Planar optical wave loop

Abstract

Provided is a planar lightwave circuit capable of suppressing signal light degradation due to stray light.　A planar lightwave circuit (200) according to the present disclosure comprises: a substrate (201); a cladding (202) formed on the substrate and having a thickness thinner than or equal to 25 μm; one or more cores (203) embedded in the cladding; and a first absorption layer (204) which is formed on the cladding and which is an absorptive ND filter. In several examples, a light-shielding groove (401) formed in the absorption layer and the cladding may further be included and a second absorption layer (402) is formed on the inner surface thereof. Furthermore, in several examples, scatterers (601) formed inside the cladding and fine pits (801) formed in the absorption layer and the cladding may be included as additional components for causing stray light wandering within the cladding to efficiently enter and to efficiently be absorbed in the absorption layer.

Description

Technical Field

[0001] The present disclosure relates to a planar optical waveguide circuit.

Background Art

[0002] A silica-based planar optical waveguide circuit (Planar Lightwave Circuit: hereinafter referred to as PLC) that has been developed in the field of optical communication is an embedded waveguide in which a core is embedded inside a cladding, and is manufactured using a glass film formation technique and a semiconductor microfabrication technique (for example, see Non-Patent Document 1). Such a PLC has the feature that functions such as light branching, wavelength multiplexing / demultiplexing, and optical path switching can be realized on a single chip depending on the shape of the core, and thus it has been put into practical use as an optical circuit for optical communication devices.

[0003] In recent years, in addition to infrared light used in optical communication, PLCs have also come to handle light in the visible light wavelength region, such as an RGB coupler that multiplexes light with wavelengths corresponding to the three primary colors (red, green, and blue in the wavelength region of visible light) (for example, see Patent Document 1), and a multiplexing circuit for light used for atomic cooling of an optical lattice clock. Along with this, not only a structure in which an optical fiber often used in optical communication is optically coupled to the incident end of the PLC, but also a structure in which a laser diode (hereinafter referred to as LD) is directly optically coupled has increased.

[0004] Figure 1 is a schematic top view showing the structure of an optical device 100 in which LD120a-c are directly optically coupled to a PLC110. Here, as an example, the optical device 100 is shown as an RGB coupler. The optical device 100 has a structure in which LD120a, which emits light with a wavelength of 640 nm (corresponding to red), LD120b, which emits light with a wavelength of 520 nm (corresponding to green), and LD120c, which emits light with a wavelength of 450 nm (corresponding to blue), are directly optically coupled to each of the cores 111a-c installed on the incident end side of the PLC110. The light of each wavelength incident to the PLC110 from each of the LD120a-c is branched at each of the demultiplexers 112a-c installed on the PLC100. One of the branched light streams is incident on the multiplexer 113, and the signal light obtained by combining the light of each wavelength guides the core 114 and is directly emitted outside the PLC 110. Meanwhile, the other branched light streams, each with a different wavelength, guide each of the cores 115a-c and are directly emitted outside the PLC 100 as monitor light.

[0005] In optical device 100 having such a configuration, at the coupling point between core 111a-c and LD120a-c, uncoupled light generated due to mode field diameter mismatch, etc., wanders within the cladding as stray light, and this stray light becomes a crosstalk component, degrading the signal light. While this problem can also occur in PLC devices with coupled optical fibers, it becomes a more serious problem in optical devices like optical device 100, which directly optical-couple the LD to the PLC and utilize direct emission from the PLC, because there is no optical fiber to act as a spatial filter.

[0006] Several techniques have been proposed to suppress the degradation of signal light due to stray light. For example, a technique is known in which light-shielding grooves are 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, the main purpose of such light-shielding grooves is to change the direction of stray light propagation, and not to suppress the stray light itself. As a result, stray light radiated outside the cladding may be reflected by the inner wall of the housing used for packaging 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 grooves, some stray light may be scattered in unexpected directions, which can be a problem.

[0007] Another example is a technique in which a light-shielding material is filled into light-shielding grooves formed in the cladding, and the light incident on the light-shielding material is absorbed (see, for example, Patent Document 2). The light-shielding material may be, for example, a silicone resin mixed with carbon black (carbon particles of nano to sub-nanometer size). Unlike the case of using only the light-shielding grooves described above, this technique can efficiently suppress stray light from entering the vicinity of the output end due to reflection and scattering, because the light-shielding material absorbs stray light. However, filling with light-shielding material is a major burden on manufacturing (e.g., prolonged lead times). Furthermore, it is difficult to apply to optical devices that are intended to be packaged, such as RGB couplers like optical device 100, from the perspective of outgassing.

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

[0009] When the signal light guiding the PLC is of the communication wavelength, a known method involves setting the electrical resistivity of the Si substrate to 0.1 Ωcm or less to allow the substrate to absorb stray light. However, since light in the visible wavelength range, such as that of the optical device 100, is hardly absorbed, most of the stray light component is radiated to the outside and can be incident near the output end due to reflection from the inner wall of the housing used for packaging.

[0010] Thus, a challenge with optical devices using PLCs, particularly RGB couplers like optical device 100, is that there is no established method for efficiently suppressing signal degradation due to stray light. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2006-178395 [Patent Document 2] International Publication No. 2011 / 065014 [Patent Document 3] Japanese Patent Application Publication No. 11-248954 [Patent Document 4] Japanese Patent Publication No. 2002-350610 [Non-patent literature]

[0012] [Non-Patent Document 1] A. Himeno, et al., “Silica-Based Planar Lightwave Circuits” J. Sel. Top. QE, vol.4, pp.913-924 (1998) [Overview of the project]

[0013] This disclosure has been made in view of the above-mentioned problems, and its purpose is to provide a planar light wave circuit that can suppress the degradation of signal light due to stray light.

[0014] To address the above-mentioned problems, this disclosure provides a planar light wave circuit comprising a substrate, a cladding formed on the substrate and having a thickness of 25 μm or less, one or more cores embedded in the cladding, and a first absorption layer formed on the cladding and being an absorption-type ND filter. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic top view showing the structure of an optical device 100 in which LD120a-c is directly optically connected to PLC110. [Figure 2] This is a schematic cross-sectional view showing the structure of the PLC200 in the first embodiment of this disclosure. [Figure 3] This table shows the relationship between the thickness of cladding 202 and the attenuation rate of light propagating within cladding 202. [Figure 4] This is a schematic cross-sectional view showing the structure of the PLC400 in a second embodiment of this disclosure. [Figure 5] This is a top view illustrating the positions in which the light-shielding groove 401 and the absorption layer 402 are formed in the PLC400 according to the second embodiment of this disclosure. [Figure 6] This is a schematic cross-sectional view showing the structure of the PLC600 in the third embodiment of this disclosure. [Figure 7] This is a top view illustrating the position where the scatterer 601 is formed in the PLC600 in the second embodiment of the present disclosure. [Figure 8] This is a schematic cross-sectional view showing the structure of the PLC800 in a second embodiment of the present disclosure. [Modes for carrying out the invention]

[0016] Hereinafter, various embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and redundant descriptions may be omitted. The materials and numerical values are for illustrative purposes and are not intended to limit the technical scope of the present disclosure. The following description is an example, and as long as it does not deviate from the gist of an embodiment of the present disclosure, some configurations can be omitted, modified, or implemented with additional configurations.

[0017] The PLC according to the present disclosure includes an absorption layer formed on a cladding that constitutes a waveguide. By absorbing stray light with this absorption layer, it becomes possible to suppress the degradation of signal light due to the stray light.

[0018] The absorption layer is an absorption-type Neutral Density (hereinafter referred to as ND) filter. Generally, an absorption-type ND filter has a structure in which an absorption layer and a dielectric layer are alternately laminated, and exhibits a high absorption rate for light incident in a direction perpendicular to the lamination direction (see, for example, Patent Document 4). On the other hand, the PLC according to the present disclosure is set to efficiently absorb light incident obliquely with respect to the incident surface of the absorption layer. This is because when stray light generated in the PLC enters the absorption layer, it frequently becomes oblique with respect to the incident surface.

[0019] In some examples, the PLC according to the present disclosure may further include light-shielding grooves formed in the absorption layer and the cladding. The absorption layer is formed at least partially on the inner surface of this light-shielding groove. A PLC having such a configuration can absorb stray light more efficiently because the effective area of the absorption layer increases. In addition, since it is possible to suppress the stray light from being radiated from the light-shielding groove to the outside of the cladding, it is also possible to suppress the radiated stray light from being incident near the emission end due to reflection on the inner wall of the housing for packaging.

[0020] Furthermore, in some examples, the PLC according to this disclosure may include additional components for efficiently directing stray light wandering within the cladding into and absorbing the absorption layer. Such components may be scatterers formed within the cladding or fine pits (holes) formed in the cladding and the absorption layer.

[0021] Various embodiments of this disclosure are described in detail below with reference to the drawings.

[0022] (First embodiment) Figure 2 is a schematic cross-sectional view showing the structure of the PLC200 in the first embodiment of the present disclosure. As shown in Figure 2, the PLC200 in this embodiment includes a substrate 201, a cladding 202 formed on the substrate, a core 203 embedded inside the cladding 202, and an absorption layer 204 formed on the cladding 202.

[0023] As described above, the absorption layer 204 is an absorption-type ND filter, and for example, an absorption layer to which Si and Nb are applied and a dielectric layer to which SiO2 and Nb2O5 are applied are alternately stacked. It may be a multilayer film. Furthermore, for example, sputtering can be applied to form the absorption layer 204.

[0024] The thickness of cladding 202 is controlled to 25 μm or less. This is intended to efficiently absorb the stray light incident at an oblique angle, as described above.

[0025] Figure 3 is a table showing the relationship between the thickness of cladding 202 and the attenuation rate of light propagating within cladding 202. The attenuation rates in Figure 3 are those of light with the wavelengths of the three primary colors (640 nm, 520 nm, and 450 nm) obtained by the cutback method, relative to PLC200 with the absorption layer 204 deposited. As an example, a comparison of the attenuation rates when the cladding 202 thickness is 12 μm and 25 μm is shown. As shown in Figure 3, when the cladding 202 thickness is 12 μm, the attenuation rates of light with the wavelengths of the three primary colors are high, at -7.82, -6.45, and -3.26 dB / mm, respectively. As the thickness of cladding 202 increases, these attenuation rates decrease, and when the cladding 202 thickness is 25 μm, they become -0.39, -0.42, and -0.49 dB / mm. If the thickness of the cladding 202 is increased beyond this point, effective absorption by the absorption layer 204 cannot be expected. Therefore, in this embodiment, the thickness of the cladding 202 is controlled to 25 μm or less.

[0026] In the PLC200 configured in this way, stray light generated at the incident end due to uncoupled light, etc., is efficiently absorbed by the absorption layer 204. Therefore, it is possible to suppress the degradation of signal light due to stray light.

[0027] (Second embodiment) Figure 4 is a schematic cross-sectional view showing the structure of the PLC400 in a second embodiment of the present disclosure. As shown in Figure 4, the PLC400 in this embodiment further includes, in addition to the configuration of the PLC200 shown in Figure 2, a light-shielding groove 401 formed in the cladding 202 and the absorption layer 204, and an absorption layer 402 at least partially formed on the inner surface of the light-shielding groove 401. The light-shielding groove 401 is formed in a position that does not affect the signal light guiding the core 203. Also, as shown in Figure 4, the absorption layer 402 is at least partially formed on the inner surface of the light-shielding groove 401.

[0028] In PLC200, the positions where the light-shielding grooves 401 and the absorption layer 402 are formed are determined according to the difference in relative refractive index between the cladding 202 and the core 203, the width of the core 203, the thickness of the core 203, and the wavelength of the light being guided. For example, if the difference in relative refractive index between the cladding 202 and the core 203 is 1%, the width of the core 203 is 2 μm, the thickness of the core 203 is 2 μm, and the wavelength of the light being guided is 640 nm, then the positions where the light-shielding grooves 401 and the absorption layer 402 are formed should be at least 5 μm away from the core 203.

[0029] The absorption layer 402 is an absorption-type ND filter, similar to the absorption layer 204 in Figure 2. Furthermore, although Figure 4 depicts the absorption layer 402 being formed only on a portion of the inner surface of the light-shielding groove 401, the area in which it is formed may be partial or the entire surface.

[0030] In the PLC400 having this configuration, the ND filter is also formed in the thickness direction, and the area occupied by the ND filter is expanded. As a result, the effective absorption area of ​​stray light increases, making it possible to absorb stray light within the cladding 202 more efficiently.

[0031] In addition, forming an ND filter (absorption layer 402) on the inner surface of the light-shielding groove 401 also has the effect of suppressing the radiation of stray light from within the cladding 202 to the outside of the cladding. As in the conventional technology described above, if only a light-shielding groove 401 is formed without the absorption layer 402, when stray light is incident on the inner surface of the light-shielding groove 401 at an angle greater than the critical angle, this stray light is not totally reflected and is radiated to the outside of the cladding 202. As described above, this radiated stray light is incident near the output end due to reflection from the inner wall of the housing for packaging, etc., and as a result, can cause degradation of the signal light. However, in the PLC400, since the stray light is absorbed by the absorption layer 402 before it is radiated to the outside of the cladding, it is possible to suppress the degradation of the signal light caused by stray light radiated to the outside of the cladding 202.

[0032] Considering the effects of stray light radiated outside the cladding 202, it is desirable that the light-shielding groove 401, including the absorption layer 402, be formed between the incident end face and the side face of the PLC 400, as shown in Figure 5, in order to suppress the direct radiation of uncoupled light (depicted by dashed arrows in Figure 5) generated at the incident end from the side face of the PLC 400.

[0033] (Third embodiment) Figure 6 is a schematic cross-sectional view showing the structure of the PLC600 in a third embodiment of the present disclosure. As shown in Figure 6, the PLC600 in this embodiment further includes a plurality of scatterers 601 formed inside the cladding 202, in addition to the configuration of the PLC200 shown in Figure 2.

[0034] From the viewpoint of scattering stray light, it is desirable that the scatterers 601 be minute and numerous, and that they be scattered within the cladding 202. For example, the dimensions of the scatterers 601 may be 10 μm or less in both width and thickness. Also, from the viewpoint of scattering stray light, it is desirable that the shape of the scatterers be polyhedrons.

[0035] The scattering material 601 is made of a material that has a refractive index difference with the cladding 202 and has the effect of scattering light. For example, the scattering material 601 may be a metal or a dielectric material that has a refractive index difference with the cladding 202. Alternatively, the scattering material 601 may be made of the same material as the core 203. In such cases, when patterning to form the optical circuit of the PLC 600, the patterning should be performed in such a way that the portion that will become the scattering material 601 is left.

[0036] Because the scatterers 601 are scattered within the cladding 202, stray light within the cladding 202 is scattered, and more of this stray light is incident on the absorption layer 204. As a result, the stray light is efficiently absorbed by the absorption layer 204. Therefore, in the PLC600 including the scatterers 601, the degradation of signal light due to stray light is efficiently suppressed.

[0037] The scatterer 601 is formed in a position that does not affect the light guiding through the core 203, similar to the light-shielding groove 401 and absorption layer 402 in the PLC 400. This position is determined according to the difference in relative refractive index between the cladding 202 and the core 203, the width of the core 203, the thickness of the core 203, and the wavelength of the guided light.

[0038] Furthermore, when the optical device to which PLC600 is applied is an RGB coupler as shown in Figure 1, the areas where the density of stray light is high are near the optical coupling portion between LD120a-c and core 111a-c and near the output portion of the demultiplexer 112a-c. Therefore, when the scatterer 601 is applied to an RGB coupler, it is desirable to install it near the optical coupling portion between LD120a-c and core 111a-c and near the output portion of the demultiplexer 112a-c, as shown by the dashed lines in Figure 7.

[0039] Furthermore, the multiplexer 113 may be equipped with a discard port 701 for absorbing or emitting unwanted light that is generated separately from the signal light and is unrelated to the function of the optical device. In such a case, as shown in Figure 7, it is possible to replace the termination of the unwanted light by installing a scatterer 601 in the discard port 701.

[0040] (Fourth embodiment) Figure 8 is a schematic cross-sectional view showing the structure of the PLC800 in a second embodiment of the present disclosure. As shown in Figure 8, the PLC800 in this embodiment further includes a plurality of pits 801 formed in the cladding 202 and the absorbing layer 204, in addition to the configuration of the PLC200 shown in Figure 2. The pits 801 are formed in positions that do not affect the signal light guiding the core 203, similar to the light-shielding grooves 401 and the scatterers 601.

[0041] The formation of the pits 801 causes stray light within the cladding 202 to be scattered, similar to the scatterer 601 shown in Figure 6, resulting in more stray light being incident on the cladding 204 than on the absorption layer 204. From this perspective, unlike the light-shielding grooves 401 shown in Figure 4, it is desirable that the pits 801 be minute and numerous, scattered throughout the cladding 202. For example, the hole diameter of the pits 801 may be 10 μm or less.

[0042] Furthermore, from the viewpoint of scattering stray light, it is desirable that the shape of the pit 801 be such that the hole diameter gradually decreases in the depth direction (thickness direction, from the top surface toward the interface with the substrate 201) of the cladding 202. For example, if the hole shape of the pit 801 is circular, it is desirable that the shape of the pit 801 be a cone shape with its tip facing toward the interface with the substrate 201.

[0043] In the PLC800 with this configuration, stray light scattered by the pit 801 is efficiently incident on and absorbed by the absorption layer 204, and as a result, degradation of the signal light can be suppressed.

[0044] Furthermore, if the optical device to which PLC800 is applied is an RGB coupler as shown in Figure 1, then, similar to PLC600, the areas where the density of stray light is high are near the optical coupling portion between LD120a-c and core 111a-c and near the output portion of demultiplexer 112a-c. Therefore, when pit 801 is applied to an RGB coupler, it is desirable to install it near the optical coupling portion between LD120a-c and core 111a-c and near the output portion of demultiplexer 112a-c, similar to the scatterer 601 of PLC600.

[0045] In addition, similar to the PLC600, the multiplexer 113 may be equipped with a discard port 701 for absorbing or emitting unwanted light that is generated separately from the signal light and is not related to the function of the optical device. In such a case, it is also possible to replace the termination of the unwanted light by installing a pit 801 at the position corresponding to the discard port 701 in Figure 7. [Industrial applicability]

[0046] As described above, the PLC according to this disclosure is configured so that the absorption layer can efficiently absorb stray light wandering within the cladding and stray light radiated to the outside of the cladding, and as a result it is possible to suppress the degradation of signal light due to such 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 visible light wavelengths such as RGB couplers.

Claims

1. A plane light wave circuit, circuit board and A cladding formed on the aforementioned substrate, having a thickness of 25 μm or less, One or more cores embedded within the cladding, A first absorption layer formed on the cladding, which is an absorption-type ND filter, wherein the absorption-type ND filter is a multilayer film in which layers to which Si and Nb are applied and layers to which SiO2 and Nb2O5 are applied are alternately stacked, the first absorption layer, A plane light wave circuit equipped with the following features.

2. Light-shielding grooves formed in the cladding and the absorbing layer, A second absorption layer, which is an absorption-type ND filter, is formed on at least a portion of the inner surface of the light-shielding groove, A plane light wave circuit according to claim 1, comprising:

3. The plane light wave circuit according to claim 2, wherein the light-shielding groove and the second absorption layer are formed near the incident end face into which light is incident.

4. The plane light wave circuit according to claim 1, further comprising scatterers scattered within the cladding and formed at positions that do not affect the signal light guiding one or more cores.

5. The planar light wave circuit according to claim 4, wherein the scattering body is a metal, a dielectric, or the same material as the one or more cores.

6. The aforementioned one or more cores are multiple cores, The aforementioned plane light wave circuit is Multiple demultiplexers connected to the aforementioned multiple cores, A multiplexer that combines the light split by the aforementioned multiple demultiplexers, A discard port that absorbs unwanted light generated by the aforementioned multiplexer, Furthermore, The planar light wave circuit according to claim 4, wherein the scattering body is formed near the incident end faces of the plurality of cores, near the exit ends of the plurality of demultiplexers, and in the discard port.

7. The plane light wave circuit according to claim 1, further comprising a plurality of pits formed from the upper surface of the absorption layer to the inside of the cladding, and positioned so as not to affect the signal light guiding the one or more cores.

8. The aforementioned one or more cores are multiple cores, The aforementioned plane light wave circuit is Multiple demultiplexers connected to the aforementioned multiple cores, A multiplexer that combines the light split by the aforementioned multiple demultiplexers, A discard port that absorbs unwanted light generated by the aforementioned multiplexer, Furthermore, The planar light wave circuit according to claim 7, wherein the pits are formed near the inlet end faces of the plurality of cores, near the outlet ends of the plurality of demultiplexers, and in the discard port.

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