Long-period fiber grating barometric sensor based on mode conversion and temperature compensation

Abstract

The invention discloses a long-period fiber bragg grating barometric sensor based on mode conversion and temperature compensation. The long-period fiber bragg grating barometric sensor comprises a long-period fiber bragg grating, the PVA coating is coated on the outer surface of the long-period fiber grating, and the thickness of the PVA coating is configured to enable the effective refractive index of a cladding mode to be in a mode conversion interval; the outer surface of the PVA coating is coated with the PDMS coating, and the PDMS coating is configured to respond to external air pressure change and change the refractive index of the PDMS coating; and the fiber bragg grating and the long-period fiber bragg grating are connected in series in the same optical fiber light path, and the fiber bragg grating is used for sensing the environment temperature and performing temperature compensation on the air pressure measurement result of the long-period fiber bragg grating. The outer surface of the long-period fiber grating is coated with the PVA coating, the outer surface of the PVA coating is coated with the PDMS coating, and the fiber bragg grating is connected in series, so that the cross sensitivity influence of temperature change on an air pressure measurement structure is effectively solved, and the measurement precision and reliability of the sensor are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of fiber optic sensing technology, and in particular to a long-period fiber optic barometric pressure sensor based on mode conversion and temperature compensation. Background Technology

[0002] Accurate measurement of air pressure is crucial in many fields, such as weather monitoring and aircraft altitude determination. Traditional barometric pressure sensors, such as resonant cylinder type and capacitive type, exhibit limitations in complex environments. Fiber optic barometric pressure sensors, due to their high sensitivity and resistance to electromagnetic interference, have become a research hotspot. Long-period fiber gratings (LPFGs) are a novel type of passive fiber optic device. They can be fabricated using various methods, including ultraviolet (UV) exposure, which utilizes the photosensitivity of the fiber to create a spatial phase grating within the fiber core by irradiating it with UV light, causing periodic changes in the refractive index; mechanical writing, which uses mechanical means to etch periodic structures onto the fiber; and femtosecond laser writing, which is also widely used to precisely fabricate the desired grating structure within the fiber using the high energy of a femtosecond laser. LPFGs can couple the core fundamental mode energy to the cladding mode and are extremely sensitive to changes in external parameters such as temperature, pressure, and refractive index, making them a promising candidate for applications in optical communication and sensing.

[0003] Chinese patent CN113189048B discloses a "long-period fiber optic grating NH3 gas sensor based on graphene oxide / cellulose acetate". By adsorbing NH3 through graphene oxide / cellulose acetate, the refractive index of the composite film is changed, which further changes the coupling efficiency between the cladding and the core of the long-period fiber optic grating, thereby affecting the output signal of the grating and achieving the detection of NH3 gas.

[0004] However, the LPFG-based sensors described above still face two inherent challenges when dealing with simultaneous changes in multiple physical quantities in the environment: (1) LPFGs are sensitive to multiple parameters such as temperature, strain, and refractive index simultaneously, with temperature drift being the most significant source of interference affecting measurement accuracy. The aforementioned schemes only focus on refractive index changes caused by gas adsorption, so their measurement results are easily affected by temperature fluctuations in real environments, leading to decreased accuracy; (2) For the measurement of physical quantities such as air pressure, the sensitivity of traditional LPFGs or schemes with simple coating of sensitive materials is insufficient to meet the requirements of high-precision applications. Therefore, it is urgent to propose a long-period fiber optic grating pressure sensor based on mode conversion and temperature compensation to solve the problems existing in the above-mentioned technologies. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides a long-period fiber Bragg grating barometric pressure sensor based on mode switching and temperature compensation. By coating the outer surface of the long-period fiber Bragg grating with a PVA coating, the effective refractive index of the cladding mode is placed within the mode switching range. A PDMS coating is then applied to the outer surface of the PVA coating to respond to changes in external air pressure and alter its refractive index. Furthermore, a fiber Bragg grating is connected in series to effectively resolve the cross-sensitivity effect of temperature changes on the barometric pressure measurement structure, significantly improving the sensor's measurement accuracy and reliability.

[0006] To achieve the above objectives, according to one aspect of the present invention, a long-period fiber Bragg grating barometric pressure sensor based on mode conversion and temperature compensation is provided, comprising: Long-period fiber gratings; A PVA coating is applied to the outer surface of the long-period fiber grating, the thickness of which is configured to keep the effective refractive index of the cladding mode within the mode transition range. A PDMS coating is applied to the outer surface of the PVA coating, the PDMS coating being configured to change its refractive index in response to changes in external air pressure; And a fiber Bragg grating connected in series with the long-period fiber grating in the same fiber optic path, used to sense ambient temperature and perform temperature compensation on the air pressure measurement results of the long-period fiber grating.

[0007] As an embodiment of this application, the mode conversion range is the parameter working range in which the effective refractive index of the cladding mode changes the most with the thickness of the PVA coating.

[0008] As one embodiment of this application, the thickness of the PVA coating in the mode transition range is 100nm-300nm.

[0009] As one embodiment of this application, the PDMS coating is a pressure-sensitive material with a porous structure, and its refractive index increases with increasing external air pressure.

[0010] As an embodiment of this application, the core radius and cladding radius of the long-period fiber grating are set to ranges of 3.5-4.5 μm and 55-65 μm, respectively.

[0011] As an embodiment of this application, the core refractive index and cladding refractive index of the long-period fiber grating are set to 1.4501 and 1.4449, respectively.

[0012] As an embodiment of this application, the phase matching relationship of the resonant peak of the long-period fiber grating is as follows:

[0013] in, This indicates the resonance peak of a long-period fiber grating; Indicates the effective refractive index of the core mold; Indicates the effective refractive index of the cladding mode. This indicates the period of a long-period fiber grating.

[0014] As one embodiment of this application, the wavelength of the long-period fiber grating undergoes a blue shift as the external air pressure increases.

[0015] As one embodiment of this application, the fiber Bragg grating undergoes temperature compensation via a compensation matrix, calculated using the following formula:

[0016] in, , These are the pressure sensitivity and temperature sensitivity of long-period fiber Bragg gratings, respectively. Temperature sensitivity of fiber Bragg gratings; This represents the wavelength shift of the fiber Bragg grating. This represents the wavelength shift of a long-period fiber grating. This refers to the change in temperature. This represents the change in air pressure.

[0017] The beneficial effects of this invention are as follows: (1) The present invention coats a PVA coating on the outer surface of a long-period fiber grating. The thickness of the PVA coating is configured such that the effective refractive index of the cladding mode is in the mode switching range. The thickness of the PVA coating in the mode switching range is 100nm-300nm. This makes the effective refractive index of the cladding mode extremely sensitive to changes in the external environment within the mode switching range. It converts the slight refractive index change caused by air pressure into a drastic shift in the resonant wavelength, thereby achieving an order-of-magnitude improvement in air pressure sensitivity and enabling the detection of extremely weak air pressure changes.

[0018] (2) Based on the refractive index sensitive characteristics of long-period fiber gratings, this invention can achieve the measurement of various physical parameters by combining sensitive materials. By coating a PDMS coating on the outer surface of the PVA coating, the PDMS coating is configured to respond to changes in external air pressure and change its refractive index, thereby causing the resonant wavelength of the long-period fiber grating to drift, thus realizing air pressure measurement.

[0019] (3) By connecting a fiber Bragg grating in series with the long-period fiber grating in the same fiber optical path, the present invention can sense the ambient temperature and use the matrix algorithm matrix to perform temperature compensation on the air pressure measurement results of the long-period fiber grating. This can eliminate the cross-sensitivity effect of temperature change on the air pressure measurement results in real time and effectively, and significantly improve the measurement accuracy and reliability of the sensor in complex temperature change environments. Attached Figure Description

[0020] Figure 1 This is a schematic cross-sectional view of the long-period fiber optic barometric pressure sensor based on mode conversion and temperature compensation provided in an embodiment of the present invention. Figure 2 This is another cross-sectional structural diagram of the long-period fiber optic barometric pressure sensor based on mode conversion and temperature compensation provided in an embodiment of the present invention. Figure 3 This is a graph showing the change in effective refractive index of the cladding mode with coating thickness in a long-period fiber optic barometric pressure sensor based on mode conversion and temperature compensation provided in an embodiment of the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] Reference Figures 1-3 The first aspect of the present invention provides a long-period fiber optic barometric pressure sensor based on mode conversion and temperature compensation, comprising: The long-period fiber grating (LPFG)1 has a grating period preferably set to 364 μm and a grating length preferably set to 3 cm.

[0023] A PVA (polyvinyl alcohol) coating 2 is applied to the outer surface of the long-period fiber grating 1, the thickness of which is configured to keep the effective refractive index of the cladding mode within the mode transition range. A PDMS (polydimethylsiloxane) coating 3 is coated on the outer surface of the PVA coating 2, the PDMS coating 3 being configured to change its refractive index in response to changes in external air pressure; And a fiber Bragg grating (FBG) connected in series with the long-period fiber grating 1 in the same fiber optic path, used to sense the ambient temperature and perform temperature compensation on the air pressure measurement results of the long-period fiber grating 1.

[0024] This application employs an all-fiber structure, primarily comprising a long-period fiber grating (LPFG) and a fiber Bragg grating (FBG). It achieves high-performance sensing by coating functional polymer coatings (PVA and PDMS) using a dip-coating method, eliminating the need for complex mechanical structures, circuits, or expensive optical components. This structure boasts a relatively simple fabrication process, low cost, and ease of integration with existing fiber optic systems, enabling large-scale applications. It effectively solves the technical problems of high cost and difficulty in existing demodulation methods, which limit their widespread application, and promotes the development of high-sensitivity LPFG barometric pressure sensors.

[0025] Specifically, the long-period fiber grating 1 is a passive optical device capable of coupling the forward-propagating core and cladding modes in a single-mode fiber and coupling optical signals of specific wavelengths into the fiber cladding. This is manifested in the transmission spectrum as a resonance peak appearing at a specific wavelength position. The phase matching relationship of the resonance peaks of the long-period fiber grating 1 is as follows:

[0026] in, This represents the resonance peak of long-period fiber grating 1; Indicates the effective refractive index of the core mold; Indicates the effective refractive index of the cladding mode; This represents the period of long-period fiber grating 1. The effective refractive index of the core mode is relatively stable with changes in the refractive index of the external environment; fluctuations in the external refractive index only affect the effective refractive index of the cladding mode. When the environmental refractive index increases, The value also increases accordingly, and the long-period fiber grating 1 A blue shift occurs. Therefore, the long-period fiber grating 1 can be used as a refractive index sensor.

[0027] Based on the refractive index-sensitive characteristics of the long-period fiber grating 1, combined with a sensitive material, the measurement of various physical parameters can be achieved. Preferably, this application selects a PDMS coating 3 with a porous structure as the pressure-sensitive material. Its volume and refractive index change significantly with changes in external air pressure, causing the resonant wavelength of the long-period fiber grating 1 to drift accordingly, thereby realizing air pressure measurement.

[0028] This application utilizes chemically stable materials such as PDMS and PVA, which offer advantages such as resistance to electromagnetic interference and corrosion, making them suitable for long-term operation in harsh environments. Furthermore, this design principle is not only applicable to barometric pressure sensing; with slight modifications (such as changing the sensitive material), it can be applied to high-sensitivity measurements of various parameters, including humidity, stress, and chemical substances, demonstrating excellent application scalability.

[0029] To further increase the sensitivity of the sensor, this application introduces a high-refractive-index PVA coating. Specifically, the core radius and cladding radius of the long-period fiber grating 1 are set to 3.5-4.5 μm and 55-65 μm, respectively; the core refractive index and cladding refractive index of the long-period fiber grating 1 are set to 1.4501 and 1.4449, respectively, and the refractive index of air is set to 1.

[0030] This application also calculates the effective refractive index of the cladding mode. The characteristic curves of PVA coating 2 with varying thickness are shown in the calculation results. Figure 3As shown in the figure, the curve's trend is non-linear. The curve's slope is greatest when the thickness of PVA coating 2 is in the 100nm-300nm range; this range is called the mode-switching range, which is the operating range where the effective refractive index of the cladding mode changes most significantly with the PVA coating thickness. Theoretically, when the external air pressure increases, the PDMS coating volume increases, its density decreases, and its refractive index decreases accordingly. When testing the sensor's pressure response under pressurized conditions, the thickness of PVA coating 2 was controlled around 250nm. At this point, the increased external air pressure causes a blue shift in wavelength, and the sensor's operating range is tuned to the mode-switching range.

[0031] In addition to its good response to air pressure, the PDMS coating 3 is also affected by temperature, which can interfere with its air pressure response characteristics, resulting in a temperature-pressure cross-sensitivity effect and thus impacting the accuracy of air pressure measurement. To avoid the influence of temperature on the measurement results, this application connects a fiber Bragg grating in series with the long-period fiber grating 1 in the same fiber optic path. The fiber Bragg grating is temperature compensated using a compensation matrix, calculated as follows:

[0032] in, , These are the pressure sensitivity and temperature sensitivity of the long-period fiber grating 1, respectively. Temperature sensitivity of fiber Bragg gratings; This represents the wavelength shift of the fiber Bragg grating. This represents the wavelength shift of the long-period fiber grating 1. This refers to the change in temperature. This represents the change in air pressure.

[0033] This application, by connecting a fiber Bragg grating in series in the same fiber optical path as the long-period fiber Bragg grating 1, can sense the ambient temperature and use a compensation matrix to perform temperature compensation on the air pressure measurement results of the long-period fiber Bragg grating 1. This can eliminate the cross-sensitivity effect of temperature changes on the air pressure measurement results in real time and effectively, and significantly improve the measurement accuracy and reliability of the sensor in complex temperature change environments.

[0034] The above description is merely a selection of preferred embodiments of this disclosure and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in the embodiments of this disclosure is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the above-described inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in the embodiments of this disclosure.

Claims

1. A long period fiber grating pressure sensor based on mode conversion and temperature compensation, characterized in that, The application relates to a long-period fiber grating, a PVA coating layer coated on the outer surface of the long-period fiber grating, a PDMS coating layer coated on the outer surface of the PVA coating layer, and a fiber Bragg grating connected in series with the long-period fiber grating in the same fiber optical path. The mode conversion interval is a parameter working interval in which the effective refractive index of a cladding mode changes most with the thickness of the PVA coating layer. The thickness of the PVA coating layer in the mode conversion interval ranges from 100 nm to 300 nm. The PDMS coating layer is a gas pressure sensitive material with a porous structure, and the refractive index of the PDMS coating layer increases with the increase of external air pressure. The core radius and the cladding radius of the long-period fiber grating range from 3.5 to 4.5 um and from 55 to 65 um respectively.

2. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, The core refractive index and the cladding refractive index of the long-period fiber grating are 1.4501 and 1.4449 respectively.

3. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, The phase matching relationship of the resonance peak of the long-period fiber grating is as follows:

4. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, The wavelength of the long-period fiber grating is blue-shifted with the increase of external air pressure.

5. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, The fiber Bragg grating is temperature compensated through a compensation matrix, and the calculation formula is as follows:

6. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, ​ 7. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, ​ wherein, represents a resonance peak of the long-period fiber grating; represents an effective refractive index of the core mode; represents an effective refractive index of the cladding mode, represents a period of the long-period fiber grating.

8. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, ​ 9. The long-period fiber grating pressure sensor based on mode conversion and temperature compensation according to claim 1, wherein, ​ wherein, , are the air pressure sensitivity and the temperature sensitivity of the long-period fiber grating, respectively; is the temperature sensitivity of the fiber Bragg grating; is the wavelength shift amount of the fiber Bragg grating; is the wavelength shift amount of the long-period fiber grating; is the temperature change amount; is the air pressure change amount.

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