A Few-Mode Fiber Bragg Grating Selective Filter

Abstract

The present invention provides a few-mode fiber grating selective filter. It consists of an incident optical fiber, an optical fiber circulator, a 1×N optical switch, a photon lantern A, a few-mode optical fiber, a grating area, a photon lantern B, a coupler and an output optical fiber. One end of the incident optical fiber is connected to a light source, and the other end is connected to the a port of the optical fiber circulator. The b port of the optical fiber circulator is connected to the 1×N optical switch. After the optical switch is connected to any optical path, the light is transmitted to the photon lantern A. The photon lantern excites different modes of the few-mode optical fiber according to the different pigtails incident at the multi-path end. After passing through the grating area, the light that meets the conditions is reflected, and the rest of the light continues to be transmitted to realize the filtering function. The reflected light is output through the c port of the optical fiber circulator, and the transmitted light is transmitted to the output optical fiber through the photon lantern B and the coupler. The present invention can be used for selective filtering, and different wavelengths can be filtered out according to the different effective refractive indices of the transmission mode of the few-mode optical fiber. It can be widely used in the field of optical communications.

Description

Technical Field

[0001] The invention relates to a few-mode optical fiber grating selective filter, belonging to the field of optical communication. Background Art

[0002] In recent years, with the rapid development of optical communication systems, it has become increasingly important to use communication spectrum resources more flexibly and efficiently, which requires optical filters to be able to achieve independent adjustment of wavelength, bandwidth and wavelength interval. Fiber optic filters are widely used in wavelength selection, optical add / drop multiplexing, pulse compression, spectrum shaping and fiber optic sensing.

[0003] Fiber Bragg grating is a passive filter device formed by axially periodically modulating the refractive index of the optical fiber core through a certain method. It is widely used in the field of optical fiber communication due to its small size, low cost, low insertion loss and easy realization of all-optical fiber network. Fiber Bragg grating is an excellent filter device in optical fiber filter due to its excellent frequency selection characteristics.

[0004] With the development of wavelength division multiplexing systems, traditional single wavelength filters can no longer meet the demand. Tunable filters can be used for demultiplexing, signal demodulation, etc. in wavelength division multiplexing systems, playing an extremely important role in them. In the patent with application number 2013201022672, a grating-type tunable filter is proposed, in which two optical fibers, two gratings, two reflectors, a beam expander and a polarization rotation element are arranged along the optical path, one of which is a rotatable reflector, and the incident angle of the two gratings is changed by the rotatable reflector to achieve wavelength selection. This invention uses a spatial grating as a filter, and achieves filtering of different wavelengths by controlling the angles of the two gratings, but this invention requires a high-precision rotating device, and a large loss will be generated when coupling the spatial light into the optical fiber, which is not suitable for use in an all-fiber system.

[0005] The patent application number 2017106791787 proposes a tunable multi-channel filter based on a silicon-based graphene Bragg grating structure. It mainly uses a single-layer graphene as a mid-infrared waveguide and an applied voltage as a modulation method to achieve the local variability of the optical properties of graphene, thereby achieving a periodic change in the effective refractive index and achieving the purpose of 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 applied voltage jointly affect the optical parameters of the graphene surface. If the periodicity of the effective refractive index on the graphene is broken by introducing a mismatched structure or an applied voltage, a defect resonance mode can be generated. At this time, the structure can be regarded as a Fabry-Perot cavity to achieve a broadband filtering effect. This method has the characteristics of tunability, but requires a complex preparation process and has the problem of coupling with the optical fiber system.

[0006] The patent with application number 2014101873448 proposes an all-fiber filter. The optical fiber used is a microstructured optical fiber. The entire system includes a light source, a single-mode optical fiber, a spectrum analyzer, a microstructured optical fiber, and a magnetic field generating and tuning device. A mask method is used to prepare a tilted grating on the germanium-doped core of the microstructured optical fiber, and a magnetic fluid is filled in the micropores of the microstructured optical fiber. The portion filled with the magnetic fluid corresponds to the position of the tilted grating and is greater than the length of the engraved fiber grating to ensure that the magnetic fluid covers the entire fiber grating area. When light passes through the microstructured optical fiber, the magnitude of the magnetic field strength generated by the magnetic field generating and tuning device is adjusted. The change in the magnetic field changes the refractive index of the magnetic fluid in the optical fiber, causing the effective refractive index of the fundamental mode and the effective refractive index of the cladding mode of the microstructured optical fiber to change, and the fundamental mode resonant wavelength and the cladding mode resonant wavelength to drift, changing the resonant wavelength of the fiber tilted grating, thereby obtaining a magnetically controlled tunable filter that changes with the external magnetic field. The optical fiber required for this method is an optical fiber of a specific structure, which is costly, has a large transmission loss, and requires an additional control method, which increases the complexity of the operation. Summary of the invention

[0007] The object of the present invention is to provide a few-mode fiber grating selective filter which has a simple and compact structure, is easy to operate, has low loss and does not require external modulation.

[0008] The object of the present invention is achieved in that:

[0009] like Figure 1 As shown, the filter is composed of an incident optical fiber 1, an optical fiber circulator 9, a 1×N optical switch 2, a photon lantern A3, a few-mode optical fiber 4, a grating area 5, a photon lantern B6, a coupler 7, an output optical fiber 8 and an output optical fiber 13. One end of the incident optical fiber 1 is connected to the light source, and the other end is connected to the a port of the optical fiber circulator 9. The b port of the optical fiber circulator 9 is connected to the 1×N optical switch 2. The optical switch transmits light to the photon lantern A3 after connecting any optical path. The photon lantern excites different transmission modes in the few-mode optical fiber 4 according to the different pigtails incident at the multi-path end; after passing through the grating area 5, the light that meets the conditions is reflected, and the rest of the light continues to be transmitted to realize the filtering function. The reflected light is output from the output optical fiber 13 through the c port of the optical fiber circulator 9, and the transmitted light is converted into the fundamental mode of the single-mode optical fiber again through the photon lantern B6, and finally transmitted to the output optical fiber 8 through the coupler 7.

[0010] The working process of the filter is as follows:

[0011] When the light emitted by the light source is transmitted to the 1×N optical switch through the incident optical fiber, the optical switch controls the incident light to enter the multi-path end pigtail of the few-mode optical fiber with the required filtered wavelength. Each pigtail at the multi-path pigtail end of the photon lantern corresponds to a mode of the few-mode optical fiber, that is, each pigtail corresponds to a filtered wavelength. The mode excited by the photon lantern continues to transmit along the few-mode optical fiber. When it reaches the grating area, the light that meets the phase matching condition is reflected. The reflection wavelength of the grating is:

[0012] λ B =2n eff Λ

[0013] In the above formula, λ B is the Bragg reflection wavelength, n eff is the effective refractive index of the mode, and Λ is the period of the grating. It can be seen from the formula that when the grating period is fixed, the reflected wavelength is affected by the effective refractive index. The core of the few-mode fiber can transmit multiple modes, and each mode has a different effective refractive index. Therefore, selective filtering can be achieved by selecting the excited mode.

[0014] Due to the reversibility of the optical path, the reflected light is transmitted in the reverse direction, and after passing through the photon lantern A, it is converted into the fundamental mode of the single-mode optical fiber, transmitted along the optical switch to the optical fiber circulator, and output from the optical fiber circulator port b and port c. This port can be used as the output end of the bandpass filter.

[0015] The transmitted light continues to be transmitted along the few-mode fiber. When it is transmitted to the photon lantern B, the transmitted mode is converted into the fundamental mode of the pigtail with lower loss through the photon lantern B and continues to be transmitted. Since the multi-channel input ends of the mode-selective photon lantern are mostly heterogeneous single-mode fibers, it is necessary to use a fused taper coupler to enable each heterogeneous single-mode fiber to be coupled with the single-mode fiber for communication. The transmitted light is finally output through the output fiber, and this output end can be used as the output end of the band-stop filter.

[0016] In order to facilitate the integration of the filter into the existing communication system, the input optical fiber and the output optical fiber described in the present invention are both single-mode optical fibers.

[0017] The coupler described in the present invention is made of a fused taper. The photon lantern used is a mode-selective photon lantern, and each pigtail incident at the multi-path end can excite a specific mode of the few-mode optical fiber. Most of the multi-path pigtail ends of this photon lantern are heterogeneous single-mode optical fibers, that is, the core and cladding sizes of each pigtail are different. Therefore, the multi-path pigtail of the optical switch used in the present invention and the multi-path pigtail of the photon lantern should match each other, that is, the core diameter and cladding diameter are consistent, and both are single-mode optical fibers.

[0018] In the present invention, the optical switch has the same number of multi-path tail fibers as the photon lantern A, that is, the same number of modes that can be transmitted by the few-mode optical fiber, and the excitation of each mode in the few-mode optical fiber can be controlled individually.

[0019] In order to reduce the loss of the entire filter, the multi-path end pigtails of the photon lanterns A and B described in the present invention are both single-mode optical fibers, which only support one mode of transmission; the single-path end pigtails are few-mode optical fibers, which can support multiple modes; the single-path pigtails of the photon lanterns A and B match the few-mode optical fibers used, and the core diameter, cladding diameter and the number of transmission modes supported are consistent.

[0020] In the present invention, in order to reduce the mutual influence between various modes, the few-mode optical fiber used is a weak inter-mode coupling few-mode optical fiber, which can also be called a low inter-mode crosstalk few-mode optical fiber. The characteristic of this optical fiber is that each mode in the core can be transmitted independently and is not affected by other modes.

[0021] The grating region of the present invention can be a single grating or an array composed of multiple gratings. The grating used is composed of one or more of fiber Bragg grating, chirped fiber grating, sampled fiber grating, and apodized fiber grating.

[0022] The beneficial effects of the present invention are:

[0023] The present invention has an all-fiber structure with low connection loss and is easy to integrate with existing communication systems. The filter uses the transmission mode of the few-mode fiber as a selective filtering channel, does not require an external control field, and is simple to operate. The device can be prepared using traditional fiber grating processing equipment, which is convenient for mass production. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the selective filter structure.

[0025] Figure 2 is a schematic diagram of the selective filter measurement device.

[0026] Figure 3 It is the mode that 6-mode optical fiber can transmit.

[0027] Figure 4 It is LP 01 Spectra when the mode is incident; (a) transmission spectrum, (b) reflection spectrum.

[0028] Figure 5 It is LP 11 Spectra when the mode is incident; (a) transmission spectrum, (b) reflection spectrum.

[0029] Figure 6 It is LP 21 Spectra when the mode is incident; (a) transmission spectrum, (b) reflection spectrum.

[0030] Figure 7 It is LP 02Spectra when the mode is incident; (a) transmission spectrum, (b) reflection spectrum.

[0031] Figure 8 It is LP 31 Spectra when the mode is incident; (a) transmission spectrum, (b) reflection spectrum.

[0032] Fig. 9 It is LP 12 Spectra when the mode is incident; (a) transmission spectrum, (b) reflection spectrum.

[0033] In the figure: 1 is the incident optical fiber; 2 is the 1×N optical switch; 3 is the photon lantern A; 4 is the few-mode optical fiber; 5 is the grating area; 6 is the photon lantern B; 7 is the coupler; 8 is the output optical fiber; 9 is the optical fiber circulator; 10 is the selective filter; 11 is the spectrometer, 12 is the light source, and 13 is the output optical fiber. DETAILED DESCRIPTION

[0034] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0035] Taking six-mode optical fiber as an example, in this embodiment, the modes that can be transmitted by the few-mode optical fiber 4 are as follows: Figure 3 As shown, including LP 01 , L.P. 11 , L.P. 21 , L.P. 02 , L.P. 31 , L.P. 12 Six modes, the few-mode fiber core diameter is 25.6μm and the cladding diameter is 125μm. Figure 2 A schematic diagram of a measurement device based on a few-mode fiber Bragg grating selective filter is given. The device is composed of a light source 12, an incident optical fiber 1, a selective filter 9, an output optical fiber 8, an output optical fiber 13, and a spectrometer 11. The structure of the selective filter is shown in FIG. Figure 1 As shown, it is formed by sequentially connecting an incident optical fiber 1, an optical fiber circulator 9, a 1×N 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 output optical fiber 8, and an output optical fiber 13. Because the few-mode optical fiber used is a six-mode optical fiber, N=6 in this embodiment, and the photon lantern multi-path end has 6 pigtails, and each incident port can excite one mode of the few-mode optical fiber.

[0036] The coupler is made by fusion taper. The photon lantern A and photon lantern B used in this embodiment are the same 6-mode photon lanterns. The photon lantern is a heterogeneous core photon lantern developed by this laboratory, which is made by adding a sleeve, assembling rods, and taper. The single-channel end of the photon lantern is a few-mode output pigtail, and its pigtail structure is the same as the few-mode optical fiber used; the multi-channel end contains 6 heterogeneous single-mode optical fibers, each of which corresponds to the excitation of a transmission mode in the few-mode optical fiber.

[0037] The grating region in this embodiment adopts a fiber Bragg grating structure. The grating is prepared on a few-mode fiber using a phase mask method. The fiber is loaded with hydrogen in a high-pressure environment before grating writing. Then, a mask with a period of 1060nm is used to write the grating. The transmission and reflection spectra of each mode after preparation are shown in FIG. Figure 4-Figure 9 As shown, Figure 4 It is LP 01 Spectra when the mode is incident, (a) and (b) are the reflection spectrum and transmission spectrum respectively; Figure 5 It is LP 11 Spectrum at mode incidence; Figure 6 It is LP 21 Spectrum at mode incidence; Figure 7 It is LP 02 Spectrum at mode incidence; Figure 8 It is LP 31 Spectrum at mode incidence; Fig. 9 It is LP 12 The spectrum of the incident mode. Since the effective refractive index of each mode is different, the reflected wavelength is different, thus forming a multi-channel filter.

[0038] The working process of the filter is as follows: the broadband light emitted by the light source 12 is transmitted to the fiber circulator through the incident optical fiber 1, input through port a, and output through port b. The output light is controlled by the 1×6 optical switch 2 and transmitted to any pigtail at the multi-way end of the photon lantern A3, and the photon lantern A3 excites a specific mode of the few-mode optical fiber 4. When the excited light passes through the grating area 5, the forward transmission light and the backward transmission light that meet the phase matching condition are coupled, and the coupled light is transmitted back along the few-mode optical fiber 4 to the photon lantern A3. Since the optical path is reversible, the reflected light is transmitted to the fiber circulator 9 through the optical switch 2 after the photon lantern A3, input from port b, and output from port c. The output light is received by the spectrometer 11, which can be used as the output end of the bandpass filter. The unreflected light continues to be transmitted along the few-mode fiber. After passing through the photon lantern B6, the transmission mode is converted into the single-mode fiber fundamental mode transmission of the multi-path end pigtail of the photon lantern B. After passing through the coupler 7, the output light is coupled to the traditional single-mode fiber and received by the spectrometer 11. The output end can be used as the output end of the band-stop filter. The effective refractive index of each mode in the few-mode fiber is different, so the reflected wavelength is different. By controlling the excitation mode through the optical switch, different wavelengths can be filtered out to achieve a selective filtering function.

Claims

1. A few-mode fiber Bragg grating selective filter, characterized in that: It consists of an incident optical fiber, an optical fiber circulator, a 1×N optical switch, a photon lantern A, a few-mode optical fiber, a grating area, a photon lantern B, a coupler and an output optical fiber; one end of the incident optical fiber is connected to a light source, and the other end is connected to a port of the optical fiber circulator; the b port of the optical fiber circulator is connected to the 1×N optical switch; the optical switch transmits light to the photon lantern A after connecting any optical path; the photon lantern excites different modes in the few-mode optical fiber according to the different pigtails incident at the multi-path end; the light emitted by the light source is transmitted to the photon lantern A after passing through the optical fiber circulator and the 1×N optical switch, and excites a specific mode of the few-mode optical fiber through any pigtail of the photon lantern A; when the excited light passes through the grating area, the light that meets the phase matching condition is coupled and transmitted back along the few-mode optical fiber to the photon lantern A, and is output from the c port of the optical fiber circulator; this port can be used as the output end of a bandpass filter; the light that is not reflected continues to be transmitted along the few-mode optical fiber, is converted into the fundamental mode of the pigtail at the multi-path end after passing through the photon lantern B, and is output after passing through the coupler; this output end can be used as the output end of a bandstop filter.

2. The few-mode fiber Bragg grating selective filter according to claim 1, characterized in that: The incident optical fiber and the output optical fiber are both single-mode optical fibers.

3. The few-mode fiber Bragg grating selective filter according to claim 1, characterized in that: The 1×N optical switch multi-channel end pigtail matches the multi-channel end pigtail of the photon lantern A, that is, the core diameter and cladding diameter are consistent.

4. The few-mode fiber Bragg grating selective filter according to claim 1, characterized in that: The photon lantern is a mode-selective photon lantern, and each pigtail incident on the multi-path end can excite a specific mode of the few-mode optical fiber.

5. The few-mode fiber Bragg grating selective filter according to claim 1, characterized in that: The multi-path end pigtails of the photon lanterns A and B are both single-mode optical fibers, supporting only one mode of transmission; the single-path end pigtails are few-mode optical fibers, supporting multiple modes; the single-path end pigtails of the photon lanterns A and B match the few-mode optical fibers used, that is, the core diameter, cladding diameter and the number of modes supported for transmission are consistent.

6. The few-mode fiber Bragg grating selective filter according to claim 1, characterized in that: The few-mode optical fiber is a weakly inter-mode coupled few-mode optical fiber, and each mode in the fiber core can be transmitted independently and is not affected by other modes.

7. The few-mode fiber Bragg grating selective filter according to claim 1, characterized in that: The grating area is composed of no less than one grating.

8. The few-mode fiber grating selective filter according to claim 1, characterized in that: The grating area is composed of one or more of a fiber Bragg grating, a chirped fiber grating, a sampled fiber grating, and an apodized fiber grating.

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