Optical modulation element and optical modulator

Abstract

To provide a light modulation element that can suppress DC bias voltage to be applied.SOLUTION: A light modulation element 100 in the present invention includes a substrate formed of a material that is not lithium niobate, a lithium niobate film including a first optical waveguide 11 and a second optical waveguide 12 forming a Mach-Zehnder type optical waveguide 10 on the substrate and connecting a branch part 15 and a coupling part 16, and a first electrode 25 and a second electrode 26 that apply an electric field to the first optical waveguide 11 and the second optical waveguide 12. A first ridge part 11 and a second ridge part 12 have a cross-sectional constant shape that is orthogonal to the length direction. At least one of the first ridge part 11 and the second ridge part 12 includes a cross-sectional non-constant shape, which is different from the cross-sectional constant shape. The light output in the case where the DC bias voltage to be applied between the first electrode and the second electrode is 0 (V) is smaller than the maximum value of the light output in the case where the DC bias voltage is varied in a predetermined range.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to an optical modulation element and an optical modulator. [Background technology]

[0002] With the spread of the Internet, communication traffic has increased dramatically, and the importance of optical fiber communication has become very important. Optical fiber communication converts electrical signals into optical signals and transmits the optical signals through optical fibers, and has the advantages of operating over a wide frequency band, low loss, and high noise resistance.

[0003] An optical modulator converts an electrical signal into an optical signal. For example, Patent Documents 1 and 2 describe a Mach-Zehnder type optical modulator in which an optical waveguide is formed by Ti (titanium) diffusion near the surface of a lithium niobate single crystal substrate. Patent Document 2 also describes correcting the operating point drift of the optical modulator. The optical modulators described in Patent Documents 1 and 2 operate at high speeds of 40 Gb / s or more, but have a long overall length of around 10 cm.

[0004] In contrast, Patent Document 3 describes a Mach-Zehnder type optical modulator using a c-axis oriented lithium niobate film. The optical modulator using the lithium niobate film is smaller and has a lower driving voltage than an optical modulator using a lithium niobate single crystal substrate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2004-37695 A [Patent Document 2] Patent No. 4164179 [Patent Document 3] JP 2019-45880 A [Patent Document 4] Patent No. 2817295 [Patent Document 5] Japanese Patent Application Publication No. 5-297332 [Patent Document 6] Japanese Patent Application Publication No. 5-297333 [Patent Document 7] JP 2008-52103 A Summary of the Invention [Problem to be solved by the invention]

[0006] Optical modulators using lithium niobate have a large extinction ratio and can operate at high frequencies, so they are used for long-distance communications such as between cities. Optical modulators using indium phosphide are also expected to be used for long-distance communications, because they can operate at high frequencies. On the other hand, in recent years, short- and medium-distance communications such as within and between data centers have also increased, and for such applications, it is necessary to reduce the size of the optical modulator. If the phase modulation section becomes shorter with the miniaturization of the optical modulator, the voltage (half-wave voltage) for shifting the phase by π increases, and the DC bias voltage applied to adjust the operating point voltage increases.

[0007] A method has been disclosed in which the lengths of two optical waveguides constituting a phase modulation section are made asymmetrical to generate a phase difference in a state where no DC bias voltage is applied between the two optical waveguides, and in this state, the operating point voltage is shifted (Patent Documents 4 to 6). Patent Document 4 discloses an optical modulator in which one of the two optical waveguides is curved to have a different length from the other in order to shift the operating point voltage. The same document also discloses an optical modulator in which the positions of the branching section and the coupling section are spaced apart from the center line of the two optical waveguides to shift the operating point voltage, and the angle changes (opening angles) before and after branching for the two optical waveguides are made uniform to equalize the intensity of branched light. Patent Documents 5 and 6 disclose optical modulators in which the lengths of two optical waveguides are made asymmetrical for the same purpose as Patent Document 4, and the distance between the waveguides is kept constant. In the methods disclosed in Patent Documents 4 to 6, the shape of the optical waveguide becomes complicated, which increases design constraints, making further miniaturization difficult.

[0008] Patent Document 7 discloses that at least a part of one of a pair of optical waveguides and a part of the other optical waveguide facing it have different widths, but this is due to a Ti-diffused waveguide. In Patent Document 7, the reason why the opposing parts of the two optical waveguides have different widths is to suppress coupling of propagating light when the optical waveguides are brought close to each other, and there is no description about operating point control.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide an optical modulation element that can be easily manufactured while keeping the applied DC bias voltage low even when the element size is small, and an optical modulator equipped with the same. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides the following means.

[0011] (1) An optical modulation element according to one aspect of the present invention comprises: a substrate made of a material other than lithium niobate; a lithium niobate film constituting a Mach-Zehnder type optical waveguide formed on one main surface of the substrate, the lithium niobate film having a first ridge portion and a second ridge portion functioning as a first optical waveguide and a second optical waveguide connecting a branch portion and a coupling portion, respectively; a first electrode that applies an electric field to the first optical waveguide; and a second electrode that applies an electric field to the second optical waveguide, wherein the first ridge portion and the second ridge portion have fixed cross-sectional shape portions having the same cross-sectional shape perpendicular to a longitudinal direction, and at least one of the first ridge portion and the second ridge portion has an irregular cross-sectional shape portion having a different cross-sectional shape from the fixed cross-sectional shape portion, and an optical output obtained when a DC bias voltage applied between the first electrode and the second electrode is set to 0 (V) is smaller than a maximum optical output obtained when the DC bias voltage is changed within a predetermined voltage range.

[0012] (2) In the light modulation element described in (1) above, it is preferable that the non-regular cross-sectional shape portion and the regular cross-sectional shape portion have a width different from each other.

[0013] (3) In the light modulation element described in (2) above, it is preferable that the non-regular cross-sectional shape portion and the regular cross-sectional shape portion have a different cross-sectional area.

[0014] (4) In the light modulation element described in any one of (1) to (3) above, the portion with an irregular cross-sectional shape is preferably disposed in a region that does not overlap either the first electrode or the second electrode.

[0015] (5) In the optical modulation element described in any one of (1) to (4) above, it is preferable that the optical output obtained when the DC bias voltage is set to 0 (V) is 85% or less of the difference between the maximum and minimum optical outputs obtained when the DC bias voltage is changed within a predetermined voltage range.

[0016] (6) An optical modulator according to an aspect of the present invention includes the optical modulation element according to any one of (1) to (5) above. Effect of the Invention

[0017] According to the present invention, it is possible to provide an optical modulation element that can be easily manufactured while keeping the applied DC bias voltage low, and an optical modulator including the same. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a block diagram of an optical modulator according to an embodiment. [Diagram 2] FIG. 2 is a plan view of a light modulation element according to an embodiment. [Diagram 3] FIG. 2 is a plan view of an optical waveguide according to one embodiment. [Figure 4] FIG. 2 is a cross-sectional view of a light modulation element according to one embodiment. [Diagram 5] FIG. 2 is a cross-sectional view of a light modulation element according to one embodiment. [Figure 6] FIG. 4 is a diagram showing the relationship between the applied voltage and the output of an optical modulator according to one embodiment. [Figure 7A]FIG. 2 is a diagram for explaining optical modulation by an example of an optical modulator according to an embodiment. [Figure 7B] 11A and 11B are diagrams for explaining optical modulation by another example of an optical modulator according to an embodiment. [Figure 7C] 11A and 11B are diagrams for explaining optical modulation by another example of an optical modulator according to an embodiment. [Figure 8A] 7B is a diagram showing the relationship between the applied voltage and the extinction ratio of the optical modulator shown in FIG. 7A. [Figure 8B] FIG. 7C is a diagram showing the relationship between the applied voltage and the extinction ratio of the optical modulator shown in FIG. 7B. [Figure 8C] FIG. 7D is a diagram showing the relationship between the applied voltage and the extinction ratio of the optical modulator shown in FIG. 7C. [Figure 9] 1 is a graph showing optical output characteristics of optical modulators according to an example and a comparative example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present invention will be described in detail below with reference to the drawings as appropriate. The drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the features of the present invention easier to understand, and the dimensional ratios of each component may differ from the actual ones. The materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and may be modified as appropriate within the scope of the effects of the present invention.

[0020] First, the directions are defined. One direction on one surface of the substrate (base material) Sb is the x-direction, and the direction perpendicular to the x-direction is the y-direction. The x-direction is, for example, the direction in which the first optical waveguide 11 extends. The z-direction is perpendicular to one surface of the substrate Sb. The z-direction is perpendicular to the x-direction and the y-direction. Hereinafter, the +z direction may be expressed as "up" and the -z direction as "down". Up and down do not necessarily coincide with the directions in which gravity is applied.

[0021] 1 is a block diagram of an optical modulator 200 according to the first embodiment. The optical modulator 200 includes an optical modulation element 100, a driving circuit 110, a DC bias application circuit 120, and a DC bias control circuit 130. The control unit 140 of the optical modulator 200 includes the driving circuit 110, the DC bias application circuit 120, and the DC bias control circuit 130.

[0022] The optical modulation element 100 converts an electrical signal into an optical signal. in The output light L is generated according to the modulation signal Sm. out Convert to.

[0023] The drive circuit 110 applies a modulation voltage Vm corresponding to the modulation signal Sm to the optical modulation element 100. The DC bias application circuit 120 applies a DC bias voltage Vdc to the optical modulation element 100. The DC bias control circuit 130 monitors the output light Lout and controls the DC bias voltage Vdc output from the DC bias application circuit 120. By adjusting this DC bias voltage Vdc, an operating point Vd, which will be described later, is controlled.

[0024] Fig. 2 is a plan view of the light modulation element 100 as viewed from the z direction. Fig. 3 is a plan view of the optical waveguide 10 of the light modulation element 100 as viewed from the z direction. Fig. 4 is a cross section taken along X1-X1' in Fig. 2. Fig. 5 is a cross section taken along X2-X2' in Fig. 2.

[0025] The optical modulation element 100 includes a substrate Sb made of a material different from lithium niobate, and a lithium niobate film (oxide film) 40 formed on one main surface of the substrate Sb. The lithium niobate film 40 has a first ridge portion 11 and a second ridge portion 12 protruding to the side opposite to the substrate Sb. The first ridge portion 11 and the second ridge portion 12 respectively constitute a Mach-Zehnder type optical waveguide 10 and function as a first optical waveguide and a second optical waveguide connecting a branch portion 15 and a coupling portion 16. Hereinafter, the first optical waveguide may be referred to as the first optical waveguide 11 using the reference numeral 11, and similarly, the second optical waveguide may be referred to as the second optical waveguide 12 using the reference numeral 12. The optical modulation element 100 further includes a first electrode 25 for applying an electric field to the first optical waveguide 11, and a second electrode 26 for applying an electric field to the second optical waveguide 12.

[0026] At least an AC power supply 31 (drive circuit 110) that applies a modulated voltage between the first electrode 25 and the second electrode 26, and a DC power supply 33 (DC bias application circuit 120) that applies a DC bias voltage between the first electrode 25 and the second electrode 26 are connected. Here, a case is illustrated in which the first electrode 25 is divided into an AC first electrode 21 for connecting to the AC power supply 31 and a DC first electrode 23 for connecting to the DC power supply 33, and the second electrode 26 is divided into an AC second electrode 22 for connecting to the AC power supply 31 and a DC second electrode 24 for connecting to the DC power supply 33. The AC first electrode 21 and the DC first electrode 23 may be integrated together or may be separated as separate bodies. The AC second electrode 22 and the DC first electrode 24 may be integrated together or may be separated as separate bodies. Hereinafter, the AC first electrode 21, the AC second electrode 22, the DC first electrode 23, and the DC second electrode 24 may be referred to as the electrode 21, the electrode 22, the electrode 23, and the electrode 24, respectively.

[0027] The light modulation element 100 includes a substrate Sb. The substrate Sb may be any substrate on which an oxide film 40 such as a lithium niobate film (LN 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 substrate Sb is not particularly limited. The lithium niobate film has the property of being easily formed as a c-axis oriented epitaxial film on substrates Sb of various crystal orientations. Since the crystals constituting the c-axis oriented lithium niobate film have three-fold symmetry, it is desirable that the underlying substrate Sb also has the same symmetry, and a c-plane substrate is preferable in the case of a sapphire single crystal substrate, and a (111) plane substrate is preferable in the case of a silicon single crystal substrate.

[0028] The optical waveguide 10 is a path through which light propagates. The optical waveguide 10 has, for example, a first optical waveguide 11, a second optical waveguide 12, an input path 13, an output path 14, a branching section 15, and a coupling section 16. The first optical waveguide 11 and the second optical waveguide 12 shown in FIG. 3 are configured to extend in the x direction except for the vicinity of the branching section 15 and the vicinity of the coupling section 16, but are not limited to such a configuration. The first optical waveguide 11 and the second optical waveguide 12 shown in FIG. 3 have approximately the same length. The branching section 15 is between the input path 13 and the first optical waveguide 11 and the second optical waveguide 12. The input path 13 is connected to the first optical waveguide 11 and the second optical waveguide 12 via the branching section 15. The coupling section 16 is between the first optical waveguide 11 and the second optical waveguide 12 and the output path 14. The first optical waveguide 11 and the second optical waveguide 12 are connected to an output path 14 via a coupling portion 16 .

[0029] The optical waveguide 10 includes a first optical waveguide 11 and a second optical waveguide 12 which are ridge portions protruding from a first surface 40a of the lithium niobate film 40. The first surface 40a is the upper surface of the lithium niobate film 40 other than the ridge portion. The two ridge portions (first ridge portion, second ridge portion) protrude in the z direction from the first surface 40a and extend along the optical waveguide 10. In this embodiment, the first ridge portion functions as the first optical waveguide 11, and the second ridge portion functions as the second optical waveguide 12.

[0030] In the optical modulation element of the present invention, the first ridge portion and the second ridge portion have a fixed cross-sectional shape portion having the same cross-sectional shape perpendicular to the length direction (light propagation direction) of the optical waveguide, and at least one of the first ridge portion and the second ridge portion has an irregular cross-sectional shape portion having a different cross-sectional shape from the fixed cross-sectional shape portion. Here, the first ridge portion and the second ridge portion are configured such that even when the DC bias voltage is 0V, the light propagating through the two optical waveguides generates a phase difference at the coupling portion. In other words, even if at least one of the first ridge portion and the second ridge portion has an irregular cross-sectional shape portion having a different cross-sectional shape from the fixed cross-sectional shape portion, if the two propagating lights do not generate a phase difference when the DC bias voltage is 0V, it does not fall under the optical modulation element of the present invention. Hereinafter, the irregular cross-sectional shape portion that generates a phase difference at the coupling portion of the light propagating through the two optical waveguides when the DC bias voltage is 0 V may be referred to as an asymmetric portion.

[0031] The optical waveguide 10 shown in Figures 2 and 3 is configured such that only the second ridge portion 12 has an irregular cross-sectional shape portion 12b as an asymmetric portion, but both the first ridge portion 11 and the second ridge portion 12 may have an irregular cross-sectional shape portion. The optical waveguide 10 has a configuration in which the second ridge portion 12 has an unshaped cross-sectional portion 12b in addition to a shaped cross-sectional portion 12a, and the first ridge portion 11 has only a shaped cross-sectional portion 11a.

[0032] In this embodiment, as a typical example, a case will be described in which only one of two ridges has an irregular cross-sectional shape portion, the irregular cross-sectional shape portion has a rectangular cross-sectional shape that differs from the regular cross-sectional shape portion only in width and has the same height (thickness) with reference to Figures 2 to 5. In this case, the regular cross-sectional shape portion and the irregular cross-sectional shape portion have mutually different cross-sectional shapes (cross-sectional areas). In addition, both or one of the fixed cross-sectional shape portion and the non-fixed cross-sectional shape portion may have a different (varying) width in the height direction (z direction) of the cross-sectional shape (for example, the cross-sectional shape is triangular or trapezoidal). In this specification, including such cases, the "width" of the fixed cross-sectional shape portion and the non-fixed cross-sectional shape portion means the width in the direction parallel to the main surface of the substrate Sb at a position that is half the maximum value in the height direction (z direction) of the cross-sectional shape. Here, the cross section in the cross-sectional shape is a cross section perpendicular to the length direction (light propagation direction) of the optical waveguide, as described above.

[0033] The characteristics of the cross-sectionally irregular portion of this example will be described using the symbols shown in FIGS. 4 and 5. As for the width, the width W2 of the cross-sectionally irregular portion 12b of the second ridge portion 12 is equal to the width W2 of the cross-sectionally irregular portion 12a of the second ridge portion 12. 0 and the width W1 of the entire first ridge portion 11 0 (W2>W2 0 =W1 0 The height (thickness) of the second ridge portion 12 (the shaped cross-sectional portion 12a and the unshaped cross-sectional portion 12b (height: Ha)) is the same as that of the first ridge portion 11 (height: Hb).

[0034] The cross-sectional shape of the non-uniform cross-sectional shape portion 12b is not particularly limited as long as it is different from the cross-sectional shape of the uniform cross-sectional shape portion 12a. Even if the cross-sectional shapes of the non-uniform cross-sectional shape portion and the uniform cross-sectional shape portion are similar to each other, the cross-sectional shapes are different from each other as long as the above-mentioned phase difference occurs. Furthermore, there is no particular limitation on the length L of the cross-sectionally irregular portion 12b, and it can be determined appropriately according to the desired phase difference.

[0035] By providing the irregular cross-sectional portion as an asymmetric portion, a difference in group velocity occurs between the first optical waveguide and the second optical waveguide, which generates a phase difference. From this viewpoint, the cross-sectional shape of the ridge of the irregular cross-sectional portion is not particularly limited, and examples thereof include a rectangle, a trapezoid, a triangle, and a semicircle. The size (L or W2 in FIG. 3) and the number of the irregular cross-sectional portion are also not particularly limited, and may be single or multiple. When multiple irregular cross-sectional portions are provided, their shapes, sizes, etc. may be the same or not. The irregular cross-sectional portion may be provided in only one optical waveguide, or may be provided in both optical waveguides. The end of the irregular cross-sectional portion in the length direction may be tapered. By providing the irregular cross-sectional shape portion, it is possible to reduce the DC bias voltage for controlling the operating point by making the optical output smaller than the maximum value at a DC bias voltage of 0 V. The cross-sectional shape and size of the ridge of the irregular cross-sectional shape portion, the number of irregular cross-sectional shape portions, etc. may be set according to the required shift amount of the operating point.

[0036] The cross-sectional shape of the cross-sectionally fixed portion of first ridge portion 11 and second ridge portion 12 may be any shape capable of guiding light, and may be, for example, rectangular, trapezoidal, triangular, semicircular, etc. The width in the y direction of the two ridge portions is preferably 0.3 μm or more and 5.0 μm or less, and the height of the two ridge portions (protruding heights Ha, Hb from first surface 40a) is preferably, for example, 0.1 μm or more and 1.0 μm or less. The ridge portions are made of the same material as the lithium niobate film 40.

[0037] There is no restriction on the position of the irregular cross-sectional shape portion on the optical waveguide, but from the viewpoint of more accurately applying an electric field to the first optical waveguide 11 and the second optical waveguide 12, it is preferable that the irregular cross-sectional shape portion is formed at a position that does not overlap the first electrode 25 and the second electrode 26 when viewed in a plane from the z direction.

[0038] The lithium niobate film 40 is, for example, a c-axis oriented lithium niobate film. The lithium niobate film 40 is, for example, an epitaxial film epitaxially grown on a substrate Sb. An epitaxial film is a single crystal film whose crystal orientation is aligned by the underlying substrate. An epitaxial film is a film having a single crystal orientation in the z direction and the xy in-plane direction, and the crystals are aligned in the x-axis, y-axis, and z-axis directions. Whether the film formed on the substrate Sb is an epitaxial film or not can be verified by, for example, checking the peak intensity and pole at the orientation position in 2θ-θ X-ray diffraction. The lithium niobate film 40 is, for example, a SiO 2 Alternatively, the lithium niobate film may be provided via a metal layer.

[0039] Specifically, when measured by 2θ-θX-ray diffraction, all peak intensities other than the target plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the target plane. For example, when the lithium niobate film 40 is a c-axis oriented epitaxial film, the peak intensities other than the (00L) plane are 10% or less, preferably 5% or less, of the maximum peak intensity of the (00L) plane. Here, (00L) is a general designation for equivalent planes such as (001) and (002).

[0040] Moreover, the conditions for checking the peak intensity at the orientation position described above only indicate orientation in one direction. Therefore, even if the above conditions are met, if the crystal orientation is not uniform within the plane, the intensity of the X-rays will not increase at a specific angle position, and no pole will be observed. For example, when the lithium niobate film 40 is a lithium niobate film, LiNbO 3 Since LiNbO has a trigonal crystal structure, the single crystal 3There are three poles of (014). In the case of lithium niobate, it is known that epitaxial growth occurs in a so-called twin state, in which crystals rotated 180° around the c-axis are symmetrically bonded. In this case, two of the three poles are symmetrically bonded, resulting in six poles. In addition, when a lithium niobate film is formed on a silicon single crystal substrate with a (100) plane, 4x3=12 poles are observed because the substrate has four-fold symmetry. In this disclosure, a lithium niobate film epitaxially grown in a twin state is also included in the epitaxial film.

[0041] The composition of lithium niobate is Li x NbA y O z A is an element other than Li, Nb, and O. x is 0.5 or more and 1.2 or less, and preferably 0.9 or more and 1.05 or less. y is 0 or more and 0.5 or less. z is 1.5 or more and 4.0 or less, and preferably 2.5 or more and 3.5 or less. The element A is, for example, K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, or Ce, and two or more of these elements may be combined.

[0042] The lithium niobate film 40 has a thickness of, for example, 2 μm or less. The thickness of the lithium niobate film 40 refers to the thickness of the portion other than the ridge portion. If the lithium niobate film 40 is too thick, the crystallinity may decrease. The thickness of the lithium niobate film 40 is, for example, about 1 / 10 or more of the wavelength of the light used. If the lithium niobate film 40 is thin, the light is less confined, and the light leaks into the substrate Sb or the buffer layer 30. If the lithium niobate film 40 is thin, the change in the effective refractive index of the optical waveguide 10 may be small even when an electric field is applied to the lithium niobate film 40.

[0043] The electrodes 21 and 22 are electrodes that apply a modulation voltage Vm to the optical waveguide 10. The electrode 21 is an example of a first electrode, and the electrode 22 is an example of a second electrode. A first end 21a of the electrode 21 is connected to a power source 31, and a second end 21b is connected to a termination resistor 32. A first end 22a of the electrode 22 is connected to the power source 31, and a second end 22b is connected to the termination resistor 32. The power source 31 is a part of a drive circuit 110 that applies a modulation voltage Vm to the optical modulation element 100.

[0044] The electrodes 23 and 24 are electrodes that apply a DC bias voltage Vdc to the optical waveguide 10. A first end 23a of the electrode 23 and a first end 24a of the power supply 24 are connected to a power supply 33. The power supply 33 is a part of a DC bias application circuit 120 that applies the DC bias voltage Vdc to the optical modulation element 100.

[0045] 2, the line width and spacing of the electrodes 21 and 22 arranged in parallel are made wider than in reality for ease of viewing. Therefore, the length (interaction length) of the overlapping portion between the electrode 21 and the first optical waveguide 11 and the length (interaction length) of the overlapping portion between the electrode 22 and the second optical waveguide 12 appear to be different, but these lengths (interaction lengths) are substantially the same. Similarly, the length (interaction length) of the overlapping portion between the electrode 23 and the first optical waveguide 11 and the length (interaction length) of the overlapping portion between the electrode 24 and the second optical waveguide 12 are substantially the same.

[0046] Furthermore, when a DC bias voltage Vdc is superimposed on the electrodes 21 and 22, the electrodes 23 and 24 do not need to be provided. Also, ground electrodes may be provided around the electrodes 21, 22, 23, and 24.

[0047] The electrodes 21, 22, 23, and 24 are on the lithium niobate film 40 with the buffer layer 30 interposed therebetween. The electrodes 21 and 23 can each apply an electric field to the first optical waveguide 11. The electrodes 21 and 23 are each located, for example, at a position overlapping with the first optical waveguide 11 in a plan view from the z direction. The electrodes 21 and 23 are each located above the first optical waveguide 11. The electrodes 22 and 24 can each apply an electric field to the second optical waveguide 12. The electrodes 22 and 24 are each located, for example, at a position overlapping with the second optical waveguide 12 in a plan view from the z direction. The electrodes 22 and 24 are each located above the second optical waveguide 12.

[0048] The buffer layer 30 is between the optical waveguide 10 and the electrodes 21, 22, 23, and 24. The buffer layer 30 covers and protects the ridge portion. The buffer layer 30 also prevents the light propagating through the optical waveguide 10 from being absorbed by the electrodes 21, 22, 23, and 24. The buffer layer 30 has a lower refractive index than the lithium niobate film 40. The buffer layer 30 is made of, for example, SiO 2 , Al 2 O 3 , MgF 2 , La 2 O 3 , ZnO, HfO 2 , MgO, Y 2 O 3 , CaF 2 , In 2 O 3 etc. or mixtures thereof.

[0049] The chip size of the optical modulation element 100 is, for example, 100 mm 2 The chip size of the optical modulation element 100 is 100 mm 2 If the optical modulation element has the following properties, it can be used as an optical modulation element for a data center.

[0050] The light modulation element 100 can be fabricated by a known method, for example, using semiconductor processes such as epitaxial growth, photolithography, etching, vapor deposition, and metallization.

[0051] The optical modulation element 100 converts an electrical signal into an optical signal. in The output light L out First, the modulation operation of the light modulation element 100 will be described.

[0052] Input light L input from input path 13 in The light propagates through the first optical waveguide 11 and the second optical waveguide 12 after branching. The phase difference between the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12 is zero when the light branches.

[0053] Next, a voltage is applied between the electrodes 21 and 22. For example, differential signals having the same absolute value, opposite polarities, and no phase shift may be applied to the electrodes 21 and 22, respectively. The refractive indices of the first optical waveguide 11 and the second optical waveguide 12 change due to the electro-optic effect. For example, the refractive index of the first optical waveguide 11 changes by +Δn from a reference refractive index n, and the refractive index of the second optical waveguide 12 changes by -Δn from the reference refractive index n.

[0054] The difference in refractive index between the first optical waveguide 11 and the second optical waveguide 12 generates a phase difference between the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12. The light propagating through the first optical waveguide 11 and the second optical waveguide 12 join together at the output path 14 to produce output light L. out The output light L out is a superposition of the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12. out The intensity of the input light L changes according to an odd-number multiple of the phase difference between the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12. For example, when the phase difference is an even multiple of π, the lights are constructive, and when it is π, the lights are destructive. In this manner, the optical modulation element 100 controls the input light L in response to the electrical signal. in The output light L out is modulated to.

[0055] A modulation voltage Vm corresponding to a modulation signal is applied to the electrodes 21 and 22 for applying a modulation voltage of the optical modulation element 100. The voltage applied to the electrodes 23 and 24 for applying a DC bias voltage, that is, the DC bias voltage Vdc output from the DC bias application circuit 120, is controlled by a DC bias control circuit 130. The DC bias control circuit 130 adjusts the operating point Vd of the optical modulation element 100 by controlling the DC bias voltage Vdc. The operating point Vd is a voltage that is the center of the modulation voltage amplitude.

[0056] The optical modulation curve by the optical modulation element 100 will be described with reference to FIG. 6. FIG. 6 is a diagram showing the relationship between the DC bias voltage and the output for the optical modulator 200 according to the first embodiment and the optical modulator having a conventional configuration in which the length and shape of the two branched optical waveguides are uniform, that is, the optical modulator having a configuration in which the optical waveguide does not have a non-uniform cross-sectional shape portion that generates a phase difference. The horizontal axis of FIG. 6 is the DC bias voltage applied to the electrodes 23 and 24, and the vertical axis is the normalized output from the optical modulation element 100. The output is normalized to "1" when the phase difference between the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12 is zero. The solid line shows the characteristics of the optical modulator having the conventional configuration, and the dashed line shows the characteristics of the optical modulator of the first embodiment.

[0057] When the first ridge portion and the second ridge portion do not have any irregular cross-sectional shape portion and are the same length, there is no phase difference between the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12. Therefore, at least in a state where no voltage is applied (Vdc=0), the light of the same phase that has passed through the two optical waveguides interferes with each other at the coupling portion 16 and reinforces each other, and the output of the optical modulation element 100 becomes a maximum value.

[0058] In contrast, in the configuration in which at least one of the first ridge portion and the second ridge portion has an irregular cross-sectional shape portion as in this embodiment, the group velocities of the light propagating through the first optical waveguide 11 and the light propagating through the second optical waveguide 12 become asymmetric, and a phase difference occurs between these lights. Therefore, even when the applied DC bias voltage is 0V (Vdc=0), the output of the optical modulation element 100 does not reach the maximum value, but becomes smaller than the maximum value, as a result of the light having different phases passing through each of the first and second ridge portions interfering with each other at the coupling portion 16. In other words, in the configuration in which at least one of the first and second ridge portions has an irregular cross-sectional shape portion, the operating point Vd is shifted to the 0V side, based on the configuration having no irregular cross-sectional shape portion and the same length. The example shown in FIG. 6 is a case in which the operating point Vd shifts to the 0V side by (1 / 2)Vπ, and the operating point Vd' is approximately 0 (V).

[0059] This allows the linear bias voltage Vdc applied to the electrodes 23 and 24 to be set to approximately 0 (V) in order to control the operating point. Furthermore, correction of the operating point due to DC drift or the like can be handled within a smaller voltage range.

[0060] As the applied voltage is increased, the output from the optical modulation element 100 gradually decreases from its maximum value, and reaches a minimum at a certain voltage. The voltage at which the output from the optical modulation element 100 reaches a minimum is the null-point voltage Vn. The half-wave voltage (half-wave phase modulation voltage) is the voltage for making the phase difference of light 180° in a Mach-Zehnder type optical modulator, and the voltage width over which the output from the optical modulation element 100 goes from maximum to minimum corresponds to the half-wave voltage Vπ. When a voltage exceeding the null-point voltage Vn is applied, the output from the optical modulation element 100 changes periodically. The output from the optical modulation element 100 repeats maximum and minimum values ​​at every half-wave voltage Vπ.

[0061] As a result, in the optical modulation element 100 of this embodiment, whether the optical output is smaller than the maximum value can be determined by applying a DC bias voltage to the electrodes 23 and 24, monitoring the output from the modulation element, and comparing it with the output value in a state where no voltage is applied (Vdc=0). Specifically, the maximum and minimum values ​​can be determined by gradually increasing the applied DC bias voltage and measuring the point where the optical output reverses from the minimum value to increase and the point where the optical output reverses from the maximum value to decrease. A modulation signal may be applied to the electrodes 21 and 22. The maximum and minimum values ​​of the intensity of the optical output can be determined by plotting the maximum and minimum values ​​of the optical output at each DC bias voltage.

[0062] 2, there are two Vπ: a half-wave voltage Vπ(RF) at electrodes 21 and 22 to which modulation voltage Vm is applied, and Vπ(DC) at electrodes 23 and 24 to which a DC bias voltage is applied. To measure the maximum and minimum values, a voltage is applied to electrodes 23 and 24 in the range of 2×Vπ(DC). The maximum value in this range is defined as the maximum value, and the minimum value as the minimum value. When a DC bias voltage Vdc is superimposed on electrodes 21 and 22, electrodes 23 and 24 do not need to be provided, and Vπ(RF) and Vπ(DC) will have the same value.

[0063] The half-wave voltage Vπ of the optical modulation element 100 varies depending on the configuration of the optical modulation element 100. The half-wave voltage Vπ varies depending on, for example, the length of the electrodes 21, 23 on the first optical waveguide 11, the length of the electrodes 22, 24 on the second optical waveguide 12, etc. Here, the length of the first electrodes 21, 23 and the length of the second electrodes 22, 24 are lengths in the light propagation direction. In the case of FIG. 2, it is the length of the part of the electrodes 21, 23 that overlaps with the first optical waveguide 11, or the length of the part of the electrodes 22, 24 that overlaps with the second optical waveguide 12. This length is called the interaction length. If the interaction length is long, the half-wave voltage Vπ becomes small, and if the interaction length is short, the half-wave voltage Vπ becomes large. When attempting to reduce the size of the optical modulation element 100, the interaction length becomes shorter and the half-wavelength voltage Vπ becomes larger; however, by shifting the operating point voltage Vdc toward 0 V as in this embodiment, the DC bias voltage applied to the electrodes 23 and 24 can be kept low.

[0064] The DC bias application circuit 120 controls the operating point voltage Vd of the optical modulation element 100. The operating point voltage Vd is the midpoint between the minimum value (Vmin) and maximum value (Vmax) of the applied voltage. The difference between the minimum value (Vmin) and maximum value (Vmax) of the applied voltage is the applied voltage width Vpp.

[0065] The operating point voltage Vd may vary depending on the temperature of the operating environment. If the operating point voltage Vd varies during use, it is corrected by the DC bias control circuit 130. The DC bias control circuit 130, for example, out Based on the branched light Lb branched from the output of the power amplifier 11, the fluctuation of the operating point voltage Vd is corrected.

[0066] The optical modulation in the case of the optical modulation element 100 exhibiting the modulation curve shown in Fig. 6 will be described with reference to Fig. 7A. The horizontal axis of Fig. 7A is the DC bias voltage applied to the optical modulation element 100, and the vertical axis is the intensity of the optical output at the applied voltage. In this case, if the operating point Vd' is set so that the shift amount of the operating point voltage is (Vn-0.5Vπ), the DC bias voltage can be set to approximately 0 (V). For example, if the applied voltage width Vpp of the modulation voltage Vm is a half-wave voltage Vπ(RF), a modulation voltage Vm in the range of (-1 / 2)Vπ(RF) to (1 / 2)Vπ(RF) is applied to the optical modulation element 100. As shown in FIG. 7A, the optical output from the optical modulation element 100 is maximum when the modulation voltage Vm is (-1 / 2)Vπ(RF), is minimum when the modulation voltage Vm is (1 / 2)Vπ(RF), and is 50% of the maximum output when the modulation voltage Vm is 0V.

[0067] Similarly, using Figure 7B, we will explain the optical modulation of the optical modulation element 100 in which the operating point Vd' is set so that the shift amount of the operating point voltage is (Vn-0.25Vπ) and the applied voltage width Vpp of the modulation voltage Vm is controlled as (1 / 4) wavelength voltage (1 / 2)Vπ(RF). In this case, if the shift amount of the operating point voltage is set to (Vn-0.25Vπ), the operating point Vd' can be set to a DC bias voltage of approximately 0 (V). A modulation voltage Vm corresponding to a range from (-1 / 4)Vπ(RF) to (1 / 4)Vπ(RF) is applied to the optical modulation element 100. As shown in FIG. 7B, the optical output from the optical modulation element 100 is maximum when the modulation voltage Vm is (-1 / 4)Vπ(RF), is minimum when the modulation voltage Vm is (1 / 4)Vπ(RF), and is 15% of the maximum output when the modulation voltage Vm is 0V (Vd').

[0068] Similarly, using Figure 7C, we will explain the optical modulation of the optical modulation element 100 in which the operating point Vd' is set so that the shift amount of the operating point voltage is (Vn-0.75Vπ) and the applied voltage width Vpp of the modulation voltage Vm is controlled as (1 / 4) wavelength voltage (1 / 2)Vπ(RF). In this case, if the shift amount of the operating point voltage is set to (Vn-0.75Vπ), the operating point Vd' can be set to a DC bias voltage of approximately 0 (V). A modulation voltage Vm corresponding to a range from (-1 / 4)Vπ(RF) to (1 / 4)Vπ(RF) is applied to the optical modulation element 100. As shown in FIG. 7C, the optical output from the optical modulation element 100 is maximum when the modulation voltage Vm is (-1 / 4)Vπ(RF), is minimum when the modulation voltage Vm is (1 / 4)Vπ(RF), and is 85% of the maximum output when the modulation voltage Vm is 0V (Vd').

[0069] The modulation signal of the high frequency voltage is controlled by, for example, the driving circuit 110. The band of the modulation element is 60 GHz or more. If the frequency band of the modulation element is 60 GHz or more, it is easy to handle high speed modulation.

[0070] Fig. 8 is a diagram showing the relationship between the applied voltage and the extinction ratio of the optical modulator 200 according to this embodiment. The horizontal axis of Fig. 8 is the DC bias voltage applied to the optical modulation element 100, and the vertical axis is the ratio of the output light Lout at the applied voltage to the output light Lout at the null point voltage Vn. The extinction ratio is the ratio of the maximum value to the minimum value of the output light Lout in the applied voltage range.

[0071] The extinction ratios of the optical modulation elements 100 shown in FIGS. 7A to 7C will be described with reference to FIGS. 8A to 8C. As shown in FIG. 8A, the optical modulation element 100 in FIG. 7A exhibits the maximum extinction ratio (about 25 dB) among those in FIGS. 7A to 7C. On the other hand, the optical modulation element 100 in FIG. 7C exhibits the smallest extinction ratio (about 3 dB) among those in FIGS. 7A to 7C, as shown in FIG. 8C. Moreover, the optical modulation element 100 in FIG. 7B exhibits an extinction ratio (about 22 dB) between those in FIGS. 7A to 7C, as shown in FIG. 8B.

[0072] As shown in FIG. 8C, the extinction ratio is small in the region where the amount of light of the output light Lout of the optical modulation element 100 is sufficiently large. In this way, under the condition that the applied voltage width Vpp is the same, when the operating point Vd is set at a position away from the null point voltage Vn, the amount of light is larger but the extinction ratio is smaller than when the operating point Vd is set near the null point voltage Vn. However, the extinction ratio required for an optical modulator for a data center is smaller than that for an optical modulator for long-distance communication, and is about 3 dB. Therefore, by setting the operating point Vd to 85% or less of the maximum optical output, the extinction ratio can be made 3 dB or more even when Vpp is smaller than Vπ.

[0073] Although the case where Vd is smaller than Vn is illustrated here, it may be larger than Vn. In that case, the operating point can be shifted by appropriately changing the shape, length (L), and width (W2) of the asymmetric portion in the present invention.

[0074] As described above, the optical modulation element 100 and the optical modulator 200 according to the first embodiment can be driven at a low voltage and can be used in the high frequency band.

[0075] As described above, the optical modulation element 100 according to this embodiment configures a Mach-Zehnder type optical waveguide, and has the first optical waveguide 11 and the second optical waveguide 12 connecting the branching section 15 and the coupling section 16, and the cross-sectional shapes of the two optical waveguides are different. As a result, the group velocities of the light traveling in the two optical waveguides become asymmetric, and a phase difference occurs between them.

[0076] This phase difference exists even when no electric field is applied to the two optical waveguides, and the light of different phases interferes with each other at the coupling section 16, partially canceling out, causing the optical output to be smaller than the maximum value. As a result, the operating point voltage can be shifted toward 0V. By adjusting the amount of shift in the wavelength range of the light used so that the operating point voltage is reduced, the increase in the applied voltage can be suppressed.

[0077] The photoelectric modulation element 100 of this embodiment does not require adjustment of the length of each optical waveguide to generate a phase difference between the first optical waveguide 11 and the second optical waveguide 12. This makes it possible to avoid the problem of the optical waveguide having a complicated shape and being difficult to manufacture. EXAMPLES

[0078] Examples of the present disclosure are illustrated below, but the present disclosure is not limited to the following examples. It is clear that a person skilled in the art can come up with various modified or amended examples within the scope of the ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present disclosure.

[0079] (Comparative Example) The structures shown in Figures 2 and 4 (structures without irregular cross-sectional shapes) were actually fabricated as prototypes using the following procedure. The substrate material was sapphire. A lithium niobate film with a thickness of 1.5 μm was formed on the surface of the substrate by sputtering. Next, a ridge was formed by forming a mask using resist and dry etching using Ar plasma. The cross-sectional shape of the ridge was rectangular, with a ridge width W1 0 and W2 0 The thickness was 1.0 μm and the ridge height was 0.4 μm. Next, the thickness was 0.8 μm and the material was LaAlO 3 The buffer layer was formed by deposition. Then, it was planarized by CMP. Finally, the first and second electrodes were formed by photolithography and gold plating.

[0080] The interaction length of the electrodes 21 and 22 was 8.5 mm, and the interaction length of the electrodes 23 and 24 was 5.0 mm. The modulation characteristics were evaluated using light with a wavelength of 1310 nm. At that time, Vπ(RF) was 8.3 (V), Vπ(DC) was 14.1 (V), and the maximum extinction ratio was 25 dB. Furthermore, the optical output was measured when a DC bias voltage was applied to the electrodes 23 and 24. The results are shown in the graph of FIG. 9. A modulation signal may be applied to the electrodes 21 and 22. By plotting the maximum and minimum values ​​of the optical output at each DC bias voltage, the maximum and minimum values ​​of the intensity of the optical output can be determined.

[0081] (Example) As shown in FIG. 5, a part of the second ridge portion had an irregular cross-sectional shape, and the other part was exactly the same as in the comparative example. 6, the cross-sectional shape of the irregular cross-sectional part of the second ridge is rectangular, and the shape in plan view from the z direction is also rectangular with a length L in the x direction of 0.53 mm and a width W2 in the y direction of 1.15 μm. In Example 1, it can be seen that the optical output intensity is 50% of the maximum value when no DC bias voltage is applied, and the operating point voltage is shifted by an amount equivalent to (1 / 2)Vπ.

[0082] The cross-sectional shape of the irregular cross-sectional shape portion of the second ridge portion in Example 2 was also rectangular, and the shape in plan view from the z direction was also rectangular, with the length L in the x direction being 0.63 mm, and the width W2 in the y direction being 1.15 μm. The cross-sectional shape of the irregular cross-sectional shape portion of the second ridge portion in Example 3 was also rectangular, and the shape in plan view from the z direction was also rectangular, with the length L in the x direction being 0.48 mm, and the width W2 in the y direction being 1.15 μm. The calculation results of the light output characteristics obtained in Examples 2 and 3 are shown in the graph of FIG. 9. The horizontal axis of the graph indicates the DC bias voltage (V), and the vertical axis of the graph indicates the light output intensity. The solid line corresponds to the output characteristics obtained in the comparative example, the dashed line corresponds to the output characteristics obtained in Example 2, and the dashed line corresponds to the output characteristics obtained in Example 3.

[0083] In the case of the comparative example, when no DC bias voltage is applied, the optical output intensity is at its maximum value, and no shift in the operating point voltage is observed. In contrast, in Example 2, when no DC bias voltage is applied, the optical output intensity is 15% of the maximum value, and it is understood that the operating point voltage has shifted by an amount equivalent to (3 / 4)Vπ. Also, in Example 3, when no DC bias voltage is applied, the optical output intensity is 85% of the maximum value, and it is understood that the operating point voltage has shifted by an amount equivalent to (1 / 4)Vπ.

[0084] In the case of Example 3 where the optical output intensity is 85% of the maximum value, an extinction ratio of 3bB can be obtained by setting the DC bias voltage of 0 (V) as the operating point and setting the applied voltage width Vpp to (1 / 2)Vπ (a modulation voltage Vm in the range of -(1 / 4)Vπ to +(1 / 4)Vπ). The shift amount of the operating point voltage can be designed arbitrarily in accordance with the required driving conditions of the modulator, but when it is desired to obtain a large extinction ratio with a small applied voltage width Vpp, it is preferable to set the shift amount of the operating point voltage so that the optical output intensity is in the range of 0% to 50% of the maximum value when no DC bias voltage is applied. The shift amount of the operating point voltage can be changed by adjusting the size, shape, etc. of the irregular cross-sectional shape portion. In the above embodiments 1 to 3, the shape of the irregular cross-sectional shape portion of the second ridge portion in plan view from the z direction is also rectangular, and the shift amount of the operating point voltage is changed by changing the length L in the x direction, but the shift amount of the operating point voltage may also be changed by changing the width W2 in the y direction, or by changing both the length L in the x direction and the width W2 in the y direction. In addition, the cross-sectional shape perpendicular to the length direction of the irregular cross-sectional shape portion may be adjusted. According to the present invention, it is possible to realize an optical modulation element that can be easily manufactured while keeping the DC bias voltage low even when the element size is small. [Explanation of symbols]

[0085] 10 Optical waveguide 11 First optical waveguide (first ridge portion) 11a Regular cross-sectional shape part 12 Second optical waveguide (second ridge portion) 12a Regular cross-sectional shape part 12b Irregular cross-sectional shape 13 Input Path 14 Output path 15 Branch 16 Joint 21, 22, 23, 24 electrodes 25 1st electrode 26 2nd electrode 30 Buffer Layer 31 AC power supply 32 Termination resistor 33 DC power supply 40 Oxide Film 40a Page 1 100 Light Modulation Element 110 Drive circuit 120 DC bias application circuit 130 DC bias control circuit 140 Control section 200 Optical Modulator L in Input light L out Output Light Lb branch light Vd Operating point voltage Vn null point voltage Vπ Half-wave voltage Vpp Applied voltage range

Claims

1. A substrate made of a material different from lithium niobate; a lithium niobate film that constitutes a Mach-Zehnder type optical waveguide formed on one main surface of the substrate and has a first ridge portion and a second ridge portion that function as a first optical waveguide and a second optical waveguide connecting a branch portion and a coupling portion, respectively; a first electrode that applies an electric field to the first optical waveguide; a second electrode that applies an electric field to the second optical waveguide; the first ridge portion and the second ridge portion have a shaped cross-sectional portion having the same cross-sectional shape perpendicular to the longitudinal direction, and one of the first ridge portion and the second ridge portion has a non-shaped cross-sectional portion having a different cross-sectional shape from the shaped cross-sectional portion, the cross-sectional irregular portion is configured to generate a phase difference of less than π / 2 or a phase difference of greater than π / 2 and less than π between the light propagating through the first optical waveguide and the light propagating through the second optical waveguide even when a DC bias voltage applied between the first electrode and the second electrode is 0 (V), each of the first electrode and the second electrode has a modulation voltage application electrode for applying a modulation voltage corresponding to a modulation signal, and a DC bias voltage application electrode disposed apart from the modulation voltage application electrode for applying a DC bias voltage to adjust an operating point; the cross-sectionally irregular portion is disposed between the modulation voltage application electrode and the DC bias voltage application electrode in a plan view, an operating point voltage is determined by the non-uniform cross-sectional shape portion and the DC bias voltage; an optical output obtained when a DC bias voltage applied between the first electrode and the second electrode is set to 0 (V) is smaller than a maximum optical output obtained when the DC bias voltage is changed within a predetermined voltage range; When the operating point voltage is Vd, the half-wave voltage is Vπ, and the null point voltage is Vn, the operating point voltage Vd is determined by the DC bias voltage and the non-regular cross-sectional shape portion so as to be in a range of Vn-Vπ<Vd<Vn-Vπ / 2 or in a range of Vn-Vπ / 2<Vd<Vn, An optical modulation element, characterized in that, when the applied voltage width of the modulation voltage is Vpp, Vπ / 2≦Vpp<Vπ.

2. The light modulation element according to claim 1 , wherein the non-regular cross-sectional shape portion and the regular cross-sectional shape portion have widths different from each other.

3. 3. The light modulation element according to claim 1, wherein the non-regular cross-sectional shape portion and the regular cross-sectional shape portion have a cross-sectional area different from each other.

4. An optical modulation element according to any one of claims 1 to 3, characterized in that the optical output obtained when the DC bias voltage is set to 0 (V) is 85% or less of the difference between the maximum and minimum optical outputs obtained when the DC bias voltage is changed within a predetermined voltage range.

5. An optical modulator comprising the optical modulation element according to any one of claims 1 to 4.

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