Highlighting the 1995 Annual Meeting of the Optical Society of America (OSA, Washington, DC) in Portland, OR, was a panel discussion entitled "Advances in compact and efficient visible sources," cosponsored by the OSA's Lasers Technical Group and Nonlinear Optics Technology Group. James Kafka of Spectra-Physics Lasers (SPL, Mountain View, CA) and Martin Fejer of Stanford University (Palo Alto, CA) jointly chaired the standing-room-only session, in which panelists described current obstacles and challenges and predicted developments over the next five years.
Kafka opened the discussion with a review of predictions regarding compact visible lasers. He noted that in 1994 solid-state lasers had been expected to displace much of conventional gas laser technology in the visible laser market (output power <1 W), but, in fact, according to Laser Focus World estimates, growth in solid-state visible devices was flat in 1994, with the conventional technologies still thriving in total revenues and especially in unit sales.Several panelists then pointed out that, in general, solid-state alternatives to low-power visible gas lasers are currently more expensive and must, therefore, offer more than just a one-for-one replacement technology. Benefits of such solid-state systems include compactness, efficiency, minimal heat dissipation, and vibration. Visible solid-state lasers have been most successful in applications that demand these features.
Shigeo Kubota of Sony Corp. (Tokyo, Japan) presented results of harmonic generation with both CW and Q-switched Nd:YAG lasers. He expressed concern over degradation problems associated with nonlinear materials for short-wavelength UV generation and stated his goal of producing 1 W of 213-nm output at 10% conversion efficiency. I myself reviewed high-power (>1 W), high-repetition rate (>10 kHz) harmonic generation of Q-switched Nd-doped lasers, in particular for OEM applications, and described recent results obtained at SPL with diode-pumped Nd:YVO4. For green-light generation with such devices, power increases will depend primarily on efficient scaling of the diode-pumped lasers. I expect that 20-25-W, Q-switched IR sources (and, therefore, ~10 W of green) should be available within five years at prices that could be supported by OEM users.
Prospects for diode-pumped lasers
Suzanne Lau of Uniphase (San Jose, CA) reviewed all commercial green solid-state lasers and described the twin challenges of providing good performance at a good price and of helping customers to climb the learning curve associated with a switch to solid-state diode-pumped lasers. In five years she expects to see smaller, cheaper, higher-power devices and shorter wavelengths.
Bill Risk of IBM Almaden (San Jose, CA) noted that the Coherent (Santa Clara, CA) 10-mW, 430-nm D3 laser is the only solid-state blue "catalog item" currently available based on frequency doubling. He described a possible window of opportunity for such sources that is likely to remain open until blue diode lasers are widely available. He also pointed out several technical barriers to these devices, including complexity, cost, and manufacturability of bulk and quasi-phase-matched (QPM) materials and related coatings.
Upconversion fiber lasers are the "crazy uncles" of the field, stated Tim Gosnell of Los Alamos National Laboratories (Los Alamos, NM), who reviewed wavelengths and output powers that have been demonstrated recently, including powers up to 300 mW and wavelengths from 381 to 635 nm. Gosnell highlighted the potential simplicity of such systems, but also pointed out that the cost of development is particularly high because the most promising results have been obtained with sophisticated ZBLAN fibers.
Reinhart Engelmann of the Oregon Graduate Institute (Beaverton, OR) surveyed recent progress in blue-green diode lasers. While the panel generally agreed that, eventually, "this would be the way to do it," Engelmann presented a plot of demonstrated lifetime versus calendar time that satirically suggested a maximum achievable diode lifetime of only about one hour! He then explained that certain specifics related to improved crystal growth would be among the keys to development of reliable blue-green diode lasers and compared the relevant properties of II-VI and III-V materials.
Panelists also presented their "wish lists" for the researchers in the audience, including requests for a good manufacturing process for QPM materials, new nonlinear materials that would allow still-shorter wavelengths into the UV, tabulations of oscillator strengths and cross sections for upconversion materials, better understanding of problems in the growth of BBO, the availability of bulk or QPM material that would allow for the generation of >1 W at wavelengths below 266 nm with industrial lifetimes, and, finally, means to greatly reduce the defect formation that limits the lifetime of blue-green diode lasers.—William L. Nighan Jr.
WILLIAM L. NIGHAN JR. is manager of the Advanced Product Technology Group at Spectra-Physics Lasers, POB 7013, Mountain View, CA 94039-7013.