Ultrashort Laser Pulse Phenomena, 2nd Edition

Imaging 583

are redirected toward the detector. The backscattered signal beam and the ortho-

gonally polarized retro-reflected reference are sent into a nonlinear crystal cut

for type II phase matched SHG. Assuming the reference has a uniform transverse

intensity profile, the second harmonic contains the image information contained

in the fundamental beam. The time of arrival of the reference ultrashort pulse

determines the depth d at which a cross section through the object is imaged into

the CCD, as illustrated in Fig. 13.2.

The ultrashort pulse source used for the preliminary tests was a fs dye laser

operating at 620 nm [5]. Urea crystals were chosen for these tests as being the

only phase-matchable type II crystals at that wavelength. Unfortunately ...