CN117148491A - Refractive index modulated multistage phase shift grating structure, preparation method and optical coupler - Google Patents
Refractive index modulated multistage phase shift grating structure, preparation method and optical coupler Download PDFInfo
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Abstract
The invention discloses a refractive index modulated multistage phase shift grating structure, a preparation method and an optical coupler, wherein the multistage phase shift grating structure is an axially arranged fiber Bragg grating, and comprises the following components: at least one section of uniform fiber grating section and a phase modulation grating section group with both ends connected with the uniform fiber grating section; the phase modulation grating segment group comprises at least two stages of phase modulation grating segments, and each stage of phase modulation grating segments are in embedded arrangement step by step; the effective refractive indexes of the uniform fiber grating segments and each level of phase modulation grating segments are different; after the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is N pi, wherein n= (0.3-0.7) +N, and N is a natural number. The multistage phase shift grating structure provided by the invention provides multistage phase modulation grating sections by carrying out multistage effective refractive index modulation in the uniform fiber grating, and improves the accuracy and the transmissivity of the optical fiber to the transmission wavelength in the reflection spectrum.
Description
Technical Field
The invention belongs to the technical field of fiber gratings, and particularly relates to a refractive index modulated multistage phase shift grating structure, a preparation method and an optical coupler.
Background
The fiber bragg gratings are mutually coupled for the mode meeting the phase matching condition, the light with a specific wavelength is reflected, and the light with other wavelengths is transmitted, so that a narrow-band filter is formed, and the fiber bragg gratings are widely applied to reflectors, filters and sensing elements, and particularly can be applied to optical couplers.
With the development of optical fiber technology, the common fiber bragg grating can not meet the requirements of optical fiber paths in various optical devices, and research and application of various gratings with special structures have been greatly broken through. The phase shift fiber grating is used as a special fiber Bragg grating, and a mutation of half grating period is introduced into the continuous fiber Bragg grating. The introduction of the phase shift points causes the spectral characteristics of the phase shift fiber bragg grating to be different from those of the fiber bragg grating, a very narrow transmission window is formed in a reflection band, and the whole grating forms an oscillation cavity. Introducing a phase shift achieves narrowband filtering. From the phase angle analysis, pi phase abrupt change is brought to become pi phase shift fiber Bragg grating.
The phase shift fiber grating has high quality wavelength selectivity, low insertion loss, no relation with polarization state and wide application. As for research and application of the phase shift grating, for example, patent CN106094085a discloses a phase shift grating manufacturing method and a phase shift grating, the method includes: forming a first layer of template on a substrate; photoetching a first layer of template according to a preset mask plate, and splitting the first layer of template into a plurality of separated first bosses, wherein the bottoms of first gaps between the first bosses are exposed substrates; forming a second layer of template on the outer surface of the first boss and the exposed surface of the substrate; etching the second layer of template, and splitting the second layer of template into a plurality of separated second bosses; the first bosses are positioned between the adjacent second bosses; removing the first boss; coating photoresist in the gaps between the second bosses to form a first photoresist soft template; carrying out holographic exposure and development on the first photoresist soft template, forming a second gap on the first photoresist soft template, and exposing a substrate at the bottom of the second gap; etching the exposed part of the substrate and removing the second boss to form the grating. The phase shift grating manufacturing method provided by the embodiment of the scheme is high in efficiency.
For another example, patent CN107037583a provides a method, an apparatus and a system for demodulating the center wavelength and the phase shift amount of the phase shift grating, and calculates by using the reflection spectrum data of the phase shift grating to be demodulated to obtain the center wavelength and the phase shift amount of the phase shift grating to be demodulated, so as to solve the problem that the center wavelength and the phase shift amount of the phase shift grating cannot be effectively demodulated, and further, the response difference of the center wavelength and the phase shift amount to the external physical quantity change can be used for sensing.
However, the prior art as described above is generally of a size that introduces a phase shift by controlling the pitch of the slots. This requires precise control of the thickness of the mask, which often results in misalignment of the narrow bands in practical applications.
Therefore, how to design a reasonable grating structure on an optical fiber, obtain a precise and controllable phase shift grating, and provide a preparation method of the phase shift grating, which is convenient and feasible to operate, so as to meet the application requirements of optical elements in different scenes is a problem to be solved urgently by those skilled in the art.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a refractive index modulated multistage phase shift grating structure, a preparation method and an optical coupler, wherein the multistage phase shift grating structure is an axially arranged fiber Bragg grating, and comprises the following components: at least one section of uniform fiber grating section and a phase modulation grating section group with both ends connected with the uniform fiber grating section; the phase modulation grating segment group comprises at least two stages of phase modulation grating segments, and each layer of phase modulation grating segments are arranged in a step-by-step embedded mode; the effective refractive indexes of the uniform fiber grating segments and each level of phase modulation grating segments are different; after the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is N pi, wherein n= (0.3-0.7) +N, and N is a natural number. The multistage phase shift grating structure provided by the invention provides multistage phase modulation grating sections by carrying out multistage effective refractive index modulation in the uniform fiber grating, and improves the accuracy and the transmissivity of the optical fiber to the transmission wavelength in the reflection spectrum.
In a first aspect, the present invention provides a refractive index modulated multi-stage phase shift grating structure, the multi-stage phase shift grating structure being an axially arranged fiber bragg grating, comprising:
at least one section of uniform fiber grating section and a phase modulation grating section group with both ends connected with the uniform fiber grating section;
the phase modulation grating segment group comprises at least two stages of phase modulation grating segments, and each stage of phase modulation grating segments are in embedded arrangement step by step;
the effective refractive indexes of the uniform fiber grating segments and each level of phase modulation grating segments are different;
after the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is N pi, wherein n= (0.3-0.7) +N, and N is a natural number.
Further, the ratio of the length of the phase modulation grating segment group to the total length of the multi-stage phase shift grating structure is not more than 1/10.
Further, after the multistage phase shift grating structure, the transmittance of the target transmission wavelength is not lower than 70%.
Further, the positional relationship between the phase modulation grating segment group and the uniform fiber grating segment in the multistage phase shift grating structure is specifically expressed as:
wherein L is 1 L is the length of one of the uniform fiber grating segments separated by a group of phase modulated grating segments 2 R is the length of another section of the uniform fiber grating segment separated by the phase modulation grating segment group 1 For uniformity of L in fiber grating section 1 Reflectivity of the segment where R 2 For uniformity of L in fiber grating section 2 The reflectivity of the section, L is the equivalent length of the multi-stage phase shift grating structure,the length of the mth-level phase modulation grating section is M is the total number of stages of the phase modulation grating section,in order to homogenize the modulation period of the fiber grating segment,in order to homogenize the effective refractive index of the fiber grating segment,for the initial effective refractive index of the fiber,is the coupling efficiency of the fiber core.
Further, the number of grids in each stage of phase modulation grating section is not less than 1/4 modulation period, and the grids in each stage of phase modulation grating section are continuously arranged;
each stage of phase modulation grating segments are hierarchically divided according to the gradual embedding, and two grating lengths respectively delta L are formed between the end point of the phase modulation grating segment of any current stage and the adjacent end point of the phase modulation grating segment of the next stage n,1 、ΔL n,2 And DeltaL n,1 ：ΔL n,2 =1:(0.5~2)。
Further, the effective refractive index of each stage of phase modulation grating section in the phase modulation grating section group is constant, the effective refractive index of each grid in each stage of phase modulation grating section is consistent, and the effective refractive index of each adjacent phase modulation grating section is changed step by step.
Further, after the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is specifically expressed as follows:
wherein,for the target transmitted wavelength phase shift after the phase modulation grating segment group,in order to correct the coefficient of the light,in order to homogenize the effective refractive index of the fiber grating segment,for the effective refractive index of the first order phase modulated grating segment,the effective refractive index of the mth-level phase modulation grating segment, M is the total number of stages of the phase modulation grating segment,in order to homogenize the modulation period of the fiber grating segment,the length of the grating segment is modulated for the mth level of phase.
In a second aspect, the present invention further provides a method for preparing a refractive index modulated multi-stage phase shift grating structure, which specifically includes the following steps:
presetting a temperature difference at a preset position of an optical fiber to prepare a uniform optical fiber grating section;
and on the uniform fiber grating section, controlling parameters of laser pulse, and writing the phase modulation grating section group based on shielding to prepare the multistage phase shift grating structure.
Further, the preset temperature difference is specifically expressed as:
wherein,in order to preset the temperature difference,is the thermo-optic coefficient of the optical fiber,is a uniform fiber grating sectionIs used for the optical fiber, the effective refractive index of the polymer,is the initial effective refractive index of the fiber.
In a third aspect, the present invention also provides an optical coupler comprising: an optical fiber comprising a multi-level phase shift grating structure, wherein the multi-level phase shift grating structure is a refractive index modulated multi-level phase shift grating structure as described above.
The invention provides a refractive index modulated multistage phase shift grating structure, a preparation method and an optical coupler, which at least comprise the following beneficial effects:
(1) According to the invention, the effective refractive index of the optical fiber is modulated in a multistage manner by progressive embedding, and phase shift is introduced into the conventional uniform fiber Bragg grating, so that the precise preset target transmission wavelength is generated in the reflection spectrum. The multi-stage progressive embedded effective refractive index modulation is used for superposing the phase shift quantity caused by the multi-stage phase modulation grating section, and through multi-dimensional modulation, the design requirement on the phase shift fiber grating can be met, the influence of parameter errors on the transmission wavelength of the reflection spectrum in the grating writing and preparing process can be reduced, and meanwhile, the phase shift grating with higher transmissivity can be obtained.
(2) According to preset target transmission wavelength and transmittance, and by combining with the attribute parameters of the optical fiber, the effective refractive indexes of all stages of phase modulation grating segments in the multistage phase shift grating structure are accurately controlled, so that the grating optical fiber meeting the application requirements of optical elements in different scenes is obtained.
(3) In the preparation process of the multistage phase shift grating structure, the electric disturbance is carried out on partial areas of the obtained uniform fiber grating section, the optimal writing capacity (such as pulse duration, repetition frequency and power, so as to obtain the multistage phase shift grating structure with customized characteristics) is sought, the effective refractive index of the specific area is caused to be modulated, the modes of a multistage treated area and an untreated area are obtained, and the effective refractive index difference is formed, so that the multistage phase shift grating structure is prepared.
(4) The magnitude of the change of the external physical quantity in the writing process is determined through the target transmission wavelength of the reflection spectrum, so that the temperature difference can be preset, controlled and monitored, and the multistage phase shift grating structure is prepared.
Drawings
FIG. 1 is a schematic diagram of a refractive index modulated multi-level phase shift grating structure according to the present invention;
FIG. 2 is a schematic flow chart of a method for fabricating a refractive index modulated multi-level phase shift grating structure according to the present invention;
FIG. 3 is a schematic diagram of a structure for preparing a phase modulation grating segment set under a preset temperature difference according to an embodiment of the present invention;
fig. 4 is a graph of the results of transmittance testing of a multi-level phase shift grating structure provided by the present invention.
Detailed Description
In order to better understand the above technical solutions, the following detailed description will be given with reference to the accompanying drawings and specific embodiments. It will be apparent that the described embodiments are only some, but not all, embodiments of the invention. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
The terminology used in the embodiments of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used in this application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise, the "plurality" generally includes at least two.
It should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a product or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such product or apparatus. Without further limitation, an element defined by the phrase "comprising one … …" does not exclude the presence of other like elements in a commodity or device comprising such element.
The phase shift grating is a non-uniform grating, and a certain phase shift area is usually introduced into a specific part of a conventional fiber Bragg grating, for example, pi phase shift is introduced into the conventional Bragg grating, so that a narrow-band peak can be generated in a reflection spectrum, and a single-mode laser output is generated.
The number, arrangement position, modulation period, etc. of the phase shift regions in the phase shift grating will have an effect on the spectrum transmitted through the fiber bragg grating. Meanwhile, in order to ensure that the obtained phase-shift Bragg grating has accurate spectral characteristic sensing characteristics, the optical fiber can be modulated with effective refractive index in a multistage progressive embedding mode, so that a phase-shift fiber Bragg grating structure with a specific target transmission wavelength is provided.
Thus, as shown in fig. 1, the present invention provides a refractive index modulated multi-stage phase shift grating structure, which is an axially arranged fiber bragg grating, comprising:
at least one section of uniform fiber grating section and a phase modulation grating section group with both ends connected with the uniform fiber grating section;
the phase modulation grating segment group comprises at least two stages of phase modulation grating segments, and each stage of phase modulation grating segments are in embedded arrangement step by step;
the effective refractive indexes of the uniform fiber grating segments and each level of phase modulation grating segments are different;
after the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is N pi, wherein n= (0.3-0.7) +N, and N is a natural number.
According to the multistage phase shift grating structure provided by the invention, the multistage embedded phase modulation grating segment group is obtained by modulating the effective refractive index in the uniform fiber grating segment, so that the set phase shift of the target transmission wavelength in the reflection spectrum is realized. The uniform fiber grating section is a conventional Bragg grating, and the modulation period and the effective refractive index of each grid in the uniform fiber grating section are the same. The phase modulation grating segment group comprises a plurality of stages of phase modulation grating segments, and the effective refractive indexes of the phase modulation grating segments are different from those of the uniform fiber grating segments.
The wavelength of a fiber bragg grating is primarily dependent on the grating period and the effective refractive index at which the coupling mode occurs. According to the invention, the effective refractive index of the optical fiber is modulated in a progressive embedded mode, and phase shift is introduced into the conventional uniform fiber Bragg grating, so that the accurate preset target transmission wavelength is generated in the reflection spectrum.
The multi-stage progressive embedded effective refractive index modulation is used for superposing the phase shift quantity caused by the multi-stage phase modulation grating section, so that the design requirement on the phase shift fiber grating can be met, the influence of parameter errors in the grating writing preparation process on the accuracy of the transmission wavelength of the reflection spectrum can be reduced, and the phase shift grating with higher transmissivity can be obtained.
The ratio of the length of the phase modulation grating segment group to the length of the multi-stage phase shift grating structure is not greater than 1/10. The effective refractive index modulation is carried out in the uniform optical fiber section, and the amplitude of the modulation is smaller, so that the length of the phase modulation grating section group is smaller, which is more than 1 order of magnitude lower than the length of the original fiber Bragg grating.
Meanwhile, in order to ensure that the transmission of the obtained multistage phase shift grating structure meets the requirement, the transmission rate of the target transmission wavelength is not lower than 70% after the multistage phase shift grating structure is set.
For a multi-level phase shift grating structure incorporating a set of phase modulated grating segments, the target transmission wavelength is in a uniform fiber grating segment, its reflectivity for the target transmission wavelengthThe following formula is provided:
wherein,and l is the length of the uniform fiber grating section for the coupling coefficient.
Thus, the transmittance T of the target transmission wavelength can be expressed as:
in addition, the coupling coefficientThe determination may be made by the following equation:
wherein,in order to uniformly modulate the refractive index of the fiber with the fiber grating segment,for the coupling efficiency of the fiber core,transmitting a wavelength for the target.
Therefore, for the multistage phase shift grating structure of the present invention, the positional relationship between the phase modulation grating segment group and the uniform fiber grating segment in the multistage phase shift grating structure under the condition of meeting the transmittance index is specifically expressed as:
wherein L is 1 L is the length of one of the uniform fiber grating segments separated by a group of phase modulated grating segments 2 R is the length of another section of the uniform fiber grating segment separated by the phase modulation grating segment group 1 For uniformity of L in fiber grating section 1 Reflectivity of the segment where R 2 For uniformity of L in fiber grating section 2 The reflectivity of the section, L is the equivalent length of the multi-stage phase shift grating structure,the length of the mth-level phase modulation grating segment is M, the total number of stages of the phase modulation grating segment，In order to homogenize the modulation period of the fiber grating segment,in order to homogenize the effective refractive index of the fiber grating segment,for the initial effective refractive index of the fiber,is the coupling efficiency of the fiber core.
The number of grids in each stage of phase modulation grating section is not less than 1/4 modulation period, and the grids in each stage of phase modulation grating section are continuously arranged;
that is, each stage of phase modulation grating section can be 1/4 modulation period, can be half modulation period, and can be integer number of modulation period. Of course, any of the modulation periods may be longer than 1/4 of the modulation period. The number of grids in each stage of phase modulation grating section is limited, so that the effect of the modulation of the refractive index of each stage on the whole multi-stage phase modulation grating structure can be ensured, and the accuracy can be improved through multi-dimensional modulation.
Each stage of phase modulation grating segments are hierarchically divided according to the gradual embedding, and two grating lengths respectively delta L are formed between the end point of the phase modulation grating segment of any current stage and the adjacent end point of the phase modulation grating segment of the next stage n,1 、ΔL n,2 And DeltaL n,1 ：ΔL n,2 =1:(0.5~2)。
Introducing phase shifts at different locations in the uniform fiber grating segment will have an effect on maximum transmission. The introducing position of the phase shift starts from the initial end of the uniform fiber grating section, and in the process of gradually moving to the central end, the peak position of the transmission wavelength is unchanged, but the maximum transmittance gradually becomes deep, and after passing through the central end, the peak position of the transmission wavelength still does not change, but the maximum transmittance gradually becomes shallow.
Therefore, in order to ensure that the maximum transmittance is within a desirable range, the ratio of two lengths formed between adjacent end points of the phase modulation grating segments of the next stage of the end points of the phase modulation grating end of the current stage is 1/2-2 times, thereby ensuring that the depth of the transmission window (i.e. the position where the target transmission wavelength is located) still reaches low loss.
As shown in fig. 1, in the diagram of the multistage phase shift grating structure, AB and GH are uniform fiber grating segments, and BG is a group of phase modulation grating segments. After passing through the phase modulation grating segment group, namely after passing through the end point B and the end point G, the target transmission wavelength forms a phase difference to generate a phase shift quantity.
After the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is specifically expressed as:
wherein,for a target amount of transmitted wavelength phase shift for a group of phase modulated grating segments,for the target transmission wavelength, deltal is the length of the group of phase modulated grating segments,the effective refractive index is the weight of the phase modulated grating segment group.
In the diagram of the multi-stage phase shift grating structure, as shown in fig. 1, the group of phase modulation grating segments of BG includes multi-stage phase modulation grating segments, for example, BC and FG are phase modulation grating segments of the same stage, CD and EF are phase modulation grating segments of another same stage, and DE is a phase modulation grating segment of another stage.
The effective refractive index of each stage of the phase-modulated grating segment set is constant, e.g., the effective refractive indices of BC and FG are constant, and the effective refractive indices of CD and EF are constant.
The effective refractive indexes of the grids of each stage of phase modulation grating section are consistent, and the effective refractive indexes of the adjacent phase modulation grating sections are changed step by step. For example, the effective refractive index of each adjacent phase modulation grating segment is in a gradual change trend from BC and FG levels to CD and EF levels to DE levels.
After the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is specifically expressed as:
wherein,for the target transmitted wavelength phase shift after the phase modulation grating segment group,in order to correct the coefficient of the light,in order to homogenize the effective refractive index of the fiber grating segment,the effective refractive index of the mth-level phase modulation grating segment, M is the total number of stages of the phase modulation grating segment,in order to homogenize the modulation period of the fiber grating segment,the length of the grating segment is modulated for the mth level of phase.
The multistage phase shift grating structure provided by the invention only modulates the effective refractive index of the uniform fiber Bragg grating, does not change the modulation period in the uniform fiber Bragg grating, and then provides multistage phase modulation grating sections, thereby ensuring the controllability of the phase shift grating.
As shown in fig. 1, taking three-level phase modulation grating segments as an example, the effect of modulating the effective refractive index of each level of phase modulation grating segments on the amount of phase shift of the entire phase modulation grating segment group is described.
At this time, the grating segments are phase-modulatedThe total number of layers M is 3,to uniform the modulation period of the fiber grating segment (i.e. the modulation period in points a to B and G to H),for the modulation period of the first-stage phase modulation grating segment (i.e. the modulation period in points B to C and F to G),for the modulation period of the 2 nd-level phase modulation grating segment (i.e. the modulation period in points C to D and E to F),the modulation period of the grating segment (i.e., the modulation period in the D-point to E-point) is phase modulated at stage 3.Is the length of the mth level phase modulated grating segment, wherein,for the length of the 1 st-stage phase modulation grating segment (i.e. for BC, FG),for the length of the 2 nd-stage phase modulation grating segment (i.e. the length of CD, EF),the length of the grating segment (i.e., the length of DE) is modulated for the 3 rd order phase.
The effective refractive index from the left end point of the optical fiber structure to the point A is the initial effective refractive index of the optical fiberThe points A to B are uniform fiber grating segments, and the effective refractive index of AB is the effective refractive index of the uniform fiber grating segments, namelyThe point B to the point C are the 1 st-stage phase modulation grating segments, and the effective refractive index of BC is the effective refractive index of the 1 st-stage phase modulation grating segments, namelyThe points C to D are 2 nd-level phase modulation grating segments, and the effective refractive index of CD is the effective refractive index of 2 nd-level phase modulation grating segments, namelyThe points D to E are 3 rd-level phase modulation grating segments, and the effective refractive index of DE is 3 rd-level phase modulation grating segments, namely。
After ED section, E point to F point are 2 nd-stage phase modulation grating section, and EF effective refractive index is 2 nd-stage phase modulation grating section effective refractive index, namelyThe point F to the point G are the 1 st-stage phase modulation grating section, and the effective refractive index of FG is the effective refractive index of the 1 st-stage phase modulation grating section, namelyThe points G to H are uniform fiber bragg grating segments, and the effective refractive index of GH is the effective refractive index of the uniform fiber bragg grating segments, namelyThe effective refractive index from the H point to the right end point of the optical fiber is the initial effective refractive index of the optical fiber。
After the three-stage phase modulation grating segments are subjected to phase modulation grating segment groups, the phase shift of the target transmission wavelength in the reflection spectrum is specifically expressed as follows:
as shown in fig. 2, the present invention further provides a method for preparing a refractive index modulated multi-stage phase shift grating structure, which specifically includes the following steps:
presetting a temperature difference at a preset position of an optical fiber to prepare a uniform optical fiber grating section;
after the temperature difference is preset, preparing uniform fiber grating segments, and exposing by adopting laser pulse which can select high-frequency CO 2 Laser exposure, ultraviolet laser irradiation may be selected.
The temperature difference is preset to be equal to the temperature difference between the temperature of the fiber grating section and other positions of the optical fiber.
And on the uniform fiber grating section, controlling parameters of laser pulse, and writing the phase modulation grating section group based on shielding to prepare the multistage phase shift grating structure.
Parameters of the laser pulse include laser pulse duration, laser repetition frequency, power, etc. The parameters of the laser pulse are controlled to enable the laser passing through the shielding plate and the mask plate to act on the uniform fiber grating section part of the optical fiber, so that the grating structure of the multistage phase modulation grating section group with customized characteristics is obtained. The size and shape of the mask plate correspond to those of the phase modulation grating segment groups, and the size and laser transmission condition of the shielding plate correspond to the lengths, effective refractive indexes and the like of the multistage phase modulation grating segments.
When the Bragg grating is written by the optical fiber, the laser pulse can be adopted for irradiation, and in the laser pulse exposure process, the external effect is applied, so that the periodic regulation and control of the effective refractive index of the optical fiber can be realized. Of the external physical quantities, temperature and stress variation are the most fundamental factors. The magnitude of the change of the external physical quantity is determined by the target projection wavelength in the reflection spectrum, so that the temperature and other conditions can be preset, controlled and monitored, and the multistage phase shift grating structure is prepared.
The multistage phase shift grating is prepared based on a shielding mode and by adopting a mask, can integrate the advantages of easiness in operation and good stability in a mask process, and overcomes the defect of inflexibility in writing. Introducing partial periodic dielectric disturbance into a uniform fiber grating section, seeking optimal writing capacity (such as pulse duration, repetition frequency and power, so as to obtain a multistage phase shift grating structure with customized characteristics), causing effective refractive index modulation of a specific region, obtaining modes of a multistage treated region and an untreated region, and forming effective refractive index difference, so as to prepare the multistage phase shift grating.
As shown in fig. 3, after the fiber bragg grating acts on the temperature field, the effect of temperature on the grating can be analyzed by the fiber bragg grating reflection wavelength formula.
In analyzing the influence of the temperature field in the fiber bragg grating, the reflection wavelength formula from the fiber bragg grating is required. Namely:
wherein,is the reflection wavelength of the fiber Bragg grating, n is the effective refractive index of the fiber,is a grating period;
and (3) deriving the temperature T according to the optical fiber Bragg grating reflection wavelength formula to obtain:
the change in the effective refractive index of the fiber can be expressed as:
wherein,is the thermo-optic coefficient of the optical fiber;
the variation of the grating period can be expressed as:
wherein,is the thermal collision coefficient of the optical fiber material;
based on the modulation principle of the effective refractive index, the temperature difference is preset, specifically expressed as:
wherein,in order to preset the temperature difference,is the thermo-optic coefficient of the optical fiber,in order to homogenize the effective refractive index of the fiber grating segment,is the initial effective refractive index of the fiber.
Parameters of the laser pulse (such as laser pulse duration, laser repetition frequency, power and the like) are controlled so that the laser transmitted through the shielding plate and the mask plate acts on the uniform fiber grating segment part on the optical fiber, thereby obtaining the grating structure of the multistage phase modulation grating segment group with customized characteristics.
The optical fiber absorbs photons emitted by the laser pulse, producing a change in refractive index that is largely dependent on the photosensitive properties of the optical fiber itself. The photosensitivity of the optical fiber itself includes refractive index, absorption spectrum, density, external physical quantity (such as strain, temperature, etc.) of the optical fiber, especially the core. The optical fiber is irradiated by the laser pulse, and the photosensitive characteristic of the optical fiber is permanently changed.
On the uniform fiber grating section, controlling the parameters of laser pulse, and writing the phase modulation grating section group based on shielding to prepare the phase modulation grating section group, which comprises the following steps:
the parameters of the laser pulse and the parameters of the shielding plate are preset, so that the effective refractive index modulation in the uniform grating section meets the requirements of the phase modulation grating section group, and the method is specifically expressed as follows:
wherein,for the length of the mth level phase modulated grating segment,for the effective refractive index of the mth order phase modulated grating segment,for the initial effective refractive index of the fiber,as the upper limit of the wavelength of the laser light,is the lower limit of the wavelength of the laser light,as a function of the laser absorption variation, anAnd parametersIn relation to each other,is the wavelength of the laser light,the strain of the optical fiber, N is the pulse of the laserThe number of the impulses is counted,for the transmission wavelength of interest,for phase modulating the length of the set of grating segments.
The specific formula of the function will be different due to the difference of laser selection, and therefore, no further limitation is made. Before writing the phase modulation grating segment group, the laser type to be adopted is selected, and then the laser type can be determined by experimental dataSpecific formulas of the functions.
As shown in fig. 4, a multi-level phase shift grating structure according to the present invention was subjected to transmittance testing. The multistage phase shift grating structure has stage number of 3, target transmission wavelength of 1064nm, and after phase modulation grating segment group, the phase shift of target transmission wavelength in reflection spectrum is 1/2 pi. From the test data, the transmittance of the target transmission wavelength is approximately 100%.
In a third aspect, the present invention also provides an optical coupler comprising:
an optical fiber comprising a multi-level phase-shift grating structure, wherein the multi-level phase-shift grating structure is a refractive index modulated multi-level phase-shift grating as described above.
The invention provides a refractive index modulated multistage phase shift grating structure, a preparation method and an optical coupler, which at least comprise the following beneficial effects:
(1) According to the invention, the effective refractive index of the optical fiber is modulated in a multistage manner by progressive embedding, and phase shift is introduced into the conventional uniform fiber Bragg grating, so that the precise preset target transmission wavelength is generated in the reflection spectrum. The multi-stage progressive embedded effective refractive index modulation is used for superposing the phase shift quantity caused by the multi-stage phase modulation grating section, and through multi-dimensional modulation, the design requirement on the phase shift fiber grating can be met, the influence of parameter errors on the transmission wavelength of the reflection spectrum in the grating writing and preparing process can be reduced, and meanwhile, the phase shift grating with higher maximum transmittance can be obtained.
(2) According to preset target transmission wavelength and transmittance, and by combining with the attribute parameters of the optical fiber, the effective refractive indexes of all stages of phase modulation grating segments in the multistage phase shift grating structure are accurately controlled, so that the grating optical fiber meeting the application requirements of optical elements in different scenes is obtained.
(3) In the preparation process of the multistage phase shift grating structure, electric disturbance is carried out on partial areas of the obtained uniform fiber grating section, the optimal writing capacity (such as pulse duration, repetition frequency and power are obtained, so that the multistage phase shift grating structure with customized characteristics is obtained), the effective refractive index of a specific area is caused to be modulated, the modes of a multistage treated area and an untreated area are obtained, and the effective refractive index difference is formed, so that the multistage phase shift grating structure is prepared.
(4) The magnitude of the change of the external physical quantity in the writing process is determined through the target transmission wavelength of the reflection spectrum, so that the temperature difference can be preset, controlled and monitored, and the multistage phase shift grating structure is prepared.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. It is therefore intended that the following claims be interpreted as including the preferred embodiments and all such alterations and modifications as fall within the scope of the invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention also include such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.
Claims (10)
1. A refractive index modulated multi-level phase shift grating structure, wherein the multi-level phase shift grating structure is an axially aligned fiber bragg grating, comprising:
at least one section of uniform fiber grating section and a phase modulation grating section group with both ends connected with the uniform fiber grating section;
the phase modulation grating segment group comprises at least two stages of phase modulation grating segments, and each stage of phase modulation grating segments are in embedded arrangement step by step;
the effective refractive indexes of the uniform fiber grating segments and each level of phase modulation grating segments are different;
after the phase modulation grating segment group, the phase shift amount of the target transmission wavelength in the reflection spectrum is N pi, wherein n= (0.3-0.7) +N, and N is a natural number.
2. The refractive index modulated multi-stage phase shift grating structure of claim 1 wherein the ratio of the length of the group of phase modulated grating segments to the total length of the multi-stage phase shift grating structure is no greater than 1/10.
3. The refractive index modulated multi-stage phase shift grating structure of claim 2, wherein the transmission of the target transmission wavelength is not less than 70% after passing through the multi-stage phase shift grating structure.
4. The refractive index modulated multi-stage phase shift grating structure of claim 3, wherein the positional relationship between the group of phase modulated grating segments and the uniform fiber grating segments in the multi-stage phase shift grating structure is specifically expressed as:
；
wherein L is 1 L is the length of one of the uniform fiber grating segments separated by a group of phase modulated grating segments 2 R is the length of another section of the uniform fiber grating segment separated by the phase modulation grating segment group 1 For uniformity of L in fiber grating section 1 Reflectivity of the segment where R 2 For uniformity of L in fiber grating section 2 The reflectivity of the section, L is the equivalent length of the multi-stage phase shift grating structure,for the length of the mth level phase modulation grating segment, M is the total number of stages of the phase modulation grating segment,/>For the modulation period of the uniform fiber grating section, +.>For homogenizing the effective refractive index of the fiber grating section, < >>For the initial effective refractive index of the fiber, +.>Is the coupling efficiency of the fiber core.
5. The refractive index modulated multi-stage phase shift grating structure of claim 2, wherein the number of grids in each stage of the phase modulation grating section is not less than 1/4 modulation period, and the grids in each stage of the phase modulation grating section are arranged in succession;
each stage of phase modulation grating segments are hierarchically divided according to the gradual embedding, and two grating lengths respectively delta L are formed between the end point of the phase modulation grating segment of any current stage and the adjacent end point of the phase modulation grating segment of the next stage n,1 、ΔL n,2 And DeltaL n,1 ：ΔL n,2 =1:(0.5~2)。
6. The refractive index modulated multi-stage phase shift grating structure of claim 2 wherein the effective refractive index of each stage of the group of phase modulation grating segments is constant and the effective refractive index of each of the grids in each stage of phase modulation grating segments is uniform and the effective refractive index of each adjacent phase modulation grating segment varies stepwise.
7. The refractive index modulated multi-stage phase shift grating structure of claim 6 wherein the amount of phase shift of the target transmission wavelength in the reflection spectrum after the group of phase modulated grating segments is expressed as:
；
wherein,for the target transmitted wavelength phase shift after the phase modulation grating segment group, < >>For correction factor, +.>For homogenizing the effective refractive index of the fiber grating section, < >>Is the effective refractive index of the mth-level phase modulation grating section, M is the total number of stages of the phase modulation grating section, +.>For the modulation period of the uniform fiber grating section, +.>The length of the grating segment is modulated for the mth level of phase.
8. A method for preparing a refractive index modulated multi-stage phase shift grating structure, comprising the steps of:
presetting a temperature difference at a preset position of an optical fiber to prepare a uniform optical fiber grating section;
and on the uniform fiber grating section, controlling parameters of laser pulse, and writing the phase modulation grating section group based on shielding to prepare the multistage phase shift grating structure.
9. The method for manufacturing a refractive index modulated multi-level phase shift grating structure according to claim 8, wherein the predetermined temperature difference is specifically expressed as:
；
wherein,for the preset temperature difference, & lt & gt>Is the thermo-optic coefficient of the optical fiber, < >>In order to homogenize the effective refractive index of the fiber grating segment,is the initial effective refractive index of the fiber.
10. An optical coupler, comprising: an optical fiber comprising a multi-level phase-shift grating structure, wherein the multi-level phase-shift grating structure is a refractive index modulated multi-level phase-shift grating structure as defined in any one of claims 1-7.
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