LASERS phy

Question 1

What is LASERS

Answer 1

Light Amplifying by Stimulated Emission of Radiations

Question 2

what is absorption

Answer 2

When an atom in the state E1 absorbs an incident photon of energy hν (= E2- E1) and makes a transition to higher energy state E2
the process is known as induced absorption or simply absorption.
A + hν = A* where A is an atom in the lower state and A* is the excited state of the atom.

Question 2

explain in detail on how lasers work (step1)

Answer 2

Excitation of atoms from lower
energy state to a higher energy state by supplying energy from an external
source is called pumping.the most commonly used ones are optical, electrical,
thermal or chemical techniques. For example, Solid state lasers usually employ
optical pumping from high energy xenon flash lamps (e.g.Ruby, Nd:YAG). Gas
lasers use an AC or DC electrical discharge through the gas medium, electron
beam bombardment or a chemical reaction. The DC electrical discharge is most
common for ‘small’ gas lasers (e.g., Helium-neon, Argon ion)…

Question 3

what is the basic theory given by Einstein on which lasers work

Answer 3

if an electron in an exited state is collided
by a photon of the light energy, then the exited electron would drop down to a lower
energy state and emit a photon of the same energy and wavelength that would move in
phase and direction as that of the colliding photon

Question 3

what are the properties of Lasers

Answer 3

Monochromatic
Coherent in nature
Highly directional
Low convergence
Travel as a narrow beam
Spreads very little
Does not fade even after long distance

Question 4

what is population inversion

Answer 4

The process of achieving a large number of atoms in excited state than ground state is called “Population Inversion”.

Question 4

what are Requisites of a laser system

Answer 4

1)pumping mechanisms in order to achieve population inversions
2)An active medium with a suitable set of energy levels to support laser action. A
medium in which light gets amplified is called an active medium. The medium may be solid liquid or gas. Out of the different atoms in the medium only small fraction of the atoms are responsible for stimulated
emission and consequent light amplification.
3) An optical cavity or resonator to introduce optical feedback and so maintain
the gain of the system overcoming all losses

Question 4

explain in detail on how lasers work (step2)

Answer 4

The process which leads to emission of stimulated photons after
establishing the population inversion is referred to as lasing. The transition
from the metastable state back to the ground state is the ‘lasing’ transition,In order to sustain laser action, one has to
confine the laser medium and the pumping mechanism in a special way that
should promote stimulated emission rather than spontaneous emission. This is
achieved by bounding the laser medium between two mirrors as shown in figure
below.On one end of the active medium is the high reflectance mirror (100%
reflecting) or the rear mirror and on the other end is the partially reflecting or
transmissive mirror or the output coupler. The laser emanates from the output
coupler, as it is partially transmissive. Stimulated photons can bounce back
and forward along the cavity, creating more stimulated emission as they go. In
the process, any photons which are either not of the correct frequency or do not
travel along the optical axis are lost

Question 5

Basic principles on which Lasers work

Answer 5

1. Absorption
2. Spontaneous Emission
3. Stimulated Emission

Question 5

what is Spontaneous Emission

Answer 5

The excited state is highly unstable and atoms always seek out the least available energy state. When atom in the excited state E2 comes down to a lower energy state E1 by emitting a photon of energy hν (= E2- E1) without the aid of any external agency, the process is termed as spontaneous emission.
A* —–>A + hν
The direction of the emitted photons is random and radiation is incoherent.

Question 5

what is Stimulated Emission

Answer 5

When a photon of right energy is incident on an atom in the excited state, the incident photon stimulates the atom to make downward transition. The photon thus emitted will have same phase and energy as that of the incident photon. This process is known as stimulated emission
A* + hν——>A + 2hν
The importance of this interaction is that the two photons emerging out will travel in
the same direction, with exactly same energy and perfectly in phase. This is the
interaction responsible for the generation of laser beam

Question 5

types of Metastable state

Answer 5

there are two types of Metastable state
1) three energy level atom
2) four energy level atom

Question 6

Population inversion explain in detail

Answer 6

For good lasing action to take place, the population of atoms must be more in an excited state than in the ground state. Under normal thermal equilibrium conditions, the population is more in the lower energy state than in the excited state. This condition is to be reversed and it is known as population inversion This can be achieved by some artificial means known as pumping [the process of raising the energy level using an external source of energy] energy in to the active medium

Question 6

Conditions or drawbacks for laser action

Answer 6

1. It is difficult to maintain the atoms in their excited states until they are stimulated to emit the photon. The excited atoms have a natural tendency to drop back to their ground state due to the spontaneous emission.
2. Atoms in their ground state undergo absorption by using photons from the
   beam that is being built, there by hindering the process of continuous stimulation and emission.

Question 6

explain Three energy level atom transition

Answer 6

Metastable state is a lower excited energy state in which atom stays more time than the excited state.
In a lasing material, the atoms originally in the ground state are pumped in to
the excited state using an external source of energy
this is a short lived state [about10-8 sec] and due to spontaneous emission, the excited state decays rapidly to the lower excited state. This lower excited state is referred as metastable state
In metastable state, the atom stays longer time [about 10-3 sec] than the excited
state. Stimulated emission can occur by passing a photon of right energy during the
transition of atoms from metastable state to the ground state. This is basically a three level atom lasing action and used in a ruby laser system. The laser produced from this lasing action is a pulsed laser

Question 7

planks radiation formula

Answer 7

8pi h v^3/c^3 x (1/e^-E2-E1 -1)
here E2-E1 is hv/KT

Question 7

Lasers are classified into what types

Answer 7

1 solid type
2 gas state
3 liquid dye lasers
4 solid state diode lasers(semiconductor laser)

Question 7

The very first p-n junction laser was built using?

Answer 7

The very first p-n junction laser was built using GaAs gallium-arsenide (infrared type) and GaAsP gallium- arsenide-phosphide (visible) semi conducting materials

Question 7

what is a LED

Answer 7

If electrons and holes are injected across a p-n junction an incoherent light
emission occurs at the junctions due to their spontaneous recombination, and called as LED.

Question 7

explain the basics of a semiconductor laser

Answer 7

If electrons and holes are injected across a p-n junction an incoherent light
emission occurs at the junctions due to their spontaneous recombination, and called as LED. But in the case of semiconductor lasers, we have to achieve population inversion to get coherent light from the p-n junction. In a p-n junction, with a large forward bias, electrons and holes are injected into and across the transition region. With increasing current, the conduction band will have a large concentration of electrons while the valance band will have concentration of holes. When these population densities become large enough, population inversion is achieved. This region about the p-n junction is termed as inversion region.