Besides distinct features on RF/optical signal generation, optoelectronic oscillators (OEOs) have also been rapidly developed as emerging techniques towards sensing, measurement, and detection. In this paper, we start with the conceptual architecture and the analytical model of OEOs. Then, three operation principles behind sensing, measurement, and detection applications are categorized, including the variation on the time delay of loop, the passband reconfiguration of microwave photonic filter in loop, and the oscillation gain from injection locking, which clearly clarify the X-to-frequency mapping (X denotes target parameter or signal) for supporting practical solutions and approaches. Next, a comprehensive review to advances in OEO-based sensing, measurement, and detection applications is presented, including length change and distance measurement, refractive index estimation, load and strain sensing, temperature and acoustic sensing, optical clock recovery, and low-power RF signal detection. As a new application example, a novel approach for in-line position finding is proposed. When a long fiber Bragg grating inserted into OEO is locally heated to slightly broaden its reflection spectrum, the target position heated is mapped into the oscillating frequency shift, according to the first operation principle. A sensitivity of 254.66 kHz/cm is obtained for position finding in the experiment. Afterward, solutions for calibration and stabilization are briefly introduced, which enable us to improve the accuracy and reliability. Finally, features and future prospects on the sensing, measurement, and detection applications are discussed, such as compact and integrated OEOs.
A microwave photonic synthetic aperture radar is proposed. Photonic-assisted microwave frequency doubling is used to generate a linearly-frequency-modulated radar signal in the transmitter end. In the receiver end, photonic de-chirping is employed to process the reflection signal. The proposed coherent microwave photonic radar is experimentally demonstrated and evaluated in a microwave anechoic chamber through a series of imaging experiments. The imaging results verify that the microwave photonic synthetic aperture radar works well and show a potential of microwave photonic techniques for high-resolution and fast radar imaging in the future.
We report a novel method to generate a stable and frequency-hopping-free microwave signal based on a mutually injection-locked dual-wavelength single-longitudinal-mode fiber laser and an optoelectronic oscillator (OEO), with the mutual injection locking realized by sharing an optical path consisting of a polarization modulator and a polarization-maintaining phase-shifted fiber Bragg grating. The two wavelengths from the fiber laser are injected into the OEO to lock the generated microwave signal, while the microwave signal from the OEO is fed back into the fiber laser to injection lock the two wavelengths. Thanks to the mutual injection locking, the operation stability of the fiber laser and the OEO are substantially improved. A microwave signal at 11.8 GHz with a phase noise of -105 dBc/Hz at a 10-kHz offset frequency is generated. A stable operation of the system without frequency shifting and hopping is demonstrated.
An ultra-wideband microwave photonic phase shifter implemented by a joint use of a polarization modulator and a polarization-maintaining fiber Bragg grating that can provide a full 360° tunable range over 30-40 GHz is demonstrated.
A photonic-assisted microwave channelizer with improved channel characteristics based on spectrum-controlled stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. In the proposed system, N lightwaves from a laser array are multiplexed and then split into two paths. In the upper path, the lightwaves are modulated by a microwave signal with its frequency to be measured. In the lower path, for each lightwave, the wavelength is shifted to a specific shorter wavelength via carrier-suppressed single-sideband modulation and the spectrum is then shaped. The wavelength-shifted and spectrum-shaped lightwaves are used to pump a single-mode fiber to trigger SBS. Thanks to the SBS effect, multiple gain channels at the N wavelengths are generated. The channel profile of each channel, determined by the designed spectral shape of the pump source, is improved with a flat top and a reduced shape factor. The characteristics including the bandwidth, channel spacing, and channel profile can be controlled by adjusting the spectral shape of the pump source. A proof-of-concept experiment is performed. A microwave channelizer with a shape factor less than 2, a tunable channel bandwidth of 40, 60, or 90 MHz, and a tunable channel spacing of 50, 70, or 80 MHz, is demonstrated.
A microwave photonic (MWP) radar with a fiber-distributed antenna array for three-dimensional (3D) imaging is proposed and demonstrated for the first time. Photonic frequency doubling, wavelength-division multiplexing and radio-over-fiber techniques are employed for radar signal generation, replication, and distribution. Based on the delay-dependent beat frequency division, parallel de-chirp processing is completed in the center office (CO), leading to multi-channel 2D ISAR imaging and further 3D reconstruction. The influence of the fiber transmission delay is discussed and the phase noise caused thereby is compensated in 3D imaging algorithm, improving the coherence between channels. An experiment of a Ku-band MWP radar with a transmitter (Tx) and 16 equivalent receivers (Rxs) is conducted and 3D imaging of three trihedral corner reflectors is achieved with a range resolution of 7.3 cm, a cross-rage resolution of 5.6 cm and an elevation resolution of 0.85°. The results verify the capability of MWP radar in high-resolution 3D imaging.
A novel multiband fusion method based on a modified RELAX algorithm (MRA) is proposed. In the MRA, a maximum difference criterion is applied to the singular values of echoes’ Hankel matrix to improve the accuracy of calculating the number of scattering centers. In addition, a transformation process is put forward to merge the frequency-dependent factor (FDF) term, which is in the geometrical theory of diffraction (GTD) model, into the phase term of the echo to simplify the model. Moreover, a matching relationship for the correspondence between the FDF values and the cost function is established to effectively increase the estimated precision of the FDF. Therefore, based on the MRA, exact estimations of the incoherent errors among subband echoes are achieved, and then a fused full-band echo (FBE) with high estimation accuracy is produced. Using all the fused FBEes, high-resolution, anti-non-Gaussian colored clutter microwave imaging is realized. The effectiveness of the proposed method is validated with the simulated and real-data experimental results in a simulated non-Gaussian colored clutter environment.
Today, wide-open, high-resolution Doppler frequency shift (DFS) estimation is essential for radar, microwave/millimeter-wave, and communication systems. Using photonics technology, an effective approach is proposed and experimentally demonstrated, providing a high-resolution and frequency-independent solution. In the approach consisting of two cascaded opto-electronic modulators, DFS between the transmitted microwave/ millimeter-wave signal and the received echo signal is mapped into a doubled spacing between two target optical sidebands. Subsequently, the DFS is then estimated through the spectrum analysis of a generated low-frequency electrical signal, with an improved resolution by a factor of 2. In experiments, DFSs from -90 to 90 KHz are successfully estimated for microwave/millimeter-wave signals at 10, 15, and 30 GHz, where estimation errors keep lower than +/- 5e-10 Hz. For radial velocity measurement, these results reveal a range from 0 to 900 m/s (0 to 450 m/s) and a resolution of 1e-11 m/s (5e-12 m/s) at 15-GHz (30-GHz) frequency band.
A practical two-dimensional beam steering solid-state system based on the synthesis of one-dimensional wavelength tuning and a one-dimensional optical phased array is demonstrated and investigated. The system incorporates an integrated multiple-channel-interference widely tunable laser, an integrated 32-channel optical phased array, an offline phase error correction unit, and home-made control electronics. The introduction of the integrated tunable laser avoids the traditional bulky light source fed into the optical phased array, making the architecture promising to be miniaturized. In addition, a calibration method based on particle swarm optimization is proposed and proved to be effective to correct the phase errors existing in the arrayed channels and improve the emitted far-field quality. Other practical aspects, such as high-speed control and cost, are taken into the consideration of the system design as well. Under the control of home-made electronics, the laser exhibits a tuning range of 50 nm with a 44 dB side-mode suppression ratio, and the system presents the characteristics of low divergence (0.63∘×0.58∘), high side-lobe suppression ratio (>10dB), and high-speed response (<10µs time constant) in an aliasing-free sweeping range of 18∘×7∘.
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