JP2001180971A - Noncrystalline optical material and optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical material which is superior in transmissivity for short-wavelength light and high in a refractive index. SOLUTION: The noncrystalline optical material is produced to have a refractive index of 1.8 or more and the following composition, excluding oxygen, in mass %: Te; 10-95, Ba; 0-20, Nb; 0-20, La; 0-20, Y; 0-20, Gd; 0-20, Na; 0-20, Ti; 0-20, Pb; 0-20, W; 0-20, Ge; 0-20, K; 0-20, Ta; 0-20, Zr; 0-20, Hf; 0-20, Sr; 0-20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical material having a high refractive index suitable for use in a lens or the like for high-density recording, and an optical element formed thereby.

[0002]

2. Description of the Related Art When recording is performed on an optical recording medium such as a magneto-optical disk or a DVD-RAM using a laser beam, the smaller the laser beam spot diameter, the higher the density of the recording. In order to reduce the spot diameter, use of light having a short wavelength and NA (Numerical Ap
spots may be created by condensing objectives with large ertures. It is also important to use an optical material having a high transmittance for short-wavelength light in order to increase the light power of the spot and to perform stable writing and reading on the optical disk. It is practically important that an objective lens having a large NA is made of an optical material having a high refractive index in order to exhibit high optical performance. However, at present, there is no lens material that has a high transmittance and a high refractive index over all visible light and is practically usable.

[0003] For example, Japanese Patent Application Laid-Open No. 11-149655,
No. 1-161999 describes that a single crystal of tellurium dioxide having a high refractive index is used for an objective lens or a solid immersion lens for high-density recording by selecting an axial direction, but the production is complicated. Not practical. In addition, tellurium dioxide crystals are slightly yellowish and have poor transmittance of light having a short wavelength of 400 nm or less (blue light).

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical material which has an excellent transmittance of short-wavelength light and a high refractive index.

[0005]

Means for Solving the Problems The object of the present invention is as follows. The components other than oxygen are as follows: Te: 10 to 95% by mass, Ba: 0 to 20% by mass, Nb: 0 to 20% by mass,
La: 0 to 20% by mass, Y: 0 to 20% by mass, Gd: 0
-20% by mass, Na: 0-20% by mass, Ti: 0-20
Mass%, Pb: 0 to 20 mass%, W: 0 to 20 mass%,
Ge: 0 to 20% by mass, K: 0 to 20% by mass, Ta: 0
-20% by mass, Zr: 0-20% by mass, Hf: 0-20
% By mass, Sr: 0 to 20% by mass, and the refractive index is 1.
An optical element made of an amorphous optical material having a content of 8 or more, an optical element made of an amorphous material containing tellurium, and 60% by mass or more of the constituents of the amorphous material excluding oxygen ,
Wherein less than 95% by mass is tellurium. A method for producing a spherical lens in which an amorphous optical material is melted, passed through a plate or net having holes, and dropped freely to form a spherical lens. The sphere formed by melting the amorphous material, passing through a plate or net with holes, and free-falling, in that it was formed by technology or injection molding, and that chromatic aberration was corrected using the diffraction effect Being a spherical lens formed as a shape,
Achieved by

That is, the inventor of the present invention has considered the use of tellurite in the form of glass to be used for an optical element, and has reached the present invention.

Hereinafter, the present invention will be described. As a raw material, tellurium dioxide is preferable to tellurium trioxide which decomposes at a high temperature and is unstable. Usually, tellurium dioxide is 450 ° C
It melts in the vicinity and becomes a molten liquid having a very low viscosity at about 500 ° C. In order to prevent the high refractive index inherent in tellurium dioxide from being reduced, an element used for a ferroelectric material such as Ba (barium), Ta (tantalum), La (lanthanum), Zr (zircon), etc. is added, and melt cooling is performed. When you do
Since the crystallization is delayed and the supercooled state can be obtained, an amorphous optical material can be obtained. At this time, the compounding element may be contained in the form of an oxide or in the form of a compound with tellurium oxide. Since the compounding element has a color in the oxide state, the content is preferably up to about 20% by mass of the constituent components excluding oxygen from the viewpoint that the resulting material does not have excessive absorption. It is preferable that tellurium accounts for 60% by mass or more and less than 95% by mass of the components other than oxygen.

As the compounding elements, Ba, Nb, La,
Y, Gd, Na, Ti, Pb, W, Ge, K, Ta, Z
r, Hf, and Sr, and it is necessary to appropriately select the type and content of the element according to the application.

An alkali metal element such as Na or K or an alkaline earth metal element such as Sr or Ba is blended in an amount of about 10% by mass of the constituents excluding oxygen so that the thickness is 5 mm.
The total transmittance near 400 nm can be improved to 75% or more, which is preferable.

The amorphous optical material of the present invention has a glass transition temperature of about 470 ° C., which is about 100 ° C. lower than that of general optical glass, and can be formed by a glass molding technique. That is, a primary processed product called a preform is prepared, heated and softened, and pressed by an optical surface mold to produce an optical element such as an optical lens.

In the amorphous optical material of the present invention, the viscosity after melting greatly fluctuates with a slight temperature difference, and the viscosity is several mPa · s.
The flow rate is reduced to about s to improve the fluidity, and the optical element can be formed by injection molding using a sealed optical surface mold.

Examples of the optical element of the present invention include a high-density optical recording objective lens and its constituent elements, and a high-resolution imaging lens such as a digital still camera.

In general, an optical material having a high refractive index has a disadvantage in optical design that the dispersion is large and the Abbe number is small, so that the refractive index difference at a plurality of wavelengths is large and the chromatic aberration is large. For example, in a semiconductor laser or the like, the instantaneous oscillation wavelength is single, but the wavelength fluctuates by about 5 to 20 nm over time, so that an optical element that ideally condenses laser light at a certain wavelength is realized. Also,
Chromatic aberration may occur due to wavelength fluctuation, and an ideal condensed spot may not be obtained. The amorphous optical material of the present invention has a very high refractive index and a small Abbe number of about 20, but is apt to cause such chromatic aberration largely in optical design.

Therefore, utilizing the fact that the dispersion has a sign opposite to that of refraction in the diffraction phenomenon, a diffraction groove is formed on the optical surface of the optical element so that the spot of the first-order diffracted light at a certain wavelength and the spot at the fluctuating wavelength can be used. It is preferable to cancel the chromatic aberration at a plurality of wavelengths by matching the spots of the second or higher order diffracted light. Since the amorphous optical material of the present invention has a very high refractive index, the curvature of the optical surface can be reduced, so that the diffraction groove can be easily formed on an optical element or a mold thereof, and the element center of the diffraction groove can be formed. Since the incident angle of the light beam does not change greatly from the periphery to the outer periphery, the diffraction efficiency can be maintained high, and the light amount loss can be reduced. Further, even when the shape of the diffraction groove is complicated to reduce or increase the high-order diffracted light, the processing is easy and a uniform effect can be expected from the center to the outer periphery of the element.

The amorphous optical material of the present invention has a large change in viscosity with respect to the melting temperature, and the viscosity drops sharply at 500 ° C. or higher. Can be freely dropped through. At this time, the optical material forms droplets of various amounts depending on the size of the pores, the wettability with the material constituting the pores and the viscosity at that time due to surface tension, and falls freely. If the optical material is set so as to be slowly cooled during the drop, the droplet becomes spherical due to surface tension, whereby a spherical lens can be produced.

Since the shape of the droplet is slightly deformed by the air resistance during the fall, for a highly accurate application in which sphericity is a problem, the falling atmosphere can be reduced or graphite can be used if the droplet is large. By rolling the slope of the gutter, shape accuracy close to a true sphere can be obtained. If a preform such as a glass mold is manufactured, sufficient spherical precision can be obtained by free fall in a normal air atmosphere.

When a spherical shape is formed by this free fall, a container filled with silicon oil or the like heated to about 250 ° C. is placed at the drop point, and the ball is completed by slow cooling while avoiding cracks due to rapid cooling. Can be recovered. Also, filling the cooling path in the course of the fall with cooling air is very effective in shortening the fall distance and reducing the size of the entire apparatus.

The hole for forming a droplet using the amorphous optical material of the present invention may be formed in the bottom of the container. To improve the volume accuracy of the droplet, a pipe-shaped hole is required to increase the length of the hole. Is preferably provided on the outlet side of the hole so that droplets are formed from the nozzle. Furthermore, when the droplet just leaves the nozzle, the atmospheric pressure on the inflow side with respect to the hole is increased to push it out of the nozzle, and then the pressure is reduced. can do. Further, if the volume accuracy of the droplet is not required and the volume of the spherical lens is 50 mm ³ or less, it may be merely passed through a net using a rather thick line instead of the hole.

In the formation of a spherical lens by free fall, forming a hole or a net with the amorphous optical material of the present invention and a material which is hardly wettable, maintaining a constant viscosity or temperature of the molten optical material, It is important that the member or mesh having holes for holding is heated while controlling the temperature, and that the size of the holes or mesh is optimized. Also, if you want to increase the drop volume, for example, set the temperature low to increase the viscosity of the melt.However, passing through the hole only by gravity may take time, making it difficult to cut from the nozzle. It is effective to pressurize the inflow side atmosphere by forcing it out. According to the method described above, a highly accurate spherical lens can be efficiently mass-produced at very low cost.

[0020]

EXAMPLES Hereinafter, the present invention will be described with reference to examples, but embodiments of the present invention are not limited thereto.

Example 1 (Glass molded lens) 6% by mass of sodium, 4% by mass of potassium, 12% by mass of barium, 78% by mass of tellurium, the remainder being composed of oxygen.
8. A high refractive index glass having a dispersion of 20.6 and a glass transition point of 467 ° C. was obtained. This glass material was cut, ground, and polished to easily obtain a spherical primary processed product (preform) having a diameter of 4 mm. This preform was heated to 490 ° C. to soften it, and then press-molded with a mold having an aspherical optical surface, to obtain a pickup objective lens for an optical disk having an NA of 0.85. The cooling time after forming was 4 minutes, and a series of molding cycles from heating was 6 minutes.

The obtained objective lens has a maximum normal angle of the optical surface of 47 °, which is a gentle curved surface of about 10 ° as compared with 55.8 ° when optical glass having a refractive index of 1.7 is used. The aspherical optical surface of the mold can be easily processed with high precision, and the on-axis thickness of the lens is reduced from 2.8 mm to 2.0 mm, which is advantageous for reducing the size and weight. In addition, since the aspherical optical surface is loose and shallow, it has a shape closer to a spherical surface, so that the lens performance is 1.5 times as large as a processing error or an arrangement error such as a tilt of the optical surface or a tilt of the entire lens. As described above, the advantage that deterioration is difficult was realized.

That is, the wavefront aberration on the axis deteriorates with respect to the surface tilt of 3 minutes in the comparative optical glass having a refractive index of 1.7.
Although about 0.12λ also occurred and could not be put to practical use at all, in the objective lens using the high refractive index glass of the present invention,
0.07λ or less. In the image height characteristics, the wavefront aberration of the comparative glass objective lens deteriorates to 0.065λ at an image height of 30 μm, whereas the objective lens of the present invention using the high refractive index glass has an image height of 0.040λ or less. In this case, the pit signal on the optical disk can be sufficiently detected, and can be put to practical use.

Example 2 (Injection-molded lens) The same high refractive glass as in Example 1 was used as a glass material and heated to 530 ° C. to reduce the viscosity to 10 mPa · s. Injection was performed into the configured lens cavity to form a lens. The mold temperature immediately before the injection was 500 ° C., and cooling was performed at a rate of 30 ° C./min after the injection. When the mold temperature reached 300 ° C., the mold was divided and the molded lens inside was taken out.

The transfer accuracy of the optical surface shape of the mold was 100 nm or less, and the lens element at the forefront of the imaging zoom lens of a digital still camera having a surface roughness Rz of 30 nm or less was formed. Because the refractive index is high, the optical surface has a gentler curvature and the thickness of the lens can be reduced,
It has been found that the amount of aberration of the light beam passing through this lens can be reduced, so that the subsequent lens group for correcting this can be reduced in number and simplified, and can greatly contribute to the reduction in size and weight of the entire lens. Further, since the tolerance can be relaxed because the lens curvature is loose, a lens design with higher performance and higher accuracy can be made by using the same tolerance as the conventional one. In addition, since the color is almost colorless, the color reproducibility, which is a problem particularly in a steel image, is faithful to the real thing, and an image which does not impair the texture can be obtained.

The injection molding method using an amorphous optical material of the present invention does not require the production of a preform as in a glass mold, so the molding cycle is slightly longer than that of a glass mold, but the total molding cost is reduced. it can.

Example 3 (Correction of Chromatic Aberration Utilizing Diffraction Effect) The amorphous optical material of the present invention used in the above-mentioned example, and a light source wavelength of 405 nm was used.
An objective lens having a wavelength variation range of ± 10 nm and an NA of 0.85 was designed. Since this objective lens is a single lens, the wavefront aberration at the focal position on the optical axis is deteriorated by 0.030 to 0.040 λ due to optical design due to wavelength fluctuation of ± 10 nm. On the other hand, when the chromatic aberration is corrected by forming a diffraction groove on the optical surface, the deterioration of the wavefront aberration is reduced by 0.004 to 0.00.
6, which was a practically acceptable level.

Embodiment 4 (Production of Spherical Lens) First, FIG. 1 shows a conceptual diagram of an apparatus used in this embodiment. In the figure, an amorphous optical material 2 in a molten state poured from a melting furnace 1 into a container 3 having holes 4.
Forms a drop 5 by passing through a hole 4 and falls freely in a dropping device 6 comprising a cylindrical cavity, and a blower 8
Through the cooling zone 7 in which an airflow of a predetermined temperature is circulated, and reaches the recovery bath 9 filled with the silicon oil 10 heated to, for example, 250 ° C. by the heater 13. The container 3 is maintained at a melting temperature by the heater 11 so as to maintain the viscosity of the poured optical material 2 at the set viscosity. The air flow circulated by the blower 8 is cooled by contacting the radiator plate 14 on the inflow side where the temperature is high due to the droplets, is heated to a predetermined temperature by the heater 12 through the blower 8, and is blown into the cavity. .

In this embodiment, a 120 mm square and a depth of 40 m
3mm diameter and 5mm length at the bottom of m graphite container
Were opened at 10 mm intervals, that is, 100 holes in total of 10 × 10 were heated at 580 ° C. in a nitrogen atmosphere. Here, the amorphous optical material of the present invention containing 10% by mass of sodium, 6% by mass of barium, and 84% by mass of tellurium in a mass composition excluding oxygen was melted at 580 ° C. and poured. The optical material forms a droplet by a hole and has a diameter of 3.8 mm while freely falling.
After falling 15 m, it rushed into a container filled with silicone oil heated to 280 ° C. and accumulated at the bottom of the container. A cooling area in which hot air of 400 ° C. was circulated from below to above was provided in the falling path, and the falling speed was reduced and cooling was performed. With the addition of 100 ml of the optical material, about 3500 spherical lenses could be obtained within one minute.

Since no pressure was applied to the inflow side of the hole, a small amount of the optical material remained in the hole, but there was no fusion between the falling balls.

[0031]

The amorphous optical material of the present invention has a refractive index of 2.0.
Having the above, the Abbe number is around 20, and has an extremely high refractive index and uniform dispersion. Further, since there is no absorption near 400 nm, it has a high transmittance even for a 405 nm semiconductor laser light which has recently been demanded for practical use in the field of high-density optical recording. In addition, since it is completely glassy, it does not partially crystallize and scatter transmitted light, there is no need to select a principal axis direction or the like, and it is completely homogeneous and has no anisotropy. Further, the Knoop hardness is also about 600, which is almost the same as that of the most common optical glass and is hardly damaged, and the polishing of the optical surface is easy. In addition, the environment resistance is stable, and burns and discoloration are less likely to occur as compared with ordinary optical glass.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an apparatus used in an embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Melting furnace 2 Amorphous optical material 4 Hole 5 Drop 6 Dropping device 7 Cooling-down area 8 Blower 9 Recovery bath 11, 12, 13 Heater

Continued on the front page F-term (reference) 4G062 AA04 BB11 DA01 DA10 DB01 DC01 DD01 DE01 DF01 DF02 DF03 DF04 EA01 EA10 EB01 EB02 EB03 EB04 EC01 EC02 EC03 EC04 ED01 EE01 EF01 EF02 EF03 EF04 FC03 EB01 FB02 FC03 FC03 FC04 FD01 FD02 FD03 FD04 FE01 FF01 FG01 FG02 FG03 FG04 FH01 FH02 FH03 FH04 FJ01 FJ02 FJ03 FJ04 FK01 FK02 FK03 FK04 FL01 GA01 GB01 GC01 GD04 GD05 GD06 GD07 GH07 H01 H03 JJ10 KK01 KK03 KK04 KK05 KK07 KK10 MM04 NN02

Claims (9)

[Claims] The composition of components other than oxygen is Te: 1.
0 to 95% by mass, Ba: 0 to 20% by mass, Nb: 0 to 2
0% by mass, La: 0 to 20% by mass, Y: 0 to 20% by mass, Gd: 0 to 20% by mass, Na: 0 to 20% by mass, T
i: 0 to 20% by mass, Pb: 0 to 20% by mass, W: 0 to 0%
20% by mass, Ge: 0 to 20% by mass, K: 0 to 20% by mass, Ta: 0 to 20% by mass, Zr: 0 to 20% by mass, H
f: 0 to 20% by mass, Sr: 0 to 20% by mass,
An amorphous optical material having a refractive index of 1.8 or more. 2. An optical element comprising the amorphous optical material according to claim 1. 3. An optical element comprising an amorphous material containing tellurium. 4. The optical element according to claim 3, wherein 60% by mass or more and less than 95% by mass of the constituents of the amorphous material except oxygen are tellurium. 5. The optical element according to claim 2, wherein the optical element is formed by a glass molding technique. 6. The optical element according to claim 2, wherein the optical element is formed by injection molding. 7. The method according to claim 2, wherein chromatic aberration is corrected using a diffraction effect.
An optical element according to item 1. 8. A method for producing a spherical lens, wherein the amorphous optical material according to claim 1 is melted, passed through a plate or net having holes, and dropped freely to form a spherical shape. 9. The spherical lens according to claim 3, wherein the amorphous material is a spherical lens formed by melting the amorphous material, passing it through a plate or net having holes, and free-falling the amorphous material. Optical element.