Can optical fibers measure the geomagnetic field?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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How does it sense tiny changes in the Earth’s magnetic field?

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The optical fiber itself is composed of high-purity silicon dioxide (\text{SiO}_2), which is an isotropic, non-magnetic medium and inherently possesses anti-electromagnetic interference (EMI) characteristics. Therefore, bare optical fibers cannot directly produce a noticeable response to static or weak magnetic fields.

The geomagnetic field intensity on the Earth’s surface is extremely weak, typically ranging between 30\ \mu\text{T} \sim 60\ \mu\text{T} (approximately 0.3 \sim 0.6\ \text{Gauss}). To utilize optical fibers to sense minute changes in the geomagnetic field, physical engineering primarily relies on the “magneto-mechanical-optical” conversion mechanism or the Faraday magneto-optical effect.

I. Core Sensing Principle: Magnetostriction and Fiber Coupling (“Magneto-Mechanical-Optical” Transduction)

This is currently the most sensitive and mature technological path for fiber optic measurement of weak magnetic fields (including geomagnetic field monitoring):

  1. Magneto-Mechanical Conversion (Magnetostrictive Materials):

    • High-permeability, high-sensitivity magnetostrictive materials (such as amorphous magnetic alloys like Metglas, and magnetostrictive materials like Terfenol-D) are processed into substrates, coatings, or thin ribbons.
    • When the external magnetic field (e.g., geomagnetic field disturbances) changes minutely, the magnetic domains within the magnetostrictive material deflect, causing the material to undergo slight dimensional changes macroscopically (generating magnetostriction strain \varepsilon_{\text{mag}} ).

  2. Mechanical-Optical Conversion (Fiber Sensing Unit):

    • Coupling with Fiber Bragg Grating (FBG): Fiber Bragg gratings are tightly bonded or electroplated onto the magnetostrictive material. The minute strain from the material is directly transmitted to the fiber core, altering the effective refractive index n_{\text{eff}} and grating period \Lambda of the grating, causing a shift in the reflected center wavelength:
      \frac{\Delta \lambda_B}{\lambda_B} = (1 - p_e) \cdot \varepsilon_{\text{mag}}
      (where p_e is the effective photoelastic coefficient).
    • Coupling with Fiber Interferometer (MZI / Michelson / Sagnac): The optical fiber serves as an interference arm, wound around a magnetostrictive cylinder or adhered to an alloy ribbon. Magnetostriction strain causes changes in the fiber length, which in turn alters the phase \Delta \phi of the transmitted light. Interferometric demodulation can achieve extremely high strain resolution (down to sub-pico-strain levels), enabling the sensing of magnetic field fluctuations as low as \text{nT} or even \text{pT}.

  3. AC Modulation of Static Geomagnetic Field (Bias Magnetic Field Technique):

    • Due to the quadratic nonlinearity of the magnetostrictive effect near zero magnetic field (\varepsilon \propto B^2), it is insensitive to weak, near-zero magnetic fields.
    • To achieve high-sensitivity detection of the geomagnetic field, the sensor typically employs an additional weak AC modulation magnetic field (Dither Field) of a known frequency or a fixed DC bias. This shifts the operating point to the linear region where the slope of the magnetostrictive curve (piezomagnetic coefficient d_{33} = \frac{\partial \varepsilon}{\partial B}) is maximized. This allows the minute drift of the geomagnetic field to modulate the AC carrier, significantly improving the signal-to-noise ratio.

II. Auxiliary Mechanism: Faraday Magneto-Optical Effect (Polarization Modulation)

Another principle is the Faraday effect (magneto-gyration):

  • When linearly polarized light passes through a magneto-optical medium along the direction of the magnetic field, its plane of polarization rotates by an angle:
    \theta = V \cdot B \cdot L
    (where V is the Verdet constant of the medium, B is the magnetic induction, and L is the optical path length).
  • Technical Limitations: Conventional silica optical fibers have a very low Verdet constant in the optical communication bands (e.g., at 1550\ \text{nm}) (only about 0.5 \sim 1\ \text{rad}/(\text{T}\cdot\text{m}) ). Relying solely on ordinary silica fibers makes it difficult to effectively resolve geomagnetic field variations in the microtesla (\mu\text{T}) range. Therefore, if this effect is used to measure weak magnetic fields, it is necessary to fuse in special terbium-doped magneto-optical fibers or micro-magneto-optical crystals (such as TGG, BIG, etc.) in the optical path, combined with a polarization interference optical circuit with high extinction ratio.

III. Key Engineering Challenges

When practically measuring minute changes in the geomagnetic field, fiber optic sensors must address the following physical interferences:

  1. Cross-Temperature Sensitivity: Both FBG and fiber interferometers are extremely sensitive to temperature due to the thermo-optic effect and thermal expansion (temperature drift is usually much greater than the wavelength/phase drift caused by weak geomagnetic disturbances). Reference differential compensation optical paths or high-precision dual-wavelength demodulation must be employed to eliminate temperature drift.
  2. Vibration and Stress Isolation: The magnetostrictive structure must utilize a stress-free packaging design to prevent external mechanical loads from directly acting on the optical fiber.

Official Product Information

Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s current core product lines focus on fiber Bragg grating sensing for physical quantities such as temperature, strain, stress, displacement, and pressure in industrial-grade and extremely harsh environments. Geomagnetic field sensors and magnetic sensing units based on the special integration of magnetostrictive materials are not part of Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®)'s existing core standard product portfolio.