Coherent light generators – Particular resonant cavity – Distributed feedback
Reexamination Certificate
2002-03-28
2004-09-21
Wong, Don (Department: 2828)
Coherent light generators
Particular resonant cavity
Distributed feedback
C372S043010, C372S045013, C372S046012
Reexamination Certificate
active
06795478
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to vertical-cavity surface-emitting lasers (VCSELs) and, in particular, to current confinement in VCSELs and single transverse mode operation 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.
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by at least two laser cavity mirrors, and an optical gain medium in the laser cavity. The gain medium amplifies electromagnetic waves (light) in the cavity, i.e. provides optical gain, by the phenomenon known as stimulated emission. In semiconductor lasers, a semiconductor active region serves as the gain medium. 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 industrial and scientific applications and can be built with a variety of structures and semiconductor materials.
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.
Laser gain (or optical gain) is a measure of how well a gain medium such as an active region amplifies photons by stimulated emission. The primary function of the active region in a semiconductor laser is to provide sufficient laser gain to permit lasing to occur. The active region may employ various materials and structures to provide a suitable collection of atoms or molecules capable of undergoing stimulated emission at a given lasing wavelength, so as to amplify light at this wavelength. The active region may comprise, for example, a superlattice structure, or a single or multiple quantum well (MQW) structure.
Amplification by stimulated emission in the active region of a semiconductor laser is described as follows. The active region contains some electrons at a higher, excited state or energy level, and some at a lower, resting (ground) state or energy level. The number and percentage of excited electrons can be increased by pumping the active region with a pumping energy, such as an electrical current or optical pump. Excited electrons spontaneously fall to a lower state, “recombining” with a hole. Both radiative and non-radiative recombination events occur in the active region. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron's energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon, and will also be in phase with the original photon. Thus, when the photons produced by spontaneous electron transition interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.)
Amplification by stimulated emission requires more photons to be produced by stimulated emission than to be absorbed by lower-state electrons. This condition, known as population inversion, occurs when there are more excited (upper lasing level) electrons than ground-state (lower lasing level) electrons. If there were more lower state than upper state electrons, then more photons would be absorbed by the lower energy electrons (causing upward excitations) than would be produced by stimulated emission. When there is a population inversion, however, enough electrons are in the excited state so as to prevent absorption by ground-state electrons from sabotaging the amplification process. Thus, when population inversion is achieved, stimulated emission will predominate over stimulated absorption, thus producing amplication of light (optical gain). If there is population inversion, lasing is possible, if other necessary conditions are also present.
Population inversion is achieved by applying a sufficient pumping energy to the active region, to raise enough electrons to the excited state. In this manner, an active region amplifies light by stimulated emission. Various forms of pumping energy may be utilized to excite electrons in the active region and to achieve population inversion and lasing. For example, semiconductor lasers of various types may be electrically pumped (EP), by a DC or alternating current. Optical pumping (OP) or other pumping methods, such as electron beam pumping, may also be used. EP semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. As a result of the potential applied, charge carriers (electrons and holes) are injected from opposite directions into an active region. This gives rise to an increase in spontaneous generation of photons, and also increases the number of excited state electrons so as to achieve population inversion.
In a semiconductor laser, an active region is sandwiched between the cavity mirrors, and pumped with a pumping energy to cause population inversion. Photons are spontaneously emitted in the active region. Some of those photons travel in a direction perpendicular to the reflectors of the laser cavity. As a result of the ensuing reflections, the photons travel through the active region multiple times, being amplified by stimulated emission on each pass through the active region. Thus, photons reflecting in the cavity experience gain when they pass through the active region. However, loss is also experienced in the cavity, for example less than perfect (100%) reflectivity of the cavity mirrors introduces loss by absorption, scattering, or even extraction of the output laser beam, which can be about 1% of the coherent cavity light.
Therefore, for lasing to occur, there must be not only gain (amplification by stimulated emission) in the active region, but a enough gain to overcome all losses in the laser cavity as well as allow an output beam to be extracted, while still allowing laser action to continue. The minimum gain provided the active region that will permit lasing, given the cavity losses, is the threshold lasing gain of the laser medium. The wavelength range over which the gain spectrum of the active region exceeds this threshold gain helps define the transverse extent of the optical cavity. (For EP lasers, the lowest drive current level at which the output of the laser results primarily from stimulated emission rather than spontaneous emission is referred to as the lasing threshold current.) When the active region provides the threshold lasing gain, there will be a sufficient amount of radiative recombinations stimulated by photons, so that the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. This causes coherent light to build up in the resonant cavity formed by the two mirrors, a portion of which passes through one of the mirrors (the “exit” mirror) as the output laser beam.
Because a coherent beam makes multiple passes through the optical cavity, an interference-induced longitudinal mode structure or wave is observed. The wave along the laser cavity is a standing EM wave
Anselm Klaus Alexander
Hwang Wen-Yen
Zheng Jun
Applied Optoelectronics, Inc.
Kinsella N. Stephan
Nguyen Dung
Wong Don
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