JPS63191123A - Optical modulator - Google Patents

Abstract

PURPOSE:To reduce electrostatic capacity between electrodes and to attain rapid responseness by arranging electrodes on the same plane in stead of opposite electrodes in an optical modulator utilizing Franz-Keldysh (FK) effect. CONSTITUTION:A GaInAP layer 2 to be an optical guide layer, an n-GaInAsP layer 3 to be a light absorbing layer, an n-InP layer 4 to be a clad layer, and an n-GaInAsP layer 5 to be a cap layer are formed on an Si-InP substrate. Then, Zn or the like is diffused so as to be expanded from the surface of the layer 5 up to the layer 3 to form a p-type area 6. A p-type electrode (Au/Zn/Au) 7 and an n-type electrode (Au/AuGe) 8a are respectively formed on the p-type area 6 and the layer 5 in the semiconductor layer constitution. Consequently, a modulating pulse can be impressed between a pair of electrodes on the surface of the semiconductor layer structure, so that the capacity between the electrodes can be reduced and rapid responsiveness can be attained.

Description

[Detailed Description of the Invention] [Summary] In an optical modulator that uses the so-called Franz Keldysch (PK) effect, which is a phenomenon in which applying an electric field to an optical waveguide increases the absorption of low-energy light passing through it, Instead of the conventional counter electrodes, the electrodes are placed on the same surface to reduce the capacitance between the electrodes and achieve high-speed response.

[Industrial application field]

The present invention relates to a PK type optical modulator with improved high-speed response.

Semiconductor lasers are used as light sources in optical communication systems and optical information processing systems.

In the modulation of laser light, when direct modulation is performed at high speeds, it is not possible to obtain an optical output waveform that matches the waveform of the input current of the laser, and relaxation oscillations occur in the optical output waveform.

Therefore, an FK type external optical modulator in which the upper limit of the temperature change rate is determined only by the RC time constant is considered to be promising.

[Conventional technology]

FIGS. 2(1) and 2(2) are cross-sectional views of a conventional PK type optical modulator.

FIG. 2(1) is a cross-sectional view in the optical axis direction, and FIG. 2(2) is a cross-sectional view in the direction perpendicular thereto.

The semiconductor layer structure of the optical modulator is formed like a shell.

In the figure, GaInAsP N 2 is used as a light guide layer and n-Galn is used as a light absorption layer on an n-1nP substrate 1.
An AsP layer 3, an n-1nP layer 4 as a cladding layer, and an n-GaInAsP layer 5 as a cap layer are formed.

Next, from the surface of the n-GalnAsP layer 5,
Diffuse Zn, etc. so that it reaches the nAsP layer 3 to form a p-type head region 6
form.

An n-side electrode 7 is formed on the p-type head region 6 of the semiconductor layer structure having the above structure, and an n-side electrode 8 is formed on the back surface of the substrate.

The waveguide is formed using a rib guide structure that utilizes the effective refractive index difference due to the difference in the thickness of the optical guide layer.

Modulation is performed in the FK type optical modulator having the above structure.

For example, light enters the light guide layer from the left end, passes through the light guide layer, and exits from the right end.

For example, the negative side of a modulation power source PPG (pulse pattern generator) is connected to the n-side electrode 7, and the positive side is connected to the n-side electrode so that a reverse bias is applied between both electrodes.

While a reverse bias is applied between both electrodes, a reverse bias is applied to the pn junction formed between the p-type head region 6 and the n-GaInAsP layer 3, and a depletion layer expands within the n-GaInAsP layer 3. There is a locally large electric field here (~10S10
5V') is applied, the light is absorbed, and the intensity of the emitted light decreases. In this case, in order to reduce the light intensity by 10 dB or more, the voltage of the modulation power source PPG needs to be 3 to 4 V.

[Problem that the invention seeks to solve]

Since conventional PK type optical modulators employ opposing electrodes, the capacitance between the electrodes is large and high-speed response is poor.

[Means for solving problems]

The solution to the above problem is to use a semiconductor layer structure including a light guide layer, a light absorption layer of one conductivity type, and a cladding layer of one conductivity type formed in sequence on a substrate of one conductivity type, and the light absorption from the surface of the semiconductor layer structure. This is achieved by an optical modulator having a pair of electrodes formed on a region of other conductivity type that is formed to reach the layer and on the surface of the semiconductor layer structure.

A plurality of regions of the other conductivity type may be formed in the optical axis direction, and a plurality of pairs of electrodes may be formed to increase the light absorption effect.

In addition, in this case, if the pair of electrodes is made into a comb-shaped electrode in which comb teeth are arranged alternately, the electrodes can be manufactured in the wafer process without having to connect the individual electrodes by bonding or the like. is easy.

[Effect]

The present invention aims at increasing speed by reducing the capacitance between the electrodes by making it possible to apply a modulating pulse between a pair of electrodes formed on the surface of a semiconductor layer structure.

〔Example〕

FIGS. 1(1) to 1(3) are a cross-sectional view and a plan view of a PK type optical modulator of the present invention.

FIG. 1 (1) is a sectional view in the optical axis direction, (2) is a sectional view in the direction perpendicular to the optical axis direction, and (3) is a plan view.

The semiconductor layer structure of the optical modulator is formed like a shell.

In the figure, a GaInAsP layer 2 is placed on a 5l-1nP substrate 1 as a light guide layer, and an n-GaInA layer is used as a light absorption layer.
An sP layer 3, an n-InPn continuous layer as a cladding layer, and n-Ga1nAsP@5 as a cap layer are formed.

The carrier concentration and thickness of each layer are set as shown below, for example.

Drawing number Eyebrows Substance name Carrier concentration Thickness cm
-”) (μm) 2 Guide layer GaInAsP (2~5
)X 10" 0.3-0.4 (undoped) 3 Absorption layer n-Ga1nAsP 2X10"
0.24 Craft layer n-InP
2X10” 0.85 Cap layer n
-GaInAsP lXl018 0.2 Next, from the surface of the aInAsP layer 5 to n-Galn
A p-type region 6 is formed by diffusing Zn or the like so as to reach the AsP layer 3.

A p-side electrode (Au/Zn/Au) 7 is placed on the p-type region 6 of the semiconductor layer structure having the above structure, and an n-GaInAsP layer 5 is placed on the p-side electrode (Au/Zn/Au).
An n-side electrode (Au/AuGe) 8A is formed on top.

The waveguide is formed using a rib guide structure that utilizes the effective refractive index difference due to the difference in the thickness of the optical guide layer.

The modulation operation of the FK type optical modulator having the above structure is similar to that of the conventional example.

The modulation speed is determined by the product of the resistance R of the 50Ω coaxial cable connected from the PPG to the optical modulator and the capacitance C of the optical modulator.
r sec, ), but it was confirmed that the present invention can modulate at 10 Gbps or more due to the reduction in interelectrode capacitance.

〔Effect of the invention〕

As described above in detail, according to the present invention, the electrode capacitance of the optical modulator can be reduced and high-speed response can be improved.

[Brief explanation of the drawing]

FIGS. 1 (1) to (3) are a sectional view and a plan view of an FK type optical modulator of the present invention, and FIGS. 2 (1) and (2) are sectional views of a conventional FK type optical modulator. In the figure, 1 is a 5l-InP substrate, 2 is a light guide layer, which is a GaInAsP layer, 3 is a light absorption layer, which is an n-GalnAsP layer, and 4 is a cladding layer, which is an n-1nP II! , 5 is n-GaI in the Kya-nobu layer.
nAsP layer, 6 is a p-type region, 7 is an n-side electrode 7. 8.8A is the n-side electrode according to the present invention.

Claims (3)

[Claims] (1) A semiconductor layer structure including a light guide layer, a light absorption layer of one conductivity type, and a cladding layer of one conductivity type sequentially formed on a substrate of one conductivity type, and a structure in which the light absorption layer is reached from the surface of the semiconductor layer structure. An optical modulator comprising a pair of electrodes formed on a region of a different conductivity type formed on a semiconductor layer structure and on a surface of the semiconductor layer structure. (2) The optical modulator according to claim 1, wherein a plurality of the regions of different conductivity types are formed, and the pair of electrodes consists of a plurality of sets. (3) The optical modulator according to claim 2, wherein the pair of electrodes are comb-shaped electrodes in which comb teeth are arranged alternately.