A method of asynchronous optical sampling based on free-running lasers with no requirement on the repetition rates is presented. The method is based on the a posteriori determination of the delay between each pair of pulses. A resolution better than 400 fs over 13 ns total delay scan is demonstrated. In addition to the advantages of conventional asynchronous sampling techniques, this method allows a straightforward implementation on already-existing laser systems using a fiber-based setup and an appropriate acquisition procedure. (C) 2012 Optical Society of America