Optical modulator

Details Optical modulator Optical modulator

2002-11-22

2003-12-09

Epps, Georgia (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S245000, C385S008000

Reexamination Certificate

active

06661557

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator for converting an electric signal into an optical or light signal, that is used in optical communication field, and more particularly, to an improvement in an optical absorption type semiconductor optical modulator.
2. Description of the Related Art
One example of the prior optical absorption type semiconductor optical modulator having a double-hetero structure is shown in
FIG. 7
in outline construction thereof. The illustrated optical modulator comprises: an n-type clad layer (a layer of n-type InP in this example)
2
that is a generally rectangular substrate of 200 &mgr;m×300 &mgr;m in this example; a stripe-like modulation layer (a layer of non-doped InGaAsP in this example)
3
that is formed on the top surface of the n-type clad layer
2
at substantially the central portion thereof in the direction of the minor side (in the width direction); a stripe-like p-type clad layer (a layer of p-type InP in this example)
4
that is formed on the top surface of the modulation layer
3
; semi-insulating (SI) buried layers (layers each of SI-InP in this example)
5
that are formed on the top surface of the n-type clad layer
2
at the both sides of a stripe-like lamination consisting of the modulation layer
3
and the p-type clad layer
4
in such manner that they are in contact with the corresponding side surfaces of the stripe-like lamination of the layers respectively and that they have the same height as that of the lamination of the layers; a p-electrode
7
that is formed on the top surface of the p-type clad layer
4
; insulation layers
6
that are formed on the top surfaces of the buried layers
5
at the both sides of the p-electrode
7
, respectively; a bonding pad
8
that is formed on the top surface of one of the insulation layers
6
; and an n-electrode
1
that is formed on the bottom surface of the n-type clad layer
2
.
The stripe-like lamination consisting of the modulation layer
3
and the p-type clad layer
4
(the widths of the modulation layer
3
and the p-type clad layer
4
are 2 &mgr;m and the lengths thereof are 200 &mgr;m in this example) are formed by forming the modulation layer
3
of a predetermined thickness on the whole top surface of the n-type clad layer
2
, then forming the p-type clad layer
4
of a predetermined thickness on the whole top surface of the modulation layer
3
, thereafter forming a stripe-like mask that is elongate in the direction of the minor side of the p-type clad layer
4
on the top surface of the p-type clad layer
4
at the central portion thereof by masking process, and removing by etching the modulation layer
3
and the p-type clad layer
4
except for the portions thereof covered by the stripe-like mask. After that, the buried layers
5
are formed on the top surface of the n-type clad layer
2
(except for the portion thereof on which the stripe-like lamination of the modulation layer
3
and the p-type clad layer
4
has been formed) until the height of the buried layers
5
becomes same as that of the stripe-like lamination of the layers. Then, the insulation layer
6
(for example, a layer of SiO
2
) is formed on all of the top surface of the stripe-like lamination of the layers and the top surfaces of the buried layers
5
, and only a portion of the insulation layer
6
corresponding to the top surface of the stripe-like lamination of the layers is removed by etching. A conductive material (conductor) is deposited on the top surface of the stripe-like lamination exposed as a result of the above etching process by use of a technique such as evaporation, sputtering or the like to form the p-electrode
7
and at the same time, the bonding pad
8
is formed on the top surface of the insulation layer
6
in position.
Further, in
FIG. 7
, though thickness of the n-type clad layer
2
is shown such that it is approximately twice the thickness of the buried layer
5
, in practice, thickness of the n-type clad layer
2
is set to 70-80 &mgr;m and substantially corresponds to thickness of the optical modulator.
The optical modulator constructed as described above modulates light that is incident from one end surface of the modulation layer
3
by varying a voltage applied between the p-electrode
7
and the n-electrode
1
and emits the modulated light from the other end surface of the modulation layer
3
into the outside. For example, when the positive terminal of a signal source is connected to the n-electrode
1
of the optical modulator, the negative terminal of the signal source is connected to the p-electrode
7
(in fact, the bonding pad
8
) of the optical modulator, and a desired ON/OFF (discontinuous) electric signal (or a pulse signal) is generated from the signal source, then a voltage (electric field) applied between the p-electrode
7
and the n-electrode
1
varies. As a result, the incident light that propagates through the modulation layer
3
is modulated whereby there can be generated a ON/OFF (discontinuous) light (optical) signal or optical signal the strength or intensity of which varies corresponding to the desired electric signal applied between the p-electrode
7
and the n-electrode
1
.
In the optical modulator constructed as described above, if the p-n junction (position at which the p-type impurity and the n-type impurity are balanced) should not be created in the p-type clad layer
4
(that is, the p-n junction is created in the modulation layer
3
), there occurs a problem that an optical loss grows larger. In practice, in case of forming the p-type clad layer
4
on the top surface of the modulation layer
3
, it is a possibility that the p-type impurity is diffused into the modulation layer
3
, and in such case, the p-n junction will be created in the modulation layer
3
. In order to prevent such difflusion of the p-type impurity, in the prior art, between the modulation layer
3
and the p-type clad layer
4
is interposed a buffer layer formed of the same material (InP in this example) as that of the non-doped clad layer, thereby preventing the p-n junction from being created in the modulation layer
3
.
FIG. 8
illustrates an end surface of one example of the prior optical modulator in which a buffer layer
9
is interposed between the modulation layer
3
and the p-type clad layer
4
, and shows only the n-type clad layer
2
, the modulation layer
3
, the buffer layer
9
, the p-type clad layer
4
and the semi-insulating buried layer
5
that are regions constituted by semiconductor materials. In the illustrated optical modulator, a stripe-like lamination of three-layer structure that comprises the stripe-like modulation layer
3
, the buffer layer
9
formed on the top surface of the stripe-like modulation layer
3
, and the p-type clad layer
4
formed on the top surface of the buffer layer
9
is formed on the top surface of the n-type clad layer
2
that is the n-type InP substrate. One example of thickness of each of these layers is as follows. As shown in
FIG. 8
, thickness of the n-type clad layer
2
is 70-80 &mgr;m, thickness of the modulation layer
3
is 0.27 &mgr;m, thickness of the buffer layer
9
is 0.10 &mgr;m, and thickness of the p-type clad layer
4
is 2.00 &mgr;m. Therefore, thickness of the semi-insulating buried layer
5
is the sum (2.37 &mgr;m) of the modulation layer
3
(0.27 &mgr;m), the buffer layer
9
(0.10 &mgr;m) and the p-type clad layer
4
(2.00 &mgr;m).
With the construction as described above, in case of forming the p-type clad layer
4
, it is a strong possibility that the p-type impurity will be difflused into the buffer layer
9
, but it is not diff-used into the modulation layer
3
. As a result, the p-n junction is created in the buffer layer
9
, and there is no possibility that it gets in the modulation layer
3
. Accordingly, the shortcoming that optical loss grows larger can be eliminated. Further, in
FIG. 8
, a direction in thickness from the p-type clad layer
4
that is the uppermost layer of the optical modu

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