Planar lightwave circuit

Abstract

Provided is a planar lightwave circuit capable of suppressing emission of stray light to an emission side in order to suppress deterioration of signal light due to the stray light.　A planar lightwave circuit (400) according to the present disclosure comprises: a core (410) including a distortion part (411) that is distorted in the vertical direction in a plane parallel to the main surface of the planar lightwave circuit (400) with respect to the propagation direction of signal light; a first light-shielding groove (430a) that is disposed on the emission side with respect to the distortion part (411) in the propagation direction of the signal light, and that is configured to reflect stray light drifting in a clad (420) of the planar lightwave circuit (400); and a second light-shielding groove (430b) that reflects the stray light reflected by the first light-shielding groove (430a) so as not to have a vector component on the emission side.

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 communications 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] 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, respectively. 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, uncoupled light generated due to mismatches in mode field diameters at the coupling portions between the cores 111a-c and the LDs 120a-c can become stray light wandering within the cladding. Such stray light can become a crosstalk component and degrade the signal light. In particular, in a structure such as the optical device 100 in which the LDs 120a-c are directly optically coupled to the PLC 110, interference between the emitted light from the core 114 or the cores 115a-c and the stray light can cause a problem in that the shape of the emitted beam can be deformed.

[0006] Furthermore, the degradation of the signal light caused by such stray light becomes even more noticeable as noise when the optical device is packaged together with a monitor photodiode (hereinafter referred to as PD).

[0007] Several techniques have been proposed to suppress the 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 interface between the glass and air is utilized to suppress the incidence of stray light near the output end (see, for example, Patent Document 2). Another example is a technique in which a light-shielding material is filled in the light-shielding groove formed in the cladding and the light incident on the light-shielding material is absorbed (see, for example, Patent Document 3). The light-shielding material can be, for example, a silicone resin mixed with carbon black (nano- to subnano-sized carbon particles).

[0008] 2A and 2B are top views schematically illustrating the structures of PLCs 200a-c according to the prior art. (a) shows a configuration in which light-shielding grooves 230a are arranged across the entire cladding 220, (b) shows a configuration in which rectangular light-shielding grooves 220b are arranged diagonally, and (c) shows a configuration in which light-shielding grooves 220c are formed as small openings near the output portion. When an optical fiber is connected to the output side of the PLCs 200a-c according to the prior art, it is sufficient to suppress stray light coupling to the core 210, thereby achieving sufficient suppression of signal light degradation. However, when the output side is a spatial output, the PLCs 200a-c cannot control the destination of the stray light that has undergone multiple reflections, which can cause problems in downstream processes such as chip assembly, due to the inability to control the destination of the stray light.

[0009] FIG. 3 shows the results of a simulation of the propagation behavior of stray light in a PLC with obliquely arranged rectangular light-shielding grooves, such as PLC 200b. In this simulation, a three-dimensional model of a PLC with obliquely arranged rectangular light-shielding grooves was created, and the beam size and radiation angle of the input signal light were set. The propagation behavior of the generated stray light was calculated based on Snell's law, and the results are shown as two-dimensional results viewed from above. Note that in this simulation, the model simulates an RGB coupler, with signal light in each of the red, green, and blue wavelength bands incident from three input terminals and propagating in the y direction within the model. As shown in FIG. 4, in a PLC with obliquely arranged rectangular light-shielding grooves, such as PLC 200b according to the prior art, some stray light reaches the vicinity of the output. Thus, in a prior art PLC, when the light-shielding grooves are arranged to cause multiple reflections of the stray light, a significant amount of uncontrollable stray light may be emitted toward the output side.

[0010] International Publication No. 2017 / 142078 Japanese Patent Application Laid-Open No. 4-333829 International Publication No. 2011 / 065014

[0011] A. Himeno, et al., “Silica-Based Planar Lightwave Circuits” J. Sel. Top. QE, vol.4, pp.913-924 (1998)

[0012] The present disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a planar lightwave circuit that can suppress the radiation of stray light to the output side in order to suppress degradation of signal light due to the stray light.

[0013] In response to the above-described problems, the present disclosure provides a planar lightwave circuit comprising: a core including a distorted portion that is distorted in a direction perpendicular to the propagation direction of the signal light in a plane parallel to the main surface of the planar lightwave circuit; a first light-shielding groove that is arranged on the output side of the distorted portion in the propagation direction of the signal light and is configured to reflect stray light wandering within the clad of the planar lightwave circuit; and a second light-shielding groove that reflects the stray light reflected by the first light-shielding groove so that it does not have a vector component on the output side.

[0014] 1A and 1B 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.

[0033] FIGS. 1A and 1B are top views schematically illustrating the structures of PLCs 200a-c according to conventional technology, where (a) shows a configuration in which light-shielding grooves 230a are arranged across the entire cladding 220, (b) shows a configuration in which rectangular light-shielding grooves 220b are arranged obliquely, and (c) shows a configuration in which light-shielding grooves 220c formed as small openings are arranged near the output portion.

[0034] FIGS. 1A and 1B are diagrams illustrating the results of a simulation of the propagation behavior of stray light in a PLC having rectangular light-shielding grooves arranged obliquely, as in PLC 200b.

[0035] FIGS. 1A and 1B are top views schematically illustrating the structure of a PLC 400 according to a first embodiment of the present disclosure.

[0036] FIGS. 1A and 1B are top views schematically illustrating the structure of a PLC 500 in which light-shielding grooves 530a and 530b are arranged at positions closer to the input side than a distortion portion 411.

[0037] FIGS. 1A and 1B are top views schematically illustrating the structure of a PLC 600 according to a second embodiment of the present disclosure. 7A and 7B are diagrams schematically showing the structure of a PLC 700 according to a third embodiment of the present disclosure, where (a) is a top view and (b) is a cross-sectional view taken along line VIIb-VIIb. FIG. 7B is a top view schematically showing the structure of a PLC 800 according to a fourth embodiment of the present disclosure. FIG. 7C is a top view schematically showing the structure of a PLC 900 according to a fifth embodiment of the present disclosure. FIG. 7D is a top view schematically showing the structure of a PLC 1000 according to a sixth embodiment of the present disclosure.

[0015] 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.

[0016] Generally, stray light occurs in PLCs at optical connections between the PLC and LD, between the PLC and optical fiber, and at loss-causing locations within the circuit (e.g., multiplexing circuits, bending offsets, etc.). Among these, stray light generated at optical connections is known to be particularly strong. In particular, at optical connections between the PLC and LD, the MFD mismatch is large, so several tens of percent of the signal light becomes stray light. Unlike the PLCs 200a-c of the prior art, the PLC of this embodiment has a detailed defined shape of the light-shielding groove in order to efficiently suppress radiation of this stray light toward the output side.

[0017] First Embodiment Fig. 4 is a top view schematically illustrating the structure of a PLC 400 according to a first embodiment of the present disclosure. Note that in Fig. 4, stray light is indicated by a light solid line. As shown in Fig. 1, the PLC 400 is an embedded waveguide circuit including a core 410 and a clad 420, and includes a first light-shielding groove 430a and a second light-shielding groove 430b formed in the clad 420 to reflect stray light. The core 410 further includes a bent portion 411 that curves from the light propagation direction (x direction in Fig. 3) toward a direction perpendicular to the light propagation direction (y direction in Fig. 3) with respect to a plane parallel to the main surface of the PLC 400. Furthermore, the first light-shielding groove 430a is disposed closer to the output side in the light propagation direction (x direction in FIG. 3 ) than the distorted portion 411 of the core 410, and the second light-shielding groove 430b is configured to reflect stray light reflected by the first light-shielding groove 430a so as to have no vector component on the output side. The first light-shielding groove 430a and the second light-shielding groove 430b can be formed by etching using an ion beam, for example.

[0018] Let the diffraction angle of the incident signal light be θ. Here, the diffraction angle θ corresponds to the maximum angle at which stray light (light not optically coupled to the core 410) generated at the incident portion can be emitted, and depends on the wavelength and MFD of the input signal light. For example, the diffraction angle θ is approximately 10 degrees when the signal light is single-mode visible light.

[0019] In the PLC 400 according to this embodiment, as described above, the core 410 includes a distortion section 411 that distorts the light in the direction (y direction) perpendicular to the light propagation direction (x direction) within a plane parallel to the major surface of the PLC 400. With this configuration, the signal light propagates while being offset in the direction (y direction) perpendicular to the light propagation direction (x direction). The position of the distortion section 411 in the x direction may be any position depending on the design. However, from the viewpoint of suppressing the emission of stray light to the output side, it is desirable to position the distortion section 411 as close to the input end as possible. In practice, it is necessary to secure areas required for polishing near the input end, width conversion of the core 410, mode filtering, etc., and therefore it is preferable to position the distortion section 411 as close to the input end as possible, taking these areas into consideration.

[0020] 5 is a top view showing a schematic structure of a PLC 500 in which light-shielding grooves 530a and 530b are arranged at positions closer to the incident side than the distortion section 411. As shown in FIG. 5, in the PLC 500, the light-shielding grooves 530a and 530b are arranged near both side surfaces of the core 410. However, when the light-shielding grooves 530a and 530b are arranged at such positions, stray light may be scattered by the edges of the light-shielding grooves 530a and 530b, resulting in uncontrollable stray light. For this reason, the light-shielding groove (corresponding to the first light-shielding groove 430a in the case of the PLC 400) that first reflects stray light generated near the incident section needs to be arranged closer to the output side than the distortion section 411 in the propagation direction (x direction).

[0021] Furthermore, from the viewpoint of efficiently blocking stray light generated near the incident portion, the first light-shielding groove 430a is configured to encompass the spread of stray light determined by the diffraction angle θ and the distance in the propagation direction (x direction) from the incident end.

[0022] Since the first light-shielding groove 430a utilizes total reflection, it is not possible to set a large reflection angle. Therefore, stray light reflected by the first light-shielding groove 430a has a vector component on the output side to a certain extent. For this reason, the PLC 400 further includes a second light-shielding groove 430b that reflects the stray light reflected by the first light-shielding groove 430a so that it does not have a vector component on the output side.

[0023] The PLC 400 having such a configuration can more efficiently prevent stray light generated near the incident end from being emitted to the output side than the PLCs 200a to 200c according to the conventional technology.

[0024] In the above description, the PLC 400 is described as including two light-shielding grooves (the first light-shielding groove 430a and the second light-shielding groove 430b), but the number of light-shielding grooves to be arranged may be set arbitrarily depending on the design.

[0025] 6 is a top view schematically illustrating a structure of a PLC 600 according to a second embodiment of the present disclosure. As shown in FIG. 6, the PLC 600 has a configuration in which the first light-shielding grooves 430 a and the second light-shielding grooves 430 b in the PLC 400 described in the first embodiment are replaced with first light-shielding grooves 630 a and second light-shielding grooves 630 b, which are curved grooves.

[0026] In the PLC 400 according to the first embodiment, the first light-shielding groove 430a and the second light-shielding groove 430b are described as being straight grooves. However, as the transmission distance of stray light reflected by each groove increases, the beam of the reflected stray light spreads, which may result in an increase in chip size or may limit the area in which the light-shielding grooves can be arranged. In response to such an event, the PLC 600 has the first light-shielding groove 630a and the second light-shielding groove 630b as curved grooves, thereby suppressing the spread of the propagating stray light.

[0027] The first light-shielding groove 630a is configured so that the surface onto which stray light enters is concave so as to focus stray light generated at the incident portion. The first light-shielding groove 630a is formed as a curved groove within a range that satisfies the total reflection condition. The first light-shielding groove 630a is also configured so that the reflected stray light propagates through the cladding while focusing, and reaches a desired beam width at the second light-shielding groove 630b. Meanwhile, the shape (curvature) of the second light-shielding groove 630b is designed so that the focusing effect of the first light-shielding groove 630a and the diffusion effect of the second light-shielding groove 630b are balanced (collimated). The shape of the second light-shielding groove 630b need not necessarily have a constant curvature, as long as it can focus stray light within a desired range. Furthermore, the surface of second light-shielding groove 630b onto which stray light is incident may be either convex or concave, depending on the location where the incident stray light (reflected by first light-shielding groove 630a) is focused. Furthermore, if the stray light does not impose any restrictions on the chip size or layout within the range of propagation within PLC 600, it is not necessary to achieve a perfect collimated state.

[0028] In this way, in the PLC 600 having the first light-shielding groove 630 a and the second light-shielding groove 630 b, which are curved grooves, it is possible to guide stray light to a desired position without expanding the range of the stray light, and to suppress the stray light from being emitted toward the output surface. However, when stray light is guided to the input portion of the PLC 600, it is desirable to configure the PLC 600 so that the stray light is not incident on the input portion so as not to be coupled to the LD or optical fiber as reflected back light.

[0029] 7A and 7B are diagrams schematically illustrating the structure of a PLC 700 according to a third embodiment of the present disclosure, in which (a) is a top view and (b) is a cross-sectional view taken along line VIIb-VIIb. As shown in Fig. 7, the PLC 700 further includes a light-shielding film 750 formed on the upper surface of the cladding 420 and on the inner surfaces of the first and second light-shielding grooves 630a and 630b in the PLC 600 described in the second embodiment.

[0030] The light-shielding film 750 may be a metal such as Cr or Nb, or a multilayer film containing a metal and a dielectric. The light-shielding film 750 may be formed by an existing film-forming technique (e.g., sputtering). A method for forming the light-shielding film on the inner surfaces of the first and second light-shielding grooves 630a and 630b includes, for example, a sputtering method in which the PLC 700 serving as the substrate is positioned at an angle relative to the sputtering target surface and the substrate is rotated while the film is formed. However, to simplify the manufacturing process, the light-shielding film 750 may be formed on only one of the inner surfaces of the first and second light-shielding grooves 630a and 630b. In this configuration, the rotation of the substrate during the film formation described above may be unnecessary. In this case, however, it is preferable that the light-shielding film 750 be formed on the inner surface of the input side of each light-shielding groove.

[0031] If the thickness of the light-shielding film 750 is excessively thin, stray light may be transmitted, and a sufficient effect of attenuating stray light may not be obtained. On the other hand, if the thickness of the light-shielding film 750 is excessively thick, deformation and peeling may occur due to increased internal stress generated in the light-shielding film 750. Therefore, it is preferable that the thickness of the light-shielding film 750 be appropriately set by design based on the optical and mechanical characteristics. For example, if the light-shielding film 750 is made of Cr, it is preferable that the thickness of the light-shielding film 750 be controlled to be approximately in the range of 100 to 1000 nm.

[0032] Even with the PLC 700 having such a configuration, it is possible to efficiently suppress stray light generated near the incident portion from being emitted to the output portion. The PLC 700 can also suppress scattered light caused by the edges and surface roughness of the first light-shielding groove 630a and the second light-shielding groove 630b, thereby further improving the stray light suppression effect. Furthermore, since light is attenuated as the number of shielding reflections increases, the number of light-shielding grooves can be reduced compared to the PLCs 400 and 600 according to the first and second embodiments. Furthermore, if the light-shielding film 750 is made of metal, it is not necessary to satisfy the total reflection condition, allowing stray light to be guided more freely.

[0033] 8 is a top view schematically illustrating a structure of a PLC 800 according to a fourth embodiment of the present disclosure. As shown in Fig. 8, the PLC 800 corresponds to, for example, the PLC 600 according to the second embodiment, in which a plurality of incident sections are provided.

[0034] PLC800 includes two cores 410a, 410b, two distortion portions 411a, 411b formed in each of cores 410a, 410b, first light-shielding grooves 630a, 630b arranged on the output side in the x direction of each of distortion portions 411a, 411b, and second light-shielding grooves 630ba, 630bb that reflect stray light reflected by each of first light-shielding grooves 630aa, 630ab so that it does not have a vector component on the output side.

[0035] Even with the PLC 800 having such a configuration, it is possible to suppress stray light generated near the entrance portion of each of the cores 410a and 410b from being radiated to the exit side.

[0036] In the PLC 800, the cores 410a and 410b are connected to the multiplexing circuit 860, and the signal light output from the multiplexing circuit 860 propagates through the core 810 and is output to the outside. However, this configuration is merely an example and is not intended to limit the structure of the PLC 800. For example, each of the cores 410a and 410b may be directly connected to an output end.

[0037] 9 is a top view schematically illustrating a structure of a PLC 900 according to a fifth embodiment of the present disclosure. As illustrated in FIG. 9 , the PLC 900 includes a core 910 on the output side of the multiplexing circuit 860 in the PLC 900 of the fourth embodiment, which includes a second distortion portion 911, a third light-shielding groove 930a disposed on the output side of the distortion portion 911 in the x-direction, and a fourth light-shielding groove 930b that reflects stray light reflected by the third light-shielding groove 930a so as to eliminate vector components on the output side. In other words, in the PLC 900, the output side of the multiplexing circuit 860 includes cores 410a and 410b, each including distortion portions 411a and 411b, connected to the input end, and components similar to those of the first light-shielding grooves 630aa and 630ab and the second light-shielding grooves 630ba and 630bb.

[0038] The PLC 900 having such a configuration can suppress radiation of not only stray light generated near the incident portion but also stray light generated due to the multiplexing circuit 860 to the output side.

[0039] 10 is a top view schematically illustrating the structure of a PLC 1000 according to a sixth embodiment of the present disclosure. As shown in FIG. 10, the PLC 1000 further includes fifth light-shielding grooves 1030 a, b in a region of the cladding 420 in the PLC 900 according to the fifth embodiment, closer to the output side than the multiplexing circuit 860. However, the fifth light-shielding grooves 1030 a, b do not necessarily have to be located closer to the output side than the multiplexing circuit 860, as long as they are located closer to the output side than the first light-shielding groove 630 a.

[0040] 10 also shows a configuration in which two fifth light-shielding grooves 1030a, 1030b are arranged one by one across the linear portion of the core 910. However, this is intended to be an example and is not intended to limit the number or positions of the fifth light-shielding grooves.

[0041] A PLC 1000 having such a configuration can suppress radiation to the output side of not only stray light generated near the input portion and stray light caused by elements installed in the circuit, but also stray light caused by manufacturing errors.

[0042] As described above, in the PLC according to the present disclosure, the core and the light-shielding film are configured to efficiently suppress stray light wandering within the cladding from being emitted to the output side. As a technology that can more efficiently suppress degradation of signal light caused by stray light than conventional technology, 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. A planar lightwave circuit comprising: a core including a distorted portion that is distorted in a direction perpendicular to the propagation direction of a signal light in a plane parallel to a main surface of the planar lightwave circuit; a first light-shielding groove that is arranged on the output side of the distorted portion in the propagation direction of the signal light and is configured to reflect stray light wandering within the cladding of the planar lightwave circuit; and a second light-shielding groove that reflects the stray light reflected by the first light-shielding groove so that it does not have a vector component on the output side.

2. The planar lightwave circuit according to claim 1, wherein the first light-shielding groove and the second light-shielding groove are curved grooves, the first light-shielding groove is configured to collect stray light while propagating it through the cladding so that the reflected stray light has a desired beam width in the second light-shielding groove, and the second light-shielding groove is configured so that the collecting effect of the first light-shielding groove and the diffusing effect of the second light-shielding groove are balanced.

3. The planar lightwave circuit according to claim 1, further comprising a light-shielding film formed on an upper surface of said cladding and on inner surfaces of said first light-shielding groove and said second light-shielding groove.

4. The planar lightwave circuit according to claim 1, further comprising: a multiplexing circuit connected to the core; a second distortion section formed in the core connected to the output side of the multiplexing circuit, the second distortion section being distorted in a direction perpendicular to the propagation direction of the signal light in a plane parallel to the main surface of the planar lightwave circuit; a third light-shielding groove arranged on the output side of the second distortion section in the propagation direction of the signal light, and configured to reflect the stray light wandering within the cladding of the planar lightwave circuit; and a fourth light-shielding groove that reflects the stray light reflected by the third light-shielding groove so that it does not have a vector component on the output side.

5. The planar lightwave circuit according to claim 1, further comprising a fifth light-shielding groove disposed on the output side of said first light-shielding groove.

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