CN104849779A - Optical element capable of generating long-distance Bessel light beam - Google Patents

Abstract

The invention discloses an optical element capable of generating long-distance Bessel light. The optical element is a plane lens, and the refractive index of the plane lens changes linearly along the radial direction. The linear gradient of the refractive index along the radial direction makes the planar lens have a line-focusing effect similar to that of an axicon, and the incident beam is line-focused to generate a Bessel beam. The surface of the element is flat and more wear-resistant, and there is no problem of becoming a domed axicon after wear. The modulation of the refractive index and thickness minimizes the processing requirements; changing the material thickness and changing the refractive index parameters can be linearly changed to produce Bessel Bessel beams with different non-diffraction distances are modulated by the non-diffraction distance of the beam. Therefore, the optical element of the present invention is an optical element that generates Bessel light more simply, has more operability, is more flexible, and has higher beam quality.

Description

An Optical Element Capable of Generating Long-Distance Bessel Beams

technical field

The invention relates to an optical element capable of generating long-distance Bessel light beams, belonging to the fields of optical transmission transformation and optical design.

Background technique

Bessel beam is a special beam with unique properties in the field of transmission, such as extremely small central spot, high central light intensity, no divergence within a certain range, and self-reconstruction when encountering obstacles. These characteristics of Bessel beams make it widely used in the fields of particle trapping, optical tension, optical imaging, optical guidance, high-precision orientation or collimation, etc. The research on Bessel beams has long been a hot spot in the field of beam transformation.

Axicon is a conical axisymmetric optical element, because the Bessel beam produced by the axicon method has stronger operability, higher energy conversion efficiency, and better beam quality. Currently, axicons are often used to generate Bessel beams. However, this The method has high requirements on the base angle of the axicon (under normal conditions, a 1° axicon can only produce an approximate Bessel light of 24.4 cm with a He-Ne laser at a 4 mm aperture), and due to processing limitations, it is difficult to make the base angle of the axicon It is extremely small (0.5° is already very small), which limits the increase of the non-diffraction distance of the Bessel beam, and is greatly limited in practical applications (such as laser ranging). Furthermore, the central cone angle of the axicon is very vulnerable to damage, and after damage, it becomes a domed axicon, which will reduce the beam quality and affect the experimental results. Therefore, it is particularly practical to design lenses that can generate long-distance Bessel light and are wear-resistant.

Contents of the invention

The object of the present invention is to provide a wear-resistant optical element capable of generating long-distance Bessel light.

In order to achieve the above object, the present invention adopts the following technical solutions:

An optical element capable of generating long-distance Bessel beams, the optical element is a plane lens, and the refractive index of the plane lens changes linearly along the radial direction.

The plane lens is in the form of a soft film.

The plane lens is a plane lens made of radial gradient refractive index material.

After adopting the above scheme, an optical element capable of generating long-distance Bessel beams of the present invention, due to the linear gradient change of the refractive index of the element, is vertically incident from different positions, and the optical path experienced by the outgoing light is different, and the plane wave after vertical incidence is equal to the phase The surface is a conic surface, which makes the geometric model of the plane lens of the present invention similar to that of the traditional axicon and has the same line focusing effect, and the incident light beam can generate a Bessel beam after being line focused. The surface of the element is flat and more wear-resistant, and there is no problem of becoming a domed axicon after wear. The modulation of the refractive index and thickness minimizes the processing requirements; changing the material thickness and changing the refractive index parameters can be linearly changed to produce Bessel Bessel beams with different non-diffraction distances are modulated by the non-diffraction distance of the beam. Therefore, the optical element of the present invention is an optical element that generates Bessel light more simply, has more operability, is more flexible, and has higher beam quality.

The plane lens in the present invention is a plane lens made of radial gradient refractive index material.

Further, the flat lens in the present invention can be in the shape of a soft film, and a film with a linear gradient of refractive index is attached to the exit surface of the optical system, and the outgoing light is long-distance Bessel light, which directly generates Bessel light without additional modulation.

In addition, the optical element of the present invention can be used in multi-group stacking (to ensure that the centers coincide), and multi-group stacking can generate Bessel beams with different distances. When in use, multiple groups of optical elements of the present invention (same or different in thickness) with the same gradient of refractive index change can be used for stacking, and multiple groups of optical elements of the present invention (same or different in thickness) with different gradients of refractive index change can also be used for stacking. overlay.

Description of drawings

Fig. 1 is the structural representation of optical element of the present invention;

Fig. 2 is the schematic diagram of the equal refractive index surface of the optical element of the present invention;

Fig. 3 is the geometrical analysis figure that optical element of the present invention produces Bessel light;

Fig. 4 is the light intensity distribution figure of the long-distance Bessel light that optical element of the present invention produces;

FIG. 5 is a schematic diagram of Bessel light at different distances modulated by combinations of optical elements of the present invention with different parameters.

Detailed ways

The structure and principle of the optical element of the present invention will be further described in detail below in conjunction with the accompanying drawings.

An optical element capable of generating long-distance Bessel light beams of the present invention, the optical element is a columnar structure as a whole, the incident surface and the outgoing surface are both flat and parallel to each other plane lens, and the refractive index of the plane lens is in the radial direction Linear gradient change. Specifically, the planar lens can be made of radially graded refractive index materials, as mentioned in the document [Zhang Fengying, Making Plastic Gradient Refractive Index Rods by Photocopolymerization. Optical Technology, 1988 (03): p. 7-11] Using the photocopolymerization method, in addition to the refractive index distribution of formula (1), gradient refractive index rods with various distribution curves can also be manufactured.” According to this document, radial gradient refractive index materials can be prepared by photocopolymerization.

In the present invention, the refractive index of the plane lens changes linearly along the radial direction, which means that the plane lens has the same refractive index at the same distance from the center position. "Radial" refers to the refractive index distribution characteristics, which has nothing to do with the shape. The plane lens Can be circular or non-circular.

As a preferred embodiment, as shown in Figure 1, a kind of optical element that can produce long-distance Bessel light beam of the present invention, this optical element is cylindrical structure as a whole, incident surface 11 and exit surface 12 are plane and The circular plane lenses 100 are parallel to each other, and the refractive index of the circular plane lenses 100 decreases linearly along the radial direction.

As shown in Figure 2, the equal refractive index surface 13 of the circular plane lens 100 is a cylindrical surface along the optical axis direction, and the refractive index variation parameter of the circular plane lens 100 is: Among them, n ₀ is the refractive index at the center of the element, r is the distance from the center, and g is the gradient change coefficient of the refractive index.

Since the refractive index of the circular plane lens 100 changes linearly along the radial direction, the optical path experienced by the outgoing light is different from different positions when it is vertically incident, and the equiphase plane after the plane wave is vertically incident is a conical surface, which makes the circular plane lens 100 The geometric model is similar to the traditional axicon and has the same line focusing effect.

FIG. 3 shows the geometric analysis of parallel light incident on the circular plane lens 100, and the prismatic area is the non-diffraction range of the Bessel beam. In this embodiment, the thickness of the circular plane lens 100 is uniform, but the refractive index decreases linearly with the increase of the distance from the axis, resulting in different phase changes of the outgoing beams, and the equal phase planes are deflected inwardly to generate Bessel beams. The light intensity distribution of this Bessel beam is shown in Figure 4.

From geometrical optics we get the formula for the non-diffraction range of the Bessel beam produced by the element Among them, R is the incident radius of the beam, D is the thickness of the element, and g is the gradient change coefficient of the refractive index. From this formula, it can be seen that the thickness of the element and the refractive index parameter have the same modulation effect on Bessel light, and reducing the thickness D or the refractive index parameter can increase the diffraction-free distance.

The amplitude transmittance of the optical element of the present invention is t(r)=exp(-i·k·g·D·r), wherein, i is an imaginary number, k is the wave vector of the incident beam, and g is the refractive index gradient of the plane lens Coefficient of variation, D is the thickness of the plane lens, r is the distance from the center of the plane lens. The diffraction-free range of Bessel beams produced by modulating parallel light is When multiple groups of optical elements of the present invention are superimposed on each other (guaranteed center coincidence), because the elements are plane lenses, they can be closely connected to each other, and their amplitude transmittance is equal to the product of the amplitude transmittance of each element, which is equivalent to As the parameter (g·D) of a single element increases, the diffraction-free range of the Bessel beam generated decreases accordingly.

When in use, multiple groups of optical elements of the present invention (same or different in thickness) with the same gradient of refractive index change can be used for stacking, and multiple groups of optical elements of the present invention (same or different in thickness) with different gradients of refractive index change can also be used for stacking. overlay. As shown in FIG. 5 , in this embodiment, two optical elements of the present invention with the same thickness and different refractive index gradients are superimposed and used to generate Bessel light with different non-diffraction distances.

The optical element of the present invention can be in the form of a soft film, and a film with a linear gradient of refractive index is attached to the exit surface of the optical system. The exit light is long-distance Bessel light, and the Bessel light is directly generated without additional modulation.

Claims (3)

1. can produce an optical element for long distance Bessel light beam, it is characterized in that: this optical element is planar lens, and the refractive index of this planar lens radially linearly graded.

2. a kind of optical element that can produce long distance Bessel light beam according to claim 1, is characterized in that: described planar lens is soft film-form.

3. a kind of optical element that can produce long distance Bessel light beam according to claim 1 and 2, is characterized in that: described planar lens is radial gradient index material plane lens.