Quasi-Phase-Matched Supercontinuum Generation in Photonic Waveguides

Abstract

Supercontinuum generation in integrated photonic waveguides is a versatile source of broadband light, and the generated spectrum is largely determined by the phase-matching conditions. Here we show that quasi-phase-matching via periodic modulations of the waveguide structure provides a useful mechanism to control the evolution of ultrafast pulses during supercontinuum generation. We experimentally demonstrate quasi-phase-matched supercontinuum to the TE20 and TE00 waveguide modes, which enhances the intensity of the SCG in specific spectral regions by as much as 20 dB. We utilize higher-order quasi-phase-matching (up to the 16 th order) to enhance the intensity in numerous locations across the spectrum. Quasi-phase-matching adds a unique dimension to the design-space for SCG waveguides, allowing the spectrum to be engineered for specific applications. Supercontinuum generation (SCG) is a χ(3) nonlinear process where laser pulses of relatively narrow bandwidth can be converted into a continuum with large spectral span [1][2][3]. SCG has numerous applications, including self-referencing frequency combs [4][5][6], microscopy [7], spectroscopy [8], and tomography [9]. SCG is traditionally accomplished using bulk crystals or nonlinear fiber, but recently, "photonic waveguides" (on-chip waveguides produced using nanofabrication techniques) have proven themselves as a versatile platform for SCG, offering small size, high nonlinearity, and increased control over the generated spectrum [10][11][12][13][14][15][16][17][18][19]. The spectral shape and efficiency of SCG is determined by the input pulse parameters, the nonlinearity of the material, and the refractive index of the waveguide, which determines the phasematching conditions. Specifically, when phase-matching between a soliton and quasi-continuous-wave (CW) light is achieved, strong enhancements of the intensity of the supercontinuum spectrum can occur in certain spectral regions. These spectral peaks are often referred to as a dispersive waves (DWs) [2,[20][21][22], and they are often crucial for providing sufficient spectral brightness for many applications. The soliton-DW phase-matching condition is typically satisfied by selecting a material with a favorable refractive index profile and engineering the dimensions of the waveguide to provide DWs at the desired wavelengths [15,16]. However, there are limitations to the refractive index profile that can be achieved by adjusting only the waveguide cross-section. Quasi-phase-matching (QPM) takes a different approach, utilizing periodic modulations of the material nonlinearity to achieve an end-result similar to true phase-matching [23][24][25][26]. QPM is routinely employed to achieve high conversion efficiency for nonlinear * danhickstein@gmail.com FIG. 1.a,b) Quasi-phase-matching (QPM) of supercontinuum generation in on-chip photonic waveguides can be achieved via waveguide-width modulation (a) or claddingmodulation (b). c) When the effective index of the soliton in the TE00 mode ("soliton index") intersects the ...

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