Patent Application
20240329316
The present invention relates to an optical device.
Communication traffic has been remarkably increased with widespread Internet use, and optical fiber communication is becoming significantly important. The optical fiber communication is a technology that converts an electric signal into an optical signal and transmits the optical signal through an optical fiber and has the characteristics of wide bandwidth, low loss, and resistance to noise.
Patent document 1 (JP 2007-114253) discloses a waveguide type light splitting element, which splits the light incident from two input-side optical fibers to multiple output-side optical fibers.
In the light splitting element in patent document 1, the transmission loss is reduced by tilting a sidewall of a coupling waveguide located in a forestage of a multiplexed portion. However, it is expected to further reduce the transmission loss during multiplexing.
The present invention is completed in view of the above problems, and its object is to provide an optical device capable of reducing transmission loss during multiplexing light.
The present application provides an optical device, comprising: a substrate; and at least two optical waveguides formed on the substrate and facing to a multiplexed portion on the substrate in a light propagation direction, the optical waveguide comprises a lamellar portion and a protruding portion which protrudes from the lamellar portion, in a cross section perpendicular to the light propagation direction, the thickness of the lamellar portion between the protruding portions of the at least two optical waveguides is greater than the thickness of the lamellar portion at the side of a side surface of the protruding portion of the optical waveguides, which does not face to other optical waveguides.
In the optical device according to the present invention, since in a cross section perpendicular to the light propagation direction, the thickness of the lamellar portion between the protruding portions of the at least two optical waveguides is greater than the thickness of the lamellar portion at the side of a side surface of the protruding portion of the optical waveguide, which does not face to other optical waveguides, the light propagating through the at least two optical waveguides tends to propagate between the optical waveguides when approaching the multiplexed portion, so that the optical loss on the outer sides of the optical waveguides is reduced, thus reducing the light transmission loss during multiplexing light.
Further, in the optical device according to the present invention, preferably, in the cross section perpendicular to the light propagation direction, the thickness of the lamellar portion between the protruding portions of the at least two optical waveguides is less than ½ of the distance from a top end of the protruding portion to the substrate. Therefore, the light transmission loss can be further reduced.
Further, in the optical device according to the present invention, preferably, the optical device further comprises a protection layer formed adjacent to the optical waveguides.
Further, in the optical device according to the present invention, preferably, the optical waveguides are formed of an oxide containing lithium.
Further, in the optical device according to the present invention, preferably, the oxide containing lithium is epitaxial lithium niobate or lithium tantalate.
Further, in the optical device according to the present invention, preferably, in the cross section perpendicular to the light propagation direction, an angle formed between the substrate and the side surface of the protruding portion of the optical waveguide, which does not face to other optical waveguides, is 85° or more. Therefore, the light transmission loss on the outer sides of the at least two optical waveguides during multiplexing light can be further avoided.
The optical device according to the present invention can effectively reduce the light transmission loss during multiplexing light.
The optical devices involved in the embodiments will be described below. In the description of each figure, the same or equivalent elements are marked with the same reference number, and sometimes repeated description is omitted.
Referring to
In the optical device 1, in a case that H1 of the lamellar portion between the optical waveguides is small, when the input light respectively from the optical waveguides 21, 22 are multiplexed at the multiplexed portion 40, optical loss is easy to occur since the lamellar portion between the optical waveguides is thin. Oppositely, in this embodiment, the height H1 of the portion 202b located between the optical waveguides 21, 22 is set to be greater than the height H3 of 202a, that is, H1>H3. Therefore, compared with the case where H1<H3, the light propagating through the optical waveguides 21, 22 tends to propagate between the optical waveguides (an area between the optical waveguide 21, 22) when approaching the multiplexed portion 40, so that the optical loss on the outer sides of the optical waveguides 21, 22 is reduced. Therefore, the light transmission loss during multiplexing light can be reduced.
Further, in the X-Z cross section perpendicular to the light propagation direction, the distance from the top end of the protruding portion 201 to the substrate 10 is set as H2. In the optical device 1 according to this embodiment, H1≤½*H2. By setting H1 to be less than or equal to ½*H2, compared with the case where H1>½*H2, the light tends more greatly to propagate along the extension direction of the optical waveguides 21, 22 in the X-Z cross section. Therefore, the light transmission loss can be further reduced.
Referring to
Optical waveguides 21, 22, 29 are not particularly limited as long as they are made of an electro-optical material. The optical waveguides are preferably formed of an oxide containing lithium. The optical waveguides are more preferably composed of epitaxial lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). This is because lithium niobate or lithium tantalate has a large electro-optical constant and is suitable as a constituent material for optical devices such as light modulators. Hereafter, the structure of the present invention will be described below in detail in a case that the optical waveguides 21, 22, 29 are lithium niobate film.
The substrate 10 is not particularly limited as long as it has a lower refractive index than the lithium niobate film, but it is preferable a substrate on which a lithium niobate film can be formed as an epitaxial film, and a sapphire single crystal substrate or a silicon single crystal substrate is preferable. The crystal orientation of the single crystal substrate is not particularly limited. The lithium niobate film has properties such as being easily formed as a c-axis-oriented epitaxial film with respect to single crystal substrates of various crystal orientations. Since the c-axis oriented lithium niobate film has three-fold symmetry, it is desirable that the underlying single crystal substrate also has the same symmetry. In the case of a sapphire single crystal substrate, a c-plane substrate is preferred, and in the case of a silicon single crystal substrate, a (111) plane substrate is preferred.
Here, the epitaxial film is a film oriented in alignment with the crystal orientation of the underlying substrate or underlying film. When the film plane is defined as the XY plane and the film thickness direction is defined as the Z axis, the crystals are aligned and oriented along the X, Y and Z axes. For example, the epitaxial film can be verified by first confirming the intensity at the orientation position by 2θ-θ X-ray diffraction and secondly confirming the pole.
Specifically, first, when measurement is performed by 2θ-θ X-ray diffraction, the peak intensity of all peaks other than the target surface is 10% or less, preferably 5% or less, of the maximum peak intensity of the target surface. For example, in a c-axis oriented epitaxial film of lithium niobate, the peak intensity of planes other than the (00L) plane is 10% or less, preferably 5% or less of the maximum peak intensity of the (00L) plane. (00L) is a generic term for equivalent planes such as (001) and (002).
Secondly, poles must be observed in the measurement. Under the condition where the peak intensities are measured at the first orientation position, only the orientation in a single direction is proved. Even if the first condition is satisfied, in the case of nonuniformity in the in-plane crystalline orientation, the X-ray intensity is not increased at a particular angle, and poles cannot be observed. Since LiNbO3 has a trigonal crystal system, single-crystal LiNbO3 (014) has 3 poles. For the lithium niobate film, it is known that crystals rotated by 180° about the c-axis are epitaxially grown in a symmetrically-coupled twin crystal state. In this case, three poles are symmetrically-coupled to form six poles. When the lithium niobate film is formed on a single-crystal silicon substrate having a (100) plane, the substrate has four-fold symmetry, and 4×3=12 poles are observed. In the present invention, the lithium niobate film epitaxially grown in the twin crystal state is also considered to be an epitaxial film.
The lithium niobate film has a composition of LixNbAyOz. A denotes an element other than Li, Nb, and O, wherein x ranges from 0.5 to 1.2, preferably 0.9 to 1.05, y ranges from 0 to 0.5, and z ranges from 1.5 to 4, preferably 2.5 to 303.5. Examples of the element A include K, Na, Rb, Cs, Be, Mg, Ca, Sr, Ba, Ti, Zr, Hf, V, Cr, Mo, W, Fe, Co, Ni, Zn, Sc, and Ce, alone or a combination of two or more of them.
The lithium niobate film is preferably formed using a film formation method, such as sputtering, CVD or sol-gel process. Application of an electric field in parallel to the c-axis of the lithium niobate that is oriented perpendicular to the main surface of the substrate can change the optical refractive index in proportion to the electric field.
In the case of the single-crystal substrate made of sapphire, the lithium niobate film can be directly epitaxially grown on the sapphire single-crystal substrate. In the case of the single-crystal substrate made of silicon, the lithium niobate film is epitaxially grown on a clad layer (not illustrated). The clad layer (not illustrated) has a refractive index lower than that of the lithium niobate film and should be suitable for epitaxial growth. As a formation method for the lithium niobate film, there is known a method of thinly polishing or slicing the lithium niobate single crystal substrate. This method has an advantage that characteristics same as those of the single crystal can be obtained and can be applied to the present invention.
The difference between embodiment 2 and embodiment 1 is that there are two or more (in this case, three) optical waveguides 23, 24, 25 in embodiment 2. Hereafter, description will be made to the difference between embodiment 1 and embodiment 2.
Referring to
In the optical device 1A, in a case that H1 of the lamellar portion between the optical waveguides is small, when the input light respectively from the optical waveguides 23, 24, 25 are multiplexed at the multiplexed portion 40A, optical loss is easy to occur since the lamellar portion between the optical waveguides is thin. Oppositely, in this embodiment, the height H1 of the portion 203b located between the optical waveguides 23˜25 is set to be greater than the height H3 of 203a, that is, H1>H3. Therefore, compared with the case where H1<H3, the light propagating through the optical waveguides 23˜25 tends to propagate between the optical waveguides (an area between the optical waveguides 23, 24 and between the optical waveguides 24, 25) when approaching the multiplexed portion 40A, so that the optical loss on the outer sides of the optical waveguides 23, 25 is reduced. Therefore, the light transmission loss during multiplexing light can be reduced.
Further, in the X-Z cross section perpendicular to the light propagation direction, the distance from the top end of the protruding portion 201A to the substrate 10A is set as H2. In the optical device 1A according to this embodiment, H1≤½*H2. By setting H1 to be less than or equal to ½*H2, compared with the case where H1>½*H2, the light tends more greatly to propagate along the extension direction of the optical waveguides 23˜25 in the X-Z cross section. Therefore, the light transmission loss can be further reduced. In addition, referring to
The difference between embodiment 3 and embodiment 1 is that angles between side surfaces of optical waveguides 26, 27 and the substrate in embodiment 3 are different. Hereafter, description will be made to the difference between embodiment 1 and embodiment 3.
Referring to
Angles α formed between the substrate 10B and the side surfaces 261, 272, of the protruding portions 201B of the optical waveguides 26, 27 which do not face to other optical waveguides, are 85° or more and 90° or less. Moreover, the side surfaces 262, 271 of the optical waveguides 26, 27 which face to other optical waveguides may also be inclined to the substrate. As a result, the light propagating through the optical waveguides 26, 27 further tends to propagate between the optical waveguides (an area between the optical waveguides 26, 27) when approaching the multiplexed portion 40. Therefore, the light transmission loss during multiplexing light can be further reduced. Here, ideally, it is expected that the angles α formed between the side surfaces 261, 272 of the optical waveguides 26, 27 and the substrate 10B are 90°. From the perspective of manufacturing errors and manufacturing ease, the angle is preferably 85° or more.
Examples 1 and 2 were made according to the structure of the optical device 1 in embodiment 1, and example 3 was made according to the structure of the optical device 1C in embodiment 3. A comparative example was made according to the structure of the optical device illustrated in
A transmission loss test of light passing through the multiplexed portion was performed on the examples and the comparative example. The experimental results are as shown in Table 1.
From Table 1, it can be seen that according to the comparison between examples 1, 2 and the comparative example, the transmission loss can be significantly reduced, through that the height H1 of the portion between the two optical waveguides is greater than the height H3 of the portion at the side of the side surface of the protruding portion of the optical waveguide, which does not face to other optical waveguides. In addition, according to the comparison between example 2 and example 3, the transmission loss can be further reduced by configuring the side surfaces of the protruding portions of the optical waveguides 21, 22, which do not face to other optical waveguides, to be perpendicular to the substrate.
Although the present invention has been described in detail in conjunction with the embodiments with reference to the drawings, it can be understood that the above description does not limit the present invention in any form. The optical devices of the present invention may include, for example, a directional coupler, a multimode interference coupler, a Y-junction coupler, and various interferometers.
Those skilled in the art can make variations and changes to the present invention as needed without deviating from the substantive spirit and scope of the present invention, all of variations and changes still fall within the scope of protection of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 202310332220.3 | Mar 2023 | CN | national |
