Internal reflection interferometric semiconductor laser apparatus

A semiconductor laser apparatus comprising: a semiconductor laser chip with a first light-emitting facet and a second light-emitting facet containing an interference effect-creating means therein for creating an interference effect within the resonator(s) thereof, a laser light-reflecting means for returning a part of the laser light emitted from the first light-emitting facet of said semiconductor laser chip to the first light-emitting facet, and a mounting base on which said semiconductor laser chip and said laser light-reflecting means are fixed in a parallel manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a semiconductor laser apparatus which attains 
laser oscillation with a stabilized oscillation wavelength. 
2. Description of the Prior Art 
Semiconductor laser devices which are mass produced can attain laser 
oscillation at a low threshold current and obtain considerably 
satisfactory results in characteristics such as the fundamental transverse 
mode, the single longitudinal mode, long life, etc., but they have 
problems with regard to a stabilized oscillation wavelength (i.e., the 
stabilized longitudinal mode) in that the oscillation wavelength varies 
continuously or discontinuously depending upon a variation in temperature 
and/or current, resulting in optical output noise which is noticeable when 
the laser devices are exposed to light and/or a reflected laser light from 
the laser devices. 
In order to eliminate the above-mentioned problems and to stabilize the 
longitudinal oscillation mode, the suppression of mode hopping (i.e., 
changes in the longitudinal oscillation mode) has been tried over a wide 
range of temperatures by the following approaches: 
The first approach for the suppression of mode hopping is the use of 
diffraction grating type lasers such as distributed feedback (DFB) lasers, 
distributed Bragg reflector (DBR) lasers, etc., which have diffraction 
gratings in the waveguide. These laser devices are excellent in wavelength 
selectivity due to the periodicity of the diffraction gratings, so that 
they can attain the stabilization of the longitudinal mode over a wide 
range of temperatures. However, their production process is complicated 
and it is difficult to use some semiconductor laser materials (e.g., 
materials which are readily oxidized). 
Second, compound resonator type lasers including cleaved coupled cavity 
lasers (in which two semiconductor lasers are coupled by their facets 
and/or which are separated into two laser-operating areas by an etching 
technique) are used. The two semiconductor lasers operate independently, 
resulting in the synchronization of their wavelengths, making possible the 
stabilization of the longitudinal mode. However, their operation relies 
upon the skill of skilled workers, and moreover small changes in the 
spacing between the two laser devices cause changes in the longitudinal 
mode, resulting in optical output noise. 
Third, the reflective index of each of both facets of a semiconductor laser 
device is made high to increase the internal optical density of the 
device, resulting in the suppression of the non-oscillation mode, thereby 
attaining stable oscillation in a longitudinal mode notwithstanding 
changes in temperatures. 
Fourth, the concentration of Te, etc., to be doped into the n-cladding 
layer of a semiconductor laser device is set at a high level resulting in 
a supersaturated absorber by which loss-gratings are formed, whereby the 
suppression of the non-oscillation mode is attained and stable laser 
oscillation is attained in a longitudinal mode notwithstanding changes in 
temperatures. 
Although the third and fourth approaches provide stable oscillation in a 
longitudinal mode regardless of changes in temperatures to a certain 
extent, mode hopping occurs not only when temperatures rise beyond a 
certain value, but also when temperatures fall beyond a certain value. 
That is, mode-hopping hysteresis occurs, causing a limitation in the range 
of temperatures and/or currents used. 
Fifth, internal reflector interferometric lasers having a reflecting area 
inside thereof are used. These laser devices have a waveguide which 
provides one or more regions having different effective refractive index 
in which reflection occurs resulting in an interference effect. Such an 
interference effect results in the stabilization of a longitudinal mode. 
In order to create a difference in the refractive index, the thickness of 
the active layer must be made uneven and/or semiconductor crystal 
materials having a different composition ratio must be used for the 
formation of the laser devices. 
The sixth approach is that two or more waveguides having different 
effective cavity lengths are optically coupled therebetween, resulting in 
an interference effect, which makes possible the stabilization of a 
longitudinal mode. Examples of these laser devices are OEMI lasers (I. H. 
A. Fattah et al., Appl. Phys. Lett., 41(2), 112, 1982) etc. 
Although the fifth and sixth approaches allow the stabilization of a 
longitudinal mode at a temperature in the range of several degrees to 
several tens of degrees, it is impossible to stabilize the longitudinal 
mode with reproducibility over a wider range of temperatures. That is, 
although the suppression of a limited number (e.g., 4 or 5) of the 
longitudinal modes which are adjacent to the oscillation longitudinal mode 
is possible, the internal reflectivity of these laser devices are so 
insufficient that a greater number of adjacent modes cannot be suppressed, 
making it difficult to put them into practical use. 
Seventh, external resonator type lasers are used, wherein laser light 
emitted from one of the facets of the laser device is reflected by an 
external reflector to return to the facet of the laser device, resulting 
in an interference effect between the external mode based on the distance 
from the facet of the laser device to the external reflector (i.e., the 
external cavity length) and the longitudinal mode based on the distance 
from one facet of the laser device to the other thereof (i.e., the 
internal cavity length of the laser device). The stabilization of a 
longitudinal mode can be attained by utilizing such an interference 
effect. The selectivity of the longitudinal mode in the seventh approach 
depends upon the external cavity length and the amount of reflected light. 
In order to increase the amount of reflected light, a semiconductor laser 
apparatus shown in FIG. 3 has been proposed, wherein a lens 2 is disposed 
between the laser device 1 and the external reflector 3 in such a manner 
that laser light emitted from the laser device is incident upon the 
external reflector 3 through the lens 2 and then reflected to return to 
the laser device 1 through the lens 2. The laser device 1 is fixed on a 
mounting base 4. Since the external cavity length (L) unavoidably becomes 
long due to the above-mentioned structure, the external mode interval 
.DELTA..lambda.e (=.lambda..sub.0.sup.2 /2 L, wherein .lambda..sub.0 is 
the central wavelength) becomes small, so that stable oscillation in a 
longitudinal mode cannot be attained. Although laser light emitted from 
the laser device can be returned to the laser device by a concave 
reflector, the production process of such a laser apparatus is complicated 
and it is difficult to position the facet of the laser device at the 
center of the concave reflector. 
The inventors of this invention designed a semiconductor laser apparatus in 
which a plane reflector is positioned to face one facet of the laser 
device in a parallel manner at the distance of 100 .mu.m therebetween 
(i.e., with an external cavity length of 100 .mu.m), and stable laser 
oscillation was attained in a longitudinal mode over a range of about 
10.degree. C. In order to further expand the temperature range, the 
inventors tried to shorten the external cavity length and measured changes 
in the oscillation wavelength when temperatures are changed at an optical 
output power of 3 mW, resulting in the characteristic curve (FIG. 4) 
showing the relationship between the temperatures and the oscillation 
wavelengths. FIG. 4 indicates that mode hopping successively occurs from 
one longitudinal mode to the adjacent longitudinal mode in each of the A 
and B zones of oscillation wavelengths, which makes the stabilization of 
laser oscillation in a longitudinal mode impossible. More particularly, 
FIG. 4 indicates that mode hopping successively occurs from the initial 
longitudinal mode to the adjacent longitudinal mode at the .circle.a 
points at which the temperature rises and the other mode hopping occurs to 
return to the initial longitudinal mode at the .circle.b points at which 
the temperature falls. That is, mode hopping hysteresis phenomenon arises 
with changes in temperature. 
As mentioned above, semiconductor lasers which stably oscillate in a single 
longitudinal mode over a wide range of temperatures and which are readily 
produced have not yet been proposed at present. 
SUMMARY OF THE INVENTION 
The semiconductor laser apparatus of this invention, which overcomes the 
above-discussed and numerous other disadvantages and deficiencies of the 
prior art, comprises a semiconductor laser chip with a first 
light-emitting facet and a second light-emitting facet containing an 
interference effect-creating means therein for creating an interference 
effect within the resonator(s) thereof, a laser light-reflecting means for 
returning a part of the laser light emitted from the first light-emitting 
facet of said semiconductor laser chip to the first light-emitting facet, 
and a mounting base on which said semiconductor laser chip and said laser 
light-reflecting means are fixed in a parallel manner. 
The interference effect-creating means is, in a preferred embodiment, 
composed of an area of the active layer having a different thickness in 
the central area of the resonator. 
The interference effect-creating means is, in a preferred embodiment, 
composed of resonators having different cavity lengths in which a part of 
the waveguide in one resonator is common to that of the waveguide in the 
other resonator from the second light-emitting facet to the branching 
portion and the other parts of the waveguides in the resonators are 
branched at said branch portion. 
Thus, the invention described herein makes possible the objects of (1) 
providing a semiconductor laser apparatus which stably oscillates in a 
single longitudinal mode over a wide range of temperatures; (2) providing 
a semiconductor laser apparatus which attains stable laser oscillation 
with a desired oscillation wavelength by precise control of temperatures 
and/or currents; (3) providing a semiconductor laser apparatus in which 
optical output noise due to unstable longitudinal modes can be suppressed; 
and (4) providing a semiconductor laser apparatus which stably attains a 
single longitudinal mode regardless modulation arising.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The semiconductor laser apparatus of this invention comprises: 
an internal reflector interferometric laser chip having an internal 
reflection-creating means for creating an internal reflection at at least 
one portion within the resonator thereof so as to suppress the adjacent 
longitudinal modes resulting in the stabilization of the main longitudinal 
mode, or a semiconductor laser chip having a laser oscillation area 
composed of two or more coupled waveguides to create an interference 
effect due to the difference in their waveguide length, thereby attaining 
the suppression of the adjacent longitudinal modes so as to stabilize the 
main longitudinal mode, and 
a laser light-reflecting means for returning a part of the laser light 
emitted from one facet of said semiconductor laser chip to said facet. 
The principle of the stabilization of a longitudinal mode according to this 
invention is explained by reference to FIGS. 5(A) to 5(D) as follows: 
FIG. 5(A) shows the spectra near the oscillation threshold value of a 
conventional ordinary semiconductor laser device, indicating that the main 
oscillation mode &lt;0&gt; is successively shifted to the adjacent modes &lt;+1&gt;, 
&lt;+2&gt;, . . . positioned at the longer wavelength side as temperatures rise. 
FIG. 5(B) shows the spectra near the oscillation threshold value of a 
conventional internal reflector interferometric laser device, indicating 
that stable oscillation in the longitudinal mode &lt;0&gt; is allowed to 
continue until mode hopping is carried out to the longitudinal mode &lt;+4&gt; 
even though the gain peak is shifted with a variation in temperature since 
the adjacent longitudinal modes &lt;+1&gt;, &lt;+2&gt; and &lt;+3&gt; are suppressed by the 
interference effect. FIG. 5(C) shows the spectra near the oscillation 
threshold value of a conventional ordinary semiconductor laser device 
provided with an external resonator, indicating that although the external 
mode &lt;+16&gt; ranks next to the main mode &lt;0&gt;, since the adjacent modes &lt; 
+1&gt;, &lt;+2&gt;, . . . , and &lt;+15&gt; are insufficiently suppressed, mode hopping 
occurs as the gain peak is shifted, resulting in the characteristic curve 
shown in FIG. 4. Semiconductor laser devices in which two or more 
waveguides are coupled therebetween exhibit the same characteristics as in 
FIGS. 4 and 5(C). FIG. 5(D) shows the spectra near the oscillation 
threshold value of a semiconductor laser apparatus of this invention, 
indicating the the suppression of the longitudinal modes from &lt;+1&gt; to 
&lt;+15&gt; can be effectively achieved due to both the interference effect of 
the semiconductor laser chip and the interference effect of the external 
resonator, and stable oscillation in the main oscillation mode &lt;0&gt; is 
allowed to continue until mode hopping is carried out to the longitudinal 
mode &lt;+16&gt; even though the gain peak is shifted with a variation in 
temperature. 
As mentioned above, this invention provides a semiconductor laser apparatus 
which stably oscillates in a single longitudinal mode over a wide range of 
temperatures. 
EXAMPLE 1 
FIG. 1(A) shows a semiconductor laser apparatus of this invention, which 
comprises a semiconductor laser chip 10 and an external reflector 23 both 
of which are fixed on a single mounting base 21 in such a manner that the 
light-emitting rear facet 25 of the semiconductor laser chip 10 faces the 
reflecting face 24 of the external reflector 23 in a parallel manner at a 
limited distance therebetween. 
The semiconductor laser chip 10 is produced as follows: On a p-GaAs 
substrate 11, an n-GaAs current blocking layer 12 is formed by liquid 
phase epitaxy. Then, a V-striped channel 19 is formed in the current 
blocking layer 12 in a manner to reach the substrate 11, resulting in a 
current path. On the current blocking layer 12 including the V-channel 19, 
a p-GaAlAs cladding layer 13, a GaAlAs active layer 14 for laser 
oscillation, an n-GaAlAs cladding layer 15, and an n-GaAs cap layer 16 for 
achieving an ohmic contact with an electrode are successively formed by 
liquid phase epitaxy, resulting in a double-heterostructure multi-layered 
crystal for laser oscillation. Then, an n-sided electrode 17 and a p-sided 
electrode 18 are formed on the upper face of the cap layer 16 and the back 
face of the substrate 11, respectively. The thickness of the portion of 
the active layer 14 corresponding to the waveguide indicated by dotted 
line 20 shown in FIG. 1(A) is different from that of the other portion of 
the active layer 14 (e.g., the thickness of said portion of the active 
layer 14 is thinner than that of the other portion thereof), causing an 
internal reflection. 
The semiconductor laser chip 10 having the above-mentioned structure is 
fixed on the mounting base 21 by a solder such as In, etc., in such a 
manner that the light-emitting rear facet 25 thereof faces the reflecting 
face 24 of the external reflector 23 in a parallel manner with a distance 
of approximately 24 .mu.m therebetween. 
When current is injected into the semiconductor laser chip 10 through the 
n-sided electrode 17 and the p-sided electrode 18, it flows through the 
current path (i.e., the striped channel 19) alone and laser oscillation 
begins within the portion of the active layer 14 positioned above the 
current path. An internal reflection arises in the portion of the active 
layer 14 indicated by the dotted line 20. Laser light emitted from the 
light-emitting rear facet 25 of the semiconductor laser chip 10 is 
reflected by the reflecting face 24 of the reflector 23 so as to return to 
the semiconductor laser chip 10. 
The oscillation wavelengths were measured with changes in temperatures in 
the case where the output power at the front facet of the semiconductor 
laser chip 10 was 3 mW, resulting in the characteristic curve (FIG. 1(B)) 
showing the relationship between the temperatures and the oscillation 
wavelengths, indicating that the oscillation wavelegnth is stable at 
temperatures even over 45.degree. C., and moreover laser oscillation when 
temperatures rise is achieved in the same longitudinal mode as that when 
temperatures fall, that is mode hopping hysteresis does not occur. 
EXAMPLE 2 
FIG. 2 shows another semiconductor laser apparatus of this invention, which 
is the same structure as in Example 1 except that the semiconductor laser 
chip is provided with two waveguides which are different in their length. 
The semiconductor laser chip 100 is produced as follows: On an n-GaAs 
substrate 31, an n-GaAlAs cladding layer 32, a GaAlAs active layer 33, a 
p-GaAlAs cladding layer 34, and an n-GaAlAs cap layer 35 are successively 
formed by liquid phase epitaxy. Then, a SiO.sub.2 film 36 is coated on the 
surface area of the cap layer 35 except for the portions corresponding to 
a striped current path to be formed by the succeeding process, resulting 
in a striped channel 44 reaching the cap layer 35 and consisting of a 
rectilinear channel 40, which runs between the rear facet and the front 
facet, and a branching channel 41, which branches in the central area of 
the rectilinear channel 40 and runs with a gentle curve to the side facet. 
Then, Zn is diffused into the striped channel 44, resulting in a p-reverse 
layer 38, from the n-cap layer 35 to the p-cladding layer 34, which 
functions as a current path. Then, a p-sided electrode (not shown) is 
formed on the striped channel 44 and the oxide film 36 and an n-sided 
electrode 39 is formed on the back face of the GaAs substrate 31. 
The semiconductor laser chip 100 having the above-mentioned structure is 
fixed on a mounting base 22 by a solder such as In, etc., in such a manner 
that the light-emitting rear facet of the semiconductor laser chip 100 
faces the reflecting face 24 of the external reflector 23 in a parallel 
manner. 
When current is injected into the semiconductor laser chip 100 through the 
p-sided electrode (not shown) and the n-sided electrode 39, it flows 
through the current path (i.e., the Zn-diffusion area 38) alone and laser 
oscillation begins in each of the resonators formed within the areas of 
the active layer 33 positioned below the striped rectilinear channel 40 
and the striped branching channel 41. Although the front facet of the 
resonator corresponding to the striped rectilinear channel 40 is common to 
that of the resonator corresponding to the striped branching channel 41, 
the cavity length from the front facet to the rear facet is different 
therebetween so that the interference effect shown in FIG. 5(B) arises, 
which allows the same stabilized longitudinal mode as in Example 1. 
This invention can provide the same results as shown in FIG. 5(D) even when 
the interference effect shown in FIG. 5(B), which is achieved by the 
semiconductor laser chip and by which mode hopping is suppressed with a 
narrow pitch, is exchanged for the interference effect shown in FIG. 5(C), 
which is achieved by the external resonator and by which mode hopping is 
suppressed with a wide pitch. Selected reflective indexes of the 
light-emitting rear facet of the semiconductor laser chip and/or the 
reflecting face of the external reflector should allow the more noticeable 
stabilization of a longitudinal mode. 
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.