Semiconductor laser

Details Semiconductor laser Semiconductor laser

2000-10-18

2002-11-26

Leung, Quyen (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

Reexamination Certificate

active

06487227

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 with an improved dielectric film formed on an end (front or rear) facet through which oscillating light passes.
2. Description of the Related Art
If a semiconductor laser is operated with the facets of the semiconductor crystal exposed, the semiconductor crystal facets will be oxidized and therefore the semiconductor laser will be gradually deteriorated. To prevent this problem, it is known, as shown in Japanese Unexamined Patent Publication No. 6(1994)-112679, for instance, that a transparent single-layer dielectric film, which consists of aluminum oxide (Al
2
O
3
), etc., is formed on an end facet of a semiconductor laser through which oscillating light passes.
In the case of such a dielectric film, the reflectance of the laser end facet varies with the thickness of the dielectric film. An example of the change characteristic of the reflectance is shown in FIG.
7
. Hence, it becomes possible to set the reflectance of the laser end facet to a desired value by controlling the film thickness. Note that in
FIG. 7
the axis of abscissas represents a ratio of optical length (refractive index×thickness) to wavelength &lgr;, not the film thickness itself.
As is well known in prior art, if in semiconductor lasers the oscillating light emitted therefrom is reflected at the facets, etc., of other optical components and incident again as return light, the oscillation will become unstable, causing problems of noise, etc. To prevent problems with return light, setting the reflectance of the end facet of the semiconductor laser relatively low (in the order of 10 to 25%) is effective. For this purpose, a method of controlling the thickness of the above-mentioned dielectric film can be applied.
However, in the case where the above-mentioned single-layer dielectric film is formed on the laser end facet, it is considerably difficult to accurately set the reflectance of the laser end facet to a desired value, because, as is seen from
FIG. 7
, the reflectance of the end facet varies sharply with a change in the film thickness.
On the other hand, a curved line b of
FIG. 6
shows a wavelength dispersion example of the reflectance of the end facet of a semiconductor laser with a single-layer Al
2
O
3
film formed. As illustrated in the figure, the reflectance of the laser end facet varies sharply with a change in the wavelength. In many cases, semiconductor lasers in a certain oscillating wavelength band (e.g., 700 to 1100 nm, etc.) are fabricated by common processing. However, if the wavelength dispersion of the reflectance of the laser end facet is great as described above, semiconductor lasers that can be fabricated by common processing will be restricted to a considerably narrow oscillating wavelength band.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-mentioned circumstances. Accordingly, it is the primary object of the present invention to provide a semiconductor laser which is capable of minimizing a change in the reflectance of an end facet, through which oscillating light passes, with respect to changes in the refractive index and thickness of a dielectric film formed on the end facet, and accurately setting the reflectance of the end facet to a desired value.
To achieve this end and in accordance with an important aspect of the present invention, there is provided a semiconductor laser comprising: a first transparent dielectric film formed on at least either a front or rear facet through which oscillating light passes; a second transparent dielectric film formed on the first transparent dielectric film; and a third transparent dielectric film formed on the second transparent dielectric film; wherein the following relationships are satisfied:
0.09
&lgr;≦n
1
d
1
≦
0
.
15
&lgr;
0.20
&lgr;≦n
2
d
2
≦
0
.
22
&lgr;
0.225
&lgr;≦n
3
d
3
≦
0
.
245
&lgr;
1.58
≦n
1
≦
1
.
64
2.0
≦n
2
≦
2
.
4
1.44
≦n
3
≦
1
.
46
where&lgr; is the oscillating wavelength, n
1
, n
2
, and n
3
are the refractive indices of the first, second, and third dielectric films with respect to the oscillating wavelength, and d
1
, d
2
, and d
3
are the thicknesses of the first, second, and third dielectric films.
For example, aluminum oxide (Al
2
O
3
), titanium dioxide (TiO
2
), tantalum pentoxide (Ta
2
O
5
), and silicon dioxide (SiO
2
), which will be described later, are known as transparent dielectrics that are transparent to oscillating light emitted from a general semiconductor laser. In the present invention, the dielectric film materials are not limited to these. For instance, this invention is capable of employing a material containing aluminum (Al) and oxygen (o) and ranging from 1.58 to 1.64 in refractive index, a material containing titanium (Ti) and oxygen (o) and ranging from 2.2 to 2.4 in refractive index, a material containing titanium (Ta) and oxygen (o) and ranging from 2.0 to 2.2 in refractive index, and a material containing silicon (Si) and oxygen (o) and ranging from 1.44 to 1.46 in refractive index.
In accordance with another important aspect of the present invention, there is provided a semiconductor laser comprising: a first transparent dielectric film formed on at least either a front or rear facet through which oscillating light passes, the first transparent dielectric film consisting of aluminum oxide (Al
2
O
3
); a second transparent dielectric film formed on the first transparent dielectric film, the second transparent dielectric film consisting of titanium dioxide (TiO
2
) or tantalum pentoxide (Ta
2
O
5
); and a third transparent dielectric film formed on the second transparent dielectric film, the third transparent dielectric film consisting of silicon dioxide (SiO
2
); wherein the following relationships are satisfied:
0.09
&lgr;≦n
1
d
1
≦
0
.
15
&lgr;
0.20
&lgr;≦n
2
d
2
≦
0
.
22
&lgr;
0.225
&lgr;≦n
3
d
3
≦
0
.
245
&lgr;
where&lgr; is the oscillating wavelength, n
1
, n
2
, and n
3
are the refractive indices of the first, second, and third dielectric films with respect to the oscillating wavelength, and d
1
, d
2
, and d
3
are the thicknesses of the first, second, and third dielectric films.
The refractive index n1 of Al
2
O
3
ranges between 1.58 and 1.64, the refractive index n2 of TiO
2
ranges between 2.0 and 2.4, the refractive index n2 of Ta
2
O
5
ranges between 2.0 and 2.2, and the refractive index n3 of SiO
2
ranges between 1.44 and 1.46.
Note that the aforementioned construction is particularly effective when applied to the case where the reflectance of the laser end facet is set relatively low (in the order of 10 to 25%) to eliminate the aforementioned problems with return light.
Reducing a change in the reflectance of the laser end facet with respect to changes in the refractive index and thickness of the dielectric film is equivalent to reducing reflectance distribution by wavelength, for the following reasons. From this fact it follows that if a dielectric film consisting of layers having a small change in the reflectance with respect to wavelength change is formed on the laser end facet, the object of the present invention is to be achieved.
Since there is a relationship of nd=&lgr;/4 between the refractive index n and thickness d of a single-layer dielectric film and the wavelength &lgr; of light at which the reflectance is the lowest, the following relationship is established:
(
n+&dgr;n
)(
d+&dgr;d
)=(&lgr;+&dgr;&lgr;)/4
in which &dgr;n is a variation in the refractive index n, &dgr;d is a variation in the thickness d, and &dgr;&lgr; is a change in the wavelength &lgr;. From this fact, if the layer construction of a dielectric film is formed so that the reflectances at wavelengths &lgr; and (&lgr;+&dgr;&lgr;) are much the same, essentially the same reflectances can be obtained within the variation &dgr;n in the refractive index and the variation &dgr;d in the thickness. Thus, the object of

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