Muguang Wang

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In this paper, a direct-detection quasi-distributed acoustic sensing (QDAS) system using dual linear-frequency-modulation pulses is proposed, which achieves high-sensitivity sensing. Microwave photonic technology is used to generate two identical linear-frequency-modulation (LFM) pulses with a certain time interval as a probe signal. Compared with the previously proposed QDAS system based on a single LFM pulse, within the threshold of fiber nonlinear effects, the power of the probe signal is increased while avoiding the use of a matching interferometer. Experimental results show that under the same probe power, the received carrier power is increased by 6 dB. In addition, an improved phase-shift integration (IPSI) algorithm is proposed, combined with the empirical mode decomposition algorithm, which significantly improves the demodulation signal-to-noise ratio compared with the Hilbert algorithm. The noise floor is greatly suppressed, achieving a level of −90.6 dB rad ² /Hz@10kHz.

To overcome the detection bottleneck of the free-running optoelectronic oscillator (OEO), which is constrained by linear response and intrinsic noise in magnetic field sensing, a high-responsivity sensing system based on a quasi-synchronized OEO with unidirectional radio frequency (RF) injection is proposed and experimentally demonstrated. By introducing an external RF signal into an OEO loop, quasi-synchronization between the OEO and the injected source is achieved. Theoretical analysis derives the beat frequency characteristics in the quasi-synchronized regime and reveals a square-root scaling law for responsivity enhancement. In the proof-of-concept experiment, the sensing unit consists of a fiber Bragg grating (FBG) bonded to a giant magnetostrictive material. Magnetic-field-induced strain shifts the reflection spectrum of the FBG, which is converted into loop delay variations by the linearly chirped FBG in the OEO, thereby modulating the OEO oscillation frequency. Experimental results show that the proposed quasi-synchronized OEO exhibits a remarkable responsivity enhancement in the weak magnetic field region. Relative to the isolated OEO, the enhancement factor of the quasi-synchronized OEO reaches 6.7 times at 0.5 mT and further improves to 13.4 times when employing second-harmonic beat frequency demodulation. Moreover, the detection resolution is improved from 0.558 mT in the isolated OEO to 14.48 μT in the quasi-synchronized scheme, corresponding to nearly a 38.5 times improvement. The minimum detectable magnetic field is evaluated to be 1.13 μT / Hz ^1/2 , and equivalent magnetic field measurement accuracy is approximately 383 μT. The proposed quasi-synchronized OEO sensing scheme, thus, offers significant advantages for weak magnetic field detection and demonstrates strong potential for precision magnetic field measurements. Owing to the universality of the square-root enhancement mechanism, it provides a versatile and effective approach for high-responsivity detection of other weak perturbations.

Microwave photonic (MWP) technology is widely used in signal generation and processing, which helps optical sensors achieve higher measurement resolution and sensitivity. In this Letter, MWP technology is used to improve the performance of distributed acoustic sensors based on an ultra-weak fiber Bragg grating array. Specifically, a linear-frequency-modulation (LFM) pulse is used as a probe signal, effectively improving the excitation threshold of fiber nonlinearity. Distributed positioning and quantitative measurement of vibration are achieved through a simple direct-detection scheme. Based on the non-matching interference, the echo beating frequency after chirp removal is reduced to an integral multiple of the LFM probe repetition rate, which helps to reduce the bandwidth of the receiving end. Compared with existing demodulation methods, the demodulation method combining phase shift integration and empirical mode decomposition reduces the noise floor by at least 7.6 dB.

Spectral self-imaging (SSI) has been considered as a key element for energy-preserving control of optical frequency comb. Reported all-optical SSI may overcome the limitation of processing speed; however, an extra comb is required as the pump source and other intrinsic issues exist as well, including central frequency deviation, spectral envelope distortion, high system cost, and/or low power-efficiency. Based on self-generated pump technology and RZ-multilevel cross-phase modulation, we propose a practical all-optical solution for distortion-free SSI with improved power-efficiency and reduced system cost. Partial power of an initial comb is spectrally broadened via self-phase modulation in a highly nonlinear fiber. The red-shifted and/or blue-shifted spectra that are not overlapped with the initial comb are utilized for pump signal, named self-generated pump technology here. The broadened spectrum is tailored using two arrayed waveguide gratings, and then converted to RZ-multilevel pump signal according to the frequency-to-time mapping relationship. Via cross-phase modulation, the RZ-multilevel pump signal imposes RZ-multilevel phase profile to the probe signal (the initial comb), leading to distortion-free SSI with reduced pump power by a factor of duty-cycle. Meanwhile, as extra pump comb and programmable spectral shaper are no longer required, the system cost is reduced greatly. The feasibility of proposed SSI system is demonstrated experimentally. Distortion-free SSIs with FSR division factors 2 (s=1, m=2), 3 (s=2, m=3) and 4 (s=1, m=4) are achieved successfully, associated with improved power-efficiencies of 10.5 times, 4.9 times and 2.7 times. Besides the intrinsic fundamental interest, this practical all-optical SSI can find important applications in high-resolution spectroscopy, and high-speed communications and signal processing.

High-accuracy sensing of alternating magnetic fields (AMF) is crucial for applications in fields such as the Industrial Internet of Things (IIoT) and medical diagnostics. This paper proposes an AMF sensing scheme based on an optoelectronic oscillator (OEO) with an embedded Mach-Zehnder interferometer, where magnetic-field-induced phase variations are converted into frequency shifts of the OEO output. A magnetic field sensitive fiber-optic sensing unit is made up of a section of optical fiber, a giant magnetostrictive material, and an aluminum alloy sensitizing skeleton. This cantilever-structured unit exploits mechanical resonance to amplify the magnetostrictive strain, enhancing the magneto-mechanical coupling efficiency. This sensing unit is embedded within an OEO loop incorporating a Mach-Zehnder interferometer, thereby achieving efficient conversion from magnetic field to the phase of optical signals and subsequently to the phase of microwave signals. By acquiring the output microwave signal from the OEO and performing offline phase demodulation, the AMF information can be accurately measured. Experimental results demonstrate that the proposed scheme achieves an AMF sensing sensitivity as high as 3 nT/Hz^1/2@10 kHz validated over a frequency range of 300 Hz to 16 kHz. This technology provides a robust solution for the requirements of high-sensitivity dynamic AMF sensing in medical fields, IIoT, and facility safety monitoring.

With strong unpredictability, high-dimensional microwave chaotic signals are significant for secure communication. However, conventional electronic techniques face challenges in generating high-dimensional chaotic signal with large bandwidth. In this paper, a dual-loop optoelectronic oscillator with cascaded phase modulators is proposed for high-dimensional microwave chaotic signal generation. In the system, phase modulation is converted into nonlinear intensity modulation by using a Mach-Zehnder interferometer to achieve chaotic oscillation. The dual-loop structure causes numerous characteristic frequencies, enhancing high-dimensional dynamics. Its mathematical model based on the Ikeda equation is established and used for dynamics analysis. An experimental comparison demonstrates that the proposed system generates high-dimensional chaotic signal with a broadband spectrum and low time-delay signature. The nonlinear dynamics are further investigated through numerical simulations, including bifurcation route and dynamics performances, which demonstrate the high-dimensional dynamical characteristics and enhanced unpredictability of the generated chaotic signal.

A photonic-assisted microwave joint radar and communication (JRC) system is proposed and experimentally demonstrated, in which differential chaos shift keying (DCSK) is employed to modulate the phase of a single-pulse linear frequency modulation (LFM) waveform. This system leverages the large bandwidth afforded by microwave photonic technology and utilizes the randomness and confidentiality of chaotic sequences to achieve incoherent secure communication and enhance radar range detection capabilities. An experiment achieving a maximum communication data rate of 250 Mbit/s and a maximum chaotic sequence rate of 1 Gchip/s. By optimizing the spreading factor, the system achieves a peak sidelobe ratio (PSLR) of 24.25 dB and a fine range resolution of 7.3 cm.