JP2018040829A - Interlayer coupler - Google Patents

Abstract

An interlayer coupler that has a simple structure and is easy to manufacture and can be realized with a practical length is provided. An interlayer coupler (100) is an interlayer coupler that couples layers by propagating a light distribution transitioned from a first waveguide (10) to a second waveguide (20), the first waveguide (10) and the second waveguide (100). The waveguide 20 includes a first taper 11 that decreases in width from the waveguide toward the tip, and a second taper 12 that tapers in a more gentle taper shape from the end of the first taper 11 toward the tip. The first taper 11 has a tapered shape with a high refractive index, and the second taper 12 has a tapered shape that gradually changes the refractive index based on the effective refractive index lowered by the first taper 11. [Selection] Figure 1

Description

  The present invention relates to an interlayer coupler that connects wirings across layers in an optical circuit chip incorporating optical wirings.

In recent years, research on silicon photonics, which can use mature process technology in silicon LSI, has enabled the realization of low-loss optical waveguides (hereinafter referred to as waveguides) even at extremely fine and sharp turns. . For this reason, it is becoming possible to reduce the size and power consumption of transmission / reception modules and systems for optical communication and to introduce and integrate optical wiring into silicon LSIs. As a candidate for such a waveguide, a silicon (Si) fine wire waveguide that can be formed on a silicon-on-insulator (SOI) substrate by a relatively simple technique is promising. It is known that a silicon fine wire or a silicon rib structure functions as a low-loss waveguide for light with a wavelength of 1.3 to 1.5 μm used for optical communication.
It is important to improve the degree of integration of the optical waveguide circuit in order to keep costs down while responding to increasing signal volume and communication speed requirements. Therefore, it is necessary not only to arrange the waveguides in parallel but also to provide a location where the waveguides intersect each other.

  In Patent Document 1, one of a lower cladding layer formed on a substrate, a first optical waveguide formed on the lower cladding layer, and the lower cladding layer partitioned by the first optical waveguide is provided. A second optical waveguide having a tip formed toward the side surface of the first optical waveguide, the tip of which is narrowed in a tapered shape, and the other of the lower cladding layer partitioned by the first optical waveguide In addition, there is described an optical waveguide circuit including a third optical waveguide having a distal end formed opposite to the distal end portion of the second optical waveguide and having the distal end portion tapered.

JP 2009-204736 A

In an optical waveguide coupling structure using a diffraction grating (such as a grating coupler having a metal mirror) that extracts light in the chip vertical direction to optically couple optical waveguides, the structure is complicated and fabrication is not easy. High cost is inevitable.
On the other hand, in a multilayer optical circuit, in order to ignore unintended signal crosstalk beyond the layers, it is desirable that the distance between the upper and lower waveguides is 1 μm or more between layers (interlayer thickness).
For example, an interlayer coupler that broadens the electromagnetic field distribution and couples the upper and lower waveguides by making the waveguide shape tapered at the tip portion has been proposed (see comparative example described later). Since it can be realized with a tapered waveguide, there is a serious drawback that a very long element length of about 5 cm is required for an interlayer distance of 1 μm or more although it is a simple structure. Since such a very long element length is required, a multilayer optical circuit including the interlayer coupler has not been put into practical use.

  This invention is made | formed in view of such a situation, and makes it a subject to provide the interlayer coupler which is easy to manufacture with a simple structure and can be implement | achieved by short length.

  In order to solve the above-described problem, an interlayer coupler according to the present invention includes a first waveguide, and a second waveguide disposed at a predetermined interlayer distance from the first waveguide, wherein the first waveguide is provided. An interlayer coupler that propagates a light distribution transitioned from a waveguide to the second waveguide and couples the layers, wherein at least one of the first waveguide and the second waveguide is from the waveguide to the tip. And a second taper that tapers in a more gradual taper shape from the end of the first taper toward the tip.

  According to the present invention, it is possible to provide an interlayer coupler that can be easily manufactured with a simple structure and can be realized with a short length.

It is a perspective view which shows the structure of the interlayer coupler which concerns on embodiment of this invention. It is a figure which shows the structure of the interlayer coupler which concerns on this embodiment, (a) is the top view, (b) is the side view. It is a figure which shows the structure of the interlayer coupler of the c-Si waveguide (1st waveguide) / InP waveguide (2nd waveguide) of the interlayer coupler which concerns on this embodiment, (a) is the top view , (B) is a side view thereof. It is a perspective view which shows the structure of the interlayer coupler of the comparative example of the interlayer coupler which concerns on this embodiment. It is a figure which shows the interlayer coupler of the comparative example of the interlayer coupler which concerns on this embodiment, (a) is the top view, (b) is a side view of the interlayer coupler of the said comparative example. It is a figure which shows distribution of the effective refractive index of the interlayer coupler of this embodiment, and the interlayer coupler of a conventional structure, (a) is a top view of the interlayer coupler of a comparative example, (b) is distribution of effective refractive index, (C) is a plan view of the present embodiment. It is a figure which shows the relationship between the element length L and the interlayer distance D for obtaining the coupling efficiency of about 95% at the time of applying the interlayer coupler of this embodiment and the interlayer coupler of a comparative example to an injector. It is a figure which shows the structure of the interlayer coupler of the modification of the interlayer coupler which concerns on this embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a perspective view showing a configuration of an interlayer coupler according to an embodiment of the present invention. 2A is a top view of the interlayer coupler, and FIG. 2B is a side view of the interlayer coupler.
[Constitution]
As shown in FIGS. 1 and 2, the interlayer coupler 100 is arranged in parallel with the first waveguide 10 formed on the substrate 1 and the first waveguide 10 with an interlayer distance (interlayer thickness) D apart. A second waveguide 20.
The interlayer coupler 100 propagates the light distribution (electromagnetic field distribution) shifted from the first waveguide 10 to the second waveguide 20 to couple the layers. The interlayer coupler 100 is a directional coupler that is used when one waveguide is connected to another layer when it is desired to cross the waveguides connecting the functional components.
The first waveguide 10 is a c-Si waveguide made of, for example, c-Si (Crystal silicon), and the second waveguide 20 is made of, for example, a-Si: H made of a-Si (amorphous silicon): H. It is a waveguide. Here, when a-Si: H is used as the material of the second waveguide 20, a-Si: H can be laminated at a low temperature, so that the optical functional element circuit and the electronic circuit connected to the second waveguide 20 are damaged. It is preferable because it is not present. Since there is no such thermal restriction, the first waveguide 10 uses c-Si as a waveguide material. Either material of the first waveguide 10 and the second waveguide 20 may be a-Si: H.
The material of the first waveguide 10 and the second waveguide 20 is not limited to silicon (Si), and any material may be used. As will be described later, the material of one or both of the waveguides may be a compound semiconductor (for example, InP).
The substrate 1 is, for example, an SOI (Silicon-On-Insulator) substrate. The substrate 1 may be a substrate made of any material.

The first waveguide 10 tapers with a first taper (1 ^st taper) 11 whose width becomes narrower toward the tip end 10a, and a gentler taper shape from the end 11a of the first taper 11 toward the tip end 10a. comprising a second taper ⁽² nd taper) 12, a.
The second waveguide 20 tapers with a first taper (1 ^st taper) 21 whose width becomes narrower toward the tip 20a and a gentler taper shape from the end 21a of the second taper 21 toward the tip 20a. And a second taper (2 ^nd taper) 22.
In the example of FIG. 1, the first waveguide 10 and the second waveguide 20 have the same structure. The first waveguide 10 and the second waveguide 20 are arranged to face each other with an interlayer distance D apart in parallel. As shown in FIGS. 1 and 2A, the first waveguide 10 has the tip 20a disposed in the + direction of the z-axis in FIG. Arranged in the negative direction of the axis. As shown in FIGS. 1 and 2B, the lower first waveguide 10 and the upper second waveguide 20 are separated from each other by an interlayer distance D. The interlayer distance D is, for example, 1 μm. Here, considering the element length, the interlayer distance D used is considered to be D = 500 nm to 1.5 μm. Therefore, it is not limited to D = 1 μm.

  As shown in FIGS. 1 and 2A, the first waveguide 10 and the second waveguide 20 include a second taper 12 of the first waveguide 10 and a second taper 22 of the second waveguide 20. When viewed from above, they are arranged so that they face each other in the opposite direction and overlap vertically. That is, only the second taper 12 of the first waveguide 10 and the second taper 22 of the second waveguide 20 are arranged so as to overlap each other when viewed from above. In the first waveguide 10 and the second waveguide 20, the second taper 12 and the second taper 22 intersect each other so as to overlap each other when viewed from above. doing. Since the first waveguide 10 and the second waveguide 20 are separated from each other in the y-axis direction of FIG. 1, for example, another waveguide can be formed on the substrate 1. Here, the other end of the second waveguide 20 has the same structure as that of FIG. 1 and is interlayer-coupled with the same structure as that of the first waveguide 10, so that the second waveguide 20 is integrated in the LSI chip. It is possible to realize an interlayer coupler that connects the wirings across the layers in the optical circuit chip in which the optical wirings are incorporated. The other end of the first waveguide 10 or the second waveguide 20 may be connected to another device (such as an optical circuit) not shown.

As shown in FIGS. 1 and 2, the second taper 12 of the first waveguide 10 and the second taper 22 of the second waveguide 20 overlap in the vertical direction (y-axis direction) when viewed from above. Be placed. Let L be the taper length of the section from the starting point of the first taper 11 of the first waveguide 10 to the starting point of the first taper 21 of the second waveguide 20.
The width of the first waveguide 10 made of c-Si is 450 nm and the thickness is 220 nm. The width of the second waveguide 20 made of a-Si: H is 450 nm and the thickness is 220 nm.

<First taper>
The first tapers 11 and 21 have a high refractive index taper shape that allows light reflection. The reflection of light and the high refractive index will be described later. The element length between the starting point of the first taper 11 of the first waveguide 10 and the starting point of the first taper 11 of the second waveguide 20 is about 200 μm.

<Second taper>
The second tapers 12, 22 are tapered more gradually from the ends 11a, 21a of the first tapers 11, 21 toward the tips 10a, 20a.
The second tapers 12 and 22 have a tapered shape that gradually changes the refractive index based on the effective refractive index lowered by allowing the first tapers 11 and 21 to reflect. The effective refractive index is an index that indicates which refractive index is effectively felt by the entire light mode. The lower the effective refractive index, the shorter the coupling length. The bond length is the length necessary to obtain the coupling efficiency in a certain percentage. Usually, it is represented by the length of the taper length when a coupling efficiency of 70 to 80% is taken. The coupling length in the case of coupling efficiency of 95% and 80% was calculated, and the first waveguide 10 and the second waveguide 10 were manufactured.
The first waveguide 10 and the second waveguide 20 are arranged so that the second tapers 12 and 22 face each other in the opposite direction and overlap each other, and the second light distribution shifted from the second taper 12 of the first waveguide 10 is the second. The layers are coupled to the second taper 22 of the waveguide 20.
The taper shape of the first taper 11 of the first waveguide 10 and the taper shape of the first taper 11 of the second waveguide 20 may be symmetrical (same shape) as shown in FIG. It may be asymmetric as shown.

  As described above, the interlayer coupler 100 has a double taper type interlayer coupler structure in which the first waveguide 10 and the second waveguide 20 include the first taper 11, 21 and the second taper 12, 22.

<Light reflection>
The reflection of light will be described.
In the present specification, the reflection of light interferes with the “electromagnetic field of light” at “discontinuous points of refractive index distribution and sudden change points (that is, points where the shape changes to a taper shape and taper points)”, To go backwards. By the way, due to the change in the refractive index distribution due to the taper shape, other light escapes (radiation), mode conversion (if there are multiple stable electromagnetic field distributions (modes) inherent to the waveguide shape, Transition phenomenon).
Among the effects of reflection, radiation, and mode conversion, the effect of reflection is dominant in this taper shape. That is, in the case of a tapered shape having a strong inclination such as the first tapers 11 and 21, reflection is more intense than radiation. Furthermore, in the case of a c-Si / a-Si: H waveguide, there is only one stable electromagnetic field distribution (light distribution). For this reason, limiting the first tapers 11 and 21 is only “reflection”.
However, when a waveguide other than c-Si / a-Si: H is used, reflection / radiation / mode conversion can all be a rate-limiting factor. Therefore, the term “adiabatic change” is used to mean that these three reflection / radiation / mode conversions can be ignored. The expression defining the first tapers 11 and 21 is defined as “the shortest one in the range in which the electromagnetic field distribution is adiabatically expanded to a desired size (the size of the starting points of the second tapers 12 and 22)”. be able to.
From the above, the role of the first tapers 11 and 21 is to “expand the electromagnetic field distribution to the desired size (the size of the starting points of the second tapers 12 and 22) at the shortest distance”. The first tapers 11 and 21 reflect the electromagnetic field distribution to the size of the starting point of the second tapers 12 and 22, threshold values for radiating and mode conversion (the threshold value is indicated by a relative difference in signal strength (power ratio), For example, each enlargement is −30 dB or less.
The advantage of this structure is that the portion that does not contribute to the coupling is shortened as much as possible by the first tapers 11 and 21. In that sense, it is described as “with the shortest distance”. When the taper length is shortened, the above-mentioned reflection (radiation) increases. Therefore, the shortest in this case means the shortest in a range where the above-described reflection (radiation) is allowable.

Light is well confined in the waveguide, but reflection always occurs when there is a strong refractive index difference in the waveguide. Therefore, a mode is adiabatically changed by forming a taper in the waveguide and gradually changing the refractive index by this taper. The taper changes the mode of light with no reflection, and the mode does not change adiabatically unless the angle (profile) of the slope of the taper is very gentle.
In the interlayer coupler, unlike in the case of a single-layer waveguide, a certain amount of overlap of the light field is required between the opposing waveguides. The angle of the taper slope needs to be considerably relaxed.

<High refractive index of the first taper>
The high refractive index of the first tapers 11 and 21 will be described.
High refractive index means that the refractive index is approximately 1.5 to 2 apart between the waveguide core and the cladding. The high refractive index is derived from the material and has a limit. The upper limit of the high refractive index is practically considered to be about 4 of Ge.

<Taper length>
The length of the taper length will be described.
The coupling length in the interlayer coupler 100 refers to the second tapers 12 and 22. This is because the first tapers 11 and 21 do not contribute to the coupling. Therefore, the element length of the interlayer coupler 100 is as follows.
Element length = first taper + second taper = first taper + coupling length In the interlayer coupler of the comparative example described later, since the entire element contributes to coupling, the coupling length = element length.

[Configuration: Waveguide asymmetric type]
FIG. 3 is a diagram showing a configuration of an interlayer coupler of c-Si waveguide (first waveguide) / InP waveguide (second waveguide), and FIG. 3 (a) is a top view of the interlayer coupler. FIG. 3 and FIG. 3B are side views of the interlayer coupler.
As shown in FIG. 3, the interlayer coupler 100A uses an InP waveguide 30 (second waveguide) instead of the a-Si: H waveguide 20 (second waveguide) shown in FIGS. . Although the III-V compound semiconductor InP is used as the material of the second waveguide here, other compound semiconductors may be used.
The first waveguide 10 tapers with a first taper (1 ^st taper) 11 whose width becomes narrower toward the tip end 10a, and a gentler taper shape from the end 11a of the first taper 11 toward the tip end 10a. comprising a second taper ⁽² nd taper) 12, a.
The width of the first waveguide 10 made of c-Si is 450 nm (see FIG. 3A), and the thickness of the first waveguide 10 is 220 nm (see FIG. 3B). The width of the first waveguide 10 at the starting point of the second taper 12 (the width of the end portion 11a of the first taper 11) W _tip2 is 250 nm, and the width of the first waveguide 10 at the end point of the second taper 12 (the first waveguide) The width 10 tip 10a) W _tip1 is 150 nm.

The second waveguide 30 has a first taper (1 ^st taper) 31 that decreases in width toward the tip 30a, and a gentler taper shape from the end 31a of the second taper 21 toward the tip 30a. narrowing second tapered comprises a ⁽² nd taper) 32, a.
The width of the second waveguide 30 made of InP is 3000 nm (see FIG. 3A), and the thickness of the second waveguide 30 is 270 nm (see FIG. 3B). The width of the second waveguide 30 at the starting point of the second taper 32 (width of the end portion 31a of the first taper 31) W _tip2 is 250 nm, and the width of the second waveguide 30 at the end point of the second taper 12 (second waveguide) 30 Width of tip 30a) W _tip1 is 150 nm.

  As shown in FIG. 3B, the first waveguide 10 and the second waveguide 30 are arranged to face each other in parallel with a distance D between them. The interlayer distance D is, for example, 1 μm. As shown in FIG. 3, in the first waveguide 10, when the tip 10a is arranged in the z-axis direction of FIG. 1, the tip 30a of the second waveguide 30 is arranged in the -direction of the z-axis of FIG. .

  As shown in FIG. 3A, the first waveguide 10 and the second waveguide 30 are such that the second taper 12 of the first waveguide 10 and the second taper 32 of the second waveguide 30 are from above. It is arranged so that the top and bottom overlap. That is, only the second taper 12 of the first waveguide 10 and the second taper 32 of the second waveguide 30 are arranged so as to overlap each other when viewed from above. The first waveguide 10 and the second waveguide 30 are arranged such that the second taper 12 and the second taper 32 overlap with each other when viewed from above, but spatially, the interlayer distance D is separated. doing. Since the first waveguide 10 and the second waveguide 30 are separated from each other in the y-axis direction of FIG. 3, an interlayer that connects the wirings across the layers in the optical circuit chip in which the optical wiring into the LSI chip is incorporated. A coupler can be realized.

  Let L be the taper length of the section from the starting point of the first taper 11 of the first waveguide 10 to the starting point of the first taper 31 of the second waveguide 30.

As shown in Table 1 and FIG. 3A, the taper length L1 of the first taper 11 of the first waveguide 10 made of c-Si is 10 μm, and the taper length L2 of the second taper 12 is 150 μm. The taper length L3 of the first taper 31 of the second waveguide 30 made of a-Si: H is 40 μm, and the taper length L2 of the second taper 32 is 150 μm. Thus, the taper length L1 of the first taper 11 and the taper length L3 of the first taper 31 are different depending on the material of the waveguide. This is because the high refractive index function of the first tapers 11 and 31 is effectively exhibited. On the other hand, the taper length L2 of the 2nd tapers 12 and 32 is equal in both. In other words, by making the taper shapes of the second tapers 12, 32 equal (both W _tip1 and W _{tip 2} are made equal), the overlapping degree of the second taper 12 and the second taper 32 in the vertical direction is equalized. To. This is for making the light propagation of the second tapers 12, 32 uniform.
As shown in Table 1, it was confirmed that the device length of the interlayer coupler 100A was 200 μm and the waveguide width dependency was ± 50 μm.

[Action]
Hereinafter, the operation of the interlayer coupler 100 configured as described above will be described.
The basic concept of the present invention will be described.
Upper and lower waveguides having a tapered taper whose width becomes narrower toward the tip are arranged in parallel so that the tapers face each other and overlap. In the interlayer coupler having such a taper, the signal light guided in the first waveguide is gradually weakened in the tapered portion so that the light confinement is gradually weakened in the first waveguide having a large refractive index. Propagation becomes impossible and the light distribution (electromagnetic field distribution) gradually shifts from the first waveguide to the second waveguide. Since the first waveguide and the second waveguide are arranged so that the tips are arranged in opposite directions and the tapers face each other and overlap each other, the light distribution transitioned from the first waveguide gradually has an optical confinement effect. It is sucked in the taper portion of the second waveguide that becomes larger, and is partially coupled to the second waveguide having a high refractive index. As described above, when the first waveguide is narrowed by the taper, the light cannot be confined and spreads as a light distribution. Since the second waveguide having the taper is provided at the opposite position in the state where the light distribution is expanded, the expanding light is sucked into the second waveguide. The taper is provided in order to broaden the light distribution without reflection in a place where the light distribution must be widened. By the way, as will be described later in the comparative example, in order to broaden the light distribution without reflection in a place where the light distribution has to be widened, the taper must be very gentle, and a long element length that is not suitable for practical use is necessary. It was.

  In the interlayer coupler including a waveguide having a taper, in order for light to propagate (transition) to an adjacent layer, when the refractive index of two layers is close, the transition is fast. Focused on. That is, in the comparative example described later, the taper function is set only from the viewpoint of spreading without reflection. On the other hand, the present inventors allow a certain decrease in the effective refractive index at a place where the refractive indexes of the two layers are separated so that light does not originally propagate (transition) to the adjacent layer. . On the other hand, by allowing the lowering of the effective refractive index, the two layers that actually contribute to the coupling continue to have close refractive indices.

  According to the present invention, the waveguide includes a first taper that becomes narrower from the waveguide toward the tip, and a second taper that tapers from the end of the first taper to the tip with a more gradual taper shape. The first taper has a taper shape with a high refractive index, and the second taper has a taper shape in which the refractive index gradually changes based on the effective refractive index lowered by the first taper.

Next, a comparative example will be described.
4 and 5 are views showing a conventional interlayer coupler as a comparative example. 4 is a perspective view showing the configuration of the interlayer coupler of the comparative example, FIG. 5A is a top view of the interlayer coupler of the comparative example, and FIG. 5B is the interlayer coupler of the comparative example. It is a side view. 4 corresponds to FIG. 1 of the present embodiment, and FIGS. 5A and 5B correspond to FIGS. 2A and 2B of the present embodiment.
As shown in FIGS. 4 and 5B, the interlayer coupler 2 having a conventional structure is separated from the first waveguide 3 formed on the substrate 1 by an interlayer distance D in parallel with the first waveguide 3. And a second waveguide 5 arranged. The first waveguide 3 is a c-Si waveguide made of c-Si, for example, and the second waveguide 5 is an a-Si: H waveguide made of a-Si: H, for example.
The first waveguide 3 includes a taper 4 that tapers toward the tip 3a, and the second waveguide 5 includes a taper 6 that tapers toward the tip 5a.
The first waveguide 3 and the second waveguide 5 have the same structure. The first waveguide 3 and the second waveguide 5 are arranged to face each other in parallel with a distance D between them. The interlayer distance D is, for example, 1 μm.

As shown in FIG. 4 and FIG. 5A, the first waveguide 3 and the second waveguide 5 are such that the taper 4 of the first waveguide 3 and the taper 6 of the second waveguide 5 are viewed from above. Are arranged so that the top and bottom overlap. Since the first waveguide 3 and the second waveguide 5 are separated from each other in the y-axis direction in FIG. 4, another waveguide can be formed on the substrate 1, for example.
Let L be the taper length of the section from the starting point of the taper 4 of the first waveguide 3 to the starting point of the taper 6 of the second waveguide 5. The width of the first waveguide 3 made of c-Si is 450 nm and the thickness is 220 nm. The second waveguide 5 made of a-Si: H has a width of 450 nm and a thickness of 220 nm.

Next, this embodiment will be described in comparison with a comparative example.
FIG. 6 is a diagram showing the distribution of the effective refractive index (Effective Index) of the interlayer coupler 100 of the present embodiment and the interlayer coupler 2 of the conventional structure, and FIG. 6 (a) shows the interlayer coupler 2 of the comparative example. FIG. 6B is a plan view of the embodiment 100, and FIG. 6B is a plan view of the embodiment 100. FIG.
A thin broken line (first waveguide 3) and a thick broken line (second waveguide 5) in FIG. 6B indicate the effective refractive index of the comparative example. In the comparative example, the effective refractive index changes in a large range of about “1.4-2.4”. Along with this, the proximity value of the effective refractive index is as high as about “2” (see symbol b in FIG. 6B). Furthermore, since the effective refractive index changes in a large range of about “1.4-2.4”, the adjacent portion is also small (see reference c in FIG. 6).

A thin solid line (first waveguide 10) and a thick solid line (second waveguide 20) in FIG. 6B indicate the effective refractive index of the present embodiment.
In order for light to propagate to the next layer, the transition is fast if the effective refractive indices of the two layers are close. In the comparative example, if an attempt is made to continue where the effective refractive indexes of the two layers are close to each other (region), a very long element length is required. If there are few places where the refractive indexes of the two layers are close to each other, the transition is little.

  In the present embodiment, the first waveguide 10 and the second waveguide 20 have a first taper 11, 12 having a tapered shape with a high refractive index allowing light reflection on the waveguide side of the second taper 12, 22. Form. The first tapers 11 and 12 are where the amount of movement per unit length increases, that is, where there is almost no coupling, the taper is gently changed to change the refractive index, and a certain degree of reflection is allowed. The taper shape has a high refractive index. For this reason, in the area | region of the 1st taper 11 and 12 shown to code | symbol a1 and a2 of FIG.6 (b), reflection generate | occur | produces by a strong refractive index difference, and an effective refractive index falls significantly. However, in the second taper 12, 12 following the first taper 11, 12, the mode is adiabatically changed by slowly changing the refractive index. The second tapers 12 and 22 of the present embodiment and the tapers 4 and 6 (see FIGS. 4 and 5) of the waveguides 3 and 5 of the comparative example are tapered so that the refractive index changes gently, and the transitioned light distribution is obtained. Although it is the same in terms of coupling, in this embodiment, as shown in FIG. 6 (b), the second taper 12, 22 has an effective refractive index lowered by allowing the first taper 11, 12 to reflect. Based on this, the refractive index is gradually changed. In other words, in the present embodiment, the portions that are hardly coupled are ignored by the high taper shape of the first taper 11, 12, and the second taper 12, 22 is coupled at a time.

  As described above, in the comparative example, since the coupling is gradually performed with a taper in a state where the effective refractive index is high, the coupling efficiency to the adjacent layer per unit length is low. In contrast, in the present embodiment, the effective refractive index of the second tapers 12 and 22 is originally low, so that the coupling efficiency for transferring to the adjacent layer per unit length is high. Since the coupling efficiency transferred to the adjacent layer is high, the total length is shortened.

This embodiment has the following two points (1) and (2).
(1) As shown by the symbol b in FIG. 6B, the proximity value of the effective refractive index is lower in this embodiment than in the comparative example. In general, the lower this value, the shorter the bond length.
(2) As shown by reference numeral c in FIG. 6B, the use of the second tapers 12 and 22 having a gently tapered shape enlarges the adjacent portion and lengthens the connectable region.
As described above, in this embodiment, the proximity value of the effective refractive index is lowered, and the proximity location where the refractive indexes of the two layers are close is increased.

FIG. 7 shows an element length L for obtaining a coupling efficiency of about 95% when the interlayer coupler of this embodiment and the interlayer coupler of the comparative example are applied to an injector (corresponding to the emitter of FIG. 6). It is a figure which shows the relationship between and the interlayer distance D. FIG. FIG. 7 shows the element length L required for the interlayer distance D in the interlayer coupler of this embodiment and the interlayer coupler of the comparative example.
As shown in FIG. 7, the element length L of the interlayer coupler of the comparative example (see symbol □ in FIG. 7) increases exponentially with respect to the interlayer distance D, and 5 cm for the interlayer distance D of 1 μm. A degree is necessary. On the other hand, the interlayer coupler of the present embodiment (see symbol ◇ in FIG. 7) can be realized at about 200 μm.

[Example]
The c-Si waveguide / InP waveguide interlayer coupler 100A shown in FIG.
In FIG. 3, when L1 = 10 μm, L2 = 150 μm, and L3 = 40 μm, a coupling efficiency of about 95% was obtained. In this case, the refractive indexes of the two differ by about 0.4. Therefore, the second taper 12 of the first waveguide 10 and the second taper 32 of the second waveguide 30 are asymmetric and L1 ≠ L3.

  In the interlayer coupler 100 of FIGS. 1 and 2, the first-stage taper length L1 with respect to the interlayer distance D is such that the interlayer distance D = 0.2-2 μm in order to obtain a coupling efficiency of about 95%. It confirmed that it was about 1 micrometer-20 micrometers. In this configuration, theoretically, a coupling efficiency of 95% or more is possible.

[Modification]
FIG. 8 is a diagram showing a configuration of a modified interlayer coupler.
As shown in FIG. 8A, the interlayer coupler 100B of the modified example is a curved taper in which the first taper 11B of the first waveguide 10B and the first taper 21B of the second waveguide 20B are convex outward. It is formed into a shape. It is also possible to form a tapered shape that is curved inward.

  As shown in FIG. 8B, in the interlayer coupler 100C according to another modification, the first taper 11C of the first waveguide 10C includes a taper 111 and a taper 112. The first taper 21 </ b> C of the second waveguide 20 </ b> C includes a taper 121 and a taper 122.

  As described above, the interlayer coupler 100 according to the present embodiment is an interlayer coupler that propagates the light distribution transitioned from the first waveguide 10 to the second waveguide 20 and couples the layers. The waveguide 10 and the second waveguide 20 have a first taper 11 that decreases in width from the waveguide toward the tip, and a first taper that tapers in a more gradually tapered shape from the end of the first taper 11 toward the tip. The first taper 11 has a high-refractive index taper shape that allows reflection of guided light, and the second taper 12 allows the first taper 11 to reflect. Based on the lowered effective refractive index, it has a tapered shape that gradually changes the refractive index.

As a result, the portion that is hardly coupled is ignored by the high-refractive index tapered shape of the first taper 11 and 12, and accordingly, the second taper 12 and 22 realize the proximity portion of the effective refractive index in a wide range. As a result, interlayer transmission of 1 μm is possible with a short element length of about 200 μm (about 1/250 of the conventional one). Since the interlayer coupler 100 is capable of transmission between thick films with a simple structure, it is expected to be practically used as an interlayer transmission device for multilayer optical circuits that can be easily manufactured.
Further, if the element length is about 1 μm, it can be performed by CAD design, so that it is much easier to manufacture than an optical waveguide coupling structure using a diffraction grating (such as a grating coupler).

The present invention is not limited to the above-described embodiments, and includes other modifications and application examples without departing from the gist of the present invention described in the claims.
In the present embodiment, the first waveguide 10 (see FIGS. 1 and 2) and the second waveguides 20 and 30 both have first tapers 11 and 21 and second tapers 12, 22, and 32. Although described, only one of the first waveguide 10 or the second waveguides 20 and 30 (for example, only the first waveguide 10) may have a first taper and a second taper. In this case, the second waveguide may be a taper such as a taper 6 (see FIGS. 4 and 5) of the second waveguide 5 of the comparative example.
Further, the taper shapes of the first taper 11 and the second taper 12 of the first waveguide 10 and the taper shapes of the first taper 21 and the second taper 22 of the second waveguide 20 may be different. Different taper shapes are different in slope angle (profile) of each taper, curved surface (curved) taper shape shown in FIG. 8A in one waveguide, and a plurality of tapers shown in FIG. 8B. (Taper element) is used.
Further, in the present embodiment, the second taper 12 of the first waveguide 10 and the second tapers 22 and 32 of the second waveguides 20 and 30 are opposed to each other in the opposite direction when viewed from above. However, the second taper 12 of the first waveguide 10 and the second tapers 22 and 32 of the second waveguides 20 and 30 are, for example, at a predetermined angle. It may be a structure overlapping (other than 180 °). By adopting such a structure, the direction of the waveguide can be changed in the interlayer coupler of the present invention.

Further, the above-described exemplary embodiments have been described in detail for easy understanding of the present invention, and are not necessarily limited to those having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. . Further, it is possible to add, delete, and replace other configurations for a part of the configuration of each exemplary embodiment.
Moreover, in the said embodiment, although the name called the interlayer coupler was used, this is for convenience of explanation and a name may be a directional coupler etc.

1 substrate 10 first waveguide 11,11A, 11B, 11C, 21,21A, 21B, 21C first taper ⁽¹ st taper)
12, 22, 32 the second taper ⁽² nd taper)
20, 30 Second waveguide 100, 100A, 100B, 100C Interlayer coupler 111, 112, 121, 122 Plural taper shape of first taper D Interlayer distance (interlayer thickness)
L Element length (taper length)

Claims (7)

A first waveguide; and a second waveguide disposed at a predetermined interlayer distance from the first waveguide; and a light distribution transitioned from the first waveguide is propagated to the second waveguide. An interlayer coupler for joining layers,
At least one of the first waveguide and the second waveguide is
A first taper that decreases in width from the waveguide toward the tip;
The interlayer coupler according to claim 1, further comprising: a second taper that tapers with a gentler taper shape from the end of the first taper toward the tip. The first taper has a tapered shape with a high refractive index,
2. The interlayer coupler according to claim 1, wherein the second taper has a tapered shape that gradually changes the refractive index based on the effective refractive index lowered by the first taper. The first waveguide and the second waveguide are:
The second tapers are arranged so as to face each other in opposite directions and overlap each other, and the light distribution transitioned from the second taper of the first waveguide is coupled to the second taper of the second waveguide so that an interlayer is formed. The interlayer coupler according to claim 1 or 2, wherein the interlayer coupler is coupled. 3. The first taper according to claim 1, wherein the first taper expands reflection, radiation, and mode conversion to a size of a starting point of the second taper within a range below a threshold value. Interlayer coupler. The interlayer coupler according to claim 1, wherein the interlayer distance is in a range of 0.5 μm to 1.5 μm. 3. The interlayer coupler according to claim 1, wherein the first taper has one or a plurality of taper shapes. 4. The interlayer coupler according to claim 1, wherein the first taper has a linear or curved taper shape.

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