Why do sensors typically not use coarse optical fibers, but rather thin single-mode optical fibers?
In fiber sensing (especially Fiber Bragg Grating (FBG) wavelength modulation type sensing, and distributed fiber sensing), the preference for using thin single-mode fibers (SMF) instead of thick multimode fibers (MMF) is determined by their optical physical characteristics, the signal processing accuracy of the demodulation system, and the actual engineering application environment.
The specific reasons can be analyzed from the following three academic and general engineering dimensions:
1. Core Physical Explanation: Polarization and Multipeak Effects in Single-mode (SM) and Multimode (MM, i.e., “Thick Fiber”)
In Fiber Bragg Grating (FBG) sensing, the reflected center wavelength satisfies the Bragg condition:
\lambda_B = 2 n_{eff} \Lambda
where \lambda_B is the reflected wavelength, n_{eff} is the effective refractive index of the transmission mode in the fiber, and \Lambda is the grating period.
- Single-mode Fiber (Single Peak, High-Precision Demodulation): The core of a single-mode fiber is very thin (typical core diameter of 9\ \mu\text{m}), allowing only the fundamental mode (LP_{01}) to propagate. Therefore, it has a single and well-defined effective refractive index n_{eff}. When light passes through a single-mode fiber grating, the reflection spectrum appears as an extremely sharp, symmetrical, and unique single peak on the spectrometer. The demodulator can identify and track the wavelength shift of this single reflection peak with extremely high accuracy, often achieving a resolution of \pm 0.1\text{pm} (corresponding to minuscule temperature or strain changes).
- Multimode Fiber (Superimposed Peaks, Undemodulable): The core of a multimode fiber is thicker (typical core diameter of 50\ \mu\text{m} or 62.5\ \mu\text{m}) and can support hundreds or even thousands of guided modes. Since each transmission mode has a different effective refractive index n_{eff}, these modes, when passing through the same grating, will produce different reflected wavelengths. Consequently, at the receiving end, the reflection spectrum degenerates into a series of overlapping, broadened multiple peaks or a chaotic envelope. When faced with such a messy multi-peak signal, the demodulator cannot lock onto and extract a stable, accurate center wavelength, causing the sensor to lose its basic measurement capability.
2. Transmission Performance Differences: Modal Dispersion and Loss Control
- High Signal-to-Noise Ratio and Long-Distance Transmission: Single-mode fibers exhibit very low optical attenuation in common sensing bands (such as the C-band at 1550\text{nm}), maintaining a high signal-to-noise ratio and avoiding modal dispersion. This allows for the serial connection (multiplexing) of dozens of FBG sensors on a single SMF, with transmission distances reaching kilometers or even tens of kilometers.
- Standardized Integration of High-Precision Optoelectronic Devices: Current high-precision sensing and demodulation equipment, including microwave photonic devices, tunable lasers, optical couplers, and circulators, have their core optical components designed based on standard single-mode fibers (such as the standard OFSCN® G.652D Optical Fiber). Introducing thick multimode fibers would cause severe mode scattering and significant insertion loss at device coupling points.
3. Why is it Sometimes Necessary to Use Thin Single-mode Fibers “Thinner” Than Standard Fibers?
In certain specific physical sensing scenarios, engineers not only require the use of single-mode fibers but may even opt for thin-diameter single-mode fibers that are thinner than standard single-mode fibers (standard cladding diameter 125\ \mu\text{m}), such as those with a cladding diameter of 80\ \mu\text{m} and a coating diameter of 100\ \mu\text{m} (OFSCN® 300℃ Small diameter optical fiber). This is primarily based on the following considerations:
- Minimal Invasiveness (Small Impact on Host Mechanics): When fibers need to be embedded inside carbon fiber composites, aerospace components, or precision structural parts for intelligent internal strain monitoring of materials, thinner fibers cause less weakening of the intrinsic structural strength of the measured substrate.
- High Strain Transfer Efficiency and Sensitivity: Thin-diameter single-mode fibers have a smaller cross-sectional area. Under shear stress or bending forces, external strain can be transmitted more directly and rapidly through the coating and cladding to the grating core, thereby improving the response speed and measurement sensitivity for high-frequency dynamic strain sensing.
- Excellent Mechanical Bending Flexibility: Thinner glass claddings can withstand smaller bending radii (low bending loss, better mechanical fatigue resistance), making them ideal for use in confined spaces, miniaturized packaging, or for 3D shape sensing.
Official Related Product References
To meet the demands for high-precision sensing and tolerance to extreme environments mentioned above, Dachen Yongsheng (OFSCN®) has developed and manufactured a series of single-mode fibers and grating products suitable for high-precision fiber sensing:
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OFSCN® G.652D Optical Fiber
Standard G.652D single-mode fiber (core 9\ \mu\text{m}, cladding 125\ \mu\text{m}), commonly used as the basic transmission medium for various standard fiber optic sensors.
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OFSCN® 300℃ Small diameter optical fiber
High-temperature resistant, small-diameter single-mode polyimide fiber (core 9\ \mu\text{m}, cladding 80\ \mu\text{m}, outer coating only 100\ \mu\text{m}), capable of operating in environments from -270\text{℃} to 350\text{℃}. It is an ideal choice for highly flexible bending and embedded intelligent material sensing.
