A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

Integrated optical frequency comb technologies

Midinfrared spectroscopy is a universal way to identify chemical and biological substances. Indeed, when interacting with a light beam, most molecules are responsible for absorption at specific wavelengths in the mid-IR spectrum, allowing to detect and quantify small traces of substances. On-chip broadband light sources in the mid-infrared are thus of significant interest for compact sensing devices. In that regard, supercontinuum generation offers a mean to efficiently perform coherent light conversion over an ultrawide spectral range, in a single and compact device. This work reports the experimental demonstration of on-chip two-octave supercontinuum generation in the mid-infrared wavelength, ranging from 3 to 13 μm (that is larger than 2500 cm-1) and covering almost the full transparency window of germanium. Such an ultrawide spectrum is achieved thanks to the unique features of Ge-rich graded SiGe waveguides, which allow second-order dispersion tailoring and low propagation losses over a wide wavelength range. The influence of the pump wavelength and power on the supercontinuum spectra has been studied. A good agreement between the numerical simulations and the experimental results is reported. Furthermore, a very high coherence is predicted in the entire spectrum. These results pave the way for wideband, coherent, and compact mid-infrared light sources by using a single device and compatible with large-scale fabrication processes.

On-Chip Mid-Infrared Supercontinuum Generation from 3 to 13 μm Wavelength.

We experimentally demonstrate supercontinuum (SC) generation in a germanium-on-silicon waveguide. This waveguide exhibits propagation loss between 1.2 dB/cm and 1.35 dB/cm in the 3.6 µm–4.5 µm spectral region for both transverse electric (TE) and transverse magnetic (TM) polarizations. By pumping the waveguide with ∼200 fs pulses at 4.6 µm wavelength, we generate a mid-infrared (IR) SC spanning nearly an octave from 3.39 µm to 6.02 µm at the −40 dB level. Through numerical analysis of the evolution of the SC, we attribute the current limit to further extension into the mid-IR mainly to free-carrier absorption.

https://hal.archives-ouvertes.fr/hal-03101507/document

Mid-infrared supercontinuum generation in a low-loss germanium-on-silicon waveguide

Efficient complementary metal-oxide semiconductor-based nonlinear optical devices in the near-infrared are in strong demand. Due to two-photon absorption in silicon, however, much nonlinear research is shifting towards unconventional photonics platforms. In this work, we demonstrate the generation of an octave-spanning coherent supercontinuum in a silicon waveguide covering the spectral region from the near- to shortwave-infrared. With input pulses of 18 pJ in energy, the generated signal spans the wavelength range from the edge of the silicon transmission window, approximately 1.06 to beyond 2.4 μm, with a −20 dB bandwidth covering 1.124–2.4 μm. An octave-spanning supercontinuum was also observed at the energy levels as low as 4 pJ (−35 dB bandwidth). We also measured the coherence over an octave, obtaining , in good agreement with the simulations. In addition, we demonstrate optimization of the third-order dispersion of the waveguide to strengthen the dispersive wave and discuss the advantage of having a soliton at the long wavelength edge of an octave-spanning signal for nonlinear applications. This research paves the way for applications, such as chip-scale precision spectroscopy, optical coherence tomography, optical frequency metrology, frequency synthesis and wide-band wavelength division multiplexing in the telecom window.

Octave-spanning coherent supercontinuum generation in silicon on insulator from 1.06 μm to beyond 2.4 μm

We demonstrate coherent supercontinuum generation spanning over an octave from a silicon germanium-on-silicon waveguide using ∼200fs pulses at a wavelength of 4 µm. The waveguide is engineered to provide low all-normal dispersion in the TM polarization. We validate the coherence of the generated supercontinuum via simulations, with a high degree of coherence across the entire spectrum. Such a generated supercontinuum could lend itself to pulse compression down to 22 fs.

/pdf/mid-infrared-supercontinuum-generation-in-silicon-germanium-36v56imkmq.pdf

Alberto DellaTorre

Papers

Mid-infrared supercontinuum generation in silicon-germanium all-normal dispersion waveguides