Coherent light generators – Particular resonant cavity – Specified cavity component
Patent
1994-05-24
1995-08-22
Davie, James W.
Coherent light generators
Particular resonant cavity
Specified cavity component
372 46, 372 50, 372 92, H01S 308
Patent
active
054447313
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser array. It can be used in numerous fields such as optical telecommunications, optical pumping, treatment of materials, etc.
2. Prior Art
The increase in the power of semiconductor lasers cannot be obtained merely by increasing the active surface, because instabilities of modes and emitter filaments which are poorly controlled occur. It is therefore necessary to define elementary emitters formed by active areas having small lateral dimensions. The power increase is then obtained by multiplying said elementary emitters.
A considerable beam quality improvement is obtained by ensuring a coherence of the optical phase between the emitters by coupling them together. The simplest coupling is obtained by evanescent wave (i.e. by covering the evanescent parts of the modes of two adjacent elementary emitters). This type of coupling favours the oscillation of two adjacent emitters in phase opposition, which leads to a multilobe radiation pattern. In the case of multistrip lasers, where the lightwave propagates parallel to the semiconductor surface, various means have been proposed for ensuring an inphase coupling, which produces an emission in a single lobe, perpendicular to the exit face (generally obtained by cleaving). This method is e.g. described in the article by J. Katz et al "Diffraction coupled phase locked semiconductor laser array" published in Appl. Phys. Lett., 42, (7), Apr. 1, 1983, pp. 554-556 and L. J. Mawst et al entitled "CW high-power diffraction-limited-beam operation from resonant optical waveguide arrays of diode lasers" published in Appl. Phys. Lett., 58 (1), Jan. 7, 1991, pp. 22-24.
The recent development of vertical cavity lasers emitting by the surface makes it possible to envisage matrixes of emitter points instead of an emitting line, as well as an emission perpendicular to the surface of the semiconductor chip and not on the edge, without involving technical complications. Such matrixes are in particular described in the article by M. Orenstein et al entitled "Two-dimensional phase-locked arrays of vertical-cavity semiconductor lasers by mirror reflectivity modulation" published in Appl. Phys. Lett., 58 (8), Feb. 25, 1991, pp. 804-806.
In general, a matrix of lasers of this type comprises a conductive substrate and, on said substrate, a lower mirror, an active layer (optionally surrounded by confinement layers) and an upper mirror formed by a matrix of reflecting areas (e.g. a matrix of 4.times.4 areas). The coupling between the different vertical elements is obtained by diffraction at the edges of the reflecting areas. The complete structure behaves like a plurality of lasers (16 in the case of a 4.times.4 matrix) slightly coupled together.
Although such structures are interesting in certain respects, they still suffer from the disadvantage of leading to a multilobe beam, which is therefore only slightly directional. This disadvantage is due to the fact that the coupling produced leads to a phase opposition between adjacent elementary lasers, which causes an anti-symmetric radiation pattern.
SUMMARY OF THE INVENTION
The object of the invention is to obviate this disadvantage. To this end, it proposes a structure which is once again an array of vertical cavities, but whose emission beam is now monolobe instead of multilobe. This result is essentially obtained by a novel structure of the upper mirror. According to the invention, said mirror comprises a plurality of high reflectivity areas surrounded by lower reflectivity areas and also comprises metallized areas serving as current leads, said metallized areas being positioned above the lower reflectivity areas. The higher reflectivity areas can be organized in matrix form.
Thus, in the structure according to the invention, the injected current density is not homogeneous and has a distribution such that the current density maxima correspond to the reflectivity minima of the upper mirror. The applicant has observed that under these
REFERENCES:
M. Orenstein et al, "Two-dimensional phase-locked arrays of vertical-cavity semiconductor lasers by mirror reflectivity modulation", Appl. Phys. Lett 58(8) 25 Feb. 1991 pp. 804-806.
D. G. Deppe et al, "Phase-coupled two-dimensional Al.sub.x Ga.sub.1-x As-GaAs vertical-cavity surface-emitting laser array", Appl. Phys. Lett. 56 (21), May 21, 1990, pp. 2089-2091.
H. J. Yoo et al, "Phase-Locked Two-Dimensional Arrays of Implant Isolated Vertical Cavity Surface Emitting Lasers", Electronic Letters, 8th Nov. 1990, vol. 26, No. 23 pp. 1944-1946.
D. G. Deppe et al, "Bistability in an AlAs-GaAs-InGaAs Vertical-Cavity Surface-emitting laser", Appl. Phys. Lett 58(23) 10 Jun. 1991, pp. 2616-2618.
Commissariat a l''Energie Atomique
Davie James W.
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