CN116540351A - Double-core hollow anti-resonance optical fiber polarization beam splitter - Google Patents
Double-core hollow anti-resonance optical fiber polarization beam splitter Download PDFInfo
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- 230000010287 polarization Effects 0.000 title claims abstract description 132
- 239000013307 optical fiber Substances 0.000 title abstract description 33
- 239000000835 fiber Substances 0.000 claims abstract description 114
- 238000005253 cladding Methods 0.000 claims abstract description 53
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims abstract description 44
- 239000010453 quartz Substances 0.000 claims abstract description 33
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- 239000012510 hollow fiber Substances 0.000 description 8
- 230000002829 reductive effect Effects 0.000 description 7
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- 239000011521 glass Substances 0.000 description 6
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- XHGGEBRKUWZHEK-UHFFFAOYSA-L tellurate Chemical compound [O-][Te]([O-])(=O)=O XHGGEBRKUWZHEK-UHFFFAOYSA-L 0.000 description 5
- 238000004891 communication Methods 0.000 description 4
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- 239000000758 substrate Substances 0.000 description 1
- SITVSCPRJNYAGV-UHFFFAOYSA-L tellurite Chemical compound [O-][Te]([O-])=O SITVSCPRJNYAGV-UHFFFAOYSA-L 0.000 description 1
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- G—PHYSICS
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
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- G—PHYSICS
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- 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
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- 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
- G02B6/02214—Optical fibres with cladding with or without a coating tailored to obtain the desired dispersion, e.g. dispersion shifted, dispersion flattened
- G02B6/02285—Characterised by the polarisation mode dispersion [PMD] properties, e.g. for minimising PMD
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
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Abstract
The invention discloses a double-core hollow anti-resonance optical fiber polarization beam splitter, which comprises a cladding region and a fiber core region; the cladding region comprises 8 quartz glass tubes with different sizes and internally nested circles or anti-resonance walls, and the quartz glass tubes are arranged in a round shape in a compact manner, and asymmetry in the x and y directions is increased by introducing semicircular semi-elliptic quartz glass tubes so as to achieve the effect of increasing double refraction. Meanwhile, the fiber core is divided into two symmetrical fiber cores, an energy coupling mechanism between modes of the two cores is introduced, and 4 supermode refractive indexes and coupling lengths of two polarization directions of the two cores are adjusted. In addition, an anti-resonance wall is nested in the semicircular semi-elliptical tube, two quartz-air interfaces are brought by adding one quartz layer, the effective refractive index of a tube mode is changed by adjusting the position of the anti-resonance wall, the effects of reducing the loss of a fiber core fundamental mode and increasing the loss of a high-order mode are achieved, and finally the beam splitter with good single-mode characteristics and good polarization beam splitting performance is provided.
Description
Technical Field
The invention relates to the technical field of polarization beam splitters, in particular to a hollow anti-resonance optical fiber polarization beam splitter with a double-core structure.
Background
With the continuous development of social informatization, the transmission of network information is increasingly advanced, and the gradual development of optical fiber communication technology is promoted. The optical device is an important component of optical fiber communication, and the optical fiber beam splitter is a passive device for realizing optical signal splitting, combining and distributing, and is an optical device which is indispensable in wavelength division multiplexing, optical fiber local area networks and certain measuring instruments. The main function of the polarizing beam splitter is to split one light beam into two mutually orthogonal polarized light beams. The polarization beam splitter based on the traditional optical fiber also has the characteristic of wavelength dependence, and the working band is single, which greatly limits the application range. Due to the unique structure of the hollow fiber, light can be limited to be transmitted in the hollow fiber, and the hollow fiber is not limited by the damage threshold of the material. The technology of the hollow anti-resonance optical fiber is developed rapidly, so that the total loss of 0.13dB/km in the O band can be realized, and the technology is superior to that of a solid optical fiber; but also has the effective single-mode characteristic required by long-distance data transmission, and becomes the most promising substitute of solid optical fiber. Because of the characteristics of simple structure, low loss, wide bandwidth and the like, the hollow anti-resonance optical fiber technology also reaches the early commercialization stage, and can be applied to a plurality of other fields such as optical communication, high-power pulse transmission, infrared transmission and the like. Therefore, research on beam splitters based on hollow anti-resonance optical fibers is developed, a novel polarization beam splitter is designed to solve the technical problems of narrow bandwidth, low extinction ratio, long length and the like of the traditional polarization beam splitter, and the novel polarization beam splitter has very important significance for promoting application and development of the hollow anti-resonance optical fibers.
There is currently a beam splitter developed for hollow-core antiresonant fibers, one of which (H.Q.Jia, X.Wang, T.T.Zhao, Z.J.Tang, Z.G.Lian, S.Q.Lou, and X.Z. generating. Ultrawideband splitter with single-mode polarization beam splitter based on dual-cavity-core anti-resonantfiber [ J ]. Applied optics.2021, 60:9781-9789.): an elliptical tube is introduced into the cladding, the middle large fiber core is divided into A, B fiber cores, and two small tubes are nested in the elliptical tube, so that the dependence of the coupling length on the wavelength is reduced, and the ultra-wideband is obtained. The numerical calculation result shows that the length of the device is 8.15cm, the working bandwidth is 370nm, and the working wavelength is 1.28-1.65 mu m. However, this document does not effectively change the refractive index of the tube mode, resulting in a longer final device length.
Another version (Y.T.Ni, J.H.Yuan, S.Qiu, G.Y.Zhou, C.M.Xia, X.Zhou, B.B.Yan, Q.Wu, K.R.Wang, X.Z.Sang, and c.x.yu.dual hose-core negative curvature fiber polarization beam splitter covering the O +e+s+c+ Lcommunication band [ J ]. Journal of the Optical Society of America b.2022,39 (9): 2493-2501.): eight round quartz glass tubes which are closely arranged are wound around the periphery of the fiber core, small tubes are nested inside six tubes, and the rest two tubes are not nested; two small tubes are introduced in the x-axis direction to divide the air core into two, two symmetrical cores a and B. Simulation results show that the length of the polarization beam splitter is 4.1cm, the broadband of 400nm is realized, the O+E+S+C+L communication wave bands are covered, and the polarization beam splitter has good single-mode characteristics. However, the large quartz tube nested with the small quartz tube in the document has limited changeable parameters, and cannot optimize more structural parameters so that the coupling length in the y polarization direction is always larger than the coupling length in the x polarization direction, and the coupling length does not change greatly with the wavelength, thus resulting in longer final device length.
Yet another scheme (C.J.Wang, H.J.Du, S.G.Li, J.S.Li, X.J.Meng, Z.Y.Yin, X.X.Ma, C.Wang, "Wide bandwidth and short-length polarization beam splitter based on tellurite glass dual hollow-core anti-resonantfiber," Optik-International Journal for Light and Electron optics.280 (2023)): eight closely arranged round tellurate glass tubes are wound on the periphery of the fiber core, small tubes are nested inside the six tubes, and two tubes in the rest y-axis direction are not nested; two internally nested small tubes in the x-axis direction of a circular tellurate glass tube divide the air core into two. To prevent the glass tube from hanging in the air, two small circular tellurate glass tubes are introduced in the x-axis direction as intermediaries for connection with the cladding. Simulation results show that the length of the polarization beam splitter is 2.122cm, the broadband of 400nm is realized, the single-mode performance can be realized within the bandwidth range of 1.26-1.66 mu m, and the polarization extinction ratio is up to-292.83 dB. However, the tellurate glass has very high nonlinear characteristic and wide infrared transmission range, but the asymmetry in the x and y directions is insufficient, and the polarization extinction ratio is not high.
Disclosure of Invention
The invention aims to solve the technical problems and provides a double-core hollow anti-resonance optical fiber polarization beam splitter, which increases asymmetry in the x and y directions by introducing a semicircular semi-elliptic quartz glass tube so as to achieve the effect of increasing double refraction; dividing the fiber core into two symmetrical fiber cores, introducing an energy coupling mechanism between modes of the two cores, and adjusting the 4 supermode refractive indexes of the double cores and the coupling lengths of the two polarization directions; in addition, an anti-resonance wall is nested in a semicircle semi-ellipse, two quartz-air interfaces are brought by adding one quartz layer, and the effective refractive index of a pipe mode is changed by adjusting the position of the anti-resonance wall, so that the effects of reducing the loss of a fiber core fundamental mode and increasing the loss of a high-order mode are achieved; finally, the beam splitter with good single-mode characteristics and good polarization beam splitting performance is provided.
In order to solve the technical problems, the invention adopts the following technical scheme:
the double-core hollow anti-resonance optical fiber polarization beam splitter comprises an outer quartz support ring, a middle fiber core region and a cladding region between the outer quartz support ring and the middle fiber core region, wherein the fiber core region is an air fiber core region and comprises two symmetrical fiber cores A and B; the cladding region comprises cladding round pipes positioned at two ends of the y direction, semicircular semi-elliptical pipes positioned at two ends of the x direction and large round pipes closely arranged between the adjacent cladding round pipes and the semicircular semi-elliptical pipes, the diameters of the cladding round pipes and the large round pipes are d0 and d1 respectively, a second round pipe with the diameter of k x d0 and a first round pipe with the diameter of k x d1 are respectively nested inside the cladding round pipes, d0 is smaller than d1, the semicircular diameter of the semicircular semi-elliptical pipe is d2, the minor axis of the semicircular is d2, the major axis of the semicircular semi-elliptical pipe is d2/hudu, and an anti-resonance wall with the length of d2 is nested at the connecting position of the semicircular and the semicircular.
By introducing a semicircular semi-elliptical tube, the asymmetry in the x and y directions is increased so as to achieve the effect of increasing the birefringence; meanwhile, the fiber core is divided into two symmetrical fiber cores A and B, an energy coupling mechanism between modes of the two cores is introduced, and 4 supermode refractive indexes of the two cores and coupling lengths of the two polarization directions are adjusted; in addition, an anti-resonance wall is nested in the semicircular semi-elliptical tube, two quartz-air interfaces are brought by adding one quartz layer, the effective refractive index of a tube mode is changed by adjusting the position of the anti-resonance wall, the effects of reducing the loss of a fiber core fundamental mode and increasing the loss of a high-order mode are achieved, and finally the beam splitter with good single-mode characteristics and good polarization beam splitting performance is provided.
The technical scheme of the invention is further improved as follows: the thickness of the large circular tube, the cladding circular tube, the semicircular semi-elliptical tube, the first circular tube, the second circular tube and the antiresonant wall is the same, and the thickness is 0.55-0.56 mu m.
The technical scheme of the invention is further improved as follows: the large round tube, the cladding round tube, the semicircular semi-elliptic tube, the first round tube, the second round tube and the antiresonance wall are made of pure quartz glass with the refractive index of 1.458, and the other spaces are air with the refractive index of 1.
The technical scheme of the invention is further improved as follows: the diameter d1 of the large circular tube is 16.36-16.4 mu m.
The technical scheme of the invention is further improved as follows: the diameter d0 of the cladding circular tube is 15.3-15.5 mu m.
The technical scheme of the invention is further improved as follows: the value range of k is 0.56-0.68.
The technical scheme of the invention is further improved as follows: in the semicircular semi-elliptical tube, the value range of d2 is 15.89-15.94 mu m, and the range of the ratio hudu is 0.387-0.395.
By adopting the technical scheme, the technical progress of the invention mainly comprises the following three points:
(1) The coupling length of the two polarization directions is different enough, the coupling length of the y polarization direction is always larger than the coupling length of the x polarization direction, and the coupling length is not changed greatly with the wavelength.
The two symmetrical fiber cores A, B of the double-core hollow anti-resonance fiber polarization beam splitter provided by the invention are all air, so that the refractive index of the air is used as the upper limit of the refractive index of the respective fiber core modes by the two fiber cores A, B. The structure of the hollow anti-resonance optical fiber polarization beam splitter introduces a semicircular semi-elliptical tube, ensures the asymmetry in two orthogonal directions of the hollow anti-resonance optical fiber polarization beam splitter, and introduces high birefringence. Meanwhile, due to the fact that the double cores are introduced, the circular center symmetry of the traditional optical fiber is broken, the originally degenerated x-polarized light and y-polarized light are degenerated, and 4 non-degenerated modes exist in the waveguide, wherein the modes are respectively as follows: the optical fiber comprises an x-polarization even mode, an x-polarization odd mode, a y-polarization even mode and a y-polarization odd mode, wherein the odd modes refer to the opposite directions of mode electric fields in two fiber cores, and the even modes refer to the same directions of mode electric fields in the two fiber cores. The odd mode and the even mode under the same polarized light have the same polarization direction, the coupling effect occurs between the odd mode and the even mode under the same polarized light due to interference, the odd mode and the even mode under different polarized light have different polarization directions, and the coupling effect does not occur between the odd mode and the even mode. After the fiber core region is electrified, 4 nondegenerate state modes, namely supermodes, namely transmission constants of an odd mode and an even mode are different due to structural asymmetry in two polarization directions, and x-polarized light and y-polarized light are periodically transferred between the fiber core A and the fiber core B. The shortest distance required when light polarized in either x and y directions is completely coupled from one fiber core to the other is the coupling length, and the calculation formula is thatThe structure must make the coupling length of two polarization directions enough in the operation bandIs a difference in (a) between the two.
The cladding region includes 8 quartz tubes and is symmetrical up, down, left and right, so that the area of the core A, B is the same. The second circular tube is nested in the cladding circular tube in the y direction, and the anti-resonance wall is nested in the semicircular semi-elliptical tube in the x direction, so that the coupling length in the y polarization direction is always larger than the coupling length in the x polarization direction. Meanwhile, the nested second round tube and the anti-resonance wall can form two quartz-air interfaces, the coupling inhibition effect between the core die and the tube die is enhanced, and therefore energy is limited to the fiber core. All quartz tubes, including large round tube, clad round tube, semi-circular semi-elliptical tube, first round tube and second round tube, and nested antiresonance walls, have uniform wall thickness, and due to antiresonance effect, the thickness of the tube determines the resonant wavelength lambda res The calculation formula isIn order to obtain lower losses, the selected band should be the antiresonant band. In the antiresonant wavelength region, the loss is lower, the effective refractive indexes of four supermodes which are coupled in two fiber cores are not greatly different, the shortest distance required when light in any polarization direction is completely coupled to the other fiber core by one fiber core is also relatively stable, the coupling length is not greatly changed along with the change of the wavelength, thereby overcoming the dependence on the wavelength, enabling the working wavelength of the beam splitter to be wider, and further increasing the working bandwidth of the beam splitter.
The technical scheme of increasing the wall thickness of all quartz tubes and anti-resonance walls can increase the area of all quartz tubes and anti-resonance walls in a cladding region at the same time, reduce the area of a fiber core region and influence the effective refractive indexes of four supermodes. The resonance wavelength is increased, and the antiresonant window of the antiresonant optical fiber is influenced, so that the window shifts to a longer wavelength along with the increase of the thickness, thereby being unfavorable for the widening of the working wavelength of the device and the increase of the bandwidth. And the lower limit of the mode refractive index of the 4 supermodes of the two polarization directions of the fiber core is raised simultaneously, and the coupling length of the two polarization directions is shortened. In summary, this technical solution is not beneficial to realizing that the coupling lengths of the two polarization directions have enough difference, and the coupling length of the y polarization direction is always larger than the coupling length of the x polarization direction, and the coupling length does not change greatly with the wavelength.
The technical scheme of increasing the diameter of the cladding circular tube in the y polarization direction can reduce the area of the two fiber cores A, B and can influence the asymmetry of the two polarization directions. The diameter of the cladding circular tube is the minimum value in all quartz tube diameters, so that the effect of limiting the light energy on the fiber cores is minimum, the coupling strength between the fundamental modes in the two fiber cores is enhanced, and the coupling length is reduced. However, increasing the diameter of the cladding circular tube not only reduces the lower limit of the mode refractive index of four supermodes in two polarization directions of the fiber core region, but also increases the loss of the fundamental mode of the fiber core. The technical scheme is not beneficial to realizing that the coupling length of two polarization directions has enough difference, the coupling length of the y polarization direction is always larger than the coupling length of the x polarization direction, and the coupling length is not greatly changed along with the wavelength.
The technical solution of increasing the ratio k affects the asymmetry of the two polarization directions. Increasing the ratio k not only reduces the lower mode index limit of the 4 supermodes for both polarization directions of the core, but also shortens the coupling length for both polarization directions. The technical scheme is not beneficial to realizing that the coupling length of two polarization directions has enough difference, the coupling length of the y polarization direction is always larger than the coupling length of the x polarization direction, and the coupling length is not greatly changed along with the wavelength.
The technical scheme of the invention can realize that the coupling lengths of the two polarization directions have enough difference, the coupling length of the y polarization direction is always larger than the coupling length of the x polarization direction, and the coupling length is not changed greatly along with the wavelength.
(2) The effective refractive index of the pipe mode is matched with the refractive index of the higher-order mode of the fiber core at the working wavelength, so that the good single-mode characteristic of the hollow anti-resonance fiber polarization beam splitter is achieved.
Anti-resonant walls are introduced to enhance the light confinement of the fiber cladding region. The light confinement capacity of the cladding region is proportional to the number of antiresonant walls, and thus, the light confinement capacity of the fiber cladding region is enhanced by adding the nested tubes (first and second tubes) and antiresonant walls. The ratio of the lowest Higher Order Mode (HOM) loss to the highest fundamental Film (FM) loss of the core, referred to as the higher order mode extinction ratio (home), is used to describe the proposedThe single-mode performance of the output optical fiber is calculated as followsThe larger the HOMER value, the better the single mode characteristic. The anti-resonance wall is movable, has more regulation and control performance on the distribution of energy in the tube relative to the circular nested tube, changes the effective refractive index in the tube, is consistent with the refractive index of the high-order mode of the fiber core, and generates mode coupling so as to increase the loss value of the high-order mode of the fiber core. Molecular increase in the HOMER value as a ratio for measuring the single mode characteristics of the beam splitter device; while suppressing coupling of the mandrel to the tube die, the denominator in the HOMER value is reduced and this ratio must be increased.
A semicircular semi-elliptical tube is introduced in the x polarization direction, the elliptical curvature is larger than that of a circle, and the large curvature can lead to small transverse area and large longitudinal area of the quartz tube. Selecting a circle with smaller curvature near the fiber core region, so that energy leakage of the fiber core region to a cladding region outside the tube can be avoided; the elliptic shape with large curvature is selected far away from the fiber core region, and the elliptic curvature is adjustable, so that the coupling between a pipe mode and a cladding mode outside the pipe and a high-order mode of the fiber core is facilitated, and the coupling inhibition can be increased, so that the molecular increase, the denominator decrease and the ratio increase in the single-mode model HOMER value of the measuring beam splitter device are necessarily caused. The anti-resonance wall supporting ring is introduced between the quartz glass tubes, so that the loss can be reduced, and the supporting effect on the optical fiber is achieved, so that the optical fiber is easier to prepare.
The technical scheme of increasing the semicircular diameter in the semicircular semi-elliptical tube in the x polarization direction can reduce the area of two fiber cores and simultaneously narrow the coupling channels of the two fiber cores. As a quartz tube which is close to the core region and divides the core region into two, the loss of the fundamental mode of the core is greatly affected. Increasing the diameter of the semicircle reduces the loss of the higher order modes of the core, which has a great influence on the single mode characteristics of the device. The technical scheme is not beneficial to realizing the matching of the effective refractive index of the pipe mode and the refractive index of the higher-order mode of the fiber core at the working wavelength, and achieves the good single-mode characteristic of the hollow anti-resonance optical fiber polarization beam splitter.
The technical scheme of increasing the length ratio hudu of the semi-elliptical long axis to the semi-circular diameter in the semi-circular semi-elliptical tube in the x polarization direction can increase the transverse area of the tube and reduce the elliptical curvature, thereby affecting the effective refractive index of the tube mode and increasing the loss of the high-order mode of the fiber core. The technical scheme is beneficial to realizing matching of the effective refractive index of the pipe mode and the refractive index of the higher-order mode of the fiber core at the working wavelength, thereby achieving good single-mode characteristic of the hollow anti-resonance fiber polarization beam splitter.
The technical proposal of increasing the diameter of the large circular tube not only reduces the area of two fiber cores, thereby changing the loss of the fundamental mode of the fiber cores; in addition, loss of the higher order modes of the core is increased, thereby raising the HOMER lower limit. The technical scheme is not beneficial to realizing the matching of the effective refractive index of the pipe mode and the refractive index of the higher-order mode of the fiber core at the working wavelength, thereby achieving the good single-mode characteristic of the hollow anti-resonance optical fiber polarization beam splitter.
According to the technical scheme, the effective refractive index of the pipe mode and the refractive index of the higher-order mode of the fiber core can be matched at the working wavelength, so that the good single-mode characteristic of the hollow anti-resonance fiber polarization beam splitter is achieved.
(3) At a wavelength of 1.55 mu m, the energy of two fiber cores is converted by utilizing a dual-core coupling mechanism and little change of the coupling length along with the wavelength: only x-polarized light is present in one core and only y-polarized light is present in one core at the shortest distance.
The length Z of the beam splitter needs the coupling length ratio to meet certain conditions, and the calculation formula is thatWhere m and n are integers and are positive integers of opposite parity, the beam splitter length is z=ml x ＝nL y . When the CLR is 1:2 or 2:1, the performance of the polarization beam splitter is optimal, x polarized light and y polarized light in the fiber core can be sufficiently separated, so that the polarization beam splitting function is realized, and the length of the beam splitter is shortest. In the antiresonant wavelength region, under the tight package of 8 quartz tubes, the 4 supermode refractive indexes of the fiber core basic mode are not greatly changed along with the wavelength, the coupling lengths of the two polarization directions are also not greatly changed along with the wavelength, and the wavelength dependence is overcome. Thus, in the operating band, the CLRThe ratio is very stable and does not change much with wavelength.
According to the theory of the coupling mode of the dual-core optical fiber, when light is incident into the fiber core, light waves propagate in the form of sine and cosine trigonometric functions and are transmitted in the form of P in To represent the incident power of the fundamental mode in the core, and after Z length transmission, the output power calculation formulas in the corresponding x and y polarization directions in the core A and the core B are as follows When P in When the power is 1, the power of the output end is normalized power. In the case of the control variable, the fixed wavelength is 1.55 μm and the coupling length ratio CLR is fixed at 2 again, so that at a very short transmission length Z, a mutual conversion of the energy of the two cores can be achieved at a short transmission distance, with only x-polarized light in one core and only y-polarized light in one core.
The polarization extinction ratio ER (PER) is used to describe the separation capability of different polarization states at one output port after a light wave has been transmitted for a distance in a fiber core, and can be defined as the normalized power ratio between one polarization and another polarization in the same fiber core, and the calculation formula is(/>Respectively the power of the x-polarized and y-polarized light output in a certain core). When->ER, when>0; when->ER, when<0. The greater the absolute value of ER, the better the effect of polarized light separation in both directions. In fact, when ER is above 20dB or below-20 dB, it means that the power of one polarization state is at least the other polarization100 times the state, two different polarization states in the fiber can be separated completely effectively. Thus, the wavelength range covered by |ER|gtoreq.20 dB is defined as the bandwidth of the polarizing beam splitter. Since the CLR ratio is stable in the operating band and does not vary much with wavelength, polarized light in both directions of the operating band can be well separated.
According to the technical scheme, at the wavelength of 1.55 mu m, the energy of two fiber cores is converted by utilizing a double-core coupling mechanism and the coupling length which do not change greatly with the wavelength: only x-polarized light is in one core and only y-polarized light is in the shortest distance in one core.
In summary, the hollow anti-resonant fiber polarization beam splitter with double cores provided by the invention introduces double refraction by utilizing the structure asymmetry in the x and y directions, so that the 4 supermode refractive indexes of the double cores are consistent along with the slope of a wavelength curve, the coupling lengths of the two polarization directions are enough different, the wavelength dependence is overcome, and the bandwidth of a device is wider. The effective refractive index of the tube mode is matched with the refractive index of the high-order mode of the fiber core at the working wavelength by utilizing the semicircular semi-elliptic quartz cladding tube of the nested antiresonant wall in the x-polarization direction, so that the good single-mode characteristic of the hollow antiresonant fiber polarization beam splitter is achieved.
Drawings
FIG. 1 is a schematic cross-sectional structure of an embodiment of the present invention;
FIG. 2 is a graph of the coupling length of the x-polarization direction and the y-polarization direction and the coupling length ratio thereof as a function of wavelength according to an embodiment of the present invention;
FIG. 3 is a graph of normalized power versus transmission length for an embodiment of the present invention;
FIG. 4 is a graph of extinction ratio versus wavelength for a dual hollow fiber polarizing beam splitter of an embodiment of the invention at a length of 3.8 cm;
the quartz support ring comprises a quartz support ring body, a cladding region, a large circular tube body, a first circular tube body, a semicircular semi-elliptical tube body, a semi-circular semi-elliptical tube body, an anti-resonance wall body, a cladding circular tube body, a second circular tube body and a first circular tube body.
Detailed Description
In order to more clearly illustrate the technical solution of the embodiments of the present invention, the following description will make a brief description of the drawings of the embodiments. It will be apparent to those skilled in the art that the drawings in the following description are one embodiment of the invention and that other drawings may be derived from them without the inventive effort.
Compared with the scheme that an elliptic pipe is introduced into a cladding in the prior art so as to divide a middle large fiber core into A, B fiber cores, the double-core anti-resonance optical fiber polarization beam splitter provided by the invention introduces a semicircular semi-elliptic pipe embedded with an anti-resonance wall, the curvatures of two ends of the semicircular semi-elliptic pipe are different, the curvatures of a semicircle and a semi-ellipse can be respectively changed, the effective refractive index of a pipe die can be effectively changed, the coupling length is not greatly changed along with the wavelength, the energy conversion distance of the two fiber cores is shortened, and the length of the beam splitter is reduced. Compared with the scheme that the cladding is eight tightly arranged round quartz glass tubes in the background art, the semi-circular semi-elliptical tube structure of the nested antiresonant wall is flexible and has a plurality of changeable parameters, and the length of the beam splitter can be very short by overcoming the wavelength dependence mechanism of the coupling length. Compared with the scheme that the substrate material is tellurate in the background technology, the invention adopts quartz material and ensures that the coupling length does not change greatly with the wavelength through structural asymmetry, so polarized light in two directions of an operation wave band can be well separated.
Examples
As shown in fig. 1, a specific structure of one embodiment of the dual hollow anti-resonant fiber polarization beam splitter proposed by the present invention is given. Comprising an outer quartz support ring 1 and a central core region and a cladding region 2 therebetween. The quartz support ring 1, the cladding round tube 7, the large round tube 3, the semicircular semi-elliptical tube 5, the nested second round tube 8, the first round tube 4 and the antiresonance wall 6 are made of pure quartz glass with the refractive index of 1.458; the other space is air with a refractive index of 1. The cladding region 2 comprises 2 cladding round tubes 7 with the diameter d0, 4 large round tubes 3 with the diameter d1 and 2 semi-elliptical semi-round tubes 5 with the semi-circular diameter d2, semi-elliptical short axis d2 and long axis d 2/hudu; the second round tube 8 with the diameter of k x d0 is nested in the round tube 7 of the cladding, the first round tube 4 with the diameter of k x d1 is nested in the large round tube 3, and the anti-resonance wall 6 with the length of d2 is nested in the semi-circular semi-elliptical tube 5; the thicknesses of the second circular tube 8, the first circular tube 4 and the antiresonance wall 6 which are nested inside are consistent with the thicknesses of the cladding circular tube 7, the large circular tube 3 and the semicircular semi-elliptic tube 5 to be t1. The diameter d0 of the cladding round tube 7 ranges from 15.3 to 15.5 μm, d0=15.3 μm is taken in this example; the diameter d1 of the large round tube 3 ranges from 16.36 to 16.4 mu m, and d1=16.4 mu m is adopted in the embodiment; in the semicircular semi-elliptical tube 5, the diameter d2 of the semicircle ranges from 15.89 to 15.94 μm, and d2=15.8 μm is adopted in the embodiment; the minor axis of the semi-ellipse is the same as the diameter d2 of the semicircle, the length ratio hudu of the major axis to the diameter of the semicircle is in the range of 0.387-0.395, and hudu=0.395 is taken in the embodiment; the diameter ratio k of the second round tube 8 to the cladding round tube 7 ranges from 0.56 to 0.68, and k=0.56 is adopted in the embodiment; all quartz tubes and antiresonant wall thicknesses t1 range from 0.44 to 0.64 μm, with t1=0.52 μm in this example.
The specific technical scheme of this patent can be described as:
the double hollow anti-resonance fiber polarization beam splitting provided by the invention consists of 8 quartz tube anti-resonance cladding regions with different sizes and shapes and two air fiber core regions with the same size. The thickness of the tube determines the resonant wavelength lambda due to the antiresonant effect res The calculation formula is(m is an arbitrary integer, n is the refractive index of quartz), λ when m=1, the thickness t1 of the tube is 0.52 μm res For lower losses, an operating band of 1.2-1.8 μm is chosen. The diameter d0 of the cladding circular tube 7 in the y polarization direction is 15.3 mu m, and the diameter ratio k range of the nested second circular tube 8 and the large circular tube 7 is 0.56; in the semicircular semi-elliptical tube 5 in the x-polarization direction, the diameter d2 of the semicircle is 15.8 μm, the minor axis of the semi-ellipse is the same as the diameter d2 of the semicircle, and the length ratio hudu of the major axis to the diameter of the semicircle is 0.395. An anti-resonance wall 6 with a length d2 is nested in the semicircular semi-elliptic tube 5. Four supermodes, namely an x-polarization odd mode, an x-polarization even mode, a y-polarization odd mode and a y-polarization, are excited after the fiber core region is electrifiedAnd an even mode. The asymmetry of the two polarization directions causes the odd and even mode transmission constants to be different, and the x-polarized light and the y-polarized light are periodically transferred between the first fiber core and the second fiber core. The shortest distance required when incident light of a certain polarization is completely coupled to another fiber core by one core region is the coupling length, and the calculation formula is +.>(/>Propagation constants of even and odd modes respectively representing x-polarization direction and y-polarization direction; />Effective refractive indexes of an even mode and an odd mode respectively representing an x polarization direction and a y polarization direction; lambda is the refractive index of the pure quartz glass for the wavelength of incident light). The embodiment of the inventionAt an operating wavelength of 1.6-2.2 μm->The operating wavelength is 3.5-4.4 mu m. When the coupling length ratio meets a certain condition, the calculation formula is +.>M and n in the formula are integers and the parity of the m and n are opposite, when the coupling length ratio is 1:2 or 2:1, the x polarized light and the y polarized light of the two polarized light signals in the optical fiber can be sufficiently separated, so that the polarization beam splitting function is realized, and the length of the beam splitter is shortest. The CLR of this example was 3.5 to 4.4 μm in operating wavelength. The ratio of the lowest higher-order mode (HOM) loss to the highest fundamental Film (FM) loss of the fiber core is increased by introducing semicircular semi-elliptical tubes with different curvatures on the left and right sides and nested anti-resonance walls, and the ratio is the higher-order mode extinction ratio (HOMER) which is used for describing the single-mode performance of the proposed optical fiber and is calculated as->The larger the HOMER value, the better the single mode characteristic. When the value is greater than 100, it is generally considered that the optical fiber can achieve single mode transmission. The home value of this embodiment is relatively high in the operating band. The HOMER values in this example are all above 100 in the 1.2-1.8 μm band.
The length Z of the beam splitter needs the coupling length ratio to meet certain conditions, and the calculation formula is thatWhere m and n are integers and are positive integers of opposite parity, the beam splitter length is z=ml x ＝nL y . When the CLR is 1:2 or 2:1, the performance of the polarization beam splitter is optimal, x polarized light and y polarized light in the fiber core can be sufficiently separated, so that the polarization beam splitting function is realized, and the length of the beam splitter is shortest. In the antiresonant wavelength region, under the tight package of 8 quartz cladding pipes, the 4 supermode refractive indexes of the fiber core basic mode have little change along with the wavelength, the coupling lengths of the two polarization directions have little change along with the wavelength, and the wavelength dependence is overcome. Therefore, the ratio of CLR is stable in the operating band, does not change greatly with the wavelength and floats up and down at 2:1.
According to the theory of the coupling mode of the dual-core optical fiber, when light is incident into the fiber core, light waves propagate in the form of sine and cosine trigonometric functions and are transmitted in the form of P in To represent the incident power of the fundamental mode in the core, and after Z length transmission, the output power calculation formulas in the corresponding x and y polarization directions in the core A and the core B are as follows When P in When the power is 1, the power of the output end is normalized power. In this example, the energy of the two cores is fixed at a wavelength of 1.55 μm with a controlled variable, and the coupling length ratio CLR is fixed at 2 and Z is 3.8cmThe amounts are converted to each other at a short transmission distance, with only x-polarized light in one core and only y-polarized light in one core.
The polarization extinction ratio ER (PER) is used to describe the separation capability of different polarization states at one output port after a light wave has been transmitted for a distance in a fiber core, and can be defined as the normalized power ratio between one polarization and another polarization in the same fiber core, and the calculation formula is(/>Respectively the power of the x-polarized and y-polarized light output in a certain core). When->ER > 0 when; when->ER < 0. The greater the absolute value of ER, the better the effect of polarized light separation in both directions. In fact, when ER is above 20dB or below-20 dB, it means that the power of one polarization state is at least 100 times that of the other polarization state, and that the two different polarization states in the fiber can be completely effectively separated. Thus, the wavelength range covered by |ER|gtoreq.20 dB is defined as the bandwidth of the polarizing beam splitter. In this embodiment, the CLR ratio is stabilized at about 2, and the change with wavelength is not large, so that polarized light in two directions of 1460-1750 nm wavelength bands can be well separated.
The influence of the above structural parameters on the invention can be known:
(1) t1 is the wall thickness common to all quartz tubes (including large round tube 3, clad round tube 7, semicircular semi-elliptical tube 5, first round tube 4, second round tube 8) and nested antiresonant walls. Increasing t1 will produce the following technical effects: (1) the resonant wavelength is increased and the antiresonant window of the antiresonant fiber is affected, and the window shifts to longer wavelengths with increasing thickness. (2) The lower mode refractive index limit of the 4 supermodes of the two polarization directions of the fiber core is raised simultaneously, and the coupling length of the two polarization directions is shortened. (3) In addition, increasing t1 increases the loss of the fundamental core mode and decreases the loss of the higher order core mode, thereby lowering the HOMER lower limit. Conversely, decreasing t1 has the opposite technical effect.
(2) The cladding in the y-polarization direction is composed of a circular quartz tube with a diameter d 0. Adjusting d0 will produce the following technical effects: (1) the d0 is set as the minimum value of the diameters of all quartz cladding pipes, so that the effect of limiting the light energy to the fiber cores is minimum, and the coupling strength between the fundamental modes in the two fiber cores is enhanced, so that the coupling length is reduced. (2) Increasing d0 reduces the area of both cores, simultaneously reduces the lower mode index limit of the 4 supermodes for both polarization directions of the cores, and shortens the coupling length for both polarization directions. (3) Furthermore, increasing d0 increases loss of the higher order modes of the core and thus increases the HOMER lower limit. Conversely, reducing d0 has the opposite technical effect.
(3) The cladding in the x polarization direction is formed by a semicircular semi-elliptical tube. The diameter of the semicircle is d2, the minor axis of the semicircle is the same as the diameter d2 of the semicircle, and the ratio hudu of the major axis to the diameter of the semicircle is the same. Adjusting d2 and hudu will produce the following technical effects: (1) increasing d2 reduces the area of the two cores, reduces the lower limit of the mode refractive index of the 4 supermodes of the two polarization directions of the cores, and shortens the coupling length of the two polarization directions. (2) The coupling channels of the two fiber cores are narrowed simultaneously, and the loss of the base film of the fiber cores and the loss of the high-order mode are reduced simultaneously. (3) In addition, increasing hudu increases the lateral area of the tube, increasing the coupling length of both polarizations, while increasing the loss of the higher order modes of the core. Conversely, reducing d2 has the opposite technical effect.
(4) The diameter of the round quartz tube between the x polarization direction and the y polarization direction is d1, namely a large round tube. Adjusting d1 will produce the following technical effects: (1) increasing d1 reduces the area of the two cores, reduces the lower limit of the mode refractive index of the 4 supermodes of the two polarization directions of the cores, and shortens the coupling length of the two polarization directions. (2) Furthermore, increasing d1 increases loss of the higher order modes of the core and thus increases the HOMER lower limit. Conversely, reducing d1 has the opposite technical effect.
(5) k is the diameter ratio of all inner nested quartz tubes to the large round quartz tube. Adjusting k will produce the following technical effects: (1) increasing k changes the effective refractive index of the pipe mode, so that the effective refractive index of the pipe mode is matched with the higher-order mode of the fiber core, thereby increasing the loss of the higher-order mode of the fiber core, inhibiting the coupling of the base film of the fiber core, and further increasing the lower limit of the HOMER. (2) Further, increasing k lowers the lower mode index limit of the 4 supermodes for both polarization directions of the core, and the coupling length for both polarization directions becomes shorter. Conversely, decreasing k has the opposite technical effect.
In summary, the hollow anti-resonant fiber polarization beam splitter with double cores provided by the invention can introduce double refraction by utilizing the structural asymmetry in the x and y directions, so that the refractive indexes of 4 supermode modes of the double cores are consistent with the slope of a wavelength curve, the coupling lengths of the two polarization directions are different enough, the wavelength dependence is overcome, the coupling length is not changed greatly along with the change of the wavelength, and the bandwidth of a device is wider. The effective refractive index of the tube mode is matched with the refractive index of the high-order mode of the fiber core at the working wavelength by utilizing the semicircular semi-elliptic quartz cladding tube of the nested antiresonant wall in the x-polarization direction, so that the good single-mode characteristic of the hollow antiresonant fiber polarization beam splitter is achieved.
As shown in FIG. 2, the coupling length of the x-polarization direction and the y-polarization direction of the dual hollow fiber polarization beam splitter based on the anti-resonance mechanism and the coupling length ratio thereof are changed along with the wavelength. It can be seen that the y polarized light increases and decreases with increasing wavelength, the x polarized light increases gradually with increasing wavelength, the coupling length of the x polarization is smaller than that of the y polarization, and the coupling length ratio decreases gradually with increasing wavelength and fluctuates up and down around 2, so that the coupling length changes little with the change of wavelength.
As shown in fig. 3, fig. 3 is a graph of normalized power versus transmission length of the dual hollow fiber polarization beam splitter based on anti-resonance mechanism provided by the invention when the wavelength is 1.55 μm. From fig. 3, it can be obtained that the difference between the x-polarized power and the y-polarized power is maximum when the length is 3.8 cm.
As shown in FIG. 4, FIG. 4 is a graph showing the relationship between extinction ratio and HOMER value of the dual hollow fiber polarization beam splitter based on anti-resonance mechanism provided by the invention when the length is 3.8 cm. From FIG. 4, it can be obtained that at a beam splitter length of 3.8cm, the extinction ratio reaches a minimum of-325 dB at a wavelength of 1550 nm. In the range of 1460-1750 nm, the polarization extinction ratio is smaller than-20 dB, so that the separation of polarized light and the beam splitting of two polarized light can be realized. Therefore, the bandwidth of the dual hollow fiber polarization beam splitter of the present embodiment is 290nm. The HOMER values were all above 100, indicating good single mode characteristics.
The above examples are only illustrative of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention, and various modifications and improvements made by those skilled in the art to the technical solution of the present invention should fall within the scope of protection defined by the claims of the present invention without departing from the spirit of the design of the present invention.
Claims (7)
1. The utility model provides a hollow anti-resonance fiber polarization beam splitter of twin-core, includes outer quartz support ring (1) and the fiber core district in middle part and cladding district (2) between the two, its characterized in that: the fiber core area is an air fiber core area and comprises two symmetrical fiber cores A and B; the cladding region (2) comprises cladding round tubes (7) positioned at two ends of the y direction, semicircular semi-elliptical tubes (5) positioned at two ends of the x direction and large round tubes (3) tightly arranged between the adjacent cladding round tubes (7) and the semicircular semi-elliptical tubes (5), the diameters of the cladding round tubes (7) and the large round tubes (3) are d0 and d1 respectively, a second round tube (8) with the diameter of k x d0 and a first round tube (4) with the diameter of k x d1 are nested inside the cladding round tubes, d0 is smaller than d1, the semicircular diameter of the semicircular semi-elliptical tubes (5) is d2, the minor axis of the semicircular is d2, the major axis of the semicircular semi-elliptical tube is d2/hudu, and an anti-resonance wall (6) with the length of d2 is nested at the connecting position of the semicircular semi-elliptical tube and the semicircular semi-elliptical tube.
2. The dual-core hollow-core antiresonant fiber polarization beam splitter of claim 1, wherein: the thickness of the large circular tube (3), the cladding circular tube (7), the semicircular semi-elliptical tube (5), the first circular tube (4), the second circular tube (8) and the antiresonant wall (6) is the same, and the thickness is 0.55-0.56 mu m.
3. The dual-core hollow-core antiresonant fiber polarization beam splitter of claim 1, wherein: the large round tube (3), the cladding round tube (7), the semicircular semi-elliptical tube (5), the first round tube (4), the second round tube (8) and the antiresonance wall (6) are made of pure quartz glass with the refractive index of 1.458, and other spaces are air with the refractive index of 1.
4. The dual-core hollow-core antiresonant fiber polarization beam splitter of claim 1, wherein: the diameter d1 of the large circular tube (3) is 16.36-16.4 mu m.
5. The dual-core hollow-core antiresonant fiber polarization beam splitter of claim 1, wherein: the diameter d0 of the cladding circular tube (7) is 15.3-15.5 mu m.
6. The dual-core hollow-core antiresonant fiber polarization beam splitter of claim 1, wherein: the value range of k is 0.56-0.68.
7. The dual-core hollow-core antiresonant fiber polarization beam splitter of claim 1, wherein: in the semicircular semi-elliptical tube (5), the d2 has a value range of 15.89-15.94 mu m and the ratio hudu has a range of 0.387-0.395.
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