What is the difference in the reflected signal between a 5 mm grating and a 10 mm grating?
The grating length of a Fiber Bragg Grating (FBG) is a fundamental parameter that significantly influences the characteristics of the reflected optical signal. In our OFSCN® product line, we typically offer lengths of 3 mm, 5 mm, 8 mm, and 10 mm (with 10 mm being the default for many standard sensors).
The primary differences between a 5 mm grating and a 10 mm grating in terms of the reflected signal are as follows:
1. Reflectivity
Reflectivity is directly related to the grating length. Generally, for the same refractive index modulation depth, a 10 mm grating will have higher reflectivity than a 5 mm grating. A longer grating provides more “periods” for the light to interact with, strengthening the constructive interference at the Bragg wavelength.
2. Spectral Bandwidth (3dB Bandwidth)
The grating length is inversely proportional to the spectral width of the reflection peak.
- 5 mm Grating: Will result in a wider spectral peak (broader bandwidth).
- 10 mm Grating: Will result in a narrower and sharper spectral peak.
A narrower bandwidth (from a longer grating) often allows for higher precision in wavelength peak detection by the interrogator, but it requires a more stable environment to avoid signal distortion.
3. Side Mode Suppression Ratio (SMSR)
The SMSR (the ratio of the main peak power to the power of the largest side lobe) is also affected by the grating length and the apodization profile. Generally, longer gratings allow for more effective apodization, which can lead to a better (higher) Side Mode Suppression Ratio, resulting in a “cleaner” signal.
Summary of OFSCN® Standards
In our OFSCN® Polyimide Fiber Bragg Gratings and other bare FBG products, these parameters are customizable:
- Grating Length: 3 mm, 5 mm, 10 mm (Custom lengths available).
- Reflectivity: Typically >70% (Customizable based on length).
- SMSR: Default ≥15 dB.
Product Visualization:
Below are standard images of our FBG sensors and strings showing the physical scale of these components:
For detailed technical specifications, you can refer to:
OFSCN® Polyimide Fiber Bragg Gratings / Fiber Bragg Grating Strings (bare)