Optical modulator

Abstract

Provided is an optical modulator in which degradation of an extinction ratio is suppressed and a temperature drift phenomenon is suppressed. An optical modulator includes: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and a control electrode for controlling light waves propagating through the optical waveguide, in which the optical waveguide has one or more Mach-Zehnder type optical waveguides (A1 to A3 and B1 to B3), the control electrode has DC electrodes (C1 to C4) which apply DC bias, a feeder electrode which feeds DC bias to the DC electrode crosses one of two branched waveguides of the Mach-Zehnder type optical waveguide, and a first dummy electrode (a dotted line E5 or E6) is provided at a specific position on the other one of the two branched waveguides, which is a specific position symmetrical in relation to a position at which the feeder electrode crosses the one of the two branched waveguides.

Claims

1. An optical modulator comprising; a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and a control electrode for controlling light waves propagating through the optical waveguide, wherein the optical waveguide has at least two Mach-Zehnder type optical waveguides disposed side by side with each other, the control electrode has a DC electrode which applies DC bias to the optical waveguide, a feeder electrode which feeds DC bias to the DC electrode crosses one of two branched waveguides of one of the two Mach-Zehnder type optical waveguides, a first dummy electrode which neither applies DC bias to the optical waveguide as the DC electrode nor feeds DC bias to the DC electrode as the feeder electrode is provided at a specific position on the other one of the two branched waveguides, which is a specific position symmetrical in relation to a position at which the feeder electrode crosses the one of the two branched waveguides, a part of the one of the two Mach-Zehnder type optical waveguides is crossed by the feeder electrode and the first dummy electrode, and a second dummy electrode which neither applies DC bias to the optical waveguide as the DC electrode nor feeds DC bias to the DC electrode as the feeder electrode is provided at a specific position on the other one of the two Mach-Zehnder type optical waveguides, which is a specific position symmetrical in relation to a position at which the feeder electrode and the first dummy electrode cross the one of the two Mach-Zehnder type optical waveguides.

2. The optical modulator according to claim 1, wherein in the feeder electrode or the dummy electrode which crosses the optical waveguide, a width of a portion which crosses the optical waveguide is made to be narrower than a width of each of portions in front of and behind the portion which crosses the optical waveguide.

3. An optical modulator comprising: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and a control electrode for controlling light waves propagating through the optical waveguide, wherein the optical waveguide has at least one Mach-Zehnder type optical waveguide, the control electrode has a DC electrode which applies DC bias to the optical waveguide, a feeder electrode which feeds DC bias to the DC electrode crosses one of two branched waveguides of the Mach-Zehnder type optical waveguide, a first dummy electrode which neither applies DC bias to the optical waveguide as the DC electrode nor feeds DC bias to the DC electrode as the feeder electrode is provided at a specific position on the other one of the two branched waveguides, which is a specific position symmetrical in relation to a position at which the feeder electrode crosses the one of the two branched waveguides, the feeder electrode has a parallel feeder electrode portion which is disposed parallel to a drawing direction of the Mach-Zehnder type optical waveguide, and a third dummy electrode, which neither applies DC bias to the optical waveguide as the DC electrode nor feeds DC bias to the DC electrode as the feeder electrode, having approximately the same shape is formed at a position symmetrical to the parallel feeder electrode portion with respect to a central axis which is parallel to the drawing direction and becomes an axis of symmetry of the Mach-Zehnder type optical waveguide.

4. The optical modulator according to claim 1, wherein the dummy electrode is electrically connected to the DC electrode.

5. The optical modulator according to claim 3, wherein the dummy electrode is electrically connected to the DC electrode.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a diagram for describing the configuration of a DC electrode of the related art.

(2) FIG. 2 is a diagram for describing a first example relating to an optical modulator according to the present invention.

(3) FIG. 3 is a diagram for describing a second example relating to the optical modulator according to the present invention.

(4) FIG. 4 is a diagram for describing a portion at which a branched waveguide and a feeder electrode intersect one another, in relation to the optical modulator according to the present invention.

(5) FIG. 5 is a diagram for describing a third example relating to the optical modulator according to the present invention.

(6) FIG. 6 is a diagram for describing a fourth example relating to the optical modulator according to the present invention.

DESCRIPTION OF EMBODIMENTS

(7) Hereinafter, an optical modulator according to the present invention will be described in detail by using preferred embodiments.

(8) As shown in FIG. 2, the optical modulator according to the present invention includes: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; and a control electrode for controlling light waves propagating through the optical waveguide, in which the optical waveguide has one or more Mach-Zehnder type optical waveguides (A1 to A3, and B1 to B3), the control electrode has DC electrodes (C1 to C4) which apply DC bias, a feeder electrode which feeds DC bias to each of the DC electrodes crosses one of two branched waveguides of the Mach-Zehnder type optical waveguide, and a first dummy electrode (a dotted line E5 or E6) is provided at a specific position on the other one of the two branched waveguides, which is a specific position symmetrical in relation to a position at which the feeder electrode crosses the one of the two branched waveguides.

(9) As the substrate which is used in the optical modulator according to an aspect of the present invention, a substrate having an electro-optic effect, such as a single crystal of any one of LiNbO.sub.3, LiTaO.sub.5, and PLZT (lead lanthanum zirconate titanate), a semiconductor such as InP, or a polymer, can be suitably used. In particular, LiNbO.sub.3 and LiTaO.sub.5 which are frequently used in an optical modulator may be used. The optical modulator according to an aspect of the present invention is more suitably applied to an optical modulator using an X-cut type substrate. However, even in a Z-cut type substrate, in a case where a propagation loss occurs due to a control electrode or a case where a temperature drift phenomenon occurs, the optical modulator according to an aspect of the present invention may be applied thereto.

(10) The optical waveguide is formed on the substrate. The optical waveguide which is formed on the substrate is formed by thermally diffusing titanium (Ti) or the like onto, for example, a LiNbO.sub.3 substrate (an LN substrate). Further, a ridge type optical waveguide in which concavity and convexity are formed on a substrate along an optical waveguide can also be used. As a pattern shape of the optical waveguide, various shapes such as a nested waveguide which is provided with at least one Mach-Zehnder type waveguide and made by combining, for example, a plurality of Mach-Zehnder type waveguides can be adopted according to the use of the optical modulator.

(11) The control electrode can be formed by forming an electrode pattern of Ti.Au on the surface of the substrate and by using a gold plating method or the like. A dummy electrode in an aspect of the present invention is difficult applied to a modulation electrode which applies a RF modulation signal, and may be applied to only a DC electrode.

(12) The DC electrode is composed of a hot electrode (C2 or C4) and a ground electrode (C1 or C3), as shown in FIG. 2. A ground electrode is separately disposed between the Mach-Zehnder type optical waveguides adjacent to each other, as shown by a symbol D of FIG. 2. Due to such a configuration, an electric field generated by the DC electrode provided in the Mach-Zehnder type optical waveguide is prevented from affecting the DC electrode of the other optical waveguide.

(13) In FIG. 2, the dummy electrodes (dotted lines E5 and E6) is provided at a part of the feeder electrode of the hot electrodes (C2 and C4), thereby allowing the propagation loss of the guided light to be set to approximately the same level between the branched waveguides A1 and A2, or between the branched waveguides B1 and B2. The dummy electrode can be provided in not only the hot electrode (C2 or C4), but also the ground electrode (C1 or C3).

(14) FIG. 3 is a diagram for describing a second example relating to the optical modulator according to the present invention. The optical waveguide has at least two Mach-Zehnder type optical waveguides (A1 to A3 and B1 to B3) disposed side by side with each other, in which a part of the one of the two Mach-Zehnder type optical waveguides (A1 to A3) is crossed by the feeder electrode and the dummy electrode, and a second other dummy electrode (a dotted line E7) is provided at a specific position on the other one of the two Mach-Zehnder type optical waveguides (B1 to B3), which is a specific position symmetrical in relation to a position at which the feeder electrode and the dummy electrode cross the one of the two Mach-Zehnder type optical waveguides.

(15) By adopting the configuration as shown in FIG. 3, the propagation loss of each other can be set to approximately the same level between the Mach-Zehnder type optical waveguides (A1 to A3) and the other Mach-Zehnder type optical waveguides (B1 to B3). Further, it is possible to maintain the extinction ratio when the two Mach-Zehnder type optical waveguides are combined, at high quality.

(16) Further, as a method of reducing the propagation loss of the guided light by the feeder electrode or the dummy electrode, as shown in FIG. 4 which is an enlarged view of a portion shown by a dotted line E8 of FIG. 3, the width of a portion at which the feeder electrode (or the dummy electrode) crosses each of the optical waveguides (A1 and A2) may be set to be narrower than the width of each of portions in front of and behind the portion which crosses the optical waveguide, as indicated by an arrow F.

(17) FIGS. 5 and 6 are diagrams for describing third and fourth examples relating to the optical modulator according to the present invention. Specifically, the feeder electrode has a parallel feeder electrode portion (E9 or E11) which is disposed parallel to a drawing direction of the Mach-Zehnder type optical waveguide, and a third dummy electrode (E10 or E12) having approximately the same shape is formed at a position symmetrical to the parallel feeder electrode portion with respect to a central axis which is parallel to the drawing direction and becomes an axis of symmetry of the Mach-Zehnder type optical waveguide. Due to this configuration, it is possible to suppress the temperature drift phenomenon and to suppress the degradation of the extinction ratio.

(18) In FIG. 5, the dummy electrode is provided on the ground electrode (C3) side, and in FIG. 6, the dummy electrode is configured on the hot electrode (C4) side. Usually, the dummy electrodes may be electrically connected to the DC electrodes (C1 to C4). Due to such a configuration, it is also possible to stabilize the potential of the dummy electrode. Further, the dummy electrode may be connected to either of the hot electrode or the ground electrode. However, in order to maintain the symmetry of the electric field which is formed by the DC electrode, the dummy electrode may be connected to the same type of electrode as the feeder electrode forming an asymmetrical shape, which becomes a cause of providing the dummy electrode.

INDUSTRIAL APPLICABILITY

(19) As described above, according to the present invention, it is possible to provide an optical modulator in which degradation of an extinction ratio is suppressed and a temperature drift phenomenon is suppressed.

REFERENCE SIGNS LIST

(20) A1 to A3, B1 to B3: optical waveguide

(21) C1 to C4: DC electrode

(22) V1 to V4: DC bias voltage

(23) D: ground electrode

(24) F: recessed portion