JP7309503B2 - isolator and optical transmitter - Google Patents

Description

The present disclosure relates to isolators and optical transmitters.

An isolator using a magneto-optical effect that exerts a non-reciprocal effect on the traveling direction of light has been proposed (see, for example, Patent Document 1).

Japanese Patent Application Laid-Open No. 2003-302603

Conventionally, the choice of nonreciprocal materials that produce a nonreciprocal effect that can be used for isolators has been limited due to the magnitude of light absorption loss and high cost.

It is an object of the present disclosure to provide isolators and optical transmitters that can expand the choice of materials that can be selected as non-reciprocal materials.

An isolator according to an embodiment of the present disclosure comprises a first waveguide and a second waveguide overlying a substrate having a substrate surface and positioned at least partially side-by-side along said substrate surface. The first waveguide has a first end and a second end, and ports for inputting and outputting electromagnetic waves at the first end and the second end, respectively. The second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends. In the isolator, when the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz).
where the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θ _F [degrees/μm], and the insertion loss allowed for the isolator is x [dB],
θ _F /α × x < 45 [degrees]
The non-reciprocal material is used.

An optical transmitter according to an embodiment of the present disclosure includes a light source, an optical modulator, and an isolator. The optical modulator modulates light emitted from the light source based on a signal to be transmitted. The isolator is arranged downstream of the light source. The isolator comprises a first waveguide and a second waveguide positioned over a substrate having a substrate surface and positioned at least partially side-by-side along the substrate surface. The first waveguide has a first port for inputting light from the light source at a first end, and a second port for outputting light from the light source at a second end different from the first end. have. The second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends. When the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz)
, the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θ _F [degrees/μm], and the insertion loss allowed for the isolator is x [dB], the optical transmitter is:
θ _F /α × x < 45 [degrees]
The non-reciprocal material is used.

An optical transmitter according to an embodiment of the present disclosure includes a light source, a driver, and an isolator. The driving section 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 comprises a first waveguide and a second waveguide positioned over a substrate having a substrate surface and positioned at least partially side-by-side along the substrate surface. The first waveguide has a first port for inputting light from the light source at a first end, and a second port for outputting light from the light source at a second end different from the first end. have. The second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends. When the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz)
, the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θ _F [degrees/μm], and the insertion loss allowed for the isolator is x [dB], the optical transmitter is:
θ _F /α × x < 45 [degrees]
The non-reciprocal material is used.

According to embodiments of the present disclosure, isolators and optical transmitters can be provided that can expand the selection of materials that can be selected as non-reciprocal materials.

1 is a perspective view of an isolator according to one embodiment; FIG. 2 is a plan view of the isolator of FIG. 1; FIG. FIG. 3 is a cross-sectional view taken along the line AA of FIG. 2; 4 is a graph showing coupling coefficients for electromagnetic waves traveling in a first direction; 4 is a graph showing coupling coefficients for electromagnetic waves traveling in a second direction; FIG. 11 is a perspective view of an isolator according to another embodiment; 1 is a block diagram showing a schematic configuration of an optical transmitter according to one embodiment; FIG. FIG. 11 is a block diagram showing a schematic configuration of an optical transmitter according to another embodiment;

A free-space isolator is known as an isolator using a nonreciprocal material that produces a nonreciprocal effect. A free-space isolator is constructed such that the polarization direction is tilted by 45 degrees due to the Faraday effect while linearly polarized light is transmitted through a non-reciprocal material placed between two polarizers. The two polarizers are arranged with their transmission axes tilted by 45 degrees to each other so as to transmit light propagating in one direction and not transmit light propagating in the opposite direction.

In order to exhibit the function of a free-space isolator, a material is required that keeps the loss of light within the required specifications until the angle of rotation of the polarization direction reaches 45 degrees. However, there are few kinds of non-reciprocal materials that satisfy these conditions, and they are expensive. Therefore, the present disclosure is a waveguide isolator, and provides a nonreciprocal directional coupler by adjoining one of two waveguides constituting the directional coupler with a nonreciprocal material exhibiting nonreciprocity. to form This makes it possible to construct an isolator using a non-reciprocal material with a relatively large absorption coefficient α.

In the present disclosure, when the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz) (1)
defined by This disclosure is
θ _F /α × x < 45 [degrees] (2)
It is possible to provide an isolator using a non-reciprocal material with a relatively large absorption coefficient α such that where θ _F [degrees/μm] is the Faraday rotation angle per thickness of the non-reciprocal material. Also, x [dB] is the insertion loss allowed for the isolator.

Embodiments of the present invention are described below with reference to the drawings. Note that the diagrams used in the following description are schematic. The dimensions, ratios, etc. on the drawings do not necessarily match the actual ones.

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

For the following description the x-axis, y-axis and z-axis directions are defined respectively. As shown in FIGS. 1 to 3, the x-axis direction is the direction in which the first waveguide 20 and the second waveguide 30 extend. The positive direction of the x-axis is the direction from one end (first end 201) of the first waveguide 20 to the other end (second end 202). The y-axis direction is a direction along the substrate surface 50a of the substrate 50 and a direction crossing 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 second waveguide 30 when 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 in which the first waveguide 20 and the second waveguide 30 are arranged when viewed from the substrate 50 .

Substrate 50 may be constructed from a variety of materials. For example, the substrate 50 may be made of a material selected from materials including metal conductors, semiconductors such as silicon, glass, resin, and the like. The substrate 50 can take various shapes. For example, the substrate 50 may have a rectangular shape having two sides extending in the x-axis direction and the y-axis direction and being long in the x-axis direction.

A first clad 61 common to the first waveguide 20 and the second waveguide 30 is formed on the substrate surface 50 a of the substrate 50 . The first core 21 , the second core 31 and the non-reciprocal member 32 are arranged on the upper surface 61 a 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 surrounded and topped by a second clad 62 formed on the first clad 61 . The first clad 61 and the second clad 62 can be collectively called the clad 60 . The first core 21 , the second core 31 and the non-reciprocal member 32 are surrounded by the clad 60 . The first waveguide 20 includes a first core 21 and a portion of the clad 60 adjacent to the first core 21 . The second waveguide 30 includes a second core 31 and a non-reciprocal member 32 and a portion of the clad 60 adjacent 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 including a dielectric. The first core 21 and the second core 31 are also called dielectric lines. The relative permittivity of the first core 21 and the second core 31 may be higher than that of the clad 60 . The first clad 61 and the second clad 62 forming the clad 60 may be made of the same dielectric material. The first clad 61 and the second clad 62 may be configured integrally. 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 that of air. Leakage of electromagnetic waves from the first waveguide 20 and the second waveguide 30 is suppressed by making the dielectric constants of the first core 21, the second core 31, and the clad 60 higher than the dielectric constant of air. can be As a result, loss due to electromagnetic waves radiated from the isolator 10 to the outside can be reduced.

The first core 21 and the second core 31 may be made of silicon (Si), for example. The clad 60 may be made of quartz glass (SiO ₂ ), for example. The dielectric constants of silicon and fused silica are about 12 and about 2, respectively. Silicon can propagate electromagnetic waves having near-infrared wavelengths from 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, they can propagate electromagnetic waves having a wavelength of 1.3 μm band or 1.55 μm band used in optical communication with low loss.

Materials for 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 of the isolator 10 of the present disclosure can be obtained. A portion of the second cladding 62 may be air without a layer of a particular material. Air is a dielectric.

The dielectric constants 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 vary along the z-axis direction. For example, the dielectric constant of the first core 21 may be distributed such that it is highest in the central portion in the z-axis direction and decreases toward 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 a graded-index 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 fall within the band of light from ultraviolet to infrared. When the wavelength of electromagnetic waves is included in the wavelength band of light, the isolator 10 is also called an optical isolator. Also, the electromagnetic wave band may be a wavelength band such as a wavelength of 1.55 μm, which is used in silicon photonics.

The first waveguide 20 and the second waveguide 30 according to this embodiment can propagate TE mode electromagnetic waves. An electromagnetic wave that has no electric field component in the direction of propagation is called a TE mode. In the waveguide, the electric field of the TE mode electromagnetic wave oscillates horizontally with respect to the substrate 50 . Electromagnetic waves in TE mode are also called TE waves. In a device or system utilizing isolator 10, first waveguide 20 and second waveguide 30 may be designed to propagate TE mode electromagnetic waves. Therefore, the polarization direction of the electromagnetic waves 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 a waveguide condition in single mode. When the first waveguide 20 and the second waveguide 30 satisfy the waveguide conditions in a single mode, the waveforms of signals propagating through the first waveguide 20 and the second waveguide 30 are less likely to collapse. The isolator 10 that combines the first waveguide 20 and the second waveguide 30 satisfying the waveguiding conditions in a single mode can be suitable for optical communications.

Non-reciprocal member 32 comprises a non-reciprocal material. Non-reciprocal materials include magnetic materials. The non-reciprocal member 32 satisfies the requirements of equation (2) above. Any nonreciprocal material can be adopted as long as it satisfies the formula (2) and the effects of the present disclosure can be obtained. The non-reciprocal material employed for the non-reciprocal member 32 may have the property of absorbing electromagnetic waves.

テンソルの各要素は、複素数で表されてよい。各要素の添え字として用いられている数字は、ｘ軸、ｙ軸及びｚ軸に対応してよい。要素として複素数を有し、比誘電率を表すテンソルは、複素比誘電率テンソルともいう。 The dielectric constant of the non-reciprocal member 32 can be represented by a tensor as in Equation (3).

Each element of the tensor may be represented by a complex number. The numbers used as subscripts for each element may correspond to the x-, y-, and z-axes. A tensor that has complex numbers as elements and represents a dielectric constant is also called a complex dielectric constant tensor.

The first waveguide 20 extends longer in the x-axis direction than the second waveguide 30 . The first waveguide 20 has a first end 201 and a second end 202 on the negative and positive sides of the x-axis, respectively. The first waveguide 20 has a first port 211 and a second port 212 through which electromagnetic waves are input and output at a first end 201 and a second end 202, respectively. An electromagnetic wave input from the first port 211 to the first waveguide 20 travels along the x-axis toward the second port 212 . An electromagnetic wave input from the second port 212 to the first waveguide 20 travels along the x-axis toward the first port 211 . Each of the first port 211 and the second port 212 may be configured as an end face, or may be configured as a coupler that is connected to an external device and capable of propagating electromagnetic waves.

The second waveguide 30 has a first end 301 and a second end 302 on the negative and positive sides of the x-axis, respectively. In other words, the second waveguide 30 has two ends. The number of second waveguides 30 is not limited to one, and may be two or more.

In the second waveguide 30 , the non-reciprocal member 32 is positioned on the positive direction side of the y-axis with respect to the second core 31 . That is, the non-reciprocal member 32 is located on the side opposite to the side facing the first waveguide 20 of the second core 31 . The nonreciprocal member 32 may be in contact with the second core 31 over the entirety of the second core 31 along the x-axis direction. Alternatively, the non-reciprocal member 32 may be in contact with the second core 31 at a portion along the x-axis direction of the second core 31 .

The first waveguide 20 and the second waveguide 30 may be positioned along each other in at least part of the extending direction. The first waveguide 20 and the second waveguide 30 may be positioned parallel to each other in at least part of the extending direction. Either or both the first waveguide 20 and the second waveguide 30 may have a linear structure. The first waveguide 20 and the second waveguide 30 can be easily formed on the substrate 50 by having such simple structures.

The first core 21 of the first waveguide 20 and the second core 31 of the second waveguide 30 in the portion along which the first waveguide 20 and the second waveguide 30 are aligned, each other's electromagnetic field (evanescent field) can be combined. Two waveguides lying along each other are also referred to as parallel waveguides. In parallel waveguides, an electromagnetic wave input into one waveguide can migrate to the other waveguide while propagating in that waveguide. That is, electromagnetic waves propagating in the first waveguide 20 can be transferred to the second waveguide 30 . Electromagnetic waves propagating in the second waveguide 30 can pass into the first waveguide 20 .

In parallel waveguides, the parameter representing the proportion of electromagnetic waves that pass from one waveguide to the other is called the coupling coefficient. The coupling coefficient is assumed to be 0 if no electromagnetic wave is transferred from one waveguide to the other. The coupling coefficient is assumed to be 1 if all electromagnetic waves pass from one waveguide to the other. It is assumed that the coupling coefficient can be a value greater than or equal to 0 and less than or equal to 1. The coupling coefficient can be determined based on the shape of each waveguide, the distance between each waveguide, the length along which the waveguides are along each other, and the like. For example, the coupling coefficient can be higher as the shapes of each waveguide are more similar. For the distance between each waveguide, the coupling coefficient can change depending on the distance that the electromagnetic wave propagates in the waveguide. That is, in parallel waveguides, the coupling coefficient can be different depending on the position along the direction in which the waveguides extend. The maximum value of the coupling coefficient can be determined based on the shape of each waveguide, the distance between each waveguide, or the like. The maximum value of the coupling coefficient can be a value of 1 or less.

In parallel waveguides, the coupling coefficient is zero at the beginning of the section where the waveguides are along each other. The length from the starting point to the position where the coupling coefficient becomes the maximum value is also called the coupling length. If the length along which the waveguides are along each other is equal to the coupling length, the coupling coefficient at the end of the section along which the waveguides are along each other may be at a maximum value. The coupling length can be determined based on the shape of each waveguide, the distance between each waveguide, or the like.

In the second waveguide 30 , the electromagnetic waves transferred from the first waveguide 20 propagate in the second waveguide 30 in the same direction as in the first waveguide 20 . In the second waveguide 30, when an 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 It can be reflected and go in the opposite direction.

The electromagnetic wave input from the first end 201 of the first waveguide 20 to the first waveguide 20 via the first port 211 is generated at the second end of the first waveguide 20 extending along the x-axis. 202. The direction from the first end 201 to the second end 202 is also called the first direction. The electromagnetic wave propagating in the first waveguide 20 is divided into the second We can move to the waveguide 30 . The coupling coefficient when the electromagnetic wave propagates in the first waveguide 20 in the first direction is also referred to as the first coupling coefficient.

The electromagnetic wave input from the second end 202 of the first waveguide 20 to the first waveguide 20 via the second port 212 is generated at the first end of the first waveguide 20 extending along the x-axis. 201. The direction from the second end 202 to the first end 201 is also called the second direction. The electromagnetic wave propagating in the first waveguide 20 is divided into the second It propagates to waveguide 30 . The coupling coefficient when the electromagnetic wave propagates in the first waveguide 20 in the second direction is also referred to as the second coupling coefficient.

The second waveguide 30 has nonreciprocity because the second core 31 is adjacent to the nonreciprocity member 32 on the positive direction side of the y-axis. Having non-reciprocity means that the effect received by propagating light differs depending on the propagation direction of the light. 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. The first coupling coefficient and the second coupling coefficient may differ from each other if the propagation characteristics of the second waveguide 30 differ based on the propagation direction of the electromagnetic wave. That is, the non-reciprocal member 32 can have the first coupling coefficient and the second coupling coefficient different.

The non-reciprocity of the second waveguide 30 due to the non-reciprocity member 32 is exhibited by applying 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 through the second waveguide 30 are configured to cross each other. In this embodiment, the polarization direction of the TE mode electromagnetic wave propagating through the second waveguide 30 is substantially parallel to the substrate surface 50a of the substrate 50 (that is, the y-axis direction). In this case, by applying an external magnetic field having a component in the z-axis direction, the non-reciprocity of the non-reciprocal member 32 is exhibited. When the magnitude of the external magnetic field is constant, the non-reciprocity is maximized by applying an external magnetic field substantially in the z-axis direction.

When the non-reciprocal member 32 is a ferromagnetic material, the non-reciprocal member 32 develops non-reciprocity without applying an external magnetic field. When the polarization direction of the electromagnetic wave propagating through 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 approximately in the z-axis direction.

When one waveguide of the parallel waveguides has nonreciprocity, 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 a value 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 a value close to 1. . Since the maximum value of the coupling coefficient is different for each propagation direction of the electromagnetic wave, the transmittance of the electromagnetic wave can be different for each propagation direction of the electromagnetic wave. In FIGS. 4 and 5, the horizontal axis and the vertical axis respectively represent the traveling distance of the electromagnetic wave in the parallel waveguide and the coupling coefficient.

If the second waveguide 30 has non-reciprocity, the coupling coefficient between the first waveguide 20 and the second waveguide 30 may differ according to the propagation direction of the electromagnetic waves. That is, if the second waveguide 30 is non-reciprocal, the first coupling coefficient of the isolator 10 can differ 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 part of the electromagnetic wave transferred to the second waveguide 30 among the input electromagnetic waves reaches the second end 302 . I can. The electromagnetic wave that reaches the second end 302 of the second waveguide 30 is not output from the second port 212 of the first waveguide 20, and is radiated to the outside from the second end 302 or reflected at the second end 302. I can. When the first coupling coefficient is large, a proportion of the electromagnetic waves that are input to the first waveguide 20 and reach the second end 302 after moving to the second waveguide 30 may increase. In this case, the ratio of the electromagnetic wave output from the second port 212 to the electromagnetic wave input to the first waveguide 20 may be reduced. 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 reduced. 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. If the first coupling coefficient is large, the transmittance for electromagnetic waves propagating in the first direction may be low. On the other hand, when the first coupling coefficient is small, the proportion of the electromagnetic wave transferred to the second waveguide 30 may be small, so the transmittance of the electromagnetic wave propagating in the first direction may be high.

The electromagnetic wave that is input from the second port 212 to the first waveguide 20 and propagates in the second direction can be subjected to the same action as the electromagnetic wave that propagates in the first direction is given by the isolator 10 according to the second coupling coefficient. . Due to this action, part of the electromagnetic wave propagating in the second direction can reach the first end 301 of the second waveguide 30 . If the second coupling coefficient is large, the transmittance for electromagnetic waves propagating in the second direction may be low. When the second coupling coefficient is small, the transmittance for electromagnetic waves propagating in the second direction can be high.

If the first coupling coefficient and the second coupling coefficient are different, the transmittance for electromagnetic waves propagating in the first direction may be different from the transmittance for electromagnetic waves propagating in the second direction. In other words, the isolator 10 can function to facilitate the propagation of electromagnetic waves in one direction and to hinder the propagation of electromagnetic waves in the opposite direction by making the first coupling coefficient and the second coupling coefficient different. When the second coupling coefficient is greater than the first coupling coefficient, the isolator 10 can function to facilitate propagation of electromagnetic waves in the first direction and to inhibit propagation of electromagnetic waves in the second direction. When the first coupling coefficient and the second coupling coefficient are approximately 0 and approximately 1, respectively, the difference between the transmittance for electromagnetic waves propagating in the first direction and the transmittance for electromagnetic waves propagating in the second direction can be increased. . As a result, the functionality of the isolator 10 can be improved.

If one of the parallel waveguides has nonreciprocity, the coupling length of the parallel waveguides for electromagnetic waves propagating in the first direction can be different from the coupling length of the parallel waveguides for electromagnetic waves propagating in the second direction. For example, as shown in FIG. 4, the coupling length for electromagnetic waves propagating in the first direction in isolator 10 can be represented as _L1 . For example, as shown in FIG. 5, the coupling length for electromagnetic waves propagating in the second direction in isolator 10 can be represented as _L2 . Isolator 10 may be configured such that L ₁ and L ₂ are different.

If the length (L in FIG. 2) along which the two waveguides follow each other in parallel waveguides is equal to the coupling length, the coupling coefficient can reach a maximum value. For example, in parallel waveguides having the relationship shown in the graph of FIG. 4, if the length L along which the two waveguides are along each other is L ₁ , the coupling coefficient can reach a maximum value. If the length L along which the two waveguides are along each other is equal to twice the coupling length, the coupling coefficient can be a local minimum. For example, in parallel waveguides having the relationship shown in FIG. 4, if the length L along which the two waveguides are along each other is 2L ₁ , the coupling coefficient can be a local minimum.

The relationship shown in the graph of FIG. 4 can be repeated in regions where the electromagnetic wave travels longer. That is, if the length L along which the two waveguides are along each other is an odd multiple of L ₁ , the coupling coefficient can be maximal. If the length L along which the two waveguides are along each other is an even multiple of L ₁ , the coupling coefficient may be a local minimum. Even in parallel waveguides having the relationship shown in FIG. 5, the coupling coefficient is maximum when the length L along which the two waveguides are along each other is an odd multiple of _L2 and when it is an even multiple of _L2 , respectively. values and local minima. L ₁ and L ₂ are lengths that can be the shortest coupling lengths in parallel waveguides, and are also called unit coupling 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 of the first waveguide 20 and the second waveguide 30 along each other may be substantially the same as an odd multiple of the unit coupling length for 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 extend may be substantially the same as an even multiple of the unit coupling length for electromagnetic waves propagating in the first direction. By doing so, the first coupling coefficient can be reduced. By doing so, the second coupling coefficient may be larger than the first coupling coefficient.

As described above, the isolator 10 according to the present embodiment includes the first waveguide 20 and the nonreciprocal second waveguide 30 while using the nonreciprocal material defined by Equation (2). Due to the configuration, it is easy to propagate electromagnetic waves in the first direction, and it is difficult to propagate electromagnetic waves in the second direction. Furthermore, since the electromagnetic wave mainly propagates through the first waveguide 20 and the second waveguide 30 , absorption by the nonreciprocal member 32 is smaller than when the electromagnetic wave propagates inside the nonreciprocal member 32 . Therefore, isolator 10 functions well as an isolator. As a result, it becomes possible to fabricate the isolator 10 with a material that satisfies the formula (2), so that the choice of materials that can be selected as the non-reciprocal material can be expanded. In addition, since the isolator 10 uses a silicon waveguide, it is expected that it can be mounted on a small chip together with various other functions using silicon photonics technology.

(Example)
A simulated embodiment of the isolator 10 is described below.

The wavelength of the electromagnetic wave propagating through the isolator 10 is assumed to be 1.55 μm. For the first waveguide 20 and the second waveguide 30, the width in the y direction of the first core 21 and the second core 31 is 400 nm, and the height in the z direction is 390 nm. The length L of the first waveguide 20 and the second waveguide 30 along each other is 250 μm. It is assumed that the first waveguide 20 and the second waveguide 30 have a dielectric constant of about 12. The dielectric constant of the cladding 60 around the first waveguide 20 and the second waveguide 30 is assumed to be approximately two.

であるものとする。 The complex dielectric constant tensor of the nonreciprocal material of the nonreciprocal member 32 is

shall be

According to simulations, under the above conditions, the effective refractive index of the first waveguide 20 is
_neff = 2.64445
becomes. In addition, the effective refractive index n _eff1 → ₂ of the electromagnetic wave propagating in the first direction of the second waveguide 30 and the effective refractive index n _eff2 → ₁ of the electromagnetic wave propagating in the second direction are respectively:
_neff1 → ₂ = 2.6436 + 0.02272i
_neff2 → ₁ = 2.64432 + 0.02272i
became.

As a result of the simulation, when the length L along which the first waveguide 20 and the second waveguide 30 are along each other is 250 μm, the first coupling coefficient has a value close to approximately zero. Also, the second coupling coefficient was a value close to approximately one. Also, in the isolator 10, the electromagnetic wave mainly propagates through the first waveguide 20 and the second waveguide 30, so the loss due to absorption by the non-reciprocal member 32 is small. As a result, the isolator 10 that propagates electromagnetic waves in the first direction with high transmittance and reduces the propagation of electromagnetic waves in the second direction with low transmittance in a non-reciprocal material with high absorption that satisfies the formula (2). It has been verified that it is possible.

FIG. 6 shows an isolator 11 according to another embodiment. In the isolator 11 , the second waveguide 30 includes a coupling portion 30 a that couples with the first waveguide 20 and two non-coupling portions 30 b and 30 c that do not couple with the first waveguide 20 .

A coupling portion 30 a of the second waveguide 30 is a portion that is close to and substantially parallel to the first waveguide 20 . At the coupling portion 30a, the second waveguide 30 and the first waveguide 20 are coupled in the electromagnetic field. The second waveguide 30 is adjacent to the nonreciprocal member 32 on the positive y-axis side at the coupling portion 30a. The non-reciprocal member 32 may at least partially abut the bond 30a. The length L of the portion where the first waveguide 20 and the second waveguide 30 are along and coupled to each other may be defined by the length of the coupling portion 30a.

The positive and negative sides of the coupling portion 30a along the x-axis of the second waveguide 30 are bent in a direction away from the first waveguide 20 (positive direction of the y-axis). The second waveguide 30 is gently bent so as not to cause reflection or the like at the bent portion. The positive side and negative side portions of the coupling portion 30 a in the x-axis direction are non-coupling portions 30 b and 30 c that do not participate in coupling with the first waveguide 20 . The second waveguide 30 is not coupled with the first waveguide 20 in the non-coupling portions 30b and 30c. The non-bonded portions 30b, 30c may or may not be adjacent to the non-reciprocal member 32.

A first end 301 and a second end 302 located at the distal end of the non-bonding portion 30b and the non-bonding portion 30c may have end surfaces tapered toward the distal end. The tapered portions of the end faces of the first end 301 and the second end 302 can be longer than the wavelength of the electromagnetic wave propagating through the second waveguide 30 . The first end 301 and the second end 302 have a tapered cross-section taken along a plane parallel to the xy plane. The first end 301 and the second end 302 may also taper in the z-axis direction toward the tip. The tapered shape can be rephrased as a tapered shape that narrows toward the tip.

The rest of the configuration of the isolator 11 in FIG. 6 is the same as that of the isolator 10 shown in FIGS. 1 to 3, so the same constituent elements are denoted by the same reference numerals and descriptions thereof are omitted.

With the configuration as described above, in the isolator 11 of FIG. It propagates towards the second port 212 without transitioning to the second waveguide 30 at 30a. However, part of the electromagnetic wave propagating through the first waveguide 20 may transfer to the second waveguide 30 at the coupling portion 30 a and proceed to the second end 302 . If the second end 302 is a flat surface, the electromagnetic waves can be at least partially reflected at the second end 302 back to the coupling portion 30a. When the electromagnetic wave returning to the coupling portion 30a moves to the first waveguide 20 again and proceeds to the first port 211 side, the electromagnetic wave source (light source) or element (optical element) located beyond the first port 211 is damaged. may have adverse effects such as

In the isolator 11, the second end 302 located at the tip of the non-coupled portion 30c of the second waveguide 30 is tapered, so that a large portion of the electromagnetic wave is not reflected at the second end 302, The light is radiated to the outside from the second waveguide 30 . This prevents the electromagnetic wave source (light source) or element (optical element) connected to the first port 211 of the isolator 11 from being adversely affected by the electromagnetic waves returning from the second end 302 of the second waveguide 30 compared to the isolator 10 . can be reduced.

Further, in the isolator 11, the first end 301 located at the tip of the non-coupling portion 30b is tapered, so that more of the electromagnetic wave traveling from the coupling portion 30a toward the non-coupling portion 30b is directed to the first end 301. It is radiated to the outside at end 301 . As a result, the electromagnetic wave incident from the second port 212 of the first waveguide 20 is coupled to the second waveguide 30 at the coupling portion 30a, and the rate of reflection at the first end 301 is reduced. As a result, the electromagnetic wave reflected at the first end 301 travels through the second waveguide 30 in the first direction, is further reflected at the second end 302, is transferred to the first waveguide 20 at the coupling portion 30a, and is transferred to the first waveguide 20. It is possible to reduce the possibility of adversely affecting the electromagnetic wave source (light source) or the like located ahead of the port 211 .

As described above, according to the isolator 11 shown in FIG. 6, in addition to the effects of the isolator 10 shown in FIGS. be able to. As a result, the electromagnetic wave that has been reflected and returned to the first port 211 is emitted from the first port 211 of the first waveguide 20 to form an electromagnetic wave source (light source) or another element (optical element). less likely to adversely affect

The isolators 10, 11 of the present disclosure can be applied to optical transmitters. An embodiment of an optical transmitter 70 employing isolators 10, 11 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 .

The light source 71 may employ a laser light source. 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 a 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. A transmission signal is converted into an analog signal by the DAC 72 . The driver 73 modulates the light emitted from the light source 71 by driving the optical modulator 74 based on the transmission signal converted into the analog signal. Various modulation schemes such as amplitude modulation and phase modulation can be adopted as modulation schemes for modulating light. As the optical modulator 74, for example, a Mach-Zehnder optical modulator formed by silicon photonics technology can be used. Optical modulator 74 may be formed on the same substrate 50 as isolator 75 .

Isolator 75 may employ an isolator according to the present disclosure. For example, the isolator 75 can be the isolator 10 shown in FIGS. 1-3 or the isolator 11 shown in FIG. 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 optical modulator 74 and transmits light directed to the outside of the optical transmitter 70 . The isolator 75 suppresses light that has returned to the optical transmitter 70 from the outside due to reflection or the like from propagating to the optical modulator 74 side. As a result, the isolator 75 reduces the risk of externally incident light entering the light source 71 and causing adverse effects such as damage to the light source 71 . The isolator 75 may be arranged between the light source 71 and the optical modulator 74, unlike in FIG. In this case, the isolator 75 can reduce the risk of light returning from the outside and return light reflected inside the optical modulator 74 entering the light source 71 .

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

The optical transmitter 80 includes a signal modulating section 81 , a driver 82 that is a driving section, a light source 83 and an isolator 84 . The signal modulator 81 converts the transmission signal into a binary modulated signal. The driver 82 controls ON/OFF of driving of the light source 83 based on the modulated signal output from the signal modulator 81 . Isolator 84 may employ an isolator according to the present disclosure. For example, the isolator 84 can be the isolator 10 shown in FIGS. 1-3 or the isolator 11 shown in FIG. The isolator 84 is arranged on the light emitting side of the light source 83 and transmits light directed to the outside of the optical transmitter 80 . The isolator 84 reduces the risk that the light that has returned to the optical transmitter 80 from the outside due to reflection or the like will enter the light source 83 and have an adverse effect such as damage on the light source 83 .

Although the embodiments of the present disclosure have been described with reference to the drawings and examples, it should be noted that those skilled in the art can easily make various variations or modifications based on the present disclosure. Therefore, it should be noted that these variations or modifications are included within the scope of this disclosure. For example, functions included in each component or each step can be rearranged so as not to be logically inconsistent, and multiple components or steps can be combined into one or divided. is. Although the embodiments of the present disclosure have been described with a focus on the apparatus, the embodiments of the present disclosure may also be implemented as a method including steps performed by each component of the apparatus. Embodiments according to the present disclosure can also be implemented as a method, a program, or a storage medium recording a program executed by a processor provided in an apparatus. It should be understood that these are also included within the scope of the present disclosure.

Descriptions such as “first” and “second” in the present disclosure are identifiers for distinguishing the configurations. Configurations that are differentiated in descriptions such as "first" and "second" in this disclosure may interchange the numbers in that configuration. For example, a first port can exchange the identifiers "first" and "second" with a second port. The exchange of identifiers is done simultaneously. The configurations are still distinct after the exchange of identifiers. Identifiers may be deleted. Configurations from which identifiers have been deleted are distinguished by codes. The description of identifiers such as “first” and “second” in this disclosure should not be used as a basis for interpreting the order of the configuration or the existence of lower numbered identifiers.

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

The arrangement of the waveguides on the substrate in each of the isolator embodiments described above is merely exemplary. For example, in each of the above-described embodiments, the orientation of the waveguides in the portions where the plurality of waveguides are adjacent to form parallel waveguides is parallel to the longitudinal side of the substrate. However, the shape of the substrate and the orientation and arrangement of the waveguides are not limited to this, and can be set arbitrarily within the range in which the effects of the present disclosure can be obtained. For example, a plurality of waveguides can be arranged not only in the direction along the substrate surface, but also in the direction perpendicular to the substrate surface.

In the above embodiments, an isolator and an optical transmitter for propagating TE mode electromagnetic waves have been described. However, the present disclosure can also be applied to isolators and optical transmitters that propagate TM mode electromagnetic waves. When the present disclosure is applied to an isolator and an optical transmitter that propagate TM mode electromagnetic waves, 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 embodiments. For example, when the present invention is applied to a TM mode isolator and an optical transmitter, the non-reciprocal member is arranged adjacent to the direction perpendicular to the substrate surface of the second waveguide (corresponding to the z-axis direction in the above embodiment). can be Also, 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 in the above embodiment).

Reference Signs List 10, 11, 70, 80 optical transmitter 20 first waveguide 21 core 201 first end 202 second end 211 first port 212 second port 30 second waveguide 31 core 32 non-reciprocal member 30a coupling portion 30b, 30c non-coupling portion 301 first end 302 second end 50 substrate 60 clad 61 first clad 62 second clad 71 light source 72 DAC (digital-to-analog converter)
73 driver 74 optical modulator 75 isolator 81 signal generator 82 driver (driving unit)
83 light source 84 isolator

Claims (11)

over a substrate having a substrate surface, comprising a first waveguide and a second waveguide positioned at least partially side-by-side along the substrate surface;
the first waveguide has a first end and a second end, and has a port for inputting and outputting an electromagnetic wave at each of the first end and the second end;
the second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends;
When the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz)
and the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θ _F [degrees/μm], and the insertion loss allowed for the isolator is set to a predetermined value x [dB],
θ _F /α × x < 45 [degrees]
An isolator characterized by using the non-reciprocal material. 2. The isolator as recited in claim 1, wherein said first waveguide and said second waveguide comprise silicon. 3. The isolator according to claim 1, wherein a polarization direction of an electromagnetic wave input to said first end is parallel to said substrate. 4. The isolator according to any one of claims 1 to 3, wherein electromagnetic waves in said first waveguide are propagated in a single mode. The coupling coefficient between the first waveguide and the second waveguide when the electromagnetic wave input from the second end propagates toward the first end is 5. The isolator according to any one of claims 1 to 4, wherein the coupling coefficient is greater than the coupling coefficient between the first waveguide and the second waveguide when propagating towards the second end. 6. The isolator according to claim 1, wherein both ends of said second waveguide are bent in a direction away from said first waveguide. a light source;
an optical modulator that modulates light emitted from the light source based on a signal to be transmitted;
an isolator arranged downstream of the light source,
The isolator is
over a substrate having a substrate surface, comprising a first waveguide and a second waveguide positioned at least partially side-by-side along the substrate surface;
The first waveguide has a first port for inputting light from the light source at a first end, and a second port for outputting light from the light source at a second end different from the first end. have
the second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends;
When the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz)
and the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θ _F [degrees/μm], and the insertion loss allowed for the isolator is set to a predetermined value x [dB],
θ _F /α × x < 45 [degrees]
An optical transmitter using said non-reciprocal material. a light source;
a driving unit that drives the light source based on a signal to be transmitted;
an isolator arranged on the light emission side of the light source,
The isolator is
over a substrate having a substrate surface, comprising a first waveguide and a second waveguide positioned at least partially side-by-side along the substrate surface;
The first waveguide has a first port for inputting light from the light source at a first end, and a second port for outputting light from the light source at a second end different from the first end. have
the second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends;
When the transmittance of the nonreciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the nonreciprocal material is T=exp(−αz)
and the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θ _F [degrees/μm], and the insertion loss allowed for the isolator is set to a predetermined value x [dB],
θ _F /α × x < 45 [degrees]
An optical transmitter using said non-reciprocal material. 1. A method of manufacturing an isolator comprising providing on a substrate having a substrate surface a first waveguide and a second waveguide positioned at least partially side by side along the substrate surface, the method comprising:
the first waveguide has a first end and a second end, and has a port for inputting and outputting an electromagnetic wave at each of the first end and the second end;
the second waveguide has nonreciprocity by adjoining a nonreciprocity material exhibiting nonreciprocity in at least part of the direction in which the second waveguide extends;
When the transmittance of the non-reciprocal material is T and the thickness is z [μm], the absorption coefficient α [dB/μm] of the non-reciprocal material is
T = exp(-αz)
where the Faraday rotation angle per thickness of the non-reciprocal material is expressed as θF [degrees/μm], and the insertion loss allowed for the isolator is x [dB],
θ _F. /α × x < 45 [degrees]
A method of manufacturing an isolator, characterized by using the non-reciprocal material. A method of manufacturing an optical transmitter comprising providing a light source, an optical modulator that modulates light emitted from the light source based on a signal to be transmitted, and an isolator arranged downstream of the light source. There is
10. A method of manufacturing an optical communication device, wherein the isolator is manufactured by the method of manufacturing an isolator according to claim 9. A method for manufacturing an optical transmitter, comprising providing a light source, a driving unit for driving the light source based on a signal to be transmitted, and an isolator arranged on the light emitting side of the light source,
10. A method of manufacturing an optical communication device, wherein the isolator is manufactured by the method of manufacturing an isolator according to claim 9.

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