The present invention relates to an optical waveguide using a rare earth oxide.
In recent years, with the aim of a Si-based photonic integrated circuit (Si photonics integrated circuits), integration of optical elements such as an optical waveguide, a light emitting element such as a laser, an optical amplifier, and a light detecting element on a Si substrate has progressed. Si-based photonic integrated circuits are considered one of the most promising and low-cost technologies to achieve, for example, high-speed and low-power consumption interconnections in data centers.
Rare earth doped materials are one of promising solutions for realizing Si-based optical amplifiers or lasers for realizing Si photonics integrated circuits. Rare earth-based materials are widely used as active media for optical amplifiers or lasers. Particularly, an erbium-doped fiber amplifier (EDFA) is a main optical amplifier used for long-distance optical fiber communication. In order to introduce the EDFA technology into a Si chip, an erbium-doped optical waveguide amplifier (EDWA) is employed. However, since the concentration of a rare earth element that can be added to a solid material generally used in this type of technology is low, only an optical gain of one-digital dB/cm or less is achieved. This requires very long devices to achieve a necessary optical gain and is not feasible on Si chips.
In contrast to the EDWA described above, a thin film of a single-crystal rare earth oxide (REO) has a higher Er concentration and a higher crystal quality than any other materials, and can be epitaxially grown on Si substrates. Thus, a REO is a promising material for application to Si based photonic integrated circuit technology.
Here, for example, in order to realize a laser, an optical amplifier, or the like on a Si substrate, an optical waveguide structure using the REO and having a low propagation loss and a high optical confinement factor is desirable. In order to ensure light propagation in an optical waveguide, a core with a higher refractive index, which is usually surrounded by a cladding with a lower refractive index, is important. In a conventional Er-doped amorphous or polycrystalline materials, the following two optical waveguide structures are known.
First, there is a configuration in which a material (Al2O3) with a high refractive index is used as a core doped with Er, and a material with a low refractive index is used as a cladding (Non Patent Literature 1). In this type of optical waveguide, typically, a lower cladding is made of SiO2, and an upper cladding is made of SiO2 or air.
Second, there is a configuration in which a material with a high refractive index (for example, SiN) is used as a core, and a low refractive index material doped with Er is used as an upper cladding (Non Patent Literature 2).
In the case of a single-crystal REO thin film, a high-quality REO cannot be grown on SiO2, and is grown only on single-crystal Si. Thus, neither of the two optical waveguide structures described above can be applied. A most straightforward waveguide structure is a Si core with an upper cladding made of a REO. It is conceivable that this structure is manufactured by growing a REO thin film on a silicon on insulator (SOI) substrate and sequentially patterning the grown REO thin film and a surface silicon layer of the SOI substrate through etching process or the like.
However, the above-described optical waveguide structure has two problems. First, since a refractive index (about 1.6 to 2.0) of the REO is much lower than a refractive index (about 3.45) of Si, waveguide modes are mostly concentrated in the layer of Si. Thus, an optical confinement factor (ΓREO) of the layer of the REO is usually very small, typically less than 10%. A mode gain (gmodal) of the EDWA is related to both a material gain (gmaterial) and the optical confinement factor and can be indicated by “gmodal=ΓREO×gmaterial”. For the optical waveguide structure described above, an achievable mode gain is restricted by the low optical confinement factor.
Furthermore, a REO is known to be very difficult to etch, and a REO processing method has not been established. Therefore, the above-described optical waveguide structure cannot be manufactured at the present time.
As described above, it is not easy to manufacture an optical waveguide using a REO that can be integrated with a high-performance Si-based optical amplifier, a laser, or the like, and, therefore, there is a problem that an optical waveguide for realizing an optical amplifier, a laser, or the like cannot be easily manufactured.
Embodiments of the present invention have been made to solve the above problems, and an object thereof is to easily manufacture an optical waveguide for realizing an optical amplifier, a laser, and the like.
An optical waveguide according to embodiments of the present invention includes: a cladding layer; a Si layer made of single-crystal Si and formed on the cladding layer; a REO layer made of a single-crystal rare earth oxide and formed on the Si layer; and a stripe-shaped cap layer formed on the REO layer and extending in a direction in which light is guided.
As described above, according to embodiments of the present invention, since only the cap layer, instead of REO layer, is processed, an optical waveguide for realizing an optical amplifier, a laser, and the like can be easily manufactured.
Hereinafter, an optical waveguide according to an embodiment of the present invention will be described below with reference to
The cladding layer 101 is formed on a substrate 111. The cladding layer 101 may be made of, for example, a silicon oxide. The Si layer 102 is made of single-crystal Si and is formed on the cladding layer 101. A surface of the Si layer 102 may be, for example, a (in) plane or a (100) plane. For example, a well-known silicon on insulator (SOI) substrate may be used, a silicon substrate portion of the SOI substrate may be the substrate 111, a buried insulating layer may be the cladding layer 101, and a surface Si layer may be the Si layer 102. In this case, the cladding layer 101 generally has a thickness of 2 μm or more, and the Si layer 102 has a thickness of 50 to 200 nm.
The REO layer 103 is made of a single-crystal rare earth oxide, and is formed on the Si layer 102. The REO layer 103 may be made of, for example, (ErxGd1-x)2O3. The REO layer 103 may have a thickness of 50 to 200 nm.
The cap layer 104 is formed on the REO layer 103. The cap layer 104 may be made of a material transparent to light to be guided. For example, the cap layer 104 may be made of SiN or Si. The cap layer 104 has a stripe shape (mesa shape) extending in a direction in which light is guided. For example, in a case where the cap layer 104 is made of SiN, the cap layer may have a thickness of 300 to 500 nm.
According to the above-described embodiment, the REO layer 103 is formed on the Si layer 102, but since the cap layer 104 with a refractive index higher than that of air is further formed thereon, the guided mode moves to the cap layer 104 side. As a result, the center of guided light mode is in the vicinity of an interface between the Si layer 102 and the REO layer 103 in the region where the cap layer 104 is formed. The optical waveguide according to the embodiment may be optically connected to, for example, a Si optical waveguide including a Si core formed by patterning the Si layer 102 in another region of the cladding layer 101.
The following is a more detailed explanation. In the optical waveguide according to the above-described embodiment, TM-polarized light is propagated (guided), and an electric field component thereof is a component in a direction perpendicular to the surface of the substrate in (cladding layer 101). A wavelength range of the guided light of the optical waveguide is around the S band, the C band, and the L band of the wavelength band used in optical communication.
A width of the cap layer 104 is important to achieve a low propagation loss.
As illustrated in
Next, a method for manufacturing an optical waveguide according to the embodiment will be described with reference to
First, as illustrated in
Next, (ErxGd1-x)2O3 is epitaxially grown on the Si layer 102 according to a well-known molecular beam epitaxy method to form the REO layer 103 as illustrated in
Next, as illustrated in
Next, as illustrated in
Next, by using the resist pattern 202 as a mask, the SiN layer 201 is etched by known reactive ion etching to form the cap layer 104 as illustrated in
The optical waveguide according to the present invention is not limited to the above-described materials and dimensions. For example, the REO layer is not limited to (ErxGd1-x)2O3, and may be made of a rare earth oxide having another composition. Embodiments of the present invention can also be applied to SiO2, barium titanate (BaTiO3), and the like instead of the REO. The cap layer is not limited to SiN, and may be made of other transparent materials. Examples thereof include SiO2, TiO2, and Si. As described above, the optimized width of the cap layer that minimizes the propagation loss may be determined through numerical simulation when respective conditions such as a material and a thickness are determined.
As the refractive index of the cap layer increases, the optical confinement factor can also be increased. This may be realized by replacing SiN with Si. As an example,
As illustrated in
As described above, according to embodiments of the present invention, since the cap layer is provided on the REO layer, an optical waveguide for realizing an optical amplifier, a laser, and the like can be easily manufactured.
In the optical waveguide according to embodiments of the present invention, by utilizing a mesa structure made of a commonly used material such as SiN or Si on a REO layer grown on a Si layer, the problems of a low confinement factor and manufacturing difficulty in the conventional optical waveguide can be solved. In order to achieve a low propagation loss, a width of the cap layer is appropriately optimized according to each parameter of the optical waveguide. The optical confinement factor in the REO layer can be further increased by using a higher refractive index cap layer. The structure of the optical waveguide according to the embodiments of present invention is general and can be applied to other functional materials grown on a Si substrate.
With the optical waveguide structure according to embodiments of the present invention, a low-loss optical waveguide with a large optical confinement factor for the REO layer grown on the Si layer can be realized. With this optical waveguide, the interaction between propagated light (guided light) and rare earth ions can be significantly enhanced, and thus a high-performance optical waveguide device based on rare earth ions, such as an optical waveguide type optical amplifier, a laser, and an on-chip optical quantum memory, can be realized.
The optical waveguide according to embodiments of the present invention can be manufactured by processing only the cap layer without requiring REO etching. Currently established general method for manufacturing a semiconductor device can be utilized for forming the cap layer. Therefore, it is extremely easy to manufacture the optical waveguide according to the present invention. The optical waveguide structure of embodiments of the present invention can also be applied to other functional materials grown on Si, and can realize a heterointegrated photonic device that cannot be realized by Si itself.
The present invention is not limited to the embodiment described above, and it is obvious that many modifications and combinations can be made by a person skilled in the art within the technical idea of the present invention.
This application is a national phase entry of PCT Application No. PCT/JP2021/007028, filed on Feb. 25, 2021, which application is hereby incorporated herein by reference.
Filing Document | Filing Date | Country | Kind |
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PCT/JP2021/007028 | 2/25/2021 | WO |