# Patent

## Title
Optical glass for polarizing optical system, production process therefor and polarizing beam splitter

## Abstract

An optical glass for polarizing optical system having a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (1):
composition (1): when represented in terms of wt. % of oxides:

SiO2: 17.0–27.0% (35.5–57.0 mol %)

Li2O+Na2O+K2O: 0.5–5.0% (0.7–20.0 mol %)

PbO: 72.0–75.0% (39.1–45.0 mol %)

As2O3+Sb2O3: 0.1–3.0% (0.1–2.0 mol %).

## Background
RELATED APPLICATIONS
This is a division of application Ser. No. 09/368,892 filed Aug. 5, 1999, U.S. Pat. No. 6,432,854, which is a continuation-in-part of application Ser. No. 08/532,693, filed Oct. 6, 1995, now abandoned, which is a 371 of PCT/JP95/00164, filed Feb. 7, 1995.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical glass for a polarizing optical system which is suitably usable for a polarizing optical system such as polarizing beam splitter and spatial light modulator for effecting polarizing modulation, and has an extremely small photoelastic constant, a process for producing such an optical glass for polarizing optical system, and a polarizing beam splitter utilizing the optical glass for polarizing optical system.
2. Related Background Art
In recent years, the utilization of a “polarizing characteristic”, as one of the factors constituting optical information, has rapidly been developed in various fields such as the field of liquid crystal. Along with such development in the utilization of the polarizing characteristic, in an optical system utilizing polarized light, i.e., a polarizing optical system, the importance of high-precision control of the polarizing characteristic constituting optical information has been increased year by year. Based on the increase in the above-mentioned importance, it has earnestly been desired to further improve the precision or accuracy in the control of the polarizing characteristic.
Among various optical elements constituting a polarizing optical system (such as substrate and prism), it is usual to use a material having an optical isotropy especially for some optical elements which are required to retain the polarizing characteristic. The reason for this is that when an optical element comprising a material having an optical anisotropy is used, the phase difference (optical path difference) between the ordinary ray and the extraordinary ray perpendicular to the ordinary ray will be changed during their passage through such a material, with respect to light which has been transmitted by the optical element, and therefore the polarizing characteristic cannot be retained in such a case.
In general, a glass which has sufficiently been subjected to annealing has an optical isotropy and also has various characteristics better than those of other materials in view of its durability, strength, transmittance, refractive index, cost, etc., and therefore such a glass is widely used for optical elements which should retain the polarizing characteristic. Particularly, borosilicate glass (e.g., a borosilicate glass mfd. by Schott Co., Germany, trade name: “BK7”) is inexpensive and excellent in durability, and also has little dispersion. Therefore, the borosilicate glass is widely used in many polarizing optical systems.
However, even when the above-mentioned conventional optical glass for polarizing optical system is used for the optical elements, a certain optical anisotropy based on a photoelastic effect can be induced in the optical element, under the application of a mechanical external stress or a thermal stress to the optical element. Accordingly, when the conventional optical glass is used for the optical element for a polarizing optical system, the polarizing characteristic of optical information can be changed on the basis of the “induced optical anisotropy” as described above. Therefore, in such a case, it is difficult for the polarizing optical system to exhibit a desired performance.
It is considered that the mechanical external stress and the thermal stress as described above are developed mainly in the following situation.
Thus, it is considered that the “mechanical external stress” is mainly developed in a step of processing a glass (such as cutting, the bonding or joining of the glass with another material, and film formation on the surface of a glass), or often a step of assembling a glass into an optical system (such as holding of the glass by a jig or holding device, and the adhesion of the glass to another member). In addition, it is considered that the “thermal stress” is developed by the production of heat in the interior of a glass (such as heat production based on the absorption of light energy), or the production of heat outside a glass (e.g., that based on heat production in a peripheral device) Further, when a glass is caused to contact or is joined with another material having a thermal expansion coefficient different from that of the glass, it is considered that a stress is developed along with the above-mentioned production of heat.
As described above, when a polarizing optical system is constituted by using an optical element, it has been difficult to completely obviate the action of the mechanical external stress or the thermal stress. Accordingly, when the conventional optical glass for polarizing optical system is used for such an optical system, it is extremely difficult to avoid the induction of the optical anisotropy based on the above-mentioned mechanical external stress or thermal stress.
An object of the present invention is to provide an optical glass for polarizing optical system, which does not substantially impair the polarizing characteristic of optical information, even under the action of a mechanical external stress or a thermal stress.
Another object of the present invention is to provide an optical glass for polarizing optical system, which is capable of controlling its refractive index in a desirable manner.
SUMMARY OF THE INVENTION
As a result of earnest study, the present inventors have found that the polarizing characteristic of optical information in an optical glass for polarizing optical system (under the action of a mechanical external stress or a thermal stress) may desirably be evaluated by using a “photoelastic constant based on the value of birefringence or double refraction (under the application of a stress) measured by a photoelasticity modulation method”. The optical glass for polarizing optical system according to the present invention is based on the above discovery and characterized by a photoelastic constant C thereof in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm.
An optical glass for polarizing optical system according to the present invention has a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (1):
composition (1): when represented in terms of wt. % of oxides:

SiO2: 17.0–27.0% (35.5–57.0 mol %)

Li2O+Na2O+K2O: 0.5–5.0% (0.7–20.0 mol %)

PbO: 72.0–75.0% (39.1–45.0 mol %)

As2O3+Sb2O3: 0.1–3.0% (0.1–2.0 mol %).

Another optical glass for polarizing optical system according to the present invention has a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (2):
composition (2): when represented in terms of mol %:

SiO2: 40.0–54.0 mol %

R2O (R: alkali metal): 0.5–9.0 mol %

PbO: 43.0–45.5 mol %

As2O3+Sb2O3: 0.1–1.5 mol %; and

the composition (2) further containing fluorine in the following range when represented in terms of mol %:

fluorine/oxygen (F/O) ratio: 0.1–18.0.

A further optical glass for polarizing optical system according to the present invention has a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm/2/N] with respect to a wavelength of 633 nm, the optical glass having the following composition (3):
composition (3) when represented in terms of mol %:

SiO2: 40.0–54.0 mol %

R2O (R: alkali metal): 0.5–9.0 mol %

RF: 0–16.0 mol %

R2SiF6: 0–3.3 mol %

PbO+PbF2: 43.0–45.5 mol %

PbF2: 0–10.0 mol %

As2O3+Sb2O3: 0.1–1.5 mol %; and

the composition (3) further containing fluorine in the following range in terms of mol %:

fluorine/oxygen (F/O) ratio: 0.1–18.0.

A process for producing an optical glass for polarizing optical system according to the present invention comprises:
changing the ratio of PbO in a lead-containing optical glass to control the photoelastic constant C thereof to provide an optical glass for polarizing optical system having a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm,
the optical glass having the following composition (1):
composition (1): when represented in terms of wt. % of oxides:

SiO2: 17.0–27.0% (35.5–57.0 mol %)

Li2O+Na2O+K2O: 0.5–5.0% (0.7–20.0 mol %)

PbO: 72.0–75.0% (39.1–45.0 mol %)

As2O3+Sb2O3: 0.1–3.0% (0.1–2.0 mol %).

Another process for producing an optical glass for polarizing optical system according to the present invention comprises:
changing the fluorine/oxygen (F/O) ratio of a fluorine-containing optical glass so as to regulate the refractive index thereof while retaining the photoelastic constant C of the optical glass in the range of substantially zero to provide an optical glass for polarizing optical system having a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm,
the optical glass having the following composition (2):
composition (2): when represented in terms of mol %:

SiO2: 40.0–54.0 mol %

R2O (R: alkali metal): 0.5–9.0 mol %

PbO: 43.0–45.5 mol %

As2O3+Sb2O3: 0.1–1.5 mol %; and

the composition (2) further containing fluorine in the following range when represented in terms of mol %:

fluorine/oxygen (F/O) ratio: 0.1–18.0.

A further process for producing an optical glass for polarizing optical system according to the present invention comprises:
changing the fluorine/oxygen (F/O) ratio of a fluorine-containing optical glass so as to regulate the refractive index thereof while retaining the photoelastic constant C of the optical glass in the range of substantially zero to provide an optical glass for polarizing optical system having a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to a wavelength of 633 nm,
the optical glass having the following composition (3):
composition (3) when represented in terms of mol %:

SiO2: 40.0–54.0 mol %

R20 (R: alkali metal): 0.5–9.0 mol %

RF: 0–16.0 mol %

R2SiF6: 0–3.3 mol %

PbO+PbF2: 43.0–45.5 mol %

PbF2: 0–10.0 mol %

As2O3+Sb2O3: 0.1–1.5 mol %; and

the composition (3) further containing fluorine in the following range in terms of mol %:

fluorine/oxygen (F/O) ratio: 0.1–18.0.

In general, when a force is applied to a transparent substance having homogeneity and isotropy such as glass so as to develop a stress therein, an optical anisotropy is induced in the transparent substance, and the transparent substance is caused to have a birefringence property in a similar manner as in a certain kind of crystalline substance. Such a phenomenon is called an “photoelastic effect”. The refractive index of a transparent substance in which a stress has been developed, may be represented by a so-called “(refractive) index ellipsoid”, and the principal refractive index axis of the refractive index ellipsoid coincides with the principal stress axis.
In general, when the principal refractive indices are denoted by n1, n2, and n3, and the principal stresses are denoted by σ1σ2, and σ3 (those having the common subscript are those having the same direction), these principal refractive indices and principal stresses satisfy the following relationship.

n1=n0+C1σ1+C2(ρ2+σ3)

n2=n0+C1σ2+C2(σ3+σ1)

n3=n0+C1σ3+C2(σ1+σ2)  <Equation 1>

In a case where light is incident on the transparent substance having such a refractive index, when a coordinate is defined so that the direction of the incident light is the same as that of the above σ3, the incident light is separated into two linearly polarized light components respectively having σ1 and σ2 directions (namely, linearly polarized light components respectively having planes of vibration which are perpendicular to each other). On the other hand, when light emerges from the transparent substance, in a case where the refractive index in the respective directions of the principal stresses (n1, n2) are different from each other, an optical path difference (phase difference) Δφ represented by the following equation is provided between these two linearly polarized light components.




  
    
      
        
          
            
              Δφ
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                    2
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              =
              
                
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                    2
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                      C
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                      C
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                        σ
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                        1
                      
                    
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                  ·
                  1
                
              
            
          
        
        
          
            
              
                
                  =
                  
                    
                      
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                          2
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                      ·
                      C
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          Equation
          ⁢
          
            
          
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In the above Equation 2, λ denotes the wavelength of light, and l (“el”) denotes the light transmission thickness of the transparent substance. The constant C=C1–C2 in the above Equation is called “photoelastic constant”.
According to the present inventor's knowledge, the value of the photoelastic constants C of conventional optical glasses which have been used for polarizing optical systems are large. For example, the value of the above constant C=2.78 [10−8 cm2/N] (wavelength λ=633 nm) was obtained in the case of the commercially available borosilicate glass “BK7” (Schott Co.) as described hereinabove. In the case of the borosilicate glass having such a large photoelastic constant C, the optical anisotropy induced by the thermal stress or mechanical external stress, and the optical path difference Δφ based on the anisotropy, naturally become certain values which are not negligible.
On the contrary, in the case of the above-mentioned optical glass for polarizing optical system according to the present invention, the photoelastic constant C is in the range of substantially zero, with respect to a wavelength of 633 nm. The term “a photoelastic constant C in the range of substantially zero” used herein refers to a condition such that the influence of the optical path difference due to optical anisotropy, which is provided when the glass is used for a polarizing optical system, is within a negligible extent with respect to the entirety of the above optical system. The photoelastic constant C is in the range of −0.2 to +0.5 (preferably −0.1 to +0.3) [10−8 cm2/N] with respect to incident light having a wavelength of 633 nm.
FIG. 1 is a graph showing a relationship between the fluorine/oxygen (F/O) ratio in a composition of the optical glass for polarizing optical system according to the present invention wherein the photoelastic constant C becomes substantially zero for a wavelength of incident light (633 nm), and the refractive index of the glass. Further, FIG. 2 is a graph showing variation in the photoelastic constant C along with a change in the above F/O ratio in the above-mentioned glass composition.
As shown in FIGS. 1 to 2, in the refractive index of the optical glass according to the present invention, a certain linearity may be established with respect to the F/O ratio, and it is observed that the photoelastic constant C of the glass becomes substantially zero irrespective of the F/O ratio. According to the present inventors knowledge, the photoelastic constant C is dependent on the lead ion content in the optical glass but is not dependent on the amount of fluorine ions introduced into the glass, and therefore it is assumed that a phenomenon such that the photoelastic constant C becomes substantially zero is established in the glass composition according to the present invention.
FIG. 3 is a graph showing transmission spectra of one composition series of the optical glass according to the present invention at a depth (thickness) of 10 mm. As shown in FIG. 3, it is recognized that the transmittance of blue light is increased by introducing fluorine into a glass composition. According to the present inventors' investigation, it is recognized that the tendency of an increase in the blue light transmittance becomes marked as the F/O ratio is increased, and along with such an increase, the absorption edge (limit of absorption on the shorter wavelength side) is also shifted to the shorter wavelength side.
Further, As2O3 and/or Sb2O3 is essentially contained in the optical glass according to the present invention in an amount of 0.1 to 3.0 wt. % (0.1–2.0mol %). Since the optical glass according to the present invention contains As2O3 and/or Sb2O3 which is capable of functioning as a defoaming agent in amount of 0.1 to 3.0 wt. % (0.1–2.0 mol %), quite a high internal transmittance with respect to light having a wavelength of 400 nm or more can be achieved.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing a relationship between the F/O ratio in a composition of the optical glass according to the present invention, and the refractive index nd as a change in the physical property of the glass.
FIG. 2 is a graph showing a relationship between the F/O ratio in a composition of the optical glass according to the present invention, and a photoelastic constant C as a physical property of the glass.
FIG. 3 is a graph showing a spectral transmission spectra of the optical glasses at a thickness of 10 mm, which were prepared in Examples 1, 3 and 5 as described hereinafter.
FIG. 4 is a schematic perspective view showing an example of the optical system for measuring the photoelastic constant C of the optical glass according to the present invention.
FIG. 5 is a schematic sectional view showing an example of the holding device for applying a stress to a sample glass, which is usable in the optical system of FIG. 4.
FIG. 6 is a schematic sectional view for illustrating a state of a light beam which is incident on the dielectric multilayer film constituting a polarizing beam splitter according to the present invention.
FIG. 7 is a schematic sectional view showing an example of the structure of the polarizing beam splitter according to the present invention.
FIG. 8 is a schematic sectional view showing an example of the structure of the first dielectric multilayer film 13 and the second dielectric multilayer film 23 according to the present invention.
FIG. 9 is a graph for comparing the transmittance characteristics based on the structures of conventional polarizing beam splitters.
FIG. 10 is a schematic sectional view showing an example of the structure of the first dielectric multilayer film 13 and the second dielectric multilayer film 23 according to a third structure embodiment of the present invention.
FIG. 11 is a schematic sectional view showing an example of the structure of another polarizing beam splitter (fourth structure embodiment) according to the present invention.
FIG. 12 is a graph for illustrating the transmittance characteristic of the dielectric multilayer film constituting the polarizing beam splitter of the first structure embodiment according to the present invention.
FIG. 13 is a graph for illustrating the incident angle dependence of the transmittance characteristic of the dielectric multilayer film constituting the polarizing beam splitter of the first structure embodiment according to the present invention.
FIG. 14 is a graph for illustrating the transmittance characteristic of the dielectric multilayer film constituting the polarizing beam splitter of the second structure embodiment according to the present invention with respect to the P-polarized light component.
FIG. 15 is a schematic sectional view showing an example of the structure of a further polarizing beam splitter (third structure embodiment) according to the present invention.
FIG. 16 (Table 1) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. 1 to 4) for a polarizing optical system according to the present invention, which were prepared in Example 1.
FIG. 17 (Table 2) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. 5 to 8) for a polarizing optical system according to the present invention, which were prepared in Example 1.
FIG. 18 (Table 3) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. 9 to 12) for a polarizing optical system according to the present invention, which were prepared in Example 1.
FIG. 19 (Table 4) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. 13 to 14) for a polarizing optical system according to the present invention, which were prepared in Example 1.
FIG. 20 (Table 5) is a table showing the data of refractive index of optical glasses for a polarizing optical system according to the present invention, etc., which were measured in Example 2.
FIG. 21 (Table 6) is a table showing the data of degree of birefringence of optical glasses for a polarizing optical system according to the present invention, etc., under the application of a predetermined stress, which were measured in Example 3.
FIG. 22 is a schematic view showing an example of the structure of a projector utilizing a polarizing beam splitter according to the present invention.
FIG. 23 is a schematic sectional view showing an example of the structure of an optical system for measuring the extinction ratio or illuminance non-uniformity of a polarizing beam splitter which has been constituted by using the optical glass for polarizing optical system according to the present invention.
FIG. 24 is a photograph showing illuminance non-uniformity which was provided when a polarizing beam splitter constituted by using the optical glass for polarizing optical system according to the present invention was evaluated by using the measurement optical system of FIG. 23.
FIG. 25 is a photograph showing illuminance non-uniformity which was provided when a polarizing beam splitter constituted by using a conventional optical glass was evaluated by using the measurement optical system of FIG. 23.
FIG. 26 (Table 7) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. 21 to 24) for a polarizing optical system according to the present invention, which were prepared in Example 1.
FIG. 27 (Table 8) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. 25 to 27) for a polarizing optical system according to the present invention, which were prepared in Example 1.
FIG. 28 is a graph showing a correlation between the lead oxide (PbO) content in the optical glass for polarizing optical system according to the present invention provided in Example 1, and the photoelastic constant C thereof.
FIG. 29 is a schematic sectional view showing an example of the basic structure of a projector system utilizing a polarizing beam splitter which has been constituted by using the optical glass for polarizing optical system according to the present invention.
FIG. 30 (Table 9) is a table showing the compositions and data of various physical properties of optical glasses (Sample Nos. A, B and C), which were prepared in Example 6.
FIG. 31 is a graph showing a correlation between the wavelengths and the internal transmittance of the optical glasses (Sample Nos. A, B and C).
FIG. 32 (Table 10) is a table showing the results of measurement of photoelastic constants C of the optical glass according to the present invention and various commercially-available optical glasses, which were measured in Example 8.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinbelow, the present invention will be described in detail, with reference to the accompanying drawings as desired.
(Photoelastic Constant C)
The optical glass for polarizing optical system according to the present invention is characterized in that the photoelastic constant C thereof is in the range of −0.2 to +0.5 (substantially zero) [10−8 cm2/N] with respect to light having a wavelength of 633 nm. The photoelastic constant C preferably be in the range of −0.1 to +0.3 [10−8 cm2/N] with respect to light having a wavelength of 633 nm.
In the present invention, the optical path difference Δφ is measured by measuring birefringence (or double refraction) by use of light having a known wavelength λ under a condition such that a known uniaxial stress σ2 is applied to a sample having a known size of l (el) so as to satisfy a relationship of σ1=σ3=0 in the <Equation 1> and <Equation 2> as described hereinabove. Based on the thus determined optical path difference Δφ, is possible to determine a photoelastic constant C=C1–C2 according to the above <Equation 2>. With respect to the details of such a method for measuring the “photoelastic constant C”, an instruction manual attached to a birefringence measuring apparatus ADR-150LC as described hereinafter; or Etsuhiro Mochida “Optical Technique Contact ,” Vol.27, No.3, page 127 (1989) may be referred to.
FIG. 4 is a schematic view showing the arrangement of optical elements in a measurement system for measuring the above-mentioned photoelastic constant C (birefringence measuring apparatus, trade name: ADR-150LC mfd. by Oak Seisakusho Co.). In FIG. 4, the “Sample S” is sandwiched between and held by a sample holder for applying a uniaxial stress to the sample, as shown in a schematic sectional view of FIG. 5, whereby the birefringence may be measured while applying a predetermined stress to the sample. Referring to FIG. 5, the sample holder comprises: a pair of metal blocks 37a and 37b (dimensions: (40 to 50 mm)×(30 to 40 mm), thickness: 25 to 30 mm) for holding a sample 36 therebetween; and a load cell 38 (diameter 20 mm, thickness: 9.5 mm, trade name: 9E01-L32–100K mfd. by Nihon Denshi-Sanei K.K.) disposed in the metal block 37a. When the load cell 38 is arranged in this manner, the value of the stress to be applied to the sample may be monitored.
The sizes of above-mentioned sample 36 are 10 mm×15 mm×20 mm, the dimensions of the stress plane are 10 mm×20 mm, the dimensions of the light transmission plane are 15 mm×20 mm, and the length of the light transmission thickness is 10 mm.
(Glass Composition)
In the optical glass for polarizing optical system according to the present invention, fluorine is not an essential component. However, the glass may preferably contain fluorine in view of a large latitude or degree of freedom in the refractive index (a large latitude in selecting the refractive index) of a composition for providing a photoelastic constant C of substantially zero, and/or in view of a relatively large transmittance of light in a shorter wavelength region (wavelength: about 400–480 nm).
(Embodiment Containing No Fluorine)
An optical glass for polarizing optical system according to the present invention (in an embodiment not containing fluorine) has the following composition, when represented in terms of oxide wt. %.

SiO2: 17.0–27.0% (35.5–57.0 mol %)

Li2O+Na2O+K2O: 0.5–5.0% (0.7–20.0 mol %)

PbO: 72.0–75.0% (39.1–45.0 mol %)

As2O3+Sb2O3: 0.1–3.0% (0.1–2.0 mol %)

The above amount of SiO2 may more preferably be 22.0–26.0%. The amount of (Li2O+Na2O+K2O) may more preferably be 0.5–3.0%. The amount of PbO may more preferably be 73.0–75.0% (39.6–45.0 mol %). The amount of (As2O3+Sb2O3) may more preferably be 0.2–0.5%.
In the optical glass for polarizing optical system according to the present invention (in an embodiment not containing fluorine), the above contents of the respective components are preferred for the following reasons. (PbO)
As described above, the photoelastic constant C of a glass has a tendency to largely depend on the PbO content. More specifically, there is a tendency such that as the PbO content is increased, the value of the photoelastic constant C is decreased, and the value of the photoelastic constant C becomes zero in a certain content, and thereafter becomes a negative value. When such a characteristic of PbO is utilized, the PbO content may preferably be used for regulating the value of the photoelastic constant C of the glass to substantially zero. According to the present inventors' knowledge, it is assumed that the reason for the change in the photoelastic constant C depending on the PbO content is that the state of the coordination of lead ions is changed along with an increase in the PbO content. The term “a photoelastic constant C in the range of substantially zero” used herein refers to a condition such that the influence of the optical path difference due to optical anisotropy of the glass according to the present invention, which is provided when the glass is used for a polarizing optical system, is within a negligible extent with respect to the entirety of the above polarizing optical system. More specifically, the photoelastic constant C is in the range of −0.2 to +0.5 [10−8 cm2/N] with respect to light having a wavelength of 633 nm. In order to obtain an optical glass having a photoelastic constant C in such a range, e.g., it is preferred to adopt a PbO content in the range of 73–75 wt. %.
According to the present inventors' experiment, it has been found that the photoelastic constant C can be made substantially zero even when a glass composition not containing lead oxide is used. However, when such a glass composition not containing lead oxide is caused to have a photoelastic constant C in the range of substantially zero, the resultant glass has a relatively large thermal expansion coefficient and also is more liable to be broken, and therefore such a glass should carefully be applied to a polarizing optical system.
(SiO2)
SiO2 is a glass forming component in the optical glass according to the present invention, and it may preferably be contained in an amount of 17 to 27 wt. %. When the SiO2 content exceeds 27 wt. %, the above-mentioned PbO content is liable to decrease so as to deviate from the preferred range of the content thereof, and the photoelastic constant C tends to be large.
(Alkali Metal Component)
The alkali metal component such as Na2O and/or K2O and/or Li2O has a function of lowering the glass melting temperature and glass transition temperature, and of improving the stability to devitrification. From such a viewpoint, the alkali metal content (when two or more kinds of alkali metal are contained, the total of those contents) may preferably be 0.5 wt. % or more. On the other hand, when the content exceeds 5.0 wt. %, the chemical durability of the glass can be impaired considerably.
(Defoaming Agent)
As2O3 and/or Sb2O3 {As2O3, Sb2O3 or (As2O3+Sb2O3)} capable of functioning as a defoaming agent, is essentially contained in the optical glass according to the present invention in an amount of 0.1 to 3.0 wt. % (0.1–2.0 mol %). When the content of the defoaming agent (when two or more kinds of defoaming agents are contained, the total of those contents; e.g., the total amount of (As2O3+Sb2O3)) exceeds 3 wt. %, the resistance to devitrification, transmission spectrum characteristic, etc., of the glass are lowered. On the other hand, when the content of the defoaming agent (AS2O3 and/or Sb2O3) is less than 0.1 wt. %, the internal transmittance with respect to light having a wavelength of 400 nm or more is lowered. The amount of the defoaming agent may more preferably be 0.2–0.5 wt. %.
(Embodiment Containing Fluorine)
An optical glass for polarizing optical system according to the present invention (in an embodiment containing fluorine) has the following composition, when represented in terms of mol %.

SiO2: 40.0–54.0 mol %

R2O (R: alkali metal): 0.5–9.0 mol %

PbO: 43.0–45.5 mol %

As2O3+Sb2O3: 0.1–1.5mol %

Fluorine/oxygen (F/O) ratio: 0.1–18.0

Another optical glass for polarizing optical system according to the present invention (in an embodiment containing fluorine) has the following composition, when represented in terms of mol %.

SiO2: 40.0–54.0 mol %

R2O (R: alkali metal): 0.5–9.0 mol %

RF: 0–16.0 mol %

R2SiF6: 0–3.3 mol %

PbO+PbF2: 43.0–45.5 mol %

PbF2: 0–10.0 mol %

As2O3+Sb2O3: 0.1–1.5 mol %

fluorine/oxygen (F/O) ratio: 0.1–18.0

In the optical glass for polarizing optical system according to the present invention, the above contents of the respective components are preferred for the following reasons.
(Lead Ion)
The lead ion may preferably be used mainly for the purpose of controlling the photoelastic constant C. In general, the photoelastic constant C of a glass composition system containing lead ions tends to depend on the content of the lead ions. A value of the photoelastic constant C of substantially zero may easily be obtained when the lead ion content (calculated in terms of PbO) is 43.0–45.5 mol % (more preferably, 44.0–45.5 mol %).
(Fluorine)
It is observed that when fluorine is introduced into the optical glass composition according to the present invention, the refractive index of the glass is decreased, and further, the absorption edge of the transmission spectrum is shifted to the shorter wavelength side.
The means for introducing fluorine into a glass composition is not particularly limited. For example, it is possible to introduce fluorine into the glass composition by using a fluoride (such as KF, K2SiF6 and/or PbF2) as a raw material for the glass. According to the present inventors' knowledge, fluorine may be introduced into the glass in an amount of 16.0 mol %, 3.3 mol %, and 10.0 mol %, respectively, when each of KF, K2SiF6, and PbF2 is used alone as a raw material for the glass. When the amount of such a component exceeds the amount thereof which can suitably be introduced into the glass, crystals can be precipitated due to excess fluorine. On the other hand, when plural kinds of fluorides are used as a raw material for the glass in a mixture or combination, it is possible to increase the fluorine/oxygen (F/O) ratio to 18.0. The (F/O) ratio may more preferably be 5.0–18.0.
(SiO2)
SiO2 is a glass forming oxide in the optical glass according to the present invention. In the optical glass according to the present invention, the SiO2 content may preferably be 40.0 mol % or more. On the other hand, in order not to decrease the lead ion content as described above for providing a preferred photoelastic constant C to deviate the lead ion content from a preferred range thereof, the SiO2 content may preferably be 54.0 mol % or less. The SiO2 content may more preferably be 45–53 mol %.
(Alkali Metal Oxide)
An alkali metal oxide such as Li2O and/or Na2O and/or K2O has an effect of lowering the melting temperature and glass transition temperature of a glass, and of improving the stability of the glass to the devitrification. In order to make the above effect sufficient, the content thereof (when plural kinds of the alkali metal oxides are contained in the glass, the total content thereof; e.g., total amount of Li2O+Na2O+K2O) may preferably be 0.5 mol % or more. On the other hand, when the alkali metal oxide content exceeds 9.0 mol %, the decrease in the chemical durability of the glass becomes marked. The alkali metal oxide content may preferably be 2.0–9.0 mol %.
(Defoaming Agent)
As2O3 and/or Sb2O3 {AS2O3, Sb2O3or (As2O3+Sb2O3)} capable of functioning as a defoaming agent, is essentially contained in the optical glass according to the present invention in an amount of 0.1 to 1.5 mol %. When the content of the defoaming agent (when two or more kinds of defoaming agents are contained, the total of those contents; e.g., the total amount of (As2O3+Sb2O3)) exceeds 1.5 mol %, the resistance to devitrification, transmission spectrum characteristic, etc., of the glass are lowered. On the other hand, when the content of the defoaming agent (As2O3 and/or Sb2O3) is less than 0.1 mol %, the internal transmittance with respect to light having a wavelength of 400 nm or more is lowered. The amount of the defoaming agent may more preferably be 0.2–0.5 mol %.
(Production Process)
As described above, the present invention may provide an optical glass for polarizing optical system having a photoelastic constant C in the range of substantially zero with respect to incident light having a wavelength in the visible region. As described above, it is possible to arbitrarily regulate the refractive index, as long as the glass composition falls within the above-mentioned preferred range thereof.
The process for producing the optical glass for polarizing optical system according to the present invention is not particularly limited. For example, the optical glass for polarizing optical system according to the present invention may easily be produced by using oxide, fluoride, carbonate, nitrate, etc., as raw materials corresponding to the above-mentioned components, weighing and mixing them to provide a formulated raw material, heating the formulated raw material to 1000 to 1300° C. to be melted and subjecting the formulated raw material to clarification and stirring to be homogenized, casting the resultant mixture into a preheated metal mold, and then gradually cooling or annealing the resultant mixture. However, at this time, if an excess amount (e.g., 5.0 mol % in terms of the content thereof) of the nitrate is used, the above-mentioned effect of the introduction of fluorine in the present invention tends to be reduced.
(Polarizing Optical System)
The above-mentioned optical glass for polarizing optical system according to the present invention may be applied to many optical elements by utilizing the characteristic thereof. The range or latitude of the application of the optical glass for polarizing optical system according to the present invention is not particularly limited, but the optical glass may particularly preferably be utilized for an optical element which is required to have a high-precision polarizing characteristic, such as polarizing beam splitter and read-out transparent substrate for a spatial light modulator.
(Beam Splitter)
Hereinbelow, there will be specifically described an embodiment wherein the optical glass for polarizing optical system according to the present invention is applied to a polarizing beam splitter.
The above polarizing beam splitter typically includes embodiments as described below.
(Embodiment 1)
A polarizing beam splitter comprising a dielectric multilayer film formed on a light-transmissive substrate (or base material), wherein:
the above dielectric multilayer film comprises a first dielectric multilayer film and a second dielectric multilayer film respectively having two different design reference wavelengths λ1 and λ2;
Each of the first and second dielectric multilayer films comprises an alternate layer, each of which comprises a laminate (or multilayer structure) comprising a two-layer basic cycle including a high-refractive index substance and a low-refractive index substance having an optical film thickness of λ1/4 or λ2/4 at each reference wavelength of λ1 or λ2, which is repetitively disposed or formed inn cycles (n: an arbitrary integer); and a thin film adjusting layer disposed on each of both sides of the alternate layer and comprising each one of the high-refractive index substance and the low-refractive index substance having an optical film thickness of λ1/8 or λ2/8; and
the alternate layer of the first dielectric multilayer film and the alternate layer of the second dielectric multilayer film respectively comprise combinations of different substances from each other.
(Embodiment 2)
A polarizing beam splitter according to the above Embodiment 1, wherein the alternate layer of the first dielectric multilayer film comprises a combination of TiO2 as the high-refractive index substance and SiO2 as the low-refractive index substance; and the alternate layer of the second dielectric multilayer film comprises a combination of TiO2 as the high-refractive index substance and Al2O3 as the low-refractive index substance.
(Embodiment 3)
A polarizing beam splitter according to the above Embodiment 1, wherein the alternate layer of the first dielectric multilayer film comprises a combination of TiO2 as the high-refractive index substance and SiO2 as the low-refractive index substance; and the alternate layer of the second dielectric multilayer film comprises a combination of ZrO2 as the high-refractive index substance and MgF2 as the low-refractive index substance.
(Embodiment 4)
A polarizing beam splitter according to the above Embodiment 1, wherein the alternate layer of the first dielectric multilayer film and the alternate layer of the second dielectric multilayer film are immersed or disposed in a liquid medium having substantially the same refractive index as that of the light-transmissive substrate.
In the polarizing beam splitter according to the present invention having the above structure, there are selected an arrangement thereof and substances to be used for the high-refractive index layer and low-refractive index layer constituting the alternate layer of the dielectric multilayer film such that they do not narrow the band width of a wavelength range to be used, even when the incident angle of a light beam to the dielectric multilayer film is somewhat changed.
In general, in order to conduct polarizing separation over a wide band, it is preferred to increase the band width for separating a P-polarized light component and an S-polarized light component with respect to the wavelength of a light beam which is to be incident on a polarizing separation film. In order to satisfy such a condition, it is preferred that the incident light beam is caused to be incident on the polarizing separation film in accordance with the Snell Is law so as to provide a design incident angle in the neighborhood of the Brewster's angle, which is an angle for providing the maximum polarizing separation between the P-polarized light component and the S-polarized light component.
The above dielectric multilayer film structure comprises the first and second dielectric multilayer films respectively having design reference wavelengths different from each other. In general, such a structure is designed so as to provide different incident angles for light beams which are to be incident on the first and second dielectric multilayer films, respectively. In addition, it is preferred to select the high-refractive index substance and low-refractive index substance constituting the first and second dielectric multilayer films so that the following Brewster's conditions (1) and (2) are made different from each other. For example, it is preferred that one of the alternate layers of the dielectric multilayer film comprises a combination of TiO2 as the high-refractive index substance and SiO2 as the low-refractive index substance, and the other of the alternate layers of the dielectric multilayer film comprises a combination of TiO2 as the high-refractive index substance and Al2O3 as the low-refractive index substance.
For the respective design reference wavelengths λ1, λ2 (λ1≠λ2), and a design reference incident angle θ, the corresponding incident angles are denoted by θ1 and θ2, respectively. For each of the set of the above conditions, the Brewster's condition is represented by the following equation (1) or (2).

λ1>λ2

λ1, θ1; nH1/COS θH1=nL1/COS θL1  (1)

λ2, θ2; nH2/COS θH2=nL2/COS θL2  (2)

    
    
        θ1; Angle of incidence of light when the light emerging from the light-transmissive substrate 1 is incident on the boundary between the first dielectric multilayer film and the light-transmissive substrate 1.
        θ2; Angle of incidence of light when the light emerging from the light-transmissive substrate 2 is incident on the boundary between the second dielectric multilayer film and the light-transmissive substrate 2.
    
    


nH1, nL1; Refractive indices of the high-refractive index substance layer and the low-refractive index substance layer constituting the alternate layer of the first dielectric multilayer film at the design reference wavelength λ1.
nH2, nL2; Refractive indices of the high-refractive index substance layer and the low-refractive index substance layer constituting the alternate layer of the second dielectric multilayer film at the design reference wavelength λ2.
θH1, θL1; Angle of incidence of light which emerges from each of the high-refractive index substance layer and the low-refractive index substance layer and is incident on the boundary, in the alternate layer of the first dielectric multilayer film at the design reference wavelength λ1.
θH2, θL2; Angle of incidence of light which emerges from each of the high-refractive index substance layer and the low-refractive index substance layer and is incident on the boundary, in the alternate layer of the second dielectric multilayer film at the design reference wavelength λ2.
FIG. 6 is a view for illustrating the state of the incidence of a light beam which is to be incident on the above dielectric multilayer film, when it emerges from the high-refractive index substance layer and the low-refractive index substance layer and is incident on the boundary. In FIG. 6, the θi, θHi, and θLi respectively correspond to the first and second dielectric multilayer films (i=1 and 2).
It is preferred that the film thicknesses of the high-refractive index substance layer, the low-refractive index substance layer, and the adjusting layer to be used for the alternate layer of the dielectric multilayer film according to the present invention are λ/4, λ/4, and λ/8, respectively. However, these film thicknesses to be actually formed can also be determined experimentally in a trial-and-error manner, and therefore these thicknesses can be somewhat different from the above design values.
The above “adjusting layer” is a layer having a function of reducing a ripple which can occur in the transmittance of the P-polarized light component. When a large ripple occurs, the wavelength range wherein the polarizing beam splitter is usable may undesirably be limited.
In order to compare the above embodiment of the polarizing beam splitter according to the present invention with another one, there will be briefly described the transmittance characteristic of another polarizing beam splitter. Such a polarizing beam splitter has basically the same structure as that shown in FIG. 7, wherein the alternate layers of the first and second dielectric multilayer films use the same combinations of a high-refractive index substance layer of TiO2 and a low-refractive index substance layer of SiO2. FIG. 9 is a graph showing the incident angle dependence of the transmittance characteristic of such a polarizing beam splitter.
Referring to FIG. 9, in the case of the design reference incident angle of 45 degrees, a band wherein the P/S polarizing separation ratio is high is 160 nm (denoted by a solid line in FIG. 9). On the other hand, when the incident angle is shifted by ±2.5 degrees to 42.5 degrees or 47.5 degrees, the band width becomes 90 nm (denoted by a dotted line of FIG. 9 and an alternate long and short dashed line). As shown in FIG. 9, a polarizing beam splitter using a dielectric multilayer film comprising only one combination may provide a wide wavelength range to be used wherein the S- and P-polarized light components can be separated from each other. However, in such a structure, a desired wavelength band width is extremely narrowed only when the angle of incidence of light to be incident on the dielectric multilayer film is shifted to a small extent.
On the contrary, in the polarizing beam splitter according to the above-mentioned embodiment of the present invention, the band width to be used therefor may be extremely broadened while retaining the separation ratio between the P-polarized light component and the S-polarized light component, even when the angle of incidence of a light beam to be incident on the dielectric multilayer film is somewhat shifted or deviated. In addition, it is possible to increase the latitude or degree of freedom in the arrangement of an optical system into which the polarizing beam splitter has been assembled.
(Process for Constituting dielectric Multilayer Film)
Hereinbelow, there will be described a process for constituting the dielectric multilayer film of the polarizing beam splitter according to the present invention.
FIG. 7 shows a structure wherein a first dielectric multilayer film 3 and a second dielectric multilayer film 4 are respectively formed or disposed on a prisms 1 and 2 as light-transmissive substrates, and are joined with each other by the medium of an adhesive layer 5.
In the structure shown in FIG. 8, a first dielectric multilayer film 13 and a second dielectric multilayer film 23 are sequentially disposed or formed on a light-transmissive substrate 1. Another light-transmissive substrate is further bonded onto the upper side thereof.
FIG. 11 shows a structure wherein dielectric multilayer films 3 and 4 are disposed on both sides of a flat glass plate 2 as a light-transmissive substrate, and further the resultant laminate is immersed in a liquid medium 6 having substantially the same refractive index as that of the glass. When such a structure is adopted, substantially the same performance as that of the structure of FIG. 7 may be provided.
(Embodiments of Structure of Polarizing Beam Splitter)
There is described a first embodiment of the structure of the polarizing beam splitter according to the present invention.
FIG. 7 shows the structure of a polarizing beam splitter wherein a prism 1 (on which a laminate of an adjusting layer 1C and an alternate layer 13 of a first dielectric multilayer film 3 is disposed, as shown in FIG. 8), is joined with prism 2 (on which a laminate of an adjusting layer 2C and an alternate layer 23 of a second dielectric multilayer film 4 is disposed, as shown in FIG. 8) by an optical adhesive 5.
In this embodiment of the structure, the prisms 1 and 2 have a refractive index ns=1.84. Further, the optical adhesive has a refractive index nb=1.52. FIG. 7 shows reflected light R and transmitted light T when a light beam is incident was at an angle of 45 degrees. The transmitted light T includes an S-polarized light component Ts and a P-polarized light component Tp.
Referring to FIG. 10, the alternate layer 13 of the first dielectric multilayer film has a design reference wavelength, λ1=680 nm, and has a structure such that a TiO2 layer 11 as a high-refractive index substance having nH1=2.38, and an Al2O3 layer 12 as a low-refractive index substance having nL1=1.65 are alternately disposed in an optical film thickness of λ1/4, respectively.
On the other hand, the alternate layer 23 of the second dielectric multilayer film has a design reference wavelength λ2=420 nm, and has a structure such that a TiO2 layer 21 as a high-refractive index substance having nH2=2.38, and an SiO2 layer 22 as a low-refractive index substance having nL1=1.47 are alternately disposed in an optical film thickness of λ2/4, respectively.
In addition, an adjusting layer 1C or 2C having a film thickness of λ1/8 or λ2/8, respectively, is disposed between the above-mentioned alternate layer 13 or 23 of the first or second dielectric multilayer film, and the prism 1 or prism 2.
In the polarizing beam splitter having the above structure, there is supposed a case wherein the angle of incidence of a light beam is shifted or deviated by ±2.5 degrees from the design reference angle of 45 degrees.
In this case, the low-refractive index substance 12 and the high-refractive index substance 11 used in the alternate layer 13 of the first dielectric multilayer film corresponding to a higher angle side (i.e., corresponding to a shorter wavelength side in terms of the wavelength to be used) are selected so that the above-mentioned Brewster's condition (1) is satisfied at an angle of θ1=47.5 degrees at which a light beam emerging from the light-transmissive substrate 1 is incident on the boundary between the light-transmissive substrate 1 and the first dielectric multilayer film 13. In this embodiment of the structure, TiO2 was selected as the high-refractive index layer 11, and Al2O3 was selected as the low-refractive index layer 12, as the combination of materials or substances constituting the alternate layer 13 of the first dielectric multilayer film.
On the other hand, the low-refractive index substance 22 and the high-refractive index substance 21 used in the alternate layer 23 of the second dielectric multilayer film corresponding to a lower angle side (i.e., corresponding to a longer wavelength side in terms of the wavelength to be used) are selected so that the above-mentioned Brewster's condition (2) is satisfied at an angle of θ2=42.5 degrees at which a light beam emerging from the light-transmissive substrate 2 is incident on the boundary between the light-transmissive substrate 2 and the first dielectric multilayer film 23. In this embodiment of the structure, TiO2 was selected as the high-refractive index layer 21, and SiO2 was selected as the low-refractive index layer 12, as the combination of materials or substances constituting the alternate layer 23 of the second dielectric multilayer film.
FIG. 12 is a graph showing the transmittance characteristics Tp, Ts of the P-polarized light component and S-polarized light component in the dielectric multilayer film structure of the above-mentioned first structure embodiment, and transmittance characteristics at incident angles of 42.5 degrees, 45 degrees and 47.5 degrees, respectively.
Hereinbelow, the incident angle dependence of the transmittance of P- and S-polarized light components in the polarizing beam splitter having the above-mentioned structure of the dielectric multilayer of the first structure embodiment is compared with that of the polarizing beam splitter (Comparative Example) having the characteristic as shown in FIG. 9 as described above.
Referring to FIG. 9, in a case where the multilayer structure of the Comparative Example is used, when the incident angle is shifted by few degrees (e.g., by about ±2.5 degrees) from the design reference angle in the wavelength range of from 480 nm to 570 nm, the band width X thereof becomes 90 nm which is a very narrow band.
On the contrary, in the first structure embodiment according to the present invention of which characteristic is shown in FIG. 12, a high polarizing separation property (Ts/Tp) of 0.1% or less is provided in the wavelength range of from 460 nm to 620 nm. In this embodiment, even when the incident angle is shifted by ±2.5 degrees from the design reference angle of incidence, the band width X is maintained at a broad band of 160 nm.
FIG. 13 is a graph showing the incident angle dependence of the transmittance of P-polarized light components at a longer wavelength λ=620 nm in the above-mentioned first structure embodiment according to the present invention.
As shown in FIG. 13, in the polarizing beam splitter of this structure embodiment, the band width of the transmittance characteristic can be considerably broadened even in consideration of the incident angle dependence thereof, as compared with that of the polarizing beam splitter of Comparative Example having the characteristic as shown in FIG. 9 wherein TiO2 and SiO2 are used for the combination of the same kind of substances, as the alternate layers constituting the first and second dielectric multilayer films.
According to the present inventors' knowledge, it is assumed that the reason for the provision of such a good characteristic in the present invention is that the film forming substances for the respective dielectric multilayer films are selected so that the alternate layer of the first dielectric multilayer film capable of causing a decrease in the longer wavelength side of the transmittance of the P-polarized light component, satisfies the Brewster's condition (1) at 47.5 degrees; and that the alternate layer of the first second dielectric multilayer film capable of causing a decrease in the shorter wavelength side of the transmittance, satisfies the Brewster's condition (2) at 42.5 degrees.
Thus, when the polarizing beam splitter having the structure according to the present invention is used, it is possible to considerably broaden the band width in the wavelength to be used, and to provide a polarizing beam splitter having a high degree of freedom in the incident angle of light.
(Second Embodiment of Structure of Polarizing Beam Splitter)
Next, there is described a second embodiment of the structure of the polarizing beam splitter according to the present invention.
The dielectric multilayer film structure of the second structure embodiment is basically the same as that of the first structure embodiment, except that the combination of substances to be used for the dielectric multilayer film is different from that used in the first embodiment.
Referring to FIGS. 7 and 8, the second structure embodiment has a polarizing beam splitter structure wherein a light-transmissive substrate 1 having thereon a laminate of an adjusting layer 1C and an alternate layer 13 of a first dielectric multilayer film 3, is joined with a light-transmissive substrate 2 having thereon a laminate of an adjusting layer 2C and an alternate layer 23 of a second dielectric multilayer film 4, by an optical adhesive 5. The light-transmissive substrates 1 and have a refractive index ns=1.52.
In this structure embodiment, the alternate layer 13 of the first dielectric multilayer film has a design reference wavelength λ1=700 nm, and has a structure such that a TiO2 layer as a high-refractive index substance having nH1=2.38, and an SiO2 layer as a low-refractive index substance having nL1=1.47 are alternately disposed in an optical film thickness of λ1/4, respectively.
The alternate layer 23 of the second dielectric multilayer film has a design reference wavelength of 430 nm, and has a structure such that a ZrO2 layer as a high-refractive index substance having nH2=2.02, and an MgF2 layer as a low-refractive index substance having nL2=1.37 are alternately disposed in an optical film thickness of λ2/4, respectively.
In addition, an adjusting layer 1C or 2C having a film thickness of λ1/8 or λ2/8, respectively, is disposed between the above-mentioned alternate layer 13 or 23 of the first or second dielectric multilayer film, and the prism 1 or 2.
In the polarizing beam splitter having the above structure, when the angle of incidence of a light beam is shifted or deviated by ±4 degrees from the design reference angle of 52 degrees in the neighborhood of the design reference angle, the low-refractive index substance 12 and the high-refractive index substance 11 used in the alternate layer 13 of the first dielectric multilayer film corresponding to a higher angle side (i.e., corresponding to a shorter wavelength side in terms of the wavelength to be used) are selected so that the above-mentioned Brewster's condition (1) is satisfied at an incident angle of 56 degrees as the angle of a light beam with respect to the normal of the film surface. In this embodiment of the structure, TiO2 was selected as the high-refractive index layer 11, and SiO2 was selected as the low refractive index layer 12, as the combination of materials or substances constituting the alternate layer 13 of the first dielectric multilayer film.
On the other hand, the low-refractive index substance 22 and the high-refractive index substance 21 used in the alternate layer 23 of the second dielectric multilayer film corresponding to a lower angle side (i.e., corresponding to a longer wavelength side in terms of the wavelength to be used) are selected so that the above-mentioned Brewster's condition (2) is satisfied at an incident angle of a light beam of 48 degrees. In this embodiment of the structure, ZrO2 was selected as the high-refractive index layer 21, and MgF2 was selected as the low-refractive index layer 22, as the combination of materials or substances constituting the alternate layer 23 of the second dielectric multilayer film.
FIG. 14 is a graph showing the transmittance characteristics the P-polarized light component and S-polarized light component in the dielectric multilayer film structure of the above-mentioned first structure embodiment, and transmittance characteristics at incident angles of 48 degrees, 52 degrees and 56 degrees, respectively.
Hereinbelow, the incident angle dependence of the transmittance of P- and S-polarized light components in the polarizing beam splitter having the above-mentioned structure of the dielectric multilayer of the second structure embodiment of the present invention is compared with that of the above-mentioned polarizing beam splitter (Comparative Example) having the characteristic as shown in FIG. 9.
Referring to FIG. 9, in a case where the multilayer structure of the Comparative Example is used, when the incident angle is shifted by few degrees (e.g., by about ±2.5 degrees) from the design reference angle in the wavelength range of from 480 nm to 570 nm, the band width X thereof becomes 90 nm which is a very narrow band to be used.
On the contrary, in the second structure embodiment according to the present invention of which characteristic is shown in FIG. 14, a high polarizing separation between the P-polarized light component and S-polarized light component is provided in the wavelength range of from 460 nm to 620 nm. In this embodiment, even when the incident angle is shifted by ±4 degrees from the design reference angle of incidence, the band width X is maintained at a broad band of 170 nm.
As shown in FIG. 14, in the polarizing beam splitter of this structure embodiment, the band width of the transmittance characteristic can be considerably broadened even in consideration of the incident angle dependence thereof, as compared with the polarizing beam splitter of Comparative Example wherein TiO2 and SiO2 are used for the combination of the same kind of substances, as the alternate layers constituting the first and second dielectric multilayer films.
According to the present inventors' knowledge, it is assumed that the reason for the provision of such a good characteristic in the present invention is that the film forming substances for the respective dielectric multilayer films are selected so that the alternate layer of the first dielectric multilayer film capable causing a decrease in the longer wavelength side of the transmittance of the P-polarized light component, satisfies the Brewster's condition (1) at 56 degrees; and that the alternate layer of the first dielectric multilayer film capable of causing a decrease in the shorter wavelength side of the transmittance, satisfies the Brewster's condition (2) at 48 degrees.
Thus, when the design reference wavelengths and the combination of the high-refractive index substance and the low-refractive index substance constituting the first and second dielectric multilayer films are made different from each other, it is possible to considerably broaden the band width in the wavelength to be used, and to provide a high-band width polarizing beam splitter having a high degree of freedom in the incident angle of light and having a high polarizing separation ratio S/P.
(Third Embodiment of the Structure of Polarizing Beam Splitter)
FIGS. 15 and 10 show a third embodiment of the structure of the polarizing beam splitter according to the present invention.
This structure embodiment is an example of the modification of the polarizing beam splitter according to the present invention in the arrangement thereof. Referring to FIG. 15, on a light-transmissive substrate 1, a first dielectric multilayer film 3 and a second dielectric multilayer film 4 are sequentially disposed or laminated, and another light-transmissive substrate 2 is further disposed thereon by the medium of an adhesive layer 5.
The structure of FIG. 15 has an advantage such that the film formation of the low-refractive index layer and high-refractive index layer may be accomplished at one time or in one batch. In other words, when the structure arrangement of the third embodiment of the polarizing beam splitter is used, the film formation of the dielectric multilayer film may be accomplished at one time or in one batch, and therefore the resultant productivity may be increased.
(Fourth Embodiment of the Structure of Polarizing Beam Splitter)
The above-mentioned FIG. 11 shows a fourth embodiment of the structure of the polarizing beam splitter according to the present invention.
Referring to FIG. 11, the polarizing beam splitter of this structure embodiment has as structure wherein a substrate of a transparent flat plate 2 is used as a light-transmissive substrate, a first dielectric multilayer film 3 and a second dielectric multilayer film 4 are disposed on both sides of the substrate of a transparent flat plate 2, and further the resultant laminate is immersed in a liquid medium 6 having substantially the same refractive index as that of the substrate of the transparent flat plate 2. For example, it is preferred to use ethylene glycol (refractive index=1.43), benzene(refractive index=1.51), etc.
In general, when a prism is used as a light-transmissive substrate, there is a possibility that birefringence can occur due to the non-uniformity in the material constituting the interior of the prism. Further, it is known that there can be a case wherein the state of polarization is changed and the characteristic of a linearly polarized light is deteriorated, when a beam of light passes through a light-transmissive substrate. In such a case, the problem of the birefringence in the light-transmissive substrate may be solved by adopting a structure using a liquid medium as in the above structure embodiment.
In addition, it is not necessary to use an expensive prism in the polarizing beam splitter having the above-mentioned structure of this fourth structure embodiment. As a result, it is possible to simplify the structure of an optical system, and to reduce the cost thereof, etc.
The meanings of the reference numerals used in the above FIGS. 6 to 15 are as follows.
1: First light-transmissive substrate (prism)
2: Second light-transmissive substrate (prism)
3: First dielectric multilayer film
4: Second dielectric multilayer film
5: Adhesive layer
6: Liquid media
11: High-refractive index substance having an optical film thickness of λ1/4
12: Low-refractive index substance having an optical film thickness of λ1/4
13: Alternate layer comprising a high-refractive index substance and a low-refractive index substance each having an optical film thickness of λ1/4
1C: Adjusting layer having an optical film thickness of λ1/8
21: High-refractive index substance having an optical film thickness of λ2/4
22: Low-refractive index substance having an optical film thickness of λ2/4
23: Alternate layer comprising a high-refractive index substance and a low-refractive index substance each having an optical film thickness of λ2/4
2C: Adjusting layer having an optical film thickness of λ2/8
(Example of Application of Polarizing Beam Splitter)
Hereinbelow, there is described an example wherein the polarizing beam splitter according to the present invention is applied to a projector.
FIG. 22 is a schematic view showing an example of the structure of a multi-color or full-color projector utilizing a polarizing beam splitter 40 according to the present invention (With respect to the details of such a projector, e.g., U.S. Pat. No. 4,127,322 may be referred to). The projector of this type is required to have a characteristic such that it can provide an image with a high contrast. In order to easily provide a high contrast, it is particularly preferred to use a polarizing beam splitter 40 having a high extinction ratio and being capable of suppressing the occurrence of non-uniformity in illuminance (that is, a polarizing beam splitter using an optical glass according to the present invention having a photoelastic constant C of substantially zero). The meanings of the reference numeral used in FIG. 22 are as follows:
15A, 15B, 15C: Optical valve (such as liquid crystal device)
24A, 24B, 24C: CRT
40: Polarizing beam splitter
41, 42: Dichroic mirror
43: Lens
44: Screen
45: Arc discharge tube
46: Spherical lens
47: Condenser/collimator lens
48: First optical axis
49: Glass cube
RA, RB, RC: Respective colors.
FIG. 29 is a schematic sectional view showing a basic example of the structure of the projector system using the polarizing beam splitter (PBS) according to the present invention. In this embodiment of FIG. 29, along an optical path, there are arranged a light source lamp, an IR-cutting filter, a UV-cutting filter, a condenser lens, the above-mentioned PBS, a liquid crystal (LC) device, (PBS), a projection lens, and a screen.
Hereinbelow, the present invention will be specifically described with reference to Examples, by which the present invention should not be limited.
EXAMPLES
Example 1
As respective raw materials for constituting respective glass compositions, there were provided corresponding oxides, carbonates, nitrates, etc. After these raw materials were highly refined in an ordinary manner, they were weighed (total weight of each batch: 100 to 500 g) in a box of which temperature had been set to room temperature, so as to provide respective ratios (wt. %) as shown in FIG. 26 (Table 7) and FIG. 27 (Table 8) (wt. percents shown in the above FIGS. 26 to 28 were 100% in total).
The thus formulated raw materials were melted in a platinum crucible at 1000–1300 degrees by use of an electric furnace in the atmospheric air, and then the resultant mixture was subjected to clarification and stirring to be homogenized in an ordinary manner. Thereafter, the resultant mixture was casted into a metal mold (made of stainless steel) which had been preheated to 300–450 degrees in advance, and then gradually cooled or annealed, whereby seven kinds of optical glasses (Sample glass Nos. 21 to 27) for polarizing optical system were prepared.
With respect to each of the thus prepared glasses (No. 21 to 27), a photoelastic constant C for light having a wavelength of λ=633 nm, and a linear expansion coefficient were measured. At this time, the photoelastic constant C was obtained by the above-mentioned photoelastic modulation method, while using light having a wavelength of λ=633 nm, and the respective glass samples having a light transmission thickness of l (el)=10 mm as shown in the above-mentioned Equations (1) and (2). The thus obtained results are shown in FIGS. 26 to 28 (Tables 7 to 8).
As shown in the above-mentioned Tables, this Example provided optical glasses for polarizing optical system having various kinds of compositions for providing a photoelastic constant C of substantially zero (C=−0.12 to 0.41).
FIG. 28 is a graph wherein the abscissa denotes the lead oxide (PbO) content and the ordinate denotes the photoelastic constant C, with respect to the each of the glasses (No. 21 to 27) as described above. In view of the graph of FIG. 28, it may be understood that the photoelastic constant C is decreased almost linearly along with an increase in the lead oxide content, and the constant becomes zero at a certain point and thereafter becomes a negative value.
With respect to a borosilicate glass “BK7” as a comparative example which has widely been used for conventional optical systems, the ratios of the components, and the measurement results of the photoelastic constant C for light having a wavelength of λ=633 nm, and the linear expansion coefficient are shown in FIG. 27 (Table 8).
In view of these FIGS. 26–28 (Table 7–8), it may be understood that the photoelastic constants C of the optical glass according to the present invention (Sample Nos. 21–27) are much smaller than that of the conventional glass “BK7”, and particularly, the optical glasses of Nos. 24 to 26 had a photoelastic constant C in an extremely small range (−0.07 to +0.10).
In addition, the linear expansion coefficients of the optical glasses of Nos. 21–27 according to the present invention are at substantially the same level as that of the “BK7”. Accordingly, it may be understood that even when the optical glasses of Nos. 21–27 according to the present invention are used instead of the “BK7”, holders for holding the optical glass, or other optical elements are not adversely affected by a difference in the thermal expansion coefficients therebetween.
Example 2
The degrees of the birefringence of the Sample glass Nos. 22, 24 and 25 prepared in Example 1, and the commercially available borosilicate glass BK7 (mfd. by Schott Co., Germany) were measured by use of an apparatus as shown in FIGS. 4 and 5 under the application of a stress of about 30 N/cm2.
More specifically, a sample of each of the glasses having a known size l (el)=10 mm was used for the measurement, the birefringence thereof was measured by using light having a known wavelength of λ=633 nm under the application of a known uniaxial stress σ2 for providing a relationship of σ1=σ3=0 in the above-mentioned Equations (1) and (2), whereby an optical path difference Δφ (nm/cm) per 1 cm of the sample glass was obtained. The thus obtained measurement results are shown in FIG. 21 (Table 6) and in the following table.
No. of sample glass: No. 24
Stress: 31.0 N/cm2 
Degree of birefringence: 3.10 nm/cm
As shown in the above FIG. 21 (Table 6), the optical glass for polarizing optical system showed an extremely small value, as compared with that of the commercially available borosilicate glass BK7.
Example 3
The refractive indices of the Sample glass Nos. 21 to 27 prepared in Example 1, and the commercially available borosilicate glass BK7 (mfd. by Schott Co., Germany) were measured by use of a commercially available apparatus for measuring refractive index, while using light having a wavelength of λ=587.6 nm, and a sample of each glass having a light transmittance thickness of l (el)=10 mm.
The thus obtained measurement results are shown in FIG. 20 (Table 5).
Example 4
As respective raw materials for constituting respective glass compositions, there were provided corresponding oxides, fluorides, carbonates, nitrates, etc. They were weighed (total weight of each batch: 100 to 500 g) in a box of which temperature had been set to room temperature so as to provide respective ratios (wt. %) as shown in FIG. 16 (Table 1), FIG. 17 (Table 2), FIG. 18 (Table 3) and FIG. 19 (Table 4), and mixed with each other thereby to provide a formulated raw material. The above FIGS. 16 to 19 (Tables 1, 2, 3 and 4) show ratios of the respective components calculated in terms of mol % and wt. % (percents shown in the respective batch were 100% in total).
The thus formulated raw materials were melted in a platinum crucible at 1000–1300 degrees by use of an electric furnace in the atmospheric air, and then the resultant mixture was subjected to clarification and stirring to be homogenized in an ordinary manner. Thereafter, the resultant mixture was casted into a metal mold (made of stainless steel) which had been preheated to 300–450 degrees in advance, and then gradually cooled or annealed, whereby 14 kinds of optical glasses (Sample glass Nos. 1 to 14) for a polarizing optical system were prepared.
With respect to each of the thus prepared glasses (Nos. 1 to 14), a refractive index nd, a transmission spectrum at a thickness of 10 mm (wavelength corresponding to a transmittance of 80%), and a photoelastic constant C for light having a wavelength of λ=633 nm were measured. At this time, the photoelastic constant C was calculated by using the birefringence under the application of a stress obtained by the above-mentioned photoelastic modulation method, while using light having a wavelength of λ=633 nm, and the respective glass samples having a light transmission thickness of l (el)=10 mm as shown in the above-mentioned Equations (1) and (2). The thus obtained results are shown in FIGS. 16 to 19 (Tables 1, 2, 3 and 4).
As shown in the above-mentioned tables, this Example provided optical glasses for polarizing optical system having various kinds of compositions for providing a photoelastic constant C of substantially zero (C=+0.01 to 0.04).
Example 5
A polarizing beam splitter (as shown in FIG. 7, the first embodiment of the structure) which had been constituted by using the optical glass for polarizing optical system (Sample No. 24) prepared in Example 1 as the material for the prisms 1 and 2, was evaluated by using an evaluation optical system shown by the schematic view of FIG. 23. The polarizing film of the polarizing beam splitter used herein was designed so as to provide a central wavelength of λ=540 nm corresponding to the wavelength of green.
More specifically, a polarizing beam splitter 61 was illuminated with the light emitted from a xenon lamp 62 as a light source, the image of the xenon lamp 62 was projected onto a screen 64 by way of a mirror 63, and the resultant non-uniformity in the illuminance on the screen 64 was evaluated by use of a photograph taken by a camera. The results of the evaluation are shown in the photograph of FIG. 24, wherein a ghost image can be recognized. As shown in FIG. 24, very little non-uniformity was observed when the polarizing beam splitter using the optical glass according to the present invention having a photoelastic constant C of substantially zero was used.
On the other hand, non-uniformity was measured by using a polarizing beam splitter having the same structure as that described above in the same manner as in the above procedure, except that a conventional optical glass (borosilicate glass BK7, mfd. by Schott Co.) was used instead of the above-mentioned optical glass according to the present invention. As a result, marked non-uniformity in the illuminance was observed as shown in the photograph of FIG. 25.
Example 6
Three kinds of optical glasses (Sample glass Nos. A, B and C) each having the composition indicated in FIG. 30 (Table 9) were prepared by the same manner as that mentioned in Example 1. With respect to each of the thus prepared glasses, an internal transmittance at a thickness of 10 mm, a wavelength corresponding to a transmittance of 80%, and a photoelastic constant C for light having a wavelength of λ=633 nm were measured. The results thus obtained are shown in FIG. 30 (Table 9) and FIG. 31.
As shown in FIG. 30 (Table 9) and FIG. 31, the optical glasses Nos. A and B according to the present invention which contain As2O3 or Sb2O3 have quite a high internal transmittance with respect to light having a wavelength of 400 nm or more. On the other hand, the internal transmittance of the comparative glass No. C which neither contains As2O3 nor Sb2O3 is low as compared with those of the present optical glasses Nos. A and B.
Example 7
With respect to each of the optical glass according to the present invention and the typical commercially-available optical glasses indicated in FIG. 32 (Table 10), a photoelastic constant C for light having a wavelength of λ=633 nm was measured by the same manner as that mentioned in Example 1. The results thus obtained are shown in FIG. 31 (Table 10).
As shown in FIG. 31 (Table 10), it will be understood that most of the commercially-available optical glasses have a photoelastic constant of 2×10−8 cm2/N or larger. More specifically, the most popular optical glass “BK7” has a photoelastic constant of about 2.85×10−8 cm2/N, and the optical glass “FC3” has a photoelastic constant of 4.09×10−8 cm2/N.
Further, the fluoride-phosphate type optical glass “PDC6” has a relatively small photoelastic constant of 0.52×10−8 cm2/N. However, this glass cannot provide a photoelastic constant of substantially zero in a visible light region, and therefore, this glass is not optimum for polarizing optical system.
On the other hand, the optical glass according to the present invention has an extremely small photoelastic constant that is 0.01×10−8 cm2/N which has never been achieved by the commercially-available optical glasses as the prior art.
As described hereinabove, the present invention provides an optical glass for polarizing optical system having a photoelastic constant C in the range of −0.2 to +0.5 [10−8 cm2/N] which is substantially zero with respect to a wavelength of 633 nm.
As described above, the optical glass for polarizing optical system according to the present invention has an excellent characteristic such that it cause substantially no optical path difference based on an optical anisotropy, even when there occurs a mechanical external stress or a thermal stress. Accordingly, when the glass according to the present invention is applied to an optical element for a polarizing optical system, the polarizing characteristic of optical information may be well retained by substantially obviating the effect of the mechanical external stress or the thermal stress.
In an embodiment wherein the optical glass for polarizing optical system according to the present invention does not contain fluorine, an optical glass for polarizing optical system having a photoelastic constant C of substantially zero may easily be accomplished by selecting the composition ratio of PbO. Accordingly, it is possible for the glass according to the present invention to provide substantially no optical anisotropy, even when there occurs a mechanical external stress or a thermal stress in the glass.
In addition, in the present invention, when the fluorine/oxygen (F/O) ratio is selected, it is also possible to produce an optical glass for polarizing optical system which is capable of increasing or decreasing the refractive index thereof within a predetermined range while retaining the photoelastic constant C to substantially zero. As described above, according to the present invention, it is possible to easily provide an optical glass or an optical element (or an optical component) utilizing such a glass which has a refractive index suitable for the purpose of the use thereof while retaining a good polarizing characteristic. Accordingly, in the present invention, the degree of freedom or possibility in the optical design may be greatly enhanced.
Further, the optical glass according to the present invention contains As2O3 and/or Sb2O3 which is capable of functioning as a defoaming agent in amount of 0.1 to 3.0 wt. % (0.1–2.0 mol %). Thus quite a high internal transmittance with respect to light having a wavelength of 400 nm or more can be achieved as discussed above.
Therefore, according to the present invention, the latitude in the selection of an “optical thin film” which is to be determined on the basis of the refractive index of glass, is broadened, and the selection of the optical thin film is facilitated. In addition, the present invention enables an improvement in the transparency (or degree of coloring) at the wavelength corresponding to visible light, and therefore the optical glass may be applied to a larger number of optical elements. The optical glass according to the present invention may particularly preferably be used for a polarizing beam splitter or a read-out transparent substrate for a spatial light modulator which is required to have a high-precision polarizing characteristic.

## Claims

1. A polarizing beam splitter comprising:
a light-transmissive substrate; and
a dielectric multilayer film disposed on the substrate, the dielectric multilayer film comprising, at least, a first dielectric multilayer film and a second dielectric multilayer film having two design reference wavelengths λ1 and λ2, respectively, which are different from each other, each of the first and second dielectric multilayer films comprising:
an alternate layer comprising an n-cycle (n: an integer) laminate of a basic cycle of a two-layer structure of a high-refractive index layer and a low-refractive index layer each having an optical film thickness of λ1/4 or λ2/4 respectively at the reference wavelength λ1 or λ2; and
a thin-film adjusting layer comprising each one of the high-refractive index layer or the low-refractive index layer having an optical film thickness of λ1/8 or λ2/8 disposed on both sides of the alternate layer;

wherein the alternate layers of the first and second dielectric multilayer films respectively comprise different combinations of substances.

2. The polarizing beam splitter according to claim 1, wherein the light-transmissive substrate comprises an optical glass for polarizing optical system having a photoelastic constant C in the range of substantially zero with respect to a wavelength in a range of about 0.4 μm to about 3.0 μm.

3. The polarizing beam splitter according to claim 1, wherein
the alternate layer of the first dielectric multilayer film comprises a combination of a high-refractive index substance of TiO2 and a low-refractive index substance of SiO2; and
the alternate layer of the second dielectric multilayer film comprises a combination of a high-refractive index substance of TiO2 and a low-refractive index substance of Al2O3.

4. The polarizing beam splitter according to claim 1, wherein
the alternate layer of the first dielectric multilayer film comprises a combination of a high-refractive index substance of TiO2 and a low-refractive index substance of SiO2; and
the alternate layer of the second dielectric multilayer film comprises a combination of a high-refractive index substance of ZrO2 and a low-refractive index substance of MgF2.

5. The polarizing beam splitter according to claim 1, wherein the first and second dielectric multilayer films are disposed in a liquid medium having substantially the same refractive index as that of the light-transmissive substrate.

6. A polarizing beam splitter comprising
a light-transmissive substrate; and
a dielectric multilayer film disposed on the substrate, the dielectric multilayer film comprising:
a first dielectric multilayer film comprising an alternate layer which includes a first high-refractive index layer and a first low-refractive index layer each having a design reference wavelength λ1; and
a second dielectric multilayer film comprising an alternate layer which includes a second high-refractive index layer and a second low-refractive index layer each having a design reference wavelength λ2;

wherein the design reference wavelength λ2 is different from the design reference wavelength λ1, and wherein the alternate layers of the first and second dielectric multilayer films respectively comprise different combinations of substances.

7. The polarizing beam splitter according to claim 6, wherein
the alternate layer of the first dielectric multilayer film comprises an n-cycle (n: an integer) laminate of a basic cycle of a two-layer structure of the first high-refractive index layer having an optical film thickness of λ1/4 and the first low-refractive index layer having an optical film thickness of λ1/4; and
the alternate layer of the second dielectric multilayer film comprises an n-cycle (a: an integer) laminate of a basic cycle of a two-layer structure of the second high-refractive index layer having an optical film thickness of λ2/4 and the second low-refractive index layer having an optical film thickness of λ2/4.

8. The polarizing beam splitter according to claim 6, wherein the first high-refractive index layer, the first low-refractive index layer, the second high-refractive index layer, and the second low-refractive index layer respectively comprise different combinations of at least three substances.

9. The polarizing beam splitter according to claim 8, wherein
one of the first high-refractive index layer and the first low-refractive index layer comprises a first substance and the other of the first high-refractive index layer and the first low-refractive index layer comprises a second substance;
one of the second high-refractive index layer and the second low-refractive index layer comprises the first substance and the other of the second high-refractive index layer and the second low-refractive index layer comprises a third substance; and
the first substance, the second substance, and the third substance are different from one another.

10. The polarizing beam splitter according to claim 8, wherein
one of the first high-refractive index layer and the first low-refractive index layer comprises a first substance and the other of the first high-refractive index layer and the first low-refractive index layer comprises a second substance;
one of the second high-refractive index layer and the second low-refractive index layer comprises a third substance and the other of the second high-refractive index layer and the second low-refractive index layer comprises a fourth substance; and
the first substance, the second substance, the third substance, and the fourth substance are different from one another.