Planar lightwave circuit and production method

Abstract

The present disclosure provides a planar lightwave circuit that enables a reduction in coupling loss upon connection to an optical element. Specifically, provided is a planar lightwave circuit characterized in that: the thickness of part of a core layer varies symmetrically with respect to the center of a core cross-section obtained by horizontally cutting the core layer in the thickness direction thereof; and the center position of each input port of two or more optical waveguides is located at the same height from the bottom surface of a lower cladding layer, or the center position of each output port of two or more optical waveguides is located at the same height from the bottom surface of the lower cladding layer.

Description

Planar lightwave circuits and manufacturing methods

[0001] The present disclosure relates to a planar lightwave circuit, and in particular to a structure of a planar lightwave circuit that can convert the spot diameters of the input and output ports of the planar lightwave circuit, reduce coupling loss when connecting to an optical element, and align the center heights of the core cross sections of all ports, and a method for manufacturing the same.

[0002] Planar lightwave circuits having planar optical waveguides made of silica-based materials, silicon, or semiconductors are often connected to other optical elements or optical fibers, posing a challenge in terms of improving the optical coupling efficiency at the connection. To address this challenge, a common approach has been to widen or narrow the core width of the optical waveguide at the optical input / output port and its vicinity in the planar lightwave circuit compared to the core width of the other optical waveguides. This approach allows the mode field diameter in the core width direction to be aligned with that of the other optical elements or optical fibers, thereby improving the optical coupling efficiency.

[0003] However, even with the above-mentioned techniques, there is a limit to the improvement in coupling efficiency. One reason for this is the expanding wavelength range input to planar lightwave circuits. Traditionally, planar lightwave circuits have been used as optical communication devices only in the communication wavelength range. However, in recent years, there have been many reports of planar lightwave circuits used in the visible range, and even reports of inputting light from the visible range to the near-infrared range into the same planar lightwave circuit. Planar lightwave circuits that handle such broadband light have a limit to the core width that can be fabricated, which can result in excitation of higher-order modes, a deterioration in optical coupling efficiency, or both.

[0004] Another method is to widen or narrow the core thickness, rather than the core width, of the optical waveguide at the optical input / output port of the planar lightwave circuit and its surrounding area. The coupling efficiency can be improved by aligning the mode field diameter in the core thickness direction with that of other optical elements or optical fibers. Furthermore, by combining this with the aforementioned method of changing the core width, further improvements in optical coupling efficiency are expected.

[0005] Patent Document 1 proposes a structure for connecting an optical fiber and a semiconductor element using a tapered optical waveguide, which has a structure that absorbs thermal expansion and contraction, as a separate component. This proposal makes it possible to input a high-power beam output from a semiconductor element into optical fibers with different diameters. However, this proposal requires different components and an additional process for connecting the components, which results in more work and higher costs than fabricating them on the same wafer.

[0006] Patent Document 2 proposes a tapered structure created by using a core material with a lower softening temperature than the lower cladding layer and hot-pressing the core layer. This proposal achieves a tapered structure in the thickness direction. However, this proposal requires a hot-pressing technique different from conventional photolithography. Furthermore, depending on the material, a very high softening temperature of around 1000°C is required. This makes it difficult to achieve. Patent Document 3 proposes a method of changing the core thickness by etching the lower cladding layer. This proposal achieves an optical waveguide with different core thicknesses. However, this proposal has the problem that the height of the core center changes depending on the core thickness. This problem also exists in Patent Document 2.

[0007] Patent Document 3 proposes a method of changing the core thickness by etching the lower cladding layer. This proposal realizes optical waveguides with different core thicknesses. However, this proposal has the problem that the height of the core center changes depending on the core thickness. This problem also occurs in Patent Document 2.

[0008] Not limited to the above-mentioned patent documents, conventional methods of changing the core thickness within the same wafer generally change the thickness only above or below the center of the core. As a result, when cores of different thicknesses are used for each optical input / output port, the height of the center of the core differs for each port. Therefore, when connecting to optical elements such as existing fiber blocks and fiber arrays, which have the same height at the center of the core, there is a problem of large connection loss.

[0009] Japanese Patent Application Laid-Open No. 2001-83374 Japanese Patent Application Laid-Open No. 2005-55576 Japanese Patent No. 6112606

[0010] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a planar lightwave circuit that can convert the spot diameters of the input and output ports of a planar lightwave circuit, reduce coupling loss when connecting to an optical element, and align the center heights of the core cross sections of all ports.

[0011] According to one embodiment of the present disclosure, there is provided a planar lightwave circuit having two or more optical waveguides each comprising a lower cladding layer, a core layer deposited on the lower cladding layer, and an upper cladding layer deposited on the core layer, wherein the thickness of a portion of the core layer varies symmetrically with respect to the center of a core cross section cut horizontally in the thickness direction of the core layer, and the central positions of the input ports of the two or more optical waveguides are located at the same height from the bottom surface of the lower cladding layer, or the central positions of the output ports of the two or more optical waveguides are located at the same height from the bottom surface of the lower cladding layer.

[0012] According to one embodiment of the present disclosure, there is provided a method for manufacturing the above-mentioned planar lightwave circuit, the method including the steps of depositing a lower cladding layer on a substrate, etching a portion of the lower cladding layer parallel to, perpendicular to, or obliquely to the thickness direction, depositing a core layer on the lower cladding layer, etching a portion of the core layer parallel to, perpendicular to, or obliquely to the thickness direction, etching the lower cladding layer and the core layer into a desired optical waveguide pattern, and depositing an upper cladding layer on the core layer.

[0013] 1A and 1B are diagrams showing a planar lightwave circuit according to one embodiment, where (a) is a top view of the planar lightwave circuit, (b) is a cross-sectional view of a portion (A-A') of an optical waveguide of the planar lightwave circuit, and (c) is a side view of the planar lightwave circuit as viewed from the direction in which light propagates. A diagram showing a cross-section of a portion (A-A') of an optical waveguide of a planar lightwave circuit according to another embodiment. A diagram showing a cross-section of a portion (A-A') of an optical waveguide of a planar lightwave circuit according to another embodiment. A diagram showing an optical fiber module including a planar lightwave circuit according to one embodiment, where (a) is a top view of the optical fiber module and a cross-sectional view of a portion (A-A') of an optical waveguide of the planar lightwave circuit, and (c) is a side view of the planar lightwave circuit as viewed from the direction in which light propagates. A diagram showing an outline of a method for fabricating a planar lightwave circuit according to an embodiment of the present disclosure.

[0014] The embodiments described below are examples of the present disclosure, and the invention of the present disclosure is not limited to the following embodiments. In the description of the present embodiments, the substrate side may be referred to as "bottom" and the direction in which the cladding material and core material are deposited as "top."

[0015] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. FIGS. 1(a) to 1(c) are diagrams illustrating a planar lightwave circuit 100 according to an embodiment of the present disclosure. FIG. 1(a) is a top view of the planar lightwave circuit 100 as viewed from a direction perpendicular to the light propagation direction (i.e., the Z direction in FIG. 1(a)). FIG. 1(b) is a cross-sectional view of a portion (A-A') of the planar lightwave circuit 100 of FIG. 1(a) as viewed from the X direction. FIG. 1(c) is a side view of the planar lightwave circuit 100 of FIG. 1(a) as viewed from a direction horizontal to the light propagation direction (the Z direction) (B-B').

[0016] The planar lightwave circuit 100 includes a plurality of optical waveguides 101. The optical waveguides 101 are composed of a core layer 103, a lower cladding layer 105, and an upper cladding layer 107, and extend along the direction of light propagation. The optical waveguides 101 include a plurality of (e.g., N) input ports for inputting light and a plurality of (e.g., M) output ports for outputting light to optical elements including space or optical fibers. Here, N and M can be 1 or greater.

[0017] The planar lightwave circuit 100 can be made of, for example, a silica-based material and silicon. The core layer 103 can be made of, for example, germanium-doped SiO. The lower cladding layer 105 can be made of, for example, pure SiO without doping, and the upper cladding layer 107 can be made of SiO doped with boron or phosphorus.

[0018] The refractive indices of the core layer 103, the lower cladding layer 105, and the upper cladding layer 107 can be controlled by changing the concentration of the dopant. The refractive index of the upper cladding layer 107 can be adjusted to be the same as that of the lower cladding layer 105. The concentration of germanium doped into the core layer can be adjusted so that the refractive index of the core layer 103 is higher than those of the lower cladding layer 105 and the upper cladding layer 107. For example, the relative refractive index difference between the core layer 103 and each of the cladding layers 105 and 107 can be 0.2% to 4.0%. As an example, in this embodiment, the relative refractive index difference is 1%.

[0019] 1(b) shows a cross-sectional view of a portion (A-A') near the output port 111 in the planar lightwave circuit 100 of FIG. 1(a). For convenience, FIG. 1(b) shows only a portion near the output port 111 as an example, but a similar structure can also be formed near the input port 109. The optical waveguide 101 of the planar lightwave circuit according to this embodiment has a structure in which the thickness of the core layer 103 in a portion (e.g., 103b) near the output port 111 is thinner than the thickness of other portions (e.g., 103a) of the core layer 103.

[0020] The lower cladding layer 105 is etched in the thickness direction (Y direction) to thin a portion of the lower cladding layer 105. Next, the core layer 103 is deposited, and then the core layer 103 is thinned by etching in a manner symmetrical to the lower cladding layer 105. Thereafter, the upper cladding layer 107 is laminated on the core layer 103. By using the above method, it is possible to create a core layer 103 having different thicknesses for each core portion, and it is possible to make the amount of change in thickness above the center of the core (P in FIG. 1(b)) equal to the amount of change in thickness below the center of the core.

[0021] The core layer 103 and the lower cladding layer 105 can be etched in the thickness direction as shown in Fig. 1(b), but it is also possible to change the cladding pattern of the lower cladding layer 105 by tilting the substrate and etching obliquely. The optical waveguide 101 of the planar lightwave circuit in which the lower cladding layer 105 and the core layer 103 are obliquely etched will be described later with reference to Fig. 3.

[0022] 1(c) is a side view of the planar lightwave circuit 100 of FIG. 1(a) viewed from a direction horizontal to the direction of light propagation (Z direction). The heights of the centers of the cores of the input ports 109 (h in FIG. 1(c)) are formed to be equal. The heights of the centers of the cores of the output ports 111 are also formed to be equal. In other words, the centers of the input ports 109 are located on a straight line at the same height from the bottom surface of the lower cladding layer 105, and the centers of the output ports 111 are located on a straight line at the same height from the bottom surface of the lower cladding layer 105. Furthermore, the heights between the input ports 109 and the output ports 111 may also be formed to be equal.

[0023] The thickness of the core layer (t in FIG. 1(c)) of each input port 109 and output port 111 differs for each port, because it differs depending on the shape of the optical fiber to which it is output, the desired beam diameter, and the wavelength of the light to be output. Because the height (h) of the center of the core is the same, even when connecting to an existing fiber block, connection loss due to misaligned core heights can be reduced.

[0024] 2 and 3 show examples of structures other than the structure of A-A' shown in Fig. 1(b) near the output port 111. The structures shown in Fig. 2 and 3 make it possible to change the core thickness with lower loss than the structure shown in Fig. 1(b).

[0025] The structure of the planar lightwave circuit according to the embodiment shown in Figure 2 differs in the shape of the core layer and cladding layer near the input / output ports. The core layer 103 in this embodiment has more steps than the core layer in Figure 1(b) (i.e., the core layer near the input / output ports in this embodiment has a stepped structure). In Figure 2, the core layer 103 is shown as having a structure in which the thickness changes twice along the Y direction, but the number of changes is not limited to two, and the core layer 103 may be fabricated so that the number of changes is greater (e.g., three or four times).

[0026] The manufacturing method will be described in detail later, but briefly, first, the lower cladding layer 105 is deposited on the substrate 113. Next, a portion of the lower cladding layer 105 is thinned by etching in the thickness direction (Y direction). By repeating patterning and etching of the lower cladding layer 105, the thickness of the lower cladding layer 105 can be changed in stages. Next, the core layer 103 is deposited. By repeating patterning and etching of the core layer 103, the core layer 103 is etched symmetrically to the lower cladding 105, and the thickness of the core layer 103 can be changed in stages. Furthermore, the upper cladding layer 107 is laminated on the core layer 103.

[0027] By using the above-described technique, for example, it is possible to fabricate an optical waveguide 101 in which the thickness of the core layer 103 is stepped and the upper and lower sides are symmetrical with respect to the core center (P). Furthermore, the amount of change in the core thickness above the core center (P) can be made equal to the amount of change in the core thickness below the core center (P). Therefore, since the heights (h) of the core centers of the input port 109 and / or the output port 111 are equal, connection loss due to misaligned core heights can be reduced even when connecting to an existing fiber block.

[0028] FIG. 3 shows another example of a structure in which the thickness of the core layer 103 changes continuously. First, a lower cladding layer 105 is deposited on a substrate 113. Next, a portion of the lower cladding layer 105 is thinned by oblique etching at a certain angle relative to the thickness direction (Y direction). By performing oblique etching, the thickness of the lower cladding layer 105 can be changed continuously. Next, a core layer 103 is deposited. The core layer 103 is etched symmetrically to the lower cladding 105, so that the thickness of the core layer 103 can be changed continuously. Furthermore, an upper cladding layer 107 is laminated on the core layer 103.

[0029] The above technique makes it possible to create, for example, a tapered structure in the thickness direction in which the core thickness 103 changes continuously. Furthermore, the amount of change in core thickness above the core center (P) can be made equal to the amount of change in core thickness below the core center (P). Therefore, because the heights (h) of the centers of the input and / or output ports are equal, connection loss due to misaligned core heights can be reduced even when connecting to an existing fiber block.

[0030] The structure of this embodiment will be described in more detail below with reference to Fig. 4. Fig. 4(a) is a top view of the planar lightwave circuit 100 shown in this embodiment and an optical fiber 401 connected to the input port 109 of the planar lightwave circuit (hereinafter referred to as the "optical fiber module 400"), as viewed from a direction perpendicular to the direction of light propagation in the planar lightwave circuit.

[0031] 4(b) is a cross-sectional view of a portion (A-A') of the planar lightwave circuit 100, and FIG. 4(c) is a side view of the output port of the planar lightwave circuit 100 in FIG. 4(a) when viewed from a direction horizontal to the direction in which light propagates. The planar lightwave circuit 100 is formed on a substrate 113 and includes a plurality of optical waveguides 101. The optical waveguides 101 are composed of a core layer 103, a lower cladding layer 105, and an upper cladding layer 107.

[0032] 4(a), the optical waveguide 101 includes a plurality of input ports (e.g., 109a to 109d), a plurality of output ports (e.g., 111a to 111e), and a plurality of waveguide sections disposed between the plurality of input ports and the plurality of output ports and configured to connect the input and output ports. The plurality of input ports 109a to 109d can be connected to input optical fibers 401a to 401d, respectively.

[0033] As an example, the input optical fibers 401a to 401d are a first input optical fiber 401a that inputs light with a wavelength of 600 nm, a second input optical fiber 401b that inputs light with a wavelength of 800 nm, a third input optical fiber 401c that inputs light with a wavelength of 1400 nm, and a fourth input optical fiber 401d that inputs light with a wavelength of 1600 nm. The wavelengths of light shown above are merely examples and may be changed depending on the field or application in which the optical fiber module 400 is used.

[0034] Each of the input optical fibers 401a to 401d has a core diameter suitable for handling each wavelength. As an example, the first input optical fiber 401a and the second input optical fiber 401b use optical fibers with a core diameter of 3 μm, and the third input optical fiber 401c and the fourth input optical fiber 401d use optical fibers with a core diameter of 7 μm.

[0035] As an example, the output ports 111a to 111e are a first output port 111a provided for monitoring light input from the first input optical fiber 401a, a second output port 111b provided for monitoring light input from the second input optical fiber 401b, a third output port 111c provided for monitoring light input from the third input optical fiber 401c, a fourth output port 111d provided for monitoring light input from the fourth input optical fiber 401d, and a fifth output port 111e provided for outputting a combined light of the light input from the first input optical fiber 401a to the fourth input optical fiber 401d.

[0036] As an example, the core thicknesses and widths of the first input port 109 a, the second input port 109 b, the fifth output port 111 e for outputting the combined light, the first output port 111 a, and the second output port 111 b are all 2 μm, while the core thicknesses and widths of the other input / output ports (e.g., the third input port 109 c, the fourth input port 109 d, the third output port 111 c, and the fourth output port 111 d) are all 6 μm. The above core thicknesses are those that enable output of light of each wavelength and combined light of wavelengths 600 nm, 800 nm, 1400 nm, and 1600 nm in approximately the fundamental mode.

[0037] Next, a method for manufacturing the planar lightwave circuit of this embodiment will be described with reference to Fig. 5. Fig. 5 is a diagram showing an overview of the method for manufacturing the planar lightwave circuit 100, and the diagrams of each step show a cross-sectional view of the planar lightwave circuit taken along line A-A' in Fig. 4(a).

[0038] (Lower Cladding Layer) First, the lower cladding layer 105 is deposited on the substrate 113 (Step 1). The lower cladding layer 105 is deposited by flame deposition and then heat-treated. In this embodiment, the thickness of the lower cladding layer 105 is 20 μm. The substrate 113 is a 6-inch non-doped silicon wafer with a thickness of 1 mm.

[0039] Next, the lower cladding layer 105 is etched (step 2). In this embodiment, the objective is to reduce the thickness of the core layer 103 near five locations: the first input port 109a and the second input port 109b, which input light with wavelengths of 600 nm and 800 nm; the fifth output port 111e, which outputs the combined light; and the first output port 111a and the second output port 111b, which monitor light with wavelengths of 600 nm and 800 nm. Photolithographic patterning is performed on the lower cladding layer within a range of 200 μm from each of the five port end faces in a direction parallel to the light propagation direction (Z direction) and 2 μm in a direction perpendicular to the light propagation direction (Y direction). Next, etching is performed in the thickness direction (Y direction) by 2 μm (see FIG. 4(b)). The resist is then removed, completing the lower cladding layer 105 etched by 2 μm in areas other than the above-mentioned ranges. The above etching methods and etching directions are merely examples, and any other etching methods and directions that have the same effect may be used.

[0040] (Core Layer) Next, the core layer 103 is deposited on the etched lower cladding layer 105 (Step 3). In this embodiment, the core layer 103 is deposited to a thickness of 6 μm using a film deposition apparatus (see FIG. 4(b)). The core layer 103 is then heat-treated at 1000° C. to 1400° C. for planarization. Additionally and / or alternatively, if planarization is insufficient in the heat treatment step, the core layer 103 may be polished.

[0041] Next, the end faces of the five ports are patterned by photolithography on the entire core layer 103, except for a 200 μm area parallel to the light propagation direction and a 2 μm area perpendicular to the light propagation direction. Then, as shown in FIG. 4(d), etching is performed in the thickness direction (Y direction) by 2 μm (Step 4). This completes the core layer 103 with a 2 μm etched thickness within the above range (see FIG. 4(d)). Etching does not have to be performed in the thickness direction. Note that ports other than the five ports (e.g., the third input port 109c, the fourth input port 109d, the third output port 111c, and the fourth output port 111d) use a 6 μm-thick core, and therefore do not require an etching process.

[0042] Next, a method for fabricating the optical waveguide 101 will be described. Photolithography and etching are performed on the core layer 103 to form the desired optical waveguide (step 5). In this embodiment, the optical waveguide 101 is fabricated to have a width of either 2 μm or 6 μm, depending on the wavelength of the input light. The optical waveguide is patterned by photolithography to have a width of 2 μm for a total of five end faces, including the first input port 109a and the second input port 109b, which input light with wavelengths of 600 nm and 800 nm, the fifth output port 111e, which outputs the combined light, and the first output port 111a and the second output port 111b, which monitor light with wavelengths of 600 nm and 800 nm. The five end faces are then etched to a depth of 2 μm or more in the thickness direction (Y direction).

[0043] The structure of the waveguide, including the optical multiplexing method and core width, may be an optical waveguide structure combining a Y-branch and a cross optical waveguide as shown in Figure 4(a), or may be another waveguide structure. Furthermore, when changing the core width, a tapered structure or another structure may be used.

[0044] Next, the upper cladding layer 107 is deposited on the core layer 103 (step 6). The upper cladding layer 107 is deposited by flame deposition and then heat-treated. In this embodiment, the thickness of the upper cladding layer 107 is 20 μm or more.

[0045] Finally, the stacked planar lightwave circuit 100 is cut with a dicing machine so that the core end faces of the input and output ports are exposed at the end faces of the planar lightwave circuit. The end faces of the planar lightwave circuit 100 are then polished to smooth the surfaces of the input and output end faces. Fiber blocks are then connected to the input and output ports using a UV adhesive, completing the optical fiber module 400 shown in FIG. 4( a).

[0046] Therefore, according to this embodiment, it is possible to provide a planar lightwave circuit that can convert the spot diameters of the input and output ports of the planar lightwave circuit, reduce the coupling loss when connecting to an optical element, and align the center heights of the core cross sections of all ports.

[0047] Additional Considerations The foregoing description of embodiments of the present invention has been presented for purposes of illustration and is not intended to be exhaustive or to be limited to the precise form disclosed. Those skilled in the art will recognize that many modifications and variations are possible in light of the above disclosure.

[0048] Finally, the language used herein has been selected primarily for readability and instructional purposes, and may not have been selected to delineate or limit the subject matter of the invention. Accordingly, it is intended that the scope of the invention be limited not by this detailed description, but rather by the appended claims. Accordingly, the disclosure of embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the claims.

[0049] 100 Planar lightwave circuit 101 Optical waveguide 103 Core layer 105 Lower cladding layer 107 Upper cladding layer 109 Input port 111 Output port 113 Substrate 400 Optical fiber module 401a to 401d Input optical fibers

Claims

1. A planar lightwave circuit having two or more optical waveguides each comprising a lower cladding layer, a core layer deposited on said lower cladding layer, and an upper cladding layer deposited on said core layer, wherein the thickness of a portion of said core layer varies symmetrically with respect to the center of a core cross section cut horizontally in the thickness direction of said core layer, and the centers of the input ports of said two or more optical waveguides are located at the same height from the bottom surface of said lower cladding layer, or the centers of the output ports of said two or more optical waveguides are located at the same height from the bottom surface of said lower cladding layer.

2. A planar lightwave circuit according to claim 1, wherein the centers of said input ports and said output ports are all located on the same straight line parallel to the bottom surface of said lower cladding layer.

3. A planar lightwave circuit according to claim 1, wherein the thickness of said symmetrically varying portion of said core layer changes stepwise or continuously two or more times.

4. The planar lightwave circuit according to claim 1, wherein the cross-sectional shapes of said input port and said output port are rectangular.

5. A method for fabricating a planar lightwave circuit, comprising the steps of: depositing a lower cladding layer on a substrate; etching a portion of the lower cladding layer in a direction parallel to, perpendicular to, or at an oblique angle to the thickness direction; depositing a core layer on the lower cladding layer; etching a portion of the core layer in a direction parallel to, perpendicular to, or at an oblique angle to the thickness direction; etching the lower cladding layer and the core layer into a desired optical waveguide pattern; and depositing an upper cladding layer on the core layer.

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