Why do certain “colors” (wavelengths) disappear after the light passes through the grating? What does that “dip” in the spectrum represent?
In the context of Fiber Bragg Gratings (FBG), the Transmission Spectrum is a functional map of the light that “survives” the journey through the grating.
1. Why do certain “colors” disappear?
The “disappearance” of specific wavelengths is caused by the physical principle of Bragg Reflection. An FBG is essentially a periodic microstructure (a series of refractive index changes) inscribed into the fiber core.
When a broad-spectrum light source is launched into the fiber, most wavelengths pass through undisturbed. However, when a wavelength matches the Bragg condition (\lambda_B = 2 \cdot n_{eff} \cdot \Lambda), the microscopic reflections from each periodic change in the grating interfere constructively. This causes that specific wavelength to be reflected back toward the source rather than continuing forward. Therefore, from the perspective of an observer at the far end of the fiber, that specific “color” has been removed or filtered out.
2. What does the “dip” represent?
The “dip” in the transmission spectrum (also called a “notch”) represents the Optical Power Loss at the Bragg wavelength.
- Center of the Dip: This corresponds to the Bragg Wavelength. Its position shifts if the fiber is subjected to temperature changes or mechanical strain, which is the fundamental basis for FBG sensing.
- Depth of the Dip: This indicates the Reflectivity of the grating. A deeper dip means a higher percentage of light at that wavelength was reflected back. For example, in an OFSCN® Polyacrylate Fiber Bragg Grating (bare), the typical reflectivity is around 70%, but it can be customized.
- Width of the Dip: This is the 3dB Bandwidth, which typically ranges from 0.1nm to 0.3nm for standard sensors, depending on the grating length and the strength of the index modulation.
Related Technology
This characteristic is utilized in various OFSCN® products to ensure high-precision measurements. For instance, the spectral characteristics are strictly controlled in products like the OFSCN® Standard Femtosecond Fiber Bragg Gratings, which can operate in extreme environments from -270°C to 800°C.
If you are looking at a spectrum on an interrogator, you are likely seeing the Reflection Spectrum (the peaks), which is the mirror image of the “dips” seen in a Transmission Spectrum.