UV-Visible Lasers Based on Rare-Earth Ions
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ernative Laser Sources for Visible and UV Wavelengths Among the most mature technologies for the generation of UV and visible laser light are various gas lasers, many of which are commercially available. 1 These include continuous-wave (cw) lasers such as Ar-ion and HeCd, and pulsed lasers such as copper vapor lasers (CVLs) for
MRS BULLETIN/SEPTEMBER 1999
visible emission and various excimer Systems for UV. Research continues to improve these populär Systems.2 Some of these, such as some excimer lasers, are rather energy efficient, but others, such as Ar, are quite inefficient. These lasers are available for several specific wave lengths, but are not broadly tunable. All have the disadvantages of any gas laser: relatively large size and fragility. SolidState lasers can be much more compact and rugged. The most straightforward way to appropriate the advantages of RE-doped solid-state lasers is to frequency-shift the Output of a successful NIR laser such as Nd:YAG. Such techniques rely on non linear optical crystals that can sum the frequencies of two photons to give one high-frequency photon. This approach requires high intensities and has been used for many years to obtain pulses of coherent light at 532, 355, and 266 n m (frequency-doubling, -tripling, a n d -quadrupling, respectively). In recent years, however, much progress has been made in the frequency-conversion of cw Nd:YAG lasers, resulting in commercial devices at 532 nm. 1 Very impressive research results have been obtained even at 266 nm, such as the 1-W Output reported by Oka et al. 3 These workers obtained the requisite high intensities with cw lasers by incorporating a nonlinear crystal inside the laser resonator itself to double the light to 532 n m , then another in a separate optical resonator to frequencydouble this light to 266 nm. It is possible to get a few more wave lengths by combining the approach just described with Raman shif ting in a solidstate material. In this technique, the frequency of the light is shifted by the frequency of a vibrational mode of the Raman active material. The process actually improves the beam quality of the
laser b e a m , but care must be used to avoid distortions of the pulse's temporal shape and pulse-to-pulse instabilities. Good results have been obtained in shif ting Nd 3 + output to wavelengths near 580 nm. 4 ' 5 Very broad wavelength tunability may be achieved by a different nonlinear frequency-shifting device: an optical parametric oscillator (OPO). In this tech nique, a high-frequency photon is incident on a properly oriented nonlinear crystal such that the crystal splits the photon into two lower-frequency pho tons. A wavelength-selective resonator cavity controls how the energy is divided between these two, thus giving tunable Output wavelengths. Beginning with 1064 nm from a NdrYAG laser, one gets visible or UV light by first frequencytripling the light to 355 nm in the traditional way as already described, then using an OPO to produce tunable light at longer wavelengths. Commercial Sys tems of
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