WO2016101470A1 - 滤模光纤 - Google Patents

滤模光纤 Download PDF

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Publication number
WO2016101470A1
WO2016101470A1 PCT/CN2015/077615 CN2015077615W WO2016101470A1 WO 2016101470 A1 WO2016101470 A1 WO 2016101470A1 CN 2015077615 W CN2015077615 W CN 2015077615W WO 2016101470 A1 WO2016101470 A1 WO 2016101470A1
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refractive index
mode
high refractive
core
index dielectric
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PCT/CN2015/077615
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English (en)
French (fr)
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陈明阳
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江苏大学
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Priority to US15/126,247 priority Critical patent/US20170082796A1/en
Publication of WO2016101470A1 publication Critical patent/WO2016101470A1/zh

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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
    • G02B6/02004—Optical fibres with cladding with or without a coating characterised by the core effective area or mode field radius
    • 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
    • G02B6/0238—Longitudinal structures having higher refractive index than background material, e.g. high index solid rods
    • 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
    • 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
    • G02B6/02319—Plurality of longitudinal structures extending along optical fibre axis, e.g. holes characterised by core or core-cladding interface features
    • G02B6/02333—Core having higher refractive index than cladding, e.g. solid core, effective index guiding
    • 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
    • G02B6/02347—Longitudinal structures arranged to form a regular periodic lattice, e.g. triangular, square, honeycomb unit cell repeated throughout cladding
    • G02B6/02352—Complex periodic lattices or multiple interpenetrating periodic lattices, e.g. unit cell having more than two materials, partially internally coated holes, for multiple bandgaps
    • 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
    • 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
    • G02B6/02347—Longitudinal structures arranged to form a regular periodic lattice, e.g. triangular, square, honeycomb unit cell repeated throughout cladding
    • 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
    • G02B6/02385—Comprising liquid, e.g. fluid filled holes

Definitions

  • the invention relates to the field of optical fiber communication, in particular to a microstructured optical fiber having the function of selectively filtering the transmission mode in the core.
  • the information capacity of single-mode fiber has been greatly improved by techniques such as time division multiplexing, wavelength division multiplexing, and polarization multiplexing, which are close to the limit.
  • time division multiplexing time division multiplexing
  • wavelength division multiplexing wavelength division multiplexing
  • polarization multiplexing polarization multiplexing
  • small-mode fibers can also achieve large-mode single-mode transmission, dispersion compensation, ultrashort pulse transmission, and nonlinear applications by selectively exciting higher-order modes. Therefore, how to effectively manipulate and control the transmission mode in the mode-less fiber becomes the key to the application of these technologies.
  • Optical filters are instruments used to select wavelengths, which can select the desired wavelength from a wide range of wavelengths. It can be used for wavelength selection, noise filtering of optical amplifiers, gain equalization, optical multiplexing/demultiplexing, and is an important device in WDM systems. Similarly, in mode-less fiber applications, a filter is also required, that is, a device capable of selectively filtering out one or some modes of the fiber.
  • the separation of modes can be achieved based on a dual-core fiber coupled with a single-mode fiber and a small-mode fiber, but it is difficult to avoid coupling between different modes [Opt. Fiber Technol., 2011, 17(5): 490-494]. Multiple modes of fiber can be used to achieve multiplexing and separation of multiple modes, and its mode field deformation is more serious [Opt. Express, 2010, 18(5): 4709-4716]. Mode separation can also be achieved by using a waveguide structure, but the structure is relatively complicated [Opt. Express, 2013, 21(15): 17904-17911, Opt. Express, 2013, 21(17): 20220-20229].
  • the bending loss of the higher-order mode is larger, and the bending loss of the lower-order mode is smaller.
  • a certain loss of one or some higher-order modes in the optical fiber can be generated, thereby achieving the purpose of the filter mode.
  • it is desired to filter out the fundamental mode or the low-order mode while retaining the high-order mode it is difficult to implement in this way.
  • the bandgap can appear in the low refractive index region. Therefore, with the bandgap fiber structure, it is possible to make the fundamental mode of the fiber out of the bandgap.
  • the high-order mode is placed within the band gap to achieve the effect of filtering out the fundamental mode.
  • its ability to bind to higher-order modes is weaker.
  • the leakage loss of higher-order modes is also often large [Opt. Express, 2010, 18(9): 8906-8915].
  • Fiber structures that effectively filter out specific patterns in the fiber while maintaining low-loss transmission in other modes have not been reported.
  • the technical solution of the present invention is: a microstructured optical fiber comprising a core and a cladding, the cladding layer comprising a matrix material, a high refractive index dielectric column and a high refractive index dielectric ring.
  • the high refractive index dielectric pillar surrounds the core and is periodically disposed within the matrix material.
  • the core is located at the center of the fiber.
  • the high refractive index dielectric ring is located outside the matrix material, high refractive index dielectric column. The relationship between the refractive indices satisfies: n out >n clad , n core >n clad ,n rod >n clad .
  • n core , n rod , n clad , n out are the refractive indices of the core, the high refractive index dielectric column, the matrix material, and the high refractive index dielectric ring, respectively.
  • r core is the core radius and ⁇ 0 is the operating wavelength.
  • the transfer mode of the microstructure region composed of the high refractive index dielectric column and the matrix material forms a supermode group.
  • the effective refractive index of the lowest-order mode in the super-mode group and the effective refractive index formed by the highest-order mode are the super-mode group intervals.
  • the effective refractive index of at least one core mode is in the super-mode group interval, and the leakage loss is greater than other core modes in which the effective refractive index is not in the super-mode group interval.
  • the refractive index of the high refractive index dielectric ring is above the supermode group interval in the operating wavelength range.
  • the high refractive index dielectric ring is a ring having an inner diameter d in , and its center is the same as the center of the core.
  • Definition L is the set of distances between the center of the core and the center of each high refractive index dielectric column, where the maximum is Lmax . Then, the distance between the high refractive index dielectric ring and the high refractive index dielectric column is required to satisfy: d in -L max -r rod ⁇ 4 ⁇ m.
  • r rod is the radius of the high refractive index dielectric column.
  • the center distance S between the core and the high refractive index dielectric column adjacent thereto satisfies: S-(r core +r rod ) ⁇ 3 ⁇ m and S-(r core +r rod) ⁇ 8 ⁇ m.
  • n out of the high refractive index dielectric ring satisfies an effective refractive index n ceff of the lowest order mode in the supermode group: n out - n ceff > 0.0005.
  • the combination of different dielectric columns allows simultaneous filtering of multiple modes on the same fiber.
  • This requires that the high refractive index dielectric column consists of 2-3 different types of high refractive index dielectric columns, and the same type of high refractive index dielectric columns constitute 1-3 regions; the total number of regions formed is 2-6; In one region, the period, diameter, and refractive index of the high refractive index dielectric column are the same, and all of the high refractive index dielectric columns are centered in a sector-shaped region centered on the center of the core.
  • the fiber cross-sectional distribution has axis symmetry.
  • the core uses a high refractive index liquid.
  • a high refractive index liquid column is used instead of a high refractive index dielectric column or a high refractive index liquid is used as a core.
  • the present invention proposes that the mode of selectively filtering out the optical fiber can be realized by coupling the core mode with the microstructure cladding mode and forming a filter mode mechanism through the high refractive index ring.
  • the fundamental mode of the dual mode fiber can be filtered out so that the fiber can transmit only a single higher order mode, or one or more higher order modes of the mode fiber can be selectively filtered out. While filtering out the corresponding modes, other modes in the fiber can maintain low loss transmission, thereby enabling effective control of modes in the fiber.
  • the filter mode of the optical fiber of the invention is simple: by connecting the mode-less fiber to the fiber of the invention, a specific mode of filtering in the mode-less fiber can be realized without performing other on the fiber. deal with.
  • FIG. 1 is a schematic cross-sectional structural view of an optical fiber of the present invention
  • FIG. 2 is a schematic cross-sectional view of another optical fiber of the present invention composed of two columns of different refractive index dielectric columns;
  • FIG. 3 is a schematic cross-sectional view of another optical fiber of the present invention composed of two columns of different sizes of high refractive index dielectric columns and having two axes of symmetry;
  • 4 is a mode field distribution diagram of the optical fiber of Embodiment 1, wherein 4 (a) is a LP 01 mode; and 4 (b) is an LP 11 mode;
  • Example 5 is a graph showing effective refractive index and leakage loss as a function of wavelength in a core mode in Example 1;
  • FIG. 6 is a graph showing the energy variation of the core mode of the core according to the transmission distance in Embodiment 1;
  • FIG. 7 is a graph showing a relationship between a core mode leakage loss and an inner diameter of a high refractive index dielectric ring in Embodiment 1;
  • FIG. 8 is a graph showing a relationship between a leakage loss of a core mode and a refractive index of a high refractive index dielectric ring in Embodiment 1;
  • 9(a) is the core mode effective refractive index curve
  • 9(b) is the core mode leakage loss curve
  • Figure 10 is a schematic view showing a cross-sectional refractive index distribution of an optical fiber in Embodiment 3;
  • Figure 11 is a graph showing leakage loss versus wavelength for the core mode in Example 3.
  • 1 is a matrix material
  • 2 is a high refractive index dielectric column
  • 3 is a high refractive index dielectric ring
  • 4 is a core.
  • Figure 1 shows a schematic structural view of an optical fiber of the present invention.
  • the cladding layer comprises a matrix material 1, a high refractive index dielectric column 2 and a high refractive index dielectric ring 3.
  • the high refractive index dielectric column 2 is periodically arranged on the matrix material 1, and the high refractive index dielectric ring 3 is located outside the high refractive index dielectric column 2 and the matrix material 1.
  • the core 4 is located at the center of the fiber.
  • the refractive index of the high refractive index dielectric column 2, the high refractive index dielectric ring 3, and the core 4 is higher than that of the matrix material 1.
  • the high refractive index dielectric column 2 is arranged on the low refractive index matrix material 1, so that any high refractive index dielectric column 2 can independently conduct the fundamental mode when the distance between the high refractive index dielectric columns 2 is sufficiently far. Assuming that the high refractive index dielectric column 2 supports only single mode transmission, when the two high refractive index dielectric columns 2 are arranged on the matrix material 1 at a relatively close distance, the fundamental modes are coupled to form two supermodes, one of which The effective refractive index of the supermode is higher than that of the fundamental mode, and the effective refractive index of the other supermode is lower than that of the fundamental mode.
  • the filter mode principle of the optical fiber of the present invention is that the high refractive index core 4 is arranged on the low refractive index matrix material 1 to form a step type fiber structure.
  • the parameter relationship between the core 4 and the matrix material 1 is satisfied: V>2.405, here That is, the core can transmit a fundamental mode and a certain number of higher order modes.
  • the high refractive index dielectric column 2 is arranged on the low refractive index matrix material 1 to form a supermode group. Therefore, by the coupling theory, if the effective refractive index of a certain mode in the core matches the effective refractive index of the supermode formed by the high refractive index dielectric column 2, the two modes will undergo energy coupling.
  • the core mode is referred to as a suppressed mode, that is, the mode is a conduction mode in a step fiber composed of the core 4 and the matrix material 1, and after the introduction of the high refractive index dielectric column 2 and the high refractive index dielectric ring 3, It is suppressed and becomes a mode with a large transmission loss. If the high refractive index dielectric column is far from the high refractive index dielectric ring 3, since the high refractive index dielectric column 2 is surrounded by the low refractive index matrix material 1, a certain refractive index guiding mechanism is formed, and the leakage loss is also small.
  • the high refractive index dielectric column 2 When the high refractive index dielectric column 2 is large, the number of supermodes formed thereof is also large. Thus, it forms a super-module group having a small effective refractive index difference in an interval around the effective refractive index of the single high refractive index dielectric column 2 mode. Since the effective refractive index of the lowest order mode is the largest and the effective refractive index of the highest order mode is the smallest, the interval between the effective refractive index of the lowest order mode and the effective refractive index of the highest order mode in the supermode group can be defined as a supermode. Group interval. Thus, when a core mode is in the effective refractive index range of the supermode group, the core mode is coupled to the supermode therein.
  • this core mode can be coupled with one or more super-modes in a wide wavelength range, thereby forming Broadband filter mode effect. It is generally required that the number N of layers of the high refractive index dielectric column 2 satisfies: N ⁇ 3, thereby ensuring that the number of high refractive index dielectric columns 2 is sufficiently large to form a sufficient supermode, and at the same time, the core 4 and the high refractive index dielectric ring The distance of 3 is also far enough to ensure a low loss in the transmission mode in the core.
  • the function of the high refractive index dielectric ring 3 is to make the supermode unable to satisfy the total internal reflection condition and generate leakage loss.
  • the high refractive index dielectric ring 3 is not present, since the refractive index of the core 4 and the high refractive index dielectric column 2 are both larger than that of the matrix material 1, the supermode formed by the core mode and the high refractive index dielectric column is formed. Both are conduction modes, and these modes are all transmitted based on the refractive index guiding mechanism, and there is no leakage loss.
  • the core mode When the effective refractive index of the core mode is above the super-mode group interval, since the refractive index of the core 4 itself is higher than that of the matrix material 1, the core mode satisfies the refractive index guiding mechanism, and thus the loss of the core mode is small.
  • the reason why there may be loss is that the refractive index of the high refractive index dielectric ring 3 may be higher than the effective refractive index of the core mode, resulting in leakage loss.
  • the leakage loss decreases as the effective refractive index of the core mode increases, because the difference in the effective refractive index between the core mode and the super mode increases, and the refractive index guiding effect is enhanced.
  • the core mode effective refractive index When the core mode effective refractive index is below the supermode group interval, the core mode can still be bound in the core region, because the effective refractive index of the supermode and core modes formed by the high refractive index dielectric column 2 is certain. The difference is that the coupling effect is weak, so the core mode is still transmitted in the vicinity of the core, and the high refractive index dielectric ring is far away from the core, so that the leakage effect on the core mode is weak.
  • the effective refractive index of the core mode when the effective refractive index of the core mode is located in the super-mode group interval, the mode loss is increased, and the filtering effect can be achieved.
  • the effective refractive index of the higher-order mode is below the super-mode group interval, the leakage loss is low and can still be transmitted in the optical fiber. If the fiber has only a fundamental mode and a higher-order mode, then in this way, only a single high-order mode of the fiber can be realized without the transmission of the fundamental mode.
  • the mode loss increases.
  • the mode whose fundamental mode and other effective refractive index are above the super-mode group interval can be conducted by the refractive index-guided transmission mode, and the mode whose effective refractive index is below the super-mode group interval can be transmitted in the band gap type conduction mode. Therefore, in this case, one or several modes in the mode-less optical fiber can be selectively filtered out.
  • the presence of the high refractive index dielectric ring 3 is to introduce losses into the supermode group. It needs to be closer to the high refractive index dielectric column, thereby destroying the mode field of the supermode and introducing losses.
  • the distance between the high refractive index dielectric ring 3 and the high refractive index dielectric column 2 is required to satisfy: d in - L max - r rod ⁇ 4 ⁇ m.
  • the refractive index of the high refractive index dielectric ring 3 needs to be higher than the supermode group interval, so that the supermode cannot form an effective refractive index guiding transmission mechanism and achieve the purpose of the filter mode.
  • the refractive index n out of the high refractive index dielectric ring 3 satisfies: n out >n ceff , where n ceff is the effective refractive index of the lowest order mode of the super mode group.
  • the core 4 and the high refractive index dielectric column 2 are not too far apart; The relatively regular mode field can still be maintained, and the core 4 and the high refractive index dielectric column 2 cannot be too close.
  • the center distance S between the core and the high refractive index dielectric column 2 adjacent thereto satisfies: 8 ⁇ m ⁇ S - (r core + r rod ) ⁇ 3 ⁇ m.
  • the refractive index n out of the high refractive index dielectric ring 3 and the effective refractive index n ceff of the lowest order mode in the supermode group satisfy: n out -n ceff > 0.0005.
  • changing the period, diameter, or refractive index of the high refractive index dielectric column can achieve adjustment of the refractive index interval of the super mode group. Therefore, if it is necessary to filter two or more modes, two or more types of high refractive index dielectric columns can be used to form the filter structure.
  • the same type of high refractive index dielectric columns should be gathered together and distributed in 1-3 regions. At the same time, the larger the area where the supermode overlaps with the core mode, the better the coupling effect and the higher the pattern loss.
  • the type of high refractive index dielectric column is not suitable, generally 2-3 types are suitable, and the total number of regions formed is 2-6.
  • the period, diameter, and refractive index of the high refractive index dielectric column from the inside to the outside of the layer should be the same in a certain region, that is, in a certain sector region,
  • the high refractive index dielectric columns are identical. Arranging the high refractive index dielectric column in an axisymmetric manner can effectively improve the filter mode effect in the case where the optical fiber is bent, and the transmission core mode has good symmetry.
  • Figure 2 shows an optical fiber structure consisting of two columns of high-refractive-index dielectrics of different sizes to filter out two core modes.
  • Figure 3 shows another fiber structure consisting of two columns of high-refractive-index dielectrics of different sizes, with four high-refractive-index dielectric column regions forming an axisymmetric distribution with two axes of symmetry.
  • the program can be sensitively changed to achieve online adjustable filter mode.
  • the method is: changing the high refractive index dielectric column into a high refractive index liquid column.
  • the refractive index of the liquid can be adjusted by controlling the temperature of the liquid, thereby changing the refractive index interval of the super-mode group, thereby achieving the purpose of filtering out different modes. If a fiber is divided into multiple segments, and the corresponding ambient temperature of each segment is different, so that the filtering mode is different, the purpose of filtering multiple modes on one fiber can be achieved.
  • the invention can also adopt a liquid core, and changing the liquid temperature can change the effective refractive index of the fiber mode, thereby moving different modes to the super-mode group interval, thereby achieving the effect of filtering out multiple modes.
  • Embodiment 1 is a diagrammatic representation of Embodiment 1:
  • Figure 1 shows a schematic cross-sectional view of one embodiment of the invention.
  • the matrix material 1 was taken as pure quartz, the refractive index difference between the high refractive index dielectric column 2 and the matrix material 1 was 0.029, and the refractive index difference between the high refractive index dielectric ring 3 and the matrix material 1 was 0.01. The difference in refractive index between the core 4 and the matrix material 1 was 0.0075.
  • the high refractive index dielectric column 2 has a period of 7.75 ⁇ m and a core diameter of 12 ⁇ m.
  • High refractive index dielectric column 2 layers N 3.
  • the mode field distributions of the fundamental mode (LP 01 mode) and the higher order mode (LP 11 mode) of the above embodiment are as shown in FIG. It can be seen that the fundamental mode field is coupled to the region of the high refractive index dielectric column 2 to form a high leakage loss.
  • the mode field distribution characteristics of the higher-order modes are similar to those in the all-solid-state bandgap fiber. Although a small amount of energy is transmitted on the high-refractive-index dielectric column 2 adjacent to the core, the main energy is still concentrated in the core region. .
  • the leakage loss of the LP 01 mode is 79 dB/m, while the leakage loss of the LP 11 mode is 0.005 dB/m.
  • Fig. 5(a) shows the super mode group interval formed by the core mode effective refractive index curve and the high refractive index dielectric column.
  • A01 is the effective refractive index curve of LP 01 mode
  • A11 is the effective refractive index curve of LP 11 mode
  • A2 is the upper boundary of the super mode group
  • A3 is the lower boundary of the super mode group
  • A4 is the high refractive index dielectric column.
  • the upper edge of the band gap At the wavelength of 1.55 ⁇ m, the number of supermodules reaches more than 30.
  • the effective refractive index of the core mode is in the supermode group interval, there is always a supermode equal to or close to its effective refractive index, thereby realizing the core mode and super Strong coupling of the mode.
  • the effective refractive index of the supermode is distributed on both sides of the effective refractive index of the fundamental mode of the high refractive index dielectric column.
  • n reff is indeed in the middle of the cladding mode.
  • the parameters of the high refractive index dielectric column 2 and the core 4 can be selected such that the effective refractive index of the suppressed mode in the core 4 is equal to the effective refractive index of the fundamental mode of the high refractive index dielectric column 2. This ensures that the core is inhibited from coupling to the cladding mode, and the wavelength of the core is suppressed. The effective refractive index of the core is still in the supermode group interval, so that the fiber has a wider filter mode operating bandwidth.
  • Figure 5(b) is a plot of leakage loss versus wavelength for core mode.
  • B01 is the leakage loss curve of the LP 01 mode
  • B11 is the leakage loss curve of the LP 11 mode. It can be seen from Fig. 5(a) and (b) that when the fundamental mode is in the super-mode group interval, the leakage loss can reach 20dB/m or more, and when it is far away from the super-mode group interval, the leakage loss is rapidly reduced.
  • the leakage loss is lower in the distance from the super-mode group, and the leakage loss increases as the wavelength increases. This is related to the expansion of the mode field to the cladding after the wavelength is increased.
  • the fiber can maintain only the LP 11 mode in the wavelength range of 1.503 ⁇ 1.61 ⁇ m. .
  • FIG. 6 it is an energy variation curve of the core fundamental mode with the transmission distance; wherein C1 is the core fundamental mode energy curve in Embodiment 1, and C2 is when the high refractive index dielectric ring 3 is absent, that is, the cladding is only The energy curve of the core fundamental mode when composed of the high refractive index dielectric column 2 and the matrix material 1.
  • the fundamental mode energy will decrease as the transmission distance increases, thereby achieving the purpose of filtering, as shown by the C1 curve in FIG.
  • the energy transfer curve of the optical fiber fundamental mode is C2. It can be seen from the figure that the fiber fundamental mode and the cladding mode are only partially coupled. It can be seen that the core suppression mode does not need to be fully coupled with the supermode, and the filter mode can be achieved. Thereby, the broadband filter mode capability of the optical fiber is ensured.
  • the effect of the high refractive index dielectric column 2 can be seen by comparing the loss of the core mode in the presence and absence of the high refractive index dielectric column 2. If the high refractive index dielectric column 2 is not present, the leakage loss of its LP 01 and LP 11 modes is 8.8 ⁇ 10 -9 and 6 ⁇ 10 -6 dB / m. In the presence of the high refractive index dielectric column 2, the leakage loss of the LP 01 mode reaches 79 dB/m, and the leakage loss of the LP 11 mode is 0.005 dB/m. It can be seen that the presence of the high refractive index dielectric column 2 increases both modes of loss, but the coupling of the LP 01 mode to the cladding mode results in an increase in the magnitude of the loss far exceeding that of the LP 11 mode.
  • FIG. 7 it is the relationship between the core mode leakage loss and the inner diameter of the high refractive index dielectric ring 3 in Example 1; wherein D01 is the leakage loss curve of the LP 01 mode; and D11 is the leakage loss curve of the LP 11 mode.
  • the inner diameter of the high refractive index dielectric ring 3 has a direct influence on the fundamental mode loss. When the inner diameter of the high refractive index dielectric ring 3 is small, the LP 01 mode loss is always large, and when the inner diameter value is increased to a certain value, Its LP 01 mode loss will decrease rapidly.
  • the energy loss of the LP 01 mode is achieved by coupling with the super mode, and the loss of the super mode is derived from the high refractive index of the high refractive index dielectric ring 3 adjacent thereto.
  • the refractive index guiding mechanism of the super mode is destroyed, so that effective light transmission cannot be achieved.
  • the leakage loss of the higher order mode always decreases as the inner diameter of the high refractive index dielectric ring 3 increases. This is because the higher-order mode is not coupled to the super-mode, and its mode field is far from the high-refractive-index dielectric ring 3. Therefore, the inner diameter of the high-refractive-index dielectric ring 3 is small, and the high-order mode cannot be transmitted.
  • FIG. 8 it is the relationship between the leakage loss of the core mode in Embodiment 1 and the refractive index of the high refractive index dielectric ring 3; wherein E01 is the leakage loss curve of the LP 01 mode; E11 is the leakage loss of the LP 11 mode. curve.
  • the refractive index of the high refractive index dielectric ring 3 determines the loss of the fundamental mode. When the refractive index of the high refractive index dielectric ring 3 is above the supermode group interval, the leakage loss of the fundamental mode is large. When the refractive index of the high refractive index dielectric ring 3 reaches a certain value, the further increase of the value has substantially no effect on the leakage loss of the fundamental mode.
  • the refractive index of the high refractive index dielectric ring 3 must be higher than the refractive index of the super mode group, thereby forming a filter mode mechanism.
  • the refractive index of the high refractive index dielectric ring 3 has little effect on the loss of the higher order mode when it changes in the vicinity of the supermode group.
  • Embodiment 2 is a diagrammatic representation of Embodiment 1:
  • the matrix material 1 is taken as pure quartz, the refractive index difference between the high refractive index dielectric column 2 and the matrix material 1 is 0.029, and the high refractive index dielectric ring 3 and the matrix material 1 are The refractive index difference is 0.01. The difference in refractive index between the core 4 and the matrix material 1 was changed to 0.0105.
  • the high refractive index dielectric column 2 has a period of 7.75 ⁇ m and a core diameter of 12 ⁇ m.
  • High refractive index dielectric column 2 layers N 3.
  • the core structure can support the four modes of LP 01 , LP 11 , LP 21 and LP 02 .
  • FIG. 9 the core mode effective refractive index and the core mode leakage loss as a function of wavelength in the second embodiment
  • FIG. 9( a ) is a core mode effective refractive index curve and a high refractive index dielectric column. Supermodel group interval.
  • Figure 9(b) is a plot of leakage loss versus wavelength for core mode.
  • F01 is the effective refractive index curve of LP 01 mode
  • F11 is the effective refractive index curve of LP 11 mode
  • F21 is the effective refractive index curve of LP 21 mode
  • F02 is the effective refractive index curve of LP 02 mode
  • F30 is the super mode group On the upper boundary
  • F40 is the lower boundary of the supermode group
  • F50 is the upper edge of the band gap of the high refractive index dielectric column
  • G01 is the leakage loss curve of the LP 01 mode
  • G11 is the leakage loss curve of the LP 11 mode
  • G21 is the LP 21 mode.
  • G02 is the leakage loss curve of the LP 02 mode.
  • the leakage loss of the LP 11 mode of the fiber is 20 dB/m or more when it is in the super-mode group interval, and the leakage loss is rapidly reduced when it is far away from the super-mode group interval.
  • the LP 01 mode it is in the index guiding area and the leakage loss is very low.
  • the LP 21 and LP 02 modes they are in the band gap conduction region, but their mode fields are expanded in the cladding region, and the loss is large. If the loss of the LP 11 mode is required to be greater than 20 dB/m, and the mode loss is less than 0.1 dB/m in other modes, the operating wavelength range of the fiber can be above 70 nm.
  • Embodiment 3 is a diagrammatic representation of Embodiment 3
  • Figure 10 shows a schematic cross-sectional view of another embodiment of the invention.
  • the parameters are the same as in the second embodiment.
  • This structure reduces the leakage loss of the two modes of LP 21 and LP 02 by increasing the number of high refractive index dielectric columns.
  • FIG. 11 it is the leakage loss curve of the core mode in the third embodiment; wherein, H01 is the leakage loss curve of the LP 01 mode; H11 is the leakage loss curve of the LP 11 mode, and H21 is the LP 21 mode. Leakage loss curve; H02 is the leakage loss curve of the LP 02 mode.
  • the LP 11 mode still has a high leakage loss, while the losses of the LP 21 and LP 02 modes are reduced. If the loss of the LP 11 mode is required to be greater than 20 dB/m, and the mode loss is less than 0.1 dB/m in other modes, the operating wavelength range of the fiber can be over 200 nm.

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Abstract

一种滤模光纤，包层由基质材料（1）、高折射率介质柱（2）和高折射率介质环（3）组成，高折射率介质柱（2）周期性排布于低折射率基质材料（1）上，高折射率介质环（3）位于低折射率基质材料（1）、高折射率介质柱（2）的外侧，纤芯（4）位于光纤的中心，使高折射率介质柱（2）形成超模群，并与纤芯（4）模发生耦合，再以高折射率环使超模群获得高的泄露损耗，从而形成有效的滤模机制的方法，实现选择性地滤除光纤的模式，少模光纤仅传输单个高阶模，在滤除相应模式的同时，光纤中其它模式可以保持低损耗传输。

Description

滤模光纤 技术领域
本发明涉及光纤通信领域，具体为一种具有选择性滤除纤芯中传输模式功能的微结构光纤。
背景技术
单模光纤的信息容量通过时分复用、波分复用、偏振复用等技术获得了极大的提高，已经接近于极限。近年来，采用多芯光纤或少模光纤，通过不同的纤芯或同一光纤中不同的模式来实现传输不同信息的方法，即所谓的空分复用技术，引起了人们广泛的兴趣。空分复用系统中，每路信号载荷着不同的信息，且每路信号均可通过时分复用、波分复用技术实现大容量通信。采用少模光纤，以不同的模式传输不同的信息的方法，即所谓的模分复用技术，可以成倍地提高光纤的传输容量。除了在模分复用技术方面的应用以外，少模光纤还可以通过选择性激发高阶模的方法，实现大模场单模传输、色散补偿、超短脉冲传输以及非线性应用等。因此，如何有效地操纵、控制少模光纤中的传输模式，成为这些技术应用的关键。
光滤波器是用来进行波长选择的仪器，它可以从众多的波长中挑选出所需的波长。它可以用于波长选择、光放大器的噪声滤除、增益均衡、光复用/解复用，是波分复用系统中的重要器件。相似的，在少模光纤应用中，也需要有滤模器，即能够选择性地滤除光纤中的某个或某些模式的器件。
基于单模光纤与少模光纤耦合的双芯光纤可以实现模式的分离，但难以避免不同模式之间的耦合【Opt.Fiber Technol.,2011,17(5):490-494】。采用多芯光纤可以实现多种模式的复用与分离，其缺点其模场变形比较严重【Opt.Express,2010,18(5):4709-4716】。采用波导结构，也可实现模式分离，但结构相对复杂【Opt.Express,2013,21(15):17904-17911，Opt.Express,2013,21(17):20220-20229】。
利用高阶模的弯曲损耗较大，而低阶模的弯曲损耗较小的特点，通过弯曲的方法，可以使光纤中的某个或某些高阶模产生大的损耗，从而实现滤模的目的。但如果想要滤除基模或低阶模而保留高阶模，则难以用这种方法实现。对于光子带隙光纤，由于传输模式需处于带隙内才能传输，而带隙可以出现在低折射率区，因此，采用带隙光纤结构，有可能可以使光纤的基模处于带隙之外，而让高阶模处于带隙之内，从而实现滤除基模的效果。然而，由于这种结构下的带隙一般较窄，导致其对高阶模的束缚能力较弱，因 而高阶模的泄露损耗也往往很大【Opt.Express,2010,18(9):8906-8915】。
能够有效滤除光纤中特定的模式而又保持其它模式低损耗传输的光纤结构，目前尚未见报道。
发明内容
针对现有技术的不足，本发明的目的是提供一种具有选择性滤除纤芯中模式功能的微结构光纤。
本发明的技术方案为：一种微结构光纤，包括纤芯和包层，所述包层包括基质材料、高折射率介质柱和高折射率介质环。所述高折射率介质柱围绕纤芯并周期性排布于基质材料内。所述纤芯位于光纤的中心。所述高折射率介质环位于所述基质材料、高折射率介质柱的外侧。其折射率之间的关系满足：nout>nclad,ncore>nclad,nrod>nclad。纤芯与基质材料的参数关系满足：V>2.405，这里
Figure PCTCN2015077615-appb-000001
这里，ncore,nrod,nclad,nout分别为纤芯、高折射率介质柱、基质材料和高折射率介质环的折射率。rcore为纤芯半径，λ0为工作波长。
高折射率介质柱和基质材料组成的微结构区域可传输的模式形成超模群。超模群中最低阶模式的有效折射率的和最高阶模式的有效折射率形成的区间为超模群区间。在工作波长范围内，至少有一个纤芯模式的有效折射率处于超模群区间，其泄露损耗大于有效折射率未处于超模群区间的其它纤芯模式。
作为本发明的进一步改进方案，在工作波长范围内，高折射率介质环的折射率处于超模群区间以上。
作为本发明的进一步改进方案，高折射率介质环为一圆环，其内径为din，其圆心与纤芯中心位置相同。定义L为纤芯中心与每一个高折射率介质柱的中心之间的距离的集合，其中的最大值为Lmax。则高折射率介质环与高折射率介质柱之间的距离需满足：din-Lmax-rrod<4μm。这里rrod为高折射率介质柱的半径。
作为本发明的改进方案，要求：高折射率介质柱的层数N满足：N≥3。
作为本发明的进一步改进方案，要求：纤芯和与之相邻的高折射率介质柱之间的中心距离S满足：S-(rcore+rrod)≥3μm且S-(rcore+rrod)≤8μm。
作为本发明的进一步改进方案，要求：高折射率介质环的折射率nout与超模群中最 低阶模式的有效折射率nceff之间满足：nout-nceff>0.0005。
以不同介质柱的组合可以实现在同一根光纤同时滤除多个模式。这要求：高折射率介质柱由2-3种不同类型的高折射率介质柱组成，且同一类型的高折射率介质柱组成1-3区域；形成的区域总数为2-6个；在同一个区域内，高折射率介质柱的周期、直径和折射率均相同，且所有的高折射率介质柱中心位于以纤芯中心为圆心的扇形区域内。
作为一种优化方案，所述光纤横截面分布具有轴对称性。
作为一种优化方案，所述纤芯采用高折射率液体。
作为实现可调滤模的一种方案，以高折射率液体柱替代高折射率介质柱或以高折射率液体作为纤芯。
本发明的技术效果为：本发明提出通过纤芯模式与微结构包层模发生耦合，再经高折射率环形成滤模机制的方法，可以实现选择性地滤除光纤的模式。例如，可以滤除双模光纤的基模，从而使光纤可以仅传输单个高阶模，也可以选择性地滤除少模光纤中的某一个或多个高阶模。在滤除相应模式的同时，光纤中其它模式可以保持低损耗传输，从而实现了对光纤中模式的有效控制。将多根具有不同滤模功能的光纤串接，即可以滤除多个光纤模式，从而使少模光纤仅传输单个高阶模。与其它滤模方法相比，本发明光纤的滤模方法简单：通过将少模光纤与本发明光纤连接的方法，即可实现滤除少模光纤中传输的特定模式，而无需对光纤做其它处理。
附图说明
图1为一种本发明光纤的横截面结构示意图；
图2为由两种不同尺寸高折射率介质柱组成的另一种本发明光纤的横截面结构示意图；
图3为由两种不同尺寸高折射率介质柱且具有两个对称轴的组成的另一种本发明光纤横截面结构示意图；
图4为实施例1光纤的模场分布图，其中，4(a)为LP01模；4(b)为LP11模；
图5为实施例1中纤芯模式的有效折射率和泄露损耗随波长变化曲线；
图6为实施例1中纤芯基模随传输距离的能量变化曲线；
图7为实施例1中纤芯模式泄露损耗与高折射率介质环的内径的关系曲线；
图8为实施例1中纤芯模式的泄露损耗与高折射率介质环的折射率的关系曲线；
图9为实施例2中纤芯模式有效折射率与纤芯模式的泄露损耗随波长变化曲线；
其中，9(a)为纤芯模式有效折射率曲线；9(b)为纤芯模式的泄露损耗曲线；
图10为实施例3中光纤的横截面折射率分布示意图；
图11为实施例3中纤芯模式的泄露损耗随波长变化曲线。
图中，1为基质材料，2为高折射率介质柱，3为高折射率介质环，4为纤芯。
具体实施方式
图1给出了本发明光纤的一种结构示意图。其中，包层包括基质材料1、高折射率介质柱2和高折射率介质环3。高折射率介质柱2周期性地排布于基质材料1上，高折射率介质环3位于高折射率介质柱2和基质材料1外侧。纤芯4位于光纤的中心。其中高折射率介质柱2、高折射率介质环3、纤芯4的折射率均高于基质材料1。
高折射率介质柱2排布于低折射率的基质材料1上，因而，当高折射率介质柱2之间距离足够远时，任一高折射率介质柱2都可以独立地传导基模。假设高折射率介质柱2仅支持单模传输，则两个高折射率介质柱2排布于基质材料1上时距离较近时，其基模会发生耦合，形成两个超模，其中一个超模的有效折射率高于基模，而另一个超模的有效折射率低于基模。同理，若三个相同的高折射率介质柱2排布于基质材料1上，则可以形成三个超模。相类似的，当在基质材料1上排布更多的高折射率介质柱2时，其产生的超模的有效折射率分布在高折射率介质柱2的基模有效折射率周围，形成一个超模群。
本发明光纤的滤模原理是：高折射率纤芯4排布于低折射率的基质材料1上，形成阶跃型光纤结构。纤芯4与基质材料1的参数关系满足：V>2.405，这里
Figure PCTCN2015077615-appb-000002
即纤芯可以传输基模和一定数量的高阶模。高折射率介质柱2排布于低折射率的基质材料1上，形成超模群。因此，由耦合理论，若纤芯中的某个模式的有效折射率与高折射率介质柱2形成的超模的有效折射率匹配，则两个模式将发生能量耦合。即使两者不完全匹配，只要其有效折射率相近，也可发生部分的能量耦合。因此，这就使得纤芯模式可以耦合到包层。当高折射率介质柱2形成的超模的有效折射率低于高折射率介质环3的折射率时，则由于无法满足全内反射条件，此超模将发生强的泄漏，因此达到滤除此纤芯模式的目的。称此纤芯模式为受抑制模式，即此模式在由纤芯4和基质材料1组成的阶跃光纤中是传导模，而在引入高折射率介质柱2和高折射率介质环3后，受到抑制，成为传输损耗很大的模式。若高折射率介质柱距离高折射率 介质环3较远，则由于高折射率介质柱2周围为低折射率的基质材料1，形成一定的折射率引导机制，其泄漏损耗也较小。
当高折射率介质柱2较多时，其形成的超模数量也较多。由此，其在单个高折射率介质柱2模式的有效折射率附近的一个区间内形成有效折射率差值很小的一个超模群。由于最低阶模式的有效折射率最大，而最高阶模式的有效折射率最小，因此，可定义超模群中最低阶模式的有效折射率的和最高阶模式的有效折射率形成的区间为超模群区间。这样，当一个纤芯模式处于这个超模群的有效折射率区间时，纤芯模式就会与其中的超模发生耦合。由于超模群的有效折射率形成数量很多、排布很密的一个区间，这就使得这个纤芯模式可以在宽波长范围内都有可能与其中的一个或多个超模发生耦合，从而形成宽带的滤模效果。一般要求高折射率介质柱2的层数N满足：N≥3，从而保证高折射率介质柱2的数量足够多，从而形成足够的超模，同时，其纤芯4与高折射率介质环3的距离也足够远，以保证纤芯中的传输模式具有低的损耗。
高折射率介质环3的作用是使超模无法满足全内反射条件，产生泄露损耗。由光纤理论，若高折射率介质环3不存在，则由于纤芯4和高折射率介质柱2的折射率均大于基质材料1，因而，纤芯模及高折射率介质柱形成的超模都是传导模，这些模式均基于折射率引导机制传输，不存在泄露损耗。
当纤芯模式有效折射率处于超模群区间以上时，由于纤芯4折射率本身高于基质材料1，其纤芯模式满足折射率引导机制，因而，纤芯模式的损耗较小。其可能存在损耗的原因是：高折射率介质环3的折射率可能会高于纤芯模式的有效折射率，从而导致泄露损耗。其泄露损耗随着纤芯模式有效折射率的增大而减小，这是因为纤芯模式与超模的有效折射率差别增大，折射率引导效应增强。
当纤芯模式有效折射率位于超模群区间以下时，纤芯模仍然可以被束缚在纤芯区传输，原因是高折射率介质柱2形成的超模与纤芯模式的有效折射率有一定差别，其耦合效应弱，因而纤芯模式仍然在纤芯及附近传输，而高折射率介质环与纤芯距离较远，因而其对纤芯模的泄漏作用较弱。
由此可见，当纤芯基模的有效折射率位于超模群区间时，其模式损耗增大，可以实现滤除的目的。而当其高阶模的有效折射率位于超模群区间以下时，其泄露损耗较低，仍可以在光纤中传输。若光纤只存在基模和一个高阶模，则通过这种方式可以实现光纤只存在单个高阶模而无基模的传输。
若纤芯某一个或几个高阶模的有效折射率位于超模群区间时，其模式损耗增大，可 以实现滤除的目的。而其基模及其它有效折射率位于超模群区间以上的模式可以以折射率引导型的传输方式传导，其有效折射率位于超模群区间以下的模式可以以带隙型传导的方式传输。因此，这种情况下可以实现选择性地滤除少模光纤中的一个或几个模式。
高折射率介质环3的存在是为了对超模群引入损耗。其需要与高折射率介质柱距离较近，从而破坏超模的模场，引入损耗。一般地，高折射率介质环3与高折射率介质柱2之间的距离需满足：din-Lmax-rrod<4μm。
高折射率介质环3的折射率需要高于超模群区间，从而使超模无法形成有效的折射率引导传输机制，实现滤模的目的。在工作波长范围内，高折射率介质环3的折射率nout满足：nout>nceff，这里nceff为超模群最低阶模式的有效折射率。
为使纤芯中的受抑制模式与高折射率介质柱2形成的超模能够形成强的耦合，纤芯4与高折射率介质柱2距离不能太远；而为使得纤芯中的传输模式仍能保持较规则的模场，纤芯4与高折射率介质柱2距离不能太近。综合而言，一般要求纤芯和与之相邻的高折射率介质柱2之间的中心距离S满足：8μm≥S-(rcore+rrod)≥3μm。
为保证受抑制模式的高损耗，一般要求高折射率介质环3的折射率nout与超模群中最低阶模式的有效折射率nceff之间满足：nout-nceff>0.0005。
如图2-3所示，改变高折射率介质柱的周期、直径或折射率均可以实现对超模群的折射率区间的调节。因此，若需要对两个以上模式进行滤除，可采用两种或多种类型的高折射率介质柱组成滤模结构。为在一个折射率区间内形成模式数量足够多的超模群，同一类型高折射率介质柱应聚集在一起，且分布在1-3个区域内。同时，超模与纤芯模重叠的区域越大，其耦合效果越好，形成的模式损耗也越高。因此，为有效滤模，高折射率介质柱的类型不宜多，一般以2-3类为宜，且形成的区域总数为2-6个。显然，为使超模成为滤除纤芯模式的通道，应在一定区域内，自内外向各层的高折射率介质柱的周期、直径、折射率相同，即在一定的扇形区域内，其高折射率介质柱是相同的。以轴对称方式排布高折射率介质柱可以有效提高光纤发生弯曲等情形下的滤模效果，并使传输的纤芯模式具有良好的对称性。图2给出了一种光纤结构，由两种不同尺寸的高折射率介质柱组成，可实现滤除两个纤芯模式。图3给出了另一种光纤结构，由两种不同尺寸的高折射率介质柱组成，其中高折射率介质柱区域有4个，形成具有两个对称轴的轴对称分布。
本方案可以灵敏变化，实现在线可调滤模。其方法是：高折射率介质柱改成高折射率液体柱。采用对温度敏感的高折射率液体，通过控制液体温度即可调节液体的折射率，从而改变其超模群的折射率区间，即可实现滤除不同模式的目的。若将一根光纤分为多段，每段对应的环境温度不同，从而使其滤除的模式也不同，即可实现在一根光纤上滤除多个模式的目的。
本发明也可以采用液体纤芯，改变液体温度即可使光纤模式的有效折射率发生改变，从而将不同的模式移动至超模群区间，达到滤除多个模式的效果。
下面详细阐述本发明的具体实施方式中的实施例。
实施例一：
图1给出了本发明的一种实施例的横截面示意图。基质材料1取为纯石英，高折射率介质柱2与基质材料1的折射率差为0.029，高折射率介质环3与基质材料1的折射率差为0.01。纤芯4与基质材料1的折射率差为0.0075。高折射率介质柱2的周期为7.75μm，纤芯直径为12μm。高折射率介质柱2层数N＝3。
上述实施例的基模(LP01模)和高阶模(LP11模)的模场分布如图4所示。可见，基模模场被耦合到高折射率介质柱2排布的区域，从而形成高的泄露损耗。而高阶模的模场分布特性与全固态带隙光纤中的模式场相似，虽然也有少量能量会在与纤芯相邻的高折射率介质柱2上传输，但其主要能量仍然集中在纤芯区。LP01模的泄露损耗达到79dB/m，而LP11模的泄露损耗为0.005dB/m。
图5(a)为纤芯模式有效折射率曲线与高折射率介质柱形成的超模群区间。其中，A01为LP01模的有效折射率曲线，A11为LP11模的有效折射率曲线，A2为超模群的上边界，A3为超模群的下边界，A4为高折射率介质柱的带隙上边缘。在1.55μm波长处，超模数量达到30个以上，因而，当纤芯模式的有效折射率处于超模群区间时，总有超模与其有效折射率相等或相近，从而实现纤芯模与超模的强耦合。前面曾指出，超模的有效折射率分布于高折射率介质柱基模的有效折射率两侧。事实上，只存在单个高折射率介质柱2时，在1.55μm波长处，其基模的有效折射率为nreff＝1.45499。从图5(a)可见，nreff确实处于包层模的中间区域。因此，可以选择高折射率介质柱2和纤芯4的参数使得纤芯4中受抑制模式的有效折射率与高折射率介质柱2的基模有效折射率相等。这样可以保证纤芯受抑制模式与包层模发生耦合，且波长变化时，纤芯受抑制模式的有效折射率仍处于超模群区间，从而使光纤具有较宽的滤模工作带宽。
图5(b)为纤芯模式的泄露损耗随波长变化曲线。其中，B01为LP01模的泄露损耗曲线；B11为LP11模的泄露损耗曲线。由图5(a)和(b)可见：当基模处于超模群区间时，其泄露损耗可达达到20dB/m以上，而在远离超模群区间时，其泄露损耗迅速减小。对于LP11模，在远离超模群区间，其泄露损耗较低，其泄露损耗随着波长的增加而增大。这与波长增加后模场向包层扩展有关。若以模式损耗大于20dB/m为滤除该模式的基本要求，而以模式损耗小于0.1dB/m为模式传输的条件，则此光纤可以在1.503～1.61μm波长范围内保持只传输LP11模。
如图6所示，为纤芯基模随传输距离的能量变化曲线；其中C1为实施例1中的纤芯基模能量曲线，C2为高折射率介质环3不存在时，即包层仅由高折射率介质柱2和基质材料1组成时，纤芯基模的能量曲线。对于本实施例1的光纤，其基模能量将随传输距离的增加而减小，从而实现滤除的目的，如图6中C1曲线所示。而当高折射率介质环3不存在时，即包层仅由高折射率介质柱2和基质材料1组成时，光纤基模的能量传输曲线为C2。由图可见，光纤基模与包层模仅发生部分耦合。由此可见，纤芯受抑制模式不需要与超模发生完全耦合，即可达到滤模的目的。由此，保证了光纤的宽带滤模能力。
高折射率介质柱2的作用可以通过比较存在与不存在高折射率介质柱2时纤芯模式的损耗来看出。若高折射率介质柱2不存在，则其LP01和LP11模的泄露损耗为8.8×10-9和6×10-6dB/m。而高折射率介质柱2存在时，LP01模的泄露损耗达到79dB/m，而LP11模的泄露损耗为0.005dB/m。可见，高折射率介质柱2的存在使得两模式的损耗均增大，但LP01模与包层模的耦合导致其损耗增加量级远远超过LP11模。
如图7所示，为实施例1中纤芯模式泄露损耗与高折射率介质环3的内径的关系曲线；其中D01为LP01模的泄露损耗曲线；D11为LP11模的泄露损耗曲线。高折射率介质环3的内径对基模损耗有直接的影响，当高折射率介质环3的内径较小时，其LP01模损耗总是很大，而当其内径值增加到一定值以后，其LP01模损耗将迅速减小。其原因是LP01模的能量损耗是通过与超模的耦合而实现的，而超模的损耗来源于与其相邻的高折射率介质环3的高折射率。显然，当高折射率介质环3与高折射率介质柱2的距离达到较小的值以后，超模的折射率引导机制即被破坏，从而无法实现有效的光传输。而相对比的，高阶模的泄露损耗总是随高折射率介质环3的内径的增大而减小。这是因为高阶模没有与超模发生耦合，其模场距离高折射率介质环3较远，因而，高折射率介质环3的内径再小也不会导致高阶模无法传输。
如图8所示，为实施例1中纤芯模式的泄露损耗与高折射率介质环3的折射率的关 系曲线；其中E01为LP01模的泄露损耗曲线；E11为LP11模的泄露损耗曲线。高折射率介质环3的折射率大小决定了基模的损耗。当高折射率介质环3的折射率位于超模群区间以上时，基模的泄露损耗很大。当高折射率介质环3的折射率达到一定值以后，其值进一步增大对基模的泄露损耗基本没有影响。这说明高折射率介质环3的折射率须高于超模群的折射率，从而形成滤模机制。另一方面，高折射率介质环3的折射率在超模群区间及附近变化时对高阶模的损耗影响很小。
实施例二：
同样按照图1给出的一种光纤实施例结构，基质材料1取为纯石英，高折射率介质柱2与基质材料1的折射率差为0.029，高折射率介质环3与基质材料1的折射率差为0.01。纤芯4与基质材料1的折射率差改变为0.0105。高折射率介质柱2的周期为7.75μm，纤芯直径为12μm。高折射率介质柱2层数N＝3。纤芯结构可以支持LP01、LP11、LP21、LP02四个模式。
如图9所示，为实施例2中纤芯模式有效折射率与纤芯模式的泄露损耗随波长变化曲线；图9(a)为纤芯模式有效折射率曲线与高折射率介质柱形成的超模群区间。图9(b)为纤芯模式的泄露损耗随波长变化曲线。F01为LP01模的有效折射率曲线，F11为LP11模的有效折射率曲线，F21为LP21模的有效折射率曲线，F02为LP02模的有效折射率曲线，F30为超模群的上边界，F40为超模群的下边界，F50为高折射率介质柱的带隙上边缘；G01为LP01模的泄露损耗曲线；G11为LP11模的泄露损耗曲线，G21为LP21模的泄露损耗曲线；G02为LP02模的泄露损耗曲线。
由两图可见：此光纤的LP11模处于超模群区间时，其泄露损耗达到20dB/m以上，而在远离超模群区间时，其泄露损耗迅速减小。对于LP01模，其处于折射率引导区域，泄露损耗很低。对于LP21、LP02两个模式，其处于带隙传导区，但其模场在包层区扩展，其损耗较大。若要求LP11模的损耗大于20dB/m，而以其它模式损耗小于0.1dB/m为模式传输的条件，则此光纤的工作波长范围可以达70nm以上。
实施例三：
图10给出了本发明的另一种实施例的横截面示意图。其参数与实施例二相同，差别是高折射率介质柱2层数N＝4。此结构通过增加高折射率介质柱数量，以减小LP21、LP02两个模式的泄露损耗。如图11所示，为实施例3中纤芯模式的泄露损耗随波长变化曲线；其中，H01为LP01模的泄露损耗曲线；H11为LP11模的泄露损耗曲线，H21为LP21 模的泄露损耗曲线；H02为LP02模的泄露损耗曲线。LP11模仍具有高的泄露损耗，而LP21、LP02两个模式的损耗得到减小。若要求LP11模的损耗大于20dB/m，而以其它模式损耗小于0.1dB/m为模式传输的条件，则此光纤的工作波长范围可以达200nm以上。
上述附图和说明仅为说明性示意图，并不对本发明的保护范围形成限制。应理解，这些实施例只是为了举例说明本发明，而非以任何方式限制本发明的范围。

Claims (10)

  1. 一种滤模光纤，包括纤芯(4)和包层，其特征在于：所述包层包括基质材料(1)、高折射率介质柱(2)和高折射率介质环(3)；
    所述纤芯(4)位于光纤的中心，所述高折射率介质柱(2)围绕纤芯(4)并周期性排布在基质材料(1)内，所述高折射率介质环(3)环绕在所述基质材料(1)、高折射率介质柱(2)的外侧，其折射率之间的关系满足：nout>nclad,ncore>nclad,nrod>nclad；纤芯(4)与基质材料(1)的参数关系满足：V>2.405，这里
    Figure PCTCN2015077615-appb-100001
    关系式中，ncore,nrod,nclad,nout分别为纤芯(4)、高折射率介质柱(2)、基质材料(1)和高折射率介质环(3)的折射率，rcore为纤芯半径，λ0为工作波长；
    所述高折射率介质柱(2)和基质材料(1)组成的微结构区域可传输的模式形成超模群，超模群中最低阶模式的有效折射率和最高阶模式的有效折射率形成的区间为超模群区间，在工作波长范围内，至少有一个纤芯模式的有效折射率处于超模群区间，其泄露损耗大于有效折射率未处于超模群区间的其它纤芯模式。
  2. 根据权利要求书1所述的一种滤模光纤，其特征在于：在工作波长范围内，所述高折射率介质环(3)的折射率处于超模群区间以上。
  3. 根据权利要求书2所述的一种滤模光纤，其特征在于：所述高折射率介质环(3)为一圆环，所述高折射率介质环(3)的内径为din，其圆心与纤芯中心位置相同；定义L为纤芯中心与每一个高折射率介质柱(2)的中心之间的距离的集合，其中的最大值为Lmax，则高折射率介质环(3)与高折射率介质柱(2)之间的距离需满足：din-Lmax-rrod<4μm，这里rrod为高折射率介质柱(2)的半径。
  4. 根据权利要求书1所述的一种滤模光纤，其特征在于：所述高折射率介质柱(2)的层数N满足：N≥3。
  5. 根据权利要求书1所述的一种滤模光纤，其特征在于：所述纤芯(4)和与之相邻的高折射率介质柱(2)之间的中心距离S满足：S-(rcore+rrod)≥3μm且S-(rcore+rrod)≤8μm。
  6. 根据权利要求书3所述的一种滤模光纤，其特征在于：所述高折射率介质环(3)的折射率nout与超模群中最低阶模式的有效折射率nceff之间满足：nout-nceff>0.0005。
  7. 根据权利要求书1所述的一种滤模光纤，其特征在于：所述高折射率介质柱(2) 由2-3种不同类型的高折射率介质柱组成，且同一类型的高折射率介质柱组成1-3区域；形成的区域总数为2-6个，在同一个区域内，高折射率介质柱(2)的周期、直径和折射率均相同，且所有的高折射率介质柱(2)中心位于以纤芯中心为圆心的扇形区域内。
  8. 根据权利要求书7所述的一种滤模光纤，其特征在于：所述光纤横截面分布具有轴对称性。
  9. 根据权利要求书1所述的一种滤模光纤，其特征在于：所述纤芯(4)采用高折射率液体。
  10. 根据权利要求书1所述的一种滤模光纤，其特征在于：以高折射率液体柱替代高折射率介质柱。
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