JP2017083607A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical modulator capable of producing an optical output signal with little nonlinearity while minimizing an increase in manufacturing cost and overall device cost and degradation in modulation characteristics.SOLUTION: Modulation electrodes comprise a signal electrode S1 disposed between a pair of branch waveguides, and two ground electrodes (G1, G2) disposed to sandwich the pair of branch waveguides therebetween. An optical modulator is configured such that, when combining light beams modulated by a pair of modulation waveguides of each branch waveguide, light intensities of the light beams differ by an intensity ratio (1:α), and each ground electrode has an extension (G10, G20) that extends from a portion thereof to partially overlap above a modulation waveguide located near the ground electrode such that depths of modulation by the modulation electrodes for the pair of modulation waveguides of each branch waveguide differ by a predetermined ratio (1:s).SELECTED DRAWING: Figure 11

Description

  The present invention relates to an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating light propagating through the optical waveguide.

In recent years, in the fields of optical communication and optical measurement, an optical waveguide is formed on a substrate having an electro-optic effect such as lithium niobate (LN), and a modulation electrode for modulating a light wave propagating in the optical waveguide is formed. The optical modulators used are often used.
In addition, in order to improve the utilization efficiency of the optical spectrum, multi-level modulation and analog modulation such as PAM (Pulse Amplitude Modulation), QAM (Quadrature Amplitude Modulation), and OFDM (Orthogonal Frequency Division Multiplexing) have been studied.

One method for obtaining a multilevel optical signal is to drive a push-pull drive type Mach-Zehnder modulator (MZM) that applies a drive voltage in reverse phase with a multilevel electrical signal.
FIG. 1 shows a configuration example of a conventional symmetrical MZM. An input optical signal is branched into two optical waveguides by a symmetric demultiplexer B1 such as a directional coupler, a multimode optical interference element (MMI), or a Y-shaped 1 × 2 coupler. A phase change of + θ and −θ is given to these optical waveguides by a signal electrode (Signal Electrode) disposed therebetween and two ground electrodes (Ground Electrode) disposed so as to sandwich the optical waveguide. Here, 2θ = (π / Vπ) · V, V is a drive voltage, and Vπ is an applied voltage that changes the phase of an optical signal propagating through the optical waveguide by a half wavelength. In the optical waveguide, portions where the phase of the light wave propagating through the optical waveguide is modulated by the electric field formed by the signal electrode and the ground electrode are referred to as modulation waveguide portions (PM1, PM2).

  The optical signal modulated in each optical waveguide is adjusted in phase by −π / 2 and + π / 2 by a bias electrode, and then a directional coupler, MMI, Y-shaped 1 × 2 coupler. Are combined by a symmetric combiner M1 and the like and output as an output optical signal. At this time, the electric field E of the output optical signal is sin (θ) as shown in FIG. 2, and the light intensity output is a sine-square curve with respect to the drive voltage V. In the optical waveguide, a portion where a predetermined phase adjustment is performed by the bias electrode is referred to as a bias adjustment unit (Bias 1 to 2).

  FIG. 2 shows a response curve of the output optical signal with respect to the drive voltage in the MZM of FIG. As shown in the figure, the response curve to the drive voltage has a non-linear sine function, so when driving with a multi-valued electrical signal, ideal output optical signals with equal intervals obtained when the response curve is a linear triangular wave Deviation occurs.

  In Patent Document 1, in order to make the response curve with respect to the drive voltage closer to a triangular wave, the modulation degree of each modulation waveguide section is adjusted to be different, so that a plurality of modulation signals are used, or the modulation corresponding to each modulation waveguide section is used. It is disclosed that the length of an electrode (in particular, a signal electrode) is set to be different for each modulation waveguide section.

  However, when a plurality of modulation signals are used, the external circuit for driving the optical modulator becomes complicated and the cost of the entire apparatus increases. In addition, setting the modulation electrodes to have different lengths tends to cause variations in the characteristics of each product, leading to an increase in manufacturing costs. In addition, since the modulation electrode is formed with a height of several tens of μm in order to cope with high frequency modulation, internal stress due to a difference in thermal expansion is likely to occur between the substrate and the modulation electrode. For this reason, if the length of the modulation electrode is different corresponding to each modulation waveguide portion, the internal stress applied to the substrate is likely to be uneven, resulting in temperature drift and deterioration of modulation characteristics.

JP 2012-252260 A

Y. Yamaguchi, S. Nakajima, A. Kanno, T. Kawanishi, M. Izutsu, and H. Nakajima, “Single Mach-Zehnder Modulator with Active Y-branch for Higher than 60 dB Extinction-Ratio Operation,” Proc. ECOC2013 , London, UK, paper P.2.15, 2013.

  The problem to be solved by the present invention is to solve the above problems, and to obtain an optical output signal in which nonlinearity is suppressed while suppressing an increase in manufacturing cost and overall cost of the apparatus, and deterioration of modulation characteristics. An optical modulator is provided.

In order to solve the above problems, the optical modulator of the present invention has the following technical features.
(1) In an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating light propagating through the optical waveguide,
The optical waveguide is a Mach-Zehnder type waveguide having two branched waveguide portions,
Each branch waveguide section includes a branch section for branching light input to the branch waveguide section, two modulation waveguide sections for modulating each light branched by the branch section, and the 2 And a combining unit that combines the lights modulated by the two modulation waveguide units,
The modulation electrode is composed of a signal electrode disposed between the two branch waveguide portions and two ground electrodes disposed so as to sandwich the two branch waveguide portions,
The light intensity at the time of synthesizing each light modulated by the two modulation waveguide sections in each branch waveguide section is different from each other and has a predetermined intensity ratio,
The two modulation waveguide sections in each branch waveguide section are arranged in a part of the ground electrode and in proximity to the ground electrode so that the degree of modulation by the modulation electrode differs from each other by a predetermined ratio. An overhanging portion is provided so as to partially overlap the upper side of the modulated waveguide portion.

(2) In the optical modulator according to (1), the overhanging portion is characterized in that the thickness of the electrode is thinner than the other part of the ground electrode.

(3) In the optical modulator according to (2), the overhanging portion is provided on the upper side of the modulation waveguide portion without a buffer layer, and the modulation waveguide is provided in a part of the overhanging portion. A through hole is formed along the portion.

(4) In the optical modulator according to any one of (1) to (3),
The intensity ratio of the light during the synthesis is 1: α (where α is a numerical value greater than 1),
The ratio of the modulation degree is 1: β (where β is a numerical value greater than 1) based on the modulation waveguide portion that propagates light with high light intensity,
When the numerical value α is approximately 5, the numerical value β is approximately 2, and when the numerical value α is approximately 9, the numerical value β is approximately 3.

  Note that the expression “substantially” in the present invention means that even when the actual numerical value α and numerical value β are slightly deviated from ideal values (for example, α = 5, β = 2), the light modulation is performed. When used as a container, it means that it can be tolerated to the extent that there is no practical problem. In addition, even if the numerical value β deviates from the ideal value, there may be a case where a modulation characteristic with improved linearity can be obtained by slightly deviating the numerical value α from the ideal value. The numerical value is expressed as “approximately”.

  In the optical modulator of the present invention, an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating light propagating through the optical waveguide, The optical waveguide is a Mach-Zehnder type waveguide having two branch waveguide portions. Each branch waveguide portion includes a branch portion for branching light input to the branch waveguide portion, and a branch portion. There are provided two modulation waveguide sections for modulating each branched light, and a combining section for combining the lights modulated by the two modulation waveguide sections. It consists of a signal electrode arranged between the waveguide sections and two ground electrodes arranged so as to sandwich the two branch waveguide sections, and is modulated by the two modulation waveguide sections in each branch waveguide section. If the light intensity when combining each light In the two modulation waveguide sections in each branch waveguide section, a part of the ground electrode is formed so that the modulation degree by the modulation electrode differs from each other by a predetermined ratio. Since the overhanging portion is provided so as to partially overlap the modulation waveguide portion arranged close to the ground electrode, the number of modulation signals applied to the modulation electrode can be suppressed, and the modulation electrode Can be set to the same length. Thereby, the increase in the manufacturing cost of an optical modulator and the cost of the whole apparatus can be suppressed. In addition, it is possible to provide an optical modulator that can suppress the occurrence of temperature drift and the deterioration of modulation characteristics and can obtain an optical output signal in which nonlinearity is suppressed.

  In the present invention, a state where the output light from the optical modulator corresponding to the level of the drive voltage deviates from the amplitude or proportionality of the light intensity is expressed as “non-linear”. A response curve having a shape close to a triangular wave, which is a linear periodic function, is called “nonlinear” as a response curve having a high “linearity” and a different shape.

It is a figure which shows the structural example of the conventional Mach-Zehnder modulator. FIG. 2 is a diagram illustrating a response curve of an output optical signal with respect to a drive voltage in the Mach-Zehnder modulator of FIG. 1. It is a figure explaining the basic composition of the optical modulator which realizes the present invention. FIG. 4 is a diagram illustrating a modulation state when the intensity ratio is set to 1: 5 and the modulation factor ratio 1: 2 in the optical modulator of FIG. 3. FIG. 5 is a diagram illustrating a response curve with respect to a driving voltage by the optical modulator of FIG. 4. FIG. 4 is a diagram illustrating a modulation state when the intensity ratio is set to 1: 9 and the modulation factor ratio 1: 3 in the optical modulator of FIG. 3. It is a figure which shows the response curve with respect to the drive voltage by the optical modulator of FIG. It is a conceptual diagram explaining the example which comprised the modulation electrode by one signal electrode regarding the optical modulator which concerns on this invention. It is a top view explaining the positional relationship of the modulation | alteration electrode used for the optical modulator of FIG. 8, and an optical waveguide. FIG. 10 is a cross-sectional view taken along one-dot chain line A-A ′ in FIG. 9. It is a top view explaining the specific characteristic of the modulation electrode used for the optical modulator of FIG. It is sectional drawing in the dashed-dotted line A-A 'of FIG. It is a figure explaining the application example regarding the overhang | projection part of the ground electrode of FIG. It is a top view concerning the optical modulator of the present invention. It is sectional drawing in the dashed-dotted line A-A 'of FIG. It is a top view explaining the application example which concerns on the optical modulator of this invention.

Hereinafter, the optical modulator of the present invention will be described in detail.
FIG. 3 is a diagram for explaining the basic configuration of an optical modulator for realizing the optical modulator of the present invention. The optical modulator includes a substrate 1 having an electro-optic effect, an optical waveguide (O10 to O15) formed on the substrate, and a modulation electrode for modulating light propagating through the optical waveguide. The optical waveguide is a Mach-Zehnder type waveguide having two branch waveguide sections, and each branch waveguide section includes a branch section (B22, B23) for branching light input to the branch waveguide section. , Two modulation waveguide sections (PM11 and PM12, PM13 and PM14) in which each of the lights branched by the branch section is modulated, and a combining section that combines the lights modulated by the two modulation waveguide sections (M22, M23) are provided.

  An optical modulator having a response curve composed of a plurality of Fourier series components is realized by integrating Mach-Zehnder modulators in parallel. The coefficient of the Fourier series can be adjusted by the light intensity ratio α, the coefficient sign can be adjusted by the bias state of the Mach-Zehnder interferometer, and each order component can be adjusted by the light modulation degree ratio β. In other words, if the number of Fourier series terms in the response curve is two, a parallel Mach-Zehnder modulator with two parallels and three parallel Mach-Zehnder modulators should have a structure with a predetermined light intensity ratio and light modulation degree ratio. It ’s fine. In the case of a triangular wave, the mathematical Fourier series expansion formula is composed of odd-order components of a sine wave, and the non-linearity is greatly improved even when approximation is performed using two Fourier series terms. In order to efficiently increase the linearity of the response curve with a small number of series terms, an even-order series term may be used regardless of the mathematical Fourier series expansion. Compared with the case of an odd-order term, an increase in optical loss occurs essentially, and the approximation near the apex (turning point) deteriorates, but the ripple (overshoot amount, deficiency amount) can be reduced, Linearity can be greatly improved. In particular, the use of a secondary component is effective in improving linearity with a small number of series terms.

  In the optical modulator of FIG. 3, a branching section B21 is provided to divide the incident light Lin into two branching waveguide sections. The branching section B21 is a symmetric duplexer that branches the light intensity ratio evenly to 1: 1. Each modulation waveguide section is provided with a Mach-Zehnder type optical waveguide, and each includes a branch section (B22, B23) and a combining section (M22, M23). The light branched at the branching portions (B22, B23) and then modulated at each modulation waveguide portion is combined at the combining portion so that the intensity ratio of the light is 1: α (where α is a numerical value greater than 1). Synthesized.

  The branched light is modulated by the respective modulation waveguide portions (PM11 to PM14). The ratio of the modulation degree in each modulation waveguide part is 1: β (however, based on the modulation degree of the modulation waveguide part (PM11 or PM14) that propagates light with high light intensity (light of intensity α) at the time of synthesis. , Β is a numerical value greater than 1. Specifically, in the modulation waveguide portion PM11 and the modulation waveguide portion PM12, the ratio of the modulation degree is 1: β. A modulation electrode (not shown) is disposed in the vicinity of the modulation waveguide portion. Specifically, as shown in FIG. 1, a modulation electrode composed of a signal electrode and a ground electrode is disposed. However, it is not preferable to prepare a modulation signal source independent of individual signal electrodes corresponding to each modulation waveguide section, because this increases the product cost of the entire optical modulator. The configuration of the modulation electrode will be described in detail later. In addition, the requirement of the intensity ratio of the light combined in the combining unit (M22, M23) is 1: α, but here, in order to simplify the explanation and the equations in the drawing, the branching unit (B22, The description will be made assuming that the intensity ratio of the light branched in B23) is 1: α. The same applies to FIG.

  In addition, each branch waveguide section has modulation waveguide sections (PM11 and PM14, PM12 and PM13) having the same modulation degree (1 or β), and these are modulated in opposite phases. The light modulated by each modulation waveguide section is adjusted to a predetermined phase by a bias adjustment section (Bias 11 to 23) formed by a bias electrode (not shown). Further, the light is multiplexed at a predetermined intensity ratio by the multiplexing units (M22, M23, and M21) to form output light Lout.

  In FIG. 4, the intensity ratio (1: α) of light input to the two modulation waveguide portions in each branch waveguide portion is set to 1: 5, and the ratio of the modulation degree by the modulation waveguide portion (1: β ) Is a diagram for explaining the modulation state of the optical modulator when set to 1: 2. As the light wave passes through each branch part (B21 to B23), modulation waveguide part (PM11 to PM14), bias adjustment part (Bias11 to 23), and multiplexing part (M22 to 23, M21), it is shown in FIG. It becomes the modulation state of the mathematical formula. Looking at the equations in FIG. 4, in the modulation waveguide section (PM11, PM14), a primary Fourier series component corresponding to the triangular wave signal is formed. Similarly, in the modulation waveguide section (PM12, PM13), a second-order Fourier series component corresponding to the triangular wave signal is formed. By combining these signals with a predetermined phase adjustment, it is possible to obtain an optical output signal in which nonlinearity is suppressed. The result is shown in FIG.

  In FIG. 6, the intensity ratio (1: α) of light input to the two modulation waveguide portions in each branch waveguide portion is set to 1: 9, and the ratio of the modulation degree by the modulation waveguide portion (1: β ) Is a diagram for explaining the modulation state of the optical modulator when set to 1: 3. As the light wave passes through each branch part (B21 to B23), modulation waveguide part (PM11 to PM14), bias adjustment part (Bias11 to 23), and multiplexing part (M22 to 23, M21), it is shown in FIG. It becomes the modulation state of the mathematical formula. Looking at the equations in FIG. 6, in the modulation waveguide section (PM11, PM14), a primary Fourier series component corresponding to the triangular wave signal is formed. Similarly, in the modulation waveguide portion (PM12, PM13), a third-order Fourier series component corresponding to the triangular wave signal is formed. By combining these signals with a predetermined phase adjustment, it is possible to obtain an optical output signal in which nonlinearity is suppressed. The result is shown in FIG.

  4 and 6 exemplify the method of adding the second-order and third-order Fourier series components to the first-order Fourier series components, but in the optical modulator according to the present invention, the light intensity ratio and the modulation degree ratio are adjusted. Further, it is possible to additionally add higher-order Fourier series components. Furthermore, in FIG.4 and FIG.6, although the method of arrange | positioning four modulation | alteration waveguide parts in parallel and adding two Fourier series components was shown, more modulation | alteration waveguide parts are arrange | positioned in parallel, It is also possible to add three or more Fourier series components. In addition to the triangular wave signal, the obtained signal waveform can be close to a sawtooth waveform, a rectangular signal suitable for digital response, or a multi-step function.

Next, the configuration of the modulation electrode that applies an electric field to the modulation waveguide portion of the optical waveguide will be described.
First, as the substrate 1 used for the optical modulator, a material having an electro-optic effect such as Pockels effect or Kerr effect, and a substrate obtained by combining these materials can be used. In particular, a material having a high Pockels effect is preferable. Specific examples include materials such as lithium niobate, lithium tantalate, and electro-optic polymer.
The optical waveguide can be formed by thermally diffusing a high refractive index substance such as Ti on the surface of the substrate 1 by a thermal diffusion method or a proton exchange method.

The substrate 1 is formed with a signal electrode and a ground electrode for modulating a light wave propagating through the modulation waveguide section, and a bias electrode for adjusting the phase of the light wave before and after the modulation. These electrodes can be formed by forming a Ti / Au electrode pattern on the surface of the substrate 1 and using a gold plating method or the like. Further, if necessary, a buffer layer such as a dielectric SiO ₂ may be provided on the surface of the substrate 1 after forming the optical waveguide, and an electrode may be formed on the buffer layer.

  In the optical modulator of the present invention, the modulation electrode for applying an electric field to the modulation waveguide section is composed of one signal electrode S1, as shown in FIG. Specifically, an electric field is applied to the two modulation waveguide portions (PM11, PM12) by the signal electrode S1 and the ground electrode G1. Further, an electric field is applied to the other two modulation waveguide portions (PM13, PM14) by the signal electrode S1 and the ground electrode G2.

  Two adjacent modulation waveguide sections (PM11 and PM12, PM13 and PM14) need to be modulated with different modulation degrees (modulation degree ratio 1: β). In FIG. 9, the modulation degree is adjusted by adjusting the positional relationship between the modulation electrode (the signal electrode S1 and the ground electrodes (G1, G2)) and the optical waveguide (O11 to O14).

  FIG. 9 is a plan view showing the arrangement of the modulation electrodes, while FIG. 10 shows a cross-sectional view taken along one-dot chain line A-A ′ in FIG. 9. In general, the intensity distribution of the electric field between the signal electrode S1 and the ground electrode (G1 or G2) becomes stronger as the signal electrode S1 is approached. For this reason, it is preferable that the modulation waveguide portion having a high modulation degree is arranged in the vicinity of the signal electrode. Even in the vicinity of the ground electrode (G1 or G2), there is a portion where a part of the electric field is locally concentrated on the end portion of the ground electrode, and thus there is a portion where the electric field strength is slightly higher than the periphery. In FIG. 9, Δφ indicates the degree of modulation in the modulation waveguide portions O12 and O13, and s indicates the ratio of the modulation degree (s = 1 / β) when the modulation degree of the modulation waveguide O12 or O13 is used as a reference. Show.

  As shown in FIG. 10, the distance from the modulation waveguide portion and the signal electrode S1 is set to the distance g1 for the optical waveguide O12 and to the distance g2 for the optical waveguide O11. Thereby, it becomes possible to set the modulation degree in the modulation waveguide portions of the optical waveguides O11 and O12 to different values. However, the phase direction of modulation is in phase. Thus, in order to change the degree of modulation depending on the distance from the signal electrode, the thickness of the substrate 1 is set to 50 μm or more, preferably 100 μm or more. The thicker the substrate is, the more easily the electric lines of force formed by the signal electrodes spread within the substrate, and the difference between the high and low electric field strengths becomes larger and the ratio of the modulation degree can be further increased. Is possible. In FIGS. 9 and 10, the description is centered on the X-cut type substrate. However, even when a Z-cut type substrate is used, the modulation degree of each modulation waveguide section is similarly adjusted by adjusting the distance from the signal electrode. Can be adjusted.

  Since the optical modulator is driven in a high frequency band, it is necessary to achieve speed matching between the light propagating through the optical waveguide and the microwave signal propagating through the signal electrode. As one of the means, the thickness of the substrate 1 is set to 30 μm or less, and further to 10 μm or less. When using such a thin substrate, since many lines of electric force coming out of the signal electrode are concentrated in the substrate, even if the distance between the modulation waveguide portion and the signal electrode changes, There is little change in the degree of modulation in the modulation waveguide section. As a result, in the method as shown in FIG. 9, it is difficult to increase the ratio of the modulation degree (for example, β ≧ 5).

  If an optical waveguide is disposed in the vicinity of the signal electrode S1 or the ground electrode G1 or G2, the modulation efficiency (effective overlap integral Γ between the signal electric field and the light electric field) can be increased to some extent by a so-called edge effect. However, when the buffer layer is absent or thin, a part of the light wave propagating through the optical waveguide is absorbed and scattered, resulting in light loss. The light propagating through each optical waveguide is preset so that the light intensity ratio (1: α) becomes a predetermined value. However, when light loss due to the modulation electrode occurs, the balance of the light intensity ratio is lost, and there is a problem that a desired signal waveform cannot be obtained. In order to obtain a desired light intensity ratio, it is necessary to comprehensively consider the light branching ratio of the branching section, the optical loss of the optical waveguide including the modulation waveguide section, and the combining ratio in the combining section that combines the light. is necessary.

  The magnitude of the modulation efficiency Γ of each modulation waveguide portion is indicated by Γa and Γb. In the case of the arrangement as shown in FIG. 9, since Γa <Γb, there is an arrangement structure in which the ratio of the light modulation degree is 1: β with reference to the optical waveguide portion that propagates light with high light intensity. sell. However, when an optical waveguide is disposed in the vicinity of the signal electrode S1 or the ground electrode G1 or G2, the optical loss dependency and the optical waveguide position dependency of the modulation efficiency Γ and the optical waveguide shape dependency are extremely large. Yield reduction due to the problem is a big problem. In order to realize the characteristics of the optical modulator in which the nonlinearity is suppressed with the structure of FIGS. 9 and 10 with high reproducibility, the manufacturing process requires extremely high reproducibility.

  In order to solve such a problem, the present inventors have found a modulation electrode structure as shown in FIG. 12 is a cross-sectional view taken along one-dot chain line A-A ′ in FIG. 11. The structure of the modulation electrode used in the optical modulator of the present invention has a ground electrode (G1 or G1) in order to weaken the strength of the electric field applied to the optical waveguide (modulation waveguide portion) arranged farther from the signal electrode. The projecting part (G10, G20) projecting from G2) is provided. A part of the branch waveguide portions O11 and O14 is provided below the projecting portions (G10 and G20) projecting from the ground electrode (G1 or G2), and the branch waveguide portions O11 and O14 are connected to the signal electrode S1 and the ground. It is provided at a position separated from the electrode G1 or G2. With this structure, the dependency of the modulation efficiency Γ due to the edge effect on the optical waveguide position dependency and the optical waveguide shape dependency can be greatly reduced.

  When using a thin substrate as shown in FIG. 12, the branched optical waveguide should be disposed on the ground electrode side at a distance of about half or more of the thickness of the substrate from the end of the overhanging portion (G10, G20). Thus, since the modulation efficiency Γ of the branched optical waveguide below the overhanging portion can be reduced by one digit or more, the modulation ratio β can be effectively adjusted. In addition, by making the thickness of the overhanging portion thinner than the main body portion of the ground electrode, it is possible to suppress a significant change in the characteristic impedance of the modulation electrode.

  There is no need to change the width sw of the signal electrode along the optical waveguide. However, when the characteristic impedance of the modulation electrode due to the overhang is suppressed slightly, or when the distance between the signal electrode and the ground electrode changes due to the overhang and the electric field strength is adjusted. For example, the width sw of the signal electrode can be changed. FIG. 11 shows an example in which the width sw of the signal electrode in the section facing the overhanging portions (G10, G20) is narrowed to prevent the characteristic impedance from being lowered.

  Since the overhang portion is formed only on the ground electrode side, the symmetry of the modulation electrode changes and internal stress due to thermal expansion may be unevenly distributed. However, since the thickness of the overhanging portion is thin, it is possible to reduce the magnitude of the generated internal stress, and it is possible to suppress the occurrence of temperature drift due to the internal stress as in Patent Document 1.

  Since the overhang portions (G10, G20) are provided on the upper portion of the optical waveguide, they cause light loss. For this reason, including the difference between the optical loss of the optical waveguide arranged in the vicinity of the signal electrode S1 and the optical loss of the optical waveguide arranged in the vicinity of the ground electrode (G1, G2), it is multiplexed at the multiplexing unit. The light intensity ratio can be 1: α. In particular, as shown in FIG. 13, by providing a through hole in a part of the overhanging portion, it is possible to reduce light loss while suppressing a change in electric field strength.

  Note that a buffer layer can be disposed below the modulation electrode including the overhang, and in this case, light loss due to the entire electrode can be reduced. Here, an example is shown in which the branching waveguide and the electrode are linear, but they may be bent or meandering. The formation of the overhang portions (G10, G20) is not limited to one place, and may be formed at a plurality of places. By forming in multiple places, it is also possible to reduce the signal frequency dependence of the modulation ratio β, which is caused by the loss difference between the sections with and without the overhangs (G10, G20). It is.

  FIG. 14 is a plan view showing an embodiment of the optical modulator of the present invention, and FIG. 15 is a cross-sectional view taken along one-dot chain line A-A ′ in FIG. 14. As shown in FIG. 14, the signal electrode S1 and the ground electrodes G1 and G2 constitute a modulation unit, and light propagating through the optical waveguide (O11 to O14) is modulated by the modulation signal Sin input to the signal electrode S1. A part of the ground electrode G1 also serves as a bias electrode, and the phase of light propagating through the optical waveguide is adjusted by applying a bias voltage (DC bias).

  FIG. 16 is a plan view showing an application example of the optical modulator of the present invention. In the optical modulator described above, the method of adjusting the light intensity ratio is based on the light branching ratio of the branching unit, but the branching unit is always used as a symmetric demultiplexer to modulate the light intensity after branching. It is also possible to adjust with an attenuator (Attenuator) and set to a desired light intensity ratio. The attenuator may be omitted, and the active Y branch shown in Non-Patent Document 1 may be adopted for the branching section. Furthermore, an active Y branch may be employed in the combining unit.

  By variably adjusting the light intensity ratio α, even if the modulation degree ratio β deviates from the ideal state due to manufacturing errors, branch circuits, band characteristics of electrode characteristics, etc., the light intensity ratio α is adjusted to a straight line. It is also possible to supplement sex. It should be noted that the effect when the modulation factor β is significantly deviated from the integer value is large near the apex (turning point) of the triangular wave and relatively small at the straight line portion of the triangular wave. Since the approximation of the triangular wave is Fourier series, it goes without saying that the modulation factor ratio β and the light intensity ratio α are preferably predetermined integers. In many applications, the peak of the triangular wave is used in analog modulation applications. The characteristics of the straight line part are emphasized from the vicinity. Complementing linearity by adjusting the light intensity ratio α is a useful method. The characteristics such as the shape and effective refractive index of each branch waveguide do not need to be the same. For example, the waveguide diameter may be changed or bent, or the impurity concentration may be changed. As a result, deterioration of characteristics (optical crosstalk) due to coupling of guided light between the optical waveguides is reduced.

  As described above, according to the present invention, an optical modulator capable of obtaining an optical output signal in which nonlinearity is suppressed while suppressing an increase in manufacturing cost, overall device cost, and deterioration in modulation characteristics. Can be provided.

DESCRIPTION OF SYMBOLS 1 Board | substrate O10-O15 Optical waveguide B21-B23 Branch part PM11-PM14 Modulation waveguide part Bias11-Bias23 Phase adjustment part M21-M23 Combined part S1 Signal electrode G1-G2 Ground electrode G10-G20 Overhang part 9 Buffer layer 10 Adhesion Layer 11 Support substrate 12 Buffer layer

Claims (4)

In an optical modulator comprising a substrate having an electro-optic effect, an optical waveguide formed on the substrate, and a modulation electrode for modulating light propagating through the optical waveguide,
The optical waveguide is a Mach-Zehnder type waveguide having two branched waveguide portions,
Each branch waveguide section includes a branch section for branching light input to the branch waveguide section, two modulation waveguide sections for modulating each light branched by the branch section, and the 2 And a combining unit that combines the lights modulated by the two modulation waveguide units,
The modulation electrode is composed of a signal electrode disposed between the two branch waveguide portions and two ground electrodes disposed so as to sandwich the two branch waveguide portions,
The light intensity at the time of synthesizing each light modulated by the two modulation waveguide sections in each branch waveguide section is different from each other and has a predetermined intensity ratio,
The two modulation waveguide sections in each branch waveguide section are arranged in a part of the ground electrode and in proximity to the ground electrode so that the degree of modulation by the modulation electrode differs from each other by a predetermined ratio. An optical modulator comprising: an overhanging portion that projects so as to partially overlap an upper side of the modulated waveguide portion. The optical modulator according to claim 1.
The overhanging portion is an optical modulator characterized in that the electrode is thinner than other portions of the ground electrode. The optical modulator according to claim 2.
The overhanging portion is provided on the upper side of the modulation waveguide portion without a buffer layer, and a through hole is formed in a part of the overhanging portion along the modulation waveguide portion. Light modulator. The optical modulator according to any one of claims 1 to 3,
The intensity ratio of the light during the synthesis is 1: α (where α is a numerical value greater than 1),
The ratio of the modulation degree is 1: β (where β is a numerical value greater than 1) based on the modulation waveguide portion that propagates light with high light intensity,
An optical modulator characterized in that when the numerical value α is approximately 5, the numerical value β is approximately 2, and when the numerical value α is approximately 9, the numerical value β is approximately 3.

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