Optical element

An aspect of the present invention is an optical element that combines light waves using a spatial optical system, including a housing section, an optical waveguide element, a combine section, and a collimator section is provided. The housing section has a storage space and is formed by joining two or more members together. The optical waveguide element is provided in the storage space and emits at least two light waves. The combine section is provided outside the storage space and combines the light waves emitted from the optical waveguide element outside the housing section using a spatial optical system. The collimator section is connected to the housing section, holds the combine section, and includes a light focusing section configured to focus a light wave combined by the combine section and an optical fiber to which the light wave focused by the light focusing section is introduced.

TECHNICAL FIELD

The present invention relates to an optical element that combines light waves using a spatial optical system.

BACKGROUND ART

In recent years, as an optical element that enables high-speed (100 Gbps or the like) and high-capacity communication, dual polarization-quadrature phase shift keying (DP-QPSK) described in Patent Literature No. 1 has been put into practical use. The above-described optical element includes a substrate on which two QPSK modulation units (having a waveguide structure called a nested structure) are formed in parallel, a light focusing element, a polarization combine element (Patent Literature Nos. 2 and 3), and the like. In addition, the above-described optical components are disposed in a housing section formed of a metal such as stainless steel. The housing section is made up of a box portion and a lid portion that covers the box portion and is air-tightly sealed.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In a case in which the housing section that stores the optical components is air-tightly sealed, generally, seam welding which is a type of electric resistance welding is used. In seam welding, joule heat which is generated near a portion being welded due to the electric conduction of the portion being welded using roller electrodes is used. During the seam welding, locally-generated joule heat creates a temperature distribution, the accompanying heat shrinkage warps the housing section of the optical element, and the adjusted positions of the optical components such as the substrate, the light-focusing element, and the polarization combine element disposed in the housing in advance are misaligned, which creates a concern that the characteristics may deteriorate. In addition, there is another concern that the same problem may be caused due to a temperature change in the ambient environment of the optical element.

Therefore, an object of the present invention is to provide an optical element of which the characteristics do not deteriorate even in a case in which the temperature changes.

Solution to Problem

According to an aspect of an optical element of the present invention, there is provided an optical element that combines light waves using a spatial optical system, including a housing section, an optical waveguide element, a combine section, and a collimator section. The housing section has a storage space therein and is formed by joining two or more members together. The optical waveguide element is provided in the storage space and emits at least two light waves. The combine section is provided outside the storage space and combines the light waves emitted from the optical waveguide element outside the housing section using a spatial optical system. The collimator section is connected to the housing section, holds the combine section, and includes light focusing section configured to focus a light wave combined by the combine section and an optical fiber to which the light wave focused by the light focusing section is introduced.

In the optical element, the optical waveguide element is provided in the storage space of the housing section and the combine section is disposed outside of the storage space. Therefore, it becomes possible to attach the combine section after two or more members configuring the housing section are joined together. Therefore, even in a case in which the members of the housing section are joined together using a method in which heat generated from seam welding is used, the combine section is combined after the members are joined together and thus there is no case in which the position misalignment of the combine section is caused due to the influence of heat generated during the joining of the members. In addition, since the combine section is provided outside of the storage space of the housing section in this configuration, it is possible to decrease the size of the housing section as much as the space of the combine section and the space required for attaching the combine section. Therefore, an increase in the length of the housing section is prevented and, even in a case in which heat is added to the housing section, the warping of the housing section can be suppressed. As described above, in the optical element, there are no cases in which the characteristics deteriorate even when the temperature changes.

In addition, in the optical element, it becomes possible to closely dispose the combine section and the collimator section along the same axis outside of the storage space of the housing section. Therefore, compared with a case in which the combine section is disposed in the storage space of the housing section and the collimator section is disposed outside of the storage space of the housing section, in the optical element, even in a case in which the temperature changes, it is possible to suppress the positions of the combine section and the collimator section being misaligned and to prevent the degradation of the characteristics.

The combine section is capable of including a polarization rotation element that rotates a polarization plane of at least one of the light waves emitted from the optical waveguide element and a polarization combine element that polarization-combines, from among the light waves emitted from the optical waveguide element, a light wave having a polarization plane not rotated by the polarization rotation element and a light wave having a polarization plane rotated by the polarization rotation element, or light waves having polarization planes rotated by the polarization rotation element.

In this case, the polarization rotation element and the polarization combine element are disposed outside of the storage space of the housing section and it becomes possible to further decrease the size of the housing section. In addition, since it becomes possible to further decrease the size of the housing section, even in a case in which the temperature changes, the warping of the housing section is further suppressed and the degradation of the characteristics can be suppressed.

The optical element is capable of further including a polarization rotation element that rotates a polarization plane of at least one of the light waves emitted from the optical waveguide element. In this case, the combine section polarization-combines, from among the light waves emitted from the optical waveguide element, a light wave having a polarization plane not rotated by the polarization rotation element and a light wave having a polarization plane rotated by the polarization rotation element, or light waves having polarization planes rotated by the polarization rotation element.

Therefore, since it is possible to hold the combine section using the collimator section and to dispose the polarization rotation element at an appropriate position, the degree of freedom for design improves.

It is preferable that the collimator section further includes a spacer section having at least two contact surfaces with which the combine section is brought into contact and the combine section is fixed to the contact surfaces of the spacer section. In this case, when the combine section is fixed to the contact surfaces of the spacer section with an adhesive, it becomes possible to ensure a wide adhesion area and it is possible to increase the adhesion strength. Therefore, it is possible to prevent the position misalignment and dropping of the combine section.

It is preferable that the collimator section further includes a restraining member that restrains a position misalignment of the combine section with respect to the contact surfaces of the spacer section. In this case, it is possible to more reliably fix the combine section to the spacer section using the restraining member.

The restraining member is preferably a leaf spring or a filler. In this case, the combine section can be easily and reliably fixed to the spacer section using the leaf spring or the filler.

It is preferable that the collimator section includes a first collimator section to which the combine section and the light focusing section are fixed and a second collimator section to which the optical fiber is fixed. In this case, it becomes possible to more accurately introduce the light waves focused by the light focusing section into the optical fiber by adjusting the positional relationship between the first collimator section and the second collimator section.

Advantageous Effects of Invention

According to the aspect of the present invention, there are no cases in which characteristics deteriorate even when the temperature changes.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described using a DP-QPSK modulator as an example with reference to the accompanying drawings.

As illustrated inFIG. 1, an optical modulator (optical element)1includes a modulation unit10, a combine section20, and a collimator section30. The modulation unit10outputs two light waves. Specifically, the modulation unit10includes a substrate (optical waveguide element)11, a housing section12, and a lens section13. A light wave is input to the substrate11through an optical fiber2. The substrate11is, for example, formed of a material having an electro-optic effect such as lithium niobate (LiNbO3). The substrate11modulates the light wave input from the optical fiber2. On the substrate11, for example, a waveguide structure in which two QPSK modulation units having an optical waveguide structure that is called a nested structure in which two Mach-Zehnder (hereinafter, expressed as “MZ”) optical waveguides are disposed in parallel and the respective MZ optical waveguides are embedded in a branch waveguide of a larger MZ optical waveguide are disposed in parallel is formed. When a driving signal is added to a modulation electrode formed on the substrate11, two modulated and multiplexed light waves are emitted from the substrate11. The configuration of the substrate11is not limited to a configuration in which two QPSK modulation units having a nested structure are formed in parallel and may be any configuration as long as two or more light waves are emitted. For example, a BPSK modulation unit which carries out binary modulation and includes one MZ optical waveguide may be employed or a modulation unit which carries out multilevel (binary or more) modulation may be employed.

The lens section13collimates the two light waves emitted from the substrate11and emits the light waves toward the combine section20. In more detail, two lens portions13athat respectively collimate the two light waves emitted from the substrate11are provided in a holding substrate13bof the lens section13. However, it is also possible to provide the lens portion13athat collimates one of the two light waves emitted from the substrate11and the lens portion13athat collimates the other light wave on separate substrates.

The housing section12includes a storage space X therein. The substrate11and the lens section13are provided in the storage space X of the housing section12. As illustrated inFIG. 2, the housing section12includes a box portion (member)12aand a lid portion (member)12b. The box portion12ahas a cuboid box shape with one open surface. The lid portion12bis disposed so as to cover the open portion of the box portion12a. The box portion12ais air-tightly sealed with the lid portion12b. As a material for the box portion12aand the lid portion12b, for example, it is possible to use a metal such as stainless steel. The box portion12aand the lid portion12bare joined together using a method in which heat generated from seam welding is used. While the housing section12formed of the box portion12aand the lid portion12bhas been described, the housing section12may also be formed by joining two or more members together through seam welding or the like.

In addition, in the box portion12a, a window12cfor guiding the light waves emitted from the substrate11outside of the housing section12is provided. As a material for the window12c, for example, sapphire can be used. The substrate11is fixed to the box portion12ausing, for example, an adhesive or the like. Meanwhile, it is also possible to make a portion with which the substrate11is brought into contact thicker than any other portion on a surface to which the substrate11is fixed and fix the substrate onto the portion having a greater thickness so as to be formed into a table shape.

As illustrated inFIGS. 1 to 3(a) and3(b), the combine section20includes a polarization rotation element21and a polarization combine element22. The polarization rotation element21rotates the polarization planes of the two light waves emitted from the substrate11and puts the two light waves into a state in which the polarization planes thereof are inclined to each other at 90 degrees. The polarization rotation element21may rotate the polarization plane of one of the two light waves emitted from the substrate11by 90 degrees or may rotate the polarization plane of one of the two light waves by 45 degrees and rotate the polarization plane of the other light wave by 45 degrees.

The polarization combine element22polarization-combines the two light waves the polarization planes of which are inclined to each other at 90 degrees by the polarization rotation element21together using a spatial optical system and emits the two light waves to the same light path. As the polarization combine element22, for example, yttrium•vanadate crystals can be used.

The combine section20is disposed in a first holder31of the collimator section30. Specifically, the combine section20is attached to a spacer section25fixed to the inner wall of the first holder31having a cylindrical shape. The spacer section25includes two contact surfaces25awith which the combine section20is brought into contact. The spacer section25is fixed to the first holder31using, for example, an adhesive or YAG welding. The combine section20is fixed to the contact surfaces25aof the spacer section25using, for example, an adhesive.

The collimator section30is connected to the housing section12and holds the combine section20. Specifically, the collimator section30includes the spacer section25, the first holder (first collimator section)31, a second holder (second collimator section)32, a light focusing lens (light focusing section)33, and an optical fiber3. The first holder31has a cylindrical shape. The combine section20is fixed to the inner wall of the first holder31through the spacer section25as described above and, furthermore, the light focusing lens33is directly fixed to the inner wall of the first holder31. As a material for the first holder31, for example, stainless steel can be used. The first holder31is fixed to the housing section12through YAG welding.

The light focusing lens33focuses the light waves combined by the polarization combine element22and introduces the light waves into the optical fiber3. The light focusing lens33is fixed to the first holder31by carrying out, for example, YAG welding on a frame portion provided on the outer circumference of the light focusing lens33and the first holder31.

A ferrule35and a fiber holder36are provided at the front end portion of the optical fiber3. The front end portion of the optical fiber3is inserted into a hole portion provided in the ferrule35. The ferrule35is held by the fiber holder36. As a material for the ferrule35, for example, zirconia can be used. In addition, as a material for the fiber holder36, for example, stainless steel can be used.

The second holder32has a cylindrical shape. As a material for the second holder32, for example, stainless steel can be used. Inside the second holder32, the fiber holder36of the optical fiber3is disposed. The fiber holder36is fixed to the second holder32through, for example, YAG welding. The first holder31and the second holder32are joined together through, for example, YAG welding.

Here, the order of combining the combine section20and the collimator section30to the housing section12will be described in detail. First, the combine section20and the light focusing lens33are attached to the inside of the first holder31. Next, the first holder31is brought into contact with the end portion of the housing section12on the light wave emission side and the position thereof is determined with respect to the housing section12so that the two light waves emitted from the substrate11are emitted from the center of the light focusing lens33through the combine section20. In addition, the first holder31is fixed to the housing section12through, for example, YAG welding.

Next, the front end portion of the optical fiber3inserted into the second holder32is brought into contact with the first holder31and the position of the optical fiber3is adjusted so that the front end portion of the optical fiber3is located at the focal position at which the light waves emitted from the light focusing lens33are coupled to the optical fiber3maximally. In this state, the fiber holder36and the second holder32are bonded to each other through, for example, YAG welding. Finally, the second holder32and the first holder31are joined together through, for example, YAG welding.

The present embodiment is configured as described above, the substrate11is provided in the storage space X of the housing section12, and the combine section20is disposed outside of the storage space X. Therefore, it becomes possible to attach the combine section20to the housing section12after the box portion12aand the lid portion12b, which configure the housing section12, are joined together. Therefore, even in a case in which the box portion12aand the lid portion12bare joined together by a method in which heat generated from seam welding is used, there is no case in which the position misalignment of the combine section20is caused due to the influence of heat generated during the joining of the box portion12aand the lid portion12bby attachment of the combine section20to the housing section12after the box portion12aand the lid portion12bare joined together. In addition, since the combine section20is provided outside of the storage space X of the housing section12in this configuration, it is possible to decrease the size of the housing section12as much as the space of the combine section20and the space required for attaching the combine section20. Therefore, an increase in the length of the housing section12is prevented and, even in a case in which heat is added to the housing section12, the warping of the housing section can be suppressed. As described above, in the optical modulator1, there are no cases in which the characteristics deteriorate even when the temperature changes.

In addition, in the optical modulator1, it becomes possible to adjacently dispose the combine section20and the collimator section30along the same axis outside the storage space X of the housing section12. Therefore, compared with a case in which the combine section20is disposed in the storage space X of the housing section12and the collimator section30is disposed outside the storage space X of the housing section12, in the optical modulator1according to the present embodiment, even in a case in which the temperature changes, it is possible to suppress the positions of the combine section20and the collimator section30being misaligned and to prevent the degradation of the characteristics.

The combine section20includes the polarization rotation element21and the polarization combine element22. The polarization rotation element21and the polarization combine element22are disposed outside of the storage space X of the housing section12. As described above, when the polarization rotation element21and the polarization combine element22are disposed outside of the storage space X of the housing section12, it becomes possible to further decrease the size of the housing section12. In addition, since it becomes possible to further decrease the size of the housing section12, even in a case in which the temperature changes, the warping of the housing section12is further suppressed and the degradation of the characteristics can be suppressed. Furthermore, in the case of the configuration of the related art in which the polarization rotation element21and the polarization combine element22are provided in the housing section, the thickness of the bottom part of the housing section12outside of the surface to which the substrate11is fixed becomes thin in order to ensure the space for disposing the polarization rotation element21and the polarization combine element22. As a result, in the portion of the bottom part having a thin thickness (thin portion) in the configuration of the related art, the influence of the temperature change becomes significant; however, in the present embodiment, the influence of the temperature change is also suppressed.

The collimator section30includes the first holder31to which the combine section20and the light focusing lens33are fixed and the second holder32to which the optical fiber3is fixed. Therefore, it becomes possible to more accurately introduce the light waves focused by the light focusing lens33into the optical fiber3by adjusting the positional relationship between the first holder31and the second holder32.

In the spacer section25to which the combine section20is attached, the two contact surfaces25awith which the combine section20is brought into contact are provided. Therefore, in a case in which the combine section20is fixed to the contact surfaces25aof the spacer section25through adhesion, it becomes possible to ensure a wide adhesion area and it is possible to increase the adhesion strength. Therefore, it is possible to prevent the position misalignment and dropping of the combine section20.

For example, the housing section12, the first holder31, and the second holder32are formed of the same metallic material such as stainless steel and are joined together through YAG welding. In this case, since the housing section12, the first holder31, and the second holder32are formed of the same material, the linear expansion coefficients thereof become identical to each other and it is possible to produce the optical modulator1which is hard to be affected by thermal strain caused by a temperature change.

Next, modification examples of the fixing of the combine section20to the spacer section25will be described. First, a first modification example will be described. As illustrated inFIG. 4, in an optical modulator1A of the first modification example, similar to the embodiment described above, the combine section20is brought into contact with the two contact surfaces25aof the spacer section25and is fixed to the contact surfaces using an adhesive. Furthermore, a leaf spring (restraining member)40that presses the combine section20toward the contact surfaces25aand restrains the position misalignment of the combine section20with respect to the contact surfaces25ais provided in the first holder31. In this case, the combine section20can be easily and more reliably fixed to the spacer section25.

Next, a second modification example will be described. As illustrated inFIG. 5, in an optical modulator1B of the second modification example, similar to the embodiment described above, the combine section20is brought into contact with the two contact surfaces25aof the spacer section25and is fixed to the contact surfaces using an adhesive. Furthermore, in the first holder31, a gap between the spacer section25and the first holder31is filled with a filler (restraining member)50such as an optical sealing material. When the gap is filed with the filler50, the position misalignment of the combine section20with respect to the contact surfaces25ais restrained. As described above, the combine section20can be easily and more reliably fixed to the spacer section25.

Thus far, an embodiment and a variety of modification examples of the present invention have been described, but the present invention is not limited to the embodiment and the modification examples. For example, for the substrate11in the optical waveguide element used in the modulation unit10, it is also possible to use a semiconductor material having an electroabsorption effect or an organic material having an electro-optic effect. The combine section20includes the polarization rotation element21and the polarization combine element22, but it is also possible to combine the polarization rotation element21into the substrate11or provide the polarization rotation element in the housing section12. In this case, it is possible to dispose the polarization rotation element21at an appropriate position such as the substrate11and the degree of freedom for design improves.

Generally, the lens section13is fixed to the bottom surface of the storage space X of the housing section12, but it is also possible to fix the lens section13to the emission end section of the substrate11. In this case, it is possible to further decrease the thin portion in the bottom part of the housing so as to decrease the size of the optical modulator and to suppress the influence of a temperature change.

As the polarization combine element22in the combine section20, a polarization beam splitter (PBS) may be used. In this case, like the embodiment described above, it is possible to decrease the size of the polarization combine element22in the optical axis direction of light waves (to shorten the length of the element) compared with a case in which yttrium•vanadate crystals and the like are used as the polarization combine element22. Therefore, it is possible to further decrease the size of the optical modulator.

The combine section20has been described as a DP-QPSK modulator having a polarization multiplexing function obtained by using the polarization rotation element21and the polarization combine element22; however, in the case of a modulator employing a modulation method in which a polarization multiplexing function is not required, it is also possible to use other light combine elements in place of the polarization combine element22. In this case, the polarization rotation element21is not required.

The position misalignment of the combine section20is prevented using the leaf spring40or the filler50, but it is also possible to restrain the position misalignment of the combine section20using other restraining members.

The light focusing lens33is fixed to the first holder31, but it is also possible to fix the light focusing lens to the second holder32.

The above-described materials for the respective sections and the joining methods such as YAG welding are simple examples and the present invention is not limited to those exemplified. In addition, the optical modulator in which two light waves are emitted from the substrate11has been described, but it is also possible to use an optical modulator in which three or more light waves are emitted.

INDUSTRIAL APPLICABILITY

According to the aspect of the present invention, there are no cases in which the characteristics deteriorate even when the temperature changes.

REFERENCE SIGNS LIST

22polarization combine element

X storage space