Recent Progress in Microwave Photonic Magnetic Field Sensing Technology （Invited）

Magnetic field sensing is essential in applications such as power equipment monitoring， geomagnetic exploration， biomedical diagnostics， and intelligent sensing infrastructures. Although electrical magnetic sensors， including Hall-effect， magnetoresistive， fluxgate， and superconducting quantum interference device sensors， have achieved technical maturity， their deployment in complex or distributed environments is often constrained by electromagnetic interference， bulky shielding requirements， and limited scalability. By contrast， microwave photonic approaches exploit the immunity， flexibility， and low cost of optical fibers together with the high resolution and wide bandwidth of microwave-domain processing， offering a promising route toward high-performance magnetic field sensing. This paper reviews the fundamental principles and recent progress of microwave photonic magnetic field sensing technologies， with emphasis on their demodulation mechanisms and system architectures.Two microwave photonic sensing routes have emerged as representative approaches. The first is based on the Optoelectronic Oscillator （OEO）， in which the fiber-optic magnetic field sensing unit is embedded within the OEO feedback loop. In this configuration， magnetic-field-induced optical perturbations are translated into shifts in the oscillation frequency or phase of OEO. Owing to the intrinsically high-Q and low phase noise of OEOs， even minute wavelength shifts from fiber Bragg grating Fabry-Perot cavities bonded to magnetostrictive materials can be amplified into measurable frequency shifts at the MHz to GHz level. Furthermore， Mach-Zehnder interferometer assisted OEO architectures enable alternating magnetic field sensing by converting magnetostrictive phase modulation into frequency or phase variations of the OEO oscillation signal， allowing sensitivities approaching the sub-nT/Hz1/2 level in the kHz magnetic field band. Responsivity enhancement strategies， such as the Vernier effect in dual loop OEOs and prestressed magnetostrictive packaging， have been demonstrated to improve the scale factor by more than an order of magnitude without modifying the single loop OEO configuration.The second route is based on Microwave Photonic Filters （MPFs）. In this scheme， an optical carrier is modulated by an electro-optic modulator and subsequently passes through a magnetic field sensing unit. After photodetection， the magnetic field information is retrieved by tracking the shift of the filter passband， typically using a vector network analyzer. MPF-based magnetic field sensing systems naturally support multi-tap and reconfigurable architectures， and can be applied to both static and alternating magnetic field measurements. Advanced implementations incorporating Fabry-Perot interferometers， polarization-maintaining fibers and cascaded fiber rings， MPF-based schemes enable enhanced responsivity， dual-parameter sensing， and quasi-distributed or vector magnetic field measurement capabilities.To render microwave photonic magnetic field sensing systems practical for engineering applications， advanced post-processing techniques are essential to mitigate nonidealities originating from both magnetic transducers and microwave photonic components. Magnetostrictive and magnetic fluid elements inherently exhibit hysteresis and temperature-dependent behavior， while fiber packaging may introduce mechanical vibrations that manifest as excess phase noise. In addition， instabilities in lasers and photodetectors can induce power and wavelength fluctuations. Model-based hysteresis compensation approaches， including Preisach and Prandtl-Ishlinskii families and their dynamic or separated loop extensions， have been shown to substantially suppress nonlinear errors and restore near-linear relationship between magnetic field and frequency. In parallel， data-driven techniques based on recurrent or feedforward neural networks， support vector machines， and hybrid model-network frameworks enable online learning of the complex coupling among magnetic field， temperature， and packaging-induced stress. These techniques enable effective suppression of nonlinear errors and adaptive decoupling of magnetic field， temperature， and packaging-induced disturbances， thereby enhancing long-term stability and measurement fidelity.In summary， microwave photonic magnetic field sensing technology combines the low cost and lightweight of fibers with the high accuracy of microwave-domain signal processing. At the hardware level， magnetic field demodulation can be achieved through microwave photonic architectures， including high-Q OEOs and tunable multi-tap MPFs. At the digital level， hysteresis can be effectively compensated using advanced signal processing and algorithms. Most reported systems still rely on discrete optoelectronic components. This reliance increases susceptibility to optical path loss and environmental perturbations， which in turn degrades demodulation accuracy and long-term stability. Future efforts should therefore focus on monolithic or hybrid photonic microwave integration to realize low loss， low cost， and compact on-chip sensing and demodulation architectures. Through coordinated control of the optical and microwave domains， this technology offers a versatile route toward high sensitivity magnetic field measurement. With continued advances in integrated microwave photonic platforms， intelligent signal processing algorithms， and novel magnetic transduction materials， microwave photonic magnetic field sensing is expected to evolve toward higher sensitivity and stronger environmental adaptability. These developments will also promote improved system integration and support future applications in power equipment monitoring， geomagnetic exploration， and biomedical magnetic detection.