Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-02-27
2004-03-02
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06700912
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a semiconductor laser element, a semiconductor laser apparatus and a method of manufacturing the apparatus, and more particularly to a multiple transverse mode high-output semiconductor laser element which is large in width of the active region or width of the light emitting portion, an apparatus using the laser element and a method of manufacturing the apparatus.
2. Description of the Related Art
Nowadays semiconductor lasers have been put into practice in various fields. Especially, wide-stripe semiconductor lasers having an oscillation wavelength in the 0.7 to 1.6 &mgr;m band has come to be in wide use with increase in output power as a pumping light source for a solid sate laser, a fiber amplifier, a fiber laser and the like, as a primary light source for second harmonic generation, as a light source for forming an image by a laser-thermal system on a thermal recording medium, for instance, in printing, as a medical light source, and a light source for laser material processing and a soldering. For these applications, that the semiconductor laser is of a high output power is very important.
A multiple transverse mode high-output semiconductor laser which is not smaller than about 10 &mgr;m in width of the light emitting portion and is not shorter than several thousands of hours in guaranteed life has been put into practice. For example, such a high-output semiconductor laser can operate continuously at an output of 1.5 W with a width of light emitting portion of about 50 &mgr;m. For example, a semiconductor laser which comprises an InGaAsP quantum-well, an InGaP optical waveguide layer and an AlGaAs clad layer and is 50 &mgr;m in stripe width and 810 nm in oscillation wavelength has been empirically proved to be sufficiently practicable at 1.5 W. In this case, high reliability at high output power is realized by virtue of a high peak light density and a light exit face temperature lowering effect obtained by increase in thickness of layers which can be realized by use of an aluminum-free active layer and an optical waveguide layer whose electric resistance is reduced by doping.
As techniques of obtaining high reliability at high output power, there have been known, for instance, a technique in which the light exit end face is specially processed or is applied with a protective layer (IEEE J. Selected Topics in Quantum Electronics, vol. 5, p. 832 (1999)) and a technique in which the absorbance index near the light exit end face is reduced (D. F. Welch, W. Streifer, R. L. Thornton, and T. Paoli: Electron. Lett. vol. 23, p. 525 (1987)).
As for a multiple transverse mode high-output semiconductor laser which is not smaller than about 50 &mgr;m in stripe width, among those which make laser oscillation at 0.87 &mgr;m, there have been reported a laser which is 100 &mgr;m in stripe width and the catastrophic optical damage of which is 11.3 W, and a laser which is 200 &mgr;m in stripe width and the catastrophic optical damage of which is 16.5 W (Electronics Letters, vol. 34, No. 2, p. 184 (1998)).
Each of the semiconductor lasers has a light emitting region (active region) which is substantially of a single layer and the light distribution in the direction normal to the active layer is confined in a micro space in the semiconductor which is as small as a half of the wavelength. Accordingly, the light density is high and since heat is generated in a narrow region, temperature elevation at the light exit end face is large, which limits increase in output power.
Methods in which a plurality of active regions are provided in a direction normal to the respective growth layers of a semiconductor laser have been proposed. In “Appl. Phys. Lett. vol. 41, p. 499 (1982)”, there is disclosed a method in which, in a full-face electrode type laser of a width of 100 &mgr;m, three double heterostructures (DH) are superposed one on another with a P
+
N
+
-tunnel junction intervening therebetween. In this structure, the active layers are spaced from each other by at least 2 &mgr;m, and the space between the active layers is larger than the wavelength, whereby the light density distribution is enlarged. However, when the growth layer side is fused to a heat sink for continuous oscillation, heat dissipation is feasible only in one direction and accordingly heat dissipation from the three active layers each forming a heat generating region is limited, which results in larger temperature elevation in active layers remote from the heat sink and in deterioration in reliability.
In the method disclosed in “Appl. Phys. Lett. vol. 42, p. 850 (1983)”, output power of a laser is increased by providing a plurality of active layers in an optical waveguide region which is thickened to at least 2 &mgr;m. In this case, though peak light intensity can be lowered, the structure merely results in a pulse-driven full-face electrode laser of a width of 250 &mgr;m and hardly contributes to suppressing temperature elevation at the light exit end face during continuous oscillation or heat dissipation. Further, in Japanese Unexamined Patent Publication No. 4(1992)-157777, there is disclosed a semiconductor laser which is arranged to pump a solid state laser at a higher output power by superposing a pair of wide stripe chips, with an electrode intervening therebetween, provided with a stripe-like light radiating portion at the center thereof. However, this arrangement is disadvantageous in that since a pair of PN-junctions are superposed to form a PNPN-junction, it is difficult to uniformly excite the two laser chips in a controlled manner.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object of the present invention is to provide a high-output semiconductor laser element which is highly reliable and higher in maximum optical power.
Another object of the present invention is to provide a high-output semiconductor laser apparatus which is highly reliable and is increased in maximum optical output.
Still another object of the present invention is to provide a method of easily manufacturing such a high-output semiconductor laser apparatus.
In accordance with a first aspect of the present invention, there is provided a high-output semiconductor laser element comprising a plurality of laser structures, each comprising at least one active layer interposed between a P-type clad layer and a N-type clad layer, which are superposed on a substrate one on another with a P
+
N
+
-tunnel junction intervening between each pair of the laser structures, the active region of each of the laser structures being not smaller than 10 &mgr;m and not larger than 80 &mgr;m in width, the distance h between the active layers which are most distant from each other in the active layers of the laser structures being not larger than the width W of the active region which is the widest in the laser structures, and the width of said semiconductor laser element being not smaller than W+2h.
It is preferred that heat sinks be provided on both sides of the semiconductor laser element, the laser structure side and the substrate side.
When heat sinks are provided, the thickness of the semiconductor laser element is preferably not larger than 100 &mgr;m and more preferably not larger than 80 &mgr;m.
Though the high-output semiconductor laser element in accordance with the first aspect of the present invention has a plurality of laser structures or active regions, the peak light intensity at the light exit end face during high power operation can be reduced since the active regions are separated from each other, whereby deterioration of the light exit end face due to photochemical reaction and the like can be suppressed. Further, by limiting the width of the active region of each laser structure to 80 &mgr;m, the heat flow parallel to the active layers can be effectively used or the non-light emitting portion can be effectively used as a heat dissipation path, whereby temperature elevation at the end face can be supp
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