JP2809216B2 - Nonlinear optical waveguide - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nonlinear optical waveguide which is a component of an optical switch used in fields such as optical fiber communication and optical information processing.

[0002]

2. Description of the Related Art To increase the speed of an optical communication system or an optical information processing system, a transmission line, a multiplexing / demultiplexing circuit, and a logic circuit do not require an "optical-electrical" or "electrical-optical" conversion circuit. It is considered necessary to build an all-optical system.
To construct such an all-optical system, a light control element capable of high-speed operation is required.

Conventionally, a method of performing light control by an electric signal (electric-light control) has been used in a light control element. In recent years, as a method expected to have higher speed, a method of performing light control by light has been proposed. (Light-light control) is drawing attention.
In particular, if an optical-optical control switch (optical-optical switch) capable of operating at high speed can be used for an optical demultiplexing circuit (optical demultiplexer) in an optical communication system, a large capacity can be realized by a time division multiplexing method. Is a big breakthrough.

[0004] The performance required for putting the optical-optical switch into practical use is not limited to the above-described high-speed performance, but also includes a wide range such as low switching energy, high repetition operation, and compact size. In particular, the switching energy is required to be within the range of the light pulse energy that can be reached by the semiconductor laser, the fiber amplifier, or the semiconductor laser amplifier.

In realizing the required performance, the first problem is that the figure of merit of the nonlinear optical effect “χ (3) / τα (χ), which is the driving principle of the optical-optical switch.
(3): magnitude of non-linearity, τ: response time, α: signal loss) are generally substantially constant. That is, it is considered difficult to obtain a nonlinear optical effect that satisfies both large nonlinearity and high speed at the same time.

When the nonlinear optical effect is roughly classified into a non-resonance excitation type and a resonance excitation type, first, the non-resonance excitation type is expected to have high speed, but has a problem that the nonlinearity is small. At present, it is considered to be quite difficult to utilize the nonlinear optical effect due to non-resonant excitation at a practical level of switching energy. On the other hand, in the resonance excitation type, the relaxation of electrons actually excited in the nonlinear optical medium is slow, which is a problem in realizing high-speed operation, but the nonlinearity is large. This point is of great appeal in practical use, and various methods for realizing high-speed operation, which have solved the problem of slow electron relaxation, have been proposed. Here, a conventional example in which a highly efficient resonance-excited nonlinear optical effect is used in a nonlinear optical waveguide which is a component of an optical-optical switch will be described.

Japanese Patent Application Laid-Open No. 2-193128 discloses an optical-optical switch utilizing a nonlinear refractive index change caused by a decrease in carrier density due to incident control light in a semiconductor optical amplifying medium. The configuration of this optical switch is
It is represented as in FIG. A Mach-Zehnder interferometer is constituted by 3 dB couplers 22 and 23 composed of fibers.
The light is branched at 2 and interferes with the coupler 23. The nonlinear optical waveguide 21 is inserted into one arm of the Mach-Zehnder interferometer, and the phase difference between the two interfering light waves determines which of the ports 27 and 28 the signal light is output from.

FIG. 4A is a diagram showing an element structure of the nonlinear optical waveguide 21. As shown in FIG. After a non-doped InGaAsP stripe 3 and a non-doped InP 4 are formed on an n-doped InP substrate 2 by selective growth, p-doped InP 5 is formed.
And n-doped InP6 to form a block layer,
A doped nGaAs cap layer 7 is formed. For the crystal growth, all organic vapor phase epitaxy (MOVP)
E) method is used. Then, a SiO ₂ insulating film 8 is formed on the surface.
Is formed, and only the portion immediately above the stripe 3 is removed by wet etching. Further, an electrode 9 is formed on the front surface, the wafer is polished, the electrode 1 is formed on the back surface, the wafer is cleaved, and both end surfaces are subjected to anti-reflection coating. The optical waveguide portion made of InGaAsP has an absorption edge wavelength of 1.50 μm, a thickness of 0.3 μm, a width of 1 μm, and a length of 300 μm. The nonlinear optical waveguide 1 showed a gain by injecting a bias current, and the gain peak wavelength was 1.47 μm.

The control light pulse is input from the port 26, passes through the wavelength selection coupler 24, and is input to the nonlinear optical waveguide 21. The control light wavelength is set in the gain region of the nonlinear optical waveguide 21. Therefore, when the control light pulse is incident on the nonlinear optical waveguide 21, the number of carriers is reduced along with the amplification of the control light, and a change in the refractive index is caused. At this time, the signal light passing through the nonlinear optical waveguide 21 undergoes a nonlinear phase shift. When the state in which the signal light is emitted from the port 27 is set to the initial state, the signal light emission port is switched to the port 28 due to a change in the refractive index in the nonlinear optical waveguide 21. Thus, the switching operation of the signal light by the control light becomes possible.

[0010]

FIG. 4B is a diagram showing the carrier density distribution before (A) and after (B) the control light pulse passes through the nonlinear optical waveguide 21. As shown in FIG. The horizontal axis indicates a position along the propagation direction. Since the control light pulse energy is small in the vicinity of the incident portion, the amount by which the carrier density decreases in exchange for the control light pulse amplification is small. The control light is amplified to some extent as it propagates, and then saturates the gain. In that case, the carrier density decreases to a size (N0) at which neither gain nor absorption is exhibited for the control light. Therefore, the amount of carriers reduced by contributing to the control light amplification in the entire nonlinear optical waveguide is as shown in FIG.
(B) It is represented by the area of the region sandwiched between A and B. It is considered that the amount of nonlinear phase shift received by the signal light is almost proportional to this area.

As can be seen from the above, in the conventional nonlinear optical waveguide, carriers remain in an inversion distribution near the incident portion. The carriers in this portion are wasted without contributing to the nonlinear refractive index change. Therefore, conventionally,
In order to obtain a sufficient nonlinear refractive index change, a high injection current amount was required.

[0012] Further, when the injection current amount is high, the requirement for the reflectance of the antireflection coating required to suppress laser oscillation in the optical amplification medium (laser medium) becomes severe. Also occurs.

An object of the present invention is to solve the above-mentioned problems and to provide a nonlinear optical waveguide capable of obtaining a sufficient nonlinear refractive index change with a small amount of injected current.

[0014]

In order to achieve the above object, a nonlinear optical waveguide according to the present invention exhibits optical amplification with respect to control light and causes a change in a nonlinear refractive index with the optical amplification. An optical waveguide comprising an amplifying medium, in which a phase shift is caused by a non-linear refractive index change with the input of the control light with respect to the input signal light, and a plurality of electrodes for injecting a current into the semiconductor optical amplifying medium When,
For the plurality of electrodes, a plurality of current injection means for independently injecting a current, and, the plurality of electrodes,
The injection current amount of the plurality of current injection means is set so that the current density at these electrodes is increased stepwise from the entrance side from the entrance side of the semiconductor optical amplification medium. It is characterized by having been done.

In the above case, it is desirable that the stepwise increase in the current density in the plurality of electrodes is monotonic increase in accordance with the optical amplification of the control light.

Further, the semiconductor optical amplification medium may have a multiple quantum well structure. In this case, the multiple quantum well structure may have a structure in which the band gap wavelength is shifted stepwise from the entrance side to the longer wavelength side. further,
The shift of the bandgap wavelength to the longer wavelength side may be set according to the shift of the gain peak wavelength to the shorter wavelength side with an increase in current density in the plurality of electrodes.

According to the present invention as described above, a plurality of electrodes are provided along the light propagation direction from the entrance side of the semiconductor optical amplifying medium, and the current is independently injected for each electrode. Therefore, the current density at each electrode can be set freely. This feature has the following effects.

In optical amplification in a semiconductor optical amplification medium, light intensity increases exponentially and rapidly with propagation, and thereafter, the gain is saturated. Therefore, the amplification degree is low and the light intensity is low near the incident part. In the present invention, the current density of each electrode is set so as to gradually increase from the entrance side, so that the carrier density is low near the incident portion and gradually increases as light propagates. As a result, the gain is saturated even in the vicinity of the incident portion where the light intensity is low, and the injected carriers contribute to the optical amplification almost without waste. Therefore, unlike the related art, the carriers remain in the population inversion in the vicinity of the incident portion, and are not wasted without contributing to the change in the refractive index.

In the case where the current density of the electrode on the entrance side is set to be low as described above, the total injection current amount is lower than that of the conventional one because the injection current amount on the entrance side is lower. Lower. Actually, the current density of the electrode on the exit side is set to be the same as that of the conventional one.

[0020]

Next, an embodiment of the present invention will be described with reference to the drawings.

<First Embodiment> FIG. 1A is a view showing a structure of a nonlinear optical waveguide according to a first embodiment of the present invention. On an n-doped InP substrate 2, a non-doped InGaAsP stripe 3 and a non-doped InP
After forming P4, p-doped InP5 and n-doped IP
A block layer is formed from nP6, and p-doped In
A GaAs cap layer 7 is formed. Here, metal organic vapor phase epitaxy (MOVPE) is used for crystal growth.

After the cap layer 7 is formed, the surface is
_{(2) An} insulating film 8 is formed, and only a portion immediately above a stripe (or a channel, but referred to as an optical waveguide in the following description) 3 is removed by wet etching. further,
An electrode 9 is formed on the surface, and a groove is formed so as to divide the electrode 9 into three regions 11 to 13 in the light propagation direction. Each groove (groove between the regions 11 to 13) dividing the electrode 9 has, for example, a width of 10 μm and a depth of non-doped In.
As far as the P layer 4, the regions 11 to 13 are electrically insulated.

After the electrodes 9 are formed as described above, the wafer is polished to form the electrodes 1 on the back surface of the wafer, and after the wafer is cleaved, non-reflective coatings are applied to both end surfaces to form a nonlinear optical waveguide. Complete.

The electrode 9 of the nonlinear optical waveguide formed as described above has a current source 14 for each of the regions 11 to 13.
To 16 are respectively connected. As a result, carrier injection into the stripe 3 can be performed at a desired current injection amount for each of the regions 11 to 13. That is, the carrier density in the optical waveguide 3 can be set to a desired carrier density for each region.

The non-doped InGaAsP3 exhibits a gain with respect to control light of a predetermined wavelength when a current is injected into the electrode 9 (regions 11 to 13), and causes a nonlinear refractive index change due to amplification of the control light. It is an optical medium. In this embodiment, the stripe 3 has, for example, an absorption edge wavelength of 1.50 μm, a thickness of 0.3 μm, and a width of 1 μm.
m and length are 300 μm. The wavelength range of light in which a gain is caused by current injection in the stripe 3 is defined as a gain region, and other wavelengths (wavelengths of light in which no gain is generated).
The range is called a transparent area.

The feature of the nonlinear optical waveguide of this embodiment is that the electrode 9
And carrier injection into the optical waveguide 3 is performed in each of the regions 11 to
In this case, the current density in each of the regions 11 to 13 is set to J1 to J3, and the carrier density is set to N1 to N3. Jl <J2 <J
3, each carrier density is Nl <N2 <
N3 is set. More specifically,
Focusing on the fact that the energy of the control light is amplified as it propagates through the optical waveguide 3, the carrier density is low on the incident side,
As the carrier density increases toward the emission side,
The carrier density is set according to the energy amplification of the control light.

Next, the principle of operation of the nonlinear optical waveguide as a light control element will be described.

The control light having a pulse width of 1 ps set in a gain region with a wavelength of 1.47 μm and the signal light set in a transparent region with a wavelength of 1.55 μm enter the optical waveguide 3. FIG. 1B shows the carrier density distribution in the nonlinear optical waveguide before (A) and after (B) the control light pulse at this time. In the figure, the horizontal axis indicates a position along the propagation direction.

Since the current density in the region 11 of the electrode 9 is set low according to the energy of the control light at the time of incidence, the carrier density in the optical waveguide 3 due to the injection of carriers from the region 11 is low. Therefore, the incident control light pulse saturates the gain even near the incident part.

Since the current densities of the regions 12 and 13 of the electrode 9 are set so as to increase stepwise according to the amplification of the energy of the control light, the optical waveguide 3 is formed by injecting carriers from these regions 12 and 13. The carrier density in each region also increases stepwise in each region. Therefore, the control light pulse saturates the gain in each of the regions corresponding to the regions 12 and 13 of the optical waveguide 3. As described above, in the optical waveguide of the present embodiment, the control light travels while saturating the gain in almost the entire area of the optical waveguide 3.

As described above, after passing the control light pulse, the carrier density reaches the carrier density N0 at which almost no absorption or gain is exhibited in almost the entire area of the optical waveguide 3. Thus, the injected carriers contribute to the amplification of the control light almost without waste. Therefore, it is possible to reduce the total amount of current injected into the nonlinear optical waveguide, and to efficiently give a sufficient nonlinear phase shift to the signal light.

<Embodiment 2> FIG. 2A is a view showing the structure of a nonlinear optical waveguide according to a second embodiment of the present invention. On the n-doped InP substrate 2, non-doped InGaAs / InG
aAsP (1.15 μm composition) multiple quantum well (MQW)
The structure is substantially the same as the structure of the first embodiment shown in FIG. 1 except that the structure 14 and the non-doped InP4 are formed. In the drawings, the same components are denoted by the same reference numerals.

The MQW structure 14 has a structure in which 20 InGaAs well layers and 20 InGaAsP barrier layers are alternately stacked. Prior to the growth of MQW structure 14, I
The SiO ₂ previously deposited on the nP substrate 2 is patterned to form a mask 15 as shown in FIG. The width of the stripe opening for growing the MQW structure 14 is, for example, 2 μm (W1), and the mask width on both sides of the stripe opening is, for example, 5 μm (W
2), 10 μm (W3) in region 12, 1 in region 13
5 μm (W4). Here, the length of each of the regions 11 to 13 is 150 μm.

After growing the MQW structure 14, the mask 15
Is removed, and p-doped In is performed in the same manner as in the first embodiment.
After forming a block layer of P5 and n-doped InP6 and further forming a p-doped InGaAs cap layer 7, an electrode 9 is formed and divided into regions 11 to 13. Here, too, the MOVPE method is used for crystal growth.

The nonlinear optical waveguide according to the present embodiment has a carrier density in the optical waveguide described in the above-described first embodiment such that the carrier density is low on the incident side and increases as it goes to the output side. In addition to the setting according to the energy amplification of the control light, the following features are provided.

That is, in the present embodiment, as the current density at which the peak wavelength of the gain is injected increases (that is, the current density increases in the order of the regions 11, 12, and 13),
In general, attention is paid to shifting to the short wavelength side, and by adopting a configuration in which the bandgap wavelength of the optical waveguide including the multiple quantum well structure 14 is shifted to the long wavelength side in multiple steps according to this wavelength shift. The shift of the peak wavelength of the gain to the shorter wavelength side is prevented.

Specifically, I formed by selective growth
The nGaAs / InGaAsP multiple quantum well structure 14
The thickness of the InGaAs layer is different in the regions 11, 12, and 13. This is because the mask width on both sides of the stripe is different in the regions 11, 12, and 13. Each region 11 to 1 of the optical waveguide having the multiple quantum well structure 14
3, the peak wavelength of photoluminescence was measured. 48 μm, 1.50 in region 12
μm in the region 13 and 1.52 μm in the region 13, and as the band gap wavelength advances from the incident surface side (regions 11, 12, 1).
(In the order of 3).

Since the band gap wavelength of the optical waveguide having the multiple quantum well structure 14 depends on the mask width formed on both sides of the stripe, a desired band gap wavelength shift can be obtained by changing the mask width. Can be set. This allows a bandgap wavelength shift in accordance with the shift of the peak wavelength of the gain to the shorter wavelength side.

Next, the principle of operation of the nonlinear optical waveguide as a light control element will be described. In the following description, as in the case of the first embodiment, the wavelength is 1.4.
It is assumed that control light having a pulse width of 1 ps set in a gain region of 7 μm and signal light set in a transparent region having a wavelength of 1.55 μm enter the optical waveguide.

The nonlinear optical waveguide exhibits a gain when a current is injected. The current densities in the regions 11, 12, 13 are Jl <J, as in the first embodiment.
2 <J3. Therefore, the carrier density is Nl <N
2 <N3. Thus, also in this embodiment, similarly to the first embodiment, the control light travels while saturating the gain in almost the entire area of the optical waveguide. Therefore, the injected carriers contribute to the amplification of the control light almost without waste, and it is possible to give a sufficient nonlinear phase shift to the signal light while keeping the total injection current amount to the nonlinear optical waveguide low. Becomes

At this time, the peak wavelength of the gain shifts to the shorter wavelength side as the injected current density increases. In the nonlinear optical waveguide of this embodiment, the band gap of the optical waveguide having the multiple quantum well structure 14 is previously determined. Since the wavelength is shifted to the longer wavelength side in the order of the regions 11, 12, and 13, the gain peak wavelength in each of the regions 11 to 13 is all l.l. 47 μm. Therefore, the peak wavelength of the gain coincides with the wavelength of the control light in each of the regions 11 to 13, and the nonlinear phase shift (nonlinear refractive index change) with high efficiency is achieved.
Will be obtained.

<Other Embodiments> The First and Second Embodiments
The optical path switching type optical-optical switch (optical control switch element) shown in FIG. 3 described above can be configured using the nonlinear optical waveguide of the embodiment. That is, a Mach-Zehnder interferometer is constituted by the 3 dB couplers 22 and 23 made of fiber, and the signal light enters from the port 25, is branched by the coupler 22, and interferes with the coupler 23.
Due to the phase difference between the two interfering light waves, the signal light
8 is determined. Nonlinear optical waveguide 21 described in the first embodiment or the second embodiment
Is inserted into one arm of the Mach-Zehnder interferometer.

The control light pulse is input from the port 26, passes through the wavelength selection coupler 24, and is input to the nonlinear optical waveguide 21. The control light pulse whose wavelength is set in the gain region enters the nonlinear optical waveguide 21 and causes a change in the refractive index. At this time, the signal light passing through the nonlinear optical waveguide 21 undergoes a nonlinear phase shift. When the state in which the signal light is emitted from the port 27 is the initial state, the signal light emission port is switched to the port 28 due to a change in the refractive index in the nonlinear optical waveguide 21. Thus, the switching operation of the signal light by the control light becomes possible.

As described above, the optical waveguide portion is made of InGaAsP or InGaAs / InGaAsP multiple quantum well, and
Although the description has been made by taking a nonlinear optical waveguide having a cladding as an example, according to the present invention, the same effect can be obtained even when a nonlinear optical waveguide made of another material is used, such as when a material that can be formed on a GaAs substrate is used. Can be obtained.

[0045]

As described above, according to the present invention,
In a non-linear optical waveguide utilizing a change in the refractive index accompanying the amplification of the control light, the operation of the change in the refractive index associated with the amplification of the light can be performed with a smaller amount of injection current than in the related art, so that a low energy light control element can be realized.

Further, since the amount of injected current can be reduced, the effect that the requirement for the reflectance of the antireflection coating required for suppressing laser oscillation in the optical amplification medium (laser medium) is reduced. is there.

[Brief description of the drawings]

FIG. 1A is a diagram showing a device structure of a nonlinear optical waveguide according to a first embodiment of the present invention, and FIG. 1B is a diagram showing a carrier density distribution in the waveguide.

FIG. 2A is a diagram showing an element structure of a nonlinear optical waveguide according to a second embodiment of the present invention, and FIG. 2B is a diagram showing an example of a mask used when performing crystal growth of an optical waveguide. It is.

FIG. 3 is a diagram showing a configuration of an optical path switching type optical switch using a nonlinear optical waveguide.

4A is a diagram showing a device structure of a conventional nonlinear optical waveguide, and FIG. 4B is a diagram showing a carrier density distribution in the waveguide.

[Explanation of symbols]

1, 9 electrode 2 n-doped InP substrate 3 non-doped InGaAsP stripe 4 non-doped InP layer 5 p-doped InP layer for current block 6 n-doped InP layer for current block 7 p-doped InP cap layer 8 SiO ₂ insulating film 11 to 13 region 14 InGaAs / InGaAsP multiple quantum well 15 mask 21 nonlinear optical waveguide 22 3 dB coupler 23 3 dB coupler 24 wavelength selective coupler 25 signal light input port 26 control light input port 27 signal light output port 28 signal light output port

Claims (5)

(57) [Claims] 1. An optical amplifier for controlling a control light,
An optical waveguide comprising a semiconductor optical amplifying medium in which a nonlinear refractive index change is caused by the optical amplification, wherein an optical waveguide in which a phase shift is caused by a nonlinear refractive index change caused by input of the control light with respect to input signal light; A plurality of electrodes for injecting a current into the optical amplification medium; anda plurality of current injection units each independently injecting a current into the plurality of electrodes, wherein the plurality of electrodes are The injection current amounts of the plurality of current injection means are set so that the current density at these electrodes is increased stepwise from the entrance side, being provided from the entrance side of the semiconductor optical amplification medium along the light propagation direction. A nonlinear optical waveguide. 2. The nonlinear optical waveguide according to claim 1, wherein the stepwise increase in the current density in the plurality of electrodes is a monotonic increase in accordance with the optical amplification of the control light. Nonlinear optical waveguide. 3. The nonlinear optical waveguide according to claim 1, wherein the semiconductor optical amplification medium has a multiple quantum well structure. 4. The nonlinear optical waveguide according to claim 3, wherein the multiple quantum well structure has a structure in which a band gap wavelength is shifted stepwise from an entrance side to a longer wavelength side. Wave path. 5. The nonlinear optical waveguide according to claim 4, wherein the shift of the bandgap wavelength to a longer wavelength side shifts a gain peak wavelength to a shorter wavelength side with an increase in current density in the plurality of electrodes. A nonlinear optical waveguide set according to: