A fiber-optic pressure sensing method based on orbital angular momentum modes

Abstract

The application discloses a kind of optical fiber pressure sensing methods based on orbital angular momentum mode, it is measured 10ps walk-off distance, and utilize ellipticity or concave angle as intermediate quantity to establish the connection between 10ps walk-off distance and pressure value, realize the measurement of pressure size, provide a kind of feasible scheme for the application of OAM mode in optical fiber pressure sensing.

Description

Technical Field

[0001] This invention relates to fiber optic pressure sensing methods, specifically to a fiber optic pressure sensing method based on orbital angular momentum modes. Background Technology

[0002] One of the advantages of OAM (orbital angular momentum)-based sensing is its multiplexing capability. Different OAM modes can be used as independent sensing channels, allowing multiple pressure sensors to be multiplexed within the same optical fiber. Each OAM mode represents a different sensing channel, enabling simultaneous pressure measurements at different points along the fiber. Furthermore, OAM modes are less susceptible to certain common interferences, such as temperature variations and bending, which can affect other types of fiber optic sensors. This is because OAM modes are based on the spatial phase distribution of light, rather than its intensity or polarization, making them less sensitive to certain environmental disturbances. OAM-based sensing can also be implemented non-invasively, as the optical fiber itself is the sensor. This non-invasiveness is particularly useful in applications where direct contact with the object being measured is undesirable or impossible.

[0003] Fiber optic pressure sensors employing the OAM (Optical Angle, AM) mode are compact and lightweight, making them suitable for remote and distributed sensing applications. Their ability to multiplex multiple sensors within a single fiber gives them a significant advantage in monitoring pressure changes over long distances, allowing simultaneous monitoring of pressure variations at different points. This distributed sensing capability is highly valuable in applications such as structural health monitoring and oil and gas pipeline monitoring.

[0004] Although the application of OAM mode in fiber optic pressure sensing is a promising research area, it is still in the early stages of development, and how to achieve pressure measurement remains an unsolved problem. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide an optical fiber pressure sensing method based on orbital angular momentum mode (OAM). It is the first to propose using the 10ps walk-off distance of the OAM mode in the optical fiber as the sensing measurement value. The optical fiber deforms due to pressure, and this deformation causes a significant change in the walk-off distance of the OAM mode. By measuring this characteristic distance, the degree of optical fiber deformation can be analyzed, thereby realizing the measurement of the pressure magnitude.

[0006] Technical solution: The fiber optic pressure sensing method based on orbital angular momentum mode described in this invention includes:

[0007] (1) Determine the optical fiber and the OAM mode in the optical fiber. The optical fiber is a hollow optical fiber with a central atmospheric hole.

[0008] (2) Apply uniform pressure to the optical fiber using a square pressure block to cause it to deform into an ellipse. Use the elliptic eccentricity as an intermediate value to obtain multiple sets of elliptic eccentricities and their corresponding pressure values ​​and 10ps walk-off distances.

[0009] (3) The first fitting formula is obtained by fitting the ellipse eccentricity and the 10ps walk-off distance, and the second fitting formula is obtained by fitting the ellipse eccentricity and the pressure value.

[0010] (4) Measure the 10ps walk-off distance, obtain the corresponding elliptic eccentricity according to the first fitting formula, and then obtain the corresponding pressure value according to the second fitting formula.

[0011] Furthermore, the optical fiber structure has several air hole layers outside the central atmospheric hole, and the air hole layers are composed of several small air holes arranged in a ring.

[0012] Furthermore, the OAM mode in the optical fiber is formed by the superposition of the parity and even components of the vector mode HE or EH, as shown in formula (1):

[0013]

[0014] Where l is the topological charge, j is the π / 2 phase difference between the even and odd modes of the same vector mode, m is the radial index, the superscript ± indicates the left-hand or right-hand circular polarization direction, the subscript ± indicates the right-hand or left-hand rotation direction of the phase wavefront, even indicates the even component, and odd indicates the odd component.

[0015] Select a vector mode of order 4 or higher for OAM mode.

[0016] Furthermore, the 10ps walk-off distance L 10ps The calculation formula is:

[0017]

[0018] Where c is the speed of light in a vacuum, and 10 ps is time. The effective refractive index for even modes, The effective refractive index for odd modes.

[0019] Furthermore, both the first and second fitting formulas were fitted using the cftool tool in MATLAB.

[0020] As an optional alternative, the fiber optic pressure sensing method based on orbital angular momentum mode described in this invention includes:

[0021] (1) Determine the optical fiber and the OAM mode in the optical fiber. The optical fiber is a hollow optical fiber with a central atmospheric hole.

[0022] (2) Apply non-uniform pressure to the optical fiber using a cylindrical pressure block to cause a change in the concave angle. Use the concave angle as an intermediate value to obtain multiple sets of concave angles and their corresponding pressure values ​​and 10ps walk-off distance.

[0023] (3) The first fitting formula is obtained by fitting the concave angle and the 10ps walk-off distance, and the second fitting formula is obtained by fitting the concave angle and the pressure value.

[0024] (4) Measure the 10ps walk-off distance, obtain the corresponding concave angle according to the first fitting formula, and then obtain the corresponding pressure value according to the second fitting formula.

[0025] Furthermore, the optical fiber structure has several air hole layers outside the central atmospheric hole, and the air hole layers are composed of several small air holes arranged in a ring.

[0026] Furthermore, the OAM mode in the optical fiber is formed by the superposition of the parity and even components of the vector mode HE or EH, as shown in formula (1):

[0027]

[0028] Where l is the topological charge, j is the π / 2 phase difference between the even and odd modes of the same vector mode, m is the radial index, the superscript ± indicates the left-hand or right-hand circular polarization direction, the subscript ± indicates the right-hand or left-hand rotation direction of the phase wavefront, even indicates the even component, and odd indicates the odd component.

[0029] Select a vector mode of order 4 or higher for OAM mode.

[0030] Furthermore, the 10ps walk-off distance L 10ps The calculation formula is:

[0031]

[0032] Where c is the speed of light in a vacuum, and 10 ps is time. The effective refractive index for even modes, The effective refractive index for odd modes.

[0033] Furthermore, both the first and second fitting formulas were fitted using the cftool tool in MATLAB.

[0034] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0035] This invention establishes a relationship between the 10ps walk-off distance and the pressure value by measuring the 10ps walk-off distance and using the elliptic eccentricity or concave angle as intermediate quantities, thereby realizing the measurement of the pressure value and providing a feasible solution for the application of OAM mode in fiber optic pressure sensing. Attached Figure Description

[0036] Figure 1 This is a flowchart of an optical fiber pressure sensing method based on orbital angular momentum mode according to an embodiment of this application;

[0037] Figure 2 This is a schematic diagram of the uniformly compressed elliptical deformation of the annular optical fiber in an embodiment of this application;

[0038] Figure 3 The uniformly compressed ring fiber in the embodiments of this application is in EH 3,1 Elliptical eccentricity and 10ps walk-off distance in the mode;

[0039] Figure 4 The uniformly compressed ring fiber in the embodiments of this application is in EH 3,1 Elliptical eccentricity and pressure value under the mode;

[0040] Figure 5 This is a schematic diagram of the uniformly compressed elliptical deformation of a photonic crystal fiber in an embodiment of this application;

[0041] Figure 6 These are the ellipticity and 10ps walk-off distance of the uniformly stressed photonic crystal fiber in different modes in the embodiments of this application;

[0042] Figure 7 This is a schematic diagram of the uneven compression deformation of the annular optical fiber in an embodiment of this application;

[0043] Figure 8 These are the concave angle and 10ps walk-off distance of the non-uniformly compressed ring fiber in different modes in the embodiments of this application. Detailed Implementation

[0044] The invention will now be further described with reference to the accompanying drawings.

[0045] Example 1

[0046] like Figure 1 A fiber optic pressure sensing method based on orbital angular momentum mode, comprising:

[0047] (1) Determine the optical fiber and the OAM mode in the optical fiber;

[0048] OAM-based sensing fibers require stable transmission of a large number of OAM modes; ordinary single-mode and few-mode fibers cannot meet this requirement. Furthermore, to ensure significant elliptical deformation under stress, hollow fibers with a central atmospheric aperture should be used. Figure 2 The diagram shows the simplest fiber optic structure that meets the requirements. The central atmospheric aperture has a radius r1 = 8 μm. Outside the aperture is a high-refractive-index ring core with an outer diameter r2 = 10 μm and a thickness of 2 μm. The ring core is made of silicon dioxide doped with germanium dioxide at a concentration of 50%, and its refractive index is 1.5172 at 1.55 μm. Outside the ring core is a silicon dioxide cladding with a radius of 62.5 μm, and its refractive index is 1.444 at 1.55 μm.

[0049] In addition, optical fibers can also be used. Figure 5 The photonic crystal fiber shown is in Figure 2 Based on the shown ring fiber, two air-hole layers are added. Each air-hole layer consists of several small air holes with a radius of 1 micrometer arranged in a ring. This fiber is suitable for transmitting a large number of OAM modes, but it is also more prone to elliptical deformation under pressure.

[0050] The OAM mode in an optical fiber is formed by the superposition of the parity and even components of the vector mode HE or EH, as shown in formula (1):

[0051]

[0052] Where l is the topological charge, j is the π / 2 phase difference between the even and odd modes of the same vector mode, m is the radial index, the superscript ± indicates the left-hand or right-hand circular polarization direction, the subscript ± indicates the right-hand or left-hand rotation direction of the phase wavefront, even indicates the even component, and odd indicates the odd component.

[0053] Because the vector mode electric field distributions constituting OAM modes of orders 1 to 3 have too few lobes and too large gaps between lobes, the variation in their 2π walk-off distance is not only related to fiber deformation but also introduces errors. Therefore, vector modes of order 4 and above are selected for OAM modes; in this embodiment, EH is used. 3,1 The pattern is illustrated with examples.

[0054] (2) Figure 2 and Figure 5 As shown, uniform pressure is applied to the optical fiber using a square pressure block to cause it to undergo elliptical deformation. Using the elliptic eccentricity as an intermediate value, multiple sets of elliptic eccentricities and their corresponding pressure values ​​and 10ps walk-off distances are obtained.

[0055] Figure 3 The figure shown is a graph showing the relationship between the obtained elliptic eccentricity of the annular fiber and the 10 ps walk-off distance. Figure 4 The figure shown is a graph showing the relationship between the elliptic eccentricity of the annular fiber and the pressure value.

[0056] Figure 6 The figure shown is a graph showing the relationship between the elliptic eccentricity of the obtained photonic crystal fiber and the 10ps walk-off distance (in the subsequent fitting steps, a ring fiber under uniform pressure is used as an example, and the graph showing the relationship between the elliptic eccentricity of the photonic crystal fiber and the pressure value is not given).

[0057] Depend on Figure 3 and Figure 6 It can be seen that the eccentricity of the ellipse increases, and the 10ps deviation distance L 10psThe performance declined. Testing revealed that hollow fiber should be selected over solid fiber. Although some high-refractive-index core solid fibers can transmit OAM mode, after elliptical deformation, the mode in these fibers completely degenerates into LP mode, no longer meeting the requirements.

[0058] Measurement of 10 ps walk-off distance: The effective refractive indices of the odd and even modes of the vector mode in an optical fiber are ideally very close, with the refractive index difference typically less than 2 × 10⁻⁶. -6 When the circular symmetry of an optical fiber is broken, the refractive index difference between odd and even modes of the same order OAM mode will increase significantly, causing mode walk-off during propagation.

[0059] 10ps walk-off distance L 10ps The calculation formula is:

[0060]

[0061] Where c is the speed of light in a vacuum, and 10 ps is time. The effective refractive index for even modes, The effective refractive index for odd modes.

[0062] (3) The first fitting formula is obtained by fitting the eccentricity of the ellipse and the 10ps walk-off distance, and the second fitting formula is obtained by fitting the eccentricity of the ellipse and the pressure value. The first and second fitting formulas are both fitted using the cftool tool in MATLAB.

[0063] Will Figure 3 10ps walk-off distance L 10ps By fitting a function to the elliptic eccentricity, the relationship between the elliptic eccentricity and the 10ps walk-off distance L is obtained. 10ps The relational formula (i.e., the first fitting formula):

[0064]

[0065] In equation (3), y represents the elliptic eccentricity, and x represents the 10ps walk-off distance L. 10ps .

[0066] Will Figure 4 By fitting a function between the elliptic eccentricity and the pressure value, the relationship between the elliptic eccentricity and the pressure is obtained (i.e., the second fitting formula):

[0067] y = -9.953x 6 +60.88x 5 -104x 4 +81.37x 3 -32.9x 2 +6.701x-0.5352 (4)

[0068] In equation (4), y represents the pressure magnitude and x represents the eccentricity of the ellipse.

[0069] It should be noted that different optical fibers have different characteristics, and the fitting formulas are also different. Formulas (3) and (4) are only applicable to different optical fibers. Figure 2 The optical fiber shown meets the above structural parameters. However, different optical fibers achieve a 10ps walk-off distance L. 10ps The method for obtaining the fitting formula after determining the eccentricity of the ellipse is the same.

[0070] (4) Measure the 10ps walk-off distance, obtain the corresponding elliptic eccentricity according to the first fitting formula, and then obtain the corresponding pressure value according to the second fitting formula.

[0071] Example 2

[0072] This second embodiment is basically the same as the first embodiment, except that:

[0073] The intermediate quantity is replaced by the elliptic eccentricity in Example 1 with the concave angle θ. When applying pressure to the optical fiber, a solid cylindrical pressure block made of a harder material than the optical fiber diameter is used. This cylindrical pressure block applies uneven pressure to the optical fiber, causing it to deform inwards. Figure 7 As shown. Figure 8 In OAM mode, the 10ps walk-off distance is related to the concave angle θ. The larger the concave angle θ, the lower the 10ps walk-off distance.

[0074] This invention is applicable to all optical fibers in OAM mode with a transmission order greater than 4. Because different optical fibers have different structures and materials, the forces causing elliptical deformation or changes in concave angle also differ. Furthermore, the calculated 10ps walk-off distance L for ring cores of different sizes and thicknesses... 10ps They are also different. Therefore, for different optical fibers, the method of this invention needs to be used for targeted data acquisition and formula fitting.

Claims

1. A fiber optic pressure sensing method based on orbital angular momentum mode, characterized in that, include: (1) Determine the optical fiber and the OAM mode in the optical fiber. The optical fiber is a hollow optical fiber with a central atmospheric hole. (2) Apply uniform pressure to the optical fiber using a square pressure block to cause it to deform into an ellipse. Use the elliptic eccentricity as an intermediate value to obtain multiple sets of elliptic eccentricities and their corresponding pressure values ​​and 10ps walk-off distances. (3) The first fitting formula is obtained by fitting the ellipse eccentricity and the 10ps walk-off distance, and the second fitting formula is obtained by fitting the ellipse eccentricity and the pressure value. (4) Measure the 10ps walk-off distance, obtain the corresponding elliptic eccentricity according to the first fitting formula, and then obtain the corresponding pressure value according to the second fitting formula.

2. The fiber optic pressure sensing method according to claim 1, characterized in that, In the optical fiber structure, several air hole layers are provided outside the central atmospheric pore, and the air hole layers are composed of several small air holes arranged in a ring.

3. The fiber optic pressure sensing method according to claim 1, characterized in that, The OAM mode in an optical fiber is formed by the superposition of the parity and even components of the vector mode HE or EH, as shown in formula (1): Where l is the topological charge, j is the π / 2 phase difference between the even and odd modes of the same vector mode, m is the radial index, the superscript ± indicates the left-hand or right-hand circular polarization direction, the subscript ± indicates the right-hand or left-hand rotation direction of the phase wavefront, even indicates the even component, and odd indicates the odd component. Select a vector mode of order 4 or higher for OAM mode.

4. The fiber optic pressure sensing method according to claim 3, characterized in that, 10ps walk-off distance L 10ps The calculation formula is: Where c is the speed of light in a vacuum, and 10 ps is time. The effective refractive index for even modes, The effective refractive index for odd modes.

5. The fiber optic pressure sensing method according to claim 1, characterized in that, Both the first and second fitting formulas were fitted using the cftool tool in MATLAB.

6. A fiber optic pressure sensing method based on orbital angular momentum mode, characterized in that, include: (1) Determine the optical fiber and the OAM mode in the optical fiber. The optical fiber is a hollow optical fiber with a central atmospheric hole. (2) Apply non-uniform pressure to the optical fiber using a cylindrical pressure block to cause a change in the concave angle. Use the concave angle as an intermediate value to obtain multiple sets of concave angles and their corresponding pressure values ​​and 10ps walk-off distance. (3) The first fitting formula is obtained by fitting the concave angle and the 10ps walk-off distance, and the second fitting formula is obtained by fitting the concave angle and the pressure value. (4) Measure the 10ps walk-off distance, obtain the corresponding concave angle according to the first fitting formula, and then obtain the corresponding pressure value according to the second fitting formula.

7. The fiber optic pressure sensing method according to claim 6, characterized in that, In the optical fiber structure, several air hole layers are provided outside the central atmospheric pore, and the air hole layers are composed of several small air holes arranged in a ring.

8. The fiber optic pressure sensing method according to claim 6, characterized in that, The OAM mode in an optical fiber is formed by the superposition of the parity and even components of the vector mode HE or EH, as shown in formula (1): Where l is the topological charge, j is the π / 2 phase difference between the even and odd modes of the same vector mode, m is the radial index, the superscript ± indicates the left-hand or right-hand circular polarization direction, the subscript ± indicates the right-hand or left-hand rotation direction of the phase wavefront, even indicates the even component, and odd indicates the odd component; the OAM mode selects a vector mode of order 4 or above.

9. The fiber optic pressure sensing method according to claim 8, characterized in that, 10ps walk-off distance L 10ps The calculation formula is: Where c is the speed of light in a vacuum, and 10 ps is time. The effective refractive index for even modes, The effective refractive index for odd modes.

10. The fiber optic pressure sensing method according to claim 6, characterized in that, Both the first and second fitting formulas were fitted using the cftool tool in MATLAB.

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