Using the frequency-resolved optical gating method, researchers at Argonne National Laboratory in Illinois have measured the phase and amplitude of 530-nm light pulses produced by self-amplified spontaneous emission in the Advanced Photon Source free-electron laser. They presented their findings, which have applications in the development of x-ray free-electron lasers, in the Dec. 2 issue of Physical Review Letters.Observations of the pulses indicated that energy chirp in the electron beam injected into the undulators generates chirp in the laser output. Conditioning the electron beam may enable free-electron lasers to yield transform-limited pulses, the researchers suggest.