When I stretch the fiber, how does the spacing of that internal “mirror” change?
When you stretch an optical fiber containing a Fiber Bragg Grating (FBG), the internal “mirror” structure—which consists of a periodic modulation of the refractive index—changes in two primary physical ways:
1. Physical Grid Expansion (The Geometric Effect)
When mechanical longitudinal strain is applied to the fiber, the physical distance between the grating planes (the grating period, denoted as \Lambda) increases. Imagine a spring being pulled; the spacing between the coils widens. Since the reflected wavelength (\lambda_B) is directly proportional to this period (\lambda_B = 2n_{eff}\Lambda), any physical stretching causes a “Red Shift” (the reflected light moves toward a longer wavelength).
2. The Photo-elastic Effect (The Refractive Index Effect)
As the fiber is stretched, its diameter slightly decreases (Poisson’s effect), and the density of the glass changes. This alters the effective refractive index (n_{eff}) of the fiber core. In standard silica fibers, the photo-elastic effect actually works against the geometric expansion slightly, but the net result of stretching is always a positive shift in the Bragg wavelength.
Technical Performance of OFSCN® Sensors
In industrial applications, simply stretching a bare fiber is often impractical. DCYS (OFSCN) utilizes these principles to create high-performance sensors that translate structural movement into precise wavelength shifts.
For example, the OFSCN® Alloy Tube Packaged Fiber Bragg Grating strain sensor uses a seamless metal tube to protect the grating while ensuring that 100% of the external strain is transferred to the internal “mirror” structure without the creep issues found in traditional glue-based sensors.
Standard Product Images:
Key Capabilities:
- Sensitivity: Typically around 1.2 pm per micro-strain (\mu\varepsilon).
- Range: Standard models support up to 6,000 \mu\varepsilon, with specialized versions like the OFSCN® Ultra-Large Range Fiber Bragg Grating Strain Sensor capable of measuring even larger displacements.
- EMI Immunity: Since the measurement is based on the spacing of light reflection rather than electrical resistance, it is completely immune to electromagnetic interference.
If you are looking into how this applies to a specific engineering project, feel free to share your requirements.