Intuitively, multimode (MM) waveguides (WGs) with significant sidewall roughness and large Δn are expected to exhibit a wide modal distribution and a large propagation loss due to excessive scattering and inter-mode coupling. Surprisingly, we observe near-SM operation regardless of the launching conditions and low propagation loss despite significant sidewall roughness and large Δn. Results constitute the first experimental observation of Marcuse's theoretical projections made almost half a century ago [1] and have prime importance for the newly evolved field of high-power photonics based on large-core WGs [2].

Various models exist for describing and calculating sidewall-roughness-induced loss (RIL) in single-mode (SM) WG whereas the case of multimode (MM) WGs, featuring highly complex dynamics due to inter-mode coupling, is rarely addressed. The sidewall roughness of a WG acts as a perturbation of its refractive index profile and may introduce weak coupling between different WG modes or between WG and radiation modes incurring propagation loss. In principle, coherent scattering of light is introduced when the spatial frequency of a sinusoidal undulation in the WG structure is equal to the difference between the propagation constants (β) of two modes. Therefore the modal and loss dynamics in MM WGs are set by the spatial frequency content of sidewall roughness.

In this work, highly MM Yb/Al-codoped fused-silica WGs were fabricated using silica-on-silica technology [3] and their sidewall roughness analyzed using white-light interferometry (WLI) showing relatively short correlation length of 5-7µm. Fluorescence streak imaging (FLI) was used to trace the rate of decay in guided power along the WG, interestingly showing non-exponential behavior. M² measurements show near-Gaussian distribution of the WG output beam (M²=1.3-1.5) with no dependence on the launching beam size and position. Transient and equilibrium regions of loss dynamics are clearly observed, with propagation loss as low as 0.5dB/cm despite significant roughness RMS (0.3µm) and large Δn≈0.5. Different transient-equilibrium behavior was observed for curved WGs. Results shed new light on the difference between the physical models describing sidewall roughness effects in SM and MM WGs and may find further application in designing high-power photonic devices based on large-core WGs.

1.            Marcuse, D., Mode conversion caused by surface imperfections of a dielectric slab waveguide. Bell System Technical Journal, 1969. 48(10): p. 3187-3215.

2.            Atar, G., et al. Large-Core Fused-Silica Waveguides for High Power Applications in Advanced Solid State Lasers. 2015. Berlin, Germany: Optical Society of America.

3.            Atar, G., et al., Fabrication and characterization of large-core Yb/Al-codoped fused silica waveguides using dry etching. Optical Materials, 2014. 38: p. 265-271.