Transcript Laser
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. Many wavelengths 2. Multidirectional 3. Incoherent 1. Directional 2. Coherent 3. Monochromatic 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 NdYAG). Flash lamps are the most common power source. The Nd-YAG laser emits infrared light at 1.064 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. HeliumNeon (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: 694.3 nm (IR) 3 level system: absorbs green/blue Gain Medium: crystal of aluminum oxide (Al2O3) with small part of atoms of aluminum is replaced with Cr3+ 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 632.8 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 (3000-10000) mW Low Level Lasers –Medical Lasers –Soft Lasers –Subthermal –Energy (1-500) mW –Therapeutic (Cold) lasers produce maximum output of 90 mW or less (600-1000) 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-aX 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.