Fiber Bragg grating Sensor (FBGS) is characterized by the periodic modulation of the refractive index in the core of an optical fiber. This modulation causes the FBG to reflect a range of wavelengths of the incident light and transmit the remaining wavelength band. Such gratings are intrinsic sensing elements that can be photo-inscribed into the optical fiber usually by exposing its core to an interference pattern from an ultraviolet laser. However, there is a set of other methods for this purpose.
Interrogation system for such sensors can operate based on the multiplexing of several FBGs in a single fiber, when these sensors operate as a quasi-distributed sensing network. Following this idea, grating-based sensors have been employed in a wide variety of applications in sensing and communications, including the sensing of temperature, from the date of its discovery and also to measure strain or refractive index, to name a few.
Fiber Bragg Grating Sensors Advantages
- FBG sensors are passive.
- Optical fibers are non conductive, so lightning will not destroy FBG sensors with an electrical surge.
- FBG sensors are immune to EMI.
- Fiber sensor instruments (aka interrogators) have a range of well over 30 km and a capacity for more than 80 sensors per fiber and 16 fibers. That’s a total of >1280 sensors per demodulation instrument.
- FBGs respond quickly to even slight temperature variations.
- FBGs can be spaced at 1 cm intervals along a fiber that is only 155 microns in diameter.
- FBG sensors are made of silica (i.e., glass). They do not corrode.
- Multiplexing dozens of FBGs in series in one fiber saves the cost of a home run lead to each sensor. Also, varying FBG sensor lead lengths does not impact sensor calibration.
- Micron Optics sensor interrogation instruments have built-in calibration artifacts that last for the life of the instrument. The FBG sensors each have a digitally encoded identity that does not change. So once a system is installed and sensor zero points are recorded, no further calibration is required. Ever
- Optical fiber is amazingly robust. Our FBG gages have been tested to >100 million cycles of +/-3,000 microstrain with no degradation of the measurement.
- Some FBG strain gages can measure up to ~ 30,000 microstrains (i.e., 3% elongation).
- Again, multiplexing is the key. A single, small fiber can connect 10s of gages to the interrogator.
- FBGs measure directly strain and temperature. Tranducer packaging around FBGs makes measurement of other properties possible like pressure, acceleration, displacement, chemical presence, etc. All of these sensors, no matter what they measure, are measured by the same interrogator.
- Because fibers are so small, they can be embedded in structures built with carbon fibers, glass fibers, concrete and steel, etc.
- Optical components like the FBGs themselves and those used to build the interrogators, are Telcordia qualified for a >25 year lifetime. Telcordia is a set of standards established by the telecom industry for critical equipment deployed in harsh field applications.
- Commercial quality FBG-based temperature sensors are available now for the -200°C to 300°C range, and promising prototypes have been shown to operate in 1,000 hour tests at 750°C. Materials like sapphire FBGs are under development for even higher temperatures.
Fiber Bragg Grating Sensors Application
Fiber Bragg granting sensors are used for a number of applications across many industries. some of the applications are listed below
- Optical telecommunications and optical sensors
- Fiber lasers
- Fiber amplifiers
- Fiber bragg filters
- Wavelength Division multiplexer/de-multiplexer
- Dispersion compensation monitoring
- Optical layer monitoring
- Humidity sensors
- Static and dynamic strain monitoring
- Length measurements in kilometer range
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