CN114660705B - Method for writing optical fiber Bragg grating surface by surface based on high-refractive index matching liquid - Google Patents

Abstract

A method for writing fiber Bragg gratings face by face based on high-refractive index matching liquid, setting the repetition frequency and power parameters of the femtosecond laser, and the moving distance of the three-dimensional electric translation stage, namely the written grating length parameters and moving speed; select a section on the optical fiber The area is used as the optical fiber grating area to be written, wipe it clean with alcohol, place the optical fiber grating area to be written on a glass slide, fix the two ends of the optical fiber with optical fiber clamps, and immerse the optical fiber on the glass slide with a refractive index matching solution >1.5. The writing area is covered with a cover glass, the refractive index matching liquid and the optical fiber form a plano-concave cylindrical lens, and the refractive index of the concave cylindrical lens is adjusted by the refractive index matching liquid; the femtosecond laser is turned on, and the fiber grating is written face by face. The distribution of the refractive index modulation area of the Bragg grating written by the surface-by-surface method in the present invention is more symmetrical in the fiber core, thereby greatly reducing the loss of the grating, and has the advantages of simple structure, high working efficiency, low loss and wide applicability.

Description

Method of writing fiber Bragg grating surface by surface based on high refractive index matching liquid

technical field

The invention belongs to the technical field of grating production, and in particular relates to a method for writing optical fiber Bragg gratings surface by surface based on a high refractive index matching liquid 12 .

Background technique

Femtosecond laser has extremely narrow pulse width and high peak power. The interaction with materials is mainly based on nonlinear effects such as multiphoton absorption and avalanche ionization. It can be used to process a variety of materials, especially transparent media. Therefore, femtosecond laser has a wide range of applications in the field of fiber grating preparation.

Fiber Bragg gratings are widely used in fiber lasers, fiber optic communications and fiber optic sensing. Fiber Bragg Grating has the advantages of low insertion loss, strong anti-electromagnetic interference ability, small size, light weight, and easy realization of wavelength division multiplexing. Fiber gratings can be used as fiber internal mirrors to realize all-fiber lasers, which have the advantages of compact structure and good stability, which greatly promotes the development of fiber lasers. The emergence of fiber gratings has promoted the development of optical fiber communication and optical fiber sensing.

Fiber Bragg grating writing methods can be divided into two categories: internal writing method and external writing method. The internal writing method is to use the light beam to form an interference standing wave in the photosensitive fiber to realize the periodic refractive index modulation along the fiber axis. Hill et al. used the internal writing method to prepare the world's first fiber grating. The external writing method is to focus the laser from the side of the fiber to the core to form a grating, including holographic interference method, phase mask method, direct writing method, etc. The holographic interferometry has extremely high requirements on the stability of the two-beam interference device, which increases the difficulty and cost of grating preparation. In contrast, the grating preparation technology of the phase mask method is relatively mature. When the laser passes through the phase mask, it will generate zero-order and symmetrically distributed ±1-order diffracted lights. The zero-order diffracted light is generally suppressed, and the ±1st-order diffracted light with high diffraction efficiency undergoes double-beam interference to form interference fringes to realize periodic refractive index modulation in the fiber. However, the fiber grating prepared by the phase mask method depends on the period of the phase mask, so the flexibility is poor, and the mask is expensive. The femtosecond laser direct writing method uses a microscopic objective lens to focus the femtosecond laser into the fiber, and writes the periodic structure of the grating one by one by precisely controlling the movement of the fiber.

At present, there are many methods to realize the preparation of fiber grating by femtosecond laser direct writing. Among them, the point-by-point direct writing method generally uses a microscopic objective lens with a high numerical aperture to focus, which can induce the refractive index modulation area of the fiber to be smaller than 1 μm, but there is asymmetry in the horizontal and vertical directions. This localized asymmetric refractive index modulation region leads to a strong birefringence effect and a large insertion loss in the prepared grating. The line-by-line direct writing method uses a laser perpendicular to the fiber axis to scan the fiber core to prepare the periodic structure of the grating one by one. This method reduces the polarization-dependent loss of the prepared FBG to a certain extent, and the insertion loss is still relatively large. The line-by-line method needs to describe the grating structure one by one, which is time-consuming and is not conducive to the large-scale production of fiber gratings. Therefore prior art still needs to improve and improve.

Contents of the invention

The technology to be solved by the present invention is to overcome the shortcomings of the existing fiber grating preparation methods, and provide a method for writing fiber Bragg gratings surface by surface based on high refractive index matching liquid 12 with reasonable design, high work efficiency, low loss and wide applicability.

The technical solution adopted to solve the above technical problems is: a method of writing fiber Bragg gratings surface by surface based on high refractive index matching liquid. The platform is equipped with an optical switch, a plane mirror reflection group, an aperture, a plane mirror, a microlens, and a three-dimensional micro-processing platform. The micro-lens is provided with a CCD camera. The three-dimensional micro-processing platform is a three-dimensional electric displacement stage with a three-dimensional Manual adjustment frame, the three-dimensional right-angle manual adjustment frame is equipped with glass slides, optical fiber fixtures and LED lights. The LED lights are located under the glass slides. The CCD camera and three-dimensional electric translation stage are connected to the computer. One end is connected to an optical spectrum analyzer; the method comprises the following steps:

S1: Setting parameters

Set the repetition frequency and power parameters of the femtosecond laser, the moving distance of the three-dimensional electric stage, and the grating length parameters and moving speed written immediately, and the moving speed is set to 321.4 μm/s;

S2: Select a section of the optical fiber to be written and peel off the coating layer or the coated layer as the area of the optical fiber to be written with the grating, wipe the area of the optical fiber with the grating to be written with alcohol, and place the area of the optical fiber to be written with the grating on a glass slide. Both ends of the optical fiber are fixed with optical fiber clamps, and the area to be written on the optical fiber is immersed in a refractive index matching solution with a refractive index >1.5 on the glass slide, covered with a cover glass, and the refractive index matching solution and the optical fiber form a plano-concave cylindrical lens. The refractive index of the concave cylindrical lens is adjusted by the refractive index matching liquid;

S3: Adjust the initial state of the optical fiber to be written by adjusting the three-dimensional right-angle manual adjustment frame, that is, the fiber core can be clearly distinguished by the microlens, so that the femtosecond laser is aligned with the fiber core during the grating writing process;

S4: Writing Fiber Bragg Grating Surface by Surface

Turn on the femtosecond laser, and after the power of the femtosecond laser is stable, the three-dimensional electric translation stage is controlled by the computer, so that the three-dimensional electric translation stage moves along the axis of the optical fiber, and the fiber Bragg grating can be written surface by surface by turning on the optical switch, which can be detected in real time by the spectrum analyzer The written optical fiber grating spectrum, then turn off the optical switch and the three-dimensional electric translation stage, after the writing is completed, take out the written optical fiber grating and wipe it clean.

As a preferred technical solution, in the step S1, the repetition frequency of the femtosecond laser is set to 200 Hz, and the power is set to 0.9-6 μW; the moving distance of the three-dimensional electric translation stage 17 is set to 100 ~6000 μm.

As a preferred technical solution, the adjustment range of the refractive index of the plano-concave cylindrical lens in the step S2 is 1.51-1.78.

The beneficial effects of the present invention are as follows:

(1) The distribution of the refractive index modulation region of the Bragg grating written by the face-by-face method in the present invention is more symmetrical than that of the Bragg grating written by the point-by-point method in the prior art, thereby greatly reducing the polarization-dependent loss and insertion loss;

(2) The Bragg grating refractive index modulation area written by the surface-by-surface method of the present invention is more evenly distributed in the fiber core, which avoids the formation of a local chemical fiber core refractive index modulation area by the Bragg grating written by the point-by-point method in the prior art, and can suppress cladding mode response;

(3) The surface-by-surface grating writing method adopted in the present invention can control the shape and size of the grating refractive index modulation area by controlling the different refractive indices of the refractive index matching liquid and the pulse energy of the femtosecond laser.

(4) The method of writing gratings by the face-to-face method adopted in the present invention changes the depth and width of the structure coverage by adjusting the refractive index of the optical fiber, as well as the size of the refractive index change and the grating period, and can also realize optical fibers with various complex structures such as chirped gratings. Raster writing.

(5) The surface-by-surface method of the present invention is simple to prepare Bragg gratings, and can realize surface-to-surface writing in various types of optical fibers, and is a universal method for surface-by-surface writing of Bragg gratings.

(6) The present invention can write Bragg gratings face by face in optical fibers without removing the coating layer, and improve the mechanical properties of the gratings.

Description of drawings

Fig. 1 is a schematic structural diagram of a device for writing fiber Bragg gratings surface by surface based on a high refractive index matching liquid according to the present invention.

FIG. 2 is a schematic structural diagram of the three-dimensional micromachining platform 8 in FIG. 1 .

Fig. 3 is the spectrogram of the fiber grating written by the face-to-face method.

FIG. 4 is a schematic diagram of surface-to-surface writing of gratings under different high-refractive-index matching liquids 12 .

Fig. 5 is a photomicrograph and spectrum comparison diagram of a grating written by the point-by-point method with a refractive index of 1.45 and by the surface-by-surface method with a refractive index of 1.7.

Fig. 6 The spectrogram of the ultrashort grating pair written by the face-to-face method.

Among them: femtosecond laser 1; optical switch 2; plane mirror reflection group 3; aperture 4; CCD camera 5; plane mirror 6; microlens 7; three-dimensional micro-processing platform 8; computer 9; broadband light source 10; spectrometer 11; High refractive index matching liquid 12; optical fiber to be written 13; optical fiber fixture 14; LED lamp 15; three-dimensional right-angle manual adjustment frame 16;

Implementation

The present invention will be described in further detail below with reference to the drawings and examples, but the present invention is not limited to the following embodiments.

Example

In Figures 1 and 2, the present embodiment is based on a device for writing fiber Bragg gratings surface-to-face with a high-refractive-index matching liquid. An optical platform 18 is provided with a femtosecond laser 1, and an optical switch 2 is installed on the optical platform 18 along the laser emission direction. , plane mirror reflection group 3, diaphragm 4, plane mirror 6, microlens 7, three-dimensional micromachining platform 8, CCD camera 5 is installed on the microlens 7, CCD camera 5 is used for real-time monitoring grating writing process, three-dimensional micromachining platform The processing platform 8 is a three-dimensional electric displacement table 17 on which a three-dimensional right-angle manual adjustment frame 16 is installed. On the three-dimensional right-angle manual adjustment frame 16, a glass slide and an optical fiber fixture 14 and an LED light 15 are installed. The LED light 15 is located below the glass slide, and the optical fiber The optical fiber 13 to be written is clamped on the clip, the writing area of the optical fiber 13 to be written is located on the glass slide, and the refractive index matching liquid is dripped on the glass slide, so that the refractive index matching liquid is completely immersed in the writing area of the optical fiber 13 to be written, and the CCD The camera 5 and the three-dimensional electric translation platform 17 are connected to the computer 9, the image collected by the CCD camera 5 is displayed by the computer 9, and the three-dimensional electric translation platform 17 is controlled by the computer 9 at the same time, one end of the optical fiber 13 to be written is connected to the broadband light source 10, and the other end is connected to the spectral analysis In the process of writing fiber Bragg grating face to face, the spectrum of the fiber Bragg grating is monitored in real time by a spectrum analyzer.

The method for writing fiber Bragg gratings surface by surface based on high refractive index matching liquid in this embodiment includes the following steps:

S1: Setting parameters

The repetition frequency of the femtosecond laser is set to 200 Hz, the power is set to 0.9-6 μW, the moving distance of the three-dimensional electric translation stage 17 is 100-6000 μm, and the moving speed is 321.4 μm/s;

S2: Select an area on the optical fiber to peel off the coating layer or have the coating layer as the area of the optical fiber to be written grating, wipe the area of the optical fiber to be written with the grating, and place the area of the optical fiber to be written on the glass slide. Fix the end with an optical fiber clamp 14, immerse the area of the optical fiber to be written on the glass slide with a refractive index matching liquid of >1.5, cover with a cover glass, and the refractive index matching liquid and the optical fiber form a plano-concave cylindrical lens, thereby realizing beam shaping , making the focal volume wider in the transverse direction of the optical fiber. Different refractive index matching liquids are equivalent to plano-concave cylindrical lenses with different refractive powers. They have the ability of the focal volume of the spot to expand the beam in the transverse direction of the optical fiber. The formed flat The refractive index of the concave cylindrical lens is adjusted by the refractive index matching liquid, and the refractive index adjustment range of the plano-concave cylindrical lens is 1.51-1.78;

In this embodiment, refractive index matching liquids with a refractive index of 1.50, 1.60, and 1.70 are selected. The femtosecond laser can form different sizes of light spots in the lateral beam expansion of the optical fiber through the refractive index matching liquid, as shown in Figure 3;

S3: adjust the initial state of the optical fiber 13 to be written by adjusting the three-dimensional right-angle manual adjustment frame 16, that is, the fiber core of the optical fiber can be clearly distinguished under the microlens 7, so that the femtosecond laser is aligned with the fiber core during the grating writing process;

S4: Writing Fiber Bragg Grating Surface by Surface

Turn on the femtosecond laser 1, and after the power of the femtosecond laser 1 is stabilized, control the three-dimensional electric translation stage 17 through the computer 9, so that the three-dimensional electric translation stage 17 moves along the axial direction of the optical fiber, and then turn on the optical switch to write the fiber Bragg grating surface by surface. The microlens 7 and the CCD camera 5 record the microscopic appearance of the grating written point by point, and detect the optical fiber grating spectrum written in real time through the spectrum analyzer, as shown in Figure 4, and then turn off the optical switch 2 and the three-dimensional electric translation stage 17, and the writing is completed , take out the inscribed fiber grating and wipe it clean.

In order to verify the beneficial effects of the present invention, the inventor used the prior art femtosecond laser point-by-point method based on the refractive index matching liquid with a refractive index of 1.45 to write gratings and the present invention based on the refractive index matching liquid with a refractive index of 1.7. The gratings were compared, and the repetition frequency of the two femtosecond lasers was set to 200 Hz, the power was set to 1 μW, and the length of the written gratings were both 3 mm; a grating micrograph and spectral comparison diagram were formed, as shown in Figure 5.

The method of writing the grating in the refractive index matching liquid with a refractive index of 1.45 is point-by-point writing, which cannot be written side by side, and the diameter D of the refractive index modulation area of the grating is about 1 μm. In contrast, in the refractive index matching liquid with a refractive index of 1.70, the grating can be written side by side, and the diameter D of the refraction modulation area of the grating increases to 8 μm in the lateral direction. It can be concluded that at the same grating transmission depth (about -17.1 dB), the insertion loss and short-wave loss of the grating written by point-by-point method are -0.71dB and -3.09dB respectively, while the grating written in 1.70 refractive index matching liquid The insertion loss and short-wave loss of the grating can be reduced to -0.21dB and -0.87dB, which is lower than the method of writing fiber Bragg grating surface by surface based on the high refractive index matching liquid in the prior art.

Example

This embodiment provides a method for writing ultra-short gratings and ultra-short grating pairs face-to-face based on a high-refractive index matching liquid, wherein the repetition frequency of the laser is 200 Hz, and the power is 6 μW. Turn on the femtosecond laser 1, and wait for the power of the laser to stabilize The three-dimensional electric translation platform 17 is controlled by the computer 9, so that the three-dimensional electric translation platform 17 moves axially along the optical fiber, and the grating length is set to 100 μm. After writing an ultra-short grating, the three-dimensional electric translation platform 17 is controlled to move 3000 μm along the optical fiber. Write the second ultra-short grating with a length of 100 μm; then turn off the optical switch 2 and the three-dimensional electric translation stage 17, and the writing is completed, and take out the written optical fiber grating. Other operation steps are identical with embodiment 1.

So far, the writing of the ultrashort grating pair is completed, and the intrinsic F-P interference cavity can be formed by two gratings. The ultrashort grating pair written by the method of the present invention has excellent coherence characteristics, and the extinction ratio can reach more than 15 dB, as shown in Figure 6.

Claims (3)

1. A method for writing an optical fiber Bragg grating surface by surface based on a high-refractive index matching liquid is characterized by comprising the following steps: the device applied by the method is that a femtosecond laser (1) is arranged on an optical platform (18), an optical switch (2), a plane mirror reflection group (3), a diaphragm (4), a plane mirror (6), a micro lens (7) and a three-dimensional micro processing platform (8) are arranged on the optical platform (18) along the laser emergent direction, a CCD camera (5) is arranged on the micro lens (7), a three-dimensional right angle manual adjusting frame (16) is arranged on the three-dimensional micro processing platform (8) which is a three-dimensional electric displacement platform (17), a glass slide, an optical fiber clamp (14) and an LED lamp (15) are arranged on the three-dimensional right angle manual adjusting frame (16), the LED lamp (15) is positioned below the glass slide, the CCD camera (5) and the three-dimensional electric displacement platform (17) are connected with a computer (9), one end of an optical fiber (13) to be inscribed is connected with a broadband light source (10), and the other end of the optical fiber is connected with a spectrum analyzer (11); the method comprises the following steps:

s1, setting parameters

Setting repetition frequency and power parameters of femto-second laser, and setting the moving distance of a three-dimensional electric displacement table (17), namely the length parameters and moving speed of a inscribed grating, wherein the moving speed is 321.4 mu m/s;

s2, selecting a section of area on the optical fiber (13) to be inscribed, stripping off a coating layer or taking the coating layer as an optical fiber grating area to be inscribed, wiping the optical fiber grating area to be inscribed with alcohol, placing the optical fiber grating area to be inscribed on a glass slide, fixing two ends of the optical fiber by an optical fiber clamp (14), immersing the optical fiber grating area to be inscribed on the glass slide by using an index matching liquid with the index of refraction being more than 1.5, covering the glass slide, covering the index matching liquid and the optical fiber to form a plano-concave cylindrical lens, and regulating the index of refraction of the plano-concave cylindrical lens formed by the index matching liquid;

s3, adjusting an initial state of the optical fiber (13) to be inscribed by adjusting the three-dimensional right-angle manual adjusting frame (16), namely clearly distinguishing an optical fiber core under the microlens (7), so that femtosecond laser is aligned to the optical fiber core in the grating inscription process;

s4, writing fiber grating face by face

The method comprises the steps of starting a femtosecond laser (1), controlling a three-dimensional electric displacement table (17) through a computer (9) after the power of the femtosecond laser (1) is stable, enabling the three-dimensional electric displacement table (17) to move along the axial direction of an optical fiber, starting a light beam (2) to write an optical fiber Bragg grating one by one, detecting the spectrum of the written optical fiber grating in real time through a spectrum analyzer (11), then closing the light switch (2) and the three-dimensional electric displacement table (17), taking out the written optical fiber grating and wiping the same.

2. The method for writing the fiber bragg grating surface by surface based on the high-refractive index matching liquid as claimed in claim 1, wherein the method comprises the following steps: in the step S1, the repetition frequency of the femtosecond laser is set to be 200 Hz, and the power is set to be 0.9-6 mu W; the moving distance of the three-dimensional electric displacement table (17), namely the length of the inscribed grating, is set to be 100-6000 mu m.

3. The method for writing the fiber bragg grating surface by surface based on the high-refractive index matching liquid as claimed in claim 1, wherein the method comprises the following steps: the refractive index adjusting range of the plano-concave cylindrical lens in the step S2 is 1.51-1.78.

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