JP2007072369A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator having low driving voltage and capable of performing modulation at high speed. <P>SOLUTION: The optical modulator has: a substrate 1 having an electrooptical effect; an optical waveguide 3 for guiding light formed on the substrate 1; and a traveling wave electrode 4 which is formed on one surface side of the substrate 1 and comprises a center conductive body 4a and grounded conductive bodies 4b and 4c for applying a high frequency electric signal modulating an optical phase, wherein the traveling wave electrode 4 includes an interaction part for modulating the optical phase by applying the high frequency electric signal, and a feed-through part for input for applying the high frequency electric signal from an external circuit to the interaction part, and also includes an impedance converter having a characteristic impedance higher than the characteristic impedance of the interaction part between the interaction part and the feed-through part for input. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention belongs to the field of optical modulators having a low driving voltage and capable of high-speed modulation.

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.

  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 prior art, and a perspective view thereof is shown in FIG. FIG. 8 is a cross-sectional view taken along the line A-A ′ of FIG. 7. The following discussion holds true for z-cut substrates as well.

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. 8, S is the width of the central conductor 4a, which is about 6 μm to 20 μm. On the other hand, W is a gap (or CPW gap) between the center conductor 4a and the ground conductors 4b and 4c.

In this 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. Further, SiO ₂ buffer layer 2 is equivalent refractive index of the high frequency electric signal _{n m} (or microwave equivalent refractive index _{n m)} the optical waveguide 3a, 3b by approximating the effective refractive index _{n o} of the light propagating the light It plays an important role in expanding the modulation band.

  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. 9 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. 9, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

  FIG. 10 shows the relationship between the product Vπ · L of the half-wave voltage Vπ and the length L of the interaction portion and the gap W of the CPW. In addition, as the gap W of CPW, about 20 micrometers-40 micrometers are used now. When the gap W of the CPW is narrowed, the strength of the high frequency electric field that interacts with the light propagating through the interaction optical waveguides 3a and 3b increases. Therefore, as shown in this figure, when the gap W of the CPW is narrowed, the product Vπ · L becomes small. An LN optical modulator with a lower driving voltage can be realized as the product Vπ · L is lower. The drive voltage when driving the LN optical modulator at a speed of 10 Gbps or more is about 5 to 6 V, and it is desired that the drive voltage is as low as possible. Therefore, it is desirable that the gap W of the CPW is narrow from the viewpoint of the driving voltage.

It shows the relationship between the microwave equivalent refractive index n _m and CPW gaps W high-frequency electric signal in FIG. 11. It shows the interaction optical waveguides 3a, the equivalent refractive index of the light propagating through _{3b n o (n o ≒ 2.2} ) also in FIG.

When the gap W of the CPW is narrowed, the high frequency electric signal generated between the center conductor 4a and the ground conductors 4b and 4c feels a lot of the SiO ₂ buffer layer 2 having a low relative dielectric constant of about 4, so that the microwave equivalent refractive index n _m can be reduced (note that the relative dielectric constant of the x-cut LN substrate 1 is about 35).

In general, the microwave equivalent refractive index n _m is greater than the equivalent refractive index n _o of the light, is a major limiting factor in the operation of the LN optical modulator at a high speed and a wide band. Therefore, to drive the LN optical modulator in faster than 10Gbps becomes essential to close the microwave equivalent refractive index n _m in the light of the equivalent refractive index n _o. From this point of view, it is desirable that the gap W of the CPW is narrow.

As described above, although from the viewpoint while reducing the driving voltage close to the microwave equivalent refractive index n _m in the light of the equivalent refractive index n _o is the CPW gap W was found to be narrow it is desirable, in the prior art Problems that occur when the gap W of the CPW is narrowed to 15 μm or less will be described below.

Figure 12 shows the CPW gaps W as a variable for the center electrode 4a and the ground electrodes 4b, (corresponding to _{Z 3} in the following Figure 14) the characteristic impedance Z of the CPW traveling wave electrode 4 made of 4c. When the CPW gap W is narrowed, the characteristic impedance Z is remarkably reduced to 30Ω or less, and impedance mismatching occurs with an external signal source of almost 50Ω. That is, there arises a problem that the power reflectance (so-called S ₁₁ ) of the high-frequency electric signal is deteriorated.

  Next, this will be considered in more detail. FIG. 13 shows a top view of the CPW traveling wave electrode 4 comprising the center conductor 4a and the ground conductors 4b and 4c constituting the x-cut LN optical modulator shown in FIG.

  Here, I is an input feed for connecting a connector core wire (not shown) (or a gold ribbon or a gold wire) for applying a high frequency electrical signal (microwave) from an external signal source (not shown) to the CPW traveling wave electrode 4. Through part, II is a connection part (or input side connection part) between the input feedthrough part I and the interaction part III, III is an interaction part where an electric signal and light interact, and IV is an output feedthrough part V And an interaction part III (or output side connection part). The output feedthrough portion V is connected to a connector core wire (or a gold ribbon or a gold wire) or a terminating resistor (not shown).

  FIG. 14 shows an equivalent circuit of the x-cut LN optical modulator shown in FIG. Here, 5 is an external signal source such as an electric driver, and 6 is an additional resistor (impedance is Rg) of the external signal source. Reference numerals 7 to 11 respectively correspond to equivalent lines from the input feedthrough portion I to the output feedthrough portion V. Specifically, 7 is an input feedthrough section I, 8 is an input side connection section II, 9 is an interaction section III, 10 is an output side connection section IV, and 11 is a line of an output feedthrough section V. . Reference numeral 12 denotes a termination resistor.

Further, Z _{1 to} Z ₅ are characteristic impedances from the input feed-through portion I to the output feed-through portion V. Specifically, Z ₁ is the input feed-through portion I (or line 7), Z _2. an input-side connecting portion II (or the line 8), _{Z 3} is the interaction portion III (or the line 9), _{Z 4} is output connection portion IV (or the line 10), _{Z 5} is output feed-through portion V (or This corresponds to the characteristic impedance of the line 11). Z _L is the resistance value of the termination resistor 12.

  Next, problems in terms of impedance mismatch and modulation band will be considered for the conventional x-cut LN optical modulators shown in FIGS.

In FIG. 14, Z _in is an input impedance when the x-cut LN optical modulator is viewed from the external signal source 5 and the additional resistor 6 (impedance Rg). _{That, Z in} the characteristic impedance _{Z 1} of the input feed-through portion I, the characteristic impedance _{Z 2} of the input-side connecting portion II, the interaction portion III of the characteristic impedance _{Z 3,} the output-side connecting portion characteristic impedance _{Z 4} of IV, the output Characteristic impedance Z ₅ of feedthrough part V and resistance value Z _L of termination resistor 12 are synthesized based on the concept of subordinate connection of transmission lines in consideration of the length of each part and the equivalent refractive index of the electric signal propagating through each part. It can be said to be impedance. 13 in the figure represents the boundary between the external signal source 5 and the additional resistor 6 and the input feedthrough I.

Lowering the driving voltage, consider the case where the CPW gaps W in order to close the microwave equivalent refractive index n _m for the light of the equivalent refractive index n _o and narrow as 15μm or less. In this case, the characteristic impedance _{Z 3} of the interaction portion III is as low as 30Ω or less.

Now, in the prior art, the characteristic impedance of other lines 7,8,10,11, i.e. the characteristic impedance _{Z 1} of the input feed-through portion I, the input-side connecting portion II characteristic impedance _{Z 2,} the output-side connecting portion IV characteristic impedance Z _4, characteristic impedance Z ₅ of the output feed-through portion _V, and the resistance value Z _L of the termination resistor 12 was equal to the characteristic impedance Z ₃ of all the interaction portion III.

As a result, the real part Re (Z _in ) of the input impedance Z _{in when} the x-cut LN optical modulator is viewed from the additional resistor 6 of the external signal source 5 hardly depends on the frequency f as shown by the solid line in FIG. , consistent with the characteristic impedance _{Z 3} of the interaction portion III, it was as low as 30Ω or less.

Accordingly, the optical modulation index (power modulation index) | m | ² is caused by impedance mismatch between the input impedance Z _in and the additional resistor 6 (impedance Rg) of the external signal source 5, as shown in FIG. It deteriorated rapidly with the frequency f, and it was extremely difficult to ensure 10 GHz as the 3 dB optical modulation band.

When Z _in (in this case, Z _in = Z ₃ ) is 30Ω or less, the power reflectivity (S ₁₁ ) of the high-frequency electric signal is higher than −10 dB (actually, it is reflected by a slight impedance mismatch). The high frequency electrical signal is superimposed and deteriorates to about -5 dB). In such a case, since the reflected high-frequency electric signal returns to the external signal source 5, there is also a problem that the jitter of the modulated optical pulse is increased.

  As described above, in the optical modulator according to the related art in which each part constituting the CPW traveling wave electrode such as the feed-through part and the interaction part on the input side and the output side has the same characteristic impedance, the driving voltage is reduced. At the same time, if the gap of the CPW traveling wave electrode is narrowed in order to bring the microwave equivalent refractive index closer to the equivalent refractive index of light, impedance mismatching with an external circuit occurs. There was a problem that it deteriorated rapidly.

  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 made up of a center conductor and a ground conductor for applying a high frequency electrical signal for modulating the phase of the light, and the traveling wave electrode applies the high frequency electrical signal In the optical modulator, the traveling wave electrode includes an interaction unit for modulating the phase of the light and an input feedthrough unit for applying the high-frequency electrical signal from an external circuit to the interaction unit. Further, an impedance conversion unit having a characteristic impedance higher than a characteristic impedance of the interaction unit is further provided between the interaction unit and the input feedthrough unit.

  An optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect, wherein a characteristic impedance of the impedance converter is 50Ω or less.

  An optical modulator according to a third aspect of the present invention is the optical modulator according to the first aspect, characterized in that a characteristic impedance of the impedance converter is 50Ω or more.

  An optical modulator according to a fourth aspect of the present invention is the optical modulator according to any one of the first to third aspects, wherein the traveling-wave electrode converts the high-frequency electric signal obtained by modulating the phase of the light at the interaction section. An output feedthrough portion for taking out from the action portion is provided, and a characteristic impedance of the impedance conversion portion is higher than a characteristic impedance of the output feedthrough portion.

  An optical modulator according to a fifth aspect of the present invention is the optical modulator according to the fourth aspect, further comprising a termination resistor electrically connected to the output feedthrough portion, wherein the resistance value of the termination resistor is It is characterized by being approximately equal to the characteristic impedance of the interaction part.

  The optical modulator according to claim 6 of the present invention is characterized in that, in the optical modulator according to claim 4 or 5, the characteristic impedance of the output feedthrough section and the characteristic impedance of the interaction section are equal. .

  In the present invention, since the impedance conversion unit is provided in front of the interaction unit, the input impedance of the optical modulator viewed from the external signal source is higher than the characteristic impedance of the interaction unit in the traveling wave electrode in the optical modulator. it can. Accordingly, impedance mismatch between the external signal source and the traveling wave electrode can be reduced, so that the optical modulation band and the power reflectivity of the high frequency electric signal can be improved.

  According to the first aspect of the present invention, the input impedance of the optical modulator can be made higher than the characteristic impedance of the interaction portion in the traveling wave electrode in the optical modulator.

  According to the second aspect of the present invention, since the characteristic impedance of the impedance converter is set to 50Ω or less, the input impedance as the optical modulator can be gradually changed with respect to the frequency.

  According to the third aspect of the present invention, since the characteristic impedance of the impedance converter is set to 50Ω or more, the input impedance as the optical modulator can be quickly increased with respect to the frequency.

  According to the fourth and fifth aspects, electrical reflection from the terminating resistor can be suppressed.

According to the sixth aspect of the present invention, impedance matching is achieved by equalizing the characteristic impedance of the output feedthrough section and the characteristic impedance of the interaction section, and the power reflectivity (S ₁₁ ) of the high-frequency electrical signal input to the optical modulator. Can be realized to be −5 dB or less over a desired light modulation band. Accordingly, it is possible to perform a wide-band light modulation of a traveling wave electrode type that is not a resonator electrode type.

  Hereinafter, embodiments of the present invention will be described. However, since the same numbers as those in the conventional embodiments shown in FIGS. 7 to 16 correspond to the same function units, the description of the function units having the same numbers is omitted here. To do.

FIG. 1 shows a top view of the CPW traveling wave electrode 4 used in this embodiment. Also in this embodiment, as in the prior art, I is an input feed-through section, II is an input-side connection section, III is an interaction section, IV is an output-side connection section, and V is an output feed-through section. The output feed-through portion V is also connected to a connector core wire (or gold ribbon or gold wire) (not shown) or a terminal resistor. In addition to the same configuration as those of the prior art, it is added impedance transformation portion VI of length L ₆ in the embodiment of the present invention shown in FIG.

FIG. 2 shows a cross-sectional view as an x-cut LN optical modulator at BB ′ of the interaction part III in FIG. Compared with the conventional technique in which the CPW gap W shown in FIG. 8 is as wide as about 20 to 40 μm, in the embodiment of the present invention shown in FIG. 2, the CPW gap W ′ is set to an extremely narrow value of about 15 μm or less. If the gap W 'of CPW thus narrow, microwave equivalent refractive index n _m of the interaction optical waveguides 3a, the light equivalent refractive index of the propagating through 3b n high-frequency electric signals can be reduced and at the same drive voltage as described above despite the advantage of close to _o, the characteristic impedance Z ₃ of the interaction portion III becomes 30Ω or less.

FIG. 3 shows a cross-sectional view of the impedance conversion section VI as a x-cut LN optical modulator at CC ′ (note that the SiO ₂ buffer layer 2 may not be provided). The CPW gap W ′ in the impedance conversion section VI is set to be about 45 μm to 1000 μm, which is wider than the CPW gap W ′ in the interaction section III.

FIG. 4 shows an equivalent circuit of the present embodiment. As in the prior art shown in FIG. 14, Z ₁ is an input feed-through section I (or line 7), Z ₂ is an input side connection section II (or line 8), and Z ₃ is an interaction section III (or line 9). , _{Z 4} are output connection portion IV (or the line 10), _{Z 5} is a characteristic impedance of the output feed-through portion V (or the line 11), the impedance conversion section VI of the characteristic impedance _{Z 6} in this embodiment (Or line 14) is added.

In the impedance converter VI, the width S ′ of the center conductor 4a may be about 6 μm to 20 μm, as in the prior art. However, the combination of the width S ′ of the center conductor 4a and the gap W ′ of the CPW is impedance conversion. As long as the characteristic impedance Z ₆ parts VI can be higher than the characteristic impedance Z ₃ of the interaction portion III, there are various combinations.

4 represents a boundary between the external signal source 5 and the additional resistor 6 (impedance Rg) and the input feedthrough portion I. In FIG. 4, Z _in ′ is an input impedance when the x-cut LN optical modulator of this embodiment is viewed from the external signal source 5 and the additional resistor 6 of the external signal source 5.

_{That, Z in} 'the characteristic impedance _{Z 1} of the input feed-through portion I, the characteristic impedance _{Z 6} of the impedance conversion section VI, the characteristic impedance _{Z 2} of the input-side connecting portion II, the interaction portion III of the characteristic impedance _{Z 3,} the output the characteristic impedance of the side connecting portion IV Z _4, it said synthesized characteristic impedance in the concept of cascaded transmission line to Z _L of the output feed-through portion characteristic impedance Z ₅ of _V, and the terminating resistor 12.

As described above, in this embodiment, since the CPW gap W is narrowed to 15 μm or less, the characteristic impedance Z ₃ of the interaction part III is as low as 30Ω or less.

Impedance transformation portion VI is the input impedance _{Z in} '(precisely, the input impedance _{Z in'} real part Re of the _{(Z in} ')) characteristic of the interaction portion III impedance _{Z 3} as viewed from an external signal source 5 and additional resistor 6 It is provided in order to improve the deterioration of the modulation characteristics of light due to the high-frequency electric signal.

Next, the relationship between the characteristic impedances Z _{1 to} Z ₆ will be considered. In FIG. 4, when the length of the input feedthrough I is as short as about 100 μm or less, the characteristic impedance Z ₁ is derived from the external signal source 5 and the additional resistor 6 of the external signal source from the x-cut LN optical modulation of this embodiment. It has been confirmed that the input impedance Z _in 'seen by the instrument is hardly affected.

Next, in the present invention, the characteristic impedance Z ₆ of the impedance conversion unit VI is set higher than the characteristic impedance Z ₃ of the interaction unit III, and the length L ₆ of the impedance conversion unit is set to the input impedance Z _in ′ (to be precise, The real part Re (Z _in ')) of the input impedance Z _in ' is set to be higher than the characteristic impedance Z ₃ of the interaction part III. The input connection portion II of the characteristic impedance _{Z 2,} the interaction portion III of the characteristic impedance _{Z 3,} the output-side connecting portion IV characteristic impedance _{Z 4,} output feed-through portion V of the characteristic impedance _{Z 5,} and terminating resistor _{Z L} May all be set equal. In particular, the interaction portion III of the characteristic impedance _{Z 3,} the output-side connecting portion characteristic impedance _{Z 4} of IV, output feed-through portion characteristic impedance _{Z 5} of V, and when equal terminating resistor _{Z L,} from the terminating resistor _{Z L} Electrical reflection can be suppressed.

As shown in FIG. 5, in this embodiment, the real part Re (Z _in ′) of the input impedance Z _in ′ when the x-cut LN optical modulator is viewed from the additional resistor 6 of the external signal source 5 is a solid line in FIG. 5. As shown in the figure, the frequency f is set to be higher than the characteristic impedance Z ₃ (30Ω or less in this case) of the interaction part III when the frequency f increases. As can be seen from FIG. 5, the impedance mismatch with the impedance Rg of the additional resistor 6 of the external signal source 5 is greatly improved with the frequency f as compared with the optical modulator according to the prior art.

In designing the impedance converter VI, the characteristic impedance and length of each part (I, II, III, IV, V, and VI) of the traveling wave electrode 4 and the equivalent refractive index of the high-frequency electric signal propagating through each part are used. In general, it has been confirmed that the length L ₆ of the impedance converter VI is preferably about several tens of μm to several mm.

Since the impedance mismatch between the real part Re (Z _in ′) of the input impedance Z _in ′ and the impedance Rg of the additional resistor 6 of the external signal source 5 is greatly improved, as a result, as shown in FIG. The light modulation index (power modulation index) | m | ^{2 with} respect to the modulation frequency f is also greatly improved as compared with the prior art of FIG. Note that the | m | ² curve shown in FIG. 6 has no dip in the desired modulation band and has a smooth curve.

As can be seen from FIG. 5 and FIG. 6, as a feature of the present invention, the real part Re (Z _in ′) of the input impedance Z _in ′ and the light modulation index (power modulation index) | m | ² draw a smooth curve. It can be seen that broadband optical modulation is possible. This is different from the characteristic that only a certain frequency is efficient as in the resonator electrode type.

The power reflectivity of the high-frequency electric signal (so-called microwave S ₁₁ ) is practically insufficient at −5 dB, and a characteristic with a smaller S ₁₁ than −10 dB or −15 dB is required. According to the embodiment, there is an advantage that smooth light modulation characteristics having no dip can be obtained at −10 dB or less within a desired light modulation band for a set bit rate (for example, 10 GHz with respect to 10 Gbps).

  Accordingly, since the high-frequency electric signal reflected and returned from the x-cut LN optical modulator to the external signal source 5 can be suppressed, an optical modulation pulse with little jitter can be obtained.

Although set to be higher than 'the real part Re _{(Z in} the') characteristic impedance _{Z 3} of the interaction portion of the input impedance _{Z in} is in the above, the input impedance _{Z in} 'the absolute value of _{| Z in'} | may be set to be higher than the characteristic impedance Z _3.

When the characteristic impedance Z ₆ of the impedance converter VI is 50Ω or less, the input impedance Z _in ′ (exactly the real part Re (Z _in ′ of the input impedance Z _in ′) is effectively improved with respect to the increase in the frequency f. )) Is disadvantageous in that the length L ₆ of the impedance converter VI needs to be several hundred μm (for example, 300 μm) or more, but the input impedance Z with respect to the frequency f is disadvantageous. There is an advantage that the change of _in 'is relatively gradual.

On the other hand, if the characteristic impedance _{Z 6} of the impedance transformation portion VI is more than 50Ω, even equal to or less than the length _{L 6} as the number 100μm impedance converter VI (e.g. 300 [mu] m), the input impedance _{Z in} conjunction with the frequency f becomes higher '( To be precise, there is an advantage that the real part Re (Z _in ')) of the input impedance Z _in ' can be rapidly increased, but there is a disadvantage that the change of the input impedance Z _in 'is large with respect to the frequency f. .

Therefore, in view of the requirements and advantages and disadvantages set forth herein, such as the dimensions and characteristics of the optical modulator, the characteristic impedance Z ₆ of the impedance transformation unit VI 50 [Omega below, or with set to at least 50 [Omega, its length The length L ₆ may be determined.

  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 traveling wave electrodes such as an asymmetric coplanar strip (ACPS) and a symmetric coplanar strip (CPS), or a lumped constant electrode may be used. . In addition to the Mach-Zehnder type optical waveguide, it goes without saying that other optical waveguides such as directional couplers and straight lines may be used as the optical waveguide.

  Also, when two interacting optical waveguides of a Mach-Zehnder optical waveguide are arranged with the central conductor in between, if the widths of the two interacting optical waveguides are made different, the DC and the dynamic extinction ratio will deteriorate even if they are close to each other. It can be avoided.

  In the above embodiment, the crystal orientation of x-cut, y-cut or z-cut, 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 may be used.

  As described above, the optical modulator according to the present invention has an effect that the RF modulation performance can be greatly improved, and is useful as an optical modulator having a high driving speed and a low driving voltage.

The top view about the traveling wave electrode in embodiment of this invention Sectional drawing in the B-B 'line of embodiment of this invention Sectional drawing in the C-C 'line of embodiment of this invention 1 is an equivalent circuit diagram of an optical modulator according to an embodiment of the present invention. The figure explaining the relationship between _Zin 'and f of the optical modulator which concerns on this invention The figure explaining the relationship between | m | ² and f of the optical modulator according to the present invention. Perspective view of an optical modulator according to the prior art Sectional drawing in the A-A 'line | wire of the optical modulator which concerns on a prior art The figure explaining operation | movement of the optical modulator based on a prior art The figure explaining the relation between Vπ · L and W of the optical modulator according to the prior art The figure explaining the relationship between _nm and W of the optical modulator which concerns on a prior art The figure explaining the relationship between Z and W of the optical modulator which concerns on a prior art Top view of a prior art traveling wave electrode Equivalent circuit diagram of a conventional optical modulator The figure explaining the relationship between _Zin and f of the optical modulator which concerns on a prior art The figure explaining the relationship of | m | ² and f of the optical modulator which concerns on a prior art

Explanation of symbols

1: x-cut LN substrate (substrate, LN substrate)
2: SiO ₂ buffer layer (buffer layer)
3: Optical waveguide 3a, 3b: Optical waveguide of the interaction part (optical waveguide)
4: traveling wave electrode 4a: center conductor 4b, 4c: ground conductor 5: external signal source 6: additional resistor 7: line representing input feedthrough part I 8: line representing input side connection part II 9: interaction part Line 10 representing line III: Line representing output side connection part IV 11: Line representing output feedthrough part V 12: Terminating resistor 13: Boundary between external signal source 5 and additional resistor 6 and optical modulator 14: Impedance conversion Line representing part VI

Claims (6)

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 and modulating the phase of the light A traveling wave electrode comprising a central conductor and a ground conductor of
The traveling wave electrode modulates the phase of the light by applying the high frequency electrical signal, and an input feedthrough for applying the high frequency electrical signal from an external circuit to the interaction portion In the optical modulator comprising a unit,
The traveling wave electrode further includes an impedance conversion unit having a characteristic impedance higher than a characteristic impedance of the interaction unit between the interaction unit and the input feedthrough unit. .   The optical modulator according to claim 1, wherein a characteristic impedance of the impedance converter is 50Ω or less.   The optical modulator according to claim 1, wherein a characteristic impedance of the impedance converter is 50Ω or more.   The traveling wave electrode further includes an output feed-through unit for taking out the high-frequency electric signal whose phase of the light is modulated by the interaction unit from the interaction unit, and a characteristic impedance of the impedance conversion unit is The optical modulator according to claim 1, wherein the optical modulator has a characteristic impedance higher than that of the output feedthrough portion.   5. The optical modulator according to claim 4, further comprising a termination resistor electrically connected to the output feedthrough portion, wherein a resistance value of the termination resistor is substantially equal to a characteristic impedance of the interaction portion. .   6. The optical modulator according to claim 4, wherein the characteristic impedance of the output feedthrough section is equal to the characteristic impedance of the interaction section.

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