Rare-earth laser produces tunable UV output

Rare-earth laser produces tunable UV output

Roland Roux

A grou¥of re searchers at the Université de Lyon (Lyon, France) has de monstrated a high-efficiency all-solid-state ultraviolet-emitting laser. The tunable system is based on cerium-doped lithium lutetium fluoride (Ce:LiLuF4) and produces 2-mJ pulses at around 308 nm.1

The basis for the work is investigation of new approaches to medical and biological applications--specifically for corneal shaping, angioplasty, and DNA sequence measurements, as well as environmental remote sensing. Lasers operating in the near-ultraviolet (near-UV) region--between 200 and 350 nm--are well suited to such applications; the lasers must be continuously tunable across at least part of this wavelength range and ideally should be all-solid-state devices.

The Ce:LiLuF4 system demonstrated at Lyon takes advantage of the interconfiguration optical transition (5d<->4f) of some rare-earth ions, such as Ce3+ in crystalline environments like fluoride and oxides. The two-stage system incorporates a fourth-harmonic Nd:YAG laser that longitudinally pumps a cerium-doped lithium strontium aluminum fluoride (Ce:LiSAF) crystal at 266 nm. Output from the Ce:LiSAF laser at 290 nm--an absorption peak of Ce:LiLuF4--then pumps the Ce:LiLuF4 crystal (see figure on p. 26, bottom).

The Nd:YAG pum¥laser (Spectra-Physics GCR 230) can deliver 110-mJ pulses with a pulsewidth of 4 ns and repetition rate of 20 Hz--it includes the Nd:YAG oscillator, three amplifiers, and two nonlinear crystals for second- and fourth-harmonic conversion of the fundamental output.

Optimum spot size

The 266-nm pum¥laser output is focused through a 20-cm-focal-length focusing lens to produce the 1.5-mm spot size required to reach the Ce:LiSAF crystal threshold--this wavelength also coincides with a minimum absorption of Ce:LiLuF4. The Ce:LiSAF laser oscillator is based on a classical flat/flat resonator with an intracavity Suprasil prism as a dispersive element to allow modest wavelength tunability between 285 and 300 nm. This range is sufficient for numerous applications.

The output from the Ce:LiSAF crystal is directed by a dichroic HR mirror to pum¥the Ce:LiLuF4 platelet crystal. This crystal is mounted in a 25-cm-long laser cavity with two flat/flat mirrors and can be pumped with its c-axis horizontal or vertical--corresponding to and s polarization, respectively.

The dichroic-cavity input mirror is highly reflective (>99%) between 300 and 370 nm and highly transmissive (>95%) at the pum¥wavelength. The pum¥beam is focused behind the crystal with a spherical lens of focal length 18.5 cm placed about 5 cm from the input dichroic. The pum¥beam spot diameter onto the crystal is on the order of 0.5 mm. The best laser performance is obtained when the c axis of the crystal is placed horizontally, that is, for -polarized excitation and emission. These conditions correspond, in principle, to a lower absorption coefficient and lower emission intensity than with s polarization.

Tests showed that with -polarized pumping and using a mirror output coupler for 23% and 60% transmission around 308 nm, the best performance was obtained with the 60% transmission coupler. The laser threshold was 0.5 mJ with the 23% coupler and 0.85 mJ with the 60% one. This threshold was low enough that the laser could operate without the cavity mirrors. The maximum laser efficiency achieved was 55%, which, according to the researchers, is the highest efficiency ever reported for similar UV solid-state lasers. The output wavelength could be tuned between 307.6 and 313.5 nm and 324 and 328.5 nm around the two emission peaks. The laser linewidth at FWHM was typically 0.35 nm, and the maximum energy delivered was 2 mJ at 308 nm.

Material advantages

The researchers note that Ce:LiLuF4 seems to be more resilient than Ce:LiSAF. The former can support an energy density of about 4 J/cm2 without damage or the visual appearance of solarization effects, whereas damage usually occurs around 1 J/cm2 with Ce:LiSAF. In addition, the thermal effects in Ce:LiLuF4 also seem less than in Ce:LiSAF. Hence, Ce:LiLuF4 is a very attractive laser crystal, although lasing has not been obtained over its whole emission domain. This means that further improvement can be expected by working with thinner crystals or with lower dopant levels to reduce reabsorption losses, rather than by addressing more specifically the excited-state absorption processes and solarization effects, which, although not seen by the eye, certainly exist because they have been seen previously.

REFERENCE

1. P. Rambaldi, R. Moncorgé, J. P. Wolf, C. Pedrini, and J. Y. Gesland, "Efficient and stable pulsed laser operation of Ce:LiLuF4 around 308 nm," submitted to Optics Communications. Rambaldi is at the Laboratoire de Spectroscopie Ionique et Moleculaire (LASIM), UMR 5579 CNRS, Université de Lyon, 69622 Villeurbanne; Moncorgé, Wolf, and Pedrini are at the Laboratoire de Physico-Chimie des Materiaux Luminescents, UMR 5620 CNRS, Université de Lyon 1, 69622 Villeurbanne; and Gesland is at the Université du Maine-Cristallogenèse, URA 807 CNRS, 72085 LeMans, France.