# Laser Principles

Laser light is drastically different from normal light. Learn about laser principles and the characteristics of different wavelengths.

## What is Light?

Light is a type of electromagnetic wave, and these waves have standard wavelengths. Starting from the longest, these wavelengths can be divided into radio waves, infra-red rays, visible rays, ultra-violet rays, X-rays, and gamma rays.

## What is Color?

When light hits an object, the wavelengths that get reflected by the object are taken in by the human eye (retina). When this occurs, we recognize the reflected wavelengths as the color of the object.

The refractive index differs depending on the wavelength, therefore light is split. As a result, we are able to recognize a wide variety of colors. For example, an apple reflects red wavelengths of light (600 to 700 nm) and absorbs all other wavelengths of light.

Black objects absorb all light and thus appear black.

## What are Visible Rays?

Some electromagnetic waves fall within the range of wavelengths that can be seen by humans. These are called visible rays.

On the short wavelength side, visible rays measure from 360 to 400 nm. On the long wavelength side, they measure from 760 to 830 nm. Wavelengths that are shorter or longer than visible rays cannot be seen by the human eye.

## Differences between ordinary light and laser beams

Here's how laser light and normal light differs:

1. 1) Lasers emit light beams with high directivity, meaning the component light waves travel together in a straight line with almost no spreading apart. Ordinary light sources emit light waves that spread apart in all directions.
2. 2) The light waves in a laser beam are all the same color, a property known as monochromaticity. Ordinary light, such as the light from a fluorescent bulb, is generally a mixture of several colors that combine and appear white as a result.
3. 3) As the light waves in a laser beam travel, they oscillate with their peaks and troughs in perfect synchronization, a characteristic known as coherence. When two laser beams are superimposed on each other, the peaks and troughs of the light waves in each beam neatly reinforce each other to generate an interference pattern.
Directivity
(light waves travel in straight line)
Monochromaticity Coherence
Ordinary light
Laser beam

## The Origin of the Word Laser

The word LASER is an acronym, which stands for “Light Amplification by Stimulated Emission of Radiation”.

## Laser Principles

When atoms and molecules absorb external energy, they move from a low energy state to a high energy state. This high energy state is described as an excited state.

Atoms that enter an excited state are unstable and will immediately attempt to return to a low energy state. This is called transition.

When transition occurs, light equal to the energy difference between states is emitted. This phenomenon is called natural emission. The emitted light then collides with other atoms that are in a similar excited state, inducing transition in the same manner. Light that has been induced to emission is called stimulated emission.

## Types of Lasers

Lasers can broadly be divided into 3 major types: Solid-state, Gas, and Liquid.

The optimal laser will differ depending on the desired processing application.

Solid-state Gas Nd:YAG Fundamental wavelength (1064 nm) Second-harmonic (532 nm)(Green laser) Third-harmonic (355 nm)(UV laser) YAG (Yttrium Aluminum Garnet) Universal marking applications Used for fine marking and processing, silicone wafers, plastics and reflective metals etc Used for micro-processing, LCD repair and also plastic and reflective metal marking Nd:YVO4 (1064 nm) YVO4 (Yttrium Vanadate) Used in applications needing high peak power and extremely stable beam power Yb: Fiber (1090 nm) Yb (Ytterbium) High average power and excellent cooling efficiency. Good for marking on metals and plastics LD: (650 to 905 nm) Semiconductor lasers (GaAs, GaAIAs, GaInAs) CO2 (10.6μm) Widely used for marking labels, etching plastics and resins as well as processing and cutting He-Ne (630 nm) (red) is common Most commonly found in measurement devices. Excimer (193 nm) Uses a combination of inert gas and hydrogen gas to create a shorter UV wavelength.Most commonly used for optometry to vaporize the lens of human eyes. Argon (488 to 514 nm) Used primarily in scientific applications and biomedical related research. Dye (330 to 1300 nm) Used more widely in scientific applications.Dye are energized by laser light to produce florescent light.

### CO2 Laser

CO2 lasers are mainly used for machining and marking applications.
CO2 lasers emit invisible infrared beams, traditionally with a wavelength of 10.6 μm. N2 gas serves to increase the energy level of CO2, and He gas serves to stabilize the CO2 energy level.

### YAG laser(Nd：YAG)

YAG lasers are used for general-purpose marking applications on plastic and metal targets and for machining applications.
YAG lasers emit invisible near-IR beams with a wavelength of 1064 nm.

#### What is YAG?

YAG is a solid with the crystalline structure of Y (yttrium), A (aluminum) and G (garnet). Through doping of a light-emitting element, in this case Nd (neodymium), a YAG crystal will enter the excitation state via absorption of light from a laser diode.

### YVO4 laser (Nd：YVO4)

YVO4 lasers are used for ultra-fine marking and machining applications.
YVO4 lasers emit invisible near-IR beams with a wavelength of 1064 nm, like the YAG laser.

#### What is YVO4?

YVO4 is a solid with the crystalline structure of Y (yttrium), V (vanadium) and O4 (oxide), or Y (yttrium) VO4 (vanadate). Through doping of a light-emitting element, in this case Nd (neodymium), a YVO4 crystal will enter the excitation state via absorption of light from a laser diode.

## Laser wavelength analysis

### Wavelength: 10600 nm

CO2 lasers have a 10,600 nm wavelength, which is the longest found in most industrial systems. Compared to YAG, YVO4 and Fiber laser wavelengths, the CO2 wavelength is 10x longer.

As the name implies, CO2 lasers are generated through the stimulation of CO2 gas.

#### Typical characteristics of 10600 nm wavelength range lasers

• Not absorbed well by metals
• Melting and burning occurs due to the long wavelength and heat transfer
• Can process transparent objects like glass and PET
• Generally can't produce contrast or discoloration

### Wavelength: 1064 nm

IR (infrared) light contains the most versatile wavelengths for laser processing. As the name implies, IR light contains the wavelengths longer than visible red (i.e. longer than 780 nm).

#### Typical characteristics of 1064 nm wavelength range lasers

• Capable of processing multiple materials (including resins and metals)
• Cannot process transparent objects (like glass) since the laser light passes straight through
• Easily creates contrast on resins

### Wavelength: 532 nm

Second Harmonic Generation (SHG) lasers use a 532 nm wavelength. This laser light is visible to humans, appearing green, and is produced by transmitting a 1064 nm wavelength through a nonlinear crystal. As the light passes through the crystal, it's wavelength is reduced by half. A YVO4 medium is normally used because the characteristics of the beam are well suited for intricate processing.

#### Typical characteristics of 532 nm wavelength range lasers

• High absorption rates in materials that do not react well with typical IR wavelengths and those that reflect IR light (such as gold and copper)
• Intricate processing is possible due to a smaller beam spot than IR lasers
• Transparent objects are typically not able to be processed
• High peak power and limited heat transfer make 532 nm lasers ideal for micro machining and intricate designs

### Wavelength: 355 nm

Third Harmonic Generation (THG) lasers have a 355 nm wavelength, which falls into the ultraviolet (UV) range of light. A YVO4 or YAG laser is used to produce a fundamental wavelength (1064 nm) that gets transferred through a nonlinear crystal to reduce the wavelength to 532nm. That light is transferred through a second nonlinear crystal to reduce the wavelength to 355 nm.

#### Typical characteristics of 355 nm wavelength range

• UV light has an extremely high absorption rate in most materials and does not apply excessive amounts of heat.
• A very small beam spot enables very fine processing
• Most non-KEYENCE UV lasers require optical crystal replacements, which affects overall running costs.

## Laser Oscillation Fundamentals

Here's a breakdown of the laser emission process

### 1. Absorption

When atoms and molecules absorb light energy, electrons within the atoms go from a state of low energy (ground state) to a state of high energy. As the energy increases, the electrons transfer from their normal orbits to exterior orbits. This state of increasing energy is called excitation.

Atom state

### 2. Natural Emission

Excited electrons rise in energy levels relative to the amount of energy absorbed. After a period of time, high-energy electrons will attempt to return to a low energy state by emitting energy. At this time, light is emitted.

This phenomenon is called natural emission.

### 3. Stimulated Emission

When incoming light interacts with high-energy electrons, the electron drops to a lower energy state and emits light of the same energy, phase and movement direction as the incoming source. In other words, a single injected photon produces a phenomenon where it becomes two photons. This is called stimulated emission.

Light produced from stimulated emission possess uniform energy, phases and movement direction. Thus, producing a multitude of light with stimulated emission allowing for the creation of strong light with those three elements set uniformly.

Laser light is created by amplifying injected light using the phenomenon of stimulated emission. As a result, it possesses the characteristics of being (1) monochromatic, (2) coherent, and (3) highly directional.

### 4. Population Inversion State

In order to oscillate a laser beam using natural emission, it is necessary to create an environment where the number of electrons in a high energy state is overwhelmingly higher than electrons in a low energy state. This is called a population inversion state.

In other words, when the amount of naturally emitted light exceeds the absorbed light, it becomes possible to effectively create a laser beam.

#### Electrons in a Population Inversion State

• = Numerous high-energy electrons
• = Few high-energy electrons

### 5. Laser Oscillation

When a single electron naturally emits light in a population inversion state, that light causes another electron to naturally emit light. This results in a chain reaction that increases the amount of light produced and creates a strong beam. This is how laser oscillation works.

## Laser Oscillation Tube Elements

### Three elements of a laser oscillation tube

All laser oscillation tubes are comprised of the following three elements:

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