JPH0644082B2 - Flat lens - Google Patents

Description

The present invention relates to a flat plate lens (hereinafter, referred to as a flat plate microlens) in which a gradient index lens is integrally formed in a transparent substrate.

[Description of Prior Art]

As shown in FIG. 9, the flat microlens 1 is made of glass.
A transparent substrate 2 having a flat surface, such as plastic, has a refractive index gradient lens 3 integrally embedded therein.
The lens 3 is a plane whose one refraction surface coincides with the surface of the substrate 2, has an optical axis in the normal direction of the substrate surface, and its refractive index distribution gradually decreases at the maximum in the optical axis direction toward the surface and toward the deep part. In addition, the distribution shape has a maximum at the center in the direction orthogonal to the optical axis and gradually decreases toward the side edge.

Further, the planar shape includes a circular shape as shown in FIG. 10 and a line shape as shown in FIG.

Explaining a typical method of producing the above-mentioned flat plate microlens, as shown in FIG. 12, first, the surface of the glass substrate 2 is covered with an ion permeation preventive mask 5 made of a metal thin film or the like, and For example, a circular minute opening 6 similar to the planar shape of the lens to be obtained is provided. Then, the substrate mask surface is dipped in a molten salt 7 containing cations, such as thallium (Tl) ions, which contribute to increasing the refractive index of the substrate glass. As a result, the cations in the molten salt 7 diffuse into the substrate through the opening 6 of the mask 5, and after the ion diffusion process for a certain time, the concentration of the cation is highest near the opening toward the deep and peripheral portions. A gradually decreasing ion concentration distribution is formed, and this refractive index gradient portion, that is, the lens 3 is formed by this ion concentration distribution. This flat plate microlens functions as a lens due to the refractive index distribution created by the substance diffused in the substrate. Therefore, the size of the refractive index distribution region can be treated in the same manner as a general lens.

[Problems to be solved by the invention]

In the conventional flat plate microlens, as shown in FIG. 13, the diffusion area of the substance that contributes to the increase of the refractive index on the substrate surface is the lens diameter 2a, and the diffusion region of the substance in the optical axis direction is the lens thickness. It is believed that d / a = 1.0, that is, the cross-sectional shape is a nearly perfect semi-circle, and that the best optical characteristics are exhibited when the shape is a perfect semi-circle. Was focused on how close to a perfect semicircle. (For example, M. Oikawa,
K. Iga and T. Sanada, ； “Distributed-index planer m
icrolens arrey prepared from deep electronigratio
n ”, Electron. Lett., 17 (13) 452-454 (1981)) However, as a result of the experiments conducted by the present inventors, in a flat plate microlens having a gentle refractive index gradient formed by ion diffusion, When the cross-sectional shape is approximated to a perfect semi-circle, a misalignment occurs between the focal point 8A of the incident ray 8 near the optical axis and the focal point 9A of the incident ray 9 near the peripheral edge of the ray incident on the lens, and the aberration increases. The number of apertures that can be effectively used as a lens is small, and when the lens cross-sectional shape has a flatness within a certain range (hereinafter, d / a is referred to as a flatness), the above aberration is minimized and the lens is effective. We have found that the numerical aperture can be increased.

It should be noted that, in FIGS. 2, 3, 2, 4, and 13, the light rays are described as being refracted at the contour surface of the lens portion (described later in detail) due to ion diffusion for convenience of notation. There is. However, the light rays are essentially refracted by the difference in the refractive index, and specifically, they are continuously refracted at the portion of the refractive index gradient.

The present invention has been completed based on the above findings.

[Means for solving problems]

In a transparent lens, a plate-shaped lens integrally formed with a lens having an optical axis in the normal direction of the substrate surface and having a refractive index gradient that gently changes toward the optical axis direction and a direction orthogonal to the optical axis, Let d be the lens thickness on the optical axis,
When the lens diameter on the substrate surface is 2a, d / a is 0.46 or
Within the range of 0.78.

[Work]

When the cross-sectional shape of the gradient index lens portion is made flat as described above, the lens aberration becomes smaller than in the case of a substantially semi-circular shape, and the effective numerical aperture of the lens can be correspondingly increased as shown in a specific example described later. .

〔Example〕

The present invention will be described below in detail based on the embodiments shown in the drawings.

FIG. 1 is a sectional view of a flat plate microlens 10 according to the present invention, in which a refractive index gradient lens 12 is provided in a transparent glass substrate 11.
Are integrally formed. This lens 12 has an optical axis 13 in the direction normal to the substrate surface, and the concentration of diffused ions is maximized near the intersection 14 of this optical axis 13 and the substrate surface, and the refractive index is also maximized. It has a refractive index distribution in which the refractive index gradually decreases with the concentration of diffused ions.

The refractive index gradient lens 12 is a sodium ion containing cations, for example, thallium ions, which increase the refractive index of the substrate glass through the opening of the mask provided on the surface of the glass substrate by the ion diffusion method described above. ,
It is formed by the concentration distribution of ions that have penetrated into the glass by being exchange-diffused with ions such as potassium. As described above, when a lens is formed by diffusing thallium ions in glass, there is a phenomenon that a clear boundary surface is shown between a region where the ions are diffused and a region where the ions are not diffused. Hereinafter, this boundary surface will be referred to as a diffusion front.

When the cut surface of the flat plate microlens obtained as described above is observed, the diffusion front 12A can be seen. This diffusion front 12A is seen because the thallium ions change relatively sharply. The hardness of the glass in the portion where the thallium ions are diffused is lowered and the progress of ion diffusion is promoted. Therefore, unlike general ion diffusion, the concentration of thallium ions changes sharply in the vicinity of the boundary between the region where thallium ions are diffused and the region where thallium ions are not diffused. Optically observing this part, it was several percent higher than the maximum concentration of thallium ions in the glass substrate.
A boundary surface is observed in the lowered part.

Then, the above-mentioned diffusion front, generally speaking, the cross section is observed, and the contour curve 12A recognized as the boundary line with the bulk portion of the substrate is hereinafter referred to as the substrate-inside outer surface of the lens 12.

As described above, the optical properties of a flat plate microlens having a refractive index gradient depending on the thallium ion concentration in the glass are determined by the refractive index difference given by the thallium ions and the shape of the refractive index distribution.

In general, the numerical aperture showing the condensing power of a lens increases in this case as the maximum refractive index difference given by thallium ions increases, but if the shape of the distribution is not appropriate, the aberration will be large and the aperture can be used effectively as a lens. The number is too big to be taken.

In this case, this maximum refractive index difference is almost proportional to the thallium ion concentration in the glass, and in the ion exchange, the alkali ions contained in the substrate glass and the thallium ions are replaced by approximately 1: 1 and the refractive index distribution is changed. Are formed by the concentration of alkali ions exchanged in the glass substrate.

However, the amount of alkali ions contained in the substrate glass is limited in terms of chemical stability and weather resistance of the glass.

Therefore, in order to obtain a lens having a large effective numerical aperture, it is necessary to set the contour shape of the region having the refractive index distribution to an appropriate shape. The contour shape of the lens can be controlled by changing the ratio of the lens radius a to be manufactured and the radius rm of the opening of the mask. here,
Let a be the desired lens radius, rm be the aperture radius of the mask (half-width in the case of a line aperture), and a / rm be the mask aperture ratio.

Generally, the diffusion spreads isotropically from the opening of the mask in the direction of the substrate surface and in the direction of the depth. When the above-mentioned aperture ratio is set to 5 or more, the cross-sectional shape of the diffusion front becomes almost semicircular,
This mode of diffusion can almost be regarded as point diffusion.

On the other hand, when the aperture ratio is decreased (that is, the aperture of the mask is made sufficiently large with respect to the lens radius), the diffusion mode changes from point diffusion to an intermediate state between point diffusion and surface diffusion. At this time, the cross-sectional shape of the diffusion front changes from a semicircular shape to a flattened shape. Then, as a result of repeated experiments on the shape of the region having the above-described refractive index distribution in the flat plate microlens, that is, the relationship between the lens shape and the effective numerical aperture, the diameter of the lens 12 on the substrate surface was determined.
2a, where d is the thickness of the lens on the optical axis, the effective numerical aperture of the lens depends on the value of d / a, and the effective numerical aperture of the lens has a maximum value near this specific value where d / a is 1.0 or less. And d
It has been found that the lens aberration increases and the effective numerical aperture decreases as / a deviates in the larger and smaller specific values.

That is, when the cross-sectional shape of the lens finally obtained approaches a substantially semicircular shape (d / a≈1.0) by increasing the above-mentioned ion diffusion processing time, etc., as shown in FIG. A positive spherical aberration occurs in which the focal point 15A position of the light beam incident near the lens portion is further away from the lens surface than the focal point 15B position of the paraxial light beam, and the effective numerical aperture of the lens is reduced.

Then, as the value of d / a (flatness) becomes smaller, the aberration becomes smaller, but when the flatness becomes too small, the lens outer surface in the center of the lens becomes flat as shown in FIG. Since the light ray incident on the peripheral portion having a large curvature is largely bent with respect to the light ray incident on the near portion, the negative spherical aberration is increased and the effective numerical aperture of the lens is also decreased. Then, as defined by the present invention, the lens outer surface inside the substrate has a flatness ratio d / a of 0.46.
By setting the curved surface so that it falls within the range of 0.78 to 0.78, the aberration due to the shift of the focal point 15 position of the paraxial light and the peripheral light becomes small as shown in FIG. 2, and a large effective numerical aperture of approximately 0.1 or more is obtained. Can be achieved, especially d / a of 0.5 to 0.6
A range of 9 is preferred.

Next, a suitable method for manufacturing the flat plate microlens according to the present invention will be described.

As shown in FIG. 5, the surface of the substrate 10 made of a transparent glass plate is first covered with an ion permeation preventive mask 16 made of a metal thin film or the like, and the mask 16 is provided with an ion diffusion opening 17 by using a well-known patterning technique. The planar shape of the opening 17 is similar to the planar shape of the lens to be obtained, for example, if the lens is a circular lens, the opening 17 is circular, and if it is line-shaped, it is linear, and the intervals are appropriately set according to the lens arrangement pattern. Place it.

Here, the size of the mask aperture 17 is very important, and if the aperture 17 is too small, the cross-sectional shape of the lens obtained by the subsequent ion diffusion process becomes a semicircle, and the effective numerical aperture of the lens is as described above. Get smaller. If the mask aperture 17 is too large, the flatness of the cross-sectional shape of the lens obtained will be too small, and the effective numerical aperture will be small.

And the radius of the mask opening (in the case of line-shaped opening, half width)
Where rm is the lens radius to be obtained and the mask aperture ratio a / rm is set within the range of 1.75 to 4.5, the d / a value is within the range of 0.46 to 0.78 and the aberration is small. As a result, a lens having a large effective numerical aperture can be obtained. Particularly, by setting a / rm within the range of 1.9 to 3.3, a lens having an effective numerical aperture of about 0.16 or more can be obtained.

Next, the substrate 11 provided with the mask having the opening set as described above is subjected to the ion diffusion treatment. This ion diffusion treatment may be the same as the conventional method. As an example, the mask surface side of the substrate is immersed in a molten salt such as sulfate or nitrate containing a cation such as thallium (Tl) that increases the refractive index of the substrate glass. If the ion addition treatment temperature is too low, the diffusion coefficient is small and it takes too much time to obtain a lens of a desired size, and if the treatment temperature is too high, the substrate glass is thermally deformed, so the glass transition temperature (Tg) It is desirable to carry out ion diffusion treatment within a range of 50 ° C above and below.

Although the case where glass is used as the substrate has been described above as an example, plastic or the like can also be used. For example, in the case of molding a flat plate microlens with plastic, first, a monomer that forms a polymer having a relatively low refractive index is partially polymerized to form a gel-like substrate, and the surface of the gel substrate is formed into a predetermined shape. A mask material having an opening of a size is provided, and a monomer forming a polymer having a relatively high refractive index is diffused into the substrate through the opening, and then the whole is heat treated to complete the polymerization. You can take the way.

Next, specific examples of the present invention will be described.

Specific example 1 mol% SiO ₂ 60%, B ₂ O ₃ 4%, ZnO 15%, K ₂ O 8%, Na
48 mm x 48 m consisting of glass with a composition of ₂ O 13%
Four m × 2 mm substrates were prepared.

After depositing a Ti film with a thickness of 1 μm as an ion permeation preventive mask on these substrate glass surfaces by sputtering, a photolithography method and a hydrofluoric acid-based etchant are used to make the diameter 10 μm to 100 μm at intervals of 10 μm. provided stepwise varied with ten circular apertures, respectively, immersed mask surface of the glass substrate, Tl ₂ sO ₄ 60 mol%, in the molten salt heated dissolved混塩of ZnSO ₄ 40 mol% to 490 ° C. Then, the ion exchange treatment is performed for 2 hours, 4 hours, and 8 hours.
5 kinds of time, 12.5 hours, 17 hours.

After the ion exchange, the surface of the substrate was polished to remove the Ti film and the surface was smoothed, and the characteristics of the gradient index lens formed in the substrate were measured.

The results are shown in Tables 1 to 5 by ion exchange treatment time.
Shown in the table.

In Tables 1 to 5, the focal length f of the lens is the wavelength 633n.
When He-Ne laser light of m was incident from the opposite side of the diffusion surface, the distance from the substrate surface where the light was condensed and the maximum brightness was measured.

NAf represents the numerical aperture as a value obtained by dividing the radius of the lens by the focal length. However, this numerical aperture cannot be used effectively over the entire range when the lens has aberrations.

Therefore, as shown in FIG. 6, a grid-striped pattern 18 having a distance of 2 mm was arranged 60 mm in front of the flat plate microlens 10, and the image forming padan by the lens was observed with a microscope 19 of 2.5 × 5.

According to this observation, when the lens has an aberration, the area of the lens having the aberration does not contribute to image formation, so that the angle of view becomes small. The half-width sine (Sin) of this angle of view is shown in Tables 1 to 5 as the effective numerical aperture NAP. FIG. 7 is a graph showing the relationship between the lens flatness d / a and the effective numerical aperture NAP. From the graph, it can be seen that the effective numerical aperture of the flat microlens has a maximum when the aspect ratio d / a of the lens is around 0.58.
This is because the spherical aberration of the lens changes from negative to positive in the vicinity of the above point, and the aberration becomes minimum. The flatness ratio d / a is 0.5
To obtain an effective numerical aperture of 0.16 or more within the range of 0.69,
It can be seen that an effective numerical aperture of about 0.1 or more can be obtained within the range of 0.46 to 0.78.

Figure 8 shows the mask aperture ratio and the effective numerical aperture NAP of the lens.
The relationship with is shown in the graph. The horizontal axis of the graph in the figure represents the ratio of the lens diameter 2a to the mask opening diameter 2rm.

From the figure, an effective numerical aperture of about 0.1 or more is obtained within the range of a / rm of 1.75 to 4.5, and particularly a large effective numerical aperture of about 0.16 or more is obtained within the range of a / rm of 1.9 to 3.3. I understand.

Specific Example 2 Three glass substrates having the same composition as in Specific Example 1 were prepared, T films were attached to the surfaces of these substrates as ion diffusion preventing masks, then openings with a diameter of 400 μm were provided in the mask films, and these substrates with masks were prepared. 16 hours each at 490 ℃ molten salt,
A flat plate microlens was manufactured by immersing it for 144 hours and 576 hours.

The diameter 2a of the lens formed in the substrate is 0.6 mm, 0.9 mm, 1.6 mm, and the lens thickness d for the above processing times, respectively.
Is 0.1mm, 0.25mm, 0.6mm, focal length is 2.5mm, 2.25mm, 6.7mm
And the effective numerical aperture NAP observed from the checkered pattern
Was 0.12, 0.2, 0.12.

The flatness d / a of each lens is 0.33, 0.56, 0.
75, the ratio of the lens radius a to the mask aperture radius rm, that is, the mask aperture ratio a / rm was 1.5, 2.25, 4.0.

From the above results, it was confirmed that the aberration of the lens can be minimized and the NA can be maximized by setting the aspect ratio to about 0.56 even for a lens having a diameter of about 0.9 mm.

〔The invention's effect〕

According to the present invention, it is possible to obtain a flat plate microlens with a small aberration and a large effective numerical aperture.

INDUSTRIAL APPLICABILITY The flat plate microlens according to the present invention is useful in a wide range of applications such as general image transmission and optical system for condensing / collimating a light beam.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a flat plate microlens of the present invention, FIG. 2 is a sectional view showing a condensing function of the lens of FIG. 1, and FIG. 3 is a case where the flatness of the lens section is too small. FIG. 4 is a sectional view showing a condensing state, FIG. 4 is a sectional view showing a condensing state when the flatness of the lens is too large, and FIG. 5 shows a mask aperture and a lens diameter when manufacturing the lens according to the present invention. FIG. 6 is a sectional view for explaining the relationship, FIG. 6 is a perspective view schematically showing a method for measuring the effective numerical aperture of the lens, and FIG. 7 is a lens flatness ratio d.
/ A and the effective numerical aperture NAP of the lens, Fig. 8 shows the relationship of the effective numerical aperture of the lens to the ratio of the diameter of the aperture of the mask provided on the substrate surface at the time of ion diffusion treatment and the obtained lens diameter. The graph which shows, FIG. 9 is sectional drawing of the conventional flat plate micro lens, FIG.10 and FIG.11 is the top view which shows the lens shape example of a flat plate micro lens, FIG.12 is sectional drawing which shows the example of the method of an ion diffusion process. FIG. 13 is a cross-sectional view for explaining the relationship between the lens diameter and the lens thickness in the conventional flat plate microlens. 1,10 …… Flat microlens, 2,11 …… Substrate 3,12 …… Gradient index lens 5,16 …… Ion permeation prevention mask 6,17 …… Mask opening, 7 …… Molten salt 13 …… Light Axis, 15,15A, 15B …… Focus 18 …… Pattern, 19 …… Microscope

Claims (2)

[Claims] 1. A substance, which has an optical axis in the direction normal to the substrate surface and which contributes to an increase in the refractive index, is ion-diffused in a substantially parallel glass substrate in the direction normal to the substrate surface. By doing so, in a flat lens in which a region having a gently changing refractive index gradient and acting as a lens is integrally formed, a diffusion region of the substance on the optical axis is defined as a lens thickness d, and A flat lens, wherein d / a is within a range of 0.46 to 0.78 when a lens diffusion diameter is 2a for the substance. 2. A flat lens according to claim 1, wherein the ratio of d to a is within the range of 0.5 ≦ d / a ≦ 0.69.