Semiconductor laser device

Details Semiconductor laser device Semiconductor laser device

2002-08-12

2004-11-30

Harvey, Minsun Oh (Department: 2828)

Coherent light generators

Particular active media

Semiconductor

C372S045013

Reexamination Certificate

active

06826217

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser device used for optical disk apparatus such as a CD-R/RW drive, a DVD-RAM drive, a MD drive and the like.
2. Description of Related Art
It has been tried to increase the recording speed of optical recording apparatuses, and, for example, CD-drives of 16-time speed recording have been put into practical use. In such an optical recording apparatus of a high recording speed, it is necessary to momentarily start up high output laser light emitting. As examples of laser devices capable of fulfilling such required characters, there are ridge type semiconductor laser devices.
FIG. 3
is a schematic sectional view of a conventional ridge type semiconductor laser device, showing a section perpendicular to the longitudinal direction of the device.
On a substrate
31
, a lower cladding layer
32
, an active layer
33
and an upper first cladding layer
34
are stacked in this order. On the upper first cladding layer
34
, a ridge-shaped upper second cladding layer
37
is formed so as to extend in the longitudinal direction of the device. On both sides of the upper second cladding layer
37
, current blocking layers
36
are formed. On the upper second cladding layer
37
and the current blocking layers
36
, a contact layer
38
is formed in ohmic contact with the upper second cladding layer
37
.
On the lower surface of the substrate
31
and the upper surface of the contact layer
38
, n-side electrodes and p-side electrodes (not shown) are formed respectively.
At the time of laser light emission, electrons recombine with holes in the active layer
33
to emit light. When the substrate
31
and the contact layer
38
are formed of GaAs and the lower cladding layer
32
, the active layer
33
, the upper first cladding layer
34
and the upper second cladding layer
37
are formed of AlGaAs based materials, the laser light can pass portions formed of AlGaAs based materials. Within these portions, the laser light is distributed in the form of a beam. The sectional form of a laser beam LB
3
at an end face of the device is shown in FIG.
3
.
FIG. 4
is an enlarged schematic sectional view showing the structure in the vicinity of the active layer
33
of the ridge type semiconductor laser device of FIG.
3
.
Between the lower cladding layer
32
and the active layer
33
, a lower beam enlargement layer
41
is provided. Between the active layer
33
and the upper first cladding layer
34
, an upper beam enlargement layer
42
is provided. By setting the refractive indexes of the lower beam enlargement layer
41
and the upper beam enlargement layer
42
to suitable values respectively, a laser beam LB
3
is enlarged by suitable widths in upward and downward directions from the active layer
33
respectively. Usually, the upper and the lower beam enlargement layers
41
and
42
are so designed that the laser beam LB
3
can be enlarged by substantially equal widths both in the upward and downward directions from the active layer
33
respectively.
The distance L
3
between the lower surface of the lower cladding layer
32
and the lower surface of the active layer
33
is so set that the lower end of the laser beam LB
3
is positioned in the vicinity of the lower surface of the lower cladding layer
32
. Similarly, the distance L
4
between the upper surface of the active layer
33
and the upper surface of the upper second cladding layer
37
is so set that the upper end of the laser beam LB
3
is positioned in the vicinity of the upper surface of the upper second cladding layer
37
. The distances L
3
and L
4
are substantially equal to each other. When the substrate
31
and the contact layer
38
are formed of materials capable of absorbing the laser beam LB
3
, the length from the lower end to the upper end of the laser beam LB
3
(hereinafter referred to as “longitudinal beam diameter”) is set to be shorter than the distance between the lower surface of the lower cladding layer
32
and the upper surface of the upper second cladding layer
37
. Thus, the laser beam LB
3
can be prevented from being absorbed.
The current injection from the upper second cladding layer
37
into the active layer
33
occurs through the lower surface of the upper second cladding layer
37
. Therefore, in the active layer
33
, the recombination of electrons and carriers of holes causes light emission in a region E
3
(hereinafter referred to as “light emission region E
3
”) having a width substantially equal to the width S
3
of the lower surface of the upper second cladding layer
37
(hereinafter referred to as “injection current width S
3
”). In this case, carriers are not consumed uniformly throughout the light emission region E
3
but consumed in a larger amount in a portion in the vicinity of the center, in the direction of the width, of the light emission region E
3
.
Though the amount of carrier consumption is nonuniform as mentioned above, the carrier density in the light emission region E
3
can be kept uniform to some extent by diffusion of carriers (especially minority carriers). However, when the operation current is enhanced in order to obtain a high optical output, carriers cannot be sufficiently complemented by diffusion in case of the injection current width S
3
being large. As a result, the carrier density becomes nonuniform in the direction of the width of the light emission region E
3
, and consequently the brightness distribution becomes nonuniform. That is, in the light emission region E
3
, the portion of light emission at a high brightness moves to a portion having a high carrier density, and consequently, the optical output becomes unstable. Since laser devices having unstable optical output cannot be used for optical recording and optical reading, the maximum rated optical output is determined within a range in which the optical output is stable.
By narrowing the injection current width S
3
, the problem of the nonuniform carrier density can be solved. That is, by making the injection current width S
3
smaller than the distance of the carrier diffusion in the active layer
33
(carrier diffusion length), the portion in which a large amount of carriers are consumed is surely supplemented with minority carriers, so that uniform light emission in the light emission region E
3
can be obtained. With such a structure, the optical output can be stabilized, whereby the maximum rated optical output of the laser device can be increased.
However, as the injection current width S
3
is made narrower, the upper surface width D
3
of the upper second cladding layer
37
also becomes narrow. The reason is that, since the side surfaces of the upper second cladding layer
37
are formed with a given inclination angle, the injection current width S
3
and the upper surface width D
3
cannot be separately changed in case of the thickness of the upper second cladding layer
37
being fixed. Since the current flows through the boundary surface between the contact layer
38
and the upper second cladding layer
37
, the area of the boundary surface between the contact layer
38
and the upper second cladding layer
37
becomes small when the upper surface width D
3
becomes narrow, so that the resistance value of the laser device rises.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor laser device capable of raising the maximum rated optical output without increasing the device resistance.
A semiconductor laser device according to the present invention includes a lower cladding layer, an active layer and an upper first cladding layer stacked in this order on a compound semiconductor substrate, a ridge-shaped upper second cladding layer provided on the upper first cladding layer, current blocking layers provided at the sides of the upper second cladding layer, and a contact layer provided on the upper second cladding layer. The distance between the upper surface of the active layer and the upper surface of the upper second cladding layer is shorter t

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