A radiation-resistant quasi-distributed apodized fiber Bragg grating humidity sensor

Abstract

This invention relates to the field of fiber optic grating humidity sensor processing technology, specifically to a radiation-resistant quasi-distributed apodized fiber optic grating humidity sensor. To address the low sensitivity of existing FBG sensors in humidity testing under radiation, a novel radiation-resistant quasi-distributed apodized fiber optic grating humidity sensor is provided. This sensor employs femtosecond laser direct writing technology, and its components consist of multiple optical vernier sensors, each composed of two ultra-short apodized fiber grating pairs. A humidity-sensitive material is formed on one of the two ultra-short apodized fiber grating pairs (FBG-FP), thereby achieving relative humidity measurement with high humidity detection sensitivity.

Description

Technical Field

[0001] This invention relates to the field of fiber optic grating humidity sensor processing technology, specifically to a radiation-resistant quasi-distributed apodized fiber optic grating humidity sensor. Background Technology

[0002] In harsh radiation environments such as space, nuclear industry, and nuclear power plants, long-term stable sensing technologies are often required for online monitoring of physical parameters such as temperature, strain, and relative humidity to improve the safety and reliability of equipment operation. Traditional electronic sensors are susceptible to electromagnetic fields, high temperatures, and ionizing radiation, which can lead to malfunctions during operation. Compared to traditional sensors, fiber Bragg grating (FBG) sensors offer advantages such as resistance to electromagnetic interference, high temperature resistance, high sensitivity, and fast response, making them a potential candidate for use in radiation environments. However, when FBGs are integrated into a radiation environment, radiation can cause point defects and changes in material density, resulting in measurement errors. Therefore, to ensure accurate measurement of physical parameters by FBG sensors in radiation environments, it is necessary to study the radiation characteristics of FBG sensors to improve their measurement accuracy.

[0003] Because uniform fiber Bragg gratings (FBGs) have many high-intensity sidelobes in their reflection spectrum, large wavelength spacing is required to reduce interference between adjacent channels when used for quasi-distributed measurements, resulting in small system capacity and severely limiting the application of FBGs in the sensing field. Fiber Bragg grating apodization can reshape the reflection spectrum of FBGs and suppress their sidelobes. Compared with uniform FBGs, using apodized FBGs can improve the signal-to-noise ratio of quasi-distributed sensing systems and increase the system's sensor capacity. Therefore, this study investigates femtosecond laser direct-writing technology for apodized FBGs to improve the sensing performance of fiber Bragg gratings. Summary of the Invention

[0004] In order to solve the problem of low sensitivity of existing FBG sensors when used for humidity testing in a radiation environment, this invention provides a new radiation-resistant quasi-distributed apodized fiber optic grating humidity sensor.

[0005] This invention is achieved using the following technical solution:

[0006] A radiation-resistant quasi-distributed apodized fiber Bragg grating humidity sensor is disclosed. The sensor is written using femtosecond laser direct writing technology and its components consist of multiple optical vernier sensors, each composed of two pairs of ultra-short apodized fiber Bragg gratings. A humidity-sensitive material is formed on one of the two ultra-short apodized fiber Bragg grating pairs, thereby realizing the measurement of relative humidity with high humidity detection sensitivity.

[0007] Principle Explanation: By coating a moisture-sensitive material onto a bare fiber Bragg grating (FBG), the material expands due to water molecule adsorption, causing mechanical strain in the FBG and resulting in a shift in its Bragg wavelength. This allows the FBG to be used as a humidity sensor. Humidity detection is affected by the coating material and thickness. To achieve high-sensitivity humidity testing, an optical vernier scheme is employed. Ultrashort fiber grating pairs (FBG-FPs) are fabricated in radiation-resistant fiber using femtosecond laser direct writing technology. These FBG-FPs are then cascaded to construct a Fabry-Perot interferometer, forming an optical vernier effect fiber optic sensor with enhanced sensitivity. Furthermore, the optical vernier effect is a method that can significantly improve measurement accuracy and has been widely used in fiber optic sensing in recent years. Similar to the working principle of a vernier caliper, the optical vernier effect consists of two interferometers with similar free spectral ranges. The reflection spectrum of the sensor is the superposition of the reflection spectra of the two interferometers. Through the double filtering of the two interferometers, the superimposed spectrum will have a periodic envelope. The period of the envelope is several times that of the original sensor period. When the spectrum of the interferometer cavity of the sensing interferometer shifts, the envelope will shift by several times. By monitoring the response of the envelope, the sensitivity of the sensor can be improved.

[0008] For a single FP interferometer, the intensity of the interference fringes can be expressed as:

[0009]

[0010] In the formula I r1 and I r2 These are the reflected light intensities at the two reflective interfaces, and φ is the phase shift between the two reflected beams. The phase shift α of the sensing cavity and the phase shift β of the reference cavity are respectively:

[0011]

[0012]

[0013] The reflection spectra of two FP interferometer cavities can be simplified as follows:

[0014] I S =A + Bcosα (4)

[0015] I R =C + Dcosβ (5)

[0016] in:

[0017] A = I S1 +I S2 =I1R1+I1(1-R1)(1-k)R2 (6)

[0018] C = I R1 +I R2=I²R³ + I²(1-R³)(1-k)R⁴ (7)

[0019]

[0020]

[0021] Where k is the resonant cavity loss during transmission, and R1, R2, R3, and R4 are the reflection coefficients of the interfaces at M1, M2, M3, and M4, respectively. I1 and I2 are the light intensities incident on the sensing FBG-FP cavity and the reference FBG-FP cavity after passing through the coupler. The expression for the light intensity of the output light obtained by connecting two FBG-FP cavities in parallel is:

[0022]

[0023] The FSR of the spectra of the sensing FBG-FP cavity and the reference FBG-FP cavity are:

[0024]

[0025]

[0026] By selecting an appropriate cavity length, the free spectral ranges of the two FBG-FPs can be made close but not unequal, thus achieving the vernier effect. The expression for the free spectral range through the parallel vernier effect is:

[0027]

[0028] Compared to a single FBG-FP cavity, the vernier effect results in a larger spectral drift, with a wavelength drift amplification factor M of:

[0029]

[0030] To improve sensitivity, a Fabry-Perot interferometer (FPI) fiber optic sensor was constructed using femtosecond laser direct-writing of ultrashort fiber gratings (FBGs) based on the vernier effect. Using a femtosecond laser direct-writing platform, adoped ultrashort FBGs were etched into the core of a single-mode fiber using a line-by-line method. During FBG writing, the first adoped FBG1 was first etched into the single-mode fiber using a weak-coupling line-by-line method. Then, a second adoped FBG2 was etched using a translation stage that moved and fixed the fiber, thus forming the first FBG-FP pair (FBG-FP) cascaded from two FBGs. When FPIs with different cavity lengths are written along a single fiber, to reduce the impact of multiple FPIs and mixed FPIs on the overall interference spectrum of the sensor, the distance between adjacent FBG-FP pairs must be greater than the coherence length of the light source. Therefore, the second FBG-FP pair was etched a certain distance from the first FBG-FP pair, thus forming two cascaded FBG-FPI pairs. In a cascaded fiber optic interferometer (FPI), one FPI serves as the reference unit and the other as the sensing unit. When the free spectral ranges (FSRs) of the two fiber optic interferometers satisfy a specific matching relationship, a vernier effect will be generated, amplifying the strain sensitivity of a single interferometer.

[0031] Furthermore, the humidity-sensitive material is a polyimide humidity-sensitive material. By coating polyimide into FBG-FP, high-sensitivity humidity measurement is achieved.

[0032] Furthermore, in the process of writing ultrashort apodized fiber gratings (FBGs) using femtosecond laser direct writing technology, the fiber sample is fixed on a high-precision displacement platform. The femtosecond laser pulse is focused inside the fiber through an oil-immersion microscope objective. Refractive index-matching oil is used to fill the gap between the fiber and the oil mirror to eliminate aberrations caused by the surface curvature of the cylindrical fiber. The microscopic image of the fiber is observed in real time using a CCD. Simultaneously, with the assistance of the CCD, the femtosecond laser focus point is aligned with the center of the fiber core using a high-precision translation stage, and the position of the focal spot within the fiber is precisely controlled. Using a modulated position edge-weak coupling line-by-line method, the length of the lines written in the fiber core is sequentially increased and decreased according to a Gaussian function to write the ultrashort apodized FBG.

[0033] Furthermore, by using femtosecond laser direct writing technology to directly write FBG-based fiber optic Fabry-Perot tandem humidity sensors, simultaneous measurement of humidity at multiple points can be achieved.

[0034] The beneficial effects of this invention are as follows: Compared with the prior art, the fiber grating using femtosecond laser direct writing technology has the advantages of flexible design of parameters such as reflectivity and grating length and radiation resistance. At the same time, this design can effectively control the Fabry-Perot cavity length, thereby flexibly modulating the signal contrast. Attached Figure Description

[0035] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0036] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0037] Figure 1 This is a schematic diagram of the overall structure of the sensor described in this invention;

[0038] Figure 2 This is a schematic diagram of the optical vernier effect;

[0039] Figure 3 This is a schematic diagram of a Brie-Perot interferometer optical vernier effect sensor based on femtosecond laser direct-write ultrashort fiber grating pairs;

[0040] Figure 4 A schematic diagram showing the formation of a humidity-sensitive material on one of the FBG-FPs;

[0041] Figure 5 A schematic diagram of the spectrum of a single FPI (FBGs)1;

[0042] Figure 6 A schematic diagram of the spectrum of a single FPI(FBGs)2;

[0043] Figure 7 Here are schematic diagrams of the spectra of two FBG-FPs;

[0044] Figure 8 This is a schematic diagram of the vernier effect produced by the spectrum when two FBG-FPs are connected in series.

[0045] Figure 9 This is a schematic diagram illustrating the response of a vernier sensor composed of an ultrashort fiber grating (FPI) under different humidity conditions.

[0046] Figure 10 This is a schematic diagram illustrating the sensitivity of a vernier sensor composed of an ultrashort fiber grating (FPI) under different humidity conditions.

[0047] Figure 11 This is a schematic diagram of the spectrum of a quasi-distributed humidity sensor composed of FPIs built based on three ultrashort fiber grating pairs with different center wavelengths. Detailed Implementation

[0048] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0049] In this description, it should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. It should also be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joint" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms according to the specific circumstances.

[0050] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0051] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0052] like Figure 1 As shown, a radiation-resistant quasi-distributed apodized fiber grating humidity sensor is described. The sensor is written using femtosecond laser direct writing technology, and its components consist of multiple optical vernier sensors composed of two ultra-short apodized fiber grating pairs (FBG-FP). A humidity-sensitive material is formed on one of the FBG-FP pairs, thereby realizing the measurement of relative humidity with high humidity detection sensitivity.

[0053] Principle Explanation: By coating a moisture-sensitive material onto a bare fiber Bragg grating (FBG), the material expands due to water molecule adsorption, causing mechanical strain in the FBG. This leads to a shift in the FBG's Bragg wavelength, thus using the FBG as a humidity sensor. Humidity detection is affected by the coating material and thickness. To achieve high-sensitivity humidity testing, an optical vernier scheme is employed. Ultrashort fiber grating pairs (FBG-FPs) are fabricated in radiation-resistant fiber using femtosecond laser direct writing technology. These FBG-FPs are then cascaded to form a Fabry-Perot interferometer, creating an optical vernier effect fiber optic sensor with enhanced sensitivity. Furthermore, the optical vernier effect is a method that can significantly improve measurement accuracy and has been widely used in fiber optic sensing in recent years. Similar to the working principle of a vernier caliper, the optical vernier effect consists of two interferometers with similar free spectral ranges. The sensor's reflection spectrum is the superposition of the reflection spectra of the two interferometers. Figure 2As shown, through the dual filtering of two photodetectors (FPs), the superimposed spectrum will exhibit a periodic envelope. The period of the envelope is several times that of the original sensor period. When the spectrum of the sensing FP interferometer cavity shifts, the envelope will shift by several times. By monitoring the response of the envelope, the sensitivity of the sensor can be improved.

[0054] For a single FP interferometer, the intensity of the interference fringes can be expressed as:

[0055]

[0056] In the formula I r1 and I r2 These are the reflected light intensities at the two reflective interfaces, and φ is the phase shift between the two reflected beams. The phase shift α of the sensing cavity and the phase shift β of the reference cavity are respectively:

[0057]

[0058]

[0059] The reflection spectra of two FP interferometer cavities can be simplified as follows:

[0060] I S =A + Bcosα (4)

[0061] I R =C + Dcosβ (5)

[0062] in:

[0063] A = I S1 +I S2 =I1R1+I1(1-R1)(1-k)R2 (6)

[0064] C = I R1 +I R2 =I²R³ + I²(1-R³)(1-k)R⁴ (7)

[0065]

[0066]

[0067] Where k is the resonant cavity loss during transmission, and R1, R2, R3, and R4 are the reflection coefficients of the interfaces at M1, M2, M3, and M4, respectively. I1 and I2 are the light intensities incident on the sensing FP cavity and the reference FP cavity after passing through the coupler. The expression for the light intensity of the output light obtained by connecting two FP cavities in parallel is:

[0068]

[0069] The FSR of the spectra of the sensing FP cavity and the reference FP cavity are:

[0070]

[0071]

[0072] By selecting an appropriate cavity length, the free spectral ranges of the two FPs can be made close but not unequal, thus achieving the vernier effect. The expression for the free spectral range through the parallel vernier effect is:

[0073]

[0074] Compared to a single FP cavity, the vernier effect results in a larger spectral drift, with a wavelength drift amplification factor M of:

[0075]

[0076] To improve sensitivity, a Fabry-Perot interferometer (FPI) fiber optic sensor was constructed using femtosecond laser direct-writing of ultrashort fiber gratings (FBGs) based on the vernier effect. Using a femtosecond laser direct-writing platform, apodized ultrashort FBGs were etched into the core of a single-mode fiber using a line-by-line method. During FBG writing, the first apodized FBG1 was first etched into the single-mode fiber using a weakly coupled line-by-line method. Then, a second apodized FBG2 was etched by moving a translation stage that held the fiber in place, thus forming the first FBG pair (FBG-FP) cascaded from two FBGs. When FPIs with different cavity lengths are written along a single fiber, to reduce the impact of multiple and mixed FPIs on the overall interference spectrum of the sensor, the distance between adjacent FBG-FP pairs must be greater than the coherence length of the light source. Therefore, the second FBG-FP pair was etched a certain distance from the first FBG-FP pair, thus forming two cascaded FBG-FPIs, as shown below. Figure 3 As shown, in a cascaded FPI, one FPI serves as the reference unit and the other as the sensing unit. When the free spectral ranges (FSRs) of the two fiber optic interferometers satisfy a specific matching relationship, a vernier effect will be generated, amplifying the strain sensitivity of a single interferometer.

[0077] The FSRFPI of the interference spectrum for each FBG-FPI is:

[0078]

[0079] Furthermore, the humidity-sensitive material is a polyimide humidity-sensitive material, which is obtained by coating one of the FBG-FPs, such as... Figure 4As shown, high-sensitivity humidity measurement is achieved. When affected by relative humidity, the humidity-sensitive material expands, and the RI of the humidity-sensitive material also changes, causing a shift in the resonant wavelength of the reflectance spectrum. By forming a vernier effect through the cascaded FSRs of two FBG-FPI1 and FBG-FPI2, the sensing response characteristics are improved by the envelope shift formed by their superimposed spectra.

[0080] The interference spectrum of the FBG-FPI sensor occurs within the bandwidth of the ultrashort FBG, such as... Figure 5 and 6 As shown in Figure 7, due to the difference in cavity length, there are slight differences between the two spectra. When FPI(FBGs) and FPI(FBGs) are connected in series, a vernier effect occurs in the spectrum, such as... Figure 8 As shown.

[0081] The humidity response of a vernier effect fiber optic sensor formed by a cascade of two ultrashort fiber Bragg pairs (FBG-FPIs) was investigated. The sensing region of the sensing unit FPI1 in the cascaded FPI sensor was coated with a polyimide humidity-sensitive material, while the reference unit FPI2 was unaffected by applied humidity. During RH experiments, the temperature was kept constant, and the relative humidity response was measured within the range of 20% RH to 95% RH. With increasing relative humidity, the envelope resonant wavelength shifted towards longer wavelengths, and the relative humidity sensitivity was 10.48 pm / % RH, which is 7.38 times that of a single FBG-FPI (10.48 / 1.42 = 7.38). Figure 9 and 10 As shown.

[0082] Furthermore, in the process of writing ultrashort apodized fiber gratings (FBGs) using femtosecond laser direct writing technology, the fiber sample is fixed on a high-precision displacement platform. The femtosecond laser pulse is focused inside the fiber using an oil-immersion microscope objective. Refractive index-matching oil is used to fill the gap between the fiber and the oil mirror to eliminate aberrations caused by the curvature of the cylindrical fiber surface. The microscopic image of the fiber is observed in real time using a CCD. Simultaneously, with the assistance of the CCD, the femtosecond laser focus point is aligned with the center of the fiber core using a high-precision translation stage, and the position of the focal spot within the fiber is precisely controlled. Using a line-by-line method with weak edge coupling at the modulation position, the length of the lines written in the fiber core is sequentially increased and decreased according to a Gaussian function to write the ultrashort apodized FBG. The femtosecond pulse at the center of the fiber core generates a series of modulation points, which are tiny voids formed by regions with higher refractive indices surrounded by regions with lower refractive indices. Excessive laser power induces larger microvoids and increased density areas in the fiber core material, leading to changes in refractive index characteristics.

[0083] In practical implementation, using femtosecond laser direct writing technology to directly write onto an FBG-based fiber optic Fabry-Perot tandem humidity sensor enables simultaneous measurement of humidity at multiple points. For example... Figure 11The image shows a three-point series humidity sensor composed of three sets of four FBGs (two FBGs forming a Fabry-Perot interferometer cavity, totaling two Fabry-Perot interferometer cavities; these two interferometer cavities, connected in series, form a single humidity sensor based on the vernier effect). This sensor can simultaneously measure humidity data at three points.

[0084] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the present invention. Although detailed descriptions have been provided with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and they should all be covered within the protection scope of the claims.

Claims

1. A radiation-resistant quasi-distributed apodized fiber Bragg grating humidity sensor, characterized in that, The sensor is written using femtosecond laser direct writing technology and its components consist of multiple optical vernier sensors, each composed of two pairs of ultra-short apod fiber gratings. Moisture-sensitive material is formed on one of the two ultra-short apod fiber grating pairs, FBG-FP.

2. The radiation-resistant quasi-distributed apodized fiber Bragg grating humidity sensor according to claim 1, characterized in that, The humidity-sensitive material is a polyimide humidity-sensitive material.

3. The radiation-resistant quasi-distributed apodized fiber Bragg grating humidity sensor according to claim 2, characterized in that, In the process of writing ultrashort apodized fiber gratings (FBGs) using femtosecond laser direct writing technology, the fiber sample is fixed on a high-precision displacement platform. The femtosecond laser pulse is focused inside the fiber through an oil-immersion microscope objective. Refractive index matching oil is used to fill the gap between the fiber and the oil mirror to eliminate aberrations caused by the curvature of the cylindrical fiber surface. The microscopic image of the fiber is observed in real time through a CCD. At the same time, with the assistance of the CCD, the femtosecond laser focusing point is aligned with the center of the fiber core through a high-precision translation stage, and the position of the focal spot in the fiber is precisely controlled. The edge weak coupling line-by-line method of modulation position is used to sequentially increase and decrease the length of the lines written in the fiber core according to a Gaussian function to write the ultrashort apodized FBG.

4. A radiation-resistant quasi-distributed apodized fiber Bragg grating humidity sensor according to claim 3, characterized in that, Using femtosecond laser direct writing technology to directly write FBG-based fiber optic Fabry-Perot series humidity sensors can achieve simultaneous measurement of humidity at multiple points.

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