Lasers that emit ultrashort pulses of light can be used for numerous applications including micromachining, microscopy, laser eye surgery, spectroscopy and controlling chemical reactions. However, 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 these distortions, researchers at the Georgia Institute of Technology have developed the first device to directly measure complex ultrashort light pulses in space and time at and near the focus.
Called SEA TADPOLE (Spatial Encoded Arrangement for Temporal Analysis by Dispersing a Pair of Light E-fields), the new device uses the concept of interferometry to measure a pulse in space and time. Two pulses, one reference and one unknown, are sent through optical fibers. The fibers are mounted on a scanning stage so that the pulses can be measured at many locations around the focus. The pulses are crossed and an interference pattern is recorded for each color of the pulse at each location with a digital camera. The patterns are 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 changes in space and time as it is focused.
Researchers tested the device by measuring ultrashort pulses focused by various lenses, since each lens can cause different complex distortions. To validate the measurements, simulations of pulses propagating through the experimental lenses were performed. Results showed that a common plano-convex lens displayed chromatic and spherical aberrations, whereas more expensive aspheric and doublet lenses exhibited mostly chromatic aberrations.