Magnetic field sensor can detect magnetic field information in the environment, and plays an important role in geological exploration, power transmission, aerospace and other fields. As an advanced magnetically sensitive nanomaterial, MHD not only exhibits rich magneto-optical properties (such as refractive index tunability and birefringence effect), but also seamlessly integrates with optical fiber because of its liquid fluidity, which shows broad application potential in the field of optical magnetic field sensing. In recent years, MHD fiber magnetic field sensor has been widely concerned by researchers at home and abroad because of its strong anti-electromagnetic interference ability, corrosion resistance, high safety and support for remote monitoring.
At present, the common magnetic field sensor structures of MHD fiber include conical fiber, photonic crystal fiber filled with MHD [8], single-mode-coreless single-mode fiber and long-period fiber grating. These sensors are demodulated by two main methods: power value detection and wavelength offset detection, so as to achieve magnetic field measurement. However, the sensor based on power value detection is easily affected by the power fluctuation of the light source, which may cause the measurement error to increase. Sensors based on wavelength offset detection rely on spectrometers to measure wavelength changes, which not only increases the cost, but also requires larger optical analysis equipment. In addition, existing sensors often offer only a single point of measurement capability.
To solve these problems, a dual-channel tapered fiber magnetic field sensing system based on Time division multiplexing (TDM) technology is proposed in this paper. The system is designed to overcome the limitations of existing technology and provide a more accurate, multi-point magnetic field measurement solution.
Principle of dual channel conical fiber magnetic field sensing system
The transmission, reception, photoelectric conversion and data processing of pulsed light are performed by the phase-sensitive optical time domain reflectometer (φ-OTDR) located on the left of the image. Due to the high energy of the initial pulse when the φ-OTDR device sends the test pulse, the receiver may not be able to accurately identify or process the returned signal in a short period of time. In order to solve this problem, a delay fiber is connected to the output of the OTDR. The specific working process is as follows: The pulsed light generated by the φ-OTDR device is first passed through the delayed fiber to reduce the impact of the initial pulse energy on subsequent signal processing.
The pulsed light is then coupled to port 2 of the circulator, transmitted through the internal optical path of the circulator, and output from port 3 of the circulator. Next, the pulsed light enters coupler 1 (OC1), where 1% of the pulsed light is allocated to sensing channel 1 consisting of OC1 and OC2, while 99% of the light is transmitted to sensing channel 2 consisting of OC3 and OC4. In sensing channel 1, the pulsed light is returned to OC2 after passing through the sensing unit (SU), where 99% of the light continues to circulate in sensing channel 1, and 1% of the light is transmitted back to the φ-OTDR via the circulator. Similarly, in sensing channel 2, light also follows the same path to cycle. The trajectory of the pulsed light is shown by the arrows in the figure. Pulsed light is cycled many times in the sensing channel, and each time it passes through the magnetic field SU, it will experience a certain loss.
Stability test
First, in a non-magnetic field environment, the pulse slope of the sensing system and the output optical power of the laser were repeated for 30 times to obtain the average attenuation slope of the system, as shown in FIG. 4(a). It can be seen that the average output optical power of the laser is 1.21 mW, and the standard deviation is 0.051 6 mW, which is equivalent to 4.26% of the average. In 30 repeated experiments, the average attenuation slopes of sensor channel 1 and channel 2 are -11.57 dB/km and -18.117 dB/km respectively, and the corresponding standard deviations are 0.109 dB/km and 0.124 dB/km, accounting for 0.942% and 0.684% of their respective mean values, respectively. This shows that even if the power of the light source fluctuates, the system still shows good stability and the measurement results are reliable.
Secondly, the sensor channels 1 and 2 were placed under a constant magnetic field intensity of 5 mT to evaluate the response stability of the magnetic field sensing system. The experimental results are shown in FIG. 4(b). It can be seen that the average attenuation slope of sensing channel 1 is -14.85 dB/km, and the standard deviation is 0.131 dB/km, accounting for 0.882% of the average value. The average attenuation slope of sensor channel 2 is -30.94 dB/km, and the standard deviation is 0.315 dB/km, accounting for 1.02% of the mean value. These data prove that the response of the sensor system under the influence of magnetic field has high consistency and stability.
An innovative dual-channel tapered fiber magnetic field sensing system based on Time division multiplexing (TDM) technology significantly improves the multiplexing capability of fiber magnetic field sensing systems. The system detects the attenuation rate of pulsed light in the sensing channel precisely and combines TDM technology to realize the simultaneous measurement of multi-point magnetic field.
Compared with the traditional MHD fiber magnetic field sensor, the system not only has stronger reuse capability, but also has higher tolerance to the power fluctuation of the light source. The experimental results show that the magnetic field sensitivity of the two sensing channels reaches -1.09 dB/(km•mT) and -3.466 dB/(km•mT) respectively in the field intensity range of 3~14 mT and 2~7 mT. These data show that the system can provide high precision measurement results over a wide range of magnetic fields.
The sensor system has many advantages: simple production process, strong reuse ability, excellent anti-electromagnetic interference performance, good stability, support for remote monitoring and so on. Therefore, it is particularly suitable for applications requiring remote multi-point magnetic field monitoring, such as power transmission lines, large mechanical devices and scientific research fields, showing broad application prospects.