914 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 22, NO. 12, JUNE 15, 2010 Development of Broadband Single-Mode Cr-Doped Silica Fibers Yi-Chung Huang, Member, IEEE, Jau-Sheng Wang, Yen-Sheng Lin, Ting-Chien Lin, Wei-Lun Wang, Yu-Kuan Lu, Szu-Ming Yeh, Hsin-Hui Kuo, Sheng-Lung Huang, Senior Member, IEEE, and Wood-Hi Cheng, Fellow, IEEE Abstract—The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A single-mode characteristic of SMCDSF was observed when the propagation wavelengths were longer than 1310 nm. The transmission loss was about 8 dB/m at 1550 nm. The successful fabrication of SMCDSFs may facilitate the possibility for utilizing the SMCDSFs as a new generation broadband fiber amplifier to cover the bandwidths in the whole 1300- to 1600-nm range of low-loss windows of silica fibers. Index Terms—Broadband, fiber amplifier, fiber design and fab- rication, single-mode fibers. I. INTRODUCTION E RBIUM-DOPED fiber amplifiers (EDFAs) are widely credited with enabling the huge transmission bandwidth and span distance of optical fiber communications during the last decade of the 20th century. Because EDFAs can effectively boost the amplitude of optical pulses traveling in optical fiber without converting those pulses to electrical signals, they can extend the range of the fiber-optic link by many thousands of kilometers. The well-known EDFA provides gains in the -band, the -band, and the -band, which totaled 140-nm usable spectral bands. The other types of fiber amplifiers, such as praseodymium (Pr)-doped and thulium (Tm)-doped [1], [2] operate gain in the -band and in the -band, respec- tively. However, the gain bandwidths of the current Er-doped, Tm-doped, and Pr-doped fiber amplifiers cannot fully cover the whole 1300- to 1600-nm range with a single fiber amplifier. The Manuscript received January 10, 2010; revised March 08, 2010; accepted March 28, 2010. Date of publication April 08, 2010; date of current version June 03, 2010. This work was supported in part by the Department of Industrial Technology of MOEA under the Contract 97-EC-17-A-07-S1-025, in part by the National Science Council under the Contract NSC97-2221-E-110-48-MY3, and in part by the MOE Program of the Aim for the Top University Plan. Y.-C. Huang is with the Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 106, Taiwan, and also with the Department of Photonics, National Sun Yat-Sen University, Kaohsiung, 804, Taiwan (e-mail: oris@mail.nsysu.edu.tw). J.-S. Wang, T.-C. Lin, W.-L. Wang, Y.-K. Lu, S.-M. Yeh, and W.-H. Cheng are with the Department of Photonics, National Sun Yat-Sen University, Kaoh- siung, 804, Taiwan (e-mail: whcheng@mail.nsysu.edu.tw). Y.-S. Lin is with the Department of Electronic Engineering, I-Shou Univer- sity, Kaohsiung County, 840, Taiwan. H.-H. Kuo is with the Department of Electrical Engineering, National Uni- versity of Kaohsiung, Kaohsiung, 811, Taiwan. S.-L. Huang is with the Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 106, Taiwan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2010.2047390 transition-metal-doped materials, such as Ni and Cr ions [3], [4], have shown 300-nm broadband emissions. Recently, a Cr:YAG crystal fiber has been fabricated by the use of a codrawing laser-heated pedestal growth (LHPG) method [5] or a drawing-tower technique [6], [7]. The Cr-doped fibers (CDFs) have demonstrated broadband emissions in the whole 1.3- to 1.6- m range. However, the growth of core diameter below 10 m by the LHPG method was difficult, and the uniformity of the core diameter varied greatly along the growth direc- tion. The drawing-tower technique equipped with rod-in-tube (RIT) design, by contrast, could provide better uniformity and smaller core diameter of CDFs [6], [7]. Recently, a single-mode Cr-doped silica fiber (SMCDSF) has been fabricated using the RIT when the wavelength of transmission light was longer than 1400 nm [8]. However, it cannot cover the typical optical communication band of 1310 nm. In this study, we improve the fabrication of an SMCDSF by employing the RIT method with the additional modifica- tion of using a smaller diameter Cr : YAG single crystal rod. A single-mode characteristic of SMCDSF was demonstrated when the wavelengths of transmission light were longer than 1310 nm, which covered the typical optical communication band. A high- resolution transmission electron microscope (HRTEM) was em- ployed to examine the microstructure of the core in SMCDSFs. The images of HRTEM showed very low densities of nano-par- ticales in the area about 2.5 m away from the center of the core. In comparison with the previous works on multimode CDFs [6], [7], the transmission loss was improved about 8 dB/m at 1550 nm. The fluorescence spectrum showed two broadbands of 800–1200 nm and 1200–1600 nm which were attributed to Cr and Cr ions, respectively. The successful fabrication of SMCDSFs may be one step forward towards the achievement of utilizing the SMCDSFs as ultra-broadband fiber-optical am- plifiers to cover the bandwidths in the whole 1300- to 1600-nm range of low-loss and low-dispersion windows of silica fibers and a broadband source for enabling high resolution in optical coherence tomography (OCT). II. FABRICATION OF SINGLE-MODE Cr-DOPED SILICA FIBERS A commercial grade YAG rod doped with 0.25 wt% Cr used as a core rod was obtained from CASIX. The initial dimen- sion of the Cr-doped YAG crystal rod had a length of 0.03 m and a diameter of 500 m. The Cr-doped YAG crystal was grown into a diameter of a 290 m with a length of 0.12 m by the LHPG method. A CO laser beam created a molten zone on the top of the source rod. The seed crystal was dipped into the molten zone and slowly withdrawn, pulling the growing crystal from 1041-1135/$26.00 © 2010 IEEE