Method and apparatus for measuring topological charge of partially coherent fractional vortex beam

An apparatus for measuring a topological charge of a partially coherent fractional vortex beam, comprising:

• a light source, configured to emit a partially coherent beam;
• a spatial light modulator, configured to generate a partially coherent fractional vortex beam, that is, a to-be-measured fractional vortex beam;
• a displacement platform, configured to:
• move a scattering sample, and perform a horizontal and vertical movement overlap scan;
• a detector, placed in diffraction area, and configured to acquire diffraction light intensity;
• a non-transitory computer-readable medium that comprises a computer program, the computer program being executed by a processor to implement the steps of a method for measuring a topological charge of a partially coherent fractional vortex beam;
• and
• a rotating frosted glass sheet,
• wherein the method comprises:
• irradiating the to-be-measured fractional vortex beam to a scattering sample, performing a horizontal and vertical movement overlap scan, and acquiring diffraction light intensity by using a detector placed in a diffraction area;
• calculating a to-be-measured electric field and performing iterative update by using a multimode stacked diffraction algorithm to minimize an error between the diffraction light intensity acquired by the detector and diffraction light intensity calculated according to the to-be-measured electric field, to obtain a target to-be-measured electric field of the to-be-measured fractional vortex beam;
• reconstructing a cross-spectral density function of the to-be-measured fractional vortex beam according to the target to-be-measured electric field, and obtaining optical field information of the to-be-measured fractional vortex beam;
• performing reverse transmission calculation according to the cross-spectral density function and the optical field information of the to-be-measured fractional vortex beam to obtain a source field vortex phase distribution diagram, and observing a magnitude and a sign of a topological charge of the to-be-measured fractional vortex beam according to the source field vortex phase distribution diagram;
• emitting a beam to the rotating frosted glass sheet;
• collimating the beam by a lens after diffusion by the rotating frosted glass sheet;
• changing a degree of coherence of the beam by a size of a spot irradiated on the rotating frosted glass sheet;
• passing the beam through the spatial light modulator to generate the partially coherent fractional vortex beam, wherein a degree of coherence of the generated partially coherent light field is large when the spot irradiated on the rotating frosted glass sheet is small and the beam on a rear surface of the spatial light modulator is a measurement object;
• focusing the partially coherent fractional vortex beam and irradiating on a resolution board, after being transmitted by a distance;
• placing the resolution board on a displacement platform, wherein a displacement distance is controlled by using a MATLAB software, the displacement platform is displaced by 10 steps in both an x direction and a y direction, with a step size of 20 μm, and a size of a spot on the resolution board is approximately 150 um;
• acquiring a diffraction light intensity at 13 cm away from the resolution board by a camera;
• obtaining main modes of the partially coherent fractional vortex beam after the acquired diffraction light intensity is introduced into a multi-mode stacked diffraction imaging;
• retransmitting the beam to a plane of the spatial light modulator in the main modes;
• and
• obtaining the topological charge by examining phase discontinuity.