Planar lightwave circuit

Abstract

Provided is a planar lightwave circuit capable of suppressing degradation of signal light due to stray light. A planar lightwave circuit according to the present disclosure is an embedded-type planar lightwave circuit in which a core that guides signal light and cladding that covers the core are formed on a substrate, said planar lightwave circuit comprising: a light-shielding groove that is formed in the cladding at a position at which there is no effect on the signal light; and a light-shielding film which is formed on the outermost surface of the cladding and on a side surface in the light-shielding groove that is closest to the input side of the planar lightwave circuit, wherein the light-shielding film is configured to absorb the wavelength of the signal light.

Description

Planar light wave circuit

[0001] The present disclosure relates to planar lightwave circuits.

[0002] Silica-based planar lightwave circuits (PLCs), which have been developed in the field of optical communications, are embedded waveguides in which a core is embedded inside a cladding, and are manufactured using glass deposition technology and semiconductor microfabrication technology (see, for example, Non-Patent Document 1). Such PLCs have the advantage that they can achieve functions such as branching of light, multiplexing / demultiplexing of wavelengths, and switching of optical paths on a single chip depending on the shape of the core, and therefore are being put into practical use as optical circuits for optical communication devices.

[0003] Furthermore, in recent years, PLCs have begun to handle not only the infrared light used in optical communications, but also light in the visible wavelength region, such as RGB couplers (see, for example, Patent Document 1) that combine light with wavelengths corresponding to the three primary colors (red, green, and blue in the visible wavelength region), and optical combining circuits used for atomic cooling in optical lattice clocks. Accordingly, in addition to structures in which optical fibers, which are often used in optical communications, are optically coupled to the input end of PLCs, structures in which laser diodes (hereinafter referred to as LDs) are directly optically coupled are also on the rise.

[0004] FIG. 1 is a top view schematically illustrating the structure of an optical device 100 in which LDs 120a-c are directly optically coupled to a PLC 110. Here, the optical device 100 is illustrated as an RGB coupler by way of example. The optical device 100 has a structure in which an LD 120a that emits light having a wavelength of 640 nm (corresponding to red), an LD 120b that emits light having a wavelength of 520 nm (corresponding to green), and an LD 120c that emits light having a wavelength of 450 nm (corresponding to blue) are directly optically coupled to cores 111a-c installed on the input end side of the PLC 110. The light having each wavelength input to the PLC 110 from each of the LDs 120a-c is branched by each of the branching filters 112a-c installed in the PLC 110. One of the branched lights is input to multiplexer 113, and the signal light obtained by multiplexing the lights of each wavelength is guided through core 114 and directly emitted outside PLC 110. On the other hand, the other branched lights having each wavelength are guided through cores 115a-c, respectively, and directly emitted outside PLC 110 as monitor lights.

[0005] In the optical device 100 having such a configuration, at the coupling portion between the cores 111a-c and the LDs 120a-c, uncoupled light generated due to mismatch in mode field diameter or the like can become stray light wandering within the cladding, and this stray light can become a crosstalk component, degrading the signal light. Such a problem can also occur in PLC devices coupled with optical fibers, but in optical devices such as the optical device 100, which directly optically couple the LD to the PLC and utilize the direct emission from the PLC, the problem is more serious because there is no optical fiber that acts as a spatial filter.

[0006] Several techniques have been proposed to suppress degradation of signal light due to such stray light. For example, a technique is known in which a light-shielding groove is formed in the cladding of a PLC and reflection at the glass-air interface is used to suppress stray light from entering the vicinity of the output end. However, such a light-shielding groove is primarily intended to redirect the stray light, and does not suppress the stray light itself. Therefore, stray light emitted outside the cladding may be reflected by the inner wall of the packaging housing and enter the vicinity of the output end, causing degradation of the signal light. In addition, depending on the surface roughness of the side surface of the light-shielding groove, some stray light may be scattered in unexpected directions.

[0007] Another known example is a technology in which a light-shielding groove formed in a cladding is filled with a light-shielding material to absorb light incident on the light-shielding material (see, for example, Patent Document 2). The light-shielding material can be, for example, a silicone resin mixed with carbon black (nano- to subnano-sized carbon particles). Unlike the above-mentioned light-shielding groove alone, this technology uses the light-shielding material to absorb stray light, thereby efficiently preventing stray light from entering the vicinity of the output end due to reflection or scattering. However, filling with a light-shielding material can cause a significant manufacturing burden (such as prolonging the construction period). Furthermore, it is difficult to apply this technology to optical devices that are intended to be packaged, such as RGB couplers like the optical device 100, due to the risk of outgassing.

[0008] As another example, a technique is known in which a metal film is formed on the surface of the cladding of a PLC, and the metal film absorbs stray light (see, for example, Patent Document 3). However, since most of the stray light is reflected at the interface between the glass part of the PLC and the metal film, high absorption efficiency cannot be expected.

[0009] When the signal light guided through the PLC is light of a communication wavelength, a method is known in which the electrical resistivity of the Si substrate is set to 0.1 Ω cm or less, causing the substrate to absorb stray light. However, since light in the visible wavelength range, such as that of optical device 100, is hardly absorbed, most of the stray light components are radiated to the outside and may be reflected by the inner wall of the packaging case and enter the vicinity of the output end.

[0010] As described above, optical devices using PLC, particularly RGB couplers such as the optical device 100, have a problem in that no method has been established to efficiently suppress degradation of signal light due to stray light.

[0011] JP 2006-178395 A International Publication No. 2011 / 065014 JP 11-248954 A JP 2002-350610 A

[0012] A. Himeno, et al., “Silica-Based Planar Lightwave Circuits” J. Sel. Top. QE, vol.4, pp.913-924 (1998)

[0013] The present disclosure has been made in consideration of the above-mentioned problems, and an object of the present disclosure is to provide a planar lightwave circuit that can suppress degradation of signal light due to stray light.

[0014] In response to the above-described problems, the present disclosure provides an embedded planar lightwave circuit in which a core through which signal light is guided and a clad covering the core are formed on a substrate, the planar lightwave circuit comprising: a light-shielding groove formed at a position within the clad that does not affect the signal light; and a light-shielding film formed on the outermost surface of the clad and on each of the side surfaces of the light-shielding groove closest to the input side of the planar lightwave circuit, the light-shielding film being configured to absorb the wavelength of the signal light.

[0015] 2(a) and 2(b) are top views schematically illustrating the structure of an optical device 100 in which LDs 120a-c are directly optically coupled to a PLC 110.

[0026] FIG. 2(a) is a top view, and FIG. 2(b) is a cross-sectional view taken along the line IIb-IIb, showing a schematic structure of a PLC 200 according to the present disclosure.

[0027] FIG. 2(b) is an enlarged view of the vicinity of a light-shielding groove 201 shown in FIG. 2(b).

[0028] FIG. 2(b) is a view schematically illustrating the formation of a light-shielding film 204 by a sputtering method.

[0029] FIG. 2(b) is a top view, and FIG. 2(b) is a cross-sectional view taken along the line VIb-VIb, showing a schematic structure of a PLC 500 according to a second embodiment of the present disclosure.

[0029] FIG. 2(a) is a top view, and FIG. 2(b) is a cross-sectional view taken along the line VIb-VIb, showing a schematic structure of a PLC 600 according to the present disclosure.

[0016] Various embodiments of the present disclosure will be described in detail below with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant description may be omitted. Materials and numerical values ​​are for illustrative purposes only and are not intended to limit the technical scope of the present disclosure. The following description is an example, and some configurations may be omitted or modified, or additional configurations may be added, as long as they do not deviate from the gist of one embodiment of the present disclosure.

[0017] A PLC according to the present disclosure includes a light-shielding groove formed in a cladding, and a light-shielding film formed on the outermost surface of the cladding and on the side of the light-shielding groove closest to the input side of the PLC. In a PLC according to the present disclosure having such a configuration, stray light spreading throughout the entire cladding is absorbed and attenuated by the light-shielding film formed on the outermost surface of the cladding, and stray light generated in the input section is absorbed and attenuated by the light-shielding film formed on the side of the light-shielding groove closest to the input side of the PLC. This provides the effect of suppressing degradation of signal light due to stray light.

[0018] In some examples, the PLC according to the present disclosure may further include a non-film-formed region on the top surface of the cladding, where the light-shielding film is not formed, in a region corresponding to an input / output portion of the signal light. The PLC according to the present disclosure having such a configuration has the characteristic that the region corresponding to the input / output portion of the signal light can be visually confirmed during optical inspection while maintaining the effect of absorbing and attenuating stray light.

[0019] As another example, the PLC according to the present disclosure may further include a protective film laminated on the light-shielding film formed on the outermost surface of the cladding. The protective film is an inorganic compound such as a carbide, nitride, or oxide, and has the function of preventing scratches from being introduced into the light-shielding film 204 during cleaning, cleaning processes, and inspection processes of the PLC.

[0020] 2A and 2B are diagrams showing a schematic structure of a PLC 200 according to the present disclosure, in which (a) is a top view and (b) is a cross-sectional view taken along the line IIb-IIb. As shown in Fig. 2, the PLC 200 includes a light-shielding groove 201 formed in a cladding 203, and a light-shielding film 204 formed on the outermost surface of the cladding 203 and on each of the side surfaces of the light-shielding groove 201 closest to the input side of the PLC 200.

[0021] As shown in Fig. 2(b), the light-shielding groove 201 is formed so that its bottom surface is the outermost surface of the substrate 202. The light-shielding groove 201 is formed at a position that does not affect the signal light that is guided within the PLC 200. Although Fig. 2 depicts a configuration in which one light-shielding groove 201 is formed, any number of light-shielding grooves 201 may be formed depending on the design.

[0022] The light-shielding film 204 is made of a material that absorbs stray light generated by the PLC 200. For example, if the PLC 200 handles signal light in the visible wavelength range, the light-shielding film 204 may be made of chromium (Cr), which absorbs light in the visible wavelength range and has high adhesion to quartz (SiO2), a typical cladding material. However, this is merely an example and does not limit the material of the light-shielding film 204. In practice, the material of the light-shielding film 204 may be any material that absorbs stray light, such as a single layer of another metal or an ND filter composed of a laminate of a metal and a dielectric.

[0023] If the thickness of the light-shielding film 204 is excessively thin, stray light may be transmitted, and a sufficient stray light attenuation effect may not be obtained. On the other hand, if the thickness of the light-shielding film 204 is excessively thick, deformation and peeling may occur due to increased internal stress generated in the light-shielding film 204. Therefore, it is preferable that the thickness of the light-shielding film 204 be appropriately set by design based on the optical and mechanical characteristics. For example, if the light-shielding film 204 is made of Cr, it is preferable that the thickness of the light-shielding film 204 be controlled to be approximately in the range of 100 to 1000 nm.

[0024] In addition, it is preferable that the light-shielding film 204 is configured to have a low reflectance of stray light. For example, if a material with a high reflectance of light in the visible wavelength range, such as gold (Au), is used for the light-shielding film 204, the stray light is reflected and returns to the cladding 203, making it impossible to efficiently attenuate the stray light.

[0025] FIG. 3 is an enlarged view of the vicinity of the light-shielding groove 201 shown in FIG. 2( b). As shown in FIG. 3, the light-shielding film 204 is formed on the outermost surface of the cladding 203 and on each of the side surfaces of the light-shielding groove 201 closest to the input side of the PLC 200. This configuration makes it possible to efficiently attenuate stray light wandering within the cladding 203 of the PLC 200. More specifically, since the stray light is mostly uncoupled light due to a mismatch in the mode field diameter at the input section of the PLC 200, the stray light generated at such an input section is efficiently absorbed by the light-shielding film 204 formed on the side surface of the light-shielding groove 201 closest to the input side of the PLC 200. Furthermore, stray light within the cladding 203 often spreads throughout the cladding 203, although the strength varies due to multiple reflections and the like. However, such stray light spreading throughout the entire cladding 203 is efficiently absorbed by the light-shielding film 204 formed on the outermost surface of the cladding 203. Based on this principle, the stray light generated in the PLC 200 is efficiently attenuated.

[0026] The light-shielding film 204 may additionally be formed on the bottom surface of the light-shielding groove 201 and / or on other side surfaces of the light-shielding groove 201 other than the side surface closest to the input side of the PLC 200. Such a configuration makes it possible to more efficiently attenuate stray light that spreads throughout the cladding. However, if the light-shielding film 204 is not formed on the side surface of the light-shielding groove 201 closest to the input side of the PLC 200, and the light-shielding film 204 is formed only on the side surface of the light-shielding groove 201 closest to the output side of the PLC 200, stray light may be radiated into space at the edge or roughness of the light-shielding groove 201. For this reason, the light-shielding film 204 needs to be formed at least on the side surface of the light-shielding groove 201 closest to the input side of the PLC 200.

[0027] Furthermore, from the viewpoint of efficiently attenuating stray light, it is preferable that the light-shielding film 204 be formed on the entire top surface of the cladding 203 in the PLC 200 and on the entire side of all the formed light-shielding grooves 201 that is closest to the input side of the PLC 200.

[0028] Such a light-shielding film 204 can be formed by, for example, a physical vapor deposition (PVD) method such as vacuum deposition or sputtering, a chemical vapor deposition (CVD) method such as plasma CVD or photo CVD, etc. However, this is an explanation intended as an example and does not limit the method of forming the light-shielding film 204.

[0029] FIG. 4 is a schematic diagram illustrating the formation of the light-shielding film 204 by sputtering. For example, to form the light-shielding film 204 in the form shown in FIGS. 2B and 3 using sputtering, the substrate PLC 200 may be positioned at an angle relative to the main surface of the sputtering target 401 (the surface from which constituent particles are emitted by momentum exchange). In this case, if the width and depth of the light-shielding groove are W and H, respectively, the angle of inclination θ (the angle of the main surface of the PLC 200 relative to the main surface of the sputtering target 401) during film formation must be greater than or equal to arctan(H / W). This is because, when a film is formed over the entire side of the light-shielding groove 201 closest to the input side of the PLC 200, the corners of the light-shielding groove 201 located on the output side of the PLC 200 block the incidence of incoming particles. However, as described above, if the light-shielding film 204 is thin, light absorption will be insufficient, so in practice, it is more preferable that the tilt angle θ during film formation be in the range of 10 degrees to 45 degrees, or arctan (H / W) to 45 degrees. This is because, assuming a groove depth of 20 μm and a groove width of 100 μm, for example, arctan (H / W) will be about 10 degrees, and the film thickness formed on the side surface of the light-shielding groove 201 closest to the input side of the PLC 200 will be relatively thinner than the film thickness formed on the outermost surface of the cladding 203.

[0030] On the other hand, in order to form the light-shielding film 204 on the bottom surface of the light-shielding groove 201 and / or on other side surfaces of the light-shielding groove 201 other than the side surface closest to the input side of the PLC 200, in the configuration shown in Figure 4, the film formation can be performed while rotating the PLC 200 that serves as the base material.

[0031] The above explanation has been given in the context of the sputtering method being used as the film formation method, but it is similar when other film formation methods are used.

[0032] 5 is a top view showing a schematic structure of a PLC 500 according to a second embodiment of the present disclosure. As shown in FIG. 5, in addition to the configuration of the PLC 200 described above, the PLC 500 further includes non-deposition regions 501 a-c, 502 on the outermost surface of the cladding 203, in a range where the signal light input to each of the cores 111 a-c does not reach the outermost surface of the cladding 203, and in a range ±10 μm from the center of the waveguide of the core 114 and 20 μm inward from the output end face of the PLC 200.

[0033] In the above description, it has been stated that the light-shielding film 204 can be formed over the entire top surface of the cladding 203 in the PLC 200. However, if the light-shielding film 204 is formed on the top surface of the cladding 203 at the input / output portion for signal light, another problem may arise in that optical inspection becomes difficult. For this reason, the PLC 500 further includes non-film-formed regions 501a-c, 502 where the light-shielding film 204 is not formed in regions of the top surface of the cladding 203 corresponding to the input / output portion for signal light.

[0034] The non-coated regions 501a-c corresponding to the input section are defined as a range in which the signal light does not reach the outermost surface of the cladding 203. On the other hand, the non-coated region 502 corresponding to the output section is defined as the minimum range that can be viewed with an optical microscope commonly used in optical inspection. The PLC 500 including the non-coated regions 501a-c, 502, whose ranges are defined in this way, has the characteristic that the position of the waveguide can be viewed (optical inspection) while maintaining the effect of suppressing stray light.

[0035] The non-film-formation regions 501a-c and 502 can be formed by existing patterning techniques such as photolithography, lift-off, and etching.

[0036] 6A and 6B are diagrams showing a schematic structure of a PLC 600 according to the present disclosure, in which (a) is a top view and (b) is a cross-sectional view taken along the line VIb-VIb. As shown in Fig. 6, the PLC 600 includes, in addition to the configuration of the PLC 200 described above, a protective film 601 laminated on the light-shielding film 204 formed on the outermost surface of the cladding 203.

[0037] The protective film 601 has the function of preventing scratches from being introduced into the light-shielding film 204 during the cleaning and inspection processes of the PLC 600. In particular, since mechanical scrubbing by brushing may be used during the cleaning process of the PLC 600, the protective film 601 is preferably made of an inorganic compound having a relatively high Vickers hardness, such as a carbide, nitride, or oxide. Among these, SiO2 is a promising material for the protective film 601 because it can be easily used in general optical circuit fabrication techniques, such as silicon photonics.

[0038] In addition, in Figure 6, the protective film 601 is depicted as not being formed on the side and bottom surfaces of the light-shielding groove 201, but this is a depiction intended for illustrative purposes, and in reality, the protective film 601 may also be formed on the side and bottom surfaces of the light-shielding groove 201.

[0039] The PLCs 200, 500, and 600 according to the present disclosure are configured so that the light-shielding film can efficiently absorb stray light wandering within the cladding 203, and as a result, it is possible to suppress degradation of signal light due to the stray light. When the output side of the PLCs 200, 500, and 600 according to the present disclosure is connected to a fiber, very little stray light is coupled to the fiber, which is particularly important when the output side is a spatial emission type.

[0040] The PLC according to the present disclosure is configured so that the light-shielding film can efficiently absorb stray light wandering within the cladding, thereby suppressing degradation of signal light due to the stray light. Such a PLC is expected to be applied not only to infrared light used in optical communication systems, but also to optical devices that handle wavelengths in the visible light range, such as RGB couplers.

Claims

1. An embedded planar lightwave circuit having a core for guiding signal light and a cladding covering the core formed on a substrate, the planar lightwave circuit comprising: a light-shielding groove formed in a position within the cladding that does not affect the signal light; and a light-shielding film formed on the outermost surface of the cladding and on each of the side surfaces of the light-shielding groove closest to the input side of the planar lightwave circuit, the light-shielding film being configured to absorb the wavelength of the signal light.

2. The planar lightwave circuit according to claim 1, wherein the light-shielding film is also formed on the bottom surface of the light-shielding groove, on a side surface of the light-shielding groove other than the side closest to the input side of the planar lightwave circuit, or on both of them.

3. The planar lightwave circuit according to claim 1, further comprising non-film-formed regions on the outermost surface of the cladding in a range where the signal light input to the core on the input side does not reach the outermost surface of the cladding, and in a range of ±10 μm from the center of the waveguide of the core on the output side and 20 μm inward from the output end face of the planar lightwave circuit.

4. The planar lightwave circuit according to claim 1, further comprising a protective film laminated on the light-shielding film formed on the outermost surface of the cladding, the protective film being an inorganic compound selected from the group consisting of carbide, nitride, and oxide.

5. The planar lightwave circuit according to claim 4, wherein said protective film is SiO2.

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