Origin and modal nature of the cladding-like resonance

Identify the precise origin and modal nature of the broad, low-intensity cladding-like mode resonance observed on the shorter-wavelength side of the Bragg resonance in femtosecond-laser-inscribed waveguide Bragg gratings.

Background

The measured transmission and reflection spectra of the integrated waveguide Bragg gratings contain a broad, low-intensity notch at shorter wavelengths than the principal Bragg resonance. The paper characterizes this feature as cladding-like but notes that the grating is written directly within the waveguide, making its precise modal origin uncertain. The authors discuss localized positive and negative refractive-index changes associated with micro- and nanovoids as a possible explanation, but do not conclusively identify the resonance’s origin or modal character.

References

This additional feature is characteristic of a cladding-like mode resonance; however, since the grating is inscribed directly within the waveguide, the precise origin and modal nature of this resonance cannot be conclusively identified at this stage.

— Compact Fully Integrated Alignment-Free and Linearly Polarized Ho$^{3+}$ Single-Frequency Waveguide Laser  (2610.09435 - Jacob et al., 7 Oct 2026) in Section 3.1, “Integration of grating planes into pre-written waveguides”