Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2001-05-14
2003-05-06
Paladini, Albert W. (Department: 2827)
Coherent light generators
Particular active media
Semiconductor
C372S043010, C372S102000
Reexamination Certificate
active
06560259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to sources of electromagnetic laser radiation and, in particular, to unipolar semiconductor quantum cascade (QC) lasers and fabrication thereof.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., “Possibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,”
Soviet Physics Semiconductors
, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (by a DC or AC current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide.
Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation is output perpendicular to the wafer surface. One type of SEL is the vertical cavity surface emitting laser (VCSEL). The VCSEL structure usually consists of an active (gain) region sandwiched between two distributed Bragg reflector (DBR, or mirror stack) mirrors. The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). Other types of VCSELs sandwich the active region between metal mirrors. The area between the reflective planes is often referred to as the resonator, or resonance cavity.
Semiconductor diode lasers are attractive as sources of optical energy in industrial and scientific applications. For example, semiconductor diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Also, semiconductor diode lasers are monolithic devices that do not require combining a resonance cavity with external mirrors and other structures to generate a coherent output laser beam. Further, the continuous development of semiconductor lasers in the last two decades has significantly improved their maximum output power to the kilowatt range, spanning wavelengths of more than 10 &mgr;m. Semiconductor lasers are now widely used in industrial processing, telecommunications, data storage, and the like. Despite these improvements, however, semiconductor diode lasers still have a relatively low power output, as compared to other, conventional types of laser devices.
Semiconductor diode lasers, including quantum well lasers, are bipolar semiconductor laser devices. A diode laser typically has n-type layers on one side, and p-type layers on the other side, of an undoped active or core region. Such bipolar laser devices rely on transitions between energy bands in which conduction band electrons and valence band holes, injected into the active region through a forward-biased p-n junction, radiatively recombine across the bandgap. Thus, in diode lasers, the bandgap of the available active region materials essentially determines, and limits, the lasing wavelength. For example, the longer the laser wavelength needed, the smaller the required material bandgap, and vice versa. Unfortunately, the characteristics of small bandgap materials can make it difficult, expensive, or impractical to obtain lasing operation at certain desired wavelengths, such as mid-infrared (mid-IR or MIR) wavelengths.
Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced. Some of those photons travel in a direction perpendicular to the reflective planes of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times. When those photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflective planes tends to increase. A typical laser includes a small difference in reflectivity between its mirrors. The primary laser output is emitted through the reflective plane having lower reflectivity.
The aforementioned QC was initially described in U.S. Pat. No. 5,457,709, which is incorporated herein by reference in its entirety. See also U.S. Pat. Nos. 5,509,025, 5,901,168, and U.S. Pat. No. 6,055,257, which are incorporated herein by reference in their entireties. Unlike diode lasers, QC lasers are unipolar, that is, they are based on one type of carrier (typically electrons in the conduction band), which make inter-subband transitions between energy levels created by quantum confinement. In a unipolar semiconductor laser, electronic transitions between conduction band states arise from size quantization in the active region heterostructure. The inter-subband transitions are between excited states of coupled quantum wells for which resonant tunneling is the pumping mechanism.
A single active region unipolar semiconductor laser is possible, but multiple active regions may be used as well. QC lasers, for example, typically comprise an active region having a plurality (e.g., 25) of essentially identical undoped active regions, sometimes referred to as radiative transition (RT) regions. Each active (RT) region comprises a plurality of semiconductor layers, and has quantum well regions interleaved with barrier regions, to provide two or more coupled quantum wells. These coupled quantum wells have at least second and third associated energy states for the charge carriers (e.g. electrons). The second energy state is of lower energy than the third energy state, which correspond to second and third wavefunctions, respectively. The energy difference between the third and the second energy states determines the laser emission wavelength. The energy difference between second and third energy states is in turn determined by the arrangement of all the coupled quantum wells in the active region. The arrangement includes the number of quantum wells, the thickness of each individual quantum well, and the energy height and thickness of each energy barrier layer between two neighboring quantum wells.
A multilayer carrier injector or injection region, sometimes referred to as an “injection/relaxation” (I/R) or “energy relaxation” region, is disposed between any two adjacent active regions. Thus, a given active region is separated from an adjoining one by an I/R region. The I/R region, like the active region, also typically comprises a plurality of semiconductor layers. Each active region-I/R region pair (i.e., each RT-I/R pair) may also be referred to as a “repeat unit.” At least some of the layers in each I/R region are doped, and in any case, the I/R regions as well as the active regions are unipolar. The aforementioned U.S. Pat. No. 5,457,709 discloses a technique for designing a QC laser that uses the inter-subband transition between energy levels of a coupled quantum well structure and an I/R regio
Applied Optoelectronics, Inc.
Kinsella N. Stephan
Paladini Albert W.
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