CN106932107A - A kind of topological charge measurement apparatus based on far field construction principle - Google Patents

Abstract

A topological charge measurement device based on the principle of far-field diffraction belongs to the field of information optics technology, and solves the problem of complex detection optical paths of the existing topological charge measurement device. Said device: the continuous laser light enters the beam splitter sequentially through the polarizer, the first beam focusing lens and the beam expanding lens. The beam splitter splits the incident laser light into two beams, one of which is reflected out of the device and the other of which is transmitted into the spatial light modulator. A spatial light modulator converts incident laser light into a vortex beam. The converted laser light is split into two beams by a beam splitter through which one beam is transmitted out of the device and the other beam is reflected by it to an aperture. Between the aperture and the photodetector, a blocking object and a second focusing lens are sequentially arranged. Far-field diffraction occurs to the laser beam incident on the shield. The occluder is opaque, and the part that coincides with the incident laser light is fan-shaped, with a central angle of 30°, and its apex is located on the central axis of the incident laser light. The invention is suitable for measuring the topological charge of the vortex beam.

Description

A topological charge measurement device based on the principle of far-field diffraction

technical field

The invention relates to a topological charge measuring device, which belongs to the technical field of information optics.

Background technique

Existing topological charge measurement devices for vortex beams are usually based on the principle of interference or anti-vortex demodulation. These two types of topological charge measurement devices require many types of optical devices, a large number, complex detection optical paths, and high costs. To a large extent, the practical application of orbital angular momentum detection technology is limited.

Contents of the invention

The invention proposes a topological charge measurement device based on the far-field diffraction principle in order to solve the problem of complex detection optical paths of the existing topological charge measurement device.

The topological charge measurement device based on the far-field diffraction principle of the present invention includes a continuous laser, a polarizer 1, a first beam focusing lens 2, a beam expander lens 3, a beam splitter 4, a spatial light modulator 5, an aperture 6, The shield 7, the second beam focusing lens 8 and the photodetector 9; the laser light emitted by the continuous laser is filtered by the polarizer 1, the beam of the first beam focusing lens 2 and the beam expansion of the beam expander lens 3 are injected into the branch in sequence. A beam splitter 4, the beam splitter 4 divides the incident laser light into two beams, one beam of laser light is reflected from the measuring device by the beam splitter 4, and the other beam of laser light is transmitted into the spatial light modulator 5 through the beam splitter 4;

The spatial light modulator 5 is used to modulate the phase of the incident laser light, converting it from a Gaussian beam to a vortex beam;

The laser beam converted by the spatial light modulator 5 returns to the beam splitter 4 in the original path, and is divided into two beams by the beam splitter 4. One beam of laser light is transmitted out of the measuring device through the beam splitter 4, and the other beam of laser light is transmitted by the beam splitter 4. The beam splitter 4 reflects to the diaphragm 6;

After the reflected laser light from the beam splitter 4 is filtered by the aperture 6 and blocked by the shield 7 in sequence, it undergoes far-field diffraction, and after being condensed by the second condensing lens 8, it enters into the photodetector 9 at the focal point. on the focal plane;

The shield 7 is opaque, and its part overlapping with the incident laser light is fan-shaped, the central angle of the fan is 30°, and its apex is located on the central axis of the incident laser light.

Preferably, the polarizer 1 is a Glan Taylor prism.

Preferably, the focal length of the first focusing lens 2 is 3 cm, and the focal length of the beam expanding lens 3 is 15 cm.

Further, the focal length of the second focusing lens 8 is 40 cm.

Preferably, the spatial light modulator 5 is a reflective spatial light modulator.

Preferably, the photodetector 9 is a CCD detector.

Preferably, the continuous laser is a 632nm helium-neon laser.

Preferably, the measuring device further includes a positive L-order spiral phase plate 10, L is a positive integer and less than or equal to 5; the positive L-order spiral phase plate 10 is arranged between the diaphragm 6 and the shield 7 on the light path.

The spatial light modulator 5 is pre-written with a topological charge beam phase hologram, and the aperture 6 is used to filter the high-order diffraction components in the vortex beam, thereby purifying the quality of the vortex beam.

After the vortex beam passes through the obstruction 7, its amplitude changes, and is transmitted according to far-field diffraction, and imaged at infinity.

The second focusing lens 8 is used to image the vortex beam transmitted by far-field diffraction on its focal plane and receive it through the photodetector 9. The number of bright spots of the image in the photodetector 9 is the topological charge of the vortex beam .

The topological charge measurement device of the present invention modulates the phase of the laser light to be measured by a spatial light modulator to convert it into a vortex beam to be measured, and sets an obstruction on the optical path of the vortex beam to make the vortex beam Far-field diffraction occurs. Compared with the existing topological charge measurement device based on the principle of interference or anti-vortex demodulation, the topological charge measurement device of the present invention requires fewer types and fewer optical devices, the detection optical path is relatively simple, and the cost is also low , can effectively promote the practical application of orbital angular momentum detection technology.

Description of drawings

In the following, the topological charge measurement device based on the principle of far-field diffraction according to the present invention will be described in more detail based on the embodiments and with reference to the accompanying drawings, wherein:

Fig. 1 is the optical path schematic diagram of the topological charge measuring device based on the principle of far-field diffraction described in Embodiment 1, wherein, 11 is a continuous laser;

Fig. 2 is a schematic diagram of the fan-shaped part where the occluder and the vortex beam overlap mentioned in the first embodiment;

Fig. 3 is a schematic diagram of the optical path of the topological charge measurement device based on the far-field diffraction principle described in the eighth embodiment.

In the figures, the same parts are given the same reference numerals. The drawings are not to scale.

detailed description

The topological charge measurement device based on the principle of far-field diffraction according to the present invention will be further described below in conjunction with the accompanying drawings.

Embodiment 1: This embodiment will be described in detail below with reference to FIG. 1 and FIG. 2 .

The topological charge measurement device based on the far-field diffraction principle described in this embodiment includes a continuous laser, a polarizer 1, a first beam focusing lens 2, a beam expander lens 3, a beam splitter 4, a spatial light modulator 5, and an aperture 6 , barrier 7, second focusing lens 8 and photodetector 9; the laser light emitted by the continuous laser is filtered by the polarizer 1, the beam of the first focusing lens 2 and the beam expansion of the beam expanding lens 3 are injected into A beam splitter 4, the beam splitter 4 divides the incident laser light into two beams, one beam of laser light is reflected from the measuring device by the beam splitter 4, and the other beam of laser light is transmitted into the spatial light modulator 5 through the beam splitter 4;

The spatial light modulator 5 is used to modulate the phase of the incident laser light, converting it from a Gaussian beam to a vortex beam;

The laser beam converted by the spatial light modulator 5 returns to the beam splitter 4 in the original path, and is divided into two beams by the beam splitter 4. One beam of laser light is transmitted out of the measuring device through the beam splitter 4, and the other beam of laser light is transmitted by the beam splitter 4. The beam splitter 4 is reflected to the diaphragm 6; the reflected laser light from the beam splitter 4 is filtered by the diaphragm 6 and shielded by the barrier 7 in sequence, then undergoes far-field diffraction, and is focused by the second focusing lens 8 Into the focal plane of the photodetector 9 at the focal point; the shield 7 is opaque, and the part that coincides with the incident laser is fan-shaped, the central angle of the fan is 30°, and its apex is located on the central axis of the incident laser.

The expression for the vortex beam is:

in, is the Gouy phase; is the Rayleigh length; ω ₀ is the beam waist radius; is the beam radius; is the normalization coefficient; is the associated Laguerre polynomial, l and p are the characteristic quantum numbers of the characterization mode; the radial quantum number p indicates that the number of concentric halos on the beam section is p+1, and the angular vector number l indicates the order of the phase singularity , that is, the phase changes by 2lπ around the phase singularity. The phase factor exp(ilθ) indicates that the beam has a helical structure, i is the imaginary unit, k is the wave vector, r is the radius of the polar coordinates, and z is the transmission distance.

Fig. 2 is a schematic diagram of the fan-shaped part where the occluder and the vortex beam overlap, wherein the white part is the vortex beam, and the black part is the occluder.

The overall amplitude transmittance function of the sector is:

The amplitude variation of the vortex beam is:

The far-field diffraction of the vortex beam is equivalent to performing a Fourier transform, and the transformed amplitude is:

The transformed amplitude is squared to obtain the light intensity distribution in the far field. The simulation diagram is obtained by software simulation, and the number of bright light spots in the simulation diagram is the number of topological charges in the vortex light.

The topological charge measurement device based on the principle of far-field diffraction described in this embodiment is based on the theory that the vortex beam has orbital angular momentum, and uses regular occluders to make the vortex beam undergo far-field diffraction, and through the beamforming lens and CCD The detector captures the far-field diffracted vortex beam, and judges the topological charge of the vortex beam through the number of bright spots of the vortex light image. Compared with the prior art, the topological charge measurement device based on the far-field diffraction principle described in this embodiment can realize the simple detection of the topological charge of the vortex beam, and can more accurately measure the topological charge in the signal light, and is suitable for Quantum communication, laser detection and optical signal detection and other fields.

Embodiment 2: This embodiment further limits the topological charge measurement device based on the principle of far-field diffraction described in Embodiment 1.

In the topological charge measurement device based on the principle of far-field diffraction described in this embodiment, the polarizer 1 is a Glan-Taylor prism.

In this embodiment, a Glan-Taylor prism is used as a polarizer. The Glan-Taylor prism is a birefringent polarizing device made of natural calcite crystal, and its main component is rhombohedral crystal of CaCO ₃ . Input a beam of unpolarized light, you can get a beam of linearly polarized light. Compared with other polarizers, its transmittance and polarization purity are higher. In this embodiment, the GCL-070215 Glan Taylor prism is selected. In this embodiment, a beam splitter of the GCC-401021 model is selected.

Embodiment 3: This embodiment further limits the topological charge measuring device based on the principle of far-field diffraction described in Embodiment 1.

In the topological charge measurement device based on the far-field diffraction principle described in this embodiment, the focal length of the first focusing lens 2 is 3 cm, and the focal length of the beam expanding lens 3 is 15 cm.

In this embodiment, a GCL-010217 lens is used as the first focusing lens, and a GCL-010212 lens is used as the beam expanding lens.

Embodiment 4: This embodiment further limits the topological charge measuring device based on the principle of far-field diffraction described in Embodiment 1.

In the topological charge measurement device based on the far-field diffraction principle described in this embodiment, the spatial light modulator 5 is a reflective spatial light modulator.

In this embodiment, a silicon-based liquid crystal spatial light modulator is selected. The spatial light modulator is a reflective spatial light modulator with a resolution of 600×600. It only changes the phase of light and does not change the intensity and polarization state of light. .

Embodiment 5: This embodiment further limits the topological charge measurement device based on the principle of far-field diffraction described in Embodiment 3.

In the topological charge measurement device based on the principle of far-field diffraction described in this embodiment, the focal length of the second focusing lens 8 is 40 cm.

In this embodiment, a lens of model GCL-010214 is used as the second focusing lens, and a diaphragm of model GCD-5701M is selected.

Embodiment 6: This embodiment further limits the topological charge measurement device based on the principle of far-field diffraction described in Embodiment 1.

In the topological charge measurement device based on the far-field diffraction principle described in this embodiment, the photodetector 9 is a CCD detector.

In this embodiment, a GCI-050104 model CCD detector is used.

Embodiment 7: This embodiment further limits the topological charge measurement device based on the principle of far-field diffraction described in Embodiment 1.

In the topological charge measurement device based on the far-field diffraction principle described in this embodiment, the continuous laser is a 632nm helium-neon laser.

The 632nm He-Ne laser in this embodiment has excellent power stability and frequency stability, and outputs linearly polarized Gaussian light with a wavelength of 632nm, the transverse mode is TEM ₀₀ , and the beam divergence angle is less than 1mrad.

Embodiment 8: This embodiment will be described in detail below in conjunction with FIG. 3 . This embodiment is to further limit the topological charge measurement device based on the principle of far-field diffraction described in any one of embodiments 1 to 7.

The topological charge measurement device based on the principle of far-field diffraction described in this embodiment also includes a positive L-order spiral phase plate 10, L is a positive integer and less than or equal to 5, and the positive L-order spiral phase plate 10 is arranged on the aperture 6 On the light path between the object 7 and the occluder.

The positive L-order spiral phase plate of this embodiment is used to judge whether the topological charge of the vortex beam is positive or negative, and the specific judgment method is:

Step 1, using the topological charge measurement device based on the far-field diffraction principle described in Embodiment 1 to obtain the topological charge of the vortex beam;

Step 2: Set a positive L-order spiral phase plate 10 on the optical path between the diaphragm 6 and the shield 7. When the number of bright spots of the image in the photodetector 9 increases, the topological charge of the vortex beam obtained in step 1 The number is positive, and when the number of bright spots of the image in the photodetector 9 decreases, the topological charge of the vortex beam obtained in step 1 is negative.

The topological charge measurement device based on the far-field diffraction principle described in this embodiment judges whether the topological charge of the vortex beam is positive or negative by adding a positive L-order spiral phase plate, and obtains the orbital angle of the vortex beam more comprehensively. momentum information.

Although the invention is described herein with reference to specific embodiments, it should be understood that these embodiments are merely illustrative of the principles and application of the invention. It is therefore to be understood that numerous modifications may be made to the exemplary embodiments and that other arrangements may be devised without departing from the spirit and scope of the invention as defined by the appended claims. It shall be understood that different dependent claims and features described herein may be combined in a different way than that described in the original claims. It will also be appreciated that features described in connection with individual embodiments can be used in other described embodiments.

Claims (8)

1. A topological charge measuring device based on a far-field diffraction principle is characterized by comprising a continuous laser, a polarizer (1), a first beam-focusing lens (2), a beam-expanding lens (3), a beam splitter (4), a spatial light modulator (5), a diaphragm (6), a shield (7), a second beam-focusing lens (8) and a photoelectric detector (9);

laser emitted by the continuous laser sequentially passes through the filtering of the polarizer (1), the beam converging of the first beam converging lens (2) and the beam expanding of the beam expanding lens (3) and then is emitted into the beam splitter (4), the beam splitter (4) divides the incident laser into two beams, one beam of laser is reflected out of the measuring device by the beam splitter (4), and the other beam of laser is transmitted into the spatial light modulator (5) through the beam splitter (4);

the spatial light modulator (5) is used for modulating the phase of incident laser and converting the incident laser from a Gaussian beam into a vortex beam;

the laser original path converted by the spatial light modulator (5) returns to the beam splitter (4) and is divided into two beams by the beam splitter (4), one beam of laser is transmitted out of the measuring device through the beam splitter (4), and the other beam of laser is reflected to the diaphragm (6) by the beam splitter (4);

reflected laser from the beam splitter (4) is subjected to far-field diffraction after being sequentially filtered by the diaphragm (6) and shielded by the shielding object (7), and is converged by the second beam condensing lens (8) and then is incident on a focal plane of the photoelectric detector (9) at a focal point;

the shade (7) is opaque, the portion of the shade coinciding with the incident laser is a sector, the central angle of the sector is 30 DEG, and the vertex of the sector is located on the central axis of the incident laser.

2. The topological charge measuring device based on the far-field diffraction principle of claim 1, characterized in that the polarizer (1) is a Glan Taylor prism.

3. The far-field diffraction principle-based topological charge measuring device according to claim 1, wherein the focal length of the first condenser lens (2) is 3cm, and the focal length of the expander lens (3) is 15 cm.

4. The topological charge measuring device based on the far-field diffraction principle as claimed in claim 1, characterized in that the spatial light modulator (5) is a reflective spatial light modulator.

5. The topological charge measuring device based on the far-field diffraction principle according to claim 3, characterized in that the focal length of the second condenser lens (8) is 40 cm.

6. The topological charge measuring device based on far-field diffraction principle according to claim 1, characterized in that the photodetector (9) is a CCD detector.

7. The far-field diffraction based topological charge measuring device of claim 1, wherein said continuous laser is a 632nm helium-neon laser.

8. The far-field diffraction principle-based topological charge measuring device according to any one of claims 1 to 7, further comprising a positive L-order spiral phase plate (10), L being a positive integer and less than or equal to 5;

a positive L-order spiral phase plate (10) is arranged on the light path between the diaphragm (6) and the shutter (7).