JP4666931B2 - Optical isolator - Google Patents

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 installing a flat Faraday rotator between two polarizers, and storing these three components in a cylindrical magnet via respective component holders. 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 optical elements of a Faraday rotator and a polarizer and a cuboid magnet are 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 of a Faraday rotator 16 and polarizers 17 and 18 and a cuboid magnet 19 are arranged on a flat mounting board 20. 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.
JP-A-10-227996

  However, as shown in Patent Document 1 of FIG. 5, in the optical isolator 15 in which the optical elements of the Faraday rotator 16 and the polarizers 17 and 18 and the rectangular magnet 19 are arranged on a flat mounting board 20, There is no description of the bonding method between each of the optical elements 21 and the flat mounting substrate 20 or between the magnet 19 and the flat mounting substrate 20, and depending on the bonding method, the optical element may drop off, the crack bonding strength may decrease, and the optical characteristics. There is a problem that the deterioration occurs.

  Specifically, since the optical element 21 and the magnet 19 are fixed to a narrow area of the surface of the single mounting substrate 20, when a bonding agent that melts and fixes at a high temperature is used, the thermal expansion coefficient of the magnet 19 is One of the causes is that it is larger than the optical element 21 and affects the adjacent optical element 21 to generate a tensile stress.

The present invention has been made in view of the above problems, and an optical isolator in which an optical element including a Faraday rotator and a polarizer and a magnet are integrated on the upper surface of a flat mounting substrate via a bonding agent. The bonding agent is separated into a region bonded to the optical element and a region bonded to the magnet on the upper surface of the mounting substrate, and the Faraday rotator and the polarizer are interposed between the optical element and the mounting substrate. And a bonding agent having a thermal expansion coefficient equal to or smaller than the smaller one of the mounting substrate, and between the magnet and the mounting substrate, whichever is smaller of the magnet and the mounting substrate A bonding agent having a thermal expansion coefficient equal to or lower than the thermal expansion coefficient is interposed .

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

  Furthermore, a groove is formed between the bonding region of the optical element of the mounting substrate and the bonding region of the magnet of the mounting substrate.

According to the configuration of the present invention, an optical isolator having a configuration in which a magnet and an optical element are bonded to a common mounting substrate using a bonding agent that melts and bonds at a high temperature. The bonding agent is configured not to contact each other, and a bonding agent having a thermal expansion coefficient equal to or smaller than the smaller one of the Faraday rotator, the polarizer, and the mounting substrate is provided between the optical element and the mounting substrate. By interposing a bonding agent having a thermal expansion coefficient equal to or lower than the smaller one of the magnet and the mounting board between the magnet and the mounting board , the stress on the optical element is relieved. It is possible to solve the problems of characteristic deterioration, cracks, and dropout of the optical element.

  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.

  As shown in the figure, an optical isolator 10 according to the present invention includes a mounting substrate 6 having bonding agents 5a and 5b formed on the upper surface, an optical element 1 including a polarizer 3, a Faraday rotator 2, and a polarizer 4, and a magnet 7. The magnet 7 is bonded to the mounting substrate 6 via the bonding agent 5a, and the optical element 1 is bonded to the mounting substrate 6 via the bonding agent 5b.

  The material of the mounting substrate 6 is selected by a mounting method in which the optical isolator 10 is mounted on the semiconductor laser module. For example, when it is fixed to the submount of the semiconductor laser module by YAG welding, a metal such as stainless steel, Kovar, or permalloy is selected. In the case of mounting by solder, a material such as the metal, ceramic, glass or the like is used, and the mounting surface is plated with, for example, Cr as a base.

  The optical element 1 includes a plate-shaped polarizer 3, a Faraday rotator 2, and a polarizer 4, and the bottom surface of these optical elements 1 is bonded to the mounting substrate 6 with a low melting point glass 5b with high accuracy.

Bonding agent 5a, the low-melting glass is used in 5b, they may be the same material or different composition material, other metal oxides to SiO ₂ is a glass material, for example, lead oxide and phosphorus oxide and zinc oxide The mixture is adjusted so that its melting point is lowered, and a cream-like one with a solvent is applied on the mounting substrate 6 in advance and the solvent is removed by temporary baking, or the upper surface of the mounting substrate 6 is removed. A preform formed in the shape of a flat plate having substantially the same size as the regions 5a and 5b is used. By bonding the low melting point glass 5b and the optical element 1 formed on the mounting substrate 6 in this way and the low melting point glass 5a and the magnet 7 in close contact with each other by firing in a high temperature furnace of 300 to 420 degrees for several seconds to several minutes. can do.

  Next, the optical element 1 of the optical isolator 10 and the configuration thereof will be described.

  The transmission polarization direction of the polarizer 3 is set in a direction parallel to one side (referred to as a reference side) parallel to the upper surface of the mounting substrate 6, and the transmission polarization direction of the other polarizer 4 is The angle is set to 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 7 are arranged on both sides of the Faraday rotator 2.

  As a material of the magnet 7, for example, a material made of samarium cobalt is suitable. The magnets 7 are arranged on both sides of the optical element 1, and the magnetic poles are arranged in the magnet 7 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.

Here, the thermal expansion coefficient of the Faraday rotator 2 is about 10 × 10 ⁻⁶ / ° C., the thermal expansion coefficient of the polarizing glass is about 6.5 × 10 ⁻⁶ / ° C., and the thermal expansion coefficient of the magnet 7. Is about 13 × 10 ⁻⁶ / ° C., and the present invention has been conceived in order to firmly mount these members having different thermal expansion coefficients on the common mounting board 6 and obtain desired optical characteristics. is there. That is, in the configuration of the present invention, since the bonding agents 5a and 5b formed on the mounting substrate 6 are not in contact with each other, the tensile stress due to the magnet 7 is not applied to the optical element 1, particularly the Faraday rotator 2, and the good characteristics are obtained. Invented to show.

  Here, in order to obtain a stronger bonding state, it is better to use a low-melting glass having an optimum thermal expansion coefficient. The stress generated in the Faraday rotator 2 and the bonding agent 5 can be caused not only by pulling by the magnet 7 via the bonding agent 5 but also by thermal contraction of the bonding agent 5 itself. When using low-melting-point glass, the material properties are weak against tensile stress, and when tensile stress is applied, the glass breaks and leads to deterioration of joint strength. It is desirable from the viewpoint of bonding strength that a low melting point glass having a thermal expansion coefficient equal to or smaller than that of a member having a low thermal expansion coefficient is interposed. Therefore, the Faraday rotator 2 and the mounting substrate 6 are joined with a low melting point glass having a thermal expansion coefficient equal to or lower than that of the Faraday rotator 2 or the mounting substrate 6 that has a smaller thermal expansion coefficient. It is desirable to do. As will be described later, this is also preferable from the viewpoint of residual stress on the Faraday rotator that affects the characteristics of the optical isolator. In addition, it is desirable to interpose a low-melting glass having a thermal expansion coefficient equal to or lower than the smaller one of the magnet 7 and the mounting board 6 in the bonding of the magnet 7 and the mounting board 6. As a result of such selection, low melting point glass having different material composition may be used for the bonding agent 5a and the bonding agent 5b.

  FIG. 2 is a sectional view for explaining the effect of the present invention.

  FIG. 2 is a cross-sectional view taken along the line AA through the magnet and the Faraday rotator shown in FIG. 1. FIG. 2A shows the bonding agent 5 formed on the entire surface of the mounting substrate 6, and the magnet 7 and the Faraday rotator. 2 shows a state where both are fixed by the bonding agent 5. FIG. 2B shows a state in which the bonding agents 5a and 5b are formed in different regions on the mounting substrate 6 without contacting each other, the magnet 7 is fixed by the bonding agent 5a, and the optical element 1 is fixed by the bonding agent 5b. Show. Since glass is weak against tensile stress, the thermal expansion coefficient and the fixing temperature are optimally set so that the stress with the bonding partner does not exceed the allowable range.

  The object expands with increasing temperature and contracts with decreasing temperature. The amount of shrinkage per unit length caused by a change in temperature of 1 ° C. is called a thermal expansion coefficient, and this is represented by α. The stress at each joint begins to occur near the fixing temperature (approximately equal to the glass transition temperature Tg) of the bonding agents 5, 5a, 5b, and the residual stress increases as the temperature decreases from here. The arrows in the magnet 7 and the Faraday rotator 2 in the figure indicate the direction in which each member contracts when the temperature drops.

  Further, it has been found that the extinction ratio of the Faraday rotator 2 among the optical elements 1 is greatly reduced by the residual stress, particularly the tensile stress. The extinction ratio of the Faraday rotator 2 is a ratio of power indicating how much of the incident linearly polarized light rotates while maintaining the linearly polarized light.

  From the above consideration, in the configuration of FIG. 2A, the magnet 7 having a large thermal expansion coefficient contracts greatly with the temperature drop from the fixing temperature of the bonding agent 5 to the normal temperature, and the bonding agent in the direction of arrow F in the figure. Since the Faraday rotator 2 is pulled through 5, there arises a problem that the extinction ratio of the Faraday rotator is lowered and cracks are generated in the low melting point glass 5. On the other hand, in FIG. 2B of the present invention, since the bonding agents 5a and 5b are separated, the Faraday rotator 2 is not easily affected by the contraction of the magnet 7. 2 and the bonding agent 5 are less likely to crack.

  Although this invention demonstrated the case where low melting glass was used as joining agent 5a, 5b, it is not restricted to this, For example, the joining agent hardened | cured at temperature higher than normal temperature, for example, a thermosetting type resin adhesive, solder, The same effect can be obtained for the joining with the brazing material. In particular, when the low melting point glass 5 described in the present embodiment is used as a bonding agent, pretreatment such as plating of the optical elements 2, 3, and 4 is unnecessary, and the number of man-hours can be reduced, or compared with bonding by resin. As a result, it is possible to achieve an effect such as realizing a very firm fixation, eliminating characteristic deterioration due to high temperature and high humidity, and realizing a highly reliable optical isolator.

  FIG. 3 is a perspective view showing a second embodiment of the optical isolator of the present invention.

  A groove 8 is formed in the mounting substrate 6 between the bonding agents 5a and 5b formed on the mounting substrate 6 so as not to contact each other. The groove 8 is formed across the upper surface of the mounting substrate 6. The optical element 1 is bonded to the plane surrounded by the two grooves 8, and the magnet is fixed to the two planes outside the two grooves 8, 8. Thereby, there exists an effect which prevents reliably the defect by the contact which tends to generate | occur | produce especially at the time of baking of bonding agent 5a, 5b. This embodiment also has the same effect as the first embodiment, and has a configuration in which the optical element 1 is more difficult to receive tensile stress from the magnet 7.

  As described above, according to the configuration of the present invention, in the optical isolator configured to bond the magnet 7 and the optical element 1 to the common mounting substrate 6 at a high temperature using a bonding agent, the magnet 7 The optical element 1 is bonded with the bonding agent 5b with the bonding agent 5a, and the bonding agent 5a and the bonding agent 5b are configured not to contact each other with certainty, so that the stress on the optical element 1 can be reduced. The problems of device characteristic deterioration, cracks, and dropout can be solved.

As an example of the present invention, an optical isolator C according to the present invention, in which a bonding agent between a magnet and a mounting substrate and an bonding element between an optical element and a mounting substrate are made of materials having different coefficients of thermal expansion, was manufactured as a prototype, and FIG. The optical isolator B shown in FIG . 1 was compared with the bonding strength and characteristics. 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 applied to both surfaces of the polarizer and the Faraday rotator. The thermal expansion coefficient of the Faraday rotator is 10 × 10 ⁻⁶ / ° C.

For both optical isolators B and C , a zirconia substrate is used as a mounting substrate material, and a low-melting glass is coated on the upper surface in advance. The thermal expansion coefficient of zirconia 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 . In the optical isolator B, one kind of low-melting glass having a thermal expansion coefficient of 8 × 10 ⁻⁶ / ° C. was used for bonding the magnet and the mounting substrate and bonding the optical element and the mounting substrate . The optical isolator C is slightly different from the zirconia of the mounting board for bonding the magnet and the mounting board.
A low-melting glass with a low thermal expansion coefficient of 9.5 × 10 ⁻⁶ / ° C. is used, and the optical expansion coefficient of the polarizer with a small thermal expansion coefficient is 6.5 × among the optical elements for bonding the optical element and the mounting substrate. using 10 ^-6 / ° C. some thermal expansion coefficient than the smaller 6.0 × ^{10 -6} / ℃ low melting point glass, using two kinds of low melting point glass or more.

The mounting substrate has a width W = 3 mm, a length D = 1.5 mm, and a thickness t = 0.3 mm. The mounting substrate B used for the optical isolator B has a width W = 1 mm and a length D = A first low melting glass of 1.5 mm is applied, a second low melting glass of width W = 0.8 mm and length D = 1.5 mm is applied to both sides of the first low melting glass region, One low melting glass and the second low melting glass are separated by a width of about 0.2 mm. Two 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.

The optical isolators B and C have the same trial production conditions, and the reference side of each optical element of the polarizer, the Faraday rotator, and the polarizer, and the magnet are bonded to the mounting substrate through a low melting point glass. The bonding was performed at a melting temperature of 380 ° C. for 1 minute for the low-melting glass.

Table 1 shows the isolation characteristics of five prototype optical isolators, the compression shear strength of the magnet and the mounting substrate, the compression shear strength of the optical element and the mounting substrate, and the average value thereof. The compression direction was the direction of the optical axis Z.

The isolation characteristic of the optical isolator C is inferior to that of the optical isolator B by about 4 dB, but clears a general isolation characteristic of 35 dB or more. In terms of bonding strength, the bonding strength of both the magnet and the optical element is 3.6 to 4.2 N / mm on average with respect to the optical isolator B. ² It was confirmed that the bonding strength was improved by the present invention.

  By the above trial manufacture, it is possible to provide an optical isolator having stable optical characteristics, easy assembly, less man-hours, and excellent in reliability without dropping, cracking, and characteristic deterioration of optical elements.

Numerical analysis of thermal stress of optical isolator was performed. As analysis conditions, the thermal expansion coefficient of the Faraday rotator 2 is 10.0 × 10 ⁻⁶ (1 / ° C.), the thermal expansion coefficients of the polarizers 3 and 4 are 6.34 × 10 ⁻⁶ (1 / ° C.), The thermal expansion coefficient of the magnet 5 is 6.5 × 10 ⁻⁶ (1 / ° C.) in the optical axis direction, 13.0 × 10 ⁻⁶ (1 / ° C.) in the direction perpendicular to the optical axis, and the thermal expansion coefficient of the bonding agent 5. Of 9.0 × 10 ⁻⁶ (1 / ° C.) and 6.0 × 10 ⁻⁶ (1 / ° C.), the melting point of the bonding agent 5 is 380 ° C., and the heat when the temperature is lowered to 20 ° C. I saw the stress. The material of the mounting substrate 6 at this time was alumina, and its thermal expansion coefficient was 7.1 × 10 ⁻⁶ (1 / ° C.).

As an analysis pattern, the optical isolator of FIG. 2 (a) has a thermal expansion coefficient of 9.0 × 10 ⁻⁶ (1 / ° C.) (Pattern 1), and the optical isolator of FIG. 2 (a). The thermal expansion coefficient of the agent 5 is 6.0 × 10 ⁻⁶ (1 / ° C.) (pattern 2), and the thermal expansion coefficient of the bonding agent 5 is 9.0 × 10 ⁻⁶ (1 in the optical isolator of FIG. 2B). / ° C.) (Pattern 3) and 4 patterns with a thermal expansion coefficient of 6.0 × 10 ⁻⁶ (1 / ° C.) (Pattern 4) of the bonding agent 5 were analyzed using the optical isolator of FIG. The analysis results are shown in Table 2. FIG. 4 shows the stress distribution for the patterns 1 and 2.

From the above analysis, it was found that the stress on the Faraday rotator is lower when the thermal expansion coefficient of the bonding agent 5 is lower. Also in this case, it was confirmed that the separation of the bonding agents 5a and 5b shown in FIG. 2B is effective in reducing the stress of the Faraday rotator 2 and effective in cracking and deterioration of characteristics.
[Brief description of drawings]

It is a perspective view which shows embodiment of the optical isolator of this invention. It is sectional drawing explaining the effect of this invention. It is a perspective view which shows 2nd Embodiment of the optical isolator of this invention. It is a perspective view of stress distribution of a stress analysis result. It is a figure which shows the structure of the conventional optical isolator miniaturized.

Explanation of symbols

1, 2: Optical elements 10, 11, 15: Optical isolator 2, 16: Faraday rotator 3, 4, 17, 18: Polarizer 5: Bonding agent 6, 20: Mounting substrate 7, 19, 22: Magnet 8: groove

Claims (3)

In an optical isolator in which an optical element including a Faraday rotator and a polarizer and a magnet are integrated on a flat mounting board upper surface via a bonding agent, the bonding agent is bonded to the optical element on the mounting board upper surface. Between the optical element and the mounting substrate, the heat of the Faraday rotator, the polarizer, and the mounting substrate, which is smaller than the thermal expansion coefficient, whichever is smaller. A bonding agent having an expansion coefficient is interposed, and a bonding agent having a thermal expansion coefficient equal to or lower than the smaller one of the magnet and the mounting board is interposed between the magnet and the mounting board. An optical isolator characterized by comprising: Claim 1 Symbol placement of the optical isolator, wherein said bonding agent is made of a low-melting-point glass. The optical isolator according to claim 1 or 2 , wherein a groove is formed between a bonding region of the optical element of the mounting substrate and a bonding region of the magnet of the mounting substrate.

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