# An Introduction to Nonlinear Integrated Photonics: Structures and Devices
## 4 Nonlinear Photonics Devices
### All-Optical Processing
#### 4.2.3 All-Optical Signal Regeneration  
Most of the work completed so far on all-optical signal regeneration has exploited the nonlinear properties of optical fibers [178; 179] or semiconductor optical amplifiers (OAS) [180; 181; 182]. Figure 14 shows the experimental setup for characterization of a polarization-independent differential phase shift keying (DPSK) regenerative wavelength converter [181]. A 10 Gb/s DPSK data stream at \(\lambda_{\text{DATA}}\) is generated by modulating the output of a tunable laser (TL) with a Mach-Zehnder modulator (MZM) driven by a bit pattern  
Figure 12: Schematic of an all-optical four-channel processor for NRZ to RZ format conversion to be integrated in an InP monolithic chip. MMI1 splits the signal and MMI2 recombines the signals from the two arms, which have a different phase shift. Operation wavelength is around 1570 nm, which is close to the gain peak of SOAs. Reproduced with permission from [177], 1943-0655 © 2023 IEEE. Figure 13: Image by a metallographic microscope of the fabricated InP monolithic chip according to the schematic in Figure 11. Reproduced with permission from [177], 1943-0655 © 2023 IEEE. generator (BPG). The modulator output is then coupled with an amplified spontaneous emission (ASE) noise loading stage, constituted by an erbium-doped fiber amplifier (EDFA) and an optical filter (OF) with 1 nm bandwidth. Two TL pumps are coupled with the noisy data and injected into the SOA: the output converted data at \(\lambda_{\text{FWM}}\) are selected, and DPSK demodulation is made by using a standard delay-interferometer (DI). The dual co-polarized pumps allow FWM independence from input signal polarization and wavelength. In the system, which is suitable for photonic integration, wavelength conversion in a range up to 6 nm was measured [181]. SOAs, however, are limited in speed due to carrier dynamics, and other attempts were made of realizing an integrated all-optical regenerator by means of other devices. One of the earliest results was obtained by [PERSON]'s group exploiting the strong Kerr nonlinearity of As\({}_{2}\)S\({}_{3}\) chalcogenide glass waveguides (n\({}_{2}\)-2 \(\times\) 10\({}^{-14}\) cm\({}^{2}\)/W) [183]. The operation principle of the integrated 2R regenerator was based on a combination of nonlinear SPM-induced spectral broadening followed by spectral filtering; the device was constituted by a low loss As\({}_{2}\)S\({}_{3}\) rib waveguide with integrated Bragg gratings. A nonlinear power transfer curve was demonstrated using 1.4 ps pulses, enough to suppress noise in an amplified transmission link [183]. Shortly thereafter, an SOI waveguide device was demonstrated to perform optical regeneration based on a Si nanowaveguide as the nonlinear medium for the SPM process integrated with a bandpass filter consisting of a waveguide ring resonator [184]. Compared with the chalcogenide device, which required \(\sim\)50 W peak power, the SOI device operated at peak powers lower than 5 W. Since then, many other excellent results have been achieved; here, only a few of them are mentioned. On-chip four-level pulse-amplitude modulation (PAM-4) wavelength conversion and signal regeneration were demonstrated in a silicon waveguide, exploiting degenerate FWM [36]. Using a silicon nanowire waveguide, [PERSON] et al. [185] experimentally demonstrated three-wavelength regeneration based on the clock-pump four-wave-mixing scheme. A limitation of silicon devices lies in nonlinear absorption (two-photon absorption, TPA): it can, however, be circumvented by using reverse-biased p-i-n diode structures implemented across the waveguide [186]. With this approach, phase regeneration for DPSK signals using a Si waveguide with a reverse-biased p-i-n junction as nonlinear medium and continuous-wave pumping was demonstrated, with high wavelength conversion efficiency [187].