Optical isolator and optical part having heat-resistant anti-reflection coating

Abstract: An improvement is proposed relating to an optical isolator for bonding of the polarizer and analyzer of polarizing glass to the respective holder rings for position adjustment. The improvement comprises: (a) forming a metallized layer of a specific composition over at least two of the side surfaces of each of the polarizer and analyzer on the area of the surface excepting for the linear areas of 50 to 150 µm width from the top and bottom surfaces of the polarizer or analyzer; and (b) bonding the polarizer and analyzer to the respective holder rings by soldering using a solder alloy with intervention of the metallized layer between the polarizer or analyzer and the holder ring. The invention further provides an optical part having, on at least one of the surfaces, a heat-resistant anti-reflection coating film which is a double-layered film consisting of a thin film of the titanium oxide TiO x (x = 1.9 to 2.2) as the underlayer of high refractive index and a thin film of silicon dioxide as the overlayer of low refractive index.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical isolator. More particularly, the invention relates to an optical isolator provided with a polariser and analyser of polarising glass to utilise the Faraday effect. An optical isolator is constructed from a polariser and analyser as well as a Faraday rotator and permanent magnet installed therebetween and these parts are built in a holder. While several particular modifications are known for an optical isolator, it is usual to use polarizing glass for the polariser and analyser in a compact optical isolator in view of the possibility of accomplishing a high quenching ratio and a small-thickness design. Several commercial products of polarising glass are available on the market including Polacore (a product by Corning Glass Works Corp.). Document JP 03 259 110 A discloses an optical isolator as specified in the preamble of claim 1, directed to the problem of peeling of metallic films and thereby improving the bonding of the polariser, analyser and Faraday rotator to the magnet. Document JP 02 301 714 A discloses an arrangement in which metallisation is provided at the peripheral part of a Faraday rotator, but not of a polariser or an analyser. Metallisation is not provided at the central part of the Faraday rotator. Japanese Utility Model Kokai 5-96830 discloses a structure of an optical isolator, as is illustrated in Figure 7 of the accompanying drawing, consisting of a polarizer 1, analyser 7 and Faraday rotator 8 built in a tubular holder 6 and a tubular magnet 9 in which a holder ring 2 is used in order to facilitate exact position adjustment of the polarizer 1, analyzer 7 and Faraday rotator 8. A diaphragm 10 is provided to limit the light incident on the polarizer 1. It is a known method to use an adhesive of an organic polymeric resin for bonding and fixing the polarizer 1 and analyzer 7 to the respective holder rings 2 and the Faraday rotator 8 to the tubular magnet 9. When the polarizer 1, analyzer 7 and Faraday rotator 8 are bonded to the holder rings 2 and to the tubular magnet 9 by using an adhesive, however, the adhesive has a relatively large thermal expansion coefficient so that a great displacement from the exact setting of the inclination angle is sometimes caused by the variations in the ambient temperature. This phenomenon unavoidably results in a deviation of the optical axis once optimized by adjustment due to the variation in the ambient temperature. In addition, volatile organic matters are emitted from the adhesive to cause contamination and degradation of the laser as the light source decreasing the long-term reliability of the optical isolator. As a solution for the above mentioned problem of low long-term reliability of the optical isolator, it is known that the polarizer, analyzer and Faraday rotator are bonded and fixed to the respective holding parts by the use of a solder or brazing alloy or low melting-point glass. As the solder alloy for this purpose, gold-based solder alloys free from lead are used in recent years in consideration of the problem of environmental pollution due to heavy metals such as lead. In bonding and fixing the polarizer, analyzer and Faraday rotator to the holding parts by soldering, one of the very important requirements is that the difference in the thermal expansion coefficients should be as small as possible between the holding parts and each of the polarizer, analyzer and Faraday rotator because, if the difference in the thermal expansion coefficients is great, cracks are possibly formed in the polarizer, analyzer and Faraday rotator due to the thermal stress resulting in complete loss of the performance of the optical isolator. This phenomenon is particularly remarkable in the bonding and fixing works between a holder ring and a polarizer and analyzer made from polarizing glass. Following methods are known to solve the problem due to crack formation mentioned above. (1) A method is disclosed in Japanese Patent Kokai 5-11215 in which the optical element and the holding part are bonded with intervention of a spacer having an intermediate thermal expansion coefficient. The material of the spacer is preferably a ceramic and the spacer has a configuration of a circular ring, optionally, provided with a slit or cutout, a configuration to match the metallized part for soldering in the optical element or a square configuration with a circular openwork at the center, optionally, divided into portions. (2) Japanese Utility Model Kokai 6-4735 discloses a method in which the holding part is provided with a slit. (3) Japanese Patent Kokai 6-167675 teaches forming the holding part from an alloy having a thermal expansion coefficient close to that of the polarizing glass such as Fe-32Ni alloy and Fe-42Ni alloy. The inventors have tried to test the practicability of the above described first to third methods for bonding and fixing of a polarizer and analyzer of polarizing glass to a holder ring by soldering and found a problem that, though without formation of cracks, the quenching ratio is not sufficiently high in the polarizer and analyzer after bonding so that the optical isolator using the same cannot work at a high quenching ratio.The reason therefor is, like the reason for the above mentioned crack formation, presumably the thermal stress due to the difference in the thermal expansion coefficients between the polarizer and/or the analyzer and the holder rings. Although cracks are not formed in the above mentioned first to third methods by virtue of the decrease in the thermal stress in the polarizer and analyzer, namely, relaxation of the thermal stress is still incomplete not to ensure a high quenching ratio as an optical isolator.Following are further known methods for decreasing the thermal stress to accomplish a high quenching ratio of the polarizer and analyzer. (4) Japanese Patent Kokai 6-34861 teaches a method in which the side surfaces of the optical element are metallized and the side surface of the optical element and the holding part are bonded with intervention of the metallized layer. (5) Japanese Patent Kokai 6-67119 discloses a method in which the surfaces of the optical element, excepting the surfaces to serve for the light beam transmission, are provided with a metallized layer patterned in at least four concentrical rings with regular spaces between the adjacent rings, of which the overall metallized area is in the range from 5 to 25% based on the area for the incident light into the optical element. The inventors have tried to test the practicability of the above described fourth and fifth methods for bonding and fixing of a polarizer and analyzer to holder rings by soldering and found a problem that the distribution of the quenching ratio is not uniform within the plane as is illustrated in Figure 6 and the quenching ratio decreases from the center toward the bonded parts. This result leads to a consequence that optical isolators cannot be so compact as to be smaller than a certain limit and accordingly the costs thereof cannot be decreased as desired. The reason therefor is as follows. It would be a possible way for obtaining an optical isolator of a high quenching ratio by using a polarizer and analyzer bonded to the holder rings by the above mentioned fourth or fifth method to use a polarizer and analyzer having a sufficiently larger surface area than the light-transmitting surfaces. Namely, the optical isolator can exhibit desirable performance by utilizing the portions of a high quenching ratio only fully isolated from the bonding parts. This method, however, has a problem that the compactness of the optical isolator has a lower limit and the optical isolator cannot be more compact than the limit. In view of the expensiveness of available commercial products of polarizing glass in general such as Polacore mentioned above, on the other hand, it is an important requirement that the area of the polarizer and analyzer should be as small as possible in order to minimize the costs of optical isolators. Accordingly, it is almost impossible by the conventional technology to have compatibility between these requirements for compactness and inexpensiveness of optical isolators with a high quenching ratio. The above described attempts undertaken by the inventors have led to a conclusion that a quenching ratio sufficiently high for an optical isolator could hardly be accomplished by the first to third methods only teaching selection of the materials and configurations of the holder rings although the problem of crack formation could be solved thereby. Further, a compact and inexpensive optical isolator having a high quenching ratio can hardly be obtained by utilizing the above described first to fifth methods either singly or in combination because of the great non-uniformity in the distribution of the quenching ratio within the plane although the quenching ratio can be partially high enough. While the overall stress by bonding can be reduced by decreasing the bonding area, the disclosure in Japanese Patent Kokai 6-67119 teaches that the bonding area cannot be decreased below a certain level in order to ensure a bonding strength suitable for practical use of the optical isolator. Thus, it is a conclusion that none of or no combinations of the above described first to fifth methods could satisfy all of the requirements for an optical isolator relative to a high quenching ratio, high bonding strength, compactness and inexpensiveness simultaneously. SUMMARY OF THE INVENTION The present invention accordingly has an object to provide an improvement in an optical isolator comprising a polariser, an analyser, a Faraday rotator, a holder to contain the polarizer, analyser and Faraday rotator and holder rings for the position adjustment of the parts. Thus the present invention provides an optical isolator comprising a polariser, an analyser, a Faraday rotator and a permanent magnet built in a holder having holder rings for position adjustment of the polariser and analyser, wherein a metallised layer is formed on at least two surfaces of the polariser and analyser through which an optical path does not pass through the optical isolator in use and the polariser and analyser are bonded to the corresponding holder rings by soldering using a solder alloy with intervention of the corresponding metallised layer between the polariser or analyser and the corresponding holder ring, characterised in that the polariser and analyser are made from polarising glass, the holder rings are made from a Kovar alloy or iron-nickel alloy, and the or each said metallised layer is formed over the area of the corresponding surface except for linear zones having a width of 50 µm to 150 µm adjacent the surfaces of the polariser or analyser through which an optical path passes when the optical isolator is in use. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a perspective view of a polariser or analyser having metallised surfaces according to the invention; Figure 2 is an illustration of the construction with a polariser or analyser bonded to the holder ring by soldering by a cross-sectional view; Figure 3 is an illustration of the measuring position of the quenching ratio on the polariser or analyser shown in Figure 1 by a perspective view; Figure 4 is a diagram showing the quenching ratio of the polariser or analyser in the inventive optical isolator within plane; Figure 5 is an illustration of the metallised surface areas on a polariser or analyser in Comparative Example 1; Figure 6 is a diagram showing the quenching ratio of the polariser or analyser in a conventional optical isolator within plane; and Figure 7 is a schematic cross-sectional view of an optical isolator showing the structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is described above, the present invention relates to an improvement in the bonding way of a polariser and/or analyser as the components of an optical isolator to the holder rings by soldering. As a result of their extensive investigations, the inventors have arrived at an unexpected discovery that, when a polariser and analyser in an optical isolator are made from polarizing glass such as a commercial product of Polacore (supra), the polarisability of the polarizer and analyzer is a phenomenon ascribable only to the very thin surface layers thereof so that, if such a layer is free from concentration of stress, degradation in the quenching ratio never takes place. While, in the present invention, the surface area to be metallized is limited to the side surfaces excepting linear areas of 100 µ m width from the top and bottom surfaces, this is because thickness of the surface layer of a polarizing glass body having an influence on the polarizability of the glass body cannot exceed 100 µ m from the surface. Accordingly, it is essential that the areas on the side surfaces close to the top and bottom surface layers having polarizability are free from metallization. When this requirement is satisfied, a compact and low-cost optical isolator can be constructed with a high quenching ratio and very narrow distribution range of the quenching ratio within the plane. Figure 1 of the accompanying drawing is a perspective view of a polarizer 11 or analyzer 17 in the configuration of a rectangular parallelepiped or rectangular plate having six surfaces 21, 21, 21, 21, 15, 15, of which at least two of the four side surfaces 21 are subjected to metallization and the grained areas 14 on the side surface 21 defined by excepting the upper and lower very narrow linear areas of each 100 µ m width are the areas for actual metallization. The material forming the polarizer and analyzer is preferably polarizing glass which is not particularly limitative provided that the surface thereof has polarizability as exemplified by the commercial product of Polacore ( supra ). The thickness of the polarizer 11 or analyzer 17 is preferably in the range from 0.3 mm to 0.6 mm. The polarizer 11 or analyzer 17 illustrated in Figure 1 has a thickness of 0.5 mm. It is necessary that at least two of the four side surfaces 21 of the polarizer 11 or analyzer 17 are metallized excepting the narrow linear zones of 100 µ m width from the top and bottom surfaces 15 and the metallized area 14 is preferably at least two thirds large of the overall area of the side surface 21. When the metallized area 14 is too small, the bonding strength cannot be high enough not to accomplish high reliability of the optical isolator. As is mentioned above, it is necessary that metallization is effected on at least two of the four side surfaces 21 or, namely, on the two opposite surfaces 21 or on all of the four side surfaces 21 excepting the top and bottom surfaces 15. In Figure 1, two metallized areas 14 are on the two opposite side surfaces 21. The overall area of metallization in the polarizer 11 or analyzer 17 illustrated in Figure 1 is at least 0.56 mm 2 which is two thirds of maximum metallizable area on two opposite side surfaces 21, i.e, 1.4×0.3×2 ×2/3 mm2, and cannot exceed 1.68 mm 2 which is the sum of the metallizable areas 14 on all of the four side surfaces 21, i.e. 1.4×0.3×4 mm 2 . When the overall metallized area available for bonding is within the above mentioned range, failure in bonding in the optical isolator does not take place in an impact test and a high quenching ratio at the center and a lowest quenching ratio within the plane which is not too low can be obtained irrespective of the metallized area for bonding. The method for the metallization is not particularly limitative and, for example, a three-layered structure consisting of chromium, platinum and gold having a thickness of 0.5 to 5 µ m can be formed by the method of vacuum vapor-phase deposition, sputtering or chemical vapor-phase deposition. Figure 2 is a schematic illustration of a polarizer 11 or analyzer 17 bonded to a holder ring 12 by using a solder alloy 13 on the metallized areas 14 according to the invention by a cross sectional view. The solder or brazing alloy used in bonding of the polarizer or analyzer to the respective holder rings is not particularly limitative including, preferably, a gold-tin alloy and gold-germanium alloy. These solder alloys are preferable in respects of little troubles due to crack formation, high bonding strength to withstand the impact test of the optical isolator and absence of any adverse influences on the quenching ratio at the center and the lowest quenching ratio within the plane to ensure high performance of the optical isolator if not to mention that these alloys are free from the problem of environmental pollution due to toxicity of heavy metals such as lead contained in the solder alloy. The material of the holder ring is preferably a Kovar alloy or an iron-nickel alloy because these alloys have a thermal expansion coefficient close to that of the polarizing glass, e.g., 6.5×10 -6 /K, not to cause troubles due to crack formation after bonding by soldering. The above mentioned Kovar alloy is an alloy consisting, for example, of 29% by weight of nickel, 17% by weight of cobalt and the balance of iron and having a thermal expansion coefficient of 4.7x10 -6 /K. Examples of suitable iron-nickel alloy includes the Fe-32Ni alloy consisting of 68% by weight of iron and 32% by weight of nickel and having a thermal expansion coefficient of 6.5 x 10 -6 /K and Fe-42Ni alloy consisting of 58% by weight of iron and 42% by weight of nickel and having a thermal expansion coefficient of 6.5 x 10 -6 /K. Incidentally, a grade of stainless steel SUS 340 has a thermal expansion coefficient of 20 x 10 -6 /K so that the stainless steel is less preferable as the material of the holder rings than the above mentioned Kovar alloy and iron-nickel alloys. Figure 4 is a plot showing the distribution of the quenching ratio within the plane of the polariser or analyser prepared in Example 1 described below which indicates that the quenching ratio even at the measuring point in the proximity of the soldered areas is approximately equal to the sufficiently large value at the centre suggesting that the polariser and analyser can be very compact. This means that an optical isolator which is so compact and inexpensive exhibits a so high quenching ratio hardly obtained in the prior art can be manufactured according to the present invention. In the following, some embodiments of the present invention are described by way of Examples. Example 1. A polarizer and an analyzer of optical glass were subjected to metallization to form a 3 µ m thick metallized layer having a three-layered structure of chromium, platinum and gold in the following manner. Figure 1 illustrates a perspective view of a polarizer 11 or analyzer 17 provided with a metallized layer 14 shown by the grained area. The polarizer 11 or analyzer 17 was made from polarizing glass (Polacore, supra) and had dimensions of 1.4 mm by 1.4 mm by 0.5 mm. Two opposite side surfaces 21 were metallized each on the 1.4 mm by 0.30 mm wide area leaving the 0.10 mm wide narrow linear zones from the top and bottom surfaces 15, 15 as is shown in the figure so that the overall metallized area was 0.84 mm 2 . Figure 2 schematically illustrates a cross sectional view of a construction consisting of the polarizer 11 or analyzer 17 of the polarizing glass Polacore and a holder ring 12 of a Kovar alloy bonded together by using a gold-tin solder alloy 13 at a temperature of 300 °C in an electric furnace. The thus prepared optical isolator was subjected to evaluation tests to find that it was free from cracks as inspected with an optical microscope and could withstand the impact test undertaken under the conditions of 2000 G and 0.3 msecond. Table 1 below summarizes the overall metallized area in mm 2 , quenching ratio at the center point P and lowest quenching ratio within the plane which is the lowest value of the quenching ratios obtained by measuring the quenching ratio along the chain line L on the top surface of the polarizer 11 or analyzer 17 shown in Figure 3. Example 2. The experimental procedure was substantially the same as in Example 1 except that the holder ring 12 was made from a Fe-32Ni alloy instead of the Kovar alloy. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1 below. Example 3. The experimental procedure was substantially the same as in Example 1 except that the holder ring 12 was made from a Fe-42Ni alloy instead of the Kovar alloy. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1 below. Example 4. The experimental procedure was substantially the same as in Example 1 except that the metallization was effected on the 1.4 mm by 0.2 mm areas leaving linear areas of 0.15 mm width from the top and bottom surfaces unmetallized. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1 below. Example 5. The experimental procedure was substantially the same as in Example 1 except that the metallization was effected on all of the four side surfaces each over a 1.4 mm by 0.3 mm area leaving 0.15 mm wide linear areas from the top and bottom surfaces unmetallized. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1 below. Example 6. The experimental procedure was substantially the same as in Example 1 except that the gold-tin solder alloy was replaced with a gold-germanium solder alloy and soldering was performed at 380 °C. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1 below. Comparative Example 1. The experimental procedure was substantially the same as in Example 1 except that metallization was effected only on the four equilateral right-angled triangular corner areas 14A of the top surface 15 as is shown in Figure 5 instead of the two rectangular areas 14 on the side surfaces. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1 below. Comparative Example 2. The experimental procedure was substantially the same as in Example 1 except that metallization was effected allover the 1.4 mm by 0.5 mm wide areas of two opposite side surfaces 21 without leaving the 0.1 mm wide linear zones unmetallized. The optical isolator was free from cracks and could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1. Comparative Example 3. The experimental procedure was substantially the same as in Example 1 except that the holder ring 12 was prepared from stainless steel SUS 304 instead of the Kovar alloy. The results of the evaluation tests of the optical isolator were that cracks were found in the polarizer and analyzer although the optical isolator could withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1. Comparative Example 4. The experimental procedure was substantially the same as in Example 1 except that metallization was effected over the 1.4 mm by 0.10 mm wide areas on the two opposite side surfaces 21, 21 leaving 0.20 mm wide linear zones from the top and bottom surfaces 15, 15 unmetallized. The results of the evaluation tests of the optical isolator were that, although no cracks were found in the polarizer and analyzer, the optical isolator could not withstand the impact test. The results obtained by the measurement of the quenching ratio are shown in Table 1. Metallized area, mm 2 Quenching ratio, center, dB Lowest quenching ratio within plane, dB Example 1 0.84 52.4 50.0 2 0.84 51.5 50.9 3 0.84 51.9 49.2 4 0.56 51.9 50.2 5 1.68 52.1 49.9 6 0.84 51.4 50.3 Comparative Example 1 0.32 38.0 24.0 2 1.40 43.2 32.3 3 0.84 34.2 21.8 4 0.28 51.8 50.1 Figure 4 shows the distribution of the quenching ratio within the plane as measured in Example 1 along the center line L shown in Figure 3 on the top surface 15 of the polarizer 11 or analyzer 17, from which it is understood that the quenching ratio is uniform allover the surface with a value at the solder-bonded portions approximately equal to the large value at the center point P. This result suggests that an optical isolator can be constructed with very compact polarizer and analyzer. The results of Examples 1 and 6 indicate that the gold-tin alloy and gold-germanium alloy as the solder alloy are preferred in respect of absence of the troubles due to crack formation and the large quenching ratio of the optical isolator which can withstand the impact test by virtue of the good bonding strength therewith. Comparison of the results obtained in Examples 1, 2 and 3 and those obtained in Comparative Example 3 is suggestive of the importance of a small difference in the thermal expansion coefficient between the polarising glass of the polariser or analyser and the material of the holder ring, of which the thermal expansion coefficients were given before. Further, comparison of the results obtained in Examples 1 and 4 and those obtained in Comparative Example 4 indicates that the optical isolator can withstand the impact test when the overall metallised surface area is 0.56 mm 2 or larger and the quenching ratio at the centre and the lowest quenching ratio within the plane do not depend on the overall metallised surface area.

Claims: An optical isolator comprising a polariser (11), an analyser (17), a Faraday rotator and a permanent magnet built in a holder having holder rings (12) for position adjustment of the polariser and analyser, wherein a metallised layer (14) is formed on at least two surfaces (21) of the polariser and analyser through which an optical path does not pass through the optical isolator in use and the polariser and analyser are bonded to the corresponding holder rings by soldering using a solder alloy (13) with intervention of the corresponding metallised layer between the polariser or analyser and the corresponding holder ring, characterised in that the polariser and analyser are made from polarising glass, the holder rings are made from a Kovar alloy or iron-nickel alloy, and the or each said metallised layer is formed over the area of the corresponding surface except for linear zones having a width of 50 µm to 150 µm adjacent the surfaces (15) of the polariser or analyser through which an optical path passes when the optical isolator is in use. An optical isolator according to claim 1, wherein the solder alloy is a gold-tin alloy or a gold-germanium alloy. An optical isolator according to claim 1 or 2, wherein the or each metallised layer is at least two thirds of the overall area of the corresponding surface. An optical isolator according to any one of the preceding claims, wherein the metallised layer has a multilayered structure consisting of chromium, platinum and gold. An optical isolator according to any one of the preceding claims, wherein the or each metallised layer has a thickness in the range from 0.5 µ m to 5 µ m. An optical isolator according to any one of the preceding claims, wherein the or each said linear area has a width in the range from 80 µ m to 120 µ m.

Inventors: RYUO TOSHIHIKO, SHIONO YOSHIYUKI, TANNO MASAYUKI, WATANABE TOSHIAKI, RYUO, TOSHIHIKO, SHIONO, YOSHIYUKI, TANNO, MASAYUKI, WATANABE, TOSHIAKI
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1996
Main classification: G02B27
IPC codes: G02F1, G02B1