LASER Introduction to Laser Properties of laser Spontaneous

LASER §Introduction to Laser §Properties of laser § Spontaneous and stimulated emission §Population inversion §Pumping methods §He-Ne laser- construction & working §Recording and reconstructing of hologram by using He-Ne laser.

Introduction to LASER- Light Amplification by Stimulated Emission of Radiation Light - the electromagnetic radiation which covers the range from ultraviolet region to the infrared region Light is monochromatic (Mono means one and Chroms means colour) in nature. Means Light having single colour or single wavelength Light is coherent in nature. All the wavelength are in same phase

Properties of LASER • The laser beam is perfectly coherent in nature i. e. All the wavelength are in one phase • The laser beam is almost perfectly monochromatic i. e. Beam having single wavelength • The laser beam shows Strong un-directionality i. e. The laser emits light only in one direction so light travel to long distance without spreading. • The laser beam shows extreme brightness i. e. The laser is much more intense and brighter than other sources.

quanta having different wavelengths, different phase. Thus, such a radiation is incoherent and has a broad spectrum.

Population Inversion • The concentration of active atoms occupying an energy state is known as population of that state Let N 1 and N 2 are the populations of lower energy state E 1 and Higher energy state. E 2 respectively. • In normal or equilibrium state the lower energy level is densely populated than the higher energy level i. e. N 1 >> N 2.

Definition: population of higher excited state must be greater than that of lower energy state. Hence making N 2 >> N 1 is called as population inversion.

Pumping Method

He-Ne Laser Construction He-Ne gas laser is automatic gas laser in winch Helium and Neon gases are mixed m the ratio of 10 : 4.

• It consist of Fused quartz tube which is of 10 to 100 cm by length and 2 to 8 mm in the diameter. the tube mixture of He and Ne gas. • Helium gas is filled with the pressure of 1 mm of mercury while neon gas is filled under the pressure of 0. 1 cm mercury. There is majority of He atoms (85%) and minority of neon atoms (l 5%). • Helium is just used for selective pumping of the upper laser level of neon. • One end of the tube is closed by fully reflecting plate. And other end has partially reflecting plate. • • He-Ne gas laser is commonly used as low power laser, Hence no cooling arrangements are required. Gas lasers are more stable and continuous type of lasers.

• Lasing transition takes place between energy level of neon, even though helium goes into early excitation. • The quantity of He is very much large compared with neon. They absorb energy from accelerating electrons for going into higher energy state. • On collision with neon atom they transfer their energy to neon atoms. And achieve population inversion of neon atom causing laser transition. • Energy level diagram for Re-Ne laser is shown in Fig.

Energy level diagram of He-Ne laser

Advantages i) It is low cost, more stable and continuous type of lasers. (ii) It is simple in construction. (iii) It is low power laser. (iv) Illuminates over the broad spectrum of 0. 63 to 100 Micrometer giving power output of 0. 5 to 10 mwatt. It can be used for laboratory and classroom purpose.

Holography