JP3957190B2 - Variable optical attenuator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a variable optical attenuator that can easily attenuate light by applying a voltage.
[0002]
[Prior art]
In a DWDM (Dense Wavelength Division Multiplexing) system or the like, for example, an EDFA (Er Doped Fiber Amplifier) is used to amplify an optical signal propagating in an optical fiber, and an optical fiber. A technique for attenuating the signal light propagating therethrough to an arbitrary light intensity is also required.
[0003]
As a variable optical attenuator for attenuating signal light, one using a rib-type waveguide has been put into practical use (see Non-Patent Document 1). This is because a p-type semiconductor region in which a p-type impurity is introduced so as to sandwich the rib and a n-type impurity introduced into a part of a rib-type waveguide formed on an SOI (Silicon on Insulator) substrate. A semiconductor structure is provided to form a PIN structure. In this variable optical attenuator, a forward current is passed through a PIN structure provided in a part of the rib to generate free carriers and to attenuate signal light guided through the rib.
[0004]
FIG. 7 is a plan view (a) and a cross-sectional view (b) showing the configuration of the conventional variable optical attenuator described above. In this variable optical attenuator, for example, a ridge is formed on a silicon layer 702 formed on a lower clad 701 made of an insulating substrate such as quartz to form a core 703, and a clad layer 704 on both sides of a part of the core 703 is formed. , P-type impurity introduction portion 705 and n-type impurity introduction portion 706 are formed. Metal pads 707 and 708 are connected to the p-type impurity introduction portion 705 and the n-type impurity introduction portion 706, respectively.
[0005]
The core 703 is formed thicker than the cladding layer 704, and the effective refractive index of the core 703 is relatively larger than the effective refractive index of the cladding layer 704. This difference in effective refractive index makes it possible to confine light in the core 703, and the core 703 functions as a waveguide. For example, the core 703 has a width of 4 μm and a height of about 4 μm, and the cladding layer 704 has a thickness of about 2 μm. The portion of the core 703 on the cladding layer 704 has a height of 2 μm. The light guided through such a rib-type waveguide has the maximum intensity at the core 703 and propagates in a state of spreading about several μm in the cladding layer 704.
[0006]
In such a variable optical attenuator having a rib-type waveguide structure, a voltage is applied in a direction in which a current flows from the metal pad 707 to the metal pad 708, whereby the p-type impurity introduction portion 705 and the n-type impurity introduction portion 706 are applied. Light passing through the sandwiched area can be attenuated. By applying a voltage as described above, holes are injected into the core 703 from the p-type impurity introducing portion 705, electrons are injected from the n-type impurity introducing portion 706, and these injected carriers are converted into the core. By absorbing the light propagating through 703, the light propagating through the core 703 is attenuated.
[0007]
The amount of carriers injected into the core 703 becomes an amount corresponding to the magnitude of the voltage, and the amount of attenuation can be varied by varying the applied voltage.
Here, the portion into which the impurity is introduced absorbs light even when no voltage is applied. Therefore, the p-type impurity introduction portion 705 and the n-type impurity introduction portion 706 are formed at a predetermined distance from the core 703.
[0008]
On the other hand, in recent years, in order to fabricate an optical integrated circuit with a further improved degree of integration, a waveguide having a core of a silicon fine wire whose cross-sectional dimension is as small as 0.2 to 0.5 μm has been developed. Yes. This is because the core and the cladding layer are not formed on the silicon layer on the lower cladding, but, for example, as shown in the perspective view of FIG. 8, the upper surface of the lower cladding 801 made of an insulator such as silicon oxide or silicon nitride is used. Further, a core 802 made of a thin silicon wire is formed, and the core 802 is covered with an upper clad 803 made of an insulator such as silicon oxide or silicon nitride. By using the silicon thin wire as the core in this way, it is possible to configure a smaller optical integrated circuit than in the case of using the rib-type waveguide described above.
[0009]
[Non-Patent Document 1]
"Proceedings of the SPIE" The International Society for Optical Engineering.vol4293, p1-9 (2001)
[0010]
[Problems to be solved by the invention]
However, in the above-mentioned waveguide made of silicon thin wire, the waveguide made of silicon thin wire is composed of a core made of silicon fine wire and a clad made of silicon oxide covering the core, instead of a clad layer made of the same material as the core. Therefore, an impurity introduction region for injecting carriers cannot be formed in the cladding around the core. As described above, there is a problem that it is difficult to configure an optical attenuator using the above-described impurity introduction portion using a silicon thin wire.
[0011]
Here, by disposing a silicon layer into which impurities are introduced so as to be in contact with both sides of the core, it is possible to provide a layer for injecting carriers for forming a variable optical attenuator. However, in this case, since there is almost no difference in refractive index between the region into which the impurity is introduced and the core, the region into which the impurity is introduced is arranged in a region with high light intensity in the waveguide. As described above, when a region into which an impurity is introduced exists in a region having a large light intensity in the waveguide, light is absorbed and attenuated in this region even if no voltage is applied. In this case, for example, the signal light cannot be propagated without being attenuated.
[0012]
The present invention has been made to solve the above problems, and is a waveguide in which a silicon fine wire is used as a core and is covered with a clad such as an insulator, and an arbitrary amount of light attenuation is given by electric control. It is an object of the present invention to provide a variable optical attenuator.
[0013]
[Means for Solving the Problems]
A variable optical attenuator according to the present invention includes a lower clad layer, a core made of silicon formed on the lower clad layer, an n-type carrier supply unit in contact with a part of the core, and the n-type carrier supply unit. Formed on the n-type carrier supply unit and the p-type carrier supply unit, the p-type carrier supply unit facing the core opposite to each other, the upper clad layer formed on the lower clad layer so as to cover the core The n-type carrier supply part is made of silicon to which oxygen or nitrogen is added in addition to the n-type impurity, and the p-type carrier supply part is made of oxygen or nitrogen in addition to the p-type impurity. It is made up of added silicon.
In this variable optical attenuator, even in the region where the n-type carrier supply unit and the p-type carrier supply unit are formed, the periphery of the core is made of a material having a lower refractive index, and light is confined in the core. ing.
[0014]
In the variable optical attenuator, the upper clad layer may be made of silicon to which oxygen or nitrogen is added. At this time, the n-type carrier supply unit is a region where n-type impurities are introduced into a part of the upper cladding layer, and the p-type carrier supply unit is a region where p-type impurities are introduced into a part of the upper cladding layer. You may make it be.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[Embodiment 1]
FIG. 1 is a plan view showing a configuration example of a variable optical attenuator according to the first embodiment of the present invention. 2 and 3 are cross-sectional views schematically showing a cross section of the variable optical attenuator shown in FIG. 1, FIG. 2 is a cross-sectional view taken along line AA ′ of FIG. 1, and FIG. 1 shows a cross section taken along line BB ′. Hereinafter, the variable optical attenuator according to the present embodiment will be described with reference to FIGS.
[0016]
In the variable optical attenuator in the present embodiment, first, a lower clad layer 102 made of silicon oxide is provided on a substrate 101 made of single crystal silicon, and a core 103 constituting a waveguide is formed on the lower clad layer 102. Is formed. The core 103 is a thin silicon wire having a cross-sectional view, a width and a thickness of about 0.2 to 0.5 μm.
[0017]
An upper clad layer 104 made of silicon to which oxygen is added (for example, polysilicon) is formed so as to cover the upper portion and both side surfaces of the core 103, and constitutes a waveguide together with the core 103. The refractive index of silicon decreases when oxygen is added (WCLai et.al "Growth and Characterization of PECVD Semi-insulating Polysilicon Films and Resistors" Journal of Electronic Materials, vol.19, p.419-423, 1990.). Therefore, the core 103 is covered with the upper cladding layer 104 having a lower refractive index than the core 103.
[0018]
In addition, in the variable optical attenuator of the present embodiment, the p-type carrier supply unit 105 into which the p-type impurity is introduced and the n-type impurity are introduced into the upper clad layer 104 which is a part of the waveguide. An n-type carrier supply unit 106 is formed so as to sandwich the core 103. A p-side electrode 107 is provided in contact with the p-type carrier supply unit 105, and an n-side electrode 108 is provided in contact with the n-type carrier supply unit 106.
The p-type carrier supply unit 105 can be formed by introducing an impurity such as boron into a part of the upper cladding layer 104 by, for example, an ion implantation method. Similarly, the n-type carrier supply unit 106 can be formed by introducing phosphorus, arsenic, or the like by, for example, ion implantation.
[0019]
According to the variable optical attenuator shown in FIGS. 1, 2, and 3 having the above-described configuration, the p-type carrier supply unit 105, the core 103, and the n-type carrier are supplied to a partial region of the waveguide centering on the core 103. A PIN diode structure including the portion 106 is provided. Therefore, by applying a forward voltage between the p-side electrode 107 and the n-side electrode 108, electrons and holes can be injected into the core 103, which is a silicon thin wire sandwiched between them.
[0020]
As described above, by applying a voltage in the forward direction, holes are injected into the core 103 from the p-type carrier supply unit 105 and electrons are injected from the n-type carrier supply unit 106, and these are injected. The carrier absorbs the light propagating through the core 103 to attenuate the light propagating through the core 103. Further, the amount of carriers injected into the core 103 from the p-type carrier supply unit 105 and the n-type carrier supply unit 106 also changes by changing the magnitude of the voltage applied to the p-side electrode 107 and the n-side electrode 108. In addition, the above-described light attenuation can be made variable.
[0021]
In addition, since the p-type carrier supply unit 105 and the n-type carrier supply unit 106 made of silicon to which oxygen is added have a refractive index smaller than that of the core 103, light is confined in the core 103 even in these regions. It has become. In other words, in the variable optical attenuator in the present embodiment, the p-type carrier supply unit 105 and the n-type carrier supply unit 106 that supply carriers to the core 103 are arranged outside the region where the light intensity is high. As a result, in this variable optical attenuator, the guided light is transmitted by the p-type carrier supply unit 105 and the n-type carrier supply unit 106 in a state where no voltage is applied to the p-side electrode 107 and the n-side electrode 108. Attenuation is suppressed.
[0022]
[Embodiment 2]
Next, a second embodiment of the present invention will be described. FIG. 4 is a plan view showing a configuration example of the variable optical attenuator in the present embodiment. 5 and 6 are cross-sectional views schematically showing a cross section of the variable optical attenuator shown in FIG. 4, FIG. 5 shows a cross section taken along line AA ′ of FIG. 4, and FIG. 4 shows a cross section taken along line BB ′. Hereinafter, the variable optical attenuator in the present embodiment will be described with reference to FIGS. 4, 5, and 6.
[0023]
In the variable optical attenuator according to the present embodiment, first, a lower clad layer 202 made of silicon oxide is provided on a substrate 201 made of single crystal silicon, and a core 203 constituting a waveguide is formed on the lower clad layer 202. Is formed. The core 203 is a thin silicon wire having a cross-sectional view, a width and a thickness of about 0.2 to 0.5 μm.
[0024]
Further, in the variable optical attenuator of the present embodiment, a p-type carrier supply unit 205 made of silicon containing oxygen and p-type impurities introduced into a partial region of the core 203, and an oxygen-containing and n-type The n-type carrier supply unit 206 made of silicon into which the impurity is introduced is formed so as to sandwich the core 203. Note that a p-side electrode 207 is provided in contact with the p-type carrier supply unit 205, and an n-side electrode 208 is provided in contact with the n-type carrier supply unit 206.
[0025]
Further, for example, an upper clad layer 204 made of polyimide having a refractive index lower than that of silicon is formed so as to cover the core 203, and constitutes a waveguide together with the core 203. The upper clad layer 204 is not limited to the above polyimide, but a material having a lower refractive index than silicon, such as inorganic materials such as silicon oxide (SiO ₂ ) and silicon oxynitride (SiON), and organic materials such as epoxy and silicone resin. The same applies to the use of. In the above-described embodiment, the upper clad layer and each carrier supply unit are made of silicon containing oxygen. However, in this embodiment, the upper clad layer and each carrier supply unit are made of different materials. Consists of.
[0026]
Here, as shown in FIG. 5, the upper cladding layer 204 is also formed in a region where the p-type carrier supply unit 205 and the n-type carrier supply unit 206 are formed. In a region sandwiched between the p-type carrier supply unit 205 and the n-type carrier supply unit 206 of the core 203, at least a portion exposed from the p-type carrier supply unit 205 and the n-type carrier supply unit 206 of the core 203 is the upper cladding. Covered by layer 204. 4 and 5 show an example in which the upper clad layer 204 is formed so as to cover a part of the p-type carrier supply unit 205, the n-type carrier supply unit 206, the p-side electrode 207, and the n-side electrode 208. ing.
[0027]
According to the variable optical attenuator shown in FIGS. 4, 5, and 6 having the above configuration, the p-type carrier supply unit 205, the core 203, and the n-type carrier are supplied to a partial region of the waveguide centering on the core 203. A PIN diode structure including the portion 206 is provided. Therefore, by applying a forward voltage between the p-side electrode 207 and the n-side electrode 208, electrons and holes can be injected into the core 203, which is a silicon thin wire sandwiched between them.
[0028]
As described above, by applying a voltage in the forward direction, holes are injected from the p-type carrier supply unit 205 and electrons are injected from the n-type carrier supply unit 206 to the core 203. The carrier absorbs the light propagating through the core 203 to attenuate the light propagating through the core 203. Also, by changing the magnitude of the voltage applied to the p-side electrode 207 and the n-side electrode 208, the amount of carriers injected into the core 203 from the p-type carrier supply unit 205 and the n-type carrier supply unit 206 also changes. In addition, the above-described light attenuation can be made variable.
[0029]
In addition, since the p-type carrier supply unit 205 and the n-type carrier supply unit 206 made of silicon to which oxygen is added have a refractive index smaller than that of the core 203, light is confined in the core 203 even in these regions. It has become. In other words, in the variable optical attenuator according to the present embodiment, the p-type carrier supply unit 205 and the n-type carrier supply unit 206 that supply carriers to the core 203 are arranged outside the region where the light intensity is high. As a result, in this variable optical attenuator, the guided light is transmitted by the p-type carrier supply unit 205 and the n-type carrier supply unit 206 in a state where no voltage is applied to the p-side electrode 207 and the n-side electrode 208. Attenuation is suppressed.
[0030]
In the above description, the p-type carrier supply unit and the n-type carrier supply unit are formed using silicon to which oxygen is added. However, the p-type carrier supply unit and the n-type carrier using silicon to which nitrogen is added. The same applies if the carrier supply unit is formed. By adding nitrogen to silicon, the refractive index can be reduced. Further, silicon to which nitrogen is added may be used for the upper clad layer. Further, silicon to which oxygen or nitrogen is added may be single crystal silicon, polycrystalline silicon, or amorphous silicon.
[0031]
【The invention's effect】
As described above, in the present invention, the n-type carrier supply unit is made of silicon to which oxygen or nitrogen is added in addition to the n-type impurity, and the p-type carrier supply unit is added to the p-type impurity in addition to oxygen or oxygen. Since it is made of silicon to which nitrogen is added, even in the region where these are formed, the periphery of the core is made of a material having a lower refractive index, and light is confined in the core.
As a result, according to the present invention, it is possible to provide a variable optical attenuator that can provide an arbitrary amount of optical attenuation by electrical control in a waveguide in which a silicon thin wire is used as a core and is covered with a clad such as an insulator. Is obtained.
[Brief description of the drawings]
FIG. 1 is a plan view showing a configuration example of a variable optical attenuator in an embodiment of the present invention.
FIG. 2 is a cross-sectional view schematically showing a cross section of the variable optical attenuator shown in FIG.
3 is a cross-sectional view schematically showing a cross section of the variable optical attenuator shown in FIG. 1. FIG.
FIG. 4 is a plan view showing a configuration example of a variable optical attenuator according to another embodiment of the present invention.
5 is a cross-sectional view schematically showing a cross section of the variable optical attenuator shown in FIG. 4. FIG.
6 is a cross-sectional view schematically showing a cross section of the variable optical attenuator shown in FIG. 5. FIG.
7A and 7B are a plan view and a cross-sectional view showing the configuration of the conventional variable optical attenuator described above.
FIG. 8 is a perspective view schematically showing a waveguide configuration using silicon fine wires.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 101 ... Substrate, 102 ... Lower clad layer, 103 ... Core, 104 ... Upper clad layer, 105 ... P type carrier supply part, 106 ... N type carrier supply part, 107 ... P side electrode, 108 ... N side electrode

Claims (3)

A lower cladding layer;
A core made of silicon formed on the lower cladding layer;
An n-type carrier supply unit in contact with a part of the core;
A p-type carrier supply unit which is opposed to the n-type carrier supply unit via the core and is in contact with the core;
An upper cladding layer formed on the lower cladding layer so as to cover the core;
At least an electrode formed on each of the n-type carrier supply unit and the p-type carrier supply unit,
The n-type carrier supply unit is composed of silicon to which oxygen or nitrogen is added in addition to n-type impurities,
The p-type carrier supply unit is composed of silicon to which oxygen or nitrogen is added in addition to p-type impurities ,
The variable optical attenuator, wherein the n-type carrier supply unit and the p-type carrier supply unit confine light guided in a core having a refractive index smaller than that of the core . The variable optical attenuator according to claim 1, wherein
The variable optical attenuator, wherein the upper clad layer is made of silicon to which oxygen or nitrogen is added. The variable optical attenuator according to claim 2, wherein
The n-type carrier supply part is a region where an n-type impurity is introduced into a part of the upper clad layer,
The variable optical attenuator, wherein the p-type carrier supply unit is a region where a p-type impurity is introduced into a part of the upper clad layer.

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