JP4420202B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator characterized by optically coupling optical waveguides formed on different surfaces of the same substrate.

In recent years, external modulators such as optical waveguide devices have been widely used in the field of optical communications and the like for the purpose of enabling high-speed switching. As such an external modulator, lithium niobate (LiNbO ₃ ; hereinafter referred to as LN) having characteristics such as high frequency characteristics, low insertion loss, and high extinction ratio is used as a substrate, and Ti or the like is heated on the substrate. A waveguide type optical modulator having an optical waveguide formed by diffusing has been put into practical use.

Conventionally, when the function of an optical modulator is improved or added, a device such as setting a longer working length between an optical waveguide and a control electrode for applying an electric field to the waveguide, or providing a different electrode for each function Was done. For example, there are a method of increasing the working length to reduce the driving voltage, a method of separating electrodes for high frequency signals and DC bias in consideration of the convenience of modulation control.
In addition, the optical modulator includes a light intensity modulator, a phase modulator, a polarization modulator, and the like, and various optical circuits are formed by combining a plurality of these.

These cause an increase in the size of the optical modulator itself, and cause problems such as an increase in manufacturing cost and a limitation in the size of a device using the optical modulator. In addition, when a plurality of electrodes are arranged on a substrate having a limited size, there is a problem that an electrode having a sufficient length cannot be arranged because of electrode wiring.
On the other hand, in Patent Document 1 below, it is possible to suppress an increase in the length of the substrate by forming a plurality of light modulation portions in parallel on the same plane of the substrate and folding back the light wave on the end side of the substrate. Proposed. However, when a plurality of light modulation units are arranged in parallel as in Patent Document 1, the lateral width of the substrate is widened, and the increase in manufacturing cost and the limitation of the application place are not solved.
JP 2001-350046 A

  The problem to be solved by the present invention is to solve the above-mentioned problems, to provide a highly integrated and compact optical modulator, and to reduce driving voltage, suppress chirping, and polarization The object is to provide an optical modulator having various functions such as independence.

In order to solve the above-mentioned problem, the invention according to claim 1 is directed to a substrate made of a material having an electro-optic effect, a surface-side optical waveguide formed on the surface of the substrate, and a back surface formed on the back surface of the substrate. Side-side optical waveguide, front-side control electrode for controlling the phase of the light wave propagating in the front-side optical waveguide, and back-side control for controlling the phase of the light wave propagating in the rear-side optical waveguide And at least one end surface of the substrate, the optical waveguide formed on the different surface has an exit and an entrance, and the light wave emitted from the exit is incident on the entrance. The optical modulator is characterized in that a folding element is arranged close to the one end face.
In the present invention, the “front surface” and the “back surface” mean that when a light wave emitted from one surface of the substrate is incident on the other surface, the surface from which the light wave is emitted is referred to as “front surface”, and the surface on which the light wave is incident is referred to as “back surface” "

  According to a second aspect of the present invention, in the optical modulator according to the first aspect, at least a part of the front-side control electrode or the rear-side control electrode is on a plate-like body different from the substrate. It is formed, and the plate-like body is disposed in proximity to the substrate.

  According to a third aspect of the present invention, in the optical modulator according to the first aspect, the front-side control electrode and the rear-side control electrode are electrically connected to each other, and the front-side control electrode advances. The high-frequency signal to be transmitted continues to travel through the back side control electrode.

  According to a fourth aspect of the present invention, in the optical modulator according to the third aspect, the electrical connection is in contact with a line penetrating the substrate, a line formed on a side surface of the substrate, or the substrate. It is performed by any of the electrically-conductive members to be performed.

  In the invention according to claim 5, in the optical modulator according to claim 1, the front-side optical waveguide and the back-side optical waveguide have at least two optical waveguides that are optically connected to each other, The optical fiber is modulated such that the modulation by the front-side control electrode and the modulation by the back-side control electrode for the light wave propagating through the two optical waveguides cause a change in the refractive index of the same sign for each optical waveguide. It is characterized in that the general connection, the arrangement of the optical waveguide on each surface and the control electrode, and the shape of the control electrode are set.

  Further, in the invention according to claim 6, in the optical modulator according to claim 1, the front-side optical waveguide and the back-side optical waveguide have at least two optical waveguides that are optically connected to each other, The optical fiber is modulated such that the modulation by the front-side control electrode and the modulation by the back-side control electrode with respect to the light wave propagating through the two optical waveguides cause a change in refractive index of opposite sign to each optical waveguide. It is characterized in that the general connection, the arrangement of the optical waveguide on each surface and the control electrode, and the shape of the control electrode are set.

  Further, in the invention according to claim 7, in the optical modulator according to claim 1, the front-side optical waveguide and the back-side optical waveguide have at least one optical waveguide that is optically connected to each other, For the light wave propagating through the optical waveguide, the optical waveguide on each surface and the control surface are different so that the polarization plane mainly modulated by the front-side control electrode and the polarization plane mainly modulated by the back-side control electrode are different. The arrangement with the electrode and the shape of the control electrode are set.

  According to an eighth aspect of the present invention, in the optical modulator according to any one of the fifth to seventh aspects, the optical waveguide to be optically connected has different polarization directions on the front surface side and the back surface side. It is characterized by passing through an area.

  According to the first aspect of the present invention, by forming the optical waveguide that is folded back at the one end surface of the substrate on the front surface and the back surface of the substrate, the length of the optical modulator can be suppressed and the optical waveguide can be made compact. Since the control electrode formed on the surface can arbitrarily control the phase of the light wave propagating through the optical waveguide, an optical modulator in which various functions are integrated can be provided.

According to the invention according to claim 2, a part of the front side control electrode or the back side control electrode is formed on a plate-like body different from the substrate on which the optical waveguide and other electrodes are formed. The manufacturing process for forming electrodes on the substrate itself can be simplified, and the state of the electric field applied to the optical waveguide on the substrate can be adjusted by adjusting the positional relationship between the substrate and the plate-like body. .
Moreover, a wide variety of optical modulators can be realized by changing the electrode pattern formed on the plate-like body.

  According to the invention of claim 3, the front side control electrode and the back side control electrode are electrically connected to each other, and the high frequency signal traveling through the front side control electrode continues to travel through the back side control electrode. Therefore, it is possible to lengthen the action length of the high-frequency signal traveling through the electrode and the light wave propagating through the optical waveguide without changing the size of the optical modulator itself.

  According to the invention of claim 4, the electrical connection between the front-side control electrode and the back-side control electrode is brought into contact with the line penetrating the substrate, the line formed on the side surface of the substrate, or the substrate. Since it is performed by any one of the conductive members, it is possible to omit complicated work such as connecting the front-side control electrode and the back-side control electrode with a thin line. In particular, in the case of a line penetrating the substrate, it is possible to prevent the wiring of the control electrode from becoming complicated.

  According to the fifth aspect of the present invention, the front-side optical waveguide and the back-side optical waveguide have at least two optical waveguides that are optically connected to each other, and the surface side with respect to the light wave that propagates through the two optical waveguides Since the modulation by the control electrode and the modulation by the back-side control electrode cause a change in the refractive index of the same sign for each optical waveguide, it is possible to reduce the drive voltage.

  According to the invention of claim 6, the front-side optical waveguide and the back-side optical waveguide have at least two optical waveguides that are optically connected to each other, and the front-side of the light wave propagating through the two optical waveguides When the modulation by the control electrode and the modulation by the back side control electrode cause a change in the refractive index of the opposite sign for each optical waveguide, chirping by the control electrode occurs on one substrate surface However, it is possible to correct the generated chirping on the other substrate surface, and light modulation with suppressed chirping is possible.

  According to the invention of claim 7, the front-side optical waveguide and the back-side optical waveguide have at least one optical waveguide that is optically connected to each other, and the front-side optical wave propagates through the optical waveguide. Since the polarization plane mainly modulated by the control electrode is different from the polarization plane mainly modulated by the back-side control electrode, different polarization planes of the same light wave are mainly modulated on the front surface and the back surface of the substrate. It is possible to perform modulation independent of the polarization plane even for a light wave having a polarization plane.

  According to the invention according to claim 8, since the optical waveguide to be optically connected passes through the substrate regions having different polarization directions on the front surface side and the back surface side, the optical connection and the optical waveguide on each surface are performed. When designing the arrangement of the control electrodes and the shape of the control electrodes, it is possible to perform more various designs.

Hereinafter, the present invention will be described in detail using preferred examples.
The present invention includes a substrate made of a material having an electro-optic effect, a surface-side optical waveguide formed on the surface of the substrate, a back-side optical waveguide formed on the back surface of the substrate, and the inside of the surface-side optical waveguide. A front-side control electrode for controlling the phase of the propagating light wave, and a back-side control electrode for controlling the phase of the light wave propagating in the back-side optical waveguide, and at least on one end surface of the substrate The optical waveguide formed on the different surface has an exit port and an entrance port, and the folding element is arranged close to the one end surface so that the light wave emitted from the exit port enters the entrance port. This is an optical modulator.

FIG. 1 shows a schematic diagram of an optical modulator according to the present invention. Optical waveguides 2-1 and 2-2 are formed on the surface of a substrate 1 having an electro-optic effect. As the shape of the optical waveguide, various shapes can be selected depending on the type of the optical modulator. Here, the description will focus on the Mach-Zehnder type optical waveguide.
An optical waveguide (not shown) is formed on the back surface of the substrate 1, and the shape of the optical waveguide on the back surface side is basically the same as that on the front surface side in FIG. Then, one Mach-Zehnder type optical waveguide is formed by optically coupling the front side optical waveguide and the rear side optical waveguide.

The substrate constituting the optical modulator is composed of a material having an electro-optic effect, for example, lithium niobate, lithium tantalate, PLZT (lead lanthanum zirconate titanate), and a quartz-based material. These single crystal materials are composed of an X-cut plate, a Y-cut plate, and a Z-cut plate. In particular, lithium niobate (LN) is used because it is easily constructed as an optical waveguide device and has a large anisotropy. It is preferable to use it.
The optical waveguide on the substrate can be formed by diffusing Ti or the like on the substrate surface by a thermal diffusion method or a proton exchange method.

Reference numeral 3 denotes a folding element for optically coupling the front side optical waveguide and the rear side optical waveguide.
As the folding element, a GRIN lens as disclosed in Patent Document 1 can be used. In addition, a reflecting member such as a mirror or a prism can also be used.

FIG. 2 is a cross-sectional view taken along one-dot chain line AA ′ in FIG.
In FIG. 2A, optical waveguides 2-1 and 2-2 are formed on the surface side of the substrate 1, and a buffer layer 4 formed of SiO ₂ or the like, and a modulation electrode constituting a control electrode. 5 and a ground electrode 6 are formed.
On the other hand, optical waveguides 2-3 and 2-4 are formed on the back side of the substrate 1, and a buffer layer 4 and DC bias electrodes 7 and 8 are further formed.

  Then, the folding element 3 causes the optical waveguide “2-1” to be changed to “2-4”, the optical waveguide “2-2” to be changed to “2-3”, or the optical waveguide “2-1” to be the same. Propagating through the front-side optical waveguide when the front-side optical waveguide and the rear-side optical waveguide are optically connected to “2-3” and from the optical waveguide “2-2” to “2-4”. The light wave to be subjected to modulation control by a high-frequency signal traveling through the modulation electrode 5 is further subjected to bias control by the DC bias electrodes 7 and 8 when propagating through the back-side optical waveguide.

  As a result, the convenience of modulation control by separating the high-frequency signal electrode and the DC bias electrode is improved, and since the optical path of the light wave is folded, the size of the optical modulator can be made compact. .

In FIG. 2 (b), the DC bias electrodes 7 and 8 on the back surface in FIG. 2 (a) are formed on another insulating plate-like body, and are close to (in close contact with or slightly separated from) the back surface of the substrate 1. Are arranged.
Thus, since the back electrode is formed on another member different from the substrate, the manufacturing process for forming the electrode on the substrate itself can be simplified, and the positional relationship between the substrate and the plate-like body is adjusted. Thus, the state of the electric field applied to the optical waveguide on the substrate can be adjusted.

  Although FIG. 2B shows an example of the DC bias electrode on the back surface, the present invention is not limited to this, and only a part of the front-side control electrode or the back-side control electrode is formed on another plate-like body. It is also possible to arrange them close to each other. Also, by preparing a plurality of types of electrode patterns to be formed on the plate-like body, it is possible to change the electrode patterns as necessary to realize a wide variety of optical modulators.

Next, an example in which the front side control electrode and the back side control electrode are electrically connected will be described.
3 uses a Z-cut plate as the substrate 1, and on the surface side of the substrate 1, the optical waveguides 2-1, 2-2, the buffer layer 4, the modulation electrode 5, and the ground electrode 6 are provided as in FIG. Is formed. On the other hand, on the back side of the substrate 1, optical waveguides 2-3 and 2-4, a buffer layer 4, and a modulation electrode 5 ′ and a ground electrode 6 ′ similar to those on the front side are formed.

  Furthermore, the polarization inversion of the substrate is performed so that the polarization direction of the substrate 1 has different polarization directions in the formation region of the front-side optical waveguide and the formation region of the back-side optical waveguide. The arrow shown in FIG. 3A indicates the polarization direction of the substrate 1. Then, the optical waveguides 2-1 to 2-4 and optical waveguides 2-2 to 2-3 are optically connected by the folding element. The front surface side modulation electrode 5 is connected to the rear surface side modulation electrode 5 ', and the front surface side ground electrode 6 is connected to the rear surface side ground electrode 6'.

  In the optical modulator of FIG. 3 (a), when a light wave is introduced into the optical waveguide and a high frequency signal is applied to the modulation electrode 5, the light wave propagates in the optical waveguide from the substrate surface to the back surface. Since the high-frequency signal travels through the electrode 5, an electric field can be continuously applied to the light wave, and the high-frequency signal traveling through the electrode and the optical waveguide propagate without changing the size of the optical modulator itself. It becomes possible to lengthen the action length with the light wave.

  Further, in FIG. 3A, the modulation by the front side control electrode and the modulation by the back side control electrode with respect to the propagating light wave are set so as to cause the refractive index change of the same sign for each optical waveguide. Has been. Such a modulation state is also referred to as “in-phase state”. Further, when the modulation causes a change in the refractive index of the opposite sign for each optical waveguide, it is also referred to as a “reverse phase state”.

Specifically, paying attention to the light wave propagating through the optical waveguide 2-2, the polarization direction of the substrate in the formation region of the optical waveguide 2-2 is upward as indicated by an arrow, and a positive voltage is applied to the modulation electrode 5. In this case, an electric field opposite to the polarization direction is generated. On the other hand, in the optical waveguide 2-3, the polarization direction of the substrate in the formation region of the optical waveguide 2-3 is downward as indicated by an arrow, and when a positive voltage is applied to the modulation electrode 5 ′, An electric field opposite to the polarization direction is generated. That is, when the light wave propagating through the optical waveguide 2-2 on the front surface side of the substrate propagates through the optical waveguide 2-3 on the back surface side of the substrate, a positive voltage is applied to the modulation electrode for the propagating light wave. When applied, an electric field opposite to the polarization direction of the substrate in the optical waveguide formation region is always applied.
In this way, both the front and back sides of the substrate perform the same operation with respect to light waves, so that the effective action length for light waves can be expanded without changing the size of the substrate, and the drive voltage can be reduced. It becomes possible to do.

FIG. 3B shows an example in which an X-cut plate is used for the substrate 1.
On the surface side of the substrate 1, optical waveguides 2-1, 2-2, a buffer layer 4, a modulation electrode 5, and a ground electrode 6 are formed as in FIG. On the other hand, on the back side of the substrate 1, optical waveguides 2-3 and 2-4, a buffer layer 4, and a modulation electrode 5 ′ and a ground electrode 6 ′ similar to those on the front side are formed. The polarization direction of the substrate 1 is the left direction indicated by the arrow in FIG.

  Then, the optical waveguides 2-1 to 2-4 and optical waveguides 2-2 to 2-3 are optically connected by the folding element. The front surface side modulation electrode 5 is connected to the rear surface side modulation electrode 5 ', and the front surface side ground electrode 6 is connected to the rear surface side ground electrode 6'.

Focusing on the light wave propagating through the optical waveguide 2-2, the polarization direction of the substrate in the formation region of the optical waveguide 2-2 is leftward as indicated by an arrow, and when a positive voltage is applied to the modulation electrode 5, An electric field in the same direction as the polarization direction is generated. On the other hand, in the optical waveguide 2-3, the polarization direction of the substrate in the formation region of the optical waveguide 2-3 is leftward as indicated by an arrow, and when a positive voltage is applied to the modulation electrode 5 ′, An electric field in the same direction as the polarization direction is generated. That is, when the light wave propagating through the optical waveguide 2-2 on the front surface side of the substrate propagates through the optical waveguide 2-3 on the back surface side of the substrate, a positive voltage is applied to the modulation electrode for the propagating light wave. When applied, an electric field in the same direction as the polarization direction of the substrate in the optical waveguide formation region is always applied.
Therefore, similarly to FIG. 3A, in the case of the X-cut plate of FIG. 3B, the modulation by the front-side control electrode and the modulation by the back-side control electrode with respect to the propagating light wave are in the same phase state. It is set to become.

FIG. 3C shows an example where a Z-cut plate is used for the substrate 1.
On the surface side of the substrate 1, optical waveguides 2-1, 2-2, a buffer layer 4, a modulation electrode 5, and a ground electrode 6 are formed as in FIG. On the other hand, on the back side of the substrate 1, optical waveguides 2-3 and 2-4, a buffer layer 4, and a modulation electrode 5 ′ and a ground electrode 6 ′ similar to those on the front side are formed. The polarization direction of the substrate 1 is upward as indicated by an arrow in FIG.

  Then, the optical waveguides 2-1 to 2-4 and optical waveguides 2-2 to 2-3 are optically connected by the folding element. The front surface side modulation electrode 5 is connected to the rear surface side modulation electrode 5 ', and the front surface side ground electrode 6 is connected to the rear surface side ground electrode 6'.

When attention is paid to the light wave propagating through the optical waveguide 2-2, the polarization direction of the substrate in the formation region of the optical waveguide 2-2 is upward as indicated by an arrow, and when a positive voltage is applied to the modulation electrode 5, An electric field in the direction opposite to the polarization direction is generated. On the other hand, in the optical waveguide 2-3, the polarization direction of the substrate in the formation region of the optical waveguide 2-3 is upward as indicated by an arrow, and when a positive voltage is applied to the modulation electrode 5 ′, An electric field in the same direction as the polarization direction is generated. That is, when the light wave propagating through the optical waveguide 2-2 on the front surface side of the substrate propagates through the optical waveguide 2-3 on the back surface side of the substrate, a positive voltage is applied to the modulation electrode for the propagating light wave. When applied, an electric field in a direction different from the polarization direction of the substrate in the optical waveguide formation region is applied to the front surface side and the back surface side of the substrate.
Therefore, in the case of FIG. 3C, the modulation by the front-side control electrode and the modulation by the back-side control electrode with respect to the propagating light wave are set to be in a reverse phase state.

In the reverse phase state as shown in FIG. 3C, the connection method of each optical waveguide is changed from the optical waveguide 2-1 to the same 2-3 in FIG. 3A and FIG. By connecting from 2-2 to 2-4, a similar reverse phase state can be realized.
Further, by adjusting the shape of the control electrode and the arrangement with respect to the optical waveguide, the direction of the electric field can be changed to be in a reverse phase state.

For example, on the surface side of FIG. 3C, since the arrangement and shape of the control electrode with respect to the optical waveguide are different in each optical waveguide, the strength of the electric field applied to each optical waveguide 2-1, 2-2 The direction is different (the direction is usually the reverse direction, which means that the direction is different from this reverse direction), and chirping is performed when the modulated light waves are combined. Will be generated.
However, such a reverse phase state causes chirping in the reverse direction on the back surface of the substrate with respect to chirping generated on the front side of the substrate, and as a result, chirping is suppressed in the entire optical modulator. The Rukoto.

Next, an example in which the present invention is used as a polarization-independent optical modulator will be described.
In FIG. 4, the optical waveguide 10, the buffer layer 4, the modulation electrode 5, and the ground electrode 6 are formed on the surface side of the substrate 1. On the other hand, on the back side of the substrate 1, an optical waveguide 10 ′, a buffer layer 4, and a modulation electrode 5 ′ and a ground electrode 6 ′ similar to those on the front side are formed.
Then, the optical waveguide 10 is optically connected to the same 10 ′ by a folding element. The front surface side modulation electrode 5 is connected to the rear surface side modulation electrode 5 ', and the front surface side ground electrode 6 is connected to the rear surface side ground electrode 6'.

Focusing on the light wave propagating through the optical waveguide 10, when a high frequency signal is applied to the modulation electrode 5, an electric field perpendicular to the substrate surface is applied to the propagating light wave and propagates in the optical waveguide 10 '. In doing so, an electric field in a direction parallel to the substrate surface is applied.
As a result, it is possible to perform modulation control on both polarization planes (polarization planes perpendicular to and parallel to the substrate plane) of light waves. That is, it is possible to perform modulation independent of the polarization plane even for a light wave having a random polarization plane.

  Since the modulation efficiency also depends on the relationship between the polarization direction of the substrate and the direction of the electric field, when the structure as shown in FIG. 4 is used, either the X-cut plate or the Z-cut plate is used for the substrate 1. However, in order to adjust the modulation efficiency on each side of the substrate to be the same, it affects the working length, which is the length at which the electric field of the control electrode acts on the light wave propagating in the optical waveguide, and the strength and direction of the electric field. It is preferable to adjust the arrangement of the optical waveguide and the control electrode that gives the light and the shape of the control electrode.

Next, a method of electrical connection to the front surface side control electrode and the back surface side control electrode will be described.
FIG. 5 shows a connection method using a via hole 21 that is a line penetrating the substrate 1 from the front surface to the back surface. The via hole 21 is connected to the ends of the modulation electrode 20-1 that applies an electric field to the optical waveguide 2-1 and the modulation electrode 20-2 that applies an electric field to the optical waveguide 2-2.
Similar to FIG. 5, an optical waveguide and a modulation electrode are formed on the back surface side of the via hole 21, and the start point of the modulation electrode on the back surface side is connected to the via hole that is a line penetrating the substrate 1.

Further, FIG. 6 shows that a line connecting the front surface side and the back surface side is formed on the side surface of the substrate 1.
6A shows a structure in which a line 22 is formed on a side surface parallel to the longitudinal direction of the substrate 1, and FIG. 6B shows a structure in which a line 23 is formed on the folding element 3 side of the substrate 1.
Further, as shown in FIG. 7, instead of the line 22 shown in FIG. 6A, a conductive member 24 (which may be formed by forming a conductive portion on the surface of an insulator) is disposed on the side surface of the substrate 1. It is also possible to make electrical connection with the back surface by the conductive member. In addition, it is possible to use a part of a part such as a casing surrounding the substrate 1 such that the conductive member also serves as a support function of the substrate 1.

In the electrical connection method shown above, what is important when connecting the front-side control electrode and the back-side control electrode of the substrate 1 is that, in connecting the modulation electrode, the high-frequency signal traveling through the modulation electrode, the optical waveguide, This is to consider that the light wave propagating through the folding element 3 is synchronized.
That is, it is necessary to set the length of the connection line, the shape of the modulation electrode, and the like so that the timing at which the electric field generated by the high-frequency signal acts on the light wave is the same on the front surface and the back surface of the substrate.

Further, as another method for applying a high frequency signal to the front surface side control electrode and the back surface side control electrode of the substrate, as shown in FIG. 30-1 and 30-2, the signal line 30-1 is connected to the modulation electrode on the front surface side, and the signal line 30-2 is connected to the modulation electrode on the back surface side.
Also in this case, in order to synchronize the high-frequency signal traveling through the modulation electrode and the light wave propagating through the optical waveguide and the folding element 3, the length of the signal line is set so as to delay the signal line 30-2 for a predetermined time. It is necessary to adjust.

  Another method for adjusting the timing at which the electric field generated by the high-frequency signal acts on the light wave is to adjust the optical path length (physical length, refractive index in the optical path, etc.) of the portion related to the folding of the light wave. There is a method of synchronizing with each other and a method of correcting the time difference by adjusting the light wave and the modulation signal of the part related to the folding when there is a time difference between the modulation signal and the light wave until the folding.

Note that when the modulation signal is always operated at a constant frequency, the modulation signal becomes the same signal every fixed period, so that synchronization can be achieved even when the timing of the modulation signal and the light wave is shifted. That is, the timing point of the modulated signal and the light wave is not a single point, but appears every period of the modulated signal.
Further, when the modulation signal is operated at a constant frequency, a new effect can be obtained by shifting the modulation signal by a half cycle and matching the timing with the light wave.
For example, when the timing of the modulation signal matches that of the light wave, modulation capable of suppressing chirping is possible as described with reference to FIG. In addition, when the modulation signal is shifted by a half period and the timing is matched with the light wave, the refractive index change of the optical waveguide on the back surface is also in the same phase as the front surface, and a reduction in driving voltage can be realized.
Therefore, when the modulation signal has a constant frequency, the timing point of the modulation signal and the light wave appears for each period, the polarization direction of the substrate having the electro-optic effect, the positional relationship between the optical waveguide and the control electrode, etc. By adjusting the timing of the modulation signal and the light wave without changing the signal, an in-phase state or a reverse-phase state can be realized.

Next, application examples of the optical modulator according to the present invention will be described.
FIG. 9 shows a configuration in which a more multifunctional optical modulator is formed by forming and superposing optical modulators on two substrates 40 and 40 ′.
As shown in FIG. 2, an optical waveguide 41, a modulation electrode 43, and a ground electrode 44 are formed on the front surface side of the substrate 40, and an optical waveguide 42 and DC electrodes 45 and 46 are formed on the back surface side.
On the other hand, an optical waveguide 47 is formed on the front surface side of the substrate 40 ', and an optical waveguide 48, a modulation electrode 49, and a ground electrode 50 are formed on the back surface side.

After the optical waveguide and each electrode are formed on the substrate 40 and the substrate 40 ′, the two are overlapped and fixed, so that the DC electrodes 45 and 46 on the back surface side of the substrate 40 become electrodes on the front surface side of the substrate 40 ′. It is possible to serve as both.
With this configuration, it is possible to integrate a plurality of optical modulators in a compact manner, and it is possible to omit a part of the electrode manufacturing process by sharing the electrodes. A modulator can be provided.

  Note that the multifunctional optical modulator as shown in FIG. 9 is not limited to the exemplified ones. For example, a shared electrode may be configured by a combination of a modulation electrode and a ground electrode, or a plurality of folding elements may be provided. It is also possible to make various changes such as sharing with other substrates. Further, when sharing the folded element, the light wave emitted from the optical waveguide 41 of the substrate 40 is introduced into the optical waveguide 48 of the substrate 40 ′, and the light wave emitted from the optical waveguide 42 is similarly introduced into the optical waveguide 47. It is also possible to return the light wave across the substrate.

Next, a method of extracting a part of the light wave propagating in the optical modulator as reference light is shown in FIGS.
FIG. 10A is a schematic diagram viewed from the front side of the optical modulator, and FIG. 10B is a schematic diagram viewed from the back side. Branch waveguides 61-1 and 61-2 are formed as optical waveguides on the front side of the substrate 60. Reference light is emitted from the end of the branch waveguide 61-1 toward the outside of the optical modulator. It is configured.

  On the other hand, the branch waveguide 61-2 is provided with the phase modulation electrode 63 and is optically connected to the optical waveguide 62 on the back surface side by a folding element 64. On the back side, an intensity modulation unit 63 connected to the optical waveguide 62 is formed, and the light wave phase-modulated on the front side is further intensity-modulated.

FIG. 11 is a schematic view seen from the front side of the substrate 70, and an optical waveguide in which a directional coupler 72 is connected to a Mach-Zehnder type optical waveguide 71 is formed on the substrate 70.
One of the light waves emitted from the directional coupler 72 is emitted to the outside as reference light, and the other light wave is introduced to the back surface side by the folding element 73.

  As shown in FIGS. 10 and 11, a reference for monitoring the wavelength and intensity of a light wave propagating in the optical modulator by using a branching waveguide or a directional coupler that branches a part of the optical waveguide. Light can be easily extracted.

  As described above, according to the present invention, an optical modulator having a high degree of integration and a compact optical modulator and having various functions such as driving voltage reduction, chirping suppression, and polarization independence can be achieved. Can be provided.

1 is a schematic diagram of an optical modulator according to the present invention. 1 is a cross-sectional view taken along one-dot chain line A-A ′ in FIG. 1, where FIG. It is a figure which shows the example of the optical modulator which concerns on this invention, (a) And (b) shows the thing of an in-phase state, (c) shows the thing of a reverse phase state. It is a figure which shows a polarization-independent type optical modulator. It is a figure which shows the 1st example of the electrical connection method of the optical modulator which concerns on this invention. It is a figure which shows the 2nd example of the electrical connection method of the optical modulator which concerns on this invention. It is a figure which shows the 3rd example of the electrical connection method of the optical modulator which concerns on this invention. It is a figure which shows the 4th example of the electrical connection method of the optical modulator which concerns on this invention. It is a figure which shows the example which accumulates the optical modulator which concerns on this invention. It is a figure which shows the 1st example of the method of taking out reference light from the optical modulator which concerns on this invention, (a) is schematic of front side, (b) shows schematic of back side. It is a figure which shows the 2nd example of the method of taking out reference light from the optical modulator which concerns on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Board | substrate 2-1, 2-2, 2-3, 2-4, 10 Optical waveguide 3 Folding element 4 Buffer layer 5, 5 'Modulation electrode 6, 6' Ground electrode

Claims (8)

  A substrate made of a material having an electro-optic effect, a surface-side optical waveguide formed on the surface of the substrate, a back-side optical waveguide formed on the back surface of the substrate, and a light wave propagating in the surface-side optical waveguide A front-side control electrode for controlling the phase; and a back-side control electrode for controlling the phase of the light wave propagating in the back-side optical waveguide, the at least one end surface of the substrate being the different surface The optical waveguide formed in the above has an exit and an entrance, and the folding element is disposed close to the one end surface so that a light wave emitted from the exit is incident on the entrance. Light modulator.   2. The optical modulator according to claim 1, wherein at least a part of the front-side control electrode or the back-side control electrode is formed on a plate-like body different from the substrate, and the plate-like body is attached to the substrate. An optical modulator characterized by being arranged in the vicinity of.   2. The optical modulator according to claim 1, wherein the front-side control electrode and the rear-side control electrode are electrically connected to each other, and the high-frequency signal traveling through the front-side control electrode continues to be the rear-side control electrode. An optical modulator characterized by proceeding through.   4. The optical modulator according to claim 3, wherein the electrical connection is performed by any of a line penetrating the substrate, a line formed on a side surface of the substrate, or a conductive member in contact with the substrate. An optical modulator characterized by that.   2. The optical modulator according to claim 1, wherein the front-side optical waveguide and the back-side optical waveguide have at least two optical waveguides that are optically connected to each other, and the optical modulator propagates through the two optical waveguides. The optical connections and the optical waveguides on each surface are adjusted so that the modulation by the front-side control electrode and the modulation by the back-side control electrode cause a change in refractive index of the same sign for each optical waveguide. And an arrangement of the control electrode and the shape of the control electrode are set.   2. The optical modulator according to claim 1, wherein the front-side optical waveguide and the back-side optical waveguide have at least two optical waveguides that are optically connected to each other, and the optical modulator propagates through the two optical waveguides. The optical connections and the optical waveguides on each surface are such that the modulation by the front-side control electrode and the modulation by the rear-side control electrode cause a change in the refractive index of the opposite sign to the respective optical waveguides. And an arrangement of the control electrode and the shape of the control electrode are set.   The optical modulator according to claim 1, wherein the front-side optical waveguide and the back-side optical waveguide have at least one optical waveguide that is optically connected to each other, and with respect to a light wave that propagates through the optical waveguide, The arrangement of the optical waveguide and the control electrode on each surface and the control electrode so that the polarization plane mainly modulated by the front-side control electrode and the polarization plane mainly modulated by the back-side control electrode are different. An optical modulator having a set shape. 8. The optical modulator according to claim 5, wherein the optically connected optical waveguide passes through a substrate region having different polarization directions on the front surface side and the back surface side. Modulator.

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