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Compact, remote optical waveguide magnetic field sensing using double-pass Faraday rotation-induced optical attenuation

Yunlong Guo, John Canning, Zenon Chaczko, and Gang-Ding Peng

Open Access Open Access

Abstract

Compact, magnetic field, $B$ sensing is proposed and demonstrated by combining the two Faraday rotation elements and beam displacement crystals within a micro-optical fiber circulator with a fiber reflector and ferromagnets to allow high contrast attenuation in an optical fiber arm. Low optical noise sensing is measured at $\lambda = {1550}\;{\rm nm}$ as a change in attenuation, $\alpha$, of optical light propagating through the rotators and back. The circulator’s double-pass configuration, using a gold mirror as a reflector, achieves a magnetic field sensitivity $s = \Delta \alpha \!/\!\Delta B = ({0.26}\;{\pm}\;{0.02})\;{\rm dB}\!/\!{\rm mT}$ with a resolution of $\Delta B = {0.01}\;{\rm mT}$, over a detection range $B = {0 - 89}\;{\rm mT}$. The circulator as a platform provides direct connectivity to the Internet, allowing remote sensing to occur. The method described here is amenable to multisensor combinations, including with other sensor technologies, particularly in future integrated waveguide Faraday optical circuits and devices, extending its utility beyond point magnetic field sensing applications.

© 2024 Optica Publishing Group

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Corrections

18 March 2024: Corrections were made to the author listing and the abstract.

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Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (7)
Fig. 1.
Fig. 1. Inside a commercially packaged circulator: (a) shows the three beam displacers and two circular permanent ferromagnets surrounding the rotator crystals embedded in semicylinder Kovar; (b) shows the structure of (a) after extracting the two ferromagnets and the Faraday rotators within; and (c) shows the removed half ferromagnet and Faraday rotating crystal.

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Fig. 2.
Fig. 2. Working principle of a commercial circulator illustrating the complex micro-optical assembly pathways through the three optical ports, two ferromagnets, four rotator units, and three polarizing displacement crystals held in place by an engineered Kovar frame within the device. The top represents the pathway of the incoming signal and transmission through the circulator, while the bottom shows the return pathway, separated for optical clarity. The light is returned to the circulator by adding a gold-coated mirror at Port 2.

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Fig. 3.
Fig. 3. Circulator-based magnetic field sensor setup schematic and (inset) image. The field magnet is fixed into an optimal position to the circulator elements such that ${B_f}$ is parallel to optical propagation through the rotators. A gold coating was placed on the fiber end-face at Port 2 to act as the broadband reflector that provides the double-pass propagation through the device.

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Fig. 4.
Fig. 4. Optical output power, $P$, from Port 3 measured in mW generated from a power meter as a function of magnet position, $x$, moving towards and away from the circulator at $x = {0}$ (see Fig. 3) and $y = {9.5}\;{\rm mm}$. $P(x)$ arises from at least two contributions with the applied magnetic field, $B(x)$, as a function of position $x$.

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Fig. 5.
Fig. 5. Shows the optical transmitted power, $P$(mW), measured directly from Port 3, against the magnet axial position, $y$, along the circulator. The 0 position is at $y = {19.5}\;{\rm mm}$ from Port 1; the black line and symbols denote the response from Port 3 with the magnet moving along one direction, the ${-}y$-axis; and open squares represent the response from Port 3 with the magnet moving in the reverse direction, the ${+}y$-axis.

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Fig. 6.
Fig. 6. Calibration plot for the combined circulator and magnet sensor allowing conversion of the measured attenuation to magnetic field: (a) shows $P$ (mW) from Port 3 as a function of $B$ (mT); (b) the plot of $B$ against ${\boldsymbol \alpha}$ is approximately linear, consistent with an exponential change in signal transmission with applied magnetic field. The inverse of the slope provides the sensor sensitivity: $s = \Delta \alpha\!/\!\Delta B$.

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Fig. 7.
Fig. 7. Optical fiber magnetic sensor measurements of an unknown magnet positioned at different locations. Within Fig. 5, the unknown magnet is black with positions around it identified. The measured magnetic field of the sensor ${B_C}$ and the magnetometer ${B_M}$ for each point is shown on the $y$ axis. These positions, P1 to P7, are points that map the magnetic field around the unknown magnet.

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Tables (1)

Tables Icon

Table 1. Sensitivity, s , Resolution, Δ B , and Dynamic Range, R , of Various Magnetic Fields, B , Sensing Approaches in the Literature Compared against Our Data

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Equations (6)

Equations on this page are rendered with MathJax. Learn more.

Δ θ = V e f f lB s .
Δ θ = 4 V e f f tB s ,
Δ θ = 4 V e f f t ( B s − B f ) .
P ( x ) = P 0 + ( P max − P o ) ( f ( 1 + e x − x 01 s 1 ) + 1 − f 1 + e x − x 02 s 2 ) ,
B = α 0 + α / s .
s = Δ α / Δ B .
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