CN211956009U - An optical system with an annular light spot - Google Patents

Abstract

The utility model belongs to the field of laser processing, and discloses an optical system of an annular light spot, which comprises a collimation unit, a phase modulation unit and a focusing unit sequentially arranged along an optical path, wherein the phase modulation unit comprises at least one spiral phase plate located in the optical path; The beam can be modulated by adding a helical phase factor through the helical phase plate located in the optical path, so that the energy distribution of the laser beam becomes a circular distribution. Further, the spiral phase plate can be arranged on the rotation axis, and the state of being in the optical path and the state of being located outside the optical path can be switched by rotating around the rotation axis, so that a variable annular light spot can be realized by switching each spiral phase plate. By improving the components of the optical system and the way of cooperation between the components, the utility model can obtain an annular light spot with energy distribution concentrated at the edge, larger diameter and longer focal depth, and the size of the further light spot can be adjusted. It is suitable for various laser processing fields such as laser cutting.

Description

An optical system with an annular light spot

technical field

The utility model belongs to the field of laser processing, and more particularly relates to an optical system of an annular light spot.

Background technique

In the light spot of ordinary Gaussian energy distribution, most of the energy is concentrated in the center, while the energy of the light spot with annular energy distribution is distributed around, and the energy in the center is low. The light spot with this energy distribution has great advantages in the fields of laser cutting, marking, cladding and micromachining.

When laser processing is performed using a spot with a Gaussian energy distribution, if the center energy is too high, ablation and other phenomena may occur. However, due to insufficient energy at the edge, when deep processing such as laser cutting is performed, the deeper the processing depth, the more insufficient the energy at the edge. , resulting in uneven cutting section. These problems can be effectively solved by using the light spot with annular energy distribution. Compared with the Gaussian distribution, the annular distribution is more evenly distributed in energy, and the energy of the annular light spot is distributed at the edge, which can effectively avoid the edge when cutting thick plates by laser. The problem of uneven section caused by insufficient energy. The main method for generating a ring beam in the prior art is a computational holography method. According to the principle of computational holography, a spatial light modulator is used to generate a ring beam, and a ring beam with arbitrary spatial distribution can be generated by designing a holographic pattern. This method has a certain damage threshold, and the conversion efficiency of the ring beam is not high due to the loss of energy and the diffraction of the beam.

Utility model content

In view of the above defects or improvement requirements of the prior art, the purpose of the present invention is to provide an optical system with an annular light spot, especially an optical system with a variable annular light spot, in which the components of the optical system and the gaps between the components are adjusted. By improving the way of cooperation, etc., the annular light spot with energy distribution concentrated on the edge, larger diameter and longer focal depth can be obtained; and by further setting the rotation axis to adjust the number of inserted light paths and the number of topological charges, the light spot can be changed. The diameter of the light spot can be adjusted, and it is suitable for various laser processing fields such as laser cutting. The laser beam is collimated by the collimating unit and incident from one end of the spiral phase plate (such as the plane end) in the phase modulation unit. A ring-shaped light spot with a ring-shaped energy distribution is obtained. The utility model can generate an annular light spot through a spiral phase plate, which is of great significance in laser processing technology, and can further independently adjust the number of spiral phase plates to realize variable topological charge of the annular focusing spot, and can be processed according to different materials. Flexibility to adjust needs.

In order to achieve the above object, according to one aspect of the present utility model, an optical system of annular light spot is provided, which is characterized in that it includes a collimation unit, a phase modulation unit and a focusing unit arranged along the optical path in sequence, wherein,

The collimating unit is used for collimating the input laser to obtain a collimated laser beam;

The phase modulation unit includes at least one spiral phase plate located in the optical path; any one of the spiral phase plates includes two end surfaces, one of which is a plane structure, and the other end surface is an opposite surface with a spiral shape step; and, for For any spiral phase plate, the central axis of the spiral phase plate coincides with the beam center of the collimated laser beam, and the laser beam can be incident from the plane end face of the corresponding spiral phase plate or can be incident from the stepped end face of the corresponding spiral phase plate, A helical phase factor is added to the helical phase plate for phase modulation, so that the energy distribution of the laser beam becomes a circular distribution; the phase modulation unit as a whole can adjust the phase of the collimated laser beam through the action of the helical phase plate. Modulation is performed to obtain a laser beam whose phase is changed;

The focusing unit is used for focusing the phase-changed laser beam to obtain an annular light spot.

As a further preference of the present invention, a laser is further provided before the collimating unit, the laser output from the laser is used as the input laser of the collimating unit, and the laser output from the laser is a laser with a Gaussian distribution of light intensity.

As a further preference of the present invention, there are multiple spiral phase plates, and for any spiral phase plate whose topological charge number is 1, the phase change around the optical axis is 2π×1.

As a further preference of the present invention, the collimating unit includes a lens group used in conjunction.

As a further preference of the present invention, the focusing unit includes a lens group used in conjunction with it.

According to another aspect of the present invention, an optical system with variable annular light spot is provided, which is characterized by comprising a collimation unit, a phase modulation unit and a focusing unit sequentially arranged along the optical path, wherein,

The collimating unit is used for collimating the input laser to obtain a collimated laser beam;

The phase modulation unit includes several helical phase plates located on the rotating shaft, and these helical phase plates can be rotated around the rotating shaft to switch the state of being located in the optical path and being located outside the optical path; any one of the helical phase plates includes two. End faces, one end face is a plane structure, and the other end face is an opposite face with a spiral shape step; and, for any spiral phase plate, when it is rotated to the state of being located in the optical path, the central axis of the spiral phase plate is the same as that of the spiral phase plate. The beam centers of the collimated laser beams are coincident, and the laser beams can be incident from the plane end face of the corresponding helical phase plate or can be incident from the stepped end face of the corresponding helical phase plate, and the helical phase factor is added to the helical phase plate for phase modulation. , so that the energy distribution of the laser beam becomes an annular distribution; the phase modulation unit as a whole can modulate the phase of the collimated laser beam through the action of the spiral phase plate, so as to emit a laser beam whose phase is changed;

The focusing unit is used for focusing the phase-changed laser beam to obtain a ring-shaped light spot;

The variable annular light spot optical system can realize the variable annular light spot by switching the states of each spiral phase plate in the optical path and outside the optical path.

As a further preference of the present invention, a laser is further provided before the collimating unit, the laser output from the laser is used as the input laser of the collimating unit, and the laser output from the laser is a laser with a Gaussian distribution of light intensity.

As a further preference of the present invention, there are multiple spiral phase plates, and for any spiral phase plate whose topological charge number is 1, the phase change around the optical axis is 2π×1.

As a further preference of the present invention, the collimating unit includes a lens group used in conjunction.

As a further preference of the present invention, the focusing unit includes a lens group used in conjunction with it.

Through the above technical solutions conceived by the present utility model, compared with the prior art, the present utility model uses an optical element such as a spiral phase plate for the first time to solve the problem that the focused spot with Gaussian energy distribution is concentrated in energy and the edge energy is insufficient during laser processing. Produces a circular focused spot. The utility model applies the conventional optical element in the prior art—the spiral phase plate for the first time to the related application of laser processing to generate annular light spots; and further, the adjustment can be achieved by controlling the number of spiral phase plates inserted into the optical path. The topological charge of the final output laser beam. However, the present invention uses a spiral phase plate to generate a ring beam, which is easy to implement. The step height of the spiral phase plate is usually in the order of microns, and the initial beam is expanded by the beam expansion system, and there is basically no divergence. The phase plate basically does not attenuate the light intensity of the beam, but only changes the phase of the beam. For a light source with good monochromaticity, the purity of the ring beam generated is higher.

The utility model utilizes spiral phase plates to construct a phase modulation unit. These spiral phase plates are consistent with the prior art and are transparent plates with a fixed refractive index. Rotation steps); helical phase plates whose thickness varies with azimuth angle. After the laser beam passes through the spiral phase plate, the phase of the outgoing beam changes accordingly due to the different optical paths at different azimuth angles, and a spiral phase factor exp(ilθ) is added to the outgoing beam, where l is the helical phase plate. The topological charge number, i represents the complex sign, thus becoming a ring beam.

When the refractive index, thickness parameters of the spiral phase plate and the wavelength of the incident light are determined, the topological charge of the obtained annular beam is also determined, which cannot be flexibly adjusted; therefore, the present utility model further proposes a more preferred annular beam. Optical system with variable spot size. That is to say, since a helical phase plate can only generate an annular light spot with a topological charge after the refractive index and thickness parameters are determined, the present invention can flexibly switch a certain helical phase plate to be located in the optical path by designing the rotation axis. In this way, based on the superposition principle of the topological charges of the ring beam, multiple helical phase plates are used to generate ring focused spots with different topological charges, thereby realizing the variable size of the ring spot.

The spiral phase plate in the phase modulation unit needs to keep the light beam incident from the flat end (or step end) of the spiral phase plate; when the spiral phase plate in the phase modulation unit is inserted into the optical path (that is, rotated to a state in the optical path) , the center of the spiral phase plate must be aligned with the center of the laser beam. The laser beam with Gaussian distribution is incident from the plane end face of each helical phase plate or the stepped end face of each helical phase plate, the center of the beam is aligned with the center of the helical phase plate, the phase of the outgoing beam is changed, and a helical phase factor is added, and the energy distribution changes. It is a ring-shaped distribution, and finally a ring-shaped light spot is obtained by focusing.

The size of the annular light spot obtained by the optical system of the variable annular light spot is affected by the topological charge, and the larger the topological charge, the larger the energy concave area of the obtained annular light spot and the larger the diameter of the light spot. By arranging several helical phase plates on the rotating shaft, the utility model can superimpose the helical phase plates inserted into the system to change the topological charge and realize variable and adjustable annular light spot. That is to say, based on the present invention, by adjusting the number of the spiral phase plates located in the optical path, the spot size can be adjusted. The phase modulation unit in the variable annular spot optical system is fixed on a rotating shaft by several helical phase plates. inside and outside the optical path). When any one spiral phase plate is inserted into the optical path, the center of the spiral phase plate can be aligned with the collimation unit and the focusing unit in the system.

The topological charge of the ring beam satisfies the superposition principle, that is to say, the size of the topological charge of the ring beam satisfies the summation relationship. For example, two ring beams with the same topological charge can be superimposed into a ring beam with twice the topological charge, For another example, if two ring beams with the same topological charge value and opposite sign are superimposed, a beam with zero topological charge number (that is, a plane light field) is formed; therefore, the topological charge of the ring beam can be changed by stacking multiple helical phase plates. Count l, and then change the size of the annular light spot obtained by the final focusing.

Specifically, the utility model has the following characteristics:

(1) Under the same conditions, the beam is added with a helical phase factor, and the focused annular spot has a larger radius and a longer focal depth than the ordinary focused spot. And the characteristics of high edge energy make it can effectively avoid adverse effects such as ablation in laser processing, the material is more uniformly heated, and the processing quality is better;

(2) The annular light spot is generated by the spiral phase plate, which can be regarded as pure phase modulation, the change of amplitude can be ignored, the energy conversion rate is high, and the loss is reduced, which is suitable for high-power laser processing;

(3) The annular light spot produced by the spiral phase plate has higher purity, stronger stability, and is not easily affected by optical effects such as diffraction;

(4) According to the needs of actual work, the number of helical phase plates in the system can be adjusted independently, and the number of topological charges can be changed by stacking multiple helical phase plates, thereby changing the size of the annular focusing spot, and realizing the continuous size of the annular spot. Adjustable, can meet the cutting requirements of different plate thicknesses;

(5) The system has good anti-misalignment characteristics, and the change of the spot size, the beam shift, and the focus shift will not affect the light field of the annular energy distribution;

(6) When used in laser additive manufacturing or laser surface treatment, the topological charge of the light spot can be changed to realize the adjustment of the width of the light spot without affecting the uniformity of the light field distribution, and the effect of the uniformity effect can be obtained.

In summary, using the optical system of the present invention, the laser beam is collimated into parallel light after passing through the collimating unit, and then a ring beam is obtained through phase modulation, and finally, the focusing unit is used to focus into a high-energy annular spot, which can be used for laser light. Cutting, marking, cladding, micro-processing and other fields. The utility model can generate an annular light spot through a spiral phase plate, which is of great significance in laser processing technology, and can independently adjust the number of spiral phase plates to realize the variable topological charge of the annular focusing spot, and can process different materials according to its own needs. Flexible adjustment to the needs of the laser beam; it can effectively solve the current negative problems such as uneven laser plate cutting section and ablation caused by the high center energy of the Gaussian beam and insufficient edge energy, and the spot size can be adjusted. High utilization rate, suitable for high-power laser processing.

Description of drawings

FIG. 1 is a schematic diagram of a variable annular light spot optical system in Embodiment 1 of the present invention.

2 is a schematic structural diagram of a spiral phase plate in the variable annular spot optical system of the present invention.

3 is a schematic diagram of a collimated Gaussian beam according to Embodiment 1 of the present invention.

4 is a phase diagram of a helical phase plate with a topological charge of 1 in the phase modulation unit according to Embodiment 1 of the present invention.

5 is a phase diagram of a helical phase plate with a topological charge of 2 in the phase modulation unit according to Embodiment 1 of the present invention.

6 is a focused spot obtained only by a collimating focusing unit when the light beam does not pass through any spiral phase plate in Embodiment 1 of the present invention.

7 is a cross-sectional light field distribution diagram on the focal plane after passing through a helical phase plate with a topological charge number of 1 in Embodiment 1 of the present invention.

FIG. 8 is a cross-sectional light field distribution diagram on the focusing plane after passing through two spiral phase plates with topological charge numbers 1 and 1 respectively in Embodiment 1 of the present invention.

9 is a cross-sectional light field distribution diagram on the focusing plane after passing through three helical phase plates with topological charge numbers of 1, 1 and 2 respectively in Embodiment 1 of the present invention.

10 is a schematic diagram of a variable annular light spot optical system in Embodiment 2 of the present invention.

1, 2 and 10 have the following meanings: 1 is the incident laser, 2 is the collimation unit, 3 is the switching shaft, 4 is the helical phase plate, 5 is the focusing unit, 6 is the focusing surface, 7 It is a scanning galvanometer, 8 is a motor connected with the scanning galvanometer 7 , 9 is another scanning galvanometer, and 10 is a motor connected with the scanning galvanometer 9 .

Detailed ways

In order to make the purpose, technical solutions and advantages of the present utility model more clearly understood, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention, and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as there is no conflict with each other.

Example 1:

The schematic diagram of the variable annular spot optical system provided by the first embodiment of the present invention is shown in FIG. 1 , and the whole system is divided into a collimation unit, a phase modulation unit and a focusing unit. The laser beam is collimated into parallel light by the lens group of the collimating unit, and then incident from the plane end of the spiral phase plate in the phase modulation unit, and the outgoing beam becomes a ring beam with a topological charge equal to that of the spiral phase plate. Finally, the light spot with annular energy distribution is obtained by focusing the lens group in the focusing unit.

Figure 2 is a schematic diagram of the structure of the spiral phase plate in the variable annular spot optical system of the present invention. The spiral phase plate is an optical element in the prior art. In the prior art, in order to facilitate processing, the spiral phase plate is The plate is made into steps, and according to the topological charge of the spiral phase plate and the refractive index of the material, the thickness of the steps of the spiral phase plate can be calculated directly with reference to the prior art (for example, Wu Mingbo, "Study on the Measurement of the Topological Charge of the Vortex Beam" etc.); the thickness of the spiral phase plate is generally in the order of microns, which can be ignored, so the influence of the phase plate on the light intensity can be ignored.

FIG. 3 is a schematic diagram of the collimated Gaussian beam according to the first embodiment of the present invention, and the axis of the Gaussian beam is on a straight line with the axes of the collimating mirror and the focusing mirror.

As shown in FIG. 1, the present utility model can adjust the number of phase plates inserted into the optical path system by rotating independently by fixing several spiral phase plates on a rotating shaft, that is, the spiral phase plates in the optical path It can be flexibly switched according to actual needs, for example:

Figure 4 is a phase simulation diagram of a spiral phase plate with a topological charge of 1 in the phase modulation unit of the first embodiment of the present invention, and a corresponding phase factor will be added to the light beam after passing through the phase plate.

Figure 5 is a phase simulation diagram of a spiral phase plate with a topological charge of 2 in the phase modulation unit of the first embodiment of the present invention, and a corresponding phase factor will be added to the light beam after passing through the phase plate.

As shown in FIG. 6, the first embodiment of the present invention is shown. When the light beam does not pass through any spiral phase plate, only through the focusing spot obtained by the collimating focusing unit, it can be seen that the light field distribution of the focusing spot is a Gaussian distribution, The spot radius is about 0.1mm.

As shown in FIG. 7, the first embodiment of the present utility model is shown. The number of topological loads after a block is 1 (n=1 at this time; of course, n can also be preset to be equal to other integers, including positive integers and negative integers) The cross-sectional light field distribution diagram of the position of the focusing surface 6 after the spiral phase plate is installed. It can be seen that through the phase modulation of the spiral phase plate, a focused spot with a circular energy distribution with low center energy and high edge energy is obtained. The topological charge is 1, the phase change is 2π, and the spot radius is about 0.2mm.

Figure 8 shows the first embodiment of the present invention, after passing through two helical phase plates with topological charges of 1 and 1, respectively, the cross-sectional light field distribution diagram of the position of the focusing surface 6 . Through the superposition of the topological charges of the helical phase plate, the topological charges of the annular light spot can be superimposed accordingly, and the final topological charge of the annular light spot is 2 (equal to 1 plus 1, the topological charge satisfies the superposition principle), and the phase changes The amount of 4π, the radius of the spot is about 0.35mm.

Figure 9 shows the first embodiment of the present utility model, after passing through three spiral phase plates with topological charges of 1, 1 and 2 respectively, the cross-sectional light field distribution diagram of the position where the focusing plane 6 is located. The topological charge of the obtained ring spot is 4 (equal to 1 plus 1 plus 2, the topological charge satisfies the superposition principle), the phase change amount is 8π, and the radius of the light spot is about 0.5 mm. Two ring beams with the same topological charge can be superimposed to obtain a ring beam with twice the topological charge. The topological charge of several spiral phase plates in the phase modulation unit of the present invention should follow this rule.

Example 2:

The schematic diagram of the variable annular spot system provided by the second embodiment of the present invention is shown in Figure 10. After the laser beam passes through the collimation unit and the phase modulation unit, it passes through two scanning galvanometers, and finally passes through the focusing objective lens. The principle is the same as that of Embodiment 1. By scanning with two galvanometers, the focus position can be changed for marking.

In the optical path of Example 2, the energy distribution of the light field on the focusing surface is consistent with that of Example 1. Due to the high stability of the annular light spot generated by the spiral phase plate, the characteristic of the annular distribution can still be maintained at the position deviated from the focal point. In the process of high-speed moving marking by scanning the galvanometer, the output of annular energy can be better maintained. Compared with the application in the vertical space of the first embodiment, the second embodiment is mainly applied in the horizontal space.

In general, the optical system of the variable annular spot in the present invention can obtain an annular energy distribution spot with low light intensity loss, adjustable spot size, low center energy, and high edge energy, which can be used for laser cutting, marking, melting, etc. Covering and other laser processing fields. Effectively solve the current negative problems such as uneven laser plate cutting section and ablation caused by the high center energy of the Gaussian beam and insufficient edge energy. Laser high power processing.

For the plane end face and the stepped end face of the helical phase plate, when they are located in the optical path, the laser incidence direction needs to be consistent. For example, the first helical phase plate laser can be incident from the stepped face, and all subsequent helical phase plates are It should be consistent with the placement direction of the first spiral phase plate, so that the laser is incident from the stepped end face of the corresponding spiral phase plate.

The above-mentioned embodiments are described in detail based on the optical system of the optimal technical solution of the present invention, the variable annular spot. concept, the present invention can also realize the optical system of fixed annular light spot, the optical system of fixed annular light spot also includes collimation unit, phase modulation unit and focusing unit arranged along the optical path in sequence, and the collimating unit and focusing unit can be completely referenced In the optical system of the variable annular spot, only the phase modulation unit needs to be adjusted. At this time, the phase modulation unit will not set the rotation axis, but fixedly set the number of preset spiral phase plates in the optical path (that is, make the number preset of the helical phase plates they can always be in the optical path). These helical phase plates are at least one helical phase plate, and certainly can also be a plurality of helical phase plates.

In addition, the collimating unit and the focusing unit in the optical system of the present invention use a combined lens group to reduce aberrations. The topological charge number of each helical phase plate can be preset, including each helical phase plate located on the rotating shaft. Taking Fig. 1 as an example, the topological charges l of the five helical phase plates shown in Fig. 1 are n respectively. , n, 2n, 4n, 8n, where n is equal to 1; for any spiral phase plate with a topological charge of l, the phase change around the optical axis is 2π×l. Referring to the prior art, the spiral phase plate can calculate the thickness of the spiral phase plate according to the material refractive index and topological charge of the spiral phase plate, average it to each step, calculate the height of each step, and carry out the corresponding production and processing. , self-prepared spiral phase plate, the higher the machining accuracy, the better, the height of each step is generally in the order of microns; of course, it can also be purchased. The collimating unit and the focusing unit in the optical system of the present invention can be a general conventional collimating focusing lens.

Those skilled in the art can easily understand that the above are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements and replacements made within the spirit and principles of the present invention Improvements, etc., should be included within the protection scope of the present invention.

Claims (10)

1. An optical system of annular light spots is characterized by comprising a collimation unit, a phase modulation unit and a focusing unit which are sequentially arranged along an optical path, wherein,

the collimation unit is used for collimating the input laser to obtain a collimated laser beam;

the phase modulation unit comprises at least one spiral phase plate positioned in an optical path; any one spiral phase plate comprises two end faces, wherein one end face is of a plane structure, and the other end face is an opposite face with a spiral step; for any spiral phase plate, the central axis of the spiral phase plate is overlapped with the beam center of the collimated laser beam, the laser beam can be incident from the plane end face of the corresponding spiral phase plate or can be incident from the step end face of the corresponding spiral phase plate, and a spiral phase factor is added through the spiral phase plate to perform phase modulation, so that the energy distribution of the laser beam is changed into annular distribution; the phase modulation unit can modulate the phase of the collimated laser beam through the action of the spiral phase plate, so that the laser beam with the changed phase is emitted;

the focusing unit is used for focusing the laser beam with the changed phase, so that an annular light spot is obtained.

2. The optical system as claimed in claim 1, wherein a laser is further disposed before the collimating unit, the laser emitted from the laser is used as the input laser of the collimating unit, and the laser emitted from the laser has a gaussian distribution of light intensity.

3. The optical system of claim 1, wherein the spiral phase plate is plural, and for any one spiral phase plate with topological charge number l, the phase of one revolution around the optical axis is changed to 2 π x l.

4. The optical system of claim 1, wherein the collimating unit comprises a cooperating lens group.

5. The optical system of claim 1, wherein the focusing element comprises a cooperating lens group.

6. An optical system of variable annular light spots is characterized by comprising a collimation unit, a phase modulation unit and a focusing unit which are sequentially arranged along an optical path, wherein,

the collimation unit is used for collimating the input laser to obtain a collimated laser beam;

the phase modulation unit comprises a plurality of spiral phase plates positioned on a rotating shaft, and the spiral phase plates can switch the states of being positioned in an optical path and being positioned outside the optical path by rotating around the rotating shaft; any one spiral phase plate comprises two end faces, wherein one end face is of a plane structure, and the other end face is an opposite face with a spiral step; when any one spiral phase plate is rotated to be positioned in the light path, the central axis of the spiral phase plate is overlapped with the beam center of the collimation laser beam, the laser beam can be incident from the plane end face of the corresponding spiral phase plate or can be incident from the step end face of the corresponding spiral phase plate, and a spiral phase factor is added to the laser beam through the spiral phase plate for phase modulation, so that the energy distribution of the laser beam is changed into annular distribution; the phase modulation unit can modulate the phase of the collimated laser beam through the action of the spiral phase plate, so that the laser beam with the changed phase is emitted;

the focusing unit is used for focusing the laser beam with the changed phase so as to obtain an annular light spot;

the variable annular light spot optical system can realize variable annular light spots by switching the states of the spiral phase plates which are positioned in the light path and outside the light path.

7. The optical system as claimed in claim 6, wherein a laser is further disposed before the collimating unit, the laser emitted from the laser is used as the input laser of the collimating unit, and the laser emitted from the laser has a gaussian distribution of light intensity.

8. The optical system of claim 6, wherein the spiral phase plate is plural, and for any one spiral phase plate with topological charge number l, the phase of one revolution around the optical axis is changed to 2 π x l.

9. The optical system of claim 6, wherein the collimating unit comprises a cooperating lens group.

10. The optical system of claim 6, wherein the focusing unit comprises a cooperating lens group.

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