To make a laser we need three things :
the block diagram can be drawn as
The atom stays at the higher level for a certain duration and decays to the lower stable ground level spontaneously, emitting a photon, with a wavelength decided by the difference between the upper and the lower energy levels. This is referred to as natural or spontaneous emission and the photon is called spontaneous photon. The spontaneous emission or fluorescence has no preferred direction and the photons emitted have no phase relations with each other, thus generating an incoherent light output (Fig.4). But it is not necessary that the atom is always de-excited to ground state. It can go to an intermediate state, called metastable state with a radiation less transition, where it stays for a much longer period than the upper level and comes down to lower level or to the ground state. Since period of stay of atoms in the metastable state is large, it is possible to have a much larger number of atoms in metastable level in comparison to the lower level so that the population of metastable state and the lower or ground state is reversed. i.e. there are more atoms in the upper metastable level than the lower level. This condition is referred to as population inversion. Once this is achieved, laser action is initiated in the following fashion. The atom in the metastable state comes down to the ground state emitting a photon. This photon can stimulate an atom in the metastable state to release its photon in phase with it. The photon thus released is called stimulated photon. It moves in the same direction as the initiating photon, has the same wavelength and polarization and is in phase with it, thus producing amplification. Since there are a large number of initiating photons, it forms an initiating electromagnetic radiation field. An avalanche of stimulated photons is generated, as the photons traveling along the length of the active medium stimulates a number of excited atoms in the metastable state to release their photons. This is referred to as the stimulated emission. These photons are fully reflected by the rear reflector (100% reflective) and the number and consequently the intensity of stimulated photons increases as they traverse through the active medium, thus increasing the intensity of radiation field of stimulated emission. At the output coupler, a part of these photons are reflected and the rest is transmitted as the laser output. This action is repeated and the reflected photons after striking the rear mirror, reach the output coupler in the return path. The intensity of the laser output increases as the pumping continues. When the input pumping energy reduces, the available initiating and subsequently the stimulated photons decrease considerably and the gain of the system is not able to overcome the losses, thus laser output ceases. Since the stimulation process was started by the initiating photons, the emitted photons can combine coherently, as all of them are in phase with each other, unlike in the case of spontaneous emission and coherent laser light is emitted (Fig.5). 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. If the input energy is given by say a flash lamp, the output will be a pulsed output and the laser is called a pulsed laser. If equilibrium can be achieved between the number of photons emitted and the number of atoms in the metastable level by pumping with a continuous arc lamp instead of a flash lamp, then it is possible to achieve a continuous laser output, which is called continuous wave laser.
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.
- Active material
- Pump source
- Resonator
the block diagram can be drawn as
Fig. 2: Basic Laser system
A representative laser system is shown in Figure (2). It consists of three basic parts.
- An active medium with a suitable set of energy levels to support laser action.
- A source of pumping energy in order to establish a population inversion.
- An optical cavity or resonator to introduce optical feedback and so maintain the gain of the system overcoming all losses.
- Excitation or pumping mechanism: Absorption of the energy by the atoms, electrons, ions or molecules as the case may be, of the active medium is a primary requisite in the generation of laser. In order to excite these elements to higher energy levels, an excitation or pumping mechanism is necessary. It is well known that under the equilibrium state, as per Boltzman?s conditions, higher energy levels are much less populated than the lower energy levels. One of the requirements of laser action is population inversion in the levels concerned. i.e. to have larger population in the upper levels than in the lower ones. Otherwise absorption will dominate at the cost of stimulated emission. There are various types of excitation or pumping mechanisms available, the most commonly used ones are optical, electrical, thermal or chemical techniques, which depends on the type of the laser gain medium employed. For example, Solid state lasers usually employ optical pumping from high energy xenon flash lamps (e.g., ruby, Nd:YAG) or from a second pump laser or laser diode array (e.g., DPSS frequency doubled green lasers). Gas lasers use an AC or DC electrical discharge through the gas medium, or external RF excitation, electron beam bombardment, or a chemical reaction. The DC electrical discharge is most common for 'small' gas lasers (e.g., helium-neon, argon ion, etc.). DC most often pumps semiconductor lasers current. Liquid (dye) lasers are usually pumped optically.
- Optical resonator: Optical resonator plays a very important role in the generation of the laser output, in providing high directionality to the laser beam as well as producing gain in the active medium to overcome the losses due to, straying away of photons from the laser medium, diffraction losses due to definite sizes of the mirrors, radiation losses inside the active medium due to absorption and scattering etc. In order to sustain laser action, one has to confine the laser medium and the pumping mechanism in a special way that should promote stimulated emission rather than spontaneous emission. In practice, photons need to be confined in the system to allow the number of photons created by stimulated emission to exceed all other mechanisms. This is achieved by bounding the laser medium between two mirrors . On one end of the active medium is the high reflectance mirror (100% reflecting) or the rear mirror and on the other end is the partially reflecting or transmissive mirror or the output coupler. The laser emanates from the output coupler, as it is partially transmissive. Stimulated photons can bounce back and forward along the cavity, creating more stimulated emission as they go. In the process, any photons which are either not of the correct frequency or do not travel along the optical axis are lost.
Laser action
Interaction of electromagnetic radiation with matter produces
absorption and spontaneous emission. Absorption and spontaneous emission
are natural processes. For the generation of laser, stimulated emission
is essential. The population inversion exists between
upper and lower levels among atomic systems, it is possible to realize
amplified stimulated emission and the stimulated emission has the same
frequency and phase as the incident radiation?. In electronic, atomic, molecular or ionic systems
the upper energy levels are less populated than the lower energy levels
under equilibrium conditions. Pumping mechanism excites say, atoms to a
higher energy level by absorption .The atom stays at the higher level for a certain duration and decays to the lower stable ground level spontaneously, emitting a photon, with a wavelength decided by the difference between the upper and the lower energy levels. This is referred to as natural or spontaneous emission and the photon is called spontaneous photon. The spontaneous emission or fluorescence has no preferred direction and the photons emitted have no phase relations with each other, thus generating an incoherent light output (Fig.4). But it is not necessary that the atom is always de-excited to ground state. It can go to an intermediate state, called metastable state with a radiation less transition, where it stays for a much longer period than the upper level and comes down to lower level or to the ground state. Since period of stay of atoms in the metastable state is large, it is possible to have a much larger number of atoms in metastable level in comparison to the lower level so that the population of metastable state and the lower or ground state is reversed. i.e. there are more atoms in the upper metastable level than the lower level. This condition is referred to as population inversion. Once this is achieved, laser action is initiated in the following fashion. The atom in the metastable state comes down to the ground state emitting a photon. This photon can stimulate an atom in the metastable state to release its photon in phase with it. The photon thus released is called stimulated photon. It moves in the same direction as the initiating photon, has the same wavelength and polarization and is in phase with it, thus producing amplification. Since there are a large number of initiating photons, it forms an initiating electromagnetic radiation field. An avalanche of stimulated photons is generated, as the photons traveling along the length of the active medium stimulates a number of excited atoms in the metastable state to release their photons. This is referred to as the stimulated emission. These photons are fully reflected by the rear reflector (100% reflective) and the number and consequently the intensity of stimulated photons increases as they traverse through the active medium, thus increasing the intensity of radiation field of stimulated emission. At the output coupler, a part of these photons are reflected and the rest is transmitted as the laser output. This action is repeated and the reflected photons after striking the rear mirror, reach the output coupler in the return path. The intensity of the laser output increases as the pumping continues. When the input pumping energy reduces, the available initiating and subsequently the stimulated photons decrease considerably and the gain of the system is not able to overcome the losses, thus laser output ceases. Since the stimulation process was started by the initiating photons, the emitted photons can combine coherently, as all of them are in phase with each other, unlike in the case of spontaneous emission and coherent laser light is emitted (Fig.5). 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. If the input energy is given by say a flash lamp, the output will be a pulsed output and the laser is called a pulsed laser. If equilibrium can be achieved between the number of photons emitted and the number of atoms in the metastable level by pumping with a continuous arc lamp instead of a flash lamp, then it is possible to achieve a continuous laser output, which is called continuous wave laser.
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