Responsivity-enhanced magnetic field sensing with a quasi-synchronized optoelectronic oscillator

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∕Hz¹∕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.