Introduction to Lasers

Learning Objectives

• Define the characteristics of lasers compared to ordinary light.
• Explain the basic principle and conditions for the operation of lasers.
• Understand absorption, spontaneous, and stimulated emission in a two-level system.
• List the properties of lasers.

Historical Context

• Discovery of Light: Early humans relied on natural light sources until the discovery of fire.
• Artificial Light Sources: Over time, various artificial light sources were developed.
• Invention of Lasers: Physicists contributed to the creation of devices that produce powerful beams of light, leading to the invention of lasers.

Importance of Lasers

• Applications: Used in barcode scanners, intruder detection, projection pointers, printers, optical tweezers, cutting materials, body art removal, and eye surgery.
• Unique Characteristics: Differ from ordinary light, allowing for intense, focused beams.

Definition of Lasers

• Acronym: Laser stands for Light Amplification by Stimulated Emission of Radiation.
• Characteristics: Produces an intense beam of monochromatic and coherent light.

Basic Principles of Lasers

• Einstein's Theory: Proposed in 1916 and later developed by Gordon Gould in 1957.
• First Working Laser: The first optical laser was invented by Theodore May Mann in 1960, using ruby as the medium.
• Laser Mediums: Various materials, including solids like ruby, gases like xenon, helium, and semiconductors can be used as laser mediums.

Working Principles of Lasers

Key Processes

1. Absorption

   • Atoms possess different energy states (ground state E0 and excited state Ex).
   • Electromagnetic energy (photons) can be absorbed by electrons, causing them to jump to a higher energy state.

2. Spontaneous Emission

   • Excited electrons return to ground state E0 by emitting photons.
   • Emitted photons are incoherent (no phase correlation).
   • Lifetime of excited atoms is typically around 10^-8 seconds; metastable states can last much longer.

3. Stimulated Emission

   • An external photon can stimulate an excited atom to emit an additional photon of the same energy, resulting in coherent light.

4. Population Inversion

   • More electrons must be in the excited state than in the ground state for laser action.
   • Achieved through optical pumping, where photons excite ground state atoms to higher energy states.
   • Thermal agitation can also help achieve population inversion.

Characteristics of Laser Light

1. Monochromaticity

   • Laser beams consist of a single color/wavelength, unlike ordinary light which is a mix of wavelengths.

2. Coherence

   • Laser light is coherent, meaning wavelengths are in phase in space and time.
   • Can travel long distances without dispersion.

3. Directionality

   • Laser light is emitted as a narrow beam in a specific direction with minimal spreading.
   • Ordinary light bulbs emit in multiple directions.

4. Sharp Focus

   • Lasers can be focused sharply, enhancing their practical applications.

Conclusion

• Lasers represent a significant technological advancement, with applications in various fields.
• Understanding the principles of lasers allows for greater appreciation and utilization of their unique characteristics.