A method for controlling the orbital angular momentum of a random light beam using non-uniform perturbations

Abstract

The present invention discloses a method for controlling the angular momentum of a random light beam orbit by using non-uniform disturbance. The method comprises: a laser generates a stable laser beam, and the size of the beam spot is adjusted so that it can cover a holographic image; a digital hologram with non-uniform random vortices is loaded on a spatial light modulator; a hologram with non-uniform random disturbance is generated by changing a random function; and the angular momentum value of the low-coherence light beam orbit can be controlled by changing the angle parameter in the random function matrix.

Description

Technical Field

[0001] The present invention provides a method for controlling the orbital angular momentum of a random light beam using non-uniform disturbances. The method can regulate and measure the orbital angular momentum of a vortex light beam under a non-uniform disturbance environment, and can be applied to the fields of sensing atmospheric turbulence, free space communication systems, channel multiplexing in optical communications, and optical detection. Background Art

[0002] An optical vortex is a spirally distributed wave with a phase singularity, and its light intensity distribution diagram is a lifebuoy-shaped hollow ring structure. An optical vortex has a spiral phase-distorted wavefront, which is represented by the exponential term exp(ilθ), where θ is the rotation azimuth, l represents the topological charge, and the counterclockwise torsional topological charge is integer, while the clockwise torsional topological charge is negative. Vortex beams that have been widely studied include Laguerre-Gaussian beams, high-order Bessel beams, and hypergeometric Gaussian beams.

[0003] Controlling and measuring the orbital angular momentum of optical vortices is of great significance to the application of vortex beams. The control and measurement of fully coherent vortex beams are already very complete. The measurement and control of the orbital angular momentum of partially coherent vortex beams (PCVB) are still insufficient. PCVB is a low-coherence beam with severe spiral phase deformation. This makes the control and measurement scheme based on the wavefront phase ineffective. In addition, the intensity distribution of PCVB also loses the distribution characteristics of phase singularities. Therefore, it is also difficult to identify the orbital angular momentum information of PCVB beams based on the intensity distribution characteristics.

[0004] In previous technologies, the research on PCVB mainly considers isotropic coherence destruction, and there is a lack of research on PCVB with non-uniform destruction in all directions. Since non-uniform coherence destruction is closer to the real disturbance environment, the measurement and control of non-uniform PCVB is of great significance to improving the accuracy of vortex sensing in atmospheric turbulence, realizing free space communication systems, OAM channel multiplexing in optical communications, and optical detection.

[0005] There are two main solutions for achieving non-uniform control of vortex beams in the prior art. The first one, the literature (Alperin SN, Niederriter RD, Gopinath JT, et al. Quantitative measurement of the orbital angular momentum of light with a single, stationary lens [J]. Optics Letters, 2016, 41 (21): 5019-5022.) proposed: using a tilted cylindrical lens to generate non-uniform distorted light, and then identifying the topological number of the beam through the singularity features in the light intensity distribution. This method is simple in device and can also be applied to the measurement of fractional topological charge, but it cannot be used for low-coherence vortex beams. The second method is proposed in the literature (Chen J, Liu X, Yu J, et al. Simultaneous determination of the sign and the magnitude of the topological charge of a partially coherent vortex beam [J]. Applied Physics B, 2016, 122 (7): 1-12.): using two vertically placed cylindrical lenses to asymmetrically control the PCVB light intensity, and then obtaining its topological charge value by observing the distribution characteristics of the beam spatial correlation function. This method can control and measure the orbital angular momentum of PCVB, but the device is complex, and the processed PCVB beam is isotropically coherently destroyed, and the spatial distribution function of the PCVB beam cannot be controlled. Summary of the invention

[0006] The present invention aims to overcome the shortcomings of the prior art and provide a method for controlling the orbital angular momentum of a random light beam using non-uniform perturbations, thereby realizing the regulation of the orbital angular momentum of the light beam under non-uniform coherence destruction, and making the measurement and regulation of the orbital angular momentum of non-uniform perturbations vortex light feasible.

[0007] To achieve the above-mentioned purpose, the technical solution adopted by the present invention provides a method for controlling the orbital angular momentum of a random light beam by using non-uniform disturbance, and the control steps are as follows: a laser generates a stable laser beam, a computer is used to prepare a hologram of a non-uniform random vortex light beam, and the laser beam spot size is adjusted so that it can completely cover the hologram on the spatial light modulator; a pinhole aperture is used to select the first-order diffraction light reflected by the spatial light modulator; the first-order diffraction light is incident on a cylindrical lens at an angle of 45° to the horizontal plane; the light beam passing through the cylindrical lens is incident on a camera, and after taking a photo and recording, the angular momentum value of the light beam orbit is calculated; a computer is used to continuously change the angle parameters in the random function matrix in the digital hologram placed in the spatial light modulator, and a camera is used to record the average light intensity distribution under the corresponding angle parameters; a quantitative relationship between the angle parameters and the numerical value of the orbital angular momentum of the light beam can be obtained through calculation, thereby realizing the measurement and control of the orbital angular momentum;

[0008] The spatial correlation function of the non-uniform random vortex light is:

[0009]

[0010] Where W(r1, r2) is the cross-spectral density function of spatial points r1 and r2, r0 is the position of the disturbance point on the optical axis cross section, P(r0) is the probability density function, r1 and r2 are any two spatial positions on the optical axis cross section, and U is the light field wave function. is the conjugate of the light field wave function; the spatial light field generated by the hologram takes r0 as the disturbance center; the probability density function P(r0) of the disturbance center is a multivariate normal random function in the xy plane, and its variance matrix is

[0011]

[0012] where c x is the root mean square of the disturbance range in the x direction, c y is the root mean square of the disturbance range in the y direction, rho is the disturbance correlation in the x and y directions, and the variance matrix sigma satisfies the semi-positive definite condition.

[0013] The perturbation parameter of the variance matrix sigma is By changing the angle parameter in the random function matrix The orbital angular momentum value of the non-uniform random vortex light can be periodically and quantitatively changed;

[0014] Due to the application of the above technical solution, the method for controlling the orbital angular momentum of a random light beam by using non-uniform disturbance provided by the present invention has the following advantages:

[0015] 1. The present invention proposes a method for controlling the orbital angular momentum of a random light beam using non-uniform perturbations.

[0016] 2. The optical path of the device adopted in the present invention can digitally control disturbances in different directions, and the orbital angular momentum can be accurately measured and easily controlled; the simple optical path structure improves the accuracy of the experiment, and the orbital angular momentum information can be directly obtained through the light intensity, which has application value in the fields of optical communication and atmospheric turbulence sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the structure of a device for controlling the orbital angular momentum of a random light beam using non-uniform disturbances provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] The present invention will be further described in detail below in conjunction with examples. The following examples 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 examples.

[0019] like Figure 1 As shown, it is a schematic diagram of the structure of a device for controlling the orbital angular momentum of a random light beam using non-uniform disturbance provided in this embodiment: it includes a laser 1; a spatial light modulator 2; a computer 3; a pinhole aperture 4; a cylindrical lens 5; a camera 6; and a computer 7;

[0020] In this embodiment, a hologram of a non-uniform random vortex beam is prepared in a computer 3; by changing the angle parameter contained in the sigma matrix in the random function P(r0) to produce a hologram with non-uniform random disturbance; connect the computer 3 to the spatial light modulator 2; turn on the laser 1 to produce a stable laser beam; adjust the spot size of the laser 1 beam so that it can completely cover the hologram of the spatial light modulator 2; use the pinhole aperture 4 to select the first-order diffraction light in the reflected beam of the spatial light modulator 2; make the first-order diffraction light pass through the cylindrical lens 5 at an angle of 45° to the horizontal plane; use the camera 6 to capture the average light intensity distribution after passing through the cylindrical lens 5; calculate the orbital angular momentum through the computer 7; change the angle parameters in the hologram in the spatial light modulator 2 Use camera 6 to record The computer 7 is used to calculate the orbital angular momentum at each angle, and finally the angle parameter can be measured. A quantitative relationship with the value of the angular momentum of the light beam's orbit; through this relationship, the value of the angular momentum of the low-coherence light beam's orbit can be controlled.

Claims

1. A method for controlling the orbital angular momentum of a low-coherence light beam using non-uniform perturbations, characterized in that: The laser beam is injected into the spatial light modulator, and the hologram loaded by the spatial light modulator generates non-uniform random vortex light; the spatial correlation function of the non-uniform random vortex light is: Where W(r1, r2) is the cross-spectral density function of spatial points r1 and r2, r0 is the position of the disturbance point on the optical axis cross section, P(r0) is the probability density function, r1 and r2 are any two spatial positions on the optical axis cross section, and U is the light field wave function. is the conjugate of the light field wave function; the spatial light field generated by the hologram takes r0 as the disturbance center; the probability density function P(r0) of the disturbance center is a multivariate normal random function in the xy plane, and its variance matrix is where c x is the root mean square of the disturbance range in the x direction, c y is the root mean square of the disturbance range in the y direction, rho is the disturbance correlation in the x direction and the y direction, and the variance matrix sigma satisfies the semi-positive definite condition; the disturbance parameter of the variance matrix sigma is By changing the angle parameter in the variance matrix sigma The orbital angular momentum value of the non-uniform random vortex light can be periodically and quantitatively changed; each The average light intensity distribution under the value, the angle parameters can be measured by calculation A quantitative relationship with the value of the angular momentum of the light beam's orbit; through this relationship, the value of the angular momentum of the low-coherence light beam's orbit can be controlled.

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