Unit II Lasers and Their Applications | PHY 110 Engineering Physics |B.tech

 

Lasers and Their Applications

1. Fundamentals of Lasers

• Laser stands for Light Amplification by Stimulated Emission of Radiation.
• A laser generates coherent, monochromatic light through a process of stimulated emission of radiation from a gain medium.
• Coherence: Laser light is coherent in both time (same frequency, phase) and space (same direction).
• Monochromatic: Laser light typically consists of a single wavelength or color.
• Directionality: Laser light travels in a very narrow, focused beam with low divergence.

2. Energy Levels in Atoms (Key to Laser Operation)

• Atoms have discrete energy levels where electrons exist.
• The energy levels in atoms or ions are quantized, meaning electrons can only occupy specific energy states.
  • Ground state: The lowest energy level.
  • Excited state: Higher energy levels where electrons can be promoted via absorption of energy (e.g., photon absorption or electrical discharge).
• Energy Gap: The difference in energy between two states ($E_2-E_1$) determines the wavelength of the photon emitted during transitions between energy levels.

3. Radiation Interaction with Matter

• Radiation Interaction: When light (electromagnetic radiation) interacts with atoms or molecules, three key processes can occur:
  1. Absorption: An atom absorbs a photon, which excites an electron to a higher energy state.
  2. Emission: An electron moves from a higher energy state to a lower one, emitting a photon.
  3. Scattering: Photon direction and energy are altered by the material.

4. Absorption of Light

• Absorption: Occurs when a photon’s energy matches the energy gap between two levels of the atom. This causes the atom’s electron to jump from a lower to a higher energy state.
  • Absorption condition: $E_{\text{photon}} = E_2 - E_1$
• This process is critical for exciting atoms in laser media to higher energy levels.

5. Spontaneous Emission of Light

• Spontaneous emission occurs when an electron in an excited state returns to a lower energy state, releasing a photon in the process.
  • This photon is emitted in a random direction and is not coherent with the initial light.
  • Spontaneous emission is governed by the Einstein A coefficient, $A_{21},$ which defines the rate of spontaneous photon emission.

6. Stimulated Emission of Light

• Stimulated emission occurs when an excited atom or molecule is hit by a photon of the same energy as the transition between the two energy levels. This causes the atom to drop to a lower energy state, releasing a second photon identical to the first.
  • This photon is coherent with the incoming photon (same frequency, phase, and direction).
  • Key process in lasers: Laser light is produced through stimulated emission, which amplifies the photon flux.

7. Einstein A and B Coefficients

• Einstein Coefficients relate to the interaction of light with matter. They describe the rates of different processes:
  • A Coefficient ($A_{21}$): The rate of spontaneous emission of a photon.
  • B Coefficient ($B_{12}$): The rate of absorption of a photon.
  • B Coefficient ($B_{21}$): The rate of stimulated emission.
• The relationship between the coefficients is given by the equation:

$$\frac{B_{21}}{B_{12}} = \frac{g_2}{g_1}$$

where $g_1$ and $g_2$are the statistical weights (degeneracies) of the energy levels $E_1$ and $E_2$

8. Metastable State

• A metastable state is an excited state where the atom or molecule can remain for a relatively long period before spontaneously returning to a lower energy state.
  • Crucial for lasers: The long lifetime of metastable states allows a higher population of atoms in the excited state, which is necessary for population inversion (essential for lasing to occur).

9. Population Inversion

• Population inversion refers to a condition where more atoms or molecules exist in the excited state than in the ground state.
  • Normal conditions: Most atoms are in the ground state, but for lasing to occur, we need more atoms in excited states than in the ground state.
  • Achieved through pumping: By applying external energy (optical, electrical, or chemical), we excite more atoms to the higher energy levels than to the lower ones, creating a situation conducive to stimulated emission.

10. Laser Cavity

• The laser cavity is the region where light amplification takes place. It consists of:
  1. Gain Medium: The material that provides the medium for stimulated emission (solid, liquid, or gas).
  2. Mirrors: There are typically two mirrors at the ends of the cavity:
     • One mirror is highly reflective, reflecting the light back into the gain medium.
     • The other mirror is partially transparent, allowing a portion of the amplified light to exit as the laser beam.
• Resonance: The laser cavity is designed to ensure that photons bounce back and forth between the mirrors, stimulating more emission.

11. Excitation Mechanisms

• Pumping Mechanism: Atoms in the gain medium are excited to higher energy states through various methods:
  1. Optical Pumping: A light source (e.g., flashlamp or another laser) excites the atoms or molecules.
  2. Electrical Discharge: A current is passed through a gas (e.g., in He-Ne lasers), exciting the atoms.
  3. DC Pumping: Direct current is applied to a solid-state laser (e.g., Nd
     laser).
  4. Chemical Pumping: Chemical reactions excite the gain medium.

12. Types of Lasers

• Nd Laser:
  • Medium: Neodymium ions ($\text{Nd}^{3+}$) doped into a YAG (Yttrium Aluminum Garnet) crystal.
  • Wavelength: 1064 nm (infrared).
  • Applications: Used in materials processing, medical surgeries (e.g., laser eye surgery), and laser spectroscopy.
• He-Ne Laser:
  • Medium: A mixture of helium and neon gases.
  • Wavelength: Typically 632.8 nm (red).
  • Applications: Commonly used in laboratory experiments, barcode scanners, and holography.
• Semiconductor Laser (Diode Laser):
  • Medium: Semiconductor materials like Gallium Arsenide (GaAs).
  • Wavelength: Varies, typically in the infrared or visible spectrum.
  • Applications: Optical communication, CD/DVD players, laser pointers, and telecommunications.

13. Lasing Process

1. Excitation: Atoms in the gain medium are excited to a higher energy state through pumping (optical or electrical).
2. Population Inversion: More atoms are in the excited state than in the ground state.
3. Stimulated Emission: Photons stimulate the emission of other photons from excited atoms.
4. Light Amplification: Photons bounce between the mirrors, stimulating further emission and building up the intensity of the light.
5. Coherent Light Output: A portion of the amplified light exits through the partially reflective mirror as a coherent laser beam.

14. Applications of Lasers

• Holography:
  • Holography is a technique used to create three-dimensional images by recording the interference pattern of light.
  • Process: A laser beam is split into two parts:
    • One beam illuminates the object, and the reflected light is directed onto a photographic plate.
    • The other beam is the reference beam, which creates an interference pattern with the object beam.
  • Applications: Holographic storage, art, security (anti-counterfeiting), and medical imaging.
• Other Applications:
  • Medical: Laser surgeries (e.g., LASIK for eyes), cancer treatment, and skin treatments.
  • Industry: Laser cutting, engraving, welding, and material processing.
  • Communication: Fiber-optic communication, where lasers carry data over long distances.
  • Military: Laser rangefinders, targeting systems, and directed-energy weapons.
  • Entertainment: Laser light shows, laser projectors, and visual effects.
  • Research: Spectroscopy, optical trapping, and laser-based measurements in scientific studies.