JPH07108796B2 - Method for manufacturing base glass substrate for gradient index lens - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a method for producing a base glass substrate suitable for producing an axial gradient index lens.

[Description of Prior Art]

As an effective method for correcting spherical aberration that is difficult to avoid with a spherical single lens made of optical glass with a uniform refractive index, a spherical lens with a refractive index distribution that changes in the optical axis direction (Referred to as mold lens) is proposed.

The axial gradient index lens has, for example, a transparent plate-like body whose refractive index monotonically decreases in the thickness direction and which has a uniform refractive index in a plane perpendicular to the optical axis. The spherical aberration can be suppressed to a very small value.

As a base material for manufacturing the above axial direction gradient index lens, a gradient index glass material having a precisely controlled refractive index distribution over the thickness required for spherical surface processing is required.

The most practical method for producing the gradient index glass material is the ion exchange method. In the ion exchange method, a glass body containing a monovalent cation is brought into contact with a medium such as a molten salt containing another monovalent cation, and the monovalent cation in the glass and another monovalent cation in the medium are contacted with each other. Are exchanged, and the concentration distribution of monovalent cations is given in the glass body to form a refractive index distribution. Usually, a glass body having a refractive index distribution is produced by forming a concentration distribution of monovalent cations having a large effect of increasing the refractive index in glass such as Li ions, Cs ions, and Tl ions.

However, when an axial gradient index glass material is produced by the above ion exchange treatment, it takes a very long time to obtain the required refractive index profile due to the slow ion diffusion rate. Is not practical.

Therefore, an electric field application ion exchange method in which a DC electric field is applied between both surfaces of the glass substrate to accelerate the movement of ions is effective.

By applying an electric field, the movement of ions in the glass substrate is promoted only in one direction (from the anode side to the cathode side), so compared to the case of ion exchange without applying an electric field due to only the diffusion phenomenon, It is also possible to form completely different ion concentration distributions.

As a method of applying an electric field, as shown in Japanese Patent Application No. 60-265918, a medium containing a monovalent cation other than Tl ions is brought into contact with one surface of a glass substrate containing Tl ₂ O as a component,
By applying a DC electric field having the medium side as an anode to both surfaces of the glass substrate, the Tl ion concentration in the substrate can be linearly changed to manufacture the axially gradient index glass material. For example, in the case of the comparative example described later, Tl ₂ O, Na ₂ O, K ₂ O
A glass substrate containing 8.2, 9.6, and 2.2 mol% as a component, respectively, and KNO ₃ as a medium on the anode side are brought into contact with each other to carry out an ion exchange by applying an electric field, whereby M ₂ O shown in FIG.
The concentration distribution can be obtained. Since Tl ₂ O1 mol%, corresponding to an increase [Delta] n = 0.014 of the refractive index, Tl ₂ O in FIG. 8
The portion where the concentration increases linearly has a refractive index distribution which also linearly increases. Therefore, M ₂ O shown in FIG.
A glass plate having a concentration distribution can be used as an axial refractive index distribution material.

[Problems to be Solved by the Invention]

However, in the case of FIG. 8, the M ₂ O concentration changes extremely rapidly at two locations (hereinafter referred to as discontinuous planes). On the discontinuous surface, the physical properties such as the density, the coefficient of thermal expansion, and the glass transition temperature of the glass substrate change abruptly, so stress tends to concentrate during each step of the electric field application ion exchange, which may cause the clutch or deformation of the glass substrate. It is harmful because

[Means for solving problems]

According to the present invention, one surface of a glass substrate containing monovalent cations is
A molten salt containing a monovalent cation different from at least the ions is brought into contact, and a DC electric field having the molten salt side as an anode is applied to both surfaces of the glass substrate to obtain a refractive index in the thickness direction of the glass substrate. In the method for producing a base glass substrate for a gradient index lens that forms a distribution, the glass substrate contains, as its component, two or more of Tl ₂ O, Na ₂ O, and K ₂ O, and these components The mol% content between (1) 0.1 × (Tl ₂ O + Na ₂ O + K ₂ O) ≦ Tl ₂ O (2) 0.45 × (Tl ₂ O + Na ₂ O + K ₂ O) ≧ Na ₂ O (3) 0.3 × ( Tl ₂ O + Na ₂ O + K ₂ O) ≦ K ₂ O, and the total amount of Tl ₂ O, Na ₂ O and K ₂ O is mol%, (4) 5 ≦ (Tl ₂ O + Na ₂ O + K ₂ O ) ≦ 40, the present invention provides a method for manufacturing a preform glass substrate for a gradient index lens.

As the glass system in the present invention, a glass system such as silicate, aluminosilicate, borosilicate, phosphate is used, but mainly composed of SiO ₂ from the viewpoint of chemical durability and molten salt resistance. A glass-based glass is desirable. Considering chemical durability, meltability, molten salt resistance, refractive index, Abbe number, etc., trivalent components such as B ₂ O ₃ , Al ₂ O ₃ , MgO, CaO, BaO, SrO, ZnO, PbO It is possible to add a divalent component such as, a tetravalent component such as TiO ₂ , ZrO ₂ , and GeO ₂ .

As monovalent components, in addition to Tl ₂ O, Na ₂ O, K ₂ O, Li ₂ O, Cs ₂ O, Rb ₂
Needless to say, O and the like can be added. Also, Tl ₂
The total amount of O, Na ₂ O and K ₂ O is preferably selected in the range of 5 to 40 mol%. When the above value is less than 5 mol%, the meltability of glass deteriorates and the difference in refractive index after the electric field application ion exchange treatment becomes small. When the above-mentioned value exceeds 40 mol%, the glass becomes unstable and is not suitable.

[Work]

The three types of ions of Tl ⁺ , Na ⁺ , and K ⁺ have a large difference in mobility in glass, and Na ⁺ is very mobile. When moving monovalent cations in glass by the force of the electric field, Na ^{+, which} is easy to move, moves preferentially, and slow-moving Tl ⁺ supplements the shortage of Na ⁺ to satisfy the condition of electrical neutrality. Follow in shape. For this reason Tl
^In some cases, the ⁺ concentration becomes higher than that in the original glass, and furthermore, a discontinuous surface in which the Tl ⁺ concentration changes extremely rapidly is formed (see Comparative Example 2). This discontinuity is Tl ⁺ −K ⁺ , Na ⁺ −K
^It can also occur with a combination of ⁺ (see Comparative Examples 1, 2, and 3).

However, according to the research conducted by the present inventors, the mobility of each ion greatly changes depending on its concentration. Therefore, Tl ⁺ , Na
By properly selecting the ion concentration of ⁺ and K ⁺ , it is possible to avoid the generation of discontinuity due to the electric field application ion exchange operation.

A discontinuous surface is not generated by the electric field application ion exchange operation, and the composition range useful as a substrate for the axial gradient index material is as follows.

First, if the Tl ₂ O concentration in the glass substrate is too low, the refractive index difference after the electric field-applied ion exchange tends to be small, and the linear concentration distribution shape of Tl ₂ O tends to be slow. The condition of (Tl ₂ O + Na ₂ O + K ₂ O) ≦ Tl ₂ O must be satisfied.

Also, if Na ₂ O in the substrate is too high, the concentration distribution of Tl ₂ O by the electric field applied ion exchange is becomes smooth, Na ₂ O
And K ₂ O concentration change rapidly, creating a discontinuous surface. Therefore, 0.45 × (Tl ₂ O + Na ₂ O + K ₂ O) ≧ Na ₂ O is required.

Furthermore, if the amount of K ₂ O in the substrate is too small, a discontinuous surface where the Tl ₂ O concentration changes rapidly is likely to occur, so 0.3 × (Tl ₂ O + Na ₂ O + K ₂ O) ≤ K ₂ O Is required.

〔Example〕

 Hereinafter, the present invention will be described in detail based on examples.

FIG. 1 is a longitudinal sectional view of an apparatus for performing an electric field application ion exchange treatment. In the figure, 1 is a glass substrate, and a side wall 1A is integrally formed around the substrate 1 to form a container. The container space is filled with the cathode-side molten salt 2A. A circular electrode 3A made of titanium is immersed in the molten salt 2A. Reference numeral 6 is a cover plate for preventing evaporation of the molten salt. Further, the lower surface of the glass substrate 1 is in contact with the anode side molten salt 2B contained in the quartz container 7. An electrode 3B made of platinum is immersed in the molten salt 2B.

The entire apparatus described above is maintained near the glass transition temperature of the glass substrate 1, and a DC electric field is applied through the lead wires 5A and 5B so that the cathode side electrode 3A is negative and the anode side electrode 3B is positive to perform ion diffusion treatment. To do. By applying an electric field, the above-mentioned monovalent cations penetrate into the lower surface of the glass substrate from the molten salt 2B on the anode side, and the monovalent cations in the glass substrate are expelled from the upper surface of the glass substrate into the molten salt 2A on the cathode side. Ion exchange of the substrate is performed.

As the glass substrate 1, a disk-shaped glass substrate having a diameter of 60 mm and a thickness of 5 mm and having the composition shown in Table 1 was used.

(M ₂ O represents Na ₂ O, K ₂ O, Tl ₂ O) Anode-side molten salt 2B is a single salt of potassium nitrate, and cathode-side molten salt 2A is sodium sulfate, potassium sulfate, or zinc sulfate. A mixed salt mixed in a molar ratio of: 20: 60 was used.

Using the glass substrates having various M ₂ O compositions, the electric field application ion exchange treatment was performed under the conditions shown in Table 2. After the ion exchange treatment, only the glass substrate portion was taken out of the apparatus and gradually cooled, or the temperature was temporarily raised by about 50 ° C. to relax stress and then gradually cooled.

In the case of Examples 1, 2, 3, 4, and 5, the Tl ₂ O concentration was linearly changed over the thickness of 0.4 to 3 mm by the electric field applied ion exchange treatment, and the change of the M ₂ O concentration was smooth and non-uniform. It was possible to fabricate an axial gradient index material without a continuous surface.

On the other hand, in Comparative Examples 1, 2, and 3 using the glass substrate having a composition outside the scope of the present invention, one or two discontinuous surfaces were generated.

〔The invention's effect〕

In the substrate glass material according to the present invention, the Tl ₂ O concentration changes linearly over a part or the whole of the thickness direction by performing the electric field application ion exchange treatment. In addition, the axial direction gradient index material having a smooth M ₂ O concentration distribution and no discontinuous surface can be obtained.

[Brief description of drawings]

FIG. 1 is a longitudinal sectional view of an apparatus for performing an electric field applied ion exchange treatment, and FIGS. 2 to 6 are thickness directions of M ₂ O components after an electric field applied ion exchange treatment of a glass substrate of an embodiment of the present invention. FIGS. 7 to 9 are graphs showing the concentration distribution, and FIGS. 7 to 9 are graphs showing the concentration distribution on the comparative glass substrate outside the present invention. 1 …… Glass substrate 1A …… Glass substrate side wall 2A …… Cathode side molten salt 2B …… Anode side molten salt 3A …… Cathode 3B …… Anode 4 …… DC power supply 5A …… Lead wire 5B …… Lead wire 6… … Quartz lid 7 …… Quartz container 8 …… Quartz ring

Claims (1)

[Claims] 1. A single electric field of a glass substrate containing monovalent cations is brought into contact with a molten salt containing monovalent cations different from at least the ions to form a direct current electric field with the molten salt side serving as an anode. Applying to both surfaces of the glass substrate, in the method of manufacturing a base material glass substrate for gradient index lens for forming a refractive index distribution in the thickness direction of the glass substrate, the glass substrate as a component, Tl ₂ O, Na ₂ O or K ₂ O is contained, and the mol% content between these components is (1) 0.1 × (Tl ₂ O + Na ₂ O + K ₂ O) ≦ Tl ₂ O (2) 0.45 × ( Tl ₂ O + Na ₂ O + K ₂ O) ≧ Na ₂ O (3) 0.3 × (Tl ₂ O + Na ₂ O + K ₂ O) ≦ K ₂ O, and the total amount of Tl ₂ O, Na ₂ O, K ₂ O Is a mol%, and (4) 5 ≦ (Tl ₂ O + Na ₂ O + K ₂ O) ≦ 40 is provided, which is a base material glass substrate for a gradient index lens.