Semiconductor laser array apparatus

A semiconductor laser array apparatus comprising a single substrate and a plurality of semiconductor laser devices that are disposed with a given pitch on the substrate, some of the semiconductor laser devices constituting laser devices of a laser oscillation-operating area of the semiconductor laser array apparatus and the other semiconductor laser devices constituting laser devices of the non-laser oscillation-operating areas that are positioned outside of the laser oscillation-operating area, wherein the oscillation threshold current level of at least one of the semiconductor laser devices of each of the non-laser oscillation-operating areas that is the closest to the laser oscillation-operating area is higher than those of the semiconductor laser devices of the laser oscillation-operating area.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
This invention relates to a semiconductor laser array apparatus in which 
the oscillation threshold current level of the semiconductor laser devices 
of the non-laser oscillation-operating areas is set to be so high that 
laser oscillation of the semiconductor laser devices of the non-laser 
oscillation-operating areas can be suppressed and laser oscillation in the 
laser oscillation-operating area alone can be achieved, resulting in a 
stabilized near-field pattern. 
2. Description of the prior art 
In recent years, high-output power semi-conductor laser array apparatuses 
in which a plurality of semiconductor laser devices are disposed with a 
certain gap therebetween on a single substrate have been developed. These 
semiconductor laser array apparatuses are produced by liquid phase epitaxy 
by the use of substrates with grooves so that light beams and current can 
be regulated. 
FIG. 4 shows a conventional synchronous phase-type semiconductor laser 
array apparatus, which is produced as follows: On a p-GaAs substrate 1, an 
n-Ga.sub.0.9 Al.sub.0.1 As current blocking layer 2, and an n-GaAs current 
blocking layer 3 are successively formed by liquid phase epitaxy. Then, a 
plurality of grooves 4 are formed in a parallel manner, corresponding to 
the laser oscillation-operating area 5, in such a way that they reach the 
GaAs substrate 1 through the current blocking layers 2 and 3. Then, on the 
n-GaAs current blocking layer 3, a p-Ga.sub.1-x Al.sub.x As cladding layer 
6, a p-(or n-)Ga.sub.1-y Al.sub.y As active layer 7, an n-Ga.sub.1-x 
Al.sub.x As cladding layer 8, and an n-GaAs cap layer 9 are successively 
formed by liquid phase epitaxy. A p-side electrode 11 and n-side electrode 
12 are formed on the bottom face of the GaAs substrate 1 and the top 
surface of the n-GaAs cap layer 9, respectively. Current is injected into 
the active layer 7 through the grooves 4, resulting in optical waveguides 
within the active layer 7 that correspond to the grooves 4. The area 
including each of the grooves 4 constitutes an individual semiconductor 
laser device, and light beams that are oscillated by these laser devices 
are coupled therebetween, resulting in high-output power laser beams. 
However, in the laser oscillation-operating area 5 in which the grooves 4 
are disposed, the top surface of the p-Ga.sub.1-x Al.sub.x As cladding 
layer 6 is bent, resulting in a bend of the active layer 7 formed thereon. 
Thus, the semiconductor laser devices become unsymmetrical, which makes 
weak the optical coupling between the adjacent semiconductor laser devices 
so that the semiconductor laser devices individually oscillate laser 
beams. That is, it is difficult for all of the semiconductor laser devices 
to attain uniform oscillation in the state that they are optically coupled 
therebetween. 
To overcome this problem, semiconductor laser array apparatus shown in 
FIGS. 5 and 6 have been proposed. The semiconductor laser array apparatus 
of FIG. 5 has a plurality of grooves 4 with a certain pitch on the entire 
surface of a p-GaAs substrate 1 so that a uniform active layer 7 can be 
obtained. Moreover, this apparatus has an n-GaAs current blocking layer 26 
between the p-GaAs substrate 1 and the n-Ga.sub.0.9 Al.sub.0.1 As current 
blocking layer 2 at the outside of the laser oscillation-operating area 5 
so that current cannot be injected into the portions of the active layer 7 
at the outside of the laser oscillation-operating area 5. The 
semiconductor laser array apparatus of FIG. 6 also has a plurality of 
grooves 4 with a certain pitch on the entire surface of the p-GaAs 
substrate 1. This apparatus has a proton-injected region 10 in both the 
n-GaAs cap layer 9 and the n-Ga.sub.1-x Al.sub.x As cladding layer 8 at 
the outside of the laser oscillation-operating area 5 so that the 
injection of current into the active layer 7 can be prevented. 
However, these apparatus shown in FIGS. 5 and 6 have the grooves 4 in the 
non-laser oscillation-operating areas outside of the laser 
oscillation-operating area 5, and accordingly there is a possibility that 
optical coupling can be attained between the adjacent semiconductor laser 
devices in the non-laser oscillation-operating areas. Thus, when current 
injected into the active layer 7, leaks in the lateral direction, the 
semiconductor laser devices of the non-laser oscillation-operating areas 
attain laser oscillation and/or attain optical coupling therebetween, 
resulting in a bad influence on the oscillation mode of the laser 
oscillation-operating area 5. 
SUMMARY OF THE INVENTION 
The semiconductor laser array apparatus of this invention, which overcomes 
the above-discussed and numerous other disadvantages and deficiencies of 
the prior art, comprises a single substrate and a plurality of 
semiconductor laser devices that are disposed with a given pitch on said 
substrate, some of the semiconductor laser devices constituting laser 
devices of a laser oscillation-operating area of said semi-conductor laser 
array apparatus and the other semi-conductor laser devices constituting 
laser devices of the non-laser oscillation-operating areas that are 
positioned outside of said laser oscillation-operating area, wherein the 
oscillation threshold current level of at least one of the semiconductor 
laser devices of each of said non-laser oscillation-operating areas that 
is the closest to said laser oscillation-operating area is higher than 
those of the semiconductor laser devices of said laser 
oscillation-operating area. 
In a preferred embodiment, at least one of the semiconductor laser devices 
of each of said non-laser oscillation-operating areas that is the closest 
to said laser oscillation-operating area contains single or plural grooves 
that are formed in a discontinuous manner in the direction from one facet 
to the other. 
In a preferred embodiment, at least one of the semiconductor laser devices 
of each of said one-laser oscillation-operating areas that is the closest 
to said laser oscillation-operating area contains single or plural grooves 
that are constricted in given portions. 
In a preferred embodiment, the reflectivity of the portions of the facets 
that correspond to said non-laser oscillation-operating areas is lower 
than that of the portion of one facet that corresponds to said laser 
oscillation-operating area. The portions of the facets with a lower 
reflectivity are coated by a single layer, whereas said portion of the 
facet with a higher reflectivity is coated by a multi-layer. 
Thus, the invention described therein makes possible the objective of 
providing a semiconductor laser array apparatus in which even when current 
leaks into the non-laser oscillation-operating areas, laser oscillation of 
the semiconductor laser devices in the non-laser oscillation-operating 
areas is suppressed and laser oscillation in the laser 
oscillation-operating area alone is attained, resulting in a stabilized 
synchronous-oscillation mode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention provides a semiconductor laser array apparatus in which the 
oscillation threshold current level of the semiconductor laser devices in 
the non-laser oscillation-operating areas is set to be higher than that of 
the semiconductor laser devices in the laser oscillation-operating area, 
and accordingly even when current leaks in the lateral direction, the 
laser devices of the non-laser oscillation-operating areas oscillate no 
laser, but only the laser devices of the laser oscillation-operating area 
oscillate laser, so that a stabilized synchronous-oscillation mode can be 
attained. 
EXAMPLE 1 
FIG. 1A shows a semiconductor laser array apparatus of this invention, 
which is produced as follows: On a p-GaAs substrate 1, an n-Ga.sub.0.9 
Al.sub.0.1 As current blocking layer (a first current blocking layer) 2 
with a thickness of 0.7 .mu.m and an n-GaAs current blocking layer (a 
second current blocking layer) 3 with a thickness of 0.1 .mu.m are 
successively formed by liquid phase epitaxy. Then, a plurality of grooves 
4 and 4a with a width of 4 .mu.m each, a depth of 1.0 .mu.m each, and a 
pitch of 5 .mu.m each are formed on the entire surface of the second 
current blocking layer 3 by photolithography and an etching technique in 
such a way that they reach the p-GaAs substrate 1 through both the n-GaAs 
current blocking layer 3 and the n-Ga.sub.0.9 Al.sub.0.1 As current 
blocking layer 2. As shown in FIG. 1B, the grooves 4 that are positioned 
corresponding to the laser oscillation-operating area 5 are formed into a 
striped shape from one facet to the other, whereas the grooves 4a that are 
positioned in the non-laser oscillation-operating areas at the outside of 
the laser oscillation-operating area 5 are formed in a discontinuous 
manner in the direction from one facet to the other. At least the closest 
groove 4a to the laser oscillation-operating area 5 is formed in such a 
discontinuous manner. In this example, two grooves 4a that are outside of 
the laser oscillation-operating area 5 are formed in a discontinuous 
manner in the direction from one facet to the other. The length l.sub.1 of 
each piece of the groove 4a is, for example, 20 .mu.m and the distance 
l.sub.2 between the adjacent pieces of the groove 4a is, for example, 10 
.mu.m. 
Then, as shown in FIG. 1A, on the second current blocking layer 3 including 
the grooves 4 and 4a, a p-Ga.sub.1-x Al.sub.x As cladding layer 6 having a 
thickness of 0.2 .mu.m in the area other than the grooves 4 and 4a, a 
p-(or n-)Ga.sub.1-y Al.sub.y As active layer 7 with a thickness of 0.08 
.mu.m, an n-Ga.sub.1-x Al.sub.x As cladding layer 8 with a thickness of 
1.0 .mu.m, and an n-GaAs cap layer 9 with a thickness of 1.5 .mu.m are 
successively formed. Then, protons are injected into both the GaAs cap 
layer 9 and the p-Ga.sub.1-x Al.sub.x As cladding layer 8 at the outside 
of the laser oscillation-operating area 5 by an ion-injecting technique in 
such a manner that the protons reach the cladding layer 8 through the cap 
layer 9, resulting in a high resistance layer 10. Then, a p-side electrode 
11 and an n-side electrode 12 are formed on the bottom face of the p-GaAs 
substrate 1 and the top surface of the n-GaAs cap layer 9, respectively, 
resulting in a semiconductor laser array apparatus. 
Even when current injected into this laser array apparatus leaks into the 
areas other than the laser oscillation-operating area 5, because of the 
discontinuous grooves 4a positioned outside of the laser 
oscillation-operating area 5, the amount of light that is to be absorbed 
toward the substrate 1 at the boundaries among the pieces of the grooves 
4a increases, resulting in an increase in optical internal losses, which 
causes difficulties in laser oscillation of the semiconductor laser 
devices containing the said grooves 4a. As a result, laser oscillation is 
attained only in the laser oscillation-operating area 5, and optical 
coupling is not achieved between the waveguide of the laser 
oscillation-operating area and the adjacent waveguide of the non-laser 
oscillation-operating area outside of the said laser oscillation-operating 
area 5. Thus, a stabilized synchronous oscillation can be attained. 
EXAMPLE 2 
FIG. 2A shows another embodiment laser array apparatus of this invention, 
which is produced as follows: In the same way as described in Example 1, 
on a p-GaAs substrate 1, an n-Ga.sub.0.9 Al.sub.0.1 As current blocking 
layer (a first current blocking layer) 2 and an n-GaAs current blocking 
layer (a second current blocking layer) 3 are successively formed by 
liquid phase epitaxy. Then, a plurality of grooves 24 and 24a are formed 
on the second current blocking layer 3 in such a way that they reach the 
substrate 1 through the current blocking layers 3 and 2. As shown in FIGS. 
2B, the grooves 24 that are positioned corresponding to the laser 
oscillation-operating area 5 are formed into a striped shape from one 
facet to the other, whereas the grooves 24a that are positioned in the 
non-laser oscillation-operating areas outside of the laser 
oscillation-operating area 5 are formed so that they are constricted in 
given portions. At least the closest groove 24a to the laser 
oscillation-operating area 5 is formed in a constricted manner. In this 
example, two grooves 24a that are outside of the laser 
oscillation-operating area 5 are formed in a constricted manner; for 
example, the width of the constricted portions of each groove 24a is 2 
.mu.m and the width of the other portions thereof is 4 .mu.m. The depth of 
the constricted portions thereof is the same as or shallower than that of 
the other portions thereof. These grooves are not disposed over the entire 
surface of the substrate 1, but the grooves 24 and 24a that are positioned 
outside of the laser oscillation-operating area 5 are disposed over the 
area with the same width as that of the laser oscillation-operating area 
5. Accordingly, there are no grooves 24 and 24a in the areas in the 
vicinity of both sides of the substrate 1. The portions of the active 
layer 7 that are formed above the grooves 24 in the outer sides are bent, 
but this phenomenon has no influence on the formation of the portion of 
the active layer of the laser oscillation-operating area 5. Therefore, the 
portion of the active layer 7 that corresponds to the laser 
oscillation-operating area 5 can by uniformly formed. 
Thereafter, as shown in FIG. 2A, on the Ga.sub.1-y Al.sub.y As active layer 
7, an n-Ga.sub.1-x Al.sub.x As cladding layer 8 and an n-GaAs cap layer 9 
are successively formed in the same way as described in Example 1. Then, 
the portions of the n-GaAs cap layer 9 and the n-Ga.sub.1-x Al.sub.x As 
cladding layer 8 that correspond to the non-laser oscillation-operating 
areas are removed by a chemical etching technique in a manner to reach the 
cladding layer 8 through the cap layer 9. Then, a Si.sub.3 N.sub.4 film 25 
is formed on the exposed surfaces of the cap layer 9 and the cladding 
layer 8, after which a p-side electrode 11 and an n-side electrode 12 are 
formed on the bottom face of the p-GaAs substrate 1 and the top surface of 
the n-GaAs cap layer 9, respectively, resulting in a semiconductor laser 
array apparatus. 
Even when current injected into this laser array apparatus leaks into the 
non-laser oscillation-operating areas, because of the constricted portions 
of the grooves 24a positioned outside of the laser oscillation-operating 
area 5, the amount of light that is to be absorbed at the shoulder 
portions of the constricted grooves 24a increases and optical internal 
losses are increased, thereby attaining the same effect as that of Example 
1. 
The method for confining current within the laser oscillation-operating 
area 5 of the active layer 7 is not limited to that of the above-mentioned 
example, but the inner-current confining method, the oxidation film stripe 
method, planer stripe method, and so on can be employed, which also attain 
the same effect as the above-mentioned. 
EXAMPLE 3 
FIG. 3A shows another semiconductor laser array apparatus of this 
invention, which is produced in the same way as that of Example 1, except 
for the following: The grooves 34 by which optical waveguides are formed 
within the active layer 7 have a symmetrically branching structure, as 
shown in FIG. 3B, which is composed of parallel stripes with a width of 4 
.mu.m each and a pitch of 5 .mu.m that are positioned from both facets to 
the middle of the wafer and branching portions that attain optical 
coupling in the middle of the wafer between the corresponding parallel 
stripes. Moreover, in the areas at the outside of the laser 
oscillation-operating area 5, there is an n-GaAs current blocking layer 26 
between the p-GaAs substrate 1 and the n-Ga.sub.0.9 Al.sub.0.1 As current 
blocking layer 2, which prevents current from being injected into the 
portions of the active layer 7 that are positioned above the said current 
blocking layers 26, resulting in non-laser oscillation-operating areas 
that are positioned outside of the laser oscillation-operating area 5. 
Moreover, Al.sub.2 O.sub.3 films 35 with an optical thickness of 
.lambda./4 (.lambda. is a light wavelength) are formed on both facets by 
the ordinary electron beam method. Then, on the portion of the Al.sub.2 
O.sub.3 film 35 one facet that corresponds to the laser 
oscillation-operating area 5, an amorphous silicon film 36 with an optical 
thickness of .lambda./4, an Al.sub.2 O.sub.3 film 35 with an optical 
thickness of .lambda./4, and an amorphous silicon film 36 with an optical 
thickness of .lambda./4 are successively formed by the lift-off method 
using resist films therein, followed by forming an Al.sub.2 O.sub.3 film 
37 with an optical thickness of .lambda./2 on the amorphous silicon film 
36, resulting in a semiconductor laser array apparatus with a reflectivity 
of 2-4% in one single-layered facet, a reflectivity of 95% in the 
multi-layered portion of the other facet that corresponds to the laser 
oscillation-operating area 5, and a reflectivity of 2-4% in the other 
portions of the other facet that correspond to the non-laser 
oscillation-operating areas. 
Because of the above-mentioned structure, mirror losses of the optical 
waveguides in the non-laser oscillation-operating areas become large and 
accordingly even when current leaks into the said areas, laser oscillation 
never arises, resulting in the same effect as that of Example 1. 
In the above-mentioned examples, the semiconductor laser devices of the 
non-laser oscillation-operating areas outside of the oscillation-operating 
area 5 are designed so that the grooves are discontinued or constricted in 
given portions or the reflectivity of the facets corresponding thereto are 
lowered, which makes the oscillation threshold current level high. 
Accordingly, even when current leaks into the non-laser 
oscillation-operating areas, laser oscillation is attained by only the 
semiconductor laser devices of the laser oscillation-operating area 5. 
However, the oscillation threshold current level of the semiconductor 
laser devices that are positioned outside of the laser 
oscillation-operating area 5 can be, of course, made high by other 
methods. 
Although the above-mentioned examples only disclose GaAs-GaAlAs 
semiconductor laser array apparatuses, this invention is applicable to 
semiconductor laser array apparatuses of the InGaAsP-InP systems or the 
like. Moreover, this invention is applicable to semiconductor laser array 
apparatuses in which the oxidation film stripe method, the planer stripe 
method using a diffusion technique, or the like is used as a 
current-confining method. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.