A method for improving the reflectivity of an ultra-weak reflection fiber grating by high-temperature annealing

Abstract

The application discloses a method for improving reflectivity of ultra-weak reflection fiber grating by high-temperature annealing, which comprises the following steps: step 1, preparing the ultra-weak reflection fiber grating on the optical fiber by using a femtosecond laser direct writing method, wherein the laser energy E of the femtosecond laser is greater than or equal to 7nj; step 2, performing high-temperature annealing on the ultra-weak reflection fiber grating, so that the reflectivity of the ultra-weak reflection fiber grating is improved to a target reflectivity, the annealing temperature T is 700-1000 DEG C, and the annealing time is not less than 8h. The method improves the reflectivity of the ultra-weak reflection fiber grating by the way of high-temperature annealing.

Description

TECHNICAL FIELD

[0001] The present application relates to fiber grating sensing technology, and in particular to a method for improving the reflectivity of ultra-weak reflection fiber grating by high-temperature annealing. BACKGROUND

[0002] Ultra-weak reflection grating array (UWFBGs) refers to a grating array formed by a plurality of ultra-weak reflection fiber gratings on the same optical fiber, and the reflectivity of each ultra-weak reflection fiber grating is very small (0.1% or less). Ultra-weak reflection grating array has been widely used in recent years for its distributed high-precision measurement, such as detection of small heat sources, distributed large strain detection, distributed pipeline gas leakage monitoring, and distributed vibration monitoring.

[0003] Traditional ultra-weak reflection fiber gratings are prepared by using an ultraviolet phase mask method, but the ultra-weak reflection fiber gratings prepared by using this method have a temperature-dependent phenomenon called "thermal decay", that is, the ultra-weak reflection fiber gratings are erased at high temperatures, which limits the application of ultra-weak reflection fiber gratings in high-temperature sensing fields (> 350℃).

[0004] In recent years, ultra-weak reflection fiber gratings prepared by using a femtosecond laser direct writing method have become a research hotspot due to their high-temperature stability and high flexibility, and they can remain stable at ultra-high temperatures.

[0005] The detection sensitivity and detection accuracy of ultra-weak reflection fiber gratings are related to their reflectivity. When preparing ultra-weak reflection fiber gratings by using a femtosecond laser direct writing method, technicians generally increase the laser energy to improve the reflectivity of the ultra-weak reflection fiber gratings, but this method cannot always improve the reflectivity of the ultra-weak reflection fiber gratings. When the laser energy is increased to a certain value, the reflectivity of the ultra-weak reflection fiber gratings tends to be stable and no longer increases with the increase of the laser energy.

[0006] Therefore, there is an urgent need to develop another method for improving the reflectivity of ultra-weak reflection fiber gratings. SUMMARY

[0007] In order to solve the above problems of the prior art, the present application provides a method for improving the reflectivity of ultra-weak reflection fiber gratings by high-temperature annealing.

[0008] The technical problem to be solved by the present application is solved by the following technical scheme:

[0009] A method for improving the reflectivity of ultra-weak reflection fiber gratings by high-temperature annealing, comprising the following steps:

[0010] Step 1: preparing an ultra-weak reflection fiber grating on an optical fiber by using a femtosecond laser direct writing method, wherein the laser energy E of the femtosecond laser is greater than or equal to 7nj;

[0011] Step 2: high-temperature annealing is performed on the ultra-weak reflection fiber grating, so that the reflectivity of the ultra-weak reflection fiber grating is increased to a target reflectivity, the annealing temperature T is 700-1000℃, and the annealing time is not less than 8h.

[0012] Further, in step 2, when the high-temperature annealing is performed on the ultra-weak reflection fiber grating, a heating stage and a constant temperature stage are included, in the heating stage, the ultra-weak reflection fiber grating is heated from room temperature to the annealing temperature, and the heating speed of the heating stage is 1-5℃ / min; in the constant temperature stage, the ultra-weak reflection fiber grating is maintained at the annealing temperature, and the maintenance time of the constant temperature stage is 2-11h.

[0013] Further, the annealing temperature T is 700℃, 800℃, 900℃ or 1000℃.

[0014] Further, the laser energy E is between 7-11nj.

[0015] Further, the laser energy E is 7nj, 9nj or 11nj.

[0016] Further, the fiber is a single-mode fiber, and a plurality of ultra-weak reflection fiber gratings are uniformly distributed on the fiber in the axial direction to form an ultra-weak reflection grating array.

[0017] Further, the method further comprises the following steps:

[0018] Step 3: cooling the ultra-weak reflection fiber grating from the annealing temperature to room temperature.

[0019] Further, in step 3, natural cooling or temperature-controlled cooling is adopted for the ultra-weak reflection fiber grating.

[0020] Further, the method further comprises the following steps:

[0021] Step 4: multiple heating and cooling treatments are performed on the ultra-weak reflection fiber grating between a first temperature and a second temperature to verify the reflectivity stability thereof.

[0022] Further, the first temperature is 100℃, and the second temperature is 800℃.

[0023] The present application has the following beneficial effects: the method of the present application performs high-temperature annealing treatment on the ultra-weak reflection fiber grating prepared by the femtosecond laser direct writing method with laser energy E≥7nj, so as to increase the reflectivity of the ultra-weak reflection fiber grating, thereby improving the detection sensitivity, detection accuracy and repeatability of the ultra-weak reflection fiber grating. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 A method for improving the reflectivity of ultra-weak reflection fiber grating by high-temperature annealing.

[0025] Figure 2 A raw distance domain spectrum of five groups of ultra-weak reflection grating arrays in a grating array sample provided by the present application.

[0026] Figure 3 Raw wavelength domain spectrum graphs of the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in a grating array sample provided by the present application.

[0027] Figure 4 Final distance domain spectrum graphs of the five groups of ultra-weak reflection grating arrays in a grating array sample provided by the present application before annealing and after 13h annealing at 700℃.

[0028] Figure 5 A relationship curve between the annealing time and the reflectivity when the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in a grating array sample provided by the present application are annealed at 700℃.

[0029] Figure 6 A relationship spectrum between the ambient temperature and the reflectivity after the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in a grating array sample provided by the present application are annealed at 700℃.

[0030] Figure 7 Final distance domain spectrum graphs of the five groups of ultra-weak reflection grating arrays in a grating array sample provided by the present application before annealing and after 13h annealing at 800℃.

[0031] Figure 8 A relationship curve between the annealing time and the reflectivity when the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in a grating array sample provided by the present application are annealed at 800℃.

[0032] Figure 9 A relationship spectrum between the ambient temperature and the reflectivity after the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in a grating array sample provided by the present application are annealed at 800℃.

[0033] Figure 10The final distance domain spectrum of the five groups of ultra-weak reflection grating arrays in the grating array sample provided by the present application before annealing and after annealing at 900 DEG C for 13h.

[0034] Figure 11 The relationship curve between annealing time and reflectivity when the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in the grating array sample provided by the present application are annealed at 900 DEG C.

[0035] Figure 12 The relationship spectrum between ambient temperature and reflectivity after the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in the grating array sample provided by the present application are annealed at 900 DEG C.

[0036] Figure 13 The final distance domain spectrum of the five groups of ultra-weak reflection grating arrays in the grating array sample provided by the present application before annealing and after annealing at 1000 DEG C for 13h.

[0037] Figure 14 The relationship curve between annealing time and reflectivity when the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in the grating array sample provided by the present application are annealed at 1000 DEG C.

[0038] Figure 15 The relationship spectrum between ambient temperature and reflectivity after the second group of ultra-weak reflection grating arrays, the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays in the grating array sample provided by the present application are annealed at 1000 DEG C. DETAILED DESCRIPTION

[0039] The present application will be described in detail below with reference to the accompanying drawings and examples, wherein the same or similar reference numerals are used throughout the drawings to represent the same or similar elements or elements having the same or similar functions. The examples described below by reference to the accompanying drawings are exemplary and are intended to explain the present application, and cannot be understood as limiting the present application.

[0040] In the description of the present application, it is to be understood by the terms "length", "width", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings, which are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as limiting the present application.

[0041] In addition, the terms "first", "second", "third" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features referred to. Therefore, the features defined with "first", "second", "third" can explicitly or implicitly include one or more of the features. In the description of the present application, the meaning of "multiple" is two or more, unless otherwise specifically limited.

[0042] In the present application, unless otherwise specifically defined and limited, the terms "mounting", "connection", "connection", "fixing", "setting" and the like should be broadly understood, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0043] Embodiment one

[0044] As shown in Figure 1 A method for improving the reflectivity of an ultra-weak reflection fiber grating by high-temperature annealing, comprising the following steps:

[0045] Step 1: An ultra-weak reflection fiber grating is prepared on an optical fiber by a femtosecond laser direct writing method, and the laser energy E of the femtosecond laser is greater than or equal to 7nj;

[0046] Step 2: High-temperature annealing is performed on the ultra-weak reflection fiber grating to increase the reflectivity of the ultra-weak reflection fiber grating to a target reflectivity, and the annealing temperature T is 700-1000℃, and the annealing time is not less than 8h.

[0047] High-temperature annealing preparation of regenerated fiber grating is a technology which utilizes stress relief effect during high-temperature annealing to erase original fiber grating with poor thermal stability on the fiber and to regenerate fiber grating with good thermal stability at the same position. The technology is generally applied to original fiber grating prepared by ultraviolet phase mask method to make up for the defect of "thermal decay" of the original fiber grating prepared by the method. However, the reflectivity of the regenerated fiber grating is lower than that of the original fiber grating.

[0048] The inventors of the patent creatively found that some fiber gratings with good thermal stability prepared by femtosecond laser direct writing method are not only not erased during high-temperature annealing, but their reflectivity is improved instead.

[0049] Based on the creative finding of the inventors, the present patent method is to perform high-temperature annealing on super-weak reflection fiber gratings prepared by femtosecond laser direct writing method with laser energy E≥7nj to improve the reflectivity of the super-weak reflection fiber gratings, thereby improving the detection sensitivity, detection accuracy and repeatability of the super-weak reflection fiber gratings.

[0050] In the embodiment, the fiber is a single-mode fiber, and a plurality of super-weak reflection fiber gratings are uniformly distributed along the axial direction of the fiber to form a super-weak reflection grating array, and the original reflectivity of each super-weak reflection fiber grating is very small (0.1% or less).

[0051] In step 2, the high-temperature annealing of the super-weak reflection fiber grating includes a heating stage and a constant temperature stage. In the heating stage, the super-weak reflection fiber grating is heated from room temperature to the annealing temperature, and the heating rate of the heating stage is 1-5℃ / min. In the constant temperature stage, the super-weak reflection fiber grating is maintained at the annealing temperature, and the maintenance time of the constant temperature stage is 2-11h.

[0052] In the embodiment, the laser energy E is between 7-11nj, and preferably, the laser energy E is 7nj, 9nj or 11nj; and the annealing temperature T is 700℃, 800℃, 900℃ or 1000℃.

[0053] Preferably, the method further comprises the following steps:

[0054] Step 3: cooling the super-weak reflection fiber grating from the annealing temperature to room temperature.

[0055] Specifically, in step 3, the super-weak reflection fiber grating can be naturally cooled or temperature-controlled cooled, wherein the cooling rate of the temperature-controlled cooling is 0.5-1.5℃ / min.

[0056] Preferably, the method further comprises the following steps:

[0057] Step 4: The ultra-weak reflection fiber grating is subjected to multiple temperature rising and falling treatments between a first temperature and a second temperature to verify the stability of its reflectivity.

[0058] Preferably, the first temperature is 100℃ and the second temperature is 800℃, and 100-800℃ can cover the temperature range of the ultra-weak reflection fiber grating for high-temperature sensing.

[0059] In this embodiment, the number of times of temperature rising and falling treatment of the ultra-weak reflection fiber grating is twice.

[0060] Embodiment Two

[0061] This embodiment is used to verify the method described in Embodiment One.

[0062] First, five groups of ultra-weak reflection grating arrays are prepared on a single-mode optical fiber by femtosecond laser direct writing to obtain a grating array sample, wherein the first group of ultra-weak reflection grating arrays is prepared by a laser energy of 3nj, the second group of ultra-weak reflection grating arrays is prepared by a laser energy of 5nj, the third group of ultra-weak reflection grating arrays is prepared by a laser energy of 7nj, the fourth group of ultra-weak reflection grating arrays is prepared by a laser energy of 9nj, and the fifth group of ultra-weak reflection grating arrays is prepared by a laser energy of 11nj.

[0063] The different groups of ultra-weak reflection grating arrays are completely the same in other parameters except for the laser energy (or initial reflectivity), such as grating period, grating length, grating spacing, center wavelength and grating order.

[0064] Among the five groups of ultra-weak reflection grating arrays, the first group of ultra-weak reflection grating arrays and the second group of ultra-weak reflection grating arrays serve as a control group, and the third group of ultra-weak reflection grating arrays, the fourth group of ultra-weak reflection grating arrays and the fifth group of ultra-weak reflection grating arrays serve as a test group.

[0065] In this embodiment, the number of ultra-weak reflection fiber gratings contained in each group of ultra-weak reflection grating arrays is n = 6, and the grating period of all the ultra-weak reflection fiber gratings is pitch = 1.07μm, the grating length is l = 1mm, the grating spacing is d = 5mm, and the grating order is K = 2.

[0066] Then, the grating array sample is connected to a light scattering reflectometer for data acquisition to simultaneously obtain the original distance domain spectrum of the five groups of ultra-weak reflection grating arrays.

[0067] In this embodiment, the optical backscattering reflectometer is an Optical Backscatter Reflectometer (OBR) 4600 from Luna Innovations, with a sweep frequency range of 1525–1610 nm and a sampling resolution of 10 μm. The laser incident direction is from the fifth group of ultra-weak reflection grating arrays to the first group of ultra-weak reflection grating arrays to prevent the reflected light formed by the fifth group of ultra-weak reflection grating arrays from interfering with the first few groups of ultra-weak reflection grating arrays.

[0068] like Figure 2 As shown, as a control group, the first group of ultra-weak reflection grating arrays array1, prepared with laser energy E=3nj, did not show an increase in Rayleigh scattering signal in the original range domain spectrum. The second group of ultra-weak reflection grating arrays array2, prepared with laser energy E=5nj, began to show an increase in Rayleigh scattering signal in the original range domain spectrum. As a test group, with the increase of laser energy E, the third group of ultra-weak reflection grating arrays array3, array4, array4, and array5, prepared with laser energy E=7nj, also showed a more significant increase in Rayleigh scattering signal in the original range domain spectrum.

[0069] like Figure 3 As shown, by performing a fast Fourier transform on the regions corresponding to the second set of ultra-weak reflection grating arrays array2, array3, array4, and array5 on the original distance domain spectrum, grating reflection peaks with reflectivities of -70dB, -45dB, -31dB, and -25dB can be seen in the original wavelength domain spectrum of the four arrays.

[0070] Next, by setting the heating program of the tube furnace, the grating array sample was heated from room temperature to an annealing temperature of 700°C at a heating rate of 5°C / min and maintained stably. The total annealing time was 13 hours. During the annealing process, the light scattering reflectometer was used to collect data on the grating array sample every 15 minutes to simultaneously obtain the real-time range domain spectra of five sets of ultra-weak reflection grating arrays.

[0071] In this embodiment, the tube furnace is EST12 / 300B tube furnace of CARBOLITE GERO company, the cavity length (TL) of the tube furnace is 300 mm, and the total length (AL) of the five sets of ultra-weak reflection grating arrays is 150 mm, so as to ensure that the five sets of ultra-weak reflection grating arrays are completely inside the tube furnace and located at the center position of the tube furnace when the grating array sample is placed, and the left and right ends of the grating array sample are heat-insulated by quartz wool to maintain a constant temperature field.

[0072] As shown in Figure 4 FIG. 6, in the high-temperature annealing at 700℃, the temperature rising time of the temperature rising stage is about 2.25h, and the maintaining time of the temperature maintaining stage is about 10.75h; as a control group, the Rayleigh scattering signal of the first set of ultra-weak reflection grating array array1 cannot be seen in the final distance domain spectrum after annealing for 13h, and the Rayleigh scattering signal enhancement of the second set of ultra-weak reflection grating array array2 in the final distance domain spectrum disappears after annealing for 13h, indicating that the second set of ultra-weak reflection grating array array2 is erased by high temperature; as a test group, the Rayleigh scattering signals of the third set of ultra-weak reflection grating array array3, the fourth set of ultra-weak reflection grating array array4 and the fifth set of ultra-weak reflection grating array array5 in the final distance domain spectrum are improved to different degrees after annealing for 13h.

[0073] As shown in Figure 5 FIG. 7, all real-time distance domain spectra of the second set of ultra-weak reflection grating array array2, the third set of ultra-weak reflection grating array array3, the fourth set of ultra-weak reflection grating array array4 and the fifth set of ultra-weak reflection grating array array5 within 13h are transformed into real-time wavelength domain spectra by fast Fourier transform, and then the relationship curve between annealing time and reflectivity is constructed by taking annealing time as the horizontal axis and reflectivity in the real-time wavelength domain spectrum as the vertical axis. It can be seen from the relationship curve that the reflectivity of the second set of ultra-weak reflection grating array array2 decreases to the same as the fiber noise after annealing for 2-4h, and the reflectivity of the third set of ultra-weak reflection grating array array3, the fourth set of ultra-weak reflection grating array array4 and the fifth set of ultra-weak reflection grating array array5 are slowly improved after 2h of rapid improvement, and finally tend to be stable, and the final reflectivity improvement amplitude is 7.8dB, 3.7dB and 2.2dB respectively.

[0074] After 13 hours, the grating array sample was placed in the tube furnace. The cooling program of the tube furnace was set to cool the grating array sample from the annealing temperature of 700°C to room temperature at a cooling rate of 1°C / min. No data acquisition was required on the grating array sample during the cooling stage.

[0075] Finally, by setting the heating and cooling programs of the tube furnace, the grating array sample is subjected to two heating and cooling processes between 100-800℃ in 100℃ increments. The light scattering reflectometer is used to collect data on the grating array sample every 100℃ to simultaneously obtain real-time range-domain spectra of the five sets of ultra-weak reflection grating arrays. Then, all real-time range-domain spectra are converted into real-time wavelength-domain spectra by fast Fourier transform to obtain the reflectivity of the third ultra-weak reflection grating array array3, the fourth ultra-weak reflection grating array array4, and the fifth ultra-weak reflection grating array array5 every 100℃. This constructs the relationship spectrum between ambient temperature and reflectivity of the third ultra-weak reflection grating array array3, the fourth ultra-weak reflection grating array array4, and the fifth ultra-weak reflection grating array array5 during each heating and cooling process.

[0076] like Figure 6 As shown, during the two heating up and cooling down processes, the reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) remained constant, with virtually no significant change. This indicates that the increase in reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) after high-temperature annealing at 700℃ is permanent.

[0077] The reason for using a 700℃ high-temperature annealing process for 13 hours on the grating array sample in this embodiment is to facilitate observation of the reflectance changes of the five sets of ultra-weak reflection grating arrays on the grating array sample during long-term high-temperature annealing. In fact, as long as the annealing time is not less than 8 hours, the reflectance of the third set of ultra-weak reflection grating arrays (array3), the fourth set of ultra-weak reflection grating arrays (array4), and the fifth set of ultra-weak reflection grating arrays (array5) can remain stable after high-temperature annealing.

[0078] Example 3

[0079] This embodiment is used to verify the method described in Embodiment 1.

[0080] This embodiment uses the same grating array sample as in Embodiment 1, and performs high-temperature annealing on the grating array sample in the same way. The only difference from Embodiment 1 is that the annealing temperature in this embodiment is 800℃.

[0081] like Figure 7 As shown, in the high-temperature annealing at 800℃, the heating time is approximately 2.5 hours and the isothermal time is approximately 10.5 hours. As a control group, after annealing for 13 hours, the first group of ultra-weak reflection grating arrays (array1) still did not show an increase in Rayleigh scattering signal in the final range domain spectrum. However, after annealing for 13 hours, the enhanced Rayleigh scattering signal in the second group of ultra-weak reflection grating arrays (array2) disappeared in the final range domain spectrum, indicating that the second group of ultra-weak reflection grating arrays (array2) was erased by high temperature. As a test group, after annealing for 13 hours, the Rayleigh scattering signals in the third group of ultra-weak reflection grating arrays (array3), the fourth group of ultra-weak reflection grating arrays (array4), and the fifth group of ultra-weak reflection grating arrays (array5) were improved to varying degrees in the final range domain spectrum.

[0082] like Figure 8 As shown, the real-time distance-domain spectra of the second, third, fourth, and fifth ultra-weak reflection grating arrays (array2, array3, array4, and array5) within 13 hours were converted into real-time wavelength-domain spectra using Fast Fourier Transform. A graph showing the relationship between annealing time and reflectivity was then constructed, with annealing time as the horizontal axis and reflectivity in the real-time wavelength-domain spectra as the vertical axis. The graph shows that after annealing for 2-4.5 hours, the reflectivity of the second ultra-weak reflection grating array (array2) decreased to approximately the same level as the fiber noise floor. In contrast, the reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) increased rapidly for the first 2 hours, then slowed down and eventually stabilized, with final reflectivity increases of 10.7 dB, 5.6 dB, and 3.8 dB, respectively.

[0083] like Figure 9As shown, during the two heating up and cooling down processes, the reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) remained constant, with virtually no significant change. This indicates that the increase in reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) after high-temperature annealing at 800℃ is permanent.

[0084] The reason for using a 13-hour high-temperature annealing process at 800℃ on the grating array sample in this embodiment is to facilitate observation of the reflectance changes of the five sets of ultra-weak reflection grating arrays on the sample during prolonged high-temperature annealing. In fact, as long as the annealing time is not less than 8 hours, the reflectance of the third ultra-weak reflection grating array (array3), the fourth ultra-weak reflection grating array (array4), and the fifth ultra-weak reflection grating array (array5) can remain stable after high-temperature annealing.

[0085] Example 4

[0086] This embodiment is used to verify the method described in Embodiment 1.

[0087] This embodiment uses the same grating array sample as in Embodiment 1, and performs high-temperature annealing on the grating array sample in the same way. The only difference from Embodiment 1 is that the annealing temperature in this embodiment is 900℃.

[0088] like Figure 10 As shown, in the high-temperature annealing at 900℃, the heating time is approximately 3 hours and the isothermal time is approximately 10 hours. As a control group, after annealing for 13 hours, the first group of ultra-weak reflection grating arrays (array1) still did not show an increase in Rayleigh scattering signal in the final range domain spectrum. However, after annealing for 13 hours, the enhanced Rayleigh scattering signal in the second group of ultra-weak reflection grating arrays (array2) disappeared in the final range domain spectrum, indicating that the second group of ultra-weak reflection grating arrays (array2) was erased by high temperature. As a test group, after annealing for 13 hours, the Rayleigh scattering signals in the third group of ultra-weak reflection grating arrays (array3), the fourth group of ultra-weak reflection grating arrays (array4), and the fifth group of ultra-weak reflection grating arrays (array5) were improved to varying degrees in the final range domain spectrum.

[0089] like Figure 11As shown, after the fast Fourier transform of all real-time distance domain spectrograms of the second group of ultra-weak reflection grating array array2, the third group of ultra-weak reflection grating array array3, the fourth group of ultra-weak reflection grating array array4 and the fifth group of ultra-weak reflection grating array array5 within 13h into real-time wavelength domain spectrograms, the annealing time is taken as the horizontal axis, and the reflectivity in the real-time wavelength domain spectrogram is taken as the vertical axis, to construct a relationship curve between the annealing time and the reflectivity. It can be seen from the relationship curve that the reflectivity of the second group of ultra-weak reflection grating array array2 is reduced to the same as the fiber noise within less than 2h of annealing, while the third group of ultra-weak reflection grating array array3, the fourth group of ultra-weak reflection grating array array4 and the fifth group of ultra-weak reflection grating array array5 are slowly improved after the first 2h of rapid improvement, and finally tend to be stable, and the final reflectivity improvement amplitude is 17.8dB, 8.8dB and 5.4dB respectively.

[0090] As shown in Figure 12 As shown, during the two heating (1st and 2nd heating up) and cooling (1st and 2nd cooling down) processes, the reflectivity of the third group of ultra-weak reflection grating array array3, the fourth group of ultra-weak reflection grating array array4 and the fifth group of ultra-weak reflection grating array array5 remains constant, and there is basically no obvious change, which shows that after the high-temperature annealing of the third group of ultra-weak reflection grating array array3, the fourth group of ultra-weak reflection grating array array4 and the fifth group of ultra-weak reflection grating array array5 at 900℃ for 13h, the reflectivity is permanently improved.

[0091] The reason why the grating array sample in this embodiment is subjected to high-temperature annealing at 900℃ for 13h is to facilitate the observation of the reflectivity change of the five groups of ultra-weak reflection grating arrays on the grating array sample during long-time high-temperature annealing. In fact, as long as the annealing time is not less than 8h, the reflectivity of the third group of ultra-weak reflection grating array array3, the fourth group of ultra-weak reflection grating array array4 and the fifth group of ultra-weak reflection grating array array5 after high-temperature annealing can remain stable.

[0092] Example Five

[0093] This embodiment is used to verify the method described in Example One.

[0094] This embodiment uses the same grating array sample as Example One, and the high-temperature annealing of the grating array sample is also carried out in the same way. The difference between this embodiment and Example One is that the annealing temperature of this embodiment is 1000℃.

[0095] As shown inFigure 13 As shown, in the high-temperature annealing at 1000℃, the heating time is approximately 3.25 hours and the isothermal time is approximately 9.75 hours. As a control group, after annealing for 13 hours, the first group of ultra-weak reflection grating arrays (array1) still did not show an increase in Rayleigh scattering signal in the final range domain spectrum. However, after annealing for 13 hours, the enhanced Rayleigh scattering signal in the second group of ultra-weak reflection grating arrays (array2) disappeared in the final range domain spectrum, indicating that the second group of ultra-weak reflection grating arrays (array2) was erased by the high temperature. As a test group, after annealing for 13 hours, the Rayleigh scattering signals in the third group of ultra-weak reflection grating arrays (array3), the fourth group of ultra-weak reflection grating arrays (array4), and the fifth group of ultra-weak reflection grating arrays (array5) were improved to varying degrees in the final range domain spectrum.

[0096] It is noteworthy that two very obvious Rayleigh scattering signal enhancements appeared in front of the second set of ultra-weak reflection grating arrays (array2). After observing the extraneous peak spectrum, it was found that these two Rayleigh scattering signal enhancements did not originate from the first set of ultra-weak reflection grating arrays (array1), but rather from the overall spectrum signal enhancement. It is speculated that this was caused by the combined effect of changes in internal fiber stress and softening of the glass structure under high temperature conditions.

[0097] like Figure 14 As shown, the real-time distance-domain spectra of the second, third, fourth, and fifth ultra-weak reflection grating arrays (array2, array3, array4, and array5) within 13 hours were converted into real-time wavelength-domain spectra using Fast Fourier Transform. A curve was then constructed showing the relationship between annealing time and reflectivity, with annealing time as the horizontal axis and reflectivity in the real-time wavelength-domain spectra as the vertical axis. The curve shows that after annealing for 5 hours, the reflectivity of the second ultra-weak reflection grating array (array2) decreased to approximately the same level as the fiber noise floor. In contrast, after a rapid initial boost of 4 hours, the reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) slowed down and eventually stabilized, with final reflectivity increases of 16.0 dB, 9.0 dB, and 5.5 dB, respectively.

[0098] It is worth noting that the reflectivity improvement brought about by high-temperature annealing of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) at 1000℃ is not significantly different from that of high-temperature annealing at 900℃. In fact, the reflectivity improvement brought about by high-temperature annealing of the third ultra-weak reflection grating array (array3) at 1000℃ is even lower than that of high-temperature annealing at 900℃. It is speculated that this may be because 1000℃ is close to the melting temperature of optical fiber, which slightly damages the grating structure of the third ultra-weak reflection grating array (array3).

[0099] like Figure 15 As shown, during the two heating up and cooling down processes, the reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) remained constant, with virtually no significant change. This indicates that the increase in reflectivity of the third, fourth, and fifth ultra-weak reflection grating arrays (array3, array4, and array5) after high-temperature annealing at 1000℃ is permanent.

[0100] The reason for using a 13-hour high-temperature annealing process at 1000℃ on the grating array sample in this embodiment is to facilitate observation of the reflectance changes of the five sets of ultra-weak reflection grating arrays on the sample during prolonged high-temperature annealing. In fact, as long as the annealing time is not less than 8 hours, the reflectance of the third ultra-weak reflection grating array (array3), the fourth ultra-weak reflection grating array (array4), and the fifth ultra-weak reflection grating array (array5) can remain stable after high-temperature annealing.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the embodiments of the present invention and not to limit them. Although the embodiments of the present invention have been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the embodiments of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for increasing the reflectivity of a super-weak reflection fiber grating by high-temperature annealing, characterized in that, The method comprises the following steps: Step 1: preparing an ultra-weak reflection fiber grating on an optical fiber by using a femtosecond laser direct writing method, the laser energy E is between 7-11 nj, the optical fiber is a single-mode optical fiber, and a plurality of ultra-weak reflection fiber gratings are uniformly distributed on the optical fiber in the axial direction to form the ultra-weak reflection fiber grating array, and the original reflectivity of each ultra-weak reflection fiber grating is 0.1% or below; Step 2: annealing the ultra-weak reflection fiber grating at high temperature to increase the reflectivity of the ultra-weak reflection fiber grating to a target reflectivity, the annealing temperature T is 700-1000℃, and the annealing time is not less than 8h; In step 2, when the ultra-weak reflection fiber grating is annealed at high temperature, it comprises a heating stage and a constant temperature stage, in the heating stage, the ultra-weak reflection fiber grating is heated from room temperature to the annealing temperature, and the heating rate of the heating stage is 1-5℃ / min; in the constant temperature stage, the ultra-weak reflection fiber grating is maintained at the annealing temperature.

2. The method of claim 1, wherein, The maintenance time of the constant temperature stage is 2-11h.

3. The method of claim 1, wherein, The annealing temperature T is 700℃, 800℃, 900℃ or 1000℃.

4. The method of claim 1, wherein, The laser energy E is 7nj, 9nj or 11nj.

5. The method of claim 1, wherein, The method further comprises the following steps: Step 3: cooling the ultra-weak reflection fiber grating from the annealing temperature to room temperature.

6. The method of claim 5, wherein, In step 3, the ultra-weak reflection fiber grating is naturally cooled or temperature-controlled cooled.

7. The method of claim 5, wherein, The method further comprises the following steps: Step 4: performing multiple heating and cooling treatments between the first temperature and the second temperature on the ultra-weak reflection fiber grating to verify the reflectivity stability thereof.

8. The method of claim 7, wherein, The first temperature is 100℃, and the second temperature is 800℃.

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