Polarizing glass sheet set for optical isolator and method for manufacturing optical element for optical isolator

Abstract

A method of manufacturing a polarizing glass sheet includes subjecting, while heating, a glass preform sheet containing metal halide particles to down-drawing, to thereby provide a glass member having stretched metal halide particles dispersed in an aligned manner in a glass matrix, and subjecting the glass member to reduction treatment to reduce the stretched metal halide particles, to thereby provide a polarizing glass sheet. A shape of the glass preform sheet during the down-drawing satisfies a relationship of the following expression:
L.sub.1/W.sub.1≥1.0
where L.sub.1 represents a length between a portion in which a width of the glass preform sheet has changed to 0.8 times an original width and a portion in which the width of the glass preform sheet has changed to 0.2 times the original width W.sub.0, and W.sub.1 represents a length equivalent to 0.5 times the original width W.sub.0 of the glass preform sheet.

Claims

1. A method of manufacturing an optical element for an optical isolator, the method comprising: a preparation step of preparing: a first polarizing glass sheet having a rectangular shape and having stretched metal particles dispersed in an aligned manner in a glass matrix, a stretching direction of the metal particles being substantially parallel to one side of the first polarizing glass sheet; and a second polarizing glass sheet having a rectangular shape and having stretched metal particles dispersed in an aligned manner in a glass matrix, a stretching direction of the metal particles forming an angle of about 45° with respect to one side of the second polarizing glass sheet; a bonding step, after the preparation step, of bonding the first polarizing glass sheet and the second polarizing glass sheet to each other through intermediation of a Faraday rotator so as to hold the Faraday rotator therebetween, to thereby manufacture an optical element base material for an optical isolator; and a cutting step of cutting the optical element base material for an optical isolator, to thereby provide an optical element for an optical isolator, wherein the preparing of the first polarizing glass sheet and the second polarizing glass sheet comprises preparing one of the first polarizing glass sheet and the second polarizing glass sheet to be free of a cutout portion, and forming at least one cutout portion in another of the first polarizing glass sheet and the second polarizing glass sheet.

2. The method of manufacturing an optical element for an optical isolator according to claim 1, wherein the cutout portion formed in the another of the first polarizing glass sheet and the second polarizing glass sheet is capable of being utilized for distinguishing between the first polarizing glass sheet and the second polarizing glass sheet.

3. A polarizing glass sheet set for an optical isolator, the polarizing glass sheet set comprising: a first polarizing glass sheet having a rectangular shape and having stretched metal particles dispersed in an aligned manner in a glass matrix, a stretching direction of the metal particles being substantially parallel to one side of the first polarizing glass sheet; and a second polarizing glass sheet having a rectangular shape and having stretched metal particles dispersed in an aligned manner in a glass matrix, a stretching direction of the metal particles forming an angle of about 45° with respect to one side of the second polarizing glass sheet, wherein one of the first polarizing glass sheet and the second polarizing glass sheet is free of a cutout portion, and another of the first polarizing glass sheet and the second polarizing glass sheet comprises at least one cutout portion that extends an entire thickness of the another of the first polarizing glass sheet and the second polarizing glass sheet.

4. The polarizing glass sheet set for an optical isolator according to claim 3, wherein the first polarizing glass sheet and the second polarizing glass sheet have a substantially square shape.

5. The polarizing glass sheet set for an optical isolator according to claim 3, wherein the cutout portion of the another of the first polarizing glass sheet and the second polarizing glass sheet has a triangular shape with a corner portion of the another of the first polarizing glass sheet and the second polarizing glass sheet being an apex of the triangular shape.

6. The polarizing glass sheet set for an optical isolator according to claim 3, wherein the cutout portion of the another of the first polarizing glass sheet and the second polarizing glass sheet is formed so as to be positioned in a corner portion of the another of the first polarizing glass sheet and the second polarizing glass sheet.

7. A polarizing glass sheet set for an optical isolator, the polarizing glass sheet set comprising: a first polarizing glass sheet having a rectangular shape and having stretched metal particles dispersed in an aligned manner in a glass matrix, a stretching direction of the metal particles being substantially parallel to one side of the first polarizing glass sheet; and a second polarizing glass sheet having a rectangular shape and having stretched metal particles dispersed in an aligned manner in a glass matrix, a stretching direction of the metal particles forming an angle of about 45° with respect to one side of the second polarizing glass sheet, wherein the first polarizing glass sheet and the second polarizing glass sheet are free from any attachment to each other, and wherein one of the first polarizing glass sheet and the second polarizing glass sheet is free of a cutout portion, and another of the first polarizing glass sheet and the second polarizing glass sheet comprises at least one cutout portion.

8. The polarizing glass sheet set for an optical isolator according to claim 7, wherein the first polarizing glass sheet and the second polarizing glass sheet have a substantially square shape.

9. The polarizing glass sheet set for an optical isolator according to claim 7, wherein the cutout portion of the another of the first polarizing glass sheet and the second polarizing glass sheet has a triangular shape with a corner portion of the another of the first polarizing glass sheet and the second polarizing glass sheet being an apex of the triangular shape.

10. The polarizing glass sheet set for an optical isolator according to claim 7, wherein the cutout portion of the another of the first polarizing glass sheet and the second polarizing glass sheet is formed so as to be positioned in a corner portion of the another of the first polarizing glass sheet and the second polarizing glass sheet.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic front view for illustrating a down-drawing step of a glass preform sheet according to one embodiment of the present invention.

(2) FIG. 2 is a schematic plan view of a polarizing glass sheet for illustrating a measurement method for polarizing axis deviation and an extinction ratio in Examples.

(3) FIG. 3A is a schematic plan view for illustrating an example of a polarizing glass sheet having a cutout portion formed in a corner portion.

(4) FIG. 3B is a schematic plan view for illustrating an example of a polarizing glass sheet having a cutout portion formed in a corner portion.

(5) FIG. 3C is a schematic plan view for illustrating an example of a polarizing glass sheet having a cutout portion formed in a corner portion.

(6) FIG. 4 is a schematic plan view for illustrating an example of a polarizing glass sheet having a plurality of cutout portions.

(7) FIGS. 5A and 5B are schematic plan views for illustrating examples of a polarizing glass sheet having a cutout portion that is asymmetric with respect to a diagonal line.

(8) FIG. 6 is a schematic plan view for illustrating an example of a polarizing glass sheet having a cutout portion formed at a position not including a corner portion.

(9) FIG. 7A is a schematic plan view for illustrating an example of a polarizing glass sheet having a plurality of cutout portions.

(10) FIG. 7B is a schematic plan view for illustrating an example of a polarizing glass sheet having a plurality of cutout portions.

(11) FIG. 7C is a schematic plan view for illustrating an example of a polarizing glass sheet having a plurality of cutout portions.

(12) FIG. 8 is a schematic perspective view for illustrating a method of manufacturing an optical element base material for an optical isolator.

(13) FIG. 9A is an explanatory view for illustrating a cutout direction of a polarizing glass sheet.

(14) FIG. 9B is an explanatory view for illustrating a cutout direction of a polarizing glass sheet.

DESCRIPTION OF EMBODIMENTS

First Aspect of Present Invention

(15) Now, embodiments of the present invention are described in detail with reference to the drawings.

(16) (Preparation of Glass Preform Sheet)

(17) First, a glass preform sheet serving as a down-drawing base material is prepared. As a glass forming the glass preform sheet, a glass having a predetermined viscosity in a temperature range (e.g., 480° C. or more), in which metal halide particles are sufficiently softened and deformed in the glass, is selected. With this, the metal halide particles can be stretched to a desired length. As such glass, there is given borosilicate glass.

(18) The glass preform sheet can be manufactured as described below. First, raw materials are blended so that a desired glass composition is obtained. In order to precipitate metal halide particles in a glass matrix in a later stage, the raw materials contain a halogen element material and a metal element material. As the halogen element, chlorine, bromine, or iodine can be used. However, iodine has a large environmental load, and hence it is preferred that chlorine or bromine be used. Further, as the metal element, it is preferred that silver or copper be used from the viewpoint that a desired extinction ratio is obtained easily. Silver bromide has a melting point lower than that of silver chloride and is liable to be spheroidized in a down-drawing step. Therefore, when silver is used as the metal particles, it is preferred that chlorine be used as the halogen element.

(19) Next, the raw materials are melted at a predetermined temperature until the raw materials become homogeneous. Then, the molten glass is formed into a sheet. The glass formed into a sheet is subjected to heating treatment, for example, at from 600° C. to 700° C., to thereby precipitate metal halide particles in a glass matrix. There is no particular limitation on the atmosphere during heating treatment, and an air atmosphere may be used. Then, the resultant is subjected to processing, such as cutting and polishing, as necessary, to thereby provide a glass preform sheet having a predetermined width.

(20) The width of the glass preform sheet is appropriately selected in accordance with the dimensions of an intended polarizing glass sheet. For example, the width of the glass preform sheet is preferably 2.5 times or more, more preferably times or more, still more preferably 10 times or more, particularly preferably 12 times or more, most preferably 15 times or more the width of the intended polarizing glass sheet. There is no particular limitation on an upper limit. However, when the width is excessively large, the polarizing axis deviation in a polarizing glass sheet surface is liable to increase. Thus, the width of the glass preform sheet is preferably 50 times or less, more preferably 30 times or less, still more preferably 25 times or less the width of the intended polarizing glass sheet. Specifically, the width of the glass preform sheet is preferably from 100 mm to 500 mm, more preferably from 120 mm to 300 mm, still more preferably from 150 mm to 250 mm.

(21) There is no particular limitation on the thickness of the glass preform sheet. However, when the thickness is excessively small, the mechanical strength of the polarizing glass sheet is liable to decrease. Meanwhile, when the thickness is excessively large, the thickness of the polarizing glass sheet increases, with the result that the light transmittance is liable to decrease, and the device is liable to increase in size. In view of the foregoing, the thickness of the glass preform sheet is preferably from 10 times to 50 times, more preferably from 12 times to 30 times, still more preferably from 15 times to 25 times the thickness of the intended polarizing glass sheet. Specifically, the thickness of the glass preform sheet is preferably from 0.5 mm to 10 mm, more preferably from 1 mm to 5 mm.

(22) (Down-Drawing of Glass Preform Sheet)

(23) The glass preform sheet is subjected to down-drawing while being heated, to thereby provide a glass member having stretched metal halide particles dispersed in an aligned manner in a glass matrix. FIG. 1 is a schematic front view for illustrating a down-drawing step of the glass preform sheet according to this embodiment. A glass preform sheet 1 is heated to be softened by heat-generating elements 2 and stretched by a tension roller 3. With this, metal halide particles 4 are also stretched in a direction of a down-drawing direction D, and a glass member 5 in which stretched metal halide particles 4′ are dispersed in an aligned manner in a glass matrix is obtained. In FIG. 1, the heat-generating elements 2 have a cylindrical shape and are each installed in a direction perpendicular to the drawing sheet. Further, a plurality of heat-generating elements 2 are also installed on a front surface side and a back surface side of the glass preform sheet 1 (not shown). It is preferred that each heat-generating element 2 be arranged, for example, in a tower shape.

(24) In the down-drawing step of the glass preform sheet 1, a shape of the glass preform sheet 1 during the down-drawing satisfies a relationship of the following expression (1).
L.sub.1/W.sub.1≥1.0  (1)
In the expression, L.sub.1 represents a length between a portion a in which a width of the glass preform sheet 1 has changed to 0.8 times an original width W.sub.0 and a portion b in which the width of the glass preform sheet 1 has changed to 0.2 times the original width W.sub.0 (softened and deformed portion S.sub.1), and W.sub.1 represents a length equivalent to 0.5 times the original width W.sub.0 of the glass preform sheet 1.

(25) In the expression (1), L.sub.1/W.sub.1 is more preferably 1.2 or more, still more preferably 1.5 or more, particularly preferably 1.8 or more, most preferably 2 or more. When L.sub.1/W.sub.1 is excessively small, the polarizing axis deviation in the polarizing glass sheet is liable to increase. There is no particular limitation on an upper limit. However, when L.sub.1/W.sub.1 is excessively large, a facility is liable to increase in size. Therefore, practically, L.sub.1/W.sub.1 is preferably 10 or less, more preferably 5 or less.

(26) The value of the length L.sub.1 of the softened and deformed portion S.sub.1 is appropriately selected so as to satisfy the relationship of the expression (1). Specifically, the value of L.sub.1 is preferably 60 mm or more, more preferably 100 mm or more, still more preferably 120 mm or more, particularly preferably 150 mm or more. When the value of L.sub.1 is excessively small, the polarizing axis deviation in the polarizing glass sheet is liable to increase.

(27) In another embodiment of the present invention, in the down-drawing step of the glass preform sheet 1, a shape of the glass preform sheet 1 during the down-drawing satisfies a relationship of the following expression (2).
L.sub.2/W.sub.1≥0.5  (2)
In the expression, L.sub.2 represents a length between a portion a in which a width of the glass preform sheet 1 has changed to 0.8 times an original width W.sub.0 and a portion c in which the width of the glass preform sheet 1 has changed to 0.5 times the original width W.sub.0 (softened and deformed portion S.sub.2), and W.sub.1 represents a length equivalent to 0.5 times the original width W.sub.0 of the glass preform sheet 1.

(28) In the expression (2), L.sub.2/W.sub.1 is more preferably 0.5 or more, more preferably 0.8 or more, particularly preferably 1.0 or more. When L.sub.2/W.sub.1 is excessively small, the polarizing axis deviation in the polarizing glass sheet is liable to increase. There is no particular limitation on an upper limit. However, when L.sub.2/W.sub.1 is excessively large, a facility is liable to increase in size. Therefore, practically, L.sub.2/W.sub.1 is preferably 20 or less, more preferably 10 or less.

(29) The value of the length L.sub.2 of the softened and deformed portion S.sub.2 is appropriately selected so as to satisfy the relationship of the expression (2). Specifically, the value of L.sub.2 is preferably 30 mm or more, more preferably 50 mm or more, still more preferably 60 mm or more, particularly preferably 75 mm or more. When the value of L.sub.2 is excessively small, the polarizing axis deviation in the polarizing glass sheet is liable to increase.

(30) It is only necessary that a distance between centers in the down-drawing direction D of the heat-generating element 2 in an uppermost stage and the heat-generating element 2 in a lowermost stage (hereinafter referred to as “length of a heat-generating portion”) be appropriately adjusted in accordance with the width W.sub.0 of the glass preform sheet 1. For example, the length of the heat-generating portion is preferably 1.5 times or more, more preferably 2 times or more, still more preferably 2.5 times or more the width W.sub.0 of the glass preform sheet 1. There is no particular limitation on an upper limit. However, when the length of the heat-generating portion is excessively large, an energy loss is caused. Therefore, the length of the heat-generating portion is preferably 10 times or less, more preferably 8 times or less the width W.sub.0 of the glass preform sheet 1. Specifically, the length of the heat-generating portion is preferably from 250 mm to 1,000 mm, more preferably from 300 mm to 800 mm or more, still more preferably from 400 mm to 800 mm.

(31) The glass preform sheet 1 is heated so that the viscosity thereof reaches preferably from 10.sup.7 dPa.Math.s to 10.sup.11 dPa.Math.s, more preferably from 10.sup.8 dPa.Math.s to 10.sup.10 dPa.Math.s, still more preferably from 10.sup.8.5 dPa.Math.s to 10.sup.9.5 dPa.Math.s in the softened and deformed portion S.sub.1 of the glass preform sheet 1. When the viscosity of the glass preform sheet 1 in the softened and deformed portion S.sub.1 is excessively low, the viscosity of the metal halide particles 4 also decreases, and the metal halide particles 4 are spheroidized, with the result that the stretched metal halide particles 4′ having a desired length are not likely to be obtained. Meanwhile, when the viscosity of the glass preform sheet 1 in the softened and deformed portion S.sub.1 is excessively high, the glass preform sheet 1 is not sufficiently softened and deformed, and the shape thereof during the down-drawing is less likely to satisfy the relationship of the expression (1). Further, in some cases, there is a risk in that the glass preform 1 may be broken.

(32) (Reduction of Glass Member)

(33) The glass member 5 obtained as described above is subjected to reduction treatment to reduce the stretched metal halide particles 4′, to thereby provide stretched metal particles. The reduction treatment is performed by heating, for example, in a hydrogen atmosphere. In general, it is only necessary that only the stretched metal halide particles 4′ located in a surface layer (for example, depth of from 10 μm to 100 μm, further from 20 μm to 80 μm) of the glass member 5 be reduced to be changed to the stretched metal particles.

(34) The extinction wavelength range of the polarizing glass sheet varies depending on the length of the stretched metal particles. Therefore, it is only necessary that the length of the stretched metal particles be appropriately adjusted in accordance with an intended extinction wavelength range. The length of the stretched metal particles is appropriately adjusted within a range of, for example, from 50 nm to 300 nm, further from 80 nm to 200 nm. Further, the aspect ratio of the stretched metal particles is appropriately adjusted within a range of, for example, from 5 to 20, further from 8 to 15.

(35) The glass member 5 subjected to the reduction treatment is subjected to processing, such as cutting, to thereby provide a polarizing glass sheet having desired dimensions. As necessary, a functional film, such as a reflection preventing film formed of a dielectric multi-layer film, may be formed on the surface of the polarizing glass sheet.

(36) (Polarizing Glass Sheet)

(37) The dimensions of the polarizing glass sheet measure, for example, preferably 5 mm square or more, more preferably 10 mm square or more, still more preferably 15 mm square or more, particularly preferably 20 mm square or more. As described above, in recent years, a manufacturing method involving manufacturing a large optical isolator through use of a large polarizing glass sheet and a Faraday rotator and cutting the optical isolator into chips each measuring from 0.5 mm square to 2.0 mm square has been adopted. Therefore, as the polarizing glass sheet becomes larger, mass manufacturing can be achieved, with the result that cost can be reduced. However, when the polarizing glass sheet is excessively large, the in-plane polarizing axis deviation increases, and a yield is liable to decrease. Therefore, the dimensions of the polarizing glass sheet measure preferably 40 mm square or less, more preferably 30 mm square or less.

(38) There is no particular limitation on the thickness of the polarizing glass sheet. However, when the thickness is excessively small, the mechanical strength of the polarizing glass sheet is liable to decrease. Meanwhile, when the thickness is excessively large, the light transmittance is liable to decrease, and the device is liable to increase in size. In view of the foregoing, the thickness of the polarizing glass sheet is preferably from 0.05 mm to 1 mm, more preferably from 0.1 mm to 0.5 mm.

(39) In a direction perpendicular to the down-drawing direction D, the angle variation (polarizing axis deviation) of the stretched metal particles at the width of 8 mm of the polarizing glass sheet falls within preferably 0.0065°/mm, more preferably 0.0060°/mm, still more preferably 0.0055°/mm, particularly preferably 0.0050°/mm. When the polarizing axis deviation of the polarizing glass sheet is excessively large, the extinction ratio variation in the polarizing glass sheet surface is liable to increase, and a yield is liable to decrease.

(40) The extinction ratio of the polarizing glass sheet is preferably 40 dB or more, more preferably 45 dB or more, still more preferably 50 dB or more at a wavelength of 1,310 nm and/or 1,550 nm of an infrared laser. The extinction ratio is calculated by the expression (3).
Extinction ratio (dB)=10×log.sub.10(P.sub.1/P.sub.2)  (3)

(41) P.sub.1=Maximum quantity of transmitted light

(42) P.sub.2=Minimum quantity of transmitted light

(43) The in-plane variation of the extinction ratio of the polarizing glass sheet at the width of 8 mm in a direction perpendicular to the down-drawing direction falls within preferably ±5 dB, more preferably ±3 dB, still more preferably ±2.5 dB, particularly preferably ±2 dB.

(44) The polarizing glass sheet obtained as described above is used as an optical isolator by being bonded to a Faraday rotator having substantially the same dimensions. Specifically, two polarizing glass sheets are bonded to each other so as to hold the Faraday rotator, and the resultant is used as an optical isolator by being cut to desired dimensions (e.g., from 0.5 mm square to 2.0 mm square) as necessary. In order to enhance performance, an optical isolator may be manufactured by alternately stacking a plurality of Faraday rotators and three or more polarizing glass sheets.

Second Aspect of Present Invention

(45) The polarizing glass sheet of the present invention has a configuration in which stretched metal particles are dispersed in an aligned manner in a glass matrix. The polarizing glass sheet can be classified into two kinds depending on the stretching direction of the metal particles. That is, there are a polarizing glass sheet (first polarizing glass sheet) in which the stretching direction of the metal particles is substantially parallel to one side, and a polarizing glass sheet (second polarizing glass sheet) in which the stretched metal particles are dispersed in an aligned manner in a glass matrix and the stretching direction of the metal particles forms an angle of about 45° with respect to one side. In the second aspect of the present invention in this description, unless otherwise stated, the simple term “polarizing glass sheet” refers to both the first and second polarizing glass sheets.

(46) The polarizing glass sheet of the present invention has a rectangular shape. In particular, it is preferred that the polarizing glass sheet have a substantially square shape.

(47) The polarizing glass sheet of the present invention comprises one or more cutout portions. There is no particular limitation on the shape of a boundary line between the cutout portion and the non-cutout portion, which characterizes the form of the cutout portion. From the viewpoint of the ease of forming the cutout portion, it is preferred that the boundary line therebetween have a straight line shape or a curved shape. When the boundary line has a curved shape, it is desired that the curved shape be an arc-shaped curve from the viewpoint of the ease of processing.

(48) Through arrangement of the cutout portion, it becomes possible to distinguish between the first polarizing glass sheet and the second polarizing glass sheet based on the presence or absence of the cutout portion, the difference in features of the cutout portions, and the like. As the shape of the cutout portion, for example, there may be preferably given (1) a triangular shape, e.g., an isosceles right triangle or a scalene right triangle, (2) a rectangular shape, e.g., a square or a rectangle, and (3) a shape, such as a fan shape, surrounded by two adjacent sides of the polarizing glass sheet and a curved line connecting points on the two sides. Those cutout portions may be formed at any positions in the polarizing glass sheet.

(49) It is preferred that the cutout position be a corner portion of the polarizing glass sheet from the viewpoint of preventing chipping during cutting or handling of the polarizing glass sheet and from the viewpoint of the yield at a time when an element is obtained by cutting and separating an optical element base material for an optical isolator. FIGS. 3A to 3C are illustrations of examples of the polarizing glass sheet 10 having one cutout portion formed in a corner portion. FIG. 3A is an illustration of the polarizing glass sheet 10 having one cutout portion 11 in an isosceles right triangle shape. FIG. 3B is an illustration of the polarizing glass sheet 10 having one cutout portion 12 in a square shape. FIG. 3C is an illustration of the polarizing glass sheet having one cutout portion 13 in a fan shape.

(50) The number of the cutout portions is not limited to one, and a plurality of cutout portions 11, 11 may be formed as illustrated in FIG. 4.

(51) The front and the back of the polarizing glass sheet of the present invention can be distinguished from each other by appropriately setting the feature of the cutout portion. When the front and the back of the polarizing glass sheet are distinguished from each other through use of one cutout portion, it is only necessary that the shape and position of the cutout portion be adjusted. When the front and the back of the polarizing glass sheet are distinguished from each other based on the shape of the cutout portion, it is only necessary that the shape of the cutout portion be set to be, for example, asymmetric with respect to a diagonal line of the polarizing glass sheet, which passes through the cutout portion. FIGS. 5A to 5B are illustrations of examples of the polarizing glass sheet 10 having one cutout portion that is asymmetric with respect to a diagonal line A. FIG. 5A is an illustration of the polarizing glass sheet 10 having one cutout portion 14 in a scalene right triangle shape, and FIG. 5B is an illustration of the polarizing glass sheet 10 having one cutout portion 15 in a rectangle shape. Further, when the front and the back of the polarizing glass sheet are distinguished from each other based on the position of the cutout portion, it is only necessary that the cutout portion be formed, for example, at a position not including a corner portion of the polarizing glass sheet. FIG. 6 is an illustration of the polarizing glass sheet 10 having one cutout portion 16 in a rectangle shape formed at a position away from a corner portion. When the front and the back of the polarizing glass sheet are distinguished from each other through use of two or more cutout portions, the shape, dimensions, cutout position, and the like of the cutout portion are set to be different between the two or more cutout portions, to thereby allow the front and the back of the polarizing glass sheet to be distinguished from each other. FIGS. 7A to 7C are illustrations of examples of the polarizing glass sheet having two cutout portions. FIG. 7A is an illustration of the polarizing glass sheet 10 in which cutout portions 11 and 17 having isosceles right triangle shapes of different dimensions are formed in two adjacent corner portions. FIG. 7B is an illustration of the polarizing glass sheet 10 in which the cutout portion 12 having a square shape and the cutout portion 15 having a rectangle shape are formed in opposed corner portions. FIG. 7C is an illustration of the polarizing glass sheet 10 having the cutout portion 12 in a square shape formed in a corner portion and the cutout portion 16 in a rectangle shape formed at a position not including a corner portion.

(52) Even when the front and the back of the glass sheet can be distinguished from each other based on only the feature of one cutout portion, the arrangement of a plurality of cutout portions is not excluded. In this case, the shape, dimensions, cutout position, and the like of the cutout portions may be set to be different or may not be set to be different.

(53) The ratio of the area of the polarizing glass sheet main portion of the polarizing glass sheet of the present invention is preferably 94% or more, more preferably 98% or more, particularly preferably 99% or more with respect to the entire area obtained by combining the main portion and the cutout portion. A region in which the cutout portion is formed cannot be used for manufacturing an optical element for an optical isolator. Therefore, as the area excluding the cutout portion becomes smaller, the manufacturing yield of the optical element for an optical isolator decreases. Thus, it is desired that the area of a region excluding the cutout portion be set to be maximized as long as a range required for the distinction can be left.

(54) A polarizing glass sheet set for an optical isolator of the present invention comprises a first polarizing glass sheet and a second polarizing glass sheet. The present invention comprises the case where the first and/or second polarizing glass sheets are formed of a plurality of polarizing glass sheets, as well as the case where the first polarizing glass sheet and the second polarizing glass sheet are each formed of one polarizing glass sheet.

(55) The distinction between the first and second polarizing glass sheets through use of the cutout portion in the polarizing glass sheet set of the present invention is described below. The distinction between the front and the back of each polarizing glass sheet through use of the cutout portion is as described above, and hence the description thereof is omitted here.

(56) For distinguishing between the first and second polarizing glass sheets through use of the cutout portion, there are the case where the cutout portion is formed in only one of the polarizing glass sheets and the case where the cutout portion is formed in both the polarizing glass sheets. The case where the cutout portion is formed in only one of the polarizing glass sheets is as described above, and hence the description thereof is omitted here.

(57) When the cutout portion is formed in both the first and second polarizing glass sheets, it is only necessary that the shape, dimensions, cutout position, number, and the like of cutout portions be set to be different. When there is a difference in any one of the shape, dimensions, cutout position, number, and the like of cutout portions, the first polarizing glass sheet and the second polarizing glass sheet can be easily distinguished from each other. Those features may also be adopted in combination.

(58) When it is not necessary to distinguish between the first and second polarizing glass sheets, in other words, when the first polarizing glass sheet and the second polarizing glass sheet can be distinguished from each other by any means other than the cutout portion, the features of the cutout portions between the first and second polarizing glass sheets may be the same.

(59) In the polarizing glass sheet set for an optical isolator of the present invention, when the first and second polarizing glass sheets are bonded to each other, the ratio of the area of a region in which the cutout portions are not present in both the polarizing glass sheets is preferably 94% or more, more preferably 98% or more, particularly preferably 99% or more with respect to the entire area. A region in which the cutout portion is present in at least one of the polarizing glass sheets cannot be used for manufacturing an optical element for an optical isolator. Therefore, as the area of a region in which the cutout portion is not present in both the first and second polarizing glass sheets becomes smaller, the manufacturing yield of the optical element for an optical isolator decreases. Thus, it is desired that the area of a region in which the cutout portion is not present be set to be maximized as long as a range required for the distinction can be left.

(60) A method of manufacturing an optical element base material for an optical isolator of the present invention comprises a preparation step, a bonding step, and a cutting step.

(61) In the preparation step, a first polarizing glass sheet and a second polarizing glass sheet, which are processed to have substantially the same shape and substantially the same dimensions, and a Faraday rotator are prepared. As the first and/or second polarizing glass sheet, a polarizing glass sheet having a cutout portion is used. The polarizing glass sheet having a cutout portion is as described above, and hence the description thereof is omitted here. Further, a method of manufacturing a polarizing glass sheet is described later.

(62) In the bonding step, as illustrated in FIG. 8, a first polarizing glass sheet 10a and a second polarizing glass sheet 10b are bonded to each other so as to hold a Faraday rotator 20 therebetween, to thereby manufacture the optical base material for an optical isolator. In FIG. 8, there are illustrated stretched metal particles 42. The description of the cutout portion is omitted. In order to enhance the performance of the optical isolator, a plurality of Faraday rotators and three or more polarizing glass sheets may be alternately stacked to manufacture an optical base material for an optical isolator. In the bonding step, it is extremely important to distinguish between the front and the back of each polarizing glass sheet, distinguish between the first and second polarizing glass sheets, and bond the first and second polarizing glass sheets to the Faraday rotator so that the polarizing glass sheets are aligned in proper directions and arranged properly. In view of the foregoing, in the present invention, the presence or absence of the cutout portion of the polarizing glass sheet, and the shape, dimensions, cutout position, number, and the like of cutout portions can be utilized for the above-mentioned distinction.

(63) In the cutting step, the optical isolator base material obtained in the bonding step is cut to predetermined dimensions (e.g., from 0.3 mm square to 2.0 mm square). Thus, the optical element for an optical isolator can be obtained.

(64) Next, a preferred method of manufacturing a polarizing glass sheet to be used in the present invention is described.

(65) (Preparation of Glass Preform Sheet)

(66) The same method and conditions as those of the first aspect of the present invention can be employed.

(67) (Down-Drawing of Glass Preform Sheet)

(68) The same method and conditions as those of the first aspect of the present invention can be employed.

(69) (Reduction of Glass Member)

(70) The same method and conditions as those of the first aspect of the present invention can be employed. As illustrated in FIG. 9A, the first polarizing glass sheet 10a can be obtained by cutting a glass member into a rectangular shape having a side parallel to the down-drawing direction D. Further, as illustrated in FIG. 9B, the second polarizing glass sheet 10b can be obtained by cutting a glass member into a rectangular shape having a side that forms an angle of 45° with respect to the down-drawing direction D. In FIG. 9A and FIG. 9B, there are illustrated a glass member 102 after the reduction treatment and the stretched metal particles 42.

(71) Further, as necessary, a functional film, such as a reflection preventing film formed of a dielectric multi-layer film, may be formed on the surface of the cut polarizing glass sheet.

(72) (Polarizing Glass Sheet)

(73) The dimensions and thickness of the polarizing glass sheet are the same as those of the first aspect of the present invention.

(74) In a direction perpendicular to the down-drawing direction D, the angle variation (polarizing axis deviation) of the stretched metal particles at the width of 8 mm of the polarizing glass sheet falls within preferably 0.01°/mm, more preferably 0.008°/mm, still more preferably 0.007°/mm, particularly preferably 0.005°/mm. When the polarizing axis deviation of the polarizing glass sheet is excessively large, the extinction ratio variation in the polarizing glass sheet surface is liable to increase, and a yield is liable to decrease.

(75) The extinction ratio and the in-plane variation of the extinction ratio of the polarizing glass sheet are the same as those of the first aspect of the present invention.

(76) (Formation of Cutout Portion)

(77) One or more cutout portions are formed in the polarizing glass sheet obtained as described above. The shape, dimensions, cutout position, number, and the like of the cutout portions are as described above, and hence the description thereof is omitted here. The cutout portion can be formed by various methods such as a dicer, a laser, etching, a core drill, and sand blasting.

EXAMPLES

(78) Now, a method of manufacturing a polarizing glass sheet according to the first aspect of the present invention is described by way of Examples. However, the present invention is not limited to Examples below. Further, the polarizing glass sheet manufactured here can be preferably used for manufacturing a polarizing glass sheet, a polarizing optical glass sheet set for an optical isolator, and an optical element for an optical isolator according to the second aspect.

(79) Examples and Comparative Examples in the present invention are shown in Table 1.

(80) TABLE-US-00001 TABLE 1 Comparative Comparative Example 1 Example 2 Example 1 Example 2 Length L.sub.1 (mm) of softened 140 186 73 61 and deformed portion S.sub.1 Length L.sub.2 (mm) of softened 48 64 36 22 and deformed portion S.sub.2 Length W.sub.1 (mm) equivalent to 85 90 75 65 0.5 times width of glass preform sheet L.sub.1/W.sub.1 1.65 2.07 0.97 0.94 L.sub.2/W.sub.1 0.56 0.71 0.48 0.34 Length (mm) of heat-generating 400 400 250 150 portion Polarizing axis deviation (°/mm) 0.0060 0.0050 0.0125 0.0225 Extinction 1,310 nm P0 50 48 46 42 ratio P1 40 43 32 31 (dB) P2 42 42 37 26 In-plane ±5 ±3 ±7 ±8 variation 1,550 nm P0 54 55 47 41 P1 61 61 61 55 P2 62 60 58 52 In-plane ±4 ±3 ±7 ±7 variation

(81) Each sample was manufactured and evaluated as described below.

(82) (a) Manufacturing of Glass Preform Sheet

(83) A raw material batch was prepared so as to provide borosilicate glass (softening point: 650° C.) containing, in terms of mass %, 60% of SiO.sub.2, 18% of B.sub.2O.sub.3, 8.5% of Al.sub.2O.sub.3, 2% of Li.sub.2O, 2.5% of Na.sub.2O, 9% of K.sub.2O, 0.3% of Ag, and 0.5% of Cl. The raw material batch was melted and formed into a sheet shape. The sheet-shaped glass was subjected to heat treatment at 675° C. for 2 hours, to thereby precipitate silver chloride particles in the glass. Then, the sheet-shaped glass was processed to provide a glass preform sheet having a width of 170 mm and a thickness of 5 mm.

(84) (b) Down-Drawing Step of Glass Preform Sheet

(85) The glass preform sheet was subjected to down-drawing while being heated in the vicinity of temperature corresponding to a viscosity of 10.sup.9 dPa.Math.s through use of an apparatus corresponding to FIG. 1, to thereby provide a glass member (width: 17 mm) having stretched silver chloride particles dispersed in an aligned manner in a glass matrix. Down-drawing conditions are shown in Table 1.

(86) (c) Reduction Treatment Step of Glass Member

(87) The glass member obtained as described above was polished so as to have a thickness of 0.2 mm and then was subjected to reduction treatment in a hydrogen atmosphere at 450° C. for 24 hours. As a result, the stretched silver chloride particles present in a surface layer of the glass member were reduced to provide stretched silver particles. After that, the glass member was cut into a piece measuring 10 mm square, to thereby provide a polarizing glass sheet.

(88) (d) Evaluation of Characteristics of Polarizing Glass Sheet

(89) The polarizing axis deviation and the extinction ratio in the polarizing glass sheet were measured as described below. FIG. 2 is a schematic plan view of a polarizing glass sheet for illustrating a measurement method for each characteristic. P0 represents the center of the polarizing glass sheet, and P1 and P2 respectively represent positions 4 mm away from P0 to the left and right in a direction perpendicular to the down-drawing direction.

(90) The polarizing glass sheet was placed on a rotary stage, and oscillation light from an near-infrared range wavelength oscillation laser (wavelength: 1,310 nm and 1,550 nm) was converted into linearly polarized light through a Glan-Thompson prism and radiated to P0, P1, and P2. The intensity of the near-infrared light having been transmitted through the polarizing glass sheet was measured through use of an optical power meter while the rotary stage was rotated around each measurement point. Angles at which the measured light intensity became maximum and minimum were read.

(91) The characteristics of the polarizing glass sheet were evaluated under the condition that, at P1 and P2, an angle at which the light intensity became minimum was defined as a polarizing axis angle (angle of stretched silver particles) at each position, and a value obtained by dividing the difference between the polarizing axis angles by 8 mm was defined as the polarizing axis deviation.

(92) At P0, P1, and P2, the ratio between the maximum value and the minimum value of the light intensity (corresponding to the ratio between the maximum value and the minimum value of a transmitted light quantity) was determined, and the extinction ratio was calculated by the expression (2). The in-plane variation of the extinction ratio was determined by the following expression (4).
In-plane variation=±(Maximum value of extinction ratio-minimum value of extinction ratio)/2  (4)

(93) As is apparent from Table 1, in Examples 1 and 2, in the down-drawing step of the glass preform sheet, the length (L.sub.1/W.sub.1) of the softened and deformed portion S.sub.1 of the glass preform sheet with respect to the length equivalent to 0.5 times that of the glass preform sheet was as large as 1.65 or more, and the length (L.sub.2/W.sub.1) of the softened and deformed portion S.sub.2 of the glass preform sheet with respect to the length equivalent to 0.5 times that of the glass preform sheet was as large as 0.56 or more. Therefore, the polarizing axis deviation of the polarizing glass sheet was as small as 0.0060°/mm or less at the width of 8 mm. Further, the in-plane variation of the extinction ratio was as small as within ±5 dB. Meanwhile, in Comparative Examples 1 and 2, L.sub.1/W.sub.1 was as small as 0.97 or less, and L.sub.2/W.sub.1 was as small as 0.48 or less. Therefore, the polarizing axis deviation of the polarizing glass sheet was as large as 0.0125°/mm or more at the width of 8 mm. Further, the in-plane variation of the extinction ratio was as large as ±7 dB or more, and the value of the extinction ratio was less than 40 dB in a part of the surface.

REFERENCE SIGNS LIST

(94) 1 glass preform sheet 2 heat-generating element 3 tension roller 4 metal halide particle 4′ stretched metal halide particle 5 glass member 10 polarizing glass sheet 10a first polarizing glass sheet 10b second polarizing glass sheet 20 Faraday rotator 11, 12, 13, 14, 15, 16, 17 cutout portion 102 glass member after reduction treatment 42 stretched metal particle