DOI: 10.1007/s00340-004-1594-3 Appl. Phys. B 79, 449–455 (2004) Lasers and Optics Applied Physics B j. kong 1, ✉ d.y. tang 1 j. lu 2 k. ueda 2 Spectral characteristics of a Yb-doped Y 2 O 3 ceramic laser 1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore 2 Institute for Laser Science, University of Electro-Communication, Japan Received: 8 August 2003/Revised version: 23 April 2004 Published online: 29 July 2004 • © Springer-Verlag 2004 ABSTRACT We report the experimental observation of ran- dom wavelength emission and intensity-dependent central- wavelength shift in a diode-pumped Yb 3+ -doped Y 2 O 3 ceramic laser. We show experimentally that, like conventional lasers, the emission of the laser has fixed well-defined transverse modes; however, its instantaneous emission wavelengths change ran- domly with time. The central wavelength of the laser emission also shifts with the intracavity light intensity. A model was de- veloped to describe the spectral behavior of Yb 3+ -doped lasers. We show that the observed random wavelength emission and central lasing wavelength shift of the laser could be well ex- plained based on the strong reabsorption of light in the gain medium. PACS 42.55.Rz; 42.60.Mi; 42.55.Xi 1 Introduction Recently, studies of Yb 3+ -doped solid-state lasers have attracted considerable attention. Yb 3+ -doped lasers have several advantages over Nd 3+ -based lasers: owing to the fact that the Yb 3+ ions have a very simple electronic structure with only two manifolds separated by about 10 000 cm −1 , ef- fects such as cross relaxation, excited-state absorption, and up-conversion are absent in the lasers. Also, the Yb 3+ ions have a very small quantum defect and therefore the lasers have very low thermal load. However, Yb 3+ -doped materi- als are ‘quasi-three-level’ systems. Since the lower laser level of the Yb 3+ -doped lasers is located in the ground-state mani- fold and has an energy of only a few hundreds of cm −1 , the thermal population of the lower laser level induces consid- erable reabsorption losses at the laser wavelength. Although the effect of reabsorption on the laser performance has been studied previously [1–6], and it was also noticed that a free- running Yb 3+ -doped laser tends to oscillate at longer wave- lengths rather than the peak gain wavelength, e.g. a free- running highly doped Yb:YAG laser usually firstly oscillates at 1048 nm instead of 1030 nm [7], the effect of reabsorption on the laser oscillation wavelength has not yet been well ad- ✉ Fax: +65/67904161, E-mail: j_kong@pmail.ntu.edu.sg dressed. In a previous paper we have reported the laser emis- sion of a diode-pumped Yb 3+ -doped Y 2 O 3 ceramic laser [8]. The Yb 3+ -doped Y 2 O 3 ceramic sample used in our experi- ment was supplied by the Konoshina Chemical Co. Ltd., Japan. It is a ceramic disk of 3 mm in thickness and 20 mm in diameter and has a Yb 3+ -doping concentration of 8 at. %. In our previous experiment we found that the fluorescence spectrum of the ceramic sample has two clear emission peaks: a stronger one centered at 1031.2 nm and a relatively weaker one around 1076 nm, which is quite different from that of the Yb:YAG single crystal, where usually only one peak cen- tered at 1030 nm is visible. The fluorescence spectrum of the Yb:Y 2 O 3 ceramic sample also exhibits a very broad spectral tail extending to 1200 nm. Based on the observed fluores- cence spectrum of this sample, it seems that the laser emission should be at 1031.2 nm. However, the actual emission wave- length of the ceramic laser was around 1078 nm, which is not only far away from 1031.2 nm, the stronger fluorescence peak of the ceramic sample, but also slightly away from the weaker peak of 1076 nm. To understand this abnormal prop- erty we have further conducted experiments to investigate the emission wavelength of the laser. In this paper we report our experimental and simulation results. We show experimentally that although the output beams of the laser have well-defined transverse modes, its emission wavelengths randomly change with time and shift to longer wavelengths as a whole with the increase of intracavity light intensity. In this sense we have realized an intrinsically random wavelength emission solid- state laser. Furthermore, a model has also been developed to explain the laser spectral behavior observed in the Yb:Y 2 O 3 ceramic laser. We show that the light-emission feature of the laser could be well understood based on the strong reabsorp- tion of the lasing light in the gain medium. 2 Experiment The laser used in our experiment is schematically shown in Fig. 1. The laser cavity consists of two concave mirrors. The mirror M1 is high reflection (HR) coated in a broadband spectrum from 1030 nm to 1100 nm and anti- reflection (AR) coated at 940 nm; it has a radius of curvature (ROC) of 1000 mm. To study the effect of reabsorption losses on the laser emission wavelength two different output mir- rors were used alternately. One (M2) is a concave mirror with