Multi-stage phase-shifted grating structure having refractive index modulation function, preparation method, and optical coupler

Abstract

Disclosed in the present invention are a multi-stage phase-shifted grating structure having a refractive index modulation function, a preparation method, and an optical coupler. The multi-stage phase-shifted grating structure is an axially arranged fiber Bragg grating, and comprises at least one uniform fiber grating section and a phase modulation grating section group having both ends connected to the uniform fiber grating section; the phase modulation grating section group comprises at least two stages of phase modulation grating sections, and the stages of phase modulation grating sections are arranged in a manner of embedding stage by stage; the effective refractive index of the uniform fiber grating section is different from that of each stage of phase modulation grating section; and after passing through the phase modulation grating section group, the phase shift amount of a target transmission wavelength in a reflection spectrum is nπ, wherein n is equal to(0.3 to 0.7)+N, and N is a natural number. According to the multi-stage phase-shifted grating structure of the present invention, by performing multiple stages of effective refractive index modulation in the uniform fiber grating, and providing multiple stages of phase modulation grating sections, the accuracy and transmittance of a transmission wavelength in a reflection spectrum on the optical fiber are improved.

Description

Refractive index modulated multi-level phase-shift grating structure, preparation method and optical coupler Technical Field

[0001] The present invention belongs to the technical field of optical fiber gratings, and in particular relates to a refractive index modulated multi-level phase-shift grating structure, a preparation method and an optical coupler. Background Art

[0002] Fiber Bragg gratings couple modes that meet phase matching conditions. Light of a specific wavelength is reflected, while light of other wavelengths is transmitted, thus forming a narrowband filter. It is widely used in reflectors, filters, and sensor elements, especially in optical couplers.

[0003] With the advancement of fiber optic technology, conventional fiber Bragg gratings (FBGs) are no longer sufficient for the optical fiber paths used in various optical devices. Significant breakthroughs have been achieved in the research and application of various specialized grating structures. Among these, the phase-shifted fiber Bragg grating (PBG), a specialized fiber Bragg grating (FBG), introduces a half-period abrupt shift in the continuous FBG. The introduction of this phase shift results in spectral characteristics distinct from those of the FBG, creating an extremely narrow transmission window within the reflection band, with the entire grating forming an oscillation cavity. The phase shift enables narrowband filtering. From a phase perspective, this results in a π phase shift, resulting in a π phase-shifted fiber Bragg grating (FBG).

[0004] Phase-shifted fiber Bragg gratings (FBGs) offer high-quality wavelength selectivity, low insertion loss, and are polarization-independent, making them widely applicable. Regarding the research and application of phase-shifted gratings, patent CN106094085A, for example, provides a method for fabricating a phase-shifted grating and a phase-shifted grating. The method comprises: forming a first template layer on a substrate; performing photolithography on the first template layer according to a preset mask to separate the first template layer into a plurality of separate first projections, with the bottom of the first gaps between the first projections exposing the substrate; forming a second template layer on the outer surfaces of the first projections and the exposed surface of the substrate; etching the second template layer to separate the second projections into a plurality of separate second projections, with the first projections positioned between adjacent second projections; removing the first projections; coating the gaps between the second projections with photoresist to form a first photoresist soft template; holographically exposing and developing the first photoresist soft template to form a second gap in the first photoresist soft template, exposing the substrate at the bottom of the second gap; and etching the exposed portion of the substrate to remove the second projections to form a grating. The phase-shift grating manufacturing method provided in the embodiment of this solution is highly efficient.

[0005] For example, patent CN107037583A provides a method, device, and system for demodulating the center wavelength and phase shift of a phase-shift grating. The system uses the reflection spectrum data of the phase-shift grating to be demodulated to calculate the center wavelength and phase shift of the phase-shift grating to be demodulated, thereby solving the problem of being unable to effectively demodulate the size of the center wavelength and phase shift of the phase-shift grating. Furthermore, the difference in the response of the center wavelength and phase shift to changes in external physical quantities can be used for sensing applications.

[0006] However, the above-mentioned prior art generally introduces the phase shift size by controlling the gap spacing, which requires precise control of the mask thickness, but in actual applications, deviations often occur, resulting in narrowband misalignment.

[0007] Therefore, how to design a reasonable grating structure on the optical fiber, obtain a precise and controllable phase-shift grating, and provide an easy-to-operate and feasible phase-shift grating preparation method to meet the application requirements of optical components in different scenarios is an urgent problem to be solved by technical personnel in this field. Summary of the Invention

[0008] To address the deficiencies in the aforementioned prior art, the present invention provides a refractive index modulated multi-level phase-shifted grating structure, a preparation method, and an optical coupler. The multi-level phase-shifted grating structure is an axially arranged fiber Bragg grating (FBG) comprising: at least one uniform fiber grating segment and a phase-modulation grating segment group connected to the uniform fiber grating segment at both ends. The phase-modulation grating segment group includes at least two levels of phase-modulation grating segments, each layer of which is arranged in a stepwise embedded manner. The effective refractive index of the uniform fiber grating segment and each level of the phase-modulation grating segment are different. After passing through the phase-modulation grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is nπ, where n = (0.3-0.7) + N, where N is a natural number. The multi-level phase-shifted grating structure of the present invention achieves multi-level phase-modulation grating segments by performing multi-level effective refractive index modulation in the uniform fiber grating, thereby improving the accuracy and transmittance of the optical fiber for the transmission wavelength in the reflection spectrum.

[0009] In a first aspect, the present invention provides a refractive index modulated multi-level phase-shifted grating structure, wherein the multi-level phase-shifted grating structure is an axially arranged fiber Bragg grating, comprising:

[0010] At least one uniform fiber Bragg grating segment, and a phase modulation grating segment group connected to the uniform fiber Bragg grating segment at both ends;

[0011] The phase modulation grating segment group includes at least two levels of phase modulation grating segments, and the phase modulation grating segments of each level are arranged in a step-by-step embedded manner;

[0012] The effective refractive index of the uniform fiber Bragg grating segment and each level phase modulation grating segment are different;

[0013] After passing through the phase-modulated grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is nπ, where n=(0.3~0.7)+N, and N is a natural number.

[0014] Furthermore, the ratio of the length of the phase modulation grating segment group to the total length of the multi-level phase-shift grating structure is no more than 1 / 10.

[0015] Furthermore, after passing through the multi-level phase-shift grating structure, the transmittance of the target transmission wavelength is not less than 70%.

[0016] Furthermore, the positional relationship between the phase modulation grating segment group and the uniform fiber grating segment in the multi-level phase-shifted grating structure is specifically expressed as follows:

[0017]

[0018] Wherein, L1 is the length of one section of the uniform fiber Bragg grating segment separated by the phase modulation grating segment group, L2 is the length of the other section of the uniform fiber Bragg grating segment separated by the phase modulation grating segment group, R1 is the reflectivity of the section where L1 is located in the uniform fiber Bragg grating segment, R2 is the reflectivity of the section where L2 is located in the uniform fiber Bragg grating segment, and L is the equivalent length of the multi-level phase shift grating structure. is the length of the mth phase modulation grating segment, M is the total number of phase modulation grating segments, is the modulation period of the uniform fiber Bragg grating segment, is the effective refractive index of the uniform fiber Bragg grating segment, is the initial effective refractive index of the optical fiber, is the coupling efficiency of the optical fiber core.

[0019] Furthermore, the number of grids in each phase modulation grating segment is not less than 1 / 4 of the modulation period, and the grids in each phase modulation grating segment are arranged continuously;

[0020] Each level of phase modulation grating segment is divided into levels by embedding step by step. The end point of any current level phase modulation grating segment and the adjacent end point of the next level phase modulation grating segment form two grating lengths ΔL respectively. n,1 , ΔL n,2 , and ΔL n,1 :ΔL n,2 =1:(0.5~2).

[0021] Furthermore, the effective refractive index of each phase modulation grating segment in the phase modulation grating segment group is constant, the effective refractive index of each grid in each phase modulation grating segment is consistent, and the effective refractive index of each adjacent phase modulation grating segment changes step by step.

[0022] Furthermore, after the phase modulation grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is specifically expressed as:

[0023]

[0024] in, is the target transmission wavelength phase shift after the phase modulation grating segment group, is the correction factor, is the effective refractive index of the uniform fiber Bragg grating segment, is the effective refractive index of the first-stage phase modulation grating segment, is the effective refractive index of the mth-order phase modulation grating segment, M is the total number of phase modulation grating segments, is the modulation period of the uniform fiber Bragg grating segment, is the length of the m-th phase modulation grating segment.

[0025] In a second aspect, the present invention further provides a method for preparing a refractive index modulated multi-level phase-shift grating structure, wherein the method comprises the following steps:

[0026] A uniform fiber grating segment is prepared by presetting a temperature difference at a pre-set position of the optical fiber;

[0027] On a uniform fiber grating segment, the parameters of the laser pulse are controlled, and a phase-modulated grating segment group is written based on shielding to prepare a multi-level phase-shifted grating structure.

[0028] Furthermore, the preset temperature difference is specifically expressed as:

[0029]

[0030] in, is the preset temperature difference, is the thermo-optical coefficient of the optical fiber, is the effective refractive index of the uniform fiber Bragg grating segment, is the initial effective refractive index of the optical fiber.

[0031] In a third aspect, the present invention further provides an optical coupler, comprising: an optical fiber containing a multi-level phase-shift grating structure, wherein the multi-level phase-shift grating structure is the multi-level phase-shift grating structure with refractive index modulation as described above.

[0032] The present invention provides a refractive index modulated multi-level phase-shift grating structure, a preparation method, and an optical coupler, which have at least the following beneficial effects:

[0033] (1) The present invention performs multi-level modulation of the effective refractive index of the optical fiber by progressive embedding, introducing a phase shift in a conventional uniform fiber Bragg grating, so that a precisely preset target transmission wavelength is generated in the reflection spectrum. The multi-level progressive embedding of the effective refractive index modulation, superimposed on the phase shift caused by the multi-level phase modulation grating segment, can meet the design requirements of the phase-shifted fiber Bragg grating through multi-dimensional modulation, while reducing the influence of parameter errors in the grating writing and preparation process on the accuracy of the reflection spectrum transmission wavelength. At the same time, a phase-shifted grating with higher transmittance can also be obtained.

[0034] (2) According to the preset target transmission wavelength and transmittance, and combined with the property parameters of the optical fiber itself, the effective refractive index of each phase modulation grating segment in the designed multi-level phase shift grating structure is precisely controlled to obtain a grating fiber that meets the application requirements of optical components in different scenarios.

[0035] (3) During the preparation of the multi-level phase-shifted grating structure, electrical perturbations are performed on a portion of the uniform fiber grating segment to seek the best writing capability (such as pulse duration, repetition frequency, and power, so as to obtain a multi-level phase-shifted grating structure with customized characteristics), causing the effective refractive index of a specific area to be modulated, and obtaining a multi-level pattern of treated and untreated areas, forming an effective refractive index difference, thereby preparing a multi-level phase-shifted grating structure.

[0036] (4) The target transmission wavelength of the reflection spectrum is used to determine the magnitude of the change in the external physical quantity during the writing process, thereby realizing temperature difference preset, control and monitoring, thereby preparing a multi-level phase-shift grating structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG1 is a schematic structural diagram of a refractive index modulated multi-level phase-shift grating structure provided by the present invention;

[0038] FIG2 is a schematic flow chart of a method for preparing a refractive index modulated multi-level phase-shift grating structure provided by the present invention;

[0039] FIG3 is a schematic structural diagram of a phase modulation grating segment group prepared under a preset temperature difference according to an embodiment of the present invention;

[0040] FIG4 is a diagram showing the transmittance test results of the multi-level phase-shift grating structure provided by the present invention. DETAILED DESCRIPTION

[0041] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0042] The terms used in the embodiments of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The singular forms "a," "an," "the," and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms, and unless the context clearly indicates otherwise, "a plurality" generally includes at least two.

[0043] It should also be noted that the terms "include," "comprises," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a product or device comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such product or device. In the absence of further limitations, an element defined by the phrase "comprises a..." does not exclude the presence of other identical elements in the product or device comprising the element.

[0044] Phase-shifted gratings are non-uniform gratings, which are usually created by introducing a certain phase shift region into a specific part of a conventional fiber Bragg grating. For example, introducing a π phase shift into a conventional Bragg grating can produce a narrowband peak in the reflection spectrum, thereby generating a single-mode laser output.

[0045] The number of phase-shift regions in a phase-shifted grating, their placement, and modulation period all affect the spectrum transmitted through the fiber Bragg grating. Considering the feasibility of fiber Bragg grating fabrication and ensuring the resulting phase-shifted Bragg grating has precise spectral sensing properties, the effective refractive index of the optical fiber can be modulated through multi-stage progressive embedding to create a phase-shifted fiber Bragg grating structure with a specific target transmission wavelength.

[0046] Therefore, as shown in FIG1 , the present invention provides a refractive index modulated multi-level phase-shifted grating structure, wherein the multi-level phase-shifted grating structure is an axially arranged fiber Bragg grating, comprising:

[0047] At least one uniform fiber Bragg grating segment, and a phase modulation grating segment group connected to the uniform fiber Bragg grating segment at both ends;

[0048] The phase modulation grating segment group includes at least two levels of phase modulation grating segments, and the phase modulation grating segments of each level are arranged in a step-by-step embedded manner;

[0049] The effective refractive index of the uniform fiber Bragg grating segment and each level phase modulation grating segment are different;

[0050] After passing through the phase-modulated grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is nπ, where n=(0.3~0.7)+N, and N is a natural number.

[0051] The multi-level phase-shifted grating structure proposed in the present invention achieves a set of phase-modulated grating segments by modulating the effective refractive index within a uniform fiber Bragg grating segment, thereby achieving a set phase shift of the target transmission wavelength in the reflection spectrum. The uniform fiber Bragg grating segment is a conventional Bragg grating, in which each grid has the same modulation period and effective refractive index. The phase-modulated grating segment group includes multiple levels of phase-modulated grating segments, each with a different effective refractive index, which also differs from the effective refractive index of the uniform fiber Bragg grating segment.

[0052] The wavelength of a fiber Bragg grating (FBG) depends primarily on the grating period and the effective refractive index of the coupling mode. This invention modulates the effective refractive index of the optical fiber through progressive embedding, introducing a phase shift in a conventional uniform fiber Bragg grating (FBG), resulting in a precisely preset target transmission wavelength in the reflection spectrum.

[0053] The multi-level progressively embedded effective refractive index modulation, superimposed on the phase shift caused by the multi-level phase modulation grating segment, can not only meet the design requirements for the phase-shifted fiber Bragg grating, but also reduce the influence of parameter errors in the grating writing and preparation process on the accuracy of the reflection spectrum transmission wavelength, while obtaining a phase-shifted grating with higher transmittance.

[0054] The ratio of the length of the phase-modulation grating segment group to the length of the multi-level phase-shifted grating structure is no greater than 1 / 10. When modulating the effective refractive index in a uniform fiber segment, the modulation amplitude is generally small. Therefore, the length of the phase-modulation grating segment group is also small, at least an order of magnitude shorter than the length of the original fiber Bragg grating.

[0055] At the same time, in order to ensure that the transmittance of the obtained multi-level phase-shift grating structure meets the requirements, it is set that the transmittance of the target transmission wavelength is not less than 70% after the multi-level phase-shift grating structure.

[0056] For the multi-level phase-shifted grating structure with phase modulation grating segment group, the target transmission wavelength is in the uniform fiber grating segment, and its reflectivity for the target transmission wavelength is The formula is as follows:

[0057]

[0058] in, is the coupling coefficient, and l is the length of the uniform fiber Bragg grating segment.

[0059] Therefore, the transmittance T of the target transmission wavelength can be expressed as:

[0060]

[0061] In addition, the coupling coefficient It can be determined by the following formula:

[0062]

[0063] in, The refractive index modulation of the optical fiber by the uniform fiber Bragg grating segment is is the coupling efficiency of the optical fiber core, is the target transmission wavelength.

[0064] Therefore, for the multi-level phase-shifted grating structure of the present invention, when the transmittance index is met, the positional relationship between the phase modulation grating segment group and the uniform fiber grating segment in the multi-level phase-shifted grating structure is specifically expressed as follows:

[0065]

[0066] Wherein, L1 is the length of one section of the uniform fiber Bragg grating segment separated by the phase modulation grating segment group, L2 is the length of the other section of the uniform fiber Bragg grating segment separated by the phase modulation grating segment group, R1 is the reflectivity of the section where L1 is located in the uniform fiber Bragg grating segment, R2 is the reflectivity of the section where L2 is located in the uniform fiber Bragg grating segment, and L is the equivalent length of the multi-level phase shift grating structure. is the length of the mth phase modulation grating segment, M is the total number of phase modulation grating segments, is the modulation period of the uniform fiber Bragg grating segment, is the effective refractive index of the uniform fiber Bragg grating segment, is the initial effective refractive index of the optical fiber, is the coupling efficiency of the optical fiber core.

[0067] The number of grids in each phase modulation grating segment is not less than 1 / 4 of the modulation period, and the grids in each phase modulation grating segment are arranged continuously;

[0068] That is, each phase-modulation grating segment can be 1 / 4 of a modulation period, half a modulation period, or an integer number of modulation periods. Of course, any value greater than 1 / 4 of a modulation period is also possible. Limiting the number of gratings in each phase-modulation grating segment ensures that the modulation of each refractive index has an effect on the entire multi-level phase-shift grating structure, while also improving precision through multi-dimensional modulation.

[0069] Each level of phase modulation grating segment is divided into levels by embedding step by step. The end point of any current level phase modulation grating segment and the adjacent end point of the next level phase modulation grating segment form two grating lengths ΔL respectively. n,1 , ΔL n,2 , and ΔL n,1 :ΔL n,2 =1:(0.5~2).

[0070] Introducing phase shifts at different locations within a uniform fiber Bragg grating segment affects the maximum transmittance. Starting from the beginning of the uniform fiber Bragg grating segment and gradually moving toward the center, the peak position of the transmission wavelength remains unchanged, but the maximum transmittance gradually deepens. Beyond the center, the peak position of the transmission wavelength remains unchanged, but the maximum transmittance gradually becomes shallower.

[0071] Therefore, in order to ensure that the maximum transmittance is within a desirable range, the ratio of the lengths of the two segments formed between the adjacent endpoints of the phase modulation grating segment of the next stage at the endpoint of the current stage needs to be between 1 / 2 and 2 times, thereby ensuring that the depth of the transmission window (i.e., the location of the target transmission wavelength) still achieves low loss.

[0072] As shown in Figure 1, in the diagram of the multi-level phase-shifted grating structure, AB and GH are uniform fiber Bragg grating segments, while BG is a phase-modulated grating segment group. After passing through the phase-modulated grating segment group, that is, after passing through endpoints B and G, the target transmission wavelength forms a phase difference, resulting in a phase shift.

[0073] After the phase modulation grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is specifically expressed as:

[0074]

[0075] in, is the target transmission wavelength phase shift of the phase-modulated grating segment group, is the target transmission wavelength, ΔL is the length of the phase modulation grating segment group, is the weighted effective refractive index of the phase modulation grating segment group.

[0076] As shown in Figure 1, in the diagram of the multi-level phase-shift grating structure, the phase-modulation grating segment group of BG includes multi-level phase-modulation grating segments, for example, BC and FG are phase-modulation grating segments of the same level, CD and EF are another phase-modulation grating segment of the same level, and DE is another phase-modulation grating segment of another level.

[0077] The effective refractive index of each phase modulation grating segment of the phase modulation grating segment group is constant, for example, the effective refractive indexes of BC and FG are constant, and the effective refractive indexes of CD and EF are constant.

[0078] The effective refractive index of each grid in each phase modulation grating segment is consistent, and the effective refractive index of each adjacent phase modulation grating segment changes step by step. For example, from BC and FG levels to CD and EF levels, and then to DE level, the effective refractive index of each adjacent phase modulation grating segment in each level changes step by step.

[0079] After the phase modulation grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is specifically expressed as:

[0080]

[0081] in, is the target transmission wavelength phase shift after the phase modulation grating segment group, is the correction factor, is the effective refractive index of the uniform fiber Bragg grating segment, is the effective refractive index of the mth-order phase modulation grating segment, M is the total number of phase modulation grating segments, is the modulation period of the uniform fiber Bragg grating segment, is the length of the m-th phase modulation grating segment.

[0082] The multi-level phase-shift grating structure provided by the present invention only modulates the effective refractive index of the uniform fiber Bragg grating without changing the modulation period of the uniform fiber Bragg grating, thereby providing multi-level phase modulation grating segments and ensuring the controllability of the phase-shift grating.

[0083] As shown in FIG1 , taking three levels of phase modulation grating segments as an example, the effect of modulating the effective refractive index of each level of phase modulation grating segment on the phase shift of the entire phase modulation grating segment group is explained.

[0084] At this time, the total number of layers M of the phase modulation grating segment is 3, is the modulation period of the uniform fiber Bragg grating segment (i.e., the modulation period from point A to point B and from point G to point H), is the modulation period of the first-level phase modulation grating segment (i.e., the modulation period from point B to point C and from point F to point G), is the modulation period of the second-level phase modulation grating segment (i.e., the modulation period from point C to point D and from point E to point F), is the modulation period of the third-level phase modulation grating segment (i.e., the modulation period from point D to point E). is the length of the m-th phase modulation grating segment, where is the length of the first-level phase modulation grating segment (i.e., the length of BC and FG), is the length of the second-level phase modulation grating segment (i.e., the length of CD and EF), is the length of the third-level phase modulation grating segment (i.e., the length of DE).

[0085] From the left end point of the optical fiber structure, the effective refractive index from the left end point of the optical fiber to point A is the starting effective refractive index of the optical fiber. , point A to point B is a uniform fiber Bragg grating segment, and the effective refractive index of AB is the effective refractive index of the uniform fiber Bragg grating segment, that is , point B to point C is the first-order phase modulation grating segment, and the effective refractive index of BC is the effective refractive index of the first-order phase modulation grating segment, that is, , point C to point D is the second-level phase modulation grating segment, and the effective refractive index of CD is the effective refractive index of the second-level phase modulation grating segment, that is, , point D to point E is the third-level phase modulation grating segment, and the effective refractive index of DE is the effective refractive index of the third-level phase modulation grating segment, that is, .

[0086] After the ED segment, points E to F are the second-stage phase modulation grating segment, and the effective refractive index of EF is the effective refractive index of the second-stage phase modulation grating segment, that is, , point F to point G is the first-order phase modulation grating segment, and the effective refractive index of FG is the effective refractive index of the first-order phase modulation grating segment, that is, , point G to point H is a uniform fiber Bragg grating segment, and the effective refractive index of GH is the effective refractive index of the uniform fiber Bragg grating segment, that is The effective refractive index from point H to the right end of the optical fiber is the starting effective refractive index of the optical fiber. .

[0087] The phase shift of the target transmission wavelength in the reflection spectrum of the three-level phase modulation grating segment after passing through the phase modulation grating segment group is specifically expressed as:

[0088]

[0089] As shown in FIG2 , the present invention further provides a method for preparing a refractive index modulated multi-level phase-shift grating structure. The method comprises the following steps:

[0090] A uniform fiber grating segment is prepared by presetting a temperature difference at a pre-set position of the optical fiber;

[0091] After the temperature difference is preset, a uniform fiber grating segment is prepared, which also requires exposure using laser pulses. The laser pulses can be high-frequency CO2 laser exposure or ultraviolet laser irradiation.

[0092] The temperature difference is preset to the temperature difference between the uniform fiber Bragg grating section temperature and the temperature at other locations of the fiber.

[0093] On a uniform fiber grating segment, the parameters of the laser pulse are controlled, and a phase-modulated grating segment group is written based on shielding to prepare a multi-level phase-shifted grating structure.

[0094] Laser pulse parameters include pulse duration, repetition rate, and power. These parameters are controlled so that the laser light transmitted through the baffle and mask acts uniformly on the fiber grating segments, resulting in a grating structure with customized characteristics, consisting of a multi-level phase-modulated grating segment group. The size and shape of the mask correspond to the phase-modulated grating segment group, while the size and laser transmission characteristics of the baffle correspond to the length and effective refractive index of the multi-level phase-modulated grating segments.

[0095] When writing Bragg gratings on optical fibers, laser pulses can be used for irradiation. Simultaneously applying external forces during the laser pulse exposure process allows for periodic regulation of the fiber's effective refractive index. Among external physical quantities, temperature and stress changes are the most fundamental. By determining the magnitude of the change in external physical quantities through the target wavelength in the reflection spectrum, it is possible to preset, control, and monitor parameters such as temperature, thereby fabricating a multi-level phase-shifted grating structure.

[0096] The multi-level phase-shift grating of the present invention is fabricated using a masking method, combining the advantages of masking technology, such as ease of operation and good stability, while overcoming the disadvantages of inflexible writing. By introducing partial periodic dielectric perturbations into a uniform fiber grating segment, optimal writing capabilities (such as pulse duration, repetition frequency, and power) are sought to achieve a multi-level phase-shift grating structure with customizable characteristics. This modulates the effective refractive index in specific regions, creating a multi-level pattern of treated and untreated areas, and forming an effective refractive index difference, thereby fabricating a multi-level phase-shift grating.

[0097] As shown in FIG3 , after the fiber Bragg grating acts on the temperature field, the effect of temperature on the grating can be analyzed using the fiber Bragg grating reflection wavelength formula.

[0098] To analyze the influence of temperature field in fiber Bragg grating, we need to use the fiber Bragg grating reflection wavelength formula. That is:

[0099]

[0100] in, is the reflection wavelength of the fiber Bragg grating, n is the effective refractive index of the fiber, is the grating period;

[0101] As mentioned above, the fiber Bragg grating reflection wavelength formula is derived with respect to temperature T to obtain:

[0102]

[0103] The change in the effective refractive index of an optical fiber can be expressed as:

[0104]

[0105] in, is the thermo-optical coefficient of the optical fiber;

[0106] The change of grating period can be expressed as:

[0107]

[0108] in, is the thermal collision coefficient of the optical fiber material;

[0109] Based on the modulation principle of the effective refractive index, the preset temperature difference is specifically expressed as:

[0110]

[0111] in, is the preset temperature difference, is the thermo-optical coefficient of the optical fiber, is the effective refractive index of the uniform fiber Bragg grating segment, is the initial effective refractive index of the optical fiber.

[0112] By controlling the parameters of the laser pulse (such as laser pulse duration, laser repetition frequency and power, etc.), the laser light passing through the baffle and mask acts on a uniform fiber grating segment on the optical fiber, thereby obtaining a grating structure of a multi-level phase modulation grating segment group with customized characteristics.

[0113] The change in refractive index caused by optical fiber absorption of photons emitted by a laser pulse primarily depends on the fiber's inherent photosensitivity. These include the fiber's refractive index, particularly the core's absorption spectrum, density, and external physical quantities such as strain and temperature. When a fiber is irradiated by a laser pulse, its photosensitivity undergoes a permanent change.

[0114] On a uniform fiber Bragg grating segment, the parameters of the laser pulse are controlled, and a phase-modulated grating segment group is written based on the occlusion to prepare the phase-modulated grating segment group, which specifically includes the following steps:

[0115] The parameters of the laser pulse and the shielding plate are preset so that the effective refractive index modulation in the uniform grating segment meets the requirements of the phase modulation grating segment group, which is specifically expressed as:

[0116]

[0117]

[0118] in, is the length of the m-th phase modulation grating segment, is the effective refractive index of the m-th phase modulation grating segment, is the initial effective refractive index of the optical fiber, is the upper limit of the laser wavelength, is the lower limit of the laser wavelength, is the laser absorption variation function, and With parameters Related, is the wavelength of the laser, is the strain of the optical fiber, N is the number of laser pulses, is the target transmission wavelength, is the length of the phase modulation grating segment group.

[0119] The specific formula of the function will vary depending on the laser selection, so no further restrictions are made. Before writing the phase modulated grating segment group, first select the type of laser to be used, and then determine it through experimental data. The specific formula of the function.

[0120] As shown in Figure 4, transmittance testing was conducted on a multi-level phase-shift grating structure proposed in the present invention. The multi-level phase-shift grating structure has three levels, and the target transmission wavelength is 1064 nm. After phase modulation of the grating segments, the phase shift at the target transmission wavelength in the reflection spectrum is 1 / 2π. The test data shows that the transmittance at the target transmission wavelength is close to 100%.

[0121] In a third aspect, the present invention further provides an optical coupler, comprising:

[0122] An optical fiber containing a multi-level phase-shifted grating structure, wherein the multi-level phase-shifted grating structure is the multi-level phase-shifted grating with refractive index modulation as described above.

[0123] The present invention provides a refractive index modulated multi-level phase-shift grating structure, a preparation method, and an optical coupler, which have at least the following beneficial effects:

[0124] (1) The present invention uses progressive embedding to perform multi-level modulation of the effective refractive index of the optical fiber, introducing a phase shift in a conventional uniform fiber Bragg grating, so that a precisely preset target transmission wavelength is generated in the reflection spectrum. The multi-level progressive embedding of the effective refractive index modulation, combined with the phase shift caused by the multi-level phase modulation grating segments, can meet the design requirements for the phase-shifted fiber Bragg grating through multi-dimensional modulation, while reducing the impact of parameter errors in the grating writing and preparation process on the accuracy of the reflection spectrum transmission wavelength. At the same time, a phase-shifted grating with higher maximum transmittance can also be obtained.

[0125] (2) According to the preset target transmission wavelength and transmittance, and combined with the property parameters of the optical fiber itself, the effective refractive index of each phase modulation grating segment in the designed multi-level phase shift grating structure is precisely controlled to obtain a grating fiber that meets the application requirements of optical components in different scenarios.

[0126] (3) During the preparation of the multi-level phase-shifted grating structure, electrical disturbances are performed on a portion of the uniform fiber grating segment to seek the best writing capability (such as pulse duration, repetition frequency, and power, so as to obtain a multi-level phase-shifted grating structure with customized characteristics), causing the effective refractive index of a specific area to be modulated, and obtaining a multi-level pattern of treated and untreated areas, forming an effective refractive index difference, thereby preparing a multi-level phase-shifted grating structure.

[0127] (4) The target transmission wavelength of the reflection spectrum is used to determine the magnitude of the change in the external physical quantity during the writing process, thereby realizing temperature difference preset, control and monitoring, thereby preparing a multi-level phase-shift grating structure.

[0128] Although preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic inventive concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the invention. Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the invention. Thus, the present invention is intended to include such changes and modifications as fall within the scope of the claims and their equivalents.

Claims

1. A refractive index modulated multi-level phase-shift grating structure, characterized in that: The multi-level phase-shifted grating structure is an axially arranged fiber Bragg grating. After the multi-level phase-shifted grating structure, the transmittance of the target transmission wavelength is not less than 70%, including: At least one uniform fiber grating segment, and a phase modulation grating segment group connected to the uniform fiber grating segment at both ends; The phase modulation grating segment group includes at least two levels of phase modulation grating segments, and the phase modulation grating segments of each level are arranged in a step-by-step embedded manner, and the ratio of the length of the phase modulation grating segment group to the total length of the multi-level phase-shift grating structure is not greater than 1 / 10; The effective refractive indexes of the uniform fiber grating segment and each level of phase modulation grating segment are different; After the phase modulation grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is nπ, where n=(0.3~0.7)+N, and N is a natural number; The positional relationship between the phase modulation grating segment group and the uniform fiber grating segment in the multi-level phase shift grating structure is specifically expressed as: ； Wherein, L1 is the length of one section of the uniform fiber grating segment separated by the phase modulation grating segment group, L2 is the length of another section of the uniform fiber grating segment separated by the phase modulation grating segment group, R1 is the reflectivity of the section where L1 is located in the uniform fiber grating segment, R2 is the reflectivity of the section where L2 is located in the uniform fiber grating segment, L is the equivalent length of the multi-level phase shift grating structure, is the length of the mth phase modulation grating segment, M is the total number of phase modulation grating segments, is the modulation period of the uniform fiber Bragg grating segment, is the effective refractive index of the uniform fiber Bragg grating segment, is the initial effective refractive index of the optical fiber, is the coupling efficiency of the optical fiber core.

2. The refractive index modulated multi-level phase-shifted grating structure according to claim 1, characterized in that: The number of grids in each phase modulation grating segment is not less than 1 / 4 of the modulation period, and the grids in each phase modulation grating segment are arranged continuously; Each phase modulation grating segment is divided into levels by embedding step by step. The end point of any current phase modulation grating segment and the adjacent end point of the next phase modulation grating segment form two grating lengths of ΔL respectively. n,1 , ΔL n,2 , and ΔL n,1 :ΔL n,2 =1:(0.5~2).

3. The refractive index modulated multi-level phase-shift grating structure according to claim 1, characterized in that: The effective refractive index of each level of phase modulation grating segments in the phase modulation grating segment group is constant, the effective refractive index of each grid in each level of phase modulation grating segments is consistent, and the effective refractive index of each adjacent phase modulation grating segment changes step by step.

4. The refractive index modulated multi-level phase-shifted grating structure according to claim 3, characterized in that: After the phase modulation grating segment group, the phase shift of the target transmission wavelength in the reflection spectrum is specifically expressed as: ； in, is the phase shift of the target transmission wavelength after the phase modulation grating segment group, is the correction factor, is the effective refractive index of the uniform fiber Bragg grating segment, is the effective refractive index of the mth phase modulation grating segment, M is the total number of phase modulation grating segments, is the modulation period of the uniform fiber Bragg grating segment, is the length of the m-th phase modulation grating segment.

5. A method for preparing a refractive index modulated multi-level phase-shift grating structure, characterized in that: The method of preparing a refractive index modulated multi-level phase-shift grating structure as claimed in any one of claims 1 to 4 comprises the following steps: Preset a temperature difference at a preset position of the optical fiber to prepare a uniform fiber grating segment; On a uniform fiber grating segment, the parameters of the laser pulse are controlled, and a phase-modulated grating segment group is written based on shielding to prepare a multi-level phase-shifted grating structure.

6. The method for preparing a refractive index modulated multi-level phase-shift grating structure according to claim 5, characterized in that: The preset temperature difference is specifically expressed as: ； in, is the preset temperature difference, is the thermo-optical coefficient of the optical fiber, is the effective refractive index of the uniform fiber Bragg grating segment, is the initial effective refractive index of the optical fiber.

7. An optical coupler, characterized in that: include: An optical fiber containing a multi-level phase-shift grating structure, wherein the multi-level phase-shift grating structure is a multi-level phase-shift grating structure with refractive index modulation as described in any one of claims 1-4.

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