13

May

Laser beam’s wavelength determines its ‘color’

May/June 2013 Volume 6 Issue 3

Ronald D Schaeffer - PhotoMachining

Ronald D. Schaeffer
CEO, PhotoMachining, Inc.
rschaeffer@photomachining.com

Lasers operate across a spectrum of light, from the far-infrared (IR) through the visible and into the deep-ultraviolet (UV). Wavelength greatly influences light’s absorption by a specific material and the mechanism by which energy is transferred into that material.

These factors affect processing results.

Figure 1 depicts the electromagnetic spectrum. Each portion of the spectrum represents a different form of radiation, including visible, UV and IR light, microwaves, radio waves, X-rays and gamma rays.

All electromagnetic radiation travels at the speed of light: 299,792,458 m/sec. Each form of light has an associated wavelength and frequency, based on the equation

c = ??, where

c is the speed of light in m/sec.;

lambda (?) is the wavelength in meters;

and nu (?) is the frequency (waves that pass a point per second), given in s-1.

From the left side of Figure 1, the wavelength increases when moving right while the frequency decreases, such that the speed of light remains constant.

laser-wavelength-chart.jpg

Figure 1: Laser wavelengths fall on the electromagnetic spectrum between the far-infrared and the deep-ultraviolet.

Depending on what’s done to light, it can behave as a wave or particles (photons). With respect to material removal, such as when laser cutting or drilling, we are more interested in the “particle” behavior of light. A laser’s wavelength determines how a beam interacts with a workpiece material. Long-wavelength (low-frequency) photons have lower energy than short-wavelength (high-frequency) photons.

On the high-energy side of the electromagnetic spectrum, UV lasers—in principle—generate ample photonic energy to interact with a material’s outer electrons. This leads to a photochemical bond-breaking mechanism of the first order and, subsequently, ejection of material. On the low-energy side of the spectrum, IR lasers remove material via a first-order thermal mechanism.

Some materials absorb light uniformly across the wavelength spectrum. Polyimide is a good example. Other materials—Teflon, for instance—absorb well in the IR range but not in the UV range, and vice versa. How well a material absorbs light is the most important criterion when lasing, which makes it important to choose the laser color the workpiece material absorbs best.

4-in-1 laser

One laser able to deliver colors from across the lasing spectrum is the Nd:YAG (neodymium-doped yttrium-aluminum garnet) and its derivatives. Nd:YAGs have a fundamental wavelength of 1,064nm, in the IR range. Because it’s a first-order thermal process, the IR 1,064nm wavelength couples well with most metals and is often used for laser micromachining, as well as welding, soldering and joining.

Lasers for joining typically are of the continuous-wave variety or have long pulse lengths in order to maximize heat input. Lasers for machining usually emit pulsed beams, which increase peak power on target.

Because of its fairly short pulse length and high pulse energy, an Nd:YAG’s fundamental output can be focused into a nonlinear crystal. The result is a 532nm wavelength, which is twice the frequency of the fundamental wavelength, and falls in the green area of the visible spectrum.

The conversion efficiency when doubling the frequency is not 100 percent. It’s usually closer to 50 percent. So, there’s a price to pay in terms of total laser output power, but the availability of green photons is helpful for some applications. For example, they couple well with some metals, notably copper-containing compounds. Green is also useful for processing thin films on visibly transparent glass or polymers.

One drawback of a 532nm laser is its beam can damage the human eye, because the wavelength falls in the middle of the visible spectrum. So, extreme care must be taken when using it, including wearing the proper safety glasses.

Scribing solar panels is an example of how two lasers that are the same except for their wavelengths can be used in combination. Solar panels are scribed numerous times. The first layer (P1) is a transparent conductive-oxide (TCO) coating scribed with an IR laser having a fundamental wavelength (1,064nm). Then, for increased efficiency, a green laser is focused through both the glass and TCO to process the P2 (amorphous silicon) and P3 (metallic conductor) layers.

Frequency tripling results in UV photons at a 355nm wavelength. This is a good all-around wavelength that couples well with many materials, including metals, ceramics, polymers and other dielectrics. This wavelength is reasonably gentle on optics, and operating a 355nm laser requires no extraordinary safety precautions. Commercial lasers are available with outputs from a couple of watts to more than 50w, with pulse lengths down into the femtosecond range.

Frequency quadrupling yields UV photons at 266nm. This wavelength couples better with almost all materials compared to a 355nm wavelength. Commercially available 266nm lasers come with only a few watts of output power, however, and the optics (external and internal) degrade faster than the optics of 355nm units. In general, 266nm lasers are only available with nanosecond pulse lengths.

By using well-known and understood conversion methods, users can get four different wavelengths from one laser. A caveat, though, is that total output power declines about 50 percent with each conversion step. That will affect the decision about which wavelength to choose for a specific application. µ

— R. Schaeffer