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Complicated Pulses Measured

Photonics.com
May 2008
ATLANTA, May 12, 2008 -- A device that reveals spherical aberrations in optical components could help biologists, chemists, and other nonlaser scientists easily measure complicated ultrashort laser pulses.

Lasers that emit ultrashort pulses of light are used for applications including micromachining, microscopy, laser eye surgery, spectroscopy and controlling chemical reactions. But the quality of the results is limited by distortions caused by lenses and other optical components that are part of the experimental instrumentation.

To better understand the distortions, researchers at the Georgia Institute of Technology developed the first device to directly measure complex ultrashort light pulses in space and time at and near the focus. Measuring the pulse at the focus is important because that’s where the beam is most intense and where researchers typically utilize it. Knowing how the light is distorted allows researchers to correct for the aberrations by changing a lens or using a pulse shaper or compressor to manipulate the pulse into the desired form.SeaTadpole.jpg
Georgia Tech physics professor Rick Trebino and graduate student Pam Bowlan with Sea Tadpole, a device that allows nonlaser scientists to easily measure complicated ultrashort pulses. (Georgia Tech Photo by Gary Meek)
“Researchers have always measured the pulse immediately as it exited the laser, so they didn’t realize the extent to which the pulse became distorted by the time it reached the focus after traveling through the optics and lenses in the system,” said Rick Trebino, a professor in Georgia Tech’s School of Physics and Georgia Research Alliance Eminent Scholar in Ultrafast Optical Physics.

It is difficult to measure ultrashort pulses because they typically last between a few femtoseconds and a picosecond, which are 10-15 and 10-12 of a second and faster than the response time of the fastest electronics.

“The light comes out as a train of extremely short bursts. The laser crams all of the energy of a continuous laser into a few femtoseconds, which creates really intense laser pulses,” said graduate student Pam Bowlan.

To achieve the highest possible intensity of the laser, the pulse must be as small as possible in space and as short as possible in time. However, focused pulses nearly always have distortions in time that vary significantly from point to point in space due to lens aberrations in focusing optics.

To address those issues, the new device, called Sea Tadpole (spatial encoded arrangement for temporal analysis by dispersing a pair of light e-fields), allows researchers to measure complicated ultrashort pulses simultaneously in space and time as they go through the focus.

“A lot of chemists and biologists use ultrafast lasers, so it was important that our device be easy to use because nonlaser scientists don’t want to spend all day measuring their laser pulses,” Bowlan said.

The research team -- which also included former graduate students Pablo Gabolde and Selcuk Akturk -- used the concept of interferometry to measure a pulse in space and time. Two pulses, one reference and one unknown, were sent through optical fibers. The fibers were mounted on a scanning stage so that the pulses could be measured at many locations around the focus.

The pulses were crossed and an interference pattern was recorded for each color of the pulse at each location with a digital camera. The patterns were used to determine the shape of the unknown pulse in space and time and to create movies showing how the intensity and color of the pulse changed in space and time as it focused.

“Because the laser pulses enter Sea Tadpole through optical fibers, which only collect a very small portion of the light, the device naturally measures pulses with high spatial resolution and can measure them at a focus spot size smaller than a micron,” said Bowlan. To further improve the spatial resolution of the device, the research team began to use specialized fibers, called near-field scanning optical microscopy fibers, which can resolve features smaller than the wavelength of the light.

The researchers tested the device by measuring ultrashort pulses focused by various lenses, since each lens can cause different complex distortions. To validate the measurements, Bowlan performed simulations of pulses propagating through the experimental lenses. Results showed that a common plano-convex lens displayed chromatic and spherical aberrations, whereas more expensive aspheric and doublet lenses exhibited mostly chromatic aberrations.

Spherical aberrations occur when the light that strikes the edges of the lens gets focused to a different point than the light that strikes the center, creating a larger, inhomogeneous focused spot size. Chromatic aberrations occur because the many colors in the laser travel at different speeds and do not stay together in space and time as the pulse passes through glass components in the experimental setup, such as lenses. As a result, each color arrives at the focus at a different time, creating a rainbow of colors in the electric field images.

Aberrations can drastically increase the pulse length, which decreases the laser intensity. A lower intensity forces researchers to increase the power of the laser, increasing the possibility of damaging the sample. Aberrations can also yield odd pulse and beam shapes at the focus, which complicate the interpretation of the experiment or application.

“Our system tells researchers what types of aberrations are present in instrumentation, which then allows them to test different lenses in the instrumentation setup or use a pulse shaper to create the desired pulse at the focus that’s free of distortions,” Bowlan said.

The device was described in a presentation at the Conference on Lasers and Electro-Optics (CLEO) in San Jose, Calif., on May 8.

The research was funded by the National Science Foundation and published in the August 2007 issue of the journal Optics Express.

For more information, visit: www.gatech.edu


GLOSSARY
aspheric
Not spherical; an optical element having one or more surfaces that are not spherical. The spherical surface of a lens may be slightly altered so as to reduce spherical aberration. Aspheric surfaces are frequently, but not necessarily, surfaces of revolution about the lens axis.
chromatic
Having the property of color.
distortion
A general term referring to the situation in which an image is not a true-to-scale reproduction of an object. The term also is used to connote the temporal alteration of the signal's waveform shape. There are many types of distortion. See also anamorphic distortion; curvilinear distortion; keystone distortion; panoramic distortion; perspective distortion; radial distortion; stereoscopic distortion; tangential distortion; wide-angle distortion.
doublet
1. A compound lens consisting of two elements. If there is an air space between the elements it is called an "air-spaced doublet.'' If the inner surfaces are cemented together, it is called a "cemented doublet.'' 2. Two spectral lines produced by a transition between a common state and two states differing only in total angular momentum.
interferometry
The study and utilization of interference phenomena, based on the wave properties of light.
lens
A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
light
Electromagnetic radiation detectable by the eye, ranging in wavelength from about 400 to 750 nm. In photonic applications light can be considered to cover the nonvisible portion of the spectrum which includes the ultraviolet and the infrared.
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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