This is referred to as natural or spontaneous emission and the photon is called spontaneous photon. This regime is called Q-switched operation and can be conceptualized as a two-mirror cavity with an optical … Population inversion though is the primary condition, but in itself is not sufficient for producing a laser. These photons are in phase with the triggering photon and also travel in its direction. Lasing process takes place in these two levels. This photon can stimulate an atom in the metastable state to release its photon in phase with it. For the generation of laser, stimulated emission is essential. Under these conditions the material always acts as an absorber of incident photons. Since there are a large number of initiating photons, it forms an initiating electromagnetic radiation field. This transition will produce a laser beam of wavelength 9.6μm. Rate of stimulated absorption, R12 (abs), from level 1 to 2 is given as: Where B12 is the Einstein's coefficient for stimulated absorption and has the units as cm3/s2J, N1 is the population in the ground state and ρ is the energy density per unit frequency of the incoming photons. The result would be an intense burst of coherent radiation. It can be a crystal, solid, liquid, semiconductor or gas medium and can be pumped to a higher energy state. This process is represented by the equation N2* + CO2 = CO2* + N2. If the system is to act as a laser, an incident photon must have a higher probability of causing stimulated emission than of being absorbed i.e. The state of lowest energy is called the ground state; all other states have more energy than the ground state and are called excited states. This is referred to as the stimulated emission. This action is repeated and the reflected photons after striking the rear mirror, reach the output coupler in the return path. 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. This lower energy state may be either the ground state or still one of the excited states but having lower energy level. A source of pumping energy in order to establish a population inversion. It is called stimulated absorptions because of the fact that the atoms absorb the incident energy at certain frequencies only. 2. Optical resonator: Two concave mirrors form a resonant cavity. The probability of transitions involving metastable levels is relatively low. In certain materials, there are energy levels, which has the spontaneous lifetime of the order of microseconds to a few milliseconds. 6. It consists of a quartz tube 5 m long and 2.5 cm in the diameter. As soon as population inversion is reached, any of the spontaneously emitted photon will trigger laser action in the tube. 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. The second condition that requires higher value of r necessitates the use of an additional supply of large amount of energy of correct wavelength to excite the desired transition. Laser- inherently a Quantum Mechanical device its operation depends on the existence of photons. The photon thus released is called stimulated photon. Principle of Laser action . Four level lasers is an improvement on a system based on three level systems. Its construction is simple and the output of this laser is continuous. The filling gas within the discharge tube consists of around 10–20% carbon dioxide (CO 2… As there are certain losses of the emitted photons within the material itself in addition to spontaneous emission, one has to think about the geometry that can overcome these losses and there is overall gain. Power output: The power output from this laser is about 10kW. It is used for treatment of liver and lung diseases. Once the atom or molecule has been produced in its excited state, there is a probability that it will emit radiation again and return to a lower energy state. state takes place by collision with cold helium atoms. 1.1.7 Carbon-Dioxide (CO2) Laser Lasing action in a CO2 molecule was first demonstrated by C. Patel in 1964.He transmitted an electric discharge pulse through pure CO2 gas in a laser tube, and got a small laser output. Now N2 molecules in the excited state collide with CO2 atoms in ground state and excite to higher electronic, vibrational and rotational levels. CO2 = Carbon dioxide atoms in ground state CO2* = Carbon dioxide atoms in excited state N2 = Nitrogen molecule in ground state. 5. As we all know that atoms and molecules can exist only in certain energy states. It is purely a statistical phenomenon related with time and space and is dependent on the lifetime of the excited state. An active medium with a suitable set of energy levels to support laser action. But stimulated emission has the same phase, direction, spectral and polarization properties as the stimulating field and both are indistinguishable in all aspects. This will produce a laser beam of wavelength 10.6μm. One can find out this ratio of spontaneous to stimulated emission using one of the following equations: where ρ is the radiation energy density and is equal to Nhn, N being the number of photons of frequency n per unit volume and k is Boltzmann's constant. Each excited state, of which there are many, has a fixed amount of energy over and above that of the ground state. In fact it is this stimulated emission, under certain conditions as explained in the earlier section that comes out of the laser device as laser. 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.