JPH0728104A - Light modulation element - Google Patents

Abstract

PURPOSE:To provide a light modulation element which can make modulation of the intensity of signal light with turning on and off of the excited light. CONSTITUTION:An n-type AlInAs layer 43 is provided on the mirror index display (111) plane of an n-type InP board 41, and a distortion MQW structure 45 composed of an A1InAs barrier layer/InGaAs well layer is furnished on the layer 43, and a p-type AlInAs layer 47 is prepared on this MQW structure 45. The well layer should be a lattice mismatching well layer which is not in lattice matching with the board 41. The p-and n-type layers 43, 47 are formed relative to the AlInAs barrier layer in such a way as making lattice matching with the board 41. Thereby absorption and transmission of the signal light can be generated by varying the band gap wavelength of the distortion MQW structure through utilization of the internal electric field generated in the distortion MQW structure by piezo effect resulting from crystal structure and the electric field generated in the distortion MQW structure owing to the light excitation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator capable of modulating the intensity of signal light by turning on and off control light, and for example, optical switching, optical signal processing, and optical information processing equipment. The present invention relates to an optical modulator that can be used as a signal gate element.

[0002]

2. Description of the Related Art An optical modulator for spatially intensity-modulating propagating signal light is used for optical switching, optical information processing, and the like.
Among these elements, in particular, the element capable of controlling the modulation by light is capable of all-optical optical signal processing,
It has the advantage that the characteristics of light can be utilized. By the way, an element whose light transmission-absorption state changes by optical excitation or such a material is generally called a "saturable absorber". In this saturable absorber, when the excitation intensity when optically exciting the saturable absorber exceeds a certain threshold value, the place where the light was absorbed until then becomes to transmit the light. This property can be one means for achieving the above-mentioned object of controlling intensity modulation of signal light by light. As a specific example, a nonlinear etalon disclosed in Document I (Photonic Switching (photonic switching) vol.3, pp.15-21 (1989)) is known. FIG. 3 is a cross-sectional view (a diagram cited from FIG. 5 of Document I) schematically showing the configuration of the nonlinear etalon element disclosed in Document I. This device comprises an AlGaAs multilayer mirror 13 on an n-GaAs substrate 11, and a multiple quantum well (MQW) layer 15 composed of an i-GaAs layer and an AlGaAs layer as a nonlinear medium on the multilayer mirror 13. This MQW layer 15 was provided with p-AlGaAs 17. This element is used by inputting signal light and pumping light when necessary. This element acts as a nonlinear Fabry-Perot resonator due to the nonlinear refractive index change in the GaAs MQW nonlinear medium that occurs when pumping light (control light) is introduced. Then, the signal light is turned on and off depending on whether or not the nonlinear refractive index change in the GaAs MQW nonlinear medium satisfies the resonance condition given by the AlGaAs multilayer mirror 13. Specifically, when the above resonance condition is satisfied, the signal light passes through this element.

[0003]

However, in the conventional optical modulator, the device characteristics are: sensitive to the film thickness uniformity of each layer of the AlGaAs multilayer mirror ;: Fabry-Perot condition easily fluctuates depending on temperature; There is a problem in that it is impossible to monitor the state of intensity modulation: if the intensity modulation of the signal light is also attempted electrically, it is impossible.

In many cases, many devices of this type are arranged in parallel and driven. For example, elements are often used in a one-dimensional array or a two-dimensional array due to the requirements of the system architecture. However, because of the above problems (1) to (3), optical and thermal interference easily occurs between the elements in the array, which is a problem in application.

This application has been made in view of the above points, and therefore the first object of the present invention is to provide
An object of the present invention is to provide a novel light modulation element that can solve the above problems. The second object of the present invention is to
An object of the present invention is to provide a novel light modulation element that can solve the above problems.

[0006]

In order to achieve the first object, according to the optical modulator of the present invention, a single crystal substrate having a zincblende (Zincblende: ZB) structure and a mirror index of the substrate are used. An optical waveguide portion provided on the upper side of the display (111) plane and including the strained quantum well structure as a light absorption layer. Here, including that the optical waveguide portion includes the strained quantum well structure as a light absorption layer means that the optical waveguide portion is configured only by the strained quantum well structure, and the optical waveguide portion includes the strained quantum well structure and the other This means that it may be composed of any other material (for example, a light guide layer).

In the present invention, the strained quantum well structure can be manufactured, for example, as follows. A compressive or tensile strain is generated in the well layer to form a lattice-mismatched well layer that is not lattice-matched to the single crystal substrate. The barrier layer is formed so as to be lattice-matched with the single crystal substrate. The lattice mismatch between the lattice mismatch well layer and the lattice matching barrier layer is utilized and these layers are grown within the critical thickness. As a result, a quantum well structure having a strain therein, that is, a strained quantum well structure is obtained. Incidentally, in manufacturing the strained quantum well structure, the barrier layer may be subjected to compressive or tensile strain contrary to the above example.

Further, in order to achieve the second object, the optical modulation element of the present invention has, in addition to the above-mentioned predetermined substrate and predetermined optical waveguide section, a zinc blende structure for the optical waveguide section. And a layer for applying an electric field parallel to the <111> direction. In practicing the present invention, a layer for applying an electric field is formed by sandwiching the optical waveguide.
It is preferable to use the type semiconductor layer and the n-type semiconductor layer. Of course, in this case, typically, a voltage is applied to each of these p-type and n-type semiconductor layers by applying a voltage to a separately provided electrode, and thereby an electric field is applied to the optical waveguide portion. Become. The zincblende structure single crystal substrate may be a zinc blende structure semiconductor single crystal substrate. In this case, one of the p-type semiconductor layer and the n-type semiconductor layer as a layer for applying the above-mentioned electric field can be replaced with the semiconductor single crystal substrate of the zinc blende structure. The layer for applying an electric field may be an electrode for applying voltage (for example, a Schottky electrode) provided in contact with the optical waveguide portion. Further, the single crystal substrate in the present invention includes not only the substrate itself but also a substrate having an epitaxial growth layer on the substrate.

[0009]

The optical modulator of the present invention comprises a single crystal substrate having a zinc blende structure and an optical waveguide portion provided on the upper side of the mirror index display (111) surface of the substrate and including a strained quantum well structure as a light absorption layer. And with. In this configuration, the zinc blend structure <
The piezo effect is caused by the asymmetry of the crystal structure in the 111> direction. Here, assuming that the strained quantum well structure causes strain in the well layer, the inside of the well layer in the strained quantum well structure is less than <due to the induced piezoelectric effect.
The strain in the 111> direction generates an electric field (hereinafter, “internal electric field” or “piezo electric field”). The bandgap wavelength in the quantum well structure in this state is hereinafter referred to as λ _A. Here, if the signal light is light having a wavelength of λ _A > λ _B and light that is sufficiently weakly excited (meaning light that does not form a space charge electric field that affects the piezoelectric field by itself), this signal light is used. Are absorbed in the strained quantum well structure. on the other hand,
When excitation light (control light) is input to this light modulation element, optical carriers are generated in the well layer by this optical excitation, and this forms a space charge distribution due to an internal electric field. Also, this space charge distribution forms an electric field, and this electric field acts as a screening electric field for the internal electric field, thus reducing the internal electric field. Since the internal electric field decreases, the bandgap wavelength in the quantum well structure at this time becomes λ _C , which is shorter than λ _A. If the band gap wavelength λ _C and the wavelength λ _{B of the} signal light satisfy λ _B > λ _C (designed to have such a relationship), the device of the present invention is excited by the excitation light. The signal light is transmitted in the open state. Therefore, the intensity modulation of the signal light is realized by the absorption of the signal light when there is no optical pumping and the transmission of the signal light when optically pumped. In the configuration of the present invention, since the multi-layer mirror layer is not used and the non-linear Fabry-Perot resonator is not used, the conventional problem can be avoided. In addition, the state of intensity modulation can be monitored by adding an inductance inside or outside the element and measuring the carrier attenuation caused by the excitation light.

Further, in the case of a constitution including a predetermined layer for applying a predetermined electric field, by applying a voltage to this layer, the screening electric field can be formed instead of generating the screening electric field by photoexcitation. Therefore, it becomes possible to electrically modulate the signal light. Also, the state of intensity modulation can be monitored by monitoring the electric field intensity via this layer.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the optical modulator of the present invention will be described below with reference to the drawings. However, the drawings used in the description merely show the dimensions, shapes, and positional relationships of the respective constituents to the extent that the present invention can be understood. Further, in each drawing, the same components are denoted by the same reference numerals, and duplicate description thereof will be omitted in some cases.

1. Description of Basic Principle The operation principle of the light modulation element of the present invention will be described with reference to an example in which an InP substrate is used as a single crystal substrate of a zinc blende structure.

As a basic example of the optical modulator of the present invention, I
On the mirror index display (111) surface of the nP substrate, AlIn
Consider a device having a multiple quantum well structure including an As barrier layer / InGaAs well layer and having a strain therein (hereinafter, a strained MQW structure). However,
It is assumed that the strained MQW structure is formed by causing compressive or tensile strain in the well layer. In this element,
Due to the asymmetry (strain) of the crystal structure in the <111> direction of the zinc blende structure, a piezoelectric field in the <111> axis direction of the zinc blende structure is induced in the well layer. The band structure of the strained MQW structure with and without the excitation light is changed as described below with reference to FIGS. 1 (A) to 1 (C). In FIGS. 1A to 1C, 21 is a well layer and 23 is a barrier layer.

First, FIG. 1A shows a band structure of a strained MQW structure when the device is not optically excited. Due to the piezoelectric field 25 generated in the well layer by providing the strained MQW structure on the predetermined substrate surface, the band structure is not a flat band state but has a potential gradient. As a specific example, considering the case where the lattice mismatch between the well layer 21 and the barrier layer 23 is, for example, 0.8%, the piezoelectric field having a magnitude of 84 KV / cm in the <111> direction inside the well layer. Is induced. And since the photo-excitation is not performed here, this piezo electric field is applied to the quantum well, so the electric field is applied vertically to the quantum well. Instead, a potential gradient is generated. In the element having the band structure in which the potential gradient is generated as described above, the energy is lower than that at the absorption edge when the element is in the flat band state, and FIG.
When the signal light 27 having a higher energy than the absorption edge in the band state of 7 and a sufficiently weak excitation is input, the signal light 27 is absorbed in the element (one mode of modulation of the signal light occurs). .

When a device having a band structure in the state as shown in FIG. 1A is excited by light (excitation light) 29 having a higher energy than the absorption edge in the flat band state of the device, the strained MQW structure is obtained. Then, as shown in FIG. 1B, an electron (e) and a hole (h), which are photocarriers, are
Electrons are induced in the conduction band and holes are induced in the valence band. These electrons and holes move in mutually opposite directions by the internal electric field (piezoelectric field) 25. Then, as shown in FIG. 1C, these electrons and holes form a space charge distribution. This space charge distribution is due to the electric field 31
To form. This electric field 31 acts as an electric field for piezo electric field screening (hereinafter referred to as “screening electric field”). The piezo electric field 25 is the above screening electric field 31.
Is reduced by (may be zero),
Since the electric field applied to the quantum well of the strained MQW structure decreases, the band structure of the strained MQW structure changes in the direction approaching the state of the flat band as shown in FIG. Therefore, the effective energy at the absorption edge in the strained MQW structure increases (the bandgap wavelength becomes shorter), so that light having a shorter wavelength than before the photoexcitation can be transmitted through the MQW structure (signal light). The other aspect of the modulation of. When the excitation light is blocked, the screening electric field is canceled after relaxation of the excited electrons and holes, and only the piezoelectric field is applied to the quantum well again. Therefore, in the strained MQW structure, as shown in FIG. The signal light is absorbed as described above. By doing so, the intensity of the signal light can be modulated by turning on / off the pumping light.

2. More Practical Example of Element Next, a practical example of the element in carrying out the present invention will be described. FIG. 2 is a perspective view showing the light modulation element.

The device of this example comprises an n-type InP substrate 41 as a single crystal substrate of zinc blende structure. And this I
An n-type AlInAs layer 43 is provided on the mirror index display (111) surface of the nP substrate 41.
3 has a strained MQW structure 45 composed of an AlInAs barrier layer / InGaAs well layer, and this strained MQW structure 4
On top of this, a p-type AlInAs layer 47 is provided. InGaAs
The well layer is a lattice mismatch well layer that does not lattice match with the substrate 41. p and n type AlInAs layers 43 and 47 and AlI
The nAs barrier layer is formed so as to be lattice-matched with the substrate 41. Further, in the device of this example, the n-type AlInAs layer 4
3. The pin structure 49 composed of the strained MQW structure 45 and the p-type AlInAs layer 47 has a ridge shape. Further, both sides of the ridge-shaped pin structure 49 are filled with a buried layer, for example, a semi-insulating InP buried layer 51. Furthermore, a first electrode 53 is provided on the back surface of the substrate 41,
A second electrode 55 is provided on the p-type AlInAs layer 47 and the buried layer 51 so as to extend therethrough.

In the element shown in FIG. 2, the optical waveguide portion is substantially composed of the strained MQW structure 45. The layers for applying an electric field to the optical waveguide part are p-type and n-type AlInAs layers 4
It is composed of 3, 47. However, p and n type AlIn
A voltage is applied to the As layers 43 and 47 by the substrate 41, the first electrode 53, and the second electrode 55. In this element,
The signal light and the excitation light are respectively put into the ridge-shaped pin structure 49. These lights may enter from one end in the longitudinal direction of the ridge-shaped pin structure 49, or the pin structure 4
9 p-side layer 47 side or n-side layer 43 (substrate 41)
You can enter from the side. However, ridge-shaped pin structure 4
It is considered that the effect can be achieved by inserting signal 9 into one end from one end in the longitudinal direction in that the signal light is absorbed. In addition, it may be possible that the pumping light and the signal light may enter from different paths. Practically, the signal light and the pumping light can be introduced into the device, for example, in a state where they are multiplexed, for example, via a fiber optical system or various waveguides.

In the element shown in FIG. 2, the elements shown in FIGS.
The intensity of the signal light can be controlled by turning on / off the pumping light according to the principle described using. However, in the element of FIG. 2, the intensity of signal light can be modulated by an electric signal. That is, in the element structure of FIG. 2, use two states, that is, when a reverse bias voltage is applied between the first and second electrodes 53 and 55 and when an electric signal is not applied between these electrodes 53 and 55. Can modulate the intensity of the optical signal. However, applying a reverse bias voltage between the first and second electrodes 53 and 55 reduces the internal electric field (piezoelectric field) of the MQW structure 45 (including the case where the electric field is zero).
A voltage is applied so that a directional electric field is generated. When a reverse bias voltage is applied between the first and second electrodes 53 and 55 in this way, the internal electric field (piezoelectric field) is reduced, so that the effective band gap of the strained MQW structure 45 widens as in the case of photoexcitation. Signal light can be transmitted. Therefore, the element shown in FIG.
It becomes an element (electroabsorption optical modulator) that can modulate not only the intensity of signal light with excitation light but also an electric signal. In this case, the element is a normally-off type element.

Although the embodiments of the optical modulator of the present invention have been described above, the present invention is not limited to the above embodiments.

For example, in the above description, an example of an element having a buried structure is shown as a more practical example of the element. This is because the lateral confinement of light can be achieved by doing so, and it is effective in reducing the loss of the optical signal. However, the present invention is not limited to the embedded type in principle. For example, some of the structures known in the field of semiconductor lasers can be applied.

Further, in the above-mentioned embodiment, InP is used as the substrate.
The substrate was used and the QW structure was composed of AlInAs / InGaAs. However, the substrate can be widely used as long as it is a single crystal substrate having a zinc blende structure. However, a semiconductor single crystal substrate having a zinc blende structure is preferable in consideration of manufacturing convenience, for example, a case where a strained MQW structure is manufactured, a case where a semiconductor layer is used as an electric field application layer, and the like. The QW structure can also be made of an appropriate material according to the substrate. As another example of the substrate and the QW structure, for example, (a). When GaAs is used as the substrate and the QW structure is composed of InGaAs / GaAs, (b). GaAs is used as the substrate and the QW structure is composed of ZnSSe / ZnS. In this case, (c). The case where an InP substrate is used as the substrate and the QW structure is made of AlInGaAs / InGaAs can be mentioned. In such a case, the same effect as that of the embodiment can be obtained. In the case of adopting the above-mentioned configurations (a) to (c) and configuring the device having the buried structure shown in FIG. 2, the buried layer is, for example, AlGaAs or InGaP in (a), It can be composed of ZnS in b) and InP or AlIn (Ga) As in (c). A II-VI group compound semiconductor substrate can also be used as the substrate.

[0023]

As is apparent from the above description, according to the optical modulator of the present invention, an internal electric field generated in the strained MQW structure due to the piezo effect due to the crystal structure and optical excitation cause distortion in the strained MQW structure. By changing the bandgap wavelength of the distorted MQW structure using the electric field generated, absorption and transmission of signal light can be generated. Therefore, an all-optical modulator can be obtained without using a nonlinear Fabry-Perot resonator using a multilayer mirror layer.
Therefore, it is possible to avoid the problem when the nonlinear Fabry-Perot resonator using the multilayer mirror layer is used. Further, the modulation intensity can be monitored by adding an inductance inside or outside the element and measuring the attenuation of the carrier generated by the excitation light. Further, in the configuration in which the layer for applying the electric field is further provided, the modulation of the intensity of the signal light can be electrically controlled.

The optical modulator of the present invention can be utilized as an optical signal switch of an optical information processing system involving intensity modulation of an optical signal by an optical clock, for example. For example, by using the optical modulation element of the present invention as the Exchange-Box of each node when optically achieving perfect shuffle, it is possible to reconfigure the multistage configuration with pump light that is an optical clock. A parallel processing system is obtained. Thus, the element of the present invention can be used as an optical gate element in optical switching and optical information processing.

[Brief description of drawings]

1 (A) to 1 (C) are explanatory views of the basic principle of an optical modulation element of the present invention, and a band structure of a strained MQW structure with and without pumping light entering the element. It is a figure showing a change of.

FIG. 2 is a diagram showing an example of a practical element structure when implementing the present invention.

FIG. 3 is a diagram for explaining a conventional light modulation element.

[Explanation of symbols]

 21: Well layer 23: Barrier layer 25: Internal electric field (piezoelectric field) 27: Signal light 29: Excitation light 31: Screening electric field 41: n-InP substrate 43: n-AlInAs layer 45: Strained MQW structure 47: p-AlInAs Layer 49: Ridge-shaped pin structure 51: Buried layer 53: First electrode 55: Second electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takeshi Takamori 1-7-12 Toranomon, Minato-ku, Tokyo Oki Electric Industry Co., Ltd.

Claims (4)

[Claims] 1. A single crystal substrate having a zinc blende structure, and an optical waveguide portion provided above the Miller index indicating (111) plane of the substrate and including a strained quantum well structure as a light absorption layer. Characteristic light modulator. 2. The light modulation element according to claim 1, further comprising a layer for applying an electric field parallel to the <111> direction of the zinc blende structure to the optical waveguide portion. Light modulator. 3. The light modulation element according to claim 1, wherein the zincblende structure single crystal substrate is a zinc blende structure semiconductor single crystal substrate. 4. The light modulation element according to claim 1, wherein the layer for applying the electric field includes a p-type semiconductor layer and an n-type semiconductor layer sandwiching the optical waveguide portion (provided that one of these layers is The single crystal substrate may be used.).