Semiconductor optical device

Details Semiconductor optical device Semiconductor optical device

1998-03-02

2001-05-01

Sanghavi, Hemang (Department: 2501)

Coherent light generators

Particular active media

Semiconductor

C372S045013, C372S046012, C372S102000, C385S014000, C385S131000

Reexamination Certificate

active

06226310

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a semiconductor optical device in which a light modulator and a semiconductor laser are integrated, and a fabricating method thereof. More particularly, the invention relates to a semiconductor optical device employed in trunk line optical communication system and a fabricating method thereof.
BACKGROUND OF THE INVENTION
Conventionally, a semiconductor optical device comprising an electro-absorption type light modulator and a semiconductor laser that are integrated on a substrate has been used as a light source for digital optical communication of 2.5 Gb/s to 40 Gb/s.
FIG. 28
is a schematic perspective view illustrating a principal part of a conventional semiconductor optical device.
FIG. 29
is an enlarged schematic perspective view illustrating a structure of a light modulator region of the semiconductor optical device.
Referring to
FIG. 28
, a semiconductor optical device
1
has a laser region
2
, a light modulator region
3
, and an isolation region
4
between the laser region
2
and the light modulator region
3
. An electrode
15
is placed at the light modulator region
3
, and an electrode
16
is placed at the laser region
2
.
The laser region
2
has a diffraction grating and constitutes a so-called distributed feedback laser. In this distributed feedback laser, light wavelength easily varies due to light reflected at a light emitting facet, i.e., return light. For this reason, a window region
5
having no optical waveguide is usually provided continuously with the light modulator region
3
.
More specifically, as shown in
FIG. 29
, in the light modulator region
3
, a buffer layer
7
serving as a lower cladding layer, a first light confinement layer
8
, an active layer
9
, a second light confinement layer
10
, a first upper cladding layer
11
, and a second upper cladding layer
12
are successively disposed on a substrate
6
. In the window region
5
, a buried layer
17
, a hole trap layer
18
, and the second upper cladding layer
12
are successively disposed on the substrate
6
. A contact layer
13
and an insulating film
14
are disposed on the second upper cladding layer
12
. In other words, the window region
5
has no active layer
9
and no optical waveguide.
By providing the window region
5
, a spot diameter of light having passed through the light modulator region
3
spreads. That is, the light advances radially. Therefore, the rate of light that is reflected at a light emitting facet
19
, i.e., a facet of the window region
5
, and returns to the optical waveguides in the light modulator region
3
and the laser region
2
is reduced. As a result, variations in light wavelength are suppressed.
By the way, the light having passed through the light modulator region
3
goes into the window region
5
. At this time, the following problem arises from the radial spread of the light.
There is a case in which the light radially spreading leaks out of the window region
5
, that is, the light radially spreading goes from the window region
5
into a region
20
comprising, for example, a buried polyimide layer, which is adjacent to the side portion of the window region
5
. In such a case, since the window region
5
and the region
20
usually comprise different materials, the light is reflected at the interface of the window region
5
and the region
20
, which causes irregular reflection of the light in the window region
5
, leading to deterioration of a beam shape of light that is emitted from the emitting facet
19
. This means that the loss of the light at the window region
5
increases. Incidentally, in a conventional example, the loss of the light became about 50%. In the case where the loss of the light is considerable as described above, connection to an optical system cannot be sufficiently performed when the semiconductor optical device
1
is used in optical communication.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a semiconductor optical device in which irregular reflection of light in a window region is prevented, thereby suppressing deterioration of the beam shape of emitted light.
Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter; it should be understood, however, that the detailed description and specific embodiment are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art from this detailed description.
According to a first aspect of the present invention, a semiconductor optical device includes a semiconductor laser region for producing laser light and having a first optical waveguide mesa structure, the first optical waveguide mesa structure including a first optical waveguide portion including a first active layer and a diffraction grating, and first current blocking layers adjacent to both sides of the first optical waveguide portion; a light modulator region for modulating the laser light produced by the semiconductor laser region and having a second optical waveguide mesa structure, the second optical waveguide mesa structure being continuous with the first optical waveguide mesa structure and including a second optical waveguide portion comprising a second active layer, and second current blocking layers adjacent to both sides of the second optical waveguide portion; a window region for radially propagating the laser light modulated by the light modulator region and having a mesa-shaped window structure, the window structure being continuous with the second optical waveguide mesa structure; and a semiconductor substrate on which the semiconductor laser region, the light modulator region, and the window region are integrated; wherein the mesa width of the window structure is larger than the mesa width of the second optical waveguide mesa structure. Therefore, even when laser light is radially propagated from the light modulator region, the light is prevented from leaking out of the window structure. In other words, the laser light radially propagated is prevented from reaching the interface of the window structure and a buried layer adjacent to the window structure. Consequently, the laser light can be prevented from being reflected at the interface of the window structure and the buried layer, thereby preventing irregular reflection of the laser light in the window structure. As a result, deterioration of the beam shape of laser light emitted from the window structure can be suppressed, resulting in a semiconductor optical device in which satisfactory connection to an optical system is possible.
According to a second aspect of the present invention, in the semiconductor optical device of the first aspect of the invention, the window structure is continuous with the second optical waveguide mesa structure; and the mesa width of the window structure gradually increases corresponding to the radiant angle of the laser light radially propagated. Accordingly, the laser light radially propagated in the window structure is prevented all the more from reaching the interface of the window structure and the buried layer. Thus, the laser light can be reliably prevented from being reflected at the interface of the window structure and the buried layer, resulting in no irregular reflection of the laser light in the window structure. Consequently, deterioration of the beam shape of laser light emitted from the window structure can be reliably suppressed, resulting in a semiconductor optical device in which satisfactory connection to an optical system is possible.
Further, since the laser light radially propagated is prevented from leaking out of the window structure by gradually increasing the mesa width of the window structure corresponding to the radiant angle of the laser light, the size of the window structure is reduced to a minimum. As a result, an increase in electric capacity with an increase in size of the window structure can be suppressed, which does no

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