Optical waveguides – Integrated optical circuit
Reexamination Certificate
1998-11-25
2002-11-12
Font, Frank G. (Department: 2877)
Optical waveguides
Integrated optical circuit
C385S131000, C372S044010, C359S346000
Reexamination Certificate
active
06480640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an integrated optical circuit device and a method for producing the same, and more specifically, to a method for coupling a plurality of optical elements. In particular, the present invention relates to an integrated optical circuit device incorporating a semiconductor laser section and an optical waveguide section having different near fields from each other, as well as a method for producing the same.
2. Description of the Related Art
Conventionally, integrated optical circuit device incorporating a semiconductor laser element (also referred to as the “semiconductor laser section”) and an optical waveguide element (also referred to as the “optical waveguide section”) having different near fields from each other are produced by a hybrid method which involves separately fabricating a semiconductor laser element and an optical waveguide element having different near fields and then combining the two elements. However, the hybrid method requires very accurate positioning and therefore is not highly industrially applicable, i.e., is not highly adaptable to being efficiently produced.
In order to solve this problem, there has been proposed a method which integrally forms a semiconductor laser element and an optical waveguide element on the same substrate.
For example, Japanese Laid-open Publication No. 63-182882 discloses a structure Including an active layer
2
and a cladding layer
3
deposited on an InP substrate
1
as shown in
FIG. 14. A
portion of the deposited active layer
2
and the cladding layer
3
is etched away, so that a buffer layer
5
, an optical waveguide layer
6
, and a protective layer
7
are integrally formed in the etched portion. An electrode
8
is formed over the remaining portion of the active layer
2
and the cladding layer
3
so as to function as a semiconductor laser section.
FIG. 15
shows another conventional example of an integrated semiconductor laser and optical waveguide disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 2, No. 2, p.88 (1990 February), which includes a semiconductor laser section
13
and an optical waveguide section
11
which are integrally formed with a tapered section
12
interposed between, such that the tapered section
12
provides a gradually varying light distribution.
However, the structure shown in
FIG. 14
inevitably results in the formation of a section
9
between the active layer
2
and the optical waveguide layer
6
which lacks the function of light confinement in a vertical direction. Since light cannot be confined along the vertical direction in the section
9
, unwanted light radiation occurs not directing towards the optical waveguide layer
6
, thereby decreasing the coupling efficiency to the optical waveguide layer
6
.
Furthermore, since the active layer
2
and the optical waveguide layer
6
are formed through separate growth processes, it is difficult to align the active layer
2
and the optical waveguide layer
6
along the height direction within the controllability of available crystal growth technology. For example, in the case where the optical waveguide layer
6
is formed by a common metal organic chemical vapor deposition method (hereinafter ter referred to as the “MOCVD method”), its thickness and position along the height direction may be offset from design values by about 5% to 10%. In general, it is desirable to form an optical waveguide layer so as to have a thickness of several &mgr;m to facilitate coupling with external elements (e.g., with an optical fiber), and this results in a buffer layer (underlying the optical waveguide layer) having a thickness of several &mgr;m. In such cases, the total positional offset along the height direction may be as great as 0.1 to 0.5 &mgr;m, and the thickness of the section
9
emerging between the active layer
2
and the optical waveguide layer
6
may also be on the order of &mgr;m, resulting in a coupling loss of about 1 dB (aside from coupling losses due to mode mismatching). The above-described positional offset, or light radiation in a section lacking the light confinement function, becomes especially remarkable in elements having a small light distribution width, e.g., semiconductor lasers.
On the other hand, in the structure shown in
FIG. 15
, since the semiconductor laser section
13
and the optical waveguide section
11
are coupled via the tapered section
12
providing a gradually varying light distribution, substantially no light radiation occurs between the two sections
13
and
11
so that highly efficient optical coupling may be obtained. However, the formation of the tapered section
12
requires several (e.g., three or more, for the illustrated structure) etching processes and regrowth processes. Such complexity in the production process, as well as degradation in the crystallinity of the tapered section
12
and the optical waveguide section
11
during the regrowth process, detracts from the theoretical effects of this technique so much that no industrial applications of this technique have been reported heretofore.
In addition, the structure in
FIG. 15
is also susceptible to some propagation loss due to loss of free carriers within the optical waveguide section
11
because the optical waveguide layer of the optical waveguide section
11
and the active layer of the semiconductor laser section
13
are formed of the same material, indicative of insufficient characteristics as an optical waveguide section
11
.
SUMMARY OF THE INVENTION
An integrated optical circuit device of the invention includes: a first optical element section including first and second element regions deposited along a thickness direction; a second optical element section formed away from the first optical element section; and a multimode interference region provided between the first and second optical element sections, the multimode interference region including a buried portion formed along a light propagation direction. The first and second optical element sections are optically coupled to each other via the multimode interference region.
The buried portion of the multimode interference region may be arranged so as to have a length such that light outgoing from the first optical element section reaches via translation the second optical element section while retaining a light distribution shape at the time of outgoing from the first optical element section.
A width of the buried portion of the multimode interference region along a direction perpendicular to the light propagation direction may change in one of a gradual manner and a stepwise manner.
A thin interface region may be formed between the first optical element section and the multimode interference region.
According to another aspect of the present invention, an integrated optical circuit device includes: a first optical element section including first and second element regions deposited along a thickness direction and having mutually different light distribution widths; a multimode interference region formed in a position along a propagation direction of light outgoing from the first optical element section, the multimode interference region including a buried portion which has a multimode waveguide structure along each of the thickness direction and an in-plane direction which is perpendicular to the thickness direction; and a second optical element section formed away from the first optical element section with the multimode interference region interposed therebetween. The first and second optical element sections are optically coupled to each other via propagation of the light outgoing from the first optical element section through the multimode interference region to the second optical element section.
In the above-mentioned structure of the integrated optical circuit device, the second optical element section may include a mesa structure which is linearly aligned with the first optical element section and the buried portion of the multimode interference region.
Furthermore, the buried p
Kawanishi Hidenori
Shimonaka Atsushi
Font Frank G.
Mooney Michael P
Morrison & Foerster / LLP
Sharp Kabushiki Haisha
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