Cascade lasing of molecular HBr in the four micron region pumped by a Nd:YAG laser

Due to a narrow window of high atmospheric transmission near 4 mum there is a great deal of interest for a scalable laser energy source in this spectral region. We propose a concept combining the advantages of solid-state and gas laser technology. It takes advantage of a coincidence of a Nd:YAG laser line and an overtone transition of the molecule HBr. Tuning a Q-switched Nd:YAG laser to 1.3391 mum allows us to excite the v (0 --> 3), J(4 --> 5) vibrational-rotational transition of HBr. To stabilize the pump frequency, a diode laser locked to this HBr transition seeds the Nd:YAG laser. Once excited, HBr can potentially lase in three subsequent steps to the ground state, two of which were observed experimentally, emitting up to three photons in the 4-mum region. We present theoretical and experimental results demonstrating the operational principle of this laser system. The comparison of experiment and theory suggests: 1) that intracavity CO2 can force the system to lase on only those transitions that are within the atmospheric transmission window, and 2) the existence of amplified spontaneous emission driven by pure rotational transitions.