Thermal measuring and testing – Determination of inherent thermal property
Reexamination Certificate
2001-06-05
2004-09-21
Wong, Don (Department: 2828)
Thermal measuring and testing
Determination of inherent thermal property
C372S046012
Reexamination Certificate
active
06793388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser, and more particularly to a semiconductor laser having a so-called simplified antiresonant reflecting optical waveguide (hereinafter referred to as S-ARROW) structure, which is constructed so as to confine a basic lateral mode light between a pair of high refractive index layers, each extending with a gap therebetween.
The present invention relates also to a method of fabricating a semiconductor laser having the S-ARROW structure.
2. Description of Related Arts
A semiconductor laser has been predominantly used as a light source for use in optical communication, a light source for use in an optical disc and the like by use of property that a laser light emitted therefrom can be collected up to a diffraction limitation. However, all light radiated from the semiconductor laser can never be collected up to the diffraction limitations thereof. Only light having well matched phases is collected at a light emission end of the semiconductor laser. The semiconductor laser in a condition of being capable of emitting such a light is known as one performing light emission in a basic lateral mode. Under condition that light of various phase is being emitted mixedly, in other words, under condition that high ordered lateral mode light is being mixedly emitted, the light cannot be collected up to a diffraction limitation.
It has been widely known that the foregoing basic lateral mode operation is more stable as an area of a light emission section is made smaller so that it is difficult for the high order lateral mode light to be mixedly present. For this reason, a size of a waveguide path in the semiconductor laser is set to 1 &mgr;m or less in a direction of a thickness thereof, and to about 2 to about 4 &mgr;m in a direction in parallel with a light emission layer thereof. It has been well known by experience that a semiconductor laser device emitting a light in a basic lateral mode more stably can be manufactured with a high yield as a width of the waveguide path in the direction in parallel with the light emission layer is made narrower.
However, if the area of the light emission section is made small by narrowing the lateral width of the waveguide path, an increase in light density in a light emission end of the semiconductor laser is inevitably brought about. The increase in the light density in the light emission end incurs deterioration of materials constituting the semiconductor laser, resulting in reduction in the life of the device.
In other words, the stabilization of the basic lateral mode (the small section area of the waveguide path) and an increase in light output (the large section area of the waveguide path) are mutually incompatible. Accordingly, to overcome this limitation is an important problem in researching and developing existing semiconductor lasers.
As disclosed in IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 8, August 1998, the S-ARROW structure has been proposed as one idea to solve the problem. The S-ARROW structure has a light emission width of about 6 &mgr;m, and the width of the light emission section of this structure can be set to be 1.5 to 2 times as large as that of the conventional structure, so that it is anticipated that the maximum light output will be improved.
The reason why the semiconductor laser adopting the S-ARROW structure emits light in the basic lateral mode will be explained below.
A cross section of the principal portion of the semiconductor laser having the S-ARROW structure, that is, a shape in a cross section perpendicular to the waveguide direction, is shown in FIG.
18
A. This semiconductor laser comprises an n-GaAs substrate
39
; a lower clad layer
38
made of n-InGaP, formed on the n-GaAs substrate
39
; a SCH (separate-cofinement-heterostructure) structural layer
37
made of InGaAsP, which includes an InGaAs quantum well activation layer; upper clad layers
36
and
32
made of p-InGaP; an etching stop layer
35
made of n-GaAs; a current blocking layer
34
made of n-AlInP; guide portions
33
of a thickness of e.g. 0.25 &mgr;m, made of n-GaAs; and a contact layer
31
made of p-GaAs.
In the above-described structure, since GaAs constituting the above-described guide portion
33
possesses a refractive index higher than that of a periphery of the guide portions
33
, an equivalent refractive index in a direction in parallel with the SCH structural layer
37
has a distribution high in the guide portion
33
and low in other portions, as shown in FIG.
18
B.
In such a waveguide structure, with respect to the width A of each of the two guide portions
33
, a dimension is selected so that while only light in a basic lateral mode is confined between these guide portions
33
, light in a high ordered lateral mode is not confined therebetween but escapes outside each of the guide portions
33
. According to the literature cited above, the width A of the guide portions
33
is set to 0.85 &mgr;m, and the width B of the groove formed by the guide portions
33
is set to 6.5 &mgr;m.
Due to the effect of the current blocking layer
34
, a current producing a gain of the laser is injected only between the two guide portions
33
, and the gain relevant to the laser light is generated only between the guide portions
33
.
Accordingly, only the light in the basic lateral mode is confined between the guide portions
33
, and hence a sufficient gain can be obtained. On the other hand, since the light in the high order lateral mode is not confined between the guide portions
33
, a gain cannot be obtained. As a natural consequence of such a fact, the light in the basic lateral mode is given priority in emission, and the semiconductor laser operates in a stable basic lateral mode until the high light output.
Heretofore, however, the fabrication of the semiconductor laser having the S-ARROW structure has inevitably shown a low yield for the following reason, causing a serious problem in mass production of the semiconductor lasers. To explain this reason, a method of fabricating the semiconductor laser having the S-ARROW structure will first be described with reference to
FIGS. 19
to
22
.
As shown in
FIG. 19
, on the n-GaAs substrate
39
, there are sequentially grown the lower clad layer
38
made of n-InGaP, the SCH (Separate-confinement-heterostructure) structural layer
37
made of InGaAsP, which includes the InGaAs quantum well activation layer, the upper clad layer
36
made of p-InGaP, the etching stop layer
35
made of n-GaAs, the current blocking layer
34
made of n-AlInP, and the guide portion
33
having a thickness of 0.25 &mgr;m, which is made of n-GaAs by use of an organometallic growth method.
The GaAs layer is partially removed by a photolithography step and an etching step while leaving the part of the GaAs layer functioning as the guide portion
33
, thus obtaining the sectional structure shown in FIG.
20
.
Furthermore, as shown in
FIG. 21
, a resist pattern
40
is formed at portions other than that corresponding to a groove having the width B (see
FIG. 18
) by use of a photolithography step.
Thereafter, by use of the resist pattern
40
as a mask, the semiconductor layers including the guide portion
33
, the current blocking layer
34
and the etching stop layer
35
are sequentially etched so as to be removed until the upper clad layer
36
made of p-InGaP is exposed. Thus, the sectional structure shown in
FIG. 22
, in which a pair of resist patterns
40
are formed, is obtained.
Thereafter, the resist pattern
40
is removed, and then the upper clad layer
32
made of p-InGaP and the contact layer
31
made of p-GaAs are formed by a crystal growth method, thus obtaining the semiconductor laser having the structure shown in FIG.
18
.
In the conventional method described above, the photolithography steps were performed two times. The pair of resist patterns
40
had a size of about 01 &mgr;m and were required to be coincident with each other with a very high precision. If this precision is low, the widths
Nguyen Tuan N.
Wong Don
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