Microwave photonics (MWP) is an interdisciplinary and cutting-edge technology that combines microwave and lightwave to generate, transport, manipulate, and measure wide-band radio-frequency (RF) signals. This field has become a focal point of research in recent years. Compared with traditional electronic systems, MWP systems offer advantages such as wide bandwidth, lightweight, low loss, and anti-electromagnetic interference. These unique benefits make MWP highly attractive for various applications including radar, electronic warfare, sensing, measuring, and communication systems. Representative demonstrations include microwave photonic radar system, ultra-wideband radio on fiber (ROF) transmission, high precision optical vector network analyzer (OVNA), ultra-low phase noise RF signal generation, and ultra-wideband RF receiver for electronic warfare. However, these impressive demonstrations are mostly bulky systems composed of discrete fiber components, which still have drawbacks such as large size, high power consumption, large mass, and costliness. They are also sensitive to environmental perturbations like vibrations and temperature variations. Therefore, there is an urgent need to reduce the size of MWP systems for their large-scale applications. Benefiting from the rapid advancements of optoelectronic integration technologies, researchers are dedicated to developing various integrated MWP chips. As known, the key requirements for high-performance MWP systems include high RF gain, low noise figure (NF), and large spurious-free dynamic range (SFDR). These can be translated to the need for low loss, high optoelectronic conversion efficiency, low noise, and high linearity in integrated MWP chips. However, none of the photonic integration platforms such as indium phosphide (InP), silicon on insulator (SOI), silicon nitride (Si₃N₄), silicon dioxide (SiO₂), and lithium niobite (LiNbO₃) can fulfill all these requirements. Given the inherent limits, numerous integrated MWP chips fabricated on these photonic integration platforms have been developed according to various application requirements. Furthermore, with the increasing complexity of microwave photonic application scenarios and the fast developments of optoelectronic integration technologies, some high-performance integrated MWP chips and modules adopting heterogeneous or hybrid integrations have begun to emerge. In this context, we aim to showcase the latest research progress in the field of integrated MWP chips. This includes MWP transceiver chips, MWP signal generation chips, MWP filtering chips, MWP beamforming chips, MWP frequency measurement chips, and programmable MWP chips. We also look forward to future development trends in integrated MWP technologies.

The MWP transceiver, capable of transmitting and receiving broadband RF signals, is a key module in radar and wireless communication systems. Over the past decade, some integrated MWP transceiver chips have been demonstrated. Recently, except for the external laser, monolithic integrated SOI MWP transmitter and receiver chips have been demonstrated in Fig. 2. An MWP receiver with RF frequency down-conversion capability has been reported using LiNbO₃ and Si₃N₄ hybrid integration (Fig. 3). Additionally, by adopting micro-assembly of lasers, modulators, photodetectors, and other electronic chips, an MWP transceiver module has been demonstrated in Fig. 4. However, the RF gain, SFDR, and NF of the module still need improvement. For low phase noise single-frequency RF signal generation, integrated optoelectronic oscillators (OEOs) are rapidly developing. Initially, tunable optical filter chips are inserted into OEO loops to achieve tunable RF signal generation. To overcome mode competition in the cavity, very high-quality factor (Q) micro-ring resonators (MRRs) and parity-time (P-T) symmetry mechanisms shown in Figs. 5 and 7 are adopted in OEOs to ensure single-mode oscillation. Furthermore, a compact OEO module has been demonstrated using hybrid integration and micro-assembly as shown in Fig. 6. However, the phase noise of the integrated OEO is still limited by the loop length and can be improved by inserting a long fiber outside of the module. Time domain synthesis (TDS) shown in Fig. 8 and spectral shaping and wavelength-to-time mapping (SS-WTT) shown in Fig. 9 have been proposed for arbitrary RF signal generation. Specially designed optical spectral shaping chips such as linear chirped waveguide Bragg gratings (LC-WBGs), LC-WBG assisted Mach-Zehnder interferometers (MZIs), LC-WBG assisted Sagnac, linear chirped grating-assisted contra-directional couplers (LCGA-CDCs) shown in Fig. 10 have been proposed to generate well-known linearly chirped microwave waveforms (LCMWs), which are very useful in microwave photonic radar systems. Moreover, Fourier-domain mode-locked (FDML) technology has been adopted in the OEO loop as shown in Fig. 11, which can obtain LCMW with a large time-bandwidth product (TBWP). Microwave photonic filters with frequency resolution down to several tens of MHz have been demonstrated by optimizing high Q MRRs or using the stimulated Brillouin scattering (SBS) effect in nonlinear waveguides such as As₂S₃ (Fig. 12). To enhance the limited out-of-band RF rejection ratio induced by the residual phase of adjacent resonance in MRRs, amplitude and phase manipulating methods such as unbalanced double sideband modulation, dual optical carriers and dual MRRs are proposed (Fig. 13). Furthermore, monolithic and hybrid integrated microwave photonic filters have been demonstrated on the InP and SOI platforms (Fig. 14). Although the basic functionalities of microwave photonic filters have been well verified, their RF gain, NF, frequency resolution, and stability still need improvement. The microwave photonic beamforming chip is a key component in an optical controlled phase array antenna (PAA) system. To reduce loss, various specially designed low loss waveguides such as SiO₂, SOI, Si₃N₄, and thin film lithium niobite (TFLN) have been used to construct low loss optical switchable delay lines (OSDLs) (Fig. 15). However, these OSDLs can only achieve discrete delay switching. Hence, various dispersion components such as MRRs, WBGs, asymmetric MZIs, and subwavelength gratings (SWGs) have been proposed to achieve continuous delay tuning. Then, by combining the above two methods, some hybrid optical tunable delay line (OTDL) chips have been demonstrated in Fig. 17. In addition, wideband microwave photonic beamforming systems with multi-channel OTDL chips have demonstrated good directional transmission and receiving capabilities (Fig. 18). On the other hand, by utilizing frequency-to-power mapping and frequency-to-time mapping mechanisms, specially designed MRR-based chips are used for achieving microwave photonic frequency measurements. Compared with frequency-to-power mapping, frequency-to-time mapping can achieve complex microwave signal measurements such as multi-frequency, chirp, and even frequency-hopping RF signals. Recently, monolithic integrated microwave photonic frequency measurement chips have emerged (Figs. 22 and 23). In addition to the application-specific MPW chips mentioned above, inspired by field programmable gate arrays (FPGA) in microelectronics, research on MWP signal processing chips with programmable and reconfigurable characteristics has also rapidly developed. The main solutions include the use of tunable MZI array networks shown in Fig. 24 and tunable MRR (MDR) array networks shown in Fig. 25. By properly configuring the driving signals applied on these chips, the light propagation paths in the networks can be well manipulated to achieve various MPW signal processing functions such as filtering, delay, wavelength multiplexing, differentiation, phase shifting, arbitrary signal generation, and frequency conversion. These programmable MWP chips are very suitable for the rapid development of new chip prototypes, which can greatly reduce the development time and costs.

Various types of MWP chips, such as MWP transceiver chips, MWP signal generation chips, MWP filtering chips, MWP beamforming chips, MWP frequency measurement chips, and programmable MWP chips, have developed rapidly. The completeness and scale of various on-chip MWP functional systems have been increased, significantly reducing the size, mass, and power consumption of MWP systems. However, there is still a certain gap between the core performance indicators (RF gain, SFDR, and NF) of MWP chips and real application scenarios. In the future, mature heterogeneous integration and optoelectronic hybrid integration technologies should be developed to leverage the advantages of various materials, breaking through key technologies such as efficient coupling of optical or electrical interfaces, suppression of optical, electrical, and thermal crosstalk, and improving stability. On this basis, multifunctional, multi-channel, and highly integrated MWP chips could be developed to meet the various requirements for high-performance radar, warfare communication, sensing, and measurement systems in the future.