacturing and gluing and polishing concepts for fibre holder and the tests and results on the IFU deployment system.