JP4926985B2 - Light modulator - Google Patents

Light modulator Download PDF

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JP4926985B2
JP4926985B2 JP2008008535A JP2008008535A JP4926985B2 JP 4926985 B2 JP4926985 B2 JP 4926985B2 JP 2008008535 A JP2008008535 A JP 2008008535A JP 2008008535 A JP2008008535 A JP 2008008535A JP 4926985 B2 JP4926985 B2 JP 4926985B2
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electrode
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crystal
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JP2009169188A (en
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敏夫 伊藤
生剛 八木
和夫 藤浦
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日本電信電話株式会社
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Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator that changes the phase of light by an electrical signal using an electro-optic crystal.

Conventionally, various optical functional parts using electro-optic crystals have been put into practical use. These optical functional parts utilize the fact that when a voltage is applied to the electro-optic crystal, the refractive index of the crystal changes due to the electro-optic effect. For example, in a KTa 1-x Nb x O 3 (0 <x <1, hereinafter referred to as KTN) crystal, the relative permittivity diverges in the temperature region during the phase transition from the paraelectric to the ferroelectric. 2nd order electro-optic effect that is proportional to the square of the dielectric constant is extremely large.

  An optical phase modulator using an electro-optic crystal changes the phase of light by changing the speed of light passing through the crystal by changing the refractive index of the crystal. The structure of the modulator includes, for example, a parallel plate type, a parallel strip type, a comb electrode type, and a through electrode type. The comb-shaped electrode has the smallest electrode width of about 1 μm.

  FIG. 1 shows a conventional multilayer electrode structure. When an electrode is formed on a substrate 101 made of a semiconductor or glass, it is generally a multi-layered electrode. Typically, an electrode having a three-layer structure of Ti electrodes 102a and 102b in contact with the substrate 101, Pt electrodes 103a and 103b, and Au electrodes 104a and 104b is used. Ti is used as an adhesive because it has excellent adhesion. Other metals having excellent adhesion include Mo, Al, Cr, and the like. However, since Mo is inferior in water resistance, it lacks reliability, Al is inferior in alkali resistance, and Cr has an environmental problem.

  Au is the most excellent electrode material because of its low electrical resistivity. Other metals with low electrical resistivity include Ag and Cu. In any case, Au is superior in that it causes a heat resistance change by film oxidation. Pt has a large work function and functions as a block layer so that electrons do not enter the semiconductor from Au. As described above, the Ti / Pt / Au layer functions in order as an adhesive layer / block layer / electrode body.

  When Ti is not used for the layer in contact with the semiconductor or the glass substrate, the electrode may be peeled off. FIG. 2 shows a photograph of the substrate surface when an electrode composed of a Pt / Ti / Au layer is produced. 2A and 2C show the case where a patterned electrode is formed on a glass substrate. FIGS. 2B and 2D show the case where a patterned electrode is formed on a silicon substrate. Electrodes were formed on each substrate with patterns of various sizes and shapes. Among these, an electrode formed on the right side from the arrow indicated by reference numeral 111 in FIG. 2C (a white circled portion), and an electrode formed on the right side from the arrow indicated by reference numeral 112 in FIG. ) Shows a portion peeled off from the substrate (a portion that appears to be fading away when seen in the figure). Although the pattern of this portion is a pattern having a width of 2 μm, peeling occurs. Therefore, when Ti is not used for the layer in contact with the substrate, it is difficult to form a comb electrode having a width of 1 μm.

International Publication No. 06/137408

  On the other hand, in an optical modulator using a KTN crystal, when Ti is used for a layer in contact with the substrate made of the KTN crystal, there is a problem that electrons are injected into the KTN crystal and cause a deflection action (see, for example, Patent Document 1). ). The deflection action is effective when used as a deflector, but is not necessary when used as a modulator, and it is preferable that the position of the light beam transmitted through the optical modulator does not vary.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical modulator having an electrode structure that is highly reliable and does not cause a deflection action.

The present invention, in order to achieve the above object, an invention according to claim 1, have a electro-optical effect, KTa 1-x Nb x O 3 (0 <x <1) der Ru electrooptical A pair of electrodes formed opposite to the surfaces of the electro-optic crystal parallel to each other in order to apply an electric field perpendicular to the optical axis of light transmitted through the electro-optic crystal. pair, before Symbol a work function greater than Pt of the electro-optic crystal metal in contact with the work function the electro-optical crystals, characterized in that the order from the surface in contact with the electro-optical crystal Pt, Ti and Au are laminated And

The electro-optical crystal, K some K is substituted with Li 1-y Li y Ta 1 -x Nb x O 3 (0 <x <1,0 <y <1) is preferable.

As described above, according to the present invention, the electrode pair, electric work function of the metal in contact with the optical crystal is a work function greater than Pt of the electro-optic crystal, Pt from the surface in contact with the electro-optic crystal in order, Ti Since Au and Au are laminated , the carrier injection efficiency is reduced and the deflection action is reduced. In addition, since the adhesiveness between the electrode pair and the electro-optic crystal is good, a highly reliable modulator can be configured.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. According to Patent Document 1, when Ti is used for a layer in contact with a substrate made of a KTN crystal, ideal ohmic contact is realized, and injection efficiency is maximized. As the work function of the electrode material increases, the Schottky contact is approached and the carrier injection efficiency decreases. That is, conduction electron injection is suppressed, and the deflection action is reduced. Since the work function of the KTN crystal is 5.0 eV, when the carrier contributing to the electric conduction of the electro-optic crystal is an electron, the work function of the electrode material of the layer in contact with the substrate is 5.0 eV or more. Is preferred.

  As an electrode material having a work function of 5.0 eV or more, Co (5.0), Ge (5.0), Au (5.1), Pd (5.12), Ni (5.15), Ir (5 .27), Pt (5.65), Se (5.9) can be used. However, since Au has poor processability when used as an adhesive layer, other materials are more preferable. Moreover, an alloy using a plurality of the above materials may be used, or a plurality of materials may be laminated.

Further, as an electro-optic crystal having a large second-order electro-optic constant, K 1-y Li y Ta 1-x Nb x O 3 (0 <x <1, 0 <y <1, hereinafter referred to as KLTN) is used. You can also. The work function of the KLTN crystal is also 5.0 eV. By using the above material for the layer in contact with the substrate, injection of conduction electrons is suppressed and the deflection action is reduced.

  FIG. 3 shows a light intensity modulator according to an embodiment of the present invention. It is a light intensity modulator combining an optical phase modulator, a polarizer, and an analyzer. As shown in FIG. 3A, a positive electrode 2 and a negative electrode 3 are formed on opposite surfaces of a substrate 1 made of KTN crystal. A polarizer 4 is disposed on the incident side of the substrate 1 and an analyzer 5 is disposed on the exit side. Of the electric field components of the light that has passed through the polarizer 4, let Ex be the component parallel to the x axis and Ey be the component parallel to the y axis. When the polarization angle of the polarizer 4 is 45 degrees with respect to the x-axis of the substrate 1, Ex = Ey.

  Changes in the phases of Ex and Ey when a voltage V is applied between the positive electrode 2 and the negative electrode 3 are given by equations (1) and (2), respectively. When the polarization angle of the analyzer 5 is 45 degrees with respect to the x-axis of the substrate 1, the intensity of the outgoing light that has passed through the analyzer 5 is given by the following equation.

  If Ex and Ey are equal,

Is given by the following equation.

  In this way, as shown in FIG. 3B, the intensity of the emitted light that has passed through the analyzer 5 can be modulated between 0% and 100% according to the voltage V.

  Two types of Pt and Ti are prepared as electrode materials for the light intensity modulator shown in FIG. Carriers contributing to electrical conduction in the KTN crystal are electrons. FIG. 4 shows the operating characteristics of the light intensity modulator of the electrode material Ti. As the applied voltage increases, the intensity of the emitted light changes, but it can be seen that the ratio of the light intensity during on / off (hereinafter referred to as the extinction ratio) is deteriorated. This is because when a voltage is applied to the electro-optic crystal, a space charge is generated inside the electro-optic crystal, and an electric field is tilted in the direction of voltage application, so that the beam is deflected between the vertically polarized light and the horizontally polarized light. This is because a deviation angle occurs. As the voltage applied to the crystal increases, the deviation angle between the vertically polarized light and the horizontally polarized light increases, and the extinction ratio deteriorates as shown in FIG.

  FIG. 5 shows the operating characteristics of the light intensity modulator of the electrode material Pt. Since Pt has a larger work function of the electrode material than Ti and is larger than the work function of the KTN crystal, the efficiency of carrier injection from the electrode to the KTN crystal decreases. The lower the carrier injection efficiency, the smaller the gradient of the electric field, and the smaller the deflection effect. When a voltage of 58 V is applied between the positive and negative electrodes, the polarization direction of the emitted light rotates 90 degrees with respect to the polarization direction of the incident light. As the applied voltage between the positive electrode 2 and the negative electrode 3 increases, the emitted light repeatedly turns on and off.

  FIG. 6 shows a multilayer structure of the electrode according to the first example. A two-layer electrode of Pt electrodes 12a and 12b in contact with the substrate 11 made of KTN crystal and Au electrodes 13a and 13b is used. Pt has a work function larger than that of a substrate made of a KTN crystal, the carrier injection efficiency is reduced, and no deflection action occurs.

  In addition, the KTN crystal has minute irregularities on the substrate surface compared to the semiconductor substrate and the glass substrate, and is fixed so that Pt enters the substrate, so that the adhesion is good and the electrode is hardly peeled off. A specific example will be described with reference to the second embodiment. Therefore, it is not necessary to use Ti for the layer in contact with the substrate, and a highly reliable optical modulator can be configured.

  FIG. 7 shows a multilayer structure of an electrode according to Example 2. A Pt electrode 22a, b in contact with the substrate 21 made of KTN crystal, a Ti electrode 23a, b, and an Au electrode 24a, b are used in a three-layer structure. Since Pt has poor adhesion to metal, Ti was used to ensure adhesion to Au. Further, since Pt also functions as a block layer, electrons from Ti and Au are prevented from entering the KTN crystal.

  FIG. 8 shows a photograph of the substrate surface when the electrode according to Example 2 was produced. An electrode made of a Pt / Ti / Au layer is formed with a thickness of 100/70/200 nm by EB vapor deposition. The symbol “.55” indicates an electrode composed of a plurality of patterns having a width of 0.55 μm, and the symbol “.65” indicates an electrode composed of a plurality of patterns having a width of 0.65 μm. Although it is partially deformed, no peeling occurs. The symbol “0.7” indicates an electrode composed of a plurality of patterns having a width of 0.70 μm, and the electrode having a width of 0.7 μm or more is neither peeled nor deformed. Only the electrode having a width of 0.50 μm and having a reference sign “0.5” causes the peeling. Accordingly, since an electrode having a width exceeding 0.7 μm can be formed, a comb-shaped electrode can be formed without any problem.

It is sectional drawing which shows the electrode structure of the conventional multilayer structure. It is a figure which shows the board | substrate surface when producing the electrode which consists of a Pt / Ti / Au layer. It is a figure which shows the light intensity modulator concerning one Embodiment of this invention. It is a figure which shows the operating characteristic of the light intensity modulator of electrode material Ti. It is a figure which shows the operating characteristic of the light intensity modulator of electrode material Pt. 1 is a cross-sectional view showing a multilayer structure of an electrode according to Example 1. FIG. 6 is a cross-sectional view showing a multilayer structure of an electrode according to Example 2. FIG. It is a figure which shows the substrate surface when the electrode concerning Example 2 is produced.

Explanation of symbols

11, 21, 101 Substrate 12, 22, 103 Pt electrode 13, 24, 104 Au electrode 23, 102 Ti electrode

Claims (2)

  1. Possess an electro-optic effect, an electro-optical crystal Ru KTa 1-x Nb x O 3 (0 <x <1) der,
    An electrode pair formed to face the surfaces of the electro-optic crystal parallel to each other in order to apply an electric field perpendicular to the optical axis of the light transmitted through the electro-optic crystal;
    The electrode pair, before Symbol a work function greater than Pt of the electro-optic crystal metal in contact with the work function the electro-optic crystal, it sequentially from the surface in contact with the electro-optical crystal Pt, Ti and Au are laminated An optical modulator characterized by.
  2. The electro-optic crystal is K 1-y Li y Ta 1-x Nb x O 3 (0 <x <1, 0 <y <1) in which a part of K is substituted with Li. The optical modulator according to claim 1.
JP2008008535A 2008-01-17 2008-01-17 Light modulator Active JP4926985B2 (en)

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