Analysis of the working principle of fiber Bragg grating sensor

For decades, electrical sensors have served as standard equipment for measuring physical and mechanical phenomena. Although they are ubiquitous in test and measurement, as electrified devices, they have inherent drawbacks, such as loss during signal transmission, susceptibility to electromagnetic noise, and more. These defects can make the use of electrical sensors quite challenging or even completely unsuitable in some special applications. Fiber optic sensors are an excellent solution to these application challenges, using light beams instead of electrical currents and standard optical fibers instead of copper wires as the transmission medium.

For decades, electrical sensors have served as standard equipment for measuring physical and mechanical phenomena. Although they are ubiquitous in test and measurement, as electrified devices, they have inherent drawbacks, such as loss during signal transmission, susceptibility to electromagnetic noise, and more. These defects can make the use of electrical sensors quite challenging or even completely unsuitable in some special applications. Fiber optic sensors are an excellent solution to these application challenges, using light beams instead of electrical currents and standard optical fibers instead of copper wires as the transmission medium.

In the past two decades, the development of optoelectronics and a large number of innovations in the optical fiber communication industry have greatly reduced the price and improved the quality of optical components. By adjusting the economies of scale in the optics industry, fiber optic sensors and fiber optic instrumentation have expanded from the research phase of laboratory experiments to practical applications in the field, such as building structural health monitoring applications.

Introduction to Fiber Optic Sensors

From a basic principle, a fiber optic sensor changes one or several properties of the light wave it propagates, such as intensity, phase, polarization state, and frequency, according to changes in the external environmental parameters being tested. Extrinsic (hybrid) fiber optic sensors only use optical fibers as the transmission medium for light waves between the device and the sensing element, while intrinsic fiber optic sensors use the fiber itself as the sensing element.

At the heart of fiber-optic sensing technology is the fiber C, a thin glass line in which light waves can propagate. Optical fiber is mainly composed of three parts: core, cladding and buffer coating. The cladding layer can reflect the stray light waves emitted by the fiber core back into the fiber core, so as to ensure that the light waves have the lowest transmission loss in the fiber core. The principle of this function is that the optical index of the core is higher than that of the cladding, so that total internal reflection occurs when the light wave propagates from the core to the cladding. The outermost protective layer provides protection from damage to the optical fiber caused by the external environment or external force. Moreover, multiple protective layers can be used according to different strengths and protective procedures required.

Figure 1. Cross-sectional view of a typical fiber

Fiber Bragg Grating (FBS) Sensors

Fiber Bragg grating sensors are the most frequently used and widest range of fiber optic sensors, which can change the wavelength of the reflected light according to changes in ambient temperature and/or strain. Fiber Bragg gratings use holographic interferometry or phase masking to expose a small section of light-sensitive fiber to a light wave with a periodic distribution of light intensity. In this way, the optical refractive index of the fiber is permanently changed according to the intensity of the light wave to which it is irradiated. The periodic changes in the refractive index of light caused by this method are called fiber Bragg gratings.

When a broad-spectrum light beam is propagated to the fiber Bragg grating, each small section of the optical fiber after the refractive index is changed will only reflect a light wave of a specific wavelength, which is called the Bragg wavelength, as shown in the following equation ( 1) shown in. This characteristic makes fiber Bragg gratings reflect only one specific wavelength of light, while all other wavelengths are propagated.

In equation (1), λb is the Bragg wavelength, n is the effective refractive index of the fiber core, and Λ is the length of the separation between the gratings, called the grating period.

Figure 2. How Fiber Bragg Grating Sensors Work

Because the Bragg wavelength is a function of the spacing length between the gratings (Λ in equation (1)), fiber Bragg gratings can be produced with different Bragg wavelengths, so that different fiber Bragg gratings can be used to reflect specific wavelengths of light waves.

Figure 3. Fiber Bragg Grating Perspective View

The change of strain and temperature will simultaneously affect the effective optical refractive index n of the fiber Bragg grating and the grating period Λ , resulting in the change of the wavelength of the reflected light wave of the grating. The variation of the reflection wavelength of a fiber Bragg grating with strain and temperature can be approximated by the relationship in equation (2):

where Δλ is the change in reflected wavelength and λo is the original reflected wavelength.

The first expression before the plus sign on the right shows the effect of a change in strain on the reflected wavelength. where pe is the strain optical sensitivity coefficient, and ε is the strain effect on the grating. The second expression after the plus sign shows the effect of a change in temperature on the wavelength. where αΛ is the thermal expansion coefficient and αn is the temperature optical sensitivity coefficient. αn reflects the effect of light refractive index due to temperature change and αΛ reflects the change of grating period caused by the same temperature change.

Because fiber Bragg gratings are affected by strain and temperature changes at the same time, these two factors should be considered at the same time and analyzed separately when calculating the change of reflected wavelength. When making temperature measurements, fiber Bragg gratings must remain completely unaffected by strain. You can use an FBG temperature sensor packaged specifically for this purpose, which ensures that the properties of the fiber Bragg grating inside the package are not coupled to any external bending, tensile, compressive or torsional strain. In this case, the thermal expansion coefficient αΛ of the glass is usually negligible in practice; therefore, the change of the reflected wavelength due to temperature change can be mainly determined by the temperature optical sensitivity coefficient αn of the fiber.

Fiber Bragg grating strain sensors are somewhat procedurally more complex because both temperature and strain affect the reflected wavelength of the sensor. In order to make a correct measurement, it is necessary to compensate for the effect of temperature on the fiber Bragg grating during the test. To achieve this compensation, it can be done using a FBG temperature sensor that is in good thermal contact with the FBG strain sensor. After obtaining the test results, the second expression to the right of the plus sign can be eliminated from equation (2) by simply subtracting the wavelength change measured by the FBG temperature sensor from the wavelength change measured by the FBG strain sensor, so that This compensates for the effects of temperature changes during strain testing.

The process of installing fiber Bragg grating strain sensors is similar to the process of installing traditional electrical strain sensors, and FBG strain sensors are available in many different types and installation methods, including epoxy, weldable, bolt-on and Embedded type.

inquiry method

Since fiber Bragg gratings can be implanted with different specific reflection wavelengths, it can be used to implement good wavelength division multiplexing (WDM) techniques. This feature makes it possible to daisy-chain multiple different sensors with specific Bragg wavelengths on a single long-distance fiber. Wavelength division multiplexing assigns each FBG sensor a specific wavelength range for its use in the broad spectrum of available optics. Due to the inherent wavelength characteristics of fiber Bragg gratings, even if the light intensity is lost and attenuated due to the bending and transmission of the fiber medium during the transmission process, the results measured by the sensor can still remain accurate.

The operating wavelength range of each individual fiber Bragg grating sensor and the total wavelength range that can be interrogated by the wavelength interrogator determine the number of sensors that can be attached to a single fiber. In general, FBG strain sensors are generally assigned an operating wavelength range of about 5 nanometers, while FBG temperature sensors are assigned an operating wavelength of about 1 nanometer because the wavelength changes caused by strain changes are more pronounced than those caused by temperature changes. Scope. And because the test range provided by a common wavelength interrogator is about 60 to 80 nanometers, the number of sensors attached to an optical fiber can generally vary from 1 to 80. Of course, this is based on the reflection wavelength of each sensor. Regions in the spectral range do not overlap on the basis of (Fig. 4). Therefore, when selecting FBG sensors, it is necessary to carefully select the nominal wavelength and the operating wavelength range to ensure that each sensor has its own independent operating wavelength region.

Figure 4. Each FBG sensor attached to the same fiber must have its own operating wavelength range

A typical FBG sensor will have an operating wavelength range of several nanometers, so the optical interrogator must be able to perform measurements with a resolution of a few picometers or less, C, a rather small order of magnitude. There are several ways to interrogate the FBG grating sensor. Interferometers are commonly used laboratory equipment that can provide fairly high resolution spectral analysis. However, these instruments are generally very expensive, bulky and not strong enough, so in some applications involving field monitoring of various structures, such as wind turbine blades, bridges, water pipes, and dams, these instruments are not used. Be applicable.

A more robust approach is the introduction of charge-coupled devices (CCDs) and fixed dispersive cells, generally referred to as wavelength position switching.

In this method, the FBG sensor (or series of FBG sensors) is illuminated with a broad-spectrum light source. These reflected beams pass through a dispersive unit, which distributes the reflected beams of different wavelengths to different positions on the CCD surface. As shown in Figure 5 below.

Figure 5. Interrogation of FBG optical sensors using wavelength position conversion method

This method can quickly and simultaneously measure all FBG sensors attached to the fiber, but it only provides very limited resolution and signal-to-noise ratio (SNR). For example, if we want to achieve a resolution of 1 picometer in the 80 nanometer wavelength range, then we need a linear CCD device containing 80,000 pixels, which is already larger than what is currently available on the market. The best linear CCD devices (as of July 2010) perform more than 10 times higher. In addition, because the energy of a broad-spectrum light source is spread over a wide range of wavelengths, the energy of the reflected beam from the FBG can be very small, sometimes even making measurements difficult.

The most popular method is to use a tunable Far-Pert filter to create a laser source with high energy and fast frequency sweep to replace the traditional broad-spectrum light source. Tunable laser sources concentrate energy in a narrow wavelength range, providing a high-energy light source with a high signal-to-noise ratio. The high optical power provided by this architecture makes it possible to use a single fiber to mount multiple optical channels, which can effectively reduce the cost of multi-channel interrogators and reduce the complexity of the system. An interrogator based on this tunable laser architecture can scan a narrow spectral band over a relatively large wavelength range. On the other hand, a photodetector will synchronize with this scan and measure the reflected light from the FBG sensor. Laser beam. When the laser wavelength emitted by the tunable laser matches the Bragg wavelength of the FBG sensor, the photodetector can measure the corresponding response. The wavelength of the tunable laser when this response occurs corresponds to the temperature and/or strain measured at the FBG sensor at this time, as shown in Figure 6.

Figure 6. Interrogation of the FBG optical sensor using the tunable laser source method

Interrogation using this method can achieve an accuracy of about 1 picometer, which corresponds to an accuracy of about 1.2 microstrain (FBG strain sensor) or about 0.1 degrees Celsius (FBG temperature sensor) for conventional FBG sensors. Because of the high optical power of the tunable laser source method relative to other methods, this interrogation method can also be used in measurement applications with longer fiber lengths (over 10 km).

Advantages of FBG Optical Sensors

By using light waves instead of electrical currents and standard optical fibers instead of copper wires as the transmission medium, FBG optical sensing addresses many of the challenges and difficulties encountered with electrical sensing. Both fiber optics and FBG optical sensors are insulators, have passive electrical properties, and are immune to electromagnetically induced noise. Interrogators with high optical power tunable laser sources can perform measurements over long distances with very low data loss rates or even zero loss. At the same time, different from the electrical sensor system, one optical channel can complete the test of multiple FBG sensors at the same time, which greatly reduces the size, weight and complexity of the test system.

In some applications with harsh external environmental conditions, some commonly used electrical sensors, such as foil strain gauges, thermocouples, and vibrating wire sensors have been difficult to use or even failed. Optical sensors are a very ideal solution. . Because optical sensors are used and mounted similarly to these traditional electrical sensors, transitioning from an electrical to an optical test solution is relatively straightforward. Having a good understanding of how fiber optics and FBGs work will help you better embrace optical testing techniques and harness all the advantages this new technology has to offer.

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