Optical modulator

An optical modulator includes a substrate having an electro-optic effect, an optical waveguide formed on the substrate, a modulation part which modulates light waves propagating through the optical waveguide, and a light receiving element which detects the light waves propagating through the optical waveguide. As the modulation part, a first modulation part and a second modulation part, which respectively modulate light waves into which input light branches, are provided. As the light receiving element, a light receiving element for the first modulation part and a light receiving element for the second modulation part are provided. The light receiving elements are disposed such that their positions in a light propagation direction are shifted from each other by an amount corresponding to one light receiving element, or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No. 2016-104774 filed May 26, 2016, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical modulator and in particular, to an optical modulator having a plurality of light receiving elements to correspond to a plurality of modulation parts.

Description of Related Art

With the progress of high frequency and large capacity of an optical communication system, increases in the performance and integration density of an optical modulator which is used for the optical communication systems are being advanced.

Further, downsizing of a substrate for configuring an optical modulator is also being advanced according to a demand for downsizing of the optical modulator. However, the high performance, the high integration density, and the downsizing of the optical modulator are conflicting demands, and therefore, a devisal for achieving all of these demands is required.

With respect to such an optical modulator, the following inventions have been proposed.

For example, Japanese Laid-open Patent Publication No. 2015-18193 discloses an optical modulator having a structure in which light outputted from an optical waveguide and entering into a reflecting groove is reflected by a reflecting film on the bottom surface of the reflecting groove to head for upward, whereby light traveling in the optical waveguide is received by a light receiving element fixed to the surface of a substrate.

For example, Japanese Laid-open Patent Publication No. 2015-194517 discloses an optical modulator having a structure in which a light receiving element is disposed so as to overlay an output waveguide configuring a Mach-Zehnder waveguide and configured so as to receive two radiated lights which are radiated from a Y-junction of the Mach-Zehnder waveguide and two or more light receiving parts are formed to be spaced apart from each other on a single light receiving element substrate.

The optical modulator according to Example 1 of the related art has a first modulation part M(#1) and a second modulation part M(#2) which respectively modulate light waves into which input light branches. Each of the modulation parts M is configured by using an optical waveguide2formed on a substrate1having an electro-optic effect, and a control electrode3for controlling light waves propagating through the optical waveguide2by a control signal. The control electrode3is configured with an RF electrode3ato which an RF signal (a modulation signal) which is a type of control signal is applied, DC electrodes3band3cto which a DC signal (bias voltage) which is a type of control signal is applied, or the like.

The optical waveguide2configuring the modulation part M has a structure in which a sub-Mach-Zehnder waveguide is disposed in a nested type at an arm portion of a main Mach-Zehnder waveguide. The light waves (signal light) modulated in the modulation part M are guided to the outside of the substrate through an output waveguide21which is connected to a Y-junction of the main Mach-Zehnder waveguide in the modulation part M.

Radiated light waveguides22for propagating radiated light which is radiated from the Y-junction of the main Mach-Zehnder waveguide in the modulation part M are provided on both sides of the output waveguide21of each of the modulation parts M. Then, a light receiving element4is disposed so as to overlay the output waveguide21and the radiated light waveguides22on both sides of the output waveguide21. In this example, as the light receiving elements4, a light receiving element4(#1) for the modulation part M(#1) and a light receiving element4(#2) for the modulation part M(#2) are provided. Each of the light receiving elements4has a light receiving part41for receiving light waves from each of the radiated light waveguides22and is bonded and fixed to a predetermined position on the substrate1with an adhesive42.

In the optical modulator according to Example 1 of the related art, the respective modulation parts M are disposed side by side in a width direction of the substrate1with the positions in a length direction (a light propagation direction) of the substrate1aligned with each other. Further, also with respect to the respective light receiving elements4, similarly, they are disposed side by side in the width direction of the substrate1with the positions in the length direction of the substrate1aligned with each other. As the light receiving element4, a light receiving element in which the length of one side is in a range of about 0.2 mm to 0.5 mm and the diameter of the light receiving part41is in a range of about 30 μm to 150 μm is used. In order to improve the workability of bonding work of the light receiving element4to the substrate1or the reliability of adhesion, it is necessary to increase the distance (D11) between the light receiving elements4(#1) and4(#2) adjacent to each other to some extent (in a range of about 0.1 mm to 0.2 mm). Accordingly, it is necessary to increase the distance (D12) between the optical waveguide2on the optical modulation area M(#1) side and the optical waveguide2on the optical modulation area M(#2) side, and this results in an increase of the length (a chip width W) in the width direction of the substrate1.

FIG. 2is a plan view for describing an optical modulator according to Example 2 of the related art.

The optical modulator according to Example 2 of the related art has a first modulation part M(#1) and a second modulation part M(#2), which respectively modulate light waves into which input light having a wavelength1branches, and a third modulation part M(#3) and a fourth modulation part M(#4), which respectively modulate light waves into which input light having a wavelength λ2branches. Each of the modulation parts M is configured by using the optical waveguide2formed on the substrate1having an electro-optic effect, and the control electrode3(the RF electrode3a, the DC electrodes3band3c, or the like) for controlling light waves propagating through the optical waveguide2by a control signal.

The optical waveguide2configuring the modulation part M has a structure in which a sub-Mach-Zehnder waveguide is disposed in a nested type at an arm portion of a main Mach-Zehnder waveguide. The light waves (signal light) modulated in the modulation part M are guided to the outside of the substrate through the output waveguide21which is connected to the Y-junction of the main Mach-Zehnder waveguide in the modulation part M.

A monitoring waveguide24which extracts and propagates some of the light waves propagating through the output waveguide21for monitoring is provided on one side of the output waveguide21of each of the modulation parts M. Then, the light receiving element4is disposed so as to overlay the monitoring waveguide24. In this example, as the light receiving elements4, a light receiving element4(#1) for the modulation part M(#1), a light receiving element4(#2) for the modulation part M(#2), a light receiving element4(#3) for the modulation part M(#3), and a light receiving element4(#4) for the modulation part M(#4) are provided. Each of the light receiving elements4has the light receiving part41for receiving light waves from the monitoring waveguide24and is bonded and fixed to a predetermined position on the substrate1with the adhesive42.

In the optical modulator according to Example 2 of the related art, the respective modulation parts M are disposed side by side in the width direction of the substrate1with the positions in the length direction (the light propagation direction) of the substrate1aligned with each other. Further, also with respect to the respective light receiving elements4, similarly, they are disposed side by side in the width direction of the substrate1with the positions in the length direction of the substrate1aligned with each other. Looking at the modulation parts M (#1) and M (#2), since a sufficient distance can be provided between the light receiving elements4(#1) and4(#2), the optical waveguide2on the modulation part M(#1) side and the optical waveguide2on the modulation part M(#2) can be disposed close to each other. The same applies to the modulation parts M(#3) and M(#4). However, looking at the modulation parts M(#2) and M(#3), in order to secure a distance in a range of about 0.1 mm to 0.2 mm between the light receiving elements4(#2) and4(#3), it is necessary to increase the distance (D13) between the optical waveguide2on the modulation part M(#2) side and the optical waveguide2on the modulation part M(#3) side, and this results in an increase of the length (the chip width W) in the width direction of the substrate1.

An object of the present invention is to solve the above problem and provide an optical modulator with a reduced chip width in an optical modulator having a light receiving element for each of a plurality of modulation parts.

In order to solve the above problem, an optical modulator according to the present invention has the following technical features.

(1) An optical modulator includes: a substrate having an electro-optic effect; an optical waveguide formed on the substrate; first and second modulation parts which respectively modulate light waves propagating through the optical waveguide; and a light receiving element which detects the light waves propagating through the optical waveguide, in which as the light receiving element, a first light receiving element for the first modulation part and a second light receiving element for the second modulation part are provided, and the first light receiving element and the second light receiving element are disposed to be shifted from each other in a light propagation direction.

(2) In the optical modulator according to the above (1), the first modulation part and the second modulation part are disposed to be shifted from each other in the light propagation direction, and the first light receiving element and the second light receiving element are disposed to be shifted from each other in the same light propagation direction as a direction in which the first modulation part and the second modulation part are shifted from each other.

(3) In the optical modulator according to the above (1) or (2), each of the first light receiving element and the second light receiving element has two or more light receiving parts.

(4) In the optical modulator according to the above (3), with respect to each of the first modulation part and the second modulation part, an output waveguide which guides the light waves modulated in the modulation part, and radiated light waveguides which guide radiated light from the modulation part to both sides of the output waveguide are provided, the respective radiated light waveguides on both sides of the output waveguide are bent so as to be parallel to each other, and the first light receiving element and the second light receiving element are respectively disposed at sections in which the radiated light waveguides on both sides of the output waveguide are parallelized, and are disposed to be shifted from each other in the light propagation direction.

According to the optical modulator according to the present invention, in an optical modulator provided with a substrate having an electro-optic effect, an optical waveguide formed on the substrate, first and second modulation parts which respectively modulate light waves propagating through the optical waveguide, and a light receiving element which detects the light waves propagating through the optical waveguide, as the light receiving element, a first light receiving element for the first modulation part and a second light receiving element for the second modulation part are provided, and the first light receiving element and the second light receiving element are disposed to be shifted from each other in a light propagation direction. Therefore, it is possible to provide an optical modulator with a reduced chip width.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1is a plan view for describing an optical modulator according to Example 1 of the related art.

FIG. 2is a plan view for describing an optical modulator according to Example 2 of the related art.

FIG. 3is a plan view for describing an optical modulator according to a first example of the present invention.

FIG. 4is a plan view for describing an optical modulator according to a second example of the present invention.

FIG. 5is a plan view for describing an optical modulator according to a third example of the present invention.

FIG. 6is a plan view for describing an optical modulator according to a fourth example of the present invention.

FIG. 7is a plan view for describing an optical modulator according to a fifth example of the present invention.

FIG. 8is a plan view for describing an optical modulator in which the fourth example and the fifth example are combined.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an optical modulator according to the present invention will be described in detail.

The optical modulator according to the present invention includes, as a basic configuration, a substrate1having an electro-optic effect, an optical waveguide2formed on the substrate1, a plurality of modulation parts M which respectively modulate light waves propagating through the optical waveguide2, and a light receiving element4which detects the light waves propagating through the optical waveguide2, as shown inFIG. 3, for example.

As the substrate1, it is acceptable if it is a substrate such as quartz or a semiconductor, on which an optical waveguide can be formed, and in particular, a substrate using a single crystal such as LiNbO3(lithium niobate), LiTaO3(lithium tantalate), or PLZT (lead lanthanum zirconate titanate), which is a substrate having an electro-optic effect, can be suitably used.

The optical waveguide2which is formed on the substrate1is formed by thermally diffusing a high refractive index substance such as titanium (Ti) onto a LiNbO3substrate (an LN substrate), for example. Further, a rib type optical waveguide in which grooves are formed on both sides of a portion serving as an optical waveguide, or a ridge type waveguide in which an optical waveguide portion is made in a convex shape can also be used. Further, it is also possible to apply the present invention to an optical circuit in which optical waveguides are formed on different waveguide substrates such as PLC and these waveguide substrates are jointed and integrated.

A control electrode3for controlling light waves propagating through the optical waveguide2by a control signal is provided on the substrate1. The control electrode3includes an RF electrode3aconfiguring a modulation electrode, a ground electrode (not shown) surrounding the RF electrode3a, DC electrodes3band3cfor applying a DC signal, or the like. These control electrodes3can be formed by forming an electrode pattern made of Ti.Au on the surface of the substrate and by a gold plating method or the like. Further, as necessary, it is also possible to provide a buffer layer such as dielectric SiO2on the surface of the substrate after the formation of the optical waveguide.

The optical waveguide2configuring the modulation part M has a structure in which a sub-Mach-Zehnder waveguide is disposed in a nested type at an arm portion of a main Mach-Zehnder waveguide. The light waves (signal light) modulated in the modulation part M are guided to the outside of the substrate through an output waveguide21which is connected to a Y-junction of the main Mach-Zehnder waveguide in the modulation part M.

The light receiving element4is disposed close to the output waveguide21, and a radiated light waveguide22for leading radiated light, which is radiated from the Y-junction, to the light receiving element4, or a monitoring waveguide24for leading some of the light waves of the output waveguide21to the light receiving element4is provided. The radiated light waveguide22guides the radiated light which is not guided to the output waveguide21, to the light receiving element4as a monitoring signal. To this end, the radiated light can be led to the radiated light waveguide22by a coupler structure such as an MMI (multimode interference type) coupler, an asymmetric three-branch structure, or providing a radiated light guide in the vicinity of a Y-branch structure. The monitoring waveguide24distributes some of the light waves of the output waveguide21as a monitoring signal by a TAP coupler, thereby guiding it to the light receiving element4.

FIG. 3is a plan view for describing an optical modulator according to a first example of the present invention.

The optical modulator of this example has a first modulation part M(#1) and a second modulation part M(#2) which respectively modulate light waves into which input light branches.

The radiated light waveguides22propagating the radiated light which is radiated from the Y-junction of the main Mach-Zehnder waveguide in the modulation part M are provided on both sides of the output waveguide21of each of the modulation parts M. Then, the light receiving element4is disposed so as to overlay the output waveguide21and the radiated light waveguides22on both sides of the output waveguide21. In this example, as the light receiving elements4, a light receiving element4(#1) for the modulation part M(#1) and a light receiving element4(#2) for the modulation part M(#2) are provided. Each of the light receiving elements4has two light receiving parts41for receiving light waves from the respective radiated light waveguides22and is bonded and fixed to a predetermined position on the substrate1with an adhesive42. The monitoring signals received by the two light receiving parts41are electrically combined in the light receiving element or outside the light receiving element. In this way, it is possible to improve the bias point shift between the modulation curves of an output signal and a monitoring signal and further increase the signal intensity of the monitoring signal.

The modulation part M(#1) and the modulation part M(#2) are disposed side by side in a width direction of the substrate1with the positions in a length direction (a light propagation direction) of the substrate1aligned with each other. Further, the light receiving element4(#1) and the light receiving element4(#2) are disposed such that their positions in the light propagation direction are shifted from each other by an amount corresponding to one light receiving element, or more. Therefore, even if the distance between the optical waveguide2on the modulation part M(#1) side and the optical waveguide2on the modulation part M(#2) side is narrowed, the light receiving element4(#1) and the light receiving element4(#2) do not overlap each other.

In this manner, due to disposing the respective light receiving elements4so as to be shifted from each other by an amount corresponding to one light receiving element, or more, in the length direction of the substrate1, the optical waveguide2on the modulation part M(#1) side and the optical waveguide2on the modulation part M(#2) side can be disposed close to each other, and thus the length (a chip width W) in the width direction of the substrate1can be shortened.

FIG. 4is a plan view for describing an optical modulator according to a second example of the present invention.

The optical modulator according to the second example is a modification example of the optical modulator according to the first example. In the optical modulator according to the first example, a distance D21from a starting end portion of the radiated light waveguide22(the Y-junction of the Mach-Zehnder waveguide) to the light receiving element4is different between the light receiving element4(#1) and the light receiving element4(#2). In general, the distance D22between the radiated light waveguides22increases with distance from the starting end portion, and therefore, the distance D22between the radiated light waveguides22at the disposition position of the light receiving element4(#1) and the distance D22between the radiated light waveguides22at the disposition position of the light receiving element4(#2) become different from each other. For this reason, in the optical modulator according to the first example, there is a concern that a difference may occur in a light receiving characteristic between the light receiving elements4.

Therefore, in the optical modulator according to the second example, the modulation part M(#1) and the modulation part M(#2) are disposed to be shifted from each other in the light propagation direction, and the light receiving element4(#1) and the light receiving element4(#2) are disposed to be shifted from each other in the same light propagation direction as the direction in which the modulation part M(#1) and the modulation part M(#2) are shifted from each other. In this example, the modulation part M(#1) and the light receiving element4(#1) are disposed to be shifted by an amount corresponding to one light receiving element, or more, further toward the downstream side in the light propagation direction than the modulation part M(#2) and the light receiving element4(#2). Further, the shift amount and direction between the modulation part M(#1) and the modulation part M(#2) coincide with the shift amount and direction between the light receiving element4(#1) and the light receiving element4(#2). Therefore, the distance D21from the starting end portion of the radiated light waveguide22(the Y-junction of the Mach-Zehnder waveguide) to the light receiving element4is the same at the light receiving element4(#1) and the light receiving element4(#2).

In this way, the distance D22between the radiated light waveguides22at the disposition position of the light receiving element4(#1) and the distance D22between the radiated light waveguides22at the disposition position of the light receiving element4(#2) coincide with each other, and therefore, it is possible to uniformize the light receiving characteristic between the light receiving elements4. Further, the waveguide structure from the Y-junction to the light receiving element4can be made to be the same between the light receiving elements4, and therefore, the light receiving characteristic is further uniformized.

Further, the light receiving element4(#1) and the light receiving element4(#2) are disposed to be shifted from each other by an amount corresponding to one light receiving element, or more, and therefore, the optical waveguide2on the modulation part M(#1) side and the optical waveguide2on the modulation part M(#2) side can be disposed close to each other. Therefore, the length (the chip width W) in the width direction of the substrate1can be shortened.

FIG. 5is a plan view for describing an optical modulator according to a third example of the present invention.

The optical modulator according to the third example is another modification example of the optical modulator according to the first example.

The modulation part M(#1) and the modulation part M(#2) are disposed side by side in the width direction of the substrate1with the positions in the length direction (the light propagation direction) of the substrate1aligned with each other. Further, the light receiving element4(#1) is disposed to be shifted by an amount corresponding to one light receiving element, or more, further toward the downstream side in the light propagation direction than the light receiving element4(#2). The respective radiated light waveguides22on both sides of the output waveguide21are bent so as to be parallel to each other, and the light receiving element4is disposed at the parallelized section.

In this way, the distance D22between the radiated light waveguides22at the disposition position of the light receiving element4(#1) and the distance D22between the radiated light waveguides22at the disposition position of the light receiving element4(#2) coincide with each other, and therefore, it is possible to uniformize the light receiving characteristic between the light receiving elements4. Further, the light receiving element4(#1) and the light receiving element4(#2) are disposed to be shifted from each other by an amount corresponding to one light receiving element, or more, and therefore, the optical waveguide2on the modulation part M(#1) side and the optical waveguide2on the modulation part M(#2) side can be disposed close to each other. Therefore, the length (the chip width W) in the width direction of the substrate1can be shortened.

At the light wave output-side end portion of the substrate1, It is desirable to bend the radiated light waveguide22in a direction in which the distance between itself and the output waveguide21increases such that the optical axes of the signal light which is output from the output waveguide21and the radiated light which is output from the radiated light waveguide22do not overlap each other. In this way, interference between the signal light (On light) and the radiated light (Off light) can be suppressed.

FIG. 6is a plan view for describing an optical modulator according to a fourth example of the present invention.

The optical modulator of this example has a first modulation part M(#1) and a second modulation part M(#2), which respectively modulate light waves into which input light having a wavelength λ1branches, and a third modulation part M(#3) and a fourth modulation part M(#4), which respectively modulate light waves into which input light having a wavelength λ2branches. The modulation parts M(#1) and M(#2) and the modulation parts M(#3) and M (#4) do not need to be necessarily formed on the same substrate1and may be respectively formed on different substrates.

The monitoring waveguide24which extracts and propagates some of the light waves propagating through the output waveguide21for monitoring is provided on one side of the output waveguide21of each of the modulation parts M. Then, the light receiving element4is disposed so as to overlay the monitoring waveguide24. In this example, as the light receiving elements4, the light receiving element4(#1) for the modulation part M(#1), the light receiving element4(#2) for the modulation part M(#2), a light receiving element4(#3) for the modulation part M(#3), and a light receiving element4(#4) for the modulation part M(#4) are provided. Each of the light receiving elements4has a single light receiving part41for receiving light waves from the monitoring waveguide24and is bonded and fixed to a predetermined position on the substrate1with the adhesive42.

The modulation parts M(#1) to M(#4) are disposed side by side in the width direction of the substrate1with the positions in the length direction (the light propagation direction) of the substrate1aligned with each other. Further, the light receiving element4(#1) and the light receiving element4(#3) are disposed to be shifted by an amount corresponding to one light receiving element, or more, further toward the downstream side in the light propagation direction than the light receiving element4(#2) and the light receiving element4(#4). Therefore, even if the distance between the optical waveguide2on the modulation part M(#2) side and the optical waveguide2on the modulation part M(#3) side is narrowed, the light receiving element4(#2) and the light receiving element4(#3) do not overlap each other.

In this way, it is possible to narrow not only the space between the modulation parts M (#1) and M (#2) and the space between the modulation parts M(#3) and M(#4) but also the space between the modulation parts M(#2) and M(#3), and therefore, the length (the chip width W) in the width direction of the substrate1can be shortened.

FIG. 7is a plan view for describing an optical modulator according to a fifth example of the present invention.

The optical modulator of this example has the first modulation part M (#1) and the second modulation part M (#2), which respectively modulate light waves into which input light branches.

The radiated light waveguides22propagating the radiated light which is radiated from the Y-junction of the main Mach-Zehnder waveguide in the modulation part M are provided on both sides of the output waveguide21of each of the modulation parts M. Then, a single light receiving element4is disposed so as to overlay the output waveguide21of each modulation part M and the radiated light waveguides22on both sides of the output waveguide21. That is, in this example, the single light receiving element4is shared by the modulation parts M(#1) and M(#2). The light receiving element4has four light receiving parts41for receiving light waves from the respective radiated light waveguides22and is bonded and fixed to a predetermined position on the substrate1with the adhesive42.

The modulation part M(#1) and the modulation part M(#2) are disposed to be shifted from each other in the light propagation direction. Then, in the light receiving element4which is shared by the modulation part M(#1) and the modulation part M(#2), the light receiving part41for the modulation part M(#1) and the light receiving part41for the modulation part M(#2) are disposed to be shifted from each other in the same light propagation direction as the direction in which the modulation part M(#1) and the modulation part M(#2) are shifted from each other. In this example, the shift amount and direction between the modulation part M(#1) and the modulation part M(#2) coincide with the shift amount and direction between the light receiving part41for the modulation part M(#1) and the light receiving part41for the modulation part M(#2). Therefore, a distance D22from the starting end portion of the radiated light waveguide22(the Y-junction of the Mach-Zehnder waveguide) to the light receiving part41is the same at the light receiving element4(#1) and the light receiving element4(#2).

In this manner, the single light receiving element4is shared by the modulation part M (#1) and the modulation part M (#2), whereby the number of components on the substrate1can be reduced. Further, the optical waveguide2on the modulation part M(#1) side and the optical waveguide2on the modulation part M(#2) side can be disposed close to each other, and therefore, the length (the chip width W) in the width direction of the substrate1can be shortened. Furthermore, the disposition of the respective light receiving parts41in the light receiving element4is matched with the manner in which the modulation parts M(#1) and M(#2) are shifted from each other, and therefore, it is also possible to uniformize the light receiving characteristic between the light receiving parts41.

The configuration of the fifth example can be used in combination with the configuration of the other example. For example, as shown inFIG. 8showing an optical modulator combined with the configuration of the fourth example, a single light receiving element4may be shared by the modulation parts which modulate light waves having different wavelengths. InFIG. 8, the light receiving element4(#2, #3) which is shared by the modulation part M(#2) for a light wave having a wavelength of λ1and the modulation part M(#3) for a light wave having a wavelength of λ2is provided. In this shared light receiving element4(#2, #3), it is desirable that a light shielding structure43for preventing signal mixing between monitoring signals is provided between the light receiving parts41for each modulation part. As the light shielding structure43, for example, an embedded type electrode or a groove structure can be used.

The present invention has been described above on the basis of the examples. However, the present invention is not limited to the contents described above, and it goes without saying that design changes can be appropriately made within a scope which does not depart from the gist of the present invention.

As described above, according to the present invention, in an optical modulator having a light receiving element for each of a plurality of modulation parts, an optical modulator with a reduced chip width can be provided.