Optical element and method for manufacturing same

Abstract

The present invention provides an optical element having a grating coupler that can be removed in a post-process for evaluating characteristics of the optical element, and a method for manufacturing the optical element. Specifically, the present invention proposes a method for manufacturing an optical element having a removable grating coupler for evaluating the characteristics of the optical element, the method including: a step for forming, at both ends of the optical element, a first grating coupler for incidence of inspection light and a second grating coupler for emission of inspection light; and a step for removing the first grating coupler and the second grating coupler after evaluation of the characteristics of the optical element.

Description

Optical Element and Method for Manufacturing the Same

[0001] The present disclosure relates to an optical element and a method for manufacturing the same.

[0002] Nonlinear optical devices using periodically poled lithium niobate (PPLN) and the like are expected to have wide applications in the fields of optical communication and photonic quantum computers. In the manufacturing process of PPLN, evaluation of optical characteristics is an essential item. Conventionally, for optical characteristic evaluation, it is common to fabricate a smooth optical end face shape and perform evaluation from the end face of the optical element. However, PPLN is a difficult-to-process material, and there is a problem that processing the optical end face shape requires additional processes. How to simplify this evaluation process is an issue for mass production.

[0003] Japanese Patent Application Laid-Open No. 2019-086385

[0004] T. Kashiwazaki, et al., “Fabrication of low-loss quasi-single-mode PPLN waveguide and its application to a modularized broadband high-level squeezer,” Appl. Phys. Lett. 119, 251104 (2021).

[0005] The present disclosure has been devised to solve the above problems, and provides an optical element having a removable grating coupler for use in characteristic evaluation of the optical element, and a method for manufacturing the same.

[0006] Specifically, according to one embodiment, there is provided a method for manufacturing an optical element having a removable grating coupler for characteristic evaluation of the optical element, the method comprising: forming a first grating coupler for inspection light incidence and a second grating coupler for inspection light emission at both ends of the optical element; and removing the first grating coupler and the second grating coupler after the characteristic evaluation of the optical element.

[0007] In another embodiment, the manufacturing method is provided in which the removal step is one of the following: removing the formed grating coupler by chemical etching; physically separating a portion of the optical element on which the grating coupler is formed; or depositing a material identical to the grating coupler on the entire surface of the optical element on which the grating coupler is formed.

[0008] Furthermore, according to another embodiment, a manufacturing method is provided in which the grating cycle of the first grating coupler is different from that of the second grating coupler.

[0009] Furthermore, according to another embodiment, a manufacturing method is provided for an optical element using periodically polarized reversal lithium niobate (PPLN).

[0010] Specifically, according to one embodiment, an optical device is provided comprising an optical element and a cladding layer formed on the optical element, wherein the cladding layer has an uneven shape on both ends of the surface opposite to the surface on which the optical element is located.

[0011] According to another embodiment, when one end of an optical element having the above-mentioned uneven shape is designated as a first uneven region and the other end as a second uneven region, an optical device is provided in which the uneven period of the first uneven region is different from the uneven period of the second uneven region.

[0012] Furthermore, according to another embodiment, an optical device is provided in which the optical element is an optical element using periodically polarized reversal lithium niobate (PPLN).

[0013] Furthermore, according to another embodiment, an optical device is provided in which the cladding layer is a layer using SiO2.

[0014] (a) is a schematic diagram of the fabrication of a mechanically formed optical element, (b) is an enlarged top view of an optical element having a removable grating coupler, and (c) is a side view of the optical element. This is a schematic diagram of an inspection that shows that the optical properties of an optical element can be evaluated on a wafer by a grating coupler formed on the optical element, with (a) showing the case when a linear optical element is inspected and (b) showing the case when a nonlinear optical element is inspected. This is a diagram showing the fabrication and removal process of a removable grating coupler according to one embodiment, with (a) showing the state before the grating coupler is fabricated, (b) showing the state after the grating coupler is fabricated and (c) showing the state after the grating coupler is removed. This is a diagram showing the fabrication and removal process of a removable grating coupler according to one embodiment, with (a) showing the state before the grating coupler is fabricated, (b) showing the state after the grating coupler is fabricated and (c) showing the state after the grating coupler is removed. This figure shows the process of manufacturing and removing a removable grating coupler according to one embodiment, where (a) is the state before manufacturing the grating coupler, (b) is the state after manufacturing the grating coupler, and (c) is the state after removing the grating coupler. This is a flowchart showing the entire process of manufacturing an optical element, including a characterization step, according to one embodiment.

[0015] In recent years, silicon photonics (SiPh) have attracted attention as a technology for realizing small optical circuits at low cost, and research and development of SiPh optical circuits are actively being conducted (see Patent Document 1). Since the mounting and inspection processes account for a large portion of the manufacturing cost of optical transceivers, it is desirable to inspect SiPh optical circuits on the wafer, select good products, and then mount them as modules in order to reduce the cost of optical transceivers.

[0016] This disclosure provides an optical element structure and a method for manufacturing the same, which enable efficient evaluation of optical properties by providing a removable inspection grating coupler on the optical element without processing the cross-section of the optical element in order to evaluate the optical properties of the optical element.

[0017] Figure 1(a) shows a schematic diagram of the fabrication of mechanically formed optical elements. First, a ZnO-doped PPLN layer 20 is directly bonded onto a LiTaO3 wafer substrate 10. After bonding, multiple optical elements 30 are formed on the wafer using a diamond polishing machine and a resin mechanical saw 5.

[0018] Here, the optical element 30 is, as a non-limiting example, a nonlinear optical element, and specifically, an optical element 30 including a PPLN ridge waveguide 3. The fabricated optical element 30 can be used in a phase-sensitive optical amplifier or a wavelength converter.

[0019] The PPLN layer 20 has a thickness of approximately 8.0 μm (in the z direction), a width of approximately 8.6 μm (in the x direction) for each optical element, and a length of approximately 45 mm (in the y direction) for each optical element. The periodically polled pattern is created by an electrical polling process with a polling period of approximately 18 μm. The sidewall angle of this optical element is approximately 87°, which is higher than that of a typical dry-etched optical element, allowing for more efficient optical coupling in the symmetric mode of the optical fiber (see Non-Patent Literature 1).

[0020] Figure 1(b) is an enlarged top view of an optical element having a removable grating coupler as described herein, and Figure 1(c) is a side view of the optical element 30. For improved visibility, the wafer substrate 10 is omitted from Figures 1(b) and (c).

[0021] Removable grating couplers 40, described later, may be formed at both ends of the optical element 30. More specifically, a first grating coupler 40a for receiving inspection light may be formed on one end of the optical element 30, and a second grating coupler 40b for emitting inspection light may be formed on the other end.

[0022] The grating coupler 40 can be formed on the optical element 30 by a combination of photolithography and dry etching or wet etching. In a non-limiting example, the grating coupler 40 can be formed using polymer materials or oxides (e.g., SiO2).

[0023] A grating coupler is designed to efficiently input and output light between an optical element and an external optical fiber or free-space light. By forming a grating coupler with a periodic structure (grating) on ​​the optical element, the direction of light propagation and coupling efficiency can be controlled. Furthermore, by appropriately designing the period of the grating, the grating coupler can selectively couple / emit only specific wavelengths.

[0024] Figures 2(a) and 2(b) are schematic diagrams of an inspection process that shows how the optical properties of an optical element can be evaluated on a wafer using a grating coupler formed on the optical element. As shown in Figure 2(a), when evaluating a linear optical element, inspection light of frequency f incident on the first grating coupler 40a propagates within the optical element 30 and is emitted from the second grating coupler 40b. Since the optical element is linear, the emitted inspection light is of frequency f. The optical properties of the optical element are evaluated by measuring the coupling efficiency, insertion loss, or polarization dependence of the emitted inspection light relative to the incident inspection light.

[0025] On the other hand, as shown in Figure 2(b), when evaluating a nonlinear optical element, the inspection light of frequency f incident on the first grating coupler 40a propagates within the optical element 30 and is emitted from the second grating coupler 40b. Because the optical element is nonlinear, the frequency of the emitted inspection light is different from the frequency f of the incident inspection light. For example, the frequency of the emitted inspection light can be any value such as 2f or 0.5f. The optical properties of the optical element are evaluated by measuring the coupling efficiency, insertion loss, or polarization dependence of the emitted inspection light relative to the incident inspection light.

[0026] The grating coupler 40 formed on the surface of the optical element 30 allows the inspection light to be input from an oblique angle to the optical propagation direction of the optical element 30. Furthermore, it has the advantage of being able to evaluate multiple optical elements in a short time simply by moving the inspection light or the wafer substrate 10, for example, in the x direction.

[0027] The following describes the structure of a removable grating coupler formed on an optical element and its manufacturing method.

[0028] (Embodiment 1) Figure 3 is a diagram showing the process of manufacturing and removing a removable grating coupler according to the first embodiment. Figure 3(a) is a side view of the optical element showing the state before manufacturing the grating coupler, Figure 3(b) is a side view showing the state after manufacturing the grating coupler, and Figure 3(c) is a side view showing the state after removing the grating coupler. Furthermore, the process shown in Figure 3 corresponds to the second step S2 and the fourth step S4 of the flowchart 100 in Figure 6, which shows the entire process of manufacturing the optical element, including the characterization step.

[0029] Referring to Figure 1(a), the optical element 30 can be manufactured by the optical element manufacturing method described above S1 (see Figures 3(a) and 6). Then, the grating coupler pattern is patterned on both ends of the optical element 30, for example, by photolithography. Next, the first grating coupler 40a and the second grating coupler 40b are manufactured by depositing SiO2, for example S2 (see Figure 3(b)).

[0030] The grating coupler 40 can be formed using polymer materials or oxides (e.g., SiO2) as non-limiting examples, but any material with the same refractive index as the optical element is acceptable.

[0031] The grating period in the first grating coupler 40a may be the same as or different from the grating period in the second grating coupler 40b. For example, when inspecting a linear optical element as shown in Figure 2(a), the same grating period can be used for efficient evaluation. On the other hand, when inspecting a nonlinear optical element as shown in Figure 2(b), the inspection can be performed more effectively by changing the grating periods in the first grating coupler 40a and the second grating coupler 40b to match the frequencies of the incident and emitted inspection light.

[0032] Furthermore, the formed grating coupler 40 only needs to be able to connect with the optical element 30 with a certain degree of efficiency, and there is no need to design an optimal structure for the grating coupler.

[0033] S3. The characteristics of the optical element 30 on the wafer are evaluated by measuring the inspection light that is incident on the first grating coupler 40a, propagates through the optical element 30, and is emitted from the second grating coupler 40b.

[0034] After inspection, the grating coupler 40 is removed (S4). In the case of organic substances such as resists like SiO2 and SU-8, they can be removed with an appropriate organic solvent such as hydrofluoric acid or a sulfuric acid-based remover.

[0035] Finally, the cross-section of the optical element 30 is polished to form a clean optical end surface S5. Each optical element may be diced into chips as needed.

[0036] The above-described process for manufacturing and removing the removable grating coupler allows for easy characterization of the optical element on the wafer substrate. Furthermore, compared to conventional configurations (see Patent Document 1) in which a separately provided optical characteristic inspection circuit is diced from this circuit, the optical element according to this embodiment does not affect the optical characteristics of the final product because the core layer of the optical element is not processed.

[0037] (Embodiment 2) Figure 4 shows the process of manufacturing and removing a removable grating coupler according to the second embodiment. Figure 4(a) is a side view of the optical element showing the state before manufacturing the grating coupler, Figure 4(b) is a side view of the grating coupler after manufacturing the grating coupler, and Figure 4(c) is a side view of the optical element showing the state after the grating coupler has been removed. Furthermore, the process shown in Figure 4 corresponds to the second step S2 and the fourth step S4 of the flowchart 100 in Figure 6, which shows the entire process of manufacturing the optical element, including the characterization step.

[0038] In this embodiment, steps S1 (Figure 4(a)) for fabricating the optical element 30 and step S2 (Figure 4(b)) for forming the grating coupler 40 on the optical element 30 are the same as in the first embodiment described above, so their explanation is omitted and only the process of removal will be described.

[0039] S4: removing the grating coupler 40 after inspecting the optical characteristics. As shown in FIG. 4(c), removal of the grating coupler 40 can be achieved by physically cutting out both ends of the optical element 30 on which the grating coupler 40 is formed using a mechanical saw or the like.

[0040] S5: forming a clean optical end face by polishing the cross section of the optical element 30 finally. Note that each optical element may be diced into chips as needed.

[0041] (Embodiment 3) FIG. 5 is a diagram illustrating the fabrication and removal process of a removable grating coupler according to the third embodiment. FIG. 5(a) is a side view of the optical element showing a state before fabrication of the grating coupler, FIG. 5(b) shows a state after fabrication of the grating coupler, and FIG. 5(c) shows a state after removal of the grating coupler. Further, the process shown in FIG. 5 corresponds to the second step S2 and the fourth step S4 in the flowchart 100 of FIG. 6, which shows the entire process of fabricating an optical element including a characteristic evaluation step.

[0042] In the present embodiment, step S1 of fabricating the optical element 30 (FIG. 5(a)) and step S2 of forming the grating coupler 40 on the optical element 30 (FIG. 5(b)) are the same as those in the first embodiment and the second embodiment described above. Therefore, description thereof is omitted, and only the removal process will be described.

[0043] S4: removing the grating coupler 40 after inspecting the optical characteristics. As shown in FIG. 5(c), removal of the grating coupler 40 can be achieved by further performing vapor deposition on the entire surface of the optical element 30 including the grating coupler 40 using the same material as that of the grating coupler 40.

[0044] The deposited layer serves as a cladding layer 50 in the optical element 30. Since the cladding layer 50 is deposited with a uniform thickness on the entire surface of the optical element 30, the upper surface of the cladding layer 50 has the same uneven pattern as that of the grating coupler 40.

[0045] However, if the thickness of the cladding layer 50 is made sufficiently larger than the thickness of the grating coupler 40, the uneven pattern has almost no effect on the optical characteristics of the optical element 30.

[0046] Finally, in S5, a clean optical end face is formed by polishing the cross section of the optical element 30 including the cladding layer 50. Note that each optical element may be diced into chips as needed.

[0047] (Embodiment 4) As shown in FIG. 5(c), an optical element (e.g., PPLN) finally manufactured according to the third embodiment includes an optical element 30 and a cladding layer 50 formed on an upper surface of the optical element 30, and the cladding layer 50 has uneven shapes at both ends of the surface opposite to the surface where the optical element 30 is located.

[0048] As a non-limiting example, the cladding layer 50 may be formed using a polymer material or an oxide (e.g., SiO2), and may be formed of the same material as the grating coupler 40.

[0049] In the cladding layer 50, the grating period of the first uneven region 50a resulting from the first grating coupler 40a and the grating period of the second uneven region 50b resulting from the second grating coupler 40b may be the same. This is because when inspecting the linear optical element shown in FIG. 2(a), this originates from the pattern in which the first grating coupler 40a and the second grating coupler 40b are formed with the same grating period.

[0050] In addition, in the cladding layer 50, the grating period of the first uneven region 50a resulting from the first grating coupler 40a and the grating period of the second uneven region 50b resulting from the second grating coupler 40b may be different. This is because when inspecting the nonlinear optical element shown in FIG. 2(b), this originates from the fact that the grating period of the first grating coupler 40a and the grating period of the second grating coupler 40b are different in accordance with the frequency of incident inspection light and the frequency of outgoing inspection light.

[0051] (Additional Considerations) The foregoing description of embodiments of this disclosure is provided for illustrative purposes only and is not intended to be exhaustive or to limit to the exact forms disclosed. Those skilled in the art will understand that many modifications and changes are possible in light of the above disclosure.

[0052] Finally, the language used herein has been selected primarily for readability and instructional purposes, and may not be selected to describe or limit the subject matter of the invention. Therefore, the scope of the invention is intended to be limited by the appended claims, not by this detailed description. Accordingly, the disclosure of embodiments of the invention is intended to illustrate, not limit, the scope of the invention as described in the claims.

[0053] This disclosure can be applied to techniques for inspecting optical elements.

[0054] 1 On-wafer optical element 5 Mechanical saw 10 Wafer substrate 20 PPLN layer 30 Optical element 40a, b Grating coupler 50 Cladding layer 50a, b Uneven region

Claims

1. A method for manufacturing an optical element having a removable grating coupler for characterization of the optical element, comprising: forming a first grating coupler for the incidence of inspection light and a second grating coupler for the emission of inspection light at both ends of the optical element; and removing the first grating coupler and the second grating coupler after characterization of the optical element.

2. The manufacturing method according to claim 1, wherein the removal step is one of the following: removing the formed grating coupler by chemical etching; physically separating a part of the optical element on which the grating coupler is formed; or depositing a material identical to the grating coupler on the entire surface of the optical element on which the grating coupler is formed.

3. The manufacturing method according to claim 1 or 2, wherein the grating period of the first grating coupler is different from the grating period of the second grating coupler.

4. The manufacturing method according to claim 1 or 2, wherein the optical element is an optical element using periodically polarized reversal lithium niobate (PPLN).

5. An optical device comprising: an optical element; and a cladding layer formed on the optical element, wherein the cladding layer has an uneven shape on both ends of the surface opposite to the surface on which the optical element is located.

6. The optical device according to claim 5, wherein when one of the ends of the cladding layer on which the uneven shape is formed is designated as a first uneven region and the other end as a second uneven region, the uneven period of the first uneven region is different from the uneven period of the second uneven region.

7. The optical device according to claim 5 or 6, wherein the optical element is an optical element using periodically polarized reversal lithium niobate (PPLN).

8. The optical device according to claim 5 or 6, wherein the cladding layer is a layer using SiO2.

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