Optical modulator

Abstract

To provide an optical modulator equipped with an optical waveguide structure having a high refractive index. [Solution] The optical modulator comprises an optical waveguide structure extending along a first direction, the optical waveguide structure comprising a first III-V compound semiconductor layer of a first conductivity type, a second III-V compound semiconductor layer of a second conductivity type, a core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, and a periodic structure layer, the first III-V compound semiconductor layer being disposed between the periodic structure layer and the core layer, the periodic structure layer comprising a first portion and a second portion arranged alternately in the first direction, the first portion having a refractive index greater than that of the first III-V compound semiconductor layer, and the second portion having a refractive index smaller than that of the first portion.

Description

Technical Field

[0001] The present disclosure relates to an optical modulator.

Background Art

[0002] Patent Document 1 discloses a Mach-Zehnder type optical modulator. This optical modulator includes a substrate formed of indium phosphide (InP). Two arm waveguides are formed on the substrate. In each arm waveguide, a lower contact layer, a lower cladding layer, a core layer, an upper cladding layer, and an upper contact layer are laminated in this order. The lower contact layer and the lower cladding layer are formed of n-type InP doped with silicon. The core layer has a multiple quantum well structure. The upper cladding layer is formed of p-type InP doped with zinc. The upper contact layer is formed of p-type indium gallium arsenide (InGaAs) doped with zinc.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the optical modulator of Patent Document 1, since the arm waveguide is formed of a III-V compound semiconductor, the refractive index of the arm waveguide cannot be increased.

[0005] The present disclosure provides an optical modulator having an optical waveguide structure with a high refractive index.

Means for Solving the Problems

[0006] An optical modulator according to one aspect of the present disclosure comprises an optical waveguide structure extending along a first direction, the optical waveguide structure comprising a first III-V compound semiconductor layer of a first conductivity type, a second III-V compound semiconductor layer of a second conductivity type, a core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, and a periodic structure layer, wherein the first III-V compound semiconductor layer is disposed between the periodic structure layer and the core layer, the periodic structure layer comprises a first portion and a second portion arranged alternately in the first direction, the first portion having a refractive index greater than that of the first III-V compound semiconductor layer, and the second portion having a refractive index less than that of the first portion. [Effects of the Invention]

[0007] According to this disclosure, an optical modulator is provided that has an optical waveguide structure having a high refractive index. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 is a schematic plan view showing an optical modulator according to one embodiment. [Figure 2] Figure 2 is a schematic plan view showing a part of the optical modulator shown in Figure 1. [Figure 3] Figure 3 is a cross-sectional view along the line III-III in Figure 2. [Figure 4] Figure 4 is a cross-sectional view along the line IV-IV in Figure 2. [Figure 5] Figure 5 is a schematic cross-sectional view showing a modified example of the optical waveguide structure. [Figure 6] Figure 6 is a schematic plan view showing a part of the optical modulator shown in Figure 1. [Figure 7] Figure 7 is a cross-sectional view along the line VII-VII in Figure 6. [Figure 8] Figure 8 is a graph showing examples of group refractive indices when the pitch is changed. [Figure 9] Figure 9 is a graph showing examples of group refractive indices when the thickness of the upper cladding layer is varied. [Figure 10]Figure 10 is a graph showing examples of group refractive indices when the thickness of the lower cladding layer is varied. [Figure 11] Figure 11 is a graph showing examples of group refractive indices when the core layer thickness is changed. [Modes for carrying out the invention]

[0009] [Description of Embodiments in this Disclosure] First, the embodiments of this disclosure will be listed and explained.

[0010] (1) An optical modulator according to one embodiment comprises an optical waveguide structure extending along a first direction, the optical waveguide structure comprising a first III-V compound semiconductor layer of a first conductivity type, a second III-V compound semiconductor layer of a second conductivity type, a core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, and a periodic structure layer, the first III-V compound semiconductor layer disposed between the periodic structure layer and the core layer, the periodic structure layer comprising a first portion and a second portion arranged alternately in the first direction, the first portion having a refractive index greater than that of the first III-V compound semiconductor layer, and the second portion having a refractive index smaller than that of the first portion.

[0011] According to the above optical modulator, the refractive index of the optical waveguide structure can be increased by a periodic structure layer having a high refractive index.

[0012] (2) In (1) above, the first portion may have a width smaller than the width of the first III-V compound semiconductor layer.

[0013] In this case, the periodic structure layer can confine light in the width direction.

[0014] (3) In (1) or (2) above, the first portion may include silicon.

[0015] (4) In any one of (1) to (3) above, the second part may contain voids or silicon oxide.

[0016] (5) In any one of (1) to (4) above, the first part may be arranged at a pitch smaller than the wavelength of light propagating through the core layer in the first direction.

[0017] In this case, light can propagate through the periodic structure layer in the first direction.

[0018] (6) In any one of (1) to (5) above, the first III-V compound semiconductor layer may have a thickness smaller than the thickness of the core layer.

[0019] In this case, light is more likely to leak from the core layer through the first III-V compound semiconductor layer to the periodic structure layer.

[0020] (7) In any one of (1) to (6) above, the thickness of the second III-V compound semiconductor layer may be 0.4 μm or less.

[0021] In this case, a high group refractive index can be obtained.

[0022] (8) In any one of (1) to (7) above, the optical modulator may further include a substrate, and the periodic structure layer may be disposed between the substrate and the first III-V compound semiconductor layer. [[ID=३०]]

[0023] (9) In any one of (1) to (8) above, the optical waveguide structure may include a first region, a second region and a third region, the first region, the second region and the third region may be arranged along the first direction, the first region may be located at the end of the optical waveguide structure in the first direction, the second region may be located between the first region and the third region, the periodic structure layer may comprise the first portion and the second portion in the third region, the periodic structure layer may comprise the third portion extending in the first direction in the first region, the third portion may have a refractive index greater than that of the first III-V compound semiconductor layer, the periodic structure layer may comprise the first portion, the second portion and the fourth portion extending in the first direction in the second region, the fourth portion may have a tapered shape with a width that decreases from the first region toward the third region.

[0024] In this case, light can propagate through the periodic structure layer from the first region to the third region.

[0025] (10) In (9) above, the first III-V compound semiconductor layer may have a first tapered portion in the first region, and the first tapered portion may have a width that increases from the first region toward the third region, the core layer may have a second tapered portion in the first region, and the second tapered portion may have a width that increases from the first region toward the third region, each of the first tapered portion and the second tapered portion may have a tip on the third portion of the periodic structure layer, and in the first direction, the tip of the second tapered portion may be located closer to the second region than the tip of the first tapered portion.

[0026] In this case, in the first region, light can propagate from the third portion of the periodic structure layer through the first tapered portion to the second tapered portion.

[0027] [Details of the embodiments of this disclosure] Embodiments of the present disclosure will be described in detail below with reference to the attached drawings. In the description of the drawings, the same reference numerals are used for identical or equivalent elements, and redundant descriptions are omitted. The drawings show, as necessary, X-axis, Y-axis, and Z-axis directions that intersect with each other. For example, the X-axis, Y-axis, and Z-axis directions are orthogonal to each other.

[0028] Figure 1 is a schematic plan view showing an optical modulator according to this embodiment. The optical modulator 1 can generate a modulated signal by modulating the intensity or phase of light in optical communication, for example. The optical modulator 1 may include an input port P1, a plurality of Mach-Zehnder modulation units MZ1, MZ2, MZ3, MZ4, MZ5, an optical filter F1, a plurality of optical couplers C1, C2, C3, C4, C5, C6, C7, and a plurality of output ports P2. The input port P1, the plurality of Mach-Zehnder modulation units MZ1 to MZ5, the optical filter F1, the plurality of optical couplers C1 to C7, and the plurality of output ports P2 may be provided on a substrate 11.

[0029] The substrate 11 extends along the X-axis and Y-axis directions. The main surface of the substrate 11 may be substantially rectangular in shape. The main surface of the substrate 11 has an edge 11a extending in the Y-axis direction and an edge 11b extending in the Y-axis direction. Edge 11b is located on the opposite side of the substrate 11 from edge 11a in the X-axis direction. An input port P1 and a plurality of output ports P2 may be provided on edge 11a. The input port P1 is located in the middle of the plurality of output ports P2.

[0030] Input port P1 is optically coupled to optical filter F1 via an optical waveguide. Optical filter F1 is, for example, a 1-input, 1-output optical component. Optical filter F1 is optically coupled to optical coupler C1 via an optical waveguide. Optical coupler C1 is, for example, a 1-input, 2-output MMI (Multi-Mode Interface) coupler. Optical coupler C1 is optically coupled to multiple (for example, two) optical couplers C2 via multiple (for example, two) optical waveguides. Each optical coupler C2 is, for example, a 1-input, 2-output MMI coupler. Each optical coupler C2 is optically coupled to optical couplers C3 and C4 via multiple (for example, two) optical waveguides. Each of optical couplers C3 and C4 is, for example, a 1-input, 2-output MMI coupler.

[0031] The optical coupler C3 is optically coupled to the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ1 by multiple (e.g., two) optical waveguides. A modulation electrode E1a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ1. A modulation electrode E1b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ1.

[0032] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ1 are optically coupled to the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ3, respectively, by a plurality of (e.g., two) optical waveguides. A heater H3a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ3. A heater H3b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ3.

[0033] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ3 are optically coupled to an optical coupler C5 by a plurality (e.g., two) of optical waveguides. The optical coupler C5 is, for example, a 2-input 1-output MMI coupler. The optical coupler C5 is optically coupled to the first arm waveguide of the Mach-Zehnder modulation unit MZ5 by an optical waveguide. A heater H5a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ5. One optical coupler C3, one Mach-Zehnder modulation unit MZ1, one Mach-Zehnder modulation unit MZ3, and one optical coupler C5 constitute one sub-Mach-Zehnder modulator.

[0034] The optical coupler C4 is optically coupled to the first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ2 by multiple (e.g., two) optical waveguides. A modulation electrode E2a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ2. A modulation electrode E2b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ2.

[0035] The first and second arm waveguides of the Mach-Zehnder modulation unit MZ2 are optically coupled to the first and second arm waveguides of the Mach-Zehnder modulation unit MZ4, respectively, by a plurality of (e.g., two) optical waveguides. A heater H4a is provided on the first arm waveguide of the Mach-Zehnder modulation unit MZ4. A heater H4b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ4. One optical coupler C4, one Mach-Zehnder modulation unit MZ2, one Mach-Zehnder modulation unit MZ4, and one optical coupler C6 constitute one sub-Mach-Zehnder modulator.

[0036] The first and second arm waveguides of the Mach-Zehnder modulation unit MZ4 are optically coupled to an optical coupler C6 by a plurality (e.g., two) of optical waveguides. The optical coupler C6 is, for example, a 2-input, 1-output MMI coupler. The optical coupler C6 is optically coupled to the second arm waveguide of the Mach-Zehnder modulation unit MZ5 by an optical waveguide. A heater H5b is provided on the second arm waveguide of the Mach-Zehnder modulation unit MZ5.

[0037] The first arm waveguide and the second arm waveguide of the Mach-Zehnder modulation unit MZ5 are optically coupled to an optical coupler C7 by multiple (e.g., two) optical waveguides. The optical coupler C7 is, for example, a 2-input, 2-output MMI coupler. The optical coupler C7 is optically coupled to multiple (e.g., two) output ports P2 by multiple (e.g., two) optical waveguides.

[0038] Optical modulator 1 includes four sub-Mach-Zehnder modulators. Light is input to input port P1 and then output from output port P2 via Mach-Zehnder modulators MZ1 to MZ5. The light is converted into four different signal lights by the four sub-Mach-Zehnder modulators. The four signal lights are combined by two optical couplers C7. The combined signal lights are output from two of the four output ports P2. Mach-Zehnder modulators MZ1 and MZ2 may include arm waveguides containing III-V compound semiconductors. The refractive index of the arm waveguides can be changed by applying a high-frequency voltage between electrodes E1a, E1b, E2a, E2b and the ground electrode. On the other hand, Mach-Zehnder modulators MZ3 to MZ5 may include arm waveguides containing silicon. The refractive index of the arm waveguide can be changed by heating the arm waveguide with heaters H3a, H3b, H4a, H4b, H5a, and H5b. Heaters H3a, H3b, H4a, H4b, H5a, and H5b may also be conductive patterns for resistive heating.

[0039] Figure 2 is a schematic plan view showing a part of the optical modulator in Figure 1. Figure 2 shows a part of the first arm waveguide of the Mach-Zehnder modulation section MZ1. Figure 3 is a cross-sectional view along line III-III in Figure 2. Figure 4 is a cross-sectional view along line IV-IV in Figure 2. As shown in Figures 2 to 4, the first arm waveguide of the Mach-Zehnder modulation section MZ1 includes an optical waveguide structure WS extending along the X-axis direction (first direction). The X-axis direction may also be the direction of light propagation. The second arm waveguide of the Mach-Zehnder modulation section MZ1, and the first and second arm waveguides of the Mach-Zehnder modulation section MZ2 may each include an optical waveguide structure WS.

[0040] The optical waveguide structure WS comprises a periodic structure layer 110, a first III-V compound semiconductor layer 120 of a first conductivity type (e.g., n-type), a core layer 130, and a second III-V compound semiconductor layer 140 of a second conductivity type (e.g., p-type). The second conductivity type is the opposite conductivity type to the first conductivity type. The periodic structure layer 110 may be placed between the substrate 11 and the first III-V compound semiconductor layer 120. The first III-V compound semiconductor layer 120 may be placed between the periodic structure layer 110 and the core layer 130. The core layer 130 may be placed between the first III-V compound semiconductor layer 120 and the second III-V compound semiconductor layer 140.

[0041] The substrate 11 may comprise a silicon substrate 100 and a silicon oxide layer 102 on the silicon substrate 100. The silicon oxide layer 102 is disposed between the silicon substrate 100 and the periodic structure layer 110. The thickness of the silicon substrate 100 may be 100 μm or more. The thickness of the silicon oxide layer 102 may be 2 μm or more.

[0042] The periodic structure layer 110 comprises a first portion 110a and a second portion 110b arranged alternately in the X-axis direction. The first portion 110a and the second portion 110b may be arranged periodically in the X-axis direction. The first portion 110a has a refractive index greater than that of the first III-V compound semiconductor layer 120. The first portion 110a may contain silicon. The first portion 110a may be a portion filled with silicon. The second portion 110b has a refractive index less than that of the first portion 110a. The refractive index of the second portion 110b may be less than that of the first III-V compound semiconductor layer 120. The second portion 110b may contain voids (air) or silicon oxide. The second portion 110b may be a portion occupied by air or a portion filled with silicon oxide.

[0043] The first portion 110a may be arranged in the X-axis direction with a pitch PT1 smaller than the wavelength of light propagating through the core layer 130 (e.g., 1.55 μm). The pitch PT1 may be between 0.3 and 0.33 μm. The pitch PT1 is the sum of the length of the first portion 110a in the X-axis direction and the length of the second portion 110b in the X-axis direction. The length of the first portion 110a in the X-axis direction may be the same as or different from the length of the second portion 110b in the X-axis direction.

[0044] The first portion 110a and the second portion 110b may have a width W1 smaller than the width W2 of the first III-V compound semiconductor layer 120, the core layer 130, and the second III-V compound semiconductor layer 140. The width W1 is the length of the first portion 110a and the second portion 110b in the Y-axis direction. The width W1 may be between 0.3 and 1.0 μm. The first portion 110a and the second portion 110b may have a thickness T110a. The thickness T110a is the length of the first portion 110a and the second portion 110b in the Z-axis direction. The thickness T110a may be between 0.1 and 0.5 μm.

[0045] The periodic structure layer 110 may further comprise a base layer 110c extending along the X-axis. The first portion 110a and the second portion 110b are positioned between the base layer 110c and the first III-V compound semiconductor layer 120. An example of the material for the base layer 110c is the same as the example of the material for the first portion 110a. The base layer 110c may have a thickness T110c that is smaller than the thickness T110a. The thickness T110c may be between 0 and 0.2 μm.

[0046] The periodic structure layer 110 may further comprise a plurality of extending portions 110d extending along the X-axis. In the Y-axis direction, the first portion 110a and the second portion 110b are positioned between the plurality of extending portions 110d. Each extending portion 110d is positioned between the base layer 110c and the first III-V compound semiconductor layer 120. Examples of materials for the extending portions 110d are the same as examples of materials for the second portion 110b.

[0047] The first III-V compound semiconductor layer 120 may be supported by a plurality of support layers 112 extending along the X-axis. Examples of materials for the support layers 112 are the same as examples of materials for the first portion 110a. In the Y-axis direction, each extended portion 110d is located between the support layer 112 and the first portion 110a. Each extended portion 110d is a region enclosed by the support layer 112, the base layer 110c, the first portion 110a, and the first III-V compound semiconductor layer 120. In the Y-axis direction, the distance W3 between the plurality of support layers 112 is greater than the width W1 of the first portion 110a and the second portion 110b, and less than the width W2 of the second III-V compound semiconductor layer 140. The distance W3 corresponds to the width of the base layer 110c.

[0048] The periodic structure layer 110 may have a thickness T110 that is smaller than the thickness T120 of the first III-V compound semiconductor layer 120. The thickness T110 may be between 0.1 and 0.5 μm. The thickness T110 is the sum of the thickness T110a of the first portion 110a and the second portion 110b and the thickness T110c of the base layer 110c.

[0049] The first III-V compound semiconductor layer 120 may contain at least one of indium phosphide (InP), indium gallium arsenide phosphide (InGaAsP), aluminum indium gallium arsenide (AlInGaAs), gallium arsenide (GaAs), or aluminum gallium arsenide (AlGaAs). An example of an n-type dopant is silicon (Si). The first III-V compound semiconductor layer 120 may have a width W2 that is greater than the distance W3 between the multiple support layers 112 in the Y-axis direction. The first III-V compound semiconductor layer 120 may have a thickness T120 that is smaller than the thickness T130 of the core layer 130. The thickness T120 of the first III-V compound semiconductor layer 120 may be 0.1 to 0.4 μm.

[0050] The core layer 130 contains an undoped III-V compound semiconductor. The core layer 130 may have a multiple quantum well structure or it may be a bulk layer. Examples of III-V compound semiconductors include InGaAsP and AlInGaAs. The core layer 130 has a thickness T130. The thickness T130 may be 0.1 to 0.5 μm.

[0051] The second III-V compound semiconductor layer 140 may contain at least one of InP, InGaAsP, AlInGaAs, GaAs, or AlGaAs. An example of a p-type dopant is zinc (Zn). The thickness T140 of the second III-V compound semiconductor layer 140 may be greater than the thickness T120 or greater than the thickness T130. The thickness T140 of the second III-V compound semiconductor layer 140 may be 0.1 to 0.4 μm.

[0052] The optical waveguide structure WS may be covered with an insulating film such as a silicon oxide film or benzocyclobutene (BCB). The insulating film may have an opening on the second III-V compound semiconductor layer 140. An electrode E1a (see Figure 1) is provided in this opening. Electrode E1a is connected to the second III-V compound semiconductor layer 140. A ground electrode is connected to the first III-V compound semiconductor layer 120. A voltage is applied between electrode E1a and the ground electrode. For example, a DC reverse bias voltage and an AC voltage are superimposed and applied between electrode E1a and the ground electrode. As a result, an electrical signal flows between electrode E1a and the ground electrode. This electrical signal changes the refractive index of the first III-V compound semiconductor layer 120, the core layer 130, and the second III-V compound semiconductor layer 140. The change in refractive index modulates the phase of the light propagating through the optical waveguide structure WS.

[0053] Light propagating through the core layer 130 may also propagate through the periodic structure layer 110, the first III-V compound semiconductor layer 120, and the second III-V compound semiconductor layer 140.

[0054] According to the optical modulator 1, the refractive index of the optical waveguide structure WS can be increased by the periodic structure layer 110 having a high refractive index. Furthermore, the optical modulator 1 can obtain a higher group refractive index (e.g., 7 or higher) compared to an optical waveguide structure that does not include the periodic structure layer 110. A high group refractive index allows for a large phase shift. Therefore, the lengths of electrodes E1a, E1b, E2a, and E2b in the X-axis direction (see Figure 1) can be shortened, and the modulation bandwidth of the optical modulator 1 can be widened.

[0055] When the width W1 of the first portion 110a is smaller than the width W2 of the first III-V compound semiconductor layer 120, the periodic structure layer 110 can confine light in the width direction (Y-axis direction).

[0056] When the first portion 110a is arranged in the X-axis direction with a pitch PT1 smaller than the wavelength of light propagating through the core layer 130, light can propagate through the periodic structure layer 110 in the X-axis direction. Since the periodic structure layer 110 has a high refractive index, the speed of light propagating through the periodic structure layer 110 is reduced. This generates slow light.

[0057] If the first III-V compound semiconductor layer 120 has a thickness T120 that is smaller than the thickness T130 of the core layer 130, then light propagating through the core layer 130 is more likely to seep out from the core layer 130 through the first III-V compound semiconductor layer 120 to the periodic structure layer 110.

[0058] A high group refractive index can be obtained when the thickness T140 of the second III-V compound semiconductor layer 140 is 0.4 μm or less.

[0059] The optical waveguide structure WS can be fabricated as follows. First, a second III-V compound semiconductor layer 140, a core layer 130, and a first III-V compound semiconductor layer 120 are sequentially stacked on the surface of a III-V compound semiconductor substrate. The III-V compound semiconductor substrate is, for example, an InP substrate or a GaAs substrate. Each layer is formed, for example, by MOCVD (Metal Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy).

[0060] Next, multiple small pieces are formed by cutting the III-V compound semiconductor substrate along the scribe lines formed on its surface.

[0061] On the other hand, optical waveguides and periodic structure layers 110 are formed by patterning the silicon layer located on the surface of the SOI (Silicon On Insulator) substrate.

[0062] Next, small pieces are bonded to the SOI substrate so that the first III-V compound semiconductor layer 120 is bonded to the periodic structure layer 110. Bonding is performed, for example, by a surface activation bonding method using nitrogen plasma. After exposing the surface of the SOI substrate and the surface of the small piece to nitrogen plasma, the surface of the small piece is brought into contact with the surface of the SOI substrate. Then, the SOI substrate and the small piece are heated while a load is applied.

[0063] Next, small pieces of III-V compound semiconductor substrate are removed, for example, by wet etching. Alternatively, before bonding, ions such as hydrogen ions may be injected into the surface of the III-V compound semiconductor substrate to form a release layer, and after bonding, the release layer may be heated and removed. Once the release layer is removed, the III-V compound semiconductor substrate is peeled off.

[0064] Next, the second III-V compound semiconductor layer 140, the core layer 130, and the first III-V compound semiconductor layer 120 are fabricated by photolithography and etching. This forms the optical waveguide structure WS. Subsequently, an insulating film is formed to cover the optical waveguide structure WS. Then, an opening is formed in the insulating film by photolithography and etching. Finally, an electrode E1a is formed in the opening by lift-off.

[0065] Figure 5 is a schematic cross-sectional view showing a modified example of the optical waveguide structure. The optical waveguide structure WSa shown in Figure 5 has the same configuration as the optical waveguide structure WS, except that the width W2 is smaller than the distance W3. In this case, when forming the insulating film covering the optical waveguide structure WSa, the insulating film is also formed within the extended portion 110d. Because the optical waveguide structure WSa has a small width W2, it has high optical confinement in the width direction (Y-axis direction).

[0066] Figure 6 is a schematic plan view showing a portion of the optical modulator in Figure 1. Figure 6 shows the end of the optical waveguide structure WS. Figure 7 is a cross-sectional view along line VII-VII in Figure 6. As shown in Figure 6, the optical waveguide structure WS may include a first region WS1, a second region WS2, and a third region WS3. The first region WS1, the second region WS2, and the third region WS3 are arranged along the X-axis direction. The direction from the first region WS1 to the third region WS3 is the X-axis direction. The first region WS1 is located at the end of the optical waveguide structure WS in the X-axis direction. The second region WS2 is located between the first region WS1 and the third region WS3.

[0067] The periodic structure layer 110 may comprise a first portion 110a and a second portion 110b in the third region WS3.

[0068] The periodic structure layer 110 may include a third portion 110e extending in the X-axis direction within the first region WS1. The third portion 110e has a refractive index greater than that of the first III-V compound semiconductor layer 120. Examples of the material for the third portion 110e are the same as examples of the material for the first portion 110a. The width and thickness of the third portion 110e may be the same as those of the first portion 110a.

[0069] The third section 110e is connected to an optical waveguide WG between the first arm waveguide of the Mach-Zehnder modulation section MZ1 and the first arm waveguide of the Mach-Zehnder modulation section MZ3. An example of the material for the optical waveguide WG is the same as the example of the material for the first section 110a. The width and thickness of the optical waveguide WG may be the same as those of the first section 110a.

[0070] The periodic structure layer 110 may comprise a first portion 110a, a second portion 110b, and a fourth portion 110f extending in the X-axis direction in the second region WS2. The fourth portion 110f partially overlaps with the first portion 110a and the second portion 110b. In the second region WS2, the first portion 110a and the second portion 110b are arranged alternately in the X-axis direction. The pitch PT1 in which the first portion 110a is arranged may increase from the first region WS1 towards the third region WS3. The fourth portion 110f has a tapered shape with a width that decreases from the first region WS1 towards the third region WS3. The fourth portion 110f may be triangular when viewed from the Z-axis direction. At the boundary between the first region WS1 and the second region WS2, the width of the fourth portion 110f may be the same as the width of the first portion 110a. At the boundary between the second region WS2 and the third region WS3, the fourth portion 110f may have a tip. The thickness of the fourth portion 110f may be the same as the thickness of the first portion 110a.

[0071] The first III-V compound semiconductor layer 120 may have a first tapered portion 122 in the first region WS1. The first tapered portion 122 has a width that increases from the first region WS1 toward the third region WS3. The first tapered portion 122 may be triangular when viewed from the Z-axis direction. The first tapered portion 122 may have a tip 122t located on the third portion 110e of the periodic structure layer 110. The tip 122t of the first tapered portion 122 may overlap with the third portion 110e of the periodic structure layer 110 when viewed from the Z-axis direction.

[0072] The core layer 130 may have a second tapered portion 132 in the first region WS1. The second III-V compound semiconductor layer 140 may have a second tapered portion 142 in the first region WS1. The second tapered portions 132 and 142 have a width that increases from the first region WS1 toward the third region WS3. The second tapered portions 132 and 142 may be triangular when viewed from the Z-axis direction. The second tapered portions 132 and 142 may each have tips 122t and 142t located on the third portion 110e of the periodic structure layer 110. The tips 122t and 142t of the second tapered portions 132 and 142 may overlap with the third portion 110e of the periodic structure layer 110 when viewed from the Z-axis direction. In the X-axis direction, the tips 132t and 142t of the second tapered portions 132 and 142 may be located closer to the second region WS2 than the tip 122t of the first tapered portion 122.

[0073] If the periodic structure layer 110 comprises a first portion 110a, a second portion 110b, and a fourth portion 110f in the second region WS2, then light can propagate through the periodic structure layer 110 from the first region WS1 to the third region WS3.

[0074] When the first III-V compound semiconductor layer 120 has a first tapered portion 122 and the core layer 130 has a second tapered portion 132, light can propagate in the first region WS1 from the third portion 110e of the periodic structure layer 110 through the first tapered portion 122 to the second tapered portion 132.

[0075] The following describes various experiments conducted to evaluate the optical modulator 1. The experiments described below are not intended to limit the scope of this invention.

[0076] (Calculation of group refractive index) The group refractive index was calculated for the optical waveguide structure WSa shown in Figure 5 using simulation calculations (FDTD method: Finite Difference Time Domain method). The optical waveguide structure used in the simulation calculations is as follows: Periodic structure layer 110: Silicon layer, Part 1, 110a: Silicone part, Second portion 110b: void portion, Group III-V compound semiconductor layer 120 (lower cladding layer): n-type InP layer, Core layer 130: Multiple quantum well structure containing InGaAsP, IIIII-V compound semiconductor layer 140 (upper cladding layer): p-type InP layer, Width W1: 0.8 μm, Width W2: 1.3 μm, Thickness T110a: 0.18 μm, Thickness T110: 0.22 μm, Pitch PT1: 0.3 μm or 0.33 μm (the length of the silicon portion is the same as the length of the void portion).

[0077] The results of the simulation calculations are shown in Figures 8 to 11. In each figure, the horizontal axis represents wavelength (μm), and the vertical axis represents the group refractive index.

[0078] Figure 8 is a graph showing examples of group refractive index when the pitch is changed. E1 shows the result when the pitch PT1 is 0.3 μm. E2 shows the result when the pitch PT1 is 0.33 μm. As shown in Figure 8, changing the pitch PT1 changes the wavelength range in which a high group refractive index can be obtained. Therefore, it can be seen that a high group refractive index can be obtained in the desired wavelength range by changing the pitch PT1.

[0079] Figure 9 is a graph showing examples of group refractive index when the thickness of the upper cladding layer is varied. E3 shows the result when the upper cladding layer is 0.4 μm thick. E4 shows the result when the upper cladding layer is 0.5 μm thick. E4 shows the result when the upper cladding layer is 0.6 μm thick. As shown in Figure 9, the maximum value of the obtained group refractive index changes when the thickness of the upper cladding layer is changed. Therefore, it can be seen that a high group refractive index can be obtained by setting the thickness of the upper cladding layer to 0.4 μm or less.

[0080] Figure 10 is a graph showing examples of group refractive indices when the thickness of the lower cladding layer is varied. E6 shows the result when the thickness of the lower cladding layer is 0.25 μm. E7 shows the result when the thickness of the lower cladding layer is 0.3 μm. As shown in Figure 10, a high group refractive index can be obtained even when the thickness of the lower cladding layer is varied.

[0081] Figure 11 is a graph showing examples of group refractive index when the thickness of the core layer is changed. E8 shows the result when the thickness of core layer 130 is 0.398 μm. E9 shows the result when the thickness of core layer 130 is 0.432 μm. As shown in Figure 11, a high group refractive index can be obtained even when the thickness of core layer 130 is changed.

[0082] The group refractive index was calculated for a mesa waveguide that does not include the periodic structure layer 110 by simulation calculation (FDTD method). The structure of the mesa waveguide used in the simulation calculation is as follows. The width of the mesa waveguide is the same as the width of the lower cladding layer, core layer, and upper cladding layer. Lower cladding layer: n-type InP layer, Core layer: Multiple quantum well structure containing InGaAsP, Upper cladding layer: p-type InP layer,

[0083] Simulation calculations showed that the group refractive index of the mesa waveguide was between 3.6 and 3.8. The equivalent refractive index of the core layer of the mesa waveguide is approximately the same as that of core layer 130 of the optical waveguide structure WSa described above. The amount of phase change is proportional to the group refractive index and the length of the electrodes. The group refractive index of the optical waveguide structure WSa described above is about 10, which is about 2.5 times that of the mesa waveguide. Therefore, in the optical waveguide structure WSa, the length of the electrodes can be shortened to about 1 / 2.5 compared to the mesa waveguide.

[0084] While preferred embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above embodiments.

[0085] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims, not in the sense described above, and all modifications within the sense and scope equivalent to the claims are intended to be included. [Explanation of symbols]

[0086] 1…Optical modulator 11… Circuit board 11a... Edge 11b...Rest 100...Silicon substrate 102...Silicon oxide layer 110…periodic structure layer 110a...first part 110b…Second part 110c...base layer 110d...extension part 110e…3rd part 110f…4th part 112…Support layer 120…First III-V compound semiconductor layer 122...First tapered section 122t...tip 130... Core Layer 132...Second tapered section 132t...tip 140…2nd III-V compound semiconductor layer 142...Second tapered section 142t...tip C1... Optical coupler C2... Optical coupler C3... Optical coupler C4... Optical coupler C5... Optical coupler C6... Optical coupler C7... Optical coupler E1a…electrode E1b…electrode E2a…electrode E2b…electrode F1... Light filter H3a... Heater H3b... Heater H4a... Heater H4b... Heater H5a... Heater H5b... Heater MZ1...Mach-Zehnder modulation section MZ2...Mach Zehnder Modulation Unit MZ3...Mach-Zehnder modulation section MZ4...Mach-Zehnder modulation section MZ5...Mach Zehnder Modulation Unit P1...Input port P2…Output port PT1…Pitch W1…width W2...Width W3... Distance WG…Optical waveguide WS…Optical waveguide structure WS1…1st area WS2…Second area WS3…Third area WSa...Optical waveguide structure

Claims

1. It comprises an optical waveguide structure extending along the first direction, The optical waveguide structure is, A first-type conductivity first-group III-V compound semiconductor layer, A second-conductivity type 2III-V compound semiconductor layer, A core layer disposed between the first III-V compound semiconductor layer and the second III-V compound semiconductor layer, Periodic structural layer, Equipped with, The first III-V compound semiconductor layer is disposed between the periodic structure layer and the core layer, The periodic structure layer comprises a first portion and a second portion that are alternately arranged in the first direction, The first portion has a refractive index greater than that of the first III-V compound semiconductor layer. The second part is an optical modulator having a refractive index smaller than that of the first part.

2. The optical modulator according to claim 1, wherein the first portion has a width smaller than the width of the first III-V compound semiconductor layer.

3. The optical modulator according to claim 1 or claim 2, wherein the first portion comprises silicon.

4. The optical modulator according to claim 1 or claim 2, wherein the second portion comprises a void or silicon oxide.

5. The optical modulator according to claim 1 or 2, wherein the first portion is arranged in the first direction at a pitch smaller than the wavelength of light propagating through the core layer.

6. The optical modulator according to claim 1 or claim 2, wherein the first III-V compound semiconductor layer has a thickness smaller than the thickness of the core layer.

7. The optical modulator according to claim 1 or claim 2, wherein the thickness of the second III-V compound semiconductor layer is 0.4 μm or less.

8. Further equipped with a circuit board, The optical modulator according to claim 1 or claim 2, wherein the periodic structure layer is disposed between the substrate and the first III-V compound semiconductor layer.

9. The optical waveguide structure includes a first region, a second region, and a third region. The first region, the second region, and the third region are arranged along the first direction, The first region is located at the end of the optical waveguide structure in the first direction, The second region is located between the first region and the third region. The periodic structure layer comprises the first portion and the second portion in the third region. The periodic structure layer comprises a third portion extending in the first direction within the first region, the third portion having a refractive index greater than that of the first III-V compound semiconductor layer. The optical modulator according to claim 1 or 2, wherein the periodic structure layer comprises, in the second region, the first portion, the second portion, and the fourth portion extending in the first direction, and the fourth portion has a tapered shape with a width that decreases from the first region toward the third region.

10. The first III-V compound semiconductor layer has a first tapered portion in the first region, and the first tapered portion has a width that increases from the first region toward the third region. The core layer has a second tapered portion in the first region, and the second tapered portion has a width that increases from the first region toward the third region. Each of the first tapered portion and the second tapered portion has its tip on the third portion of the periodic structure layer, The optical modulator according to claim 9, wherein in the first direction, the tip of the second tapered portion is located closer to the second region than the tip of the first tapered portion.

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