Ultrahigh Sensitivity Surface Plasmon Resonance Magnetic Field Sensor Based on D-Shape Four-Hole Fiber
The conventional magnetic sensors are either bulky or require complex setups. The optical fiber-based magnetic field sensors address these challenges. However, these sensors often have low sensitivity, so a surface plasmon resonance (SPR) sensor was developed to solve the issue. This paper explores one of the most recent designs, which uses a side-polished D-shape fiber with four holes (DFHF-SPR). Due to its high accuracy and simple design, the new sensor could be advantageous in areas like geological exploration, mining, and detecting tiny magnetic particles in medical research.
The DFHF-SPR magnetic field sensor features a side-polished D-shape fiber with four air holes and an Aluminum (Al) wire for SPR generation. The structure of the sensor was designed and tested using advanced software, i.e., COMSOL, to ensure the best performance. The optimized DFHF-SPR sensor enhances magnetic field detection by adjusting key parameters. The best design includes a core diameter (d₀) of 18 μm, an air hole diameter of 40 μm, and an Al wire diameter of 0.8 μm. The DFHF-SPR magnetic field sensor enhances sensitivity and simplifies Magnetic Fluids (MF) filling. The MF (SS-F10C) refractive index varies from 1.4352 to 1.4383 in a 30-200 Oersted (Oe) range. The Al wire enables strong energy transfer, SPR coupling, and oxidation resistance.
When light travels through the fiber core, some escape into the surrounding magnetic fluid. Because the glass (SiO₂) has a higher ability to bend light than the fluid, it creates an evanescent wave at the boundary. This wave then interacts with tiny free-moving electrons on the surface of the aluminum wire, causing them to vibrate and generate a surface plasma wave. When both waves synchronize, energy transfers to the aluminum wire, leading to a noticeable drop in light intensity. Since the magnetic fluid’s properties change with the strength of the magnetic field, the specific wavelength at which the energy transfer happens alters. The sensor can accurately measure magnetic field strength by analyzing these wavelength shifts.
The sensor is tested using a 100W tungsten halide light source (350–1400 nm) and an Ocean Optics USB2000 and Optical Spectrum Analyzer (OSA) (400–2200 nm). Multi Mode Fibres (MMFs) are fused using an optical fiber fusion machine. The temperature is maintained at 24.3°C, and a DC power source controls the magnetic field. Light intensity and RW shifts are recorded on a PC via the OSA. This configuration achieves a peak sensitivity of 176,000 pm/mT at 35 Oe, with a resolution of 5.68 nT and a maximum Figure of Merit (FOM) of 7.93 mT⁻¹. The sensor operates effectively in the 35-150 Oe range, maintaining high sensitivity, i.e., ≥1333.33 pm/mT. Temperature compensation via FBG ensures stability, and ±5% fabrication errors cause minimal variation.
The DFHF-SPR sensor has redefined magnetic field detection technology with unmatched sensitivity, precision, and efficiency. Its high stability and cost-effective design make it a game-changer in sensing technology. In the future, the sensor will pave the way for innovative advancements and enhance accuracy across various scientific, technological, and industrial applications.