JP2023024843A - ring resonator - Google Patents

Abstract

To provide a ring resonator which features a simple configuration, low loss, and low noise.SOLUTION: A ring resonator (10) is provided, comprising a substrate (11) and a ring waveguide unit (12) configured to guide light circularly on the substrate (11), the ring waveguide unit (12) having a diffraction grating unit (18) having a plurality of recesses (17) and protrusions (16) formed periodically along an outer periphery thereof.SELECTED DRAWING: Figure 1

Description

The present invention relates to ring resonators, and more particularly to ring resonators with diffraction gratings.

A ring resonator is an optical device in which, for example, an input waveguide and an output waveguide are arranged adjacent to a closed optical waveguide. Various forms of closed waveguides are conceivable, but a ring-shaped (annular) optical waveguide (ring waveguide) will be described below as an example. A ring waveguide has a plurality of resonance points, and light having a wavelength (frequency) that satisfies resonance conditions is extracted from light input from the input waveguide and output from the output waveguide. That is, the ring resonator functions as a wavelength filter with a high Q value, for example.

A ring resonator may also be used to generate harmonics. For example, the second harmonic generation element disclosed in Patent Document 1 is a bulk-type SHG (Second Harmonic Generation) element, and uses a nonlinear optical crystal that generates the second harmonic. Two resonator mirrors and a reflecting surface are arranged in the nonlinear optical crystal, the two resonator mirrors and the reflecting surface form a ring resonator structure, and light circulates in this ring resonator structure. A second harmonic generated in the nonlinear optical crystal is extracted from one of the resonator mirrors.

For example, Japanese Patent Laid-Open No. 2002-200001 is known as a document related to loss improvement of a ring resonator. The whistle-shaped ring resonator filter disclosed in US Pat. A coupler was used for coupling. It is stated that this can enhance the coupling of light input to the ring resonator.

JP-A-05-173214 Japanese Patent Publication No. 2018-527746

In recent years, there have been an increasing number of mobile bodies equipped with sensing systems such as LiDAR (Light Detection and Ranging). A transmission/reception device that uses light as a signal is an essential device for such a sensing system, and one of the key components of the transmission/reception device is a multiplier. A multiplier is generally a component that multiplies the frequency of an input signal by an integer and outputs it. In the case of an optical signal, the frequency of light is multiplied by an integer (the wavelength is 1/integer).

On the other hand, devices such as transmitting/receiving devices for sensing systems are required to have low loss and low noise, and it is also important that they be small, lightweight, and easy to mount. As described above, although multipliers using ring resonators are known, it would be convenient if a ring resonator with a relatively simple structure having these characteristics could be realized.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a ring resonator which has a simple configuration and which can achieve low loss and low noise.

In order to solve the above-mentioned problems, the ring resonator of the present invention comprises a substrate and a ring waveguide section for guiding light in an annular manner on the substrate, the ring waveguide section having a plurality of concave portions along the outer circumference. and a diffraction grating portion in which convex portions are formed periodically.

In such a ring resonator of the present invention, the second harmonic can be generated in the diffraction grating portion while the input light circulates around the ring resonator. The use of a ring resonator composed of diffraction gratings may provide effects such as low loss and low noise.

In one aspect of the present invention, the ring waveguide portion is made of a nonlinear optical material.

Moreover, in one aspect of the present invention, the shape of the convex portion is any one of a blaze shape, a pillar shape, and a slant shape.

Further, in one aspect of the present invention, the convex portion has a slant shape, and the convex portion is inclined in the propagation direction of the light propagating through the optical waveguide.

In one aspect of the present invention, an input waveguide arranged adjacent to the ring waveguide section and an output waveguide arranged adjacent to the ring waveguide section are further included.

In one aspect of the present invention, a plurality of the ring waveguide portions are arranged adjacent to each other, and an input waveguide is arranged adjacent to the ring waveguide portion at one end and the ring waveguide at the other end. and an output waveguide positioned adjacent to the section.

In one aspect of the present invention, a readout waveguide disposed adjacent to at least one of the ring waveguide portions for reading out an optical signal; and an optical switch provided in the readout waveguide for transmitting or blocking the optical signal. and further including.

Further, in one aspect of the present invention, the plurality of ring waveguide portions adjacent to each other are arranged such that the concave portions and the convex portions are engaged with each other.

Further, in one aspect of the present invention, the convex portion of each of the plurality of ring waveguide portions has a slant shape, and the light propagation directions of the plurality of ring waveguide portions adjacent to each other are opposite to each other. It's becoming

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a ring resonator that has a simple configuration and can achieve low loss and low noise.

1(a) is a schematic plan view and (b) is a schematic cross-sectional view showing the structure of a ring resonator 10 according to a first embodiment. FIG. (a) is a conceptual diagram for explaining the action of the ring resonator 10 according to the first embodiment, (b) is a diagram showing variations in the shape of the projections of the diffraction grating and their optical characteristics, and (c) is a slant. It is a conceptual diagram explaining the shape parameter of the convex part of a shape. FIG. 5 is a schematic plan view showing the structure of a ring resonator 20 according to a second embodiment; FIG. 11 is a schematic plan view showing the structure of a ring resonator 30 according to a third embodiment;

(First embodiment)
BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or equivalent constituent elements, members, and processes shown in each drawing are denoted by the same reference numerals, and duplication of description will be omitted as appropriate. In the following description, a mode in which the ring resonator of the present invention is applied to a multiplier that generates a second harmonic will be described as an example. As an example, the ring resonator according to the present embodiment is used in a sensing system such as LiDAR mounted on mobile bodies such as motorcycles, automobiles, and aircraft.

FIG. 1(a) is a schematic plan view showing the structure of the ring resonator 10 in this embodiment, and FIG. 1(b) is a ring resonator in this embodiment cut along line AA shown in FIG. 1(a). 2 is a schematic cross-sectional view showing the structure of the vessel 10. FIG. As shown in FIGS. 1A and 1B, the ring resonator 10 includes a substrate 11, a ring waveguide section 12, an input waveguide 13, an output waveguide 14, and couplers 20-1 and 20-2. there is The ring resonator 10 is configured using an optical integrated circuit as an example, and each of the ring waveguide section 12, the input waveguide 13, the output waveguide 14, and the couplers 20-1 and 20-2 is formed by an optical waveguide. there is FIG. 1 schematically shows the structure of the ring resonator 10, and the dimensions and angles in the drawing do not represent the actual dimensions of the optical element.

The substrate 11 is a supporting member for forming each optical waveguide. Although the material of the substrate 11 is not particularly limited, silicon dioxide (SiO ₂ ) is used in the ring resonator 10 of this embodiment.

Input waveguide 13 is optically coupled to ring waveguide section 12 by coupler 20 - 1 to guide input light Pin to ring waveguide section 12 . Light that is not coupled to the ring waveguide portion 12 out of the input light Pin is output to the outside of the ring resonator 10 as output light Pout'.
The output waveguide 14 is optically coupled to the ring waveguide section 12 by the coupler 20-2, and outputs the light generated in the ring waveguide section 12 as the output light Pout.

The form of the couplers 20-1 and 20-2 is not particularly limited, but in this embodiment, an evanescent optical coupler is used as an example. In the following description, the couplers 20-1 and 20-2 are collectively referred to as the "coupler 20".

The ring waveguide portion 12 includes a diffraction grating portion 18 having convex portions 16 and concave portions 17 formed along the outer periphery. The material for forming the ring waveguide section 12 is not particularly limited, but the ring resonator 10 in this embodiment is formed using titanium oxide (TiO ₂ ), which is an example of a nonlinear optical material. However, the material forming the ring waveguide portion 12 may be any nonlinear optical material, and is not limited to titanium oxide. As an example, the shape of the convex portion 16 in the present embodiment is a slant shape, which will be described later. In this embodiment, the input waveguide 13 and the output waveguide 14 are also made of titanium oxide, but the input waveguide 13 and the output waveguide 14 may be made of a material different from that of the ring waveguide section 12. good.

As an example, the diffraction grating section 18 (that is, the ring waveguide section 12) is made of a nonlinear optical material having a second-order nonlinear susceptibility χ ⁽²⁾ in the range of 7×10 ⁻¹² m/W or less. Materials with χ ⁽²⁾ greater than 7×10 ⁻¹² m/W are difficult to handle and expensive, and are not suitable for use in optical devices such as ring resonators.

Next, with reference to FIG. 2, the ring waveguide section 12 will be described in more detail. FIG. 2(a) is a diagram showing only the ring waveguide portion 12 extracted from FIG. 1(a). As shown in FIG. 2(a), of the input light Pin input from the input waveguide 13, the light having the angular frequency ω is coupled to the ring waveguide section 12 by the coupler 20-1 (not shown) and is indicated by D1. orbit in the direction. The light of angular frequency ω is partly converted into light of angular frequency 2ω by the action of the diffraction grating portion 18 in the process of circulating the ring waveguide portion 12 . That is, part of the light with the angular frequency ω is doubled to the light with the angular frequency 2ω and converted into the second harmonic.

That is, the ring waveguide section 12 acts as a so-called ring resonator and has a unique resonant wavelength (frequency) derived from the waveguide length of the ring waveguide section 12 and the like. In the case of the ring waveguide section 12, this resonant angular frequency is set to ω. Then, the light with the angular frequency ω coupled to the ring waveguide section 12 is wavelength-converted by the diffraction grating section 18 arranged in the ring waveguide section 12 in the process of circulating the ring waveguide section 12, and part of the light is It becomes the second harmonic of the angular frequency 2ω. Light having an angular frequency close to 2ω and outside the band of the ring waveguide section 12 is emitted from the ring waveguide section 12 and output as output light Pout'.

Further, in this embodiment, as shown in FIG. 2A, local plasmons LP are generated at the acute angle portion of the upper side of the slant-shaped convex portion 16 . This local plasmon LP enhances the intensity of the second harmonic circulating in the ring waveguide section 12 (local field enhancement).

Here, the shape of the convex portion 16 forming the diffraction grating portion 18 will be described in more detail. As shown in FIG. 2B, representative shapes of the convex portion 16 include a blaze shape, a pillar shape, and a slant shape. The blaze shape is also called a sawtooth shape, and refers to a saw tooth-like shape. The pillar shape is a columnar shape composed of two sides protruding in the normal direction of a circle C (see FIG. 2(c)) forming the outer edge of the ring waveguide portion 12 and an upper side orthogonal to the two sides. The slant shape consists of two sides inclined by a predetermined angle with respect to the normal direction of the circle C forming the outer edge of the ring waveguide portion 12, and an upper side connected to one end of each of the two sides.

The shape of the slant-shaped convex portion 16 will be described in more detail with reference to FIG. 2(c). FIG. 2(c) is an enlarged view of the slant-shaped convex portion 16. As shown in FIG. As shown in FIG. 2(c), the convex portion 16 has side sides S1 and S2 and an upper side U formed to intersect a circle C forming the outer edge of the ring waveguide portion 12. As shown in FIG. A point P, which is an intersection of the side S1 and the upper side U, is a point (local field) that is the source of the local plasmons LP described above.

Each of the sides S1 and S2 is inclined at an angle θ with respect to the normal N to the circle C forming the outer edge of the ring waveguide portion 12 . However, the inclination angle of the side S2 may be different from the inclination angle of the side S1. The size of the projection 16 is represented by the width d, which is the length of the portion of the projection 16 along the circle C, and the height h2 of the side S1 in the normal N direction of the circle C. As shown in FIG.

Here, an example of the size of each part of the ring waveguide part 12 will be described. The inner diameter R of the ring waveguide portion 12 shown in FIG. 2A is 150 μm or less, and the width W of the annular portion of the ring waveguide portion 12 excluding the diffraction grating portion 18 is about 1.0 μm to 37.5 μm. The height h1 in the cross-sectional direction of the ring waveguide portion 12 shown in (b) is 10 μm or less. The width d of the protrusions 16 is about 0.25 μm, the height h2 is about 0.70 μm, the pitch (interval) P between adjacent protrusions 16 is about 700 nm to 800 nm, and the inclination angle θ is about 30 degrees to 60 degrees. be. As an example, the pitch P of the convex portions 16 is defined by the interval between the intersections of the side S1 and the circle C, as shown in FIG. 2(c).

Next, the optical characteristics of each shape of the convex portion 16 will be described. FIG. 2(b) shows the optical characteristics of the blaze shape, pillar shape, and slant shape. The propagation loss shown in FIG. 2(b) means the loss received by the propagating light when it circulates in the ring waveguide section 12, and the local electric field means the strength of the local field that generates the local plasmons LP described above. . As shown in FIG. 2(b), the blaze shape has a small propagation loss and a moderate local electric field. The pillar shape has a relatively large propagation loss and a relatively small local electric field. The slant shape has a small propagation loss and a large local electric field. Although any shape of the convex portion 16 shown in FIG. 2B may be adopted according to the required optical characteristics, the ring resonator 10 of the present embodiment is based on the premise that local plasmon LP is used. It has a slant shape.

As shown in FIG. 2(b), the slant shape has excellent optical properties in both propagation loss and local electric field. Therefore, in the present embodiment, focusing on this optical characteristic, the slant-shaped convex portion 16 is adopted, and further loss reduction is attempted. That is, local plasmons LP are generated by utilizing the fact that the local electric field is large, and light circulating in the ring waveguide section 12 is amplified by the local plasmons LP. In other words, the local plasmons LP according to the present embodiment have such intensity that the propagation loss of the ring waveguide section 12 in the ring resonator 10 does not matter.

The ring resonator 10 configured as described above can generate the second harmonic by the action of the diffraction grating section 18 . In addition, noise is removed by the filtering action of the ring resonator 10, so the generated second harmonic is low noise. Furthermore, since the generated second harmonic is amplified by the local plasmon LP, a low-loss ring resonator can be realized. Moreover, since the ring resonator 10 is formed by optical integrated circuit technology, it can be made small, lightweight, and low in height.

Here, a method for manufacturing the ring resonator 10 will be briefly described. First, a silicon dioxide substrate is prepared. Next, a titanium oxide film is formed on the substrate using a sputtering method or the like. Next, the titanium oxide film is processed using known techniques such as photolithography and etching to form the ring waveguide section 12, the input waveguide 13, and the output waveguide . The ring resonator 10 is manufactured by the above method.

As described in detail above, according to the ring resonator of the present embodiment, it is possible to provide a ring resonator that has a simple configuration and can achieve low loss and low noise.

(Second embodiment)
The ring resonator 20 in this embodiment will be described with reference to FIG. This embodiment is a form in which a plurality of ring waveguide portions 12 are arranged in the first embodiment. Therefore, similar configurations are denoted by the same reference numerals, and detailed description thereof is omitted.

As shown in FIG. 3, the ring resonator 20 includes a substrate 11, an input waveguide 13, a plurality of (n in FIG. 3) ring waveguide sections 12-1, 12-2, 12- 3, . . . , 12-n (hereinafter collectively referred to as “ring waveguide portion 12”), and an output waveguide 14 are included. , 12-n form a CROW (Coupled Ring-resonating Optical Waveguide).

The input waveguide 13 is coupled to the ring waveguide section 12-1 via a coupler (not shown). The output waveguide 14 is coupled to the ring waveguide section 12-n via a coupler (not shown). As shown in FIG. 3, each of the ring waveguide portions 12 is arranged so that the inclination directions of the projections 16 are the same, and the projections 16 and the recesses 17 of the adjacent ring waveguide portions 12 are meshed with each other. are placed. Here, the reason why the convex portion 16 and the concave portion 17 are arranged so as to mesh with each other is that it is expected to increase the light coupling efficiency. The convex portion 16 and the concave portion 17 of the wave path portion 12 may be spaced apart from each other.

In the ring resonator 20 of this embodiment, the light propagating through each ring waveguide portion 12 is directed in the opposite direction between adjacent ring waveguide portions 12 . That is, in the ring waveguide section 12-1, the propagating light circulates in the direction indicated by symbol D1 (clockwise direction), and in the ring waveguide section 12-2, the direction indicated by symbol D2 (counterclockwise direction), In the ring waveguide portion 12-3, it circulates in the direction indicated by symbol D1 (clockwise direction), . . . , in the direction indicated by symbol D1 (clockwise direction) in the ring waveguide portion 12-n. . . , 12-n, the propagation direction of the circulating light is the same as the inclination direction of the projections 16, but the ring waveguides 12-2, . In 12-4, . It should be noted that whether the propagation direction of the light circulating in the ring waveguide portion 12 is the same direction (that is, clockwise direction) or the opposite direction (that is, counterclockwise direction) as the inclination direction of the convex portion 16 depends on the ring waveguide. Although there may be a slight difference in the speed of light circulating in the portion 12, it is believed to have little effect on the basic characteristics, particularly the generation of local plasmons LP.

In the ring resonator 20 as well, when the input light Pin with an angular frequency ω is input to the input waveguide 13, a second harmonic with an angular frequency of 2ω is generated in each of the ring waveguide sections 12, and from the output waveguide 14 It is output as output light Pout. Light having an angular frequency close to 2ω and outside the band of the ring waveguide section 12 is emitted from the ring waveguide section 12 and output as output light Pout'.

The ring resonator 20 configured as described above functions as a delay device. That is, since each ring waveguide section 12 delays the circulating light, a certain time delay occurs between the input light Pin and the output light Pout. This delay can be adjusted by the number of ring waveguide sections 12 . The adjustment may be performed by selecting the ring waveguide section 12 in which the output waveguide 14 is arranged. In the present embodiment, the case where n is an odd number has been described as an example, but it may of course be an even number.

As described above, the ring resonator according to the present embodiment can also provide a ring resonator that has a simple configuration and can achieve low loss and low noise. In particular, the ring resonator 20 in this embodiment functions as a delay device capable of adjusting the delay time.

(Third embodiment)
The ring resonator 30 in this embodiment will be described with reference to FIG. This embodiment is a form in which the readout waveguide 19 is arranged in the second embodiment. Therefore, similar configurations are given the same reference numerals, and detailed description thereof is omitted.

As shown in FIG. 4, in the ring resonator 30, the ring waveguide section 12-1 has the readout waveguide 19-1, the ring waveguide section 12-2 has the readout waveguide 19-2, and the ring waveguide section 12- A readout waveguide 19-3 is coupled to 3, . There is an optical switch 15-1 in the middle of the readout waveguide 19-1, an optical switch 15-2 in the middle of the readout waveguide 19-2, and an optical switch 15-3 in the middle of the readout waveguide 19-3. , . . . , optical switches 15-n are arranged in the middle of the readout waveguides 19-n. Output light Pout1 from the readout waveguide 19-1, output light Pout2 from the readout waveguide 19-2, output light Pout3 from the readout waveguide 19-3, . are output as output light Poutn. Hereinafter, the readout waveguides 19-1 to 19-n will be collectively referred to as "readout waveguides 19", and the optical switches 15-1 to 15-n will be collectively referred to as "optical switch 15". Pout1 to Poutn are collectively referred to as "output light Pouti".

In the ring resonator 30 configured as described above, by turning on (transmission) or off (blocking) the optical switch 15, whether or not the light circulating in the ring waveguide section 12 is read from the readout waveguide 19, That is, it is possible to control whether or not to output the output light Pouti. In this embodiment, the configuration in which the readout waveguides 19 are arranged in all the ring waveguide portions 12 has been exemplified and explained. good too.

Further, the arrangement (coupling) position of the readout waveguide 19 with respect to the ring waveguide section 12 is not limited to the position shown in FIG. In addition, although FIG. 4 exemplifies a form in which the readout waveguide 19 is arranged above or below the ring waveguide portion 12, it is not limited to this, and may be arranged obliquely at a position deviated from the upper or lower portion. Furthermore, the optical switch 15 is not essential in the present embodiment, and may be omitted if the output light Pouti is to be constantly output from each readout waveguide 19 .

As described above, the ring resonator according to the present embodiment can also provide a ring resonator that has a simple configuration and can achieve low loss and low noise. In particular, the ring resonator 30 in this embodiment can control extraction of light circulating in each of the plurality of ring waveguide sections 12 .

In each of the above-described embodiments, the case where the harmonic generated by the ring waveguide portion 12 is the second harmonic has been described. By changing the size (width d, height h2) of the portion 16, the arrangement pitch P, the inclination angle θ, and the like, it is possible to generate higher-order harmonics.

The present invention is not limited to the above-described embodiments, but can be modified in various ways within the scope of the claims, and can be obtained by appropriately combining technical means disclosed in different embodiments. is also included in the technical scope of the present invention.

10, 20, 30... Ring resonator 11... Substrate 12, 12-1, 12-2, 12-3,..., 12-n... Ring waveguide section 13... Input waveguide 14... Output waveguide 15, 15-1, 15-2, 15-3, . . . , 15-n... Optical switch 16... Convex part 17... Concave part 18... Diffraction grating part 19, 19-1, 19-2, 19-3,... , 19-n... Readout waveguides 20, 20-1, 20-2... Coupler C... Circles D1, D2... Direction LP... Local plasmon Pin... Input light Pout, Pout', Pouti, Pout1, Pout2, Pout3, . . , Poutn... Output light θ... Angle

Claims (9)

a substrate;
A ring waveguide section for guiding light circularly on the substrate,
A ring resonator, wherein the ring waveguide portion has a diffraction grating portion in which a plurality of concave portions and convex portions are periodically formed along the outer circumference. A ring resonator according to claim 1,
A ring resonator, wherein the ring waveguide portion is made of a nonlinear optical material. 3. The ring resonator according to claim 1 or 2,
A ring resonator, wherein the shape of the projection is one of a blaze shape, a pillar shape, and a slant shape. A ring resonator according to claim 3,
The convex portion has a slant shape,
A ring resonator according to claim 1, wherein said convex portion is inclined in a propagation direction of light propagating through said optical waveguide. A ring resonator according to any one of claims 1 to 4,
an input waveguide arranged adjacent to the ring waveguide section;
and an output waveguide disposed adjacent to the ring waveguide portion. A ring resonator according to any one of claims 1 to 4,
A plurality of said ring waveguide portions are arranged adjacent to each other,
an input waveguide arranged adjacent to the ring waveguide portion at one end;
and an output waveguide disposed adjacent to the ring waveguide portion at the other end. A ring resonator according to claim 6,
a readout waveguide disposed adjacent to at least one ring waveguide portion for reading out an optical signal;
and an optical switch provided in the readout waveguide for transmitting or blocking the optical signal. 8. The ring resonator according to claim 6 or 7,
A ring resonator, wherein the plurality of adjacent ring waveguide portions are arranged such that the concave portions and the convex portions are engaged with each other. A ring resonator according to any one of claims 6 to 8,
the convex portion of each of the plurality of ring waveguide portions has a slant shape,
A ring resonator, wherein the light propagation directions of the plurality of adjacent ring waveguide portions are opposite to each other.

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