Optical isolator and method for fabricating the same

Abstract: A polarizer (7), a Faraday rotator (9) and an analyzer (10) are fixed by fused glass applied on an outer periphery of the polarizer to be contacted with the Faraday rotator and on an outer periphery of the analyzer to be contacted with the Faraday rotator. No adhesive exists between contact planes of the polarizer and the Faraday rotator and those of the Faraday rotator and the analyzer.

Description: This invention relates to an optical isolator to be used for an optical communication system, an optical information processing system, etc. in accordance with magneto-optic effect, and a method for fabricating the same. One type of a conventional optical isolator comprises a polarizer, a 45 degree - Faraday rotator, and an analyzer which are fixed by applying optic adhesive between the polarizer and the Faraday rotator and between the Faraday rotator and the analyzer, and a magnet which is fixed to an outer periphery of the Faraday rotator by use of adhesive. In operation, incident light is supplied to the polarizer of the optical isolator, so that a polarization component of the incident light coinciding with a polarization plane of the polarizer is passed through the polarizer to be supplied to the Faraday rotator, from which a light having a polarization rotated by 45 degrees is supplied. The light having the polarization rotated by 45 degrees is passed through the analyzer, so that output light is supplied from the analyzer of the optical isolator. On the contrary, return light propagating in the direction opposite to that of the incident light is stopped by the polarizer, after it is passed through the analyzer and the Faraday rotator. The detailed structure and operation will be explained later. According to the conventional optical isolator, however, there is a disadvantage in that anti-humidity property is not so high, because the polarizer, the Faraday rotator and the analyzer are fixed on optical planes by the optic adhesive, so that it is not preferable to be used under high temperature and high humidity. Japanese Patent Publication No. 1-200 223 discloses a conventional optical isolator of the type defined in the first part of claim 1. In the case of this prior art device, Faraday rotator, analyzer and polarizer are fixed together by soldering. Accordingly, it is an object of the invention to provide an optical isolator having improved anti-humidity property, and a method for fabricating the same. It is a further object of the invention to provide an optical isolator which can be used under high temperature and high humidity, and a method for fabricating the same. It is a still further object of the invention to provide an optical isolator which operates with stabilized characteristics even under high temperature and high humidity, and a method for fabricating the same. According to a first feature of the invention, an optical isolator, comprises: a polarizer for propagating light having a predetermined polarization; a Faraday rotator for rotating the predetermined polarization of the light by a predetermined angle; an analyzer for propagating light having a polarization rotated relative to a polarization plane of the polarizer by the predetermined angle; and a magnet provided on an outer periphery of the Faraday rotator to apply a magnetic field to the Faraday rotator; wherein the polarizer and the Faraday rotator are fixed together by a first glass applied on at least one of outer peripheries of the polarizer and the Faraday rotator; and the Faraday rotator and the analyzer are fixed together by a second glass applied on at least one of outer peripheries of the Faraday rotator and the analyzer. According to a second feature of the invention, a method of fabricating an optical isolator is as claimed in claim 4 or 5. BRIEF DESCRIPTION OF THE DRAWINGS This invention will be explained in more detail in conjunction with appended drawings, wherein; Figs. 1A and 1B are a front view and a cross sectional view, respectively, showing a conventional optical isolator; Figs. 2A and 2B are a front view and a cross sectional view, respectively, showing an optical isolator of a preferred embodiment according to the invention; and Figs. 3A to 3D are perspective views showing a method for fabricating an optical isolator of a preferred embodiment according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before explaining an optical isolator of a preferred embodiment according to the invention, the aforementioned conventional optical isolator will be explained in Figs. 1A and 1B. The conventional optical isolator comprises a polarizer 13, a 45 degree - Faraday rotator 14, an analyzer 15, and a magnet 16, wherein the polarizer 13, the 45 degree - Faraday rotator 14, and the analyzer 15 are fixed on their facing optical planes by use of optic adhesive 17, and the magnet 16 is fixed on an outer periphery of the 45 degree - Faraday rotator 14 by use of adhesive 18. In this conventional optical isolator, the 45 degree - Faraday rotator 14 has a length along light axis, so that light to be propagated therethrough is rotated in its polarization plane in accordance with Faraday effect by 45 degrees, and the rotation of polarization plane is dependent on non-reciprocity property of Faraday effect, in which the rotational direction of incident light polarization is determined by an inherent property of the Faraday rotator 14, and a state of parallel or non-parallel between light propagation direction and a magnetic field. This Faraday rotator 14 is often made in an optical communication system of YIG (yttrium iron garnet) or (RBi) IG (Bi-substituted rare earth iron garnet). In operation, input light (solid line arrow) is supplied to the polarizer 13, so that light having a polarization component in coincidence with a polarization plane of the polarizer 13 is selected to be supplied to the 45 degree - Faraday rotator 14. Thus, light which is propagated through the 45 degree - Faraday rotator 14 has a polarization rotated relative to the input light by 45 degrees. Here, it is assumed that a polarization plane of the analyzer 15 is set to be inclined relative to the polarization plane of the polarizer 13 by 45 degrees. Under this assumption, light supplied from the Faraday rotator 14 is propagated through the analyzer 15 without any loss. Thus, output light (solid line arrow) is supplied from the analyzer 15. On the other hand, when return light (dot line arrow) is supplied to the analyzer 15, light having a polarization which is rotated relative to the polarization plane of the polarizer 13 by 45 degrees is selected to be supplied to the 45 degree - Faraday rotator 14, the light having the polarization rotated relative to the polarization plane of the polarizer 13 by 45 degrees is propagated through the Faraday rotator 14 to be further rotated in accordance with non-reciprocity property thereof by 45 degrees. Thus, only light having a polarization rotated by 90 degrees is supplied to the polarizer 13, so that no return light is propagated through the polarizer 13. Consequently, all the return light is interrupted by the optical isolator. However, the conventional optical isolator has the disadvantage as explained before. Next, an optical isolator of a preferred embodiment according to the invention will be explained in Figs. 2A and 2B. The optical isolator comprises a polarizer 7, a 45 degree - Faraday rotator 9, an analyzer 10, and a magnet 12, wherein the polarizer 7 and the Faraday rotator 9 are fixed on their optic planes by use of a low melting point glass 8 surrounding an end portion 7a of the polarizer 7, and the Faraday rotator 9 and the analyzer 10 are also fixed on their optic planes by use of a low melting point glass 8 surrounding an end portion 10a of the analyzer 10, while the magnet 12 is fixed on an outer periphery of the polarizer 7, the Faraday rotator 9 and the analyzer 10 thus fixed by use of a low melting point glass 6. In this optical isolator, a polarization plane of the analyzer 10 is arranged to be rotated relative to that of the polarizer 7, and the 45 degree - Faraday rotator 9 is made of a thick film of (GdBi)₃ Fe₅O₁₂ (Bi-substituted gadolinium iron garnet), a thick film of (GdBi)₃ (FeAlGa)₅O₁₂ (Bi-substituted gadolinium iron aluminium gallium garnet), etc. obtained, for instance, by liquid phase epitaxy. In general, (RBi) IG (Bi-substituted rare earth iron garnet) is large in Faraday rotation coefficient and small in saturation magnetization, so that a small sized optical isolator can be realized by use of it. In this preferred embodiment, an optical isolator having a diameter of 4mm and a length of 3mm is realized. In this preferred embodiment, the low melting point glass 6 has a melting point lower than that of the low melting point glass 8. The assembly of the polarizer 7, the 45 degree - Faraday rotator 9 and the analyzer 10 fixed beforehand and the magnet 12 is carried out at a temperature lower than the melting point of the low melting point glass 8 and higher than that of the low melting point glass 6. Under this process condition, the disassembling of the polarizer 7, the Faraday rotator 9 and the analyzer 10 is avoided. In this preferred embodiment, anti-reflection films for air are provided on light input and output planes of the polarizer 7, the 45 degree - Faraday rotator 9, and the analyzer 10. In operation, input light (solid line arrow) is supplied to the polarizer 7 of the optical isolator, so that output light (solid line arrow) having a polarization rotated relative to a polarization plane of the polarizer 7 by 45 degrees is supplied from the analyzer 10 of the optical isolator, in the same principle as explained in the conventional optical isolator. On the other hand, return light (dot line arrow) is interrupted not to be propagated through the optical isolator. Then, a method for fabricating an optical isolator of a preferred embodiment according to the invention will be explained in Figs. 3A to 3D. In Fig. 3A, a polarizer substrate having a plurality of polarizers 7 each defined by cutting lines 11 and having an end portion 7a, a 45 degree - Faraday rotator substrate having the plurality of 45 degree - Faraday rotators 9 each defined by cutting lines 11, and an analyzer substrate having the plurality of analyzers 10 each defined by cutting lines and having an end portion 10a are assembled by using first and second half-sintered grid shaped low melting point glass tablets 8A which are respectively interposed between the polarizer and Faraday rotator substrates and between the Faraday rotator and analyzer substrates, such that the end portions 7a and 10a of the polarizer and analyzer substrates are accommodated in openings of the grid shaped low melting point glass tablets 8A. Thus, an assembled structure is obtained, as shown in Fig. 3B, in accordance with the fixing of the polarizer, 45 degree - Faraday rotator and analyzer substrates by thermal treatment applied to the glass tablets 8A at a temperature of 450° C. In Fig. 3C, the assembled structure is cut to be separated by the cutting lines 11. In this separated structure, the analyzer 10 is fabricated in advance to have a polarization plane rotated relative to a polarization plane of the polarizer 7 by 45 degrees. In addition, the polarizer 7, the 45 degree - Faraday rotator 9 and the analyzer 10 are stacked, such that the analyzer 10 coincides in rotational direction of polarization plane with the 45 degree - Faraday rotator 9, and anti-reflection films for air are provided on facing optical planes of the polarizer 7, the 45 degree - Faraday rotator 9 and the analyzer 10. As shown in Fig. 3A, the polarizer substrate and the analyzer substrate have the end portions 7a and 10a which are defined by grid shaped grooves. The grooves are provided to correspond to the cutting lines 11 in accordance with the shaving by use of a precise blade saw having a thickness of less than 0.2mm. A depth of the grooves is smaller than a thickness of the grid shaped low melting point glass tablets 8A, so that the grid shaped low melting point glass tablets 8A are well contacted with the contacting portions of the polarizer, 45 degree - Faraday rotator and analyzer substrates. In this preferred embodiment, the depth of the grooves for defining the end portions 7a and 10a is 0.3mm, and the thickness of the glass tablets 8A is 0.35mm. The dimensions described here and other dimensions as shown in Fig. 3A are as follows. W₁ = 1.9mm W₂ = 0.2mm W₃ = 0.6mm t₁ = 1.3mm t₂ = 0.35mm t₃ = 0.35mm h = 0.30mm A width of the grooves for defining the end portions 7a and 10a is determined dependent on the width W₃ of frames of the glass tablets 8A. In assembling each member as shown in Fig. 3A, the glass tablets 8A are required to be handled in such a manner that the glass tablets 8A are not broken, when they are mounted on the analyzer substrate, and subsequently on the 45 degree - Faraday rotator substrate by means of an automatical assembling apparatus. In Fig. 3C, the aforementioned separated structure is obtained in accordance with the separation of the assembled structure along the cutting lines 11 positioned over the central lines of the frames of the glass tablets 8A by use of a blade having a thin width. Thus, optical isolator elements each have the polarizer 7, the 45 degree - Faraday rotator 9 and the analyzer 10 which are assembled by the low melting point glasses 8 fixed on the outer peripheries of the end portions 7a and 10a thereof between the polarizer and Faraday rotator substrates and between the Faraday rotator and analyzer substrates. A size of this optical isolator element is 1.7mm in side and 3mm in length. In Fig. 3D, this optical isolator element is fixed on the central axis of the magnet 12 to the magnet 12 by use of a low melting point glass 6 (see Figs. 2A and 2B) applied between an outer periphery of the optical isolator element and an inner periphery of the magnet 12. The low melting point glass 6 has a melting point lower than that of the low melting point glass 8, as explained before. Thus, the optical isolator is fabricated, as shown in Figs. 2A and 2B. In this optical isolator, there are advantages in that anti-humidity property is improved, and stabilized characteristics of making it possible to be used under high temperature and high humidity are obtained, because the low melting point glasses 8 are melted on the outer peripheries of the end portions of the polarizer 7 and the analyzer 10 to fix the polarizer 7, the 45 degree - Faraday rotator 9 and the analyzer 10. The 45 degree - Faraday rotator 9 may be provided with the end portions having concave peripheries for adhesive on the both sides thereof into which the low melting point glasses 8 are applied in place of the polarizer 7 and the analyzer 10. Further, the dimensions as described before may be changed dependent on the design of an optical isolator. In this preferred embodiment, although the low melting point glass tablet 8A is used, it may be replaced by low melting point glass filled into grooves for defining the end portions of the polarizer 7 and the analyzer 10. Although the invention has been described with respect to specific embodiment for complete and clear disclosure, it is understood that modifications and alternative constructions may occur to one skilled in the art without departing from the scope of the claims appended hereto.

Claims: An optical isolator, comprising: a polarizer (7) for propagating light having a predetermined polarization; a Faraday rotator (9) for rotating said predetermined polarization of said light by a predetermined angle; an analyzer (10) for propagating light having a polarization rotated relative to a polarization plane of said polarizer by said predetermined angle; and a magnet (12) provided on an outer periphery of said Faraday rotator (9) to apply a magnetic field to said Faraday rotator; characterized in that said polarizer (7) and said Faraday rotator (9) are fixed together by a first glass (8) applied on at least one of outer peripheries of said polarizer (7) and said Faraday rotator (9); and said Faraday rotator (9) and said analyzer (10) are fixed together by a second glass (8) applied on at least one of outer peripheries of said Faraday rotator (9) and said analyzer (10). An optical isolator, according to claim 1, wherein: said magnet is fixed on an outer periphery of said Faraday rotator thereto by a third glass (6). An optical isolator, according to claim 2, wherein: said first to third glasses are low melting point glasses, the melting point of said first and second glasses is higher than that of said third glass. A method of fabricating an optical isolator, comprising the steps of: providing a polarizer substrate, a Faraday rotator substrate, and an analyzer substrate; providing first and second grid shaped glass tablets which are half sintered; forming grooves on a surface of said polarizer and a surface of said analyzer, said grooves coinciding with a pattern of said grid shaped glass tablets, and said surfaces of said polarizer and said analyzer being for contact with said Faraday rotator; interposing said first grid shaped glass tablet between said polarizer and said Faraday rotator, and said second grid shaped glass tablet between said Faraday rotator and said analyzer, said first and second grid shaped glass tablets being positioned in said grooves on said surfaces of said polarizer and said analyzer, thereby fixing said polarizer, said Faraday rotator, and said analyzer to provide an assembled structure by melting said first and second grid shaped glass tablets; cutting said assembled structure along said grooves by a cutting margin narrower than a width of said grooves to provide a plurality of optical isolator elements, each of said optical isolator elements being composed of a polarizer, a Faraday rotator and an analyzer, said polarizer and said Faraday rotator being fixed on at least one of outer peripheries thereof by a glass cut from said first grid shaped glass tablet, and said Faraday rotator and said analyzer being fixed on at least one of outer peripheries thereof by a glass cut from said second grid shaped glass tablet; and providing a magnet on an outer periphery of said Faraday rotator of said optical isolator element. A method of fabricating an optical isolator, comprising the steps of: providing a polarizer substrate, a Faraday rotator substrate, and an analyzer substrate; forming grooves on a surface of said polarizer and a surface of said analyzer, said grooves being of a grid shaped pattern, and said surfaces of said polarizer and said analyzer being for contact with said Faraday rotator; filling glass into said grooves of said polarizer and said analyzer, thereby fixing said polarizer, said Faraday rotator, and said analyzer to provide an assembled structure by melting said glass; cutting said assembled structure along said grooves by a cutting margin narrower than a width of said grooves to provide a plurality of optical isolator elements, each of said optical isolator elements being composed of a polarizer, a Faraday rotator and an analyzer, said polarizer and said Faraday rotator being fixed on at least one of outer peripheries thereof by said glass filled into said grooves of said analyzer, and said Faraday rotator and said analyzer being fixed on at least one of outer peripheries thereof by said glass filled into said grooves of said analyzer; and providing a magnet on an outer periphery of said Faraday rotator of said optical isolator element.

Inventors: OHKAWARA KAZUHIDE, OHKAWARA, KAZUHIDE
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: G02F1
IPC codes: G02F1, G02B27