JPH07104107A - Fresnel lens - Google Patents

Abstract

PURPOSE:To provide the Fresnel lens which is excellent in diffraction efficiency by a lens structure having the decreased parts where working is made difficult by a laser beam diameter having a finite diameter. CONSTITUTION:The annular bands of the Fresnel lens 1 formed with the plural annular bands constituting diffraction gratings to such shapes as to be gradually narrower in the width in an outer peripheral direction are formed by dividing the annular bands to plural annular band groups m1, m2 in the outer peripheral direction. The phase differences at the focal points of diffracted light are set larger in the integer times value the nearer the annular band groups m2 in the outer peripheral direction. Then, the problems that annular band widths are narrower the nearer the outer peripheral direction and that the accuracy of working is not obtainable in the case of the specified integer times value with the conventional constitution are solved by feasibility of formation of the annular bands to the large width meeting the integer times values with every annular band group.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a Fresnel lens used as a micro optical element of an optical application system such as an optical information device such as an optical disk.

[0002]

2. Description of the Related Art FIG. 3 shows a Fresnel lens F having a conventional structure.
A partial sectional view of L in the diameter direction is shown. In FIG. 3, a plurality of annular zones 3 are formed concentrically around the optical axis 30.
The positions and shapes of the respective ring zones 31, 32, ... Are formed so that the phase difference of the light passing through the focal planes through 1, 32. That is, the focal length of the lens is f,
The wavelength of the light is λ, the refractive index of the material forming the lens is ns,
Assuming that the radial distance from the optical axis 30 is R, the boundary position Rm between the m-th ring zone 32 and the (m-1) th ring zone 31 is
The phase difference between the light 34 reaching the focus through the optical axis 30 and the light 36 reaching the focus through the position Rm is given by the following equation (1) so that the phase difference is an integral multiple of the wavelength.

[Equation 1] Further, the phase difference at the focal point between the light 36 passing through the m-th ring zone 32 and the light 35 passing through the (m-1) th ring zone 31 passes, for example, on the innermost side of each ring zone 31, 32. When calculated for light, it becomes as shown in the following formula (2), which is an integral multiple of the wavelength, and in this case, it is 1.

[Equation 2] Further, the height hm at the radial displacement rm from the position Rm of the m-th ring zone 32 is given by the following expression (3). hm = (λ + a−b) / (ns−f / b) ... (3)

[Equation 3] Further, the width dm of the m-th ring zone 32 is given by the following equation (4). dm = Rm-Rm-1 (4) As shown in the example of the position and shape of the ring zone 32, the width of each ring zone formed concentrically around the optical axis 30 becomes more distant in the outer circumferential direction. Narrows. When this is applied to a specific example, the ring zone width dm changes as shown in FIG.
This example is a case of a Fresnel lens that focuses laser light having a wavelength of 0.6328 μm at a position with a focal length of 5 mm.

The Fresnel lens having a minute size configured as described above can be manufactured by a method such as an electron beam drawing method, a laser beam drawing method, or a machining method. Above all, the laser beam drawing method, which can perform highly accurate processing in the atmosphere, is drawing attention. This method applies photoresist on a lens substrate formed on a transparent plane,
A desired shape is formed by a thin film of photoresist on the surface of the lens substrate by exposing with a laser beam and developing. As shown in FIG. 5, while rotating the lens substrate 20 coated with photoresist, a circular exposure pattern is formed by exposing with a laser beam 21 that is narrowed down, and further the lens substrate 20 is sent in a predetermined direction. As a result, a concentric exposure pattern can be obtained. By developing this, the lens substrate 20
A Fresnel lens having a plurality of ring zones formed thereon can be manufactured. Since the speed at which the photoresist dissolves in the developing solution depends on the exposure amount of the laser beam 21, by controlling the exposure amount of the laser beam 21, the digging amount of the photoresist in the depth direction is controlled. The Fresnel lens obtained by this technique is extremely thin and small, and because it is formed of a thin film, it has the excellent characteristics that there is almost no change in focal length due to temperature changes in the surroundings.

[0004]

However, in the Fresnel lens, when trying to increase the numerical aperture in order to improve the light-collecting performance, the width of the ring zone is inevitably narrowed, which is shown in the above formula for forming the ring zone. As described above, there is a problem that in the annular zone portion in the outer circumferential direction where the annular zone width is narrowed in the radial direction, there is a limit to the processing accuracy related to the beam diameter of the laser beam, which seriously affects the diffraction efficiency as a Fresnel lens. It was That is, FIG.
In the design cross section of the Fresnel lens shown in Fig. 3, when the serrated annular zones 31, 32, 33 are processed by a laser beam having a finite thickness, the annular zones 31 ... Edge sagging at the boundary between
The actual processed shape becomes the shape shown by the broken line in the figure. In the conventional example of calculating the ring width shown in FIG. 4, when the laser beam diameter is 1.5 μm, m = 10
In the above rings, the laser beam diameter is 1 with respect to the width of the rings.
Since it occupies 0% or more, the shape accuracy cannot be obtained in the area of about 10% of the ring zone. The present invention was devised in view of the above conventional problems, and it is an object of the present invention to provide a Fresnel lens having excellent diffraction efficiency with a lens structure having a small number of portions that are difficult to process due to a laser beam diameter having a finite thickness. To aim.

[0005]

In order to achieve the above object, the first means adopted by the present invention is a diffraction grating for diffracting incident light and condensing it on a focus at a predetermined distance. A plurality of ring zones are formed concentrically on the lens substrate surface, and the phase difference at the focus of the diffracted light from the adjacent ring zones is the value obtained by multiplying the wavelength of the incident light by a predetermined integer value. Further, in a Fresnel lens in which the respective ring zones are formed in a shape and a position such that the intervals of the diffraction grating are gradually narrowed in the outer peripheral direction, the ring zones have an integer value within the ring zone group of one. A Fresnel lens having a constant value and being formed by being divided into a plurality of orbicular zones that have a larger integer value and are larger between adjacent orbicular zones Has been done. Further, the second means adopted by the present invention is that a plurality of orbicular zones forming a diffraction grating for diffracting incident light and condensing the diffracted light on focal points separated by a predetermined distance are concentrically formed on the lens substrate surface. The phase difference of the diffracted light from the adjacent orbicular zones at the focal point becomes a value obtained by multiplying the wavelength of the incident light by a predetermined integer value, and the interval of the diffraction grating becomes gradually narrower in the outer peripheral direction. In a Fresnel lens in which each of the above-mentioned ring zones is formed in such a shape and position, the cross section of the ring zone is a substantially triangular shape inclined in the outer peripheral direction, and the diffraction grating spacing is such that the base of the triangle shape is the ring zone width. The plurality of orbicular zones are divided into a plurality of orbicular zone groups in the outer diameter direction, and the annular zone groups in the outer circumferential direction are formed to have a width and height such that the integer value becomes larger. And is configured as a Fresnel lens.

[0006]

According to the present invention, in a Fresnel lens formed in a shape in which the width of a plurality of annular zones forming a diffraction grating is gradually narrowed in the outer peripheral direction, the annular zones are composed of a plurality of annular zone groups in the outer peripheral direction. Are formed separately, and the integer multiple of the phase difference at the focus of the diffracted light is set to be larger for the ring zone in the outer peripheral direction. Therefore, in the conventional configuration, when the integer multiple value is constant, the annular zone width becomes narrower toward the outer peripheral direction, and the accuracy of machining cannot be obtained. It can be solved by increasing the width of the zone. This configuration corresponds to claim 1. The formation of the annular zones divided into the above annular zones is such that, in a Fresnel lens in which approximately triangular annular zones are continuous in a sawtooth shape, the triangular base is set to have an annular zone width in the circumferential zone. This is realized by gradually increasing the width and height of the ring zone by 2 times, 3 times, ... According to the integer multiple value. This configuration corresponds to claim 2.

[0007]

Embodiments of the present invention will be described below with reference to the accompanying drawings for the understanding of the present invention. still,
The following example is an example embodying the present invention and does not limit the technical scope of the present invention. Figure 1
FIG. 2 is a partial cross-sectional view of a Fresnel lens according to an embodiment of the present invention in the diametrical direction, and FIG. 2 is an enlarged cross-sectional view of the ring zone group B position shown in FIG. In FIG. 1, the Fresnel lens 1 is formed on the lens substrate 2 by performing a development process after exposing the photoresist coated on the lens substrate 2 to a ring pattern by a laser drawing method. A ring group m ₁ and a second ring group m ₂ are formed. In the first ring zone group m ₁ closer to the central axis 3, the phase difference at the focus when the light incident on the Fresnel lens 1 passes through the adjacent ring zones 11, 12, ... The widths of the ring zones 11, 12, ... Are set so as to be one time the wavelength of the emitted light. Further, in the second ring zone group m ₂ closer to the outer circumference, the ring zones 21 in which the lights incident on the Fresnel lens 1 are adjacent to each other,
22 is adjusted so that the phase difference at the focal point when it is focused on the focal point through 22 ... Is twice the wavelength of the incident light.
Widths of 1, 22, ... Are set. The first ring group m
₁ is formed in the same shape as the ring zone shown in the conventional example. Therefore, the description of the conventional example is applied to the configuration of the first ring zone group m ₁ . As shown in the enlarged view of FIG. 2, the first ring is formed from the central axis 3 of the annular zones 11, 12 ... The zone group m ₁ is defined by α in the outer peripheral direction from the zone α in the case where the _second zone group m ₂ is not formed as in the conventional configuration.
A second ring zone group m ₂ is formed by combining two of the + 1st, α + 2nd ... Zones 21, 22.

[0008] By forming a second annular zone group m ₂ in a first outer circumferential direction of the annular zone group m ₁ as described above, annular zone adjacent the second annular zone group m ₂ 21, 22 Since the phase difference at the focal point of the light passing through is twice the wavelength of the light,
The heights of the ring zones 21, 22, ... That belong to the ring zone group m ₂ are set as follows. In FIG. 2, the height hm at the radial displacement rm ₂ from the boundary position Rm ₂ between the innermost ring zone 21 of the second ring zone group m ₂ and the ring zone 25 of the β-th ring zone 26.
₂ is given by the following equation. When 0 ≦ rm ₂ <Rm ₁ −Rm ₁ −1, hm ₂ = hm ₁ −1 (rm ₁ = rm ₂ ) + hm ₁ (rm
₁ = 0) Rm ₁ −Rm ₁ −1 ≦ rm ₂ <Rm ₁ +1 −Rm ₁ −1
Then, hm ₂ = hm ₁ (rm ₁ = rm ₂ ), where m ₂ = 0,1,2 ... m ₂ = (m ₁ −mα) / 2−1 As described above, the second ring group by m ₂ is formed, it is possible to reduce configure the number of zones in the area of the annular zone group m ₂ of the second, the annular boundary shape accuracy of the region in the annular width is reduced to obtain The problem in the conventional configuration which does not exist is improved. In the above embodiment, the phase difference at the focal point of the light passing through the adjacent ring zones of the ring zones 21, 22 ... Which belong to the second ring group m ₂ is set to twice the wavelength of the light. , This can be an integral multiple such as 3 times or 4 times. Further, the number of ring zones is not limited to two as in the above-described embodiment, and more can be set. Furthermore, the ring zone does not necessarily have to be a perfect circle depending on the intended use of the lens,
The elliptical shape does not impair the effects of the present invention.

[0009]

Since the present invention is configured as described above, there is a problem that in the conventional configuration, the annular zone width becomes narrower toward the outer peripheral direction and the machining accuracy cannot be obtained. It can be solved by setting a large band width. The formation of the annular zones divided into the above annular zones is such that, in a Fresnel lens in which approximately triangular annular zones are continuous in a sawtooth shape, the triangular base is set to have an annular zone width in the circumferential zone. It is realized by increasing the width and height of the ring zone by an integral multiple.

[Brief description of drawings]

FIG. 1 is a schematic view showing a configuration of a Fresnel lens according to an example of the present invention without a partial cross section in a diameter direction.

FIG. 2 is a partially enlarged sectional view of the same lens.

FIG. 3 is a partial cross-sectional view showing a configuration of a Fresnel lens according to a conventional configuration.

FIG. 4 is a graph showing changes in the annular zone width of the same lens.

FIG. 5 is a schematic view showing a method of manufacturing a Fresnel lens.

FIG. 6 is a schematic diagram illustrating a problem of forming a minute annular zone according to a conventional configuration.

[Explanation of symbols]

1 ... the Fresnel lens 2 ... Lens substrates 11 to 13 ... first annular 21-23 ... second annular m ₁ ... first annular zone group m ₂ ... second annular zone group

Claims (2)

[Claims] 1. A plurality of orbicular zones that form a diffraction grating for diffracting incident light and condensing them on a focal point separated by a predetermined distance are concentrically formed on the lens substrate surface, and The phase difference of the diffracted light at the focal point becomes a value obtained by multiplying the wavelength of the incident light by a predetermined integer value, and the above-mentioned shape and position are such that the intervals of the diffraction grating are gradually narrowed in the outer peripheral direction. A Fresnel lens having an annular zone, wherein the annular zone is formed by being divided into a plurality of annular zone groups satisfying the following conditions (1) and (2). (1) The value of the integer value is constant in the ring zone group of 1. (2) There is a difference in the above-mentioned integer value between the adjacent ring zones, and the larger the ring zone group in the outer peripheral direction is. 2. A plurality of orbicular zones that form a diffraction grating for diffracting incident light and condensing them on a focal point separated by a predetermined distance are formed concentrically on the lens substrate surface, and The phase difference of the diffracted light at the focal point becomes a value obtained by multiplying the wavelength of the incident light by a predetermined integer value, and the above-mentioned shape and position are such that the intervals of the diffraction grating are gradually narrowed in the outer peripheral direction. In a Fresnel lens in which an annular zone is formed, the annular zone has a substantially triangular cross-section inclined in the outer peripheral direction, and the plurality of annular zones are formed at diffraction grating intervals with the base of the triangular zone as the annular zone width. A Fresnel lens, characterized in that the band is divided into a plurality of ring groups in the outer diameter direction, and the ring group in the outer circumferential direction has a shape in which the integer value increases.