 Hello and welcome to the lecture on optical source laser learning outcomes by the end of the session student will be able to illustrate laser basic concept pause the video here and enlist the different optical sources three main optical light sources available are first is wideband continuous spectra sources that is incandescent lamp second is monochromatic incoherent source that is LED light emitting diodes and third is monochromatic coherent sources that is laser in this lecture we are going to concentrate on the basic concept of the third optical source that is laser what does laser stands for the optical source is often considered to be the active component in an optical fiber communication system its fundamental function is to convert electrical energy in the form of current into optical energy that is in the light energy in an efficient manner which allows the light output to be effectively launched or coupled into the optical fiber strictly speaking laser is a active device which amplifies light hence the derivation of the term laser as an acronym for light amplification by stimulated emission of radiations the operation of the device may be described by the formation of an electromagnetic standing wave within an optical cavity or resonator which provides an output of monochromatic highly coherent radiation which means laser is a narrow light beam of a single wavelength that is monochromatic in which each wave is in phase that is coherent with other waves laser is a device which produce an intense concentrated and highly parallel beam of coherent light to gain an understanding of light generating mechanism within the laser it is necessary to consider both the fundamentals of atomic concepts and the device structure is necessary atom consist of nucleus and electron if an interacting photon is energetic enough it may be absorbed by an atom rising to an excited state in 1917 it was pointed out by Einstein that an excited atom can return to a lowest state via two distinctive mechanisms those mechanisms are first is spontaneous emission and second is stimulated emission figure number one illustrate two energy level atomic system where an atom is initially in lower energy state when a photon with enough energy that is band gap energy is incident on an atom it may be excited into higher energy state through the absorption of the photon this process is sometime referred to as stimulated absorption alternately when an atom is initially in the higher energy state it can make a transition to the lower energy state providing the emission of a photon this emission process can occur in two waves by spontaneous emission or by stimulated emission we will see the spontaneous emission first the spontaneous emission process is illustrated in the figure number two which shows an electron in excited state an excited state electron releasing photon during transition from higher state to the lower state in spontaneous emission the atom returns to the lower energy state from higher energy state or excited state in an entirely random manner this random nature of spontaneous emission process where light is emitted by electromagnetic by electronic transitions from a large number of atoms giving incoherent radiation that is each wave has a different phase with the other waves each similar emission process in semiconductor provides the basic mechanism for light generation within the LED which is also an optical source stimulated emission in stimulated emission when a photon having an energy equal to the energy difference between the stood state interacts with the atom in the upper energy state causing it to return to the lower state with the creation of a second photon which is shown in figure number three the photon produced by stimulated emission is generally an identical energy to the one which caused it and hence the light associated with them is of the same frequency the light associated with the stimulating and stimulated photon is in phase and has same polarization therefore in contrast to the spontaneous emission coherent radiation is obtained furthermore this means that when an atom is stimulated to the light energy emission by an incident wave the liberated energy can add to the wave in a constructive manner providing amplification population inversion under the condition of thermal equilibrium the lower energy level of the two level atomic system contains more atoms in than the upper energy level this situation which is normal for structures at room temperature however to achieve optical amplification it is necessary to create a non-equilibrium distribution of atoms such that the population of the upper energy level is greater than that of the lower energy level this condition is known as population inversion this process is achieved using an external energy source and that energy source is referred as pump in order to achieve population inversion it is necessary to extract atoms into the upper energy level and hence obtain a non-equilibrium distribution the two level system discussed above does not lend itself to a suitable population inversion the population inversion however may be obtained in a systems with three or four energy levels the three energy level diagrams for such systems is shown in figure number four to aid attainment of the population inversion system display a central metastable state in which the atoms spend an unusually long time in orders of millisecond to seconds the three level system shown in figure number four consists of a ground level even a metastable level e2 and a third level above the metastable level is e3 which suitable pumping the electrons in some of the atoms may be excited from the ground level even into the higher level e3 since e3 is a normal level the electrons will rapidly decay by non-radiative process to either e2 or directly to the even level the metastable level even exhibits a much longer lifetime than level e3 which allows a large number of atoms to accumulate at level even over a period the density of the atoms in the metastable state increases above those in the ground state and a population inversion is obtained between these two levels stimulated emission and hence lasing can then occur creating a radiative electron transition between levels e2 and even to generate laser beam three conditions must be satisfied that is first stage population inversion stimulated emission and third is pumping of atoms from lower energy state to higher energy level the light amplification in laser occurs when a photon colliding with an atom in the excited energy state causes the stimulated emission of second photon and then both these photons release two more continuation of this process effectively creates creates avalanche multiplication and when the electromagnetic waves associated with these photons are in phase amplified coherent emission is obtained to achieve laser action it is necessary to contain photons within the laser beam and maintain the condition for coherence this is accomplished by placing plane or curved mirrors at either end of the amplifying medium as shown in figure number five the optical cavity formed is more analogous to an oscillator than an amplifier as it provides positive feedback of the photons by reflection at the mirrors hence the optical signal is fed back many times while receiving amplification as it passes through the medium the structure acts as a fabric aerotraisonator although the amplification of the signal from a single pass through the medium is quite small but after multiple passes the net gain can be large and lasing takes place the types of pumping source the optical pumping source this pumping method is suitable for liquid and solid laser because they have wide absorption bands the second is electric pumping this pumping method is suitable for gas laser because they have narrow absorption band and the third is pumping through chemical reaction these pumping sources are used to achieve population inversion the types of laser laser classification is based on the type of laser active medium the three types are solid liquid and gas in solid laser ruby is an example and aluminum granite in liquid laser dyes are there in gas lasers helium neon organ or carbon dioxide these are the some of the examples of laser materials these are the references thank you