Continuous-wave broadly tunable Cr 2+ :ZnSe laser pumped by a thulium fiber laser Alphan Sennaroglu a, * , Umit Demirbas a , Nathalie Vermeulen b , Heidi Ottevaere b , Hugo Thienpont b a Laser Research Laboratory, Department of Physics, Koc ¸ University, Rumelifeneri Yolu, Sariyer 34450, Istanbul, Turkey b Vrije Universiteit Brussel (VUB), Department of Applied Physics and Photonics (IR-TONA), B-1050 Brussels, Belgium Received 24 February 2006; received in revised form 25 May 2006; accepted 22 June 2006 Abstract We describe a compact, broadly tunable, continuous-wave (cw) Cr 2+ :ZnSe laser pumped by a thulium fiber laser at 1800 nm. In the experiments, a polycrystalline ZnSe sample with a chromium concentration of 9.5 · 10 18 cm 3 was used. Free-running laser output was around 2500 nm. Output couplers with transmissions of 3%, 6%, and 15% were used to characterize the power performance of the laser. Best power performance was obtained with a 15% transmitting output coupler. In this case, as high as 640 mW of output power was obtained with 2.5 W of pump power at a wavelength of 2480 nm. The stimulated emission cross-section values determined from laser threshold data and emission measurements were in good agreement. Finally, broad, continuous tuning of the laser was demonstrated between 2240 and 2900 nm by using an intracavity Brewster cut MgF 2 prism and a single set of optics. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.55.f; 42.55.Rz; 42.60.Pk; 42.60.Lh 1. Introduction When divalent transition metal ions such as Cr 2+ or Fe 2+ are introduced into chalcogenide hosts, strong absorption and emission bands are formed in the mid-infrared and depending on the ion-host combination, lasing action can be obtained in the 2–4 lm wavelength range. One of the most important members of this class of infrared solid-state lasers is Cr 2+ :ZnSe. As was first demonstrated by De Loach et al. [1,2], broadly tunable laser emission can be obtained from Cr 2+ :ZnSe between 2000 and 3100 nm in cw case [3], and between 1880 and 3100 nm during pulsed operation [4]. To date, gain-switched [1], continuous-wave (cw) [5], and mode-locked [6] operations have been demonstrated. Important applications of Cr 2+ :ZnSe lasers include medi- cine [7], atmospheric imaging [8], vibrational spectroscopy [9–11], and optical excitation of mid-infrared lasers and optical parametric oscillators [12]. The broad absorption band of the Cr 2+ :ZnSe gain med- ium extending from 1500 to 2100 nm makes it possible to use several alternative laser systems as pump sources (absorption peak = 1775 nm, width (FWHM) = 365 nm [13–15]. Some of the pump lasers that have been used to date include Co 2+ :MgF 2 [1,2,16–18], Tm:YALO [5], Tm: YLF [19], NaCl:OH  [20], Ho:YAlO [21], Er:YAG [10], laser diodes [22–24], Er-doped fiber lasers [8,9,25–28], Ba(NO 3 ) 2 and BaWO 4 Raman lasers [29,30], and KTi- OPO 4 (KTP) optical parametric oscillators (OPO) [31]. A number of important criteria need to be met in choos- ing the optimum pump source for practical Cr 2+ :ZnSe sys- tems, such as commercial availability, possibility of room temperature and cw operation, and long-term stability. In addition, ion–ion interaction at high doping concentrations also places constraints on the choice of the optimum pump- ing wavelength. To understand this further, we prepared several Cr 2+ :ZnSe samples by diffusion doping [15] and 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.06.064 * Corresponding author. Tel.: +90 212 3381429; fax: +90 212 3381559. E-mail address: asennar@ku.edu.tr (A. Sennaroglu). www.elsevier.com/locate/optcom Optics Communications 268 (2006) 115–120