We propose a data acquisition trigger signal, suitable for free-space, asynchronous optical sampling (ASOPS) systems, based on Laser Pulse Coherence (LPC). At 1 GHz laser pulse repetition frequencies and higher, this method becomes an attractive alternative to established techniques, as it lacks the necessity of high peak pulse intensity, maximising the total laser power available for measurements. Additional advantages are the large unamplified signal generated, ease of initial alignment, and greatly reduced beam pointing stability errors. Employing a LPC trigger source, we realise a sub 30 fs trigger timing error for our ASOPS system.