JPH0616121B2 - Fresnel lens and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a Fresnel lens which can be used in the field of optical information processing using coherent light, or in the field of measurement and control of optical application, and a manufacturing method thereof.

Prior Art FIG. 4 shows a conventional Fresnel lens and its manufacturing method.
Photoresist 2 on glass substrate 1 as shown in FIG. 4 (a).
Spin coating. Next, as shown in FIG. 4 (b), exposure and development are performed on the photoresist 2 by using a mask and a mask having a Fresnel lens pattern which has been prepared in advance. The spot diameter of 1.5 μm was obtained with the Fresnel lens manufactured by the above method (for example, Showa 58).
IEICE General Conference 1029).

Problems to be Solved by the Invention By the way, in the fields of optical information processing, optical applied measurement control, etc., a lens is often used in combination with a polarizing plate, and an element having both functions of diffraction and polarization has been desired.

With the conventional method of manufacturing a Fresnel lens as described above, a Fresnel lens whose diffraction characteristic depends on the polarization direction cannot be manufactured.

Means for Solving the Problems The present invention provides a substrate containing lithium niobate or lithium tantalate as a main component, so that the diffraction characteristics of outgoing light with respect to incident light from the surface of the substrate have polarization direction dependence, The above problem is solved by using a Fresnel lens including a refractive index changing portion formed by ion exchange on the surface of the substrate.

Further, the present invention, a step of forming a mask pattern on the substrate surface containing lithium niobate or lithium tantalate as a main component, and using the mask pattern as a mask, the diffraction characteristics of the emitted light with respect to the incident light from the substrate surface The above problem is solved by providing a Fresnel lens manufacturing method including a step of forming a refractive index changing portion on the surface of the substrate by an ion exchange method so as to have polarization direction dependency.

Function The present invention is to manufacture a Fresnel lens by using the ion exchange method in which only the refractive index for extraordinary light changes in the substrate. By making the diffraction characteristic dependent on the polarization direction, the lens has two characteristics of diffraction and polarization. The function can be realized by one component.

Example The radius at which the phase difference of the focused spherical wave of the incident plane wave on the Fresnel lens is an integral multiple of π is equal to the radius of the boundary circle of the Fresnel lens. Therefore, let the radius of the mth boundary circle from the center be R
If m, then equation (1) holds.

Here, f is the focal length, and k is the wave number. A black-and-white Fresnel lens pattern mask made of Cr was formed on the glass according to the equation (1). The specification of the Fresnel lens manufactured by this is that the wavelength is 0.6328 μm, the element diameter is 3 mm, the focal length is 4.6 mm, and the minimum line width of the pattern is 6 μm.

Next, the Fresnel lens manufacturing process using the mask
Shown in the figure. First, as shown in Fig. 1 (a), lithium niobate 5
An Al film 6 was formed on the X surface of the substrate by electron beam evaporation.
Next, as shown in FIG. 1B, a photoresist 2 was applied and patterned on the resist using a Fresnel lens pattern. Next, as shown in FIG. 1C, the Al film 6 is etched with a phosphoric acid-based etching solution. Finally, as shown in FIG. 1 (d), ion exchange is performed in benzoic acid (240 ° C.) for 1.5 hours, and then the Al film 6 is removed. The ion exchange time for forming the refractive index changing portion 8 by ion exchange in this way was determined as follows.

The fraction in lithium niobate is k ₁ , the fraction in the refractive index changing portion 3 due to the same ion exchange is k ₂ , and the thickness of the refractive index changing portion 8 is d.
Then, the phase difference φ is given by equation (2).

Here, Δn is the refractive index difference between the refractive index changing portion 8 and the lithium niobate 5, and K ₀ is the wave number in vacuum. When trying to change this phase difference by π, the thickness d = λ /
(2Δn). The wavelength λ of the He-Ne laser is 0.6
If the refractive index difference is 328 μm and the refractive index difference is 0.128, the thickness d is 2.5 μm, and the ion exchange time required for this is the second.
From the relationship between the ion exchange time and the ion exchange depth shown in the figure, it will be 1.5 hours.

He-Ne laser light was made incident on the prepared Fresnel lens 9, and the extinction ratio of vertically polarized light (extraordinary light direction) and horizontal polarized light (ordinary light direction) was measured and found to be 30 dB. When the light intensity distribution on the focal plane was measured, the spot size was 6 μm. As described above, the Fresnel lens of this embodiment has many advantages such as small size, light weight, and low manufacturing cost.

Another embodiment of the Fresnel lens of the present invention is shown in FIG.
As shown in FIG. 3 (a), an Al film 6 on the lithium niobate 5 serving as a mask for ion exchange is formed in a sawtooth shape, and the ion exchange method is applied to the lithium niobate 5 as shown in FIG. 2 (b). The refractive index changing portion 8 was blazed. Thus, by blazing the Fresnel lens 9, it is possible to form an image with an efficiency of almost 100%, and the extinction ratio is 40d.
It was B. Although LiNbO ₃ was used for the substrate in the examples, LiTaO ₃ may be used.

Effects of the Invention According to the Fresnel lens of the present invention and the manufacturing method thereof,
Since the Fresnel lens has a diffraction direction dependency of the diffraction characteristic, it is possible to make the Fresnel lens compact, in which the two functions of the diffraction function and the polarization function can be configured by one element.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a manufacturing process of a Fresnel lens according to an embodiment of the present invention, FIG. 2 is a diagram showing a relation between ion exchange time and ion exchange depth, and FIG. 3 is another diagram of the present invention. FIG. 4 is a view for explaining the manufacturing process of the Fresnel lens of the embodiment, FIG.
The figure is a figure for demonstrating the manufacturing process of the conventional Fresnel lens. 5: Lithium niobate, 8: Refractive index changing part, 9:
Fresnel lens.

Claims (5)

[Claims] 1. A substrate containing lithium niobate or lithium tantalate as a main component, and ion exchange is performed on the surface of the substrate so that diffraction characteristics of outgoing light with respect to incident light from the surface of the substrate have polarization direction dependence. A Fresnel lens comprising the formed refractive index changing portion. 2. The Fresnel lens according to claim 1, wherein an X plate is used as the substrate containing lithium niobate or lithium tantalate as a main component. 3. The thickness d of the refractive index changing portion and the wavelength λ of the incident light.
2. The Fresnel lens according to claim 1, wherein there is a relationship of d = λ / (2 · Δn) between the refractive index change portion and the refractive index difference Δn between the substrate and the substrate. 4. A step of forming a mask pattern on the surface of a substrate containing lithium niobate or lithium tantalate as a main component, and using the mask pattern as a mask, diffraction characteristics of outgoing light with respect to incident light from the substrate surface And a step of forming a refractive index changing portion on the surface of the substrate by an ion exchange method so as to have polarization direction dependency. 5. The method for producing a Fresnel lens according to claim 4, wherein the ion exchange method is carried out in benzoic acid at a temperature of 160 ° C. to 250 ° C.