1. Basics

1.1 Definition

The word laser is actually an acronym which stands for

Light Amplification by Stimulated Emission of Radiation.

Lasers are used for a variety of applications:

• Research
  • Study of mechanisms at interfaces
  • Detection of single molecules
  • Structure of matter
• Medical
  • Retinal photocoagulation
  • Sight correction
  • General surgery
  • Photodynamic Therapy (PDT)
• Commercial
  • Supermarket checkout scanners
  • Determining site boundaries for construction
  • Light Show
• Industrial
  • Marking
  • Cutting
  • Welding

1.2 Properties

Laser light has three properties that differentiate it from ordinary light: it is monochromatic, directional, and coherent.

• Monochromatic: The light emitted from a laser is monochromatic; that is, it is of one color/wavelength. In contrast, ordinary white light is a combination of many colors (or wavelengths) of light.
• Directional: Lasers emit light that is highly directional; that is, laser light is emitted as a relatively narrow beam in a specific direction. Ordinary light, such as from a lightbulb, is emitted in many directions away from the source.
• Coherent: The light from a laser is said to be coherent, which means that the wavelengths of the laser light are in phase in space and time. Ordinary light can be a mixture of many wavelengths.

1.3 Operating Principles

Lasers consist of a lasing medium, excitation mechanism, optical resonator, and an output coupler.

• Lasing medium:
  The lasing medium of a laser is a substance that emits light in all directions and can be a gas, liquid, solid, or semiconducting material.
• Excitation mechanism:
  The excitation mechanism of a laser is the source of energy used to excite the lasing medium. Excitation mechanisms typically used are electricity from a power supply, a flashtube, lamp, or the energy from another laser.
• Optical resonator :
  A laser's feedback mechanism is used to reflect light from the lasing medium back into itself and typically consists of two mirrors at each end of the lasing medium. As the light is bounced between the mirrors, it increases in strength, resulting in amplification of the energy from the excitation mechanism in the form of light.
• Output coupler:
  The output coupler of a laser is usually a partially transparent mirror on one end of the lasing medium that allows some of the light to leave the lasing medium in order that the light be used for the production of the laser beam. The output coupler is usually part of the optical resonator.

1.4 Types of Lasers

There are many types of lasers available for research, medical, industrial, and commercial uses. Lasers are often described by the kind of lasing medium they use: Gas, Liquid (dye), Solid state, or Semiconductor.

Lasers are also often characterized by duration of laser light emission. A continuous wave (CW) laser is a laser which emits a steady beam of light, whereas a pulsed laser emits laser light in an off-and-on or pulsed manner. A Q-switched laser is a pulsed laser which contains a shutter-like device that does not allow emission of laser light until opened. Energy is built up in a Q-switched laser and released by opening the shutter-like device to produce a single very short and intense laser pulse.