RF channelization is regarded as one of the most effective approaches to relieve the difficulty of processing wideband signals in microwave and millimeter-wave receivers. In this paper, a novel coherent optical RF channelizer with broad instantaneous bandwidth and large in-band interference suppression is proposed and demonstrated based on a multichannel photonic image-reject mixer (IRM). A Ku-band RF signal with an instantaneous bandwidth of 5 GHz is sliced into five consecutive subchannels with 1-GHz instantaneous bandwidth by the proposed channelizer. The in-band interference suppression, which relies on the image-rejection ratios of the IRM, is about 25 dB. To the best of our knowledge, the proposed channelizer achieves the highest in-band interference suppression in such wide instantaneous bandwidth.

Cognitive radio is considered as a possible disruptive force to improve the spectral resource efficiency through sensing and interacting with the environment. Traditional electrical technologies to implement the cognitive radio face challenges in terms of bandwidth, resolution, and speed. In this paper, the concept and architecture of broadband cognitive radio systems enabled by photonics are proposed. Key microwave photonic techniques for the architecture are reviewed, including the photonics-based spectrum sensing, the photonic arbitrary waveform generation, the photonics-based self-interference cancellation processing, and the microwave photonic dechirp processing and radar imaging. A preliminary demonstration of a cognitive radar system enabled by photonics is performed. The future possible research directions on this topic are discussed.

A microwave channelizer based on a photonic dual-output image-reject mixer (IRM) is proposed and demonstrated. By introducing the dual-output IRM based on the balanced Hartley structure, $2N$ channels can be produced by using the optical frequency combs (OFCs) with $N$ comb lines. The required electrical hybrids and the optical filters are also reduced by half. In addition, the channelization efficiency is doubled, and the in-band interferences, including the image and the nonlinear mixing spurs, are greatly suppressed. A proof-of-concept experiment is performed. By applying a pair of OFCs with three comb lines, an RF signal with a 6 GHz instantaneous bandwidth is successfully divided into six channels with 1 GHz bandwidth. The in-band interference is suppressed by more than 25 dB for all channels.

A photonics-based multi-band linearly frequency-modulated (LFM) waveform generator with reconfigurable center frequency, bandwidth and time duration is proposed and demonstrated. By introducing two coherent optical frequency combs (OFCs) with a frequency shift and different free spectral ranges (FSRs) as multi-frequency optical LOs, a set of LFM signals with different center frequencies will be generated if one of the combs is modulated by an intermediate-frequency (IF) LFM signal. The center frequencies of the generated RF-LFM signals can be flexibly tuned by adjusting the frequency shift between the two OFCs. In addition, by introducing a series of proper time delays to the LFM signals and combining them, a frequency-stepped LFM signal can be generated. Furthermore, when the bandwidth of the IF-LFM signal equals the difference of the comb FSRs, and the time duration of IF-LFM signal equals the time delay of the consecutive channels, a LFM signal with both bandwidth and time duration multiplied can be obtained. With N comb lines, the maximum achievable time-bandwidth product (TBWP) is N × N times of the applied IF LFM signal. A proof-of-concept experiment is carried out. A set of LFM signals with frequencies ranging from L to Ka bands are generated. By introducing proper time delays, a frequency-stepped LFM signal with frequency steps between 10 GHz and 20 GHz is also produced. In addition, LFM signals with the bandwidth and time duration multiplied by 2 and 5 are realized (4-GHz bandwidth, 2-μs time duration and 10-GHz bandwidth, 5-μs time duration), respectively. Correspondingly, the TBWPs are increased by 4 and 25 times.

A distributed multiple-input multiple-output chaotic radar based on wavelength-division multiplexing technology (WDM) is proposed and demonstrated. The wideband quasi-orthogonal chaotic signals generated by different optoelectronic oscillators (OEOs) are emitted by separated antennas to gain spatial diversity against the fluctuation of a target’s radar cross section and enhance the detection capability. The received signals collected by the receive antennas and the reference signals from the OEOs are delivered to the central station for joint processing by exploiting WDM technology. The centralized signal processing avoids precise time synchronization of the distributed system and greatly simplifies the remote units, which improves the localization accuracy of the entire system. A proof-of-concept experiment for two-dimensional localization of a metal target is demonstrated. The maximum position error is less than 6.5 cm.

Recent developments in photonics-based microwave image-reject mixers (IRMs) are reviewed with an emphasis on the pre-filtering method, which applies an optical or electrical filter to remove the undesired image, and the phase cancellation method, which is realized by introducing an additional phase to the converted image and cancelling it through coherent combination without phase shift. Applications of photonics-based microwave IRM in electronic warfare, radar systems and satellite payloads are described. The inherent challenges of implementing photonics-based microwave IRM to meet specific requirements of the radio frequency (RF) system are discussed. Developmental trends of the photonics-based microwave IRM are also discussed.