JP2009128694A - Optical switching element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical switching element with low power consumption. <P>SOLUTION: The optical switching element 1 includes: a substrate 8; a cladding layer 9; an optical waveguide layer 10; and an electric field applying section. The cladding layer 9 is disposed on the substrate 8. The optical waveguide layer 10 is disposed on the cladding layer 9, includes a pair of optical waveguides 2, 3 having optical coupling sections 4 and extending with a distance between each other, and has a refractive index larger than that of the cladding layer 9. The electric field applying section is installed for applying an electric field to the pair of optical waveguides 2, 3 and the cladding layer 9 in the optical coupling sections 4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical switch element, and more particularly to an optical switch element having a semiconductor layer.

  Wavelength division multiplexing (hereinafter referred to as “WDM”) optical communication realizes high-capacity and high-speed optical communication by transmitting multiplexed signal light through a single optical fiber. As the wavelength of the multiplexed signal light, for example, a 1.55 μm band and a 1.3 μm band are used. In this WDM optical communication, an optical switch element is required for demultiplexing the multiplexed signal light by a demultiplexer and outputting it to a designated place. As a conventional optical switch element, for example, there is one disclosed in Patent Document 1. The optical switch element will be described below.

  FIG. 14 is a top view schematically showing a configuration of a conventional optical switch element. 15 is a schematic cross-sectional view taken along line XV-XV in FIG.

  Referring to FIG. 14, the optical switch element 101 has optical waveguides 102 and 103. The optical waveguides 102 and 103 are provided close to each other over a predetermined length in the optical coupling unit 104. The optical waveguides 102 and 103 form a directional coupler. That is, in the optical coupling unit 104, light propagating through one of the optical waveguides 102 and 103 can be coupled with the other adjacent one. An input port 105 is provided on the end surface of the optical waveguide 102 on one side of the optical coupling unit 104. On the other side of the optical coupling unit 104, output ports 106 and 107 are provided on the end faces of the optical waveguides 102 and 103, respectively.

  Referring to FIG. 15, an optical switching element 101 includes an n + GaAsAl lower cladding layer 109 formed on an n + GaAs substrate 108, and an nGaAs optical waveguide formed on the lower cladding layer 109 and having optical waveguides 102 and 103 thereon. Layer 110. On each of the optical waveguides 102 and 103, an upper cladding layer 111 and an upper electrode 112 are sequentially provided. A lower electrode 113 is provided on the lower surface of the substrate 108. The upper cladding layer 111 has a configuration in which a p + GaAsAl layer 115 is laminated on an nGaAsAl layer 114, and a pn junction is formed at the interface between the nGaAsAl layer 114 and the p + GaAsAl layer 115.

Next, the operation of the optical switch element 101 will be described.
When no voltage is applied between the upper electrode 112 and the lower electrode 113, the signal light input from the input port 105 propagates through the optical waveguide 102 toward the optical coupling unit 104. Then, in the optical coupling unit 104 that is a directional coupler, the signal light is coupled to the optical waveguide 103 adjacent to the optical waveguide 102. The combined signal light propagates from the optical coupling unit 104 through the optical waveguide 103 and is output from the output port 107.

  On the other hand, when a voltage is applied between the upper electrode 112 and the lower electrode 113, the equivalent refractive index of the optical waveguide 102 and the optical waveguide 103 is increased in the optical coupling unit 104. Therefore, the optical waveguide 102 and the optical waveguide Each optical path length with 103 becomes longer. At this time, the signal light input from the input port 105 is coupled to the optical waveguide 103 adjacent to the optical waveguide 102 in the optical coupling unit 104 which is a directional coupler after propagating through the optical waveguide 102. As described above, since the optical path length is long in the optical coupling unit 104, the signal light continues to propagate through the optical waveguide 103 of the optical coupling unit 104, and the signal light is coupled to the optical waveguide 102 again in the optical coupling unit 104. To do. The signal light coupled to the optical waveguide 102 propagates from the optical coupling unit 104 through the optical waveguide 102 and is output from the output port 106.

  As described above, in the optical switch element 101, an optical switching operation is realized by freely selecting the output port destination of the signal light by applying a voltage between the upper electrode 112 and the lower electrode 113.

Next, the principle of switching of the optical switch element 101 will be described in more detail.
The above-described optical switch element 101 uses a change in the equivalent refractive index of the optical waveguides 102 and 103 due to the plasma effect. That is, the average carrier density in the optical waveguides 102 and 103 and the periphery of the optical waveguide changes due to a change in the voltage applied between the upper electrode 112 and the lower electrode 113, and is proportional to the amount of change in the carrier density. As a result, the equivalent refractive index changes. This change in carrier density is caused by a change in the thickness of a depletion layer formed by a pn junction provided at the interface between the nGaAsAl layer 114 and the p + GaAsAl layer 115.

  When no voltage is applied between the upper electrode 112 and the lower electrode 113, the depletion layer does not reach the optical waveguides 102 and 103, so that carriers are present in the optical waveguides 102 and 103 and the optical waveguide layer 110 around them. Existing.

  On the other hand, when a lower potential than the lower electrode 113 is applied to the upper electrode 112, a reverse bias is applied to the pn junction, thereby increasing the thickness of the depletion layer at the interface between the nGaAsAl layer 114 and the p + GaAsAl layer 115, A depletion layer extends into the optical waveguides 102 and 103. Since almost no carriers are present in the depletion layer, the average carrier density of the optical waveguides 102 and 103 and their periphery is lowered, and the equivalent refractive index of the optical waveguides 102 and 103 is increased. For this reason, the optical path lengths of the optical waveguides 102 and 103 in the optical coupling unit 104 are increased, and optical switching is performed.

  Similarly, in the optical switch element of Patent Document 2, an optical switching operation is performed by a reverse bias to the pn junction. This optical switching element has a structure in which an optical waveguide portion of signal light having an acceptor impurity is sandwiched between a p-type contact layer and an n-type contact layer. With this structure, a pn junction is formed at the interface between the optical waveguide and the n-type contact layer.

Next, the outline of the manufacturing method of the optical switch element 101 is demonstrated.
First, a substrate preparation step for forming an element substrate is performed. Specifically, the lower clad layer 109, the optical waveguide layer 110, the nGaAsAl layer 114, and the p + GaAsAl layer 115 are laminated in this order on the substrate 108 by a film formation process by epitaxial growth, so that the device is used. A substrate is prepared. During epitaxial growth, the carrier density is adjusted by changing the impurity concentration in the layer by using Si or Sn as a donor impurity. In the optical waveguide layer 110 and the nGaAsAl layer 114, the carrier density is about 2 × 10 ¹⁷ to 1 × 10 ¹⁸ cm ^−3, and in the p + GaAsAl layer 115, the carrier density is about 1 × 10 ¹⁶ to 1 × 10 ¹⁷ cm ^−3. ing.

  Next, using a sputtering method or a vapor deposition method, a metal film to be the upper electrode 112 and a metal film to be the lower electrode 113 are respectively formed on the surface on the p + GaAsAl layer 115 side and the surface on the substrate 108 side of the element substrate. Is done.

  Next, an optical waveguide forming process for forming the optical waveguides 102 and 103 is performed. Specifically, after a resist is applied on the metal film to be the upper electrode 112, electron beam exposure and photolithography are performed to form a resist pattern having a pattern corresponding to the optical waveguides 102 and 103. The

Next, the resist pattern is used as a mask, and an etching process such as ICP (Inductively Coupled Plasma) etching, reactive ion etching, or reactive ion beam etching is performed, so that the metal film to be the upper electrode 112 and p + GaAsAl The layer 115, the nGaAsAl layer 114, and a part of the optical waveguide layer 110 in the thickness direction are etched in this order. Thus, the optical switch element 101 is obtained.
JP 63-121826 A JP 2003-215646 A

  In the conventional optical switch element 101, when a voltage is applied between the upper electrode 112 and the lower electrode 113 in order to change the output port destination of the signal light, the n-type semiconductor of the nGaAsAl layer 114 is changed to the p + GaAsAl layer 115. Current due to minority carriers flows to the p-type semiconductor. For this reason, in the switching operation of the optical switch element 101, a current that flows in the thickness direction of the substrate 108 is generated between the upper electrode 112 and the lower electrode 113, so that the power consumption of the optical switch element 101 increases. It was.

  Further, since the refractive index difference between each of the lower cladding layer 109 and the upper cladding layer 111 and the optical waveguide layer 110 is small, the signal light propagating through the optical waveguides 102 and 103 is transmitted to the lower cladding layer 109 and the upper cladding layer 111. Therefore, there is a problem that the propagation loss of the optical switch element 101 becomes large.

  In addition, there is a problem that it is necessary to perform epitaxial growth requiring a difficult technique in the film forming process.

  In addition, four layers are etched in the etching process. For this reason, in order to perform etching with high efficiency and high accuracy, four types of etching conditions corresponding to the respective layers must be combined in the etching process. In addition, since the number of layers to be etched is large, a mask having high etching resistance must be used. For these reasons, there has been a problem that the etching process becomes difficult.

  The present invention has been made in view of the above problems, and a main object thereof is to provide an optical switch element with low power consumption. Another object of the present invention is to provide an optical switch element having a small propagation loss. Still another object of the present invention is to provide an optical switch element capable of performing each of the film forming process and the etching process more easily.

  The optical switch element of the present invention includes a substrate, an insulating layer, a semiconductor layer, and an electric field applying unit. The insulating layer is provided on the substrate. The semiconductor layer includes a pair of protrusions that are provided on the insulating layer and have at least one region that runs parallel to each other at a distance from each other, and has a refractive index that is larger than the refractive index of the insulating layer. The electric field applying unit is provided to apply an electric field to a region including at least a part of at least one of the pair of protrusions and at least a part of the insulating layer.

  According to the optical switch element of the present invention, since the current flow caused by the electric field generated by the electric field applying unit is inhibited by the insulating layer, the power consumption can be reduced. In addition, since it is not necessary to have a structure in which the semiconductor layer in which the optical waveguide is formed is sandwiched between other semiconductor layers that are epitaxially related to this semiconductor layer, the region of the semiconductor layer that sandwiches the semiconductor layer is suppressed so that the propagation loss of the optical waveguide is suppressed. The material can be selected. Further, since it is not necessary to form the semiconductor layer by epitaxial growth, the film forming process can be performed more easily. In addition, since the number of semiconductor layers can be reduced, the etching process can be performed more easily.

  Preferably, in the above optical switch element, the electric field applying unit includes an electrode provided on at least a part of at least one of the pair of protrusions. Thereby, it is possible to perform control to change the optical path length in at least a part of at least one of the pair of protrusions. A switching operation can be performed by this control.

  In the above optical switch element, preferably, the electric field applying unit includes a voltage generating unit for applying a voltage corresponding to a switching state of the optical switch element to the electrode. Thereby, the switching state of the optical switch element can be controlled by controlling the voltage generated from the voltage generator.

In the above optical switch element, the semiconductor layer preferably has an impurity concentration of 1 × 10 ¹⁵ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less. As a result, the switching operation of the optical switch element can be performed more reliably.

  Preferably, in the above optical switch element, the electric field applying unit applies the electric field in at least one region in which the pair of protrusions run parallel to each other with a space therebetween. Thus, a directional coupler type optical switch element can be configured.

  Alternatively, in the above-described optical switch element, preferably, at least one region in which the pair of protrusions run parallel to each other with a space therebetween is a pair of regions forming a Mach-Zehnder optical interferometer. . Thus, a Mach-Zehnder type optical switch element can be configured.

  As described above, according to the optical switch element of the present invention, effects such as reduction in power consumption can be obtained.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
The optical switch element according to the present embodiment is formed on an SOI (Silicon On Insulator) substrate in which a silicon oxide (SiO ₂ ) film and a single crystal silicon (Si) film are sequentially stacked on a silicon (Si) substrate. It has a directional optical coupler. First, the configuration of this optical switch element will be described. FIG. 1 is a top view schematically showing the configuration of the optical switch element according to Embodiment 1 of the present invention. FIG. 2 is a schematic sectional view taken along line II-II in FIG.

  Referring to FIGS. 1 and 2, optical switch element 1 of the present embodiment mainly includes substrate 8, cladding layer 9, optical waveguide layer 10, and electric field application unit.

The substrate 8 is a substrate made of silicon (n + Si) containing, for example, P (phosphorus) doped at 1 × 10 ¹⁸ cm ⁻³ as a high concentration n-type impurity. The clad layer 9 is an insulating layer formed on the substrate 8 and is, for example, a SiO ₂ layer having a thickness of 1 μm. The optical waveguide layer 10 is a semiconductor layer laminated on the cladding layer 9 and has a refractive index higher than that of the cladding layer 9. The material of the optical waveguide layer 10 is preferably a p-type or n-type semiconductor having an impurity concentration of 1 × 10 ¹⁵ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less, for example, doped with 1 × 10 ¹⁶ cm ⁻³ . Single-crystal silicon (p-Si) containing B (boron) as a p-type impurity. The optical waveguide layer 10 has the optical waveguides 2 and 3 as a pair of protrusions. The optical waveguides 2 and 3 have an optical coupling portion 4 that is at least one region that is adjacent to each other with a space therebetween and that runs in parallel over a predetermined length.

  The electric field application unit may apply an electric field to a region including each of the optical waveguides 2 and 3 in the optical coupling unit 4 and the cladding layer 9 positioned below each of the optical waveguides 2 and 3 in the optical coupling unit 4. It is provided so that it can.

  Specifically, the electric field application unit includes an upper electrode 11, a lower electrode 12, a conductive wire 13, and a voltage power supply device (voltage generation unit) 14. The upper electrode 11 is provided on each of the optical waveguides 2 and 3 in the optical coupling unit 4. The lower electrode 12 is provided on the surface of the substrate 8 opposite to the surface on which the cladding layer 9 is provided. Conductive wire 13 electrically connects each of upper electrode 11 and lower electrode 12 to voltage power supply device 14. The voltage power supply device 14 can apply a voltage corresponding to the switching state of the optical switch element 1 (the selection state of the output port from which the signal light is output) between the upper electrode 11 and the lower electrode 12. Each potential of the upper electrode 11 and the lower electrode 12 is provided so that it can be controlled independently. With the above configuration, the electric field applying unit can apply an electric field in the optical coupling unit 4.

  Further, the optical switch element 1 has an input port 5 and output ports 6 and 7. Each of the input port 5 and the output port 6 is provided on one end surface of one and the other of the optical waveguide 2. The output port 7 is provided on the end face of the optical waveguide 3 that is located on the same side as the output port 6 with respect to the optical coupling portion 4.

  Next, an outline of the operating principle of the optical switch element of the present embodiment will be described. The signal light propagating through each of the optical waveguides 2 and 3 propagates while being influenced by the equivalent refractive index of each of the optical waveguides 2 and 3. These equivalent refractive indexes are obtained from the refractive indexes of the optical waveguides 2 and 3 and the peripheral optical waveguide layer 10.

  When a potential of +5 V is applied to the upper electrode 11 and −5 V is applied to the lower electrode 12, the equivalent refractive index of each of the optical waveguides 2 and 3 in the optical coupling unit 4 is lowered, and the optical path length of the optical coupling unit 4 is short Become. At this time, the signal light incident from the input port 5 propagates through the optical waveguide 2 and then couples with the optical waveguide 3 adjacent to the optical waveguide 2 in the optical coupling portion 4 of the directional coupler. The signal light coupled to the optical waveguide 3 propagates from the optical coupling unit 4 through the optical waveguide 3 and is output from the output port 7.

  On the other hand, when a potential of −5 V is applied to the upper electrode 11 and +5 V is applied to the lower electrode, the equivalent refractive index of the optical waveguides 2 and 3 in the optical coupling unit 4 increases, and the optical path length of the optical coupling unit 4 increases. . At this time, the signal light incident from the input port 5 propagates through the optical waveguide 2 and then couples with the optical waveguide 3 adjacent to the optical waveguide 2 in the optical coupling portion 4 of the directional coupler. Since the optical path length of the optical coupling unit 4 is increased as described above, the signal light propagates through the optical waveguide 3 of the optical coupling unit 4 after that, and the signal light is again transmitted to the optical waveguide 2 in the optical coupling unit 4. Join. The signal light coupled to the optical waveguide 2 propagates through the optical waveguide 2 from the optical coupling unit 4 and is output from the output port 6.

  As described above, in the optical switch element 1, the optical path lengths of the optical waveguide 2 and the optical waveguide 3 in the optical coupling unit 4 are changed by changing the voltage applied between the upper electrode 11 and the lower electrode 12. Thus, the output port destination of the signal light is selected.

  Next, the principle of switching of the optical switch element 1 of the present embodiment will be described in more detail. The optical switch element 1 uses a change due to the plasma effect of the equivalent refractive index of the optical waveguides 2 and 3. That is, the amount of change in the equivalent refractive index of the optical waveguides 2 and 3 is the average carrier density in the optical waveguides 2 and 3 and the periphery thereof due to the change in the voltage applied between the upper electrode 11 and the lower electrode 12. Is proportional to the amount of change. This carrier density is controlled by the thickness of the depletion layer of the optical switch element 1.

  FIG. 3 is a schematic cross-sectional view showing the thickness distribution of the depletion layer when the electric field of the optical switch element in Embodiment 1 of the present invention is zero. Referring to FIG. 3, when voltage power supply device 14 is in the off state, depletion layers 19a and 19b are formed with substantially equal thicknesses at the interfaces between cladding layer 9 and optical waveguide layer 10 and substrate 8, respectively. Has been. The thickness of the depletion layer 19a affects the signal light propagating through the optical waveguides 2 and 3, respectively. Since the depletion layer 19b is very thin because the impurity concentration of the substrate 8 is high, the influence on the propagation of the signal light can be ignored.

  FIG. 4 is a schematic cross-sectional view showing the thickness distribution of the depletion layer when the electric field of the optical switch element in the first embodiment of the present invention is in one direction. Referring to FIG. 4, when a potential of +5 V is applied to upper electrode 11 and −5 V is applied to lower electrode 12, depletion layer 20 a and the interface between clad layer 9, optical waveguide layer 10, and substrate 8 are applied. Each of 20b is formed. In the optical coupling part 4, the thickness of the depletion layer 20a formed at the interface between the cladding layer 9 and the optical waveguide layer 10 becomes thinner than the state where no voltage is applied (the state shown in FIG. 3). For this reason, the influence of the depletion layer 20a received by the signal light propagating through the optical waveguides 2 and 3 in the optical coupling unit 4 is small. Since carriers exist outside the region of the depletion layer 20a, the equivalent refractive index of the optical waveguides 2 and 3 is lowered, and the optical path length in the optical coupling unit 4 is shortened.

  FIG. 5 is a schematic cross-sectional view showing the thickness distribution of the depletion layer when the electric field of the optical switch element in the first embodiment of the present invention is in another direction. Referring to FIG. 5, when a potential of −5 V is applied to upper electrode 11 and +5 V is applied to lower electrode 12, depletion layer 21 a and the interface between clad layer 9, optical waveguide layer 10, and substrate 8 are applied. Each of 21b is formed. In the optical coupling part 4, the depletion layer 21 a is thicker than the state where no voltage is applied (the state shown in FIG. 3). For this reason, since the depletion layer 21a spreads to the optical waveguides 2 and 3, the signal light propagating through each of the optical waveguides 2 and 3 in the optical coupling portion 4 is greatly affected by the depletion layer 21a. Since almost no carriers are present in the depletion layer 21a, the equivalent refractive index of the optical waveguides 2 and 3 is increased, and the optical path length in the optical coupling unit 4 is increased.

  Next, the outline of the manufacturing method of the optical switch element 1 of this Embodiment is demonstrated. 6, FIG. 7, FIG. 9 and FIG. 10 are schematic cross-sectional views sequentially showing the first to fourth steps of the method for manufacturing the optical switch element in the first embodiment of the present invention. FIG. 8 is a top view schematically showing a second step of the method of manufacturing the optical switch element in the first embodiment of the present invention. Note that the P1-P1 line in FIG. 8 corresponds to the cross-sectional positions in FIG. 6, FIG. 7, FIG. 9, and FIG.

Referring to FIG. 6, first, a substrate preparation process for preparing SOI substrate 15 is performed. Specifically, a 1 μm thick cladding layer 9 and a 250 nm thick optical waveguide layer 10 are formed on a 600 μm thick substrate 8 doped with 1 × 10 ¹⁸ cm ⁻³ of P (phosphorus). The SOI substrate 15 is prepared by stacking in order. During the lamination, the carrier density of the optical waveguide layer 10 is adjusted by changing the impurity concentration of the optical waveguide layer 10.

  Referring mainly to FIGS. 7 and 8, metal film 16 and lower electrode 12 are formed on SOI substrate 15 (FIG. 6) by sputtering or vapor deposition. Each of the metal film 16 and the lower electrode 12 is made of Ti (titanium) and Au (gold).

  Next, an optical waveguide forming process for forming the optical waveguides 2 and 3 is performed. First, a resist is applied to the metal film 16 on the SOI substrate 15. Next, direct writing with an electron beam or photolithography is performed to form a resist pattern 18 having a pattern corresponding to the optical waveguides 2 and 3. In the case of direct drawing with an electron beam, for example, a process condition in which an electron beam irradiation current is 0.1 nA and an electron beam dose time per dot is 4.5 sec can be used. In the case of photolithography, for example, a resist pattern can be formed under conditions where the transfer time is about 10 seconds.

Next, using the resist pattern 18 as a mask, an etching process such as ICP etching, reactive ion etching, or reactive ion beam etching is performed, so that the metal film 16 and a part of the optical waveguide layer 10 in the thickness direction are etched. Is done. When reactive ion etching is used, process conditions corresponding to the material of the film to be etched are used. As the etching conditions for the metal film 16, for example, an etching gas is NF ₃ gas, an etching pressure is 0.1 Pa, and an RF (Radio Frequency) power is 200 W. As the etching conditions for the optical waveguide layer 10, for example, the etching gas is a mixed gas of chlorine (Cl ₂ ) gas 25 sccm and nitrogen (N ₂ ) gas 10 sccm, the etching pressure is 0.1 Pa, and the RF power is 200 W. It is done.

Referring to FIG. 9, upper electrode 11 is formed from metal film 16 by the etching described above. Next, the resist pattern 18 is removed by oxygen (O ₂ ) ashing.

  Referring to FIG. 10, upper electrode 11 and optical waveguides 2 and 3 are formed by the above etching process.

  Referring again to FIG. 2, voltage power supply device 14 is connected to each of upper electrode 11 and lower electrode 12 by conducting wire 13. Thus, the optical switch element 1 of the present embodiment is obtained.

  According to the present embodiment, as shown in FIG. 2, since the cladding layer 9 existing between the substrate 8 and the optical waveguide layer 10 is an insulator, a voltage is generated between the upper electrode 11 and the lower electrode 12. Is applied, no current flows between the substrate 8 and each of the optical waveguides 2 and 3. Therefore, power consumption in the switching operation is reduced.

In the conventional structure (FIG. 15), the nGaAs optical waveguide layer 110 (FIG. 15) is sandwiched between the lower clad layer 109 and the upper clad layer 111 which are in an epitaxial relationship with the nGaAs optical waveguide layer 110. In the present embodiment, it is possible to select the material of the region sandwiching the optical waveguide layer 10 so as to reduce the propagation loss of signal light propagating through the optical switch element 1 without considering the ease of epitaxial growth. . Specifically, a material (for example, SiO ₂ ) having a refractive index sufficiently lower than the refractive index of the optical waveguide layer 10 can be selected as the material of the clad layer 9 provided on the substrate 8 side of the optical waveguide layer 10. .

  Further, unlike the conventional structure, it is not necessary to form the cladding layer 9 and the optical waveguide layer 10 by epitaxial growth, so that the film forming process can be performed more easily.

  In addition, since the number of layers stacked on the substrate 8 in the manufacturing process is two and the number of layers is small, the film forming process and the etching process are simplified.

The optical waveguide layer 10 preferably has an impurity concentration of 1 × 10 ¹⁵ cm ⁻³ or more and 1 × 10 ¹⁸ cm ⁻³ or less. As a result, the thickness of each depletion layer of the optical waveguides 2 and 3 having a thickness of about 0.25 μm using a voltage of about 10 V, which is often easily obtained in a normal semiconductor device, is approximately in the thickness direction as shown in FIG. It can be spread all over or almost zero as shown in FIG. Therefore, the optical switch element 1 can be switched more reliably. The reason why the thickness of the optical waveguides 2 and 3 is selected to be about 0.25 μm is that the propagation mode of the signal light can be easily changed to the single mode. Further, when the impurity concentration of the optical waveguide layer 10 is larger than the above range, there is a problem that a voltage necessary for making the depletion layer thick as shown in FIG. 5 becomes too high. Further, when the impurity concentration is smaller than the above range, there is a problem that a voltage required to make the depletion layer sufficiently thin as shown in FIG. 4 becomes too high.

  A depletion layer 19a (FIG. 3) is formed between the cladding layer 9 and the optical waveguide layer 10, and the thickness of the depletion layer 19a is as shown in the depletion layers 20a (FIG. 4) and 21a (FIG. 5) by voltage application. As long as it can be controlled, the material of the substrate 8 may be different from the material exemplified above. For example, a p-type semiconductor doped with a high concentration of In (indium) or Ga (gallium), an n-type semiconductor doped with a high concentration of P (phosphorus) or As (arsenic), Cu (copper), W (Tungsten), TiN (titanium nitride) or Al (aluminum) alloy conductors can be used.

  Further, as long as a depletion layer 19a is formed between the cladding layer 9 and the optical waveguide layer 10 and the thickness of the depletion layer 19a can be controlled by voltage application, the material of the optical waveguide layer 10 is different from the materials exemplified above. Also good. For example, an n-type semiconductor doped with As (arsenic) or a p-type semiconductor doped with B (boron), Ga (gallium), or In (indium) can be used.

  Moreover, the optical switch element 1 can also be comprised using the board | substrate 8 which consists of a p-type semiconductor, and the optical waveguide layer 10 which consists of an n-type semiconductor. However, in this case, the operation of the optical switch element 1 is opposite to that described above with respect to the applied voltage.

  The value of the potential applied to each of the upper electrode 11 and the lower electrode 12 is not limited to −5 V or +5 V described above, and the depletion layer 19 a formed at the interface between the cladding layer 9 and the optical waveguide layer 10. Any potential can be used as long as the layer thickness can be changed in the range of 0 μm to 0.25 μm in the optical coupling portion 4.

Further, the impurity concentration of the optical waveguide layer 10 is not limited to 10 × 10 ¹⁶ cm ⁻³ exemplified above, and the thickness of the depletion layer 19a (FIG. 3) is set to 0 μm to 0.25 μm in the optical coupling portion 4 by voltage application. Any concentration that can be changed within the range is acceptable. For this purpose, the impurity concentration may be, for example, 1 × 10 ¹⁵ or more and 1 × 10 ¹⁸ cm ⁻³ or less.

The impurity concentration of the substrate 8 is not limited to 1 × 10 ¹⁸ cm ⁻³ exemplified above, and the influence of the depletion layer 19b (FIG. 3) on the signal light propagating through each of the optical waveguides 2 and 3 is sufficiently small. Any impurity concentration may be used. For this purpose, the impurity concentration may be, for example, 1 × 10 ¹⁸ cm ⁻³ or more and 1 × 10 ²² cm ⁻³ or less.

(Embodiment 2)
The optical switch element of this embodiment has a Mach-Zehnder optical interferometer. First, the configuration of this optical switch element will be described. FIG. 11 is a top view schematically showing the configuration of the optical switch element according to Embodiment 2 of the present invention. 12 is a schematic cross-sectional view taken along the line XII-XII in FIG. With reference to FIGS. 11 and 12, the optical switch element 51 of the present embodiment includes an optical waveguide layer 10s and an electric field applying unit having upper electrodes 72 and 73.

  The optical waveguide layer 10s is laminated on the cladding layer 9, and is made of the same material as the optical waveguide layer 10 (FIG. 2). The optical waveguide layer 10s has optical waveguides 2s and 3s as a pair of protrusions. The optical waveguides 2 s and 3 s have 3 dB directional couplers 4 a and 4 b that are a pair of regions that are close to each other with a space therebetween and run in parallel over a predetermined length. Each of the 3 dB directional couplers 4 a and 4 b has a function of equally dividing signal light input from one optical waveguide and branching it to two optical waveguides, and a pair of 3 dB directional couplers 4a and 4b form a Mach-Zehnder optical interferometer.

  The electric field applying unit includes the upper electrodes 72 and 73 on the optical waveguides 2s and 3s, respectively, in the arm portion 60 located in the region sandwiched between the pair of 3dB directional couplers 4a and 4b. ing. Thereby, the electric field applying unit can apply an electric field to a region including each of the optical waveguides 2s and 3s and the clad layer 9 positioned below each of the optical waveguides 2s and 3s. Each of the input port 5 and the output port 6 is provided on one end surface of one and the other of the optical waveguide 2s. The output port 7 is provided on the end face of the optical waveguide 3 s that is located on the same side as the output port 6 with respect to the arm portion 60.

  Since the configuration other than the above is substantially the same as the configuration of the first embodiment described above, the same or corresponding elements are denoted by the same reference numerals, and description thereof is not repeated.

Next, the principle of the switching operation of the optical switch element according to the present embodiment will be described.
The signal light incident from the input port 5 propagates through the optical waveguide 2s and reaches the 3 dB directional coupler 4a. Then, the signal light is branched into two to the optical waveguides 2s and 3s by the 3 dB directional coupler 4a. The two branched signals propagate through the optical waveguides 2s and 3s in the arm portion 60, respectively. When the upper electrodes 72 and 73 are set to the same potential, the optical path lengths of the optical waveguides 2s and 3s are equal in the arm portion 60. Therefore, these two signals are incident on the 3 dB directional coupler 4 b in phase with each other, and are coupled by the 3 dB directional coupler 4 b through the optical waveguide 3 s. The combined signal light is output from the 3 dB directional coupler 4b through the optical waveguide 3s at the output port 7.

  On the other hand, when the voltage power supply 14 applies a potential of −10 V only to the upper electrode 72 with the substrate 8 as a reference, and the potential of the upper electrode 73 remains zero, the equivalent refraction of the optical waveguide 3 s in the arm portion 60. The rate does not change, and only the equivalent refractive index of the optical waveguide 2s increases. Therefore, if the value of the applied voltage is optimally adjusted, the optical path 2s and the optical waveguide are made longer by making the optical path length of the optical waveguide 2s longer than the optical path length of the optical waveguide 3s by the half wavelength of the signal light in the arm portion 60. The optical path difference ΔL with respect to 3 s can be set to the half wavelength of the signal light. In this case, the two signals branched by the 3 dB directional coupler 4 a are incident on the 3 dB directional coupler 4 b in opposite phases. Therefore, the two signal lights are coupled by the 3 dB directional coupler 4b through the optical waveguide 2s. The combined signal light is output from the 3 dB directional coupler 4b through the optical waveguide 2s at the output port 6.

  As described above, by controlling the voltage application by the voltage power supply device 14, the output port destination of the signal light is freely changed and the optical switching operation is performed.

  The optical path difference ΔL provided when signal light is output from the output port 6 is not limited to the half wavelength described above.

(N is an integer equal to or greater than 0).
As long as this equation is satisfied, the voltage application point can be any one or both of the upper electrodes 72 and 73. Further, the positive / negative of the applied potential with respect to the substrate 8 is arbitrary. When a voltage is applied to both the upper electrodes 72 and 73, the above equation can be satisfied by adjusting the voltage between the upper electrodes 72 and 73.

  In addition, since the basic principle for changing the equivalent refractive index in the above is the same as that in the first embodiment, the description thereof will not be repeated.

Next, an outline of a method for manufacturing the optical switch element 51 will be described.
First, an SOI substrate similar to the SOI substrate 15 shown in FIG. 6 of Embodiment 1 is prepared. A metal film 16 to be the upper electrodes 72 and 73 is formed on the SOI substrate.

  Referring to FIG. 13, resist pattern 67 is formed on metal film 16. The pattern shape of the resist pattern 67 corresponds to the pattern shapes of the optical waveguides 2s and 3s. Since the technique used for forming the resist pattern is the same as that in the first embodiment, the description thereof will not be repeated. Further, an optical switch element 51 shown in FIG. 11 is obtained through an etching process substantially similar to that of the first embodiment.

  According to the present embodiment, since the clad layer 9 existing between each of the upper electrodes 72 and 73 and the substrate 8 is an insulator, there is a potential difference between each of the upper electrodes 72 and 73 and the substrate 8. Even if there is, no current flows between each of the upper electrodes 72 and 73 and the substrate 8. Therefore, power consumption in the switching operation is reduced.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The present invention can be applied particularly advantageously to an optical switch element having a semiconductor layer.

It is a top view which shows roughly the structure of the optical switch element in Embodiment 1 of this invention. It is a schematic sectional drawing in alignment with the II-II line of FIG. It is a schematic sectional drawing which shows the thickness distribution of a depletion layer in case the electric field of the optical switch element in Embodiment 1 of this invention is zero. It is a schematic sectional drawing which shows the thickness distribution of a depletion layer in case the electric field of the optical switch element in Embodiment 1 of this invention is one direction. It is a schematic sectional drawing which shows the thickness distribution of a depletion layer in case the electric field of the optical switch element in Embodiment 1 of this invention is another direction. It is a schematic sectional drawing which shows the 1st process of the manufacturing method of the optical switch element in Embodiment 1 of this invention. It is a schematic sectional drawing which shows the 2nd process of the manufacturing method of the optical switch element in Embodiment 1 of this invention. It is a top view which shows roughly the 2nd process of the manufacturing method of the optical switch element in Embodiment 1 of this invention. It is a schematic sectional drawing which shows the 3rd process of the manufacturing method of the optical switch element in Embodiment 1 of this invention. It is a schematic sectional drawing which shows the 4th process of the manufacturing method of the optical switch element in Embodiment 1 of this invention. It is a top view which shows roughly the structure of the optical switch element in Embodiment 2 of this invention. It is a schematic sectional drawing in alignment with the XII-XII line | wire of FIG. It is a schematic sectional drawing which shows 1 process of the manufacturing method of the optical switch element in Embodiment 2 of this invention. It is a top view which shows the structure of the conventional optical switch element roughly. It is a schematic sectional drawing in alignment with the XV-XV line | wire of FIG.

Explanation of symbols

  1, 51 Optical switch element, 2, 2s, 3, 3s Optical waveguide, 4 Optical coupling part, 4a, 4b Directional coupler, 5 Input port, 6, 7 Output port, 8 Substrate, 9 Clad layer, 10, 10s Optical waveguide layer, 11 Upper electrode, 12 Lower electrode, 13 Conductor, 14 Voltage power supply, 15 SOI substrate, 16 Metal film, 18, 67 Resist pattern, 19a, 19b, 20a, 20b, 21a, 21b Depletion layer, 60 arm Part, 72, 73 Upper electrode.

Claims (6)

A substrate,
An insulating layer provided on the substrate;
A semiconductor layer that is provided on the insulating layer and includes a pair of protrusions having at least one region that are parallel to each other with a space therebetween, and having a refractive index that is greater than a refractive index of the insulating layer;
An optical switch element comprising: an electric field applying unit for applying an electric field to a region including at least a part of at least one of the pair of protrusions and at least a part of the insulating layer.   The optical switch element according to claim 1, wherein the electric field applying unit includes an electrode provided on at least a part of at least one of the pair of protrusions.   3. The optical switch element according to claim 1, wherein the electric field application unit includes a voltage generation unit for applying a voltage corresponding to a switching state of the optical switch element to the electrode. The semiconductor layer is made of either a p-type semiconductor and n-type semiconductor, with a least 1 × 10 ¹⁵ cm ^-3 to 1 × 10 ¹⁸ cm ^-3 of impurity concentration, according to any of claims 1 to 3 Optical switch element.   The optical switch element according to claim 1, wherein the electric field applying unit applies an electric field in the at least one region. The at least one region is a pair of regions forming a Mach-Zehnder optical interferometer;
The optical switch element according to claim 1, wherein the electric field applying unit applies an electric field in a region sandwiched between the pair of regions.

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