Optical modulator

In an optical modulator, lights that have been branched by an input optical branching section are input via curved waveguides to a plurality of optical modulation sections arranged in parallel on the same substrate. In the optical modulation sections, optical branching sections of an MZ type optical waveguide are arranged shifted to an output side in the longitudinal direction (x direction) of the substrate, corresponding to an arrangement of input ends of signal electrodes. As a result, even if the input ends of the signal electrodes of the respective optical modulation sections are arranged side by side with a predetermined spacing on one side face of the substrate, input light can be applied to the respective optical modulation sections at low loss, without incurring an increase in the drive voltage.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2009-029003, filed on Feb. 10, 2009, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment relates to an optical modulator which is an optical waveguide device used in optical communication, and is configured with a plurality of Mach-Zehnder (MZ) type modulation sections arranged in parallel on a single substrate.

BACKGROUND

An optical waveguide device that uses an electro-optic crystal such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO2), is formed by forming a metal film of titanium (Ti) or the like on a part of a crystal substrate, to be thermally defused, or to be patterned, after which it is proton exchanged or the like in benzoic acid, to form an optical waveguide, and thereafter an electrode is provided in the vicinity of the optical waveguide. As such an optical waveguide device that uses an electro-optic crystal, there is known for example an optical modulator as illustrated inFIG. 1.

InFIG. 1, an optical waveguide formed on a substrate100comprises; an input waveguide121, an optical branching section122, a pair of branching waveguides123and124, an optical multiplexing section125, and an output waveguide126. A signal electrode131and an earth electrode132are provided on the pair of branching waveguides123and124, to form a coplanar electrode. In the case where a Z-cut substrate is used, in order to use the refractive index variation due to the electric field in the Z direction, the signal electrode131and the earth electrode132are arranged directly above the optical waveguides. More specifically, the electrodes are patterned with the signal electrode131on the branching waveguide123, and the earth electrode132on the branching waveguide124. Here in order to prevent the light that is propagated through the branching waveguides123and124from being absorbed by the signal electrode131and the earth electrode132, a buffer layer (not illustrated in the figure) is provided between the substrate100, and the signal electrode131and the earth electrode132. For the buffer layer, an oxide silicon (SiO2) or the like of 0.2 to 2 μm thickness is used.

In the case where such an optical modulator is driven at high speed, the output end P4of the signal electrode131is connected to the earth electrode132via a resistance (not illustrated in the figure) to make a travelling wave electrode, and a microwave electric signal RF is applied from the input end P3of the signal electrode131. At this time, due to the electric field generated between the signal electrode131and the earth electrode132, the refractive indices of the branching waveguides123and124respectively change as +na and −nb, so that the phase difference of the light propagated on the branching waveguides123and124changes. Therefore, a light Lin input to the input port P1is intensity modulated by Mach-Zehnder (MZ) interferometer, and modulation light Lout is output from the output port P2. The range INT illustrated by the arrow in the figure, represents the part in which light and electric field interact, and in the following description, this is referred to as the “interaction portion”. Furthermore, the longitudinal direction of the substrate10(the propagation direction of the light in the interaction portion INT) is the x direction, and the direction perpendicular to the x direction is the y direction. By changing the cross-section shape of the signal electrode131to control the effective refractive index of the microwave electric signal RF, and by matching propagation speeds of the light and the microwave electric signal with each other, high speed optical response characteristics can be obtained.

Furthermore, due to the variety of recent optical modulation formats (for example multi-valued modulation format, optical polarization division multiplexing format, and the like), there are many cases where signals corresponding to a desired optical modulation format are generated, by combining a number of conventional optical modulators such as illustrated inFIG. 1(for example, refer to Japanese Laid-open Patent Publication No. 2008-122786).

In the above described configuration where a plurality of optical modulators are combined, in order to reduce the size of the overall optical modulator, it is effective to integrate respective optical modulators on a single chip (substrate). In the following description, individual optical modulators integrated on a single chip is referred to as “an optical modulation section”.

More specifically, the optical modulator illustrated inFIG. 2is a configuration example of where two optical modulation sections120A and120B are arranged in parallel on a single substrate100. The optical modulation sections120A and120B, similarly to the configuration illustrated inFIG. 1, each have an MZ type optical waveguide, a signal electrode, and an earth electrode. Furthermore, as the optical waveguide for inputting light to the respective optical modulation sections120A and120B, an input waveguide111connected to one input port P1which is one end face of the substrate100, is branched into two curved waveguides113A and113B by an optical branching section112, and the respective curved waveguides113A and113B are connected to input waveguides121A and121B of the respective optical modulation sections.

In the above described configuration, considering to apply electric signals RFAand RFBfrom the outside to signal electrodes131A and131B of the respective optical modulation sections120A and120B, electrode input terminals are provided in a package (not illustrated in the figure) for accommodating the substrate100. If electrode input terminals respectively corresponding to the optical modulation sections120A and120B are placed side by side on the side face on one side of the package, mounting of the substrate100can be facilitated, and the mounting footprint made small. In this case, for the respective signal electrodes131A and131B on the substrate100, electrode pads formed near each of input ends P3Aand P3Bare arranged side by side on one side (the lower side in the figure) of the opposite side faces of the substrate100.

In the electrode pads near the respective input ends P3Aand P3B, in order to connect to the outside (the electrode input terminals of the package) with wire bonding or the like, it is necessary to have a certain amount of spacing. Therefore, as illustrated at the top ofFIG. 2, the location of the start points of the interaction portions INTAand INTBin the respective optical modulation sections120A and120B are displaced by the spacing dxEof the input ends P3Aand P3Bof the respective signal electrodes131A and131B. As a result, the length of the interaction portion INTBof one optical modulation section120B becomes shorter than the length of the interaction portion INTAof the other optical modulation section120A, and hence the drive voltage on the optical modulation section120B side rises.

Instead of making the interaction portion INTBof the optical modulation section120B short, then as illustrated at the center ofFIG. 2, the size of the substrate100is extended in the lengthwise direction (x direction), and the location of the interaction portions INTAand INTBof the respective optical modulation sections120A and120B are moved in the x direction corresponding to the spacing dxEof the input ends P3Aand P3Bof the respective signal electrodes131A and131B, so that the length of the respective interaction portions INTAand INTBcan be made equal. Here, the output ends P4Aand P4Bof the respective signal electrodes131A and131B are arranged side by side on the other face (the upper face in the figure) of the opposite side faces of the substrate100. However this configuration incurs a lengthening of the size of the package and enlargement of the optical modulator. Furthermore, the number of chips (substrates100) that can be cut from a single chip is reduced, so that manufacturing costs are increased.

In order to arrange the electrode pads of the respective signal electrodes131A and131B side by side on one side of the substrate100without shortening the interaction portion or extending the substrate size in the longitudinal direction, then as illustrated at the bottom ofFIG. 2, it is necessary to shorten the length dx0from the end face (hereunder referred to as the input end face) where the input port P1of the substrate100is located, to the optical branching sections122A and122B of the respective optical modulation sections120A and120B. However, in this case, the radii of curvature of the respective curved waveguides113A and113B becomes small. Therefore the bend loss (radiation loss) in the curved waveguides113A and113B is increased, and the input light intensity of the respective optical modulation sections120A and120B is reduced.

SUMMARY

According to an aspect of the invention, an optical modulator includes a plurality of optical modulation sections which are arranged in parallel on a same substrate having an electro-optic effect, in the respective optical modulation sections, a Mach-Zehnder type optical waveguide is formed on the substrate, and a signal electrode and an earth electrode are provided along a pair of branching waveguides that are disposed between an optical branching section and an optical multiplexing section of the Mach-Zehnder type optical waveguide, that perform modulation of light that propagates on the Mach-Zehnder type optical waveguide by applying an electric signal corresponding to modulation data to the signal electrode that is a travelling wave electrode. This optical modulator includes; an input optical branching section that branches light input from a single input port arranged on an input end face of the substrate into a plurality of lights, and a plurality of curved waveguides that guide the lights branched by the input optical branching section to input ends of the Mach-Zehnder type optical waveguides of the respective optical modulation sections. Furthermore, in the signal electrodes of the respective optical modulation sections, each of the input ends to which the electric signal is applied are arranged side by side with a predetermined spacing on one side face of opposite side faces that intersect with the input end face of the substrate. Moreover, the optical branching sections of the respective optical modulation sections are arranged shifted to the output side in a first direction parallel with the opposite side faces of the substrate, corresponding to the arrangement of the input ends of the signal electrodes.

DESCRIPTION OF EMBODIMENTS

Hereunder is a detailed description of embodiments of the invention, with reference to the drawings.

FIG. 3is a plan view illustrating a configuration of an optical modulator according to a first embodiment.

InFIG. 3, in the optical modulator of this embodiment, on a single crystal substrate10of, for example, LiNbO3or LiTaO2having an electro-optic effect, there is arranged in parallel, two optical modulation sections20A and20B. For the substrate10, here a Z-cut crystal is used, and on one end face thereof (the right end face in the figure) is arranged a single input port P1that is common with the respective optical modulation sections20A and20B, and on the other end face (the left end face in the figure) there is arranged two output ports P2Aand P2Brespectively corresponding to the optical modulation sections20A and20B. In this embodiment also, the propagation direction (the longitudinal direction of the substrate10) of the light in the interaction portions INTAand INTBof the respective optical modulation sections20A and20B is the x direction, and the direction perpendicular to the x direction is the y direction.

One end of one input waveguide11is connected to the input port P1. The other end of the input waveguide11is connected to the input end of an input optical branching section12, and the input ends of the optical modulation sections20A and20B are respectively connected to the two output ends of the input optical branching section12via curved waveguides13A and13B. The input optical branching section12branches the input light into two at a required intensity ratio and outputs this. The respective curved waveguides13A and13B are made gentle approximate S shapes, and each have different lengths corresponding to the displacement of the position of later mentioned optical branching sections22A and22B of the respective optical modulation sections20A and20B. However, the radii of curvature of the curved waveguides13A and13B are approximately the same.

The optical modulation section20A is formed on the surface portion located on the upper side in the figure of the substrate10, and is provided with: a Mach-Zehnder (MZ) type optical waveguide comprising an input waveguide21A, an optical branching section22A, a pair of branching waveguides23A and24A, an optical multiplexing section25A, and an output waveguide26A; and a signal electrode31A and an earth electrode32A that are patterned along the pair of branching waveguides23A and24A.

The input waveguide21A has one end connected to the curved waveguide13A, and to the other end is connected the input end of the optical branching section22A.

In the optical branching section22A, light propagated on the input waveguide21A is branched into two at an intensity ratio of 1:1. To the two output ends of the optical branching section22A is respectively connected each end of the pair of branching waveguides23A and24A. Regarding the arrangement of the optical branching section22A on the substrate10, in the x direction the spacing to the input end face (the right end face where the input port P1is positioned) of the substrate10is dx0, and in the y direction the spacing to the input optical branching section12is dyA. For the positions of the optical branching sections in this description, the branching point (intersection of the Y branch) is made the reference.

In the pair of branching waveguides23A and24A, the branching waveguide23A positioned on the upper side in the figure, and the branching waveguide24A positioned on the lower side in the figure, are parallel with the x direction.

In the optical multiplexing section25A, the two input ends are respectively connected to the other ends of the branching waveguides23A and24A, and the lights propagated on the branching waveguides23A and24A are multiplexed into one. To the one output end of the optical multiplexing section25A is connected one end of the output waveguide26A.

The output waveguide26A outputs the light that has been multiplexed by the optical multiplexing section25A, to the outside from the output port P2Aconnected to the other end.

The signal electrode31A is formed along directly above the branching waveguide23A.

The earth electrode32A is separated from the signal electrode31A, and also is formed to include the portion along directly above the branching waveguide24A. Here the earth electrode32A formed between the branching waveguide24A and a later described branching waveguide23B of the optical modulation section20B, is common to a later described earth electrode32B of the optical modulation section20B.

The signal electrode31A constitutes a travelling wave electrode in which an output end P4Aleading to one side face (the side face on the upper side in the figure) of the opposite side faces parallel with the longitudinal direction of the substrate10, is connected to the earth electrode32A via a resistor (not illustrated in the figure), and a microwave electric signal RFA(electric signal corresponding to modulation data) is applied from the input end P3Aleading to the other side face (the side face on the lower side in the figure) of the substrate10.

For the signal electrode31A and the earth electrode32A, these may be formed on the substrate10(optical waveguide) via a buffer layer that uses SiO2or the like (not illustrated in the figure). By providing a buffer layer, the light propagated within the branching waveguides23A and24A can be prevented from being absorbed by the signal electrode31A and the earth electrode32A.

The optical modulation section20B is formed on the surface portion located on the lower side in the figure of the substrate10, and is provided with: a Mach-Zehnder (MZ) type optical waveguide comprising an input waveguide21B, an optical branching section22B, a pair of branching waveguides23B and24B, an optical multiplexing section25B, and an output waveguide26B; and a signal electrode31B and an earth electrode32B that are patterned along the pair of branching waveguides23B and24B.

The input waveguide21B has one end connected to the curved waveguide13B, and to the other end is connected the input end of the optical branching section22B.

In the optical branching section22B, light propagated on the input waveguide21B is branched into two at an intensity ratio of 1:1. To the two output ends of the optical branching section22B is respectively connected each end of the pair of branching waveguides23B and24B.

Regarding the arrangement of the optical branching section22B on the substrate10, in the x direction the spacing to the input end face of the substrate10is dx0+dx1, and in the y direction the spacing to the input optical branching section12is dyB. In other words, in this relative arrangement of the optical branching section22A of the optical modulation section20, the position of the optical branching section22B of the optical modulation section20B, is displaced in the x direction by dx1from the optical branching section22A to the output end face side (the left end face where the output ports P2Aand P2Bare positioned) of the substrate10, and is displaced in the y direction by dyBto the opposite side to the optical branching section22A with the input optical branching section12as a reference, so that dyBbecomes longer than dyA. That is, for the optical branching sections22A and22B of the respective optical modulation sections20A and20B, the spacing in the x direction is dx1, and the spacing in the y direction is dyA+dyB(where dyA<dyB). The spacing dx1in the x direction can be set corresponding to the spacing dxEof the input ends P3Aand P3Bof the respective signal electrodes31A and31B. Here dx1is made equal to dxE.

In the pair of branching waveguides23B and24B, the branching waveguide23B positioned on the upper side in the figure, and the branching waveguide24B positioned on the lower side in the figure, are parallel with the x direction.

In the optical multiplexing section25B, the two input ends are respectively connected to the other ends of the branching waveguides23B and24B, and the lights propagated on the branching waveguides23B and24B are multiplexed into one. To the one output end of the optical multiplexing section25B is connected one end of the output waveguide26B.

The output waveguide26B outputs the light that has been multiplexed by the optical multiplexing section25B, to the outside from the output port P2Bconnected to the other end.

The signal electrode31B is formed along directly above the branching waveguide23B.

The earth electrode32B is separated from the signal electrode31B, and also is formed to include the portion along directly above the branching waveguide24B. Here the earth electrode32B formed between the branching waveguide23B and the branching waveguide24A of the optical modulation section20A, is common to the earth electrode32A of the optical modulation section20A.

The signal electrode31B constitutes a travelling wave electrode in which an output end P4Bleading to one side face (the side face on the upper side in the figure) of the opposite side faces parallel with the longitudinal direction of the substrate10, is connected to the earth electrode32B via a resistor (not illustrated in the figure), and a microwave electric signal RFB(electric signal corresponding to modulation data) is applied from the input end P3Bleading to the other side face (the side face on the lower side in the figure) of the substrate10. Also for the signal electrode31B and the earth electrode32B, these may be formed on the substrate10(optical waveguide) via a buffer layer in the same way as for the case described above.

In the above manner, the signal electrodes31A and31B of the respective optical modulation sections20A and20B are provided with each of their input ends P3Aand P3Barranged on one side face of the opposite side faces of the substrate10. Since the electrode pads are each formed in the vicinity of the respective input ends P3Aand P3B, the predetermined spacing dxEbetween the respective electrode pads for connecting to the outside by wire bonding or the like is ensured. Furthermore, also for the output ends P4Aand P4Bof the respective signal electrodes31A and31B, similar to the input ends P3Aand P3B, these may be provided arranged on the other side face of the opposite side faces of the substrate10, with the electrode pads for termination formed with the spacing dxE. By arranging the input ends P3Aand P3Band the output ends P4Aand P4Bof the respective signal electrodes31A and31B in this manner, testing and mounting of the substrate10is facilitated.

In the optical modulator configured as described above, the optical branching sections22A and22B in the MZ type optical waveguides of the respective optical modulation sections20A and20B are arranged displaced by dx1in the x direction, and are arranged in the y direction non symmetrically on either side of the input optical branching section12with the spacings to the input optical branching section12different to each other. As a result, between the input optical branching section12and the optical branching sections22A and22B are respectively connected by the curved waveguides13A and13B with a large radius of curvature. At this time, if the size of the substrate10is to be made similar to the aforementioned case at the top inFIG. 2, the spacing dx0between the input end face of the substrate10and the optical branching section22A of the optical modulation section20A becomes shorter than that for the case at the top inFIG. 2, and the spacing dx0+dx1between the input end face of the substrate10and the optical branching section22B of the optical modulation section20B becomes similar to or longer than that for the case at the top inFIG. 2. Furthermore, since the length of the interaction portion INTBof the optical modulation section20B, is made the same as the length of the interaction portion INTAof the optical modulation section20A, the drive voltages of the respective optical modulation sections20A and20B become basically the same level.

The radii of curvature of the curved waveguides13A and13B depend on the positions of the respective optical branching sections22A and22B. In the case where the spacing dx1in the x direction of the optical branching sections22A and22B is equal to the spacing dxEof the input ends P3Aand P3Bof the respective signal electrodes31A and31B, each of the radii of curvature becomes a maximum. Preferably the radius of curvature of the curved waveguide13A and the radius of curvature of the curved waveguide13B are set so as to be equal to each other.

Next is a description of the operation of the optical modulator according to a first embodiment.

At first, output light from a single light source or the like (not illustrated in the figure) is input to the input waveguide11from the input port P1of the present optical modulator. The input light Lin is input to the input optical branching section12through the input waveguide11, and bifurcated according to a required intensity ratio. One of the branched lights output from the input optical branching section12is input to the input waveguide21A of the optical modulation section20A through the curved waveguide13A, and the other branched light is input to the input waveguide21B of the optical modulation section20B through the curved waveguide13B.

At this time, the light input to the respective optical modulation sections20A and20B is attenuated due to the bend loss in the curved waveguides13A and13B. However, in the configuration of the present optical modulator, the relative positions of the input optical branching section12and the respective optical modulation sections20A and20B on the substrate10, is devised to correspond to the spacing dxEof the input ends P3Aand P3Bof the respective signal electrodes31A and31B, so that the radii of curvature of the respective curved waveguides13A and13B become relatively large values. Therefore attenuation of the input light due to the bend loss is only slight or practically nonexistent.

In the optical modulation section20A (20B), the light input to the input waveguide21A (21B) is bifurcated in the optical branching section22A (22B) according to an intensity ratio of 1:1, and respectively sent to the branching waveguides23A and24A (23B and24B). Here, by applying a microwave electric signal RFA(RFB) from the input end P3A(P3B) of the signal electrode31A (31B), light that has been modulated corresponding to the level of the microwave electric signal RFA(RFB) passes through the output waveguide26A (26B) and is output from the output port P2A(P2B) to the outside.

In the above manner, according to the optical modulator of the first embodiment, even if two of the optical modulation sections20A and20B are arranged in parallel on a single substrate10, and the input ends P3Aand P3Bof each of the signal electrodes31A and31B are arranged side by side with a spacing dxEon one side face of the substrate10, the input light of the respective optical modulation section20A and20B can be applied at low loss without incurring an increase in drive voltage or an enlargement in substrate size. As a result, mounting of the substrate10on the package is facilitated, and the mounting footprint is also small. Therefore cost reduction and miniaturization of the optical modulator becomes possible.

In the first embodiment, the example is shown for where the respective optical branching sections22A and22B are arranged so that the spacing dx1in the x direction of the optical branching sections22A and22B of the respective optical modulation sections20A and20B becomes the same as the spacing dxEof the input ends P3Aand P3Bof the respective signal electrodes31A and31B. However the invention is not limited to this. For example, even if the spacing dx1of the optical branching sections22A and22B becomes narrower than the spacing dxEof the respective input ends P3Aand P3B, in the case where the radii of curvature of the curved waveguides13A and13B can be made sufficiently large, the spacing dx1of the respective optical branching sections22A and22B may be determined to match a value for the radius of curvature where it is possible to suppress the bend loss in the curved waveguides13A and13B to below a predetermined value. In this case, as illustrated inFIG. 4, for the signal electrode31A of the optical modulation section20A, the input leader line portion from the input end P3Aup until on the branching waveguide23A (the start point of the interaction portion INTA) may be wired at an incline with an angle with respect to the y direction. As a result, while keeping the spacing of the input ends P3Aand P3Bof the respective signal electrodes31A and31B at dxE(>dx1), the length in the x direction of the substrate10can be shortened, so that it is possible to realize an even smaller optical modulator.

Furthermore, in the first embodiment, since the lengths of the input leader line portions from the input ends P3Aand P3Bof the respective signal electrodes31A and31B up to on the branching waveguides23A and23B are different, then even if the electric signals RFAand RFBare applied simultaneously to the input ends P3Aand P3B, the modulation lights output from the respective optical modulation section20A and20B cannot be synchronized. In order to synchronize these, the timing of the electric signals RFAand RFBmay be delayed to correspond to the difference in the length of the input leader line portions of the respective signal electrodes31A and31B, or as illustrated inFIG. 5, the input leader line portion of the signal electrode31B may be bent, so that the length of this portion becomes equal to the length of the input leader line portion of the signal electrode31A. In the example ofFIG. 5, regarding the spacing of the input ends P3Aand P3Bof the respective signal electrodes31A and31B, by ensuring the necessary dxEin the connection to the outside, and bending the input leader line portion of the signal electrode31B, the spacing in the x direction of the start points of the respective interaction portions INTAand INTBbecomes dxE′ which is narrower than dxE. In this case, the spacing dx1in the x direction of the respective optical branching sections22A and22B may be made so as to be equal to dxE′.

Next is a description of a second embodiment.

FIG. 6is a plan view illustrating a configuration of an optical modulator according to the second embodiment. Parts the same as or corresponding to the configuration of the first embodiment illustrated inFIG. 3are denoted by the same reference symbols and description is omitted. The same also applies for subsequent other embodiments.

InFIG. 6, the point where the configuration of the optical modulator of this embodiment is different to the case of the first embodiment, is that instead of the optical branching sections22A and22B and the optical multiplexing sections25A and25B that are configured using the Y branching waveguide, there is provided optical branching sections27A and27B and optical multiplexing sections28A and28B that use a 2×2 optical coupler having two light input ends and two light output ends.

More specifically, in the optical branching section27A of the optical modulation section20A, the input waveguide21A is connected to the light input end that is the closer to the input optical branching section12of the two light input ends, and the input light Lin that is bifurcated by the input optical branching section12passes through the curved waveguide13A and the input waveguide21A and is applied to the aforementioned light input end. One end of each of the branching waveguides23A and24A is respectively connected to the two light output ends of the optical branching section27A, and the light for which the input light to the light input end has been bifurcated at an intensity ratio of 1:1 is output to the branching waveguides23A and24A from each light output end. The remaining light input end of the optical branching section27A is unused.

In the optical multiplexing section28A of the optical modulation section20A, the other ends of the branching waveguides23A and24A are respectively connected to the two light input ends, and the light that is propagated on the respective branching waveguides23A and24B is multiplexed into one, and then the multiplexed light is bifurcated and output. The output waveguide26A is connected to one light output end of the optical multiplexing section28A, and the modulation light of the optical modulation section20A passes through the output waveguide26A and is output from the output port P2Ato the outside. The other light output end of the optical multiplexing section28A is left open.

Moreover, in the optical branching section27B of the optical modulation section20B, the input waveguide21B is connected to the light input end that is the closer to the input optical branching section12of the two light input ends, and the input light Lin that is bifurcated by the input optical branching section12passes through the curved waveguide13B and the input waveguide21B and is applied to the aforementioned light input end. One end of each of the branching waveguides23B and24B is respectively connected to the two light output ends of the optical branching section27B, and the light for which the input light to the light input end has been bifurcated at an intensity ratio of 1:1 is output to the branching waveguides23B and24B from each light output end. The remaining light input end of the optical branching section27B is unused.

In the optical multiplexing section28B of the optical modulation section20B, the other ends of the branching waveguides23B and24B are respectively connected to the two light input ends, and the light that is propagated on the respective branching waveguides23B and24B is multiplexed into one, and then the multiplexed light is bifurcated and output. The output waveguide26B is connected to one light output end of the optical multiplexing section28B, and the modulation light of the optical modulation section20B passes through the output waveguide26B and is output from the output port P2Bto the outside. The other light output end of the optical multiplexing section28B is left open.

Also in the optical modulator configured as described above, a similar operational effect to the case of the first embodiment can be obtained. Moreover, for the optical branching sections27A and27B that use the 2×2 optical coupler, the light input end closer to the input optical branching section12, of the two light input ends is connected to the curved waveguides13A and13B via the input waveguides21A and21B. Therefore, the radii of curvature of the respective curved waveguides13A and13B can be made large, so that it is possible to apply input light to the respective optical modulation section20A and20B at a lower loss. Moreover, instead of making the radii of curvature of the respective curved waveguides13A and13B large, the spacing dx0from the input end side of the substrate10up to the optical branching section22A can also be made short, and hence it is possible to further miniaturize the optical modulator.

In the above second embodiment, the relationship between the light input to the optical branching section27A in the optical modulation section20A and the light output from the optical multiplexing section28A is a cross-port state, whereas the relationship between the light input to the optical branching section27B in the optical modulation section20B and the light output from the optical multiplexing section28B is a through-port state. Therefore, there is the possibility that the extinction ratios of the modulation lights of the respective optical modulation sections20A and20B are different. In order to avoid this, for example as illustrated inFIG. 7, in the optical branching section27B of the optical modulation section20B, the curved waveguide13B may be connected via the input waveguide21B to the light input end that is further from the input optical branching section12, of the two light input ends, so that the respective optical modulation sections20A and20B become the same cross-port state. That is, in the respective optical branching section27A and27B, light is applied to the light input end positioned on the same side, of the two light input ends, and in the respective optical multiplexing sections28A and28B, light is taken out from the light output end positioned on the same side, of the two light output ends, so that the extinction ratio of the modulation lights of the respective optical modulation sections20A and20B can be made the same.

Next is a description of a third embodiment.

FIG. 8is a plan view illustrating a configuration of an optical modulator according to the third embodiment.

InFIG. 8, in the optical modulator of this embodiment, for example in the aforementioned first embodiment illustrated inFIG. 3, DC electrodes33A and33B are provided separate to the signal electrodes31A and31B of the respective optical modulation sections20A and20B, and by adjusting the bias voltage applied to the DC electrodes33A and33B, the voltage to switch off the optical output of the respective optical modulation sections20A and20B is held constant.

Furthermore, as the optical waveguides for multiplexing the modulation light output from the output waveguides26A and26B of the respective optical modulation section20A and20B into one, and outputting this, curved waveguides14A and14B, an output optical multiplexing section15, and an output waveguide16are formed. The configuration of this optical modulator is a form that includes two child MZ interferometers within a parent MZ interferometer that is formed between the input port P1and the output port P2, giving a configuration corresponding to an optical modulation format such as differential quadrature phase shift keying (DPQSK) or the like. Here, in the parent MZ interferometer also, DC electrodes33A and33B for adjusting the bias voltage are provided.

More specifically, the branching waveguides23A,24A,23B, and24B, of the respective optical modulation section20A and20B are each extended on the output side in order to arrange the DC electrodes33A and33B between the finish point of the respective interaction portions INTAand INTBand the optical multiplexing sections25A and25B. Furthermore, in the signal electrode31B of the optical modulation section20B, the patterning is changed so that the output end P4Bis drawn out to the side face on the same side as the input end P3B. Moreover, the output waveguides26A and26B of the respective optical modulation sections20A and20B are each extended to the outside in order to arrange the DC electrodes34A and34B of the parent MZ interferometer.

The DC electrode33A is formed along directly above the extension portion of the branching waveguide24A for example, and one end is drawn out to one side face (the side face on the upper side in the figure) of the substrate10. Furthermore, the DC electrode33B is formed along directly above the extension portion of the branching waveguide23B for example, and one end is drawn out to the other side face (the side face on the lower side in the figure) of the substrate10. Regarding the relative arrangement of the respective DC electrodes33A and33B in the x direction, similar to the arrangement of the respective optical branching sections22A and22B, the DC electrode33B is moved by dx1to the output side from the DC electrode33A. Preferably the lengths in the x direction of the DC electrodes33A and33B (the lengths of the portions overlapping the branching waveguides24A and23B) are made approximately equal.

The DC electrode34A is formed along directly above the extension portion of the output waveguide26A for example, and one end is drawn out to one side face (the side face on the upper side in the figure) of the substrate10. Furthermore, the DC electrode34B is formed along directly above the extension portion of the output waveguide26B, and one end is drawn out to the other side face (the side face on the lower side in the figure) of the substrate10. Regarding the relative arrangement of the respective DC electrodes34A and34B in the x direction, similar to the arrangement of the respective DC electrodes33A and33B, the DC electrode34B is moved by dx1to the output side from the DC electrode34A. Preferably also the lengths in the x direction of the DC electrodes34A and34B (the lengths of the portions overlapping the branching waveguides24A and23B) are made approximately equal.

The earth electrodes32A and32B of the respective optical modulation sections20A and20B are extended to the output side corresponding to the DC electrodes33A,33B,34A, and34B, and are each separated from the respective DC electrodes33A,33B,34A, and34B.

The curved waveguide14A is connected between the output waveguide26A of the optical modulation section20A and one input end of the output optical multiplexing section15. Furthermore, the curved waveguide14B is connected between the output waveguide26B of the optical modulation section20B and the other input end of the output optical multiplexing section15. The curved waveguides14A and14B become a gentle approximate S shape, and each have different lengths corresponding to the arrangement of the output optical multiplexing section15described later. However, the radii of curvature of the curved waveguides14A and14B are approximately the same.

In the output optical multiplexing section15, the modulation light that has passed through the curved waveguides14A and14B and is then applied to the respective input ends is multiplexed into one and output. One end of the output waveguide16is connected to one output end of the output optical multiplexing section15. In the output waveguide16, the light that has been multiplexed by the output optical multiplexing section15is output to the outside from the output port P2connected to the other end.

The arrangement on the substrate10of the output optical multiplexing section15is such that in the y direction, the spacing to the optical multiplexing section25A of the optical modulation section20A is dyB, and the spacing to the optical multiplexing section25B of the optical modulation section20B is dyA. That is, the relative arrangement of the input optical branching sections12,22A, and22B on the input side, and the relative arrangement of the respective output optical multiplexing sections15,25A, and25B on the output side, have a symmetrical relationship in relation to a central point of the parent MZ interferometer. As a result, the optical path lengths of the pair of branch arms positioned between the input optical branching section12and the output optical multiplexing section15in the parent MZ interferometer, are equal to each other. For the positions of the optical multiplexing sections, the multiplexing point (the intersection of the Y shape waveguide) is made the reference.

In the optical modulator of the above configuration, for example the intensity of the modulation light in the optical modulation sections20A and20B is monitored using an output monitor or the like (not illustrated in the figure), and the bias voltage applied to the respective DC electrodes33A and33B corresponding to the monitor result is feedback controlled, to thereby keep the voltage for switching off the light output of the optical modulation sections20A and20B constant. Furthermore, by adjusting the bias voltage applied to the DC electrodes33A and33B, the phase difference of the respective modulation lights that are multiplexed by the output optical multiplexing section15can be made a desired state. Moreover, since the optical path lengths of the pair of branch arms in the parent MZ interferometer are made equal to each other, the characteristic wavelength dependence in the signal light that has been multiplexed by the output optical multiplexing section15can also be reduced. Consequently, in the present optical modulator, it is possible to stabilize and output signal light corresponding to the optical modulation format of DQPSK or the like.

In the third embodiment, the application example is shown for where the DC electrodes or the like are added to the configuration of the first embodiment. However a similar application is also possible in the configuration of the second embodiment. Furthermore, the modulation light in the optical modulation sections20A and20B, is multiplexed into one by the output optical multiplexing section15, and output. However similarly to the case of the first and second embodiments, the modulation light of the optical modulation sections20A and20B may of course also be output separately.

Next is a description of a fourth embodiment.

In the above mentioned first through third embodiments, the description was for an optical modulator in which two optical modulation sections were arranged in parallel on a single substrate. However the present invention is also applicable to a parallel arrangement of three or more optical modulation sections. Therefore, in the fourth embodiment, an application example is described for where, for example, four optical modulation sections are arranged in parallel on the same substrate.

FIG. 9is a plan view illustrating a configuration of an optical modulator according to the fourth embodiment.

InFIG. 9, in the optical modulator of this embodiment, four optical modulation sections20A,20B,20C, and20D are arranged in parallel on a single crystal substrate10having an electro-optic effect. This optical modulator is provided with one input port P1and one output port P2, and one end of a single input waveguide11is connected to the input port P1. The other end of the input waveguide11is connected to an input end of an input optical branching section12, and to the two output ends of the input optical branching section12are respectively connected input ends of optical modulation sections12AB and12CD via curved waveguides13AB and13CB. To the two output ends of the input optical branching section12AB are respectively connected the input ends of the optical modulation sections20A and20B via curved waveguides13A and13B. To the two output ends of the input optical branching section12CD are respectively connected the input ends of the optical modulation sections20C and20D via curved waveguides13C and13D.

The optical modulation sections20A and20B and the optical modulation sections20C and20D, have a similar configuration to the optical modulation sections20A and20B in the aforementioned first embodiment. Furthermore, the relative positional relationship of the optical modulation section20B and the optical modulation section20C is similar to the relative positional relationship of the optical modulation section20A and the optical modulation section20B in the first embodiment.

That is, in the respective optical modulation sections20A to20D, the input ends P3Ato P3Dof each of the signal electrodes31A to31D are arranged side by side with spacings dxEon one side face, of the opposite side faces of the substrate10, and the output ends P4Ato P4Dare arranged side by side on the other side face of the substrate10. Moreover, the relative arrangement of the optical branching sections22A to22D on the substrate10, in the x direction corresponds to the arrangement spacing dxEof the input ends of the respective signal electrodes31A to31D, with the optical branching section22B displaced by dx1to the output side from the optical branching section22A, the optical branching section22C displaced by dx1to the output side from the optical branching section22B, and the optical branching section22D displaced by dx1to the output side from the optical branching section22C. In the y direction, the positions of the respective optical branching sections22A and22B are displaced by dyAand dyBto either side of the input optical branching section12AB (dyA<dyB), and the positions of the respective optical branching sections22C and22D are displaced by dycand dyDto either side of the input optical branching section12CD (dyc<dyD). Moreover, the positions of the respective optical branching section12AB and12CD are displaced by dyABand dyCDto either side of the input optical branching section12(dyAB<dyCD).

Furthermore, as optical waveguides for multiplexing into one and outputting the respective modulation lights output from the output waveguides26A to26D of the respective optical modulation sections20A to20D, there are formed curved waveguides14A to14D, and14AB and14CD, output optical multiplexing sections15AB,15CD, and15, and an output waveguide16.

The respective curved waveguides14A and14B connect between the output waveguides26A and26B of the optical modulation section20A and20B, and the input ends of the output optical multiplexing section15AB. The respective curved waveguides14C and14D connect between the output waveguides26C and26C of the optical modulation section20C and20D, and the input ends of the output optical multiplexing section15CD. The respective curved waveguides14AB and14CD connect between the output ends of the output optical modulation section15AB and15CD, and the input ends of the output optical multiplexing section15. Each of the curved waveguides14A to14D,14AB, and14CD have a gentle approximate S shape, and each have different lengths corresponding to the arrangement of the respective output optical multiplexing sections15AB,15CD, and15. However, the radii of curvature of the respective curved waveguides14A to14D, and the radii of curvature of the curved waveguides14AB and14CD, are each approximately the same.

The output optical multiplexing section15AB multiplexes into one, the modulation light that has passed through the curved waveguides14A and14B and been applied to the respective input ends, and output this to the curved waveguide14AB. The output optical multiplexing section15CD multiplexes into one, the modulation light that has passed through the curved waveguides14C and14D and been applied to the respective input ends, and output this to the curved waveguide14CD. The output optical multiplexing section15further multiplexes into one, the modulation light that has passed through the curved waveguides14AB and14CD and been applied to the respective input ends, and output this to the output waveguide16. In the output waveguide16, the light that has been multiplexed by the output optical multiplexing section15is output to the outside from the output port P2connected to the other end.

The relative arrangement of the output optical multiplexing sections15,15AB, and15CD have a symmetrical relationship to the relative arrangement of the respective input optical branching sections12,12AB, and12CD on the input side. As a result, the optical path length of the two pairs of (four) branch arms positioned between the input optical branching section12and the output optical multiplexing section15are equal to each other.

According to the optical modulator as described above, even if the four optical modulation sections20A to20D are arranged in parallel on a single substrate10, and the input ends P3Ato P3Dof each of the signal electrodes31A to31D are arranged side by side with spacings dxEon one side face of the substrate10, the input lights to the respective optical modulation sections20A to20D can be applied with low loss without incurring an increase in drive voltage or enlargement in substrate size. Furthermore, since the optical path lengths of the branch arms corresponding to the respective optical modulation sections20A to20D, positioned between the input optical branching section12and the output optical multiplexing section15are made equal to each other, the characteristic wavelength dependence in the signal light that has been multiplexed by the output optical multiplexing section15can also be reduced.

In the fourth embodiment, the modulation lights in the respective optical modulation sections20A to20D are multiplexed into one in the output optical multiplexing sections15AB,15CD, and15, and output. However it is also possible for the modulation lights of the respective optical modulation section20A to20D to be output separately, or for the signal lights that have been multiplexed by the respective output optical multiplexing sections15AB and15CD to be output separately.