# Patent

## Title
Semiconductor laser with reduced heat loss

## Abstract

A semiconductor laser having a semiconductor chip (1) which contains an active layer (5) and emits radiation in a main radiating direction (6). The active layer (5) is structured in a direction perpendicular to the main radiating direction (6) in order to reduce heating of the semiconductor chip (1) by spontaneously emitted radiation (10).

## Background
RELATED APPLICATIONS
This patent application claims the priority of German Patent Application 103 39 980.1-54 filed Aug. 29, 2003, the disclosure content of which is hereby incorporated by reference.
FIELD OF THE INVENTION
The invention relates to a semiconductor laser having a semiconductor chip which contains an active layer and emits radiation in a main radiating direction.
BACKGROUND OF THE INVENTION
The efficiency and output power of semiconductor lasers are crucially limited by the heat arising as a result of radiative and non-radiative recombinations of the injected charge carriers.
In order to improve the heat dissipation, optoelectronic components are often provided with a heat sink. In this case, the temperature increase ΔT in the active zone is given by ΔT=Rth·Pv, where Rth is the thermal resistance between the active zone and the heat sink and Pv is the power loss. A temperature increase generally brings about a lower amplification in the case of a semiconductor laser. In order to reduce such a temperature increase which is disadvantageous for the efficiency of the semiconductor laser, it is advantageous to reduce the thermal resistance Rth or the power loss Pv.
By way of example, Kusznetsov et. Al., IEEE Journ. Sel. Topics in Quantum Electronics 5 (3), 561 (1999) discloses improving the thermal linking of the heat sink to the active zone by removal of a substrate that is typically arranged between the active zone and the heat sink. The thermal resistance Rth and the temperature increase ΔT are thereby reduced.
The power loss Pv arising within the active layer is crucially determined by the non-radiative recombination of charge carriers by means of defects or by means of Auger processes, and by radiative recombinations of charge carriers by spontaneous emission. A high recombination rate leads to a short lifetime of the induced charge carriers and thus to a high laser threshold, a low efficiency and output power. It is desirable, therefore, to minimise these types of recombination of charge carriers. The non-radiative recombination of charge carriers by means of defects can be influenced by the quality of the epitaxy. By contrast, the non-radiative recombination of charge carriers by means of Auger processes is difficult to influence.
The spontaneous emission of radiation can be influenced by structures of the order of magnitude of the light wavelength, for example by microresonators or photonic crystals. A detailed presentation of the mode of operation and the methods for production of photonic crystals is contained in the document T. F. Krauss, R. M. De La Rue, Prog. Quant. Electr. 23 (1999) 51-96, the content of which is hereby incorporated by reference.
The influencing of the spontaneous emission by a microresonator is disclosed in the document Y. Hanamaki, H. Akiyama, Y. Shiraki, Semicond. Sci. Technol. 14 (1999) 797-803. U.S. Pat. No. 5,420,880 describes reducing the laser threshold of a surface-emitting semiconductor laser by means of reducing the coupling-out of spontaneously emitted radiation.
Furthermore, WO 98/43329 discloses a vertically emitting laser in which the radiation emitted spontaneously or in stimulated fashion from a first volume region of an active medium in a transverse direction is utilised for pumping a second volume region surrounding the first volume region.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a semiconductor laser having a comparatively small degree of heating during operation.
This and other objects are attained in accordance with one aspect of the present invention directed to a semiconductor laser having a semiconductor chip which contains an active layer and emits radiation in a main radiating direction. The active layer is structured in a direction perpendicular to the main radiating direction in order to reduce heating of the semiconductor chip by spontaneously emitted radiation.
A first embodiment of the invention provides for the active layer of the semiconductor laser to be structured in a direction perpendicular to the main radiating direction in such a way that the coupling-out of radiation that is emitted spontaneously within the active layer is improved. Since the proportion of the spontaneously emitted radiation which is coupled out from the semiconductor chip is not reabsorbed in the semiconductor chip, the proportion does not bring about any disadvantageous heating of the semiconductor chip.
A second embodiment of the invention provides for the active layer to be structured in a direction perpendicular to the main radiating direction in such a way as to reduce the spontaneous emission of radiation in the active layer in this direction. In contrast to the first embodiment of the invention, in which the disadvantageous heating of the semiconductor chip is reduced by increasing the coupled-out proportion of spontaneously emitted radiation, the aim of this embodiment is actually to reduce the production of spontaneously emitted radiation. This embodiment is based on the fact that spontaneous emission is not a purely intrinsic material property, but rather may also depend on the structure of the active medium and the surroundings.
The semiconductor laser may be, for example, a disc laser (VECSEL—Vertical External Cavity Surface Emitting Laser) in which the active layer has the form of a mesa. The mesa structure may be produced e.g. by means of a photolithographic patterning in conjunction with an etching method. The mesa structure spatially delimits the active layer in a direction perpendicular to the main radiating direction of the laser, so that, in the active layer, spontaneously emitted radiation that is emitted in this direction can leave the semiconductor chip after a comparatively short path through the active medium through the side walls of the mesa.
In the case of an optically pumped disc laser in which a pump radiation source radiates pump radiation into a partial region of the active layer, the width of the mesa in a direction perpendicular to the main radiating direction of the semiconductor laser is approximately as large as the width of the optically pumped region. Since the absorption within the optically pumped region is only low on account of the population inversion during laser operation, this measure reduces the reabsorption of spontaneously emitted radiation.
The side walls of the mesa preferably run obliquely with respect to the main radiating direction of the laser. This prevents the side walls from forming a resonator perpendicularly to the main radiating direction of the laser.
Furthermore, it is advantageous if the side walls of the mesa have a reflection-reducing layer. This not only reduces an undesirable effect of the side walls as a resonator transversely with respect to the main radiating direction, but also improves the coupling-out of spontaneously emitted radiation from the semiconductor chip.
One example of the second embodiment of the invention provides a disc laser with an active layer that has the form of a mesa, the side walls of the mesa forming a resonator in such a way as to reduce the spontaneous emission in the active layer. For this purpose, the resonator is preferably dimensioned such that radiation having the emission wavelength is not capable of propagation. In this case, in contrast to the first embodiment of the invention described previously, it is advantageous if the side walls of the mesa have a reflection-increasing layer for the purpose of amplifying the resonator effect.
A further example of the second embodiment of the invention provides a semiconductor laser in which the active layer has a region provided for optical pumping by a pump radiation source, the optically pumped region being surrounded by a region having, in a direction perpendicular to the main radiating direction of the laser, a periodic structure that forms a photonic crystal in which radiation having the emission wavelength is not capable of propagation.
The periodic structure is for example a lattice-type arrangement of cutouts. The said cutouts may be filled with a material that has a refractive index which differs from that of the active layer.
The resonator or the photonic crystal alters the photonic state density in the active layer in such a way as to reduce the probability of the spontaneous emission of radiation in the direction perpendicular to the main radiating direction.
A further advantageous reduction of the heating of the semiconductor chip may be achieved by virtue of the fact that the semiconductor laser contains a heat sink. The latter may be arranged for example on a side of a substrate that is remote from the active layer, the layer sequence of the semiconductor laser being applied to the substrate. An originally present substrate of the semiconductor chip is preferably removed, so that a residual thin-layer system containing the active layer, in particular, is in direct contact with the heat sink.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a diagrammatic illustration of a cross section through a first example of a first embodiment of a semiconductor laser according to the invention,
FIG. 2 shows a diagrammatic illustration of a cross section through a second example of the first embodiment of a semiconductor laser according to the invention,
FIG. 3a shows a diagrammatic illustration of a cross section through a second example of a second exemplary embodiment of a semiconductor laser according to the invention,
FIG. 3b shows a diagrammatic sectional view of the active layer of the third exemplary embodiment from above, and
FIG. 4 shows a simulation of the output power of a semiconductor laser as a function of the pump power, the intensity and the coupling-out of the spontaneously emitted radiation having been varied.
FIG. 5 shows a first example of the second embodiment of a semiconductor laser according to the invention.



DETAILED DESCRIPTION OF THE DRAWINGS
Identical or identically acting elements are provided with the same reference symbols in the figures.
The semiconductor laser illustrated in FIG. 1 is a surface-emitting optically pumped semiconductor laser with an external resonator (VECSEL). The semiconductor laser has a semiconductor chip 1 containing a substrate 2, a reflector 3, and an active layer 5. The reflector 3, which is preferably a Bragg reflector, forms, together with an external reflector 4, the resonator of the semiconductor laser. The semiconductor laser emits radiation in the main radiating direction 6.
The active layer 5 may, for example, have a quantum well structure and contain in particular III-V compound semiconductor materials. A region 7 of the active layer 5 is optically pumped by a pump radiation source 8. The active layer 5 has the form of a mesa. In a direction perpendicular to the main radiating direction 6 of the semiconductor laser, the mesa has a smaller cross section than the rest of the semiconductor chip 1. The mesa structure is produced for example by means of a photolithographic patterning and a subsequent etching process.
Radiation 10 that is spontaneously emitted within the region 7, into which the pump radiation source 8 radiates pump radiation 9, and propagates in the direction of the side walls 11 of the mesa reaches the side walls 11 after a comparatively short path through regions 12 of the active layer 5 that are not optically pumped and therefore absorb. Within the optically pumped region 7, by contrast, spontaneously emitted radiation 10 can propagate in a manner virtually free of absorption, since a population inversion is present there on account of the pump process. The shape of the active layer 5 as a mesa structure reduces the non-inverted regions 12 in a direction perpendicular to the main radiating direction, so that the absorption of spontaneously emitted radiation is advantageously reduced in these regions 12.
Particularly preferably, the mesa is approximately as wide as the optically pumped region 7 in a direction perpendicular to the main radiating direction 6. The width of the non-inverted regions 12 that can absorb spontaneously emitted radiation 10 is therefore reduced to a minimum.
In order to further improve the coupling-out of spontaneously emitted radiation 10 from the semiconductor chip 1, the side walls 11 of the mesa are advantageously provided with an antireflective layer 13. This reduces the back-reflection on account of the difference in refractive index of the active layer 5 with respect to the surroundings.
The reduced absorption of spontaneously emitted radiation 10 reduces the thermal loading on the semiconductor laser. In order to better dissipate heat loss that occurs nevertheless, the semiconductor laser may furthermore contain a heat sink 14. By way of example, the semiconductor chip 1 may be mounted onto the heat sink 14 by the rear side of the semiconductor substrate 2. The thermal resistance between the active layer 5 and the heat sink 14 may advantageously be reduced by removing the substrate 2. Such a semiconductor laser is illustrated in FIG. 2.
The second example of the first embodiment of a semiconductor laser according to the invention as illustrated in FIG. 2 differs from that illustrated in FIG. 1 furthermore by virtue of the fact that the side walls 15 of the mesa run obliquely with respect to the main radiating direction 6 of the semiconductor laser. The oblique side walls 15 advantageously do not form a resonator and thus prevent a build-up of oscillations of the laser in a direction perpendicular to the main radiating direction 6. In this exemplary embodiment, too, the side walls 15 of the mesa are preferably provided with an antireflective layer 13.
The second embodiment of the invention, the aim of which is actually to reduce the production of the spontaneous emission, may be realised for example in the example illustrated in FIG. 5 by virtue of the fact that the side walls 11 of the mesa form a resonator which reduces the spontaneous emission in a direction perpendicular to the main radiating direction. In this embodiment of the invention, it may even be advantageous to provide the side walls 11 of the mesa with a reflection-increasing layer 16.
A second example of the second embodiment of the invention is illustrated in FIGS. 3a and 3b. Apart from the configuration of the active layer 5, the semiconductor laser illustrated in FIG. 3a corresponds to the semiconductor laser illustrated in FIG. 1. The active layer 5 is patterned in such a way that the optically pumped region 7, in a direction perpendicular to the main radiating direction 6 of the laser, is surrounded by a region 17 having a lattice structure that forms a photonic crystal in which radiation having the emission wavelength is not capable of propagation.
The lattice structure is realised for example by a region 17 of the active layer 5 that surrounds the optically pumped region 7 being provided with a periodic arrangement of cutouts 18. This is illustrated from above in the sectional view of the active layer 5 as illustrated in FIG. 3b. The cutouts 18 may also be filled by a material having a refractive index that differs from the refractive index of the surrounding semiconductor material.
The results of a simulation calculation as shown in FIG. 4 once again illustrate the fundamental idea of the two embodiments of the invention. The illustration shows simulations of the output power PL of a semiconductor laser as a function of the power of the pump radiation source PP.
In the case of the simulation of the curve 19 illustrated with square symbols, it was assumed that spontaneously emitted radiation arising in the semiconductor laser is not coupled out. Given a pump power of above approximately 17 W, the semiconductor laser exhibits a severe dip in the output power on account of thermal overheating.
The curve 20 illustrated with circular symbols was simulated under the assumption that 50% of the spontaneously emitted radiation is coupled out from the semiconductor chip. This improved coupling-out of spontaneously emitted radiation corresponds to the basic idea of the first embodiment of the invention. The curve 20 illustrates that the output power does not dip until at a higher pump power and, therefore, it is also possible to achieve a higher output power.
The curve 21 illustrated with triangular symbols illustrates the principle of the second embodiment of the invention. In this simulation, it was assumed that although spontaneously emitted radiation is not coupled out from the semiconductor chip, the spontaneously emitted radiation is reduced overall by 50%. In this case, too, it is possible to achieve higher output powers than in the case of the curve 19. In the case of the simulated curve 21, given the same pump power, the output power of the semiconductor laser is greater than in the case of the curve 20 since the laser threshold is also lowered overall as a result of the reduction of the spontaneous emission. Therefore, the laser activity also already commences at a lower pump power.
The invention is not restricted by the description on the basis of the exemplary embodiments. Rather, the invention encompasses any new feature and also any combination of features, which in particular comprises any combination of features in the patent claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.

## Claims

1. A semiconductor laser having a semiconductor chip which contains an active layer and emits radiation in a main radiating direction, wherein the active layer is structured in a direction perpendicular to the main radiating direction to reduce heating of the semiconductor chip by spontaneously emitted radiation, and the active layer has the form of a mesa, the mesa comprising side walls having a reflection-reducing layer.

2. The semiconductor laser according to claim 1, wherein the semiconductor laser is a disc laser.

3. The semiconductor laser according to claim 2, wherein the disc laser has an external resonator.

4. The semiconductor laser according to claim 1, wherein the active layer has a region provided for optical pumping by a pump radiation source.

5. The semiconductor laser according to claim 4, wherein the active layer has the form of a mesa, and the width of the mesa in a direction perpendicular to the main radiating direction of the semiconductor laser is approximately as large as the width of the optically pumped region.

6. The semiconductor laser according to claim 1, wherein the laser contains a heat sink.

7. The semiconductor laser according to claim 6, wherein no substrate is contained between the heat sink and the active layer.

8. The A semiconductor laser comprising a semiconductor chip which contains an active layer and emits radiation in a main radiating direction, wherein the active layer is structured in a direction perpendicular to the main radiating direction to reduce heating of the semiconductor chip by spontaneously emitted radiation;
wherein the active layer has the form of a mesa, the mesa comprising side walls that extend parallel or obliquely to the main radiating direction, and the side walls comprising a reflection-reducing layer.