Laser cutting technology

Laser fundamentals

Very versatile technology in which a very small spot can be obtained. Smaller the spot, higher the power density. The laser beam heats up the material; the electromagnetic waves at low frequency (visible range) can interact just with electrons. Laser beam = Heat source. The interaction depends on the frequency of the beam but also on the structure of the material itself (different absorption). Laser beam can interact only with the electrons in the material because the heavier nuclei are not able to follow the high laser frequency.

Internal absorption

How deep can the beam go inside the material? The incident beam is very focused. A part will be reflected: I_R. A part will be transmitted inside the material: I_t. If the material is very transparent, the beam could even go outside (we don't want that, we want heat to be dissipated). The dissipation follows an exponential law; Beer-Lambert Law.

I_t(z) = I_t(0) e^-2z = AI_t e^-2z

If we calculate I_t at a depth = s = ¹/

I_t(z=s) = I_t(0) . e^-2s = I_t(0) . e^-2 = 0.135 I_t(0)

I_t(z=2s) = I_t(0) e^-2z = I_t(0) e^-4 = 0.018 I_t(0)

At a depth equal to s - already 86% of the power has been absorbed. At a depth of 2s - only 1.8% of power is remained, almost 99% has been absorbed.

Laser fundamentals: Very versatile technology in which a very small spot can be obtained. Smaller the spot, higher the power density. The laser beam heats up the material; the electromagnetic waves at low frequency (visible range) can interact just with electrons. Laser beam = Heat source. The interaction depends on the frequency of the beam but also on the structure of the material itself (different absorptions). Laser beam can interact only with the electrons in the material because the heavier nuclei are not able to follow the high laser frequency.

Internal absorption

How deep can the beam go inside the material? The incident beam is very focused. A part will be reflected: I_R. A part will be transmitted inside the material: I_t. If the material is very transparent, the beam could even go outside (we don't want that, we want heat to be dissipated). The dissipation follows an exponential law; Beer-Lambert Law.

I_t(z) = I_t(0) e^-2αz = ΔI_t e^-2αz

If we calculate I_t at a depth = δ = 1/α

I_t(z=δ) = I_t(0) e^-2αδ = I_t(0) e^-2 = 0.135 I_t(0)

I_t(z=2δ) = I_t(0) e^-2αz = I_t(0) e^-4 = 0.018 I_t(0)

At a depth equal to δ - already 86% of the power has been absorbed. At a depth of 2δ - only 1.8% of power is remained, almost 99% has been absorbed.

Applications of laser

• Cutting
• Welding
• Heat-treatments
• Drilling
• Marking
• Milling
• Cleaning
• Measurements

Light as electromagnetic field

An electromagnetic field is made by an electric field perpendicular to a magnetic field.

E_y(z,t) = E_y0 · sin(ωt − yz + φ)

• ω = angular velocity [rad/s] ω = 2πf
• φ = phase [rad]
• λ = wavelength [cm]

λ · f = V velocity of propagation of the laser beam inside a given medium [m/s]. If the medium is vacuum: V = c = 3 · 10⁸ m/s

Spectrum of electromagnetic waves

Our interest: from UV to near infrared (10 μm, industrial laser)

Light can be considered in two ways:

• Electromagnetic wave
• Beam made of particles having given energy (quantum).

A quantum of energy is called photon. E_f = energy of a photon = h . f = h . ^c/_λ

h = Planck constant = 6.63 · 10^-34 Js

The smaller the wavelength, the higher the energy associated to a single photon. So smaller λ, deeper it will go inside the material.

Laser

Laser: Light Amplification by Stimulated Emission of Radiation

One wavelength, one phase, one direction (with low divergence).

Difference ≠ Different wavelengths, different phases, big divergence.

Light amplification and stimulated emission need an active medium (gas).

1. Population I