JPH0430115A - Semiconductor optical modulator - Google Patents

Abstract

PURPOSE:To lower driving voltage without impairing an extincition ratio charac teristic by arranging a p type and an n type semiconductor layers of high-dope and high refractive index to form a part of a core part to cover a non-dope layer as the electric field impression layer of a core layer from the upper and lower sides. CONSTITUTION:A p type and an n type semiconductor guide layers 10, 12 whose refractive indexes are larger than a p type and an n type clad layers 3, 5 are arranged so as to put the non-dope semiconductor layer 11 of the core layer in between. By arranging the semiconductor layer of the high refractive index to constitute a part of the core part, the light confinement coefficient of a light control function semiconductor layer can be made large, and a light control function such as optical modulation, etc., can be made to act highly efficiently without necessitating to lengthen the length of a waveguide, and the semiconductor optical modulator of low driving voltage is obtained without impairing the extinction ratio characteristic.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field 1) The present invention relates to a semiconductor optical modulator that is used in the fields of optical communication and optical information processing and is capable of high-speed operation at low driving voltage.

[Prior art l] The basic structure of conventionally used electroabsorption semiconductor optical modulators is that a non-doped core layer is placed as an electric field application layer between p-type and n-type cladding layers with high impurity concentrations. There are things that exist. Optical functions such as light modulation are made possible by applying an external electric field to the electric field application layer of these elements.

In this type of modulation element, the magnitude of the drive voltage is an important characteristic that determines the performance of the modulator, as well as the upper limit frequency (bandwidth) that can be operated, and it is desirable that the drive voltage be as small as possible and the band should be wide.

FIG. 6 is a perspective view showing an example of a conventional optical modulator.

Here, 1 is an electrode made of Au or the like, and 2 is a cap layer for obtaining ohmic contact between the semiconductor material and the electrode 1. 3 is an n-type semiconductor layer, 4 is a non-doped semiconductor layer, 5 is an n-type semiconductor layer, and 6 is an n-type semiconductor substrate, and these layers 3, 4, and 5.6 form an optical waveguide. The n-type semiconductor layer 3 and the n-type semiconductor layers 5 and 6 become a cladding part, and the non-doped semiconductor layer 4 becomes a core part. 7 is an electrode.

In this case, if a negative external voltage (reverse bias) ■ is applied to the electrode 1 with respect to the electrode 7, depending on the magnitude of the voltage V,
An electric field is applied to the non-doped layer 4, and the absorption coefficient α of this layer changes.

FIG. 7 is a diagram schematically showing the relationship between the electric field strength E of the non-doped layer 4 and the magnitude α of its absorption coefficient.0 In normal operation, as shown in FIG. It is true that α is thick (and the propagation loss for light becomes large. Therefore, when light 8 of a constant intensity is incident from the end face of the core part 4, the output light 9 whose intensity is modulated according to the voltage V is transmitted to the optical modulator. It is emitted from.

FIG. 8 shows the relationship between the thickness d of the non-doped layer 4, the spot size wJ3 in the depth direction of the optical waveguide, and the optical confinement coefficient F of the non-doped layer (core portion) 4 in an example of a conventional semiconductor optical modulator. As a specific example, In is used as p-type and n-type semiconductor layers (cladding parts) 3 and 5.
AρAs is used, and the non-doped layer 4 is InGaAs/
As can be seen from Figure 8, which shows a calculation example using an InAl2As multiple quantum well layer, when d is made smaller,
There is a relationship in which r becomes smaller; in particular, when d becomes less than 0.2 μm, the spot size W increases rapidly, and at the same time, the size of r also decreases significantly.

Normally, in order to realize an optical modulator with low driving voltage, it is considered to reduce d. As a result, the electric field strength E can be strengthened for the same externally applied voltage V, so that the change in the absorption coefficient α becomes large and low voltage operation becomes possible. However, when d is decreased, the element capacitance increases and the upper limit operating frequency decreases. Therefore, in order to prevent this, the waveguide width W and the waveguide length may be reduced.

L Problems to be Solved by the Invention] However, in an actual optical modulator, the effective absorption coefficient of the optical waveguide has a magnitude of r×α, so as can be seen from the relationship in FIG. When it is less than 0.2 μm, r becomes significantly small, the extinction ratio (on/off ratio of light) characteristics deteriorate, and there is a limit to the improvement of the driving voltage. Also,
In a structure in which d is small, if the length of the waveguide is increased in order to improve the extinction ratio, the element capacitance increases, which has the disadvantage of significantly deteriorating the frequency characteristics.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a semiconductor optical modulator with a low driving voltage by solving one or more of the drawbacks.

1 Means for Solving the Problems 1 In order to achieve such objects, the present invention includes a semiconductor substrate, a first semiconductor substrate of a first conductivity type disposed on the substrate, and constituting a cladding portion of an optical waveguide. a semiconductor layer, an electric field application layer disposed on the first semiconductor layer and composed of a semiconductor layer with a low impurity concentration, and a second conductivity type semiconductor layer disposed on the electric field application layer and constituting a cladding portion. a semiconductor optical modulator configured to include at least two semiconductor layers, which are disposed between the first semiconductor layer and the electric field application layer, have the first conductivity type, and have a refractive index of the first semiconductor layer and the first semiconductor layer; a third semiconductor layer having a refractive index greater than that of the semiconductor layer; and a third semiconductor layer disposed between the second semiconductor layer and the electric field application layer, having the second conductivity type and having a refractive index of the second conductivity type semiconductor layer. It is characterized by comprising at least one of the fourth semiconductor layers having a refractive index greater than .

Here, the above-mentioned 1. Second. At least one of the third and fourth semiconductor layers can have a multiple quantum well structure or a multilayer structure.

L effect] In the present invention, the p-type, n-type semiconductor layer, which is electrically conductive and has a high refractive index and which forms the optically conductive core portion, is formed by the above-described third. By arranging the fourth semiconductor layer to sandwich the non-doped layer as the electric field application layer above and below, the optical confinement coefficient of the non-doped layer can be increased (
Therefore, the driving voltage can be lowered without increasing the waveguide length and without damaging the extinction ratio characteristics.

[Embodiment J] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing the configuration of an embodiment of the present invention.

Note that the same reference numerals are given to the same components as in the conventional example.

In FIG. 1, 1 is an electrode, 2 is a cap layer, 3 is a p-type semiconductor layer, 5 is an n-type semiconductor layer, 6 is an n-type semiconductor substrate, and 7
is an electrode. Furthermore, IO is a p-type semiconductor guide layer,
11 is a non-doped semiconductor layer, 12 is an n-type semiconductor guide layer, and an electrode 7 is arranged on one main surface of the substrate 6;
Layer 5, 12.11.10.3.2.1 on the same other main surface
are arranged in this order. P-type and n-type semiconductor layers 3 and 5 constitute a cladding section. Guide layers 10 and 1
2 and the non-doped semiconductor layer 11 constitute a core portion, and an electric field is applied to the non-doped layer 11 by an external voltage. 1st
In the figure, the non-doped layer 11 is formed relatively thin in order to reduce the driving voltage, but since the guide layers 1° and 12 are placed above and below it, the spot size of the optical waveguide does not expand. , the optical confinement coefficient of the non-doped layer 11 can be increased.

In FIG. 2, n-type InP is used as the semiconductor substrate 6, and the electrode 1
layer 2 is a P0-type InGaAs layer, layer 3 is a p-type InP layer, and layer 10 is a p-type InGaAs layer.
Layer 1 is a multiple quantum well (MQW) layer in the 1 and 5 μm band using InGaAsP/InP material as the P layer and layer 11.
n-type 1 nGaAsP layer as layer 2, n-type In as layer 5
Regarding the case of a semiconductor optical modulator having a structure in which Au electrodes are used as the P layer and the electrode 7, for example, the guide layers 1o and 1
The thickness ag+ and dgx of 2 are made equal (dg+=d
An example of calculation of the relationship between the thickness d of the non-doped layer 11 and the spot size when gt:dg) is shown.

FIG. 3 shows an example of calculation of d and the optical confinement coefficient F at that time.

As can be seen from Figures 2 and 3, when d is increased for a certain size 6, the spot size W becomes smaller, and the spot size W becomes thicker (as the ads become thinner). , this tendency becomes remarkable, and the effect of the present invention becomes greater.If dg is made thicker than necessary (then, as can be seen from the relationship in FIG. (As a result, the effect of the present invention is lost.

In the embodiment shown in FIG. 1, the non-doped layer 11 is sandwiched between the guide layers 1013 and 12 from above and below, but as shown in FIG. The effects of the present invention can also be obtained by arranging only the shaped semiconductor guide layer 16 adjacent to the non-doped layer 14 or 15.

Alternatively, as the guide layer 10, 12, 13 or 16, a multilayered semiconductor layer made of two or more types of materials may be used. Furthermore, a semiconductor layer having an MQW structure can be used as the guide layer 10, 12, 13 or 16, and in this case, the absorption effect of the guide layer can be reduced, so that the propagation loss of the guide layer can be reduced.

Effects of the Invention 1 As explained above, in the present invention, by arranging a high refractive index semiconductor layer constituting a part of the core portion adjacent to a semiconductor layer having a light control function, It is possible to increase the optical confinement coefficient of the layer, thereby allowing optical control functions such as optical modulation to operate with high efficiency without the need to increase the waveguide length, thereby impairing the extinction ratio characteristic. A semiconductor optical modulator with low driving voltage can be realized without any problems.

Although the present invention has been described in the case of an electro-absorption semiconductor modulator, the present invention is also applicable to an optical detector that utilizes absorption coefficient control as the optical functionality of a non-doped semiconductor layer, or a quantum confined Stark modulator. The effects of the present invention can be effectively exhibited and utilized even when applied to optical waveguide functional devices such as optical phase modulators and optical switches that utilize refractive index control using the Franz Keldeitssch effect, Pockels effect, etc. is self-evident.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing one embodiment of the invention, FIGS. 2 and 3 are characteristic diagrams for explaining the invention in detail, and FIGS. 4 and 5 are other embodiments of the invention. FIG. 6 is a perspective view showing a conventional example, and FIGS. 7 and 8 are diagrams explaining characteristics of the conventional example. 1.7... Electrode, 2... Cap layer, 3... N-type semiconductor layer (cladding layer), 4... Non-doped semiconductor layer (core layer) 5... N-type semiconductor layer (cladding layer) ), 6... N-type semiconductor substrate, 8... Incident light, 9... Outgoing light,

Claims (1)

[Scope of Claims] 1) A semiconductor substrate, a first semiconductor layer of a first conductivity type disposed on the substrate and constituting a cladding portion of an optical waveguide, and a first semiconductor layer disposed on the first semiconductor layer and having a low an electric field application layer formed of a semiconductor layer with an impurity concentration; and an electric field application layer disposed on the electric field application layer,
A semiconductor optical modulator configured to include at least a second semiconductor layer of a second conductivity type constituting a cladding portion, disposed between the first semiconductor layer and the electric field application layer,
a third semiconductor layer having the first conductivity type and having a refractive index greater than the refractive index of the first semiconductor layer;
disposed between the semiconductor layer and the electric field application layer, and the second
A semiconductor optical modulator comprising at least one of a fourth semiconductor layer having a conductivity type and a refractive index greater than the refractive index of the second conductivity type semiconductor layer. 2) The semiconductor optical modulator according to claim 1, wherein at least one of the first, second, third, and fourth semiconductor layers has a multiple quantum well structure. 3) The semiconductor optical modulator according to claim 1, wherein at least one of the first, second, third, and fourth semiconductor layers has a multilayer structure.