# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 4 Nonlinear Photonics Devices
### Nonlinear Sources  
To generate SC from a chalcogenide glass fiber, a short pulse laser (i.e., fs laser) is often employed as the pump source operating at the NIR region where dispersion of chalcogenide glasses is positive. In SC generation under NIR pumping, the resultant spectra usually show limited bandwidth due to the limited pump power and materials dispersion. To this end, optical pumping at wavelengths longer than the zero-dispersion wavelength (ZDW) is often applied, where the dispersion of the chalcogenide glasses becomes negative. The SC generation is this case becomes much more efficient as the soliton effect becomes dominant, such as soliton splitting and self-frequency shift, which leads to much larger broadening of the SC spectra [192]. An experimental demonstration of a tunable continuum source in silicon photonic wires (SPWs) with a power-dependent broad spectrum was reported in [193]. As can be seen in Figure 15, a spectral broadening of more than 350 nm was observed upon propagation of ultrashort 1.3 \(\upmu\)m wavelength optical pulses in a 4.7 mm long single-mode waveguide. Supercontinuum white light generation in plasmonic nanostructures, depending on the nonlocality of the electron response, has also been theoretically investigated [194]. Mode-locked lasers were initially used for comb generation. Thereafter, comb emission was demonstrated in continuous-wave (cw) laser-pumped resonators through cascaded third-order parametric processes [195]. In such Kerr resonators, a first pair of sidebands is generated around the pump frequency by cavity modulation instability or degenerate four-wave mixing (FWM); subsequently, cascaded four-wave mixing processes lead to formation, around the pump frequency, of a uniform frequency comb, where self- and cross-phase modulation act to compensate for the unequal cavity mode spacing induced by the group velocity dispersion (GVD) [196]. Because of the relatively low strength of third-order nonlinearity, generation of Kerr combs requires small interaction volumes and high-Q resonators. For these reasons, small resonators are particularly suited to reach broadband comb generation with quite moderate pump power [62]. Advancements in the fabrication technology of optical micro-cavities may enable realizing ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates in the form of compact and integrated devices. In this framework, demonstration of planar high-Q resonators, compatible with silicon technology [197; 198], has revealed a unique opportunity for these devices to provide entirely new capabilities for photonic-integrated technologies. It can be recalled that electro-optic (EO) modulation, in materials with second-order nonlinearity, provides an interesting alternative to Kerr (third-order nonlinearity) resonators for generation of OFCs; the electrical controllability of EO combs guarantees greater versatility and also excellent comb stability and phase coherence. A microring resonator, with loaded Q-1.5 \(\times\) 10\({}^{6}\), may, therefore, be fabricated in a thin-film lithium niobate (TFLN) platform, which is also characterized by ultra-low-loss optical waveguides [199]. This solution is depicted in Figure 16; a microwave signal, with modulation frequency equal to the free spectral range (FSR) of the optical resonator, couples light between different resonator modes. The input cw laser light is, therefore, modulated, giving rise to sidebands at the modulation frequency, which are then recirculated to be modulated again. At the output, an EO comb can be measured that spans more frequencies than the entire telecommunications L-band (over 900 comb lines spaced about 10 gigahertz apart), with the prospect of enabling octave-spanning by proper dispersion engineering [199]. A disadvantage of this single-resonator device is the low (\(\sim\)0.3%) comb conversion efficiency due to the low coupling efficiency of the light from the input waveguide to the microring EO-driven resonance.