LASERS A laser is a device that produce a very narrow intense beam of monochromatic coherent light. The emitted beam is a nearly perfect plane wave. The action of a laser is based on quantum theory.

STIMULATED EMISSION See Fig1. (a) this is a resonant situation. we have seen that energy hf corresponds to the energy difference between an occupied energy level of the atom and an available excited state it may of course jump spontaneously to the Lower state with the emission of a hf Fig.1

Photon. However ,if a photon with This same energy strikes the excited atom ,it can stimulate the atom to make the transition sooner to the lower state see Fig. (b) This phenomenon is called Stimulated emission, and it can be seen that not only do we still have the original photon, but also a second one of the hf Fig.1

Same frequency as a result of the atom’s transition And these two photons are exactly in phase ,and they are moving in the same direction ,this is how coherent light is produced in a laser. Hence the name “laser”, which is an acronym for light amplification by stimulated emission of radiation .

Normally ,most atoms are in the lower state so incident photons will mostly be absorbed in order to obtain the coherent light from stimulate emission, two conditions must be satisfied. First: the atoms must be excited to the higher state. an inverted population is needed one in which more atoms are in the upper state than in the lower state .

Second: The higher state must be a Metastable state -- a state in which the electrons remain longer than usual so that the transition to the lower state occurs by stimulated emission rather than spontaneously. For now, we assume that the atoms have been exited to an upper state. See Fig2.is a schematic diagram of a laser: the “lasting” material is a

Long narrow tube at the end of which are two mirrors ,one of which is partially transparent (perhaps 1 or 2 percent ).some of these is the atom shown on the far left in Fig 2. Mirror Partially transparent mirror Fig.2

If the emitted photons strikes another atom in the exited state, it stimulates this atom to emit a photon of the same frequency, moving in the same direction ,and in phase with it. these two photons then move on to strike other atoms causing more stimulated emission. As the process continues, the number of photons multiplies. When The photons strike the end mirrors, most are reflected

Back ,and as they move in the opposite direction ,they continue to stimulate more atoms to emit photons. As the photons move back and forth between the mirrors a small percentage passes through the partially transparent mirror at one end. These photons make up the narrow coherent external laser beam.

In a ruby laser, the lasing material is a ruby rod consisting of with THE RUBY LASER In a ruby laser, the lasing material is a ruby rod consisting of with a small percentage of aluminum ( ) atoms replaced by chromium ( )atoms. The atoms are the ones involved in lasing. The atoms are excited by strong flashes of light of wavelength 550nm, which corresponds to a photon energy of 2.2ev.shown as Fig.3 , the atoms are excited from

State to state . this process is called optical pumping. The atoms quickly decay either back to or to the intermediate state ,which is metastable with a lifetime of about s atoms can be forced into the state than are in the state. Thus we have the inverted population needed for lasing . As soon as a few atoms in the state

jump down to , they emit photons that produce stimulated emission of the other atoms and the lasing action begins. A ruby laser thus emits a beam whose photons have energy 1.8ev and a wavelength of 694.3nm(for “ruby-red” light) Fig.3 Fig.3

Helium-Neon LASER In a helium-neon laser, the lasing material is a gas, a mixture of about 15 percent He and 85 percent Ne. In this laser, the atoms are excited by applying a high voltage to the tube so that an electric discharge takes place within the gas. In the process ,some of the He atoms

are raised to the metastable state shown in Fig.4 which corresponds to a jump of 20.61ev.Now Ne atoms have an excited state that is almost exactly The same energy above the ground State, 20.66ev. The atoms do collision Helium Neon 20.61ev 20.66ev 1. 96ev Fig.4

Not quickly return to the ground state by spontaneous emission, but instead often give their energy to a Neon atom when they collide. In such a collision, the He drops to the ground state and the Neon atom is excited to the state .In this manner, the state in Neon-which is Metastable- becomes more populated Than the level.

CONCAVE MIRRORS AND BREWSTER’S WINDOWS Four commonly used configurations Are shown in Fig.5.The hemispherical arrangement at the Center, with a concave mirror at one end only, has its center of curvature at the center of the reflecting surface of the plan mirror . The spherical Mirror arrangement has the two

Centers of curvature falling together at the center point C of the configuration. Hemispherical Plane parallel Fully silvered Partly silvered Spherical Confocal C Fig.5

The confocal arrangement has the two centers at the centers of the opposite mirror faces. One mirror is usually fully silvered, and the other is partially silvered or fully silvered with a clear spot at its center. By Titling these plates or beveling the ends of a solid laser to the polarizing angle ,the windows or ends will have a 100 percent transmission for light whose electric

Vector is parallel to the plane of incidence see Fig.6 gas Brewster Window Fully silvered Polarized light Partly silvered (a) solid (b) Fig. 6

The normal component is partially reflected at each interface with traversal of the laser. The laser beam is thereby polarized, as with a pile of plates. The polarizing angle is given by where n is the refractive index of the medium. .

FREQUENCY DOUBLING In 1961 four physicists at the University of Michigan focused a beam from a ruby laser emitting 3-kw pulses of red light of wavelength 6943Ǻ on to a quartz crystal, thereby producing an observable number of photons of half the wavelength, of 3471.5Ǻ This new wavelength, which lies in the ultraviolet region of the spectrum,

is exactly double the frequency of the laser’s red light. The possibility that this was fluorescent light could be ruled out since it was emitted in a highly directional beam parallel to the incident light. The classical explanation of these phenomena involves ionization of the loosely bound valence electrons, which in many Crystals are shared by other atoms in

The bonding of the structure. An atom giving up an electron to its neighbor leaves it with a net positive charge, and the neighbor with an extra electron has a net negative charge. As light waves pass through, these ions respond to the associated electric and magnetic fields by being set in to vibration with the source frequency. When the incident light intensity is extremely high, as it is in a laser beam, the induced atomic vibrations are nonlinear in their response, just as they are with loud sounds, and higher harmonics are generated. The second harmonic is far more

intense than higher modes. From the point of View of quantum theory, when two photons interact with matter, both energy and momentum are conserved in producing a single photon.

OTHER LASERS Other types laser include :chemical lasers, in which the energy input comes from the chemical reaction of highly reactive gases; dye lasers ,whose frequency is tunable; gas lasers, capable of high power output in the infrared; rare-earth solid- state lasers such as the high-power neodymium: YAG laser; and the junction laser in which the transitions occur between the bottom of the conduction band and the upper part of the valence band .

LASER SAFETY Laser light varies in intensity from a milliwatt for an inexpensive He-Neon laser to many kilowatts for a laser. Laser injuries have been few, and their dangers highly debatable. However, the greatest danger is the inadvertent direction of an undiverged laser beam directly in to the eye. The weak beam from a continuous He-Neon laser is probably of little danger, since eyelids can close upon sudden exposure. More intense beams, and especially pulsed beams, can cause injury,

Due primarily to the ability of the eye to focus the parallel beam onto a small area of the retina . Good safety practice in the presence of high- powered lasers involves the use of filtering glasses and shields and awareness that a laser beam incident upon a specular reflecting surface can redirect the beam undiminished in intensity.

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