BASIC PRINCIPLES OF MEDICAL LASERS leactur 7 Dr.khitam Y. Elwasife special Topics 2019-2020 Layout Fundamentals of Laser • Introduction– Properties of Laser Light– Basic Components of Laser– Basic laser operation– Types of Lasers– Laser Applications Principles – of Medical Lasers Types of Medical Lasers– Laser: Medical Applications– Laser: Surgery and Diagnostics– Laser Hazards– Laser Safety– LASER STAND FOR LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION Introduction LASER Light Amplification by Stimulated Emission of Radiation. •An optical source that emits photons in a coherent beam. •optical lasers, a device which produces any particles or electromagnetic radiations in a coherent state is called “Laser”, e.g., Atom Laser. •In most cases “laser” refers to a source of coherent photons i.e., light or other electromagnetic radiations. It is not limited to photons in the visible spectrum. There are 3 x-ray lasers, infrared lasers, UV lasers etc. Properties of Laser Light • The light emitted from a laser is monochromatic, that is, it is of one color/wavelength. In contrast, ordinary white light is a combination of many colors • Lasers emit light that is highly directional, that is, laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as from a light bulb, is emitted in many directions away from the source. • The light from a laser is said to be coherent, which means that the wavelengths of the laser light are in phase in space and time. Ordinary light can be a mixture of many wavelengths. Ordinary Light vs. Laser Light Ordinar Laser y Light Light Basic Concepts: Laser is a narrow beam of light of a single wavelength (monochromatic) in which each wave is in phase (coherent) with other near it. Laser apparatus is a device that produce an intense concentrated, and highly parallel beam of coherent light. Basic theory for laser (Einstein 1917) : 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 return to a lowest state via two distinctive mechanisms: emission and من تلقاء نفسها Spontaneous stimulated emission. Spontaneous emission: Each electron can drop back spontaneously to the ground state emitting photons. Emitted photons no incoherent. It varies in phase from point to point and from moment to moment. e.g. emission from tungsten lamp. : emission تحفيز Stimulated 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 Excited state hν Ground state Absorption: 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 E absorption. 2 hn=E2 – E1 E1 According to Boltzmann's statistics, if a sample has a large number of atoms, No, at temperature T, then in thermal equilibrium the number of atoms in energy states E1 and E2 are: -E /kT N1 = No e 1 -E /kT N2 = No e 2 If E1 < E2 Then N1 > N2 If E1 < E2 and N1 < N2 This is called انقﻻب."population inversion" Population inversion: Generally electrons tends to (ground state). What would happen if a actual percentage of atoms could 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 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 . غير مستقرMetastable By shining white light into this gas many atoms can be raised, through resonance, from the ground state to excited states. Population Inversion E1 = Ground state, E2 = Excited state (short life time ns), E3 = Metastable state (long life time from ms to s). Life times 10-9 sec E3 -3 E2 10 -1 sec Output hn =5500 Ao (amplification) E1 Excitation Pumping sources: Optical pumping: suitable for liquid and solid laser because they have wide absorption bands. Electric pumping: suitable for gas laser because they have narrow absorption band. Chemical reaction. characteristic properties of Laser Beam Coherent (in phase) Monochromatic (single wavelength) Collimated (highly parallel) Intense (Concentrated) -directionality, -brightness. Physical Properties of Laser 1. Energy- the amount of work done – measured in joules 2. Power- Rate of energy – measured in joules per second or Watts (1J/s =1 W) 3. Irradiance- power density- the power of the laser per unit area. 4. Fluence - energy density- amount of energy transfer per unit area - irradiance multiplied by the exposure time ( j/cm2)- more important in determining laser effect on tissues than total energy transfer. USES OF LASER TECHNOLOGY INCLUDING: SCIENCE MACHINING COMMUNICATIONS SECURITY/MILITARY MEDICINE Basic Components of Laser Active Medium Major determining factor of the wavelength of operation and other properties of laser. •Hundreds of different gain media in which laser operation has been done. •The gain medium may be solid crystals such as ruby or, liquid dyes, gases like CO2 or Helium/Neon, and semiconductors such as GaAs. Pumping Mechanism •The pump source is the part that provides energy to produce a population inversion. •Pump sources include electrical discharges, flash lamps, arc lamps, light from another laser, chemical reactions and even explosive devices. •The type of pump source used principally depends on the gain medium. Optical Resonator Its simplest form is two parallel mirrors placed around the gain medium. •Light from the medium produced by the spontaneous emission is reflected by the mirrors back into the medium where it may be amplified by stimulated emission. •One of the mirrors reflects essentially 100% of the laser light while the other reflects less than 100% of the laser light and transmits the other. Basic Components of Laser Laser system consists of three important parts. 1. Active medium or laser medium 2. An energy source (referred to as the pump or pump source) 3. An optical resonator consisting of a mirror or system of mirrors 6 To generate laser beam three processes must be satisfied:- Population inversion. Stimulated emission. Pumping source. COLLIMATED BEAM MEDIUM MIRROR PUMP 1- exitation 2- photone emission 3- amplification 4- lazer beam and Basic Principles of Light Emission Absorption When the electrons drop from the high energy states to lower energy states, then they will emit light at a wavelength that has an energy (and a corresponding wavelength) equal to the difference between the two energy states. This is known as an emission spectrum. •Absorption: •Absorption is a condition in which something takes in another substance. It is a physical or chemical phenomenon or process, in which atoms, molecules, or ions enter in the inner part (called "bulk") of a gas, liquid, or solid material Absorption is the process by which the energy of the photon is taken up by another. Emission Photon Absorption Photon Absorption: A photon with frequency v hits an atom at rest (left), and excites it to higher energy level (E2) while the photon is absorbed. Threshold Condition The lasing threshold is the lowest excitation level at which a laser's output is occur by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation A laser action will be happen if the beam increases in intensity during a round trip, Usually, additional losses in intensity occur, such as absorption, In general, the laser will lase if, in reflection a round trip This is called achieving Threshold. Gain > Loss The threshold of lasing is where the laser gain is greater than all the losses. Types of Laser Lasers are usually classified in terms of their active (lasing) medium. Major types are: • Solid-state lasers • Semiconductor Lasers • Dye Lasers • Gas Lasers Types of Lasers • Solid-state .("or neodymium:yttrium-aluminum garnet "YAG ياقوت material (such as ruby 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 pn 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 lases in the visible and IR. Argon lases in the visible and UV. CO2 lasers emit light in the far- infrared (10.6 micro m), and are used for cutting hard materials. A dye laser A dye laser uses a gain medium consisting of an organic dye, which is a carbon-based, soluble stain that is often fluorescent,. The dye is mixed with a solvent, allowing the molecules to diffuse evenly throughout the liquid. The dye solution may be spread through a dye cell, or streamed through open air using a dye jet. A high energy source of light is needed to 'pump' the liquid beyond its lasing threshold. Mirrors are also needed to oscillate the light produced by the dye’s fluorescence, which is amplified with each pass through the liquid. The output mirror is normally around 80% reflective, while all other mirrors are usually more than 99.9% reflective. The dye solution is usually spread at high speeds, to help avoid triplet absorption and to decrease degradation of the dye. A prism or diffraction grating is usually mounted in the beam path, to allow tuning of the beam.