IMPROVED OPERATION OF A CW YLF LASER

Abstract

We developed a cw laser in the 2pm region operating at liquid-nitrogen (77 K) and at dry ice temperature (210K) pumped by the 514 pm line of ~ r + laser. Also we observed two walenghts (670 rnrn. 890 mm) that are significantly correlated with laser emission. Introduction. The Holmium emission at 2.06 pn has long been the basis for the development of efficient solid-state lasers. Laser action has been reported for difTerent host crystals. both in pulsed and in cw regime: room temperature a s well a s 77 K operation has been described /1 8/. Recently we developed a highly emcient cw laser employing a LiYF4 crystal doped with Er, Tm and Ho. with the purpose of obtaining a low-noise tunable source for spectroscopic applications in the 2 pn wavelength region. The main results achieved in the improvement of our laser are described in the first part of this paper while in the second part we studied the visible an infrared fluorescence from the higher energy levels of Ho and Er, which are populated by upconversion processes. The motivation of this work was to improve the understanding of the basic mechanisms of energy transfer and eventually to provide guidelines for the design of a better laser device. Experimental Apparatus. The LiW4 crystal, doped with 0.72 Er, 0.072 Tm and 0.003 Ho. is placed in a cryostat inside an astigmatically compensated resonator. The pump lght is the 514 n m line of an Ar laser, which is injected into the YL.F resonator by a ZnSe beam splitter. The crystal is placed with its C axis paralIel to both the pump and the infrared polarkation. Its dimensions are 7 x 3.5 mm and it is 2.5 mm thick. The absorption coefficient at 514 n m is about 6 cm-l (at the temperature of 77 K). The resonator has a total length of about 1 meter and a folded structure with two concave mirrors (R = 5 and 8 cm) and a flat output mirror, all clamped to a super-invar base to insure a good thermal and mechanical stability /9/. The curved mirrors and the crystal are kept inside the vacuum chamber of the cryostat, which is terminated by a Brewster window. An additional side window allows the direct observation of the fluorescence from the crystal in a direction orthogonal to the laser propagation. The fluorescence at 670 or 890 nrn is selected by a small monochromator and detected by a cooled photomultiplier with S1 spectral response. A mechanical chopper and lock-in amplifier are used for signal processing. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19917104 JOURNAL DE PHYSIQUE IV