# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 4 Nonlinear Photonics Devices
### All-Optical Processing
#### 4.2.2 Frequency Conversion
In 2017, a new structure was proposed, where the field was mainly confined in the silicon core based on silica-filled vertical-dual slots; a spectrally flattened near-zero anomalous group-velocity dispersion covering the whole telecom wavelength range was possible, and, in a wavelength conversion experiment, a \(3\) dB bandwidth of \(76\) nm was measured, which was significantly broadened compared to the corresponding strip waveguide [162]. More recently, the problem of data transmission over the C band (\(1530\)-\(1565\) nm) closely approaching the nonlinear Shannon capacity limit began to be considered. One of the novel ways to circumvent this limit is to extend WDM beyond the C band, e.g., to the O band (\(1260\)-\(1360\) nm). It has been shown that a proper design of an SOI waveguide allows to shift wavelengths from the whole C band into the O band by FWM-based all-optical signal processing, without the need of changing pump laser wavelengths [163]. One year later, the same research group reported the experimental achievement of optical wavelength conversion over the C-to-O-band using multi-modal FWM in highly nonlinear SOI waveguides [164]. The authors claimed that it was an important step towards ultra-high-bandwidth optical communication networks, which utilize the entire low-attenuation infrared wavelength range of a standard single-mode fiber; as a further step towards a complete electronic-photonic-integrated circuit (EPIC), they envisaged the co-integration of the nonlinear signal processor with conventional transmitter- and receiver-units [164]. In order to face the difficulty of satisfying the energy and phase matching conditions in a microresonator, an interesting proposal was made by [PERSON] et al. [165], who designed and fabricated a PhC microresonator exhibiting a constant free spectral range. An InGaP cavity, \(200\)\(\upmu\)m long and \(650\) nm wide, was heterogeneously integrated on an SOI PIC. The structure is shown in Figure 11; the nanobeam cavity is fabricated on top of the Si waveguide using e-beam lithography and hole drilling by inductive coupling plasma (ICP) etching. A stimulated FWM with a \(-12\) dB signal to idler conversion was measured. Table 1 presents the characteristics of this PhC device in comparison to ring resonators in different materials (Q\({}_{\text{avg}}\) is the geometrical average of the loaded Q for the different resonance modes; \(\eta_{\text{NL}}\) is the nonlinear FWM conversion efficiency). It clearly appears that the PhC on the SOI device, even if with lower conversion efficiency, has the advantage of requiring lower pump power and a remarkably smaller footprint [165]. Figure 11: Photonic crystal (PhC) resonator integrated on an SOI platform on Si substrate: 3D sketch (**left**); YZ cross-section with the structure’s layers (**center**); BCB is benzocyclobutene, the adhesive used for bonding; SEM image of the InGaP cavity, \(650\) nm wide, \(290\) nm thick (**right**). Reproduced with modifications from [165] under Creative Commons license. In order to increase the bandwidth density of on-chip interconnects without increasing the number of waveguides, waveguide crossings, and chip footprint, an option is to exploit mode-division-multiplexing (MDM) in conjunction with WDM [44]. A device based on multiple microring resonators in an SOI wafer, having 0.11 mm\({}^{2}\) footprint, operated with multiple co-propagating 10 Gb s\({}^{-1}\) communication signals and reached up to 60 Gb s\({}^{-1}\) of aggregate bandwidth. It was claimed that, properly designed with five spatial modes and 87 WDM channels, the device could be able to support an aggregate data rate up to 4.35 Tb s\({}^{-1}\)[44]. Using polarization-division-multiplexing PDM, orthogonal-frequency-division-multiplexing OFDM transmission, and a polarization multiple-input multiple-output (MIMO) system at the receiver, a 16 \(\times\) 52.5 Gb s\({}^{-1}\) transmission over 4160 km of standard single-mode fiber was demonstrated with 50 GHz WDM channel spacing [45].