CN112068239A - A single stress element fiber tilted fiber Bragg grating and its preparation method - Google Patents

Abstract

The invention provides a single stress element optical fiber inclined fiber Bragg grating and a preparation method thereof. The single stress element optical fiber is a special fiber with a single stress element in the cladding. It is characterized in that the excimer laser, reflector, beam expander, cylindrical lens, mask plate and single stress element optical fiber are sequentially placed into the writing optical path, and then the optical fiber is adjusted using the refractive index matching liquid, microscope and optical fiber rotating fixture to make the optical fiber. The angle between the stress element and the grating plane in the fiber core meets the required single stress element fiber tilted fiber Bragg grating requirement. The invention can be used for the preparation of single stress element optical fiber inclined fiber Bragg grating, and can be widely used in the technical field of optical fiber devices.

Description

A single stress element fiber tilted fiber Bragg grating and its preparation method

(1) Technical field

The invention relates to a single stress element optical fiber inclined fiber Bragg grating and a preparation method thereof, belonging to the technical field of optical fiber devices.

(2) Background technology

As an important component of fiber Bragg gratings, tilted fiber Bragg gratings (tilted fiber Bragg gratings) have their own unique characteristics. Different from ordinary fiber Bragg gratings, the grating plane of tilted fiber Bragg gratings is no longer perpendicular to the fiber axis, but has a certain tilt angle. The introduction of inclination strengthens the coupling from the core mode of the forward transmission to the cladding mode and the radiation mode of the backward transmission. Based on this feature, tilted fiber Bragg gratings have a wide range of applications in optical filtering, fiber sensing, gain flatteners, and polarization-related devices.

The tilted fiber Bragg grating is routinely prepared by the phase mask method. There are two typical UV laser exposure orientations. The relative positions of the UV laser, the fiber Bragg grating phase mask and the fiber are shown in Figure 2. (1) As shown in Figure 2(a1), when the ultraviolet laser 2-1 irradiates the fiber Bragg grating phase reticle 2-2 vertically, the fiber Bragg grating phase reticle 2-2 and the fiber 2-3 to be written are relatively parallel, and As shown in Figure 2(a2), when the grating lines 2-4 in the fiber Bragg grating phase reticle are not perpendicular to the fiber to be written 2-3, the distribution of the refractive index modulation in the fiber core is similar to the grating built in the fiber Bragg grating phase reticle. The line distribution is the same, and it is not perpendicular to the fiber, and the tilted fiber Bragg grating can be prepared. (2) As shown in Figure 2 (b1), when the ultraviolet laser 2-1 does not irradiate the fiber Bragg grating phase reticle 2-2 vertically, the fiber Bragg grating phase reticle 2-2 and the fiber 2-3 to be written are relatively parallel, And as shown in Figure 2(b2), when the grating lines 2-4 in the fiber Bragg grating phase reticle are relatively perpendicular to the fiber 2-3 to be written, the distribution of the refractive index modulation in the fiber core is the same as that in the fiber Bragg grating phase reticle. If the grating lines have the same distribution and are not perpendicular to the fiber, a tilted fiber Bragg grating or a chirped tilted fiber Bragg grating can be prepared.

Since the tilted fiber Bragg grating is written obliquely into the cylindrical core, the grating surface is an ellipse in the axial direction. Therefore, each grid is symmetrical about the plane where the long axis of its grating surface is located. Therefore, although the tilted fiber Bragg grating has directionality, and the response of the cladding mode resonance peak is different when the bending direction is at different angles to the grating surface, it can only resolve the bending in the semicircular direction, and the fiber orientation is about the grating surface. When the long axis is symmetrical to the left and right bending, there is no difference in the response degree of the spectrum, that is, only the bending of 0°-180° can be resolved, and the bending resolution of the full range of 360° cannot be achieved. Therefore, this symmetry will affect the usage scenarios of tilted fiber Bragg gratings.

In order to solve this problem, the present invention breaks the uniform distribution of the refractive index in the fiber core by introducing a single stress element into the fiber cladding, so that the tilted fiber Bragg grating written in a specific direction is no longer symmetrical, and further expands the tilted fiber Bragg Raster usage scenarios.

(3) Contents of the invention

The purpose of the present invention is to provide a single stress element fiber tilted fiber Bragg grating and a preparation method thereof.

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

The components of the single-stressed element fiber tilted fiber Bragg grating include a single-stressed element fiber and a tilted fiber Bragg grating written in its core; its preparation method is to use a uniform fiber Bragg grating mask, by adjusting the rotation angle of the mask, and Adjust the relative positions of the stress element and the fiber core, and write tilted fiber Bragg gratings with different grating tilt angles and stress element positions.

The single stress element fiber is a special fiber with a stress element arranged around the core.

The stress element is a material whose thermal expansion coefficient is not equal to the thermal expansion coefficient of the quartz matrix.

The preparation method includes the following steps

1) Put the excimer laser, reflecting mirror, beam expander, cylindrical lens, mask plate and single stress element fiber into the writing optical path in sequence;

2) Rotate the mask to select the grating tilt angle;

3) Adjust the angle between the stress element in the fiber and the grating surface by using U-shaped groove, cladding refractive index matching liquid, microscope and fiber rotation fixture;

3) Write a tilted fiber Bragg grating in the fiber core using an ultraviolet laser;

4) Recoat or encapsulate a single stress element fiber tilted fiber Bragg grating.

In the preparation method, the inclination angle of the grating and the angle between the stress element and the grating surface will affect the grating spectrum.

The tilted fiber Bragg grating of the present invention is based on a single stress element fiber, which is unique in the stress-induced refractive index profile of the fiber core. A single-mode fiber, as shown in Figure 1(a), is axisymmetric about the x-axis and y-axis of the cross-section, and is center-symmetric about the center O of the cross-section, so the core of the single-mode fiber is The stress conditions are uniform, which also makes the refractive index distribution of the fiber cladding and core layer uniform. This uniformity keeps the polarization state of the guided mode in the core unchanged, and the written tilted fiber Bragg grating also has uniformity on each grating plane. For polarization-maintaining fibers, such as the panda-type polarization-maintaining fiber shown in Figure 1(b), the optical fiber is axisymmetric about the x-axis and y-axis of the cross-section, but not about the center O of the cross-section. The center is symmetrical, the stress units are symmetrically distributed in the cladding on both sides of the core, and the two will apply stress to the core, so that the uniformity is broken, but due to the symmetrical distribution of the stress units, the refractive index distribution of the core is caused It is also symmetrical. This symmetry allows polarization-maintaining fibers to transmit two orthogonal polarization modes. The single stress element fiber based on the present invention, the cross section of the fiber is shown in Figure 1(c), the schematic diagram of the radial refractive index distribution of the cross section is shown in Figure 9, and the fiber model is shown in Figure 10, where 10-1 is the fiber The cladding, 10-2 is the stress element, and 10-3 is the fiber core. Such an optical fiber is symmetrical in its cross-section about the x-axis of the cross-section, but asymmetrical about the y-axis, and also asymmetric about the center O of the cross-section. A single stress element only applies stress in one general direction to the core, resulting in a symmetrical distribution of the refractive index of the core about the line connecting the center of the stress element and the center of the core. This makes the single stress element fiber core have the same birefringence effect as the polarization-maintaining fiber core, but different from the polarization-maintaining fiber core, the refractive index distribution has two axes of symmetry, and the birefringence of one axis-symmetric refractive index distribution is not uniform. effect, which results in the transmission of one polarization state and one distortion state in the core. This asymmetric design in the state of slanted two-dimensional distribution of the core refractive index, only maintains part of the polarization of the transmission mode. Combined with the tilted fiber Bragg grating, when the long axis of the grating surface is orthogonal to the line connecting the stress element center and the fiber geometric center, the asymmetry of the single-stress fiber tilted fiber Bragg grating can be reflected. Enables omnidirectional bending sensing in a two-dimensional plane.

The single stress element fiber tilted fiber Bragg grating prepared by the present invention needs to be improved on the basis of the two preparation methods shown in Fig. 2, and it is necessary to ensure the relative positions of the stress element, the fiber core and the ultraviolet laser as shown in Fig. 2 (a3) As shown in (b3), the long axis of the tilted fiber Bragg grating surface is orthogonal to the line connecting the center of the fiber and the center of the stress element, so that the asymmetry of the tilted fiber Bragg grating of the single stress fiber can be reflected to the greatest extent.

The present invention has at least the following several outstanding beneficial effects:

(1) The present invention provides an asymmetric fiber grating and a preparation method thereof by providing a tilted fiber Bragg grating based on a single stress element fiber, which is suitable for the preparation of an asymmetric fiber grating in which the fiber core is located at the geometric center of the fiber .

(2) The single stress element tilted fiber Bragg grating prepared by the present invention can realize omnidirectional bending sensing in a two-dimensional plane.

(2) The single stress element tilted fiber Bragg grating prepared by the present invention breaks the symmetry of the common tilted fiber Bragg grating and expands the application scene.

(4) Description of drawings

Figure 1 is a schematic cross-sectional view of a single-mode fiber, a polarization-maintaining fiber, and a single-stress element fiber, (a) is a single-mode fiber, (b) is a panda polarization-maintaining fiber, and (c) is a single-stress element fiber.

Figure 2 is a schematic diagram of two tilted fiber Bragg grating writing methods, (a1) and (b1) are schematic diagrams of the relative positions of the ultraviolet laser, mask and optical fiber in the two methods, respectively, (a2) and (b2) are distributed as two The schematic diagram of the relative position of the mask and the optical fiber in this method, (a3) and (b3) are the schematic diagrams of the relative positions of the stress element, the fiber core and the irradiation direction of the ultraviolet laser after replacing the optical fiber with a single stress element fiber, respectively, 2- 1 is the ultraviolet laser, 2-2 is the fiber Bragg grating phase mask, 2-3 is the fiber to be written, 2-4 is the grating line in the fiber Bragg grating phase mask, and 2-5 is the replacement of the fiber with a single stress After replacing the fiber with the stress element, 2-6 are the core of the fiber after replacing the fiber with a single stress element fiber.

Figure 3 is a schematic diagram of a system for writing single stress element fiber fiber Bragg grating and asymmetric fiber Bragg grating, 3-1 is an excimer laser, 3-2 is a mirror part, 3-3 is a beam expander part, and 3-4 is a Cylindrical lens part, 3-5 is a small gantry, 3-6 is a CCD component with integrated light source, 3-7 is a fiber Bragg grating phase mask part, 3-8 is a U-shaped groove, and 3-9 is a fiber rotating fixture part , 3-10 is a single stress element fiber, 3-11 is a broadband light source, and 3-12 is a spectrometer.

Figure 4 is a schematic diagram of the required components for the operation of determining the relative positions of the fiber core and the stress element in a single stress element fiber, 4-1 is a small gantry, 4-2 is a CCD assembly with an integrated light source, and 4-3 is an optical fiber rotating fixture , 4-4 is a single stress element fiber, 4-5 is a U-shaped groove, 4-6/4-7 is a Z-axis lift table, 4-8 is an adapter plate, and 4-9 is an XY stage.

Figure 5 is a schematic diagram of the operation of determining the relative positions of the core and the stress element in the single stress element fiber, 5-1 is the U-shaped groove, 5-2 is the matching liquid whose refractive index is the same as the refractive index of the single stress element fiber cladding, 5 -3 is the single stress element fiber cladding, 5-4 is the single stress element fiber core, 5-5 is the stress element in the single stress element fiber, 5-6 is the ultraviolet laser, h is the single stress element fiber core and The distance from the edge of the stress element.

Figure 6 is a schematic diagram of the axial direction of the fiber after adjusting the relative positions of the inner core and the stress element of the single stress element fiber, 6-1 is the U-shaped groove, and 6-2 is the matching liquid whose refractive index is the same as the refractive index of the single stress element fiber cladding , 6-3 is the single stress element fiber cladding, 6-4 is the single stress element fiber core, 6-5 is the stress element in the single stress element fiber, 6-6 is the ultraviolet laser, h is the single stress element fiber fiber The maximum distance between the core and the edge of the stress element.

Fig. 7 is a schematic structural diagram of a fiber Bragg grating phase reticle, 7-1 is a rotating disk base, 7-2 is a rotating disk, 7-3 is a fiber Bragg grating phase reticle, and 7-4 is a fiber Bragg grating phase reticle In the grid area, 7-5 is a single stress element fiber, and 7-6 is an XYZR four-axis displacement stage.

Figure 8 is a schematic structural diagram of a single stress element fiber tilted fiber Bragg grating, (a) is the end view of the single stress element fiber, (b) is the side view of the single stress element fiber, 8-1 is the single stress element fiber cladding, 8- 2 is the stress element, 8-3 is the fiber core of a single stress element, and 8-4 is the written tilted fiber Bragg grating.

FIG. 9 is a schematic diagram of a one-dimensional refractive index distribution in a radial cross-section of an optical fiber end of a single stress element.

Figure 10 is a model of a single stress element fiber, 10-1 is a single stress element fiber cladding, 10-2 is a stress element, and 10-3 is a single stress element fiber core.

(5) Specific implementation methods

The present invention will be further described below in conjunction with specific embodiments.

Embodiment 1: A single stress element fiber tilted fiber Bragg grating and its preparation method Preferably, in this embodiment, the diameter of the stress element can be 32 microns, and the distance between its geometric center and the optical fiber geometric center can be 26 microns.

The inclined fiber Bragg grating writing system used in the present invention is shown in Figure 3, and the preparation steps are as follows:

The inclined fiber Bragg grating writing system used in the present invention is shown in Figure 3. The difference between the two methods is the exposure orientation of the ultraviolet laser. The general main preparation steps are as follows:

Step 1: Set the writing parameters of the excimer laser 3-1, and adjust the relative positions of the reflecting mirror part 3-2, the beam expander part 3-3 and the cylindrical lens part 3-4 in the writing system to collimate the ultraviolet laser beam so that the It can accurately focus on the core position of the single stress element fiber 3-10 to be written;

Step 2: According to the prepared fiber Bragg grating type, select a fiber Bragg grating phase reticle with appropriate parameters, and place it on the reticle holder to form the fiber Bragg grating phase reticle parts 3-7. According to the difference of the prepared fiber Bragg grating, Select an exposure orientation in Figure 2, and adjust the fiber Bragg grating phase mask according to the relative positions of the ultraviolet laser, the fiber Bragg grating phase mask, and the single stress element fiber shown therein;

Step 3: Completely remove the fiber Bragg grating area to be written on the single stress element fiber and the coating layer embedded in the U-groove area on the left and right sides, and put it into the fiber rotating fixture 3-9 to fix it. Two U-shaped grooves 3-8 that can be embedded in the single-stressed element fiber are formed, and the matching liquid that is consistent with the refractive index of the single-stressed element fiber cladding is dropped into the groove;

Step 4: First use the small gantry 3-5 to move the CCD assembly 3-6 of the integrated light source to just above the U-shaped groove 3-8 on one side, turn on the light to observe the relative position of the stress element in the fiber and the fiber core, and rotate the same forward and reverse. Rotate the optical fiber clamp 3-9 on the side, and stop when the stress element is at the back side of the fiber core with respect to the incident direction of the ultraviolet laser, and the maximum distance from the fiber core appears, record the scale of the rotating clamp at this time, and repeat on the other side do the same;

Step 5: At this time, the relative position of the stress element and the fiber core has been determined, and the relative positions of the two can be adjusted by rotating the fiber rotating fixture 3-9 according to the scale at the same time to meet the needs of preparing different fiber Bragg gratings;

Step 6: Connect one end of the single stress element fiber 3-10 to the broadband light source 3-11, and the other end to the spectrometer 3-12, turn on the excimer laser 3-1, and after collimating and expanding the beam and compressing it into a narrow parallel light, Exposure is performed through the fiber Bragg grating phase mask part 3-7, and the writing process is monitored in real time through the spectrometer 3-12, and the exposure is stopped when the required spectrum is reached;

Step 7: Disconnect the single stress element fiber 3-10 and the broadband light source 3-11 from the spectrometer 3-12, open the fiber rotation fixture 3-9, remove the single stress element fiber 3-10, and package it.

Both methods for preparing tilted fiber Bragg gratings of single-stressed element fibers require the determination of the exact positions of the core and stress elements within the single-stressed element fiber, which are performed in steps 3 and 4. Taking one side as an example, the detailed operations are as follows: As shown in Figure 4, after pre-processing the single stress element fiber 4-4 and placing it on the fiber rotation fixture 4-3, it is convenient for the Z-axis translation stage 4-7 to convert the U The groove 4-5 is raised to encapsulate the single stress element optical fiber 4-4, and then a liquid matching the refractive index of the cladding of the single stress element fiber is dropped into the U-shaped groove 4-5. Use a small gantry 4-1 to move the CCD assembly 4-2 of the integrated light source directly above the U-shaped groove 4-5, and turn on the light to observe the relative position of the stress element in the fiber and the fiber core. The Z-axis lift table 4-6 can adjust the height of the optical fiber in step 1; the adapter plate 4-8 can ensure that the optical fiber rotating fixture 4-3 and the U-shaped groove 4-5 move with the XY stage 4-9 at the same time.

The method of determining the position of the core and the stress element in the single stress element fiber is shown in Figure 5. Initially, as shown in Figure 5(a), the core 5-4 and the stress element 5-5 of the single stress element fiber will appear random initial position, the edge distance h between the two can be calibrated by the pixel points observed on the CCD. Rotate the optical fiber rotating fixture 4-3 in Figure 4, and h will also change accordingly. When h reaches the maximum value h _max , and the single stress element fiber core 5-4, stress element 5-5 and ultraviolet laser 5-6 are three When the relative position of the individual is shown in Figure 5(b), the relative position of the single stress element fiber core 5-4 and the stress element 5-5 can be determined. At this time, the result viewed from the side is shown in Fig. 6. The line connecting the geometric centers of the optical fiber core 6-4 of the single stress element and the stress element 6-5 will be parallel to the optical platform where the writing system is located. Using the same operation for the other side, after calibrating the position of the fiber core and stress element of the single stress element, the fiber rotating fixture can be rotated to meet the relative position requirements when preparing the inclined fiber Bragg grating.

The difference between the two kinds of tilted fiber Bragg gratings that can prepare single stress element fiber, the main difference lies in the choice of exposure orientation in step 2 and step 5, that is, two different UV lasers, single stress element fiber core and stress element. relative position.

Orientation 1: The UV laser is normally incident on the fiber Bragg grating phase mask. As shown in Fig. 2(a1)/(a2)/(a3), the UV laser 2-1 is normally incident on the fiber Bragg grating phase mask 2-2, and the single stress element fiber 2-3 is attached with the fiber Bragg grating phase mask 2 -2, there is an included angle between the grating region 2-4 in the fiber Bragg grating phase mask and the single stress element fiber 2-3, and the single stress element 2-5 in the single stress element fiber is located directly behind the fiber core 2-6. The operation of adjusting the angle between the grating lines of the fiber Bragg grating phase reticle and the single stress element fiber is shown in Figure 7. The fiber Bragg grating phase reticle 7-3 is fixed on the rotating disk 7-2, and can pass the edge of the rotating disk 7-2. The scale of , determines the rotation angle of the fiber Bragg grating phase reticle. The rotation angle will also be the included angle between the fiber Bragg grating phase reticle grating line 7-4 and the single stress element fiber 7-5, that is, the included angle between the grating grating surface and the fiber core. And as shown in Figure 2(a3), the location of the stress element should be right behind the fiber core.

Azimuth 2: Ultraviolet laser oblique incident fiber Bragg grating phase mask. As shown in Fig. 2(b1)/(b2)/(b3), the ultraviolet laser 2-1 is obliquely incident on the fiber Bragg grating phase mask 2-2, and the single stress element fiber 2-3 is attached with the fiber Bragg grating phase mask 2-2, the inner grating region 2-4 of the fiber Bragg grating phase mask is perpendicular to the single stress element fiber 2-3, and the single stress element 2-5 in the single stress element fiber is located directly above or directly below the fiber core 2-6. The operation of adjusting the angle between the fiber Bragg grating phase mask and the UV laser is shown in Figure 7. Rotate the XYZR four-axis stage 7-6. The rotation angle is the angle at which the UV laser enters the fiber Bragg grating phase mask, and the fiber is also tilted for this purpose. The angle between the Bragg grating grating region and the fiber core. At this time, it is also necessary to adjust the position of the fiber. As shown in Figure 4, adjust the XY two-axis displacement stages 4-9 on the left and right sides, so that the single stress element fiber can be attached to the fiber Bragg grating phase mask. And as shown in Figure 2(b3), the position of the stress element should be directly above or below the fiber core.

The tilted fiber Bragg grating based on the single stress element fiber prepared by the present invention is shown in FIG. 8 , (a) is the end view of the single stress element fiber, and (b) is the side view of the single stress element fiber.

In the specification and drawings, there have been disclosed typical embodiments of the invention. The present invention is not limited to these exemplary embodiments. The specific terms are used in a generic and descriptive sense only and are not intended to limit the scope of protection of the present invention.

Claims (5)

1. A single stress element optical fiber tilt fiber Bragg grating and a preparation method thereof are characterized in that: the single stress element optical fiber tilt fiber Bragg grating comprises a single stress element optical fiber and a tilt fiber Bragg grating written in a fiber core of the single stress element optical fiber; the preparation method comprises the steps of using a uniform fiber Bragg grating mask plate, and writing inclined fiber Bragg gratings with different grating inclination angles and stress element positions by adjusting the rotation angle of the mask plate and the relative positions of the stress element and the fiber core.

2. The single stress element fiber tilt fiber bragg grating and the preparation method thereof according to claim 1, wherein: the single stress element optical fiber is a special optical fiber with a stress element arranged around the fiber core.

3. The single stress element fiber tilt fiber bragg grating and the method for manufacturing the same according to claim 1 and claim 2, wherein: the stress element is a material with a thermal expansion coefficient not equal to that of the quartz substrate.

4. A single stress element optical fiber tilt fiber Bragg grating and a preparation method thereof are characterized by comprising the following steps:

1) sequentially placing an excimer laser, a reflector, a beam expander, a cylindrical lens, a mask plate and a single stress element optical fiber into a writing light path in sequence;

2) rotating the mask plate and selecting the inclination angle of the grating;

3) adjusting the included angle between the stress element in the optical fiber and the grating surface by using a U-shaped groove, cladding refractive index matching liquid, a microscope and an optical fiber rotating clamp;

3) writing inclined fiber Bragg grating in the fiber core by using ultraviolet laser;

4) and then coating or packaging the single stress element optical fiber tilt fiber Bragg grating.

5. The single stress element fiber tilt fiber bragg grating and the preparation method thereof according to claim 4, wherein: in the preparation method, the grating spectrum can be influenced by the inclination angle of the grating and the included angle between the stress element and the grating surface.

Images