Optical modulation element

Abstract

The present invention provides an optical waveguide element that reduces the propagation loss of light and suppresses DC drift. The optical waveguide element (1) includes: a substrate (10); an optical waveguide formed of a thin film formed on the substrate (10), having a ridge portion as a protruding portion and a flat portion having a film thickness thinner than that of the ridge portion. The optical waveguide includes: a first waveguide portion having a first ridge width (W1) and a first flat film thickness (T sb1 ); a second waveguide portion having a second ridge width (W2) and a second flat film thickness (T sb2 ); a first intermediate waveguide portion connected to the first waveguide portion, having a third ridge width (W 1C ) and a first flat film thickness (T sb1 ); a second intermediate waveguide portion connected to the second waveguide portion, having a fourth ridge width (W 2C ) and a second flat film thickness (T sb2 ), the first waveguide portion, the first intermediate waveguide portion, the second intermediate waveguide portion, and the second waveguide portion are arranged in this order, the second flat film thickness (T sb2 ) is thicker than the first flat film thickness (T sb1 ), and the third ridge width (W 1C ) is wider than the fourth ridge width (W 2C ).

Description

Technical Field

[0001] The present invention relates to an optical waveguide element and an optical modulation element, and particularly to a waveguide structure of a Mach-Zehnder type optical modulation element. Background Art

[0002] With the popularization of the Internet, the communication volume has increased dramatically, and optical fiber communication is of great importance. Optical fiber communication is a communication method that converts an electrical signal into an optical signal and transmits the optical signal through an optical fiber, and has the characteristics of a wide bandwidth, low loss, and strong noise resistance.

[0003] As a method of converting an electrical signal into an optical signal, a direct modulation method using a semiconductor laser and an external modulation method using an optical modulator are known. Although the direct modulation does not require an optical modulator and has a low cost, it has a limit in high-speed modulation, and the external optical modulation method is used in high-speed and long-distance applications.

[0004] As an optical modulator, a Mach-Zehnder type optical modulator in which an optical waveguide is formed near the surface of a lithium niobate single crystal substrate by Ti (titanium) diffusion has been put into practical use (for example, refer to Patent Document 1). The Mach-Zehnder type optical modulator uses an optical waveguide (Mach-Zehnder optical waveguide) having a structure of a Mach-Zehnder interferometer that divides light emitted from one light source into two, passes each through a different path, and then overlaps again to cause interference. High-speed optical modulators of 40 Gb / s or more have been commercialized, but the total length up to about 10 cm has become a significant drawback.

[0005] On the other hand, Patent Document 2 discloses a Mach-Zehnder type optical modulator using a c-axis oriented lithium niobate film. Compared with an optical modulator using a lithium niobate single crystal substrate, the optical modulator using a lithium niobate film can be significantly miniaturized and have a lower driving voltage.

[0006] Patent Document 3 describes a ridge type optical waveguide element having: a flat portion including a waveguide layer formed on a substrate and made of a lithium niobate film, the waveguide layer having a predetermined thickness; and a ridge portion protruding from the flat portion. Since the thickness of the flat portion of the ridge type optical waveguide element is less than 0.4 times the wavelength of light propagating in the ridge portion, even if the ridge width is reduced, the propagation loss can be suppressed low. In addition, regarding the optical waveguide structure, Patent Document 4 describes that in order to connect an input / output waveguide portion formed of a ridge type optical waveguide and a main portion of an optical switch formed of a high mesa type optical waveguide, an input / output tapered waveguide portion is provided between the input / output waveguide portion and the main portion of the optical switch to change the waveguide shape step by step.

[0007] Prior Art Documents

[0008] Patent Documents

[0009] Patent Document 1: Japanese Patent No. 4485218 Gazette

[0010] Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006 - 195383 Gazette

[0011] Patent Document 3: Japanese Unexamined Patent Application Publication No. 2017 - 129834 Gazette

[0012] Patent Document 4: Japanese Patent No. 3816924 Gazette Summary of the Invention

[0013] Problems to be Solved by the Invention

[0014] In the case of processing a lithium niobate film epitaxially grown on a substrate into a ridge shape to form an optical waveguide, as described in Patent Document 3, the height of the ridge is sufficiently ensured, and the film thickness of the flat portion extending to the left and right of the ridge is thinned. Thus, light confinement can be enhanced. When a voltage is applied between a pair of electrodes, a sufficient electric field can be applied to the optical waveguide, and the half - wavelength voltage Vπ can be reduced. In addition, the half - wavelength voltage Vπ is the difference V1 - V2 between the voltage V1 at which the light output is maximum and the voltage V2 at which the light output is minimum, and the drive voltage is proportional to the half - wavelength voltage Vπ. Therefore, reducing the half - wavelength voltage Vπ means a low drive voltage.

[0015] However, when a DC bias is applied to such an optical waveguide, the DC drift is large, so there is a problem that the life of the optical modulation element is short. The DC drift is the change over time of the operating point of the light output. Usually, the operating point of the light output is adjusted by a DC bias to be the average of the maximum light output and the minimum light output. However, in the case of thinning the flat film thickness to ensure the height of the ridge, there is a problem that the change over time of the operating point is large, and even when a large DC bias is applied, the time (life) until the operating point cannot be adjusted is short.

[0016] Therefore, an object of the present invention is to provide an optical waveguide element capable of reducing the light propagation loss and suppressing DC drift, and an optical modulation element using the optical waveguide element. Another object of the present invention is to provide an optical modulation element capable of achieving both low drive voltage and suppression of DC drift.

[0017] Means for Solving the Problems

[0018] The inventors of the present application repeatedly conducted in - depth research on the structure of an optical modulation element capable of suppressing DC drift, and as a result, found that the DC drift depends on the flat film thickness of the ridge waveguide. By thinning the flat film thickness, the drive voltage of the optical modulation element can be reduced, but on the other hand, it becomes a cause for increasing the DC drift. It was also found that when connecting two optical waveguides with different flat film thicknesses, due to the discontinuous change in the flat film thickness, the connection loss increases, but by adjusting the ridge widths of the two optical waveguides respectively, the connection loss can be reduced.

[0019] The present invention is an invention based on such a technical insight. The present invention provides an optical waveguide element, characterized by comprising: a substrate; an optical waveguide formed of a thin film formed on the substrate, having a ridge portion as a protruding portion and a flat portion having a film thickness thinner than that of the ridge portion, the optical waveguide including: a first waveguide portion having a first ridge width (W1) and a first flat film thickness (T sb1 ); a second waveguide portion having a second ridge width (W2) and a second flat film thickness (T sb2 ); a first intermediate waveguide portion connected to the first waveguide portion, having a third ridge width (W 1C ) and the first flat film thickness (T sb1 ); a second intermediate waveguide portion connected to the second waveguide portion, having a fourth ridge width (W 2C ) and the second flat film thickness (T sb2 ), the first waveguide portion, the first intermediate waveguide portion, the second intermediate waveguide portion, and the second waveguide portion are arranged in sequence, the second flat film thickness (T sb2 ) is thicker than the first flat film thickness (T sb1 ) by (T sb2 > T sb1 ), the third ridge width (W 1C ) is wider than the fourth ridge width (W 2C ) by (W 1C > W 2C ).

[0020] According to the present invention, it is possible to reduce the connection loss generated when connecting ridge waveguides with different flat film thicknesses.

[0021] In the present invention, it is preferable that the third ridge width (W 1C ) is narrower than twice the fourth ridge width (W 2C ) (W 1C < 2 × W 2C ). According to this structure, it is possible to prevent an increase in connection loss caused by the third ridge width (W 1C ) of the first intermediate waveguide portion being too wide relative to the fourth ridge width (W 2C ) of the second intermediate waveguide portion.

[0022] In the present invention, it is preferable that the third ridge width (W 1C ) is wider than the first ridge width (W1) (W 1C > W1), and the fourth ridge width (W 2C ) is wider than the second ridge width (W2) (W 2C > W2). According to this structure, the axial offset is strong, and the effect of reducing the connection loss can be improved.

[0023] In the present invention, it is preferable that the second ridge width (W2) is wider than the first ridge width (W1) (W2 > W1). According to this structure, when the first waveguide portion is an RF portion and the second waveguide portion is a DC portion, low driving voltage can be achieved in the RF portion, and the suppression effect of DC drift can be improved in the DC portion.

[0024] In the present invention, it is preferable that the optical waveguide further includes a third intermediate waveguide portion, which is disposed between the first intermediate waveguide portion and the second intermediate waveguide portion and has a cross-sectional shape different from those of the first intermediate waveguide portion and the second intermediate waveguide portion. In this case, the third intermediate waveguide portion may also have a third flat film thickness (T sb1 ) thinner than the first flat film thickness (T sb3 ), may have a structure without a flat portion, or may not have a waveguide shape. Regardless of any structure, the connection loss generated when connecting ridge waveguides with different flat film thicknesses can be reduced.

[0025] In the present invention, it is preferable that the length (L C ) of the third intermediate waveguide portion is 3 μm or less (L C ≤ 3 μm). As long as the length of the third intermediate waveguide portion is 3 μm or less, a waveguide structure with low connection loss can be achieved regardless of the cross-sectional shape of the third intermediate waveguide portion.

[0026] In the present invention, it is preferable that the first flat film thickness (T sb1 ) is less than 0.6 μm (0 μm ≤ T sb1 < 0.6 μm), and the second flat film thickness (T sb2 ) is 0.6 μm or more (T sb2 ≥ 0.6 μm). According to this structure, low driving voltage can be achieved in the RF portion, and DC drift can be suppressed in the DC portion.

[0027] In the present invention, it is preferable that the thin film is a lithium niobate film, and the c-axis direction of the lithium niobate film is oriented in a direction perpendicular to the main surface of the substrate. According to the ridge waveguide formed of the lithium niobate film, even if the ridge width is reduced, the propagation loss can be suppressed to a low level.

[0028] In addition, the present invention provides an optical modulation element, characterized by further comprising: an optical waveguide element having the above characteristics; a signal electrode that applies an RF signal to the first waveguide portion; a bias electrode that applies a DC bias to the second waveguide portion, and the thin film is made of an electro-optic material. According to the present invention, an optical modulation element capable of reducing the connection loss generated when connecting ridge waveguides with different flat film thicknesses can be provided.

[0029] Advantages of the Invention

[0030] According to the present invention, an optical waveguide element and an optical modulation element capable of reducing light propagation loss and suppressing DC drift can be provided. Further, according to the present invention, an optical modulation element capable of achieving both low driving voltage and suppression of DC drift can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 (a) and (b) are schematic top views showing the structure of the optical modulation element according to the first embodiment of the present invention. Figure 1 (a) shows only the optical waveguide. Figure 1 (b) shows the whole optical modulation element including the traveling-wave electrode.

[0032] Figure 2 (a) and (b) are schematic cross-sectional views of the optical modulation element 1. Figure 2 (a) is a cross-sectional view of the RF part along the Figure 1 (a) and (b) X1-X1' line. Figure 2 (b) is a cross-sectional view of the DC part along the Figure 1 (a) and (b) X2-X2' line.

[0033] Figure 3 is a schematic top view and a schematic cross-sectional view showing the waveguide structure near the middle part between the RF part and the DC part.

[0034] Figure 4 is a schematic perspective view showing the waveguide structure of the middle part three-dimensionally.

[0035] Figure 5 is a schematic top view showing a case where an axial shift occurs between the optical waveguide on the RF part side and the optical waveguide on the DC part side.

[0036] Figure 6 is a schematic top view and a schematic cross-sectional view showing the optical modulation element according to the second embodiment of the present invention, and the waveguide structure near the middle part between the RF part and the DC part.

[0037] Figure 7 is a three-dimensional representation of Figure 6 the waveguide structure of the middle part shown.

[0038] Figure 8 (a) to (c) are diagrams for explaining a method of forming a waveguide structure having the Figure 6 and Figure 7 shown third intermediate waveguide part.

[0039] Figure 9 is a schematic top view and a schematic cross-sectional view showing the optical modulation element according to the third embodiment of the present invention, and the waveguide structure near the middle part between the RF part and the DC part.

[0040] Figure 10 is a schematic perspective view showing Figure 9 the waveguide structure of the intermediate portion shown.

[0041] Figure 11 (a) to (c) are diagrams for explaining Figure 9 and Figure 10 the method of forming the waveguide structure of the third intermediate waveguide portion shown.

[0042] Figure 12 (a) and (b) are schematic cross-sectional views of the optical modulation element according to the fourth embodiment of the present invention, Figure 12 (a) is a cross-sectional view of the RF section, Figure 12 (b) is a cross-sectional view of the DC section.

[0043] Figure 13 is a schematic top view showing the waveguide structure near the intermediate portion between the RF section and the DC section.

[0044] Figure 14 is a schematic cross-sectional view showing the structure of the DC section of the optical modulation element according to the fifth embodiment of the present invention.

[0045] Figure 15 is a graph showing the relationship between the flat film thickness T sb1 of the ridge waveguide of the RF section and the electric field efficiency VπL.

[0046] Figure 16 is a graph showing the relationship between the ridge width W 1C of the first intermediate waveguide portion on the RF section side and the connection loss (dB).

[0047] Figure 17 is a graph showing the relationship between the magnitude ΔW of the axial offset between the first intermediate waveguide portion and the second intermediate waveguide portion and the connection loss (dB).

[0048] Figure 18 is a graph showing the relationship between the length L C of the third intermediate waveguide portion and the connection loss (dB). DETAILED DESCRIPTION OF THE INVENTION

[0049] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0050] Figure 1 (a) and (b) are schematic top views showing the structure of the optical modulation element according to the first embodiment of the present invention, Figure 1 (a) shows only the optical waveguide, Figure 1 (b) shows the entirety of the optical modulation element including the traveling-wave electrode.

[0051] As Figure 1 (a) and (b) show, the optical modulation element 1 includes: a Mach-Zehnder optical waveguide 2 formed on a substrate 10, having first and second waveguides 2a, 2b arranged in parallel with each other; a first signal electrode 4a arranged along the first waveguide 2a; a second signal electrode 4b arranged along the second waveguide 2b; a first bias electrode 5a arranged along the first waveguide 2a; a second bias electrode 5b arranged along the second waveguide 2b. The first and second signal electrodes 4a, 4b and the first and second waveguides 2a, 2b together constitute the RF interaction part 3ir of the Mach-Zehnder type optical modulation element. In addition, the first and second bias electrodes 5a, 5b and the first and second waveguides 2a, 2b together constitute the DC interaction part 3id of the Mach-Zehnder type optical modulation element.

[0052] The Mach-Zehnder optical waveguide 2 is an optical waveguide element having a structure of a Mach-Zehnder interferometer, and has: an input waveguide 2i; a beam splitting part 2c that splits the light propagating in the input waveguide 2i; first and second waveguides 2a, 2b extending from the beam splitting part 2c and arranged in parallel with each other; a beam combining part 2d that combines the light propagating in the first and second waveguides 2a, 2b; and an output waveguide 2o that propagates the light output from the beam combining part 2d. The input light input to the input waveguide 2i is split by the beam splitting part 2c, travels separately in the first and second waveguides 2a, 2b, is combined by the beam combining part 2d, and is output as modulated light from the output waveguide 2o.

[0053] The first and second signal electrodes 4a, 4b are linear electrode patterns that overlap the first and second waveguides 2a, 2b in a top view, and both ends thereof are led out to the vicinity of the outer peripheral end of the substrate 10. That is, one ends 4a1, 4b1 of the first and second signal electrodes 4a, 4b are led out to the vicinity of the edge of the substrate 10 to form signal input ports, and a drive circuit 9a is connected to the signal input ports. In addition, the other ends 4a2, 4b2 of the first and second signal electrodes 4a, 4b are led out to the vicinity of the edge of the substrate 10 and are connected to each other via a terminal resistor 9b. Thus, the first and second signal electrodes 4a, 4b function as differential coplanar traveling wave electrodes.

[0054] In order to apply a DC voltage (DC bias) to the first and second waveguides 2a, 2b, the first and second bias electrodes 5a, 5b are provided independently of the first and second signal electrodes 4a, 4b. One ends 5a1, 5b1 of the first and second bias electrodes 5a, 5b are led out to the vicinity of the edge of the substrate 10 to form DC bias input ports, and a bias circuit 9c is connected to the DC bias ports. In the present embodiment, the formation regions of the first and second bias electrodes 5a, 5b are provided on the output end side of the Mach-Zehnder optical waveguide 2 with respect to the formation regions of the first and second signal electrodes 4a, 4b, but may also be provided on the input end side.

[0055] In this way, the first and second signal electrodes 4a and 4b constitute an RF section 3a for applying an RF signal to the first and second waveguides 2a and 2b, and the first and second bias electrodes 5a and 5b constitute a DC section 3b for applying a DC bias to the first and second waveguides 2a and 2b. No signal electrode or bias electrode is provided in the intermediate section 3c between the RF section 3a and the DC section 3b. Details will be described later, but the optical waveguide in the intermediate section 3c has a special shape for connecting the optical waveguides of the RF section 3a and the DC section 3b.

[0056] Differential signals (modulation signals) with the same absolute value but different polarities are input to one ends of the first and second signal electrodes 4a and 4b. Since the first and second waveguides 2a and 2b are made of a material having an electro-optic effect such as lithium niobate, the refractive indices of the first and second waveguides 2a and 2b change as +Δn and -Δn, respectively, due to the electric fields applied to the first and second waveguides 2a and 2b, and the phase difference between the pair of optical waveguides changes. The signal light modulated by this change in phase difference is output from the output waveguide 2o.

[0057] In this way, since the optical modulation element 1 of the present embodiment is a dual-drive type composed of a pair of signal electrodes, the symmetry of the electric fields applied to the pair of optical waveguides can be improved, and wavelength chirping can be suppressed.

[0058] Figure 2 (a) and (b) are schematic cross-sectional views of the optical modulation element 1. Figure 2 (a) is a cross-sectional view of the RF section 3a along the Figure 1 (a) and (b) X1-X1' line. Figure 2 (b) is a cross-sectional view of the DC section 3b along the Figure 1 (a) and (b) X2-X2' line.

[0059] As Figure 2 (a) and (b) show, the optical modulation element 1 has a multilayer structure in which a substrate 10, a waveguide layer 11, a protective layer 12, a buffer layer 13, and an electrode layer 14 are laminated in sequence.

[0060] The substrate 10 is, for example, a sapphire single crystal substrate, and a waveguide layer 11 made of an electro-optic material typified by lithium niobate is formed on the main surface of the substrate 10. The waveguide layer 11 has a ridge portion 11r as a protruding portion and flat portions 11s with a thinner film thickness provided on both sides of the ridge portion 11r. The ridge portion 11r constitutes the first and second waveguides 2a and 2b. The width W1 (the first ridge width) of the ridge portion 11r in the RF section 3a and the width W2 (the second ridge width) of the ridge portion 11r in the DC section 3b can be set to 0.5 to 5 μm. In the present embodiment, the width W1 of the ridge portion 11r in the RF section 3a is equal to the width W2 of the ridge portion 11r in the DC section 3b, but they may also be different.

[0061] The ridge portion 11r is the part that becomes the center of the optical waveguide. As described above, the ridge portion 11r is the upwardly protruding part. In terms of this upwardly protruding part, the film thickness of the electro-optic material film becomes thicker than the parts on the left and right, so the effective refractive index becomes higher. Therefore, light is also restricted in the left-right direction and functions as a three-dimensional optical waveguide. The shape of the ridge portion 11r only needs to be a shape capable of guiding light, and the film thickness of the electro-optic material film in the ridge portion 11r only needs to be a convex shape that is thicker than the electro-optic material films on the left and right. Therefore, it can also be a dome shape convex upward, a triangular shape, etc. The ridge portion 11r can be formed by forming a mask such as a resist on the electro-optic material film and selectively etching the electro-optic material film to form a pattern. The width, height, shape, etc. of the ridge portion 11r need to be optimized to improve the device characteristics.

[0062] Generally, the thickness of the ridge portion 11r is equal to the thickness of the electro-optic material film. The width of the ridge portion 11r (the ridge widths W1 and W2) is defined as the width of the upper surface of the ridge portion 11r. This is because the side surface of the illustrated ridge portion 11r is perpendicular to the substrate 10, but there are also cases where it is inclined. It is preferable that the inclination angle of the side surface of the ridge portion 11r is close to 90°, but it is sufficient that it is at least 70° or more. In the case where the width of the upper surface of the ridge portion 11r is set as the ridge width in this way, even when the ridge portion 11r has a trapezoidal shape, the ridge width can be clearly defined.

[0063] The flat portions 11s provided on both sides of the ridge portion 11r are portions made of an electro-optic material film that extends from the ridge portion 11r to the left and right and is thinner than the ridge portion 11r. In the present embodiment, the flat portions 11s have a substantially constant thickness, but the film thickness of the flat portion near the lower part of the ridge portion 11r is unstable, and sometimes a gentle tapered shape remains or a depression occurs. Therefore, the thickness of the flat portion 11s is not the thickness at the place where the film thickness changes gradually, but is defined as the thickness at the place where the film thickness is stable and slightly away from the lower part of the ridge portion 11r.

[0064] The thickness T of the flat portion 11s in the RF section 3a sb1(The thickness of the first flat plate) is different from the thickness T of the flat plate portion 11s in the DC portion 3b. sb2 (The thickness of the second flat plate) is different. In the present embodiment, it is preferable that the thickness T of the flat plate portion 11s in the DC portion 3b sb2 is thicker than the thickness T of the flat plate portion 11s in the RF portion 3a. sb1 In this way, by increasing the thickness of the flat plate of the ridge waveguide in the DC portion 3b, DC drift can be reduced, and long life of the optical modulation element can be achieved. In addition, by reducing the thickness of the flat plate of the ridge waveguide in the RF portion 3a and increasing the protruding height of the ridge portion 11r, light confinement can be enhanced to improve the optical modulation efficiency.

[0065] The protective layer 12 is formed in a region that does not overlap with the first and second waveguides 2a, 2b in a top view. Since the protective layer 12 covers the entire surface of the upper surface of the waveguide layer 11 where the ridge portion 11r is not formed, and the side surface of the ridge portion 11r is also covered by the protective layer 12, scattering loss due to roughness of the side surface of the ridge portion 11r can be prevented. The thickness of the protective layer 12 is substantially the same as the height of the ridge portion 11r of the waveguide layer 11. The material of the protective layer 12 is not particularly limited, and for example, silicon oxide (SiO2) can be used.

[0066] The buffer layer 13 is formed at least on the upper surface of the ridge portion 11r in order to prevent the light propagating in the first and second waveguides 2a, 2b from being absorbed by the first and second signal electrodes 4a, 4b. The buffer layer 13 is preferably made of a material having a refractive index smaller than that of the waveguide layer 11 and higher transparency than that of the waveguide layer 11. For example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, Y2O3, etc. can be used. The thickness of the buffer layer 13 on the upper surface of the ridge portion 11r may be about 0.2 to 1 μm. More preferably, the buffer layer 13 is made of a material having a high dielectric constant. In the present embodiment, the buffer layer 13 covers not only the upper surfaces of the first and second waveguides 2a, 2b, but also the entire surface of the base surface including the upper surface of the protective layer 12, but it may also be patterned to selectively cover only the vicinity of the upper surfaces of the first and second waveguides 2a, 2b. In addition, the protective layer 12 may be omitted, and the buffer layer 13 may be directly formed on the entire upper surface of the waveguide layer 11.

[0067] Regarding the film thickness of the buffer layer 13, in order to reduce the light absorption of the electrode, the thicker the better, and in order to apply a high electric field to the optical waveguide, the thinner the better. There is a trade-off relationship between the light absorption of the electrode and the applied voltage of the electrode, so it is necessary to set an appropriate film thickness according to the purpose. The higher the dielectric constant of the buffer layer 13, the more capable it is of reducing VπL (an index indicating the electric field efficiency), so it is preferred. The lower the refractive index of the buffer layer 13, the more capable it is of thinning the buffer layer 13, so it is preferred. Generally, the refractive index of a material with a high dielectric constant also becomes high, so it is very important to select a material with a high dielectric constant and a low refractive index considering the balance between the two. As an example, Al2O3 has a relative dielectric constant of about 9 and a refractive index of about 1.6, and is a preferred material. LaAlO3 has a relative dielectric constant of about 13 and a refractive index of about 1.7. In addition, LaYO3 has a relative dielectric constant of about 17 and a refractive index of about 1.7, and is a particularly preferred material.

[0068] It can also be composed of different materials Figure 2 (a) The buffer layer 13 of the RF section 3a and Figure 2 (b) The buffer layer 13 of the DC section 3b. By using a buffer layer material that can optimize the characteristics of the RF section 3a for the buffer layer 13 of the RF section 3a, and using a buffer layer material that can reduce DC drift for the buffer layer 13 of the DC section 3b, each characteristic can be optimized. As a buffer layer material that can reduce DC drift, for example, a material containing an oxide of silicon and indium can be cited.

[0069] As Figure 2 (a) shows that a first signal electrode 4a and a second signal electrode 4b are provided on the electrode layer 14 of the RF section 3a. In order to modulate the light traveling in the first waveguide 2a, the first signal electrode 4a is disposed overlapping the ridge portion 11r corresponding to the first waveguide 2a and is opposed to the first waveguide 2a via the buffer layer 13. In order to modulate the light traveling in the second waveguide 2b, the second signal electrode 4b is disposed overlapping the ridge portion 11r corresponding to the second waveguide 2b and is opposed to the second waveguide 2b via the buffer layer 13.

[0070] As Figure 2 (b) shows that a first bias electrode 5a and a second bias electrode 5b are provided on the electrode layer 14 in the DC section 3b. In order to apply a bias electric field to the light traveling in the first waveguide 2a, the first bias electrode 5a is disposed overlapping the ridge portion 11r corresponding to the first waveguide 2a and is opposed to the first waveguide 2a via the buffer layer 13. In order to apply a bias electric field to the light traveling in the second waveguide 2b, the second bias electrode 5b is disposed overlapping the ridge portion 11r corresponding to the second waveguide 2b and is opposed to the second waveguide 2b via the buffer layer 13.

[0071] As Figure 2As shown in (a) and (b), in a cross-section orthogonal to the traveling directions of the first and second waveguides 2a and 2b, the electrode structure is symmetric left and right. Therefore, it is possible to make the magnitudes of the electric fields applied to the first and second waveguides 2a and 2b from the first and second signal electrodes 4a and 4b as identical as possible to reduce wavelength chirping. Further, in the present invention, the electrode structure is not particularly limited, and it may be a so-called single-drive type electrode structure, and the presence or absence and layout of the ground electrode are also not particularly limited.

[0072] The waveguide layer 11 is not particularly limited as long as it is an electro-optic material, but is preferably made of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optic constant and is suitable as a constituent material for optical devices such as optical modulation elements. Hereinafter, the structure of the present embodiment when the waveguide layer 11 is a lithium niobate film will be described in detail.

[0073] The substrate 10 is not particularly limited as long as its refractive index is lower than that of the lithium niobate film, but is preferably a substrate on which the lithium niobate film can be formed as an epitaxial film, and is preferably a sapphire single crystal substrate or a silicon single crystal substrate. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film has a property of being easily formed as an epitaxial film with a c-axis orientation with respect to single crystal substrates having various crystal orientations. Since the c-axis oriented lithium niobate film has a three-fold symmetry, it is desirable that the single crystal substrate of the substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferred, and in the case of a silicon single crystal substrate, a (111) plane substrate is preferred.

[0074] Here, an epitaxial film is a film that is oriented in accordance with the crystal orientation of the underlying substrate or underlying film. When the in-plane of the film is set as the X-Y plane and the film thickness direction is set as the Z axis, the crystal is oriented in accordance with the X axis, Y axis, and Z axis directions. For example, first, the peak intensity at the orientation position based on 2θ-θ X-ray diffraction is confirmed, and then the pole is confirmed, whereby the epitaxial film can be proved.

[0075] Specifically, first, when performing measurement based on 2θ-θ X-ray diffraction, the peak intensity of all planes other than the plane of interest needs to be 10% or less, preferably 5% or less, of the maximum peak intensity of the plane of interest. For example, in the c-axis oriented epitaxial film of lithium niobate, the peak intensity of planes other than the (00L) plane is 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. (00L) is a display collectively representing equivalent planes such as (001) and (002).

[0076] Next, in the pole measurement, it is necessary to observe the poles. Among the conditions for confirming the peak intensity at the above-described first orientation position, only the orientation in one direction is shown. Even if the above-described first condition is satisfied, when the crystal orientations in the plane are inconsistent, the intensity of the X-ray does not increase at a specific angular position, and the poles cannot be seen. Since LiNbO3 has a trigonal crystal structure, there are three poles of LiNbO3(014) in the single crystal.

[0077] In the case of a lithium niobate film, it is known that epitaxial growth is performed in a state of so-called twin crystals in which crystals rotated 180° around the c-axis are symmetrically bonded. In this case, since the three poles become a state in which two are symmetrically bonded, the number of poles becomes six. In addition, when a lithium niobate film is formed on a (100) plane silicon single crystal substrate, the substrate has four-fold symmetry, so 4×3 = 12 poles are observed. Further, in the present invention, a lithium niobate film epitaxially grown in a twin crystal state is also included in the epitaxial film.

[0078] The composition of the lithium niobate film is Li x NbA y O z . A represents an element other than Li, Nb, and O. x is 0.5 to 1.2, preferably 0.9 to 1.05. y is 0 to 0.5. z is 1.5 to 4, preferably 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, etc., and two or more thereof can be combined.

[0079] The film thickness of the lithium niobate film is preferably 2 μm or less. This is because it is difficult to form a high-quality film when the film thickness is greater than 2 μm. On the other hand, when the film thickness of the lithium niobate film is too thin, the light confinement in the lithium niobate film becomes weak, and light leaks to the substrate 10 or the buffer layer 13. Even when an electric field is applied to the lithium niobate film, the change in the effective refractive index of the optical waveguides (2a, 2b) may become small. Therefore, the film thickness of the lithium niobate film is preferably at least about 1 / 10 of the wavelength of the light used.

[0080] As a method for forming a lithium niobate film, film formation methods such as sputtering, CVD, and sol-gel methods are preferably used. The c-axis of lithium niobate is oriented perpendicular to the main surface of the substrate 10, and an electric field is applied parallel to the c-axis. Thus, the optical refractive index changes proportionally with the electric field. When sapphire is used as the single crystal substrate, the lithium niobate film can be directly epitaxially grown on the sapphire single crystal substrate. When silicon is used as the single crystal substrate, the lithium niobate film is formed by epitaxial growth via a cladding layer (not shown). As the cladding layer (not shown), a cladding layer with a refractive index lower than that of the lithium niobate film and suitable for epitaxial growth is used. For example, when Y2O3 is used as the cladding layer (not shown), a high-quality lithium niobate film can be formed.

[0081] In addition, as a method for forming a lithium niobate film, a method of grinding or cutting a lithium niobate single crystal substrate to thin it is also known. This method has the advantage of obtaining the same characteristics as a single crystal and can be applied to the present invention.

[0082] As Figure 2 As shown in (a), the widths of the first and second signal electrodes 4a and 4b are slightly wider than the ridge width W1 of the first and second waveguides 2a and 2b formed of the ridge-shaped lithium niobate film. In order to concentrate the electric fields from the first and second signal electrodes 4a and 4b on the first and second waveguides 2a and 2b, the widths of the first and second signal electrodes 4a and 4b are preferably 1.1 to 15 times, more preferably 1.5 to 10 times, the ridge width W1 of the first and second waveguides 2a and 2b.

[0083] As Figure 2 As shown in (b), the widths of the first and second bias electrodes 5a and 5b are slightly wider than the ridge width W2 of the first and second waveguides 2a and 2b formed of the ridge-shaped lithium niobate film. In order to concentrate the electric fields from the first and second bias electrodes 5a and 5b on the first and second waveguides 2a and 2b, the widths of the first and second bias electrodes 5a and 5b are preferably 1.1 to 15 times, more preferably 1.5 to 10 times, the ridge width W2 of the first and second waveguides 2a and 2b.

[0084] Figure 2 The flat portions 11s of the first and second waveguides 2a and 2b in the DC portion 3b shown in (b) are thicker than Figure 2 the flat portions 11s of the first and second waveguides 2a and 2b in the RF portion 3a shown in (a). Thus, by increasing the thickness T of the flat film of the ridge waveguide in the DC portion 3b sb2 , the DC drift can be reduced. In addition, by reducing the thickness T of the flat film of the ridge waveguide in the RF portion 3a sb1 , the light confinement in the optical waveguide can be enhanced to improve the modulation efficiency, and driving can be performed at as low a voltage as possible.

[0085] The reason for reducing DC drift by thickening the flat film thickness in the DC section 3b is not clear, but it is speculated whether the damage generated when processing the lithium niobate film into a ridge shape affects DC drift. In order to reduce the flat film thickness (increase the protrusion height of the ridge), it is necessary to dig deeper into the upper surface of the lithium niobate film, leaving more damage on the processed surface. On the other hand, when thickening the flat film thickness (reducing the protrusion height of the ridge), the processing amount of the lithium niobate film is small, so the damage on the processed surface is small. Therefore, it is considered that DC drift is reduced and the life of the DC section 3b is extended.

[0086] By relatively increasing the protrusion height of the ridges 11r constituting the first and second waveguides 2a and 2b in both the RF section 3a and the DC section 3b, and reducing the thickness T sb1 、T sb2 (flat film thickness) of the flat portions 11s formed on both sides of the ridge 11r, the optical confinement of the light propagating in the optical waveguide can be enhanced and driven at as low a voltage as possible. However, when this waveguide structure is adopted in the DC section 3b, the DC drift becomes large and the optical modulation element cannot have a long life. On the other hand, when reducing the protrusion height of the ridges 11r in both the RF section 3a and the DC section 3b, and thickening the thickness T sb1 、T sb2 (flat film thickness) of the flat portions 11s, the DC drift can be reduced, but the half-wavelength voltage Vπ becomes high and it cannot be driven at a low voltage.

[0087] However, as in this embodiment, by making the flat film thickness of the optical waveguide in the DC section 3b different from that in the RF section 3a, relatively thickening the flat film thickness of the DC section 3b, and relatively thinning the flat film thickness of the RF section 3a, the DC drift suppression effect in the DC section 3b and the low driving voltage in the RF section 3a can be achieved at the same time.

[0088] When the flat film thickness T sb1 of the first and second waveguides 2a and 2b in the RF section 3a is different from the flat film thickness T sb1 of the first and second waveguides 2a and 2b in the DC section 3b, when simply connecting the two, due to the mismatch of the waveguide shape, the connection loss becomes large. Therefore, in this embodiment, an intermediate waveguide section for improving the waveguide matching is provided in the intermediate section 3c between the RF section 3a and the DC section 3b. Hereinafter, the structure of the intermediate waveguide section will be described in detail.

[0089] Figure 3 is a schematic top view and a schematic cross-sectional view showing the waveguide structure near the intermediate section 3c between the RF section 3a and the DC section 3b. In addition, Figure 4 is a schematic perspective view showing the waveguide structure in the intermediate section 3c three-dimensionally.

[0090] As Figure 3and Figure 4 As shown, each of the first and second waveguides 2a and 2b has: a first waveguide portion 21a which is an optical waveguide of the RF section 3a, a second waveguide portion 21b which is an optical waveguide of the DC section 3b, a first intermediate waveguide portion 22a provided on the RF section 3a side of the intermediate section 3c and connected to the first waveguide portion 21a, and a second intermediate waveguide portion 22b provided on the DC section 3b side of the intermediate section 3c and connected to the second waveguide portion 21b. That is, the first and second waveguides 2a and 2b have a structure in which the first waveguide portion 21a, the first intermediate waveguide portion 22a, the second intermediate waveguide portion 22b, and the second waveguide portion 21b are arranged in sequence from the RF section 3a side to the DC section 3b side.

[0091] The first waveguide portion 21a has a ridge width W1 and a flat film thickness T sb1 , and the second waveguide portion 21b has a ridge width W2 (=W1) and a flat film thickness T sb2 (>T sb1 ). When directly connecting the first waveguide portion 21a and the second waveguide portion 21b with different flat film thicknesses and a narrow ridge width, due to the mismatch at the boundary portion, the propagation loss of light increases. Thus, in the present embodiment, the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b are provided between the first waveguide portion 21a on the RF section 3a side and the second waveguide portion 21b on the DC section 3b side to reduce the propagation loss of light.

[0092] The first intermediate waveguide portion 22a is a ridge waveguide connected to the first waveguide portion 21a and has the same flat film thickness T as the first waveguide portion 21a sb1 , but has a ridge width W 1C (the third ridge width) wider than that of the first waveguide portion 21a. The first intermediate waveguide portion 22a is provided to increase the ridge width of the first waveguide portion 21a and has a ridge width expansion portion where the ridge width gradually increases toward the second waveguide portion 21b. In this way, at the connection position with the second intermediate waveguide portion 22b, the first intermediate waveguide portion 22a has a ridge width W 1C (>W1) wider than that of the first waveguide portion 21a.

[0093] The second intermediate waveguide portion 22b is a ridge waveguide connected to the second waveguide portion 21b and has the same flat film thickness T as the second waveguide portion 21b sb2 , but has a ridge width W 2C (the fourth ridge width) wider than that of the second waveguide portion 21b. The second intermediate waveguide portion 22b is provided to increase the ridge width of the second waveguide portion 21b and has a ridge width expansion portion where the ridge width gradually increases toward the first waveguide portion 21a. In this way, at the connection position with the first intermediate waveguide portion 22a, the second intermediate waveguide portion 22b has a ridge width W 2C (>W2) wider than that of the second waveguide portion 21b.

[0094] The ridge width W of the first intermediate waveguide portion 22a 1C is wider than the ridge width W of the second intermediate waveguide portion 22b 2C Accordingly, at the boundary position between the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b, the ridge width and the flat plate film thickness of the optical waveguide change discontinuously. When connecting the first waveguide portion 21a on the RF portion 3a side and the second waveguide portion 21b on the DC portion 3b side, which have different flat plate film thicknesses, the spot size of the light propagating in the optical waveguide is different, so the connection loss increases at the boundary position. However, by widening the ridge width W of the optical waveguide on the RF portion 3a side where the flat plate film thickness is relatively thin 1C and narrowing the ridge width W of the optical waveguide on the DC portion 3b side where the flat plate film thickness is relatively thick 2C it is possible to make the spot size of the light at the boundary between the first waveguide portion 21a and the second waveguide portion 21b consistent, and thereby reduce the connection loss.

[0095] Preferably, the ridge width W of the first intermediate waveguide portion 22a 1C is larger than the ridge width W of the second intermediate waveguide portion 22b 2C and smaller than twice the ridge width W of the second intermediate waveguide portion 22b (W 2C <W 2C <2×W 1C ). This can prevent an increase in the connection loss caused by the ridge width W of the first intermediate waveguide portion 22a 2C being too large. 1C

[0096] Generally, it is easy to form an optical waveguide with a varying ridge width. By a single patterning, an optical waveguide having a tapered shape with a continuously varying ridge width can be formed, so there is no need to form an optical waveguide with a discontinuous ridge width. However, in forming optical waveguides with different flat plate film thicknesses, two patterning processes with different processing conditions are required, and it is not easy to accurately align the two optical waveguides. Therefore, in the present embodiment, the optical waveguide on the RF portion 3a side and the optical waveguide on the DC portion 3b side, which have different flat plate film thicknesses, are processed separately, and the ridge width near the connection portion between the optical waveguide on the RF portion 3a side and the optical waveguide on the DC portion 3b side is widened to connect the two. Thereby, the connection loss can be reduced, but a ridge waveguide with a discontinuous ridge width and flat plate film thickness is formed.

[0097] Figure 5 is a schematic top view showing a case where an axial shift occurs between the optical waveguide on the RF portion 3a side and the optical waveguide on the DC portion 3b side.

[0098] ​In the case where the flat film thickness of the ridge waveguide is made different, the waveguide patterns on the RF unit 3a side (the first waveguide unit 21a and the first intermediate waveguide unit 22a) and the waveguide patterns on the DC unit 3b side (the second waveguide unit 21b and the second intermediate waveguide unit 22b) are processed separately. As Figure 5 shown, misalignment (axial shift) of the waveguide pattern sometimes occurs. However, by increasing the ridge width of the first intermediate waveguide unit 22a and the second intermediate waveguide unit 22b as in the present embodiment, it is possible to prevent a reduction in the cross-sectional area of the waveguide caused by axial shift, and to reduce the optical propagation loss caused by processing deviation of the waveguide pattern.

[0099] As described above, in the optical modulation element 1 of the present embodiment, the flat film thickness T of the optical waveguide of the DC unit 3b sb2 is larger than the flat film thickness T of the optical waveguide of the RF unit 3a sb1 Therefore, a low drive voltage can be applied to the RF unit 3a, and DC drift can be reduced.

[0100] In addition, in the optical modulation element 1 of the present embodiment, the optical waveguide (the first waveguide unit 21a) of the RF unit 3a and the optical waveguide (the second waveguide unit 21b) of the DC unit 3b having different flat film thicknesses are connected. Therefore, the first intermediate waveguide unit 22a and the second intermediate waveguide unit 22b with a changing ridge width are provided in the intermediate unit 3c between the RF unit 3a and the DC unit 3b, and the ridge width W of the first intermediate waveguide unit 22a on the RF unit 3a side 1C is wider than the ridge width W of the second intermediate waveguide unit 22b on the DC unit 3b side 2C Therefore, it is possible to match the spot size of the light propagating in the first waveguide unit 21a and the spot size of the light propagating in the second waveguide unit 21b to reduce the propagation loss.

[0101] Figure 6 is an optical modulation element according to a second embodiment of the present invention, and is a schematic top view and a schematic cross-sectional view showing a waveguide structure near the intermediate unit 3c between the RF unit 3a and the DC unit 3b. In addition, Figure 7 is a schematic perspective view showing three-dimensionally Figure 6 the waveguide structure in the intermediate unit 3c shown in

[0102] As shown in Figure 6 and Figure 7 shown, the optical modulation element 1 is characterized in that a third intermediate waveguide unit 22c is provided between the first intermediate waveguide unit 22a and the second intermediate waveguide unit 22b. The third intermediate waveguide unit 22c has: a ridge portion 11r having a two-stage structure, which has a ridge lower portion having the same ridge width W as the first intermediate waveguide unit 22a 1C and a ridge upper portion having the same ridge width W as the second intermediate waveguide unit 22b 2CThe upper part of the ridge structure; the flat plate part 11s, which is formed on both sides of the ridge part 11r of the secondary structure, and the flat plate film thickness T sb3 is formed thinner than the flat plate film thickness T of the first waveguide part 21a sb1 . In the present embodiment, the third intermediate waveguide part 22c is a ridge waveguide having a thin flat plate part 11s, but it may also be a waveguide structure in which the flat plate part 11s is completely eliminated.

[0103] Preferably, the length L of the third intermediate waveguide part 22c C is 3 μm or less. This is because when the third intermediate waveguide part 22c is too long, the propagation loss increases.

[0104] Figure 8 (a) to (c) are diagrams for explaining a method of forming a waveguide structure having the third intermediate waveguide part 22c shown in Figure 6 and Figure 7 .

[0105] As shown in Figure 8 (a) to (c), a waveguide structure having the third intermediate waveguide part 22c can be formed by locally overlapping the processing area of the waveguide pattern for forming the RF part 3a and the processing area of the waveguide pattern for forming the DC part 3b near the boundary between the two.

[0106] Specifically, first, as shown in Figure 8 (a), after covering substantially the entire surface of the formation area 25b of the DC part 3b with the mask 30b, the electro-optic material film in the formation area 25a of the RF part 3a is processed by milling or the like to form the first waveguide part 21a and the first intermediate waveguide part 22a as the waveguide pattern of the RF part 3a. At this time, the edge E of the mask 30b b is located on the front side with respect to the boundary line B and does not protrude to the side of the formation area 25a of the RF part 3a, so ridge processing is performed on the formation area 25b of the DC part 3b near the boundary line B. In the processing of the RF part 3a, for example, by extending the processing time, a thin flat plate part is formed.

[0107] Next, as shown in Figure 8 (b), after covering substantially the entire surface of the formation area 25a of the RF part 3a with the mask 30a, the electro-optic material film in the formation area 25b of the DC part 3b is processed by milling or the like to form the second waveguide part 21b and the second intermediate waveguide part 22b as the waveguide pattern of the DC part 3b. At this time, the edge E of the mask 30a a is located on the front side with respect to the boundary line B and does not protrude to the side of the formation area 25b of the DC part 3b, so ridge processing is performed on the formation area 25a of the RF part 3a near the boundary line B. In the processing of the DC part 3b, by shortening, for example, the processing time, a thick flat plate part is formed.

[0108] Thus, near the boundary line B between the formation region 25a of the RF unit 3a and the formation region 25b of the DC unit 3b, by doubly performing ridge processing, as Figure 8 (c) shows, a third intermediate waveguide portion 22c having a cross-sectional shape different from that of the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b is formed. In addition, in Figure 8 , processing is performed in the order of the RF unit 3a and the DC unit 3b. Conversely, even if processing is performed in the order of the DC unit 3b and the RF unit 3a, the same result is obtained.

[0109] According to the present embodiment, the same effect as that of the first embodiment can be achieved. That is, the ridge width W of the first intermediate waveguide portion 22a on the RF unit 3a side 1C is wider than the ridge width W of the second intermediate waveguide portion 22b on the DC unit 3b side 2C , so that the spot size of the light propagating in the first waveguide portion 21a can be matched with the spot size of the light propagating in the second waveguide portion 21b to reduce the propagation loss.

[0110] Figure 9 FIGS. are a schematic top view and a schematic cross-sectional view of the waveguide structure near the intermediate portion 3c between the RF unit 3a and the DC unit 3b of the optical modulation element according to the third embodiment of the present invention. In addition, Figure 10 is a schematic perspective view that three-dimensionally shows Figure 9 the waveguide structure in the intermediate portion 3c shown in

[0111] As shown in Figure 9 and Figure 10 , the optical modulation element 1 is characterized in that a third intermediate waveguide portion 22c is provided between the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b, and the third intermediate waveguide portion 22c is composed of an unprocessed waveguide layer 11 that does not have a ridge shape. That is, the third intermediate waveguide portion 22c is a region that does not have a waveguide shape. Other structures are the same as those of the second embodiment.

[0112] Figure 11 (a) to (b) are diagrams for explaining a method of forming a waveguide structure having Figure 9 and Figure 10 the third intermediate waveguide portion 22c shown in

[0113] As shown in Figure 11 (a) to (c), a waveguide structure having the third intermediate waveguide portion 22c can be formed by preventing the processing region of the waveguide pattern for forming the RF unit 3a and the processing region of the waveguide pattern for forming the DC unit 3b from overlapping near their boundaries.

[0114] Specifically, first, as shown in Figure 11As shown in (a), after covering substantially the entire surface of the formation region 25b of the DC section 3b with the mask 30b, the electro-optical material film within the formation region 25a of the RF section 3a is processed by milling or the like to form a first waveguide section 21a and a first intermediate waveguide section 22a, which are waveguide patterns of the RF section 3a. At this time, the edge E of the mask 30b b is located deeper than the boundary line B and extends on the side of the formation region 25a of the RF section 3a. Therefore, an unprocessed region is formed in the formation region 25a of the RF section 3a near the boundary line B. In the processing of the RF section 3a, for example, by extending the processing time, a thin flat section is formed.

[0115] Next, as Figure 11 shown in (b), after covering substantially the entire surface of the formation region 25a of the RF section 3a with the mask 30a, the electro-optical material film within the formation region 25b of the DC section 3b is processed by milling or the like to form a second waveguide section 21b and a second intermediate waveguide section 22b, which are waveguide patterns of the DC section 3b. At this time, the edge E of the mask 30a a is located deeper than the boundary line B and extends on the side of the formation region 25b of the DC section 3b. Therefore, an unprocessed region is formed in the formation region 25a of the RF section 3a near the boundary line B. In the processing of the DC section 3b, by shortening, for example, the processing time, a thick flat section is formed.

[0116] In this way, by forming an unprocessed region near the boundary line B between the formation region 25a of the RF section 3a and the formation region 25b of the DC section 3b, as Figure 11 shown in (c), a third intermediate waveguide section 22c having a cross-sectional shape different from that of the first intermediate waveguide section 22a and the second intermediate waveguide section 22b is formed.

[0117] According to the present embodiment, the same effects as those of the first embodiment can be achieved. That is, the ridge width W of the first intermediate waveguide section 22a on the RF section 3a side 1C is wider than the ridge width W of the second intermediate waveguide section 22b on the DC section 3b side 2C so that the spot size of the light propagating in the first waveguide section 21a can be made to match the spot size of the light propagating in the second waveguide section 21b to reduce the propagation loss.

[0118] Figure 12 (a) and (b) are schematic cross-sectional views of the optical modulation element according to the fourth embodiment of the present invention, Figure 12 (a) is a cross-sectional view of the RF section 3a, Figure 12 (b) is a cross-sectional view of the DC section 3b. In addition, Figure 13 is a schematic plan view showing the waveguide structure near the intermediate section 3c between the RF section 3a and the DC section 3b.

[0119] As Figure 12 (a) and (b) and Figure 13 shown, the optical modulation element 1 is characterized in that the ridge widths W1 of the first and second waveguides 2a, 2b (the first waveguide portion 21a) in the RF portion 3a and the ridge widths W2 of the first and second waveguides 2a, 2b (the second waveguide portion 21b) in the DC portion 3b are different. In the present embodiment, the ridge widths W2 of the first and second waveguides 2a, 2b (the second waveguide portion 21b) in the DC portion 3b are wider than the ridge widths W1 of the first and second waveguides 2a, 2b (the first waveguide portion 21a) in the RF portion 3a. Other structures are the same as those in the first embodiment. The optical modulation element 1 of the present embodiment can not only achieve the same effects as those in the first embodiment, but also improve the effect of reducing DC drift.

[0120] Figure 14 (a) and (b) are schematic cross-sectional views of the optical modulation element according to the fifth embodiment of the present invention, Figure 14 (a) is a cross-sectional view of the RF portion 3a, Figure 14 (b) is a cross-sectional view of the DC portion 3b.

[0121] As Figure 14 (a) and (b) shown, the optical modulation element 1 of the present embodiment is characterized in that the protective layer 12 is omitted in the DC portion 3b. That is, the DC portion 3b of the optical modulation element 1 has a multilayer structure in which the substrate 10, the waveguide layer 11, the buffer layer 13, and the electrode layer 14 are laminated in sequence. The buffer layer 13 is formed on the entire surface of the waveguide layer 11 so as to cover not only the upper surface of the ridge portion 11r but also the side surfaces. Therefore, the upper surfaces of the flat portions 11s on both sides of the ridge portion 11r are also covered by the buffer layer 13.

[0122] When the protective layer 12 is omitted in the DC portion 3b, it is preferable that the buffer layer 13 of the DC portion 3b is made of a dielectric material different from that of the buffer layer 13 of the RF portion 3a, and it is preferable that it is made of the same dielectric material as the protective layer 12 of the RF portion 3a. That is, it is preferable to form the buffer layer 13 of the DC portion 3b using the same dielectric material as the protective layer 12 of the RF portion 3a. In addition, the structure in which the protective layer 12 is omitted in the DC portion 3b is equivalent to the protective layer 12 and the buffer layer 13 of the DC portion 3b formed of the same material Figure 2 shown.

[0123] The preferred embodiments of the present invention have been described above, but the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention, and of course, these modifications are also included in the scope of the present invention.

[0124] For example, in the above-described embodiment, an example of a dual-drive type optical modulation element in which a pair of signal electrodes is provided for a Mach-Zehnder optical waveguide having a pair of optical waveguides is given. However, the present invention is not limited to such an optical modulation element, and various optical modulation elements having an RF section 3a and a DC section 3b can be targeted.

[0125] In addition, in the above-described embodiment, an optical modulation element having a pair of optical waveguides formed of a lithium niobate film epitaxially grown on a substrate 10 is given. However, the present invention is not limited to such a structure, and optical waveguides can also be formed of electro-optic materials such as barium titanate and lead zirconate titanate. However, as long as the optical waveguides are formed of a lithium niobate film, the width of the optical waveguides can be formed narrowly and the drive voltage can be reduced. On the other hand, the problem of DC drift is significant, and the effect of the present invention is obvious. In addition, as the waveguide layer 11, a semiconductor material, a polymer material, or the like having an electro-optical effect can also be used.

[0126] (Example)

[0127] (Evaluation of the flat film thickness of the RF section 3a)

[0128] The flat film thickness T of the ridge waveguide in the RF section 3a is evaluated by simulation sb1 for its influence on the electric field efficiency VπL. The thickness of the waveguide layer 11 made of a lithium niobate film is set to 1.5 μm, and the electric field efficiency VπL is obtained when the flat film thickness T of the ridge waveguide sb1 is changed. As a result, as Figure 15 shown, the flat film thickness T of the ridge waveguide sb1 is minimum at about 0.3 μm. In addition, as long as the flat film thickness T sb2 is 0.6 μm or less, VπL can be set to 2.2 Vcm or less, which is a preferable range.

[0129] (Evaluation of the flat film thickness of the DC section)

[0130] The flat film thickness T of the ridge waveguide in the DC section 3b is evaluated sb2 for its influence on DC drift. In this evaluation test, a constant bias voltage is applied to the bias electrode at a temperature of 80°C, and at the same time, the DC drift amount is measured, and the time (life) required for the DC drift amount to exceed 50% from the start of the evaluation test is measured. The thickness of the waveguide layer 11 made of a lithium niobate film at this time is set to 1.5 μm. As a result, the life at a flat film thickness T sb2 of 0.3 μm is about 1 hour, and the life at 0.6 μm is about 84 hours. On the other hand, when the flat film thickness T sb2 is 1.1 μm, the life becomes a good result of more than 1000 hours.

[0131] (Evaluation of the ridge width of the first and second intermediate waveguide sections)

[0132] In Figure 3 and Figure 4 In the waveguide structure shown, the change in the connection loss (dB) was evaluated by simulation when the ridge width W of the first intermediate waveguide portion 22a was changed. The thickness of the waveguide layer 11 made of a lithium niobate film was set to 1.5 μm, the ridge width W of the second intermediate waveguide portion 22b 1C = 2.5 μm, the flat film thickness T of the first intermediate waveguide portion 22a 2C = 0.4 μm, and the flat film thickness T of the second intermediate waveguide portion 22b sb1 = 1.1 μm. As a result, as shown in sb2 it was confirmed that the connection loss was minimized in the range of 3 to 4.5 μm for the ridge width W, and the connection loss of ridge waveguides with different flat film thicknesses could be reduced. Figure 16 As shown 1C in the range of 3 to 4.5 μm, the connection loss was minimized, and the connection loss of ridge waveguides with different flat film thicknesses could be reduced.

[0133] (Evaluation of the axial offset of the optical waveguide)

[0134] The influence of the magnitude ΔW of the axial offset of the optical waveguide shown in Figure 11 on the connection loss (dB) was evaluated by simulation. The thickness of the waveguide layer 11 made of a lithium niobate film was set to 1.5 μm, and the combinations of the ridge widths (W 1C , W 2C ) of the first and second intermediate waveguide portions 22a and 22b were set to five types: (1.2 μm, 2.5 μm), (2.5 μm, 2.5 μm), (3.8 μm, 2.5 μm), (4.2 μm, 3 μm), and (5.2 μm, 4 μm). In addition, the flat film thickness T of the first intermediate waveguide portion 22a sb1 = 0.4 μm, and the flat film thickness T of the second intermediate waveguide portion 22b sb2 = 1.1 μm.

[0135] As a result, as shown in Figure 17 it was observed that the narrower the ridge widths W 1C , W 2C of the first and second intermediate waveguide portions 22a and 22b were, the greater the tendency for the connection loss to be when there was no axial offset of the optical waveguide. In addition, the greater the axial offset, the greater the increase in the connection loss. When the ridge widths (W 1C , W 2C ) of the first and second intermediate waveguide portions 22a and 22b were (1.2 μm, 2.5 μm), the increase rate of the connection loss with respect to the axial offset was the largest. When the ridge widths (W 1C , W 2C ) of the first and second intermediate waveguide portions 22a and 22b were (5.2 μm, 4 μm), the increase rate of the connection loss with respect to the axial offset was the smallest. That is, the ridge widths W of the first and second intermediate waveguide portions 22a and 22b1C , W 2C The narrower it is, the higher the increasing rate of the connection loss is.

[0136] (Evaluation of the influence of the third intermediate waveguide section)

[0137] Evaluate through simulation Figures 6 to 11 The influence of the presence of the third intermediate waveguide section 22c shown on the connection loss (dB). Set the thickness of the waveguide layer 11 composed of a lithium niobate film to 1.5 μm, and the flat film thickness T of the first intermediate waveguide section 22a sb1 = 0.4 μm, and the flat film thickness T of the second intermediate waveguide section 22b sb2 = 1.1 μm. In addition, set the combination of the ridge widths (W 1C , W 2C ) of the first and second intermediate waveguide sections 22a and 22b to three patterns of (3.8 μm, 2.5 μm), (4.2 μm, 3 μm), and (5.2 μm, 4 μm), and evaluate these pattern combinations Figures 6 to 8 The case where the flat film thickness is thin (here, the flat film thickness is zero) shown and the case without Figures 9 to 11 The waveguide structure shown is evaluated.

[0138] As a result, as Figure 18 shown, the longer the third intermediate waveguide section 22c is, the more the connection loss increases. In particular, the increasing rate of the connection loss of the third intermediate waveguide section 22c without Figures 9 to 11 the waveguide structure shown is higher than that of the third intermediate waveguide section 22c of the waveguide structure without Figures 6 to 8 the flat part shown. However, in any structure, as long as the length of the third intermediate waveguide section 22c is 3 μm or less, the connection loss is 0.2 dB or less. It is possible to sufficiently fabricate the third intermediate waveguide section 22c with a length of 3 μm or less, which is a level without practical problems.

[0139] Explanation of reference numerals

[0140] 1 Optical modulation element

[0141] 2 Mach-Zehnder optical waveguide

[0142] 2a First waveguide

[0143] 2b Second waveguide

[0144] 2c Beam splitting section

[0145] 2d Wave combining section

[0146] 2i Input waveguide

[0147] 2o Output waveguide

[0148] 3a RF section

[0149] 3b DC section

[0150] 3c intermediate section

[0151] 3id DC interaction section

[0152] 3ir RF interaction section

[0153] 4a First signal electrode

[0154] 4a1 One end of the first signal electrode

[0155] 4a2 The other end of the first signal electrode

[0156] 4b Second signal electrode

[0157] 4b1 One end of the first signal electrode

[0158] 4b2 The other end of the first signal electrode

[0159] 5a First bias electrode

[0160] 5a1 One end of the first bias electrode

[0161] 5b Second bias electrode

[0162] 5b1 One end of the second bias electrode

[0163] 9 Terminal resistor

[0164] 9a Driver circuit

[0165] 9c Bias circuit

[0166] 10 Substrate

[0167] 11 Waveguide layer

[0168] 11r Ridge

[0169] 11s Flat part

[0170] 12 Protective layer

[0171] 13 Buffer layer

[0172] 14 Electrode layer

[0173] 21a First waveguide section

[0174] 21b Second waveguide section

[0175] 22a First intermediate waveguide section

[0176] 22b Second intermediate waveguide section

[0177] 22c Third intermediate waveguide section

[0178] 30a mask

[0179] 30b mask.

Claims

1. An optical modulation element, characterized in that, Comprising: A substrate; An optical waveguide composed of a thin film of an electro-optic material formed on the substrate, having a ridge portion as a protruding part and a flat portion having a film thickness thinner than that of the ridge portion; A signal electrode that applies an RF signal to the optical waveguide; And A bias electrode that applies a DC bias to the optical waveguide, The optical waveguide includes: A first waveguide portion having a first ridge width and a first flat film thickness; A second waveguide portion having a second ridge width and a second flat film thickness; A first intermediate waveguide portion connected to the first waveguide portion, having a third ridge width and the first flat film thickness; and A second intermediate waveguide portion connected to the second waveguide portion, having a fourth ridge width and the second flat film thickness, The first waveguide portion, the first intermediate waveguide portion, the second intermediate waveguide portion, and the second waveguide portion are arranged in this order, The second flat film thickness is thicker than the first flat film thickness, The third ridge width is wider than the fourth ridge width, The signal electrode applies the RF signal to the first waveguide portion, The bias electrode applies the DC bias to the second waveguide portion.

2. The optical modulation element according to claim 1, wherein: The third ridge width is narrower than twice the fourth ridge width.

3. The optical modulation element according to claim 1, wherein: The third ridge width is wider than the first ridge width, and the fourth ridge width is wider than the second ridge width.

4. The optical modulation element according to claim 1, wherein: The second ridge width is wider than the first ridge width.

5. The optical modulation element according to any one of claims 1 to 4, wherein: The optical waveguide further includes a third intermediate waveguide portion provided between the first intermediate waveguide portion and the second intermediate waveguide portion, having a cross-sectional shape different from those of the first intermediate waveguide portion and the second intermediate waveguide portion.

6. The optical modulation element according to claim 5, wherein: The third intermediate waveguide portion has a third flat film thickness thinner than the first flat film thickness.

7. The optical modulation element according to claim 5, wherein: The third intermediate waveguide portion does not have the flat portion.

8. The optical modulation element according to claim 5, wherein: The third intermediate waveguide portion does not have a waveguide shape.

9. The optical modulation element according to claim 5, wherein: The length of the third intermediate waveguide portion is 3 μm or less.

10. The optical modulation element according to claim 5, wherein: The first flat film thickness is less than 0.6 μm, The second flat film thickness is 0.6 μm or more.

11. The optical modulation element according to claim 1, wherein: The thin film is a lithium niobate film, The c-axis of the lithium niobate film is oriented in a direction perpendicular to the main surface of the substrate.

Images