Measurement of Variations in Gas Refractive Index with 10-9Resolution Using Laser Speckle

Morgan Facchin, Graham D. Bruce, Kishan Dholakia

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)


Highly resolved determination of refractive index is vital in fields ranging from biosensing through to laser range finding. Laser speckle is known to be a sensitive probe of the properties of the light and the environment, but to date speckle-based refractive index measurements have been restricted to 10-6resolution. In this work we identify a strategy to optimize the sensitivity of speckle to refractive index changes, namely, by maximizing the width of the distribution of optical path lengths in the medium. We show that this can be realized experimentally by encapsulating the medium of interest within an integrating sphere. While mitigating against laser-induced heating effects, we demonstrate that variations of the refractive index of air as small as 4.5 × 10-9can be resolved with an uncertainty of 7 × 10-10. This is an improvement of 3 orders of magnitude when compared to previous speckle-based methods.

Original languageEnglish
Pages (from-to)830-836
Number of pages7
JournalACS Photonics
Issue number3
Publication statusPublished - 2022 Mar 16

Bibliographical note

Publisher Copyright:
© 2022 American Chemical Society. All rights reserved.

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Biotechnology
  • Atomic and Molecular Physics, and Optics
  • Electrical and Electronic Engineering


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