JP2005283697A - Optical isolator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate troubles in an optical isolator, such as the falling-off of an optical element, development of cracks, degradation in joint strength and the deterioration in optical characteristics. <P>SOLUTION: In the optical isolator where the optical element including a Faraday rotator and a polarizer is integrated on the top surface of a planar mounting substrate through an adhesive, at least a groove for mounting in which the Faraday rotator is inserted is formed on the top surface of the mounting substrate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical isolator used for removing return light generated when light emitted from a light source is introduced into various optical elements and optical fibers.

  In an optical communication module or the like, light emitted from a light source such as a laser light source is incident on various optical elements or optical fibers, but a part of the incident light is reflected on the end surfaces or inside of the various optical elements or optical fibers. It is scattered. A part of the reflected or scattered light tries to return to the light source as return light, and an optical isolator is used to prevent the return light.

  Conventionally, this type of optical isolator is configured by storing an optical element in which a flat Faraday rotator is installed between two polarizers in a cylindrical magnet via each component holder. It was. Normally, the Faraday rotator is adjusted to a thickness that rotates the polarization plane of light having a predetermined wavelength in the saturation magnetic field by 45 °, and the two polarizers rotate so that their transmission polarization directions are shifted by 45 ° rotation direction. Coordinated and configured.

  The optical isolator having such a configuration requires a Faraday rotator and two polarizers as separate parts and a holder for each element, which increases the number of parts and the number of assembling steps. The adjustment work is complicated and incurs high costs. In addition, it is difficult to reduce the size, and further, it is necessary to adjust the polarization plane of the optical isolator when it is incorporated in the light source module, and the mounting is complicated.

  For this reason, an optical isolator in which each optical element of a Faraday rotator and a polarizer and a rectangular magnet is installed on a flat mounting board has been proposed.

  Patent Document 1 shows a conventional miniaturized optical isolator 15 shown in FIG. 5, and its configuration will be described below.

  The optical isolator 15 has a structure in which optical elements 21 of a Faraday rotator 16 and polarizers 17 and 18 and a rectangular parallelepiped magnet 19 are arranged on a flat mounting board 20 on both sides thereof. Here, the polarizers 17 and 18 have a function of absorbing a polarization component in one direction of transmitted light and transmitting a polarization component orthogonal to the polarization component, and the Faraday rotator 16 has a saturation magnetic field strength. 1 has a function of rotating the polarization plane of light of a predetermined wavelength by about 45 degrees. The two polarizers 17 and 18 are cut out so that the plane contacting the respective substrates 20 is a reference plane, and the transmitted polarization directions are 0 degrees and 45 degrees with respect to the reference plane.

Here, in the configuration in which the outer shape of each optical element 21 is cut and formed accurately and then disposed on the mounting substrate 20, the overall size in the thickness direction of the mounting substrate 20 is reduced, and the entire size is reduced. In order to reduce the cost and increase the reliability, the shape of the main surface of each optical element is made square and the intervals between the optical elements 21 are reduced.
JP-A-10-227996

  However, as shown in Patent Document 1 in FIG. 5, in the optical isolator 15 in which the optical elements 21 of the Faraday rotator 16 and the polarizers 17 and 18 and the rectangular magnet 19 are arranged on a flat mounting substrate 20. However, depending on the bonding conditions between each of the optical elements 21 and the flat mounting substrate 20, there is a problem that the bonding strength is reduced due to dropping of the optical elements, cracks, or poor bonding.

  Further, in order to meet the recent demand for miniaturization, it is necessary to arrange the optical elements 21 as narrow as possible. However, as the distance is reduced, the bonding agent should not adhere to the surface due to surface tension. As a result, there is a problem that the bonding agent becomes a defective product that blocks the opening, which is a light transmission region of each optical element, and the yield is deteriorated.

  FIG. 6 is a side view of an excerpt of the essential parts shown to explain the problems in manufacturing the optical element holding structure and the optical isolator using the optical element holding structure. Here, when the polarizer 17 and the Faraday rotator 16 which are optical elements are fixed to the mounting substrate 20 using a bonding agent 25 made of an epoxy resin, the bonding agent 25 is subjected to surface tension and the polarizer 17 and the Faraday rotator. 16 shows a state of being attached between the two. Such a phenomenon becomes more prominent as the distance between the optical elements is reduced. When the bonding agent 25 is attracted by the surface tension between the optical elements without escape, a substantially U-shaped shape as shown in FIG. Is formed. In a severe case, the U-shaped bonding agent crawls up to the optical surface of each optical element and reaches the opening. In order to prevent this phenomenon, it is necessary to widen the interval between the optical elements, which leads to a problem that leads to an increase in the size of the optical isolator.

  Further, in the optical isolator 15 in which the optical elements 21 of the Faraday rotator 16 and the polarizers 17 and 18 and the rectangular magnet 19 are arranged on the flat mounting substrate 20 as shown in FIG. There is no description of the bonding method between the plate 21 and the flat mounting substrate 20, or between the magnet 19 and the flat mounting substrate 20. Depending on the bonding method, the optical element 21 may drop, the crack bonding strength may decrease, and the optical characteristics may deteriorate. There is a problem that occurs.

  Specifically, since the optical element 21 and the magnet 19 are fixed to a narrow area on the surface of the single mounting substrate 20 with a bonding agent, the heat of the magnet 19 is used when a bonding agent that melts and fixes at a high temperature is used. Since the expansion coefficient is larger than that of the optical element 21, the adjacent optical element 21 is affected and tensile stress is generated. In particular, when the bonding agent has a thermal expansion coefficient that is equal to or lower than the smaller one of the thermal expansion coefficients of the optical element 21 and the mounting substrate 20, the bonding agent and the Faraday rotator 16 have different thermal expansion coefficients. Another cause is that the tensile stress is concentrated on the corner of the joint surface of the Faraday rotator 16.

  In general, in order to prevent reflection with respect to incident light, each optical element is not arranged so as to be perpendicular to the incident light beam, but is arranged so as to have an angle other than that, but there is no guide portion on the mounting substrate 20. In addition, since the angle is adjusted by visual observation, the angle adjustment accuracy is very poor. Therefore, there is a problem that the characteristics of the isolator are not stable. In addition, when an alloy such as stainless steel, Kovar, or alloy is used for the mounting substrate 20 and the guideline of the optical element is formed by a groove, burrs are generated due to processing exceeding the groove width, and the optical element is caught and mounted. The surface becomes non-planar, which causes problems such as deterioration in mounting accuracy.

  The present invention has been made in view of the above problems, and in an optical isolator in which an optical element including a Faraday rotator and a polarizer is integrated with a flat plate-like mounting substrate via a bonding agent, A mounting groove into which at least the Faraday rotator can be inserted is formed on the upper surface of the mounting substrate.

  The mounting groove is formed so that each of the Faraday rotator and the polarizer of the optical element can be inserted.

  Further, the bonding agent is made of a low melting point glass.

  Further, the mounting substrate is made of a glass material, and the mounting groove is formed by cutting.

  According to the configuration of the present invention, the mounting groove into which at least the Faraday rotator can be inserted is formed on the upper surface of the mounting substrate, and the Faraday rotator is bonded to the bottom surface of the mounting groove via the bonding agent. When mounting the optical element on the mounting substrate, even if the bonding agent swells on the optical surface side of the Faraday rotator, it rises in the mounting groove, so it can be separated from the polarizer bonding agent. The raised bonding agent does not enter the opening diameter of the optical element.

  In particular, by forming a mounting groove for the Faraday rotator and mounting the Faraday rotator in the mounting groove, the distance between the corner of the Faraday rotator contacting the mounting board and the polarizer or magnet is separated. Therefore, stress concentration at the time of mounting can be avoided, and generation of cracks can be suppressed. When the mounting groove formed on the mounting substrate is formed so that each of the Faraday rotator and polarizer of the optical element can be inserted, the bonding agent rises to the optical surface side when the optical element is mounted on the mounting substrate. Is more preferable because it is possible to completely suppress.

  Furthermore, by forming the mounting groove as described above, the effect of reducing the size of the optical isolator is realized because the rise of the bonding agent and the influence of stress are reduced even when the interval between the optical elements is set narrow. Have Further, since the mounting groove can be used as an angle adjustment guide portion of the optical element, it is possible to solve the problem of unstable isolator characteristics due to poor angle adjustment accuracy.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a perspective view showing an embodiment of an optical isolator according to the present invention.

  Although the figure is configured symmetrically with respect to the A surface, the optical element 1 is shown with one magnet omitted to make it easier to understand the state of joining the mounting substrate 6.

  As shown in the figure, the optical element 10 of the present invention includes a mounting substrate 22 having three groove portions 6 traversing the upper surface, an optical element 1 including a polarizer 3, a Faraday rotator 2, and a polarizer 4, It comprises a magnet 8 disposed on both sides of the optical element 1. One end of the optical element 1 is inserted into the groove 6 through the bonding agent 7, and the end of each optical element 1 is inserted through the bonding agent 7. Are fixed to the mounting substrate 22.

  The transmission polarization direction of the polarizer 3 used in the optical element 1 is set to a direction parallel to one side (this is referred to as a reference side) parallel to the upper surface of the mounting substrate 22, and the other polarizer 4. The transmission polarization direction is set to a direction of 45 degrees with respect to the reference side. Here, by making the upper surface of the mounting substrate 6 and the reference sides of the polarizer 3 and the polarizer 4 substantially coincide with each other and fixing, the transmission polarization directions of the polarizer 3 and the polarizer 4 are not adjusted to each other. When the Faraday rotation angle of the Faraday rotator 2 is approximately 45 degrees, the best insertion loss characteristic and isolation characteristic can be obtained.

  The polarizers 3 and 4 are configured by an absorption polarizer having a function of absorbing a unidirectional polarization component of incident light, or a birefringent polarizer that separates or synthesizes a unidirectional polarization component of incident light. The The absorptive polarizer is made of, for example, polarizing glass having a structure in which ellipsoidal metal particles are dispersed in glass. This polarizing glass is a glass having polarization characteristics by arranging long stretched metal particles in one direction in the glass itself, light having a polarization plane perpendicular to the stretch direction of the metal particles is transmitted, Light with a parallel polarization plane is absorbed. For example, it is made of polarizing glass having a structure in which ellipsoidal metal particles are dispersed in glass. This polarizing glass is a glass having polarization characteristics by arranging long stretched metal particles in one direction in the glass itself, light having a polarization plane perpendicular to the stretch direction of the metal particles is transmitted, Light with a parallel polarization plane is absorbed.

  The Faraday rotator 2 is adjusted to have a thickness at which the polarization direction of light incident at room temperature rotates 45 degrees. When high isolation is required for the optical isolator 10, the rotational deviation between the polarizer 3 and the polarizer 4 is precisely adjusted to 45−α degrees with respect to the polarization rotation angle 45 + α degrees of the Faraday rotator 2. There is a method in which light is incident from the opposite direction (from the side of the polarizer 4), and the polarizer 2 is rotationally adjusted so that the transmitted light is minimized. Therefore, the transmission polarization directions of the polarizer 3 and the polarizer 4 can be cut out by shifting by 45-α degrees in advance, and for example, the transmission polarization direction of the polarizer 4 can be set to 45-α degrees with respect to the reference side. In addition, the accuracy ± α of the polarization rotation angle of the Faraday rotator is desirably about 1 degree due to the characteristics of the optical isolator, and is installed on the upper surface of the mounting substrate 6 with high accuracy.

  The Faraday rotator 2 is, for example, a bismuth-substituted garnet crystal, and the thickness thereof is set so that the polarization plane of incident light having a predetermined wavelength rotates 45 degrees. In general, in order to rotate the polarization plane, it is necessary to apply a sufficient magnetic field in the direction of the optical axis L of the incident light, and the magnets 8 are arranged on both sides of the Faraday rotator 2 (one magnet). Is omitted).

  As a material of the magnet 8, for example, a material made of samarium cobalt is suitable. The magnets 8 are arranged on both sides of the optical element 1, and the magnetic poles are arranged on the magnet 8 in such a direction that the magnetic field lines passing through the Faraday rotator 2 are maximized. Has a magnetic field intensity sufficient to rotate the polarization plane of incident light having a predetermined wavelength by 45 degrees. Further, the shape of the magnet is not limited to this, and may be one as long as the Faraday rotator satisfies a predetermined magnetic field strength, and the shape is not limited.

  The mounting substrate 22 is composed of the main substrate 9 and the sub substrate 5 disposed on the upper surface thereof, and the above-described groove portion 7 is formed on the upper surface of the sub substrate.

  The main board 9 has not only the above-described sub-board 5 on the upper surface but also a width that allows the magnets 8 to be placed on both sides of the sub-board 5. The main board 9 is mounted to mount the optical isolator 10 on the semiconductor laser module. For example, when fixing to the submount of the semiconductor laser module by YAG welding, a metal such as stainless steel, Kovar, permalloy or the like is selected, and in the case of mounting by solder, the metal or ceramic, A material such as glass is used, and the mounting surface is subjected to Au plating with a Cr base, for example, is selected.

  The groove portions 7 formed on the upper surface of the sub-substrate 5 are embedded so that the bonding agent 7 does not contact each other. The groove portion 7 can individually mount the polarizers 3 and 4 and the Faraday rotator 2 which are the optical elements 1, and the size thereof is formed so that each optical element 1 can be inserted exactly. The formation direction is formed in a direction perpendicular to the optical axis direction in FIG. 1, but is not limited to this. For example, the configuration shown in FIG. 3 is preferable.

  FIG. 3 is a perspective view showing another configuration of the sub-board used in the present invention.

  The groove 6 is formed so as to cross the upper surface of the sub-substrate 5, and the groove 6 is formed with an angle θ with respect to one side of the sub-substrate 5. As in FIG. 1, a bonding agent 7 is embedded in the groove 6 so as not to contact each other. The size of the groove 6 is formed so that each optical element 1 can be inserted just like FIG.

  When the optical element 1 is inserted and fixed along the formed groove 6, the optical element 1 can be easily fixed so as to have an angle with respect to the incident light, and the reflected light from the optical element 1 can be fixed. The influence can be reduced.

  In addition, it is desirable to use a ceramic material or a glass material for the sub-board 5, and in particular, a transparent glass material is used so that bubbles inside the bonding agent and bonding defects with the optical elements and cracks can be found and the defects can be easily identified. It is very effective to use. In addition, there are many types of glass materials, and the material having a desired thermal expansion coefficient is easily available, so that it is an optimal substrate material for the present invention. In addition, the groove 6 can be easily formed by cutting and there is almost no burr, which is the most suitable point for employing the substrate material of the present invention.

As the bonding agent 7, low melting point glass is used, which may be the same material or different composition materials, and other metal oxides such as lead oxide, phosphorus oxide and zinc oxide are mixed with SiO ₂ which is a glass material, The melting point of which is adjusted to be low, which is creamed together with a solvent, is applied in advance on the groove 6 formed on the sub-substrate 5, and the solvent is skipped by preliminary firing, or the groove 6 A preform formed in the shape of a rod of approximately the same size as is used. Bonding is performed by firing for several seconds to several minutes in a high-temperature furnace of 300 to 420 degrees with the optical element 1 in close contact with the groove portion 6 in which the low melting point glass 7 formed in the groove portion of the mounting substrate 6 is integrated. be able to.

According to the optical isolator configured as described above, the thermal expansion coefficient of the Faraday rotator 2 made of a commonly used garnet single crystal is about 10 × 10 ⁻⁶ / ° C., and the heat of the polarizer made of a glass material The expansion coefficient is about 6.5 × 10 ⁻⁶ / ° C., and the thermal expansion coefficient of the magnet 8 made of samarium cobalt rare earth is about 13 × 10 ⁻⁶ / ° C., but the groove 6 of the present invention is formed. By adopting a configuration in which the bonding agents 7 formed on the sub-substrate 5 are difficult to contact with each other, members having different thermal expansion coefficients of the optical elements 1 can be firmly mounted on the common sub-substrate 5 and optically reduced even if the size is reduced. Since the swelling of the bonding agent 7 between the optical elements due to the approach of the element 1 can also be reduced, it is easy to ensure the aperture diameter, and desired optical characteristics can be obtained. Moreover, the tensile stress caused by the magnet 8 is not applied to the optical element 1, particularly the Faraday rotator 2, and good characteristics can be exhibited.

  Here, in order to obtain a stronger bonding state, it is necessary to select a low-melting glass having an optimum thermal expansion coefficient. Specifically, low-melting glass is weak in tensile stress, and in the state where tensile stress is applied, the glass breaks and leads to deterioration in bonding strength. From the viewpoint of bonding strength, it is desirable to select a low-melting-point glass having a thermal expansion coefficient equal to or smaller than that. Therefore, a low-melting glass having a thermal expansion coefficient equal to or lower than that of the optical element 1 or the mounting substrate 6 having the smaller thermal expansion coefficient is selected for bonding the optical element 1 and the sub-substrate 5. As a result of such selection, it is also conceivable to use low-melting-point glasses having different material blends for the polarizers 3 and 4 and the Faraday rotator 2.

  FIG. 2 is a cross-sectional view illustrating the fixed state of the optical element 1.

  FIG. 2 is a cross-sectional view taken along the line AA through the magnet and the Faraday rotator shown in FIG. 1, and shows how the end portion of the optical element 1 is inserted into the groove portion 6 of the mounting substrate 22 and fixed by the bonding agent 7. Show. The width of the groove 6 is preferably 1.1 to 1.5 times the thickness of each optical element 1, and the depth is preferably 0.03 mm or more from the viewpoint of bonding strength. The bonding agent 7 does not contact each other and is formed in another region of the groove 6, so that the bonding agent 7 does not rise between the optical elements 1. Here, it is not always necessary to insert each optical element 1 into a separate groove, and it is confirmed that the same effect can be obtained by forming only the groove for fixing the Faraday rotator 2 on the mounting substrate 6. Has been. Further, by inserting and fixing only the Faraday rotator 2 in the groove 6, it is possible to prevent the characteristic deterioration of the Faraday rotator 2 that is particularly susceptible to stress.

  FIG. 4 is a diagram showing a method for manufacturing the sub-substrate 5 with the bonding agent 7 used in the present invention.

  First, as shown in FIG. 4A, a groove 13 is formed on a large mounting board 12 by cutting. Here, the thickness of the blade 11 may be determined in accordance with the width of the groove 13, or a desired groove width may be formed by processing several times using a blade having a thickness of several tens of microns. Further, the depth of the groove 13 can be easily formed by the cutting depth of the blade. In this way, a large-sized mounting substrate 12 in which a plurality of grooves 13 as shown in FIG. Next, as shown in FIG. 5C, the bonding agent 14 is applied on one surface of the large-sized mounting substrate 12, and then the bonding agent 14 attached to a place other than the groove 13 is removed with a scraper or the like. For example, when a low-melting glass is used for the bonding agent 14, a cream-like low-melting glass is applied on the mounting substrate 12 by screen printing, and the low-melting glass other than the groove is removed by a scraper and remains in the groove 13. The low melting point glass is temporarily fired and fixed to the mounting substrate 12. Next, as shown in FIG. 5D, the large-sized substrate 12 with the bonding agent is cut into small pieces having an actual substrate size. Thus, the sub-board 5 with the bonding agent can be easily manufactured by cutting.

  In the mounting substrate 22 used in the present invention, the sub-substrate 5 is disposed on the main substrate 9 and the groove 6 is formed on the upper surface of the sub-substrate 5. However, the present invention is not limited to this. The groove 6 may be formed. As a material of the mounting substrate 22 in this case, glass or ceramic is preferably used as in the sub-substrate 5.

  As described above, according to the configuration of the present invention, light that is small, has a large aperture diameter, does not easily cause cracking or dropping off of the optical element, and does not require adjustment of the mounting angle of the optical element. An isolator is realized.

  As an example of the present invention, the optical isolator shown in FIG. 1 was produced. Each component and configuration will be described below.

The polarizer used is a polar core (product name) manufactured by Corning, the size is 1 mm square and the thickness is 0.2 mm, the mounting side with the substrate is the reference side, and the polarizer 3 on the incident side is the reference side The output side polarizer 4 is set to transmit a polarization direction of 45 degrees with respect to the reference side. The thermal expansion coefficient of the polarizer is 6.5 × 10 ⁻⁶ / ° C.

The Faraday rotator was a bismuth-substituted garnet, the size was 1 mm square, the thickness was 0.4 mm, and the polarization rotation angle in the saturation magnetic field strength was 45 degrees. Both are elements that operate with respect to light having a wavelength of 1.55 μm, and antireflection films for air (n = 1) are provided on both surfaces of the polarizer and the Faraday rotator. The thermal expansion coefficient of the Faraday rotator is 10 × 10 ⁻⁶ / ° C.

A glass substrate is used as the sub-substrate material, a groove is formed on the upper surface, and low melting point glass is applied in advance. The groove is formed at an angle of 4 degrees with respect to one side of the mounting substrate. The depth of the groove is about 0.05 mm, and this groove is filled with low-melting glass. The groove width was two for the polarizer, 0.25 mm, and one Faraday rotator groove having a width of 0.5 mm was formed between the two polarizer grooves. The interval between the grooves was 0.05 mm. The thermal expansion coefficient of the glass material is 10.5 × 10 ⁻⁶ / ° C., which is almost the same as the thermal expansion coefficient of the Faraday rotator, and the Faraday rotator is not affected by the stress from the mounting substrate. Similarly, low melting point glass having a size slightly lower than 10.5 × 10 ⁻⁶ / ° C. and 8 × 10 ⁻⁶ / ° C. was selected in consideration of stress on the Faraday rotator.

  The size of the sub-board is W = 1.2 mm, length D = 1.5 mm, and thickness t = 0.3 mm. One end of the optical element is held in a state where the low melting point glass is melted by heating this mounting board to 350 degrees. Fixed in the groove. The size of the main board is W = 3 mm, length D = 1.5 mm, thickness t = 0.3 mm, the sub board and the magnet on which the optical element is mounted are mounted on the main board made of stainless steel, and the bonding surface is An optical isolator was fabricated by integrating with solder. As the magnet, two samarium cobalt magnets having a substantially rectangular parallelepiped shape having a width W = 0.8 mm, a length D = 1.4 mm, and a thickness T = 1.4 mm were used.

  Under the above prototype conditions, 50 optical isolators were manufactured and the characteristics were measured. As a result, it was confirmed that all the optical isolators had good and uniform characteristics with an effective aperture diameter of 0.9 mm or more, an insertion loss of 0.3 dB or less, and an isolation of 35 dB or more.

  Next, the reliability of the produced optical isolator was evaluated. The tests were conducted vibration test, impact test, temperature cycle test, high temperature holding test, low temperature holding test, high temperature high humidity test shown in Telcordia 1221. In all tests, the amount of change in insertion loss is ± 0.2 dB or less, A good result was obtained in which the amount of change in isolation was ± 3 dB or less.

  By the above trial manufacture, to provide an optical isolator having a large aperture diameter, stable optical characteristics, easy assembly, less man-hours, and excellent reliability without dropping, cracking, and characteristic deterioration of optical elements. Can do.

It is a perspective view which shows embodiment of the optical isolator of this invention. It is the sectional view on the AA line of FIG. 1 explaining the fixed state of the optical element of this invention. It is a perspective view which shows the structure of the sub board | substrate used for this invention. (A)-(d) is a figure which shows the manufacturing method of the sub board | substrate with a bonding agent used for this invention. It is a figure which shows the structure of the conventional optical isolator miniaturized. It is sectional drawing which shows the structure of the conventional optical isolator miniaturized.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 2: Optical element 10, 15: Optical isolator 2, 16: Faraday rotator 3, 4, 17, 18: Polarizer 7, 14, 25: Bonding agent 5, 15: Sub board | substrate 9: Main board | substrates 20, 22 : Mounting substrate 6, 13: groove 8, 19: magnet 11: blade
12: Large format mounting board

Claims (4)

In an optical isolator in which an optical element including a Faraday rotator and a polarizer is integrated on the upper surface of a flat mounting substrate via a bonding agent, at least the Faraday rotator can be inserted into the upper surface of the mounting substrate. An optical isolator characterized by comprising: 2. The optical isolator according to claim 1, wherein the mounting groove is formed so that each of a Faraday rotator and a polarizer of the optical element can be inserted. The optical isolator according to claim 1, wherein the bonding agent is made of low-melting glass. The optical isolator according to claim 1, wherein the mounting substrate is made of a glass material, and the mounting groove is formed by cutting.

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