JP2007025370A - Organic waveguide type optical modulator and optical communications system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic waveguide type optical modulator which is free of signal deterioration and can be driven at high speed at a lower voltage. <P>SOLUTION: A Mach-Zehnder optical modulator is shown as an interference type optical modulator, as an example, an optical waveguide 11 to be a passive part comprises a polymer and DAST (4-dimethylamino-N-methyl-4-stilbazolium tosylate) crystals are selectively disposed at both branched optical waveguides 11a and 11b. The organic waveguide type optical modulator 10 has a modulation part 12, to which high frequency modulation signal voltage is applied from a signal power source, a bias-applying part 13 for applying DC voltage for setting an appropriate optical bias, and ground electrodes 17 and 18. Crystal axes of the DAST crystals, disposed at the two branched waveguides, are in the same direction and an electric field formed by a bias electrode 16, and the ground electrode 17 is in a direction opposite to the direction of the crystal axes of the DAST crystals. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a waveguide type optical modulator, and more particularly, to an organic waveguide type optical modulator that applies an electro-optic effect of an organic material and is driven at a low voltage.

Due to the rapid development of the information society represented by the Internet, the demand for high-capacity and high-speed optical communication is increasing in order to exchange more information. For this reason, information transmission using light has been proposed not only for trunk-line networks, but also for LANs, network terminals and electronic devices, boards, and LSI chips.
In this optical interconnection, as a device for modulating a data signal from an electrical signal to an optical signal, there is a waveguide type optical modulator using an electro-optic effect. The electro-optic effect is a phenomenon in which the refractive index of the medium changes when an electric field is applied to the optical medium, and includes a Pockels effect that is a refractive index change proportional to the electric field and a Kerr effect that is proportional to the square of the electric field. .

As an example of a modulator used in a conventional backbone system, there is a Mach-Zehnder type light intensity modulator using an inorganic ferroelectric crystal LiNbO ₃ (LN). The optical waveguide portion is formed by diffusing Ti ions or the like into an LN crystal cut out in an appropriate orientation, thereby forming a region having a large refractive index as a core layer, and an LN crystal without Ti diffusion or a buffer layer of a silicon oxide film. The cladding layer is used.
An electric field from the outside is a traveling wave electrode made of a metal such as Au or Cu, and is applied to the modulation electrode of the planar stripline. Incident light is branched into two ports at the Y branch on the entrance side, and light propagating through the ports interacts with microwaves applied to the electrodes to cause a phase change. The light causing the phase change is superimposed on the light propagating on the other side by the Y branch on the exit side, and the combined wave causes an intensity change due to the phase change and is converted into an optical signal corresponding to the electrical signal. .

However, in optical interconnections connecting devices, boards, and LSI chips, the power supply voltage of LSI chips mounted on them is constantly decreasing, and noise is applied to mounted LSIs. The optical modulator is also required to be driven at a low voltage because it is not applied and the power consumption is low. Further, as a matter of course, there is a demand for speeding up to 40 GHz and 80 GHz, which cannot be achieved by direct driving of the LD, and the conventional optical modulator using LN has a large dielectric constant ε = 28, so it is faster. even when the electric signal was the electrode of the traveling waveform can be applied, the applied microwave refractive index of the refractive index of the _{(n o = 2.286, n e} = 2.200) that there is a shift in Therefore, there is a limit to speeding up the material.
The driving voltage is defined by the electro-optic constant and the electrode structure. For low-voltage driving, a material having a larger electro-optic constant such as LN electro-optic constant r ₃₃ = 30.8 (wavelength 633 nm) is required. It has been. Furthermore, as used in backbone systems, LN optical modulators are discrete devices, and devices that are more suitable for integration are also required.

Thus, organic crystals and organic polymer materials are expected as low-voltage and high-speed optical modulators because they have a smaller dielectric constant than inorganic materials, can be made thin, and have a large electro-optic constant. In addition, since it is an organic material, it can be processed at a low temperature and can be integrated with an optical waveguide.
As the organic polymer material, a second-order nonlinear optical material having an electro-optic effect dissolved in a polymer such as polymethyl methacrylate (PMMA) or amorphous polycarbonate (APC), or a second-order nonlinear optical material directly or There are some which are bonded to a polymer chain via a spacer atomic group, and an optical modulator using such an organic polymer material having an electro-optic effect as a core layer is known.
One example is PMMA doped with an azo dye (Journal of Optical Society of America, B4 (1987) 968).

These organic polymer materials are heated to a temperature higher than the glass transition point, and the orientation is fixed by a poling treatment such as corona charging, so that the electro-optic effect is exhibited. There is a problem that the reliability as an optical modulator is inferior because it is relaxed by temperature and changes with time.
Therefore, although heat resistance improvement by APC having a glass transition point higher than that of PMMA has been studied (Appl. Phys. Lett., 78 (2001) 3136), it is not sufficient, and heat resistance and changes with time are still not achieved. It has become a challenge.
On the other hand, with respect to organic crystals, 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST), whose structural formula is shown in FIG. 5, was developed in the Nakanishi Laboratory of Tohoku University and has an extremely large nonlinear optical constant d. ₁₁ = 1010 pm / V (λ = 1.3 μm) and electro-optic constant r ₁₁ = 75 pm / V (λ = 820 nm), and because of the low dielectric constant (ε ₁ = 5.2) peculiar to organic crystals , Low voltage, high speed light modulation and detection, millimeter wave (or submillimeter wave) generation and so on.

Since it is an organic crystal, it does not require a polarization treatment unlike an organic polymer, and is stable without change over time such as polarization relaxation. There is a report (M. Thakur et al., Appl. Phys. Lett., Vol. 74 (1999) 635) in which a DAST crystal thin film was actually formed by the improved shear method and the principle of light modulation was confirmed. However, since the working length is short, there are problems such as a small modulation depth.
Therefore, the present applicant has proposed a waveguide type optical modulator using DAST in Japanese Patent Application Laid-Open No. 2003-202533. In the present invention, a core groove is formed in a lower clad portion, an organic crystal DAST showing an electro-optic effect is embedded therein, chemical mechanical polishing is performed using a solvent that does not deteriorate the characteristics of DAST, and DAST other than the groove portion is formed. A waveguide type optical modulator is realized by removing DAST and forming DAST in the core portion.

FIG. 6 shows a conventional optical modulator disclosed in Japanese Patent Laid-Open No. 5-142504. An optical waveguide 101 is formed by thermally diffusing Ti in the Z-axis cut LN substrate 100, and each electrode is formed in the optical waveguide 101 to constitute an optical modulator.
1 shows a conventional optical modulator disclosed in Japanese Patent Laid-Open No. 5-142504. An optical waveguide 101 is formed by thermally diffusing Ti in the Z-axis cut LN substrate 100, and each electrode is formed in the optical waveguide 101 to constitute an optical modulator. Of the pair of branch waveguides 101a and 101b, the signal electrode 102 and the bias electrode 103 are formed on one branch waveguide 101a, and the ground electrode 104 corresponding to the signal electrode 102 and the bias electrode 103 is formed on the other branch waveguide 101b. ing.
FIG. 7 shows a conventional optical modulator disclosed in Japanese Patent Laid-Open No. 2003-233042. An interference-type optical waveguide 101 is formed on a substrate 100 having an electro-optic effect that is cut in the Z axis, and includes a signal electrode 102 and a bias electrode 103. The bias electrode 103 has the same absolute value and the opposite sign. It is stipulated that the configuration is such that a voltage is applied, an effective bias electric field is applied, the applied drive voltage is reduced, and the operation cost of the communication system can be reduced.
As a specific electrode, a comb-like bias electrode 103 is formed on the optical waveguide 101, and a vertical bias electrode is formed on the substrate 100.
In FIG. 7, reference numeral 104 denotes a ground electrode, 105 denotes a first bias electrode, 106 denotes a second bias electrode, 107 denotes a first electrode on the waveguide, 108 denotes a first potential supply electrode, and 109 denotes a first potential supply electrode. 2 represents an electrode on the waveguide, 110 represents a second potential supply electrode, 111 represents a power source, and 112 represents an RF signal generation source.

JP-A-5-142504 JP 2003-233042 A JP 2003-202533 A Journal of Optical Society of America, B4 (1987) 968 Appl. Phys. Lett. 78 (2001) 3136 Appl. Phys. Lett. , Vol. 74 (1999) 635

By the way, in such an interference type optical modulator, it is necessary to apply a DC bias for setting an optical bias. However, as in the case of an organic waveguide type optical modulator, a high speed exceeding 40 GHz is required. According to the optical modulation, it is difficult to superimpose and apply the high frequency modulation signal and the DC bias without deteriorating the high frequency signal, and a costly circuit or device is required. there were.
In the configuration described in Patent Document 1, since the signal electrode and the bias electrode are separated from each other, the optical modulator can be driven without a capacitor, but there is a detailed description of DC drift control using the bias electrode. However, it does not describe how to apply an effective electric field.
In the configuration described in Patent Document 2, since the complicated electrode structure is in the same plane, there is a concern that a short circuit between the electrodes may occur and the yield may be reduced, and it is difficult to effectively form a vertical electric field. On the other hand, in order to effectively apply an electric field perpendicular to the substrate to the optical waveguide and to obtain impedance matching, the LN substrate is processed, and the vicinity of the optical waveguide is formed into a convex ridge shape, with respect to the electrode on the waveguide. Although the LN of the substrate may be processed so that the potential supply electrodes are not on the same plane, there are problems in reducing the voltage and increasing the speed.

SUMMARY OF THE INVENTION An object of the present invention is to provide an organic waveguide type optical modulator capable of being driven at a high speed and a low voltage without causing signal degradation, and an object thereof.
Another object of the present invention is to provide an optical communication system that can be driven at a low voltage and has a large data transmission capacity.

  In order to achieve the above object, according to the first aspect of the invention, an electro-optic effect medium in which light can be incident and emitted, the refractive index is changed by an externally controlled electric field, and the phase of the light is changed. An optical waveguide having an organic material as a core layer and an electrode for changing the phase of light, and interfering with light propagating on a plurality of optical waveguides. Each of the plurality of optical waveguides is provided with a signal electrode for applying a modulation signal, a bias electrode for biasing an applied signal component, and a ground electrode.

  According to a second aspect of the present invention, in the organic waveguide optical modulator according to the first aspect, the organic material having the electro-optic effect is 4-dimethylamino-N-methyl-4-stilbazolium It is a rate (DAST).

  According to a third aspect of the present invention, in the organic waveguide type optical modulator according to the second aspect, the DAST crystal axis has two branching waveguides that interfere with each other and is disposed in the two branching waveguides. Are in the same direction, and the electric field formed by the bias electrode and the ground electrode is opposite to the crystal axis of the DAST.

  According to a fourth aspect of the present invention, in the organic waveguide type optical modulator according to the third aspect, the first and second bias electrodes are provided as the bias electrodes, and the absolute values of these bias electrodes are equal in sign. Applies the opposite voltage.

  According to a fifth aspect of the present invention, in the organic waveguide type optical modulator according to the third or fourth aspect, the bias electrode and the ground electrode are substantially symmetrical with respect to a center line of the two branch waveguides. It is arranged.

  According to a sixth aspect of the present invention, in the organic waveguide type optical modulator according to the fifth aspect, the signal electrode and the corresponding ground electrode are substantially symmetric with respect to the center line of the two branch waveguides. It is characterized by being arranged.

  According to a seventh aspect of the present invention, in the organic waveguide type optical modulator according to any one of the second to sixth aspects, the propagation direction of the optical waveguide is a b-axis direction of the DAST crystal, and The voltage application by the signal electrode and the bias electrode is in the a-axis direction of the DAST crystal.

  According to an eighth aspect of the present invention, in the organic waveguide type optical modulator according to any one of the second to seventh aspects, the bias electrode and the DAST disposed on the core layer are disposed on substantially the same plane. It is characterized by being.

  According to a ninth aspect of the present invention, in the organic waveguide type optical modulator according to the second aspect of the present invention, the DAST crystal axis includes two branching waveguides that interfere with each other and is disposed in the two branching waveguides. Are in the same direction, and the electric field formed by the signal electrode and the ground electrode is in the opposite direction to the crystal axis of the DAST, and is driven by push-pull drive.

  According to a tenth aspect of the present invention, in the organic waveguide optical modulator according to any one of the first to ninth aspects, the refractive index of the organic material having the electro-optic effect is controlled, and the signal electrode is It is a traveling wave type electrode.

  According to an eleventh aspect of the present invention, in the optical communication system, the organic waveguide type optical modulator according to any one of the first to tenth aspects is used as a component of the light output section.

According to the first aspect of the present invention, there is no need for a device such as a circuit for superimposing a DC bias and a high-frequency AC signal or a bias T, and there is no deterioration in the high-frequency signal. An organic waveguide type optical modulator that can be driven and has high reliability can be provided.
According to the second aspect of the present invention, it is possible to provide an organic waveguide type optical modulator that is faster, driven at a lower voltage, and has high reliability.
According to the third aspect of the present invention, since the phase difference can be doubled with a single power source and the optical bias can be set at a low voltage, the organic conductive circuit with a low driving voltage is maintained while maintaining a high speed. A waveguide type optical modulator can be provided.
According to the fourth aspect of the present invention, an optical bias at a lower voltage can be set, and an organic waveguide type optical modulator driven at a low voltage can be provided while maintaining a high speed.

According to the fifth aspect of the present invention, the distortion due to the thermal expansion of the metal electrode and the organic material can be made symmetric, the operating point shift is less likely to occur, and more reliable while maintaining high-speed optical modulation with low voltage driving. It is possible to provide an organic waveguide type optical modulator having a high height.
According to the sixth aspect of the present invention, heat generation and heat dissipation due to current are equal in the portion where the high-frequency signal is applied to the two branch waveguides, and no temperature difference occurs between the two branch waveguides. It is possible to provide an organic waveguide type optical modulator with higher reliability while maintaining high-speed optical modulation with low voltage drive.
According to the seventh aspect of the invention, since the largest electro-optic constant of DAST can be used, it is possible to provide an organic waveguide type optical modulator driven at a low voltage while maintaining a high speed. .
According to the invention described in claim 8, a bias voltage can be effectively applied, an optical bias at a lower voltage can be set, and an organic waveguide driven at a low voltage while maintaining a high speed. Type optical modulator can be provided.
According to the ninth aspect of the present invention, it is possible to provide an organic waveguide type optical modulator that can be driven at a lower voltage while maintaining high speed.
According to the invention described in claim 10, since a high-speed signal voltage can be applied, a higher-speed organic waveguide type optical modulator can be provided.
According to the eleventh aspect of the present invention, it is possible to provide an optical communication system that can be driven at a low voltage and has a large data transmission capacity.

Hereinafter, a first embodiment of the present invention will be described with reference to FIG.
First, before describing the specific configuration, consideration will be given to the characteristics and usage modes of DAST.
The bulk crystal of DAST takes a rhomboid plate-like crystal form that is large in the a and b axis directions and small in the c axis direction. In the case of a DAST thin film crystal, there are a and b axes in the film plane, and the c axis is perpendicular to the film. DAST having such a geometric structure can easily form a and b axes in a plane parallel to the substrate.
Largest electro-optic constant of DAST are the r _11, the light is propagated b axis, preferably an electric field is applied to the a-axis direction. Therefore, the electric field formed by the bias electrode and the ground electrode (and the electric field formed by the signal electrode and the ground electrode) is horizontal to the substrate, and a reverse electric field is applied to each branch waveguide with the simple structure shown below. be able to. Further, by forming an electrode embedding groove on the substrate, a bias electrode and a ground electrode can be formed in substantially the same plane as the DAST optical waveguide, and a more effective electric field can be formed.

In X-cut substrate of a conventional LN, thus, it is possible to form an electrode in the form sandwiching the optical waveguide, since the dielectric constant is large, the effective refractive index n _m of the microwave to the refractive index n ₀ of the light On the other hand, the band is limited by the speed mismatch between the microwave and the light wave, and increasing the speed is an issue.
On the other hand, the dielectric constant of DAST is as small as ε ₁ = 5.2, and the materials for forming the cladding layer around the electrode are polyimide resin, epoxy resin, PMMA, acrylate ester resin, silicon having a low dielectric constant. A traveling-wave electrode with a large electric field confinement without using a band limit due to mismatch of the speed of the microwave and the light wave by using an inorganic film having a low dielectric constant such as SiO ₂ or a resin such as resin or cyanurate resin. Can be formed.

On the other hand, DAST has an absorption coefficient between wavelengths of 750 nm and 1600 nm of about 0.5 cm ^−1, and it is not preferable to use DAST for the part where light is simply demultiplexed and combined, because transmission loss increases. Therefore, only a portion necessary for the refractive index change due to the electro-optic effect is formed by DAST, and the other portions are formed by an optical waveguide with less optical loss, thereby providing a low-loss optical modulator. .
This as a low loss optical waveguide, polyimide resin or epoxy resin, PMMA, acrylate resin, silicone resin, objects or SiO ₂ or Si made of resin, such as cyanurate resin, an optical waveguide using an inorganic film such as SiN, Furthermore, an optical waveguide using a photonic crystal having a periodic fine structure may be used.
In addition, when all of the optical waveguide has a field effect, such as LN, an electric field is applied to the optical waveguide even in an unexpected part depending on the arrangement of the electrodes, resulting in a phase change. By arranging them only, the phase change can be easily controlled.

In addition, “the DAST disposed on the bias electrode and the core layer is substantially in the same plane” is defined by the branched optical waveguide in which the DAST is disposed in an electrode arrangement in which a voltage is applied in a lateral direction parallel to the substrate. It is shown that a bias electrode and a ground electrode, or a first bias electrode and a second bias electrode are formed in the surface to be formed.
The branched optical waveguide has a thickness of about several to several tens of μm, and is not exactly a plane, but the plane defined by the two branched optical waveguides, the bias electrode and the ground electrode, or the first A state in which the bias electrode and the second bias electrode intersect is indicated.
Therefore, the thicknesses of the bias electrode and the ground electrode, or the first bias electrode and the second bias electrode are not defined by the thickness of the optical waveguide, and the upper and lower surfaces of the branched optical waveguide are used for impedance matching. If the two planes defined by the above and the bias electrode and the ground electrode, or the first bias electrode and the second bias electrode intersect, they may be formed thicker above or below.

With such a configuration, since a large electric field formed on the bias electrode is applied to DAST, an optical bias can be set at a lower voltage.
Further, since the bias electrode and the ground electrode are arranged symmetrically with respect to the center line of the two branch waveguides, the metal electrode is formed in the portion where the phase change occurs due to DAST of both branch optical waveguides. The strain due to thermal expansion of the organic material can be made symmetric, and the operating point shift hardly occurs. As used herein, “symmetric with respect to the center line of the two branch waveguides” means a cross section at the center line that is parallel to the optical waveguides on the input and output sides and that is the center of symmetry of the two branch optical waveguides. On the other hand, the bias electrode and the ground electrode are arranged symmetrically, and the shape and area thereof are formed to be equal.

Similarly, the traveling wave type electrode is composed of a traveling wave signal electrode for inputting a high-frequency pulse signal and a ground electrode. The traveling wave signal electrode and the ground electrode are symmetrical in area and shape with respect to the two branch waveguides. With the arrangement, the heat generated by the high-frequency signal current is equal and the temperature difference does not occur in the portion where the phase change occurs due to DAST of both branch optical waveguides, so that the operating point shift called temperature drift does not occur.
In the conventional examples described in Patent Documents 1 and 2, either the bias electrode and the ground electrode or the signal electrode and the ground electrode are asymmetric with respect to the center line AA ′ of the two branching waveguides. A point shift is likely to occur.

Hereinafter, a first embodiment of the present invention will be specifically described with reference to FIG.
Here, as an example of the interference type optical modulator, a Mach-Zehnder type is shown, and the optical waveguide 11 that is a passive portion is formed of a polymer, and both branched optical waveguides 11a that are active portions with a refractive index change by an external electric field, A DAST is selectively arranged at 11b.
The optical path lengths are assumed to be equal.
The organic waveguide type optical modulator 10 includes a modulation unit 12 to which a high frequency modulation signal voltage is applied from a signal power source, and a bias application unit 13 to apply a DC voltage in order to set an appropriate optical bias.
The CW light from the semiconductor laser enters from the incident-side optical waveguide 11 and is divided into two branch optical waveguides 11a and 11b by the Y branch, and when passing therethrough, the high frequency modulation signal voltage applied to the signal electrode 14 A phase difference is generated by the organic crystal DAST that is an electro-optic effect medium selectively disposed in the branching waveguides 11a and 11b.

Next, a DC voltage corresponding to an optical bias is applied, and the light that has caused the phase change is superimposed on the light propagating through the other side by the Y branch on the exit side, and the combined wave changes in intensity due to the phase change. And is converted into an optical signal corresponding to the electrical signal.
In this example, the bias applying unit is formed at the subsequent stage of the optical modulator 10, but the reverse is also possible.
The bulk crystal of DAST takes a rhomboid plate-like crystal form that is large in the a and b axis directions and small in the c axis direction. Further, as described in Non-Patent Document 3, in the case of a DAST thin film crystal, there are a and b axes in the film plane, and the c axis is perpendicular to the film.
DAST having such a geometric structure can easily form a and b axes in a plane parallel to the substrate 15. Since the electrooptic constant r ₁₁ having the largest DAST is used, it is preferable that the light propagates in the b-axis, an electric field is applied in the a-axis direction, and the polarization plane of incident light is incident so as to be parallel to the a-axis.
Therefore, the DAST optical waveguide 11 is grounded to the bias electrode 16 so that the electric fields formed by the bias electrode 16 and the ground electrode 17 and the traveling wave type signal electrode 14 and the ground electrode 18 are also parallel to the substrate 15. It is preferable to arrange the electrodes 17 and the shape sandwiched between the signal electrode 14 and the ground electrode 18 (FIG. 1).

Further, a bias electrode 16 is disposed at the center of the branch optical waveguides 11a and 11b, and ground electrodes 17 and 18 are disposed outside the branch optical waveguides 11a and 11b, thereby applying a DC electric field in the opposite direction to each DAST. Since the phase change can be doubled, the optical bias can be set with a lower voltage.
Further, the bias electrode 16 and the ground electrode 17 are arranged symmetrically with respect to the center line AA ′ of the two branch waveguides 11a and 11b.
On the other hand, similarly, with respect to the modulation unit 12, the traveling wave signal electrode 14 is disposed at the center of the branch optical waveguide, and the ground electrode 18 is disposed outside the branch optical waveguides 11 a and 11 b. Can be applied, and a double phase change can be caused by the push-pull operation, which enables driving at a lower voltage.

The signal electrode 14 is a traveling wave electrode provided with a 50Ω termination resistance, and has an electrode structure that can cope with high-speed light modulation. Further, the signal electrode 14 and the ground electrode 18 are arranged symmetrically with respect to the center line AA ′ of the two branch waveguides 11a and 11b.
The bias application unit 13 may monitor the light intensity in order to cope with a temperature change and a DC drift, and may change the DC voltage by a feedback circuit.
A polyimide substrate is used as the substrate 15, and a polyimide resin serving as a cladding layer of the optical waveguide 11 is spin-coated on the substrate 15. Furthermore, using a Mach-Zehnder waveguide pattern mask, reactive ion etching using photolithography and oxygen gas was performed to form a core groove.
Next, a polyimide resin having a large refractive index serving as a core is formed in the groove to form a low-loss optical waveguide. Further, by using lithography and etching, the polyimide serving as the core is removed only in a selected portion of the branched optical waveguide where DAST is selectively formed, and a core groove having an equal length is formed again.
Here, DAST was crystal-grown by a slow cooling method using methanol as a solvent. Regarding the crystal orientation, the DAST single crystal is embedded in a core groove so that the waveguide direction is the b-axis, the a-axis is in a plane parallel to the substrate 15, and the c-axis is the direction perpendicular to the substrate 15. Crystals were formed. At this time, crystal grown DAST exists in addition to the core groove. Therefore, unnecessary portions of DAST are removed by polishing.

Polishing can be performed using a urethane foam pad, Supreme RN-H (Rodel), using silica abrasive particles to remove DAST other than the core groove and leave DAST only in a specific core groove. . The length of the DAST selectively arranged in the branch optical waveguide was 15 mm and the thickness was 6 μm.
Here, the DAST layer is formed by a slow cooling method using a seed crystal, but the DAST thin film may be formed by an improved shear method as described in non-patent literature.
This upper layer was formed by spin-coating the same polyimide as the lower cladding layer to form an upper cladding layer having a thickness of 2 μm.
Next, the Au electrode was formed to a thickness of 8 μm by plating, and the bias electrode 16, the ground electrodes 17 and 18, and the signal electrode 14 were formed. The signal electrode 14 and the ground electrodes 17 and 18 were each provided with a terminal resistance of 50Ω to form a traveling wave type signal electrode.

At this time, as shown in FIG. 1, the bias electrode 16 and the signal electrode 14 were formed in the center of the branch waveguides 11a and 11b, and the ground electrodes 17 and 18 were formed outside the branch waveguides 11a and 11b.
In this way, both the modulation unit 12 and the bias application unit 13 can apply a reverse electric field to the DAST and cause a double phase change, so that the optical bias application and modulation can be performed at a lower voltage. Is possible.
Further, the bias electrode 16 and the ground electrode 17 are arranged symmetrically with respect to the center line AA ′ of the two branch waveguides 11a and 11b, so that the distortion due to the thermal expansion of the metal electrode and the organic material can be made symmetrical. Further, since the signal electrode 14 and the ground electrode 18 are also arranged symmetrically with respect to the center line AA ′ of the two branch waveguides 11a and 11b, a phase change occurs due to DAST of both branch optical waveguides 11a and 11b. In this portion, heat generated by the high-frequency signal current is equal, and no temperature difference is generated, so that an operating point shift called temperature drift does not occur.

Thus, a Mach-Zehnder type organic waveguide type optical modulator 10 was produced. When a modulation signal is input between electrodes via a coaxial cable while a laser beam having a wavelength of 1.3 μm is incident, a low DC voltage of +1 V is supplied as an optical bias setting, and a low driving voltage of 1.2 V is used as the modulation voltage. The optical modulator could be driven by voltage.
Since a traveling wave type electrode is used, high-speed light intensity modulation of 40 GHz can be provided. Further, since the modulation unit 12 performs a push-pull operation, optical modulation without chirp can be realized.
In FIG. 1, reference numeral 19 denotes a signal power source, and 20 denotes a bias power source.

FIG. 2 shows a second embodiment. Note that the same parts as those in the above embodiment are denoted by the same reference numerals, and unless otherwise specified, description of the configuration and functions already described is omitted, and only the main part will be described (the same applies to other embodiments below).
This embodiment is different from the first embodiment in that a separate bias power source is connected to the bias electrode of the bias applying unit 13 so that the potentials are equal in absolute value and opposite in sign. ing. Other configurations are the same as those of the first embodiment.
A first bias electrode 21 is disposed at the center of the branch optical waveguide, and a second bias electrode 22 is disposed outside the branch optical waveguides 11 a and 11 b so as to face the first bias electrode 21. In FIG. 2, reference numeral 23 denotes a first bias power source, and 24 denotes a second bias power source.

Of V _{= V} 0/2 to the first bias electrode 21, by applying a potential of V = _-V 0/2 to the second bias electrode 22, each branch waveguide 11a the potential difference in the first embodiment similarly _{V 0} , 11 b can be applied with a reverse DC electric field, and a double phase change can be generated. Therefore, the voltage is as low as ± 0.5 V, which is a half voltage of the first embodiment. The optical bias can be set with a DC voltage.
Also in this embodiment, the bias electrodes 21 and 22 are symmetrically arranged with respect to the center line AA ′ of the two branching waveguides 11a and 11b, and the signal electrode 14 and the ground electrode 18 are also symmetrically arranged.
Here, the V _{= V} 0/2 to the first bias electrode 21, gave the potential of V = _-V 0/2 to the second bias electrode 22, the first bias electrode 21 V = _-V 0/2 The potential of V = V _0/2 may be applied to the second bias electrode 22.

FIG. 3 shows a third embodiment.
3A is a top view of the organic waveguide type optical modulator 10, and FIG. 3B is a cross-sectional view of the bias applying unit 13 taken along line BB ′.
In the first embodiment, the bias electrode 16 and the ground electrode 17 are formed on the clad of the optical waveguide 11. However, in the present embodiment, the bias electrode 16 and the ground electrode 17 are branched waveguides 11a in which DAST is arranged. , 11b and substantially in the same plane.
As shown in FIG. 3B, the bias electrode 16 and the ground electrode 17 are formed deeper than the plane defined by the lower surfaces of the two branch optical waveguides. In FIG. 3A, hatched portions indicate “deeply formed portions” of the bias electrode 16 and the signal electrode 14.
As the substrate 15, a Si substrate on which a thermal oxide film was formed was used, an epoxy resin was used as the cladding layer 25, and an epoxy resin having a higher refractive index than that of the cladding layer 25 was used as a core layer other than DAST.

The optical waveguide 11 was formed by lithography and etching according to a known technique, and then a core groove for selectively disposing DAST was formed.
Here, as in the first embodiment, DAST was crystal-grown by a slow cooling method using methanol as a solvent. Regarding the crystal orientation, the DAST single crystal was formed by the seed crystal with the core groove embedded so that the waveguide direction was the b-axis and the a-axis was in a plane parallel to the substrate. At this time, DAST other than the core groove can be removed by polishing, and the DAST can be left only in the specific core groove.
Here, the length of the DAST selectively disposed in the branch optical waveguide is 12 mm and the thickness is 6 μm. The same epoxy resin as the lower clad layer was formed to a thickness of 2 μm as the upper clad layer.

Next, the bias electrode 16 and the ground electrode 17 are formed on both sides of the two branch optical waveguides 11a and 11b on which DAST is arranged so as to be substantially in the same plane. At this time, the traveling waveform signal electrode 14 and the ground electrode 18 are also formed simultaneously.
First, a portion of the bias electrode 16 indicated by hatching in FIG. 3A and a groove corresponding to the ground electrode 17, the signal electrode 14, and the ground electrode 18 are formed by lithography and etching using a known technique.
The groove depth was formed about 2 μm deep from the bottom of the DAST optical waveguide so that the branched optical waveguides 11a and 11b, the bias electrode 16 and the ground electrode 17 were substantially in the same plane. Au was buried in the electrode grooves by vapor deposition, and unnecessary Au was removed by damascene method to form a bias electrode 16 having a thickness of 10 μm, a ground electrode 17, a traveling waveform signal electrode 14, and a part of the ground electrode 18.
“The bias electrode and the DAST arranged in the core layer are substantially in the same plane” is defined by the branch optical waveguide in which the DAST is arranged in an electrode arrangement in which a voltage is applied in the lateral direction parallel to the substrate 15. The bias electrode 16 and the ground electrode 17 are formed in a plane (in substantially the same plane indicated by a one-dot chain line in FIG. 3B).

Next, in FIG. 3A, the electrode at the portion that intersects the branched optical waveguides 11a and 11b, the T-shaped lead portion that intersects the Y-branch on the light emission side of the bias electrode 16, and the T-shaped lead portion of the signal electrode 14 The terminal electrode was formed on the upper clad layer, and the terminal end of the signal electrode 14 was provided with a 50Ω termination resistor to form a traveling wave type electrode.
Thus, a Mach-Zehnder type organic waveguide type optical modulator 10B was produced. The bias electrode 16 and the ground electrode 17, the signal electrode 14 and the ground electrode 18 are arranged symmetrically with respect to the center line AA ′ of the two branch optical waveguides 11a and 11b.
When a modulation signal is input between electrodes via a coaxial cable while a laser beam having a wavelength of 1.3 μm is incident, an electric field can be applied more effectively because the bias electrode 16 and DAST are in substantially the same plane. The optical bias can be set with a DC voltage as low as +0.8 V, the optical modulator can be driven with a low driving voltage, and since the traveling wave type electrode is used, high-speed light intensity modulation of 40 GHz can be performed.

FIG. 4 shows a fourth embodiment.
It is an example of the optical communication system using the organic waveguide type optical modulator mentioned above. In this example, an optical interconnection between a plurality of ICs is realized on a board made of glass epoxy.
The optical interconnection is used for a high-speed signal line, and a low-speed signal line is not shown, but an electric wiring is used as usual.
An optical communication system 30 includes an optical output unit 31 for converting an electrical signal from an IC into an optical signal and outputting the optical signal, an optical transmission path 32 for transmitting the optical signal, and converting the transmitted optical signal into an electrical signal. It comprises an optical input unit 33 for this purpose.
Here, the light output unit 31 includes a light source 34 formed of a semiconductor laser, an optical waveguide 35 that guides CW light from the light source 34 to the organic waveguide optical modulator 10, an electric circuit that serially processes an electric signal from the IC, and an organic circuit. An electric signal processing unit 36 composed of a driver for driving the waveguide type optical modulator 10, an organic waveguide type optical modulator 10 for converting an electric signal from the electric signal processing unit 36 into an optical signal, and electric between them It consists of electrical wiring 39 for transmitting signals. The organic waveguide type optical modulator 10 uses the same organic waveguide type optical modulator as that shown in the first embodiment.

On the other hand, the optical input unit 33 again receives a signal received by the light receiving element 37 for receiving the transmitted signal and an amplifier for amplifying the signal from the light receiving element 37 and the light receiving element 37 as necessary. The electric signal processing unit 38 made of an electric circuit for converting the bit into the bit and the electric wiring 39 for transmitting an electric signal between them.
A single mode optical fiber was used as the optical transmission line. Here, a single-mode quartz optical fiber is used as the optical transmission line, but depending on the application, such as a short-distance transmission at a relatively low speed, a multi-mode optical fiber or POF, or in-board optical communication The optical waveguide made of quartz or resin can be used as appropriate.
In this embodiment, each of the boards 40 and 41 is provided with two optical outputs and an optical input to enable bidirectional optical transmission. However, the present invention is not limited to this. It is not limited to a simple configuration.

An optical communication system having at least an optical output unit using the organic waveguide type optical modulator 10 and performing only one-way transmission having only a pair of optical output units and an optical input unit may be used. Further, in this example, an optical communication system between different boards is shown, but an IC may be in the same board.
Here, only one organic waveguide type optical modulator is formed. However, a plurality of optical waveguide optical modulators may be arrayed as necessary to enable wider-band communication.
4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) which has a very large electro-optic constant and a small dielectric constant and is an organic crystal in the light output portion 31 Is used as a medium for the electro-optic effect, so that the signal electrode for applying the modulation signal is separated from the bias electrode for biasing the applied signal component by separating the signal electrode for applying the modulation signal from the high speed, low voltage drive and high reliability. The organic waveguide type optical modulation that can set the optical bias with a low bias voltage by applying the electric field formed by the two in the opposite direction to the crystal axis of the DAST crystal disposed in the two branching waveguides that interfere with each other By using the optical device, it is possible to provide an optical communication system that can be driven at a low voltage and has a large data transmission capacity.

It is an outline top view of an organic waveguide type optical modulator in a 1st embodiment of the present invention. It is a general | schematic top view of the organic waveguide type optical modulator in 2nd Embodiment. It is a figure which shows the organic waveguide type optical modulator in 2nd Embodiment, (a) is a schematic plan view, (b) is sectional drawing in the B-B 'line of (a). It is a block diagram of the optical communication system in 4th Embodiment. It is a figure which shows the structural formula of DAST. It is a general | schematic top view of the conventional optical modulator. It is a general | schematic top view of the other conventional optical modulator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Organic waveguide type optical modulator 11 Optical waveguide 11a, 11b Branching waveguide 14 Signal electrode 16 Bias electrode 17, 18 Ground electrode 21 1st bias electrode 22 2nd bias electrode

Claims (11)

An optical waveguide that has an organic material in the core layer as an electro-optic effect medium that changes the refractive index by an externally controlled electric field and changes the phase of the light. An interference type organic waveguide optical modulator that interferes with light propagating on a plurality of optical waveguides.
An organic waveguide type optical modulator comprising a signal electrode for applying a modulation signal, a bias electrode for biasing a signal component to be applied, and a ground electrode for each of the plurality of optical waveguides. The organic waveguide optical modulator according to claim 1,
The organic material having an electro-optic effect is 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST). The organic waveguide type optical modulator according to claim 2,
The DAST has two branching waveguides that interfere with each other, the DAST crystal axes arranged in the two branching waveguides are in the same direction, and the electric field formed by the bias electrode and the ground electrode is An organic waveguide type optical modulator characterized by being opposite to the crystal axis. The organic waveguide type optical modulator according to claim 3,
An organic waveguide type optical modulator having first and second bias electrodes as the bias electrodes, and applying a voltage having the same absolute value and opposite signs to these bias electrodes. In the organic waveguide type optical modulator according to claim 3 or 4,
An organic waveguide type optical modulator, wherein the bias electrode and the ground electrode are disposed substantially symmetrically with respect to a center line of the two branch waveguides. The organic waveguide type optical modulator according to claim 5,
An organic waveguide type optical modulator, wherein the signal electrode and a ground electrode corresponding to the signal electrode are arranged substantially symmetrically with respect to a center line of the two branch waveguides. In the organic waveguide type optical modulator according to any one of claims 2 to 6,
The propagation direction of the optical waveguide is the b-axis direction of the DAST crystal, and the voltage application by the signal electrode and the bias electrode is the a-axis direction of the DAST crystal. Light modulator. In the organic waveguide type optical modulator according to any one of claims 2 to 7,
An organic waveguide type optical modulator, wherein the bias electrode and the DAST arranged in the core layer are arranged in substantially the same plane. The organic waveguide type optical modulator according to claim 2,
The DAST includes two branch waveguides that interfere with each other, the crystal axes of the DAST arranged in the two branch waveguides are in the same direction, and the electric field formed by the signal electrode and the ground electrode is An organic waveguide type optical modulator characterized in that it is in a direction opposite to a crystal axis and is driven by push-pull driving. In the organic waveguide type optical modulator according to any one of claims 1 to 9,
An organic waveguide type optical modulator, wherein a refractive index of an organic material having an electro-optic effect is controlled, and the signal electrode is a traveling wave type electrode.   The optical communication system which uses the organic waveguide type optical modulator in any one of Claim 1 thru | or 10 as a component of an optical output part.

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