JP6216217B2 - Mode conversion element and optical waveguide element - Google Patents

Description

  The present invention relates to a substrate-type optical waveguide element used in, for example, optical fiber communication, and more particularly to a mode conversion element and an optical waveguide element that perform mode conversion.

Currently, the amount of information transmitted by optical communication is steadily increasing. In order to cope with such an increase in the amount of information, measures such as an increase in signal speed and an increase in the number of channels by wavelength division multiplexing are being promoted. In particular, in the next-generation 100 Gbps digital coherent transmission technology aiming at high-speed information communication, in order to double the amount of information per unit time, polarization multiplexing that places information on two polarized waves with orthogonal electric fields The method is used.
The modulation method for high-speed communication including polarization multiplexing requires a complicated optical modulator, which causes problems such as an increase in size and cost of the apparatus. In response to these problems, an optical modulator using a substrate-type optical waveguide using silicon, which has advantages such as easy processing, downsizing by integration, and low cost by mass production, has been studied.

However, the polarization multiplexing in such a substrate type optical waveguide has the following problems. In general, a substrate-type optical waveguide has an asymmetric shape in the width direction parallel to the substrate and the height direction perpendicular to the substrate. For this reason, there are two types of modes: a mode having substantially only an electric field component in the width direction (hereinafter referred to as TE mode) and a mode having substantially only an electric field component in the height direction (hereinafter referred to as TM mode). The characteristics such as the effective refractive index are different from the polarization mode.
Of these modes, the basic TE mode and the basic TM mode are often used. Here, the basic TE mode is a mode (TE ₀ ) having the largest effective refractive index in the TE mode, and the basic TM mode is a mode (TM ₀ ) having the largest effective refractive index in the TM mode. .
When optical modulation operation is performed for these modes having different characteristics, it is difficult with only a single substrate type optical waveguide device. When a substrate-type optical waveguide device optimized for each mode is required, a great effort is required in the development of the substrate-type optical waveguide device.

As a method for solving this problem, there is a method in which the basic TE mode is used as input light to a desired substrate type optical waveguide element optimized for the basic TE mode, and the output is polarized to the basic TM mode. It is done. Here, polarization conversion refers to conversion from the basic TE mode to the basic TM mode, or from the basic TM mode to the basic TE mode. In order to perform the above operation, a substrate type optical waveguide element that performs polarization conversion on the substrate is required.
As a technique for performing such polarization conversion on a substrate, there is a technique in which a basic TE mode is converted into a high-order TE mode, and then the high-order TE mode is converted into a basic TM mode. Here, the higher order TE mode represents a TE mode (TE ₁ ) having the second highest effective refractive index.
Such a conversion element requires two elements: an element that converts the basic TE mode into a higher order TE mode and an element that converts the higher order TE mode into a basic TM mode. The present invention relates to an element that converts the basic TE mode into a higher-order TE mode.
The conversion between the basic mode and the higher order mode including the conversion from the basic mode to the higher order mode and the conversion in the opposite direction is referred to as higher order mode conversion.

There exists a thing of a nonpatent literature 1 as a polarization conversion element.
As illustrated in FIG. 27, the polarization conversion element 210 described in Non-Patent Document 1 includes cores 211 and 212 and a clad 215. Part of the cores 211 and 212 in the length direction is arranged side by side to form a directional coupler 218. The clad 215 has a lower clad 217 and an upper clad 216. The upper clad 216 is provided on the cores 211 and 212 and the lower clad 217.
The cores 211 and 212 in the directional coupler 218 are waveguides having a rectangular cross section with different widths (hereinafter referred to as rectangular waveguides).
The widths of the cores 211 and 212 are designed so that the effective refractive index of the higher-order TE mode of the input-side core 211 and the effective refractive index of the basic TE mode of the output-side core 212 are close to each other at a certain wavelength. . As a result, phase matching occurs and the high-order TE mode of the input-side core 211 is coupled to the basic TE mode of the output-side core 212 with high efficiency, so that high-order mode conversion is possible.
In the technique of Non-Patent Document 1, a high-order TE mode is converted to a basic TE mode. However, since a reverse process also holds in a passive optical waveguide, the directional coupler converts a high-order TE mode into a high-order TE mode. It also has a function of converting to the TE mode. That is, the directional coupler is an element that performs higher-order mode conversion.
Non-Patent Documents 2 and 3 disclose directional couplers that handle coupling in the same mode.

Daoxin Dai and John E. Bowers, “Novel concept for ultracompact polarization splitter-rotator based on silicon nanowires,” Optics Express, Vol. 19, Issue 11, pp. 10940-10949 (2011) S.V.Burke, "Spectral index method applied to two nonidentical closely separated rib waveguides," IEE Proceedings, Vol. 137, Pt. J, No. 5 (1990) G.B.Cao, et.al., "Directional couplers realized on Silicon-On-Insulator," IEEE Photonics Technology Letters, Vol. 17, No. 8, pp1671-1673 (2005)

In the directional coupler of the conversion element described in Non-Patent Document 1, since the mode handled in each optical waveguide is different, the amount of change in the effective refractive index when the wavelength changes is different for each optical waveguide. For this reason, there is a problem that high-efficiency conversion over a wide wavelength band cannot be expected.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a mode conversion element and an optical waveguide element capable of highly efficient mode conversion over a wide wavelength band.

In order to solve the above problems, the present invention includes a lower clad and an optical waveguide region formed on the lower clad and having a higher refractive index than the lower clad, and the optical waveguide regions are formed apart from each other. And a slab part that is thinner than the rib part and extends in the width direction of the rib part, and one of the pair of rib parts is an input side optical waveguide. And the other is an output side optical waveguide, and at least a part of the input side and output side optical waveguides are arranged in parallel to form a directional coupler, and the directional coupler is formed from the input side optical waveguide. The output-side optical waveguide can be coupled between modes of different orders, and the slab portion has a thickness direction dimension perpendicular to the width direction in a cross section perpendicular to the length direction of the rib portion. Smaller than the rib part and the directionality An intermediate region formed by connecting the input-side optical waveguide and the output-side optical waveguide to each other in at least a part of the coupler, and an output-side optical waveguide among the input-side and output-side optical waveguides A mode conversion element having an outward extension region extending outward in the width direction from the outer side.
The input side optical waveguide is guided by an nth (n-1) order mode which is a TE mode having an effective refractive index nth (n is a natural number), and the output side optical waveguide is mth (m is A natural number, m> n), which is a TE mode with a large effective refractive index. (M−1) The next mode is guided, and the TE mode of the input side optical waveguide and the TE mode of the output side optical waveguide can be coupled. Preferably there is.
In the mode conversion element of the present invention, a bent waveguide is disposed at one or both ends in the longitudinal direction of each of the input-side optical waveguide and the output-side optical waveguide, and the bent waveguide is connected to the input-side optical waveguide. The bent waveguides connected to the output side optical waveguide may approach each other toward the directional coupler, or may be separated from each other in a direction away from the directional coupler.
Either or both of the input-side optical waveguide and the output-side optical waveguide may be formed with a tapered optical waveguide in which the width of the rib portion and / or the slab portion decreases as it goes in the extending direction. .
Either or both of the input-side optical waveguide and the output-side optical waveguide may be formed with a tapered optical waveguide in which the width of the rib portion and / or slab portion increases in the extending direction. .
It is preferable that the cores of the input side optical waveguide and the output side optical waveguide have the same height.
Preferably, a zero-order mode is guided in the input-side optical waveguide, and a first-order mode is guided in the output-side optical waveguide.
In the mode conversion element of the present invention, it is preferable that the slab part and the rib part are made of Si, and the lower clad and the upper clad are made of SiO ₂ .

The present invention provides an optical waveguide device having the mode conversion device and a high-order polarization conversion device connected to the output-side optical waveguide.
The present invention provides a DP-QPSK modulator including the mode conversion element.
The present invention provides a polarization diversity coherent receiver including the mode conversion element.
The present invention provides a polarization diversity system including the mode conversion element.

In the present invention, the input-side optical waveguide has a structure having a slab portion only inward (half-rib waveguide structure), and the output-side optical waveguide in which a mode having a different order from the input side is guided is provided on both sides. This is a structure having a slab portion on the side (rib waveguide structure).
Because the input side optical waveguide uses a semi-rib waveguide structure with a slab part only on one side (output side optical waveguide side), the electric field oozes out to the other side (outside side) that is not involved in coupling Therefore, the coupling to the adjacent output side optical waveguide becomes strong.
In addition, since a rib waveguide structure with a small effective refractive index change with respect to the wavelength is adopted for the output side optical waveguide, the difference in change in the effective refractive index with respect to the wavelength between the mode on the input side and the higher order mode on the output side. Can be compensated.
Therefore, mode conversion can be performed with high efficiency over a wide wavelength band.

The mode conversion element of the present invention has a structure in which the input-side and output-side optical waveguides share a slab portion (intermediate region). In this structure, a large amount of electric field oozes out from the slab portion, so that the coupling to the adjacent waveguide is strengthened, so that the coupling efficiency can be increased over a wide wavelength band as compared with the conversion element having the rectangular waveguide.
This contributes to realizing high-efficiency mode conversion over a wide wavelength band.

1 shows an embodiment of a mode conversion element of the present invention, where (a) is a plan view and (b) is a cross-sectional view taken along line II of (a). (A) It is sectional drawing of the optical waveguide which is a rib waveguide. (B) It is a graph which shows the relationship between the effective refractive index of fundamental TE mode, and the width | variety of a slab part. (A) It is sectional drawing of the optical waveguide which has an optical waveguide area | region which has a rib part and a slab part. (B) It is a graph which shows the relationship between the width | variety of a slab part, and the effective refractive index of fundamental TE mode. It is a graph which shows the relationship between the effective refractive index of fundamental TE mode and high order TE mode, and the wavelength of light in a rectangular waveguide. It is a graph which shows the relationship between the effective refractive index of fundamental TE mode and high order TE mode, and the wavelength of light in a rib waveguide. It is a graph which shows the relationship between the effective refractive index of fundamental TE mode and high order TE mode, and the wavelength of light in a half rib waveguide. It is a graph which shows the relationship between the effective refractive index of a fundamental mode and a higher-order mode, and an effective refractive index wavelength variation | change_quantity in a rectangular waveguide. It is a graph which shows the relationship between the effective refractive index of a fundamental mode and a higher order mode, and an effective refractive index wavelength variation | change_quantity in a rib waveguide. It is a graph which shows the relationship between the effective refractive index of a fundamental mode and a higher-order mode, and an effective refractive index wavelength variation | change_quantity in a half rib waveguide. It is the graph which showed the relationship between the effective refractive index of high order TE mode, and an effective refractive index wavelength variation | change_quantity. It is a graph which shows the relationship between the wavelength of light, and the absolute value of (delta). It is a graph which shows the relationship between the space | interval of an optical waveguide, and a coupling coefficient. It is a graph which shows the relationship between the wavelength of light, and a coupling coefficient. It is a graph which shows the relationship between the wavelength of light, and coupling efficiency. It is a graph which shows the relationship between the wavelength of light, and coupling efficiency. An example of the mode conversion element using a rectangular waveguide is shown, (a) is a top view, (b) is sectional drawing which follows the XVI-XVI line of (a). An example of the mode conversion element using a rib waveguide is shown, (a) is a top view, (b) is sectional drawing which follows the XVII-XVII line of (a). An example of the mode conversion element using a half rib waveguide is shown, (a) is a top view, (b) is sectional drawing which follows the XVIII-XVIII line of (a). It is a top view which shows the modification of the mode conversion element of this invention. It is a top view which shows an example of the optical waveguide element using an example of the mode conversion element of this invention. The high-order polarization conversion element used for the optical waveguide element of the previous figure is shown, Comprising: (a) is a top view, (b) is sectional drawing. It is a top view which shows the other example of the optical waveguide element using an example of the mode conversion element of this invention. The high-order polarization conversion element used for the optical waveguide element of the previous figure is shown, (a) is a plan view, (b) is a sectional view taken along line XXIIIb-XXIIIb in (a), and (c) is It is sectional drawing which follows the XXIIIc-XXIIIc line of (a). It is a schematic diagram which shows an example of a DP-QPSK modulator. It is a schematic diagram which shows an example of a polarization diversity coherent receiver. It is a schematic diagram which shows an example of a polarization diversity system. An example of the conventional mode conversion element is shown typically, (a) is a perspective view, (b) is sectional drawing which follows the XXVII-XXVII line.

Hereinafter, based on a preferred embodiment, the present invention will be described with reference to the drawings.
1A and 1B show a mode conversion element 10 according to a first embodiment of a mode conversion element of the present invention, wherein FIG. 1A is a plan view and FIG. 1B is a cross-sectional view.
In FIG. 1A, the light guide direction is the vertical direction, which is the length direction of the optical waveguides 11 and 12. In FIG. 1B, the light guiding direction is a direction perpendicular to the paper surface. In the following description, in the cross section perpendicular to the light guiding direction, the dimension in the direction in which the input side optical waveguide 11 and the output side optical waveguide 12 face each other is referred to as the width, and the dimension in the direction perpendicular to the facing direction is the height. That's it. In FIG. 1B, the width is a dimension in a direction parallel to the substrate S, and the height is a dimension in a direction perpendicular to the substrate S. Hereinafter, the positional relationship of each structure may be described with the height direction (above FIG. 1B) as the upper side and the opposite direction as the lower side. In the plan view of FIG. 1A and the like, the slab portion 4 is shaded.

As shown in FIG. 1, the mode conversion element 10 includes an optical waveguide 1 having an optical waveguide region 2 and a clad 5.
The optical waveguide region 2 has a pair of rib portions 3 (thick portions) formed apart from each other and a slab portion 4 (thin plate portion) formed extending in the width direction of the rib portions 3.
The rib part 3 and the slab part 4 consist of the same material, and are integrally formed.
The optical waveguide region 2 is made of a material having a higher refractive index than that of the clad 5, preferably Si (silicon). The optical waveguide region 2 can be formed by processing the uppermost silicon (Si) layer of an SOI (Silicon on insulator) substrate made of Si—SiO ₂ —Si.

The rib portion 3 is formed to protrude upward from the upper surface 4 a of the slab portion 4. One of the pair of rib portions 3 is the input side optical waveguide 11, and the other rib portion 3 is the output side optical waveguide 12. is there.
The width of the output side optical waveguide 12 in the illustrated example is larger than the width of the input side optical waveguide 11.
The relationship between the height H1 of the input-side optical waveguide 11 and the height H2 of the output-side optical waveguide 12 is not particularly limited, and H1> H2, H1 = H2, or H1 <H2 may be satisfied. It is desirable that the difference is not extremely large. In the illustrated example, the heights H1 and H2 are equal to each other.

At least a part of the length direction of the input side optical waveguide 11 and the output side optical waveguide 12 is arranged in parallel with each other to constitute a directional coupler 18 (asymmetric directional coupler).
The optical waveguides 11 and 12 in the directional coupler 18 in the illustrated example are linear, are formed at regular intervals in the length direction, and are parallel to each other.

The slab portion 4 is formed thinner than the rib portion 3, and is formed between the input side optical waveguide 11 and the output side optical waveguide 12 by connecting them to each other, and from the output side optical waveguide 12 to the width. And an output-side outward extending region 15 extending outward in the direction.
The output-side outward extension region 15 is an outward extension region formed only in the output-side optical waveguide 12 among the input-side and output-side optical waveguides 11 and 12.
Since the slab portion 4 extending outward is not formed in the input side optical waveguide 11, the outer surface 11b of the input side optical waveguide 11 is a surface on which the slab portion 4 is not formed (non-slab portion forming surface). ).
The slab part 4 (regions 13 and 15) may be formed in at least a part of the waveguide direction in the directional coupler 18. That is, the slab part 4 may be formed only in a part of the length direction of the rib part 3 in the directional coupler 18, or may be formed over the entire length of the rib part 3. In the illustrated example, the slab portion 4 is formed over the entire length of the rib portion 3.

The slab part 4 is formed thinner than the rib part 3.
The thickness is a dimension in a direction perpendicular to the width direction of the rib portion 3 in a cross section perpendicular to the length direction of the rib portion 3. That is, it is a dimension in a direction orthogonal to the length direction and the width direction of the rib portion 3.

The clad 5 includes a lower clad 7 and an upper clad 6 provided on the optical waveguide region 2 and the lower clad 7. The lower clad 7 is made of, for example, an SiO ₂ layer of an SOI substrate.
The mode conversion element 10 may be a substrate type optical waveguide element in which the optical waveguide 1 is formed on the substrate S.

The optical waveguide region 2 has a structure (rib waveguide) in which one of the two rib portions 3 has the slab portions 4 in both the width directions, and the other rib portion 3 is one in the width direction (inward side). ) Only has a slab portion 4 (half-rib waveguide). For this reason, the optical waveguide region 2 can be referred to as a “half-rib / rib waveguide”.
A waveguide having a structure having the slab portion 4 has an electric field that spreads also in the slab portion, so that light confinement is weaker than that of a rectangular waveguide (for example, the cores 211 and 212 in FIG. 27), and soaks into adjacent rib portions. More out.

FIG. 2B shows the effective refractive index of the fundamental TE mode in the optical waveguide region 2A (rib waveguide) (see FIG. 2A) having the slab portions 4 in both the width directions of the rib portion 3, and the optical waveguide. It is a graph which shows the relationship with the width | variety W1 of the area | region 2A. In this example, the width of the rib portion 3 is 400 nm.
The effective refractive index is related to confinement in the optical waveguide region, and the light confinement becomes stronger as the value of the effective refractive index of the optical waveguide region is larger. When the width of the slab portion is larger, light confinement becomes stronger, so that the effective refractive index of the optical waveguide region tends to increase.
As shown in FIG. 2B, the effective refractive index hardly changes when the width of the optical waveguide region is 2 μm or more. This is because the electric field of the basic TE mode is mainly distributed in the rib portion of the optical waveguide region, so if the width of the slab portion becomes sufficiently large, the influence of the change in the width of the slab portion on the electric field of the basic TE mode is very large. It shows that it becomes smaller.
Therefore, in this example, by setting the width of the optical waveguide region to 2 μm or more, the optical waveguide region can be regarded as a general rib waveguide. However, even when the width of the optical waveguide region is smaller than 2 μm, the light penetrates into the slab portion when the width of the optical waveguide region is larger than the width of the rib portion. For this reason, the effect of the present invention is exhibited even with such a waveguide structure. This is not limited to the optical waveguide structure shown in FIG. 2 (a) or the mode to be handled, but also holds in general optical waveguide structures and modes.

FIG. 3B shows the effective refractive index of the basic TE mode in the half-rib waveguide (see FIG. 3A) having the slab part 4 on one side in the width direction of the rib part 3, and the width W2 of the slab part 4. It is a graph which shows the relationship.
As shown in FIG. 3B, the effective refractive index hardly changes in the range where the width of the slab portion 4 is 1 μm or more. This is because the electric field of the basic TE mode is mainly distributed in the rib portion of the optical waveguide region, so if the width of the slab portion becomes sufficiently large, the influence of the change in the width of the slab portion on the electric field of the basic TE mode is very large. It shows that it becomes smaller.
Therefore, in this example, the optical waveguide region can be regarded as a half-rib waveguide by setting the width of the slab portion to 1 μm or more. However, even if the width of the slab portion is smaller than 1 μm, if it is larger than 0 μm, light oozes out into the slab portion. For this reason, the effect of the present invention is exhibited even with such a waveguide structure. This is not limited to the optical waveguide structure shown in FIG. 3A or the mode to be handled, but also holds in general optical waveguides and modes.

Next, the effect of this embodiment will be described.
The coupling efficiency T of the directional coupler is expressed by the following equation when the input side optical waveguide and the output side optical waveguide are not too close.

  Here, F and q are represented by the following equations, respectively.

  δ is expressed by the following equation.

L represents the length of the optical waveguide in the directional coupler (see FIG. 1A), ΔN _I represents the effective refractive index difference between the modes to be coupled when two waveguides exist independently, and λ Represents the wavelength of light.

From equations (1) to (4), in order to enable highly efficient coupling of the coupling target modes of the input-side optical waveguide and the output-side optical waveguide, the difference in effective refractive index of these modes is expressed by the equation It is necessary to adjust so that δ in (4) becomes smaller in comparison with the coupling coefficient χ. It is preferable that the effective refractive index of the input-side optical waveguide and the effective refractive index of the output-side optical waveguide are approximately the same. The effective refractive index is related to light confinement and can be adjusted by changing the size of the waveguide. Specifically, the height can be adjusted by changing the height of the rib portion, the height of the slab portion, and the waveguide width.
In particular, if the number of times of etching is reduced in the substrate-type waveguide and the manufacturing process is simplified, the height of the rib part and the height of the slab part should be the same in the input / output side optical waveguide. The effective refractive index is preferably adjusted by changing the waveguide width.

  χ represents the strength of coupling between the two waveguides, which is called a coupling coefficient and is obtained by the following equation.

Each parameter of Expression (5) is defined as follows (assuming that the basic TE mode of the input-side optical waveguide 11 and the higher-order TE mode of the output-side optical waveguide 12 are coupled).
N ₁ : Refractive index distribution of the optical waveguide region cross section when only the input side optical waveguide exists N: Refractive index distribution Ei (i = 1) of the optical waveguide region cross section when the input side optical waveguide and the output side optical waveguide exist , 2): electric field vector of the mode guided through the waveguide i (waveguide 1 is the input side optical waveguide, and waveguide 2 is the output side optical waveguide)

  Since Equation (5) integrates the inner product of the electric fields of both modes in the cross section of the output optical waveguide, the coupling coefficient χ increases as the light oozes out of the optical waveguide, and the coupling becomes stronger.

Hereinafter, it will be described that the wavelength dependence of the asymmetric directional coupler depends on the coupling strength, that is, the coupling coefficient χ, based on the equations (1) to (5).
Expression (1) representing the coupling efficiency T is composed of F and sin ² (qL), and each value varies depending on the wavelength.
As shown in the equation (2), F depends on the ratio of δ proportional to the effective refractive index difference to the coupling coefficient χ, and approaches F as the coupling coefficient χ increases, and the coupling efficiency increases.
In particular, in the case of an asymmetric directional coupler, even if the structure of the optical waveguide is determined so that the effective refractive index difference ΔN _I (∝δ) becomes sufficiently small at a certain wavelength, the effective refractive index difference increases as the wavelength changes. There is. When the coupling coefficient χ is small, the influence of the deviation becomes significant from the equations (1) and (2), and the wavelength dependency of the coupling efficiency increases. Therefore, from the viewpoint of reducing the wavelength dependence of the asymmetric directional coupler, it is preferable that χ is large.
In addition, since the effective refractive index difference ΔN _I (） δ) increases as the change in wavelength increases, if the coupling coefficient χ is large, the value of F is maintained high over a wide wavelength band from the equations (2) and (4). Can lead to expansion of the wavelength band.
The term sin ² (qL) indicates that the coupling efficiency T depends on the length L of the optical waveguide in the directional coupler, and light is transferred from one waveguide to the other by L. It expresses that. The length at which light moves most at a certain wavelength is called the coupling length (Lc) and is expressed by the following equation.

If L = Lc at a certain wavelength, sin ² (qL) in equation (1) becomes 1, and the coupling efficiency increases. From equation (6), Lc depends on χ and δ and varies depending on the wavelength (see equation (4)), so it is inevitable that sin ² (qL) becomes smaller than 1 depending on the wavelength, If the change in δ is reduced, χ is increased, or both, the influence of δ is reduced and the wavelength dependency is improved.
From the above, it can be seen that when the change in δ with respect to the wavelength is small and the coupling coefficient χ is large, the coupling efficiency can be maintained high over a wide band.
A case where δ is smaller than χ is referred to as “phase matching” (or simply “phase matching”), and when this is satisfied, the modes can be coupled.
The conversion of the modes handled in the present invention is TE _i and TE _j (i ≠ j), TM _i and TM _j (i ≠ j), TE _i and TM _j (i and j may be the same or different). Includes conversion.

In the mode conversion element 10, the input-side optical waveguide 11 through which the basic TE mode is guided has a structure (semi-rib waveguide) in which the slab portion 4 is provided only inward, and the output through which the higher-order TE mode is guided. The side optical waveguide 12 has a structure (rib waveguide) having slab portions 4 on both sides.
Since the input side optical waveguide 11 has a structure in which the slab portion 4 is provided only on one side (output side optical waveguide 12 side), the electric field oozes out to the other side (outside side) that is not involved in the coupling. As a result, the coupling to the adjacent output-side optical waveguide 12 becomes strong.
Further, since the output side optical waveguide 12 employs a rib waveguide structure with a small effective refractive index change with respect to the wavelength, it is possible to compensate for the difference in the effective refractive index change with respect to the wavelength of the basic TE mode and the higher order TE mode. it can.
For this reason, a small δ can be maintained with respect to the wavelength change. Therefore, mode conversion can be performed with high efficiency over a wide wavelength band.

  Further, the mode conversion element 10 has a structure in which the optical waveguides 11 and 12 share the slab portion 4 (intermediate region 13). In this structure, since the electric field penetrates into the slab portion, the coupling to the adjacent waveguide is strengthened, and the coupling efficiency can be increased over a wide wavelength band as compared with the conversion element having the rectangular waveguide. This contributes to realizing high-efficiency mode conversion over a wide wavelength band.

Next, the characteristics of the asymmetric directional coupler using the half rib / rib waveguide, the rectangular waveguide, the rib waveguide, and the half rib waveguide are compared.
First, the change in the effective refractive index when the wavelength is changed will be described.
Asymmetric directional couplers handle coupling between different modes. When the modes are different, the change amount of the effective refractive index when the wavelength is changed is not necessarily the same.
This is a problem that does not occur in a symmetric directional coupler intended to perform coupling between the same modes by waveguides having the same structure, and can be said to be a problem peculiar to an asymmetric directional coupler.

<Calculation Example 1>
(Rectangular waveguide)
A simulation was performed on the conversion element 210 shown in FIG.
The conversion element 210 includes cores 211 and 212 and a clad 215. Part of the cores 211 and 212 in the length direction is arranged side by side to form a directional coupler 218. The clad 215 includes a lower clad 217 and an upper clad 216 provided on the cores 211 and 212 and the lower clad 217.
The cores 211 and 212 in the directional coupler 218 are waveguides having a rectangular cross section (rectangular waveguide).
The upper clad and the lower clad are made of SiO ₂ (refractive index: 1.44), and the core is made of Si (refractive index: 3.47).
The height of the cores 211 and 212 was 220 nm, and the width of the core 212 was 400 nm.
The interval between the cores 211 and 212 was 500 nm. At a wavelength of 1580 nm, the width of the core 212 was determined so that the effective refractive index of the basic TE mode of the core 211 and the higher order TE mode of the core 212 were close. Specifically, the width of the core 211 was 836 nm.
FIG. 4 shows the relationship between the effective refractive index of the basic TE mode guided through the core 212 and the higher-order TE mode guided through the core 211 and the wavelength of light.

<Calculation Example 2>
(Rib waveguide)
A mode conversion element 60 shown in FIG. 17 was produced.
The mode conversion element 60 is different from the mode conversion element 10 shown in FIG. 1 in that the slab portion 4 has an input side outward extending region 14 extending outward in the width direction from the input side optical waveguide 11.
The height of the rib portions (optical waveguides 11 and 12) was 220 nm, the width of the rib portions (only the input side optical waveguide 11) was 400 nm, and the height of the slab portion was 95 nm.
The distance between the optical waveguides 11 and 12 was 500 nm.
The upper clad and the lower clad are made of SiO ₂ (refractive index: 1.44), and the optical waveguide region is made of Si (refractive index: 3.47).
The width of the output-side optical waveguide 12 was determined so that the effective refractive index of the basic TE mode of the input-side optical waveguide 11 and the higher-order TE mode of the output-side optical waveguide 12 were close at a wavelength of 1580 nm. Specifically, the width of the output side optical waveguide 12 was 959 nm.
FIG. 5 shows the relationship between the effective refractive index of the basic TE mode and the higher order TE mode and the wavelength of light.

<Calculation Example 3>
(Semi-rib waveguide)
A mode conversion element 50 shown in FIG. 18 was produced.
The mode conversion element 50 differs from the mode conversion element 10 shown in FIG. 1 and the like in that it has an optical waveguide region 52 instead of the optical waveguide region 2. In the optical waveguide region 52, the slab portion 4 does not include the outward extension region 15, and is formed only from the intermediate region 13.
The optical waveguide region 52 has a structure (semi-rib waveguide) having the slab portion 4 only on one side in the width direction of the rib portion 3, and a part in the length direction of the input side optical waveguide 11 and the output side optical waveguide 12. Are arranged in parallel to each other to form a directional coupler 58 (asymmetric directional coupler).
The width of the output-side optical waveguide 12 was determined so that the effective refractive index of the basic TE mode of the input-side optical waveguide 11 and the higher-order TE mode of the output-side optical waveguide 12 were close at a wavelength of 1580 nm. Specifically, the width of the output side optical waveguide 12 was set to 905 nm.
Other conditions were the same as those in Calculation Example 2.
FIG. 6 shows the relationship between the effective refractive index of the basic TE mode and the higher order TE mode and the wavelength of light.

  4 to 6, it can be seen that in any of Calculation Examples 1 to 3, the effective refractive index difference (∝δ) increases as the wavelength deviates from 1580 nm. A large effective refractive index difference is a factor that decreases the coupling efficiency T due to a change in wavelength.

In general, when the waveguide width is determined so that the effective refractive indexes are equal in the same waveguide structure, the change in the effective refractive index due to the wavelength change is higher than the higher-order TE mode (the waveguide having the same waveguide width). Mode with a low effective refractive index) is larger than a TE mode of a smaller order.
The effective refractive index is related to the confinement of the electric field in the core. If this value is the same, the confinement is considered to be about the same. In such a situation, higher order TE modes have a larger electric field spread than lower order modes, so that confinement of light in the waveguide and immersion outside the waveguide when the wavelength is changed. The change in the output is large, and the effective refractive index change related to the ratio of the electric field distributed to the core and the clad becomes large.

FIGS. 7 to 9 show fundamental mode and higher order mode conversion elements of Calculation Examples 1 to 3 (Calculation Example 1: Rectangular waveguide, Calculation example 2: Rib waveguide, Calculation example 3: Half-rib waveguide), respectively. It is a graph which shows the relationship between the effective refractive index (wavelength 1580nm) of a mode, and the effective refractive index wavelength variation | change_quantity in 1580nm.
Here, the effective refractive index wavelength variation is an absolute value of a value obtained by differentiating the effective refractive index with respect to the wavelength.

  In evaluating the effective refractive index wavelength variation, the effective refractive index was changed by changing only the waveguide width. This method is the same as the design method for setting the effective refractive index of the mode to be coupled with an asymmetric directional coupler. The refractive index condition, the height of the waveguide, and the height of the slab are the same as those in Calculation Examples 1 to 3.

7 to 9, it can be seen that with the same effective refractive index, the higher-order TE mode always has a larger effective refractive index wavelength variation than the basic TE mode.
For this reason, when an asymmetric directional coupler that couples different modes using the same waveguide structure is formed, the difference in effective refractive index of both modes with respect to wavelength change becomes a problem.
Therefore, in the present invention, this problem is solved by using a waveguide in which the fundamental TE mode is guided as a half-rib waveguide and a waveguide in which the higher-order TE mode is guided as a rib waveguide.

The reason why the above problem can be solved by adopting this structure is as follows.
When the waveguide structure is different, if the waveguide width is determined so that the effective refractive index is the same in the same mode, the effective refractive index change when the wavelength changes is smaller in the waveguide structure with weaker confinement. Become.
When a rib waveguide is compared with a half-rib waveguide, the rib waveguide has slabs on both sides, so the difference in refractive index between the core (optical waveguide region) and the clad on both sides viewed from the center of the waveguide is small. And the confinement becomes weaker.
Therefore, the rib waveguide is a waveguide that is less confined than the half-rib waveguide, and the effective refractive index change amount with respect to the wavelength is small. As an example, FIG. 10 shows the relationship between the effective refractive index and the effective refractive index wavelength variation of each of the higher-order TE modes of the rib waveguide (see FIG. 17) and the half-rib waveguide (see FIG. 18). A graph is shown. Here, the effective refractive index is adjusted by changing the width of the waveguide (the width of the rib portion).

FIG. 10 shows that the change in the effective refractive index with respect to the wavelength is smaller in the rib waveguide than in the half-rib waveguide.
Therefore, in the asymmetric directional coupler, the waveguide in which the basic TE mode is guided is a half-rib waveguide, and the waveguide in which the higher-order TE mode is guided is a rib waveguide having a small effective refractive index change with respect to the wavelength. This makes it possible to compensate for the difference in effective refractive index change with respect to the wavelengths of the basic TE mode and the higher-order TE mode. As a result, it is possible to maintain a smaller δ with respect to the wavelength change than the conventional technology over a wide wavelength band. This is the great effect of the structure according to the present invention.

Another significant effect of the present invention is that it can have a large coupling coefficient χ.
The mode conversion element of the present invention has a rib waveguide and a half rib waveguide structure sharing a slab structure. In such a structure, since an electric field oozes out to the slab portion, the coupling to the adjacent waveguide is strong, and the coupling coefficient is improved as compared with, for example, the conversion element having a rectangular waveguide described in Non-Patent Document 1. .

On the other hand, in the rib waveguide, since there are slab portions on both sides of the rib portion, a large leaching occurs in a portion that does not contribute to the coupling, which is disadvantageous compared to a structure without slab portions on both sides (see FIG. 18). It becomes. However, a rib waveguide guided by a higher-order TE mode has slab portions, which are high refractive index regions, on both sides, and light is distributed here, so that it is more effective for the waveguide width than a half-rib waveguide. The refractive index increases.
Therefore, when the effective refractive index is made the same as that of the basic TE mode of the half-rib waveguide, the waveguide of the rib waveguide in which the higher-order TE mode is guided as compared with the half-rib waveguide in which the higher-order TE mode is guided. The width becomes narrower. For this reason, confinement is weakened, so that the penetration into the adjacent waveguide is increased and the coupling is strengthened. As a result, the same degree of coupling as that of the half-rib waveguide having a large coupling coefficient χ is possible.
Thus, the structure in which the effective refractive index is smaller with respect to the waveguide width in the basic TE mode in which the waveguide width is smaller than the waveguide in which the higher-order TE mode is guided is a conventional technique ( For example, the coupling coefficient χ can be improved as compared with the conversion element described in Non-Patent Document 1, and as a result, high coupling efficiency in a wide band is possible.

  Next, a specific comparison between the half rib / rib waveguide employed in the present invention, the rectangular waveguide (Calculation Example 1, see FIG. 16), and the Half Rib Waveguide (Calculation Example 3, see FIG. 18). I do. The rib waveguide (Calculation Example 2, see FIG. 17) is not considered because the coupling efficiency is inferior to that of the half-rib waveguide.

<Calculation Example 4>
(Half rib / rib waveguide)
A simulation was performed on the mode conversion element 10 shown in FIG. The height of the rib portions (optical waveguides 11 and 12) was 220 nm, the width of the rib portions (only the input side optical waveguide 11) was 400 nm, and the height of the slab portion was 95 nm.
The width of the output side optical waveguide 12 was determined so that the effective refractive index of the basic TE mode of the input side optical waveguide 11 and the higher order TE mode of the output side optical waveguide 12 were close. Other conditions were the same as those in Calculation Example 2.
Table 1 shows the effective refractive index obtained by the simulation by the finite element method (FEM). Table 1 also shows the width of the output-side optical waveguide 12.

  As will be described later, when the coupling coefficient χ is taken into consideration, the effective refractive index difference is small in all the calculation examples and is about the same for each calculation example.

The effective refractive index varies depending on the wavelength, and in particular, the amount of change with respect to the wavelength of light is not equal for the effective refractive index of different modes of optical waveguides having different widths.
From equation (4), since the effective refractive index difference is proportional to δ, δ increases or decreases with changes in wavelength.
FIG. 11 shows the basic TE mode and output side light of the input side optical waveguide for the half rib / rib waveguide (see FIG. 1), the rectangular waveguide (see FIG. 16), and the half rib waveguide (see FIG. 18). It is a graph which shows the relationship between the absolute value of (delta) calculated from the difference of the effective refractive index with the high order TE mode of a waveguide, and the wavelength of light.
From this figure, it can be seen that the half rib / rib waveguide can minimize δ. As described above, the reason why δ can be minimized in the case of a half rib / rib waveguide is that the waveguide has an asymmetric structure.

Next, the coupling coefficient χ in the asymmetric directional coupler is compared for each waveguide.
The coupling coefficient χ was determined as follows.
The coupling length Lc is obtained from the difference ΔNs between the two effective refractive indexes obtained when obtaining the mode of the cross section of the directional coupler (see FIG. 1B, FIG. 16B and FIG. 18B). be able to.
The effective refractive index difference ΔNs is obtained in a mode distribution (so-called super mode) in which the fundamental TE mode is guided in the input-side optical waveguide and the higher-order mode is guided in the output-side optical waveguide. The bond length Lc can be expressed by Equation (7) using ΔNs. For theoretical considerations, see Okamoto's book "Basics of Optical Waveguide" (Corona).

  From equations (6) and (7), the coupling coefficient χ is expressed by the following equation.

ΔNs can be obtained by simulation using a finite element method (FEM).
FIG. 12 shows the gap (gap) between the input-side optical waveguide and the output-side optical waveguide for the half-rib / rib waveguide (see FIG. 1), rectangular waveguide (see FIG. 16), and half-rib waveguide (see FIG. 18). ) And the coupling coefficient χ of the asymmetric directional coupler. The wavelength of light was 1580 nm.
From this figure, in the case of the half rib / rib waveguide and the half rib waveguide, the coupling coefficient χ is larger than that of the rectangular waveguide. This is because the half-rib / rib waveguide and the half-rib waveguide have a slab portion between the waveguides, so that light spreads out of the rib portion compared to the rectangular waveguide.
The structure using the leaching of light into the slab part is particularly useful in a highly confined Si waveguide having a large refractive index difference between the optical waveguide region and the cladding.
The half-rib / rib waveguide and the half-rib waveguide were almost equivalent in terms of the coupling coefficient χ.

Next, for each waveguide structure, the relationship between the coupling efficiency T of the asymmetric directional coupler and the wavelength of light was examined.
The distance between the input side optical waveguide and the output side optical waveguide was 500 nm, and FEM was used for the simulation.
FIG. 13 shows the relationship between the coupling coefficient χ and the wavelength of light. From this figure, it can be seen that the half-rib / rib waveguide has a larger coupling coefficient χ than the rectangular waveguide. It can also be seen that the semi-rib waveguide can obtain a comparable coupling coefficient χ in comparison with the half-rib / rib waveguide.
Based on this, the coupling efficiency T will be considered below.

FIG. 14 and FIG. 15 are graphs showing the relationship between the coupling efficiency T of the asymmetric directional coupler that couples the basic TE mode and the higher order TE mode and the wavelength of light for each waveguide structure. FIG. 15 is an enlarged view of a part of FIG.
The length L of the optical waveguide of the directional coupler was the coupling length Lc when the wavelength was 1580 nm. Note that L may be longer or shorter than Lc.
The coupling length Lc is 23.5 μm for the half-rib / rib waveguide, 144.7 μm for the rectangular waveguide, and 22.3 μm for the half-rib waveguide.
The minimum value of the coupling efficiency is about −15.8 dB for the rectangular waveguide and about −1.3 dB for the half-rib waveguide in the wavelength band from 1520 nm to 1640 nm, whereas the half value employed in the present invention. The increase / decrease width of the coupling efficiency in the rib / rib waveguide is greatly improved to about −0.78 dB.
That is, the half rib / rib waveguide showed high coupling efficiency over a wide wavelength band.
This is because the half-rib / rib waveguide can compensate for the effective refractive index difference by using different structures for the input-side optical waveguide and the output-side optical waveguide, and the coupling coefficient is increased by the structure sharing the slab part. This is due to two reasons that it was possible.

Since the directional coupler 18 (see FIG. 1) using the half rib / rib waveguide has a high coupling coefficient χ, the coupling length Lc is shortened, and the asymmetric directional coupler by the rectangular waveguide (see FIG. 16) is used. The size can be reduced in comparison.
This is because the coupling length Lc in the rectangular waveguide in the above example is 144.7 μm, whereas the coupling length Lc in the half rib / rib waveguide is 23.5 μm, which is significantly shortened. It is clear from that.

  Δ proportional to the effective refractive index difference is not only caused by a change in wavelength, but also caused by a manufacturing error such as a change in the waveguide width and slab height. Therefore, the half rib / rib waveguide having a high coupling coefficient χ has an advantage that it can cope with a manufacturing error as compared with a rectangular waveguide, a rib waveguide, or the like.

In an optical waveguide (particularly an optical waveguide made of silicon), a loss may occur due to scattering of guided light due to side wall roughness caused by the manufacturing process. In particular, in an asymmetric directional coupler, if the width of the optical waveguide is reduced in order to increase the coupling, the amount of light that oozes out increases, so that it is susceptible to side wall roughness and loss is likely to occur.
On the other hand, since the half rib / rib waveguide has a slab portion, the area of the side wall portion is smaller than that of a structure without a slab portion (such as a rectangular waveguide), so that this loss can be reduced.

In the above example, conversion between the basic mode and the higher-order TE mode is assumed, but the present invention can also convert between other modes. For example, if the TE mode having the (i + 1) th highest effective refractive index is TE _i , TE _i , TE _j and δ proportional to the effective refractive index of the mode of (i, j> = 0 and i ≠ j) are combined. By making it smaller than the coefficient χ, efficient mode conversion is possible.
Also, assuming that the TM mode with the i-th highest effective refractive index is TM _(i−1) , in the present invention, the effective refractive index of the mode of TM _i , TM _j, and (i, j> = 0 and i ≠ j). By making δ proportional to the difference smaller than the coupling coefficient χ, efficient mode conversion is possible.

The present invention can also cope with mode multiplexing. Mode multiplexing refers to propagating different TE modes in the same waveguide.
Mode multiplexing can also be supported because in the case of an asymmetric directional coupler, the effective refractive index is not necessarily the same between the optical waveguides except for the mode to be coupled.
When the effective refractive index difference between the optical waveguides is large, δ increases and the coupling efficiency decreases.
That is, when the basic TE mode is input to the input-side optical waveguide and the basic TE mode is also input to the output-side optical waveguide at the same time (the basic TE ′ mode for distinction), at the output end of the output-side optical waveguide, An output in which the high-order TE mode in which the basic TE mode is converted and the basic TE ′ mode is multiplexed can be obtained.

In the illustrated example, in the directional coupler 18, excellent conversion characteristics (coupling efficiency T and the like) are obtained by designing the widths of the input side optical waveguide 11 and the output side optical waveguide 12. Not only the width of the waveguides 11 and 12 but also the optimum conversion characteristics can be obtained by optimizing the height. Further, the conversion characteristics can be improved by optimizing both the width and height of the optical waveguides 11 and 12.
For example, as a result of optimization, an excellent conversion characteristic can be obtained when one or both of the height and the width of the input side optical waveguide and the output side optical waveguide are different from each other.

In the present invention, the (n-1) th order mode, which is the TE mode having an effective refractive index nth (n is a natural number), is guided to the input side optical waveguide, and the mth ( m is a natural number, and it is preferable that (m-1) order mode which is a TE mode having a large effective refractive index is guided by m> n).
For example, it is preferable that a zero-order mode is guided in the input-side optical waveguide, and a first-order mode is guided in the output-side optical waveguide.

Hereinafter, the present invention will be specifically described with reference to examples.
<Example 1>
The mode conversion element 10 of Example 1 has the structure shown in FIG.
In the mode conversion element 10 of this embodiment, the middle SiO 2 layer of the SOI substrate is used as the lower cladding, and the Si layer is used as the optical waveguide region 2. After the optical waveguide region 2 is formed, a SiO ₂ layer can be provided as an upper clad. The refractive index of SiO ₂ that is the cladding material was 1.44, and the refractive index of Si that was the material of the optical waveguide region was 3.48.
In this mode conversion element 10, the coupling coefficient χ can be increased over a wide wavelength range, and a high coupling efficiency T can be obtained.

<Example 2>
FIG. 19 shows a mode conversion element 10 </ b> A that is a modification of the mode conversion element 10.
In the mode conversion element 10A, bent waveguides 8a and 8b are formed at both ends of the input side optical waveguide 11 of the directional coupler 18, and bent waveguides 9a and 9b are formed at both ends of the output side optical waveguide 12, respectively. Has been.
On one side of the waveguide direction, the bent waveguide 8a connected to the input-side optical waveguide 11 and the bent waveguide 9a connected to the output-side optical waveguide 12 approach each other toward the directional coupler 18. Yes.
On the other side in the waveguide direction, the bent waveguide 8b connected to the input side optical waveguide 11 and the bent waveguide 9b connected to the output side optical waveguide 12 are separated from each other in a direction away from the directional coupler 18. .
By forming the bent waveguides 8a, 8b, 9a, and 9b, the two waveguides can be gradually approached / separated, and unnecessary light reflection can be suppressed.
The bent waveguides 8a, 8b, 9a, and 9b may be provided in only one of the input side optical waveguide 11 and the output side optical waveguide 12, or one side of the input side optical waveguide 11 or the output side optical waveguide 12. And may be provided only on the other side. When a bent waveguide is not provided, the optical waveguide can be extended linearly from each optical waveguide of the directional coupler 18.

<Example 3>
FIG. 20 shows an example of an optical waveguide element in which the mode conversion element 10 and the high-order polarization conversion element 101 are combined.
An input-side waveguide connected to the input-side optical waveguide 11 is a first port 11a, and a waveguide connected to the input-side of the output-side optical waveguide 12 is a second port 12a. The third port 12 b on the output side of the output side optical waveguide 12 is connected to the high-order polarization conversion element 101.
As shown in FIG. 21, the high-order polarization conversion device 101 includes a core 102, a lower clad 103 having a refractive index lower than that of the core, and an upper clad 104 having a refractive index lower than that of the core 102. Since the upper clad 104 and the lower clad 103 have different refractive indexes, higher-order polarization conversion is possible. The core 102 is made of Si, for example. The lower cladding 103 is made of, for example, SiO ₂ . The upper clad 104 is made of air, for example.
As shown in FIG. 21A, the core 102 is formed in a tapered shape whose width continuously decreases in the light guiding direction of the optical waveguide 11.

  As shown in FIG. 20, a tapered region 19 in which the width of the slab portion 4 gradually decreases in the direction extending from the output side optical waveguide 12 is formed on the output side of the output side optical waveguide 12. Yes. In the taper region 19, the width of the slab portion 4 gradually decreases, and the end portion 19 a in the extending direction loses the slab portion 4, and the optical waveguide region 2 has a rectangular cross section.

In the taper region 19 in the illustrated example, the width of the slab portion 4 decreases in the extending direction, while the width of the rib portion 3 is constant in the length direction, but in the tapered region, the width of the rib portion extends in the extending direction. The taper may gradually decrease.
That is, the taper region may have a tapered shape in which the width of the rib portion and / or the slab portion decreases as it goes in the extending direction.
In the illustrated example, the tapered region 19 is formed in the output side optical waveguide 12, but may be formed in the input side optical waveguide 11. That is, the tapered region 19 can be formed in one or both of the input side optical waveguide 11 and the output side optical waveguide 12.
The taper region 19 preferably has a shape in which the width of the rib portion and / or the slab portion gradually decreases over the entire length in the extending direction. However, the taper region 19 has a constant width portion or an extending direction in a part of the length direction. A portion where the width increases may be included.

The taper region 19 is not limited to a shape in which the width of the rib portion and / or slab portion decreases in the extending direction, but may be a shape in which the width of the rib portion and / or slab portion increases in the extending direction in the same direction. Good.
In addition, the taper area | region 19 can be formed in any one or both of the optical waveguides 11 and 12 also about Example 1,2.

In this optical waveguide element, the basic TE mode can be converted into a high-order TE mode by the asymmetric directional coupler 18, and the high-order TE mode can be converted into the basic TM mode by the high-order polarization conversion element 101.
Since the high-order polarization conversion element 101 does not affect the basic TE mode, if the basic TE mode is input to the first port 11a and the second port 12a at the same time, from the output side of the high-order polarization conversion element 101 An output in which the basic TE mode and the basic TM mode are combined is obtained. Thus, it can be used as an element for performing polarization multiplexing.

<Example 4>
FIG. 22 shows another example of the optical waveguide element in which the mode conversion element 10 and the high-order polarization conversion element 111 are combined.
The high-order polarization conversion element 111 is connected to the third port 12 b on the output side of the output side optical waveguide 12.
As shown in FIG. 23, in the high-order polarization conversion device 111, the core 112 is composed of a lower core 114 and an upper core 113, and the width of the upper core 113 or the width of the lower core 114 is a light guide of the optical waveguide 11. It is formed in a tapered shape that continuously decreases in the wave direction.
The starter 118 has three or more modes in which the effective refractive index decreases in the order of the basic TE mode, the higher-order TE mode, and the basic TM mode. The end unit 119 has three or more modes in which the magnitude of the effective refractive index decreases in the order of the basic TE mode, the basic TM mode, and the higher order TE mode.
The core shape of the optical waveguide 1 between the start portion 118 and the end portion 119 has a vertically asymmetric structure in which the width of the upper core 113 and the width of the lower core 114 are different.
The high-order polarization conversion element 111 can perform polarization conversion between the high-order TE mode of the start unit 118 and the basic TM mode of the end unit 119.
In the high-order polarization conversion element 111, the upper cladding 114 and the lower cladding 113 may have the same refractive index. The lower clad 113 and the upper clad 114 are made of, for example, SiO ₂ . The optical waveguide region 2 is made of Si, for example.
The high-order polarization conversion element 111 is formed with a wide lower core 114. The lower core 114 can be formed integrally with the slab portion 4 of the mode conversion element 10.

In this optical waveguide element, the basic TE mode can be converted into a high-order TE mode by the asymmetric directional coupler 18, and the high-order TE mode can be converted into a basic TM mode by the high-order polarization conversion element 111.
Since the high-order polarization conversion element 111 does not affect the basic TE mode, if the basic TE mode is input to the first port 11a and the second port 12a at the same time, from the output side of the high-order polarization conversion element 111 An output in which the basic TE mode and the basic TM mode are combined is obtained. Thus, it can be used as an element for performing polarization multiplexing.

<Example 5>
(DP-QPSK modulator)
The mode conversion element of the present invention is described in Reference Document 1 (P. Dong, C. Xie, L. Chen, LL Buhl, and Y.-K. Chen, “112-Gb / s Monolithic PDM-QPSK Modulator in Silicon,” European. Dual Polarization-Quadrature Phase (DP-QPSK) as disclosed in Conference and Exhibition on Optical Communication, Vol. 1, p. Th.3.B.1, June 16, 2012) (Shift Keying).
FIG. 24 schematically shows an example of the DP-QPSK modulator. The DP-QPSK modulator 20 has independent QPSK signals for both the basic TE mode and the basic TM mode by utilizing the fact that two modes of a basic TE mode and a basic TM mode can exist in a normal optical waveguide. DP-QPSK modulation is performed. Specifically, the light input in the basic TE mode from the input unit 21 is branched into two optical waveguides 22 and 22, modulated into QPSK signals by the QPSK modulators 23 and 23, and then one side of the optical waveguides 24 and 24. The basic TE mode is converted to the basic TM mode by the polarization conversion element 25, the two modes are combined on the same optical waveguide by the polarization beam combiner, and signals independent of the basic TE mode and the basic TM mode are output. To the unit 26.

<Example 6>
(Polarization diversity coherent receiver)
The polarization conversion element of the present invention is described in Reference 2 (C. Doerr et al., “Packaged Monolithic Silicon 112-Gb / s Coherent Receiver,” IEEE Photonics Technology Letters, Vol. 23, pp. 762-764, 2011). Can be used for a coherent receiver on a Si optical waveguide of a polarization multiplexed signal that simultaneously transmits the basic TE mode and the basic TM mode.
FIG. 25 schematically shows an example of a polarization diversity coherent receiver. This coherent receiver 30 connects a polarization multiplexed signal optical waveguide 31 that transmits the basic TE mode and the basic TM mode at the same time to a polarization conversion element 32 that can simultaneously perform polarization conversion and polarization beam splitter. A basic TE mode signal is branched to one of 33 and 33, and a basic TE mode signal converted from the basic TM mode is branched to the other of the optical waveguides 33 and 33. A semiconductor laser light source generally used as the local light 34 uses only one polarized wave, for example, an output of a basic TE mode (local). In the case of using such a light source, conventionally, polarization conversion of local light is necessary.
However, in this coherent receiver 30, since the signal light becomes a signal in the basic TE mode after polarization separation, polarization conversion of local light becomes unnecessary. The signal light and the local light are output from the coupling unit 36 via the optical multiplexing unit 35.
When an optical waveguide type structure is used for the polarization conversion element 32, the coupling part 36 has a polarization separation function for coupling light to the outside of the element, such as an inverted taper mode field converter coupled from the side of the substrate. It is possible to use a coupler without it. For example, reference 3 (Qing Fang, et al., “Suspended optical fiber-to-waveguide mode size converter for silicon photonics,” Optics Express, Vol. 18, Issue 8, pp. 7763-7769 (2010 The structure of the reverse taper type disclosed in)) can be disclosed.

<Example 7>
(Polarization diversity method)
The polarization conversion element of the present invention is disclosed in Reference 4 (Hiroshi Fukuda et al., “Silicon photonic circuit with polarization diversity,” Optics Express, Vol. 16, Issue 7, pp. 4872-4880 (2008)). Use polarization multiplex transmission in which the basic TE mode and the basic TM mode are transmitted at the same time, or use elements to give the same operation to both modes when one polarization is transmitted randomly If so, it can be used to implement a polarization diversity scheme.
In the polarization diversity method 40 shown in FIG. 26, the polarization multiplexed signal optical waveguide 41 that transmits the basic TE mode and the basic TM mode at the same time is changed to a polarization conversion element 42 that can simultaneously perform polarization conversion and polarization beam splitter. The basic TE mode signal is branched to one of the optical waveguides 43 and 43, and the basic TE mode signal converted from the basic TM mode is branched to the other of the optical waveguides 43 and 43. The signal light of the basic TE mode operated by the elements 44 and 44 is synthesized from the optical waveguides 45 and 45 by the polarization conversion element 46, and the optical signal of the polarization multiplexed signal in which the basic TE mode and the basic TM mode are transmitted simultaneously. Output to the waveguide 47.

As the polarization conversion element 42, the polarization conversion element of the present invention capable of performing polarization conversion and polarization beam splitter at the same time can be used as in the polarization diversity coherent receiver.
As the polarization conversion element 46, as in the DP-QPSK modulator, the polarization conversion element of the present invention capable of simultaneously performing polarization conversion and polarization beam combiner can be used.

  As mentioned above, although this invention has been demonstrated based on suitable embodiment, this invention is not limited to the above-mentioned example, Various modifications are possible in the range which does not deviate from the summary of this invention.

DESCRIPTION OF SYMBOLS 1 ... Optical waveguide, 2 ... Optical waveguide area, 3 ... Rib part, 4 ... Slab part 4, 5 ... Cladding, 6 ... Upper clad, 7 ... Lower clad, 10 ... Mode conversion element, 11 ... Input side optical waveguide, 11b The outer surface of the input side optical waveguide, 12 the output side optical waveguide, 13 the intermediate region, 15 the output side outward extending region, and 18 the asymmetric directional coupler.

Claims (12)

A lower clad, and an optical waveguide region formed on the lower clad and having a higher refractive index than the lower clad,
The optical waveguide region has a pair of rib portions formed apart from each other, and a slab portion formed thinner than the rib portions and extending in the width direction of the rib portions,
One of the pair of rib portions is an input side optical waveguide, the other is an output side optical waveguide,
The input side and output side optical waveguides are at least partially arranged in parallel to form a directional coupler,
The directional coupler is capable of coupling between modes having different orders from the input side optical waveguide to the output side optical waveguide,
The slab portion has a dimension in the thickness direction perpendicular to the width direction in a cross section perpendicular to the length direction of the rib portion, which is smaller than the rib portion, and in at least a part of the directional coupler, An intermediate region formed by connecting them to each other between the input side optical waveguide and the output side optical waveguide;
A mode conversion element comprising: an outward extension region extending outward only in the width direction from only the output side optical waveguide of the input side and output side optical waveguides. In the input side optical waveguide, a (n−1) th order mode which is a TE mode having an effective refractive index nth (n is a natural number) is guided,
The output side optical waveguide is guided by an (m−1) th order mode which is a TE mode having an effective refractive index of mth (m is a natural number, m> n),
The mode conversion element according to claim 1, wherein the TE mode of the input-side optical waveguide and the TE mode of the output-side optical waveguide can be coupled.   A curved waveguide is disposed at one or both ends in the longitudinal direction of each of the input-side optical waveguide and the output-side optical waveguide, and is connected to the curved waveguide connected to the input-side optical waveguide and the output-side optical waveguide. 3. The mode conversion element according to claim 1, wherein the bent waveguides approach each other toward the directional coupler or are separated from each other in a direction away from the directional coupler.   One or both of the input-side optical waveguide and the output-side optical waveguide is formed with a tapered optical waveguide in which the width of the rib portion and / or the slab portion decreases as it goes in the extending direction. The mode conversion element according to any one of claims 1 to 3.   One or both of the input-side optical waveguide and the output-side optical waveguide are formed with tapered optical waveguides in which the width of the rib portion and / or slab portion increases in the extending direction. The mode conversion element according to any one of claims 1 to 3.   The mode conversion element according to claim 1, wherein the input side optical waveguide and the output side optical waveguide have the same core height.   The mode conversion element according to any one of claims 1 to 6, wherein a zero-order mode is guided in the input-side optical waveguide, and a primary mode is guided in the output-side optical waveguide. An upper clad covering the slab part, the rib part, and the lower clad;
The mode conversion element according to claim 1, wherein the slab part and the rib part are made of Si, and the lower clad and the upper clad are made of SiO ₂ .
  An optical waveguide device comprising the mode conversion device according to claim 1 and a high-order polarization conversion device connected to the output-side optical waveguide.   A DP-QPSK modulator comprising the mode conversion element according to claim 1.   A polarization diversity coherent receiver comprising the mode conversion element according to claim 1.   A polarization diversity system comprising the mode conversion element according to claim 1.

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