GO through a supermarket checkstand and, chances are, your purchases will be scanned by a laser. Watch TV and see advertisements for excimer laser surgery to correct nearsightedness. The list goes on, for laser technology has infiltrated modern life. But this hardly means that laser research and development is complete. Now, Lawrence Livermore National Laboratory laser scientists are engaged in two directions of research: advancing to more and more difficult applications, and refining current technology so that lasers can be made ever more efficient, reliable, and cost effective. For consumers, the progress of this work will be marked by seeing previously exotic applications become commercially feasible.
The move toward smaller but more powerful, more reliable, and less expensive lasers has taken a jump with the discovery of Ce:LiSAF, a laser crystal developed under the terms of a Cooperative Research and Development Agreement (CRADA) between the Laboratory and VLOC, a Division of II-VI Inc (formerly Lightning Optical Corporation) in Tarpon Springs, Florida. Ce:LiSAF is the nomenclature for a crystal made of cerium embedded, or doped, in a host medium consisting of lithium strontium aluminum fluoride (LiSrAlF6). It is an optical crystal, emitting ultraviolet light in a range of wavelengths that make the laser tunable.
The new cerium laser crystal is a significant product for two reasons. First, it provides the ability to generate ultraviolet light directly, compared to previous methods that were far more complicated, less predictable, and worked only under restrictive conditions. Ultraviolet light is desirable for applications that require finely focused, high-intensity beams or for sensing materials with ultraviolet absorption bands. Of the various kinds of laser light--infrared, visible, and ultraviolet--ultraviolet is the most difficult to obtain because it consists of the highest energy wavelengths. The capability of generating ultraviolet light simply and directly will extend laser applications.
The older, usual method for generating tunable (variable color) ultraviolet laser light is to take available, longer wavelengths and use various means to step them up through intermediate wavelengths until the ultraviolet portion of the energy spectrum is reached. This delicate process is called frequency conversion. The figure at right compares the frequency conversion required for the new laser crystal with an existing commercial approach for generating ultraviolet light. In the laser using the Ce:LiSAF crystal, input energy from a Nd:YAG (neodymium-doped yttrium aluminum garnet--a commonly used crystal) laser undergoes nonlinear frequency conversion twice. That light is beamed through the Ce:LiSAF crystal, and ultraviolet light is the result. In the existing commercial approach, one beam pumped through a Nd:YAG crystal undergoes frequency conversion; the resulting light is used as input energy for a nonlinear optical parametric oscillator. A second beam from the Nd:YAG crystal goes through two frequency conversions, is combined with the output from the oscillator, and is mixed in an optical parametric amplifier. The resulting light must then go through frequency conversion before ultraviolet light is attained. With two fewer critical frequency conversion steps, the Ce:LiSAF-based method results in more reliable and efficient generation of ultraviolet light, with less energy lost along the way.
The second reason why the new crystal is significant is that it makes an ultraviolet solid-state laser system commercially feasible. Generating laser light is not simple because much of the energy put into the laser system ends up as heat. Yet light energy gain must be larger than the losses if lasing is to occur. Therefore, in every part of the laser system, the objective is to maximize energy gains while minimizing the losses. The cerium laser crystal, which was specifically designed to be an amplifying agent in a solid-state system, generates useful ultraviolet wavelengths so simply that it makes possible a compact, robust, and cost-effective laser system.

Crystal Properties
In the Ce:LiSAF crystal, cerium is the light emitter while lithium strontium aluminum fluoride serves as the crystalline host and preserves the favorable optical properties of cerium. The atomic structure of cerium provides the property of transitioning to the ultraviolet wavelengths when it is bombarded with light. This is an advantageous property, and scientists have been investigating cerium's potential as an ultraviolet laser since the early 1980s. However, early demonstrations were so discouraging that cerium was all but dropped from active experimentation and commercial consideration. What was happening was that cerium-doped crystals had developed two major energy-loss problems: solarization and excited-state absorption. Solarization is the loss of transparency in a crystal-- it becomes colored--from ultraviolet radiation, like sunlight. Excited-state absorption causes input energy to turn into heat instead of laser light, which also can be debilitating to the laser.
To make use of cerium's ultraviolet properties, a complementary host medium was needed to provide it with stability against ultraviolet radiation and the effects of excited-state absorption. Over the years, Laboratory scientists investigated different host media and built up a body of data about them. In the mid-1980s, Stephen Payne and co-workers invented the LiSAF laser host medium for use with chromium-doped infrared lasers. This very successful material, used in commercial laser designs, was selected for trials with cerium.

CRADA Partnering
For the experiments, making the crystals was an important, but difficult and time-consuming first step. Much can go wrong during the process, resulting in damaged, contaminated, or flawed pieces with the wrong optical characteristics. Fortunately, the Laboratory scientists had an opportunity to enter into a CRADA with the scientists at VLOC, who had expertise in growing crystals and manufacturing them with high yield. In addition, VLOC provided a range of facilities for precision crystal cutting and polishing and a means for purifying chemicals for the crystal growth process. The closeness of the Laboratory-VLOC collaboration, in which laser-crystal analysis and fabrication cycles were carefully coordinated, eventually led to the successful production of the cerium laser crystal on a routine basis.
The new cerium laser offers efficiencies approaching 50%, which means that extracted laser light is about half of the energy that was input into the laser. This value is in the range of efficiencies for commercial laser products at nonultraviolet wavelengths. Furthermore, this material is from 10 to 100 times more efficient than other types of solid-state laser material that have been studied in the past. Finally, because the ultraviolet light is provided in a wavelength range corresponding to approximately 10% of the bandwidth, the ability to tune, or shift, to another wavelength for particular applications is an additional critical feature that this laser provides.






Applications
Because it is straightforward technology, Ce:LiSAF is expected to usher in a new era of laser applications. It is particularly well suited to remote sensing environmental applications because many targeted molecules, including ozone and aromatic compounds, have characteristic absorption bands in the ultraviolet. Already, a cerium laser has been deployed to remotely detect ozone and sulfur dioxide in the environment.
The U.S. Army is considering its use to monitor the presence of tryptophan, a common component of biological weapons. Another potential military use could be to secure wireless communications links between infantry units over short distances of approximately 1 kilometer on a battlefield. Because ultraviolet light from a cerium laser can be tuned to attenuate, or taper off, around 1 kilometer from the source, it can be detected only by receivers within less than about 10 kilometers of the transmitter. This feature makes remote detection of the communication signals (for example, with a satellite or behind enemy lines) impossible.
The power, simplicity, and reproducibility of Ce:LiSAF will change traditionally difficult, expensive, and sensitive applications into commercially feasible ones. Because of this crystal, tunable ultraviolet lasers may move rapidly from the domain of scientific research laboratories into industry.

Key Words: Ce:LiSAF, cerium crystal, R&D 100 Award, tunable ultraviolet laser, solid-state laser.

For further information contact: Chris Marshall (510) 422-9781 (cmarshall@llnl.gov).


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