JPH0747496B2 - Method for producing glass having refractive index distribution - Google Patents

Description

The present invention relates to a method for producing glass having a refractive index distribution.

[Prior Art] Conventionally, for example, in a multimode optical fiber, when there is a discontinuous refractive index difference between the core and the clad, mode conversion is likely to occur, and bending loss is large. Therefore, this problem has been solved by an optical fiber in which the refractive index of the core part is distributed in a parabolic manner.

As a method of distributing the refractive index, a method of changing the components of the reaction gas to distribute the refractive index when stacking the core portion by the CVD method, a glass rod doped with ions such as Tl ⁺ having a large electronic polarizability, Immerse it in a melt such as KNO ₃ and
^{It is known that +} and K ⁺ are ion-exchanged by diffusion so that the concentration of doped ions is distributed parabolicly, and that the concentration of ions is distributed by electrolysis.

[Problems to be Solved by the Invention]

In the CVD method, the flow rate of the reaction gas is controlled to give a distribution, but the control requires an extremely advanced technique. Further, in the method utilizing ion exchange, it is necessary to combine ions having different diffusion rates, which requires a lot of time, and it is difficult to control the distribution. Further, in the method of changing the composition of the glass to give an ion distribution by electrolysis, it is difficult to give an electrolytic gradient, and it is extremely difficult to give a desired refractive index distribution.

An object of the present invention is to solve these problems of the prior art and to provide a method for producing glass having a refractive index distribution.

[Means for Solving the Problems]

The method for producing a glass having a refractive index distribution of the first aspect of the present invention which achieves the above object comprises pressure-treating the glass at a temperature lower than the glass transition point to form a glass having a higher density in the surface layer portion of the glass. .

The method for producing a glass having a refractive index distribution of the second aspect of the present invention is to produce a treated glass having a uniformly increased density by pressure-treating the glass at a temperature near the glass transition point or a temperature higher than the glass transition point, This treated glass is heat-treated at a temperature lower than the glass transition point to form a glass having a higher density in the central portion.

In the first and second aspects of the present invention, a high pressure device such as an anvil type or a belt type or hot isostatic pressing (HI
P) Pressurize the glass using the equipment.

Quartz glass (Tg ~ 1100 ° C), BK7 as the glass to be treated
(Tg to 560 ° C.) and the like, which are difficult to crystallize by heat treatment, are preferred. The shape of the glass to be treated is not particularly limited, but it is necessary to uniformly apply pressure from the surroundings so as not to destroy the glass to be treated. The processing pressure varies depending on the high-pressure device, and is 50 GPa at maximum for anvil-type or belt-type high-pressure devices and about 1 GPa at maximum for HIP devices, and the device used is determined according to the maximum refractive index value in the required refractive index distribution.

Further, in order to obtain a processed glass which is less aging and has good transparency, it is desirable that the processing temperature (Tp) is about 0.7 ≦ Tp / Tg ≦ 1.3 (temperature is K) with respect to the glass transition point (Tg). When Tp / Tg is less than 0.7, the temperature range that can be used stably becomes narrow, and when it is more than 1.3, crystal analysis occurs during pressure treatment, depending on the glass type.
Transparency drops.

First, in the first aspect of the present invention, a glass sample having a uniform composition is pressed by a high-pressure device and then heated to a temperature lower than the glass transition point. Alternatively, conversely, a glass sample heated to a temperature lower than the glass transition point may be pressed. Since the viscosity of the sample is high in the temperature range lower than the glass transition point, the pressure applied to the sample is not uniform, and the pressure decreases as it goes from the surface layer portion to the central portion. However, since there is a certain degree of viscosity, the difference in pressure between the surface layer portion and the central portion becomes smaller with time. Although it depends on the type and size of glass, Tp / Tg, and the desired refractive index gradient, a silica glass of about 1 cm ³ becomes almost uniform at a temperature of Tp / Tg = 0.9 at a holding time of about 5 minutes. Therefore, if the temperature is cooled to room temperature until the pressure difference becomes saturated in a holding time of 5 minutes or less, a glass having a higher density in the surface layer portion, that is, a larger refractive index in the surface layer portion can be obtained. Further, by changing the processing pressure, a glass having a larger refractive index gradient can be obtained as the pressure becomes higher and a glass having a smaller refractive index gradient can be obtained as the processing time becomes longer at a constant processing time.

Next, in the second aspect of the present invention, a glass sample having a uniform composition, which is pressurized by a high-pressure device, is heated to a predetermined temperature range above the glass transition point, or while being heated to a temperature near the glass transition point, The treatment time is sufficiently long to alleviate the non-uniformity of pressure applied to the glass sample. Then, there is no pressure difference between the surface layer portion and the central portion of the glass sample, and the density is increased uniformly, that is, a homogeneous treated glass having a larger refractive index than that before the treatment is obtained. When the treatment pressure is changed, the refractive index also changes, and the higher the pressure, the higher the refractive index of the glass is obtained. Therefore, it is possible to obtain a homogeneous treated glass having an arbitrary refractive index higher than that before the treatment.

Then, the obtained treated glass is subjected to a heat treatment again at a temperature lower than the glass transition point under a pressure lower than the treatment pressure,
The density of the treated glass relaxes from the surface layer and becomes smaller. At this time, the central portion of the treated glass is less relaxed in the density due to the heat treatment and remains at a higher density. Therefore, when cooling is performed in this state, a processed body having a higher density in the central portion, that is, a refractive index distribution larger in the central portion is obtained.

The glasses obtained by the first invention and the second invention are both compositionally uniform and have no difference in transmission characteristics even if there is a refractive index distribution due to pressure. That is, while the conventional distributed index glass has a different transmittance depending on the location, the present invention is characterized in that the transmittance is uniform.

Examples of the present invention will be described below.

〔Example〕

Example 1 4 cm square quartz glass was subjected to hot isostatic pressing (HIP) treatment at 150 MPa and 1000 ° C. for 30 minutes. The obtained quartz glass block was cut to measure the refractive index distribution. The measurement results are shown in FIG. 1 in relation to the refractive index n and the distance L from the center of the block. The refractive index decreases from the surface layer (1.4632) toward the inside (1.4626), and the change in the refractive index is 0.04%.

Example 2 A 4 cm square quartz glass was subjected to a hot isostatic pressing (HIP) treatment at 150 MPa and 1200 ° C. for 2 hours to obtain a homogeneous glass to be treated, which was then held in the atmosphere at 900 ° C. for 30 minutes. The refractive index distribution of the obtained quartz glass is shown in FIG. 2 as in FIG. The refractive index increased from the surface layer (1.4613) toward the inside (1.4634), and the change in refractive index was 0.14%.

〔The invention's effect〕

As described above, according to the present invention, glass having a refractive index distribution can be obtained in a short time. That is, 1) When a glass having a uniform composition is heated at a temperature lower than the glass transition point and pressurized by a high pressure device, a treated product having a higher refractive index in the surface layer portion is obtained. For example, rod-shaped lenses having the same function as concave lenses can be obtained from cylindrical glass.

2) Homogeneous treated glass having an arbitrary refractive index higher than that before the treatment by changing the treatment conditions when pressure treatment is performed near the glass transition point or until the density becomes uniform at the above temperature and then cooled. Can be obtained.

3) When the treated glass obtained in 2) is heat-treated again and then cooled, the surface is thermally relaxed to obtain a treated body having a high refractive index in the central portion. For example, a rod lens having the same function as a convex lens can be obtained from cylindrical glass.

Therefore, in any case, glass having an arbitrary refractive index distribution can be produced. In particular, since the refractive index is changed only by changing the density, other optical characteristics are hardly changed, and the difference in transmission characteristics due to the location, which was impossible with the conventional CVD method or the method using ion exchange, is eliminated. be able to.

[Brief description of drawings]

FIG. 1 and FIG. 2 are graphs showing the refractive index distribution of the silica glass obtained by the first and second inventions, respectively.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Saburo Kose 3-4-8 Satsukioka, Ikeda-shi, Osaka (72) Inventor Hiroshi Yamashita 18-8 Doiuchi, Sasabe, Kawanishi-shi, Hyogo (72) Inventor Kinoshita Actually, Ikeda City, Osaka Prefecture Midorigaoka 1-chome 2-17-106

Claims (2)

[Claims] 1. A method for producing a glass having a refractive index distribution, which comprises subjecting a glass to a pressure treatment at a temperature lower than a glass transition point to form a glass having a higher density in a surface layer portion of the glass. 2. A glass which is treated at a temperature near or above the glass transition point to increase its density homogeneously, and the treated glass is heat-treated at a temperature lower than the glass transition point so that the central portion has a higher density. A method for producing glass having a refractive index distribution, characterized in that: