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Bhadra, 2 S. M. M. Ali, 1 S. S. A. Damanhuri, 1 H. Ahmad, 3 and Sulaiman Wadi Harun 1,3 1 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia; Corresponding author: swharun@um.edu.my 2 Fiber Optics and Photonics Division, Central Glass and Ceramic Research Institute, CSIR, 196, Raja S.C. Mullick Road, Kolkata 700 032, India 3 Department of Physics, Photonics Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia Received 31 August 2012 ABSTRACT: A 1.9 lm single mode laser is demonstrated by employing a newly developed large mode area Tm 3þ /Yb 3þ co-doped air-clad photonic crystal fiber (PCF) in conjunction with 931 nm pumping for the first time. The PCF has an Yb 3þ and Tm 3þ ion concentrations of about 16  10 19 and 4  10 19 ions/cc, respectively. The laser produces a maximum output power of 4.6 mW with an efficiency slope of 0.70% at a multimode pump power of 2.3 W with a 2.7 m long PCF in a linear cavity with two fiber Bragg grating. The threshold of the input pump power is obtained at around 1.3 W. V C 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1124–1124, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27523 Key words: two micron laser; Thulium Ytterbium co-doped fiber; fiber laser 1. INTRODUCTION Recently, Thulium-doped fiber laser (TDFL) operating at near 2 lm has attracted much interest because of the possibility of combining high efficiency, high output power, and eye-safe operation in addition to introducing specific applications associ- ated with this wavelength, such as remote sensing and biomedi- cal applications [1]. The TDFL uses Thulium-doped fiber (TDF) as a gain medium, which can be efficiently pumped at 800 nm, 1200 nm, or 1600 nm [2]. Due to the so-called cross- relaxation process, an efficient 2 lm laser operation can be achieved by using 3 F 4 ! 3 H 6 pump transition of Thulium ion near 800 nm. In this process, two ground-level Thulium ions can be excited to the upper lasing level of the 3 F 4 ! 3 H 6 transi- tion by absorbing only one pumped photon near 800 nm, which suggests that one excited Tm 3þ ion at the 3 H 4 level can generate two Tm 3þ ions at the 3 F 4 upper lasing level [3]. However, the availability of high-power diodes for this wavelength range is rather scarce which makes them very costly. Pumping TDFs at 1200 or 1600 nm is complicated due to the need for an interme- diate laser source, as high-power laser diodes are not commer- cially available for this wavelength. Therefore, Yb-sensitized Tm-doped fibers are gaining more importance [4] as 980 nm pump sources are more widely available. Tm 3þ has a level ( 3 H 5 ), which is (quasi-) resonant with the excited Yb 3þ level ( 2 F 5/2 ) and thus permits the energy transfer. Recently, large-mode-area (LMA) air-clad silica fibers have become a popular choice for high-power fiber laser systems [5, 6]. They are designed for efficient coupling of pump light, reduction of nonlinear effects, high conversion of pump light and high heat-handling capacity. In this article, a fiber laser is demonstrated using an LMA Tm 3þ /Yb 3þ co-doped air-clad photonic crystal fiber (PCF) as the gain medium. Lasing is achieved at 1900 nm region using a fiber Bragg grating (FBG) as a wavelength selective filter in the linear cavity resonator. 2. EXPERIMENT Figure 1 shows the experimental setup of the proposed LMA Tm 3þ /Yb 3þ co-doped air-clad PCF laser with two FBGs to es- tablish a laser cavity. The PCF was fabricated by stacking capil- laries around a solid Tm 3þ /Yb 3þ co-doped rod. The doped rod forms the central active core while the capillaries provide the air clad. The Yb 2 O 3 þ Tm 2 O 3 co-doped rod was produced by the conventional modified chemical vapor deposition (MCVD) pro- cess coupled with solution doping technique. Thin-walled capil- laries of suitable diameters are stacked around the solid doped rod inside a thick silica jacketing tube to form the final air-clad fiber perform. The perform is drawn to produce a LMA PCF with a fiber diameter of 128 lm, core diameter of 5.8 lm, and bridge width of 0.58 lm as shown in the inset of Figure 1. The Yb 3þ and Tm 3þ ion concentrations in the PCF are measured to be about 16  10 19 and 4  10 19 ions/cc, respectively. The PCF is pumped by a multimode 931 nm pump via a multimode combiner (MMC) to generate an ASE centered at 1900 nm. The ASE oscillates in the linear cavity to lase at the peak wavelength of the overlapping spectrum between the two FBGs. Both FBGs operate at the same center wavelength of 1901.6 nm but different reflectivities of 99.6% and 50%. The 99.6% and 50% FBGs have a 3 dB spectral bandwidth of 1.5 nm and 0.6 nm, respectively. The spectrum and power of the output laser are extracted from the output port of the 50% FBG and measured using an optical spectrum analyzer (OSA) and power meter. The experiment is carried out at various PCF lengths and all components are fusion spliced. The thermal opti- cal effect is observed in this experiment as the input power reaches its threshold level. The reason is maybe due to the inter- rupted heat dissipation caused by coupling loss and the low heat Figure 1 Schematic diagram of the laser cavity. Inset shows the microscope image of cleaved end face for the PCF. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com] 1124 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 55, No. 5, May 2013 DOI 10.1002/mop