CN100468008C - Transverse stress sensing system and implementation method of photonic crystal fiber with grating written - Google Patents

Abstract

An optical fiber sensing system for detecting transverse stress and strain. The output light of the laser light source is coupled into a zero-birefringence or high-birefringence photonic crystal fiber after passing through a polarization controller. The photonic crystal fiber is written with a long-period grating and placed on a In the unit, it is convenient to feel the applied external lateral stress, and the spectral detector connected to the output terminal detects the transmission spectrum, and sends the detected information to the signal processing and display unit to obtain the detection result of the lateral stress. Or the output light of the laser light source passes through the optical circulator, and then couples into the photonic crystal fiber with the Bragg grating written on it. The photonic crystal fiber is placed in the force unit, and the signal light reflected from the Bragg grating passes through the optical circulator and then output to the spectral detector. , detect the reflection spectrum, and send the detected information to the signal processing and display unit to obtain the detection result of the transverse stress.

Description

Transverse stress sensing system and implementation method of photonic crystal fiber with grating written

technical field

The present invention relates to an accurate and sensitive optical fiber sensing system to measure transverse stress or strain, in particular a grating-written photonic crystal fiber based on a grating-written zero-birefringence photonic crystal fiber or a high-birefringence photonic crystal fiber Lateral stress sensing system and implementation method.

Background technique

The safety and quality monitoring of buildings is mainly to detect the stress and deformation under the action of external force. The general detection method is to install a large number of distributed strain gauges or other strain-sensing fibers in the building materials. Since the composite structure composed of fiber materials and building materials can properly improve the strength of building materials, the use of composite fiber materials for building loads Deformation detection is a relatively common method. Embed optical fibers in building materials or buildings to form intelligent composite structures (also called smart composite structures). When the building materials or buildings are deformed, the embedded optical fibers will feel the corresponding stress and strain. Its optical characteristics (mainly transmission characteristics) can change immediately, so the detection of the optical signal transmitted in the optical fiber can monitor the deformation and load changes in the building in real time, so as to realize the safety monitoring of the building. This method can not only be used for the detection of buildings, but also has wide application prospects in the fields of machinery, biomedicine, aerospace and other fields. The use of optical fiber for stress sensing and detection not only has the inherent advantages of optical fiber, such as anti-electromagnetic interference, high mechanical strength, etc., but also can realize distributed detection, and the sensitivity is very high.

Optical fiber stress sensing and detection technology has experienced nearly 20 years of development, and there are many kinds of stress sensors based on ordinary silica optical fibers. Fiber optic stress sensors can be divided into longitudinal stress sensors and transverse stress sensors according to the direction of the detected stress. According to the basic working principle, they can be divided into two categories, one is fiber optic stress sensors, and the other is grating stress sensors. The optical fiber type stress sensor is that the optical fiber deforms under the action of external force, changes the transmission characteristics of its mode, and detects the transmitted light to obtain the change of deformation or external force. The grating type stress sensor uses the period (longitudinal) or symmetry (transverse) of the fiber grating (including fiber Bragg grating FBG and long period grating LPG) to change with the external force, thereby changing the transmission spectrum or reflection spectrum of the grating to realize the external force change. detection.

The detection of longitudinal stress by fiber-optic strain sensors is generally based on the interference between multiple modes in dual-mode fibers or few-mode fibers. When the fiber is stretched, the phase relationship between different modes changes, so the optical field (intensity) at the output end changes accordingly. According to the periodicity of the intensity change, the phase change can be obtained, thereby obtaining the deformation or stress of the fiber. When the ordinary circular fiber is subjected to transverse stress, the optical path difference and coherence of the two polarization components of the back-reflected light are generally detected to determine the longitudinal position of the transverse stress applied to the fiber, but it is difficult to determine the magnitude of the stress.

Since the spectral characteristics of fiber gratings are very sensitive to the fiber structure and grating period, since the grating came out in the early 1990s, sensors based on Bragg gratings and long-period gratings ( Including stress sensor and temperature sensor) have been widely researched and applied. When the fiber is elongated by longitudinal tension, the period of the grating written in the core region will increase accordingly. For Bragg gratings, the peak reflection wavelength shifts to longer wavelengths; for long-period gratings, the transmission spectrum shifts to longer wavelengths. When the fiber is subjected to lateral pressure, it may be assumed that the pressure is applied to the side of the fiber along the x direction, then the size of the fiber in the x direction will shrink, while the size in the y direction will expand. For round fibers, there will be inherent birefringence; for birefringent fibers, the birefringence properties will change. The reflection spectrum or transmission spectrum of the Bragg grating or long-period grating written in this birefringent fiber is split, and two reflection peaks or two sets of transmission spectra appear, which correspond to two sets of modes with orthogonal polarization directions. . When the transverse stress applied to the fiber changes, the birefringence characteristics of the fiber change immediately. Generally, the change of the transverse stress is proportional to the wavelength shift of the grating spectrum. Therefore, the detection of the two reflection peak wavelengths of the Bragg grating The change of transverse stress can be detected in time by detecting the movement of two sets of transmission spectra of the long-period grating.

"Fiber Bragg Grating Lateral Strain Sensor System" Chinese patent CN1155798C introduces a transverse strain sensor system based on Bragg gratings in ordinary optical fibers, and can simultaneously measure temperature or force at different locations.

The longitudinal strain sensitivity of optical fiber is generally 0.8×10 ^-6 μ ε ^-1 , and the temperature sensitivity is 6×10 ^-6 ℃ ^-1 ; the laboratory sensitivity can reach 0.344nm/(N .mm ^-1 ); the laboratory sensitivity of transverse stress sensing and detection using long-period gratings has reached 50nm/(N.mm ^-1 ).

The sensitivity of the optical fiber strain sensor is determined by the main material of the optical fiber, quartz (SiO ₂ ). The Young's modulus of SiO ₂ is very large, generally exceeding 70GPa (the specific value is related to the doping in the silica fiber, the undoped cladding is about 72GPa, and the 3% Ge-doped is about 70.8GPa), under the action of external force, its The deformation is generally very small, so the process requirements for the optical fiber stress sensor are relatively high. In order to change the single dependence of the sensitivity of the optical fiber transverse stress sensor on the Young's modulus of the quartz material, people try to change the optical fiber structure, thereby changing the deformation of the optical fiber under the action of external force, so as to improve the accuracy of optical fiber transverse stress sensing and detection and sensitivity.

The use of Bragg gratings in multi-core optical fibers for transverse stress detection is a recent research work, but it has also shown its attractive advantages. The sensitivity of transverse stress sensors using 4-core optical fibers has reached 0.24nm/(N. mm ^-1 ).

The transverse stress detection by using side-hole fiber and the Bragg grating in it has received more attention. It has not only been confirmed theoretically and experimentally that the sensitivity of this fiber to transverse stress can reach 2.0nm/(N.mm ^-1 ) Above, and also researched its dynamic measurement range, it can realize high-precision pressure measurement with 0.03MPa resolution in the range of 0-38.08MPa.

The present invention uses a novel photonic crystal fiber (PCF: Photonic Crystal Fiber), writes a Bragg grating or a long-period grating in it, and senses the transverse stress (strain).

In the photonic crystal fiber cladding area, a large number of air holes are arranged longitudinally. According to the light guiding mechanism, it can be divided into two categories, namely, refractive index light guiding and photonic band gap (PBG: Photonic Band Gap) light guiding. The core of a typical refractive index light-guiding photonic crystal fiber is solid silica, and the cladding is a porous structure. The air hole in the cladding reduces the effective refractive index of the cladding, thereby satisfying the total reflection (TIR: Total Internal Reflection) condition, and the beam is bound to the core for transmission. The cladding region of the photonic bandgap light-guiding fiber is a periodic structure, and the photonic bandgap generated by it can bind the light beam to the core region of the fiber for transmission. The periodic cladding structure of photonic bandgap optical fiber is a two-dimensional photonic crystal, and the refractive index only changes periodically in the cross section. Along the longitudinal direction of the fiber, the refractive index is uniform, and the light is not restricted when it is transmitted along the longitudinal direction. But the Bragg reflection of the transverse periodic structure will generate transverse resonance, forming a frequency domain (wavelength) band gap. If a line defect is introduced to destroy the periodicity of the two-dimensional photonic crystal, a defect mode will be generated in the photonic band gap of the cladding structure, and can be trapped in the core region and transmitted along the fiber. This new light guiding mechanism can realize light guiding in the low refractive index region of the optical fiber (such as the air core). This type of optical fiber cannot guide light based on total reflection. Many of its new features can be widely used in optical fiber sensing and optical fiber communication. .

So far, people have used various materials such as pure silica, non-quartz glass (such as sulfide glass, Schott glass) and polymers to prepare photonic crystal fibers. There are many other new properties of photonic crystal fiber, such as endless single mode, large mode field area single mode fiber, highly nonlinear fiber, high birefringence fiber, dispersion controlled fiber, and so on.

In the refractive index photonic crystal fiber, if the size of the air hole is different along different directions, or the shape of the hole is elliptical instead of circular, or the position of the air hole is asymmetrical, high birefringence can be obtained. The birefringence of these high-birefringence photonic crystal fibers can be an order of magnitude higher than that of conventional Panda fibers. Ning Guan reported a high birefringence photonic crystal fiber, which maintains polarization in the range of 480nm to 1620nm, and the polarization crosstalk is better than -25dB, and the crosstalk in the range of 1300nm to 1620nm is only about -45dB, even if the fiber bending radius is only 10mm Crosstalk doesn't worsen either. Crystal Fiber A/S (PCF products are more comprehensive after the acquisition of Blazephotonics) provides high-birefringence photonic crystal fibers with a length of more than 100m, and the polarization coupling is better than 30dB, and the temperature coefficient of birefringence is significantly lower than that of ordinary high-birefringence fibers. These properties can be used to develop sensors with novel properties.

At present, the research on temperature and stress sensing using photonic crystal fiber has begun, but because the application of photonic crystal fiber has just started, and the detection of transverse stress is relatively difficult, so there is still no transverse stress sensor based on photonic crystal fiber. related reports. We have studied the deformation of photonic crystal fiber and ordinary fiber under the action of external force. The results show that the air hole is beneficial to enhance the strain of the fiber under external force. The larger the hole, the greater the strain. If the number of air holes is increased, the strain will increase further. The strain has a strong dependence on the structure of the photonic crystal fiber. Therefore, the best strain parameters can be obtained by selecting different photonic crystal fibers, which not only reduces the difficulty of detection, improves the detection sensitivity, but also obtains the best transverse stress. Detection conditions.

Accompanying drawing 5, 6, 7 are three kinds of typical zero birefringence photonic crystal fibers, write Bragg grating (or long-period grating) in it, when the fiber is not subject to any stress, the two polarization states of the fundamental mode are degenerate (accompanying drawing 8), there is a loss peak at the position corresponding to the resonance wavelength on the transmission spectrum; when the photonic crystal fiber is subjected to lateral pressure, the fiber produces birefringence, and the original loss peak splits into two, and the wavelength interval corresponding to the two loss peaks It is proportional to the birefringence of the optical fiber, therefore, detecting the wavelength interval between the loss peaks on the transmission spectrum can sense and detect the stress applied in the transverse direction of the optical fiber.

Accompanying drawing 9, 10, 11, 12 are four kinds of typical high birefringence photonic crystal fibers, write Bragg grating (or long-period grating) in it, respectively correspond to the transmission spectra of two fundamental modes with orthogonal polarization directions. Two loss peaks (wavelength positions corresponding to the resonant wavelengths of the two polarization states). If the transverse pressure applied to the photonic crystal fiber increases the birefringence of the fiber, the wavelength interval between the original two loss peaks increases under the pressure, and the increase is proportional to the transverse stress; if applied to the photonic crystal fiber The transverse pressure above reduces the birefringence of the fiber, and the wavelength interval between the original two loss peaks decreases under the pressure, and the decrease is proportional to the transverse pressure. Therefore, detecting the wavelength interval between the loss peaks on the transmission spectrum can sense and detect the stress applied in the transverse direction of the optical fiber.

The above application is written in the transverse stress (strain) sensing system of the photonic crystal fiber with grating, which is realized by detecting the transmission spectrum. For the system with Bragg grating, the change of the reflection spectrum can be detected. The same principle can realize the transverse stress (strain) ) detection.

The present invention—transverse stress and strain sensing system using photonic crystal fiber with gratings, utilizes the higher sensitivity of photonic crystal fibers to external forces, adopts zero birefringence photonic crystal fibers or high The birefringent photonic crystal fiber can detect the stress applied in the transverse direction of the fiber in real time and with high precision.

For further research, based on the above contents, a distributed transverse stress (strain) sensing and detection system or sensor network can be realized to monitor large-scale overall strain conditions of large buildings in real time.

Contents of the invention

An object of the present invention is to provide a zero-birefringence photonic crystal fiber transverse stress (strain) sensing and detection system written with a Bragg grating, further realizing a distributed sensing system or sensor network, which is used for measuring and has nothing to do with the direction of force transverse stress or strain.

Another object of the present invention is to provide a high birefringence photonic crystal fiber transverse stress (strain) sensing system written with Bragg gratings, further realizing a distributed sensing system or sensor network for measuring direction-dependent transverse stress or strain.

Another object of the present invention is to provide a zero-birefringence photonic crystal fiber transverse stress (strain) sensing system written with a long-period grating to further realize a distributed sensing system or sensor network for measuring direction-dependent transverse stress or strain.

Another object of the present invention is to provide a high birefringence photonic crystal fiber transverse stress (strain) sensing system written with a long-period grating to further realize a distributed sensing system or sensor network for measuring direction-dependent transverse stress or strain.

An optical fiber sensing system for detecting transverse stress and strain, mainly including a laser light source, an ordinary single-mode optical fiber, a zero-birefringence photonic crystal fiber or a high-birefringence photonic crystal fiber, a Bragg grating, an optical circulator, a force unit, and a spectral detector , signal processing and display unit, the output light of the laser light source passes through the optical circulator, and then couples into the zero-birefringence or high-birefringence photonic crystal fiber. The photonic crystal fiber is written with Bragg gratings and placed in the force-bearing unit, which is convenient for feeling the application The external transverse stress, the signal light reflected from the Bragg grating is output to the spectrum detector after passing through the optical circulator, and the reflection spectrum is detected, and the detected information is sent to the signal processing and display unit to obtain the detection result of the transverse stress.

An optical fiber sensing system for detecting transverse stress and strain, mainly including a laser light source, an ordinary single-mode optical fiber, a zero-birefringence photonic crystal fiber or a high-birefringence photonic crystal fiber, a long-period grating, a polarization controller, a force unit, and a spectral detection device, signal processing and display unit, the output light of the laser light source is coupled into the zero-birefringence or high-birefringence photonic crystal fiber after the polarization controller, the photonic crystal fiber is written with a long-period grating, and placed in the force unit, which is convenient Feeling the applied external lateral stress, the spectral detector connected to the output terminal detects the transmission spectrum, and sends the detected information to the signal processing and display unit to obtain the detection result of the lateral stress.

The transverse stress sensing system and implementation method of the photonic crystal fiber written with grating have the following steps:

First, select a photonic crystal fiber with zero birefringence or high birefringence, fabricate a Bragg grating or a long-period grating in it, and determine the resonant wavelength of the grating by measuring the transmission spectrum of the grating;

Second, select the working band of the laser so that it can cover the resonant wavelength of the grating used in the sensing system, and determine the laser light source and photodetector used in the corresponding system, as well as active and passive components such as polarization controllers and optical circulators;

Third, the photonic crystal fiber with the grating is placed in the stress unit, which is convenient for applying external transverse stress to the fiber;

Fourth, after the output light of the laser light source passes through the polarization controller, it is coupled into the photonic crystal fiber;

Fifth, connect the optical path of the sensing system and adjust it precisely to make the system response, sensitivity, and accuracy the best;

Sixth, use an optical circulator to connect the reflected light to a spectral detector to detect the reflected spectrum; or directly connect the far end of the optical fiber to the spectral detector to detect the transmitted spectrum;

Seventh, debug the signal processing and display unit, process the detection signal and display it on the terminal equipment, so as to realize the real-time detection of lateral stress.

The principle of the invention is to use the relationship between the transmission characteristics of the grating in the photonic crystal fiber and the transverse deformation of the fiber to sense and detect the transverse stress (strain). Transverse stress causes transverse strain in the optical fiber, which not only changes the symmetry of the optical fiber, but also changes the shape of the air hole in the optical fiber, and changes the refractive index of the quartz material due to pressure, thereby changing the transmission characteristics of the optical fiber, mainly changing the transmission of each mode constant and mode indices, thus changing the birefringence of the fiber. Writing Bragg gratings or long-period gratings in zero-birefringence photonic crystal fibers, under the action of transverse stress (accompanying drawing 14), produces linear birefringence, and there is no longer only one resonant wavelength on the grating reflection spectrum or transmission spectrum, but split into two resonant wavelengths corresponding to the two polarization states, reflection peaks appear at these two resonant wavelengths (accompanying drawing 19, accompanying drawing 20), the wavelength interval is proportional to the fundamental mode mode birefringence of the optical fiber, thus proportional to External transverse stress applied to an optical fiber. Therefore, real-time monitoring of transverse stress can be realized by detecting the change of the interval between two reflection peak wavelengths or transmission loss peak wavelengths.

The two fundamental modes of the high-birefringence photonic crystal fiber (Fig. 13) have inherent birefringence, which is much higher than that of the traditional birefringence fiber. If Bragg gratings or long-period gratings are written in it, even if there is no external transverse stress, the reflection spectrum peak Or the loss peaks of the transmission spectrum are also at two separate resonance wavelengths (Fig. 21), the wavelength separation is proportional to the birefringence of the fiber. When the external stress is applied to the fiber transversely along the slow axis of the fiber (the force F in the vertical direction of the accompanying drawing 15), the birefringence is reduced, thereby reducing the wavelength interval; when the external stress is applied along the fast axis of the fiber (the force F in the horizontal direction of the accompanying drawing 15 Force F), its transverse strain and refractive index distribution are shown in Figure 16 and Figure 17, the birefringence increases and the wavelength interval increases. Therefore, detecting the change of the interval between two reflection peak wavelengths or transmission loss peak wavelengths can realize real-time monitoring of the magnitude and direction of the transverse stress.

The solution of the present invention mainly has several key technologies.

First, the selection of zero birefringence photonic crystal fiber and high birefringence photonic crystal fiber. Due to the imperfection of the manufacturing process, it is generally impossible to obtain an optical fiber without birefringence at all, but the present invention uses the birefringence characteristics of the optical fiber to perform transverse stress sensing and detection. Therefore, the inherent birefringence of the optical fiber caused by process defects Problems can be avoided.

Second, the posture adjustment of the high birefringence photonic crystal fiber is beneficial to detect the direction of external stress. In high birefringence photonic crystal fibers, the change of mode birefringence is related to the direction of external force. Therefore, the direction of external transverse stress can be determined by adjusting the attitude of the fiber.

Third, the writing of Bragg gratings. Whether it is a zero-birefringence or high-birefringence photonic crystal fiber sensor system, the present invention requires writing Bragg gratings therein, which is the current frontier technology. The structure of photonic crystal fiber is relatively complex, especially the technology of writing Bragg gratings in pure silica photonic crystal fiber is more difficult. At present, there are only a few experimental reports in the world, mainly two methods of traditional phase mask ultraviolet exposure and two-photon absorption.

Fourth, the introduction of long-period gratings. Whether it is a zero-birefringence or high-birefringence photonic crystal fiber sensor system, the present invention needs to introduce a long-period grating into it, which is the current frontier technology. However, compared to the difficulty of writing Bragg gratings, it is relatively easy to introduce long-period gratings into photonic crystal fibers. The long-period grating can be written in the photonic crystal fiber by using the traditional amplitude mask ultraviolet exposure method, or the equivalent long-period grating.

Fifth, the detection of the reflection spectrum or transmission spectrum of the grating is the detection of the change of the resonance wavelength. By detecting the change of the resonant wavelength interval, the detection of the magnitude and direction of the transverse stress is realized.

The technical effect of the invention can be reflected in the actual sensor application. Application of a zero-birefringence photonic crystal fiber transverse stress (strain) sensor with a grating, because the optical fiber used does not have inherent birefringence, no matter from which direction the transverse stress is applied, the resulting transverse deformation and refractive index change will affect the output optical signal The effects are all the same, so it is not possible to distinguish which direction the transverse stress comes from. This sensor system can only be used for direction-independent transverse stress detection. Application of a high birefringence photonic crystal fiber transverse stress (strain) sensor with a grating, because the optical fiber used has a high intrinsic birefringence, the influence of transverse deformation and refractive index changes caused by transverse stress applied in different directions on the output optical signal different, so the sensor system can be used for direction-dependent transverse stress detection.

The technical effect of applying a grating-written zero-birefringence or high-birefringence photonic crystal fiber transverse stress (strain) sensor can also be improved by the following means.

First, when selecting a zero-birefringence photonic crystal fiber, consider its single-mode characteristic, especially the decisive factor of the fiber structure to the single-mode characteristic. If the structure is sensitive to deformation under external force, the sensitivity of the sensor can be improved.

Second, when selecting a high-birefringence photonic crystal fiber, consider its single-mode characteristic, especially the decisive factor of the fiber structure to the single-mode characteristic. If the structure is sensitive to deformation under external force, the sensitivity of the sensor can be improved.

Third, when selecting a high-birefringence photonic crystal fiber, consider its birefringence characteristics, especially the decisive factor for the birefringence characteristics of the fiber structure. If the structure is more sensitive to deformation under the action of external force, the change of birefringence is also more sensitive, which can improve the sensitivity of the sensor.

Fourth, when selecting a high-birefringence photonic crystal fiber, consider its posture under lateral pressure, mainly the direction of the force. By adjusting the posture of the fiber, you can find the most sensitive direction to the external force, thereby improving the sensitivity of the sensor.

Fifth, when using a wavelength detector or a spectral detector, if the wavelength resolution is high, or the responsivity is high, or the sensitivity is high, the sensitivity of the sensor system can be improved.

Sixth, the improvement of the functions of other signal processing parts of the sensor system is also conducive to improving the technical effect of the sensor.

Seventh, the improvement of the functions of other parts of the sensor system and the improvement of device performance are conducive to improving the technical effect of the sensor system.

So far, the transverse stress (strain) sensing system using the grating-written photonic crystal fiber has been given and introduced. These and other objects and advantages of the invention will become apparent to those skilled in the art upon consideration of the detailed description of the invention and the accompanying drawings. Obviously, those skilled in the art can easily modify, change, change, use and apply the present invention, and all those modifications, changes, changes, uses and applications that do not depart from the essence of the present invention are included in the present invention.

Description of drawings

Figure 1 is a block diagram of a transverse stress transmission system using a photonic crystal fiber written with a grating;

Figure 2 is a block diagram of a transverse stress reflective system using a photonic crystal fiber with a grating;

Fig. 3 The way of introducing the lateral periodic pressure into the long-period grating;

Figure 4 Pressure unit structure;

Figure 5 Triangular period zero birefringence photonic crystal fiber structure;

The zero-birefringence photonic crystal fiber structure of Fig. 6 tetragonal period;

Figure 7. Zero-birefringence photonic crystal fiber structure with honeycomb period;

The mode field distribution of the fundamental mode in the zero birefringence photonic crystal fiber of Fig. 8 triangular period;

Fig. 9 the first high birefringence photonic crystal fiber structure;

Fig. 10 The second high birefringence photonic crystal fiber structure;

Fig. 11 The third high birefringence photonic crystal fiber structure;

Fig. 12 the fourth high birefringence photonic crystal fiber structure;

The mode field distribution of the fundamental mode in the high birefringence photonic crystal fiber corresponding to Fig. 13 and Fig. 10;

Fig. 14 Schematic diagram of force exerted on zero-birefringence photonic crystal fiber;

Fig. 15 Fig. 10 corresponds to a schematic diagram of the stress on the high birefringence photonic crystal fiber;

Fig. 16 The strain distribution of the optical fiber corresponding to Fig. 10 after being stressed along the fast axis direction;

Figure 17 shows the refractive index distribution of the optical fiber corresponding to Figure 10 after being stressed along the fast axis direction;

The relationship between the long-period grating period and the resonant wavelength in the photonic crystal fiber of Fig. 18 triangular period;

Figure 19 Transmission spectrum of long period grating in zero birefringence photonic crystal fiber;

Figure 20 Reflection spectrum and transmission spectrum of fiber Bragg grating in zero-birefringence photonic crystal fiber;

Figure 21 Reflection and transmission spectra of Bragg gratings in high birefringence photonic crystal fibers.

Detailed ways

In order to illustrate the present invention more clearly, the present invention will be further described below in conjunction with examples of implementation and accompanying drawings, but the protection scope of the present invention should not be limited thereby.

Example 1

A long-period grating transverse stress (strain) sensing system is introduced into the photonic crystal fiber, a fiber sensing system for detecting transverse stress and strain, which mainly includes a laser light source, an ordinary single-mode fiber, a polarization controller, a zero-birefringence photonic crystal fiber or High birefringence photonic crystal fiber, long period fiber grating, force unit, photodetector, signal processing and display unit. The output light band of the laser light source should be able to cover the resonant wavelength of the long period grating.

As shown in accompanying drawing 1, the optical fiber in the figure is a photonic crystal fiber with zero birefringence or high birefringence, which is stressed in the pressure unit shown in accompanying drawing 4, forming periodic microbends, 1 is the optical fiber, 2 is the base of the pressure unit, There are periodic grooves, 3 is the upper cover of the pressure unit, and 4 is the position of the optical fiber in the pressure unit; a long-period grating is introduced into the optical fiber in the manner shown in Figure 3. The light is transmitted to the transverse stress detection area through the ordinary single-mode optical fiber, and the light passes through the optical fiber under the action of the transverse stress, and is output to the optical detector (wavelength or spectrum detection) to detect the change of its transmission spectrum or resonance wavelength; after signal sampling and processing , display, and output the graphics and data results of the transverse stress test.

When the photonic crystal fiber is subjected to lateral pressure in a specially designed and manufactured stress unit, periodic microbending occurs, and longitudinal modulation is introduced into the photonic crystal fiber through periodic microbending to form an equivalent long-period grating. Parameters such as the modulation depth of the long-period grating will change with the transverse stress, thereby changing the transmission characteristics of the grating, so the change of the external stress can be detected.

Example 2

Transverse stress (strain) sensing system with Bragg grating written in photonic crystal fiber, a fiber sensing system for detecting transverse stress and strain, mainly includes laser light source, ordinary single-mode fiber, zero birefringence photonic crystal fiber or high birefringence Photonic crystal fiber, Bragg grating, force unit, optical circulator, light detector, signal processing and display unit. The output light band of the laser light source should be able to cover the resonant wavelength of the Bragg grating.

As shown in accompanying drawing 2, the optical fiber in the figure is a photonic crystal fiber with zero birefringence or high birefringence, and a Bragg grating is written therein; the light is transmitted to the transverse stress detection area through a circulator and an ordinary single-mode optical fiber, and the light passes through under the action of the transverse stress Optical fiber; it is reflected to the circulator by the Bragg grating, and output from one port of the circulator to the photodetector (wavelength or spectrum detection) to detect the position change and the wavelength interval of the peak wavelength of the reflection spectrum, and the photodetector detects the two polarization states corresponding to The wavelength interval between the resonant wavelengths changes, and the change is proportional to the transverse stress; after signal sampling, processing, and display, the graphics and data results of the transverse stress detection are output.

Because the change of the inherent birefringence of the high birefringence photonic crystal fiber is related to the direction of the external force, changing the direction of the transverse stress will change the birefringence of the fiber, thereby changing the change of the peak wavelength interval of the reflection spectrum of the Bragg grating written in it. Therefore, by Signal sampling, processing, and display can obtain the results of the magnitude and direction of the transverse stress.

Claims (8)

1. An optical fiber sensing system for detecting transverse stress and strain, mainly including laser light source, common single-mode optical fiber, zero birefringence photonic crystal fiber or high birefringence photonic crystal fiber, Bragg grating, optical circulator, force unit, spectrum Detector, signal processing and display unit, it is characterized in that: the output light of laser light source passes through optical circulator, then couples into zero-birefringence or high-birefringence photonic crystal fiber, and Bragg grating is written on the photonic crystal fiber, and is placed on the stressed In the unit, it is convenient to feel the applied external transverse stress. The signal light reflected from the Bragg grating passes through the optical circulator and then output to the spectrum detector to detect the reflection spectrum, and send the detected information to the signal processing and display unit to obtain the transverse stress. Stress test results. 2. An optical fiber sensing system for detecting transverse stress and strain, mainly including a laser light source, an ordinary single-mode optical fiber, a zero-birefringence photonic crystal fiber or a high-birefringence photonic crystal fiber, a long-period grating, a polarization controller, a force unit, Spectrum detector, signal processing and display unit, it is characterized in that: laser light source output light is coupled into zero birefringence or high birefringence photonic crystal fiber after polarization controller, writes long-period grating on photonic crystal fiber, and is placed on In the stress unit, it is convenient to feel the applied external lateral stress. The spectral detector connected to the output terminal detects the transmission spectrum, and sends the detected information to the signal processing and display unit to obtain the detection result of the lateral stress. 3. The optical fiber sensing system according to claim 2, characterized in that: the longitudinal modulation is introduced in the photonic crystal fiber by transverse periodic microbends to form an equivalent long-period grating. 4. The optical fiber sensing system according to claim 2, characterized in that: when the photonic crystal optical fiber is subjected to lateral pressure in the stressed unit, periodic microbending occurs to form a long-period grating. 5. The optical fiber sensing system according to claim 1, characterized in that: the laser light source output light band covers the resonant wavelength of the Bragg grating. 6. The optical fiber sensing system according to claim 2, characterized in that: the laser light source output light band covers the resonant wavelength of the long period grating. 7. The optical fiber sensing system according to claim 1, characterized in that: an optical circulator is used to couple the optical signal reflected by the Bragg grating to the spectral detector. 8. The optical fiber sensing system according to claim 1 or 2, characterized in that: the spectral detector detects the change in wavelength interval between the resonance wavelengths corresponding to the two polarization states, and the change is proportional to the transverse stress.

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