LASERS
What is a laser?
Laser stands for Light Amplification by the Stimulated Emission of Radiation. Lasers radiate photons that are identical in phase direction and amplitude. They produce a beam that is in one single direction that is intense and polarized. 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.
How a laser works
Lasers are involved in the emission
and absorption of radiation within the atomic or molecular structure of
material. Electrons are bound to an atom or molecule in order to occupy as
many possible energy levels as possible. The lowest is known as the ground
state. It is Quantum mechanics that dictate which energy levels are
possible for a given system. If an atom is in the ground state it will stay
there until excited by external forces. An electron in an excited state
will radiate a photon in order to return to a lower energy level because
it must conserve energy. This is due to the first law of thermodynamics that
states that energy cannot be created or destroyed. Movement from one
energy level to another either happens when the atom absorbs or emits a photon.
When there is a transition to a lower level it is known as radiative decay. The
two types are known as spontaneous emission and stimulated emission.
Spontaneous emission occurs naturally when a external disruption is applied to
the excited atom. Stimulated emission happens when an electron in a
excited state, energy E2 can be stimulated to decay and energy E1 by
the interaction of a photon of energy h
=
E2-E1. This photon then hits the electron which then decays E1 producing two
identical photons. A laser takes advantage of absorption, spontaneous and
stimulated emissions to create conditions that are favorable for light
amplification (also known as optical gain). *Electrons can also decay by
other processes such as collisions with other particles know as a process called
loss that competes with the gain.*
Population Inversion
The populations of electrons in E2 and E1 are denoted by N2 and N1. When material is in equilibrium, Boltzmann statistics describes the system and almost all particles are in ground state. If enough light of energy is supplied the population can be driven until N2 and N1 are equal. Under these circumstances, the rates of absorption and stimulated emission are equal.
Argon Laser
The argon atom is pumped into the 4p energy level by collision with electrons. A first atom ionizes and a second excites it from ground state (energy level 1) to the fourth energy level directly. The 4p ions then decay down to the 4s either by spontaneously or by stimulation by a photon of energy. The wavelength of the photon is between 400 and 600 nm. The ion then decays from the 4s to ground state emitting an ultraviolet photon about 74 nm.
Helium-Neon Laser
The most common and inexpensive gas laser, the helium-neon laser is constructed to operate in the red at 632.8 nm and green at 543.5 nm and in the infrared at 1523 nm. One of the excited levels of helium very close to a level in neon so close in fact that upon collision of a helium and a neon atom, the energy can be transferred from the helium to the neon atom.
*Helium-neon lasers are common in the introductory physics laboratories, but they can still be dangerous! According to Garmire, an unfocused 1-mW Helium-Neon laser has a brightness equal to the sun's rays and is just as dangerous to stare at directly.*
Ruby Laser
The ruby laser is the first type of laser actually constructed, first demonstrated by T. H. Maiman. The ruby mineral (corundum) is aluminum oxide with a small amount of chromium which gives it its characteristic pink or red color by absorbing green and blue light. The ruby laser is used as a pulsed laser, producing red light at 694.3 nm. After receiving a pumping flash from the flash tube, the laser light emerges for as long as the excited atoms persist in the ruby rod, which is typically about a millisecond.
Beam Expanders
This is a type of lens used in our show to increase the diameter of the laser beam.. In most cases, the term is taken to mean a telescope designed to take a small-diameter collimated beam input beam and produce a larger diameter collimated output beam, thus reducing the divergence of the beam. The beam expander consists of a negative input lens and a positive objective. The expansion factor is the ratio of the focal length of the two lenses.
LINKS
How CD players and MP3 players work
Firework Display and pyrotechnics
Further Information
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