CONFERENCE REVIEW: Solid-state lasers catch a lift at ASSL
The OSA Advanced Solid-State Lasers (ASSL) Topical Meeting has long been the best-kept secret in the laser industry, at least for those interested in the cutting edge of solid-state laser technology.
The OSA Advanced Solid-State Lasers (ASSL) Topical Meeting has long been the best-kept secret in the laser industry, at least for those interested in the cutting edge of solid-state laser technology. Each year, the world leaders of the solid-state laser community gather in an intimate setting to enjoy both formal presentations and extensive informal discussions with three days of open access to students and researchers alike. This year the added bonus was alpine skiing at the meeting site, Davos, Switzerland. The first meeting of the new millennium, held February 13-16, was also the first ASSL Topical Meeting to be held outside the United States.
Despite the difficulties of arranging a meeting of more than 250 people, it went off without a hitch thanks to the tireless efforts of the OSA staff and to the excellent management of the cochairs Ursula Keller of the Swiss Federal Institute of Technology (Zurich, Switzerland) and Chris Marshall of Lawrence Livermore National Laboratory (Livermore, CA). Approximately 80% of the attendees were from the USA and Europe, with the remainder from Japan, the Middle East, and Asia/Australia.
The conference began with a series of presentations on high-power diode-pumped lasers, starting with an invited paper on the application of such technology to the future of laser materials processing from the University of Stuttgart (Stuttgart, Germany). The following talks reflected escalating laser power and oscillated between Californian and German groups, but the record power reported was 3 kW from Toshiba (Tokyo, Japan). The laser community truly knows no borders. A paper by TRW (Redondo Beach, CA) on a 900-W high-brightness diode-pumped Nd:YAG phase-conjugated laser described 900-W output with diffraction-limited beam quality—a record for laser brightness.
At an evening session, Ralph Hutcheson remembered with deep affection one of the founders of our industry, Arthur Schawlow, Nobel Prize winner and inventor of the optical maser. Günter Huber provided an entertaining look at the history of crystal growth and showed a finely tuned balance of European and American humor. It is important to note that we "laser jocks" owe our every advance to such crystal growers who often spend a lifetime to create a better laser or nonlinear crystal for our use. In particular, materials like Nd:YVO4, KTP, and LBO have brought about a revolution.
Fiber lasers took a session of their own, featuring an invited paper from David Richardson (University of Southampton; Southampton, England), who covered the power-scaling problems and their solution through increased core diameter and mode control through index gradient. Mid-IR fiber lasers showed additional promise: in a paper titled "10-mJ cladding-pumped Tm-doped silica fiber laser operating in gain-switched mode," researchers from the University of Manchester (Manchester, England) described Tm-doped fibers with Q-switched energies up to 10 mJ but pulse durations quite long (100 ns) and rather unstable, which limit peak power and efficacy. Also of interest was a poster paper from Soreq (Yavne, Israel), which described 880-nm pumping of conventional bulk-crystal YAG lasers and showed potential for achieving fiber-laser performance from bulk-crystal lasers if lower-level absorption losses can be overcome.
Self-doubling materials such as the YCOB family appeared in several papers and posters with a variety of impressive diode-to-green conversion efficiencies (for example, a paper from Macquarie University (Sydney, Australia); however, these materials remain at relatively low power levels. It will be interesting to see if these materials scale in power or are hampered by gray-tracking problems, as suggested in a paper from Osaka University (Osaka, Japan). A paper from ACREO (Norrkoping, Sweden) titled "Novel design for diode-pumped miniature lasers" gave an interesting glimpse into the future with a laser on a semiconductor chip. Such chip-scale devices could find their way into all manner of future microelectronics devices, making diode-pumped lasers more like diodes themselves.
The UV session was dominated by researchers from Japan, who have developed materials like CLBO and Gd:YCOB, a material presented last year that offers non-critical phase matching for tripling and gives 30% conversion efficiency of 1064-nm light to 355 nm. Unfortunately materials problems continue to plague UV crystals, as described in a paper, also from Osaka University, on photoinduced damage in Gd:YCOB and its circumvention.
This year's session on novel applications opened with an invited paper from MRC Systems and the University of Heidelberg (Heidelberg, Germany) on laser neurosurgery and the use of modelocked lasers to remove fine layers of tissue with minimal thermal effects. The elimination of the heat-affected zone in these processes will likely extend to many commercial medical and industrial applications in years to come.
Many technologies that have made their debut at the ASSL meeting have quickly found their way into commercial products—single-frequency operation induced by an acousto-optical modulator (ASSL '97), the passively modelocked SESAM (ASSL '96), and the periodically poled lithium niobate optical parametric oscillator (ASSL '97), to name a few.
It was refreshing to see that at least one commercial company—Spectra Physics Lasers (Mountain View, CA)—demonstrated its gratitude with a cash gift to the conference, an effort that helped defray some of the additional costs of holding the meeting in Europe. The lion's share of the funding was provided by NASA, the US Air Force Office of Scientific Research, the Swiss National Science Foundation, the Swiss Office of Education & Science, and the US Night Vision & Electronic Sensors Directorate. These organizations were the pioneers of our diode-pumped industry, providing the funding to critical crystal growers and diode manufacturers that spawned the early devices for research.
The exhibitors were undaunted by the challenges of shipping their booths to Switzerland and participated enthusiastically in the lively technical discussions, business, and networking that are so much a part of ASSL. Coherent (Santa Clara, CA) gave a good showing of its new diode technology and promised continued support of the national laboratories for development. Raicol Crystals (Yehud, Israel) showed continued support of gray-track-resistant KTP and also provided an RTP Q-switch that was discussed in a postdeadline paper on high-repetition-rate applications. It reported the highest repetition rate operation of any electro-optic Q-switch to date, at 75 kHz, with low insertion loss.
The Advanced Solid State Lasers 2001 Topical Meeting will be held in Seattle, WA, January 28-31.
Larry Marshall is president of Light Solutions, 1212 Terra Bella Ave., Mountain View, CA 94043, and a Laser Focus World editorial advisor.