# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
###### Abstract
As the input intensity is increased, the field inside the cavity also increases, lowering the absorption of material and thus increasing the field intensity further. If the intensity of the incident field is subsequently lowered, the field inside the cavity tends to remain large because the absorption of the material system has again been reduced. However, even if single bistable switches have already been demonstrated on different platforms, it is worth noting that the next big challenge is realization of a complex system where several bistable switches are connected in tandem and in parallel [19; 20; 21; 22; 23; 24; 25; 26; 27]. Nowadays, almost all data flow, including internet, phone calls, etc., goes through fiber optic transmission lines, and the field of communications continues to expand to higher data rates and shorter delays to allow more capacity. The demands of the modern world are for high-speed optical communication and interconnection. The increase in the amount of data available presents both opportunities and problems, leading to \"big data\" (i.e., large, diverse sets of information that grow at ever-increasing rates) issues. Storing and handling big data may be difficult using traditional techniques; such a new challenge may be faced by using photonic devices for massive parallel processing and nonlinear photonic devices for ultrafast handling [28; 29; 30]. Today, in optical networks, data are encoded on photons for transmission, while information processing is often carried out through optical-electrical-optical (OEO) conversions. In the last 20 years, with the aim to assist/replace some of the electronic modules used in network routers, optical signal processing systems (OSPs) have been investigated. OSP refers to a broad range of techniques with the aim to process and manipulate the signal, i.e., amplitude, phase, wavelength, and polarization of optical waves, directly in the optical domain. Optical signal processing techniques employ a wide range of devices and various nonlinearities to achieve multiple network functionalities. Widely used functionalities demonstrated in nonlinear photonic circuits (PICs) include wavelength [31; 32; 33; 34; 35] and format conversions [36; 37], routing [38; 39], phase-sensitive amplification [40], optical multiplexing and demultiplexing [41; 42; 43; 44; 45], optical memory [46; 47], all-optical tunable delay [48; 49], and signal regeneration [50; 51]. Thus far, most of the existing OSP research has relied on third-order nonlinearities, such as four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM) [52; 53; 54; 55; 56; 57; 58; 59]. Current optical networks are mostly based on time-division-multiplexing (TDM) and wavelength-division-multiplexing (WDM). In the former, multiple relatively low-bit-rate streams of data with the same carrier frequency are interleaved to create a single high-bit-rate stream, while the latter involves simultaneous propagation of multiple data signals, each at a different wavelength in a single optical line. Wavelength-division-multiplexing (WDM) techniques offer very effective utilization of the fiber bandwidth directly in the wavelength domain. A first issue of current optical networks is wavelength-blocking. In order to overcome it, a fundamental piece is represented by wavelength conversion devices. They can be obtained using different nonlinear effects, such as FWM or XPM [31; 32; 33; 34; 35]. Another issue is to increase transmission bandwidth, and an option is to combine TDM and WDM. In this process, demultiplexing an ultra-high-data-rate time-multiplexed signal to speeds receivable through electronics is achieved by wavelength conversion. Specifically, information multiplexed in the time domain through optical time demultiplexing (OTDM) can be converted into parallel lower-data-rate wavelength or spatial channels.