In quasi-distributed sensing, what limits the minimum distance between two adjacent gratings？
In quasi-distributed fiber Bragg grating (FBG) sensing systems, the minimum distance between two adjacent FBG sensors is primarily limited by spectral separation and the physical length of the grating.
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Spectral Separation: Each FBG reflects a specific Bragg wavelength. To ensure that an optical interrogator can uniquely identify and track each FBG, their Bragg wavelengths must be sufficiently separated in the optical spectrum. This separation must account for:
- FBG Bandwidth: The full width at half maximum (FWHM) of each FBG’s reflection spectrum.
- Expected Wavelength Shift: The maximum anticipated wavelength shift due to the measured parameters (e.g., strain or temperature). If these shifts cause the spectra of adjacent FBGs to overlap, it becomes impossible to accurately distinguish their individual responses.
- Interrogator Wavelength Range and Resolution: The total number of FBGs that can be multiplexed on a single fiber is limited by the interrogator’s available wavelength scanning range, and the resolution dictates how closely wavelengths can be packed.
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Physical Grating Length: While spectral separation is often the dominant factor for the sensing distance, the FBGs themselves have a physical length (typically a few millimeters to several centimeters). Physically, two FBGs cannot be placed closer than their individual lengths, plus any necessary lead-in/lead-out fiber and packaging.
For applications requiring compact, closely spaced strain measurements, OFSCN provides solutions like the OFSCN® Polymer-encapsulated Fiber Bragg Grating Strain Sensor (0.7mm/1.2mm diameter). These sensors are designed with small diameters to facilitate integration in tight spaces.
Here is an example of an FBG strain sensor:
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