1. Introduction

Simultaneous physical parameters measurement is a hotspot and problem in physics. Cho et al. [1] proposed a micro-electromechanical systems-based magnetometer-accelerometer for mobile electronic systems. Guan et al. [2] used a superstructure fiber Bragg grating to measure strain and temperature simultaneously, which had an application in structure health monitoring area. Simultaneous physical parameters measurement is not only an efficient method of measurement, but also has many additional advantages such as saving space, decreasing transmission data, and low cost.

Acoustics and magnetism have common application areas, especially under water. For an underwater vehicle, magnetometer is used as a navigator and sonar is used to avoid obstacles. For ocean bottom earthquake monitoring, the motions of ocean floor are always along with sound and magnetic anomaly. For exploration of marine mineral resources, using marine streamer is a common way to analysis the structure of ocean bottom strata, and magnetometer is the best instrument of exploring ferromagnetic minerals. So a sensor with the function of simultaneous acoustic and magnetic measurement has a broad application prospect in marine science.

Many groups of optical fiber sensing have developed the techniques of fiber acoustic sensors and fiber magnetic sensors. Løveseth et al. [3] used fiber distributed feedback (DFB) lasers as acoustic sensors in 1999. Foster et al. [4] presented a micro-engineered silicon fiber laser hydrophone of high sensitivity. Du et al. [5] used a nanothick silver diaphragm based distributed Bragg reflector (DBR) fiber laser as microphone, which achieved high accuracy. For magnetic field measurement, fiber optic magnetometers were mostly based on Faraday effect [6] or fiber device covered with magnetostrictive jacket, such as nickel, metallic glass, in 1980s [7–9 ]. With the develop of nanotechnology, magnetic materials have become more intelligent and easier to be integrated with optical fiber. For example, filling magnetic fluid (MF) into photonic crystal fiber (PCF) or glass capillary or using MF as cladding are hotspots in recent years [10–14 ]. Fiber acoustic sensors and fiber magnetic field sensors have better performance than before, and have been more practical, which provides a solid foundation for the research on simultaneous acoustic and magnetic measurement. But in general, the response of sound can be considered as an AC signal, and the response of magnetic field is usually at low frequency or ultra-low frequency, which can be considered as a DC signal. It sets up an obstacle for simultaneous measurement of the two physical parameters. For most demodulation system, wide frequency band demodulation is always a technical problem owing to many factors, for example, 1/f noise for low frequency demodulation, sampling rate limited by electronic devices for high frequency demodulation. An effective method is to provide an active AC carrier for DC magnetic signal, so that the acoustic signal and magnetic signal can be demodulated simultaneously in frequency domain using a “high” frequency demodulator. An active AC current combining with a magnetic field signal can generate an AC Lorentz force [15]. Thanks to the immunity to electromagnetic interference of fiber sensing, an optoelectronic hybrid fiber optic sensor may be a promising solution to this problem.

In this paper, we will present an optoelectronic hybrid fiber optic acoustic and magnetic sensor (FOAMS) based on fiber laser sensing. It has advantages that efficient, low cost, and fit for long-time monitoring.

2. Principle

As an exploratory study, we designed an optoelectronic hybrid fiber laser sensor for simultaneously measuring acoustic pressure and magnetic flux intensity in the air. As shown in Fig. 1(a) , a metal beam was clamped onto a U-type polymethyl methacrylate (PMMA) support through two nonmagnetic bolts. Using PMMA to make the support was just taking electric insulation and low-cost into account. Both the beam and the support have a very small hole about 0.5 mm in the middle. A distributed feed back (DFB) fiber laser was put through them and fixed by glue at the edges of the two holes. To avoid bending, a prestrain was applied on the DFB fiber laser in the assembling process. As shown in Fig. 1(b), two electric wires were fixed along the two sides of the support and welded with the two ends of the beam. At the bottom of the support, the two wires and the fiber were bundled together, protected by a rubber sheath, and constituted an optoelectronic hybrid cable. The material of metal beam was beryllium bronze, which had good elasticity and conductivity. When connecting the wire terminal 1 (red line) and 2 (green line) to a current source, there will be a well distributed current field through the beam.

 

Fig. 1 Schematic view of (a) the designed fiber acoustic and magnetic sensor, (b) its front view, and (c) force analysis of the beam with the applied acoustic pressure and magnetic field.

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As shown in Fig. 1(c), the beam will be deformed when pressures are applied on its surface, and the maximum deflection is at its center in the z direction. The deformation will lead to the strain of the DFB fiber laser ε(t), which makes a shift of its center wavelength. It can be expressed as

For a rectangular clamped beam, the static pressure sensitivity of the fiber laser sensor can be approximately calculated by [16]

Owing to the property of wave, the acoustic pressure acting on the beam is actually the differential pressure between the up and down surface. A single-frequency acoustic signal can be expressed by ΔP_A(ω_it), and ω_i represents the angular frequency. A broad frequency band acoustic signal can be expressed by ∑ΔP_A(ω_it), thus ω_i represents a discrete frequency point among the frequency band. As shown in Fig. 1(c), only the y-axial component of the magnetic field could be measured. So the FOAMS is a component sensor for magnetic field measurement. Under a static magnetic field, Lorentz force applied on a conductor can be expressed by BI(t)a, and I(t) is an active current. The surface density of the Lorentz force ΔP_B(t) can be expressed as BI(t)/b. Taking I(t) as a single-frequency signal, it can be expressed as I ₀sin(ω ₀ t), where ω ₀ is an arbitrary angular frequency out of the frequency band of sound to be measured. The dynamic range of the sensor is simultaneously determined by the amplitude of the acoustic pressure and magnetic flux intensity. Unwanted phenomena such as frequency multiplication or even different frequency will come out if one of them is too large in some special testing environment. Thus I ₀ turns to be a very important parameter to balance the detecting capability of the sensor between sound and magnetic field.

The acoustic signal and magnetic field signal are both applied onto the surface of the beam. But they could be well distinguished in frequency domain. On one hand, the frequency of the applied current is a single and adjustable frequency signal. The changes of the intensity of static magnetic field can only induce the changes of output amplitude at the applied frequency point in frequency domain. But acoustic signals in practical applications are usually within a “broad” frequency band comparing with the single frequency point. On the other hand, acoustic signals to be measured are usually within a frequency band in practical applications. For example, undersea earthquake is usually lower than 200 Hz. The applied current is controllable for an operator or a preset program, a suitable frequency point could always be found.

3. Experiment and results

A FOAMS was fabricated in the size of 6 cm × 4 cm × 1 cm. The length between the fixed points of the fiber laser is about 60 mm. Since the natural frequency of fiber laser is very low at the level of ~Hz, the frequency response of the sensor is mainly determined by the metal beam. The effective size of the beam is 2 cm × 1 cm × 0.3 mm. Analyzed by the software of SolidWorks Simulation, the first three orders natural frequency of the designed beam is respectively 2189 Hz, 4674 Hz, and 6019 Hz.

The experiment setup is shown in Fig. 2 . A speaker controlled by signal generator A was used to produce a sine acoustic wave. And the amplitude and the frequency of the acoustic pressure were adjustable. A magnetic field generator based on Helmholtz coils produced a static uniform magnetic field. Signal generator B was used as a sinusoidal voltage source, of which the output voltage could be displayed on the screen. The resistor was also adjustable. The fiber laser demodulator consisted of a 980 nm pumping light source, some fiber devices like fiber coupler and wavelength division multiplex (WDM), a photoelectric detector, a DAQ, and so on. The interrogation of FOAMS was achieved by phrase-generated carrier (PGC) demodulation with the help of a PZT driving fiber Michelson interferometer. The detectable frequency band of the demodulator was from 20 Hz to 2000 Hz. The demodulation results could be displayed through the computer in time domain and frequency domain.

 

Fig. 2 The experiment setup. The red lines represent fibers, and blue lines represent electric wires. The direction of the arrows represents the transmissions of signals. The reference acoustic sensor and magnetometer are not given, because they were at the same place to the FOAMS when did comparative experiments.

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Firstly, taking the FOAMS as an acoustic sensor, a piezoelectric microphone (BK 4189) was used as a reference. And taking the FOAMS as a magnetometer, a magnetometer based on Hall effect (SG-42 digital Tesla-meter) was used as a reference. Although the comparison method cannot rule out the effects based on the sensors themselves, roughly accurate results can be achieved, which is shown in Fig. 3 . The red one is a fitting curve of the acoustic pressure sensitivity. The maximum fluctuation at low frequency band is more than 3 dB, which is probably owing to near-field effect. The average value from 200 Hz to 1200 Hz is −164.7 dB (0 dB re 1 pm/μPa). Comparing to the acoustic sensitivity, the magnetic field sensitivity (the blue curve) got a flatter frequency response. To void the effects of the electric wires, the amplitude of the current was set at 100 mA all through the experiment. The magnetic field sensitivity was about 0.6 dB ± 0.5 dB (0 dB re 1 pm/ (T·A)). Based on the PGC demodulation, the resolutions of the acoustic pressure and magnetic flux intensity were respectively ~mPa and ~μT.

 

Fig. 3 The frequency response curves of the acoustic pressure sensitivity and the magnetic field sensitivity.

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The beam is the only sensing element in this structure. And the acoustic pressure and magnetic field intensity both show a linear relationship to the strain of fiber laser. So their response curves are of the same shape, when taking FOAMS as acoustic sensor and magnetic field sensor respectively.

An experiment on simultaneous acoustic and magnetic measurement was carried out. Assuming the acoustic signal to be measured was lower than 1000 Hz, a 1200 Hz carrier-current was applied into the FOAMS. As shown in Fig. 4 , when the FOAMS using as a magnetometer was monitoring the environment, a broken-in acoustic signal at 500 Hz could also be detected. From Figs. 4(a)-4(c), the amplitude of the acoustic signal increased higher, but it had little effect on the response of the magnetic signal. In Fig. 4(d), the new generated frequency points such as 1000 Hz and 1500 Hz was the frequency multiplication of the acoustic signal, and 700 Hz might be a different frequency by mechanics or the program of the demodulation system. Figures 4(e)-4(g) could also prove that the increasing magnetic field intensity had little influence on the acoustic measurement. Owing to the limitation of the magnetic field generator, there was no obvious generated frequency point except for a short peak at 1400 Hz in Fig. 4(h).

 

Fig. 4 The simultaneous acoustic and magnetic signals in frequency domain. From (a) to (d) is a stable magnetic field (1200 Hz) with an increasing acoustic pressure (500 Hz). From (e) to (h) is a stable acoustic field (500 Hz) with an increasing magnetic field intensity (1200 Hz).

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4. Conclusion

In conclusion, an optoelectronic hybrid fiber optic acoustic and magnetic sensor was proposed. An experiment on the FOAMS was carried out, and the testing results showed that simultaneously measuring acoustic pressure and the magnetic flux intensity was realized. The acoustic pressure sensitivity was about −164.7 dB (0 dB re 1 pm/μPa) and the magnetic field sensitivity was 0.6 dB (0 dB re 1 pm/ (T•A)). The resolution and the dynamic range of FOAMS could be further improved through some optimal designs.