A wireless passive optical fiber sensing system and method based on laser crystal and an automatic inspection system

Abstract

This application discloses a wireless passive fiber optic sensing system, method, and automatic inspection system based on a laser crystal. The wireless passive fiber optic sensing system includes a light source module, a demodulation module, and a wireless passive fiber optic grating sensing module. The light source module and demodulation module are located at the active end, while the wireless passive fiber optic grating sensing module is located at the passive end. The light source module emits a first pump laser. The wireless passive fiber optic grating sensing module is placed on the object under test and wirelessly transmits both invisible light of a specific wavelength related to the measured physical quantity and an indicator laser in the visible light band back to the demodulation module. The demodulation module receives the reflected light from the wireless passive fiber optic grating sensing module and resolves the wavelength of the light to obtain the magnitude of the measured physical quantity on the wireless passive fiber optic grating sensing module. This application solves the problem of alignment difficulties caused by the invisible optical axis at the passive end by using laser crystal end-face pumping and dual visible light assistance, thereby improving alignment efficiency and reducing the dependence on mechanical adjustment precision.

Description

Technical Field

[0001] This application belongs to the field of fiber optic grating wireless sensing technology, and more specifically, relates to a wireless passive fiber optic sensing system, method and automatic inspection system based on laser crystal. Background Technology

[0002] A fiber grating is a structure created within the core of a single-mode fiber using ultraviolet or femtosecond lasers to produce a periodic refractive index distribution (with a period of approximately several hundred nanometers and a total length of approximately 10 millimeters). Its function is to selectively reflect light of specific wavelengths. For example, a grating capable of reflecting a wavelength of 1550 nm will, when incident with broadband light (such as 1520 nm to 1570 nm), only reflect the 1550 nm wavelength back along its original path, while the remaining wavelengths of laser light pass through the grating with almost no loss and continue propagating along the fiber core.

[0003] The sensing principle of fiber Bragg gratings is based on the shift in their reflected wavelength as external physical quantities change. When the grating is subjected to tension, temperature, or other forces, its refractive index period changes, causing the originally reflected wavelength (e.g., 1550 nm) to shift (e.g., to 1551 nm). By accurately measuring the shift in the reflected wavelength, the magnitude of the physical quantity (such as strain or temperature) causing this change can be deduced. Depending on the packaging design, fiber Bragg gratings can be used to measure various physical parameters such as temperature and stress.

[0004] Figure 1 This demonstrates a typical contact-based connection scheme for traditional fiber Bragg grating temperature sensing systems: multiple fiber Bragg grating sensors are connected in series and attached to the surface of the object being measured (such as a high-voltage electrical cabinet), and then directly connected to a demodulator via fiber optic patch cords. The demodulator integrates a broadband light source, a circulator, and a demodulation module. By analyzing the wavelength shift of the light reflected from the sensors (e.g., from 1550nm to 1551nm), the corresponding temperature change can be calculated.

[0005] The working process of the fiber Bragg grating temperature sensing system is as follows: Broadband light source 1 within demodulator 10 emits broadband light ranging from 1520nm to 1570nm, which enters fiber 4 via circulator 2 and then sequentially passes through temperature sensors 5, 6, and 7 connected in series. Each sensor's grating is designed to reflect different center wavelengths (e.g., 1530nm, 1540nm, and 1550nm respectively). The specific wavelength light signals reflected by each sensor return along the original path and are sent to demodulation module 3 via circulator 2.

[0006] When the temperature at the location of a sensor (such as temperature sensor 5) changes, its grating period changes accordingly, causing a shift in the reflected wavelength (e.g., from 1530nm to 1531nm). The demodulation module 3 can determine the real-time temperature value of the measuring point by detecting the amount of this wavelength change and combining it with the pre-calibrated wavelength-temperature relationship.

[0007] The application of this technology in the power industry presents a physical safety risk: the sensing system requires laying fiber optic cables from the work area to the high-voltage area, and this connection line is permanent. If the fiber optic cable contains metal components (as is the case with most fiber optic cables on the market), high voltage may be conducted to the work area along the metal, leading to a risk of electric shock. Workers are highly likely to mistakenly use such metal-containing fiber optic cables during installation or maintenance, further increasing the safety risk.

[0008] Figure 2 The wireless fixed single-beam fiber optic sensing scheme presented here presents challenges in terms of flexibility and alignment. This scheme transmits signals through two fixed fiber optic collimators (8, 9), but the emitted laser beam is only about 1 mm in diameter and is a single beam, requiring strict coaxial alignment at both ends to establish communication. Due to the extremely small spatial alignment tolerance, the system relies on a high-precision mechanical adjustment mechanism, resulting in complex installation and debugging, and difficulty in moving the system after completion. This makes it unsuitable for mobile inspections or scenarios requiring frequent changes in detection points. Particularly challenging is the fact that the optical axis of the passive fiber optic collimator 9 is invisible, making precise alignment of its 1 mm diameter beam extremely difficult.

[0009] In summary, the existing technology has the following main drawbacks:

[0010] (1) Safety risks: The contact method may mistakenly use optical cables containing metal components to connect the high-voltage area and the work area, which poses a risk of high-voltage breakdown and electric shock to personnel, resulting in poor safety.

[0011] (2) Complex installation and poor flexibility: The optical axis alignment of the wireless fixed solution is extremely difficult, requiring a high-precision mechanical mechanism with small tolerance. The installation and debugging are complicated and lack flexibility, making it unsuitable for mobile scenarios.

[0012] (3) Alignment difficulties: The optical axis of the passive end in the wireless scheme is not visible, which leads to alignment difficulties.

[0013] (4) Short transmission distance: because Figure 2 The two optical axes are difficult to align, which limits the spacing in practical applications to 2 to 5 meters, failing to meet the monitoring needs of the power industry at distances of tens of meters. Summary of the Invention

[0014] In view of the shortcomings of the prior art, the purpose of this application is to provide a wireless passive fiber optic sensing system based on laser crystal, which aims to solve the problem of alignment difficulties caused by the invisible optical axis of the passive end in the prior art.

[0015] This application provides a wireless passive fiber optic sensing system based on laser crystals, including a light source module, a demodulation module, and a wireless passive fiber optic grating sensing module. The light source module and demodulation module are located at the active end, while the wireless passive fiber optic grating sensing module is located at the passive end. The light source module is used to emit a first pump laser. The wireless passive fiber optic grating sensing module is used to be placed on the object under test and wirelessly transmits invisible light of a specific wavelength related to the measured physical quantity back to the demodulation module. The demodulation module is used to receive the reflected light from the wireless passive fiber optic grating sensing module and perform wavelength resolution to obtain the magnitude of the measured physical quantity on the wireless passive fiber optic grating sensing module.

[0016] Furthermore, the wireless passive fiber Bragg grating sensing module includes a laser crystal light-emitting unit, a beam splitter, a fiber Bragg grating sensor, and a fiber collimator. The first end of the beam splitter is connected to the fiber collimator via a single-mode fiber, the second end of the beam splitter is connected to the laser crystal light-emitting unit via a single-mode fiber, and the third end of the beam splitter is connected to the fiber Bragg grating sensor via a single-mode fiber. The laser crystal light-emitting unit generates a second laser beam based on a first pump laser emitted from a pump source. The second laser beam passes through the beam splitter and enters the fiber collimator, and exits from the fiber collimator to form a first spatial beam. When the first spatial beam and the second spatial beam are aligned, broadband light within a specific wavelength range enters the fiber collimator, and the broadband light that reaches the second half of the beam splitter enters the fiber Bragg grating sensor.

[0017] Furthermore, the laser crystal light-emitting unit includes an off-axis parabolic mirror and a laser crystal; the off-axis parabolic mirror is used to focus the first pump laser onto the laser crystal; the laser crystal uses a crystal doped with rare earth element ions as a gain medium, absorbs energy from the first pump laser to cause dysprosium ions to transition from the ground state to the excited state and emit a second laser through a transition from a high energy level to a low energy level.

[0018] Among them, the doped rare earth element ions include praseodymium ions (Pr³). + ), samarium ions (Sm³) + ), terbium ions (Tb³) + ) or dysprosium ions (Dy³ + It is one or a combination of rare earth element ions such as , , and .

[0019] Preferably, when the first pump laser is blue light with a wavelength of 444 nm and the second laser is yellow light with a wavelength of 579.3 nm, the laser crystal is a borate crystal doped with dysprosium ions.

[0020] Furthermore, the laser crystal light-emitting unit also includes a high-reflection mirror, a low-reflection mirror, and a focusing mirror; the high-reflection mirror is used to have high transmittance for the first pump laser and high reflectivity for the second laser, allowing the pump light to enter the laser oscillation cavity efficiently, while reflecting the generated laser photons back to the gain medium for amplification; the low-reflection mirror is used to couple a portion of the laser output from the resonant cavity to form a usable laser beam; the focusing mirror is used to focus the laser beam and allow it to enter the optical fiber.

[0021] Furthermore, the pumping system of the laser crystal adopts an end-face pumping method: the first pump laser can be incident from a wide range of angles, focused by an off-axis parabolic mirror onto the laser crystal, and output as the second laser.

[0022] This application also provides a wireless passive fiber optic sensing method based on laser crystals, including:

[0023] The first pump laser is focused onto the laser crystal to generate the second laser;

[0024] After being split, the second laser enters the fiber collimator and is emitted from the fiber collimator to form the first spatial beam.

[0025] The broadband light and visible light are combined and then passed through a circulator, and then through a single-mode fiber and out of the fiber collimator to form a second spatial beam.

[0026] The visible first spatial beam is aligned with the visible second spatial beam by moving the fiber collimator.

[0027] Furthermore, the laser crystal uses a crystal doped with rare earth element ions as the gain medium. It absorbs energy from the first pump laser, causing the rare earth element ions to transition from the ground state to the excited state and emit a second laser through a transition from a high energy level to a low energy level.

[0028] Furthermore, the method for aligning the first spatial beam with the second spatial beam is as follows:

[0029] (1) Acquire images that show the fiber collimator and the laser emitted to the fiber collimator;

[0030] (2) Determine whether the H, S, and V of each pixel in the image are within the laser reference range, identify the pixels within the laser reference range as the target laser, and determine the position of the target laser in the image;

[0031] Among them, the red laser reference range (650nm): H∈[0°,10°]∪[350°,360°], S>50, V>200; the yellow laser reference range (579.3nm): H∈[30°,45°], S>50, V>200.

[0032] (3) Find the contour features in the image that correspond to the standard contour of the fiber optic collimator, and determine the position of the fiber optic collimator in the image based on the found contour features that correspond to the standard contour of the fiber optic collimator.

[0033] (4) Detect whether the center of the target laser in the image coincides with the center of the fiber collimator in the image; if yes, the fiber collimator and the laser emitted to the fiber collimator are aligned; if no, adjust the position of the fiber collimator or adjust the position of the laser emitted to the fiber collimator, and proceed to step (1).

[0034] In this application, the active end is not directly connected to the passive end optical fiber. Instead, a pump laser excites the passive end laser crystal to generate a visible light signal to assist in alignment. Wireless sensing is then achieved through broadband light and fiber optic grating reflection. It supports wireless sensing over distances of 2-50 meters or even longer, making it suitable for high-risk environments such as power and chemical industries.

[0035] This application also provides an automated inspection system based on the aforementioned wireless passive fiber optic sensing system, comprising: a light source module, a demodulation module, a wireless passive fiber optic grating sensing module, a camera, a two-axis galvanometer module, a robotic arm, and an AGV (Automated Guided Vehicle). The AGV (Automated Guided Vehicle) has a wireless passive fiber Bragg grating (FBG) sensing module mounted on the object under test. A light source module, demodulation module, camera, two-axis galvanometer module, and robotic arm are all mounted on the AGV. During operation, after the AGV stops, the camera identifies the position of the FBG sensing module. The two-axis galvanometer module deflects the first pump laser beam emitted from the light source module and directs it into the FBG sensing module. The FBG sensing module wirelessly transmits a second laser beam—a specific wavelength of invisible light related to the measured physical quantity—along with a second laser beam in the visible light band. The robotic arm moves to align the fiber collimator at its end with this beam until the second spatial beam emitted from the collimator coincides with the first spatial beam from the FBG sensing module, reaching the demodulation module. The demodulation module analyzes the wavelength of the light and demodulates the physical values ​​at each grating sensor to obtain the magnitude of the measured physical quantity on the FBG sensing module.

[0036] In this application, an AGV, drone, or robot is used to carry the active end. The pump light direction is adjusted by a galvanometer, and the passive end yellow laser is identified by a camera. The robotic arm automatically completes the alignment of the fiber collimator and collects sensor data.

[0037] This application has significant advantages over traditional solutions:

[0038] (1) Large alignment tolerance: This application solves the problem of alignment difficulty caused by the invisibility of the optical axis of the passive end by using laser crystal end face pumping (pump light can be incident from multiple angles with extremely large angle tolerance) and dual visible light (579.3nm yellow light and 650nm red light) assistance, thereby improving alignment efficiency; it also reduces the dependence on mechanical adjustment accuracy, and the AGV parking accuracy (±10mm) does not affect its use.

[0039] (2) High safety: This application adopts a wireless passive design, avoiding the risk of high-voltage electric shock, and is suitable for high-risk scenarios such as power. The passive end (installed on the high-voltage side) does not require power supply, batteries, or any electronic circuits. Its light source energy is provided entirely by the active end in the air. This fundamentally eliminates the risks of electric shock, sparks, lightning strikes, etc., which is an essential safety advantage compared to any "wireless" solution that requires local power supply (such as Zigbee, Wi-Fi sensing).

[0040] (3) High flexibility: This application eliminates the physical fiber optic connection and can be equipped with AGVs and robots to achieve automatic inspection. It supports long-distance sensing of 2 meters to 50 meters (the traditional wireless solution is only 2 meters to 5 meters).

[0041] (4) Easy installation and maintenance: This application does not require a complex and precise mechanical adjustment mechanism, which reduces the difficulty of deployment and debugging, and improves the system reliability and scenario adaptability. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the structure of a traditional fiber Bragg grating temperature sensing system;

[0043] Figure 2 This is a schematic diagram of the structure of a wireless fixed single-beam fiber optic sensing system provided by existing technology.

[0044] Figure 3 This is a schematic diagram of the structure of the wireless passive fiber optic sensing system provided in this application;

[0045] Figure 4 This is a schematic diagram of the structure of the laser crystal light-emitting unit provided in this application;

[0046] Figure 5 This is a schematic diagram of the remote air-isolated end-face pump structure provided in this application;

[0047] Figure 6 This is a schematic diagram of the wireless passive fiber optic sensing system provided in this application applied to an intelligent vehicle to achieve automatic inspection. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0049] Regarding the third point in the background technology, "the optical axis of the passive end is not visible, which leads to alignment difficulties," to make it easier to understand, let's take an example: Suppose two people, A and B, are 50 meters apart, each holding a flashlight. Now, the axes of the two flashlights need to be aligned in the air to achieve information exchange.

[0050] Scenario 1: If neither of the two people turns on their flashlights, the probability of the two flashlights, which are 50 meters apart, aligning their axes is almost zero.

[0051] Scenario 2: If both people have their flashlights on, it is easy for them to align the axes of the flashlights by aligning the two beams.

[0052] Scenario 3: If one of the people, say person B, is standing in an area that is flammable and explosive and cannot carry batteries, then their flashlight will not have a beam. If only person A's flashlight has a beam, it will be very difficult for A and B to align the axes of their flashlights. This situation is what was mentioned earlier as "in the wireless solution, the optical axis of the passive end is not visible, which leads to alignment difficulties".

[0053] Scenario 4: If person B is standing in a flammable and explosive area and their flashlight has no batteries, but person A uses a device to remotely transmit energy to person B, turning on person B's light and giving person B's flashlight a beam of light, then the two flashlights can be easily aligned.

[0054] To solve the problem of "the optical axis of the passive end being invisible, leading to alignment difficulties" mentioned above, this application adopts the fourth solution: that is, energy is transmitted from the active end to light up the "lamp" of the passive end, the light of the "lamp" is coupled into the optical fiber of the passive end, and then emitted from the optical fiber collimator 9. In this way, the difficulty of aligning the two is greatly reduced when there are beams in both the optical fiber collimator 8 and the optical fiber collimator 9.

[0055] The common scenario of remotely lighting a "lamp" is when sunlight shines on a photovoltaic panel, which then converts the light energy into electrical energy to power the lamp. However, the light emitted by ordinary lamps scatters in all directions, making it difficult to couple into single-mode optical fiber. To couple into single-mode fiber, highly directional light, such as laser light, is required. Laser crystals are a perfect "lamp" that fits this application perfectly.

[0056] A laser crystal is a core gain medium used to generate laser light, typically composed of doped active ions (such as Nd³⁺). + Yb³ +Ti³ + Lasers are composed of matrix crystals (such as YAG, YVO4, BeAl2O4, etc.). Their working principle involves amplifying the optical signal through stimulated emission, thereby outputting a high-brightness, highly monochromatic, and highly directional laser. Common laser crystals include Nd:YAG (neodymium-doped yttrium aluminum garnet), widely used in industry and medicine; Yb:YAG, suitable for high-efficiency lasers; and Ti:Sapphire (titanium sapphire), used in ultrafast lasers. Their performance depends on the activating ions, matrix crystal characteristics, and doping concentration. In lasers, the crystal absorbs energy through optical pumping and is a key material for laser generation.

[0057] The pumping methods for laser crystals mainly include the following:

[0058] (1) End Pumping: The pump light enters the crystal perpendicularly or obliquely from one end face. The energy is concentrated and the efficiency is high. It is suitable for outputting high-power, high-beam-quality lasers and is often used in small solid-state lasers.

[0059] (2) Side Pumping: The pump light is irradiated from multiple directions on the side of the crystal. Generally, multiple LD arrays are used to surround the crystal. It is suitable for high power output, but the beam quality is slightly weaker and the heat dissipation requirements are higher.

[0060] (3) Back-pumping: The pump light is incident from the back of the crystal, which is usually used in special optical structure designs.

[0061] (4) Fiber-coupled pumping: The pump light is introduced through the fiber, which has high coupling efficiency and is convenient for system integration.

[0062] The above methods can be selected based on the laser's power, beam quality, and system structure requirements.

[0063] This application provides a wireless passive fiber optic grating sensing device and method that requires no physical contact, has a large spatial alignment tolerance, and enables visualization of the passive optical path. It can solve the technical problems existing in the prior art, such as the risk of electric shock, difficulty in alignment, short wireless sensing distance (only 2 to 5 meters), complex debugging, and inability to move and inspect.

[0064] This application uses a laser crystal as a passive "lamp". For ease of explanation, the laser crystal specifically used is a dysprosium ion (Dy³) laser. +Doped borate crystals, abbreviated as Dy:GMB crystals, were used to pump a 444nm blue LD and construct a plano-cavity laser using the Dy:GMB crystal, resulting in a yellow laser with a wavelength of 579.3nm. The maximum output power was 161mW, and the slant efficiency was 3.7%. Compared to traditional nonlinear frequency doubling for visible light lasers, this method utilizes rare-earth element praseodymium ions (Pr³⁺). + ), samarium ions (Sm³) + ), terbium ions (Tb³) + ), Dysprosium ions (Dy³) + Laser crystals that directly output visible light are prepared by doping with substances such as Dy³. For yellow-light band lasers, the crystal is doped with Dy³. + More suitable. Using Dy³ prepared by the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. + A laser with a wavelength of 579.3nm was obtained by using a GMB crystal and pumping it with a 444nm blue LD to build a flat-cavity laser.

[0065] To suit the application scenarios of this application, the laser crystal employs an improved end-face pumping method, with the specific structure shown below: a 444nm blue pump laser is emitted from tens of meters away towards a large-aperture (5mm~300mm) off-axis parabolic mirror, the diameter of which can be customized, such as 100mm. The off-axis parabolic mirror can focus the incoming parallel beam to its own focal point. One end of a laser crystal with a diameter of 3mm and a length of 20mm is placed at the focal point of the off-axis parabolic mirror. After the pump laser is focused onto the crystal, it causes the activated particles in the crystal to achieve a population inversion distribution. A resonant cavity is constructed from a high-reflection mirror and a low-reflection mirror (the high-reflection mirror also serves as the pump light input mirror, with coating parameters for anti-reflection and anti-reflection at 444nm wavelength, achieving a measured transmittance of 96% at 444nm; and high reflectivity at 579nm wavelength, with a reflectivity ≥99.8%). The low-reflection mirror is the output coupling mirror, with a coated mirror that has a transmittance of 1%~2% at 579nm wavelength). A laser crystal in a population-inverted state generates stimulated emission of light within the resonant cavity, causing photons to oscillate back and forth. The low-reflection mirror outputs a portion of the laser light, which is then focused by a focusing lens and coupled into a single-mode fiber, ultimately achieving the output and transmission of 579.3nm yellow laser light.

[0066] This application employs visual alignment, which greatly reduces the difficulty of alignment. Specifically, the passive end emits a 579.3nm yellow laser (visible), and the active end emits a 650nm red light (visible). Both are visible beams, which are easy to identify with the naked eye or a camera, significantly reducing the difficulty of alignment.

[0067] The laser crystal-based wireless passive fiber optic sensing system provided in this application adds a laser crystal-based "lamp" to the traditional coaxial transmit-receive optical path between two fiber collimators to generate a visible beam for the passive end, facilitating alignment. The structure of this laser crystal-based wireless passive fiber optic sensing system is as follows: Figure 3 As shown, specifically, the wireless passive fiber optic sensing system based on laser crystals includes: a light source module 501, a demodulation module 502, and a wireless passive fiber optic grating sensing module 503. The light source module 501 and demodulation module 502 are located at the active end, while the wireless passive fiber optic grating sensing module 503 is located at the passive end. The light source module 501 is used to emit and transmit pump laser light. The wireless passive fiber optic grating sensing module 503 is used to be placed on the object under test and wirelessly transmit light of a specific wavelength related to the measured physical quantity back to the demodulation module 502 (to achieve wireless sensing of the measured physical quantity). The demodulation module 502 is used to receive the reflected light from the module 503 and analyze the light wavelength through the wavelength demodulator built into the module 502, thereby knowing the change in the light wavelength, that is, obtaining the magnitude of the measured physical quantity on the module 503.

[0068] The light source module 501 includes a pump light source 21, which is used to emit pump laser and form a spatial beam 22.

[0069] The demodulation module 502 includes a single-mode fiber 33, a circulator 34, a single-mode fiber 35, a wavelength division multiplexer 36, a light source 37, a visible light source 38, a fiber optic grating demodulation module 39, and a single-mode fiber 40. The light source 37 is used to emit broadband light in the wavelength range of 1520~1570nm. The visible light source 38 is used to emit 650nm visible light. The two beams of light are combined by the wavelength division multiplexer 36, enter the single-mode fiber 35, pass through the circulator 34, enter the single-mode fiber 33, and are emitted from the fiber collimator 32 to form a spatial beam 31, in which the 650nm visible light portion can be seen by the human eye. The fiber optic grating demodulation module 39 is used to demodulate and analyze the wavelength of the returned light to determine the magnitude of the measured physical quantity.

[0070] The wireless passive fiber Bragg grating sensing module 503 includes a laser crystal light-emitting unit 23, a single-mode fiber 24, a beam splitter 25, a single-mode fiber 26, a fiber Bragg grating sensor 27, a single-mode fiber 28, and a fiber collimator 29. The first end of the beam splitter 25 is connected to one end of the single-mode fiber 28, the second end of the beam splitter 25 is connected to one end of the single-mode fiber 24, the third end of the beam splitter 25 is connected to one end of the single-mode fiber 26, the other end of the single-mode fiber 28 is connected to the fiber collimator 29, the other end of the single-mode fiber 24 is connected to the laser crystal light-emitting unit 23, and the other end of the single-mode fiber 26 is connected to the fiber Bragg grating sensor 27. The laser crystal light-emitting unit 23... 3. A yellow laser is generated from the blue pump laser emitted by the pump source 21. The yellow laser travels along the single-mode fiber 24, then passes through the beam splitter 25 and enters the single-mode fiber 28. It is emitted from the fiber collimator 29 to form a spatial beam 30. When the beam 30 emitted from the fiber collimator 29 and the beam 31 emitted from the fiber collimator 32 are aligned, broadband light in the wavelength range of 1520nm~1570nm enters the fiber collimator 29 from the fiber collimator 32, travels along the fiber 28 to the beam splitter 25, and then half of the broadband light enters the single-mode fiber 26. The single-mode fiber 26 is equipped with encapsulated fiber optic grating sensors 27, 271, and 272.

[0071] The working process of the wireless passive fiber optic sensing system based on laser crystal provided in this application is as follows:

[0072] Pump source 21 emits a blue pump laser with a wavelength of 444nm, forming a spatial beam 22 with a diameter of 3mm. The spatial beam 22 enters the laser crystal light-emitting unit 23 to produce yellow laser. Since the pump laser is reflected onto the laser crystal from different angles to the off-axis parabolic mirror, thus producing yellow laser, the alignment tolerance of the laser crystal light-emitting unit 23 is very large. The yellow laser travels along the single-mode fiber 24, then passes through a beam splitter 25 with a splitting ratio of 50% / 50% and enters the single-mode fiber 28, and then exits from the fiber collimator 29 to form a spatial beam 30.

[0073] Broadband light in the wavelength range of 1520~1570nm is emitted from light source 37; 650nm visible light is emitted from visible light source 38; the two beams are combined by wavelength division multiplexer 36, enter single-mode fiber 35, pass through circulator 34, enter single-mode fiber 33, and exit from fiber collimator 32 to form a second spatial beam 31, the 650nm visible light portion of which is visible to the human eye. Because both the first spatial beam 30 and the second spatial beam 31 are visible beams, the fiber collimator 32 can be easily moved until beams 30 and 31 are aligned. After alignment, broadband light in the wavelength range of 1520nm~1570nm enters fiber collimator 29 from fiber collimator 32, travels along fiber 28 to splitter 25, and then half of the broadband light enters single-mode fiber 26. Single-mode fiber 26 has encapsulated fiber Bragg grating sensors 27, 271, and 272. For example, sensor 27 reflects a wavelength of 1530nm, sensor 271 reflects a wavelength of 1540nm, and sensor 272 reflects a wavelength of 1550nm. This light of three wavelengths is reflected back to splitter 25 by the fiber Bragg gratings at the sensors, with one path traveling through single-mode fiber 28 to fiber collimator 29.

[0074] Three wavelengths of light are emitted from fiber collimator 29 and then return to fiber collimator 32. The three wavelengths of light plus the yellow laser light enter the single-mode fiber 33 from fiber collimator 32, pass through circulator 34 and then enter the single-mode fiber 40, and are transmitted to fiber optic demodulation module 39. The demodulation module identifies the three wavelength values ​​and can deduce the physical values ​​to be measured at fiber optic sensors 27, 271 and 272, such as temperature values, based on the pre-calibrated values.

[0075] This application innovatively proposes a design for "lighting" from a distance: relative to... Figure 2 The original spatial optical alignment scheme based on two fiber collimators in single-mode fiber: aligning two beams with a diameter of 1 mm in space requires 4-axis adjustment. This is mainly because the optical axis of the passive end is invisible, making alignment extremely difficult. Therefore, a direct approach was adopted... Figure 2 The existing wireless sensing solutions are essentially immobile, requiring only alignment adjustments and fixing in place, offering no flexibility whatsoever. Furthermore, in practice, the spacing between two fiber optic collimators is typically only a few millimeters, resulting in a very short wireless range. This application, however, utilizes a laser crystal as a passive "lamp," employing Dy³... +A borate-doped crystal (Dy:GMB) is used as the laser medium. Pumped by a 444nm blue laser LD, it generates a 579.3nm yellow laser, serving as the visible light source for the passive end (the end under test), making the optical axis of the fiber collimator visible. This laser crystal employs a plano-cavity laser structure, allowing the pump light to enter from multiple angles. After reflection by an off-axis parabolic mirror, the light is focused onto the laser crystal, achieving high-tolerance pump alignment (no strict limitation on the active end position is required; the pump laser emission source can still operate with an offset of + / -100 mm, as long as the pump laser is directed towards a large-aperture off-axis parabolic mirror).

[0076] In this application, the structure of the laser crystal light-emitting unit 23 is as follows: Figure 4 As shown, it includes: an off-axis parabolic mirror, a laser crystal, a high-reflection mirror, a low-reflection mirror, and a focusing mirror. The off-axis parabolic mirror is used to focus the pump laser emitted from a distant point onto the laser crystal. The laser crystal, at its core, utilizes doped dysprosium ions (Dy³⁺). + Gadolinium magnesium borate crystals (Dy³) + Using GMB as the gain medium, this crystal exhibits strong absorption of blue light near 444 nm (absorption efficiency up to 55%). After absorbing energy, Dy³ + Ions transition from the ground state to the excited state, ultimately emitting 579.3 nm yellow light through a transition from a high energy level to a low energy level. The high-reflectivity mirror and low-reflectivity mirror consist of two plane mirrors. The high-reflectivity mirror has high transmittance (96%) for the 444 nm pump light and high reflectivity (≥99.8%) for the 579 nm laser. Its function is to allow the pump light to efficiently enter the laser oscillation cavity while reflecting the generated laser photons back to the gain medium for amplification. The low-reflectivity mirror has a transmittance of 1%~2% for the 579 nm laser. Its function is to couple a portion of the laser output from the resonant cavity to form a usable laser beam. The laser beam is focused by a focusing lens and enters the optical fiber.

[0077] This application innovatively employs a remote, air-pumped end-face method for long-distance wireless sensing applications, which is significantly different from the short-range end-face pumping design commonly found in traditional lasers. To ensure that the pump light can still effectively excite the passive laser crystal after traveling tens of meters, we have made targeted improvements to the crystal pumping system. The core of this improvement lies in using a large-aperture off-axis parabolic mirror for focusing, achieving high-tolerance end-face pumping, such as... Figure 5As shown, the following explains why pumping can be achieved with high tolerance: For example, a large-aperture off-axis parabolic mirror has an aperture of 100 mm and a focal length of 150 mm. When the pump laser is emitted from 15 meters away, the distance of 15000 mm / the focal length of 150 mm is 100 times the amplitude. Since the crystal diameter is 3 mm, even if the pump source is offset by 3 * 100 mm = 300 mm at the far end, the pump laser will still be absorbed by the laser crystal after being focused by the off-axis parabolic mirror. This design allows the pump light to act on the laser crystal (such as a Dy:GMB crystal) from multiple angles and over a wide range, rather than relying on strict end-face precision incidence. Therefore, this structure has a large tolerance for the incident position and angle of the pump light, significantly reducing the alignment difficulty.

[0078] The laser crystal-based wireless passive fiber optic sensing system provided in this application can be installed on intelligent vehicles (such as AGVs, drones, robotic dogs, and other robots) to achieve automatic inspection. Figure 6 As shown, the specific working process of automatic inspection is illustrated below, taking AGV as an example:

[0079] A wireless passive fiber Bragg grating sensor module 503 is installed on the object to be measured. For example, if there are three high-voltage switchgear cabinets in a substation that require temperature measurement, three wireless passive fiber Bragg grating sensor modules 503 are installed on these three switchgear cabinets respectively, with each grating sensor attached to the point where temperature measurement is required. An AGV (Automated Guided Vehicle) 507 is set as a stopping point in front of each switchgear cabinet, typically 2-50 meters away from the wireless passive fiber Bragg grating sensor module 503. When inspection is required, the AGV 507 stops at this point. The industry standard for AGV 507 parking positioning accuracy is generally + / -10mm, plus an angular offset of + / -1°. At this accuracy, achieving alignment of two 1mm beams with the passive optical axis invisible using a traditional two-fiber collimator scheme is very difficult.

[0080] After the AGV 507 comes to a stop, the camera 504 identifies the position of the wireless passive fiber optic grating sensing module 503. Then, the two-axis galvanometer module 505 deflects the 444nm visible blue pump light beam emitted from the blue light source 21 and directs it into the laser crystal light-emitting unit 23 of the wireless passive fiber optic grating sensing module 503. As mentioned earlier, the pump laser can be reflected onto the laser crystal from different angles when it is directed at the off-axis parabolic mirror, thus generating yellow laser light. Therefore, the alignment tolerance of the laser crystal light-emitting unit 23 is very large, so the stopping accuracy of the AGV does not affect the reception of the laser crystal.

[0081] After the light beam reaches each grating, the corresponding wavelength of light is reflected back and finally directed towards the AGV vehicle through the fiber collimator of the wireless passive fiber optic grating sensing module 503. Because the light beam is a visible yellow laser with a wavelength of 579.3nm, its spatial position is easily identifiable. The moving robotic arm 506 aligns the fiber collimator 32 installed at the end of the robotic arm 506 with the light beam until the red light beam emitted from the fiber collimator 32 coincides with the yellow light beam from the wireless passive fiber optic grating sensing module 503. The light beam from the wireless passive fiber optic grating sensing module 503 enters through the fiber collimator 32 and reaches the demodulation module. The demodulation module can demodulate the physical values ​​at each grating sensor, such as temperature values.

[0082] The wireless passive fiber optic sensing system provided in this application is used in the process of automatic inspection of embodied intelligent vehicles. The visual recognition and positioning of dual laser spots (579.3nm yellow laser and 650nm red laser) is the core and key step of automatic alignment. Compared with conventional workshop environments, automatic inspection scenarios have problems such as large fluctuations in background lighting and complex interference sources (such as reflections from high-voltage electrical cabinets, equipment coatings, and strong ambient light), which place extremely high demands on the robustness of the visual recognition algorithm.

[0083] However, the two commonly used visual recognition algorithms have significant limitations in the automatic inspection scenario of this application, as follows:

[0084] (1) Threshold segmentation method after converting RGB color image to grayscale image

[0085] The fundamental flaw of this method is its lack of color-dimensional filtering capability; it can only perform single-dimensional segmentation based on brightness (weighted grayscale values ​​in a grayscale image, calculated using the formula below). It is easily affected by background interference and cannot distinguish between different colored light spots. Its core process involves converting the RGB image to a grayscale image, and then extracting the bright areas using a fixed or adaptive threshold. The grayscale value calculation formula is as follows: Gray =0.299R+0.587G+0.114B; Under this calculation logic, laser spots of different colors and background interference may have similar grayscale values, causing confusion between the target spot and the interference area. Taking the dual laser spot and white wall reflection commonly seen in automatic inspection scenarios of this application as an example: the grayscale value of high-brightness yellow laser (R=220,G=220,B=0) ≈0.299×220+0.587×220=194.9; the grayscale value of high-brightness red laser (R=220,G=0,B=0) ≈0.299×220≈65.8; the grayscale value of medium-brightness white wall reflection (R=120,G=120,B=120) =0.299×120+0.587×120+0.114×120=120. It is evident that the grayscale value of the reflection from the white wall falls between that of the red and yellow laser spots, making precise differentiation impossible.

[0086] (2) Deep learning algorithms (such as YOLO series algorithms) Deep learning algorithms are suitable for the rapid identification of irregular objects, but they face challenges such as high engineering application costs and weak scene adaptability in the automatic inspection scenario of this application. Such algorithms need to complete model training based on a large number of labeled samples, while automatic inspection scenarios have diverse environments (such as significant differences in equipment layout and lighting conditions in different substations). If samples are collected and models are trained again for each type of scenario, it will generate extremely high manpower and time costs, making it difficult to meet the flexible deployment requirements of industrial-grade automatic inspection.

[0087] To address the aforementioned issues, and considering the core characteristics of the dual laser spot in this application—① high relative brightness, making it easy for pixels to reach saturation after imaging; ② strong monochromaticity, with color purity far exceeding that of background interference sources—

[0088] This application proposes a multi-channel joint threshold segmentation algorithm based on the HSV color space to achieve accurate identification of dual laser spots in complex scenes. The specific principle and process are as follows:

[0089] The RGB format image captured by the camera is converted pixel by pixel to the HSV color space. The HSV color space decomposes color attributes into three independent dimensions: hue (H), saturation (S), and lightness (V). The conversion process uses a single pixel as the basic unit. The calculation logic for lightness (V) is as follows: V = max (R,G,B) / 255; This parameter only reflects the brightness characteristics of the pixel and is completely unrelated to the color attribute; Hue (H) reflects the color category of the pixel, and Saturation (S) reflects the purity of the color.

[0090] Based on the spectral characteristics of the dual laser spots in this application, a precise channel threshold range is set:

[0091] Red laser (650nm): H∈[0°,10°]∪[350°,360°], S>50, V>200;

[0092] Yellow laser (579.3nm): H∈[30°,45°], S>50, V>200.

[0093] Precise extraction of dual laser spots is achieved through joint screening using H+S+V three channels:

[0094] (1) Use the H channel to lock the hue range of red and yellow lasers and eliminate background interference from other colors;

[0095] (2) Use the S channel to filter low-saturation background noise (such as white wall reflections and equipment diffuse reflections).

[0096] (3) Use the V channel to extract the bright spot area, and combine the Otsu method (OTSU) adaptive threshold algorithm to adjust the threshold parameters according to the real-time brightness distribution of the image to adapt to the lighting fluctuation scene.

[0097] This algorithm, through a dual filtering mechanism of "color + brightness," can completely distinguish red and yellow laser spots from background interference sources. Even in automatic inspection scenarios with varying lighting conditions and complex interference, it can maintain extremely high recognition accuracy and robustness, providing a reliable visual positioning basis for precise beam alignment. This sensing system includes fiber optic collimator 32 and fiber optic collimator 29. Fiber optic collimator 32 transmits red light, and fiber optic collimator 29 receives the red light transmitted from fiber optic collimator 32; fiber optic collimator 29 transmits yellow light, and fiber optic collimator 32 receives the yellow light transmitted from fiber optic collimator 29.

[0098] The embodied intelligent vehicle control system of this application stores the standard profile features of the fiber optic collimator 32 and the fiber optic collimator 29. By comparing the standard profile features of the fiber optic collimators on an image, the presence of identical profile features indicates the existence of a corresponding fiber optic collimator on the image.

[0099] In this application, the specific method for aligning and adjusting the second spatial beam 31 emitted by the fiber optic collimator 32 and the first spatial beam 30 emitted by the fiber optic collimator 29 is as follows:

[0100] (1) Acquire an image that shows the fiber collimator and the laser emitted to the fiber collimator;

[0101] (2) Determine whether the H, S, and V of each pixel in the image are within the laser reference range. Pixels within the laser reference range are identified as the target laser, and the position of the target laser in the image is determined.

[0102] Among them, the red laser reference range (650nm): H∈[0°, 10°]∪[350°, 360°], S>50, V>200; the yellow laser reference range (579.3nm): H∈[30°, 45°], S>50, V>200.

[0103] (3) Find the contour features in the image that correspond to the standard contour of the fiber optic collimator, and determine the position of the fiber optic collimator in the image based on the found contour features that correspond to the standard contour of the fiber optic collimator.

[0104] (4) Detect whether the center of the target laser in the image coincides with the center of the fiber collimator in the image; if yes, the fiber collimator and the laser emitted to the fiber collimator are aligned; if no, adjust the position of the fiber collimator or adjust the position of the laser emitted to the fiber collimator, and proceed to step (1).

[0105] In the sensing system of this application, sensing and detection are performed only after all fiber optic collimators are aligned with the laser emitted to the fiber optic collimators.

[0106] In this embodiment, the fiber optic collimator 32 needs to be aligned with the yellow laser emitted by the fiber optic collimator 29, and the fiber optic collimator 29 needs to be aligned with the red light emitted by the fiber optic collimator 32.

[0107] In this application's sensing system, after all fiber optic collimators and the laser emitted to them are aligned, the control light source 37 emits broadband light in the wavelength range of 1520nm to 1570nm. This broadband light enters fiber optic collimator 29 from fiber optic collimator 32, travels along fiber optic 28 to beam splitter 25, and then half of the broadband light enters single-mode fiber 26. The fiber optic grating sensors 27, 271, and 272 on the single-mode fiber 26 reflect the corresponding wavelengths. The reflected beams then enter fiber optic collimator 32 and reach the demodulation module. The demodulation module can demodulate the physical values ​​at each grating sensor, such as temperature values.

[0108] In summary, this application has several advantages over the prior art (CN120063345B):

[0109] (1) Lower alignment difficulty: Existing technologies require the acquisition of three-dimensional point clouds of the metal disk and the first fiber collimator through a 3D industrial camera, the calculation of the optical axis position using a visual matching algorithm, and then the alignment of the second fiber collimator with the first fiber collimator through a two-dimensional linear module and a dual-axis galvanometer, which is a complex process. In contrast, this application uses a laser crystal as a passive end "lamp" to generate a 579.3nm yellow laser, and the active end emits a 650nm red light, both of which are visible beams and are easy to identify with the naked eye or a camera, greatly reducing the alignment difficulty.

[0110] (2) Longer sensing distance: Existing 3D industrial cameras are expensive and have limited working distances, typically only 2 to 5 meters, with the price increasing further and accuracy decreasing. Therefore, the patented solutions can only provide wireless fiber optic sensing solutions with a range of 2 to 5 meters, which is limited in distance. This application supports wireless sensing at distances of 2 to 50 meters or even longer, making it suitable for high-risk environments such as power and chemical industries, and meeting the need for sensing at greater distances.

[0111] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A wireless passive fiber optic sensing system based on laser crystals, characterized in that, It includes a light source module (501), a demodulation module (502), and a wireless passive fiber Bragg grating sensing module (503); the light source module (501) and the demodulation module (502) are located at the active end, and the wireless passive fiber Bragg grating sensing module (503) is located at the passive end; The light source module (501) is used to emit the first pump laser (22); The wireless passive fiber optic grating sensing module (503) is used to be placed on the object being measured and to wirelessly transmit invisible light of a specific wavelength related to the measured physical quantity back to the demodulation module (502). The demodulation module (502) is used to receive the reflected light from the wireless passive fiber optic grating sensing module (503) and to analyze the wavelength of the light, thereby obtaining the magnitude of the measured physical quantity on the wireless passive fiber optic grating sensing module (503). The wireless passive fiber optic grating sensing module (503) includes a laser crystal light-emitting unit (23), a beam splitter (25), a fiber optic grating sensor (27), and a fiber collimator (29). The first end of the beam splitter (25) is connected to the fiber collimator (29) via a single-mode fiber, the second end of the beam splitter (25) is connected to the laser crystal light-emitting unit (23) via a single-mode fiber, and the third end of the beam splitter (25) is connected to the fiber grating sensor (27) via a single-mode fiber. The laser crystal light-emitting unit (23) generates a second laser based on the first pump laser emitted by the pump light source (21). The second laser passes through the beam splitter (25) and enters the fiber collimator (29), and is emitted from the fiber collimator (29) to form a first spatial beam (30). When the first spatial beam (30) and the second spatial beam (31) are aligned, broadband light within a specific wavelength range enters the fiber collimator (29) and the second half of the broadband light reaches the splitter (25) and enters the fiber optic grating sensor (27).

2. The wireless passive fiber optic sensing system as described in claim 1, characterized in that, The laser crystal light-emitting unit (23) includes an off-axis parabolic reflector and a laser crystal; The off-axis parabolic mirror is used to focus the first pump laser onto the laser crystal; The laser crystal uses a crystal doped with rare earth element ions as a gain medium. It absorbs energy from the first pump laser, causing the rare earth element ions to transition from the ground state to the excited state and emit a second laser through a transition from a high energy level to a low energy level.

3. The wireless passive fiber optic sensing system as described in claim 2, characterized in that, Doped rare earth element ions include praseodymium ions (Pr³). + ), samarium ions (Sm³) + ), terbium ions (Tb³) + ) or dysprosium ions (Dy³ + One or more of the following combinations; When the first pump laser is blue light with a wavelength of 444nm and the second laser is yellow light with a wavelength of 579.3nm, the laser crystal is a borate crystal doped with dysprosium ions.

4. The wireless passive fiber optic sensing system as described in claim 2, characterized in that, The laser crystal light-emitting unit (23) also includes a high-reflection mirror, a low-reflection mirror, and a focusing mirror; The high-reflectivity mirror is used to have high transmittance to the first pump laser and high reflectivity to the second laser, allowing the pump light to enter the laser oscillation cavity efficiently, while reflecting the generated laser photons back to the gain medium for amplification. The low-reflection mirror is used to couple out a portion of the laser from the resonant cavity and form a usable laser beam. The focusing lens is used to focus the laser beam and guide it into the optical fiber.

5. The wireless passive fiber optic sensing system as described in claim 2, characterized in that, The pumping system of the laser crystal adopts an end-face pumping method: the first pump laser can be incident from a wide range of angles, focused by an off-axis parabolic mirror onto the laser crystal, and output as the second laser.

6. A wireless passive fiber optic sensing method based on the wireless passive fiber optic sensing system according to any one of claims 1-5, characterized in that, include: The first pump laser is focused onto the laser crystal to generate the second laser; After being split, the second laser enters the fiber collimator and is emitted from the fiber collimator to form the first spatial beam. The broadband light and visible light are combined and then passed through a circulator, and then through a single-mode fiber and out of the fiber collimator to form a second spatial beam. The visible first spatial beam is aligned with the visible second spatial beam by moving the fiber collimator.

7. The wireless passive fiber optic sensing method as described in claim 6, characterized in that, The laser crystal uses a crystal doped with rare earth element ions as a gain medium. It absorbs energy from the first pump laser, causing dysprosium ions to transition from the ground state to the excited state and emit a second laser through a transition from a high energy level to a low energy level.

8. The wireless passive fiber optic sensing method as described in claim 6, characterized in that, The method for aligning the first spatial beam with the second spatial beam is as follows: (1) Acquire images that show the fiber collimator and the laser emitted to the fiber collimator; (2) Determine whether the H, S, and V of each pixel in the image are within the laser reference range, identify the pixels within the laser reference range as the target laser, and determine the position of the target laser in the image; (3) Find the contour features in the image that correspond to the standard contour of the fiber optic collimator, and determine the position of the fiber optic collimator in the image based on the found contour features that correspond to the standard contour of the fiber optic collimator. (4) Detect whether the center of the target laser in the image coincides with the center of the fiber collimator in the image; if yes, the fiber collimator and the laser emitted to the fiber collimator are aligned; if no, adjust the position of the fiber collimator or adjust the position of the laser emitted to the fiber collimator, and proceed to step (1).

9. An automatic inspection system based on the wireless passive fiber optic sensing system according to any one of claims 1-5, characterized in that, include: The light source module (501), demodulation module (502), wireless passive fiber optic grating sensing module (503), camera (504), two-axis galvanometer module (505), robotic arm (506) and AGV (507); The wireless passive fiber optic grating sensing module (503) is mounted on the object to be measured; The light source module (501), the demodulation module (502), the camera (504), the two-axis galvanometer module (505), and the robotic arm (506) are all mounted on the AGV (507); During operation, after the AGV (507) comes to a complete stop, the camera (504) identifies the position of the wireless passive fiber Bragg grating sensing module (503). The two-axis galvanometer module (505) deflects the first pump laser beam emitted from the light source module (501) and then directs it into the wireless passive fiber Bragg grating sensing module (503). The wireless passive fiber Bragg grating sensing module (503) wirelessly transmits a second laser beam, consisting of a specific wavelength of invisible light and a visible light band related to the measured physical quantity, back to the AGV (507). 7) The optical fiber collimator at the end of the mechanical arm (506) is aligned with the light beam by moving the mechanical arm (506) until the second spatial beam emitted from the optical fiber collimator coincides with the first spatial beam from the wireless passive fiber grating sensing module (503) and then reaches the demodulation module (502); the demodulation module (502) realizes the analysis of the light wavelength and obtains the magnitude of the measured physical quantity on the wireless passive fiber grating sensing module (503) by demodulating the physical values ​​at each grating sensor.

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