Combined Software Tools Model Effects of Optics on Ultrashort Pulses
Researchers and optical systems designers at Fraunhofer Institut für Angewandte Optik und Feinmechanik in Jena, Germany, faced a short and theoretical problem, but one with lengthy and real consequences: Laser pulses with durations of less than 100 fs are important in multiphoton laser scanning microscopy and ultrafast laser micromachining, but analytically predicting the effect of real-world optics on them was possible only under certain restrictive situations.
Ray-tracing and wave optical propagation software tools were combined to simulate the propagation of ultrashort laser pulses through complex optical systems, such as a microscope objective (top) and an aspheric lens (bottom). In these examples, which show the region ±200 µm from the focus, the original laser pulse duration was 50 fs. Courtesy of Ulrike Fuchs.
"It was not possible to calculate the impact of a focusing optic consisting of more than one lens and not being achromatic," explained Ulrike Fuchs, a researcher at the institute.
The investigators have developed a solution by combining ray-tracing and wave optical calculations, splitting the ultrashort pulse problem into two parts and solving each with a different software package. The technique can handle achromatic lenses, diffractive optical elements, reflective optics and less-than-ideal laser pulses. Moreover, it works even as the optics become more complicated and does so without much of a penalty, Fuchs said.
"The effort of the calculation of the propagation of laser pulses through optical systems is nearly independent of the number of elements within the system," she explained.
In the procedure, the propagation is first calculated by ray tracing, which decomposes the pulse into its spectral components. Then wave optical methods determine the propagation into the focal region, an area in which the geometric optics used in ray tracing are not valid. Such techniques have been used to calculate monochromatic point-spread functions of optical systems, but Fuchs said that extending them to polychromatic ultrashort pulses requires accounting for a number of details.
In the May 16 issue of Optics Express, the researchers demonstrated that this combination approach works, using ray-tracing software from Zemax Development Corp. of Bellevue, Wash., and wave optical simulation software from LightTrans GmbH of Jena. Any software could have been used, they said.
They looked at three optical systems in the work and showed that their computed outcome matched real results. For instance, their technique predicted that a 24-fs pulse centered at 800 nm and traveling through a microscope objective would lengthen significantly -- and it does. They also calculated the amount and kind of pulse distortion caused by the optics.
They uncovered two side effects from the focusing of ultrashort pulses. One is a boundary wave pulse, or forerunner pulse, that appears to be an extra pulse originating from the system aperture. This has been identified in other numerical calculations and has been observed in experimental setups. The second is a stationary intensity distribution along the optical axis, which has not been directly identified in experiments but which may be behind experimental reports of streaks.
Fuchs believes that there is no reason why the two-step method could not be accomplished by a single piece of software, and Frank Wyrowski, the founder of LightTrans, said that the company's VirtualLab 3 optics software will model the propagation of ultrashort pulses. The software is slated for release in the fall.
For their part, the Fraunhofer researchers will continue to use two packages to meet their lens design requirements. Their goals involve the inclusion of nonlinear interactions between the pulse and materials, such as glass, that are manipulated by the focused pulse. Such interactions must be taken into account in the design of optical systems, Fuchs said.
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