Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-01-20
2001-02-27
Healy, Brian (Department: 2874)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S044010, C372S045013, C372S050121
Reexamination Certificate
active
06195375
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a self-pulsation type semiconductor laser which realizes multi-modes by causing self-pulsation.
2. Description of the Related Art
A semiconductor laser is used as a light source for an optical disk apparatus etc. At this time, how to suppress the so-called “returning light noise” generated by part of the light reflected from the optical disk striking the semiconductor laser is important.
As one type of semiconductor laser designed to suppress this returning light noise, there is known a so-called “self-pulsation type semiconductor laser” which realizes multi-modes by causing self-pulsation of the semiconductor laser.
FIG. 7
is a cross-sectional view of an example of the configuration of a self-pulsation type semiconductor laser of the related art.
Note that here, a case is shown where an AlGaInP based material is used for configuring the self-pulsation type semiconductor laser.
As shown in
FIG. 7
, a self-pulsation type semiconductor laser
10
is comprised of an n-type GaAs substrate
11
on which are successively stacked an n-type AlGaInP clad layer
12
, a GaInP active layer
13
, a p-type AlGaInP clad layer
14
, a p-type GaInP intermediate layer
15
, and a p-type GaAs cap layer
16
.
The upper layer portion of the p-type AlGaInP clad layer
14
, the p-type GaInP intermediate layer
15
, and the p-type GaAs cap layer
16
have mesa-type stripe shapes extending in one direction.
Namely, a stripe portion
17
is formed by the upper layer of the p-type AlGaInP clad layer
14
, the p-type GaInP intermediate layer
15
, and the p-type GaAs cap layer
16
.
An n-type GaAs current narrowing layer
18
is buried at the portions at the two sides of the stripe portion
17
, by which a current narrowing structure is formed.
A p-side electrode
19
such as a Ti/Pt/Au electrode is provided on the p-type GaAs cap layer
16
and the n-type GaAs current narrowing layer
18
.
On the other hand, an n-side electrode such as an AuGe/Ni/Au electrode is provided on the other surface of the n-type GaAs substrate
11
.
FIG. 8
is a rough graph of the distribution of the refractive index of the self-pulsation type semiconductor laser
10
shown in FIG.
7
.
Here, a refractive index distribution parallel to the direction of a pn junction of the self-pulsation type semiconductor laser
10
and perpendicular to a longitudinal direction of the resonator (hereinafter this direction will be referred to as a “horizontal direction”) is shown in correspondence with FIG.
7
.
As shown in
FIG. 8
, the self-pulsation type semiconductor laser
10
has a refractive index in the horizontal direction of a high refractive index n
1
at a portion corresponding to the stripe portion
17
and of a low refractive index n
2
at portions corresponding to the two sides of the stripe portion
17
, that is, a so-called “step shaped” refractive index distribution.
In this way, in the self-pulsation type semiconductor laser
10
, light is guided in the horizontal direction by changing the refractive index in the horizontal direction in a step manner.
In this case, a refractive index difference &Dgr;n (=n
1
−n
2
) between a portion corresponding to the stripe portion
17
and portions corresponding to the two sides of it is set to be not more than 0.003, whereby the confinement of light in the horizontal direction by the GaInP active layer
13
is eased.
During operation of the self-pulsation type semiconductor laser
10
configured as above, as shown in
FIG. 7
, a width WP of the optical waveguide region
22
becomes larger than a width WG of a gain region
21
inside the GaInP active layer
13
. Thus, the optical waveguide region
22
outside the gain region
21
becomes a saturable absorbing region
23
.
In the self-pulsation type semiconductor laser
10
, the change of the refractive index in the horizontal direction is reduced so as to increase the amount of seepage of light in the horizontal direction and increase the interaction between the light and the saturable absorbing region
23
inside the GaInP active layer
13
so as to realize self-pulsation. Accordingly, it is necessary to secure a sufficient saturable absorbing region
23
.
Turning now to the problem to be solved by the present invention, as explained above, the self-pulsation type semiconductor laser
10
of the related art has a so-called “ridge configuration” as shown in FIG.
9
and provides a saturable absorbing region (SAR) at the two sides of the optical waveguide inside the active layer to realize self-pulsation.
In this case, as shown in
FIG. 9
, when a gain region (a width of which is G) inside an active layer generated by the spread of the current is made as narrow as possible and the optical waveguide spot size (a width of which is P) is made conversely made relatively large to fulfill a relationship of P>G, the amount of difference functions as a saturable absorbing region to generate self-pulsation.
For this, specifically, the refractive index difference &Dgr;n of the waveguide is made an intermediate guide of about 0.005 to 0.001 between an index guide and a gain guide to fulfill this relationship.
In such a semiconductor laser of the related art, however, since the width of the saturable absorbing region is determined by the delicate balance between the spread of light and the spread of current, there is instability such as a poor yield of the laser generating self-pulsation, a suppressed self-pulsation at the time of high temperature operation due to an increased current diffusion and narrower saturable absorbing region, and similarly a suppressed self-pulsation at the time of high output operation due to an increased current diffusion and narrower saturable absorbing region.
Especially, at a high temperature and high output, the so-called pulsation stops and the problem of noise arises.
Furthermore, a self-pulsation type semiconductor laser generally has a current threshold Ith considerably higher (about 1.5 times) that in an ordinary index guide type or a gain guide type. Also, depending on the system, sharp rising kinks are caused in the so-called L-I characteristic near the current threshold Ith. These have proven to be obstacles in applying the laser.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a self-pulsation type semiconductor laser having a high yield in manufacture and capable of performing stable self-pulsation even when operating at a high temperature and at a high output.
According to a first aspect of the present invention, there is provided a self-pulsation type semiconductor laser, comprising a first conductivity type first clad layer, an active layer formed on the first clad layer; a second conductivity type second clad layer formed on the active layer; a current narrowing structure comprising first conductivity type current narrowing layers buried at portions at the two sides of a stripe portion formed in the second clad layer; saturable absorbing regions formed on the two sides of the active layer inside a laser resonator; and a holding mechanism for holding a predetermined refractive index difference &Dgr;n of a waveguide in a horizontal direction parallel to a pn junction and perpendicular to a longitudinal direction of the resonator within a range capable of continuously generating pulsation; a thickness d of the second clad layer outside the waveguide being set to a small value capable of suppressing the spread in the horizontal direction of a current introduced to the active layer through the stripe portion to close to a stripe width W of a bottom portion of the stripe portion.
According to a second aspect of the present invention, there is provided a self-pulsation type semiconductor laser, comprising a first conductivity type first clad layer; an active layer formed on the first clad layer; a second conductivity type second clad layer formed on the active layer; a current narrowing structure comprising first conductivity type current narrowing layers buried at portions at the tw
Healy Brian
Sonnenschein Nath & Rosenthal
Sony Corporation
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