CN111552024A - Special-shaped core fiber grating preparation technology based on 120-degree included angle reflection exposure superposition - Google Patents

Abstract

The invention provides a device and a method for preparing a special-shaped core fiber grating based on reflection exposure superposition with an included angle of 120 degrees, wherein the device comprises a phase mask plate, a quartz slice, a fiber support plate and fiber matching liquid, wherein one surface of the fiber support plate is provided with a V-shaped groove plated with a metal reflection film and used for placing an optical fiber to be inscribed, the quartz slice covers the fiber support plate, the phase mask plate covers the quartz slice, and the fiber matching liquid is used for filling a gap between the V-shaped groove on the fiber support plate and the optical fiber to be inscribed. The method comprises the steps of device installation, device adjustment, optical fiber preparation, grating inscription, preparation completion and the like, can effectively improve the light intensity distribution of an inscription beam in the cross section of the optical fiber, and is suitable for grating inscription of various special-shaped core optical fibers.

Description

Fabrication technology of special-shaped core fiber grating based on 120-degree angle reflection and exposure stacking

(1) Technical field

The invention relates to the technical field of fiber grating preparation, in particular to a fiber grating preparation technology suitable for special-shaped core fibers.

(2) Background technology

Fiber grating refers to a grating structure formed by periodically changing the refractive index on the fiber core by certain technical means. Fiber Bragg gratings (FBGs) written on single-mode fibers are the most mature fiber gratings currently studied. This type of grating has excellent optical narrowband filtering and sensing properties, and has been widely used in the field of optoelectronics and fiber sensing. . In recent years, in order to meet the needs of communication and sensing, many special fibers have been designed and produced, such as few-mode fibers, multi-core fibers, ring-core fibers, multi-clad fibers, etc. Increasing year by year, such as: Wei Ying (Wei Ying, Jiao Xingxing. Experimental study on the sensing characteristics of polarization-maintaining fiber Bragg gratings [J]. Infrared and Laser Engineering, 2008(37): 107-110.) pointed out that in the polarization-maintaining fiber The FBGs written on the fast axis and the slow axis have different sensitivity to lateral loads, which can be applied to fields such as weighing; Bao Weijia (Bao Weijia. Research on New Fiber Bragg Grating Vector Strain Sensor [D]. Northwestern University, 2018.) Researched writing FBG on multi-clad fiber and realized vector strain measurement; E. Lindley (LINDLEY E, MIN S-S, LEON-SAVAL S, et al. Demonstration of uniform multicorefiber Bragg gratings [J]. Optics Express, 2014, 22 (25):31575.) carried out the research on the FBG writing technology of multi-core fiber, which can be used to suppress the noise signal in space exploration, and has a very important application prospect in the field of astrophotonics.

The phase mask method is a widely used method for writing fiber gratings. This method usually uses ultraviolet light to irradiate the phase mask to form diffraction fringes, and uses the ±1st order diffraction fringes to laterally expose the photosensitive fiber to prepare FBG. The method greatly reduces the coherence requirement of the light source, and the Bragg wavelength of the prepared FBG only depends on the fringe period of the phase mask, which reduces the difficulty of the grating preparation process. As the most commonly used FBG preparation method, the phase mask method based on ultraviolet laser has laid the foundation for the practical and industrialization of FBG.

The phase mask method based on ultraviolet laser has also been tried to write gratings for special fibers. However, because the structure of special fibers is very different from that of single-mode fibers, some special fibers belong to special-shaped core fibers, that is, the waveguide core of such fibers. Not in the center of the fiber, but distributed throughout the fiber interface. Therefore, when a phase mask is used to prepare fiber gratings, due to the cylindrical lens effect of the fiber itself, it is difficult for the ±1st order diffracted beams emitted from the mask to form a uniform light intensity distribution on the entire fiber cross section, so it is difficult to prepare high-quality fiber gratings. fiber grating.

To solve the above problems, there are currently two methods available:

(1) Literature (LINDLEY E, MIN S-S, LEON-SAVAL S, et al.Demonstration of uniformmulticore fiber Bragg gratings[J].Optics Express,2014,22(25):31575.) proposed an improved method: select A section of quartz capillary with a suitable size, grind one side of the capillary to a certain thickness and then polish it, insert the optical fiber to be written into the capillary, and make the diffracted beam of the phase mask irradiate from the side of the capillary to eliminate the cylindrical lens effect of the optical fiber itself.

(2) The patent (authorization number: CN 106249348B) proposes a method for writing an apodized fiber grating, which proposes to rotate the optical fiber to be written while writing the grating, so as to eliminate the asymmetry of the photosensitive refractive index modulation caused by the large core diameter.

In the above method (1), the process of side-throwing the quartz capillary is complicated, and the processing time is long, and in order to allow the optical fiber to be inserted into the capillary, the inner diameter of the capillary must be larger than the diameter of the optical fiber, and the air filled between the two will still affect the The writing result has an impact; in method (2), the fiber to be written is rotated while the FBG is writing, and the slight asymmetry of the optical path or the slight vibration of the rotating motor will affect the writing effect of the FBG.

(3) Contents of the invention

The purpose of the present invention is to provide a special-shaped core fiber grating preparation technology based on 120-degree angle reflection exposure stacking. This technology can evenly write the distribution of the light beam in the fiber and improve the writing quality of the grating.

The object of the present invention is achieved in this way:

A grating preparation device suitable for special-shaped core optical fibers is invented. The device is composed of a phase mask plate, a quartz sheet, an optical fiber carrier plate and an optical fiber matching liquid. Covered on the optical fiber carrier, the phase mask is covered on the quartz sheet, and the optical fiber matching liquid is used to fill the gap between the V-shaped groove on the optical fiber carrier and the optical fiber to be written;

For the above-mentioned phase mask, various phase masks for FBG writing can be selected;

The above-mentioned quartz flakes are not less than the phase mask in length and width, and have a thickness of 50 microns. They are made of quartz glass and have high transmittance in the ultraviolet wavelength band used for writing, which can transmit the diffracted beam generated by the phase mask to the lower layer without damage. Optical fiber carrier, while protecting the phase mask from being polluted by the optical fiber to be written and the optical fiber matching liquid on the optical fiber carrier;

The above-mentioned optical fiber carrier has a length and width not less than the quartz flakes, and the material used has high transmittance in the ultraviolet wavelength band for writing. The central area of one side of the carrier has a V-shaped groove. The two sides are coated with metal reflective film, which has high reflectivity in the ultraviolet light band for writing. The angle between the two sides of the groove is 120 degrees. The groove depth H and groove width W are determined by the diameter D of the fiber cladding to be written. The calculation formula is:

The above-mentioned optical fiber matching solution uses water as a solvent and glycerin as a solute, and has high transmittance for writing in the ultraviolet wavelength band. The refractive index consistent with the optical fiber cladding can be obtained by precisely controlling the ratio of water and glycerol.

A method for preparing a grating suitable for a special-shaped core fiber is invented, comprising the following steps:

Step 1, device installation: place the grating preparation device of the present invention on the grating writing platform, and adjust the writing beam to vertically irradiate the phase mask of the grating preparation device from top to bottom;

Step 2, device adjustment: remove the phase mask and the quartz sheet of the grating preparation device, adjust the position and height of the optical fiber carrier, so that the writing ultraviolet beam is focused in the V-shaped groove of the optical fiber carrier;

Step 3, fiber preparation: strip the coating layer of the area to be written on the optical fiber to be written, the stripping length is greater than the length of the V-shaped groove of the optical fiber carrier, wipe it with alcohol, put it into the V-shaped groove, and drop an appropriate amount of optical fiber to match liquid, gently move the fiber to be written, so that the fiber matching liquid completely fills the gap between the V-groove and the fiber to be written, cover the silica sheet on the fiber carrier, and ensure that the air between the two is excluded, and the phase The mask plate is covered on the quartz sheet, and the optical fiber to be written is fixed by the optical fiber fixture, and the optical fiber to be written is connected to the grating and the on-line monitoring system;

Step 4, grating writing: start the laser for grating writing, use the grating writing online monitoring system to observe the grating writing effect, when the grating meets the writing requirements, turn off the laser and the grating writing online monitoring system;

Step 5, complete the preparation: disconnect the optical fiber from the grating writing online monitoring system, remove the phase mask and the quartz sheet of the grating preparation device, take out the optical fiber from the V-shaped groove of the optical fiber carrier, and complete the grating preparation.

Compared with the prior art, the advantages of the present invention are:

(1) The use of optical fiber matching liquid eliminates the convergence effect after the beam enters the optical fiber, so that the distribution area of the writing beam on the cross-section of the optical fiber is wider and the light intensity distribution is more uniform;

(2) The reflective film of the V-shaped groove improves the utilization rate of the writing beam, realizes the irradiation of the optical fiber from three directions, and further improves the distribution and intensity of the writing beam in the cross-section of the optical fiber;

(3) The writing device can be produced by using the existing micro-machining technology, with short production cycle and high machining accuracy;

(4) The operation steps of the writing process are simple, the rapid writing of the grating can be realized, and the consistency of the grating is guaranteed;

(4) Description of drawings

Fig. 1 is the device structure schematic diagram of the present invention;

Fig. 2 is a part of optical fiber types applicable to the present invention: (a) a schematic diagram of the structure of a single-mode fiber, (b) a schematic diagram of the structure of a multi-mode fiber, (c) a schematic diagram of the structure of a ring core fiber, (d) a schematic diagram of the structure of a multi-core fiber;

3 is a schematic diagram of the principle of irradiating the optical fiber with the writing beam in the V-shaped groove;

Fig. 4 is the top view of embodiment 2;

Fig. 5 is the side view of embodiment 2;

In the figure: 1- Grating preparation device for special-shaped core fiber, 101- Phase mask, 102- Quartz sheet, 103- Fiber carrier, 104- Fiber matching liquid, 2- Optical platform, 3- Excimer laser, 4- -Aperture, 5-beam expander, 6-reflector, 7-cylindrical lens, 8-three-axis stage, 9-fiber to be written, 10-fiber clamp, 11-fiber V-groove connector, 12-grating Writing online monitoring system, 13-writing beam.

(5) Specific implementation methods

The present invention proposes a special-shaped core fiber grating preparation technology based on 120-degree angle reflection and exposure stacking. Fig. 2 lists some of the applicable fibers of the present invention. Preparation of gratings for mode and multimode fibers. The selection of the optical fiber has no influence on the preparation device and preparation method of the present invention, so the specific optical fiber type is not specified in the following examples.

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

Example 1:

In this embodiment, the structure and working principle of the grating preparation device of the present invention will be described with reference to FIG. 1 and FIG. 3 .

宽度

式中D代表光纤包层直径，例如，针对单模光纤，其包层直径D＝125±5微米，取D＝130微米，计算可知槽深不小于140微米，槽宽不小于485微米。As shown in FIG. 1, the grating preparation device 1 of the present invention includes a phase mask 101, a quartz sheet 102, an optical fiber carrier 103 and an optical fiber matching liquid 104. The phase submerged plate 101 is covered on the quartz sheet 102, and the quartz sheet 102 is covered with On the optical fiber carrier 103; the phase mask 101 can be selected from various phase masks for writing FBG. The length and width of the quartz sheet 102 are not less than the phase mask 101, the thickness is 50 microns, and the material is quartz glass. With high transmittance, the function of the quartz sheet 102 is to separate the phase mask 101 from the optical fiber carrier 103 by a certain distance, which can protect the phase mask 101 from the optical fiber to be written 9 and the optical fiber matching liquid 104 on the optical fiber carrier 103. contamination, and can also ensure that the optical fiber 9 to be written is parallel to the phase mask 101 and the spacing is within the diffraction distance of the phase mask 101 . The length and width of the optical fiber carrier plate 103 are not less than the quartz flakes 102, and the thickness is 2 mm. The material used has high transmittance in the ultraviolet wavelength band used for writing. The grating period of the template 101 is in the same direction, and the angle between the two sides of the groove is 120 degrees. If the V-shaped groove can just fit the fiber, that is, the circular section of the fiber cladding is tangent to the three sides of the V-shaped groove, through a simple geometric relationship It can be seen from the calculation that the depth of the V-shaped groove is required

width

In the formula, D represents the diameter of the fiber cladding. For example, for a single-mode fiber, the cladding diameter is D=125±5 microns. Taking D=130 microns, the calculation shows that the groove depth is not less than 140 microns and the groove width is not less than 485 microns.

The two sides of the V-shaped groove in the optical fiber carrier plate 103 are coated with a metal reflective film, which can reflect nearly 100% of the inscribed ultraviolet light back into the V-shaped groove. By precisely controlling the refractive index of the fiber matching liquid to be consistent with the refractive index of the fiber cladding, the V-shaped groove can be regarded as the same homogeneous material. As shown in Figure 3, the writing beam and the reflected beams on both sides of the V-shaped groove are equivalent to three The beam with an included angle of 120 degrees illuminates the optical fiber at the same time, so the uniform beam distribution can be effectively improved, and the quality of the grating preparation can be improved.

The optical fiber matching liquid used in the device uses water as the solvent and glycerin as the solvent, and the ratio of the two is precisely adjusted to obtain the refractive index consistent with the optical fiber cladding, which is used to fill the gap between the optical fiber to be written and the V-shaped groove. It can be seen from the absorption spectrum characteristics of water and glycerin that the absorption bands of water and glycerin are both below 230nm, and the ultraviolet light of 248nm wavelength is used as the writing light source, which will not be absorbed by the fiber matching liquid.

Example 2

In this embodiment, the steps of preparing gratings by using the grating preparation device of the present invention will be described with reference to FIG. 4 and FIG. 5 .

Step 1, optical path construction: in order to realize the grating preparation, in addition to the grating preparation device 1 of the present invention, the required components include: an optical platform 2, an excimer laser 3, a diaphragm 4, a beam expander 5, a mirror 6, a column The lens 7 , the three-axis displacement stage 8 , the optical fiber clamp 10 , the optical fiber V-shaped connector 11 and the grating writing online monitoring system 12 . The excimer laser 3 emits a high-energy ultraviolet writing beam 13, the diaphragm 4 is used to take out a light spot with uniform light intensity from the beam 13 emitted by the excimer laser 3, and the beam expander 5 is used to expand the light spot obtained by the diaphragm 4 to To meet the needs of grating writing, the mirror 6 is used to adjust the writing beam from the horizontal direction to the vertical direction, to meet the needs of the grating preparation device 1, and the cylindrical lens 7 compresses the writing beam 13 in the direction of the optical fiber section to improve the energy density. The device 1 is fixed on the three-axis displacement stage 8, and the position and height of the grating preparation device 1 can be adjusted by adjusting the three-axis displacement stage 8. The optical fiber clamps 10 are distributed on both sides of the three-axis displacement stage 8, and are used for fixing the optical fiber 9 to be written. , to avoid the position change of the optical fiber during the writing process from affecting the grating writing quality, the optical fiber V-shaped connector 11 is used to connect the optical fiber 9 to be written to the grating writing online monitoring system 12, and the grating writing online monitoring system 12 is used for real-time monitoring of the optical fiber 9 to be written. The optical table 2 is used to fix the above-mentioned optical components to avoid the influence of environmental vibration on the grating writing;

Step 2, parameter adjustment: start the excimer laser 3, set the pulse frequency and energy, remove the phase mask 101 and the quartz slice 102 on the grating preparation device 1, adjust the position and height of the grating preparation device 1 through the three-axis stage 8 , so that the writing beam 13 is focused in the V-shaped groove of the optical fiber carrier 103;

Step 3, placing the optical fiber: strip the coating layer of the to-be-written area on the optical fiber 9 to be written, the stripping length is greater than the length of the V-shaped groove of the optical fiber carrier 103, wipe it with alcohol, and put it into the V-shaped groove of the optical fiber carrier 103 , drop an appropriate amount of optical fiber matching liquid 104, gently move the optical fiber 9 to be written, so that the optical fiber matching liquid 104 completely fills the gap between the V-shaped groove and the optical fiber 9 to be written, and cover the quartz sheet 102 on the optical fiber carrier plate 103 , make sure to remove the air between the two, cover the phase mask 101 on the quartz sheet 102, fix the optical fibers 9 to be written at both ends of the grating preparation device 1 with the optical fiber clamp 10, and use the optical fiber V-shaped connector 11 to connect the optical fibers to be written. 9. Access the grating writing online monitoring system 12;

Step 4, write grating: increase the output energy of the excimer laser 3 to perform grating writing, and use the grating writing online monitoring system 12 to observe the grating writing effect. When the grating meets the writing requirements, turn off the excimer laser 3 and the grating writing online monitoring system 12 ;

Step 5, complete the preparation: open the optical fiber V-shaped connector 11, open the optical fiber clamp 10, remove the phase mask 101 and the quartz sheet 102, take out the optical fiber 9 from the optical fiber carrier plate 103, and complete the grating preparation.

Claims (6)

1. a special-shaped core fiber grating preparation device based on 120 degree angle reflection exposure stacking, it is characterized in that: comprise phase mask plate, quartz sheet, optical fiber carrier plate and optical fiber matching liquid, the optical fiber carrier plate has V-shaped groove on one side, with For placing the optical fiber to be written, the quartz sheet is covered on the optical fiber carrier, the phase mask is covered on the quartz sheet, and the optical fiber matching liquid is used to fill the gap between the V-shaped groove on the optical fiber carrier and the optical fiber to be written. 2. The optical fiber carrier according to claim 1 is characterized in that: the material used has high transmittance in the ultraviolet wavelength band for writing, and the center position of one side of the carrier has a V-shaped groove, and the direction of the groove is the same as that of the phase mask grid period. Consistently, both sides of the groove are coated with a metal reflective film, which has high reflectivity in the ultraviolet light band for writing. The angle between the two sides of the groove is 120 degrees. The groove depth H and groove width W are determined by the diameter D of the fiber cladding to be written. Calculate The formula is:

3. Quartz flake according to claim 1, is characterized in that: length and width are not less than phase mask plate, thickness is 50 microns, material is quartz glass, has high transmittance in the ultraviolet wave band for writing, can phase mask. The diffracted beam generated by the template is transmitted to the lower fiber carrier without damage, while protecting the phase mask from being polluted by the fiber to be written and the fiber matching liquid on the fiber carrier. 4. The phase mask according to claim 1, characterized in that: various phase masks for FBG writing can be selected. 5. The optical fiber matching solution according to claim 1, characterized in that: using water as a solvent and glycerol as a solute, it has high transmittance for writing the ultraviolet wavelength band, and is obtained by accurately controlling the ratio of water and glycerol to be consistent with the optical fiber cladding. the index of refraction. 6. A special-shaped core fiber grating preparation method based on 120-degree included angle reflection and exposure stacking, is characterized in that: comprising the following steps: Step 1: place the grating preparation device of the present invention on the grating writing platform, and adjust the writing beam to vertically irradiate the phase mask of the grating preparation device from top to bottom; Step 2: remove the phase mask and the quartz sheet of the grating preparation device, adjust the position and height of the optical fiber carrier, so that the writing ultraviolet beam is focused in the V-shaped groove of the optical fiber carrier; Step 3: Strip off the coating on the area to be written on the optical fiber to be written. The stripping length is longer than the length of the V-shaped groove of the optical fiber carrier. After wiping it with alcohol, put it into the V-shaped groove, drop an appropriate amount of fiber matching liquid, and lightly Gently move the fiber to be written, so that the fiber matching liquid completely fills the gap between the V-shaped groove and the fiber to be written, cover the quartz sheet on the fiber carrier, make sure to remove the air between the two, and cover the phase mask on the fiber carrier. On the quartz sheet, use the optical fiber fixture to fix the optical fiber to be written, and connect the optical fiber to be written to the grating and write the online monitoring system; Step 4: Start the laser for grating writing, use the grating writing online monitoring system to observe the grating writing effect, when the grating meets the writing requirements, turn off the laser and the grating writing online monitoring system; Step 5: Disconnect the optical fiber from the grating writing online monitoring system, remove the phase mask and the quartz sheet of the grating preparation device, and take out the optical fiber from the V-shaped groove of the optical fiber carrier to complete the grating preparation.

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