Spectral phase measurement of complex pulses using delay controlled fringe free interferometry technique

The shortcomings in complex femtosecond pulse measurement of conventional spectral phase interferometry for direct electric-field reconstruction (SPIDER) are commented in this paper. To solve the problem, we propose an improved version of SPIDER, where single replica of the unknown pulse upconverts synchronously with two frequency-shifted narrow-banded long pulses. With the introduction of a suitable small delay between the upconverted pulses, the spectral phase of the unknown pulse can be directly calculated from the fringe-free spectra and Fourier-transform filtering is not required. The results of numerical simulation show that the accuracy of the new method in complex pulse measurement is higher than conventional SPIDER.