JP4491530B2 - Optical isolator - Google Patents

Description

  The present invention relates to an optical isolator suitable as a passive optical device such as an optical communication module, a semiconductor laser module, and an optical amplifier mainly used in an optical communication system.

  An optical isolator is one of the main devices in an optical communication system using an optical fiber. In an optical communication module, a semiconductor laser module, an optical amplifier, etc., the optical isolator returns to the laser diode light source side by reflection from optical components in the optical path system. It is used for preventing light and preventing the occurrence of light resonance inside the optical amplifier. Such an optical isolator is arranged between a laser diode light source and an optical fiber, or between two optical fibers, and includes two polarizers (one of which is also called an analyzer) and a Faraday rotator. Each optical element is combined and a magnet for saturation magnetization of the Faraday rotator is disposed in the vicinity of the Faraday rotator.

  The structure of a specific polarization-dependent optical isolator will be explained. For example, in the case of an optical isolator called a “single-stage type”, the two polarizers are arranged so that the relative angles of the polarization directions of the two polarizers differ from each other by about 45 degrees. The two Faraday rotators having a thickness of about 45 degrees at a predetermined wavelength in a saturation magnetic field are arranged between the polarizers in a saturated magnetic field. On the other hand, light in the reverse direction has a function of blocking it with high loss characteristics (reverse direction loss).

In recent years, there has been a strong demand for these optical isolators to be reduced in price by downsizing and mass production. As a measure to meet such demands, the number of assembly steps in manufacturing is reduced, or optical adjustment between optical elements is performed. Various means for reducing this work have been devised. As one of the measures, there has been proposed an optical isolator having a structure in which a plate-shaped polarizer, a Faraday rotator, and a rectangular parallelepiped magnet are installed on the surface of a plate-shaped mounting substrate (see, for example, Patent Document 1). .)
JP-A-10-227996 (page 2-4, FIG. 1)

  FIG. 5 is a perspective view showing an example of such an optical isolator 100. First, before assembling the optical isolator 100, a polarizer 1a and a 45-degree Faraday rotator 2 (hereinafter simply referred to as a Faraday rotator if necessary) so that the polarization direction is set in a predetermined direction. Each optical element of a polarizer 1b (also referred to as an analyzer) whose polarization direction is set to be 45 degrees different from that of the polarizer 1a in the direction of rotation of the Faraday rotator 2 is cut into a flat plate shape. Further, after forming a Cr film layer on the side surfaces (surfaces excluding the optical surface) of the polarizers 1a and 1b and the Faraday rotator 2 by a vacuum deposition method, an Au film layer is further formed thereon and soldered. A metal film layer is formed so that bonding can be performed.

  On the other hand, a metal film layer of Ni / Au is formed on one surface of the flat mounting substrate 4 and one surface of the rectangular magnets 3 and 3 by a wet method so that soldering is possible. Then, each optical element and the magnets 3 and 3 are bonded to the mounting substrate 4 in a reducing atmosphere at a constant temperature using a solder material. Thereafter, the magnets 3 and 3 are magnetized in parallel with the light incident direction in the optical isolator 100. The optical isolator 100 is configured as described above.

  In order to optically couple the optical isolator 100 with an optical component such as an optical fiber or a lens, as shown in FIG. 6, for example, as shown in FIG. 6, the optical isolator is installed as an example of an installation device. In general, the optical isolator 100 is joined to the base 5). As a joining method, a joining method is often used in which a solder material (not shown) is arranged between the mounting substrate 4 and the mounting base 5 and the solder material is melted and solidified.

  However, in order to solder the mounting substrate 4 and the mounting base 5 to each other, solder bonding is performed on the surface 4b of the mounting substrate opposite to the optical element mounting surface 4a of the mounting substrate 4 over the entire area on the surface 4b. When the metal film layer is formed and solder bonding is performed, there is a problem that the reverse loss of the optical isolator 100 is remarkably reduced after bonding. In particular, when the materials of the mounting board 4 and the mounting base 5 are different and the thermal expansion coefficient of the mounting base 5 is significantly smaller than the thermal expansion coefficient of the mounting base 4, there is a problem that the reverse loss is significantly reduced. there were.

  Accordingly, the present invention has been made in view of the above-described problems, and an object thereof is to join an optical isolator in which an optical element is installed on a surface of a mounting substrate, and the mounting substrate is joined to an installation device by soldering. However, by reducing the thermal stress strain applied to the Faraday rotator at the time of joining, it is possible to obtain a stable optical characteristic by suppressing the reduction in reverse loss, and to provide a high-quality and highly reliable optical isolator It is to be.

In order to solve the above-mentioned problems, an optical isolator according to claim 1 of the present invention is an optical isolator in which each optical element of two polarizers and one Faraday rotator is installed on the surface of a mounting substrate. In
Solder bonding is performed on the surface of the mounting substrate on the opposite side by forming a metal film layer that can be soldered only on a partial area on the surface of the mounting substrate on the side opposite to the optical element mounting surface of the mounting substrate A non-formed region that does not have a metal film layer capable of
Furthermore, the planar shape of the metal film layer is circular, and the center point thereof corresponds to the center position on the installation surface of the Faraday rotator with respect to the mounting substrate,
Further, a solder material is disposed so as to face the metal film layer and is installed in the optical isolator installation device, and the solder material is heated to be joined to the installation device. It is an optical isolator.

The optical isolator according to claim 2 of the present invention is characterized in that the mounting substrate is made of any one material of ferritic stainless steel, austenitic stainless steel, copper tungsten alloy, tungsten, and oxide ceramics. It is an optical isolator.

  According to the optical isolator of the present invention, by limiting the metal film layer that can be soldered to be formed on the surface of the mounting substrate opposite to the optical element mounting surface of the mounting substrate to a partial shape and region, Even if the material of the optical isolator mounting board and the installation device are different and there is a difference in the thermal expansion coefficient between them, the thermal stress generated in the Faraday rotator when soldering the optical isolator mounting board to the installation device Distortion can be reduced. Accordingly, it is possible to realize an optical isolator with suppressed optical loss and stable optical characteristics even after being joined to the installation apparatus.

  Further, the planar shape of the metal film layer that can be soldered and formed in a partial region is a circular shape, and the center point of the circular shape is made to correspond to the center position on the installation surface of the Faraday rotator with respect to the mounting substrate. The direction of the thermal stress applied to the Faraday rotator can be made equiangular in a concentric manner starting from the center position. Accordingly, since the thermal stress is equally applied to the Faraday rotator with an equal size in a circular shape in the same direction, it is possible to reduce the thermal stress distortion generated in the Faraday rotator.

  Furthermore, the material of the mounting substrate is any one of ferritic stainless steel, austenitic stainless steel, copper tungsten alloy, tungsten, and oxide ceramics, so that the thermal expansion coefficient of the mounting substrate is changed to the thermal expansion coefficient of each optical element. Since it can be approximated, it becomes possible to further reduce the thermal stress distortion at the time of soldering due to the difference in thermal expansion coefficient.

  Hereinafter, embodiments of an optical isolator according to the present invention will be described in detail with reference to FIGS. 1 is a perspective view showing the appearance of the optical isolator of the present invention, FIG. 2 is a plan view of the optical isolator when FIG. 1 is viewed from the direction of arrow U, and FIG. 3 is a mounting substrate for the optical isolator of FIG. FIG. 4 is a plan view showing an example of a planar shape of a metal film layer that can be soldered and formed on a surface opposite to the optical element mounting surface in FIG. 4, and FIG. 4 is a plan view of the metal film layer that can be soldered in FIG. It is a schematic perspective view which shows the relative positional relationship of a center point and the center position in the installation surface of the Faraday rotator with respect to an attachment board | substrate. The same parts as those of the conventional optical isolator 100 are denoted by the same reference numerals, and redundant description is omitted or simplified.

  A metal film layer is formed and formed on one surface 4a of the mounting substrate 4 so that solder bonding is possible. The region where the metal film layer is formed on the optical element installation surface 4a is at least as large as the region where the optical elements and the magnets 3 and 3 are installed.

  Next, a metal film layer (not shown) is formed using a sputtering method and a masking method so that solder bonding can be performed on the side surfaces (surfaces excluding the optical surface) of the polarizers 1a and 1b and the Faraday rotator 2. A solder layer (not shown) is formed thereon by vapor deposition.

  Polarizers 1a and 1b are flat glass polarizers, including a type in which dielectric particles are encapsulated in a glass substrate and a type in which dielectrics are laminated on a glass substrate, both of which are orthogonal to the polarization direction. For example, a “trade name polar core” manufactured by Corning Co., Ltd. can be used.

  On the other hand, the Faraday rotator 2 uses a single crystal plate such as a bismuth-substituted rare earth iron garnet manufactured by a liquid phase epitaxial growth method (LPE method), and the incident light is applied when a saturation magnetic field is applied in the light incident direction. In order to rotate the polarization direction of the light exactly 45 degrees around the optical axis, it is configured to have a predetermined thickness with respect to the light traveling direction.

  Then, on the surface 4a of the mounting substrate 4, one side of each side surface is arranged in the order of the polarizer 1a, the Faraday rotator 2, and the polarizer 1b in order from the light incident direction (the direction of arrow I in FIGS. 1 and 2). Each optical element is installed as a reference. At that time, the polarizers 1a and 1b are arranged so that their polarization directions are relatively different from each other by 45 degrees. Further, as shown in FIG. 2, each optical element is installed on the surface 4a so as to have a predetermined inclination angle θ with respect to the light incident direction. By installing each optical element obliquely so as to have an inclination angle θ, it is possible to prevent light reflected by the optical surface of each optical element from returning to the laser diode light source.

  Next, in a state where the optical elements are arranged on the surface 4a, the optical elements and the mounting substrate 4 are inserted into an electric furnace and heated to a temperature equal to or higher than the melting temperature of the solder layer in an inert gas. As a result, the solder layer formed on the side surface of each optical element is melted, and the mounting substrate 4 and each optical element are joined. While the solder layer is melted, a certain load is applied to each optical element. In order to prevent deterioration of the characteristics of the optical element due to thermal stress distortion that occurs at the joint between each optical element (particularly the Faraday rotator 2) and the mounting substrate 4 when the solder layer is melted and solidified, Both the optical element and the mounting substrate 4 are gradually cooled to room temperature. The inert gas is preferably nitrogen that is a reducing atmosphere.

  Next, a metal film layer is formed on one surface of the rectangular magnets 3 and 3 by a wet method, and a solder layer is further formed on the metal film layer. Thereafter, the magnets 3 and 3 are disposed on the surface 4a. Then, the solder layer is melted in the electric furnace, and the magnets 3 and 3 are joined to the mounting substrate 4. After the joining, the magnets 3 and 3 are magnetized in parallel with the light incident direction to obtain the optical isolator 1.

  As the magnets 3 and 3, for example, a samarium cobalt (Sm-Co) -based magnet, a neodymium (Nd-Fe-B) -based magnet, a ferrite magnet, or the like can be used. In particular, a samarium-cobalt magnet has high magnetic characteristics. Therefore, it is suitable for downsizing, and is optimal because it is excellent in temperature stability and oxidation resistance.

Moreover, as a material which comprises the attachment board | substrate 4, any one material of ferritic stainless steel, austenitic stainless steel, copper tungsten alloy, tungsten, and oxide ceramics is mentioned. In particular, it is desirable to use stainless steel SUS430 (magnetic material) which is a plastic material. Although SUS430 is not a non-magnetic material, it is not easily affected by a magnetic field and has a thermal expansion coefficient that is very close to that of each of the optical elements. Therefore, SUS430 is optimal as a substrate material for installing the optical elements. Also, stainless steel SUS304 (nonmagnetic material, thermal expansion coefficient: about 16 × 10 ⁻⁶ / ° C.), which is a plastic material having a relatively large thermal expansion coefficient, can be used. Further, tungsten steel, tungsten alloy (for example, Cu20W80) or the like is also suitable as a plastic material other than the stainless steel having a low thermal expansion coefficient. Further, oxide ceramics such as zirconia ceramics and alumina ceramics which are brittle materials may be used. In particular, zirconia ceramics have high toughness, high rigidity, high strength, and high weatherability, and are resistant to changes in the thermal environment. Furthermore, their thermal expansion coefficients (9.2 × 10 ^-6 / ° C) are polarizers 1a and 1b. This is desirable because it is an intermediate value of the thermal expansion coefficient of the Faraday rotator 2.

  The metal film layer is preferably composed of three layers of Cr / Pt / Au. The Cr layer formed in the first layer is formed in order to ensure adhesion with the optical element, and the Pt layer formed in the second layer is formed in order to prevent diffusion of the solder layer. The Au layer formed on the third layer promotes the wettability of the solder layer, forms a bonding layer for the solder layer, and enables stable soldering. Note that the metal film layer only needs to be able to ensure sufficient adhesion strength, so not only Cr / Pt / Au but also a metal film layer of another configuration such as Cr / Ni / Au or Ti / Pt / Au may be used. good. Alternatively, a Ni / Au metal film layer may be used.

  Further, for example, Au / Sn can be used for the solder layer, and the Au / Sn film formed by vapor deposition is Au / Sn eutectic solder (80% / 20%) which is a vapor deposition material. ) Melting point (280 degrees). The thickness is set to about 5 μm. The solder layer is not limited to Au / Sn, and a eutectic solder material having another composition such as Au / Ge may be used as a solder layer having a melting temperature higher than that of Au / Sn. In place of the solder layer, a solder stay may be melted and solidified on the metal film layer, and the optical elements and the magnets 3 and 3 may be bonded onto the surface 4a.

  The optical surfaces of the polarizers 1a and 1b and the Faraday rotator 2 prevent Fresnel reflection caused by the refractive index difference between the air layer interposed between the optical elements and the optical elements. In order to reduce optical insertion loss, it is desirable to apply a non-reflective coating (not shown) for the air layer.

  Next, on the surface 4b of the mounting substrate 4 opposite to the optical element installation surface 4a, as shown in FIG. 3, a metal film that can be soldered only in a partial region of the entire region of the surface 4b. The layer 6 is formed so that its planar shape is circular. Since the metal film layer 6 is not formed in the other region on the surface 4b, the region 4b remains as a non-formed region having no metal film layer that can be soldered.

  The center point Cl in the circular shape of the metal film layer 6 and the center position Cf on the installation surface 2a of the Faraday rotator 2 with respect to the mounting substrate 4 are positioned so as to be substantially on the same line in the vertical direction as shown in FIG. Corresponding.

  The optical isolator 1 is installed by disposing a solder material (not shown) between the mounting substrate 4 and the installation device (mounting base 5) as shown in FIG. 6 so as to face the metal film layer 6. It is installed in the equipment. Thereafter, the optical isolator 1 and the installation apparatus are locally heated, whereby a solder material (not shown) is heated and melted and solidified, and the optical isolator 1 is joined to the installation apparatus.

  As described above, the metal film layer 6 that can be soldered and formed on the surface 4b is not extended over the entire area of the surface 4b, but limited to a partial shape and area, so that the mounting board 4 and the metal substrate layer 6 are installed. Even if there is a difference in the coefficient of thermal expansion between the materials used for the installation device, it is possible to reduce the thermal stress distortion generated in the Faraday rotator 2 when the mounting substrate 4 is soldered to the installation device. Become. Accordingly, it is possible to realize an optical isolator with suppressed optical loss and stable optical characteristics even after being joined to the installation apparatus.

  Next, the optical isolator 1 according to the present invention shown in FIG. 1 and the conventional optical isolator 100 shown in FIG. 5 for comparison were manufactured, and the characteristics of the reverse loss were compared. The purpose of this embodiment is to compare the characteristics of the reverse loss of the optical isolator accompanying the change in the area of the metal film layer 6 region that can be soldered on the surface 4b of the mounting substrate 4. The only difference in configuration between the optical isolator 1 and the optical isolator 100 is the area change. Therefore, the presence or absence of the tilt angle θ, which is another difference, is matched to the configuration of the optical isolator 100.

First, the configuration of the optical isolator 1 will be described. Stainless steel SUS430 (coefficient of thermal expansion: about 10.4 × 10 ⁻⁶ / ° C.) was used as the mounting substrate 4 and a flat plate having a thickness of 0.25 mm was cut into a width of 3.5 mm and a depth of 2.0 mm. Then, a Ni / Au plated metal film layer capable of soldering is formed on the surface of the mounting substrate 4 by a wet method. At that time, the Ni / Au plated metal film layer is formed over the entire area on the optical element installation surface 4a, and on the surface 4b of the mounting substrate opposite to the one optical element installation surface 4a, φ1. Etching with a potassium cyanide solution using a masking method so that the Ni / Au plated metal film layer 6 is formed in a 7 mm circular shape and only the Ni plating remains on the same surface other than the φ1.7 mm circular shape. went.

  Next, two polarizers 1a and 1b (both 1.0 mm square and 0.2 mm thick) and 45 degrees Faraday rotator 2 (1.0 mm square and thick) whose polarization directions are set to be 45 degrees different from each other 0.45 mm) was prepared, and a Cr / Pt / Au metal film layer was formed on the side surfaces of these optical elements by using a masking method and a sputtering method in combination. In addition, rectangular parallelepiped magnets 3 and 3 are prepared, and a Ni / Au metal film layer is formed on one surface thereof by a wet method so that soldering is possible. Next, these optical elements and magnets 3 and 3 were arranged on the surface 4a and integrated, and then the magnets 3 and 3 were magnetized to produce the optical isolator 1.

  Meanwhile, the configuration of the conventional optical isolator 100 will be described. The difference in configuration between the optical isolator 100 and the optical isolator 1 is only the area of the Ni / Au plated metal film layer 6 that can be soldered on the surface 4b. The formation region of the Ni / Au plating metal film layer 6 on the surface 4b in the optical isolator 100 is assumed to extend over the entire region on the surface 4b. Since other configurations are the same, description thereof is omitted.

  Magnets 3 and 3 were samarium cobalt magnets having dimensions of 0.85 mm width × 2.0 mm depth × 1.4 mm height.

  For the optical isolator 1 and the optical isolator 100 manufactured as described above, first, the peak value (evaluation wavelength range 1550 nm ± 30 nm) of the reverse loss in the optical isolator alone before being bonded to the installation device was evaluated at room temperature.

  As a result, the peak value of the reverse loss was 43.0 dB in the optical isolator 1 according to the present invention, and 44.2 dB in the conventional optical isolator 100.

Next, after fixing two optical isolators to the mounting base 5 as an example of the installation apparatus as shown in FIG. 6, the peak value of each reverse loss was measured. First, as the mounting base 5, an Fe-Ni-Co alloy "trade name Kovar" (coefficient of thermal expansion: approx. 4.5 x ^10-6 / ° C) is cut into a flat plate with a width of 4.0mm x depth of 2.5mm x thickness of 0.25mm. Two pieces were prepared, and a metal film layer and a solder layer were formed on one surface 5 a of the mounting base 5. Then, after the optical isolator 1 or the optical isolator 100 was soldered on the one surface 5a, the peak value of the reverse loss was evaluated at room temperature in the evaluation wavelength range of 1550 nm ± 30 nm.

  As a result, the peak value of the reverse loss was 43.0 dB in the optical isolator 1 according to the present invention, and 32.3 dB in the conventional optical isolator 100.

  From the above, the optical isolator 1 according to the present invention is a device with a placement device made of a material having a thermal expansion coefficient different from that of the mounting substrate of the optical isolator by limiting the solder joint area with the placement device. It has been found that even if solder bonding is performed, the reverse loss is prevented from being reduced and the optical characteristics are stable.

  Further, in the optical isolator 1, the planar shape of the solderable metal film layer formed on the surface 4b is circular, and the center point Cl of the circular shape is attached to the mounting substrate 4 on the installation surface 2a of the Faraday rotator 2 Therefore, the direction of the thermal stress applied to the Faraday rotator 2 can be made concentric and equidistant from the center position Cf. Therefore, it was also found that the thermal stress was applied to the Faraday rotator evenly in the same direction and circularly in the same direction, so that the influence of the thermal stress strain accompanying the solder joint to the Faraday rotator 2 was not observed. .

  In the above-described embodiments and examples, the configuration of the single-stage type (two polarizers and one 45-degree Faraday rotator) optical isolator has been described as an example, but the present invention is limited to this configuration. The above implementation is possible even for a so-called 1.5-stage type (3 polarizers and 2 45 degree Faraday rotators) or a 2-stage type (4 polarizers and 2 Faraday rotators) optical isolator. The same effects as those of the embodiment and the embodiment can be obtained.

  By using the optical isolator of the present invention for passive optical devices such as optical communication modules, semiconductor laser modules, and optical amplifiers used in optical communication systems, it is possible to prevent return light to the laser diode light source, Generation of light resonance can be prevented.

The perspective view which shows the external appearance of the optical isolator of this invention. The top view of an optical isolator when FIG. 1 is seen from the arrow U direction. The top view which shows an example of the planar shape of the metal film layer which can be soldered and formed on the surface on the opposite side to the optical element installation surface in the attachment board | substrate of the optical isolator of FIG. FIG. 4 is a schematic perspective view showing a relative positional relationship between the center point of the metal film layer capable of soldering in FIG. 3 and the center position on the installation surface of the Faraday rotator with respect to the mounting substrate. The perspective view which shows an example of the conventional optical isolator. The perspective view which shows the joining state of an optical isolator and the apparatus for installation.

Explanation of symbols

1 Optical isolator
1a, 1b Polarizer 2 Faraday rotator 3 Magnet 4 Mounting board 5 Mounting base 6 Metal film layer that can be soldered

Claims (2)

In an optical isolator in which each optical element of two polarizers and one Faraday rotator is installed on the surface of a mounting substrate,
Solder bonding is performed on the surface of the mounting substrate on the opposite side by forming a metal film layer that can be soldered only on a partial area on the surface of the mounting substrate on the side opposite to the optical element mounting surface of the mounting substrate A non-formed region that does not have a metal film layer capable of
Furthermore, the planar shape of the metal film layer is circular, and the center point thereof corresponds to the center position on the installation surface of the Faraday rotator with respect to the mounting substrate,
Further, a solder material is disposed so as to face the metal film layer and is installed in the optical isolator installation device, and the solder material is heated to be joined to the installation device. An optical isolator. 2. The optical isolator according to claim 1 , wherein the mounting substrate is made of any one material of ferritic stainless steel, austenitic stainless steel, copper tungsten alloy, tungsten, and oxide ceramics.

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