High-sensitivity polarization maintaining optical fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating

Abstract

The invention discloses a high-sensitivity polarization maintaining optical fiber multi-parameter simultaneous demodulation method based on a Brillouin dynamic grating. According to the method, simultaneous demodulation of the temperature, the fast axis pressure and the slow axis pressure is realized by using a relational expression # imgabs0 #. In the formula, # imgabs 1 # is a pressure sensing coefficient of birefringence frequency shift when force is applied along a fast axis, # imgabs 2 # is a pressure sensing coefficient of birefringence frequency shift when force is applied along a slow axis, # imgabs 3 # is a temperature sensing coefficient of birefringence frequency shift, # imgabs 4 # is a pressure sensing coefficient of Brillouin frequency shift when force is applied along the fast axis, # imgabs 5 # is a pressure sensing coefficient of Brillouin frequency shift when force is applied along the slow axis, and # imgabs 4 # is a pressure sensing coefficient of Brillouin frequency shift when force is applied along the slow axis. # 6 is a temperature sensing coefficient of Brillouin frequency shift, # 7 is a pressure sensing coefficient of Brillouin line width when force is applied along a fast axis, # 8 is a pressure sensing coefficient of Brillouin line width when force is applied along a slow axis, # 9 is a temperature sensing coefficient of Brillouin line width, and # 6 is a temperature sensing coefficient of Brillouin line width. DeltaP1, DeltaP2 and DeltaT are pressure variation when force is applied along the fast axis, pressure variation when force is applied along the slow axis and temperature variation respectively.

Description

Technical Field

[0001] The present invention relates to the field of optical fiber technology, and in particular to a high-sensitivity polarization-maintaining optical fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating. Background Art

[0002] Distributed fiber-optic sensing technology based on Brillouin scattering is widely used in fields such as dams, bridges, roads, and medical applications due to its advantages in temperature and strain measurement, such as high precision, a large measurement range, and high spatial resolution. However, traditional distributed Brillouin fiber-optic sensing can only measure temperature and strain, with low measurement accuracy and problems such as cross-sensitivity. To address these issues, researchers have proposed fiber-optic sensing based on Brillouin dynamic grating (BDG). This effectively separates the pump light, probe light, and reflected light, improving measurement accuracy and spatial resolution while enabling simultaneous temperature and strain sensing. Typically, two pump beams with the same polarization state are injected into an optical fiber, which stimulates a coherent acoustic field through the stimulated Brillouin scattering (SBS) effect, which then forms a BDG. BDG technology addresses the shortcomings of traditional distributed Brillouin fiber-optic sensing by separating BDG generation and detection into independent processes. This allows for shorter probe pulse widths, improving spatial resolution, and resulting in a narrower Brillouin gain spectrum.

[0003] Civil structures and large-scale machinery are inevitably subject to lateral pressure generated by their own weight and external impacts during construction and use. Therefore, a reliable and efficient lateral pressure sensor is needed in actual production. In addition, temperature is also an important physical quantity to be monitored, and in many cases, it is necessary to achieve simultaneous measurement of the two. Brillouin frequency shift is often used to sense temperature or pressure in fiber-optic distributed sensing based on Brillouin scattering, but it is sensitive to both temperature and pressure, making it difficult to achieve simultaneous measurement of temperature and pressure. To solve this problem, researchers have proposed Brillouin dynamic grating generation and detection based on a single laser. In order to achieve high-precision demodulation of temperature and strain, researchers have proposed a single polymer diaphragm fiber Bragg grating sensor for simultaneous measurement of pressure and temperature. Although existing research has been able to perform high-precision simultaneous demodulation of temperature and pressure, the simultaneous demodulation of the three variables of temperature, fast-axis pressure, and slow-axis pressure has not yet been achieved. To this end, the present invention proposes a high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating.

[0004] Because polarization-maintaining fiber is sensitive to temperature, fast-axis pressure, and slow-axis pressure, this technology can be applied to high-precision fiber gyroscope ring detection. Furthermore, the sensor and demodulation method proposed in this invention provide valuable insights into the simultaneous, distributed monitoring of temperature and pressure in different directions during the construction and operation of civil structures and large machinery. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating to solve the problems of low sensitivity and narrow measurement range in the prior art and to achieve polarization-maintaining fiber multi-parameter simultaneous demodulation.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: Step 1: A high-sensitivity, polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic gratings was designed. The cross-section of the photonic crystal fiber amplifier includes a cladding and a core. The characteristic feature is that the introduction of an elliptical air hole destroys the original symmetric structure of the photonic crystal fiber.

[0007] Furthermore, the material of the optical fiber is SiO2.

[0008] Step 2: When pressure is applied to the sensing fiber, it undergoes elastic deformation, causing an elasto-optic effect that changes the fiber's birefringence. When the ambient temperature changes, the sensing fiber deforms, and the restrained deformation generates thermal stress, which also changes the fiber's birefringence. This change in birefringence leads to a change in the birefringence frequency shift. The change in birefringence frequency shift due to temperature, fast-axis pressure, or slow-axis pressure can be expressed as: in: is the pressure sensing coefficient, ΔP is the pressure change, is the temperature sensing coefficient, and ΔT is the temperature change.

[0009] Step 3: When the designed high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature is affected by temperature, fast-axis pressure or slow-axis pressure, the Brillouin frequency shift will change. The change in Brillouin frequency shift caused by temperature, fast-axis pressure or slow-axis pressure can be expressed as: in: is the pressure sensing coefficient, is the temperature sensing coefficient.

[0010] Step 4: When the designed high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature is affected by temperature, fast-axis pressure, or slow-axis pressure, the Brillouin linewidth will change. The change in Brillouin linewidth caused by temperature, fast-axis pressure, or slow-axis pressure can be expressed as: in: is the pressure sensing coefficient, is the temperature sensing coefficient.

[0011] When the temperature is fixed, stress analysis is performed on the photonic crystal fiber amplifier in step 1 when pressure is applied along the fast axis, slow axis, and different directions. The changes in birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth under the corresponding pressure can be obtained. When the pressure is fixed, the operating temperature of the fiber model is changed from 0°C to 100°C in steps of 10°C. The changes in birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth under the corresponding temperature can be obtained. Substituting the changes in birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth under different pressures and the changes in birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth under different temperatures into equations (1), (2), and (3), the changes in birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth with temperature under different pressures can be obtained.

[0012] Step 5: The designed optical fiber sensor satisfies the following relationship with the calculated birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth along temperature, fast axis pressure, and slow axis pressure: in: is the pressure sensing coefficient of the birefringence frequency shift when force is applied along the fast axis, is the pressure sensing coefficient of the birefringence frequency shift when force is applied along the slow axis, is the temperature sensing coefficient of the birefringence frequency shift, is the pressure sensing coefficient of the Brillouin frequency shift when force is applied along the fast axis, is the pressure sensing coefficient of the Brillouin frequency shift when force is applied along the slow axis, is the temperature sensing coefficient of the Brillouin frequency shift, is the pressure sensing coefficient of the Brillouin line width when force is applied along the fast axis, is the pressure sensing coefficient of the Brillouin line width when the force is applied along the slow axis, is the temperature sensing coefficient of the Brillouin linewidth, ΔP1, ΔP2, and ΔT are the pressure change when force is applied along the fast axis, the pressure change when force is applied along the slow axis, and the temperature change, respectively. Solving equation (4) yields the relationship between fast axis pressure, slow axis pressure, and temperature and the birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth, which allows for simultaneous demodulation of multiple parameters, specifically expressed as: By substituting specific parameters into equation (5), it is possible to verify whether the three variables are all related to the birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth and achieve demodulation.

[0013] In the above technical solution, the present invention provides a high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating, which has the following beneficial effects:

[0014] When a transverse pressure of 0 to 30 MPa and a temperature of 0 to 100°C are applied, the pressure sensitivity of the photonic crystal fiber along its fast axis is -1.961 GHz / MPa, the pressure sensitivity along its slow axis is 1.356 GHz / MPa, and its temperature sensitivity is 0.105 MHz / °C. This represents a -957 MHz / MPa improvement over the currently optimal photonic crystal fiber structure.

[0015] According to the relationship between temperature, fast axis pressure, slow axis pressure and birefringence frequency shift, Brillouin frequency shift and Brillouin linewidth, the present invention can realize simultaneous demodulation of multiple parameters. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a system flow chart of a high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating in a specific embodiment.

[0017] Figure 2 This is a schematic cross-sectional view of a high-sensitivity fiber optic sensor for monitoring pressure and temperature based on Brillouin dynamic gratings, according to a specific embodiment. In the figure: 1 - substrate material, 2 - outermost row of large elliptical air holes, 3 - inner row of small elliptical air holes, 4 - circular air holes; a1 - major axis of the large elliptical air hole, b1 - minor axis of the large elliptical air hole, a2 - major axis of the small elliptical air hole, b2 - minor axis of the small elliptical air hole, d1 - diameter of the photonic crystal fiber, d2 - diameter of the circular air hole, d3 - distance between the large elliptical air hole and the fiber core.

[0018] Figure 3 This is a block diagram of the experimental principle of a high-sensitivity optical fiber sensor capable of monitoring pressure and temperature based on Brillouin dynamic grating in a specific embodiment.

[0019] Figure 4a and Figure 4b These are respectively changes in the cross section of the optical fiber when pressure is applied along the fast axis and the slow axis by a high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature in a specific embodiment.

[0020] Figure 5a and Figure 5bThese are respectively diagrams showing the relationship between the birefringence frequency shift and temperature and pressure when pressure is applied in the fast axis direction or the slow axis direction while the temperature is changed in a specific embodiment of a high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature.

[0021] Figure 6a and Figure 6b They are respectively diagrams showing the relationship between the Brillouin frequency shift and temperature and pressure when the pressure is applied in the fast axis direction or the slow axis direction while the temperature is changed in a specific embodiment of the high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature.

[0022] Figure 7a and Figure 7b These are respectively diagrams showing the relationship between the Brillouin linewidth and temperature and the change in pressure when the pressure is applied in the fast axis direction or the slow axis direction while the temperature is changed in a specific embodiment of a high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature.

[0023] Figure 8 This is the error between the simulation value and the actual applied temperature, fast-axis pressure, and slow-axis pressure when a high-sensitivity fiber optic sensor based on Brillouin dynamic grating capable of monitoring pressure and temperature applies pressure along both the fast and slow axes while changing the temperature in a specific embodiment. DETAILED DESCRIPTION

[0024] The present invention will be further described in detail below with reference to the accompanying drawings and through examples. The following examples are intended to explain the present invention but the present invention is not limited to the following examples.

[0025] Figure 2 Shown is a schematic cross-sectional view of a high-sensitivity fiber optic sensor for monitoring pressure and temperature based on a Brillouin dynamic grating according to the present invention. Substrate material 1 is SiO2. The cross-section of this photonic crystal fiber amplifier exhibits a multi-layered structure. The core region is surrounded by a layer of smaller air holes, and the cladding incorporates a pair of larger air holes symmetrically along the longitudinal axis. The major and minor axes of the larger elliptical air hole are a1 and b1, respectively, while those of the smaller elliptical air hole are a2 and b2, respectively. The diameter of the photonic crystal fiber is d1, the diameter of the circular air hole is d2, and the distance between the larger elliptical air hole and the core is d3.

[0026] In this embodiment, the parameters of the high-sensitivity optical fiber sensor for monitoring pressure and temperature based on Brillouin dynamic grating are as follows: the major axis a1 and minor axis b1 of the large elliptical air hole are 80 μm and 40 μm, respectively; the major axis a2 and minor axis b2 of the small elliptical air hole are 6 μm and 3 μm, respectively; the diameter d1 of the photonic crystal fiber is 125 μm; the diameter d2 of the circular air hole is 3 μm; and the distance d3 between the large elliptical air hole and the fiber core is 30 μm.

[0027] The experimental principle diagram of the high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature in this embodiment is as follows: Figure 3 As shown:

[0028] In this Brillouin dynamic grating system, two pump beams are injected into a polarization-maintaining fiber along the x-axis under the control of a polarization controller, "writing" the dynamic fiber Bragg grating (FBG). A narrow-linewidth semiconductor laser is used to generate probe light. This light is first modulated into pulses by an electro-optical modulator 1, then amplified by an erbium-doped fiber amplifier and a polarization controller to propagate it along the y-axis. It then passes through a circulator and a polarization beam splitter into a polarization-maintaining fiber for "reading" the fiber Bragg grating spectrum. The thick pink lines in the figure all utilize polarization-maintaining fiber, while the rest utilize single-mode fiber. Optical power meter 1 is used to detect the BOTDA signal. Optical power meter 2 monitors the power of the pump and probe beams 1 and the time delay between them. Optical power meter 3 measures the reflected signal from the dynamic grating, thereby reading the birefringence spectrum.

[0029] In this embodiment, the high-sensitivity optical fiber sensor based on Brillouin dynamic grating capable of monitoring pressure and temperature is subjected to stress along the fast axis and slow axis at a pressure of 30 MPa. The change in the optical fiber cross section is as follows: Figure 4a and 4b As shown, observe Figure 4a and 4b We can get:

[0030] The high-sensitivity optical fiber sensor based on Brillouin dynamic grating for monitoring pressure and temperature of the present invention introduces an elliptical air hole, thereby destroying the original symmetric structure of PCF, making its structure different in the fast axis and slow axis directions, thereby generating a birefringence effect Δn=|n x eff -n y eff |, where n x eff is the effective refractive index of the x polarization state, n y eff is the effective refractive index of the y polarization state.

[0031] In this embodiment, the high-sensitivity optical fiber sensor based on Brillouin dynamic grating capable of monitoring pressure and temperature applies pressure along the fast axis and the slow axis, and the birefringence frequency shift is as follows: Figure 5a and 5b As shown, observe Figure 5a and 5b We can get:

[0032] The high-sensitivity fiber optic sensor for monitoring pressure and temperature based on Brillouin dynamic gratings described in the present invention exhibits a linear relationship between the birefringence frequency shift and temperature or pressure when transverse pressure is applied along the fast axis and the temperature is varied. The birefringence frequency shift increases slowly with increasing temperature but gradually decreases with increasing pressure. Furthermore, when transverse pressure is applied along the slow axis and the temperature is varied, the birefringence frequency shift exhibits a linear relationship with temperature or pressure, increasing with increasing temperature and pressure.

[0033] In this embodiment, the high-sensitivity optical fiber sensor based on Brillouin dynamic grating capable of monitoring pressure and temperature applies pressure along the fast axis and the slow axis. The Brillouin frequency shift is as follows: Figure 6a and 6b As shown, observe Figure 6a and 6b We can get:

[0034] The high-sensitivity fiber optic sensor for monitoring pressure and temperature based on Brillouin dynamic gratings described in the present invention exhibits a linear relationship between the Brillouin frequency shift and temperature or pressure when transverse pressure is applied along the fast axis and the temperature is varied, increasing with increasing temperature and pressure. Furthermore, the Brillouin frequency shift exhibits a linear relationship between the Brillouin frequency shift and temperature or pressure when transverse pressure is applied along the slow axis and the temperature is varied, increasing with increasing temperature but decreasing with increasing pressure.

[0035] In this embodiment, the high-sensitivity optical fiber sensor based on Brillouin dynamic grating capable of monitoring pressure and temperature applies pressure along the fast axis and the slow axis. Figure 7a and 7b As shown, observe Figure 7a and 7b We can get:

[0036] The high-sensitivity fiber optic sensor for monitoring pressure and temperature based on Brillouin dynamic gratings described in the present invention exhibits a linear relationship between the Brillouin linewidth and temperature or pressure when transverse pressure is applied along the fast axis and the temperature is varied, increasing with increasing temperature and pressure. Furthermore, the Brillouin linewidth exhibits a linear relationship between the Brillouin linewidth and temperature or pressure when transverse pressure is applied along the slow axis and the temperature is varied, increasing with increasing temperature but gradually decreasing with increasing pressure.

[0037] In this embodiment, the high-sensitivity fiber optic sensor based on Brillouin dynamic grating that can monitor pressure and temperature applies pressure along the fast axis and slow axis simultaneously. When the temperature is changed, the error between the simulation value and the actual applied temperature, fast axis pressure and slow axis pressure is shown in FIG7 . Figure 8 We can get:

[0038] The maximum error values ​​of fast axis pressure, slow axis pressure and temperature are 0.78MPa, 0.89MPa and 0.88℃ respectively, and the demodulation error is kept within 1MPa or 1℃. Figure 8 Statistical analysis of the demodulation error values ​​for all simulation cases revealed that the mean errors for fast-axis pressure, slow-axis pressure, and temperature were 0.21 MPa, 0.31 MPa, and 0.30°C, respectively, and the standard deviations were 0.15 MPa, 0.21 MPa, and 0.21°C, respectively. This preliminarily validates the effectiveness of the proposed method.

[0039] The above is only one embodiment of the present invention, not all or the only embodiment. Any equivalent transformation of the technical solution of the present invention made by ordinary technicians in this field after reading the specification of the present invention is covered by the claims of the present invention.

Claims

1. A high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating, characterized in that: The following steps are involved: Step 1: A high-sensitivity fiber optic sensor for monitoring pressure and temperature based on Brillouin dynamic gratings was designed. The sensor's characteristics include: a novel edge-hole-type polarization-maintaining photonic crystal fiber amplifier with a multi-layered cross-section, where the core is surrounded by a layer of smaller air holes and the cladding has a pair of larger air holes symmetrically introduced along the longitudinal axis; Step 2: When the designed high-sensitivity fiber optic sensor based on Brillouin dynamic grating that can monitor pressure and temperature is affected by temperature, fast-axis pressure, or slow-axis pressure, the birefringence frequency shift will change. The change in birefringence frequency shift caused by temperature, fast-axis pressure, or slow-axis pressure can be expressed as: in: is the pressure sensing coefficient, ΔP is the pressure change, is the temperature sensing coefficient, ΔT is the temperature change; Step 3: When the designed high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature is affected by temperature, fast-axis pressure or slow-axis pressure, the Brillouin frequency shift will change. The change in Brillouin frequency shift caused by temperature, fast-axis pressure or slow-axis pressure can be expressed as: in: is the pressure sensing coefficient, is the temperature sensing coefficient; Step 4: When the designed high-sensitivity optical fiber sensor based on Brillouin dynamic grating that can monitor pressure and temperature is affected by temperature, fast-axis pressure, or slow-axis pressure, the Brillouin linewidth will change. The change in Brillouin linewidth caused by temperature, fast-axis pressure, or slow-axis pressure can be expressed as: in: is the pressure sensing coefficient, is the temperature sensing coefficient; Step 5: The designed optical fiber sensor satisfies the following relationship with the calculated birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth along temperature, fast axis pressure, and slow axis pressure: in: is the pressure sensing coefficient of the birefringence frequency shift when force is applied along the fast axis, is the pressure sensing coefficient of the birefringence frequency shift when force is applied along the slow axis, is the temperature sensing coefficient of the birefringence frequency shift, is the pressure sensing coefficient of the Brillouin frequency shift when force is applied along the fast axis, is the pressure sensing coefficient of the Brillouin frequency shift when force is applied along the slow axis, is the temperature sensing coefficient of the Brillouin frequency shift, is the pressure sensing coefficient of the Brillouin line width when force is applied along the fast axis, is the pressure sensing coefficient of the Brillouin line width when the force is applied along the slow axis, is the temperature sensing coefficient of the Brillouin linewidth, ΔP1, ΔP2, and ΔT are the changes in pressure when a force is applied along the fast axis, the changes in pressure when a force is applied along the slow axis, and the changes in temperature, respectively; According to formula (4), the relationship between fast axis pressure, slow axis pressure and temperature and birefringence frequency shift, Brillouin frequency shift and Brillouin linewidth can be obtained, based on which simultaneous demodulation of multiple parameters can be achieved, which is specifically expressed as By substituting specific parameters into equation (5), it is possible to verify whether the three variables are all related to the birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth and achieve demodulation.

2. The high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating according to claim 1, characterized in that: The introduction of the elliptical air hole in step 1 destroys the original symmetrical structure of the photonic crystal fiber.

3. The high-sensitivity polarization-maintaining fiber multi-parameter simultaneous demodulation method based on Brillouin dynamic grating according to claim 1, characterized in that: In the step 1, the optical fiber material is pure SiO2.

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