A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation".The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.
A laser differs from other sources of light in that it emits light which is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar. Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum. Alternatively, temporal coherence can be used to produce ultrashort pulses of light with a broad spectrum but durations as short as a femtosecond.
Basic Principles of Lasers
To explain the process of light amplification in a laser requires an understanding of the energy transition phenomena in the atoms of its active medium. They include: spontaneous emission, stimulated emission/absorption and non-radiative decay. The theory of quantum mechanics states that the electrons of atoms can take different energy states, E1, E2, E3, for example, with E1<E2<E3.
Spontaneous Emission
By quantum mechanics the lower energy level is more stable than higher energy levels, so electrons tend to occupy the lower level. Those electrons in higher energy levels decay into lower levels, with the emission of EM radiation. This process is called spontaneous emission. The radiation emitted is equal to the energy difference between the two levels.
E2 - E1 = hn0
๏ Where E2 is the upper energy level
E1 is the lower energy level
h is Plank’s constant
n0 is frequency of the radiated EM wave.
E1 is the lower energy level
h is Plank’s constant
n0 is frequency of the radiated EM wave.
Stimulated Emission
This is crucial if lasing is to occur. Suppose the atoms of the active medium are initially in E2. If external EM waves with frequency n0 that is near the transition frequency between E2 and E1 is incident on the medium, then there is a finite probability that the incident waves will force the atoms to undergo a transition E2 to E1. Every E2-E1 transition gives out an EM wave in the form of a photon. We call this stimulated emission since the process is caused by an external excitation. The emitted photon is in phase with the incident photon, has the same wavelength as it and travels in the same direction as the incident photon.
Stimulated Absorption
If the atom is initially in the ground level E1, the atom will remain in this level until it gets excited. When an EM wave of frequency n0 is incident on the material, there is a finite probability that the atom will absorb the incident energy and jump to energy level E2. This process is called Stimulated Absorption.
Non-Radiative Decay
Note that the energy difference between the two levels can decay by non-radiative decay. The energy difference can change into kinetic energy or internal energy through collisions with surrounding atoms, molecules or walls.
CO2 Laser
The CO2 laser (carbon dioxide laser) is a molecular gas laser with emission in the long-wavelength infrared spectral region. It is based on a gas mixture as the gain medium, which contains carbon dioxide (CO2), helium (He), nitrogen (N2), and possibly some hydrogen (H2), oxygen (O2), water vapor and/or xenon (Xe). Such a laser is electrically pumped via an electrical gas discharge, which can be operated with DC current, with AC current (e.g. 20–50 kHz) or in the radio frequency (RF) domain.
Although direct excitation of CO2 molecules into the upper laser level is possible, it has proven to be most efficient to use a resonant energy transfer from nitrogen molecules. Here, nitrogen molecules are excited by the electric discharge into a metastable vibrational level and transfer their excitation energy to the CO2 molecules when colliding with them. The exited CO2 molecules then largely participate in the laser transition. Helium serves both to depopulate the lower laser level and to remove the heat. Other constituents such as hydrogen or water vapor can help (particularly in sealed-tube lasers) to reoxidize carbon monoxide (CO, formed in the discharge) to carbon dioxide.
Principle
The active medium is a gas mixture of CO2, N2 and He. The laser transition takes place between the vibrational states of CO2 molecules.
Energy states of CO2 molecules
Carbon dioxide (Co2) is a symmetric molecule (O=C=O) and it has three modes of vibration:
- Symmetric stretching.
- Bending.
- Antisymmetric stretching is shown in the figure.
Symmetric stretching mode
In this mode of vibration, carbon atoms are at rest and both oxygen atoms vibrate simultaneously along the axis of the molecule departing or approaching the fixed carbon atoms.
Bending mode
In this mode of vibration, oxygen atoms and carbon atoms vibrate perpendicular to molecular axis.
Asymmetric stretching mode
In this mode of vibration, oxygen atoms and carbon atoms vibrate asymmetrically, i.e., oxygen atoms move in one direction while carbon atoms in the other direction.
Construction of Carbon Dioxide Laser
it consists of a tube 5m long and 2 cm in diameter. The discharge is produced by d.c excitation. The resonant cavity is formed of confocal silicon mirrors coated with aluminum. Pressured He is about 7 Torr, P (N2)~ 1.2 Torr, and P (CO2)~0.33 torr. The gain of E(0,0,1) – E (1,0,0) transition is higher, so laser oscillator at 10.6 µm.
To obtain an oscillation of 9.0 µm, some selective device such as a diffraction grating is placed in the cavity. The maximum power of this laser is about 50 – 60 w/m. CO2 lasers are widely used in the industries.
Working of Carbon Dioxide Laser
To have a CO2 laser, a mixture of Co2 and N2 in the ratio of about 0.8:1 is filled in a gas discharge tube. Also, helium is part of the mixture. Helium is more than N2 in the mixture. Co2 molecule act as an active or laser medium and N2 molecule help in achieving the population inversion in the same way as helium is used in He-Ne laser.
When an electric discharge is passed through the tube, the number of electrons is emitted, which pumps nitrogen molecules to V = 1 state.
In CO2 laser, the energy difference between the vibrational energy levels of Nitrogen and carbon dioxide is very small ( i.e. about 0.3 eV ) and hence there are a large number of electrons in the gas discharge tube having the energy of more than 0.3 eV. The energy levels of CO2 molecules are shown in the figure.
The vibrational state for N2 is a metastable state. Therefore, it provides a longer time for the collision between the excited N2 molecules and atom molecule of Co2 in the ground state.
As a result of this, the Co2 molecules are excited to the energy state E (0,0,1). Hence, population inversion is achieved as the number of CO2 molecules in the energy state E (0,0,1) are much more than the Co2 molecules in other vibrational states like E (1,0,0), E (0,0,2), and E (0,1,0).
Transition through stimulated emission process from energy level E (0,0,1) to the energy level E (1,0,0)and E (0,2,0) give rise to infrared Photons of wavelength10 µm and 9.6 µm respectively.
The function of adding Helium in the mixture of Co2 and N2 is many folds.
- It influences the lifetime of the energy label and does not spoil the population inversion.
- Helium has high thermal conductivity and so helps to conduct heat away to the walls of the tube keeping CO2 cold. In other words, helium helps to depopulate the lower level.
Advantages of Carbon Dioxide Laser
- The construction of CO2 laser is simple.
- The output of this laser is continuous.
- It has high efficiency.
- It has very high output power.
- The output power can be increased by extending the length of the gas tube.
Disadvantages of Carbon Dioxide Laser
- The contamination of oxygen by carbon monoxide will have some effect on laser action.
- The operating temperature plays an important role in determining the output power of laser.
- The corrosion may occur at the reflecting plates.
- Accidental exposure may damage our eyes since it is invisible (infrared region) to our eyes.
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