Significance Modern radar systems play crucial roles across various applications including imaging, high-resolution remote sensing, and surveillance. Among radar modulation schemes, linear frequency-modulated (LFM) waveform radar stands out due to its capability to maintain expansive instantaneous bandwidth and high power concurrently. Furthermore, its distinctive dechirp reception technique simplifies data acquisition and processing at the receiver end, requiring only a low-speed analog-to-digital converter (ADC) with a sampling rate below 1 GSa/s, thereby facilitating swift and potentially real-time operations. However, the electrical generation of LFM signals has encountered limitations. Multistage frequency upconversion introduces considerable in-band distortion and temporal jitter, constraining practical applications. Recent strides in microwave photonics technologies have consequently fostered various photonics-based LFM signal generation techniques. These approaches capitalize on photonics'distinct advantages, such as ultra-wide bandwidth, flat response, and immunity to electromagnetic interference. They have been substantiated through experimental validation. Progress The frequency-to-time mapping method (Fig. 1) shapes a short optical pulse using a spectral shaper and then maps it using a dispersion element. Once the components are selected, the generated LFM signal typically has a fixed center frequency and bandwidth, with a time-bandwidth product (TBWP) of approximately 100. Microwave photonic frequency multiplication (Fig. 2) and spectrum stitching (Fig. 4) methods significantly enhance TBWP. These techniques involve either multiplying narrow-band intermediate frequency LFM (IF-LFM) signals or seamlessly stitching them together in the optical domain to create a wideband signal. However, these methods typically struggle to generate signals across a one-octave frequency range and require additional control loops to maintain phase continuity at stitching points. Additionally, the Fourier domain mode-locked optoelectronic oscillator (FDML-OEO) (Fig. 6) has been proposed as an alternative to microwave sources. This oscillator utilizes a time-variant optical bandpass filter that rapidly scans with a period equal to the round-trip delay time of the laser ring cavity, allowing for the selection and oscillation of longitudinal modes sequentially. Despite its ability to produce frequency-chirped optical pulses with bandwidths exceeding 40 GHz, the FDML-OEO faces challenges such as relatively poor linearity and limited temporal period. To enhance the reconfigurability of the generated signals, a promising method involves employing heterodyne beating between two optical signals-one from a continuous wave (CW) laser source and the other from a frequency-swept laser source. Typically, techniques such as optical phase locking or injection locking are used to ensure phase coherence between these laser sources. When a semiconductor laser undergoes external optical injection, it can exhibit various nonlinear dynamic states. Under suitable injection conditions, the laser operates in a period-one (P1) oscillation state. At this point, in addition to the light at the injection frequency, the output also generates two asymmetric sideband signals, whose frequencies are determined by the intensity and detuning frequency of the injected light. According to this mechanism, reconfigurable LFM signals can be generated by flexibly controlling the intensity and detuning frequency of the injected light (Fig. 5). Compared to other schemes, the advantage of using a P1 resonant light source lies in its ability to convert light intensity variations directly into frequency variations, thereby enabling wide-range tuning of the output light frequency simply by adjusting the injection light intensity. Our research introduces a novel approach for generating LFM signals utilizing photonic methods, specifically through the heterodyne beating of two phase-locked tunable lasers in real-time (Fig. 8). This configuration comprises two integrated tunable self-injection locked lasers operating in a master-slave configuration alongside an optical phase-locked loop (OPLL). In our proposed scheme, the master laser is thermally tuned to generate broadband LFM optical signals. A piezoelectric transducer (PZT) integrated into the slave laser enables rapid tuning, facilitating real-time and high-precision phase locking with the master laser driven by the OPLL. By concurrently managing the parameters of both lasers and the reference signal within the OPLL, we achieve the generation of reconfigurable LFM microwave signals following photoelectric conversion. Conclusions and Prospects The generation of wideband chirped microwave signals using photonic technology has demonstrated several advantages, including high frequency, large bandwidth, easy tunability, and immunity to electromagnetic interference. This technology is increasingly recognized as crucial for overcoming electronic bottlenecks and achieving high-resolution, multifunctional radar systems. Microwave photonic radars based on these signal generation techniques have gradually transitioned from initial system demonstrations to practical applications. To meet the requirements of operational systems, future developments in signal generation should prioritize the following areas: 1) Suppressing frequency drift of optical components. The optical frequency band is 4-5 orders of magnitude higher than the microwave frequency band. Even slight frequency drifts in the light source and optical components can lead to significant changes in the microwave signal. Therefore, techniques such as optical phase locking or other negative feedback loops should be employed to suppress frequency drift in lasers, filters, and other optical components, ensuring the stability of the microwave signal; 2) Improving optoelectronic conversion efficiency. Despite the advantageous features such as high frequency and broad bandwidth offered by photonic-based LFM signal generation, substantial energy loss during the conversion between optical and microwave frequencies remains a critical concern. The use of multiple stages of optical or electrical amplifiers unavoidably diminishes the signal-to-noise ratio, thereby exacerbating signal degradation. Consequently, it is imperative to boost the efficiency of electro-optical modulators and photodetectors and to integrate high-efficiency optoelectronic devices to bolster system link gain; 3) System integration. Current signal generation systems typically comprise disparate optical components, contributing to a bulky system footprint and heightened power consumption, while also rendering them susceptible to external environmental disturbances. Therefore, it is crucial to maximize the benefits of photonic integration and optoelectronic hybrid integration. This involves consolidating multiple optical or optoelectronic components onto a unified platform or adopting hybrid integration approaches to reinforce system robustness and mitigate size, weight, and power (SWaP) concerns. With technological advancements and in-depth research, these issues are poised to be effectively addressed, which promotes the greater role of microwave photonic signal generation technology in radar systems.
