JPH0583889B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is used in optical communication systems, etc., and can modulate light at high speed with low voltage and low loss. The present invention relates to an integrated waveguide type optical device using possible semiconductor materials.

(Prior art and its problems) With the full-scale commercialization of optical communication systems in recent years, there has been a need for optical devices with various functions and higher performance. Examples of such devices include optical switches that switch optical paths and optical modulators that change the propagation state of light. Among these devices, devices that utilize electro-optic effects have excellent characteristics and are widely used. Waveguide optical devices such as optical switches or optical modulators that utilize such electro-optic effects include those using dielectric materials such as LiNbO ₃ and semiconductor materials such as GaAs and InP. There is. Although waveguide type optical devices using dielectric materials have advantages such as very low light propagation loss and low voltage operation, they are not suitable for monolithic integration with light sources and the like. In contrast, waveguide optical devices using semiconductor materials can be monolithically integrated with light-emitting and light-receiving devices, and are important for realizing future optical integrated circuits.

Conventionally, a directional coupler type modulator/switch has been used as a semiconductor waveguide type optical device that utilizes such an electro-optic effect. A directional coupler has two waveguides, and switches between the two waveguides using changes in the propagation state of guided light due to the electro-optic effect. If we pay attention only to this, we can treat it as a phase modulator. Here, the basic structure and operation of the phase modulator will be explained using the phase modulator as an example. Figure 1 is an example.
FIG. 3 is a diagram showing the basic structure of a phase modulator using InGaAsP/InP-based materials. Although this is the simplest structure, such a structure causes a very large light propagation loss. This will be discussed in detail later. Figure 1 is a cross-sectional view of the phase modulator taken along a plane perpendicular to the light propagation direction, n ⁺ -
n-InGaAsP guide layer 4 and p ⁺
A p ⁺ diffusion layer of -InGaAsP is stacked in a rib shape as shown in FIG. 1, and an electrode 2 is provided thereon. When the refractive index of the p ⁺ diffusion layer 3 is _n3 , the refractive index of the guide layer ₄ is n4, and the refractive index of the substrate 5 is _n5 , generally there is a relationship of _n4 > _n3 and _n4 > _n5 . In addition, as shown in Figure 1, since the guide structure has a rib shape, in the horizontal direction, the effective refractive index is higher in the lower part of the electrode than in the parts on both sides of the guide layer, even with the same refractive index. a little expensive.
Therefore, the light incident on the n-InGaAsP guide layer 4 is confined in both the vertical and horizontal directions, forming a waveguide and propagating three-dimensionally. Here, a reverse bias voltage signal 1 is applied to the electrode 2.
Then, a depletion layer spreads in the guide layer 4, which is a low carrier concentration layer, and an electric field is applied.The propagation state of the light propagating through the guide layer 4 changes due to the electro-optic effect caused by the electric field, and the emitted light becomes the reverse bias voltage signal 1. shows the change in phase according to . Phase modulation is performed in the manner described above.

However, the waveguide type optical device having such a structure and utilizing the electro-optic effect has the following problems. Immediately above and below the guide layer 4, there is a thick p ⁺ diffusion layer 3 with very high light absorption and a low resistance substrate 5 with a high carrier concentration, so most of the light seeping out from the guide layer is absorbed by the p ⁺ diffusion layer. 3. It is absorbed by the low resistance substrate 5 and becomes a propagation loss. In order to increase the electric field strength and improve the modulation efficiency, the guide layer 4 can be made thinner, but if the composition is constant, the thinner the guide layer 4 is, the more light seeps out of the guide layer. Since the propagation loss will also increase, the guide layer thickness cannot be made so thin if the propagation loss is taken into account. Furthermore, if the thickness of the guide layer is increased, the propagation loss is reduced, but an effective electric field cannot be applied, and the voltage required to obtain the desired phase change becomes large. Therefore, in order to obtain an effective electric field, the guide layer cannot be made very thick. There is a trade-off between voltage and propagation loss, as described above when the guide layer thickness becomes thinner, the voltage decreases but the propagation loss increases, and when the guide layer thickness becomes thicker, the propagation loss decreases but the voltage increases. Therefore, with such a structure, it was not possible to reduce both voltage and propagation loss.

Next, as another conventional example, we will explain an example in which a buffer layer with low light absorption is provided between the guide layer and the electrode and between the guide layer and the substrate in order to create a structure to reduce propagation loss. . FIG. 2 is a diagram for explaining a phase modulator provided with a buffer layer for reducing propagation loss. Also, here we will explain the case where InGaAsP/InP, which is an actual semiconductor material, is used. FIG. 2 is a cross-sectional view of the phase modulator taken along a plane perpendicular to the light waveguide direction. On the n ⁺ -InP substrate 16, an n-InP buffer layer 15, an n ^- -InGaAsP guide layer 14, an n ^-
-InP buffer layer 13, p ⁺ -InP diffusion layer 12, and electrode 11 are laminated as shown in FIG. n ^- −
The refractive index of the InGaAsP guide layer 14 is n ₁₄ , and the others are p ⁺
Letting the refractive indices of the -InP ₁₂ , n ^- -InP buffer layer 13 and n ^- -InP buffer layer 15 to be n ₁₂ , n ₁₃ and n ₁₅ , respectively, n ₁₄ > n ₁₂ = n ₁₃ = n ₁₅ . In addition, in the horizontal direction, the rib-type structure effectively creates a difference in refractive index. The light that has entered the guide in this manner is guided three-dimensionally within the guide layer 14. The modulation method is the same as in the previous example, and the electrode 11
A reverse bias voltage signal 10 is applied between the n ⁺ -InP substrate 16 and the low resistance n + -InP substrate 16, and a change in the propagation state caused by the electro-optic effect is utilized.

In this way, the buffer layer 1 is placed above and below the guide layer 14.
3 and 15, the guide layer 14
The light seeping into the buffer layers 13 and 15 with lower light absorption located above and below the guide layer 14 is hardly lost, and the light leaks to the buffer layers 13 and 15 up to the n ⁺ -InP substrate located outside the buffer layers 13 and 15. Only a small amount of light that seeps out through 15 becomes a propagation loss. Therefore, by providing a buffer layer, the propagation loss can be made smaller than before, and it can be said that the thicker the buffer layer is, the smaller the propagation loss is. However, buffer layer 1
By providing 3 and 15, the thickness to which the electric field is applied increases, so the applied voltage also increases accordingly.In the end, considering the two points of loss and voltage, even if a buffer layer is provided, it is possible to achieve low loss and It cannot be said that a low voltage element can be realized.

Another conventional example is a structure for low voltage and low loss, in which a low resistance layer with a very thin electrode for applying an electric field is provided below the guide layer, and a layer with low light absorption is provided below the guide layer. Explain about. FIG. 3 is a diagram for explaining a phase modulator having a structure for low voltage and low loss. Also, here we will explain the case where InGaAsP/InP is used as the material. FIG. 3 is a cross-sectional view of the phase modulator taken along a plane perpendicular to the light waveguide direction. High resistance InP with low carrier concentration and low light absorption
n ⁺ -InP low resistance layer 25, n ^- - on the substrate 26
InGaAsP guide layer 24, n ^- -InP buffer layer 2
3. The p ⁺ -InP diffusion layer 22 and the electrodes 21 and 28 are the third
They are stacked as shown in the figure. The light incident on the guide layer 24 passes through the n ^- -InGaAsP guide layer 24 in the vertical direction.
and n ^- -InP buffer layer 23, n ⁺ -InP low resistance layer 3
Due to the difference in refractive index with 5, the wave is confined in the horizontal direction by forming a rib shape, and the wave is guided three-dimensionally. The light seeping out from the guide layer 24 goes upward to the buffer layer 23, downward to the low resistance layer 25, and to the substrate 26.
It spreads to. Since the upper buffer layer 23 is a layer with a low carrier concentration, it absorbs almost no light, and regarding light seepage downward, the light is absorbed by the high carrier concentration immediately below the guide layer 24. Only the n ⁺ -InP low resistance layer 25 is the layer, and the low carrier concentration substrate 26 below it absorbs almost no light. In addition, since the low resistance layer 25 is a very thin layer, only a small portion of the light seeping out below the guide layer 24 is absorbed by the low resistance layer 25 . Therefore, it can be said that the propagation loss in a waveguide type optical element having such a structure is extremely small. Furthermore, phase modulation is performed by applying a reverse bias voltage signal 20 to the electrode 21, but since there is a low resistance layer 25 that serves as an electrode for applying an electric field directly under the guide layer 24, the voltage in the guide layer 24 can be effectively adjusted. Since an electric field can be applied, the voltage required to perform phase modulation using an electric field is smaller than in the structure shown in FIG. 2 in which voltage is applied from outside the buffer layer.

However, when such semiconductor materials are used, the electro-optic effect in the semiconductor is small, so
Even if you try to lower the voltage by reducing the thickness of the guide layer and optimizing the parameters, at best
The voltage limit was 10V, and it was difficult to lower the voltage to the TTL level that allowed high-speed modulation.

(Object of the invention) The object of the invention is to eliminate the above-mentioned drawbacks,
The object of the present invention is to provide a semiconductor-integrated waveguide optical device that is capable of high-speed modulation operation with low loss and low voltage, and has the potential to play a part in future optical integrated circuits.

(Structure of the Invention) According to the present invention, there is provided a semiconductor waveguide optical element that controls guided light by applying an electric field to the guided light propagating in a guide layer, wherein the guided light is controlled by applying an electric field to the guided light propagating in the guide layer. A low-resistance layer that is extremely thin compared to the field distribution of the guided light is provided adjacent to the guide layer on at least one of both sides of the direction, and further a layer that is thinner than the guide layer is provided in succession to the low-resistance thin layer. A relatively thick layer with a low refractive index and low light absorption is provided, an electrode for applying the electric field serving as a modulation means for the guided light is provided on the low resistance thin layer, and a relatively thick layer with low light absorption is provided. An electrically semi-insulating layer is used as one of the layers, and the low-resistance thin layer adjacent to the semi-insulating layer is used as an active layer.
An integrated waveguide type optical device is obtained, characterized in that a FET is formed and the FET is used as a control means for applying an electric field to the guide layer of the waveguide type optical device.

(Detailed Description of Configuration) The present invention solves the problems of the prior art by adopting the above-described configuration. First, in the third conventional example, which aims to achieve low loss and low voltage, an electrically semi-insulating layer is used as a relatively thick layer with low light absorption, and an adjacent thin layer with low resistance is used as the active layer of the FET. can be used on the same substrate.
Integrate FET and waveguide type optical device. In this way, since the same substrate and the same low-resistance layer (active layer in the FET) are used, even if the FET is integrated, there is almost no manufacturing effort.
Furthermore, this FET is used to apply an electric field to the guide layer of the waveguide type optical element, and the guided light in the guide is controlled by the electric field. Therefore, the control signal can be applied to the gate of the FET. In this way, since the electric field is applied to the guide layer using the amplification effect of the FET, the voltage applied to the guide does not change, but the drive voltage of the FET can be low, and the apparent voltage can be lowered. Therefore, compared to the conventional semiconductor waveguide type optical device, which has limitations in terms of voltage reduction, by adopting this configuration, voltage reduction to the TTL level can be realized. Furthermore, the loss is as low as in the conventional example, and by using this configuration, a semiconductor-integrated waveguide optical device capable of high-speed modulation with low loss and low voltage can be obtained.

(Example) Examples of the present invention will be described in detail below with reference to the drawings. FIG. 4 is a diagram showing one embodiment of the present invention. This embodiment will be explained using an InGaAsP/InP-based semiconductor material, and FIG. 4 shows a diagram of light waveguide when the integrated waveguide type optical device of the present invention is applied to a phase modulator as an example. A cross-sectional view thereof taken in a plane perpendicular to the direction is shown. On a semi-insulating InP substrate 36 with low light absorption, an n ⁺ -InP low resistance layer (active layer) 35, an n ^- -InGaAsP guide layer 34,
The n ^{- -InP} buffer layer 33 is grown, and p ⁺
- Diffuse the InP layer 32. Thereafter, a part of the active layer 35 for forming an FET is exposed by etching or the like. A part of the exposed active layer is electrically isolated by etching and applying an insulating film 40, and then a drain electrode 37, a gate electrode 38, and a source electrode 39 are attached to form a MIS-FET. Regarding the guide, an electrode 31 is attached on the p ⁺ -InP diffusion layer 32 and etched into a rib shape to form a three-dimensional waveguide. Finally, the drain electrode 37 and guide electrode 31 are wired. The manufacturing process described here is just an example, and an FET can be formed on the low resistance layer 35, and an electric field can be applied to the guide layer 34 of the waveguide type optical element formed on the other side of the low resistance layer 35 using the FET. There is no need to specify the process if the structure is such that the voltage can be applied.

A TTL level high-speed signal 41 is applied to the gate electrode 38
When input, the input signal is amplified by the FET and can be taken out between the drain 37 and the source 39.
By connecting this, an electric field sufficient to control the guided light propagating through the guide layer can be applied to the guide layer 34, and modulation according to the input signal becomes possible. By adopting the configuration shown in FIG. 4 in this manner, a waveguide type optical element capable of performing high-speed modulation at the TTL level can be obtained.

FIG. 5 shows an equivalent circuit of an integrated waveguide type optical device according to the present invention. Since the semiconductor waveguide type optical element 57 is a reverse bias device, it is indicated by the symbol of a diode. Input signal 5 to gate terminal 55
8 is input. When the input signal 58 is at a high level, the FET 56 is turned on and a current flows between the drain 53 and the source 54, and there is almost no potential difference between them, so no voltage is applied to the guide layer of the waveguide optical element 57. Furthermore, when the input signal 58 is at a low level, the FET 56 is in the OFF state, and almost no current flows between the drain 53 and the source 54, and the voltage determined by the OFF resistance of the FET, the bias resistance 52, and the bias voltage 51 is the waveguide light source. In addition to element 57, an electric field can be applied to the guide layer. 59 is a waveguide type optical element 5 by input signal 58.
This shows the electric field applied to 7. In this way, depending on the signal to the gate 55, the waveguide type optical element 5
An electric field can be applied to the guide layer 7, and the guided light can be controlled by a TTL level signal input to the gate. Therefore, according to the present invention, it is possible to obtain an integrated waveguide type optical device that can perform high-speed modulation at a lower voltage than ever before.

In addition, here we have shown an example in which an FET is integrated into a phase modulator using InGaAsP/InP-based semiconductor materials, but GaAAs/GaAs-based semiconductor materials may be used, and waveguide type optical devices are particularly important. It does not need to be a phase modulator, and may be a directional coupler or the like. Furthermore, confinement of light in the horizontal direction in the waveguide structure is not limited to the rib type; it may also be achieved by forming grooves in the substrate or low resistance layer.
Alternatively, this may be done by using an embedded structure. In addition to the electro-optic effect as a control means for guided light, there is also the Franz-Kjeldetsch effect, in which the basic absorption edge shifts to longer wavelengths when a reverse bias is applied to a pn junction or Schottki junction, and the depletion layer effect caused by an electric field. The present invention is also applicable to a waveguide type optical element that utilizes a carrier depletion effect that utilizes expansion.

Furthermore, even if a thin low-resistance layer (active layer of the FET) is placed on the guide layer, if a semi-insulating layer is used as the guide layer, the active layer can be used to form the FET.

(Effects of the Invention) As described in detail above, according to the present invention, it is possible to obtain a semiconductor integrated waveguide optical device that not only has low loss but also can perform high-speed modulation at a lower voltage. This will greatly contribute to the realization of future optical integrated circuits.

[Brief explanation of the drawing]

Figure 1 is a diagram for explaining the basic structure and operation of a phase modulator using InGaAsP/InP semiconductor materials, and Figure 2 is a diagram showing a buffer layer in the phase modulator shown in Figure 1. Figure 3 is a diagram for explaining a conventional example in which an extremely thin layer for applying an electric field is provided on one side of the guide layer in order to achieve low loss and low voltage. FIG. 4 is a diagram for explaining an embodiment of the integrated waveguide type optical device according to the present invention, and FIG. 5 is a diagram showing the relationship between the operation of the FET and the operation of the waveguide type optical device using the equivalent circuit of FIG. 4. FIG. In the figure, 1, 10, 20 are reverse bias signals for modulation, 2, 11, 21, 28, 31 are electrodes,
3, 12, 22, 32 are p ⁺ diffusion layers, 13, 15,
23, 33 is ⁿ -buffer layer, 4, 14, 24,
34 is a guide layer, 5 and 16 are n ⁺ substrates, and 26 is n ^-
substrate, 25 and 35 are n ⁺ -low resistance layers (active layer of FET), 36 is a semi-insulating substrate, 37 is a drain electrode,
38 is a gate electrode, 39 is a source electrode, 40 is an insulating film, 41 is a TTL level input signal, 51 is a bias voltage, 52 is a bias resistor, 53 is a drain terminal, 54 is a source terminal, 55 is a gate terminal,
56 is an FET, 57 is a reverse bias device which is a waveguide optical element, 58 is an input signal, and 59 is an electric field signal applied to the waveguide optical element in response to the input signal.

Claims (1)

[Claims] 1 A semiconductor waveguide optical element that controls the guided light by applying an electric field to the guided light propagating in the guide layer, wherein the guided light is provided on at least one of both sides of the guide layer in the electric field application direction. A low-resistance layer that is extremely thin compared to the field distribution of wave light is provided adjacent to the guide layer, and a layer that is continuous with the low-resistance thin layer has a refractive index lower than that of the guide layer and has less light absorption. In a semiconductor waveguide optical element, a relatively thick layer is provided, and an electrode for applying the electric field serving as a modulation means for the guided light is provided to the thin layer with low resistance. A FET is formed using an electrically semi-insulating layer as one layer, and the low-resistance thin layer adjacent to the semi-insulating layer is used as an active layer, and the FET is connected to the waveguide optical element. An integrated waveguide type optical device characterized in that the integrated waveguide type optical device is configured as means for controlling the application of an electric field to the guide layer.