Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-12-09
2002-10-22
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S045013, C372S066000
Reexamination Certificate
active
06470039
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser and a production method thereof, and particularly to a self pulsation type semiconductor laser and a production method thereof.
FIG. 16
is a schematic sectional view, seen along the direction perpendicular to the resonator length direction, of a related art inner stripe type semiconductor laser.
Layers are epitaxially grown in sequence on the entire surface of a substrate
1
made from n-type GaAs: a first cladding layer
2
made from n-type Al
0.5
Ga
0.5
As, an active layer
3
made from Al
0.15
Ga
0.85
As, a second cladding layer
4
made from p-type Al
0.5
Ga
0.5
As, and a heavily doped contact layer
5
made from p-type GaAs. The stacked layers are selectively etched from the contact layer
5
side up to a depth reaching the inner portion of the second cladding layer
4
, to form two grooves
8
, thereby forming a stripe-like ridge
7
extending in the direction perpendicular to the paper plane of
FIG. 16
between the grooves
8
. In this case, the depth of the groove
8
is selected such that the second cladding layer
4
having a specific thickness “d” remains under the grooves
8
.
A current constriction layer
6
made from n-type GaAs is grown in such a manner as to bury the grooves
8
.
A first electrode
9
is formed on the contact layer
5
and the current constriction layer
6
in such a manner as to be in ohmic-contact therewith, and a second electrode
10
is formed on the back surface of the substrate
1
in such a manner as to be in ohmic-contact therewith.
In the semiconductor laser having such a configuration, the active layer
3
is divided into a gain region
11
, two saturable absorption regions
12
on both sides of the gain region
11
, and two outside regions
13
on both sides of the saturable absorption regions
12
.
A current, which is restrictively supplied to the stripe-like ridge
7
by the effect of the current constriction layer
6
, is injected in the gain region
11
of the active layer, with a result that a gain necessary for laser oscillation occurs only in the gain region
11
of the active layer
3
.
The saturable absorption region
12
does not undergo current injection, and acts as a light saturable absorber which does not absorb light when the light intensity increases to some extent and becomes a transparent body.
The saturable absorber, therefore, acts as a Q switch, which is capable of adjusting the ratio of light effused from the gain region
11
to the saturable absorption region
12
by selecting the width “W” of the gain region
11
and the thickness “d” of each of portions, on both sides of the stripe-like ridge
7
, of the second cladding layer
4
. The output of the laser light is periodically changed by adjusting the ratio of light effused from the gain region
11
to the saturable absorption region
12
, to thus constitute a self pulsation type semiconductor laser.
A light distribution region upon operation is schematically shown by a chain line “a” in FIG.
16
.
Such a self pulsation laser, which is low in coherence of laser light and also low in a so-called optical feedback induced noise due to an unstable laser oscillation state caused by return of light, having been emitted from the laser, to the laser again, is useful as an optical disk light source or a high-speed LAN (Local Area Network) light source.
It is experientially known that the above-described self pulsation laser is obtained by selecting the width “W” of the gain region
11
at a narrow value (generally, 5 &mgr;m or less), and setting a difference &Dgr;n in effective refractive index between the gain region
11
and the saturable absorption region
12
at a small value (generally, &Dgr;n≦0.01) by adjusting the thickness “d” of each of the portions, on both the sides of the stripe-like ridge
7
, of the second cladding layer
4
. However, since the allowable ranges of the width “W” and the thickness “d” are narrow, it is difficult to adjust the width “W” and the thickness “d” at the etching step for forming the grooves
8
.
Accordingly, it is difficult to sustain the self pulsation at a high light output, for example, 10 mW, and also it is difficult to sustain the self pulsation at a high operational temperature, for example, 70° C. Further, it is difficult to produce a self pulsation laser with a high production yield.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a self pulsation type semiconductor laser capable of sustaining the self pulsation at a high light output and/or at a high operational temperature, and to provide a method of producing the semiconductor laser.
Another object of the present invention is to provide a self pulsation type semiconductor laser which is produceable with a high production yield, and to provide a method of producing the semiconductor laser.
A semiconductor laser according to the present invention basically includes a first cladding layer, an active layer, a second cladding layer, and a current constriction layer.
The active layer may be formed in such a manner that it has a gain region which is defined as a current injection region by the current constriction means and which is capable of acquiring an optical gain by current injection thereto; a saturable absorption region in which there occurs light effusion thereto; and an outside region in which there little occurs light effusion thereto. The active layer may be also formed in such a manner that it has only a gain region, and a saturable absorption layer, which has a saturable absorption region disposed at such a position as to allow the region to absorb light from the gain region and also has an outside region disposed outside the saturable absorption region in such a manner as to be in contact therewith, is provided, separately from the active layer, in at least one of the first and second cladding layers.
In each of these configurations, an effective band gap of the saturable absorption region may be larger than that of the outside region.
A method of producing a semiconductor laser according to the present invention basically includes steps of sequentially growing a first cladding layer, an active layer, and a second cladding layer on a substrate, and forming a current constriction means.
The current constriction means can be formed in accordance with a related art method.
In this method, the active layer may be formed in such a manner that it has a gain region which is defined as a current injection region by the current constriction means and which is capable of acquiring an optical gain by current injection thereto; a saturable absorption region in which there occurs light effusion thereto; and an outside region in which there little occurs light effusion thereto. The active layer may also be formed in such a manner that it has only a gain region, and a saturable absorption layer, having a saturable absorption region, disposed at such a position as to allow the region to absorb light from the gain region, in which there occurs light effusion thereto, and also having an outside region, being in contact with the saturable absorption region, in which there little occurs light effusion thereto, is provided separately from the active layer.
In each of these methods, an effective band gap of the saturable absorption region may be larger than that of the outside region.
According to the semiconductor laser of the present invention having the above-described configuration, it is possible to sustain the self pulsation at a high light output and/or a high operational temperature.
With respect to a self pulsation type semiconductor laser, it is known that the function of a saturable absorption region can be made higher by making the carrier lifetime in the saturable absorption region shorter than the carrier lifetime in a gain region and/or by making the differential gain in the saturable absorption region larger than the differential gain in the gain region (see H. Kawaguchi, Appl. Phys. Lett., 45(12)pp. 1264 (1984); M. Ueno and R. Lang, J. Appl. Phys., 58(4)
Ip Paul
Menefee James
Sonnenschein Nath & Rosenthal
Sony Corporation
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