Semiconductor laser device and a method for driving the same

A semiconductor laser device comprising a laser-oscillating optical waveguide composed of a control region which functions to absorb light and main regions which function to oscillate laser light, said control region being positioned in the center portion of said optical waveguide and said main regions being positioned on both ends of said control region, wherein said laser device further comprises a shunting means by which the ratio of the current Ig flowing to said control region to the total current It injected into said laser device is set to meet the inequality (1): ##EQU1## wherein Lg is the length of said control region and Lt is the length of said optical waveguide.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor laser device achieving an 
incoherent length for use as a light source in video discs, analog optical 
communication, fiber-gyros, etc., and a method for driving the same. 
2. Description of the Prior Art 
In general, semiconductor laser devices which function not only to record 
signals on optical discs but also to read them are of a DRAW (direct read 
after write) type. These semiconductor laser devices are required to 
achieve as high an output power operation as possible so as to make 
recording of signals easy. For this purpose, the front facet for emitting 
light therefrom is coated with a low-reflection index film and the rear 
facet is coated with a high-reflection index film, so that a large amount 
of light can be emitted from the front facet, thereby achieving high 
output power operation. This approach is advantageous in that the 
differential efficiency from the front facet is improved and high output 
power can be produced by a small amount of driving current. However, the 
reflection index of the light-emitting facet of such a laser device is so 
low that light reflected by an optical disc returns to the laser device 
and attains an optical coupling with light produced within the laser 
device, resulting in an external resonator positioned between the front 
facet of the laser device and the optical disc face. The length of the 
external cavity varies to a great extent with movement of the optical disc 
face, which causes reflected light-induced noise, resulting in errors in 
reading the signals, which makes serious problems in practical use. In 
order to prevent the reflected light-induced noise, a method by which the 
coherent length of a semiconductor laser device is shortened enough so 
that the formation of an external resonator between the optical disc face 
and the laser device does not occur is known. According to this method, 
gain guided laser devices or index guided laser devices with a lowered 
refractive index are used so that laser oscillation in a 
multi-longitudinal mode can be obtained or the width of each element of 
the multi-longitudinal mode can be enlarged by self-pulsation to thereby 
shorten the coherent length, causing the instability of the transverse 
mode, which makes high output power operation difficult. 
SUMMARY OF THE INVENTION 
The semiconductor laser device of this invention, which overcomes the 
above-discussed and numerous other disadvantages and deficiencies of the 
prior art, comprises a laser-oscillating optical waveguide composed of a 
control region which functions to absorb light and main regions which 
function to oscillate laser light, said control region being positioned in 
the center portion of said optical waveguide and said main regions being 
positioned on both ends of said control region, wherein said laser device 
further comprises a shunting means by which the ratio of the current Ig 
flowing to said control region to the total current It injected into said 
laser device is set to meet the inequality (1): 
##EQU2## 
wherein Lg is the length of said control region and Lt is the length of 
said optical waveguide. 
In a preferred embodiment, the control region is separated from said main 
regions by shallow grooves located above said optical waveguide, said 
shallow grooves being connected to deep grooves located in parallel to 
said optical waveguide at a certain distance from said optical waveguide 
in a zigzag manner with regard to the main region, the control region, and 
the other main region. 
In a preferred embodiment, the shunting means is an external shunt resistor 
or a built-in shunt resistor. 
The method for driving semiconductor laser devices of this invention, which 
also overcomes the above-discussed and numerous other disadvantages and 
deficiencies of the prior art, comprises controlling the amount of current 
injected into each of said control region and said main regions by a 
shunting means in such a manner that when said laser devices achieve low 
output power operation, said current flowing to said control region is 
maintained at a low level whereby said control region becomes a saturable 
absorption region, and when said laser devices achieve high output power 
operation, said current flowing to said control region is maintained at a 
high level. 
In a preferred embodiment, the control region is separated from said main 
regions by shallow grooves located above said optical waveguide, said 
shallow grooves being connected to deep grooves located in parallel to 
said optical waveguide at a certain distance from said optical waveguide 
in a zigzag manner with regard to the main region, the control region, and 
the other main region. 
In a preferred embodiment, the shunting means is composed of external shunt 
resistors, one of which is selected by a switching means to accommodate 
therethrough the current to said control region. 
Thus, the invention described herein makes possible the objects of (1) 
providing an incoherent semiconductor laser device which can be reproduced 
by the control of current flowing to a saturable absorption region, which 
is formed in a portion of the optical waveguide, by means of a shunting 
means; (2) providing an incoherent semiconductor laser device in which 
noise derived from reflected light does not occur, so than when the laser 
device is used as a light source in video discs, analog optical 
communication, fiber-gyros, etc., stable images and/or signals can be 
obtained; (3) providing a method for driving the semiconductor laser 
device by which the laser device is driven with a stable transverse mode 
even at a high output power operation and with a shortened coherent length 
at a low output power operation, and moreover it is driven without the 
occurence of noise due to reflected light, so that the laser device can be 
useful as an information processing light source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Example 1 
FIGS. 1(A) to 1(C) show a semiconductor laser device of this invention, 
which comprises an optical waveguide 1 with the internal cavity length Lt 
which functions as the laser oscillation-operating area. The optical 
waveguide 1 is composed of a control region (or a saturable absorption 
region) 2 with the length Lg positioned in the center portion and two main 
regions 3 positioned on both ends of the control region 2. The control 
region 2 is separated from each of the two main regions 3 by shallow 
grooves 4 which are formed above the optical waveguide 1. The 
semiconductor laser device is also provided with deep grooves 5 which are 
located in parallel to the optical waveguide 1 at a certain distance from 
the optical waveguide 1 in a manner to connect to the grooves 4. This 
semiconductor laser device further comprises a p-semiconductor substrate 
6, an n-current blocking layer 13, a p-cladding layer 7 for confining 
carriers therein, a p- (or n- or non-doped) active layer 8 for laser 
oscillation, an n-cladding layer 9 for confining carriers therein, and an 
n-cap layer 10 for achieving an ohmic contact in that order, resulting in 
a double-heterostructure multi-layered crystal. An n-sided electrode 11 
and a p-sided electrode 12 are disposed on the upper face of the cap layer 
10 and the back face of the semiconductor substrate 6, respectively, and 
are connected to an external circuit. Moreover, a V-channeled stripe 100 
is formed in the semiconductor substrate 6 through the n-current blocking 
layer 13 so as to open a current path, resulting in a VSIS (V-channeled 
substrate inner stripe) semiconductor laser device. 
The optical waveguide 1, which functions as a laser oscillation-operating 
area and both ends of which form the facets for emitting laser light 
therefrom, exists within the portion of the active layer positioned above 
the V-channel 100. The grooves 4 are positioned above the optical 
waveguide 1 as shown in FIG. 1(B) and composed of two shallow grooved 
portions which reach the interface between the cap layer 10 and the 
n-cladding layer 9 via the n-sided electrode 11 and the cap layer 10. The 
grooves 5 are positioned outside of the optical waveguide 1 as shown in 
FIG. 1(C) and composed of three deep grooved portions which reach the 
interface between the p-cladding layer 7 and the current blocking layer 13 
via the n-sided electrode 11, the n-cladding layer 9, the active layer 8 
and the p-cladding layer 7. Two of the three grooved portions of the 
grooves 5 located between the facets and the grooves 4 and the other 
grooved portion located between the grooves 4 are disposed in a zigzag 
manner with regard to the optical waveguide 1. Thus, the control region 2 
is electrically connected to each of the two main regions 3 by the 
portions of the active layer 8 and the portions of the n-cladding layer 9 
positioned below the grooves 4. The active layer 8 is separated from the 
n-cladding layer 9 by the grooves 5. It is preferable that the coupling 
resistance in the connection portion at which the control region 2 is 
connected to the main regions 3 is high. 
FIG. 2 shows an equivalent network of the semiconductor laser device shown 
in FIGS. 1(A) to 1(C), wherein Rs corresponds to the contact resistance 
between the electrodes 11 and 12 and the laser device (i.e., the series 
resistance of the laser device), Rh corresponds to the above-mentioned 
coupling resistance, a and b correspond to the electrode terminals of the 
control region 2 and the main region 3, respectively, and c corresponds to 
the common terminal disposed on the substrate 6. This semiconductor laser 
device is mounted on a three-terminal type stem. By connecting a shunt 
resistor Rg to the terminal a, the total current It injected into this 
laser device can be branched into Ig and It-Ig, which flow into the 
control region 2 and the main region 3, respectively. If the shunt 
resistor Rg is put into the stem, a two-terminal stem can be used as the 
stem on which the laser device is mounted. By changing the resistance of 
the resistor Rg so as to change the current ratio Ig/It, the inventors 
determined the oscillation longitudinal mode of the laser device and found 
the fact that when Ig/It and Lg/Lt meet the inequality (1), self-pulsation 
occurs and the spectral width of the longitudinal mode is enlarged up to 
0.2 .ANG.-2 .ANG., which will come to as short as 3 cm-0.3 cm in the 
coherent length. 
##EQU3## 
Although the oscillation threshold current Ith must be at the minimum value 
when Ig/It=Lg/Lt, it will increase by only 5-15 mA according to the 
inequality (1). Such an increase in the oscillation threshold current has 
no effect on the shortening of the coherent length. When Ig/It&lt;0.01, the 
control region 2 becomes a complete absorption region so that the laser 
device cannot attain laser oscillation. 
FIG. 5 is a characteristic curve showing the relationship between the 
current ratio Ig/It and the width of a longitudinal mode at an optical 
output power of 3 mW. The ratio Ig/It varies with changes in the 
resistance of the external resistor Rg. The longitudinal mode width 
continuously varies from 2 to 0.2 .ANG. in the Ig/It range of 0.02 to 
0.13. The oscillation wavelength .lambda. of this laser device was 780 nm, 
the coupling resistance Rh of this laser device was 500.OMEGA., and the 
contact resistance Rs between the electrodes and this laser device was 
5.OMEGA.. 
FIG. 6 is a characteristic curve showing the relationship between the 
current I and the optical output power L at the Ig/It values of 0, 0.05, 
and 0.2, which indicates that the dual stable state does not occur when 
Ig/It&gt;0.01. 
In the above-mentioned example, a p-GaAs substrate was used as the 
semiconductor substrate 6, an n-GaAs was used as the current blocking 
layer 13, a p-Ga.sub.0.5 Al.sub.0.5 As was used as the p-cladding layer 7, 
a Ga.sub.0.85 Al.sub.0.15 As was used as the active layer 8, an 
n-Ga.sub.0.5 Al.sub.0.5 As was used as the cladding layer 9, an n-GaAs was 
used as the cap layer 10, and an Au--Ge/Al and an Au--Zn, respectively, 
were used as the n-sided electrode 11 and the p-sided electrode 12. The 
internal cavity length Lt was 250 .mu.m and the length Lg of the center 
portion (i.e., the control region 2) of the optical waveguide 1 was 50 
.mu.m. In order to meet the inequality (1), Ig/It was set to be greater 
than 0.01 but to be less than 0.13 (i.e., 0.01&lt;Ig/It&lt;0.13). 
Example 2 
FIGS. 3(A), 3(B) and 3(C) show another semiconductor laser device of this 
invention. FIG. 4 shows an equivalent network of the semiconductor laser 
device shown in FIGS. 3(A) to 3(C), which is the same as described in 
Example 1 except that the resistor Rg is composed of a built-in resistance 
bridge 14 disposed within the laser device. Current is fed to the control 
region 2 through the narrow and long resistance bridge 14 which is 
positioned on the n-cladding layer 9. The resistance bridge 14 is 
constituted by a groove which has the same depth as the grooves 4 and 
which reaches the interface between the cap layer 10 and the n-cladding 
layer 9. The resistance of the resistance bridge 14 (i.e., the resistance 
of the resistor Rg) can be selected to be a desired value by the control 
of the width and length of the resistance bridge 14 and/or the thickness 
of the n-cladding layer 9. In order to obtain the current ratio 
represented by the inequality (1), the resistor Rg is selected to meet the 
inequality (2). 
##EQU4## 
As a stem on which this semiconductor laser device is mounted, an ordinary 
two-terminal type stem can be employed if wires 15 and 16 through which 
current is fed to the control region 2 and the main regions 3 are bonded 
to a common terminal of the stem. 
When the width of the resistance bridge 14 and the length thereof were 7 
.mu.m and 10 .mu.m, respectively, and the thickness of the n-cladding 
layer 9 was 2 .mu.m, the resistance Rg was 150.OMEGA.. The oscillation 
threshold current of the semiconductor laser device was 65 mA and a 
multi-mode oscillation with an enlarged longitudinal mode width of 2 .ANG. 
was attained at an optical output power of 3 mW. The oscillation spectra 
are shown in FIG. 7(A) and the enlarged oscillation spectrum of a single 
element of the multi-longitudinal mode is shown in FIG. 7(B). 
In the above-example, materials used for the substrate 6, the current 
blocking layer 13, the p-cladding layer 7, the active layer 8, the 
n-cladding layer 9, the cap layer 10, and the n- and p- sided electrodes 
11 and 12 were same as those of Example 1. The lengths Lt and Lg were 250 
.mu.m and 50 .mu.m, respectively. Ig/It were set to be 0.01&lt;Ig/It&lt;0.13 to 
meet the inequality (1). 
Although Examples 1 and 2 disclosed a GaAs-GaAlAs semiconductor laser 
device alone, they can be applied to semiconductor laser devices of 
InGaAsP systems and other semiconductor materials. The optical waveguide 
structure is not limited to a VSIS type, but any kind of optical waveguide 
structure can be used in this invention. Moreover, the position, the 
length, etc., of the control region are not limited to the conditions 
described in Examples 1 and 2. 
Example 3 
This example discloses a method for driving semiconductor laser devices 
according to this invention, wherein the semiconductor laser device shown 
in FIGS. 8(A), 8(B) and 8(C) was used. This semiconductor laser device is 
the same as that shown in FIGS. 1(A) to 1(C) except that the front facet 
for emitting light therefrom was covered with a dielectric film 44 of 
Al.sub.2 O.sub.3.Si.sub.3 N.sub.4 or the like which has a thickness of 
about .lambda./4 (.lambda. is the oscillation wavelength) and which has a 
reflection index of 2-8%, and that the rear facet was covered with a 
dielectric film 55 composed of alternate layers which consist of 
low-refractive index films of Al.sub.2 O.sub.3.Si.sub.3 N.sub.4 or the 
like (the thickness of each film being about .lambda./4) and 
high-refractive index films of Si or the like (the thickness of each film 
being about .lambda./4). The reflective index of the dielectric film 55 is 
in the range of 70 to 95%. 
FIG. 9 shows an equivalent network and an operation circuit of the 
semiconductor laser device shown in FIGS. 8(A) to 8(C), wherein Rs, Rh, a, 
b, and c are the same as those shown in FIG. 2 of Example 1. This laser 
device is mounted on a three-terminal type stem. R.sub.1 and R.sub.2 are 
the resistances for controlling a current which flows to the control 
region 2, and R.sub.1 is set to be smaller than R.sub.2 (i.e., R.sub.1 
&lt;R.sub.2). R.sub.1 and R.sub.2 are shunt resistors connected to the 
terminal a by a switch Sw such that the total current It can be branched 
into Ig and Im(=It-Ig) which flow to the control region 2 and the main 
region 3, respectively. R.sub.1 can be, of course, composed of a variable 
shunt resistor Rg for obtaining an incoherent length described in Example 
1. In this case, R.sub.2 is omitted. 
FIG. 10 shows a characteristic curve showing the relationship between the 
current Im and the optical output power L, wherein the curve l.sub.1 is 
the I-L characteristics attained when a large amount of Ig flowed into the 
control region 2 via R.sub.1 by means of the switch Sw and the curve 
l.sub.2 is the I-L characteristics attained when a small amount of Ig 
flowed to the control region 2 via R.sub.2 by means of the switch Sw. When 
the amount of Ig is small, the control region 2 becomes a light absorption 
region, which causes an increase in the threshold current Ith, resulting 
in self-pulsation within the laser device. Thus, the oscillation 
longitudinal mode attained by the laser device is a multi-mode, the width 
of each element of which is broad, 0.1-2 .ANG.. This phenomenon is shown 
in FIG. 11(A) illustrating the oscillation spectra in reading signals. In 
this case, since the coherent length of the semiconductor laser device 
becomes as short as 3 mm-90 mm, even when the semiconductor laser device 
is used as a means for reading signals of an optical disc, an external 
resonator is not formed between the semiconductor laser device and the 
optical disc, and accordingly noise due to reflected light from the 
optical disc never occurs. On the other hand, when the recording of 
signals at which a high output power operation is required is carried out, 
noise due to reflected light almost never occurs. FIG. 11(B) shows the 
oscillation spectra in recording signals. 
This semiconductor laser device is operated as follows: The current Im 
flowing to the main regions 3 is maintained at a fixed level (Imo) and the 
current Ig is injected into the control region 2 via one of the shunt 
resistors R.sub.1 and R.sub.2, the selection of which can be readily 
conducted by means of the switch Sw, so that the operation conditions that 
are suitable to the recording and reading of signals can be easily set up. 
It is not necessarily essential to the operation of the laser device that 
the current Im is maintained at a fixed level. As described in Example 1, 
the inventors tried to determine the oscillation longitudinal mode of this 
semiconductor laser device by changing the value of the shunt resistor so 
as to change the current ratio Ig/It, and observed that when Ig/It and 
Lg/Lt meet the inequality (1), self-pulsation occurs and the spectral 
width is enlarged to 0.1 .ANG.-2 .ANG., which will result in a coherent 
length of as short as 3 mm-90 mm. Although the oscillation threshold 
current Ith must be at the minimum value when Ig/It=1, it will become only 
about 5-15 mA according to the inequality (1). Such an increase in the 
oscillation threshold current has no effect on the shortening of the 
coherent length. When Ig/It&lt;0.01, the control region 2 becomes a complete 
absorption region so that the laser device cannot attain laser 
oscillation. 
Although the above-mentioned example disclosed a GaAs-GaAlAs semiconducter 
laser device alone, InGaAsP/InP systems can also be used. Moreover, the 
structure of the optical waveguide is not limited to a VSIS type, but any 
kind of optical waveguide structure can be used in this invention. The 
position, the length, etc., of the control region are not limited to the 
conditions described in Example 3. 
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.