An Introduction to Nonlinear Integrated Photonics: Structures and Devices

Abstract

The combination of integrated optics technologies with nonlinear photonics, which has led to growth of nonlinear integrated photonics, has also opened the way to groundbreaking new devices and applications. In a companion paper also submitted for publication in this journal, we introduce the main physical processes involved in nonlinear photonics applications and discuss the fundaments of this research area. The applications, on the other hand, have been made possible by availability of suitable materials with high nonlinear coefficients and/or by design of guided-wave structures that can enhance a material's nonlinear properties. A summary of the traditional and innovative nonlinear materials is presented there. Here, we discuss the fabrication processes and integration platforms, referring to semiconductors, glasses, lithium niobate, and two-dimensional materials. Various waveguide structures are presented. In addition, we report several examples of nonlinear photonic integrated devices to be employed in optical communications, all-optical signal processing and computing, or in quantum optics. We aimed at offering a broad overview, even if, certainly, not exhaustive. However, we hope that the overall work will provide guidance for newcomers to this field and some hints to interested researchers for more detailed investigation of the present and future development of this hot and rapidly growing field.

Keywords: all-optical communications; all-optical computing; all-optical digital devices; all-optical signal processing; all-optical signal regeneration; integrated photonics; microcomb generation; nonlinear photonics; optical materials; photonic structures; photonics devices; signal amplification and frequency conversion; supercontinuum generation.

Figure 10

Output spectra from a single-mode silicon photonic wire waveguide with a cross-section of 220 nm × 445 nm and length of L = 4.2 mm fabricated on a SOI. pump (λp~1435 nm), and signal laser sources were multiplexed and launched into the waveguide using a tapered fiber in copropagating configuration; several signal wavelengths (λs = 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.

Figure 1

Schematic representation of various channel waveguides. Adapted from [63] under Creative Commons License 3.0.

Figure 2

Different types of optical microcavities: (a) micro-ring resonator, (b) micro-toroid resonator, (c) micro-sphere resonator. Adapted from [76] under Creative Commons License 3.0.

Figure 3

Structure of a photonic crystal waveguide, with air holes represented by blue and red circles. Simulations were made by considering standard silicon-on-insulator air-bridge PhCW of 220 nm thickness, lattice period 400 nm, and radius R of the first air-holes row 100 nm. By adjusting the radius R2 and lattice positions ΔX of the second air-holes rows, a very wide flat band larger than 50 nm could be obtained. Reproduced from [81] under Creative Commons License 3.0.

Figure 4

BIC waveguide structure based on polymer strip waveguides (WG1 and WG2) onto a lithium niobate film. Here, WG1 and WG2 are 500 nm thick and LN film is 300 nm thick. (a) Cross-section of the structure; (b) normalized electric field distribution of the TE continuous mode; (c) and (d) TM leaky modes. Under proper conditions, the coupling between the TE continuum modes and the TM bound modes can lead to well-confined BIC modes (see (e,f)). Reproduced from [109] under Creative Commons License.

Figure 5

Schematic illustration of meta-atom, 1D chain, 2D metasurface, and 3D metamaterial. Inserts are the representation of the parameter space for permittivity ε and permeability μ and the typical examples of applications of metamaterials. Adapted from [102] under Creative Commons License 3.0.

Figure 6

Schematics of AlGaAs platforms and waveguide geometries. (a–c) 3-layer platform with strip-loaded, nanowire, and half-core waveguides, respectively. (d–f) 2-layer platforms with suspended nanorib, suspended nanowire, and AlGaAs-OI waveguides, respectively. (g–i) Multi-layer platform with multi-quantum-well waveguide, modulated-χ⁽²⁾ waveguide, and Bragg-reflector waveguide, respectively. Reproduced from [120] under Creative Commons License.

Figure 7

Fabrication of a microring in a 350 nm thick a-SiC film with silica cladding of 3 µm (bottom) and 2 µm (top). (a) Schematic process flow; (b) SEM micrograph of the microring having 100 µm diameter; (c) higher magnification SEM image of the coupling area between the ring and a ridge waveguide. Reprinted with permission from Xing et al., ACS Photonics [134]. Copyright 2019, American Chemical Society.

Figure 8

Schematic of a Si-PIC packaged with a multi-channel quasi-planar-coupled (QPC) fiber-array, a hybrid-integrated laser source based on a micro-optic bench (MOB), a vertically integrated electronic integrated circuit (EIC), and a thermo-electric cooler. Electrical connections between the PIC and the printed circuit board (PCB) are made by wire-bonds, while the connections between the PIC and EIC are made using copper pillar bumps (CPBs). Reproduced from [137] under Creative Commons (CC-BY) license.

Figure 9

Schematic illustration of an all-optical switch made of an add–drop microring resonator. The dotted line represents the transmittance spectrum of a cold cavity. The solid line is the transmittance when inputs are applied. A resonant shift occurs due to the optical Kerr effect. Two different wavelengths, λ₁ and λ₂, are used for the operation. On the left column λ₁ will drop (high) when only λ₁ is inputted (high).) On the middle column λ₂ will not drop (low) when only λ₂ is inputted (high). On the right column λ₂ will drop (high) when both λ₁ and λ₂ are inputted (high). As a result, λ₂ can be switched off and on by turning λ₁ signal on and off. Reprinted with permission from [22] © The Optical Society.

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.

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.

Figure 14

Sketch of the experimental setup for characterization of a SOA-based polarization-independent all-optical regenerator for DPSK data. The principles of operations are schematized within the two dashed boxes. (TL: tunable lasers; MZM: Mach–Zehnder modulator; BPG: bit pattern generator; EDFA: Erbium-doped fiber amplifier; PC: polarization controller; VOA: variable optical attenuator; OF: optical filter; DI: delay line.) Reproduced from [181] under Creative Commons license.

Figure 15

Supercontinuum generation in a 4.7 mm long silicon photonic wire waveguide for several input central wavelengths at P 0 ≈ 1W. The inset shows that spectral broadening increases as λ₀ approaches the ZGVD wavelength of 1290 nm. Reprinted with permission from [193] © The Optical Society.

Figure 16

EO comb generator using a microring resonator. (a) Sketch of a bulk EO comb generator based on an EO (χ⁽²⁾) phase modulator inside a Fabry–Pérot resonator. A cw laser beam enters the resonator and an optical frequency comb is generated at the output. (b) EO comb generation principle; the microwave modulating signal has frequency equal to FSR of the Fabry–Pérot resonator. (c) Integrated EO comb generator, where a microring replaces the Fabry–Pérot resonator. The cw signal from input waveguide is coupled into the microring and EO-modulated at a frequency matching the FSR of the ring. The generated comb is then coupled back to the waveguide. Reproduced with permission from [199] © Springer Nature Ltd.