A method for measuring the topological charge of fractional-order correlated vortex beams

Abstract

The invention discloses a method for measuring the topological charge of a fractional-order correlated vortex beam. The method comprises: a laser generates fractional-order correlated vortex light, and the size of the beam spot is adjusted so that it can cover a holographic phase plate; the cross spectral density function (CCF) of the fractional-order correlated vortex light is measured, and the position of the fractional topological charge (FTC) gap on the cross section of the correlated vortex light to be measured in the function space is observed; a digital holographic phase plate is loaded on a spatial light modulator and its azimuth is adjusted so that the CCF gap generated by the digital holographic phase plate coincides with the FTC gap of the correlated vortex to be measured; a pinhole aperture is used to select the first-order diffraction light reflected by the spatial light modulator; the phase difference of the digital holographic phase plate is changed; the overlap of the FTC gap in the CCF of the light field to be measured and the gap of the phase plate is presented as: gap appears-gap disappears-gap appears; the phase difference corresponding to these three phenomena is used to calculate the decimal part of the FTC, and the final value of the fractional topological charge of the correlated vortex light is obtained in combination with the integer topological charge part.

Description

Technical Field

[0001] The present invention provides a method for measuring the topological charge of a fractional-order correlated vortex beam. The device can measure the fractional part of the topological charge value in a fractional-order correlated vortex beam and can be applied to the fields of optical image coding, optical communication, particle manipulation, optical trapping and optical detection. Background Art

[0002] An optical vortex is a spiral distribution wave with a phase singularity, and a dark area appears in the middle of the intensity distribution. The spiral phase distorts the wavefront, which is represented by the exponential term exp(ilθ), where θ is the rotation azimuth and l is the topological charge. Under the condition of near-axial propagation, each photon in the optical vortex has The topological charge of represents Planck's constant. Laguerre-Gaussian beams, higher-order Bessel beams, and hypergeometric Gaussian beams are some well-known vortex beams.

[0003] The vortex state of the vortex beam can be considered as fractional order based on the integer value, where the fractional vortex state is the superposition of the integer vortex beam state. Fractional vortex light has a spiral phase structure, zero central light intensity, a unique radial gap in the spot, and a certain topological charge (Orbital Angular Momentum, abbreviated as OAM). These characteristics make fractional vortex light have broad application prospects in nonlinear optics, biomedicine, optical communications and other fields.

[0004] Correlated vortex light is a random light field formed by the light vortex passing through atmospheric turbulence. This light source is closer to the light source in the real environment, and its application in actual production and life is of great significance. We call this random light beam partially coherent vortex light (PCVB). Fractional-order partially coherent vortex light, because it contains a fractional part of the topological charge, greatly increases the carrying capacity of vortex light information, and is a new direction for optical image coding and optical communication.

[0005] At present, there are several methods for measuring fractional topological charge (FTC). For example, the interference intensity diagram between the vortex beam and its conjugate beam is obtained by cascading Mach-Zehnder interferometers to measure FTC; another method is to use the dynamic angle double slit method to measure the FTC of the vortex beam. The optical paths used in these methods are relatively complex, which is difficult to implement in practical applications, and most of them are only applicable to a specific vortex beam.

[0006] The literature Hosseini-Saber MA, Akhlaghi EA, Saber A. Diffractometry based vortex beams fractional topological charge measurement [J]. Optics Letters, 2020, 45 (13): 3478-3481. reports a method for measuring the topological charge of a fractional vortex beam. This method uses a phase plate with a certain thickness to measure the fractional part of the topological charge of a fractional vortex beam by controlling its tilt angle. However, this method relies on the diffraction intensity distribution characteristics of the vortex beam and can only measure the topological charge of a fully coherent vortex light. In a disturbed environment, the coherence of the light field decreases and the structural features of the intensity diffraction pattern disappear. Therefore, this method cannot measure the fractional part of the topological charge of the fractional-order correlated vortex light. In addition, the tilt angle and rotation angle of the phase plate used in this technology are easily affected by environmental disturbances, making it difficult to achieve quantitative control, and are prone to large experimental errors, affecting the measurement accuracy. The above are the main problems that have not yet been solved in the measurement of the topological charge of fractional vortex light. Summary of the invention

[0007] The present invention aims to overcome the shortcomings of the prior art and provide a method for measuring the topological charge of a fractional-order correlated vortex light beam. The device provided by the present invention writes a phase plate into a hologram, and quantitatively controls the spatial distribution of the correlation function of the outgoing light field by changing the azimuth angle and phase difference of the hologram. The topological charge of the fractional-order correlated vortex light is measured by using the distribution characteristics of the second-order correlation function of the light field.

[0008] The technical solution to achieve the purpose of the present invention is to provide a method for measuring the topological charge of a fractional-order correlated vortex beam, comprising the following steps:

[0009] (1) The laser generates fractional-order correlated vortex light, and the beam spot size is adjusted so that it can completely cover the holographic phase plate; the cross-spectral density function (CCF) of the fractional-order correlated vortex light field is measured, and the position of the FTC notch on the cross section of the beam to be measured in the function space is observed; the digital holographic phase plate is loaded on the spatial light modulator and its azimuth is adjusted so that the CCF notch generated by the digital holographic phase plate coincides with the FTC notch of the PCVB;

[0010] (2) Use a pinhole aperture to select the first-order diffraction light reflected by the spatial light modulator; change the phase difference of the digital holographic phase plate; make the overlap of the FTC notch and the phase plate notch in the measured light field CCF show the following changes: notch appears - notch disappears - notch reappears, and record the phase difference loaded by the digital holographic phase plate when these three changes occur, recorded as phi1, phi2, phi3; use the formula:

[0011]

[0012] The fractional part l′ of FTC is calculated; combined with the integer topological charge part, it is the final value of the fractional topological charge of the associated vortex light.

[0013] The beneficial effects of the present invention are:

[0014] 1. The present invention proposes a method for measuring the topological charge of a fractional-order correlated vortex beam.

[0015] 2. The optical path of the device used in the present invention can be digitally controlled, and the phase adjustment can be quantitatively controlled. The optical path structure is simple, which reduces the experimental error caused by the rotating phase plate, improves the accuracy and feasibility of the experiment, and has application value in the fields of optical communication and imaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a schematic diagram of the structure of a device for measuring fractional-order correlated vortex optical topological charge provided in an embodiment of the present invention.

[0017] Figure 2 It is a holographic phase plate image rotated at a certain azimuth angle. DETAILED DESCRIPTION

[0018] The present invention will be further described in detail below in conjunction with the embodiments. The following embodiments are explanations of the present invention and are also preferred application forms of the present invention, but the present invention is not limited to the following embodiments.

[0019] like Figure 1 As shown, it is a schematic diagram of the structure of a device for measuring the topological charge of a fractional-order correlated vortex beam provided in this embodiment: it includes a laser 1; a spatial light modulator 2; a computer 3; a pinhole diaphragm 4; a continuous-shooting camera 5; and a computer 6;

[0020] like Figure 2 As shown, it is a schematic diagram of the digital holographic phase plate provided in this embodiment, part a contains phase information, part b does not contain phase information, and the phase plate can be rotated by an angle of azimuth.

[0021] In this embodiment, holographic phase plate images of different azimuth angles are prepared in computer 3; computer 3 is connected to spatial light modulator 2; laser 1 is turned on to generate fractional-order correlated vortex light; the spot size of the laser beam 1 is adjusted so that it can completely cover the holographic phase plate of spatial light modulator 2; the azimuth angle of the holographic phase plate on spatial light modulator 2 is adjusted by computer 3, and the CCF gap generated by the digital holographic phase plate and the FTC gap of PCVB are observed in CCF space; the azimuth angle of the holographic phase plate is adjusted until the CCF gap generated by the digital holographic phase plate coincides with the FTC gap generated by PCVB; the pinhole aperture 4 is used to select the first-order diffraction light in the reflected light beam of spatial light modulator 2; The computer 6 controls the continuous shooting camera 5 to take pictures to obtain the image after the gap overlaps, and records the azimuth angle of the holographic phase plate at this time; changes the phase difference of the holographic phase plate under the azimuth angle, and the diffraction pattern in the CCF space also changes, and records the CCF pattern under the phase difference; repeats the above-mentioned photographing steps, and saves the CCF patterns under different phase differences; as the phase difference changes, the overlap of the FTC gap and the phase plate gap in the CCF pattern of the light field to be measured presents the following changes: gap appears-gap disappears-gap reappears; records the phase difference corresponding to the photo of this phenomenon, and calculates the decimal part l′ of FTC by formula (1.1); combined with the integer topological charge part, it is the final value of the fractional topological charge of the associated vortex light.

Claims

1. A method for measuring the topological charge of a fractional-order correlated vortex beam, comprising a laser (1), a spatial light modulator (2), a computer (3), a pinhole aperture (4), a continuous-shooting camera (5), and a computer (6); characterized in that: (A) preparing a holographic phase plate image for generating different azimuth angles in a computer (3); connecting the computer (3) to a spatial light modulator (2), and loading the prepared hologram into the spatial light modulator (2); Turning on the laser (1) to generate fractional-order correlated vortex light, adjusting the spot size of the laser (1) output light beam so that it can completely cover the holographic phase plate of the spatial light modulator (2); adjusting the azimuth angle of the holographic phase plate on the spatial light modulator (2) by means of the computer (3) so that the cross-density spectrum function (CCF) gap generated by the digital holographic phase plate coincides with the fractional topological charge (FTC) gap of the partially coherent vortex light (PCVB); (B) using a pinhole aperture (4) to select first-order diffraction light from the reflected light of the spatial light modulator (2); the first-order diffraction light reflected by the spatial light modulator (2) is incident on a continuous shooting camera (5), and the continuous shooting camera (5) is controlled by the computer (6) to take a picture to obtain an image after the gaps overlap; and the azimuth angle of the holographic phase plate at this time is recorded; replacing a phase plate hologram with a different phase difference in the computer (3) to generate a diffraction pattern with a different phase, repeating the above-mentioned photographing steps, and saving the picture; (C) Change the phase difference of the digital holographic phase plate; make the overlapping part of the FTC gap and the phase plate gap in the measured light field CCF show the following changes: gap appears - gap disappears - gap reappears; record the phase difference loaded by the digital holographic phase plate when these three changes occur, and calculate the decimal part of FTC; combined with the integer topological charge part, it is the final value of the fractional topological charge of the associated vortex light.

2. The method for measuring the topological charge of a fractional-order correlated vortex beam according to claim 1, characterized in that: The computer (3) in the device controls the digital holographic phase plate generated by the spatial light modulator (2), and uses the computer (3) to adjust the azimuth angle of the holographic plate so that the CCF gap generated by the holographic phase plate coincides with the FTC gap of the PCVB, and then replaces the holographic plate with a different phase difference at the azimuth angle to take pictures.

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