JPH04152320A - Optical modulator made of linbo3 - Google Patents

Abstract

PURPOSE:To lower a high-frequency surface resistance and a driving voltage by forming electrodes consisting of oxide superconducting films on an electrooptical crystal substrate which consists of an LiNbO3 single crystal of a (010) crystal bearing and has an optical waveguide. CONSTITUTION:The straight optical waveguide 3 consisting of Ti is provided on the surface of the electrooptical crystal substrate 1 consisting of the LiNbO3 single crystal of the (010) bearing. Further, the oxide superconducting layer formed by formulating the metallic Er, Ba and Cu of Y materials to a prescribed compsn. is formed on the substrate 1, by which the electrodes 2 are formed. There is no need for forming buffer layers and the matching of lattice constants is good if the LiNbO3 substrate 1 of the (010) crystal bearing is used in such a manner. The growth of an epitaxial thin film on the substrate is, therefore, possible. The presence of non-superconducting phases is decreased in this way and the high-frequency surface resistance is lowered. The driving voltage is thus lowered.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an external modulator in optical communications.

[Prior Art] As the transmission speed and capacity of optical communication systems increase, it becomes necessary to increase the frequency of optical modulation, and high-frequency optical modulation systems are being studied. This method can be roughly divided into a direct modulation method using a semiconductor laser and an external modulation method using an optical modulator using an electro-optic crystal such as LiNbO3. The direct modulation method using a semiconductor laser is a method of high-frequency modulating the emitted laser light itself and extracting the modulated laser light.The frequency limit has currently been extended to more than 10 GHz, but the essential problem of frequency chirping is There are unavoidable problems. Frequency chirping is a phenomenon in which the frequency spreads during intensity modulation, and is particularly problematic in high frequency ranges. In this regard, external modulation methods without frequency chirping are being reconsidered.

Optical modulation using the external modulation method involves providing an optical waveguide within an electro-optic crystal substrate, and performing phase modulation using an effective interaction between a light wave and an electric field (electro-optic effect) using an electrode in the vicinity of the optical waveguide. Therefore, materials with large electro-optic effects are
The light modulation efficiency also increases, and materials with this large electro-optic effect include oxide dielectric materials such as LiNbO3 and compound semiconductor materials such as gallium arsenide, which have high efficiency (high electro-optic constant) and light guide. The LINb03 optical modulator has been widely studied from the viewpoint of ease of wave path formation. When a push-pull operation is performed to improve efficiency, there are two possible configurations of an optical modulation element using this LINb03 single crystal, as shown in FIGS. 5 and 6, based on the electro-optical constants of the crystal. In Figures 5-6, (1) and (LA
) indicates an electro-optic crystal substrate, (2) an electrode, (3) an optical waveguide, and (4) a buffer layer. Figure 5 shows (001)
An optical waveguide (3
), a buffer layer (4) (S102) and an electrode (2) are formed, and FIG.
An optical waveguide (3) and an electrode (2) are formed using an O3 single crystal as a substrate.
). Further, in both cases, the electric field is applied in the Z direction (in the direction of the arrow in the figure), and is perpendicular to the traveling direction of the light wave. When comparing these two types of configurations,
First, regarding the modulation efficiency, assuming the same electrode spacing, electrode width, and electrode length, and ignoring differences in field distribution of light waves and modulated waves in each configuration, the required modulation voltage is approximately equal. However, (010)
In the electrode configuration with the orientation L+Nb0a, the electrode capacitance increases in the lumped constant compared to the (001) orientation, so the modulation band becomes narrower. In the case of a traveling waveform, since the band is determined by the electrode length, the two bands are equal, but the characteristic impedance becomes low and the required modulation power increases. In either case, the (001) orientation of L+NbO3 has higher efficiency. Therefore, a (001) oriented LiN b Os substrate has conventionally been used.

There are two types of I-r N b Oa optical modulators in the (ooB direction), a traveling wave type and a resonant type, due to differences in electrode structure, and each type has advantages and disadvantages. For example, the bandwidth can be wide, or the operating voltage can be as high as around several tens of volts, and for example, if the electrode length is 8.2
A drive voltage of 1.7 V was required to perform phase modulation of 1 radian at a frequency of 17 Gt (z) using a modulator with an electrode spacing of 15 μι and a wavelength of 1.3 μι. The optical modulator has a narrow bandwidth but can perform optical modulation in a high frequency band, which is difficult to do with traveling waves, at a low operating voltage by reducing loss by using an electrode structure that resonates high frequencies in the electrode part.For example, In a resonant electrode modulator with an electrode length of 18 r AIll and an electrode spacing of 50 μl, 17 CII
To perform phase modulation of 1 radian at the frequency of Z, 5.2
It was possible to operate with a driving voltage of about V. Therefore, resonant electrode type optical modulators are more successful in operating at low voltage and power than traveling wave type optical modulators.

However, even in resonant electrode type optical modulators, the low operating voltage and modulation efficiency reported to date still do not reach a satisfactory level.

Therefore, in order to improve the modulation efficiency of the − layer, the present inventors
Attempts are being made to reduce loss in the electrodes by changing the electrodes of the optical modulator from AI to superconducting materials.

However, as stated in Journal of Applied Physics, Vol. 63, page 4591, YBa2Cu30. The oxide superconducting film becomes a polycrystalline film due to a mismatch in lattice constants, and as a result, the high-frequency surface resistance was not as low as expected.

[Problem to be solved by the invention] As explained above in the conventional example, the L of the (001) direction
The problem with the INbO3 optical modulator is that even if it uses a resonant electrode structure, the driving voltage is high, so in order to put it into practical use and popularize it, it is necessary to increase the Q value of the circuit and increase the modulation efficiency. It is. The purpose of the present invention is to propose a LINbO3 optical modulator that can be driven at a lower voltage for the purpose of increasing Q and improving modulation efficiency.

[Means for Solving the Problems] The present invention provides L I N b 03 with (010) crystal orientation.
an electro-optic crystal substrate having an optical waveguide made of a single crystal;
The present invention relates to an optical modulator comprising an electrode made of an oxide superconducting film formed on the electro-optic crystal substrate.

[Effect j When using an L+Nb0a substrate with (001) orientation,
It becomes necessary to form a buffer layer. However, oxide superconducting materials tend to react with the oxide buffer layer, leading to film deterioration. Also, because the matching of case and planned number is poor, the film quality is polycrystalline rather than epitaxial. The high frequency surface resistance also does not become very low. On the other hand, (020)
Using an L r N b 03 oriented substrate not only eliminates the need to form a buffer layer, but also allows epitaxial thin film growth on the substrate due to good lattice constant matching. Therefore, the presence of non-superconducting phases is small and a thin film with sufficiently low high-frequency surface resistance can be obtained.

FIG. 2 shows the relationship between surface resistance and frequency of A1 and ErBa2Cu30 at 77. In the figure, the solid line is E r
Theoretical value of Ba 2 Cu30 y, - dotted chain line is (001
) orientation on a 1.1NbO substrate with a total of Ba2Cu30. The measured value, the two-dot chain line is L iN b O3 in the (010) direction
Measured value of ErBa2Cu30 on the substrate, dotted line is A1
Shows the measured values of the electrodes. From Figure 2, (oio) direction 1
It is clear that the surface resistance of ErBa2Cu3o on the iN b O3 substrate is close to the theoretical value.

As a result, the modulation efficiency of optical modulators with superconducting electrodes has been significantly increased from several tens of minutes to several hundreds of modulation power, and the modulation voltage has also been significantly increased from several minutes to several tens of minutes. Improved. This improvement differed depending on the electrode structure of the optical modulator, and was larger for the resonant electrode structure than for the traveling waveform.

[Example] The present invention will be described below with reference to the drawings, but the present invention is not limited by the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention.

In FIG. 1, (1) is an electro-optic crystal substrate, (2) is a superconducting electrode, and (3) is an optical waveguide.

The electro-optic crystal substrate (1) only needs to have the property that its refractive index changes depending on an electric field, and its shape and size are not particularly limited. In view of the above characteristics, the material is LiTa0
, BaTiO3, ZnO1 (Pb, La) (Zr, T
j) Crystal materials having an electro-optic effect such as Os, Ba2NaNb5015, and ceramics thereof are used.

The superconducting electrode (2) is made of a superconducting film.

It is preferable that the superconducting film has low surface resistance characteristics in order to efficiently transmit signals.

Therefore, it is preferable to use a copper-based, nickel-based, vanadium-based, bismuth-based, or titanium-based superconducting oxide. Specific examples include E"~Ba-Cu-〇 system, Bj
-8r-Cs-Cu-0 system, Tj-Ba-Ca-Cu-
Examples include superconducting oxides such as 0 series.

The thickness of the superconducting film is preferably 500 angstroms or more in view of the penetration depth of the signal electric field.

Methods for forming superconducting films include CVD method, sputtering method,
Examples include vapor deposition method, sol-gel method, and screen printing method.

The other configurations are similar to conventional optical modulators and are not particularly limited.

Hereinafter, the present invention will be specifically explained based on Examples.
The present invention is not limited to such embodiments.

Example 1 An electro-optic crystal substrate ([) was fabricated using an L+NbO3 single crystal substrate with a (010) orientation. A linear optical waveguide (3) with a width of 6 μm and a length of 18 mm was formed on the surface of the obtained electro-optic crystal substrate by a Ti electron beam evaporation method to a film thickness of 35 nm.
It was formed with After that, 1050℃ in an argon atmosphere.
C. for 7 hours to diffuse Ti into L+NbO3 to form an optical waveguide.

A superconducting electrode (2) was formed by the ICB method (ion cluster beam method). The superconducting material used was the Y-based material Er Ba 2 Cu 30 from the viewpoint of ease of film formation, and the metals Er, Ba, and Cu were each mixed to have a composition of 1.23. The evaporation rate was controlled, and the evaporation conditions were: the temperature of the electro-optic crystal substrate (1) was 640°C, the degree of vacuum was on the order of 10'' Torr, the accelerating voltage was 2 kV, and the evaporation time was 05 hours to form an oxide superconducting layer with a thickness of 03 μm. did. The superconducting transition temperature Tc of the obtained superconducting film was 89. Also, the high frequency surface resistance was 10-5 ohms at 77K, lOG Hz. The measurement results of this surface resistance are shown in FIG. 2 by the two-dot chain line.

The resulting superconducting film was then processed into an electrode that resonates at a desired frequency. The electrode processing was performed by applying a resist, exposing, developing, and pattern etching using photolithography technology.

The optical modulation characteristics of the optical modulator manufactured in this way at 77K are shown by the solid line in FIG.

In the figure, 16 GHz to 18 GHz modulation frequency characteristics (
At 17.2 GHz), the operating voltage required for 1 radian phase modulation was sufficiently low at 8 V, and good photoconductive properties were also confirmed.

Comparative Example 1 Same as Example 1 except that the orientation of the single crystal substrate was (001).
An optical modulator was fabricated in the same manner as above. The superconducting transition temperature Tc of the superconducting film of the obtained optical modulator was 80, which was inferior to 89 in Example 1.

In addition, the high frequency surface resistance is 77, 10Gt (z 2 ×
It exhibited a considerably larger surface resistance than Example 1, i.e., 3 ohms. In FIG. 2, the measurement results of this surface resistance are shown by the dashed-dotted line.

Comparative Example 2 A conventional optical modulator was manufactured by forming an A1 electrode having the same shape as in Example 1 to a thickness of 1.8 μM at room temperature by electron beam evaporation. The characteristics of the obtained optical modulator were measured under the same conditions as in Example 1. As a result, the driving voltage required for phase modulation of 1 radian was 7V. The optical modulation characteristics of the obtained optical modulator are shown by dotted lines in FIG.

From FIG. 3, it can be seen that the optical modulator of Example 1 was able to achieve a voltage reduction of 1/3 to 1/4 compared to that of Comparative Example 2.

Example 2 An optical modulator was produced in the same manner as in Example 1 except that the electrode structure was a traveling wave electrode. 17G of the obtained optical modulator
The drive voltage required for 1 radian of phase modulation at Hz was 9 volts. Furthermore, the relationship between the modulation voltage and frequency of the obtained optical modulator is shown by a solid line in FIG.

Comparative Example 3 An optical modulator was produced in the same manner as in Example 2, except that the electrodes were AI electrodes. The driving voltage required for phase modulation of 1 radian at 17 GHz of the obtained optical modulator was 20 V. Furthermore, the relationship between the modulation voltage and frequency of the obtained optical modulator is shown by the dotted line in FIG.

From FIG. 4, it can be seen that the driving voltage of the optical modulator of Example 2 is the same as that of Comparative Example 3.
It can be seen that it is reduced to less than 1/2 of that of .

[Effects of the Invention] As explained above, according to the optical modulator of the present invention, by forming electrodes of an oxide superconductor on a (010) crystal orientation single crystal substrate having an optical waveguide, the optical modulator can be improved. This has the effect that driving voltage can be reduced.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of an example of the present invention, FIG. 2 is a graph showing the relationship between surface resistance and frequency, and FIG. 3 is a graph showing the relationship between modulation depth and frequency between Example 2 and Comparative Example 2. , FIG. 4 is a graph showing the relationship between modulation depth and frequency in Example 2 and Comparative Example 3, and FIG. 5 is a graph showing the relationship between modulation depth and frequency in Example 2 and Comparative Example 3.
O3 single crystal is bonded to the substrate, optical waveguide, buffer layer (Sin
,, ') and electrodes, FIG. 6 shows an optical modulator in which an optical waveguide and electrodes are formed using a (010) oriented LiN b O3 single crystal as a substrate. (Main symbols in the drawings) (1): Electro-optic crystal substrate (2): Electrode (3): Optical waveguide representative Masu Oiwa 1: Electro-optic crystal substrate 2: Electrode 6: Optical waveguide fan 2 Diagram: Theoretical value 11---: ht main electrode---: Example 1 1---: Comparative example 1 Wave number (Hz) 3 times' (010) LiNbO3 :Af1 Frequency ((3Hz) 4 years National modulation frequency ( GH2) Age 5 'A' 6 area

Claims (1)

[Claims] (1) An optical device comprising an electro-optic crystal substrate having an optical waveguide made of a LiNbO_3 single crystal with a (010) crystal orientation, and an electrode made of an oxide superconducting film formed on the electro-optic crystal substrate. modulator.