Engineering Physics: Laser notes for 1st semester

Engineering Physics: Laser notes for 1st semester

In an earlier blog, we have discussed Interference and Diffraction. Now we will make notes of Laser.

LASER:

Light Amplification by Stimulated Emission of Radiation (LASER) is a device that generates light by a technique known as Stimulated Emission.

Classification of LASER light:

1. Highly directionality 
2. Highly intense
3. Highly coherence
4. Highly monochromatic

• Highly directionality: laser light travels in a single direction and it can be expressed as

Divergence(ϕ) = arc/ radius

                           = d2 -d1/S2 - S1

The divergence of laser light is 1/10³ which is very small. hence, we can say that laser light is highly directional.

• Highly intense: When two photons each of amplitude are in phase with other. Then the resultant amplitude of two photons is 2a and the intensity is 4a² whereas in laser many numbers of photons are in phase with each other, the amplitude of the resulting wave becomes na and hence the intensity of the laser is proportional to n²a². Therefore, the  1mW He-Ne laser is highly intense than the sun.
• Highly coherence: coherence is when the waves emitted from two sources having the same frequency and constant phase difference.

All the emitted photons are in a phase of the laser radiation.

• Highly monochromatic: All the photons emitted between discreet energy levels have the same wavelength in laser radiation.

Types of LASER:

1. Solid LASER (Ruby laser)
2. Liquid LASER (Europium chelate laser)
3. Gas LASER (He-Ne)
4. Dry LASER (Rhodamine 6G)
5. Semiconductor LASER (GaAs)
6. Chemical LASER (HF)

Stimulated Absorption:

The process of excitation of an atom into an excited state from the ground state by absorbing the incident photons is known as Stimulated Absorption.

It depends upon the number of atoms present in the ground state and the energy density of photons

        Stimulated absorption rate = B12(V)N1

The energy of photons, 

      (∆E)=E2-E1=hV

Spontaneous Emissions:

The process of de-excitation of the atom itself from the excited state by emitting a photon into the ground state after its lifetime.

It depends upon no. of atoms.

        Spontaneous emission rate = A21N2

Factors of spontaneous emissions:

• Radiation emitted by the atom is incoherent 
• Radiation emitted by the atom is poly-monochromatic
• Radiation emitted by the atom is less directional 
• Radiation emitted by the atom is less intense

Stimulated Emissions:

The process of de-excitation of the atom itself from the excited state by interacting with an additional photon within its lifetime by emitting an additional photon into the ground state.

It depends upon no. of atoms and the energy density of photons.

             Stimulated emission rate = B21N2(V)

Factors of Stimulated emissions:

• Radiation emitted by the atom is coherent 
• Radiation emitted by the atom is monochromatic 
• Radiation emitted by the atom is highly directional 
• Radiation emitted by the atom is highly intense

Einstein Co-efficient:

The relation b/w the three co-efficient I.e stimulated absorption, spontaneous emission, and Stimulated emission rate is established by Einstein co-efficient.

N1- Number of atoms per unit volume with energy E1.

N2- Number of atoms per unit volume with energy E2.

(V)- Density of photons

Upward Transition

Stimulated absorption rate = B12N1(V)

Where B12 is the Einstein coefficient of stimulated absorption.

Downward Transition

Spontaneous emission rate = A21N2

Where A21  is the Einstein coefficient of spontaneous emission.

Stimulated emission rate = B21N2(V)

Where A21  is the Einstein coefficient of stimulated emission.

At equilibrium, 

        B12N1(V) = A21N2 + B21N2(V)

Population inversion:

The no. of atoms present in the excited state(N2) is greater than the no. of atom present in the ground state(N1).

or,

The population of the excited state is greater than the population of the ground state is known as Population inversion.

              Ni = Noe^(-Ei/KT)

Where,

No- the population of the ground state

K- Boltzmann constant

Pumping: The process by which population inversion is achieved is known as pumping.

Components of laser:

• Energy Source
• Active medium
• Optical cavity(resonator)

Energy Source - Population inversion must be in the active medium to get laser emission.

Active medium - Active medium is a medium where we create population inversion to get stimulated emission of radiation.

Optical cavity (resonator) - The mirror constitutes the optical cavity when the active medium is enclosed b/w a fully reflective mirror and partially reflected mirror.

1. Excitation mechanism - This mechanism is used to excite atoms from the ground state to the excited state.

• Optical Pumping (Solid laser)
• Electrical discharge Pumping (Gas laser)
• Chemical Pumping (Chemical laser)
• Injection current Pumping (Semiconductor laser)

Notes: He-Ne emits continuous laser radiation.

Construction of He-Ne gas laser:

The He-Ne gas is taken in the ratio of 10:1 in the discharge tube.

The fully reflected mirror and partially reflected mirror are placed on either end of the discharge tube. He-Ne gas laser generally uses 80cm length and 1/2cm radius of the discharged tube.

We will update questions and answers in the coming days.

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