OPTICAL MODULATOR

Abstract

To prevent deterioration in an extinction ratio due to asymmetry between a pair of optical waveguides.

An optical modulator has a Mach-Zehnder optical waveguide including mutually parallel first and second waveguides and a signal electrode for controlling the phase of light propagating in the Mach-Zehnder optical waveguide. The first and second waveguides have a first section in which the second waveguide has a line width smaller than that of the first waveguide and a second section in which the first waveguide has a line width smaller than that of the second waveguide. The first section and the second section are replaced with each other in curved parts.

Claims

1. An optical modulator comprising: a Mach-Zehnder optical waveguide having an input waveguide, a demultiplexer demultiplexing light propagating in the input waveguide, first and second waveguides extending in parallel from the demultiplexer, a multiplexer multiplexing lights propagating in the first and second waveguides, and an output waveguide propagating light output from the multiplexer; and a signal electrode controlling the phase of light propagating in the Mach-Zehnder optical waveguide, wherein the first and second waveguides have a first section in which the second waveguide has a line width smaller than that of the first waveguide and a second section in which the first waveguide has a line width smaller than that of the second waveguide, and the first section and the second section are replaced with each other in curved part.

2. The optical modulator as claimed in claim 1, wherein the first and second waveguides have an even number of the curved parts.

3. The optical modulator as claimed in claim 1, wherein the first and second waveguides have first to third straight parts provided mutually in parallel, a first curved part connecting the first and second straight parts, and a second curved part connecting the second and third straight parts, the first section is provided in the first and third straight parts, the second section is provided in the second straight part, and the first and second sections are replaced with each other in the first and second curved parts.

4. The optical modulator as claimed in claim 1, wherein the waveguide is a ridge waveguide wherein a lithium niobate film formed on a substrate is shaped into a ridge.

5. The optical modulator as claimed in claim 2, wherein the first and second waveguides have first to third straight parts provided mutually in parallel, a first curved part connecting the first and second straight parts, and a second curved part connecting the second and third straight parts, the first section is provided in the first and third straight parts, the second section is provided in the second straight part, and the first and second sections are replaced with each other in the first and second curved parts.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a schematic plan view illustrating a configuration of an optical modulator according to a first embodiment of the present invention, and particularly illustrating only an optical waveguide.

[0020] FIG. 2 is a schematic plan view illustrating the configuration of the optical modulator according to the first embodiment of the present invention, and particularly illustrating the entire of the optical modulator including traveling-wave electrodes.

[0021] FIGS. 3A and 3B are schematic views for explaining elimination of asymmetry between the pair of optical waveguides.

[0022] FIGS. 4A to 4C are schematic cross-sectional views of the optical modulator 1 illustrated in FIGS. 1 and 2, wherein FIG. 4A is a cross-sectional view taken along the line Xa-Xa in FIGS. 1 and 2, FIG. 4B is a cross-sectional view taken along the line Xb-Xb in FIGS. 1 and 2, and FIG. 4C is a cross-sectional view taken along the line Xc-Xc in FIGS. 1 and 2.

[0023] FIGS. 5A and 5B are schematic plan views each illustrating a configuration of an optical modulator according to a second embodiment of the present invention, wherein FIG. 5A illustrates only an optical waveguide, and FIG. 5B illustrates the entire of the optical modulator including traveling-wave electrodes.

[0024] FIGS. 6A and 6B are schematic cross-sectional views of the optical modulator illustrated in FIGS. 5A and 5B, wherein FIG. 6A is a cross-sectional view taken along the line Xa-Xa in FIGS. 5A and 5B, and FIG. 6B is a cross-sectional view taken along the line Xb-Xb in FIGS. 5A and 5B.

[0025] FIG. 7 is a graph showing a relationship between a ratio of a length L.sub.A2 of a second line width constant section Z.sub.A2 to a length L.sub.A1 of a first line width constant section Z.sub.A1 and an extinction ratio of the optical modulator, wherein the horizontal axis thereof indicates L.sub.A2/L.sub.A1, and the vertical axis thereof indicates the extinction ratio at an operating wavelength of 1550 nm.

MODE FOR CARRYING OUT THE INVENTION

[0026] Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings.

[0027] FIGS. 1 and 2 are schematic plan views each illustrating the configuration of an optical modulator according to a first embodiment of the present invention. FIG. 1 illustrates only an optical waveguide, and FIG. 2 illustrates the entire configuration of the optical modulator including traveling-wave electrodes.

[0028] As illustrated in FIGS. 1 and 2, an optical modulator 1 according to the present embodiment includes a Mach-Zehnder optical waveguide 2 formed on a substrate 10 and having first and second waveguides 5A and 5B provided in parallel to each other, a signal electrode 21 provided so as to overlap the first waveguide 5A in a plan view, a first ground electrode 22 provided so as to overlap the second waveguide 5B in a plan view, and a second ground electrode 23 provided on the side opposite to the first ground electrode 22 with respect to the signal electrode 21.

[0029] The Mach-Zehnder optical waveguide 2 is an optical waveguide having a Mach-Zehnder interferometer structure. The Mach-Zehnder optical waveguide 2 has, in the order from an optical input port 1i toward an optical output port 1o, an input waveguide 3 constituted by a single optical waveguide, a demultiplexer 4 for demultiplexing light propagating in the input waveguide 3, a pair of parallel waveguides 5 (first and second waveguides 5A and 5B) extending from the demultiplexer 4, a multiplexer 6 for multiplexing lights propagating in the first and second waveguides 5A and 5B, and an output waveguide 7 constituted by a single optical waveguide and propagating light output from the multiplexer 6.

[0030] The input waveguide 3 extending from the optical input port 1i is connected to one ends of the first and second waveguides 5A and 5B through the demultiplexer 4, and the other ends of the first and second waveguides 5A and 5B are connected to the output waveguide 7 through the multiplexer 6. An input light S.sub.in input to the optical input port 1i is input to the demultiplexer 4 through the input waveguide 3 to be demultiplexed. The demultiplexed lights travel through the first and second waveguides 5A and 5B and then multiplexed at the multiplexer 6. The multiplexed light travels through the output waveguide 7 and is output from the optical output port 1o as a modulated light S.sub.out.

[0031] In the present embodiment, the Mach-Zehnder optical waveguide 2 is constituted by a combination of a straight part and a curved part. The first and second waveguides 5A and 5B of the Mach-Zehnder optical waveguide 2 have mutually parallel first to third straight parts 5S.sub.1 to 5S.sub.3, a first curved part 5C.sub.1 connecting the first straight part 5S.sub.1 and second straight part 5S.sub.2 together and a second curved part 5C.sub.2 connecting the second straight part 5S.sub.2 and the third straight part 5S.sub.3 together. The first and second curved parts 5C.sub.1 and 5C.sub.2 are formed into concentric half circles so as to turn the traveling direction of the optical waveguide by 180°. As described above, the first and second waveguides 5A and 5B have a folded structure obtained by alternately combining the straight and curved parts.

[0032] The first to third straight parts 5S.sub.1 to 5S.sub.3 of the first and second waveguides 5A and 5B of the Mach-Zehnder optical waveguide 2 extend in the longitudinal direction (Y-direction) of the substrate 10, and the optical input port 1i and the optical output port 1o are provided respectively at one and the other end sides of the substrate 10 in the longitudinal direction. In the present embodiment, the first and second waveguides 5A and 5B preferably have the even number of the curved parts 5C. That is, the first and second waveguides 5A and 5B preferably have 2n+1 (n is a positive integer) straight parts 5S and 2n curved part 5C. Thus, the entire length of the first waveguide 5A and the entire length of the second waveguide 5B can be made equal at the plurality of the curved parts 5C to make it possible to improve symmetry between the first and second waveguides 5A and 5B.

[0033] In the configuration described above, the input light S.sub.in input to the optical input port 1i is then input to one end of the first straight part 5S.sub.1. The input light S.sub.in then travels from the one end of the first straight part 5S.sub.1 toward the other end thereof, turns back at the first curved part 5C.sub.1, travels from one end of the second straight part 5S.sub.2 toward the other end thereof in the direction opposite to that in the first straight part 5S.sub.1, turns back at the second curved part 5C.sub.2, and travels from one end of the third straight part 5S.sub.3 toward the other end thereof in the same direction as in the first straight part 5S1. The modulated light S.sub.out is output from the optical output port 1o.

[0034] The signal electrode 21 (first control electrode) is provided along a part of the first waveguide 5A and is positioned between the first and second ground electrodes 22 and 23 in a plan view. The first ground electrode 22 (second control electrode) is provided along a part of the second waveguide 5B. One end of the signal electrode 21 serves as an RF signal input port, and the other end thereof is connected to the first and second ground electrodes 22 and 23 through a terminal resistor 24. Thus, the signal electrode 21 and first and second ground electrodes 22, 23 function as coplanar traveling-wave electrodes.

[0035] The RF signal input port of the signal electrode 21 is input with an electrical signal (modulated signal). The first and second waveguides 5A and 5B are made of an electrooptic material typified by lithium niobate, so that applying an electric field to the first and second waveguides 5A and 5B makes the refractive indices of the first and second waveguides 5A and 5B change like +Δn and −Δn, respectively, causing a change in phase difference between the pair of optical waveguides. A signal light modulated by the change in phase difference is output from the optical output port lo. When phase difference between two lights propagating respectively in the first and second waveguides 5A and 5B is large, the two lights cancel each other, resulting in an OFF state. When the phase difference is small, the intensities of the two lights are enhanced, resulting in an ON state. An extinction ratio refers to a ratio between light intensity when the optical modulator 1 is in an ON state and light intensity when it is in an OFF state. That is, a high extinction ratio is obtained at a combination of a brighter ON state and a darker OFF state, and a low extinction ratio is obtained at a combination of a darker ON state and a brighter OFF state.

[0036] For application of DC bias, a pair of bias electrodes (not illustrated) may preferably be provided at positions overlapping respectively the first and second waveguides 5A and 5B in a plan view. One ends of the pair of bias electrodes each serve as an input port for DC bias. The pair of bias electrodes may be provided on the side close to the optical input port 1i or optical output port 1o of the Mach-Zehnder optical waveguide 2 with respect to the signal electrode 21. In place of providing the bias electrode, a modulated signal on which DC bias is already superimposed may be input to the RF signal input port.

[0037] The Mach-Zehnder optical waveguide 2 according to the present embodiment is featured in that the mutually parallel first and second waveguides 5A and 5B branched from the input waveguide 3 each differ in line width depending on the position in the longitudinal direction.

[0038] The parallel waveguides 5 (first and second waveguides 5A and 5B) have, in the order from the optical input port 1i side toward the optical output port 1o side, a first uniform width section Z.sub.A1, a first width transition section Z.sub.B1, a second uniform width section Z.sub.A2, a second width transition section Z.sub.B2, and the first uniform width section Z.sub.A1. The first straight part 5S.sub.1 and third straight part 5S.sub.3 each constitute the first uniform width section Z.sub.A1, and the second straight part 5S.sub.2 constitutes the second uniform width section Z.sub.A2. The first curved part 5C.sub.1 constitutes the first width transition section Z.sub.B1, and the second curved part 5C.sub.2 constitutes the second width transition section Z.sub.B2.

[0039] The first uniform width section Z.sub.A1 (first section) is a section in which the first and second waveguides 5A and 5B each have a uniform line width and in which the second waveguide 5B has a line width smaller than that of the first waveguide 5A. The second uniform width section Z.sub.A2 (second section) is a section in which the first and second waveguides 5A and 5B each have a uniform line width and in which the first waveguide 5A has a line width smaller than that of the second waveguide 5B. The first width transition section Z.sub.B1 is a section in which the line widths of the first and second waveguides 5A and 5B vary. The second width transition section Z.sub.B2 is a section in which the line widths of the first and second waveguides 5A and 5B vary. In the uniform width section, the line width need not necessarily be completely uniform and may slightly vary as compared to that in the width transition section.

[0040] Line widths W.sub.a1 and W.sub.a3 of the first waveguide 5A in the first uniform width section Z.sub.A1 only need to be larger than the second waveguide 5B (W.sub.a1<W.sub.b1, W.sub.a3<W.sub.b3) in the first uniform width section Z.sub.A1 and need not be larger than a line width W.sub.a2 of the first waveguide 5A in the in the second uniform width section Z.sub.A2. That is, the line width W.sub.a1 of the first waveguide 5A in the first uniform width section Z.sub.A1 and the line width W.sub.a2 thereof in the second uniform width section Z.sub.A2 may have a relation of W.sub.a1<W.sub.a2. Such a relation may apply to the second waveguide 5B.

[0041] The entire length (L.sub.1+L.sub.5) of the first uniform width section Z.sub.A1 is preferably equal to the entire length (L.sub.3) of the second uniform width section Z.sub.A2. That is, the length (L.sub.1+L.sub.5) of a part of the first waveguide 5A that has a line width larger than that of the second waveguide 5B and the length (L.sub.3) of a part of the first waveguide 5A that has a line width smaller than that of the second waveguide 5B are preferably equal to each other, and the length (L.sub.3) of a part of the second waveguide 5B that has a line width larger than that of the first waveguide 5A and the length (L.sub.1+L.sub.5) of a part of the second waveguide 5B that has a line width smaller than that of the first waveguide 5A are preferably equal to each other. Further, the entire length of the first waveguide 5A in the first and second width transition sections Z.sub.B1 and Z.sub.B2 and the entire length of the second waveguide 5B in the first and second width transition sections Z.sub.B1 and Z.sub.B2 are preferably equal to each other. However, depending on the level of the extinction ratio required for an optical modulator, the lengths of the first uniform width section Z.sub.A1 and second uniform width section Z.sub.A2 need not be completely equal and may be slightly different.

[0042] FIGS. 3A and 3B are schematic views for explaining elimination of asymmetry between the pair of optical waveguides.

[0043] When a waveguide part 11 on the substrate 10 is processed so as to have a ridge shape to form a plurality of mutually parallel straight waveguides extending in the Y-direction, the line width varies among the straight waveguides due to a manufacturing process. For example, as illustrated in FIG. 3A, out of six straight waveguides SL.sub.1 to SL.sub.6 extending in the Y-direction, the straight waveguide SL.sub.1, which is closest to one end side (origin point P.sub.0) in the X-direction, may have a largest line width W.sub.SL1, and as the position of the straight waveguide becomes more distant from the one end side in the X-direction, the line width may be reduced. That is, the line widths W.sub.SL1 to W.sub.SL6 of the six straight waveguides SL.sub.1 to SL.sub.6 have a relation of W.sub.SL1>W.sub.SL2>W.sub.SL3>W.sub.SL4>W.sub.SL5>W.sub.SL6. In this case, the line widths of the two straight waveguides selected from among these straight waveguides and adjacent to each other in the X-direction are always different. This makes the effective refractive indices of the two straight waveguides different, with the result that the extinction ratio of an optical modulator deteriorates.

[0044] However, as illustrated in FIG. 3B, when one end e.sub.11 of the straight waveguide SL.sub.1 and one end e.sub.14 of the straight waveguide SL.sub.4 are connected through a curved waveguide CL.sub.1, one end e.sub.12 of the straight waveguide SL.sub.2 and one end e.sub.13 of the straight waveguide SL.sub.3 are connected through a curved waveguide CL.sub.2, the other end e.sub.23 of the straight waveguide SL.sub.3 and the other end e.sub.26 of the straight waveguide SL.sub.6 are connected through a curved waveguide CL.sub.3, and the other end e.sub.24 of the straight waveguide SL.sub.4 and the other end e.sub.25 of the straight waveguide SL.sub.5 are connected through a curved waveguide CL.sub.4, sections in which the line width is relatively large and those in which the line width is relative small can be included in a single optical waveguide. Then, as illustrated in FIG. 3, in each of the first and second waveguides 5A and 5B, the entire length of the section having a line width larger than the line width of the counterpart waveguide and the entire length of the section having a line width smaller than the line width of the counterpart waveguide are made substantially equal, whereby asymmetry between the first and second waveguides 5A and 5B can be eliminated. Thus, the effective refractive indices of the pair of optical waveguides can be made to coincide with each other, thereby making it possible to improve the extinction ratio of the optical modulator 1.

[0045] FIGS. 4A to 4C are schematic cross-sectional views of the optical modulator 1 illustrated in FIGS. 1 and 2. FIG. 4A is a cross-sectional view taken along the line Xa-Xa in FIGS. 1 and 2, FIG. 4B is a cross-sectional view taken along the line Xb-Xb in FIGS. 1 and 2, and FIG. 4C is a cross-sectional view taken along the line Xc-Xc in FIGS. 1 and 2.

[0046] As illustrated in FIGS. 4A to 4C, the optical modulator 1 according to the present embodiment has a multilayer structure including a substrate 10, a waveguide layer 11, a protective layer 12, a buffer layer 13, an insulating layer 14, and an electrode layer 15 which are stacked in this order.

[0047] The substrate 10 is, e.g., a sapphire substrate, and the waveguide layer 11 made of a lithium niobate film, is formed on the surface of the substrate 10. The waveguide layer 11 has the first and second waveguides 5A and 5B (ridge waveguides) each formed by a ridge part 11r. The width of each of the first and second waveguides 5A and 5B can be set to, e.g., about 1 μm. The waveguide layer 11 is not particularly limited as long as it is made of an electrooptic material and is preferably made of lithium niobate (LiNbO.sub.3). This is because lithium niobate has a large electrooptic constant and is thus suitable as the constituent material of an electrooptic device such as an optical modulator. When the width of the ridge part 11r is not uniform in the Z-axis direction, the average value thereof is defined as the line width.

[0048] The protective layer 12 is formed in an area not overlapping the first and second waveguides 5A and 5B in a plan view. The protective layer 12 covers the entire surface of the waveguide layer 11 excluding portions where the ridge parts 11r are formed, and the side surfaces of the ridge parts 11r are also covered with the protective layer 12, so that a scattering loss caused due to the roughness of the side surfaces of the ridge parts 11r can be prevented. The thickness of the protective layer 12 is substantially equal to the height of each of the ridge parts 11r of the waveguide layer 11. There is no particular restriction on the material of the protective layer 12 and, for example, silicon oxide (SiO.sub.2) may be used.

[0049] The buffer layer 13 is formed on the upper surface of the ridge parts 11r constituting the first and second waveguides 5A and 5B so as to prevent light propagating in the first and second waveguides 5A and 5B from being absorbed by the signal electrode 21 and first ground electrode 22. The buffer layer 13 is preferably formed of a material having a lower refractive index and a higher transparency than the waveguide layer 11. The thickness of the buffer layer 13 immediately above the ridge parts 11r is preferably 0.3 μm or more and 3 μm or less. The film thickness of the buffer layer 13 is preferably as large as possible in order to reduce light absorption of an electrode and preferably as small as possible in order to apply a high electric field to the first and second waveguides 5A and 5B. The light absorption and applied voltage of an electrode have a trade-off relation, so that it is important to select a material having a high dielectric constant and a low refractive constant in consideration of balance therebetween.

[0050] In the present embodiment, although the buffer layer 13 covers not only the upper surfaces of the respective first and second waveguides 5A and 5B, but also the entire underlying surface including the upper surface of the protective layer 12, it may be patterned so as to selectively cover only the vicinity of the upper surfaces of the first and second waveguides 5A and 5B. Further, the buffer layer 13 may be directly formed on the upper surface of the waveguide layer 11 with the protective layer 12 omitted.

[0051] The insulating layer 14 is provided to form a level difference on the lower surface of the traveling-wave electrodes. Openings (slits) are formed in areas of the insulating layer 14 that overlap the respective first and second waveguides 5A and 5B to expose the upper surface of the buffer layer 13 therethrough. A part of the electrode layer 15 is embedded in the openings, whereby the level differences are formed respectively on the lower surfaces of the signal electrode 21 and first ground electrode 22. The thickness of the insulating layer 14 is preferably 1 μm or more. When the thickness of the insulating layer 14 is 1 μm or more, an effect brought about by formation of the level difference on the lower surfaces of the respective signal electrode 21 and first ground electrode 22 can be obtained.

[0052] The electrode layer 15 is provided with the signal electrode 21, first ground electrode 22, and second ground electrode 23. The signal electrode 21 (first control electrode) is provided so as to overlap the ridge part 11r corresponding to the first waveguide 5A in order to modulate light traveling inside the first waveguide 5A and is opposed to the first waveguide 5A through the buffer layer 13. The first ground electrode 22 (second control electrode) is provided so as to overlap the ridge part 11r corresponding to the second waveguide 5B in order to modulate light traveling inside the second waveguide 5B and is opposed to the second waveguide 5B through the buffer layer 13. The second ground electrode 23 is provided on the side opposite to the first ground electrode 22 with respect to the signal electrode 21.

[0053] The signal electrode 21 has a two-layer structure and has an upper layer part 21a formed on the electrode layer 15 and a lower layer part 21b embedded in an opening penetrating the insulating layer 14. The lower layer part 21b of the signal electrode 21 is positioned at the end portion of the upper layer part 21a of the signal electrode 21 on the side close to the first ground electrode 22. Accordingly, the lower surface of the lower layer part 21b of the signal electrode 21 is positioned on the side closer to the first ground electrode 22 than the lower surface of the upper layer part 21a. With such a configuration, the lower surface of the lower layer part 21b of the signal electrode 21 contacts the upper surface of the buffer layer 13 above the first waveguide 5A to cover the first waveguide 5A through the buffer layer 13. The lower surface of the upper layer part 21a of the signal electrode 21 is positioned above the lower surface of the lower layer part 21b and does not contact the buffer layer 13.

[0054] The first ground electrode 22 also has a two-layer structure and has an upper layer part 22a formed on the electrode layer 15 and a lower layer part 22b embedded in an opening penetrating the insulating layer 14. The lower layer part 22b of the first ground electrode 22 is positioned at the end portion of the upper layer part 22a of the first ground electrode 22 on the side close to the signal electrode 21. Accordingly, the lower surface of the upper layer part 22a of the first ground electrode 22 is positioned on the side closer to the signal electrode 21 than the lower surface of the lower layer part 22b. With such a configuration, the lower surface of the lower layer part 22b of the first ground electrode 22 contacts the upper surface of the buffer layer 13 above the second waveguide 5B to cover the second waveguide 5B through the buffer layer 13. The lower surface of the upper layer part 22a of the first ground electrode 22 is positioned above the lower surface of the lower layer part 22b and does not contact the buffer layer 13.

[0055] The second ground electrode 23 is provided on the side opposite to the first ground electrode 22 with respect to the signal electrode 21. Although the second ground electrode 23 has a single-layer structure including only a conductor provided in the electrode layer 15, it may have a two-layer structure like the signal electrode 21 and first ground electrode 22.

[0056] As described above, a pair of optical waveguides sometimes have asymmetry in terms of the line width due to a manufacturing process, and this may result in deterioration in the extinction ratio. However, in the first uniform width section Z.sub.A1, the line widths W.sub.a1 and W.sub.a3 of the first waveguide 5A are made larger than the line widths W.sub.b1 and W.sub.b3 of the second waveguide 5B as illustrated in FIGS. 4A and 4C and, conversely, in the second uniform width section Z.sub.A2, the line width W.sub.b2 of the second waveguide 5B is made larger than the line width W.sub.a2 of the first waveguide 5A as illustrated in FIG. 4B to make the length of the first uniform width section Z.sub.A1 and the length of the second uniform width section Z.sub.A2 substantially equal to each other. This can enhance symmetry in terms of the line width between a pair of waveguides to improve the extinction ratio.

[0057] The following describes in detail the configuration of the optical modulator 1 when the waveguide layer 11 is made of a lithium niobate film.

[0058] While the substrate 10 is not particularly limited in configuration as long as it has a refractive index lower than that of a lithium niobate film, a substrate for which a lithium niobate film is used as an epitaxial film is preferable, and a sapphire single crystal substrate or a silicon single crystal substrate is particularly preferable. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film is easily formed as a c-axis oriented epitaxial film for single crystal substrates with various crystal orientations. The c-axis oriented lithium niobate film has three-fold symmetry and, thus, an underlying single crystal substrate desirably also has the same symmetry. As the sapphire single crystal substrate, a c-plane substrate is preferable. As the silicon single crystal substrate, a (111)-plane substrate is preferable.

[0059] The term “epitaxial film”, as used herein, refers to a film having the crystal orientation of the underlying substrate or underlying film. Assuming that the film surface extends in X-Y plane and that the film thickness direction is Z-axis direction, the crystal of the epitaxial film is uniformly oriented along the X-, Y-, and Z-axes. For example, an epitaxial film can be confirmed by, firstly, measuring the peak intensity at the orientation position by 2θ-θ X-ray diffraction and, secondly, observing poles.

[0060] Specifically, firstly, in the 2θ-θ X-ray diffraction measurement, all the peak intensities except for the peak intensity on a target surface must be 10% or less, preferably 5% or less, of the maximum peak intensity on the target surface.

[0061] For example, in a c-axis oriented epitaxial lithium niobate film, the peak intensities except for the peak intensity on a (00L) surface are 10% or less, preferably 5% or less, of the maximum peak intensity on the (00L) surface. The (00L) is a general term for (001), (002) and other equivalent surfaces.

[0062] Secondly, poles must be observable in the measurement. Under the condition where the peak intensities are measured at the first orientation position, only the orientation in a single direction is proved. Even if the first condition is satisfied, in the case of nonuniformity in the in-plane crystalline orientation, the X-ray intensity does not increase at a particular angle, and poles cannot be observed. Since LiNbO.sub.3 has a trigonal crystal system structure, single crystal LiNbO.sub.3 (014) has 3 poles. For the lithium niobate film, it is known that crystals rotated by 180° about the c-axis are epitaxially grown in a symmetrically coupled twin crystal state. In this case, three poles are symmetrically coupled to form six poles. When the lithium niobate film is formed on a silicon single crystal substrate having a (100) plane, the substrate has four-fold symmetry, and 4×3=12 poles are observed. In the present invention, the lithium niobate film epitaxially grown in the twin crystal state is also considered to be an epitaxial film.

[0063] The lithium niobate film has a composition of LixNbAyOz. A denotes an element other than Li, Nb, and O, wherein x ranges from 0.5 to 1.2, preferably 0.9 to 1.05, y ranges from 0 to 0.5, and z ranges from 1.5 to 4, preferably 2.5 to 3.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, alone or a combination of two or more of them.

[0064] The lithium niobate film preferably has a film thickness of 2 μm or less. This is because a high-quality lithium niobate film having a thickness more than 2 μm is difficult to form. The lithium niobate film having an excessively small thickness cannot completely confine light in it, allowing the light to penetrate through the substrate 10 and/or the buffer layer 13. Application of an electric field to the lithium niobate film may therefore cause a small change in the effective refractive index of the optical waveguide. Thus, the lithium niobate film preferably has a film thickness that is at least approximately one-tenth of the wavelength of light to be used.

[0065] The film thickness of the lithium niobate film in the electric field applying region is preferably 1 μm or more and particularly preferably 1.4 μm or more. In a case where the wavelength of light is 1550 nm for optical communication systems, the lithium niobate film having a film thickness of less than 1 μm undergoes abrupt increase in the half wavelength voltage Vπ, making it difficult to set the half wavelength voltage Vπ to 3V or less which is a practical voltage value. That is, the lithium niobate film having a small thickness cannot completely confine light in it, whereby an electrooptic effect effectively decreases. On the other hand, the lithium niobate film having a film thickness of 1.5 μm or more can sufficiently confine light, so that even if the film thickness is increased some more, Vπ hardly changes. As described above, when the film thickness of the lithium niobate film is set to 1 μm or more, a drive voltage and a propagation loss can be reduced.

[0066] The lithium niobate film is preferably formed using a film formation method, such as sputtering, CVD or sol-gel process. Application of an electric field in parallel to the c-axis of the lithium niobate that is oriented perpendicular to the main surface of the substrate 10 can change the optical refractive index in proportion to the electric field. In the case of the single crystal substrate made of sapphire, the lithium niobate film can be directly epitaxially grown on the sapphire single crystal substrate. In the case of the single crystal substrate made of silicon, the lithium niobate film is epitaxially grown on a clad layer (not illustrated). The clad layer (not illustrated) has a refractive index lower than that of the lithium niobate film and should be suitable for epitaxial growth. For example, a high-quality lithium niobate film can be formed on a clad layer (not illustrated) made of Y.sub.2O.sub.3.

[0067] As a formation method for the lithium niobate film, there is known a method of thinly polishing or slicing the lithium niobate single crystal substrate. This method has an advantage that characteristics same as those of the single crystal can be obtained and can be applied to the present invention.

[0068] As described above, the optical modulator 1 according to the present embodiment has the Mach-Zehnder optical waveguide 2 including the mutually parallel first and second waveguides 5A and 5B branched from the single input waveguide 3 and the traveling-wave electrodes that applies an electric field to the first and second waveguides 5A and 5B. The first and second waveguides 5A and 5B have the first uniform width section Z.sub.A1 in which the line width of the second waveguide 5B is larger than that of the first waveguide 5A, the second uniform width section Z.sub.A2 in which the line width of the first waveguide 5A is larger than that of the second waveguide 5B, and first and second width transition sections Z.sub.B1 and Z.sub.B2each provided between the first and second uniform width sections Z.sub.A1 and Z.sub.A2 and each having a varied waveguide line width. The length of the first uniform width section Z.sub.A1 and the length of the second uniform width section Z.sub.A2 are equal to each other, so that asymmetry between the pair of optical waveguides due to a manufacturing process can be eliminated. This prevents background light outside the operating wavelength from being brought in a guided state when the operating wavelength of input/output light is in an OFF state to thereby prevent deterioration in the extinction ratio.

[0069] FIGS. 5A and 5B are schematic plan views each illustrating the configuration of an optical modulator according to a second embodiment of the present invention. FIG. 5A illustrates only an optical waveguide, and FIG. 5B illustrates the entire configuration of the optical modulator including a traveling-wave electrode. FIGS. 6A and 6B are schematic cross-sectional views of the optical modulator 1 illustrated in FIGS. 5A and 5B. FIG. 6A is a cross-sectional view taken along the line Xa-Xa in FIGS. 5A and 5B, and FIG. 6B is a cross-sectional view taken along the line Xb-Xb in FIGS. 5A and 5B.

[0070] As illustrated in FIGS. 5A and 5B, the optical modulator 1 according to the present embodiment is featured in that the first and second waveguides 5A and 5B of the Mach-Zehnder optical waveguide 2 are each constituted by a straight part. Further, as shown in FIGS. 6A and 6B, the optical modulator 1 has a multilayer structure including a substrate 10, a waveguide layer 11, a protective layer 12, a buffer layer 13, an insulating layer 14, and an electrode layer 15 which are stacked in this order. This structure is the same as that of the first embodiment.

[0071] In the present embodiment, the first uniform width section Z.sub.A1 is a section in which the first and second waveguides 5A and 5B each have a uniform line width and in which the second waveguide 5B has a line width smaller than that of the first waveguide 5A. The second uniform width section Z.sub.A2 is a section in which the first and second waveguides 5A and 5B each have a uniform line width and in which the first waveguide 5A has a line width smaller than that of the second waveguide 5B. The first width transition section Z.sub.B1 is a section in which the line widths of the first and second waveguides 5A and 5B vary. The second width transition section Z.sub.B2 is a section in which the line widths of the first and second waveguides 5A and 5B vary.

[0072] As in the first embodiment, the entire length (L.sub.1+L.sub.5) of the first uniform width section Z.sub.A1 is preferably equal to the entire length (L.sub.3) of the second uniform width section Z.sub.A2. That is, the length (L.sub.1+L.sub.5) of a part of the first waveguide 5A that has a line width larger than that of the second waveguide 5B and the length (L.sub.3) of a part of the first waveguide 5A that has a line width smaller than that of the second waveguide 5B are preferably equal to each other, and the length (L.sub.3) of a part of the second waveguide 5B that has a line width larger than that of the first waveguide 5A and the length (L.sub.1+L.sub.5) of a part of the second waveguide 5B that has a line width smaller than that of the first waveguide 5A are preferably equal to each other. Further, the entire length of the first waveguide 5A in the first and second width transition sections Z.sub.B1 and Z.sub.B2 and the entire length of the second waveguide 5B in the first and second width transition sections Z.sub.B1 and Z.sub.B2 are preferably equal to each other. However, depending on the level of the extinction ratio required for an optical modulator, the lengths of the first uniform width section Z.sub.A1 and second uniform width section Z.sub.A2 need not be completely equal and may be slightly different.

[0073] According to the present embodiment, it is possible to enhance symmetry between the first and second waveguides 5A and 5B to thereby improve the extinction ratio.

[0074] In conventional optical modulators, one of the first and second waveguides 5A and 5B constituting the Mach-Zehnder optical waveguide 2 may have a line width slightly larger than the other one thereof over the entire length, and this results in deterioration in the extinction ratio due to longitudinally nonuniform distribution (asymmetry) of the line width of an optical waveguide. However, in the present embodiment, in each of the first and second waveguides 5A and 5B, the length of a part having a line width larger than the line width of the counterpart waveguide and the entire length of a part having a line width smaller than the line width of the counterpart waveguide are made equal, whereby it is possible to enhance symmetry between the first and second waveguides 5A and 5B to thereby improve the extinction ratio.

[0075] While the preferred embodiments of the present invention has been described, the present invention is not limited to the above embodiments, and various modifications may be made within the scope of the present invention, and all such modifications are included in the present invention.

[0076] For example, although a single drive type optical modulator is taken as an example in the above embodiment, the electrode structure of the optical modulator is not particularly limited, and the present invention may be applied to a so-called dual drive type optical modulator. Further, although the optical modulator has a single channel structure in which a single Mach-Zehnder optical modulator is provided on the substrate 10 in the above embodiment, it may have a multi-channel array structure provided with a plurality of Mach-Zehnder optical modulators.

EXAMPLE

[0077] Influences of a difference in the line length between the first uniform width section Z.sub.A1 and the second uniform width section Z.sub.A2 of the first and second waveguides 5A and 5B on the extinction ratio were verified.

[0078] Assuming that an optical path length difference Δnd.sub.1=5000 nm when the length of the first uniform width section Z.sub.A1 is L.sub.A1 and that an optical path length difference Δnd.sub.2=5000 nm×L.sub.A2/L.sub.A1 when the length of the second uniform width section Z.sub.A2 is L.sub.A2, the relation between the extinction ratio (Ext ratio), which is obtained when the optical path length difference of the entire Mach-Zehnder optical waveguide=Δnd.sub.12 (Δnd.sub.1-Δnd.sub.2) and an operating wavelength=1550 nm, and L.sub.A2/L.sub.A1 is illustrated in the graph of FIG. 7.

[0079] As can be seen from FIG. 7, when the specification of the extinction ratio of an optical modulator is 28 dB or more, the required specification is satisfied under the condition of L.sub.A2/L.sub.A1>0.5. That is, when the optical path length difference Δnd is 5000 nm in the uniform width section, a desired extinction ratio can be obtained when L.sub.A2/L.sub.A1>0.5 is satisfied, and thus L.sub.A1=L.sub.A2 need not be satisfied.

REFERENCE SIGNS LIST

[0080] 1: optical modulator [0081] 1i: optical input port [0082] 1o: optical output port [0083] 2: Mach-Zehnder optical waveguide [0084] 3: input waveguide [0085] 4: demultiplexer [0086] 5: parallel waveguide [0087] 5A: first waveguide [0088] 5B: second waveguide [0089] 5C.sub.1: first curved part [0090] 5C.sub.2: second curved part [0091] 5S.sub.1: first straight part [0092] 5S.sub.2: second straight part [0093] 5S.sub.3: third straight part [0094] 6: multiplexer [0095] 7: output waveguide part [0096] 10: substrate [0097] 11: waveguide part [0098] 11r : ridge part [0099] 12: protective layer [0100] 13: buffer layer [0101] 14: insulating layer [0102] 15: electrode layer [0103] 21: signal electrode (first control electrode) [0104] 21a : upper layer part of signal electrode [0105] 21b : lower layer part of signal electrode [0106] 22: first ground electrode (second control electrode) [0107] 22a : upper layer part formed on first ground electrode [0108] 22b : lower layer part formed on first ground electrode [0109] 23: second ground electrode [0110] 24: terminal resistor [0111] CL.sub.1-CL.sub.4: curved waveguide [0112] SL.sub.1-SL.sub.6: straight waveguide [0113] S.sub.in: input light [0114] S.sub.out: modulated light (output light) [0115] Z.sub.A1: first uniform width section (first section) [0116] Z.sub.A2: second uniform width section (second section) [0117] Z.sub.B1: first width transition section [0118] Z.sub.B2: second width transition section