12-10-2012, 03:50 PM
LASER
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INTRODUCTION OF LASER
Laser is the abbreviation for “Light amplification by stimulated emission of radiation”. Laser light is emitted when many atoms undergo similar energy transitions at the same time. This is achieved by promoting a large number of atoms to an energy level above the ground state. As an electron in one of the excited atoms jumps down from its higher energy level it emits a photon. As this photon travels past another atom in an excited state, it causes the electron in this atom to jump down to the lower level.
The laser was first demonstrated by the American Theodore Maiman in 1960 using a ruby laser. The stimulated emission of radiation was initially postulated by Einstein in 1917.
A laser consists of an active medium which is placed between two mirrors. The arrangement of the mirrors is called a laser cavity. On of the mirrors is semi-transparent to release the laser that is generated in the cavity. Energy must be supplied from ex. flash bulb. The supplement of energy in the form of a for instance flash bulb is called pumping. The active medium consists of atoms or molecules.
Normally almost all of the atoms are in their lowest energy level, the so called ground state. From there they can be transferred to a higher, excited energy level through absorption of a light quantum (a photon) with the energy ∆E = hv. The upper state is often very short-lived (microsecond to nanosecond) and the atom returns to the ground state during the emission of radiation. Two radiation processes can happen; spontaneous emission or stimulated emission. The spontaneous emission consist of a random emission of photons in random direction is responsible for the excited state being so short-lived. The stimulated emission can happen if the atom is shone with radiation that has the frequency that is equal to the transition (v). The stimulated photons are emitted with the same frequency as that of the stimulating photons. The probability for the decay of an excited atom is equal to the probability for absorption of a photon by an atom in its ground state and being excited. The stimulated emission can be regarded as a negative absorption.
CHARACTERISTICS OF LASERS
Laser light has three unique characteristics, which make it different than "ordinary" light. It is:
• Monochromatic
• Directional
• Coherent
Monochromatic means that it consists of one single color or wavelength. Even through some lasers can generate more than one wavelength, the light is extremely pure and consists of a very narrow spectral range.
Directional means that the beam is well collimated (very parallel) and travels over long distances with very little spread.
Coherent means that all the individual waves of light are moving precisely together through time and space, i.e. they are in phase.
Monochromaticity
The energy of a photon determines its wavelength through the relationship E = hc/λ, where c is the speed of light, h is Planck's constant, and λ is wavelength. In an ideal case, the laser emits all photons with the same energy, and thus the same wavelength, it is said to be monochromatic. The light from a laser typically comes from one atomic transition with a single precise wavelength. So the laser light has a single spectral color and is almost the purest monochromatic light available.
Beam diameter
It is very interesting to note that, the intensity of laser light is not same throughout the cross section of the beam. This is because of the fact that the cavity also controls the trans-verse modes, or intensity cross sections. The ideal beam has a symmetric cross section: The intensity is greater in the middle and tails off at the edges. This is called the Transverse Electromagnetic Mode (TEM 00) output as shown in the figure. The subscripts n and m (0 and 0 in this case) in the TEMnm are correlated to the number of nodes in the x and y directions. A theoretical TEM 00 beam has a perfect Gaussian profile. Detailed discussion on modes is given in the next section. Lasers can produce many other TEM modes, which would be discussed in later sections. In general, one can say that laser beams have a symmetric intensity profile. i.e. if we run across the beam, the intensity is minimum at the edge and as we move towards the center it increases and is maximum at the center and then it falls in a similar fashion as on the other side, where from we started. In fact, we can start at any point on the rim of the laser beam and the result will be same, as discussed earlier.
Directionality and beam divergence
One of the important properties of laser is its high directionality. The mirrors placed at opposite ends of a laser cavity enables the beam to travel back and forth in order to gain intensity by the stimulated emission of more photons at the same wavelength, which results in increased amplification due to the longer path length through the medium. The multiple reflections also produce a well-collimated beam, because only photons traveling parallel to the cavity walls will be reflected from both mirrors. If the light is the slightest bit off axis, it will be lost from the beam. The resonant cavity, thus, makes certain that only electromagnetic waves traveling along the optic axis can be sustained, consequent building of the gain.
Brightness
While summing up the discussion on monochromaticity (narrow line width) and directionality (low divergence) of laser, radiance of laser cannot be missed out. It is defined as the power emitted per unit surface area per unit solid angle. The units are watts per square meter per steradian. A steradian is the unit of solid angle, which is three-dimensional analogue of conventional two-dimensional (planar) angle expressed in radians.
PRINCIPLE OF A LASER
A laser usually comprises an optical resonator (laser resonator, laser cavity) in which light can circulate (e.g. between two mirrors), and within this resonator a gain medium (e.g. a laser crystal), which serves to amplify the light. Without the gain medium, the circulating light would become weaker and weaker in each resonator round trip, because it experiences some losses, e.g. upon reflection at mirrors. However, the gain medium can amplify the circulating light, thus compensating the losses if the gain is high enough. The gain medium requires some external supply of energy – it needs to be “pumped”, e.g. by injecting light (optical pumping) or an electric current (electrical pumping → semiconductor lasers). The principle of laser amplification is stimulated emission.