In recent years, global big data and network traffic have experienced explosive growth, and signal processing faces significant challenges in capacity and energy consumption. Currently, more than 90% of data information is transmitted via optical waves. As long as information processing is conducted using electronic devices, optical-electrical-optical (O-E-O) conversion is required. Compared to the processing speed of electronic devices, the bandwidth of optical signal transmission is significantly large. The signal should be demultiplexed into multiple low-rate signals, which can then be processed in the electrical domain to facilitate optical signal processing. However, demultiplexing and subsequent processing will increase the required number of O-E-O conversion devices, resulting in higher system complexity, costs, and energy consumption. On the other hand, optical nonlinear effects feature ultrafast response, large bandwidth, and parallelism, which can directly process high-speed optical signals. Nonlinear optical signal processing (NOSP) is the process of employing optical nonlinear effects to process information. If efficient enough, NOSP has the potential to significantly reduce the cost and power consumption of network information exchange and processing. Unfortunately, NOSP usually requires high-power lasers since photons are bosons and the interactions between photons are usually weak. The interaction between light and matter should be enhanced to reduce power consumption. In recent years, the development of semiconductor integration technology has promoted the development of photonic devices toward integration. Photonic integrated devices feature low cost, low power consumption, light weight, high stability, and small size, which are beneficial for realizing more complex functional devices. Additionally, photonic integrated devices can localize the optical field in a very small area, greatly enhancing the interaction between light and matter. Meanwhile, integrated material platforms with high refractive indices have high nonlinear coefficients, making them suitable for developing NOSP devices and applications. In the early development of optical communication technology, communication capacity improvement relied on time division multiplexing. Researchers employed nonlinear effects to develop ultrafast optical switches for demultiplexing time division multiplexed signals. With the invention of optical amplifiers and the popularity of wavelength division multiplexing technology, NOSP applications such as all-optical wavelength conversion, format conversion, all-optical logic, and all-optical signal regeneration have emerged. The optical network is evolving toward more flexible and efficient transparent optical network requirements, and NOSP is also developing toward more advanced functions, such as constellation aggregation or disaggregation, advanced modulation format regeneration, efficient spectral bandwidth allocation, and spectral shifting. NOSP devices mainly adopted bulk materials in the early days, among which high nonlinear fibers (HNLFs), periodically poled lithium niobate (PPLN), and semiconductor optical amplifiers (SOA) caught wide attention. HNLF is mature in fabrication technology, enabling ultra-low loss transmission where nonlinear effects can accumulate. The nonlinear effects can be significant after traveling down the HNLF over considerable lengths (usually several hundred to thousands of meters). Issues related to HNLF include large size, dispersion fluctuation, vulnerability to environmental effects in polarization state, limitations in nonlinear conversion bandwidth, and low threshold of stimulated Brillouin scattering, which restrict the applications. Lithium niobate as a second-order nonlinear material has high nonlinear efficiency. However, due to phase mismatch between the fundamental and second harmonic waves, periodic poling of lithium niobate is usually required to achieve quasi-phase matching and improve conversion efficiency. The photorefractive effect of PPLN may degrade the phase matching condition at high pump power. Additionally, the optical waves involved in NOSP usually operate in the same wavelength range, and achieving NOSP with PPLN requires cascading second-order processes. SOA is an optoelectronic device that exhibits strong nonlinear effects due to the interaction between light and carriers. It is characterized by high efficiency, small size, and low cost, but also has limitations such as limited response rate and high noise levels. In recent years, various low-loss photonic integration platforms have emerged, with the rapid development of integrated NOSP devices. Photonic integrated devices have small mode field areas, providing extremely high energy densities. Meanwhile, integrated materials usually have high nonlinear coefficients, giving them inherent nonlinear advantages. In addition, the development of heterogeneous integration technology combines the advantages of various materials, allowing for the manufacture of various optoelectronic functional modules on the same chip using different materials. Typical integrated nonlinear material systems include silicon, hydex, silicon nitride, aluminum gallium arsenide, thin-film lithium niobate, and chalcogenide glasses. Wide bandgap compound semiconductor materials such as gallium nitride and silicon carbide have been widely adopted in new energy vehicles, 5G, aerospace, and other fields. They have recently caught the attention of optical nonlinearities. Additionally, integrated nonlinear devices based on gallium phosphide have also been reported, and the ultra-high nonlinearities of materials such as two-dimensional materials and organic polymers are gradually gaining attention.

We review the development and application of nonlinear optical signal processing devices. The advantages and disadvantages of various optical nonlinear effects utilized in signal processing are discussed, with the unique requirements of nonlinear optical signal processing devices highlighted. To overcome the low efficiency of nonlinear optical signal processing devices, we summarize methods for enhancing efficiency from material and structural innovation aspects. Meanwhile, an overview of the nonlinear performance of diverse integrated material platforms is presented. Various structural characteristics for improving efficiency are compared in detail in terms of baud rate, nonlinear bandwidth, manufacturability, integration density, and power efficiency. Optical frequency combs can deliver multi-beam coherent light, and their integration with all-optical signal processing technology has the potential for more sophisticated signal processing capabilities.

The combination of nonlinear and integrated technologies provides an effective way to manipulate light and drive the development of nonlinear photonic functional devices. Nonlinear integrated photonics has shown great potential in optical signal processing, with the prospect of reducing system power consumption and cost. Currently, low nonlinear efficiency remains a major challenge for various NOSP applications. While new materials and structures continue to emerge, there is still significant room for improvement in nonlinear applications. We focus on the merits, challenges, and typical applications of nonlinear integrated devices in optical signal processing, including optical frequency combs, wavelength conversion for optical communication networks, format conversion, regeneration, optical computing, and quantum light source applications. With the continuous performance improvement of various functional devices on the chip, integrated optical signal processing technology is expected to yield significant breakthroughs in the near future. For example, with the development and maturity of optical frequency comb technology, multiple coherent beams of light can be provided via simple devices, and the interaction strength between light and matter can be further enhanced by material development and the design of novel structures to improve nonlinear efficiency. Heterogenous integration and on-chip amplifiers have also rapidly developed, and it is expected that integrated optical combs, amplifiers, modulators, delay lines, and nonlinear units can be built on a single chip. Finally, this enables a wider range of NOSP functionalities to be realized and thus may revolutionize optical communication, computing, and quantum optics applications.