Scattering media for reconstructive wavemeters, interrogators and sensors

Abstract

Coherent light scattering processes can lead to speckle pattern formation by complex multi-wave diffraction. This phenomenon seems chaotic and purely stochastic, but speckle motifs are highly deterministic and unique. The speckle pattern can be reproduced for a given input optical wavelength and electric field profile, providing sufficient system stability is achieved, thus realising industrial expectations of low-cost, small footprint, stable and high-resolution optical sensing systems. In this thesis, the concept of a reconstructive speckle pattern scattering medium-based device is discussed along with system engineering approaches, performance and stability tests, measurement reconstruction techniques and characteristics of several selected media: multi-mode fibre, laser-written free space 3D scattering chips and flat fibre-inscribed scattering matrices. The experiments demonstrated the flat fibre medium, which combined long-term intrinsic stability of the 3D chip and high-resolution measurement of the multimode fibre, resulting in a Raspberry Pi-controlled device acting as a wavemeter in range 1040-1056 nm with 0.1 nm resolution limited by the laser tuning step and 15.05 dB signal-to-noise ratio. Additionally, the system performance as a strain interrogator (0-200 microstrains), polarimeter (0-20°), magnetic field sensor (0-75 mT) and displacement meter (0-180 microns) with respective resolutions up to 4 microstrains, 0.04°, 8.37 mT and 1.25 microns and temporal stability of at least 144 h was confirmed. The demonstrated fine resolutions were achieved by developing dimensionality reduction machine-learning algorithms (singular value decomposition and principal component analysis), which overcome the speckles correlation limit. The presented devices realised a proof-of-concept for low-cost, compact and stable sensing solutions with the potential to compete with existing systems, yet further research is required for full system integration and application specific environmental testing.

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Identifiers

ORCID for Przemyslaw Ludwik Falak: orcid.org/0000-0002-2970-0700

ORCID for Gilberto Brambilla: orcid.org/0000-0002-5730-0499

ORCID for Timothy Lee: orcid.org/0000-0001-9665-5578

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