ABSTRACT
We present what we believe to be the first measurement of the spectral properties of a soft x-ray laser seeded by a high-order harmonic beam. Using an interferometric method, the spectral profile of a seeded Ni-like krypton soft x-ray laser (32.8 nm) generated by optical field ionization has been experimentally determined, and the shortest possible pulse duration has been deduced. The source exhibits a Voigt spectral profile with an FWHM of 3.1+/-0.3 mA, leading to a Fourier-transform pulse duration of 4.7 ps. This value is comparable with the upper limit of the soft x-ray pulse duration determined by experimentally investigating the gain dynamics, from which we conclude that the source has reached the Fourier limit. The measured bandwidth is in good agreement with the predictions of a radiative transfer code, including gain line narrowing and saturation rebroadening.