EP-4737961-A1 - PLANAR LIGHTWAVE CIRCUIT

EP4737961A1EP 4737961 A1EP4737961 A1EP 4737961A1EP-4737961-A1

Abstract

A planar lightwave circuit according to the present disclosure includes: a substrate (1), a cladding layer (2) provided on the substrate (1); a core (3) embedded in the cladding layer (2) to form a waveguide; and a depressed region (210) in which a plurality of depressed portions (5) formed in the cladding layer (2) are arranged two-dimensionally in an in-plane direction of the cladding layer (2).

Inventors

GO TAKASHI
TAKESHITA MAKOTO
YANAGISAWA MASAHIRO
OGAWA DAISUKE

Assignees

NTT Innovative Devices Corporation

Dates

Publication Date: 20260506
Application Date: 20240730

Claims (11)

A planar lightwave circuit, comprising: a substrate; a cladding layer provided on the substrate; a core embedded in the cladding layer to form a waveguide; and a depressed region in which a plurality of depressed portions formed in the cladding layer are arranged two-dimensionally in an in-plane direction of the cladding layer.
The planar lightwave circuit according to claim 1, wherein the depressed portions are air layers.
The planar lightwave circuit according to claim 2, wherein an angle formed by a normal of a side wall surface of the depressed portions and a normal of a substrate surface of the substrate is different from 90°.
The planar lightwave circuit according to claim 3, wherein a normal direction of the side wall surface of the depressed portions is a direction oriented from an interior to an exterior of the cladding layer, a normal direction of the substrate surface of the substrate is a direction oriented from the substrate toward the cladding layer, and an angle formed by the normal direction of the side wall surface and the normal direction of the substrate surface is less than 90°.
The planar lightwave circuit according to claim 1, wherein a shape of the depressed portions in a plan view is a circular shape.
The planar lightwave circuit according to claim 1 or 2, wherein an arrangement of the plurality of depressed portions in the depressed region is a hexagonal packing arrangement.
The planar lightwave circuit according to claim 2, wherein a shape of the depressed portions in a plan view is a circular shape, an arrangement of the plurality of depressed portions in the depressed region is a hexagonal packing arrangement, and when a diameter of the circular shape is represented by D and an arrangement pitch of the depressed portions is represented by P, the diameter D and the arrangement pitch P satisfy Formula 1 below. 0.5 ⋅ P ≤ D < P
The planar lightwave circuit according to claim 7, wherein the diameter D and the arrangement pitch P satisfy Formula 2 below. 3 2 ⋅ P ≤ D < P
The planar lightwave circuit according to claim 2, wherein a normal direction of a side wall surface of the depressed portions is a direction oriented from an interior to an exterior of the cladding layer, a normal direction of a substrate surface of the substrate is a direction oriented from the substrate toward the cladding layer, and an angle formed by the normal direction of the side wall surface and the normal direction of the substrate surface is less than 90°, a shape of the depressed portions is a circular shape in a plan view and an arrangement of the depressed portions in the depressed region is a hexagonal packing arrangement, the angle formed by the normal direction of the side wall surface and the normal direction of the substrate surface is represented using a deviation angle θ (≠ 0°) from the angle as 90° - θ, a position of a central axis of the core relative to the substrate is represented by an initial position y 0 , a refractive index of the cladding layer is represented by n clad , a diameter of the circular shape is represented by D, an arrangement pitch of the depressed portions is represented by P, and a repetitive pitch of the depressed portions is represented by P' = (√3)·P, respectively, and when an initial angle of a travel angle of stray light relative to the substrate surface in an i-th cladding layer (where i is an integer greater than or equal to 1) in the depressed region is represented by ϕ 0 = 0° and a position of the stray light relative to the substrate is represented by y i , a length of the depressed region in a direction along a propagation direction of the stray light is longer than i min ·P', where i min denotes a minimum i for which the position y i derived from Formulas 3 to 5 below is zero or less. ϕ ′ i = sin − 1 n clad ⋅ sin ϕ i − 1 + θ − θ ϕ i = θ − sin − 1 sin θ − ϕ ′ i / n clad y i = y i − 1 − D ⋅ tan ϕ ′ i − P ′ − D ⋅ tan ϕ i
The planar lightwave circuit according to claim 2, wherein a part of or all of the plurality of depressed portions that configure the depressed region have different sizes in either or both of a plan view and a sectional view relative to the substrate.
The planar lightwave circuit according to claim 10, wherein the depressed region is segmented into a plurality of regions, the sizes of the depressed portions differ and an arrangement pitch of the depressed portions differ in a part or all of the plurality of segmented regions.

Description

Technical Field The present disclosure relates to a planar lightwave circuit. Background Art Planar lightwave circuits (PLCs) fabricated on substrates using optical waveguides enable integration of diverse interferometers such as optical filters, optical multiplexers/demultiplexers, or optical switches onto a single chip and are realized using various material systems. For example, planar lightwave circuits composed of silica-based optical waveguides are widely used in practical applications primarily in the field of optical communications because of their low optical loss, stability, and/or reliability. Citation List Patent Literature Patent Literature 1 Japanese Patent Laid-Open No. 2010-32921Patent Literature 2 Japanese Patent Laid-Open No. H9-5548 Summary of Invention In known planar photonic circuits, there is room for improvement in suppressing or reducing crosstalk caused by stray light propagating through a cladding layer. An object of the present disclosure is to provide a planar lightwave circuit capable of improving an effect of suppressing or reducing crosstalk caused by light (for example, stray light) propagating in a cladding layer. A planar lightwave circuit according to an aspect of the present disclosure includes: a substrate, a cladding layer provided on the substrate; a core embedded in the cladding layer to form a waveguide; and a depressed region in which a plurality of depressed portions formed in the cladding layer are arranged two-dimensionally in an in-plane direction of the cladding layer. The planar lightwave circuit of one aspect of the present disclosure, it is possible to improve an effect of suppressing or reducing crosstalk caused by light (for example, stray light) propagating through the cladding layer. Brief Description of Drawings [Figure 1] Figure 1 is a top view schematically illustrating an optical circuit configured with a planar lightwave circuit technique for fabricating a silica-based optical waveguide on a silicon substrate.[Figure 2] Figure 2 is a partial sectional view partially illustrating a cross section along a cutting line X-X' depicted in Figure 1.[Figure 3] Figure 3 is a top view schematically illustrating an optical circuit that is a non-limiting example of a planar lightwave circuit with a light-shield groove group according to an embodiment.[Figure 4] Figure 4 is a partial enlarged view for describing details of each light-shield groove groups depicted in Figure 3.[Figure 5] Figure 5 is a partial sectional view partially illustrating a cross section along a cutting line X-X' depicted in Figure 4.[Figure 6] Figure 6 is a partial sectional view partially illustrating a cross section along a cutting line Z-Z' depicted in Figure 4.[Figure 7] Figures 7(a) to 7(h) are each a top view schematically illustrating an example of an arrangement of deep grooves in a light-shield groove group.[Figure 8] Figure 8 is a diagram illustrating a detailed example of how stray light across a plurality of deep grooves in the light-shield groove group according to the embodiment.[Figure 9] Figure 9 is a graph illustrating an example of a positional change in a y-axis direction of stray light when the stray light propagating in a z-axis direction travels through the light-shield groove group according to the embodiment.[Figure 10] Figure 10 is a sectional view schematically illustrating an example of a light-reception element mounted on an end face of an optical circuit according to the embodiment.[Figure 11] Figure 11 is a diagram illustrating an example of an arrangement pattern corresponding to a configuration in which a diameter of deep grooves is changed in a part of or all of multiple segmented regions in the example of the deep groove arrangement pattern depicted in Figure 7(d).[Figure 12] Figure 12 is a sectional view schematically illustrating an example of a finished shape of grooves near a processing limit of the embodiment.[Figure 13] Figure 13 is a top view schematically illustrating an example of a test circuit fabricated in order to confirm an effect of a light-shield groove group according to an example.[Figure 14] Figure 14 is a graph illustrating an example of a result of evaluating a transmittance spectrum of the test circuit with respect to a wavelength according to the example. Description of Embodiment Hereinafter, an embodiment and an example of the present disclosure will be described in detail with reference to the drawings. The same or similar reference signs indicate the same or similar elements, and repeated descriptions thereof may be omitted. In the following descriptions, when materials and numerical values are described, they are examples, and the present disclosure may be implemented by additionally or alternatively using other materials and numerical values. [Overview] Figure 1 is a top view schematically illustrating an optical circuit 100 configured with a planar lightwave circuit technique of fabricating a silica-based optical waveguide on