JPH0792526B2 - Grating lens - Google Patents

Description

TECHNICAL FIELD The present invention relates to a grating lens,
In particular, the present invention provides a grating lens capable of converging a diverging spherical wave into a converging spherical wave and having a high NA, which is excellent in optical characteristics.

2. Description of the Related Art Microlenses are important components of optical application systems such as compact disc CDs, laser discs, and optical communication systems. Among them, a grating lens such as a micro-Fresnel lens has a feature that it can reproduce an arbitrary wavefront by changing the interval of the grooves of the grating portion when the lens thickness is several μm or less, and is attracting attention.

As a conventional example, there is a grating lens (Fresnel lens) as shown in FIG. 7 (T.Shiono et.al:“Com
puter-controlled electron-beom writing system fo
r thin film micro-optics ", J.Vac.Sci.Technol.B, 5,
1, PP.33-36 (Jan. 1987)). That is, on the substrate 1,
A grating portion 7 having a sawtooth cross section as shown in FIG. 7 is formed. By processing the grating portion 7 into a sawtooth shape, a phenomenon of refraction is added in addition to diffraction, and the light collection efficiency is improved.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention When the conventional grating lens shown in FIG. 7 is of a type that converts a converging spherical wave into a diverging spherical wave, as shown in FIG. Λ
The first-order diffraction efficiency, which indicates the ratio of converging incident light, gradually decreased with decreasing (toward the outer peripheral portion). However, the curve shown in FIG. 8 shows that the incident angle θ of each grating is with respect to the used wavelength λ and the refractive index n of the grating portion. Is the Bragg angle θ _B given by ## EQU2 ## In other words, it is the value when the numerical aperture NA on the incident side is equal to that on the outgoing side. The grating period for which the first-order diffraction efficiency is 60% or more is Λ ≧ 2.8λ, and this grating period corresponds to the outermost circumference of the grating lens with numerical aperture NA = 0.18 on the incident and exit sides. That is, the efficiency of the outer peripheral portion of the grating lens with NA> 0.18 drops to 60% or less, and there is a problem in that the light collection efficiency deteriorates.

Also in manufacturing, it is difficult to form a good saw-tooth shape in a region where the period of the outer peripheral portion is small, and the shape tends to be disturbed.

The present invention has been made in view of the above points, and particularly provides a grating lens capable of a high NA having excellent optical characteristics.

Means for Solving the Problems The present invention, in order to solve the above problems, comprises at least a substrate and a grating portion formed on the substrate,
The center part of the grating part has a sawtooth cross section,
The peripheral portion of the grating portion has a rectangular cross section.

Effect According to the present invention, the cross section of the central portion of the grating having a relatively large period is formed into a sawtooth shape, and the cross section of the peripheral portion having a relatively small period is formed into a rectangular shape. To improve the light condensing characteristics of.

Example FIGS. 1 and 2 are a sectional view and a plan view, respectively, showing the structure of a grating lens according to an example of the present invention. For example, on a transparent substrate 1 such as glass or synthetic resin, for example CM
The grating portion 2 formed of an electron beam resist such as S or PMMA is provided. The grating unit 2
Cycle as it will periphery concentrically or concentric elliptical to the lens effect is Yes in the smaller, central portion 2 _A
The cross-section in a so-called sawtooth shape as shown in FIG. 1, the peripheral portions 2 _B cross-section, is a rectangular shape. A known electron beam drawing method was used as a method for processing the grating portion 2. That is, the substrate 1 is coated with an electron beam resist, an exposure amount distribution is given using an electron beam drawing apparatus so as to correspond to the film thickness distribution of the grating lens as shown in FIG. Then, the film thickness of the resist was changed to manufacture. The depth h _A of the groove of the grating portion central portion 2 _A of this time, the used wavelength lambda, relative refractive index n of the grating portion, for example, _{h A λ / (n-1} ) and to the groove of the peripheral portion 2 _B The depth of h _B ≠ ≠
It was a h _A, for example, h _B 1.5h _A. In this embodiment, for example, λ = 0.6328 μm, n = 1.6, and for example, h _A = 1.05 μm, h _B =
It was set to 1.5 μm. FIG. 3 shows the first-order diffraction efficiency of the grating lens of the present embodiment when the Bragg angle is incident. Minimum period Λ of 2 _A in central part of grating
The maximum period Λ ₂ between ₁ and the peripheral portion 2 _B is, for example, approximately 2.5λ = 1.6μ
m. By doing so, as can be seen from FIG. 3, by forming a rectangular grating even in the peripheral portion where the diffraction efficiency deteriorates as in the conventional example,
Efficiency has improved. In the grating lens of the present embodiment, the grating period at which the first-order diffraction efficiency is 60% or more is Λ ≧ 1.55λ = 0.98 μm, and this grating period is almost the same as the numerical aperture NA = 0.32 on the incident side and the emitting side. Corresponds to the outermost circumference of. Therefore, in the grating lens of the present example, the light-collecting efficiency of 60% or more could be realized in the entire lens area with NA ≦ 0.32 on the incident side and the emitting side.

FIG. 4 shows an example of using the grating lens of this embodiment. For example, incident light 5 of a divergent spherical wave emitted from a light source of λ = 0.6328 μm enters the grating portion 2 from the substrate 1 side, is diffracted or refracted at this portion 2, and becomes an emitted light 6 of a convergent spherical wave. It is converted and focused on the focal point 3. In the present embodiment, since the Bragg angle is incident, the incident angle θ and the emission angle θ ₂ in the air are θ = θ ₂ . Here, the numerical aperture NA is NA = sin θ = using the diffraction angle of the outermost circumference of the lens.
It is given by r'sin θ ₁ = sin θ ₂ . However, the refractive index of the substrate was n '.

FIG. 5 shows the incident angle dependence of the diffraction efficiency of the peripheral portion 2 _B of the grating portion in the grating lens of the present embodiment, for example, in the period Λ = 2λ = 1.3 μm, h _B = 1.5 μm. As can be seen from the figure, when the incident angle θ is the Bragg angle, for example, θ _B = 9 °, the efficiency is the highest and reaches 80% or more. The smaller the period, the steeper the incident angle dependence.
The maximum appeared at θ = θ _B, which was period dependent. Therefore, in the grating lens of the present embodiment, the incident wave is more divergent spherical wave than plane wave or convergent spherical wave.
It was discovered that the light collection efficiency is high, and that the effect is particularly large when the incident angle is the Bragg angle, that is, when the NAs on the incident side and the outgoing side are equal.

FIG. 6 shows the dependence of the first-order diffraction efficiency of the grating peripheral portion 2 _{B on} the groove depth h _{B at} the period of the grating lens of this embodiment, for example, Λ = 2λ = 1.3 μm, θ = θ _B = 9 °. It is shown. We have found that the groove depth is approximately h _B = λ.
It was discovered that the efficiency becomes maximum when the thickness is an odd multiple of /2(n-1)0.5 μm. It was also found that the half-value widths of the respective peaks are almost the same and are 2 _W λ / 4 (n-1).
Therefore, the groove depth h _B of the peripheral portion 2 _B of the grating is
For any integer m It was more efficient to satisfy. However, the maximum value of the efficiency depends on the period, and the maximum value was high when Λ ≦ 3λ.

In this embodiment, the case where the numerical aperture NA on the entrance side and the exit side was 0.34 was described, but when NA ≧ 0.18, the efficiency was higher than that of the conventional example and there was an effect.

Also on the manufacturing surface, it was difficult to form a good saw-tooth shape in a region where the period of the outer periphery of the grating lens is small, but in the case of this embodiment, a good shape can be formed because of the rectangular grating. It was

The sawtooth shape and the rectangular shape of the cross section of the grating described in the present embodiment are the shapes as shown in FIG. 1. For example, it is effective if there is a shape close to that shown in FIG.

Although the grating lens of this embodiment is manufactured by the electron beam drawing method, it may be manufactured by using a focused ion beam or a mechanical processing such as a CNC lathe. It is also possible to form a mold by using the grating portion 2 produced by these as a master and to form it by a duplication method using a synthetic resin such as transparent epoxy, UV curable resin, PMMA, etc., which is inexpensive mass production. Can also be realized. Also, in this embodiment,
I mentioned a lens with a circular grating shape,
It is also effective for an elliptical elliptical grating lens and a linear cylindrical grating lens.

EFFECTS OF THE INVENTION According to the present invention as described above, there is an effect that it is possible to realize a grating lens having a high NA, which enables high efficiency even in the peripheral portion of the lens and has a good condensing characteristic.

[Brief description of drawings]

1 and 2 are a cross-sectional view and a plan view, respectively, showing the structure of a grating of an embodiment of the present invention, and FIG. 3 is a relation between the first-order diffraction efficiency and the grating period of the grating lens of the embodiment of the present invention. FIG. 4 is a diagram showing an example of using the grating lens of one embodiment of the present invention, FIG.
6 and 6 are diagrams showing the relationship between the first-order diffraction efficiency of the grating lens of one embodiment of the present invention, the incident angle θ ₁ and the groove depth h _B, and FIGS. 7 and 8 are conventional examples, respectively. FIG. 5 is a diagram showing the relationship between the sectional structure of the grating, diffraction efficiency, and the grating period. 1 ... Substrate, 2 ... Grating part.

Claims (7)

[Claims] 1. A substrate and a grating portion formed on the substrate, wherein a central portion of the grating portion has a saw-tooth cross section, and a peripheral portion of the grating portion has a rectangular cross section. Grating lens to do. 2. The depth of the groove at the center of the grating is
The grating lens according to claim 1, wherein the depth is different from the depth of the groove in the peripheral portion of the grating portion. 3. A minimum period Λ _{1 at} the central part of the grating part
Is not less than 2.5 times the used wavelength λ, and the grating lens according to claim 1. 4. A maximum period Λ ₂ around the grating portion.
Is less than or equal to 2.5 times the used wavelength λ, and the grating lens according to claim 1. 5. A groove depth h in the peripheral portion of the grating portion.
_B is the wavelength λ used, the refractive index n of the grating part, and an arbitrary integer m The grating lens according to claim 1, characterized in that: 6. The grating lens according to claim 1, wherein the numerical aperture NA on the incident side is equal to that on the emitting side. 7. The grating lens according to claim 6, wherein the numerical aperture NA on the incident side and the emitting side is 0.18 or more.