Folded cavity solid-state laser

Details Folded cavity solid-state laser Folded cavity solid-state laser

2002-03-26

2004-09-14

Wong, Don (Department: 2828)

Coherent light generators

Particular resonant cavity

Distributed feedback

C372S093000, C372S043010

Reexamination Certificate

active

06792026

ABSTRACT:

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Available
REFERENCE TO MICROFICHE APPENDIX
Not Available
TECHNICAL FIELD OF INVENTION
This invention relates to semiconductor diode lasers, more specifically, to solid-state semiconductor diode lasers, which have multilayered vertical optical cavities that comprise a substrate, an electrically pumped double-heterostructure light emitting diode active-region, and an optically pumped solid-state active medium; all being components typically disposed between two feedback providing contra-positioned light reflecting structures.
BACKGROUND OF THE INVENTION
Solid-state semiconductor diode lasers have numerous applications in fields as varied as the automobile industry, medicine, scientific instrumentation, and telecommunications. Solid-state semiconductor diode lasers, specifically solid-state semiconductor diode lasers having a multilayered vertical optical cavity (i.e., vertical orientation that is perpendicular to the substrate of the semiconductor diode), which have become widely known, typically, as (VCSELs) “Vertical Cavity Surface Emitting Lasers”.
Categorically, VCSEL light sources for use in gigabit-Ethernet network applications have been adopted in a remarkably short amount of time. Because VCSELs have a reduced threshold current, a circular output beam, and are inexpensive to manufacture at very high-volumes is what makes present day VCSELs particularly suitable for multimode optical fiber “Local-Area Networks” (i.e., LANs). 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, but minimal modal discrimination that allows emission into multiple transverse optical-modes. Such multi-mode VCSELs make ideal local area network laser light sources.
However, because they are inexpensive to manufacture, new VCSELs that can singularly emit at the fundamental optical transverse mode are ever increasingly being sought-out for emerging applications including long and short-haul data-communications using single-mode optical fiber, barcode scanning, laser printing, optical read/write data-heads, and modulation spectroscopy. Achieving single-mode operation in selectively oxidized VCSELs is a challenging task, simply because the inherent index confinement within these high-performance lasers is very large. VCSELs have optical-cavity lengths on the order of one-wavelength and, therefore operate within a single longitudinal optical-mode.
Nevertheless, because of their relatively large cavity diameters (i.e., roughly “5.0” to “20.0” micrometers), these lasers will usually operate in multiple transverse optical-modes. Wherein, each transverse optical-mode will possess a unique wavelength and what is typically called a transverse spatial intensity profile (i.e., intensity pattern). 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 mode) is desired.
In general, pure fundamental mode emission (i.e., a spatial intensity pattern seen as a simple circular shaped dot) within a selectively oxidized VCSEL can be attained by increasing the optical loss to higher-order transverse optical-modes relative to that of the previously mentioned fundamental lower-order transverse optical-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 will operate in a single transverse optical-mode have recently been developed.
Furthermore, the previously mentioned strategies are based either on introducing loss that is relatively greater for higher-order optical-modes, thereby relatively increasing gain for the fundamental transverse optical-mode, or on directly creating greater gain for the fundamental transverse optical-mode. Increased modal loss for higher-order optical-modes has been demonstrated by three different techniques. The first approach to modal discrimination uses an etched-surface relief structure on the periphery of the top facet, which selectively reduces the reflectivity of the top mirror for the higher-order transverse optical-modes. The advantage of this technique is that the etched ring around the edge of the cavity in the top quarterwave mirror-stack assembly can be produced during the VCSEL's fabrication using conventional dry etching, or be post processed on a completed VCSEL die using focused ion-beam etching. The disadvantages, however, of an etched-surface relief structure is that it requires careful alignment to the oxide aperture or it can increase the optical scattering loss for the fundamental transverse optical-mode, as manifested by the relatively low (i.e., less than 2-mW) single-mode output powers that have been already reported. Therefore, it would be more desirable to introduce mode-selective loss into a VCSEL's epitaxial structure to avoid extra fabrication steps and problems with self-alignment and loss thereof.
Moreover, two such techniques are the use of tapered oxide apertures and extended optical cavities within VCSELs. The first approach pursued at Sadia National Laboratories (i.e., Albuquerque, N. Mex.) is typically called gain-guided design and it is predicated on designing the profile of the oxide aperture tip to preferentially increase loss for higher-order transverse optical-modes. For example, the aperture-tip profile can be produced by tailoring the composition of (AlGaAs) “Aluminum-Gallium-Arsenide” layers, which are specifically oxidized to create an electro-optic aperture within the before mentioned VCSEL's internal cavity. A VCSEL containing a tapered oxide structure whose tip is vertically and precisely positioned at the null of the longitudinal optical standing wave that occurs within the laser can produce greater than 3-mW of single transverse optical-mode output, and greater than 30-dB of side-mode suppression. Creating this kind of structure, however, requires a detailed understanding of the specifics regarding the oxidizing process and how it is implemented using specific materials, or tapered oxide structures improperly produced will cause additional loss rather than additional gain for the fundamental transverse optical-mode.
In addition, the second approach pursued at the University of Ulm (i.e., Ulm, Germany) is typically called index-guided design and it is predicated on designing a way to increase modal discrimination by extending the optical cavity length of VCSELs, thereby increasing the diffraction loss for the higher-order transverse optical-modes. Researchers at the University of Ulm (i.e., Ulm, Germany) have reported single fundamental optical-mode operation up to 5-mW using a VCSEL designed with a “4” micrometer thick cavity spacer inserted into its vertical optical cavity. The problem, however, is that by using even-longer cavity spacers we begin to introduce multiple longitudinal optical-modes (i.e., causing what is sometimes called spatial hole burning) into the laser's system, however, single fundamental lower-order transverse optical-mode operation up to nearly 7-mW has been demonstrated using this approach. It is interesting to note that VCSELs containing multiple wavelength cavities do not appear to suffer any electrical penalty, however, careful design is required to balance the trade-offs between the modal selectivity over a VCSEL's transverse and longitudinal optical-modes.
Finally, by manipulating the modal gain of a device rather than its loss can also produce single fundamental transverse optical-mode VCSELs. A technique to spatially aperture laser gain has been recently developed at Sadia National Laboratories. The essential aspect of a VCSEL designed using this approach is found in the VCSEL's lithographically defined gain region, which is produced by an intermixing of the VCSEL's quantum-well active-region at the lateral peri

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