What are those small ripples next to the main signal peak? Will they interfere with my measurement results?
Those “small ripples” you see next to the main signal peak are called Sidelobes, and their intensity relative to the main peak is quantified by the Sidelobe Suppression Ratio (SLSR).
1. What are Sidelobes and SLSR?
When a Fiber Bragg Grating (FBG) is manufactured, the periodic variation of the refractive index in the fiber core has a finite length. This abrupt start and end of the grating structure cause interference patterns that manifest as smaller secondary peaks (ripples) on either side of the primary reflection peak.
The Sidelobe Suppression Ratio (SLSR) is the power difference (usually measured in dB) between the peak of the main reflection signal and the peak of the highest sidelobe.
2. Will they interfere with your measurement results?
In most standard sensing applications, minor sidelobes do not cause issues. However, they can interfere with your results in two specific scenarios:
- Multi-sensor Multiplexing: If you have multiple FBG sensors on a single fiber string with narrow wavelength spacing, the sidelobes of one sensor can overlap with the main peak of an adjacent sensor, leading to measurement noise or “crosstalk.”
- Peak Tracking Accuracy: If the SLSR is too low (meaning the ripples are too high), some FBG interrogators may struggle to lock onto the true center of the main peak, especially if the signal is weakened or distorted by physical strain.
3. How to Minimize Sidelobes
To achieve high-precision measurements, it is common to use Apodized FBGs. Apodization is a technique where the intensity of the refractive index modulation is gradually increased and then decreased along the length of the grating, rather than starting and stopping abruptly. This “smooths out” the interference and significantly suppresses the sidelobes.
At OFSCN (Beijing Dacheng Yongsheng Technology Co., Ltd.), we specialize in high-quality FBG components. For example, our FBG products are designed with high SLSR to ensure clear signal identification even in complex multiplexing environments.
Product Visualization:
Below is a typical representation of a high-quality FBG spectrum where sidelobes are effectively suppressed:
Related Products:
For high-performance sensing, you can explore our standard FBG products:
OFSCN® Fiber Bragg Grating (FBG)
If you are seeing exceptionally high ripples that are fluctuating, it might not be a characteristic of the FBG itself but could be caused by non-uniform strain or clamping issues on the sensor. Feel free to share your specific installation environment if you’d like to troubleshoot further.