Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2002-09-16
2004-04-13
Ip, Paul (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S045013
Reexamination Certificate
active
06721348
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a vertical-cavity surface-emitting laser (VCSEL) comprising one or more quantum well layers and first and second mirror means defining a laser cavity therebetween.
BACKGROUND OF THE INVENTION
A VCSEL is a semiconductor laser device including one or more semiconductor layers exhibiting an appropriate band gap structure to emit light in a desired wavelength range perpendicularly to the one or more semiconductor layers. Typically, the thickness of a corresponding semiconductor layer is in the range of a few nanometers. In the case of a multi-quantum well laser, the thickness and the strain created during the formation of the stack of semiconductor layers having, in an alternating fashion, a different gap determine the position of the energy level in the quantum wells of the conduction bands and valence bands defined by the layer stack. The position of the energy levels defines the wavelength of the radiation that is emitted by recombination of an electron-hole-pair confined in the respective quantum wells. Contrary to edge emitting semiconductor laser devices, the current flow and the light propagation occurs in a vertical direction with respect to the semiconductor layers. Above and below the semiconductor layers respective mirrors, also denoted as top and bottom mirrors, wherein the terms “top” and “bottom” are exchangeable, are provided and form a resonator to define an optical cavity. The laser radiation established by the resonator is coupled out through that mirror having the lower reflectivity.
Although VCSEL devices suffer from relatively low output power due to their small laser cavity, VCSELs are steadily gaining in importance in a variety of technical fields, since a VCSEL device exhibits a number of advantages when compared to a conventional double heterostructure laser diode, also referred to as edge-emitting lasers. First, a large number of VCSEL devices can be fabricated and entirely tested on the initial substrate, so that a significant reduction in manufacturing costs is obtained compared to edge-emitting lasers. Second, the overall volume of a single VCSEL device is reduced by a factor of about 10-100 compared to the double heterostructure laser diode. Third, due to the extremely small volume of the gain region that is defined in the vertical direction by the thickness of the semiconductor layers having in alternating fashion a different band gap, the current for operating the VCSEL device is in the range of a few milliampers, whereby a high efficiency of conversion of current into light is achieved. Fourth, a further VCSEL device exhibits a relatively low beam divergence, which allows a high coupling efficiency to other optical components, such as optical fibers, without the necessity of additional converging optical elements.
In order to take full advantage of the characteristics of a relatively small gain volume of VCSEL devices, the mirrors defining the laser cavity and usually provided in the form of Bragg mirrors must exhibit a high reflectivity owing to the small optical length of the gain volume, since resonator losses are inversely proportional to the resonator length, good electrical conductivity, since at least a portion of the injection current is lead through the layer stack of the Bragg mirror, and a low thermal resistance to conduct the heat generated in the gain region to the periphery of the VCSEL device. Presently a huge variety of VCSEL devices formed on a gallium arsenide (GaAs) substrate comprising gallium arsenide-aluminum arsenide (GaAs/AlAs) based Bragg mirrors are commercially available, wherein the Bragg mirrors substantially fulfill the above-mentioned criteria. These commercially available VCSEL devices are adapted to operate in a wavelength range of about 850-980 nm. Laser devices operating in this wavelength range are suitable for a variety of applications, including short-haul applications in data communication systems. For usage of VCSEL devices in combination with long-haul fiber optical cables, the operation wavelength of the VCSEL devices has to be increased to about 1.3-1.55 &mgr;m, since the typically employed optical fibers exhibit their dispersion minimum and their absorption minimum, respectively, at the wavelength of 1.3 &mgr;m and 1.55 &mgr;m. Modification of standard VCSEL devices operating in the wavelength range below 1 &mgr;m, however, for the required long wavelength range is not a straightforward development, since formation of adequately defined quantum wells with materials that are lattice-matched to GaAs require a highly-strained semiconductor layer, thereby rendering the VCSEL devices obtained as unreliable. Therefore, commonly employed semiconductor compounds that are lattice-matched to indium phosphide are used for semiconductor laser devices operating at a wavelength of 1.3-1.55 &mgr;m, wherein, however, the complex technology developed for GaAs-based substrates may not be directly transferred to indium-phosphide (InP) based VCSEL devices.
In view of the above-identified problems, a great effort has been made to realize a VCSEL device operating in the long wavelength range and exhibiting a relatively high output power combined with a good temperature stability. In order to meet these requirements, some serious problems have to be solved.
First, traditional indium-phosphide based materials used as an gain region in a long wavelength VCSEL device do not provide sufficient contrast in refractive index to allow the fabrication of highly reflective distributed Bragg reflectors required for the proper operation of the VCSEL device. Due to the reduced volume of the gain region of the resonator combined with not high-enough reflective mirrors, an increased operating current is required to obtain a stimulated emission. To date, therefore, only pulsed operation at room temperature is achieved.
Second, photon absorption by free charge carriers, i.e., by charge carriers that can “freely” move within the conduction band or the valency band, increases with wavelength of the photons as well as with the charge carrier concentration. In particular, in a semiconductor laser relatively high charge carrier concentrations are required which will, in combination with the longer wavelength of the photons, therefore limit the maximum achievable mirror reflectivity owing to the increased absorption of the semiconductor layers forming the Bragg reflectors.
Third, as is known from edge-emitting lasers GalnAsP (gallium/indium/arsenic/phosphorous) has a poorer gain versus temperature performance than a GaAs-based gain region due to a reduced carrier confinement and increased Auger recombination.
To overcome these technological challenges, two approaches have been proposed. The first approach involves hybrid structures that use InGaAsP/InP or InGaAs/InGaAlAs quantum wells/barriers-based gain regions and mirrors formed by depositing dielectrics or growing semiconductor materials by epitaxy. Presently, the most promising long wavelength VCSEL devices have been manufactured by wafer fusion of a wafer bearing an InGaAsP gain region and a wafer bearing AlGaAs based distributed Bragg reflectors. The wafer fusion technique, however, requires multiple substrates and is difficult to accomplish on a full wafer basis. Consequently, it is very difficult to establish a reliable fabrication process on the basis of this technology.
The second approach proposes the formation of a complete VCSEL structure in a single step by epitaxial growth. To this end, new materials appropriate for emitting at long wavelengths have been directly grown on a GaAs substrate so that AlGaAs distributed Bragg reflectors may be used in combination with these new materials forming the gain region. Emission at 1.3 &mgr;m has been shown with GalnNAs, GaAsSb quantum wells and InGaAs quantum dots. Promising wells have been obtained by using antimonide-based distributed Bragg reflectors that are lattice-matched to indium phosphide.
“Electrically-pumped single epitaxial VCSELs at 1.55 &mgr;m with Sb-based mirrors”, by E. Hall
Almuneau Guilhem
Moser Michael
Avalon Photonics AG
Fattibene Arthur T.
Fattibene Paul A.
Fattibene & Fattibene
Ip Paul
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