Short and sweet
There has been extraordinary progress in the generation of light pulses since the invention of the laser in 1960. Researchers have developed laser systems that produce near-infrared (NIR) pulses shorter than 5 fswhich corresponds to only a few optical cyclesand techniques to produce even shorter pulses are being explored (see oemagazine, July 2001, page 8). Femtosecond pulses have revolutionized diverse disciplines of science and technology ranging from optical spectroscopy to biomedical imaging to data communications.
A laser pulse is an electromagnetic wave packet. It is completely characterized by its electric field
which is usually described by an envelope A(t), a carrier or center frequency
Being faster by several orders of magnitude than the fastest electronics, femtosecond pulses have to be characterized optically. The basic idea is to correlate the pulse to be measured with itself or an optical signal derived from it. To do so, we measure the correlation signal by probing the geometrical length L over which the pulses overlap while changing the length of one arm of the interferometer. The corresponding delay L/c is an estimate of the pulse duration (see figure). In the case of a second-order nonlinearity (e.g., second-harmonic frequency generation), the recorded signal, after averaging over the fringes, is given by
Unfortunately, the detailed pulse shape cannot be inferred from S. From the FWHM of S,
Several techniques have been developed to obtain A(t) and
Spectral phase interferometry for direct electric-field reconstruction (SPIDER), uses interference in the spectral domain. Two identical, time-delayed pulses with different center frequencies are created from a single pulse using difference frequency generation, for example, and then overlapped in a spectrometer. From the observed spectral interference pattern one can calculate A(t) and
Phase and intensity from correlation and spectrum only (PICASO) records the pulse spectrum and cross-correlations of the pulse with a replica modified by a known linear element (dispersive plate and/or attenuator) in one arm of the correlator. A computer algorithm then retrieves the pulse parameter that fits the data best.
These techniques are aimed at obtaining A(t) and