We have now demonstrated and characterized gas-filled hollow-core fiber lasers based on population inversion from
acetylene (<sup>12</sup>C<sub>2</sub>H<sub>2</sub>) and HCN gas contained within the core of a kagome-structured hollow-core photonic crystal fiber.
The gases are optically pumped via first order rotational-vibrational overtones near 1.5 μm using 1-ns pulses from an
optical parametric amplifier. Transitions from the pumped overtone modes to fundamental C-H stretching modes in both
molecules create narrow-band laser emissions near 3 μm. High gain resulting from tight confinement of the pump and
laser light together with the active gas permits us to operate these lasers in a single pass configuration, without the use of
any external resonator structure. A delay between the emitted laser pulse and the incident pump pulse has been observed
and is shown to vary with pump pulse energy and gas pressure. Furthermore, we have demonstrated lasing beyond 4 μm
from CO and CO<sub>2</sub> using silver-coated glass capillaries, since fused silica based fibers do not transmit in this spectral
region and chalcogenide fibers are not yet readily available. Studies of the laser pulse energy as functions of the pump
pulse energy and gas pressure were performed. Efficiencies reaching ~ 20% are observed for both acetylene and CO<sub>2</sub>.
An optically pumped overtone HBr laser is investigated experimentally and theoretically. The frequency tuning and
stabilization of the Nd:YAG pump laser is described. Results of HBr laser emission are presented. The simulation shows
promising features of both pulsed and cw pumped systems concerning efficiency, frequency tuning and heat dissipation.
A concept combining the advantages of solid-state and gas-laser technologies is being developed to realize a scalable, efficient 4-micron laser. A Nd:YAG laser is tuned to 1.3391 microns by temperature-tuning and cavity selection. The addition of diode seeding permits excitation of a specific v = 3 rotational state of HBr. Lasing can potentially occur in three subsequent steps to the ground state, emitting numerous lines in the 4-micron region, which enhances efficiency and spectral coverage. Using up to 25-mJ pump energy, two elements of the possible three-level cascade have been observed. We have observed emission selectivity due to intracavity carbon dioxide and inferred the presence and contributory influence of pure-rotational stimulated emission, which may have importance to the overall behavior of other similar molecular pulsed lasers (and even continuous-wave HX lasers). We present theoretical and experimental results demonstrating the operational principle and utility of this laser system.
We demonstrate the use of an Nd:YAG laser to optically pump an HBr gas laser, a concept combining the advanctages of solid-state and gas-laser technologies. A Q-switched Nd:YAG laser, tuned to 1.34 μm and frequency-stabilized, excites the v=3, J=5 state of HBr in a third-overtone transition. A diode laser locked to the HBr transition provides the seed signal for the Nd:YAG laser. Higher efficiencies than previously obtained for similar systems are expected in this concept because of the possibility of cascade lasing, which we have observed in our experiments and predicted in our simulations. Pump pulses of approximately 400-ns duration and 20-mJ energy are directed along the axis of a 1-m containing 15-torr HBr. In the currently unoptimized configuration, approximately 6 laser lines at about 4 μm are observed. The observed laser spectrum suggests collisional pumping for some lines and that intracavity atmospheric CO<sub>2</sub> is a factor. We have developed and used a kinetic model, which shows good agreement with the experimental results. There is evidence of pure rotatioal lasing. Amplifiers are being added to increase the pump energy to 1 J.
Due to a narrow window of high atmospheric transmission near 4 microns, there is a great deal of interest for a scalable laser energy source in this spectral region. We propose a concept that combines the advantages of solid-state and gas laser technology. A Nd:YAG laser is tuned to 1.3391 microns by inserting an intracavity etalon and raising the operating temperature of the laser rod to 85 degree(s)C. This allows us to excite the v (0 →3), J (4 → 5) vibrational-rotational transition of HBr. To stabilize the 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, emitting three photons in the 4-micron region. We present theoretical and experimental results demonstrating the operational principle of this laser system.