A beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution

Abstract

The present invention discloses a beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution. Through the angular quadratic phase distribution of the phase element, different orbital angular momentum modes contained in the incident beam are separated to different spatial positions, and the range of the orbital angular momentum modes that can be split is greatly increased on the basis of ensuring a certain beam splitting interval. The present invention is based on a single phase element, has a compact structure, is simple to operate, can be applied to a variety of orbital angular momentum beams, and has high practicality in practical applications.

Description

Technical Field

[0001] The present invention relates to the field of optoelectronic technology, and in particular to a beam orbit angular momentum mode beam splitter based on angular quadratic phase distribution Background Art

[0002] A vortex beam is a structured beam that carries orbital angular momentum (OAM), also known as an OAM beam. Its complex amplitude expression contains a spiral phase term in the form of exp(ilθ), where l is the angular quantum number, also known as the topological charge, which can be any integer and is the eigenvalue of OAM; θ is the angular coordinate. Since different OAM modes are mutually orthogonal to form an infinite-dimensional Hilbert space, OAM beams have great application potential in ultra-large-capacity optical communications, optical particle manipulation, holographic encryption, remote sensing detection, and quantum technology. An OAM beam can carry multiple different OAM components at the same time. Accurately separating different OAM modes in a beam of light for OAM mode recognition and measurement is an important basis for the application of OAM beams. For example, when using the rotational Doppler effect of an OAM beam to detect the rotation speed of a rotating body, it is necessary to accurately separate and extract the target OAM mode in the echo signal before calculating the rotation speed; in an OAM multiplexing ultra-large-capacity optical communication system, it is necessary to effectively separate different OAM modes of the beam to achieve signal decoding, etc.

[0003] In recent years, scholars at home and abroad have carried out research on pattern recognition methods for optical beam OAM, and proposed a variety of different technical solutions including interferometry, mode conversion, rotational Doppler, and computational holography. However, although these methods can realize OAM mode detection, the required optical systems are relatively complex; in terms of OAM mode spatial separation technology, the commonly used one is the OAM mode beam splitting technology based on coordinate transformation. The system mainly includes a coordinate transformation grating, a phase compensation grating and two lenses. This means that the spatial separation of different OAM modes using this method requires four phase diffraction optical devices to be modulated in sequence. The cumbersome optical system and limited dynamic range of OAM mode measurement limit its practical application. In recent years, a simpler method has emerged to achieve angular separation of different OAM modes using a single phase diffraction optical device. However, this scheme cannot take into account both the OAM mode resolution and the maximum separable OAM mode range at the same time, and it also has limitations for the spatial beam splitting of the more common Laguerre-Gaussian OAM beam. Summary of the invention

[0004] In view of this, the present invention discloses a beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution, and further discloses a beam orbital angular momentum mode measurement method and a beam orbital angular momentum mode beam splitting system based on the beam orbital angular momentum mode beam splitter.

[0005] The present invention discloses a beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution. Based on the optical symmetry transformation and the focusing principle of the quadratic phase lens, it has an annular angular quadratic phase distribution and combines multiple annular phases into a single phase modulation element, thereby spatially separating different OAM modes of the incident light beam and further realizing OAM mode measurement of the incident light.

[0006] The present invention discloses a method for measuring the orbital angular momentum mode of a light beam based on angular quadratic phase distribution. The method adopts the orbital angular momentum mode beam splitter of the light beam based on angular quadratic phase distribution of the present invention to diffract different OAM modes contained in an incident light beam to different spatial positions on an observation plane. The OAM mode of the incident light beam is calculated by measuring the angular coordinates corresponding to the center position of the diffraction spot on the observation plane.

[0007] The present invention discloses a beam orbit angular momentum mode splitting system based on angular quadratic phase distribution, which is composed of the beam orbit angular momentum splitter, aperture stop, planar array detector and host of the present invention, wherein:

[0008] The light beam orbital angular momentum mode beam splitter of the present invention has an angular quadratic phase distribution, and is used to convert different incident orbital angular momentum modes into spatial modes with different rotation angles, so that the diffracted light intensities corresponding to different modes are located at different spatial positions;

[0009] The aperture stop is placed in the optical path behind the beam orbital angular momentum beam splitter to limit the aperture angle of the outgoing light and reduce the influence of the aberration caused by the excessive incident phase gradient on the light intensity distribution on the observation plane after beam splitting;

[0010] The planar array detector is used to receive and measure the light intensity distribution after beam splitting, and transmit it to the host;

[0011] The host is used to analyze and process the light intensity distribution after beam splitting and output the beam splitting result.

[0012] The present invention has the following beneficial effects:

[0013] (1) The present invention is a beam orbital angular momentum mode splitter based on angular quadratic phase distribution, which combines multiple annular phases into a single phase element, greatly increasing the range of OAM modes that can be split without sacrificing the angular spacing after mode splitting, and has wide practicality.

[0014] (2) The present invention is a beam orbital angular momentum mode splitting system based on angular quadratic phase distribution, which breaks through the requirements of existing mode splitting devices on the light field distribution of the incident light and can be applied to OAM mode splitting of incident light beams with different light field distributions. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The phase distribution of the light beam orbital angular momentum mode beam splitter based on the angular quadratic phase distribution of the present invention;

[0016] Figure 2 It is a beam orbit angular momentum mode beam splitting system device of the present invention with angular quadratic phase distribution, wherein: 1-the beam orbit angular momentum mode beam splitter based on angular quadratic phase distribution of the present invention, 2-aperture stop, 3-array detector, 4-host;

[0017] FIG3(a) is a simulation result of a diffraction spot on an observation plane after mode splitting using the angular quadratic phase distribution beam OAM mode splitting system of the present invention when the incident beam contains three different OAM modes (l=1, l=6, l=16);

[0018] FIG3(b) is an experimental measurement result of the diffraction spot on the observation plane after mode splitting using the angular quadratic phase distribution beam OAM mode splitting system of the present invention when the incident beam contains three different OAM modes (l=1, l=6, l=16);

[0019] FIG4(a) is a simulation result of a diffraction spot on an observation plane after mode splitting using the angular quadratic phase distribution beam OAM mode splitting system of the present invention when the incident beam contains three different OAM modes ( / =-8, l=6, l=16);

[0020] FIG4(b) is an experimental measurement result of the diffraction spot on the observation plane after mode splitting using the angular quadratic phase distribution beam OAM mode splitting system of the present invention when the incident beam contains three different OAM modes (l=-8, l=6, l=16); DETAILED DESCRIPTION

[0021] The present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0022] The present invention is a beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution, which can focus parallel beams with different incident angles at different lateral positions on the focal plane similar to the converging lens with traditional radial quadratic phase distribution. Different OAM modes contained in the incident beam have different angular phase gradients, and can be converged to different angular positions after being modulated by the phase element with angular quadratic phase distribution. The beam OAM mode beam splitter is composed of three concentric annular phase elements with angular quadratic phase distribution, and the phase expression of each phase element is:

[0023] r j-Δr j <r<r j +Δr j

[0024] Where j = 1, 2, 3, ... represents different annular phase elements, θ and r are coordinate variables of the polar coordinate system; r j and Δr j are the center radius and half width of the annular region corresponding to the phase element respectively; α j is the quadratic coefficient of the angular coordinate θ, which is used to control the angular spacing between the diffraction spots corresponding to adjacent OAM modes on the observation plane after beam splitting and the maximum range of separable OAM modes; parameter β j Used to adjust the focusing effect of the diffraction spot in the radial direction; j As a mode shift variable, the range of separable OAM modes can be shifted. Each annular phase element has an independent modulation effect on the incident beam, and its focal length satisfies:

[0025]

[0026] When the OAM mode with an angular quantum number of l is incident, the phase expression of the light field modulated by the phase element is:

[0027]

[0028] For different modes of OAM beam incident, the diffraction spot on the observation plane rotates only in the angular direction, and the rotation angle satisfies

[0029]

[0030] Therefore, different incident OAM modes are separated to different angular positions, while the radial focusing position remains fixed. The beam OAM mode splitter is composed of three concentric annular phase elements arranged in sequence according to the radius. The designed phase distribution is shown in the attached figure. Figure 1 As shown. Each phase element has the same focal length and an independent OAM mode splitting range and radial focusing position, so the OAM modes in different ranges can be separated to different radial positions. Thus, the two-dimensional spatial splitting of the incident OAM mode can be achieved by the OAM mode beam splitter based on angular quadratic phase distribution of the present invention.

[0031] The present invention provides a method for measuring the orbital angular momentum mode of a light beam based on angular quadratic phase distribution. An incident light beam containing different OAM modes is spatially separated by a light beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution. Then, the angular coordinates and radial coordinates of the center position of the diffraction spot after beam splitting are measured. The OAM mode carried by the incident light beam can be calculated, thereby realizing the measurement of the OAM mode of the incident light beam.

[0032] The present invention provides a beam orbital angular momentum mode splitting system based on angular quadratic phase distribution, such as Figure 2 As shown, it is equipped with the beam orbital angular momentum mode beam splitter, aperture stop, planar array detector and host of the present invention, wherein:

[0033] The optical beam orbital angular momentum mode beam splitter of the present invention has an angular quadratic phase distribution, which is used to convert different incident OAM modes into spatial modes with different rotation angles, so that the diffraction light intensities corresponding to different OAM modes are located at different spatial positions; the aperture stop is placed in the optical path behind the optical beam orbital angular momentum mode beam splitter, so as to limit the aperture angle of the outgoing light and reduce the influence of the aberration caused by the excessive incident phase gradient on the intensity distribution of the light spot after splitting; the planar array detector is used to receive and measure the intensity distribution after splitting, and transmit it to the host; the host is used to analyze and process the intensity distribution after splitting, and output the splitting result.

[0034] The following briefly introduces the beam splitting effect of a beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution of the present invention in combination with two embodiments.

[0035] Example 1: Spatial splitting of an incident beam carrying three OAM modes (l=1, l=6, l=16)

[0036] In this embodiment, based on a beam orbital angular momentum mode splitting system based on angular quadratic phase distribution of the present invention, we spatially split the incident light carrying three OAM (l=1, l=6, l=16) mode components, and observe and analyze the diffraction spot after splitting. Figure 3(a) is the diffraction spot after splitting obtained by simulation, and Figure 3(b) is the diffraction spot after splitting obtained by experiment. The experimental results are very close to the simulation results, and the spatial distribution of the focused light intensity is basically the same, which proves that the present invention can effectively realize the spatial splitting of different OAM modes.

[0037] Example 2: Spatial splitting of an incident beam carrying three OAM modes (l = -8, l = 6, l = 16)

[0038] In this embodiment, based on a beam orbital angular momentum mode splitting system based on angular quadratic phase distribution of the present invention, we spatially split the incident light carrying three OAM (l = -8, l = 6, l = 16) mode components, and observe and analyze the diffraction spot after splitting. Figure 4 (a) is the diffraction spot after splitting obtained by simulation, and Figure 4 (b) is the diffraction spot after splitting obtained by experiment. The experimental results are very close to the simulation results, and the spatial distribution of the focused light intensity is basically the same, which proves that the present invention can effectively realize the spatial splitting of different OAM modes.

[0039] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A beam orbital angular momentum mode beam splitter based on angular quadratic phase distribution, characterized in that: (1) The beam orbital angular momentum mode beam splitter is composed of three concentric annular phase elements with angular quadratic phase distribution. The phase expression of each phase element is: Where j = 1, 2, 3, ... represents different annular phase elements, θ and r are polar coordinate system variables, r j and Δr j are the center radius and half width of the annular region corresponding to the phase element, respectively; (2)α j is the quadratic coefficient of the angular coordinate θ, which is used to control the angular spacing between the diffraction spots corresponding to adjacent orbital angular momentum modes on the observation plane after beam splitting and the maximum range of the separable orbital angular momentum modes; (3) Parameter β j Used to adjust the focusing effect of the diffraction spot in the radial direction; (4)l j As a mode shift variable, it can shift the range of separable orbital angular momentum modes; (5) Each phase element has an independent modulation effect on the incident light beam, and its focal length satisfies: (6) For different modes of incident orbital angular momentum beams, the diffraction spot on the observation plane rotates only in the angular direction, and the rotation angle satisfies:

2. A method for measuring the orbital angular momentum pattern of an incident light beam based on the orbital angular momentum pattern beam splitter according to claim 1, wherein an incident light beam containing different orbital angular momentum patterns is spatially separated by a beam orbital angular momentum pattern beam splitter based on angular quadratic phase distribution, and then the angular coordinates and radial coordinates of the center position of the diffraction spot after beam splitting are measured, so that the orbital angular momentum pattern carried by the incident light beam can be calculated, thereby realizing the measurement of the orbital angular momentum pattern of the incident light beam.

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