By Patricia Daukantas
Wow, there are so many exciting sessions here at Frontiers in Optics/Laser Science XXV (FiO/LS), it’s hard to select the ones to write about! This won’t be the last OPN blog post on the subject, I can assure you.
Wednesday night we had three parallel sessions of postdeadline papers, with some interesting findings.
Federico Furch and his colleagues at Colorado State University (U.S.A.) reported their demonstration of the first all-diode-pumped soft-X-ray laser. Although numerous applications demand compact soft-X-ray sources, previous “tabletop” X-ray lasers have been pumped by flash lamps, so their repetition rates have been limited to 10 Hz or thereabouts.
Furch’s team found that cryogenically cooled Yb:YAG made a good gain material for the laser system’s two amplifiers. The system achieved lasing at the 18.9-nm line of nickel-like molybdenum by focusing three collinear pulses at grazing incidence onto a solid Mo target. Other specs: 8.5-ps pulses with 1 J of energy at 10 Hz. The researchers noted that hitting the target with more than 700 mJ was required to get the laser line. Next on their platter: significantly increasing the energy and repetition rate of the diode-pumped system, and trying to get the output into the 13- to 14-nm range by replacing the Mo target with one of cadmium or silver.
Also at the postdeadline session, it was a pleasure to hear a talk by Sheng Liu of the University of Maryland, Baltimore County (U.S.A.), an OSA student chapter leader who participated in my September OPN cover story on ethical careers. Liu and his collaborators, who include OSA Past President Anthony M. Johnson, are studying the electron dynamics inside quantum cascade lasers (QCLs), which are important in such applications as trace gas detection in the atmosphere. They set up a pump probe to generate femtosecond pulses at 3 to 12 mm and found that the signal transmission depends on whether the pulses’ polarization matches the TM mode of the QCL.
Ronald Holzlöhner and his colleagues at the European Southern Observatory (ESO) in Garching, Germany, reported experimental results that were even better than the figure that got printed in the postdeadline-paper book. ESO has been working on laser-guide-star (LGS) adaptive-optics (AO) systems to give their ground-based telescopes better “seeing” by decreasing blur caused by atmospheric turbulence. LGS systems require a light source resonant with the D2 line of sodium at 589 nm, which is the familiar “yellow glow” of sodium; narrow linewidth; 20 or 30 W of power in a stable beam; and rugged, reliable turnkey operation (because astrophysicists aren’t necessary laser tinkerers). Dye lasers were first used for LGS AO, but they aren’t the easiest to operate.
The ESO team built a Raman fiber amplifier using the observatory’s proprietary technology for suppressing stimulated Brillouin scattering. The amplifier has less than 1.5 MHz bandwidth, which is important because narrow bandwidth is necessary for efficiently exciting sodium atoms high in the atmosphere. After frequency doubling in an external cavity, the system put out 28 W with a conversion efficiency of 82 percent. (The original paper said 26.5 W, but the team got a better result after submitting the paper.) The output is more than sufficient for LGS projects, and the technology is being transferred to industry.
Many of Wednesday’s AO presentations, both in FiO and in the concurrent topical meeting on AO, focused on ophthalmic applications of AO, especially in imaging of the living retina.
Thursday wraps up FiO/LS and the Fall Optics & Photonics Congress topical meetings with a full slate of sessions lasting until dinnertime. Back to the conference now....