Laser part 2.pdf

Laser Light Amplification by Stimulated Emission of Radiation  Laser Generators Power Supply Lasing Medium - gas, solid or liquid material that generates laser light Pumping Device - creates population inversion essential for laser operation Optical Resonant Cavity - chamber wher...

Laser Light Amplification by Stimulated Emission of Radiation  Laser Generators Power Supply Lasing Medium - gas, solid or liquid material that generates laser light Pumping Device - creates population inversion essential for laser operation Optical Resonant Cavity - chamber where population inversion occurs that contains reflecting surfaces  Types of lasers Solid-state lasers have lasing material distributed in a solid matrix (such as ruby). Flash lamps are the most common power source. 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 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 infrared and are used for cutting hard materials. Solid-state Laser  Example: Ruby Laser  Operation wavelength: 694.3 nm  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.  Ruby Laser  A ruby laser is a solid-state laser that uses a ruby crystal as its gain medium.  It was the first type of laser invented, and was first operated by Theodore H. "Ted" Maiman at Hughes Research Laboratories on 1960-05-16.  The ruby mineral is aluminum oxide with a small amount(about 0.05%) of chromium which gives its characteristic pink or red color by absorbing green and blue light.  The ruby laser is used as a pulsed laser, producing red light at 694.3 nm.  Working of ruby laser  Ruby laser is based on three energy levels. The upper energy level E3 short-lived, E1 is ground state, E2 is metastable state with lifetime of 0.003 sec. When a flash of light falls on ruby rod, radiations of wavelength 5500 nm are absorbed by Cr3+ which are pumped to E3. The ions after giving a part of their energy to crystal lattice decay to E2 state undergoing radiation less transition. In metastable state , the concentration of ions increases while that of E1 decreases. Hence ,population inversion is achieved. A spontaneous emission photon by Cr3+ ion at E2 level initiates the stimulated emission by other Cr3+ ions. Mirrors at each end reflect the photons back and forth ,containing this process of stimulated emission and amplification The photons leave through the partially silvered mirror at one end , this is laser light.  Helium-Neon Lasers A helium-neon laser, is a type of small gas laser. HeNe lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics. Its usual operation wavelength is 632.8 nm, in the red portion of the visible spectrum. It operates in Continuous Working (CW) mode. The Helium-Neon laser was the first continuous laser.  Pulsed vs. Continuous Laser  Adjusting pulse rate changes average power which affects the treatment time if a specified amount of energy is required  Continuous lasers operate with a stable average beam power. In most higher-power systems, one is able to adjust the power.  With pulsed laser treatment times may be exceedingly long to deliver same energy density with a continuous wave laser Parameters  Laser –Wavelength –Output power – Intensity  Wavelength  Nanometers (nm)  Longer wavelength (lower frequency) = greater penetration  Wavelength is affected by power  Power  Output Power –Watts or milliwatts (W or mW) –Important for laser safety Intensity ◼ Power Density (intensity) –W or mW/ cm2 – Takes into consideration – actual beam diameter – Beam diameter determines power density  Dosage  Dosage reported in Joules per square meter (J/m2)  One Joule is equal to one watt per second  Dosage is dependent on ◦ Output of the laser in Watts ◦ Time of exposure in seconds ◦ Beam surface area of laser in m2  Dosage  After setting the pulse rate, which determines average power of laser, only treatment time per m2 needs to be calculated TA = (E /Pav) x A TA = treatment time for a given area E = joules of energy per m2 Pav = Average laser power in watts A = beam area in m2  Depth of Penetration Laser depth of penetration depends on type of laser energy delivered “Direct effect” occurs from absorption of photons “Indirect effect” produced by chemical events caused by interaction of photons emitted from laser and the tissues  Depth of Penetration Absorption of HeNe occurs within first 2-5 mm of soft tissue with an indirect effect of up to 8-10 mm GaAs (compound of the elements gallium and arsenic). which has a longer wavelength directly absorbed at depths of 1-2 cm and has indirect effect up to 5 cm – Better for treating deeper tissues  The use of Lasers Science – precise measurements, spectroscopy Medicine –eye surgery Industry – cutting and welding, guidance systems Arts – etching Telecommunications (fiber optics) Consumer – CDs, DVDs, laser lights Laser Treatment & Diagnostics  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.  Clinical Applications Wound healing Immunological responses Inflammation Scar tissue Pain Bone healing Calculate the treatment time to deliver 10000 J/m2 with a 0.0004 W average power GaAs laser with a 0.000007 cm2 beam area.  During a preparation for an open heart operation, the ultrasonic used with frequency 2 MHz. If the average distance between the probe and the heart is 30 mm, calculate the time required for ultrasound wave to reach the heart. Calculate the wavelength of the ultrasound ( speed of sound in human tissue is 1500 m/s) t = x / v = (30 × 10-3) / 1500 = 2 × 10-5 s λ= v / f = 1500 / (2×106) = 75×10-5 m  A wave that has a vertical distance of 32 cm from a trough to crest , a frequency of 2.4 Hz and a horizontal distance of 48 cm from crest to nearest trough , Determine the amplitude ,periodic time , wavelength and speed of such wave.  Blood has an acoustic impedance of 1.6 x 106 kg m-2 s-1 and ultrasound travels through it at a speed of 1570 m s-1. Calculate the density of blood.  A person standing between two vertical cliffs and 640 m away from the nearest cliff shouted. He heard the 1st echo after 4 seconds and the second echo 3 seconds later. Calculate (i) the velocity of sound in air and (ii) the distance between the cliff.  Solution Velocity of sound v = = = 320 m s-1 Distance the second cliff, d = = = 1120 m Therefore the distance between the cliffs, D = 640 + 1120 = 1760 m A wave machine in a swimming pool produces 10 waves each second. The waves travel 30 metres along the pool in 6 seconds. a What is the frequency of the waves? b Calculate the speed of the waves. c Calculate the wavelength of the waves. In the following diagram the waves move from left to right and then stop. The motion of the waves lasts for four seconds. a What is the wavelength of the waves? b What is the frequency of the waves? c What is the speed of the waves?