Nonlinear propagation of negatively chirped pulses: maximizing the intensity at the output of a fiber probe

Summary form only given. When a short (100-fs) pulse in the 800-nm spectral region propagates in a fiber, normal dispersion induces a positive chirp in the pulse that exits the fiber. A grating pair may be used to pre-compensate the fiber dispersion, so that in the linear regime (low power) a transform-limited pulse may be obtained at the fiber output. A number of applications are envisaged for fiber probes, which require ultrashort pulses to appear at the exit face of the fiber; examples include time-resolved near-field scanning (NSOM) pump-probe experiments and two-photon fluorescence (TPF) excited through an NSOM tip. Because the signal in these applications involves the nonlinear excitation of the system under study, one requires that the peak intensity of the pulse at the fiber output be maximized. At moderate to high powers, though, self-phase modulation (SPM) acts to narrow the bandwidth of a negatively chirped pulse, thus increasing the pulse duration and lowering the expected TPF signal. Nevertheless, because the TPF increases with the square of the peak intensity, it can be maximized by increasing the energy per pulse as long as the pulsewidth increases sublinearly with pulse fluence. In the experiment, we have measured the pulse duration and two-photon fluorescence intensity as a function of the average pump power to see if we can gain in signal by going to higher powers at the expense of increased pulse width.