Fiber Bragg Gratings
|Central Wavelength (CW), nm||530 - 2300|
|Wavelength Tolerance, nm||< 0.1|
|Reflectivity, %||0.1 … 99.9|
|Bandwidth (FWHM), nm||0.03 - 3|
|Sidelobe Suppression Ratio (SLSR), dB||> 20|
|FBG Length, mm||0.1 - 50|
|Coating||Acrylate; Polyimide; No coating|
|Temperature range (acrylate), °C||-20 … +85|
|Temperature range (polyimide), °C||-200 … +350|
|Connectors||FC/PC; FC/APC; any other|
Types of FBGs
Three Methods of FBGs Inscription
Currently, there are three known methods of FBGs inscription. Optromix is able to write fiber Bragg gratings by all 3 techniques, using 4 available laser stands.
Interferometric method. When fiber core is illuminated with a fringe pattern created by a phase mask (in this case FBG period is defined by a phase mask period)
Direct point-by-point method. When each FBG “pitch” is formed as a result of the nonlinear fs laser pulse absorption (in this case FBG period is defined by laser pulse repetition rate and by velocity of a fiber translation along its axis)
Continuous core-scanning method. When fiber position is modulated in transverse plane (in this case FBG period is defined by modulation frequency and by velocity of a fiber translation along its axis)
Fiber Bragg Gratings
Meaning and Principle
FBG (fiber Bragg grating) is a periodic circuit created in a fiber core. It has a capability to reflect certain emission wavelengths and transmit the others. The refraction index periodic variation in the fiber core causes the reflection generating a wavelength-specific dielectric mirror. Hence, fibre Bragg grating performs as an inline fiber optic filter blocking or reflecting specific wavelengths.
Fiber Bragg gratings may serve as:
- Sensing elements in FBG sensors
- High Reflector (HR) and Output Coupler (OC) mirrors in laser cavities
- Spectral filters (optical fiber filters)
- Dispersion compensators in telecommunications.
FBG spectral specifications are based on the periodic change of fiber core refractive index at different lengths and depend on the optical heterogeneity diffraction.
A fiber Bragg grating can be of different lengths from 0.1 to 50 mm.
The length of periodic fiber optic grating changes when mechanical compression and tension are applied; hence, the reflected wavelengths are adjusted as well.
FBG fabrication is a complex process. Bragg grating is usually written into a single mode fiber with an acrylate or polyimide coating. Unlike a multimode fiber, a single mode fiber has a much smaller core diameter.
Acrylate fiber is inexpensive, easy to clean up (with a mechanical or thermal stripper), but it is suitable only for regular operating conditions (it can withstand constant exposure to -20 … +85°C temperature range, and for a short period of time up to + 100°C).
Polyimide fiber is significantly more expensive, it is more difficult to clean up (with an electric arc or concentrated sulfuric acid), it is suitable for more aggressive environments with operating temperature -200 … +350°C. Some polyimide high temperature optical fibers can withstand temperatures up to 800°C.
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