JPH07215732A - Optical glass for polarization optical system and controlling method for photoelastic constant - Google Patents

Abstract

PURPOSE:To obtain an optical glass free from damaging the polarization property of optical information even under thermal stress or dynamic external stress and to enable to control the photoelastic constant of the optical glass by blending to have specific compound for the optical glass for polarization optical system. CONSTITUTION:The optical glass having the photoelasticity of substantially zero to the wavelength of incident light is obtained by having the composition consisting of 17.0-27.0% SiO2, 0.5-5.0% Li2O+Na2O+K2O, 73.0-75.0 PbO and 0-3.0% As2O3+Sb2O3 expressed in terms of oxide by wt.% in the optical glass for polarization optical system. And by changing the composition ratio of PbO contained in the lead glass, the photoelastic constant of the optical glass can be controlled.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical glass for a polarization optical system used in a polarization optical system such as a spatial light modulator for performing polarization modulation or a polarization beam splitter, and a method for controlling its photoelastic constant.

[0002]

2. Description of the Related Art In recent years, the number of means for controlling the polarization characteristics of optical information has increased, and the field of application of optical systems utilizing polarized light, that is, polarization optical systems, is acceleratingly expanding. When the polarization optical system is applied, it is important to control the polarization characteristics of the optical information with high accuracy, but at present, it cannot be said that the performance required is necessarily satisfied, and the control accuracy of the polarization characteristics is improved. Further improvement is desired.

Of optical components such as a substrate and a prism which constitute a polarization optical system, an optically isotropic material is used for an optical component at a portion where the polarization characteristic originally possessed by optical information needs to be preserved. Has been. This is because light transmitted through a material having optical anisotropy (that is, having birefringence) has a phase difference (optical path difference) between a principal ray and an extraordinary ray orthogonal to the principal ray that passes through the material. This is because the polarization characteristics of the optical information cannot be preserved because they change as compared with those before.
Generally, glass that has been sufficiently annealed has optical isotropy, and there are many glasses that are superior to other materials in terms of durability, strength, transmittance, refractive index, and price. Such glass is often used in optical components for which polarization characteristics should be preserved. In particular, borosilicate glass (for example, BK7 = a symbol of German Schott) has excellent durability and has a small dispersion, and is therefore widely used in polarizing optical systems.

[0004]

However, even in the above-mentioned conventional optical glass for polarizing optical system, the optical anisotropy caused by the photoelastic effect is induced under thermal stress or mechanical external stress. There is a problem that the polarization characteristics of optical information are changed due to optical anisotropy, and it becomes difficult for the polarization optical system to obtain desired performance.

These thermal stresses and mechanical external stresses are mainly generated when joining glass and other materials, when forming a film on the glass surface, when the heat generated by the light energy absorbed by the glass cannot be ignored, and in peripheral equipment. Occurs when the glass is bonded to a material having a different coefficient of thermal expansion when it is internally or externally heated, or when it comes into contact with a jig when it is incorporated into an optical system. Therefore, it is unavoidable that thermal stress or mechanical external stress acts on the glass when the polarization optical system is used, and that optical anisotropy is unavoidable in the conventional glass.

An object of the present invention is to provide an optical glass for a polarizing optical system which does not impair the polarization information of optical information even under thermal stress or mechanical external stress.

[0007]

In general, when a force is applied to an isotropic transparent body such as glass to generate a stress, optical anisotropy is generated in the transparent body and a certain crystal is produced. It becomes birefringent like the body. This is called the photoelastic effect. The refractive index of the transparent body when stress is generated can be represented by a so-called refractive index ellipsoid, and at this time, the main refractive index axis of the refractive index ellipsoid coincides with the main stress axis. Generally, assuming that the main refractive indices are n ₁ , n ₂ , and n ₃ , and the main stresses are σ ₁ , σ ₂ , and σ ₃ (those having common subscripts are in the same direction), the following equation Such a relationship is established.

## EQU1 ## n ₁ = n ₀ + C ₁ σ ₁ + C ₂ (σ ₂ + σ ₃ ) n ₂ = n ₀ + C ₁ σ ₂ + C ₂ (σ ₃ + σ ₁ ) n ₃ = n ₀ + C ₁ σ ₃ + C ₂ ( σ ₁ + σ ₂ ) where C ₁ and C ₂ are constants specific to the wavelength of light and the material of the transparent body.

When light is incident on such a transparent body, if the coordinates are taken so that the direction is the same as σ ₃ , the incident light is in the σ ₁ and σ ₂ directions, that is, the vibrating planes are orthogonal to each other. Divided into two linearly polarized light. Since the refractive index (n ₁ , n ₂ ) in each principal stress direction is different when the light is emitted from the transparent body, an optical path difference (phase difference) Δφ represented by the following equation occurs between these two linearly polarized lights. .

## EQU2 ## Δφ = (2π / λ) (n ₂ −n ₁ ) · l = (2π / λ) (C ₁ −C ₂ ) (σ ₂ −σ ₁ ) · l = (2π / λ) · C -((Sigma) _2- (sigma) ₁ ) * l Here, (lambda) is the wavelength of light and l is the light transmission thickness of a transparent body.
C = C ₁ -C ₂ is called photoelastic constant.

Conventionally, the value of the photoelastic constant C of the optical glass used in the polarization optical system is large.
In K7, a value of 2.78 (wavelength λ = 633 nm) is obtained. For this reason, the optical anisotropy induced by thermal stress or mechanical external stress, and the optical path difference Δφ based on the optical anisotropy become values that cannot be ignored.

Therefore, the present inventor considers that if the glass having a photoelastic constant C is substantially zero, optical anisotropy hardly occurs even under thermal stress or mechanical external stress. As a result of diligent research on an optical glass in which C is substantially zero, it is noted that in the glass containing lead oxide (PbO), the photoelastic constant C greatly depends on the content of lead oxide. The present invention has been accomplished.

Therefore, the present invention firstly provides an "optical glass for a polarizing optical system, characterized in that the photoelastic constant is substantially zero with respect to the wavelength of incident light."

Secondly, "Optical glass for polarizing optical system characterized by having the following composition in terms of oxide% by weight: SiO ₂ 17.0 to 27.0% Li ₂ O + Na ₂ O + K ₂ O 0.5 to 5.0% PbO 73.0 to 75.0% As ₂ O ₃ + Sb ₂ O ₃₀ to 3.0% ”.

Thirdly, "Pb contained in lead glass
A method for controlling a photoelastic constant of an optical glass for a polarizing optical system, which comprises controlling a photoelastic constant of the lead glass by changing a composition ratio of O.

[0014]

The reason for limiting the composition range of each component in the optical glass for polarizing optical system of the present invention is as follows. As mentioned above, PbO is the photoelastic constant C of glass.
Greatly depends on the PbO content, and specifically, as the PbO content increases, the value of the photoelastic constant C decreases,
It is used to control the value of the photoelastic constant C to substantially zero by utilizing the fact that it takes a negative value after it becomes zero at a certain fixed amount. The reason that the value of the photoelastic constant C changes depending on the content of PbO is considered to be that the coordination state of lead ions changes as the content thereof increases. The term "substantially zero" as used herein means a value at which the influence of the entire polarization optical system due to the optical path difference based on optical anisotropy can be ignored when the optical glass of the present invention is used in the polarization optical system. As an example, the photoelastic constant C is -0.1 for light with a wavelength of 500 to 650 nm.
It should be in the range of ~ 0.1 [10 ^-8 cm ² / N]. An optical glass having a photoelastic constant C in this range has a PbO content of 73 to 75.
It is realized by setting the content within the range of wt%.

SiO ₂ is the glass composition in the optical glass of the present invention, and is required to be 17% by weight or more, but 27% by weight.
When it is above, the above-mentioned content of PbO will fall outside the predetermined range, and the photoelastic constant will become large.

Alkali metal components such as Na ₂ O + K ₂ O + Li ₂ O have the effects of lowering the melting temperature and glass transition temperature of the glass and increasing the stability of the glass against devitrification.
It is necessary to be 5% by weight or more, but if it exceeds 5% by weight, the chemical durability is significantly impaired. As ₂ O ₃ + Sb ₂ O ₃ can be added if necessary, but if it exceeds 3% by weight, devitrification resistance and spectral transmission characteristics of the glass are impaired.

In the optical glass for polarizing optical system of the present invention, oxides, carbonates, nitrates, etc. corresponding to respective elements are used as raw materials for the respective components, and these are weighed and mixed in a desired ratio to prepare a raw material for preparation. It can be easily produced by heating it to 1,000 to 1,300 ° C. to melt it, refining it, stirring it to homogenize it, then casting it in a preheated mold and slowly cooling it.

Although it is possible to make the photoelastic constant C substantially zero by using glass having a composition not containing lead oxide, the polarizing optical system has properties such as large thermal expansion coefficient and easy cracking. Careful application to Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto.

[0019]

[Examples] As raw materials for each component, oxides, carbonates, nitrates, etc. corresponding to each element were prepared, highly purified, and then weighed so that the ratios shown in Table 1 and Table 2 were obtained. , And mix it to prepare raw material, which is 1,000-1,300
A polarized optical system fused optical glass was manufactured by heating to ℃, melting in an electric furnace, clarification, stirring, homogenization, casting in a preheated mold and slow cooling. The numerical values in Table 1 and Table 2 show the component ratio by weight% in terms of oxide, and the total is 100%.

Regarding the glass thus obtained,
The photoelastic constant and the linear expansion coefficient for light with a wavelength λ = 633 nm were measured. The photoelastic constant C is σ ₁ = σ ₃ in the above equation using light of known wavelength λ and sample of known size 1.
It was calculated by measuring the optical path difference Δφ with a known uniaxial stress σ ₂ of = 0 being applied to the sample. The measurement results are shown in Tables 1 and 2. In addition, a graph in which the horizontal axis represents the content of lead oxide (PbO) and the vertical axis represents the photoelastic constant is shown in FIG. It can be understood that the photoelastic constant decreases almost linearly with an increase in the content of lead oxide, takes a value of zero at a certain point, and takes a negative value thereafter.

As a comparative example, the component ratio of BK7, which has been widely used in the polarization optical system, and the wavelength λ =
Table 2 shows the measurement results of the photoelastic constant and the linear expansion coefficient for light of 633 nm. The photoelastic constants of the optical glasses of Nos. 1 to 7 are much smaller than those of BK7, and in particular, the photoelastic constants of the optical glasses of Nos. 4 to 6 are values within a range that can be considered to be substantially zero. I understand.
Moreover, since the linear expansion coefficient of the optical glass of Nos. 1 to 7 is almost the same level as that of BK7, even when the optical glass of Nos. 1 to 7 is used instead of BK7, the jigs and other optical parts are There is no effect due to the difference in coefficient of thermal expansion.

[Table 1]

[Table 2]

[0022]

As described in detail above, according to the present invention, by selecting the composition ratio of PbO, it is possible to realize an optical glass for a polarization optical system whose photoelastic constant is substantially zero. It is possible to prevent optical anisotropy from substantially occurring even when the glass of the present invention is subjected to thermal stress or mechanical external stress. Therefore, by applying the glass of the present invention to the optical component of the polarization optical system, the influence of thermal stress, mechanical external stress, etc. can be eliminated and the polarization characteristic of optical information can be maintained. In particular, the optical glass for a polarization optical system of the present invention is suitably used for a polarization beam splitter, a transparent substrate on the reading side of a spatial light modulator, etc., which requires highly accurate polarization characteristics.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the content of lead oxide and the photoelastic constant.

Claims (3)

[Claims] 1. An optical glass for a polarization optical system having a photoelastic constant within a range of substantially zero with respect to the wavelength of incident light. 2. The optical glass for a polarizing optical system according to claim 1, wherein the optical glass for a polarizing optical system has the following composition in a weight percentage of oxide. Note SiO ₂ 17.0 to 27.0% Li ₂ O + Na ₂ O + K ₂ O 0.5 to 5.0% PbO 73.0 to 75.0% As ₂ O ₃ + Sb ₂ O ₃ 0 to 3.0% 3. A method for controlling a photoelastic constant of an optical glass for a polarizing optical system, which comprises controlling a photoelastic constant of the lead glass by changing a composition ratio of PbO contained in the lead glass.