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Laser physics

A helium-neon laserKastler-Brossel Laboratory at Univ. Paris 6. The pink-orange glow running through the center of the tube is from the electric discharge which inadvertently produces incoherent light, just as in a neon tube. That glowing plasma however also acts as the gain medium through which the internal beam passes as it is reflected in between the two mirrors. Laser radiation output from the front mirror can be seen to produce a tiny (about 1mm in diameter) intense spot on the screen to the right. Although it is a deep and pure red color, spots of laser light are so intense that cameras are typically overexposed and distort their color, often appearing more white. demonstration at the

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 by either absorbing or 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.

Spectrum of a helium neon laser illustrating its very high spectral purity (limited by the measuring apparatus). The .002 nm bandwidth of the lasing medium is well over 10,000 times narrower than the spectral width of a light-emitting diode (whose spectrum is shown here for comparison), with the bandwidth of a single longitudinal mode being much narrower still.

The optical resonator is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser. The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting back on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption. If the gain (amplification) in the medium is larger than the resonator losses, then the power of the recirculating light can rise exponentially. But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power the net gain (gain times loss) reduces to unity and the gain medium is said to be saturated. In a continuous wave (CW) laser, 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 applied pump power is too small, the gain will never be sufficient to overcome the resonator losses, and laser light will not be produced. 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 in a spatial mode supported by the resonator will pass more than once through the medium and receive substantial amplification.

The beam in the cavity and the output beam of the laser, when travelling in free space (or a homogenous medium) rather than waveguides (as in an optical fiber laser), can be approximated as a Gaussian beam in most lasers; such beams exhibit the minimum divergence for a given diameter. However some high power lasers may be multimode, with the transverse modes often approximated using Hermite-Gaussian or Laguerre-Gaussian functions. It has been shown that unstable laser resonators (not used in most lasers) produce fractal shaped beams. Near the beam "waist" (or focal region) it is highly collimated: the wavefronts are planar, normal to the direction of propagation, with no beam divergence at that point. However due to diffraction, that can only remain true well within the Rayleigh range. The beam of a single transverse mode (gaussian beam) laser eventually diverges at an angle which varies inversely with the beam diameter, as required by diffraction theory. Thus, the "pencil beam" directly generated by a common helium-neon laser would spread out to a size of perhaps 500 kilometers when shone on the Moon (from the distance of the earth). On the other hand the light from a semiconductor laser typically exits the tiny crystal with a large divergence: up to 50°. However even such a divergent beam can be transformed into a similarly collimated beam by means of a lens system, as is always included, for instance, in a laser pointer whose light originates from a laser diode. That is possible due to the light being of a single spatial mode. This unique property of laser light, spatial coherence, cannot be replicated using standard light sources (except by discarding most of the light) as can be appreciated by comparing the beam from a flashlight (torch) or spotlight to that of almost any laser.

The mechanism of producing radiation in a laser relies on stimulated emission, where energy is extracted from a transition in an atom or molecule. This is a quantum phenomenon discovered by Einstein who derived the relationship between the A coefficient describing spontaneous emission and the B coefficient which applies to absorption and stimulated emission. However in the case of the free electron laser, atomic energy levels are not involved; it appears that the operation of this rather exotic device can be explained without reference to quantum mechanics.

World Humanitarian Day

On 11 December 2008, the United Nations General Assembly made history when it adopted the Swedish-sponsored GA Resolution A/63/L.49 on the Strengthening of the Coordination of Emergency Assistance of the United Nations, that amongst other important humanitarian decisions, decided to designate 19 August as World Humanitarian Day (WHD). The Resolution gives, for the first time, a special recognition to all humanitarian and United Nations and associated personnel who have worked in the promotion of the humanitarian cause and those who have lost their lives in the cause of duty and urges all Member States, entities of the United Nations within existing resources, as well as the other International Organizations and Non-Governmental Organizations to observe it annually in an appropriate way. It marks the day on which the then Special Representative of the Secretary-General to Iraq, Sérgio Vieira de Mello and 21 of his colleagues tragically made the ultimate sacrifice in the cause of duty following the bombing of the UN Headquarters in Baghdad.


The designation of 19 August as World Humanitarian Day is the outcome of the relentless efforts of the Sérgio Vieira de Mello Foundation and his family working closely with the Ambassadors of France, Switzerland, Japan and Brazil in both Geneva and New York to table and steer the draft Resolution through the General Assembly. The Foundation conveyed its deep gratitude to the United Nations General Assembly and all Member States for the worthy gesture of recognition that has ensured that the tragic loss of Vieira de Mello and his 21 colleagues and all humanitarian personnel who have made the ultimate sacrifices in relieving the suffering of victims of humanitarian crises have not been in vain.

A national of Brazil, Sérgio Vieira de Mello dedicated a lifetime spanning over thirty years in the United Nations, serving in some of the most challenging humanitarian situations in the world to reach the voiceless victims of armed conflict, alleviate their suffering and draw attention to their plight. His death together with 21 colleagues on 19 August 2003 in Baghdad, deprived the victims of armed conflict worldwide of a unique humanitarian leader of unmatched courage, drive and empathy who championed their cause fearlessly and etched their plight on the world map. The tragic event also robbed the humanitarian community of an outstanding humanitarian leader and intellectual whose thinking, philosophy, dynamism and courage inspired all and remains a timeless legacy for coming generations to emulate.

Mindful of this legacy, in 2006 the Vieira de Mello family and a group of close friends founded the Sergio Vieira de Mello Foundation dedicated to continue his unfinished mission of encouraging dialogue between communities and relieving the plight of victims of humanitarian crises. The Foundation is dedicated to supporting initiatives and efforts to promote dialogue for peaceful reconciliation and co-existence between peoples and communities divided by conflict through an annual Sergio Vieira Mello Award, an Annual Sergio Vieira Mello Memorial Lecture, a Sergio Vieira de Mello Fellowship and advocating for the security and independence of humanitarian actors, wherever they may be operating and whomever they may be operating for. The Foundation views the World Humanitarian Day as a befitting tribute to all humanitarian personnel who have made the ultimate sacrifices to make the world a better place for all victims of humanitarian crises and an encouragement to all their serving colleagues to aspire to even greater heights in accomplishing that laudable goal.


The Sérgio Vieira de Mello Foundation is committed to working closely with all Governments, the United Nations, International Organizations and Non-Governmental Organizations to give the Word Humanitarian Day a meaningful observance every year. The UN Office for the Coordination of Humanitarian Affairs is leading efforts to plan and guide the observance of the Day that will be commemorated world wide by Governments, the United Nations and International Humanitarian Organizations and NGOs. The Day will be commemorated for the first time on 19 August 2009. Details of the organization and the range of events that will mark the Day will be posted in this section as they become available.

Sex Machines Museum Wanna Visit !

Three BDSM masks on display in the Sex Machines Museum.

Sex Machines Museum (abbreviated SMM) is a sex museum in Prague, Czech Republic which has a collection of sex devices. Established in 2002, it is located near the Old Town Square. The official website of the Sex Machines Museum describes itself as "an exposition of mechanical erotic appliances, the purpose of which is to bring pleasure and allow extraordinary and unusual positions during intercourse." It is the only sex museum in the world solely dedicated to sex machines.

The three floored museum has a collection of approximately 200 gadgets, many of which are accompanied by flexible dummies for better understanding. Some of the appliances were made as early as the 16th century. Its collections include body harnesses and "copulation tables" the purpose of which were to facilitate unconventional, even weightless, sex positions, instruments for the stimulation of "penile, scrotal, anal, vaginal and clitoral tissue" including a vibrator, wicked finger-spikes, "coercive" chairs designed for "absolute domination", an Asian "Magic Box" palanquin which has sliding peepholes, throne chairs with a hole in the seat to facilitate oral sex, chastity belts with clawed teeth which dates back to the 1580s, iron corsets etc. There is an anti-masturbation appliance for boys displayed in the museum which was made in France during the 1920s. It contained an electronic ring which was placed on the penis. The ring automatically switched on when there was an erection so that the boy's parents could become aware. Shoes worn by ancient Greek prostitutes are displayed in the museum. These shoes had the sentence "follow my steps" engraved on the soles so that they could leave an imprint on the ground. It also has a collection of erotic clothing. The art gallery in SMM has collection of images pertaining to human sexuality. There is a theatrette in the museum which shows some of the world's earliest pornographic films directed in Spain during the 1920s.

After the opening of the museum, city officials in Prague criticized it for what they viewed to be its "disagreeable" content. This increased the popularity of the Sex Machines Museum among tourists.