Optical integrated circuit

Abstract

This optical integrated circuit is provided with a substrate having a substrate surface, a first waveguide and a second waveguide for propagating electromagnetic waves along the substrate surface, and a non-reciprocal member positioned on the side away from the substrate surface of the first waveguide and the second waveguide. At least a part of the non-reciprocal member is located in the range of the mode field diameter of an electromagnetic wave in the TM mode in the first waveguide. The non-reciprocal member is not located in the range of the mode field diameter of the electromagnetic wave in the TM mode in the second waveguide.

Description

Optical Integrated Circuit

[0001] The present disclosure relates to integrated optical circuits.

[0002] As described in Patent Document 1, an optical isolator is known that uses a magneto-optical material Ce:YIG as a waveguide layer.

[0003] International Publication No. 2007 / 083419

[0004] An optical integrated circuit according to an embodiment of the present disclosure includes a substrate having a substrate surface, first and second waveguides for propagating electromagnetic waves along the substrate surface, and a nonreciprocal member located on the first and second waveguides away from the substrate surface, wherein at least a portion of the nonreciprocal member is located within a range of a mode field diameter of the electromagnetic wave in TM mode in the first waveguide, and the nonreciprocal member is not located within a range of a mode field diameter of the electromagnetic wave in TM mode in the second waveguide.

[0005] 2 is a cross-sectional view showing an example of the configuration of an optical integrated circuit according to an embodiment of the present invention, and is a diagram showing an example of the intensity distribution of an electromagnetic wave in TM mode propagating through a waveguide, and is a graph showing the intensity of each point on the Y axis in the intensity distribution of FIG.

[0006] (Configuration Example of Optical Integrated Circuit 1) As shown in FIG. 1 , an optical integrated circuit 1 according to one embodiment includes a substrate 50, a box layer 52, an insulating layer 54, a waveguide 20, and a nonreciprocal member 40. The substrate 50 has a substrate surface 50A. The box layer 52 is located on the substrate surface 50A. The waveguide 20 is located on the box layer 52. The waveguide 20 extends in the depth direction of the page, i.e., along the Z-axis direction. The insulating layer 54 is located on the box layer 52 and the waveguide 20. The nonreciprocal member 40 is located on the insulating layer 54.

[0007] The substrate 50 may be configured to include a conductor such as metal, a semiconductor such as silicon, glass, or resin, etc. In this embodiment, the substrate 50 is made of silicon (Si), but is not limited to this and may be made of various other materials.

[0008] The waveguide 20 is surrounded by a box layer 52 and an insulating layer 54. The waveguide 20 is also referred to as a core. The box layer 52 and the insulating layer 54 are also referred to as a clad. The core and the clad may be configured to include a dielectric. The waveguide 20 is also referred to as a dielectric line. The waveguide 20 as a core is located along the substrate surface 50A and propagates electromagnetic waves along the substrate surface 50A. In other words, the electromagnetic waves propagate through the waveguide 20 as a core in the direction in which the waveguide 20 extends.

[0009] The materials of the core and the cladding are determined so that the relative dielectric constant of the core is greater than that of the cladding. In other words, the materials of the core and the cladding are determined so that the refractive index of the cladding is smaller than that of the core. In this way, the electromagnetic wave propagating through the core can be totally reflected at the boundary with the cladding. As a result, the loss of the electromagnetic wave propagating through the core can be reduced.

[0010] The relative dielectric constants of the core and the cladding may be made larger than that of air. By making the relative dielectric constants of the core and the cladding larger than that of air, leakage of electromagnetic waves from the optical integrated circuit 1 can be suppressed. As a result, loss due to radiation of electromagnetic waves from the optical integrated circuit 1 to the outside can be reduced.

[0011] In this embodiment, the material of the waveguide 20 as the core is silicon (Si), but is not limited to this and may be various other materials. The material of the box layer 52 and the insulating layer 54 as the cladding may be silica glass or silicon oxide film (SiO ２ ), but is not limited to this and may be various other materials. The relative dielectric constants of silicon and quartz glass are approximately 12 and approximately 2, respectively. Silicon can propagate electromagnetic waves having near-infrared wavelengths of approximately 1.2 μm to approximately 6 μm with low loss. When the waveguide 20 is made of silicon, it can propagate electromagnetic waves having wavelengths in the 1.3 μm or 1.55 μm band used in optical communications with low loss.

[0012] The waveguide 20 includes a first waveguide 21 and a second waveguide 22. The first waveguide 21 and the second waveguide 22 are both located on a box layer 52. The insulating layer 54 has a recess 30 defined by a side surface 31, a side surface 32, and a bottom surface 33. The recess 30 is located such that the bottom surface 33 extends along the first waveguide 21. The recess 30 may be formed, for example, by etching the insulating layer 54.

[0013] The nonreciprocal member 40 is formed by being deposited on the substrate 50 with the recess 30 formed in the insulating layer 54. The portion of the nonreciprocal member 40 deposited inside the recess 30 corresponds to the first nonreciprocal member 41. The portion of the nonreciprocal member 40 deposited outside the recess 30 corresponds to the second nonreciprocal member 42. In other words, the nonreciprocal member 40 includes the first nonreciprocal member 41 and the second nonreciprocal member 42. The first nonreciprocal member 41 is located on the bottom surface 33 of the recess 30 of the insulating layer 54. The second nonreciprocal member 42 is located on the portion of the insulating layer 54 that does not have the recess 30.

[0014] In this embodiment, Ce:YIG (cerium-substituted yttrium iron garnet) is used as the material of the nonreciprocal member 40. A transparent magnetic material such as a partially substituted YIG material, such as Bi:YIG (bismuth-substituted YIG), may also be used as the material of the nonreciprocal member 40. A ferromagnetic material, such as FeCo, FeNi, or CoPt, or a material containing a ferromagnetic material, may also be used as the material of the nonreciprocal member 40. A dielectric material composited with magnetic nanoparticles, such as a nanogranular material, may also be used as the material of the nonreciprocal member 40. The nonreciprocal member 40 is not limited to these, and various other magnetic materials may also be used.

[0015] The distance between the upper surface of the first waveguide 21 and the lower surface of the first nonreciprocal member 41 is represented by D1. The distance between the upper surface of the second waveguide 22 and the lower surface of the second nonreciprocal member 42 is represented by D2.

[0016] TM mode electromagnetic waves are electromagnetic waves whose electric field amplitude direction coincides with the normal direction to the substrate surface 50A of the substrate 50 when the electromagnetic waves propagate along the substrate surface 50A. When TM mode electromagnetic waves propagate in the Z-axis direction in the waveguide 20 illustrated in FIG. 1, the electric field amplitude direction coincides with the Y-axis direction. Conversely, TE mode electromagnetic waves are electromagnetic waves whose electric field amplitude direction is orthogonal to the propagation direction of the electromagnetic wave and coincides with the direction along the substrate surface 50A when the electromagnetic waves propagate along the substrate 50. When TE mode electromagnetic waves propagate in the Z-axis direction in the waveguide 20 illustrated in FIG. 1, the electric field amplitude direction coincides with the X-axis direction.

[0017] Assume that a TM mode electromagnetic wave propagates through the waveguide 20 illustrated in FIG. 1 . FIG. 2 illustrates the intensity distribution of the electromagnetic wave in a cross section perpendicular to the propagation direction of the TM mode electromagnetic wave propagating through the waveguide 20. The horizontal axis of FIG. 2 represents the X-axis direction, i.e., the coordinate along the substrate surface 50A in the cross section perpendicular to the propagation direction of the electromagnetic wave. The vertical axis of FIG. 2 represents the Y-axis direction, i.e., the coordinate normal to the substrate surface 50A. H represents the length of the waveguide 20 in the Y-axis direction, i.e., the height of the waveguide 20. W represents the length of the waveguide 20 in the X-axis direction, i.e., the width of the waveguide 20.

[0018] The intensity of an electromagnetic wave corresponds to the magnitude of the energy of the electric field component of the electromagnetic wave. In the intensity distribution image of Figure 2, the closer a pixel color is to black, i.e., the darker it is, the greater the intensity of the electromagnetic wave at the position corresponding to that pixel. Conversely, the closer a pixel color is to white, i.e., the lighter it is, the weaker the intensity of the electromagnetic wave at the position corresponding to that pixel.

[0019] 3 is a graph showing the intensity of the TM mode electromagnetic wave at each point on a line that passes through the center of the cross section of the waveguide 20 in FIG. 2 and extends in the Y-axis direction, i.e., the height direction of the waveguide 20. The horizontal axis of the graph in FIG. 3 represents the coordinate in the Y-axis direction, i.e., the coordinate in the height direction of the waveguide 20. The center of the height direction of the waveguide 20 corresponds to the point where the coordinate in the Y-axis direction is 0. The vertical axis of the graph in FIG. 3 represents the power, i.e., intensity, of the TM mode electromagnetic wave at each coordinate in the Y-axis direction. In FIG. 3, the range corresponding to the width of the waveguide 20 is represented by H. In the graph in FIG. 3, the position where the intensity of the electromagnetic wave is at a minimum value corresponds to the surface of the waveguide 20. Therefore, the distance between two positions where the intensity of the electromagnetic wave is at a minimum value in the graph in FIG. 3 corresponds to the width (H) of the waveguide 20.

[0020] The intensity of the electromagnetic wave decreases outside the waveguide 20 as it moves away from the surface of the waveguide 20. The intensity of the TM mode electromagnetic wave outside the waveguide 20 decreases according to a Gaussian distribution. As a propagation characteristic of the TM mode electromagnetic wave in the waveguide 20, the mode field diameter (MFD) in the Y-axis direction, which is the amplitude direction of the electric field of the TM mode electromagnetic wave, is defined. The mode field diameter in the Y-axis direction is the distance from the maximum intensity of the electromagnetic wave propagating through the waveguide 20 where the intensity of the electromagnetic wave is 1 / e ２ In other words, the mode field diameter in the Y-axis direction is defined as the distance between two positions where the intensity of the electromagnetic wave decreases to 1 / e of the maximum intensity. ２ The length of the range in which the mode field diameter is equal to or greater than 1 / 2. Note that e is the value representing the base of the natural logarithm, i.e., Napier's constant. In Figure 3, the range corresponding to the mode field diameter in the Y-axis direction is represented as MFD.

[0021] The energy of an electromagnetic wave is 1 / e of the maximum intensity of the electromagnetic wave. ２ The electromagnetic wave energy is effectively distributed within the range described above. In other words, the mode field diameter corresponds to the length of the range in which the electromagnetic wave energy is effectively distributed. Furthermore, the range of the mode field diameter corresponds to the range in which the electromagnetic wave energy is effectively distributed. The range of the mode field diameter in the Y-axis direction corresponds to the range in which the electromagnetic wave energy propagating through the waveguide 20 is effectively distributed in the Y-axis direction.

[0022] The mode field diameter in the Y-axis direction is determined based on the width direction length W of the waveguide 20 in a cross section perpendicular to the extension direction of the waveguide 20, and the height direction length H. The mode field diameter in the Y-axis direction is further determined based on the refractive index of the waveguide 20, the refractive index of the box layer 52 and the insulating layer 54 positioned to surround the waveguide 20, and the wavelength of the electromagnetic wave propagating through the waveguide 20.

[0023] The range in which the TM mode electromagnetic waves propagating through the waveguide 20 are effectively distributed in the Y-axis direction is also referred to as the range of the TM mode field diameter. The presence of the nonreciprocal member 40 within the range of the TM mode field diameter causes nonreciprocity that cannot be ignored to appear in the waveguide 20 through which the TM mode electromagnetic waves propagate. Conversely, the absence of the nonreciprocal member 40 within the range of the TM mode field diameter causes the nonreciprocity that appears in the waveguide 20 through which the TM mode electromagnetic waves propagate to be zero or negligibly small. A nonreciprocity of zero means that no nonreciprocity appears.

[0024] When nonreciprocity is to be exhibited in the waveguide 20 through which the TM mode electromagnetic wave propagates, the optical integrated circuit 1 is configured so that at least a portion of the nonreciprocal member 40 is located within the range of the mode field diameter of the TM mode of the waveguide 20. Specifically, the optical integrated circuit 1 may be configured so that the distance from the surface of the waveguide 20 away from the substrate surface 50A, i.e., the upper surface, to the nonreciprocal member 40 is equal to or less than half the value obtained by subtracting the height of the waveguide 20 from the mode field diameter of the TM mode of the waveguide 20. In this manner, the effective distribution of the energy of the TM mode electromagnetic wave propagating through the waveguide 20 extends to the nonreciprocal member 40. As a result, nonreciprocity is exhibited in the waveguide 20 through which the TM mode electromagnetic wave propagates.

[0025] When nonreciprocity is not to be exhibited in the waveguide 20 through which the TM mode electromagnetic wave propagates, or when the nonreciprocity exhibited in the waveguide 20 is made negligible, the optical integrated circuit 1 is configured so that the nonreciprocity member 40 is not located within the range of the mode field diameter of the TM mode of the waveguide 20. Specifically, the optical integrated circuit 1 may be configured so that the distance from the surface of the waveguide 20 away from the substrate surface 50A, i.e., the upper surface, to the nonreciprocity member 40 is greater than half the value obtained by subtracting the height of the waveguide 20 from the mode field diameter of the TM mode of the waveguide 20. In this way, the effective distribution of energy of the TM mode electromagnetic wave propagating through the waveguide 20 does not extend to the nonreciprocity member 40. As a result, nonreciprocity is not exhibited in the waveguide 20 through which the TM mode electromagnetic wave propagates.

[0026] The optical integrated circuit 1 according to this embodiment is configured to impart nonreciprocity to the first waveguide 21. In this case, at least a portion of the first nonreciprocity member 41 located on the side of the first waveguide 21 away from the substrate surface 50A is located within the range of the mode field diameter of the TM mode of the first waveguide 21. In other words, the distance from the upper surface of the first waveguide 21 to the first nonreciprocity member 41, i.e., D1, may be equal to or less than half the value obtained by subtracting the height of the first waveguide 21, i.e., H, from the mode field diameter of the TM mode of the first waveguide 21.

[0027] The nonreciprocal member 40 includes a first nonreciprocal member 41. Therefore, the nonreciprocal member 40 including the first nonreciprocal member 41 is located within the range of the mode field diameter of the TM mode of the first waveguide 21.

[0028] The optical integrated circuit 1 according to this embodiment is configured so as not to exhibit nonreciprocity in the second waveguide 22. In this case, the second nonreciprocity member 42 located on the side of the second waveguide 22 away from the substrate surface 50A is not located within the range of the mode field diameter of the TM mode of the second waveguide 22. In other words, the distance D2 from the upper surface of the second waveguide 22 to the second nonreciprocity member 42 may be greater than half the value obtained by subtracting the height H of the second waveguide 22 from the mode field diameter of the TM mode of the second waveguide 22. The value obtained by subtracting the height H of the second waveguide 22 from the mode field diameter of the TM mode of the second waveguide 22 is represented as TH1 in the graph of FIG. 3 .

[0029] Furthermore, in order to prevent nonreciprocity from occurring in the second waveguide 22, the first nonreciprocal member 41 is positioned outside the range of the mode field diameter of the TM mode of the second waveguide 22. Therefore, the nonreciprocal member 40 including the first nonreciprocal member 41 and the second nonreciprocal member 42 is not positioned within the range of the mode field diameter of the TM mode of the second waveguide 22.

[0030] As described above, when both the first waveguide 21 that exhibits nonreciprocity and the second waveguide 22 that does not exhibit nonreciprocity are implemented in the optical integrated circuit 1, the distance between the nonreciprocity member 40 and the waveguide 20 is appropriately set. In this way, the nonreciprocity exhibited in each of the first waveguide 21 and the second waveguide 22 is controlled as intended by the design. As a result, the optical integrated circuit 1 can operate as intended by the design.

[0031] In order to further reduce the nonreciprocity of the TM mode occurring in the second waveguide 22, the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that reaches the second nonreciprocity member 42 may be further reduced.

[0032] For example, a criterion for whether the energy of the electromagnetic wave propagating through the waveguide 20 is sufficiently attenuated outside the waveguide 20 may be that the energy is attenuated to less than 1% of the maximum value of the energy of the electromagnetic wave propagating through the waveguide 20. An attenuation of 1% of the energy corresponds to 20 dB (decibels), which is a standard commonly used in the field of optical fibers.

[0033] When the above-mentioned criteria are applied, the second nonreciprocity member 42 is not located in a range where the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that leaks outside the second waveguide 22 is 1% or more of the maximum value of the energy of the electromagnetic wave in the second waveguide 22. In other words, the distance from the upper surface of the second waveguide 22 to the second nonreciprocity member 42, i.e., D2, may be a value greater than half the value obtained by subtracting the height of the second waveguide 22, i.e., H, from the range where the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that leaks outside the second waveguide 22 is 1% or more of the maximum value of the energy of the electromagnetic wave in the second waveguide 22. The value of half of the value obtained by subtracting the height of the second waveguide 22, i.e., H, from the range in which the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that leaks outside the second waveguide 22 is 1% or more of the maximum value of the energy of the electromagnetic wave inside the second waveguide 22 is represented as TH2 in the graph of FIG. 3 .

[0034] The first nonreciprocity member 41 is not located in a range where the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that leaks outside the second waveguide 22 is 1% or more of the maximum value of the energy of the electromagnetic wave in the second waveguide 22. Therefore, the nonreciprocity member 40 is not located in a range where the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that leaks outside the second waveguide 22 is 1% or more of the maximum value of the energy of the electromagnetic wave in the second waveguide 22.

[0035] 1 / e, which is the measure of attenuation that defines the mode field diameter ２ is approximately 13.5%, which is greater than 1%. Therefore, the range of the mode field diameter is narrower than the range in which the energy of the TM mode electromagnetic wave propagating through the second waveguide 22 that leaks outside the second waveguide 22 is 1% or more of the maximum energy of the electromagnetic wave inside the second waveguide 22.

[0036] The nonreciprocity member 40 is not positioned in a range where the energy leaked outside the second waveguide 22, which does not exhibit nonreciprocity, is 1% or more of the maximum value, thereby further reducing the nonreciprocity exhibited in the second waveguide 22. This reduces the possibility that nonreciprocity not intended in the design of the optical integrated circuit 1 will be exhibited in the second waveguide 22. As a result, the optical integrated circuit 1 can operate as intended in the design.

[0037] (Application Example of Optical Integrated Circuit 1) The optical integrated circuit 1 may include an element that functions as an isolator. The isolator may include at least one nonreciprocal line. The isolator may include a first waveguide 21 that exhibits nonreciprocity as the nonreciprocal line. The isolator may include a second waveguide 22 that does not exhibit nonreciprocity as the reciprocal line. The isolator may be used in combination with a configuration that transmits electromagnetic waves. The isolator may be applied to an optical switch, an optical transceiver, or a data center. The isolator may be applied to, for example, an electromagnetic wave transmitter. The electromagnetic wave transmitter includes an isolator and a light source. The electromagnetic wave transmitter inputs an electromagnetic wave from the light source to the isolator and outputs the electromagnetic wave from the isolator to a receiver. The isolator is configured so that the transmittance of the electromagnetic wave propagating from the light source to the receiver is greater than the transmittance of the electromagnetic wave propagating from the receiver to the light source. This makes it difficult for electromagnetic waves to be incident on the light source. As a result, the light source can be protected.

[0038] The light source may be, for example, a semiconductor laser such as a laser diode (LD) or a vertical cavity surface emitting laser (VCSEL). The light source may include a device that emits electromagnetic waves of various wavelengths, not limited to visible light. The light source may be formed on the substrate 50 together with the isolator. The light source may input electromagnetic waves in TE mode or TM mode to the isolator.

[0039] The electromagnetic wave transmitter may further include a modulator and a signal input unit. The modulator modulates the electromagnetic wave by changing its intensity. The modulator may be located between the isolator and the receiver, rather than between the light source and the isolator. The modulator may, for example, pulse-modulate the electromagnetic wave. The signal input unit accepts input of a signal from an external device or the like. The signal input unit may include, for example, a D / A converter. The signal input unit outputs a signal to the modulator. The modulator modulates the electromagnetic wave based on the signal acquired by the signal input unit.

[0040] The light source may include a modulator and a signal input section. In this case, the light source may output a modulated electromagnetic wave and input it to the isolator.

[0041] The electromagnetic wave transmitter may be mounted on the substrate 50. The light source may be mounted so as to be connected, via a modulator, to a branching section that branches and inputs an electromagnetic wave to at least one nonreciprocal line. The light source may be mounted so as to be connected to the branching section without a modulator. The receiver may be mounted so as to be connected to a multiplexing section that multiplexes the electromagnetic wave input from the at least one nonreciprocal line without a modulator. The receiver may be mounted so as to be connected to the multiplexing section via a modulator. In this case, the modulator may be mounted so as to be connected to the multiplexing section.

[0042] The optical integrated circuit 1 may be formed by silicon photonics technology.

[0043] Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art can make various modifications or alterations based on the present disclosure. Therefore, it should be noted that these modifications or alterations are included in the scope of the present disclosure. For example, the functions included in each component can be rearranged so as not to be logically inconsistent, and multiple components can be combined or divided into one.

[0044] In the present disclosure, descriptions such as "first" and "second" are identifiers for distinguishing the configuration. In the present disclosure, the configurations distinguished by descriptions such as "first" and "second" can have their numbers interchanged. For example, the first waveguide 21 can have its identifiers "first" and "second" interchanged with the second waveguide 22. The identifiers are interchanged simultaneously. The configurations remain distinguished even after the identifiers are interchanged. Identifiers may be deleted. A configuration from which an identifier has been deleted is distinguished by a symbol. The identifiers "first" and "second" in the present disclosure should not be used solely to interpret the order of the configurations or to justify the existence of an identifier with a smaller number.

[0045] In this disclosure, the X-axis, Y-axis, and Z-axis are provided for convenience of explanation and may be interchanged. The configurations according to this disclosure have been described using an orthogonal coordinate system formed by the X-axis, Y-axis, and Z-axis. The positional relationship between the components according to this disclosure is not limited to an orthogonal relationship.

[0046] 1 Optical integrated circuit 20 Waveguide (21: first waveguide, 22: second waveguide) 30 Recess (31, 32: side surface, 33: bottom surface) 40 Non-reciprocal member (41: first non-reciprocal member, 42: second non-reciprocal member) 50 Substrate (50A: substrate surface, 52: box layer, 54: insulating layer)

Claims

1. An optical integrated circuit comprising: a substrate having a substrate surface; a first waveguide and a second waveguide for propagating electromagnetic waves along said substrate surface; and a nonreciprocal member located on the side of said first waveguide and said second waveguide away from said substrate surface, wherein at least a portion of said nonreciprocal member is located within a range of a mode field diameter of electromagnetic waves in TM mode in said first waveguide, and said nonreciprocal member is not located within a range of a mode field diameter of electromagnetic waves in TM mode in said second waveguide.

2. An optical integrated circuit as claimed in claim 1, wherein said nonreciprocal member is not located in a range in which the energy of the TM mode electromagnetic wave propagating through said second waveguide that leaks outside said second waveguide is 1% or more of the maximum value of the energy of the electromagnetic wave in said second waveguide.

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