Subcategory: Physics (not Nanoscience)
Harry Burton - Delaware State University
Co-Author(s): C. Debardelaben, P. Chrostoski, A. Bachavala, W. Amir, and T. A. Planchon, Delaware State University
Active optics, such as Spatial Light Modulators (SLM), have been widely used as a means of correcting for atmospheric turbulence in ground-based telescopes. Since their conceptualization, they have seen increasing popularity as a method of altering the spatial distribution (profile) of light sources. In high-intensity laser systems, it has been used to correct for wavefront aberrations [1,2], which maximizes the laser intensity at the focus of the laser for plasma physics experiments. However, since laser-matter interaction happens within an interaction volume – not just the focal plane – a method is needed to optimize the interaction volume instead of a single transverse plane. Here, the method developed uses a genetic algorithm (GA) [3,4]. This type of learning algorithm is useful to find a solution that has no analytical form. For focusing a laser, what is desired is the characteristic of the spatial phase that can generate a specific volume shape. The experimental setup starts with a 488 nm cw laser, which reflects from a 512×512 pixel SLM and forms a reference focal volume very similar to a Gaussian beam. The GA finds the best solution for the phase on its own, by obtaining optical feedback from a CCD camera; rather than us assigning a target phase. We used the similarity between CCD image planes to generate a longitudinally elongated focal volume, and different focal volume shapes by using different fitness functions. The resulting wavefronts were then measured by inserting the SLM within a spatial interferometer. In conclusion, for the specific purpose of controlling the characteristics of the interaction volume, we developed an approach using a genetic algorithm (GA) with fitness function simultaneously based on the intensity profiles of the laser at five (or more) different transverse planes. By using this strategy, we were able to obtain convergence of the algorithm towards a self-reconstructing beam (Bessel beams), and different shapes based on minimization of energy, size or similitude between profiles. This approach has potential application in several fields, like optical microscopy, or optical beam trapping.
References:  Chanteloup, J., et al., Nearly diffraction-limited laser focal spot obtained by use of an optically addressed light valve in an adaptive-optics loop, Optics Letters, 1998, 23, p 475
 Bahk, S., P. Rousseau, T. Planchon, et al., Generation and characterization of the highest laser intensities (10^22 W/cm2), Optics Letters, 2004, 29, p 2837
 Pal, S., et al., Artificial intelligence programming with LabVIEW: genetic algorithms for instrumentation control and optimization, Computer Methods and Programs in Biomedicine, 1995, 47, p73
 Planchon, T., W. Amir, et al., ‘Adaptive correction of a tightly focused, high-intensity laser beam by use of a third-harmonic signal generated at an interface’, Optics Letters, 2006, 31, p. 2214
Funder Acknowledgement(s): This work was supported, in part, by the National Institute of Health NIH NIGMS IDeA program (grant #P20 GM103446), the National Science Foundation (NSF-CREST grant #1242067), and the National Aeronautics and Space Administration (NASA URC 5 grant # NNX09AU90A).
Faculty Advisor: Thomas Planchon, firstname.lastname@example.org
Role: My responsibility was in utilizing the Genetic Algorithm needed to modulate the beam focus, as well as developing both the interferometry setup and LabVIEW VI necessary to perform the experiment.