Note: Descriptions are shown in the official language in which they were submitted.
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3ACKGROUND OF THE INVENTION
Field of the Invention
T~e pres~nt invention relates to a semiconductor
laser.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematically enlarged cross-sectional
view of a prior art semiconductor laser;
Fig. 2 is a graph indicating a noise level relative
to a forward current of a semiconductor laser and a
power thereof;
Fig. 3 is a schematically enlarged cross-sectional
view showing an embodiment of a semiconductor laser
according to the present invention;
Fig. 4 is a graph indicating a relation between a
threshold value current density and a width of a removed-
away portion formed in a light absorbing layer; and
Fig. 5 is a graph indicating a distribution of a
light coming from an active layer.
DescriPtion of the Prior Art
¦ Generally, prior art semiconductor lasers are roughly
¦ classified into the refractive index-guiding type one
and the gain-guiding type one in accordance with its
light and carrier confinement mechanism in the longitudinal
mode thereof.
As a semiconductor laser of refractive index-guiding
type, there is proposed such one as, for example, shown
in ~ig. 1. Referring to Fig. 1, this semiconductor laser
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is forme~ such that on a GaAs substrate 1 of, for example,
N type there are formed a first cladding layer 2 of N
type AQzGal zAs, an active layer 3 of P or N type AQxGal xAs,
a se~ond cladding layer 4 of P type AQzGal zAs~ a light
absorbing layer 5 of N type AQyGal yAs buried into the
second cladding layer 4 and a capping layer 6 of P type
with high impurity concentration. The light absorbing
layer S has a removed-away portion Sa of stripe-shape
having a width W which is provided by removing away the
light absorbing layer 5 at its, for example, central por- -
tion in the direction perpendicular to the sheet of
paper of Fig. 1. The composition of this light absorbing
layer S is selected such that its forbidden band width is
smaller than that of the active layer 3 and its refractive
ir.dex for the light oscillated out from the light emission
region of the active layer 3 is higher than that of the
light emission region, namely, the active layer 3. That
is, in the compositions of the active layer 3 and the
light absorbing layer 5, they are selected so as to
establish the condition of x > y.
In Fig. l, reference numerals 7 and 8 designate
electrodes which are deposited on the capping layer 6
and the substrate l in ohmic-contact therewith, respec-
tively.
In this structure of the semiconductor laser, if a
predete_mined forward voltage is applied between the
electrodes 7 and 8, a light emission is carried out in
the active layer 3 of which the carrier and light are
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confined by the first and sec~nd cladding layers 2 and
. In this case, a distance d2 between the active layer
3 and the light absorbing layer 5 is selected to be a
distance through which the light oscillated out.f~om the
active layer 3 is able to reach the light absorbing layer
5, for example, in a range from 0.3 to 0.4 ~m. If the
distance d2 is selected as described above, the light
introduced to the light absorbing layer 5 from the
active layer 3 is absorbed by the light absorbing layer
5, whereby between the portion below this light absorb-
ing layer 5 and the portion corresponding to the removed-
away portion 5a of stripe-shape at the center of the
light absorbing layer 5 in which the light is hardly
absorbed, there is formed a difference of effective
refractive index, namely, a difference an of built-in
refractive index in the lateral direction. In this case,
the width W of the removed-away portion 5a of stripe-shape
of the light absorbing layer 5 is gelected in a range
from 5 to 8 ~ so that also on the basi of the selection
of the distance d2, a difference ansnl - n2 between a
refractive index nl of the portion (hereinafter referred
to as the central postion) of the active layer 3
opposing to the removed-away portion 5a of the light
absor~ing layer 5 and refractive index n2 of the portions
of the active layer 3 at both sides of the central
portion can ~e presented as, for example, +10 2 to ~10 3.
As described a~ove, in the active layer 3, the confinement
effect for the light oscillation in the lateral direction
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is produced in the central portion opposing to the removed-
away portion 5a, in which the light emission region is
restricted.
As the semiconductor laser of refractive index-
guiding type, it is not limited to the structure in which
the light absorbing layer 5 is buried into the second
cladding layer 4 but the similar operation to the above
can be carried out for such a semiconductor laser is
pos~ible, which is known as a C.S.P (channeled substrate
planar) disclosed iIl a publicated document of Japanese
patent application unexamined No. 143787/1977 and in
which the substrate 1, for example, is used as the light
absorbing layer.
As mentioned above, although the semiconductor laser
of refractive index-guiding type is constructed as shown
in Fig. 1, the semiconductor laser of this structure
can be modified into the gain-guiding type semiconductor
laser, too. That is, in the ~tructure of Fig. 1, since
the light absorbing layer 5 is selected to be a conductive
type which is different from that of the second cladding
layer 4, there is then an effect that a thyristor struc-
ture of P-N-P-N is formed in the portion in which the
light absorbing layer ~ exists to thereby limit the
current path. As a result, the semiconductor laser
of gain-guiding type can be formed by using this light
absorbing layer 5 as the current limit region. That is,
in a qualitative standpoint, a strong current concen-
tration in the stripe portion is produced by reducing
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the width w of the removed-away portion Sa as compared
with that of the semiconductor laser of the afore-mentioned
refractive index-guiding type or the thickness dl of the
active layer 3 is increased or the distance d2 between the
active layer 3 and the current limit region (that is, the
light absorbing layer 5) is increased so that the effect
of absorbing the light from the active layer 3 by the
light absorbing layer 5 can be reduced. Thus, a negative
refractive index changing amount -Qne provided by the
injected carrier becomes dominant as compared with the
changing amount ~n of the built-in refractive index so
that the difference ~n (where ~n = ~n - ~ne) between the
refractive index of the central portion of the active layer
3 and the refractive index of the both side portions lies
in a range from -10 2 to -10 3. Thus the semiconductor
laser of the gain-guiding type can be made.
However, both of the semiconductor laser of refrac-
tive index-guiding type and the semiconductor laser of
gain-guiding type have merits and demerits, respectively.
Accordingly, when each of them is used as the writing
ar.d/or reading light source of, for example, the video
disc, each proposes a practical problem.
More speclfically, in the semiconductor laser of
the refractive index-guiding type, since its longitudinal
mode is the single mode, when it is used as the writing
and/or reading light source in, for example, an optical
video disc and so on, there is a defect that a mode
hopping noise is caused by a returned light. Fig. 2 is
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a graph showing measured results of a mode hopping noise
caused by a forward current relative to the semiconductor
laser. In Fig. 2, a curve 21 indicates measured results
of noise produced in the semiconductor laser of refractive
index-guiding type and as will be clear therefrom, the
mode hopping noise is produced. On the other hand, in
the semiconductor laser of refractive index-guiding type,
a so-called beam waist position exists near the light
end face of the light emission region. There is then an
advantage that in practical use, the focal position can
be set easily. Further, since a far distant image on
the cross-section in the direction parallel to the junction
or a so-called far field pattern is symmetrical with each
other in the right and left direction, there is then an
advantage that similarly in, for example, the practical
use, it is easy to obtain a reading and/or writing light
of spot-shape having a small distortion. As compared
therewith, in the semiconductor laser of gain-guiding
type, the beam waist position exists at the inside of
about 20 ~m from the light end face of the light emission
region, the far field pattern is frequently asymmetrical
with each other in the right and left direction, and
the astigmatism is large whereby the spot distortion
becomes relatively large. As will be clear from a noise
characteristic shown by a curve 22 in Fig. 2, the noise
level of the semiconductor laser of the gain-guiding
type is high as compared with the noise level shown by
the curve 21 of the semiconductor laser of refractive
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index-guiding type. ~owever, since the longitudinal
mode thereof is the multi-mode, there is then an advan-
tage that no mode hopping noise is caused by ~he returned
light.
OBJECTS AND SUMM~RY OF THE INVENTION
Accordingly, it is an object of this invention to
provide a semiconductor device having a so-called interme-
diate characteristic between those of the refractive
index-guiding type semiconductor laser and the gain- -
guiding type semiconductor laser ~nd which can utilize
the advantages of the both and which can complement the
defects thereof so that a noise can be reduced.
It is another object of this invention to provide a
semiconductor device which is suitable as a writing and/or
reading light source in, for example, an optical video
disc or digital audio disc.
According to one aspect of the present invention,
there is provided a semiconductor laser comprising a
substrate on which a first cladding layer, an active
layer, a second cladding layer and a light absorbing
layer for limiting a current path and for absorbing a
light oscillated out from said active layer are
se~uentially formed in contact to one another, wherein
said light absorbing layer is provided with a removed-
away portion of stripe-shape for forming said current
path, wherein a width W of said removed-away portion is
selected in a range from 1 to 4 ~m, a thickness dl of
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said active layer is selected so as to satisfy the
condition of d1 > 500A and a distance d2 between said
active layer and said light absorbing layer is selected
in a range from 0.2 to 0.7 ~m.
These and other objects, features and advantages of
the semiconductor laser according to the present inven-
tion will become apparent from the following detailed
description of the preferred embodiment taken in
conjuction with the.accompanying drawings! throughout
which like reference numerals designate the same elements
and parts.
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DESCRIPTION OF THE PRE~ERRED EMBODIMENT
Now, an embodiment of a semiconductor laser according
to the present invention will hereinafter be described
with reference to Fig. 3 and the followings. In this
embodiment, similarly to the semiconductor laser described
in connection with Fig. 1, on a GaAs substrate 31 of a
conductive type, for example, N type, there are sequentially
formed a first cladding layer 32 of N type made of AQzGal zAs,
an active layer 33 of P type or N type made of AQxGal xAs,
a second cladding layer 34 made of A~zGal zAs with the
conductive type different from that of the first cladding
layer 32 or P type, a light absorbing layer 35 made of
A~yGal yAs having an effect for limiting a current path
and which can absorb the light coming from the active
layer 33 and a capping layer 36 of high impurity concen-
tration and of conductive type P same as that of the second
cladding layer 34.
The light absorbing layer 35 is of a conductive type
different from the second cladding layer 34, for example,
N type in this embodiment and in order that the light
absorbing layer 35 is opposed through the second cladding
layer 34 to the active layer 33 with a distance d2, the
light absorbing layer 35 is buried into, for example,
the second cladding layer 34. Further, the light absorb-
ing layer 35 has at its, for example, central portion aremoved-away portion 35a of stripe-shape with a predeter-
mined width and which is extended in the direction
perpendicular to the sheet of paper of Fig. 3.
The compositions of the first and second cladding
layers 32 and 34 are selected to satisfy the condition
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of z > x so that the forbidden band widths thereof become
larger than that of the active layer 33. Further, the
composition of the light absorbing layer 35 is selected
such that the forbidden band width thereof becomes
smaller than that of the active layer 33 and that the
refractive inde~ thereof with respect to the light
oscillated from the light emission region of the active
layer 33 becomes higher than that of the light emussion
region, or the active layer 33. In other words, in the
compositions of the active layer 33 and the light absorb-
ing layer 35, their compositions are selected so as to
satisfy the condition of x > y. Electrodes 37 and 38
are respectively deposited on the capping layer 36 and
the substrate 31 in ohmic-contact therewith.
In this structure, the width W of the removed-away
portion 35a of the light absorbing layer 35 is selected
in a range from 1 to 4 1~m, preferably 2 to 4 ~m. The
thickness dl of the active layer 33 is selected so as
to satisfy the condition of dl > 500A, preferably
1500A > d1 > 700A. Further, the distance d2 between
the active layer 33 and the light absorbing layer 35
is selected in a range from 0.2 to 0.7 ~m, preferably
0.3 to 0.5 ~m.
In the semiconductor laser of this structure, a
noise characteristic relative to a forward current I
flowing between the both electrodes 37 and 38 when the
forward voltage is applied between the electrodes 37
and 38 becomes as shown by a curve 23 in Fig. 2. In
Fig. 2, a curve 24 indicates a relation between the
current I and the power P.
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As will be clear from the comparison of the curve
23 with the curve 22, the noise level of the semiconductor
laser of the pxesent invention is reduced as compared
with that of the gain-gulding type semiconductor laser.
That is, if now the semiconductor laser of the present
invention is operated by the power of, for example, 5 mW
by which a current Il is flowed, the difference between
the curves 22 and 23 becomes about 5 dBm. In other words,
when the semiconductor laser is operated at the power
of 5 mW, according to this invention, it is possible to
reduce the noise by about 5 dBm as compared with the
semiconductor laser of gain-guiding type. Furthermore,
in the semiconductor laser of the present invention, as
will be clear from the curve 23, there is produced no
mode hopping noise which is seen just in the curve 21.
More particularly, according to the structure of the
present invention, it may be considered that self-excited
oscillation is produced by the light and carrier confine-
ment which is intermediate between those of the prior
art refractive index-guiding type and gain-guiding type
ones. In other words it may be considered that the
semiconductor laser of the present invention is operated
in a range in which the afore-mentioned effective
refractive index difference ~n is extremely small positive
or negative value as compared with the semiconductor
lasers of refractive index-guiding type and gain-guiding
type.
Further, according to the semiconductor laser of
the present invention, the symmetric property of the far
field pattern is improved as compared with the prior art
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semiconductor laser of gain-guiding type and the diameter
of beam spot thereof is made small, thus the threshold
current being reduced substantially. It can be understood
that based upon these merits the semiconductor laser
of this invention is operated as the intermediate semi-
conductor laser between the refractive index-guiding
type and the gain-guiding type ones.
The afore-described removal of mode hopping noise,
low noise, reduction of threshold current and improve-
ment of far field pattern can be achieved by particulariz-
ing three of the width W of the removed-away portion 35a
of the light absorbing layer 35, the distance d2 between
the active layer 33 and the light absorbing layer 35
and the thickness d1 of the active layer 33 simultaneously.
More specifically, the width W is selected in a range
from 1 to 4 ~m, preferably 2 to 4 ~m, the thickness d
O
is selected so as to satisfy the condition of dl > 500A
o o
(preferably 1500A > dl > 700A) and the distance d2 is
selected in a range from 0.2 to 0.7 ~m (preferably in a
range from 0.3 to 0.5 ~m).
These width W, the thickness d1 and the distance
d2 will respectively be described below.
Now, the width W of the removed-away portion 35a
of the light absorbing layer 35 will be considered.
Since the current concentration is weakened as the width
W is increased as earlier noted, the refractive index
change ¦~ne¦ by the injection of the carrier becomes
small. If the width W is increased more, the change
~n of the built-in refractive index becomes dominant so
that the semiconductor of this invention is opera~ed as
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the refractive index-guiding type one. In this case, the
width W is one of the ~actors which determines the
threshold current Ith for the oscillation of the semi-
conductor laser. Since the threshold curren~ Ith is given
as the product by a threshold current density Jth and
the area of the light emission, when the width W is
increased, the width of the light emission region, or
its area is increased, whereby the threshold current Ith
is increased. On the other hand, since the relation
between the threshold current density and the width W
i5 shown in Fig. 4, when the width W becomes large, the
threshold current density Jth becomes small. Therefore,
the threshold current Ith can be suppressed to be small
if the width W is in a certain range. Accordingly, in
order to reduce the threshold current Ith, the selection
of the width W becomes an important factor.
~ he thickness dl of the active layer 33 will be
described. If the thickness dl thereof is increased,
the area of the light emission area is increased and
this leads to the increase of the threshold current Ith
as mentioned before. Accordingly, in view of reducing
the threshold current Ith, it is preferable that the
thickness dl is not so large. Now, the permeation of
light from the active layer 33 to the first and second
cladding layers 32 and 34 will be considered. As shown
in Fig. 5, when the thickness dl is small, the permeation
of light presents a steep distribution as shown by a
curve 51 in Fig. 5, while when the thickness dl is large,
it presents a gentle distribution as shown by a curve 52
in Fig. 5. Accordingly, as the thickness dl is increased,
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the light absorbing effect by the light absorbing layer
35 is decreased so that the semiconductor laser of this
invention can be operated as the gain-guiding type one,
or it becomes possible to avoid the mode hopping noise
from being produced.
Further, also the distance d2 between the active
layer 33 and the light absorbing layer 35 is one of the
factors which determines the effect of the light absorb-
ing layer 35 for absorbing the light from the active
layer 33. If the distance d2 is small, the light
absorbing e~fect is large so that the refractive index-
guiding type characteristic becomes dominant in the
semiconductor laser. If on the other hand the distance
d2 is increased, the light absorbing effect is decreased
or lost so that the semiconductor laser of this invention
becomes the gain-guiding type one.
As describéd above, by the fact that the width W,
the thickness dl and the distance d2 are particularized
at the same time, the occurrence of the mode hopping
noise can be avoided, the noise level can be reduced as
compared with the semiconductor laser of the gain-guiding
type, the threshold current Ith is low as compared with
the threshold current Ithg of the gain-guiding type one
and the far field pattern can be improved.
As set forth above, according to the semiconductor
laser of the present invention, the mode hopping noise
can be prevented from being produced and as compared
with the semiconductor laser of the gain-guiding type,
the noise and the threshold current can be reduced
and the far field pattern can be improved. Accordingly,
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when the semiconductor laser of this invention is used
as a writing and/or reading light source of, for example,
the video disc and the digital audio disc, there are
brought a great deal of advantages that the resolution
S and S/N (signal-to-noise) ratio can be improved and
that the optical system can be simplified and so on.
The above description is given on a single prererred
embodiment of the invention, but it will be apparent that
many modifications and variations could be effected by
one skilled in the art without departing from the spirits
or scope of the novel concepts of the invention, so that
the scope of the invention should be d~termined by the
appended claim only.
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