Vortex beam phase dynamics process measuring device based on hollow-core optical fiber

Abstract

The invention discloses a vortex beam phase dynamics process measuring device based on a hollow-core optical fiber, which ensures that the spiral phase wavefront of a vortex beam is kept highly complete in the transmission process by utilizing the excellent mode retention capability of the hollow-core optical fiber, and introduces a fundamental mode reference beam to generate coaxial interference with a vortex beam to be measured, so as to measure the vortex beam phase dynamics process. The forming process of an interference pattern is recorded through a high-frame-rate camera, data processing is carried out on recorded interference data, phase information of a vortex light beam to be measured is obtained through inversion, and therefore measurement of the phase field dynamics process of the vortex light field is achieved. According to the method, the detection of the topological charge of the vortex beam and the measurement of the phase dynamics process of the vortex beam can be simply and efficiently realized, an idea is provided for solving the problems of vortex beam detection and limited channel capacity of an optical communication system, and a key technical foundation is laid for engineering implementation of a new generation of high-dimensional optical communication systems.

Description

Technical Field

[0001] The present invention relates to the field of vortex beams, and particularly to a measuring device for the phase dynamics process of vortex beams based on a hollow-core fiber. Background Art

[0002] With the rapid development of optical communication, vortex beams have played a huge role in the space-division multiplexing communication network. It is a special optical field with a helical phase wavefront and a phase singularity. Each photon in the beam carries an orbital angular momentum of lh, where l is the topological charge number or the order of the orbital angular momentum, and h is the Planck constant. A vector vortex beam is a new type of beam that simultaneously has a non-uniform polarization state and a helical phase structure within the beam cross-section. It has different polarization states at different positions on the same wavefront and exhibits many novel characteristics in the interaction with matter, which enables it to be widely applied in fields such as particle manipulation, microscopy imaging, material processing, and communication. Compared with traditional optical fibers, due to its unique structure and low-refractive-index cladding, a hollow-core fiber has ultra-low transmission loss and excellent mode-holding ability, can effectively suppress the nonlinear effects and mode coupling problems in traditional optical fibers, and ensure the integrity of the vortex beam during transmission.

[0003] In recent years, many new methods have been proposed for the detection of vortex beams. Methods such as using the relationship between the phase distribution characteristics after the transmission of vortex light and the topological charge number, elliptical apertures, phase-shift methods, Dammann gratings, and novel Dammann gratings can all detect vortex beams. However, the experimental devices used in these methods have complex structures, low detection sensitivity for high-order vortex beam modes, resulting in limited channel capacity of the optical communication system, and are susceptible to environmental influences. For example, turbulence, temperature changes, or mechanical vibrations will introduce phase distortions, thereby reducing the detection accuracy. In addition, existing methods can only detect the topological charge number of vortex beams, and there is no complete set of detection methods for their phase dynamics process, resulting in the research on the phase dynamics of vortex beams remaining only at the theoretical stage.

[0004] To further deepen the research on vortex beams, it is necessary to study their phase dynamics process. For this purpose, a system and method that can solve the above problems are required. Summary of the Invention

[0005] Based on the deficiencies of the above-mentioned existing technologies, the present invention provides a measuring device for the phase dynamics process of vortex beams based on a hollow-core fiber, in order to simply and efficiently achieve the detection of the topological charge number of vortex beams and the measurement of the phase dynamics process of vortex beams, thereby providing an idea for solving the problems of vortex beam detection and limited channel capacity of the optical communication system, and laying a key technical foundation for the engineering implementation of a new generation of high-dimensional optical communication systems.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A measurement device for the phase dynamics process of a vortex beam based on a hollow-core fiber according to the present invention is characterized by comprising: a laser light source, a half-wave plate, a polarization beam splitter, a phase modulator, an expansion lens group, a mirror, a spatial modulator, a coupling lens, a hollow-core fiber, a high-frame-rate camera, and a computer;

[0008] The laser light source emits a fundamental mode beam, which is decomposed into two orthogonally polarized beams, denoted as S light and P light, after passing through the half-wave plate and the polarization beam splitter in sequence. By adjusting the polarization direction angle between the half-wave plate and the polarization beam splitter, the intensity ratio between the S light and the P light can be adjusted;

[0009] After the phase modulator applies an additional phase to the S light to become a fundamental mode reference beam, the fundamental mode reference beam is expanded by the expansion lens group and then reflected by the mirror to the high-frame-rate camera;

[0010] The spatial modulator modulates the P light to generate a vortex beam, which enters the coupling lens and is coupled into the hollow-core fiber for propagation. The vortex beam after passing through the hollow-core fiber interferes coaxially with the reflected beam at the high-frame-rate camera. After the high-frame-rate camera records the formation process of the interference pattern of the vortex beam frame by frame, it is sent to the computer, and then the computer demodulates each frame pattern to invert the phase dynamics process of the vortex beam.

[0011] The measurement device for the phase dynamics process of a vortex beam according to the present invention is also characterized in that the computer inversely calculates the phase dynamics process of the vortex beam according to the following steps:

[0012] Step 1. Obtain the number l of the orbital angular momentum of the vortex beam by using Equation (1):

[0013] (1)

[0014] In Equation (1), and are the electric field expressions of the vortex beam and the fundamental mode reference beam respectively, represents the interference fringe intensity of an interference pattern of one frame, represents the angle between the vortex beam and the polar axis in the polar coordinate system, is the wave number of light, is the wavelength, and θ is the angle between the fundamental mode reference beam and the optical axis in the xOz plane in the rectangular coordinate system;

[0015] Step 2. When the vortex beam and the fundamental mode reference beam undergo coaxial interference, according to the additional phase introduced by the fundamental mode reference beam, the phase relationship between the vortex beam and the fundamental mode reference beam is obtained using Equation (2) for obtaining a frame of phase pattern of the vortex beam:

[0016] (2)

[0017] In Equation (2), and are the phase of the vortex beam and the phase of the fundamental mode reference beam, respectively; represents the imaginary unit;

[0018] Step 3. After sequentially splicing the frame phase patterns and quantities frame by frame according to the recorded order, the phase dynamics process of the vortex beam is obtained.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] 1. The present invention integrates the detection of the topological charge number of the vortex beam and the research on the phase dynamics in one experimental device, realizing the structural compactness of the detection device.

[0021] 2. By combining with a hollow-core fiber, the present invention enables the vortex beam to maintain its original characteristics to the greatest extent during propagation, overcomes the interference of the external environment on the vortex beam, and makes the detection result more accurate.

[0022] 3. By combining with a computer deep learning algorithm, the present invention overcomes the problems of high system complexity, long recognition time, and limited recognition mode order in the traditional method, improves the pattern recognition efficiency, and simplifies the system device.

[0023] 4. By studying the phase dynamics of the vortex beam, the present invention understands the establishment process of the vortex beam at a deeper level, laying a key technical foundation for solving the problem of limited channel capacity in optical communication systems. Brief Description of the Drawings

[0024] Figure 1 is a schematic structural diagram of the present invention based on the measurement of the phase dynamics process of the vortex beam;

[0025] Figure 2 is a schematic step diagram of the present invention for measuring the phase dynamics process of the vortex beam;

[0026] Figure 3 is a schematic diagram for judging the number of orbital angular momenta l of the vortex beam provided by an embodiment of the present invention.

[0027] Reference numerals in the figure: 1001 - laser light source, 1002 - half - wave plate, 1003 - polarization beam splitter, 1004 - phase modulator, 1005 - beam expander lens group, 1006 - mirror, 1007 - spatial modulator, 1008 - coupling lens, 1009 - hollow - core fiber, 1010 - high - frame - rate camera, 1011 - computer. Specific implementation mode

[0028] To make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further elaborates on the present invention in detail with reference to specific embodiments and the accompanying drawings.

[0029] In this embodiment, a measurement device for the phase dynamics process of vortex beams based on a hollow - core fiber, as Figure 1 shown, includes: a laser light source 1001, a half - wave plate 1002, a polarization beam splitter 1003, a phase modulator 1004, a beam expander lens group 1005, a mirror 1006, a spatial modulator 1007, a coupling lens 1008, a hollow - core fiber 1009, a high - frame - rate camera 1010, and a computer 1011;

[0030] The laser light source 1001 is used to generate a Gaussian fundamental - mode beam. After passing through the half - wave plate 1002 and the polarization beam splitter 1003 in sequence, it is decomposed into two orthogonally polarized beams, the S - polarized light and the P - polarized light; and by adjusting the polarization - direction angle between the half - wave plate 1002 and the polarization beam splitter 1003, the intensity ratio between the S - polarized light and the P - polarized light can be adjusted.

[0031] The S - polarized light enters the phase modulator 1004 to apply an additional phase and becomes a fundamental - mode reference beam. Applying an additional phase to it can ensure that the interference pattern with the vortex beam rotates, enhancing the pattern recognition. Thereafter, the fundamental - mode reference beam is expanded by the beam expander lens group 1005. At this time, the phase of the fundamental - mode reference beam can be approximately regarded as a plane, and then it is reflected by the mirror 1006 to the high - frame - rate camera 1010;

[0032] After the spatial modulator 1007 modulates the P - polarized light, a vortex beam is generated and enters the coupling lens 1008 to be coupled into the hollow - core fiber 1009 for propagation. Since the hollow - core fiber can effectively suppress the nonlinear effects and mode - coupling problems in traditional fibers and can well prevent the influence of changes in the external environment on the vortex beam, it can ensure that the helical phase front of the vortex beam remains highly intact during transmission. The vortex beam after passing through the hollow - core fiber 1009 interferes coaxially with the reflected beam at the high - frame - rate camera 1010. After the high - frame - rate camera 1010 records the formation process of the interference pattern of the vortex beam frame by frame, it is sent to the computer 1011, and then the computer 1011 demodulates each frame pattern to invert the phase dynamics process of the vortex beam.

[0033] In this embodiment, asFigure 2 As shown, a device for measuring the phase dynamics process of a vortex beam based on a hollow-core fiber. The computer 1011 inversely calculates the phase dynamics process of the vortex beam according to the following steps:

[0034] Step 1. Use Equation (1) to obtain the value l of the orbital angular momentum of the vortex beam:

[0035] (1)

[0036] In Equation (1), and are the electric field expressions of the vortex beam and the fundamental mode reference beam respectively, represents the interference fringe intensity of a frame of interference pattern, represents the angle between the vortex beam and the polar axis in the polar coordinate system, is the wave number of light, is the wavelength, and θ is the angle between the fundamental mode reference beam and the optical axis in the xOz plane in the rectangular coordinate system;

[0037] Using the sum-to-product formula and letting , Equation (1) can be simplified to:

[0038] (2)

[0039] Figure 3 gives the light intensity distribution of the interference field of the value l of the orbital angular momentum of the vortex beam calculated according to Equation (2). The number of crosshairs at the center of its interference pattern is the same as the absolute value of the order l of the vortex beam, and the opening direction of the crosshairs is determined by the sign of l.

[0040] Step 2. When the vortex beam and the fundamental mode reference beam undergo coaxial interference, according to the additional phase introduced by the fundamental mode reference beam, use Equation (2) to obtain the phase relationship between the vortex beam and the fundamental mode reference beam, for obtaining a frame of phase pattern of the vortex beam:

[0041] (3)

[0042] In Equation (3), and are the phase of the vortex beam and the phase of the fundamental mode reference beam respectively; represents the imaginary unit;

[0043] Step 3. Coaxially interfere the fundamental mode reference beam with the vortex beams at different orbital angular momentum values. Run a demodulation program on a computer to demodulate the interference pattern, invert the phase patterns corresponding to the vortex beams at different orbital angular momentum values, and compare them with the patterns in the database. After splicing the frame phase patterns and quantities frame by frame in the order recorded in each frame, obtain the phase dynamics process of the vortex beam.

Claims

1. A vortex beam phase dynamics process measurement device based on hollow core optical fiber, characterized in that: include: Laser light source (1001), half-wave plate (1002), polarization beam splitter (1003), phase modulator (1004), beam expansion lens group (1005), reflector (1006), spatial modulator (1007), coupling lens (1008), hollow core optical fiber (1009), high frame rate camera (1010) and computer (1011); The laser light source (1001) emits a fundamental mode light beam, and after passing through the half-wave plate (1002) and the polarization beam splitter (1003) in sequence, the light beam is decomposed into two orthogonal polarized light beams, which are recorded as S light and P light. The intensity ratio between the S light and the P light is adjusted by adjusting the angle of the polarization direction between the half-wave plate (1002) and the polarization beam splitter (1003); After the phase modulator (1004) applies an additional phase to the S light, the S light becomes a fundamental mode reference beam, and after the fundamental mode reference beam is expanded by the beam expansion lens group (1005), it is reflected by the reflector (1006) to the high frame rate camera (1010); After the spatial modulator (1007) modulates the P light, a vortex light beam is generated, and the vortex light beam enters the coupling lens (1008) and is coupled to the hollow-core optical fiber (1009) for propagation. The vortex light beam after passing through the hollow-core optical fiber (1009) performs coaxial interference with the reflected light beam at the high-frame rate camera (1010), and the high-frame rate camera (1010) records the formation process of the interference pattern of the vortex light beam frame by frame and sends it to the computer (1011), so that the computer (1011) demodulates each frame pattern to invert the phase dynamic process of the vortex light beam.

2. A vortex beam phase dynamics process measurement device according to claim 1, characterized in that: The computer (1011) inverts the phase dynamics process of the vortex beam according to the following steps: Step 1. Use equation (1) to obtain the orbital angular momentum l of the vortex beam: (1) In formula (1), and are the electric field expressions of the vortex beam and the fundamental mode reference beam, respectively. represents the interference fringe intensity of a frame of interference pattern, represents the angle between the vortex beam and the polar axis in the polar coordinate system, is the light wave number, is the wavelength, θ is the angle between the fundamental mode reference beam and the optical axis in the xOz plane in the rectangular coordinate system; Step 2. When the vortex beam and the fundamental mode reference beam coaxially interfere, the phase relationship between the vortex beam and the fundamental mode reference beam is obtained using equation (2) according to the additional phase introduced by the fundamental mode reference beam, which is used to obtain a frame phase pattern of the vortex beam: (2) In formula (2), and are the phase of the vortex beam and the phase of the fundamental mode reference beam respectively; represents an imaginary unit; Step 3. After splicing the frame phase pattern and quantity frame by frame in the order of recording each frame, the phase dynamic process of the vortex beam is obtained.

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