JP5304209B2 - Spot size converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spot size converter, facilitating its manufacture, low cost, and having a favorable light containment effect. <P>SOLUTION: This spot size converter 11 is equipped with: a clad 15 formed on a substrate 13; and a core 17 buried in the clad. The spot size converter has an input/output end face 25. The core is tapered toward the input/output end face along a light propagation direction 27. The clad is provided with groove parts 31a and 31b formed along the light propagation direction on both sides of the core for exposing a substrate surface 13a extending to the input/output end face. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a spot size converter used for optical connection between a Si wire waveguide and an external optical system such as a laser diode or an optical fiber.

In recent years, a technique using Si as an optical waveguide material has attracted attention. Among these techniques, there is known an optical waveguide called Si thin-wire waveguide, in which Si is used as a core and SiO ₂ having a refractive index much smaller than that of Si is used as a cladding.

  Since the Si fine wire waveguide has a very large refractive index difference between the core and the clad, it is possible to strongly confine light in the core. As a result, by using the Si fine wire waveguide, for example, it is possible to manufacture a small-sized curved waveguide with a bending radius reduced to about 1 μm, and so on, so that an optical device having a very fine submicron order size is produced. It is possible.

  For this reason, the Si wire waveguide is attracting attention as a technology that has the potential to fuse Si electronic devices and optical devices on the same chip.

  By the way, in order to connect an Si optical waveguide and an external optical element such as a laser diode (hereinafter also referred to as LD) or an optical fiber, a spot size converter is disposed between them. This is because the diameter of the core of the Si wire waveguide is very small as compared with a general external optical element such as an LD or an optical fiber. Therefore, by using a spot size converter, it is necessary to reduce the spot size of light input from the LD to the Si wire waveguide, or to increase the spot size of light output from the Si wire waveguide to the optical fiber. is there.

  Various proposals have been made for such spot size converters. For example, a spot size converter in which the width of the optical waveguide is reduced in a tapered manner toward the input / output end face side, that is, a spot size converter is known (for example, see Patent Documents 1 to 4). Also, a spot size converter is known in which the thickness of the optical waveguide is reduced in a tapered manner toward the input / output end face side, that is, the spot size converter is reduced (see, for example, Patent Document 5 or 6).

  Here, in such a spot size converter, when the input / output end face of the spot size converter is separated from the end of the core on the input / output end face side, the input / output end face and the end of the core The light spreads between them, and there is a risk of light leaking to the substrate on which the core and the cladding are formed. As a result, for example, when light is input from the LD to the Si thin wire waveguide, there arises a problem that the coupling efficiency is deteriorated.

Therefore, in order to solve this problem, a structure in which the optical confinement effect is enhanced by including an optical waveguide with a second core, that is, a double core structure is known (for example, see Patent Document 7). ).
JP 2002-162528 A JP 2000-235128 A US Pat. No. 6,684,011 JP 2003-207684 A Japanese Patent Laid-Open No. 9-15435 JP 2005-326876 A JP-A-7-63935

  However, the spot size converter having such a double core structure has a problem that it is difficult to manufacture and the manufacturing cost increases.

  The present invention has been made in view of these problems. Accordingly, an object of the present invention is to provide a spot size converter that is easy to manufacture and low in cost and has a good light confinement effect.

  As a result of repeated research, the inventors of this application have found that light can be efficiently confined by providing groove portions formed by partially removing the clad on both sides of the core.

  In order to achieve the above object, the spot size converter of the present invention has the following features.

That is, the spot size converter according to the present invention includes a clad formed on a substrate and a core embedded in the clad. The spot size converter according to the present invention has an input / output end face. The core tapers toward the input / output end face along the light propagation direction. Further, in the clad, grooves that expose the substrate surface are formed on both sides of the core along the light propagation direction so as to extend to the input / output end surfaces. Further, a lateral groove along the substrate surface is formed in the peripheral clad portion including the core from the groove portion toward the core side.

  By providing such a technical feature, the spot size converter of the present invention differs from the above-described spot size converter according to the prior art in that the substrate surface is formed on the clad without forming a double core structure. A good light confinement effect can be obtained only by providing the groove to be exposed. Therefore, according to the present invention, it is possible to provide a spot size converter that is easy to manufacture and can confine light efficiently at a low cost as compared with the prior art.

Hereinafter, a spot size converter according to an embodiment and a reference example of the present invention will be described with reference to the drawings. Each drawing merely schematically shows the shape, size, and arrangement relationship of each component to the extent that the present invention can be understood. Therefore, the configuration of the present invention is not limited to the illustrated configuration example.

< Reference example >
In the reference example , a spot size converter including a clad formed on a substrate and a core embedded in the clad will be described.

FIG. 1A is a plan view schematically showing a main part of a spot size converter according to a reference example of the present invention. FIG. 1B shows the structure shown in FIG. 1A cut in the thickness direction of the substrate along the line II in FIG. 1A, that is, the direction orthogonal to the light propagation direction. It is a figure which shows a cross section.

In the actual spot size converter, in the structure shown in FIG. 1 (A), the entire core is embedded in the clad. In FIG. 1 (A), the characteristic part according to the reference example is formed. For the sake of clarity, a portion of the cladding is omitted.

In addition, the spot size converter according to the reference example is disposed between an external optical element such as an LD or an optical fiber and the Si thin wire waveguide. Then, spot size conversion is performed between the external optical element and the Si thin wire waveguide for input light input from the external optical element to the Si thin wire waveguide or output light output from the Si thin wire waveguide to the external optical element. . For this reason, at least a region in which the input light or output light propagates in the spot size converter is formed as a region transparent to the input light and output light.

The spot size converter 11 according to the reference example is formed on a substrate 13 and includes an optical waveguide portion 19 including a clad 15 and a core 17.

In the spot size converter 11 according to this reference example , the input light input from the external optical element to the Si thin wire waveguide, or the output light output from the Si thin wire waveguide to the external optical element is input / output end face 25, Therefore, it is input or output from the input / output port.

In this reference example , as shown in FIG. 1, the spot size converter 11 causes the input light 23 emitted from the LD 21 as an external optical element to correspond to the spot size of the Si wire waveguide (not shown). A configuration example for spot size conversion will be described. The Si fine wire waveguide is disposed, for example, on the side facing the LD 21 with the spot size converter 11 interposed therebetween, and is provided continuously and integrally with the above-described optical waveguide portion 19 (not shown).

  In the spot size converter 11, the input / output end face 25 faces the exit end face 21a of the LD 21 that emits the emitted light, that is, the input light 23 to the spot size converter 11, and preferably, for example, 1 to 2 μm. They are spaced apart.

  The substrate 13 is preferably a rectangular parallel plate, for example. The material constituting the substrate 13 is preferably single crystal Si, for example.

The clad 15 is disposed over the upper surface 13a of the substrate 13, that is, the entire substrate surface 13a. The material constituting the clad 15 is preferably SiO ₂ , for example.

  The core 17 is provided so as to be embedded in the clad 15 and extends linearly.

Further, the refractive index of the core 17 is higher than the refractive index of the clad 15. More specifically, in this reference example, it is preferable to set the refractive index of the core 17 to, for example, about 3.5, and the refractive index of the cladding 15 to, for example, about 1.5 to 1.7. Due to this refractive index difference, the input light 23 input to the optical waveguide portion 19 from the input / output end face 25 is confined in the core 17, and as a result, propagates in the core 17 along the extending direction of the core 17. Therefore, in the spot size converter 11 according to this reference example , the extending direction of the core 17 is a light propagation direction 27 indicated by an arrow.

  In the spot size converter 11, the thickness T1 of the core 17 is preferably set to 0.3 μm, for example. The thickness T2 of the clad 15 between the core 17 and the substrate 13 is preferably 2 μm, for example, and the thickness T3 of the clad 15 above the core 17 is preferably 2 μm, for example. .

  The core 17 is tapered toward the input / output end face 25 along the extending direction, that is, the light propagation direction 27. By forming the core 17 in such a shape, that is, in a tapered shape, the input light 23 from the LD 21 is spot-size converted while propagating through the core 17.

  Here, when the core 17 is tapered for the purpose of spot size conversion of the input light 23, the thickness T1 of the core 17 gradually decreases toward the input / output end face 25 or the light propagation direction 27. Any shape may be employed in which the width along the substrate surface 13a, that is, the width W1 along the short direction 29 gradually decreases toward the input / output end surface 25 side.

  When the core 17 has a shape in which the thickness T1 gradually decreases toward the input / output end face 25 side, the thickness T1 becomes 0 at the tip of the core 17 on the input / output end face 25 side. In this case, the preferable value of the thickness T1 of the core 17 described above is the maximum value of the thickness T1, and the opposite end facing the input / output end face 25, that is, the interface with the Si wire waveguide (not shown). The thickness at 17b.

  Further, when the core 17 has a shape in which the width W1 gradually decreases toward the input / output end face 25, the width W1 becomes zero at the tip of the core 17 on the input / output end face 25 side. In the configuration example shown in FIG. 1A, a case is employed in which the width W1 along the short direction of the core 17 is gradually narrowed toward the input / output end face 25 side.

  In addition, it is preferable that the tip portion 17a of the core 17 on the input / output end face 25 side coincides with the surface position of the input / output end face 25 in order to input the input light 23 from the LD 21 to the core 17 efficiently. However, the distal end portion 17a and the input / output end face 25 may be slightly separated according to the spot size of the LD 21, ease of manufacture, or other design. The spot size of the LD 21 means the half width along the short direction 29 of the peak of the light intensity of the input light emitted from the LD 21. Further, FIG. 1A shows a configuration example in which the distal end portion 17a and the input / output end face 25 are separated from each other.

Further, in the spot size converter 11 according to the reference example , grooves 31 a and 31 b are formed in the clad 15.

  The groove portions 31 a and 31 b are provided on both sides of the core 17 along the light propagation direction 27 with the core 17 interposed therebetween, and extend to the input / output end face 25. And the groove parts 31a and 31b have exposed the substrate surface 13a from the bottom face in these 31a and 31b.

Such groove portions 31 a and 31 b have a refractive index lower than that of the clad 15 because the refractive index is the same as that of the atmosphere, that is, about 1. Therefore, by forming the groove portions 31a and 31b, in the spot size converter 11 according to the reference example , the portion including the optical waveguide portion 19 sandwiched between the groove portions 31a and 31b, that is, the core 17 and the core 17, is included. In the clad portion 15 a, the input input light 23 can be strongly confined from both sides along the short direction 29. As a result, by inputting the input light 23 to the portion of the optical waveguide portion 19 consisting of the core 17 and the peripheral cladding portion 15a, that is, the effective optical waveguide portion 19a, the spot size converter 11 according to this reference example uses the substrate Therefore, spot size conversion can be performed with a good coupling efficiency without light leaking to 13.

In the spot size converter 11 according to the reference example , the grooves 31 a and 31 b may be embedded using a material having a refractive index lower than that of the cladding 15.

Here, it has been found by simulation that a suitable value corresponding to the spot size of the LD 21 exists in the width between the groove portions 31a and 31b, that is, the width W2 along the short direction 29 of the effective optical waveguide portion 19a. . That is, in the spot size converter 11 according to the reference example , the width along the short direction 29 between the groove portions 31a and 31b, that is, the width W2 of the effective optical waveguide portion 19a is set to 70% or less of the spot size of the LD 21 at the maximum. Is preferred. Details of this simulation will be described later.

Thus, unlike the spot size converter according to the prior art, the spot size converter 11 according to the reference example does not form a double core structure, and only the grooves 31a and 31b are provided in the clad 15, so that a good light can be obtained. The confinement effect can be obtained. Therefore, in this reference example, it is possible to provide a spot size converter that is easy to manufacture and can confine light efficiently at a low cost as compared with the prior art.

In addition, the spot size converter 11 according to such a reference example can be easily manufactured by preparing a well-known SOI substrate, for example.

That is, first, an SOI substrate in which a single crystal Si layer, a SiO ₂ film, and a Si film are stacked in this order is prepared. Then, the core 17 described above is formed from the remaining portion of the Si film by partially removing the Si film using a known etching technique. Thereafter, the core 17 is buried on the entire surface of the SOI substrate, and an SiO ₂ film is deposited. As a result, the optical waveguide section 19 including the clad 15 made of SiO ₂ and the core 17 embedded in the clad 15 is formed on the single crystal Si substrate 13 as described above. . Further, both side regions of the core 17 along the light propagation direction 27 of the clad 15 are removed from the both side regions to the input / output end surface 25 using a known etching technique until the substrate surface 13a is exposed. As a result, the removed regions are referred to as the above-described grooves 31a and 31b.

Here, the inventors of the present invention use BPM (Beam Propagation Method) in order to evaluate the characteristics of the spot size converter according to this reference example and to confirm the optimum values of various dimensions. And simulated.

  2 to 8 show the results of simulation by BPM, respectively. Hereinafter, each of these results will be described.

In addition, unless otherwise indicated, each simulation which obtained each result of these FIGS. 2-8 is the following about the spot size converter 11 of the reference example by the structural example shown to FIG. 1 (A) and (B). Each condition was set and performed.

  That is, in each simulation, the thickness T2 of the lower clad 15 of the core 17 was set to 2 μm, and the thickness T3 of the upper clad 15 of the core 17 was set to 2 μm.

In each simulation, the spot size converter 11 of the reference example is connected to a Si wire waveguide having a core having a thickness of 0.3 μm and a width of 0.3 μm perpendicular to the thickness direction and the light propagation direction. This was done assuming that it will be used. Therefore, the thickness T1 of the core 17 of the spot size converter 11 is set to 0.3 μm. Then, the maximum value of the width W1 along the short direction of the core 17 having a tapered shape, that is, the width W1 at the interface 17b with the core of the Si wire waveguide (not shown) was set to 0.3 μm.

  Each simulation shows the coupling efficiency when the spot size of the LD 21 is 3 μm and the input light 23 having a wavelength of 1.31 μm emitted from the LD 21 is input from the input / output end face 25 to the effective optical waveguide portion 19a. ing.

  Moreover, the vertical axis | shaft of each figure has shown the coupling efficiency of the spot size converter 11 with the relative value of light intensity. That is, the light intensity of the input light 23 emitted from the LD 21 is set to 1, and the relative value of the light intensity when the input light 23 is spot size converted by the spot size converter 11 is shown. In each figure, there is a figure showing the result that the relative light intensity value exceeds 1. This is considered to be an error due to a change in the shape of the input light 23 due to spot size conversion.

  First, FIG. 2 is a diagram showing the relationship between the coupling efficiency and the length L1 of the core 17 along the light propagation direction 27, that is, the tapered length L1 of the tapered shape, in the spot size converter 11. In FIG. 2, the horizontal axis indicates the length L1 of the core 17 in units of μm. In the simulation for obtaining the result of FIG. 2, the width between the groove portions 31a and 31b, ie, the width W2 along the short direction 29 of the effective optical waveguide portion 19a was set to 2 μm.

  A curve I in FIG. 2 shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 0 μm, that is, the tip end portion 17a matches the surface position of the input / output end face 25. A curve II in FIG. 2 shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 10 μm. Curve III in FIG. 2 shows the result when the distance L2 between the tip 17a and the input / output end face 25 is 20 μm.

From the results of FIG. 2, it is clear that in the spot size converter 11, when the taper length L1 is 20 μm or more, the obtained coupling efficiency does not change greatly at each separation distance L2. Therefore, from this result, in the spot size converter 11 according to the reference example , the taper length L1 is preferably set to at least 20 μm.

  Next, FIG. 3 is a diagram illustrating the relationship between the coupling efficiency and the width W2 along the short direction 29 of the effective optical waveguide portion 19a in the spot size converter 11. In FIG. 3, the horizontal axis indicates the width W2 of the effective optical waveguide portion 19a in units of μm. In the simulation that obtained the result of FIG. 3, the taper length L1 was set to 30 μm.

  A curve I in FIG. 3 shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 0 μm, that is, the tip end portion 17a matches the surface position of the input / output end face 25. A curve II in FIG. 3 shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 5 μm. A curve III in FIG. 3 shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 10 μm. Further, a curve IV in FIG. 3 shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 15 μm. A curve V in FIG. 3 shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 20 μm.

From the results of FIG. 3, in the spot size converter 11, when the spot size of the LD 21 is 3 μm, the coupling obtained when the width W2 of the effective optical waveguide portion 19a is larger than 2 μm at each separation distance L2. It can be seen that the efficiency is significantly reduced. Therefore, from this result, in the spot size converter 11 according to the reference example , when the spot size of the LD 21 is 3 μm, for example, the width W2 of the effective optical waveguide portion 19a is preferably set to 2 μm or less at the maximum.

Next, FIGS. 4A and 4B are diagrams for evaluating the effects of the grooves 31a and 31b of the spot size converter 11 according to the reference example .

  FIG. 4A shows the amount of shift when coupling the LD to the both sides along the short direction and the coupling efficiency in a spot size converter according to the prior art, that is, a spot size converter in which no groove is formed. It is a figure which shows the relationship.

FIG. 4B is a diagram showing the relationship between the amount of shift and the coupling efficiency when the LD 21 is shifted to both sides along the short direction 29 in the spot size converter 11 according to the reference example . 4B, the width between the grooves 31a and 31b, that is, the width W2 of the effective optical waveguide portion 19a is set to 2 μm. 4B, unlike the configuration shown in FIG. 1, in the simulation, the one side surface of the core 17 extending from the tip end portion 17a and orthogonal to the substrate surface 13a is placed in the light propagation direction 27. The spot size converter 11 is assumed to have a tapered configuration by making the other side surface an inclined surface with respect to the one side surface.

4A and 4B, the spot size converter 11 according to the reference example assumed as a sample is different from the spot size converter according to the conventional technique in terms of the configuration. There is only presence or absence. 4A and 4B, the length of the core along the light propagation direction, that is, the taper length was set to 20 μm.

  4 (A) and 4 (B), the horizontal axis represents the central axis along the light propagation direction of the LD spot, and the short direction along the light propagation direction of the core of the spot size converter. The point where the center coincides is set to 0, and the deviation amount of the LD along the short direction from this point is graduated in units of μm. In this horizontal axis, the amount of deviation when the LD is shifted from the above-described zero point to one side along the short direction is a positive value, and the amount of deviation when the LD is shifted to the other side is Negative values are shown.

  Further, curve I in FIG. 4A shows a spot size converter according to the prior art, in which the separation distance between the tip portion on the input / output end face side of the core and the input / output end face is 0 μm, that is, the tip portion is the surface of the input / output end face. The result when matched with the position is shown. Further, a curve II in FIG. 4A shows the result when the separation distance between the core tip and the input / output end face is 5 μm. In addition, a curve III in FIG. 4A shows the result when the separation distance between the tip portion of the core and the input / output end face is 10 μm. Further, a curve IV in FIG. 4A shows the result when the separation distance between the core tip and the input / output end face is 15 μm. Further, a curve V in FIG. 4A shows the result when the separation distance between the tip portion of the core and the input / output end face is 20 μm.

4B indicates that the distance L2 between the tip 17a and the input / output end face 25 is 0 μm, that is, the tip 17a is the surface of the input / output end face 25 in the spot size converter 11 according to the reference example. The result when matched with the position is shown. A curve II ′ in FIG. 4B shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 5 μm. Further, a curve III ′ in FIG. 4B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 10 μm. Further, a curve IV ′ in FIG. 4B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 15 μm. A curve V ′ in FIG. 4B shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 20 μm.

As is apparent from the results of FIGS. 4A and 4B, the spot size converter 11 according to the reference example performs spot size conversion according to the conventional technique at the separation distance between the tip end portion and the input / output end face of each core. Compared to the vessel, it shows good coupling efficiency. Therefore, from this result, it was confirmed that the input light 23 can be efficiently confined in the effective optical waveguide portion 19a by providing the groove portions 31a and 31b.

4A and 4B, in the spot size converter 11 according to the reference example and the spot size converter according to the prior art, the positions of the LD and the spot size converter along the short direction. It can be seen that the effect on the coupling efficiency due to the deviation is comparable. However, as described above, the spot size converter 11 according to the reference example has improved coupling efficiency in comparison with the spot size converter according to the prior art in the separation distance between the tip portion and the input / output end face of each core. Therefore, even when the LD is displaced along the short side direction, a good coupling efficiency can be obtained.

Next, FIG. 5 shows the amount of deviation and coupling efficiency when the LD 21 is shifted to both sides along the short direction 29 in the spot size converter 11 according to the reference example , as in FIG. It is a figure which shows a relationship. In the simulation that obtained the result of FIG. 5, the width between the groove portions 31a and 31b, that is, the width W2 of the effective optical waveguide portion 19a was set to 1 μm. Moreover, in the simulation which obtained the result of this FIG. 5, taper length L1 was 20 micrometers.

  In FIG. 5, the horizontal axis indicates that the central axis along the light propagation direction of the LD spot coincides with the center of the spot size converter along the light propagation direction of the core in the short direction. The deviation amount of the LD 21 along the short direction from this point is graduated in units of μm. On the horizontal axis, the shift amount when the LD 21 is shifted from the above-described zero point to one side along the short direction 29 is a positive value, and the shift amount when the LD 21 is shifted to the other side. Is shown as a negative value.

5 indicates that, in the spot size converter 11 according to the reference example , the separation distance L2 between the tip 17a and the input / output end face 25 is 0 μm, that is, the tip 17a is made to coincide with the surface position of the input / output end face 25. The result is shown. Further, a curve II in FIG. 5 shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 5 μm. A curve III in FIG. 5 shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 10 μm. A curve IV in FIG. 5 shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 15 μm. A curve V in FIG. 5 shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is set to 20 μm.

Comparing the result of FIG. 5 with the result of FIG. 4B already described, in the spot size converter 11 according to the reference example , the width W2 of the effective optical waveguide portion 19a is shortened to 1 μm, and thus each separation distance L2 It can be seen that the coupling efficiency obtained is improved.

  Further, in the simulation obtained from the result of FIG. 5, the width W2 of the effective optical waveguide portion 19a of the spot size converter 11 is shortened, so that the input light 23 is incident in the short direction 29 of the input / output end face 25. The substantial width along is shortened. As a result, the deviation of the LD 21 along the short direction 29 has a greater effect than in the simulation in which the result of FIG. 4B is obtained, that is, when the width W2 of the effective optical waveguide portion 19a is 2 μm. Yes. However, as is apparent from the results of FIG. 5, the spot size converter 11 having the width W2 set to 1 μm is not affected by this deviation if the deviation amount of the LD 21 is in the range of −0.5 to 0.5 μm. It is almost unaffected and shows good coupling efficiency. Since it is easy to position the LD 21 having a spot size of 3 μm with respect to the effective optical waveguide portion 19a having a width W2 of 1 μm while keeping the amount of deviation within a range of −0.5 to 0.5 μm. Even in the spot size converter 11 with the width W2 of 1 μm, it can be said that good coupling efficiency can be obtained.

Next, FIGS. 6A and 6B are diagrams for evaluating the effects of the grooves 31a and 31b of the spot size converter 11 according to the reference example .

  FIG. 6A shows a shift amount and coupling efficiency when the LD is shifted up and down along the thickness direction of the substrate in a spot size converter according to the prior art, that is, a spot size converter in which no groove is formed. It is a figure which shows the relationship.

FIG. 6B is a diagram showing the relationship between the amount of shift and the coupling efficiency when the LD 21 is shifted up and down along the thickness direction of the substrate 13 in the spot size converter 11 according to the reference example . is there. 4B, the width between the grooves 31a and 31b, that is, the width W2 of the effective optical waveguide portion 19a is set to 2 μm.

6A and 6B, in the simulation, the spot size converter 11 according to the reference example assumed as a sample and the spot size converter according to the prior art are structurally different. There is only presence or absence. 6A and 6B, the length of the core along the light propagation direction, that is, the taper length was set to 20 μm. As already described, in this simulation, the wavelength of the input light emitted from the LD is 1.31 μm.

  6A and 6B, the horizontal axis represents the central axis along the light propagation direction of the LD spot, and the center of the spot size converter in the thickness direction along the light propagation direction of the core. The point at which these coincide with each other is set to 0, and the deviation amount of the LD along the thickness direction from this point is graduated in units of μm. On the horizontal axis, the amount of deviation when the LD is shifted upward from the above-described zero point along the thickness direction is a positive value, and the amount of deviation when the LD is shifted downward is a negative value. Show.

  Further, curve I in FIG. 6A shows a spot size converter according to the prior art, in which the separation distance between the tip portion on the input / output end face side of the core and the input / output end face is 0 μm, that is, the tip portion is the surface of the input / output end face. The result when matched with the position is shown. Further, a curve II in FIG. 6A shows the result when the separation distance between the core tip and the input / output end face is 5 μm. Further, a curve III in FIG. 6A shows the result when the separation distance between the tip portion of the core and the input / output end face is 10 μm. A curve IV in FIG. 6 (A) shows the result when the separation distance between the tip of the core and the input / output end face is 15 μm. A curve V in FIG. 6A shows the result when the separation distance between the tip end portion of the core and the input / output end face is 20 μm.

In addition, in the spot size converter 11 according to the reference example , the curve I ′ in FIG. The result when matched with the position is shown. A curve II ′ in FIG. 6B shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 5 μm. A curve III ′ in FIG. 6B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 10 μm. A curve IV ′ in FIG. 6B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 15 μm. A curve V ′ in FIG. 6B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 20 μm.

As is apparent from the results of FIGS. 6A and 6B, the spot size converter 11 according to the reference example performs spot size conversion according to the prior art at the separation distance between the tip end portion and the input / output end face of each core. Compared to the vessel, it shows good coupling efficiency. Therefore, it was confirmed that the input light 23 can be efficiently confined in the effective optical waveguide portion 19a by providing the groove portions 31a and 31b.

Further, from the results of FIGS. 6A and 6B, in the spot size converter 11 according to the reference example and the spot size converter according to the related art, the positional deviation along the thickness direction between the LD and the spot size converter. It can be seen that the influence on the coupling efficiency is about the same. However, as described above, the spot size converter 11 according to the reference example has improved coupling efficiency in comparison with the spot size converter according to the prior art in the separation distance between the tip portion and the input / output end face of each core. Therefore, even when the LD is displaced along the thickness direction, good coupling efficiency can be obtained.

Next, FIGS. 7A and 7B are diagrams for evaluating the effects of the grooves 31a and 31b of the spot size converter 11 according to the reference example .

FIGS. 7A and 7B show the same simulation as that obtained with the above-described FIGS. 6A and 6B along the wavelength of the input light from the LD and the light propagation direction of the core. It was obtained by changing the length, that is, the taper length. That is, in this simulation, the wavelength of the input light from the LD was 1.49 μm. In this simulation, the taper length of the core is 10 μm in each of the spot size converter according to the prior art and the spot size converter 11 according to the reference example . Other configurations are the same as those of the spot size converter according to the prior art and the spot size converter 11 according to the reference example , which are used as samples in the simulations obtained in FIGS. 6A and 6B described above.

  Further, in FIGS. 7A and 7B, the horizontal axis indicates the amount of deviation of the LD along the thickness direction of the LD in units of μm, as in FIGS. 6A and 6B.

  Further, curve I in FIG. 7A shows a spot size converter according to the prior art, in which the separation distance between the tip portion on the input / output end face side of the core and the input / output end face is 0 μm, that is, the tip portion is the surface of the input / output end face. The result when matched with the position is shown. In addition, a curve II in FIG. 7A shows the result when the separation distance between the tip portion of the core and the input / output end face is 5 μm. A curve III in FIG. 7A shows the result when the distance between the tip of the core and the input / output end face is 10 μm. Further, a curve IV in FIG. 7A shows the result when the separation distance between the core tip and the input / output end face is 15 μm.

7B shows a curve I ′ in the spot size converter 11 according to the reference example. The distance L2 between the tip 17a and the input / output end face 25 is 0 μm, that is, the tip 17a is the surface of the input / output end face 25. The result when matched with the position is shown. Further, a curve II ′ in FIG. 7B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 5 μm. A curve III ′ in FIG. 7B shows the result when the separation distance L2 between the tip end portion 17a and the input / output end face 25 is 10 μm. In addition, a curve IV ′ in FIG. 7B shows the result when the separation distance L2 between the distal end portion 17a and the input / output end face 25 is 15 μm.

As is clear from FIGS. 7A and 7B, even when the wavelength of the input light is 1.49 μm, the spot size according to the reference example is different in the separation distance between the tip end portion and the input / output end face of each core. The converter 11 shows good coupling efficiency compared to a spot size converter according to the prior art. Therefore, it was confirmed that the input light 23 can be efficiently confined in the effective optical waveguide portion 19a by providing the groove portions 31a and 31b.

7A and 7B, in the spot size converter 11 according to the reference example and the spot size converter according to the prior art, the positional deviation along the thickness direction between the LD and the spot size converter. It can be seen that the influence on the coupling efficiency is about the same.

Here, in an optical transmission / reception system that transmits and receives an optical signal using a single optical fiber, such as an optical subscriber system, in many cases, the wavelength is 1.31 μm as an upstream optical signal from the subscriber side to the station. In addition, an optical signal having a wavelength of 1.49 μm is used as a downstream optical signal from the station to the subscriber side. As is clear from the results of FIGS. 6A and 6B and FIGS. 7A and 7B described above, the spot size converter 11 according to the reference example has the input light and wavelength of 1.31 μm. It was found that good coupling efficiency can be obtained for both of 1.49 μm input light. Therefore, the spot size converter 11 according to this reference example can perform spot size conversion with good coupling efficiency for both the upstream optical signal and downstream optical signal in the optical transmission / reception system described above. it can.

Next, FIGS. 8A and 8B are views showing the relationship between the width W2 along the short direction 29 of the effective optical waveguide portion 19a and the coupling efficiency in the spot size converter 11 according to the reference example . It is. 8A and 8B, the horizontal axis indicates the width W2 of the effective optical waveguide portion 19a in units of μm.

FIG. 8A shows the result when the input light 23 from the LD 21 having a spot size of 3 μm is input to the spot size converter 11 according to the reference example . In the simulation obtained this result, the length L1 of the core 17 along the light propagation direction 27, that is, the taper length L1 is set to 30 μm, and the separation distance L2 between the tip 17a of the core 17 and the input / output end face 25 is set. Was set to 5 μm.

FIG. 8B shows the result when the input light 23 from the LD 21 having a spot size of 6 μm is input to the spot size converter 11 according to the reference example . In the simulation that obtained this result, the length L1 of the core 17 along the light propagation direction 27, that is, the taper length L1 is set to 70 μm, and the separation distance L2 between the tip 17a of the core 17 and the input / output end face 25 Was set to 5 μm.

  A curve I in FIG. 8A shows the result when the thickness T2 of the lower clad 15 of the core 17 is set to 1 μm and the thickness T3 of the upper clad 15 of the core 17 is set to 1 μm. Further, a curve II in FIG. 8A shows a result when the thickness T2 is set to 2 μm and the thickness T3 is set to 2 μm. Further, a curve III in FIG. 8A shows a result when the thickness T2 is set to 3 μm and the thickness T3 is set to 3 μm.

  A curve I ′ in FIG. 8B shows the result when the thickness T2 of the lower cladding 15 of the core 17 is set to 2 μm and the thickness T3 of the upper cladding 15 of the core 17 is set to 2 μm. . In addition, a curve II ′ in FIG. 8A shows the result when the thickness T2 is set to 3 μm and the thickness T3 is set to 3 μm.

  8A, in the spot size converter 11, when the spot size of the LD 21 is 3 μm, the width W2 of the effective optical waveguide portion 19a is within 2 μm at each thickness T2 and T3 of the clad 15. If it exists, it turns out that favorable coupling efficiency is obtained.

  8B, in the spot size converter 11, when the spot size of the LD 21 is 6 μm, the width W2 of the effective optical waveguide portion 19a is 2 at each thickness T2 and T3 of the cladding 15. It can be seen that good coupling efficiency can be obtained when the thickness is within ˜4 μm.

Therefore, as apparent from the results of FIGS. 8A and 8B, in the spot size converter 11 according to the reference example , in order to obtain a good coupling efficiency, the width W2 of the effective optical waveguide portion 19a is set to It can be said that it is preferable to set it to 70% or less of the spot size of the LD 21 at the maximum.

<First Embodiment >
In the first embodiment, a spot size converter in which the configuration of the grooves 31a and 31b described in the above-described reference example (see FIG. 1B) is changed from the above-described configuration example will be described.

In the first embodiment , in order to obtain the above-described light confinement effect more efficiently, the spot size converter 11 (see FIGS. 1A and 1B) according to the reference example described above is further added. A typical configuration is provided. Since other components and operational effects are the same as those of the reference example , common components are denoted by the same reference numerals, and redundant description thereof is omitted.

FIG. 9 is a schematic cross-sectional view for explaining the first embodiment . The cross section shown in FIG. 9 corresponds to a cross section cut in the thickness direction of the substrate along the line II shown in FIG.

In the spot size converter 35 according to the first embodiment , grooves 31 a and 31 b are formed in the peripheral cladding portion 15 a including the core 17.

  The lateral grooves 37a and 37b are formed along the substrate surface 13a toward the core 17 side.

In such a spot size converter 35 according to the first embodiment, in addition to the light confinement effect from both sides along the short direction 29 by the grooves 31a and 31b, the lateral grooves 37a and 37b are further effective. A light confinement effect from the lower side along the thickness direction of the substrate 13 can be obtained between the optical waveguide portion 19 a and the substrate 13. Therefore, with the configuration of the spot size converter 35 according to the first embodiment, the light confinement effect of the spot size converter according to the reference example described above can be obtained more efficiently.

<Second Embodiment >
In the second embodiment, the configuration of the grooves 31a and 31b described in the reference example described above or the first embodiment (see FIGS. 1B and 9) is changed from these configuration examples described above. A spot size converter will be described.

In the second embodiment , in order to obtain the above-described light confinement effect more efficiently, the spot size converter 11 (see FIGS. 1A and 1B) according to the reference example described above, or the first The spot size converter 35 according to the embodiment is further provided with an additional configuration. Since other components and operational effects are the same as those of the reference example , common components are denoted by the same reference numerals, and redundant description thereof is omitted. Here, a configuration example in which the second embodiment is applied to the spot size converter 11 according to the reference example described above is illustrated and described.

FIG. 10 is a schematic cross-sectional view for explaining the second embodiment . Note that the cross section shown in this figure corresponds to a cross section taken in the thickness direction of the substrate along the line II shown in FIG.

In the spot size converter 39 according to the second embodiment, the exposed surface 15b of the peripheral cladding portion 15a, that is, the exposed surface 15b of the effective optical waveguide portion 19a is covered with the reflective film 41.

  The reflective film 41 is preferably provided with a thickness of, for example, about 0.1 μm using, for example, single crystal Si having a refractive index of about 3.5.

In such a spot size converter 39 according to the second embodiment , since the effective optical waveguide portion 19a is covered with the reflective film 41, the light input into the effective optical waveguide portion 19a is in the short direction. The reflective film 41 is confined from both sides along the line 29 and from the upper side along the thickness direction of the substrate 13. Therefore, with the configuration of the spot size converter 39 according to the second embodiment, the light confinement effect of the spot size converter according to the reference example or the first embodiment described above can be obtained more efficiently.

Here, in FIG. 10, the reflective film 41 is orthogonal to the substrate surface 13a exposed as the exposed surface 15b of the peripheral cladding portion 15a, that is, the upper surface 15ba facing the substrate surface 13a, and the inner wall surfaces of the grooves 31a and 31b, and The structural example which covers the whole surface of both side surface 15bb and 15bc along the light propagation direction 27 is shown. However, in the spot size converter 39 according to the second embodiment , the reflective film 41 is a part of the exposed surface 15b of the peripheral cladding portion 15a, for example, the upper surface, depending on the ease of manufacturing, manufacturing cost, and other designs. It is good also as a structure which covers only 15ba. In this case, light leakage to the upper surface 15ba is suppressed by the confinement effect by the reflective film 41, and light leakage to both sides along the short direction 29 is suppressed by the confinement effect by the grooves 31a and 31b.

(A) is the top view which represented roughly the principal part of the spot size converter by the reference example of this invention. (B) is sectional drawing cut out along the II line | wire about the structure shown to (A). It is a figure which shows the relationship between taper length L1 and coupling efficiency in the spot size converter by the reference example of this invention. It is a figure which shows the relationship between the width W2 of an effective optical-waveguide part, and coupling efficiency in the spot size converter by the reference example of this invention. (A) And (B) is a figure for evaluating the effect of the groove part of the spot size converter by a reference example . It is a figure for evaluating the effect of the groove part of the spot size converter by the reference example of this invention. (A) And (B) is a figure for evaluating the effect of the groove part of the spot size converter by the reference example of this invention. (A) And (B) is a figure for evaluating the effect of the groove part of the spot size converter by the reference example of this invention. (A) And (B) is a figure which shows the relationship between the width W2 of an effective optical-waveguide part and coupling efficiency in the spot size converter by the reference example of this invention. It is a schematic cross-sectional view illustrating a first embodiment of the present invention. It is a schematic sectional drawing explaining 2nd Embodiment of this invention.

Explanation of symbols

11, 35, 39: Spot size converter 13: Substrate 15: Clad 17: Core 19: Optical waveguide portion 19a: Effective optical waveguide portion 21: LD
23: input light 25: input / output end face 27: light propagation direction 29: short direction 31a, 31b: groove 37a, 37b: lateral groove 41: reflective film

Claims (3)

A spot size converter comprising a clad formed on a substrate and a core embedded in the clad,
The spot size converter has an input / output end face,
The core is tapered toward the input / output end face along the light propagation direction,
In the clad, grooves are formed on both sides of the core along the light propagation direction so as to expose the substrate surface, extending to the input / output end surface,
A spot size converter characterized in that a lateral groove along the substrate surface is formed in the peripheral cladding part including the core from the groove part toward the core side.   The spot size converter according to claim 1,
  The core and the clad are each made of one material.
A spot size converter characterized by that. The spot size converter according to claim 1 or 2,
A spot size converter characterized in that an exposed surface of the peripheral cladding portion is coated with a reflective film.