Laser Principles

What is Light?

Light is a type of "electromagnetic wave". "Electromagnetic waves" follow a standard of "wavelength" and starting from those of long wavelength, can be divided into radio waves, infra-red rays, visible rays, ultra-violet rays, X-rays, and gamma rays.

What is Colour?

As wavelengths of light hit an object, wavelengths that are reflected without being absorbed by the object are taken in by the human eye (retina). When this occurs, we recognise these wavelengths as the "colour" of the object. The refractive index differs depending on the wavelength, therefore light is split. As a result, we are able to recognise a wide variety of "colours". For example, an apple (receiving day-light, which includes specific light rays that enable humans to see the colour red,) 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 is Visible Light?

Electromagnetic waves that are within the range of wavelengths that can be seen by humans are called "visible rays". On the short wavelength side, visible rays measure 360 to 400 nm, and they measure 760 to 830 nm on the long wavelength side. Wavelengths that are shorter or longer than "visible rays" cannot be seen by the human eye.

What is Visible Light?

Differences Between Ordinary Light and Laser beams

This is where regular lights (lamps, etc.) and lasers differ. asers emit beams of light with high directivity, which means that 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. The light waves in a laser beam are all the same colour (a property known as monochromaticity). Ordinary light (such as the light from a fluorescent bulb) is generally a mixture of several colours that combine and appear white as a result.
As the light waves in a laser beam travel, they oscillate with their peaks and troughs in perfect synchronisation, 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.

  Ordinary light Laser light
Directivity
(Straightness)
LightbulbLightbulb LaserLaser
Monochromaticity Non-uniform wavelengthsNon-uniform wavelengths Uniform wavelengthsUniform wavelengths
Coherence Non-uniform phaseNon-uniform phase The peaks and troughs are aligned.The peaks and troughs are aligned.

Laser Etymology

The term “laser” originated as an acronym for “light amplification by stimulated emission of radiation”.

Laser Principles

When atoms (molecules) absorb external energy, they move from a low level (low energy state) to a high level (high energy state). This state is described as an excited state.
This excited state is one that is unstable and in this state, the atoms will immediately attempt to return to a low energy state. This is called transition.
When this occurs, light that is equivalent to the energy difference is emitted. This phenomenon is called natural emission. The emitted light collides with other atoms that are in a similar excited state, inducing transition in the same manner. This light that has been induced to emission is called stimulated emission.

Laser Principles

Laser Types

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

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

Solid-state

Nd: YAG
YAG (Yttrium Aluminium Garnet)

Standard wavelength (1064 nm)

  • General-purpose marking

Second harmonic (532 nm) (Green laser)

  • Soft marking on silicon wafers, etc.
    Used for detailed marking and processing

Third harmonic (355 nm) (UV laser)

  • Used for ultra-detailed processing such as LCD marking, repair processing, and VIA hole processing
    Liquid crystal repair processing: Cutting the coating pattern during repairs
    VIA hole processing: Drilling holes in PCBs
YAG Laser (Nd: YAG)
YAG lasers are used for general-purpose marking and for processing such as marking and trimming not only plastic materials but also metal materials. With a near-infrared light wavelength of 1064 nm, these lasers cannot be seen by the human eye.
YAG is a crystalline structure of yttrium (Y), aluminium (A), and garnet (G). Through doping of a light-emitting element, in this case neodymium ion (Nd), the YAG crystal will enter the excited state via absorption of light from a lamp or laser diode.
Nd: YVO4 (1064 nm)
YVO 4 (Yttrium Vanadate)
  • Small character marking
    High peak power at high Q-switch frequencies
    Good energy conversion efficiency
YVO4 laser (Nd: YVO4)
YVO4 lasers are commonly used for detailed marking applications such as marking small characters and other processing tasks. With a light wavelength similar to that of YAG lasers (1064 nm), YVO4 lasers cannot be seen by the human eye.
YVO4 lasers are solid lasers with a crystal structure of yttrium (Y), vanadium (V), and oxide (O4). When this structure is doped with a neodymium ion (Nd) light-emitting element, applying LD light from the end of the structure creates a state of excitation.
Yb: Fibre (1090 nm)
Yb (Ytterbium)
  • High-output marking
    Extremely large amplification medium surface area for easy high output
    Miniaturisation possible thanks to high cooling efficiency and simplified cooling mechanisms
LD (650 to 905 nm)
  • Semiconductor laser (GaAs, GaAlAs, GaInAs)

Gas

CO2 (10.6 μm)
  • Processing machines, marking applications, laser scalping
CO2 laser
CO2 lasers are commonly used in processing machines and for marking applications.
With an infrared light wavelength of 10.6 μm, these lasers cannot be seen by the human eye. CO2 lasers include not only CO2 gas inside the completely sealed oscillation tube but also specific amounts of N2 (nitrogen) and He (helium).
This characteristic gives CO2 lasers the nickname of “sealed type” lasers. The nitrogen (N2) increases the energy of the CO2, while helium (He) steadily lowers the energy to a more stable state.
Standard He-Ne (630 nm)
  • Measurement systems (profile measurement, etc.)
    This is the most popular type of laser.
    With a low output, these lasers are commonly used for profile measurements, etc.
Excimer (193 nm)
  • Semiconductor exposure equipment, eye care
    Excimer lasers generate light with a relatively simple structure that mixes inert gas with halogen gas.
    As a deep ultraviolet (DUV) laser, the absorption rate is incredibly high.
    (Such lasers are used in eye care to perform corrections by evaporating the crystalline lens and focusing the retina.
Argon (488 to 514 nm)

  • Scientific applications
    Available in a variety of colours, argon lasers are mainly used in laboratories, such as biotechnology labs.

Liquid

Dye (330 to 1300 nm)
  • Scientific applications
    Using laser light to excite dye results in dye fluorescence.

Wavelength Characteristics

CO2 Laser Marker
Wavelength 10600 nm :
Commonly used for marking paper, plastic, glass, and ceramic.
This wavelength is also absorbed by transparent targets, enabling use for marking on film and other objects.
The high output of this wavelength enables gate cutting of moulded products, cutting of PET sheets, etc.
  • YVO4 Laser Marker
  • YAG Laser Marker
  • Fibre Laser Marker
Wavelength 1064 nm (Fibre: 1090 nm):
(Standard wavelength)
Commonly used for marking metal, plastic, and ceramic.
This wavelength provides good colouration on plastic, enabling high-visibility marking.
YVO4, YAG, and fibre lasers all have different light characteristics even though the wavelengths are similar. This is due to different mediums and oscillation methods. Each is used for different applications according to the target and purpose. YVO4 lasers have a high peak power and short pulse width, allowing for high-quality detailed marking and processing. Utilising heat from a long pulse width, fibre lasers are good for black-annealed marking and deep marking into metal. Finally, even with inferior processing quality, YAG lasers are ideal for welding and other applications requiring a large amount of heat.
Green Laser Marker
Wavelength 532 nm:
(SHG wavelength)
Generally speaking, the shorter the wavelength of a laser, the higher its energy and the higher its absorption rate with the material.
Although YAG and YVO4 laser light is not easily absorbed, green laser light is suitable for use with hard-to-mark materials.
UV Laser Marker
Wavelength 355 nm:
(THG wavelength)
This laser has a wavelength that is even shorter than SHG in the UV region.
UV lasers offer high absorption rate regardless of material and cause minimal heat stress. This minimises product damage and enables high-contrast marking.

Laser Oscillation Principles

This section introduces the principles leading up to laser light oscillation.

1. Excitation

When light is provided from an external source, the electrons in atoms absorb the light, thereby changing their energy state from ground (lowest energy) to excited (higher energy). As the energy increases, the electrons move from their normal orbit to a more distant orbit. This increase in energy is referred to as “excitation”.

Atom state
Atom in its ground state
Atom in its ground state
Atom in its excited state
Atom in its excited state
Electron state
Electron state

2. Natural emission

Electrons in the excited state will vary in their increased energy levels depending on the amount of energy absorbed. Electrons with increased energy will tend to stabilise after a period of relaxation in which the increased energy is released in an attempt to return to a low-energy state. As this occurs, light with the same energy as the released energy is emitted. This is known as natural emission.

Atom state
Atom state
Electron state
Electron state

3. Stimulated emission

As shown in the figures below, when light passes through an electron with the same level of energy, additional light photons with the exact same energy, phase, and direction are created. With stimulated emission, for every light photon that passes through an electron, two photons are emitted. This is known as stimulated emission.
Because the stimulated emission light has the same energy, phase, and direction of travel as the incident light, stimulating and releasing a large amount of light makes it possible to create a strong light that adopts these three factors. Laser light is created by utilising stimulated emission to amplify incident light. As such, laser light is monochromatic (because the energies of the light must be the same), coherent (because the phases are aligned), and highly directive (because the direction of travel is aligned).

Atom state
Atom state
Electron state
Electron state

4. Population inversion

To oscillate laser light using stimulated emission, the density of high-energy electrons must first be overwhelmingly increased over that of low-energy electrons. This is known as population inversion. Ensuring that the number of light photons emitted exceeds the number of light photons absorbed enables effective creation of laser light.

Electron population inversion
Electron population inversion
  • = Numerous high-energy electrons
  • = Few high-energy electrons

5. Laser oscillation

When one electron emits light through natural emission during population inversion, that light initiates stimulated emission from another electron, and as the number of light photons increase as each electron stimulates nearby electrons, strong light is created. This is referred to as laser oscillation.

Electron population inversion
Electron population inversion
A: Natural emission B: Stimulated emission

Structure of Laser Oscillation Tubes

Three laser elements

Laser oscillation tubes consist of the following three elements.

  1. Laser medium
  2. Excitation source
  3. Amplifier
Three laser elements
  1. Laser medium
  2. Excitation source
  3. Amplifier
Three laser elements

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