Laser physics

The gain medium of a laser is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission. It can be of any state: gas, liquid, solid or plasma. The gain medium absorbs pump energy, which raises some electrons into higher-energy ("excited") quantum states. Particles can interact with light both by absorbing photons or by emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved and the amount of stimulated emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. By itself, this makes an optical amplifier. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.
The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design.
The optical cavity, a type of cavity resonator, contains a coherent beam of light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be saturated. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification.
The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an optical fiber laser), are, at best, low order Gaussian beams. However this is rarely the case with powerful lasers. If the beam is not a low-order Gaussian shape, the transverse modes of the beam can be described as a superposition of Hermite-Gaussian or Laguerre-Gaussian beams (for stable-cavity lasers). Unstable laser resonators on the other hand, have been shown to produce fractal shaped beams.[4] The beam may be highly collimated, that is being parallel without diverging. However, a perfectly collimated beam cannot be created, due to diffraction. The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a helium-neon laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon. By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well.
Although the laser phenomenon was discovered with the help of quantum physics, it is not essentially more quantum mechanical than other light sources. The operation of a free electron laser can be explained without reference to quantum mechanics.

LASER DESIGNS

Main article: Laser construction
A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.
Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.
The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

Terminology

The word laser was originally spelled LASER and is an acronym for light amplification by stimulated emission of radiation. The word light in this phrase is used in the broader sense, referring to electromagnetic radiation of any frequency, not just that in the visible spectrum. Hence there are infrared lasers, ultraviolet lasers, X-ray lasers, etc. Because the microwave equivalent of the laser, the maser, was developed first, devices that emit microwave and radio frequencies are usually called masers. In early literature, particularly from researchers at Bell Telephone Laboratories, the laser was often called the optical maser. This usage has since become uncommon, and as of 1998 even Bell Labs uses the term laser.[2]
The back-formed verb to lase means "to produce laser light" or "to apply laser light to."[3] The word "laser" is sometimes used to describe other non-light technologies. For example, a source of atoms in a coherent state is called an "atom laser."

ELECTROMAGNETIC RADIATIONS

A laser is a device that emits light (electromagnetic radiation) through a process called stimulated emission. Laser light is usually spatially coherent, which means that the light either is emitted in a narrow, low-divergence beam, or can be converted into one with the help of optical components such as lenses. More generally, coherent light typically means the source produces light waves that are in step. They have the same frequencies and identical phase[1]. The coherence of typical laser emission is a distinctive characteristic of lasers. Most other light sources emit incoherent light, which has a phase that varies randomly with time and position. Typically, lasers are thought of as emitting light with a narrow wavelength spectrum ("monochromatic" light). This is not true of all lasers, however: some emit light with a broad spectrum, while others emit light at multiple distinct wavelengths simultaneously.

IBM Drops Employee Co-Pay for Primary Care Visits

In what's seen as a highly unusual move, IBM says it will stop requiring employees to shell out a $20 co-payment when they see a primary care physician.

The company says the decision will save costs by encouraging employees to be seen and treated by primary care physicians sooner, thus reducing the likelihood of later expensive visits to emergency departments and specialists, theWall Street Journalreported.

The policy change "is designed to encourage people to get fixed early. .... We'd rather diagnose a situation and deal with it quickly as opposed to it becoming chronic," said Randy MacDonald, senior vice president for human resources.

IBM's decision is "very unusual," said Helen Darling, president of the National Business Group on Health, a trade group representing large employers. "The number of employers who cover primary-physician visits without a co-pay is miniscule," she told theWall Street Journal.

IBM is one of the largest employers in the United States, and its actions sometimes begin new trends.

California Gives $230 Million for Stem Cell Research

Embryonic stem cells are the focus of only four of 14 projects that received $230 million in grants from California's stem cell research program. The other projects use less controversial adult stem cells or conventional drugs designed to kill cancer stem cells, which are believed to give rise to tumors.

Wednesday's announcement about the funding to state universities and companies is seen as tacit acknowledgement that it will be a long time before the full potential of human embryonic stem cells in treating human diseases is achieved,The New York Timesreported.

Recipients of the grants are supposed to have a therapy ready for initial human testing within four years.

People don't care about what type of stem cells are used as long as researchers find treatments for diseases such as cancer and AIDS, according to officials of the 10-year, $3 billion program that was launched by California in 2004,The Timesreported.

FDA Panel Recommends First Non-Drug Asthma Treatment

A new technology from a small California-based company should be approved as the first non-drug treatment for asthma, a U.S. Food and Drug Administration advisory panel recommended Wednesday.

Asthmatx's Alair System employs bronchial thermoplasty, which uses radiofrequency wave-generated heat to burn away lung tissue that impairs breathing and causes wheezing and coughing spasms, theAssociated Pressreported.

The radiofrequency waves are delivered via a catheter controlled by a respiratory specialist. The procedure, performed over three sessions of a half hour each, is appropriate only for adult patients with severe asthma that doesn't respond to drug treatment.

The FDA panel voted six to one to recommend approval of the new system under certain conditions, including long-term safety monitoring of patients, theAPreported. The FDA usually follows the advice of its advisory panels.

The Alair System is already approved in Europe. If the FDA does approve the system, it may be available in the United States in the first half of 2010.

Dental Costs Lowest in Georgia and Ohio

Dental care for people in Georgia and Ohio costs almost $150 less than the U.S. average of $607 a year, says a federal government study released Thursday.

The average annual expenditure for dental care in Georgia was $466, while in Ohio it was $474, said the latestNews and Numbersfrom the Agency for Healthcare Research and Quality.

Among the other findings from the analysis of average annual dental expenditures in the 10 states with the highest populations in 2006:
The highest proportion of residents with dental expenses (52.5 percent) was in Michigan and the lowest was in Texas (30 percent).
The national average for out-of-pocket payment for dental care was 49 percent. People in Florida paid more (62.5 percent) and those in Pennsylvania paid less (42 percent).
Nationally, private insurers paid 43 percent of all dental expenditures.

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Chinese Drywall Contains Higher Chemical Content

Chinese-made drywall has higher amounts of some chemicals than American-made drywall, say U.S. government agencies that have investigated reports of health problems, foul smells and corrosion by owners of homes with the Chinese product.

The Environmental Protection Agency and other departments have analyzed the drywall and say further study is needed to determine if there's a direct link between the problems and the wallboard, theAssociated Pressreported.

During the peak of the U.S. housing boom, materials became scarce, and construction companies imported millions of pounds of Chinese-made drywall, which ended up in thousands of homes.