Surface plasmon resonance (SPR)-based highly integrated optical fiber sensor for simultaneous detection of temperature, magnetic field, methane, relative humidity, voltage and hydrogen

Technical Summary: SPR-Based Highly Integrated Optical Fiber Sensor for Simultaneous Six-Parameter Detection

Problem Statement
Industrial safety production, particularly in environments where methane and hydrogen coexist with complex physical conditions, requires real-time monitoring of multiple parameters including temperature, magnetic fields, humidity, voltage, and gas concentrations. Traditional electrochemical or semiconductor sensors pose significant explosion risks in these high-risk environments due to their inherent electromagnetic properties. While optical fiber sensors offer a safer alternative due to their immunity to electromagnetic interference and small size, existing multi-parameter sensors face limitations. Current solutions often rely on cascaded longitudinal structures, which suffer from high signal loss and low sensitivity, or transverse structures that are difficult to fabricate with high integration density. There is a need for a highly integrated, single-probe sensor capable of simultaneously detecting six distinct physical quantities with high sensitivity and stability.

Methodology
The authors propose a novel Surface Plasmon Resonance (SPR)-based optical fiber sensor utilizing a 12-hole photonic crystal fiber (PCF) as the substrate. The sensor design involves a three-sided polishing process to expose the gold-coated air holes to the external environment.

• Structure: The sensor features a 125 µm fiber diameter with air holes of diameter $d$ and a specific distance $l_1$ from the core. Gold films (thickness $h$) are deposited in alternately arranged air holes.
• Sensing Mechanism: Six distinct air holes are filled with specific sensitive materials, each responsive to a target parameter:
  • Temperature (T): Polydimethylsiloxane (PDMS).
  • Magnetic Field (H): Magnetic Fluid (MF).
  • Methane (CH₄): Cryptophane-A and ultraviolet-cured fluorosiloxane complex (Crypt-A/UVCFS).
  • Relative Humidity (RH): Polyvinyl alcohol (PVA).
  • Voltage (U): E7 nematic liquid crystal (E7NLC).
  • Hydrogen (H₂): Palladium-tungsten trioxide (Pd-WO₃).
• Simulation and Optimization: The sensor performance was analyzed using the Finite Element Method (FEM) via COMSOL software. The study focused on the fundamental mode (FM) and its coupling with Surface Plasmon Polariton (SPP) modes. Structural parameters ($d$, $l_1$, and $h$) were optimized to maximize the average sharpness (AS) of the six resonance loss peaks while minimizing total transmission loss (SL).
• Demodulation: A sensitivity matrix was constructed to decouple the six parameters. The system solves for variations in T, H, CH₄, RH, U, and H₂ based on the shifts of the six distinct resonance wavelengths (RW1–RW6).

Key Contributions

1. First-of-its-Kind Integration: The paper claims this is the first optical fiber sensor to achieve simultaneous integrated detection of six physical quantities (T, H, RH, U, CH₄, H₂) within a single probe structure.
2. Optimized Multi-Channel Design: By utilizing a 12-hole fiber with alternating gold-coated holes filled with different sensitive materials, the design achieves distinct resonance peaks for each parameter, avoiding the high loss associated with cascaded sensor arrays.
3. Robust Matrix Inversion: The study demonstrates that the sensitivity matrix is well-conditioned (condition number = 71.22), ensuring that the inversion process for demodulating the six parameters is stable and insensitive to measurement noise.
4. Manufacturing Tolerance: The design exhibits strong robustness, with sensitivity fluctuations remaining below 6% even when structural parameters deviate by ±3% from the design values, indicating feasibility for practical fabrication.

Results
Numerical simulations and theoretical analysis yielded the following performance metrics within specific operating ranges:

• Operating Ranges: Temperature (20–40°C), Magnetic Field (35–135 Oe), Methane (0–2%), Relative Humidity (30–70%), Voltage (180–260 V), and Hydrogen (0–3%).
• Sensitivities: The optimized sensor achieved average sensitivities of:
  • Temperature: -4.9 nm/°C
  • Magnetic Field: 0.85 nm/Oe
  • Methane: -41.3 nm/%
  • Relative Humidity: -4.85 nm/%RH
  • Voltage: 3.95 nm/V
  • Hydrogen: -60.05 nm/%
• Spectral Characteristics: Six distinct loss peaks were observed in the transmission spectrum. Each peak corresponds to a specific parameter, with the resonance wavelength shifting linearly in response to changes in the target variable while other variables remain constant.
• Comparison: Compared to existing multi-parameter sensors (referenced as [9], [11], [30], [31]), this work demonstrates superior integration density and competitive or superior sensitivity levels for the respective parameters.

Significance
The authors state that this research provides a breakthrough in integration density within the field of optical fiber sensing. By successfully realizing the simultaneous detection of six diverse physical and chemical parameters in a single, compact device, the sensor addresses the critical need for monitoring complex, high-risk industrial environments. The paper highlights the sensor's potential application in environmental monitoring, industrial production status detection, and geological exploration, particularly in scenarios where multiple elements change simultaneously and safety is paramount. The work establishes a new framework for high-density multi-parameter sensing using SPR principles in photonic crystal fibers.