Stimulated absorption occurs when a photon strikes an atom with just exactly the proper energy to induce an electronic transition between two energy states. It is used to perform microsurgery and bloodless operations. 2. Excited atoms can loose their energy not only by spontaneous emission, but also by induced or stimulated emission and therefore the emission output of the system consists of spontaneous and stimulated emissions. This non-equilibrium condition is known as called population inversion. Most lasers are based on 3 or 4 level energy level systems, which depends on the lasing medium. These photons are in phase with the triggering photon and also travel in its direction. Once this is achieved, laser action is initiated in the following fashion. This implies that in order to create population inversion, one must look for non-thermal equilibrium system and thus the need for special laser materials. 4. High power CO2 laser finds applications in material processing, welding, drilling, cutting soldering etc. Study Material, Lecturing Notes, Assignment, Reference, Wiki description explanation, brief detail, CO2 Molecular gas laser: Principle, Construction, Working, Characteristics, Advantages, Disadvantages and Applications, It consists of a quartz tube 5 m long and 2.5 cm in the diameter. Wavelength of output: The wavelength of output is 0.6μm and 10.6μm. An active medium with a suitable set of energy levels to support laser action. CO2 = Carbon dioxide atoms in ground state CO2* = Carbon dioxide atoms in excited state N2 = Nitrogen molecule in ground state. This is referred to as natural or spontaneous emission and the photon is called spontaneous photon. An optical cavity or resonator to introduce optical feedback and so maintain the gain of the system overcoming all losses. The process is known as pumping. A carbon dioxide molecule has a carbon atom at the center with two oxygen atoms attached, one at both sides. These levels are known as METASTABLE levels. Considering a case of ordinary bulb having a filament temperature of about 5000K and emitting radiation in the wavelength range of 0.6 micron corresponding to frequency of 5 x 10 14 Hz, the probability of stimulated emission is approximately one hundredth of that of the spontaneous emission. This condition is referred to as population inversion. Wavelength of output: The wavelength of output is 0.6μm and 10.6μm. In sealed lasers, this takes place as the helium atoms strike the walls of the container. Pumping mechanism excites say, atoms to a higher energy level by absorption (Figs.3a and 3b). In CO 2 molecular gas laser, transition takes place between the vibrational states of Carbon dioxide molecules. Laser … The probability of stimulated emission is proportional to the intensity of the energy density of external radiation and the induced emission has a firm phase relationship with it, unlike spontaneous emission. When an electric discharge occurs in the gas, the electrons collide with nitrogen molecules and they are raised to excited states. W. K. Koechner, Solid State Laser Engineering, Spriger-Verlag, London. We may conclude that, laser action is preceded by three processes, namely, absorption, spontaneous emission and stimulated emission - absorption of energy to populate upper levels, spontaneous emission to produce the initial photons for stimulation and finally, stimulated emission for generation of coherent output or laser. Its construction is simple and the output of this laser is continuous. (BS) Developed by Therithal info, Chennai. On the other hand in a four level laser, the material is pumped to level 4, which is a fast decaying level, and the atoms decay rapidly to level 3, which is a metastable level. For a system to work as a laser one requires that stimulated emission should exceed photon absorption; it leads us to the following two conditions: First condition cannot be achieved under thermal equilibrium conditions. This process is stimulated emission. Amplification. Since the excited level of nitrogen is very close to the E5 level of CO2 atom, population in E5 level increases. Considering an ideal material with only two non-degenerate energy levels, where absorption, spontaneous emission and stimulated emission takes place, one can arrive at the following conclusion. The contamination of oxygen by carbon monoxide will have some effect on laser action. Though the laser action will continue as long as the energy is given to the active medium, it may be stated that pulsed laser is obtained if the population inversion is available in a transient fashion and continuous wave (CW) laser is possible if the population inversion is maintained in a steady-state basis. It consists of three basic parts. It is mostly used in neuro surgery and general surgery. Under these conditions the material always acts as an absorber of incident photons. These systems are shown in figs 3a and 3b. In other words, it must have a metastable state to support stimulated emission. Spontaneous emission is completely isotropic. 5. It is mostly used in neuro surgery and general surgery. Three types of processes are possible for a two-level atomic system. The operating temperature plays an important role in determining the output power of laser. Stimulated processes like stimulated absorption, or stimulated emission require incoming photons of the right frequency, whereas spontaneous emission can take place in the absence of incoming photon also. The contamination of oxygen by carbon monoxide will have some effect on laser action. This regime is called Q-switched operation and can be conceptualized as a two-mirror cavity with an optical … At the most the excited state population N2 (t) reaches a steady state at t → ∞, and the highest proportion of atoms that can exist in the excited state N2/Ntotal<1/2. 1. Lasing process takes place in these two levels. 5. The rate of spontaneous emission, R21 (spon), from level 2 to 1 is given as: One can observe that this spontaneous decay of the upper level takes place in the absence of an electromagnetic field and the rate is proportional to the population of that level and thus does not depend on the intensity of the excitation source. Therefore CO 2 gas laser is considered the type of molecular gas laser.. This discharge tube is filled with gaseous mixture of CO2(active medium), helium and nitrogen with suitable partial pressures.