Fiber Optic Strain Sensors Technology and Applications

Nowadays, different types of fiber optic strain sensors have attracted attention from all over the world. Fiber Bragg Grating (FBG) has become the most widespread technique, directly applicable to bridges, concrete, and dams for strain measurement.

To create the actual strain sensor, the optical fiber is inscribed during production with a Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.

The material interferences are placed at certain intervals. When the fiber is stretched or compressed and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa. Not only does the reflected light take a little longer or shorter to travel back when the Fiber Bragg Grating is under strain, but the wavelength that is reflected also changes. In scientific terms, the Fiber Bragg Grating has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.

For measurements, the optical fiber needs to be connected to a so-called interrogator; it continuously sends out light in different wavelengths, one at a time, thus covering a wide spectrum.  In order to ensure the safety of personal and public property, the precise and real-time monitoring of strain becomes more and more important in all kinds of engineering applications, such as chemical plants, gas stations, power stations, bridges, tunnels, oil pipelines, etc.. In general, these application environments full of poisonous gas, intense radiation, and elevated temperature are dangerous to human health, so safe and efficient remote monitoring of strain is of great significance. Compared with conventional electrical sensing methods, an optical fiber strain sensor is more suitable for present applications because of its compact size, high sensitivity, multiplexing capability, immunity to electromagnetic interference, high-temperature tolerance, and resistance to harsh environments.

Fiber optic strain sensors are welded directly to the surface of the metal structure (pipes, beams, etc.), and it has a protective silicone cover. Fiber optic strain sensors are durable and stable, widely used for civil engineering constructions, particularly they reinforce concrete structures exceptionally well.

If you would like to purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com  or +1 617 558 98 58

Interrogation techniques for FBG Sensor Arrays

FBG sensors are very suitable for sensing and data acquisition, where sensor arrays can be multiplexed using similar techniques that have been applied to fiber-optic sensors like wavelength-division multiplexing (WDM), spatial-division-multiplexing (SDM), and time-division-multiplexing (TDM) as they can be directly implemented in the fiber without changing the diameter of the fiber. This feature makes FBG sensors suitable for a wide range of applications.

The main problem with the TDM system is that the sensors must be placed sufficiently far apart because the pulse returning from the adjacent sensors must be able to reach and get detected separately. In WDM systems, different sensors have the nominal central wavelength, and other sensors are separated by a few nanometers. WDM interrogation is available in two topologies i.e., series and parallel. The parallel approach is easier to implement but the series topology allows the optical power from the sensing FBG array to be used much more efficiently than parallel topology.

The number of sensors that you can incorporate within a single fiber depends on the wavelength range of operation of each sensor and the total available wavelength range of the interrogator. Because typical interrogators provide a measurement range of 60 to 80 nm, each fiber Bragg grating array of sensors can usually incorporate anywhere from one to more than 80 sensors – as long as the reflected wavelengths do not overlap in the optical spectrum. Be careful when selecting the nominal wavelengths and ranges for the FBG sensors in an array to ensure that each sensor operates within a unique spectral range.

Major limitations in interrogating FBG sensor arrays are the cross-talk, spectral shadowing, and interference. For all the interrogation approaches, some crosstalk between adjacent sensors seems to be unavoidable. The use of a serial array of FBG sensors with the same central wavelength results in the crosstalk between sensors. The amount of light reflected by the FBG sensors located nearest to the source will affect the amount of the optical power reaching and be returned from gratings further from the source. The lower the peak reflectivity of the FBGs is, the smaller the effect is. Another source of the crosstalk in a TDM serial array of identical FBG sensors arises from multiple reflections between FBGs. This can lead to pulses arriving simultaneously at the detector having undergone a direct reflection from a sensor element and also having experienced a number of multiple reflection paths between FBGs.

A tunable fiber Bragg grating (FBG) optical filters

FBGs in optical filtersIn order to make the fiber Bragg grating tunable (it means controlling the reflected wavelength), the Bragg grating period must be controllable. This is achieved by one of several methods. For example, the application of a stretching force elongates the fiber, which thus changes its period (mechanical tuning). Mechanical tuning of the FBG results in a faster and large response in terms of wavelength shift.  The application of heat elongates the fiber, thus changing its period (thermal tuning). Changing the temperature of the FBG results in a slow wavelength shift; hence, a small tuning range is achieved.  FBG final applications are in fiber dispersion compensation, in gain flattening of erbium-doped fiber amplifiers, and in add-drop multiplexers/demultiplexers. However, the fiber used to make an FBG should be free of imperfections as well as microscopic variations of the refractive index.

Tunable FBG is a valuable option for all applications that require flexibility in center wavelength (and/or frequency). The tuning setup consists of the mechanical assembly where the FBG is applied to. The mechanics induce strain to the FBG, shifting its center wavelength homogeneously and chirp-free.

Common FBGs have the flat top response, low cost, and low insertion loss that meets the requirement of add/drop FBG multiplexers in the optical WDM fiber transmission systems. However, a great deal of research interest in FBGs involves the property of being tunable in both the Bragg wavelength and bandwidth. Recent studies have shown large tuning ranges of 110 nm in the Bragg wavelength and more than 10 nm in the 3-dB bandwidth for uniform FBGs. Given such desirable tunability, FBG is becoming a very flexible and “smart” optical fiber component. Bandwidth-tunable FBG optical filters have been studied in many applications, such as tunable dispersion compensation, phased array antenna, and temperature-independent fiber grating sensing.

The important parameters of light sources for FBG interrogators are optical power, tuning range, tuning speed, and continuous tuning capabilities. Among the tunable laser sources are DFB lasers, multi-section distributed Bragg reflector lasers, and external cavity lasers.

Chirped Fiber Bragg Gratings (CFBG) for high-speed fiber optic communications systems

A chirp is a linear variation in the grating period, that can be added to the refractive index profile of the grating. The reflected wavelength fluctuates with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the ability to add dispersion—especially, different wavelengths reflected from the grating will be subject to different delays.

A non-uniform resonance wavelength along the length of the grating in a CFBG can be accomplished by varying the period or by varying the average effective refractive index. The average refractive index can be changed using different methods, for example, changing the amplitude of the reflective index modulation profile or variation the fiber in the region of the grating length. The chirped FBG was manufactured with the usage of a chirped phase mask to generate a variation in the period of the refractive index.

Chirped fiber Bragg gratings have been widely used for dispersion compensations in high-speed fiber optic communications systems because they are able to retard pulsed light depending on its wavelength. Experience has proven that ideas in one field find applications in another. Actually, this type of optical device has been attracting significant attention in the fiber optic sensing community, in high sensitivity sensors or wavelength discriminators in interrogation systems.

There are two prevailing fields of application of chirped FBG: measurement of curvature based on chirped fiber Bragg gratings and new interrogation system, written in an Erbium-doped fiber. The increasing demand for measurement of curvatures has stimulated the appearance of few sensing systems that depend on the intrinsic characteristics of fiber Bragg gratings. A curvature measurement technique using a smart composite consists of two chirped fiber Bragg gratings. The two gratings are embedded on the opposite sides of the composite laminate and serve as curvature sensors and as wavelength discriminators enabling a temperature-independent intensity-based scheme for the measurement of the radius of curvature.

FBG interrogation relies on the usage of the edge filtering concept applied to a chirped fiber Bragg grating written in an erbium-doped fiber as the processing element. Through the combination of the photon amplification of the erbium-doped fiber and of the distributed wavelength reflection characteristics of the chirped FBG, it becomes possible to reach different reading sensitivities and amplification of the remote sensing signal. The ability of chirped FBG has also been employed successfully in the development of interrogation techniques. One of these techniques uses the group-delay in a Sagnac loop interferometer and another the spectra response of broadband chirped gratings.