Quasi-distributed Sensing Technology

Since it is flexible to functionalize each fiber Bragg grating as a specific sensor with a package design, this enables a quasi-distributed sensing system for measuring temperature, strain, pressure, vibration, acceleration, and seismic signals. This quasi-distributed snesing system may be configured to measure several measurands, such as localized temperature and pressure values along a steam or gas pipeline. One of the advantages lies in the exact location(s) one can specifically monitor it, such as a potential leakage from a valve or pipe tube or hot spots from high-power transformer/generator end-windings. On the other hand, the small size and low-mass of fiber Bragg grating elements are more suitable for dynamic thermal or mechanical event monitoring, such as trasient structural instability, transient thermal ramping, and dynamic pressure and acosutic signal from a combustion process etc.

Fiber Bragg Grating

A fiber Bragg grating (FBG) consists of micro Bragg reflectors in a short segment of optical fiber core that reflects particular wavelength of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength specific dielectric mirror.  The light source could be a tunable laser or a broadband light source, and the reflected or transmitted spectra are normally analyzed with an optical sensing interrogator. Each FBG has a unique Bragg resonant wavelength that is determined by 2×n×L, where n is effective refractive index in the fiber core, and L is grating periodicity or pitch size.


Fiber Bragg grating is created by "inscribing" periodic or aperiodic variation of refractive index into the fiber core using high-power ultraviolet (UV) laser or femtosecond IR laser. Two main processes are used: interference and masking. The method that is preferable depends on the type of grating to be manufactured. UV laser phase mask method is used to inscribe FBGs from photosensitive specialty fibers, while femtosecond laser could manufacture a FBG from pure silica fiber core. It is understood that a grating is a telecom filter device, which can normally be used as an inline optical filter, or as a wavelength-specific reflector for wavelength multiplexing or de-multiplexing telecom applications. It is not a sensor if without a further engineering in thermal post-treatment, calibration, package, and hardware integration.


Fiber Bragg Grating Sensor

It is clear that a FBG can be used either as a temperature sensor if the strain effect is negligible or as a strain sensor if the thermal effect is negligible. Owing to the thermal effect can be removed from strain response that the thermal compensated strain sensor has been functionalized for measuring strain, flow, vibration, displacement, load, and seismic/acoustic etc. signals.  On the other hand, a fiber Bragg grating is not a sensor in the point view of engineering. As a sensor it should have reliable performance and accuracy that can be comparable to conventional electric sensors, such as thermocouple or pressure gage. In reality, a FBG has to experience an engineering process to satisfy the requirements as a sensor. This process includes, but not limited to, residual strain and hydrogen release, calibration, package, drift, accuracy, and reliability tests. In addition, long-term sensing performance evaluation and qualification are also required to ensure its reliability for 10-20 years' operation.