Light modulation element

Abstract

The present invention provides an optical modulator that can achieve both low driving voltage and DC drift suppression. The optical modulator (1) comprises: a substrate (10); and an optical waveguide, which is composed of an electro-optical material film formed on the substrate (10), and has a ridge portion (11r) as a protruding portion and a flat plate portion (11s) having a film thickness thinner than the ridge portion (11r). The optical waveguide includes: a first waveguide portion having a first ridge width (W1) and a first flat plate film thickness (T sb1 ), to which an RF signal is applied; a second waveguide portion having a second ridge width (W2) and a thickness (T sb1 ) different second plate film thickness (T sb2 ), a DC bias is applied.

Description

Technical Field

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

[0002] With the rapid growth of the internet, communication volume has increased dramatically, placing a significant emphasis on fiber-optic communications. Fiber-optic communications convert electrical signals into optical signals and transmit them through optical fibers. They offer wide bandwidth, low loss, and strong noise immunity.

[0003] Known methods for converting electrical signals into optical signals include direct modulation using semiconductor lasers and external modulation using optical modulators. Direct modulation, while not requiring an optical modulator and cost-effective, has limitations in high-speed modulation. Therefore, external optical modulation is used for high-speed, long-distance applications.

[0004] As an optical modulator, a Mach-Zehnder optical modulator (MZM) is used, in which an optical waveguide is formed near the surface of a lithium niobate single crystal substrate by diffusion of Ti (titanium) (see, for example, Patent Document 1). A Mach-Zehnder optical modulator uses an optical waveguide (MZM) with a Mach-Zehnder interferometer structure, which splits light emitted from a single light source into two, passes through different paths, and then overlaps to cause interference. While high-speed optical modulators exceeding 40 Gb / s are commercially available, their overall length, approximately 10 cm, is a significant drawback.

[0005] In contrast, Patent Document 2 discloses a Mach-Zehnder optical modulator using a c-axis oriented lithium niobate film. Compared to optical modulators using a lithium niobate single crystal substrate, optical modulators using a lithium niobate film can achieve significant miniaturization and lower driving voltage.

[0006] Patent Document 3 describes a ridge-type optical waveguide element comprising a slab portion having a waveguide layer composed of a lithium niobate film formed on a substrate, the waveguide layer having a predetermined thickness, and a ridge portion protruding from the slab portion. The thickness of the slab portion of this ridge-type optical waveguide element is less than 0.4 times the wavelength of light propagating through the ridge portion, thereby minimizing propagation loss even when the ridge width is reduced. Furthermore, Patent Document 4 describes an optical waveguide structure in which, in order to connect an input / output waveguide portion composed of a ridge-type optical waveguide to an optical switch main unit composed of a high-mesa optical waveguide, an input / output tapered waveguide portion is provided between the input / output waveguide portion and the optical switch main unit, thereby causing the waveguide shape to change in a stepwise manner.

[0007] Prior art literature

[0008] Patent Literature

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

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2006-195383

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2017-129834

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

[0013] Technical problem to be solved by the invention

[0014] When forming an optical waveguide by processing a lithium niobate film epitaxially grown on a substrate into a ridge shape, as described in Patent Document 3, light confinement can be enhanced by ensuring a sufficient ridge height and reducing the thickness of the flat plate portions extending to the left and right of the ridge. When a voltage is applied between a pair of electrodes, a sufficient electric field is applied to the optical waveguide, reducing the half-wave voltage Vπ. Furthermore, the half-wave voltage Vπ is the difference (V1-V2) between the voltage V1 at maximum light output and the voltage V2 at minimum light output. The driving voltage is proportional to the half-wave voltage Vπ. Therefore, reducing the half-wave voltage Vπ means lowering the driving voltage.

[0015] However, applying a DC bias to this type of optical waveguide results in significant DC drift, shortening the lifespan of the optical modulator. DC drift refers to the temporal variation of the operating point of the optical output. Typically, the operating point of the optical output is adjusted using a DC bias to achieve the average of the maximum and minimum optical outputs. However, when the thickness of the flat plate is reduced to ensure a high ridge height, the temporal variation of the operating point is significant. Even with a high DC bias, the time until the operating point becomes unadjustable (i.e., the lifespan) is short.

[0016] Therefore, an object of the present invention is to provide an optical modulator that can achieve both low drive voltage and DC drift suppression. Another object of the present invention is to provide an optical waveguide element that can reduce light propagation loss and suppress DC drift, and an optical modulator using the same.

[0017] Technical solutions to technical problems

[0018] The inventors of this application have conducted extensive research on optical modulator structures that can suppress DC drift. They discovered that DC drift depends on the slab thickness of the ridge waveguide. While reducing the slab thickness can reduce the drive voltage of the optical modulator, this also increases DC drift. They also discovered that connecting two optical waveguides with different slab thicknesses increases connection loss due to the discontinuous change in slab thickness. However, adjusting the ridge width of each waveguide individually reduces this connection loss.

[0019] The present invention is based on this technical knowledge. The optical modulator of the present invention comprises: a substrate; and an optical waveguide composed of an electro-optical material film formed on the substrate, the optical waveguide having a ridge portion as a protruding portion and a slab portion having a film thickness thinner than the ridge portion, wherein the optical waveguide includes: a first waveguide portion having a first ridge width (W1) and a first slab film thickness (T sb1 ), to which an RF signal is applied; a second waveguide portion having a second ridge width (W2) and a second slab film thickness (T sb2 ), a DC bias is applied.

[0020] According to the present invention, the slab film thickness of the first waveguide portion constituting the RF portion is different from the slab film thickness of the second waveguide portion constituting the DC portion. Therefore, the driving voltage of the RF portion can be reduced while suppressing DC drift in the DC portion.

[0021] In the present invention, it is preferred that the second flat film thickness (T sb2 ) is thicker than the first flat film (T sb1 )Thickness(T sb2 >T sb1 ). According to this structure, a low driving voltage can be achieved in the RF part, and DC drift can be suppressed in the DC part.

[0022] 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, the DC drift suppression effect can be improved in the DC portion.

[0023] In the present invention, it is preferred that the first flat film thickness (T sb1 ) is less than 0.6μm(0μm≦T sb1 <0.6μm), the second flat plate film thickness (T sb2 ) is 0.6 μm or more (T sb2 ≧0.6 μm). This structure can realize a lower driving voltage in the RF section and suppress DC drift in the DC section.

[0024] In the present invention, the electro-optical material film is preferably a lithium niobate film, with the c-axis of the lithium niobate film oriented perpendicular to the principal surface of the substrate. A ridge waveguide formed of the lithium niobate film can minimize propagation loss even when the ridge width is reduced, enabling miniaturization and lowering of the driving voltage of the optical modulator. Furthermore, by increasing the thickness of the slab film of the second waveguide portion as described above, DC drift can be suppressed.

[0025] The optical modulator of the present invention preferably further comprises: a signal electrode for applying the RF signal to the first waveguide; and a bias electrode for applying the DC bias to the second waveguide. Furthermore, the optical waveguide is preferably a Mach-Zehnder optical waveguide comprising: an input waveguide; a beam splitting portion for splitting light propagating through the input waveguide; first and second waveguides extending from the beam splitting portion and arranged parallel to each other; a beam combining portion for combining light propagating through the first and second waveguides; and an output waveguide for transmitting light output from the beam combining portion. According to the present invention, a long-life Mach-Zehnder optical modulator with reduced drive voltage and suppressed DC drift can be realized.

[0026] Effects of the Invention

[0027] The present invention provides an optical modulator capable of achieving both low drive voltage and DC drift suppression. Furthermore, the present invention provides an optical waveguide element capable of reducing light propagation loss and suppressing DC drift, and an optical modulator using the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 (a) and (b) are schematic plan views showing the structure of the light modulator according to the first embodiment of the present invention. Figure 1 (a) shows only the optical waveguide, Figure 1 (b) shows the entire optical modulation element including the traveling wave electrodes.

[0029] Figure 2 (a) and (b) are schematic cross-sectional views of the light modulation element 1. Figure 2 (a) is along Figure 1 (a) and (b) are cross-sectional views of the RF portion along the X1-X1' line, Figure 2 (b) is along Figure 1 (a) and (b) are cross-sectional views of the DC portion along the X2-X2' line.

[0030] Figure 3 1 and 2 are a schematic top view and a schematic cross-sectional view showing the waveguide structure near the intermediate portion between the RF portion and the DC portion.

[0031] Figure 4It is a rough three-dimensional diagram showing the waveguide structure in the middle portion.

[0032] Figure 5 It is a schematic plan view showing a state where an axial misalignment occurs between the optical waveguide on the RF section side and the optical waveguide on the DC section side.

[0033] Figure 6 1 and 2 are a schematic plan view and a schematic cross-sectional view showing a waveguide structure near an intermediate portion between an RF portion and a DC portion in an optical modulation element according to a second embodiment of the present invention.

[0034] Figure 7 It is a three-dimensional representation Figure 6 A rough three-dimensional view of the waveguide structure in the middle part is shown.

[0035] Figure 8 (a) to (c) are used to illustrate the Figure 6 and Figure 7 FIG. 1 is a diagram showing a method for forming the waveguide structure of the third intermediate waveguide section.

[0036] Figure 9 1 and 2 are a schematic plan view and a schematic cross-sectional view showing a waveguide structure near an intermediate portion between an RF portion and a DC portion in an optical modulation element according to a third embodiment of the present invention.

[0037] Figure 10 It is a three-dimensional representation Figure 9 A rough three-dimensional view of the waveguide structure in the middle part is shown.

[0038] Figure 11 (a) to (c) are used to illustrate the Figure 9 and Figure 10 FIG. 1 is a diagram showing a method for forming the waveguide structure of the third intermediate waveguide section.

[0039] Figure 12 (a) and (b) are schematic cross-sectional views of a light modulator according to a fourth embodiment of the present invention. Figure 12 (a) is a cross-sectional view of the RF portion, Figure 12 (b) is a cross-sectional view of the DC portion.

[0040] Figure 13 It is a schematic plan view showing the waveguide structure near the intermediate portion between the RF section and the DC section.

[0041] Figure 14 It is a schematic cross-sectional view showing the structure of a DC portion of an optical modulation element according to a fifth embodiment of the present invention.

[0042] Figure 15 is the slab film thickness T of the ridge waveguide in the RF section sb1 A graph showing the relationship between ΔV and electric field efficiency VπL.

[0043] Figure 16 is the ridge width W of the first intermediate waveguide portion on the RF portion side. 1C Graph of its relationship with connection loss (dB).

[0044] Figure 17 Graph showing the relationship between the magnitude ΔW of the axis misalignment between the first intermediate waveguide portion and the second intermediate waveguide portion and the connection loss (dB).

[0045] Figure 18 is the length L of the third intermediate waveguide section C Graph of its relationship with connection loss (dB). DETAILED DESCRIPTION

[0046] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0047] Figure 1 (a) and (b) are schematic plan views showing the structure of the light modulator according to the first embodiment of the present invention. Figure 1 (a) shows only the optical waveguide, Figure 1 (b) shows the entire optical modulation element including the traveling wave electrodes.

[0048] like Figure 1 As shown in (a) and (b) of FIG, , this optical modulator 1 includes a Mach-Zehnder optical waveguide 2 formed on a substrate 10 and having first and second waveguides 2a and 2b arranged parallel to 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; and a second bias electrode 5b arranged along the second waveguide 2b. The first and second signal electrodes 4a and 4b, together with the first and second waveguides 2a and 2b, constitute an RF interaction portion 3ir of the Mach-Zehnder optical modulator. Furthermore, the first and second bias electrodes 5a and 5b, together with the first and second waveguides 2a and 2b, constitute a DC interaction portion 3id of the Mach-Zehnder optical modulator.

[0049] The Mach-Zehnder optical waveguide 2 is an optical waveguide element having the structure of a Mach-Zehnder interferometer. It comprises an input waveguide 2i; a beam splitting section 2c that splits the light propagating through the input waveguide 2i; first and second waveguides 2a and 2b extending parallel to each other from the beam splitting section 2c; a beam combining section 2d that combines the light propagating through the first and second waveguides 2a and 2b; and an output waveguide 2o that transmits the light output from the beam combining section 2d. The input light into the input waveguide 2i is split by the beam splitting section 2c and propagates through the first and second waveguides 2a and 2b, respectively. The light is then combined by the beam combining section 2d and output as modulated light from the output waveguide 2o.

[0050] The first and second signal electrodes 4a and 4b are linear electrode patterns that overlap with the first and second waveguides 2a and 2b when viewed from above. Their ends are extended to near the outer periphery of the substrate 10. Specifically, one end 4a1 and 4b1 of the first and second signal electrodes 4a and 4b are extended to near the edge of the substrate 10, forming a signal input port, which is connected to the drive circuit 9a. Furthermore, the other ends 4a2 and 4b2 of the first and second signal electrodes 4a and 4b are extended to near the edge of the substrate 10 and connected to each other via a terminal resistor 9b. Thus, the first and second signal electrodes 4a and 4b function as differential, coplanar traveling-wave electrodes.

[0051] To apply a DC voltage (DC bias) to the first and second waveguides 2a and 2b, first and second bias electrodes 5a and 5b are independently provided along with first and second signal electrodes 4a and 4b. One end 5a1 and 5b1 of the first and second bias electrodes 5a and 5b is extended near the edge of the substrate 10 to form a DC bias input port, which is connected to a bias circuit 9c. In this embodiment, the first and second bias electrodes 5a and 5b are formed closer to the output end of the Mach-Zehnder optical waveguide 2 than the first and second signal electrodes 4a and 4b, but may alternatively be formed closer to the input end.

[0052] Thus, the first and second signal electrodes 4a and 4b constitute the RF section 3a, which applies an RF signal to the first and second waveguides 2a and 2b. The first and second bias electrodes 5a and 5b constitute the DC section 3b, which applies a DC bias to the first and second waveguides 2a and 2b. Neither signal electrodes nor bias electrodes are provided in the intermediate section 3c between the RF section 3a and the DC section 3b. While details will be described later, the optical waveguide in the intermediate section 3c has a unique shape that connects the optical waveguide of the RF section 3a to the optical waveguide of the DC section 3b.

[0053] A differential signal (modulation signal) with the same absolute value but different signs (negative and positive) is input to one end of the first and second signal electrodes 4a and 4b. The first and second waveguides 2a and 2b are made of a material with an electro-optical effect, such as lithium niobate. Therefore, the electric field applied to the first and second waveguides 2a and 2b causes the refractive index of the first and second waveguides 2a and 2b to change by +Δn and -Δn, respectively, thereby changing the phase difference between the pair of optical waveguides. Signal light modulated by this phase difference is output from the output waveguide 2o.

[0054] As described above, the optical modulation element 1 of the present embodiment is a dual-drive type composed of a pair of signal electrodes. Therefore, the symmetry of the electric field applied to the pair of optical waveguides can be improved, and wavelength chirp can be suppressed.

[0055] Figure 2(a) and (b) are schematic cross-sectional views of the light modulation element 1. Figure 2 (a) is along Figure 1 (a) and (b) are cross-sectional views of the RF portion 3a along the X1-X1' line, Figure 2 (b) is along Figure 1 (a) and (b) are cross-sectional views of the DC portion 3b taken along the X2-X2' line.

[0056] like Figure 2 As shown in (a) and (b) of FIG. 1 , 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 stacked in this order.

[0057] The substrate 10 is, for example, a sapphire single crystal substrate. A waveguide layer 11 composed of an electro-optical material, typically lithium niobate, is formed on the main surface of the substrate 10. The waveguide layer 11 includes a protruding ridge 11r and thinner slab portions 11s disposed on either side of the ridge 11r. The ridge 11r forms the first and second waveguides 2a and 2b. The width W1 (first ridge width) of the ridge 11r in the RF portion 3a and the width W2 (second ridge width) of the ridge 11r in the DC portion 3b can be 0.5 to 5 μm. In this embodiment, the width W1 of the ridge 11r in the RF portion 3a and the width W2 of the ridge 11r in the DC portion 3b are equal, but they may be different.

[0058] The ridge 11r is the portion that becomes the center of the optical waveguide. As mentioned above, the ridge 11r refers to a portion that protrudes upward. The thickness of the electro-optical material film in the portion that protrudes upward becomes thicker than that of the portions on the left and right, and therefore the effective refractive index becomes higher. Therefore, it is possible to confine light in the left and right directions and function as a three-dimensional optical waveguide. The shape of the ridge 11r can be any shape that can waveguide light, and the thickness of the electro-optical material film in the ridge 11r can be a convex shape that is thicker than the thickness of the electro-optical material films on the left and right. Therefore, it can also be an upwardly convex dome shape, a triangular shape, etc. The ridge 11r can be formed by forming a mask such as a resist on the electro-optical material film and selectively etching the electro-optical material film to perform pattern drawing. The width, height, shape, etc. of the ridge 11r need to be optimized to improve device characteristics.

[0059] Typically, the thickness of the ridge 11r is equal to the thickness of the electro-optical material film. The width of the ridge 11r (ridge width W1, W2) is defined as the width of the upper surface of the ridge 11r. This is because the side surface of the ridge 11r shown in the figure is perpendicular to the substrate 10, but there is also a case of inclination. Preferably, the inclination angle of the side surface of the ridge 11r is close to 90°, but it can be at least 70°. In the case where the width of the upper surface of the ridge 11r is set as the ridge width in this way, the ridge width can be clearly defined even if the ridge 11r has a trapezoidal shape.

[0060] The flat plate portions 11s, located on either side of the ridge 11r, extend leftward and rightward from the ridge 11r and are composed of an electro-optical material film that is thinner than the ridge 11r. In this embodiment, the flat plate portions 11s actually have a certain thickness. However, the thickness of the flat plate film near the base of the ridge 11r is unstable, sometimes leaving a gentle taper or depression. Therefore, the thickness of the flat plate portions 11s is not defined as the thickness at the point where the film thickness changes transitionally, but rather as the thickness at the point where the film thickness stabilizes slightly away from the base of the ridge 11r.

[0061] The thickness T of the flat plate portion 11s in the RF portion 3a sb1 (First flat plate film thickness) and the thickness T of the flat plate portion 11s in the DC portion 3b sb2 (Second flat plate film thickness) is different. In this embodiment, it is preferable that the thickness T of the flat plate portion 11s in the DC portion 3b is sb2 Than the thickness T of the flat plate portion 11s in the RF portion 3a sb1 By increasing the thickness of the ridge waveguide slab in the DC section 3b, DC drift can be reduced, thereby extending the life of the optical modulator. Furthermore, by reducing the thickness of the ridge waveguide slab in the RF section 3a and increasing the protruding height of the ridge portion 11r, light confinement can be enhanced, thereby improving light modulation efficiency.

[0062] The protective layer 12 is formed in an area that does not overlap with the first and second waveguides 2a and 2b when viewed from above. The protective layer 12 covers the entire area of ​​the upper surface of the waveguide layer 11 where the ridge 11r is not formed. The side surfaces of the ridge 11r are also covered by the protective layer 12, thereby preventing scattering losses caused by the roughness of the side surfaces of the ridge 11r. The thickness of the protective layer 12 is approximately the same as the height of the ridge 11r of the waveguide layer 11. The material of the protective layer 12 is not particularly limited; for example, silicon oxide (SiO2) can be used.

[0063] The buffer layer 13 is formed at least on the upper surface of the ridge 11r to prevent light propagating through the first and second waveguides 2a and 2b from being absorbed by the first and second signal electrodes 4a and 4b. The buffer layer 13 is preferably made of a material with a lower refractive index and higher transparency than the waveguide layer 11. For example, Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, or Y2O3 can be used. The thickness of the buffer layer 13 on the upper surface of the ridge 11r can be approximately 0.2 to 1 μm. The buffer layer 13 is more preferably made of a material with a high dielectric constant. In this embodiment, the buffer layer 13 covers not only the upper surfaces of the first and second waveguides 2a and 2b, but also the entire surface of the base, including the upper surface of the protective layer 12. However, the buffer layer 13 can also be patterned to selectively cover only the vicinity of the upper surfaces of the first and second waveguides 2a and 2b. Alternatively, the protective layer 12 may be omitted, and the buffer layer 13 may be formed directly on the entire upper surface of the waveguide layer 11 .

[0064] As for 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. The light absorption of the electrode and the applied voltage of the electrode are in a relationship of mutual increase and decrease, therefore, it is necessary to set an appropriate film thickness according to the purpose. The higher the dielectric constant of the buffer layer 13, the lower the VπL (an indicator of electric field efficiency) can be, so it is preferred. The lower the refractive index of the buffer layer 13, the thinner the buffer layer 13 can be, so it is preferred. Generally, the refractive index of a material with a high dielectric constant also becomes higher. Therefore, it is very important to consider the balance between the two and select a material with a high dielectric constant and a low refractive index. As an example, Al2O3 has a relative dielectric constant of about 9 and a refractive index of about 1.6, which 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, which is a particularly preferred material.

[0065] Can also be made of different materials Figure 2 (a) The buffer layer 13 of the RF portion 3a and Figure 2 (b) The buffer layer 13 of the DC section 3b. The buffer layer 13 of the RF section 3a uses a buffer layer material that can optimize the characteristics of the RF section 3a, and the buffer layer 13 of the DC section 3b uses a buffer layer material that can reduce DC drift, thereby optimizing each characteristic. Examples of buffer layer materials that can reduce DC drift include materials containing silicon oxide and indium oxide.

[0066] like Figure 2As shown in FIG. 1 (a), a first signal electrode 4a and a second signal electrode 4b are provided on the electrode layer 14 of the RF unit 3a. The first signal electrode 4a is provided to overlap with the ridge portion 11r corresponding to the first waveguide 2a, and faces the first waveguide 2a via the buffer layer 13, in order to modulate light propagating in the first waveguide 2a. The second signal electrode 4b is provided to overlap with the ridge portion 11r corresponding to the second waveguide 2b, and faces the second waveguide 2b via the buffer layer 13, in order to modulate light propagating in the second waveguide 2b.

[0067] like Figure 2 As shown in (b), a first bias electrode 5a and a second bias electrode 5b are provided on the electrode layer 14 in the DC section 3b. The first bias electrode 5a is provided so as to overlap with the ridge portion 11r corresponding to the first waveguide 2a in order to apply a bias electric field to light propagating through the first waveguide 2a, and faces the first waveguide 2a via the buffer layer 13. The second bias electrode 5b is provided so as to overlap with the ridge portion 11r corresponding to the second waveguide 2b in order to apply a bias electric field to light propagating through the second waveguide 2b, and faces the second waveguide 2b via the buffer layer 13.

[0068] like Figure 2 As shown in (a) and (b) of FIG, the electrode structure is bilaterally symmetrical in a cross section perpendicular to the travel direction of the first and second waveguides 2a and 2b. Therefore, the magnitudes of the electric fields applied to the first and second waveguides 2a and 2b by the first and second signal electrodes 4a and 4b, respectively, can be made as equal as possible, thereby reducing wavelength chirp. Furthermore, in the present invention, the electrode structure is not particularly limited and may be a so-called single-drive type electrode structure. The presence or absence of a ground electrode and its layout are also not particularly limited.

[0069] The waveguide layer 11 is not particularly limited as long as it is an electro-optical material, but is preferably composed of lithium niobate (LiNbO3). This is because lithium niobate has a large electro-optical constant and is suitable as a material for optical devices such as light modulators. The following describes in detail the structure of this embodiment in which the waveguide layer 11 is a lithium niobate film.

[0070] 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 capable of forming the lithium niobate film as an epitaxial film, 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 the property of being easily formed as a c-axis oriented epitaxial film relative to single crystal substrates of various crystal orientations. The c-axis oriented lithium niobate film has a three-fold symmetry, and therefore, it is desirable that the underlying single crystal substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferably used, and in the case of a silicon single crystal substrate, a (111) plane substrate is preferably used.

[0071] Here, an epitaxial film is a film whose crystal orientation is uniformly oriented relative to the underlying substrate or base film. When the film surface is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals are uniformly oriented along the X, Y, and Z axes. For example, by first confirming the peak intensity at the orientation position based on 2θ-θ X-ray diffraction, and then confirming the pole point, it can be confirmed that the film is an epitaxial film.

[0072] Specifically, when performing measurements based on 2θ-θ X-ray diffraction, the peak intensities of all planes other than the target plane must be less than 10%, and preferably less than 5%, of the maximum peak intensity of the target plane. For example, in a c-axis-oriented epitaxial film of lithium niobate, the peak intensities of all planes other than the (00L) plane must be less than 10%, and preferably less than 5%, of the maximum peak intensity of the (00L) plane. (00L) is a general term for equivalent planes such as (001) and (002).

[0073] Next, in the pole measurement, it is necessary to observe the pole. The conditions for confirming the peak intensity at the first orientation position described above only indicate orientation in one direction. Even if the first condition is met, if the crystal orientation is inconsistent within the plane, the X-ray intensity will not increase at a specific angle position, and the pole cannot be observed. LiNbO3 has a trigonal crystal structure, so there are three poles in the LiNbO3 (014) single crystal.

[0074] In the case of lithium niobate films, it is known that epitaxial growth occurs in a so-called twinned state, where crystals are symmetrically bonded by rotating 180° about the c-axis. In this case, three poles are symmetrically bonded to two of them, resulting in six poles. Furthermore, when a lithium niobate film is formed on a (100)-plane silicon single crystal substrate, the substrate is quadratically symmetrical, resulting in 4 × 3 = 12 poles. Furthermore, in the present invention, epitaxially grown lithium niobate films in a twinned state are also included in epitaxial films.

[0075] The composition of 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. Element A includes K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, Ce, and the like, and a combination of two or more thereof is also acceptable.

[0076] The thickness of the lithium niobate film is preferably 2 μm or less. This is because, when the film thickness is greater than 2 μm, it is difficult to form a high-quality film. On the other hand, if the lithium niobate film is too thin, the light confinement in the lithium niobate film becomes weak, and light may leak into the substrate 10 or the buffer layer 13. Even if an electric field is applied to the lithium niobate film, the change in the effective refractive index of the optical waveguide (2a, 2b) may become small. Therefore, the lithium niobate film is preferably at least 1 / 10 the wavelength of the light used.

[0077] As a method for forming a lithium niobate film, it is preferred to utilize a film forming method such as a sputtering method, a CVD method, or a sol-gel method. The c-axis of the lithium niobate is oriented perpendicularly to the main surface of the substrate 10, and an electric field is applied parallel to the c-axis, whereby the optical refractive index changes in proportion to the electric field. When sapphire is used as a single crystal substrate, the lithium niobate film can be directly epitaxially grown on the sapphire single crystal substrate. When silicon is used as a single crystal substrate, the lithium niobate film is formed by epitaxial growth via a cladding (not shown). As the cladding (not shown), a cladding having a lower refractive index than that of the lithium niobate film and suitable for epitaxial growth is used. For example, when Y2O3 is used as a cladding (not shown), a high-quality lithium niobate film can be formed.

[0078] Another known method for forming a lithium niobate film is polishing or dicing (slicing) a thinned lithium niobate single crystal substrate. This method has the advantage of achieving properties similar to those of a single crystal and is therefore applicable to the present invention.

[0079] like 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 lithium niobate film formed into a ridge shape. To concentrate the electric field 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, and more preferably 1.5 to 10 times, the ridge width W1 of the first and second waveguides 2a and 2b.

[0080] like 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 lithium niobate film formed into a ridge shape. To concentrate the electric field 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, and more preferably 1.5 to 10 times, the ridge width W2 of the first and second waveguides 2a and 2b.

[0081] Figure 2 The slab portions 11s of the first and second waveguides 2a and 2b in the DC portion 3b shown in FIG. Figure 2The slab portions 11s of the first and second waveguides 2a and 2b in the RF portion 3a shown in (a) are formed thick. Thus, by increasing the slab film thickness T of the ridge waveguide in the DC portion 3b, sb2 , it is possible to reduce DC drift. In addition, by reducing the thickness T of the ridge waveguide in the RF section 3a, sb1 , which can strengthen the confinement of light propagating in the optical waveguide and improve the modulation efficiency, and can be driven at the lowest possible voltage.

[0082] The reason why DC drift is reduced by increasing the thickness of the flat film in DC section 3b is unclear, but it is speculated that damage incurred when machining the lithium niobate film into the ridge shape affects DC drift. Reducing the thickness of the flat film (increasing the protruding height of the ridge) requires digging deeper beneath the upper surface of the lithium niobate film, leaving more damage on the machined surface. On the other hand, increasing the thickness of the flat film (decreasing the protruding height of the ridge) reduces the amount of machining required on the lithium niobate film, resulting in less damage to the machined surface. Therefore, it is believed that DC drift is reduced, extending the life of DC section 3b.

[0083] The protruding height of the ridge portion 11r constituting the first and second waveguides 2a and 2b is relatively increased in both the RF portion 3a and the DC portion 3b, and the thickness T of the flat plate portions 11s formed on both sides of the ridge portion 11r is reduced. sb1 、T sb2 (flat film thickness) can enhance the confinement of light propagating in the optical waveguide and enable driving at the lowest possible voltage. However, when this waveguide structure is used in the DC section 3b, the DC drift increases, and the life of the optical modulator cannot be extended. On the other hand, in both the RF section 3a and the DC section 3b, the protruding height of the ridge 11r is reduced, and the thickness T of the flat section 11s is increased. sb1 、T sb2 (flat film thickness), the DC drift can be reduced, but the half-wave voltage Vπ becomes high, making it impossible to drive at a low voltage.

[0084] However, as in the present embodiment, by making the planar film thickness of the optical waveguide in the DC portion 3b different from that in the RF portion 3a, the planar film thickness of the DC portion 3b is relatively thickened, and the planar film thickness of the RF portion 3a is relatively thinned, it is possible to achieve both the DC drift suppression effect in the DC portion 3b and the low driving voltage in the RF portion 3a.

[0085] The slab film thickness T of the first and second waveguides 2a and 2b in the RF section 3a is sb1 The thickness T of the first and second waveguides 2a and 2b in the DC portion 3b is sb1In different situations, simply connecting the two will result in increased connection loss due to waveguide shape mismatch. Therefore, in this embodiment, an intermediate waveguide section is provided in the intermediate portion 3c between the RF section 3a and the DC section 3b to improve waveguide matching. The structure of the intermediate waveguide section is described in detail below.

[0086] Figure 3 3a and 3b are a schematic top view and a schematic cross-sectional view showing a waveguide structure in the vicinity of the intermediate portion 3c between the RF portion 3a and the DC portion 3b. Figure 4 It is a schematic perspective view showing the waveguide structure of the intermediate portion 3c in three dimensions.

[0087] like Figure 3 and Figure 4 As shown, the first and second waveguides 2a and 2b each include a first waveguide portion 21a serving as an optical waveguide for the RF portion 3a, a second waveguide portion 21b serving as an optical waveguide for the DC portion 3b, a first intermediate waveguide portion 22a provided in the intermediate portion 3c near the RF portion 3a and connected to the first waveguide portion 21a, and a second intermediate waveguide portion 22b provided in the intermediate portion 3c near the DC portion 3b and connected to the second waveguide portion 21b. Specifically, 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 this order from the RF portion 3a side toward the DC portion 3b side.

[0088] The first waveguide portion 21a has a ridge width W1 and a slab film thickness T sb1 The second waveguide portion 21b has a ridge width W2 (= W1) and a plate film thickness T sb2 (>T sb1 When the first waveguide portion 21a and the second waveguide portion 21b, each having different film thicknesses and a narrow ridge width, are directly connected, light propagation loss increases due to mismatching at the boundary. Therefore, in this embodiment, a first intermediate waveguide portion 22a and a second intermediate waveguide portion 22b are provided between the first waveguide portion 21a on the RF portion 3a side and the second waveguide portion 21b on the DC portion 3b side to reduce light propagation loss.

[0089] The first intermediate waveguide portion 22a is a ridge waveguide connected to the first waveguide portion 21a and has the same slab film thickness T as that of the first waveguide portion 21a. sb1 , but has a ridge width W wider than that of the first waveguide portion 21a 1C (Third ridge width) The first intermediate waveguide portion 22a is provided to widen the ridge width of the first waveguide portion 21a and has a ridge width expansion portion in which the ridge width gradually increases as it goes toward the second waveguide portion 21b. Thus, at the connection position with the second intermediate waveguide portion 22b, the first intermediate waveguide portion 22a has a ridge width W wider than that of the first waveguide portion 21a.1C (>W1).

[0090] The second intermediate waveguide portion 22b is a ridge waveguide connected to the second waveguide portion 21b and has the same slab film thickness T as that of the second waveguide portion 21b. sb2 , but has a ridge width W wider than that of the second waveguide portion 21b 2C (Fourth ridge width). The second intermediate waveguide portion 22b is provided to widen the ridge width of the second waveguide portion 21b and has a ridge width expansion portion in which the ridge width gradually increases as it goes toward the first waveguide portion 21a. Thus, at the connection position with the first intermediate waveguide portion 22a, the second intermediate waveguide portion 22b has a ridge width W wider than that of the second waveguide portion 21b. 2C (>W2).

[0091] The ridge width W of the first intermediate waveguide portion 22a 1C than the ridge width W of the second intermediate waveguide portion 22b 2C Therefore, at the boundary between the first intermediate waveguide section 22a and the second intermediate waveguide section 22b, the ridge width and slab thickness of the optical waveguide vary discontinuously. When the first waveguide section 21a on the RF section 3a side and the second waveguide section 21b on the DC section 3b side, which have different slab thicknesses, are connected, the spot sizes of the light propagating through the optical waveguide differ, and thus the connection loss increases at the boundary. However, by increasing the ridge width W of the optical waveguide on the RF section 3a side, where the slab thickness is relatively thin, 1C , and reduce the ridge width W of the optical waveguide on the DC portion 3b side where the slab film thickness is relatively thick 2C , the spot size of light at the boundary between the first waveguide portion 21 a and the second waveguide portion 21 b can be made uniform, thereby reducing connection loss.

[0092] The ridge width W of the first intermediate waveguide portion 22a is preferably 1C than the ridge width W of the second intermediate waveguide portion 22b 2C larger than the ridge width W of the second intermediate waveguide portion 22b 2C 2 times smaller (W 2C <W 1C <2×W 2C ). It is possible to prevent the ridge width W of the first intermediate waveguide portion 22a from 1C Too large a value will increase the connection loss.

[0093] Generally, forming an optical waveguide with a varying ridge width is easy. A single patterning process can produce a tapered optical waveguide with a continuously varying ridge width, eliminating the need to form an optical waveguide with a discontinuous ridge width. However, forming an optical waveguide with varying slab thicknesses requires two patterning processes with different processing conditions, making accurate alignment of the two optical waveguides difficult. Therefore, in this embodiment, the optical waveguide on the RF portion 3a side and the optical waveguide on the DC portion 3b side, with their different slab thicknesses, are processed separately. The ridge width near the connection between the RF portion 3a and DC portion 3b is then widened to connect the two. This reduces connection loss, but results in a ridge waveguide with discontinuous ridge widths and slab thicknesses.

[0094] Figure 5 It is a schematic plan view showing a state in which an axial misalignment occurs between the optical waveguide on the RF portion 3 a side and the optical waveguide on the DC portion 3 b side.

[0095] In order to make the slab film thickness of the ridge waveguide different, the waveguide pattern on the RF portion 3a side (the first waveguide portion 21a and the first intermediate waveguide portion 22a) and the waveguide pattern on the DC portion 3b side (the second waveguide portion 21b and the second intermediate waveguide portion 22b) are processed separately. Figure 5 As shown, the waveguide pattern may sometimes be misaligned (axial deviation). However, by widening the ridge widths of the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b as in this embodiment, it is possible to prevent a reduction in the cross-sectional area of ​​the waveguide due to axial deviation, and reduce light propagation loss due to uneven processing of the waveguide pattern.

[0096] As described above, in the optical modulation element 1 of this embodiment, the slab film thickness T of the optical waveguide of the DC portion 3b is sb2 Than the slab film thickness T of the optical waveguide of the RF section 3a sb1 Therefore, the RF unit 3a can be driven at a low voltage and the DC drift can be reduced.

[0097] In the optical modulator 1 of this embodiment, the optical waveguide (first waveguide portion 21a) of the RF portion 3a and the optical waveguide (second waveguide portion 21b) of the DC portion 3b, which have different planar film thicknesses, are connected. Therefore, the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b with varying ridge widths are provided in the intermediate portion 3c between the RF portion 3a and the DC portion 3b. The ridge width W of the first intermediate waveguide portion 22a on the RF portion 3a side is set to 0. 1C The ridge width W of the second intermediate waveguide portion 22b on the DC portion 3b side is 2C Therefore, the spot size of the light propagating in the first waveguide portion 21 a and the spot size of the light propagating in the second waveguide portion 21 b can be matched to reduce the propagation loss.

[0098] Figure 6 1 is a schematic top view and a schematic cross-sectional view showing a waveguide structure in the vicinity of the intermediate portion 3c between the RF portion 3a and the DC portion 3b in the optical modulation element according to the second embodiment of the present invention. Figure 7 It is a three-dimensional representation Figure 6 A rough perspective view of the waveguide structure of the middle part 3c is shown.

[0099] like Figure 6 and Figure 7 As shown, 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. The third intermediate waveguide portion 22c has a two-stage ridge portion 11r having the same ridge width W as the first intermediate waveguide portion 22a. 1C The lower portion of the ridge has the same ridge width W as the second intermediate waveguide portion 22b. 2C and a flat portion 11s, which is formed on both sides of the ridge portion 11r of the two-stage structure, the flat film thickness T sb3 The thickness T is greater than the flat plate film thickness T of the first waveguide portion 21a. sb1 In this embodiment, the third intermediate waveguide portion 22 c is a ridge waveguide having a thin slab portion 11 s , but may also be a waveguide structure in which the slab portion 11 s is completely eliminated.

[0100] It is preferable that the length L of the third intermediate waveguide portion 22c is C This is because if the third intermediate waveguide portion 22c is too long, the propagation loss increases.

[0101] Figure 8 (a) to (c) are used to illustrate the Figure 6 and Figure 7 FIG. 2 is a diagram illustrating a method for forming the waveguide structure of the third intermediate waveguide portion 22 c.

[0102] like Figure 8 As shown in (a) to (c), the waveguide structure having the third intermediate waveguide portion 22c can be formed by partially overlapping the processing area for forming the waveguide pattern of the RF portion 3a and the processing area for forming the waveguide pattern of the DC portion 3b near the boundary between the two.

[0103] In detail, first, Figure 8 As shown in (a), after the substantially entire surface of the formation region 25b of the DC portion 3b is covered with a mask 30b, the electro-optical material film in the formation region 25a of the RF portion 3a is processed by milling or the like to form the waveguide pattern of the RF portion 3a, namely, the first waveguide portion 21a and the first intermediate waveguide portion 22a. At this time, the edge E of the mask 30b is bSince it is located closer to the front side than the boundary line B and does not extend beyond the formation region 25a of the RF portion 3a, ridge processing is performed on the formation region 25b of the DC portion 3b near the boundary line B. In processing the RF portion 3a, a thin flat plate portion is formed by, for example, extending the processing time.

[0104] Then, if Figure 8 As shown in (b), after the mask 30a covers the substantially entire surface of the formation region 25a of the RF portion 3a, the electro-optical material film in the formation region 25b of the DC portion 3b is processed by milling or the like to form the waveguide pattern of the DC portion 3b, namely, the second waveguide portion 21b and the second intermediate waveguide portion 22b. At this time, the edge E of the mask 30a is a Since it is located closer to the front side than the boundary line B and does not extend beyond the DC portion 3b formation region 25b side, ridge processing is performed on the RF portion 3a formation region 25a near the boundary line B. In processing the DC portion 3b, a thick flat plate portion is formed by, for example, shortening the processing time.

[0105] In this way, ridge processing is performed twice near the boundary line B between the formation region 25a of the RF portion 3a and the formation region 25b of the DC portion 3b. Figure 8 As shown in (c), 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. Figure 8 In the embodiment, the RF portion 3a and the DC portion 3b are processed in this order, but the same result can be obtained even if the DC portion 3b and the RF portion 3a are processed in this order.

[0106] According to this embodiment, the same effects as those of the first embodiment can be achieved. That is, the ridge width W of the first intermediate waveguide portion 22a on the RF portion 3a side is set to 1C The ridge width W of the second intermediate waveguide portion 22b on the DC portion 3b side is 2C Therefore, the spot size of the light propagating in the first waveguide portion 21 a and the spot size of the light propagating in the second waveguide portion 21 b can be matched to reduce the propagation loss.

[0107] Figure 9 3a and 3b are schematic top views and schematic cross-sectional views showing a waveguide structure in the vicinity of an intermediate portion 3c between the RF portion 3a and the DC portion 3b in the optical modulation element according to the third embodiment of the present invention. Figure 10 It is a three-dimensional representation Figure 9 A rough perspective view of the waveguide structure of the middle part 3c is shown.

[0108] like Figure 9 and Figure 10As shown, this 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. The third intermediate waveguide portion 22c is formed of an unprocessed waveguide layer 11 that does not have a ridge shape. In other words, the third intermediate waveguide portion 22c is a region without a waveguide shape. The remaining structure is the same as that of the second embodiment.

[0109] Figure 11 (a) to (b) are used to illustrate the Figure 9 and Figure 10 FIG. 2 is a diagram illustrating a method for forming the waveguide structure of the third intermediate waveguide portion 22 c.

[0110] like Figure 11 As shown in (a) to (c), the waveguide structure having the third intermediate waveguide portion 22c can be formed by preventing the processing area for forming the waveguide pattern of the RF portion 3a and the processing area for forming the waveguide pattern of the DC portion 3b from overlapping near their boundaries.

[0111] In detail, first, Figure 11 As shown in (a), after the substantially entire surface of the formation region 25b of the DC portion 3b is covered with a mask 30b, the electro-optical material film in the formation region 25a of the RF portion 3a is processed by milling or the like to form the waveguide pattern of the RF portion 3a, namely, the first waveguide portion 21a and the first intermediate waveguide portion 22a. At this time, the edge E of the mask 30b is b Since it is located deeper than the boundary line B and extends beyond the RF portion 3a forming region 25a, an unprocessed region is formed in the RF portion 3a forming region 25a near the boundary line B. In processing the RF portion 3a, a thin flat plate portion is formed by, for example, extending the processing time.

[0112] Then, if Figure 11 As shown in (b), after the substantially entire surface of the formation region 25a of the RF portion 3a is covered with a mask 30a, the electro-optical material film in the formation region 25b of the DC portion 3b is processed by milling or the like to form the waveguide pattern of the DC portion 3b, namely, the second waveguide portion 21b and the second intermediate waveguide portion 22b. At this time, the edge E of the mask 30a is a Since it is located deeper than the boundary line B and extends beyond the DC portion 3b forming region 25b, an unprocessed region is formed in the RF portion 3a forming region 25a near the boundary line B. In processing the DC portion 3b, a thick flat plate portion is formed by, for example, shortening the processing time.

[0113] In this way, an unprocessed area is formed near the boundary line B between the formation area 25a of the RF portion 3a and the formation area 25b of the DC portion 3b, as shown in FIG. Figure 11As shown in (c), a third intermediate waveguide portion 22c having a cross-sectional shape different from the first intermediate waveguide portion 22a and the second intermediate waveguide portion 22b is formed.

[0114] According to this embodiment, the same effects as those of the first embodiment can be achieved. That is, the ridge width W of the first intermediate waveguide portion 22a on the RF portion 3a side is set to 1C The ridge width W of the second intermediate waveguide portion 22b on the DC portion 3b side is 2C Therefore, the spot size of the light propagating in the first waveguide portion 21 a and the spot size of the light propagating in the second waveguide portion 21 b can be matched to reduce the propagation loss.

[0115] Figure 12 (a) and (b) are schematic cross-sectional views of a light modulator according to a fourth embodiment of the present invention. Figure 12 (a) is a cross-sectional view of the RF portion 3a, Figure 12 (b) is a cross-sectional view of the DC portion 3b. Figure 13 It is a schematic plan view showing the waveguide structure near the intermediate portion 3c between the RF portion 3a and the DC portion 3b.

[0116] like Figure 12 (a) and (b) and Figure 13 As shown, this optical modulator 1 is characterized in that the ridge width W1 of the first and second waveguides 2a, 2b (first waveguide portion 21a) in the RF portion 3a is different from the ridge width W2 of the first and second waveguides 2a, 2b (second waveguide portion 21b) in the DC portion 3b. In this embodiment, the ridge width W2 of the first and second waveguides 2a, 2b (second waveguide portion 21b) in the DC portion 3b is wider than the ridge width W1 of the first and second waveguides 2a, 2b (first waveguide portion 21a) in the RF portion 3a. The remaining structure is the same as in the first embodiment. The optical modulator 1 of this embodiment not only achieves the same effects as in the first embodiment but also enhances the DC drift reduction effect.

[0117] Figure 14 (a) and (b) are schematic cross-sectional views of a light modulator according to a 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.

[0118] like Figure 14As shown in Figures (a) and (b), the optical modulator 1 of this embodiment is characterized in that the protective layer 12 is omitted in the DC portion 3b. Specifically, the DC portion 3b of the optical modulator 1 has a multilayer structure in which a substrate 10, a waveguide layer 11, a buffer layer 13, and an electrode layer 14 are stacked in this order. The buffer layer 13 is formed over the entire surface of the waveguide layer 11, covering not only the top surface of the ridge portion 11r but also the side surfaces. Therefore, the top surfaces of the slab portions 11s on both sides of the ridge portion 11r are also covered by the buffer layer 13.

[0119] In the case where the protective layer 12 is omitted in the DC section 3b, the buffer layer 13 of the DC section 3b is preferably made of a dielectric material different from that of the buffer layer 13 of the RF section 3a, and is preferably made of the same dielectric material as that of the protective layer 12 of the RF section 3a. That is, the buffer layer 13 of the DC section 3b is preferably formed using the same dielectric material as that of the protective layer 12 of the RF section 3a. Figure 2 The protective layer 12 and the buffer layer 13 of the DC portion 3 b shown are equivalent.

[0120] While preferred embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention, which are naturally encompassed within the scope of the present invention.

[0121] For example, in the above embodiment, a dual-drive optical modulator element in which a pair of signal electrodes are provided for a Mach-Zehnder optical waveguide having a pair of optical waveguides is exemplified, but the present invention is not limited to such an optical modulator element and can be applied to various optical modulator elements having an RF section 3a and a DC section 3b.

[0122] Furthermore, in the above embodiment, an optical modulation element having a pair of optical waveguides formed using a lithium niobate film epitaxially grown on substrate 10 is exemplified. However, the present invention is not limited to this structure; the optical waveguides may also be formed using electro-optical materials such as barium titanate or lead zirconate titanate. However, as long as the optical waveguide is formed using a lithium niobate film, the width of the optical waveguide can be narrowed, thereby reducing the driving voltage. However, this reduces the problem of DC drift, which is a significant advantage of the present invention. Furthermore, semiconductor materials or polymer materials exhibiting electro-optical effects may also be used as the waveguide layer 11.

[0123] Example

[0124] (Evaluation of the Flat Plate Film Thickness of the RF Section 3a)

[0125] The slab film thickness T of the ridge waveguide in the RF section 3a was evaluated by simulation. sb1 The influence on the electric field efficiency VπL. The thickness of the waveguide layer 11 composed of the lithium niobate film is set to 1.5 μm, and the slab film thickness T of the ridge waveguide is determined.sb1 The electric field efficiency VπL changes. As a result, Figure 15 As shown, the ridge waveguide plate film thickness T sb1 At about 0.3μm, VπL is the smallest. In addition, as long as the flat film thickness T sb2 When the thickness is 0.6 μm or less, VπL can be reduced to 2.2 Vcm or less, which is a preferred range.

[0126] (Evaluation of Flat Film Thickness in DC Section)

[0127] Evaluation of the slab film thickness T of the ridge waveguide in the DC portion 3b sb2 The effect on DC drift. In this evaluation test, a constant bias voltage was applied to the bias electrode at a temperature of 80°C, and the DC drift was measured. The time (lifetime) required from the start of the evaluation test until the DC drift exceeded 50% was measured. The thickness of the waveguide layer 11 composed of the lithium niobate film was set to 1.5 μm. As a result, the plate film thickness T sb2 The lifespan is about 1 hour when the thickness is 0.3 μm, and about 84 hours when the thickness is 0.6 μm. sb2 When the particle size was 1.1 μm, the lifespan was a good result exceeding 1000 hours.

[0128] (Evaluation of the Ridge Widths of the First and Second Intermediate Waveguides)

[0129] exist Figure 3 and Figure 4 In the waveguide structure shown in FIG, the effect of reducing the ridge width W of the first intermediate waveguide portion 22a to 1C The thickness of the waveguide layer 11 composed of the lithium niobate film is set to 1.5 μm, and the ridge width W of the second intermediate waveguide portion 22b is set to 2C = 2.5 μm, the slab film thickness T of the first intermediate waveguide portion 22 a sb1 = 0.4 μm, the slab film thickness T of the second intermediate waveguide portion 22 b sb2 =1.1μm. As a result, Figure 16 As shown, it can be confirmed that the ridge width W 1C In the range of 3 to 4.5 μm, the connection loss is the smallest, and the connection loss of the ridge waveguide with different slab film thicknesses is reduced.

[0130] (Evaluation of Axis Misalignment of Optical Waveguide)

[0131] Evaluated through simulation Figure 11 The effect of the axis deviation ΔW of the optical waveguide on the connection loss (dB) is shown. The thickness of the waveguide layer 11 composed of the lithium niobate film is set to 1.5 μm, and the ridge width (W) of the first and second intermediate waveguide parts 22a and 22b is set to 1C、W 2C ) are set to five combinations: (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). sb1 = 0.4 μm, the slab film thickness T of the second intermediate waveguide portion 22 b sb2 =1.1μm.

[0132] The results, such as Figure 17 As shown, the ridge width W of the first and second intermediate waveguide portions 22a and 22b can be observed. 1C 、W 2C The narrower the optical waveguide, the greater the connection loss when there is no axis deviation. In addition, the greater the axis deviation, the greater the connection loss. The ridge width (W) of the first and second intermediate waveguide parts 22a and 22b is. 1C 、W 2C ) is (1.2 μm, 2.5 μm), the rate of increase of connection loss with respect to axis deviation is the largest, and the ridge width (W 1C 、W 2C ) is (5.2 μm, 4 μm), the rate of increase of connection loss with respect to axis deviation is the smallest. That is, the ridge width W of the first and second intermediate waveguide portions 22a and 22b is 1C 、W 2C The narrower it is, the higher the rate of increase in connection loss.

[0133] (Evaluation of the Influence of the Third Intermediate Waveguide Section)

[0134] Evaluated through simulation Figures 6 to 11 The influence of the presence of the third intermediate waveguide portion 22c on the connection loss (dB) is shown. The thickness of the waveguide layer 11 composed of the lithium niobate film is set to 1.5 μm, and the plate film thickness T of the first intermediate waveguide portion 22a is set to 1.5 μm. sb1 = 0.4 μm, the slab film thickness T of the second intermediate waveguide portion 22 b sb2 =1.1μm. In addition, the ridge width (W 1C 、W 2C ) are set to three patterns: (3.8μm, 2.5μm), (4.2μm, 3μm), and (5.2μm, 4μm). Figures 6 to 8 The case where the flat film thickness is thin (here, the flat film thickness is zero) and the case where there is no Figures 9 to 11 The situation of the waveguide structure shown is evaluated.

[0135] The results, such as Figure 18As shown, the longer the third intermediate waveguide portion 22c is, the greater the connection loss is. Figures 9 to 11 The third intermediate waveguide portion 22c shown does not have a waveguide structure. Figures 6 to 8 Compared to the third intermediate waveguide portion 22c of the waveguide structure without a slab shown in the figure, the rate of increase in connection loss is higher. However, in any structure, as long as the length of the third intermediate waveguide portion 22c is 3 μm or less, the connection loss is 0.2 dB or less. Third intermediate waveguide portions 22c with a length of 3 μm or less can be manufactured sufficiently, which is a level that poses no practical problems.

[0136] Description of Reference Numerals

[0137] 1 Light modulation element

[0138] 2 Mach-Zehnder waveguide

[0139] 2a First waveguide

[0140] 2b Second waveguide

[0141] 2c Beam splitter

[0142] 2d beam combining section

[0143] 2i Input waveguide

[0144] 2o output waveguide

[0145] 3a RF unit

[0146] 3b DC Division

[0147] 3c Middle

[0148] 3id DC interaction site

[0149] 3ir RF interaction unit

[0150] 4a First signal electrode

[0151] 4a1 One end of the first signal electrode

[0152] 4a2 The other end of the first signal electrode

[0153] 4b Second signal electrode

[0154] 4b1 One end of the first signal electrode

[0155] 4b2 The other end of the first signal electrode

[0156] 5a First bias electrode

[0157] 5a1 One end of the first bias electrode

[0158] 5b Second bias electrode

[0159] 5b1 One end of the second bias electrode

[0160] 9 Terminal resistor

[0161] 9a Driver circuit

[0162] 9c Bias Circuit

[0163] 10 substrate

[0164] 11 Waveguide layer

[0165] 11r ridge

[0166] 11s tablet department

[0167] 12 protective layer

[0168] 13 Buffer layer

[0169] 14 Electrode layer

[0170] 21a First waveguide section

[0171] 21b Second waveguide section

[0172] 22a First intermediate waveguide portion

[0173] 22b Second intermediate waveguide section

[0174] 22c Third intermediate waveguide section

[0175] 30a mask

[0176] 30b mask.

Claims

1. A light modulation element, characterized in that: have: substrate; as well as An optical waveguide is formed of an electro-optical material film formed on the substrate, and has a ridge portion as a protruding portion and a flat plate portion having a film thickness thinner than the ridge portion. The optical waveguide comprises: a first waveguide portion having a first ridge width and a first slab film thickness, to which an RF signal is applied; as well as A second waveguide portion having a second ridge width and a second slab film thickness different from the first slab film thickness is applied with a DC bias. The second ridge width is wider than the first ridge width.

2. The light modulation element according to claim 1, wherein The second flat film thickness is thicker than the first flat film thickness.

3. The light modulation element according to claim 1 or 2, wherein The thickness of the first flat plate is less than 0.6 μm, The second flat plate has a thickness of 0.6 μm or more.

4. The light modulation element according to claim 1 or 2, wherein The electro-optical material 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.

5. The light modulation element according to claim 1 or 2, wherein Also features: a signal electrode for applying the RF signal to the first waveguide portion; and A bias electrode applies the DC bias to the second waveguide portion.

6. The light modulation element according to claim 1 or 2, wherein: The optical waveguide is a Mach-Zehnder optical waveguide, which has: an input waveguide; a beam splitting portion that splits the light propagating in the input waveguide; first and second waveguides extending from the beam splitting portion and arranged parallel to each other; a beam combining portion that combines the light propagating in the first and second waveguides; and an output waveguide that propagates light output from the beam combining portion.

Images