What is the long-term wavelength drift stability of the sensors during static monitoring? How do you distinguish real physical changes from pseudo-signals caused by light source aging?
The long-term wavelength drift stability of Fiber Bragg Grating (FBG) sensors during static monitoring is primarily influenced by the stability of the FBG itself and the performance of the demodulation interrogator.
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FBG Sensor Stability: FBG sensors are passive, optical elements inscribed in the core of an optical fiber. Once inscribed and properly annealed (if high-temperature applications are involved), the FBG’s physical grating period is highly stable. Any wavelength shift in the FBG’s reflected peak is directly proportional to changes in its physical strain and temperature. Therefore, the FBG itself exhibits excellent inherent long-term stability, provided it is not subjected to physical degradation or extreme environments beyond its specified operating limits. For robust long-term applications, encapsulated sensors like the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor are designed to protect the grating and fiber, enhancing their long-term performance and reliability in harsh conditions.
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Distinguishing Real Physical Changes from Pseudo-Signals caused by Light Source Aging: Pseudo-signals caused by light source aging primarily originate from the FBG interrogator, not the FBG sensor itself. The FBG acts as a precise optical filter; it reflects a specific wavelength determined by its grating period and refractive index. The interrogator’s role is to accurately measure this reflected wavelength.
If the interrogator’s internal broadband light source (or components within its optical path) ages and its spectral characteristics drift, it can lead to inaccuracies in the measured FBG wavelength, creating a pseudo-signal that mimics a physical change. To distinguish between real physical changes and these pseudo-signals, high-quality FBG interrogators employ several sophisticated techniques:
- Internal Wavelength Reference: Most advanced OFSCN® Fiber Bragg Grating Interrogators incorporate an internal, highly stable wavelength reference (e.g., a gas absorption cell or a reference FBG maintained at a constant temperature) to continuously calibrate and compensate for any drift in the light source or detector array. This ensures that the measured wavelength shift is always relative to a precise, stable reference.
- Real-time Compensation Algorithms: Interrogators utilize advanced signal processing algorithms that can differentiate between broad spectral shifts (indicating light source drift) and sharp, distinct FBG wavelength shifts (indicating real physical changes).
- Regular Calibration: While interrogators are designed for long-term stability, periodic calibration against certified wavelength standards can further confirm and maintain their measurement accuracy.
- Temperature Compensation for Strain Measurements: For strain sensing, temperature is a significant cross-sensitivity that can produce pseudo-strain signals. To isolate true mechanical strain, it is common practice to use dedicated FBG temperature sensors (such as the OFSCN 800C Fiber Bragg Grating Temperature Sensor) or temperature-compensation gratings placed in an unstrained condition near the strain sensor. This allows for the mathematical removal of temperature-induced wavelength shifts from the total measured shift.
By combining intrinsically stable FBG sensors with high-precision interrogators featuring robust internal referencing and proper compensation strategies, it is possible to achieve highly accurate and reliable long-term static monitoring, effectively distinguishing real physical changes from system-level pseudo-signals.