FBG structural health monitoring

FBG has been considered as an excellent sensor element, which is currently receiving more and more research interest. In order to measure strain/temperature variations with high accuracy, the ability to detect small shifts in Bragg wavelength becomes an essential requirement for an FBG sensing system.

With the construction of high buildings and bridges, etc., in recent years, the importance of structural health monitoring technologies to assess building safety is being re-examined. In previous systems using electrical strain sensors, every sensor required power supply, and the installed sensors were easily affected by electromagnetic noise, thunderstorms, etc., causing noise components in the electrical signal measured by remote sensing and presenting a risk of degraded accuracy. Optical fibers have been used as sensors to solve these problems with a focus on optical sensing technology. Since optical sensing technology does not require supplying power to the sensor itself, it offers many advantages including long life spans with excellent corrosion resistance, excellent explosion-proofs, easy remote measurement at distances of more than 10 km with no concerns about electromagnetic noise effects, etc. In addition, the characteristics of optical fibers lend them to linear and sheet designs for extreme environments, making them ideal for disaster monitoring and structural health monitoring systems.

Some of the well-known technologies in optical fiber sensing rely on measuring changes in the frequency of Brillouin backscatter occurring in optical fibers to determine structural deformations and temperature changes. Another method uses a Fiber Bragg Grating (FBG) forming a diffraction grating at the optical fiber core as a sensor to measure changes in the center wavelength of the optical spectrum reflected from the FBG sensor as an index of the amount of strain impressed on the fiber and temperature changes.  Since FBG sensor monitors are used mainly for natural disaster and structural health monitoring they are designed to be convenient for installation, small, and lightweight. Additionally, to be able to measure small strain and temperature changes quickly, they require a high responsivity of better than 1 kHz as well as better measurement accuracy than commercially available FBG sensor monitors and our previously developed model.

Distributed Temperature Sensing (DTS) system resolution

DTS system resolutionThe main idea of the Distributed Temperature Sensing is Raman-based temperature measurement joined with Optical Time-domain Reflectometry (OTDR). A short pulse of light is enabled into the fiber. The propagating light creates Raman backscattered light at two wavelengths, from all points along with the fiber. The wavelengths of the Raman backscattered light are different from that of the forward propagating light and are named “Stokes” or “anti-Stokes”.The amplitude of the Stokes light is not very dependent on temperature, while the amplitude of the anti-Stokes light is strongly dependent on temperature.

A typically distributed sensing system is described by the spatial and temperature resolution. The spatial resolution is the minimum distance of the sensor to measure a step change in temperature along with the optical fiber. The temperature resolution is a measure of the precision to distinguish the absolute temperature. The temperature resolution depends on the measurement time and the launch pulse-recurrence rate. The laser pulse energy and duration are accurately controlled and optimized at the measurement maximum length to provide the best available temperature resolution within acceptable accuracy limits. As the sampling time is increased, the temperature resolution is improved and resolved temperatures become more accurate.

The report “Distributed Temperature Sensing (DTS) Market 2016 – 2023,” points out the key factors impacting the growth of this market and assesses its growth during the period between 2016 and 2023. Fiber optic sensing is challenging because the physical properties of light into the fiber are affected by strain, temperature, or sound. Several technologies enable local measurement – using sensors at chosen points along with the fiber (eg, fiber Bragg grating (FBG) technology for measuring localized pressure, strain, temperature, and flow) – or distributed sensing – with sensing occurring all along with the fiber.

Distributed sensing systems have been developed for the oil and gas industry to assist reservoir engineers in optimizing the well lifetime. Nowadays they find a wide variety of applications as integrity monitoring tools in process vessels, storage tanks, and piping systems offering the operator tools to schedule maintenance programs and maximize service life.

Optromix Company offers a Distributed fiber optic sensing system with high spatial and temporal profiling over large surfaces, long lengths, and at locations where conventional point sensing is not applicable or costs effective.

Superstructure fiber Bragg gratings (sampled SFBGs)

superstructure FBGsThe FBG structure can change with the use of the refractive index, or the grating period. The grating period can be uniform or graded, and either localized or distributed in a superstructure. The refractive index has two main features, the refractive index profile, and the decline. The refractive index can be uniform or apodized, and the refractive index decline is positive or zero.

A superstructure fiber Bragg grating (SFBG), also called a sampled fiber Bragg grating, is a special FBG that consists of several small FBGs placed in close proximity to one another. SFBGs have attracted attention in recent years with the discovery of techniques allowing the creation of equivalent chirp or equivalent phase shifts. The biggest advantage of an SFBG with the equivalent chirp or equivalent phase shifts is the possibility to generate gratings with a greatly fluctuating phase and amplitude by adjusting the spatial profile of the superstructure. The realization of SFBGs with the equivalent chirp or equivalent phase shifts requires only sub-millimeter precision.

 

Superstructure (sampled) fiber Bragg gratings (SFBGs) are good optical filters for optical communication and optical sensor systems, because of their comb-like filter response. The length of SFBG is conventionally limited by that of the phase mask. However, the length of the high-quality phase mask fabricated by an interferometric technique is limited to ~ 5 cm.  The main fabrication technique for long SFBGs based on scanning phase masks and trimming relative phases between FBGs. This technique permits to fabricate of long SFBGs with short phase masks. Superstructure fiber Bragg grating is a novel fiber Bragg grating. Because it is flexible, passive, low insertion loss, and small polarization dependent. It causes great interest and enthusiasm for the people in many areas. Ambient temperature and strain can both affect the sampled grating reflection spectrum.