CN113970812A - Few-mode fiber grating selective filter - Google Patents

Few-mode fiber grating selective filter Download PDF

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Publication number
CN113970812A
CN113970812A CN202111195032.8A CN202111195032A CN113970812A CN 113970812 A CN113970812 A CN 113970812A CN 202111195032 A CN202111195032 A CN 202111195032A CN 113970812 A CN113970812 A CN 113970812A
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mode
optical fiber
fiber
few
grating
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苑立波
王洪业
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Guilin University of Electronic Technology
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Guilin University of Electronic Technology
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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
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2746—Optical coupling means with polarisation selective and adjusting means comprising non-reciprocal devices, e.g. isolators, FRM, circulators, quasi-isolators
    • 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
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/264—Optical coupling means with optical elements between opposed fibre ends which perform a function other than beam splitting
    • 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
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Optical Integrated Circuits (AREA)

Abstract

The invention provides a few-mode fiber grating selective filter. The photonic lantern consists of an incident optical fiber, an optical fiber circulator, a 1 XN optical switch, a photonic lantern A, a few-mode optical fiber, a grating area, a photonic lantern B, a coupler and an emergent optical fiber. One end of an incident optical fiber is connected with a light source, the other end of the incident optical fiber is connected with an a port of an optical fiber circulator, a b port of the optical fiber circulator is connected with a 1 XN optical switch, the optical switch is communicated with any light path and then transmits light to a photon lantern A, and the photon lantern excites different modes of a few-mode optical fiber according to different tail fibers incident from multiple ends. The light meeting the conditions after passing through the grating area is reflected, the rest light is continuously transmitted, the filtering function is realized, the reflected light is output through the port c of the optical fiber circulator, and the transmitted light is transmitted to the emergent optical fiber through the photon lantern B and the coupler. The invention can be used for selective filtering, and can realize filtering of different wavelengths according to different effective refractive indexes of few-mode optical fiber transmission modes. Can be widely used in the field of optical communication.

Description

Few-mode fiber grating selective filter
Technical Field
The invention relates to a few-mode fiber grating selective filter, belonging to the field of optical communication.
Background
In recent years, with the rapid development of optical communication systems, it is becoming more and more important to utilize communication spectrum resources more flexibly and efficiently, which requires that optical filters be capable of realizing independent adjustability of wavelength, bandwidth and wavelength interval. The optical fiber filter has wide application in the aspects of wavelength selection, optical add-drop multiplexing, pulse compression, spectrum shaping, optical fiber sensing and the like.
The fiber grating is a passive filter device formed by periodically modulating the refractive index of the fiber core in the axial direction by a certain method. The optical fiber network has small volume, low cost, small insertion loss and easy realization of all-optical fiber network, and is widely applied in the field of optical fiber communication. The fiber grating is an excellent filter device in the fiber filter due to the excellent frequency selection characteristic.
With the development of wavelength division multiplexing systems, the conventional single wavelength filter has failed to meet the demand. Tunable filters are used for demultiplexing, signal demodulation, etc. of wavelength division multiplexing systems, and play an extremely important role therein. A grating-type tunable filter is proposed in the patent with application number 2013201022672, which is provided with two optical fibers, two gratings, two reflectors, a beam expanding element and a polarization rotating element along an optical path, wherein one of the reflectors is a rotatable reflector, and the incident angles of the two gratings are changed by the rotatable reflector, thereby realizing wavelength selection. The invention uses the space grating as a filter, realizes the filtering of different wavelengths by controlling the angles of the two gratings, but the invention needs a high-precision rotating device, generates larger loss when the space light is coupled into the optical fiber and is not suitable for the use of an all-fiber system.
The patent with application number 2017106791787 proposes a tunable multi-channel filter based on a silicon-based graphene bragg grating structure, which mainly uses single-layer graphene as a mid-infrared waveguide and uses an external voltage as a modulation means to realize local variability of optical characteristics of the graphene, thereby realizing periodic change of effective refractive index and achieving the purpose of serving as a bragg reflector. The reflector has a three-layer structure including a graphene layer, a silicon grating substrate and an intermediate silicon dioxide insulating layer. The grating structure and the external voltage influence optical parameters of the surface of the graphene together, if unmatched structures or external voltages are introduced, the periodicity of the effective refractive index on the graphene is broken, a defect resonant mode can be generated, and the structure can be regarded as a Fabry-Perot cavity at the moment, so that the broadband filtering effect is realized. The method has the characteristic of tuning, but needs a complex preparation process and has the problem of coupling with an optical fiber system.
The patent application No. 2014101873448 proposes an all-fiber filter. The optical fiber used is a microstructure optical fiber, the whole system comprises a light source, a single-mode optical fiber, a spectrum analyzer, the microstructure optical fiber and a magnetic field generating and tuning device, an inclined grating is prepared on the germanium-doped fiber core of the microstructure optical fiber by using a mask plate method, magnetic fluid is filled in micropores of the microstructure optical fiber, and the part filled with the magnetic fluid corresponds to the position of the inclined grating and is longer than the length of the carved optical fiber grating so as to ensure that the magnetic fluid covers the whole optical fiber grating area. When light passes through the micro-structural fiber, the intensity of the magnetic field generated by the magnetic field generating and tuning device is adjusted, the change of the magnetic field changes the refractive index of the magnetic fluid in the fiber, so that the effective refractive index of the base mode and the effective refractive index of the cladding mode of the micro-structural fiber are changed, the resonance wavelength of the base mode and the resonance wavelength of the cladding mode are shifted, and the resonance wavelength of the fiber tilt grating is changed, thereby obtaining the magnetic control tunable filter which is changed along with the external magnetic field. The optical fiber required by the method is the optical fiber with a specific structure, so that the cost is high, the transmission loss is large, an additional regulation and control mode is required, and the complexity of operation is increased.
Disclosure of Invention
The invention aims to provide a few-mode fiber grating selective filter which has simple and compact structure, easy operation, low loss and no need of external modulation.
The purpose of the invention is realized as follows:
as shown in fig. 1, the filter is composed of an incident optical fiber 1, a fiber circulator 9, a 1 xn optical switch 2, a photon lantern a3, a few-mode optical fiber 4, a grating region 5, a photon lantern B6, a coupler 7, an exit optical fiber 8 and an exit optical fiber 13. One end of an incident optical fiber 1 is connected with a light source, the other end of the incident optical fiber is connected with an a port of an optical fiber circulator 9, a b port of the optical fiber circulator 9 is connected with a 1 XN optical switch 2, the optical switch transmits light to a photon lantern A3 after being communicated with any optical path, and the photon lantern excites different transmission modes in a few-mode optical fiber 4 according to different tail fibers incident from multiple ends; after passing through the grating region 5, the light satisfying the condition is reflected, the remaining light is continuously transmitted, so as to realize the filtering function, the reflected light is output from the exit optical fiber 13 through the c port of the optical fiber circulator 9, the transmitted light converts the filtered mode into the fundamental mode of the single-mode optical fiber again through the photon lantern B6, and finally is transmitted to the exit optical fiber 8 through the coupler 7.
The filter works substantially as follows:
when light emitted by the light source is transmitted to the 1 XN optical switch through the incident optical fiber, the optical switch controls the incident light to enter the multi-path end tail fiber of the few-mode optical fiber with the wavelength needing to be filtered. Each tail fiber at the end of the multipath tail fibers of the photon lantern is corresponding to one mode of the few-mode fiber, namely each tail fiber is corresponding to one filtering wavelength. The mode excited by the photon lantern is continuously transmitted along the few-mode optical fiber, and after reaching the grating area, the light meeting the phase matching condition is reflected. The reflection wavelength of the grating is:
λB＝2neffΛ
in the above formula, λBIs the Bragg reflection wavelength, neffIs the effective index of the mode, Λ is the period of the grating. It can be seen from the formula that when the grating period is fixed, the reflection wavelength is influenced by the effective refractive index, multiple modes can be transmitted in the fiber core of the few-mode fiber, and each mode has a different effective refractive index, so that selective filtering can be realized by selecting the excited mode.
Due to the reversibility of the optical path, the reflected light is reversely transmitted, is converted into a basic mode of the single-mode optical fiber after passing through the photon lantern A, is transmitted to the optical fiber circulator along the optical switch, and is output through a port b and a port c of the optical fiber circulator. The port may serve as the output of a band pass filter.
The transmitted light continues to be transmitted along the few-mode optical fiber, and when transmitted to the photon lantern B, the transmitted mode is converted into the fundamental mode of the tail fiber through the photon lantern B and continues to be transmitted with low loss. Since most of the multi-channel input ends of the mode selection type photon lantern are heterogeneous single mode fibers, the heterogeneous single mode fibers can be coupled with the single mode fibers for communication by using a fused tapered coupler. The transmitted light is finally output through the emergent optical fiber, and the output end can be used as the output end of the band elimination filter.
In order to facilitate the integration of the filter into the existing communication system, the incident optical fiber and the emergent optical fiber are single-mode optical fibers.
The coupler is made of a fused biconical taper. The adopted photon lantern is a mode selection type photon lantern, and incidence of each tail fiber at a multi-path end can excite a specific mode of the few-mode optical fiber. The multi-path tail fiber end of the photon lantern is mostly heterogeneous single-mode fiber, namely, the fiber core and the cladding of each tail fiber are different in size, so that the multi-path end tail fiber of the optical switch used by the invention is matched with the multi-path end tail fiber of the photon lantern, namely, the fiber core diameter and the cladding diameter are consistent, and the multi-path end tail fiber and the cladding diameter are both single-mode fiber.
In the invention, the number of the optical switch and the tail fibers at the ends of the photon lantern A is the same, namely the number of the modes which can be transmitted by the few-mode optical fiber is the same, and the excitation of each mode in the few-mode optical fiber can be independently controlled.
In order to reduce the loss of the whole filter, the tail fibers at the ends of the multiple paths of the photon lantern A and the photon lantern B are single-mode fibers, and only one mode transmission is supported; the single-path end tail fiber is a few-mode fiber and can support multiple modes; the single-path tail fibers of the photon lanterns A and B are matched with the few-mode fibers, and the diameters of the fiber cores and the cladding are consistent with the number of supported transmission modes.
In the invention, in order to reduce the mutual influence among the modes, the used few-mode optical fiber is weak mode-to-mode coupling few-mode optical fiber, the optical fiber can also be called low mode-to-mode crosstalk few-mode optical fiber, and the optical fiber is characterized in that each mode in a fiber core can be independently transmitted and is not influenced by other modes.
The grating area can be a single grating or an array formed by a plurality of gratings. The grating used by the method is composed of one or more of fiber Bragg grating, chirped fiber grating, sampling fiber grating and apodization fiber grating.
The invention has the beneficial effects that:
the invention is of an all-fiber structure, has small connection loss and is convenient to integrate with the existing communication system; the filter uses the transmission mode of the few-mode optical fiber as a selective channel for filtering, and a control field is not required to be added, so that the operation is simple; the device can be prepared by using the traditional fiber grating processing equipment, and the mass production is convenient.
Drawings
Fig. 1 is a schematic diagram of a selectivity filter structure.
Fig. 2 is a schematic diagram of a selectivity filter measurement device.
Fig. 3 is a diagram of the modes that a 6-mode fiber can transmit.
FIG. 4 is LP01Spectrum at mode incidence; (a) transmission spectrum, (b) reflection spectrum.
FIG. 5 is LP11Spectrum at mode incidence; (a) transmission spectrum, (b) reflection spectrum.
FIG. 6 is LP21Spectrum at mode incidence; (a) transmission spectrum, (b) reflection spectrum.
FIG. 7 is LP02Spectrum at mode incidence; (a) transmission spectrum, (b) reflection spectrum.
FIG. 8 is LP31Spectrum at mode incidence; (a) transmission spectrum, (b) reflection spectrum.
FIG. 9 is LP12Spectrum at mode incidence; (a) transmission spectrum, (b) reflection spectrum.
In the figure: 1 is an incident optical fiber; 2 is a 1 XN optical switch; 3 is a photon lantern A; 4 is a few-mode optical fiber; 5 is a grating area; 6 is a photon lantern B; 7 is a coupler; 8 is an emergent optical fiber; 9 is a fiber circulator; 10 is a selective filter; the spectrometer 11, the light source 12 and the exit fiber 13.
Detailed Description
The invention will be further elucidated with reference to the drawings and specific embodiments, without however being limited thereto.
Taking a six-mode fiber as an example, the modes that can be transmitted by the few-mode fiber 4 in this embodiment are shown in fig. 3, including LP01，LP11，LP21，LP02，LP31，LP12Six modes, few-mode fiber core diameter 25.6 μm, cladding diameter 125 μm. Fig. 2 shows a schematic diagram of a measuring device based on a few-mode fiber grating selective filter. The device consists of a light source 12, an incident optical fiber 1, a selective filter 9, an emergent optical fiber 8, an emergent optical fiber 13 and a spectrometer 11. Selective filterThe structure of the optical fiber is shown in fig. 1, and the optical fiber comprises an incident optical fiber 1, an optical fiber circulator 9, a 1 XN optical switch 2, a photon lantern A3, a few-mode optical fiber 4, a grating region 5, a photon lantern B6, a coupler 7, an emergent optical fiber 8 and an emergent optical fiber 13 which are connected in sequence. Since the few-mode fiber used is a six-mode fiber, N is 6 in this embodiment, and the end of the photonic lantern multiple has 6 pigtails, each incident port can excite one mode of the few-mode fiber.
The coupler is made in a fused biconical taper mode. The photon lantern a and the photon lantern B used in this embodiment are the same 6-mode photon lantern. The photon lantern is a heterogeneous core photon lantern developed in the laboratory and is manufactured by adding a sleeve, a group of rods and a tapering cone. The single-path end of the photon lantern is a few-mode output tail fiber, and the tail fiber structure is the same as that of the few-mode optical fiber; the multi-path end comprises 6 heterogeneous single-mode fibers, and each single-mode fiber correspondingly excites one transmission mode in the few-mode fibers.
The grating area in this embodiment adopts a structure of a fiber bragg grating, a phase mask method is used to prepare a grating on a few-mode fiber, hydrogen is carried on the fiber in a high-pressure environment before grating writing, and then grating writing is performed by using a mask plate with a period of 1060 nm. The transmission spectrum and reflection spectrum of each mode after the preparation are shown in FIGS. 4-9, and FIG. 4 is LP01The spectrum when the mode is incident, (a) and (b) are respectively a reflection spectrum and a transmission spectrum; FIG. 5 is LP11Spectrum at mode incidence; FIG. 6 is LP21Spectrum at mode incidence; FIG. 7 is LP02Spectrum at mode incidence; FIG. 8 is LP31Spectrum at mode incidence; FIG. 9 is LP12Spectrum of the mode at incidence. The effective refractive indexes of the modes are different, so that the reflected wavelengths are different, and the multichannel filter is formed.
The filter works as follows, the broadband light emitted by the light source 12 is transmitted to the optical fiber circulator through the incident optical fiber 1, and is input through the a port and output through the b port. The output light is controlled to be transmitted to any tail fiber at the multi-path end of the photon lantern A3 after passing through the 1 x 6 optical switch 2, and a specific mode of the few-mode optical fiber 4 is excited through the photon lantern A3. When the excited light passes through the grating region 5, the forward transmission light and the backward transmission light which meet the phase matching condition are coupled, the coupled light is reversely transmitted to the photon lantern A3 along the few-mode optical fiber 4, and because the light path has reversibility, the reflected light is transmitted to the optical fiber circulator 9 through the optical switch 2 after passing through the photon lantern A3, and is input through a port b and output through a port c, and the output light is received through the spectrometer 11, and the port can be used as the output end of the band-pass filter. The light which is not reflected is continuously transmitted along the few-mode optical fiber, after passing through the photon lantern B6, the transmission mode is converted into the single-mode optical fiber fundamental mode transmission of the photon lantern B multi-path end tail fiber, the light is output and coupled to the traditional single-mode optical fiber after passing through the coupler 7, the output light is received through the spectrometer 11, and the output end can be used as the output end of the band elimination filter. Effective refractive indexes of all modes in the few-mode optical fiber are different, so that reflected wavelengths are different, the excited modes are controlled through the optical switch, different wavelengths can be filtered, and the selective filtering function is realized.

Claims (8)

1. A few-mode fiber grating selective filter is characterized in that: the photonic lantern consists of an incident optical fiber, an optical fiber circulator, a 1 XN optical switch, a photonic lantern A, a few-mode optical fiber, a grating area, a photonic lantern B, a coupler and an emergent optical fiber; one end of an incident optical fiber is connected with a light source, the other end of the incident optical fiber is connected with a port a of an optical fiber circulator, a port b of the optical fiber circulator is connected with a 1 XN optical switch, the optical switch is communicated with any light path and then transmits light to a photon lantern A, and the photon lantern excites different modes in the few-mode optical fiber according to different tail fibers incident from multiple paths of ends; after passing through the grating area, the light meeting the conditions is reflected, the rest of the light is continuously transmitted, the filtering function is realized, the reflected light is output through the port c of the optical fiber circulator, the transmitted light is converted into the fundamental mode of the single-mode optical fiber again through the photon lantern B, and finally the fundamental mode is transmitted to the emergent optical fiber through the coupler.
2. The few-mode fiber grating selective filter of claim 1, wherein: the incident optical fiber and the emergent optical fiber are both single-mode optical fibers.
3. The few-mode fiber grating selective filter of claim 1, wherein: the 1 XN optical switch multi-path end tail fiber is matched with the multi-path end tail fiber of the photon lantern A, namely the fiber core diameter and the cladding diameter are consistent in size.
4. The few-mode fiber grating selective filter of claim 1, wherein: the photon lantern is a mode selection type photon lantern, and incidence of each tail fiber at a multi-path end can excite a specific mode of the few-mode optical fiber.
5. The few-mode fiber grating selective filter of claim 1, wherein: the tail fibers at the ends of the photon lanterns A and B are single-mode fibers, and only one mode transmission is supported; the single-path end tail fiber is a few-mode fiber and can support multiple modes; the single-path end tail fibers of the photon lanterns A and B are matched with the few-mode fibers, namely the diameter of the fiber core, the diameter of the cladding and the number of supported transmission modes are consistent.
6. The few-mode fiber grating selective filter of claim 1, wherein: the few-mode optical fiber is weak mode-to-mode coupling few-mode optical fiber, and each mode in the fiber core can be transmitted independently and is not influenced by other modes.
7. The few-mode fiber grating selective filter of claim 1, wherein: the grating area is composed of at least one grating.
8. The few-mode fiber grating selective filter of claim 1, wherein: the grating area is composed of one or more of fiber Bragg grating, chirped fiber grating, sampling fiber grating and apodization fiber grating.
CN202111195032.8A 2021-10-12 2021-10-12 Few-mode fiber grating selective filter Pending CN113970812A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117420680A (en) * 2023-12-18 2024-01-19 华中科技大学 Photon lantern design method with mode-dependent loss equalization function

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN117420680A (en) * 2023-12-18 2024-01-19 华中科技大学 Photon lantern design method with mode-dependent loss equalization function
CN117420680B (en) * 2023-12-18 2024-02-23 华中科技大学 Photon lantern design method with mode-dependent loss equalization function

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