In the past few years, silicon nitride planar waveguides have become a reference platform for nonlinear
nanophotonics, especially for Kerr frequency comb generation but also for spectral broadening and supercontinuum generation. In this work, we present several spectral broadenings using waveguides with different group velocity dispersion, some of them reaching a full octave span. By means of an innovative hyperspectral near-field microscopy technique, we fully characterize the spectral change that occurs during the propagation of light in the waveguide.
Optical near-field microscopy allows the mapping of the electromagnetic field with a resolution down to a few tens of nanometers, below the diffraction limit. Such a resolution is achieved by collecting the evanescent and propagative
fields using a dielectric probe made out of a tapered optical fiber whose extremity has a 50-nm diameter. While this technique has traditionally been used in linear optics with only one or a few wavelengths, it has recently been
extended to the mapping of the optical spectrum using a spectrometer and a fast camera. In this work, we use both a visible CCD camera and an InGaAs camera for infrared measurements around the pump wavelength at 1550 nm. The pump laser is a 100-fs pulsed laser source with a peak power of about 30 kW.
An hyperspectral measurement consists in recording the optical intensity for each position (x,y) on the sample and for each wavelength: a 3D matrix P(x,y,\lambda) is therefore obtained. From this raw data, several representations can be made. The spectra can be compared from point to point, when following the waveguide, allowing to better understand the nonlinear processes at stake during the supercontinuum generation. In particular, we show that depending on the width of the waveguide, the spectral broadening is qualitatively different owing to the
different regimes of dispersion. Our setup allowed us to measure the spectrum evolution on 1 cm of propagation, leading to an octave-spanning spectrum in the case of an anomalous-dispersion waveguide. In this case, spectral feature such as dispersive waves, third-harmonic generation and self-phase modulation give very clear and obvious signatures.
Hyperspectral near-field imaging also allows to image the multi-mode propagation within larger waveguides. In the case of spectral broadening in such waveguides, different spatial modes participate to the propagation and are differently visible depending on the wavelength. Clear interference patterns can be visualized using false-color imaging representations. This technique would provide much needed characterization for the emerging nonlinear optics in multimode waveguides research area.