A photonics-based radar with generation and de-chirp processing of broadband linear frequency modulated continuous-wave (LFMCW) signal in optical domain is proposed for high-resolution and real-time inverse synthetic aperture radar (ISAR) imaging. In the proposed system, a broadband LFMCW signal is generated by a photonic frequency quadrupler based on a single integrated electro-optical modulator, and the echoes reflected from the targets are de-chirped to a low frequency signal by a microwave photonic frequency mixer. The proposed radar can operate at a high frequency with a large bandwidth, and thus achieve an ultra-high range resolution for ISAR imaging. Thanks to the wideband photonic de-chirp technique, the radar receiver could apply low-speed analog-to-digital conversion and mature digital signal processing, which makes real-time ISAR imaging possible. A K-band photonics-based radar with an instantaneous bandwidth of 8 GHz (18-26 GHz) is established and its performance for ISAR imaging is experimentally investigated. Results show that a recorded two-dimensional imaging resolution of ~2 cm × ~2 cm is achieved with a sampling rate of 100 MSa/s in the receiver. Besides, fast ISAR imaging with 100 frames per second is verified. The proposed radar is an effective solution to overcome the limitations on operation bandwidth and processing speed of current radar imaging technologies, which may enable applications where high-resolution and real-time radar imaging is required.

A scheme for photonic generation of linearly chirped microwave waveforms (LCMWs) with a large time-bandwidth product (TBWP) is proposed and demonstrated based on an optically injected semiconductor laser. In the proposed system, the optically injected semiconductor laser is operated in period-one (P1) oscillation state. After optical-to-electrical conversion, a microwave signal can be generated with its frequency determined by the injection strength. By properly controlling the injection strength, an LCMW with a large TBWP can be generated. The proposed system has a simple and compact structure. Besides, the center frequency, bandwidth, as well as the temporal duration of the generated LCMWs can be easily adjusted. An experiment is carried out. LCMWs with TBWPs as large as 1.2x10⁵ (bandwidth 12 GHz; temporal duration 10 μs) are successfully generated. The flexibility for tuning the center frequency, bandwidth and temporal duration is also demonstrated.

In this paper, a simple scheme for linear frequency-modulated (LFM) waveform generation based on a frequency-sweeping optoelectronic oscillator is proposed and demonstrated. The OEO is built up with an optically injected semiconductor laser and the oscillation frequency can be tuned by adjusting the optical injection strength. By applying an injection strength controller in the OEO for rapid frequency sweeping, an LFM microwave waveform can be generated. When the sweep period of the output frequency matches with the round-trip time of the OEO cavity, signal quality of the generated LFM waveform can be significantly enhanced by the high Q optoelectronic oscillation. In the experiment, an LFM signal with a bandwidth as large as 7 GHz, a chirp rate reaching 0.18 GHz/ns, and a time-bandwidth product (TBWP) up to 2804.2 is generated. The corresponding electrical spectrum is a frequency comb with a contrast as high as 47 dB. Based on this system, an improved scheme for extending the frequency and bandwidth of the generated LFM signal is proposed employing a polarization modulator (PolM) to implement microwave photonic frequency multiplication. With this method, an LFM waveform with a TBWP as large as 13839.1 (bandwidth 15.6 GHz; temporal period 887.12 ns) is obtained.

A photonic approach to generating dual-band linear frequency modulation (LFM) signal is proposed based on optical frequency quadrupling and polarization multiplexing. This is achieved by using an integrated polarization multiplexing dual-parallel Mach-Zehnder modulator to perform frequency quadrupling in two orthogonal polarizations independently. After optical-to-electrical conversion, two LFM signals in different frequency bands can be generated simultaneously. The proposed scheme has a very simple and compact structure. Thanks to the frequency quadrupling technique, high-frequency, and wideband LFM signals can be generated with low speed electrical devices. The central frequency, bandwidth, and temporal durationof the generated LFM signals can be easily adjusted. In the experiment, the generation of dual-band LFM signals in K-band and Ka-band (centered at 20 and 30 GHz, respectively) is demonstrated. Tunability of the central frequency, bandwidth, and time duration is also verified. The proposed signal generator is a promising candidate in dual-band multi-function radar applications.

We propose and experimentally demonstrate an approach to generating reconfigurable radar waveforms based on an optically injected semiconductor laser. In the proposed system, the semiconductor laser is operated at period-one oscillation state, in which an optical output signal containing a microwave modulation on the intensity is generated. After photo detection, a frequency-tunable microwave signal is obtained with its instantaneous frequency determined by the optical injection strength and/or the detuning frequency between the master and slave lasers. Since the dynamical behavior of a semiconductor laser evolves at a subnanosecond time scale, by properly designing a control signal to manipulate the optical injection strength, reconfigurable microwave waveforms with desired parameters can be generated for radar and other applications. In particular, the generation of continuous-wave or pulsed, linearly chirped, and frequency-hopping microwave waveforms with reconfigurable parameters are experimentally demonstrated. In addition, a high-resolution distance measurement experiment is performed to verify the feasibility of applying the proposed microwave waveform generator to radar applications.

Under proper optical injection, period-one (P1) dynamics of an optically injected semiconductor laser can be excited by undamping the relaxation resonance of the semiconductor through Hopf bifurcation. The output intensity would oscillate at a microwave frequency, i.e., the P1 frequency. After optical-to-electrical conversion, a microwave signal can be generated with its frequency determined by the detuning frequency and the injection strength. Therefore, it provides a convenient way to dynamically control the instantaneous frequency of the generated microwave signal by manipulating the injection strength. In this work, a flexible frequency-hopping (FH) microwave waveform generator by dynamic control of an optically injected semiconductor laser is proposed and demonstrated. The system has a compact structure, and the key device is a commercial semiconductor laser. The generated FH microwave waveform has a high degree of flexibility, i.e., the central frequency, bandwidth, sequence length, and frequency order can be easily adjusted. A proof-of-concept experiment is carried out. Wideband (>10 GHz) stepped linear and Costas sequence are successfully generated. Autoambiguity functions of the generated frequency-hopping sequences are also investigated. The experimental results can verify the feasibility of the proposed FH waveform generator, which may find wide applications in future radar and communications systems.