JP2000321543A - Optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a novel and useful optical modulator which is adequate for using as light flickering element of, for example, an optical communication system. SOLUTION: A substrate 11 which defines an optical waveguide 14 and consists of a ferroelectric is provided with an ultrasonic generator 12 which emits a surface acoustic wave along the progression direction of the polarized light progressing in the optical waveguide 14 in order to change the mode of the polarized light by interacting on the polarized light guided to the optical waveguide 14. The optical waveguide 14 is provided with a filter 16 which permits the passage of the polarized light selectively according to the mode of the polarized light. Further, the progressing route of the surface acoustic wave from the ultrasonic generator 12 to the optical waveguide 14 is provided with a semiconductor structure 13 which induces a two-dimensional electron gas for absorbing the surface acoustic wave. The semiconductor structure is provided with a modulating means for increasing or decreasing the two-dimensional electron gas of the semiconductor structure in order to increase or decrease the absorption quantity of the surface acoustic wave.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical modulator suitable for use as an optical flicker element for generating an optical signal in an optical communication system.

[0002]

2. Description of the Related Art Light handling solid state devices are disclosed, for example, in Herman et al., Electronics Letters, 1992.
Issued on March 26, 2016, Volume 28, Issue 7, 642-64
There is a solid state device introduced on page 4 (hereinafter referred to as Document 1).

According to this solid-state device, light guided to an optical waveguide defined on a ferroelectric substrate selectively passes through a polarizing filter provided at the entrance side of the optical waveguide, for example, TE mode light. Through the TE-pass that allows
Since only predetermined polarized light (TE mode light) is allowed to pass, it receives polarized light. The polarized light passes from the TE mode to the TM mode by receiving the interaction of surface acoustic waves from an interdigital converter (IDT), which is an acousto-electric transducer having IDTs, after the polarizing filter. Is converted.

[0004] Only the converted TM-mode light is allowed to pass through a polarizing filter, for example, a TM-pass provided at a subsequent stage. Further, the wavelength of light that undergoes mode conversion depends on the wavelength of surface acoustic waves. Therefore, according to the technique described in Document 1, by adjusting the wavelength of the surface acoustic wave, the wavelength of the light passing therethrough can be adjusted, thereby realizing an optical filter whose wavelength can be adjusted.

[0005] A solid-state device that handles surface acoustic waves includes:
For example, Applied Physics
Letters, published October 12, 1998, Vol. 73, No. 15, pages 2128-2130 (hereinafter referred to as Reference 2). This reference 2 discloses that a surface acoustic wave from an interdigital converter (IDT) incorporated on a ferroelectric substrate is used as a moving carrier of a two-dimensional electronic structure (2DES) of a semiconductor incorporated on a ferroelectric substrate. (2D electron gas) introduces attenuation.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned prior art, and has as its object to provide a new and useful optical modulator suitable for use as, for example, a light blinking element in an optical communication system. And

[0007]

<Structure> An optical modulator according to the present invention includes a substrate made of a ferroelectric material having an optical waveguide defined therein, and a polarization modulator incorporated in the substrate and guided by the optical waveguide. An ultrasonic generator that emits a surface acoustic wave along the traveling direction of the polarized light that travels through the optical waveguide to interact and change the mode of the polarized light, and is provided in the optical waveguide, and according to the mode of the polarized light. A filter that selectively allows the polarized light to pass therethrough, and a two-dimensional electron that is provided on a traveling path of the surface acoustic wave on the substrate from the ultrasonic generator to the optical waveguide and absorbs the surface acoustic wave. A semiconductor structure for generating a gas, and a modulating means provided in connection with the semiconductor structure for increasing or decreasing the two-dimensional electron gas of the semiconductor structure so as to increase or decrease the absorption of the surface acoustic wave by the semiconductor structure. And

<Operation> According to the optical modulator of the present invention, the mode of polarized light guided in the optical waveguide is converted by receiving a surface acoustic wave. Further, the intensity of light passing through the filter changes according to the mode conversion degree. From this, the two-dimensional electron gas in the semiconductor structure is increased or decreased by the modulating means provided in connection with the semiconductor structure provided on the traveling path of the surface acoustic wave from the ultrasonic generator to the optical waveguide, Since the intensity of the surface acoustic wave can be substantially changed,
The intensity of light passing through the filter can be effectively increased or decreased by the modulation means. Therefore, according to the present invention, the intensity of the light guided from the optical waveguide can be modulated by the modulation signal from the modulating means. It can be used as an element.

The semiconductor material constituting the semiconductor structure is
In general, since the refractive index is different from that of an optical waveguide formed on a ferroelectric, when a semiconductor structure is formed in the optical waveguide, the light guiding function of the waveguide is impaired. Therefore, it is desirable to form a semiconductor structure on the traveling path of the surface acoustic wave from the ultrasonic generator without impairing the light guiding action of the optical waveguide.

As one method, the optical waveguide is formed by an incident-side linear portion and an emitting-side linear portion, each of which extends along a linear path, and an S-shaped curved portion connecting the two linear portions, for example. An ultrasonic generator and the semiconductor structure may be formed near the curved portion of the substrate off the optical waveguide.

Alternatively, the ultrasonic generator and the semiconductor structure may be arranged such that a traveling direction of a surface acoustic wave from the generator with respect to a linear portion of the optical waveguide is a traveling direction of polarized light along the optical waveguide. And at an acute angle from the optical waveguide on the substrate.

With the ultrasonic generator and the semiconductor structure arranged at an acute angle as one set, two sets of the ultrasonic generator and the semiconductor structure are respectively disposed on both sides of the linear portion with respect to the linear portion. They can be arranged to be symmetric. The frequency of the surface acoustic wave from the ultrasonic generator is made different for each set, and the two sets are used alternatively according to the wavelength of the light to be handled by switching the set to be used. Can be. Also, apart from frequency switching, in one set and the other set, the modulating means provided in connection with the semiconductor structure is selectively used for an electric signal and an optical signal to be described later. ,
An embodiment in which both sets are used alternatively can be adopted.
It is also possible to realize these use modes in combination.

Further, the semiconductor structure may be formed on both sides of the linear portion so as to straddle the linear portion of the optical waveguide. A groove is formed between the linear portion of the optical waveguide on the substrate and the semiconductor structure on both sides of the linear portion to prevent unnecessary absorption of the polarized light passing through the optical waveguide by the semiconductor structure. Is preferred.

The semiconductor structure can be made of a compound semiconductor such as a GaAs-based semiconductor having a quantum well.

As the modulating means, voltage applying means for applying a voltage to the semiconductor structure to increase or decrease the free carriers in the quantum well can be used, whereby an electric signal can be adopted as a modulation signal. it can. Carriers in the quantum well whose amount is increased or decreased in response to the electric signal can freely move two-dimensionally along the quantum well, and such free carriers are called a two-dimensional electron gas. . This two-dimensional electron gas effectively absorbs energy from the surface acoustic waves as their kinetic energy. As a result, the intensity of the surface acoustic wave can be substantially controlled by the electric signal, whereby the light passing through the optical waveguide is modulated according to the electric modulation signal of the modulating means.

Light modulating means for irradiating the semiconductor structure with light can be used as the modulating means, whereby an optical signal can be adopted as a modulation signal. In order to obtain good modulation characteristics, the mobility of the free carriers constituting the two-dimensional electron gas of the quantum well is 1000 cm ² /
It is desirable to show a value equal to or higher than Vs.

When an optical modulation signal is used, an inter-band transition which is a transition between a valence band and a conduction band of a semiconductor can be used as a transition of a free carrier generated in the semiconductor by irradiation of the optical signal. In a semiconductor structure having a quantum well, an interband transition in the quantum well can be used.
When the wavelength of the light of the light modulation signal is smaller than the bandgap wavelength of the semiconductor, the irradiation of the light modulation signal can effectively increase or decrease the free carriers in the quantum well, that is, the two-dimensional electron gas. Since the two-dimensional electron gas in the quantum well effectively absorbs energy from the surface acoustic wave in the same manner as described above, the intensity of the surface acoustic wave is substantially controlled by an optical modulation signal. Thus, the light passing through the optical waveguide can be modulated. In this interband transition, either a single quantum well or a multiple quantum well can be adopted as the quantum well.

With respect to the above-described optical modulation signal, inter-subband transition can be used instead of inter-band transition.
In order to utilize this intersubband transition, a multiple quantum well including a plurality of quantum wells having different quantum well levels is employed as the quantum well. Even with light having a wavelength larger than the band gap wavelength, light having a wavelength smaller than the interband wavelength in a quantum well having a relatively shallow quantum well level excites carriers in the quantum well and is excited. The free carriers move to the quantum well having the deeper quantum well level, where the quantum well absorbs energy from the surface acoustic wave as described above. Therefore, by utilizing the intersubband transition, even light having a wavelength of energy smaller than the band gap energy of the semiconductor structure can be effectively used as a modulation signal.

In order to utilize the piezoelectric effect of the substrate, the ultrasonic generator desirably uses a surface acoustic wave generating element including an acoustoelectric conversion element having a pair of electrodes formed on the substrate. A representative example of such a surface acoustic wave generating element is an interdigital converter (IDT) having interdigital electrodes.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the illustrated embodiments. <Embodiment 1> FIG. 1 shows an embodiment 1 of an optical modulator according to the present invention.
FIG. The optical modulator 10 according to the present invention includes, for example, a rectangular substrate 11, a surface acoustic wave generating element 12 incorporated in the substrate, and a semiconductor structure 13 provided near the surface acoustic wave generating element.

The substrate 11 is made of a ferroelectric such as LiNbO ₃ . On a substrate 11 made of a ferroelectric, an optical waveguide 14 having a lower refractive index than that of the substrate is formed. The optical waveguide 14 is a rectangular substrate 1 in the example shown in FIG.
A first linear portion 14a extending from one end of the substrate toward the center thereof along one longitudinal linear path;
1 from the other end to the center thereof, the first straight portion 14
The second straight portion 14b extends in parallel to the line a, and an S-shaped curved portion 14c interconnecting the inner ends of the straight portions 14a and 14b. One linear portion 14a is on the incident side, and receives, at its outer end, light having a wavelength λ of, for example, 1.5 μm from a light source (not shown) such as a semiconductor laser.

As is well known in the art, the optical waveguide 14 is formed, for example, by forming a Ti layer having a thickness of 100 nm and a width of 7 μm on the substrate 11 by vapor deposition and optical lithography, and then forming the layer at about 1000 ° C. at 8 ° C. By performing the heat treatment for a long time, the single-mode optical waveguide 14 for the incident light can be formed.

In the vicinity of the outer end of the first linear portion 14a on the incident side, a TE pass filter 15 for guiding, for example, TE mode polarized light out of the light from the light source into the optical waveguide 14 is formed. ing. The TE pass filter 15 is
For example Y ₂ O ₃ and 50n having a 25nm thickness dimension
It is formed by vacuum-depositing a laminated body made of Al having a thickness dimension of m on a predetermined position on the substrate 11.

The TE pass filter 15 has the traveling direction of the light incident on the optical waveguide 14 as the z-axis (x, y, z).
When viewed in coordinates, only the TE mode light having a polarization plane in one of the x direction and the y direction perpendicular to the traveling direction is allowed to pass.

In the vicinity of the outer end of the second linear portion 14b on the emission side, there is formed a TM pass filter 16 which allows transmission of TM mode polarized light which is a mode different from the polarized light passing through the TE pass filter 15. The TM pass filter 16
For example, it can be formed by a conventionally well-known hydrogen ion diffusion method. According to the hydrogen ion diffusion method, a predetermined area on the substrate 11 is immersed in, for example, benzoic acid at 250 ° C. for about 15.5 hours. Thereafter, it can be formed by undergoing an annealing process at about 330 ° C.

The TM pass filter 16 has the traveling direction of the light incident on the optical waveguide 14 as the z-axis (x, y, z).
When viewed in coordinates, TM having a 90 ° rotational relationship with the polarized light passing through the TE pass filter 15 around the z-axis in the traveling direction.
Allow only mode light to pass.

Therefore, the polarized light guided into the optical waveguide 14 via the TE pass filter 15 is not substantially emitted from the optical waveguide 14 via the TM pass filter 16 unless converted into the TM mode.

The surface acoustic wave generating element 12 is arranged in the vicinity of the curved portion 14c on the substrate 11 in order to convert the mode of polarized light passing through the TE pass filter 15. Since the ferroelectric material constituting the substrate 11 exhibits piezoelectric characteristics, the surface acoustic wave generating element 12 is used to construct the ultrasonic generator 12 together with the substrate by utilizing the piezoelectric characteristics of the substrate 11.
1 is provided with a pair of electrodes 12a formed thereon. The surface acoustic wave generating element 12 is a conventionally well-known interdigital converter, and a pair of electrodes 12a facing each other are alternately arranged at intervals in the longitudinal direction of the substrate 11 (see FIG. 1). Are shown only a part of them for simplicity.).
Each electrode portion 12b can be formed, for example, with a width of 100 μm and an interval of 18 μm from each other. The pair of electrodes 12a having these electrode portions 12b is
As in 4 above, it can be formed by vapor deposition of a Ti layer and a photolithographic technique.

Between the pair of electrodes 12a, for example, about 174 MH corresponding to the wavelength λ of the input light to the optical waveguide 14.
An AC signal 17 of z is applied. Due to the application of the AC signal 17, the surface acoustic wave generating element 12
Surface acoustic waves are radiated in the arrangement direction. This surface acoustic wave is emitted in the longitudinal direction of the substrate 11,
A well-known silencer 18 is formed to absorb unnecessary elastic waves propagating in the direction opposite to the propagation direction of the elastic waves propagating along the second straight portion 14b toward the outer end thereof. ing. Also, on both sides of the second straight portion 14b,
A pair of acoustic guides 19 are provided for effectively guiding the surface acoustic wave from the surface acoustic wave generating element 12 along the second linear portion 14b. Each acoustic guide 19 has a width dimension of, for example, 0.5 mm, and is formed at an interval of, for example, 110 μm from each other.

The acoustic guide 19 includes the surface acoustic wave generating element 1.
Like the two pairs of electrodes 12a, the electrodes can be formed by vapor deposition of a Ti layer having a thickness of, for example, 150 nm and photolithography. Although the acoustic guide 19 and the silencer 18 described above can be omitted,
In order to enhance the interaction between the surface acoustic wave described below and the polarized light, it is desirable to provide them as shown in the figure.

As is well known in the art, the surface acoustic wave radiated from the surface acoustic wave generating element 12 toward the outer end of the second linear portion 14b is a ferroelectric portion receiving the surface acoustic wave, that is, The polarization of the region including the two linear portions 14b is periodically changed. In response to this periodic change in polarization, the TE mode light guided from the first linear portion 14a to the second linear portion 14b via the curved portion 14c is:
While passing through the second linear portion 14b, the plane of polarization undergoes a 90 ° rotation about the axis of travel so that TM
The mode light undergoes mode conversion. Such interaction between sound and light is well known in the above-mentioned document 1.

The polarized light converted to the TM mode by the acousto-optic interaction described above passes through the TM pass filter 16 and
The light is emitted from the optical waveguide 14. Therefore, by supplying the AC signal 17 to the surface acoustic wave generating element 12, light having a wavelength corresponding to the AC signal can be emitted from the optical waveguide. Further, by adjusting the frequency of the AC signal 17, the frequency of the emitted light can be adjusted.

In order to modulate the light radiated from the optical waveguide 14, the second linear portion 1
The semiconductor structure 13 is provided on a traveling path of the surface acoustic wave to 4b.

FIG. 2 is a sectional view schematically showing the semiconductor structure 13. The semiconductor structure 13 has a quantum well structure formed on the substrate 11 in the example shown in FIG. The quantum well structure 13 includes a quantum well having a band gap energy depth sufficient to cause an interaction with a surface acoustic wave. For example, the quantum well structure 13 includes a pair of barrier layers 13a and 13b and both barrier layers 13a and 13b. And a quantum well layer 13c between them.

The quantum well structure 13 can be formed of, for example, a GaAs compound semiconductor. As an example of the quantum well structure 13 made of a GaAs-based compound semiconductor, the lower barrier layer 13a on the substrate 11 has a thickness of 30 nm and is made of, for example, silicon as an impurity for carriers.
It can be formed of an Al _0.3 Ga _0.7 As layer added at a concentration of ¹⁰ / cm ² . The quantum well layer 13c can be formed of a GaAs layer having a thickness of 10 nm. Also,
The upper barrier layer 13b is made of Al having a thickness of 500 nm.
It can be formed with a _0.3 Ga _0.7 As layer.

In such a quantum well structure 13, carriers exhibiting a mobility of 1000 cm ² / Vs or more can be generated in the quantum well (13c) by carrier injection by applying a potential described later.

Each layer (13a to 13a) of the quantum well structure 13
The thickness of c) can be changed as needed, and the thickness of the upper barrier layer 13b is determined, for example, when the bias potential described later is 0 V, that is, the bias potential is applied to the quantum well structure 13. Is not applied so that the carrier in the quantum well (13c) is depleted.
Selected.

The quantum well structure 13 having a laminated structure
Can be formed using, for example, an epitaxial lift-off (ELO) technique as shown in the above-mentioned document 2. According to this ELO technique, the quantum well structure 13 is stacked on a GaAs substrate by, for example, an AlAs sacrificial layer using a molecular beam epitaxial growth method, and the sacrificial layer is selectively formed using hydrofluoric acid. By removing, the quantum well structure 13 can be obtained. A rectangular quantum well structure 13 having, for example, a length of 2 mm and a width of 1 mm obtained by the ELO technique is arranged on the substrate 11 with its width direction along the longitudinal direction of the substrate 11. . The quantum well structure 13 and the substrate 11 are coupled by Van der Waals force as shown in the above-mentioned document 2.

As shown in FIG. 2, an electrode 20 showing an ohmic contact is formed on a part of the barrier layer 13b above the quantum well structure 13. Also the upper barrier 13
b, for example, Ti, P
A gate electrode 22 composed of a multilayer of t and Au is formed.

Between the electrode 20 of the quantum well structure 13 and the gate electrode 22, a modulating means 23 comprising a voltage applying means is provided.
Is applied. When this modulation signal is at the 0V level, the quantum well (1
Since the carrier of 3c) is depleted, free carriers that interact with the surface acoustic wave from the surface acoustic wave generating element 12, that is, a two-dimensional electron gas having high mobility are contained in the quantum well (13c). Substantially absent.

Therefore, at this time, the TE pass filter 15
Is converted into the TM mode by the interaction with the surface acoustic wave having a frequency corresponding to the wavelength of the TE mode light from the surface acoustic wave generating element 12 as described above. As a result, The light is radiated from the optical waveguide 14 through the TM pass filter 16.

When the modulation signal from the modulation means 23 is, for example, -4
When the voltage reaches V, -4 V is applied between the electrode 20 and the gate electrode 22.
Is applied, the two-dimensional electron gas as free carriers is accumulated in the quantum well (13c) of the quantum well structure 13. These two-dimensional electron gases absorb the energy of surface acoustic waves as their kinetic energy. By this acoustic electron action, for example, when the concentration of the two-dimensional electron gas becomes, for example, about 10 ¹⁰ / cm ² , the amplitude of the surface acoustic wave from the surface acoustic wave generating element 12 toward the optical waveguide 14 becomes 1 /. As it reaches 100, it is strongly damped. When the surface acoustic wave undergoes such a strong attenuation, the TE mode light having passed through the TE pass filter 15 does not undergo the mode conversion as described above, and the TM pass filter 16 provided in the second linear portion 14b. Reach TM pass filter 16 receiving this TE mode light
Blocks the transmission of the TM mode light, so that the emission of light from the optical waveguide 14 is blocked.

As is apparent from the above description, the surface acoustic wave from the surface acoustic wave generating element 12 can be attenuated in accordance with the modulation signal from the modulating means 23, thereby being guided to the optical waveguide 14. Can emit light intermittently. This makes it possible to modulate the light guided to the optical waveguide 14 by the modulation signal.

Therefore, by applying, for example, an electric circuit for transmitting a pulse electric signal of an optical communication system as the modulating means 23, the optical modulator 10 according to the present invention can be used as an optical blinking element for generating an optical signal. And can be used effectively.

In the above-described quantum well structure 13, an example has been described in which free carriers in the quantum well 13c are depleted when the bias potential is 0V. Instead of this example, by selecting the material of the gate electrode 22 of the quantum well structure 13 or the thickness of the other barrier 13b, a bias potential of 0 V, that is, when there is no bias potential, is applied to the quantum well (13c). It can be set so that the two-dimensional electron gas is accumulated and the quantum well (13c) is depleted when a bias potential is applied. Accordingly, light can be emitted from the optical waveguide 14 when a bias potential is applied between the quantum well structure 13 and the gate electrode 22, contrary to the above-described example.

In the first embodiment, the modulating means 23 provided in association with the semiconductor structure 13 having a quantum well structure includes
An example of the voltage applying means has been described. Instead of this voltage applying means, a light irradiation means can be used.

FIG. 3 showing the following specific example, the same functional parts as in the first specific example are denoted by the same reference numerals as those shown in FIG. In each of the drawings following FIG. 3 (excluding FIGS. 5, 6, and 10), an AC signal 17 provided in connection with the surface acoustic wave generating element 12 is omitted for simplification of the drawings.

<Example 2> FIG. 3 shows an example of an optical modulator 110 using light irradiation means 123 using visible light as a modulation signal. The quantum well structure 113 receiving the visible light from the light irradiation means 123 basically has a quantum well structure (13) similar to that shown in FIG. However, the quantum well structure 113 does not require the electrode 20, the gate oxide film 21, and the gate electrode 22 provided in the quantum well structure 13.

The quantum well (13c) of the quantum well structure 113
When visible light having a wavelength shorter than the bandgap wavelength of the light from the light irradiating means 123 is applied to the upper surface of the quantum well structure 113 by the visible light excitation, the quantum well (13
An inter-band transition occurs in c), and as a result, a two-dimensional electron gas similar to that described above that absorbs the surface acoustic wave from the surface acoustic wave generating element 12 is generated in the quantum well (13c).

Accordingly, the visible light from the light irradiating means 123 is used as a modulation signal and the quantum well
The two-dimensional electron gas of the third quantum well (13c) can be increased or decreased. This makes it possible to modulate the light emitted from the optical waveguide 14 by the light modulation signal from the light irradiation means 123.

According to the light irradiating means 123 of the second embodiment, visible light can be used as a modulation signal, and a gate electrode for the quantum well structure 113 is not required.
The configuration can be simplified. This quantum well structure 1
A multiple quantum well structure in which a plurality of quantum wells 13 are provided can be adopted.

<Embodiment 3> Embodiment 3 shown in FIG. 4 is a modification of the optical modulator 120 using the light irradiating means 223 which uses far-infrared light having a longer wavelength than visible light, that is, far-infrared light having small energy. Here is an example. When such low energy light is used as a modulation signal, the surface acoustic wave
A relatively well-structured quantum well exhibiting a band gap wavelength shorter than the wavelength of visible light but longer than the wavelength of light used as a modulation signal; Enough to show the effect,
Multiple quantum well structure having deeper quantum well 2
13 can be used to utilize the inter-subband transition that occurs in this multiple quantum well.

As shown in FIG. 5, the multiple quantum well structure 213 includes a pair of barrier layers 213 arranged on the substrate 11.
a and 213b, and a main quantum well layer 213c disposed between the barrier layers, and further include a plurality of auxiliary quantum well layers 213d and a plurality of auxiliary barriers 213e alternately stacked on the upper barrier 213b. Auxiliary quantum well layer 2
13d and the auxiliary barrier 213e are laminated by repeating 10 to 100 pairs (of which two pairs are shown in FIG. 5) as necessary so as to supply a required number of carriers. Is done.

Each auxiliary barrier 213e contains, for example, 10 ¹⁰ / silicon as an impurity for generating carriers.
It can be formed of an Al _0.3 Ga _0.7 As layer having a thickness of about 30 μm, added at a concentration of cm ² . The auxiliary quantum well layer 213d can be formed of a GaAs layer having a thickness of, for example, 5 to 10 nm necessary for forming an excitation level near the barrier and forming a ground level, as described later. .

The main quantum well layer 213c has the quantum well structure 1
3 can be formed of a GaAs layer having a thickness of 10 nm similar to that of the third quantum well (13c). The upper barrier layer 213b located above the main quantum well layer has a thickness of, for example, 300 nm. It can be formed with an Al _0.3 Ga _0.7 As layer having the same. Further, the lower barrier layer 213a positioned below the primary quantum well layer 213c, for example can be formed of Al _0.3 Ga _{0 .7} As layer having a thickness of 30 nm.
An impurity such as silicon is added to the barrier layer 213a so that necessary carriers can be reliably obtained.
It is desirable to add at a concentration of ⁹ / cm ² .

Each layer (213a to 213a) of the quantum well structure 213
The thickness of the upper barrier layer 13b can be appropriately changed as needed, and the thickness of the upper barrier layer 13b can be changed as needed.
The value is set to a value sufficient to prevent impurities added to 13e from reaching the main quantum well (213c) due to thermal diffusion or the like.

The above-described quantum well structure 213 is described in Example 1.
It can be formed by the ELO technique described in the above.
As clearly shown in FIG. 5, the quantum well structure 213 has an oblique cut surface 214 for directly irradiating the auxiliary quantum well layer 213d with a modulation signal, and is exposed in connection with the formation of this surface. The main quantum well layer 213c has a lower electrode 2
15 are formed. In addition, the auxiliary barrier 2 located at the top
On the upper electrode 13e, an upper electrode 222 is formed, and a DC bias 216 of, for example, 1 V with the main quantum well layer 213c side being positive is applied between both electrodes. These electrodes 215 and 222 can be formed of, for example, Cr and Au.

FIG. 6 shows the band structure of the quantum well structure 213 under the above-described DC bias. Even if the main quantum well layer 213c of the quantum well structure 213 receives light such as far-infrared light, a transition that excites carriers occurs in the main quantum well layer 213c having a band gap wavelength shorter than the wavelength of this light. There is no.

However, each auxiliary quantum well layer 213d
When far-infrared light having an energy substantially equal to the energy between the ground level L ₁ and the excited level L ₂ formed in the auxiliary quantum well layer 213d is guided to the electron at the ground level L ₁ There is excited level L ₂ to the excitation level L _2. The excited electrons pass through the barriers (213b and 213e) and reach the main quantum well layer 213c due to the tunnel effect under the DC bias 216, and they generate the surface acoustic wave in the same manner as described above. The two-dimensional electron gas effectively acts on absorption of the surface acoustic wave from the element 12.

Therefore, the above-mentioned auxiliary quantum well layer 213d
The intersubband transition accompanied by the transition in the above allows the two-dimensional electron gas in the quantum well structure 213 to be increased or decreased by using light having energy smaller than that of visible light. Thus, the surface acoustic wave from the surface acoustic wave generating element 12 can be attenuated, and the light guided to the optical waveguide 14 can be modulated as in the first and second embodiments. The far-infrared light from the light irradiating means 223 is incident on the oblique cut surface 214 so that the vibration plane of the electric field of the far-infrared light is parallel to each auxiliary quantum well layer 213d. A two-dimensional electron gas can be generated.

In Examples 1 to 3, the optical waveguide 14 defined on the substrate 11 is provided with a curved portion 14c. The curved portion 14c prevents the optical waveguide 14 from overlapping with the quantum well structures 13, 113, and 213 having different refractive indexes, whereby the light guiding action of the optical waveguide 14 and the guided polarization and surface acoustic wave generating elements are achieved. 12 and the interaction between the surface acoustic waves from the surface acoustic wave 12 was able to be secured. However, if the optical waveguide 14 has a curved portion, light transmission loss at the curved portion may be a problem. In order to solve this problem, an example will be described in which an optical waveguide 14 that follows a straight path without a curved portion 14c is employed.

<Embodiment 4> FIG. 7 shows an optical modulator 130 according to Embodiment 4 of the present invention. The surface acoustic wave generating element 12 similar to that described above of the optical modulator 130 has a center axis C extending along the width direction of each of the interdigital electrode portions 12b, that is, a linear direction extending between the electrode portions 12b. The angle θ formed between the optical waveguide 14 and the optical waveguide 14 is arranged on one side of the optical waveguide 14 so as to form an acute angle when viewed from the TE pass filter 15, which is the light traveling direction of the optical waveguide 14, toward the TM pass filter 16. ing. Corresponding to the angular orientation of the surface acoustic wave generating element 12, the quantum well structure 13 and the silencer 18 are also angularly arranged in the same attitude as the surface acoustic wave generating element 12.

Since the surface acoustic wave radiated from the surface acoustic wave generating element 12 is propagated as a spherical wave, the polarized light passing through the optical waveguide 14 due to the angular arrangement described above and the elastic wave which gives a mode conversion to the polarized light. The interaction with the surface wave is not significantly reduced. In particular, the range in which the angle θ exceeds 0 ° and is 30 ° or less (0 ° <θ ≦ 30 °)
Does not show any substantial effect of the inclined arrangement. However, if the angle θ is greater than 30 °, the interaction is substantially affected. Therefore, it is desirable to arrange the surface acoustic wave generating elements 12 angularly so that the inclination angle θ does not exceed 30 °.

In relation to the angular arrangement of the surface acoustic wave generating element 12, the acoustic guide 19 located on the side opposite to the side where the surface acoustic wave generating element 12 is arranged when viewed from the optical waveguide 14 is connected to the surface acoustic wave generating element. It is desirable to extend the acoustic guide 19 beyond the other acoustic guide 19 on the side where the 12 is arranged toward the TE pass filter 15 in order to enhance the function of the acoustic guide.

Due to the angular arrangement of the surface acoustic wave generating element 12 described above, the optical waveguide 14 can be provided without a curved portion.
Therefore, the mode of light can be effectively converted by the surface acoustic wave without causing propagation loss due to the curved portion and without impairing the guiding action of the optical waveguide 14. Thereby, the intensity change of the surface acoustic wave due to the increase and decrease of the two-dimensional electron gas by the quantum well structure 13 can be increased, and the modulation efficiency of the light by the modulation signal can be increased, so that the light guided to the optical waveguide 14 can be effectively applied. Modulation can be applied.

<Embodiment 5> FIG. 8 shows a pair of surface acoustic wave generation elements 12, a quantum well structure 13 and a muffler 18 with respect to the angular arrangement of the surface acoustic wave generation elements 12 described according to the embodiment 4. The optical modulator 140 is shown symmetrically arranged with respect to the linear optical waveguide 14.

Multiplexed light having a wavelength of, for example, 1.5 μm and 1.3 μm is introduced into the optical waveguide 14 of the optical modulator 140. An AC signal 17 of 174 MHz is applied between a pair of electrodes 12a of one surface acoustic wave generation element 12 arranged on one side of the optical waveguide 14 so as to correspond to light of 1.5 μm. On the other hand, the other surface acoustic wave generating element 12 arranged on the other side of the optical waveguide 14 to correspond to light of 1.3 μm.
Between the pair of electrodes 12a is a 200 MHz AC signal 1
7 is applied.

As long as the two-dimensional electron gas is generated in the quantum well (13c) by the modulating means 23 associated with each quantum well structure 13 corresponding to each surface acoustic wave generating element 12, both surface acoustic wave generations are performed. Since the surface acoustic wave from the element 12 is attenuated, neither light of both wavelengths is converted from the TE mode to the TM mode. Therefore,
In this state, no light of any wavelength is emitted from the optical waveguide 14.

For example, the modulation means 23 of one quantum well structure 13 corresponding to the one surface acoustic wave generating element 12 corresponding to 1.5 μm applies
When the supply of the two-dimensional electron gas, that is, the supply of the carrier is stopped, the one surface acoustic wave generating element 1 to which the AC signal 17 of 174 MHz corresponding to 1.5 μm is applied.
Only the 1.5 μm TE mode light passing through the optical waveguide 14 undergoes the conversion to the TM mode without the surface acoustic waves from 2 being attenuated. For this reason, of the light of both wavelengths, 1.
Only light of 5 μm can be emitted from the optical waveguide 14.

Therefore, according to the optical modulator 140, the multiplexed light having two wavelengths can be selectively extracted for each of the wavelengths by the modulation signals of the pair of modulation means 23.

In the specific examples 4 and 5, the quantum well structure 13 for receiving the electric modulation signal is used as the semiconductor structure 13, but the optical modulator 13 shown in the specific examples 4 and 5 is used.
As the quantum well structure 13 of the optical modulator 140 and the optical modulator 140, the quantum well structure 113 and the quantum well structure 213 which receive the optical modulation signal as shown in the specific examples 2 and 3 can be adopted.

In each of the specific examples 1 to 5, the surface acoustic wave from the surface acoustic wave generating element 12 is radiated from the element, and then the quantum well structure 13 and the optical waveguide 14 of the substrate 11 are formed.
Since the light reaches the optical waveguide 14 after passing through the region outside the region, the distance between the regions gives a delay of 10 to 50 μs for the switching response of the output signal to the modulation signal by the modulation means 23.

<Embodiment 6> The optical modulator 15 shown in FIG.
According to 0, light modulation with excellent responsiveness can be performed without inducing the time delay described above. Optical modulator 150 of specific example 6
In the figure, a surface acoustic wave generating element 12 including a pair of electrodes 12a on a substrate 11 is formed on an optical waveguide 14 across the same. Furthermore, the semiconductor structure 13 comprises two parts 1
The portions 13-1 and 13-2 are formed on both sides of the optical waveguide 14 so as to be divided into 3-1 and 13-2 and straddle the optical waveguide 14. The lower part of the surface acoustic wave generating element 12 where the pair of electrodes 12a is provided is a part of the substrate 11 on which the optical waveguide 14 is provided.
Therefore, even if the surface acoustic wave generating element 12 is arranged across the optical waveguide 14, the optical waveguide 14 of the substrate 11 does not change in the refractive index as seen in the quantum well structure 13.

In the example of the embodiment 6, the semiconductor structure 13 has a pair of quantum well structures 13 (13) including a pair of barriers 13a and 13b and a quantum well 13c disposed therebetween.
-1 and 13-2). Each quantum well structure part 1
3-1 and 13-2 are arranged on the substrate 11 at a distance from the optical waveguide 14 as clearly shown in FIG. Between the electrode 20 and the gate electrode 22 of each quantum well structure portion 13-1 and 13-2,
An electric modulation signal from the modulating means 23 is applied in parallel.

Between the optical waveguide 14 on the substrate 11 and each of the quantum well structure portions 13-1 and 13-2, a width and a depth of, for example, 2 μm extending along the optical waveguide 14 on both sides of the optical waveguide 14. Grooves 151 having the same dimensions are formed at an interval of 8 μm from each other. This groove 151
Can be formed using, for example, an electron cyclotron resonance etching method. Although the groove 151 can be omitted, the quantum well structure 13-1
It is desirable to form the groove 151 as shown in the figure in order to reduce the component of the polarized light absorbed by the light source 13 and 13-2.

According to the optical modulator 150 of Embodiment 6, since the surface acoustic wave generating element 12 is formed on the optical waveguide 14, the quantum well structure portions 13-1 and 13-2 are formed.
The delay time with respect to the modulation signal which is received depends only on the width dimension of each quantum well structure portion 13-1 and 13-213. Therefore, similarly to the specific example 1, by setting the width of each of the quantum well structure portions 13-1 and 13-2 to 1 mm, the delay of the response of the output signal to the modulation signal can be reduced to about 1 μsec. it can. Further, the quantum well structure portions 13-1 and 13-2 and the groove 151 may extend to the same length as the guide 19. Instead of the groove 151, a low refractive index film provided on the optical waveguide 14 may be used. In this case, the quantum well structure portions 13-1 and 13
-2 rides on this film.

In the specific example 6, as the semiconductor structure 13, the quantum well structure portions 13-1 and 13-1 receiving the electric modulation signal are used.
Although the example using 3-213 is shown, the quantum well structure portions 13-1 and 13- of the optical modulator 150 shown in the specific example 6 are shown.
In the second embodiment, a quantum well structure portion similar to the quantum well structure 113 and the quantum well structure 213 receiving the optical modulation signal as shown in the specific examples 2 and 3 can be adopted.

As described above, the polarization filter provided on the output side of the optical waveguide is a TM that allows the passage of the mode-converted polarized light when the polarized light undergoes the mode conversion due to the interaction with the surface acoustic wave. A pass filter was adopted. Instead of this, a TE pass filter that allows the passage of polarized light that does not undergo mode conversion can be used as the polarizing filter provided on the output side of the optical waveguide. in this case,
The on / off relationship of the optical output signal from the optical waveguide due to the modulation signal, that is, the relationship of the output value is inverted.

When the polarized light is guided to the input end of the optical waveguide, or depending on other circumstances, the polarizing filter (15) provided on the input side can be omitted.

As the substrate made of a ferroelectric, the above-described Li
In addition to NbO ₃ , a conventionally well-known ferroelectric substrate such as quartz or LiTNaO ₃ can be appropriately selected. As a semiconductor material for a quantum well structure, for example, instead of the above-mentioned GaAs-based compound semiconductor materials such as GaAs and AlGaAs, for example, InP
Various semiconductor materials such as the compound semiconductor materials described above can be used as appropriate.

As a semiconductor structure which is a two-dimensional electron gas supply source, in addition to the above-described quantum well structure, for example, a quantum wire or HEMT element which becomes a two-dimensional electron gas supply source having high mobility of carriers may be used. it can.

[0082]

According to the present invention, as described above, the modulation means provided in connection with the semiconductor structure provided on the traveling path of the surface acoustic wave from the ultrasonic generator toward the optical waveguide, The two-dimensional electron gas of the semiconductor structure interacting with the surface acoustic wave can be increased or decreased, and the intensity of light passing through a filter provided in the optical waveguide can be increased or decreased. , The intensity of light guided from the optical waveguide can be effectively changed, and a new and useful optical modulator is provided.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a specific example 1 of an optical modulator according to the present invention.

FIG. 2 is a cross-sectional view schematically showing a quantum well structure of Example 1.

FIG. 3 is a perspective view showing a specific example 2 of the optical modulator according to the present invention.

FIG. 4 is a perspective view showing a specific example 3 of the optical modulator according to the present invention.

FIG. 5 is a cross-sectional view schematically showing a quantum well structure of Example 3.

FIG. 6 is an explanatory diagram of the band gap of the quantum well structure of Example 3.

FIG. 7 is a plan view showing Example 4 of the optical modulator according to the present invention.

FIG. 8 is a plan view showing Example 5 of the optical modulator according to the present invention.

FIG. 9 is a plan view showing Example 6 of the optical modulator according to the present invention.

FIG. 10 is a cross-sectional view taken along line XX shown in FIG.

[Explanation of symbols]

10, 110, 120, 130, 140 and 150
Optical modulator 11 Substrate 12 (Surface acoustic wave generating element) Ultrasonic generator 13, 113, 213 (Quantum well structure) Semiconductor structure 14 Optical waveguide 16 Filter 23, 123, 223 Modulation means

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2H047 KA03 KA12 NA07 PA12 QA02 QA03 RA08 2H079 AA04 AA13 BA01 BA02 CA05 DA03 DA16 DA22 EA03 EB23 HA08 KA05 KA20

Claims (13)

[Claims] 1. A substrate made of a ferroelectric substance having an optical waveguide defined therein, and the optical waveguide being integrated with the substrate and traveling through the optical waveguide to change a mode of the polarized light by interacting with polarized light guided by the optical waveguide. An ultrasonic generator that emits a surface acoustic wave along the traveling direction of the polarized light, a filter provided in the optical waveguide, and selectively allowing passage of the polarized light according to the mode of the polarized light; A semiconductor structure provided in a traveling path of the surface acoustic wave on the substrate from a vessel to the optical waveguide, and generating a two-dimensional electron gas for absorbing the surface acoustic wave; and And a modulator for increasing or decreasing the two-dimensional electron gas of the semiconductor structure so as to increase or decrease the absorption of the surface acoustic wave by the semiconductor structure. 2. The optical waveguide according to claim 1, wherein each of the optical waveguide includes an incident side linear portion and an emitting side linear portion along a linear path, and a curved portion connecting the two linear portions. 2. The light according to claim 1, wherein the light is provided in the vicinity of the curved portion of the substrate, and the ultrasonic generator is arranged such that a traveling direction of a surface acoustic wave from the generator is along the linear portion on the emission side. Modulator. 3. The optical waveguide includes a linear portion along a linear path, and the ultrasonic generator and the semiconductor structure are arranged such that a traveling direction of a surface acoustic wave from the generator has a polarization direction along the optical waveguide. The optical modulator according to claim 1, wherein the optical modulator is disposed on the substrate so as to form an acute angle with the traveling direction. 4. An optical modulator according to claim 3, wherein said ultrasonic generator and said semiconductor structure are respectively arranged symmetrically on both sides of said linear portion in a symmetrical manner with respect to said linear portion. 5. The optical waveguide includes a straight section along a straight path, the sound generator is disposed across the straight section, and the semiconductor structure is formed on both sides of the straight section across the straight section. The optical modulator according to claim 1, wherein: 6. An unnecessary light absorption of the polarized light passing through the optical waveguide by the semiconductor structure is provided between the linear portion of the optical waveguide and the semiconductor structure on both sides of the linear portion on the substrate. 4. An optical modulator according to claim 3, wherein a groove is formed. 7. The optical modulator according to claim 1, wherein the semiconductor structure is made of a compound semiconductor having a quantum well. 8. The optical modulator according to claim 7, wherein the mobility of free carriers in the quantum well indicates a value of 1000 cm ² / Vs or more. 9. The modulating means comprises voltage applying means for applying a voltage to the semiconductor structure so as to increase or decrease the free carriers in the quantum well, and an electric signal from the voltage applying means is used as a modulation signal. The optical modulator according to claim 7. 10. The modulation means comprises light irradiation means for irradiating the semiconductor structure with light to increase or decrease the free carriers in the quantum well, and an optical signal from the light irradiation means is used as a modulation signal. Item 7. An optical modulator according to Item 7. 11. The carrier transition generated in the semiconductor structure by irradiating the semiconductor structure with the optical signal is an inter-band transition caused by absorption of light having a wavelength smaller than a band gap wavelength of the semiconductor structure. Item 11. The optical modulator according to Item 10. 12. The quantum well is a multiple quantum well including a plurality of quantum wells having different quantum well levels, and the carrier transition generated in the semiconductor structure by irradiating the semiconductor structure with the optical signal. The optical modulator according to claim 10, wherein is an intersubband transition caused by absorption of light having a wavelength larger than the band gap wavelength of the semiconductor structure. 13. The optical modulator according to claim 1, wherein the ultrasonic generator is a surface acoustic wave generating element including a pair of electrodes formed on the substrate to use a piezoelectric effect of the substrate.