IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 14, JULY 15, 2006 1503 A Simple Method to Estimate the Dispersion of Silica Optical Fiber Having Unknown Refractive Index Profile Parameters Pramod R. Watekar, Seongmin Ju, and Won-Taek Han, Member, IEEE Abstract—A new method to approximately determine the disper- sion of the silica optical fiber with unknown refractive index pro- file parameters is presented. The estimated error in dispersion was about 12% and 7% for the double-core optical fiber and the con- ventional single-mode fiber, respectively. Index Terms—Mode-field diameter (MFD), mode-field spot size, optical fiber dispersion, splice-loss. I. INTRODUCTION T HE dispersion in the optical fiber with unknown refractive index (RI) profile is presently determined by using the spe- cialized equipment based on the phase-shift principle [1], [2]. In the fiber network field work, to recheck the power require- ments, it is often required to determine the dispersion param- eters of the optical fibers which were laid years ago and with unknown parameters. If a first-hand knowledge of the disper- sion of the fiber is known to the field engineer, then it is always useful to take appropriate decisions before going for the expen- sive option of using the specialized equipment; for example, a high dispersion fiber can be discarded and one with somewhat better performance can be rechecked for the performance using the specialized equipment. In this letter, we present a simple and inexpensive method to determine the dispersion in the unknown optical fiber using the splice loss measurement, which is accu- rate within 12% of the error. II. THEORY The lateral splice loss in decibels between two iden- tical fibers carrying a fundamental mode can be modeled using the relative field amplitude of the guided fundamental mode as a function of radial distance and it is given as [3] (1) Manuscript received February 16, 2006; revised April 6, 2006. This work was supported in part by the GIST Technology Initiative (GTI), South Korea, and by the Brain Korea-21 Information Technology Project, Ministry of Education and Human Resources Development, South Korea. The authors are with the Department of Information and Communications, Gwangju Institute of Science and Technology, Gwangju 500-712, South Korea (e-mail: wthan@gist.ac.kr). Digital Object Identifier 10.1109/LPT.2006.877618 where in which is the lateral offset between two identical optical fibers. Using the Gaussian field approximation (1) can be written as [1], [3] (2) where is the mode field spot size and the equation is valid for . The effective index of the optical fiber carrying single mode is related to with the following relationship [4], [5]: (3) where is the core effective RI, is the cladding RI and is the wavelength. The chromatic dispersion ( ) in the optical fiber is defined using , i.e., (4) where is the free-space velocity of light. Thus, dispersion of any optical fiber carrying a single-mode can be estimated without knowing its index-profile or radial parameters using (1) to (4) if the lateral splice-loss is known and the Gaussian type mode-field radial variation is assumed. It is noted that even for the highly non-Gaussian profile, if a condition is sat- isfied, (2) can give a correct estimation of the spectral variation of the spot size. To estimate the value of of the optical fiber using (3), apart from the , integration constant is required. The integration constant vanishes at the wavelength of the fundamental mode cutoff (LP ). For the conventional single-mode fibers (SMFs), the fundamental mode cutoff is at infinitely large wavelength; on the other hand, the finite value of the LP cutoff exists for the -profile fiber [5]. Hence, in the case of the unknown optical fiber, it may or may not have the finite fundamental mode cutoff and a generalized condition for the arbitrary profile optical fiber will be suitable which is given as . Considering the limitations of available broadband optical sources, a finite value 1041-1135/$20.00 © 2006 IEEE