JP2007033793A - Optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical modulator superior in yield of manufacture by possessing low drive voltage at a high speed and possessing small DC bias voltage. <P>SOLUTION: The optical modulator includes a substrate 1 having electric optical effect, a light guide 3 formed on the substrate, a traveling wave electrode 4 comprising a center conductor 4a and a contact conductor 4b and 4c formed on one face side of the substrate and for propagating a high-frequency electric signal modulating light, and a bias electrode comprising a center conductor 19a and contact conductors 19b and 19c formed on the above face side and applying bias voltage on light. The optical modulator comprises a high-frequency electric signal interaction part 20 for modulating an optical phase by applying the high-frequency electric signal on the traveling wave electrode in the light guide, and a bias interaction part 21 for regulating the optical phase by applying the bias voltage on the bias electrode. The bias voltage is applied on both of the center conductor of the traveling wave electrode provided on the high-frequency electric signal interaction part and the center conductor of the bias electrode provided on the bias interaction part. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention belongs to the field of optical modulators that are high in speed, low in driving voltage, low in DC bias voltage, and good in production yield.

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).

[First prior art]
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. 5 is a cross-sectional view taken along line AA ′ of FIG. The following discussion holds true for the z-cut.

In the figure, reference numeral 1 denotes an x-cut LN substrate, 2 denotes a SiO ₂ buffer layer having a thickness D that is transparent in a wavelength region used in optical communication such as 1.3 μm or 1.55 μm (note that the thickness D ranges from 200 nm to 1 μm). 3) is an optical waveguide formed by thermal diffusion at 1050 ° C. for about 10 hours after Ti is deposited on the x-cut LN substrate 1, and constitutes a Mach-Zehnder interference system (or Mach-Zehnder optical waveguide). Reference numerals 3a and 3b denote optical waveguides (or interaction optical waveguides) in a portion where an electrical signal and light interact (referred to as an interaction portion), that is, optical waveguides constituting 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 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 waveguide, not only the RF electrical signal but also the DC bias voltage changes the phase of the light. Further, SiO ₂ buffer layer 2 is effective microwave refractive index n _m of the optical waveguide 3a of the electrical signals, 3b by approximating the light of the effective refractive index n _o of propagating, an important role of expanding a light modulation band is doing.

  FIG. 6 shows a schematic view of the first prior art viewed from the top. Here, reference numeral 10 denotes a capacitor for cutting a DC component built in the electric signal source 11. Reference numeral 12 denotes an electrical terminal, 13 denotes a capacitor for cutting a DC component, and 14 denotes a DC power source for applying a DC bias voltage. Due to the two capacitors 10 and 13, the DC component from the DC power source 14 does not flow as a current.

This is the first interaction portion 15 having a traveling wave electrode 4 of a length L ₁ which is provided in the prior art there are SiO ₂ buffer layer 2 as shown in FIG. 4, DC bias to the SiO ₂ buffer layer 2 A voltage Vb is applied. However, since this SiO ₂ buffer layer 2 has high electrical resistance, it is known that a so-called DC drift occurs due to a voltage drop here. This DC drift has a great adverse effect on the reliability of the LN optical modulator.

  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 simultaneously with the RF electrical signal between the center conductor 4a and the ground conductors 4b and 4c.

  FIG. 7 is a characteristic diagram showing an example of voltage-light output characteristics of the LN optical modulator, and shows the relationship between the voltage applied to the traveling wave electrode 4 and the intensity of light output from the LN optical modulator. ing. Here, Vb is a DC bias voltage during operation. As shown in FIG. 7, the DC bias voltage Vb is normally set at the midpoint between the peak and bottom of the light output characteristic.

Shows a half-wave voltage V [pi _DC for half-wave voltage V [pi _RF and DC bias for the RF electric signal when the length L ₁ of the traveling wave electrode 4 provided on the interaction portion 15 and the variable in Figure 8. The half-wave voltage Vπ _RF for the RF electrical signal is one of the important performance indicators for the radio frequency (RF) operation in the LN optical modulator, and the smaller the value, the easier the driving at high speed. On the other hand, the half-wave voltage Vπ _{DC with} respect to the DC bias is an extremely important performance index that determines the long-term reliability of the LN optical modulator. As described later, the smaller this value, the smaller the DC drift, and the longer-term reliability. Are better.

Since this first prior art simultaneously applying an RF electrical signal and the DC bias voltage to the traveling wave electrode 4 of the interaction portion 15, half-wave voltage V [pi _DC for half-wave voltage V [pi _RF and DC bias of the RF electric signal Match. Moreover, as can be seen, the exact inverse proportion to the length L ₁ of the half-wave voltage V [pi _DC are both traveling wave electrode 4 for half-wave voltage V [pi _RF and DC bias of the RF electric signal.

As shown in FIG. 8, by increasing the length L ₁ of the interaction unit 15, two half-wave voltages Vπ _RF and Vπ _DC , which are important performance indexes of the LN optical modulator, can be reduced. . In just it is not so say it can provide excellent LN optical modulator to the length and the long-term reliability tend to drive if longer L ₁ of the interaction portion 15.

FIG. 9 shows a 3 dB light modulation band Δf with respect to the length L ₁ of the interaction unit 15 of the LN light modulator. Since the RF electrical signal propagates through the traveling wave electrode 4 and attenuates, the 3 dB optical modulation band Δf deteriorates more rapidly than it is inversely proportional to the length L ₁ of the interaction portion 15 of the LN optical modulator.

  Next, a method for improving this will be described. The 3 dB optical modulation band Δf of the LN optical modulator is approximately expressed by the following equation.

Δf = 1.4 · c ₀ / (π | n _m −n _o | · L) (1)
Here, n _m and n _o is RF electric signal and an optical equivalent refractive index of the respective propagating traveling wave electrode 4 and the interaction optical waveguides 3a, the 3b, c ₀ is the speed of light in vacuum.

Therefore, the 3 dB light modulation band Δf of the LN light modulator when the RF electric signal equivalent refractive index _nm is used as a variable is shown in FIG. In other words, as the equivalent refractive index n _m of the RF electric signal approaches the equivalent refractive index n _o of light, 3 dB optical modulation bandwidth Δf of the LN optical modulator grow rapidly. Note that in practice the RF electric signal propagating center electrode 4a in addition to the difference between the RF electric signal equivalent refractive index n _m and the light of the equivalent refractive index n _o, the ground conductor 4b, and a traveling wave electrode 4 consisting 4c propagation since there is a loss, even equal two equivalent refractive index even _(n m ₌ n _o) Δf is not larger that the more divergent.

In general, larger than the light of the equivalent refractive index n _o is 2.2 weak in the vicinity of the equivalent refractive index n _m wavelength is 1.55μm in the RF electrical signals, to improve the performance of the optical modulator the reduced importance It becomes. For this purpose, the thickness D of the SiO ₂ buffer layer 2 shown in Patent Document 2 is increased in combination with the method of increasing the thickness of the normal center conductor 4a and the ground conductors 4b and 4c shown in Patent Document 1. The method is widely used. The latter is also useful for increasing the characteristic impedance Z of the traveling wave electrode 4.

The Figure 11 shows the equivalent refractive index _{n m} of the RF electric signal in the case of a variable thickness D of the SiO ₂ buffer layer 2. As can be seen, when the thickness D of the SiO ₂ buffer layer 2 is increased, the equivalent refractive index n _m of the RF electric signal rapidly decreases.

That is, as shown in FIG. 12, when the thickness D of the SiO ₂ buffer layer 2 is increased, the 3 dB light modulation band Δf of the LN light modulator is improved. However, since the thermal expansion coefficients of the SiO ₂ buffer layer 2 and the x-cut LN substrate 1 are different, the thick SiO ₂ buffer layer 2 is easily separated from the x-cut LN substrate 1. Moreover, since the output of the driver actually used is about 5 to 6 V, the thickness D of the SiO ₂ buffer layer 2 cannot be increased excessively. Accordingly, the thickness D of the SiO ₂ buffer layer 2 is limited to about 1.3 μm. This is the same even when a z-cut LN substrate is used as the substrate. In FIG. 12, the first prior art example is indicated by symbols.

For the first conventional technique that can take the length L ₁ of the interaction portion 15 to some extent is long, 3 dB optical modulation bandwidth Δf of the LN optical modulator increases rapidly as shown by the symbol in Figure 12 Using space. However, since the thickness D of the SiO ₂ buffer layer 2 cannot be increased excessively as described above, there is a limit to the length L ₁ of the interaction portion 15 that can be set. Therefore, in FIG. 4 and FIG. 6, it is not possible to utilize all of the lengths of the two optical waveguides 3a and 3b between the branching portion and the combining portion of the Mach-Zehnder optical waveguide 3.

Next, consider the DC drift that affects the reliability of the LN optical modulator. The DC bias voltage Vb described in FIG. 7 changes with time. This is called DC drift. This DC drift has an element caused by the SiO ₂ buffer layer 2 and an element caused by the x-cut LN substrate 1 (the same applies to the z-cut LN substrate), and the former is generally larger than the latter. Further, the medium-term DC drift is determined by the SiO ₂ buffer layer 2, and the long-term DC drift is determined by the x-cut LN substrate 1.

DC, internal field strength _{E int,} DC drift [Delta] V _drift due to SiO ₂ buffer layer 2 in the case where the _B and variables in SiO ₂ buffer layer 2 in FIG. 13 _shows a _B. As can be seen from Fig, DC drift [Delta] V _drift due to SiO ₂ buffer layer _{2, B} increases the SiO ₂ buffer layer 2 DC, internal field strength _{E int, B} is increased. Therefore, in order to reduce the DC drift ΔV _{drift, B} caused by the SiO ₂ buffer layer 2, it is important to reduce the DC internal electric field strength E _{int, B} of the buffer layer 2.

FIG. 14 shows the DC drift ΔV _{drift, B} caused by the SiO ₂ buffer layer 2 with respect to the length L ₁ of the interaction portion 15. As shown in FIGS. 4 and 5, in the first prior art, a DC bias voltage is applied to the center conductor 4a and the ground conductors 4b and 4c via the buffer layer 2, and a relatively large value of 5 to 6V. The DC bias voltage Vb is applied to the SiO ₂ buffer layer 2. Therefore, in this first prior art, the DC drift ΔV _{drift, B} caused by the buffer layer 2 becomes large.

As described above, in the first prior art, the lengths of the two optical waveguides 3a and 3b of the Mach-Zehnder optical waveguide 3 cannot be fully utilized. As a result, the DC drift ΔV _drift caused by the SiO ₂ buffer layer 2 can be prevented. _{, B} is large.

[Second prior art]
FIG. 15 shows a schematic view of the second prior art, which is an attempt to solve the problem of DC drift in the first prior art, as viewed from above.

As described above, a major problem in the first prior art, that is, DC drift, was caused by a DC voltage drop in the SiO ₂ buffer layer 2 to which a DC bias voltage was applied.

Therefore, in the second prior art, the region (17) for applying the RF electric signal is separated from the region (18) for applying the DC bias voltage. As shown in FIG. 15, an RF electrical signal interaction portion 17 of length L _{2 to which} an RF electrical signal is applied, a center conductor 16a and ground conductors 16b and 16c of length L ₃ to which a DC bias voltage is applied. A DC bias interaction unit 18 having a bias electrode made of That is, as can be seen from FIG. 16 shown as a cross-sectional view at BB ′ in FIG. 15, the DC bias interaction portion 18 has the SiO ₂ buffer layer 2 existing in FIG. 4 shown as the first prior art. Absent.

Therefore, in the second prior art, there is no DC drift due to the SiO ₂ buffer layer 2, and the long-term reliability is determined by the DC drift due to the x-cut LN substrate 1 itself. In general, since the DC drift due to the x-cut LN substrate 1 itself is smaller than the DC drift due to the SiO ₂ buffer layer, it has been adopted as an effective means for improving the reliability of the LN optical modulator.

However, even in the case of the second prior art shown in FIG. 15, for DC biasing the length L ₂ and the DC bias voltage of the RF electric signal interaction portion 17 to RF electrical signal is applied is applied total length L ₃ of the interaction portion 18 is substantially determined by the two optical waveguides 3a, 3b the length of the Mach-Zehnder optical waveguide. Therefore, the shorter the length L ₃ of the DC bias interaction portion 18 which DC bias voltage is applied when increasing the length L ₂ of the RF electric signal interaction portion 17 to RF electrical signal is applied, the reverse Increasing the length L ₃ of the DC bias interaction portion 18, in turn, the length L ₂ of the RF electric signal interaction portion 17 is shortened.

FIG. 17 shows the DC bias half-wave voltage Vπ _DC with respect to the length L ₃ of the DC bias interaction section 18. When the length _{L 3} of the DC bias interaction portion 18 is short, the center conductor 16a and the ground conductor 16b, it is necessary to increase the DC bias voltage applied to 16c. As a result, the electric field strength between the center conductor 16a and the ground conductors 16b and 16c is increased, and the internal electric field strength in the x-cut LN substrate 1 is small although the DC drift due to the SiO ₂ buffer layer 2 is small. DC drift occurs in the x-cut LN substrate 1 due to the above.

Meanwhile, in order to ensure the reliability of the LN optical modulator, when the length L ₃ of the DC bias interaction portion 18, in turn, shorter length L ₂ of the RF electric signal interaction portion 17 As a result, the half-wave voltage Vπ _RF of the RF electrical signal is increased. This is shown in FIG. Since the length L ₂ of the second RF electrical signals in the prior art of the interaction portion 17 is shorter than the length L ₁ of the interaction portion 15 of the RF electric signal in the first prior art, as can be seen, The half-wave voltage Vπ _RF of the RF electrical signal is high.

In order to avoid this, it is necessary to set the thickness D of the SiO ₂ buffer layer 2 (not shown) in the RF electric signal interaction unit 17 to be thin, speed matching between the RF electric signal and light, and the characteristic impedance. It will be disadvantageous from the viewpoint.
Japanese Patent Laid-Open No. 01-091111 Japanese Patent Laid-Open No. 02-051123

As described above, in the first prior art, the DC bias voltage is also applied to the interaction portion where the RF electrical signal interacts with the light, so that the DC drift caused by the SiO ₂ buffer layer has occurred. On the other hand, in the second prior art devised to avoid the problem of the first prior art, only the DC bias is applied to the DC bias interaction portion provided independently of the RF electrical signal interaction portion. The sum of the length of the RF electrical signal interaction portion and the length of the DC bias interaction portion is determined. As a result, the length of the DC bias interaction part or the RF electric signal interaction part cannot be made sufficiently long, and the reliability deteriorates due to the high internal electric field strength in the LN substrate. Or the RF modulation performance as an LN optical modulator deteriorates.

  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 formed on the substrate for guiding light, and one surface of the substrate. A traveling wave electrode formed on the side and made of a center conductor and a ground conductor for propagating a high-frequency electric signal for modulating the light; and a center conductor and a ground conductor formed on the surface side for applying a bias voltage to the light A bias electrode comprising: a high-frequency electrical signal interaction unit for modulating the phase of the light by applying the high-frequency electrical signal to the traveling-wave electrode in the optical waveguide; and the bias electrode A bias interaction unit for adjusting the phase of the light by applying a bias voltage to the center electrode of the traveling wave electrode provided in the interaction unit for the high frequency electric signal and the bar. Wherein the bias voltage to both of the center conductor of the bias electrodes provided in the interaction portion for astigmatism is applied.

  An optical modulator according to a second aspect of the present invention is the optical modulator according to the first aspect, wherein the central conductor of the traveling wave electrode provided in the high-frequency electrical signal interaction portion and the bias interaction portion are provided. The central conductor of the provided bias electrode is electrically connected.

An RF electrical signal and a DC bias voltage are simultaneously applied to the interaction unit without the DC bias interaction unit, or only the RF electrical signal is applied to the RF electrical signal interaction unit, and the DC bias interaction unit is applied to the DC bias interaction unit. Unlike the conventional technique in which only the DC bias voltage is applied, in the invention of claim 1 of the present invention, only the interaction unit including the traveling wave electrode for simultaneously applying the RF electric signal and the DC bias voltage and the DC bias are provided. A DC bias interaction unit including a bias electrode to be applied is provided. Therefore, since the total length of the interaction part to which the DC bias voltage is applied becomes long, the DC bias half-wave voltage can be reduced. As a result, it is possible to suppress the DC drift caused by the SiO ₂ buffer layer in the interaction portion to which the RF electric signal and the DC bias voltage are applied simultaneously and the DC drift caused by the LN substrate itself. In addition, since the total length of the interaction part to which the DC bias is applied can be increased, a part of the interaction part can be allocated to the RF electric signal interaction part, and a design that reduces the high-speed response characteristics and the half-wave voltage of the RF electric signal is also possible. It becomes.

  In the second aspect of the invention, by electrically connecting the central conductor of the interaction portion to which the RF electrical signal and the DC bias voltage are simultaneously applied and the central conductor of the DC bias interaction portion to which only the DC bias is applied, the DC Since the bias power source can be shared, there is an effect that the configuration for driving the optical modulator is simplified.

  Hereinafter, embodiments of the present invention will be described. However, since the same numbers as those in the conventional embodiments shown in FIGS. 4 to 19 correspond to the same function units, the description of the function units having the same numbers is omitted here. To do.

[First Embodiment]
FIG. 1 shows a schematic top view of the first embodiment of the present invention. Similar to the first prior art shown in FIG. 6, the center electrode 4a and the ground electrodes 4b, which includes the interaction unit 20 of a length L ₄ which applies an RF electric signal and the DC bias voltage consists 4c. Moreover, as in the second prior art shown in FIG. 15, the center conductor 19a and the ground conductor 19b, the DC bias voltage interaction portion having a bias electrode length L ₅ which applies only a DC bias voltage consists 19c 21.

FIG. 2 is a sectional view taken along the line CC ′ in FIG. 22a, 22b, 22c and 22d are SiO ₂ buffer layers. These SiO ₂ buffer layers 22a, 22b, 22c, and 22d do not have to be the same as in the second prior art, but in this embodiment, by using these, the edges of the center conductor 19a and the ground conductors 19b and 19c are used. an optical waveguide 3a, close to 3b, thereby reducing the DC bias half-wave voltage V [pi _DC.

  In the present embodiment, as in the first prior art shown in FIG. 6, a capacitor 13 that cuts a DC component is used so that a DC bias can be applied between the center conductor 4a and the ground conductors 4b and 4c, and a DC power source is used. 14 are electrically connected by an electrical connection 30. The DC bias voltage from the DC power source 14 is also supplied between the center conductor 19a and the ground conductors 19b and 19c of the DC bias interaction unit 21. The central conductor 4a and the central conductor 19a may be supplied with a DC bias voltage from a DC power supply provided individually, but in this embodiment, the configuration is simplified by supplying the DC bias voltage to be applied from the same power supply. ing.

Thus, in the present embodiment, the DC bias voltage is supplied not only to the DC bias interaction unit 21 but also to the interaction unit 20 that applies the RF electrical signal. Therefore, for the DC bias voltage, the length of the interaction portion is L ₄ + L ₅ . Further, as shown in FIG. 2, in the DC bias interaction portion 21, the center conductor 19 a and the ground conductors 19 b and 19 c are in direct contact with the x-cut LN substrate 1.

For the DC bias half-wave voltage Vπ _DC , when the SiO ₂ buffer layer shown in FIG. 17 is not provided and the center conductor 19a and the ground conductors 19b and 19c are in direct contact with the x-cut LN substrate 1, FIG. This is significantly lower than when a DC bias voltage is supplied through the SiO ₂ buffer layer 2 shown. That is, the center conductor 19a and the ground conductor 19b, when 19c is in direct contact with the x- cut LN substrate 1, a DC bias half-wave voltage V [pi _DC is low, even if the length _{L 5} of the DC bias interaction portion 21 A DC bias voltage can be effectively supplied even if it is short. Therefore, by applying a DC bias voltage to the DC bias interaction unit 21, the DC bias half-wave voltage Vπ _DC as the entire optical modulator can be lowered as compared with applying a DC bias voltage only to the interaction unit 20. it can.

Thus, the total length of the interaction part for applying the DC bias voltage is longer than L ₄ + L ₅ , the first and second conventional techniques, and the necessary DC bias voltage can be set low. That is, the DC bias voltage applied to the interaction unit 20 and the DC bias interaction unit can also be lowered. Therefore, it means that both the DC drift caused by the SiO ₂ buffer layer 2 and the DC drift caused by the x-cut LN substrate in the DC bias interaction part 21 can be simultaneously reduced in the interaction part 20. Further, since the length L ₄ of the interaction unit 20 to which the RF electric signal is applied can be made as long as the first prior art, the 3 dB light modulation band Δf can be widened.

Table 1 summarizes the characteristics of the x-cut LN optical modulator according to the first embodiment of the present invention shown in FIG. 1 and the first and second prior arts shown in FIGS. 4 and 15, respectively. Yes. As can be seen from this table, in this embodiment, improved characteristics can be realized by the 3 dB optical modulation band Δf, the RF electrical signal half-wave voltage Vπ _RF , and the DC bias half-wave voltage Vπ _DC . In particular, since the low DC bias half-wave voltage V [pi _DC, there is a great advantage of high reliability.

[Second Embodiment]
FIG. 3 shows a schematic top view of the optical modulator according to the second embodiment of the present invention. Interaction of length L ₆ for applying an RF electrical signal and a DC bias voltage, which comprises the central conductor 4a and the ground conductors 4b and 4c included in the optical modulator of the first embodiment of the present invention shown in FIG. and parts 25, the center conductor 23a, the ground conductor 23b, the DC bias interaction portion 26 having a bias electrode length _{L 7} for applying a DC bias voltage consists 23c (wherein the first DC bias interaction portion in addition to called), further in the present invention, the center conductor 24a and the ground conductor 24b, DC bias interaction portion 27 having a bias electrode length _{L 8} for applying a DC bias with consisting 24c (second DC bias For example). The cross sections of the first DC bias interaction unit 26 and the second DC bias interaction unit 27 are the same as those in FIG. 2 (or FIG. 16).

  The center conductors 23a and 24a of the bias electrodes constituting the first DC bias interaction unit 26 and the second DC bias interaction unit 27 are connected by an electrical connection 31, and the RF electrical signal and the DC are connected. If the central conductor 4a of the interaction unit 25 to which the bias voltage is applied simultaneously is also connected by the electrical connection 30, it is preferable that only one DC power source 14 is required. When the center conductors 23a and 24a of the first DC bias interaction unit 26 and the second DC bias interaction unit 27 are electrically connected, the center conductor 4a and the ground conductor 4b of the interaction unit 25 are provided. It is desirable to wire while avoiding the gap between 4c.

  Also, it is convenient if the ground conductors 23b, 23c, 24b, and 24c are electrically connected to each other or directly connected to a metal housing (not shown) as a ground. Normally, the bias is DC or extremely low frequency, and thus no problem occurs even if the electrodes are routed and connected in this way. However, it goes without saying that a bias voltage may be supplied to the central conductors 4a, 23a and 24a from different power sources.

In the second embodiment of the present invention shown in FIG. 3, the total sum of the lengths of the regions to which the DC bias voltage is applied can be significantly increased to L ₆ + L ₇ + L ₈ . Alternatively, a portion of its length allocation to the interaction unit 25, it is also possible to increase the length L _6. As a result, as shown in Table 1, in the second embodiment of the present invention, not only can reduce the DC bias half-wave voltage V [pi _DC, 3 dB optical modulation bandwidth Δf can be improved.

  As described above, generally, if the length of the RF electrical signal interaction portion can be increased, the thickness of the buffer layer formed immediately below the traveling wave electrode 4 can be increased. Accordingly, the speed of the microwave and the light can be made close and the characteristic impedance of the optical modulator can be made close to the characteristic impedance of the driver, thereby improving the modulation performance.

Further, when it is reduced DC bias half-wave voltage V [pi _DC, DC bias voltage applied to the interaction portion 25 can also be lowered, DC drift due to SiO ₂ buffer layer 2 can be suppressed. Furthermore, if the sum of the DC bias interaction portions in which the central conductors 23a, 24a and the ground conductors 23b, 23c, 24b, 24c are in direct contact with the x-cut LN substrate 1 becomes long, the inside of the x-cut LN substrate 1 Therefore, the DC drift in the x-cut LN substrate 1 can be reduced.

[About each embodiment]
When a z-cut substrate is used instead of the x-cut LN substrate, a buffer layer is required immediately above the optical waveguide in the DC bias interaction section. In the present invention, since the total sum of the DC bias interaction portions can be increased, the DC bias voltage can be set low. That is, since the electric field strength in the buffer layer can be lowered, not only the DC drift in the z-cut LN substrate but also the DC drift due to the buffer layer can be reduced.

  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.

  In addition, since no RF electric field is applied to the DC bias interaction unit, there is no need to consider the characteristic impedance of the DC bias interaction unit, and the width of the central conductor of the DC bias interaction unit is the RF electrical signal interaction unit. The gap between the center conductor and the ground conductor in the DC bias interaction section should be narrower than the gap between the center conductor and the ground conductor in the RF electrical signal interaction section. It goes without saying that is possible.

  Further, 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 having a high driving yield, a low drive voltage, a low DC bias voltage, and a high manufacturing yield.

1 is a schematic top view of an optical modulator according to a first embodiment of the present invention. Sectional drawing in the C-C 'line | wire of FIG. 1 showing the optical modulator of the 1st Embodiment of this invention Schematic top view of the second embodiment of the optical modulator of the present invention. Perspective view of the first prior art Sectional drawing in the A-A 'line | wire of FIG. 4 showing 1st prior art Schematic top view of the first prior art Diagram explaining the operation of a general optical modulator General optical modulator V [pi _RF, diagram showing the relationship between the V [pi _DC and _{L 1} Diagram showing the relationship between Δf and L ₁ of a general optical modulator Diagram showing the relationship between Δf and n _m of a general optical modulator The figure which shows the relationship between _nm and D of a general optical modulator The figure which shows the relationship between (DELTA) f and D of a general optical modulator The figure explaining the internal electric field strength of a buffer layer, and DC drift resulting from a buffer layer The figure explaining the DC drift resulting from the length of the interaction part for DC bias, and a buffer layer Schematic top view of the second prior art Sectional drawing in the B-B 'line of the 2nd prior art Diagram showing the relationship between the V [pi _DC and L ₃ of a general optical modulator Diagram showing the relationship between the V [pi _RF and L ₂ of a general optical modulator The figure explaining the DC drift resulting from the length of the interaction part for DC bias, and an x-cut LN board | substrate

Explanation of symbols

1: x-cut LN substrate (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 10: capacitor 11: electrical signal source 12: electrical termination 13: capacitor 14: DC power supply 15: interaction unit 16a: center conductor 16b, 16c: ground Conductor 17: RF electrical signal interaction part 18: DC bias interaction part 19a: Center conductor 19b, 19c: Ground conductor 20: Interaction part 21: Interaction part 22a, 22b, 22c, 22d: SiO ₂ buffer layer 23a: Center conductor 23b, 23c: Ground conductor 24a: Center conductor 24b, 24c: Ground conductor 25: Interaction unit 26: Interaction unit 27: Interaction unit 30, 31: Electrical connection

Claims (2)

A substrate having an electro-optic effect; an optical waveguide formed on the substrate for guiding light; and a central conductor formed on one side of the substrate for propagating a high-frequency electrical signal that modulates the light And a traveling wave electrode composed of a ground conductor and a bias electrode formed on the surface side and composed of a center conductor and a ground conductor for applying a bias voltage to the light,
The optical waveguide has a high frequency electrical signal interaction unit for modulating the phase of the light by applying the high frequency electrical signal to the traveling wave electrode, and a bias voltage applied to the bias electrode. An optical modulator including a bias interaction unit for adjusting a phase of light;
A bias voltage is applied to both the central conductor of the traveling wave electrode provided in the high-frequency electrical signal interaction portion and the central conductor of the bias electrode provided in the bias interaction portion. Light modulator.   The light characterized in that the central conductor of the traveling wave electrode provided in the high-frequency electrical signal interaction portion and the central conductor of the bias electrode provided in the bias interaction portion are electrically connected. Modulator.

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