JP3147386B2 - Polarization-independent semiconductor optical switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization independent semiconductor optical switch which will be an important element in an optical information processing system in future optical communication systems.

[0002]

2. Description of the Related Art Optical switches are one of the key elements of future high-speed optical communication systems and optical information processing systems.
R & D is becoming active in various places.
As the optical switch, a switch using a dielectric such as LiNbO _{3 and a} switch using a semiconductor such as GaAs or InP are considered. However, the optical switch is not compatible with other optical elements such as an optical amplifier or an electronic circuit such as EET. In recent years, expectations have been increasing for semiconductor optical switches that can be integrated and that can be easily miniaturized and multi-channeled. Considering the above application fields, such a semiconductor optical switch is required to have a high speed operation, a low power consumption operation, a low voltage operation, a low loss operation, a high degree of integration, an independence on the polarization of incident light, and the like. You. Semiconductor optical switches include a total reflection switch and a Y-branch digital switch using a refractive index change caused by a band-filling effect or a free carrier plasma effect caused by current injection, and a refraction caused by an electro-optic effect caused by applying an electric field. A directional coupler switch using a change in index and a total reflection type switch using a change in refractive index due to a shift of an exciton absorption peak when an electric field is applied to a multiple quantum well are being studied.
Among these methods, the current injection type has the disadvantage that the operation speed is slow and the power consumption is large. On the other hand, the optical switch using the electro-optic effect has an advantage that the element length is longer than that of the total reflection type, but high-speed operation with low power consumption is possible. In particular, the Y-branch digital switch using the electro-optic effect has the advantage that, in addition to the high-speed and low-power operation, the switching characteristics for both TE and TM modes are equalized by increasing the length of the control electrode. are doing.

Conventionally, a current injection type Y-branch digital optical switch has been proposed by K.K. As reported by Ueki et al. In the Proceedings of the IEICE Spring National Convention, 1989, 4-253, polarization-independent switching can be expected, but the drive current was as large as 250 mA. When such a large current flows, the advantage of low power consumption is not only lost, but also
This is a major problem because the advantage of high-speed operation is impaired, and the heat generated by the flow of current causes problems in operation stability and reliability. On the other hand, the directional coupler type optical switch using the electro-optic effect has an advantage that it can operate at high speed and consume low power and has low loss. Regarding low-loss properties, E.I. Kapon
Applied Physics Letters (A) states that a low-loss optical waveguide of 0.15 dB / cm at a wavelength of 1.52 μm can be realized in a GaAs / AlGaAs system.
Applied Physics Letters, Vol. 50, No. 23 (1987).
In addition, since the electro-optic effect has almost no wavelength dependence, even if the operating wavelength is far from the band gap, the change in the refractive index is not different from the case near the band gap. Therefore, the GaAs / AlGaAs optical waveguide is very promising as a long wavelength band directional coupler type switch material. By the way, the conventional directional coupler type switch has an element length of several mm.
There was a problem that it was long. In particular, in a device that switches optical paths, such as a matrix type optical switch, the directional couplers are coupled in several stages, so that the element length of the directional coupler is equal to the length of the entire matrix optical switch in the light incident direction. Was a factor that increased dramatically. For this reason, the matrix optical switch has a very large aspect ratio. Further, when a switching characteristic independent of the TE and TM polarizations is given to the element, there is a problem that the loss increases. As described above, the operation of the conventional semiconductor optical switch is affected by the polarization state of the incident light. Therefore, the conventional semiconductor optical switch has a problem to be solved with respect to a technique for stabilizing the operation regardless of the polarization state.

[0004]

According to the present invention, at least a conductive type semiconductor cladding layer and an undoped semiconductor waveguide are provided on a zinc-blende (111) oriented conductive type semiconductor substrate. A layer, an undoped semiconductor cladding layer, a reverse conductivity type semiconductor cladding layer, and a reverse conductivity type semiconductor cap layer, which are sequentially laminated, and a waveguide mode separator for separating TE and TM modes on the substrate; Two optical switches each utilizing the electro-optic effect and connected to each branch destination of the two optical switches, and two lights for joining each one of the output optical waveguides of the two optical switches to one optical waveguide. A structure of a polarization-independent semiconductor optical switch characterized in that a junction is formed.

According to the present invention, the mode separator includes:
On a sphalerite structure (111) -oriented semiconductor substrate,
Conductivity type semiconductor cladding layer, an undoped semiconductor waveguide layer, an undoped semiconductor cladding layer, reverse-conducting type semiconductor cladding layer, a layer structure in which reverse-conducting semiconductor cap layer are sequentially laminated, or a portion of at least the undoped semiconductor cladding layer total conductivity type semiconductor cladding layer, and a conductive type having a semi <br/> conductive capping layer is an optical waveguide of the rib type formed being removed, the rib-type Y-shaped branched from the optical waveguide 2 A structure of a polarization independent semiconductor optical switch having the present optical waveguide, and having electrodes on the back surface of the two optical waveguides and the semiconductor substrate. type semiconductor substrate, the conductivity type semiconductor cladding layer, an undoped semiconductor waveguide layer, an undoped semiconductor cladding layer, reverse-conducting type semiconductor cladding layer, reverse-conducting semiconductor key -Up layer has a sequentially stacked layer structure, at least an undoped portion or all of the semiconductor clad layer, conductivity type semiconductor cladding layer, and the branch of a conductive type semiconductor capping layer is formed by removing rib optical waveguide by having a rib type 2 present optical waveguides of the Y-shaped, adopting the structure of a polarization-independent semiconductor optical switch, characterized in that it has a 2 present optical waveguide and the rear surface of the semiconductor substrate to an electrode of the rib type ing.

[0006]

Mode separator as one embodiment of a polarization-independent semiconductor optical switch constituted by mode separator of claim 1 in the present invention, when configured by the optical switch according to the optical switch to claim 2 1 is shown in FIG. This polarization independent semiconductor switch has a Y
TE having the same structure as the branch type digital optical switch,
TM mode separator 65, Y-branch type digital optical switch 6 connected to each of the branch destinations of the TE and TM mode separators
4, and an optical multiplexer 63 in which each branch of the Y-branch digital optical switch 64 alternately merges into one optical waveguide. The signal light is transmitted to the TE / TM mode separator 65 1
The undoped semiconductor waveguide layer 4 is guided from the optical waveguide side. An electric field can be applied to an arbitrary optical waveguide by electrodes provided on each optical waveguide of the two optical waveguide portions of the TE and TM mode separators 65. The two optical waveguides generated by the change in the refractive index due to the electro-optic effect can be applied. Asymmetry between the optical waveguides (ie,
Depending on the propagation constant difference), the light can be branched to an arbitrary one of the two optical waveguides. The operation principle of the asymmetric Y-branch optical waveguide will be described below.

As shown in FIG. 3, in an asymmetric Y-branch optical waveguide, for example, two single-mode optical waveguides having different widths (different propagation constants) are connected to one multi-mode optical waveguide 41. Assume. Now, assuming that the fundamental (zero-order) mode 50 is excited in the multimode optical waveguide 31 when the branch angle between the two optical waveguides is very small, the light is transmitted through the optical waveguide 52 having a large width (large propagation constant). To join. When the first mode 51 is excited, the light is coupled to the optical waveguide 43 having a small width (a small propagation constant). Therefore, by using one waveguide as a single mode optical waveguide, it is possible to always couple light to the optical waveguide 42 having a large width (a large propagation constant).

GaAs having a plane orientation of (100)
Electric field (E) in an optical waveguide fabricated on a semiconductor substrate
The change in the refractive index caused by the electro-optic effect due to the application of is Δn _TE = Γ · n _eff ³ · r ₄₁ · E / 2 for the TE mode Δn _TM = 0 for the TM mode, where n _{eff is the} effective refractive index r ₄₁ : electro-optic coefficient Γ: light confinement coefficient E: electric field strength, switching to TM mode could not be performed. However, Ga in which the plane orientation of the crystal is (111)
When the optical mode splitter is manufactured on an As semiconductor substrate, the refractive index change caused by the electro-optic effect is Δn _TE = Γ · n _eff ³ · r ₄₁ · E / (2√3) with respect to the _TE mode. Since Δn _TM = −Γ · n _eff ³ · r ₄₁ · E / √3, a positive refractive index change occurs in the TE mode by applying an electric field in the (111) direction, and the TM mode Since a negative refractive index change occurs, the asymmetric positive between the two waveguides is reversed between the TE and TM modes, and the TE and TM modes are branched to different waveguides. By changing the waveguide to which the electric field is applied, TE, TE,
It is possible to branch the TM mode.

Therefore, the TE / TM mode separator 65
, The TE and TM modes are separated, the Y-branch digital optical switch 64 performs independent switching for each of the TE and TM modes, and the optical multiplexer 63 combines the TE and TM modes again. Thus, polarization-independent optical switching is possible.

FIG. 4 shows a reference example of a polarization-independent semiconductor optical switch according to the present invention.

This polarization-independent semiconductor optical switch has the structure of a directional coupler type optical switch, and has a structure in which the complete coupling length of the TE mode is twice as long as the complete coupling length of the TM mode. A TE / TM mode separator 61; a directional coupler type optical switch 62 connected to a branch destination of the directional coupler type TE / TM mode separator 61; and a directional coupler type optical switch 62 An optical multiplexer 63 is formed in which the branches alternately merge into one optical waveguide. The signal light is guided through the undoped semiconductor waveguide layer 4 from one optical waveguide side of the directional coupler type TE / TM mode separator 61. The directional coupler type TE / TM mode separator 61
Has a structure in which the complete coupling length of the TE mode is twice as large as the complete coupling length of the TM mode, so that both the TE mode and the TM mode can be separated. As described in the above embodiment, the directional coupler type optical switch 62 uses a semiconductor substrate having a crystal plane orientation in the (111) direction, so that the electro-optical switch can be used for both TE and TM modes. It is possible to cause a change in the refractive index due to the effect. Therefore, switching can be performed in the same manner as a normal directional coupler type optical switch. At the same time, switching can be performed independently for both the TE mode and the TM mode, which also has the effect of increasing the degree of freedom in design. As mentioned above,
In the reference example , both the TE mode and the TM mode are separated by the directional coupler type TE / TM mode separator 61.
Polarization-independent optical switching is possible by performing independent switching for each of the TE and TM modes in the directional coupler type optical switch 62 and coupling the TE and TM modes again in the optical multiplexer 63. It is.

[0012]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

FIG. 1 shows a GaAs / AlGaAs according to the present invention.
FIG. 3 is a perspective view showing an embodiment of a polarization-independent semiconductor optical switch using an sY branch optical digital switch. Also,
FIG. 4 is a perspective view showing an embodiment of a polarization-independent semiconductor optical switch using a GaAs / AlGaAs directional coupler type optical switch according to the present invention.

In FIG. 1, n-AlGaAs is formed on an n-GaAs semiconductor substrate 6 having a crystal plane orientation (111).
(X = 0.3-0.5) Conductive semiconductor clad layer 5, G
aAs undoped semiconductor waveguide layer 4, AlGaAs (x =
0.3-0.5) Undoped semiconductor cladding layer 3, p-
An AlGaAs (x = 0.3 to 0.5) reverse conductivity type semiconductor cladding layer 2 and ap ⁺ -GaAs reverse conductivity type semiconductor cap layer 1 are formed.

Here, T having a stripe-shaped rib portion
The E / TM mode separator 65, the Y-branch type digital optical switch 64, and the optical multiplexer 63 are composed of the p ⁺ -GaAs reverse conductivity type semiconductor cap layer 1, p-AlGaAs (x = 0.3 to
0.5) AlGaAs (x = 0.3-0.5) for further electrical isolation from the opposite conductivity type semiconductor cladding layer 2
The waveguide pattern is formed by removing and leaving the waveguide pattern up to a part of the undoped semiconductor cladding layer. On the optical waveguide branched into two of the TE and TM mode separators 65, two of the Y-branch digital optical switches 64 are formed. An electrode 7 is formed on the branched optical waveguide and on the back surface of the n-GaAs semiconductor substrate 6. n-AlGaAs (x = 0.3-0.
5) Conductive semiconductor cladding layer 5, GaAs undoped semiconductor waveguide layer 4, AlGaAs (x = 0.3 to 0.5) undoped semiconductor cladding layer 3, p-AlGaAs (x =
0.3-0.5) Reverse conductivity type semiconductor cladding layer 2, p ⁺ −
The thickness of the GaAs reverse conductivity type semiconductor cap layer 1 is 1 to 2 μm, 0.2 μm, 0.4 μm, 1.0 μm,
It is about 0.2 μm. TE, TM mode separator 65,
The length of one optical waveguide portion of each of the two Y-branch type digital optical switches 64 is about 200 μm, 1 mm, 1 mm, and the width is 2 μm. Also, the branch angles of all Y branches are 2 mrad,
The waveguide width is about 2 μm and the length is about 3 mm. In addition, the interval between the two optical waveguides, each of which is one optical waveguide in the optical multiplexer 63 on the output side, is 100 μm. Next, GaAs /
A method for manufacturing an AlGaAs polarization independent semiconductor optical switch will be briefly described. N-G with crystal plane orientation (111)
An n-AlGaAs (x = 0.3 to 0.5) conductivity type semiconductor cladding layer 5, an GaAs undoped semiconductor waveguide layer 4, and an AlGaAs (x = 0.3) are formed on an aAs semiconductor substrate 6 by MBE or MOVPE. .About.0.5) Undoped semiconductor cladding layer 3, p-AlGaAs (x = 0.3 to
0.5) Reverse conductivity type semiconductor clad layer 2, p ⁺ -GaAs
A reverse conductivity type semiconductor cap layer 1 is sequentially grown. afterwards,
After the electrodes 7 are formed on the entire surface of the substrate, the electrodes 7 are processed into a rib shape having a width of about 2 μm by using a usual photolithography method and an etching method. Thereafter, the masking of the rib portion is performed again by using a normal photolithography method, and the AlGaAs (x = 0.3 to 0.5) undoped semiconductor cladding layer 3 other than the rib portion and the p-AlGaAs (x = 0.3 ~
0.5) Reverse conductivity type semiconductor clad layer 2, p ⁺ -GaAs
The reverse conductivity type semiconductor cap layer 1 is formed of AlGaAs (x = 0.
3 to 0.5) Reactive ion beam etching is performed to reach the undoped semiconductor cladding layer 3. Finally, an electrode 7 is formed on the back surface of the n-GaAs semiconductor substrate 6.

As described above, the plane orientation of the crystal according to the present invention is (11)
1 is an example of a configuration and a manufacturing method of an embodiment of a polarization-independent semiconductor optical switch using a GaAs / AlGaAsY branched optical digital switch, which is 1).

GaAs / Al as a reference example
A polarization-independent semiconductor optical switch using a GaAs directional coupler optical switch is shown in FIG.
1) n-AlGa on the n-GaAs semiconductor substrate 6
As (x = 0.3-0.5) conductivity type semiconductor clad layer 5, GaAs undoped semiconductor waveguide layer 4, AlGaAs
(X = 0.3-0.5) Undoped semiconductor cladding layer 3, p-AlGaAs (x = 0.3-0.5) reverse conductivity type semiconductor cladding layer 2, p ⁺ -GaAs reverse conductivity type semiconductor cap layer 1 Are formed.

Here, a directional coupler type TE / TM mode separator 61 having a stripe-shaped rib portion, a directional coupler type optical switch 62 and an optical multiplexer 63 are composed of p ⁺ -GaAs.
s reverse conductivity type semiconductor cap layer 1, p-AlGaAs (x
= 0.3 to 0.5) AlGaAs (x = 0.3) for further electrical isolation from the opposite conductivity type semiconductor cladding layer 2
.About.0.5) The waveguide pattern is formed by removing and leaving the waveguide pattern up to a part of the undoped semiconductor cladding layer, and is formed on the optical waveguide of the directional coupler type TE / TM mode separator 61 by the directional coupler type light. An electrode 7 is formed on the optical waveguide of the switch 62 and on the back surface of the n-GaAs semiconductor substrate 6. n-AlGaAs (x = 0.3-0.5) conductivity type semiconductor cladding layer 5, GaAs undoped semiconductor waveguide layer 4, AlGaAs (x = 0.3-0.5) undoped semiconductor cladding layer 3, p-AlGaAs (X = 0.3
0.5) Reverse conductivity type semiconductor cladding layer 2, p ⁺ -GaAs
The thickness of the reverse conductivity type semiconductor cap layer 1 is 1 to
2 μm, 0.2 μm, 0.4 μm, 1.0 μm, 0.2
It is about μm. The directional coupler type TE / TM mode separator 61 and the two directional coupler type optical switches 62 are 3 mm, 4 mm and 4 mm in length, 2 μm in width, 1.75 μm in waveguide spacing, and 2. 0 μm and 2.0 μm. The interval between the two optical waveguides, each of which is one optical waveguide in the optical multiplexer 63 on the output side, is 100 μm. Next, a method for manufacturing a GaAs / AlGaAs polarization independent semiconductor optical switch will be briefly described. The MBE method or the M
Using the OVPE method, n-AlGaAs (x = 0.3 to
0.5) Conductive semiconductor clad layer 5, GaAs undoped semiconductor waveguide layer 4, AlGaAs (x = 0.3-0.
5) Undoped semiconductor cladding layer 3, p-AlGaAs
(X = 0.3-0.5) reverse conductivity type semiconductor clad layer 2,
A p ⁺ -GaAs reverse conductivity type semiconductor cap layer 1 is sequentially grown. Then, after the electrode 7 is formed on the entire surface of the substrate, the electrode 7 is processed into a rib shape having a width of about 2 μm by using ordinary photolithography and etching. Thereafter, masking of the rib portion is performed again by using a normal photolithography method, and AlGaAs (x = 0.3 to 0.
5) Undoped semiconductor cladding layer 3, p-AlGaAs
(X = 0.3-0.5) reverse conductivity type semiconductor clad layer 2,
p ⁺ -GaAs reverse conductivity type semiconductor cap layer 1 is formed of AlGa
As (x = 0.3 to 0.5) As far as the undoped semiconductor cladding layer 3, it is removed by a reactive ion beam etching method. Finally, an electrode 7 is formed on the back surface of the n-GaAs semiconductor substrate 6.

The above is an example of the configuration and the manufacturing method of the polarization independent semiconductor optical switch using the GaAs / AlGaAs directional coupler type optical digital switch having the crystal orientation (111) according to the reference technology of the present invention. is there.

Next, the GaAs / AlG according to the present invention will be described.
FIG. 2 shows the optical switching state when the TE and TM modes are incident on the polarization independent semiconductor optical switch using the aAsY branch type optical digital switch. FIG.
(A) shows switching when light in the TE mode 11 is incident. FIG. 2B shows the optical power at the time of switching in the TM mode when the same voltage as in FIG. 2A is applied to each switch. FIG. 2 (a),
As can be seen from (b), light is emitted from the same emission end for any mode, and polarization-independent optical switching is possible by the present invention.

[0021]

As described above, according to the present invention, it is possible to realize a polarization-independent semiconductor optical switch that switches for incident light having an arbitrary polarization state. Note that the present invention is not limited to the above embodiment. As an embodiment, a GaAs-based polarization-independent semiconductor optical switch has been described. However, the present invention is not limited to this, and the present invention is also effective to a device using another material such as an InP-based material. Needless to say, the combination of the mode separator and the optical switch is not limited to the above embodiment.

[Brief description of the drawings]

FIG. 1 is a perspective view showing an embodiment of a polarization-independent semiconductor optical switch using a GaAs / AlGaAsY branched optical digital switch according to the present invention.

FIGS. 2A and 2B are diagrams showing TE mode switching characteristics by optical power of a polarization-independent semiconductor optical switch using a GaAs / AlGaAsY branched optical digital switch according to the present invention, and GaAs / AlGaAs according to the present invention.
FIG. 7B is a diagram showing the switching characteristics of the TM mode by the polarization-independent semiconductor optical switch using the sY branch type optical digital switch in terms of optical power.

FIG. 3 is a diagram showing a switching operation of the asymmetric Y-branch optical waveguide.

FIG. 4 shows GaAs / AlGaAs according to the reference technology of the present invention.
FIG. 2 is a perspective view showing an example of a polarization-independent semiconductor optical switch using an s-directional coupler optical switch.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 reverse conductive type semiconductor cap layer 2 reverse conductive type semiconductor clad layer 3 undoped semiconductor clad layer 4 undoped semiconductor waveguide layer 5 conductive type semiconductor clad layer 6 semiconductor substrate 7 electrode 11 TE mode 12 TM mode 41 multimode optical waveguide 42 Thick optical waveguide 43 Narrow optical waveguide 50 Fundamental (0th-order) mode 51 Primary mode 61 Directional coupler type TE / TM mode separator 62 Directional coupler type optical switch 63 Optical multiplexer 64 Y-branch digital light Switch 65 Y-branch type TE, TM mode separator

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02F 1/31 G02F 1/025 JICST file (JOIS)

Claims (2)

(57) [Claims] 1. A semiconductor cladding layer having at least a conductivity type, an undoped semiconductor waveguide layer, an undoped semiconductor cladding layer,
A reverse conductivity type semiconductor clad layer and a reverse conductivity type semiconductor cap layer are sequentially laminated, a waveguide mode separator for separating a TE mode and a TM mode from each other on the substrate, and each branch destination of the mode separator. Two optical switches each utilizing an electro-optic effect and two optical couplers each of which joins one of the output optical waveguides of the two optical switches to one optical waveguide. Ri Na is formed, the mode separator, Striped zinc blende structure (111) orientation guide
A conductive semiconductor cladding layer on a conductive semiconductor substrate;
Semiconductor waveguide layer, undoped semiconductor cladding layer, reverse conduction
The conductive semiconductor clad layer and the reverse conductive semiconductor cap layer
It has a layer structure that is stacked next to each other, and at least undoped semiconductor
Part or all of the body cladding layer, conductive semiconductor cladding
Layer and the conductive type semiconductor cap layer are removed.
A rib-shaped optical waveguide that is branched from the optical waveguide.
Having two Y-shaped rib-shaped optical waveguides,
A polarization independent semiconductor optical switch having electrodes on an optical waveguide and a back surface of a semiconductor substrate . 2. A sphalerite structure having a (111) orientation half-type conductivity.
On the conductive substrate, at least a conductive semiconductor clad layer,
Undoped semiconductor waveguide layer, undoped semiconductor cladding layer,
Reverse conductivity type semiconductor cladding layer, reverse conductivity type semiconductor cap layer
Are sequentially stacked, and a TE mode and a TM mode are provided on the substrate.
A waveguide-type mode separator for separating the mode from each other;
The electro-optic effect connected to each branch of the
Two waveguide-type optical switches using the light, and the two optical switches
A single optical waveguide is used for each of the output optical waveguides of the switch.
Two optical couplers are formed so as to be joined to each other, and the waveguide type optical switch is provided with a sphalerite structure (111) direction.
A conductive semiconductor clad layer,
Undoped semiconductor waveguide layer, undoped semiconductor cladding layer,
Reverse conductivity type semiconductor cladding layer, reverse conductivity type semiconductor cap layer
Has a layer structure sequentially laminated, at least undoped
Part or all of the semiconductor cladding layer,
And the conductive semiconductor cap layer are removed.
Y-shaped rib branched from the formed rib-type optical waveguide
It has 2 present optical waveguide type, 2 present optical Namiji及of the Li Bed type
Characterized by having electrodes on the back surface of the semiconductor substrate
Independent semiconductor optical switch.