An all-fiber weak magnetic field sensor combined with reflectance spectroscopy analysis method is proposed. The sensor uses two chirped gratings to form a full-fiber chirped grating F-P cavity at a distance and pastes it on the surface of the giant magnetostrictive material. The output reflection spectrum of the chirped grating F-P cavity is demodulated by the fringe counting method

and the length of the cavity of the chirped grating F-P cavity is changed under the effect of the external magnetic field. Thus

the sensing measurement of the magnetic field is realized. The experimental results show that average value of measurement error of this sensor is 0. 43 μT

the variance is 0. 3 μT

and the phase sensitivity of the system is 1. 62 rad/μT. In summary

the proposed fiber weak magnetic field sensor has the advantages of small size

fast response

large response bandwidth and multipoint networking. It has great application value in the measurement of weak magnetic field in the future.