<p>The characterisation technique of asynchronous cross-correlation that uses a known auxiliary reference laser to measure the duration of unknown ultrashort test laser pulses has been improved by adding a delayed copy of the reference laser pulse, creating a known time reference for autocalibration. The calibrated delay is formed by partially inserting a thin optical plate into the reference beam. This modified technique, which provides significantly higher precision than the standard asynchronous cross-correlation technique, was used to measure the duration of ultrafast pulses in the ultraviolet region at a wavelength unsuited to the use of common autocorrelation techniques. The split-beam principle can also be extended to include spectral characterisation of ultrashort pulses.</p>