Laser .

An acronym for Light Amplification by Stimulated Emission of Radiation. A laser is essentisly a thin, intense beam of coherent light. It differs from light emitted by conventionnal light bulbs in that traditional light is incoherent: light waves are radiated in all directions independently of each other, and the crests and troughs of the wave do not coincide with each other and therefore possess little energy. In a laser, all the light waves are emited in the same direction and with their crests and troughs aligned with each other. The beam thus has a great deal of energy. The light rays, as they travel along the beam, are kept sa close to being parallel with each other as possible and, as a result, the waves diverge very slightly. As an example, in 1962 a on – foot – wide laser beam was pointed at the moon and when it reached at the moon, illuminated a two-mile wide area.An ordinary light beam making the same journey would cover a 25000-mile-wide area.

Essentially, a laser is created by stimulating certain substances to emit light, which involves adding energy until the low-energy-level atoms have absorbed enough energy trigger an emission of enrgy in the form of electromagnetic radiation –i.e.. light. The first laser utilized ruby crystal rods.Today’s lasers utilize gases or liquids as the laser material.

Lasers have a wide range of uses in a variety of industries. In imaging and printing, fine beams of laser light are used to image light sensitive printing plates and imagesetter films, and lasers are used in computer laser printers to expose a charged metal plate in regions corresponding to image areas, allowing toner to adhere to it and transfer to the paper passing beneath it. Lasers are also used in optical discs, such as audio CDs and CD-ROMs, to either write or read the tiny pits in the surface of the discs which can then be translated into digital or analog data. Lasers are also used for a variety of surgical procedures.

The lasers was a successor to an earlier device called a maser, which stood for Microwave Amplification by stimulated Emission of Radiation. Once, lasers were merely the stuff of science fiction: now most homes in the United State have a least one.

DEVELOPMENT OF THE LASER
Albert Einstein, in 1917, hypothesized that light can be produced by atomic processes. An atom consists of a nucleus and one or more electrons in muotion around it. The electrons and the nucleus are related in terms of energy levels and the electrons normally occupy the lowest energy level, a ground state. Electrons can occupy higher energy levels, leaving some of the lower energy states vacant. Electrons can change from one energy state to another by the absorption or emission of light energy, via radiative transition. Electrons can absorb energy from the transfer of the energy of a photon directly to an orbital electron. The increase in the energy of the electron causes it to jump to a higher energy level; the atom is then said to be in an “xcited” state. Electrons can accept only the preciseamount of energy needed to move from one energy level to another. Only photons of the exact energy acceptable to the electron can be aboserded.Photons of similar energy will not be absorbed. Another means used to excite electron is where energy is supplied by collisions with electrons accelerated by an electric field. The result og excitation is that the absorption of energy places an electron in a higher energy level. The atom is said to be excited.

LASER ARCGITECTURE
A laser consists of a lasing medium a “pumping” system, and an optical cavity. The laser material must have a metastable state in which in atoms or molecules cab be trapped after receiving energy from the pumping system. The pumping systems imparts energy to the atoms, enabling them to be raised to an excited “metastable state” creating a population inversion. Optical pumping uses photons provided by a sources such as a gas flash lamp or another laser to transfer energy to the lasing material.The optical source must provid photons that correspond to the allowed transition levels of the lasing material .Collision pumping relies on the transfer of energy to the lasing material by collision with the atoms of the lasing material. Energies which correspond to the transition must be provided, often by electrical discharge in a pure gas ,or gas mixture. Chemical pumping systems use the binding energy released in chemical reactions to raise the lasing material to the metastable state. An optical cavity provides the amplification in the laser and selects the photons traveling in the desired direction. As the first atom or molecule in the same state via stimulated emission. If the photons are traveling in a direction that leads to the valls of the lasing material, a rod or tube, they are lost and amplification terminates. If one of the decaying atoms releases a photon parallel to the axis of the lasing rod or tube. The reflected photons pass back through the material triggering further emissions along the same path, which are reflected by mirrors at the ands of the lasing material. As amplification or gain exceeds the losses in the cavity, laser oscillation occurs, and a concentrated beam of coherent linght is formed. Lasers are commonly designated by the lasing material amployed:
Gas
Dye
Solid state
Semiconductor

Solid-state lasers employ a lasing material distributed in a solid matrix. An exaple is the Neodymium-YAG laser. The term YAG is an abbreviation for the crystal Yttrium Aluminum Granet, which serves as the host for Neodymium ions. This laser emits an infrared beam at the wavelength of 1.064 micrometers or 1.064 nanometers (1 nm = 10-9m). Internal or external devices convert the output to visible or ultraviolet wavelength.

Gas lesers use a gas or a mixture of gases within a tube. The most common gas laser uses a mixture of helium and neon (HeNe) with a primary output of 632.8 nm, which is a visible red color. It was first developed in 1961 and has proved to be the forerunner of a whole family of gas lasers. Gas lasers are similar in construction and behavior. For example, the CO2 gas laser radiates at 10.6 micrometers in the far-infrared spectrum. Argon and krypton gas lasers operate with multiple-frequency emissions principall in the visible spectra. The wavelengths of argon lasers are 488nm and 154nm.

Dye lasers use a laser medium that is a compiex organic dye in liquid solution or suspension. The major feature of these lasers is their “tunability”. Choice of the dye and its concentration allaws the production of laser light over a broad range of wavelength in or near the visible spectrum. Dye lasers commonly employ optical pumping, although some resution pumping common dye is Rhodamne 6G, which provides tenability over a 200-nm bandwidth in the red portion (620 nm) of the spectrum. Semiconductor lasers (also referred to as diode lasers) should not be confused with solid state lasres. Semiconductor devices consist of two layers of semiconductor material sandwiwh together.There lasres are small and modest power but may be built into larger arrays. A common diode laser with an amission of 840 nm.

Continuous-wave lasers operate with a stable average beam power. In higher-power systems, the power can be adjusted, but in low-power gas lasers, such as HeNe, the power level is fixed by design, and performance degrades with use. Single-pulsed (normal mode) lasers have pulse durations of a few hundred microseconds to a few millisec-onds, refered to as along pulse or normal mode. These lasers result from an intracavity delay that allows the laser media to strore a maximum of potential energy. Under optimum gain conditions, emission accurs in single pulses of 10-8 second time units.

Repetitively pulses or scanning lasers involve pulse lasers operating at a fixed (or variable) pulse rate from a few pulses to 20.000 pulses per second. The direction of a continuous wave laser can be scanned with optical systems to produce the equivalent of a repetitively pulse output. Mode-locked lasers operate as a result of the resonant modes of the optical cavity, which affect the characteristics of the output beam. When the phases of different frequency modes are synchro-nized, or “locked together”, the different modes will interfere with one another to generate a heat effect. The result is laser output as regularly spaced pulsations.