PROF. KAPARE A.K. SUBMITTED BY- DEPARTMENT OF PHYSICS Radhabai Kale Mahila Mahavidyalaya, Ahmednagar.

Definition of laser A laser is a device that generates light by a process called STIMULATED EMISSION. The acronym LASER stands for- Light Amplification by Stimulated Emission of Radiation

Absorption E1 E2

Spontaneous Emission

Stimulated Emission

Stimulated Emission

Absorption Light that falls on a piece of material will decrease exponentially.  = (N1-N2)B21(hf) n/c N1 is often more than N2 (N1 < N2) Example for tungsten  is typically 106m-1 (+ve) If we want implication,  must be –ve i.e. N2 > N1

Population Inversion Therefore we must have a mechanism where N2 > N1 This is called POPULATION INVERSION Population inversion can be created by introducing a so call metastable centre where electrons can piled up to achieve a situation where more N2 than N1 The process of attaining a population inversion is called pumping and the objective is to obtain a non-thermal equilibrium. It is not possible to achieve population inversion with a 2-state system. If the radiation flux is made very large the probability of stimulated emission and absorption can be made far exceed the rate of spontaneous emission. But in 2-state system, the best we can get is N1 = N2. To create population inversion, a 3-state system is required. The system is pumped with radiation of energy E31 then atoms in state 3 relax to state 2 non radiatively. The electrons from E2 will now jump to E1 to give out radiation.

3 states system

Therefore in a laser…. Three key elements in a laser •Pumping process prepares amplifying medium in suitable state •Optical power increases on each pass through amplifying medium •If gain exceeds loss, device will oscillate, generating a coherentoutput

Another Typical Application of Laser – Fibre Optics An example of application is for the light source for fibre optics communication. Light travels down a fibre optics glass at a speed, = c/n, where n = refractive index. Light carries with it information Different wavelength travels at different speed. This induce dispersion and at the receiving end the light is observed to be spread. This is associated with data or information lost. The greater the spread of information, the more loss However, if we start with a more coherent beam then loss can be greatly reduced.

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