JP2017072807A - Semiconductor optical waveguide, semiconductor optical modulator, and semiconductor optical modulation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve further improvement of a modulation efficiency in a semiconductor optical waveguide including a core that functions as a reverse-bias type phase modulation part.SOLUTION: A semiconductor optical waveguide comprises a high refractive index region (11) including a core (12) that is thicker than other parts (13, 14) and functions as a reverse-bias type phase modulation part. The core (12) includes a junction surface between P type semiconductor layers (12a, 13) and N type semiconductor layers (12b, 14). Portions (13, 14) other than the core in the high refractive index region (11) are formed with a plurality of holes (13a, 14a) at intervals along borderlines with the core (12).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a semiconductor optical waveguide that modulates light according to a modulation voltage applied from the outside, a semiconductor optical modulator, and a semiconductor optical modulation system.

  A semiconductor optical waveguide having a core and a clad made of a semiconductor, for example, a silicon optical waveguide is known. The silicon optical waveguide is an optical waveguide in which a silicon high refractive index region is provided inside a silica substrate, and a part or all of the high refractive index region is a core. In the silicon optical waveguide, light confinement in the core is realized by the difference in refractive index between silica and silicon.

  Also, a PIN junction (P-type semiconductor layer / I-type semiconductor layer / N-type semiconductor layer, see Non-Patent Document 1 and Patent Document 1) or PN junction (P-type semiconductor layer / N) in the high refractive index region of the silicon optical waveguide. By forming the bonding surface of the type semiconductor layer (see Patent Document 2), the phase of light output from the silicon optical waveguide can be changed according to the voltage applied to both ends of the bonding structure. For this reason, a Mach-Zehnder type optical modulator can be configured by combining two silicon optical waveguides and adopting a PIN junction structure or a PN junction structure as the structure of at least one of the silicon optical waveguides. A silicon optical waveguide in which a PIN junction I-type semiconductor layer is replaced with an insulator layer is also known.

  In the case of an optical modulator composed of a silicon optical waveguide in which a PIN junction is formed, a voltage applied to both ends of the PIN junction structure (hereinafter referred to as “modulation voltage”) is often applied in the forward bias direction. As described above, the core of the silicon optical waveguide that applies the modulation voltage in the forward bias direction functions as a forward bias type phase modulation section. A silicon optical waveguide provided with a core functioning as a forward bias type phase modulator is referred to as a “forward bias type silicon optical modulator”. On the other hand, in the case where the junction surface of the PN junction is formed in the core, the modulation voltage is often applied in the reverse bias direction. Thus, the core of the silicon optical waveguide that applies the modulation voltage in the reverse bias direction functions as a reverse bias type phase modulation section. A silicon optical waveguide having a core functioning as a reverse bias type phase modulation unit is referred to as a “reverse bias type silicon optical modulator”. Note that a reverse bias type silicon optical modulator often employs a configuration in which a constant reverse bias voltage is applied to both ends of a PN junction structure, and then a modulation voltage whose one-side amplitude is equal to or less than the reverse bias voltage is applied. .

  Non-Patent Document 1 describes a forward-bias type silicon optical modulator in which a PIN junction surface is formed in a silicon optical waveguide. The forward-bias type silicon optical modulator can easily increase the amount of change in the carrier density in the core of the silicon optical waveguide, as compared with the reverse-bias type silicon optical modulator. For this reason, in the forward bias type silicon modulator, the phase of the light guided through the core can be sufficiently changed without increasing the amplitude of the modulation voltage. In other words, the modulation voltage can be lowered.

  On the other hand, in a forward-bias type silicon optical modulator, since a current flows between both semiconductor layers of a PIN junction, a traveling wave electrode cannot be adopted as an electrode for applying a modulation voltage. Further, the forward bias type silicon optical modulator has a slower response speed of the carrier density change to the change of the modulation voltage than the reverse bias type silicon optical modulator. For this reason, the forward bias type silicon optical modulator has a problem that it is difficult to increase the operation speed as compared with the reverse bias type silicon optical modulator. In other words, the forward bias type silicon optical modulator has a problem that it is difficult to widen the transmission band.

  Further, in the forward bias type silicon optical modulator, since a large current flows between both semiconductor layers of the PIN junction as described above, the power consumption tends to be larger than that of the reverse bias type silicon optical modulator. It has a problem.

  Patent Document 1 describes a reverse bias type silicon optical modulator in which a PN junction surface is formed in a core. The reverse bias type silicon optical modulator is advantageous in that the operation speed can be easily increased as compared with the forward bias type silicon optical modulator. This is because (1) the traveling wave electrode can be adopted as the configuration of the electrode to which the modulation voltage is applied, and (2) the response speed of the change in the carrier density with respect to the change in the modulation voltage is high. Further, since a modulation voltage in the reverse bias direction is applied to both ends of the PN junction structure, only a weak current flows between both ends of the PN junction structure. Therefore, the reverse-bias type silicon optical modulator is advantageous in that the power consumption can be easily suppressed as compared with the forward-bias type silicon optical modulator.

JP 2012-63769 A (published March 29, 2012) JP 2014-126728 A (published July 7, 2014)

S. Akiyama et. Al., OPTICS EXPRESS, Vol.20, No.3, p.2911, 2012

  However, the reverse bias type silicon optical modulator has a problem that the modulation efficiency is poor as compared with the forward bias type silicon optical modulator, that is, it is difficult to lower the modulation voltage.

  As an example of the structure of a conventional reverse bias type silicon optical waveguide, there is a structure shown in FIG. 14 (a) or (b).

  A silicon optical waveguide 500 shown in FIG. 14A includes a high refractive index region 510 including a P-type semiconductor layer 511 and an N-type semiconductor layer 512 whose side surfaces are bonded to each other. In this high refractive index region 510, the thickness of the central portion (hereinafter also referred to as “rib”) including the PN junction surface 513 is thicker than the thickness of other portions (hereinafter also referred to as “slab”). The structure to be adopted is adopted. By adopting such a configuration, the rib of the high refractive index region 510 including the bonding surface 513 of the PN junction can function as the core 514 of the silicon optical waveguide 500.

  A silicon optical waveguide 500 shown in FIG. 14B includes a high refractive index region 510 composed of a P-type semiconductor layer 511 and an N-type semiconductor layer 512 whose upper end and lower end are joined. In the high refractive index region 510, the thickness of the central portion where the P-type semiconductor layer 511 and the N-type semiconductor 512 overlap is greater than the thickness of the slab. Therefore, the central portion of the high refractive index region 510 where the P-type semiconductor layer 511 and the N-type semiconductor 512 overlap (that is, including the junction surface 513 of the PN junction) functions as a core 514 in which guided light is localized. Can be made.

  However, even if the configuration shown in FIG. 14A or the configuration shown in FIG. 14B is adopted, a part of the light guided by the silicon optical waveguide 500 is outside the core 514. It has spread to. That is, these configurations still have room for improving the modulation efficiency.

  The present invention has been made in view of the above problems, and an object thereof is to further improve modulation efficiency in a semiconductor optical waveguide having a core functioning as a reverse bias type phase modulation unit. .

  In order to solve the above problems, the present invention provides a high refractive index region composed of a P-type semiconductor layer and an N-type semiconductor layer in which side surfaces or one upper surface end and the other lower surface end are joined. The high refractive index region includes a core that is thicker than other portions and functions as a reverse bias type phase modulation unit, and the core includes the P-type semiconductor layer and the N-type semiconductor optical waveguide. A plurality of holes are periodically formed along the boundary with the core in a portion other than the core of the high refractive index region, including a bonding surface with the type semiconductor layer. .

  According to said structure, compared with the case where the said several hole is not formed in parts other than the said core of the said high refractive index area | region, the light which guides the said high refractive index area | region is formed in the said core. It is possible to concentrate in the vicinity of the junction surface (pn junction surface). For this reason, the modulation efficiency of the semiconductor optical waveguide can be further increased as compared with the case where the plurality of holes are not formed in a portion other than the core of the high refractive index region.

  In the semiconductor optical waveguide according to one embodiment of the present invention, the hole is in contact with the core, or the interval between the hole and the core is 0.25 times or less the width of the core. preferable.

  According to said structure, compared with the case where the said several hole is not formed in parts other than the said core of the said high refractive index area | region, the light which propagates the said high refractive index area | region reliably is carried out by the inside of the said core. Can concentrate. For this reason, it is possible to reliably increase the modulation efficiency of the semiconductor optical waveguide as compared with the case where the plurality of holes are not formed in a portion other than the core of the high refractive index region.

  In the semiconductor optical waveguide according to one aspect of the present invention, it is more preferable that the hole is in contact with the core.

  According to the above configuration, the light propagating through the high refractive index region can be localized in the vicinity of the joint surface of the core. For this reason, the modulation efficiency of the semiconductor optical waveguide can be substantially maximized.

  In the semiconductor optical waveguide according to one aspect of the present invention, the period of the plurality of holes and the length of the propagation direction of light propagating through the high refractive index region of each of the plurality of holes are It is preferably shorter than the effective wavelength of light propagating through the refractive index region.

  According to said structure, the interaction which arises between the light which propagates a high refractive index area | region, and the periodic structure of a hole can be suppressed. Therefore, the light can be localized near the joint surface of the core without causing an optical loss of the light propagating through the high refractive index region.

  In the semiconductor optical waveguide according to one aspect of the present invention, each of the plurality of holes has a width in a direction orthogonal to both the propagation direction of light propagating through the high refractive index region and the depth direction of the hole. The core width is preferably 0.375 to 2.5 times the width of the core.

  Each of the plurality of holes has a width in the direction perpendicular to both the propagation direction of the light propagating through the high refractive index region and the depth direction of the hole being 0.375 times or more of the width of the core. Further, it is possible to suppress light from oozing out from the core to portions other than the core in the high refractive index region, and to localize the light well in the vicinity of the joint surface of the core. Further, the width of each of the plurality of holes in the direction perpendicular to both the propagation direction of the light propagating through the high refractive index region and the depth direction of the hole is 2.5 times or less of the width of the core. As a result, it is possible to prevent the electrical resistance value of the portion other than the core in the high refractive index region from being unnecessarily increased. That is, according to said structure, light can be well localized near the joint surface of a core, suppressing the electrical resistance value of parts other than the said core of a high refractive index area | region.

  The semiconductor optical waveguide according to one aspect of the present invention preferably further includes a clad surrounding the high refractive index region, and the hole is filled with a material constituting the clad.

  According to said structure, manufacture of the said semiconductor optical waveguide can be simplified, and deterioration of the said high refractive index area | region can be prevented. For example, when the high refractive index region is made of silicon and the cladding is made of silica, the silicon constituting the high refractive index region is prevented from being oxidized and the surface of the silicon constituting the high refractive index region is prevented from being contaminated by foreign substances. be able to.

  In the semiconductor optical waveguide according to one aspect of the present invention, the high refractive index region is made of a semiconductor in which a dopant is added to silicon or a semiconductor in which a dopant is added to indium phosphorus, and the cladding is doped with a dopant in silica and indium phosphorus. It is preferable that it consists of any one of the above-mentioned semiconductor and air.

  According to said structure, in order to produce a semiconductor optical waveguide, the existing semiconductor technology can be utilized. Therefore, it is possible to suppress the cost for manufacturing the semiconductor optical waveguide.

  The semiconductor optical waveguide according to an aspect of the present invention further includes a first traveling wave electrode connected to the P-type semiconductor layer and a second traveling wave electrode connected to the N-type semiconductor layer. It is preferable.

  According to said structure, when using the said semiconductor optical waveguide as a phase modulation part, operation | movement of a phase modulation part can be sped up.

  A semiconductor optical modulator according to an aspect of the present invention is a Mach-Zehnder type semiconductor optical modulator in which an optical modulation unit is provided in at least one arm, and the semiconductor according to an aspect of the present invention is used as the optical modulation unit. It is preferable to provide an optical waveguide.

  A semiconductor optical modulation system according to an aspect of the present invention includes a semiconductor optical modulator according to an aspect of the present invention, a voltage source that applies a reverse bias voltage to the P-type semiconductor layer and the N-type semiconductor layer, It is preferable to comprise.

  According to said structure, the semiconductor optical modulator and semiconductor optical modulation system which concern on 1 aspect of this invention have an effect similar to the semiconductor optical waveguide of this invention.

  The present invention can further improve the modulation efficiency in a semiconductor optical waveguide having a core functioning as a reverse bias type phase modulation section. Further, in the semiconductor optical modulator provided with the semiconductor optical waveguide of the present invention, the modulation efficiency can be further improved.

(A) is a perspective view which shows the structure of the optical waveguide which concerns on the 1st Embodiment of this invention. (B) is a perspective view showing a configuration of a high refractive index region provided in the optical waveguide. (A) And (b) is sectional drawing which shows the structure of the said high refractive index area | region. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide based on the Example of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 1st modification of this invention. (C) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 2nd modification of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 3rd modification of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 4th modification of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 5th modification of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 6th modification of this invention. (C) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 7th modification of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 8th modification of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 9th modification of this invention. (C) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 10th modification of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 11th modification of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 12th modification of this invention. (A) is a perspective view which shows the structure of the high refractive index area | region with which the optical waveguide which concerns on the 2nd Embodiment of this invention is provided. (B) And (c) is sectional drawing which shows the structure of the said high refractive index area | region. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide based on the 2nd Example of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 13th modification of this invention. It is a perspective view which shows the structure of the optical modulator which concerns on the 3rd Embodiment of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 1st reference example of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 1st comparative example of this invention. (C) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 2nd reference example of this invention. (D) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 2nd comparative example of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 3rd comparative example of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 4th comparative example of this invention. (C) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 5th comparative example of this invention. (A) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 6th comparative example of this invention. (B) is a graph which shows the electric field distribution of the light which propagates the optical waveguide which concerns on the 7th comparative example of this invention. (A) And (b) is sectional drawing which shows the structure of the conventional optical waveguide.

  The semiconductor optical waveguide according to the present invention includes a semiconductor having a high refractive index region including side surfaces or a P-type semiconductor layer and an N-type semiconductor layer in which one upper surface end and the other lower surface end are joined. It is an optical waveguide. The high refractive index region is thicker than other portions and includes a core that functions as a reverse bias type phase modulation unit. The core includes a joint surface between the P-type semiconductor layer and the N-type semiconductor layer, and a plurality of holes are periodically formed along a boundary with the core in a portion other than the core in the high refractive index region. Is formed.

  In the present embodiment, a semiconductor optical waveguide 10 including a high refractive index region 11 in which side surfaces of a P-type semiconductor layer (12a, 13) and an N-type semiconductor layer (12b, 14) are joined will be described.

[First Embodiment]
A semiconductor optical waveguide according to a first embodiment of the present invention will be described with reference to FIGS. FIG. 1A is a perspective view showing a configuration of a semiconductor optical waveguide 10 according to the present embodiment. FIG. 1B is a perspective view showing the configuration of the high refractive index region 11 provided in the semiconductor optical waveguide 10 shown in FIG. 2A is a cross-sectional view showing the configuration of the semiconductor optical waveguide 10 shown in FIG. 1A, and is a cross section taken along the line AA ′ shown in FIG. FIG. 2B is a cross-sectional view showing the configuration of the semiconductor optical waveguide 10 shown in FIG. 1A, and is a cross section taken along the line BB ′ shown in FIG. FIG.

(Configuration of Semiconductor Optical Waveguide 10)
As shown in FIG. 1A, the semiconductor optical waveguide 10 includes a high refractive index region 11, a clad 19 composed of a lower clad 19a and an upper clad 19b, a first electrode 15, and a first signal line 16. A second electrode 17 and a second signal line 18. The refractive index of the material constituting the clad 19 is lower than the refractive index of the material constituting the high refractive index region 11. The high refractive index region 11 is formed on the upper surface of the lower cladding 19a. The upper clad 19 b is formed on the upper surface of the lower clad and the upper surface of the high refractive index region 11 so as to surround the periphery of the high refractive index region 11.

  As shown in FIG. 2B, the high refractive index region 11 includes a core 12 including a P-type semiconductor portion 12a and an N-type semiconductor portion 12b, a first semiconductor layer 13, and a second semiconductor layer 14. It has. The first semiconductor layer 13 connects the first electrode 15 and the P-type semiconductor unit 12a. The second semiconductor layer 14 connects the second electrode 17 and the N-type semiconductor portion 12b. The first electrode 15 and the second electrode 17 are electrodes for applying a modulation voltage in the reverse bias direction to both ends of the core 12. The semiconductor optical waveguide 10 employs a configuration in which a constant reverse bias voltage is applied to both ends of the core 12 and then a modulation voltage whose one-side amplitude is equal to or less than the reverse bias voltage is applied.

  The P-type semiconductor layer described in the claims is composed of the P-type semiconductor portion 12 a and the first semiconductor layer 13, and the N-type semiconductor layer is composed of the N-type semiconductor portion 12 b and the second semiconductor layer 14. Composed. The 1st semiconductor layer 13 and the 2nd semiconductor layer 14 are parts other than the core 12 of the high refractive index area | region 11 as described in a claim.

  A first signal line 16 and a second signal line 18 are formed on the surface of the upper clad 19b in the region where the first electrode 15 and the second electrode 17 are formed. The signal lines 16 and 18 are configured so that the electrodes 15 and 17 can be connected to a voltage source that outputs an electrical signal for applying a modulation voltage to the core 12.

  The voltage source is configured to apply a modulation voltage in the reverse bias direction to the semiconductor layers 13 and 14 that are portions of the high refractive index region 11 other than the core 12 of the high refractive index region 11. In other words, the voltage source is configured to apply a modulation voltage in the reverse bias direction to the core 12. The modulation voltage in the reverse bias direction is given by the sum of a constant reverse bias voltage and a modulation voltage whose one-side amplitude is equal to or less than the reverse bias voltage. The voltage source may be constituted by a single voltage source that directly outputs a modulation voltage, or a first voltage source that applies a constant reverse bias voltage, and a modulation voltage whose one-side amplitude is equal to or less than the reverse bias voltage. You may comprise by the 2nd voltage source to apply.

  The semiconductor optical waveguide 10 can modulate the light propagating through the high refractive index region 11 in accordance with the modulation voltage in the reverse bias direction supplied from the power source. That is, the semiconductor optical waveguide 10 can convert an electrical signal into an optical signal.

(High refractive index region 11)
The high refractive index region 11 is configured by a semiconductor (for example, silicon (Si) or indium phosphide (InP)). The free carrier density and refractive index of the high refractive index region 11 are controlled to desired values by adding a dopant to the base semiconductor. In the semiconductor optical waveguide 10 according to the present embodiment, the base semiconductor is silicon. Therefore, the semiconductor optical waveguide 10 according to this embodiment is the silicon optical waveguide 10. Hereinafter, the silicon optical waveguide 10 is simply referred to as an optical waveguide 10. The refractive index of the high refractive index region 11 depends on the amount of dopant added, but is about 3.5 at a wavelength of 1200 to 1600 nm used for optical communication.

As a material constituting the clad 19, silica (SiO ₂ ), InP to which a dopant is added, or the like can be employed. Moreover, the process of forming the upper clad 19b may be omitted, and a configuration in which air functions as the upper clad may be employed. In the optical waveguide 10 according to the present embodiment, the lower clad 19a and the upper clad 19b are both made of SiO _2. The refractive index of the clad 19 is about 1.4.

As shown in FIG. 2A, the thickness H _S of the first semiconductor layer 13 and the second semiconductor layer 14 is common and is thinner than the thickness H _L of the core 12 functioning as a phase modulation unit. . That is, the high refractive index region 11 is constituted by the thick core 12 and the thin semiconductor layers 13 and 14. Hereinafter, the thick core 12 is also referred to as a rib, and the thin semiconductor layers 13 and 14 are also referred to as a slab. That is, the high refractive index region 11 has a rib slab type cross-sectional shape.

  In the semiconductor optical waveguide 10, an additive-free core made of a semiconductor to which no dopant is added may be further formed on the upper surface of the core 12. The width of the additive-free core is preferably the same as the width of the core 12.

(Hole 13a, 14a)
A plurality of holes 13 a are periodically formed along the boundary with the core 12 in the first semiconductor layer 13 that is a portion of the high refractive index region 11 other than the core in the high refractive index region. Similarly, a plurality of holes 14 a are periodically formed along the boundary with the core 12 in the second semiconductor layer 14 that is a portion of the high refractive index region 11 other than the core in the high refractive index region 11. Yes.

When the high refractive index region 11 is viewed from the top, (1) the holes 13a, 14a are in contact with the core 12, or (2) the interval W _OFF between the holes 13a, 14a and the core 12 is the width W _{L of the} core 12. It is preferable that it is 0.25 times or less. The optical waveguide 10 according to the present embodiment employs a configuration in which the holes 13a and 14a are in contact with the core 12, that is, W _OFF = 0 μm. In other words, one of the four side surfaces forming the hole 13a coincides with the side surface of the core 12 (see FIG. 2B). The same applies to the hole 14a.

The width W _ap in the direction orthogonal to both the propagation direction of the light propagating through the high refractive index region 11 of the holes 13 a and 14 a and the depth direction of the holes 13 a and 14 a is 0.375 of the width W _L of the core 12. It is preferable that they are 2 times or more and 2.5 times or less. By the width _{W ap} is equal to or greater than 0.375 times the width _{W L,} and suppress the out light only is immersed in the semiconductor layers 13 and 14 from the core 12, light well in the vicinity of the junction surface 12j of the core 12 Localization Can be made. Further, the width _{W ap} is equal to or less than 2.5 times the width _{W L,} the electric resistance of the semiconductor layers 13 and 14 can be prevented from being unnecessarily increased. That is, by the width _{W ap} is equal to or less than 2.5 times 0.375 times the width _{W L,} while suppressing the electric resistance of the semiconductor layers 13 and 14, light well in the vicinity of the junction surface 12j of the core 12 Can be localized.

Each of the period L _C for forming the holes 13 a and 14 a at the boundary between the core 12 and the semiconductor layers 13 and 14 and the length L _ap of the holes 13 a and 14 a is an effective wavelength of light propagating through the high refractive index region 11. It is preferably shorter than λ _eff . The effective wavelength λ _eff is given by λ _eff = λ / N _eff where λ is the wavelength of light in vacuum and N _eff is the calculated effective refractive index of the high refractive index region 11. Thus, each cycle _{L C} and the length _{L ap} is preferably shorter than lambda _{/ N eff.}

When the period L _C and the length L _ap are sufficiently shorter than the effective wavelength λ _eff , the interaction between the light propagating through the core 12 and the holes 13 a and 14 a can be suppressed. As a result, light has an intermediate refractive index at the boundary between the core 12 and the semiconductor layers 13 and 14 between the refractive index of Si (eg, 3.48) and the refractive index of SiO ₂ (eg, 1.44). As in the case of the waveguide structure in which is formed, the light propagates through the core 12. The refractive index of the intermediate, (1) the refractive index of Si (e.g. 3.48), (2) the refractive index of SiO ₂ (e.g. 1.44) is the ratio of _{L ap} for (3) the period _{L C} opening Depends on the ratio.

  As described above, because the light propagating through the high refractive index region 11 is configured to suppress the interaction with the periodic structure of the holes 13a and 14a, the holes 13a and 14a act as a grating. There is nothing. Therefore, the optical waveguide 10 can suppress the optical loss of light propagating through the high refractive index region 11.

(First electrode 15 and second electrode 17)
The first electrode 15 and the second electrode 17 are preferably traveling wave electrodes. By employing the traveling wave electrode, the operation speed of the optical waveguide 10 can be increased. That is, the operating band of the optical waveguide 10 can be widened.

  Here, the coordinate system shown in FIG. 1A is defined as follows. (1) The axis parallel to the direction in which the core 12 is extended is taken as the y-axis. The direction of the y-axis is determined so that the direction from the front to the back in FIG. (2) The axis parallel to the thickness direction of the core 12 is taken as the z-axis. The direction of the z axis is determined so that the direction from the lower clad 19a to the upper clad 19b is a positive direction. (3) The axis parallel to the width direction of the core 12 is taken as the x-axis. The direction of the x axis is determined so that the x axis forms a right-handed system together with the y axis and the z axis described above.

  In the optical waveguide 10 according to the present embodiment, the holes 13a and 14a are configured to penetrate from the upper surface to the lower surface of the semiconductor layers 13 and 14. However, the holes 13a and 14a are not limited to the structure penetrating from the upper surface toward the lower surface. That is, part of the semiconductor layers 13 and 14 may be formed below the holes 13a and 14a.

The holes 13a and 14a may be filled with SiO ₂ constituting the upper clad 19b. That is, part or all of the insides of the holes 13a and 14a may be filled with SiO ₂ constituting the upper clad 19b, or the inside thereof may be a void.

According to this configuration, it is only necessary to deposit SiO ₂ that forms the upper clad after forming the high refractive index region 11, so that the manufacturing of the optical waveguide 10 can be simplified. Further, since the SiO ₂ is filled, the hole 13a, the side surface of 14a is covered by the SiO _2. Therefore, deterioration of the high refractive index region 11 (for example, oxidation of Si) can be prevented.

In the optical waveguide 10 according to the present embodiment, a configuration in which the entire inside of the holes 13a and 14a is filled with SiO ₂ is adopted.

In the optical waveguide 10 configured as described above, a plurality of holes 13 a and 14 a arranged periodically are formed in the semiconductor layers 13 and 14 outside the core 12. Furthermore, holes 13a, the inside of 14a SiO ₂ is filled. As a result, the substantial refractive index with respect to the light propagating through the core 12 in the boundary region between the core 12 and the first semiconductor layer 13 (the region where the holes 13a are periodically formed), and the core 12 and the first semiconductor layer 13 The refractive index with respect to the light propagating through the core 12 in the boundary region with the semiconductor layer 14 (region in which the holes 14a are periodically formed) is as follows: (1) The dopant constituting the semiconductor layers 13 and 14 It is determined by averaging the refractive index of the added Si and (2) the refractive index of SiO ₂ constituting the upper cladding 19b. Further, the effective refractive index of the optical waveguide 10 with respect to light propagating through the core 12 is calculated using these refractive indexes and the shape of the optical waveguide 10.

When the substantial refractive index of the boundary region becomes a refractive index obtained by averaging the refractive index of Si and the refractive index of SiO ₂ , the optical waveguide 10 has a rib slab type in which the holes 13a and 14a are not formed. Compared with the optical waveguide, the light propagating through the high refractive index region 11 can be confined more strongly by the core 12. Therefore, the optical waveguide 10 can increase the electric field intensity of light distributed in the depletion layer region formed in the vicinity of the joint surface 12j of the core 12, and as a result, the modulation efficiency, which is the efficiency when modulating light, is improved. Can be improved.

  The optical waveguide 10 according to the present embodiment can be manufactured using, for example, an SOI (Silicon on Insulator) substrate. In this case, the BOX layer (buried oxide layer) of the SOI substrate may be used as the lower clad 19a, and the Si layer formed on the BOX layer may be patterned into the shape of the high refractive index region 11. Each of the P-type semiconductor part 12a, the N-type semiconductor part 12b, and the semiconductor layers 13 and 14 provided in the high refractive index region 11 is realized by adding a dopant to the Si layer formed on the BOX layer. Can do. When the optical waveguide 10 is manufactured using an SOI substrate, an Si layer (not shown in FIG. 1A) exists below the lower clad 19a.

  As a process for manufacturing these optical waveguides 10, an existing process for manufacturing a semiconductor optical waveguide can be applied. In order to manufacture the optical waveguide 10, a mask pattern for patterning the outer shape of the high refractive index region 11 is formed on the holes 13a and 14a on the basis of a process for manufacturing a conventional rib slab type semiconductor optical waveguide. In addition, it is only necessary to change to a mask pattern that can be patterned. Therefore, the optical waveguide 10 can be manufactured without adding a new process to the process for manufacturing the conventional rib slab type semiconductor optical waveguide. Therefore, it is possible to prevent the manufacturing cost of the optical waveguide 10 from rising.

(First embodiment)
The optical waveguide 10 according to the first embodiment of the present invention will be described with reference to FIG. FIG. 3A is a graph showing an electric field distribution of light propagating through the optical waveguide 10 according to the present embodiment, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

In this embodiment, it is assumed that the optical waveguide 10 is manufactured using an SOI substrate in which the thickness of the Si layer formed on the BOX layer is 220 nm. Therefore, 220 nm is adopted as the thickness _HL of the core 12. The point that the thickness H _L = 220 nm is the same for the other modified examples described in this specification.

In the optical waveguide 10 of the present embodiment, parameters other than the thickness H _L are as follows.
The thickness H _S of the semiconductor layers 13 and 14 is 100 nm.
-3.48 was adopted as the refractive index of silicon forming the high refractive index region 11 (core 12 and semiconductor layers 13 and 14).
- the clad 19 as a refractive index of SiO ₂ forming was employed 1.44.
-As the width W _L of the core 12, 500 nm was adopted.
-Adopted 150 nm as the width W _ap of the holes 13a, 14a.
· Hole 13a, as the period _{L C} to form the 14a, was adopted 300 nm.
· Hole 13a, as the length _{L ap} of 14a, has adopted a 150 nm. That was adopted 50% as the aperture ratio is the ratio of _{L ap} to the period _{L C.}
-The wavelength λ of light propagating through the high refractive index region 11 is 1.55 μm.

  FIG. 3A shows the result of simulating the electric field distribution of light propagating through the high refractive index region 11 using the optical waveguide 10 configured as described above.

A numerical value averaged using the refractive index of Si, the refractive index of SiO ₂ , and the aperture ratio was adopted as the refractive index of the boundary region between the core 12 and the semiconductor layers 13 and 14. This also applies to other modified examples and other examples described in this specification.

  The effective refractive index of the high refractive index region 11 with respect to the fundamental mode light propagating through the optical waveguide 10 calculated in the optical waveguide 10 of the present example was 2.48.

  Referring to FIG. 3A, (1) the light propagating through the high refractive index region 11 is particularly well confined in the core 12 in the high refractive index region 11, and (2) the electric field strength of the light is It was found that the strong region was concentrated in the vicinity of the joint surface 12j of the core 12.

  Therefore, the optical waveguide 10 according to the present example can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region. The electric field distribution of light propagating through an optical waveguide employing an existing rib slab type high refractive index region will be described later with reference to another drawing.

(First reference example)
An optical waveguide 50 according to a first reference example of the present invention will be described with reference to FIG. FIG. 11A is a graph showing the electric field distribution of light propagating through the optical waveguide 50 according to this reference example, and the light in the cross section perpendicular to the light propagation direction, that is, the cross section along the zx plane. It is a graph which shows electric field distribution.

  The optical waveguide 50 of this reference example was obtained by replacing the high refractive index region 11 of the optical waveguide 10 of the first example with a core 52 having a rectangular cross-sectional shape. In other words, the optical waveguide 50 has a configuration in which the semiconductor layers 13 and 14 are omitted from the high refractive index region 11 of the optical waveguide 10.

  In this reference example, the thickness of the core 52 is 220 nm and the width of the core 52 is 500 nm.

  Referring to (a) of FIG. 11, it was found that the light propagating through the core 52 is well localized near the joint surface of the core 52 due to the absence of the semiconductor layers 13 and 14.

(First comparative example)
An optical waveguide 60 according to a first comparative example of the present invention will be described with reference to FIG. FIG. 11B is a graph showing the electric field distribution of the light propagating through the optical waveguide 60 according to this comparative example, and the light in the cross section perpendicular to the light propagation direction, that is, the cross section along the zx plane. It is a graph which shows electric field distribution.

  The optical waveguide 60 of this comparative example was obtained by separating the semiconductor layers 13 and 14 from the core 12 in the high refractive index region 11 included in the optical waveguide 10 of the first example. That is, in the optical waveguide 60, the core 62 and the semiconductor layers 63 and 64 constituting the high refractive index region 61 are separated from each other. In this comparative example, 1.2 μm was adopted as the distance between the core 62 and the semiconductor layers 63 and 64.

  Referring to FIG. 11B, the light propagating through the high refractive index region 61 of the optical waveguide 60 is transmitted from the core 62 even though the distance between the core 62 and the semiconductor layers 63 and 64 is as large as 1.2 μm. It was found that it leaked greatly and was localized in the semiconductor layers 63 and 64.

  In contrast, for example, the light propagating through the high refractive index region 11 of the optical waveguide 10 of the first embodiment is well localized near the joint surface of the core 12 as shown in FIG. Was. This effect is attributed to the fact that the semiconductor layers 13 and 14 are provided with holes 13a and 14a in addition to the high refractive index region 11 being a rib slab type high refractive index region.

(Second reference example)
An optical waveguide 50 according to a second reference example of the present invention will be described with reference to FIG. FIG. 11C is a graph showing the electric field distribution of light propagating through the optical waveguide 50 according to this reference example, and the light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

  The optical waveguide 50 of this reference example was obtained by changing the width of the core 52 to 400 nm in the optical waveguide 50 of the first reference example.

  Referring to (c) of FIG. 11, the light leaking from the core 52 increases as the width of the core 52 becomes narrow, but the light propagating through the core 52 is near the joint surface of the core 52. It turns out that it is well localized.

(Second comparative example)
An optical waveguide 60 according to a second comparative example of the present invention will be described with reference to FIG. FIG. 11D is a graph showing the electric field distribution of the light propagating through the optical waveguide 60 according to this comparative example, and the light in the cross section perpendicular to the light propagation direction, that is, the cross section along the zx plane. It is a graph which shows electric field distribution.

  The optical waveguide 60 of this comparative example was obtained by changing the width of the core 62 to 400 nm in the optical waveguide 60 of the first comparative example.

  Referring to FIG. 11D, as in the case of the optical waveguide 60 according to the first comparative example (FIG. 11B), the light propagating through the high refractive index region 61 of the optical waveguide 60 is the core. It was found that the material leaked greatly from 62 and was localized in the semiconductor layers 63 and 64.

In contrast to this, in the fourth embodiment to be described later, the light propagating through the high refractive index region 11 of the optical waveguide 10 is well near the joint surface of the core 12 as shown in FIG. It was localized. That is, in addition to the high refractive index region 11 is has a Ribusurabu-shaped cross section, by holes 13a, 14a are provided on the semiconductor layers 13 and 14, narrowing the width _{W L} of the core 12 to 400nm Even in this case, the optical waveguide 10 of the fourth example was found to be able to well localize the light propagating through the core 12 in the vicinity of the joint surface of the core 12.

(First modification)
An optical waveguide 10 according to a first modification of the present invention will be described with reference to FIG. FIG. 3B is a graph showing the electric field distribution of the light propagating through the optical waveguide 10 according to this modification, and the light in the cross section perpendicular to the light propagation direction, that is, the cross section along the zx plane. It is a graph which shows electric field distribution.

Optical waveguide 10 of this modification, the optical waveguide 10 of the first embodiment were obtained by changing the thickness _{H S} of the semiconductor layers 13 and 14 to 140 nm. Further, in the optical waveguide 10 of the present example, the calculated effective refractive index of the high refractive index region 11 was 2.51.

  Referring to FIG. 3B, as in the case of the optical waveguide 10 of the first embodiment, (1) the light propagating through the high refractive index region 11 is the core 12 in the high refractive index region 11 in particular. (2) It was found that the region where the electric field intensity of light is strong is concentrated in the region near the joint surface 12j of the core 12.

  Therefore, the optical waveguide 10 according to the present embodiment can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(Second modification)
An optical waveguide 10 according to a second modification of the present invention will be described with reference to FIG. FIG. 3C is a graph showing the electric field distribution of light propagating through the optical waveguide 10 according to this modification, and the light in the cross section perpendicular to the light propagation direction, that is, the cross section along the zx plane. It is a graph which shows electric field distribution.

Optical waveguide 10 of this modification, the optical waveguide 10 of the first embodiment were obtained by changing the thickness _{H S} of the semiconductor layers 13 and 14 to 150 nm. Further, in the optical waveguide 10 of the present example, the calculated effective refractive index of the high refractive index region 11 was 2.52.

  Referring to FIG. 3B, as in the case of the optical waveguide 10 of the first embodiment, (1) the light propagating through the high refractive index region 11 is the core 12 in the high refractive index region 11 in particular. (2) It was found that the region where the electric field intensity of light is strong is concentrated in the region near the joint surface 12j of the core 12.

  Therefore, the optical waveguide 10 according to the present embodiment can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(Third comparative example)
An optical waveguide 70 according to a third comparative example of the present invention will be described with reference to FIG. FIG. 12A is a graph showing the electric field distribution of light propagating through the optical waveguide 70 according to this comparative example, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

The optical waveguide 70 of this comparative example, the optical waveguide 10 of the first embodiment were obtained by changing the thickness _{H S} of the semiconductor layers 13 and 14 to 160 nm. The high refractive index region 71 of the optical waveguide 70 is a member corresponding to the high refractive index region 11 of the optical waveguide 10. In addition, the effective refractive index of the high refractive index region 71 for the fundamental mode light propagating through the optical waveguide 70 calculated in the optical waveguide 70 of this example was 2.54.

  Referring to FIG. 12A, a part of the light propagating through the high refractive index region 71 passes through the boundary region between the core 72 and the semiconductor layers 73 and 74 (region where the holes 73a and 74a are formed). It was found that the semiconductor layers 13 and 14 ooze out. Therefore, the optical waveguide 70 according to this modification cannot improve the modulation efficiency as compared with the existing optical waveguide adopting the rib slab type high refractive index region.

(About the thickness of the semiconductor layers 13 and 14)
As described above, the thickness _{H S} of the semiconductor layers 13 and 14 varied in the range of 100nm or more 160nm or less. As a result, the optical waveguide 70 with a thickness H _S reaches 160nm, compared with an optical waveguide employing a high refractive index region existing Ribusurabu type, it was found that it is impossible to improve modulation efficiency.

Therefore, the thickness H _S of the semiconductor layers 13 and 14 normalized by the thickness H _L of the core 12 is preferably 68.2% or less when described in one significant digit.

The more you reduce the thickness H _S, merit light propagating the high refractive index region 11 can be strongly localized to the joint surface near the core 12 may occur. On the other hand, an excessively thin thickness H _S, the resistance value between the first electrode and the P-type semiconductor portion 12a, and the resistance between the second electrode and the N-type semiconductor portion 12b is increased The demerit that the voltage value of the modulation voltage required for driving the optical waveguide 10 becomes high arises. In consideration of these merits and demerits, the thickness H _S normalized by the thickness _HL can be appropriately selected so as to be in the range of 68.2% or less.

(Fourth comparative example)
An optical waveguide 80 according to a fourth comparative example of the present invention will be described with reference to FIG. FIG. 12B is a graph showing the electric field distribution of light propagating through the optical waveguide 80 according to this comparative example, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

The optical waveguide 80 of this comparative example, the optical waveguide 10 of the first embodiment, by changing the width _{W L} of the core 12 to 400 nm, the hole 13a formed in the semiconductor layers 13 and 14, by omitting 14a Obtained. In other words, it was obtained by omitting the holes 13a and 14a formed in the semiconductor layers 13 and 14 from the optical waveguide 10 (see FIG. 4B) according to a fourth modification described later. The high refractive index region 81 of the optical waveguide 80 is a corresponding member of the high refractive index region 11 of the optical waveguide 10. In addition, the effective refractive index of the high refractive index region 81 for the fundamental mode light propagating through the optical waveguide 80 calculated in the optical waveguide 80 of this example was 2.47.

Referring to FIG. 12B, it was found that most of the light propagating through the high refractive index region 81 of the optical waveguide 80 oozes from the core 82 into the semiconductor layers 83 and 84. Thus, a high refractive index region 81 of the existing Ribusurabu type, when the width W _L of the core 82 has adopted the high refractive index region 81 which is 400 nm, due to the light from the core 82 is oozed large The optical waveguide 80 cannot improve the modulation efficiency.

(Fifth comparative example)
An optical waveguide 80 according to a fifth comparative example of the present invention will be described with reference to FIG. FIG. 12C is a graph showing the electric field distribution of light propagating through the optical waveguide 90 according to this comparative example, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

  The optical waveguide 90 of this comparative example was obtained by omitting the holes 13a and 14a formed in the semiconductor layers 13 and 14 in the optical waveguide 10 of the first example. The high refractive index region 71 of the optical waveguide 90 is a member corresponding to the high refractive index region 11 of the optical waveguide 10. Further, the effective refractive index of the high refractive index region 91 for the fundamental mode light propagating through the optical waveguide 90 calculated in the optical waveguide 90 of this comparison was 2.56.

(Third Modification)
An optical waveguide 10 according to a third modification of the present invention will be described with reference to FIG. FIG. 4A is a graph showing the electric field distribution of light propagating through the optical waveguide 10 according to this modification, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

The optical waveguide 10 of this modification is obtained by changing the width W _L of the core 12 to 400 nm and changing the width W _ap of the holes 13a and 14a to 450 nm in the optical waveguide 10 of the first embodiment. . The effective refractive index of the high refractive index region 11 for the fundamental mode light propagating through the optical waveguide 10 calculated in the optical waveguide 10 of the present modification was 2.29.

  Referring to FIG. 4A, as in the case of the optical waveguide 10 of the first embodiment, (1) the light propagating through the high refractive index region 11 is the core 12 in the high refractive index region 11 in particular. (2) It was found that the region where the electric field intensity of light is strong is concentrated in the region near the joint surface 12j of the core 12.

  Therefore, the optical waveguide 10 according to the present embodiment can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(Fourth modification)
An optical waveguide 10 according to a fourth modification of the present invention will be described with reference to FIG. FIG. 4B is a graph showing the electric field distribution of light propagating through the optical waveguide 10 according to this modification, and light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

The optical waveguide 10 of the present modification was obtained by changing the width W _ap of the holes 13a and 14a to 150 nm in the optical waveguide 10 of the third modification. The effective refractive index of the high refractive index region 11 for the fundamental mode light propagating through the optical waveguide 10 calculated in the optical waveguide 10 of the present modification was 2.32.

  Referring to FIG. 4B, as in the case of the optical waveguide 10 of the first embodiment, (1) the light propagating through the high refractive index region 11 is the core 12 in the high refractive index region 11 in particular. (2) It was found that the region where the electric field intensity of light is strong is concentrated in the region near the joint surface 12j of the core 12.

  Therefore, the optical waveguide 10 according to the present embodiment can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(About the width of the holes 13a and 14a)
As described above, when the widths W _ap of the holes 13a and 14a constituting the holes 13a and 14a are changed to 150 nm and 450 nm, in any case, the optical waveguide 10 according to the modified example of the present invention It has been found that the modulation efficiency can be improved as compared with an optical waveguide employing a rib slab type high refractive index region.

Therefore, the width W _ap of the holes 13 a and 14 a is preferably 0.375 times or more the width W _L of the core 12.

As the width W _ap is increased, there is an advantage that the light propagating through the high refractive index region 11 can be strongly localized near the joint surface of the core 12. On the other hand, if the width W _ap is too large, the resistance value between the first electrode and the P-type semiconductor portion 12a and the resistance value between the second electrode and the N-type semiconductor portion 12b increase. There is a demerit that the voltage value of the modulation voltage required to drive the optical waveguide 10 becomes high. Taking into account and these advantages and disadvantages, the width W _ap may be appropriately selected to be in the range of more than 0.375 times the width W _L of the core 12.

(5th-7th modification)
Optical waveguides 10 according to fifth to seventh modifications of the present invention will be described with reference to FIGS. Each of (a) to (c) of FIG. 5 is a graph showing an electric field distribution of light propagating through the optical waveguide 10 according to the fifth to seventh modified examples, and with respect to the light propagation direction. It is a graph which shows electric field distribution of light in a perpendicular section, ie, a section along a zx plane.

The optical waveguide 10 of the fifth modification, in the optical waveguide 10 of the first embodiment, by changing the width W _L of the core 12 to 400 nm, the opening ratio obtained by changing to 25%. Each of the optical waveguides 10 according to the sixth modification and the seventh modification was obtained by changing the opening ratio of the optical waveguide 10 according to the fifth modification to 50% and 75%, respectively. . The effective refractive index of the high refractive index region 11 for the fundamental mode light propagating through the optical waveguide 10 calculated in each of the optical waveguides 10 according to the fifth to seventh modifications is 2.39, 2. 32, 2.26.

  Referring to FIGS. 5A to 5C, as in the case of the optical waveguide 10 of the first embodiment, (1) the light propagating through the high refractive index region 11 is in the high refractive index region 11. However, it is particularly well confined in the core 12, and (2) it was found that the region where the electric field strength of light is strong is concentrated in the region near the joint surface 12j of the core 12.

  Therefore, each of the optical waveguides 10 according to the fifth to seventh modifications can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(Eighth to tenth modifications)
The optical waveguide 10 according to the eighth to tenth modified examples of the present invention will be described with reference to FIGS. Each of (a) to (c) of FIG. 6 is a graph showing an electric field distribution of light propagating through the optical waveguide 10 according to the eighth to tenth modification examples, with respect to the light propagation direction. It is a graph which shows electric field distribution of light in a perpendicular section, ie, a section along a zx plane.

Each of the optical waveguide 10 according to the eighth to tenth modified example is characterized in that in each of the optical waveguide 10 according to the fifth to seventh modification, obtained by changing the width W _L of the core 12 to 500nm . The effective refractive index of the high refractive index region 11 for the fundamental mode light propagating through the optical waveguide 10 calculated in each of the optical waveguides 10 according to the eighth to tenth modified examples is 2.52, 2.. 48, 2.44.

  Referring to FIGS. 6A to 6C, as in the case of the optical waveguide 10 of the first embodiment, (1) the light propagating through the high refractive index region 11 is in the high refractive index region 11. However, it is particularly well confined in the core 12, and (2) it was found that the region where the electric field strength of light is strong is concentrated in the region near the joint surface 12j of the core 12.

  Therefore, each of the optical waveguides 10 according to the eighth to tenth modified examples can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(About opening ratio)
As described above, the optical waveguide 10 width _{W L} is 400 nm, in the optical waveguide 10 Metropolitan width _{W L} is 500 nm, and the aperture ratio varied in the range of 75% or less than 25%. As a result, even 500nm even 400nm width _{W L,} if the aperture ratio is in the range of 75% or less than 25% optical waveguide 10 according to a modified example of the present invention, existing Ribusurabu It was found that the modulation efficiency can be improved as compared with the optical waveguide employing the high refractive index region of the mold.

  Therefore, the opening ratio is preferably 25% or more.

  As the aperture ratio is increased, there is an advantage that the light propagating through the high refractive index region 11 can be strongly localized near the joint surface of the core 12. On the other hand, if the aperture ratio is too high, the resistance value between the first electrode and the P-type semiconductor portion 12a and the resistance value between the second electrode and the N-type semiconductor portion 12b increase, and the light guide There is a demerit that the voltage value of the modulation voltage required for driving the waveguide 10 becomes high. Considering these merits and demerits, the aperture ratio can be selected as appropriate within a range of 25% or more.

(11th-12th modifications)
The optical waveguide 10 according to the 11th to 12th modifications of the present invention will be described with reference to FIGS. Each of (a) to (b) of FIG. 7 is a graph showing an electric field distribution of light propagating through the optical waveguide 10 according to the 11th to twelfth modifications, with respect to the light guiding direction. 3 is a graph showing the electric field distribution of light in a cross section perpendicular to the zx plane.

The optical waveguide 10 according to the eleventh modification is obtained by changing the distance W _OFF between the holes 13a, 14a and the core 12 to 100 nm in the optical waveguide 10 according to the first embodiment.

The optical waveguide 10 according to the twelfth modification is obtained by changing the interval W _OFF to 100 nm in the optical waveguide 10 according to the fourth modification.

  Referring to FIGS. 7A and 7B, the optical waveguide 10 according to the first embodiment and the fourth modification is not applicable, but (1) the high refractive index region 11 is formed. The propagating light is particularly well confined in the core 12 in the high refractive index region 11, and (2) the region where the electric field strength of light is strong is concentrated in the region near the joint surface 12j of the core 12. I understood.

  Therefore, each of the optical waveguides 10 according to the 11th to 12th modified examples can improve the modulation efficiency as compared with the existing optical waveguide employing the rib slab type high refractive index region.

(Regarding the distance between the holes 13a, 14a and the core 12)
As described above, the optical waveguide 10 width _{W L} is 400 nm, in the optical waveguide 10 Metropolitan width _{W L} is 500 nm, it was changed to 100nm apart _{W OFF} from 0 nm. As a result, even 500nm even 400nm width W _L, the optical waveguide 10 according to a modified example of the present invention is different from the optical waveguide employing the high refractive index region existing Ribusurabu type, It has been found that the modulation efficiency can be improved.

Therefore, the interval W _OFF is not limited to 0 nm, and may be 100 nm. That is, it is preferable that the holes 13 a and 14 a are in contact with the core 12, or the interval W _OFF is 0.25 times or less the width W _L of the core 12. The effect obtained by providing the holes 13a and 14a in the semiconductor layers 13 and 14 is greatest when the holes 13a and 14a are in contact with the core 12, and decreases as the interval W _OFF increases.

[Second Embodiment]
In the present embodiment, the semiconductor optical waveguide 30 includes a high refractive index region 31 in which the upper surface end portion of the P-type semiconductor layer (12a, 13) and the lower surface end portion of the N-type semiconductor layer (12b, 14) are joined. Will be described. A PN junction in which the upper end portion of the P-type semiconductor layer and the lower end portion of the N-type semiconductor layer are joined in this manner is called a vertical PN junction, which is a high refractive index region including the vertical PN junction. Is called a laminated high refractive index region.

  An optical waveguide 30 according to a second embodiment of the present invention will be described with reference to FIG. FIG. 8A is a perspective view showing the configuration of the high refractive index region 31 provided in the optical waveguide 30 according to the present embodiment. FIG. 8B is a cross-sectional view showing the configuration of the semiconductor optical waveguide 30 shown in FIG. 8A, and a cross section taken along the line CC ′ shown in FIG. FIG. 8C is a cross-sectional view showing the configuration of the semiconductor optical waveguide 30 shown in FIG. 8A, and a cross section taken along the line DD ′ shown in FIG. FIG.

(Configuration of optical waveguide 30)
The optical waveguide 30 according to the present embodiment is obtained by replacing the high refractive index region 11 included in the optical waveguide 10 according to the first embodiment with a high refractive index region 31 shown in FIG. It is done. That is, members other than the high refractive index region 31 of the optical waveguide 30 are configured in the same manner as the optical waveguide 10. Accordingly, the same members as those of the optical waveguide 10 are denoted by the same member numbers, and the description thereof is omitted. Further, the abbreviations assigned to the parameters relating to the high refractive index region 31 are the same as those for the high refractive index region 11.

  As shown in FIG. 8A, the high refractive index region 31 includes a P-type semiconductor layer (32a, 33) and an N-type semiconductor layer (32b, 34). The P-type semiconductor layer includes a P-type semiconductor part 32 a and a first semiconductor layer 33. Similarly, the N-type semiconductor layer includes an N-type semiconductor portion 32 b and a second semiconductor layer 34.

  As shown in FIG. 8B, the high refractive index region 31 has a core 32 that is thicker than portions (semiconductor layers 33 and 34) other than the core in the high refractive index region. The core 32 includes, in part, a junction surface 32j (a junction surface of a PN junction) between the P-type semiconductor layers (32a, 33) and the N-type semiconductor layers (32b, 34). In other words, the core 32 having the joint surface 32j therein is formed by joining the upper surface end portion of the N-type semiconductor layer (32b, 34) and the lower surface step portion of the P-type semiconductor layer (32a, 33). The The core 32 is a portion where the P-type semiconductor layers (32a, 33) and the N-type semiconductor layers (32b, 34) overlap each other, and is a vertical PN junction.

  As shown in FIGS. 8A and 8C, holes 33 a are periodically formed along the boundary with the core 32 in the first semiconductor layer 33. Similarly, holes 34 a are periodically formed in the second semiconductor layer 34 along the boundary with the core 32.

  The optical waveguide 30 configured as described above has the same effects as the optical waveguide 10 according to the first embodiment.

(Second embodiment)
An optical waveguide 30 according to a second embodiment of the present invention will be described with reference to FIG. FIG. 9A is a graph showing an electric field distribution of light propagating through the optical waveguide 30 according to the present embodiment, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

The parameters adopted in the optical waveguide 30 of this example are as follows.
The core 32 as a thickness _{H L,} was adopted 220 nm.
The thickness H _S of the semiconductor layers 33 and 34 is 100 nm.
-3.48 was adopted as the refractive index of silicon forming the high refractive index region 31 (core 32 and semiconductor layers 33, 34).
- the clad 19 as a refractive index of SiO ₂ forming was employed 1.44.
-Adopted 500 nm as the width W _L of the core 32.
-Adopted 150 nm as the width W _ap of the holes 33a, 34a.
· Hole 33a, as the period _{L C} is the period of forming a 34a, was adopted 300 nm.
· Hole 33a, as the length _{L ap} of 34a, has adopted a 150 nm. That is, 50% was adopted as the aperture ratio that is the ratio of L _{ap to} the period LC.
-The wavelength λ of light propagating through the high refractive index region 31 is 1.55 μm.

  Referring to (a) of FIG. 9, (1) the light propagating through the high refractive index region 31 is well confined in the core 32 in the high refractive index region 31, and (2) the electric field strength of the light. It was found that the strong region was concentrated in the vicinity of the joint surface 32j of the core 32.

  Therefore, the optical waveguide 30 according to the present example was able to improve the modulation efficiency as compared with the existing optical waveguide employing the laminated high refractive index region. Note that the electric field distribution of light propagating through an optical waveguide employing a high refractive index region having an existing laminated core will be described later with reference to different figures.

(Sixth comparative example)
An optical waveguide 100 that is a comparative example of the optical waveguide 30 according to the second embodiment of the present invention will be described with reference to FIG. The optical waveguide 100 according to this comparative example was obtained by omitting the holes 33a and 34a formed in the semiconductor layers 33 and 34 of the optical waveguide 30 according to the second example.

  Referring to (a) of FIG. 13, it was found that light leaked to the semiconductor layers 33 and 34 more than in the optical waveguide 30 according to the second example.

(13th modification)
An optical waveguide 30 according to a thirteenth modification of the present invention will be described with reference to FIG. FIG. 9B is a graph showing the electric field distribution of light propagating through the optical waveguide 30 according to the present embodiment, and shows light in a cross section perpendicular to the light propagation direction, that is, a cross section along the zx plane. It is a graph which shows electric field distribution.

An optical waveguide 30 according to the present modification, the optical waveguide 30 according to the second embodiment were obtained by changing the width _{W L} of the core 32 from 500nm to 400 nm.

Referring to FIG. 9 (b), even when the change in 400nm width _{W L} from 500 nm, (1) the light propagating the high refractive index region 31, particularly the core Among high refractive index region 31 It was found that the region where the electric field intensity of light is strong is concentrated in the region near the joint surface 32j of the core 32.

  Therefore, the optical waveguide 30 according to the present modification can improve the modulation efficiency as compared with the existing optical waveguide employing the laminated high refractive index region.

(Seventh comparative example)
An optical waveguide 100 that is a comparative example of the optical waveguide 30 according to the thirteenth modification of the present invention will be described with reference to FIG. The optical waveguide 100 according to this comparative example was obtained by omitting the holes 33a and 34a formed in the semiconductor layers 33 and 34 of the optical waveguide 30 according to the thirteenth modification.

  Referring to FIG. 13B, it was found that light leaked to the semiconductor layers 33 and 34 more than in the optical waveguide 30 according to the second example.

[Third Embodiment]
A semiconductor optical modulator 1 according to a third embodiment of the present invention will be described with reference to FIG. Note that a description of the same members as those of the optical waveguide 10 according to the first embodiment is omitted. FIG. 10 is a perspective view showing the configuration of the semiconductor optical modulator 1 according to this embodiment.

  The semiconductor optical modulator 1 modulates incident light by applying a modulation electric field corresponding to a modulation signal to a PN junction made of a semiconductor. The semiconductor optical modulator 1 is a Mach-Zehnder (MZ) type optical modulator, and each of two arm parts constituting the Mach-Zehnder type optical interferometer is provided with an optical light according to the first embodiment. A waveguide 10 is provided. The semiconductor optical modulator 1 is a silicon optical modulator 1. Hereinafter, the silicon optical modulator 1 is simply referred to as an optical modulator 1.

(Configuration of optical modulator 1)
The optical modulator 1 is a Mach-Zehnder optical modulator 1 in which an optical modulation unit is provided in at least one arm, and the optical waveguide 10 or the optical waveguide 30 according to one embodiment of the present invention is used as the optical modulation unit. I have. In the present embodiment, both of the two arm portions included in the optical modulator 1 are provided with the optical modulation unit, and each of the optical modulation units includes the optical waveguide 10 according to the first embodiment. The configuration is adopted.

  The optical modulator 1 is manufactured using one SOI substrate. The SOI substrate has a structure in which a lower Si layer 20, a BOX layer 19a, and an upper Si layer are stacked. In the optical modulator 1 and the optical waveguide 10, the BOX layer 19a of the SOI substrate is used as the lower cladding. Further, by processing the upper Si layer of the SOI substrate, the cores 22, 23, 24a, 24b of the optical modulator 1 and the high refractive index region 11 of the optical waveguide 10 are formed.

The upper clad 19b is obtained by depositing SiO ₂ on the lower clad 19a, the cores 22, 23, 24a, 24b and the high refractive index region 11. The upper clad 19 b is formed so as to surround the cores 22, 23, 24 a, 24 b and the high refractive index region 11.

  Assuming that light propagates in the positive y-axis direction of the coordinate system shown in FIG. 10, the core 23 functions as an entrance-side core, and the core 22 functions as an exit-side core. The core 23 is branched into a core 24 a that is a first arm portion and a core 24 b that is a second arm portion, and then merges with the core 22.

  The first optical waveguide 10a is inserted in the middle of the first arm portion 24a, and the second optical waveguide 10b is inserted in the middle of the second arm portion 24b.

  Each of the two traveling wave electrodes (first electrode and second electrode) included in the first optical waveguide 10a has a signal line 16a extending in the light propagation direction (y-axis direction). And the signal line 18a are connected. Similarly, a signal line 16b and a signal line 18b are connected to each of the two traveling wave electrodes (first electrode and second electrode) included in the second optical waveguide 10b.

  The light modulation system according to the present embodiment includes a voltage source that applies a first modulation voltage to the signal line 16a and the signal line 18a, and applies a second modulation voltage to the signal line 16b and the signal line 18b. I have. The first modulation voltage and the second modulation voltage applied by this voltage source are both modulation voltages in the reverse bias direction.

  The optical modulator 1 configured as described above has the same effect as the optical waveguide 10. Further, the optical modulator 1 configured as described above can be manufactured by using the same process as the process of manufacturing the optical waveguide 10 on one SOI substrate. That is, it is not necessary to add a new process in order to manufacture the optical modulator 1. Therefore, the optical modulator 1 can be manufactured at the same cost as the optical waveguide 10.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  The present invention can be used for a semiconductor optical waveguide, a semiconductor optical modulator, and a semiconductor optical modulation system that modulate light according to a modulation voltage applied from the outside.

1 Optical modulator (semiconductor optical modulator)
10, 10a, 10b Optical waveguide (semiconductor optical waveguide)
11, 31 High refractive index region 12, 12a, 12b, 32 Core 12j, 32j PN junction surface 13, 33 First semiconductor layer 14, 34 Second semiconductor layer 15, 17 Traveling wave electrode 16, 16a, 16b, 18 , 18a, 18b Signal line 19 Cladding 19a Lower cladding 19b Upper cladding 20 Lower Si layer 22, 23 Optical waveguide 24a First arm portion 24b Second arm portion

Claims (10)

A semiconductor optical waveguide having a high refractive index region composed of a P-type semiconductor layer and an N-type semiconductor layer in which side surfaces or one upper surface end and the other lower surface end are joined,
The high refractive index region is thicker than other parts, and includes a core that functions as a reverse bias type phase modulation unit,
The core includes a joint surface between the P-type semiconductor layer and the N-type semiconductor layer,
In a portion other than the core of the high refractive index region, a plurality of holes are periodically formed along the boundary with the core.
A semiconductor optical waveguide characterized by the above. The hole is in contact with the core, or the interval between the hole and the core is not more than 0.25 times the width of the core.
The semiconductor optical waveguide according to claim 1. The hole is in contact with the core;
The semiconductor optical waveguide according to claim 2. The period of the plurality of holes and the length of each of the plurality of holes in the propagation direction of the light propagating through the high refractive index region are shorter than the effective wavelength of the light propagating through the high refractive index region, respectively. ,
The semiconductor optical waveguide according to claim 1, wherein: The width of each of the plurality of holes in the direction perpendicular to both the propagation direction of light propagating through the high refractive index region and the depth direction of the hole is not less than 0.375 times the width of the core. .5 times or less,
The semiconductor optical waveguide according to claim 1, wherein: Further comprising a cladding surrounding the high refractive index region;
The inside of the hole is filled with a material constituting the cladding.
The semiconductor optical waveguide according to claim 1, wherein: The high refractive index region is made of a semiconductor in which a dopant is added to silicon or a semiconductor in which a dopant is added to indium phosphide, and the cladding is made of any one of silica, a semiconductor in which a dopant is added to indium phosphide, and air. ,
The semiconductor optical waveguide according to claim 6. A first traveling wave electrode connected to the P-type semiconductor layer; and a second traveling wave electrode connected to the N-type semiconductor layer.
The semiconductor optical waveguide according to claim 1, wherein: A Mach-Zehnder type semiconductor optical modulator in which an optical modulation unit is provided in at least one arm unit, wherein the optical modulation unit includes the semiconductor optical waveguide according to any one of claims 1 to 8.
A semiconductor optical modulator. A semiconductor optical modulator according to claim 9;
A voltage source that applies a reverse bias voltage to the P-type semiconductor layer and the N-type semiconductor layer.
A semiconductor light modulation system.