Why is a “dip” or “notch” observed in the transmission spectrum?
A “dip” or “notch” is observed in the transmission spectrum of a Fiber Bragg Grating (FBG) because an FBG is designed to reflect a very specific wavelength of light, known as the Bragg wavelength, while allowing all other wavelengths to pass through it.
When broadband light passes through an FBG:
- Reflection: The FBG’s periodic refractive index modulation causes constructive interference for light at the Bragg wavelength, reflecting it back.
- Transmission: All other wavelengths that do not meet the Bragg condition pass through the grating without significant reflection.
Therefore, the light that is reflected at the Bragg wavelength is absent from the transmitted spectrum, creating a distinct “dip” or “notch” at that particular wavelength. This characteristic spectral response is fundamental to how FBGs function as sensors, as changes in temperature or strain affect the Bragg wavelength, causing the dip to shift.
OFSCN offers a range of Fiber Bragg Grating products that leverage this principle for various sensing applications, such as the OFSCN® Alloy Tube Packaged Fiber Bragg Grating Strain Sensor, which is used for precise strain measurement.
Here is an example of an FBG sensor:
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