Orbital angular momentum spectrum measuring device based on topological charge conjugate interference

Abstract

The invention belongs to the technical field of spectral measurement, and discloses an orbital angular momentum spectrum measurement device based on topological charge conjugate interference. Comprising a first non-polarized beam splitter prism, a second non-polarized beam splitter prism, a first dielectric film total reflection mirror, a first precise piezoelectric displacement platform, a second dielectric film total reflection mirror, a third dielectric film total reflection mirror, a first silver film reflection mirror, a second silver film reflection mirror, a third non-polarized beam splitter prism, a fourth dielectric film total reflection mirror, a fourth non-polarized beam splitter prism and a third silver film reflection mirror. The system comprises a first precise piezoelectric displacement platform, a second precise piezoelectric displacement platform, a fourth silver film reflector, a fifth dielectric film total reflective mirror, a second precise piezoelectric displacement platform, a sixth dielectric film total reflective mirror, a seventh dielectric film total reflective mirror, a fifth non-polarized beam splitter prism, a first CCD camera, a second CCD camera and a computer. According to the invention, the accuracy and sensitivity of measurement are improved, and the method has better precision, real-time performance and convenience.

Description

Technical Field

[0001] This invention belongs to the field of spectral measurement technology and relates to an orbital angular momentum spectrum measurement device based on topological charge conjugate interference. Background Technology

[0002] A vortex beam (VB) is a novel type of laser beam with characteristics distinct from ordinary Gaussian beams. Its energy propagates spirally rather than in a straight line. Due to its spiral wavefront and the presence of a phase singularity at the beam center, the light field distribution is a hollow ring. In cylindrical coordinates... Below, the complex amplitude expression for the wavefront of a vortex beam contains a phase spiral term. This macroscopically reflects the orbital angular momentum (OAM) of a photon. The value of the orbital angular momentum carried by each photon can be represented by the angular quantum number l, also known as the topological charge, which is the reduced Planck constant. In addition, any two vortex beams of different orders are orthogonal to each other, meaning that vortex beams also have orthogonality, which means that vortex beams of different orders transmitted coaxially can be separated from each other.

[0003] These unique properties of vortex beams make them crucial for applications in many fields, such as ultra-high-capacity optical communication, holographic storage, remote sensing, laser processing, and high-resolution imaging. In important applications like optical communication and holographic information storage, the vortex beam, acting as the information carrier, is not merely a single orbital angular momentum mode, but a multimode hybrid vortex beam containing multiple orbital angular momentum modes. Because vortex beams of different orders are orthogonal to each other, a single vortex beam can simultaneously carry different orbital angular momentum components. The energy proportions of these components determine the beam's intensity, phase, and wavefront distribution. For multimode hybrid vortex beams, even when the orbital angular momentum components are the same but their proportions differ, their intensity and wavefront distributions are completely different. The orbital angular momentum spectrum is defined as the energy ratio of the beam across its different orbital angular momentum components, reflecting some properties of the beam's orbital angular momentum. Accurate and efficient measurement of the orbital angular momentum spectrum of a vortex beam, in order to extract its encoded information or sieve it to achieve modular multiplexing, is a prerequisite for the practical application of the above-mentioned technologies.

[0004] Currently, scholars both domestically and internationally have conducted extensive research on the measurement of beam orbital angular momentum spectra and developed various beam orbital angular momentum spectrum measurement techniques. Existing beam orbital angular momentum spectrum measurement methods can generally be divided into three categories: the first category is the earliest developed diffraction measurement method, which relies on designing special diffraction gratings and analyzing the relevant properties of the diffraction field to infer the orbital angular momentum spectrum of the beam under test; the second category is the mode beam splitting method, whose core idea is to separate different OAM components in the beam under test using certain technical means, and then measure the intensity of each component separately to obtain the orbital angular momentum spectrum; the third category includes other measurement methods besides the first two categories, such as the complex amplitude derivation method, the angular coherence function method, and the grayscale algorithm. However, current orbital angular momentum spectrum measurement technology still suffers from problems such as small measurement range and large system size. Therefore, it is urgent to explore a miniaturized, compact orbital angular momentum spectrum measurement technology with a large OAM state measurement range to promote the application and development of vortex beams in ultra-large capacity optical communication, high resolution imaging, holographic storage and other fields, and at the same time add a cornerstone to the progress of cutting-edge technologies in the current optical field. Summary of the Invention

[0005] (a) Technical problems to be solved

[0006] To address the shortcomings of existing composite vortex beam orbital angular momentum spectrum measurement techniques, such as small angular quantum number range, multimode crosstalk, and poor real-time performance, this invention proposes an orbital angular momentum spectrum measurement device based on topological charge conjugate interferometry. This device utilizes two composite vortex beams with topological charge conjugates to perform coaxial interference, and then calculates the light intensity corresponding to each order of angular quantum number by angular integration of the difference interferogram. The aim is to provide a real-time, efficient, and widely applicable orbital angular momentum spectrum measurement technique.

[0007] (II) Technical Solution

[0008] To address the aforementioned technical problems, this invention provides an orbital angular momentum spectrum measurement device based on topological charge conjugate interference, comprising a first non-polarizing beam splitter prism, a second non-polarizing beam splitter prism, a first dielectric film total reflection mirror, a first precision piezoelectric displacement platform, a second dielectric film total reflection mirror, a third dielectric film total reflection mirror, a first silver film reflector, a second silver film reflector, a third non-polarizing beam splitter prism, a fourth dielectric film total reflection mirror, a fourth non-polarizing beam splitter prism, a third silver film reflector, a fourth silver film reflector, a fifth dielectric film total reflection mirror, a second precision piezoelectric displacement platform, a sixth dielectric film total reflection mirror, a seventh dielectric film total reflection mirror, a fifth non-polarizing beam splitter prism, a first CCD camera, a second CCD camera, and a computer. The composite vortex light to be tested is incident perpendicularly on the first non-polarizing beam splitter and split into two beams with orthogonal propagation directions. The fourth dielectric film total reflection mirror, the first non-polarizing beam splitter, and the second non-polarizing beam splitter are arranged at right angles. The first polarizing beam splitter, the fourth dielectric film total reflection mirror, and the fourth non-polarizing beam splitter are arranged at right angles, and the fourth dielectric film total reflection mirror is placed at a 45° angle to the optical axes on both sides of the right angle. The second polarizing beam splitter, the first dielectric film total reflection mirror, the second dielectric film total reflection mirror, the third dielectric film total reflection mirror, the first silver film reflector, the second silver film reflector, and the third non-polarizing beam splitter form the first Mach-Zehnder interference structure, and the two arm optical paths undergo even and odd number of mirror reflections, respectively. The second and third dielectric film total reflection mirrors are fixed on the first precision piezoelectric displacement platform, and the movement adjustment direction of the first precision piezoelectric displacement platform is parallel to the beam incident on the second dielectric film total reflection mirror. The first CCD camera and the second silver film reflector are respectively located at the first... The three non-polarizing beam splitters are located on both sides, with the centers of the camera's photosensitive surface, the mirror's reflecting surface, the prism's reflecting surface, and the transmission surface coaxial. The fourth polarizing beam splitter, the third silver film mirror, the fourth silver film mirror, the fifth dielectric film total reflection mirror, the sixth dielectric film total reflection mirror, the seventh dielectric film total reflection mirror, and the fifth non-polarizing beam splitter form a second Mach-Zehnder interference structure, with the two arm optical paths undergoing odd and even mirror reflections respectively. The sixth and seventh dielectric film total reflection mirrors are fixed on a second precision piezoelectric displacement platform, and the movement adjustment direction of the second precision piezoelectric displacement platform is parallel to the beam incident on the sixth dielectric film total reflection mirror. The second CCD camera and the fourth silver film mirror are located on both sides of the fifth non-polarizing beam splitter, with the centers of the camera's photosensitive surface, the mirror's reflecting surface, the prism's reflecting surface, and the transmission surface coaxial. The interference images detected by the first and second CCD cameras are transmitted to a computer for processing via a data cable.

[0009] The four silver film mirrors are placed at a 67.5° angle to the incident light, and the seven dielectric film total reflection mirrors are placed at a 45° angle to the incident light.

[0010] The composite vortex light to be tested contains one or more topological charges.

[0011] Among them, the five non-polarized beam splitters are insensitive to the polarization state of the incident light and can maintain the same beam splitting characteristics for P-polarized and S-polarized light over a wide wavelength range.

[0012] Among them, two precision piezoelectric displacement platforms are used to adjust the optical path difference between the two arms of the first and second interferometer structures, respectively. Their phase adjustment accuracy needs to reach π / 90, that is, the adjustment accuracy of the displacement platform is on the nanometer scale.

[0013] When the phase difference of the first or second Mach-Zehnder interferometer structure increases, the interferogram detected by the corresponding CCD camera exhibits periodic changes with a repetition period of 2π.

[0014] In this process, after the interferograms detected by the two CCD cameras are transmitted to the computer, they first need to be processed by difference, then by angular integration, then the corresponding light intensity is calculated according to the weight of each order of topological charge, and finally the orbital angular momentum spectrum of the composite vortex light is obtained by normalization.

[0015] (III) Beneficial Effects

[0016] The orbital angular momentum spectrum measurement device based on topological charge conjugate interferometry provided by the above technical solution has the following beneficial effects:

[0017] (1) In this invention, the mirror property of the vortex beam is utilized, that is, the angular quantum number of the vortex beam becomes the opposite of the original number after each mirror reflection. By using odd number of reflections and even number of reflections respectively, topologically charged conjugate vortex light is generated.

[0018] (2) In this invention, by taking the difference between two interferograms with different phase differences, the orbital angular momentum spectrum is calculated based on the difference image. This method is not only applicable to composite vortex beams with topological loads that are centrally symmetric, but can also be used to measure the orbital angular momentum spectrum of any asymmetric distribution of topological loads.

[0019] (3) In this invention, a polarization-insensitive beam splitter is used for beam splitting and beam combining, which can effectively improve the distortion of the interference image caused by uneven beam splitting and further improve the accuracy of measurement.

[0020] (4) In this invention, the optical path scheme of combining a dielectric film total reflection mirror and a silver film reflection mirror is selected. While satisfying the odd / even mirror reflection times, the loss during transmission is reduced, which is beneficial to improving the sensitivity of the entire measurement system.

[0021] (5) In this invention, a dual Mach-Zehnder interferometer structure is designed. Compared with the measurement scheme of a single Mach-Zehnder interferometer structure, it avoids the complicated steps of repeatedly adjusting the displacement platform and reduces the error caused by the interference of different wave trains. This dual interferometer structure makes the measurement method more accurate, real-time and convenient. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the basic structure of the orbital angular momentum spectrum measurement device based on topological charge conjugate interference, according to an embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram illustrating the method for adjusting the optical signal delay time in an embodiment of the present invention.

[0024] Figure 3 This is a schematic diagram illustrating the method of generating pulsed light signals with fixed frequency and delay in an embodiment of the present invention. Detailed Implementation

[0025] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0026] Combination Figure 1This embodiment of the orbital angular momentum spectrum measurement device based on topological charge conjugate includes: a first non-polarizing beam splitter prism 1, a second non-polarizing beam splitter prism 2, a first dielectric film total reflection mirror 3, a first precision piezoelectric displacement platform 4, a second dielectric film total reflection mirror 5, a third dielectric film total reflection mirror 6, a first silver film reflector 7, a second silver film reflector 8, a third non-polarizing beam splitter prism 9, a fourth dielectric film total reflection mirror 10, a fourth non-polarizing beam splitter prism 11, a third silver film reflector 12, a fourth silver film reflector 13, a fifth dielectric film total reflection mirror 14, a second precision piezoelectric displacement platform 15, a sixth dielectric film total reflection mirror 16, a seventh dielectric film total reflection mirror 17, a fifth non-polarizing beam splitter prism 18, a first CCD camera 19, a second CCD camera 20, and a computer 21; the device to be measured... The composite vortex light is incident perpendicularly on the first non-polarizing beam splitter 1 and splits into two beams with orthogonal propagation directions; the fourth dielectric film total reflection mirror 10, the first non-polarizing beam splitter 1, and the second non-polarizing beam splitter 2 are arranged at right angles, the first polarizing beam splitter 1, the fourth dielectric film total reflection mirror 10, and the fourth non-polarizing beam splitter 11 are arranged at right angles, and the fourth dielectric film total reflection mirror 10 is placed at a 45° angle to the optical axes on both sides of the right angle; the second polarizing beam splitter 2, the first dielectric film total reflection mirror 3, the second dielectric film total reflection mirror 5, the third dielectric film total reflection mirror 6, the first silver film mirror 7, the second silver film mirror 8, and the third non-polarizing beam splitter 9 form the first Mach-Zehnder interference structure, and the two arm optical paths undergo even and odd number of mirror reflections respectively; the second dielectric film total reflection mirror ...10, the first non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam splitter 11, the second non-polarizing beam The mass film total reflection mirror 5 and the third dielectric film total reflection mirror 6 are fixed on the first precision piezoelectric displacement platform 4. The movement adjustment direction of the first precision piezoelectric displacement platform 4 is parallel to the beam incident on the second dielectric film total reflection mirror 5. The first CCD camera 19 and the second silver film reflector 8 are located on both sides of the third non-polarizing beam splitter 9, and the centers of the photosensitive surface of the camera, the reflecting surface of the reflector, the reflecting surface of the prism, and the transmission surface are coaxial. The fourth polarizing beam splitter prism 11, the third silver film reflector 12, the fourth silver film reflector 13, the fifth dielectric film total reflection mirror 14, the sixth dielectric film total reflection mirror 16, the seventh dielectric film total reflection mirror 17, and the fifth non-polarizing beam splitter prism group 18 form a second Mach-Zehnder interference structure, and the two arm optical paths pass through odd and even numbers of times, respectively. Specular reflection; the sixth dielectric film total reflection mirror 16 and the seventh dielectric film total reflection mirror 17 are fixed on the second precision piezoelectric displacement platform 15, and the movement adjustment direction of the second precision piezoelectric displacement platform 15 is parallel to the light beam incident on the sixth dielectric film 16 total reflection mirror; the second CCD camera 20 and the fourth silver film reflector 13 are respectively located on both sides of the fifth non-polarizing beam splitter prism 18, and the centers of the photosensitive surface of the camera, the reflective surface of the reflector, the reflective surface of the prism, and the transmission surface are coaxial; the four silver film reflectors are placed at a 67.5° angle to the incident light, and the seven dielectric film total reflection mirrors are placed at a 45° angle to the incident light; the interference images detected by the first CCD camera 19 and the second CCD camera 20 are all transmitted to the computer 21 for processing via data lines.

[0027] The vortex beam has a spiral phase Each photon carries orbital angular momentum, and these conditions are satisfied by Laguerre-Gaussian beams, Bessel beams, and Bessel-Gaussian beams. In this embodiment, we will use a Laguerre-Gaussian beam as an example to explain in detail the measurement principle of orbital angular momentum spectrum. Typically, the electric field of a multimode vortex beam in cylindrical coordinates can be expressed as:

[0028]

[0029] in, This represents the Laguerre-Gaussian mode, where l is the angular quantum number (also known as the topological charge), p is the radial quantum number (which can take any non-negative integer), and ρ is the radius in cylindrical coordinates. Angle A is in cylindrical coordinates. lp This represents the intensity corresponding to each mode. When the vortex light undergoes an odd number of mirror reflections, the orientation of its electric field intensity will change from... Become Azimuth term by Become Electric field strength is Become When a vortex beam undergoes an even number of mirror reflections, its electric field distribution remains unchanged. The corresponding angular integral autocorrelation function term is:

[0030]

[0031] In the formula, <···> e The vortex represents the ensemble average, and * denotes the complex conjugate term. According to the Wiener-Khintchine theory, the Fourier transform of this autocorrelation function can be expressed as the integral of the intensity of the topologically charged vortex light field at each order.

[0032]

[0033] In the formula S represents the difference between two phase angles. l This is the normalized intensity of a vortex beam with topological charge l. After entering the MZ interferometer, the multimode vortex beam splits into two beams of equal energy and identical optical fields. One beam undergoes four mirror reflections, maintaining its angular quantum number; the other beam undergoes three mirror reflections, resulting in its angular quantum number becoming the opposite of its original value. After re-coupling and interference, the electric field expression for the output composite vortex beam is:

[0034]

[0035] In the formula, k1 and k2 represent the transmission and reflection ratios of the non-polarizing beam splitter, respectively, and β1 and β2 are the phases of the two beams after different delays. Therefore, the light intensity per unit angular direction is:

[0036]

[0037] In the formula, δ = β2 - β1 represents the optical path difference between the two arms of the interferometer. Two interference images with phase differences of δ1 and δ2 are measured respectively, ensuring δ1 + δ2 = π / 2. Then, the difference is processed, and the light intensity of the difference interference image is integrated angularly to obtain the result. Finally, by performing a weighted calculation on the integral of the angular light intensity, the intensity expression for the orbital angular momentum with topological charge value l can be obtained:

[0038]

[0039] By calculating the intensity of each order l orbital angular momentum sequentially and then normalizing it, the orbital angular momentum spectrum of the composite vortex beam under test can be obtained.

[0040] Combination Figure 2 We used a spatial light modulator to generate a 20th-order composite vortex beam with a topological charge range from -10 to 10, such as... Figure 2 As shown in (a), the intensity ratio of its topological charges is 4:0:5:1:6:3:7:1:3:1:0:2:2:4:6:0:8:9:2:4:6. The phase difference of the first Mach-Zehnder interferometer is set to 0, and the phase difference of the second Mach-Zehnder interferometer is set to π / 2. The interferogram measured by the first CCD camera 19 is shown below. Figure 2 As shown in (b), the interferogram measured by the second CCD camera 20 is as follows: Figure 2 As shown in (c). The two interferograms are input into computer 21, and after grayscale difference processing, the result is as follows. Figure 2 The image shown in (d) is a picture.

[0041] Combination Figure 3 After performing angular integration on the difference interferogram, the distribution of light intensity with azimuth angle is obtained, as shown below. Figure 3 As shown in (a). By performing a weighted calculation of the angular intensity integral corresponding to Equation (6), the intensity values ​​of the orbital angular momentum of each topological charge can be obtained. Finally, normalization is performed to obtain the orbital angular momentum spectrum, as shown in (a). Figure 3 As shown in (b), by comparing the preset intensity ratio with the measurement results, it can be found that the measurement error is less than 1%, which confirms the accuracy of the orbital angular momentum spectrum measurement method proposed in this invention.

[0042] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A device for measuring orbital angular momentum spectrum based on topological charge conjugate interference, characterized in that, The system includes a first unbiased beam splitter, a second unbiased beam splitter, a first dielectric film total reflection mirror, a first precision piezoelectric displacement platform, a second dielectric film total reflection mirror, a third dielectric film total reflection mirror, a first silver film reflector, a second silver film reflector, a third unbiased beam splitter, a fourth dielectric film total reflection mirror, a fourth unbiased beam splitter, a third silver film reflector, a fourth silver film reflector, a fifth dielectric film total reflection mirror, a second precision piezoelectric displacement platform, a sixth dielectric film total reflection mirror, a seventh dielectric film total reflection mirror, a fifth unbiased beam splitter, a first CCD camera, a second CCD camera, and a computer; the composite vortex light to be measured is incident perpendicularly on the first unbiased beam splitter. The prism splits the light beams into two beams with orthogonal propagation directions. The fourth dielectric film total reflection mirror, the first non-polarizing beam splitter, and the second non-polarizing beam splitter are arranged at right angles, as are the first polarizing beam splitter, the fourth dielectric film total reflection mirror, and the fourth non-polarizing beam splitter. The fourth dielectric film total reflection mirror is placed at a 45° angle to both optical axes of the right angle. The second polarizing beam splitter, the first dielectric film total reflection mirror, the second dielectric film total reflection mirror, the third dielectric film total reflection mirror, the first silver film mirror, the second silver film mirror, and the third non-polarizing beam splitter form the first Mach-Zehnder interference structure. The two optical paths pass through even-numbered and odd-numbered mirrors, respectively. Reflection; the second and third dielectric film total reflection mirrors are fixed on the first precision piezoelectric displacement platform, and the movement adjustment direction of the first precision piezoelectric displacement platform is parallel to the beam incident on the second dielectric film total reflection mirror; the first CCD camera and the second silver film reflector are respectively located on both sides of the third non-polarizing beam splitter prism, and the centers of the photosensitive surface of the camera, the reflecting surface of the reflector, the reflecting surface of the prism, and the transmission surface are coaxial; the fourth polarizing beam splitter prism, the third silver film reflector, the fourth silver film reflector, the fifth dielectric film total reflection mirror, the sixth dielectric film total reflection mirror, the seventh dielectric film total reflection mirror, and the fifth non-polarizing beam splitter prism form the second matrix. The Hertzsprung-Zehnder interference structure has optical paths in both arms undergoing odd and even mirror reflections, respectively. The sixth and seventh dielectric film total reflection mirrors are fixed on a second precision piezoelectric displacement platform, the direction of which is parallel to the beam incident on the sixth dielectric film total reflection mirror. The second CCD camera and the fourth silver film reflector are located on opposite sides of the fifth non-polarizing beam splitter prism, and the centers of the camera's photosensitive surface, the reflective surface of the reflector, the reflective surface of the prism, and the transmission surface are coaxial. The interference images detected by the first and second CCD cameras are transmitted to a computer for processing via a data cable.

2. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 1, characterized in that, Four silver-coated mirrors are placed at a 67.5° angle to the incident light.

3. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 2, characterized in that, Seven dielectric film total reflection mirrors are placed at a 45° angle to the incident light.

4. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 3, characterized in that, The composite vortex light under test contains one or more topological charges.

5. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 4, characterized in that, The five unpolarized beam splitters are insensitive to the polarization state of the incident light and maintain the same beam splitting characteristics for P-polarized and S-polarized light over a wide wavelength range.

6. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 5, characterized in that, Two precision piezoelectric displacement platforms are used to adjust the optical path difference between the two arms of the first and second interferometer structures, respectively. The phase adjustment accuracy reaches π / 90, that is, the adjustment accuracy of the displacement platforms is on the nanometer scale.

7. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 6, characterized in that, When the phase difference of the first or second Mach-Zehnder interferometer structure increases, the interferogram detected by the corresponding CCD camera exhibits periodic changes with a repetition period of 2π.

8. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 7, characterized in that, After the interferograms detected by the two CCD cameras are transmitted to the computer, they are first processed by difference, then by angular integration, and then the corresponding light intensity is calculated according to the weight of each order of topological charge. Finally, the normalization process is performed to obtain the orbital angular momentum spectrum of the composite vortex light.

9. The orbital angular momentum spectrum measurement device based on topological charge conjugate interference as described in claim 4, characterized in that, A 20th-order composite vortex beam with a topological charge range from -10 to 10 was generated using a spatial light modulator, with the intensity ratio of the topological charge being 4:0:5:1:6:3:7:1:3:1:0:2:2:4:6:0:8:9:2:4:

6.

10. A method for measuring orbital angular momentum spectrum based on topological charge conjugate interferometry, characterized in that, The measurement was performed using the orbital angular momentum spectrum measuring device based on topological charge conjugate interference as described in claim 9.

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