JP2021021833A - Isolator and optical transmitter - Google Patents

Abstract

To reduce unnecessary reflection at a terminal of a waveguide.SOLUTION: An isolator 10 includes, on a substrate 50, a first waveguide 20 and a second waveguide 30 that are located at least partially side by side along a substrate surface. The first waveguide 20 and the second waveguide 30 include a first core 21 and a second core 31, respectively. The first waveguide 20 has ports 211, 212 through which electromagnetic waves are input and output to and from a first end 201 and a second end 202. The second waveguide 30 has a first portion 30a at which an electromagnetic fields are combined in a near field with the first waveguide 20, and second portions 30b, 30c at which the electromagnetic fields are not combined in the near field with the first waveguide. The second waveguide 30 has a non-reciprocal member 32 that is in contact with at least part of the second core 31 of the first portion 30a. The non-reciprocal member 32 is further in contact with at least parts of the second core 31 of the second portions 30b, 30c.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to isolators and optical transmitters.

Conventionally, a waveguide type isolator has been proposed (see, for example, Patent Document 1). For example, Patent Document 1 has two waveguides between two three-branch optical couplers, one of which contains a 90 ° reciprocal phase shifter and the other of which has a 90 ° reciprocal phaser. An interferometer-type optical isolator composed of a Mach-Zehnder interferometer including a non-reciprocal phase shifter is disclosed.

JP-A-2003-302603

However, in a waveguide type isolator, when reflection of incident light occurs at the end of the waveguide, the reflected light becomes return light, which may cause problems such as damage to the light source. It is preferable that the reflection at the end of the waveguide can be reduced as much as possible.

Therefore, an object of the present invention made by paying attention to these points is to provide an isolator and an optical transmitter capable of reducing unnecessary reflection at the end of a waveguide.

The isolator of the present disclosure includes a first waveguide and a second waveguide located at least partially aligned with the substrate surface on a substrate having a substrate surface. The first waveguide includes a first core, the second waveguide includes a second core, and the first core and the second core are surrounded by a dielectric. The first waveguide has a first end and a second end, and each of the first end and the second end has a port for input / output of electromagnetic waves. The second waveguide has a first portion along the first waveguide and a second portion not included in the first portion. The second waveguide has a non-reciprocal member in contact with at least a part of the second core of the first portion. The non-reciprocal member further contacts at least a part of the second core of the second portion.

The optical transmitter of the present disclosure includes a light source, an optical modulator, and an isolator. The light modulator modulates the light emitted from the light source based on the signal to be transmitted. The isolator is arranged on the downstream side of the light source. The isolator includes a first waveguide and a second waveguide that are located at least partially aligned with the substrate surface on a substrate having a substrate surface. The first waveguide includes a first core, the second waveguide includes a second core, and the first core and the second core are surrounded by a dielectric. The first waveguide has a first port at which light from the light source is input at the first end, and a second port at which light from the light source is output at a second end different from the first end. Have. The second waveguide has a first portion along the first waveguide and a second portion not included in the first portion. The second waveguide has a non-reciprocal member in contact with at least a part of the second core of the first portion. The non-reciprocal member further contacts at least a part of the second core of the second portion.

The optical transmitter of the present disclosure includes a light source, a drive unit, and an isolator. The drive unit drives the light source based on a signal to be transmitted. The isolator is arranged on the light emitting side of the light source. The isolator includes a first waveguide and a second waveguide that are located at least partially aligned with the substrate surface on a substrate having a substrate surface. The first waveguide includes a first core, the second waveguide includes a second core, the first core and the second core are surrounded by a dielectric, and the first waveguide is a first core. It has a first port at which light from the light source is input, and a second port at which light from the light source is output at a second end different from the first end. The second waveguide has a first portion along the first waveguide and a second portion not included in the first portion. The second waveguide has a non-reciprocal member in contact with at least a part of the second core of the first portion. The non-reciprocal member further contacts at least a part of the second core of the second portion.

According to an embodiment of the present invention, it is possible to provide an isolator and an optical communication device capable of reducing unnecessary reflection at the end of a waveguide.

It is a perspective view of the isolator which concerns on one Embodiment. It is a top view of the isolator of FIG. FIG. 2 is a sectional view taken along the line AA of FIG. It is a graph which shows the coupling coefficient with respect to the electromagnetic wave traveling in the 1st direction. It is a graph which shows the coupling coefficient with respect to the electromagnetic wave traveling in the 2nd direction. It is sectional drawing of the isolator which concerns on another embodiment. It is a block diagram which shows the schematic structure of the optical transmitter which concerns on one Embodiment. It is a block diagram which shows the schematic structure of the optical transmitter which concerns on other embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The figures used in the following description are schematic. The dimensions, ratios, etc. on the drawings do not always match the actual ones.

As shown in FIGS. 1 to 3, the isolator 10 according to the embodiment includes a substrate 50, a first waveguide 20, and a second waveguide 30. The first waveguide 20 and the second waveguide 30 are arranged along the substrate surface 50a (see FIG. 3) of the substrate 50. The first waveguide 20 and the second waveguide 30 are arranged in parallel at least partially close to each other.

For the following explanation, the x-axis direction, the y-axis direction, and the z-axis direction are defined, respectively. As shown in FIGS. 1 to 3, the x-axis direction is the extending direction of the first waveguide 20. The positive direction of the x-axis is the direction from one end of the first waveguide 20 (first end 201 described later) to the other end (second end 202 described later). The y-axis direction is a direction along the substrate surface 50a of the substrate 50 and intersects with the x-axis direction. The y-axis direction may be substantially orthogonal to the x-axis direction. The positive direction of the y-axis is the direction toward the side where the second waveguide 30 is located as viewed from the first waveguide 20. The z-axis direction is a direction perpendicular to the substrate surface 50a. The z-axis direction is orthogonal to the x-axis direction and the y-axis direction. The positive direction of the z-axis is the direction on the side where the first waveguide 20 and the second waveguide 30 are arranged when viewed from the substrate 50.

Each component of the isolator 10 will be described in more detail below.

The substrate 50 can be made of various materials. For example, the substrate 50 may be made of a material selected from a material including a metal conductor, a semiconductor such as silicon, glass, or a resin. The substrate 50 can take various shapes. For example, the substrate 50 has two sides extending in the x-axis direction and the y-axis direction, and may have a rectangular shape long in the x-axis direction.

On the substrate surface 50a of the substrate 50, a first clad 61 common to the first waveguide 20 and the second waveguide 30 is formed. A first core 21, a second core 31, and a non-reciprocal member 32 are arranged on the upper surface 61a of the first clad 61. The non-reciprocal member 32 is arranged in contact with the second core 31.

As shown in FIG. 3, the first core 21, the second core 31, and the non-reciprocal member 32 are peripherally and upperly covered with a second clad 62 formed on the first clad 61. The first clad 61 and the second clad 62 can be collectively referred to as a clad 60. The first core 21, the second core 31, and the non-reciprocal member 32 are surrounded by a clad 60. The first waveguide 20 includes a first core 21 and a clad 60 in a portion close to the first core 21. The second waveguide 30 includes a second core 31 and a non-reciprocal member 32, and a clad 60 in a portion close to the second core 31 and the non-reciprocal member 32.

The first core 21, the second core 31, and the clad 60 may be configured to include a dielectric. The first core 21 and the second core 31 are also referred to as dielectric lines. The relative permittivity of the first core 21 and the second core 31 may be higher than the relative permittivity of the clad 60. The first clad 61 and the second clad 62 constituting the clad 60 may be made of the same dielectric material. The first clad 61 and the second clad 62 may be integrally configured. When the first clad 61 and the second clad 62 are integrally formed, the isolator 10 can be easily formed. The relative permittivity of the first core 21, the second core 31, and the clad 60 may be higher than the relative permittivity of air. By making the relative permittivity of the first core 21, the second core 31, and the clad 60 higher than the relative permittivity of air, leakage of electromagnetic waves from the first waveguide 20 and the second waveguide 30 is suppressed. Can be done. As a result, the loss due to the emission of electromagnetic waves from the isolator 10 to the outside can be reduced.

The first core 21 and the second core 31 may be made of, for example, silicon (Si). The clad 60 may be made of, for example, quartz glass (SiO ₂ ). The relative permittivity of silicon and quartz glass is about 12 and about 2, respectively. Silicon can propagate electromagnetic waves with near-infrared wavelengths of about 1.2 μm to about 6 μm with low loss. When the first core 21 and the second core 31 are made of silicon, electromagnetic waves having wavelengths in the 1.3 μm band or 1.55 μm band used in optical communication can be propagated with low loss.

The materials of the first core 21, the second core 31 and the clad 60 are not limited to the above materials. Any material can be used for the first core 21, the second core 31, and the clad 60 as long as the function as the isolator 10 of the present disclosure can be obtained. The portion of the second clad 62 may be air (dielectric) without providing a layer of a specific material.

The relative permittivity of the first core 21 and the second core 31 may be uniformly distributed along the z-axis direction, or may be distributed so as to change along the z-axis direction. For example, the relative permittivity of the first core 21 may be distributed so as to be highest in the central portion in the z-axis direction and decrease as it approaches the first clad 61 and the second clad 62. In this case, the first waveguide 20 can propagate electromagnetic waves on the same principle as the graded index type optical fiber.

The first core 21 and the second core 31 covered with the clad 60 propagate electromagnetic waves. In the present disclosure, electromagnetic waves may include electromagnetic waves of various wavelengths. The wavelength of the electromagnetic wave may be included in the band of light from ultraviolet light to infrared light. When the wavelength of the electromagnetic wave is included in the wavelength band of light, the isolator 10 is also referred to as an optical isolator. The electromagnetic wave band may be a wavelength band such as 1.55 μm, which is used in silicon photonics.

The first waveguide 20 and the second waveguide 30 according to the present embodiment can propagate electromagnetic waves in TE mode. An electromagnetic wave having no electric field component in the propagation direction is called TE mode. In the waveguide, the electric field of the TE mode electromagnetic wave oscillates in the horizontal direction with respect to the substrate 50. Electromagnetic waves in TE mode are also called TE waves. In a device or system utilizing the isolator 10, the first waveguide 20 and the second waveguide 30 may be designed to propagate electromagnetic waves in TE mode. Therefore, the polarization direction of the electromagnetic wave input to the isolator 10 may be parallel to the substrate 50. When the electromagnetic wave is light, the polarization direction is also called the polarization direction.

The first waveguide 20 and the second waveguide 30 may satisfy the waveguide condition in the single mode. When the first waveguide 20 and the second waveguide 30 satisfy the waveguide condition in the single mode, the waveforms of the signals propagating through the first waveguide 20 and the second waveguide 30 are less likely to be distorted. The isolator 10 that combines the first waveguide 20 and the second waveguide 30 that satisfy the waveguide condition in the single mode may be suitable for optical communication.

The first waveguide 20 extends longer in the x-axis direction than the second waveguide 30. The first waveguide 20 can have a linear shape. The first waveguide 20 is not limited to a linear shape. The first waveguide 20 may have a bent portion. In the isolator 10 illustrated in FIGS. 1 to 3, the first waveguide 20 will be described as being along the x-axis direction.

The first waveguide 20 has a first end 201 and a second end 202 on the negative side and the positive side of the x-axis, respectively. The first waveguide 20 includes first port 211 and second port 212 to which electromagnetic waves are input and output, respectively, at the first end 201 and the second end 202. The electromagnetic wave input from the first port 211 to the first waveguide 20 travels toward the second port 212 along the x-axis. The electromagnetic wave input from the second port 212 to the first waveguide 20 travels toward the first port 211 along the x-axis. Each of the first port 211 and the second port 212 may be configured as an end face of the first core 21, or may be connected to an external device and configured as a coupler capable of propagating electromagnetic waves.

The second waveguide 30 is located on the substrate 50 at least partially along the substrate surface 50a alongside the first waveguide 20. The second waveguide 30 includes a first portion 30a and a second portion 30b, 30c. The first portion 30a is a portion along the first waveguide 20. The first portion 30a of the second waveguide 30 is arranged linearly in the direction along the x-axis direction so as to be parallel to the first waveguide 20. In the first portion 30a of the second waveguide 30, the first core 31 is such that the electromagnetic field in the proximity field (evanescent field) can be coupled to the first core 21 of the first waveguide 20 to the extent that the electromagnetic field can be coupled. It is located close to the core 21.

The second waveguide 30 further includes second portions 30b and 30c in which the electromagnetic field in the near field is not coupled to the first waveguide 20. The second portions 30b and 30c are portions not included in the first portion 30a of the second waveguide 30. The second portions 30b and 30c of the second waveguide 30 may be located on one side or both sides of the first portion 30a in the longitudinal direction of the second waveguide 30. In the example shown in FIGS. 1 to 3, the second portions 30b and 30c of the second waveguide 30 are located on both sides of the first portion 30a. Hereinafter, the second portions 30b and 30c of the second waveguide 30 will be described as being located on both sides of the first portion 30a. The second portion 30b is located on the negative side of the first portion 30a on the x-axis. The second portion 30c is located on the positive side of the first portion 30a on the x-axis.

The second portions 30b and 30c of the second waveguide 30 are bent in a direction deviating from the first waveguide 20 on the side of the first portion 30a. That is, the second portions 30b and 30c are bent so as to face the positive direction of the y-axis. Between each of the first portion 30a and the second portions 30b and 30c of the second waveguide 30, there may be a portion that transitions from a state of being coupled to the first waveguide 20 to a state of not being coupled. The portion that transitions from the state of being coupled to the first waveguide 20 to the state of being uncoupled may not be included in the second portions 30b and 30c.

The second waveguide 30 has a first end 301 and a second end 302 on the second portion 30b located on the negative side of the x-axis and the second portion 30c located on the positive side, respectively. Have. In other words, the second waveguide 30 has both ends.

At the first end 301 and the second end 302 of the second waveguide 30, the second core 31 can have an end face having a tapered shape toward the tip. In other words, both ends of the second core 31 of the second waveguide 30 have a smaller diameter toward the tip. The tapered portion of the second core 31 can be longer than the wavelength of the electromagnetic wave propagating in the isolator 10. The first end 301 and the second end 302 have a tapered cross section cut by a plane parallel to the xy plane. The width of the first end 301 and the second end 302 may be further narrowed toward the tip in the z-axis direction. The tapered shape can be rephrased as a tapered shape in which the diameter decreases toward the tip.

The portion of the first waveguide 20 and the second waveguide 30 along each other is also referred to as a parallel waveguide. Electromagnetic fields in the near field between the waveguides can be coupled to the portions of the first waveguide 20 and the second waveguide 30 along each other. In a parallel waveguide, an electromagnetic wave input to one of the waveguides of the first waveguide 20 and the second waveguide 30 along each other can be transferred to the other waveguide while propagating in the waveguide. .. That is, the electromagnetic wave propagating in the first waveguide 20 can move to the second waveguide 30. The electromagnetic wave propagating in the second waveguide 30 can move to the first waveguide 20.

In a parallel waveguide, a parameter representing the ratio of electromagnetic waves moving from one waveguide to the other is called a coupling coefficient. If no electromagnetic wave is transferred from one waveguide to the other, the coupling coefficient is assumed to be zero. When all electromagnetic waves are transferred from one waveguide to the other, the coupling coefficient is assumed to be 1. The coupling coefficient can be a value of 0 or more and 1 or less. The coupling coefficient can be determined based on the shape of each waveguide, the distance between the waveguides, the length of the waveguides along each other, and the like. For example, the coupling coefficient can be higher as the shape of each waveguide approaches. For the distance between each waveguide, the coupling coefficient can vary depending on the distance the electromagnetic wave propagates in the waveguide. That is, in a parallel waveguide, the coupling coefficient may differ depending on the position along the direction in which the waveguide extends. The maximum value of the coupling coefficient can be determined based on the shape of each waveguide, the distance between each waveguide, and the like. The maximum value of the coupling coefficient can be a value of 1 or less.

In a parallel waveguide, the coupling coefficient at the starting point of the section where the waveguides are along each other is 0. The length from the start point to the position where the bond coefficient reaches the maximum value is also called the bond length. If the length of the waveguides along each other is equal to the bond length, the bond coefficient at the end of the section along which the waveguides follow each other can be a local maximum. The bond length can be determined based on the shape of each waveguide, the distance between each waveguide, and the like.

In the second waveguide 30, the electromagnetic wave transferred from the first waveguide 20 propagates in the same direction as in the first waveguide 20 in the second waveguide 30. In the second waveguide 30, the electromagnetic wave propagates in the second core 31 toward the first end 301 or the second end 302 while repeating total reflection. When the electromagnetic wave reaches the first end 301 or the second end 302, the electromagnetic wave is radiated from the first end 301 or the second end 302 or reflected by the first end 301 or the second end 302 and travels in the opposite direction. It can be radiant.

The non-reciprocal member 32 includes a non-reciprocal material. The non-reciprocal material is a material having different propagation characteristics depending on the propagation direction of the electromagnetic wave. The non-reciprocal member 32 may be configured to include a magnetic material. As an example, as the non-reciprocal member 32, a dielectric material in which magnetic nanoparticles are composited, for example, a nanogranular material can be used. The non-reciprocal material used for the non-reciprocal member 40 may have a property of absorbing electromagnetic waves.

The non-reciprocal member 32 is located on the positive side of the y-axis with respect to the second core 31 of the first portion 30a of the second waveguide 30. The non-reciprocal member 32 may be in contact with the second core 31 over the entire surface of the first portion 30a of the second waveguide 30 along the x-axis direction of the second core 31. Alternatively, the non-reciprocal member 32 may be in contact with a part of the second core 31. The non-reciprocal member 32 comes into contact with the second core 31 at the first portion 30a of the second waveguide 30, so that the second waveguide 30 has non-reciprocity as described later.

The non-reciprocal member 32 further contacts at least a part of the second core 31 of the second portions 30b and 30c of the second waveguide 30. For example, in the second portions 30b, 30c of the second waveguide 30, when the second core 31 has a plurality of sides around the extending direction of the second waveguide 30, the non-reciprocal member 32 is the second. It may touch at least one of the plurality of sides of the two cores 31.

The non-reciprocal member 32 absorbs a part of the electromagnetic wave propagating in the second waveguide 30 at the surface in contact with the second waveguide 30. When a part of the electromagnetic wave propagating through the second waveguide 30 is totally reflected at the interface where the second waveguide 30 is in contact with the non-reciprocal member 32, a part of it exudes to the non-reciprocal member 32 as an evanescent wave. .. The electromagnetic wave exuded to the non-reciprocal member 32 is at least partially absorbed by the non-reciprocal member 32.

The non-reciprocal member 32 may be in contact with the second portions 30b and 30c of the second waveguide 30 so as to cover at least a part of the first end 301 and the second end 302. The non-reciprocal member 32 covers the first end 301 and the second end 302 at least partially, so that the electromagnetic wave propagating through the second core 31 at the first end 301 and the second end 302 is at least partially covered. Be absorbed.

As shown in FIG. 1, the second core 31 of the second portions 30b and 30c of the second waveguide 30 can have a structure embedded in the non-reciprocal member 32. The non-reciprocal member 32 has a dimension in the z direction substantially equal to that of the second core 31. In this case, the non-reciprocal member 32 may be in contact with the side surfaces of the second portions 30b and 30c of the second waveguide 30 as a whole.

The electromagnetic wave input from the first end 201 of the first waveguide 20 to the first core 21 via the first port 211 is included in the first core 21 of the first waveguide 20 extending along the x-axis. , Propagate toward the second end 202. The direction from the first end 201 to the second end 202 is also referred to as the first direction. The electromagnetic wave propagating in the first core 21 reaches the second core 31 at a ratio according to the coupling coefficient based on the distance propagating in the first direction along the portion of the first core 21 along with the second core 31. Can move. The coupling coefficient when an electromagnetic wave propagates in the first direction in the first core 21 is also referred to as a first coupling coefficient.

The electromagnetic wave input from the second end 202 of the first waveguide 20 to the first core 21 via the second port 212 is included in the first core 21 of the first waveguide 20 extending along the x-axis. , Propagate toward the first end 201. The direction from the second end 202 to the first end 201 is also referred to as a second direction. The electromagnetic wave propagating in the first core 21 reaches the second core 31 at a ratio according to the coupling coefficient based on the distance propagating in the second direction along the portion of the first core 21 along with the second core 31. Can move. The coupling coefficient when an electromagnetic wave propagates in the second direction in the first core 21 is also referred to as a second coupling coefficient.

The second core 31 of the second waveguide 30 comes into contact with the non-reciprocal member 32 on the positive direction side of the y-axis, so that the second waveguide 30 has non-reciprocity. Having non-reciprocity means that the effect received by the propagating electromagnetic wave differs depending on the propagating direction of the electromagnetic wave. The second waveguide 30 may have different propagation characteristics depending on whether the electromagnetic wave propagates in the first direction or the electromagnetic wave propagates in the second direction. When the propagation characteristics of the second waveguide 30 are different based on the propagation direction of the electromagnetic wave, the first coupling coefficient and the second coupling coefficient can be different from each other. That is, the non-reciprocal member 32 can have a different first coupling coefficient and a second coupling coefficient.

The non-reciprocity of the second waveguide 30 by the non-reciprocal member 32 is manifested by the application of a magnetic field from the outside. The direction of the external magnetic field applied to the non-reciprocal member 32 and the polarization direction of the electromagnetic wave propagating in the second waveguide 30 are configured to intersect each other. In the present embodiment, the polarization direction of the electromagnetic wave in the TE mode propagating in the second waveguide 30 is substantially parallel to the substrate surface 50a of the substrate 50 (that is, in the y-axis direction). In this case, the non-reciprocity caused by the non-reciprocal member 32 is exhibited by applying an external magnetic field having a component in the z-axis direction. When the magnitude of the external magnetic field is constant, the non-reciprocity becomes the largest by applying the external magnetic field in the substantially z-axis direction.

When the non-reciprocal member 32 is a ferromagnet, the non-reciprocal member 32 exhibits non-reciprocity without applying an external magnetic field. When the polarization direction of the electromagnetic wave propagating in the second waveguide 30 is the y-axis direction, the non-reciprocal member 32 is arranged so that the magnetization direction has a component in the z-axis direction. Preferably, the non-reciprocal member 32 is arranged so that the magnetization direction is substantially the z-axis direction.

When one of the parallel waveguides has non-reciprocity, the maximum value of the coupling coefficient when the electromagnetic wave propagates in the first direction is different from the maximum value of the coupling coefficient when the electromagnetic wave propagates in the second direction. sell. For example, as shown in FIG. 4, the maximum value of the coupling coefficient between the first waveguide 20 and the second waveguide 30 when the electromagnetic wave propagates in the first direction can be configured to be close to 0. .. For example, as shown in FIG. 5, the maximum value of the coupling coefficient between the first waveguide 20 and the second waveguide 30 when the electromagnetic wave propagates in the second direction can be configured to be close to 1. .. Since the maximum value of the coupling coefficient differs depending on the propagation direction of the electromagnetic wave, the transmittance of the electromagnetic wave may differ depending on the propagation direction of the electromagnetic wave. In FIGS. 4 and 5, the horizontal axis and the vertical axis represent the traveling distance of the electromagnetic wave in the parallel waveguide and the coupling coefficient, respectively.

When the second waveguide 30 has non-reciprocity, the coupling coefficient between the first waveguide 20 and the second waveguide 30 may differ depending on the propagation direction of the electromagnetic wave. That is, when the second waveguide 30 has non-reciprocity, the first coupling coefficient of the isolator 10 may be different from the second coupling coefficient. By adjusting the magnitude of the non-reciprocity of the second waveguide 30, the second coupling coefficient can be made larger than the first coupling coefficient.

When the electromagnetic wave input from the first port 211 to the first waveguide 20 propagates in the first direction, at least a part of the input electromagnetic wave transferred to the second waveguide 30 is the second waveguide 30. It is propagated to the second part 30c of. The electromagnetic wave propagated in the second portion 30c of the second waveguide 30 is not output from the second port 212 of the first waveguide 20, but is absorbed by the second portion 30c of the second waveguide 30, or is the second end. It can be radiated to the outside from 302 or reflected at the second end 302. When the first coupling coefficient is large, the proportion of the electromagnetic waves input to the first waveguide 20 that move to the second waveguide 30 and are absorbed or radiated externally in the second portion 30c can be large. In this case, the ratio of the electromagnetic waves output from the second port 212 to the electromagnetic waves input to the first waveguide 20 can be small. That is, the ratio of the intensity of the electromagnetic wave output from the second port 212 to the intensity of the electromagnetic wave input to the first port 211 can be small. The ratio of the intensity of the electromagnetic wave output from the second port 212 to the intensity of the electromagnetic wave input to the first port 211 is also referred to as the transmittance of the isolator 10 with respect to the electromagnetic wave propagating in the first direction. When the first coupling coefficient is large, the transmittance for electromagnetic waves propagating in the first direction can be low. On the other hand, when the first coupling coefficient is small, the ratio of the electromagnetic wave moving to the second waveguide 30 can be small, so that the transmittance for the electromagnetic wave propagating in the first direction can be high.

The electromagnetic wave input from the second port 212 to the first waveguide 20 and propagated in the second direction can receive the same action as the electromagnetic wave propagating in the first direction from the isolator 10 depending on the second coupling coefficient. .. By the action, a part of the electromagnetic wave propagating in the second direction is propagated to the second portion 30b of the second waveguide 30. When the second coupling coefficient is large, the transmittance for electromagnetic waves propagating in the second direction can be low. When the second coupling coefficient is small, the transmittance for electromagnetic waves propagating in the second direction can be high.

When the first coupling coefficient and the second coupling coefficient are different, the transmittance for the electromagnetic wave propagating in the first direction and the transmittance for the electromagnetic wave propagating in the second direction may be different. That is, the isolator 10 can function so as to facilitate the propagation of electromagnetic waves in one direction and make it difficult to propagate electromagnetic waves in the opposite direction by making the first coupling coefficient and the second coupling coefficient different. When the second coupling coefficient is larger than the first coupling coefficient, the isolator 10 can function to facilitate the propagation of the electromagnetic wave in the first direction and make it difficult to propagate the electromagnetic wave in the second direction. When the first coupling coefficient and the second coupling coefficient are set to about 0 and about 1, respectively, the difference between the transmittance for the electromagnetic wave propagating in the first direction and the transmittance for the electromagnetic wave propagating in the second direction can be increased. .. As a result, the function of the isolator 10 can be improved.

When one of the parallel waveguides has non-reciprocity, the bond length of the parallel waveguide with respect to the electromagnetic wave propagating in the first direction can be different from the bond length of the parallel waveguide with respect to the electromagnetic wave propagating in the second direction. For example, as shown in FIG. 4, the bond length with respect to the electromagnetic wave propagating in the first direction in the isolator 10 can be expressed as L ₁ . For example, as shown in FIG. 5, the bond length with respect to the electromagnetic wave propagating in the second direction in the isolator 10 can be expressed as L ₂ . The isolator 10 may be configured such that L ₁ and L ₂ are different.

In a parallel waveguide, if the length of the two waveguides along each other (L in FIG. 2) is equal to the bond length, the bond coefficient can be a maximum. For example, in the parallel waveguide having the relationship shown in the graph of FIG. 4, when the length L of the two waveguides along each other is L ₁ , the coupling coefficient can be a maximum value. If the length L of the two waveguides along each other is equal to twice the bond length, the bond coefficient can be minimal. For example, in the parallel waveguide having the relationship shown in FIG. 4, when the length L of the two waveguides along each other is 2L ₁ , the coupling coefficient can be a minimum value.

The relationship shown in the graph of FIG. 4 can be repeated even in a region where the traveling distance of the electromagnetic wave is long. That is, when the length L of the two waveguides along each other is an odd multiple of L ₁ , the coupling coefficient can be a maximum value. When the length L of the two waveguides along each other is an even multiple of L ₁ , the coupling coefficient can be a minimum value. Even in the parallel waveguides having the relationship shown in FIG. 5, the coupling coefficient is maximized when the length L of the _two waveguides along each other is an odd multiple of L ₂ and when it is an even multiple of L _2. It can be a value or a local minimum. L ₁ and L ₂ are lengths that can be the shortest bond length in a parallel waveguide, and are also called unit bond lengths. That is, the bond length may be an odd multiple of the unit bond length.

The first coupling coefficient and the second coupling coefficient can be adjusted by adjusting the length L of the first waveguide 20 and the second waveguide 30 along each other. The length L along which the first waveguide 20 and the second waveguide 30 are along each other may be substantially the same as an odd multiple of the unit bond length with respect to the electromagnetic wave propagating in the second direction. By doing so, the second coupling coefficient can be increased. The length L along which the first waveguide 20 and the second waveguide 30 follow each other may be substantially the same as an even multiple of the unit bond length with respect to the electromagnetic wave propagating in the first direction. By doing so, the first coupling coefficient can be reduced. By doing so, the second coupling coefficient may be made larger than the first coupling coefficient.

By being configured as described above, in the isolator 10, most of the electromagnetic waves incident from the first port 211 of the first waveguide 20 and propagating in the first direction are sent to the first portion 30a of the second waveguide 30. It propagates toward the second port 212 without moving. However, a part of the electromagnetic wave propagating in the first waveguide 20 moves to the first portion 30a of the second waveguide 30 at the portion where the first waveguide 20 and the second waveguide 30 are along each other, and the second core. 31 can propagate towards the second end 302.

The electromagnetic wave propagating from the second core 31 toward the second end 302 is generated by the non-reciprocal member 32 at least in the portion where the second core 31 of the second portion 30c of the second waveguide 30 is in contact with the non-reciprocal member 32. Partially absorbed. Therefore, the electromagnetic wave can be attenuated as it propagates through the second portion 30c.

Further, the electromagnetic wave that has reached the second end 302 of the second waveguide 30 has a tapered shape at the second end 302, so that many parts are not reflected and the second end of the second waveguide 30 is not reflected. It is radiated from 302 to the outside. In particular, when the second end 302 is covered with the non-reciprocal member 32, the electromagnetic wave radiated from the second end 302 is absorbed by the non-reciprocal member 32. Further, by making the tapered portion of the second end 302 of the second core 31 longer than the wavelength of the electromagnetic wave propagating in the second waveguide 30, the reflection of the electromagnetic wave at the second end 302 can be particularly reduced. ..

Therefore, according to the isolator 10, the electromagnetic wave incident from the first port 211 of the first waveguide 20 and transferred to the second waveguide 30 in the first portion 30a is reflected by the second end 302 and is reflected in the second waveguide 30. It is possible to suppress the return to the first portion 30a of 30. As a result, the electromagnetic wave returned to the first portion 30a moves to the first waveguide 20 again and propagates to the first port 211 side, and the electromagnetic wave source (light source) or element (optical element) connected to the first port 211 or the like. It is possible to reduce the risk of adverse effects such as damage to the device.

When the first port 211 is used as the input port of the isolator 10, the electromagnetic wave incident from the first port 211 returns to the first port 211, and the electromagnetic wave source (light source) or element (optical) connected to the first port 211. Damage to the element) is one of the most worrisome events. Therefore, the electromagnetic wave traveling in the first direction on the first waveguide 20 is non-reciprocal to the second core 31 of the second portion 30c, which is the traveling side when the electromagnetic wave travels to the first portion 30a of the second waveguide 30. The configuration in which the members 32 are in contact with each other is particularly effective in preventing the return of the input electromagnetic wave.

Further, in the isolator 10, most of the electromagnetic waves incident from the second port 212 of the first waveguide 20 and propagating in the second direction are the first in the portion where the first waveguide 20 and the second waveguide 30 are along each other. 2 Move to the first portion 30a of the waveguide 30. The electromagnetic wave transferred to the second waveguide 30 in the first portion 30a propagates through the second core 31 of the second waveguide 30 toward the first end 301.

The electromagnetic wave propagating from the second core 31 of the second waveguide 30 toward the second end 302 is not generated at the portion where the second core 31 of the second portion 30b of the second waveguide 30 is in contact with the non-reciprocal member 32. It is at least partially absorbed by the reciprocal member 32. Therefore, the electromagnetic wave can be attenuated as it propagates through the second portion 30b.

Further, the electromagnetic wave that has reached the first end 301 of the second waveguide 30 has a tapered shape at the first end 301, so that many parts are not reflected and the first end of the second waveguide 30 is not reflected. It is radiated from 301 to the outside. In particular, when the first end 301 is covered with the non-reciprocal member 32, the electromagnetic wave radiated from the first end 301 is absorbed by the non-reciprocal member 32. Further, by making the tapered portion of the first end 301 of the second core 31 longer than the wavelength of the electromagnetic wave propagating in the second waveguide 30, the reflection of the electromagnetic wave at the first end 301 can be particularly reduced. ..

Therefore, according to the isolator 10, the electromagnetic wave incident from the second port 212 of the first waveguide 20 and transferred to the second waveguide 30 in the first portion 30a is reduced from being reflected by the first end 301. be able to. Further, according to the isolator 10, the electromagnetic wave reflected by the first end 301 of the second waveguide 30 propagates to the second end 302 of the second waveguide 30, and is further reflected by the second end 302. It can be reduced. As a result, in the isolator 10, the electromagnetic wave incident from the second port 212 of the first waveguide 20 is transferred to the second waveguide 30, and is reflected in order at the first end 301 and the second end 302, and is again first. It is possible to reduce the return to the waveguide 20. Therefore, in the isolator 10, the electromagnetic wave incident from the second port 212 of the first waveguide 20 eventually proceeds to the first port 211, and the electromagnetic wave source (light source) or element (optical element) connected to the first port 211. ) Etc. can be reduced, and the risk of adverse effects such as damage can be reduced.

As described above, in the isolator 10, the non-reciprocal member 32 comes into contact with the second core 31 at both portions of the second portions 30b and 30c located on both sides of the first portion 30a of the second waveguide 30. The effect of reducing unnecessary reflection is increased.

As described above, the isolator 10 of the present disclosure can reduce unnecessary reflection at the end of the second waveguide 30. As a result, the isolator 10 uses the electromagnetic wave source (electromagnetic wave source) due to the electromagnetic waves incident from the first port 211 or the second port 212 of the first waveguide 20 being reflected in the second waveguide 30 and output from the first port 211. The adverse effect on the light source) or other elements (optical elements) can be reduced.

Further, the isolator 10 of the present disclosure also uses the non-reciprocal member 32 used for imparting non-reciprocity to the second waveguide 30 in order to reduce unnecessary electromagnetic waves. Therefore, the isolator 10 does not need to be separately provided with a special component for reducing unnecessary electromagnetic waves. Therefore, the isolator 10 of the present disclosure is simpler and cheaper because it is not necessary to add a process for installing the absorbing end as compared with the case where an absorbing material such as metal is provided at the end of the second waveguide 30. Can be configured in.

Further, in the isolator 10 of the present disclosure, since unnecessary electromagnetic waves are absorbed by the non-reflective member 32 in contact with the side surface of the second core 31 of the second waveguide 30, the member that absorbs the electromagnetic waves at the end of the second waveguide 30. It is less likely that reflection in the return direction will occur due to the difference in refractive index at the interface, as compared with the case of installing.

In the embodiments of FIGS. 1 to 3, the first core 21 of the first waveguide 20, the second core 31 of the second waveguide 30, and the non-reciprocal member 32 are covered with the second clad 62. However, as shown in the cross-sectional view of FIG. 6, the isolator 11 according to another embodiment does not have to be provided with the second clad 62. FIG. 6 is a cross-sectional view of the isolator 11 at a position corresponding to the cross-sectional view of FIG. In this case, the first core 21, the second core 31, and the non-reciprocal member 32 may come into contact with air at a portion that does not come into contact with the first clad 61. Air is a dielectric having a relative permittivity of about 1.0006. Further, in the isolator 10 according to the embodiment of FIGS. 1 to 3, the surface of the second core 31 of the second waveguide 30 on the positive direction side of the z-axis was covered with the second clad 62. In the isolator 11, the second core 31 of the second waveguide 30 may be covered with a non-reciprocal member 32 on the surface on the positive side of the z-axis. By covering the second core 31 of the second portions 30b and 30c with the non-reciprocal member 32, the isolator 11 can absorb a larger proportion of electromagnetic waves in the second portions 30b and 30c.

The isolators 10 and 11 of the present disclosure can be applied to an optical transmitter. The optical transmitter 70 according to the embodiment to which the isolators 10 and 11 are applied is shown in the block diagram of FIG.

The optical transmitter 70 includes a light source 71, a DAC 72 (Digital to Analog Converter), a driver 73, an optical modulator 74, and an isolator 75.

As the light source 71, a laser light source may be adopted. The laser light source may include a semiconductor laser such as an LD (Laser Diode). The light emitted by the light source 71 may be included in the band from infrared light to ultraviolet light. The light source 71 can continuously emit stable light.

The optical transmitter 70 receives a transmission signal to be transmitted from another device or the like. The transmission signal is converted into an analog signal by the DAC 72. The driver 73 drives the light modulator 74 based on the transmission signal converted into an analog signal to modulate the light emitted from the light source 71. As the modulation method for modulating light, various methods such as amplitude modulation and phase modulation can be adopted. As the light modulator 74, for example, a Machzenda optical modulator formed by silicon photonics technology can be used. The light modulator 74 may be formed on the same substrate 50 as the isolator 75.

The isolator 75 may employ an isolator according to the present disclosure. For example, as the isolator 75, the isolator 10 shown in FIGS. 1 to 3 or the isolator 11 shown in FIG. 6 can be used. The isolator 75 is arranged downstream of the light source 71 in the optical path of the light emitted from the light source 71. For example, as shown in FIG. 7, the isolator 75 is arranged on the light emitting side of the light modulator 74 and transmits light toward the outside of the light transmitter 70. The isolator 75 suppresses the light returned from the outside to the light transmitter 70 due to reflection or the like from propagating to the light modulator 74 side. As a result, the isolator 75 reduces the possibility that light incident from the outside enters the light source 71 or the like and adversely affects the light source 71 or the like. The isolator 75, unlike FIG. 7, may be arranged between the light source 71 and the light modulator 74. In this case, the isolator 75 can reduce the possibility that the light returned from the outside and the returned light reflected inside the light modulator 74 are incident on the light source 71.

Another embodiment of the optical transmitter is shown in FIG. The optical transmitter 70 of FIG. 7 employs an external modulation method in which the light from the light source 71 is modulated by an optical modulator 74 outside the light source 71. In the embodiment illustrated in FIG. 8, a direct modulation method that directly controls the light source is adopted.

The optical transmitter 80 includes a signal modulation unit 81, a driver 82 as a drive unit, a light source 83, and an isolator 84. The signal modulation unit 81 converts the transmission signal into a binary modulation signal. The driver 82 controls on / off of driving the light source 83 based on the modulated signal output from the signal modulation unit 81. The isolator 84 may employ an isolator according to the present disclosure. For example, as the isolator 84, the isolator 10 shown in FIGS. 1 to 3 or the isolator 11 shown in FIG. 6 can be used. The isolator 84 is arranged on the light emitting side of the light source 83 and transmits light toward the outside of the light transmitter 80. The isolator 84 reduces the possibility that the light returned from the outside to the light transmitter 80 due to reflection or the like is incident on the light source 83 and adversely affects the light source such as damage.

Although the embodiments according to the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can easily make various modifications or modifications based on the present disclosure. It should be noted, therefore, that these modifications or modifications are within the scope of this disclosure. For example, the functions included in each component or each step can be rearranged so as not to be logically inconsistent, and a plurality of components or steps can be combined or divided into one. Is. Although the embodiment according to the present disclosure has been mainly described for the device, the embodiment according to the present disclosure can also be realized as a method including steps executed by each component of the device. The embodiments according to the present disclosure can also be realized as a method, a program, or a storage medium on which a program is recorded, which is executed by a processor included in the apparatus. It should be understood that the scope of this disclosure also includes these.

In the present disclosure, the descriptions such as "first" and "second" are identifiers for distinguishing the configuration. The configurations distinguished by the descriptions such as "first" and "second" in the present disclosure can exchange numbers in the configurations. For example, the first port can exchange the identifiers "first" and "second" with the second port. The exchange of identifiers takes place at the same time. Even after exchanging identifiers, the configuration is distinguished. The identifier may be deleted. The configuration with the identifier removed is distinguished by a code. Based solely on the description of identifiers such as "first" and "second" in the present disclosure, it shall not be used as a basis for interpreting the order of the configurations and for the existence of identifiers with smaller numbers.

In the present disclosure, the x-axis, y-axis, and z-axis are provided for convenience of explanation and may be interchanged with each other. The configuration according to the present disclosure has been described using a Cartesian coordinate system composed of x-axis, y-axis, and z-axis. The positional relationship of each configuration according to the present disclosure is not limited to being orthogonal.

The arrangement of the waveguide on the substrate in each embodiment of the isolator is merely an example. For example, in each of the above embodiments, the direction of the waveguide of the portion where the plurality of waveguides are close to each other to form a parallel waveguide is parallel to the side in the longitudinal direction of the substrate. However, the shape of the substrate and the orientation and arrangement of the waveguide are not limited to this, and can be set in any way as long as the effects of the present disclosure can be obtained. For example, the plurality of waveguides can be arranged not only along the substrate surface but also in the direction perpendicular to the substrate surface.

In the above embodiment, an isolator and an optical transmitter for propagating electromagnetic waves in TE mode have been described. However, the present disclosure can also be applied to isolators and optical transmitters that propagate electromagnetic waves in TM mode. When the present disclosure is applied to an isolator and an optical transmitter that propagate electromagnetic waves in TM mode, the arrangement of the non-reciprocal member with respect to the second waveguide and the direction of the applied magnetic field are different from those of the above embodiment. For example, when the present invention is applied to a TM mode isolator and an optical transmitter, the non-reciprocal members are arranged in a direction perpendicular to the substrate surface of the second waveguide (corresponding to the z-axis direction of the above embodiment). sell. Further, at this time, the magnetic field can be applied in a direction orthogonal to the direction in which the second waveguide extends (corresponding to the y-axis direction of the above embodiment).

10, 11 Isolator 20 1st Dielectric Path 21 1st Core 201 1st End 202 2nd End 211 1st Port 212 2nd Port 30 2nd Dielectric Path 31 2nd Core 31a 1st Part 31b, 31c 2nd Part 301 1st end 302 2nd end 32 Non-reciprocal member 50 Substrate 50a Substrate surface 60 Clad 61 1st clad (dielectric)
62 Second clad (dielectric)
70, 80 Optical transmitter 71 Light source 72 DAC (Digital-to-Analog converter)
73 Driver 74 Optical modulator 75 Isolator 81 Signal generator 82 Driver (drive unit)
83 Light source 84 Isolator

Claims (9)

On a substrate having a substrate surface, a first waveguide and a second waveguide located at least partially aligned with the substrate surface are provided.
The first waveguide includes a first core, the second waveguide includes a second core, and the first core and the second core are surrounded by a dielectric.
The first waveguide has a first end and a second end, and each of the first end and the second end has a port for input / output of electromagnetic waves.
The second waveguide has a first portion along the first waveguide and a second portion not included in the first portion.
The second waveguide has a non-reciprocal member in contact with at least a part of the second core of the first portion.
The non-reciprocal member further contacts at least a part of the second core of the second portion.
Isolator. In the first part of the second waveguide, the second core of the second waveguide is located close to such that the first core of the first waveguide and the electromagnetic field in the near field are coupled. The isolator according to claim 1. The isolator according to claim 1 or 2, wherein the second core has a plurality of side surfaces around the longitudinal direction, and the non-reciprocal member is in contact with at least one surface of the second core of the second portion. The isolator according to any one of claims 1 to 3, wherein the second portion of the second waveguide is bent in a direction deviating from the first waveguide. The second portion of the second waveguide is located on one or both sides of the first portion, and the non-reciprocal member makes at least the first waveguide from the first end to the second end. The invention according to any one of claims 1 to 4, wherein when the electromagnetic wave traveling in the direction of the above moves to the first portion of the second waveguide, the electromagnetic wave traveling in the direction of is in contact with the second core of the second portion. Isolator. The isolator according to any one of claims 1 to 5, wherein both ends of the second core of the second waveguide have a diameter decreasing toward the tip end. When the electromagnetic wave input from the second end propagates toward the first end, the coupling coefficient between the first waveguide and the second waveguide is the electromagnetic wave input from the first end. The isolator according to any one of claims 1 to 6, which is larger than the coupling coefficient between the first waveguide and the second waveguide when propagating toward the second end. Light source and
An optical modulator that modulates the light emitted from the light source based on the signal to be transmitted,
It is equipped with an isolator located on the downstream side of the light source.
The isolator
On a substrate having a substrate surface, a first waveguide and a second waveguide located at least partially aligned with the substrate surface are provided.
The first waveguide includes a first core, the second waveguide includes a second core, and the first core and the second core are surrounded by a dielectric.
The first waveguide has a first port at which light from the light source is input at the first end, and a second port at which light from the light source is output at a second end different from the first end. Have and
The second waveguide has a first portion along the first waveguide and a second portion not included in the first portion.
The second waveguide has a non-reciprocal member in contact with at least a part of the second core of the first portion.
The non-reciprocal member further contacts at least a part of the second core of the second portion.
Optical transmitter. Light source and
A drive unit that drives the light source based on a signal to be transmitted,
It is provided with an isolator arranged on the light emitting side of the light source.
The isolator
On a substrate having a substrate surface, a first waveguide and a second waveguide located at least partially aligned with the substrate surface are provided.
The first waveguide includes a first core, the second waveguide includes a second core, and the first core and the second core are surrounded by a dielectric.
The first waveguide has a first port at which light from the light source is input at the first end, and a second port at which light from the light source is output at a second end different from the first end. Have and
The second waveguide has a first portion along the first waveguide and a second portion not included in the first portion.
The second waveguide has a non-reciprocal member in contact with at least a part of the second core of the first portion.
The non-reciprocal member further contacts at least a part of the second core of the second portion.
Optical transmitter.

Images