CN113777699B - Photonic crystal image transmission optical fiber and design method thereof - Google Patents

Photonic crystal image transmission optical fiber and design method thereof Download PDF

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CN113777699B
CN113777699B CN202111040708.6A CN202111040708A CN113777699B CN 113777699 B CN113777699 B CN 113777699B CN 202111040708 A CN202111040708 A CN 202111040708A CN 113777699 B CN113777699 B CN 113777699B
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fiber
diameter
photonic crystal
air
crystal image
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CN113777699A (en
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高福宇
范荣华
杨越
郑月
李立京
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Beihang University
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/02—Optical fibres with cladding with or without a coating
    • G02B6/02295—Microstructured optical fibre
    • G02B6/02314—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes
    • G02B6/02342—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by cladding features, i.e. light confining region
    • G02B6/02361—Longitudinal structures forming multiple layers around the core, e.g. arranged in multiple rings with each ring having longitudinal elements at substantially the same radial distance from the core, having rotational symmetry about the fibre axis
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B23/00—Telescopes, e.g. binoculars; Periscopes; Instruments for viewing the inside of hollow bodies; Viewfinders; Optical aiming or sighting devices
    • G02B23/24—Instruments or systems for viewing the inside of hollow bodies, e.g. fibrescopes
    • G02B23/26—Instruments or systems for viewing the inside of hollow bodies, e.g. fibrescopes using light guides
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/0012—Optical design, e.g. procedures, algorithms, optimisation routines

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Abstract

The invention discloses a photonic crystal image-transmitting fiber and a design method thereof, wherein a fiber core and a cladding of the photonic crystal image-transmitting fiber are coaxial, the fiber core is a solid core, the cladding comprises an air layer formed by air holes, the air holes are arranged in a hexagon shape integrally, the thickness of the air layer is less than or equal to 2.45 mu m, and the diameter of the air layer is less than or equal to 8.89 mu m. The optical fiber has the advantages of low visible light waveband limiting loss, superfine diameter, strong irradiation resistance and the like, can improve the resolution of the optical fiber endoscope and expand the application scene of the optical fiber endoscope when being used in the optical fiber endoscope, and can solve the problem that high-resolution imaging is difficult to realize in an irradiation environment.

Description

Photonic crystal image transmission optical fiber and design method thereof
Technical Field
The invention belongs to the technical field of special optical fibers, and particularly relates to an irradiation-resistant and ultrafine photonic crystal image transmission optical fiber in a visible light waveband and a design method thereof.
Background
Nuclear energy is a clean energy source, and is favored by people due to high power generation conversion efficiency, rich fuel reserves and wide application prospect. The defects are that the nuclear energy power generation can generate strong nuclear radiation (such as a large amount of gamma rays and the like), and once nuclear leakage occurs, huge life and property losses can be caused, and long-term damage can be caused to the environment. Therefore, the safe and reasonable utilization of nuclear energy for power generation has become one of the hot spots in global energy research.
In order to safely and controllably utilize nuclear energy for power generation, real-time imaging monitoring is needed to be carried out on the nuclear energy power generation process and the material changing process. Due to the particularity of nuclear reaction, the industrial camera can cause serious distortion of images under the environment of strong irradiation and cannot work normally. The influence of strong irradiation on the industrial camera is mainly reflected in two aspects: first, the refractive index, volume, etc. of the camera lens may be changed by the irradiation environment, resulting in a decrease in the signal-to-noise ratio and the imaging quality, and a deterioration in the structural matching. Can use doping irradiationStabilizers (e.g. CeO)2) The radiation-resistant lens solves the problem. Secondly, the image sensor (CCD/CMOS) in the digital camera is based on a doped semiconductor, electrons in the material can be converted into free electrons by separating from constraint through gamma rays in the irradiation environment, so that a false electric signal (not generated by real imaging rays) is generated, and noise caused by gamma radiation is generated; in addition, the interaction between the semiconductor crystal lattice and gamma ray produces new matter, which can form crystal lattice defect on the semiconductor to make the pixel point unable to sense light signal and become one permanent bad point on the image. Although the digital camera has high imaging resolution, an image sensor (CCD/CMOS) of the digital camera cannot normally work in real time under a strong irradiation environment; the image sensor of the analog camera is a vacuum tube, and the irradiation resistance is relatively better than that of the image sensor of the digital camera, but the resolution of the analog camera is lower and the imaging quality is poorer. In summary, to use a high-resolution image sensor (CCD/CMOS), the imaging system of the camera needs to be changed to keep the high-resolution image sensor away from the irradiation environment.
The imaging system of the optical fiber endoscope is that a target object is imaged to the incident end face of an optical fiber image transmission bundle by using an objective lens, the image of the emergent end face of the optical fiber image transmission bundle is coupled to the target face of an image sensor through a coupling mirror, and an optical signal is converted into an electric signal and is sent to a control display system to generate an image. As mentioned above, the imaging system of the optical fiber endoscope can realize that the image sensor works in the non-irradiation environment. According to research, the single optical fibers used by the existing optical fiber image transmission bundle are all step type optical fibers with doped fiber cores, and the optical fibers can generate huge transmission loss under a strong irradiation environment, so that images cannot be transmitted. The photonic crystal fiber is made of pure silica materials and has superior radiation resistance (as shown in table 1). Therefore, the problem of attenuation increase caused by the irradiation of the traditional optical fiber can be essentially solved by using the photonic crystal fiber as an image transmission beam.
TABLE 1 Photonic crystal fiber and conventional fiber irradiation experiment results
Note: irradiation environment dose rate 104Gy/h, time 16h
Figure BDA0003249110630000021
The optical fiber image transmission bundle is a core device of the optical fiber endoscope, the quality of key indexes directly determines the imaging quality and the working distance of the optical fiber endoscope, the diameter of the optical fiber determines the resolution of the image transmission bundle, and the limiting loss determines the transmittance of the image transmission bundle. The diameter of the bare fiber of the photonic crystal in the visible light wave band in the current optical fiber market is generally 125 μm, the diameter of the bare fiber of the special small-diameter photonic crystal is thinnest to 80 μm, and the requirement of manufacturing a high-resolution image transmission beam is far not met. Therefore, how to design the photonic crystal fiber with the visible light waveband, the ultra-fine property and the ultra-low loss becomes a key problem for realizing high-resolution imaging under the strong irradiation environment.
Disclosure of Invention
In order to solve the defects of the prior art, the invention provides an ultrafine photonic crystal fiber which can be used for high-resolution imaging in a visible light wave band under a strong irradiation environment and a design method thereof, and the specific technical scheme of the invention is as follows:
the utility model provides a photonic crystal image transmission fiber, photonic crystal image transmission fiber's fibre core is coaxial with the cladding, the fibre core is solid core, and the cladding includes the air bed that the air hole constitutes, the whole hexagon of being arranged of air hole, the thickness of air bed is less than or equal to 2.45 mu m, the diameter of air bed is less than or equal to 8.89 mu m.
Furthermore, the air holes are circular, the diameters of all the air holes are the same, the distances between the adjacent air holes are equal, and the three adjacent air holes are arranged in a triangular shape.
Furthermore, the photonic crystal image transmission fiber is made of pure silicon dioxide materials, has strong irradiation resistance and has the dose rate of 10 under the irradiation environment4Under the conditions of Gy/h and 16h, the attenuation caused by irradiation is less than 0.5 dB/km.
Furthermore, the ratio of the diameter of the air holes of the photonic crystal image transmission fiber to the distance between the air holes, namely the duty ratio, is more than or equal to 0.7.
Further, the photonic crystal image transmission fiber has low fiber limit lossAt 10-9In the order of dB/km.
Further, the transmission window of the photonic crystal image transmission fiber is in a visible light band.
A design method of a photonic crystal image transmission fiber comprises the following steps:
s1: determining the shape and arrangement mode of the air holes: selecting an arrangement mode of circular air holes and triangles based on production equipment of the photonic crystal fiber;
s2: determining duty cycle
Selecting a duty ratio not less than 0.7 according to a calculation formula (1) of the fiber core diameter and the air layer diameter and the influence of the fiber core diameter, the air hole diameter and the number of air hole layers of the photonic crystal image transmission fiber on the characteristics:
the calculation formula of the air layer diameter of the photonic crystal image transmission optical fiber is as follows:
Figure BDA0003249110630000031
wherein d isair layerIs the diameter of the air layer, noutIs the sum of the number of air holes and the number of air holes removed from the interior, dholeThe diameter of the air hole, and F is the duty ratio;
s3: determining the diameter range of the fiber core;
s4: determining the minimum thickness of the air layer: the diameter of the air hole and the number of the air hole layers are selected to meet the condition of limiting loss, and the minimum air layer thickness is selected.
Further, the specific process of step S3 is:
s3-1: establishing a photonic crystal image transmission optical fiber geometric model, and obtaining mode field distribution and the effective refractive index of a fundamental mode by using a finite element method;
s3-2: on the basis of a geometric model, the diameter of a fiber core is changed by removing the number of layers of air holes in the inner layer of the photonic crystal image transmission fiber, and the limiting loss and the diameter of the fiber core are calculated by the formulas (2) and (3):
the limiting loss equation:
Figure BDA0003249110630000032
wherein L isCFor the confinement loss of the fiber, i ═ x, y, x, y represent the x-polarization mode and the y-polarization mode, respectively, and Im represents the effective refractive index neffλ is the wavelength of the transmitted light;
the calculation formula of the diameter of the fiber core of the photonic crystal image transmission fiber is as follows:
Figure BDA0003249110630000033
wherein d iscoreIs the core diameter, ninRemoving the layer number of the internal air holes;
s3-3: repeating the step S3-2 to obtain a relation graph of the fiber core diameter and the limiting loss and a mode field distribution graph of different fiber core diameters;
s3-4: and (4) finding out a section in which the limiting loss changes tend to be stable along with the change of the fiber core diameter, namely the range of the fiber core diameter of the photonic crystal image transmission optical fiber, by analyzing and observing a relationship diagram between the fiber core diameter and the limiting loss obtained in the step S3-3 and a mode field distribution diagram of different fiber core diameters.
Further, the specific process of step S4 is as follows:
s4-1: the number of the air hole layers is n, and the limiting loss and the thickness of the air layer corresponding to the single-layer air hole are sequentially calculated from the fiber core to the outside in the diameter range of the fiber core;
s4-2: according to the step S4-1, in the diameter range of the fiber core, the limiting loss and the thickness of the air layer corresponding to the air holes of the two adjacent layers are calculated in sequence from the fiber core to the outside; repeating the steps until the limiting loss and the thickness of the air layer corresponding to the n layers of air holes are calculated;
s4-3: and analyzing and comparing the data obtained in the step S4-2 to determine the number of air hole layers and the diameter of the air holes.
The invention has the beneficial effects that: the optical fiber has the advantages of low limit loss of visible light wave band, superfine diameter, strong irradiation resistance and the like, can improve the resolution of the optical fiber endoscope and expand the application scene of the optical fiber endoscope when being used in the optical fiber endoscope, and can solve the problem that high-resolution imaging is difficult to realize in the irradiation environment.
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In order to illustrate embodiments of the present invention or technical solutions in the prior art more clearly, the drawings which are needed in the embodiments will be briefly described below, so that the features and advantages of the present invention can be understood more clearly by referring to the drawings, which are schematic and should not be construed as limiting the present invention in any way, and for a person skilled in the art, other drawings can be obtained on the basis of these drawings without any inventive effort. Wherein:
FIG. 1 is a diagram of analysis of key indicators of an optical fiber image bundle;
FIG. 2 is a flow chart of the design of the photonic crystal image-transmitting fiber of the present invention;
FIG. 3 is a cross-sectional view of a photonic crystal image-transmitting fiber of the present invention;
FIG. 4 is a graph of core diameter versus confinement loss at 780nm wavelength;
FIG. 5 is an enlarged view of a portion of FIG. 4;
FIG. 6 is a comparison of the mode field distributions of photonic crystal image-transmitting fibers of different core diameters, wherein (a) the mode field distribution has a core diameter of 0.68 μm; (b) a mode field distribution diagram with a fiber core diameter of 3.79 mu m;
FIG. 7 is a diagram showing a mode field distribution when the number of air hole layers is 1;
FIG. 8 is a diagram showing a mode field distribution when the number of air hole layers is 2;
FIG. 9 is a diagram showing a mode field distribution when the number of air holes is 3;
FIG. 10 is a diagram showing a mode field distribution when the number of air hole layers is 4;
FIG. 11 is a diagram showing a mode field distribution when the number of air hole layers is 5.
Detailed Description
In order that the above objects, features and advantages of the present invention can be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings. It should be noted that the embodiments of the present invention and features of the embodiments may be combined with each other without conflict.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those specifically described herein, and therefore the scope of the present invention is not limited by the specific embodiments disclosed below.
The optical fiber image transmission bundle is a core device of the optical fiber endoscope, the quality of key indexes directly determines the imaging quality and the working distance of the optical fiber endoscope, the index requirements of the photonic crystal image transmission optical fiber are given in figure 1, the resolution of the image transmission bundle is determined by the diameter of the optical fiber, and the transmittance of the image transmission bundle is determined by the limiting loss. The diameter of the bare fiber of the photonic crystal in the visible light wave band in the current optical fiber market is generally 125 μm, the diameter of the bare fiber of the special small-diameter photonic crystal is thinnest to 80 μm, and the requirement of manufacturing a high-resolution image transmission beam is far not met. Therefore, designing the photonic crystal fiber with a visible light waveband, superfine performance and ultralow loss becomes a key problem for realizing high-resolution imaging in a strong irradiation environment.
The transmission characteristic of the photonic crystal image transmission optical fiber is determined by the diameter of the fiber core and the thickness of the air hole; the thickness of the part without the air hole in the cladding only determines the mechanical strength of the photonic crystal fiber, the thickness can be very thin in the actual manufacturing process, and the lost mechanical strength can be compensated by special packaging. Thus, the air layer diameter determines the diameter of the photonic crystal fiber. The invention provides an irradiation-resistant and superfine photonic crystal image transmission fiber with visible light wave band, which has the following main structural parameters: the shape of the air holes, the arrangement mode of the air holes, the duty ratio, the diameter of the fiber core, the diameter of the air holes and the number of the air hole layers. The influence of the structural parameters on the fiber characteristic qualitative is shown in table 2, and according to the analysis of indexes in fig. 1, the diameter of the optical fiber is minimized by adjusting the structural parameters of the photonic crystal image transmission fiber under the condition of meeting low limiting loss, so as to meet the requirement for manufacturing a high-resolution image transmission beam.
TABLE 2 qualitative relationship of the influence of the structural parameters of photonic crystal image-transmitting fiber on its characteristics
Figure BDA0003249110630000051
Figure BDA0003249110630000061
The detailed design flow of the photonic crystal image transmission fiber is shown in fig. 2. Firstly, the shape and arrangement mode of the air holes are determined according to the production equipment and the manufacturing process of the photonic crystal fiber. Secondly, the larger the F, the stronger the bending resistance of the photonic crystal image-transmitting fiber (flexible use) and the larger the numerical aperture (high coupling efficiency) are obtained according to the table 2; reasonable reduction n is obtained by formula (1) and formula (3)in、noutAnd dholeAnd increasing F can reduce the air layer diameter; the index of the photonic crystal image transmission fiber provided by the invention requires that a large duty ratio is selected as much as possible. Under the duty ratio, an interval that the limiting loss changes steadily along with the change of the fiber core diameter is found, and the interval is the range of the fiber core diameter. And finally, determining the minimum thickness of the air layer under the condition of meeting the requirement on the limiting loss, if the requirement on the imaging resolution is met, directly outputting the structural parameters of the photonic crystal image transmission optical fiber, and if the requirement on the imaging resolution is not met, reselecting the duty ratio and repeating the steps until the structural parameters of the optical fiber are output until the requirement on the imaging resolution is met.
The photonic crystal image-transmitting fiber has a fiber core and a cladding which are coaxial, the fiber core is a solid core, the cladding comprises an air layer formed by air holes, the air holes are arranged in a hexagonal shape integrally, the thickness of the air layer is less than or equal to 2.45 mu m, and the diameter of the air layer is less than or equal to 8.89 mu m.
In some embodiments, the air holes are circular, all the air holes have the same diameter, the distance between adjacent air holes is equal, and three adjacent air holes are arranged in a triangle.
In some embodiments, the number of air hole layers is 2, the air hole diameter is 1.16 μm, the core diameter is 3.99 μm, the air layer thickness is 2.45 μm, and the air layer diameter is 8.89 μm.
In some embodiments, the number of air hole layers is 3, the air hole diameter is 0.68 μm, the core diameter is 3.85 μm, the air layer thickness is 2.19 μm, and the air layer diameter is 8.23 μm.
Preferably, the number of air hole layers is 4, the diameter of the air holes is 0.48 μm, the diameter of the core is 3.78 μm, the thickness of the air layer is 2.28 μm, and the diameter of the air layer is 7.94 μm.
In some embodiments, the photonic crystal image transmission fiber is made of pure silicon dioxide material, has strong irradiation resistance and has the dose rate of 10 in the irradiation environment4Under the conditions of Gy/h and 16h, the attenuation caused by irradiation is less than 0.5 dB/km.
In some embodiments, the ratio of the diameter of the air holes to the pitch of the air holes, i.e., the duty ratio, of the photonic crystal image transmission fiber is greater than or equal to 0.7.
In some embodiments, the photonic crystal image-transmitting fiber has a fiber confinement loss of less than 10-9In the order of dB/km.
In some embodiments, the transmission window of the photonic crystal image transmission fiber is in the visible light band.
A design method of a photonic crystal image transmission fiber comprises the following steps:
s1: determining the shape and arrangement mode of the air holes: selecting an arrangement mode of circular air holes and triangles based on production equipment of the photonic crystal fiber;
s2: determining duty cycle
Selecting a duty ratio not less than 0.7 according to a calculation formula (1) of the fiber core diameter and the air layer diameter and the influence of the fiber core diameter, the air hole diameter and the number of air hole layers of the photonic crystal image transmission fiber on the characteristics:
the calculation formula of the diameter of the air layer of the photonic crystal image-transmitting optical fiber is as follows:
Figure BDA0003249110630000071
wherein d isair layerIs the diameter of the air layer, noutIs the sum of the number of air holes and the number of air holes removed from the interior, dholeThe diameter of the air hole, and F is the duty ratio;
s3: determining the diameter range of the fiber core;
s4: determining the minimum thickness of an air layer; the diameter of the air hole and the number of the air hole layers determine the thickness of the air layer and the limiting loss of the optical fiber, and the diameter of the air hole and the number of the air hole layers are selected to meet the condition of limiting loss, so that the minimum thickness of the air layer is selected.
The specific process of step S3 is:
s3-1: establishing a photonic crystal image transmission optical fiber geometric model, and obtaining mode field distribution and the effective refractive index of a fundamental mode by using a finite element method;
s3-2: on the basis of a geometric model, the diameter of a fiber core is changed by removing the number of layers of air holes in the inner layer of the photonic crystal image transmission fiber, and the limiting loss and the diameter of the fiber core are calculated by the formulas (2) and (3):
the limiting loss equation:
Figure BDA0003249110630000072
wherein L isCFor the confinement loss of the fiber, i ═ x, y, x, y represent the x-polarization mode and the y-polarization mode, respectively, and Im represents the effective refractive index neffλ is the wavelength of the transmitted light;
the calculation formula of the diameter of the fiber core of the photonic crystal image transmission fiber is as follows:
Figure BDA0003249110630000081
wherein d iscoreIs the core diameter, ninRemoving the layer number of the internal air holes;
s3-3: repeating the step S3-2 to obtain a relation graph of the fiber core diameter and the limiting loss and a mode field distribution graph of different fiber core diameters;
s3-4: and finding out an interval in which the limiting loss changes tend to be stable along with the change of the fiber core diameter, namely the range of the diameter of the fiber core of the photonic crystal image transmission optical fiber, by analyzing and observing a relation graph of the fiber core diameter and the limiting loss obtained in the step S3-3 and mode field distribution graphs of different fiber core diameters.
Firstly, according to a relation graph of the core diameter and the limiting loss, the limiting loss changes to a stable approximate interval along with the change of the core diameter, and then according to a mode field distribution graph (whether the air layer completely binds light in the core is mainly observed) of different core diameters, an accurate range is obtained.
The specific process of step S4 is:
s4-1: the number of the air hole layers is n, and the limiting loss and the thickness of the air layer corresponding to the single-layer air hole are sequentially calculated from the fiber core to the outside in the diameter range of the fiber core;
s4-2: according to the step S4-1, in the diameter range of the fiber core, the limiting loss and the thickness of the air layer corresponding to the air holes of the two adjacent layers are calculated in sequence from the fiber core to the outside; repeating the steps until the limiting loss and the thickness of the air layer corresponding to the n layers of air holes are calculated;
s4-3: and analyzing and comparing the data obtained in the step S4-2 to determine the number of air hole layers and the diameter of the air holes.
For the convenience of understanding the above technical aspects of the present invention, the following detailed description will be given of the above technical aspects of the present invention by way of specific examples.
Example 1
The simulation process of the irradiation-resistant and ultra-fine photonic crystal image-transmitting fiber in the visible light band is described by taking a circular air hole, a triangular arrangement mode and a duty ratio of 0.9 as an example (the section is shown in figure 3).
(1) Determining the minimum core diameter
The diameter of the air hole is selected to be 0.2 μm (in order to observe the trend relationship between the core diameter and the loss more finely, the size of the loss value is not important at this time), the number of air hole layers is 5,
Figure BDA0003249110630000082
other structural parameters are ensured to be unchanged, the size of the fiber core diameter is only changed (realized by increasing and removing the number of inner layer air holes and increasing the fiber core diameter), and the value range of the fiber core diameter which limits the loss change to be stable is explored.
The confinement loss for a core diameter varying from 0.68 μm to 6.46 μm is shown in FIGS. 4 and 5, where it can be seen that the confinement loss variation is significant for core diameters less than 3.35 μm and very slight for core diameters greater than 3.35 μm, i.e., a minimum of 3.35 μm.
It can be seen from fig. 6(a) that when the core diameter is small (comparable to the wavelength size of the transmitted light), the air holes of the cladding layer cannot completely confine the light to the core, resulting in very large confinement loss. As shown in fig. 6(b), the air holes of the cladding layer can completely confine light to the core for transmission when the core diameter is increased to 3.79 μm.
(2) Determining the minimum thickness of the air layer
In the visible light wave band, the limit loss is ensured to be certain (10)-9dB/m) the minimum thickness of the air layer was investigated. The limit loss of the photonic crystal image transmission fiber is mainly determined by the number of air hole layers and the diameter of the air holes, and the number of the air hole layers is stronger than the diameter of the air holes for influencing the limit loss. Combining the above analysis of the minimum diameter of the fiber core, the fiber core diameter is ensured to be at least larger than 3.35 μm by reducing the number of layers in the air holes, and the limiting loss reaches a set value (10) by reducing the diameter of the air holes and increasing the number of layers-9dB/m), the change in the diameter of the air layer was observed.
As shown in Table 3, the air holes had 1 layer, and the loss was restricted to a set value of 10 when the diameter of the air holes was increased to 8.6 μm-9In dB/m order, the thickness of the air layer is 8.6 μm. By analogy, when the number of air hole layers is 5, the thickness of the air layer reaches the inflection point, namely, the number of air hole layers is 4, the diameter of the air hole is 0.48 μm, the diameter of the fiber core is 3.78 μm (larger than the minimum diameter of the fiber core), and the thickness of the air layer reaches the minimum value when the fiber core is 2.08 μm.
It is further seen from table 3 that the air layer thickness variation is large when the number of air hole layers is changed from 1 layer to 2 layers, and in comparison, the air layer thickness variation is gentle when the number of air hole layers is changed from 2 layers to 3 layers to 4 layers, which means that the parameters corresponding to the air hole structures of 2 layers, 3 layers and 4 layers can be used as the structural parameters of the ultra-fine photonic crystal image-transmitting optical fiber.
TABLE 3 air layer thickness variation Table for different air hole layer number and air hole diameter at 780nm wavelength
Figure BDA0003249110630000091
Fig. 7-11 are mode field profiles for different numbers of layers, and it can be seen that when the core diameter is larger than the minimum core diameter, the air holes of the cladding substantially completely confine the light waves to be transmitted in the core.
Based on the theoretical analysis and simulation verification, the air hole structure parameters of 2 layers, 3 layers and 4 layers can be used as the structure parameters of the superfine photonic crystal image transmission optical fiber. The optimal design result is as follows: the number of air holes is 4, the diameter of the air hole is 0.48 mu m, and the duty ratio is
Figure BDA0003249110630000101
The diameter of the air layer is 7.94 μm, the confinement loss is 9.84E-09dB/m, and the mode field distribution is shown in FIG. 11.
The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (7)

1. The photonic crystal image transmission fiber is characterized in that a fiber core and a cladding of the photonic crystal image transmission fiber are coaxial, the fiber core is a solid core, the cladding comprises air layers formed by air holes, the air holes are arranged in a hexagonal shape as a whole, the thickness of each air layer is less than or equal to 2.45 micrometers, and the diameter of each air layer is less than or equal to 8.89 micrometers;
the air holes are circular, the diameters of all the air holes are the same, the distances between every two adjacent air holes are equal, and the three adjacent air holes are arranged in a triangular shape;
the photonic crystal image transmission fiber is made of pure silicon dioxide materials, has strong irradiation resistance and has the dose rate of 10 under the irradiation environment4Under the conditions of Gy/h and 16h, the attenuation caused by irradiation is less than 0.5 dB/km.
2. The photonic crystal image transmission fiber of claim 1, wherein the ratio of the diameter of the air holes to the pitch of the air holes, i.e. the duty cycle, of the photonic crystal image transmission fiber is not less than 0.7.
3. A photonic crystal image transmitting fiber according to claim 1 or 2, wherein the photonic crystal image transmitting fiber has a fiber confinement loss of less than 10-9In the order of dB/km.
4. The photonic crystal image transmission fiber of claim 1, wherein the transmission window of the photonic crystal image transmission fiber is in the visible light band.
5. A method of designing a photonic crystal image-transmitting fiber according to any one of claims 1 to 4, comprising the steps of:
s1: determining the shape and arrangement mode of the air holes: selecting an arrangement mode of circular air holes and triangles based on production equipment of the photonic crystal fiber;
s2: determining duty cycle
Selecting a duty ratio not less than 0.7 according to a calculation formula (1) of the fiber core diameter and the air layer diameter and the influence of the fiber core diameter, the air hole diameter and the number of air hole layers of the photonic crystal image transmission fiber on the characteristics:
the calculation formula of the diameter of the air layer of the photonic crystal image-transmitting optical fiber is as follows:
Figure FDA0003610599070000011
wherein, dairlayerIs the diameter of the air layer, noutIs the sum of the number of air holes and the number of air holes removed from the interior, dholeThe diameter of the air hole, and F is the duty ratio;
s3: determining the diameter range of the fiber core;
s4: determining the minimum thickness of the air layer: the diameter of the air hole and the number of the air hole layers are selected to meet the condition of limiting loss, and the minimum air layer thickness is selected.
6. The method as claimed in claim 5, wherein the step S3 is performed by the following steps:
s3-1: establishing a photonic crystal image transmission optical fiber geometric model, and obtaining mode field distribution and the effective refractive index of a fundamental mode by using a finite element method;
s3-2: on the basis of a geometric model, the diameter of a fiber core is changed by removing the number of layers of air holes in the inner layer of the photonic crystal image transmission fiber, and the limiting loss and the diameter of the fiber core are calculated by the formulas (2) and (3):
the limiting loss equation:
Figure FDA0003610599070000021
wherein L isCFor the confinement loss of the fiber, i ═ x, y, x, y represent the x-polarization mode and the y-polarization mode, respectively, and Im represents the effective refractive index neffλ is the wavelength of the transmitted light;
the calculation formula of the diameter of the fiber core of the photonic crystal image transmission fiber is as follows:
Figure FDA0003610599070000022
wherein d iscoreIs the core diameter, ninRemoving the layer number of the internal air holes;
s3-3: repeating the step S3-2 to obtain a relation graph of the fiber core diameter and the limiting loss and a mode field distribution graph of different fiber core diameters;
s3-4: and (4) finding out a section in which the limiting loss changes tend to be stable along with the change of the fiber core diameter, namely the range of the fiber core diameter of the photonic crystal image transmission optical fiber, by analyzing and observing a relationship diagram between the fiber core diameter and the limiting loss obtained in the step S3-3 and a mode field distribution diagram of different fiber core diameters.
7. The method as claimed in claim 6, wherein the step S4 comprises the following steps:
s4-1: the number of the air hole layers is n, and the limiting loss and the thickness of the air layer corresponding to the single-layer air hole are sequentially calculated from the fiber core to the outside in the diameter range of the fiber core;
s4-2: according to the step S4-1, in the diameter range of the fiber core, the limiting loss and the thickness of the air layer corresponding to the air holes of the two adjacent layers are calculated in sequence from the fiber core to the outside; repeating the steps until the limiting loss and the thickness of the air layer corresponding to the n layers of air holes are calculated;
s4-3: and analyzing and comparing the data obtained in the step S4-2 to determine the number of air hole layers and the diameter of the air holes.
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