Characterising nonlinear waveguides by scanning near-field optical microscopy

Abstract

Scanning near-field optical microscopy (SNOM) has been applied to investigate the dispersion and nonlinear phenomena in a multimode Ta2O5 rectangular waveguide. Unlike the conventional approach of observing only the output spectra, the SNOM technique can collect the localised spectra from the evanescent field at various locations of the waveguide. This provides the visualisation of pulse evolution prior to the final development as the output light. The SNOM-acquired spectra consist of unique features which have not been observed before in previous nonlinear pulse propagation researches. These distinctive characteristics are attributed to the localised nature of the data and the multimode nonlinear pulse propagation. In order to understand the underlying physics of the experimental data, a numerical model simulating this SNOM visualisation has been developed. The simulation was based on the nonlinear Schrödinger equation, adapted for multimode pulses, and performed by the split-step Fourier algorithm.

The spectra exhibit very fine features which can be attributed to the interference of various modes with different phase modulation owing to dispersion and nonlinear effects. Accordingly, the complexity of the spectral features increase with the propagation distance and the number of contributing modes. The multimode spectra rapidly broaden at the beginning stage of the propagation, owing to the supplementary intermodal phase modulation. Unlike the single-mode case, in which the spectral broadening caused by the self-phase modulation continuously develops along the propagation distance, the broadening process in the multimode pulse is decelerated at the later distance. This is owing to the separation of the higher-order modes and consequently the influence of the cross-phase modulation on the spectral broadening is reduced.

The SNOM technique can also provide the observation of high resolution evolution of the pulse spectra. Both spectral variations along the length of the waveguide and across the waveguide are observable. Such a variation over the wavelength scale is caused by the interference of modes with different phases complexly formed by the dispersion and nonlinear effects.

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Identifiers

ORCID for William Brocklesby: orcid.org/0000-0002-2123-6712

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