Method and device for measuring orbital angular momentum spectrum of light beam based on power measurement

Abstract

The invention discloses a light beam orbital angular momentum spectrum measurement method and device based on power measurement. Reference single-mode orbital angular momentum modes of different orders are introduced to coaxially interfere with a to-be-measured light beam in sequence, then the optical power value of a reference light beam, the optical power value of the to-be-measured light beam and the optical power value of the interfered light beam are measured respectively, and the orbital angular momentum spectrum of the to-be-measured light beam can be deduced reversely. In the analysis process, the orbital angular momentum spectrum of the light beam can be measured only by using the power meter to measure the power of the light beam, the cost is low, the structure is simple, control is easy, and the method can be widely applied to the frontier fields of ultra-large-capacity optical communication, new-system laser detection, high-dimensional quantum information and the like.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and in particular to a method and device for measuring a light beam orbital angular momentum spectrum based on power measurement. Background Art

[0002] Previous studies have shown that spiral wavefronts The laser beam can carry orbital angular momentum (OAM), where l is the angular quantum number, also known as the topological charge. is the angular coordinate. The angular quantum number l is the eigenvalue of OAM, so it can also be called the OAM order. It can be any integer. The OAM value carried by each photon in the light beam is in, The different OAM modes contained in the light beam are mutually orthogonal and can form an infinite-dimensional Hilbert space. This high-dimensional characteristic makes OAM have great potential in the fields of ultra-large capacity optical communications, new laser detection, and high-dimensional quantum information technology, and has become one of the research hotspots in the laser field at home and abroad in recent years.

[0003] Different OAM modes carried by a laser beam can have different intensity ratios, and the intensity ratios of these OAM modes constitute the OAM spectrum of the beam. The OAM spectrum determines the mode field distribution, wavefront distribution and topological characteristics of the beam, and is one of the important parameters to characterize the characteristics of the beam. High-precision measurement of the OAM spectrum of a beam is the key foundation for cutting-edge applications based on OAM. There are reports on OAM spectrum measurement work at home and abroad. These schemes usually rely on area array detectors to collect the lateral light field distribution. However, area array detectors are usually expensive. The high cost restricts the widespread promotion of OAM spectrum measurement technology and limits the development of OAM applications. Therefore, it is urgent to develop a new low-cost method for measuring the OAM spectrum of a beam that does not require area array detection. Summary of the invention

[0004] In view of this, the present invention discloses a method and device for measuring the orbital angular momentum spectrum of a light beam based on power measurement.

[0005] The present invention discloses a method for measuring the orbital angular momentum spectrum of a light beam based on power measurement. For a certain OAM order l0 to be measured in the light beam to be measured, two single-mode l0-order OAM beams with the same optical power but a phase difference of π / 2 are introduced as reference beams, so that they interfere with the light beam to be measured coaxially respectively. The optical power value of the reference beam, the optical power value of the light beam to be measured, and the optical power value of the light beam obtained after the light beam to be measured interferes with the two reference beams respectively are measured by an optical power meter. The relative intensity information of the OAM mode with the order l0 contained in the light beam to be measured can be deduced. The above process is circulated to make reference single-mode OAM beams of different orders incident in sequence, and the relative intensity information of each OAM mode in the light beam to be measured can be obtained in sequence, thereby obtaining the OAM spectrum of the light beam to be measured. The above analysis process only needs to measure the optical power of the light beam, which can be completed with only one optical power meter, effectively reducing the cost of the light beam OAM spectrum measurement system.

[0006] The present invention provides a device for measuring the orbital angular momentum spectrum of a light beam based on power measurement, comprising a laser, a beam expander, a fork-shaped grating, an aperture, a depolarizing 50-50 beam splitter prism, a focusing lens, an optical power meter, and a host, wherein:

[0007] The laser is used to generate a fundamental mode Gaussian beam;

[0008] The beam expander is placed in the laser light path behind the laser and is used to adjust the spot diameter of the reference beam to match the spot size of the beam to be measured;

[0009] The fork grating is placed in the laser light path behind the beam expander and is used to convert the Gaussian beam into reference single-mode OAM beams of different orders;

[0010] The aperture is placed in the laser light path behind the fork-shaped grating, and is used to filter out the first-order diffraction light of the fork-shaped grating to obtain the required reference single-mode OAM beam;

[0011] The depolarizing 50-50 beam splitter is placed in the laser light path behind the aperture, and is used to combine and interfere the reference single-mode OAM beam with the beam to be measured;

[0012] The focusing lens is placed in the laser light path behind the depolarized 50-50 beam splitter prism, and is used to focus the outgoing light beam after interference;

[0013] The optical power meter is placed in the laser light path behind the focusing lens and is used to measure the optical power of the outgoing light beam after interference;

[0014] The host is used to analyze the optical powers of the light beam to be measured, the reference light beam and the light beam after interference measured by the optical power meter, and calculate the OAM spectrum of the light beam to be measured.

[0015] The present invention has the following beneficial effects:

[0016] (1) The present invention discloses a method for measuring the orbital angular momentum spectrum of a light beam based on power measurement, which measures the OAM spectrum of the light beam to be measured by measuring the light beam power with an optical power meter. It does not require an array detector and has low cost, and is expected to be widely promoted and applied.

[0017] (2) The light beam orbital angular momentum spectrum measurement device based on power measurement disclosed in the present invention has a simple and stable structure and can be applied to various application scenarios. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The working principle of a method for measuring the orbital angular momentum spectrum of a light beam based on power measurement of the present invention;

[0019] Figure 2 The present invention is a device for measuring the orbital angular momentum spectrum of a light beam based on power measurement, wherein: 1-laser, 2-beam expander, 3-fork grating, 4-aperture, 5-depolarizing 50-beam splitter prism, 6-focusing lens, 7-optical power meter, 8-host;

[0020] FIG3(a) shows the optical power of the 10th to -10th order reference single-mode OAM beam corresponding to the angular quantum number l of the single-mode OAM beam to be measured when the OAM spectrum of single-mode OAM beams of different orders is measured by using a method and device for measuring the orbital angular momentum spectrum of a light beam based on power measurement of the present invention, the optical power of the single-mode OAM beam to be measured, and the optical power of the single-mode OAM beam to be measured after the single-mode OAM beam to be measured interferes with the 10th to -10th order reference single-mode OAM beam respectively;

[0021] FIG3( b ) is a simulation analysis result and an experimental result of measuring the OAM spectra of single-mode OAM beams of different orders using a method and device for measuring the orbital angular momentum spectrum of a beam based on power measurement of the present invention;

[0022] FIG. 4( a ) and FIG. 4( b ) are simulation analysis results and experimental results of measuring the OAM spectrum of a multi-OAM mode mixed light beam using a light beam orbital angular momentum spectrum measurement method and device based on power measurement of the present invention. DETAILED DESCRIPTION

[0023] The present invention is described in detail below in conjunction with the accompanying drawings and embodiments.

[0024] The present invention provides a method for measuring the orbital angular momentum spectrum of a light beam based on power measurement, such as Figure 1 As shown. Let the complex amplitude distribution of the measured beam be E m It is expressed as:

[0025]

[0026] Where r is the radial coordinate, is the angular coordinate, z is the axial coordinate, l is the angular quantum number, a l is the amplitude distribution, is the additional phase of the l-order OAM component.

[0027] The optical power P of the light beam to be measured m for:

[0028]

[0029] The complex amplitude distribution E of the reference single-mode vortex light of order l0 r1 It can be expressed as:

[0030]

[0031] Among them, b l0 is the amplitude of the reference single-mode vortex beam, and the optical power P of the reference single-mode vortex beam with order l0 is r for:

[0032]

[0033] Reference single-mode OAM beam E of order l0 r1 With the measured beam E m The optical power after interference is P icos for:

[0034]

[0035] Then introduce a reference single-mode OAM beam E of order l0 r2 , which is compared with the reference single-mode OAM beam E of order l0 r1 With an additional phase delay of π / 2, E r2 It can be expressed as:

[0036]

[0037] The reference single-mode vortex light E r2 With the measured beam E m The optical power after interference is P isin for:

[0038]

[0039] When the reference beam and the measured beam have the same radial distribution, that is, When the OAM mode intensity of the measured beam order is l0, for:

[0040]

[0041] According to the above method, the reference single-mode OAM beams of different orders are interfered with the measured beam in turn, and the intensity of the measured beam at different orders is repeatedly measured to obtain the intensity ratio of each OAM component of the measured beam, that is, the OAM spectrum of the measured beam {C l}.

[0042] It should be noted that in the actual measurement process, by replacing Figure 2 The fork gratings with different transmittance distribution functions in the device shown can obtain E corresponding to different OAM orders in turn. r1 With E r2 .

[0043] The present invention provides a device for measuring the orbital angular momentum spectrum of a light beam based on power measurement, such as Figure 2 As shown, it is equipped with a laser, a beam expander, a fork grating, an aperture, a depolarizing 50-50 beam splitter prism, a focusing lens, an optical power meter, and a host, wherein: the laser is used to generate a fundamental mode Gaussian beam; the beam expander is placed in the laser optical path behind the laser, and is used to adjust the spot diameter of the beam to match the spot size of the beam to be measured; the fork grating is placed in the laser optical path behind the beam expander, and is used to convert the Gaussian beam into reference single-mode OAM beams of different orders; the aperture is placed in the laser optical path behind the fork grating, and is used to filter out the first-order diffraction light of the fork grating, The required reference single-mode OAM light beam is obtained; the depolarizing 50-50 beam splitter prism is placed in the laser light path behind the aperture, and is used to combine and interfere the reference single-mode OAM light beam with the light beam to be measured; the focusing lens is placed in the laser light path behind the depolarizing 50-50 beam splitter prism, and is used to focus the outgoing light beam after interference; the optical power meter is placed in the laser light path behind the focusing lens, and is used to measure the optical power of the outgoing light beam after interference; the host is used to analyze the optical powers of the light beam to be measured, the reference light beam and the light beam after interference measured by the optical power meter, and calculate the OAM spectrum of the light beam to be measured.

[0044] The following briefly introduces a method and device for measuring the orbital angular momentum spectrum of a light beam based on power measurement in combination with two embodiments of the present invention.

[0045] Example 1: Measuring OAM spectra of single-mode OAM beams of different orders

[0046] In this embodiment, based on the method and device for measuring the orbital angular momentum spectrum of a light beam based on power measurement of the present invention, the OAM intensity spectra of single-mode OAM light beams with angular quantum numbers of -10 to +10 are analyzed and measured in turn. FIG3(a) shows the optical power of the 10th to -10th order reference single-mode OAM light beam corresponding to the angular quantum number l = -7 of the single-mode OAM light beam to be measured, the optical power of the single-mode OAM light beam to be measured, and the optical power of the light beam after the single-mode OAM light beam to be measured interferes with the 10th to -10th order reference single-mode OAM light beam. FIG3(b) shows the OAM intensity spectra corresponding to single-mode OAM light beams of different orders obtained by experiments and numerical simulations. It can be seen that the experimental results are consistent with the simulation results.

[0047] Example 2: Measuring the OAM spectrum of a mixed beam with multiple OAM modes

[0048] In this embodiment, two beams of light to be measured with different OAM spectra are randomly generated, and then their OAM spectra are measured based on the method and device for measuring the orbital angular momentum spectrum of light beams based on power measurement of the present invention. Figures 4(a) and 4(b) show the OAM spectra of the two beams of light to be measured experimentally and the OAM spectra obtained by numerical simulation. It can be seen that the experimental results are almost completely consistent with the simulation results.

[0049] The above embodiments show that the method and device for measuring the orbital angular momentum spectrum of a light beam based on power measurement of the present invention have good performance.

[0050] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for measuring the orbital angular momentum spectrum of a light beam based on power measurement, characterized in that: For the orbital angular momentum order l0 that needs to be measured in the light beam to be measured, two single-mode orbital angular momentum beams with the same optical power but a phase difference of π / 2 are introduced as reference beams, and they are respectively coaxially interfered with the light beam to be measured. The optical power value of the reference beam, the optical power value of the light beam to be measured, and the optical power value of the light beam obtained after the light beam to be measured interferes with the two reference beams are measured by an optical power meter. The relative intensity information of the orbital angular momentum mode of order l0 contained in the light beam to be measured can be deduced. The above process is repeated to make reference single-mode orbital angular momentum beams of different orders incident in turn, and the relative intensity information of each orbital angular momentum mode in the light beam to be measured is calculated in turn, thereby obtaining the orbital angular momentum spectrum of the light beam to be measured.

2. According to the method of claim 1, let the two reference single-mode orbital angular momentum beams with the same intensity and angular quantum number be 10 and 10 respectively. r1 and E r2 , where E r2 Compared to E r1 With an additional phase delay of π / 2, if the power of the beam to be measured measured by the optical power meter is P m , with reference to the single-mode orbital angular momentum beam E r1 and E r2 The optical power is P r , the measured beam and E r1 The optical power after interference is P icos , the measured beam and E r2 The optical power after interference is P isin , then the orbital angular momentum mode intensity C of order l0 in the measured beam is l0 for: According to the above method, the reference single-mode orbital angular momentum beams of different orders are interfered with the beam to be measured in turn, and the intensity of the beam to be measured at different orders is repeatedly measured. The intensity ratio of each orbital angular momentum component of the beam to be measured can be obtained, that is, the orbital angular momentum spectrum of the beam to be measured {C l }.

3. A device for measuring the orbital angular momentum spectrum of a light beam based on power measurement, characterized in that: It is equipped with laser, beam expander, fork grating, aperture, depolarizing 50-50 beam splitter prism, focusing lens, optical power meter and host, including: The laser is used to generate a fundamental mode Gaussian beam; The beam expander is placed in the laser light path behind the laser and is used to adjust the spot diameter of the light beam to match the spot size of the light beam to be measured; The fork-shaped grating is placed in the laser light path behind the beam expander and is used to convert the Gaussian beam into reference single-mode orbital angular momentum beams of different orders; The aperture is placed in the laser light path behind the fork-shaped grating, and is used to filter out the first-order diffraction light of the fork-shaped grating to obtain the required reference single-mode orbital angular momentum beam; The depolarizing 50-50 beam splitter prism is placed in the laser light path behind the aperture, and is used to combine and interfere the reference single-mode orbital angular momentum beam with the beam to be measured; The focusing lens is placed in the laser light path behind the depolarized 50-50 beam splitter prism, and is used to focus the outgoing light beam after interference; The optical power meter is placed in the laser light path behind the focusing lens and is used to measure the optical power of the outgoing light beam after interference; The host is used to analyze the optical powers of the light beam to be measured, the reference light beam and the light beam after interference measured by the optical power meter, and calculate the orbital angular momentum spectrum of the light beam to be measured.

Images