【Repost】Understanding Grating Sensors in One Article

皇华Ameya360

【Repost】Understanding Grating Sensors in One Article [Copy link]

Grating sensor refers to a sensor that uses the principle of grating fringe to measure displacement. Grating is a dense and evenly spaced parallel engraved line on a long strip of optical glass, with a line density of 10 to 100 lines/mm. The grating fringe formed by the grating has an optical amplification effect and an error averaging effect, thus improving the measurement accuracy. 25)]Introduction:

In 1978, K. O. Hill and others from the Communications Research Center in Ottawa, Canada, first discovered the photosensitivity effect of optical fiber in germanium-doped quartz optical fiber, and used the standing wave writing method to make the world's first fiber Bragg grating. Subsequently, G. Meltz and others from the United Technologies Research Center in the United States realized the UV laser side writing technology of fiber Bragg grating (FBG), which made a breakthrough in the production technology of fiber Bragg grating. With the continuous improvement of fiber Bragg grating manufacturing technology, its application results are increasing. The entire field from fiber optic communication and fiber optic sensing to optical computing and optical information processing will undergo revolutionary changes due to the practical application of fiber Bragg grating. Fiber Bragg grating technology is another major technological breakthrough in optical fiber technology after the erbium-doped fiber amplifier (EDFA) technology. Fiber Bragg Gratings are made using the photosensitivity in optical fibers. The so-called photosensitivity in optical fibers refers to the property that when laser passes through doped optical fibers, the refractive index of the optical fiber will change accordingly with the spatial distribution of light intensity. The spatial phase grating formed in the fiber core is actually a narrowband (transmission or reflection) filter or reflector formed in the fiber core. This characteristic can be used to manufacture many fiber optic devices with unique performance. They all have a wide reflection bandwidth range, low additional loss, small size, easy coupling with optical fibers, can be integrated with other optical devices, and are not affected by environmental dust. There are many types of fiber Bragg Gratings, which are mainly divided into two categories: one is Bragg grating (also called reflection or short-period grating), and the other is transmission grating (also called long-period grating). Fiber Bragg gratings can be divided into periodic structures and non-periodic structures in terms of structure, and can also be divided into filter gratings and dispersion compensation gratings in terms of function; among them, dispersion compensation gratings are non-periodic gratings, also known as chirped gratings. At present, the application of fiber Bragg gratings is mainly concentrated in the fields of optical fiber communication and optical fiber sensors. In the field of optical fiber sensors, the application prospects of fiber Bragg grating sensors are very broad. As fiber Bragg grating sensors have the advantages of anti-electromagnetic interference, small size (standard bare fiber is 125um), light weight, good temperature resistance (the upper limit of the working temperature can reach 400℃～600℃), strong multiplexing ability, long transmission distance (the distance from the sensor to the demodulation end can reach several kilometers), corrosion resistance, high sensitivity, passive device, easy deformation, etc., they have been successfully used in aviation and aerospace fields as effective non-destructive testing as early as 1988. At the same time, fiber Bragg grating sensors can also be used in various fields such as chemical medicine, material industry, water conservancy and electricity, shipbuilding, coal mining, etc., as well as in the field of civil engineering (such as buildings, bridges, dams, pipelines, tunnels, containers, highways, airport runways, etc.) to measure the integrity and internal strain state of concrete components and structures, thereby establishing smart structures and further realizing intelligent buildings. Working principle: We know that the Bragg wavelength lB of the grating is determined by the following formula: lB=2nL ⑴ Wherein, n is the effective refractive index of the core mode; L is the grating period. 25)]When the temperature, stress, strain or other physical quantities of the environment in which the fiber Bragg grating is located changes, the period of the grating or the refractive index of the core will change, thereby changing the wavelength of the reflected light. By measuring the change in the wavelength of the reflected light before and after the physical quantity changes, the change in the physical quantity to be measured can be obtained. For example, the different refractive index changes of left-handed and right-handed polarized waves induced by the magnetic field can be used to directly measure the magnetic field. In addition, through specific technologies, it is also possible to measure stress and temperature separately and simultaneously. By coating specific functional materials (such as piezoelectric materials) on the grating, indirect measurement of physical quantities such as electric fields can also be achieved. 1. Working principle of chirped fiber Bragg grating sensor In the grating sensor system introduced above, the geometric structure of the grating is uniform, which is very effective for the fixed-point measurement of a single parameter. However, it is incapable of measuring strain and temperature at the same time or measuring the distribution of strain or temperature along the length of the grating. At this time, the chirped fiber Bragg grating sensor is a good choice. Chirped fiber Bragg gratings are used in high-bit remote communication systems due to their excellent dispersion compensation capabilities. The working principle is basically the same as that of fiber Bragg grating sensors. Under the influence of external physical quantities, the chirped fiber Bragg grating will change not only the DlB change, but also the spectrum broadening. This sensor is very useful in situations where both strain and temperature exist. Due to the influence of strain, the chirped fiber Bragg grating reflection signal will be broadened and the peak wavelength will also shift, while the temperature change will only affect the position of the center of gravity due to the temperature dependence of the refractive index (dn/dT). Therefore, by simultaneously measuring the spectral shift and broadening, strain and temperature can be measured simultaneously. 25)]2. Working principle of long period fiber grating (LPG) sensor

[color=rgb(25, 25,The period of long-period fiber grating (LPG) is generally considered to be hundreds of microns. It can couple the light from the core into the cladding at a specific wavelength. The formula is as follows: li = (n0- niclad) · L ⑵ In the formula, n0 is the refractive index of the core; niclad is the effective refractive index of the i-order axisymmetric cladding mode. The light will rapidly attenuate in the cladding due to the loss of the cladding/air interface, leaving a series of loss bands. An independent LPG may have many resonances over a wide wavelength range. The central wavelength of the resonance depends mainly on the refractive index difference between the core and the cladding. Any changes caused by strain, temperature or external refractive index changes can produce large wavelength shifts in the resonance. By detecting Dli, information about changes in external physical quantities can be obtained. The response of the resonance band of LPG at a given wavelength usually has different amplitudes, so it is suitable for building multi-parameter sensors.

Source: Internet, if infringed, please delete

Vicctoor

The sensor consists of four parts: scale grating, indicator grating, optical path system and measurement system (see figure). When the scale grating moves relative to the indicator grating, it forms light and dark overlapping grating stripes distributed roughly according to the sine law. These stripes move at the relative speed of the grating and directly illuminate the photoelectric element, and a series of electrical pulses are obtained at their output ends. Digital signal output is generated through amplification, shaping, direction identification and counting systems, directly displaying the measured displacement.

shakencity

Grating sensor refers to a sensor that uses the principle of grating fringe to measure displacement. Grating is a dense and evenly spaced parallel engraved line on a long strip of optical glass, with a line density of 10 to 100 lines/mm. The grating fringe formed by the grating has an optical amplification effect and an error averaging effect, thus improving the measurement accuracy.