Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
1999-08-18
2002-08-06
Davie, James W. (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
Reexamination Certificate
active
06430204
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device, more particularly to a semiconductor laser device applicable for an optical information processing system, as well as an optical information processing system that uses the same.
A semiconductor laser device having a conventional Fabry-Perot type resonator structure has a larger optical power density around the facets than inside the resonator, thereby the generated heat energy causes a phenomenon of catastrophic optical damage to occur and melt the semiconductor material of the laser device. This comes to limit the upper limit value of the optical output of the laser device. In order to prevent such a phenomenon, a method has been taken; the large bandgap energy assumed at the facets of the resonator is increased, thereby forming a window structure transparent to the laser light there. A high output semiconductor laser provided with such a window structure and a method for forming the same are disclosed in the related art technical document
1
: IEEE Journal of Quantum Electronics 1993, vol.29, No.6, pp.1874-1879. In the case of the semiconductor laser described in the related art technical document
1
, the bandgap energy of the active layer is set largely around the facets of the resonator, thereby suppressing effectively the optical absorption around the facets and avoiding the above mentioned phenomenon of catastrophic damage even at a high optical output time (that is, when a high current is injected into the active layer). However, the semiconductor laser described in the related art technical document
1
has been confronted with problems that the laser oscillation becomes unstable in the fundamental traverse-mode when a high current is injected into the active layer, so that a kink occurs in the optical output-current characteristics and the laser light cannot follow up the waveform of the fast pulse current from a driving power supply, thereby saturating the optical output and degrading the linearity of the efficiency. Consequently, the semiconductor laser described in the related art technical document
1
has been impossible to correspond fast and linearly to the injected current waveform up to a high optical output value. The related art technical document
1
describes no idea for solving such problems.
Techniques related to the stripe optical waveguide structure for semiconductor laser devices are disclosed, for example, in the related art technical document
2
: Japanese Patent Laid-Open No. Hei 5-291681 and the related art technical document
4
: Japanese Patent Laid-Open No. Hei 10-154843. In the related art technical document
2
, a structure with modulated stripe width is described as a structure for easier starting of self-sustained pulsating. The document
2
also describes the range for setting such a waveguide structure and the design contents. The related art technical document
3
describes how an optical waveguide structure with a modulated stripe width is employed for reducing the series resistance of the laser device, thereby suppressing heat generation. The document
3
also describes how to improve the reliability of the laser device using such an optical waveguide structure with a modulated stripe width, as well as the wavelength selectivity effective to control the longitudinal mode. However, those documents
2
and
3
do not describe anything about a method for obtaining an optical high output from the laser device and the stability of the optical output itself nor anything about the influences of the stripe structure be exerted on the fundamental traverse-mode nor anything about the guidelines of the structure for suppressing higher order modes.
On the other hand, the document
4
describes how to control the fundamental traverse-mode with respect to an optical waveguide structure with a periodically modulated stripe width, as well as the designing guidelines for such a structure. In other words, the document
4
describes a method for narrowing periodically the width of a stripe optical waveguide to be extended along the resonator, thereby making the shape of structure symmetric to the center line extended along the resonator of the stripe optical waveguide so as to secure the stability of the fundamental traverse-mode.
SUMMARY OF THE INVENTION
In the case of the conventional laser device as described in the related art technical document
1
, the stripe structure employs the stripe optical waveguide whose width is fixed in the direction along the resonator and is formed in linear shape. Even in the fundamental traverse-mode, when a high current is injected in the active layer, a kink phenomenon occurs in the optical output-current characteristics due to the unstable fundamental traverse-mode. And furthermore, this makes it impossible for the laser light to follow up linearly with the driving current when data is written in memories of an optical disk and cause the memories to be deformed and the error rate to be increased in reading.
Factors causing the traverse-mode to become unstable include a hole burning phenomenon, a beam steering phenomenon, etc. The hole burning phenomenon is a phenomenon that reduces the built-in refractive index difference to the lateral direction of an active layer due to a refractive index change to occur when carriers are injected. The beam steering phenomenon is a kind of the beam instability phenomena. The phenomenon means changes of the irradiation direction and the optical output due to an interference to be caused by the coupling of the fundamental mode with the higher order (first order) mode while the laser light resonates by making a round strip in the resonator when the propagation coefficient is changed by the density of carriers, gain profile, and operation temperature in the optical waveguide.
Hereunder, description will be made for the hole burning phenomenon with reference to a semiconductor laser device provided with a stripe laser optical waveguide structure. The hole burning phenomenon is the main factor for causing the above mentioned unstable traverse mode. While a laser light is oscillated with injected carriers (a driving current) in the stripe optical waveguide structure of a semiconductor laser device, if the injected amount of carriers is increased so as to raise the optical output, the following problem arises; when high carriers are injected, the laser light intensity is increased locally in the center of the stripe optical waveguide, thus the stimulated emission of the laser light at this site is increased remarkably. Consequently, the injected carriers are much consumed due to the recombination of injected carriers, causing the center of the distributed carriers in the stripe optical waveguide to become thin just like a hole. Such a profile of carriers affects directly the refractive index profile in the emission active layer, and further the propagation of the laser light in the active layer. Such the injected carriers then cause a minus change in the refractive index difference, forming a hole-like recess in part of the refractive index profile of the emission active layer (a high refractive index area usually generated like an almost rectangular shape according to the stripe ridge) built in by an optical waveguide, etc.
In order to solve the above conventional problems of the hole burning phenomenon, measures have been taken up to now so that the refractive index difference is increased in the lateral direction of the active layer and a narrower stripe waveguide is designed, as well as a quantum well structure is employed for the active layer, thereby suppressing the reduction of the refractive index when carriers are injected.
However, these related art techniques have not been effective for solving the latter problem with the beam steering phenomenon. Hereunder, this phenomenon will be described with reference to a semiconductor laser device provided with a stripe laser optical waveguide structure.
In order to solve the beam steering phenomenon, in consideration of propagation coefficient
Antonelli Terry Stout & Kraus LLP
Davie James W.
Hitachi , Ltd.
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