Multimode fibers (MMFs) can serve as high-precision, non-contact temperature sensors using speckle patterns, multimode interference, or distributed sensing techniques.

Principles of MMF Temperature Sensing

Multimode fibers can detect temperature changes because the speckle pattern or interference of light propagating through the fiber is highly sensitive to environmental variations, including temperature. As light travels through the MMF, temperature fluctuations alter the refractive index and physical length of the fiber, causing measurable changes in the output light pattern or spectral characteristics . This allows the fiber itself to act as a distributed temperature sensor.

Techniques and Methods

1. Deep Learning-Based Speckle Sensing By inputting the speckle pattern generated in an MMF into a Convolutional Neural Network (CNN), both the temperature and the location of heating points along the fiber can be predicted with high accuracy. Experiments have shown temperature prediction within ±1 °C and location accuracy below 1 cm, making this method suitable for hazardous or hard-to-access environments .
2. Multimode Interference (MMI) Sensing MMF sections can be used in single-mode–multimode–single-mode (SMS) structures, where the spectral dip shifts linearly with temperature. Sensitivity depends on core diameter, numerical aperture (NA), and fiber length. Larger core diameters increase temperature sensitivity, while longer fiber sections reduce it. This approach can also be designed to be strain-insensitive, improving measurement stability .
3. Temperature Principal and Anti-Principal Modes Special light states in MMFs can be engineered to either enhance or reduce sensitivity to temperature fluctuations. Anti-principal modes amplify temperature-induced changes, improving sensor resolution, while principal modes provide robustness against environmental disturbances .
4. Distributed Temperature Sensing (DTS) with MMFs MMFs with 50 µm or 62.5 µm cores and graded-index profiles are commonly used in DTS systems. Graded-index fibers reduce modal dispersion, improving signal-to-noise ratio and measurement precision over long distances. DTS systems provide continuous, real-time temperature monitoring along the entire fiber length, unlike point-based sensors .

Fiber Considerations

• Core and Cladding: The core transmits light, while the cladding confines it via total internal reflection.
• Coating: Polyimide or metal coatings are used for high-temperature or harsh environments, while polyacrylate is suitable for standard conditions .
• Core Diameter: Larger cores (e.g., 400–600 µm) increase sensitivity but may reduce spatial resolution .
• Fiber Type: Step-index or graded-index MMFs are chosen based on the application, with graded-index preferred for DTS due to reduced modal dispersion .

Applications

MMF-based temperature sensing is widely applied in:

• Industrial and hazardous environments
• Fire detection and safety monitoring
• Environmental and structural monitoring
• Healthcare and fluid dynamics studies These methods offer non-contact, high-precision, and distributed temperature measurement, often outperforming traditional fiber-optic sensors in terms of installation simplicity, cost, and robustness . In summary, multimode fibers provide versatile and accurate temperature sensing capabilities, leveraging speckle patterns, multimode interference, and distributed sensing, with advanced techniques like deep learning further enhancing precision and spatial resolution.