# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 4 Nonlinear Photonics Devices
### All-Optical Processing
#### 4.2.2 Frequency Conversion  
In FWM-based converters, the wavelength conversion from the pump to the idler is realized and the information optically encoded onto one wavelength channel can be simply copied onto another wavelength channel. Let us mention a few published results:  
1. C-band wavelength conversion in Si photonic wire waveguides with submicron cross-section was demonstrated by means of nondegenerate FWM (see Figure 10 of [157]). The nonresonant character of the FWM enabled demonstrating frequency tuning of the idler from \(\sim\)20 GHz to \(>\)100 GHz, thus covering several C-band DWDM channels. 2. Conversion bandwidths greater than 150 nm and peak conversion efficiencies of \(-\)9.6 dB were also achieved via FWM and appropriate engineering in silicon nanowaveguides [158]. Furthermore, utilizing fourth-order dispersion, wavelength conversion across telecommunication bands from 1477 nm (S-band) to 1672 nm (U-band) was demonstrated with an efficiency of \(-\)12 dB. 3. A wavelength conversion bandwidth of 190 nm with an efficiency of 21 dB, obtained by FWM in polymer (PMMA)-cladded chalcogenide (As\({}_{2}\)Se\({}_{3}\)) hybrid microwires, was achieved [159]. Wavelength conversion combined with small footprint (10 cm length), low loss (<4 dB), ease of fabrication, and the transparency of As\({}_{2}\)Se\({}_{3}\) from near-to-mid-infrared regions make the proposed device very promising. The goal of achieving a broad wavelength conversion bandwidth may be achieved acting on group-velocity dispersion (GVD); flattened GVD is also important for effective supercontinuum generation. Silicon waveguides exhibit high nonlinearity, but dispersion flattening represents a difficult task due to the strong light confinement and high waveguide dispersion. One has mainly to work on the design of the cross-sectional dimension of the waveguide, and various solutions have been proposed. Staying with a rib waveguide allows only a few degrees of freedom so that sandwich and slot structures with greater flexibility in geometrical parameters have been designed and tested. Both sandwich and slot waveguides are realized by introducing slots between silicon strips and another material to fill the slot(s); the standard silicon-on-insulator (SOI) technology, however, makes easier fabricating the vertical-slot structure than the horizontal-sandwich one. An early work was based on a hybrid strip/slot silicon waveguide, capable of producing a flattened dispersion of 0 \(\pm\) 16 ps/(nm\(\cdot\)km) over a 553-nm wavelength range (1562 to 2115 nm) [160]. Shortly  
Figure 10: Output spectra from a single-mode silicon photonic wire waveguide with a cross-section of 220 nm \(\times\) 445 nm and length of L = 4.2 mm fabricated on a SOI. pump (\(\uplambda\)p\(\sim\)1435 nm), and signal laser sources were multiplexed and launched into the waveguide using a tapered fiber in copropagating configuration; several signal wavelengths (\(\uplambda\)s = 1545.5, 1548.5, 1550.5, 1552.5, and 1555.5 nm) were used. On each side of the signal wavelength employed, newly generated satellite peaks are clearly seen. Reprinted with permission from [157] © The Optical Society. later, a horizontal double-slot Si waveguide was designed to achieve low and flattened dispersion: simulation indicated that two silica slots at the bottom of the waveguide could produce flattened dispersion from \(-26\) to \(21\) ps/(nm\(\cdot\)km) over a bandwidth of \(802\) nm, whereas a structure with two silica slots placed at both sides could achieve flattened dispersion from \(-17\) to \(23\) ps/(nm\(\cdot\)km) over a bandwidth of \(878\) nm (\(1498\) to \(2376\) nm wavelength) [161].