The name sounds quite technical; what specific physical discovery does it represent? How does it differ from a conventional grating?
The term “Bragg” in Fiber Bragg Grating (FBG) refers to the Bragg Law (or Bragg’s Law of Diffraction), formulated by Sir William Lawrence Bragg and his father Sir William Henry Bragg, who were awarded the Nobel Prize in Physics in 1915.
1. The Physical Discovery
The name represents the physical phenomenon where light (or X-rays) reflects off a periodic structure at specific angles and wavelengths. In the context of an FBG, the “grating” is a periodic variation in the refractive index of the optical fiber’s core.
When light travels through the fiber, it encounters this periodic structure. According to the Bragg condition, only a specific wavelength—known as the Bragg wavelength (\lambda_B)—is reflected back, while all other wavelengths are transmitted. The fundamental formula is:
\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 index variations).
2. Difference from a Conventional Grating
While a conventional diffraction grating (like those found in spectrometers) usually works by reflecting or transmitting light into multiple orders across a surface, a Fiber Bragg Grating is an “in-fiber” component.
The key differences are:
- Geometry: Conventional gratings are typically bulk optical components (flat or curved surfaces). An FBG is integrated directly into the core of an optical fiber (often as thin as 125μm or 255μm).
- Wavelength Selectivity: FBGs are extremely narrow-band filters. Because the “grating” is distributed over a long distance (typically 5mm to 10mm) inside the fiber, the wavelength selection is incredibly precise.
- Sensing Capability: Unlike static conventional gratings, an FBG is sensitive to its environment. If the fiber is stretched (strain) or heated (temperature), the grating period (\Lambda) or the refractive index (n_{eff}) changes, causing the reflected Bragg wavelength to shift. This is the foundation of OFSCN® sensing technology.
3. OFSCN® Technical Implementation
At Beijing Dacheng Yongsheng Technology (OFSCN), we utilize different manufacturing techniques to create these gratings depending on the application:
- UV Writing (Mask Method): Used for products like OFSCN® Polyacrylate Fiber Bragg Gratings, suitable for standard temperatures (-40℃ to 100℃).
- Femtosecond Laser Writing: Used for OFSCN® Standard Femtosecond Fiber Bragg Gratings, which can operate in extreme conditions (up to 300℃) and offer high mechanical strength.
Below is a representation of a standard FBG string:
For more information on how these gratings are integrated into industrial sensors, you can explore our technical resources at ofscn.net.