Test: Laser - Electronics and Communication Engineering (ECE) MCQ

LASER: Light Amplification by Stimulated Emission of Radiation

• Lasers have found wide applications in areas as diverse as optical communications, medical surgery, welding technology, entertainment electronics etc.
• It is used highly because of the collimated nature of the laser beams and the consequent possibility of delivering a very high energy density in a limited region of space.

Depending on the material used for the active medium, lasers are broadly classified as:
(i) conventional or gas lasers
(ii) solid-state lasers
(iii) liquid lasers
(iv) semiconductor lasers.

Gas Lasers

• Among the gas lasers, some of the most commonly used ones are Helium-Neon laser, Carbon dioxide laser and Argon- ion laser.

Solid-State Lasers

• Typical examples of solid-state lasers are Ruby lasers, Nd-glass laser etc. 
• Ruby laser consists of rods of ruby, which are Al2O3 with about 0.05% Cr with a highly polished mirror at one end and a semi-transparent mirror at the other.
• A xenon flash bulb is used to excite chromium atoms to their excited states.

MASER
Maser (Microwave Amplification by Stimulated Emission of Radiation), a device that produces and amplifies electromagnetic radiation mainly in the microwave region of the spectrum.

Ruby Maser
Ruby masers are very low-noise pre-amplifiers used in the microwave receiving systems of the Deep Space Instrumentation Facility (DSIF) and the Deep Space Network (DSN).

Ruby masers are the most sensitive and lowest noise microwave amplifiers used in the field, yet they are rugged and are not susceptible to the microscopic failures that sometimes occur in sub-micron junction devices.

• The search for more suitable materials revealed Ruby Maser, which is a crystal­line form of silica (Al2O3) with slight natural doping of chromium.
• Ruby Maser has the advantages of being solid, having suitably arranged energy levels, and being paramag­netic, which virtually means “slightly magnetic.”
• Cavity masers using ruby were developed at the Jet Propulsion Laboratory (JPL) at 960 megahertz (MHz) and 2388 MHz.

Ammonia Maser
The principle of ammonia maser is to separate the two types of molecules which have different energies.

• Ammonia Maser is such a device for generating electromagnetic waves.
• Ammonia molecule has two resonant states with a small energy difference eV.

Geometrically, the two states may be picturized as follows

The molecules make a transition from one state to another by absorption or emission of radiation.
Ruby masers are preferred because of their high-frequency stability compared to ammonia masers.

Concept:

What is population inversion:
“When more atoms exist in higher energy state compare to lower energy state”.

# What is need of population inversion in LASER.
“To perform stimulated emission”

# What is stimulated emission ?
“When on atom in excited state return to the ground state in the presence of photon by emitting another photon which is identical to it in frequency, direction, polarization and phase.

# How the population inversion in semiconductor cases diode occurs?
“By introduction of trap centres on P and n sides”.

• The laser has higher bandwidth and higher data rate.(Statement (a) is correct)  
• ​Lasers can produce high output power in fiber optic communications. The laser source is useful for high power i.e. more than 20 mW is available. (Statement (b) is correct).
• LED’s have a longer life than lasers, i.e.LED’s: 105 - 108 hours (Statement (c) is incorrect).

Laser:

• The acronym of LASER is Light Amplification by Stimulated Emission of Radiation.
• It is a device that emits coherent light through a process called stimulated emission.
• Coherent light is a light in which the electromagnetic waves maintain a fixed and predictable phase relationship with each other over a period of time.

Differentiate between Laser light and Non-Laser light:

External power efficiency is given by

where η_ex→ external power efficiency of the device
η_T → total efficiency, E_G → bandgap energy, V→ applied voltage

LASER:

• A laser is any of a class of devices that produces an intense beam of light of a very pure single color.
• The light produced by lasers is in general far more monochromatic, directional, powerful, and coherent than that from any other light source. 

Interference of light

• When two light waves interact with each other, the wave effect leads to a phenomenon called interference of light. 
• For interference the two waves have to be of the same frequency or else the interference will not be stable and the band lengths and distances will keep varying. 
• The high coherence of a laser's output is very helpful in measurement and other applications involving interference of light beams.
• Usually, a laser is used which is split to form two monochromatic sources used to demonstrate interference.

In Stimulated emission, the photon produced is of the same energy to the one which cause it. Hence, the light associated with stimulated photon is in phase and has same polarization. Therefore, in contrast to spontaneous emission, coherent radiation is obtained. The coherent radiation phenomenon in laser provides amplification thereby making laser a better optical source than LED.

The modes of laser are separated by a frequency internal δf and this separation is given by-
δf = c/2nL
Where
c = velocity of light
n = Refractive index
L = Length of the crystal.