Optical device

Abstract

An optical device is provided with a first optical waveguide (100) and a second optical waveguide (120). The first optical waveguide (100) is provided with a lower cladding layer (101) and a first core (102) composed of Si. The second optical waveguide (120) is provided with a second core (121) composed of a compound semiconductor, and a cladding layer (122) composed of silicon oxide is formed on the second core (121). The second optical waveguide (120) overlaps the first optical waveguide (100) and is formed along the first optical waveguide (100). The second core (121) is supported in contact with the top of the first core (102).

Description

Optical Devices

[0001] The present invention relates to optical devices.

[0002] Toward quantum information processing, research and development into quantum manipulation and calculation using photons is actively progressing, due to the fact that photons have weak coupling with the environment, are less susceptible to decoherence, and can operate at room temperature. Furthermore, research and development of platforms for manipulating photons was initially based on optical circuits and systems using free-space optical systems and optical fibers, which have long been commercially available.

[0003] Furthermore, toward the realization of future on-chip quantum information processing devices, research and development is accelerating on photon manipulation using optical circuits based on optical waveguides as a platform, utilizing the diverse functions provided by various optical waveguide devices. In particular, with the rapid development of Si photonics technology using Si or SiN as core materials, photon sources, photon interferometers, branching and multiplexing, wavelength filtering, phase manipulation, photon detection, etc. have been demonstrated. Furthermore, by integrating these, it has been demonstrated that basic quantum operations and calculations can be performed on optical circuits with cores made of Si or SiN.

[0004] Among these, photon sources are extremely important functional elements that generate photons, which serve as quantum information media. An ideal photon source would be able to deterministically emit single photons at any time. Previous attempts have been made to realize single-photon sources by utilizing quantum dots or two-level systems based on lattice defects in materials. However, these single-photon sources still have significant challenges, such as low optical coupling efficiency with optical waveguides, the need for cryogenic temperatures for operation, and high technological barriers to fabrication.

[0005] To address the above-mentioned issues, photon pair sources (or heralded single photon sources) are widely used as photon sources on Si or SiN optical circuits as an alternative to the ideal single photon source. Photon pair sources utilize the nonlinear optical effect (four-wave mixing) in Si optical waveguides to generate correlated photon pairs through a spontaneous parametric process using externally incident pump light.

[0006] For example, a Si optical waveguide can confine light in the optical communication wavelength range in a core cross section of the submicron order by taking advantage of the high refractive index of Si, and has a small effective mode area (A eff In addition, it has been reported that the nonlinear refractive index (n2) of Si is 100 to 200 times higher than that of glass, which is the core material of optical fibers, and the nonlinear parameter γ (=n2ω / c / A eff ) has also been reported to have a high value of about 200 (1 / W / m). This has the advantage that photon pairs can be generated with low pump light power.

[0007] Furthermore, by optimizing the core size, Si optical waveguides can be made into optical waveguides with anomalous dispersion around a wavelength of 1550 nm. Therefore, if an appropriate four-wave mixing gain spectrum is obtained using a Si optical waveguide, the wavelength of the photon pairs generated by four-wave mixing can be sufficiently separated from the pump wavelength. This has the advantage that photons at the pump wavelength, which cause noise, can be cut using a wavelength filter downstream of the photon pair source, making it easier to obtain photon pairs with less noise. In other words, the high nonlinearity (γ) of Si optical waveguides and the broadband four-wave mixing spectrum due to anomalous dispersion provide strong motivation for their widespread use as photon pair sources.

[0008] Meanwhile, optical waveguides with cores made of compound semiconductors have recently attracted attention as a new photon pair source. In particular, III-V compound semiconductors such as AlGaAs have a higher n value than Si, and are expected to realize highly efficient photon pair sources. Furthermore, compound semiconductors can be configured to have a desired band gap by controlling their composition, making it possible to suppress nonlinear loss caused by two-photon absorption, which becomes particularly pronounced when high pump light power is input. Furthermore, similar to Si optical waveguides, optical waveguides with cores made of compound semiconductors can achieve high optical confinement by using SiO2 cladding, and it is also possible to impart anomalous dispersion to the optical waveguide by controlling the structural dispersion.

[0009] Although compound semiconductor optical waveguides are thus promising as photon pair sources, it is still desirable to use Si photonics technology as a platform for constructing optical circuits other than photon pair sources. The reasons for this include the high performance of functional elements other than photon pair sources, which have already been established in optical devices for communication, excellent manufacturability that enables the realization of large-scale optical integrated circuits, and low manufacturing costs.

[0010] Based on this concept, Non-Patent Document 1 proposes a technology for integrating Si optical circuits with compound semiconductor (AlGaAs) optical waveguides. In this technology, a compound semiconductor wafer with crystal growth of AlGaAs-based materials is bonded to a wafer with a Si optical circuit formed thereon, and then the AlGaAs layer is thinned and the optical waveguide is processed to achieve the integration of an AlGaAs optical waveguide on a Si optical circuit. In Non-Patent Document 1, as shown in Fig. 1(b) and Fig. 2(b) of Non-Patent Document 1, there is no Si layer directly below the AlGaAs optical waveguide. Instead, the AlGaAs core is covered with SiO2, and the cross-sectional shape of the AlGaAs core is appropriately designed to impart anomalous dispersion.

[0011] W. Xie et al., "Silicon-integrated nonlinear III-V photonics", Photonics Research, vol. 10, no. 2, pp. 535-541, 2022.

[0012] However, in the technology of Non-Patent Document 1, there is air space directly below the AlGaAs optical waveguide, making it mechanically fragile and prone to breakage, as shown in Fig. 1(b) and Fig. 2(b) of Non-Patent Document 1. Furthermore, even if it does not lead to breakage, the core portion of the AlGaAs optical waveguide above the air space bends, making it difficult to control the distance to the buried oxide layer below, resulting in a major problem of poor dispersion controllability.

[0013] The present invention has been made to solve the above problems, and has as its object to enable a compound semiconductor optical waveguide to be more firmly integrated on a Si optical circuit.

[0014] The optical device according to the present invention comprises a lower cladding layer, a first optical waveguide formed on the lower cladding layer and having a first core made of Si, and a second optical waveguide having a second core made of a compound semiconductor, overlapping the first optical waveguide and formed along the first optical waveguide, the second core being supported in contact with the top of the first core.

[0015] The optical device according to the present invention also comprises a lower cladding layer, a first optical waveguide formed on the lower cladding layer and having a first core made of Si, a second core made of a compound semiconductor, and a second optical waveguide overlapping the first optical waveguide and formed along the first optical waveguide, and an insulating layer formed on the first core, the second core being supported in contact with the insulating layer.

[0016] As described above, according to the present invention, the second core made of a compound semiconductor of the second optical waveguide formed to overlap the first optical waveguide is supported on the first core of the first optical waveguide, so that the compound semiconductor optical waveguide can be more firmly integrated on the Si optical circuit.

[0017] Fig. 1 is a cross-sectional view showing the configuration of an optical device according to a first embodiment of the present invention. Fig. 2 is a characteristic diagram showing the dependency of dispersion in the fundamental propagation mode of the optical device according to the first embodiment of the present invention on the width of the first core 102. Fig. 3 is a cross-sectional view showing the configuration of an optical device according to a second embodiment of the present invention. Fig. 4 is a characteristic diagram showing the dependency of dispersion in the fundamental propagation mode of the optical device according to the second embodiment of the present invention on the width of the first core 102. Fig. 5 is a cross-sectional view showing the configuration of an optical device according to a third embodiment of the present invention. Fig. 6 is a characteristic diagram showing the dependency of dispersion in the fundamental propagation mode of the optical device according to the third embodiment of the present invention on the width of the first core 102'.

[0018] Hereinafter, an optical device according to an embodiment of the present invention will be described.

[0019] First Embodiment First, an optical device according to a first embodiment of the present invention will be described with reference to Fig. 1. This optical device includes a first optical waveguide 100 and a second optical waveguide 120. This optical device may include a photon pair source formed from the second optical waveguide 120. For example, a Si optical circuit is formed by the first optical waveguide 100.

[0020] The first optical waveguide 100 includes a lower cladding layer 101 and a first core 102 made of Si. The first core 102 is formed on the lower cladding layer 101. In this example, the first core 102 is formed from a convex portion of a Si layer 105 formed on the lower cladding layer 101. Recesses are formed in the Si layer 105 to sandwich the area that will become the first core 102, forming a slab 103, and the first optical waveguide 100 is a so-called rib-type waveguide. Note that a peripheral portion 104 around the rib-type waveguide formed by the first core 102 and the slab 103 has the same thickness as the first core 102.

[0021] The second optical waveguide 120 includes a second core 121 made of a compound semiconductor, and a cladding layer 122 made of silicon oxide is formed on the second core 121. The second core 121 can be made of, for example, AlGaAs (Al composition 20%). The second core 121 can also be made of a III-V group compound semiconductor such as InP, GaAs, or InGaP, or a II-VI group compound semiconductor such as ZnSe. The material and composition of the second core 121 can be adjusted appropriately to suit the desired nonlinearity.

[0022] The second optical waveguide 120 overlaps the first optical waveguide 100 and is formed along the first optical waveguide 100. The second core 121 is supported on and in contact with the first core 102. The second optical waveguide 120 has a cladding layer 122 supported on the peripheral portion 104 at its peripheral portion. A space is left on the side of the first core 102. This space can be filled with a cladding material such as silicon oxide or resin.

[0023] For the fundamental propagation mode of the second optical waveguide 120, in which light is mainly confined in the second core 121, various structures and refractive indices are set so that the anomalous dispersion of this fundamental propagation mode can be ensured even in a supermode in which the electromagnetic field distribution is coupled with the first core 102.

[0024] 2 shows the dependency of dispersion of the fundamental propagation mode on the width of the first core 102. As shown in FIG. 1, the structural parameters of each part are as follows: the width of the second core 121 is w, the height of the second core 121 is h, the width of the first core 102 is w Si , the thickness of the Si layer 105 is h Si , the thickness of the slab 103 is h SiSlab , the width of the slab 103 in the direction perpendicular to the waveguide direction is w SiRm It was decided.

[0025] In the calculation, w = 700 nm, h = 400 nm, h Si = 220 nm, h SiSlab = 110 nm, w SiRm = 5 mm. The second core 121 was made of AlGaAs (Al composition 20%). The wavelength of the guided light was calculated as 1550 nm, and the cladding layer 122 on the upper part of the second optical waveguide 120 was assumed to have a thickness of 200 nm and to be uniformly disposed so as to cover the second core 121.

[0026] Width w of the first core 102 Si It can be seen that the dispersion is a positive value and anomalous dispersion is ensured for any wavelength between 100 nm and 300 nm. Furthermore, according to the first embodiment, the first core 102 below the second core 121 has high mechanical strength because it serves as a support structure for the second core 121. Furthermore, because the second core 121, which has a high refractive index, and the first core 102 are in contact with each other, it is possible to reduce uncertainty in the optical waveguide characteristics, such as the effective refractive index, group refractive index, and dispersion, which are caused by uncertainty in the distance between them.

[0027] Furthermore, w Si If the θ is too large, higher-order propagation modes will exist in addition to the fundamental propagation mode, which will be a problem. It is preferable to adjust the dimensions of the second core 121 and the first core 102 so as to maintain single-mode properties while ensuring the desired dispersion characteristics.

[0028] Second Embodiment Next, an optical device according to a second embodiment of the present invention will be described with reference to Fig. 3. This optical device includes a first optical waveguide 100 and a second optical waveguide 120. This optical device may include a photon pair source formed from the second optical waveguide 120. For example, a Si optical circuit is formed by the first optical waveguide 100.

[0029] The first optical waveguide 100 includes a lower cladding layer 101 and a first core 102 made of Si. The first core 102 is formed on the lower cladding layer 101. In this example, the first core 102 is formed from a convex portion of a Si layer 105 formed on the lower cladding layer 101. Recesses are formed in the Si layer 105 to sandwich the area that will become the first core 102, forming a slab 103, and the first optical waveguide 100 is a so-called rib-type waveguide. Note that a peripheral portion 104 around the rib-type waveguide formed by the first core 102 and the slab 103 has the same thickness as the first core 102.

[0030] Moreover, in the second embodiment, an insulating layer 106 is provided on the first core 102. The insulating layer 106 can be made of silicon oxide (SiO2). An insulating layer 107 is formed on the peripheral portion 104. The insulating layer 107 is formed to the same thickness as the insulating layer 106 and can be made of silicon oxide (SiO2).

[0031] The second optical waveguide 120 includes a second core 121 made of a compound semiconductor, and a cladding layer 122 made of silicon oxide is formed on the second core 121. The second core 121 can be made of, for example, AlGaAs (Al composition 20%). The second core 121 can also be made of a III-V group compound semiconductor such as InP, GaAs, or InGaP, or a II-VI group compound semiconductor such as ZnSe. The material and composition of the second core 121 can be adjusted appropriately to suit the desired nonlinearity.

[0032] The second optical waveguide 120 overlaps the first optical waveguide 100 and is formed along the first optical waveguide 100. The second core 121 is supported (formed) in contact with the insulating layer 106 formed on the first core 102. The cladding layer 122 of the second optical waveguide 120 is supported on the insulating layer 107 of the peripheral portion 104 at the periphery thereof. Space is provided on the sides of the first core 102. This space can be filled with a cladding material such as silicon oxide or resin.

[0033] For the fundamental propagation mode of the second optical waveguide 120, in which light is mainly confined in the second core 121, various structures and refractive indices are set so that the anomalous dispersion of this fundamental propagation mode can be ensured even in a supermode in which the electromagnetic field distribution is coupled with the first core 102.

[0034] 4 shows the dependence of dispersion of the fundamental propagation mode on the width of the first core 102 when the thickness of the insulating layer 106 is 0 nm, 50 nm, and 100 nm. As shown in FIG. 3, the structural parameters of each part are: the width of the second core 121 is w, the height of the second core 121 is h, the width of the first core 102 is w Si , the thickness of the Si layer 105 is h Si , the thickness of the insulating layer 106 is h SiO2 , the thickness of the slab 103 is h SiSlab , the width of the slab 103 in the direction perpendicular to the waveguide direction is w SiRm It was decided.

[0035] In the calculation, w = 700 nm, h = 400 nm, h Si = 220 nm, h SiSlab = 110 nm, w SiRm = 5 mm. The second core 121 was made of AlGaAs (Al composition 20%). The wavelength of the guided light was calculated as 1550 nm, and it was assumed that the cladding layer 122 on the upper part of the second optical waveguide 120 had a thickness of 200 nm and was uniformly disposed so as to cover the second core 121.

[0036] Width w of the first core 102 Si It can be seen that the dispersion is a positive value in any range from 100 nm to 300 nm, and anomalous dispersion is ensured.

[0037] Furthermore, according to the second embodiment, the first core 102 below the second core 121 serves as a support structure for the second core 121, and therefore has high mechanical strength. Furthermore, the second core 121, which has a high refractive index, and the first core 102 are in contact with each other via the insulating layer 106, which reduces uncertainty in optical waveguide properties such as the effective refractive index, group refractive index, and dispersion due to uncertainty in the distance between them. Silicon oxide constituting the insulating layer 106 is known to be an effective adhesive layer when bonding AlGaAs and Si by hydrophilic bonding or transfer printing. Therefore, the structure of the optical device according to the second embodiment is also effective from the viewpoint of the manufacturing process.

[0038] Third Embodiment Next, an optical device according to a third embodiment of the present invention will be described with reference to Fig. 5. This optical device includes a first optical waveguide 100' and a second optical waveguide 120. This optical device may include a photon pair source formed from the second optical waveguide 120. For example, a Si optical circuit is formed by the first optical waveguide 100'.

[0039] The first optical waveguide 100' includes a lower cladding layer 101 and a first core 102' made of Si. The first core 102' is formed on the lower cladding layer 101. In this example, the first core 102' has a rectangular cross section perpendicular to the optical axis direction, and the first optical waveguide 100' is a channel type. For example, the first core 102' can be formed by removing all of the Si layer 105 in the region between the peripheral portions 104 that sandwich the portion to be the first core 102'. The peripheral portion 104 around the first optical waveguide 100' made of the first core 102' has the same thickness as the first core 102'.

[0040] Moreover, in the third embodiment, an insulating layer 106 is formed on the first core 102'. The insulating layer 106 can be made of silicon oxide (SiO2). An insulating layer 107 is formed on the peripheral portion 104. The insulating layer 107 is formed to the same thickness as the insulating layer 106 and can be made of silicon oxide (SiO2).

[0041] The second optical waveguide 120 includes a second core 121 made of a compound semiconductor, and a cladding layer 122 made of silicon oxide is formed on the second core 121. The second core 121 can be made of, for example, AlGaAs (Al composition 20%). The second core 121 can also be made of a III-V group compound semiconductor such as InP, GaAs, or InGaP, or a II-VI group compound semiconductor such as ZnSe. The material and composition of the second core 121 can be adjusted appropriately to suit the desired nonlinearity.

[0042] The second optical waveguide 120 overlaps the first optical waveguide 100' and is formed along the first optical waveguide 100'. Also in the third embodiment, the second core 121 is supported (formed) in contact with the insulating layer 106 formed on the first core 102'. In the peripheral portion of the second optical waveguide 120, the cladding layer 122 is supported on the insulating layer 107 of the peripheral portion 104. Furthermore, spaces are provided on the sides of the first core 102'.

[0043] For the fundamental propagation mode of the second optical waveguide 120, in which light is mainly confined in the second core 121, various structures and refractive indices are set so that the anomalous dispersion of this fundamental propagation mode can be ensured even in a supermode in which the electromagnetic field distribution is coupled with the first core 102'.

[0044] 6 shows the dependence of dispersion of the fundamental propagation mode on the width of the first core 102' when the thickness of the insulating layer 106 is 0 nm, 50 nm, and 100 nm. As shown in FIG. 5, the structural parameters of each part are: the width of the second core 121 is w, the height of the second core 121 is h, the width of the first core 102' is w Si , the thickness of the Si layer 105 is h Si , the thickness of the insulating layer 106 is h SiO2 , the width of the slab 103 in the direction perpendicular to the waveguide direction is w SiRm It was decided.

[0045] In the calculation, w = 700 nm, h = 400 nm, h Si = 220 nm, w SiRm= 5 mm. The second core 121 was made of AlGaAs (Al composition 20%). The wavelength of the guided light was calculated as 1550 nm, and it was assumed that the cladding layer 122 on the upper part of the second optical waveguide 120 had a thickness of 200 nm and was uniformly disposed so as to cover the second core 121.

[0046] Width w of the first core 102′ Si It can be seen that the dispersion is a positive value in any range from 100 nm to 300 nm, and anomalous dispersion is ensured.

[0047] Also in the third embodiment, the first core 102' below the second core 121 has high mechanical strength because it serves as a support structure for the second core 121. Furthermore, the second core 121, which has a high refractive index, and the first core 102' are in contact with each other via the insulating layer 106, which reduces uncertainty in optical waveguide properties such as the effective refractive index, group refractive index, and dispersion due to uncertainty in the distance between them. Silicon oxide constituting the insulating layer 106 is known to be an effective adhesive layer when bonding AlGaAs and Si by hydrophilic bonding or transfer printing. Therefore, the structure of the optical device according to the third embodiment is also effective from the viewpoint of the fabrication process.

[0048] Furthermore, since the first core 102' is a channel type, the process of stopping the etching of the Si layer to form the slab required for forming a rib-type core is not necessary, and the influence of processing errors on the errors from the design values ​​that affect the effective refractive index, group refractive index, and dispersion error can be reduced.

[0049] It goes without saying that the above-mentioned values ​​for each dimension of the second core 121 are merely examples, and they can be adjusted as appropriate to obtain desired dispersion characteristics and single mode properties.

[0050] As described above, according to the embodiment of the present invention, the second core made of a compound semiconductor of the second optical waveguide formed to overlap the first optical waveguide is supported on the first core of the first optical waveguide, so that the compound semiconductor optical waveguide can be more firmly integrated on the Si optical circuit.

[0051] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0052] 100...first optical waveguide, 101...lower cladding layer, 102...first core, 103...slab, 104...surrounding portion, 105...Si layer, 120...second optical waveguide, 121...second core, 122...cladding layer.

Claims

1. An optical device comprising: a lower cladding layer; a first optical waveguide formed on the lower cladding layer and comprising a first core made of Si; and a second optical waveguide comprising a second core made of a compound semiconductor, overlapping the first optical waveguide and formed along the first optical waveguide, wherein the second core is supported in contact with the top of the first core.

2. An optical device comprising: a lower cladding layer; a first optical waveguide formed on the lower cladding layer and comprising a first core made of Si; a second optical waveguide comprising a second core made of a compound semiconductor, overlapping the first optical waveguide and formed along the first optical waveguide; and an insulating layer formed on the first core, wherein the second core is supported in contact with the insulating layer.

3. An optical device according to claim 1 or 2, comprising a photon pair source formed from said second optical waveguide.

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