Experimental method and device for measuring orbital angular momentum spectrum of vortex beam based on coaxial interference

Abstract

The invention relates to the technical field of optical communication related calculation, in particular to an experimental method and device for measuring a vortex beam orbital angular momentum spectrum based on coaxial interferometry, and the device comprises a helium-neon laser, a beam-expanding collimation system, a light splitting modulation assembly, a spatial light modulator, a phase polarization adjustment assembly and a CCD camera. A spatial light modulator is arranged on a light path of a Gaussian beam output by the helium-neon laser, and a CCD camera is arranged at the tail end of the light path of the beam modulated by the spatial light modulator; the beam expanding and collimating system and the light splitting and modulating assembly are sequentially and coaxially arranged on a light path between the helium-neon laser and the spatial light modulator, and the phase polarization adjusting assembly is coaxially arranged on a light path between the spatial light modulator and the CCD camera. According to the invention, stable coaxial interference of the vortex beam and accurate measurement of the orbital angular momentum spectrum can be realized by using a small amount of experimental equipment, and the construction difficulty of an experimental interference light path is greatly reduced.

Description

Technical Field

[0001] The present invention relates to the field of optical communication related computing technology, and in particular to an experimental method and device for measuring the orbital angular momentum spectrum of a vortex light beam based on coaxial interferometry. Background Art

[0002] With the explosive growth of the Internet and data traffic, people have put forward higher requirements for faster and more information transmission capabilities. Compared with traditional communication methods, optical communication has higher bandwidth and longer transmission distance, which is very conducive to the realization of large-scale data transmission. Vortex beams, as a kind of light beam with a spiral phase structure, carry orbital angular momentum (OAM). Vortex beams carrying different orbital angular momentum modes are orthogonal to each other in space, which can greatly improve the channel capacity of optical communication and show great potential and development significance in the field of optical communication. Therefore, it is extremely important to accurately and efficiently identify the orbital angular momentum modes in vortex beams.

[0003] The orbital angular momentum spectrum is also called the spiral spectrum. It is similar to the frequency spectrum and represents the energy proportion of different orbital angular momentum modes in the light beam. Because the orbital angular momentum of the vortex beam is closely related to its phase structure, the interferometer method has become an effective method to calculate the orbital angular momentum spectrum of the vortex beam. In the interferometer method, the interference of the light beam requires the introduction of a reference beam. The main experimental schemes currently used for the Mach-Zehnder interferometer are as follows: Figure 2As shown. The experimental device consists of a He-Ne laser, a beam expansion and collimation system consisting of a lens with a focal length of 5 cm (L1) and a lens with a focal length of 25 cm (L2), a polarizing beam splitter (PBS), a half-wave plate (HWP), a spatial light modulator (SLM), a quarter-wave plate (QWP), a reflector (M), a beam splitter (BS), a polarizer (P), and a CCD camera (CCD). The principle is to divide the laser beam into two beams, one of which is used as a reference beam, and the other is modulated by a spatial light modulator to generate a vortex beam, and then the beams are combined to achieve interference between the vortex beam and the reference beam. In order to obtain the light field intensity required for calculation, it is necessary to shield the reference beam optical path and the measured beam optical path respectively to obtain the non-interfered reference beam intensity and vortex beam intensity. In addition, two different phase delays need to be introduced in the reference beam path or the vortex beam path to obtain the corresponding interference intensity of the reference beam and the vortex beam. Finally, the orbital angular momentum spectrum distribution of the vortex beam is calculated using the four light field intensities.

[0004] Therefore, the existing interferometry-based method for measuring the orbital angular momentum spectrum of a vortex beam has the following disadvantages: the optical path is difficult to adjust. The accurate measurement of the orbital angular momentum spectrum of a vortex beam requires that the reference beam and the vortex beam are strictly coaxial (that is, the propagation directions of the reference beam and the vortex beam are completely consistent with the center of the beam). To achieve coaxial interference between the reference beam and the vortex beam, precise adjustment of the optical path is required. If the solution of splitting and then combining the beams is adopted, the beams will pass through multiple reflective devices, resulting in the problem of exponentially increased difficulty in adjusting the optical path.

[0005] Therefore, it is necessary to provide an experimental method and device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry to solve the above problems. Summary of the invention

[0006] The present invention provides an experimental method and device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interference. By coaxially incidenting a vertically polarized Gaussian reference beam and a horizontally polarized Gaussian beam of equal intensity into a spatial light modulator, the vortex beam generated after modulation by the spatial light modulator remains coaxial with the reference beam. Therefore, the reference beam and the vortex beam can be coaxially transmitted during the entire transmission process, ensuring that the measured interference intensity must be the coaxial interference intensity, so as to solve the following shortcomings of the existing method for measuring the orbital angular momentum spectrum of a vortex beam based on interference: the difficulty of adjusting the optical path is large. The accurate measurement of the orbital angular momentum spectrum of a vortex beam requires that the reference beam and the vortex beam are strictly coaxial (that is, the propagation directions of the reference beam and the vortex beam are completely consistent with the center of the beam). To achieve coaxial interference between the reference beam and the vortex beam, precise adjustment of the optical path is required. If a solution of splitting and then combining the beam is adopted, the beam will pass through multiple reflective devices, resulting in the problem of exponentially increased difficulty in adjusting the optical path.

[0007] The present invention provides an experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry, which adopts the following technical scheme, including: A spatial light modulator is arranged on the optical path of a Gaussian beam output by a helium-neon laser, and a CCD camera is arranged at the end of the optical path of the beam modulated by the spatial light modulator; The beam expansion and collimation system and the light splitting and modulation components are coaxially arranged in the optical path between the HeNe laser and the spatial light modulator. and a phase polarization adjustment component, which is coaxially arranged on an optical path between the spatial light modulator and the CCD camera; Among them, the beam expansion and collimation system is used to expand and collimate the Gaussian beam, the splitter modulation component is used to modulate the expanded and collimated beam into a target Gaussian beam with equal light field intensity in the horizontal polarization direction and the vertical polarization direction; the spatial light modulator is used to modulate the horizontal polarization direction of the target Gaussian beam into a horizontally polarized vortex beam; and output an outgoing beam in which the horizontally polarized vortex beam and the vertically polarized target Gaussian beam are superimposed; the phase polarization adjustment component is used to introduce 0 or phase difference; and adjusting the phase-adjusted light beam to a preset light beam form, wherein the light beam forms include: a vortex light beam, a Gaussian light beam, and an interference light beam corresponding to the phase difference; the CCD camera is used to measure the light field intensity corresponding to different light beam forms.

[0008] Preferably, when the introduced phase difference is 0, the phase polarization adjustment component includes: a polarizer, which is used to adjust the phase-adjusted light beam into a vortex light beam, a Gaussian light beam or an interference light beam with a phase difference of 0; which is used to introduce a phase difference between the horizontally polarized vortex light beam and the vertically polarized target Gaussian light beam in the outgoing light beam; when the introduced phase difference is The phase polarization adjustment component includes: a quarter wave plate and a polarizer. The quarter wave plate is arranged on the optical path between the polarizer and the spatial light modulator. The quarter wave plate is used to introduce a phase polarization between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam. The phase difference.

[0009] Preferably, the beam expansion and collimation system comprises: a first lens and a second lens, the first lens and the second lens are coaxially arranged, and the focal length of the second lens is greater than the focal length of the first lens.

[0010] Preferably, the focal length of the second lens is five times the focal length of the first lens.

[0011] Preferably, the light splitting modulation component comprises: A polarization beam splitter, used for splitting the expanded and collimated light beam into a horizontally polarized light beam and a vertically polarized light beam perpendicular to each other, wherein the transmitted horizontally polarized light beam remains in the light path; and a half-wave plate, which adjusts the polarization direction of the horizontally polarized light beam after being split by the polarization beam splitter, so that the outgoing light beam is a target Gaussian beam with equal light field intensity in the horizontal polarization and vertical polarization directions.

[0012] Preferably, the spatial light modulator is a reflective modulator or a transmissive modulator. The present invention provides an experimental method for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry, which adopts the following technical scheme, including: Turn on the HeNe laser; Turning on the spatial light modulator and causing the spatial light modulator to load a corresponding phase hologram to generate a vortex light beam of a preset orbital angular momentum mode; Turn on the CCD camera; The polarizer is adjusted so that the angle between the polarization direction and the horizontal direction is 0°, and the CCD camera collects the current first light field intensity; The polarizer is adjusted so that the angle between the polarization direction and the horizontal direction is 90°, and the CCD camera collects the current second light field intensity; Adjust the polarizer so that the angle between the polarization direction and the horizontal direction is 0° to 90°, preferably 45°, and the CCD camera collects the current first interference light field intensity; A quarter wave plate is added, and the polarizer is adjusted so that the angle between the polarization direction and the horizontal direction is 0° to 90°, preferably 45°, and the CCD camera collects the current second interference light field intensity; According to the first light field intensity, the second light field intensity, the first interference light field intensity and the second interference light field intensity, orbital angular momentum spectra of the vortex light beam corresponding to different orbital angular momentum modes are obtained.

[0013] Preferably, the step of obtaining the orbital angular momentum spectrum of the vortex light beam corresponding to the orbital angular momentum mode m is:

[0014] In the formula, is the orbital angular momentum spectrum of the vortex beam corresponding to the orbital angular momentum mode m; is the expansion coefficient when the orbital angular momentum mode is m; is the sum of all orbital angular momentum modal energies contained in the vortex beam; The angle between the polarization direction of the polarizer and the horizontal direction when obtaining the first interference light field intensity and the second interference light field intensity; represents the phase difference introduced between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam; represents the intensity of the first light field; represents the intensity of the second light field; represents the intensity of the first interference light field; represents the intensity of the second interference light field is the complex amplitude of the horizontally polarized vortex beam light field; is the complex amplitude of the vertically polarized Gaussian beam light field; is the intensity of the interference light field received by the CCD camera; * indicates conjugation; It represents the phase difference between the expansion coefficient of orbital angular momentum mode m and the complex amplitude of Gaussian beam light field; represents an imaginary unit; Represents the azimuth in the polar coordinate system; represents the radius in polar coordinates; represents the phase difference term introduced by the quarter-wave plate; represents taking the real part; represents the interference term in the interference light field intensity; represents the interference term in the first interference light field intensity; Represents the interference term in the second interference light field intensity.

[0015] Preferably, before collecting the first interference light field intensity and the second interference light field intensity, the angle between the polarization direction of the polarizer and the horizontal direction is adjusted.

[0016] The beneficial effects of the present invention are: 1. The present invention utilizes the polarization selection characteristic of the spatial modulator for the modulated light beam, and coaxially inputs the vertically polarized Gaussian reference beam and the horizontally polarized Gaussian beam into the spatial light modulator, so that the vortex light beam generated after modulation by the spatial light modulator remains coaxial with the reference light beam. Therefore, it can be achieved that the reference light beam and the vortex light beam are coaxially transmitted in the entire transmission process, ensuring that the measured interference intensity must be the coaxial interference intensity, solving the problem that the vortex light beam and the reference light beam are difficult to be strictly coaxial in the traditional Mach-Zehnder interference experimental measurement. At the same time, a small amount of experimental equipment can be used to achieve stable coaxial interference of the vortex light beam and accurate measurement of the orbital angular momentum spectrum, greatly reducing the difficulty of building the experimental interference optical path.

[0017] 2. The present invention greatly improves the efficiency in light field intensity collection. The light field intensity collection of the present invention does not need to shield the reference beam path or the vortex beam path, and only needs to adjust the rotation angle of the polarizer to collect the required light field intensity; if the beam is split and measured, four light intensity patterns can be measured simultaneously, which is expected to be applied to the real-time measurement of the angular momentum spectrum of the vortex beam in the future. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 It is a schematic structural diagram of an experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to the present invention; Figure 2 A schematic diagram of an experimental device of an existing Mach-Zehnder interferometer solution in the background technology of the present invention; Figure 3 This is a light field intensity distribution diagram used for calculating the orbital angular momentum spectrum of a vortex beam carrying a single mode (and the orbital angular momentum mode is m=+7, m=-7) obtained by experimental measurement in an embodiment of the present invention; Figure 4 This is the measurement result of orbital angular momentum spectrum of vortex beam carrying single mode (orbital angular momentum mode m=+7, m=-7); Figure 5 This is the measurement result of orbital angular momentum spectrum of the vortex beam carrying dual modes (m=-4, m=5); Figure 6 This is the measurement result of orbital angular momentum spectrum of the vortex beam carrying three modes (m=-3, m=1, m=5); Figure 7 This is the measurement result of orbital angular momentum spectrum of a vortex beam carrying four modes (m=-7, m=-3, m=1, m=5).

[0020] In the figure: 1. Helium-neon laser; 2. First lens; 3. Second lens; 4. Polarization beam splitter; 5. Half-wave plate; 6. Spatial light modulator; 7. Quarter-wave plate; 8. Polarizer; 9. CCD camera. DETAILED DESCRIPTION

[0021] 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.

[0022] An embodiment of an experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry of the present invention is as follows: Figure 1 As shown, it includes: a helium-neon laser 1, a beam expansion and collimation system, a spectroscopic modulation component, a spatial light modulator 6, a phase polarization adjustment component and a CCD camera 9. The spatial light modulator 6 is arranged on the optical path of the Gaussian light beam output by the helium-neon laser 1, and the CCD camera 9 is arranged at the end of the optical path of the light beam modulated by the spatial light modulator 6; the beam expansion and collimation system and the spectroscopic modulation component are coaxially arranged on the optical path between the helium-neon laser 1 and the spatial light modulator 6 in sequence, and the phase polarization adjustment component is coaxially arranged on the optical path between the spatial light modulator 6 and the CCD camera 9, wherein The beam expansion and collimation system is used to expand and collimate the Gaussian beam. The spectroscopic modulation component is used to modulate the expanded and collimated beam into a target Gaussian beam with equal light field intensity in the horizontal polarization direction and the vertical polarization direction. The spatial light modulator 6 is used to modulate the horizontal polarization direction of the target Gaussian beam into a horizontally polarized vortex beam; and output an outgoing beam in which the horizontally polarized vortex beam and the vertically polarized target Gaussian beam are superimposed. The phase polarization adjustment component is used to introduce 0 or and adjusting the phase-adjusted light beam to a preset light beam form, wherein the light beam forms include: a vortex light beam, a Gaussian light beam, and an interference light beam corresponding to the phase difference; the CCD camera 9 is used to measure the light field intensity corresponding to different light beam forms.

[0023] Specifically, when the introduced phase difference is 0, the phase polarization adjustment component includes: a polarizer 8, which is used to adjust the phase-adjusted light beam into a vortex beam, a Gaussian beam or an interference beam with a phase difference of 0; it is used to introduce a phase difference of 0 between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing light beam; Figure 2 As shown, the phase difference introduced is When the phase polarization adjustment component comprises: a quarter wave plate 7 and a polarizer 8, the quarter wave plate 7 is arranged on the optical path between the polarizer 8 and the spatial light modulator 6, and the quarter wave plate 7 is used to introduce a phase polarization between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing light beam. The phase difference.

[0024] Specifically, the beam expansion and collimation system includes: a first lens 2 and a second lens 3, the first lens 2 and the second lens 3 are coaxially arranged, and the focal length of the second lens 3 is greater than the focal length of the first lens 2, and the focal length of the second lens 3 is five times the focal length of the first lens 2. In this embodiment, the focal length of the second lens 3 is 25 mm, and the focal length of the first lens 2 is 5 mm.

[0025] Specifically, the spectroscopic modulation component includes: a polarization beam splitter 4 and a half-wave plate 5. The polarization beam splitter 4 is used to split the expanded collimated light beam into a horizontally polarized light beam and a vertically polarized light beam that are perpendicular to each other; the half-wave plate 5 is used to modulate the corresponding light field intensities of the horizontally polarized light beam and the vertically polarized light beam that are perpendicular to each other into a target Gaussian beam with equal light field intensity.

[0026] Specifically, the spatial light modulator 6 is a reflective modulator or a transmissive modulator, such as Figure 1 As shown, in this embodiment, the spatial light modulator 6 is a reflective modulator.

[0027] In summary, if Figure 1 As shown, an embodiment of the experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry of the present invention specifically includes: a helium-neon laser (He-Ne Laser), a beam expansion and collimation system consisting of a first lens (L1) with a focal length of 5 cm and a second lens (L2) with a focal length of 25 cm, a polarizing beam splitter (Polarizing Beam Splitter, PBS), a half-wave plate (Half-Wave Plate, HWP), a spatial light modulator (Spatial Light Modulator, SLM), a quarter-wave plate (Quarter-Wave Plate, QWP), a polarizer (Polarizer, P) and a CCD camera (CCD).

[0028] An embodiment of an experimental method for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry of the present invention comprises the following specific steps: Step 1, turn on the HeNe laser 1; Step 2, turning on the spatial light modulator 6, and making the spatial light modulator 6 load the corresponding phase hologram to generate a vortex light beam of a preset orbital angular momentum mode (the vortex light beams corresponding to different orbital angular momentum modes require different phase holograms); Step 3, turn on the CCD camera 9; Step 4: Adjust the polarizer 8 so that the angle between the polarization direction and the horizontal direction is 0°, and the CCD camera 9 collects the current first light field intensity. ; Step 5: Adjust the polarizer 8 so that the angle between the polarization direction and the horizontal direction is 90°, and the CCD camera collects the current second light field intensity. ; Step 6: Adjust the polarizer 8 so that the angle between the polarization direction and the horizontal direction is 45°, and the CCD camera 9 collects the current first interference light field intensity. ; Step 7: Add a quarter wave plate 7, adjust the polarizer 8 so that the angle between the polarization direction and the horizontal direction is 45°, and the CCD camera 9 collects the current second interference light field intensity. ; Step 8: According to the first light field intensity, the second light field intensity, the first interference light field intensity, and the second interference light field intensity, the orbital angular momentum spectrum of the vortex light beam corresponding to the orbital angular momentum mode is obtained. Specifically, the steps of obtaining the orbital angular momentum spectrum of the vortex light beam corresponding to the orbital angular momentum mode is m are as follows: Step 81: According to the spiral harmonic decomposition theory, the complex amplitude of the horizontally polarized vortex beam light field can be expanded to be expressed as: (1) In the formula, is the complex amplitude of the horizontally polarized vortex beam light field; is the expansion coefficient when the orbital angular momentum mode is m; represents an imaginary unit; Represents the azimuth in the polar coordinate system.

[0029] in, It can be obtained by Fourier transform of the vortex beam light field, that is: (2) In the formula, is the expansion coefficient when the orbital angular momentum mode is m; is the complex amplitude of the horizontally polarized vortex beam light field.

[0030] The orbital angular momentum spectrum of the vortex beam corresponding to the orbital angular momentum mode m is defined as: (3) In the formula, is the orbital angular momentum spectrum of the vortex beam corresponding to the orbital angular momentum mode m; is the sum of all orbital angular momentum modal energies contained in the vortex beam (i.e., all the energy of the light field), ; Represents the radius in polar coordinates.

[0031] From formula (2) and formula (3), we know that to obtain the orbital angular momentum spectrum of the vortex beam, we need to measure the complex amplitude of the light field of the vortex beam: Or all expansion coefficients This embodiment uses the interference method to measure the orbital angular momentum spectrum of the vortex beam, which requires four light field intensities: 1) the first light field intensity of the horizontally polarized vortex beam, 2) the second light field intensity of the reference beam (vertically polarized target Gaussian beam), 3) the first interference light field intensity with a phase difference of 0 between the vortex beam and the reference beam, and 4) the additional phase difference between the vortex beam and the reference beam. The following is a formula for calculating the orbital angular momentum spectrum of the vortex light beam corresponding to different orbital angular momentum modes by using the first light field intensity, the second light field intensity, the first interference light field intensity and the second interference light field intensity. Suppose the light field complex amplitude of the vertically polarized target Gaussian light beam of the outgoing light beam passing through the spatial light modulator 6 is ( G represents a Gaussian beam, i.e., the reference beam in the present invention), the complex amplitude of the light field of the horizontally polarized vortex beam is , the angle between the polarizer 8 and the horizontal direction is The complex amplitude of the light field of the coaxial superposition beam after passing through the polarizer 8 is: (4) The CCD camera 9 receives the image when the angle between the polarizer 8 and the horizontal direction is , and the phase difference introduced between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam is The corresponding interference light field intensity is: (5) In the formula, Indicates that the angle between the polarizer 8 and the horizontal direction is , and the phase difference introduced between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam is The corresponding interference light field intensity when represents the intensity of the first light field; represents the intensity of the second light field; represents the interference term in the interference light field intensity; in, (6) (7) (8) In the formula, represents the phase difference introduced between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam; * represents conjugation; represents the phase difference term introduced by the quarter-wave plate; represents taking the real part; represents an imaginary unit; It should be noted that when a quarter wave plate 7 with a slow axis parallel to the polarization direction of the reference beam is added between the spatial light modulator 6 and the polarizer 8, , without adding a quarter wave plate .when hour, ,when hour ; Then using formula (1), It can be expressed as: (9) In the formula, represents the interference term in the interference light field intensity; It represents the phase difference between the expansion coefficient of orbital angular momentum mode m and the complex amplitude of the light field of the vertically polarized target Gaussian beam; arg represents the phase angle.

[0032] Using phase difference , And the angle between the polarization direction of the polarizer and the horizontal direction is The first interference light field intensity , the second interference light field intensity And formula (8), we have: (10) According to formula (10), we can get : (11) Substituting equation (11) into equation (3) can obtain the orbital angular momentum spectrum of the vortex beam. Therefore, a total of four light field intensities are required, namely , , , In order to maintain the best interference effect, the intensity of the second interference light field and the intensity of the first interference light field are obtained. Generally 45° is taken.

[0033] Two specific measurement examples are given below to illustrate the measurement process and practicality of this embodiment: Measurement Example 1: Measurement of orbital angular momentum spectrum of a single-mode vortex beam: In this example, the OAM spectrum (orbital angular momentum spectrum) of a vortex beam carrying a single OAM mode is measured. Figure 3 are the four light field intensities measured when carrying orbital angular momentum modes m=7 and m=-7. It can be seen that the modulation efficiency of the spatial light modulator cannot reach 100%, resulting in incomplete modulation of the horizontally polarized light. The modulated vortex beam contains some horizontally polarized Gaussian light that is not modulated. Therefore, the intensity It is polygonal rather than an ideal circular shape. The corresponding OAM spectrum measurement results are as follows Figure 4 (a) and Figure 4 As shown in (b), the theoretical value is basically consistent with the experimental value, where the energy distribution at m=0 comes from the incomplete modulation part of the spatial light modulator.

[0034] Measurement Example 2: Measurement of orbital angular momentum spectrum of multi-modal vortex beam: In this example, the OAM spectrum of a vortex beam carrying multiple orbital angular momentum modes is measured. Figure 5 The OAM spectrum measurement results of dual-mode (m=-4, m=5) vortex beams with different energy ratios are shown in Figure 2. Figure 6 The OAM spectrum measurement results of three-mode (m=-3, m=1, m=5) vortex beams with different energy ratios are shown in Figure 2. Figure 7 The OAM spectrum measurement results of four-mode (m=-7, m=-3, m=1, m=5) vortex beams with different energy ratios are shown. Among them, the preset OAM spectrum is the OAM spectrum that the vortex beam theoretically generated by the phase hologram loaded on the spatial light modulator should have. The experimentally measured OAM spectrum is consistent with the preset OAM spectrum distribution. Due to the modulation efficiency limitation of the spatial light modulator, the experimentally measured OAM spectrum energy ratio is lower than the energy ratio corresponding to the preset OAM, but the energy ratio between modes is consistent with the preset one. Since the modulation mode of the spatial light modulator adopted in this embodiment is pure phase modulation, a part of the unnecessary OAM mode energy distribution will be included in the generated multi-mode vortex beam (in the present invention, it is a smaller part of the energy distribution in the preset OAM spectrum). It can be seen from the measurement results of this example that even in the case of multi-modes with different energy ratios, the present invention still has good measurement accuracy and a large measurement range, which proves the effectiveness and accuracy of the present invention in measuring the complex OAM spectrum of multi-mode vortex beams.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry, It is characterized in that include: A helium-neon laser (1), wherein a spatial light modulator (6) is arranged on the optical path of a Gaussian light beam outputted by the laser, and a CCD camera (9) is arranged at the end of the optical path of the light beam modulated by the spatial light modulator (6); The beam expansion and collimation system and the light splitting and modulation component are coaxially arranged in sequence on the optical path between the helium-neon laser (1) and the spatial light modulator (6). and a phase polarization adjustment component, coaxially arranged on the optical path between the spatial light modulator (6) and the CCD camera (9); The beam expansion and collimation system is used to expand and collimate the Gaussian beam; the beam splitting and modulation component is used to modulate the expanded and collimated beam into a target Gaussian beam with equal light field intensity in the horizontal polarization direction and the vertical polarization direction; the spatial light modulator (6) is used to modulate the horizontal polarization direction of the target Gaussian beam into a horizontally polarized vortex beam; and output an outgoing beam that is a superposition of the horizontally polarized vortex beam and the vertically polarized target Gaussian beam; the phase polarization adjustment component is used to introduce 0 or 10 phases between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam. and adjusting the phase-adjusted light beam into a preset light beam form, wherein the light beam form includes: a vortex light beam, a Gaussian light beam, and an interference light beam corresponding to the phase difference; and the CCD camera (9) is used to measure the light field intensity corresponding to different light beam forms.

2. The experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 1, It is characterized in that When the introduced phase difference is 0, the phase polarization adjustment component comprises: a polarizer (8) for adjusting the phase-adjusted light beam into a vortex light beam, a Gaussian light beam or an interference light beam with a phase difference of 0; for introducing a phase difference of 0 between the horizontally polarized vortex light beam and the vertically polarized target Gaussian light beam in the outgoing light beam; when the introduced phase difference is When the phase polarization adjustment component comprises: a quarter wave plate (7) and a polarizer (8), the quarter wave plate (7) is arranged on the optical path between the polarizer (8) and the spatial light modulator (6), and the quarter wave plate (7) is used to introduce a phase polarization between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing light beam. The phase difference.

3. The experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 1, It is characterized in that The beam expansion and collimation system comprises: a first lens (2) and a second lens (3), wherein the first lens (2) and the second lens (3) are coaxially arranged, and the focal length of the second lens (3) is greater than the focal length of the first lens (2).

4. The experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 3, It is characterized in that The focal length of the second lens (3) is five times the focal length of the first lens (2).

5. The experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 1, It is characterized in that The optical splitter modulation components include: A polarization beam splitter (4) is used to split the expanded and collimated light beam into a horizontally polarized light beam and a vertically polarized light beam that are perpendicular to each other, wherein the transmitted horizontally polarized light beam remains in the light path; And a half-wave plate (5) is used to adjust the polarization direction of the horizontally polarized light beam after being split by the polarization beam splitter, so that the output light beam is a target Gaussian light beam with equal light field intensity in the horizontal polarization direction and the vertical polarization direction.

6. The experimental device for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 1, It is characterized in that The spatial light modulator (6) is a reflective modulator or a transmissive modulator.

7. An experimental method for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry. It is characterized in that include: Turn on the HeNe laser; Turning on the spatial light modulator and causing the spatial light modulator to load a corresponding phase hologram to generate a vortex light beam of a preset orbital angular momentum mode; Turn on the CCD camera; The polarizer is adjusted so that the angle between the polarization direction and the horizontal direction is 0°, and the CCD camera collects the current first light field intensity; The polarizer is adjusted so that the angle between the polarization direction and the horizontal direction is 90°, and the CCD camera collects the current second light field intensity; Adjust the polarizer so that the angle between the polarization direction and the horizontal direction is 0°~90°, and the CCD camera collects the current first interference light field intensity; Add a quarter wave plate and adjust the polarizer so that the angle between the polarization direction and the horizontal direction is 0°~90°, and the CCD camera collects the current second interference light field intensity; According to the first light field intensity, the second light field intensity, the first interference light field intensity and the second interference light field intensity, orbital angular momentum spectra of the vortex light beam corresponding to different orbital angular momentum modes are obtained.

8. The experimental method for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 7, It is characterized in that The steps for obtaining the orbital angular momentum spectrum of the vortex beam corresponding to the orbital angular momentum mode m are as follows: In the formula, is the orbital angular momentum spectrum of the vortex beam corresponding to the orbital angular momentum mode m; is the expansion coefficient when the orbital angular momentum mode is m; is the sum of all orbital angular momentum modal energies contained in the vortex beam; The angle between the polarization direction of the polarizer and the horizontal direction when obtaining the first interference light field intensity and the second interference light field intensity; represents the phase difference introduced between the horizontally polarized vortex beam and the vertically polarized target Gaussian beam in the outgoing beam; represents the intensity of the first light field; represents the intensity of the second light field; represents the intensity of the first interference light field; represents the intensity of the second interference light field is the complex amplitude of the horizontally polarized vortex beam light field; is the complex amplitude of the vertically polarized Gaussian beam light field; is the intensity of the interference light field received by the CCD camera; * indicates conjugation; It represents the phase difference between the expansion coefficient of orbital angular momentum mode m and the complex amplitude of Gaussian beam light field; represents an imaginary unit; Represents the azimuth in the polar coordinate system; represents the radius in polar coordinates; represents the phase difference term introduced by the quarter-wave plate; represents taking the real part; represents the interference term in the interference light field intensity; represents the interference term in the first interference light field intensity; Represents the interference term in the second interference light field intensity.

9. The experimental method for measuring the orbital angular momentum spectrum of a vortex beam based on coaxial interferometry according to claim 7, It is characterized in that Before collecting the first interference light field intensity and the second interference light field intensity, the angle between the polarization direction of the polarizer and the horizontal direction is adjusted to 45°.

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