A modulatable optical fiber polarizer based on graphene micro-nano optical fiber

Abstract

The invention discloses a modulatable optical fiber polarizer based on graphene micro-nano optical fiber, and relates to the field of optoelectronic technology. A modulatable optical fiber polarizer based on graphene micro-nano optical fiber comprises a micro-nano optical fiber, wherein a solid silica core is arranged in the micro-nano optical fiber, a plurality of air holes are opened in the micro-nano optical fiber, wherein the air holes comprise circular air holes and rectangular air holes, wherein a heterojunction is arranged in the rectangular air hole; wherein the heterojunction is arranged at two opposite sides of the rectangular air hole; wherein the heterojunction is composed of a boron nitride film and a graphene film, wherein the graphene film and the boron nitride film are arranged at intervals; wherein the thickness of the graphene film and the boron nitride film varies from 1 to 2000 layers; wherein the purpose of the invention is to provide a modulatable optical fiber polarizer based on the electro-optic modulatable property of graphene, wherein different voltages are applied to the heterojunction in the air hole of the graphene micro-nano optical fiber polarizer, thereby adjusting the polarization absorption effect.

Description

Technical Field

[0001] The invention belongs to the field of optoelectronic technology and relates to a modulatable optical fiber polarizer based on graphene micro-nano optical fiber. Background Art

[0002] Polarizers can convert incident light of any polarization state into outgoing light of a specific polarization, thereby controlling the polarization state of the incident light. Polarizers can be widely used in sensing, optical communications and other fields. In particular, in the field of optical communications, new types of optical communication technologies such as quantum communications require the use of the polarization state of light to generate and transmit information. Traditional polarizers, such as polarizers made of materials with anisotropic absorption or prism-type polarizers based on birefringence, are not conducive to the integration of optical systems. Fiber polarizers made of micro-nano optical fibers can be well integrated with optical light path systems.

[0003] Polarization-maintaining fiber is a fiber that has a strong ability to maintain the polarization state of linearly polarized incident light by introducing the birefringence effect caused by geometric factors and stress factors into traditional single-mode optical fiber. Currently, common types include elliptical, panda, bow-tie, and photonic crystal fiber types. The large birefringence coefficient of polarization-maintaining fiber leads to a large difference in the propagation constants of the two orthogonal polarization fiber core modes of the incident light, thus avoiding interference of the external environment (such as bending, pressure, or distortion) on the polarization state of the linearly polarized incident light.

[0004] Graphene is a carbon atom with sp 2 A two-dimensional atomic crystal that is hybridized and arranged in a symmetrical periodic structure has excellent optical, electrical and thermal properties; for example, graphene has bipolar transport properties and its carrier mobility is as high as 10 6 cm 2 v -1 s -1 , much higher than traditional semiconductor materials such as silicon; in terms of optical properties, its unique linear band structure brings many excellent properties. The light absorption rate of graphene in the visible light and near-infrared bands is 2.3% / layer, which makes it have great development prospects in the field of optical communications; in 2011, a research team proposed a graphene-based fiber polarizer: graphene is placed on the surface of a side-sectioned optical fiber. When a beam of circularly polarized light is incident, due to the interaction between graphene and light, the polarized light component perpendicular to the graphene surface is greatly lost, while the polarized light component parallel to the graphene surface passes through with almost no loss, and finally a beam of polarized light with a polarization direction parallel to the graphene surface is output, which shows that graphene has polarization-selective absorption; this is the first reported fiber polarization device made of graphene combined with optical fiber, which provides a basis for combining graphene with micro-nano optical fibers to make new fiber polarization devices.

[0005] In particular, when voltage is applied to graphene to change its Fermi level, the absorption of light by graphene can be modulated. When sufficient voltage is applied to make the Fermi level of graphene rise or fall relative to the Dirac point to a position half the energy of the incident photon, the incident photon cannot excite the generation of electron-hole pairs due to the Pauli blocking effect, resulting in saturable absorption, and the incident light can pass through normally. On the contrary, when the Fermi level position moves and does not meet the above conditions, the graphene electrons absorb the energy of the incident photon and produce a transition, and the incident light cannot pass through normally. This is called the electro-optical modulatable property of graphene.

[0006] Micro-nano optical fiber is a type of optical fiber with a microstructure, including some air holes arranged along the length direction of the optical fiber; the micro-nano optical fiber involved in the present invention introduces low-refractive-index air holes around a solid silica core, so that light can be mainly confined to propagate in the core; the micro-nano optical fiber does not have strict requirements on the arrangement of air holes, so it has a large design space, and can achieve excellent functions such as high birefringence, non-cutoff single mode, large mode field area and adjustable dispersion by adjusting the shape and size of the air holes; therefore, changing the shape of the air holes of the micro-nano optical fiber in a certain direction, such as changing the original circular air holes to a rectangular shape, will cause the micro-nano optical fiber to have asymmetric structure in the transverse and longitudinal directions, thereby improving its birefringence effect; at the same time, if graphene is arranged in the air holes with changed shape, since graphene does not change the original transmission mode in the micro-nano optical fiber and has polarization absorption asymmetry, a high-efficiency polarizer with high birefringence, high extinction ratio, no damage to the optical fiber structure, no damage to the original transmission mode of light, wide working band (1260nm~2000nm), and easy integration can be achieved;

[0007] Furthermore, boron nitride is introduced while setting graphene to form a heterojunction structure of graphene / boron nitride / graphene, and the polarization absorption effect of the graphene micro-nano optical fiber polarizer is modulated in combination with the aforementioned electro-optical modulatable properties of graphene; this heterojunction structure is similar to a capacitor, and by applying a driving voltage, the Fermi level of the graphene in the heterojunction can be raised or lowered relative to the Dirac point to a position half the energy of the incident photon, so that the polarization absorption of the graphene micro-nano optical fiber polarizer is weakened to almost zero, and becomes a polarization-maintaining fiber; and the thickness of a single layer of graphene and a single layer of boron nitride are both below 1 nm, and introducing a heterojunction structure composed of multi-layer graphene and multi-layer boron nitride into a micro-nano optical fiber will not change the original transmission mode in the optical fiber. Summary of the invention

[0008] (I) Technical problems to be solved by the present invention:

[0009] The purpose of the present invention is to provide a modulated optical fiber polarizer based on the electro-optical modulatable property of graphene, which can adjust the polarization absorption effect by applying different voltages to the heterojunction in the air hole of the graphene micro-nano optical fiber polarizer; the present invention is convenient to couple with the optical fiber optical path system, has a wide working frequency band, a high extinction ratio, and can be widely used in the fields of optical fiber communication and optical fiber sensing.

[0010] (II) In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions:

[0011] A modulatable optical fiber polarizer based on graphene micro-nano optical fiber comprises a micro-nano optical fiber, wherein a solid silica core is arranged in the micro-nano optical fiber, a plurality of air holes are opened in the micro-nano optical fiber, wherein the air holes comprise circular air holes and rectangular air holes, and a heterojunction is arranged in the rectangular air hole.

[0012] Preferably, the heterojunction is arranged at two opposite sides of the rectangular air hole.

[0013] Preferably, the heterojunction is composed of a boron nitride film and a graphene film, and the graphene film and the boron nitride film are arranged at an interval.

[0014] Preferably, the thickness of the graphene film and the boron nitride film ranges from 1 to 2000 layers.

[0015] Preferably, electrodes and leads are respectively provided on the graphene film, and the electrodes and leads are respectively located on two opposite sides of the rectangular air hole.

[0016] Preferably, the air holes are arranged around the silica fiber core and are distributed periodically to form an equivalent optical fiber cladding.

[0017] Preferably, the rectangular air holes are symmetrically arranged at lateral positions on both sides of the silica fiber core.

[0018] (III) The beneficial effects of the present invention include the following four points:

[0019] (1) Based on the design of the micro-nano optical fiber, the present invention changes the two transverse air holes near the core into rectangular air holes, so that the micro-nano optical fiber has an asymmetric structure in the transverse and longitudinal directions, thereby improving its birefringence effect.

[0020] (2) The present invention arranges heterojunctions consisting of continuous single-layer or multi-layer graphene films and boron nitride films on the upper and lower opposite sides of the rectangular air hole of the micro-nano optical fiber. This heterojunction structure is similar to a capacitor. The polarization absorption effect of the graphene micro-nano optical fiber polarizer can be modulated by applying a driving voltage.

[0021] (3) The present invention can modulate the polarization absorption effect of the optical fiber polarizer by changing the magnitude of the driving voltage: when no driving voltage is applied or the applied driving voltage cannot make the graphene Fermi level rise or fall to a position half of the incident photon energy relative to the Dirac point, the polarization absorption effect is "on"; on the contrary, when the applied driving voltage can make the graphene Fermi level rise or fall to a position half of the incident photon energy relative to the Dirac point, the polarization absorption effect is "off", and the optical fiber polarizer works as a polarization-maintaining fiber; by modulating the polarization absorption effect, the optical fiber polarizer can be switched between the above two working modes.

[0022] (4) The effect of the optical fiber polarizer in the present invention when the polarization absorption effect is "on" is as follows: light is transmitted in the optical fiber polarizer, and graphene interacts with the evanescent wave entering the rectangular air hole and absorbs linearly polarized light with a polarization direction parallel to its plane surface, so that the transverse polarization direction component of the transmitted light is lost to zero, while the longitudinal polarization direction component is almost unaffected and passes normally, thereby achieving polarization filtering. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of the structure of a modulatable optical fiber polarizer based on graphene micro-nano optical fiber proposed by the present invention;

[0024] Figure 2 This is a working schematic diagram of a modulatable optical fiber polarizer based on graphene micro-nano optical fiber proposed in the present invention. DETAILED DESCRIPTION

[0025] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0026] Embodiment 1:

[0027] See also Figure 1 , a modulatable optical fiber polarizer based on graphene micro-nano optical fiber, comprising a micro-nano optical fiber, wherein a solid silica core is arranged in the micro-nano optical fiber, wherein a plurality of air holes are opened in the micro-nano optical fiber, wherein the air holes include circular air holes and rectangular air holes, wherein a heterojunction is arranged in the rectangular air hole;

[0028] The heterojunction is arranged at two opposite sides of the rectangular air hole;

[0029] The heterojunction is composed of a boron nitride film and a graphene film, and the graphene film and the boron nitride film are arranged at an interval;

[0030] The thickness of the graphene film and the boron nitride film ranges from 1 to 2000 layers;

[0031] Electrodes and leads are respectively arranged on the graphene film, and the electrodes and leads are respectively located on two opposite sides of the rectangular air hole;

[0032] The air holes are arranged around the silica fiber core and are distributed periodically to form an equivalent optical fiber cladding;

[0033] The rectangular air holes are symmetrically arranged at lateral positions on both sides of the silica fiber core;

[0034] In the present invention, the cladding refractive index of the micro-nano optical fiber is reduced by setting periodically distributed air holes, and the transmitted light is confined in the core with a larger refractive index. The special thing is that: due to the existence of evanescent waves at the interface, part of the transmitted light in the core will enter the vicinity of the surface of the surrounding air holes; in the visible light and near-infrared bands, the absorption loss of graphene for linear polarized light with a polarization direction parallel to its surface plane is much greater than that of linear polarized light with other polarization directions; the transverse air holes around the core of the micro-nano optical fiber are rectangular in shape, and a graphene / boron nitride / graphene heterojunction is introduced at the transverse edge to ensure that the graphene has little absorption loss for linear polarized light with a polarization direction parallel to its surface plane;

[0035] By setting the lateral air holes around the core of the micro-nano optical fiber to rectangular air holes and introducing graphene / boron nitride / graphene heterojunctions in the rectangular air holes, this heterojunction structure is similar to a capacitor. By applying a driving voltage, the polarization absorption effect of the graphene micro-nano optical fiber polarizer can be modulated. The specific modulation effects are as follows:

[0036] ① When no driving voltage is applied or the applied driving voltage cannot make the graphene Fermi level rise or fall to half the energy of the incident photon relative to the Dirac point, that is, the polarization absorption effect is in the "on" state, graphene interacts with the evanescent wave, causing absorption loss to the light transmitted in the micro-nano optical fiber, and graphene has polarization selectivity for light absorption; the effect under the above state is: light is transmitted in the optical fiber polarizer, graphene interacts with the evanescent wave entering the rectangular air hole, and absorbs linearly polarized light with a polarization direction parallel to its plane surface, so that the transverse polarization direction component of the transmitted light is lost to zero, while the longitudinal polarization direction component is almost unaffected and passes normally, thereby achieving polarization filtering;

[0037] ②On the contrary, when the applied driving voltage can make the graphene Fermi level rise or fall relative to the Dirac point to half the energy of the incident photon, that is, the polarization absorption of graphene is in a "closed" state, the absorption loss of graphene for light is reduced to almost zero, and light can pass normally. At this time, the micro-nano optical fiber can be used as a polarization-maintaining optical fiber due to its high birefringence.

[0038] Embodiment 2:

[0039] See also Figure 2 , which is different from Example 1, the specific working process of the modulatable optical fiber polarizer based on graphene micro-nano optical fiber described in the present invention is as follows:

[0040] When a beam of incident light with circular polarization enters the modulatable optical fiber polarizer based on graphene micro-nano optical fiber described in the present invention, there are the following two propagation modes:

[0041] When no driving voltage is applied or the driving voltage applied cannot make the graphene Fermi level rise or fall to half the energy of the incident photon relative to the Dirac point, that is, the polarization absorption effect is in the "on" state, due to the asymmetry of the polarization absorption and the high extinction ratio of graphene, a beam of linearly polarized outgoing light will be obtained;

[0042] When the applied driving voltage can make the graphene Fermi level rise or fall to half the energy of the incident photon relative to the Dirac point, that is, the polarization absorption of graphene is in a "closed" state, the absorption loss of graphene to light is almost zero. Moreover, due to the high birefringence of the micro-nano optical fiber with improved air hole shape, the circular polarization state of the incident light is decoupled, that is, the propagation constants of the components in the two orthogonal polarization directions differ greatly, resulting in different propagation speeds.

[0043] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and improved concepts of the present invention within the technical scope disclosed by the present invention, and they should be covered by the protection scope of the present invention.

Claims

1. A modulatable optical fiber polarizer based on graphene micro-nano optical fiber, comprising micro-nano optical fiber, characterized in that: A solid silica core is arranged in the micro-nano optical fiber, and a plurality of air holes are opened in the micro-nano optical fiber, wherein the air holes include circular air holes and rectangular air holes, and a heterojunction is arranged in the rectangular air holes; the air holes are arranged around the silica core and are periodically distributed to form an equivalent optical fiber cladding; the rectangular air holes are symmetrically arranged at lateral positions on both sides of the silica core; The heterojunction is arranged at two opposite sides of the rectangular air hole; the heterojunction is composed of a boron nitride film and a graphene film, and the graphene film and the boron nitride film are arranged at intervals; electrodes and leads are respectively arranged on the graphene film, and the electrodes and leads are respectively located on two opposite sides of the rectangular air hole.

2. The modulatable optical fiber polarizer based on graphene micro-nano optical fiber according to claim 1, characterized in that: The thickness of the graphene film and the boron nitride film is 1-2000 layers.

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