Abstract: A pulsed laser comprises a laser rod (6), and is pumped by diode array (3) selected such that with additional components (7, 9) only a maximum of two longitudinal modes are generated between mirror (1) and output coupler (2). By slow Q-switching Q-switch (9) hoped to get a single longitudinal mode output pulse. However if on the onset of the relaxation pulse mode beating is detected by detector (28), indicating that two longitudinal modes are present, then the optical path length of the resonator is changed by quarter of one wave length, by applying an electrical signal to a pulse length control means (10), such that one of the modes is suppressed prior to the Q-switch being fully opened. The output may be amplified by amplifier arrangement (24).