GB2433498A

GB2433498A - Glass composition for poling and non-linear optical glass material, and non-linear optical element

Info

Publication number: GB2433498A
Application number: GB0706096A
Authority: GB
Inventors: Koichi Sakaguchi
Original assignee: Nippon Sheet Glass Co Ltd
Current assignee: Nippon Sheet Glass Co Ltd
Priority date: 2004-09-17
Filing date: 2005-07-01
Publication date: 2007-06-27
Anticipated expiration: 2025-07-01
Also published as: GB2433498B; GB0706096D0; WO2006030574A1; JPWO2006030574A1

Abstract

A glass composition for poling, which exhibits a non-linear optical effect by poling, has a glass transition temperature of 570{C or lower, has an OH concentration of not less than 100 and less than 1000 ppm on the mass basis, and has a concentration of a mono-valent cation except a hydrogen ion of 1/10 or less of the above OH concentration on the mass basis in terms of oxide. The above glass composition can exhibit a non-linear optical effect being equivalent to (or higher than) that of a silica glass, and also is excellent in formability.

Description

DESCRIPTION

GLASS COMPOSITION FOR POLING AND

NON-LINEAR OPTICAL GLASS MATERIAL, AND

NON-LINEAR OPTICAL ELEMENT

chnical Field

The present invention relates to a glass composition for poling showing a nonlinear optical effect by poling, to a nonlinear optical glass material obtained by poling the glass composition and to a nonlinear optical element.

Background Art

Although inorganic glass compositions generally do not exhibit a nonlinear optical effect, it is known that the glass compositions are able to develop a nonlinear optical effect by introducing a periodic polarization structure to them. Glass compositions showing a nonlinear optical effect (nonlinear optical glass materials) are expected to find application in the field of optical communication because they show an SHG: Second Harmonic Generation phenomenon, for example. The SHG is generated by a second-order nonlinear optical effect.

As materials showing a nonlinear optical effect (nonlinear optical materials), dielectric crystals are already known. However, nonlinear optical materials made of a dielectric crystal are difficult to manufacture stably, as they require a specific crystal structure, and a precise optical adjustment for phase matching is essential when they are put into practical use. Since nonlinear optical glass materials are easier for manufacturing and for phase matching than these dielectric crystals, much attention has been drawn to them.

As a method of introducing a periodic polarization structure to a glass composition, a method of imparting a semipermanent polarization structure by applying a high voltage (a large electric field) to the composition, i.e. poling, is common. In particular, thermal poling, in which a high voltage is applied while heating a glass composition, provides better durability and stability of the polarization structure.

Considerable research into poling silica glass, for use as an optical fibre for optical communication, has been conducted previously. For example, R. A. Myers, Optics Letters, vol. 16(1991), p. 1732 discloses that poling in silica glass is due to the movement of sodium ions contained in the glass, as an impurity, under an electric field. Nowadays, for silica glass it is generally accepted that a depleted layer, in which the sodium ion o concentration is relatively reduced, is formed as a trace amount of sodium ion moves in the glass composition, and a nonlinear optical effect is generated by an internal electric field developed in the formed, depleted layer.

As an example of specific methods for poling in silica glass, U.S. Pat. No. 5,239,407 discloses a method for obtaining a considerable second-order nonlinear optical effect by thermal poling in silica glass. W097/46906 discloses a method for generating a considerable nonlinear electrooptical effect in an optical fibre of silica glass and highly effective thermal poling at a high temperature of 450 C or higher and at a large electric field at 800 V/pm or greater. JP6(1994)265946A discloses a method for obtaining a material with a large nonlinear optical effect by employing a glass composition with 1000 ppm or higher of OH concentration derived from an OH group and the like, and results on silica glass are shown in the

Examples thereof.

However, since silica glass has a very high viscosity, this gives disadvantages in the capability for moulding into various shapes when using silica glass in various devices as a nonlinear optical element. It is also difficult to process silica glass by machining and laser processing.

It is an object of the present invention to provide a glass composition for poling that is able to develop a nonlinear optical effect by poling equal to, or greater than, that for silica glass and which is excellent in mouldability.

It is another object of the present invention to provide a nonlinear optical glass material formed with the glass composition for poling and a nonlinear optical element formed with the glass composition for poling.

Disclosure of Invention

A glass composition for poling of the present invention shows a nonlinear optical effect by poling, has a glass transition temperature of 570 C or lower, has an OH concentration of 100 ppm or more and less than 1000 ppm as a mass fraction, and has a concentration of monovalent cation other than hydrogen ion of 1/10 or less than the OH concentration, wherein the concentration of monovalent cation is expressed as a mass fraction expressed as its oxide.

This glass composition is able to develop a nonlinear optical effect by poling equal to, or greater than, that for silica glass and is excellent in mouldabiity.

A nonlinear optical glass material of the present invention is a nonlinear optical glass material obtained by poling the glass composition for poling of the present invention.

A nonlinear optical element of the present invention includes the nonlinear optical glass material of the present invention.

Brief Description of Drawings

FIG. 1 is a schematic view illustrating an example of the nonlinear optical element of the present invention.

Best Mode for Carrvin Out the Invention The glass composition of the present invention may be substantially free from Si02.

The glass composition of the present invention may be substantially free from A1203.

The glass composition of the present invention preferably includes the following components, indicated by mol% 20% to 60% of Zn0 20% to 60% of B203; and 0% to 50% of total bismuth oxide (expressed as Bi203).

In the glass composition of the present invention, a content of total bismuth oxide (expressed as Bi203) is preferably 0.1 mol% or higher, and more preferably 0.5 mol% or higher.

In the glass composition of the present invention, a molar ratio represented by ZnO / (ZnO + B203) is preferably in a range from 0.4 to 0.6.

The glass composition of the present invention may substantially consist of the following components, indicated by mol% 20% to 60% of Zn0 20% to 60% of B203 and 0% to 50% of total bismuth oxide (expressed as Bi203).

In the glass composition of the present invention, the content of total bismuth oxide (expressed as Bi203) is preferably 0.1 mol% or higher, and a difference ia between an absorption coefficient a45o to light with a wavelength of 450 nm and an absorption coefficient a700 to light with a wavelength of 700 nm (& = a45o -a700) is preferably less than 0.5 cm'.

Glass Transition Temperature The glass composition for poling of the present invention has a glass transition temperature of 570 C or lower. The glass transition temperature (Tg) is a temperature indexing glass viscosity. When Tg in a glass composition is 570 C or lower, it is possible to apply a regular moulding method by melting to the glass composition, and the glass composition of the present invention is excellent in mouldabiity. It is also possible to lower a melting temperature during manufacture compared to silica glass. Tg of silica glass is at about 1200 C or higher, normally.

Although the lowest limit of Tg in the glass composition of the present invention is not particularly limited, an example of the lowest limit may be at about 300 C because Pg at too low a temperature is prone to cause relaxation in the glass structure and reduction in its poling effect.

Tg may be obtained by, for example, measuring thermal expansion of the glass composition, such as a thermal expansion coefficient measurement by an apparatus for thermomechanical analysis (TM.A), and measuring thermal analysis by a differential scanning calorimeter (DSC).

OH Concentration An OH group derived from moisture and the like exists in glass compositions, and the glass composition of the present invention develops a nonlinear optical effect equal to (or more than) that for silica glass mainly by defining the OH concentration.

Poling the glass composition of the present invention makes hydrogen ions derived from OH groups move in the glass due to the electric field gradient applied by the poling as the drive power. A nonlinear optical effect (mainly a second-order nonlinear optical effect) is considered to be developed by formation of an internal electric field in the glass composition arising from movement of hydrogen ions. The hydrogen ions after movement by poling are resistant to diffusion, as a hydrogen ion has greater bond strength with an oxygen atom compared to other monovalent cations, such as a Na ion. The formed internal electric field is stable following passage of time after poling. That is, the nonlinear optical glass material and the nonlinear optical element obtained by poling the glass composition of the present invention are excellent in stability.

The glass composition for poling of the present invention has an OH concentration of 100 ppm or higher and lower than 1000 ppm expressed as a mass fraction. When the OH concentration is lower than 100 mass ppm, the number of hydrogen ions moved by poling is not enough and the internal electric field for developing a nonlinear optical effect equal to that for silica glass is not formed sufficiently. When the OH concentration is 1000 mass ppm or more, durability of the glass composition of the present invention, in which Tg is 570 C or lower, is deteriorated.

In order to have the OH concentration within the range described above, an adequate type and amount of hydrate may be employed, for example, as at least a part of raw material (starting material) for the glass composition of the present invention.

Concentration of Monovalent Cation In order to develop a nonlinear optical effect equal to (or more than) that for silica glass, it is important to provide the concentration of monovalent cation other than hydrogen ion in addition to providing the OH concentration. In the present specification, monovalent cations other than hydrogen ion, are defined as ions of alkali metal elements, such as Li, Na, K, Rb or Cs, ions of transition metal elements, such as Cu, Ag, Au or Ti, or the like.

Since these monovalent cations can move in the glass composition more easily than hydrogen ions under the electric field gradient by poling (these ions having weaker bond strength with oxygen atoms than hydrogen ions, as described above), they are considered to inhibit the movement of hydrogen ions during poling. These monovalent cations are also considered to reduce the development of a nonlinear optical effect, as they function to counteract the internal electric field formed after poling.

The glass composition for poling of the present invention has the concentration of monovalent cations, other than hydrogen ion, of 1/10 or less than the OH concentration at a mass fraction where the cation is expressed as its oxide (at a mass fraction expressed as its oxide). When the fraction value exceeds 1/10 of the OH concentration, formation of the internal electric field by hydrogen ions is inhibited and the nonlinear optical effect developed by poling is reduced.

The lowest limit of the fraction value is not particularly limited, and an example may be about 1/1000 of the OH concentration.

Preferable Composition The composition of the glass composition for poling of the present invention is not particularly limited as long as the Tg, the OH concentration and the concentration of monovalent cation except hydrogen ion satis1y the relationship described above, and an example of preferable range of composition may be as follows. Si02

The glass composition of the present invention may be substantially free from S102. In the present specification, to "be substantially free" is defined as having the content of the substance at 0.1 mol% or lower. A1203

Preferable Range of Composition The glass composition of the present invention preferably includes the following components, indicated by mol% ZnO 20% to 60%; B203 20% to 60%; and total bismuth oxide (expressed as B1203) 0% to 50%.

The reasons for limiting the composition in the preferable range of composition are described below.

Bismuth Oxide The glass composition of the present invention includes a bismuth oxide, and the range of composition of bismuth oxide is provided by total bismuth oxide expressed as Bi203 regardless of the oxidation status of bismuth in the present invention. In the following description, content of bismuth oxide" is defined as "content of total bismuth oxide expressed as B." 12 3 Bismuth oxide is an optional component having the actions of lowering the viscosity of the glass composition, i.e. decreasing Tg, and stabilizing the glass composition.

The content of bismuth oxide is preferably 0.1 mol% or higher (i.e. the bismuth oxide is preferred to be substantially included), more preferably 0.5 znol% or higher and even more preferably 5 mol% or higher. When the content is 0.5 mol% or higher, the action of lowering the viscosity of the glass composition is enhanced. When it exceeds 50 mol%, the glass composition becomes coloured and inadequate for optical use. The content of bismuth oxide is more preferably 25 mol% or lower and is even more preferably 20 mol% or lower.

Bismuth oxide has the action of increasing the refractive index of the glass composition. The more the refractive index of the glass composition is increased, the more a nonlinear optical effect is obtained. As the refractive index increases, the obtainable thirdorder nonlinear optical effect is increased. The magnitude of a second-order nonlinear optical effect and that of a third-order nonlinear optical effect are positively correlated, and thus the obtainable second-order nonlinear optical effect is considered to be increased.

When the content of bismuth oxide is 0.1 mol% or more, it is preferable that a difference Au between an absorption coefficient ao to light with a wavelength of 450 nm and an absorption coefficient a7oo to light with a wavelength of 700 nm (Au = a450 -a700) is less than 0.5 cur'. Poling enables the glass composition to develop a greater nonlinear optical effect.

There is a possibility that the electronic state of Bi in the bismuth oxide is concerned with the increase in the nonlinear optical effect developed by poling when Au is within the range described above.

In some cases, a glass composition including a bismuth oxide exhibits electronic conduction. It is considered that this phenomenon is developed by having electrons hopping among Bi having different electronic states, where the Bi also may be described as Bi ions having different oxidation status. When the glass composition exhibits electronic conduction, a current flowing during poling increases and the formation of the internal electric field by movement of the hydrogen ions is inhibited.

Thus, it is preferable to have low electronic conduction in the glass composition of the present invention.

The inventor of the present invention found that the electronic conduction and the coloured state have a correlation (i.e. the electronic conduction of the glass composition is related to the coloured state) in a glass composition including a bismuth oxide. This is considered because the electronic state of bismuth contributes to the coloured state of the glass as well as the electronic conduction. Thus, a nonlinear optical effect developed by poling can be increased by colouring the glass composition including a bismuth oxide within a predetermined range.

A glass composition including a bismuth oxide often has a broad absorption spectrum over the ranges from ultraviolet, visible towards infrared. When the glass composition mentioned above is coloured, absorption in the ultraviolet range and the visible range near the ultraviolet spectral region tends to be increased compared to absorption in the infrared range and the visible range near the infrared spectral region. Accordingly, the inventor defined Au mentioned above and found that Au was preferably in the range of less than 0.5 cm'. In a case of &i within this range, it is considered to be preferable for use as an optical material also because of the weak colouring of the glass composition.

A glass composition including a bismuth oxide and having 1a within the range above may be obtained, for example, by controlling the melting temperature while manufacturing the composition. Under the same condition of composition, as the melting temperature is higher, the obtained glass composition tends to have stronger colouring and greater electronic conduction. Thus, when manufacturing the glass composition including a bismuth oxide, it is preferable to have a melting temperature (Ti) of 1000 C or lower, more preferably of 950 C or lower and even more preferably of 800 C or lower, for example. It should be noted, though, that the specific temperatures for preferred Ti are dependent on the composition to be manufactured. The lowest limit of Ti i8 not particularly limited as long as it is a temperature at a melting point (Tm) of the glass composition to be obtained or higher, and an example of such Tm is 700 C.

At such melting temperature, however, homogenization of the composition may become insufficient or bubbles generated during melt may remain due to high viscosity of the melting glass, depending on the composition of the glass composition. In such a case, it is acceptable to melt glass materials first at a temperature T2 that is higher than Ti and then to lower the temperature to Ti. Homogenization and/or degassing of the composition can be promoted by tempering at T2 first, and it is possible to restrict colouring of the glass composition and to restrict developing electronic conduction by lowering to Ti later. It is preferred that T2 is at 1200 C or lower.

For example, when the composition of the glass composition is composed of total bismuth oxide (expressed as Bi203) at 12.5%, ZnO at 43.75% and B203 at 43.75%, indicated by mol%, Ti may be at about 800 C and T2 may be at about 1000 C. This composition corresponds to Samples 1 and 5 through 8 in the Examples. In the Examples, other examples of Ti and T2 for the glass composition composed as above are shown. ZnO

ZnO is an essential component having the actions of lowering the viscosity of the glass composition and stabilizing the glass composition.

These actions become insufficient when the content is less than 20 mol%, and stability of the glass composition is deteriorated when it exceeds 60 mol%.

The content of ZnO is more preferable to be in the range from 40 mol% to 60 mol%. 2Q3

B203 is an essential component forming a glass network structure.

When the content is less than 20 mol%, it becomes difficult to obtain a homogeneous glass composition, which does not devitrify during manufacture, and to form the glass itselt On the other hand, when the content exceeds 60 mol%, viscosity of the glass composition becomes higher and its mouldability is reduced. It is also prone to deteriorate the durability.

The content of B203 is more preferably in the range from 40 mol% to mol%.

B2O. ZnO In the glass composition of the present invention, the molar ratio represented by ZnO / (ZnO + B203) i8 more preferably in the range from 0.4 to 0.6, and thus it becomes possible to obtain a more stable glass composition. The glass composition is easily separated into phases when it is less than 0.4, and the glass composition is easily crystallised when the ratio exceeds 0.6.

Other Comvonents In the glass composition of the present invention, components other than above may be included for the purposes of controlling the refractive index, controlling viscosity, improving devitrification resistance during manufacture and the like.

Specifically, a divalent oxide of at least one selected from MgO, CaO, SrO and BaO may be included at a level of 10 mol% or less in total, preferably less than 3 niol%, and an oxide of Y203, La203, PlO2, Ta205, Nb205, SiOz, Ge02, Ga205, 1fl205 and the like may be included at a level of 5 mol% or less in total, preferably less than 3 mol%.

In addition, Ce02 may be included at a level of 5 mol% or less, preferably less than 3 mol% for clarification (degassing) during melt and for restraint on reduction of the bismuth oxide.

Components derived from the clarifier employed in the clarification as (degassing) step, such as As205, Sb205, 803, Sn02 and Fe203, further may be included at a level of 1 mol% or less in total.

Impurities derived from raw materials for industrial glass may further be included at a level of less than 0.1 mol% each. When they are included at a level of less than 0.1 mol%, their effect on physical properties of the glass composition is small and they make substantially no problems.

That means, these impurities also may be described as substantially not included.

The nonlinear optical glass material and the nonlinear optical element of the present invention are described below.

The nonlinear optical glass material of the present invention is a glass material obtained by poling the glass composition of the present invention described above.

Poling may be performed in a common method and under common conditions, and an example of such poling may be performed by applying an electric field of the magnitude approximately in a range from 1 x 106 V/rn to 4 x 10 V/rn to the objective glass composition at a temperature approximately in the range from 200 C to 500 C. It should be noted, however, that the temperature has to be at Tg or lower for the glass composition. A glass composition (a glass mould) processed in a form to be used as a nonlinear optical element may be poled, and this facilitates manufacturing a nonlinear optical element including the nonlinear optical glass material of the present invention.

The nonlinear optical element of the present invention includes the nonlinear optical glass material of the present invention. An example of such nonlinear optical element is a wavelength converter, which is provided with nonlinear optical glass materials in the forms of optical fibre and optical waveguide.

Fig. 1 shows an example of the wavelength converter. A wavelength converter 1 shown in Fig. 1 is provided with an optical wave guide 3 made of a nonlinear optical glass material of the present invention on a substrate 2. A signal light 4 incident from an end face 3a through the optical waveguide 3 has its wavelength converted by a nonlinear optical effect during transmission through the optical waveguide 3, and then exits from the other end face 3b as wavelength converted light 5.

EXAMPLES

Hereinafter, the present invention is described further in detail using the Examples. The present invention, however, is not limited to the

following Examples.

ExamDle 1 In Example 1, glass compositions having the compositions indicated in Table 1 below (Samples 1 to 4 for Examples according to the invention and Samples A and B as Comparative Examples) were fabricated by melting, and the physical properties of each glass composition (OH concentration, concentration of monovalent cation other than hydrogen ion (expressed as oxide) and Tg) were measured. The fabricated glass compositions were poled after the measurement, and the magnitude of each nonlinear optical effect developed by the poling (an SHG intensity reflecting each second-order nonlinear optical effect) was evaluated.

Method of Fabricating Samoles The specific method of fabricating each glass composition sample is as follows.

Samples I to 4 (Exam1es according to the invention) Each reagent of bismuth oxide (Bi203), zinc oxide (ZnO) and boric acid (trihydrate of H3B03) was employed for making the glass material, and the mass on melting was 400 g in total. The reason for employing trihydrate of boric acid as raw material for the B203 component was to have the OH concentration in the fabricated samples at 100 mass ppm or higher.

Next, after sufficiently blending each reagent until homogenised, the formed mixture was put into a platinum crucible and heated at 1000 C in an electric furnace. It was thus melted and left for 1.5 hours. Then, after rapidly cooling (casting) the formed molten glass to room temperature by pouring onto a stainless steel plate, glass compositions were obtained by gradually cooling the cast glass in the electric furnace (by being kept at Tg of each glass composition for 30 minutes and then cooled down to room temperature at a temperature lowering rate of 100 C per hour or lower).

The obtained glass compositions were cut and polished into a rectangular parallelepiped shape (20 mm X 30 mm x 1 mm) to make Samples 1 to 4.

-Sample A (Comparative Example)-First, a glass composition was obtained using a similar method as for Samples 1 to 4, except for employing boron oxide (B203) instead of trihydrate of H3B03 for glass material and then omitting the gradual cooling.

Next, an operation of remelting the obtained glass composition at 1000 C and then rapidly cooling was repeated twice. After the second rapid cooling, it was gradually cooled, cut and polished in the same method as Samples 1 to 4 to obtain Sample A in a rectangular parallelepiped shape. The size of Sample A was identical to those of Samples 1 to 4. The reason for employing B203 as raw material for the B203 component and for repeating a plurality of times of melting and rapid cooling was to even ensure that the OH concentration in the fabricating sample was less than 100 mass ppm.

Sample B (Comparative Example) First, Sample B in a rectangular parailelepiped shape was obtained using a similar method as Samples 1 to 4; except for further adding sodium carbonate (NazCOa) as raw material to make the content of Na20 in the glass composition to be obtained 6000 mass ppm. The size of Sample B was identical to those of Samples 1 to 4. Here, the content of Na20 is not included in the 100 mol% sum of the content of total bismuth oxide, ZnO and B203.

Physical Properties Measurement The OH concentration, the concentration of monovalent cation other than hydrogen ion (expressed as oxide) and Tg of each glass composition sample fabricated in the method above were measured.

The OH concentration was obtained by an absorption peak intensity of a wavelength near 2900 nm from infrared absorption spectrum of each glass composition obtained by JR measurement using a spectrophotometer.

The absorption peak within the range of the wave number corresponds to OH existing in each glass composition.

The concentration of monovalent cation other than hydrogen ion (expressed as oxide) was obtained by a method of chemical analysis (inductively coupled plasma spectroscopy: JCP).

Tg was obtained by thermal expansion curve of each glass composition calculated by TMA.

The results of measuring each physical property are shown in Table 1.

Poling Separated from the measurement of physical properties, each glass composition sample fabricated in the method above was poled.

First, Al (aluminium) electrodes were formed by vacuum evaporation on both 20 mm X 30 mm surfaces in each sample. Next, each sample was put into an electric furnace to be heated up to each poling temperature indicated in Table 1 after electrically connecting the formed pair of electrodes to a high voltage application circuit. While maintaining the poling temperature, a voltage of 4 kV was applied to each pair of Al electrodes. Five minutes after starting the voltage application, lowering of thetemperature was commenced and the voltage application was stopped when the temperature of each sample reached 50 C. After that, they were allowed to cool down to room temperature by leaving. Each time length of voltage application (poling time) is shown in Table 1.

SHG Intensity Measurement The SHG intensity of each sample after poling was measured using the method described below.

First, Al electrodes in each sample after poling were dissolved chemically and removed. Next, a light intensity (SGH intensity) of the wavelength of 532 nm, which is a second harmonic, was measured using an excitation light of 1064 nm in wavelength by the Maker fringe technique.

A relative value in comparison to a reference value obtained in the following manner was used for evaluating the SHG intensity of each sample. The reference value was the SGH intensity obtained by measuring, using the same method as above after poling (conditions: at a poling temperature of 280 C, with 4 kV of applied voltage, for 30 minutes to apply the voltage) a prepared silica glass (Herasil 1, manufactured by Heraeus) having the identical shape as each sample. The results of evaluation are shown in Table 1. The poling conditions mentioned above were common for silica glass poling.

TABLE 1 ____ ____ ____ ____ SampleNo. 1 2 3 4 A B ibtal Bismuth Oxide 12.5 6.3 25.0 0.0 25.0 6.3 Composition ( 1%) (expressed mo as B1203) ____ ____ ____ ____ ______ ______ ZnO 43.75 46.85 37.5 55.0 37.5 46.85 _________ B203 43.75 46.85 37.5 45.0 37.5 46.85 Concentration of OH group 530 810 460 620 90 760 (mass ppm) _____ _____ _____ _____ _______ _______ Concentration Physical of Properties Monovalent 50 50 30 50 30 6000 Cation (mass ppm) _____ _____ _____ _____ _______ _______ 470 495 419 550 419 500 Poling Thmperatuze 400 420 280 500 280 350 ( C) ____ ____ ____ ____ ______ ______ Poling Applied * . Voltage 4 4 4 4 4 4 Conditions Poling Time Length 5 30 5 15 5 5 ___________ (minutes) ______ ______ ______ ______ ________ ________ Under Under SHG Intensity 1.54 1.22 0.46 0.81 Detection Detection _________________ ____ ____ ____ ____ Limit Limit * SHG Intensity has a reference value of 1 given by silica glass (Herasil 1, manufactured by Heraeus).

As shown in Table 1, each Tg for Samples 1 to 4 was 570 C or lower and it was found that each had an excellent moulding property.

Although Samples A and B, which were Comparative Examples, had Tg at 570 C or lower, the obtained SHG intensity was so low as to be under the detection limit. It is considered that the reason for these results is that Sample A had less than 100 mass ppm of the OH concentration and Sample B had a relatively high concentration of monovalent cation other than hydrogen ion with respect to the OH concentration.

On the other hand, as high as a 1.54 fold SHG intensity was obtained at the maximum with Samples 1 to 4 compared to the SHG intensity of the silica glass, and nonlinear optical effects equal to, or more than, that for the silica glass were obtained by poling. The reason for the relatively low SHG intensity of Sample 3 compared to the rest of the Samples according to the invention is considered to be because the poling temperature was lower and the time length for poling was shorter than for the other samples.

Examvle 2 In Example 2, samples having the identical composition as Sample 1 of Example 1 but melted under different conditions during fabrication (Samples 5 to 8) were fabricated for measuring the physical properties and evaluating the SHG intensity after poling. An absorption coefficient a4o to light with a wavelength of 450 nm and an absorption coefficient a700 to light with a wavelength of 700 nm were measured to obtain &t (Aa = -(1700).

Samples 5 to 8 were fabricated in a similar method as Sample 1 of Example 1. Each melting temperature is indicated in Table 2 below. As indicated in Table 2, Sample 5 was melted at 950 C for 1.5 hours. Sample 6 was melted at 1000 C for 1.5 hours and then at 800 C for 4 hours, Sample 7 was melted at 1000 C for 1.5 hours and then at 700 C for 4 hours and Sample 8 was melted at 1100 C for 1.5 hours.

For each fabricated sample, the physical properties were measured, poling was carried out and the SHG intensity was measured after poling.

&z of each sample was calculated by an absorption coefficient obtained by Lambert-Beers law after measuring transmittance by varying the thickness of each sample. The results of measurement are shown in Table 2. The results with Sample 1 are provided again in Table 2 for convenience.

TABLE 2 ______

SamieNo. 1 5 6 7 8 Total Bismuth Oxide 12.5 12.5 12.5 12.5 12.5 Composition ( 1%) expresse as Bi203) ______ _____ ______ ______ ______ ZnO 43.75 43.75 43.75 43.75 43.75 _________ B203 43.75 43.75 43.75 43.75 43. 75 TlPFiine 1000/1.5 950/1.5 800/4 700/6 1100/1.5 Melting (C/hour) _______ _______ _______ _______ _______ Conditions T2PI'iine -1000/1.5 1000/1.5 ________ (C/hour) _______ ______ _______ _______ _______ oncentration of OH group 530 630 510 520 510 (mass ppm) ______ ______ _______ ______ _______ Concentration of Physical Monovalent 50 50 50 50 50 Properties Cation (mass ppm) _______ ______ _______ _______ _______ (os) 470 470 470 470 470 Au 0.3 0.29 0.08 0.05 2.8 ______ (cm') _____ ____ _____ _____ _____ SHG Intensity 1.54 1.73 1.85 1.54 0.72 * SHG Intensity has a reference value of 1 given by silica glass (Herasil 1, manufactured by Heraeus).

As shown in Table 2, it was confirmed that a nonlinear optical effect equal to, or more than, that for silica glass was developed with each sample by poling. In particular, as high as a 1.85 fold SHG intensity was obtained at the maximum with Samples 5 to 7, in which each Au was 0.5 cm' or less, compared to the SHG intensity of the silica glass.

The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this application are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Industrial Aimlicability According to the present invention, a glass composition for poling that is able to develop a nonlinear optical effect equal to (or more than) that for silica glass and that is excellent in its mouldabiity can be provided by poling.

Claims

CLAIMS

1. A glass composition for poling, showing a nonlinear optical effect by poling, having a glass transition temperature of 570 C or lower, having an OH concentration of 100 ppm or higher and lower than 1000 ppm expressed as a mass fraction, and having a concentration of monovalent cation other than hydrogen ion of 1/10 or less than the OH concentration, wherein the concentration of monovalent cation is expressed as a mass fraction expressed as its oxide.

2. The glass composition for poling according to claim 1, wherein the glass composition is substantially free from 5102.

3. The glass composition for poling according to claim 1, wherein the glass composition is substantially free from A1203.

4. The glass composition for poling according to claim 1, comprising the following components, indicated by mol%: ZnO 20% to 60%; B203 20% to 60%; and total bismuth oxide (expressed as Bi203) 0% to 50%.

5. The glass composition for poling according to claim 4, wherein a content of total bismuth oxide (expressed as Bi2O,) is 0.1 mol% or higher.

6. The glass composition for poling according to claim 4, wherein a content of total bismuth oxide (expressed as Bi203) is 0.5 mol% or higher.

7. The glass composition for poling according to claim 4, wherein a molar ratio represented by ZnO / (ZnO + B203) is in a range from 0.4 to 0.6.

8. The glass composition for poling according to claim 1, substantially consisting of the following components, indicated by mol%: ZnO 20% to 60%; B203 20% to 60%; and total bismuth oxide (expressed as Bi203) 0% to 50%.

9. The glass composition for poling according to claim 5, wherein a difference a between an absorption coefficient ao to light with a wavelength of 450 nm and an absorption coefficient (1700 to light with a wavelength of 700 nm (Au = a45o -u700) is less than 0.5 cm1.

10. A nonlinear optical glass material obtained by poling the glass composition for poling according to claim 1.

11. A nonlinear optical element comprising the nonlinear optical glass material according to claim 10.