A chiral fiber grating with a non-circularly symmetric cross-section

Abstract

The present invention discloses a chiral fiber grating with a non-circular symmetric cross-section, which is fabricated by twisting a fiber with a certain birefringence and a non-circular symmetric cross-section having a multiple rotational symmetry structure under completely molten conditions. The cross-section of the grating is non-circular symmetric and has a multiple rotational symmetry. The core rotates around the central axis of the grating, and the period is Λ = 100 μm to 10,000 μm; the refractive index of the core is greater than that of the cladding, and it supports the stable transmission of orbital angular momentum modes with Δn > 1×10<supgt;‑4< / supgt>; the core has a helical periodic spatial geometric structure: the circular core deviates from the central axis of the grating; or the elliptical core is located on the central axis of the grating; or the elliptical core deviates from the central axis of the grating. This chiral fiber grating solves the problems of filtering and demultiplexing of orbital angular momentum in orbital angular momentum fiber communication, and has the advantages of small volume, flexible structure, low loss, and compatibility with existing fiber communication systems.

Description

Technical Field

[0001] The present invention relates to optical fiber technology, and particularly to a chiral fiber grating with a non-circular symmetric cross-section. Background Art

[0002] With the rapid development of modern society, people's requirements for communication rate and quality are getting higher and higher, and the existing traditional single-mode fiber communication has approached the saturation state. To meet the growing demand, many fiber communication technologies have been proposed, such as wavelength division multiplexing, time division multiplexing, mode division multiplexing, and space division multiplexing. However, these technologies are limited by the communication capacity. In recent years, the orbital angular momentum (OAM) multiplexing technology has attracted a great deal of attention and research. The light beam carrying orbital angular momentum can be characterized by its phase factor where is the azimuth angle and l is the topological charge number. Theoretically, the topological charge number of orbital angular momentum can take any integer, and the orbital angular momentum modes with different topological charge numbers are orthogonal to each other. Therefore, the orbital angular momentum multiplexing technology has no limitation on the communication capacity and can meet the growing demand in the future.

[0003] The chiral fiber grating is a new type of optical waveguide structure, which refers to the structure obtained by rotating the optical fiber at a high speed under the melting condition. Different from the traditional fiber grating with refractive index modulation, it has a helical spatial structure and there are periodic geometric modulations in both the longitudinal and angular directions. In recent years, the use of chiral fiber gratings for the efficient generation of orbital angular momentum has attracted a great deal of attention, and more and more scholars have begun to focus on and study the all-fiber long-distance orbital angular momentum communication system. However, at present, the filtering and demultiplexing of orbital angular momentum are mainly based on free-space optical devices, such as spiral phase plates, fork diffraction gratings, and spatial phase modulators. In the fiber-based long-distance orbital angular momentum communication system, the introduction of free-space optical devices will increase the complexity and attenuation of the system and reduce the feasibility of the system. Therefore, there is an urgent need to develop a fiber-based device for efficiently filtering and demultiplexing orbital angular momentum modes. Summary of the Invention

[0004] The object of the present invention is to provide a chiral fiber grating with a non-circular symmetric cross-section, which solves the problems of filtering and demultiplexing of orbital angular momentum in orbital angular momentum fiber communication, and has the advantages of small volume, flexible structure, low loss, and compatibility with the existing fiber communication system.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] A chiral fiber grating with a non-circular symmetric cross-section, which is fabricated by rotating a fiber with a non-circular symmetric cross-section having a certain birefringence and a multiple rotation symmetric structure under fully molten conditions, includes a core and a cladding, wherein:

[0007] The cross-section of the chiral fiber grating is non-circular symmetric and has multiple rotational symmetries, and the order of rotational symmetry can be set as needed;

[0008] The core rotates around the central axis of the grating with a period of Λ = 100 μm to 10,000 μm;

[0009] The refractive index of the core is greater than that of the cladding and supports the stable transmission of orbital angular momentum modes, where the difference in effective refractive indices Δn between orbital angular momentum modes > 1×10 -4 ;

[0010] Δn = |n1 - n2|, where n1 is the effective refractive index of orbital angular momentum mode 1 and n2 is the effective refractive index of orbital angular momentum mode 2;

[0011] The core has a helical periodic spatial geometric structure, and the specific structure is:

[0012] The circular core deviates from the central axis of the grating, and the offset distance d = 0.5 μm to 10 μm;

[0013] Or, the elliptical core is located on the central axis of the grating, and the elliptical eccentricity e = 0.2 to 1;

[0014] Or, the elliptical core deviates from the central axis of the grating, the offset distance d = 0.5 μm to 10 μm, and the elliptical eccentricity e = 0.2 to 1.

[0015] It can be seen from the technical solutions provided by the present invention described above that the above chiral fiber grating solves the problems of filtering and demultiplexing of orbital angular momentum in orbital angular momentum fiber communication, and has the advantages of small volume, flexible structure, low loss, and compatibility with existing fiber communication systems. It can be applied to almost all fiber-based orbital angular momentum applications, such as orbital angular momentum fiber communication, orbital angular momentum fiber sensing, orbital angular momentum fiber imaging, and fiber-based orbital angular momentum manipulation. Description of the Drawings

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0017] Figure 1 The figure shows a schematic structural diagram of the cross-section of the non-circular symmetric eccentric fiber of the embodiment of the present invention;

[0018] Figure 2 It is a schematic structural diagram of the cross-section of the non-circular symmetric elliptical core fiber of the embodiment of the present invention;

[0019] Figure 3 It is a schematic structural diagram of the cross-section of the non-circular symmetric eccentric elliptical fiber of the embodiment of the present invention. Specific embodiments

[0020] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments, which does not constitute a limitation to the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the protection scope of the present invention.

[0021] A chiral fiber grating with a non-circular symmetric cross-section, which is made by rotating a fiber with a non-circular symmetric cross-section having a certain birefringence and a multiple rotation symmetry structure under completely molten conditions, including a core and a cladding, wherein:

[0022] The cross-section of the chiral fiber grating is non-circular symmetric, has multiple rotational symmetries, and the order of rotational symmetry can be set as needed, specifically any positive integer;

[0023] The core rotates around the central axis of the grating, and the period is Λ = 100 μm to 10,000 μm;

[0024] The refractive index of the core is greater than that of the cladding and supports the stable transmission of orbital angular momentum modes, where the difference in effective refractive indices Δn between orbital angular momentum modes > 1×10 -4 ;

[0025] Δn = |n1 - n2|, where n1 is the effective refractive index of orbital angular momentum mode 1 and n2 is the effective refractive index of orbital angular momentum mode 2; for example, orbital angular momentum mode 1 is Orbital angular momentum mode 2 is Δn > 1×10 -4 Can avoid crosstalk between mode 1 and mode 2;

[0026] The core has a helical periodic spatial geometric structure;

[0027] The non-circular symmetric chiral fiber grating can be obtained by rotating the following three types of fibers under completely molten conditions:

[0028] As Figure 1 shown in the structural schematic diagram of the cross-section of the non-circular symmetric eccentric optical fiber according to the embodiment of the present invention, the circular core is offset from the central axis of the optical fiber. In the figure, 1 is the core, 2 is the cladding, and the eccentricity distance d = 0.5 μm to 10 μm; the specific eccentricity distance depends on core parameters such as refractive index and radius. In order to achieve a certain birefringence B > 1×10 -4 , the birefringence of the optical fiber is increased by increasing the eccentricity distance;

[0029] Or, as Figure 2 shown in the structural schematic diagram of the cross-section of the non-circular symmetric elliptical core optical fiber according to the embodiment of the present invention, the elliptical core is located on the central axis of the optical fiber, and the ellipticity e = 0.2 to 1; where the major axis radius a is greater than the minor axis radius b. In order to achieve a certain birefringence B 1×10 -4 , the birefringence inside the optical fiber is increased by increasing the difference between the major axis radius and the minor axis radius;

[0030] Or, as Figure 3 shown in the structural schematic diagram of the cross-section of the non-circular symmetric eccentric elliptical optical fiber according to the embodiment of the present invention, the elliptical core is offset from the central axis of the optical fiber, the eccentricity distance d = 0.5 μm to 10 μm, and the ellipticity e = 0.2 to 1. In order to achieve a certain birefringence B 1×10 -4 , the birefringence inside the optical fiber is increased by increasing the eccentricity distance and increasing the difference between the major axis radius and the minor axis radius of the core;

[0031] Among them, the birefringence B |n x -n y |, where n x is the effective refractive index of the x-polarized mode; n y is the effective refractive index of the y-polarized mode; if the cross-section is circular, n x = n y , so B = 0 and there is no birefringence; if the cross-section is non-circular symmetric, n x ≠ n y , B ≠ 0; and the greater the deviation of the core in the x-direction and y-direction of the cross-section (this deviation includes two aspects, one is that the core itself is elliptical and the other is that the position of the core is offset from the center of the optical fiber), the greater B is. For the three structures in the embodiments of the present invention as described above, the embodiments of the present invention define B > 1×10 -4It is possible to effectively modulate x and y polarized light within a short distance. The larger the value of B, the more significant the modulation effect on polarized light can be considered. However, at the same time, it will also bring higher losses. The magnitude of the birefringence is also affected by the grating manufacturing process. For example, external environmental disturbances will also generate a certain birefringence in the grating. In practical applications, the magnitude of the birefringence is determined according to requirements, and it is not that the larger the better.

[0032] In addition, the heating of the cross-section non-circular symmetric chiral fiber grating can be by CO2 laser heating, or by oxy-hydrogen flame heating, or by discharge heating with a discharge electrode; the condition of complete melting can be judged by the transmission spectrum. When complete melting is achieved, the transmission spectrum of the chiral fiber grating is smooth and the insertion loss is very low; conversely, the transmission spectrum of the chiral fiber grating is not smooth and the insertion loss is very large.

[0033] In specific implementation, the parameter selection of the cross-section non-circular symmetric chiral fiber grating is based on the following phase matching condition:

[0034] β1 - β2 = 4(1 + 1)π / Λ

[0035] Among them, β1 is the effective propagation constant of the selected orbital angular momentum mode to be filtered and demultiplexed; β2 is the propagation constant of the coupled cladding mode that satisfies the matching condition; l is the topological charge number of the selected orbital angular momentum; Λ is the period of the cross-section non-circular symmetric chiral fiber grating. In specific implementation, the birefringence, effective refractive index, and propagation constant of the grating in the phase matching condition can be obtained by modeling and simulation using finite element analysis software such as COMSOL, MODESOLUTION, and mathematical software MATLAB.

[0036] The cross-section non-circular symmetric chiral fiber grating has two chiralities, namely left-handed helix and right-handed helix; cross-section non-circular symmetric chiral fiber gratings with different chiralities can achieve the filtering and demultiplexing of different chiral orbital angular momentum modes. The chirality selection of the above cross-section non-circular symmetric chiral fiber grating is based on:

[0037] If the filtering and demultiplexing are for the orbital angular momentum mode generated by the HE mode, then select a cross-section non-circular symmetric chiral fiber grating with the same helical direction as the orbital angular momentum mode;

[0038] If the filtering and demultiplexing are for the orbital angular momentum mode generated by the EH mode, then select a cross-section non-circular symmetric chiral fiber grating with the opposite helical direction to the orbital angular momentum mode;

[0039] Among them, the HE mode and the EH mode are both hybrid vector modes, where H is the magnetic field and E is the electric field. They can also be called the hybrid vector mode HE and the hybrid vector mode EH, which are common concepts in fiber optics.

[0040] By adjusting the parameters of the fiber core, the period and chirality of the helix, the cross-section non-circular symmetric chiral fiber grating can select the orbital angular momentum modes to be filtered out and demultiplexed, and theoretically a specific grating only acts on a selected orbital angular momentum mode.

[0041] In specific implementation, the orbital angular momentum mode (OAM) is linearly superposed by a pair of mutually orthogonal even and odd modes with a phase difference of π / 2, expressed as follows:

[0042]

[0043]

[0044] Where the '+' and '-' in the upper right corner of OAM represent left-handed and right-handed circular polarizations respectively; the '+' and '-' in the lower right corner of OAM represent the orbital angular momentum of left-handed and right-handed helices respectively; l is the topological charge number of the selected orbital angular momentum; m is the radial order; j is the imaginary unit;

[0045] For the orbital angular momentum composed of HE modes, its helical direction is always the same as the circular polarization direction; for the orbital angular momentum composed of EH modes, its helical direction is always opposite to the circular polarization direction. This phenomenon is classical entanglement, and such orbital angular momentum is spin-entangled orbital angular momentum. The cross-section non-circular symmetric chiral fiber grating described in the embodiments of the present invention is applicable to the filtering and demultiplexing of the orbital angular momentum composed of HE modes or the orbital angular momentum composed of EH modes. Specifically:

[0046] Taking the orbital angular momentum composed of HE modes as an example, the right-handed helical cross-section non-circular symmetric chiral fiber grating can filter out the right-handed helical orbital angular momentum mode that satisfies the phase matching condition; similarly, the left-handed helical cross-section non-circular symmetric chiral fiber grating can filter out the left-handed helical orbital angular momentum mode that satisfies the phase matching condition;

[0047] For the orbital angular momentum composed of EH modes, since its helical direction is always opposite to the circular polarization direction, the right-handed helical cross-section non-circular symmetric chiral fiber grating can filter out the left-handed helical orbital angular momentum mode that satisfies the phase matching condition, and the left-handed helical cross-section non-circular symmetric chiral fiber grating can filter out the right-handed helical orbital angular momentum mode that satisfies the phase matching condition.

[0048] In practical applications, one of the spin-entangled orbital angular momenta can be used as an information carrier.

[0049] In addition, in specific implementation, by cascading multiple cross-section non-circular symmetric chiral fiber gratings, simultaneous filtering and demultiplexing of multiple orbital angular momenta can be achieved on a single fiber.

[0050] It should be noted that the content not described in detail in the embodiments of the present invention belongs to the prior art well-known to those skilled in the art.

[0051] In summary, compared with the existing orbital angular momentum filters based on free-space devices, the cross-section non-circular symmetric chiral fiber grating described in the embodiments of the present invention has the advantages of small volume, flexible structure, low loss, and compatibility with existing fiber optic communication systems, and can be applied to almost all fiber-based orbital angular momentum applications, such as orbital angular momentum fiber optic communication, orbital angular momentum fiber optic sensing, orbital angular momentum fiber optic imaging, and fiber-based orbital angular momentum manipulation.

[0052] At the same time, the present invention has a helical spatial characteristic and a periodically modulated grating characteristic, can realize the filtering and demultiplexing of arbitrarily spin-entangled orbital angular momentum, and by selecting appropriate grating parameters and cascading multiple gratings, can realize the simultaneous filtering and demultiplexing of multiple orbital angular momenta on a single fiber.

[0053] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims. The information disclosed in the background art part of this article is only intended to deepen the understanding of the overall background art of the present invention, and should not be regarded as an admission or any form of implication that this information constitutes the prior art well-known to those skilled in the art.

Claims

1. A chiral fiber grating with a non-circular symmetric cross-section, characterized in that, The chiral fiber grating is fabricated by rotating a fiber with a non-circular cross-section having a certain birefringence and a multiple rotation symmetry structure under completely molten conditions, and includes a core and a cladding, wherein: The order of rotational symmetry of the chiral fiber grating is set as needed; The core rotates around the central axis of the grating, with a period of Λ = 100 μm to 10000 μm; The refractive index of the core is greater than that of the cladding and supports stable transmission of orbital angular momentum modes, where the difference in effective refractive index Δn between orbital angular momentum modes is > 1×10 -4 ; Δn = |n1 - n2|, where n1 is the effective refractive index of the orbital angular momentum mode 1 and n2 is the effective refractive index of the orbital angular momentum mode 2; The core has a helical periodic spatial geometric structure, and the specific structure is: The circular core deviates from the central axis of the grating, and the off-core distance d = 0.5 μm to 10 μm; Alternatively, the elliptical core is located on the central axis of the grating, and the elliptical eccentricity e = 0.2 to 1; Alternatively, the elliptical core deviates from the central axis of the grating, the off-core distance d = 0.5 μm to 10 μm, and the elliptical eccentricity e = 0.2 to 1.

2. The chiral fiber grating with a non-circular cross-section according to claim 1, wherein If the core is a structure in which the circular core deviates from the central axis of the grating, the birefringence of the fiber is increased by increasing the off-core distance; If the core is a structure in which the elliptical core is located on the central axis of the grating, the birefringence inside the fiber is increased by increasing the difference between the major axis radius and the minor axis radius; If the core is a structure in which the elliptical core deviates from the central axis of the grating, the birefringence inside the fiber is increased by increasing the off-core distance and increasing the difference between the major axis radius and the minor axis radius of the core.

3. The chiral fiber grating with a non-circular cross-section according to claim 1, wherein The heating of the chiral fiber grating with a non-circular cross-section can be carried out by heating with a CO2 laser, or by heating with a hydrogen-oxygen flame, or by heating by discharging electrodes; [[ID=X]]The condition of complete melting is judged by the transmission spectrum. When complete melting is achieved, the transmission spectrum of the chiral fiber grating is smooth and the insertion loss is very low; conversely, the transmission spectrum of the chiral fiber grating is not smooth and the insertion loss is very large.

4. The chiral fiber grating with a non-circular symmetric cross-section according to claim 1, wherein, The parameter selection of the chiral fiber grating is based on the following phase matching condition: β1 - β2 = 4(l + 1)π / Λ Wherein, β1 is the effective propagation constant of the selected orbital angular momentum mode to be filtered and demultiplexed; β2 is the propagation constant of the coupled cladding mode that satisfies the matching condition; l is the topological charge number of the selected orbital angular momentum; Λ is the period of the chiral fiber grating with a non-circular cross-section.

5. The chiral fiber grating with a non-circular symmetric cross-section according to claim 1, characterized in that, The chiral fiber grating has two chiralities, namely left-handed helix and right-handed helix; chiral fiber gratings with different chiralities can achieve the filtering of different chiral orbital angular momentum modes.

6. The chiral fiber grating with a non-circular symmetric cross-section according to claim 5, wherein, The chirality selection of the chiral fiber grating is based on: If the filtering and demultiplexing are orbital angular momentum modes composed of HE modes, a chiral fiber grating with a non-circular cross-section having the same helical direction as the orbital angular momentum mode is selected; If the filtering and demultiplexing are orbital angular momentum modes composed of EH modes, a chiral fiber grating with a non-circular cross-section having the opposite helical direction to the orbital angular momentum mode is selected; Among them, both the HE mode and the EH mode are hybrid vector modes, where H is the magnetic field and E is the electric field; By adjusting the parameters of the fiber core, the period and chirality of the helix, the cross-section non-circular symmetric chiral fiber grating can select the orbital angular momentum modes to be filtered out and demultiplexed, and theoretically a specific grating only acts on a selected orbital angular momentum mode.

7. The cross-section non-circular symmetric chiral fiber grating according to claim 1, wherein By cascading multiple cross-section non-circular symmetric chiral fiber gratings, simultaneous filtering out and demultiplexing of multiple orbital angular momenta can be achieved on a single optical fiber.

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