Abstract

We numerically investigate self-defocusing solitons in a lithium niobate (LN) waveguide designed to have a large refractive index (RI) change. The waveguide evokes strong waveguide dispersion and all-normal dispersion is found in the entire guiding band spanning the near-IR and the beginning of the mid-IR. Meanwhile, a self-defocusing nonlinearity is invoked by the cascaded (phase-mismatched) second-harmonic generation under a quasi-phase-matching pitch. Combining this with the all-normal dispersion, mid-IR solitons can form and the waveguide presents the first all-nonlinear and solitonic device where no linear dispersion (i.e. non-solitonic) regimes exist within the guiding band. Soliton compressions at 2 μm and 3 μm are investigated, with nano-joule single cycle pulse formations and highly coherent octave-spanning supercontinuum generations. With an alternative design on the waveguide dispersion, the soliton spectral tunneling effect is also investigated, with which few-cycle pico-joule pulses at 2 μm are formed by a near-IR pump.

Highlights

  • Quadratic nonlinear materials were used for the first nonlinear optics experiment known as the second harmonic generation (SHG) [1]

  • Self-defocusing soliton self-compressions as well as soliton-induced supercontinuum generations (SCGs) have been investigated and demonstrated in lithium niobate (LN) waveguides with small refractive index (RI) change [14,15,16,17], but the pump wavelength is limited to be below the zero dispersion wavelength (ZDW), which in LN is around 2 μm

  • In this paper we proposed an LN waveguide design with a large refractive index (RI) change, a design intended to extend self-defocusing soliton formation further into the mid-IR

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Summary

Introduction

Quadratic nonlinear materials were used for the first nonlinear optics experiment known as the second harmonic generation (SHG) [1]. Extending the operation range of cascaded quadratic nonlinear waveguides beyond 2 μm would be meaningful for a variety of applications such as spectroscopy and biological imaging One way around this is to use other novel quadratic nonlinear materials that naturally support self-defocusing soliton compressions in the near- and mid-IR [18], but waveguide formation has not been studied in most of them. We propose here a new LN waveguide design aiming to increase the RI change enough to substantially extend the normal dispersion regime so mid-IR self-defocusing soliton formation is possible We achieve this by a standard ridge waveguide design, similar to our recent publication [14]. We investigate a design where a small anomalous dispersion region is sandwiched between two normal dispersion regions, and show few-cycle DW formation in that region seeded by a near-IR soliton

Waveguide structure and dispersion properties
Nonlinear induced polarization and modal nonlinear susceptibilities
Cascaded quadratic nonlinearity and overall self-defocusing nonlinearity
Few-cycle self-defocusing soliton compressions beyond 2 μmum
DW generation and soliton spectral tunneling
Conclusion

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