High power semiconductor laser device and method for fabricating the same

The present invention relates to high power semiconductor laser device and method for fabricating the same utilizing ion implanting process, by which a beam steering phenomenon of an optical output due to filaments is eliminated. This elimination is achieved by a periodically varying gain given for a resonator of the semiconductor laser device. That is, this invention changes a gain distribution which causes the generation of filaments in the resonator into different distribution. According to the present invention, there is formed an insulation layer through ion implantation to an active layer to adjust current density implanted to the active layer, thereby eliminating non-uniform distribution of the light along the longitudinal direction of the resonator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to high power semiconductor laser device used 
for a light source of an Erbium Doped Fiber Amplifier (EDFA) and, more 
particularly to high power semiconductor laser device and method for 
fabricating the same utilizing ion implanting process. 
2. Description of the Conventional Art 
A semiconductor laser at a 0.98 micro-meter wavelength is used as a light 
source for the Erbium Doped Fiber Amplifier EDFA for amplifying an optical 
signal passing through an optical fiber. As an optical output from the 
above-mentioned semiconductor laser is increased, a light amplifying rate 
of EDFA becomes higher accordingly. To this end, it is highly demanded to 
fabricate the 0.98 micro-meter semiconductor laser, which is capable of 
providing high power output. Especially, it should be apparent that the 
semiconductor laser for use in the EDFA is required to further improve its 
optical output, and an optical coupling efficiency between the 
semiconductor laser and an optical fiber coupled thereto has to be 
improved for use in module. There has been greatly improved the powerful 
and reliable 0.98 micro-meter semiconductor laser and the high optical 
coupling efficiency between the high power semiconductor laser and the 
optical fiber. 
A desired 0.98 micro-meter semiconductor laser module may be made through 
the coupling of high power semiconductor laser to the optical fiber. 
However, patterns (emission patterns) of optical output emitted from the 
semiconductor laser constituting the module are varied depending upon 
varying operation conditions for the semiconductor laser. This causes an 
amount of light introduced into the optical fiber to be instantly 
attenuated, resulting in the deteriorated performance of the semiconductor 
laser module. 
FIGS. 1 to 3 illustrate steps in the fabrication of a conventional 0.98 
micro-meter ridge waveguide semiconductor laser device. 
FIG. 1 schematically shows a sectional view of the structure consisting, in 
sequence, of a compound semiconductor substrate 1, a GaInAsP graded layer 
2 for further assisting electric current flow caused by band gap 
difference between GaAs and GaInP layers on the substrate 1, a GaInP clad 
layer 3, a GaInAsP graded layer 4, a GaInAs/GaInAsP active layer 5, a 
GaInAsP graded layer 6, a GaInP clad layer 7, a GaInAsP graded layer 8, 
and a GaAs layer 9 for an ohmic contact. These layers have been grown by 
Metal Organic Vapor Phase Epitaxy (MOVPE). 
Then, as shown in FIG. 2, an insulation layer 10 of SiO.sub.2 or Si.sub.3 
N.sub.4 is formed over the resultant specimen of FIG. 2, followed by a 
well-known photolithography process. This process leaves an insulation 
layer 10 with the active layer having a constant width of 2 to 3 
micro-meters. Sequentially, wet etching or dry chemical etching process is 
further performed until a top surface of the GaInAsP graded layer 6 is 
exposed, so that ridges are made as shown in FIG. 2. In FIG. 2, the 
insulation layer 10 is removed through an appropriate etching process, 
followed by the formation of another insulation layer 10 of Si.sub.3 
N.sub.4 or SiO.sub.2 over the whole surface of the resulting structure. 
Then, electric current injection window is made to the ridge so as to 
inject electric current through the ridges. The injection is made through 
the deposited, plated p-side electrode 11 having a thickness of 2 to 3 
micro-meters and which can withstand high current. After thinning the 
substrate until the thickness of an order of 100 micro-meters is obtained, 
an n-side electrode 12 is formed, by which the conventional 0.98 
micro-meter RWG semiconductor layer is completed. 
In case electric current is injected into the conventional semiconductor 
laser fabricated through steps as described above, current injection is 
made through ridges within a cavity having width of 2 to 3 micro-meter and 
length of 800 to 1000 micro-meter. Therefore, only the active layer 5 
under the ridge can provide an optical gain, by which light is emitted. 
There are difference between the refractive indices of respective layers 
arranged in a vertical direction relative to the active layer 5, wherein 
the indices get smaller in order of the active layer 5, graded layers 6, 4 
and clad layers 7, 3. Further, the difference between the effective 
refractive index of area under the ridge and index of other area except 
said area exists, as seen in horizontal direction relative to the active 
layer 5. With these differences, the light emitted by the above-mentioned 
gain can be collected around the active layer. 
The active layer employed in the RWG semiconductor layer is configured 
depending upon both gain-guided and index-guided waveguiding properties. 
When injecting electric current, the active layer of a rectangular stripe 
shape having been formed through etching process is slightly enlarged in 
the horizontal direction by current spreading. A length of the active 
layer corresponds to that of the cavity for the RWG semiconductor laser, 
while the rectangular width is slightly larger than that of the ridge. In 
order to provide high output power from the 0.98 micro-meter semiconductor 
laser, width of the ridge is given as large as possible within a range 
wherein a sectional area (rectangle) of the active layer is sized to 
maintain a single lateral mode. The effects of significant lateral spatial 
hole burning combined with waveguides inherently sensitive to 
perturbations by injected carriers are manifested in the widespread 
observation that as they are driven to higher currents, most high power 
lasers eventually lase on multiple lateral modes. This broken condition 
causes filament having width of 1 micro-meter and length of 100 to 150 
micro-meter to be generated at random. The generation of such filaments is 
due to the attenuation of a fundamental lateral mode caused by the spatial 
hole burning phenomenon, and is further due to oscillation of higher-order 
lateral mode which has higher gain at a side along the cavity axis of the 
active layer. Lasing mode field pattern of the optical output from the 
semiconductor laser becomes varied accordingly. 
The central axle of emission pattern in the fundamental lateral mode is 
consistent with that of the cavity. However, in case the filaments occur 
at side along the cavity axis of the active layer, a beam steering 
phenomenon is induced which the central axle of emission pattern is 
deviated from the axle of the cavity to a side opposed to the generated 
filament. The 0.98 micro-meter semiconductor laser module to which the 
optical fiber optically aligned under the fundamental lateral mode 
operation is attached intends to induce varying amount of the light to be 
coupled to the optical fiber, because of the beam steering phenomenon due 
to the generated filament. This makes maximum optical output varied and 
deteriorates the stability of the optical output, resulting in the 
reduction of performance of the semiconductor laser module. 
In other words, injection of high current to the conventional semiconductor 
laser as mentioned above causes the fundamental lateral mode to be 
attenuated due to the spatial hole burning. On the other hand, under the 
above condition, higher-order lateral mode obtains high gain at a side 
along the cavity axis of the active layer. For these reasons, filaments of 
1 micro-meter in width are generated at random. Such filaments causes 
emission pattern of an optical output at the output surface to be varied. 
Although width of the active layer may be narrowed to eliminate such a 
phenomenon, the narrowed active layer provides the low optical power and 
readily induces damage to the optical output surface. 
SUMMARY OF THE INVENTION 
In relation to those problems as mentioned above, the present invention is 
to eliminate the beam steering phenomenon due to filaments generated when 
the conventional high power laser is operated under high injected current. 
For this object, the present invention performs selectively ion 
implantation to a clad layer over an active layer to adjust current 
density to be implanted in a longitudinal direction of the active layer 
and to thereby eliminate non-uniform distribution of the light along the 
longitudinal direction of the cavity. 
To achieve the above objects, there is provided high power semiconductor 
laser comprising: a compound semiconductor substrate; a first graded 
layer, a first clad layer, a second graded layer, an active layer, a third 
graded layer and a second clad layer sequentially formed on said substrate 
by using a first crystal growth; ion-implanted regions formed in said 
second clad layer, wherein the ion-implanted regions are electrically 
isolated through high temperature annealing so as to allow gain along a 
longitudinal of the active layer to be modulated with the same period as 
the ion-implanted period; a third clad layer, a fourth graded layer and 
ohmic contact layer sequentially formed on the second clad layer having 
the ion-implanted regions by using a second crystal growth, wherein these 
layers are etched until the third graded layer is exposed in order to form 
a ridge waveguide structure; an insulation layer having current injection 
window in some portion of top area of the ridge; a first conductivity 
electrode formed over the entire surface of the insulation layer; and a 
second conductivity electrode formed on the rear surface of the substrate. 
To achieve the another objects of the present invention, there is also 
provided a method fabricating high power laser, the method comprising the 
steps of: a first crystal growing for forming a first graded layer, a 
first clad layer, a second graded layer, an active layer, a third graded 
layer and a second clad layer on a compound semiconductor substrate; 
forming a first insulation pattern on the second clad layer to define 
ion-implanted regions by photolithography; implanting ions by using the 
first insulation pattern as a mask, and annealing, so that said 
ion-implanted regions allow gain along a longitudinal direction of the 
active layer to be modulated with same period as ion-implanted period; a 
second crystal growing for sequentially forming a third clad layer, a 
fourth graded layer and an ohmic contact layer on the second clad layer 
having ion-implanted regions; forming a second insulation pattern having a 
width a little wider than that of the ion-implanted regions; sequentially 
etching using the second insulation pattern as a mask until the third 
graded layer is exposed, to form a ridge waveguide structure; forming an 
insulation layer having a current injection window in some portion of top 
area of the ridge; forming a first conductivity electrode formed over the 
entire surface of the insulation layer; and forming a second conductivity 
electrode formed on the rear surface of the substrate. 
Preferably, depth of said ion-implanted regions is defined to reach the top 
of the active layer so as to prevent any increasing loss of the active 
layer, and has a 10 micro-meters long length along the resonator's length 
and width a little wider than that of the active layer, such shaped 
regions being sequentially arranged at an interval of 5 to 10 
micro-meters. 
More preferably, before said annealing, further comprising the steps of: 
removing the first insulation pattern; and forming a silicon nitride film 
on both top and rear surface of the resultant structure, to protect the 
surface degradation during annealing process.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred embodiment according to the present invention will now be 
described in detail in accordance with the accompanying drawings. 
This invention provides a new 0.98 micro-meter semiconductor laser device 
by which the beam steering phenomenon induced by filaments is eliminated. 
The object of the present invention can be accomplished by a periodic gain 
adjustment relating to a cavity of the semiconductor laser. Such 
elimination is possible in that a continuous gain distribution needed to 
generate filaments in the resonator is changed in its shape in accordance 
with the present invention. 
Implementation of gain adjustment for the 0.98 micro-meter semiconductor 
laser can be achieved through a preferred embodiment of the present 
invention given below. The preferred embodiment according to the present 
invention will be explained in detail with reference to FIGS. 4 to 8. 
FIG. 4 shows a sectional view of the structure consisting of a compound 
semiconductor substrate 1, a GaInAsP graded layer 2 for further assisting 
electric current flow caused by band gap difference between GaAs and GaInP 
layer on the substrate 1, a GaInP clad layer 3, a GaInAsP graded layer 4, 
a GaInAs/GaInAsP active layer 5, a GaInP graded layer 6, and 0.5 
micro-meter thick GaInP clad layer 13, these layers being obtained through 
the epitaxial growth process using Metal Organic Vapor Phase Epitaxy 
(MOVPE) as first crystal growth process. 
Then, as shown in FIG. 5, an insulation layer 14 of SiO.sub.2 or Si.sub.3 
N.sub.4 is formed over the resultant specimen of FIG. 4, followed by a 
well-known photolithography process for patterning. This patterning 
process is applied to the insulation layer 14, and then leaves the 
patterned insulation layers 14 which are isolated by 5 to 20 micrometer, 
respectively, each pattern having length of the order of 10 micro-meters 
in the longitudinal direction of the cavity and width a little wider than 
that of the active layer. Side sectional view of the resultant structure 
is shown in FIG. 5. Using the patterned insulation layers 14 and 
photoresist film 16 as a mask, ions of B+, Ar+, Si+, or He+ are implanted 
towards the structure, followed by the removal of the used mask. 
Next, in order to electrically isolate ion-implanted regions 15, an 
insulation layer of Si.sub.3 N.sub.4 is formed on both surface and rear of 
the structure, and then annealing is performed at the temperature of 600 
to 950.degree. C. for several seconds. It is noted that the ion-implanted 
regions may be adjusted at a constant ratio, so that the depth of 
implantation is defined to reach the top of the GaInAsP graded layer. This 
is because it is likely to induce an increase any loss of the active 
layer, in case the ion implantation is made till the active layer. 
After the removal of the insulation layer used to protect the surface 
degradation during annealing process, the second crystal growing is 
performed. By the second crystal growing, as shown in FIG. 6, a GaInP clad 
layer 17, a GaInAsP graded layer 18, and a GaAs layer 19 for ohmic contact 
are sequentially grown. 
Referring to FIG. 7, the insulation layer 20 is patterned through the 
photolithographic process, with 2 to 3 micro-meters width of the actively 
layer maintained. The patterned insulating layer 20 is used as a mask for 
subsequent wet or dry etching process. The etching process is conducted 
until the GaInAsP graded layer 6 is revealed. Thus, the demanded ridge is 
obtained from the above-described processes. 
The following sequential processes are further performed so as to be inject 
electric current into the ridges thus constructed above. To this end, the 
insulation layer 20 as shown in FIG. 7 is removed, and then another 
insulation layer 21 of Si.sub.3 N.sub.4 or SiO.sub.2 is formed over the 
whole surface of the structure. A top area of the ridge having the 
insulation layer provided thereon has electric current injection window 
provided through an appropriate window forming process. Subsequently, a 
p-side electrode 22 is deposited, and then is subjected to plating process 
for obtaining 2 to 3 micro-meter thick electrode 22, the thickness of 
which ensures that the structure withstands injected high current. 
In order to obtain an 100 micro-meters thick substrate, thinning process is 
applied to the structure, followed by the formation of an n-side electrode 
23. Thus, as shown in FIG. 8, a 0.98 micro-meters ridge waveguide 
semiconductor laser in accordance with the present invention is completed. 
In comparison to the conventional semiconductor laser device, a feature of 
the semiconductor laser device of the present invention is that gain 
modulation along a longitudinal direction of the active layer is made with 
the same period as ion-implantation period. This is because gain of the 
active layer is determined by an amount of injected electric charges. Gain 
distribution along the longitudinal direction of the active layer causes 
modulation of distribution of light inside the cavity, so that any 
generation of filaments, which are otherwise presented more than 100 
micro-meters in its length along the resonator, is essentially eliminated. 
Gain period, i.e., the ion-implanted area and non-implanted area along the 
longitudinal direction of the resonator is defined by the following 
requirements. 
Maximum value of the modulation period should be smaller than minimum 
length of filament generating in a longitudinal direction of the cavity, 
and, on the other hand, minimum value of the modulation period should be 
larger than diffusion length of electric charges which are injected 
through the non-implanted area, in case of gain modulation by the 
ion-implanted area. 
During the forming of an anti-reflection coating film on a front output 
surface and high-reflection coating film on a rear output surface, these 
processes being required in fabricating a general semiconductor laser 
device, light inside the resonator shows a smooth distribution, rather 
than uniform distribution. 
A preferred example to which the present invention is applied under said 
circumstances is as follows. Intensity of the modulation and period 
adjustment may be accomplished by controlling in proportional to light 
distribution an interval between the ion-implanted areas. For the purpose 
of eliminating non-uniform distribution of the light in a longitudinal 
direction of the resonator, electric current density injected into the 
active layer is adjusted. The preferred embodiment of the present 
invention can be applied to either some or general length of the 
resonator, regardless of gain or lateral mode distribution modulation and 
modulation intensity and period of the modulation. In case the preferred 
embodiment is applied the general length of the resonator, the modulation 
forms a standing wave with respect to both front and rear output surfaces. 
Further, it should be apparent that the preferred embodiment can be also 
applied to high power semiconductor laser devices at different 
wavelengths, as well as the 0.98 micro-meters semiconductor laser and 1.48 
micro-meters devices which are used as an excitation light source for 
optical fiber amplifying units. 
An ion implantation process is employed so as to ensure a stable optical 
output through an adjustment of electric current injected into the active 
layer. In accordance with the present invention, an insulation layer is 
formed through the thermal treatment after implanting ions towards some 
portions of the clad layer overlying the active layer. Such ion-implanted 
areas allow gain modulation along a longitudinal direction of the active 
layer to be made with the same period as ion-implantation period. This is 
because gain of the active layer is determined by an amount of injected 
electric charges. Gain distribution along the longitudinal direction of 
the active layer causes modulation of distribution of light inside the 
resonator, so that any filament generation of more than 100 micro-meters 
in its length along the resonator is essentially eliminated. 
As described above, the present invention eliminates beam steering 
phenomenon relating to an optical output due to filaments essentially 
generating from high power semiconductor laser. When one fabricates module 
including semiconductor laser device coupled to the optical fiber, it can 
be expected to have maximum optical output of the module, and to have 
features of a stable optical output regardless of operation conditions for 
the semiconductor laser device. 
While the present invention has been described with respect to certain 
preferred embodiment only, other modifications and variations may be made 
without departing from the scope of the present invention as set forth in 
the following claims.