Structure of a Fiber Bragg Grating Demodulator

A Fiber Bragg Grating (FBG) demodulator consists of an FBG sensing array, an optical interrogation system, and signal processing modules to convert wavelength shifts into measurable physical quantities.

Core Components

1. FBG Sensing Array: The FBG sensors are inscribed in the fiber core and reflect light at specific Bragg wavelengths. Changes in temperature, strain, or pressure cause shifts in the reflected wavelength, which the demodulator detects . 2. Optical Interrogation System: The demodulator typically includes one of the following:

• Tunable Laser Source: Scans across the FBG wavelength range to detect the peak reflection.
• Broadband Light Source with Tunable Filter: Measures the transmitted or reflected spectrum.
• Spectral Imaging System: Uses miniaturized fiber optic spectrometers to capture the reflected spectrum for static and dynamic strain measurements .
• Filterless Systems with PWM: Detect wavelength shifts without optical filters, suitable for harsh environments . 3. Signal Processing and Demodulation Module: The demodulator converts the optical signal into a wavelength measurement using algorithms such as:
• Cross-Correlation and Variable-Step-Size Methods: Achieve high-resolution wavelength demodulation by interpolating spectra and refining the search for the peak wavelength .
• Hilbert Transform, Fast Phase Correlation, and Karhunen–Loève Transform: Enhance computational efficiency and accuracy .
• Gaussian Curve Fitting and Wavelet Filtering: Handle distorted spectra and improve signal-to-noise ratio .

Working Principle

1. The FBG reflects light at a wavelength determined by the grating period and the refractive index of the fiber.
2. External physical changes (strain, temperature) shift the Bragg wavelength.
3. The optical interrogation system captures the reflected spectrum.
4. Signal processing algorithms identify the peak wavelength shift, which is then converted into the corresponding physical parameter (e.g., strain in microstrain or temperature in °C), .

Advantages of Modern FBG Demodulators

• High sensitivity and resolution (pm-level wavelength detection).
• Multiplexing capability for multiple FBG sensors along a single fiber.
• Resistance to electromagnetic interference and harsh environmental conditions.
• Compact and integrated designs using spectral imaging or miniaturized spectrometers . In summary, an FBG demodulator integrates optical sensing, precise wavelength interrogation, and advanced signal processing to accurately measure physical parameters from wavelength shifts, with various architectures optimized for static, dynamic, or harsh-environment applications.