High Peak Power, Passively $Q$ -switched Microlaser for Ignition of Engines

The compact (electric spark plug size), diode-pumped, passively Q-switched Nd:YAG/Cr⁴⁺:YAG microlaser was developed for ignition of engines. Output energy of 2.7 mJ per pulse and 11.7 mJ per four-pulse train with a pulsewidth of 600 ps and an M ² value of 1.2 were obtained at a pump duration of 500 ¿s. The optical-to-optical conversion efficiency was 19%. Brightness of the microlaser was calculated as 0.3 PW/ sr-cm² and optical power intensity was calculated as 5 TW/cm² at the focal point of ignition. The enhanced combustion by the microlaser ignition was successfully demonstrated in a constant-volume chamber at room temperature and atmospheric pressure. The cross section area of a flame kernel generated by laser ignition is 3 times larger than that by a conventional spark plug at 6 ms after ignition in a stoichiometric mixture (A/F 15.2) of C ₃H ₈/air, even though ignition energy of the laser is 1/3 of that of the spark plug. Hundred percent ignition was successfully demonstrated in a lean mixture of A/F 17.2 by laser ignition, where electric spark plug ignition failed.