JP2941970B2 - Resonant electrode type optical modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an external modulator for high-speed optical communication, and more particularly to an optical modulator of a resonance electrode type.

[0002]

2. Description of the Related Art As the transmission speed of an optical communication system increases, the frequency of optical modulation becomes higher, and a high-frequency optical modulation method is being studied. High-frequency optical modulation methods are roughly classified into a direct modulation method using a semiconductor laser and an external modulation method represented by a LiNbO ₃ optical modulator. The direct modulation method of a semiconductor laser modulates the laser oscillation itself at a high frequency and extracts the modulated laser light. The frequency limit has been extended to more than ten GHz at present,
This direct modulation scheme has an inherent unavoidable problem of frequency chirping. This frequency chirping is a phenomenon in which the frequency is spread in the case of intensity modulation, and is particularly problematic in a high frequency range. Therefore, an external modulation method without frequency chirping is more desirable. Hereinafter, a LiNbO ₃ optical modulator, which is a representative example of the optical modulator based on the external modulation method, will be described.

Conventionally, there are two types of external modulation type LiNbO ₃ optical modulators, a traveling waveform and a resonance type, due to differences in electrode structure, and each has advantages and disadvantages. That is, the optical modulator having the traveling waveform electrode structure can have a wide bandwidth, but the operating voltage is several tens of volts.
Higher and lower, for example, electrode length 8.2mm, electrode spacing 15μ
A modulator having a wavelength of 1.3 μm and a wavelength of 17 μm requires a voltage of 17 Vp-p to perform 1 rad phase modulation at a frequency of 17 GHz. On the other hand, when an optical modulator having a resonant electrode structure is used, optical modulation can be performed at a low operating voltage in a high-frequency band, which has a narrow bandwidth but is difficult for traveling waves. For example, a resonant electrode optical modulator having an electrode length of 18 mm and an electrode spacing of 50 μm can operate at a voltage of about 5.2 Vp-p to perform 1 rad phase modulation at a frequency of 17 GHz. As described above, the optical modulator of the resonance electrode type has succeeded in lower voltage and lower power operation than the traveling waveform.

[0004] However, even the above-mentioned optical modulator of the resonance electrode type has not yet reached a satisfactory level with the low operating voltage and modulation efficiency reported to date. More specifically, as described in the Nikkan Kogyo Shimbun on December 8, 1989, the conversion efficiency was increased by using a modulation electrode as a resonator and the Q of the circuit was increased. Ten
There is a report of modulation at low power. However, even in this report, since the operating voltage requires about 5 V, there is a problem that a dedicated GaAs driver for high voltage driving is required, and the reliability is low due to the high voltage operation.

[0005]

As described above, the problem of the optical modulator using the conventional electro-optic crystal is that the driving voltage is high even when the resonant electrode type structure is employed. For practical use and spread, it is necessary to increase the Q value of the circuit and increase the modulation efficiency.

For this purpose, it is conceivable to form an oxide superconducting thin film having excellent superconducting characteristics (low loss, high Q) on a LiNbO ₃ substrate, to form an electrode therefrom, and to achieve a low loss and low operating voltage. . Methods for forming an oxide superconducting thin film on a LiNbO ₃ single crystal substrate include a vapor deposition method, a sputtering method,
There is a method such as a CVD (Chemical Vapar Deposition) method.
However, in any method, the substrate temperature is set to 60 at the time of film formation.
Since it is necessary to hold the film at a high temperature of 0 to 900 ° C. for several hours, there is a problem that the LiNbO ₃ substrate and the constituent elements of the oxide superconducting thin film are interdiffused and the oxide superconducting thin film is deteriorated. This deterioration of the oxide superconducting thin film has a small effect on the superconducting characteristics (Tc, Jc) in the low frequency region, but the characteristics (surface resistance) in the high frequency region are sensitive to the film deterioration, so the characteristics are greatly reduced. And the modulation efficiency as an optical modulator is greatly reduced.

It is an object of the present invention to provide an optical modulator having the above-described resonant electrode structure, which has a high Q and an improved modulation efficiency and can be driven at a lower voltage.

[0008]

According to the present invention, an electrode is formed of an oxide superconducting material, and an oxide layer which does not easily react with the oxide superconducting material is formed at least between the electrode and the electro-optic crystal substrate. The present invention relates to a resonant electrode type optical modulator characterized by the following.

[0009]

The oxide superconducting material is formed between the electro-optic crystal substrate and the oxide superconducting electrode (electrode formed of the oxide superconducting material) by forming an oxide that does not easily react with the oxide superconducting material into a film. Prevents the superconducting properties of the electrode from deteriorating due to the diffusion reaction of the constituent elements with LiNbO ₃ .

Further, when the film formed between the electro-optic crystal substrate and the oxide superconducting electrode has a lower refractive index than the optical waveguide in the electro-optic crystal, the light in the optical waveguide is guided by the waveguide. It will pass through without being leaked outside while being preserved.

Further, the frequency band is restricted by forming the oxide superconducting electrode portion into a resonance type structure. However, the optical modulator is reduced due to two advantages of reducing conductor loss and improving resonance characteristics (Q value). The voltage can be increased, and the modulation efficiency is greatly improved as compared with the traveling waveform electrode optical modulator.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The electrodes in the optical modulator of the present invention are made of an oxide superconducting material.

The oxide superconducting material is not particularly limited. However, since a high-frequency signal is applied to the superconducting electrode, the surface resistance at high frequency is lower, and the Tc (critical temperature) is 77 K or more from the viewpoint of a cooling system. preferable.

Specific examples of such a superconducting material include:
For example ErBa ₂ Cu ₃ O _y as any Y-based, Bi-Sr-
Examples thereof include Ca-Cu-O-based materials and Tl-Ba-Ca-Cu-O-based materials. Among them, ErBa ₂ Cu ₃ O _y is preferable from the viewpoint of easy film formation.

It is desirable that the electrode made of the superconducting material resonates at a desired frequency in order to increase the light modulation efficiency. Therefore, it is preferable that at least a desired frequency is converted into a wavelength, and that is λ, the electrode is formed in parallel with the optical waveguide with a length of λ / 2.

The electro-optic crystal substrate used in the present invention comprises:
There is no particular limitation on crystals and ceramics having an electro-optic effect. Specific examples of the electro-optic crystal substrate include, for example, LiNbO ₃ , LiTaO ₃ , BaTiO ₃ , ZnO, α-Quar
tz.

Further, an optical waveguide having a refractive index different from that of the substrate is formed on the electro-optic crystal substrate by titanium diffusion, external diffusion or ion exchange. However, since the optical waveguide of the external diffusion and the proton exchange raises only the extraordinary refractive index of the substrate, the propagation of the light wave is limited to biaxial polarization. For this reason, orthogonal Nicol-type light modulation is impossible, but the influence of optical damage is smaller than that of titanium diffusion, and a light wave of high intensity can be propagated.

In the optical modulator according to the present invention, an oxide layer that does not easily react with the oxide superconducting material is formed at least between the electro-optic crystal substrate and the oxide superconducting electrode.

The oxide layer is not particularly limited as long as it is a layer made of a material that does not easily react with the superconducting material. The term “not easily reacted” means that the Tc, Jc (critical current density), resistance-temperature characteristics, and particularly, surface resistance-frequency characteristics of the superconducting material are not reduced. Further, a material having a refractive index lower than that of the optical waveguide in the electro-optic crystal (approximately 2.1 or less is a guide) is preferable because light passes through the waveguide without leaking out of the waveguide.

Specific examples of the oxide constituting the oxide layer include, for example, SrTiO ₃ , ZrO ₂ , Y ₂ O ₃ , Mg
O, LaGaO ₃ , LaAlO ₃ , YAlO ₃ , Y ₂ BaC
such as uO _5, NdGa O ₃ based material and the like.

Since the oxide layer needs to apply a high-frequency signal to the electrode, the dielectric loss due to the high-frequency electric field decreases as the relative dielectric constant decreases and as the thickness decreases. Therefore, the relative permittivity is preferably 40 or less at the operating temperature of the superconducting light modulator of the present invention, and the thickness is also preferably 1 μm or less.

The optical modulator of the present invention can be manufactured, for example, as follows.

First, an optical waveguide is formed on an electro-optic crystal substrate, and then an oxide layer is formed by an ICB (cluster ion beam) method, an EB vapor deposition method, a sputtering method, a laser ablation method, a CVD method, and the like. ICB method, EB
Vapor deposition, sputtering, laser ablation, CVD
An oxide superconducting thin film is formed by a sol-gel method or the like. Then, the optical modulator of the present invention is manufactured by processing the thin film into an electrode that resonates at a desired frequency by using a normal photoengraving technique.

[Examples 1 and 2 and Comparative Example 1] Three optical modulators A-1 (Example 1) and B-1 having a cross section shown in FIG.
(Example 2) and C-1 (Comparative Example 1) were produced. In the figure, 1
Is an electro-optic crystal substrate, 2 is an optical waveguide, 3 is an oxide layer, 4
Is a superconducting electrode.

First, Z-cut LiNbO _ThreeSingle crystal substrate (element
18mm in length) and a 35nm-thick light guide by electron beam evaporation of metallic Ti
A wave path was formed.

Next, Al, Mg, S
r, Ti metal is melted in a crucible and, while introducing oxygen or ozone, deposited under the conditions of a substrate temperature of 670 ° C., a degree of vacuum of 10 ⁻⁴ Torr, an acceleration voltage of 2 kV, and a deposition time of 2 to 4 hours. An insulating oxide layer having a thickness of 0.1 to 0.2 μm as shown in Table 1 was formed. Table 1 shows the type, lattice constant, relative permittivity, and film thickness of the formed oxide layer. Note that Al ₂ O ₃ is a material that reacts with the oxide superconducting electrode. The light refractive indices are 2.29 for SrTiO ₃ , 1.7 for MgO, and 1.75 for Al ₂ O ₃ .
Therefore, MgO and Al ₂ O ₃ can be formed directly on the waveguide because they are lower than the refractive index of the waveguide. Since SrTiO ₃ has a large refractive index and increases light scattering when formed directly on a waveguide,
It is desirable to form another low refractive index thin film on the SrTiO ₃ and the waveguide.

[0027]

[Table 1] Next, on the oxide layer, the deposition rates of metals Er, Ba, and Cu were controlled so as to have a composition of 1/2/3, and at the substrate temperature shown in Table 2, ErBa _{2 was} obtained by the same ICB method as described above. Cu ₃ O _y
An oxide superconducting thin film was formed. Table 2 shows the Tc (superconducting transition temperature) and the film thickness of the ErBa ₂ Cu ₃ O _y superconducting thin film formed on the oxide layer.

[0028]

[Table 2] Then, the superconducting thin film formed in this manner is processed into a shape of an electrode that resonates at a desired frequency by applying resist, exposing, developing, and pattern-etching using photolithography technology. -1, B-1 and C-1 were prepared.

FIG. 2 shows the configuration of the thus-obtained resonant electrode type LiNbO ₃ optical modulator of the present invention. In the figure, 3 indicates an oxide layer, 4 indicates a superconducting electrode, 5 indicates a modulation signal source, and 6 indicates an optical fiber. In the superconducting resonance electrode, a coplanar line having both ends short-circuited is set on the optical waveguide, and a modulation signal is input from the center of the electrode by a power supply coplanar line. Also, to lengthen the E / O interaction, the resonator length is set so that 17 GHz is the second harmonic, and the center electrode is 5 μm, which is larger than the width of the optical waveguide, in order to increase the impedance of the electrode coplanar line. The electrodes were narrowed and the electrode interval was widened to 50 μm.

Next, the optical modulators A-1, B-1 and C-
The optical modulation characteristics of 1 at 77K were measured.

As a result, although A-1 (Example 1) had a high relative dielectric constant of SrTiO _{3 as} an oxide layer material and a large dielectric loss,
The modulation efficiency as a modulator was improved because of the good superconducting properties of the electrodes. On the other hand, the superconducting property of the electrode B-1 (Example 2) was slightly different from that of the electrode A-1, but the relative dielectric constant was low at 9.1 and the dielectric loss was at a level that did not cause much problem. , And showed the best characteristics. B-1
In the 16 GHz to 18 GHz modulation frequency characteristic (17.2 GHz), the operating voltage required to modulate 1 rad phase was sufficiently low at 1.5 V, and good optical transmission characteristics for both TM mode light and TE mode light were confirmed. FIG. 3 shows the relationship between the modulation voltage and the frequency of B-1. C-1 (Comparative Example 1) had a low relative dielectric constant, but had high reactivity between the superconducting electrode and the oxide layer, so that good superconducting properties could not be obtained.

Comparative Example 2 A conventional Al electrode having the same shape and a thickness of 1.8 μm was formed and formed at room temperature by an EB (electron beam) evaporation method instead of the superconducting electrode formed in the above embodiment.

When the modulation frequency characteristic of the obtained optical modulator D-1 was measured, the operating voltage required to modulate the 1 rad phase at 17.2 GHz was 5.2 V. FIG. 3 shows the relationship between the modulation voltage and the frequency.

From FIG. 3, it can be seen that the optical modulator of the present invention in which the electrodes are made superconducting is 1/3 to 1/3 in comparison with the optical modulator using the Al electrode.
It can be seen that it can be driven with a voltage as low as 1/4.

Comparative Example 3 An optical modulator E-1 was manufactured in exactly the same manner as in Example 2 except that no oxide layer was formed.

The superconducting characteristics of the superconducting electrode of the obtained optical modulator were lower than those obtained by forming an oxide layer.
The temperature characteristics of the resistance of this superconducting electrode were semiconductor-like, and Tc was several tens of K lower than that of the superconducting electrode provided with the oxide layer. X-ray analysis confirmed that the decrease in the superconducting properties was due to the reaction between the ErBa ₂ Cu ₃ O _y superconducting electrode and the LiNbO ₃ substrate during film formation. Further, when the optical transmission was examined using this optical modulator, it was found that the TM mode light was not transmitted, so that only the TE mode light could be used, and the function as the optical modulator was extremely low.

In the above embodiment, MgO or SrTiO ₃ was used for the oxide layer, but Y ₂ O ₃ , LaGaO ₃ , LaAlO ₃ ,
It was confirmed from the same experiment as in the above example that the same effect was obtained with lO ₃ , ZrO ₂ , and NdGaO ₃ .

Examples 3 to 4 and Comparative Example 4 A copper-diffused optical waveguide having a width of 6 μm and a length of 18 mm was formed on a z-cut LiTaO ₃ single crystal substrate by copper vapor deposition and subsequent heat diffusion. However, since the Curie temperature of the LiTaO ₃ crystal is lower than the diffusion temperature of the copper element, when forming an optical waveguide, after depositing copper on the substrate surface, an electric field is applied vertically downward from the substrate surface and lower than the Curie temperature. Hold the substrate at the temperature,
Copper element diffusion was performed.

Next, an oxide layer and an oxide superconducting electrode were formed in the same manner as in Examples 1 and 2 and Comparative Example 1, and optical modulators A-2, B-2 and C-2 were fabricated. When the evaluation was performed, the optical modulators A-1, B-1, and C-1 (Examples 1 and 2,
The same results as in Comparative Example 1) were obtained.

Comparative Example 5 A conventional Al electrode having the same shape and a thickness of 1.8 μm was formed and formed at room temperature by an EB (electron beam) evaporation method instead of the superconducting electrode formed in the above embodiment.

When the modulation frequency characteristic of the obtained optical modulator D-2 was measured, the operating voltage required to modulate the 1 rad phase at 17.2 GHz was 6.8 V. FIG. 4 shows the relationship between the modulation voltage and the frequency.

FIG. 4 shows that the optical modulator of the present invention in which the electrodes are made superconducting is one-third or less of the optical modulator using the Al electrode.
It can be seen that it can be driven with a voltage as low as 1/4.

[0043]

According to the optical modulator of the present invention, electrodes are formed of an oxide superconducting material, and high-efficiency and low-voltage operation is possible. In addition, since an oxide layer that does not easily react with the oxide superconducting material is formed at least between the electro-optic crystal substrate and the oxide superconducting electrode, it is necessary to form electrodes without deteriorating the high-frequency characteristics of the oxide superconducting material. Is possible.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing an optical modulator according to the present invention.

FIG. 2 is a plan view illustrating a configuration of an optical modulator according to the present invention manufactured in an example.

FIG. 3 is a graph showing a relationship between a modulation voltage and a frequency of an optical modulator.

FIG. 4 is a graph showing a relationship between a modulation voltage and a frequency of an optical modulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Electro-optic crystal substrate 2 Optical waveguide 3 Oxide layer 4 Superconducting electrode 5 Modulation signal source 6 Optical fiber

────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takashi Mizuochi 5-1-1 Ofuna, Kamakura-shi Mitsubishi Electric Corporation Communication Systems Laboratory (72) Inventor Tadashi Kitayama 5-1-1 Ofuna, Kamakura-shi Mitsubishi Electric (72) Inventor Makoto Utsunomiya 8-1-1, Tsukaguchi-Honmachi, Amagasaki-shi Mitsubishi Materials Corporation Materials Research Laboratory (56) References JP-A-1-284826 (JP, A) JP Hei 2-237082 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) G02F 1/03 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A resonant electrode type light, wherein an electrode is formed of an oxide superconducting material, and an oxide layer hardly reacting with the oxide superconducting material is formed between at least the electrode and the electro-optic crystal substrate. Modulator. 2. The optical modulator according to claim 1, wherein the oxide layer has a lower refractive index than an optical waveguide in the electro-optic crystal.