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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The Principle of Operation of Laser
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.
Key considerations about laser production and its properties
- The directional properties of the laser beam can be attributed to the physics of the stimulated emission process that restricts the emission of the stimulated photon in the same direction as that of the exciting photon.
- The coherence of the laser beam is related to both the temporal phase correlations of the electromagnetic field which comprises the laser beam and the different positions in space over which these correlations remain constant.
- The laser beam is restricted to emit only at one particular wavelength of a very small spread which lies at the center of the band of frequencies encountered in spontaneous emission. These frequencies, in turn cause emission at the same frequency so that the extremely narrow beam divergence is achieved. The laser beam is intense because the rate of emission of energy is much higher in the laser than in a hot body.
- There are 3 classes of gain media: gas, liquid and solid. Gas lasers exhibit narrow wave regions where there is an appreciable optical gain. This is due to their sharp spectroscopic transitions. Liquid lasers have broad regions for optical gain corresponding roughly to their fluorescence. Solid-state lasers can have either narrow or broad gain regions depending upon the nature of the fluorescence.
- The pumping mechanism can be classified as optical and electronic. In the optical pumping, a coherent laser can be used for exciting the laser medium to its excited state. Arc lamps & Tungsten lamps are generally employed in continuous lasers while flash lamps are used in pulsed lasers. In electronic pumping, a discharge is created in the gain medium which excites the population inversion.
- The resonator provides the means to control the laser by adjusting the losses experienced by the cavity. The resonant cavity plays a vital part in the laser operation. Photons which do not propagate nearly along the cavity axis tent to be lost, quickly passing out the sides of the medium, which accounts for the high degree of collimation of the laser beam. Although the medium acts to amplify the wave, the optical feedback provided by the cavity converts the system into an oscillator. The energy decay within a cavity is expressed in terms of the Q-factor (quality factor). If the cavity is disrupted e.g. by displacing the mirror, laser action ceases. This can be done deliberately to delay oscillation in the cavity. This is known as Q-switching.
Types of Lasers and their various Applications in Medical Field
- Ruby Laser – It gives out a red light. Used in applications where high energy pulses are required. Patients with retinal detachment are treated with ruby laser. In this treatment, the retina is welded to the choroid with the heat generated. The ruby laser can easily pass through the pupil, cornea, vitreous humor and lens to reach the retina.
- Helium-Neon Laser –This laser employs a gaseous active medium in which the atoms of one gas (Neon) pump themselves up through collisions with the excited atoms of another (Helium). In contrast to ruby laser, a Helium-Neon laser can operate at several wavelengths. Besides this, the line width of the radiation emitted is much smaller than the ruby laser. He-Ne laser can be used for measurement of visual acuity. It is a useful tool for the ophthalmologist in deciding about the necessity of performing cataract surgery on the patient.
- Argon laser – It gives blue-green light. It can be transmitted through clear fluids without any heating. It can also go through glass fibres. It can be transmitted through glass fibres to a desired area. Red colour tissue can easily absorb the argon laser. Hence, blood vessels can absorb argon laser and change it into heat. This results in photocoagulation of blood protein. Argon laser is utilized in neurosurgery, microsurgery and a number of applications for gastrointestinal surgery.
- Carbon dioxide laser – A carbon dioxide laser can kill tissues by destroying their cells. The cells are destroyed by the steam formation in them. The area that is vaporized by the laser is both localised and without any kind of combustion. This results into very little damage to the surrounding tissues. It is commonly used in surgeries for: sickness of the nose, pharynx, larynx, trachea & oral cavity and treatment of vaginal and cervical neoplasia.
- Laser diode –This laser features an active medium that is a semiconductor. It bears similarity to that found in LEDs (light emitting diodes). The most common laser diode is made from p-n junction that is powered by an injected electric current. Diode laser is commonly used in dentistry.
- Excimer laser – It operates primarily in the ultraviolet spectral region. It is used for improving vision by controlled ablation of the cornea.
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