Lasers (contd)

Metastable state #

• Required for population inversion
• Has a longer lifetime $10^{-6} \rightarrow 10^{-3}s$
• Created between the energy levels where we want to control and hold the atoms.
• We do stimulated emission of atoms at the metastable state.

Pumping mechanisms for population inversion #

1. Optical pumping
   1. Light is used for stimulated absorption.
   2. Used in solid lasers.
   3. Eg: Ruby, Nd:YAG, Nd:Glass lasers
2. Electrical discharge pumping
   1. Used in gas lasers.
   2. Eg: He-Ne, CO2, Argon ion laser
3. Chemical pumping
   1. . Excitation due to chemical excitation
   2. Eg: HF, DF lasers
4. Injection current pumping
   1. Used in semiconductors
   2. Injection of current through junction causes inversion among minority charge carriers
   3. Eg: InP, GaAs.

Laser system #

1. Active medium: contains atoms to be excited
2. Optical cavity: Two mirrors for reflection and constructive interference for amplification.
3. Pumping mechanism: Used to excite atoms

3 level laser system #

Let there be three energy levels $E_0<E_1<E_2$

1. Pumping from $E_0 \rightarrow E_2$
2. $E_2\rightarrow E_1$ metastable state
3. Population inversion is achived between $E_0$ and $E_1$
4. Lasing action $E_1\rightarrow E_0$

4 level laser system #

Let there be four energy levels $E_0<E_1<E_2<E_3$

1. Pumping from $E_0 \rightarrow E_3$
2. Rapid decay from $E_3 \rightarrow E_2$
3. Population inversion is achived between $E_1$ and $E_2$
4. Stimulated lasing action $E_2\rightarrow E_1$
5. Gets back to the ground state $E_1\rightarrow E_0$

Various Lasers #

He-Ne laser #

• Helium gets excited by electron collision
• Neon gets excited due to collision with He.
• He:Ne=10:1

Merits #

1. Operated continuously
2. Highly monochromatic
3. Highly stable
4. Output can be tuned to any certain available λ
5. Separate cooling isn’t needed

Demerits #

Very low compared to lasers such as a ruby laser.

Applications #

1. Holograms
2. Industries
3. Communication
4. Printers
5. Laboratories
6. Scanners

Semiconductor laser #

ND:YAG laser #

Applications of lasers #

1. Communication
   1. Optical communication
   2. LIDAR: light detection and ranging
   3. High speed photography
   4. Holography
2. Medical application
   1. Cancer treatment
   2. Surgeries
3. Industrial
   1. Cutting, Welding, Drilling
   2. Electronics
4. Scientific research
   1. Spectroscopy
   2. Photochemistry
   3. Non-linear optics
5. Laser induced nuclear fusion