Isolator, light source apparatus, optical transceiver, optical switch, optical amplifier, and data center

An isolator includes a first waveguide with a linear shape and a second waveguide with an annular shape on a substrate including a substrate surface, the first waveguide being positioned along the substrate surface. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Japanese Patent Application No. 2018-222905 filed on Nov. 28, 2018, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an isolator, a light source apparatus, an optical transceiver, an optical switch, an optical amplifier, and a data center.

BACKGROUND

In a known configuration, an isolator whose transmittance differs depending on the propagation direction of electromagnetic waves includes a non-reciprocal phase device. For example, see patent literature (PTL) 1.

CITATION LIST

Patent Literature

SUMMARY

An isolator according to an embodiment of the present disclosure includes a first waveguide with a linear shape positioned on a substrate surface of a substrate, and a second waveguide with an annular shape positioned adjacent the first waveguide. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends.

An isolator according to an embodiment of the present disclosure includes a first waveguide and a second waveguide. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The second waveguide is positioned along the first waveguide. The second waveguide has an annular shape. The first waveguide and the second waveguide are coupled so that, at any operating frequency, a coupling coefficient when electromagnetic waves inputted from the second end propagate towards the first end is greater than a coupling coefficient when electromagnetic waves inputted from the first end propagate towards the second end.

A light source apparatus according to an embodiment of the present disclosure includes an isolator and a light source. The isolator includes a first waveguide with a linear shape and a second waveguide with an annular shape on a substrate including a substrate surface, the first waveguide being positioned along the substrate surface. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends. The light source optically connects to the port.

A light source apparatus according to an embodiment of the present disclosure includes an isolator and a light source. The isolator includes a first waveguide and a second waveguide. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The second waveguide is positioned along the first waveguide. The second waveguide has an annular shape. The first waveguide and the second waveguide are coupled so that at any operating frequency, a coupling coefficient when electromagnetic waves inputted from the second end propagate towards the first end is greater than a coupling coefficient when electromagnetic waves inputted from the first end propagate towards the second end. The light source optically connects to the port.

An optical transceiver according to an embodiment of the present disclosure includes an isolator and a light source. The isolator includes a first waveguide with a linear shape and a second waveguide with an annular shape on a substrate including a substrate surface, the first waveguide being positioned along the substrate surface. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends. The light source optically connects to the port.

An optical transceiver according to an embodiment of the present disclosure includes an isolator and a light source. The isolator includes a first waveguide and a second waveguide. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The second waveguide is positioned along the first waveguide. The second waveguide has an annular shape. The first waveguide and the second waveguide are coupled so that at any operating frequency, a coupling coefficient when electromagnetic waves inputted from the second end propagate towards the first end is greater than a coupling coefficient when electromagnetic waves inputted from the first end propagate towards the second end. The light source optically connects to the port.

An optical switch according to an embodiment of the present disclosure includes an isolator. The isolator includes a first waveguide with a linear shape and a second waveguide with an annular shape on a substrate including a substrate surface, the first waveguide being positioned along the substrate surface. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends.

An optical switch according to an embodiment of the present disclosure includes an isolator. The isolator includes a first waveguide and a second waveguide. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The second waveguide is positioned along the first waveguide. The second waveguide has an annular shape. The first waveguide and the second waveguide are coupled so that at any operating frequency, a coupling coefficient when electromagnetic waves inputted from the second end propagate towards the first end is greater than a coupling coefficient when electromagnetic waves inputted from the first end propagate towards the second end.

An optical amplifier according to an embodiment of the present disclosure includes an isolator. The isolator includes a first waveguide with a linear shape and a second waveguide with an annular shape on a substrate including a substrate surface, the first waveguide being positioned along the substrate surface. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends.

A data center according to an embodiment of the present disclosure communicates via a device that includes an isolator. The isolator includes a first waveguide with a linear shape and a second waveguide with an annular shape on a substrate including a substrate surface, the first waveguide being positioned along the substrate surface. The first waveguide and the second waveguide each include a core and a cladding. The first waveguide includes a first end, a second end, and a port at each of the first end and the second end for input and output of electromagnetic waves. The core of the second waveguide includes a non-reciprocal member in at least a portion of a cross-section intersecting a direction in which the second waveguide extends.

DETAILED DESCRIPTION

In an isolator that includes a non-reciprocal phase device, the isolator overall becomes large, as does insertion loss, when non-reciprocity is small in a waveguide that includes a non-reciprocal member. Demand exists for providing sufficient non-reciprocity to a waveguide that includes a non-reciprocal member to reduce insertion loss while achieving a smaller size.

Various embodiments are described below in detail with reference to the drawings.

First Embodiment

As illustrated inFIG.1, an isolator10according to the first embodiment includes, on a substrate50having a substrate surface50a, a linear first waveguide20positioned along the substrate surface50aand an annular second waveguide30.

The substrate50may be configured to include a conductor, such as a metal; a semiconductor, such as silicon; glass; resin; or the like. In the first embodiment, two directions that are parallel to the substrate surface50aand perpendicular to each other are designated the x-axis direction and the y-axis direction, and the direction perpendicular to the substrate surface50ais designated the z-axis direction.

On the substrate50that includes the substrate surface50a, the first waveguide20and the second waveguide30are positioned along the substrate surface50aso as to overlap each other as viewed from the substrate surface50a. The first waveguide20and the second waveguide30are positioned so as not to intersect as viewed from the substrate surface50a. When the first waveguide20extends in the x-axis direction, for example, non-intersection as viewed from the substrate surface50arefers to the second waveguide30also extending in the x-axis direction. Here, stating that the second waveguide30extends in the x-axis direction refers to the annular second waveguide30overall extending in the x-axis direction and includes the case of a portion of the annular second waveguide30extending in the y-axis direction, for example.

One of the first waveguide20and the second waveguide30is in contact with the substrate surface50a. In a configuration with the first waveguide20in contact with the substrate surface50a, the second waveguide30is positioned above the first waveguide20, i.e. on the positive side in the z-axis direction. In a configuration with the second waveguide30in contact with the substrate surface50a, the first waveguide20is positioned above the second waveguide30, i.e. on the positive side in the z-axis direction. A configuration in which the first waveguide20is in contact with the substrate surface50ais described below.

The first waveguide20extends in the x-axis direction. The first waveguide20has a first end201on the negative side in the x-axis direction and a second end202on the positive side in the x-axis direction. The first waveguide20has a first port211and a second port212, for input and output of electromagnetic waves, respectively on the first end201and the second end202. The electromagnetic waves that are inputted to the first waveguide20from the first port211proceed along the x-axis towards the second port212. The electromagnetic waves that are inputted to the first waveguide20from the second port212proceed along the x-axis towards the first port211. The first port211and the second port212may be formed as the end faces of a core21or may be configured as a coupler that connects to an external apparatus and is capable of propagating electromagnetic waves.

The second waveguide30extends in the x-axis direction. The second waveguide30is annular and is optically connected without interruption. The annular shape is not particularly restricted as long as the shape is optically connected without interruption. By virtue of the second waveguide30having an annular shape, the non-reciprocity of a non-reciprocal waveguide in the second waveguide30can be increased. The annular shape of the second waveguide30can also achieve desired characteristics even when the second waveguide30extending in the x-axis direction is short. The insertion loss can thereby be reduced while achieving a smaller size of the isolator10.

The second waveguide30has an end301on the negative side in the x-axis direction and an end302on the positive side in the x-axis direction. At least a portion of the second waveguide30is coupled to the first waveguide20. An example of one second waveguide30is illustrated inFIG.1, but a plurality of second waveguides30may be provided in the x-axis direction.

The first waveguide20and the second waveguide30are positioned along each other at least at a portion thereof in the direction of extension. The first waveguide20and the second waveguide30are positioned in parallel to each other at least at a portion thereof in the direction of extension. Two waveguides positioned along each other are also referred to as a parallel waveguide. In a parallel waveguide, electromagnetic waves are inputted to one waveguide can transfer to the other waveguide while propagating. In other words, electromagnetic waves propagating in the first waveguide20can transfer to the second waveguide30. Electromagnetic waves propagating in the second waveguide30can transfer to the first waveguide20.

In a parallel waveguide, the parameter representing the proportion of electromagnetic waves that transfer from one waveguide to the other waveguide is also referred to as the coupling coefficient. When no electromagnetic waves whatsoever transfer from one waveguide to the other waveguide, the coupling coefficient is 0. When all of the electromagnetic waves transfer from one waveguide to the other waveguide, the coupling coefficient is 1. The coupling coefficient can be a value from 0 to 1. The coupling coefficient can be determined based on the shape of each waveguide, the inter-waveguide distance, the length that the waveguides lie along each other, or the like. For example, the coupling coefficient can increase as the shapes of the waveguides are more similar. As for the inter-waveguide distance, the distance between the first waveguide20and the second waveguide30, for example, may be the distance between the core21and a core31. The coupling coefficient can change in accordance with the distance over which the electromagnetic waves propagate within the waveguides. In other words, in a parallel waveguide, the coupling coefficient can differ in accordance with the position along the direction in which the waveguides extend. The local maximum of the coupling coefficient can be determined based on the shape of each waveguide, the inter-waveguide distance, or the like. The local maximum 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 lie along each other is 0. The length from the starting point to the position where the coupling coefficient reaches a local maximum is also referred to as the coupling length. When the length over which the waveguides lie along each other is equivalent to the coupling length, the coupling coefficient at the end point of the section where the waveguides lie along each other can be a local maximum. The coupling length can be determined based on the shape of each waveguide, the inter-waveguide distance, or the like.

In the second waveguide30, the electromagnetic waves that transferred from the first waveguide20also propagate in the second waveguide30in the same direction as in the first waveguide20. The electromagnetic waves propagate around the annular shape in the second waveguide30and are absorbed due to the loss of the non-reciprocal waveguide.

As illustrated inFIG.2, the first waveguide20includes the core21, a cladding22, and a cladding23. The core21and the claddings22,23extend in the x-axis direction. The cladding22is positioned on the negative side in the z-axis direction, and the cladding23is positioned on the positive side in z-axis direction, with respect to the core21. The cladding22is positioned on the side of the substrate50as seen from the core21. The cladding23is positioned on the opposite side from the substrate50as seen from the core21. The cladding23is thus positioned on the side of the second waveguide30as seen from the core21. The cladding22, the core21, and the cladding23are layered in this order as seen from the substrate50. The cladding22and the cladding23are positioned to sandwich the core21along the direction in which the first waveguide20and a portion of the second waveguide30overlap. In other words, the cladding22and the cladding23are positioned on either side of the core21along the direction in which the first waveguide20and a portion of the second waveguide30overlap. The core21may include a first surface21apositioned on the side of the substrate50and a second surface21bpositioned on the opposite side from the first surface21a. The cladding22may be positioned in contact with the first surface21a, and the cladding23may be positioned in contact with the second surface21b.

As illustrated inFIG.2, the second waveguide30includes the core31, a non-reciprocal member32, a cladding33, and a cladding34. The core31has an annular shape and extends in the x-axis direction overall. The non-reciprocal member32extends in the x-axis direction. The claddings33,34are annular claddings extending in the x-axis direction overall. The non-reciprocal member32may be positioned on the positive side in the z-axis direction with respect to the core31. The non-reciprocal member32may be positioned on the negative side in the z-axis direction with respect to the core31. The non-reciprocal member32may be positioned side by side with the core31on the negative side or the positive side in the y-axis direction.

As illustrated inFIG.2, the shapes of the core31and the non-reciprocal member32as seen in a cross-section intersecting the x-axis are configured not to have point symmetry. The shapes of the core31and the non-reciprocal member32are further configured not to have line symmetry. The core31and the non-reciprocal member32are together referred to as an asymmetric core. The asymmetric core is configured to include the core31and the non-reciprocal member32. The asymmetric core may have the non-reciprocal member32in at least a portion of a cross-section thereof intersecting the x-axis. The core31may be configured to include at least one type of dielectric. The non-reciprocal member32may be in contact with a surface of the at least one type of dielectric on the side of the substrate50or on the opposite side.

A degree of symmetry can be used as an index representing whether a cross-section of the asymmetric core is close to point symmetry. The degree of symmetry may be represented by the proportion that matches between the cross-sectional shape of the asymmetric core and a cross-sectional shape obtained by rotating the cross-sectional shape of the asymmetric core 180 degrees about a predetermined point. A cross-sectional shape with a high degree of symmetry is close to point symmetry. The asymmetric core may be configured so that the degree of symmetry of a cross-sectional shape thereof is low.

In a cross-section of the asymmetric core, the area of the core31may be configured to be greater than the area of the non-reciprocal member32. This configuration enables the majority of the electromagnetic waves to propagate inside the core31. Consequently, the loss of electromagnetic waves in the second waveguide30can be reduced.

A cross-section of the asymmetrical core may be configured so that the core31is positioned in a portion where the intensity of electromagnetic waves propagating through the asymmetric core is the greatest. This configuration enables a high-intensity portion of the electromagnetic waves to propagate inside the core31. Consequently, the loss of electromagnetic waves in the second waveguide30can be reduced.

The cladding33is positioned on the negative side in the z-axis direction, and the cladding34is positioned on the positive side in z-axis direction, with respect to the asymmetric core. The cladding33is positioned on the side of the substrate50as seen from the asymmetric core. The cladding34is positioned on the opposite side from the substrate50as seen from the asymmetric core. The cladding34is thus positioned on the side of the second waveguide30as seen from the asymmetric core. The cladding33, the asymmetric core, and the cladding34are layered in this order as seen from the substrate50. The cladding33and the cladding34are positioned to sandwich the asymmetric core. In other words, the cladding33and the cladding34are positioned on either side of the asymmetric core along the direction in which the first waveguide20and the second waveguide30overlap. The core31may include a first surface31apositioned on the side of the substrate50and a second surface31bpositioned on the opposite side from the first surface31a. The cladding33may be positioned in contact with the first surface31a, and the cladding34may be positioned in contact with the second surface31b.

The core21and the core31, along with the cladding22, the cladding23, the cladding33, and the cladding34may be configured to include a dielectric. The core21and the core31are also referred to as dielectric lines. The relative permittivity of the core21may be higher than the relative permittivity of the cladding22and the cladding23. The relative permittivity of the core31may be higher than the relative permittivity of the cladding33and the cladding34. The cladding23and the cladding33may be configured from the same dielectric material. The cladding23and the cladding33may be configured integrally. An integrated configuration of the cladding23and the cladding33facilitates formation of the isolator10. The relative permittivity of the core21and the core31, and of the cladding22, the cladding23, the cladding33, and the cladding34, may be higher than the relative permittivity of air. Leakage of electromagnetic waves from the first waveguide20and the second waveguide30can be suppressed by the relative permittivity of the core21and the core31, and of the cladding22, the cladding23, the cladding33, and the cladding34, being higher than the relative permittivity of air. Consequently, the loss due to irradiation of electromagnetic waves from the isolator10to the outside can be reduced.

The core21and the core31may, for example, be formed from silicon (Si). The cladding22, the cladding23, the cladding33, and the cladding34may, for example, be formed from quartz glass (SiO2). The relative permittivity of silicon is approximately 12, and the relative permittivity of quartz glass is approximately 2. Silicon can propagate electromagnetic waves that have a near infrared wavelength of approximately 1.2 μm to approximately 6 μm with low loss. When formed from silicon, the core21and the core31can propagate electromagnetic waves that have a wavelength in the 1.3 μm band or the 1.55 μm band used in optical communication with low loss.

The first waveguide20configured to include the core21, the cladding22, and the cladding23and the second waveguide30configured to include the asymmetric core, the cladding33, and the cladding34may satisfy waveguide conditions in a single mode. When the first waveguide20and the second waveguide30satisfy waveguide conditions in the single mode, the waveform of the signal propagating through the first waveguide20and the second waveguide30is less likely to collapse. The isolator10that combines the first waveguide20and the second waveguide30that satisfy waveguide conditions in the single mode can be suitable for optical communication.

The relative permittivity of the core21or the core31may be distributed uniformly 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 core21may be distributed so as to be highest in the central portion in the z-axis direction and to decrease towards the cladding22and the cladding23. In this case, the core21can propagate electromagnetic waves by the same principle as a graded-index optical fiber.

The electromagnetic waves that are inputted to the core21from the first end201of the first waveguide20via the first port211propagate towards the second end202in the core21of the first waveguide20that extends along the x-axis. The direction from the first end201towards the second end202is also referred to as the first direction.

The electromagnetic waves propagating in the core21propagate in the core31of the second waveguide30in a ratio corresponding to the coupling coefficient based on the distance of propagation in the first direction within the core21. The coupling coefficient when the electromagnetic waves propagate in the first direction within the core21is also referred to as the first coupling coefficient.

The electromagnetic waves that are inputted to the core21from the second end202of the first waveguide20via the second port212propagate towards the first end201in the core21of the first waveguide20that extends along the x-axis. The direction from the second end202towards the first end201is also referred to as the second direction.

The electromagnetic waves propagating in the core21propagate in the core31of the second waveguide30in a ratio corresponding to the coupling coefficient based on the distance of propagation in the second direction within the core21. The coupling coefficient when the electromagnetic waves propagate in the second direction within the core21is also referred to as the second coupling coefficient.

The asymmetric core of the second waveguide30may have different propagation characteristics when electromagnetic waves propagate in the first direction and when electromagnetic waves propagate in the second direction. When the propagation characteristics of the asymmetric core differ based on the propagation direction of electromagnetic waves, the first coupling coefficient and the second coupling coefficient can differ from each other. In other words, the non-reciprocal member32can cause the first coupling coefficient and the second coupling coefficient to differ from each other.

The non-reciprocal member32may be formed from a material that has different propagation characteristics when electromagnetic waves propagate in the first direction and when electromagnetic waves propagate in the second direction. A material that has different propagation characteristics depending on the propagation direction of electromagnetic waves is also referred to as a non-reciprocal material. The non-reciprocal member32may, for example, be configured to include a magnetic material such as magnetic garnet, ferrite, iron, or cobalt. The relative permittivity of the non-reciprocal member32can be expressed by a tensor as in Expression (1).

ɛr=[ɛ11ɛ12ɛ13ɛ21ɛ22ɛ23ɛ31ɛ32ɛ33](1)
Each element of the tensor may be represented by a complex number. The numbers used as the subscript of each element may correspond to the x-axis, y-axis, and z-axis. A tensor that has complex numbers as elements and represents relative permittivity is also referred to as a complex relative permittivity tensor.

The non-reciprocal member32may include a non-reciprocal material at a predetermined concentration. The predetermined concentration may change within the cross-section intersecting the x-axis. The predetermined concentration may change in at least a portion viewed in the polarization direction of electromagnetic waves inputted to the isolator10.

As illustrated inFIG.3, the non-reciprocal member32may be positioned so as to be inserted between the cladding33and the cladding34in a cross-section intersecting the x-axis. In the annular second waveguide30, the dimension of the cladding33and the cladding34in the y-axis direction is represented as A. In the annular second waveguide30, the dimension of the core31and the non-reciprocal member32in the z-axis direction is represented as B. In the annular second waveguide30, the dimension of the non-reciprocal member32in the y-axis direction is represented as C. For example, the non-reciprocal member32is positioned side by side with the core31along the y-axis, towards the positive side in the y-axis direction. The second waveguide30extends in the x-axis direction so that the dimension thereof in the x-axis direction becomes a predetermined length. The non-reciprocal member32has a relative permittivity expressed by a complex relative permittivity tensor.

In the second waveguide30, the difference between the phase of electromagnetic waves propagating through the second waveguide30in the first direction and the phase of electromagnetic waves propagating through the second waveguide30in the second direction is calculated by simulation. The difference between the phase of electromagnetic waves propagating in the first direction and the phase of electromagnetic waves propagating in the second direction is also referred to as the phase difference.

As illustrated inFIG.4, the phase difference can change in accordance with the value of C/A. The value of C/A represents the proportion of the asymmetric core, viewed in the z-axis direction, that is occupied by the non-reciprocal member32. When the value of C/A is near 0.5, the phase difference increases. The phase difference is adjusted by changing the value of C/A. When the phase difference is large, the non-reciprocity of the attenuation amount of electromagnetic waves increases. In other words, as the phase difference increases, the difference between the attenuation amount when electromagnetic waves propagate in the first direction and the attenuation amount when electromagnetic waves propagate in the second direction increases. The second waveguide30is configured by adjustment of the value of C/A to have a property such that the attenuation amount of electromagnetic waves differs in accordance with the propagation direction of electromagnetic waves. The property such that the attenuation amount of electromagnetic waves differs in accordance with the propagation direction of electromagnetic waves is also referred to as non-reciprocity. When the value of C/A is near 0.5, the degree of symmetry of the asymmetrical core is lower. In other words, the non-reciprocity of the second waveguide30is increased by the degree of symmetry of the asymmetrical core being decreased.

As illustrated inFIG.5, the non-reciprocal member32may be positioned so as to be inserted between the core31and the cladding34in a cross-section intersecting the x-axis. In the annular second waveguide30, the dimension of the core31in the y-axis direction is represented as A. In the annular second waveguide30, the dimensions of the non-reciprocal member32in the y-axis direction and the z-axis direction are represented as C and D. For example, the non-reciprocal member32is positioned farther on the positive side in the z-axis direction than the core31, towards the positive side in the y-axis direction within the second waveguide30. The second waveguide30extends in the x-axis direction so that the dimension thereof in the x-axis direction becomes a predetermined length. The non-reciprocal member32has a relative permittivity expressed by a complex relative permittivity tensor. In the second waveguide30, the difference between the phase of electromagnetic waves propagating through the second waveguide30in the first direction and the phase of electromagnetic waves propagating in the second direction is calculated by simulation.

As illustrated inFIG.6, the phase difference can change in accordance with the value of C/A. The relationship between the phase difference and the value of C/A can change in accordance with the value of D/B. When the value of C/A is near 0.5, the phase difference increases. The phase difference is adjusted by changing the value of C/A. When the value of D/B is large, the phase difference increases. The phase difference is adjusted by changing the value of D/B. The second waveguide30is configured by adjustment of the value of C/A and the value of D/B to have non-reciprocity. When the value of D/B increases within a predetermined range, the degree of symmetry of the asymmetrical core is lower. In other words, the non-reciprocity of the second waveguide30is increased by the degree of symmetry of the asymmetrical core being decreased.

As illustrated inFIG.7, the non-reciprocal member32may be positioned so as to be inserted between the core31and the cladding34in a cross-section intersecting the x-axis. In this case, the asymmetrical core does not have point symmetry. Accordingly, the second waveguide30has non-reciprocity.

When one waveguide in the parallel waveguide has non-reciprocity, the local maximum of the coupling coefficient when electromagnetic waves propagate in the first direction can differ from the local maximum of the coupling coefficient when electromagnetic waves propagate in the second direction. For example, as illustrated inFIG.8, a configuration can be adopted so that the local maximum of the coupling coefficient for the first waveguide20and the second waveguide30when electromagnetic waves propagate in the first direction is a value near 0. For example, as illustrated inFIG.9, a configuration can be adopted so that the local maximum of the coupling coefficient for the first waveguide20and the second waveguide30when electromagnetic waves propagate in the second direction is a value near 1. By the local maximum of the coupling coefficient differing for each propagation direction of electromagnetic waves, the transmittance of electromagnetic waves can differ for each propagation direction of electromagnetic waves. InFIGS.8and9, the horizontal axis represents the travel distance of electromagnetic waves in the parallel waveguide, and the vertical axis represents the coupling coefficient.

When the second waveguide30has non-reciprocity, the coupling coefficient for the first waveguide20and the second waveguide30differs in accordance with the propagation direction of electromagnetic waves. In other words, when the second waveguide30has non-reciprocity, the first coupling coefficient of the isolator10differs from the second coupling coefficient. The second coupling coefficient is made larger than the first coupling coefficient by adjustment of the degree of non-reciprocity of the second waveguide30.

When electromagnetic waves are inputted to the first waveguide20from the first port211and propagate in the first direction, at least a portion of the electromagnetic waves that transferred to the second waveguide30, from among the inputted electromagnetic waves, reach the end302. Among the electromagnetic waves inputted to the first waveguide20, the proportion of electromagnetic waves that transfers to the second waveguide30and reaches the end302increases when the first coupling coefficient is large. In this case, among the electromagnetic waves inputted to the first waveguide20, the proportion of electromagnetic waves that are outputted from the second port212decreases. In other words, the ratio of the intensity of electromagnetic waves outputted from the second port212to the intensity of the electromagnetic waves inputted to the first port211decreases. The ratio of the intensity of electromagnetic waves outputted from the second port212to the intensity of the electromagnetic waves inputted to the first port211is also referred to as the transmittance of the isolator10with respect to electromagnetic waves propagating in the first direction. When the first coupling coefficient is large, the transmittance with respect to electromagnetic waves propagating in the first direction decreases. On the other hand, when the first coupling coefficient is small, the proportion of electromagnetic waves that transfer to the second waveguide30decreases. The transmittance with respect to electromagnetic waves propagating in the first direction therefore increases.

The electromagnetic waves that are inputted to the first waveguide20from the second port212and propagate in the second direction experience the same effect as the effect of the isolator10on electromagnetic waves propagating in the first direction. Because of this effect, a portion of the electromagnetic waves propagating in the second direction reaches the end301of the second waveguide30. When the second coupling coefficient is large, the transmittance with respect to electromagnetic waves propagating in the second direction decreases. When the second coupling coefficient is small, the transmittance with respect to electromagnetic waves propagating in the second direction increases.

When the first coupling coefficient and the second coupling coefficient differ, the transmittance with respect to electromagnetic waves propagating in the first direction and the transmittance with respect to electromagnetic waves propagating in the second direction can differ. In other words, by the first coupling coefficient and the second coupling coefficient being different, the isolator10can function to facilitate propagation of electromagnetic waves in one direction and impede propagation of electromagnetic waves in the opposite direction. When the second coupling coefficient is larger than the first coupling coefficient, the isolator10can function to facilitate propagation of electromagnetic waves in the first direction and impede propagation of electromagnetic waves in the second direction. When the first coupling coefficient is substantially zero and the second coupling coefficient is substantially one, the difference between the transmittance with respect to electromagnetic waves propagating in the first direction and the transmittance with respect to electromagnetic waves propagating in the second direction can be increased. The functionality of the isolator10can therefore improve.

When one waveguide in the parallel waveguide has non-reciprocity, the coupling length of the parallel waveguide with respect to electromagnetic waves propagating in the first direction and the coupling length of the parallel waveguide with respect to electromagnetic waves propagating in the second direction can differ. For example, as illustrated inFIG.8, the coupling length with respect to electromagnetic waves propagating in the first direction in the isolator10can be represented as L1. For example, as illustrated inFIG.9, the coupling length with respect to electromagnetic waves propagating in the second direction in the isolator10can be represented as L2. The isolator10may be configured so that L1and L2differ.

When the length over which the two waveguides lie along each other in a parallel waveguide is equivalent to the coupling length, the coupling coefficient can attain a local maximum. For example, when the length over which two waveguides lie along each other is L1in a parallel waveguide that has the relationship illustrated in the graph ofFIG.8, the coupling coefficient can attain a local maximum. When the length over which the two waveguides lie along each other is equivalent to two times the coupling length, the coupling coefficient can attain a local minimum. For example, when the length over which two waveguides lie along each other is 2L1in a parallel waveguide that has the relationship illustrated inFIG.8, the coupling coefficient can attain a local minimum. The relationship illustrated in the graph inFIG.8can also repeat in an area where the travel distance of the electromagnetic waves has increased. In other words, the coupling coefficient can attain a local maximum when the length over which two waveguides lie along each other is an odd multiple of L1. The coupling coefficient can attain a local minimum when the length over which two waveguides lie along each other is an even multiple of L1. In a parallel waveguide that has the relationship illustrated inFIG.9as well, the coupling coefficient can attain a local maximum and the local minimum respectively when the length over which two waveguides lie along each other is an odd multiple of L2and an even multiple of L2. L1and L2are the shortest possible coupling lengths in the parallel waveguide and are also referred to as the unit coupling length. In other words, the coupling length may be an odd multiple of the unit coupling length.

The first coupling coefficient and the second coupling coefficient can be adjusted by adjustment of the length over which the first waveguide20and the second waveguide30lie along each other. The length over which the first waveguide20and the second waveguide30lie along each other may be substantially the same as an odd multiple of the unit coupling length with respect to electromagnetic waves propagating in the second direction. This configuration can increase the second coupling coefficient. The length over which the first waveguide20and the second waveguide30lie along each other may be substantially the same as an even multiple of the unit coupling length with respect to electromagnetic waves propagating in the first direction. This configuration can decrease the first coupling coefficient. The second coupling coefficient may in this way be made larger than the first coupling coefficient.

Among the electromagnetic waves that are inputted to one port of the isolator10, the amount of electromagnetic waves that are not outputted from the other port is also referred to as the attenuation amount. When the attenuation amount of electromagnetic waves is large, the transmittance of the electromagnetic waves is low. The attenuation amount of electromagnetic waves that travel in the first direction and in the second direction in the isolator10can be calculated by a simulation using the finite element method or the like.

As illustrated inFIG.10, the relationship between the attenuation amount of the electromagnetic waves propagating in the second direction and the frequency of the electromagnetic waves is represented in the graph by the solid curve labeled S12, and the relationship between the attenuation amount of the electromagnetic waves propagating in the first direction and the frequency of the electromagnetic waves is represented in the graph by the dashed curve labeled S21. The horizontal axis of the graph represents the frequency of electromagnetic waves propagating through the first waveguide20, and the vertical axis represents the attenuation amount of the electromagnetic waves. The attenuation amount of the electromagnetic waves is represented in units of decibels (dB). A curve located higher along the vertical axis represents that the attenuation amount of electromagnetic waves is small. A curve located lower along the vertical axis represents that the attenuation amount of electromagnetic waves is large.

As illustrated inFIG.10, the attenuation amount, represented by S12, of electromagnetic waves propagating in the second direction can become greater than the attenuation amount, represented by S21, of electromagnetic waves propagating in the first direction in a predetermined frequency band represented by fb1. In this case, the isolator10can function to facilitate propagation of electromagnetic waves in a predetermined frequency band from the first port211to the second port212while impeding propagation of the electromagnetic waves from the second port212to the first port211. The predetermined frequency band in which the isolator10can function to vary the attenuation amount for each propagation direction of electromagnetic waves is also referred to as the operating frequency of the isolator10. The operating frequency of the isolator10can be freely determined based on the configuration of the isolator10. In other words, the second coupling coefficient can be larger than the first coupling coefficient at any operating frequency.

The isolator10has the function of making the transmittance with respect to electromagnetic waves propagating in the first direction and the transmittance with respect to electromagnetic waves propagating in the second direction differ. This function can also be implemented by an isolator90according to a comparative example illustrated inFIG.11.

The isolator90includes an input end91, a branch coupler92, a reciprocal phase shifter93, a non-reciprocal phase shifter94, a branch coupler95, and an output end96. Electromagnetic waves inputted from the input end91branch at the branch coupler92and propagate to the reciprocal phase shifter93and the non-reciprocal phase shifter94. The electromagnetic waves undergo a phase shift in the reciprocal phase shifter93and the non-reciprocal phase shifter94, are coupled at the branch coupler95, and propagate to the output end96. The reciprocal phase shifter93and the non-reciprocal phase shifter94can be configured so that the electromagnetic waves inputted from the input end91are outputted from the output end96. On the other hand, electromagnetic waves inputted from the output end96branch at the branch coupler95and propagate to the reciprocal phase shifter93and the non-reciprocal phase shifter94. The electromagnetic waves undergo a phase shift in the reciprocal phase shifter93and the non-reciprocal phase shifter94, are coupled at the branch coupler92, and propagate to the input end91. The reciprocal phase shifter93and the non-reciprocal phase shifter94can be configured so that the electromagnetic waves inputted from the output end96are not outputted from the input end91.

In the isolator90according to the comparative example, the loss of electromagnetic waves is relatively large in the non-reciprocal phase shifter94and the branch couplers92and95. On the other hand, electromagnetic waves propagate through the core31as a general rule in the isolator10according to the first embodiment. By virtue of the second waveguide30having an annular shape in the isolator10according to the first embodiment, the non-reciprocity of the non-reciprocal waveguide in the second waveguide30can be increased. The annular shape of the second waveguide30in the isolator10according to the first embodiment can also achieve desired characteristics even when the second waveguide30extending in the x-axis direction is short. Consequently, the loss of electromagnetic waves in the isolator10according to the first embodiment can be smaller than the loss of electromagnetic waves in the isolator90according to the comparative example. The isolator10according to the first embodiment can, in other words, reduce the loss of electromagnetic waves while being compact. The first waveguide20and the second waveguide30are also referred to respectively as a reciprocal line and a non-reciprocal line in the isolator10according to the first embodiment.

In the isolator90according to the comparative example, the non-reciprocal phase shifter94and the branch couplers92and95are mounted to be connected in series, which impedes a reduction in size. On the other hand, the first waveguide20and the second waveguide30overlap in the isolator10according to the first embodiment, making it easier to reduce the size of the isolator10on the substrate50. Consequently, the isolator10according to the first embodiment can be integrated and implemented on the substrate50. The isolator10according to the first embodiment can, in other words, function to facilitate transmission of electromagnetic waves in one direction and impede transmission in the opposite direction by virtue of having an integrated configuration.

As illustrated inFIG.12, the first waveguide20may include a matching adjustment circuit25. The matching adjustment circuit25can adjust the propagation characteristics for each frequency of electromagnetic waves propagating in the first waveguide20. The matching adjustment circuit25may, for example, be provided as a structure in which the core21has a plurality of holes24, as illustrated inFIG.13. The second waveguide30is indicated inFIG.13by a virtual dashed double-dotted line. The holes24may pass through the core21in the y-axis direction. The holes24may pass through from the first surface21ato the second surface21bof the core21. The holes24may pass through the claddings22,23in the y-axis direction. The holes24may be side by side in the x-axis direction. In other words, the holes24may be side by side in the direction in which the core21extends. The number of holes24is not limited to 9. The shape of the holes24as seen from the z-axis direction is not limited to a rectangle and may be any of various shapes, such as a circle or a polygon.

The holes24may be arranged periodically in the x-axis direction. When the core21includes holes24arranged periodically in the x-axis direction, the first waveguide20including the core21can form a Bragg diffraction grating. When electromagnetic waves are inputted to the first waveguide20from the first port211, the portion of the inputted electromagnetic waves that has a wavelength satisfying the Bragg reflection condition can be reflected and return to the first port211. On the other hand, the electromagnetic waves having other wavelengths can propagate towards the second port212. In other words, the first waveguide20that includes the holes24can function as a filter for electromagnetic waves having a predetermined wavelength.

When the first waveguide20includes the matching adjustment circuit25, the attenuation amount, represented by S21, of electromagnetic waves propagating in the first direction can become greater than the attenuation amount, represented by S12, of electromagnetic waves propagating in the second direction in a predetermined frequency band represented by fb2, as illustrated inFIG.14, for example. Here, fb2may be a different frequency band than the frequency band indicated by fb1inFIG.10. By adjustment of the configuration of the matching adjustment circuit25, fb2can be set to a higher frequency band or a lower frequency band than the frequency band indicated by fb1. A description of the matter common to both the graph inFIG.14and the graph inFIG.10is omitted.

The non-reciprocal member32may be configured to have non-reciprocity when a magnetic field in a predetermined direction is applied. The non-reciprocal member32may be configured to have non-reciprocity when a magnetic field having a component in the z-axis direction is applied. The predetermined direction is not limited to the z-axis direction and may be any of various directions. The predetermined direction may be determined based on the cross-sectional shape of the asymmetric core or the degree of symmetry. The non-reciprocal member32may be configured to have non-reciprocity of a different magnitude in accordance with a change in the intensity or orientation of the magnetic field. This configuration of the isolator10enables control of whether the non-reciprocal member32has non-reciprocity, or control of the degree of non-reciprocity of the non-reciprocal member32.

As illustrated inFIG.15, the isolator10may further include a magnetic field applicator80configured to apply a magnetic field. The magnetic field applicator80may be positioned on the positive side in the z-axis direction with respect to the second waveguide30. The magnetic field applicator80may be positioned on the substrate50side of the second waveguide30, with the first waveguide20therebetween. The magnetic field applicator80may be positioned in a different manner than in the example illustrated inFIG.15. The magnetic field applicator80may be a permanent magnet, such as a ferrite magnet or a neodymium magnet. The magnetic field applicator80may be an electromagnet.

The propagation mode of electromagnetic waves in a parallel waveguide can include an even mode and an odd mode. The even mode is a mode such that the electric field of propagating electromagnetic waves faces the same direction in the waveguides forming the parallel waveguide. The odd mode is a mode such that the electric field of propagating electromagnetic waves faces opposite directions in the waveguides forming the parallel waveguide. The electromagnetic waves can propagate in the parallel waveguide based on the effective refractive index of the parallel waveguide. The effective refractive index of the parallel waveguide can be determined based on the shape of each waveguide forming the parallel waveguide, the relative permittivity of the material forming the waveguides, the propagation mode of electromagnetic waves, or the like. The effective refractive index of the parallel waveguide when the electromagnetic waves propagate in the even mode is also referred to as the even mode refractive index. The effective refractive index of the parallel waveguide when the electromagnetic waves propagate in the odd mode is also referred to as the odd mode refractive index. The even mode refractive index is represented as neven, and the odd mode refractive index is represented as nodd. The coupling length in the parallel waveguide can be represented by Expression (2) below.

L=m⁢⁢λ02·neven-nodd(2)
(where L is the coupling length, m is an odd number, and λ0is the wavelength in a vacuum)

The isolator10can be used in combination with a configuration for inputting light. In this case, the isolator10is also referred to as an optical isolator. As illustrated inFIG.16, a light source apparatus100includes the isolator10, a light source110, a lens112, and a power supply114that supplies power to the light source110. The light source110may, for example, be a semiconductor laser such as a Laser Diode (LD) or a Vertical Cavity Surface Emitting LASER (VCSEL). The light source110may be formed on the substrate50.

The lens112collects the light outputted from the light source110on the first port211of the first waveguide20in the isolator10. The shape of the lens112is not particularly restricted. A small ball lens, a biconvex lens, a plano-convex lens, or the like can be adopted as the lens112. The lens112may be configured to include a material that is light transmitting with respect to the wavelength of propagating light.

The light source110can be considered optically connected to the first port211via the lens112. The positional relationships between the light source110, the lens112, and the first port211may be fixed to prevent misalignment. The light source110, the lens112, and the first port211may be integrated on the substrate50. The light source110may input linearly polarized light whose polarization direction is the y-axis direction to the first port211. The light source apparatus100need not include the lens112. When not including the lens112, the light source apparatus100may input light emitted from the light source110directly to the first port211.

The method for inputting light from the light source110to the first port211is not limited to a method for inputting light of the light source110directly or via the lens112. The light source110may be coupled to the first port211via an optical fiber. The method for inputting light that propagates through the optical fiber into the first port211may be any of various methods, such as a method for connecting free space via a lens or the like, a method for directly abutting the exit face of the optical fiber against the first port211, or a method for using a connection waveguide120(seeFIG.17).

By including the light source110and the isolator10, the light source apparatus100can output light, outputted from the light source110, through the isolator10in the first direction. On the other hand, the isolator10in the light source apparatus100can impede propagation of light returning in the second direction, impeding the return of light to the light source110side. Light can therefore be outputted efficiently.

In the light source apparatus100, the first waveguide20may be configured to be in contact with the substrate surface50a. In other words, the first waveguide20may be positioned on the side closer to the substrate surface50athan the second waveguide30. This configuration enables the light source110integrated on the substrate50to easily be connected optically to the first port211.

As illustrated inFIG.17, the connection waveguide120may include a core121, a cladding122, and a cladding123. The relative permittivity of the core121may be substantially equal to the relative permittivity of the core21of the first waveguide20. The core121may be formed from the same material as the core21. The relative permittivity of the cladding122and the cladding123may be set lower than the relative permittivity of the core121. The relative permittivity of the cladding122and the cladding123may be substantially equal to the relative permittivity of the cladding22and the cladding23of the first waveguide20. The cladding122and the cladding123may be formed from the same material as the cladding22and the cladding23. The end face of the core121on the side in the positive direction of the x-axis is in contact with the first port211positioned on the end face of the core21on the side in the negative direction of the x-axis. The thickness of the core121in the z-axis direction may be greater than the thickness of the core21of the first waveguide20in the z-axis direction. The thickness of the core121in the z-axis direction may be substantially equal to the thickness of the core21of the first waveguide20in the z-axis direction.

The light inputted to the side of the core121in the negative direction of the x-axis may be linearly polarized light whose polarization direction is the y-axis direction. In other words, the polarization direction of light inputted to the core121from the side in the negative direction of the x-axis may be parallel to the substrate surface50a. When the light source110integrated on the substrate50is a semiconductor laser, the polarization direction of light emitted by the semiconductor laser is parallel to the substrate surface50a. A semiconductor laser is easy to integrate on the substrate50. This configuration can therefore facilitate formation of the light source apparatus100.

At the connecting portion between the core121and the core21, the width of the core121in the y-axis direction may be substantially equal to the width of the core21in the v-axis direction. When the width of the core121and the core21in the y-axis direction changes discontinuously at the connecting portion between the core121and the core21, light whose polarization direction is the y-axis direction tends to be irradiated at the connecting portion. The loss due to irradiation can be reduced at the connecting portion between the core121the core21by the width of the core121and the core21in the y-axis direction being made substantially equal.

As illustrated inFIG.18, the core121of the connection waveguide120may be tapered to become thinner in the z-axis direction as it approaches the portion connected to the core21of the first waveguide20. When light whose polarization direction is the y-axis direction is inputted to the connection waveguide120, this configuration enables the inputted light to be matched to the propagation mode of light in the core21. Non-conformance between propagation modes of light is thus less likely when light enters the core21from the core121. Consequently, the occurrence of loss when light enters the core21from the core121can be reduced.

Second Embodiment

An isolator10A according to a second embodiment is described next.

The differences between the isolator10according to the first embodiment and the isolator10A according to the second embodiment are as follows. In the isolator10according to the first embodiment, the first waveguide20and the second waveguide30are positioned along the z-axis direction. In the isolator10A according to the second embodiment, the first waveguide20and the second waveguide30are instead positioned along the v-axis direction. Since the remaining configuration is the same as that of the isolator10according to the first embodiment, a redundant explanation is omitted.

As illustrated inFIG.19, an isolator10A according to the second embodiment includes, on a substrate50having a substrate surface50a, a linear first waveguide20positioned along the substrate surface50aand an annular second waveguide30.

On the substrate50that includes the substrate surface50a, the first waveguide20and the second waveguide30are positioned along the substrate surface50aso as to be side by side in parallel with the substrate surface50a.

The first waveguide20extends in the x-axis direction and is in contact with the substrate surface50a. The first waveguide20has a first end201on the negative side in the x-axis direction and a second end202on the positive side in the x-axis direction. The first waveguide20has a first port211and a second port212, for input and output of electromagnetic waves, respectively on the first end201and the second end202. The electromagnetic waves that are inputted to the first waveguide20from the first port211proceed along the x-axis towards the second port212. The electromagnetic waves that are inputted to the first waveguide20from the second port212proceed along the x-axis towards the first port211. The first port211and the second port212may be formed as the end faces of a core21or may be configured as a coupler that connects to an external apparatus and is capable of propagating electromagnetic waves.

The second waveguide30extends in the x-axis direction and is in contact with the substrate surface50a. The second waveguide30is annular and is optically connected without interruption. The annular shape is not particularly restricted as long as the shape is optically connected without interruption. By virtue of the second waveguide30having an annular shape, the non-reciprocity of a non-reciprocal waveguide in the second waveguide30can be increased. The annular shape of the second waveguide30can also achieve desired characteristics even when the second waveguide30extending in the x-axis direction is short. The insertion loss can thereby be reduced while achieving a smaller size of the isolator10A.

The second waveguide30has an end301on the negative side in the x-axis direction and an end302on the positive side in the x-axis direction. An example of one second waveguide30is illustrated inFIG.19, but this example is not limiting, and two or more second waveguides30may instead be provided.

By virtue of the second waveguide30having an annular shape in the isolator10A according to the second embodiment, the non-reciprocity of the non-reciprocal waveguide in the second waveguide30can be increased. The annular shape of the second waveguide30in the isolator10A according to the second embodiment can also achieve desired characteristics even when the second waveguide30extending in the x-axis direction is short. Consequently, the loss of electromagnetic waves in the isolator10A according to the second embodiment can be smaller than the loss of electromagnetic waves in the isolator90according to the comparative example. The isolator10A according to the second embodiment can, in other words, reduce the loss of electromagnetic waves while being compact.

Positioning the first waveguide20and the second waveguide30so as to overlap each other as viewed from the substrate surface50a, as in the isolator10according to the first embodiment, has the advantage that the size of each waveguide is easy to adjust in the z-axis direction during production of the isolator10. On the other hand, positioning the first waveguide20and the second waveguide30so as to be side by side in parallel with the substrate surface50a, as in the isolator10A according to the second embodiment, has the advantage that the production process of the isolator10A becomes relatively simple.

The isolator10, the isolator10A, and the light source apparatus100according to the present disclosure may be mounted in an optical transceiver that has a modulation function. The isolator10and the isolator10A according to the present disclosure may be used in an optical switch or an optical amplifier. The isolator10and the isolator10A according to the present disclosure may be used in a device. A device including the isolator10or the isolator10A according to the present disclosure may be used for communication in a data center.

Although embodiments of the present disclosure have been described through drawings and examples, it is to be noted that various changes and modifications will be apparent to those skilled in the art on the basis of the present disclosure. Therefore, such changes and modifications are to be understood as included within the scope of the present disclosure. For example, the functions or the like included in the various components or steps may be reordered in any logically consistent way. Furthermore, components or steps may be combined into one or divided. While embodiments of the present disclosure have been described focusing on apparatuses, the present disclosure may also be embodied as a method that includes steps performed by the components of an apparatus. The present disclosure may also be embodied as a method executed by a processor provided in an apparatus, as a program, or as a recording medium having a program recorded thereon. Such embodiments are also to be understood as falling within the scope of the present disclosure.

The references to “first”, “second”, and the like in the present disclosure are identifiers for distinguishing between the corresponding elements. The numbers attached to elements distinguished by references to “first”, “second”, and the like in the present disclosure may be switched. For example, the identifiers “first” and “second” of the first port and the second port may be switched. Identifiers are switched simultaneously, and the elements are still distinguished between after identifiers are switched. The identifiers may be removed. Elements from which the identifiers are removed are distinguished by their reference sign. Identifiers in the present disclosure, such as “first” and “second”, may not be used in isolation as an interpretation of the order of elements or as the basis for the existence of the identifier with a lower number.

In the present disclosure, the x-axis, y-axis, and z-axis are provided for the sake of explanation and may be interchanged. Configurations according to the present disclosure have been described using an orthogonal coordinate system constituted by the x-axis, y-axis, and z-axis. The positional relationships between elements according to the present disclosure are not limited to orthogonal relationships.