Open Access
27 June 2020 Fiber positioning in microlens-fiber coupled integral field unit
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Abstract

A generic fiber positioning strategy and a fabrication path are presented for microlens-fiber-coupled integral field units (IFUs). It is assumed that microlens-produced microimages are carried to the spectrograph input through a step-index, multimode fiber, but our results apply to micropupil reimaging applications as well. Considered are the performance trades between the filling percentage of the fiber core with the microimage versus throughput and observing efficiency. A merit function is defined as the product of the transmission efficiency and the étendue loss. For a hexagonal packing of spatial elements, the merit function has been found to be maximized to 94% of an ideal fiber IFU merit value (which has zero transmission loss and does not increase the étendue) with a microlens-fiber alignment (centering) tolerance of 1-μm RMS. The maximum acceptable relative tilt between the fiber and the microlens face has been analyzed through optical modeling and found to be 0.3 deg RMS for input f-ratio slower than f  /  3.5, but it is much more relaxed for faster beams. From the acceptable tilt, we have deduced a minimum thickness of the fiber holder to be 3 mm for 5  μm clearance in hole diameter relative to the fiber outer diameter. Several options of fabricating fiber holders have been compared to identify cost-effective solutions that deliver the desired fiber positioning accuracy. Femto-second laser-drilling methods from commercial vendors deliver holes arrayed on plates with a relative position accuracy of ±1.5-μm RMS, similar diameter accuracy, and with an aspect ratio of 1:10 (diameter:thickness). One commercial vendor combines femtosecond laser-drilling with photolithographic etching to produce plates with thickness of 5 mm, but with similar (±1-μm RMS) positioning accuracy and conical entry ports. Both of these techniques are found to be moderately expensive. A purely photolithographic technique performed at Wisconsin Center for Advanced Microelectronics (a facility at the University of Wisconsin, Madison), in tandem with deep reactive ion etching, has been used to produce a repeatable recipe with 100% yield. Photolithography is more precise (0.5-μm RMS) in terms of hole positioning and similar diameter accuracy (1-μm RMS) but the plate can only have a thickness of 250  μm.

CC BY: © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
Sabyasachi Chattopadhyay, Matthew A. Bershady, Marsha J. Wolf, Michael P. Smith, and Andrew S. Hauser "Fiber positioning in microlens-fiber coupled integral field unit," Journal of Astronomical Telescopes, Instruments, and Systems 6(2), 025002 (27 June 2020). https://doi.org/10.1117/1.JATIS.6.2.025002
Received: 3 February 2020; Accepted: 16 June 2020; Published: 27 June 2020
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CITATIONS
Cited by 5 scholarly publications.
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KEYWORDS
Etching

Semiconducting wafers

Microlens

Photoresist materials

Optical lithography

Silicon

Photomasks

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