Optical Module

Abstract

The optical module has an optical branching element and an optical mixing element, wherein the optical branching element polarizes and separates the signal light including the first polarized light and the second polarized light having the polarization plane orthogonal to the polarization plane of the first polarized light into the first polarized light and the second polarized light. The optical branching element includes a birefringent material. The optical branching element causes the first polarized light incident from the first end face to travel straight along the first direction and to be emitted from the second end face. The optical branching element causes the second polarized light incident from the first end face to travel straight along the second direction and to be emitted from the second end face. The optical branching element causes the local oscillation light incident from the first end face to travel straight along the first direction and to be emitted from between the exit point of the first polarized light and the exit point of the second polarized light in the second end face.

Description

[0001] This application claims priority based on Japanese application No. 2019-165572 filed on September 11, 2019, and all the contents described in the above Japanese application are cited. Technical Field

[0002] The invention relates to an optical module. Background Art

[0003] Japanese Patent Application Publication No. 2001-033604 discloses an optical component for controlling an optical path using a polarization separation prism, which is made of a material showing birefringence, i.e., a property in which the refractive index differs for light having a vibration plane parallel to a substrate and light having a vibration plane perpendicular to the substrate.

[0004] Japanese Patent Application Laid-Open No. 2002-107579 discloses a photosynthesis and demultiplexing module having a uniaxial birefringent crystal having an incident surface for injecting a first linear polarization light and a second linear polarization light and an output surface for emitting the combined light obtained by combining the first linear polarization light and the second linear polarization light.

[0005] Japanese Patent Publication No. 63-019928 discloses a double-balanced polarization diversity receiving device in a coherent optical communication method. The receiving device includes: an optical mixer that mixes received light and local oscillation light; a polarization beam splitter that separates the mixed output light into orthogonal polarization components; and a light receiving element into which the separated polarization component light is incident. The polarization beam splitter is composed of, for example, a birefringent crystal. Summary of the invention

[0006] The present invention provides an optical module. The optical module has an optical branching element and an optical mixing element. The optical branching element contains a birefringent material and has a first end face and a second end face. Signal light including a first polarized light and a second polarized light having a polarization plane orthogonal to the polarization plane of the first polarized light is incident on the first end face. The second end face emits the first polarized light and the second polarized light respectively. The optical branching element causes the first polarized light incident from the first end face to travel straight along the first direction and be emitted from the second end face. The optical branching element causes the second polarized light incident from the first end face to travel straight along the second direction inclined relative to the first direction and be emitted from the second end face. The optical branching element causes the local oscillation light incident from the first end face with a polarization plane parallel to the polarization plane of the first polarized light to travel straight along the first direction and be emitted from between the exit point of the first polarized light and the exit point of the second polarized light in the second end face. The optical mixing element has a third end face arranged opposite to the second end face. In the third end face, the local oscillation light incident port optically coupled to the exit point of the local oscillation light of the second end face is arranged between the first incident port optically coupled to the exit point of the first polarized light of the second end face and the second incident port optically coupled to the exit point of the second polarized light of the second end face.

[0007] The present invention also provides other optical modules. The optical module has an optical combining element and an optical modulation element. The optical combining element contains a birefringent material and has a first end face and a second end face. The first end face emits a signal light including a first polarized light and a second polarized light having a polarization plane orthogonal to the polarization plane of the first polarized light. At the second end face, the first polarized light and the second polarized light are respectively incident to different incident points from each other. The optical combining element causes the first polarized light incident from the second end face to travel straight along the first direction and be emitted from the first end face. The optical combining element causes the second polarized light incident from the second end face to travel straight along the second direction inclined relative to the first direction and be emitted from a position in the first end face that overlaps with the first polarized light. The optical multiplexing element causes the non-modulated light incident from the first end face with a polarization plane parallel to the polarization plane of the first polarized light to travel straight along the first direction toward the second end face, and to be emitted from between the incident point of the first polarized light and the incident point of the second polarized light in the second end face. The optical modulation element generates the first polarized light and the second polarized light based on the linearly polarized non-modulated light. The optical modulation element has a third end face arranged opposite to the second end face. In the third end face, the non-modulated light incident port optically coupled to the exit point of the non-modulated light of the second end face is provided between the first exit port optically coupled to the incident point of the first polarized light of the second end face and the second exit port optically coupled to the incident point of the second polarized light of the second end face. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 It is a plan view schematically showing a light receiving module as the optical module according to the first embodiment.

[0009] Figure 2 yes Figure 1 An oblique view of the light branching element is shown.

[0010] Figure 3 This is a diagram for explaining polarization separation achieved by an optical branching element.

[0011] Figure 4 This is a diagram for explaining the optical path in a state where the mounting angle of the optical branching element is deviated.

[0012] Figure 5 This is a diagram for explaining the optical path in a state where the mounting angle of the optical branching element is deviated.

[0013] Figure 6 This is a graph showing the relationship between the rotation deviation caused by the mounting error of the light branching element and the parallel deviation of the beam interval of the polarized lights N1 and N2.

[0014] Figure 7 This is a graph showing the relationship between the parallel axis deviation of the beam interval between two signal lights and the coupling efficiency.

[0015] Figure 8 It is a plan view schematically showing a light modulation module as an optical module according to the second embodiment.

[0016] Fig. 9 yes Figure 8 An oblique view of the optical wave combining element shown.

[0017] Fig.10 This is a diagram for explaining the multiplexing achieved by the optical multiplexing element.

[0018] Fig.11 This is a diagram for explaining the optical path when the mounting angle of the optical multiplexing element is deviated.

[0019] Fig.12 This is a diagram for explaining the optical path when the mounting angle of the optical multiplexing element is deviated.

[0020] Fig.13 FIG. 1 is a schematic plan view showing a light receiving module as an optical module according to a comparative example.

[0021] Fig.14 This is a diagram for explaining the optical path in a state where a manufacturing error occurs in a comparative example.

[0022] Fig.15 It is a graph showing the relationship between manufacturing error and loss of coupling efficiency.

[0023] Fig.16 This is a diagram for explaining the optical path in a state where a manufacturing error occurs in a comparative example.

[0024] Fig.17 This is a diagram for explaining the optical path in a state where a mounting error occurs in a comparative example.

[0025] Fig.18 Graph showing the relationship between the rotation deviation due to the mounting error and the parallel deviation of the beam interval between two signal lights in a comparative example.

[0026] Fig.19 This is a graph showing the relationship between the parallel axis deviation of the beam interval between two signal lights and the coupling efficiency. DETAILED DESCRIPTION

[0027] In the optical receiving module that performs demodulation during coherent communication, the function of separating the polarization component lights of the signal light is required. In addition, in the optical modulation module that performs modulation during coherent communication, the function of combining the polarization component lights of the signal light is required. As elements for demodulation or modulation, semiconductor elements with InP, Si, etc. as the main constituent materials are sometimes used. In the above case, when separating or combining the polarization component lights, an optical filter using a dielectric multilayer film is usually used. At this time, the coupling optical system of the optical input part such as the optical fiber and the above-mentioned semiconductor element is composed of a spatial optical system including the above-mentioned optical filter and lens, etc. In the spatial optical system, the angle of the transmitted light becomes an important parameter of the coupling efficiency. In the coupling optical system of coherent communication, it is required to achieve high coupling efficiency and suppress the difference in coupling efficiency, that is, imbalance, between the two polarization component lights contained in the signal light.

[0028] As an optical component for separating or combining the polarization component lights of the signal light, a combiner / demultiplexer called a polarizing beam splitter (PBS) or a polarizing beam combiner (PBC) is sometimes used. The PBS or PBC is, for example, a glass component. A dielectric multilayer film is provided on the optical path of the signal light in the glass component. In the dielectric multilayer film, the PBS or PBC allows the polarization component light of one side to pass through and reflects the polarization component light of the other side, thereby separating or combining the two polarization component lights at an angle of 90 degrees. A reflector is often provided on the optical path of the polarization component light reflected from the above-mentioned glass component. In this case, the PBS or PBC further reflects the reflected polarization component light through the reflector, and adjusts the traveling direction of the polarization component light by, for example, bending it back 90 degrees.

[0029] In the optical components as described above, manufacturing errors or installation errors sometimes occur. In the above-mentioned PBS or PBC, for example, when manufacturing errors such as the deviation of the angle of the dielectric multilayer film and the deviation of the angle of the reflector are generated, the angle of the light to be transmitted is easily deviated from the design value. Therefore, the coupling efficiency difference between the two polarization component lights is easily affected by the manufacturing errors. On this basis, when installation errors such as the deviation of the installation angle of the glass component are generated, the beam spacing of the two lights to be transmitted is easily changed from the design value. Therefore, the loss of the coupling efficiency of each polarization component is easily affected by the installation error.

[0030] [Effects of the Invention]

[0031] According to one embodiment of the present invention, when separating or combining polarization components of signal light, the coupling efficiency of two polarization components can be made uniform and high coupling efficiency can be achieved.

[0032] [Description of Embodiments of the Invention]

[0033] First, the embodiments of the present invention are listed for description. An optical module involved in one embodiment has an optical branching element and an optical mixing element. The optical branching element includes a birefringent material and has a first end face and a second end face. Signal light including a first polarized light and a second polarized light having a polarization plane orthogonal to the polarization plane of the first polarized light is incident on the first end face. The second end face emits the first polarized light and the second polarized light respectively. The optical branching element causes the first polarized light incident from the first end face to travel straight along the first direction and be emitted from the second end face. The optical branching element causes the second polarized light incident from the first end face to travel straight along the second direction inclined relative to the first direction and be emitted from the second end face. The optical branching element causes the local oscillation light incident from the first end face with a polarization plane parallel to the polarization plane of the first polarized light to travel straight along the first direction and be emitted from between the exit point of the first polarized light and the exit point of the second polarized light in the second end face. The optical mixing element has a third end face arranged opposite to the second end face. In the third end face, a local oscillation light incident port optically coupled to an exit point of local oscillation light of the second end face is provided between a first incident port optically coupled to an exit point of first polarized light of the second end face and a second incident port optically coupled to an exit point of second polarized light of the second end face.

[0034] In the optical module, the optical branching element includes a birefringent material. Thus, the signal light is polarized and separated by using the difference in refraction angles of each polarization component when incident on the first end face of the optical branching element. The first polarized light and the second polarized light after polarization separation in the above manner are not reflected but emitted straight from the second end face. Therefore, each exit angle of the first polarized light and the second polarized light from the second end face becomes the same as the incident angle of the signal light to the first end face. Thus, even if a manufacturing error is assumed, it is avoided that only one of the angles of the first polarized light and the second polarized light deviates greatly from the design value. Therefore, it is possible to suppress the coupling efficiency difference between the first polarized light and the second polarized light. In addition, even if the installation angle of the optical branching element is assumed to be deviated, the beam interval of the emitted first polarized light and the second polarized light is not easy to change from the design value. Therefore, the deviation of the optical path of the first polarized light relative to the first incident port and the deviation of the optical path of the second polarized light relative to the second incident port are suppressed respectively. As described above, in the polarization separation of the signal light, the coupling efficiency of each of the first polarized light and the second polarized light can be made uniform, and high coupling efficiency can be achieved.

[0035] The optical module according to one embodiment may further include: a first collimating lens that emits the signal light as collimated light toward the first end face before the signal light enters the first end face; and a second collimating lens that emits the local oscillation light as collimated light toward the first end face before the local oscillation light enters the first end face. In this case, the optical branching element can transmit the signal light and the local oscillation light as parallel light.

[0036] In the optical module according to one embodiment, the light branching element may have a rectangular parallelepiped shape whose longitudinal direction is a direction perpendicular to the first end face.

[0037] In an optical module according to one embodiment, a polarization plane rotating material for rotating the polarization plane of the second polarized light by 90° may be provided in a region including an emission point of the second polarized light on the second end face. In this case, the polarization plane of the second polarized light passing through the optical branching element can be aligned with the polarization plane of the local oscillation light.

[0038] In the optical module according to one embodiment, the birefringent material may have uniaxiality.

[0039] In the optical module according to one embodiment, the birefringent material may include at least one of titanium dioxide, yttrium vanadate, lithium niobate, and calcium carbonate.

[0040] In the optical module according to one embodiment, the second end face may be parallel to the first end face.

[0041] The optical module involved in other embodiments has an optical combining element and an optical modulation element. The optical combining element includes a birefringent material and has a first end face and a second end face. The first end face emits a signal light including a first polarized light and a second polarized light having a polarization plane orthogonal to the polarization plane of the first polarized light. In the second end face, the first polarized light and the second polarized light are respectively incident to different incident points from each other. The optical combining element causes the first polarized light incident from the second end face to travel straight along the first direction and be emitted from the first end face. The optical combining element causes the second polarized light incident from the second end face to travel straight along the second direction inclined relative to the first direction and be emitted from a position overlapping with the first polarized light in the first end face. The optical combining element causes the unmodulated light incident from the first end face with a polarization plane parallel to the polarization plane of the first polarized light to travel straight along the first direction toward the second end face and be emitted from between the incident point of the first polarized light and the incident point of the second polarized light in the second end face. The light modulator generates the first polarized light and the second polarized light based on the linearly polarized non-modulated light. The light modulator has a third end face arranged opposite to the second end face. In the third end face, the non-modulated light incident port optically coupled to the non-modulated light exit point of the second end face is provided between the first exit port optically coupled to the first polarized light incident point of the second end face and the second exit port optically coupled to the second polarized light incident point of the second end face.

[0042] In the optical module, the optical multiplexing element includes a birefringent material. Thus, the first polarized light and the second polarized light incident on the second end face of the optical multiplexing element are refracted at different refraction angles by using the difference in refraction angles of each polarization component, and are multiplexed at the first end face. At this time, the first polarized light and the second polarized light incident from the second end face are not reflected but go straight, and are emitted from the first end face as signal light. Therefore, the emission angle of the signal light from the first end face becomes the same as the incident angles of the first polarized light and the second polarized light to the second end face. Thus, even if a manufacturing error is assumed, it is avoided that only one of the angles of the first polarized light and the second polarized light deviates greatly from the design value. Therefore, it is possible to suppress the coupling efficiency difference between the first polarized light and the second polarized light emitted as signal light. In addition, even if the installation angle of the optical multiplexing element is assumed to be deviated, the beam interval of the first polarized light and the second polarized light emitted as signal light is not easy to change from the design value. Therefore, the deviation of the optical paths of the first polarized light and the second polarized light with respect to the optical fiber etc. is suppressed. As described above, in the multiplexing of signal light, the coupling efficiency of the first polarized light and the second polarized light can be made uniform, and high coupling efficiency can be achieved.

[0043] The optical module involved in other embodiments may further include: a third collimating lens, which emits the non-modulated light as collimated light toward the first end face before the non-modulated light is incident on the first end face; a fourth collimating lens, which emits the first polarized light as collimated light toward the second end face before the first polarized light is incident on the second end face; and a fifth collimating lens, which emits the second polarized light as collimated light toward the second end face before the second polarized light is incident on the second end face. In this case, the optical multiplexing element can transmit the first polarized light, the second polarized light, and the non-modulated light as collimated light.

[0044] In an optical module according to another embodiment, the optical multiplexing element may have a rectangular parallelepiped shape whose longitudinal direction is a direction perpendicular to the first end face.

[0045] In the optical module according to other embodiments, a polarization plane rotating material for rotating the polarization plane of the second polarized light by 90° may be provided in the region of the second end face including the incident point of the second polarized light. In this case, the polarization plane of the second polarized light passing through the optical multiplexing element can be made orthogonal to the polarization plane of the first polarized light.

[0046] In an optical module according to another embodiment, the birefringent material may have uniaxiality.

[0047] In an optical module according to another embodiment, the birefringent material may include at least one of titanium dioxide, yttrium vanadate, lithium niobate, and calcium carbonate.

[0048] In the optical module according to another embodiment, the second end face may be parallel to the first end face.

[0049] [Details of the embodiments of the present invention]

[0050] Hereinafter, a specific example of the optical module of the present invention will be described with reference to the accompanying drawings. In addition, the present invention is not limited to these examples, but is shown by the claims, including the contents equivalent to the claims and all changes within the scope thereof. In the following description, the same reference numerals are marked for the same elements in the description of the accompanying drawings, and repeated descriptions are omitted.

[0051] [First embodiment]

[0052] Figure 1 It is a plan view schematically showing a light receiving module 1A as an optical module according to the first embodiment. Figure 1The optical receiving module 1A shown is an optical receiving module for coherent communication. The optical receiving module 1A causes interference between the phase-modulated signal light (Signal Beam) NA and the local oscillator light (Local Beam) LA, and demodulates the information contained in the signal light NA. The demodulated information is converted into an electrical signal and output to the outside of the optical receiving module 1A. The optical receiving module 1A has an optical input unit 10 and an optical mixing element 40. The optical receiving module 1A also has a coupling optical system that optically couples the optical input unit 10 and the optical mixing element 40. The signal light NA is polarization-separated in the coupling optical system and introduced into the optical mixing element 40.

[0053] The optical mixing element 40 is, for example, a semiconductor element whose main structural materials are InP, Si, etc. The optical mixing element 40 has an end face 43, two incident ports 41a, 41b, and one local oscillation light incident port 42. The incident port 41a, the local oscillation light incident port 42, and the incident port 41b are arranged in sequence on the end face 43. In other words, the local oscillation light incident port 42 is located between the incident port 41a and the incident port 41b. The end face 43 is an example of the third end face in the present invention. The incident port 41a is an example of the first incident port in the present invention. The incident port 41b is an example of the second incident port in the present invention. The interval between the incident port 41a and the incident port 41b is, for example, 500 μm. The polarized light N1 on one side after polarization separation is input to the incident port 41a, and the polarized light N2 on the other side after polarization separation is input to the incident port 41b. The polarized light N2 has a polarization plane orthogonal to the polarization plane of the polarized light N1. Polarized light N1 is an example of the first polarized light in the present invention. Polarized light N2 is an example of the second polarized light in the present invention. Local oscillation light LA ​​is input to the local oscillation light incident port 42. The optical mixing element 40 causes the polarized light N1 and the polarized light N2 to interfere with the local oscillation light LA. The optical mixing element 40 demodulates the information contained in the polarized light N1 and the polarized light N2 respectively through the local oscillation light LA. The optical receiving module 1A may be a coherent module further including a housing (not shown) for accommodating the optical input unit 10, the coupling optical system, and the optical mixing element 40.

[0054] The optical input unit 10 includes optical fibers 11, 12 and a lens array 13. The optical fiber 11 is a single-mode optical fiber and transmits a signal light NA. The signal light NA includes a first polarized light N1 and a second polarized light N2, which are two polarization components orthogonal to each other. In the present embodiment, the first polarized light N1 is P polarized and the second polarized light N2 is S polarized. The optical fiber 12 is a polarization-maintaining optical fiber and transmits a P-polarized local oscillation light LA. That is, the polarization plane of the local oscillation light LA ​​is parallel to the polarization plane of the first polarized light N1. The optical fibers 11 and 12 extend in the same direction as each other and are arranged in a direction intersecting the extending direction. The optical axis of the optical fiber 11 is parallel to the optical axis of the optical fiber 12.

[0055] The lens array 13 is located on one side of the extending direction of the optical fibers 11 and 12, and is opposite to both the end face of the optical fiber 11 and the end face of the optical fiber 12. The lens array 13 includes lenses 13a and 13b. The lenses 13a and 13b are arranged in the same direction as the arrangement direction of the optical fibers 11 and 12. The lens 13a is optically coupled to the optical fiber 11. The lens 13a is arranged on the optical path of the signal light NA output from the end face of the optical fiber 11, and emits the signal light NA as collimated light toward the end face 21 before the signal light NA is incident on the end face 21. The lens 13b is optically coupled to the optical fiber 12. The lens 13b is arranged on the optical path of the local oscillation light LA ​​output from the end face of the optical fiber 12, and emits the local oscillation light LA ​​as collimated light toward the end face 21 before the local oscillation light LA ​​is incident on the end face 21. The lens 13a is an example of the first collimating lens in the present invention. The lens 13b is an example of the second collimating lens in the present invention.

[0056] The coupling optical system includes an optical branching element 20 and a lens array 30. The optical branching element 20 is arranged at a position on the optical path of the signal light NA and on the optical path of the local oscillation light LA. The optical branching element 20 includes an end face 21 and an end face 22 located on the opposite side of the end face 21. The end face 21 is an example of the first end face in the present invention. The end face 22 is an example of the second end face in the present invention. The end face 22 is parallel to the end face 21. For example, an anti-reflection film is provided on each of the end faces 21 and 22. In the present embodiment, the shape of the optical branching element 20 is a rectangular parallelepiped extending in a direction orthogonal to the end faces 21 and 22 as a length direction. The end face 21 is opposite to the lens array 13. The lens 13a collimates the signal light NA toward the end face 21 and emits it, and the lens 13b collimates the local oscillation light LA ​​toward the end face 21 and emits it.

[0057] Figure 2 yes Figure 1 The optical branching element 20 is shown in a perspective view. The optical branching element 20 is arranged in a state where the normal line LN of the end face 21 is tilted at a small angle, such as 2 degrees, relative to the optical axis of the optical fiber 11, 12. Both the signal light NA and the local oscillation light LA ​​are incident on the end face 21 at the above-mentioned small angle of incidence.

[0058] Here, the light branching element 20 is composed of a birefringent material, and in one example, is composed only of a birefringent material. A birefringent material is a material having birefringence. As a birefringent material, a material having uniaxiality is used. As a specific example of a birefringent material, titanium dioxide (TiO 2 ), yttrium vanadate (YVO 4 ), lithium niobate (LiNbO 3 ), calcium carbonate (CaCo 3 ), potassium dihydrogen phosphate (KH 2 PO 4, abbreviation: KDP), quartz, etc. The birefringent material includes at least one of titanium dioxide, yttrium vanadate, lithium niobate, calcium carbonate, potassium dihydrogen phosphate and quartz. The optical branching element 20 utilizes the effect of the birefringent material to refract the incident signal light NA and the local oscillation light LA ​​at different refraction angles for each direction of the polarization plane and emit them. That is, the optical branching element 20 refracts the P polarization and the S polarization at different refraction angles. As a result, the optical branching element 20 polarizes and separates the signal light NA into polarized light N1 and polarized light N2.

[0059] Figure 3 2 is a diagram for explaining polarization separation achieved by the optical branching element 20. Figure 3 As shown, the optical branching element 20 refracts the P polarization in the signal light NA incident from the end face 21 as the polarization N1 at a refraction angle θ1. The optical branching element 20 causes the polarization N1 to go straight along the direction D1 and be emitted from the end face 22. The optical branching element 20 causes the S polarization in the signal light NA incident from the end face 21 to go straight as the polarization N2 at a refraction angle θ2. The refraction angle θ2 is different from the refraction angle θ1 by the branching angle δ. The optical branching element 20 causes the polarization N2 to go straight along the direction D2 inclined relative to the direction D1 and be emitted from the end face 22. The polarizations N1 and N2 are emitted from the end face 22 at the same output angle as the incident angle of the signal light NA to the end face 21. The direction D1 is an example of the first direction in the present invention. The direction D2 is an example of the second direction in the present invention.

[0060] Polarized lights N1 and N2 are emitted from the end face 22 respectively in a state of being branched separately with a branching amount P. The branching amount P is the same as the interval between the two incident ports 41a and 41b, for example, 500 μm. The length S1 of the optical branching element 20 in the longitudinal direction is set based on the branching amount P by the following equations (1) to (3). Equation (1) shows the relationship between the branching amount P and the length S1. Equation (2) shows the relationship between the angle θo formed by the crystal optical axis C of the optical branching element 20 and the P polarization, i.e., the normal light, and the angle θe formed by the crystal optical axis C of the optical branching element 20 and the S polarization, i.e., the extraordinary light. In equation (2), no is the refractive index of the normal light, and ne is the refractive index of the extraordinary light. Equation (3) shows the relationship between the branching angle δ of the polarized lights N1 and N2 and the angles θo and θe.

[0061] [Formula 1]

[0062] P=S1tanδ···(1)

[0063] [Formula 2]

[0064]

[0065] [Formula 3]

[0066] tanδ=tan(θ o -θ e )···(3)

[0067] exist Figure 3 . The angle θo and the angle θe are shown in FIG. When the angle θo is in the range of 47 to 49 degrees, the branching angle δ is maximum. Therefore, in this embodiment, as the light branching element 20, a structure in which the angle θo is in the range of 47 to 49 degrees, for example, 48 degrees, is used.

[0068] Refer again Figure 1 The light branching element 20 further causes the local oscillation light LA ​​incident from the end face 21 to be at the same angle as the polarized light N1, that is, Figure 3 The light branching element 20 makes the local oscillation light LA ​​to be refracted in the same direction as the polarized light N1, that is, Figure 3 The local oscillation light LA ​​goes straight in the direction D1 shown and is emitted from the end face 22. The local oscillation light LA ​​is emitted from the end face 22 at an emission angle that is the same as the incident angle to the end face 21. The local oscillation light LA ​​is emitted from between the emission point 20a of the polarized light N1 and the emission point 20b of the polarized light N2 in the end face 22. The emission point 20c of the local oscillation light LA ​​in the end face 22 is located, for example, at the midpoint between the emission point 20a and the emission point 20b.

[0069] A half-wavelength plate, i.e., a λ / 2 plate 24, is provided in the optical branching element 20. The λ / 2 plate 24 is an example of a polarization plane rotating material in the present embodiment. The λ / 2 plate 24 can be made of a flat birefringent material such as crystal. The λ / 2 plate 24 is provided in a portion of the end face 22 including the exit point 20b of the polarized light N2. This region is a region that does not include the exit points 20a and 20c of the polarized light N1 and the local oscillation light LA, respectively. The polarized light N2 passes through the λ / 2 plate 24 when it is emitted from the optical branching element 20. The λ / 2 plate 24 rotates the polarization plane of the polarized light N2 by 90 degrees. Therefore, the polarization plane of the polarized light N2 that has passed through the λ / 2 plate 24 coincides with the polarization plane of the polarized light N1 and the polarization plane of the local oscillation light LA.

[0070] The lens array 30 is disposed between the light branching element 20 and the light mixing element 40. One surface of the lens array 30 faces the end surface 22 of the light branching element 20, and the other surface of the lens array 30 faces the end surface 43 of the light mixing element 40. In other words, the end surface 22 and the end surface 43 face each other via the lens array 30.

[0071] The lens array 30 includes lenses 31, 32, and 33. The lenses 31, 32, and 33 are arranged in sequence in the same direction as the arrangement direction of the incident port 41a, the local oscillation light incident port 42, and the incident port 41b. The lens 31 is arranged on the optical path of the polarized light N1 emitted from the end face 22, and focuses the polarized light N1 on the incident port 41a. The lens 32 is arranged on the optical path of the local oscillation light LA ​​emitted from the end face 22, and focuses the local oscillation light LA ​​on the local oscillation light incident port 42. The lens 33 is arranged on the optical path of the polarized light N2 emitted from the end face 22, and focuses the polarized light N2 on the incident port 41b.

[0072] The incident port 41a is optically coupled to the exit point 20a of the polarized light N1 in the end face 22. The local oscillation light incident port 42 is optically coupled to the exit point 20c of the local oscillation light LA ​​in the end face 22. The incident port 41b is optically coupled to the exit point 20b of the polarized light N2 in the end face 22. The polarized light N1 is input to the optical mixing element 40 from the incident port 41a. The local oscillation light LA ​​is input to the optical mixing element 40 from the local oscillation light incident port 42. After the local oscillation light LA ​​is input to the optical mixing element 40, it is branched into two local oscillation lights. The polarized light N2 is input to the optical mixing element 40 from the incident port 41b. The optical mixing element 40 includes a multi-mode interference (MMI) waveguide and a photodiode optically coupled to the waveguide.

[0073] The MMI waveguide is a waveguide formed on, for example, an InP substrate. The MMI waveguide causes the polarized light N1 to optically interfere with the local light branched from the local light LA, and performs homodyne detection or heterodyne detection. Thus, the MMI waveguide separates the information contained in the polarized light N1 into a phase component that is consistent with the phase of the local light LA ​​and a phase component that differs from the phase of the local light LA ​​by 90 degrees, and performs demodulation. That is, the optical mixing element 40 demodulates two independent information regarding the polarized light N1. Similarly, the MMI waveguide causes the polarized light N2 to optically interfere with the local light branched from the local light LA, and performs homodyne detection or heterodyne detection. Thus, the MMI waveguide separates the information contained in the polarized light N2 into a phase component that is consistent with the phase of the local light LA ​​and a phase component that differs from the phase of the local light LA ​​by 90 degrees, and performs demodulation. That is, the optical mixing element 40 demodulates two independent information regarding the polarized light N2. The four independent information demodulated by the optical mixing element 40 are introduced to the outside of the optical receiving module 1A after signal processing.

[0074] The effects of the above-described light receiving module 1A will be described. First, a comparative example will be described. Fig.131 is a schematic top view showing an optical receiving module 1X as an optical module involved in a comparative example. The difference between the optical receiving module 1X and the optical receiving module 1A is that the optical branching element 20 is replaced with an optical branching element 20X. The optical branching element 20X includes an end face 21X opposite to the lens array 13 and an end face 22X opposite to the lens array 30. The end face 22X is parallel to the end face 21X. For example, an anti-reflection film is provided on each of the end faces 21X and 22X. In addition, a λ / 2 plate 24 is provided on the end face 22X, similarly to the optical branching element 20.

[0075] The optical branching element 20X is formed by bonding a triangular prism and a parallelogram prism, each made of glass. The end face 21X is a face including one of a pair of opposite sides in the parallelogram prism. The end face 22X is a face including the other of a pair of opposite sides in the parallelogram prism and one side of the triangular prism. The triangular prism is bonded to the face including one of the other pair of opposite sides in the parallelogram prism. In the parallelogram prism, a dielectric multilayer film 25X is provided on the bonding surface with the triangular prism. The dielectric multilayer film 25X allows P polarization to pass through and reflects S polarization. Thus, the optical branching element 20X polarizes and separates the polarized lights N1 and N2. If the signal light NA passes through the optical branching element 20X, the polarized light N1 goes straight and the polarized light N2 changes its optical path by 90 degrees. In the parallelogram prism, a reflective film 26X is provided on the surface parallel to the bonding surface of the triangular prism. As a result, the optical path of the polarized light N2 which is the S polarization reflected by the dielectric multilayer film 25X is further changed by 90 degrees and becomes parallel to the optical path of the polarized light N1.

[0076] Here, a case where a manufacturing error occurs in the light receiving module 1X will be described. Fig.14 and Fig.16 This is a diagram for explaining the optical path in a state where a manufacturing error occurs in a comparative example. Fig.14 and Fig.16 In FIG. 1 , the state where the manufacturing error is zero is indicated by a double-dashed line. Fig.14 As shown, for example, in the optical branching element 20X, a deviation angle Δθ1 may occur in the dielectric multilayer film 25X due to manufacturing errors. In such a case, since the dielectric multilayer film 25X and the reflective film 26X are not parallel, a deviation may occur in the optical path of the polarized light N2. As a result, the coupling efficiency in the polarized light N2 is reduced. Therefore, a difference in coupling efficiency occurs between the polarized lights N1 and N2.

[0077] Fig.15 is a graph showing the relationship between manufacturing error and loss in coupling efficiency. Fig.15In the graph, the horizontal axis shows the deviation angle Δθ1, and the vertical axis shows the loss of coupling efficiency of polarized light N2. Usually, the allowable value of coupling efficiency loss is about 0.5 dB. Therefore, Fig.15 As shown, as the manufacturing accuracy of the light branching element 20X, it is required to set the deviation angle Δθ1 to be within the range of greater than or equal to -0.05 degrees and less than or equal to 0.05 degrees.

[0078] like Fig.16 As shown, for example, in the optical branching element 20X, sometimes due to manufacturing errors, a deviation angle Δθ2 is generated in the surface formed by the parallelogram prism in the end face 22X. Or, sometimes a deviation angle Δθ3 is generated in the surface formed by the triangular prism in the end face 22X. In such a case, since the end face 21X, i.e., the incident surface, and the end face 22X, i.e., the exit surface, are not parallel, the optical path of at least one of the polarized lights N1 and N2 is deviated. As a result, the coupling efficiency of at least one of the polarized lights N1 and N2 is reduced. Therefore, a difference in coupling efficiency occurs between the polarized lights N1 and N2.

[0079] Fig.17 This is a diagram for explaining the optical path in a state where an installation error occurs in a comparative example. Fig.17 In FIG. 1 , the state where the installation error is zero is indicated by the double-dotted line. Fig.17 As shown, for example, in the optical branching element 20X, a rotation deviation Δθ4 may be generated due to an installation error. In such a case, an axial deviation ΔX is generated between the polarized light N1 and the incident port 41a, and an axial deviation ΔY is generated between the polarized light N2 and the incident port 41b. In addition, the beam interval of the polarized light N1 and N2 is enlarged by a value obtained by adding the axial deviation ΔX and the axial deviation ΔY compared to the interval between the incident ports 41a and 41b. In other words, a parallel deviation (ΔX+ΔY) of a magnitude obtained by adding the axial deviation ΔX and the axial deviation ΔY is generated. Therefore, when the parallel deviation (ΔX+ΔY) is equally distributed with respect to the polarized light N1 and N2, a parallel axial deviation (ΔX+ΔY) / 2 is generated between the polarized light N1 and the incident port 41a and between the polarized light N2 and the incident port 41b.

[0080] Fig.18 Graph showing the relationship between the rotation deviation Δθ4 caused by the mounting error in the comparative example and the parallel deviation (ΔX+ΔY) of the beam interval of the polarized lights N1 and N2. Fig.19 This is a graph showing the relationship between the parallel axis deviation (ΔX+ΔY) / 2 of the beam spacing of polarized light N1 and N2 and the coupling efficiency. Fig.18 In the graph, the horizontal axis shows the magnitude of the rotation deviation Δθ4, and the vertical axis shows the magnitude of the parallel deviation (ΔX+ΔY). Fig.19In the graph, the horizontal axis shows the magnitude of the parallel axis deviation (ΔX+ΔY) / 2 between the polarized light N1, N2 and each incident port 41a, 41b, and the vertical axis shows the magnitude of the loss of the coupling efficiency of the polarized light N1, N2. Fig.18 As shown, in the light branching element 20X, for example, when the rotation deviation Δθ4 is 5 degrees, as shown in FIG. Fig.18 As shown by the dotted line in the graph, the parallel deviation (ΔX+ΔY) becomes about 80 μm. In this case, the parallel axis deviation (ΔX+ΔY) / 2 between the polarized light N1, N2 and each incident port 41a, 41b becomes about 40 μm, so Fig.19 As shown by the dotted line in the graph, it can be seen that the coupling efficiency is reduced by about 0.6 dB. In other words, the light receiving sensitivity of each incident port 41a, 41b to the polarized light N1, N2 is reduced by about 0.6 dB.

[0081] In contrast, in the optical receiving module 1A according to the first embodiment, the optical branching element 20 includes a birefringent material. As a result, the signal light NA is polarized and separated using the difference in refraction angles of each polarization component when incident on the end face 21 of the optical branching element 20. The polarized lights N1 and N2 separated in the above manner travel without being reflected and are emitted from the end face 22. Therefore, the emission angles of the polarized lights N1 and N2 from the end face 22 are the same as the incident angle of the signal light NA to the end face 21. Therefore, even if the end faces 21 and 22 are not parallel due to manufacturing errors, when emitted from the end face 22, the optical paths of the polarized light N1 and the polarized light N2 will deviate from the design values ​​at the same angle. In addition, even if the angle θo formed by the crystal optical axis C and the traveling direction of the P polarization deviates from the above range due to other manufacturing errors, when emitted from the end face 22, the optical path of the polarized light N1 and the optical path of the polarized light N2 become parallel to each other, and only the emission points 20a and 20b at the end face 22 deviate from the design value. Thus, it is avoided that only one of the angles of the polarized light N1 and the polarized light N2 deviates greatly from the design value. Therefore, it is possible to suppress the occurrence of a difference in coupling efficiency between the polarized lights N1 and N2.

[0082] in addition, Figure 4 and Figure 5 This is a diagram for explaining the optical path in a state where the mounting angle of the optical branching element 20 is deviated. Figure 4 In FIG. 1 , the state where the installation error is zero is indicated by the double-dotted line. Figure 4As shown in FIG. 1 , it is assumed that a rotation deviation Δθ4 is generated in the light branching element 20 due to an installation error. In this case, an axial deviation ΔX is generated between the polarized light N1 and the incident port 41a, and an axial deviation ΔY is generated between the polarized light N2 and the incident port 41b. Even in the above case, the beam interval of the polarized light N1 and N2 is unlikely to change from the designed value. Figure 5 As shown, the difference between the axis deviation ΔX and the axis deviation ΔY, that is, the parallel deviation Δa, which is the difference between the axis deviation ΔX and the axis deviation ΔY, is slightly generated as the variation of the beam interval of the polarized lights N1 and N2 from the designed value. Figure 5 The diagram shows a state in which the parallel deviation Δa is equally distributed relative to the polarized lights N1 and N2 by adjusting the optical axes of the optical branching element 20 and the optical mixing element 40, that is, a state in which a parallel axis deviation Δa / 2 is generated between the polarized light N1 and the incident port 41a and between the polarized light N2 and the incident port 41b.

[0083] Figure 6 4 is a graph showing the relationship between the rotation deviation Δθ4 caused by the mounting error of the light branching element 20 and the parallel deviation Δa of the beam interval between the polarized lights N1 and N2. Figure 7 This is a graph showing the relationship between the parallel axis deviation Δa / 2 of the beam spacing of polarized light N1 and N2 and the coupling efficiency. Figure 6 In the graph of , the horizontal axis shows the magnitude of the rotation deviation Δθ4, and the vertical axis shows the magnitude of the parallel deviation Δa. Figure 7 In the graph of FIG. 1 , the horizontal axis shows the magnitude of the parallel axis deviation Δa / 2 between the polarized light N1, N2 and each incident port 41a, 41b, and the vertical axis shows the magnitude of the loss of the coupling efficiency of the polarized light N1, N2. Figure 6 As shown by the dotted line in the graph of FIG. 1 , in the light branching element 20, for example, when the rotation deviation Δθ4 is 5 degrees, the parallel deviation Δa is about 10 μm. In this case, the parallel axis deviation Δa / 2 between the polarized light N1, N2 and each incident port 41a, 41b is about 5 μm, so Figure 7 As shown by the dotted line in the graph, the loss of coupling efficiency is less than 0.1 dB. In other words, the decrease in the light receiving sensitivity of each incident port 41a, 41b to polarized light N1, N2 can be suppressed to less than 0.1 dB. Therefore, even if the rotation deviation Δθ4 caused by the installation error is the same, the loss of coupling efficiency can be suppressed compared with the comparative optical receiving module 1X.

[0084] As described above, according to the optical receiving module 1A involved in the first embodiment, even if the installation angle of the optical branching element 20 is deviated, the beam interval of the emitted polarized light N1 and N2 is unlikely to change from the design value. Therefore, the parallel axis deviation Δa / 2 of the optical path of the polarized light N1 and the optical path of the polarized light N2 relative to each of the incident ports 41a and 41b is suppressed. As described above, the coupling efficiency of each of the polarized light N1 and N2 after polarization separation from the signal light NA by the optical branching element 20 can be made uniform, and high coupling efficiency can be achieved.

[0085] In the first embodiment, the optical receiving module 1A further includes lenses 13a and 13b that emit the signal light NA and the local light LA ​​as collimated light toward the end surface 21. This allows the polarized light N1 and N2 as collimated light and the local light LA ​​to be transmitted to the optical branching element 20.

[0086] In the optical receiving module 1A according to the first embodiment, a λ / 2 plate 24 is provided in a region including an emission point 20b of polarized light N2 in the end face 22. This allows the polarization plane of polarized light N2 passing through the optical branching element 20 to be aligned with the polarization plane of local light LA.

[0087] [Second embodiment]

[0088] Next, an optical module according to a second embodiment will be described. Figure 8 It is a plan view schematically showing a light modulation module 1B as an optical module according to the second embodiment. Figure 8 The optical modulation module 1B shown is an optical modulation module for coherent communication. The optical modulation module 1B performs phase modulation on the unmodulated light LB to generate the signal light NB. In addition, the unmodulated light is a continuous light with a constant intensity. The optical modulation module 1B has an optical input / output unit 50 and an optical modulation element 80. The optical modulation module 1B also has a coupling optical system that optically couples the optical input / output unit 50 and the optical modulation element 80. The optical modulation element 80 generates two polarized lights N3 and N4 from the linearly polarized unmodulated light LB. The two polarized lights N3 and N4 are combined in the coupling optical system in such a way that their polarization planes are orthogonal to each other, and are introduced into the optical input / output unit 50 as the signal light NB. The signal light NB includes a first polarized light N3 and a second polarized light N4 as two polarization components that are orthogonal to each other. That is, the second polarized light N4 has a polarization plane that is orthogonal to the polarization plane of the first polarized light N3. In the present embodiment, the first polarized light N3 is P polarized, and the second polarized light N4 is S polarized. The polarization plane of the non-modulated light LB is parallel to the polarization plane of the first polarized light N3.

[0089] The light modulator 80 is, for example, a semiconductor element whose main structural material is InP, Si, etc. The light modulator 80 has an end face 83, two exit ports 81a, 81b, and one non-modulated light input port 82. The exit port 81a on one side, the non-modulated light input port 82, and the exit port 81b on the other side are arranged in sequence on the end face 83. In other words, the non-modulated light input port 82 is arranged between the exit port 81a and the exit port 81b. The end face 83 is an example of the third end face in the present invention. The exit port 81a is an example of the first exit port in the present invention. The exit port 81b is an example of the second exit port in the present invention. The interval between the exit port 81a and the exit port 81b is, for example, 500μm. The light modulator 80 branches the non-modulated light LB input from the non-modulated light input port 82, and modulates the polarized light N3 on one side and the polarized light N4 on the other side, respectively. The exit port 81a outputs the modulated polarized light N3. The exit port 81b outputs the modulated polarized light N4. The polarized light N3 is an example of the first signal light in the present invention. The polarized light N4 is an example of the second signal light in the present invention. The optical modulation module 1B may be a coherent module further including a housing (not shown) for housing the optical input / output unit 50, the coupling optical system, and the optical modulation element 80.

[0090] The non-modulated light LB is input to the optical modulation element 80 from the non-modulated light incident port 82. After being input from the non-modulated light incident port 82, the non-modulated light LB is branched into two non-modulated lights. The optical modulation element 80 includes an optical modulation waveguide. The optical modulation waveguide is, for example, a waveguide formed on an InP substrate. The optical modulation waveguide performs coherent modulation on one side of the non-modulated light branched from the non-modulated light LB to generate polarized light N3. Similarly, the optical modulation waveguide performs coherent modulation on the other side of the non-modulated light branched from the non-modulated light LB to generate polarized light N4. That is, the optical modulation element 80 generates two polarized lights N3 and N4 from the non-modulated light LB. The polarized lights N3 and N4 generated by the optical modulation element 80 are output from the optical modulation element 80 via the exit ports 81a and 81b, respectively. The polarized light N3 is output from the exit port 81a. The polarized light N4 is output from the exit port 81b.

[0091] The coupling optical system includes an optical combining element 60 and a lens array 70. The lens array 70 is arranged between the optical combining element 60 and the optical modulator 80. One surface of the lens array 70 is opposite to the optical combining element 60, and the other surface of the lens array 70 is opposite to the end face 83 of the optical modulator 80. The lens array 70 has lenses 71, 72, and 73. The lenses 71, 72, and 73 are arranged in sequence along the same direction as the arrangement direction of the exit port 81a, the non-modulated light entrance port 82, and the exit port 81b. The lens 71 is arranged on the optical path of the polarized light N3, and before the polarized light N3 is incident on the end face 62, the polarized light N3 is emitted toward the end face 62 as collimated light. The lens 72 is arranged on the optical path of the non-modulated light LB emitted from the end face 62, and the non-modulated light LB is focused on the non-modulated light entrance port 82. The lens 73 is disposed on the optical path of the polarized light N4 and emits the polarized light N4 toward the end surface 62 as collimated light before the polarized light N4 enters the end surface 62. The lens 71 is an example of the fourth collimating lens in the present invention. The lens 73 is an example of the fifth collimating lens in the present invention.

[0092] The optical combining element 60 is arranged at a position on the optical path of the signal light NB and on the optical path without the modulated light LB. The optical combining element 60 includes an end face 61 and an end face 62 opposite to the end face 61. The end face 61 is an example of the first end face in the present invention. The end face 62 is an example of the second end face in the present invention. The end face 62 is parallel to the end face 61. An anti-reflection film is provided on each of the end faces 61 and 62, for example. In the present embodiment, the shape of the optical combining element 60 is a rectangular parallelepiped extending in a direction orthogonal to the end faces 61 and 62 as a length direction. The end face 62 is opposite to the lens array 70. The end face 62 and the end face 83 are opposite to each other via the lens array 70.

[0093] The output port 81a of the above-mentioned light modulator 80 is optically coupled to the incident point 60a of the polarized light N3 in the end face 62. The non-modulated light input port 82 is optically coupled to the output point 60c of the non-modulated light LB in the end face 62. The output port 81b is optically coupled to the incident point 60b of the polarized light N4 in the end face 62. The polarized lights N3 and N4 are incident on the incident points 60a and 60b different from each other. The lens 71 emits the polarized light N3 as collimated light toward the end face 62. The lens 72 focuses the non-modulated light LB emitted from the end face 62. The lens 73 emits the polarized light N4 as collimated light toward the end face 62.

[0094] Fig. 9 yes Figure 8 The optical combining element 60 is shown in perspective view. The optical combining element 60 is arranged in a state where the normal line LN' of the end face 61 is tilted at a small angle, such as 2 degrees, relative to the optical axis of the light modulating element 80. The polarized lights N3 and N4 are incident on the end face 62 at the above-mentioned small angles of incidence.

[0095] Here, the optical wave combining element 60 is composed of a birefringent material, and in one example, is composed only of a birefringent material. A birefringent material is a material having birefringence. As a specific example of a birefringent material, titanium dioxide (TiO 2 ), yttrium vanadate (YVO 4 ), lithium niobate (LiNbO 3 ), calcium carbonate (CaCo 3 ), potassium dihydrogen phosphate (KH 2 PO 4 , abbreviation: KDP), quartz, etc. The birefringent material includes at least one of titanium dioxide, yttrium vanadate, lithium niobate, calcium carbonate, potassium dihydrogen phosphate and quartz. The optical combining element 60 utilizes the effect of the birefringent material to refract the incident polarized light N3 and N4 at different refraction angles for each direction of the polarization plane and emit them. That is, the optical combining element 60 refracts the P polarization and the S polarization at different refraction angles. Thus, the optical combining element 60 combines the polarized light N3 and N4 to generate a signal light NB including the P polarization and the S polarization.

[0096] Fig.10 60 is a diagram for explaining the multiplexing performed by the optical multiplexing element 60. Fig.10 As shown, the optical combining element 60 causes the polarized light N3 incident from the end face 62 to be refracted as P polarization at a refraction angle θ3. The optical combining element 60 causes the polarized light N3 to go straight toward the end face 61 along the direction D3 and to be emitted from the end face 61. The optical combining element 60 causes the polarized light N4 incident as P polarization to the half-wavelength plate, i.e., the λ / 2 plate 64 described later and converted into S polarization to be refracted at a refraction angle θ4. The refraction angle θ4 is different from the refraction angle θ3 by the combining angle δ'. The optical combining element 60 causes the polarized light N4 to go straight toward the end face 61 along the direction D4 inclined relative to the direction D3 and to be emitted from a position in the end face 62 overlapping with the polarized light N3. Direction D3 is an example of the first direction in the present invention. Direction D4 is an example of the second direction in the present invention.

[0097] Polarized light N3 and N4 are incident from the end face 62 in a state separated by a distance Q. The distance Q is the same as the interval between the two exit ports 81a and 81b. The interval between the exit ports 81a and 81b is, for example, 500 μm. The length S2 of the optical combining element 60 in the longitudinal direction is set based on the distance Q by the following equations (4) to (6). Here, equation (4) shows the relationship between the distance Q and the length S2. Equation (5) shows the relationship between the angle θo' formed by the crystal optical axis C' of the optical combining element 60 and the P polarization, i.e., normal light, and the angle θe' formed by the crystal optical axis C' of the optical combining element 60 and the S polarization, i.e., extraordinary light. no is the refractive index of the normal light, and ne is the refractive index of the extraordinary light. Equation (5) shows the relationship between the combining angle δ' of the polarized light N3 and N4 and the angles θo' and θe'.

[0098] [Formula 4]

[0099] Q=S2tanδ′···(4)

[0100] [Formula 5]

[0101]

[0102] [Formula 6]

[0103] tanδ′=tan(θ o ′-θ e ′)···(6)

[0104] exist Fig.10 , the angle θo' and the angle θe' are shown. When the angle θo' is in the range of 47 to 49 degrees, the multiplexing angle δ' is maximum. Therefore, in this embodiment, as the optical multiplexing element 60, a structure in which the angle θo' is in the range of 47 to 49 degrees, for example, 48 degrees is used.

[0105] Refer again Figure 8 The optical multiplexing element 60 further causes the non-modulated light LB incident from the end face 61 to be incident at the same angle as the polarized light N3, that is, Fig.10 The optical wave combining element 60 makes the non-modulated light LB to be refracted in the direction completely opposite to the polarized light N3, that is, Figure 3 The optical multiplexing element 60 emits the non-modulated light LB incident on the end face 61 at the same incident angle as the incident angle of the polarized light N3 and the polarized light N4 on the end face 62. The non-modulated light LB goes straight toward the end face 62 in a direction completely opposite to the direction D3 shown and is emitted from the end face 62. The non-modulated light LB is incident on the end face 61 at the same incident angle as the incident angle of the polarized light N3 and the polarized light N4 on the end face 62. The non-modulated light LB is emitted from the end face 62 at an output angle that is the same as the incident angle. The non-modulated light LB is emitted from between the incident point 60a of the polarized light N3 and the incident point 60b of the polarized light N4 on the end face 62. The output point 60c of the non-modulated light LB on the end face 62 is located, for example, at the midpoint between the incident point 60a and the incident point 60b.

[0106] In order to set the polarized light N4 incident on the end face 62 to S polarization, a half-wavelength plate, i.e., a λ / 2 plate 64, is provided in the optical multiplexing element 60. The λ / 2 plate 64 is an example of a polarization plane rotating material in this embodiment. The λ / 2 plate 64 is made of a flat birefringent material such as crystal. Figure 8 As shown, the λ / 2 plate 64 is provided in a part of the region including the incident point 60b of the polarized light N4 in the end face 62. This region is a region that does not include the incident point 60a of the polarized light N3 and the exit point 60c of the non-modulated light LB. The polarized light N4 passes through the λ / 2 plate 64 and enters the optical multiplexing element 60. The λ / 2 plate 64 rotates the polarization plane of the polarized light N4 by 90 degrees. Therefore, the polarization plane of the polarized light N4 that has passed through the λ / 2 plate 64 is orthogonal to the polarization plane of the polarized light N3 and the polarization plane of the non-modulated light LB.

[0107] The optical input / output section 50 includes optical fibers 51 and 52 and a lens array 53. The lens array 53 is opposite to the end face 61 of the optical multiplexing element 60. The lens array 53 includes a lens 53a and a lens 53b. The lens 53b is an example of the third collimating lens in the present invention. The lenses 53a and 53b are arranged in the same direction as the arrangement direction of the exit point of the signal light NB and the incident point of the non-modulated light LB in the end face 61. The lens 53a is optically coupled to the exit point of the signal light NB in ​​the end face 61. The lens 53b is optically coupled to the incident point of the non-modulated light LB in the end face 61. The lens 53a focuses the signal light NB emitted from the end face 61. The lens 53b emits the non-modulated light LB toward the end face 61 as collimated light before the non-modulated light LB is incident on the end face 61.

[0108] Optical fiber 51 is a single-mode optical fiber that transmits signal light NB. Optical fiber 52 is a polarization-maintaining optical fiber that transmits P-polarized non-modulated light LB. Optical fibers 51 and 52 are arranged in the same direction as the arrangement direction of lenses 53a and 53b. Optical fibers 51 and 52 extend in the same direction and in a direction intersecting the arrangement direction. The optical axis of optical fiber 51 is parallel to the optical axis of optical fiber 52. Lens array 53 is located on one side of the extension direction of optical fibers 51 and 52, and is opposite to both the end face of optical fiber 51 and the end face of optical fiber 52. Lens 53a focuses signal light NB toward the end face of optical fiber 51. Lens 53b parallelizes non-modulated light LB output from the end face of optical fiber 52.

[0109] As described above, the polarized lights N3 and N4 combined by the optical combining element 60 are input to the optical fiber 51 as signal light NB including P polarization and S polarization through the lens 53a of the lens array 53, and are transmitted to the outside of the optical modulation module 1B. In the optical modulation module 1B configured as described above, the optical input / output section 50 may include the optical input section 10 of the optical receiving module 1A according to the first embodiment, and the optical combining element 60 and the lens array 70, which are the coupling optical systems, may include the optical branching element 20 and the lens array 30, which are the coupling optical systems of the optical receiving module 1A according to the first embodiment. In other words, the optical modulation module 1B replaces the optical mixing element 40 and allows the optical modulation element 80 to function as the optical receiving module 1A. In other words, the optical branching element 20 can function as the optical combining element 60, or the optical combining element 60 can function as the optical branching element 20, depending on the transmission direction of the light.

[0110] The effects of the optical modulation module 1B described above are described. In the optical modulation module 1B according to the second embodiment, the optical multiplexing element 60 includes a birefringent material. As a result, the polarized lights N3 and N4 incident on the end face 62 of the optical multiplexing element 60 are refracted at different refraction angles by the difference in refraction angles of the respective polarization components, and are multiplexed at the end face 61. At this time, the polarized lights N3 and N4 incident from the end face 62 travel without being reflected, and are emitted from the end face 61 as signal light NB. Therefore, the emission angle of the signal light NB from the end face 61 is the same as the incident angles of the polarized lights N3 and N4 to the end face 62. Therefore, even if the end faces 61 and 62 are not parallel due to manufacturing errors, when emitted from the end face 61 as signal light NB, the optical path of the polarized light N3 and the optical path of the polarized light N4 deviate from the design value at the same angle. In addition, when the angle θo' formed by the crystal optical axis C' and the traveling direction of the P polarization deviates from the above range due to other manufacturing errors, when emitted from the end face 61 as the signal light NB, the optical paths of the polarized light N3 and the polarized light N4 are also parallel to each other, and only the exit points in the end face 61 deviate from the design values. Thus, it is avoided that only one of the angles of the polarized light N3 and the polarized light N4 deviates greatly from the design value. Therefore, it is possible to suppress the occurrence of a difference in coupling efficiency between the polarized lights N3 and N4 emitted as the signal light NB.

[0111] Fig.11 and Fig.12 This is a diagram for explaining the optical path when the installation angle of the optical multiplexing element 60 is deviated. Fig.11 In FIG. 1 , the state where the installation error is zero is indicated by the double-dotted line. Fig.11As shown in FIG. 1 , it is assumed that a rotation deviation Δθ4 is generated in the optical multiplexing element 60 due to an installation error. In this case, an axial deviation ΔX' is generated between the polarized light N3 and the optical fiber 51, and an axial deviation ΔY' is generated between the polarized light N4 and the optical fiber 51. In such a case, the beam interval of the polarized light N3 and N4 is not likely to change from the design value. Fig.12 As shown, the difference between the axis deviation ΔX′ and the axis deviation ΔY′, that is, the parallel deviation Δa′, which is the difference between the axis deviation ΔX′ and the axis deviation ΔY′, is slightly generated as the variation of the beam interval of the polarized lights N3 and N4 from the designed value. Fig.12 The diagram shows a state in which the parallel deviation Δa' is equally distributed relative to the polarized lights N3 and N4 by adjusting the optical axis of the optical combiner element 60 and the optical fiber 51, that is, a state in which a parallel axis deviation Δa / 2' is generated between the polarized light N3 and the optical axis of the optical fiber 51 and between the polarized light N4 and the optical axis of the optical fiber 51.

[0112] As described above, according to the optical modulation module 1B involved in the second embodiment, even if the installation angle of the optical multiplexing element 60 is deviated, the beam interval of the polarized lights N3 and N4 emitted as the signal light NB is not likely to change from the design value. Therefore, the parallel axis deviation Δa' / 2 of the polarized lights N3 and N4 with respect to the optical fiber 51 can be suppressed. As described above, in the signal light NB after being multiplexed by the optical multiplexing element 60, the coupling efficiency of each of the polarized lights N3 and N4 can be made uniform, and a high coupling efficiency can be achieved.

[0113] In the second embodiment, the optical modulation module 1B includes a lens 53b that emits the non-modulated light LB as collimated light toward the end face 61, and lenses 71 and 73 that emit the polarized light N3 and N4 as collimated light toward the end face 62. Thus, the polarized light N3 and N4 as collimated light and the non-modulated light LB can be transmitted to the optical multiplexing element 60.

[0114] In the optical modulation module 1B according to the second embodiment, a λ / 2 plate 64 for changing the phase of polarized light N4 by 90 degrees is provided in a region including the incident point 60b of polarized light N4 in the end face 62. Thus, the polarization plane of polarized light N4 passing through the optical multiplexing element 60 can be made orthogonal to the polarization plane of polarized light N3.

[0115] The above-mentioned embodiments are explanations of one embodiment of the optical module according to the present invention. The optical module according to the present invention can arbitrarily modify the above-mentioned embodiments.

[0116] For example, in the first embodiment, an example in which the optical branching element 20 is composed of only a birefringent material is described, but at least a portion of the optical branching element 20 through which the signal light NA and the local light LA ​​pass may be composed of a birefringent material, and at least a portion of the other portions may be composed of a material other than the birefringent material. The shape of the optical branching element 20 is not limited to a rectangular parallelepiped shape as long as it has end faces 21 and 22 and extends in a direction perpendicular to the end faces 21 and 22. In the second embodiment, an example in which the optical combining element 60 is composed of only a birefringent material is described, but at least a portion of the optical combining element 60 through which the polarized light N3 and N4 and the non-modulated light LB pass may be composed of a birefringent material, and at least a portion of the other portions may be composed of a material other than the birefringent material. The shape of the optical combining element 60 is not limited to a rectangular parallelepiped shape as long as it has end faces 61 and 62 and extends in a direction perpendicular to the end faces 61 and 62.

[0117] The present invention can be applied to an optical module such as a transceiver including the optical receiving module 1A according to the first embodiment and the optical modulation module 1B according to the second embodiment.

Claims

1. An optical module, comprising: An optical branching element, which polarization-separates a signal light including a first polarization component and a second polarization component whose polarization planes are orthogonal to each other into a first polarization component as the first polarization component and a second polarization component as the second polarization component and emits the local oscillation light incident parallel to the signal light, the optical branching element comprising a birefringent material, having a first end face and a second end face, the first end face being incident with the signal light including the first polarization component and the second polarization component having a polarization plane orthogonal to the polarization plane of the first polarization component, and the second end face being incident with the first polarization component and the second polarized light are emitted respectively, the optical branching element causes the first polarized light incident on the first end face to travel straight along the first direction and be emitted from the second end face, causes the second polarized light incident on the first end face to travel straight along the second direction inclined with respect to the first direction and be emitted from the second end face, causes the local oscillation light incident on the first end face with a polarization plane parallel to the polarization plane of the first polarized light to travel straight along the first direction and be emitted from between the exit points of the first polarized light and the exit points of the second polarized light in the second end face; and An optical mixing element that causes the first polarized light and the second polarized light emitted from the optical branching element to interfere with the local oscillation light emitted from the optical branching element, respectively. The optical mixing element has a third end face arranged opposite to the second end face, and in the third end face, a local oscillation light incident port optically coupled to the exit point of the local oscillation light of the second end face is arranged between the first incident port optically coupled to the exit point of the first polarized light of the second end face and the second incident port optically coupled to the exit point of the second polarized light of the second end face.

2. The optical module according to claim 1, wherein: Also features: a first collimating lens configured to emit the signal light as collimated light toward the first end face before the signal light enters the first end face; and The second collimator lens emits the local oscillation light toward the first end surface as collimated light before the local oscillation light enters the first end surface.

3. The optical module according to claim 1 or 2, wherein: The light branching element has a rectangular parallelepiped shape whose longitudinal direction is a direction perpendicular to the first end face.

4. The optical module according to claim 1 or 2, wherein: A polarization plane rotating material for rotating the polarization plane of the second polarized light by 90 degrees is provided in a region of the second end face including an emission point of the second polarized light.

5. The optical module according to claim 1 or 2, wherein: The birefringent material has uniaxiality.

6. The optical module according to claim 1 or 2, wherein: The second end surface is parallel to the first end surface.

7. An optical module, comprising: An optical combining element, comprising a birefringent material, having a first end face and a second end face, the first end face emitting signal light including a first polarized light and a second polarized light having a polarization plane orthogonal to the polarization plane of the first polarized light, the second end face allowing the first polarized light and the second polarized light to be incident at different incident points from each other, the optical combining element causing the first polarized light incident from the second end face to travel straight along a first direction and be emitted from the first end face, causing the second polarized light incident from the second end face to travel straight along a second direction inclined with respect to the first direction and be emitted from a position of the first end face overlapping with the first polarized light, causing unmodulated light incident from the first end face with a polarization plane parallel to the polarization plane of the first polarized light to travel straight along the first direction toward the second end face and be emitted from between the incident points of the first polarized light and the incident points of the second polarized light on the second end face; and An optical modulation element having a third end face arranged opposite to the second end face, wherein a non-modulated light incident port optically coupled to the non-modulated light exit point of the second end face is arranged between a first exit port optically coupled to the first polarized light incident point of the second end face and a second exit port optically coupled to the second polarized light incident point of the second end face.

8. The optical module according to claim 7, wherein: Also features: a third collimating lens configured to emit the non-modulated light as collimated light toward the first end face before the non-modulated light is incident on the first end face; a fourth collimating lens configured to emit the first polarized light as collimated light toward the second end face before the first polarized light is incident on the second end face; and A fifth collimator lens is configured to emit the second polarized light toward the second end surface as collimated light before the second polarized light is incident on the second end surface.

9. The optical module according to claim 7 or 8, wherein: The optical multiplexing element has a rectangular parallelepiped shape whose longitudinal direction is a direction perpendicular to the first end face.

10. The optical module according to claim 7 or 8, wherein: A polarization plane rotating material for rotating the polarization plane of the second polarized light by 90 degrees is provided in a region of the second end face including the incident point of the second polarized light.

11. The optical module according to claim 7 or 8, wherein: The birefringent material has uniaxiality.

12. The optical module according to claim 7 or 8, wherein: The second end surface is parallel to the first end surface.

Images