2448 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 11, NOVEMBER 2004 -Band Single-Stage EDFA With 25-dB Gain Using Distributed ASE Suppression K. Thyagarajan and Charu Kakkar, Student Member, IEEE Abstract—We propose a novel compact design for a single-stage -band erbium-doped fiber amplifier, wherein distributed sup- pression of -band amplified spontaneous emission is provided by optimized bend loss in a coaxial core fiber. Simulations show that 25-dB unsaturated gain over 30-nm bandwidth (1495–1525) nm is achievable with the designed module, using a nominal pump power of 500 mW. The noise figure of the amplifier varies between 4.5 and 8 dB from 1495 to 1525 nm. By proper designing, we have also ensured that the gain ripple over the entire 30-nm bandwidth is 3 dB. Tolerance toward accuracy in bend radius has also been calculated and it has been shown through simulations that the amplifier performance is maintained for 1-mm variation in bend diameter. Index Terms—Erbium, optical fiber amplifiers, optical fiber communication. I. INTRODUCTION O PENING up of newer bands for transmission ( -band and -band) is a potential solution for increasing the capacity of current wavelength-division-multiplexed systems. Recently, there has been a lot of activity in -band amplifiers, and several amplification schemes based on Raman amplifica- tion [1], hybrid configuration of Raman and erbium-doped fiber amplifier (EDFA) [2], and Tm-doped fluoride amplifiers [3] have been proposed to amplify signals in the -band. -band amplifiers based on silica-based erbium-doped fibers (EDFs) have also been recently reported [4], [5]. It has been shown that efficient -band EDFAs require high inversion levels along the fiber and -band amplified spontaneous emission (ASE) suppression, which otherwise depletes the population inver- sion. Ono et al. [4] have achieved 21-dB gain in -band with a nine-stage EDFA configuration, using discrete ASE filters after each stage. In [5], a two-stage -band EDFA based on a W-index fiber, with fundamental mode cutoff at 1525 nm, has been demonstrated, which provides distributed loss to -band ASE. Distributed ASE filtering has advantages in terms of a lesser number of components, increased pump efficiency, and design simplicity. In this letter, we report an efficient design for single-stage -band EDFA based on a coaxial core fiber, wherein distributed ASE filtering is achieved by winding the fiber with an optimally Manuscript received May 3, 2004; revised June 17, 2004. The work of C. Kakkar was supported by the Industrial Research and Development Unit of the Indian Institute of Technology, Delhi, India, under the High Value Research Assistantship. The authors are with the Department of Physics, Indian Institute of Tech- nology Delhi, New Delhi 110016, India (e-mail: ktrajan@physics.iitd.ernet.in; charukakkar@rediffmail.com). Digital Object Identifier 10.1109/LPT.2004.835196 Fig. 1. Refractive index profile of the designed coaxial fiber. chosen bend radius. Bend loss usually has a strong spectral vari- ation because of the variation of mode field diameter with wave- length. Coaxial fiber design provides extra degrees of freedom in terms of tuning the bend loss variation. Hence, by optimizing the fiber parameters and the bend radius, we have ensured that wavelengths above 1525 nm suffer a high ( 6 dB/m) bend loss, whereas the wavelengths below 1525 nm suffer minimal loss. In our proposed fiber, bend loss at 1530 nm is 100 times greater than that at 1490 nm, which leads to a high net gain in the -band. II. FIBER DESIGN AND PRINCIPLE OF OPERATION Fig. 1 shows the refractive index profile of the proposed fiber, which consists of two concentric cores—the inner core with large and outer core with small . Here, is defined as , , and corresponds to the re- fractive index of pure silica, calculated using Sellemeier’s equa- tion. The parameters of the two cores are so chosen that each of these supports a single azimuthally symmetric mode in the operating range of wavelengths. Coaxial fiber design has been studied for a variety of applications, including dispersion com- pensation [6], intrinsic gain flattening of EDFA [7], and intrinsic Raman gain flattening [8], and has also been experimentally re- alized [9], [10]. In such a fiber, the spectral variation of the field of the fundamental supermode can be modified by tuning the phase-matching wavelength (PMW). PMW is the wavelength at which the fundamental modes of the individual cores are phase matched. For wavelengths below PMW, the modal field is tightly confined to the inner core and as the wavelength ap- proaches the PMW, the fractional power in the outer core in- creases leading to a sharp increase in the bend loss. Thus, by optimizing the fiber parameters and the bend radius, spectral 1041-1135/04$20.00 © 2004 IEEE