Helium Neon Laser, Semiconductor Laser and Applications of Laser - Civil Engineering (CE) PDF Download

Helium-Neon Laser (Gaseous state laser)

Construction:
It consists of quartz discharge tube of length 50cm and diameter 0.5cm fitted with Brewsters windows on either side and filled with the mixture of He and Ne gas in the ratio of 10:1. It is placed between two highly parallel plane mirrors one of which is completely silvered while the other is partially silvered.

Working:
The energy level diagram for He and Ne atoms are as shown in the fig. When discharge is produced in the tube large numbers of electrons are produced. These highly energetic electrons collide with His atoms, which are abundant and excite them to energy level 2s. This type of collision is called collision of first kind and represented as follows,
e₁ + He → He^∗ + e₂
Where e₁ and e₂ are energies of electron before and after collision.
2s energy level of He is relatively metastable with the energy of about 20.61eV. 5s energy level of Neon with the energy 20.66eV is very close to 2s energy level of the Heatom. Therefore when the He atoms in the excited state 2s collide with less abundant Neon Atoms, they transfer their energies completely to neon atom in the ground state so that neon atom get excited to 5s energy level.

After transfering energy to Ne atom the He atom returns to its ground state. This type of energy transfer is called resonance transfer and collisions are called collision of second kind represented as below.
He^∗ + Ne → Ne^∗ + He
Since number of Ne atoms is less when compared to number of He atoms, within a short interval of time population inversion is created between 5s and 3p energy level. The transition from 5s to 3p results in laser beam of wavelength 6328A˚ , transition from 3p to 3s takes place in the form of spontaneous emission and transition from 3s to 2p in the form of non- radiative transitions.

Semiconductor Laser ( Injection Laser)
Light emitting diodes are basically semiconductor lasers. A widely used semiconductor laser is GaAs Laser (Gallium Arsenide).

Construction:
The figure shows a typical Semiconductor laser. It consists of a heavily doped PN junction with a depletion layer of thickness 0.1 micrometre. Diode used is a cube with each edge 0.4mm long with the junction lying horizontal as shown in the figure. The current is passed through the ohmic contacts provided to the top and bottom faces. The frontand back faces are polished and made highly parallel to each other to have a laser cavity. The other two faces are roughened.

Working:
The Diode is forward biased using an external source. Therefore electrons and holes flow across junction. Hence the current flows through the diode. The semiconductor used in LED and Semiconductor LASER is a direct band gap semiconductor. In case of a direct band gap semiconductor when a hole meets an electron it recombines with electron emitting a photon. This could be considered as the transition of electron from conduction band to valance band. When the current is low spontaneous emission is predominant. If the current is sufficiently high population inversion is achieved. The photons liberated initially due to spontaneous emissions induce further stimulated emissions. The laser cavity helps in the amplification of light. Finally this results in an avalanche of photons and hence the laser action is achieved. If the GaAs semiconductor is used then the wavelength of the laser emitted is 840nm.

Applications of Laser

Engineering applications:
The engineering applications of laser are
(1) Cutting, Drilling and Welding
(2) Measurement of pollutants in the atmosphere
(3) Holography etc.,

Laser Welding, Cutting and Drilling:
(1) Welding:
focusing a beam of laser on the welding spot does laser welding. The heat generated melts the material over a tiny area on which the beam is focused. The impurities such as oxides float on the surface of the melt. Hence when cooled the welded region becomes homogeneous solid structure. This makes the joint stronger. Since the Laser beam can be controlled with a high precision the welding is also done with high precision. Laser welding is a contact less process. Hence no foreign material can get into the welded joint. Laser welding is used in microelectronics in which components are sensitive to heat. Carbon dioxide lasers with a power output of 10 KW are used for this purpose.

(2) Cutting:
In metals Laser cutting is done with the assistance of gas blowing. A nozzle through which oxygen gas is blown surrounds the focusing part of the Laser. Hence a fine jet of gas is also focused on the cutting spot to where the Laser beam is focused. The combustion of the gas burns the metal. The oxygen jet will blow the tiny splinters along with the molten part of the metal away. The Laser beam controls the accuracy of cutting not the burning gas. Laser cutting is used in Textile industry etc., The advantages of the laser cutting are (a) High quality Cutting (b) No thermal damage and chemical change etc., Low power carbon dioxide laser is used for cutting purposes.

(3) Drilling:
Subjecting the material to pulses does Laser Drilling. The duration of the pulses will be of 0.1 ms to 1 ms. The intense heat generated over a short duration by the pulses evaporates the material locally. Hence the hole is left. Nd-YAG Laser is used in case metals but Carbon Dioxide Laser is used in case of both Metals and Non metals. The advantages of Laser drilling are (a) No tools wear out (b) Drilling can be achieved at any oblique angle (c) Very fine holes of dimension 0.2 to 0.5 mm can be drilled.

Measurement of pollutants in the atmosphere: 
There are different types of pollutants in the atmosphere. They are
(1) Gasses like (a) Oxides of Nitrogen (b) Carbon monoxide (c) Sulphur dioxide
(2) Particulate matter such as (a) dust (b) smoke etc.
The measurement of pollutants is done using Laser and is referred to as LIDAR (Light detection and ranging). The Lidar system consists of a transmitter and receiver. The laser beam is sent through the atmosphere. The receiver receives the back-scattered light. Distance of the congestion from the measuring station is calculated on the time-delay between the pulse emission and the reception of the back-scattered light. The concentration of pollutants can be mapped for different vertical sections of the atmosphere by scanning space around the station. In this method the composition of the pollutant cannot be determined. The following methods can be employed to know the composition of the pollutants. In both the methods laser beam is passed through the sample of polluted air collected from the desired region.

1. Absorption technique:
When the laser beam passes through the collected sample it undergoes absorption of various degrees depending on the pollutant. Depending on the characteristic absorption pattern of the transmitted light the composition of the atmospheric pollutant can be determined.

2. Raman scattering:
In this method the Raman-Spectrum of the transmitted light is obtained. This spectrum consists of two side bands in addition to the incident wavelength. The side bands are symmetrically spaced on both sides of the incident wavelength. The change in wavelength of the side bands is called Raman Shift. Based on Raman Shift the composition of the pollutant can be determined.