Fiber vs CO₂ vs Nd:YAG Lasers for Cutting, Welding and Marking

Scope

This article compares the three laser families used most in laser machining: ytterbium fiber lasers near 1070 nm, CO₂ lasers at 10.6 μm, and Nd:YAG lasers at 1064 nm with their harmonics. In short: fiber lasers do most metal cutting and welding, CO₂ lasers most cutting and engraving of wood, acrylic, paper, textiles and glass, and pulsed fiber or diode-pumped Nd:YAG and Nd:YVO₄ lasers most marking, with green and ultraviolet harmonics where the material needs a shorter wavelength. The sections below give the physical reasons, which come down to the wavelength, the beam quality, and how the beam is delivered.

At a glance

Wavelength and absorption

What a material absorbs decides most of the choice. Metals reflect infrared light more strongly the longer the wavelength: the free-electron (Hagen-Rubens) estimate gives iron at room temperature an absorptance of about 3.5% at 10.6 μm, and measured values near 1 μm are several times higher. Absorption rises as the metal heats and melts, and once a keyhole forms it traps the beam by multiple reflection, so the difference narrows during welding; but it governs how easily a process starts and how much power a thin sheet needs. Copper and gold reflect most near-infrared light even at 1 μm, which is why green (515 or 532 nm) and blue diode lasers are used for them, where their absorptance is several times higher again.

Organic materials and glass behave the other way. Wood, paper, leather, textiles, most plastics and all silicate glasses absorb 10.6 μm within a few micrometres to tens of micrometres of the surface, while at 1 μm clear acrylic and glass are nearly transparent and wood absorbs unevenly. A CO₂ laser therefore cuts acrylic with a melted, flame-polished edge and engraves glass, and a 1 μm laser at the same power does neither well. The ultraviolet harmonics of Nd:YAG and Nd:YVO₄ (355 and 266 nm) are absorbed strongly by nearly every material, which is why they mark plastics and cut thin films with little heat spreading.

Spot size and depth of focus

A laser beam focused with a lens of focal length $f$ from a collimated diameter $D$ reaches a spot diameter

$$d_0 \;=\; \frac{4\,M^2\,\lambda\,f}{\pi D}$$

(see Focusing a Laser Beam). The wavelength enters directly. At $f/D$ = 10 and $M^2$ = 1, a 1070 nm beam focuses to 13.6 μm and a 10.6 μm beam to 135 μm. The diffraction-limited beam parameter product, $\lambda/\pi$, is 0.34 mm·mrad at 1070 nm and 3.37 mm·mrad at 10.6 μm, a factor of ten.

The factor shows up either as a smaller spot or as a longer Rayleigh range. For the same 150 μm spot, $z_R = \pi w_0^2/(M^2 \lambda)$ is 16.5 mm at 1070 nm and 1.7 mm at 10.6 μm for beams of equal $M^2$. Multi-kilowatt fiber lasers are usually multimode, with $M^2$ well above 1, so the practical advantage is smaller than the tenfold ideal, but it remains large enough to give narrow kerfs and fast cutting of thin sheet.

Irradiance. A 2 kW beam focused to a 100 μm diameter spot has an average irradiance of $2\,\text{kW}/(\pi\,(50\ \mu\text{m})^2)$ = 2.5 × 10⁷ W/cm² within the 1/e² diameter, well above the roughly 10⁶ W/cm² at which keyhole welding begins. Reaching that spot size at 10.6 μm needs a faster lens and gives a far shorter depth of focus.

Beam delivery and efficiency

Silica fiber transmits 1 μm light with negligible loss over tens of metres, so a fiber laser's output, and the fundamental of a Nd:YAG laser, can be carried in a flexible process fiber to a robot-mounted head. Silica and ordinary glass absorb 10.6 μm, so CO₂ beams travel through free space along mirrors and are focused with zinc selenide lenses or copper mirrors, which ties the laser to a gantry or articulated arm and needs periodic alignment.

Efficiency affects running cost and cooling. A fiber laser converts roughly a third to a half of its electrical input to light, a sealed or flowing CO₂ laser roughly a tenth to a fifth, and a lamp-pumped Nd:YAG a few percent; diode pumping raised Nd:YAG and Nd:YVO₄ considerably. The rest becomes heat in the chiller.

Which laser for which job

Nd:YAG has lost most continuous-wave work to fiber lasers, which reach the same wavelength with better efficiency and beam quality. It remains where its stored energy matters: lamp-pumped pulsed welders for jewellery and spot welding deliver joules per pulse, and diode-pumped Q-switched Nd:YAG and Nd:YVO₄ lasers with harmonic generation supply the green and ultraviolet sources used in marking, scribing and micromachining.

Safety

The two wavelength regions carry different eye hazards. Light between about 400 and 1400 nm, including 1064 and 1070 nm, passes through the eye and is focused on the retina, and at 1 μm it is invisible, so a reflection can cause permanent damage without any visible warning. Light at 10.6 μm is absorbed at the front of the eye and in the skin, a hazard to the cornea and a burn hazard rather than a retinal one. Industrial systems are usually fully enclosed; open-beam work needs eyewear rated for the specific wavelength, and eyewear for one does not protect against the other.

References: W. M. Steen and J. Mazumder, Laser Material Processing (4th ed., Springer, 2010); W. Koechner, Solid-State Laser Engineering (6th ed., Springer, 2006); ANSI Z136.1, American National Standard for Safe Use of Lasers.