Flexible bags – Lifting or suspending element – With provision for positioning element in nonuse location
Reexamination Certificate
2001-10-10
2003-12-30
Healy, Brian (Department: 2874)
Flexible bags
Lifting or suspending element
With provision for positioning element in nonuse location
C385S031000, C385S088000, C385S089000, C372S092000, C372S097000, C372S098000, C438S026000, C438S027000, C438S029000
Reexamination Certificate
active
06669367
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to devices that emit electromagnetic radiation and, in particular, to a laser assembly comprising an optical fiber coupled to a semiconductor laser having a bottom laser cavity mirror and a top cavity mirror.
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. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors, one of which is serves as the exit mirror. The area between the reflective planes is often referred to as the resonator, or the Fabry-Perot resonance cavity in some cases. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in the resonant cavity formed by the two mirrors. A portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors passes through the exit mirror as the output laser beam.
Various forms of pumping energy may be utilized to cause the active region to begin to emit photons. For example, semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. EP 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 (to carry away the energy lost by the transition, so as to conserve energy). Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when an electron is in a higher energy level and a photon with an energy nearly equal to the difference between the electron's energy and a lower energy interacts with the electron. In this case, the photon may stimulate the electron to fall into the lower energy state, thereby emitting a photon. The emitted photon will have the same energy as the original photon, and, if viewed as waves, there will be two waves emitted (from the electron's atom) in phase with the same frequency. Thus, when the photons produced by spontaneous electron transition photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If a sufficient number of the electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light, and thus lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes through the exit mirror as the output laser beam.
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 output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The “vertical” direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with “up” being typically defined as the direction of epitaxial growth an also the direction of emission of the output laser beam from the “top”, exit mirror. Both EP and OP VCSEL designs are possible. VCSELs can have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, a smaller divergence angle, and scalability to monolithic laser arrays. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer and at least partially packaged to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region sandwiched between two mirrors (reflectors), such as distributed Bragg reflector (DBR) mirrors. The two mirrors may be referred to as the top (exit) DBR and the bottom DBR. Other types of VCSELs sandwich the active region between metal mirrors.
A VCSEL must have a means of pumping the active region to achieve gain. For example, in an EP VCSEL, top and bottom electrical contacts are typically provided above and below the active region, respectively, so that a pumping current can be applied through the active region. Because the optical gain is low in a vertical cavity design, the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to lase.
DBRs are typically formed of multiple pairs of layers referred to as mirror pairs. DBRs are sometimes referred to as mirror stacks. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication techniques. The layers of the DBR are quarter-wave optical-thickness (QWOT) layers of alternating high and low refractive indices, where each mirror pair contains one high and one low refractive index QWOT layer. The number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity).
The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The difference in the refractive indices of adjacent layers in a DBR is referred to as the N-contrast from layer to layer. A lower N-contrast from layer to layer provides lower reflectivity, for a given number of mirror pairs. Semiconductor material typically has lower N-contrast than dielectric, and thus requires more layers for the same reflectivity. Thus, the difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a diel
Baillargeon James N.
Hwang Wen-Yen
Lin Chih-Hsiang
Murry Stefan J.
Zheng Jun
Applied Optoelectronics, Inc.
Healy Brian
Kinsella N. Stephan
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