Special issue on microwave photonics
Microwave photonics can be used for the generation, processing, receiving and distribution of microwave signals based on optoelectronic devices, with advantages such as broad bandwidth, low loss, light weight, fast reconfigurability and immunity to electromagnetic interference. With the rapid development of the theory and technology of microwave photonics, cross research between microwave photonics and multidiscipline has been the key of its direction of development. The research status of cross research between microwave photonics and some disciplines was summarized, and prospects of cross research between microwave photonics and disciplines such as laser technology, integrated optoelectronics, quantum technology and artificial intelligence were provided.
Traditional signal and information processing technologies are relatively independent and complicated. Artificial intelligence (AI) technology introduces a processing scheme of signal conversion plus information recognition to improve the level of intelligence in system processing. However, a high intensity of signals and information in future applications demand for more efficient systems and more flexible decision-making capabilities. It was proposed that optoelectronic integration technology was promising to realize the processing of signal and information as a new processing paradigm. Taking the complementary advantages of photonic and electronic technologies in electromagnetic scales, physical advantages, and practical implementations, overall and direct processing of signal and information was achieved and had the potential to integrate deeper levels of intelligence technology. Emerging signal and information processing paradigms enabled by optoelectronic integration were reviewed. The supportive significance of optoelectronic hybrid integration on optoelectronic integration processing technology was demonstrated.
Microwave signal detection and analysis are the key technologies for electrical information systems like communication, radar, electronic warfare. With the rapid development of new information technology, microwave photonic technology combines the advantages of both lightwave and microwave, which is characterized by the advantages of large bandwidth, low loss and anti-electromagnetic interference. In this paper, a comprehensive overview of the microwave photonic measurements, especially photonic-assisted microwave frequency measurement schemes based on frequency-amplitude mapping, frequency-to-time mapping, and Optical channelization was introduced. In addition, the corresponding problems and prospects were briefly summarized.
Content integrated, broadband, large group delay devices have important applications in microwave photonic filtering, true delay phased array antenna and other fields, which can effectively reduce the system size and power consumption. In this paper, a broadband large dispersion delay chip based on silicon-based photonic integration was proposed and implemented. By using ultra-low loss waveguide structure and side wall normal vector modulation structure, on-chip integration of large dispersion waveguide grating was realized. The dispersion was about 250 ps/nm, maximum group delay was 2440 ps and the bandwidth was more than 9.4 nm. The chip is expected to be used in microwave photonics, high-speed fiber communication system and other fields.
Microresonator-based frequency combs (microcombs) are very promising for microwave photonic applications for their advantages including wide bandwidth, large line spacing, and compact volume. Recently, various microcomb-based microwave photonic systems have been reported such as high-spectral-purity microwave generation, microwave photonic signal processing, true-time-delay beamforming, etc. Outstanding performances including low phase noise, high reconfigurability, and large Nyquist bandwidth have been demonstrated, showing a bright future of microcomb-based microwave photonics.
Silicon photonic integration platform has attracted extensive attention in the field of optical communication due to its high integration and CMOS process compatibility. As one of the most important devices in optical communication system, electro-optic modulator plays a key role in loading electrical signals onto optical signals. To break the performance limitation of silicon-based modulator, the large-area bonding technology of silicon and lithium niobate and the low loss waveguide etching technology of lithium niobate can be used to achieve high-performance thin film electro-optic modulator based on heterogeneous silicon and lithium niobate platform. At present, this kind of modulator with the best performance exhibits a half-wave voltage of 3 V, a 3 dB electro-optical bandwidth of more than 70 GHz, an insertion loss of less than 1.8 dB, and an extinction ratio of more than 40 dB. In this paper, the research status of integrated modulator based on silicon and lithium niobate heterogeneous platform was compared and the structure design and fabrication process of the heterogeneous integrated thin-film modulator were introduced respectively.
Microwave photonics is an interdisciplinary subject of microwave engineering and photonics technology. It is a fused microwave and optical system that studies the interaction between optical and microwave signals in the medium and the generation, processing, transmission, and receiving of microwave signals in the optical domain. Integration is an inevitable trend of microwave photonics since the performances of current discrete devices-based microwave photonic systems are poor in terms of size, power consumption, stability and cost. The main scientific and technical issues of integrated microwave photonics were discussed, its development status and frontier research progresses were summarized, and an outlook of its future prospects was given.
The electronic warfare is a key force of the information war, whose capabilities can be enhanced based on microwave photonics (MWP) technology due to the broadband, high speed, parallelism, and compactness. Microwave photonics technology, as applied to electronic warfare systems, involves the signal generation, transmission, and processing. Firstly, the mission and capability requirements of electronic warfare were presented, and the core elements affecting the effectiveness of electronic warfare was analyzed. Secondly, the advantages of MWP applied to the electronic warfare were discussed in detail. Taking the optical beam forming as an example, MWP technology overcomes the beam-squinting effect. Finally, towards the transition from electronic warfare to electromagnetic spectrum warfare, the challenges and development trend of MWP were proposed.
The High Throughput Satellite (HTS) is the major developing and application direction of the new generation wideband communication satellite. The domestic and neighborhood market have a huge demand of HTS. The characteristics and development demands with microwave photonics of HTS were analyzed. The main research developments, especially the new developments of HTS in our country, were introduced. The applications of microwave photonics on HTS and the foreign microwave photonics payloads of HTS were studied. And a new microwave photonics payload scheme was proposed and studied. The effectiveness and feasibility of this microwave photonics payload scheme were tested. In the end, the problems and suggestion of microwave photonics HTS were analyzed.
Microwave photonic radar enables the generation and processing of broadband radar signals, which can significantly improve the range resolution of the radar system. To improve the radar angle resolution and realize flexible beam control, combining microwave photonic radar technology with array radar technology is an inevitable development trend. Previously, the optical truth delay technology is intensively investigated to achieve squint-free beam steering in broadband phased array radars, which usually face the problems of high complexity, poor flexibility, and limited delay accuracy. In recent years, the broadband radar architecture based on microwave photonic frequency multiplication and de-chirp receiving has received extensive attention. The array radar constructed based on this technology has wide operation bandwidth while enabling real-time digital compensation and processing functions, which provides a new idea for the development of broadband array radars. In this paper, the research progress of the broadband array radar based on microwave photonic frequency multiplication and de-chirp processing was reviewed. After expounding the transceiver mechanism of microwave photonic broadband radar, the method for constructing broadband phased array radar and the performance of digital beam scanning and imaging were introduced. Then, the radar array was extended to MIMO architecture. The broadband microwave photonic MIMO radar based on optical wavelength division multiplexing technology was introduced and its performance in target detection and imaging was analyzed.
Ultra-wideband uni-traveling carrier (UTC) photodetectors have broadband advantages over traditional PIN detectors, as only fast electrons are required to transport in UTC photodetectors. They will be one of the key optoelectronic devices in the sub-terahertz systems, such as 6G broadband wireless communications, terahertz imaging, ultra-wideband noise generators, etc. For the requirements of optoelectronic conversion in sub-terahertz frequency band, high-speed photo-generated carrier transport mechanics and inductive coplanar waveguide (CPW) structure were studied to improve the device bandwidth and saturation power of the photodetector. A dual-drift layer structure MUTC photodetector chip with bandwidth of 106 GHz, saturated output power of 7.3 dBm, and a CPW-optimized MUTC photodetector chip with bandwidth over 150 GHz were developed.
An actively mode-locked optoelectronic oscillator (OEO) scheme was proposed to generate high-repetition-rate microwave pulse signals based on high-order harmonic mode locking. In the proposed scheme, the bias port of the electro-optic intensity modulator in the cavity was driven by a sinusoidal signal. The frequency of the sinusoidal signal (
) was set to be
times of the free spectral range of the OEO cavity to realize fundamental (
) and harmonic (
) mode locking, where the repetition rate of the generated microwave pulse signal was equal to
. In the proof-of-concept experiment, 10th-, 50th- and 100th-order harmonic mode locking were realized, where the repetition rate of the generated microwave pulse signal was 360 kHz, 1.8 MHz and 3.6 MHz, respectively. The proposed actively mode-locked OEO scheme provides a new way to generate low-phase-noise microwave pulse signals for pulse Doppler radar application.
With the development of the modern communication system, broadband and high-frequency microwave radio frequency (RF) signals have been widely applied in the fields of radar, communication and signal processing. Based on the microwave photonic channelization, ultra-wideband RF signals were generated through dual optical frequency combs (OFCs) with different free spectrum ranges (FSRs). In the channelized synthesis system, multiple independent narrowband signal was input in each channel for up-conversion and detected by multi-heterodyne detection to reconstruct a wideband RF signal with continuous spectrum. In multi-heterodyne detection, interference suppression technique increased the highest frequency that the synthesized RF signal could reach. In the experiment, a wideband RF signal was synthesized with an instantaneous bandwidth of 4 GHz, covering a frequency range of 8.4-12.4 GHz. The experiment demonstrates an interference suppression ratio of 21 dB, indicating that the interference suppression technique increases the highest frequency of the output signal and effectively improves the spectrum utilization.
In this paper, a scheme was proposed to achieve microwave photonic real time Fourier transformation based on spectrally-discrete dispersion, which aimed for the time-frequency information analysis of microwave signals in the future 6G wireless communication. The discrete dispersion media showed periodic narrowband filtering in the intensity frequency response and discrete quadratic phase distribution in the phase frequency response. Real time frequency to time mapping of microwave signals was then obtained using the discrete dispersion media. Furthermore, the time and frequency information of a non-stationary signal could be analyzed. Since only discrete phase shifts rather than continuously-changed true time delay were required, huge equivalent dispersion could be achieved with compact device to reduce the delay. An implementation based on cascaded ring resonators was proposed in this paper to achieve discrete dispersion, where fiber loops of 4 cm were used. Highly linear FTM with a slope of 0.8 ps/MHz was obtained through numerical stimulation, which was equivalent to 5800 km standard single-mode fiber. The resolution of the FTM reached 50 MHz and the unambiguous bandwidth reached 5 GHz. Taking the linear frequency modulation (LFM) signal which was often used in the convergence of sensing and communication in 6G as an example, the short-time Fourier transform function of the system was simulated to analyze the time and frequency information of the signal. The time resolution reached 20 ns, and the speed of spectrum analysis was as high as
FTs/s, which was significantly better than the traditional DSP based schemes.
A microwave photonic frequency mixer constituted of an optically-carried local oscillator (LO) and a wavelength-division modulator was proposed. The wavelength-division modulator chip, which was consisted of a silicon phase modulator, two micro-ring filters, a photodetector, two optical couplers, and two grating couplers, was designed and fabricated. Based on the chip, a microwave photonic harmonic frequency mixer was implemented. In the experiment, an optically-carried LO was generated by double-sideband suppressed-carrier modulation at a Mach-Zehnder modulator. An RF signal from 6 to 16 GHz was successfully converted into a signal with a frequency of 33 to 23 GHz. In order to suppress the remaining mixing spurs, two solutions, i.e., increasing the rejection ratio of the micro-ring filter to decrease the intensity of the leaked optically-carried LO and introducing an optical phase shifter to correct the phase of the leaked optically-carried LO, were proposed and verified by simulation. It should be noted that the latter is simpler and more suitable for photonic integration.
Optical beam forming network is an important part in optically controlled phased array radar, which could improve the beam scanning ability with large bandwidth and direction angle. The direction of beam is usually controlled by optical switches to change relative delay of transmitting and receiving channels. Among commonly-used techniques, dispersion delay is a concision way to realize optical beam forming network. Linear dispersion is only applicable to beam forming with limited dispersion delay and channels. With the increase of delay, nonlinear dispersion delay accumulates, which distorts the beamform. Therefore, relative dispersion slope (RDS) was used as a nonlinear factor. Moreover, adjusting wavelengths of commercial lasers was raised to compensate the nonlinearity. If RDS was 0.003 nm−1, the maximum wavelength interval stretched from 0.796 nm to 0.862 nm and wavelengths shifted −0.31 nm. In this case, maximum difference between modified and commercial laser wavelengths was 0.2 nm, which was suitable for the passband of commercial wavelength division multiplexing devices. In the meantime, ratio of main to side lobe improved from 5 dB to 12.9 dB with large scanning direction. Based on the analysis, the smaller RDS value was, the less wavelengths modifications of lasers were. Therefore, RDS is a key parameter in choosing dispersion material and adjusting wavelengths of lasers. In this way, distorted beamform could be recovered. The abilities of imaging and identifying thus could be improved in phase arrayed systems.