Enhancement of OFDR Signal-to-Noise Ratio by Coated UV-Transparent Chirped Weak Fiber Bragg Grating and Its Fabrication Method

Abstract

This invention relates to the field of fiber optic distributed sensing, specifically the method for enhancing the signal-to-noise ratio (SNR) of OFDR using coated ultraviolet-transparent chirped weak fiber gratings. The method utilizes coated ultraviolet-transparent chirped weak fiber gratings as the sensing fiber in an OFDR distributed sensing system, improving the intensity and SNR of the measured optical signal. The fabrication method employs coated ultraviolet-transparent fiber, eliminating the need to remove the coating during the writing process. Furthermore, an electrically controlled adjustable aperture and a transmission wheel are used to continuously write chirped weak fiber gratings with periods ranging from large to small, small to large, or large to small and then back to large. This invention uses coated ultraviolet-transparent fiber, directly writing the fiber grating without removing the coating, thus improving manufacturing efficiency. The chirped weak fiber grating has a wider reflectance spectrum, allowing for average reflectivity and utilizing a wider range of sensing wavelengths to improve the SNR of the OFDR distributed sensing system.

Description

Technical Field

[0001] This invention belongs to the field of fiber optic distributed sensing, and specifically relates to a method for enhancing the signal-to-noise ratio of OFDR with a coated ultraviolet transparent chirped weak fiber grating and its preparation method. Background Technology

[0002] Optical frequency domain reflectance (OFDR) technology is a widely used high-precision distributed sensing technology due to its high spectral resolution, high spatial positioning accuracy, and ability to continuously sense objects. It has also received extensive attention and achieved great development in the fields of distributed temperature, distributed strain, and spatial shape sensing.

[0003] With the increasingly widespread application of OFDR distributed sensing technology, the demand for sensing density, sensing accuracy, and continuous sensing of OFDR technology has grown rapidly in recent years. Traditional methods use weak fiber Bragg grating strings as sensing fibers. Strain or temperature sensing capability is only available at locations on the fiber Bragg gratings, while ordinary fibers between the gratings lack sensing capabilities. Therefore, this remains quasi-distributed sensing and cannot achieve true distributed sensing. Furthermore, the protective coating on the fiber has a strong absorption effect on ultraviolet light. Therefore, the coating must be removed while writing the weak fiber Bragg grating strings, and then recoated after writing. The coating removal and recoating process is cumbersome, and the recoating process may damage the fiber. Additionally, the recoated portion may affect the transmission efficiency of the fiber strain window installation.

[0004] In recent years, research on fiber optic scattering phenomena has become increasingly in-depth, with spontaneous Rayleigh scattering in optical fibers attracting growing attention. The spontaneous Rayleigh scattering in optical fibers arises because the minute differences in refractive index distribution and the presence of impurities within the fiber during manufacturing can be considered as a stable and invariant random periodic equivalent weak grating. Its reflection spectrum is related to the periodic distribution of this equivalent weak grating, exhibiting a local spectral characteristic similar to a "fingerprint." When the fiber is subjected to strain or temperature, the period and refractive index of the equivalent weak grating change accordingly, causing a shift in the local "fingerprint" spectrum. Strain or temperature information can be extracted using spectral cross-correlation. However, spontaneous Rayleigh scattering in optical fibers is extremely weak, with a reflection signal of approximately -120 dB, resulting in a very low signal-to-noise ratio for OFDR distribution sensing, thus affecting the accuracy of strain and temperature sensing.

[0005] Currently, the main method to improve the signal-to-noise ratio (SNR) of OFDR distributed sensing is to expose hydrogen-loaded optical fibers with ultraviolet light to increase the intensity of the random periodic equivalent weak gratings within them. However, the fiber coating has extremely strong absorption of ultraviolet light, and the fiber cladding has a strong scattering effect on ultraviolet light. The effect of ultraviolet light on the fiber core is very weak, and currently, the maximum increase in reflected signal can be 10 to 100 times. Furthermore, ultraviolet exposure only increases the reflectivity of the random periodic equivalent weak gratings in the fiber; the disordered characteristics of its spectrum, resembling a fingerprint, remain unchanged. Therefore, the SNR of this fingerprint-like spectrum in cross-correlation remains very low, affecting the accuracy of strain or temperature extraction. Moreover, within a fixed wavelength scanning range, when a large strain or temperature is applied to the fiber, the fingerprint-like spectrum will undergo a significant shift. During spectral cross-correlation, the spectral similarity signal is greatly reduced, further decreasing the accuracy of strain or temperature extraction.

[0006] In summary, OFDR distributed sensing technology has evolved from quasi-distributed to truly distributed sensing. While spontaneous Rayleigh scattering in optical fibers has enabled distributed sensing of spatial position and strain or temperature, the intensity of spontaneous Rayleigh scattering signals is very low. Although ultraviolet exposure can improve the signal intensity to some extent, it remains low. Moreover, the spectral characteristics, similar to fingerprints, still lead to reduced spectral similarity in large-scale strain or temperature sensing and affect the signal-to-noise ratio in subsequent signal processing and sensing information extraction. Summary of the Invention

[0007] To overcome the shortcomings of existing methods for enhancing the signal-to-noise ratio (SNR) of OFDR distributed sensing technology, this invention proposes a coated ultraviolet-transparent chirped weak fiber grating (GFCG) for enhancing OFDR SNR and its fabrication method. Using a coated ultraviolet-transparent fiber reduces the absorption rate of the coating on ultraviolet light, enabling direct writing of the GFCG without stripping the coating, thus reducing the complexity and damage rate of the GFCG writing process. Writing continuous weak GFCGs within the fiber increases the intensity of reflected signals at various locations within the fiber, and the broad reflection spectrum of the chirped GFCG can average the reflected signal intensity. By utilizing a wide wavelength range of sensing light, the intensity of the sensing light signal at the end of the sensing fiber can be increased, thereby improving the signal-to-noise ratio of the OFDR distributed sensing system. Furthermore, writing chirped weak fiber gratings in the fiber instead of random periodic equivalent weak gratings for distributed Rayleigh scattering provides a definite spectral characteristic with an approximate Gaussian spectral type, exhibiting extremely high spectral similarity under strain or temperature effects, which can improve the signal-to-noise ratio of spectral cross-correlation. Moreover, if continuous chirped weak fiber gratings are written, there are no refractive index modulation "blank" intervals between fiber gratings, enabling all-fiber distributed sensing.

[0008] Technical solution of the present invention

[0009] A method for enhancing the signal-to-noise ratio of OFDR using coated ultraviolet transparent chirped weak fiber gratings is characterized by using coated ultraviolet transparent chirped weak fiber gratings as the sensing fiber of the OFDR distributed sensing system. The coated ultraviolet transparent chirped weak fiber gratings are either spatially continuous periodic linear chirped fiber gratings or spatially discontinuous periodic linear chirped fiber gratings. The periods of the coated ultraviolet transparent chirped weak fiber gratings are arranged continuously from large to small, from small to large, or from large to small and then from small to large. The coated ultraviolet transparent chirped weak fiber gratings have a wide reflectance spectrum, which can average the reflectance and utilize a wide wavelength range of sensing light to increase the intensity of the sensing light signal at the end of the sensing fiber, thereby increasing the intensity of the measured light signal of the OFDR distributed sensing system. Before measuring strain or temperature on the sensing fiber, the OFDR distributed sensing system performs a measurement to obtain the sensing fiber's signal intensity. The OFDR distributed sensing system records the approximately Gaussian-shaped reference reflection spectrum at each location. After measuring strain or temperature in the sensing fiber, it performs another measurement to obtain the approximately Gaussian-shaped measured reflection spectrum at each location in the sensing fiber. Then, it extracts the offset of the center wavelength of the reference and measured reflection spectra at each location in the sensing fiber. The process of extracting the offset of the center wavelength of the reflection spectrum at each location in the coated transparent chirped weak fiber grating is as follows: the difference between the wavelengths corresponding to the fitted center values ​​of the two approximately Gaussian-shaped reflection spectra is calculated, or the approximately Gaussian-shaped reflection spectra at each corresponding location in the coated ultraviolet transparent chirped weak fiber grating before and after strain or temperature sensing are cross-correlated. The center peak offset of the cross-correlation signal at each location is extracted. Then, based on the relationship between strain or temperature and the center wavelength offset of the reflection spectrum, the strain or temperature at each location in the sensing fiber is demodulated.

[0010] Specifically, the reflectivity of each coated layer of ultraviolet transparent chirped weak fiber Bragg grating region was designed based on the sensing distance of the OFDR distributed sensing system and the expected reflected signal intensity, with the reflectivity ranging from 1×10⁻⁶. -8 Up to 1×10 -2 .

[0011] A method for fabricating a coated ultraviolet transparent chirped weak fiber grating, characterized by the following steps:

[0012] The first step is to obtain the relationship curve between ultraviolet light irradiation time and fiber optic grating reflectivity. The broadband light source enters from one end of the optical fiber, and after transmission through the optical fiber, it enters the spectrometer. The spectrometer measures the spectrum, which is the transmission spectrum of the broadband light source through the optical fiber. Then, the power of the ultraviolet light is set, and the ultraviolet light is focused on the fiber core by a cylindrical lens, an electrically controlled adjustable aperture, and a phase mask. The fiber optic grating is etched on the fiber core, and the exposure time and the spectral depression measured by the spectrometer are recorded. The spectral depression measured by the spectrometer is the reflectivity of the etched fiber optic grating in the exposure area, thus obtaining the relationship curve between ultraviolet light irradiation time and fiber optic grating reflectivity. The optical fiber is a coated ultraviolet transparent fiber, and the coating layer does not need to be removed during the etching process.

[0013] The second step involves etching a UV-transparent, continuously chirped, weakly chirped fiber grating with a coating layer. Based on the design value of the reflectivity of the UV-transparent, chirped, weakly chirped fiber grating and the relationship curve between UV irradiation time and fiber grating reflectivity, the exposure time for etching the fiber grating within the exposure area is calculated. The fiber optic cable is mounted on four linearly arranged rotating conveyor wheels: wheel one, wheel two, wheel three, and wheel four. Two sections of fiber optic cable between the edge and center conveyor wheels are respectively suspended by weights one and two to provide a fixed tension. The fiber optic cable is mounted on rotating conveyor wheels one and four. Driven by conveyor wheels two, three, and four, the fiber optic winding discs move linearly in one direction. At each end are fiber optic winding disc one and two for releasing and collecting optical fibers, respectively. The control system is connected to the electrically adjustable aperture and conveyor wheels one, two, three, and four via cables one, two, three, and five. Under the control of the control system, a coated ultraviolet transparent chirped weak fiber grating is inscribed. The inscription of the coated ultraviolet transparent chirped weak fiber grating includes the following steps:

[0014] (a) Rotating conveyor wheel one, rotating conveyor wheel two, rotating conveyor wheel three and rotating conveyor wheel four rotate, driving the optical fiber to move to the exposure area;

[0015] (b) Before writing the UV-transparent chirped weak fiber grating at each position, adjust the orientation of the mask and the fiber, so that the period of the UV-transparent chirped weak fiber grating written on the fiber is arranged continuously from large to small, or continuously from small to large, or continuously from large to small and then from small to large. The UV-transparent chirped weak fiber grating has a wide reflectance spectrum and can average reflectivity. It can also use a wide wavelength range of sensing light to increase the intensity of the sensing light signal at the end of the sensing fiber, thereby increasing the intensity of the light signal measured by the OFDR distributed sensing system. The opening time of the electrically controlled adjustable aperture is the calculated UV irradiation time. Write the fiber grating on the fiber in the exposure area, and then close the electrically controlled adjustable aperture.

[0016] (c) Rotate conveyor wheel one, rotate conveyor wheel two, rotate conveyor wheel three and rotate conveyor wheel four to move the optical fiber to the next exposure area;

[0017] (d) Repeat (b) to (c) until the writing length reaches the design value of the UV transparent chirped weak fiber grating of the coating layer.

[0018] The advantages of this invention are:

[0019] 1. Using coated ultraviolet transparent optical fiber reduces the absorption rate of the coating to ultraviolet light, enabling direct writing of fiber gratings without removing the coating. This reduces the complexity of the fiber grating writing process and the damage rate, thereby improving the efficiency of optical fiber manufacturing.

[0020] 2. Writing continuous chirped weak fiber Bragg gratings in optical fibers can increase the intensity of reflected signals at various locations in the fiber, control the local reflectivity of the fiber according to the sensing length, and improve the signal-to-noise ratio of OFDR distributed sensing systems.

[0021] 3. Inscribing chirped weak fiber gratings in optical fibers to replace random periodic equivalent weak gratings with distributed Rayleigh scattering results in definite spectral characteristics with an approximate Gaussian spectral form. The spectral similarity under strain or temperature effects is extremely high, which can improve the signal-to-noise ratio of spectral cross-correlation.

[0022] 4. Chirped fiber gratings have a wide reflectance spectrum, which can average reflectance and utilize a wide wavelength range of sensing light, thereby improving the intensity of the sensing light signal at the end of the sensing fiber.

[0023] 4. If continuous chirped weak fiber gratings are inscribed in the optical fiber, there are no refractive index modulation "blank" intervals between fiber gratings, enabling all-fiber distributed sensing. Attached Figure Description

[0024] Figure 1 This is a schematic diagram showing the approximate "Gaussian" shift in the reflectance spectrum at a local location of a chirped fiber grating with low UV transmittance in the coating layer due to strain or temperature.

[0025] In the figure: 11. Before strain or temperature, the approximately Gaussian-type reflection spectrum at a local location of the UV-transparent chirped weak fiber grating with coating; 12. After strain or temperature, the approximately Gaussian-type reflection spectrum at a local location of the UV-transparent chirped weak fiber grating with coating.

[0026] Figure 2 This is a schematic diagram showing the shift in the cross-correlation signal of the approximately "Gaussian" type reflection spectrum at a local location of the coated ultraviolet transparent chirped weak fiber grating before and after strain or temperature treatment.

[0027] In the figure: 21. Cross-correlation signal of the approximately "Gaussian" type reflection spectrum at a local location of the UV transparent chirped weak fiber grating of the coating before strain or temperature treatment; 22. Cross-correlation signal of the approximately "Gaussian" type reflection spectrum at a local location of the UV transparent chirped weak fiber grating of the coating after strain or temperature treatment.

[0028] Figure 3 It is a curve showing the relationship between the fiber grating period and the fiber position when the periods of continuously chirped fiber gratings are arranged from large to small and then from small to large.

[0029] Figure 4 It is a curve showing the relationship between the fiber grating period and the fiber position when continuously chirped fiber gratings are arranged in descending order of period.

[0030] Figure 5 It is a curve showing the relationship between the fiber grating period and the fiber position when the periods of continuously chirped fiber gratings are arranged in ascending order.

[0031] Figure 6 This is a schematic diagram of the fabrication of a UV-transparent chirped weak fiber grating with a coating layer;

[0032] In the diagram: 61. Broadband light source; 62. Optical fiber; 63. Spectrometer; 64. Ultraviolet light; 65. Cylindrical lens focusing; 66. Electrically controlled adjustable aperture; 67. Phase mask; 68. Rotating conveyor wheel one; 69. Rotating conveyor wheel two; 610. Rotating conveyor wheel three; 611. Rotating conveyor wheel four; 612. Weight one; 613. Weight two; 614. Optical fiber winding reel one; 615. Optical fiber winding reel two; 616. Control system; 617. Cable one; 618. Cable two; 619. Cable three; 620. Cable four; 621. Cable five.

[0033] Figure 7 This is a schematic diagram of the transmission spectrum of an optical fiber before the fiber grating is inscribed.

[0034] Figure 8 This is a schematic diagram of the transmission spectrum of an optical fiber after a fiber grating has been inscribed.

[0035] Figure 9 This is a schematic diagram of a fiber Bragg grating;

[0036] In the figure: 901. Coating layer; 902. Cladding layer; 903. Fiber core.

[0037] Figure 10 This is a schematic diagram showing the relationship between the ultraviolet light irradiation time of the coating layer and the reflectivity of the fiber grating.

[0038] Figure 11 This is a schematic diagram of two consecutive ultraviolet exposure areas;

[0039] In the diagram: 111. Previous UV exposure area; 112. Next UV exposure area. Detailed Implementation

[0040] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings:

[0041] A method for enhancing the signal-to-noise ratio of OFDR using coated ultraviolet transparent chirped weak fiber gratings is characterized by using coated ultraviolet transparent chirped weak fiber gratings as the sensing fiber of the OFDR distributed sensing system. The coated ultraviolet transparent chirped weak fiber gratings are either spatially continuous periodic linear chirped fiber gratings or spatially discontinuous periodic linear chirped fiber gratings. The periods of the coated ultraviolet transparent chirped weak fiber gratings are arranged continuously from large to small, from small to large, or from large to small and then from small to large. The coated ultraviolet transparent chirped weak fiber gratings have a wide reflectance spectrum, which can average the reflectance and utilize a wide wavelength range of sensing light to increase the intensity of the sensing light signal at the end of the sensing fiber, thereby increasing the intensity of the measured light signal of the OFDR distributed sensing system. Before measuring strain or temperature on the sensing fiber, the OFDR distributed sensing system performs a measurement to obtain the sensing fiber's signal intensity. The OFDR distributed sensing system records the approximately Gaussian-shaped reference reflection spectrum at each location. After measuring strain or temperature in the sensing fiber, it performs another measurement to obtain the approximately Gaussian-shaped measured reflection spectrum at each location in the sensing fiber. Then, it extracts the offset of the center wavelength of the reference and measured reflection spectra at each location in the sensing fiber. The process of extracting the offset of the center wavelength of the reflection spectrum at each location in the coated transparent chirped weak fiber grating is as follows: the difference between the wavelengths corresponding to the fitted center values ​​of the two approximately Gaussian-shaped reflection spectra is calculated, or the approximately Gaussian-shaped reflection spectra at each corresponding location in the coated ultraviolet transparent chirped weak fiber grating before and after strain or temperature sensing are cross-correlated. The center peak offset of the cross-correlation signal at each location is extracted. Then, based on the relationship between strain or temperature and the center wavelength offset of the reflection spectrum, the strain or temperature at each location in the sensing fiber is demodulated.

[0042] Specifically, the reflectivity of each coated layer of ultraviolet transparent chirped weak fiber Bragg grating region was designed based on the sensing distance of the OFDR distributed sensing system and the expected reflected signal intensity, with the reflectivity ranging from 1×10⁻⁶. -8 Up to 1×10 -2 .

[0043] A method for fabricating a coated ultraviolet transparent chirped weak fiber grating, characterized by the following steps:

[0044] The first step is to obtain the relationship curve between ultraviolet light irradiation time and fiber optic grating reflectivity. Broadband light source 61 enters from one end of fiber optic 62, and after transmission through fiber optic 62, it enters spectrometer 63. The spectrometer measures the spectrum, which is the transmission spectrum of the broadband light source through fiber optic 62. Then, the power of ultraviolet light 64 is set, and ultraviolet light 64 is focused on the core of fiber optic 62 by cylindrical lens 65, electrically adjustable aperture 66, and phase mask plate 67. Fiber optic grating is etched on the core of fiber optic 62, and the exposure time and the spectral depression measured by spectrometer 63 are recorded. The spectral depression measured by spectrometer 63 is the reflectivity of the etched fiber optic grating in the exposure area, thus obtaining the relationship curve between ultraviolet light irradiation time and fiber optic grating reflectivity. Fiber optic 62 is a coated ultraviolet transparent fiber, and the coating layer does not need to be removed during the etching process.

[0045] The second step involves writing the UV-transparent continuous chirped weak fiber grating in the coating layer. Based on the design value of the reflectivity of the UV-transparent chirped weak fiber grating and the relationship curve between UV irradiation time and fiber grating reflectivity, the exposure time for writing the fiber grating within the exposure area is calculated. The fiber optic cable 62 is mounted on a linearly arranged set of rotating conveyor wheels 68, 69, 610, and 611. Weights 612 and 613 are respectively attached to the two segments of fiber optic cable 62 between the edge and center conveyor wheels to provide a fixed tension. The fiber optic cable 62 is mounted on the rotating conveyor wheels 68, 69, 610, and 611. Driven by 610 and rotating conveyor wheel 611, the optical fiber optic cable moves linearly in one direction. At both ends, there are fiber optic winding discs 614 and 615 for releasing and collecting optical fibers 62. The control system 616 is connected to the electrically adjustable aperture 66 and rotating conveyor wheels 68, 69, 610, and 611 via cables 617, 618, 619, 620, and 621. Under the control of the control system 616, a coated ultraviolet transparent chirped weak fiber optic grating is inscribed. The inscription of the coated ultraviolet transparent chirped weak fiber optic grating includes the following steps:

[0046] (a) Rotating conveyor wheel 1 68, rotating conveyor wheel 2 69, rotating conveyor wheel 3 610 and rotating conveyor wheel 4 611, driving optical fiber 62 to move to exposure area 111;

[0047] (b) Before writing the UV-transparent chirped weak fiber grating at each position, adjust the orientation of the mask 67 and the fiber 62 so that the period of the UV-transparent chirped weak fiber grating written on the fiber 62 is arranged continuously from large to small, or continuously from small to large, or continuously from large to small and then from small to large. The UV-transparent chirped weak fiber grating has a wide reflectance spectrum and can average reflectivity. It can also use a wide wavelength range of sensing light to increase the intensity of the sensing light signal at the end of the sensing fiber, thereby increasing the intensity of the light signal measured by the OFDR distributed sensing system. The opening time of the electrically controlled adjustable aperture 66 is the calculated UV irradiation time. Write the fiber grating on the fiber 62 in the exposure area, and then close the electrically controlled adjustable aperture 66.

[0048] (c) Rotate conveyor wheel 1 68, rotate conveyor wheel 2 69, rotate conveyor wheel 3 610 and rotate conveyor wheel 4 611 to move the optical fiber to the next exposure area 112.

[0049] (d) Repeat (b) to (c) until the writing length reaches the design value of the UV transparent chirped weak fiber grating of the coating layer.

[0050] The innovations and technical effects of this invention are as follows: Using a coated ultraviolet transparent chirped weak fiber grating reduces the absorption rate of the coating on ultraviolet light, enabling direct writing of fiber gratings without removing the coating. This reduces the complexity of the fiber grating writing process and the damage rate, improving fiber manufacturing efficiency. Writing continuous weak fiber gratings in the fiber increases the intensity of reflected signals at various locations within the fiber. Furthermore, the wide reflectance spectrum of the chirped fiber grating allows for average reflectance, utilizing a wide wavelength range of sensing light to increase the intensity of the sensing light signal at the end of the sensing fiber, thus improving the signal-to-noise ratio of the OFDR distributed sensing system. Replacing the distributed Rayleigh scattering random periodic equivalent weak grating with a chirped weak fiber grating in the fiber provides a definite approximately Gaussian spectral characteristic, exhibiting extremely high spectral similarity under strain or temperature effects, thereby improving the signal-to-noise ratio of spectral cross-correlation. Moreover, if continuous chirped weak fiber gratings are written, there are no refractive index modulation "blank" intervals between fiber gratings, enabling all-fiber distributed sensing. The technical effects of this invention are outstanding.

Claims

1. A method for enhancing the signal-to-noise ratio of OFDR using a coated ultraviolet transparent chirped weak fiber grating, characterized in that... The OFDR distributed sensing system uses coated ultraviolet-transparent chirped weak fiber gratings as the sensing fiber. These coated ultraviolet-transparent chirped weak fiber gratings can be spatially continuous periodic linear chirped fiber gratings or spatially discontinuous periodic linear chirped fiber gratings. The periods of these coated ultraviolet-transparent chirped weak fiber gratings are arranged continuously from large to small, from small to large, or from large to small and then from small to large. The wide reflectance spectrum of the coated ultraviolet-transparent chirped weak fiber gratings can average reflectance and utilize a wide wavelength range of sensing light to increase the intensity of the sensing light signal at the end of the sensing fiber, thereby increasing the intensity of the measured light signal in the OFDR distributed sensing system. Before measuring strain or temperature in the sensing fiber, the OFDR distributed sensing system performs a measurement to obtain approximately Gaussian reference reflected light at various locations in the sensing fiber. The OFDR distributed sensing system records the reflection spectrum data. After measuring strain or temperature in the sensing fiber, it performs another measurement to obtain an approximately "Gaussian" type of measured reflection spectrum at each position of the sensing fiber. Then, it extracts the offset of the center wavelength of the reference and measured reflection spectra at each position of the sensing fiber. The process of extracting the offset of the center wavelength of the reflection spectrum at each position of the coated transparent chirped weak fiber grating is as follows: the difference between the wavelengths corresponding to the fitted center values ​​of the two approximately "Gaussian" type reflection spectra is calculated, or the approximately "Gaussian" type reflection spectra at each corresponding position of the coated ultraviolet transparent chirped weak fiber grating before and after strain or temperature sensing are cross-correlated. The center peak offset of the cross-correlation signal at each position is extracted. Then, based on the relationship between strain or temperature and the center wavelength offset of the reflection spectrum, the strain or temperature at each position of the sensing fiber is demodulated.

2. The method for enhancing the signal-to-noise ratio of OFDR using a coated ultraviolet transparent chirped weak fiber grating according to claim 1, characterized in that: The reflectivity of each coated layer of the UV-transparent chirped weak fiber Bragg grating region was designed based on the sensing distance of the OFDR distributed sensing system and the expected reflected signal intensity. The reflectivity range was 1×10⁻⁶. -8 Up to 1×10 -2 .

3. A method for fabricating a coated ultraviolet transparent chirped weak fiber grating, characterized in that... The method includes the following steps: Step 1: Obtain the relationship curve between ultraviolet light irradiation time and fiber optic grating reflectivity. The broadband light source (61) enters from one end of the fiber (62), and after being transmitted through the fiber (62), it enters the spectrometer (63). The spectrum measured by the spectrometer is the transmission spectrum of the broadband light source through the fiber (62). Then, the power of the ultraviolet light (64) is set. The ultraviolet light (64) is focused by the cylindrical lens (65), the electrically controlled adjustable aperture (66), and the phase mask (67) onto the core of the fiber (62). The fiber optic grating is written on the core of the fiber (62). The exposure time and the concavity of the spectrum measured by the spectrometer (63) are recorded. The concavity of the spectrum measured by the spectrometer (63) is the reflectivity of the fiber optic grating written in the exposure area. Thus, the relationship curve between ultraviolet light irradiation time and fiber optic grating reflectivity is obtained. The fiber (62) is a coated ultraviolet transparent fiber, and the coating layer does not need to be removed during the writing process. The second step involves writing the UV-transparent continuous chirped weak fiber grating of the coating layer. Based on the design value of the reflectivity of the UV-transparent chirped weak fiber grating of the coating layer and the relationship curve between the UV irradiation time and the reflectivity of the fiber grating, the exposure time for writing the fiber grating within the exposure area is calculated. The fiber (62) is mounted on a linearly arranged rotating conveyor wheel one (68), rotating conveyor wheel two (69), rotating conveyor wheel three (610), and rotating conveyor wheel four (611). The two segments of fiber (62) between the edge and the center conveyor wheel are respectively hung with weight one (612) and weight two (613) to provide a fixed tension. The fiber (62) is mounted on rotating conveyor wheel one (68), rotating conveyor wheel two (69), rotating conveyor wheel three (610), and rotating conveyor wheel four (611). Driven by the rotating conveyor wheel four (611), the fiber optic winding disk one (614) and the fiber optic winding disk two (615) at both ends are used to release and collect the fiber optic cable (62). The control system (616) is connected to the electrically controlled adjustable aperture (66) and the rotating conveyor wheels one (68), two (69), three (610), and four (611) via cables one (617), two (618), three (619), four (620), and five (621). Under the control of the control system (616), the coated ultraviolet transparent chirped weak fiber optic grating is inscribed. The inscription of the coated ultraviolet transparent chirped weak fiber optic grating includes the following steps: (a) Rotate the first rotating wheel (68), the second rotating wheel (69), the third rotating wheel (610) and the fourth rotating wheel (611) to move the optical fiber (62) to the exposure area (111); (b) Before writing the UV transparent chirped weak fiber grating at each position, adjust the orientation of the mask (67) and the fiber (62) so that the period of the UV transparent chirped weak fiber grating written on the fiber (62) is arranged continuously from large to small or from small to large or from large to small and then from small to large. The UV transparent chirped weak fiber grating has a wide reflectance spectrum and can average reflectivity and use a wide wavelength range of sensing light to improve the intensity of the sensing light signal at the end of the sensing fiber, thereby improving the intensity of the light signal measured by the OFDR distributed sensing system. The opening time of the electrically controlled adjustable aperture (66) is the calculated UV irradiation time. Write the fiber grating on the fiber (62) in the exposure area, and then close the electrically controlled adjustable aperture (66). (c) Rotate the first conveyor wheel (68), the second conveyor wheel (69), the third conveyor wheel (610), and the fourth conveyor wheel (611) to move the optical fiber to the next exposure area (112). (d) Repeat (b) to (c) until the writing length reaches the design value of the UV transparent chirped weak fiber grating of the coating layer.

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