WO2022165959A1

WO2022165959A1 - Chirped fiber grating, preparation method therefor, and chirped fiber grating filter

Info

Publication number: WO2022165959A1
Application number: PCT/CN2021/084147
Authority: WO
Inventors: 廖常锐; 王义平; 杨凯明; 蔡智濠; 刘博男
Original assignee: 深圳大学
Priority date: 2021-02-03
Filing date: 2021-03-30
Publication date: 2022-08-11

Abstract

A preparation method for a chirped fiber grating, comprising the following steps: step S1: performing morphological analysis on a target spectrum of a chirped fiber grating, fitting and approximating the target spectrum by using a plurality of Bragg grating spectra, and decomposing the target spectrum into a plurality of Bragg grating spectra; step S2: performing parameter derivation on each decomposed Bragg grating spectrum by using a Bragg grating formula, so as to obtain preparation parameters of the chirped fiber grating; and step S3: preparing the chirped fiber grating by means of a laser according to the preparation parameters. Therefore, carrying out adaptable and flexible preparations according to spectral requirements of chirped fiber gratings is achieved. A chirped fiber grating filter, comprising a chirped fiber grating obtained and prepared by means of the foregoing preparation method, and complex filtering spectral requirements are achieved by means of a single device.

Description

Chirped fiber grating, preparation method thereof, and chirped fiber grating filter

technical field

The invention relates to the technical field of fiber gratings, in particular to a chirped fiber grating, a preparation method thereof, and a chirped fiber grating filter.

Background technique

Existing fiber grating filters are mainly obtained by preparing chirped gratings by an ultraviolet laser mask method, and a filter customization method based on cascaded long-period fiber gratings.

technical problem

The chirped grating was fabricated by UV laser mask method. The UV laser mask method is currently the mainstream method for preparing chirped fiber gratings, and high-quality chirped gratings can be prepared by the mask method. However, using the UV laser mask method has the following disadvantages: the preparation of the chirped fiber grating often requires hydrogen-carrying treatment of the fiber to increase the photosensitivity, high time cost and high experiment risk; limited by the flexibility of the mask, the prepared The parameters such as the period and chirp amount of the chirped fiber grating cannot be adjusted flexibly, and chirped gratings with different parameters need to be prepared using different masks, which is costly.

Filter customization method based on cascaded long-period fiber gratings: Long-period fiber gratings are prepared by CO ₂ laser processing, femtosecond laser processing, etc., and by cascading multiple fiber gratings with different grating periods and grating lengths to achieve specific filter spectral shape 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; the use of long-period fiber gratings to match a specific filter spectral shape requires The number of cascaded devices is large and the overall size is large, which is not conducive to high integration.

technical solutions

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.

A preparation method of a chirped fiber grating provided by the present invention comprises the following steps:

Step S1: perform topography analysis on the target spectral shape of the chirped fiber grating, use multiple Bragg grating spectra to fit and approximate the target spectral shape, and decompose the target spectral shape into multiple Bragg grating spectra;

Step S2: use the Bragg grating formula to deduce the parameters of each decomposed Bragg grating spectrum to obtain the preparation parameters of the chirped fiber grating;

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

As an embodiment of the preparation method of the chirped fiber grating provided by the present invention, the preparation method of the chirped fiber grating further comprises the following steps:

Step S4: Compare the spectrum of the prepared chirped fiber grating with the target spectral shape, if the difference between the spectrum of the chirped fiber grating and the target spectral shape exceeds a preset error value, optimize the preparation parameters, and execute the steps S3.

As an embodiment of the preparation method of 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.

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

As an embodiment of the preparation method of the chirped fiber grating provided by the present invention, the preparation parameters include grating parameters and processing parameters;

First use the Bragg grating formula to deduce the parameters of the decomposed Bragg grating spectrum to obtain the grating parameters of the chirped fiber grating; then obtain the processing parameters of the chirped fiber grating.

As an embodiment of the preparation method of the chirped fiber grating provided by the present invention, the grating parameters include the initial period and the final period of the chirped fiber grating, and the chirp amount.

As an embodiment of the preparation method of the chirped fiber grating provided by the present invention, the processing parameters of the chirped fiber grating include laser energy, laser repetition frequency, and processing speed.

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

The present invention also provides a chirped fiber grating, including an optical fiber and a chirped fiber grating formed in the optical fiber; the chirped fiber grating has grating segments with different modulation amounts.

As an embodiment of the chirped fiber grating provided by the present invention, the chirped fiber grating has different line widths/thicknesses.

As an embodiment of the chirped fiber grating provided by the present invention, the chirped fiber grating is prepared by the above-mentioned preparation method.

The present invention also provides a chirped fiber grating filter, including the above-mentioned chirped fiber grating.

beneficial effect

This application proposes for the first time a method for preparing a chirped fiber grating with a customizable spectral shape. Compared with the existing fiber grating filter preparation technology, the outstanding advantage of the present invention is that the high flexibility of laser processing is used to realize the The spectral shape of the chirped fiber grating requires flexible and flexible preparation, and the prepared chirped fiber grating has low insertion loss.

The chirped fiber grating filter obtained in the present application has high spectral quality and accurate spectral shape matching. Compared with the existing cascade long-period fiber grating filter, the chirped fiber grating filter of the present application can achieve complex filtering spectral shape requirements through a single device; Market prospects and practical significance.

Description of drawings

1 is a schematic diagram of a processing optical path system of a chirped fiber grating provided by the application;

Fig. 2 is a kind of line-by-line chirped fiber grating structural schematic diagram of the application;

Fig. 3 is a kind of point-by-point chirped fiber grating structural schematic diagram of the application;

4 is a schematic structural diagram of a line-by-line chirped fiber grating with different grating modulation amounts of the present application;

Fig. 5 is the schematic diagram of the fitting process of target spectral shape in the preparation method of the chirped fiber grating of the application;

6 is a schematic diagram of the spectral shape of the chirped fiber grating prepared by the chirped fiber grating preparation method of the present application;

Figure 7 is a schematic diagram showing the comparison between the target spectral shape and the prepared chirped fiber grating spectral shape.

Embodiments of the present invention

In order to make those skilled in the art better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only Embodiments are part of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", " Rear, Left, Right, Vertical, Horizontal, Top, Bottom, Inner, Outer, Clockwise, Counterclockwise, Axial, The orientations or positional relationships indicated by "radial direction", "circumferential direction", etc. are based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying the indicated devices or elements. It must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be construed as a limitation of the present invention.

In addition, the terms "first" and "second" are only used for descriptive purposes, and should not be construed as indicating or implying relative importance or implying the number of indicated technical features. Thus, a feature delimited with "first", "second" may expressly or implicitly include at least one of that feature. In the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise expressly and specifically defined.

In the present invention, unless otherwise expressly specified and limited, the terms "installed", "connected", "connected", "fixed" and other terms should be understood in a broad sense, for example, it may be a fixed connection or a detachable connection , or integrated; it can be a mechanical connection or an electrical connection or can communicate with each other; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two components or the interaction relationship between the two components, unless otherwise expressly qualified. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

The technical solutions of the present invention will be further described in detail below through the accompanying drawings and embodiments.

The preparation method of a kind of chirped fiber grating proposed in this application comprises the following steps:

Step S1: perform topography analysis on the target spectral shape of the chirped fiber grating, use multiple Bragg grating spectra to fit and approximate the target spectral shape by means of simulation, and decompose the target spectral shape into multiple Bragg grating spectra;

In step S1, first analyze the spectral shape of the target spectral shape (such as the filtering spectral shape) expected by the grating, use multiple Bragg grating spectra to approximate the target spectral shape, and simulate the target spectral shape by means of simulation. First, it is split into the superposition of multiple Bragg grating spectra to achieve a reasonable discretization decomposition of the target spectral shape. Multiple Bragg grating spectra are simply referred to as fitted spectral shapes. As shown in Figure 5, the solid line spectrum is any filter target spectral shape, and the dashed line represents the Bragg grating spectrum. Multiple Bragg grating spectra are used to approximate the target spectral shape. The higher the accuracy of matching the target spectral shape. The appropriate number of Bragg grating spectra can be selected according to actual needs and error range.

After the target spectral shape is decomposed in step S1, step S2 uses the Bragg grating formula to inversely derive the preparation parameters of the chirped fiber grating.

In step S2, the Bragg grating formula is the following formula (1).

Formula (1),

where n _eff is the effective refractive index constant, m and Λ are the order and period of the grating, respectively, and λ is the Bragg grating resonance wavelength (operating wavelength).

The preparation parameters in step S2 include grating parameters and processing parameters. Firstly, the Bragg grating formula is used to deduce the parameters of the decomposed Bragg grating spectrum, and the grating parameters of the chirped fiber grating are obtained; 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 amount of chirp. The processing parameters of chirped fiber grating include laser energy, laser repetition frequency, and processing speed.

Using the Bragg grating formula, the parameters of each decomposed Bragg grating spectrum are deduced, and the initial period and the final period of the chirped fiber grating can be obtained through the parameters of the Bragg grating spectrum. The function form corresponding to the chirp.

At the same time, the overall coupling coefficient range of the chirped fiber grating is first determined according to the target spectral shape, and then the coupling coefficients at different positions of the chirped fiber grating are estimated according to the difference in the reflection intensities of different Bragg gratings in the fitted spectral shape, and the actual coupling coefficient is set reasonably according to the Parameters such as laser energy and processing speed during preparation and processing.

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

In step S3, the optical fiber is fixed on the three-dimensional displacement platform during preparation, preferably, a high-magnification and large numerical aperture microscope objective 7 is used as the processing objective, 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 preparation parameters obtained by the calculation in the preparation software of the control device 9 . Figure 6 is a schematic diagram of the spectral shape of the prepared chirped fiber grating, and the parameters of the chirped fiber grating are determined by a series of Bragg gratings. The solid line in Fig. 6 represents the chirped fiber grating, and the dashed line represents the Bragg grating.

The laser in step S3 is a femtosecond laser.

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

The laser processing optical path system includes a laser 1 , an optical power attenuator 2 , a mirror 3 , a diaphragm 4 , an electronically controlled shutter 5 , a dichroic mirror 6 , a microscope objective lens 7 , and a CCD camera 10 . Laser 1 is a femtosecond laser. The displacement platform is a three-dimensional displacement platform 8 . The control device may be a computer 9 . The optical power attenuator 2 adjusts the power of the femtosecond laser through polarization selection. The reflecting mirror 3 and the dichroic mirror 6 are used to adjust the optical path. After the femtosecond laser is emitted by the laser 1, it passes through the optical power attenuator 2, the mirror 3, and the aperture 4 in turn. The appropriate laser processing power is obtained by the polarization state of the laser, and the opening and closing degree of the aperture is adjusted to obtain the appropriate spot size; The electronically controlled shutter 5 and mirror 6 reach the microscope objective lens 7 for focusing; after the laser is focused, it 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. The CCD camera 10 The processing situation can be observed in real time. The laser 1 , the electronically controlled 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.

A calculation module can be set in the computer, the calculation module has a built-in calculation program, and the target spectral shape is input in the computer 9, and the calculation module of the computer 9 can automatically decompose the target spectral shape into a plurality of Bragg grating spectra, and according to the Bragg grating formula, for The parameters of each decomposed Bragg grating spectrum are deduced, and finally the fabrication parameters of the chirped fiber grating are obtained.

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

In the present application, the chirped fiber grating is processed by femtosecond laser, the parameters can be adjusted in time, the preparation is flexible, the processing speed is fast, and the efficiency is high. Using the precision of femtosecond laser micromachining, the core of the optical fiber can be inscribed with certain regular structures, such as line arrays, point arrays, etc., so as to modulate the refractive index of the optical fiber. Using the advantages of the flexibility of fabricating fiber gratings by femtosecond laser processing, the period of the chirped fiber grating can be regularly changed under the premise of controllability, so as to obtain the chirped spectrum that meets the actual needs.

Through the above steps S1 to S3, the chirped fiber grating can be prepared.

A chirped fiber grating of the present application includes an optical fiber, and the structure of the chirped fiber grating is directly written in the optical fiber by femtosecond laser technology. The chirped fiber grating may have a line-by-line chirped fiber grating structure, as shown in Figure 2; The chirped fiber grating can also be a point-by-point chirped fiber grating structure. As shown in FIG. 3 , the chirped fiber grating of a point array is formed on the core 22 .

Chirped fiber gratings are fiber gratings with non-uniform grating periods along the axial direction of the fiber. The existing chirped fiber gratings have the same grating modulation amount. As shown in Figure 2, different grating segments composing chirped fiber gratings With the same line width/thickness, it is impossible to control the intensity of each wavelength position of the grating spectrum, and its characteristic spectrum is a flat spectrum.

The application provides a chirped fiber grating, and its grating segments have different modulation amounts. The modulation amount at different positions of the grating can be precisely controlled, and the grating of the chirped fiber grating can have different modulation amounts at different positions, so that the details of the grating spectrum can be controlled by changing the modulation amount at the corresponding position. It can be understood that the grating segments of the chirped fiber grating have different modulation amounts, and some of the grating segments may have the same modulation amount, and another part of the grating segments have different modulation amounts, and the modulation amounts of the gratings at different positions can be adjusted according to requirements; or , or grating segments at different positions have different modulation amounts. A chirped fiber grating of the present application has different line widths/thicknesses, as shown in FIG. 4 .

The specific chirp quantity can be controlled by the chirp quantity function, The essence of the chirp is the uneven change of the period, and its change rule can be in the form of a linear function, or it can change in the form of a nonlinear function such as a quadratic function.

In order to further optimize the performance of the chirped fiber grating obtained by the preparation, the preparation method of the chirped fiber grating further comprises the following steps:

Step S4: Compare the spectrum of the prepared chirped fiber grating with the target spectral shape, if the difference between the spectrum of the chirped fiber grating and the target spectral shape exceeds a preset error value, optimize the preparation parameters, and execute the steps S3: According to the optimized preparation parameters, the chirped fiber grating is prepared by laser.

By optimizing the fabrication parameters, chirped fiber gratings that better match the expected target spectral shape can be further fabricated. In order to obtain a high-quality chirped fiber grating, step S4 can be performed multiple times to realize multiple optimization iterations of the preparation parameters, and finally a chirped fiber grating filter that is highly matched with the target spectral shape within the error range can be obtained.

In step S4, by analyzing the difference between the spectrum of the chirped fiber grating and the target spectral shape, parameters such as the grating period, chirping amount, laser energy, processing speed, and laser repetition frequency used for processing and preparation are selected according to specific conditions. One or more of the optimization iterations are performed. Figure 7 is a schematic diagram showing the comparison between the target spectral shape and the prepared chirped fiber grating spectral shape.

Specifically, the difference between the two spectra can be analyzed, and different preparation parameter optimization methods can be carried out according to the following situations:

If the overall difference between the two is large in the spectral wavelength range, the grating period and chirp amount need to be replaced and iterated; in this case, the preparation parameters need to be greatly adjusted, including, the Bragg grating spectrum in step S1 may need to be adjusted. The number N is adjusted, so that the grating parameters in the preparation parameters are changed; and the preparation parameters in step S2, such as processing parameters, are also adjusted.

If there is a large difference in spectral intensity between the two as a whole, it is necessary to adjust some of the preparation parameters during the grating preparation in step S2, such as laser energy, processing speed, laser repetition frequency, etc.; the grating period, chirp amount, number of periods, etc. can be maintained Keeping the same, adjust the prepared energy and change the modulation amount of the grating for matching.

If the spectrum is only different in some areas, the processing parameters of other areas can be kept unchanged, and the processing parameters can be adjusted only for the areas with large differences.

In the present application, compared with the preparation method of the chirped fiber grating by the ultraviolet laser mask method, the preparation method of the chirped fiber grating of the present application has higher flexibility, and can adjust the target spectral shape (filtering spectral shape) according to the requirements of the special target spectral shape The spectral shape is decomposed into a plurality of Bragg grating spectra, and the required grating preparation parameters are deduced. The desired grating can be obtained directly by inputting the parameters in the control device, that is, the computer's processing and preparation software.

In addition, femtosecond laser processing to prepare chirped fiber gratings does not require additional preprocessing of the fiber, nor does it have specific requirements for the type of fiber, and the chirped fiber grating can be efficiently prepared on any type of fiber.

Existing filters based on cascaded long-period fiber gratings usually require multiple devices to be cascaded, and are difficult to match with complex filtering spectral shapes. However, by using the preparation method of the chirped fiber grating of the present application, the preparation parameters of the grating can be customized according to the target filtering spectral shape, so that a single device can meet the complex filtering requirements. A single chirped fiber grating can greatly reduce the size of the filter, which is of great significance to the improvement of device integration.

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

The invention utilizes laser processing to prepare a chirped fiber grating filter whose spectral shape can be customized, and can obtain the required filtering spectral shape by flexibly changing the preparation parameters. The filtering spectrum shape of the chirped fiber grating filter is variable, and the prepared chirped fiber grating filter has high filtering spectrum shape accuracy, high manufacturing efficiency and low manufacturing cost. Therefore, the present invention can satisfy complex filtering requirements with a single device in the field of erbium-doped fiber amplifier (EDFA) gain flattening, high-power fiber laser scattered light filtering, etc., and has good market prospect and application value.

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

Obviously, the above-described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The accompanying drawings show the preferred embodiments of the present application, but do not limit the scope of the patent protection of the present application. This application may be embodied in many different forms, rather these embodiments are provided so that a thorough and complete understanding of the disclosure of this application is provided. Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing specific embodiments, or perform equivalent replacements for some of the technical features. Any equivalent structure made by using the contents of the description and drawings of the present application, which is directly or indirectly used in other related technical fields, is also within the scope of protection of the patent of the present application.

Claims

A preparation method of chirped fiber grating is characterized in that, comprises the following steps:

Step S1: perform topography analysis on the target spectral shape of the chirped fiber grating, use multiple Bragg grating spectra to fit and approximate the target spectral shape, and decompose the target spectral shape into multiple Bragg grating spectra;

Step S2: use the Bragg grating formula to deduce the parameters of each decomposed Bragg grating spectrum to obtain the preparation parameters of the chirped fiber grating;

Step S3: According to the preparation parameters, the chirped fiber grating is prepared by laser.
The preparation method of chirped fiber grating according to claim 1, is characterized in that, also comprises the following steps:

Step S4: Compare the spectrum of the prepared chirped fiber grating with the target spectral shape, if the difference between the spectrum of the chirped fiber grating and the target spectral shape exceeds a preset error value, optimize the preparation parameters, and execute the steps S3.
The method for preparing a chirped fiber grating according to claim 2, wherein the method for optimizing the preparation parameters comprises: adjusting the number of Bragg grating spectra in step S1, and adjusting the preparation parameters in step S2 .
The method for preparing a chirped fiber grating according to claim 2, wherein the method for optimizing the preparation parameters comprises: adjusting the preparation parameters in step S2.
The method for preparing a chirped fiber grating according to claim 1, wherein the preparation parameters include grating parameters and processing parameters;

First use the Bragg grating formula to deduce the parameters of the decomposed Bragg grating spectrum to obtain the grating parameters of the chirped fiber grating; then obtain the processing parameters of the chirped fiber grating.
The method for preparing a chirped fiber grating according to claim 5, wherein the grating parameters include the initial period and the final period of the chirped fiber grating, and the chirp amount.
The method for preparing a chirped fiber grating according to claim 4, wherein the processing parameters of the chirped fiber grating include laser energy, laser repetition frequency, and processing speed.
The method for preparing a chirped fiber grating according to claim 1, wherein the laser in step S3 is a femtosecond laser.
A chirped fiber grating is characterized in that it 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.
The chirped fiber grating according to claim 9, wherein the chirped fiber grating has different line widths/thicknesses.
The chirped fiber grating according to claim 9, wherein the chirped fiber grating is prepared by the preparation method described in any one of claims 1-8.
A chirped fiber grating filter, characterized by comprising the chirped fiber grating according to any one of claims 9 to 11.