CN112068237A

CN112068237A - Cascade multi-type grating based on single stress element optical fiber and preparation method thereof

Info

Publication number: CN112068237A
Application number: CN202010930634.2A
Authority: CN
Inventors: 苑立波; 徐致远
Original assignee: Guilin University of Electronic Technology
Current assignee: Guilin University of Electronic Technology
Priority date: 2020-09-07
Filing date: 2020-09-07
Publication date: 2020-12-11

Abstract

The invention provides a cascade multi-type grating based on a single stress element optical fiber and a preparation method thereof. It is characterized by that it is formed from long-period chirp optical fibre grating and chirp optical fibre Bragg grating or inclined optical fibre Bragg grating which are cascade-connected. The long-period chirped fiber grating is obtained by one-time periodic spiral torsion; the chirped fiber bragg grating is obtained by a uniform grating mask plate by using a preparation method of the uniform fiber bragg grating. The method can be used for preparing the cascaded multi-type grating based on the single stress element optical fiber, and can be widely applied to the technical field of optical fiber devices.

Description

Cascade multi-type grating based on single stress element optical fiber and preparation method thereof

(I) technical field

The invention relates to a cascade multi-type grating based on a single stress element optical fiber and a preparation method thereof.

(II) background of the invention

The optical fiber sensor based on the Fiber Bragg Grating (FBG) can adapt to various working environments due to the characteristics of simple structure, light weight, small size and strong anti-interference capability, so that the optical fiber sensor has wide development prospect. The slanted fiber Bragg grating (TFBG) is an important component of FBG and has its uniqueness. Unlike ordinary FBG, the grating plane of TFBG is not perpendicular to the axial direction of the optical fiber any more, but has a certain inclination angle. The introduction of the inclination strengthens the coupling from a fiber core mode which is transmitted from the front direction to a cladding mode and a radiation mode which are transmitted from the back direction. Based on the characteristics, the TFBG has wide application in the fields of optical filtering, optical fiber sensing, gain flatteners, polarization related devices and the like. Chirped Fiber Bragg Grating (CFBG) is a fiber grating in which the refractive index and the grating period vary in the axial direction, and is widely used for fiber dispersion compensation and broadband filters due to its small volume, low loss, wide reflection bandwidth, and stable dispersion. Long Period Fiber Gratings (LPFGs) are an ideal all-optical band-stop transmission type filter device, which has important applications in fiber-optic communications, fiber-optic sensing and other fields.

Therefore, several fiber gratings are cascaded to one optical fiber, so that partial defects of the fiber gratings can be effectively made up, the application range of the device is improved, and multi-parameter sensing can be realized by utilizing the difference of the sensitivity of the fiber gratings to variables.

Disclosure of the invention

The invention aims to provide a cascade multi-type grating based on single stress element optical fiber and a preparation method thereof

The purpose of the invention is realized as follows:

the cascaded multi-type grating based on the single stress element optical fiber comprises a long-period chirped fiber grating and a chirped fiber Bragg grating or an inclined chirped fiber Bragg grating.

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

The stress element is a material with a thermal expansion coefficient not equal to that of the quartz substrate.

The long-period chirped fiber grating and the chirped fiber Bragg grating or the inclined chirped fiber Bragg grating need to be prepared on a single stress element fiber by using different preparation methods in sequence.

The preparation method of the long-period chirped fiber grating comprises the following steps:

1) stripping a coating layer from a region to be helically twisted of the single stress unit optical fiber;

2) placing the single stress element optical fiber without the coating layer into a machine capable of performing spiral torsion operation, and setting the pitch and the period;

3) one end of a single stress unit optical fiber is connected to a broadband light source, and the other end of the single stress unit optical fiber is connected to a spectrometer;

4) and starting the broadband light source and the spectrometer, starting torsion, and taking out the single stress element optical fiber after the procedure is finished to finish the preparation.

The preparation method of the chirped fiber Bragg grating comprises 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) adjusting the position of a stress element in the optical fiber by using a U-shaped groove, cladding refractive index matching fluid, a microscope and an optical fiber rotary clamp, so that the single stress element does not shield ultraviolet laser to irradiate the fiber core;

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

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

The invention relates to a preparation method for realizing chirp on a long-period fiber grating, in particular to a preparation method which is realized by simultaneously obtaining two fiber gratings. The first type of fiber grating is an LPFG due to a periodic helical twist in space. The second type of fiber grating is a chirped fiber grating formed by twisting a single-stress-element fiber due to the existence of a stress element.

The grating formation principle of the second fiber grating is mainly explained here. The grating formation of the chirped fiber grating is characterized by the change of the refractive index and the grating period, so that the chirping effect can be realized only by the different changes of the refractive indexes of different gratings. Specifically, with the micro-integration concept, a single stress cell fiber such as that of FIG. 3 is twisted when the core index profile has a large range of lateral variations, one of which is the structure of FIG. 4. The process of 2-a to 2-d in fig. 2 is used to represent a period of torsion, the stress cell fiber is sliced along the axial direction, and the change of the total grid refractive index of each slice is simplified into an arrow head from high refractive index to low refractive index distribution, 2-1 is the core of the stress cell fiber, 2-3 is the distribution of the core refractive index from high to low by the arrow head, taking the area shown by circle 2-2 as an example, after the stress cell fiber is helically twisted, the refractive index of the 2-2 area in the former grid can be different from that of the 2-2 area in the latter grid in the process from 2-a to 2-d. Although the optical field variation and the periodic scale of each part of the fiber core are the same in space during twisting, the refractive index modulation of each divided grid is different due to the wide range of transverse variation of the fiber core refractive index distribution. In this case, the transmission wavelength of each small local area network in the fiber core contributing to the overall grating transmission is different for the whole fiber grating, so that chirp occurs in the transmission spectrum.

The method for realizing the chirp on the fiber Bragg grating has similar principle. The grating forming characteristic of the chirped fiber Bragg grating is the change of the refractive index and the grating period, so that the chirping effect can be realized only by the different changes of the refractive indexes of different gratings. By means of the micro-integration concept, when the core refractive index profile has a large range of lateral variations, the core with lateral variations in refractive index is divided approximately equally into a plurality of grids. When ultraviolet laser is irradiated on the fiber core through a uniform grating mask plate, the light field change and the period scale of each part of the fiber core are the same in space, but the refractive index modulation of each divided grid is different due to the fact that the refractive index distribution of the fiber core has wide range of transverse change. At this time, for the whole fiber bragg grating, the reflection wavelengths contributed by each small local area grid in the fiber core to the whole bragg reflection are also different, so that a chirp phenomenon occurs in the reflection spectrum. Because the contribution of the weak reflection generated by each grid to the total Bragg reflection is related to the transverse distribution of the refractive index of the fiber core, the chirp effect can be realized only by realizing different distribution of the refractive index of the fiber core on the basis of using a uniform grating mask plate. Further, if the transverse distribution of the refractive index of the fiber core is linear, the linear chirped grating is obtained; if the grating is nonlinear, the nonlinear chirped grating can be obtained, and if the grating is not changed, the uniform standard fiber Bragg grating can be obtained, so that the fiber Bragg grating with various spectrums can be manufactured by only one uniform grating mask plate. The chirped fiber Bragg grating can be prepared by making the optical fiber parallel or not parallel to the uniform fiber Bragg grating mask plate.

The invention has at least the following outstanding advantages:

(1) the long-period chirped fiber grating and the chirped fiber Bragg grating or the inclined chirped fiber Bragg grating are cascaded, so that the structure and the preparation process of the chirped cascaded fiber grating are simplified.

(2) The invention can superpose the long-period fiber grating and the chirped fiber grating only by periodically and spirally twisting the single stress element fiber once without periodically and spirally twisting the fiber, thereby simplifying the preparation process of the long-period chirped fiber grating and reducing the size of the fiber grating.

(3) The invention can prepare the chirped fiber grating without a chirped grating mask plate, thereby saving the production cost.

(4) The invention can realize multi-parameter sensing through the cascade effect.

(5) The method of the invention is beneficial to mass production of devices.

(IV) description of the drawings

FIG. 1 is a schematic cross-sectional view of a single stress element fiber.

Fig. 2 is a schematic diagram of the positions of 4 single stress element optical fiber spirals, a to d are 4 positions in one spiral period, 2-1 is a fiber core, 2-2 is a grid for example, 2-3 shows the distribution of the refractive index of the fiber core from high to low by an arrow, and 2-4 is an optical fiber cladding.

FIG. 3 is a schematic diagram of an untwisted single-stress-element fiber, 3-1 being the fiber cladding and 3-2 being the single-stress-element, 3-3 fiber core.

FIG. 4 is a schematic diagram of a single-stress-element fiber after twisting for one period, where 4-1 is the twisted fiber cladding, 4-2 is the twisted single-stress element, and 4-3 is the twisted fiber core.

Fig. 5 is a schematic view of the axial cross-section one-dimensional refractive index profile of a single-stress-element optical fiber end.

FIG. 6 is a schematic diagram of a system for helically twisting a single stress element fiber. 6-1 is a broadband light source, 6-2 is a single stress element optical fiber, 6-3 is a machine of a spiral torsion optical fiber, and 6-4 is a spectrometer.

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

FIG. 8 is a schematic diagram of the operation of determining the relative positions of the fiber core and the stress element in the single stress element optical fiber, wherein 8-1 is a small portal frame, 8-2 is a CCD assembly of an integrated light source, 8-3 is an optical fiber rotating clamp, 8-4 is the single stress element optical fiber, 8-5 is a U-shaped groove, 8-6/8-7 is a Z-axis lifting table, 8-8 is an adapter plate, and 8-9 is an XY displacement table.

FIG. 9 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, where 9-1 is a U-shaped groove, 9-2 is a matching fluid with a refractive index consistent with that of the cladding of the single stress element fiber, 9-3 is the cladding of the single stress element fiber, 9-4 is the core of the single stress element fiber, 9-5 is the stress element in the single stress element fiber, 9-6 is ultraviolet laser, and h is the distance between the core of the single stress element fiber and the edge of the stress element.

FIG. 10 is an axial view of an optical fiber with the relative positions of the fiber core and the stress element adjusted in a single stress element optical fiber, where 10-1 is a U-shaped groove, 10-2 is a matching fluid with a refractive index consistent with that of the cladding of the single stress element optical fiber, 10-3 is the cladding of the single stress element optical fiber, 10-4 is the fiber core of the single stress element optical fiber, 10-5 is the stress element in the single stress element optical fiber, 10-6 is an ultraviolet laser, and h is the maximum distance between the fiber core of the single stress element optical fiber and the edge of the stress element.

FIG. 11 is a schematic structural diagram of the FBG phase mask, 11-1 is a base of a rotating disk, 11-2 is a rotating disk, 11-3 is the FBG phase mask, 11-4 is a gate region in the FBG phase mask, 11-5 is a single stress element fiber, and 11-6 is an XYZR four-axis displacement table.

FIG. 12 is a schematic diagram of the entire cascade system, where 12-1 is the light source, 12-2 is the long period chirped fiber grating, 12-3 is the chirped fiber Bragg grating, and 12-4 is the spectrometer.

(V) detailed description of the preferred embodiments

The invention is further illustrated below with reference to specific examples.

Example 1: a cascade multi-type grating based on single stress element optical fiber and a preparation method thereof.

In the embodiment, the long-period chirped fiber grating and the chirped fiber bragg grating are cascaded or the chirped fiber bragg grating is inclined, and the front step and the rear step are manufactured separately.

Preferably, in this embodiment, the diameter of the stress element may be 32 microns, and the geometric center of the stress element may be 26 microns away from the geometric center of the optical fiber.

The long-period chirped fiber grating preparation system is shown in fig. 6, and the preparation process is as follows:

preferably, the machine for helically twisting the optical fiber used in this embodiment is an LZM110 carbon dioxide fusion splicer.

Step 1: stripping a coating layer from a region to be helically twisted of the single stress unit optical fiber 6-2;

step 2: placing the single stress element optical fiber without the coating layer into an LZM110 carbon dioxide fusion splicer 6-3, and setting a program of the fusion splicer to customize the pitch and the period of the helix;

and step 3: one end of the single stress element optical fiber 6-2 is connected with the broadband light source 6-1, and the other end is connected with the spectrometer 6-4;

and 4, step 4: starting a broadband light source 6-1 and a spectrometer 6-4, operating a fusion splicer 6-3, starting torsion, and taking out the single stress element optical fiber 6-2 after the procedure is finished to finish the preparation of the long-period chirped fiber grating.

Then, chirped fiber bragg gratings are prepared at different positions, and a preparation system is shown in fig. 7, wherein the preparation process comprises the following steps:

step 1: setting the writing parameters of an excimer laser 7-1, and adjusting the relative positions of a reflecting mirror part 7-2, a beam expanding mirror part 7-3 and a cylindrical lens part 7-4 in a writing system to collimate an ultraviolet laser beam so that the ultraviolet laser beam can be accurately focused to the position of a fiber core of a single stress element optical fiber 7-10 to be written;

step 2: selecting an FBG phase mask with proper parameters according to the type of the prepared FBG, placing the FBG phase mask on a mask clamp to form an FBG phase mask part 7-7, selecting different exposure orientations according to different prepared FBGs, and adjusting the positions of the FBG phase mask and the single stress element optical fiber relative to the ultraviolet laser.

And step 3: completely removing the coating layers of the FBG region to be etched and the U-shaped groove regions embedded on the left side and the right side of the single stress element optical fiber, putting the single stress element optical fiber into an optical fiber rotary clamp 7-9 for fixation, respectively lifting two U-shaped grooves 7-8 capable of wrapping and embedding the single stress element optical fiber between the left clamp and the right clamp, and dripping matching liquid with the refractive index of a cladding of the single stress element optical fiber into the grooves;

and 4, step 4: firstly, moving a CCD assembly 7-6 of an integrated light source to a position right above a U-shaped groove 7-8 on one side by using a small portal frame 7-5, observing the relative position of a stress element and a fiber core in the optical fiber by opening light, rotating a rotating optical fiber clamp 7-9 on the same side forward and backward until the stress element is positioned on the rear side of the fiber core and stops when the maximum distance between the stress element and the fiber core is formed relative to the incident direction of ultraviolet laser, recording the scale of the rotating clamp at the moment, and repeatedly carrying out the same operation on the other side;

and 5: at the moment, the relative positions of the stress element and the fiber core are determined, and the relative positions of the stress element and the fiber core can be adjusted by simultaneously rotating the optical fiber rotating clamps 7-9 according to scales so as to meet the requirements of preparing different FBGs;

step 6: one end of a single stress unit optical fiber 7-10 is connected with a broadband light source 7-11, the other end of the single stress unit optical fiber is connected with a spectrometer 7-12, an excimer laser 7-1 is started, after the single stress unit optical fiber is collimated, expanded and compressed into narrow parallel light, exposure is carried out through an FBG phase mask portion 7-7, the writing process is monitored in real time through the spectrometer 7-12, and the exposure is stopped when the required spectrum is reached;

and 7: and (3) disconnecting the single stress element optical fiber 7-10 and the broadband light source 7-11 from the spectrometer 7-12, opening the optical fiber rotary clamp 7-9, taking down the single stress element optical fiber 7-10, and packaging.

The method can prepare single stress element optical fiber ordinary FBGs, chirped FBGs, inclined FBGs and other different types of FBGs, and the difference is that different relative positions of ultraviolet laser, a single stress element optical fiber core and a stress element are selected, and the single stress element optical fiber ordinary FBGs, the chirped FBGs, the inclined FBGs and the like can be obtained by performing ultraviolet laser writing along different lateral directions of the single stress element optical fiber.

In step 2, the detailed operation of adjusting the FBG phase mask is as follows: (1) and adjusting the included angle between the grating line of the FBG phase mask and the single stress element optical fiber. As shown in FIG. 11, the FBG phase mask 11-3 is fixed on the rotating disk 11-2, and the rotation angle of the FBG phase mask can be determined by the scale on the edge of the rotating disk 11-2. The rotation angle will also be the angle between the grating line 11-4 of the FBG phase mask and the single stress element optical fiber 11-5. (2) And adjusting the incident angle of the ultraviolet laser incident on the FBG phase mask. As shown in FIG. 11, the FBG phase mask is fixed on the XYZR four-axis rotary displacement table 11-6, and the included angle of the ultraviolet laser incident on the FBG phase mask can be easily adjusted by rotating the XYZR four-axis rotary displacement table 11-6.

The determination of the positions of the core and the stress element in the single stress element optical fiber is performed in step 3 and step 4, taking one side as an example, and the detailed operation is as follows: as shown in fig. 8, after the single stress element optical fiber 8-4 is subjected to the preliminary treatment and is placed in the optical fiber rotary clamp 8-3, the Z-axis displacement table 8-7 is facilitated to lift the U-shaped groove 8-5 to be embedded with the single stress element optical fiber 8-4, and then the matching fluid with the same refractive index as the cladding of the single stress element optical fiber is dripped into the U-shaped groove 8-5. And (3) moving the CCD assembly 8-2 of the integrated light source to be right above the U-shaped groove 8-5 by using a small portal frame 8-1, and observing the relative positions of the stress element and the fiber core in the optical fiber by opening light. The Z-axis lifting table 8-6 can adjust the height of the optical fiber in the step 1; the adapter plate 8-8 can ensure that the optical fiber rotary clamp 8-3 and the U-shaped groove 8-5 move along with the XY displacement table 8-9 at the same time.

The observation situation is shown in fig. 9, and when the observation starts, the fiber core 9-4 and the stress element 9-5 of the single stress element fiber are at random initial positions as shown in fig. 9(a), and the edge distance h between the two can be calibrated by using a pixel point observed on the CCD. Rotating the optical fiber rotating clamp 8-3 in fig. 4, h will also change, when h reaches the maximum value and the relative positions of the single stress element optical fiber core 9-4, the stress element 9-5 and the ultraviolet laser 9-6 are as shown in fig. 9(b), the relative positions of the single stress element optical fiber core 9-4 and the stress element 9-5 can be determined. The result of the side view is shown in fig. 10, and the geometric center line of the single stress element fiber core 10-4 and the stress element 10-5 is parallel to the optical platform where the writing system is located. The same operation is used for the other side, and after the fiber core and the stress element position of the single stress element optical fiber are calibrated, the optical fiber rotating clamp can be rotated so as to meet the relative position requirement when the specific FBG is prepared.

The finally prepared cascade multi-type grating graph 12 based on the single stress element optical fiber is shown, wherein 12-1 is a light source, 12-2 is a long-period chirped fiber grating, 12-3 is a chirped fiber Bragg grating, and 12-4 is a spectrometer.

In the description and drawings, there have been disclosed typical embodiments of the invention. The invention is not limited to these exemplary embodiments. Specific terms are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A cascade multi-type grating based on single stress element optical fiber and a preparation method thereof are characterized in that: the cascaded multi-type grating based on the single stress element optical fiber comprises a long-period chirped fiber grating and a chirped fiber Bragg grating or an inclined chirped fiber Bragg grating.

2. The cascade multi-type grating based on the single stress element optical fiber 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 cascade multi-type grating based on the single stress element optical fiber and the preparation method thereof 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 cascade multi-type grating based on single stress element optical fiber and a preparation method thereof are characterized in that: the long-period chirped fiber grating and the chirped fiber Bragg grating or the inclined chirped fiber Bragg grating need to be prepared on a single stress element fiber by using different preparation methods in sequence.

5. The cascade multi-type grating based on the single stress element optical fiber and the preparation method thereof as claimed in claim 4, wherein the preparation method of the long period chirped fiber grating comprises the following steps:

6. The cascaded multi-type grating based on the single stress element optical fiber and the preparation method thereof as claimed in claim 4, wherein the preparation method of the chirped fiber Bragg grating comprises the following steps: