While precise and fast, the down side to cutting with microsecond (ms) fiber lasers has been that the parts require a number of post-processing operations after they are cut, which add significantly to part cost, and can also damage mechanically delicate parts.

Schematic processing comparison of microsecond to femtosecond lasers.
In recent years, ultra-short femtosecond (fs) laser technology has been introduced, which produces pulses that leave no thermal fingerprint on the part. The disk-based femtosecond lasers offer sub-400 fs pulses, plus best-in-class beam quality and peak power that enable an extremely high-quality cold ablation cutting process, rather than a melt ejection process. The resulting cut therefore requires minimal post-processing, and the smaller beam size allows machining of very fine details.

In the past, fs lasers have been considered too slow for commercially viable operations. Recent studies evaluated cutting time per part and post-processing steps, demonstrating that the return on investment for a disk fs laser can, in many cases, be less than 12 months, especially for high-value components. A key aspect of realizing the fs laser’s potential is the system platform.

Femtosecond light pulses are ultra-short pulses (USPs). One fs equals 10-15 seconds and as a calibration point, a 300-fs pulse equates to a physical length of pulse of only 90 micrometers (μm). Since there is no thermal processing as there would be in nanosecond (ns) pulses, USPs have many advantages, including no heat impact, which means no thermal tension in the material and no change of material characteristics; no shock waves or structural changes; no micro-cracks, resulting in smooth, processed surfaces; no melting effects; no surface damage requiring rework or after-processing; no debris; no ejected material; and no recast layer. The figure illustrates these effects on an application using a long pulse laser (for example, μs) compared to that of an ultra-short pulse laser like an fs laser.

Femtosecond laser technology has been widely used in institutions and research centers for more than 30 years, but commercial-ready fs technology that can last in an industrial environment with a 24/7 qualification has only been around for about seven years. Originally used for wafer dicing and scribing of P1, P2, and P3 solar panels or creating channels in panels for electrodes, fs lasers are now advancing into a new wave of machining capabilities.

The fs disk laser can create unique features that were previously not possible due to quality concerns, particularly with polymers processing. The nearly cold cutting process means very fine feature sizes can be cut into the thinnest material, while still maintaining mechanical and material integrity. No internal water cooling is needed for even the smallest Nitinol diameter tube.

The industrial robustness of the disk fs laser needs to be matched to an equivalent system to deliver precise and repeatable machining, day-in and day-out. Designing precision micromachining systems may appear to be just a question of determining how much granite is needed. While there is no doubt that mechanical stiffness and isolation are required, this is simply the starting point. Determining how delicate parts and materials will be repeatedly positioned or clamped, implementing in-system part inspection, and incorporating real-time optical beam diagnostics are several other key pieces of the puzzle.

For example, the beam is directed through the system by mirrors, so maintaining optical alignment is important, but this is only the first step. Ensuring that the beam profile and power levels are maintained requires the use of optical diagnostic tools, and these tools must be in-line and non-intrusive, providing real-time information.

The femtosecond disk offers unique in-class process capability with excellent beam quality and high peak powers. To maximize the process capability for production, the laser must be integrated into a system that enables high quality and repeatable processing.

This work was done by Geoff Shannon of Amada Miyachi America and Steven Hypsh of Jenoptik. For more information, Click Here .

NASA Tech Briefs Magazine

This article first appeared in the June, 2015 issue of NASA Tech Briefs Magazine.

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