CN108363142B - Rectangular waveguide mode conversion device - Google Patents

Rectangular waveguide mode conversion device Download PDF

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CN108363142B
CN108363142B CN201810252325.7A CN201810252325A CN108363142B CN 108363142 B CN108363142 B CN 108363142B CN 201810252325 A CN201810252325 A CN 201810252325A CN 108363142 B CN108363142 B CN 108363142B
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waveguide
main
mode
auxiliary
refractive index
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CN108363142A (en
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陈明阳
曹国栋
姜文帆
杨磊
任乃飞
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Jiangsu 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
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/14—Mode converters
    • 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
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
    • G02B6/122—Basic optical elements, e.g. light-guiding paths

Abstract

The invention provides a rectangular waveguide mode conversion device, which consists of an auxiliary waveguide, a main waveguide and a cladding.The main waveguide can support M modes for transmission, and M is more than or equal to 2; the auxiliary waveguide supports only fundamental mode transmission. One end of the auxiliary waveguide is intersected with the side face of the main fiber core, and the center distance of the two waveguides at the other end is larger than one half of the sum of the widths of the two waveguides. Not only realize E12、E22The modes are mutually converted, and the modes supported in the main waveguide can be completely and efficiently converted into other modes under the same structure. The parameters between the auxiliary waveguide and the main waveguide satisfy: the effective refractive index of the fundamental mode of the auxiliary waveguide is greater than the effective refractive index of any mode in the main waveguide. The invention provides a novel rectangular waveguide mode conversion device which can realize conversion between a low-order mode and a high-order mode of a light wave mode.

Description

Rectangular waveguide mode conversion device
Technical Field
The invention relates to the field of optical fiber communication, in particular to a rectangular waveguide mode conversion device capable of realizing conversion between a low-order mode and a high-order mode.
Background
In recent years, the transmission capacity of single-mode optical fiber gradually approaches to the limit value, people gradually shift attention to few-mode optical fiber communication, and the purpose of double-expansion of the transmission capacity can be achieved by coupling different information to different modes. A few-mode waveguide device capable of realizing mode-to-mode conversion and multiplexing different modes into a few-mode optical fiber in few-mode optical fiber communication is one of the key factors determining the system performance. In recent years, with the rapid development of optical fiber communication, some optical fiber transmission systems, such as space division multiplexing transmission systems, and optical devices need to use a high-order mode for transmission, and at this time, the mode converter is used to implement the conversion operation between the modes, and the design of the mode converter determines the crosstalk and the conversion efficiency during the mode conversion process. The long period fiber grating can realize the high-efficiency conversion between the fundamental mode and the high-order mode of the few-mode fiber, and the bandwidth reaches 34nm [ IEEE photon, technol, Lett., 2015, 27(9): 1006-1009 ]. By adopting the method of long-period fiber grating cascade, the conversion between high-order modes can be realized theoretically, but the 3dB bandwidth is only about 10 nm [ Opt Express, 2014, 22(10): 11488-. Mode conversion [ Opt. Express, 2013, 21(21):25113-25119] can also be realized by adopting the Y-shaped waveguide, but the Y-shaped waveguide can only realize certain mode conversion due to the limitation of two branch sizes, so that the flexibility of the converter is limited. The mode separation can be realized by the double-core Fiber based on the coupling of the single-mode Fiber and the few-mode Fiber, but the coupling between different modes is difficult to avoid [ Opt. Fiber technol., 2011, 17(5): 490-. The multi-core optical fiber can realize multiplexing and separation of a plurality of modes, and has the defect of serious mode field deformation [ Opt. Express, 2010,18(5): 4709-4716 ]. The mode conversion of broadband [ Opt.Lett., 2007, 32(4): 328-330] can be realized by adopting the tapered photonic crystal fiber structure, but the conversion between the fundamental mode and the high-order mode of the fiber can be realized.
Generally, the conversion characteristics of the mode converter based on the mode coupling principle have strong wavelength dependence, the working bandwidth is narrow, and the uniformity of the output spectrum is poor. Multiplexing and demultiplexing of such modes also presents significant difficulties because the mode effective indices of the quadruple degenerate modes in the fiber are equal. For the application of few-mode fiber systems, there is still no effective means for implementing mode interconversion. The rectangular waveguide mode conversion device is one of the most common mode conversion devices at present, is similar to a few-mode optical fiber in size, has low insertion loss, and can better couple a mode into the optical fiber. The existing waveguide structure can also realize mode separation, but the structure is relatively complexOpt. Express,2013, 21(15): 17904-17911，Opt. Express,2013, 21(17): 20220-20229]The function is single, mode interconversion cannot be achieved, and the utilization rate of the rectangular waveguide is limited. The traditional mode converter can only realize that one mode is fixed to the other modeThe mode is switched, and the flexibility is poor.
In summary, a mode converter capable of flexibly realizing conversion between a plurality of modes and having a simple structure has yet to be developed.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a rectangular waveguide mode conversion device to solve the technical problem of conversion between modes.
The technical scheme of the invention is as follows:
a rectangular waveguide mode conversion device, characterized by: the waveguide comprises an auxiliary waveguide, a main waveguide and a cladding, wherein the auxiliary waveguide is a single-mode waveguide, the main waveguide supports high-order mode transmission, the main waveguide supports M modes, and M is more than or equal to 2; the auxiliary waveguide and the main waveguide are both rectangular straight waveguides, and the height of the main waveguide is greater than that of the auxiliary waveguide; the height-to-width ratio of the main waveguide cross section ranges from 1.067:1 to 1.3:1, and the height-to-width ratio of the auxiliary waveguide cross section ranges from 1: 1; the auxiliary waveguide intersects with the side face of the main fiber core; defining the end surface with the main waveguide and the auxiliary waveguide as a multi-core end surface, and the end surface with the main waveguide only as a single-core end surface; the intersection position of the auxiliary waveguide and the main fiber core is positioned between the middle position of the main waveguide in the axial direction and the end face of the single core; on the multi-core end face, the central distance between the auxiliary waveguide and the main waveguide is greater than one half of the sum of the widths of the auxiliary waveguide and the main waveguide; the cladding layer is coated outside the auxiliary waveguide and the main waveguide.
Preferably, the included angle between the central axis of the main waveguide and the central axis of the auxiliary waveguide is 0.011oTo 0.021o。
Preferably, the refractive index of the main waveguide
Figure DEST_PATH_IMAGE002
Assisted waveguide refractive index
Figure DEST_PATH_IMAGE004
The difference is
Figure DEST_PATH_IMAGE006
，
Figure DEST_PATH_IMAGE008
Satisfy the requirement of
Figure DEST_PATH_IMAGE010
Preferably, on the single-core end face, the end face of the auxiliary waveguide and the end face of the main waveguide are both located on the single-core end face, that is, the central axis of the bottom face of the auxiliary waveguide and the central axis of the bottom face of the main waveguide intersect at a point, and the intersection point is located on the single-core end face.
Preferably, the effective refractive index of the fundamental mode of the auxiliary waveguide
Figure DEST_PATH_IMAGE012
And effective refractive index of main waveguide mode
Figure DEST_PATH_IMAGE014
Satisfies the following conditions:
Figure DEST_PATH_IMAGE016
。
preferably, the difference in effective refractive index between the main waveguide internal modes
Figure DEST_PATH_IMAGE018
Satisfies the following conditions:
Figure DEST_PATH_IMAGE020
here, the
Figure DEST_PATH_IMAGE022
Wherein M is the total number of modes of the main waveguide,
Figure DEST_PATH_IMAGE024
is the effective refractive index of the i-th mode of the main waveguide, and has
Figure DEST_PATH_IMAGE026
。
Preferably, the refractive index of the auxiliary waveguide
Figure 541317DEST_PATH_IMAGE004
Transverse theretoWidth of the surface
Figure DEST_PATH_IMAGE028
Satisfies the following conditions:
Figure DEST_PATH_IMAGE030
wherein k is
Figure DEST_PATH_IMAGE032
And λ is the operating wavelength.
Preferably, all modes in the main waveguide can be efficiently converted in the wavelength range 1.2-1.6 μm.
The invention has the technical effects that: the invention provides a novel rectangular waveguide mode conversion device, the structure does not have a degenerate mode, and the mode in the main waveguide can be sequentially converted into a higher-order mode when being input from one end and can be sequentially converted into a lower-order mode when being input from the other end. The structure also has the advantages of good coupling effect on the optical fiber, larger manufacturing tolerance, low crosstalk, wide working wavelength range, good output energy uniformity, no polarization dependence and the like. The invention can be widely applied to few-mode optical fiber communication, sensing and other systems.
Drawings
Fig. 1 is a schematic structural diagram of a rectangular waveguide device according to embodiment 1 of the present invention.
FIG. 2 is a schematic diagram of the mode distribution of the transmission supported in the main waveguide at 1.55 μm for the rectangular waveguide of the present invention.
FIG. 3 is a diagram showing the effect of the positional relationship between the auxiliary waveguide and the main waveguide on the mode conversion, (a) input E12Modulo, (b) input E22Molding; the abscissa in the figure is the position coordinate of the auxiliary waveguide on the side surface of the waveguide, and represents that the auxiliary waveguide is from the central position of the side surface of the main waveguide to the bottom end of the side surface.
FIG. 4 (a) is a graph of effective refractive index versus wavelength for modes in the primary and secondary waveguides; in FIG. 4, (b) is E21Die sum E12The mode effective index is plotted as a function of aspect ratio.
Fig. 5 shows the effect of the angle between the central axis of the auxiliary waveguide and the central axis of the main waveguide on the mode conversion.
FIG. 6 shows light input into fundamental mode E from the auxiliary waveguide of example 1 of the rectangular waveguide of the present invention11The mode energy curve output from the main waveguide, in which the fundamental mode E of the auxiliary waveguide11Effective refractive index greater than fundamental mode E of the main waveguide11The effective refractive index of the mode.
FIG. 7 is a graph showing the mode energy curve versus wavelength for light input from the multi-core end face main waveguide and output from the single-core end face main waveguide in example 1 of the rectangular waveguide according to the present invention, in which the fundamental mode E of the auxiliary waveguide11Effective refractive index greater than fundamental mode E of the main waveguide11The effective refractive index of the mode. Wherein (a) inputs E11Modulo, (b) input E21Modulo, (c) input E12Modulo, (d) input E22Modulo, (E) input E31And (5) molding.
In the figure:
1 is an auxiliary waveguide, 2 is a main waveguide, and 3 is a cladding.
Detailed Description
The invention will be further described with reference to the following figures and specific examples, but the scope of the invention is not limited thereto.
The rectangular waveguide mode conversion device provided by the invention comprises a main waveguide 2, an auxiliary waveguide 1 and a cladding 3, wherein the main waveguide 2 can support M modes, M is more than or equal to 2, namely the main waveguide 2 supports high-order mode transmission; the invention realizes the operations of leading-in, extracting, converting and the like of modes in the main fiber core by introducing the auxiliary waveguide 1. One end of the auxiliary waveguide 1 is intersected with the side face of the main fiber core, and the center distance of the two waveguides at the other end is larger than one half of the sum of the widths of the auxiliary waveguide 1 and the main waveguide 2. The cladding 3 is coated outside the auxiliary waveguide 1 and the main waveguide 2, one side surface of the auxiliary waveguide 1 and one side surface of the main fiber core are on the same plane, namely the central axis of the bottom surface of the auxiliary waveguide 1 is intersected with the central axis of the bottom surface of the main waveguide 2 at one point. This is essentially different from the conventional Y-type waveguide mode converter, which has a different size of the auxiliary waveguide 1 and the main waveguide 2, and can realize the mode conversion in the main waveguide 2Instead of converting from the main waveguide 2 through the Y branch, the conversion efficiency is well ensured, and the waveguide connection loss and the consistency of the connection of the main waveguide 2 and the subsequent waveguide device are reduced. Meanwhile, the Y-type waveguide is limited by two branches and can only convert one fixed mode under the condition of fixed size, and the flexibility of the device is limited. Then, the bottom end of the auxiliary waveguide 1 positioned on the side surface of the main waveguide 2 can well solve the problem of realizing conversion of different modes in the same device. It is verified that if the auxiliary waveguide 1 is located at the center of the side of the main waveguide 2, some modes such as E in the main waveguide 211、E21、E31、E13Modes are well convertible, but E therein12、E22The mode cannot be converted. Therefore, we adopt a novel structure to make the auxiliary waveguide 1 located at the bottom of the side surface of the main waveguide 2, and the structure well solves the problem of E12、E22The difficulty that the modes can not be converted is solved, and simultaneously, the modes supported in the main waveguide 2 can be converted into other modes under the same structure and high efficiency.
Fig. 1 is a schematic structural diagram of a rectangular waveguide device according to embodiment 1 of the present invention. The number of the transmission modes supported by the main waveguide 2 at a wavelength of 1.55 μ M is M =6, the sections of the main waveguide 2 and the auxiliary waveguide 1 are both rectangular, the aspect ratio of the cross section of the main waveguide 2 is selected to be 1.1:1, the aspect ratio of the cross section of the auxiliary waveguide 1 is 1:1, the refractive index difference between the auxiliary waveguide 1 and the cladding 3 material is selected to be 0.01, and the central distance between the main waveguide 2 and the auxiliary waveguide 1 is selected to be 21 μ M. And satisfies the following conditions: the auxiliary waveguide 1 satisfies:
Figure 501533DEST_PATH_IMAGE016
(ii) a Wherein the content of the first and second substances,
Figure 343587DEST_PATH_IMAGE014
is the effective refractive index of the fundamental mode of the primary waveguide 2,
Figure 535534DEST_PATH_IMAGE012
is the effective refractive index of the fundamental mode of the auxiliary waveguide 1. The number of the main waveguides 2 and the auxiliary waveguides 1 is 1. For convenienceIn the expression, it is specified that the end face where the main waveguide 2 and the auxiliary waveguide 1 exist is a multi-core end face, and the end face where only the main waveguide 2 exists is a single-core end face.
Fig. 2 is a schematic diagram of the transmission mode supported by light at 1.55 μm wavelength from the main waveguide 2 of the structure described in example 1, in order to distinguish the difference between the mode transmitted in the rectangular waveguide and the transmission waveguide in the optical fiber and to more intuitively describe the mode conversion process. Fig. 3 shows the effect of the relative positional relationship between the auxiliary waveguide 1 and the side surface of the main waveguide 2 on the mode conversion, where (a) in fig. 3 is an input E12 mode, and (b) in fig. 3 is an input E22 mode, and the abscissa of (a) and (b) in fig. 3 is the position coordinates of the auxiliary waveguide 1 on the side surface of the waveguide, which indicates that the auxiliary waveguide 1 is from the center position of the side surface of the main waveguide 2 to the bottom end of the side surface. When the auxiliary waveguide 1 is at the center position of the side of the main waveguide 2, the input E12Mold E22No mode conversion occurs, but as the auxiliary waveguide 1 is away from the side center position, the mode conversion starts, and the conversion efficiency is better as the distance increases. Therefore, the conversion efficiency is the best when the auxiliary waveguide 1 is located at the bottom of the side surface of the main waveguide 2.
Fig. 4 (a) is a graph showing the effective refractive index of the modes in the main waveguide 2 and the auxiliary waveguide 1 as a function of wavelength, and fig. 4 (b) is a graph showing the effective refractive index of the E21 mode and the E12 mode as a function of the aspect ratio. When the aspect ratio of the cross section of the main waveguide 2 is 1.067:1, the effective refractive index of the modes in the main waveguide 2 and the auxiliary waveguide 1 is a curve of the relation of the effective refractive index and the wavelength. As shown in FIG. 4 (b), E21Mold and E12The mode effective refractive index is equal at an aspect ratio of 1:1, but the two modes of effective refractive index are gradually distinguished as the aspect ratio is varied.
When the aspect ratio of the cross section of the main waveguide 2 is 1:1, the existence of the degenerate mode will cause the case that one mode is input to be converted into two modes. For this purpose, the aspect ratio of the cross section of the main waveguide 2 is at least 1.067:1, so that the mode is no longer degenerate. In order to ensure the effective switching between the modes, a certain difference value is required to be satisfied between the effective refractive indexes of the transmission modes of the main waveguide 2, and the difference between the effective refractive indexes of any two adjacent modes is calculated
Figure 146644DEST_PATH_IMAGE020
Efficient conversion of modes can be achieved.
As the aspect ratio of the main waveguide 2 cross-section continues to increase to 1.3:1, E will occur at a wavelength of 1.55 μm where the supported modes within the main waveguide 2 will change12Has a mode effective refractive index greater than E22The mode effective refractive index of (1). The structure also achieves efficient mode conversion for this phenomenon, but the aspect ratio cannot be too large because it affects the mode conversion when it is connected to the optical fiber.
Fig. 5 shows the effect of the angle between the central axis of the auxiliary waveguide 1 and the central axis of the main waveguide 2 on the mode conversion. As shown in FIG. 5, the included angle is 0.011oTo 0.021oThe modes can achieve switching and below-20 dB crosstalk. When the angle is too small or too large, the mode conversion crosstalk will increase.
The main waveguide 2 is chosen to support E at a wavelength of 1.55 μm11、E21、E12、E22、E31、E13Six modes of transmission. If the auxiliary waveguide 1 has a fundamental mode E11Has an effective refractive index larger than the fundamental mode E of the main waveguide 211Effective refractive index, when a fundamental mode is input from the auxiliary waveguide 1, E is excited in the main waveguide 211The mode, and the output from the main core at the end face of the single core is shown in fig. 6.
When light is input from the multi-core end surface main waveguide 2, the result of output from the single-core end surface differs depending on the parameters of the auxiliary waveguide 1. Assuming that the main waveguide 2 can support E11、E21、E12、E22、E31、E13Six modes of transmission while assisting the fundamental mode E of the waveguide 111Has an effective refractive index larger than the fundamental mode E of the main waveguide 211The effective refractive index of (a). Then, E is inputted from the multi-core end face main waveguide 2 respectively11、E21、E12、E22、E31In the mode, the modes output from the main fiber core of the end face of the single core are respectively as follows: e21、E12、E22、E31、E13The mode, namely the mode is converted from a low order to a high order; if it is transfusedInto is E13The die will leak and no longer output from the single core end face. Similarly, if the fundamental mode E of the auxiliary waveguide 1 is selected11Is larger than the main waveguide 2E21Effective refractive index of mode smaller than that of main waveguide 2E11The effective refractive index of the mode. Then E is respectively input from the main fiber core of the multi-core end face11、E21、E12、E22、E31In the mold, the modes output from the single-core end face are respectively as follows: e11、E12、E22、E31、E13Mode, i.e. E11Mode does not transition; if the input is E13The die will leak and no longer output from the single core end face.
FIG. 7 is a graph showing a mode energy curve with respect to wavelength, in which light is input from the multi-core end-face main waveguide 2 of example 1 and output from the single-core end-face main waveguide 2, and in which the effective refractive index of the fundamental mode of the auxiliary waveguide 1 is larger than E of the main core11The effective refractive index of the mode. Wherein (a) is input E11Modulo, (b) is input E21Modulo, (c) is input E12Modulo, (d) is input E22Modulo (E) is an input E31And (5) molding. As can be seen from FIG. 7, E of input11Mold E21Mold E12Mold E22Conversion of mode to E21Mold E12Mold E22Mold E31Mode, and other modes are all very low in energy. Input E31The mode starts to leak at 1.5 μm, i.e., the primary waveguide 2 does not fully support E at wavelengths greater than 1.5 μm13The die is transferred.
In the structure, the length and the width of the cross sections of the auxiliary waveguide 1 and the main waveguide 2 are not changed along with the transmission distance, namely, the mode coupling between different waveguides is not realized in the modes of tapering and the like.
In the case of a small refractive index difference between the waveguide and the cladding 3, the rectangular waveguide device of the present invention has polarization independence, i.e., two polarization states of the same mode have the same transmission characteristics.
The main waveguide 2 and the auxiliary waveguide 1 are both rectangular waveguides, i.e. the cross-section thereof is rectangular and meets a certain aspect ratio, in other cases it is difficult to achieve mode conversion and operation with low crosstalk.

Claims (6)

1. A rectangular waveguide mode conversion device, characterized by: the waveguide structure comprises an auxiliary waveguide (1), a main waveguide (2) and a cladding (3), wherein the auxiliary waveguide (1) is a single-mode waveguide, the main waveguide (2) supports high-order mode transmission, the main waveguide (2) supports M modes for transmission, and M is more than or equal to 2; the auxiliary waveguide (1) and the main waveguide (2) are both rectangular straight waveguides, and the height of the main waveguide (2) is greater than that of the auxiliary waveguide (1); the value range of the height-to-width ratio of the cross section of the main waveguide (2) is 1.067:1 to 1.3:1, and the height-to-width ratio of the cross section of the auxiliary waveguide (1) is 1: 1; the auxiliary waveguide (1) intersects with the side face of the main fiber core; defining the end surface where the main waveguide (2) and the auxiliary waveguide (1) exist as a multi-core end surface, and the end surface where only the main waveguide (2) exists as a single-core end surface; the intersection position of the auxiliary waveguide (1) and the main fiber core is positioned between the middle position of the main waveguide (2) in the axial direction and the end face of the single core; on the multi-core end face, the central distance between the auxiliary waveguide (1) and the main waveguide (2) is greater than one half of the sum of the widths of the auxiliary waveguide (1) and the main waveguide (2); the cladding (3) is coated outside the auxiliary waveguide (1) and the main waveguide (2); refractive index n of the auxiliary waveguide (1)aAnd the width a of the cross section satisfies:
Figure FDA0002521786070000011
wherein
Figure FDA0002521786070000012
λ is the operating wavelength, naTo assist the refractive index of the waveguide (1), ncIs the refractive index of the cladding (3); on the single-core end face, the end face of the auxiliary waveguide (1) and the end face of the main waveguide (2) are both positioned on the single-core end face, namely the central axis of the bottom surface of the auxiliary waveguide (1) and the central axis of the bottom surface of the main waveguide (2) are intersected at one point, and the intersection point is positioned on the single-core end face.
2. The rectangular waveguide mode conversion device of claim 1, wherein: the included angle between the central axis of the main waveguide (2) and the central axis of the auxiliary waveguide (1) ranges from 0.011 degrees to 0.021 degrees.
3. The rectangular waveguide mode conversion device of claim 1, wherein: the refractive index n of the main waveguide (2)mAuxiliary waveguide (1) refractive index naThe difference is Deltaa＝na-nm，ΔaSatisfies 0.0065 ≥ deltaa＞0。
4. The rectangular waveguide mode conversion device of claim 1, wherein: the effective refractive index n of the fundamental mode of the auxiliary waveguide (1)fEffective refractive index n of fundamental mode of main waveguide (2)1Satisfies the following conditions: n isf＞n1。
5. The rectangular waveguide mode conversion device of claim 1, wherein: an effective refractive index difference Deltan between modes in the main waveguide (2)i＝ni+1-niSatisfies the following conditions: Δ ni≥1×10-4M-1 is more than or equal to i and more than or equal to 1, wherein M is the total number of modes of the main waveguide (2), and n isiIs the effective refractive index of the i-th mode of the main waveguide (2) and has ni＞ni+1。
6. The rectangular waveguide mode conversion device of claim 1, wherein: all modes in the main waveguide (2) can be efficiently converted in the wavelength range 1.2-1.6 μm.
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CN107272115A (en) * 2017-08-14 2017-10-20 江苏法尔胜光电科技有限公司 A kind of pattern multiplexer/demultiplexer based on three core fibres

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CN107272115A (en) * 2017-08-14 2017-10-20 江苏法尔胜光电科技有限公司 A kind of pattern multiplexer/demultiplexer based on three core fibres

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