Robust method for finding the modes in a fibre with an unknown refractive index profile

Ľubomír Škvarenina; Johanna Trägårdh; Stephen Simpson

doi:10.1117/12.2632808

19 May 2022 Robust method for finding the modes in a fibre with an unknown refractive index profile

Ľubomír Škvarenina, Johanna Trägårdh, Stephen Simpson

Proceedings Volume 12140, Micro-Structured and Specialty Optical Fibres VII; 121400M (2022) https://doi.org/10.1117/12.2632808
Event: SPIE Photonics Europe, 2022, Strasbourg, France

Conference Poster

Abstract

The number of eigenmodes in an optical fibre depends on the wavelength of the excitation laser beam as well as on the exact geometry and refractive index profile of the fibre. The latter is often proprietary information and, when available, is only specified to within manufacturing tolerances. We here present a method for obtaining the number of fibre modes, as well as their shape, which requires no knowledge about the fibre, save a very approximate core radius. The method is based on the singular value decomposition (SVD) of a set of speckle patterns, measured at the output end of the fiber, which is then expanded onto a set of orthonormal basis functions. We present two possible approaches for the field expansion, where the first approach uses a generic orthonormal basis, such as Laguerre–Gaussian or Zernike functions, and the second one is a basis-free approach where the set of speckled patterns themselves form the basis. Using a set of simulated speckles patterns, we observed that the correct number of fibre modes can be obtained through the SVD decomposition, even at high levels of additive random noise. With a slight extension, using speckle patterns obtained at multiple excitation wavelengths (or equivalently, for different lengths of the same fiber) the method can also retrieve the shape of the actual fibre modes, by forming an appropriate linear combination of SVD modes.

Citation Download Citation

Ľubomír Škvarenina, Johanna Trägårdh, and Stephen Simpson "Robust method for finding the modes in a fibre with an unknown refractive index profile", Proc. SPIE 12140, Micro-Structured and Specialty Optical Fibres VII, 121400M (19 May 2022); https://doi.org/10.1117/12.2632808

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