JP2023109239A - Optical waveguide device - Google Patents

Abstract

To provide an interference meter-type element which has an unimodal wavelength characteristic and is highly responsive to change of the index of refraction.SOLUTION: The present invention relates to an optical waveguide device having an optical waveguide core 30 on a lower clad 20 and having a detection unit 50. The detection unit includes: a first waveguide 100; a second waveguide 200 located near the first waveguide and having a slot; and a grating for shifting light which propagates in the first waveguide toward the second waveguide in at least one of the first waveguide and the second waveguide.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical waveguide element, and more particularly to an optical waveguide element that can be used as an element for detecting refractive index changes around the waveguide.

Silicon (Si) photonics has conventionally attracted attention as a platform technology for optical waveguide devices. The characteristics of Si photonics are the miniaturization and integration of optical devices such as optical waveguides and similar modulators and light receivers, and the compatibility with existing semiconductors, by using the manufacturing process of semiconductor devices such as CMOS (Complementary Metal Oxide Semiconductor). The high productivity is due to the 200 mm or 300 mm wafer process provided by diverting manufacturing technology. In addition, a Si waveguide having a Si core and a Si oxide film (SiO ₂ ) as a clad has a relative refractive index difference of up to 40%, so that a high optical confinement effect can be obtained. In particular, in a Si wire waveguide, the radius of curvature of the bent waveguide and the pitch of the parallel wiring can be reduced to the order of several microns, making it possible to miniaturize the optical circuit layout.

As an example of the use of the Si waveguide, application to an element for detecting refractive index change around the waveguide is under consideration (see, for example, Non-Patent Document 1 or Non-Patent Document 2). By using the evanescent wave near the waveguide wall surface, it is possible to detect the biological material adsorbed to the waveguide surface.

Elements used to detect biological substances can be classified into a resonator type, such as a ring resonator, which uses light resonance, and an interferometer type, such as a Mach-Zehnder interferometer, which uses interference. There is also a report that the interferometer type is advantageous for highly sensitive detection of changes in refractive index due to adsorption of biological substances.

Advanced Material Technologies vol.5, p.1901138, 2020 Sensors vol.16, p.285, 2016

However, in both the resonator type and the interferometer type, since the wavelength response characteristic is periodic, a large refractive index change causes the wavelength characteristic to change beyond the period, making it indistinguishable from a small refractive index change. There is In the resonator type, there are elements having single-peak wavelength characteristics such as Bragg gratings and one-dimensional photonic crystals, but in the interferometer type, elements with simple structures are not known.

The present invention has been made in view of the above problems. SUMMARY OF THE INVENTION An object of the present invention is to obtain an interferometer type element that has a unimodal wavelength characteristic and high responsiveness to changes in the refractive index.

In order to achieve the above object, an optical waveguide element of the present invention is an optical waveguide element formed by providing an optical waveguide core on a lower clad, and has a detection section. The detection unit includes a first waveguide, a second waveguide arranged close to the first waveguide and formed with a slot, and propagating through the first waveguide to at least one of the first waveguide and the second waveguide. and a grating for transferring light from the waveguide to the second waveguide.

Further, according to another embodiment of the optical waveguide device of the present invention, it has N (N is an integer equal to or greater than 2) detection portions. An input waveguide is connected to one end of the first waveguide provided in each detection section, and an output waveguide is connected to the other end of the first waveguide. The output waveguide connected to the k-th (k is an integer from 1 to N−1) detection section is connected to the input waveguide connected to the k+1-th detection section.

According to another embodiment of the optical waveguide device of the present invention, the first input/output waveguide, the ring-shaped waveguide, and the optical waveguide core are formed on the lower clad to form a ring resonator, , with a second input and output waveguide, and having a detection portion as described above.

An input waveguide is connected to one end of the first waveguide, an output waveguide is connected to the other end, and the input waveguide and the output waveguide are connected by a curved waveguide to form a ring waveguide. Configure. The second waveguide of the detection section constitutes the first input/output waveguide, and the second input/output waveguide is arranged close to the ring-shaped waveguide.

In the optical waveguide device of the present invention, gratings may be provided on both side surfaces of the second waveguide. Alternatively, the grating may be provided on the side surface of the first waveguide on the side of the second waveguide.

According to a further preferred embodiment of the optical waveguide element of the present invention, the optical waveguide core is covered with an upper clad made of the same material as the lower clad in the region where the detector is not provided, and the region where the detector is provided. , the optical waveguide core is not covered with an upper cladding.

The optical waveguide element of the present invention has a single-peak wavelength characteristic and high responsiveness to changes in the refractive index.

It is a schematic diagram for demonstrating a 1st waveguide element. It is a figure which shows the characteristic of a 1st waveguide element. It is a schematic diagram for demonstrating a 2nd waveguide element. It is a figure which shows the characteristic of a 2nd waveguide element. It is a schematic diagram for demonstrating a 3rd waveguide element. It is a schematic diagram for demonstrating a 4th waveguide element.

Embodiments of the present invention will be described below with reference to the drawings, but the shape, size and arrangement of each component are only schematically shown to the extent that the present invention can be understood. Further, although preferred configuration examples of the present invention will be described below, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many modifications and variations that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(Structure of first waveguide element)
An optical waveguide device (hereinafter also referred to as a first waveguide device) according to a first embodiment of the present invention will be described with reference to FIG. FIG. 1 is a schematic diagram for explaining the first waveguide element. FIG. 1A is a schematic plan view of the first waveguide element, omitting a supporting substrate and a lower clad, which will be described later, and showing only the optical waveguide core. In other schematic plan views, the supporting substrate and the lower clad are omitted, and only the optical waveguide core is shown. FIG. 1B is a cut end view of a partial region of the first optical waveguide device.

The first optical waveguide device is configured with a lower clad 20 on a support substrate 10 and an optical waveguide core 30 on the lower clad 20 .

In order to prevent light propagating through the optical waveguide core 30 from escaping to the support substrate 10, the distance between the support substrate 10 and the optical waveguide element 30, that is, the thickness of the lower clad 20 is preferably 1 μm or more. Moreover, the thickness of the optical waveguide core 30 is preferably 200 to 400 nm, which is a value capable of achieving a single mode condition in the depth direction. Here, it is assumed that the thickness of the optical waveguide core 30 is 220 nm.

Light propagates according to the planar shape of the optical waveguide core 30 to achieve a desired function.

The first waveguide element has a detection section 50 configured with a first waveguide 100 and a second waveguide 200 .

The first waveguide 100 and the second waveguide 200 are straight waveguides arranged in parallel and close to each other. The width _W1 of the first waveguide 100 is, for example, 600 nm, and the width _W2 of the second waveguide 200 is, for example, 700 nm. Also, the length L of the first waveguide 100 and the second waveguide 200 is, for example, approximately 99 μm. A gap g between the first waveguide 100 and the second waveguide 200 is, for example, 300 nm.

An input waveguide 110 is connected to one end of the first waveguide 100 . An output waveguide 120 is connected to the other end of the first waveguide 100 .

A first curved waveguide 210 is connected to one end of the second waveguide 200 . A second curved waveguide 220 is connected to the other end of the second waveguide 200 . The first curved waveguide 210 and the second curved waveguide 220 are provided so that the distance from the first waveguide 100 increases as the distance from the second waveguide 200 increases. The input waveguide 110 and the output waveguide 120 may also be curved waveguides.

A slot 202 is provided in the center of the second waveguide 200 along the longitudinal direction of the second waveguide 200 . The width W _s of the slot 202 is, for example, 150 nm. A grating 204 is formed on the side wall of the second waveguide. The grating 204 is configured, for example, in a trigonometric shape with a period Λ of 2.78 μm. The width W of the grating 204, which corresponds to twice the amplitude of the trigonometric function, is minimum near one longitudinal end and the other end and gradually increases toward the longitudinal center, The maximum width is, for example, 130 nm.

The propagation loss of light propagating through the optical waveguide core 30 tends to be lower when the optical waveguide core 30 is surrounded by an upper clad made of the same material as the lower clad 20 . Therefore, it is preferable to provide an upper clad covering the optical waveguide core 30 with SiO ₂ , which is the same material as the lower clad 20, in a portion other than the sensing region where the detection unit 50 necessary for detecting biological substances is provided.

On the other hand, in the area where the detection section 50 is provided, it is preferable not to provide the upper clad covering the optical waveguide core 30 in order to detect the refractive index change due to adsorption of the biological substance with high sensitivity.

(Operation of first waveguide element)
The operation of the first waveguide element will be explained. Light input to the input waveguide 110 mainly excites a mode in which light is concentrated in the first waveguide 100 . When phase matching between the light propagating in the first waveguide 100 and the mode propagating in the second waveguide 200 is realized by the grating 204 provided in the second waveguide 200, the light propagating in the first waveguide 100 The light is converted into light propagating through the second waveguide 200 . Between the equivalent refractive index N ₁ of light propagating through the first waveguide 100, the equivalent refractive index N ₂ of light propagating through the second waveguide 200, and the grating period Λ, the following equation (1) holds: holds.

λ=λ/(N ₁ −N ₂ ) (1)
When the refractive index around the first waveguide 100 and the second waveguide 200 changes, the equivalent refractive indices _N1 and _N2 change, so that the phase-matching wavelength λ changes. This change in phase-matching wavelength λ is detected to detect the refractive index change around the waveguide.

Here, since the slot 202 is formed in the second waveguide 200, it is more affected by the change in the refractive index inside the slot than on both side surfaces and top surface of the waveguide. Therefore, the second waveguide 200 with slots 202 is more sensitive to ambient refractive index changes than the first waveguide 100 without slots. Therefore, mainly the equivalent refractive index _N2 of the second waveguide 200 contributes to the change of the phase matching wavelength λ.

From the above equation (1), the wavelength change Δλ with respect to the change _ΔN2 in the refractive index _N2 of the second waveguide satisfies the relationship given by the following equation (2).

Δλ/λ=−ΔN ₂ /(N _1g −N _2g ) (2)
Here, N _1g and N _2g are the group refractive indices of the first waveguide 100 and the second waveguide 200, respectively. From the above equation (2), it can be seen that a large wavelength change can be obtained if N _1g -N _2g is small.

FIG. 2 is a diagram showing characteristics of the first waveguide element. In FIG. 2, the horizontal axis indicates the wavelength [unit: μm], and the vertical axis indicates the light intensity (a.u.) as the measured value. FIG. 2 shows the results of a simulation by a three-dimensional FDTD (Finite Difference Time Domain) method. In FIG. 2, curve I indicates the intensity of light input to the input waveguide 110, curve II indicates the intensity of light output from the output waveguide 120, and curve III indicates the intensity of light output from the second curved waveguide 220. shows the light intensity that will be used. Here, each numerical condition is shown in the above example.

As shown in FIG. 2, the output intensity (curve II) from the output waveguide 220 of the first waveguide element can have a well-shaped depression (peak dip) (indicated by A in FIG. 2). can. At this time, the wavelength change with respect to the refractive index change around the waveguide is 1500 nm/RIU (Refractive Index Unit), which is one or more orders of magnitude higher than that of a normal sensor.

(Manufacturing method of the first waveguide element)
The first optical waveguide element can be easily manufactured by using, for example, an SOI (Silicon On Insulator) substrate.

An example of a method for manufacturing the first waveguide element will be described below.

First, an SOI substrate composed of a support substrate layer, an _SiO2 layer, and a Si layer, which are sequentially laminated, is prepared. Next, for example, dry etching is performed to pattern the Si layer. This dry etching ends when the _SiO2 layer is reached. As a result, the basic structure of the optical waveguide including the supporting substrate 10, the lower clad 20, and the optical waveguide core 30 can be obtained.

(Structure of second waveguide element)
An optical waveguide device (hereinafter also referred to as a second waveguide device) according to a second embodiment of the present invention will be described with reference to FIG. FIG. 3 is a schematic diagram for explaining the second waveguide element. FIG. 3 is a schematic plan view of a second waveguide element;

The second waveguide element differs from the first waveguide element in the planar shape of the optical waveguide core. Since other configurations are the same as those of the first waveguide element, redundant description may be omitted.

The second waveguide element has a detection section 51 configured with a first waveguide 101 and a second waveguide 201 .

The first waveguide 101 and the second waveguide 201 are linear waveguides arranged in parallel and close to each other. The width of the first waveguide 101 is, for example, 600 nm, and the width of the second waveguide 201 is, for example, 700 nm. Also, the lengths of the first waveguide 101 and the second waveguide 201 are, for example, about 91 μm. A gap between the first waveguide 101 and the second waveguide 201 is, for example, 300 nm.

An input waveguide 110 is connected to one end of the first waveguide 101 via a tapered waveguide 134 . An output waveguide 120 is connected to the other end of the first waveguide 101 via a tapered waveguide 136 . A first curved waveguide 210 is connected to one end of the second waveguide 201 via a tapered waveguide 234 . A second curved waveguide 220 is connected to the other end of the second waveguide 201 via a tapered waveguide 236 . The first curved waveguide 210 and the second curved waveguide 220 are provided so that the distance from the first waveguide 101 increases as the distance from the second waveguide 201 increases. The length of each tapered waveguide 134, 136, 234 and 236 is, for example, 15 μm.

A slot 202 is provided in the center of the second waveguide 201 along the longitudinal direction of the second waveguide 201 . The width of slot 202 is, for example, 150 nm. Slots 204 and 206 also extend into tapered waveguides 234 and 236 connected to second waveguide 201 . The slots 204 and 206 provided in the tapered waveguides 234 and 236 are formed in a shape gradually coming to one side as the first and second curved waveguides 210 and 220 are approached.

A grating 104 is formed on the side wall of the first waveguide 101 . The grating 104 is provided only on the side wall of the first waveguide 101 on the second waveguide 201 side. The grating 104 is configured, for example, in a trigonometric shape with a period of 2.28 μm. The width W of the grating, which corresponds to twice the amplitude of the trigonometric function, is minimum near one end and the other end in the longitudinal direction, gradually increases toward the center in the longitudinal direction, and reaches a maximum. The amplitude is, for example, 130 nm.

The side wall of the first waveguide 101 on the side of the second waveguide 201 is a place where the light intensity of the mode in which the light field distribution mainly concentrates on the first waveguide 101 is high. Therefore, by providing the grating 104 on the side wall of the second waveguide 201, higher mode conversion efficiency can be obtained. Further, by providing tapered waveguides 234 and 236 between the second waveguide 201 and the first and second curved waveguides 210 and 220, and further providing slots 204 and 206 in the tapered waveguides 234 and 236, , the loss of light can be suppressed. Similar to the second waveguide 201, by providing the tapered waveguides 134 and 136 between the first waveguide 101 and the input waveguide 110 and the output waveguide 120, light loss can be suppressed.

(Operation of second waveguide element)
Characteristics of the second waveguide element will be described with reference to FIG. FIG. 4 is a diagram showing characteristics of the second waveguide element. In FIG. 4, the horizontal axis indicates the wavelength [unit: μm], and the vertical axis indicates the light intensity (a.u.) as the measured value. FIG. 4 shows the results of a simulation by the three-dimensional FDTD method, similar to FIG. In FIG. 4, curve I indicates the light intensity input to the input waveguide, curve II indicates the light intensity output from the output waveguide, and curve III indicates the light output from the second curved waveguide. showing strength. Here, each numerical condition is shown in the above example.

As shown in FIG. 4, a well-shaped dip (peak-dip) can be obtained in the output intensity (curve II) from the output waveguide of the second waveguide element. At this time, the wavelength change with respect to the refractive index change around the waveguide is 1400 nm/RIU (Refractive Index Unit), which is one or more orders of magnitude higher than that of a normal sensor.

(Third waveguide element)
An optical waveguide device (hereinafter also referred to as a third waveguide device) according to a third embodiment of the present invention will be described with reference to FIG. FIG. 5 is a schematic diagram for explaining the third waveguide element.

The third waveguide element is configured by connecting N (N is an integer equal to or greater than 2) detection units 51 in series. FIG. 5 shows an example in which N is three. Here, an example in which each detection unit 51 is configured in the same manner as the second waveguide element will be described, but the present invention is not limited to this. Each detector may be configured in the same manner as the first waveguide element.

An input waveguide 110 is connected to one end of the first waveguide provided in each detection unit 51, and an output waveguide 120 is connected to the other end. The input waveguide 110-1 connected to the first detection section 51-1 functions as the input waveguide of the third waveguide element. The output waveguide 120-k connected to the k-th (k is an integer from 1 to N−1) detection section 51-k is connected to the k+1-th detection section 51-(k+1) via the curved waveguide 140. connected to the input waveguide 110-(k+1). The output waveguide 120-N connected to the Nth detection section 51-N functions as the output waveguide of the third waveguide element.

Light output from a second curved waveguide (not shown) connected to the second waveguide of each detector 51 is used to obtain a peak-dip. The light output from this second curved waveguide may be discarded or used for monitoring.

Also, here, the output waveguide 120-k connected to the k-th detection unit 51-k is connected to the k+1-th detection unit 51-(k+1) through the curved waveguide 140, and the input waveguide 110- (k+1) has been described as an example, but the present invention is not limited to this. The output waveguide 120-k connected to the k-th detection unit 51-k may be directly connected to the input waveguide 110-(k+1) connected to the k+1-th detection unit 51-(k+1), It may be connected via a straight waveguide.

The effect of using multiple stages will be explained. The response on the slope where a linear response is obtained is expressed by T=(ax+b) ^N , where ax+b is the one-step response. where x is the wavelength change. In the case of one stage, b=0.5 assuming that the output varies between 0 and 1. Then, for N=3, the response of the linear part of T is of the form T≈3ab ² x+b ³ . For a slope that gives a linear response, b ³ =0.5, so if a=1, then T≈1.9x+0.5. As a result, with three stages, the sensitivity is approximately twice as high as that with one stage. As N increases, b approaches 1 and T≈Nb ^N−1 x+b ^N. Therefore, if N is sufficiently large, N times the sensitivity can be obtained as compared to a single stage.

(Fourth waveguide element)
An optical waveguide device (hereinafter also referred to as a fourth waveguide device) according to a fourth embodiment of the present invention will be described with reference to FIG. FIG. 6 is a schematic diagram for explaining the fourth waveguide element.

The fourth waveguide element comprises a first input/output waveguide, a ring-shaped waveguide, and a second input/output waveguide, which constitute a ring resonator. Here, an example in which the detection section 51 is configured in the same manner as the second waveguide element will be described, but the configuration is not limited to this. The detection section may be configured similarly to the first waveguide element.

An input waveguide 110 is connected to one end of a first waveguide 101 provided in the detection section 51, and an output waveguide 120 is connected to the other end thereof. An input waveguide 110 and an output waveguide 120 are connected by a curved waveguide 150 to form a ring waveguide.

A second waveguide 201 included in the detection unit 51 constitutes a first input/output waveguide.

The second input/output waveguide 300 is arranged close to the curved waveguide 150 forming a ring-shaped waveguide. This closely arranged portion 310 is a coupler structure, and adjusts the height of the resonance peak by appropriately removing part of the light circulating in the ring-shaped waveguide.

Since the light circulating in the ring-shaped waveguide repeatedly passes through the detection section 51, a multistage effect similar to that of the third waveguide element configured by connecting the detection sections 51 in multiple stages can be obtained.

REFERENCE SIGNS LIST 10 support substrate 20 lower clad 30 optical waveguide core 50, 51 detector 100, 101 first waveguide 104, 204 grating 110 input waveguide 120 output waveguide 134, 136, 234, 236 tapered waveguide 140, 150 curved waveguide 200, 201 second waveguide 202, 204, 206 slot 210 first curved waveguide 220 second curved waveguide 300 second input/output waveguide

Claims (6)

An optical waveguide element formed by providing an optical waveguide core on a lower clad,
a first waveguide;
a second waveguide positioned adjacent to the first waveguide and having a slot formed thereon;
An optical waveguide, wherein at least one of the first waveguide and the second waveguide has a detection section including a grating for transferring light propagating in the first waveguide to the second waveguide. element. An optical waveguide element formed by providing an optical waveguide core on a lower clad,
having N (N is an integer of 2 or more) detection units,
Each detector is
a first waveguide;
a second waveguide positioned adjacent to the first waveguide and having a slot formed thereon;
At least one of the first waveguide and the second waveguide has a grating for transferring the light propagating in the first waveguide to the second waveguide,
An input waveguide is connected to one end of the first waveguide provided in each of the detection units, and an output waveguide is connected to the other end of the first waveguide,
An optical waveguide device characterized in that an output waveguide connected to a k-th (k is an integer of 1 or more and N-1 or less) detection section is connected to an input waveguide connected to a k+1-th detection section. An optical waveguide element comprising a first input/output waveguide, a ring-shaped waveguide, and a second input/output waveguide, which is formed by providing an optical waveguide core on a lower clad and constitutes a ring resonator,
a first waveguide;
a second waveguide positioned adjacent to the first waveguide and having a slot formed thereon;
At least one of the first waveguide and the second waveguide has a detection unit including a grating for transferring light propagating in the first waveguide to the second waveguide,
an input waveguide is connected to one end of the first waveguide and an output waveguide is connected to the other end;
the input waveguide and the output waveguide are connected by a curved waveguide to form the ring-shaped waveguide;
The second waveguide constitutes the first input/output waveguide,
The second input/output waveguide is arranged close to the ring-shaped waveguide.
An optical waveguide device characterized by: 4. The optical waveguide device according to claim 1, wherein the grating is provided on both side surfaces of the second waveguide. 4. The optical waveguide device according to claim 1, wherein the grating is provided on a side surface of the first waveguide on the side of the second waveguide. In a region where the detection section is not provided, the optical waveguide core is covered with an upper clad made of the same material as the lower clad,
6. The optical waveguide device according to claim 1, wherein the optical waveguide core is not covered with an upper clad in a region where the detection section is provided.

Images