JP2017083608A - 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 signal electrodes (S1, S2) disposed between each pair of modulation waveguides (O11 and O12, O13 and O14), and two ground electrodes (G1-G3) disposed to sandwich each pair of modulation waveguides therebetween, where the signal electrodes are formed by bifurcating an input signal electrode S0. An optical modulator is configured such that, when combining light beams modulated by the pair of modulation waveguides of each branch waveguide, light intensities of the light beams differ by an intensity ratio, and positions of the signal electrodes and ground electrodes relative to the modulation waveguides are set up such that depths of modulation by the modulation electrodes for the pair of modulation waveguides of each branch waveguide differ by a predetermined ratio.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, multilevel 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 modulation corresponding to each modulation waveguide section is performed. 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 arranged between the two modulation waveguide portions and two ground electrodes arranged so as to sandwich the two modulation waveguide portions, and the signal electrode is , Formed by branching one input signal electrode into two,
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,
Arrangement positions of the signal electrode and the ground electrode with respect to the modulation waveguide portion in the two modulation waveguide portions in each branch waveguide portion so that the degrees of modulation by the modulation electrodes are different from each other by a predetermined ratio. Is set.

(2) In the optical modulator described in (1) above,
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 degree of 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 portions for modulating each branched light, and a combining portion for combining the lights modulated by the two modulation waveguide portions, and the modulation electrode is provided with the two modulation waveguides. It is composed of a signal electrode disposed between the waveguide portions and two ground electrodes disposed so as to sandwich the two modulation waveguide portions, and the signal electrode includes two input signal electrodes. Branching into two, and each branching waveguide section The light intensity at the time of combining the lights modulated by the two modulation waveguide portions is different from each other and has a predetermined intensity ratio. In the two modulation waveguide portions in each branch waveguide portion, The arrangement position of the signal electrode and the ground electrode with respect to the modulation waveguide portion is set so that the degree of modulation differs from each other by a predetermined ratio, so that the number of modulation signals applied to the modulation electrode can be suppressed. In addition, the length of 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 an example of arrangement | positioning of the electrode regarding the optical modulator which concerns on this invention. It is a perspective view which shows an example of the modulation electrode used for the optical modulator of this invention. It is a figure which shows sectional drawing in the dashed-dotted line A-A 'of FIG. It is a top view explaining the other Example of the optical modulator which concerns on 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 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. Note that, in an optical waveguide in which the same light intensity propagates, the same output can be obtained even if the optical modulation state is replaced and operated. For example, regarding the optical waveguide O12 and the optical waveguide O13, even when the optical modulation state (the modulation degree is the same but the phase direction is the reverse direction) is replaced between the two, the light in the same modulation state is finally combined. The same output light can be obtained. In FIG. 8 to be described later, the modulation states in the modulation waveguides PM12 and PM13 in FIG. 4 or FIG. 6 are replaced.

  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. Further, the obtained signal waveform can be approximated not only to a triangular wave signal but also to a sawtooth waveform, a rectangular signal suitable for digital response, and 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 two signal electrodes S1 and S2, as shown in FIG. However, in the case where independent modulation signals are applied to the two signal electrodes, the cost of the entire apparatus including the optical modulator increases as in Patent Document 1. In the present invention, an optical modulator with substantially one signal electrode is realized by making the improvements shown in FIGS. In FIG. 8, illustration of the intensity ratio of light branched at each branching portion is omitted. In addition, in order to obtain a desired light intensity ratio, 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 are comprehensively considered. It is necessary.

  An electric field is applied to the modulation waveguide section PM11 by the signal electrode S1 and the ground electrode G1. Further, an electric field is applied to another modulation waveguide section PM12 by the signal electrode S1 and the ground electrode G2. In the modulation waveguide portions PM11 and PM12, the opposite phase modulation is performed depending on the positional relationship between the signal electrode S1 and the ground electrodes (G1, G2).

  Further, it is necessary to modulate two adjacent modulation waveguide portions (PM11 and PM12, PM13 and PM14) with different modulation degrees (modulation degree ratio 1: β). As shown in FIGS. 9 and 10, by adjusting the positional relationship between the modulation electrode (the signal electrode S1 and the ground electrode (G1, G2)) and the optical waveguide (O11 to O14), the modulation degree is adjusted. Yes.

  FIG. 9 shows an example of a modulation electrode that applies a modulation signal to the modulation waveguide section. In order to make the explanation easy to understand, here, the optical waveguide arranged below the modulation electrodes (S0 to S2, G1 to G3) is displayed so as to overlap the modulation electrodes, and the bias shown in FIG. Illustration of electrodes and the like is omitted.

  For the two modulation waveguide portions provided in each branch waveguide portion, the same intensity of the modulation signal can be used for each branch waveguide portion. For this reason, as shown in FIG. 9, all the modulation waveguides can be obtained simply by inputting one modulation signal by branching one input signal electrode S0 into two to form signal electrodes (S1, S2). It is possible to obtain a modulation signal necessary for the unit. Further, in this structure, since the electrodes can be substantially symmetric with respect to the optical waveguide in the entire modulation waveguide portion (PM11, PM12, PM13, PM14), internal stress applied to the substrate Are less likely to cause unevenness, and temperature drift and modulation characteristics are less likely to deteriorate.

  Further, in order to realize two different modulation degrees with the modulation signal inputted to one signal electrode (S1 or S2), as shown in FIG. 10, the signal electrodes (S1, S2) for the modulation waveguide portions (O11 to O14) are used. ) And the arrangement positions of the ground electrodes (G1 to G3) are adjusted. FIG. 10 is a cross-sectional view taken along one-dot chain line AA ′ in FIG. 9. In order to change the degree of modulation, the distance (g1, g2) between the signal electrode and the ground electrode is changed corresponding to the modulation waveguide section. Yes. As the interval (g1, g2) increases, the modulation factor decreases, and the interval ratio g2 / g1 (= S) determines the ratio of the modulation factor in the two modulation waveguide portions O11 and O12 (O13 and O14). Parameter.

  In FIG. 10, the strength of the electric field applied to the optical waveguide (modulation waveguide portion) disposed between the signal electrode and the ground electrode is adjusted by adjusting the distance between the signal electrode and the ground electrode. Not limited to this, it is also possible to adjust the modulation degree by changing the distance from the signal electrode to the optical waveguide (modulation waveguide section) without changing the distance between the signal electrode and the ground electrode. It is.

  9 and 10, when the substrate 1 is formed of a thin plate having a thickness of several tens of μm or less, the reinforcing substrate 11 is bonded to the substrate 1 via the adhesive layer 10 in order to maintain the mechanical strength. Yes. In this structure, the electric signal acts on the light so that the voltage is low, which is advantageous for lowering the voltage, and the characteristic impedance is hardly lowered even if the substrate 1 is made of a material having a high relative dielectric constant. There are many advantages over the above.

  FIG. 11 is an example showing the arrangement of electrodes applied to the optical modulator of the present invention. In the optical modulator of the present invention, it is necessary to branch light at a predetermined intensity ratio into the modulation waveguide sections (O11 and O12, O13 and O14) in each branch waveguide section. There is a method of obtaining a predetermined branching ratio by adjusting the shape of the optical waveguide at the branching portion. For example, as shown in FIG. 11, the branching portions (B22, B23) are configured by symmetrical splitters, It is also possible to employ a configuration (attenuator) in which light intensity adjusting means is arranged corresponding to the modulation waveguide portion. By adjusting the DC bias voltage (DC10 to DC13), it is possible to adjust the intensity of the light wave propagating downstream of the light intensity modulating means. 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 multiplexing 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.

  FIG. 11 also illustrates an electrode configuration related to the bias electrode shown in FIG. 8, and the DC bias voltage applied to each bias electrode is indicated by a symbol “DC20, DC21, DC30”.

  When one input signal electrode S0 is branched into two and used, as shown in FIG. 11, when the characteristic impedance of the input signal electrode is Z0, the branched signal electrodes (S1, S2) are doubled. It is necessary to secure the characteristic impedance (2 * Z0). If not set in this way, impedance mismatching occurs at the branch portion of the signal electrode. In order to broaden the bandwidth of a modulator using a material having a high relative dielectric constant such as lithium niobate as a substrate, the branched signal electrodes (S1, S2) and ground electrodes (G1-G3) are as shown in FIG. It is necessary to form a thick coplanar electrode. Although it is difficult to design impedance characteristics of a thick coplanar electrode in which the distance (g1, g2) between the signal electrode and the ground electrode is asymmetrical, it is possible by a finite element method. From the viewpoint of ease of designing the microwave characteristics such as impedance matching, the distance between the signal electrode (S1, S2) and the ground electrode (G1 to G3) is the same, and the distance between the modulation waveguide portion and the signal electrode. The degree of modulation of each modulation waveguide section may be adjusted by adjusting. Further, the adjustment of the distance from the ground electrodes (G1 to G3) and the adjustment of the distance between the modulation waveguide portion and the signal electrode may be used in combination.

  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 S0-S2 Signal electrode G1-G3 Ground electrode 10 Adhesive layer 11 Support substrate

Claims (2)

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 arranged between the two modulation waveguide portions and two ground electrodes arranged so as to sandwich the two modulation waveguide portions, and the signal electrode is , Formed by branching one input signal electrode into two,
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,
Arrangement positions of the signal electrode and the ground electrode with respect to the modulation waveguide portion in the two modulation waveguide portions in each branch waveguide portion so that the degrees of modulation by the modulation electrodes are different from each other by a predetermined ratio. An optical modulator characterized in that is set. The optical modulator according to claim 1.
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 degree of 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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