JP2004258119A - Variable optical attenuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable optical attenuator with which arbitrary optical attenuation is obtained with electric control by using a waveguide comprising a silicon thin line as a core covered with a cladding such as an insulator. <P>SOLUTION: A slab 102 composed of silicon is equipped between a lower cladding layer 101 and an upper cladding layer 103. A core 102a with 0.1-0.5 μm cross sectional dimension is formed by making a part of the slab 102 thicker. An n-type carrier supplying part 104 and a p-type carrier supplying part 105 are disposed on a region of a slab 102 holding the core 102a. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is applied to a planar lightwave circuit and an element in a planar lightwave circuit such as an optical waveguide filter used in the field of optoelectronics and optical communication, which uses silicon built on an SOI substrate as an optical waveguide. And a variable optical attenuator capable of electrically controlling the waveguide light intensity.
[0002]
[Prior art]
In a DWDM (Dense Wavelength Division Multiplexing) system or the like, when an EDFA (Er Doped Fiber Amplifier) is used, an optical signal transmitted through an optical fiber is converted to an arbitrary optical intensity. Must be set. As a variable optical attenuator for attenuating the signal light, a variable optical attenuator using a rib-type waveguide has been put to practical use (see Non-Patent Document 1).
[0003]
This is because a p-type semiconductor region in which a p-type impurity is introduced into a slab sandwiching a rib and an n-type impurity in which an n-type impurity is introduced into a part of a rib-type waveguide formed on an SOI (Silicon on Insulator) substrate. And a PIN-shaped semiconductor region. In this variable optical attenuator, free carriers are generated by passing a forward current through a PIN structure provided in a part of a rib to attenuate signal light guided through the rib.
[0004]
FIG. 4 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, a ridge is formed on a silicon layer to form a portion thicker than the slabs 402 and 403, and the ridge is used as a core 401. By making the core 401 thicker than the slabs 402 and 403 on both sides, the effective refractive index of the core 401 becomes relatively larger than the effective refractive indexes of the slabs 402 and 403, and light is confined in the core 401. Become like
[0005]
The core 401 is formed to have a width of about 4 μm and a thickness of about 4 μm, for example. The slabs 402 and 403 are formed to have a thickness of about 2 μm. Therefore, the ridge 401 is higher than the slabs 402 and 403 by 2 μm. In the rib waveguide configured as described above, the guided light has the maximum intensity in the portion of the core 401 and spreads to the slabs 402 and 403 by about several μm.
[0006]
In the conventional variable optical attenuator shown in FIG. 4, a p-type impurity introduction part 404 and an n-type impurity introduction part 405 are formed in slabs 402 and 403 in a part of the above-mentioned rib waveguide. Metal pads 406 and 407 are connected to the p-type impurity introduction section 404 and the n-type impurity introduction section 405, respectively.
[0007]
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 406 to the metal pad 407, so that the p-type impurity introduction section 404 and the n-type impurity introduction section 405 are applied. Light passing through the sandwiched region can be attenuated. By applying a voltage as described above, holes enter the core 401 from the p-type impurity introduction portion 404 and electrons enter the core 401 from the n-type impurity introduction portion 405, and these carriers propagate through the core 401. By absorbing light, the light propagating through the core 401 is attenuated.
[0008]
The amount of carriers entering the core 401 is 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 where the impurity is introduced absorbs light even when no voltage is applied. Therefore, the p-type impurity introduction part 404 and the n-type impurity introduction part 405 are formed at a predetermined distance from the core 401.
[0009]
On the other hand, in recent years, in order to manufacture an optical integrated circuit with a higher degree of integration, a waveguide having a core of a silicon thin wire whose dimension in the cross-sectional direction is extremely small as 0.2 to 0.5 μm has been developed. I have. This is not to form a core and a cladding layer on the silicon layer on the lower cladding. For example, as shown in the perspective view of FIG. 5, the lower cladding 501 made of an insulator such as silicon oxide or silicon nitride is formed. Then, a core 502 made of a thin silicon wire is formed, and the core 502 is covered with an upper clad 503 made of an insulator such as silicon oxide or silicon nitride.
[0010]
Further, as shown in FIG. 6, there is a waveguide having a structure in which a core 602 is covered with two upper clads 603 and 604. In the waveguide of FIG. 6, the upper cladding 603 is configured such that the difference in the refractive index between the upper cladding 604 and the core 602 is smaller. The waveguide of FIG. 6 is used, for example, for changing the spot size.
In any of the waveguides, by using a silicon thin wire for the core, a smaller optical integrated circuit can be configured than in the case of using the rib-type waveguide described above.
[0011]
[Non-patent document 1]
"Proceedings of the SPIE" The International Society for Optical Engineering. vol4293, p1-9 (2001)
[0012]
[Problems to be solved by the invention]
However, in the above-described waveguide using a silicon thin wire, the waveguide using a silicon thin wire has a cladding layer made of a silicon thin wire and a cladding such as silicon oxide that covers the core, instead of having a cladding 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. Therefore, there is a problem that it is difficult to configure an optical attenuator using the above-described impurity introduction part.
[0013]
Here, by arranging 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 the refractive index between the region into which the impurity is introduced and the core, the region into which the impurity is introduced is disposed in a region where the light intensity is high in the waveguide. As described above, if there is a region in which impurities are introduced in a region where the light intensity is high in the waveguide, light is absorbed and attenuated in this region even when no voltage is applied. In this case, for example, the signal light cannot be propagated without being attenuated.
[0014]
The present invention has been made in order to solve the above-mentioned problems, and a waveguide in which a silicon thin wire is used as a core and this is covered with a clad such as an insulator, and an arbitrary optical attenuation is given by electric control. It is an object of the present invention to provide a variable optical attenuator.
[0015]
[Means for Solving the Problems]
The variable optical attenuator of the present invention is formed by increasing the thickness of a lower clad layer having a lower refractive index than silicon, a slab including a silicon layer formed on the lower clad layer, and a part of the slab. A core, an upper cladding layer formed on the slab covering the core, an n-type carrier supply unit provided near the core of the slab, and facing the n-type carrier supply unit via the core. At least a p-type carrier supply unit provided near the core of the slab, and electrodes respectively connected to the n-type carrier supply unit and the p-type carrier supply unit, wherein the core has a width of 0.2 to 0.5 μm. And a height of 0.2 to 0.5 μm.
In this variable optical attenuator, the core is a silicon thin wire, and in a region between the n-type carrier supply unit and the p-type carrier supply unit, when a forward voltage is applied, the n-type carrier supply unit and the p-type carrier supply unit The carrier is injected into the core from the carrier supply unit.
[0016]
In the above-mentioned variable optical attenuator, by forming the slab to have a film thickness of less than half of the core, the difference in the refractive index between the core and the periphery thereof can be further increased.
In the variable optical attenuator, the cross section of the core may be formed to be substantially square.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a perspective view (a) and a plan view (b) showing a configuration example of a variable optical attenuator according to an embodiment of the present invention. Describing the configuration of this variable optical attenuator, first, a slab 102 made of silicon is provided on a lower clad layer 101 made of an insulating material such as silicon oxide or silicon nitride.
[0018]
The slab layer 102 includes a core 102a having a ridge structure extending in a predetermined direction. The portion of the core 102a is formed to have a width of about 0.2 μm, for example, and the height of the core 102a from the interface of the lower cladding layer 101 is formed to be about 0.2 μm. Therefore, the cross section of the core 102a is formed to be substantially square. The slab layer 102 is formed to have a thickness, for example, half the height of the core 102a. The cross section of the core 102a is not limited to a square.
[0019]
This structure is such that, for example, the silicon (SOI) layer on the buried insulating layer of the SOI substrate is thinned by a general lithography technique such as photolithography or electron beam lithography and an etching technique to leave a thin region other than the core. It can be formed by fine processing. In this case, the buried insulating layer of the SOI substrate becomes the lower cladding layer 101. The pattern formed by processing becomes the core 102a, and the remaining area becomes the slab 102.
[0020]
The core 102a thus formed is covered by the upper cladding layer 103 made of a material having a lower refractive index. Therefore, the core 102a is formed below the lower clad layer 101 and the upper clad layer 103 having a smaller refractive index than the core 102a. In a region other than the core 102a, the slab 102 is sandwiched between the lower cladding layer 101 and the upper cladding layer 103.
[0021]
The upper cladding layer 103 may be made of silicon oxide or silicon nitride, similarly to the lower cladding layer 101. The upper cladding layer 103 may use an organic resin material such as a polyimide resin, an epoxy resin, and a silicone resin. Any material that has low loss due to the material, is easy to design the refractive index, has good compatibility with the fabrication process of optical devices using silicon fine wires, and has little change to the environment It may be used for the cladding layer 103.
[0022]
In the present embodiment, the n-type carrier supply unit 104 and the p-type carrier supply unit 105 are provided in the slabs 102 near both sides of the core 102a in a part of the waveguide around the core 102a in the waveguide direction. And The n-type carrier supply unit 104 and the p-type carrier supply unit 105 are respectively arranged opposite to each other near the core 102a with the core 102a interposed therebetween.
[0023]
The n-type carrier supply unit 104 and the p-type carrier supply unit 105 need only be separated from the core 102a to such an extent that they do not contribute to attenuation of light guided through the waveguide around the core 102a. Further, the n-type carrier supply unit 104 and the p-type carrier supply unit 105 may be formed in a range where carriers can be injected into the core 102a.
[0024]
The n-type carrier supply unit 104 and the p-type carrier supply unit 105 can be easily formed by using a known impurity introduction technique such as an ion implantation method, a diffusion method, and a plasma doping method.
Further, electrodes 106 and 107, which are partially exposed on the upper cladding layer 105, are connected to the n-type carrier supply unit 104 and the p-type carrier supply unit 105, respectively. In FIG. 1B, the electrodes 106 and 107 are omitted.
[0025]
In the variable optical attenuator of FIG. 1, by applying a voltage to the electrodes 106 and 107, carriers are injected into the core 102a from the n-type carrier supply unit 104 and the p-type carrier supply unit 105, and the waveguide composed of the core 102a is formed. The intensity of the guided (propagated) signal light can be attenuated. Further, by changing the magnitude of the voltage applied to the electrodes 106 and 107, the amount of carriers injected from the n-type carrier supply unit 104 and the p-type carrier supply unit 105 into the core 102a also changes. The attenuation can be made variable.
[0026]
In the variable optical attenuator having such a configuration, the slab 102 on which the n-type carrier supply unit 104 and the p-type carrier supply unit 105 are formed has a thickness which is １ ／ of the height of the core 102a. Since the light is thin, the light guided through the waveguide centered on the core 102a does not ooze into the slab 102, and concentrates on the core 102a to form a single mode. Therefore, according to the variable optical attenuator shown in FIG. 1, there is no region where the impurity is introduced in the region where the light intensity is high in the waveguide, and it is possible to propagate the signal light without attenuating. I have.
[0027]
FIG. 2 is a correlation diagram showing the relationship between the value of the relative refractive index difference Δ between the core 102a and the side portion calculated by the equivalent refractive index method and the thickness of the slab 102. FIG. 2 shows a case where the height of the core 102a is 300 nm. As is apparent from FIG. 2, when the thickness of the slab 102 is 150 nm, which is half of the core 102a, the relative refractive index difference Δ is 16%. When the thickness of the slab 102 is reduced, the relative refractive index difference can be increased.
[0028]
For example, when the thickness of the slab 102 is 1/3 of the height of the core 102a, the relative refractive index difference Δ is 25% (75 nm). When the thickness of the slab 102 is 1/6 (50 nm) of the height of the core 102a, the relative refractive index difference Δ is 35% or more. This relative refractive index difference is large enough to realize a steep bending structure that cannot be realized by a quartz-based planar circuit optical element and can be realized by a silicon thin wire. As described above, by setting the thickness of the slab 102 to a small value that is less than half the height of the core 102a, a larger difference in the refractive index can be realized.
[0029]
As described above, according to the variable optical attenuator shown in FIG. 1, a PIN diode type electronically controlled variable optical attenuating operation that fully utilizes the feature of the silicon thin wire that can realize a steep bending structure can be realized. As a result, according to the variable optical attenuator shown in FIG. 1, the optical element can be further miniaturized, and the optical element can be integrated. Further, as compared with the conventional variable optical attenuator shown in FIG. 5, the width of the core is much smaller, so that it is possible to operate at a higher speed.
[0030]
It is needless to say that the configuration of the variable optical attenuator in the present embodiment can be applied to the optical waveguide shown in FIG. 6 in which the silicon thin wire is covered with the two upper claddings. For example, it may be configured as shown in FIG. FIG. 3 is a perspective view (a) and a plan view (b) showing a configuration example of a variable optical attenuator according to another embodiment of the present invention.
[0031]
The variable optical attenuator shown in FIG. 3 will be described. First, a slab 302 made of silicon is provided on a lower clad layer 301 made of an insulating material such as silicon oxide or silicon nitride. The slab layer 302 includes a core 302a having a ridge structure extending in a predetermined direction. The portion of the core 302a is formed to have a width of about 0.2 μm, for example, and the height of the core 302a from the interface of the lower cladding layer 301 is formed to be about 0.2 μm. The slab layer 302 is formed to have a thickness, for example, half the height of the core 302a.
[0032]
The core 302a thus configured is covered with an intermediate cladding layer 313 made of a material having a lower refractive index, and these are covered with an upper cladding layer 303 made of a material having a lower refractive index than the intermediate cladding layer 313. I have. In a region other than the core 30a, the slab 302 is sandwiched between the intermediate cladding layer 313 or the upper cladding layer 303 and the lower cladding layer 301.
[0033]
Also in the variable optical attenuator of FIG. 3, an n-type carrier supply unit 304 and a p-type carrier supply unit 305 are provided on a part of the waveguide centered on the core 302 a, on the slabs 302 on both sides of the core 302 a. Have. The n-type carrier supply section 304 and the p-type carrier supply section 305 are arranged to face each other with the core 302a interposed therebetween. Further, electrodes 306 and 307 which are partially exposed on the upper cladding layer 305 are connected to the n-type carrier supply section 304 and the p-type carrier supply section 305, respectively. In FIG. 3B, the electrodes 306 and 307 are omitted.
[0034]
Also in the variable optical attenuator of FIG. 3, by applying a voltage to the electrodes 306 and 307, carriers are injected into the core 302a from the n-type carrier supply unit 304 and the p-type carrier supply unit 305, and the waveguide formed by the core 302a is formed. The intensity of the guided (propagated) signal light can be attenuated. In addition, by changing the magnitude of the voltage applied to the electrodes 306 and 307, the amount of carriers injected from the n-type carrier supply unit 304 and the p-type carrier supply unit 305 into the core 302a also changes. The attenuation can be made variable.
[0035]
Also in the variable optical attenuator shown in FIG. 3 configured as described above, the slab 302 on which the n-type carrier supply unit 304 and the p-type carrier supply unit 305 are formed has a thickness of １ ／ the height of the core 302a. Since the light is sufficiently thin, the light guided through the waveguide centered on the core 302a does not seep into the slab 302, but concentrates on the core 302a to form a single mode. Therefore, also in the variable optical attenuator shown in FIG. 3, there is no region into which the impurity is introduced in the region where the light intensity is high in the waveguide, and the signal light can be propagated without being attenuated. .
[0036]
By the way, in the above description, the slab is provided over the entire area of the waveguide, but the present invention is not limited to this. The slab was provided only in the region of the variable optical attenuator provided with the p-type carrier supply unit and the n-type carrier supply unit. In other regions, there was no slab and the core was sandwiched between the lower cladding layer and the upper cladding layer. It may have a waveguide structure.
[0037]
Further, in the above description, the cross-sectional dimension of the core is 0.2 μm, but is not limited to this. For example, if the cross section of the core is a substantially square shape having a side of 0.2 to 0.5 μm, single-mode optical confinement (waveguide structure) can be performed without polarization. The cross-sectional shape of the core is not limited to a square, but may be a vertically long or horizontally long rectangle. In this case, the core has a polarization dependency. Therefore, the cross-sectional dimension of the core may be such that the width and height are in the range of 0.2 to 0.5 μm.
[0038]
【The invention's effect】
As described above, in the present invention, a slab made of silicon is provided between the lower clad layer and the upper clad layer, and a part of the slab is thickened to have a cross-sectional dimension of 0.1 to 0.5 μm. A core is formed, and an n-type carrier supply unit and a p-type carrier supply unit are provided in a slab region sandwiching the core.
[0039]
As a result, according to the present invention, the core is a thin silicon wire, and in a region between the n-type carrier supply unit and the p-type carrier supply unit, when a forward voltage is applied, the n-type carrier supply unit and From the p-type carrier supply, carriers are injected into the core.
Therefore, according to the present invention, there is an excellent effect that a variable optical attenuator capable of giving an arbitrary optical attenuation amount by electric control can be realized by a waveguide in which a silicon thin wire is used as a core and this is covered with a cladding such as an insulator. can get.
[Brief description of the drawings]
FIG. 1 is a perspective view (a) and a plan view (b) showing a configuration example of a variable optical attenuator according to an embodiment of the present invention.
FIG. 2 is a correlation diagram showing a relationship between a value of a relative refractive index difference Δ between a core 102a and a side portion of the variable optical attenuator shown in FIG. 1 and a thickness of a slab 102.
FIG. 3 is a perspective view (a) and a plan view (b) showing a configuration example of another variable optical attenuator according to the embodiment of the present invention.
FIGS. 4A and 4B are a plan view and a sectional view showing a configuration of a conventional variable optical attenuator of a rib waveguide.
FIG. 5 is a perspective view showing a configuration of a conventional optical waveguide made of a thin silicon wire.
FIG. 6 is a perspective view showing a configuration of a conventional optical waveguide made of a thin silicon wire.
[Explanation of symbols]
101: lower cladding layer, 102: slab, 102a: core, 103: upper cladding layer, 104: n-type carrier supply unit, 105: p-type carrier supply unit, 106, 107: electrodes.

Claims (3)

A lower cladding layer having a smaller refractive index than silicon;
A slab composed of a silicon layer formed on the lower cladding layer,
A core formed by thickening a part of the slab,
An upper cladding layer formed on the slab over the core,
An n-type carrier supply unit provided near the core of the slab;
A p-type carrier supply unit provided near the core of the slab opposite to the n-type carrier supply unit via the core;
An electrode connected to each of the n-type carrier supply unit and the p-type carrier supply unit;
The variable optical attenuator, wherein the core has a width of 0.2 to 0.5 μm and a height of 0.2 to 0.5 μm. The variable optical attenuator according to claim 1,
The variable optical attenuator, wherein the slab is formed to have a thickness less than half of the core. The variable optical attenuator according to claim 1 or 2,
A variable optical attenuator, wherein a cross section of the core is formed in a substantially square shape.

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