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Coherent imager module with a large field of view for synthetic aperture interferometry applications

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journal contribution
posted on 26.02.2020, 11:58 by Jorge Garcia-Armenta, Pablo Ruiz, Jeremy Coupland, Russ Coggrave, Rob JC Middleton
Optical areal profilometry of large precision-engineered surfaces require high-resolution measurements over large fields of view. Synthetic Aperture Interferometry (SAI) offers an alternative to the conventional approach of stitching small fields of view (FOV) obtained with Coherent Scanning Interferometry (CSI) using high-NA objectives. In SAI, lowresolution digital holograms are recorded for different illumination and observation directions and they are added coherently to produce a high-resolution reconstruction over a large FOV. This paper describes the design, fabrication and characterization of a large FOV, compact and low-cost coherent imager (CI) as a building block of a coherent sensor array for a SAI system. The CI consists of a CMOS photodetector array with 1.12 µm pixel pitch, a square entrance pupil and a highly divergent reference beam that emerges from a pinhole milled with a focused ion beam on the cylindrical cladding at the tip of an optical fibre. In order to accurately reconstruct the digital holograms, the wavefront of the reference beam is estimated by localizing the reference source relative to the photodetector array. This is done using an optimization approach that simultaneously reconstructs plane waves that reach the aperture from 121 different illumination directions and guarantees a phase root-mean-squared (RMS) error of less than a fifth of the wavelength across the CI entrance pupil at a boundary of the FOV. The CI performance is demonstrated with a holographic reconstruction of a 0.110 m wide object placed at a distance of 0.085 m, i.e. a FOV = ±0.57 rad, the highest reported to date with a holographic camera.

Funding

Synthetic aperture interferometry: High-resolution optical measurement over an exceptionally large field of view : Karen Muncey

EPSRC (EP/M020940/1, EP/L014998/1), Epigem Ltd. and the UK Government’s Department for Business, Energy and Industrial Strategy (BEIS) through the UK’s National Measurement System Programmes at NPL.

History

School

  • Mechanical, Electrical and Manufacturing Engineering

Published in

Optics Express

Volume

28

Issue

6

Pages

8574 - 8588

Publisher

Optical Society of America

Version

AM (Accepted Manuscript)

Rights holder

© Optics Express

Publisher statement

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License (https://creativecommons.org/licenses/by/4.0/). Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Acceptance date

15/02/2020

Publication date

2020-03-10

Copyright date

2020

ISSN

1094-4087

Language

en

Depositor

Dr Pablo Ruiz. Deposit date: 25 February 2020

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