A frequency-doubled Nd:YAG laser was used to pump the RbTiOAsO<sub>4</sub>(RTA)/AgGaSe<sub>2</sub> (AGSE) cascade optical
parametric oscillator (OPO) to generate the 5.764 μm IR pulses, which correspond to the main absorption band of
cholesterol. A maximum average output power of 40 mW was regularly obtained at 30 Hz and a pump power of 2.5 W
with a long-term pulse-to-pulse fluctuation of ±10%.