Method and apparatus for measuring the topological charge of vortex light using fiber laser self-interference

Abstract

This invention provides a method and apparatus for measuring the topological charge of vortex beams using fiber laser self-interference, comprising: splitting a vortex laser into two paths, namely a first beam and a second beam, by a first beam splitter; the first beam, after being focused, is transmitted to a beam splitter prism, where it interferes with the second beam, which is also transmitted to the beam splitter prism; the interfering laser beam, after being combined by the beam splitter prism, is transmitted to a spot acquisition camera; the spot acquisition camera acquires an interference fringe image; and the topological charge of the vortex beam is measured based on the interference fringe image. This invention achieves the acquisition of a far-field spot, the determination of the topological charge of the vortex beam, and the measurement of the fiber laser power using a single optical path; compared with other methods that generate two central stacks as bifurcation points, this invention is simpler and avoids the situation where the bifurcation points coincide, enabling a more accurate determination of the topological charge.

Description

Technical Field

[0001] This invention relates to the field of laser measurement technology, and in particular to a method and apparatus for measuring the topological charge of vortex light by self-interference of fiber laser. Background Technology

[0002] Vortex beams possess a helical phase wavefront with a phase singularity at the center, and each photon carries orbital angular momentum. The light intensity exhibits a ring-shaped hollow distribution, making them valuable for research and application in fields such as optical communication, particle acceleration, particle manipulation, and super-resolution imaging. Currently, the methods for generating vortex light fields mainly fall into two categories. One category involves spatial structure generation, such as through metasurfaces, spatial light modulators, and helical phase plates; the other involves direct generation within optical fibers, utilizing long-period fiber gratings, photonic lanterns, and acousto-induced fiber gratings.

[0003] Vortex beams can be generated and transmitted in optical fibers. Optical fibers have waveguide mode stability, and fiber lasers have advantages such as compact structure, convenient thermal management, and high efficiency, which are conducive to realizing high-power and high-stability vortex beam output.

[0004] Measuring the phase singularity of a vortex beam is crucial for confirming the existence of optical vortices. This is especially true when the output spot of a fiber laser is ring-shaped. Since conventional optical fibers are ring waveguide structures, the ring-shaped spot represents the eigenmodes that the fiber can output. The intensity distribution of the ring alone could indicate a scalar mode, a radial / angular polarization vector mode, or a phase vortex mode. Measuring the phase singularity is necessary to confirm that the fiber is outputting a vortex beam. Summary of the Invention

[0005] To address the technical problems existing in the prior art, this invention provides a method and apparatus for measuring the topological charge of vortex light using fiber laser self-interference.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] On one hand, this invention provides a method for measuring the topological charge of vortex light using fiber laser self-interference, comprising:

[0008] The vortex laser is split into two beams by the first beam splitter, namely the first beam and the second beam;

[0009] The first beam is focused and transmitted to the beam splitter, where it interferes with the second beam, which is also transmitted to the beam splitter. The interference laser beam is then combined by the beam splitter and transmitted to the spot acquisition camera.

[0010] Interference fringe images are acquired by a spot-collecting camera;

[0011] The topological charge of the vortex light is measured based on the interference fringe image. If the interference fringe image contains positive Y-shaped fringes, it indicates that the topological charge is positive and the absolute value of the topological charge is equal to the number of branches at the bifurcation point of the positive Y-shaped fringe. If the interference fringe image contains inverted Y-shaped fringes, it indicates that the topological charge is negative and the absolute value of the topological charge is equal to the number of branches at the bifurcation point of the inverted Y-shaped fringe.

[0012] On the other hand, the present invention provides a device for measuring the topological charge of vortex light by self-interference of fiber laser, which is used to realize the above-mentioned method for measuring the topological charge of vortex light by self-interference of fiber laser, including: a vortex laser output end, a collimating lens, a first beam splitter, a second beam splitter, a focusing system, a transmission lens group, a beam splitter prism, a spot acquisition camera, and a power measurement device.

[0013] A vortex laser is output from the vortex laser output terminal. After being collimated by a collimating lens, the output vortex laser is transmitted to the first beam splitter. The first beam splitter splits the vortex laser into two beams, namely the first beam and the second beam. The first beam is focused by a focusing system and then transmitted to the beam splitter prism through a transmission mirror group. The second beam is transmitted to the second beam splitter, which splits the second beam into two beams, namely the second beam and the third beam. The second beam is transmitted to the beam splitter prism, which combines the first beam and the second beam, causing the two beams to interfere with each other. The combined interfering laser beam is transmitted to a spot acquisition camera, which acquires the interference fringe image. The third beam is transmitted to a power measurement device.

[0014] Furthermore, the vortex laser is generated by a vortex laser generation system, and the vortex laser generated by the vortex laser generation system is output through an end cap, which is coated with an anti-reflection film for the laser output band with a transmittance greater than 99.5%.

[0015] Furthermore, the first beam splitter and the second beam splitter are lenses that simultaneously possess transmission and reflection functions, including but not limited to back-polished reflectors, beam splitting prisms, and flat glass.

[0016] Furthermore, the transmission mirror assembly includes a reflector and an attenuator.

[0017] Furthermore, the attenuator is a neutral density filter or a variable attenuator, which can reduce the intensity of the focused spot, making the intensity of the two beams incident on the beam splitter similar, thereby enabling the two beams to self-interfere and obtain clear interference fringes.

[0018] Furthermore, it also includes a light-blocking plate for blocking the collimating light spot, the light-blocking plate being disposed between the first beam splitter and the second beam splitter or between the second beam splitter and the beam splitting prism.

[0019] Furthermore, the focusing system includes a focusing lens, a variable magnification beam shrinker, and a laser beam shrinker group composed of convex lenses with different focal lengths.

[0020] Furthermore, the power measurement device is used to monitor the optical power during the self-interference experiment and calculate the output power of the fiber laser based on the measurement results.

[0021] Compared with the prior art, the technical effects that this invention can produce are:

[0022] This invention collimates the vortex laser output from an optical fiber, causing interference between the collimated spot and the refocused spot. The collimated local spot can be considered a plane wave, achieving interference between the focused vortex beam and the plane wave. By observing the intensity distribution of the forked pattern measured after interference, the topological charge of the vortex beam can be determined. Furthermore, this invention achieves far-field spot acquisition, vortex beam topological charge determination, and fiber laser power measurement using a single optical path.

[0023] In this invention, the two beams generating self-interference have similar optical path lengths, and one of the beams is focused or contracted to ensure that the intensity of the two beams is similar, thus producing clear interference fringes. If the optical path lengths of the two beams differ significantly, the clarity of the interference fringes will be too low, making it impossible to determine the topological charge. Furthermore, the generated interference fringe image contains only one bifurcation point, and beams with opposite topological charge signs bifurcate in opposite directions, allowing for accurate determination of the topological charge sign. Compared to other methods that generate bifurcation points from two central stacks, this invention is simpler and avoids overlapping bifurcation points, enabling more accurate determination of the topological charge. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0025] Figure 1 A schematic diagram of a fiber laser self-interference device for measuring the topological charge of vortex light, provided as an embodiment;

[0026] Figure 2 Figure 1 shows the focused spot and interference fringe pattern of the vortex beam provided in Example 1. Figure (a) is the focused spot pattern of the vortex beam with a topological charge of +1, Figure (b) is the interference fringe pattern formed after the interference of the spot in Figure (a), Figure (c) is the focused spot pattern of the vortex beam with a topological charge of -1, and Figure (d) is the interference fringe pattern formed after the interference of the spot in Figure (c).

[0027] Figure 3 Figure 2 shows the focused spot and interference fringe pattern of the vortex beam provided in Example 2. Figure (a) is the focused spot pattern of the vortex beam with a topological charge of +1, Figure (b) is the interference fringe pattern formed after the interference of the spot in Figure (a), Figure (c) is the focused spot pattern of the vortex beam with a topological charge of -1, and Figure (d) is the interference fringe pattern formed after the interference of the spot in Figure (c).

[0028] Figure 4 The diagram shows the results of simultaneous detection of pump power and output signal power, vortex beam focused spot, and interference fringes at the output end of the vortex laser provided in Example 3. ①, ②, and ③ are the vortex beam focused spots corresponding to low pump power, medium pump power, and highest pump power, respectively, and ④, ⑤, and ⑥ are the interference fringes formed after interference of the spots corresponding to ①, ②, and ③.

[0029] Figure label:

[0030] 1. End cap; 2. Collimating lens; 3. First beam splitter; 4. Second beam splitter; 5. Focusing system; 6. Reflector; 7. Attenuator; 8. Beam splitter prism; 9. Spot acquisition camera; 10. Power measurement device. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] One embodiment provides a method for measuring the topological charge of vortex light using fiber laser self-interference, including:

[0033] The vortex laser is split into two beams by the first beam splitter, namely the first beam and the second beam;

[0034] The first beam is focused and transmitted to the beam splitter 8, where it interferes with the second beam that is also transmitted to the beam splitter 8. The interference laser beam is then combined by the beam splitter 8 and transmitted to the spot acquisition camera 9.

[0035] Interference fringe images are acquired by the spot acquisition camera 9;

[0036] The topological charge of the vortex light is measured based on the interference fringe image. If the interference fringe image contains positive Y-shaped fringes, it indicates that the topological charge is positive and the absolute value of the topological charge is equal to the number of branches at the bifurcation point of the positive Y-shaped fringe. If the interference fringe image contains inverted Y-shaped fringes, it indicates that the topological charge is negative and the absolute value of the topological charge is equal to the number of branches at the bifurcation point of the inverted Y-shaped fringe.

[0037] The optical path length of the first beam transmitted to the beam splitter 8 is equal to the optical path length of the second beam transmitted to the beam splitter 8.

[0038] Reference Figure 1 A fiber laser self-interference vortex topological charge measurement device provided in one embodiment includes a vortex laser output end, a collimating lens 2, a first beam splitter 3, a second beam splitter 4, a focusing system 5, a transmission mirror group, a beam splitter 8, a spot acquisition camera 9, and a power measurement device 10.

[0039] A vortex laser is output from the vortex laser output terminal. After being collimated by collimating lens 2, the output vortex laser is transmitted to the first beam splitter 3. The first beam splitter 3 splits the vortex laser into two beams, namely the first beam and the second beam. The first beam is focused by focusing system 5 and then transmitted to beam splitter prism 8 through transmission mirror group. The second beam is transmitted to the second beam splitter 4, which splits the second beam into two beams, namely the second beam and the third beam. The second beam is transmitted to beam splitter prism 8, which combines the first beam and the second beam, causing interference between the two beams. The combined interfering laser beam is transmitted to spot acquisition camera 9, which acquires the interference fringe image. The third beam is transmitted to power measurement device 10.

[0040] The optical path length of the first beam transmitted to the beam splitter 8 is equal to the optical path length of the second beam transmitted to the beam splitter 8.

[0041] The vortex laser is generated by a vortex laser generation system, and the vortex laser generated by the vortex laser generation system is output through end cap 1. The end cap 1 is coated with an anti-reflection film for the laser output band, with a transmittance greater than 99.5%. If the output is not using end cap 1, the tail end of the output fiber can be cut at an 8-degree angle to avoid laser reflection.

[0042] The first beam splitter 3 and the second beam splitter 4 are lenses that have both transmission and reflection functions, including but not limited to back-polished reflectors, beam splitters, and flat glass.

[0043] The transmission mirror assembly includes a reflector 6 and an attenuator 7.

[0044] The attenuator 7 is a neutral density filter or a variable attenuator. The attenuator 7 can reduce the intensity of the focused spot, so that the light intensities of the two beams incident on the beam splitter 8 are close, thereby enabling the two beams to self-interfere and obtain clear interference fringes.

[0045] The power measurement device 10 is used to monitor the optical power during the self-interference experiment and to calculate the output power of the fiber laser based on the measurement results.

[0046] In one embodiment, the fiber laser self-interference vortex topological charge measurement device further includes a light-blocking plate, which is used to block the collimated light spot. The light-blocking plate is disposed between the first beam splitter 3 and the second beam splitter 4 or between the second beam splitter 4 and the beam splitter prism 8, so that the light spot acquisition camera 9 only measures the focused far-field light spot, and the measurement of the far-field light spot and the self-interference fringes is realized in the same device.

[0047] The focusing system 5 includes a focusing lens, a variable magnification beam shrinker, and a laser beam shrinker group composed of convex lenses with different focal lengths.

[0048] Example 1:

[0049] A fiber laser self-interference device for measuring the topological charge of vortex light includes: a vortex laser output end, a collimating lens 2, a first beam splitter 3, a second beam splitter 4, a focusing system 5, a reflector 6, an attenuator 7, a beam splitter prism 8, a spot acquisition camera 9, a power measurement device 10, and a light-blocking plate.

[0050] The first beam-splitting lens 3 and the second beam-splitting lens 4 are flat glass; the focusing system 5 is a focusing lens; the attenuator 7 is a neutral density filter; and the light-blocking plate is disposed between the first beam-splitting lens 3 and the second beam-splitting lens 4.

[0051] Reference Figure 2 , Figure 2 The images provided in Example 1 show the focused spot and interference fringe patterns of the vortex beam. Figure (a) shows the focused spot of the vortex beam with a topological charge of +1, and Figure (b) shows the interference fringe pattern formed after interference of the spot in Figure (a). The interference fringes are misaligned to form Y-shaped fringes, and the inverted "Y" shape is clearly visible in Figure (b). Figure (c) shows the focused spot of the vortex beam with a topological charge of -1, and Figure (d) shows the interference fringe pattern formed after interference of the spot in Figure (c). The upright "Y" shape is clearly visible in Figure (d). The opposite opening directions of the "Y" shape indicate that the signs of the topological charge numbers are opposite, and the number of branches at the bifurcation points represents the absolute value of the topological charge number.

[0052] Example 2:

[0053] The focusing system 5 is a variable magnification beam reducer; the first beam splitter 3 and the second beam splitter 4 are back-polished reflectors to keep the optical path lengths of the first beam and the second beam equal, and the rest of the device remains consistent with the setup in Embodiment 1.

[0054] Reference Figure 3 , Figure 3 The images provided in Example 2 show the focused spot and interference fringe patterns of the vortex beam. Figure (a) shows the focused spot of the vortex beam with a topological charge of +1, and Figure (b) shows the interference fringe pattern formed after interference of the spot in Figure (a). A positive "Y" shape is clearly visible in Figure (b). Figure (c) shows the focused spot of the vortex beam with a topological charge of -1, and Figure (d) shows the interference fringe pattern formed after interference of the spot in Figure (c). An inverted "Y" shape is clearly visible in Figure (d). The opposite opening directions of the "Y" shape indicate that the signs of the topological charge numbers are opposite, and the number of branches at the bifurcation points represents the absolute value of the topological charge number.

[0055] Example 3:

[0056] The device is configured identically to that of Example 1; refer to Figure 4 , Figure 4 The image shown in Example 3 illustrates the simultaneous detection results of the pump power and output signal power of the vortex laser output end, the focused spot of the vortex beam, and the interference fringes of the spot. ①, ②, and ③ represent the focused spots of the vortex beam corresponding to low, medium, and highest pump power, respectively, while ④, ⑤, and ⑥ are the interference fringe patterns formed after interference of the spots corresponding to ①, ②, and ③. This demonstrates that the present invention can achieve far-field spot acquisition, determination of the topological charge of the vortex beam, and measurement of the fiber laser power using a single optical path.

[0057] Matters not covered in this invention are common knowledge.

[0058] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0059] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

[0060] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A device for measuring the topological charge number of vortex light using fiber laser self-interference, characterized in that, include: Vortex laser output end, collimating lens, first beam splitter, second beam splitter, focusing system, transmission mirror group, beam splitting prism, spot acquisition camera, power measurement device; A vortex laser is output from the vortex laser output terminal. After being collimated by a collimating lens, the output vortex laser is transmitted to the first beam splitter. The first beam splitter splits the vortex laser into two beams: a first beam and a second beam. The first beam is focused by a focusing system and transmitted through a transmission mirror group to a beam splitter prism. The second beam is transmitted to the second beam splitter, which splits it into two beams: a second beam and a third beam. The second beam is transmitted to the beam splitter prism, which combines the first and second beams, causing them to self-interfere. The combined interfering laser beam is transmitted to a spot acquisition camera, which captures the interference fringe image. The third beam is transmitted to a power measurement device. The topological charge of vortex light is measured based on the interference fringe image. If the interference fringe image contains positive Y-shaped fringes, it indicates that the topological charge is positive and the absolute value of the topological charge is equal to the number of branches at the bifurcation point of the positive Y-shaped fringe. If the interference fringe image contains inverted Y-shaped fringes, it indicates that the topological charge is negative and the absolute value of the topological charge is equal to the number of branches at the bifurcation point of the inverted Y-shaped fringe. The fiber laser self-interference vortex topological charge measurement device also includes a light-blocking plate for blocking the collimated light spot; the light-blocking plate is disposed between the first beam splitter and the second beam splitter or between the second beam splitter and the beam splitting prism; the transmission optical path of the first beam to the beam splitting prism is equal to the transmission optical path of the second beam to the beam splitting prism.

2. The fiber laser self-interference vortex topological charge measurement device as described in claim 1, characterized in that, The vortex laser is generated by a vortex laser generation system, and the vortex laser generated by the vortex laser generation system is output through an end cap. The end cap is coated with an anti-reflection film for the laser output band, with a transmittance greater than 99.5%.

3. The fiber laser self-interference vortex topological charge measurement device as described in claim 1, characterized in that, The first beam splitter and the second beam splitter are lenses that have both transmission and reflection functions, including a back-polished reflector, a beam splitter prism and a flat glass.

4. The fiber laser self-interference vortex topological charge measurement device as described in claim 1, characterized in that, The transmission mirror assembly includes a reflector and an attenuator.

5. The fiber laser self-interference vortex topological charge measurement device as described in claim 4, characterized in that, The attenuator is a neutral density filter or a variable attenuator. The attenuator can reduce the intensity of the focused spot, so that the light intensities of the two beams incident on the beam splitter are close, thereby enabling the two beams to self-interfere and obtain clear interference fringes.

6. The fiber laser self-interference vortex topological charge measurement device as described in claim 1, characterized in that, The focusing system includes a focusing lens, a variable magnification beam shrinker, and a laser beam shrinker group composed of convex lenses with different focal lengths.

7. The fiber laser self-interference vortex topological charge measurement device as described in claim 1, characterized in that, The power measurement device is used to monitor the optical power during the self-interference experiment and to calculate the output power of the fiber laser based on the measurement results.

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