JP2007017494A - Photonic crystal waveguide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photonic crystal waveguide in which reduction of reflection loss can be fully contrived. <P>SOLUTION: The photonic crystal waveguide 10 is composed of two first waveguide regions 23a, 23b and one second waveguide region 24. In these first and second waveguides 23a, 23b, 24, there are formed a plurality of holes 2 each having a prescribed diameter of the opening, wherein the holes 2a-2g in the first waveguide 23a are formed in the manner that the diameter of the opening is gradually made smaller toward the propagating direction of a signal light, while the holes 2g-2a in the first waveguide 23b are formed in the manner gradually made larger. Defects with no holes formed are linearly arranged along the propagating direction of the signal light, with a linear defect waveguide 6 thereby formed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photonic crystal waveguide, and more particularly to a photonic crystal waveguide using a two-dimensional photonic crystal.

  In recent years, with the explosive increase in communication demand, the transmission speed of networks has increased dramatically, and the construction of optical networks is progressing even in subscriber systems that have been constructed with conventional conductor networks. In order to reduce the cost and reduce the size of communication equipment, it is necessary to reduce the size and increase the integration of optical components used inside the communication equipment. Progressing rapidly.

  As a waveguide which is a basic structure constituting such an optical component, for example, there is a photonic crystal waveguide using a photonic crystal. A photonic crystal is a structure in which two types of materials having different refractive indexes are arranged with a period similar to the wavelength of light. In this structure, the electrons are reflected by the periodic potential distribution in the solid crystal to form a band structure with respect to the energy of the electrons. Is formed. This band structure is called a photonic band gap (PBG).

  The photonic crystal has three important features in terms of anisotropy and dispersibility, including the photonic band gap. For photonic crystal waveguides, a structure has been studied that makes use of these features of photonic crystals to achieve low loss and high interaction with a medium.

  However, in the photonic crystal waveguide, the refractive index of the photonic crystal waveguide at the entrance end and the exit end is different from the refractive index of the medium outside the photonic crystal waveguide. When signal light is incident or emitted from the photonic crystal waveguide to the outside, reflection loss occurs at the boundary surface due to the difference in refractive index. In general, all the losses including the reflection loss cause deterioration of the waveform of the transmitted signal light. Therefore, it is important to reduce the reflection loss in order to transmit the signal light without error. This reflection loss increases as the difference in refractive index increases.

  Therefore, when inputting / outputting signal light from the air to the photonic crystal waveguide or emitting signal light from the photonic crystal waveguide to the outside air, the photonic crystal guide is used. The difference between the refractive index of the medium constituting the waveguide and the refractive index of air becomes very large, and this reflection loss becomes a serious problem. For example, Patent Literature 1 and Patent Literature 2 propose a method for reducing such reflection loss.

  In Patent Document 1, an antireflection layer made of a photonic crystal is provided at an incident end and an output end of a photonic crystal waveguide. The reflection loss is reduced by setting the thickness and effective refractive index of the antireflection layer so that the reflected waves generated on the front and rear surfaces of the antireflection layer interfere with each other and cancel each other. In addition, the thickness of this antireflection layer means the thickness of the signal light in the propagation direction.

In Patent Document 2, a line defect waveguide in which a line defect is introduced into a two-dimensional photonic crystal structure in which a periodic structure is formed is used as a photonic crystal waveguide, and a thin line waveguide is used as the line defect waveguide. There has been proposed a structure in which signal light is made incident through the. In this case, a reflection loss occurs at the interface between the fine wire waveguide and the line defect waveguide. Therefore, in order to reduce this reflection loss, the shape of both sides of the line defect in the vicinity of the connection portion between the fine line waveguide and the line defect waveguide is made by cutting the circle with a straight line, and it is placed behind the photonic crystal waveguide. As it enters, the area to be cut is reduced.
JP 2003-270458 A JP 2004-101740 A

  However, the conventional photonic crystal waveguide has the following problems. First, in the photonic crystal waveguide in Patent Document 1, it is necessary to set the effective refractive index and thickness of the antireflection layer made of photonic crystal so that the reflected wave is canceled by interference. However, since the photonic crystal has a periodic structure, the thickness needs to be an integral multiple of this unit period, and therefore it is not possible to set a thickness that is a condition for completely eliminating reflection. . Thus, when the antireflection layer is not set to a desired thickness, there is a problem that the reflection loss cannot be sufficiently reduced.

  In addition, the photonic crystal waveguide disclosed in Patent Document 2 does not have a structure for reducing reflection loss with respect to the thin-line waveguide for allowing signal light to enter the line-defect waveguide. For this reason, there is a problem that the signal light receives a large reflection loss when the signal light enters the thin wire waveguide. Thus, the conventional photonic crystal waveguide has a problem that the reflection loss cannot be sufficiently suppressed.

  The present invention has been made to solve such problems, and an object of the present invention is to provide a photonic crystal waveguide in which reflection loss can be sufficiently reduced.

  The photonic crystal waveguide according to the present invention is a photonic crystal waveguide using a photonic crystal member having an incident end portion for inputting predetermined signal light and an output end portion for emitting signal light that has entered and propagated. And it is provided with a plurality of holes periodically arranged in the photonic crystal member. The plurality of holes include a series of first holes formed so that the opening areas of the holes gradually differ from the incident end along the propagation direction of the signal light.

  According to this structure, the series of first holes are formed so that the opening area of the holes gradually varies along the propagation direction of the signal light from the incident end, so that the outside of the photonic crystal waveguide is formed. The refractive index can be gradually changed as the signal light propagates with respect to the signal light that is incident on and propagates through the photonic crystal waveguide. Thereby, at the boundary between the outside and the incident end, the difference between the external refractive index and the refractive index on the incident end side of the photonic crystal waveguide can be further reduced. The reflection loss at the boundary can be further reduced.

  The plurality of holes may include a series of second holes formed such that the opening areas of the holes gradually differ along the propagation direction of the signal light propagating toward the emission end. preferable.

  In this case, with respect to the signal light propagating through the photonic crystal waveguide toward the emission end, the refractive index can be gradually changed as the signal light propagates. As a result, the difference between the refractive index of the exit end of the photonic crystal waveguide and the external refractive index can be further reduced, and as a result, the reflection loss at the boundary between the exit end and the outside can be further reduced. it can.

  When the outside is air having a refractive index of 1, in order to reduce the difference in refractive index at the interface between the outside and the incident side end, and to reduce the difference in refractive index at the interface between the emission side end and the outside, The photonic crystal member is disposed so as to be sandwiched between a pair of first waveguide regions and a pair of first waveguide regions respectively disposed on the incident side and the emission side of the signal light. And a series of first air holes are disposed in one first waveguide region of the pair of first waveguide regions, and the second waveguide extends from the incident end. The opening area of the series of first holes is gradually reduced toward the area, and the series of second holes is the other first waveguide area of the pair of first waveguide areas. The opening area of the series of second holes gradually increases from the second waveguide region toward the emission end. It may preferably be set to.

  Further, in order to propagate signal light having a wavelength within the photonic band gap as the signal light through the photonic crystal waveguide, the plurality of holes are one of a series of first holes and second holes. A plurality of third holes disposed in the second waveguide region having a predetermined constant opening area smaller than the smallest opening area, the plurality of holes being two-dimensionally formed in the photonic crystal member; The arranged and photonic crystal member preferably includes a line defect waveguide region composed of a defect region in which the holes are not formed linearly among the plurality of holes.

In this case, the signal light propagates through the line defect waveguide region.
Further, in order to propagate signal light having a wavelength in the band structure as signal light through the photonic crystal waveguide, the photonic crystal member includes a thin wire waveguide region in which a plurality of holes are arranged one-dimensionally. It is preferable that

  Furthermore, in order to control the group velocity of signal light and the interaction with the medium by the dispersibility obtained based on the band structure caused by the photonic crystal, a plurality of vacancies are formed in the thin wire waveguide region. It is preferable that they are periodically arranged over the entire region from the incident side to the emission side.

  In order to further increase the change in the refractive index and reduce the change in the refractive index at the interface, a rectangular shape is preferable as the opening shape of the holes.

Embodiment 1
A line defect waveguide using an SOI (Silicon On Insulator) substrate will be described as the photonic crystal waveguide according to the first embodiment of the present invention. As shown in FIG. 1, the photonic crystal waveguide 10 includes two first waveguide regions 23 a and 23 b and one second waveguide region 24. The first waveguide region 23a is located on the incident side in the signal light traveling direction (Z-axis direction) with respect to the second waveguide region 24, and the first waveguide region 23b is the second waveguide. It is located on the signal light emission side with respect to the region 24. The second waveguide region 24 is disposed so as to be sandwiched between the first waveguide region 23a and the first waveguide region 23b.

  A plurality of holes 2 each having a predetermined opening diameter are formed in the first waveguides 23 a and 23 b and the second waveguide 24. Each of the holes 2 is formed in the silicon layer 1 formed on the silicon substrate 21 with the silicon oxide film 22 interposed, as shown in FIGS.

  The holes 2a to 2g in the first waveguide 23a are formed so that the opening diameter gradually decreases along the light propagation direction (Z-axis direction). On the other hand, the holes 2g to 2a in the first waveguide 23b are formed so that the opening diameter gradually increases along the propagation direction (Z-axis direction) of the signal light. The opening diameter of each hole 2a-2g is, for example, hole 2a = 0.27 μm, hole 2b = 0.26 μm, hole 2c = 0.25 μm, hole 2d = 0.24 μm, hole 2e = 0. .23 μm, hole 2 f = 0.22 μm, hole 2 g = 0.21 μm. The air holes 2h in the second waveguide 24 are formed with a constant opening diameter, and the opening diameter is, for example, 0.20 μm. Each hole 2 positioned along the direction orthogonal to the Z axis is formed to have the same opening diameter as shown in FIG. 3, for example.

  As shown in FIG. 4, each hole 2 is periodically formed so that the center of the circular hole is located at the apex of a regular triangle (virtual triangular lattice). In the photonic crystal waveguide 10, this period (the length of one side of the equilateral triangle) is 0.40 μm. As shown in FIG. 5, of the holes that are to be periodically arranged as described above, a portion where the holes are not formed is referred to as a defect in the photonic crystal waveguide.

  In the photonic crystal waveguide 10, as shown in FIG. 6, the line defect waveguide 6 is formed by arranging the defects linearly along the traveling direction of the signal light. In the line defect waveguide 6, signal light (for example, wavelength = 1550 nm) having a wavelength corresponding to the level of the line defect formed in the PBG propagates along the line defect. The present photonic crystal waveguide 10 is configured as described above.

  Next, a method for manufacturing the above-described photonic crystal waveguide will be described. First, as shown in FIG. 7, a silicon layer 22 having a thickness of about 0.25 μm and a refractive index of 3.5 is interposed on a silicon substrate 21 with a silicon oxide film 22 having a thickness of about 3 μm and a refractive index of about 1.4. An SOI substrate on which 1 is formed is prepared. A resist film 30 is applied and formed on the surface of the silicon layer 1.

  Next, a resist film 30a having a pattern for forming a two-dimensional photonic crystal structure having a line defect as shown in FIG. 8 is obtained by subjecting the resist film 30 to an exposure process using a normal EB (Electron Beam). Is formed. Here, as the resist pattern, for example, a pattern for forming holes shown in FIG. 1 is formed.

  Next, as shown in FIG. 9, by using the patterned resist film 30a as a mask, the silicon layer 1 is subjected to general dry etching, thereby forming the void 2 that exposes the surface of the silicon oxide film 22. . Thereafter, as shown in FIG. 10, the remaining resist film 30a is removed by a predetermined stripping solution. Thus, the photonic crystal waveguide 10 shown in FIGS. 1 to 3 is completed.

  In the photonic crystal waveguide 10 described above, for example, predetermined signal light propagating in the air having a refractive index of 1 enters the first waveguide region 23a from the incident end 23aa of the photonic crystal waveguide 10. The signal light incident on the first waveguide region 23a passes through the line defect waveguide 6 in the first waveguide region 23a, passes through the line defect waveguide 6 in the second waveguide region, and passes through the first waveguide region 23b. The light propagates through the line defect waveguide 6 and is emitted again to the outside air from the emission end portion 23bb.

  At this time, in the first waveguide region 23a into which the signal light is incident, the opening diameter of the air holes 2 is gradually reduced along the traveling direction (Z-axis direction) of the signal light from the incident end 23aa. As a result, air (refractive index 1) positioned outside the photonic crystal waveguide 10 is propagated, enters the photonic crystal waveguide 10 from the incident end 23aa, and propagates in the photonic crystal waveguide 10. With respect to the signal light, the refractive index gradually increases as the signal light propagates in the first waveguide region 23a.

  Therefore, as compared with the case where the signal light is directly incident on the photonic crystal waveguide having the refractive index (effective refractive index) of the second waveguide region from the air having the refractive index of 1, the first and the first waveguide regions 23a. At the boundary with the (incident end portion 23aa), the difference between the refractive index of air and the refractive index of the first waveguide region 23a can be further reduced. As a result, reflection loss at the boundary between the outside and the first waveguide region 23a can be further reduced.

  In addition, at the boundary 25 between the first waveguide region 23a and the second waveguide region 24, there is almost no difference in refractive index between them, and the second waveguide region 23 can be separated from the first waveguide region 23 without causing reflection loss. Signal light can be propagated to the waveguide region 24.

  Furthermore, there is almost no difference in refractive index between the second waveguide region 24 and the first waveguide region 23b at the boundary 25, so that the first waveguide region 24 can be separated from the first waveguide region 24 without causing a reflection loss. The signal light can be propagated to the waveguide region 23b.

  In the first waveguide region 23b, in contrast to the first waveguide region 23a, the refractive index gradually decreases as the signal light propagates toward the emission end 23bb. As a result, the difference between the refractive index of air and the refractive index of the first waveguide region 23b can be further reduced at the boundary between the outside and the first waveguide region 23b (outgoing end portion 23bb). And the reflection loss at the boundary between the first waveguide region 23b and the first waveguide region 23b.

  Further, in the air holes 2 arranged in the first waveguide regions 23a and 23b of the photonic crystal waveguide described above, the opening area is made constant in the direction orthogonal to the Z-axis direction, and then the Z-axis direction Are arranged so that the opening areas are gradually different. Thereby, the change of the refractive index in the Z-axis direction can be clearly formed, and the reflection loss can be more effectively reduced.

  When the reflection loss of the photonic crystal waveguide 10 described above was evaluated, it was confirmed that the reflection loss was approximately 0 dB. On the other hand, it was found that the reflection loss is 3 dB in the photonic crystal waveguide that does not include the first waveguide regions 23a and 23b, and that the reflection loss can be significantly reduced in the photonic crystal waveguide 10 described above.

  In the photonic crystal waveguide 10 described above, the hole 2 is described as an example having a circular opening shape. The opening shape is not limited to a circle, and may be any other shape as long as the opening area can be gradually changed. In particular, when the external medium is air, the opening area of the holes arranged on the incident side in the first waveguide region 23a is increased to increase the boundary between the outside and the first waveguide region 23a. By reducing the difference in refractive index, it is possible to further reduce reflection loss when signal light is incident. Similarly, the difference in refractive index at the boundary between the outside and the first waveguide region 23b is reduced by increasing the opening area of the holes arranged on the emission side in the first waveguide region 23b. Thus, the reflection loss when the signal light is emitted can be further reduced.

  Moreover, as an opening diameter of the circular hole 2, the case where the hole smaller than the unit length (0.40 μm) of the periodic structure is described as an example, but the opening diameter is smaller than the unit length. It is good also as a structure which is set large and adjacent holes are connected. In this case, the difference in refractive index at the boundary between the outside and the first waveguide regions 23a and 23b is further reduced, thereby further reducing the reflection loss when the signal light is incident and the reflection loss when it is emitted. can do.

  Further, in the first waveguide regions 23 a and 23 b of the photonic crystal waveguide 10, the length in the Z-axis direction is set to seven periods so that seven holes 2 are arranged along the Z-axis direction. The case described above is described as an example. The length of the first waveguide regions 23a and 23b in the Z-axis direction is not limited to this period as long as a reflection loss reduction effect can be obtained.

  In addition, the signal light having a wavelength of 1550 nm has been described as an example of the signal light propagating through the photonic crystal waveguide 10, but as the signal light, signal light in a wavelength range for optical communication (about 1300 nm to 1600 nm) is used. It can be propagated.

  In the photonic crystal waveguide 10 described above, the case where the line defect waveguide region is formed in the silicon layer 1 of the SOI substrate has been described as an example, but the refractive index higher than the refractive index of the cladding layer and the cladding layer. Other materials may be used as long as the signal light is confined by the waveguide layer having a refractive index and the waveguide layer can form the photonic crystal structure.

Embodiment 2
In the above-described photonic crystal waveguide, the case where signal light having a wavelength in the photonic band gap is propagated using a line defect waveguide as an optical waveguide has been described. Here, a silicon fine wire waveguide will be described as an example of the photonic crystal waveguide. In the silicon thin wire waveguide, since a photonic crystal is formed in the optical waveguide, signal light having a wavelength in the band structure is propagated as signal light, not in the photonic band gap.

  As shown in FIG. 11, the photonic crystal waveguide 10 is constituted by a silicon fine wire waveguide 7 including two first waveguide regions 23 a and 23 b and one second waveguide region 24. A plurality of holes 3 are arranged one-dimensionally in each of the two first waveguide regions 23a and 23b. The width W of the silicon wire waveguide 7 is about 0.50 μm. As shown in FIG. 12, each hole 3 is formed in a silicon layer 1 formed on a silicon substrate 21 with a silicon oxide film 22 interposed therebetween.

  The holes 3a to 3j in the first waveguide 23a are formed so that the aperture diameter gradually decreases along the signal light propagation direction (Z-axis direction). On the other hand, the holes 3j to 3a in the first waveguide 23b are formed so that the opening diameter gradually increases along the propagation direction (Z-axis direction) of the signal light. The opening diameters of the holes 3a to 3j are, for example, hole 3a = 0.34 μm, hole 3b = 0.31 μm, hole 3c = 0.28 μm, hole 3d = 0.25 μm, hole 3e = 0. .22 μm, hole 3 f = 0.19 μm, hole 3 g = 0.16 μm, hole 3 h = 0.13 μm, hole 3 i = 0.10 μm, hole 3 j = 0.07 μm. In addition, the period of holes (the length between adjacent holes) L is 0.45 μm.

  In the photonic crystal waveguide 10 described above, as predetermined signal light, signal light having a wavelength in the band structure, for example, having a wavelength of 1300 nm propagates through air having a refractive index of 1, and the incident end of the photonic crystal waveguide 10 The light enters the first waveguide region 23a from 23aa. The signal light incident on the first waveguide region 23a propagates from the first waveguide region 23a through the second waveguide region 24 to the first waveguide region 23b, and again from the output end 23bb to the outside. It is emitted into the air.

  At this time, in the first waveguide region 23a into which the signal light is incident, the opening diameter of the air holes 2 is gradually reduced along the traveling direction (Z-axis direction) of the signal light from the incident end 23aa. As a result, air (refractive index 1) positioned outside the photonic crystal waveguide 10 is propagated, enters the photonic crystal waveguide 10 from the incident end 23aa, and propagates in the photonic crystal waveguide 10. With respect to the signal light, the refractive index gradually increases as the signal light propagates in the first waveguide region 23a.

  Therefore, as compared with the case where the signal light is directly incident on the photonic crystal waveguide having the refractive index (effective refractive index) of the second waveguide region from the air having the refractive index of 1, the first and the first waveguide regions 23a. At the boundary with the (incident end portion 23aa), the difference between the refractive index of air and the refractive index of the first waveguide region 23a can be further reduced. As a result, reflection loss at the boundary between the outside and the first waveguide region 23a can be further reduced.

  In addition, at the boundary 25 between the first waveguide region 23a and the second waveguide region 24, there is almost no difference in refractive index between them, and the second waveguide region 23 can be separated from the first waveguide region 23 without causing reflection loss. Signal light can be propagated to the waveguide region 24.

  Furthermore, there is almost no difference in refractive index between the second waveguide region 24 and the first waveguide region 23b at the boundary 25, so that the first waveguide region 24 can be separated from the first waveguide region 24 without causing a reflection loss. The signal light can be propagated to the waveguide region 23b.

  In the first waveguide region 23b, in contrast to the first waveguide region 23a, the refractive index gradually decreases as the signal light propagates toward the emission end 23bb. As a result, the difference between the refractive index of air and the refractive index of the first waveguide region 23b can be further reduced at the boundary between the outside and the first waveguide region 23b (outgoing end portion 23bb). And the reflection loss at the boundary between the first waveguide region 23b and the first waveguide region 23b.

  When the reflection loss of the photonic crystal waveguide 10 described above was evaluated, it was confirmed that the reflection loss was approximately 0 dB. On the other hand, it was found that the reflection loss is 3 dB in the photonic crystal waveguide that does not include the first waveguide regions 23a and 23b, and that the reflection loss can be significantly reduced in the photonic crystal waveguide 10 described above.

  In the first waveguide regions 23a and 23b of the photonic crystal waveguide 10, the length in the Z-axis direction is set to 10 periods so that ten holes 2 are arranged along the Z-axis direction. The case of minutes was described as an example. The length of the first waveguide regions 23a and 23b in the Z-axis direction is not limited to this period as long as a reflection loss reduction effect can be obtained.

  In the photonic crystal waveguide 10 described above, the case where the thin-line waveguide region is formed in the silicon layer 1 of the SOI substrate has been described as an example, but the refractive index higher than the refractive index of the cladding layer and the cladding layer. Other materials may be applied as long as the signal light is confined by the waveguide layer having the structure and the waveguide layer can form the photonic crystal structure.

Embodiment 3
Here, another example of a silicon fine wire waveguide will be described as a photonic crystal waveguide. As shown in FIG. 13, the photonic crystal waveguide 10 is constituted by a silicon fine wire waveguide 8 including two first waveguide regions 23 a and 23 b and one second waveguide region 24. In the photonic crystal waveguide 10, a plurality of holes 4 are arranged one-dimensionally in each of the two first waveguide regions 23 a and 23 b, and further, in the second waveguide region 24. Also, the plurality of holes 4 are arranged one-dimensionally. The width W of the silicon wire waveguide 8 is about 0.50 μm. As shown in FIG. 14, each hole 4 is formed in the silicon layer 1 formed on the silicon substrate 21 with the silicon oxide film 22 interposed.

  The holes 4a to 4g in the first waveguide 23a are formed so that the opening diameter gradually decreases along the propagation direction (Z-axis direction) of the signal light. On the other hand, the holes 4g to 4a in the first waveguide 23b are formed so that the opening diameter gradually increases along the propagation direction (Z-axis direction) of the signal light. The diameters of the holes 4a to 4g are, for example, hole 4a = 0.34 μm, hole 4b = 0.32 μm, hole 4c = 0.30 μm, hole 4d = 0.28 μm, hole 4e = 0. .26 μm, hole 4f = 0.24 μm, hole 4g = 0.22 μm, and hole 4h = 0.20 μm. The period of the holes (the length between adjacent holes) is 0.45 μm.

  In the photonic crystal waveguide 10 described above, the holes 4 are one-dimensionally arranged in each of the two first waveguide regions 23a and 23b and the one second waveguide region 24, so that the silicon fine wire waveguide is formed. A one-dimensional photonic crystal structure is formed over the entire length (full length). When signal light having a wavelength within the band structure is propagated through the photonic crystal waveguide, the group speed and medium of the signal light can be reduced due to various dispersibility obtained based on the band structure caused by the photonic crystal. It becomes possible to control the interaction.

  In other words, the bending of the band changes depending on the structure of the photonic crystal, and the bending of the band corresponds to the refractive index. Therefore, the refractive index can be variously changed depending on the structure of the photonic crystal. Thus, by changing the size of the refractive index, the group velocity of light can be changed, and the interaction with the medium can be controlled. In this case, if the light intensity of the incident signal light is large, the effect of nonlinear interaction increases, and this nonlinear interaction is used as optical signal processing. Therefore, in order to obtain a strong nonlinear interaction with a sufficiently high light intensity, it is important to reduce the reflection loss at the entrance end and the exit end.

  In the photonic crystal waveguide 10 described above, the opening diameter of the holes 4 gradually increases from the incident end 23aa along the traveling direction (Z-axis direction) of the signal light in the first waveguide region 23a where the signal light is incident. It is getting smaller. As a result, air (refractive index 1) positioned outside the photonic crystal waveguide 10 is propagated, enters the photonic crystal waveguide 10 from the incident end 23aa, and propagates in the photonic crystal waveguide 10. With respect to the signal light, the refractive index gradually increases as the signal light propagates in the first waveguide region 23a.

  As a result, the difference between the refractive index of air and the refractive index of the first waveguide region 23a is further reduced at the boundary between the outside and the first waveguide region 23a (exit end portion 23bb). The reflection loss at the boundary with one waveguide region 23a can be further reduced.

  In addition, at the boundary 25 between the first waveguide region 23a and the second waveguide region 24, there is almost no difference in refractive index between them, and the second waveguide region 23 can be separated from the first waveguide region 23 without causing reflection loss. Signal light can be propagated to the waveguide region 24.

  Furthermore, there is almost no difference in refractive index between the second waveguide region 24 and the first waveguide region 23b at the boundary 25, so that the first waveguide region 24 can be separated from the first waveguide region 24 without causing a reflection loss. The signal light can be propagated to the waveguide region 23b.

  In the first waveguide region 23b, in contrast to the first waveguide region 23a, the refractive index gradually decreases as the signal light propagates toward the emission end 23bb. As a result, the difference between the refractive index of air and the refractive index of the first waveguide region 23b is further reduced at the boundary between the outside and the first waveguide region 23b (exit end portion 23bb). The reflection loss at the boundary with the one waveguide region 23b can be further reduced.

Modified Example Next, a modified example of the above-described photonic crystal waveguide will be described. Whereas the planar shape of the holes of the photonic crystal waveguide described above is substantially circular, the photonic crystal waveguide 10 according to the modification is formed in the silicon layer 1 as shown in FIGS. 15 and 16. The planar shape of the hole 5 to be made is a rectangle. The length of each rectangular hole in the Z-axis direction is set to the same length as the diameter of each corresponding circular hole 4 formed in the above-described photonic crystal waveguide. The length of each rectangular hole 5 in the direction orthogonal to the Z-axis is set to the same length as the width W of the silicon wire waveguide. That is, in the photonic crystal waveguide 10 according to the modification, a gap is formed in the silicon fine wire waveguide.

  In particular, in the photonic crystal waveguide 10 according to the modification, the area (area) of the hole 5 is increased by making the planar shape of the hole 5 rectangular as compared to the circular hole. Thereby, the width of the refractive index change in the first waveguide regions 23a and 23b can be further increased, and the difference in refractive index at the boundary between the first waveguide regions 23a and 23b and air is further reduced. As a result, a sharp change in the refractive index at the interface can be alleviated.

  In the photonic crystal waveguide according to the modification, the length of each rectangular hole in the direction perpendicular to the Z-axis is set to the same length as the width of the silicon wire waveguide as an example. explained. As the photonic crystal waveguide, the length of the holes may be set shorter than the width of the silicon wire waveguide as long as the area of the holes can be gradually changed along the Z-axis direction. Further, the holes may be formed so that the planar shape of the holes is a square.

  The embodiment disclosed this time is merely an example, and the present invention is not limited to this. The present invention is defined by the terms of the claims, rather than the scope described above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

It is a top view of the photonic crystal waveguide which concerns on Embodiment 1 of this invention. FIG. 2 is a cross-sectional view taken along a cross-sectional line II-II shown in FIG. 1 in the same embodiment. FIG. 3 is a cross-sectional view taken along a cross-sectional line III-III shown in FIG. 1 in the same embodiment. In the same embodiment, it is a fragmentary top view for demonstrating arrangement | positioning of a void | hole. In the same embodiment, it is a fragmentary top view for demonstrating a defect. In the same embodiment, it is a top view for demonstrating a line defect waveguide. In the same embodiment, it is sectional drawing which shows 1 process of the manufacturing method of a photonic crystal waveguide. FIG. 8 is a cross-sectional view showing a step performed after the step shown in FIG. 7 in the same embodiment. FIG. 9 is a cross-sectional view showing a step performed after the step shown in FIG. 8 in the same embodiment. FIG. 10 is a cross-sectional view showing a step performed after the step shown in FIG. 9 in the same embodiment. It is a top view of the photonic crystal waveguide which concerns on Embodiment 2 of this invention. FIG. 12 is a cross sectional view taken along a cross sectional line XII-XII shown in FIG. 11 in the same embodiment. It is a top view of the photonic crystal waveguide which concerns on Embodiment 3 of this invention. FIG. 14 is a cross sectional view taken along a cross sectional line XIV-XIV shown in FIG. 13 in the same embodiment. In the same embodiment, it is a top view of the photonic crystal waveguide concerning a modification. FIG. 16 is a cross-sectional view taken along a cross-sectional line XVI-XVI shown in FIG. 15 in the same embodiment.

Explanation of symbols

  1 Silicon layer, 2, 3, 4, 5 holes, 6-line defect waveguide, 7, 8, 9 Silicon thin-line waveguide, 10 Photonic crystal waveguide, 21 Silicon substrate, 22 Silicon oxide film, 23a, 23b 1 waveguide region, 24 second waveguide region, 25 interface.

Claims (7)

A photonic crystal waveguide using a photonic crystal member having an incident end for entering predetermined signal light and an exit end for emitting signal light that has entered and propagated,
Comprising a plurality of holes periodically arranged in the photonic crystal member;
The plurality of vacancies include a series of first vacancies formed such that the opening areas of the vacancies gradually differ from the incident end along the propagation direction of the signal light.   The plurality of holes include a series of second holes formed so that the opening areas of the holes gradually differ along the propagation direction of the signal light propagating toward the emission end. The photonic crystal waveguide according to 1. The photonic crystal member is
A pair of first waveguide regions respectively disposed on an incident side and an output side of the signal light;
A second waveguide region disposed to be sandwiched between the pair of first waveguide regions,
The series of first air holes is disposed in one first waveguide region of the pair of first waveguide regions, and is directed from the incident end toward the second waveguide region. The opening area of the series of first holes is set to be gradually reduced,
The series of second air holes are arranged in the other first waveguide area of the pair of first waveguide areas, and the series of second holes are directed from the second waveguide area toward the emission end. The photonic crystal waveguide according to claim 2, wherein the opening area of the second hole is set to be gradually increased. The plurality of holes are arranged in the second waveguide region with a predetermined constant opening area smaller than the smallest opening area in the series of the first hole and the second hole. A plurality of third holes formed,
The plurality of holes are two-dimensionally arranged in the photonic crystal member,
4. The photonic crystal waveguide according to claim 3, wherein the photonic crystal member includes a line defect waveguide region including a defect region in which a plurality of holes are not formed in a linear shape. 5.   The photonic crystal waveguide according to claim 3, wherein the photonic crystal member includes a thin-line waveguide region in which a plurality of the holes are arranged one-dimensionally.   6. The photonic crystal waveguide according to claim 5, wherein, in the thin-line waveguide region, the plurality of holes are periodically arranged over the entire region from the incident side to the outgoing side of the signal light.   The photonic crystal waveguide according to claim 3, wherein each of the plurality of holes has a rectangular opening shape.

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