Magnetic field fiber sensor based on the magneto-birefringence effect of magnetic fluid

• In the paper, we provided an in-line MF-filled D-shaped fiber magnetic field sensor based on fiber Sagnac interferometer. The sensitive section is fabricated via a D-shaped microstructure inscribed in a high-birefringence fiber Sagnac loop using femtosecond laser. The optical fiber Sagnac interferometer and magnetically controlled birefringence of MF can be integrated well using the D-shaped microstructure. Experimental results show the good performance, especially high stable extinction ratio, of the Magnetic-field fiber sensor.
• And the sensitive section can be made longer to increase sensitivity, whereas the thickness of the MF film was limited in the open cavity, especially for high concentration of black-brown MF. The Sagnac fiber magnetic field sensor can even be fabricated without cascading the PMF.
• Moreover, There are several reasons for the work of our research:
• Firstly, Numerous refractive index tunability of MF-based sensors have been reported, However, the tunable range of the refractive index of MF under an external magnetic field is rather small, which limits the sensitivities of MF-based magnetic field sensors.
• Sencondly, the MF possesses a large birefringence coefficient and exhibits huge transmission loss because of the increase in the transmission length. Thus, applications based on the birefringence effect of MF are rarely reported.
• Thirdly, a sagnac fiber magnetic field sensor based on the magneto-birefringence of MF film was first proposed by Zu in 2011. The sensitivity of this sensor has been improved in 2014 from16.7 pm/Oe to 592.8 pm/Oe. But a lot of problems still exsit:
• for instance, incorporating MF film in the loop using two collimators is not favorable, and manual alignment issues which limit the application of MF. And the birefringence of MF is always accompanied by dichroism, and the output optical power will be varied by changing the applied magnetic field strength.
• The most important thing is that the sensitive section of these sensors cannot be extended to increase sensitivity, particularly in the sections with a high concentration of black-brown MF. The reason is that absorption loss occurs when the incident light directly passes through the dark reddish-brown colloidal solution (MF). This problem needs a solution for the optical applications of MF in the future.
• These scenarios contribute to the deterioration of device performance. In this study, an in-line fiber magnetic field sensor based on the magneto-birefringence effect of MF is proposed to avoid the aforementioned limitations and provide good performance. The sensitive section of this sensor can be extended to increase sensitivity, whereas the thickness of the MF film is limited in the open cavity, particularly in the section with a high concentration of black-brown MF.
• Finally, the proposed fiber magnetic sensor also has many merits such as immunity to electromagnetic interference, compactness, safety, easy fabrication and low maintenance requirements. This therefore provides a probable way forward in the development of magnetic field sensors and has attractive potential applications.

In this study, the magneto-birefringence effect of magnetic fluid (MF) is adopted to form an innovative fiber optic magnetic field sensor. The sensitive section is fabricated via a D-shaped microstructure inscribed in a high-birefringence fiber Sagnac loop with a femtosecond laser. The D-shaped microstructure facilitates good combination of the optical-fiber Sagnac interferometer with the magneto-birefringence effect of MF without suffering from absorption loss and manual alignment. Experimental results show the good performance of the magnetic field fiver sensor, particularly its high stable extinction ratio. Preliminary results are provided, and the magnetic field sensitivity of 0.0823 nm/mT can be further improved by increasing the depth and length of the D-shaped microstructure.