JP2922190B1 - Nonlinear optical semiconductor device - Google Patents

Abstract

An optical control type ultra-high-speed semiconductor optical switch using transition between sub-bands of a semiconductor realizes stable polarization-independent operation with a simple configuration. SOLUTION: A first nonlinear optical waveguide portion 12a and a second nonlinear optical waveguide portion 12b utilizing a transition between subbands of a semiconductor quantum well are connected in series with a half-wave plate 14 interposed therebetween. The power and the polarization state of the nonlinear optical semiconductor device are set such that the power incident on the first nonlinear optical waveguide section 12a as the TM mode is substantially equal to the power incident on the second nonlinear optical waveguide section 12b as the TM mode. Is set, and the control light pulse belonging to the absorption wavelength band of the transition between sub-bands is incident at a predetermined timing with respect to the signal light pulse belonging to the absorption wavelength band of the transition between sub-bands.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonlinear optical semiconductor device such as an optically controlled ultra-high-speed semiconductor optical modulator or an optically controlled ultra-high-speed semiconductor optical switch, and more particularly to a nonlinear optical device which does not depend on the polarization state of incident signal light. The present invention relates to an optical semiconductor device.

[0002]

2. Description of the Related Art In a backbone transmission system in the coming multimedia age, a large amount of information of several hundreds to several terabits per second flies over an optical fiber. To realize an optical node that can freely process such information, an ultra-high-speed optical switch controlled by ultra-short optical pulses is required. If the purpose is periodic optical demultiplexing, a physical phenomenon in which the internal state returns to the original state in the cycle of demultiplexing can be used. However, in order to switch the signal light on a bit basis, it is necessary to return to the original state within the time corresponding to one slot of the signal. As a principle of such an ultra-high-speed optical switch capable of operating in a bit mode, use of transition between subbands in a nitride semiconductor quantum well is considered (for example, Japanese Patent Application Nos. Hei 6-248451 and Hei 7).
-42014, Japanese Patent Application No. 7-102198, Japanese Patent Application No. 8
No. 31534 or N. Suzukiand N. Iizuka Japane
se Journal of Aplied Physics, vol.36, pp.L1006-L100
8,1997).

[0003] The relaxation time between subbands is about three orders of magnitude faster than the relaxation time between bands. In particular, if a quantum well of a nitride semiconductor that is a wide gap material is used,
The transition between subbands in the near infrared region, which is important in optical communication, can be realized. In addition, since the interaction between the LO phonon and the electrons is large, the relaxation time can be shortened by one digit or more compared to the GaAs-based intersubband transition. The relaxation time between subbands in a GaN quantum well is about 1.55 μm at a transition wavelength and about 1 at room temperature.
Since it is as short as 00 fs, a bit mode optical switching operation of an ultra-high-speed repetition of about 1 Tb / s can be performed by utilizing the saturation of the absorption between sub-bands. In addition, since a semiconductor is used, a small, light, and stable optical switch can be realized, and it goes without saying that mass production is possible. Furthermore, nitride semiconductors are particularly tough semiconductor materials and are resistant to temperature rise.

As described above, transition between subbands in a nitride semiconductor quantum well is a very attractive device principle for realizing an ultrahigh-speed optical switch. However, the absorption between the subbands in the conduction band of the semiconductor quantum well is mainly caused by an electric field component perpendicular to the well layer (hereinafter referred to as TM
Mode). In some cases, it is known that inter-subband absorption also occurs for a polarization component parallel to the well (hereinafter, referred to as TE mode).
Generally, the response to the TM mode is weaker, and the resonance wavelength of the absorption is generally shifted between the TE mode and the TM mode. Optical switches that use intersubband transition have such a large polarization dependence, and despite their ultra-high-speed response characteristics, optical fibers with uncertain polarization states of transmitted signal light There were problems with using it on nodes of the network.

As one method for solving this problem,
It is conceivable to use polarization diversity. However, since it is difficult to perform polarization separation or polarization rotation on a semiconductor, these parts are provided outside the semiconductor optical waveguide, and the configuration becomes complicated. In particular, when processing ultra-high-speed optical signals of the terabit class per second, it is necessary to accurately adjust the timing of the signal light pulse and the control light pulse for the two polarization branches, and to keep the timing stable even if there is a fluctuation in the external environment. This is the most difficult technique.

[0006]

As described above, in the conventional ultra-high-speed nonlinear optical semiconductor device using the intersubband transition in the nitride semiconductor quantum well, the polarization state of the signal light has a large polarization dependence. However, there is a problem with the use of fiber optic networks at nodes that are uncertain. Providing polarization diversity in order to solve this is a factor that complicates the configuration.

The present invention has been made in view of the above problems, and an object of the present invention is to utilize transitions between subbands in a semiconductor capable of performing an ultra-high-speed repetition operation of several hundred gigabits per second or more. An object of the present invention is to provide an optically controlled ultra-high-speed nonlinear optical semiconductor device that enables stable polarization-independent operation with a simple configuration.

[0008]

[Means for Solving the Problems]

(Configuration) In order to solve the above problem, the present invention employs the following configuration. That is, the present invention, the first nonlinear optical waveguide section and the second nonlinear optical waveguide section using the transitions between sub-bands of semiconductors quantum wells are connected in series across the half-wave plate, and the substrate Non-linear optical semiconductor device integrated on
And a TM mode in the first nonlinear optical waveguide portion.
Control light power and second nonlinear optical waveguide section incident as
And the control light power incident as TM mode
Power and polarization state are set so that
Control light pulses belonging to the absorption wavelength band of
Optical signal pulses belonging to the absorption wavelength band of intersubband transition
At a predetermined timing, the two nonlinear optical waveguide portions
The light is incident from one end .

Here, preferred embodiments of the present invention include the following. (1) The center wavelength of the half-wave plate should substantially match the center wavelength of the signal light. (2) The semiconductor quantum well has a group III-V semiconductor containing nitrogen in the well layer. (3) 1.55 μm is included in the absorption wavelength band of the transition between sub-bands.

[0010]

[0011]

According to the present invention, in a nonlinear optical semiconductor device, there is provided a first interface section in which two optical waveguides are arranged, a first interface section in which two nonlinear optical waveguides are arranged, A mode conversion unit comprising a first intermediate optical waveguide having a first half-wave plate inserted therein and a second intermediate optical waveguide having a second half-wave plate inserted in the middle thereof; A second nonlinear optical waveguide section in which two nonlinear optical waveguides are arranged; a second interface section in which two optical waveguides are arranged; two optical waveguides in the first interface section; A first one-to-one optical coupler coupling the two nonlinear optical waveguides of the nonlinear optical waveguide section of
A second 1 coupling the two nonlinear optical waveguides of the first nonlinear optical waveguide section with the first and second intermediate optical waveguides.
A one-to-one optical coupler, a third one-to-one optical coupler that couples the first and second intermediate optical waveguides and two nonlinear optical waveguides of the second nonlinear optical waveguide section, and the second one-to-one optical coupler; A fourth one-to-one coupling two nonlinear optical waveguides of the nonlinear optical waveguide section and two optical waveguides of the second interface section.
A semiconductor quantum well having an intersubband absorption for a signal light wavelength is used for each of the nonlinear optical waveguides including an optical coupler as a constituent element and constituting the first and second nonlinear optical waveguide portions. It is characterized by being.

Here, the power and the polarization are set so that the control light power incident on the first nonlinear optical waveguide portion as the TM mode and the control light power incident on the second nonlinear optical waveguide portion as the TM mode are substantially equal. The control light pulse whose state is set and belongs to the absorption wavelength band of the transition between sub-bands,
It is desirable that the signal light pulse is incident at a predetermined timing.

(Operation) Depending on the presence or absence of the TM mode component of the control light pulse in the first nonlinear optical waveguide portion, the intensity and phase of the TM mode component of the signal light are modulated and controlled so as to have desired values. However, the TE mode component hardly changes in both the signal light and the control light due to the polarization dependence of the transition between subbands. Thereafter, the TM mode is changed to the TE mode for both the signal light and the control light by the half-wave plate inserted into the optical waveguide.
The TE mode is converted to the TM mode. Also in the second nonlinear optical waveguide section, the intensity and phase of the TM mode component of the signal light (TE mode component before passing through the half-wave plate) are changed to the TM mode component of the control light (the half-wave plate). Is modulated and controlled to a desired value by a TE mode component before passing through, but each TE mode component (1/2)
The TM mode component before passing through the wave plate) hardly changes. As a result, the modulation and control are performed such that the TM mode component and the TE mode component of the incident signal light have a desired intensity or phase almost independently in the first nonlinear optical waveguide portion and the second nonlinear optical waveguide portion, respectively. Will be done.

Therefore, if the ratio between the TM mode component and the TE mode component of the control light pulse is controlled so that the nonlinear optical effects in the first nonlinear optical waveguide section and the second nonlinear optical waveguide section become equal, With one control light pulse, a polarization-independent optical modulation / control operation similar to the polarization diversity is realized.

Generally, the first nonlinear optical waveguide section and the second
Since there is a loss for the TE mode light in the one-wavelength plate, in order to realize the same nonlinear response in the two nonlinear optical waveguide sections, the control light power incident as the TM mode at the input section of the two nonlinear optical waveguide sections Is set so that the TE mode component of the incident control light pulse is larger than the TM mode component. Further, incomplete mode conversion caused by a shift between the wavelength of the half-wave plate and the wavelength of the control light can be compensated for by fine adjustment of each polarization mode component of the incident control light pulse.

It is well known that a passive optical device can compensate for polarization dependence by inserting a half-wave plate in the middle. However, this technique is not generally available for nonlinear optical devices. Since the principle of superposition does not hold for the non-linear phenomenon, the first non-linear optical waveguide portion and the second non-linear optical waveguide portion having different non-linearities are connected in series in this order, and are connected in series in the reverse order. The output results are different. Therefore, T
Even if a half-wave plate is inserted between the nonlinear optical waveguide portions having different responses (both are not zero) in the E mode and the TM mode, the polarization independent operation is not realized.

On the other hand, if the transition between sub-bands having little response to the TE mode light is used, a linear optical waveguide (TE mode propagation) and a nonlinear optical waveguide (TM mode propagation) sandwiching a half-wave plate. Are connected in series, so that the same result can be obtained even if the order is changed. That is, it can be said that the polarization-independent nonlinear optical response by the series connection of the present invention is realized by taking the polarization dependence of the transition between sub-bands in reverse.

By using a nitride semiconductor quantum well,
Near infrared region used in optical communication (wavelength 1.55 μm band, 1.
Transition between sub-bands (for example, 3 μm band) is easily realized. In addition, since the relaxation time between sub-bands in a nitride semiconductor is as short as about 100 fs, an ultra-high-speed response of several hundred gigabits per second or more is possible. The wavelength band can be widened in response to the short intra-subband relaxation time.

[0020]

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 schematic diagram showing a configuration of a nonlinear optical semiconductor device (optical switch) according to a first embodiment of the present invention.

This optical switch comprises a nonlinear optical waveguide element 1 utilizing a transition between subbands of a nitride semiconductor quantum well, a mode-locked semiconductor laser 2, a pulse compression unit 3, an optical TDM.
The modulation pattern generator 4, the polarization controller 5, the polarization-independent optical coupler 6, and the optical fibers 7 (7a, 7b, 7c) for connecting these and inputting / outputting signal light are the main components. . Actually, there are other components such as an optical isolator, an optical filter, a monitor PD, an optical amplifier, a control circuit, and the like. The signal light is 400 Gb /
s, and the wavelength is 1.55 μm.

The mode-locked semiconductor laser 2 has a wavelength of about 1.54 μm at 40 GHz, which is 1/10 of the period of the signal light.
An optical pulse train having a width of 2.5 ps is generated. The repetition frequency of the mode-locked semiconductor laser 2 is 400 Gb / s
(A synchronizing / stabilizing device is omitted). The output is compressed by the pulse compression unit 3 into an ultrashort light pulse having a transform limit width of about 100 fs. Optical TDM in which a Si plane lightwave circuit (PLC) and a high-speed optical modulator (gate switch) are hybrid-integrated
The modulation pattern generation unit 4 divides the ultrashort optical pulse into ten, and uses a high-speed optical modulator provided in each branch for 40G.
A predetermined pattern of 400 Gb / s is generated by modulating a predetermined pattern of b / s and combining them by shifting them by a predetermined time. The polarization of the output light from the pattern generator 4 is controlled by the polarization controller 5 and guided to the optical coupler 6. Note that the peak power of the control light pulse is amplified and controlled by an optical amplifier (not shown) so as to have a predetermined value.

On the other hand, the signal light modulated at 400 Gb / s is input from the signal input optical fiber 7a,
And is coupled to the nonlinear optical waveguide element 1 via the optical fiber 7b. The output of the nonlinear optical waveguide element 1 is output from the optical fiber 7c. It should be noted that the optical coupling system may be a butt joint or a lens system as needed. The peak power of the signal light pulse is sufficiently smaller than that of the control light.

The nonlinear optical waveguide element 1 includes a sapphire substrate 1
And a ridge optical waveguide 12 (12a, 12b) formed on a nitride semiconductor layer 11 formed on the substrate 1. A slot 15 formed at the center of the ridge optical waveguide 12 has a 15 μm thick polyimide half-wave plate. 14 has been inserted. Half
The center wavelength of the wave plate 14 is set to 1.55 μm. Since the half-wave plate 14 is thin, the insertion loss is 0.5
It can be a small value of dB or less. The front of the half-wave plate 14 is the first nonlinear optical waveguide 12a, and the rear is the second nonlinear optical waveguide 12b. Element length is about 1m
m. An anti-reflection film 15 is formed on the entrance and exit surfaces of the nonlinear optical waveguide section 12.

FIG. 2 is a sectional view schematically showing the layer structure of the nonlinear optical waveguide element 1. This nonlinear optical waveguide element 1
On a sapphire substrate 10, a low-temperature-grown AlN buffer layer 1 is formed.
6, undoped AlN lower cladding layer 17, undoped Al _0.8 Ga _0.2 N lower light guide layer 18, Al _0.8 Ga
_0.2 N / GaN multiple quantum well layer 19, undoped Al
_0.8 Ga _0.2 N upper light guide layer 20, undoped AlN
It has a structure in which the upper cladding layer 21 is laminated. The ridge optical waveguide 12 is formed by processing the upper cladding layer 21 into a ridge shape.

The multiple quantum well layer 19 has a thickness of about 1.56 n
An m-type n-type GaN well layer 22 and an undoped Al _0.8 Ga _0.2 N barrier layer 23 having a thickness of about 2 nm are alternately stacked in 100 pairs. The carrier density in the well layer 22 is about 1 ×
10 ¹⁹ cm ^-3 . Instead of this structure, a thin portion of the well layer 22 and the barrier layer 23 that is in contact with the well layer 22 is undoped, and only a central portion of the barrier layer 23 is doped with an n-type impurity. May be adopted. Alternatively, both the well layer and the barrier layer may be doped with n-type impurities. The resonance peak wavelength of the intersubband absorption is 1.54 μm. Since the absorption half width is as wide as about 0.2 μm, the transmittance of the signal light having a wavelength of 1.55 μm can be modulated by the absorption saturation by the control light having a wavelength of 1.54 μm. It is also possible to switch the wavelength multiplexed signal light at a time.

When there is no control light pulse in the first nonlinear optical waveguide section 12a, the TM component of the signal light pulse is almost absorbed by the inter-subband absorption. Since the power of the signal light pulse is small, saturation of absorption does not occur. However, when a TM mode control light pulse having a large power is incident, most of the electrons in the base sub-band are excited to the second sub-band by the inter-sub-band absorption, and significant absorption saturation occurs. Therefore, the TM component of the signal light pulse incident during or immediately after the control light pulse is incident is transmitted without being absorbed.

Since the relaxation time between sub-bands is as short as about 100 fs, 100 fs after passing the control light having a pulse width of 100 fs.
To a degree, the electrons excited in the second subband have relaxed to the original ground subband. For this reason, the absorption has returned to the original state by the time the signal light of the next time slot (one slot is 250 fs) comes. Therefore, ultra high-speed bit mode optical switching is realized for the TM component of the signal light. At this time, both the control light and the signal light are TE
The component has no absorption and no absorption saturation occurs. Since the entire length of the first nonlinear optical waveguide section 12a is as short as 500 μm, the polarization dispersion can be ignored with respect to the pulse width.

In the half-wave plate 14, conversion between the TE mode and the TM mode is performed. The light of the TE mode component that has not changed in the first nonlinear optical waveguide section 12a becomes the TM mode in the second nonlinear optical waveguide section 12b,
When there is no strong control light pulse, the signal light is absorbed, and when there is a control light pulse, the signal light is transmitted. The first nonlinear optical waveguide portion 12 is absorbed by the control light pulse and is saturated.
a is transmitted to the second nonlinear optical waveguide 12b.
Is transmitted as it is as TE mode light.

Therefore, the TM mode component of the signal light is also TE
The mode component is also transmitted in the slot where the control light pulse exists, and is absorbed in the slot where no control light pulse exists. As a result, 4
Polarization-independent optical gate switching can be realized with an ultra-high speed of 00 Gb / s. The remaining portion of the control light pulse transmitted through the two nonlinear optical waveguide portions 12a and 12b without being absorbed is an optical filter (not shown) provided at the outlet, which transmits the 1.55 μm light but blocks the 1.54 μm light. ).

In the polarization controller 5, the polarization state of the control light is controlled so that the absorption saturation in the first nonlinear optical waveguide section 12a and the second nonlinear optical waveguide section 12b become approximately the same. That is, the first nonlinear optical waveguide section 12
In order to compensate for the propagation loss of “a” and the coupling loss of the half-wave plate 14, the polarization component coupled to the TE mode in the first nonlinear optical waveguide section 12a is two times smaller than the polarization component coupled to the TM mode.
It is set to be about dB higher. Unlike the signal light, the polarization state of the control light is stable. Therefore, if the polarization controller 5 is set once at the beginning, there is no need to change the control state. Therefore, instead of the polarization controller 5, the output from the control light modulation pattern generation unit 4 is output by the polarization maintaining fiber, and the polarization maintaining fiber is connected to the optical coupler at a predetermined angle. Can be realized. Of course, if the polarization of the control light can be adjusted and controlled to some extent, the object can be achieved even when the loss of the optical waveguide with respect to the TE mode light changes due to a large temperature change or the like.

Strictly speaking, since the center wavelength of the half-wave plate 14 is 1.55 μm, the signal light is almost completely mode-converted, but the control light having the wavelength of 1.54 μm is partially mode-converted. It will remain without being. In this case as well, including this residual component, the second nonlinear optical waveguide portion 12b has T
The initial polarization state may be adjusted so that the total power incident as M-mode light has a desired value.

As a modified example, an arrangement in which the control light pulse is incident from the rear side is also conceivable. TM of control light pulse
The component contributes to the signal light control in the second nonlinear optical waveguide section 12b, and the TE component is converted to the TM mode by the half-wave plate 14, and is used for the signal light control in the first nonlinear optical waveguide section 12a. Contribute. In this case, the two nonlinear optical waveguide sections 12
The timings at which the signal light pulse and the control light pulse interact at a and 12b are slightly different. But,
Since the signal light transmission time of the signal light is shorter than the pulse width, when the control light pulse is set to be incident when the signal light pulse is incident on the first nonlinear optical waveguide portion 12a, the above-mentioned condition is obtained. Almost the same polarization-independent high-speed optical switching operation as that of the first embodiment can be realized. Two nonlinear optical waveguide sections 12 due to timing deviation
The subtle difference in the nonlinear response between a and 12b can be adjusted by the polarization of the control light. In this arrangement, an optical circulator can be used to separate the control light pulse and the signal light pulse.

FIG. 3 shows a modified example in which two optical gate switches for receiving the control light pulse and the signal light pulse from opposite sides are arranged to constitute an optical routing switch.
The pattern generation unit 4b further divides each of the ten branches into two, and one of the optical modulators performs modulation opposite to that of the other optical modulator. By multiplexing the divided 10 branches by shifting them by a predetermined time by another multiplexer, it is possible to invert one control light pattern output with the other control light pattern output. The signal light pulse is divided into two, guided to two nonlinear optical waveguides 12c and 12d each having a half-wave plate 14b inserted in an intermediate slot, and switched by control light pulses having opposite phases to output 8a. And the output 8b.

In this example, if in-phase modulation is applied to the two branches by the pattern generation section 4b, it is possible to output the same signal to both outputs. By arranging a large number of nonlinear optical waveguide elements, it is possible to branch the same signal to a plurality of branches or to output only a specific branch. Further, as shown in FIG. 4 (the control light portion is omitted for simplicity), if the non-linear optical waveguide elements 1 are arranged in a matrix with the demultiplexer 9a and the multiplexer 9b crossing each other, N Vs. N
Can be configured. However, in this case, a buffer function is required in order to prevent collision of optical pulses in the multiplexer 9b. For example, a method of connecting some of the multiplexers 9b to a buffer can be considered.

As described above, according to the present embodiment, not only the one-to-one gate switch and the one-to-two routing, but also
Even in the 1: N broadcast type or the M: N exchange type, signal light can be freely switched to a plurality of outputs in an ultra-high speed arbitrary pattern regardless of the polarization state of the signal light. In this case, the configuration is much simpler than in the case where polarization diversity switching is performed by each optical switching element.

(Second Embodiment) FIG. 5 shows a second embodiment of the present invention.
It is a figure which shows typically the structure of the nonlinear optical semiconductor device (Mach-Zehnder interferometer type optical switch) which concerns on embodiment.

In this embodiment, the nitride semiconductor laminated structure 101 formed on a sapphire substrate has
First nonlinear optical waveguide portion 112a and second nonlinear optical waveguide portion 11 utilizing intersubband transition of GaN quantum well
2b are connected in series with a half-wave plate 114 interposed therebetween, and a third nonlinear optical waveguide portion 112c using the intersubband transition of the AlGaN / GaN quantum well, and a fourth branch. The nonlinear optical waveguide portion 112d has a half
A second branch connected in series across the wavelength plate 114, an optical waveguide 102 for introducing incident signal light, and input light for equally dividing the incident signal light into a first branch and a second branch A coupler 103, a first optical multiplexing unit 104a that multiplexes the signal light and the control light incident on the first branch, and an input side of the second branch in order to maintain symmetry with the first branch. The output in which the output intensity to the two output waveguides 105a and 105b changes according to the phase difference between the provided second optical multiplexing unit 104b and the signal light propagating through the first branch and the second branch. An optical coupler 106, a control light pulse incident optical waveguide 107a, and a dummy waveguide 107b for maintaining symmetry are integrated. The first branch and the second branch are symmetrical, and the optical couplers 103 and 106 are designed so that the polarization dependence and the wavelength dependence are reduced. The control light pulse generation unit, the polarization control unit, and other peripheral units are substantially the same as those in the first embodiment, and thus description thereof is omitted.

The absorption peak wavelength of the nonlinear optical waveguide section 112 is set to 1.5 μm, the control light wavelength is set to 1.52 μm, and the signal light wavelength is set to 1.55 μm, which has relatively small absorption.
Since the absorption spectrum of the intersubband transition of the nitride semiconductor is wide, the refractive index with respect to the signal light wavelength changes due to the absorption saturation of the control light pulse, and the signal light undergoes phase modulation. The center wavelength of the half-wave plate 114 is set to 1.55 μm, which is the peak wavelength of the signal light. In the absence of the control light pulse, the signal light is output from the output waveguide 105.
a.

The signal light pulse passes through the optical waveguide 102, the input optical coupler 103, and the two optical multiplexing sections 104a and 104b, and the first non-linear optical waveguide section 112a of the first branch.
And the light enters the third nonlinear optical waveguide portion 112c of the second branch. Also, a control light pulse synchronized with the light pulse of the desired slot of the signal light enters the first branched nonlinear optical waveguide portion 112a via the control light pulse incident light waveguide 107a and the first optical multiplexing portion 104a. I do. Both the signal light and the control light undergo TM / TE mode conversion in the half-wave plate 114 and propagate through the nonlinear optical waveguide portions 112b and 112d.

The power of the TM mode component of the incident control light pulse is such that the phase change with respect to the TM signal light in the first nonlinear optical waveguide portion 112a is (1 + 2n) π (where n is an integer).
It is controlled to become. TE of incident control light pulse
The power of the mode component is adjusted by the second nonlinear optical waveguide 112.
The phase change with respect to the TM signal light at b is (1 + 2n) π
(Where n is an integer). At this time, the deviation of the wavelength of the control light from the half-wave plate 114, the loss of the portion of each branch up to the half-wave plate 114, imperfections due to the wavelength and polarization of the optical coupler, etc. And the fine adjustment of the polarization. Therefore,
Regardless of the polarization of the input signal light, the phase difference between the two branches incident on the output optical coupler 106 is π-modulated by the control light pulse, and the output destination can be switched from 105a to 105b. Other details are in accordance with the first embodiment.

As described above, also in this embodiment, the high speed,
Polarization-independent optical switching is realized. Further, it goes without saying that various modifications can be made to the present embodiment, for example, to compensate for loss by hybridly integrating a semiconductor optical amplifier in an interferometer.

Third Embodiment In an optical control type nonlinear optical switch, the wavelengths of the signal light and the control light are often shifted in order to separate the signal light and the control light. As in the case of the interferometer-type optical switch according to the second embodiment, when it is desired to use the refractive index change rather than the absorption coefficient change for the signal light,
The signal light wavelength is preferably set near the bottom of the absorption spectrum, whereas the control light wavelength is preferably set near the absorption peak from the viewpoint of switching energy. Since the absorption spectrum of the intersubband transition of the nitride semiconductor quantum well is broad, there are cases where the wavelengths of the control light and the signal light need to be considerably shifted.

In the present invention, a half-wave plate is used for mode conversion. However, if the signal light is selected so as to have a half wavelength, the control light deviates. As described above, if the shift is small, fine adjustment can be performed by inclining the polarization of the incident control light. However, if the shift is large, the polarization dependency cannot be completely compensated.

FIG. 6 shows a non-linear light according to the third embodiment of the present invention, which is designed so that polarization-independent ultra-high-speed optical modulation and control can be performed even if the wavelengths of the control light and the signal light are largely different. It is a figure which shows the structure of a semiconductor device typically. Using this optical waveguide as an element, an absorption-controlled optical switch as in the first embodiment or a phase-controlled optical switch as in the second embodiment can be configured.

The nonlinear optical semiconductor device of this embodiment is
In order, an input portion 61 having a signal light input optical waveguide 61a and a control light input optical waveguide 61b, and two nonlinear optical waveguides 6
A first nonlinear optical waveguide portion 62 in which 2a and 62b are arranged in parallel, a first intermediate optical waveguide 63a in which a first half-wave plate 64a is inserted in the middle, and a second half optical waveguide 63a in the middle. The mode converter 63 including the second intermediate optical waveguide 63b into which the one-wavelength plate 64b is inserted, two nonlinear optical waveguides 65a,
65b are arranged in parallel with the second nonlinear optical waveguide section 6
5 and an output section 66 comprising a signal light output optical waveguide 66a and a control light output optical waveguide 66b, each of which comprises a first one-to-one optical coupler 67a and a second one-to-one. Optical coupler 67b, third one-to-one optical coupler 67
c and a fourth one-to-one optical coupler 67b.

The wavelength of the signal light pulse is 1.65 μm, and the wavelength of the control light pulse is 1.48 μm. The first half
The center wavelengths of the wave plate 64a and the second half-wave plate 64b substantially match the control light wavelength and the signal light wavelength, respectively. Nonlinear optical waveguides 62a, 62b, 65a, 65b
Utilizes the nonlinear optical characteristics of nitride semiconductor quantum wells due to intersubband absorption, and the absorption peak wavelength is 1.
48 μm. Four nonlinear optical waveguides 62a, 62
b, 65a, and 65b are formed so that the propagation characteristics and the length are the same. Optical couplers 67a, 67b, 67
c and 67d are designed so that one-to-one branching substantially independent of polarization is realized for both the control light wavelength and the signal light wavelength.

First, the propagation of signal light when there is no control light pulse will be described. The signal light input from the signal light input optical waveguide 61a is split by the first one-to-one optical coupler 67a into two symmetric nonlinear optical waveguides 62a and 62b. The powers of the two split signal lights are equal and the phases are shifted by π / 2. Since the nonlinear optical waveguides 62a and 62b are symmetric, this phase relationship is equal to the second one-to-one optical coupler 67b.
At the entrance. As a result, the second one-to-one
Due to the interference in the optical coupler 67b, the signal light is changed to the second 2
The light is branched into the optical waveguide 63b into which the half-wave plate 64b is inserted. Since the center wavelength of the second half-wave plate 64b matches the signal light wavelength, almost complete TE / TM mode conversion is performed on the signal light. Next, the signal light is split by the third one-to-one optical coupler 67c into two symmetric nonlinear optical waveguides 65a and 65b. Similarly, the signal light propagated through the nonlinear optical waveguides 65a and 65b is output to the signal light output optical waveguide 66a by the interference at the fourth one-to-one optical coupler 67d.

The propagation of the control light pulse input from the control light input optical waveguide 61b is exactly the same. Due to the interference of the control light split into two by the first one-to-one optical coupler 67a at the second one-to-one optical coupler 67b, the control light pulse is changed to the first one.
Is output to the optical waveguide 63a in which the half-wave plate 64a is inserted.

Control light wavelength and first half-wave plate 64a
, The control light is also almost completely T
The switching between the E mode and the TM mode is performed. further,
The fourth control light divided into two by the third one-to-one optical coupler 67c
The control light pulse is output to the control light output optical waveguide 66b due to the interference in the one-to-one optical coupler 67d.

The power of the TM mode component of the control light pulse is set so that the phase of the signal light is π-modulated by the absorption between the sub-bands in the nonlinear optical waveguides 62a and 62b. The power of the TE mode component of the control light pulse is set so that the signal light is subjected to π phase modulation in the nonlinear optical waveguides 65a and 65b after being converted into the TM mode by the first half-wave plate 64a. ing. Nonlinear waveguide 6
The two branches at 2, 65 are symmetric, and the control light pulse is subjected to in-phase phase modulation, so that the optical path itself of the signal light does not change. Also, the nonlinear optical waveguides 62 and 65 are
No modulation is applied to the polarization component propagating as a mode. Since the peak power of the signal light is about three orders of magnitude lower than the peak power of the control light, phase modulation by the signal light itself can be ignored. Therefore, the control light pulse causes the signal light to undergo π phase modulation regardless of the polarization. In this configuration, since the signal light and the control light pass through different half-wave plates, the wavelengths of both may be different.

A Mach-Zehnder interferometer is formed by preparing two such optical waveguides and using them for each of the two branches of the interferometer. If control light is incident on only one of the branches, the presence or absence of the control light is determined. Since a phase difference of π can be given to the two branched signal lights, polarization-independent optical switching can be performed.
In addition, since the transition between sub-bands of the nitride semiconductor is used, ultra-high-speed operation can be performed at room temperature as described in the above two embodiments.

Various modifications and applications are also possible in this embodiment. For example, in the configuration of the Mach-Zehnder interferometer described above, the control light is made to enter from the opposite side to the signal light,
Modifications and applications such as performing a logical operation by guiding different control lights to the two branches are also possible.

[0054]

As described above in detail, according to the present invention, the first nonlinear optical waveguide portion and the second nonlinear optical waveguide portion utilizing the intersubband transition of the semiconductor quantum well are formed by a half wavelength. By adopting a configuration in which the plates are connected in series, an ultra-high-speed repetitive optical switching operation can be realized regardless of the polarization state of the incident signal light pulse with a simple configuration without using polarization diversity etc. be able to.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a configuration of a nonlinear optical semiconductor device (optical gate switch) according to a first embodiment.

FIG. 2 is a diagram schematically showing a cross-sectional layer structure of the nonlinear optical waveguide element 1 of the optical gate switch according to the first embodiment.

FIG. 3 is a diagram laterally showing a configuration of an optical routing switch according to a modification of the first embodiment.

FIG. 4 is a diagram showing a modified example in which the nonlinear optical waveguide element according to the first embodiment is applied to an N-to-N optical matrix switch.

FIG. 5 is a diagram schematically showing a configuration of a Mach-Zehnder interferometer type optical switch according to a second embodiment.

FIG. 6 is a diagram schematically showing a configuration of a nonlinear optical semiconductor device according to a third embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Nonlinear optical waveguide element 2 ... Mode lock semiconductor laser 3 ... Pulse compression part 4, 4b ... Optical TDM modulation pattern generation part 5 ... Polarization controller 6, 103, 106, 67a, 67b, 67c, 67d
... optical couplers 7a, 7b, 7c ... optical fibers 9a, 9b ... multiplexer / demultiplexer 10 ... sapphire substrate 11, 101 ... nitride semiconductor layers 12a, 112a, 62 ... first nonlinear optical waveguide parts 12b, 112b, 65: second nonlinear optical waveguide section 112c: third nonlinear optical waveguide section 112d: fourth nonlinear optical waveguide section 13: slot section 14, 14, b, 114, 64a, 64b: half-wave plate 16: reflection preventing film 17 and 21 ... clad layer 18, 20 ... optical guiding layer _{19 ... Al 0.8 Ga 0.2 N /} GaN multiple quantum well layer 104a, 104b ... optical multiplexer 102,105a, 105b, 107a, 107b, 6
1a, 61b, 63a, 63b, 66a, 66b ... optical waveguide

──────────────────────────────────────────────────の Continued on the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G02F 1/35 G02F 1/025 JICST file (JOIS)

Claims (2)

(57) [Claims] A first nonlinear optical waveguide portion and a second nonlinear optical waveguide portion utilizing intersubband transition of a semiconductor quantum well are connected in series with a half-wave plate interposed therebetween, and are provided on a substrate. A control optical power incident on the first nonlinear optical waveguide as a TM mode and a control optical power incident on the second nonlinear optical waveguide as a TM mode. The power and the polarization state are set to be substantially equal, and the control light pulse belonging to the absorption wavelength band of the inter-subband transition is a predetermined control light pulse with respect to the signal light pulse belonging to the absorption wavelength band of the inter-subband transition. A nonlinear optical semiconductor device characterized by being incident at one timing from one end of the two nonlinear optical waveguide portions. 2. A first interface section in which two optical waveguides are arranged, a first nonlinear optical section in which two nonlinear optical waveguides are arranged, and a first half wavelength in the middle. A mode converter consisting of a first intermediate optical waveguide with a plate inserted, a second intermediate optical waveguide with a second half-wave plate inserted in the middle, and two nonlinear optical waveguides are arranged. A second nonlinear optical waveguide section and a second nonlinear optical waveguide section in which two optical waveguides are arranged.
Interface section, two optical waveguides of the first interface section, and two optical waveguides of the first nonlinear optical waveguide section.
A first one-to-one optical coupler that couples the two nonlinear optical waveguides, and a first one-to-one optical coupler that couples the two nonlinear optical waveguides of the first nonlinear optical waveguide section and the first and second intermediate optical waveguides. 2
A one-to-one optical coupler, a third one-to-one optical coupler coupling the first and second intermediate optical waveguides and two nonlinear optical waveguides of the second nonlinear optical waveguide section, Fourth one coupling the two nonlinear optical waveguides of the two nonlinear optical waveguide sections and the two optical waveguides of the second interface section.
A semiconductor quantum well having an intersubband absorption for a signal light wavelength is used for each of the nonlinear optical waveguides including the one-to-one optical coupler as a component, and forming the first and second nonlinear optical waveguide portions. A nonlinear optical device characterized in that it is used.