JPH1152439A - Optical switch - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable optical switching of high repetition at an ultra-high speed by setting the wavelengths of control light and signal light in such a manner that a reverse refractive index fluctuation is attained with absorption saturation by using the saturation of the inter-sub-band absorption in quantized wells as principle. SOLUTION: The nonlinear optical response of the inter-sub-band absorption region 2 on a sapphire region 2 is used as the switch principle. The signal light is introduced into the inter-sub-band absorption region 2 from the end face of the substrate 1 via an optical waveguide 3. On the other hand, the control light pulse is introduced from the end face of the substrate 1 via an optical waveguide 41 (42 ) into the inter-sub-band absorption region 2. Two pieces of nonlinear optical waveguides 51 , 52 are formed in the inter-sub-band absorption region 2 with a slight deviation from the perpendicular direction with respect to an incident surface in such a manner that these waveguides are mode coupled near the input part but are separated in the output part. The outputs of the nonlinear optical waveguides 51 , 52 are outputted outside via respective output optical waveguides 61 , 62 . At this time, the wavelengths of the control light and the signal light are so set that the refractive index fluctuation by the absorption saturation is reversed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical switch of an optical control type, and more particularly to an ultra-high-speed digital optical switch utilizing transition between subbands in a semiconductor quantum well.

[0002]

2. Description of the Related Art Optical time division multiplexing (OTDM) technology and optical frequency division multiplexing (OFDM) in the coming multimedia era
In terabit optical networks that make full use of technology,
Node processing, such as signal light routing, is expected to become a technology bottleneck. In order to realize large-capacity data processing that cannot be performed by conventional electrical processing using optical nodes, it is urgently necessary to develop an ultra-high-speed and high-efficiency optically controlled optical routing switch.

As ultra-high-speed optical switches currently under research and development, there are optical switches using the nonlinearity of an optical fiber such as a non-linear loop mirror (NOLM), and a Machtz shown in FIG. There is a semiconductor optical switch of the ender interferometer type. These are basically interferometers. By shifting the phase of one of the two split signal lights by π by a nonlinear optical effect due to a strong control light pulse, the split of the signal light output from the output coupler is changed. It is to switch.

FIG. 17 is a diagram schematically illustrating a control light input power and a signal light output of such an interferometer type optical switch. Since it is an interferometer type, the optical input / output characteristics basically have periodicity, and the control light power needs to be adjusted to an appropriate operating point to shift the phase of the signal light by π. However, since the operating point fluctuates sensitively due to fluctuations in the environmental temperature, the control light power, the coupling coefficient, and the like, it is difficult to maintain a desirable optical switching operation unless a complicated stabilizing circuit or the like is added.

[0005] On the other hand, such a high-speed operation is not possible with an electrically controlled optical switch, but a digital optical switch (DOS) that is resistant to fluctuations in the operating point has been realized. FIG.
FIG. 2 is a diagram schematically showing the configuration. A plurality of electrodes are formed on the branch optical waveguide that is gradually divided into two, and the refractive index distribution of the branch optical waveguide is made uneven by reverse bias or current injection, so that the signal light has a relatively high refractive index. It is designed to be switched to a branch.

[0006] However, in the ultra-high-speed light control type, a satisfactory DOS has not been realized due to the following problems. That is, when the refractive index is reduced by the control light, the control light beam is dispersed in the cross section of the optical waveguide, so that good switching cannot be performed. When the refractive index is increased by the control light, stable optical waveguide can be achieved by the self-focusing effect. However, since the refractive index also increases for the signal light, it is difficult to separate the high-power control light from the low-power control light. Further, since the propagation characteristic of the signal light greatly changes due to the control light pulse waveform (change due to power fluctuation and temperature fluctuation), stable output signal light cannot be obtained.
Also, large wavelength chirping occurs in the signal light due to the cross-phase modulation.

[0007]

As described above, in the conventional optical switch of the optical control type capable of operating at an ultra-high speed, since the propagation characteristics of the signal light greatly change due to the temperature fluctuation and the control light power fluctuation, a stable output signal is obtained. There was a problem that light could not be obtained.

The present invention has been made in view of the above circumstances, and an object of the present invention is to enable ultra-high-speed and high-speed repetitive optical switching, and to prevent temperature fluctuation and control light power fluctuation. An object of the present invention is to provide a digital optical switch that is stable with respect to the digital optical switch.

[0009]

[Means for Solving the Problems]

(Constitution) The gist of the present invention is to use the saturation of the intersubband absorption in the quantum well as the switching principle and to set the wavelengths of the control light and the signal light so that the refractive index fluctuation due to the absorption saturation is reversed. is there.

That is, the present invention relates to an optical switch of an optical control type, in which a semiconductor quantum well layer having a plurality of sub-bands in a conduction band is provided therein, and the mode-coupling input portion gradually increases from an input portion to an output portion. A non-linear optical waveguide that is configured to branch into two, and that causes absorption saturation between subbands due to the incidence of strong light, and a signal light having a wavelength whose refractive index decreases due to the saturation of absorption between the subbands. Means for inputting to the input portion of the nonlinear optical waveguide, means for injecting control light having a wavelength at which the refractive index increases due to saturation of the intersubband absorption into the nonlinear optical waveguide so as to be biased to one side, and Means for outputting signal light from one of the two branches of the output section of the wave path.

Here, preferred embodiments of the present invention include the following. (1) The semiconductor quantum well layer is made of a group III-V semiconductor containing nitrogen. (1a) The semiconductor quantum well layer is composed of a GaN well layer and Al _x Ga
_1-x N barrier layer (1 ≧ x ≧ 0.7). (1b) All or a part of a layer constituting the semiconductor quantum well layer is doped with an n-type impurity such that electrons of 3 × 10 ¹⁸ cm ⁻³ or more are accumulated in the moving layer.

(2) The nonlinear optical waveguide has a gain for signal light. (2a) An active layer containing at least indium, gallium, and arsenic as a constituent element is formed in a part of the nonlinear optical waveguide, and a means for injecting current into the active layer is provided. (2b) In the configuration of (2a), the quantum well layer contains nitrogen
It is made of a III-V semiconductor and is formed close to the active layer. (2c) In the configuration of (2a), a current flowing in the active layer does not flow in the quantum well layer.

(3) The nonlinear optical waveguide is formed of two optical waveguides, and each input section is mode-coupled in close proximity, but each output section is separated. (4) The nonlinear optical waveguide is formed of one Y-branch waveguide on the input side and two Y-branch waveguides on the output side. Signal light is input from the input side, and control light is input from the output side.

(Function) In the present invention, the portion of the nonlinear optical waveguide through which the control light propagates has an increased refractive index for the control light but a reduced refractive index for the signal light. Only the signal light is switched to the branch where the control light does not propagate. Since this switching is stable with respect to a control light input having a predetermined power or more, digital optical switching that is resistant to temperature fluctuations and control light power fluctuations is realized.

In addition, since the power distribution of the signal light and the control light have a small overlap, the propagation characteristics of the signal light are less affected by the power change of the control light. Further, since the signal light and the control light propagate through different branches and have different wavelengths, it is easy to separate the signal light and the control light. Furthermore, since the switching principle is based on the transition between sub-bands, a very high-speed and high-repetition switching operation is possible.

If the semiconductor quantum well layer is made of a group III-V semiconductor containing nitrogen having a large discontinuity in the conduction band, it becomes possible to absorb short wavelengths between sub-bands. Optical switches that operate in important near-infrared wavelength bands are possible.

Further, since the nitride semiconductor quantum well also has a short inter-subband relaxation time, a high-speed response (≧ 1 Tb / s) is possible. Further, in the nitride semiconductor quantum well, the carrier density can be increased (10 ¹⁹ cm ⁻³ ). By setting the carrier density to 3 × 10 ¹⁸ or more, large nonlinear light absorption can be realized.

If the nonlinear optical waveguide has a gain with respect to the signal light, it is possible to compensate for the attenuation of the signal light due to intersubband absorption or the like, and it is possible to obtain an output not less than the input light. .

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. (First Embodiment) FIG. 1 is a diagram schematically showing a configuration of a main part of an optical switch according to a first embodiment of the present invention as viewed from above. FIGS. 2, 3, and 4 correspond to FIGS.
XX ′ section, YY ′ section, ZZ ′ of the optical switch of FIG.
It is a figure which shows a cross section typically. Note that the configuration of the module including the signal input / output unit is the same as that of the related art, and a description thereof will be omitted.

The optical switch of the present embodiment uses the nonlinear optical response of the intersubband absorption region 2 formed on the sapphire substrate 1 as a switching principle. The signal light is guided from the end face of the substrate 1 to the inter-subband absorption region 2 via the optical waveguide 3. On the other hand, the control pulse is led to intersubband absorption region 2 through the light guide 4 ₁ or 4 _2. In the intersubband absorption region 2, two nonlinear optical waveguides 5 ₁ ,
5 _2, in the mode coupling to the output portion proximate the input unit so as to separate, is formed slightly deviated from a direction perpendicular to the plane of incidence. The output of the nonlinear optical waveguide 5 _1, 5 ₂ are outputted to the outside through respective output optical waveguides 6 _and 62.

The portion other than the inter-subband absorption region 2 includes an undoped AlN lower cladding layer 7 and an undoped AlGa
The optical waveguide structure has an N core layer 8 and an undoped AlN upper cladding layer 9. Layers not directly related to the operation principle of the present invention, such as a low-temperature growth buffer layer necessary for forming a good nitride semiconductor layer on the sapphire substrate 1, are not described.

The optical waveguide 3 and 4. ₁ by making a ridge in the upper cladding layer 9, 4 _2, 6 _1, 6 ₂ are formed. On the other hand, the intersubband absorption region 2, a multiple quantum well nonlinear optical waveguide layer 10 made of is formed, the non-linear optical waveguide 5 ₁ by ridges formed similarly to the upper cladding in place of the core layer 8, 5 ₂ Is stipulated. The optical electric field is guided in the vertical direction so that a part thereof leaks into the cladding layers 7 and 9 centering on the core layer 8 and the multiple quantum well layer 10, and the ridge has a high equivalent refractive index in the horizontal direction. Is guided around.

The butt-coupling portions of the input optical waveguides 3, 4 ₁ , 4 ₂ and the nonlinear optical waveguides 5 ₁ , 5 ₂ are arranged so that the signal light introduced via the optical waveguide 3 is used for the two nonlinear optical waveguides 5 ₁ , 5 2.
_{2 so as} to be almost equal to each other, and the optical waveguides 4 ₁ ,
4 ₂ Most introduced is controlled light through and is configured to couple to a nonlinear optical waveguide 5 _1, 5 ₂ corresponding respectively.

In this embodiment, the tip of the optical waveguide 3 is tapered so that the signal light is efficiently coupled to the _two nonlinear waveguides 5 ₁ and 5 ₂ . The optical waveguides are configured so as to exude to substantially the same extent and be guided by two adjacent optical waveguides 4 ₁ and 4 ₂ .
The coupling coefficient between the nonlinear waveguides 5 ₁ , 5 ₂ and the output waveguides 6 ₁ , 6 ₂ is the same for both branches, so that the signal light is not input when the control light is not input. 2
The light is output almost equally from the two output optical waveguides 6 ₁ and 6 ₂ .

FIG. 5 is a conduction band diagram showing the optical waveguide structure of the intersubband absorption region 2 in the direction normal to the substrate. The nonlinear optical waveguide layers 10 sandwiched between the lower cladding layer 7 and the upper cladding layer 9 in the intersubband absorption region 2 are arranged in order from the bottom.
Undoped Al _0.7 Ga _0.3 N optical guide layer 11 ₁ , undoped AlN layer 12 ₁ , 1.56 nm (6 monolayer) n-type (carrier density 3 × 10 ¹⁸ cm ⁻³ ) GaN well layer 13 and thickness of about 2 nm (8 monolayers) a multiple quantum well layer (20 wells) composed of an undoped AlN barrier layer 14, an undoped AlN layer 12 ₂ , and an undoped Al
And a _0.7 Ga _0.3 N light guide layer 11 _2.
Two subbands 15 and 16 are formed in the conduction band of the quantum well, and the transition wavelength thereof is 1.49 μm (the energy difference is 0.83 eV).

FIG. 6 is a conduction band diagram showing an optical waveguide structure in a region other than the inter-subband absorption region 2 in the direction normal to the substrate. Core layer 8 in portion other than inter-subband absorption region 2
Is undoped AlN layer 12 ₁ in the nonlinear optical waveguide layer 10, GaN well layer 13, AlN barrier layer 14, and an undoped AlN layer 12 ₂ All undoped Al _0.5 G
a _0.5 N layer 17 has been substituted.

FIG. 7 is a diagram (dispersion curve of the sub-band in a plane parallel to the well) for explaining the inter-sub-band absorption and its relaxation process. The conduction band discontinuity of GaN and AlN is 2
Since it is close to eV, there is no concern that the electrons excited in the second sub-band 16 due to the inter-subband absorption leak out of the quantum well. When no light is incident, most of the electrons are distributed toward the bottom of the first sub-band 15. When short pulse light having a polarization component in the well normal direction and having a wavelength that resonates with the transition between sub-bands is incident, some of the electrons are instantaneously excited by the second sub-band 16 due to the absorption between the sub-bands. . In the following description, it is assumed that all light is in the TM mode polarized in the direction perpendicular to the well.

On the field base where the intensity of the incident light is strong, an instantaneous absorption saturation occurs due to a decrease in the number of electrons distributed in the first sub-band 15. This operation occurs even at room temperature or at a temperature higher than room temperature.

The electrons excited in the second sub-band 16 are relaxed to the high energy state of the first sub-band 15 by the interaction with the LO phonon (about 90 meV). In the case of a GaN well layer having inter-subband absorption in this wavelength band, its relaxation time is on the order of 100 fs. As a result, non-equilibrium electrons having a large energy are generated in the first sub-band 15, but due to the interaction between the electrons, the thermal equilibrium Fermi distribution (the Fermi level and the carrier temperature are higher than the initial state) with a time constant of about 20 fs. )
To relax. The high-energy electrons release LO phonons (about 90 meV) and relax to the original low-energy state.

When a large amount of non-equilibrium LO phonons are generated in this process, some electrons re-absorb the LO phonons, so that picosecond order tailing occurs in the energy relaxation in the sub-band. However, since the electrons once relaxed to the first sub-band 15 relax to the thermal equilibrium distribution in a short time of about 20 fs, the probability of returning to the second sub-band 16 by LO phonon reabsorption is low. Also, 2
Since the dispersion curves of the two sub-bands 15 and 16 are substantially parallel, the saturation of absorption is substantially determined by the number of electrons in the first sub-band 15, and hardly depends on the wave number (kinetic energy) that the electrons take in the sub-band.

Further, the forbidden band width of GaN is as wide as about 3.5 eV, and no inter-band absorption, including multiphoton absorption, occurs for light having a wavelength of 1.55 μm. Therefore, the time constant of the intersubband absorption saturation due to the strong light pulse is substantially determined only by the intersubband relaxation time. In other words, the absorption returns to the original state with a time constant of the order of 100 fs after the disappearance of the pulse light.

FIG. 8 is a diagram for explaining the intersubband absorption spectrum and the dispersion spectrum of the equivalent refractive index. The solid line in FIG. 8 indicates the TM mode light having an electric field component perpendicular to the well. It shows an absorption spectrum and a dispersion spectrum of an equivalent refractive index based on the absorption spectrum. The broken line in the figure
It shows the wavelength dependence of absorption and refractive index when strong light is incident. When the absorption between the sub-bands is saturated, the refractive index decreases on the long wavelength side of the absorption peak wavelength, and the refractive index increases on the short wavelength side. During such a short time (〜100 fs), the movement of the electrons can be ignored, so that the refractive index of only the portion through which light passes changes.

Here, the wavelength of the control light is set to 1.45 μm, which is the short wavelength side of the absorption peak wavelength between subbands.
It is assumed that the wavelength of the signal light is set to 1.55 μm, which is the long wavelength side of the absorption peak between subbands. Both wavelengths are within the frequency band affected by intersubband absorption and its saturation.

Now, the signal light enters from the optical waveguide 3,
Consider the case where it is incident on the nonlinear optical waveguide 5 ₁ simultaneously strong control optical pulse (pulse width 100 fs) via an optical waveguide 4 _1. Since the refractive index with respect to the control light of the nonlinear optical waveguide 5 ₁ rises due to strong control light, the control light pulse without substantially binding to the other nonlinear optical waveguide 5 ₂ propagates through the nonlinear optical waveguide 5 _1, the output optical waveguide emitted from 6 _1. This control light can be easily blocked by a wavelength filter provided outside.

On the other hand, the control light is a refractive index relative to the nonlinear optical waveguide 5 ₁ of the signal light propagating decreases, the power of the signal light pushes towards the relatively high refractive index nonlinear optical waveguide 5 ₂ Will be done. Therefore, after propagating a sufficient length, most of the signal light power nonlinear optical waveguide 5 ₂
Go, the signal light is outputted from only the output optical waveguides 6 _2.

FIG. 9 shows the relationship between the control light intensity and the output signal light intensity. The solid line shows the output of the branch where the control light propagates, and the broken line shows the output of the branch where the control light does not propagate. Nonlinear optical waveguide 5 _1, 5 ₂ of 150μm length, nonlinear optical waveguide 5 _1, 5 ₂ of 0.5μm intervals of the input unit, and 1.5μm spacing nonlinear optical waveguide 5 _1, 5 ₂ of the output unit Then, a change in the refractive index of about 0.3% is sufficient, and the energy of the control light pulse required for switching is about 10 pJ.
If the control light is strong than this well, a situation where a relatively high refractive index signal light to the nonlinear optical waveguide 5 ₂ is forced because is maintained, the signal light is output only from the output optical waveguide 6 _2. Also, since the overlap between the signal light and the control light is small,
The fluctuation of the signal light propagation characteristics (intensity, power distribution, phase, etc.) depending on the control light pulse waveform is small.

Therefore, digital optical switching which is hardly affected by fluctuations in control light pulse energy and environmental temperature, which cannot be realized by the interferometer type or the non-linear directional coupler type, is realized.

[0038] Similarly, when the control pulse is inputted only from the optical waveguide 4 _2, the signal light is outputted from the output optical waveguide _61. When the control light pulse is input to the _two waveguides 4 ₁ and 4 ₂ at the same time, the refractive index changes of the _two branches 5 ₁ and 52 are the same, so that both the output optical waveguides 6 ₁ and 6 ₂
Output signal light having substantially the same power.

As described above, the time constant of absorption saturation is of the order of 100 fs, which is the time constant of relaxation between subbands.
Since the carrier distribution of the two subbands 15 and 16 returns to the original state within 1 ps, the pattern effect (past history dependence) does not occur even in the ultra-high-speed repetition operation of 1 terabit per second. A control light pulse energy of about 10 pJ is sufficient. Therefore, optical digital switching with ultra-high speed repetition and low switching energy is realized. Of course, this optical switch operates even at a temperature higher than room temperature.

(Second Embodiment) FIG. 10 is a diagram schematically showing the configuration of an optical switch according to a second embodiment of the present invention. The optical switch according to the present embodiment is Si switch
Two planar optical circuits (PLCs) 21 fabricated on a substrate
_1, made as 21 ₂ and the nitride is formed on a sapphire substrate a semiconductor optical integrated circuit 22 of the main unit. As in the description of the first embodiment, the description of the configuration of the input / output fiber and the module is omitted.

The nitride semiconductor optical integrated circuit 22 has the same structure as the inter-subband absorption region 2 in the first embodiment.
N / Al (Ga) N multiple quantum well layer is undoped Al
It has a layer structure sandwiched between (Ga) N cladding layers. GaN
The / Al (Ga) N multiple quantum well layer is doped n-type, and its intersubband absorption wavelength is around 1.5 μm.

[0042] PLC 21 _1, the optical waveguide 24 for introducing control light optical waveguide 23 for introducing the signal light into the nitride semiconductor light integrated circuit 22 is formed. In the nitride semiconductor optical integrated circuit 22, two asymmetric nonlinear optical waveguides 25 ₁ and 25 ₂ are formed. Optical waveguide 23
Some the output of the primarily bound to the non-linear optical waveguide 25 ₁ to bind to the non-linear optical waveguide 25 _2, output as is most bind only to the non-linear optical waveguide 25 ₁ of the optical waveguide 24 is butt connected ing.

The non-linear optical waveguides 25 ₁ and 25 ₂ are formed in parallel and close proximity to each other at the incident portion and are coupled to each other, but branch off at a small angle from the middle and become independent optical waveguides at the output portion. . The non-linear optical waveguide 25 ₁ is formed wider than the non-linear optical waveguide 25 ₂ , and when a strong control light pulse is not incident, the non-linear optical waveguide 25 ₁ having a relatively large equivalent refractive index is propagated as the signal light propagates. moves toward the, output from the nonlinear optical waveguide 25 _1.

[0044] The control light PLC 21 _2, respectively nonlinear optical waveguide 25 _1, 25 ₂ and butt connected output optical waveguides 26 _1, 26 ₂ are formed. Figure 11 is a diagram schematically illustrating the W-W'cut surface of PLC 21 ₁ shown in FIG. 10. The core portions 23 and 24 of each optical waveguide are G
e made of SiO ₂ and SiO ₂ clad 2
It is surrounded by 7. The light is guided in such a manner that a part of the light leaks into the cladding 27.

The principle of the operation is almost the same as that of the first embodiment, except that the present embodiment has an asymmetric optical waveguide structure and a single input control light. Signal light is slightly longer wavelengths of TM light than the absorption wavelength intersubband, when there is no strong control light, the nonlinear waveguide 25 _1, 25 ₂ of the asymmetry as described above, the nonlinear optical waveguide 25 ₁ Output from A strong control optical pulse of the TM mode a slightly shorter wavelength than the intersubband absorption wavelength is incident on the nonlinear optical waveguide 25 ₁ through the optical waveguide 24, the equivalent refractive index is relative with respect to the nonlinear waveguide 25 ₁ of the signal light And as the signal light propagates, the nonlinear optical waveguide 2
It is pushed towards the 5 _2.

[0046] As a result, when the control pulse above a certain power is input from the optical waveguide 24, the signal light is switched to the digital towards the nonlinear optical waveguide 25 _2. Therefore,
Also in the present embodiment, optical digital switching is realized in which repetition is very fast and energy required for switching is small.

(Third Embodiment) FIG. 12 is a schematic diagram showing a schematic configuration of an optical switch according to a third embodiment of the present invention. The optical switch includes a PLC 31, a semiconductor optical integrated circuit 32 having an n-type AlN / GaN quantum well having an intersubband absorption wavelength of about 1.5 μm as an optical waveguide layer, an optical fiber 33, and optical circulators 37 ₁ , 37 ₂ , and 37 _3. Is the main component. Nitride semiconductor optical integrated circuit 3
2 has a Y-branch 35 made of a nonlinear optical waveguide,
Optical waveguide 36 ₁ bonded against the output of the Y-branch 35 in 1, 36 ₂ are formed, respectively. Y
The branch angle of the branch 35 is 3 milliradians. Optical fiber 33 is butt-coupled each waveguide 35, 36 _1, 36 _2.

In this embodiment, unlike the previous embodiment, the control light pulse and the signal light propagate in opposite directions. That is,
Input signal light optical circulator 37 ₃ and the optical fiber 33
Whereas inputted from the left side of the figure Y branch 35 via a control light optical circulator 37 ₁ or 37 _2, optical fiber 33, through the optical waveguide 36 ₁ or 36 _2, Y from the right side of FIG. It is input to branch 35. The output of the control light and the signal light is separated by the optical circulator 37 _1, 37 _2, 37 _3. The signal light is 1.6 μm, and the wavelength of the control light pulse is 1.4 μm.

[0049] For example, if the control pulse from the optical circulator 37 ₂ is input, the optical waveguide of the Y branch 35 36
Equivalent refractive index is decreased with respect to the signal light of the connected branches into _two, the signal light is concentrated on a relatively refractive index is connected to the high light waveguide 36 ₁ branch. Conversely, the optical circulator 37
_{When a} control light pulse is input from ₁ , the optical waveguide 36
_The signal light is output from the _second branch. The principle and operation of the change in the refractive index due to the saturation of the intersubband absorption are the same as those of the previous two embodiments. Therefore, also in the present embodiment, an optical digital switch with low switching energy can be realized by ultra-high-speed repetition.

(Fourth Embodiment) FIG. 13 is a schematic diagram showing a schematic configuration of an optical switch according to a fourth embodiment of the present invention. FIGS. 14 and 15 correspond to U in FIG.
It is a figure which shows the -U 'cross section and the VV' cross section typically.

This optical switch has a ridge type optical waveguide 43, 44 ₁ , 44 ₂ , 46 ₁ , 46 having an InGaAsP active layer 47 formed on a p-type InP substrate 41 as a core layer.
₂ and GaN / Al formed on the sapphire substrate 42
N multiple quantum well layer 55 and Al (Ga) N clad layer 5
Non-linear optical waveguide 45 formed by directly bonding a mesa structure made of 6 to an upper part of the InGaAsP active layer 47 to be integrated.
_1, comprising 45 ₂ and the main components. In this example,
The bonding surfaces are the AlN layer of the quantum well layer 55 and the n-type InP cladding layer 56, but other layers may be interposed.

[0052] In the non-linear optical waveguide 45 _1, 45 _2, intersubband absorption occurs by a part of the optical field that issued the active layer 47 Karashimi overlap GaN / AlN multiple quantum well layer 55. In order to inject a current into the active layer 47, an n-type InP cladding layer 50 having a thickness of 0.18 μm and an n-type InP
51 and a current block region 48 made of p-type InP 52;
Electrodes 49 and 54 are provided. Adjacent ridge 4
3,44 _1, between 44 _2, polyimide 53 is embedded. No current is injected into the GaN / AlN multiple quantum well layer 55 because of the band discontinuity between the AlN layer and the n-type InP cladding layer 56 forming the bonding surface.

The operation of this optical switch is similar to the operation of the optical switch of the above-described embodiment, except that the optical waveguides 43 and 44 are operated.
_1, 44 _2, 46 _1, 46 ₂ is that it has a gain by current injection different. When the optical waveguide has no gain as in the previous embodiment, the signal light output is considerably smaller than the input due to intersubband absorption loss, and some external compensation is required. On the other hand, if there is a gain as in the present embodiment, it is possible to obtain a signal light output equal to or higher than the input. The other points are the same as those of the previous embodiment, and an optical digital switch having low switching energy is realized by ultra-high-speed repetition.

(Modifications) The present invention is not limited to the above embodiment, but various combinations and modifications are possible. For example, a wavelength filter may be integrated in the PLC of the input / output unit, or a plurality of optical switches may be integrated on the same substrate. When a semiconductor substrate is used as in the fourth embodiment, a control light pulse light source may be integrated.

The material that causes intersubband absorption is also Al (G
a) It is not limited to the N / GaN quantum well, and the absorption wavelength between sub-bands is not limited to around 1.5 μm. The quantum well layer may have a plurality of intersubband absorption peaks by using a symmetric coupling quantum well or an asymmetric quantum well. Alternatively, the barrier layer may be thinned to have a superlattice structure, and may be used in the form of interminiband absorption rather than intersubband absorption. Also in these cases, the control light wavelength is set to a wavelength at which the refractive index increases due to absorption saturation, and the signal light wavelength is set to a wavelength at which the refractive index decreases due to absorption saturation.
In the first embodiment, the impurity doping is performed only on the well layer. However, the doping may be performed on the entire quantum well layer, or the modulation doping may be performed on all or a part of the barrier layer. The specific structure and form of the optical waveguide are not limited to the above embodiments. In addition, various modifications can be made without departing from the scope of the present invention.

[0056]

As described in detail above, according to the present invention, the switching principle uses the saturation of the absorption between sub-bands in the quantum well, and the wavelength of the control light and the signal light is changed in the refractive index fluctuation due to the absorption saturation. By setting so as to realize, ultra-high speed and high repetition optical switching is realized. In addition, since the output port does not change with respect to a control light input equal to or more than a predetermined input, stable optical digital switching is realized with respect to temperature fluctuations and control light power fluctuations.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a main part of an optical switch according to a first embodiment as viewed from above.

FIG. 2 is XX ′ of the optical switch according to the first embodiment.
The figure which shows a cross section typically.

FIG. 3 is a diagram illustrating a YY ′ optical switch according to the first embodiment;
The figure which shows a cross section typically.

FIG. 4 is a view illustrating the ZZ ′ of the optical switch according to the first embodiment;
The figure which shows a cross section typically.

FIG. 5 is a diagram schematically showing a conduction band structure of the intersubband absorption region 2 of the optical switch according to the first embodiment.

FIG. 6 is a view schematically showing a conduction band structure of an optical waveguide layer in a region other than the inter-subband absorption region 2 of the optical switch according to the first embodiment.

FIG. 7 is a schematic diagram illustrating inter-subband absorption and its relaxation process.

FIG. 8 is a diagram illustrating an intersubband absorption spectrum and a dispersion spectrum of an equivalent refractive index when strong light is not incident (solid line) and when strong light is input (dashed line).

FIG. 9 shows the relationship between the control light pulse intensity and the output signal light intensity in the optical switch of the present invention.

FIG. 10 is a diagram schematically showing a configuration of an optical switch according to a second embodiment.

FIG. 11 is a diagram illustrating the WW of the optical switch according to the second embodiment;
'A diagram schematically showing a cross section.

FIG. 12 is a schematic diagram illustrating a schematic configuration of an optical switch according to a third embodiment.

FIG. 13 is a schematic diagram showing a schematic configuration of an optical switch according to a fourth embodiment.

FIG. 14 shows UU of an optical switch according to a fourth embodiment.
'A diagram schematically showing a cross section.

FIG. 15 shows VV of the optical switch according to the fourth embodiment.
'A diagram schematically showing a cross section.

FIG. 16 is a configuration diagram of a conventional Mach-Zehnder interferometer type nonlinear optical switch.

FIG. 17 is a diagram illustrating the relationship between control light input and signal light output in a conventional interferometer type nonlinear optical switch.

FIG. 18 is a configuration diagram of a conventional digital optical switch.

[Explanation of symbols]

1,42 ... sapphire substrate 2,22,32 ... intersubband absorption regions 3,23,43 ... signal light input optical waveguide 4 _1, 4 _2, 24, 44 _1, 44 ₂ ... control light input optical waveguide 5 _1, 5 _2, 25 _1, 25 _2, 45 _1, 45 ₂ ... nonlinear optical waveguide 6 _1, 6 _2, 26 _1, 26 _2, 36 _1, 36 _2, 46
₁ , 46 ₂ ... output optical waveguide 7, 9, 27 ... cladding layer 8, 23, 24 ... core 10 ... quantum well waveguide layer 13 ... well layer 14 ... barrier layer 15 ... first subband 16 ... second Subbands 21 ₁ , 21 ₂ , 31 PLC 33 Optical fiber 35 Non-linear optical Y branch 37 ₁ , 37 ₂ , 37 ₃ Optical circulator 41 P-type InP substrate 47 Active optical waveguide 48 Current block region 49 54 ... electrode 50 ... n-type InP layer

Claims (3)

[Claims] 1. A semiconductor quantum well layer having a plurality of subbands in a conduction band therein, wherein the semiconductor quantum well layer is configured so as to be gradually branched from an mode-coupled input portion to an output portion. A nonlinear optical waveguide that causes absorption saturation between sub-bands due to the incidence of strong light; and a unit that inputs signal light having a wavelength whose refractive index decreases due to saturation of absorption between the sub-bands to an input portion of the nonlinear optical waveguide. Means for injecting control light having a wavelength at which the refractive index increases due to saturation of the inter-subband absorption into the nonlinear optical waveguide so as to be biased to one side; and from one of two branches of an output section of the nonlinear optical waveguide. Means for outputting a signal light. 2. The semiconductor quantum well layer according to claim 1, wherein said semiconductor quantum well layer comprises
2. The optical switch according to claim 1, wherein the optical switch is made of a group V semiconductor. 3. The optical switch according to claim 1, wherein said nonlinear optical waveguide has a gain for signal light.