JP2008052103A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator, operating at a high speed and having a small alpha parameter, a high extinction ratio, a low drive voltage and a low DC bias voltage. <P>SOLUTION: The modulator comprises a substrate 1, having an electro-optical effect, an optical waveguide 23 fabricated on the substrate, a traveling-wave electrode 24 fabricated on one surface of the substrate and comprising a center conductor 24a for high-frequency electrical signals and a plurality of ground conductors 24b, 24c for applying high-frequency electrical signals to modulate light. The optical waveguide is a Mach-Zehnder optical waveguide, having a plurality of interactive optical waveguides 23a, 23b, the widths of which partially differ from each other where the phase of light is modulated by applying high-frequency electrical signals to the traveling-wave electrode, wherein each of the plurality of interactive optical waveguides has tapered portions 31 to 36 where the width varies, and the length of the tapered portion is determined, in such a manner that the coupling of light between tapered portions is decreased fully and the radiation loss of light in the tapered portion is decreased fully. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention belongs to the field of optical modulators which are high in speed, have a small α parameter, have a large extinction ratio, and have a small driving voltage and DC bias voltage.

An optical waveguide and a traveling wave electrode are provided on a substrate having a so-called electro-optic effect (hereinafter, the lithium niobate substrate is abbreviated as an LN substrate) such as lithium niobate (LiNbO ₃ ) whose refractive index is changed by applying an electric field. The formed traveling wave electrode type lithium niobate optical modulator (hereinafter abbreviated as LN optical modulator) is applied to a 2.5 Gbit / s, 10 Gbit / s large capacity optical transmission system because of its excellent chirping characteristics. Yes. Recently, application to an ultra large capacity optical transmission system of 40 Gbit / s is also being studied, and it is expected as a key device.

[First prior art]
This LN optical modulator includes a type using a z-cut substrate and a type using an x-cut substrate (or y-cut substrate). Here, an x-cut substrate LN optical modulator using an x-cut LN substrate and a coplanar waveguide (CPW) traveling wave electrode is taken up as a first prior art, and a perspective view thereof is shown in FIG. FIG. 9 is a cross-sectional view taken along line AA ′ of FIG.

In the figure, 1 is an x-cut LN substrate, 2 is a transparent SiO ₂ buffer layer having a thickness of about 200 nm to 1 μm in the wavelength region used in optical communication such as 1.3 μm or 1.55 μm, and 3 is an x-cut LN. This is an optical waveguide formed by thermally diffusing Ti at 1050 ° C. for about 10 hours after depositing Ti on the substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Reference numerals 3a and 3b denote optical waveguides (or interactive optical waveguides) in a portion where an electrical signal and light interact (referred to as an interaction portion), that is, two arms of a Mach-Zehnder optical waveguide. The CPW traveling wave electrode 4 includes a central conductor 4a and ground conductors 4b and 4c.

In FIG. 9, W _a and W _b are the widths of the interaction optical waveguides 3a and 3b. In the first prior art, the widths of the two interaction optical waveguides 3a and 3b are equal (that is, W _a = W _b , for example, W _a and W _b are both 9 μm). G _wg is a distance (also referred to as a waveguide gap) between the interactive optical waveguides 3a and 3b, and is, for example, 16 μm. Δ is the horizontal distance between the edge of the center conductor 4a and the center (or center line) of the interaction optical waveguide 3b. Usually, the center conductor 4a, the ground conductors 4b and 4c, and the interaction optical waveguide 3a, Since the positional relationship with 3b is symmetric, the horizontal distance (or center line) between the edge of the center conductor 4a and the center of the interactive optical waveguide 3a is also Δ.

  Here, in FIG. 9, 18 is the center (or center line) of the center conductor 4a, and 19a and 19b are the centers (or center lines) of the interaction optical waveguides 3a and 3b.

  FIG. 10 shows a top view of only the optical waveguide 3. Here, let L be the length of the interaction optical waveguides 3a and 3b. Although FIG. 10 shows only the optical waveguide, A-A ′ is indicated at a position corresponding to A-A ′ in the perspective view of FIG. 8.

In the first prior art, a bias voltage (usually a DC bias voltage) and a high-frequency electric signal (also referred to as an RF electric signal) are superimposed and applied between the center conductor 4a and the ground conductors 4b and 4c. In the waveguides 3a and 3b, not only the RF electric signal but also the DC bias voltage changes the phase of the light. Further, the microwave effective index n _m of the interaction optical waveguides 3a of the buffer layer 2 is an electrical signal, by approximating the effective refractive index n _o of the light propagating the 3b, and important function of expanding the optical modulation band is doing.

  Next, the operation of the LN optical modulator configured as described above will be described. In order to operate this LN optical modulator, it is necessary to apply a DC bias voltage and an RF electrical signal between the center conductor 4a and the ground conductors 4b and 4c.

  The voltage-light output characteristics shown in FIG. 11 are the voltage-light output characteristics of the LN optical modulator in a certain state, and Vb is the DC bias voltage at that time. As shown in FIG. 11, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

FIG. 12 shows the product of the half-wave voltage Vπ and the length L of the interactive optical waveguide (referred to as Vπ · L, which is a measure for considering the driving voltage), the edge of the center conductor 4a, and the interactive optical waveguide. A relationship with the distance Δ from the center 19b of 3b is shown. In this calculation, the value of Δ is determined by changing the gap _Gwg between the interaction optical waveguides 3a and 3b. From FIG. 12, it can be seen that the distance Δ between the edge of the center conductor 4a and the center 19b of the interaction optical waveguide 3b should be small to some extent, and further, there exists an optimum value.

Therefore, in order to reduce the drive voltage, if the distance Δ between the edge of the center conductor 4a and the center 19b (and 19a) of the interaction optical waveguide 3b (and 3a) is reduced, the distance between the interaction optical waveguides 3a and 3b is reduced. Gap _Gwg becomes smaller. However, as shown in FIG. 13, when the gap _Gwg between the interaction optical waveguides 3a and 3b decreases, the degree of coupling between the interaction optical waveguides 3a and 3b increases remarkably, and the power when the light is turned on / off is increased. In other words, there is a problem that deterioration of the extinction ratio or chirping occurs.

[Second prior art]
In general, in order to reduce the degree of coupling between two optical waveguides, the distance _Gwg between the two optical waveguides is increased to keep the light propagating through each optical waveguide away from each other, or the two optical waveguides There is a method of suppressing mutual coupling by changing the widths of the waveguides and changing the equivalent refractive index (or propagation constant) of light propagating through the two optical waveguides.

When the former method, that is, the distance _Gwg between the two optical waveguides is increased, Vπ · L increases as described with reference to FIG. 12, and as a result, a high drive voltage is required. Therefore, in the second prior art shown as a cross-sectional view in FIG. 14, the widths W _a ′ and W _b ′ of the two interactive optical waveguides 5a and 5b are made different. FIG. 15 shows a top view of only the optical waveguide 5 of the second prior art. In FIG. 14, the x-cut LN substrate 1, the center conductor 4 a, the ground conductors 4 b and 4 c, and the buffer layer 2 are included in the cross-sectional view taken along the line BB ′ in FIG. 15.

  Δ ′ is a distance in the horizontal direction between the edge of the central conductor 4a and the center of the interactive optical waveguide 5b. Here, in FIG. 14, 20a and 20b are the centers (or center lines) of the interaction optical waveguides 5a and 5b. That is, in the second prior art, in order to reduce the drive voltage, the distance Δ ′ between the edge of the center conductor 4a and the center of the interaction optical waveguide 5b is reduced, and the two interaction optical waveguides 5a and 5b are reduced. This is a method of suppressing the coupling of light propagating through two optical waveguides by making the widths different from each other.

However, this second prior art still has unsolved major problems to be discussed below. As shown in FIG. 15, in the second prior art, the widths W _a ′ and W _b ′ of the interaction optical waveguides 5a and 5b are partially changed, and the length L ₁ in the interaction optical waveguides 5a and 5b is changed. the region first region, and a region of length L ₂ second region. In FIG. 15, 6, 7, 8, 9, 10, and 11 are tapers whose widths change, and it is necessary to provide three tapers on each of the interaction optical waveguides 5a and 5b.

Now, consider the coupling of light between the interaction optical waveguides 5a and 5b, which is the biggest problem in the second prior art. In FIG. 15, tapers 7 and 10 are regions in which the widths of the interacting optical waveguides 5a and 5b change most greatly. When the length of the taper 7, 10 and _{L T,} the taper 6, 8, 9, 11, the interaction optical waveguides 5a, the variation of the width of the 5b is half as compared with the taper 7 and 10, as a result, the length of the taper 6, 8, 9, 11 is about half of _{L T.} Thus, the interaction optical waveguides 5a, a tapered width is provided in a region which varies in 5b, the longest length L _T of the taper 7,10. The length L _T of the specific taper 7,10, hitherto only long enough to reduce the radiation loss in the taper, came is set to, for example, a length of about 6 mm.

However, this set length of 6 mm has a serious problem. 16, the length L _T is 6mm of the LN optical modulator taper 7,10 shows the results of measurement of α parameter representing the amount of chirping in the case of a variable DC bias voltage Vb. Here, the widths W _a ′ and W _b ′ in the first region of the optical waveguides of the two interaction optical waveguides 5a and 5b are 6 μm and 11 μm, respectively, and 11 μm and 6 μm in the second region, respectively. The gap G _wg ′ between the two interactive optical waveguides 5a and 5b is 14 μm. The wavelength of light used in the measurement was 1.55 μm. The length _{L 1} and length _{L 2} of the first region and the second region were respectively 10mm and 30 mm.

  As can be seen from the figure, there is a region where the value of the α parameter varies greatly with respect to the DC bias voltage Vb, and the DC bias that greatly exceeds | α | <0.1, which is a practically required specification for the α parameter. There is a value for voltage Vb.

  As a result, when the LN optical modulator is used in an actual transmission system, the DC bias voltage Vb changes due to DC drift, and when this voltage is reached, the α parameter suddenly increases. . When the α parameter is increased, an unexpected chirping occurs regardless of whether it is a zero chirp type or a pre-chirp type, and a serious situation has occurred in which the transmission error rate becomes extremely large.

  As a result of detailed studies, coupling of inverted phase light propagating through the two interacting optical waveguides 5a and 5b occurs at the tapers 7 and 10 at a certain DC bias voltage Vb, and the extinction ratio is reduced. It was found that the α parameter was suddenly changed as it deteriorated.

  As described above, when the two interactive optical waveguides are brought close to the center conductor as in the first prior art in order to reduce the driving voltage, the two interactive optical waveguides approach each other. For this reason, the light is coupled, resulting in deterioration of the high frequency modulation characteristics such as deterioration of the extinction ratio and increase of the α parameter, that is, chirping. In the second conventional technique proposed in order to solve this, the widths of the two interactive optical waveguides are made different from each other, light is coupled in a tapered region where the widths of the two interactive optical waveguides change. In addition, the extinction ratio deteriorates at a certain DC bias voltage, suddenly an unexpected α parameter is generated (that is, an unspecified chirping occurs), causing a transmission error or realizing zero chirping as an optical modulator. There was a problem that the yield when doing so varied.

  In order to solve the above problems, an optical modulator according to claim 1 of the present invention includes a substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and one of the substrates. A traveling wave electrode formed on the surface side and comprising a central conductor for high frequency electrical signals for applying a high frequency electrical signal for modulating the light and a plurality of ground conductors, and the optical waveguide includes the traveling wave A Mach-Zehnder optical waveguide having a plurality of interaction optical waveguides having at least a part of a width for modulating the phase of the light by applying the high-frequency electrical signal to an electrode, and the plurality of interaction optical waveguides are Each having a taper that is a portion where the width changes, so that light coupling between each taper of the plurality of interactive optical waveguides is sufficiently small, and radiation loss of light at the taper is As min decreases, characterized in that setting the length of the taper.

  An optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect, wherein the width relations of the plurality of interactive optical waveguides are switched in the longitudinal direction.

  An optical modulator according to a third aspect of the present invention is the optical modulator according to the first aspect, wherein the width relationship of the plurality of interactive optical waveguides is not interchanged in the longitudinal direction.

  According to a fourth aspect of the present invention, in the optical modulator according to the first to third aspects, the lengths of the two regions having different widths of the plurality of interactive optical waveguides are substantially equal to each other. It is characterized by.

  According to a fifth aspect of the present invention, in the optical modulator according to the first to third aspects, the lengths of the two regions having different widths of the plurality of interactive optical waveguides are not equal to each other. It is characterized by.

  According to a sixth aspect of the present invention, in the optical modulator according to the first to fifth aspects, the distance between the center line of the center conductor and each center line of the plurality of interactive optical waveguides is The center conductor and the plurality of ground conductors are different.

  An optical modulator according to a seventh aspect of the present invention is the optical modulator according to any one of the first to fifth aspects, wherein a center of a waveguide gap of the plurality of interactive optical waveguides and a center line of the central conductor are provided. The substrate surfaces are shifted from each other in the substrate surface direction.

  An optical modulator according to an eighth aspect of the present invention is the optical modulator according to any one of the first to seventh aspects, wherein the taper has a length of 5 μm to 5.5 mm.

  An optical modulator according to a ninth aspect of the present invention is the optical modulator according to any one of the first to seventh aspects, wherein the length of the taper is from 5 μm to 1 mm.

  In the conventional optical modulator in which the widths of the interaction optical waveguides are made different in order to suppress the coupling of light generated between the two interaction optical waveguides brought close to each other in order to reduce the drive voltage, Light coupling occurred in the tapered region where the width was changed. According to the present invention, the extinction ratio, chirping, and insertion are set by setting the length of the taper shorter than the conventional one so that light coupling does not occur in this tapered region and the radiation loss is sufficiently small. An optical modulator excellent from the viewpoint of loss can be provided.

  Hereinafter, embodiments of the present invention will be described. Since the same numbers as those in the prior art shown in FIGS. 8 to 15 correspond to the same function units, the description of the function units having the same numbers is omitted here.

[First Embodiment]
FIG. 1 is a top view of an optical waveguide 23 included in the optical modulator according to the first embodiment of the present invention. Here, 23a and 23b are interaction optical waveguides constituting the optical waveguide 23, W _a ″ and W _b ″ are the widths of the two interaction optical waveguides 23a and 23b, and the two interactions W _a ″ and W _b ″ are made different in order to suppress the coupling of light between the optical waveguides 23a and 23b. G _wg ″ is a gap between the two interactive optical waveguides 23a and 23b.

As shown in FIG. 1, in the first embodiment, the magnitude relationship between the widths W _a ″ and W _b ″ of the interaction optical waveguides 23a and 23b is set to the first region having the length L ₁ ″ and the length L. _{In the 2} ″ second region, the replacement is performed. Reference numerals 31, 32, 33, 34, 35, and 36 are tapered regions in which the widths of the two interactive optical waveguides 23a and 23b change. L _T ″ is the length of the opposing tapers 32, 35.

2 and 3 are sectional views as optical modulators at DD ′ and EE ′ in FIG. Here, FIGS. 2 and 3 are sectional views taken along line DD ′ and EE ′ in FIG. 1, and the central conductor 24a and the ground conductors 24b and 24c of the traveling wave electrode 24 and the x-cut LN substrate 1 , SiO ₂ buffer layer 2 is added. 2 and 3, 26 is the center (or center line) of the central conductor 24a, and 25a and 25b are the centers (or center lines) of the interaction optical waveguides 23a and 23b.

Δ ₁ ″ represents the horizontal distance between the edge of the center conductor 24a and the center 25a of the interaction optical waveguide 23a, and Δ ₂ ″ represents the edge of the center conductor 24a and the center 25b of the interaction optical waveguide 23b. Represents the distance in the horizontal direction.

  2 and 3, for example, the center 26 of the center conductor 24a is shifted from the surface of the x-cut LN substrate 1 in the horizontal direction. The order of the shift amount is as small as micron or submicron. Accordingly, a predetermined length (for example, about 50 μm) is provided at the boundary between the first region and the second region, and the center conductor 24a and the ground conductors 24b and 24c in the first region and the second region are linearly or gently connected to each other. By joining, deterioration of electrical characteristics can be avoided.

As shown in FIG. 1, the relationship between the widths W _a ″ and W _b ″ of the interactive optical waveguides 23a and 23b in the first embodiment is W _a in the first region of the length L ₁ ″. ''<W _b ''. Therefore, in this first region, as shown in FIG. 2, by setting Δ ₁ ″ <Δ ₂ ″ in the same manner as in the first embodiment of the present invention shown in FIG. The efficiency of the interaction of light propagating through the waveguides 23a and 23b is made equal.

On the other hand, in the second region of length L ₂ ″, W _a ″> W _b ″. Therefore, in the second region, by setting Δ ₁ ″> Δ ₂ ″ as shown in FIG. 3, the efficiency of the interaction between the high-frequency electric signal and the light propagating through the interaction optical waveguides 23a and 23b is made equal. Yes. The above is a description of one example of the present embodiment, and the principle of the present invention described below is applicable only to the magnitude relationship described above for Δ ₁ ″ and Δ ₂ ″. It can be applied to all embodiments of the present invention.

In other words, the distance delta ₁ between the center 25a of the edge and the interaction optical waveguides 23a of the center conductor 24a _'' and the distance delta ₂ between the center 25b of the edge and the interaction optical waveguide 23b of the center conductor 24a _{'delta 1} for'_'' ≠ Δ ₂ ″, but depending on the width and formation conditions of the interaction optical waveguides 23a and 23b, Δ ₁ ″ = Δ ₂ ″, and instead, the center of the gap between the interaction optical waveguides 23a and 23b. Needless to say, the center 26 of the central conductor 24a may be different.

Furthermore, since the principle of the present invention has no restrictions on L ₁ ″ and L ₂ ″, L ₁ ″> L ₂ ″, L ₁ ″ <L ₂ ″, L ₁ ″ = L ₂ ″ may be used, and in the most extreme case, L ₁ ″ = 0 or L ₂ ″ = 0 may be used as described in the second embodiment.

  Now, in the design of the taper 31, 32, 33, 34, 35, 36 of the interaction optical waveguides 23a, 23b, while suppressing the coupling of light between the interaction optical waveguides 23a, 23b in these tapered regions, It is extremely important to reduce the radiation loss that occurs when the width of the optical waveguide changes.

FIG. 4 shows the maximum absolute value of the α parameter generated when the DC bias voltage is changed with the length L _T ″ of the tapers 32 and 35 having the longest length in FIG. 1 as a variable. Here, the widths W _a ″ and W _b ″ in the first region of the two interactive optical waveguides 23a and 23b are 6 μm and 11 μm, respectively, and 11 μm and 6 μm in the second region, respectively. is there. G _wg ″ was 14 μm. The wavelength of light used in the measurement was 1.55 μm. It should be noted that the above values for W _a ″, W _b ″, G _wg ″, or the wavelength used are examples, and are not limited to these values. This is true for all embodiments of the present invention. I can say that.

As can be seen from this figure, the maximum absolute value of the α parameter greatly depends on the length L _T ″ of the tapers 32 and 35, and this is because the coupling of light between the tapers 32 and 35 depends on the α parameter. This is because it greatly affects

Since the maximum value of the absolute value of the α parameter generally required as a zero chirp optical modulator is within 0.1, the length L _T ″ of the tapers 32 and 35 from FIG. 4 is within 5.5 mm. Should. In addition, in order to make the maximum value of the absolute value of the α parameter within 0.1, it has been stated that the length L _T ″ of the tapers 32, 35 should be within 5.5 mm, but the above W _a ″ There was no significant difference in this value except for W _b ″ and G _wg ″.

Now, if the width of the optical waveguide is changed, radiation loss tends to occur in that region. Therefore, in determining the lengths of the tapers 31, 32, 33, 34, 35, and 36 in which the widths of the interactive optical waveguides 23a and 23b in FIG. FIG. 5 shows the value of radiation loss when the length L _T ″ of the tapers 32 and 35 is used as a variable. Since the radiation loss is slightly different between the taper 32 in which the width of the optical waveguide is widened and the taper 35 in which the width of the optical waveguide is narrow, FIG. 5 shows the average value of the radiation loss in the tapers 32 and 35. Since only the insertion loss as an optical modulator affects the extinction ratio, it is important to reduce the radiation loss due to the taper.

As can be seen from FIG. 5, when the widths of the interactive optical waveguides 23a and 23b change, if the length L _T ″ of the tapers 32 and 35 is set to about 5 μm to 10 μm or more, the radiation loss is sufficient. The length L _T ″ of the tapers 32 and 35 is preferably 5 μm or more. This is also true for all embodiments of the invention.

Here, FIG. 6 shows the DC bias voltage dependency of the α parameter of the LN optical modulator in which the length L _T ″ of the tapers 32 and 35 is set to 1 mm. The widths W _a ″ and W _b ″ in the first region of the optical waveguides of the two interaction optical waveguides 23a and 23b are 6 μm and 11 μm, respectively, and 11 μm and 6 μm in the second region, and G _wg ′. 'Is 14 μm, and the lengths L ₁ ″ and L ₂ ″ of the first region and the second region are 10 mm and 30 mm, respectively, as in the second prior art. The wavelength of light used in the measurement was 1.55 μm. As can be seen from the figure, the change of the α parameter with respect to the DC bias voltage is sufficiently small, and the specification of within ± 0.1 is completely satisfied. Further, it has been confirmed that the dynamic extinction ratio is also improved by suppressing the unexpected α parameter. This is also true for a z-cut LN optical modulator that is normally used when α is about -0.74.

The length L _T ″ of the tapers 32 and 35 may be longer than 1 mm as long as it is within 5.5 mm, or may be shorter than 1 mm, for example, about 700 μm. Then, the effect of suppressing the coupling of light at the tapers 32 and 35 is remarkably high. Furthermore, it has been confirmed that depending on the manufacturing conditions of the optical waveguide 23, the length of the tapers 32 and 35 can be shortened to about 500 μm or less.

As long as the coupling of light between the interaction optical waveguides 23a and 23b at the tapers 31, 32, 33, 34, 35, and 36 is sufficiently small and the radiation loss at the taper portion is sufficiently small, the length of the taper is It can be even shorter. Further, the present invention is for suppressing light coupling at the taper of the interaction optical waveguides 23a and 23b, and the size of the first region length L ₁ ″ and the second region length L ₂ ″ is small or large. There are no restrictions on the relationship. These can be said not only for this embodiment but also for all embodiments of the present invention.

[Second Embodiment]
In the first embodiment shown in FIG. 1, a case where L ₂ ″ = 0 is shown in FIG. 7 as a second embodiment of the present invention. In other words, the widths of the two interactive optical waveguides 12a and 12b are made substantially constant in the widths W _a ″ and W _b ″ in the interactive optical waveguide 12a and in the interactive optical waveguide 12b. In the case of this embodiment, the length L _T ′ ″ of the tapers 13, 14, 15, 16 may be set within 5.5 mm. Compared with the tapers 32 and 35 of FIG. 1 shown as the first embodiment, the change in the width of the interactive optical waveguides 12a and 12b is half, so that it can actually be set within 2.75 mm, for example, 350 μm. It can be as short as possible or even shorter.

  The present invention is not limited to the above-described embodiments.

[About each embodiment]
In the above description, the CPW electrode has been described as an example of the traveling wave electrode. However, it goes without saying that various other traveling wave electrodes or lumped constant type electrodes may be used. Further, although the description has been given assuming that there are two gaps of the CPW electrode, there may be more gaps.

  In the above embodiments, a so-called DC bias integrated type in which a high-frequency electric signal and a DC bias voltage are applied to the same interaction optical waveguide has been described. However, the present invention relates to a separate interaction between a high-frequency electric signal and a DC bias voltage. It goes without saying that the present invention can also be applied to a so-called DC bias separation type applied to an optical waveguide.

  In addition, since the present invention can suppress chirping caused by unexpected optical coupling between two interacting optical waveguides, it can be applied not only to a zero chirp type but also to a prechirp type optical modulator. Furthermore, in the above embodiment, the x-cut, y-cut or z-cut plane orientation, that is, the x-axis, y-axis or z-axis of the crystal is perpendicular to the substrate surface (cut plane). The surface orientation in each of the embodiments described above may be the main surface orientation, and other surface orientations may be mixed as surface orientations subordinate to these, and not only the LN substrate but also the lithium tantalum. It goes without saying that other substrates such as rates and semiconductors may be used.

  As described above, the optical modulator according to the present invention is useful as an optical modulator that is high in speed, has a small α parameter, has a large extinction ratio, is controlled in chirping, and has a small driving voltage and DC bias voltage. .

The top view of the optical waveguide which a 1st embodiment in the present invention has Sectional drawing as an optical modulator taken along line D-D 'in FIG. Sectional view as an optical modulator taken along line E-E 'of FIG. The figure explaining the principle of this invention The figure explaining the principle of this invention The figure explaining the principle of this invention The top view of the optical waveguide which the 2nd Embodiment in the present invention has 1 is a perspective view of a first conventional optical modulator. Sectional drawing in the A-A 'line of FIG. Top view of the optical waveguide of the first prior art The figure explaining operation | movement of 1st prior art Diagram showing the relationship between Vπ · L and Δ The figure which shows the relationship between the light coupling degree and _Gwg Sectional view of second conventional optical modulator Top view of second prior art optical waveguide The figure explaining the problem of the 2nd prior art

Explanation of symbols

1: x-cut LN substrate (substrate)
2: SiO ₂ buffer layer (buffer layer)
3, 5, 12, 23: Optical waveguide 3a, 3b, 5a, 5b, 12a, 12b, 23a, 23b: Interaction optical waveguide 4, 24: Traveling wave electrode (CPW traveling wave electrode)
4a, 24a: Center conductor 4b, 4c, 24b, 24c: Ground conductor 6, 7, 8, 9, 10, 11, 13, 14, 15, 16, 31, 32, 33, 34, 35, 36: Taper ( Taper area)
18, 26: Center conductor center line (center line)
19a, 19b, 20a, 20b, 25a, 25b: center line (center line, center) of the interaction optical waveguide

Claims (9)

A substrate having an electro-optic effect, an optical waveguide for guiding light formed on the substrate, and a high-frequency electric signal formed on one surface side of the substrate for modulating the light A traveling wave electrode comprising a central conductor for a high frequency electrical signal and a plurality of ground conductors, and the optical waveguide has a width for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode. A Mach-Zehnder optical waveguide having a plurality of interactive optical waveguides at least partially different from each other,
The plurality of interaction optical waveguides each include a taper that is a portion whose width changes, so that light coupling between the tapers of the plurality of interaction optical waveguides is sufficiently small, and An optical modulator characterized in that the length of the taper is set so that light radiation loss at the taper is sufficiently small.   The optical modulator according to claim 1, wherein the width relationship of the plurality of interactive optical waveguides is switched in the longitudinal direction.   The optical modulator according to claim 1, wherein the width relationship of the plurality of interactive optical waveguides is not interchanged in the longitudinal direction.   4. The optical modulator according to claim 1, wherein the lengths of the two regions having different widths of the plurality of interactive optical waveguides are substantially equal to each other. 5.   4. The optical modulator according to claim 1, wherein lengths of the two regions having different widths of the plurality of interactive optical waveguides are not equal to each other.   The distance between the center line of the center conductor and the center line of each of the plurality of interactive optical waveguides is different between the center conductor and the plurality of ground conductors. The light modulator described.   The center of the waveguide gap which the said some interaction optical waveguide has and the centerline of the said center conductor are mutually shifted | deviated and arrange | positioned at the said substrate surface direction, The Claims 1-5 characterized by the above-mentioned. Light modulator.   The optical modulator according to claim 1, wherein the taper has a length of 5 μm to 5.5 mm.   The optical modulator according to claim 1, wherein the taper has a length of 5 μm to 1 mm.

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