Performance of a Directed Infrared Countermeasure Laser System

Abstract

Two alternative approaches have been selected to generate high energy per pulse, low repetition rate tunable laser radiation in the 3-5 µm spectral region. First, a cascade OPO (optical parametric oscillator) scheme based on either LiNbO_3 / AgGaSe_2 or KTP / ZnGeS_2 crystal combinations is investigated taking into account the properties of the idler and signal waves, respectively. Second, a single OPO configuration using AgGaS_2 is analyzed. Both schemes driven by Nd:YAG pump lasers are evaluated with respect to output power and efficiency as a function of primary pump power over the whole spectral range of interest. In addition, damage thresholds of coated and uncoated AgGaS_2 and ZnGeP_2 crystals are experimentally determined at 1 µm and 2.3 µm using the small spot standard procedure, and discussed in relation to the corresponding pumping threshold. The two Nd:YAG lasers used in the present investigation are a commercially available flash lamp pumped device with a 3 ns pulse width and a diode pumped customer designed long pulse (9 ns) laser, respectively. Both devices deliver pulse energies of several 100 mJ at pulse repetition rates between 10 Hz and 100 Hz each. The beam quality issue of both the pump beam and the final OPO beam is emphasized: two-dimensional profiles and quantitative measurements of the M^2-factor are presented for various operation conditions. The impact of pump laser characteristics and nonlinear crystal quality on the performance of a mid-IR countermeasure system is addressed and identified as an area of major concern. Finally, novel technical approaches will be discussed for orientation of future R&D work.