# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
###### Abstract
This process has been achieved utilizing FWM, whereby a relatively low repetition rate pump switches out temporally multiplexed channels by converting them to new wavelengths (the idler in the FWM process) [41; 42; 43]. There are, of course, other routes that can be followed to increase transmission bandwidth and/or to develop advanced processing chips. The goal of increasing bandwidth density of on-chip interconnects without increasing the number of waveguides, waveguide crossings, and chip footprint, for instance, may be reached by exploiting mode-division-multiplexing (MDM) in conjunction with WDM [44]. Polarization-division-multiplexing (PDM) and orthogonal frequency-division-multiplexing (OFDM) are two other effective methods to increase the spectral efficiency of a communication system [45]. When signal modulation rate increases, signal degradation in the optical channel caused by dispersion, nonlinearity, and noise becomes a critical issue. Conventionally, signal regeneration in an optical system is performed through optical-electrical-optical (OEO) conversion, in which a weak and distorted signal is detected, restored in electronics, and retransmitted onto an optical fiber. Regenerators are designed to increase system performance, reduce data degradation, and enhance signal-to-noise ratio (SNR) for higher link capacity. In general, regenerators perform three signal processing functions: (1) reamplifying, (2) reshaping, and (3) retiming the signal. When the data rate is becoming higher and higher (towards 100 Gb/s), optoelectronic regeneration schemes will be very hard to implement or even impossible. Thus, all-optical regeneration, either 2R (re-amplification, re-shaping) or 3R (re-amplification, re-shaping, re-timing), has become a key technology to improve signal quality [50; 51]. Nonlinear optical sources provide an outstanding example of new possibilities offered by integrated nonlinear photonic chips, such as generation of new classes, named supercontinuum generation [60], and microcombs [61], which are capable of generating coherent, ultra-broadband light sources that cannot be produced in linear photonic systems. Supercontinuum generation is a device functionality that has important applications in many areas of photonic integrated circuits, particularly in WDM applications. As an example, it could be beneficial to use a single broadband laser source, select by filtering specific wavelength channels, and then modulate these channels instead of using a separate laser for each wavelength. Optical frequency combs (OFCs) are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally spaced spectral lines that represent precise marks in frequency. This discrete ensemble of equally spaced laser frequencies that distinguish OFCs from other light sources is the spectral counterpart of the regular train of short pulses emitted by mode-locked lasers. The OFCs solve the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection, or photonics-assisted signal processing. Readers interested to delve into the main physical processes involved in nonlinear photonics applications and the most appealing materials are referred to a companion paper [62]. The focus here is to present some examples of nonlinear integrated photonic devices, with a brief overview of the types of optical integrated structures and the most common material fabrication platforms. In the next section, planar and three-dimensional geometry structures are introduced; in the third section, material platforms are discussed, and, in the last section, advances in nonlinear integrated photonic devices that provide an idea of the high potential for practical applications and future challenges are reported and discussed.