Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-11-17
2003-01-14
Leung, Quyen (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S096000, C372S097000, C257S081000
Reexamination Certificate
active
06507595
ABSTRACT:
The present invention relates to a vertical-cavity surface-emitting laser (VCSEL) device comprising a plurality of VCSEL elements arranged on a common substrate, each VCSEL element comprising first mirror means having a first reflectivity and second mirror means having a second reflectivity at a predetermined wavelength for forming an optical resonator for said wavelength, and a laser active region disposed between said first and second mirror means.
Semiconductor laser devices steadily gain in importance in a plurality of industrial applications. In particular, in the fields of gas spectroscopy, coupling of laser light into optical fibers, and in communication systems where a high transmission rate is required, semiconductor laser devices with high spectral purity, i.e. with single mode output radiation in the longitudinal as well as the transverse directions, are highly desirable. Especially, vertical-cavity surface-emitting lasers (VCSEL) represent an important development, since the possibility of manufacturing a large plurality of such laser devices on a single semiconductor substrate provides laser devices with high efficiency and low power consumption in conjunction with low manufacturing costs. These laser devices inherently lase in a single longitudinal mode due to their small longitudinal extension of the laser active region (some hundreds of nanometers). When, however, a high output power from a single device is required, the lateral extension of this device has to be increased, thereby reducing the spectral purity of the laser output, since then the beam quality suffers from the competition of many transverse radiation modes. Accordingly, the highest possible single mode output power from a VCSEL is limited (currently the maximum value achieved is 4.8 mW), since the size of the VCSEL must remain small to restrict emission to a single fundamental transverse mode.
In order to achieve increased output power while maintaining a well-defined single transverse mode which is desirable for a variety of applications such as laser printing, material treatment, or optical pumping, so-called “phase coupled arrays” have been developed and investigated during the last years. In such a phase coupled array, usually the top or bottom surface of a laterally widely extending VCSEL is divided into a plurality of laser elements by means of a grid-like structure, typically formed of metal. The thickness of the grid bars separating adjacent laser elements are selected so as to allow the electric fields of adjacent elements to couple to each other. Since, in general, top and bottom distributed Bragg reflectors as well as the laser active region are provided common to all single laser elements, and a current is supplied to the common active region by means of the conductive grid bars, the laser elements are no longer individually addressable. Accordingly, the phase coupled array can also be considered a laterally large VCSEL device emitting in a coherent supermode, wherein the nodes of the electric field are defined by the grid structure on the top or bottom surface of the VCSEL.
In the early 1990's, phase coupled arrays were demonstrated for the first time and, in recent developments, have shown very promising behavior in pulsed operation with more than 500 mW of a single mode peak output power in, for example, an 8×8 array, as disclosed in “Applied Physical Letters” Vol. 61, 1160 (1992).
In order to provide for a phase coupled array in a VCSEL, a variety of possibilities have been practiced in the prior art.
In IEEE “Journal of Quantum Electronics,” Vol. 26, No.11, November 1990, a phase coupled array is described having a metal grid inside the cavity defining areas of low reflectivity. Subsequently, a dielectric mirror has been deposited after the formation of the metal grid and this dielectric and serves as the outcoupling mirror of the laser device. This fabrication technology, however, is quite complicated and the device exhibits during operation a mixture of the lowest order and several high order modes, so that this approach does not seem to be very promising.
In “Optics Letters,” Vol. 18, No.5, Mar. 1, 1993, a VCSEL is disclosed having a metal grid applied to the top of a complete VCSEL structure, including two semiconductor distributed Bragg reflectors. However, the reflectivity variation induced by the metal grid alone, is too low to stabilize the highest order transverse mode for CW operation.
In “Applied Physical Letters,” Vol. 60, 1535, 1992, a bottom emitting VCSEL structure is described, having a semiconductor bottom mirror and a hybrid semiconductor/gold top mirror. The reflectivity of the top mirror is fine-tuned with two different metalizations, wherein highly reflective gold is evaporated on the laser elements, while less reflective TiAu or Cr/Au is used for the grid which defines the individual laser elements. Since no light can escape through the top metals, this technology is only appropriate for bottom emitters.
In “Applied Physical Letters,” Vol. 58, 890, 1991,a VCSEL is disclosed, wherein a grid is etched into the top distributed Bragg reflector. According to this technology, no current injection is possible and, hence, the device is merely able to be operated with optical pumping.
It is, therefore, an object of the present invention to provide a VCSEL device having a high output power with a defined single transverse radiation mode, whereby the aforementioned disadvantages of the prior art are avoided.
According to the present invention, there is provided a vertical-cavity surface-emitting laser device comprising a plurality of VCSEL elements arranged on a common substrate, each VCSEL element comprising first mirror means and second mirror means each having a predefined reflectivity at a predetermined wavelength, for forming an optical resonator for said wavelength, a laser active region disposed between said first and second mirror means, and a grid layer arranged over said first mirror means, said grid layer having a plurality of openings corresponding to the respective VCSEL elements, said VCSEL device being characterized in that it comprises a contact layer having a predetermined thickness, said contact layer being interposed between and adjacent to each of said first mirror means and said grid layer, wherein an optical thickness of said contact layer and a reflectivity and an absorption of said grid layer is selected so as to provide an effective reflectivity of each of said first mirror means depending on said grid layer and being different for areas covered by the grid and areas corresponding to said grid openings.
According to the present invention, the employment of the contact layer in combination with the structure of the grid layer, i.e. the geometric structure as well as the composition and the thickness thereof, provides simple means for a significant variation of the reflectivity of the first mirror means. Due to the varying reflectivity along the lateral dimension of the VCSEL device, the loss within the cavity varies accordingly and, hence, a single transverse radiation mode is sufficiently stabilized. Therefore, the VCSEL device of the present invention allows, contrary to the prior art devices which are described to be operated only in a pulsed mode, the operation with constant current and continuous wave (cw) thereby insuringe high continuous output power exhibiting a single transverse radiation mode.
Preferably, said first and second mirror means are provided as Bragg reflectors which are common to all of said VCSEL elements. By this measure, a high density of VCSEL elements may be provided, and, at the same time, the manufacturing process for such a device is considerably simplified. Moreover, manufacturing of such devices is compatible to standard fabrication methods such as selective oxidation, mesa etching, and proton implantation.
Advantageously, the thickness of said contact layer is adjusted such that the reflectivity of said first mirror means is reduced in regions covered by the grid, compared to regions corresponding to
Brunner Marcel
di Sopra Fabrice Monti
Kapon Elyahou
Avalon Photonics
Leung Quyen
Moser Patterson & Sheridan
Rodriguez Armando
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