Principle of fiber Bragg grating (FBG)

一．Principleof Fiber Bragg Grating

Fiber Bragg grating (FBG) is a kind of special method to make therefractive index of fiber core change periodically in the axial direction, thusforming a diffraction grating in the core. Light travels forward along thefiber and reflects at every small change (where the refractive index changes),but other wavelengths of light will continue to travel along the fiber exceptfor the light that meets the phase matching conditions. The light satisfyingthe phase matching condition will return along the original fiber path.

The Bragg wavelength λ_Β is determined by the following formula:

λ_Β=2N Λ ...........(1)

Here, N is the effective refractive index of the laser propagatingin the fiber; Λ is the period of the Bragg grating.It can be seen from equation(1) that the reflection wavelength λ_Β is affected by the change of the physical or mechanicalproperties of the grating region. For example, due to the Elasto-opticaleffect, the strain on the fiber changes Λ and n. Similarly, due tothe thermo optic effect, the change of temperature will lead to the change ofN; for unconstrained fibers,λ_Β will be affected by thermal expansion and thermal contraction, asshown in equation (2). The first term on the right side of the equationdescribes the effect of strain on λβ, and the second term describes the effect of temperature on λ_Β.

Δλ_Β = λ_Β(1-ρα)Δε + λ_Β(α+ξ)ΔT .......(2)

Where, Δλ_Βis the change of Braggwavelength, ρα, α and ξ   are the elastic opticalcoefficient, thermal expansion coefficient and thermal optical coefficientrespectively, Δε is the change of strain, ΔT is the change oftemperature. For the typical grating with wavelength λ_Β≈ 1550 nm, the sensitivity of strain and temperature is about 1.2PM / με and 10 PM / º C, respectively.

It is particularly important that the two conditions of equation(2) are independent, which means that the FBG can measure the temperature byisolating the fiber from the strain, while the strain measurement withtemperature compensation can be carried out in the case of temperaturedetermination, which usually comes from another strain isolated FBG.

In addition to strain and temperature measurement, fiber Bragggrating (FBG) can also be used to measure pressure, acceleration anddisplacement by implanting transducers.

二．Multiplexingprinciple of FBG

1) WDM(wavelength division multiplexing)

The principle of wavelength division multiplexing (WDM) is verysimple. multiple gratings form a grating string and each grating has differentBragg wavelength. In the actual operation process, two methods are used toachieve this:（1）A broadband light source and a spectrometer are used fordetection.

（2）A sensitive adjustable wavelength light source and a simplephotodiode   detector are used.

The following figure(Figure 1) shows the schematic diagram of the principle of FBG multi-pointmultiplexing and demodulation composed of a sweep light source and a detector.The scanning generator is used to adjust the light source and to spread thelight source in any given wavelength range on the optical fiber. When thiswavelength is consistent with the Bragg wavelength of the fiber Bragg grating(FBG), light will be reflected to the photodetector along the fiber. At thesame time, the scanning generator provides the timing signal to the processor,which can convert the light intensity vs time information into spectralinformation. The processor will then process to identify the peaks of thisspectrum, find out their peak positions and convert them into target physicalquantities such as strain or temperature.

Figure 1   Schematic diagramof WDM equipment

a) Light source, b) scanning filter, c) scanning generator, d)coupling network of 1-4 channels, e) Bragg fiber grating array, f)photoelectric detector, g) processor, H) time change of detector on Channel 4,time ti converted to Bragg wavelength λi.

2) TDM(Time division multiplexing)

The time division multiplexing (TDM) system uses a wide-band pulselight source and distinguishes different gratings by the time it takes for thereturn signal from the grating to reach the detector. The pulse at the gratingwith a small distance from the modulation and demodulation unit is receivedfirst than that at the grating with a large distance from the modulation anddemodulation unit. The following figure shows the Bragg grating array withdifferent spacing L from the modulation and demodulation unit. The time tirequired for a pulse to return from a Bragg fiber grating with a distance of Liis determined by the following formula:

In the formula   ti = 2li c/n,C is the speed of light propagation in vacuum, and N is the refractive index ofoptical fiber.

After determining the position of the grating in the array, thelinear CCD can be used to collect spectral information to measure thewavelength drift of the FBG. Of course, a high-speed spectrometer can also beused.

Figure 2. Schematic diagram of TDM equipment

Top: the pulse from the light source (a) passes through thecoupler (b) (the coupler is also connected to the detector (c)), and istransmitted to the grating including the Bragg grating (E);

Bottom: the pulse from the light source at t0 is reflected by theBragg grating with the spacing of L1, L2 and L3 between the light source andthe modulation and demodulation unit, and returns at T1, T2 and T3,respectively.

一．Advantagesof FBG sensing technology

Compared with the traditional electronic sensor technology, thesensor based on FBG has many significant advantages:

For harsh environments

The FBG sensor is completely passive and does not use any electroniccomponents. Therefore, they can work in extreme temperature from lowtemperature to hundreds of degrees, and can work for a long time in placeswhere electronic sensors and instruments cannot work.

Anti electromagnetic interference

Another advantage of the passive characteristics of FBG sensors isthat they are not interfered by electrostatic, electromagnetic and radiosignals. So they can be installed in power stations and other places withserious electronic noise. In addition, due to their passivity, they are 100%explosion-proof and safe in nature. They can be used in almost all dangerousblasting environments.

Remote sensing

Optical fiber is a very efficient signal carrier. Therefore, theelectronic modulation and demodulation unit can be installed tens of kilometersaway from the sensor location. The traditional electronic strain measurementsystem needs to enlarge properly to prevent the noise from flooding the signal.For monitoring oil wells, lifting columns, pipelines or tunnels and other long-distance,remote building structures, this feature has unique and huge benefits. Theoptical sensor has no influence of the lead wire. Because the fiber Bragggrating sensor system is measured as the wavelength, it is not affected by thesignal attenuation, so it is impossible for the remote sensor signal to haveerrors in the process of transmission along a long fiber.

Long term stability

Another advantage of FBG sensor is its long-term stability forremote monitoring. As a passive sensor, fiber Bragg grating has thecharacteristic of zero drift, so it can be used for many years withoutrecalibration. The sensor is installed on the structure, then connected to themodulation and demodulation equipment, and data is collected every few years,then the real operation of the structure since the last reading can beobtained.   Because a modem unit can beused in many structures, the economic advantage of this technology is greatlyincreased.

Micro size

The fiber for Bragg grating is very small, and its diameter isonly about 0.15mm. Therefore, many sensors can be applied to very smalldisturbance structures. In particular, fiber optic sensor arrays can beembedded into composites to detect internal strain, temperature and damagewithout affecting the structural properties of the composites.

Multiplexing technology

A fiber can be engraved into many Bragg gratings, and amulti-channel demodulation device can demodulate hundreds of fibers at the sametime. Compared with the technology that each sensor needs a dedicated channel,using WDM technology can greatly reduce the price of intensively installedequipment. In addition, the fiber is smaller, lighter and reusable than thecable, so large-scale FBG sensors can be installed in specific applicationsthat cannot be installed due to the weight and volume limitations of the cable.

Fatigue durability

After testing the carbon fiber sample embedded in the opticalfiber sensor, it is found that after one million fatigue loads, the embeddedoptical fiber sensor will not have fatigue or bonding damage. In the future, wewill test the glass fiber material to prove that the life of the optical fibersensor embedded in the blade of the wind turbine can reach 25 years of its ownlife. For surface mounting applications, optical fiber is less likely to bedamaged by combination. Compared with most electronic sensor technologies, ithas stronger adaptability to humidity and chemicals.

Easy to install and low cost

Imagine the installation of many traditional electronic strainsensors: each sensor needs to be bonded to the structure to be tested, and thenthe bonding pad is associated with each sensor that needs to be bonded; theneach sensor needs to be welded to the pad connected with it in the field; thenthe cable needs to be welded to all pads in the field and the cable needs to besmoothed and fixed to the On a series of instruments; finally, before themeasurement begins, the bridge connected by all sensors needs to be leveled.

In contrast, using FBG strain sensors, only a few fibers need tobe bonded to the structure, and then they are connected to a FBG modem, and thereading of strain array can be obtained only by pressing a button, which can beused as a reference value for subsequent reading.