applied optics- lasers NEW

Introduction to Lasers

• Laser stands for Light Amplification by Stimulated Emission of Radiation.

• A laser device generates a monochromatic, collimated, and coherent beam of light.

• Lasers emit high-intensity radiation in a unique direction and are used in various fields such as:

  • Radio astronomy

  • Satellite communication

  • Optical fiber communication

  • Holography

  • Material testing and welding

  • Medicine

Principle of Laser Operation

Quantum Processes in Laser Action

• Stimulated Absorption: When an atom in ground state (E1) absorbs a photon, it transitions to an excited state (E2).

  • The process requires that the energy of the photon equals the energy difference between the two states (E2 - E1).

  • Number of absorption transitions (Nab) can be described by:

    • Nab = N1 B12 Q Δtwhere N1 = number of atoms in state E1, B12 = probability of absorption transition, Q = energy density of incident radiation.

Excited State Dynamics

• Atoms in the excited state (E2) tend to return to the ground state (E1) by emitting a photon.

• Two emission processes can occur:

  • Spontaneous Emission: An atom emits a photon without external influence; it is a random process.

  • Stimulated Emission: An external photon triggers the atom to emit a photon, resulting in two photons of identical frequency and direction.

  • Induced Emission: The key process for laser operation; multiplication of photons through stimulated emission leads to coherent light.

Differences Between Emission Types

• Stimulated Emission:

  • High intensity, monochromatic, and coherent.

  • Photons emitted in the same direction.

• Spontaneous Emission:

  • Less intense and incoherent, emitting photons in all directions at random.

Einstein's Coefficients

• Einstein's coefficients describe the relationship between absorption and emission processes:

  • R12 = B12 ρν N1 (rate of stimulated absorption)

  • R21 (SP) = A21 N2 (rate of spontaneous emission)

  • R21 (ST) = B21 ρν N2 (rate of stimulated emission)

Population Inversion

• Population Inversion: The condition when more atoms are in an excited state (N2) than in the ground state (N1). It is achieved through a process called pumping.

• Pumping Methods:

  1. Optical Pumping

  2. Direct Electron Excitation

  3. Inelastic Atom-Atom Collision

  4. Direct Conversion

  5. Chemical Process

Basic Components of a Laser

1. Active Medium: The material where laser action occurs.

2. Pumping Source: Provides energy to excite atoms in the active medium.

3. Optical Resonator: Composed of mirrors that amplify the light through multiple reflections.

   • One fully reflecting and one partially reflecting mirror.

Characteristics of Laser Radiation

• High Directionality: Laser light travels in a narrow beam.

• High Intensity: Energy is concentrated in a small area.

• Monochromaticity: A single frequency leads to little angular spreading.

• High Coherence: Identical phase and direction of wavefronts result in powerful energy concentrations.

Applications of Lasers

• In Communication: Efficiently transmitting data in telecommunications.

• In Manufacturing: Cutting, welding, and machining with precision.

• In Medicine: Treating various health conditions and conducting surgeries with minimal invasiveness.

• In Holography: Creating three-dimensional images through interference patterns.

Types of Lasers by Active Medium

1. Solid State Lasers: Examples include Ruby and Nd:YAG lasers.

2. Gas Lasers: Examples include He-Ne and CO2 lasers.

3. Semiconductor Lasers: Commonly used in fiber optics.

4. Liquid Dye Lasers: Utilize liquid solutions as the active medium.

5. Chemical Lasers: Rely on chemical reactions for excitation.

Key Processes in Specific Lasers

He-Ne Laser

• Active Medium: Helium and Neon gas mixture.

• Wavelength: Typically 632.8 nm.

• Applications: Used in holography and interferometric experiments.

CO2 Laser

• Active Medium: Mixture of CO2 and nitrogen.

• Wavelengths: 9.6 μm and 10.6 μm.

• Applications: Material processing and medical surgery.

Nd-YAG Laser

• Active Medium: Doped Yttrium Aluminum Garnet.

• Wavelength: 1.064 μm.

• Applications: Laser surgery and signaling applications.

Ruby Laser

• Active Medium: Ruby crystal composed of Al2O3 with Cr3+ ions.

• Wavelength: 694.3 nm.

• Applications: Manufacturing and design.

Semiconductor Laser

• Active Medium: p-n junction of semiconductor material like GaAs.

• Wavelength Range: Typically 830 nm to 850 nm.

• Applications: Telecommunications and laser printing.

Holography

• Definition: A photographic representation of a three-dimensional object through interference of coherent light.

• Uses: Data storage, non-destructive testing, and optical signal processing.

• Types: Optical, Acoustical, X-ray, and Microwave holography.

Summary

Lasers are crucial in modern technology, providing advancements in communication, manufacturing, medicine, and many other fields with their unique properties and operational principles.