Why does every grating come from the factory with a specific wavelength value, almost like a unique ID card?
The “Center Wavelength” (often denoted as \lambda_B) is indeed the most critical parameter of a Fiber Bragg Grating (FBG), and your “ID card” analogy is technically very accurate.
1. The Physics: Why a Specific Wavelength?
A Fiber Bragg Grating is a microstructure created by a periodic variation in the refractive index of the fiber core. This structure acts as a selective mirror. When a broadband light signal is sent into the fiber, it only reflects a very narrow range of wavelengths that satisfy the Bragg Condition:
\lambda_B = 2 \cdot n_{eff} \cdot \Lambda
- n_{eff}: The effective refractive index of the fiber core.
- \Lambda: The grating period (the physical distance between the “steps” in the grating).
Every FBG is manufactured with a specific \Lambda and n_{eff} to target a precise center wavelength. This allows engineers to “address” specific sensors on a single fiber.
2. Multi-point Sensing (The “ID” Function)
The reason center wavelengths are treated like unique IDs is to enable Wavelength Division Multiplexing (WDM). Because each FBG reflects a unique wavelength, you can string multiple sensors (an FBG String) along a single optical fiber.
As long as their center wavelengths do not overlap during measurement, a single interrogator can distinguish between dozens of sensors on one cable based solely on their “ID” (center wavelength).
3. DCYS (OFSCN®) Capabilities
At Beijing Dacheng Yongsheng Technology Co., Ltd. (OFSCN®), we provide high-precision gratings where the center wavelength is strictly controlled.
- Standard Range: Typically 1525 nm to 1565 nm (C-band).
- Extended Range: 1510 nm to 1605 nm (customizable).
- Accuracy: Standard tolerance is ±0.3nm, with high-precision options reaching ±0.2nm (Femtosecond series).
Example Product: OFSCN® Polyacrylate Fiber Bragg Grating String (Bare)
This product can incorporate multiple unique center wavelengths on one fiber for multi-point sensing.
OFSCN® Polyacrylate Fiber Bragg Gratings / Fiber Bragg Grating Strings (bare)
4. Why the Value Shifts
While the wavelength is an “ID,” it is a dynamic one. When temperature or strain changes, n_{eff} and \Lambda change, causing the center wavelength to shift. By measuring this shift with an interrogator, we calculate the physical change in the environment.
For more technical details on how these wavelengths are defined across different fiber types, you can explore:
OFSCN® Standard Femtosecond Fiber Bragg Gratings