Question

Describe how a LASER functions.

How does a LASER differ from a continuum source?

a brief description of each of the four steps of lasing.  

Answer 1

Laser is created when electrons in an atom absorbs energy from any external opto-electric source and get excited. These excited electons move from lower energy state to higher energy state, that is, to there excited state.

When these electons return to their ground state, they emit photons as a form of radiation.If these photons are at the same wavelength and are coherent then only laser is produced.

here,

Ground state: It is the initial lower energy level or state of the electron.

Coherent: Coherent waves are those two or more waves having constant phase difference over time. Waves are said to be in-phase only when their crest(maxima) and trough(minima) meet at same place and at same time.

Wavelength: Wavelength is the distance measured between any two consecutive crest or trough of a wave.

Laser differs from ordinary light in following ways:

1. Monochromatic: Laser is highly monochromatic, means it mainly consist of single wavelength.
2. Directionility: Laser travel in a single direction whearas ordinary light can travel in all possible directions at the same time.
3. Intensity: Lasers are more intensed then ordinary light. It means that in a laser higher number of photons pass through per unit area per unit time as compared to ordinary light.
4. Coherence: Lasers are coherent while ordinary light is not commonly coherent from the beginning.

Steps involved in lasing are:

1. Pumping;Electrons in an atom get excited to higher energy state by absorbing energy from external source.
2. Population Inversion: In general the number of electrons in a lower energy state is always greater as compared to higher energy level. To achieve a position where number of electrons in higher energy level is greater than lower energy level is called population inversion. To achieve this position firstly, pumping is done i.e. energy is given to electrons at lower energy level by which it gets excited to higher energy state, but electrons are not stable at this state so from this higher energy level they come back to a energy state between these two states where it is stable for small amount of time. This state is called metastable state. (electrons do not radiate any energy while coming from higher energy state to meta-stable state)In this state number of electrons are greater than the initial lower energy state thus population inversion is  accomplished.
3. Laser Action: Electrons present in meta-stable state decays to lower energy state by stimulated emission by which laser is produced.Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level.
4. Back to ground state:Electrons in higher energy state decays directly from higher energy state to lower energy state by spontaneous emission.Spontaneous emission is a process in which atom decays from higher energy state to lower energy state without any outside mechanism.