Laser light is monochromatic, coherent and collimated. In other words, photons tightly oscillate in step and at one single frequency. The spatial coherent property of the laser light propagating in complex media is the subject of my research. Spatial degree of coherence is what basically sets the laser light apart from the other light in nature, making possible directed energy applications. Photons both change their paths and have their coherent properties altered when they propagate through the environment. Maximizing the delivered energy in the complex medium is the focus of my research the result of which would improve data links in free space optical communications and increase efficiency of directed energy applications.
When a laser light beam propagates in maritime environment, it encounters turbulence along its path. Moving aerosols and temperature variations influence the changes in refractive index, creating optical turbulence. As a result, the light intensity fluctuates, or scintillates, on the target. The Academy grounds provide a natural maritime environment for our laser beam propagation investigations.
We initially propagated fully coherent laser beams across Sherman Field at the Academy, and obtained probability density function of intensity changes over the several hundred meters long links [j10]. Laser beam propagation analyses uses stochastic approach, so our focus is on the probability distribution determination using moments obtained from field experimental data. This approach distinguishes our work from the rest of the scientific literature.
In the next series of experiments we created spatially distributed laser beamlets with slight variation in coherence among them. When combined at the target, this light is called spatially partially coherent beam and theoretically has higher chance to propagate through complex medium with reduced scintillation. Our experiments at the Hospital Point clearly demonstrated this property [j6]. A ‘sweet spot’ of coherence, with minimal scintillation for the utilized optical link, was found. This result was a significant theoretical verification.
Electromagnetic laser beams are constructed by combining vertically and horizontally polarized scalar beams originating from the same source. We created spatially partially coherent electromagnetic beams and propagated along about 50 m corridor links. In these experiments we introduced both the varying spatial partial coherence, polarization, and changed the beam profile from flat top to ring shaped beams [j5, j6]. The experimental data demonstrated 50% scintillation reduction when electromagnetic beam is compared to the scalar beam. This work is the most cited among my laser research publications.
Flat top beams are constructed by summing Gaussian (bell shaped) beams with adequate weights in order to create the beam that has uniform intensity cross section. These beams are defined by using two parameters: coherence and flatness levels. The performance of the flat top beam when coherence and flatness parameters varied [j3] was observed over a 70 m corridor optical link. Theory predicts that the scintillation is unaffected if you add more than a specified number of Gaussian beams for a given coherence level. In this case a distinct flatness-coherence threshold relationship was measured and theoretical trends were confirmed.
Over the course of eight years, I developed a sophisticated mobile experimental test bed with range of varying beam properties. This capability facilitated consistent verification of scintillation reduction on target when spatially partially coherent beams are propagated in actual complex media. Last year my interest was shifted to scattered light underwater because the water environment offers a possibility to study complex medium. I measured scattered light intensity and normalized variance and demonstrated lower levels for the partially coherent vs fully coherent light. This property is beneficial in covert military actions. This year I had the opportunity to present these experiments at Optics Society of America Imaging and Applied Optics Congress and publish the detailed results [j2]. Since this research is unique, the presentation created a significant national and international interest and opened the possibilities for extensive collaborations.
This summer I created electromagnetic beams with the flat top profile and tested their performance underwater in the presence of moving scatterers [j1]. So far, this is the most complex environment I have worked with, and our findings point to better scintillation performance of spatially partially coherent beams in contrast to fully coherent beams. This result elevates the spatially partially coherent laser beams for possible use in underwater communication applications.
The most significant impact of my laser research are the experiments conducted in the field. By exploring the laser light performance outside the laboratory I was able to show that actual propagation and scattering of spatially partially coherent laser light in maritime environment offers a viable method in basic research of laser light in complex media.