New laser-focusing technique emerges
I am a big fan of technology that is scientifically interesting, simple and elegant in concept—and actually solves problems that would be difficult or impossible to solve otherwise. Simultaneous Spatial and Temporal Focusing (SSTF) is an example of such a technology.
SSTF reduces unwanted effects in the focal region of an ultrashort-pulse laser (USPL). Few people outside of a small academic circle have heard of the technique, but, in certain applications, it has some powerful advantages over currently accepted approaches.
The significance of SSTF is that it permits the use of ultrashort pulses, which, in turn, allow the processing of certain materials that are nearly impossible using conventional focusing techniques.
Figure 1: With Simultaneous Spatial and Temporal Focusing, the laser light’s “colors” are separated in one spatial dimension, for passing through the beam-delivery system, then are recombined at the focal point. SSTF facilitates the laser machining of cleaner features with minimal tapering. Images courtesy Colorado School of Mines.
As has been pointed out often in this column, ultrashort pulses facilitate high-precision laser micromachining. Of great interest is the use of USPLs to manufacture glass-based microfluidic medical devices.
The Boulder, Colo., area is home to an eclectic group of organizations researching USPLs, especially femtosecond lasers. The group includes the Colorado School of Mines, NIST (National Institute of Standards and Technology), University of Colorado, Colorado State University and KMLabs Inc. SSTF for micromachining was developed at the Colorado School of Mines by Professor Jeff Squier and some of his colleagues, and the technique is licensed exclusively to KMLabs for commercialization.
I discussed SSTF and its applications with Squier and Dr. Chris Wood, chief technology officer of KMLabs. An edited version of our discussion follows.
How it works. Figure 1 depicts the process. A single-pass, double-grating configuration is used to take temporally chirped beams and spatially chirp them. In this case, the incoming beam’s pulse duration is less than 100fs. In accordance with established principles of physics, the shorter a laser pulse is temporally, the larger the bandwidth must be. So, a very short femtosecond pulse always has a large bandwidth—a consequence that affects optics design for USPLs.
SSTF separates laser light “colors” in one spatial dimension, for passing through the beam-delivery system, and then recombines them at the focal point. Therefore, the separated multicolor beam can pass through the optical train—even through otherwise transparent substrates—without any attenuation or troubling side effects, like nonlinear interactions.
Figure 2a depicts the focusing geometry, and Figures 2b and 2c show a femtosecond beam without and with SSTF. Without SSTF, the beam’s intensity is sufficient to generate a supercontinuum in the material that attenuates the energy and creates undesired side effects. With SSTF, the plasma is created at just the focal point, as the intensity only reaches a critical threshold at that single (spherical) point in space.
Advantages of SSTF. As an extension of the beam’s traveling through the optical system without attenuation or side effects, SSTF can be used to focus on the back side of an otherwise transparent substrate. In most microfabrication techniques, a laser beam is focused on the front surface of a substrate and ablation proceeds from front to back. As a result, successive pulses must focus through debris and plasma produced by earlier pulses. Eventually, the beam interacts with the side walls, leading to the formation of taper and limiting the attainable aspect ratio.
Figure 2: Focusing geometry with Simultaneous Spatial and Temporal Focusing (a), and a femtosecond beam without SSTF (b) and with it (c). Note that with SSTF, the plasma is created at just the focal point.
With the SSTF technique, ablation starts on the back side of the substrate surface and ablated byproducts are ejected on the side opposite the incoming beam. Thus, it is possible to produce clean features with almost no taper and at very high aspect ratios.
Drawbacks to SSTF. Because of the gratings and multicolor light, which is also spatially separated, the beam is elliptical instead of circular when transmitted through the optical system. There is spectral spacing in one direction, and this must be addressed by using larger optics.
Moreover, simple refractive optics generally will not work well because each of the wavelengths focuses at a different point in space. Reflective optics currently are preferred when employing SSTF. Colorado School of Mines and KMLabs are collaborating to develop simple optics that will counter and correct these issues.
Availability and cost. Models have been developed that can work with existing optical systems. And, via a simple changeover process, users can switch between laser machining with and without SSTF. Commercial models are not currently available. It’s estimated that unit costs will be similar to other existing beam-shaping strategies, such as homogenization or polygon scanning.
SSTF has been demonstrated as a viable technique for the use of temporal focusing with low-NA (numerical aperture) femtosecond lasers. The technique mitigates nonlinear interactions such that the beam maintains sufficient coherence over long path lengths to allow ablation of optically transparent materials. Much higher-aspect-ratio features can be realized with this technique than with, for instance, chemical etching or even front-side ablation.
Squier and Wood said SSTF should be considered another tool for femtosecond-laser processing, especially for single-step fabrication of devices made from transparent materials. Other application areas include intraocular lenses and, perhaps, certain surgeries. µ