Semiconductor light emitting device

Details Semiconductor light emitting device Semiconductor light emitting device

1999-07-09

2002-07-02

Ip, Paul (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S043010

Reexamination Certificate

active

06414976

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light emitting device, more particularly relates to a semiconductor laser.
2. Description of the Related Art
FIG. 28
is a sectional view of an example of the configuration of a self-pulsation type semiconductor laser of the related art using a so-called a buried ridge structure.
Note that, here, a case where the self-pulsation type semiconductor laser is constituted by an AlGaAs-based material is shown.
As shown in
FIG. 28
, in this self-pulsation type semiconductor laser
10
, an n-type GaAs substrate
11
has successively stacked on it an n-type AlGaAs cladding layer
12
, an AlGaAs active layer
13
, a p-type AlGaAs cladding layer
14
, and a p-type GaAs cap layer
15
.
The upper layer portion of the p-type AlGaAs cladding layer
14
and the p-type GaAs cap layer
15
have mesa-type stripe shapes extending in one direction.
Namely, a stripe portion
16
is constituted by these upper layer portion of the p-type AlGaAs cladding layer
14
and the p-type GaAs cap layer
15
.
At the two side portions of this stripe portion
16
are buried a GaAs current narrowing layers
17
. A GaAs current narrowing structure is formed by this.
On the p-type GaAs cap layer
15
and the GaAs current narrowing layers
17
is provided a p-side electrode
18
such as for example a Ti/Pt/Au electrode.
On the other hand, on the back surface of the n-type GaAs substrate
11
is provided an n-side electrode
19
such as for example an AuGe/Ni/Au electrode.
FIG. 29
is a schematic graph of the distribution of the refractive index of the self-pulsation type semiconductor laser
10
shown in FIG.
28
.
Here, the distribution of the refractive index of the region in which light is guided in a direction parallel to a pn junction of the self-pulsation type semiconductor laser
10
and then perpendicular to a resonator length direction (hereinafter this direction will be referred to as a lateral direction) is shown in correspondence to FIG.
28
.
As shown in
FIG. 29
, the self-pulsation type semiconductor laser
10
has a so-called step-like distribution of the refractive index in the lateral direction, where the refractive index n
1
in a part corresponding to the stripe portion
16
is high and the refractive index n
2
in the part corresponding to the two sides of the stripe portion
16
is low.
By changing the refractive index in steps in the lateral direction in this way, the light is guided in the lateral direction in the self-pulsation type semiconductor laser
10
.
In this case, the difference &Dgr;n (=n
1
-n
2
) in the refractive indexes between the part corresponding to the stripe portion
16
and the parts corresponding to the two sides thereof is set to be not more than about 0.003 and the optical confinement in the lateral direction of the AlGaAs active layer
13
is eased.
At the time of operation of the self-pulsation type semiconductor laser
10
constituted in this way, as shown in
FIG. 28
, a width WP of a light waveguide region
22
becomes larger than a width WG of a gain region
21
inside the AlGaAs active layer
13
. The light waveguide region
22
at the outside of the gain region
21
becomes a saturable absorbing region
23
.
In this self-pulsation type semiconductor laser
10
, by making the change in the refractive index in the lateral direction small, the seepage of light in the lateral direction is increased. By making the interaction between the light and the saturable absorbing region
23
inside the AlGaAs active layer
13
larger, self-pulsation is realized. For this purpose, it is necessary to secure a sufficient saturable absorbing region
23
.
As explained above, the self-pulsation type semiconductor laser
10
has so-called a ridge structure as shown in
FIG. 30
, in which saturable absorbing regions are provided at the two sides of the light waveguide inside the active layer and made to perform the self-pulsation.
In this case, as shown in
FIG. 30
, when the relationship of P>G is satisfied by making the gain region inside the active layer (width thereof defined as G) created by a spread of the current as narrow as possible and conversely setting a light waveguide spot size (width thereof defined as P) relatively large, this difference acts as the saturable absorbing region and causes the self-pulsation.
This relationship is satisfied by using the refractive index difference An of the waveguide as an intermediate guide between an index guide of about 0.005 to 0.001 and a gain guide.
FIG. 31
is a perspective view of an example of the configuration of a gain guide-type semiconductor laser of the related art,
FIG. 32
is a plan view of an example of the configuration of the gain guide-type semiconductor laser of the related art, and
FIG. 33
is a sectional view of an example of the configuration of the gain guide-type semiconductor laser of the related art.
As shown in the figures, this gain guide-type semiconductor laser
30
is comprised of an n-type GaAs substrate
31
on which an n-type AlGaAs cladding layer
32
, an AlGaAs active layer
33
, a p-type AlGaAs cladding layer
34
, and a p-type GaAs cap layer
35
are successively stacked.
On the two sides of this stripe portion
36
are formed a current narrowing layers
37
given a higher resistance by ion implantation of for example B
+
ions.
On the p-type GaAs cap layer
35
and the current narrowing layer
37
is provided a p-type electrode
38
such as a Ti/Pt/Au electrode.
On the other hand, on a back surface of the n-type GaAs substrate
31
is provided an n-type electrode
39
such as an AuGe/Ni/Au electrode.
In the case of this gain guide-type semiconductor laser, from a practical viewpoint, the waveguide is constituted as a tapered waveguide forming a taper with a wide stripe width at the center portion becoming narrower near the end surface as shown in FIG.
32
.
Note that, in
FIG. 32
, L denotes the entire resonator length,
11
a taper region length,
13
a wide stripe region length at the center portion, w
1
a stripe width near the end surface, and w
3
a stripe width at the center portion.
In the gain guide-type semiconductor laser
30
having this configuration, at the time of operation, the current flows through the stripe portion
36
and flows into the active layer
33
, but since the current narrowing layer
37
is provided, the flow of the current to the two side directions of the active layer
33
is suppressed.
As a result, a light emitting region of a predetermined width is formed and laser oscillation is carried out.
In the case of a gain guide-type semiconductor laser not provided with almost any refractive index difference An in the lateral direction, since vertical multimode oscillation is carried out, the relative returned light noise characteristic is good. Also, the electrostatic withstand voltage is high, therefore there is strong surge proofness.
In the case of this gain guide-type semiconductor laser, the noise level required for the semiconductor laser is about 1% of the amount of the returned light in terms of the value of the relative intensity noise (RIN) and about −120 dB to −125 dB at the time of an output of several mW, so the laser is suitable as a light source for a CD or other optical disc.
Summarizing the problems to be solved by the invention, the semiconductor lasers explained above suffered from the following problems.
Namely, a self-pulsation type semiconductor laser with a refractive index difference &Dgr;n set near 0.003 and causing self-pulsation in a lateral region Inside the active layer as mentioned above has a relatively large astigmatism difference of about 10 odd &mgr;m and changes in the beam spread angle &thgr;// in a parallel direction of a far field pattern (FFP) due to the optical output. As a result, there is the problem that it is difficult to apply this to the optical system of an optical disc.
Further, in the case of a not illustrated index guide-type semiconductor laser, when the beam spread angle &thgr;// in the p

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