Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2000-07-22
2004-03-09
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C372S093000
Reexamination Certificate
active
06704336
ABSTRACT:
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
REFERENCE TO MICROFICHE APPENDIX
Not Applicable.
TECHNICAL FIELD OF INVENTION
This invention relates to semiconductor laser diodes, specifically to such semiconductor laser diodes, which have multilayered vertically oriented cavity structures that typically comprise of a substrate used to provide growth foundation, a (LED) “Light Emitting Diode” used as pump source and to also provide gain, and two contra-reflecting mirrors used to provide optical feedback and emission wavelength selection.
BACKGROUND OF THE INVENTION
Semiconductor laser diodes, specifically semiconductor laser diodes having a multilayered vertical cavity structure (i.e., vertical orientation that is perpendicular to the substrate of the semiconductor diode) have become widely known as (VCSELs) “Vertical Cavity Surface-emitting Lasers”. However, while the present invention uses a vertically oriented LED structure to produce fundamental photonic radiation (i.e., spontaneous stimulation) and to provide gain, its feedback providing optical cavity and the physics that occur therein, are quite different from that of VCSEL laser diodes. Therefore, the present invention should be categorized as a new kind of semiconductor laser diode.
For example, the present invention, by using only one mirror in place of two mirrors to provide feedback, the present invention has redefined vertically orientated cavity design. Inspired by the present invention's unique optical physics and design, I have named this new semiconductor laser diode, for future identification, the (FCSEL) “Folded Cavity Surface-emitting Laser”.
However, at this point I would like to digress by first describing some current trends in vertically oriented laser diode design of which, the most known and widely used is the VCSEL laser diode design. These light sources have been adopted for several applications, such as gigabit-Ethernet in a remarkably short amount of time. Due to their reduced threshold current, circular output beam, inexpensive, and high-volume manufacture, VCSELs are particularly suitable for multimode optical-fiber local-area networks (i.e., LANs).
Moreover, selectively oxidized VCSELs contain an oxide aperture within its vertical cavity that produces strong electrical and optical confinement, enabling high electrical-to-optical conversion efficiency and minimal modal discrimination; allowing emission into multiple transverse optical-modes. Such multi-mode VCSEL lasers make ideal local area network laser light sources. VCSELs that emit into a single optical transverse mode are ever increasingly being sought-out for emerging applications including data communication using single-mode optical fiber, barcode scanning, laser printing, optical read/write data-heads, and modulation spectroscopy. Consequently, achieving single-mode operation in selectively oxidized VCSELs is a challenging task, because the inherent index confinement within these high-performance laser diodes is very large. VCSELs have optical-cavity lengths approximately one-wavelength and therefore, operate within a single longitudinal optical-mode. However, because of their relatively large cavity diameters (i.e., roughly 5.0 to 20.0 micrometers), these laser diodes usually operate in multiple transverse optical-modes, where each transverse optical-mode possesses a unique wavelength and what is called a transverse spatial intensity profile (i.e., intensity pattern).
Moreover, for applications requiring small spot size or high spectral purity, lasing in a single transverse optical-mode, usually the lowest-order fundamental mode (i.e., TEM-00) is desired. In general, pure fundamental mode emission within a selectively oxidized VCSEL can be attained by increasing optical loss to higher-order transverse optical-modes relative to that of the previously mentioned fundamental mode. By selectively creating optical loss for any particular mode, we increase modal discrimination, which consequently leads to a VCSELs operation in a single transverse optical-mode. Strategies for producing VCSELs that operate in single transverse optical-mode have recently been developed.
Furthermore, these strategies are based either on introducing loss that is relatively greater for higher-order optical-modes and thereby, relatively increasing gain for the fundamental transverse optical-mode, or as an alternative, creating greater gain for the fundamental transverse optical-mode. Increased modal loss for higher-order optical-modes has been successfully demonstrated using three different techniques. The first approach to modal discrimination uses an etched-surface relief located on the periphery of the top facet that selectively reduces the reflectivity of the top mirror for higher-order transverse optical-modes. The advantage of this technique is that the ring located around the edge of the cavity, etched in the top quarterwave mirror stack assembly can be produced during the VCSEL's initial fabrication by conventional dry-etching, or it can be post processed on a completed VCSEL die using focused ion-beam etching. A disadvantage, however, of etched-surface relief is that it requires careful alignment to the oxide aperture and can increase the optical scattering loss of the fundamental transverse optical-mode, as manifested by the relatively low (i.e., less than 2 mW) single-mode output powers that have been reported. Therefore, it would be more desirable to introduce mode-selective loss into the VCSEL's structure during its epitaxial deposition to avoid extra fabrication steps and to provide self-alignment problems.
Moreover, two such techniques are the use of tapered oxide apertures and extended optical cavities within the VCSEL laser diode. The first approach, which was pursued extensively at Sandia National Laboratories (i.e., Albuquerque, N.Mex.), is predicated on designing the profile of the oxide aperture tip in order to preferentially increase loss for higher-order transverse optical-modes. The aperture-tip profile is produced by tailoring the composition of the (AlGaAs) “Aluminum-Gallium-Arsenide” layers, which are oxidized during fabrication to create an aperture located within the before mentioned VCSEL. A VCSEL containing a tapered oxide whose tip is vertically positioned at a null in the longitudinal optical standing wave can produce greater than 3-mW of single-mode output, and greater than 30-dB of side-mode suppression. Creating this structure, however, requires a detailed understanding of the oxidation process, and produces additional loss for the fundamental transverse optical-mode, as well.
In addition, a second way to increase modal discrimination is to extend the optical cavity length of VCSEL itself and thus, increase the diffraction loss for the higher-order transverse optical-modes. Researchers at the University of Ulm (i.e., Ulm, Germany) have reported single-mode operation up to 5-mW, using a VCSEL constructed with a 4-micrometer thick cavity spacer inserted within the optical-cavity. The problem, however, is that by using even-longer cavity spacers can also introduce multiple longitudinal optical-modes (i.e., what is sometimes called spatial hole burning), but single-transverse-optical-mode operation up to nearly 7-mW has been demonstrated. It is interesting to note that VCSELs containing multiple wavelength cavities do not appear to suffer any electrical penalty, although careful design is required to balance the trade-offs between the modal selectivity of the transverse and the longitudinal optical-modes.
Finally, manipulating the modal gain rather than loss can also produce single-mode VCSELs. A technique, designed to spatially aperture laser-gain independently of the oxide aperture has been developed at Sandia National Laboratories. The essential aspect of these VCSELs is the lithographically defined gain region, which is produced by an intermixing of quantum-well active regions at the lateral periphery of the VCSEL's laser cavity. Typically, fabrication processes for a typical VCSEL laser diode beg
Ip Paul
Kirkpatrick & Lockhart LLP
Nguyen Phillip
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