Why are fiber Bragg gratings not afraid of light weakening, but only of color changes?
This sentence vividly and figuratively summarizes the core physical mechanism of Fiber Bragg Gratings (FBG) — Wavelength Encoding technology.
In optical engineering and sensing technology, “not afraid of the light weakening, only afraid of the color changing” explains why FBG sensors have superior stability and anti-interference capabilities compared to traditional intensity-modulated (light intensity modulation) optical fiber sensors. We can deeply analyze this principle from the following academic and physical dimensions:
I. Core Physical Principle: Bragg Equation and Wavelength Encoding
The working basis of fiber grating sensors is the Bragg reflection principle. When broadband light enters the optical fiber and transmits to the grating region, only light that satisfies the specific Bragg condition is reflected back. The core equation is:
\lambda_B = 2 n_{\text{eff}} \Lambda
In the text, its physical variables are represented as follows:
- \lambda_B is the center wavelength of the reflected light (visually or spectrally appearing as a specific “color”).
- n_{\text{eff}} is the effective refractive index of the fiber core.
- \Lambda is the physical period of the grating.
Changes in external measurable quantities (such as temperature T or strain \varepsilon ) directly alter the fiber’s refractive index n_{\text{eff}} (through the thermo-optic effect or photoelastic effect) and the grating period \Lambda (through thermal expansion or mechanical stretching), thereby causing the center wavelength \lambda_B of the reflected light to drift. This modulation method, using wavelength (color) as the sole carrier of the physical quantity being measured, is “wavelength encoding”.
II. Why “Not Afraid of the Light Weakening”?
In practical optical fiber transmission and engineering applications, optical signals encounter various attenuations during transmission in the fiber channel:
- Transmission Loss: Exponential decay of light intensity due to long-distance transmission.
- Connection Loss: Disconnection, poor alignment, or contamination of fiber optic connector ends (e.g., FC/APC).
- Bending Loss: Radiation leakage occurring when the fiber is physically bent.
- Light Source Aging: Decline in broadband light source output power over time.
If the sensor uses “intensity encoding” (i.e., relying on reduced light intensity to determine changes in physical quantities), any light attenuation (