An active fiber based wireless passive fiber sensing system and method

Abstract

The application belongs to the technical field of fiber grating sensing, and specifically discloses a wireless passive fiber sensing system and method based on an active optical fiber, which comprises: an active end and a passive end; the active end is provided with a transmitting module and a receiving module; the passive end is provided with a receiving module, a fiber grating sensing module and a transmitting module; the transmitting module of the active end is used for transmitting a first spatial light beam; the receiving module of the passive end is used for taking the first spatial light beam as pump light to generate broadband light in the active optical fiber through a cladding pumping mode; the fiber grating sensing module of the passive end is used for reflecting light of a specific wavelength in the broadband light through a fiber grating sensing mode; the transmitting module of the passive end is used for transmitting a second spatial light beam based on the light of the specific wavelength; the receiving module of the active end is used for receiving the second spatial light beam; wherein the first spatial light beam and the second spatial light beam are not coaxial. Through the application, the alignment difficulty can be reduced and the wireless sensing distance can be improved.

Description

Technical Field

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

[0002] The principle of fiber gratings is as follows: a periodic refractive index change is modulated on the core of a single-mode fiber using an ultraviolet laser (one period is several hundred nanometers along the fiber core axis, and the length of a single grating region is approximately 10 millimeters). For example, a grating that reflects a wavelength of 1550 nm will, when broadband light in the wavelength range of 1520–1570 nm is passed through one end of the fiber, reflect the 1550 nm wavelength light back along its original path, while the remaining wavelengths will pass through the grating and continue propagating along the fiber core.

[0003] A passive fiber optic sensing system typically consists of an active end and a passive end. The active end is the end that emits an initial light beam and receives the sensing signal. The passive end is the end that receives the initial light beam, generates a sensing signal, and then emits it back; the passive end is equipped with a fiber Bragg grating sensor.

[0004] The active and passive ends are usually connected by optical fiber. This wired connection (or contact connection) lacks flexibility and poses safety hazards under certain working conditions.

[0005] To achieve wireless connectivity between the active and passive ends, existing technologies configure fiber optic collimators at both ends, with the collimators at the active and passive ends aligned using a completely coaxial alignment method. However, existing wireless passive fiber optic sensing solutions suffer from technical problems such as high alignment difficulty and short wireless sensing distance. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the purpose of this application is to reduce the alignment difficulty and increase the wireless sensing distance for wireless passive fiber optic sensing.

[0007] To achieve the above objectives, in a first aspect, this application provides a wireless passive optical fiber sensing system based on active optical fiber, the system comprising: an active end and a passive end;

[0008] The active end is equipped with a transmitting module and a receiving module; the passive end is equipped with a receiving module, a fiber Bragg grating sensing module, and a transmitting module.

[0009] The active end's transmitting module is used to emit the first spatial beam;

[0010] The passive receiving module is used to use the first spatial beam as pump light to generate broadband light in the active optical fiber through cladding pumping.

[0011] The passive fiber Bragg grating sensing module is used to reflect light of a specific wavelength in broadband light through fiber Bragg grating sensing.

[0012] The passive end's emission module is used to emit a second spatial beam based on light of a specific wavelength;

[0013] The receiving module at the active end is used to receive the second spatial beam;

[0014] The first spatial beam and the second spatial beam are not coaxial.

[0015] The active end is the end that emits the initial beam and receives the sensing signal. The passive end is the end that receives the initial beam, generates a sensing signal, and then emits it back. In this application, the passive end does not require a power supply, that is, it is passive, so the passive end in this application can also be called a passive end. The initial beam here is the aforementioned first spatial beam.

[0016] It is understandable that the first spatial beam and the second spatial beam are not coaxial, thus splitting the traditional coaxial light-receiving path into a physically independent light-emitting path and a light-receiving path. The light path containing the first spatial beam is the light-emitting path, and the light path containing the second spatial beam is the light-receiving path. Cladding pumping can reduce the alignment difficulty of the light-emitting path and increase the intensity of the sensing signal (reflected light). The higher intensity sensing signal helps to increase the wireless sensing distance. Furthermore, when the light-emitting path is aligned, the fiber optic grating senses and reflects light of a specific wavelength, making the optical axis of the passive fiber collimator visible—for example, visible to an infrared camera. The infrared camera can be used to locate the light-receiving path, effectively reducing the alignment difficulty of the light-receiving path.

[0017] Therefore, by coordinating the pump light emitted by the active end, the transmission optical path, and the active optical fiber of the passive end, as well as making the optical axis visible in the optical fiber collimator of the passive end, it is possible to reduce alignment difficulty (large spatial alignment tolerance) and increase wireless sensing distance.

[0018] The working principle of reducing the alignment difficulty of the transmitted optical path by cladding pumping is explained in detail here. Active optical fibers are generally double-clad structures with core mode field diameters in the range of a few micrometers (e.g., 5.4~6.0 micrometers) and numerical apertures in the range of 0.21~0.25. The inner cladding is polygonal (e.g., octagonal) in shape, with edge-to-edge distances on the order of hundreds of micrometers (e.g., 400 micrometers), and the numerical aperture of the inner cladding is not less than 0.46.

[0019] In the emission optical path of this application, the pump light in space is coupled into a relatively large inner cladding layer, typically with a diameter of several hundred micrometers, much larger than the few micrometers of the fiber core, rather than requiring precise coupling into a fiber core with a diameter of less than 10 micrometers as required by existing schemes. This effectively relaxes the precision requirements for spatial light alignment, allowing the emission optical path to be established within a larger alignment tolerance range, thus effectively reducing the alignment difficulty of the emission optical path.

[0020] The working principle of improving wireless sensing distance is explained in detail here. In existing solutions, broadband light is generated at the active end and transmitted to the passive end after passing through the space between the active and passive ends. At this point, the broadband light has already undergone significant attenuation, resulting in a lower intensity of the light reflected by the fiber Bragg grating sensor (i.e., the sensing signal). Consequently, when the passive end transmits the low-intensity sensing signal back to the active end, the sensing signal is difficult to transmit over a long distance in space, resulting in a short wireless sensing distance (generally less than 5 meters). In the solution of this application, the passive end generates broadband light in the active optical fiber through cladding pumping. It can be seen that the broadband light in this application is generated at the passive end, which can avoid the signal attenuation caused by the broadband light transmitting from the active end to the passive end. This increases the intensity of the light reflected by the fiber Bragg grating sensor (i.e., the sensing signal), and when the passive end transmits the sensing signal back to the active end, the sensing signal can transmit over a long distance in space, effectively improving the wireless sensing distance to tens of meters.

[0021] In one possible implementation, the active end's transmitting module is composed of a cascaded light source generating module, optical fiber, and optical fiber collimator.

[0022] In one possible implementation, the light source generation module includes: a pump light source, a visible light source, and a wavelength division multiplexer;

[0023] Pump light sources are used to generate pump light, and visible light sources are used to generate visible light.

[0024] Wavelength division multiplexers are used to combine pump light and visible light, and then deliver the combined light to an optical fiber collimator via optical fiber.

[0025] Understandably, visible light is used to indicate the direction of the optical axis of the active end fiber collimator, so as to align the optical axis of the active end fiber collimator with the passive end, so that the passive end can receive the first spatial beam.

[0026] In one possible implementation, the active end also includes: a first alignment component and / or a second alignment component.

[0027] The first alignment component includes: a first motion mechanism, a first camera, and a first control module;

[0028] The first motion mechanism is used to adjust the pose (position and attitude) of the active end's transmitting module.

[0029] The first camera is used to detect visible light from the first spatial beam;

[0030] The first control module is used to drive the first motion mechanism based on the visible light detected by the first camera until the optical axis of the fiber collimator at the active end is aligned with the beam receiving component at the passive end, and the beam receiving component at the passive end is used to receive the first spatial beam.

[0031] The beam receiving component on the passive end can be the focusing lens on the receiving module of the passive end.

[0032] It is understandable that by configuring a first motion mechanism, a first camera, and a first control module at the active end, the visible light of the first spatial beam can be detected. Then, by driving the first motion mechanism, the pose of the transmitting module at the active end can be adjusted so that the optical axis of the fiber collimator at the active end is aligned with the beam receiving component at the passive end, thereby achieving automated alignment of the transmitted optical path.

[0033] The second alignment component includes: a second motion mechanism, a second camera, and a second control module;

[0034] The second motion mechanism is used to adjust the pose of the receiving module at the active end;

[0035] The second camera is used to detect the beam of light in the second space.

[0036] The second control module is used to drive the second motion mechanism based on the second spatial beam detected by the second camera until the beam receiving component at the active end is aligned with the second spatial beam. The beam receiving component at the active end is used to receive the second spatial beam.

[0037] The beam receiving component at the active end can be a focusing lens on the receiving module of the active end.

[0038] It is understandable that by configuring a second motion mechanism, a second camera, and a second control module at the active end, the visible light of the second spatial beam can be detected. Then, by driving the second motion mechanism, the pose of the receiving module at the active end can be adjusted so that the beam receiving component at the active end is aligned with the second spatial beam, thereby achieving automated alignment of the receiving optical path.

[0039] In one possible implementation, the active receiver module is constructed by cascading a focusing lens, an optical fiber, and a demodulation module.

[0040] In one possible implementation, the passive receiving module is composed of a cascaded focusing lens, an active optical fiber, and a first single-mode optical fiber.

[0041] Among them, active optical fibers support cladding pumping.

[0042] Optionally, the focusing lens of the passive receiving module is defined as the first focusing lens, and the focal length of the first focusing lens is determined by the following formula:

[0043] ;

[0044] ;

[0045] in, Indicates the focal length of the first focusing lens; Indicates the diameter of the pump beam; This represents the distance from the exit of the first spatial beam to the first focusing lens along the horizontal axis. This indicates the total receiving angle of the inner cladding of an active optical fiber; Indicates the inner cladding diameter of an active optical fiber; This represents the vertical distance between the exit port of the first spatial beam and the optical axis of the first focusing lens (representing the maximum positioning error).

[0046] It is understandable that by appropriately selecting the focal length of the first focusing lens... By satisfying the above inequality, the maximum positioning error can be determined. Working distance and inner cladding parameters , In this case, it ensures that all pump light enters the inner cladding of the active fiber, enabling the system to have a sufficiently large alignment tolerance and effectively improving ease of use in engineering.

[0047] In one possible implementation, the passive fiber Bragg grating sensing module includes: a beam splitter, a second single-mode fiber, and multiple fiber Bragg grating sensors.

[0048] Multiple fiber Bragg grating sensors are configured on a second single-mode fiber;

[0049] The beam splitter is used to provide broadband light to the second single-mode fiber through beam splitting, and to provide the light reflected by the fiber Bragg grating sensor to the passive end's transmitter module through beam splitting.

[0050] Alternatively, the passive fiber Bragg grating sensing module includes: a circulator, a second single-mode fiber, and multiple fiber Bragg grating sensors;

[0051] Multiple fiber Bragg grating sensors are configured on a second single-mode fiber;

[0052] The circulator is used to provide broadband light to the second single-mode fiber and to provide the light reflected by the fiber Bragg grating sensor to the passive end of the transmitter module.

[0053] In one possible implementation, the passive end's transmitter module is constructed by cascading a third single-mode fiber and a fiber collimator.

[0054] In one possible implementation, the active optical fiber has a double-clad structure, which consists of an inner cladding and an outer cladding. The outer cladding is used to confine the pump light within the inner cladding, while the inner cladding wraps around the fiber core. The fiber core mode field diameter is 4 micrometers to 20 micrometers, and the edge-to-edge distance of the inner cladding is 100 micrometers to 3000 micrometers.

[0055] Secondly, this application also provides a wireless passive optical fiber sensing method based on active optical fiber, applied to the system described in the first aspect or any possible implementation of the first aspect, the method comprising:

[0056] Adjust the pose (position and attitude) of the active transmitting module so that the first spatial beam is aligned with the passive receiving module;

[0057] Adjust the orientation of the receiving module at the active end so that the second spatial beam is aligned with the receiving module at the active end.

[0058] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here.

[0059] Overall, the technical solutions conceived in this application have the following beneficial effects compared with the prior art:

[0060] (1) Enhance safety: The wireless sensing solution with all-fiber optics and no electrical connection is adopted to eliminate the risk of high voltage electric shock.

[0061] (2) Large alignment tolerance and strong engineering usability: Existing technologies use coaxial precision coupling alignment method, which has small alignment tolerance and weak engineering usability. This application uses the separation of light and light transmission paths and cladding pumping technology, combined with the design of large-size inner cladding active optical fiber, to significantly increase the alignment tolerance of the light transmission path, which can effectively reduce the precision requirements of mechanical adjustment mechanism and the complexity of installation and debugging, and has strong engineering usability.

[0062] (3) The alignment process is intuitive and the operation efficiency is high: the light of the returned sensor signal is used as a beacon, and the passive end optical axis is visualized through the camera, avoiding "blind alignment" and effectively improving the efficiency and reliability of the receiving optical path alignment process.

[0063] (4) Excellent flexibility and scalability: The system is suitable for integration into mobile platforms (such as drones), realizing the mode improvement from fixed monitoring to mobile inspection.

[0064] (5) Long transmission distance: By generating broadband light locally at the passive end, the strength of the sensing signal is effectively improved, enabling long-distance wireless sensing at tens of meters, which is higher than traditional solutions (<5 meters). Attached Figure Description

[0065] Figure 1 This is a schematic diagram of a wired passive fiber optic sensing system provided by existing technology.

[0066] Figure 2 This is a schematic diagram of a wireless passive fiber optic sensing system provided by existing technology;

[0067] Figure 3 This is a schematic diagram illustrating the working principle of the cladding pump provided in the embodiments of this application;

[0068] Figure 4 This is one of the structural schematic diagrams of a wireless passive optical fiber sensing system based on active optical fiber provided in the embodiments of this application;

[0069] Figure 5 This is the second schematic diagram of the structure of the wireless passive optical fiber sensing system based on active optical fiber provided in the embodiments of this application;

[0070] Figure 6 This is a schematic diagram of the structural parameters of the double-clad active optical fiber cross-section provided in the embodiments of this application;

[0071] Figure 7 This is a schematic diagram of the pump light being focused onto the end face of the active optical fiber by a focusing lens, as provided in an embodiment of this application.

[0072] Figure 8 This is a schematic diagram of the wireless passive optical fiber sensing system based on active optical fiber provided in the embodiments of this application applied to automatic inspection;

[0073] Figure 9 This is a flowchart illustrating the wireless passive optical fiber sensing method based on active optical fiber provided in the embodiments of this application.

[0074] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0075] 1 is a broadband light source; 2 is a circulator; 3 is a first demodulation module; 4 is an optical fiber between the circulator and the fiber Bragg grating temperature sensor; 5 is a first fiber Bragg grating temperature sensor; 6 is a second fiber Bragg grating temperature sensor; 7 is a third fiber Bragg grating temperature sensor; 8 is a first fiber collimator; 9 is a second fiber collimator; 10 is a demodulator; 11 is a laser beam; 501 is the active end transmitting module; 502 is the active end receiving module; 503 is the passive end; 21 is a pump light source; 22 is a first optical fiber; 23 is the active end fiber collimator; 24 is a first spatial beam; 25 26 is an active optical fiber; 27 is a first single-mode optical fiber; 28 is a beam splitter; 29 is a second single-mode optical fiber; 30 is a fiber optic grating sensor assembly; 31 is a third single-mode optical fiber; 32 is a passive fiber collimator; 33 is a second spatial beam; 34 is a second focusing lens; 35 is a second optical fiber; 36 is a second demodulation module; 37 is a visible light source; 38 is a wavelength division multiplexer; 39 is a first focusing lens; 291 is a first fiber optic grating sensor; 292 is a second fiber optic grating sensor; 293 is a third fiber optic grating sensor; 61 is UAV No. 1; 62 is UAV No. 2. Detailed Implementation

[0076] To facilitate a clearer understanding of the various embodiments of this application, some relevant background knowledge will be introduced as follows.

[0077] The principle of fiber optic grating sensing is as follows: when an external physical quantity changes, causing a change in the refractive index period of the grating, the wavelength that the grating can reflect will change accordingly. For example, a grating that normally reflects only 1550nm wavelength light may change if the grating region is affected by an external physical quantity, such as tension or temperature, causing the refractive index period to change and the grating to reflect only 1551nm wavelength light. Based on this principle, the magnitude of the physical quantity causing the change can be inferred from the change in the wavelength of the reflected light. Depending on the different packaging designs, some grating sensors can be used to measure temperature, while others can be used to measure changes in force.

[0078] Traditional fiber Bragg grating temperature sensing systems, such as Figure 1 As shown, a contact-type connection scheme is typically used: multiple fiber Bragg grating sensors are connected in series and attached to the point being measured (such as the surface of a high-voltage electrical cabinet), and then physically connected to the demodulator via fiber optic patch cords. The demodulator has a built-in broadband light source (wavelength range 1520-1570nm), a circulator, and a demodulation module for spectral analysis. It infers temperature changes by monitoring the reflected wavelength shift (e.g., 1550nm→1551nm). The broadband light source is typically an amplified spontaneous emission (ASE) source.

[0079] Figure 1A fiber optic grating sensing system can include the aforementioned common grating sensor strings (as passive ends) used to measure temperature, with each sensor, such as... Figure 1 The first fiber Bragg grating temperature sensor 5, the second fiber Bragg grating temperature sensor 6, and the third fiber Bragg grating temperature sensor 7 are attached to the location where the temperature needs to be measured. Then, the optical fiber (such as...) Figure 1 One end of the optical fiber 4 between the circulator and the fiber Bragg grating temperature sensor is connected to the demodulator 10 (as the active end). The demodulator 10 includes a broadband light source 1, a circulator 2, and a first demodulation module 3.

[0080] The specific sensing process is as follows: Broadband light source 1 emits broadband light in the wavelength range of 1520~1570nm, passes through circulator 2, enters optical fiber 4 between circulator and fiber Bragg grating temperature sensor, and sequentially passes through first fiber Bragg grating temperature sensor 5, second fiber Bragg grating temperature sensor 6, and third fiber Bragg grating temperature sensor 7. The grating period of each sensor is intentionally set to be different, so the reflected wavelengths are also different. For example, first fiber Bragg grating temperature sensor 5 reflects a wavelength of 1530nm, second fiber Bragg grating temperature sensor 6 reflects a wavelength of 1540nm, and third fiber Bragg grating temperature sensor 7 reflects a wavelength of 1550nm. The light of these three wavelengths is reflected back to circulator 2 and reaches first demodulation module 3. If a temperature change at a certain sensor causes a change in the grating spacing (i.e., the grating period is changed), this will cause a change in the wavelength of the light reflected back from that point. For example, if the light originally reflected from the first fiber optic grating temperature sensor 5 to the demodulation module is 1530nm, but becomes 1531nm, then the demodulation module can know the current temperature value at that point based on the corresponding data that has been calibrated beforehand.

[0081] Existing technology drawbacks – physical connection risks: If this sensing system is deployed in the power industry, fiber optic cables need to be laid from the personnel work area to the high-voltage area, and this connection line will always exist. If there are metal components inside the fiber optic cable, the high-voltage electrodes may be led to the personnel work area along the connection line (and most fiber optic cables have metal components, so workers are very likely to mistakenly use fiber optic cables containing metal components during installation and maintenance), which poses a risk of electric shock.

[0082] In existing technologies, there have been attempts to use wireless fixed single-beam fiber optic sensing solutions in the power industry, as follows: Figure 2 Using two fixed fiber optic collimators, such as Figure 2The existing wireless solution uses a first fiber collimator 8 and a second fiber collimator 9 to acquire the sensing signal. However, this method is inflexible and requires stringent alignment precision. For example, due to cost considerations, the outer diameter of the first fiber collimator 8 and the second fiber collimator 9 is approximately 3 mm, while the diameter of the emitted laser beam 11 is 1 mm. As a single beam, the first fiber collimator 8 and the second fiber collimator 9 must be perfectly aligned to the same optical axis to establish communication. Therefore, in existing wireless solutions, the spatial alignment tolerance between the fiber collimator at the laser transmitter and the fiber collimator of the fiber under test is small, requiring a precision mechanical adjustment mechanism. This solution has high precision requirements for the mechanical adjustment mechanism, is complex to install and debug, and is not easily moved after completion. Therefore, it is poorly applicable in scenarios requiring mobile inspection or frequent changes in detection points. It is particularly important to note that the alignment in the existing technology requires perfect alignment to the same optical axis. During the alignment process, no light is emitted from the passive end (without a light source) of its fiber collimator. Therefore, during the alignment process, the passive end's fiber collimator (such as...) Figure 2 The optical axis of the second fiber collimator 9 is invisible (belonging to "blind alignment") and its diameter is only 1 mm, so it is very difficult to align the first fiber collimator 8 and the second fiber collimator 9.

[0083] Another challenge is that fiber Bragg grating sensors are typically based on single-mode fiber. Multimode fibers have too many modes, which can affect wavelength demodulation. Coupled spatial light into a single-mode fiber with a core diameter of only 9µm would result in significant loss (for example,...). Figure 2 The spatial beam in the image has a diameter of 1 millimeter and an area 12,000 times that of the fiber core. In particular... Figure 2 In this wireless passive sensing scheme, broadband light is generated at the active end and transmitted to the passive end after passing through the space between the active and passive ends. At this point, the broadband light has already been significantly attenuated, resulting in a lower intensity of the light reflected by the fiber Bragg grating sensor (i.e., the sensing signal). Consequently, when the passive end transmits the low-intensity sensing signal back to the active end, the sensing signal is difficult to transmit over a long distance in space, resulting in a short wireless sensing distance (generally less than 5 meters).

[0084] In summary, the existing technologies have the following main defects: (1) the existing contact solutions have the risk of high voltage breakdown and poor safety; (2) the existing wireless solutions are difficult to align; (3) in the existing wireless solutions, the optical transmission loss is large and the wireless sensing distance is short.

[0085] To overcome the above-mentioned shortcomings, this application provides a wireless passive optical fiber sensing system and method based on active optical fiber, which can reduce the difficulty of alignment and increase the wireless sensing distance.

[0086] Active optical fiber and passive optical fiber are two key components in optical fiber communication systems. The main difference lies in whether or not they require external energy to drive them.

[0087] Passive optical fiber does not have gain functionality and serves only as a transmission medium for optical signals, such as... Figure 5 The first single-mode fiber 26, the second single-mode fiber 28, and the third single-mode fiber 30 are in the middle.

[0088] Active optical fiber is a special type of optical fiber doped with rare earth elements (such as erbium and ytterbium). When excited by external pump light, these rare earth ions undergo energy level transitions, converting the pump light of a specific wavelength into broadband light with a wider spectrum through an amplified spontaneous emission process. During the manufacturing process, trace amounts of rare earth ions (such as erbium, ytterbium, and thulium) are doped into the fiber core. The type of active optical fiber is mainly determined by the type of rare earth ions doped in its core; different ions generate and amplify light in different wavelength bands.

[0089] The active optical fiber mentioned in this application is typically erbium-doped fiber. Erbium-doped fiber is produced by doping a certain amount of trivalent erbium ions into the fiber core during the fiber manufacturing process. Erbium ions are sensitive to photons of specific wavelengths, absorbing their energy and causing a change in their energy level. For example, erbium ions can typically absorb pump light at a wavelength of 980 nm, thus generating broadband light in the 1530–1565 nm band.

[0090] There are two ways to introduce pump light into erbium-doped fiber: one is core-pumping, where 980nm pump light enters the core of the erbium-doped fiber directly through a wavelength division multiplexer (WDM) to pump the erbium ions in the core; the other is cladding-pumping, where the erbium-doped fiber has a double-clad structure. The core is doped with erbium ions and is very small (about 9 micrometers) to conduct broadband ASE light (1530nm~1565nm band). The inner cladding surrounds the core and is much larger (usually several hundred micrometers, or even irregularly shaped like a rectangle or D-shape). The inner cladding is undoped and its function is to conduct the pump light. The outermost cladding has the lowest refractive index and ensures that the pump light is confined within the inner cladding.

[0091] like Figure 3 As shown, this application employs a cladding-pumped method. The ASE light source generation process in this application is as follows: A high-power pump laser (e.g., a 980nm laser) can be easily coupled into a relatively large inner cladding (hundreds of micrometers in diameter, much larger than the 9 micrometers of the fiber core). Because of the large area of ​​the inner cladding, the coupling efficiency is very high, allowing for the injection of hundreds of watts or even kilowatts of pump power. The pump light propagates within the inner cladding but repeatedly passes through the central fiber core. Each time the pump light passes through the fiber core, it is absorbed by erbium ions in the core, thereby exciting the erbium ions and causing them to generate ASE light in the 1530nm~1565nm wavelength band that propagates within the fiber core.

[0092] 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.

[0093] In this application, the terms "first" and "second," etc., are used to distinguish different objects, not to describe a specific order of objects. For example, "first spatial beam" and "second spatial beam," etc., are used to distinguish different spatial beams, not to describe a specific order of spatial beams.

[0094] In the embodiments of this application, the terms "exemplary" or "for example" are used to indicate that something is an example, illustration, or description. Any embodiment or design that is described as "exemplary" or "for example" in the embodiments of this application should not be construed as being more preferred or advantageous than other embodiments or design. Specifically, the use of the terms "exemplary" or "for example" is intended to present the relevant concepts in a specific manner.

[0095] In the description of the embodiments of this application, unless otherwise stated, "multiple" means two or more, for example, multiple processing units means two or more processing units, multiple elements means two or more elements, etc.

[0096] The embodiments of this application are described below with reference to the accompanying drawings.

[0097] Figure 4 This is one of the structural schematic diagrams of a wireless passive optical fiber sensing system based on active optical fiber provided in the embodiments of this application. The system includes: an active end and a passive end;

[0098] The active end is equipped with a transmitting module and a receiving module; the passive end is equipped with a receiving module, a fiber Bragg grating sensing module, and a transmitting module; the transmitting module of the active end is used to transmit a first spatial beam; the receiving module of the passive end is used to use the first spatial beam as pump light to generate broadband light in the active fiber through cladding pumping; the fiber Bragg grating sensing module of the passive end is used to reflect light of a specific wavelength in the broadband light through fiber Bragg grating sensing; the transmitting module of the passive end is used to transmit a second spatial beam based on light of a specific wavelength; the receiving module of the active end is used to receive the second spatial beam; wherein, the first spatial beam and the second spatial beam are not coaxial.

[0099] It is understood that in this application, the first spatial beam and the second spatial beam are not coaxial, and the traditional coaxial light-receiving path is split into a light-emitting path (the light path where the first spatial beam is located) and a light-receiving path (the light path where the second spatial beam is located), which are physically independent of each other.

[0100] For the transmitting optical path, the pump light can be transmitted to the passive end. Using cladding pumping technology, the pump light is coupled to the large inner cladding of the active optical fiber at the passive end. Since the inner cladding size (usually hundreds of micrometers) is much larger than the single-mode fiber core (9µm), this process has a very large alignment tolerance for spatial light, which significantly reduces the positioning accuracy requirements of the transmitting end.

[0101] When the transmitting optical path is aligned, the fiber optic grating senses and reflects light of a specific wavelength, making the optical axis of the passive fiber collimator visible during the alignment process (e.g., visible to an infrared camera, and the receiving optical path can be located by the infrared camera), effectively reducing the alignment difficulty of the receiving optical path (the optical path between the transmitting module of the passive end and the receiving module of the active end).

[0102] Therefore, by transforming the complex coaxial alignment into two relatively simple alignments (transmitting optical path alignment and receiving optical path alignment), the precision requirements of the mechanical adjustment mechanism and the complexity of installation and debugging are effectively reduced.

[0103] Furthermore, as mentioned earlier, in the prior art, broadband light is generated at the active end. However, in the solution of this application, the passive end generates broadband light in the active optical fiber through cladding pumping. It can be seen that the broadband light in this application is generated at the passive end, which can avoid the signal attenuation caused by the broadband light being transmitted from the active end to the passive end. This increases the intensity of the light reflected by the fiber Bragg grating sensor (i.e., the sensing signal). As a result, when the sensing signal is transmitted back to the active end from the passive end, the sensing signal can be transmitted over a longer distance in space, effectively increasing the wireless sensing distance (up to tens of meters).

[0104] The following examples illustrate the wireless passive optical fiber sensing system based on active optical fiber provided in this application.

[0105] Figure 5 This is the second schematic diagram of the structure of a wireless passive optical fiber sensing system based on active optical fiber provided in this application embodiment. The system includes an active end and a passive end 503. The active end is configured with a transmitting module 501 and a receiving module 502. The core of this application is: an ASE broadband light source is constructed at the passive end using active optical fiber. Pump light is emitted from the active end and enters the large-diameter cladding of the active optical fiber at the passive end, i.e., cladding pumping is adopted. The large-diameter cladding receives spatial light with large tolerance. The pump light in the cladding excites the doped material in the fiber core through multiple reflections and is converted into broadband light propagating in the fiber core. It should be noted that the passive end in this application does not require a power supply, that is, it is passive, so the passive end in this application can also be called a passive end.

[0106] like Figure 5As shown, pump light in the 980nm wavelength range is emitted from pump light source 21; 650nm visible light is emitted from visible light source 37. This visible light is used to indicate the direction of the optical axis of the active end fiber collimator 23, so as to align the optical axis of the active end fiber collimator 23 with the first focusing lens 39; the two beams are combined by wavelength division multiplexer 38, pass through the first optical fiber 22, and are emitted from the active end fiber collimator 23 to form a millimeter-scale first spatial beam 24.

[0107] like Figure 5 As shown, the first spatial beam 24 passes through a focusing lens (such as...) Figure 5 The first focusing lens 39) enters the inner cladding of the active optical fiber 25.

[0108] like Figure 5 As shown, erbium ions in the core of the active fiber 25 absorb the pump light from the inner cladding and emit broadband light of 1530~1565nm that propagates along the core and enters the first single-mode fiber 26; then, after passing through a beam splitter 27 with a splitting ratio of 50% / 50%, it enters the second single-mode fiber 28.

[0109] like Figure 5 As shown, the second single-mode fiber 28 has an encapsulated fiber Bragg grating sensor assembly 29, which includes a first fiber Bragg grating sensor 291, a second fiber Bragg grating sensor 292, and a third fiber Bragg grating sensor 293. For example, the first fiber Bragg grating sensor 291 reflects a wavelength of 1530nm, the second fiber Bragg grating sensor 292 reflects a wavelength of 1540nm, and the third fiber Bragg grating sensor 293 reflects a wavelength of 1550nm. The light of these three wavelengths is reflected back to the beam splitter 27 by the fiber Bragg gratings at the corresponding sensors, splitting into two paths. One path goes through the third single-mode fiber 30 to the fiber collimator 31 at the passive end, and the other path enters the first single-mode fiber 26, then diverges into space after passing through the active fiber 25 and the first focusing lens 39.

[0110] Optionally, the beam splitter 27 can be replaced with a circulator, through which the broadband light output from the first single-mode fiber 26 is provided to the second single-mode fiber 28, and through the circulator, the light reflected by the fiber optic grating sensor assembly 29 is provided to the third single-mode fiber 30, and then reaches the fiber collimator 31 at the passive end via the third single-mode fiber 30.

[0111] like Figure 5 As shown, light of three wavelengths is emitted from the passive end fiber collimator 31 to form a second spatial beam 32 with a diameter of millimeters, which then encounters a focusing lens (such as...). Figure 5The light beam is guided into the second optical fiber 35 after passing through the second focusing lens 33. It is understood that a camera can be configured at the active end to detect the second spatial beam 32, thereby adjusting the pose of the receiving module 502 at the active end so that the beam receiving component at the active end (e.g., the second focusing lens 33 of the receiving module 502 at the active end) is aligned with the second spatial beam, thereby achieving automated alignment of the receiving optical path.

[0112] like Figure 5 As shown, light of three wavelengths enters the second optical fiber 35 and is then transmitted to the fiber grating demodulation module (i.e., Figure 5 The second demodulation module 36 in the middle) identifies three wavelength values, and can then calculate physical quantities such as temperature and deformation at the first fiber optic grating sensor 291, the second fiber optic grating sensor 292, and the third fiber optic grating sensor 293 based on the pre-calibrated values.

[0113] It is important to note that existing technologies generally require high conversion efficiencies (e.g., 30%~60%) for pump light usage, which in turn necessitates a close fit between the pump light and the double-clad fiber structure of the active fiber. Therefore, the distance between the pump light source and the double-clad fiber structure is typically shortened, rather than allowing the pump light to propagate a certain distance (e.g., 10-20 meters) before entering the double-clad fiber structure. Furthermore, the pump light generation and conversion processes are not configured in two separate devices (e.g., active and passive ends). In this application, however, the pump light is propagated a certain distance before entering the double-clad fiber structure to reduce alignment issues in the wireless transmission path. The pump light generation and conversion processes are configured at the active and passive ends respectively; for example, a 980nm pump light source is placed at the active end, while the conversion from 980nm to the 1530~1565nm band occurs at the passive end. The active optical fiber at the passive end can pump and generate broadband light (in existing technologies, broadband light is generated at the active end), increasing the intensity of the sensing signal (reflected light) (which helps to increase the wireless sensing distance). It should also be noted that the demodulation module at the active end has a receiving range spanning 50dB, and even if the pump light conversion efficiency is low (e.g., less than 30%), it can still be used in the sensing system of this application. Therefore, in this application, the coordination between the pump light emitted by the active end, the wireless transmission optical path, and the active optical fiber at the passive end is not a conventional technical means, but rather an improvement made by the applicant to existing technology through creative effort.

[0114] Figure 5 The first focusing lens 39 is a key component of the passive end 503 that receives pump light. To achieve high-tolerance alignment, the following section combines... Figure 6 and Figure 7 The parameter design of the first focusing lens 39 is illustrated by way of example.

[0115] During the alignment of the transmitted optical path, the pose (position and attitude) of the active end is adjusted. Due to limitations in positioning accuracy, positioning errors occur during this adjustment process. For example, a mobile platform (such as a drone) used to support the active end will experience positioning errors (e.g., on the order of ±100 mm) during the pose adjustment process, due to limitations in positioning accuracy. To ensure that the pump light can effectively enter the inner cladding of the active fiber 25 within a certain positioning error range, the parameters of the first focusing lens 39 need to be designed. Its optical principle is as follows: Figure 6 and Figure 7 As shown.

[0116] Figure 6 The structural parameters of a double-clad active optical fiber cross-section are shown. Among them, the core diameter of the active optical fiber is... Typical values ​​are 5–10 micrometers; inner cladding diameter of active optical fibers Typical values ​​are 100–800 micrometers; the core's total receiving angle is γ, typically 8°–24°; while the inner cladding's total receiving angle is... The typical value is 54°~120°. During the alignment of the emitted optical path, the pump light needs to be able to enter the inner cladding.

[0117] Figure 7 The upper half of the optical path shows the optical path of the pump light incident on the first focusing lens 39 when there is no positioning error and the first spatial beam 24 (including pump light) is located on the optical axis of the first focusing lens 39. Figure 7 The lower half of the optical path illustrates the optical path when the pump light is incident on the first focusing lens 39, and the first spatial beam 24 (including the pump light) is not on the optical axis of the first focusing lens 39 due to positioning error.

[0118] Assuming the diameter of the pump light is When there is no positioning error and the first spatial beam 24 (including the pump light) is located on the optical axis of the first focusing lens 39, the pump light passes through a focal length of... After the first focusing lens 39, the minimum angle between the focused pump light and the end face of the active fiber is Figure 7 middle The calculation formula is as follows:

[0119] (1);

[0120] in, This represents the minimum angle between the focused pump light and the end face of the active fiber when the first spatial beam 24 (including the pump light) is located on the optical axis of the first focusing lens 39 without positioning error. This indicates the focal length of the first focusing lens 39. This indicates the diameter of the pump light.

[0121] When a positioning error exists and the first spatial beam 24 (including the pump light) is not on the optical axis of the first focusing lens 39, the offset distance (the vertical distance between the exit port of the first spatial beam 24 and the optical axis of the first focusing lens 39) is assumed to be... The pump light emitted by the active end is at an angle (The angle between the optical axis of the first spatial beam 24 and the optical axis of the first focusing lens 39) is directed towards the first focusing lens 39, at which point the focal point position shifts in the Y-axis (vertical axis) direction. Furthermore, the angle between the focused pump light and the end face of the active fiber is the smallest. It can be approximated as:

[0122] (2);

[0123] in, This indicates the minimum angle between the focused pump light and the end face of the active fiber when there is a positioning error and the first spatial beam 24 (including the pump light) is not on the optical axis of the first focusing lens 39; This represents the angle between the optical axis of the first spatial beam 24 and the optical axis of the first focusing lens 39. This indicates the vertical distance between the exit port of the first spatial beam 24 and the optical axis of the first focusing lens 39. (representing the maximum positioning error) This represents the distance from the exit of the first spatial beam 24 to the first focusing lens 39 in the X-axis (horizontal axis) direction.

[0124] in addition It can be obtained through the following formula:

[0125] (3);

[0126] in, This indicates the offset of the focal point formed after being focused by the first focusing lens 39 in the Y-axis (vertical axis) direction.

[0127] To ensure that all pump light energy enters the inner cladding, the following two conditions must be met simultaneously: angle condition and offset condition.

[0128] Angular condition: The maximum tilt angle of the pump light after focusing ( It needs to be within the receiving angle range of the inner cladding of the optical fiber, that is:

[0129] (4);

[0130] Offset conditions: Smaller than the radius of the inner cladding ,Right now:

[0131] (5).

[0132] Substituting formula (2) into formula (4) and formula (3) into formula (5), we can obtain that the focal length of the first focusing lens 39 needs to satisfy the following two formulas (formula (6) and formula (7)) simultaneously in order for all the pump light to enter the inner cladding of the active fiber:

[0133] (6);

[0134] (7);

[0135] It is understandable that by appropriately selecting the focal length of the first focusing lens 39... To satisfy the above inequalities (Equations (6) and (7)), the maximum positioning error can be known. Working distance and inner cladding parameters , In this case, ensuring that all pump light enters the inner cladding of the active fiber ensures that the system has a sufficiently large alignment tolerance, effectively improving ease of use in engineering. For example, when the active end is carried by a mobile platform (such as a drone), by reasonably selecting the focal length of the first focusing lens 39, it is possible to flexibly adapt to mobile platforms with different positioning accuracies, thus improving ease of use in engineering.

[0136] Figure 8 This is a schematic diagram illustrating the application of the active optical fiber-based wireless passive optical fiber sensing system provided in this application to automatic inspection. Figure 8 As shown, the wireless passive optical fiber sensing system based on active optical fiber provided in this application embodiment can be installed on a mobile platform (such as two drones used in conjunction) to achieve automatic inspection.

[0137] like Figure 8 As shown, a wireless passive fiber optic grating sensing module, i.e., the passive end 503, is installed on the object under test. Figure 5 (The passive terminal 503 is the same as in the example). For example, if there are 4 high-voltage transmission towers on a hillside that need to be measured to monitor the deformation caused by snow and ice accumulation, then the 4 passive terminals 503 are installed on the 4 high-voltage towers respectively, and each grating sensor is attached to the point where the deformation needs to be measured.

[0138] like Figure 8 As shown, a drone stopping point is set in front of each high-voltage tower. The distance between the drone and the passive end 503 is tens of meters (e.g., about 20 meters). When inspection is required, the first drone 61 will run to the point and stop (e.g., hover).

[0139] like Figure 8As shown, after the UAV comes to a stop, the first UAV 61 can use its camera to locate the visible light spot (the first UAV 61 carries an active-end transmission module 501, which emits a first spatial beam 24 containing visible light, which can form a spot on the passive end 503), and then adjust its own posture so that the first spatial beam 24 is aligned with the first focusing lens 39. Thus, the transmission module 501 on the first UAV 61 can inject 980nm pump light into the active optical fiber of the passive end 503. Because the active optical fiber has a focusing lens at its front end and a large cladding size, the receiving angle is large, so the UAV's parking accuracy does not affect the reception of the pump light. That is, a certain range of parking error is allowed.

[0140] like Figure 8 As shown, after the light beam reaches each grating, the light of the corresponding wavelength is reflected back and finally directed towards the second UAV 62 through the fiber collimator of the passive end 503. The second UAV 62 is equipped with a camera 504, which can be customized to identify the wavelength band of broadband light (1520~1570nm). The second spatial beam 32 (the wavelength of which is located in the 1520~1570nm range) serves as an alignment indicator beacon. The camera can detect the second spatial beam 32 (making the optical axis of the fiber collimator of the passive end 503 visible), thereby adjusting the pose of the receiving module 502 of the active end so that the beam receiving component of the active end (e.g., the second focusing lens 33 of the receiving module 502 of the active end) is aligned with the second spatial beam. Since the optical axis of the passive end 503 is made visible during the alignment process of the receiving optical path, the alignment difficulty of the receiving optical path can be reduced, and automated alignment of the receiving optical path can be achieved within a range of tens of meters.

[0141] like Figure 8 As shown, based on the spatial position of the second spatial beam 32, the second UAV 62 adjusts its own pose (position and attitude) until the second focusing lens 33 of the receiving module 502 is aligned with the second spatial beam 32.

[0142] like Figure 8 As shown, the light beam enters from the optical fiber of the receiving module 502 and reaches the demodulation module; the demodulation module can demodulate the physical values ​​at each grating sensor, such as deformation values.

[0143] Figure 9 This is a flowchart illustrating the wireless passive optical fiber sensing method based on active optical fiber provided in the embodiments of this application, as shown below. Figure 9 As shown, this method is applied to any of the above-mentioned wireless passive optical fiber sensing systems based on active optical fiber. The method includes the following steps S101 and S102.

[0144] Step S101: Adjust the pose of the active end's transmitting module so that the first spatial beam is aligned with the passive end's receiving module.

[0145] Step S102: Adjust the pose of the receiving module of the active end so that the second spatial beam is aligned with the receiving module of the active end.

[0146] It is understandable that by coordinating the pump light emitted by the active end, the transmission optical path, and the active optical fiber of the passive end, as well as making the optical axis visible in the optical fiber collimator of the passive end, it is possible to reduce the alignment difficulty (large spatial alignment tolerance) and increase the wireless sensing distance.

[0147] In summary, the wireless passive optical fiber sensing system and method based on active optical fiber provided in this application have the following significant advantages.

[0148] (1) Intrinsically safe and highly reliable: The wireless sensing solution with all-fiber and no electrical connection completely eliminates the risk of high voltage electric shock introduced by the metal components of the optical cable, making it particularly suitable for industrial fields with extremely high safety requirements, such as power systems.

[0149] (2) It has a large alignment tolerance and is easy to use in engineering.

[0150] (3) The alignment process is intuitive and the operation efficiency is high.

[0151] (4) The optical axis of the passive fiber collimator is made visible. Whether it is manually operated with the aid of a display screen or an automatic control system based on an infrared camera (such as a drone), it can achieve rapid and accurate positioning and tracking of the optical path, which greatly improves the efficiency and success rate of mobile inspection and temporary deployment.

[0152] (5) Flexibility and scalability: The system is suitable for integration into mobile platforms (such as drones and robots), realizing the upgrade from "fixed monitoring" to "mobile inspection", enabling temporary or periodic detection of scattered, high-risk or difficult-to-lay cable areas, and expanding the application scenarios of fiber optic grating sensing technology.

[0153] (6) Long transmission distance and wide range of applicable scenarios: Through the active fiber optic design of the passive end, the limitations of existing fiber optic wireless sensing are effectively overcome, and long-distance wireless signal transmission at tens of meters level is realized, which meets the inspection needs of large-scale facilities such as substations, large bridges, and slopes.

[0154] It is understood that the various numerical designations used in the embodiments of this application are merely for the convenience of description and are not intended to limit the scope of the embodiments of this application.

[0155] 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 optical fiber sensing system based on active optical fiber, characterized in that, include: Active end and passive end; The active end is equipped with a transmitting module and a receiving module; The passive end is equipped with a receiving module, a fiber Bragg grating sensing module, and a transmitting module; The active end's transmitting module is used to emit the first spatial beam; The passive receiving module is used to use the first spatial beam as pump light to generate broadband light in the active optical fiber through cladding pumping. The passive fiber Bragg grating sensing module is used to reflect light of a specific wavelength in broadband light through fiber Bragg grating sensing. The passive end's emission module is used to emit a second spatial beam based on light of a specific wavelength; The receiving module at the active end is used to receive the second spatial beam; The first spatial beam and the second spatial beam are not coaxial.

2. The wireless passive optical fiber sensing system based on active optical fiber according to claim 1, characterized in that, The active end's transmitting module is composed of a cascaded light source generating module, optical fiber, and optical fiber collimator.

3. The wireless passive optical fiber sensing system based on active optical fiber according to claim 2, characterized in that, The light source generation module includes: a pump light source, a visible light source, and a wavelength division multiplexer; Pump light sources are used to generate pump light, and visible light sources are used to generate visible light. Wavelength division multiplexers are used to combine pump light and visible light, and then deliver the combined light to an optical fiber collimator via optical fiber.

4. The wireless passive optical fiber sensing system based on active optical fiber according to claim 3, characterized in that, The active end also includes: a first alignment component and / or a second alignment component; The first alignment component includes: a first motion mechanism, a first camera, and a first control module; The first motion mechanism is used to adjust the pose of the active end's transmitting module; The first camera is used to detect visible light from the first spatial beam; The first control module is used to drive the first motion mechanism based on the visible light detected by the first camera until the optical axis of the fiber collimator at the active end is aligned with the beam receiving component at the passive end. The beam receiving component at the passive end is used to receive the first spatial beam. The second alignment component includes: a second motion mechanism, a second camera, and a second control module; The second motion mechanism is used to adjust the pose of the receiving module at the active end; The second camera is used to detect the beam of light in the second space. The second control module is used to drive the second motion mechanism based on the second spatial beam detected by the second camera until the beam receiving component at the active end is aligned with the second spatial beam. The beam receiving component at the active end is used to receive the second spatial beam.

5. The wireless passive optical fiber sensing system based on active optical fiber according to claim 1, characterized in that, The active receiver module is composed of a cascaded focusing lens, optical fiber, and demodulation module.

6. The wireless passive optical fiber sensing system based on active optical fiber according to claim 1, characterized in that, The passive receiver module is composed of a cascaded focusing lens, an active optical fiber, and a first single-mode optical fiber. Among them, active optical fibers support cladding pumping.

7. The wireless passive optical fiber sensing system based on active optical fiber according to claim 1, characterized in that, The passive fiber Bragg grating sensing module includes: a beam splitter, a second single-mode fiber, and multiple fiber Bragg grating sensors; Multiple fiber Bragg grating sensors are configured on a second single-mode fiber; The beam splitter is used to provide broadband light to the second single-mode fiber through beam splitting, and to provide the light reflected by the fiber Bragg grating sensor to the passive end's transmitter module through beam splitting. Alternatively, the passive fiber Bragg grating sensing module includes: a circulator, a second single-mode fiber, and multiple fiber Bragg grating sensors; Multiple fiber Bragg grating sensors are configured on a second single-mode fiber; The circulator is used to provide broadband light to the second single-mode fiber and to provide the light reflected by the fiber Bragg grating sensor to the passive end of the transmitter module.

8. The wireless passive optical fiber sensing system based on active optical fiber according to claim 1, characterized in that, The passive end's transmitting module is constructed by cascading a third single-mode fiber and a fiber collimator.

9. The wireless passive optical fiber sensing system based on active optical fiber according to claim 1, characterized in that, Active optical fiber has a double-clad structure, which consists of an inner cladding and an outer cladding. The outer cladding is used to confine the pump light within the inner cladding, while the inner cladding wraps around the fiber core. The core mode field diameter is 4 micrometers to 20 micrometers, and the edge-to-edge distance of the inner cladding is 100 micrometers to 3000 micrometers.

10. A wireless passive optical fiber sensing method based on active optical fiber, characterized in that, The method, applied to the wireless passive optical fiber sensing system based on active optical fiber as described in any one of claims 1-9, comprises: Adjust the orientation of the active transmitting module so that the first spatial beam is aligned with the passive receiving module; Adjust the orientation of the receiving module at the active end so that the second spatial beam is aligned with the receiving module at the active end.

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