Method for improving reflectivity of ultra-weak reflection fiber bragg grating through high-temperature annealing

Abstract

The invention discloses a method for improving the reflectivity of an ultra-weak reflection fiber bragg grating through high-temperature annealing, which comprises the following steps of: 1, preparing the ultra-weak reflection fiber bragg grating on an optical fiber by adopting a femtosecond laser direct writing method, and enabling the laser energy E of femtosecond laser to be greater than or equal to 7nj; 2, high-temperature annealing is conducted on the ultra-weak reflection fiber grating, the reflectivity of the ultra-weak reflection fiber grating is improved to the target reflectivity, the annealing temperature T ranges from 700 DEG C to 1000 DEG C, and the annealing time is not shorter than 8 h. According to the method, the reflectivity of the ultra-weak reflection fiber bragg grating is improved in a high-temperature annealing mode.

Description

Technical Field

[0001] The present invention relates to fiber Bragg grating sensing technology, and particularly to a method for improving the reflectivity of ultra-weak reflection fiber Bragg gratings through high-temperature annealing. Background Art

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

[0003] Traditional ultra-weak reflection fiber Bragg gratings are prepared by the ultraviolet phase mask method. However, the ultra-weak reflection fiber Bragg gratings prepared by this method have a temperature-dependent phenomenon called "thermal decay", that is, the ultra-weak reflection fiber Bragg gratings will be erased at high temperatures, which limits the application of ultra-weak reflection fiber Bragg gratings in the high-temperature sensing field (>350°C).

[0004] In recent years, ultra-weak reflection fiber Bragg gratings prepared by 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 Bragg gratings are related to their reflectivity. When preparing ultra-weak reflection fiber Bragg gratings by the femtosecond laser direct writing method, technicians generally increase the reflectivity of ultra-weak reflection fiber Bragg gratings by increasing the laser energy. However, this method cannot continuously increase the reflectivity of ultra-weak reflection fiber Bragg gratings. When the laser energy is increased to a certain value, the reflectivity of ultra-weak reflection fiber Bragg gratings tends to be stable and no longer increases with the increase of laser energy.

[0006] Therefore, it is urgent to develop another method for increasing the reflectivity of ultra-weak reflection fiber Bragg gratings. Summary of the Invention

[0007] To solve the above-mentioned deficiencies of the prior art, the present invention provides a method for increasing the reflectivity of ultra-weak reflection fiber Bragg gratings by high-temperature annealing.

[0008] The technical problems to be solved by the present invention are realized through the following technical solutions:

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

[0010] Step 1: Prepare an ultra-weak reflection fiber Bragg grating on the optical fiber by the femtosecond laser direct writing method, where the laser energy E of the femtosecond laser is ≥7 nj;

[0011] Step 2: Perform high-temperature annealing on the ultra-weakly reflecting fiber grating to increase the reflectivity of the ultra-weakly reflecting fiber grating to the target reflectivity. The annealing temperature T is 700 - 1000 °C, and the annealing time is not less than 8 h.

[0012] Further, in Step 2, when performing high-temperature annealing on the ultra-weakly reflecting fiber grating, it includes a heating stage and a constant-temperature stage. In the heating stage, the ultra-weakly reflecting fiber grating is heated from room temperature to the annealing temperature, and the heating rate in the heating stage is 1 - 5 °C / min; in the constant-temperature stage, the ultra-weakly reflecting fiber grating is maintained at the annealing temperature, and the maintenance time in the constant-temperature stage is 2 - 11 h.

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

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

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

[0016] Further, the optical fiber is a single-mode optical fiber, and several ultra-weakly reflecting fiber gratings are uniformly distributed along the axis on the optical fiber to form an ultra-weakly reflecting grating array.

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

[0018] Step 3: Cool the ultra-weakly reflecting 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-weakly reflecting fiber grating.

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

[0021] Step 4: Perform multiple heating and cooling treatments on the ultra-weakly reflecting fiber grating between a first temperature and a second temperature to verify its reflectivity stability.

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

[0023] The present invention has the following beneficial effects: The method of this patent performs high-temperature annealing treatment on an ultra-weakly reflecting fiber grating prepared by a femtosecond laser direct writing method with laser energy E ≥ 7 nj to increase the reflectivity of the ultra-weakly reflecting fiber grating, thereby improving the detection sensitivity, detection accuracy and repeatability of the ultra-weakly reflecting fiber grating. Description of the Drawings

[0024] Figure 1 Block diagram of the method for improving the reflectivity of ultra-weak reflection fiber gratings by high-temperature annealing provided by the present invention.

[0025] Figure 2 Original distance-domain spectrogram of five groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention.

[0026] Figure 3 Original wavelength-domain spectrograms of the second, third, fourth, and fifth groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention.

[0027] Figure 4 Final distance-domain spectrograms of five groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention before annealing and after annealing at 700 °C for 13 h.

[0028] Figure 5 Relationship curve diagram between the annealing time and the reflectivity when the second, third, fourth, and fifth groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention are annealed at 700 °C.

[0029] Figure 6 Relationship spectrum between the ambient temperature and the reflectivity after the second, third, fourth, and fifth groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention are annealed at 700 °C.

[0030] Figure 7 Final distance-domain spectrograms of five groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention before annealing and after annealing at 800 °C for 13 h.

[0031] Figure 8 Relationship curve diagram between the annealing time and the reflectivity when the second, third, fourth, and fifth groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention are annealed at 800 °C.

[0032] Figure 9 Relationship spectrum between the ambient temperature and the reflectivity after the second, third, fourth, and fifth groups of ultra-weak reflection grating arrays in the grating array sample provided by the present invention are annealed at 800 °C.

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

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

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

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

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

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

[0039] The present invention will be described in detail below with reference to the accompanying drawings and embodiments. The examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention.

[0040] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation on the present invention.

[0041] In addition, the terms "first", "second", "third" are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", "third" may explicitly or implicitly include one or more of such features. In the description of the present invention, the meaning of "a plurality" is two or more unless otherwise specifically defined.

[0042] In the present invention, unless otherwise clearly defined and limited, the terms "installed", "connected", "connected to", "fixed", "set", etc. should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or integrated; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and may also be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0043] Embodiment 1

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

[0045] Step 1: Prepare an ultra-weak reflection fiber grating on the optical fiber by femtosecond laser direct writing method, and the laser energy E of the femtosecond laser is E≥7nj;

[0046] Step 2: Perform high-temperature annealing on the ultra-weak reflection fiber grating to increase the reflectivity of the ultra-weak reflection fiber grating to the target reflectivity. The annealing temperature T is 700-1000°C, and the annealing time is not less than 8h.

[0047] The preparation of regenerated fiber Bragg gratings by high-temperature annealing is a technique that utilizes the stress elimination effect during high-temperature annealing. After erasing the original fiber Bragg gratings with poor thermal stability on the fiber, regenerated fiber Bragg gratings with good thermal stability are regenerated at the same position. This technique is generally applied to the original fiber Bragg gratings prepared by the ultraviolet phase mask method to compensate for the defect of "thermal decay" in the original fiber Bragg gratings prepared by this method. However, the reflectivity of the regenerated fiber Bragg gratings is lower than that of the original fiber Bragg gratings.

[0048] The inventors of this patent creatively discovered that some fiber Bragg gratings with good thermal stability prepared by femtosecond laser direct writing method not only will not be erased during high-temperature annealing treatment, but their reflectivity is increased instead.

[0049] The method of this patent is precisely based on this creative discovery of the inventors. High-temperature annealing treatment is performed on the ultra-weak reflection fiber Bragg gratings prepared by femtosecond laser direct writing method with laser energy E≥7nj to increase the reflectivity of the ultra-weak reflection fiber Bragg gratings, thereby improving their detection sensitivity, detection accuracy and repeatability.

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

[0051] In step 2, when performing high-temperature annealing on the ultra-weak reflection fiber Bragg gratings, it includes a heating stage and a constant temperature stage. In the heating stage, the ultra-weak reflection fiber Bragg gratings are heated from room temperature to the annealing temperature, and the heating rate of the heating stage is 1-5°C / min; in the constant temperature stage, the ultra-weak reflection fiber Bragg gratings are maintained at the annealing temperature, and the maintenance time of the constant temperature stage is 2-11h.

[0052] In this embodiment, the laser energy E is between 7-11nj. Preferably, the laser energy E is 7nj, 9nj or 11nj; the annealing temperature T is 700°C, 800°C, 900°C or 1000°C.

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

[0054] Step 3: Cool the ultra-weak reflection fiber Bragg gratings from the annealing temperature to room temperature.

[0055] Specifically, in step 3, the ultra-weak reflection fiber Bragg gratings can be cooled naturally or by temperature-controlled cooling, where the cooling rate of the temperature-controlled cooling is 0.5-1.5°C / min.

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

[0057] Step 4: Perform multiple heating and cooling treatments on the ultra-weakly reflective fiber grating between the first temperature and the second temperature to verify its reflectivity stability.

[0058] Preferably, the first temperature is 100 °C and the second temperature is 800 °C. The temperature range of 100 - 800 °C can cover the temperature range when the ultra-weakly reflective fiber grating is used for high-temperature sensing.

[0059] In this embodiment, the number of heating and cooling treatments on the ultra-weakly reflective fiber grating is two times.

[0060] Embodiment 2

[0061] This embodiment is used to verify the method described in Embodiment 1.

[0062] First, use the femtosecond laser direct writing method to fabricate five groups of ultra-weakly reflective grating arrays on a single-mode fiber to obtain a grating array sample. Among them, the first group of ultra-weakly reflective grating arrays is fabricated with a laser energy of 3 nj, the second group of ultra-weakly reflective grating arrays is fabricated with a laser energy of 5 nj, the third group of ultra-weakly reflective grating arrays is fabricated with a laser energy of 7 nj, the fourth group of ultra-weakly reflective grating arrays is fabricated with a laser energy of 9 nj, and the fifth group of ultra-weakly reflective grating arrays is fabricated with a laser energy of 11 nj.

[0063] Between different groups of ultra-weakly reflective grating arrays, except for the different laser energies (or initial reflectivities), other parameters are exactly the same, such as the grating period, grating length, grating spacing, central wavelength, and grating order, etc.

[0064] Among the five groups of ultra-weakly reflective grating arrays, the first group of ultra-weakly reflective grating arrays and the second group of ultra-weakly reflective grating arrays are used as control groups, and the third group of ultra-weakly reflective grating arrays, the fourth group of ultra-weakly reflective grating arrays, and the fifth group of ultra-weakly reflective grating arrays are used as test groups.

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

[0066] Then, use an optical scattering reflectometer to connect to the grating array sample for data acquisition to simultaneously obtain the original distance-domain spectrograms of the five groups of ultra-weakly reflective grating arrays.

[0067] In this embodiment, the optical scattering reflectometer uses the Optical Backscatter Reflectometer (OBR) 4600 of Luna Innovations. The swept frequency range is 1525 - 1610 nm, the sampling resolution is 10 μm, and 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 previous groups of ultra-weak reflection grating arrays.

[0068] As Figure 2 shown, as the control group, the first group of ultra-weak reflection grating arrays array1 prepared with a laser energy E = 3 nj does not show an increase in Rayleigh scattering signal in the original distance-domain spectrogram. The second group of ultra-weak reflection grating arrays array2 prepared with a laser energy E = 5 nj begins to show an increase in Rayleigh scattering signal in the original distance-domain spectrogram. As the test group, with the increase of the laser energy E, the third group of ultra-weak reflection grating arrays array3 prepared with a laser energy E = 7 nj, the fourth group of ultra-weak reflection grating arrays array4 prepared with a laser energy E = 9 nj, and the fifth group of ultra-weak reflection grating arrays array5 prepared with a laser energy E = 11 nj also show more obvious increases in Rayleigh scattering signal in the original distance-domain spectrogram.

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

[0070] Next, by setting the heating program of the tube furnace, the grating array sample is heated from room temperature to the annealing temperature of 700 °C at a heating rate of 5 °C / min and maintained stable. The total annealing time is 13 h. At the same time, during the annealing process, the optical scattering reflectometer is used to collect data from the grating array sample every 15 min to simultaneously obtain the real-time distance-domain spectrograms of the five groups of ultra-weak reflection grating arrays.

[0071] In this embodiment, the tube furnace is the 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 groups of ultra-weak reflection grating arrays is 150 mm. When placing the grating array sample, it is ensured that the five groups of ultra-weak reflection grating arrays are completely inside the tube furnace and located at the exact center position of the tube furnace. The left and right ends of the grating array sample are insulated with quartz wool to maintain a constant temperature field.

[0072] As Figure 4 shown, during the high-temperature annealing at 700 °C, the heating time in the heating stage is approximately 2.25 h, and the maintaining time in the constant-temperature stage is approximately 10.75 h. As a control group, after annealing for 13 h, no increase in the Rayleigh scattering signal can still be seen in the final distance-domain spectrogram of the first group of ultra-weak reflection grating array array1, while after annealing for 13 h, the enhancement of the Rayleigh scattering signal in the final distance-domain spectrogram of the second group of ultra-weak reflection grating array array2 disappears, indicating that the second group of ultra-weak reflection grating array array2 is erased by high temperature. As a test group, after annealing for 13 h, the Rayleigh scattering signals in the final distance-domain spectrograms 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 are improved to different degrees.

[0073] As Figure 5 shown, after performing fast Fourier transform on all the 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 13 h to obtain real-time wavelength-domain spectrograms, then using the annealing time as the horizontal axis and the reflectivity in the real-time wavelength-domain spectrogram as the vertical axis, a relationship curve graph between the annealing time and the reflectivity is constructed. It can be seen from the relationship curve graph that after annealing for 2 - 4 h, the reflectivity of the second group of ultra-weak reflection grating array array2 drops to be basically the same as the fiber background noise, while after a rapid increase for 2 h, the increasing speed 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 slows down and finally stabilizes. The final increases in reflectivity are 7.8 dB, 3.7 dB, and 2.2 dB respectively.

[0074] After 13 h, continue to place the grating array sample in the tube furnace. By setting the cooling program of the tube furnace, cool the grating array sample from the annealing temperature of 700 °C to room temperature at a cooling rate of 1 °C / min. During the cooling stage, no data collection is required for the grating array sample.

[0075] Finally, by setting the heating program and cooling program of the tube furnace, the grating array sample is heated and cooled twice between 100 °C and 800 °C in steps of 100 °C. And use the light scattering reflectometer to collect data on the grating array sample every 100 °C to simultaneously obtain five groups of real-time distance-domain spectrograms of the ultra-weak reflection grating array. Then, after performing fast Fourier transform on all the real-time distance-domain spectrograms to obtain real-time wavelength-domain spectrograms, obtain 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 every 100 °C, so as to construct the relationship spectrum between the ambient temperature and 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 during each heating and cooling process.

[0076] As Figure 6 shown, during the two heating up (1st and 2nd heating up) and cooling down (1st and 2nd cooling down) processes, the reflectivities 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 remain constant and basically show no obvious change, indicating that after high-temperature annealing at 700 °C, the increase in 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 is permanent.

[0077] In this embodiment, the reason for performing high-temperature annealing treatment on the grating array sample at 700 °C for 13 h is to facilitate observing the change in the reflectivity of the five groups 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 h, the reflectivities 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 can remain stable after high-temperature annealing.

[0078] Embodiment III

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

[0080] This embodiment uses exactly the same grating array sample as in Embodiment 1, and the grating array sample is annealed at high temperature in the same manner. The difference from Embodiment 1 is only that the annealing temperature in this embodiment is 800 °C.

[0081] As Figure 7 shown, in the high-temperature annealing at 800 °C, the heating-up time is about 2.5 h, and the constant-temperature time is about 10.5 h; as a control group, after annealing for 13 h, the first group of ultra-weak reflection grating arrays array1 still does not show an increase in the Rayleigh scattering signal in the final distance-domain spectrogram, while after annealing for 13 h, the Rayleigh scattering signal enhancement in the final distance-domain spectrogram of the second group of ultra-weak reflection grating arrays array2 disappears, indicating that the second group of ultra-weak reflection grating arrays array2 is erased by high temperature; as a test group, after annealing for 13 h, the Rayleigh scattering signals in the final distance-domain spectrograms of 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 are improved to different degrees.

[0082] As Figure 8 shown, after all the real-time distance-domain spectrograms of the second group of ultra-weak reflection grating arrays array2, 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 within 13 h are fast Fourier transformed into real-time wavelength-domain spectrograms, a relationship curve graph between the annealing time and the reflectivity is constructed with the annealing time as the horizontal axis and the reflectivity in the real-time wavelength-domain spectrogram as the vertical axis. It can be seen from the relationship curve graph that after annealing for 2 - 4.5 h, the reflectivity of the second group of ultra-weak reflection grating arrays array2 drops to be basically the same as the fiber background noise, while after a rapid increase in the first 2 h, the increasing speed of 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 slows down and finally tends to be stable, and the final reflectivity increase amplitudes are 10.7 dB, 5.6 dB, and 3.8 dB respectively.

[0083] As Figure 9As shown, during the two heating-up (1st and 2nd heating up) and cooling-down (1st and 2nd cooling down) processes, the reflectivities 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 remain constant, with basically no obvious changes, indicating that after the high-temperature annealing at 800 °C, the increase in their reflectivities is permanent.

[0084] The reason for using 800 °C for the high-temperature annealing treatment of the grating array sample for up to 13 h in this embodiment is to facilitate observing the changes in the reflectivities of the five 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 h, the reflectivities 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] Embodiment 4

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

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

[0088] As Figure 10 shown, during the high-temperature annealing at 900 °C, the heating-up time is approximately 3 h, and the constant-temperature time is approximately 10 h; as a control group, after annealing for 13 h, no increase in the Rayleigh scattering signal can still be seen in the final distance-domain spectrogram for the first ultra-weak reflection grating array array1, while after annealing for 13 h, the enhancement of the Rayleigh scattering signal in the final distance-domain spectrogram for the second ultra-weak reflection grating array array2 disappears, indicating that the second ultra-weak reflection grating array array2 is erased by high temperature; as a test group, after annealing for 13 h, the Rayleigh scattering signals in the final distance-domain spectrogram for 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 are respectively enhanced to varying degrees.

[0089] As Figure 11As shown, after all the real-time distance-domain spectrograms of the second group of ultra-weak reflection grating arrays array2, 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 within 13 h are transformed into real-time wavelength-domain spectrograms by fast Fourier transform, then with the annealing time as the horizontal axis and the reflectivity in the real-time wavelength-domain spectrogram as the vertical axis, a relationship curve graph between the annealing time and the reflectivity is constructed. It can be seen from the relationship curve graph that the reflectivity of the second group of ultra-weak reflection grating arrays array2 drops to be basically the same as the fiber background noise within less than 2 h of annealing, while for 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, after a rapid increase in the first 2 h, the increasing speed slows down and finally tends to be stable, and the final increases in reflectivity are 17.8 dB, 8.8 dB, and 5.4 dB respectively.

[0090] As Figure 12 shown, during the two heating-up (1st and 2nd heating up) and cooling-down (1st and 2nd cooling down) processes, the reflectivities of 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 remain constant and basically show no obvious changes, indicating that after high-temperature annealing at 900 °C, the increases in the reflectivities of 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 are permanent.

[0091] In this embodiment, the reason for performing high-temperature annealing treatment on the grating array sample at 900 °C for up to 13 h is to facilitate observing the changes in the reflectivities of the five groups 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 h, the reflectivities of 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 can remain stable after high-temperature annealing.

[0092] Example Five

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

[0094] This embodiment uses exactly the same grating array sample as in Example One, and then performs high-temperature annealing on the grating array sample in the same manner. The difference from Example One is only that the annealing temperature in this embodiment is 1000 °C.

[0095] AsFigure 13 As shown, in the high-temperature annealing at 1000°C, the heating time is approximately 3.25 h, and the holding time is approximately 9.75 h. As a control group, after annealing for 13 h, the first group of ultra-weak reflection grating arrays array1 still shows no improvement in the Rayleigh scattering signal in the final distance-domain spectrogram, while after annealing for 13 h, the Rayleigh scattering signal enhancement in the final distance-domain spectrogram of the second group of ultra-weak reflection grating arrays array2 disappears, indicating that the second group of ultra-weak reflection grating arrays array2 is erased by high temperature. As a test group, after annealing for 13 h, the Rayleigh scattering signals in the final distance-domain spectrogram of 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 are improved to varying degrees.

[0096] It is worth noting that there are two very obvious enhancements in the Rayleigh scattering signal at the front position of the second group of ultra-weak reflection grating arrays array2. After observing the Extraneous peak spectrum, it is found that these two enhancements in the Rayleigh scattering signal do not come from the first group of ultra-weak reflection grating arrays array1, but are the improvement of the overall spectral signal, which is presumably caused by the combined effect of internal stress changes and glass structure softening in the optical fiber under high-temperature conditions.

[0097] As Figure 14 shown, after performing a fast Fourier transform on all the real-time distance-domain spectrograms of the second group of ultra-weak reflection grating arrays array2, 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 within 13 h to obtain real-time wavelength-domain spectrograms, and then using the annealing time as the horizontal axis and the reflectivity in the real-time wavelength-domain spectrogram as the vertical axis, a relationship curve graph between the annealing time and the reflectivity is constructed. It can be seen from the relationship curve graph that after annealing for 5 h, the reflectivity of the second group of ultra-weak reflection grating arrays array2 drops to be basically the same as the fiber background noise, while after a rapid increase in the first 4 h, the increasing speed of 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 slows down and finally stabilizes, and the final increases in reflectivity are 16.0 dB, 9.0 dB, and 5.5 dB respectively.

[0098] It is worth noting that during high-temperature annealing, the increase in 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 after high-temperature annealing at 1000 °C is not much different from that after high-temperature annealing at 900 °C. Even more, the increase in reflectivity of the third ultra-weak reflection grating array array3 after high-temperature annealing at 1000 °C has decreased compared to that after high-temperature annealing at 900 °C. It is speculated that 1000 °C is already close to the melting temperature of the optical fiber, causing slight damage to the grating structure of the third ultra-weak reflection grating array array3.

[0099] As Figure 15 shown, during the two heating-up (1st and 2nd heating up) and cooling-down (1st and 2nd cooling down) processes, the reflectivities 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 remain constant, with basically no obvious changes, indicating that after high-temperature annealing at 1000 °C, the increase in their reflectivities is permanent.

[0100] In this embodiment, high-temperature annealing treatment of the grating array sample at 1000 °C for up to 13 h is adopted to facilitate observing the changes in reflectivity of the five 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 h, the reflectivities 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 are not intended to limit them. Although the embodiments of the present invention have been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the embodiments of the present invention can still be modified or equivalently replaced, and these modifications or equivalent replacements cannot make the modified technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the reflectivity of ultra-weak reflection fiber grating by high temperature annealing, characterized in that: The steps include: Step 1: preparing an ultra-weak reflection fiber Bragg grating on an optical fiber by a femtosecond laser direct writing method, wherein the laser energy of the femtosecond laser is E≥7nj; Step 2: performing high temperature annealing on the ultra-weak reflection fiber Bragg grating to increase the reflectivity of the ultra-weak reflection fiber Bragg grating to a target reflectivity, the annealing temperature T is 700-1000° C., and the annealing time is not less than 8 hours.

2. The method according to claim 1, characterized in that In step 2, when the ultra-weak reflection fiber Bragg grating is subjected to high-temperature annealing, it includes a heating stage and a constant temperature stage. In the heating stage, the ultra-weak reflection fiber Bragg grating is heated from room temperature to the annealing temperature, and the heating rate of the heating stage is 1-5°C / min; in the constant temperature stage, the ultra-weak reflection fiber Bragg grating is maintained at the annealing temperature, and the maintenance time of the constant temperature stage is 2-11h.

3. The method according to claim 1, characterized in that The annealing temperature T is 700°C, 800°C, 900°C or 1000°C.

4. The method according to claim 1, characterized in that The laser energy E is between 7-11 nj.

5. The method according to claim 1 or 4, characterized in that: The laser energy E is 7nj, 9nj or 11nj.

6. The method according to claim 1, characterized in that The optical fiber is a single-mode optical fiber, and a plurality of ultra-weak reflection optical fiber gratings are evenly distributed along the axial direction on the optical fiber to form an ultra-weak reflection grating array.

7. The method according to claim 1, characterized in that The method further comprises the steps of: Step 3: Cooling the ultra-weak reflection fiber grating from the annealing temperature to room temperature.

8. The method according to claim 7, characterized in that In step 3, the ultra-weak reflection fiber grating is cooled naturally or with controlled temperature.

9. The method according to claim 7, characterized in that: The method further comprises the steps of: Step 4: subjecting the ultra-weak reflection fiber Bragg grating to multiple temperature increases and decreases between the first temperature and the second temperature to verify the reflectivity stability thereof.

10. The method according to claim 9, characterized in that The first temperature is 100°C, and the second temperature is 800°C.

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