Sensörlerde neden genellikle kalın fiber optikler yerine ince tek modlu fiber optikler kullanılır?
Fiber optic sensing (especially wavelength-modulated sensing based on Fiber Bragg Gratings (FBG) and distributed fiber optic sensing) uses thin single-mode optical fibers (SMF) instead of thick multimode optical fibers (MMF) due to its optical physical characteristics, signal processing accuracy of the demodulation system, and practical engineering application environment.
The specific reasons can be analyzed from the following three academic and general engineering dimensions:
1. Core Physical Explanation: Polarization and Multi-peak Effects in Single-mode (SM) vs. Multi-mode (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): Single-mode fiber has a very thin core (typical core diameter is 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 drift of this single reflection peak with extremely high precision, often achieving a resolution of \pm 0.1\text{pm} (corresponding to tiny temperature or strain changes).
- Multi-mode Fiber (Superimposed Peaks, Undemodulable): Multi-mode fiber has a thicker core (typical core diameter is 50\ \mu\text{m} or 62.5\ \mu\text{m} ) and can support hundreds or even thousands of guided modes. Since the effective refractive index n_{eff} differs for each transmission mode, these modes produce different reflection wavelengths when passing through the same grating. Consequently, at the receiving end, the reflection spectrum degenerates into a series of overlapping, broadened multi-peaks or a messy envelope. When faced with such a chaotic multi-peak signal, the demodulator cannot lock onto and extract a stable and accurate center wavelength, rendering the sensor incapable of basic measurement.
2. Transmission Performance Differences: Mode Dispersion and Loss Control
- High Signal-to-Noise Ratio and Long-distance Transmission: Single-mode fiber exhibits extremely low optical attenuation in common sensing bands (such as 1550\text{nm} in the C-band), maintaining a high signal-to-noise ratio without modal dispersion. This allows for the serial connection (multiplexing) of dozens of FBG sensors on a single SMF, with transmission distances ranging from kilometers to tens of kilometers.
- Standardized Integration of High-precision Optoelectronic Devices: The high-precision sensing demodulation equipment currently available, including microwave photonic devices, tunable lasers, optical couplers, and circulators, has core optical components designed based on standard single-mode fibers (such as the standard OFSCN® G.652D Optical Fiber). Introducing thick multimode fibers would result in severe mode scattering and significant insertion loss at device coupling points.
3. Why is it Sometimes Necessary to Use Smaller Diameter Single-mode Fibers than Standard Ones?
In certain specific physical sensing scenarios, engineers not only require the use of single-mode fibers but may even opt for smaller diameter single-mode fibers (with cladding diameters smaller than the standard 125\ \mu\text{m} , such as OFSCN® 300℃ Small diameter optical fiber with a cladding diameter of 80\ \mu\text{m} and a coating diameter of 100\ \mu\text{m} ). This is primarily based on the following considerations:
- Minimal Intrusiveness (Small Impact on Base Mechanics): When fibers need to be embedded within carbon fiber composites, aerospace components, or precision structural parts for internal strain monitoring of smart materials, thinner fibers cause less weakening of the intrinsic structural strength of the measured substrate.
- High Strain Transfer Efficiency and Sensitivity: Smaller 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: A thinner glass cladding can withstand smaller bending radii (low bending loss, better mechanical fatigue resistance), making it highly suitable for applications in confined spaces, micro-encapsulation, or for 3D Shape Sensing.
Official Related Product References
To meet the diverse needs for high-precision sensing and resistance to extreme environments mentioned above, Dacheng Yongsheng (OFSCN®) has developed and manufactured a series of single-mode fibers and grating products suitable for high-precision fiber optic 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 the ideal choice for highly flexible bending and embedded smart material sensing.
