Picosecond pulses from semiconductor lasers are chirped (have time-dependent wavelengths) due to the effect of free carriers on the refractive index of the laser material. For optically pumped ultrashort-cavity semiconductor film lasers, the chirp is large (>1 nm/psec) and nonlinear in time because of the very high carrier concentration (> 1020 carriers/cm3) present during the operation of the laser. Experimentally, the chirp of film lasers is measured by optical upconversion sampling of the laser pulse followed by spectral filtering. This paper presents a mathematical model of this detection scheme which is used to extract the instantaneous time-dependent laser wavelength λ(t) from the measured data. Coefficients for the linear and quadratic terms of a power-series expansion of λ(t) are obtained for two InGaAsP film lasers. These parameters are used to compute time-averaged pulse spectra, which are compared with measured spectra. A formula is presented for the compression of chirped pulses in dispersive optical media which is used for comparison with experimental pulse compression results obtained by passing the film laser pulse through a short dispersive optical fiber. Finally, the time-dependent wavelength is related to the instantaneous carrier concentration in semiconductor material.
© 1984 Optical Society of America
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