Chirped fiber grating preparation method and chirped fiber grating filter

Abstract

The present invention discloses a method for preparing a chirped fiber grating, comprising the following steps S1: performing a morphological analysis on a target spectrum of a chirped fiber grating, fitting and approximating the target spectrum using a plurality of Bragg grating spectra, and decomposing the target spectrum into a plurality of Bragg grating spectra; step S2: using a Bragg grating formula, deriving parameters for each decomposed Bragg grating spectrum to obtain preparation parameters of the chirped fiber grating; step S3: preparing the chirped fiber grating by laser according to the preparation parameters. The present invention also provides a chirped fiber grating, and a chirped fiber grating filter. The present application realizes flexible and flexible preparation according to the spectral shape requirements of the chirped fiber grating. The chirped fiber grating filter of the present application can realize complex filtering spectral shape requirements through a single device.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber gratings, and in particular to a chirped optical fiber grating, a preparation method thereof, and a chirped optical fiber grating filter. Background Art

[0002] Existing fiber grating filters are mainly obtained through the following ways:

[0003] 1. Preparation of chirped gratings using the ultraviolet laser mask method. The ultraviolet laser mask method is currently the mainstream method for preparing chirped fiber gratings. High-quality chirped gratings can be prepared by the mask method. However, the use of the ultraviolet laser mask method has the following disadvantages: the preparation of chirped fiber gratings often requires hydrogen treatment of the optical fiber to increase photosensitivity, which is time-consuming and costly, and the experimental risk is high; limited by the flexibility of the mask, the period, chirp amount and other parameters of the prepared chirped fiber grating cannot be flexibly adjusted, and chirped gratings with different parameters need to be prepared using different masks, which is costly.

[0004] 2. Customization method of filter based on cascaded long-period fiber grating: Long-period fiber grating is prepared by CO2 laser processing, femtosecond laser processing, etc., and multiple fiber gratings with different grating periods and grating lengths are cascaded to achieve specific filtering spectrum requirements. However, filters based on long-period fiber gratings have the following disadvantages: long-period fiber gratings are sensitive to external changes, so the filter stability is weak and the device packaging requirements are high; using long-period fiber gratings to match specific filtering spectrum shapes requires a large number of cascaded devices, and the overall size is large, which is not conducive to high integration. Summary of the invention

[0005] In order to overcome the problems existing in the prior art, the present invention provides a chirped fiber grating, a preparation method thereof, and a chirped fiber grating filter.

[0006] The present invention provides a method for preparing a chirped fiber grating, comprising the following steps:

[0007] Step S1: performing a morphological analysis on a target spectrum of a chirped fiber grating, fitting and approximating the target spectrum using a plurality of Bragg grating spectra, and decomposing the target spectrum into a plurality of Bragg grating spectra;

[0008] Step S2: using the Bragg grating formula, deriving parameters for each decomposed Bragg grating spectrum to obtain the preparation parameters of the chirped fiber grating;

[0009] Step S3: According to the preparation parameters, chirped fiber grating is prepared by laser.

[0010] As an implementation scheme of the method for preparing a chirped fiber grating provided by the present invention, the method for preparing a chirped fiber grating further comprises the following steps:

[0011] Step S4: Compare the spectrum of the prepared chirped fiber grating with the target spectrum. If the difference between the spectrum of the chirped fiber grating and the target spectrum exceeds a preset error value, optimize the preparation parameters and execute step S3.

[0012] As an implementation of the method for preparing the chirped fiber grating provided by the present invention, the method for optimizing the preparation parameters includes: adjusting the number of Bragg grating spectra in step S1, and adjusting the preparation parameters in step S2.

[0013] As an implementation of the method for preparing the chirped fiber grating provided by the present invention, the method for optimizing the preparation parameters includes: adjusting the preparation parameters in step S2.

[0014] As an implementation scheme of the method for preparing a chirped fiber grating provided by the present invention, the preparation parameters include grating parameters and processing parameters;

[0015] Firstly, the Bragg grating formula is used to deduce the parameters of the decomposed Bragg grating spectrum to obtain the grating parameters of the chirped fiber grating; and then the processing parameters of the chirped fiber grating are obtained.

[0016] As an implementation of the method for preparing a chirped fiber grating provided by the present invention, the grating parameters include an initial period and a final period of the chirped fiber grating, and a chirp amount.

[0017] As an implementation of the method for preparing a chirped fiber grating provided by the present invention, the processing parameters of the chirped fiber grating include laser energy, laser repetition rate, and processing speed.

[0018] As an implementation of the method for preparing a chirped fiber grating provided by the present invention, the laser in step S3 is a femtosecond laser.

[0019] The present invention also provides a chirped fiber grating, comprising an optical fiber and a chirped fiber grating formed on the optical fiber; the chirped fiber grating has grating segments with different modulation amounts.

[0020] As an implementation scheme of the chirped fiber grating provided by the present invention, the chirped fiber grating has different line widths / thicknesses.

[0021] As an implementation scheme of the chirped fiber grating provided by the present invention, the chirped fiber grating is prepared by the above-mentioned preparation method.

[0022] The present invention also provides a chirped fiber grating filter, comprising the chirped fiber grating mentioned above.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] This application proposes for the first time a method for preparing a chirped fiber grating with customizable spectral shape. Compared with the existing fiber grating filter preparation technology, the outstanding advantage of the present invention is that it utilizes the high flexibility of laser processing to achieve flexible and flexible preparation according to the spectral shape requirements of the chirped fiber grating, and the prepared chirped fiber grating has low insertion loss.

[0025] The chirped fiber grating filter obtained in this application has high spectral quality and accurate spectral shape matching. Compared with the existing cascaded long-period fiber grating filter, the chirped fiber grating filter of this application can achieve complex filtering spectral shape requirements through a single device; it can meet complex filtering requirements with a single device, and has good market prospects and practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A schematic diagram of a processing optical path system for a chirped fiber grating provided in this application;

[0027] Figure 2 A schematic diagram of a line-by-line chirped fiber grating structure of the present application;

[0028] FIG3 is a schematic diagram of a point-by-point chirped fiber grating structure of the present application;

[0029] Figure 4 A schematic diagram of a line-by-line chirped fiber Bragg grating structure with different grating modulation amounts according to the present application;

[0030] Figure 5 A schematic diagram of a target spectrum shape fitting process in the preparation method of a chirped fiber grating of the present application;

[0031] Figure 6 A schematic diagram of the spectrum of a chirped fiber grating prepared by the chirped fiber grating preparation method of the present application;

[0032] Figure 7 Schematic diagram of the comparison between the target spectrum and the prepared chirped fiber grating spectrum. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0034] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0035] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0036] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, an electrical connection, or communication with each other; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0037] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

[0038] A method for preparing a chirped fiber grating proposed in the present application comprises the following steps:

[0039] Step S1: performing a morphological analysis on a target spectrum of a chirped fiber grating, fitting and approximating the target spectrum using multiple Bragg grating spectra by simulation, and decomposing the target spectrum into multiple Bragg grating spectra;

[0040] Step S2: using the Bragg grating formula, deriving parameters for each decomposed Bragg grating spectrum to obtain the preparation parameters of the chirped fiber grating;

[0041] Step S3: According to the preparation parameters, chirped fiber grating is prepared by laser.

[0042] In step S1, the spectral shape of the target spectrum shape (such as the filter spectrum shape) expected by the grating is first analyzed, and the target spectrum shape is approximately fitted using multiple Bragg grating spectra. The target spectrum shape is first split into the superposition of multiple Bragg grating spectra by simulation, so as to achieve a reasonable discretization decomposition of the target spectrum shape. Multiple Bragg grating spectra are referred to as fitted spectra. Figure 5 As shown in the figure, the solid line spectrum is any filtered target spectrum shape, and the dotted line represents the Bragg grating spectrum. Multiple Bragg grating spectra are used to approximate the target spectrum shape. The more Bragg grating spectra there are, the higher the accuracy of fitting and matching the target spectrum shape by multiple Bragg grating spectra. The appropriate number of Bragg grating spectra can be selected according to actual needs and error range.

[0043] After step S1 realizes the decomposition of the target spectrum, step S2 uses the Bragg grating formula to reversely derive the chirped fiber grating preparation parameters.

[0044] In step S2, the Bragg grating formula is as follows (1).

[0045] Formula (1),

[0046] Among them, n eff is the effective refractive index constant, m and Λ are the order and period of the grating respectively, and λ is the resonant wavelength (operating wavelength) of the Bragg grating.

[0047] The preparation parameters in step S2 include grating parameters and processing parameters. First, the Bragg grating formula is used to derive the parameters of the decomposed Bragg grating spectrum to obtain the grating parameters of the chirped fiber grating; then the processing parameters of the chirped fiber grating are obtained. The grating parameters include the initial period and final period of the chirped fiber grating and the chirp amount. The processing parameters of the chirped fiber grating include laser energy, laser repetition rate, and processing speed.

[0048] By using the Bragg grating formula, the parameters of each Bragg grating spectrum after decomposition are derived, and the initial period and final period of the chirped fiber grating are obtained through the parameters of the Bragg grating spectrum. Therefore, the chirp amount of the chirped fiber grating and the function form corresponding to the chirp amount can be solved.

[0049] At the same time, the overall coupling coefficient range of the chirped fiber grating is first determined according to the target spectrum shape, and then the coupling coefficient of the chirped fiber grating at different positions is specifically estimated according to the reflection intensity difference of different Bragg gratings in the fitted spectrum shape, and the laser energy, processing speed and other parameters in the actual preparation process are reasonably set according to the coupling coefficient.

[0050] In step S2, the preparation parameters of the chirped fiber grating can be automatically obtained through an automatic calculation program of the software.

[0051] In step S3, the optical fiber is fixed on a three-dimensional displacement platform during preparation. Preferably, a microscope objective lens 7 with high magnification and large numerical aperture is used as a processing objective lens, and the platform is adjusted so that the femtosecond laser can be accurately focused on the starting position of the grating preparation. The chirped fiber grating can be prepared by inputting the calculated preparation parameters into the processing preparation software of the control device 9. Figure 6 The figure shows a schematic diagram of the spectrum of the prepared chirped fiber grating, the parameters of which are determined by a series of Bragg gratings. Figure 6 The solid line represents the chirped fiber Bragg grating, and the dotted line represents the Bragg grating.

[0052] The laser in step S3 is a femtosecond laser.

[0053] Figure 1 It is a schematic diagram of a processing optical path system of a chirped fiber grating preparation system according to an embodiment of the present invention, which includes a laser processing optical path system, a control device, and a displacement platform.

[0054] The laser processing optical path system includes a laser 1, an optical power attenuator 2, a reflector 3, an aperture 4, an electrically controlled shutter 5, a dichroic mirror 6, a microscope objective lens 7, and a CCD camera 10. The laser 1 is a femtosecond laser. The displacement platform is a three-dimensional displacement platform 8. The control device can be a computer 9. The optical power attenuator 2 adjusts the power of the femtosecond laser by polarization selection. The reflector 3 and the dichroic mirror 6 are used to adjust the optical path. After being emitted by the laser 1, the femtosecond laser passes through the optical power attenuator 2, the reflector 3, and the aperture 4 in sequence, and obtains the appropriate laser processing power by adjusting the laser polarization state, and adjusts the aperture switch degree to obtain the appropriate spot size; then passes through the electrically controlled shutter 5 and the reflector 6 in sequence to reach the microscope objective lens 7 for focusing; after being focused, the laser acts on the optical fiber sample placed on the three-dimensional displacement platform 8, and can be processed and prepared by controlling the movement of the three-dimensional displacement platform 8, and the CCD camera 10 can observe the processing situation in real time. The laser 1, the electric shutter 5, the three-dimensional displacement platform 8 and the CCD camera 10 of the laser processing optical path system are all connected to the control device 9. The control device adjusts the modulation amount of the chirped fiber grating at different positions by controlling the laser processing optical path system and the displacement platform.

[0055] A calculation module can be set in the computer, and the calculation module has a built-in calculation program. The target spectrum shape is input into the computer 9, and the calculation module of the computer 9 can automatically decompose the target spectrum shape into multiple Bragg grating spectra, and derive parameters for each decomposed Bragg grating spectrum according to the Bragg grating formula, and finally obtain the preparation parameters of the chirped fiber grating.

[0056] The preparation parameters can be adjusted according to requirements through the processing preparation software on the control device 9.

[0057] In this application, chirped fiber gratings are processed by femtosecond laser, and the parameters can be adjusted in time, the preparation is flexible, the processing speed is fast, and the efficiency is high. By utilizing the precision of femtosecond laser micromachining, the core of the optical fiber can be engraved with a certain regular structure, such as a line array, a point array, etc., so as to modulate the refractive index of the optical fiber. By utilizing the flexibility of femtosecond laser processing to prepare fiber gratings, the period of the chirped fiber grating can be changed regularly under the premise of being controllable, so as to obtain a chirped spectrum that meets actual needs.

[0058] Through the above steps S1 to S3, a chirped fiber grating can be prepared.

[0059] A chirped fiber grating of the present application includes an optical fiber, and the structure of the chirped fiber grating is directly written into the optical fiber by femtosecond laser technology. The chirped fiber grating can be a line-by-line chirped fiber grating structure, as shown in FIG2; 21 in the figure is the cladding of the optical fiber, 22 is the core, and the line array of chirped fiber grating is formed on the core 22. The chirped fiber grating can also be a point-by-point chirped fiber grating structure, as shown in FIG3, and the point array of chirped fiber grating is formed on the core 22.

[0060] Chirped fiber Bragg grating is a fiber Bragg grating with uneven grating period along the axial direction of the fiber. The grating modulation amount of existing chirped fiber Bragg grating is consistent, such as Figure 2 As shown, different grating segments constituting the chirped fiber grating have the same line width / thickness, and it is impossible to achieve intensity control of each wavelength position of the grating spectrum, and its characteristic spectrum is a flat spectrum.

[0061] The present application provides a chirped fiber grating, whose grating segments have different modulation amounts. The modulation amounts at different positions of the grating can be precisely controlled, so that the grating of the chirped fiber grating can have different modulation amounts at different positions, thereby controlling the detailed morphology of the grating spectrum by changing the modulation amounts at the corresponding positions. It can be understood that the chirped fiber grating, whose grating segments have different modulation amounts, may have a part of the grating segments with the same modulation amount, and another part of the grating segments with different modulation amounts, and the modulation amounts of the gratings at different positions can be adjusted according to demand; or, the modulation amounts of the grating segments at different positions may be different. The present application provides a chirped fiber grating, whose gratings have different line widths / thicknesses, such as Figure 4 shown.

[0062] The specific chirp amount can be controlled by a chirp amount function. The chirp amount is essentially an uneven change in period, and its change rule can be in the form of a linear function or in the form of a nonlinear function such as a quadratic function.

[0063] In order to further optimize the performance of the prepared chirped fiber grating, the preparation method of the chirped fiber grating further includes the following steps:

[0064] Step S4: compare the spectrum of the prepared chirped fiber grating with the target spectrum. If the difference between the spectrum of the chirped fiber grating and the target spectrum exceeds a preset error value, optimize the preparation parameters and execute step S3: prepare the chirped fiber grating by laser according to the optimized preparation parameters.

[0065] By optimizing the preparation parameters, a chirped fiber grating that is more closely matched to the expected target spectrum can be further prepared. In order to obtain a high-quality chirped fiber grating, step S4 can be performed multiple times to achieve multiple optimization iterations of the preparation parameters, and finally a chirped fiber grating filter that is highly matched to the target spectrum within an error range can be obtained.

[0066] In step S4, by analyzing the difference between the spectrum of the chirped fiber grating and the target spectrum, one or more of the parameters such as the grating period, chirp amount, laser energy, processing speed, laser repetition rate, etc. used in the processing and preparation are optimized and iterated according to the specific situation. Figure 7 Shown is a schematic diagram comparing the target spectrum shape and the prepared chirped fiber grating spectrum shape.

[0067] Specifically, the spectral differences between the two can be analyzed, and different preparation parameter optimization methods can be performed according to the following situations:

[0068] If the overall difference between the two in the spectral wavelength range is large, it is necessary to replace and iterate the grating period and chirp amount, etc.; in this case, the preparation parameters need to be adjusted significantly, including the possibility of adjusting the number N of Bragg grating spectra in step S1 to change the grating parameters in the preparation parameters; and also adjusting the preparation parameters in step S2 such as processing parameters.

[0069] If the overall spectral intensity difference between the two is large, it is necessary to adjust some preparation parameters such as laser energy, processing speed, laser repetition rate, etc. during grating preparation in step S2; the grating period, chirp amount, number of periods, etc. can be kept unchanged, and the prepared energy can be adjusted and the modulation amount of the grating can be changed to achieve matching.

[0070] If the spectrum differs only in some areas, the processing parameters of other areas can be kept unchanged, and only the processing parameters of the areas with larger differences can be adjusted.

[0071] Compared with the preparation of chirped fiber gratings by the ultraviolet laser mask method, the preparation method of chirped fiber gratings in the present application has higher flexibility, and can decompose the target spectrum into multiple Bragg grating spectra according to the special target spectrum shape (filter spectrum shape) requirements, and derive the required grating preparation parameters. The desired grating can be obtained directly by inputting the parameters in the processing and preparation software of the control device, i.e., the computer.

[0072] In addition, femtosecond laser processing to prepare chirped fiber gratings does not require additional pretreatment of the optical fiber and has no specific requirements for the type of optical fiber. Chirped fiber gratings can be efficiently prepared on any type of optical fiber.

[0073] Existing filters based on cascaded long-period fiber gratings usually require multiple devices to be cascaded, and it is difficult to match complex filter spectra. However, the preparation method of the chirped fiber grating of the present application can customize the grating preparation parameters according to the target filter spectrum, so that a single device can meet complex filtering requirements. The size of the filter can be greatly reduced by a single chirped fiber grating, which is of great significance to the improvement of device integration.

[0074] The present invention also provides a chirped fiber grating, which is prepared by the above-mentioned preparation method.

[0075] The present invention also provides a chirped fiber grating filter, comprising the chirped fiber grating mentioned above.

[0076] The present invention utilizes laser processing to prepare a chirped fiber grating filter with a customizable spectral shape, and can flexibly change the preparation parameters to obtain the required filtering spectral shape. The filtering spectral shape of the chirped fiber grating filter is variable, and the prepared chirped fiber grating filter has high filtering spectral shape accuracy, high preparation efficiency, and low preparation cost. Therefore, the present invention can satisfy complex filtering requirements with a single device in the field of optical communications, such as gain flattening of erbium-doped fiber amplifiers (EDFAs), filtering of scattered light of high-power fiber lasers, etc., and has good market prospects and application value.

[0077] Compared with the prior art, the present invention has the following beneficial effects:

[0078] This application proposes for the first time a method for preparing a chirped fiber grating with customizable spectral shape. Compared with the existing fiber grating filter preparation technology, the outstanding advantage of the present invention is that it utilizes the high flexibility of laser processing to achieve flexible and flexible preparation according to the spectral shape requirements of the chirped fiber grating, and the prepared chirped fiber grating has low insertion loss.

[0079] The chirped fiber grating filter obtained in this application has high spectral quality and accurate spectral shape matching. Compared with the existing cascaded long-period fiber grating filter, the chirped fiber grating filter of this application can achieve complex filtering spectral shape requirements through a single device; it can meet complex filtering requirements with a single device, and has good market prospects and practical significance.

[0080] Obviously, the embodiments described above are only some embodiments of the present application, rather than all embodiments. The preferred embodiments of the present application are given in the accompanying drawings, but they do not limit the scope of patent protection of the present application. The present application can be implemented in many different forms. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive. Although the present application is described in detail with reference to the aforementioned embodiments, for those skilled in the art, it is still possible to modify the technical solutions recorded in the aforementioned specific implementation methods, or to perform equivalent replacement of some of the technical features therein. Any equivalent structure made using the contents of the specification and drawings of this application, directly or indirectly used in other related technical fields, is also within the scope of patent protection of this application.

Claims

1. A method for preparing a chirped fiber grating, characterized in that: The following steps are involved: Step S1: performing a morphological analysis on a target spectrum of a chirped fiber grating, fitting and approximating the target spectrum using a plurality of Bragg grating spectra, and decomposing the target spectrum into a plurality of Bragg grating spectra; Step S2: using the Bragg grating formula, deriving parameters for each decomposed Bragg grating spectrum to obtain the preparation parameters of the chirped fiber grating; Step S3: preparing a chirped fiber grating by laser according to the preparation parameters; Step S4: comparing the spectrum of the prepared chirped fiber grating with the target spectrum. If the difference between the spectrum of the chirped fiber grating and the target spectrum exceeds a preset error value, optimizing the preparation parameters and executing step S3; The preparation parameters include grating parameters and processing parameters, the grating parameters include the initial period, final period and chirp amount of the chirped fiber grating, and the processing parameters include laser energy, laser repetition rate and processing speed; In step S4, the method for optimizing the preparation parameters includes the following: If the spectrum of the prepared chirped fiber grating is greatly different from the target spectrum in the spectral wavelength range, the initial period, final period and chirp amount of the chirped fiber grating are replaced and iterated, and the number of Bragg grating spectra in step S1 is adjusted; If the spectrum of the prepared chirped fiber grating is greatly different from the target spectrum in spectral intensity, the laser energy, processing speed and laser repetition rate are adjusted while keeping the initial period, final period, chirp amount and the number of periods between the initial period and the final period of the chirped fiber grating unchanged.

2. The method for preparing a chirped fiber grating according to claim 1, characterized in that: Firstly, the Bragg grating formula is used to deduce the parameters of the decomposed Bragg grating spectrum to obtain the grating parameters of the chirped fiber grating; and then the processing parameters of the chirped fiber grating are obtained.

3. The method for preparing a chirped fiber grating according to claim 1, characterized in that: The laser in step S3 is a femtosecond laser.

4. A chirped fiber grating filter, characterized in that: The chirped fiber grating comprises the chirped fiber grating as described in any one of claims 1 to 3.

5. The chirped fiber grating filter according to claim 4, characterized in that: The chirped fiber grating comprises an optical fiber and a chirped fiber grating formed in the optical fiber; the chirped fiber grating has grating segments with different modulation amounts.

6. The chirped fiber grating filter according to claim 5, characterized in that: The chirped fiber gratings have different line widths or thicknesses.

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