JP2001183612A - Optical modulation element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-speed optical modulation element of which the overall length is made longer to reduce the production cost by facilitating handling, working, and a production process and of which the length of an element active part is made shorter to improve the frequency band. SOLUTION: A waveguide structure 20 is provided with an active part 10 and passive waveguide parts 11a and 11b provided on both sides (or one side) in the light propagation direction of the active part, and an upper electrode layer 6 of the active part 10 and upper electrode layers 6 of passive waveguide parts 11a and 11b are separated from each other with separation grooves 13a and 13b between them, and upper electrodes 7, 12a, and 12b which are provided so as to be brought into contact with the upper electrode layer 6 of the active part 6 and upper electrode layers 6 of passive waveguide parts 11a and 11b are separated from each other with separation grooves 13a and 13b between them, and upper electrodes 12a and 12b of passive waveguide parts 11a and 11b and lower electrodes 8a and 8b are short-circuited by wiring 14a and 14b respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type light modulation device.

[0002]

2. Description of the Related Art In optical communication technology, a combination of a light source and a light modulation element is often used to generate a high-speed optical signal. As an example of the light modulation element, there is an electroabsorption-type light modulation element (light modulator) using the quantum confined Stark effect.

FIG. 3 is a perspective view of an example of a conventional lumped-constant type electro-absorption type light modulation device.

The light modulating element comprises an n-type semiconductor layer 24, a multiple quantum well layer 25 serving as a light modulating layer, and a p-type semiconductor layer 26 formed on an n-type semiconductor substrate 23. A lower electrode 27 that is in ohmic contact with the upper electrode 27 and an upper electrode 28 that is in ohmic contact with the p-type semiconductor layer 26 are formed.

[0005] Each end face constituting the light incident surface 21a on which light enters and the light exit surface 21b from which light exits is usually formed by cleavage. Reference numeral 29 denotes the length of an active portion (active portion length) to which an electric field is applied and which modulates light.

FIG. 4 is a graph showing an example of the light transmission characteristics of the electro-absorption type light modulation device shown in FIG.

The horizontal axis represents the reverse bias voltage (V), and the vertical axis represents the light transmittance (%) indicating how much light incident on the light modulation element is transmitted.

As shown in FIG. 4, when the reverse bias voltage increases, the light transmittance changes from almost 100% to almost 0%. By using such characteristics and changing the reverse bias voltage at high speed with time, the intensity of the incident light can be modulated, and an optical signal that changes at high speed can be generated.

The operating speed of such a lumped-constant type electro-absorption type optical modulator is limited by a time constant (CR time constant) determined by a product of a load resistance and a junction capacitance of the electro-absorption type optical modulator. The value of the load resistance is usually determined by the impedance of the transmission line (in general, 50Ω is often used) and is constant. Therefore, in order to improve the frequency band of the electroabsorption type optical modulation element (enlarge the frequency bandwidth), It is important to shorten the active portion length 29 and reduce the element area. However, when the element length is shortened, there is a problem that the chip size becomes smaller than the limit of handling, and a problem that the cleavage processing of the semiconductor substrate itself becomes difficult because the interval between cleavage planes is too small. Was.

FIGS. 5A and 5B are views showing another conventional electroabsorption type light modulation device, in which FIG. 5A is a top view, FIG. 5B is a sectional view taken along the line H--H in FIG. ) Is II of (a).
It is sectional drawing in a cutting line.

That is, in order to avoid the above problem, FIG.
As shown in the optical modulator shown in FIG. 1, the active portion length 29 is shortened, and high-resistance semiconductor waveguides (passive waveguide portions) 30a and 30b are regrown at both ends in the light propagation direction (optical axis direction). Also, a structure for increasing the overall element length has been used.

In FIG. 5, the same reference numerals as those in FIG. 3 denote the same components. 302a and 302b are core layers, and 301a, 303a, 301b and 303b are cladding layers of core layers 302a and 302b, respectively (that is,
(Which functions as a light confinement layer having a lower refractive index than the core layers 302a and 302b and a protective layer for the core layers 302a and 302b).

[0013]

However, the structure as shown in FIG. 5 has a problem that the regrowth is necessary, which complicates the manufacturing process and raises the manufacturing cost.

The present invention solves the above-mentioned problems of the prior art, and makes it possible to increase the total element length in order to facilitate handling, processing, and manufacturing processes and reduce manufacturing costs. Another object of the present invention is to provide a high-speed light modulation device capable of shortening the length of an element active portion in order to improve a frequency band.

[0015]

In order to achieve the above object, the present invention provides a light modulation device comprising a waveguide structure including a semiconductor multilayer film including a lower electrode layer, a light modulation layer, and an upper electrode layer; Light having a lower electrode provided in contact with the lower electrode layer, an upper electrode provided in contact with the upper electrode layer, and a light incident surface and a light output surface provided on both sides in the light propagation direction of the waveguide structure. In the modulation element, the waveguide structure includes an active portion, and a passive waveguide portion provided on both sides or one side of the active portion in the light propagation direction, wherein the upper electrode layer of the active portion, the passive portion, The upper electrode layer of the waveguide portion is separated from each other by a groove, and the upper electrodes provided in contact with the upper electrode layer of the active portion and the upper electrode layer of the passive waveguide portion respectively have grooves. Separated from each other by the passive And the upper electrode of the waveguide section, characterized in that short-circuited and the lower electrode.

With such a configuration, an optical modulator having a short active portion length and a long overall element length can be realized with a simple configuration, so that the time required for device fabrication can be greatly reduced as compared with the prior art. An optical element can be realized at low cost.

[0017]

Embodiments of the present invention will be described below in detail with reference to the drawings. In the drawings described below, those having the same functions are denoted by the same reference numerals, and the repeated description thereof will be omitted.

Hereinafter, an embodiment of the present invention will be described by taking an electro-absorption type optical modulation element used for optical communication in the 1.55 μm band as an example.

Embodiment 1 FIGS. 1A and 1B show a light modulation device according to a first embodiment of the present invention, wherein FIG. 1A is a top view, FIG. c) is a sectional view taken along the line AA of (a),
(D) is a sectional view taken along the line BB (or the line CC) of (a).

In the light modulation device of this embodiment, an n-type InAlA as a lower electrode layer is formed on a semi-insulating InP substrate 1.
s layer 2 (film thickness 0.5 μm, doping concentration 1 × 10 ¹⁹ c
m ⁻³ ), undoped (ie, i-type) InAlA
s layer (thickness 0.005 μm) and undoped InGaAs
The multiple quantum well layer 3, which is a light modulation layer having a structure in which s layers (thickness: 0.0075 μm) are laminated 12 times, an undoped InAlAs layer 4 (thickness: 0.15 μm), p
InAlAs layer 5 (film thickness 1 μm, doping concentration 5
× 10 ¹⁷ cm ⁻³ ), p-type InGa as the upper electrode layer
As layer 6 (film thickness 0.5 μm, doping concentration 2 × 10 ¹⁹
cm ^-3 ) are formed in this order. In (c),
Reference numeral 20 denotes a waveguide structure, and reference numerals 21a and 21b denote light incidence surfaces and light emission surfaces provided on both sides of the waveguide structure 20 in the light propagation direction.

As shown in FIG. 1C, an ohmic upper electrode 7 is in contact with a p-type InGaAs layer 6 as an upper electrode layer, and as shown in FIG.
Ohmic lower electrodes 8a and 8b are formed in contact with the nAlAs layer 2, and a known insulating film 9 for passivation is formed thereon.

In FIG. 1C, the portion of the waveguide structure denoted by reference numeral 10 is an active portion that operates as a modulation element, and the portion of the waveguide structure denoted by reference numerals 11a and 11b is an optical waveguide portion that does not function as an active portion, that is, a passive portion. This is a waveguide section.

As shown in (c) and (d), passive waveguide portions 11a and 11b have
Ohmic upper electrodes 12a and 12b are formed in contact with p-type InGaAs layer 6, which is an upper electrode layer, respectively.

The active portion 10 and the passive waveguide portion 11
a and 11b, respectively, between the upper electrodes 7, 12
a, 12b and p-type InGaAs layer 6 of upper electrode layer
Are removed by etching using known lithography to form separation grooves 13a and 13b.

The separation grooves 13a and 13b are filled with a passivation insulating film 9.

As shown in FIGS. 1D, 1C and 1A, the lower electrodes 8a and 8b and the passive waveguide 11
Wirings 14a and 14b are formed to connect (short-circuit) the upper electrodes 12a and 12b with the upper electrodes 12a and 12b.

In order to reduce the contact resistance of the ohmic upper electrode 7, the p-type InGaAs layer 6 in contact therewith is doped with conductive impurities at a high concentration and has a low resistance. However, the separation grooves 13a and 13b are formed. By being separated from each other, the upper electrode 7 and the upper electrodes 12a and 1a
2b is kept at a high resistance.

That is, the present light modulation element comprises a waveguide structure 20 composed of a semiconductor multilayer film including a lower electrode layer 2, a light modulation layer 3, and an upper electrode layer 6, and a lower part provided in contact with the lower electrode layer 2. In the light modulation element having the electrodes 8a and 8b, the upper electrode 7 provided in contact with the upper electrode layer 6, and the light incident surface 21a and the light emission surface 21b provided on both sides of the waveguide structure 20 in the light propagation direction, The waveguide structure 20 includes the active portion 10,
And passive waveguide portions 11a and 11b provided on both sides (or one side) of the active portion 10 in the light propagation direction.
The upper electrode layer 6 of the active portion 10 and the upper electrode layer 6 of the passive waveguide portions 11a and 11b are separated from each other by separation grooves 13a and 13b, and the upper electrode layer 6 of the active portion 10 and the passive waveguide portions 11a and 11b. The upper electrodes 7, 12a, 12b provided in contact with the upper electrode layer 6 of the
3b are separated from each other by a passive waveguide portion 11
a, 11b upper electrode 12a, 12b and lower electrode 8
a and 8b are short-circuited by the wirings 14a and 14b, respectively.

In the structure of the present embodiment, the width W of the waveguide is 2 μm, and the lengths of the separation grooves 13a and 13b (in the optical axis direction).
Since L is 5 μm, the upper electrodes 7 and 12a, 12a
The resistance between b is as high as 10 kΩ or more. this is,
The value is about 25 times that of the case where the separation grooves 13a and 13b are not formed. Note that the waveguide width W, the separation groove length L, and the like are design items and can be set as appropriate.

Next, the operation of the electro-absorption type light modulation device shown in FIG. 1 will be described.

In order to operate this light modulating element, for example, between the upper electrode 7 and the lower electrodes 8a and 8b,
A constant or time-varying reverse bias voltage is applied. This produces the quantum confined Stark effect,
According to the characteristics shown in FIG. 4 described in the related art, the light transmittance of the active portion 10 changes, and the light modulation operation is performed.

On the other hand, since the upper electrodes 12a or 12b of the passive waveguide portions 11a and 11b and the lower electrodes 8a and 8b are connected (short-circuited) by the wirings 14a or 14b, respectively, the upper electrodes 12a and 12b and the lower electrodes 12a and 12b are connected to each other. The electrodes 8a and 8b have the same potential. Therefore, no reverse bias voltage is applied to the backspacing waveguide portions 11a and 11b, and the quantum confined Stark effect does not occur.
It shows a light transmittance close to 100%.

The upper electrode 7 and the upper electrodes 12a, 12a
b, the reverse bias voltage applied between the upper electrode 7 and the lower electrodes 8a and 8b is applied as it is,
As described above, the upper electrode 7 and the upper electrode 12a or 12a
b is separated by the separation grooves 13a and 13b and kept at a high resistance, so that the flowing current is sufficiently small (electrically separated).

The passive waveguide sections 11a, 11a, 1
By appropriately setting the length of 1b to the required length, even when the length of the active portion 10 is extremely shortened, the entire length of the light modulation element is set to a length sufficient for handling and realizing a chip. Can be kept.

The upper electrode 7 in the active portion 10 and the passive waveguide portions 11a and 11b according to the present invention.
, 12a and 12b, and the separation groove structure of the upper electrode layer 6 can be easily formed by adding only one etching step in the production of the light modulation element, and the conventional regrowth shown in FIG. Compared with the method used, there is an effect that the cost for manufacturing the light modulation element can be reduced.

When such a structure is formed, it is of course effective to use a method of precisely controlling the etching depth of the isolation trenches 13a and 13b by using selective etching using different semiconductor layers.

In the present light modulating element, the length of the active portion 10 of the element can be shortened, so that the limiting factor of the operation speed of the element due to the CR time constant can be avoided, and a high-speed light modulating element can be realized. In other words, a light modulator having a short active section 10 and a long total element length can be realized with a simple configuration, so that the time required for device fabrication can be greatly reduced as compared with the conventional technique, and the cost can be reduced. Thus, a light modulation element can be realized.

Embodiment 2 FIGS. 2A and 2B show a second embodiment of the present invention, wherein FIG. 2A is a top view, FIG. 2B is a cross-sectional view taken along the line DD, and FIG.
(A) is a cross-sectional view taken along the line EE, (d) is a cross-sectional view taken along the line FF in (a), and (e) is (a).
It is sectional drawing in the GG cutting line.

The structure of the light modulation element of this embodiment is the same as that shown in FIG. 1, except for the structure of the upper electrode.

In FIG. 2, 17a, 17b and 70 are traveling wave electrodes, and 18a and 18b are ground lines. The traveling wave electrodes 17a, 17b, 70 are formed of the same material here, but may be formed of different materials. The ground lines 18a and 18b are connected to the ohmic lower electrode 8a and the ohmic upper electrode 12a of the passive waveguide portion 11a, and the ohmic electrode 8b and the passive waveguide portion 11b of the ohmic electrode 1b.
2b. That is, in the first embodiment of FIG. 1, as shown in FIG. 1D, both the upper electrodes 12a and 12b of the passive waveguide portions 11a and 11b and the lower electrodes 8a and 8b are short-circuited. In the present embodiment, each of the upper electrodes 12a and 12b of the passive waveguide portions 11a and 11b and one of the lower electrodes 8a and 8b are short-circuited.

The upper electrodes 12a and 12b of the passive waveguide portions 11a and 11b and the lower electrodes 8a and
8b may be short-circuited.

In the conventional electro-absorption type optical modulator as shown in FIG. 3, when a traveling-wave electrode is arranged, the electrode is drawn out from the end of the active portion in a direction perpendicular to the optical axis. Must be largely bent to about 90 degrees. However, in the traveling wave type electro-absorption type optical modulation element of this structure, by providing the passive waveguides 11a and 11b, as shown in FIG. The electrodes can be arranged without bending the electrodes to about 90 degrees. Therefore, at the bent portion of the traveling wave electrode, deterioration of transmission characteristics caused by the electrode structure not being continuously changed in the signal propagation direction, such as frequency dispersion, propagation loss, reflection, etc. of an electric signal propagating through the traveling wave electrode And the effect of limiting the operating speed can be avoided.

Although the present invention has been specifically described based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various changes can be made without departing from the gist of the present invention. It is.

For example, in the device configurations of the first and second embodiments, the passive waveguide 1
Although 1a and 11b are formed, a similar effect can be obtained even when formed only on one end of the active portion 10.

Further, in the above embodiment, the case where the electro-absorption type optical modulator using the quantum confined Stark effect is described as the optical modulator, but the optical modulator using other principles, for example, the Mach-Zehnder type optical modulator is used. It can also be applied to a modulation element and the like.

[0046]

As described above, according to the present invention,
When realizing a very high-speed element by shortening the active part length and avoiding the device speed limiting factor and the electrode shape limiting factor due to the CR time constant, the active part length is short with a simple configuration, and the total element length is reduced. Since a long light modulation element can be realized, there is an effect that the time required for manufacturing the element can be significantly reduced and the light modulation element can be realized at low cost as compared with the conventional technique.

[Brief description of the drawings]

FIGS. 1A and 1B are diagrams showing a light modulation element according to a first embodiment of the present invention, wherein FIG. 1A is a top view, FIG. 1B is a top view showing a partial upper layer, and FIG. 3D is a cross-sectional view taken along a line AA, and FIG. 4D is a cross-sectional view taken along a line BB (or a CC line) in FIG.

FIGS. 2A and 2B are diagrams showing a second embodiment of the present invention, wherein FIG.
Is a top view, (b) is a sectional view taken along the line D-D,
(C) is a sectional view taken along the line EE in (a), (d)
3A is a cross-sectional view taken along the line FF in FIG. 3A, and FIG. 3E is a cross-sectional view taken along the line GG in FIG.

FIG. 3 is a perspective view of an example of a conventional lumped-constant type electro-absorption light modulation element.

FIG. 4 is a graph showing an example of light transmission characteristics of the electro-absorption light modulation device of FIG.

5A and 5B are diagrams showing an electroabsorption type light modulation element of another conventional configuration example, where FIG. 5A is a top view, FIG. 5B is a cross-sectional view taken along the line HH in FIG. 5A, and FIG. It is sectional drawing in the II cutting line of (a).

[Explanation of symbols]

REFERENCE SIGNS LIST 1 semi-insulating InP substrate, 2 n-type InAlAs layer (lower electrode layer), 3 multiple quantum well layer (light modulation layer), 4 undoped InAlAs layer, 5 p-type InAlAs layer, 6
... p-type InGaAs layer (upper electrode layer), 7 ... upper electrode,
8a, 8b: lower electrode, 9: insulating film, 10: active part, 1
1a, 11b: passive waveguide portion, 12a, 12b: upper electrode, 13a, 13b: separation groove, 14a, 14b: wiring, 20: waveguide structure, 21a: light incident surface, 21b: light emission surface, 17a, 17b , 70 ... traveling wave electrode, 18a,
18b: ground line, 23: n-type semiconductor substrate, 24: n-type semiconductor layer, 25: multiple quantum well layer (light modulation layer), 26: p
Type semiconductor layer, 27 ... lower electrode, 28 ... upper electrode, 29 ...
Active part length, 302a, 302b ... core layer, 301a, 3
03a, 301b, 303b ... clad layers.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Ito 2-3-1 Otemachi, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (72) Inventor Tadao Ishibashi 2-3-3, Otemachi, Chiyoda-ku, Tokyo No. 1 Nippon Telegraph and Telephone Corporation F term (reference) 2H079 AA02 AA13 BA01 DA16 EA03 HA04 HA15

Claims (1)

[Claims] 1. A waveguide structure comprising a semiconductor multilayer film including a lower electrode layer, a light modulation layer, and an upper electrode layer; a lower electrode provided in contact with the lower electrode layer; An upper electrode provided, and a light modulation element having a light incident surface and a light emission surface provided on both sides of the waveguide structure in the light propagation direction, wherein the waveguide structure has an active portion and the light of the active portion. A passive waveguide portion provided on both sides or one side in the propagation direction, wherein the upper electrode layer of the active portion and the upper electrode layer of the passive waveguide portion are separated from each other by a groove, The upper electrode provided in contact with the upper electrode layer of the portion and the upper electrode provided in contact with the upper electrode layer of the passive waveguide portion are separated from each other by a groove, and the upper electrode of the passive waveguide portion and the lower portion are provided. Characterized by short circuit with the electrode An optical modulation element.