Planar lens

Details Planar lens Planar lens

2003-02-24

2004-11-09

Lester, Evelyn A. (Department: 2873)

Optical: systems and elements

Lens

With graded refractive index

C359S653000, C359S654000

Reexamination Certificate

active

06816319

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a planar lens for use in the art of optical communication, optical data processing, etc., and a production method thereof.
DESCRIPTION OF THE RELATED ART
To cope with the rise in capacity of communications or the rise in the amount of data to be processed in data processing, the utilization of parallelism of light has been noted as an extremely effective means. In order to utilize parallelism of light, it is desired that optical fibers, which are optical transmission means, be arranged in arrays, not to mention light source and optical detector. It is necessary that lenses, which are light coupling means, be arrayed as well.
A planar lens array having many fine lens formed in a transparent flat substrate has excellent characteristics of arrayed lens. Since such a planar lens array is prepared by photolithography, a fine lens having a diameter of not greater than 1 mm can be easily prepared. Further, these lenses can be arrayed with a high precision. Accordingly, these lenses can be easily aligned with the optic axis of other arrayed optical elements and thus are suitable for parallel optical transmission and data processing.
A representative method for producing a planar lens array is disclosed in JP-A-61-201639. A glass substrate is coated with a metallic titanium film as an ion diffusion preventive mask. The titanium film is then provided with a circular aperture for diffusion of ion using an ordinary photolithographic technique. The glass substrate thus masked is then dipped in a molten salt mixed with nitrate so that alkaline metal ions in the glass substrate are exchanged with components for increasing the refractive index of glass such as thallium (Tl) ion and silver (Ag) ion in the molten salt. In this manner, a distribution of refractive indices corresponding to the distribution of concentration of hemispherically dispersed ions is formed in the glass substrate. This distribution acts as a lens.
In addition, descriptions of graded refractive index lenses can be found in JP-A-61-26535 and JP-A-61-132541. More specifically, these documents disclose in their Examples that ion exchange took place between a glass plate in which P
2
O
5
, Na
2
O, K
2
O and Al
2
O
3
are contained and a fused salt made of silver nitrate and potassium nitrate. Further, they disclose that the distribution curve patterns showing refractive index-distributions within the refractive index distribution region were upwardly convex in shape.
SUMMARY OF THE INVENTION
When Tl is used as a refractive index-increasing component, the resulting lens can be provided with a raised numerical aperture. However, since the rate of diffusion of Tl ion in the glass substrate is small, it takes much time to prepare a lens having a diameter of not smaller than 100 &mgr;m.
Moreover, the arts disclosed in the above-cited documents JP-A-61-26535 and JP-A-61-132541 relate to treatment for exchanging ions between Ag ions and Na or/and K ions in phosphate glass.
On the other hand, when Ag is used as a refractive index-increasing component, a lens having a diameter of not smaller than 100 &mgr;m can be easily prepared because the rate of diffusion of Ag ion in the glass substrate is high.
However, the phosphate glass containing about 48 weight % P
2
O
5
, about 20 weight % Na
2
O and about 9 weight % K
2
O is used as mother glass in the documents JP-A-61-26535 and JP-A-61-132541. In this glass, phosphoric acid acts as network former, and besides, alkalis are contained in high amounts. Therefore, such glass has a drawback of being poor in chemical resistance.
The invention has been made in order to solve such problems. Therefore, an aim of the invention is to provide a planar lens having a great numerical aperture acquired by diffusion of Ag as a refractive index-increasing component into a glass substrate containing Li and having excellent chemical resistance and a method of producing the aforesaid planar lens.
The invention is intended for a hemispherical planar lens in which a refractive index-increasing component is diffused in a substantially hemispherical form or a semicylindrical (lenticular) planar lens in which a refractive index-increasing component is diffused in a substantially semicylindrical form. These planar lenses are characterized by utilizing Ag as a refractive index-increasing component and further containing in a glass substrate at least Li as a component of alkali metal to undergo ion exchange for Ag.
In the case of using Ag as a refractive index-increasing component, long-range diffusion of Ag is controlled to a greater extent as a difference between the ionic radii of Ag and an element exchanged for Ag becomes bigger. As a result, the distribution curve of refractive index comes to have a convex form in the positive direction of the coordinate axis for the refractive index n.
However, the condition of Ag diffusion is also influenced by factors other than the ionic radii of ions participating in ion exchange. For instance, the state of chemical bonds in glass is one of such factors. Specifically, the stronger the interaction between Ag and elements forming a network structure of the glass, the more greatly the diffusion of Ag is retarded.
When a refractive index distribution curve of the present planar lens is rendered convex in the positive direction of the coordinate axis for the refractive index, the numerical aperture of the lens can be enlarged for the following reason.
More specifically, the refractive index at some diffusion length is greater than that in a “linear” distribution, provided that the refractive index distribution curve is convex in the positive direction of the coordinate axis for the refractive index n within the distribution region of refractive indices of a planar lens. In other words, the refraction of light at the diffusion length satisfying the above condition becomes great and the focal length of the lens can be shortened. As a result, the numerical aperture of the lens can be enlarged. Additionally, the present planar lens is large in numerical aperture and sufficiently practicable as far as the numerical apertures thereof are of the order of 0.1.
(1) A planar lens of a first embodiment of the present invention, in which a refractive index distribution is formed by diffusing a refractive index-increasing component into a flat glass substrate in a substantially hemispherical form or semicylinder form,
wherein the refractive index-increasing component is silver, and the flat glass substrate comprises Li as an alkali metal that is subject to an ion exchange with the silver
(2) The planar lens as described in the item (1), wherein, when taking a radial distance: r from the center of the hemisphere or the semicylinder and a refractive index: n at the radial distance r as coordinate axes orthogonal to each other, a distribution curve of the refractive index: n with respect to the radial distance: r is a convex curve in the positive direction of the coordinate axis for the refractive index: n.
(3) The planar lens as described in the item (1), wherein, when the distribution of refractive indices is expressed as a function: n(r) of a radial distance from the center of the hemisphere or the semicylinder: r and a refractive index: n, the second derivative: n″(r) of the function n(r) is negative or zero.
(4) The planar lens as described in the item (1), wherein the flat glass substrate comprises:
0 to 80 mol % SiO
2
;
0 to 80 mol % B
2
O
3
;
0 to 80 mol % P
2
O
5
;
3 to 45 mol % Li
2
O;
0 to 20 mol % Na
2
O;
0 to 30 molt MgO;
0 to 30 mol % CaO;
0 to 30 mol % SrO;
0 to 30 mol % BaO;
0 to 30 mol % ZrO
2
;
0 to 30 mol % Y
2
O
3
;
0 to 20 mol % La
2
O
3
;
0 to 40 mol % Al
2
O
3
; and
0 to 20 mol % Sm
2
O
3
,
wherein the sum of SiO
2
, B
2
O
3
and P
2
O
5
is 30 to 80 mol %, and the sum of Li
2
O and Na
2
O is 3 to 65 mol %.
(5) The planar lens as described in the item (1), wherein the flat glass substrate comprises:
30 to 80 mol % SiO
2
;
0 to 60 mol % B
2
O
3
;
3 to 45 mol % Li
2
O;
0 to 20 mol % Na
2
O;
0 to 30 mol % MgO;
0

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