 LIGHT  AMPLIFICATION BY  STIMULATED  EMISSION OF  RADIATION

 Laser is a narrow beam of light of a single  wavelength (monochromatic) in which each  wave is in phase (coherent) with other near it.

1.Directional 2.Coherent 3.Monochromatic 1.Many wavelengths 2.Multidirectional 3.Incoherent

 Atom composed of a nucleus and electron cloud  If an incident photon is energetic enough, it may be absorbed by an atom, raising the latter to an excited state.  It was pointed out by Einstein in 1917 that an excited atom can be revert to a lowest state via two distinctive mechanisms:  Spontaneous Emission and  Stimulated Emission.

 Each electron can drop back spontaneously to the ground state emitting photons.  Emitted photons bear no incoherent. It varies in phase from point to point and from moment to moment.  e.g. emission from tungsten lamp.

 Each electron is triggered into emission by the presence of electromagnetic radiation of the proper frequency. This is known as stimulated emission and it is a key to the operation of laser.  e.g. emission from Laser

 Let us consider an atom that is initially in level 1 and interacts with an electromagnetic wave of frequency n. The atom may now undergo a transition to level 2, absorbing the required energy from the incident radiation. This is well-known phenomenon of absorption.

 Generally electrons tends to (ground state). What would happen if a substantial percentage of atoms could somehow be excited into an upper state leaving the lower state all empty? This is known as a population inversion. An incident of photon of proper frequency could then trigger an avalanche of stimulated photon- all in phase (Laser).

 Consider a gas enclosed in a vessel containing free atoms having a number of energy levels, at least one of which is Metastable.  By shining white light into this gas many atoms can be raised, through resonance, from the ground state to excited states.

 E1 = Ground state,  E2 = Excited state (short life time ns),  E3 = Metastable state (long life time from ms to s).

 According to the active material: solid-state, liquid, gas, or semiconductor lasers.  According to the wavelength: Infra-red (IR), Visible, Ultra-violet (UV) or X-ray Lasers.

 Solid-state lasers have lasing material distributed in a solid matrix (such as ruby or Nd- YAG). Flash lamps are the most common power source. The Nd-YAG laser emits infrared light at nm.  Semiconductor lasers, sometimes called diode lasers, are p-n junctions. Current is the pump source. Applications: laser printers or CD players.

 Dye lasers use complex organic dyes, such as Rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.  Gas lasers are pumped by current. Helium- Neon (He-Ne) lasers in the visible and IR. Argon lasers in the visible and UV. CO2 lasers emit light in the far-infrared (10.6 mm), and are used for cutting hard materials.

 Example: Ruby Laser  Operation wavelength: nm (IR)  3 level system: absorbs green/blue  Gain Medium: crystal of aluminum oxide (Al 2 O 3 ) with small part of atoms of aluminum is replaced with Cr 3+ ions.  Pump source: flash lamp  The ends of ruby rod serve as laser mirrors

 A helium–neon laser or He-Ne laser, is a type of gas laser whose gain medium consists of a mixture of helium and neon(10:1) inside of a small bore capillary tube, usually excited by a DC electrical discharge. The best-known and most widely used He-Ne laser operates at a wavelength of nm, in the red part of the visible spectrum.

 The energy or pump source of the laser is provided by a high voltage electrical discharge passed through the gas between electrodes (anode and cathode) within the tube(figure 7). A DC current of 3 to 20 mA is typically required for CW operation. The optical cavity of the laser usually consists of two concave mirrors or one plane and one concave mirror, one having very high (typically 99.9%) reflectance and the output coupler mirror allowing approximately 1% transmission.

 High Level Lasers –Surgical Lasers –Hard Lasers –Thermal –Energy ( ) mW

 Low Level Lasers –Medical Lasers –Soft Lasers –Subthermal –Energy (1-500) mW –Therapeutic (Cold) lasers produce maximum output of 90 mW or less ( ) nm light

 Treatment cover everything from the ablation of tissue using high power lasers to photochemical reaction obtained with a weak laser.  Diagnostics cover the recording of fluorescence after excitation at a suitable wavelength and measuring optical parameters.

Energy is reflected, transmitted, absorbed and scattered Lambert Beer law I = Io 10 -  X α= absorption coefficient X = thickness of material Io = incident intensity I = transmitted intensity Extinction length = 1/α = L; where 90% of the intensity is absorbed.

 Laser light waves penetrate the skin with no heating effect, no damage to skin & no side effects.  Laser light directs bio stimulation light energy to the body’s cells which convert into chemical energy to promote natural healing & pain relief.  Stimulation of wound healing – Promotes faster wound healing/clot formation –Helps generate new & healthy cells & tissue

Laser surgery to correct for (a) nearsightedness, and (b) farsightedness

 In surgery, lasers can be used to operate on small areas without damaging delicate surrounding tissue.