  A novel characterization method to measure the pulse duration of ultrafast
near-IR pulses is introduced, which uses simple tabletop optics, is relatively
inexpensive, and is expected to work in a broad wavelength range. Our
diagnostic tool quantitatively characterizes the laser pulse duration of any
near-IR wavelength assuming a Gaussian pulse shape with a linear chirp. We
negatively prechirp near-IR pulses with a home-built broadband pulse compressor
(BPC) and send this prechirped beam through a cell filled with a low-molar
solution of a fluorescent dye in a liquid. After two-photon absorption, this
dye fluoresces in the visible, and we record this visible signal as a function
of the propagation distance in the liquid cell. We calibrate the group velocity
dispersion (GVD) of our home-built BPC device against the known GVD of the
compressor of our 800 nm laser and confirm this value using geometric
considerations. Now knowing the GVD of BPC and the recorded visible signal for
various amounts of negative chirp, let us extract the smallest pulse duration
of the near-IR pulse from this visible signal. As a useful corollary, our
analysis also enables the direct measurement of the GVD for liquids and the
indirect measurement of the absorption coefficient for liquids in the near-IR
range, in contrast to indirect GVD measurements that rely on methods such as
the double derivative of the refractive index.
