Fiber Bragg Grating Sensing Technology and Its Advantages

One. Principle of Bragg Fiber Grating

The Bragg Fiber Bragg Grating (FBG) is an optical sensor that uses a strong ultraviolet laser to be recorded in the center of a standard, single-mode fiber in a spatially varying manner.

UV Beam-ultraviolet laser beam;

FBG Region-Prague fiber grating region;

Fibre Core-fiber optic center;

FBG period Λ-period of the Bragg fiber grating;

Fibre Cladding-fiber cladding;

Polymer fibre coating-polymer fibre coating

Short-wavelength ultraviolet photons have enough energy to break the high-stability silicon oxide binder, destroy the structure of the optical fiber, and slightly increase its refractive index. The interference between two continuous laser beams or the optical fiber and its shield will cause a strong spatial periodic change of the ultraviolet light, resulting in a corresponding periodic change in the refractive index of the optical fiber. The grating formed in the fiber region where this change occurs will become a wavelength-selective mirror image: light travels down the fiber and reflects at every small change, but these reflections will cause destructive interference at most wavelengths, And continuous propagation along the fiber. However, within a specific narrow-band wavelength range, there will be useful interference, which will return along the fiber.

The Bragg wavelength λΒ is determined by the following formula:

λΒ = 2neff Λ ........... (1)

Here, neff is the effective refractive index of the laser light propagating in the fiber; Λ is the period of the Bragg grating.

It can be seen from equation (1) that the reflected wavelength λB will be affected by changes in the physical or mechanical characteristics of the grating region. For example, due to the elastic-optic effect, the strain on the fiber will change Λ and neff. Similarly, due to the thermo-optic effect, the temperature change will cause the change in neff; for unconstrained fiber, Λ will be affected by thermal expansion and contraction, such as Equation (2) shows. The first term on the right side of the equation describes the effect of strain on λΒ, and the second term describes the effect of temperature on λΒ.

ΔλΒ = λΒ (1-ρα) Δε + λΒ (α + ξ) ΔT ....... (2)

In the formula, ΔλB is the change of Bragg wavelength, ρα, α and ξ respectively represent the elasto-optic coefficient, thermal expansion coefficient and thermo-optic coefficient, Δε represents the change in strain, and ΔT represents the change in temperature. For a typical grating recorded on silica with a wavelength of λB ≈ 1550 nm, the sensitivity of strain and temperature is approximately equal to 1.2 pm / με and 10 pm / ºC, respectively.

It is particularly important that the two conditions of equation (2) are independent, which means that the Bragg Fiber Grating (FBG) can isolate the fiber from the strain to perform temperature measurement; and the strain measurement with temperature compensation can be The temperature is determined, and this temperature is usually derived from another strain-isolated Bragg fiber grating (FBG).

In addition to strain and temperature measurement, Bragg fiber grating (FBG) can also be used for pressure, acceleration, displacement and other measurements by implanting transducers. Smart Fibres not only manufactures FBG sensors and transducers, but also develops and produces equipment for illuminating optical fibers and modulating and demodulating Bragg reflections.

Variety

· An oblique filter (which can be another Bragg grating) can be used to directly convert wavelength changes into light intensity changes. If the transmittance of this filter with wavelength changes is known, then the narrow-band wavelength reflected on the single-mode grating can be determined by measuring and comparing the intensity of light waves passing through and blocking. For a filter with a transmission spectrum as shown in the lower left figure below, when the Bragg wavelength increases from λ1 to λ2, the reduced transmission intensity and the reflected or blocked light wave intensity Ir will increase accordingly. This is the simplest and cheapest way to demodulate the Bragg fiber grating, but the biggest disadvantage is that it can only demodulate one grating at a time.

Using a passive filter to demodulate the Bragg fiber grating, the change in wavelength (left) is converted into a change in light intensity (right)

● A tilt filter (which can be another Bragg grating) can be used to directly convert the wavelength change into the light intensity change. If the transmittance of this filter as a function of wavelength is known, the narrow-band wavelength reflected on the single-mode grating can be determined by measuring and comparing the intensity of the light waves passing through and blocking. For a filter with a transmission spectrum as shown in the lower left figure, when the Bragg wavelength increases from λ1 to λ2, the reduced transmission intensity and the reflected or blocked light wave intensity Ir will increase accordingly. This is the simplest and cheapest way to demodulate the Bragg fiber grating, but the biggest disadvantage is that it can only demodulate one grating at a time.

● The widely used method is to illuminate the Bragg fiber grating through a narrow-band adjustable light source, which is the basis of Smart Fibres' current products. This method is further introduced in the "WDM" chapter.

☆ Wavelength Division Multiplexing (WDM)

The principle of wavelength division multiplexing (WDM) is very simple: multiple gratings form a single-mode fiber and each grating has a different Bragg wavelength. In actual operation, it is implemented by the following two methods:

â—‡ Use a broadband light source and a spectrometer for detection.

â—‡ Use a sensitive adjustable light source or a light source with a wavelength after scanning and a simple photodiode detector.

Smart Fibres's modulation and demodulation unit uses the latter method, and the schematic diagram on the right shows the working principle of this method.

Schematic diagram of the working principle of WDM equipment

Notes: a) light source, b) scan filter, c) scan generator, d) 1-4 channel coupling network, e) Bragg fiber grating array, f) photodetector, g) processor, h) channel 4 The time change of the detector on, the time ti is converted into the Bragg wavelength λλi

The scan generator is used to adjust the light source and sweep the light source in any given wavelength range propagating on the optical fiber. When this wavelength is consistent with the Bragg wavelength of the Bragg Fiber Grating (FBG), the light will be reflected along the fiber to the photodetector. At the same time, the scan generator provides the timing signal to the processor, allowing the processor to convert the light intensity vs. time information into spectral information. The processor will then process to identify the peaks of this spectrum, find their peak position and convert it to strain or temperature.

☆ Time Division Multiplexing (TDM)

A time-division multiplexing (TDM) system uses a broadband pulsed light source and distinguishes different gratings by the time it takes the light source's return signal from the grating to reach the detector. The pulses at the grating with a small distance from the modem unit are received first before the distance is larger. The figure below shows a Bragg grating array with a different distance l from the modem unit. The time ti required for a pulse returned from a Bragg fiber grating with distance li is determined by the following formula:

ti = 2li c / n where c is the speed of light propagating in a vacuum and n is the refractive index of the fiber.

After determining the position of the gratings in the array, the passive tilt filter as described above can be used to determine the wavelength of each pulse as it arrives. Of course, high-speed spectrometers can also be used.

Schematic diagram of the working principle of time division multiplexing (TDM) equipment. Top: the pulse from the light source (a) passes through the coupler (b) (this coupler is also connected to the detector (c)) and is transmitted to the grating containing the Bragg grating (e); bottom: the light source comes out at time t0 The pulse is reflected by the Bragg gratings with a spacing of l1, l2, and l3 from the modem unit, and returns at t1, t2, and t3, respectively.

Smart Fibres has been positioned since its inception to provide the most cost-effective tunable or scanning wavelength division multiplexing (WDM) technology.

three. Advantages of Bragg fiber grating sensing technology

Sensors based on Bragg Fiber Bragg Grating (FBG) have many significant advantages over traditional electronic sensor technology:

Suitable for harsh environments

The Bragg fiber Bragg grating sensor is completely passive and does not use any electronic components. Therefore, they can work at extreme temperatures ranging from low temperatures to high temperatures of several Baidus, and can work for a long time in places where electronic sensors and meters cannot work.

Anti-electromagnetic interference

Another benefit of the passive characteristics of Bragg fiber grating sensors is that they are not disturbed by static, electromagnetic and radio frequency sources. Therefore, they can be installed in places with severe electronic noise such as power stations. In addition, because they are passive, they are essentially 100% safe, and they can be used in most hazardous blasting environments.

The remote sensing fiber is a very efficient signal carrier. Therefore, the electronic modem unit can be installed tens of kilometers away from the sensor location. The traditional electronic strain measurement system needs to be properly amplified to prevent noise from overwhelming the signal. This feature has unique and huge benefits for monitoring long-distance and remote building structures such as oil wells, lifting columns, pipes or tunnels. The optical sensor has no influence of the lead. Because the Bragg fiber grating sensing system is measured as a wavelength, it is not affected by signal attenuation, so the sensor signal at the far end cannot be wrong during transmission along the longer fiber.

Long-term stability

Another advantage of the Bragg fiber Bragg grating sensor is its long-term stability for remote monitoring. As a passive sensor, the Bragg fiber grating has zero drift characteristics, so it can be used for many years without recalibration. Install the sensor on the structure, and then connect it to the modem equipment, collect data every few years, you can get the real action of the structure since the last reading. Since a modem unit can be used for many structures, this greatly increases the economic advantage of this technology.

Tiny size

The optical fiber for recording Bragg gratings is very small, only about 0.15mm in diameter. Therefore, many sensors can be applied to structures with very little disturbance. In particular, the fiber optic sensor array can be embedded in a composite material for detecting internal strain, temperature and damage without affecting the structural performance of the composite material.

Reuse technology

A fiber can be engraved with many Bragg gratings, and a multi-channel demodulation device can demodulate hundreds of fibers at the same time. Compared with the technology that each sensor requires a proprietary channel, the use of wavelength division multiplexing technology can greatly reduce the price of densely installed equipment. In addition, optical fibers are smaller, lighter than cables, and can be reused, so large-scale Bragg fiber grating sensors can be installed in specific applications that cannot be installed due to the weight and volume of the cable.

Fatigue durability

After testing the carbon fiber sample with embedded optical fiber sensor, it is found that after 1 million fatigue loadings, the embedded optical fiber sensor will not cause fatigue or joint damage. In the future, we will also conduct tests on glass fiber materials to prove that the life of the fiber optic sensor embedded in the blade of the wind turbine can reach the life of the blade of 25 years. For surface-mount applications, optical fibers are less prone to bond damage and have a stronger adaptability to humidity and chemicals than most electronic sensor technologies.

Easy to install and low cost

Imagine the situation where many traditional electronic strain sensors are installed: each sensor needs to be bonded to the structure to be tested, and then the bonding pad is associated with each sensor that needs to be bonded; then each sensor needs to be connected to the bonding pad on site Welding together; then the cable needs to be soldered to all the pads on the spot and the cable is smoothed and fixedly connected to a series of instruments; finally, before the measurement begins, the bridges connected to all sensors need to be leveled.

In comparison, using Bragg Fiber Bragg Grating strain sensors, only a few fibers need to be bonded to the structure, and they are connected to a Bragg Fiber Bragg Grating modem, and the reading of the strain array can be obtained with just one button. Reference value for subsequent reading.

Remember, the installation of the instrument requires technicians, and it is difficult and expensive to install the instrument on a specific structure. Obviously, the cost and time saved by installing optical fiber is very considerable.

Smart Fibres has been devoted to the research and development of Prague fiber gratings for many years, and has developed some solutions for the installation of various structures.