The word laser is an acronym that stands for light amplification by stimulated emission of radiation. A laser is defined as a beam of monochromatic light that carries with it a high energy. The monochromatic light can be focused on to any minute area with a fine degree of exactness.
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According to the quantum theory of energy, each atom has known characteristic energy value. If another form of energy such as heat, light or electricity is used to stimulate an atom, its electrons are displaced and raised to a high level or excited state. Once in the excited state, the atom holds the energy for only 10-8 S. After this time, the energy is spontaneously released in the form of diffused light and the electrons return to their original resting state. The spontaneous release of energy is observed in the conventional lighting which is diffused in all directions and is comprised of many wavelengths which collectively appear as white light to the human eye.
The laser action depends on the phenomenon of stimulated emission. Let’s consider an atom X as shown the diagram below in excited state which can come back to its normal or ground state by emitting a ‘’photon’’ or a light quantum whose frequency is related to the excitation energy E by the equation:
E = hv
Where, h is plank’s constant, v is frequency of emission.
This corresponds to the phenomenon referred as to as spontaneous emission. If during the period, the atom is still excited, it can be stimulated to emit if it is struck by an outside photon having precisely the energy of the one that would otherwise be emitted spontaneously. So the stimulating photon is augmented by the one released by the atom as shown in the diagram above. Key point to note is that the photon upon release falls exactly in step or in phase with the photon that stimulated its release. It is therefore, possible to realize a laser in terms of synchronization of a large number of excited atoms so that when they work together, they produce a powerful coherent wave.
Since most atoms are in the ground state, their absorption is generally far more likely than the emission but if a population inversion could be obtained i.e. with more atoms in the excited state, and incident photon of the correct frequency could trigger stimulated emission causing avalanche of coherent photons. The incident wave could continue to grow so long as the scattering processes were few and the population inversion is maintained.
To be able to achieve this, it is necessary to have an active medium in which atoms are kept in excited state and stimulated by an outside photon to emit light in particular direction. The process by means by which a medium is activated is called pumping. This entails injecting electromagnetic energy into the medium at a wavelength different from the stimulating wavelength.
The active medium is usually enclosed in a resonator box with highly reflecting walls. The photons released by the stimulated emission undergo multiple reflections and result in a coherent wave of growing strength. The laser output is obtained if the resonator box is transparent to the emitted laser beam.
In order to collect the number of high energy photons accumulating within the system, a double-mirrored resonating chamber is used to reflect the light beam so that the rays of light are super-imposed as a single high-density energy beam. The high energy stored within the resonating chamber can then be directed through the partially reflective mirror by releasing the shutter in a precisely controlled manner.
Find out more about: Fingertip Pulse Oximeter Blood Oxygen Saturation Monitor
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