Cladding mode coupling in highly localized fiber Bragg gratings: modal properties and transmission spectra

TL;DR

This paper investigates the spectral properties of inhomogeneous fiber Bragg gratings, revealing strong cladding mode resonances and their sensitivity to core modifications, which can enable advanced sensing applications.

Contribution

It provides a detailed analysis of cladding mode coupling in highly localized FBGs with inhomogeneous profiles, highlighting the potential for precise control and new sensing techniques.

Findings

Cladding mode resonances can span a full octave in the spectrum.

Coupling strength is highly sensitive to the modification position in the core.

Deeper than 20dB resonances observed in transmission spectra.

Abstract

The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs con- siderably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications.