JP7849690B2 - Optical phase modulator - Google Patents

Description

This invention relates to an optical phase modulator.

Conventionally, optical modulation elements are known in which electro-optic material is embedded in the slit portion of a slot waveguide. Figures 2A and 2B show an example of such a conventional optical phase modulator (see, for example, Non-Patent Document 1). Multiple extraction electrodes 130, periodically arranged along its longitudinal direction, are connected to the slot waveguide 120. A high-frequency voltage is applied from a metal electrode 150 to the semiconductor electrode 122 constituting the slot waveguide 120 via these extraction electrodes 130. In response, the refractive index of the electro-optic material in the slit portion 123 changes, and the light propagating through the slot waveguide 120 is phase-modulated.

Antao Chen et al., “Achieving Higher Modulation Efficiency in Electrooptic Polymer Modulator With Slotted Silicon Waveguide”, Journal of Lightwave Technology, Vol. 29, No. 21, November 1, 2011, p.3310-3318

In an optical phase modulator with the above configuration, the optical field extends to the vicinity of the metal electrode 150, weakening the confinement of light in the slit portion 123 and reducing modulation efficiency. Furthermore, optical absorption occurs at the metal electrode 150, increasing optical loss. Additionally, to minimize optical absorption at the metal electrode 150, the length L of the extraction electrode 130 must be increased, which increases the electrical resistance between the semiconductor electrode 122 and the metal electrode 150, making high-speed modulation difficult.

To solve the above-mentioned problems, one aspect of the present invention is an optical phase modulator comprising: an optical waveguide composed of a plurality of semiconductor electrodes arranged in close proximity to each other on a substrate and an electro-optic material disposed between the plurality of semiconductor electrodes; a plurality of first extraction electrodes arranged along the optical propagation direction of the optical waveguide, each electrically connected to the semiconductor electrodes; and a plurality of second extraction electrodes arranged along the optical propagation direction of the optical waveguide, each electrically connected to the first extraction electrodes, and offset from the first extraction electrodes in the optical propagation direction.

Furthermore, in another aspect of the present invention, in the above-described embodiment, each of the plurality of second extraction electrodes is offset so as to be located midway between two adjacent extraction electrodes among the plurality of first extraction electrodes.

Furthermore, in another aspect of the present invention, the first and second extraction electrodes are arranged at equal intervals.

Furthermore, in another aspect of the present invention, in the above-described aspect, the first extraction electrodes are arranged periodically such that three or more consecutive extraction electrodes form a repeating unit.

Furthermore, in another aspect of the present invention, the first extraction electrodes are arranged at unequal intervals.

Furthermore, in another aspect of the present invention, in the above-described embodiment, the thicknesses of the first and second extraction electrodes are equal to the thickness of the semiconductor electrode.

Furthermore, in another aspect of the present invention, in the above-described embodiment, the thicknesses of the first and second extraction electrodes are thinner than the thickness of the semiconductor electrode.

Furthermore, in another aspect of the present invention, in the above-described embodiment, the thickness of the second extraction electrode is thinner than the thickness of the semiconductor electrode and the first extraction electrode.

Furthermore, in another aspect of the present invention, the first and second extraction electrodes are made of semiconductors.

Furthermore, in another aspect of the present invention, the second extraction electrode is made of a semiconductor with higher electrical conductivity than the first extraction electrode.

Furthermore, in another aspect of the present invention, in the above-described aspect, a connecting electrode is further provided between the plurality of second extraction electrodes and the plurality of first extraction electrodes, for continuously connecting the plurality of second extraction electrodes to the plurality of first extraction electrodes.

According to the present invention, modulation efficiency can be improved in an optical phase modulator using a slot waveguide. Furthermore, optical loss can be reduced and high-speed modulation can be enabled.

This is a schematic diagram of an optical phase modulator according to the first embodiment of the present invention. This is a schematic diagram of an optical phase modulator according to the first embodiment of the present invention. This is a schematic diagram of a conventional optical phase modulator. This is a schematic diagram of a conventional optical phase modulator. The simulation results of the optical electric field distribution in the optical phase modulator according to this embodiment and the optical phase modulator according to a conventional configuration are shown. The simulation results of optical propagation loss in the optical phase modulator according to this embodiment and in the optical phase modulator according to a conventional configuration are shown. The simulation results of the frequency response characteristics of the optical phase modulator according to this embodiment and the optical phase modulator according to a conventional configuration are shown. This is a schematic diagram of an optical phase modulator according to a second embodiment of the present invention. This is a schematic diagram of an optical phase modulator according to a third embodiment of the present invention. This is a schematic diagram of an optical phase modulator according to a fourth embodiment of the present invention. This is a schematic diagram of an optical phase modulator according to a fourth embodiment of the present invention. This is a schematic diagram of an optical phase modulator according to a fifth embodiment of the present invention. This is a schematic diagram of an optical phase modulator according to a fifth embodiment of the present invention.

The embodiments of the present invention will be described in detail below with reference to the drawings.

Figures 1A and 1B are schematic diagrams of an optical phase modulator according to the first embodiment of the present invention. Figure 1A shows a top view of the optical phase modulator, and Figure 1B shows a cross-sectional view of the optical phase modulator using lines AA, BB, and CC in Figure 1A. The optical phase modulator 100 according to the first embodiment of the present invention includes a slot waveguide 120. The slot waveguide 120 consists of a pair of semiconductor electrodes 122 arranged in close proximity on a substrate 110, and an electro-optic polymer 124 embedded in a slit 123 between the semiconductor electrodes 122. Other electro-optic materials may be used.

In Figure 1A, the semiconductor electrode 122 extends linearly in one direction (Z-axis) on the substrate 110. Two semiconductor electrodes 122 are arranged parallel to each other via a narrow slit 123, which is filled with an electro-optic polymer 124. Light is localized near the slit 123 between the two semiconductor electrodes 122 and propagates along the longitudinal direction of the slit 123. That is, the direction of light propagation in the slot waveguide 120 is the Z-axis direction in Figure 1A. The electro-optic polymer 124 exhibits an electro-optic effect in response to an electric field. When an electric field (high-frequency electric field) is applied between the two semiconductor electrodes 122, the refractive index of the electro-optic polymer 124 filling the slit 123 between the semiconductor electrodes 122 changes. This causes phase modulation in the light propagating through the slot waveguide 120.

To apply an electric field between the two semiconductor electrodes 122, each semiconductor electrode 122 is electrically connected to a metal electrode 150 via a plurality of first extraction electrodes 130 and a plurality of second extraction electrodes 140. The plurality of first extraction electrodes 130 and the plurality of second extraction electrodes 140 are also electrodes made of semiconductor material. A modulation signal for phase-modulating light propagating through the slot waveguide 120 is input to the pair of metal electrodes 150 from a drive circuit (not shown).

Multiple first extraction electrodes 130 are arranged at predetermined intervals along the longitudinal direction of the semiconductor electrode 122. Each first extraction electrode 130 is connected to the side surface of the semiconductor electrode 122 and extends along a direction perpendicular to the longitudinal direction of the semiconductor electrode 122 (the X-axis direction). In a plan view, each first extraction electrode 130 may have a rectangular shape, for example, with a width (dimension in the Z-axis direction) W1 and a length (dimension in the X-axis direction) L1. The spacing d1 between each first extraction electrode 130 may be equal.

Multiple second extraction electrodes 140 are similarly arranged at predetermined intervals along the longitudinal direction of the semiconductor electrode 122. Each second extraction electrode 140 is connected to the end of the first extraction electrode 130 and extends in the same direction as the first extraction electrode 130 (X-axis direction). The opposite end of each second extraction electrode 140 is connected to the metal electrode 150. In a plan view, each second extraction electrode 140 may have a rectangular shape, for example, with a width W2 and a length L2. The spacing d2 between each second extraction electrode 140 may be equal.

As shown in Figure 1A, in the optical phase modulator 100 according to the first embodiment of the present invention, the plurality of first extraction electrodes 130 and the plurality of second extraction electrodes 140 are both periodically arranged along the optical propagation direction of the slot waveguide 120, but each second extraction electrode 140 is offset from the first extraction electrode 130 in the optical propagation direction of the slot waveguide 120. More specifically, each second extraction electrode 140 is connected to two adjacent first extraction electrodes 130 and is positioned exactly in the middle of the two first extraction electrodes 130 in the Z-axis direction. In other words, the plurality of first extraction electrodes 130 and the plurality of second extraction electrodes 140 are arranged with a half-period offset from each other, and the gap 142 between the second extraction electrodes 140 (the portion where no extraction electrodes exist) is adjacent to each first extraction electrode 130.

Since a first extraction electrode 130 made of semiconductor material is connected to the slot waveguide 120, the electric field of light propagating through the slot waveguide 120 spreads laterally (in the X-axis direction) beyond the slot waveguide 120 via the first extraction electrode 130. However, because each first extraction electrode 130 is adjacent to a gap 142 between second extraction electrodes 140, the spread of the optical electric field beyond the end of the first extraction electrode 130 in the lateral direction (i.e., into the gap 142) is suppressed. In other words, the lateral spread of the electric field of light propagating through the slot waveguide 120 is approximately the same as the area where the first extraction electrode 130 exists (the area enclosed by the two vertical dashed lines in Figure 1B).

As a result of suppressing the lateral spread of the optical field, the light is strongly confined by the slits 123 of the slot waveguide 120. Therefore, the interaction between the light propagating through the slot waveguide 120 and the electro-optic polymer 124 within the slits 123 becomes greater, improving the modulation efficiency in the optical phase modulator 100.

Furthermore, since the optical field does not extend to the second extraction electrode 140, it is possible to prevent an increase in optical loss in the optical phase modulator 100 due to light absorption by the metal electrode 150. Moreover, without increasing light absorption by the metal electrode 150, the metal electrode 150 can be positioned closer to the slot waveguide 120, thereby shortening the length L2 of the second extraction electrode 140. This reduces the electrical resistance between the semiconductor electrode 122 and the metal electrode 150, lowering the CR time constant of the circuit, and consequently enabling high-speed modulation in the optical phase modulator 100.

Here, the characteristics of the optical phase modulator 100 according to this embodiment are compared with those of a conventional optical phase modulator. Figures 2A and 2B are schematic diagrams of a conventional optical phase modulator 900 (see, for example, Non-Patent Document 1). Figure 2A is a top view of the optical phase modulator, and Figure 2B shows a cross-sectional view of the optical phase modulator along line AA in Figure 2A.

The optical phase modulator 900 has a configuration in which the semiconductor electrode 122 and the metal electrode 150 are connected by only one extraction electrode 130, and does not have a second extraction electrode. In this configuration, since the long extraction electrode 130 extends from the semiconductor electrode 122 to the metal electrode 150, the optical electric field spreads widely in the lateral direction (X-axis direction) along this long extraction electrode 130, and the confinement of light into the slit 123 of the slot waveguide 120 is weakened. Figure 3 shows the simulation results of the optical electric field distribution for the optical phase modulator 100 according to this embodiment and the optical phase modulator 900 according to a conventional configuration. The horizontal and vertical axes in Figure 3 correspond to the X-axis and Z-axis in Figures 1A and 2A, respectively. As shown in Figure 3, in the optical phase modulator 100 according to this embodiment, the lateral spread of the optical electric field is suppressed more than in the optical phase modulator 900, and as a result, the confinement of light into the slit 123 of the slot waveguide 120 is stronger. Therefore, the optical phase modulator 100 according to this embodiment can achieve a higher modulation efficiency than the conventional optical phase modulator 900.

Figure 4 shows the simulation results of optical propagation loss for the optical phase modulator 100 according to this embodiment and the optical phase modulator 900 with a conventional configuration. The horizontal axis in Figure 4 represents the distance L between the semiconductor electrode 122 and the metal electrode 150 (see Figure 2A), and the vertical axis represents the optical propagation loss per unit length of the slot waveguide 120. In the conventional optical phase modulator 900, the optical loss is large because the optical field extends close to the metal electrode 150. However, in the optical phase modulator 100 according to this embodiment, the range in which the optical field extends is limited to the vicinity of the first extraction electrode 130 (the optical field does not extend close to the metal electrode 150), thus reducing the optical loss.

Figure 5 shows the simulation results of the frequency response characteristics of the optical phase modulator 100 according to this embodiment and the optical phase modulator 900 with a conventional configuration. In the optical phase modulator 100 of this embodiment, the CR time constant is reduced by decreasing the electrical resistance between the semiconductor electrode 122 and the metal electrode 150, thereby achieving faster modulation operation than the conventional optical phase modulator 900.

Figure 6 is a schematic diagram of the optical phase modulator according to the second embodiment of the present invention. In Figure 6, the same reference numerals are used for components identical to those in the first embodiment described above, and their descriptions are omitted below. The optical phase modulator 200 according to the second embodiment of the present invention includes a connecting electrode 135 between the first extraction electrode 130 and the second extraction electrode 140. The connecting electrode 135 is arranged in a long, linear manner parallel to the slot waveguide 120, connecting the first extraction electrode 130 and the second extraction electrode 140. The width W3 of the connecting electrode 135 may be, for example, approximately the same as the width W1 of the first extraction electrode 130. By connecting the first extraction electrode 130 and the second extraction electrode 140 using such a connecting electrode 135, the electrical resistance between the semiconductor electrode 122 and the metal electrode 150 can be reduced.

Figure 7 is a schematic diagram of the optical phase modulator according to the third embodiment of the present invention. In Figure 7, the same reference numerals are used for components identical to those in the first embodiment described above, and their descriptions are omitted below. In the optical phase modulator 300 according to the third embodiment of the present invention, each second extraction electrode 140 is offset in the optical propagation direction of the slot waveguide 120 relative to the first extraction electrode 130, similar to the optical phase modulator 100 of the first embodiment, but it is not positioned in the middle of two adjacent first extraction electrodes 130. More specifically, while each second extraction electrode 140 of the optical phase modulator 100 was connected to two adjacent first extraction electrodes 130, in the optical phase modulator 300 of this embodiment, each second extraction electrode 140 is connected to only one first extraction electrode 130 and is positioned biased toward that single first extraction electrode 130.

However, as shown in Figure 7, in this optical phase modulator 300 as well, each first extraction electrode 130 is adjacent to the gap 142 between the second extraction electrodes 140 (the portion where no extraction electrode exists). Therefore, similar to the first embodiment, it is possible to suppress the optical field from extending beyond the end of the first extraction electrode 130 into the gap 142. Consequently, even if the offset between the first extraction electrode 130 and the second extraction electrode 140 is as shown in Figure 7, it is possible to achieve high modulation efficiency, low optical loss, and high-speed modulation operation.

Figures 8 and 9 are schematic diagrams of an optical phase modulator according to the fourth embodiment of the present invention. In Figures 8 and 9, the same reference numerals are used for components identical to those in the second embodiment described above, and their descriptions are omitted below. In the optical phase modulator 400 according to the fourth embodiment of the present invention, the multiple first extraction electrodes 130 are not spaced equally, but are arranged such that the spacing between the first extraction electrodes 130 repeats as d1, d3, d1, d3, ... (where d1 ≠ d3). Similarly, in the optical phase modulator 500 according to the fourth embodiment of the present invention, the multiple first extraction electrodes 130 are arranged such that the spacing between the first extraction electrodes 130 repeats as d1, d1, d3, d1, d1, d3, ... (where d1 ≠ d3). Thus, in this embodiment, the spacing between the first extraction electrodes 130 is unequal. The spacing between the first extraction electrodes 130 may be any unequal spacing different from that shown in Figures 8 and 9.

In the optical phase modulator of the previously described embodiment, when the spacing between the first extraction electrodes 130 is equal, light propagating through the slot waveguide 120 may be attenuated by multiple reflections due to such a periodic structure (i.e., the equally spaced first extraction electrodes 130). In the optical phase modulator of the fourth embodiment, by having unequal spacing between the first extraction electrodes 130, multiple reflections from a structure in which multiple first extraction electrodes 130 are connected can be suppressed, further reducing optical loss.

Figures 10 and 11 are schematic diagrams of an optical phase modulator according to a fifth embodiment of the present invention. In Figures 10 and 11, the same reference numerals are used for components identical to those in the first embodiment described above, and their descriptions are omitted below. In the optical phase modulator 600 according to the fifth embodiment of the present invention, the first extraction electrode 130 and the second extraction electrode 140 have a thinner thickness than the semiconductor electrode 122. Because the first extraction electrode 130 is formed thinly, the optical electric field inside the first extraction electrode 130 is reduced, and the confinement of light into the slit 123 of the slot waveguide 120 can be strengthened. Furthermore, in the optical phase modulator 700 according to the fifth embodiment of the present invention, the semiconductor electrode 122 and the first extraction electrode 130 have the same thickness, and the second extraction electrode 140 has a thinner thickness than them. Because the second extraction electrode 140 is thinner than the first extraction electrode 130, the spread of the optical electric field in the lateral direction (towards the gap 142) is further suppressed. This allows for stronger confinement of light into the slit 123 of the slot waveguide 120, similar to the optical phase modulator 600 in Figure 10.

In some embodiments, the semiconductor electrode 122, the first extraction electrode 130, the second extraction electrode 140, and the connecting electrode 135 may be composed of various semiconductor materials. For example, the semiconductor electrode 122 and the first extraction electrode 130 may be composed of InP (indium phosphide), and the second extraction electrode 140 may be composed of Si (silicon). Other III-V semiconductors may be used instead of InP. By using InP for the material in the region with a relatively large photoelectric field (i.e., the semiconductor electrode 122 and the first extraction electrode 130), optical loss can be kept low. Furthermore, by using Si, which has high electrical conductivity, for the material in the region where the photoelectric field is almost nonexistent (i.e., the second extraction electrode 140), the overall electrical resistance between the semiconductor electrode 122 and the metal electrode 150 can be reduced. Note that each electrode 122, 130, 140, and 135 may be composed of the same semiconductor material.

Although embodiments of the present invention have been described above, the present invention is not limited thereto, and various modifications are possible without departing from its essence.

100 Optical phase modulator 110 Substrate 120 Slot waveguide 122 Semiconductor electrode 123 Slit 124 Electro-optic polymer 130 First extraction electrode 135 Connecting electrode 140 Second extraction electrode 142 Gap 150 Metal electrode

Claims (11)

An optical waveguide comprising a plurality of semiconductor electrodes arranged in close proximity to each other on a substrate, and an electro-optic material disposed between the plurality of semiconductor electrodes,
A plurality of first extraction electrodes are arranged to the side of the optical waveguide along the optical propagation direction of the optical waveguide , and each is electrically connected to the semiconductor electrode,
A plurality of second extraction electrodes are arranged on the side opposite to the optical waveguide of the plurality of first extraction electrodes along the optical propagation direction of the optical waveguide, each of which is electrically connected to the first extraction electrode and positioned offset from the first extraction electrode in the optical propagation direction,
An optical phase modulator equipped with the following features. The optical phase modulator according to claim 1, wherein each of the plurality of second extraction electrodes is offset so as to be located midway between two adjacent extraction electrodes among the plurality of first extraction electrodes. The optical phase modulator according to claim 1 or 2, wherein the first and second extraction electrodes are each arranged at equal intervals. The optical phase modulator according to claim 1 or 2, wherein the first extraction electrodes are arranged periodically so that three or more consecutive extraction electrodes form a repeating unit. The optical phase modulator according to claim 1 or 2, wherein the first extraction electrodes are arranged at unequal intervals. The optical phase modulator according to any one of claims 1 to 5, wherein the thickness of the first and second extraction electrodes is equal to the thickness of the semiconductor electrode. The optical phase modulator according to any one of claims 1 to 5, wherein the thickness of the first and second extraction electrodes is thinner than the thickness of the semiconductor electrode. The optical phase modulator according to any one of claims 1 to 5, wherein the thickness of the second extraction electrode is thinner than the thickness of the semiconductor electrode and the first extraction electrode. The optical phase modulator according to any one of claims 1 to 8, wherein the first and second extraction electrodes are made of semiconductors. The optical phase modulator according to claim 9, wherein the second extraction electrode is made of a semiconductor with higher electrical conductivity than the first extraction electrode. The optical phase modulator according to any one of claims 1 to 10, further comprising connecting electrodes between the plurality of second extraction electrodes and the plurality of first extraction electrodes, for continuously connecting the plurality of second extraction electrodes to the plurality of first extraction electrodes.