A fiber-optic magnetic sensor system for the detection of weak magnetic field was constructed. Magnetic field sensing part was fabricated by bonding a section of optical fiber to amorphous metallic glass(2605SC) having large magnetostostriction effect. And with the directional coupler, all fiber type Mach-Zehnder interferometer was constructed to measure the variation of the external magnetic field by translating it into the optical phase shift in the interferometer. The signal fading problem of the interferometer, which is due to random phase drifts originated from the environment, ie, temperature fluctuation, vibrations, etc, was elliminated by feedback phase compensation. This allows the sensitivity to be maintained at the maximum by keeping the interferometer in quadrature phase condition.
The frequency reponse of metallic glass sample was found to be nearly flat in the range of 900Hz~2KHz and dc bias field for the maximum ac response was 3.5 Oe. The mechanical resonance frequency of metallic glass was found to be 5.1 KHz and nearly flat in the range of 5300~5500 Hz. The interferometer output showed good linearity over the range ±0.5 $O_e$. For 1Kz ac magnetic field the scale factor S and the minimum detectable magnetic field were measured to be 8.0rad/O{\boldmath$e$} and $3×10^{-6}$O{\boldmath$e$}/{\boldmath$\sqrt{Hz}$} at 1 Hz detection bandwidth respectively. By using the dc magnetic feedback nulling schemes, the sensing element is maintained in the state of zero magnetizion. And we have showed that the effects of the magnetic hystereis is minimized around zero magnetization. For the maeasurement of dc magnetic field, the minimum detection magnetic field were measured to be $6.9×10^{-5}$O{\boldmath$e$}/{\boldmath$\sqrt{Hz}$} (1Hz detection bandwidth) with 5.49 KHz ac dither field. The magnetic feedback nulling schemes have been employed in the fiber-optic magnetometers leading to an extended linear dynamic range and the elimination of magnetic hysteresis effect.
Another important source of noise in such a magnetometer is so-called residual signal that brings the noise at low frequency to near the detection frequency(i.e. dither frequency). The residual signal comes from the inhomogeneous response of the metallic glass to the applied magnetic field. The suppression of residual signal has been demonstrated by the proper choice of magnetic dither frequency and amplitude resulting in high sensitivity of the magnetometer. This approach, requiring precise control of the dither frequency, may not provide a stable solution to the problem when subjected to environmental perturbation that introduce changes in the mechanical properties of the metallic glass transducer.
