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Laser Basics:

Laser Light:

The light from a laser (Light Amplification by Stimulation Emission of Radiation) is brighter than any natural or man-made source, including the sun. The major difference between both, is that laser light is monochromatic (one wavelength or color), directional and coherent.

Laser Components:

All lasers contain 3 primary components to operate: active medium, excitation mechanism and the optical resonator.

1- Active medium:
Is a collection of atoms, molecules, or ions which absorb energy from an outside source and generate laser light through atomic processes. Many of the laser beam’s output characteristics, including the wavelength, are determined by the active medium. Examples of media include:

Solid Crystalline Materials: e.g. Ruby, Neodymium: YAG, Erbium-Doped Fiber
Semi-Conductor Materials (diode): e.g. gallium/arsenide; gallium/aluminum/arsenide
Liquid Dyes: utilize a flowing dye pumped by flash lamp of other lasers
Gaseous Materials: e.g. Helium-Neon, CO2, Argon, Krypton

2- Excitation Mechanism:

Is the input energy device. It could be an intense light source, an electrical current through an active gas, or in the case of dye lasers, light from another laser.

3- Optical Resonator:
Consists of two specially designed mirrors, dramatically opposed to each other. The high reflectance mirror reflects 100% of the light which strikes it, while the other mirror (called the output coupler or partially transmissive mirror) reflects less than 100% of the light. The small fraction of light which passes through the partially transmissive mirror is the beam output.

Laser Operation:

The generation of laser radiation occurs in several steps:
Step 1: Excitation mechanism or energy source causes a flash of emitted light, which is absorbed by the active medium.
Step 2: Atoms of the active medium then become excited to a higher energy level. When the number of excited atoms is greater than the number of non-excited atoms, a population inversion is created, necessary for laser action to occur.
Step 3: A small amount of energy in the form of a photon is emitted when the excited atoms return to a non-excited state. These photons, in turn, stimulate other excited atoms in the active medium to release additional photons, causing a chain reaction through the active medium, this is called stimulated emission.
Step 4: The mirrors at each end of the laser reflect emitted light back into the active medium, a process referred to as amplification. As the light inside the laser grows more and more intense, part of it escapes through the partially transmissive mirror as laser light.

Laser Categories:

Lasers are classified by their wavelength, maximum output power and by their active medium. Lasers characterized by their active medium, maybe solid, liquid or gas.

Solid State Lasers:  
•The term "solid state", as related to lasers, refers to a group of optically clear materials such as glass or a "host crystal" with an impurity dopant.
•Most common include the Ruby laser,  Nd: YAG (Neodymium: Yttrium/Aluminum/Garnet) and Erbium-Doped Fiber.
•Solid state lasers can be operated as a continuous wave (CW) or pulse mode.

Semi Conductor (Diode) Lasers:
In terms of numbers, the diode laser is the most common laser today. The three common families of diode lasers are composed of:
1. Ga AIAs (Gallium/Aluminum/Arsenide) with a wavelength output in the 750 to 950 nanometers (used in CD&CD/ROM players)
2. InGaAsP (Indium/Gallium/Arsenide/Phosphide) with a wavelength output in the 1100 – 1650 nm range (used in optical telecommunications).
Another family of diode lasers like AlGaInP (Aluminum/Gallium/Arsenide/Phosphide) operates in the visible part of the spectrum, primarily red.

Liquid (Dye) Lasers:
The common liquid lasers utilize a flowing dye as the active medium, and are primped by a flash lamp or by another laser, such as an Argon laser. They operate either as pulse or continuous wave (CW) mode.

Gas Lasers:
Gas lasers are like fluorescent light bulbs and neon signs. In a Helium-Neon (HeNe) laser, the mixture of helium and neon gas is confined to a hollow glass tube, then an electric current passes through the tube, atoms are excited and causes them to emit light. Other gas laser systems include the Carbon Dioxide (CO2) laser, and the Argon (Ar) laser. Another variation is the laser family known as the Excimer lasers ("excited dimer"), such as the Xenon-Chloride (XeCl) laser. Excimers function in the ultraviolet part of the spectrum.
Gas lasers generally operate as continuous wave (CW), with the exception of excimer lasers.

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