The basic concept of laser is first introduced by Einstein, in 1917 and which is the abbreviations of "Light Amplification by Stimulated Emission of Radiation"
A light of natural source, such as sunlight and lamp, makes spontaneous emission, while laser beam makes stimulated emission. Laser has the physical properties in common (phase, wavelength, polarization and direction) and a variety of laser is being developed and used in the fields of industry, medical treatment, R & D and advertisement.
Before discussing principles how laser operates, it can be helpful to review how the light appears.
In brief, there are three theories on how the light appears.
The first theory is blackbody radiation, the second is Cerenkov radiation,
and the last is the theory in which radiation will occur when a charged particle is accelerated.
If an object has a temperature greater than 0 K, the object emits its thermal energy as a form of electromagnetic wave.
Due to thermal energy, atoms and molecules vibrate in an object.
Atoms and molecules themselves are electrically neutral but they have both nuclei and electrons that are positively and negatively charged, respectively and separated each other by some distance.
These pairs are electric dipoles that vibrate and make electric field perturbation which produces electromagnetic wave propagating into space.
Like this, electromagnetic wave propagates into space infinitely, even though the strength decreases in inverse proportion to the distance squared.
When atoms and molecules vibrate and then produce electromagnetic waves, the frequency of standing waves must meet the boundary conditions.
This frequency is called intrinsic frequency. Because waves that do not meet the boundary conditions for standing waves will disappear by interference in an object.
Let l be a wavelength of the wave corresponding to intrinsic frequency, c the speed of light, and the size of an object L, respectively.
Then I will be, l = c L..... (1)
But, for an object of usual size, there are so many intrinsic frequencies that meet the conditions for standing waves.
So we will see a light of almost continuous spectrum. In addition, light waves of wavelengths of diverse deviation from Doppler effect that results from vibration of atoms and molecules make the light of more continuous spectrum.
For a given temperature, the energy level that atoms or molecules have in thermal motion in matter is not a single energy value but in Maxwell-Boltzman distribution.
Hence the frequencies that individual atoms can have differ from one another, and the numbers of atoms or molecules that have some frequency differ from one another.
As a result, the electromagnetic waves are in Maxwell-Boltzmann distribution.
The function of frequency in which electromagnetic waves are emitting is as follows:
r(n) = 8pnn/ccc[(Exp(hn/kT) -1)]...................(2)
The graph is as follows.
- Nd:YAG laser
Wavelength : 532nm (green color)
Classification : 4-level laser
This laser typically emits a light of wavelength of 1064nm in the infrared and, for many applications,
the infrared light is frequency-doubled in order to obtain visible light. (532nm, green)
- Ruby laser
Wavelength : 694nm (red color)
Classification : 3 level laser
Luby is Al2O3 containing a small quantity of Cr3+ion.
As Al3+ion is replaced with Cr3+ion, local crystal field is splitted and Cr3+absorbs green color and emit only red color.
- Alexandrite laser
- Ar laser
Wavelength : 488nm(blue color), 514nm(green color)
This Ar laser is used by passing current discharge through a tube containing
about 1 torr of Ar. Ar+ and Ar3+ ions go through laser transition from the excited state (excited by current
discharge) to the ground state, emit ultraviolet light at 72 nm, and are neutralized in the laser tube
(by electrode).
- He-Ne laser
Wavelength : 632.8nm
Mixture of helium and neon (1:5 mole) is used as active medium.
- CO2 CO2 laser
Wavelength : 10.6§
Emits a radiation of 9.2§ ~ 10.8§ in the infrared range.
- N2 N2 Laser
Its transfer efficiency of stimulus (337nm in the infrared range) is so high that just one penetration of
radiation of a pulse enables to make a laser beam, thus not requiring a mirror.
This laser is said to have superradiation effect.
- Organic dye laser
- Ga-As - CdS - ZnS