Multiple fields of science seek accurate characterization of the spectral phase of ultrashort pulses. Previous methods based on autocorrelation or interferometry required complex and expensive setups. Now, researchers at Michigan State University (East Lansing, MI) have developed a relatively simple technique, multiphoton intrapulse interference phase scan (MIIPS), that can characterize the spectral phase of 50-fs laser pulses with high accuracy, yielding magnitude and sign of second- and third-order phase modulation (linear and quadratic chirp) without iteration or inversion procedures.
Figure 1. In MIIPS, the shaped pulses are frequency-doubled by the SHG crystal (top). In this data obtained for near transform-limited pulses, the angles of the plots show the increase from minimal cubic phase distortion (middle) and more substantial cubic phase distortion (bottom).
"In analogy to a Wheatstone Bridge in electronics, MIIPS is based on measuring the difference between a well-known reference function and the unknown phase modu- lation in the pulse," explains Marcos Dantus of the research group. Second harmonic generation (SHG) depends on the phase function across the spectrum of the laser pulse. In the MIIPS technique, a reference phase function is scanned while changes in the second harmonic spectrum obtained from a thin SHG crystal are recorded. Changes in the wavelength for the maximum signal are used to directly determine the spectral phase modulation of the femtosecond pulse.
The group used a pulse shaper consisting of a pair of gratings, a pair of 200-mm cylindrical lenses, and a spatial light modulator to impose a reference phase function on the 50-fs pulsed output of a regeneratively amplified Ti:sapphire laser. The pulses from the amplifier first passed through the pulse shaper, then through a 0.3-mm beta barium borate crystal. A spectrometer with a cooled CCD detector captured the data. The shaper calibrated phase-delay accuracies of better than 1°.
The group is not only working on a version of MIIPS that can characterize sub-15-fs pulses but is also evaluating different phase functions to improve the determination of arbitrary higher-order phase modulation. Applications include telecom and biomedical studies.