Femtosecond Laser Filaments for Use in Sub-Diffraction-Limited Imaging and Remote Sensing

I'm pleased to introduce Professor Sokolov and his team. They were the first to use high-power lasers to detect anthrax in real time. That is to say in a nanosecond, not in several seconds.

They have then gone on to work on other problems about the filamentation of light, the focusing of light, and ways in which we can use these techniques to go beyond the standard quantum limits. We describe an experimental protocol to use femtosecond laser filaments in order to achieve sub-diffraction limited resolution at distances that would be intractable classically. Laser filaments can maintain high intensity and sub-millimeter diameter over long propagation distances.

This enables sensing, scanning, imaging spectroscopy with enhanced resolution. The laser filamentation can be generated in many media including atmosphere and water. The technique can be adapted to ocean optic studies.

It's not easy to generate the laser filamentation. One useful trick is to adjust the chirp of the pulse to achieve the necessary intensity. So today we're going to be seeing the experiments on filamentation, focusing of light into tiny fibers, and this is something which, put in the present context, helps us to visualize what we're doing with the experiments, from detecting anthrax to looking at ocean optics.

Set up the apparatus on an optical bench and follow safety precautions for a Class 4 laser. To create the filament, use a pulsed, amplified femtosecond titanium sapphire laser. Pass the laser beam through an iris that slightly clips the outer edges.

The sharp gradient in the spatial intensity profile caused by clipping the laser is known to seed filament formation. Next, pass the beam through a converging lens with a focal length greater than 200 centimeters. Help seed self-focusing by slightly tilting the lens with respect to the propagation direction.

Arrange to have an appropriate beam dump after the geometric focus of the lens. To observe a filament, operate the laser with an instantaneous output power sufficient for self-focusing in air. Look for filamentation near the geometric focus of the lens using white paper.

With the paper in the beam path, search for a diffused halo of several millimeters surrounding a flickering, bright core of about 100 micrometers. Make further observations beyond the filament. There, bright, multicolored, conical emission rings are the result of a characteristic self-phased modulation process in the air.

Multiple bright spots indicate there are multiple filaments. To eliminate the bright spots, introduce attenuation in the beam before the iris. With the proper attenuation, the bright spots in the conical emission pattern are eliminated.

Prepare to conduct a test of remote scanning with the laser. Secure a two-axis motorized translation stage in the beam path so that it translates perpendicular to the beam. Ensure that the laser beam filament is incident at the center of the stage.

Next, create a target to scan with the system. Obtain a container and place two millimeters of sand on its bottom. Put copper, aluminum, and stainless steel objects on top of the layer of sand.

Then, put another two-millimeter layer of sand on top of the metals. With the laser off, put the container in the center of the translation stage where the filamentation occurs. In order to collect data, connect the spectrometer output to the computer.

Set up the external trigger and computer control for the laser to fire a single shot. Next set up the sensor apparatus. In this case, position a spectrometer so its entrance points at the filamentation impact point on the translation stage.

Use the lens to couple light from the impact point into the spectrometer. Place the lens from one to two focal lengths from where the filamentation occurs. Trigger the laser by software and record the signal from the spectrometer.

This arrangement of copper, aluminum, and stainless steel is obscured beneath about two millimeters of sand. The spectral features of the buried metals as measured by the setup allow the creation of a composite image with copper in green, aluminum in red, and stainless steel in cyan. A non-filamented laser beam at the diffraction limit scanned over a small, printed, Texas A&M logo does not reveal recognizable text.

By contrast, a filamented laser beam scanned over the logo generates an image with discernible elements. Pulse energy and intensity are very important parameters for generating laser filamentation. Use of laser filamentation in remote spectroscopy may increase signal-to-noise ratio in remote sensing.

This technique paved the way to achieve the high spectral resolution in remote sensing. Class 4 lasers necessary for this work are dangerous. Experimenters should wear personal protective equipment and follow all safety protocols.