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Fixed and Variable Pulse Fiber Lasers for Marking Plastics

by Scott R. Sabreen, president

The Sabreen Group, Inc.

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Temporal pulse shape IPG fixed pulse length laser at 100 ns pulse length.


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Temporal pulse shape IPG MOPA laser at 4 ns pulse length.

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The emergence of Nanosecond Ytterbium Fiber lasers is one of the most significant advancements for marking, welding and cutting. Fundamentally, fiber lasers are different than other solid-state marking lasers. Both fixed and variable pulse length MOPA fiber lasers are utilized based upon the application requirements. As with many new technologies there is often confusion and unintended misrepresentation of important terminology and distinctive factors. The word “fiber” is used to identify a “fiber delivered” laser, when in actuality it is not a real “fiber laser.” The purpose of this article is to educate readers and clarify the terms “fiber” and “fiber laser.”

With fiber lasers, the active medium that generates the laser beam is dispersed within a specialized fiber optic cable. In contrast to fiber-delivered lasers, the entire path of the beam is within fiber optic cable all the way to the beam delivery optics.

“Fiber delivered” laser

A “fiber delivered” laser beam is one that is generated using conventional free space optics technology via other solid-state media such as rods, discs or slabs and is then focused down into a fiber and delivered to the workpiece via this flexible fiber optic cable. This “fiber delivery” technique has been in use with flashlamp-pumped solid state lasers for almost 30 years and does indeed reduce many problems associated with building laser systems in that the laser can be remote from the actual workstation. More recently this technique is employed by direct diode lasers where complex optical techniques are used to combine very many individual diode laser beams into a fiber for delivery to the workpiece. Although these lasers are still “free space” devices in one sense, the diode lasers can be squeezed into a much smaller “free space” and the benefits of fiber delivery are maintained.

Real fiber laser

A real fiber laser is a totally different disruptive technology that has many major attributes that none of these other laser types can match. The very essence of a real fiber laser, and this does bear repeating, is that the beam is generated in the fiber itself. This is achieved by writing optical gratings into the fiber, and these act as the partially and totally reflecting mirrors required to make up the laser resonator. This active fiber or gain medium within which the laser beam is generated is drawn down contiguously with layers of cladding. Pump light is introduced into the cladding layer by splicing larger core diameter fibers from laser diodes into the cladding and the outer cladding confines this pump light. This elegant solution to the problem of generating laser beams is scaleable simply by increasing the length of the active fiber, by increasing the number of pump diodes or for high power applications where focusability is not so important, using optical combiners to combine many beams together. In this way the average power of fiber lasers is being increased to average powers far in excess of anything available from other technologies.

Fiber lasers yield superior beam quality and brightness. One metric for beam quality is M2. The smaller the M2 value, the better the beam quality, whereas M2 = 1 is the ideal Gaussian laser beam. A laser with superior beam quality can be focused to a small spot size, which leads to high energy density. Fixed and variable pulse (MOPA) fiber lasers with pulse energy up to 1 mJ and high power density can mark many historically difficult polymers. Vanadate lasers also possess a small M2 value with shorter pulse width than fixed fiber and YAG lasers. Pulse duration influences the degree of heat and carbonization into the material. Short(er) pulse width can be advantageous for sensitive polymeric applications.

IPG Photonics, a leading developer and manufacturer of high performance fiber lasers, offers both fixed pulse (sometimes referred to as “Q-switch”) and variable short pulse (MOPA) lasers. Application develop is highly specific. The selection of which laser type to integrate is determined by the output characteristics of the laser interacting with the optimized polymer material. Reference the graph illustrations below representing Temporal Pulse Shapes of Fixed & Variable (MOPA) Pulse Length Ytterbium Fiber Lasers - IPG Photonics.1

When setting up a fixed pulse length fiber laser for marking, two inputs must be set:

  1. Pulse repetition rate (often referred to as pulse frequency).
  2. Pump power in percent. One hundred percent refers to the maximum possible electrical input to the pump diodes.

When setting up a variable short pulse MOPA fiber laser for marking, three inputs are set:

  1. Pulse duration (often referred to as pulse length).
  2. Pulse repetition rate (pulse frequency).
  3. Pump power in percent. One hundred percent refers to the maximum possible electrical input to the pump diodes.

For both graphs, the particular combination of parameter inputs controls the output properties of the laser beam, namely the pulse energy, the peak power (the highest instantaneous peak of the pulse energy, J/pulse duration) and the average power (average power in Watts = pulse energy in joules x pulse repetition rate in Hz).

All beam-steered fiber lasers are not created equal. The hardware and software components laser manufacturers incorporate into their systems makes significant difference in marking contrast, quality and speed. A primary attribute is the power density (watts/cm2) at the mark surface (which is different than the raw output power of the laser). The output mode of the laser beam is critical to the marking performance. These output modes relate to factors including the beam divergence and power distribution across the diameter of the laser beam.

Power density is a function of focused laser spot size. Focused laser spot size for any given focal length lens and laser wavelength is a function of laser beam divergence which is controlled by laser configuration, mode selecting aperture size and upcollimator (beam expander) magnification. Pulse repetition rate and peak power density are critical parameters in forming the mark and achieving the optimal contrast and speed. The arithmetic curves of power vs. pulse repetition rate are inversely proportional. High peak power at low frequency increases the surface temperature rapidly, vaporizing the material while conducting minimal heat into the substrate. As the pulse repetition increases, a lower peak power produces minimal vaporization but conducts more heat. Additional contributing factors that influence the marking contrast and quality are, of course, beam velocity and the vector line separation distance.

Scott R. Sabreen is founder and president of The Sabreen Group, Inc., which is an engineering company specializing in secondary plastics manufacturing processes – laser marking/laser welding, surface pretreatments, bonding decorating and finishing, and product security. He has been developing new technologies and solving manufacturing problems for over 30 years. Sabreen can be contacted at 972.820.6777 or www.sabreen.com or www.adhesionbonding.com.

1. Acknowledgement – Dr. Tony Hoult (IPG Photonics)