Design concept for optical fibers with enhanced SBS threshold

Abstract: We propose a criterion to predict the relative value of the stimulated Brillouin scattering (SBS) threshold in single-mode optical fibers with different refractive index profiles. We confirm our results by several representative measurements. We show that with the proper profile design one can achieve more than 3 dB increase in the SBS threshold compared to the standard single-mode optical fiber.

1. Introduction

SBS is a nonlinear effect caused by interaction between optical and acoustic waves (for an in- depth description see, e.g., [1]). For many fiber-optic applications it has a negative impact. For example, SBS has been shown to restrict performance of soliton-based long-haul transmission systems [2] and analog CATV systems [3]. In addition, SBS is an undesirable effect in the optical power delivery, high-power fiber lasers, and unrepeatered transmission where high input optical powers are required. Once the input power reaches some critical value, the amount of backscattered power increases quickly with the input power. The onset of SBS is determined by that critical input power which is called the SBS threshold (SBST) [4]. An increase in the SBST power which is highly desirable for many applications can be accomplished in several ways. For example, one can broaden the spectrum of the input signal using phase modulation to reduce its overlap with the narrow Brillouin gain spectrum [5] or impose a variation of fiber parameters such as stress [6], temperature [7], the core radius [8], or the Brillouin shift frequency [9, 10] along the fiber length. A more practical solution, however, is to control the Brillouin gain by the fiber design because fibers with different index profiles have different Brillouin gain spectra and therefore different SBS thresholds [11, 12, 13, 14].

In this paper we describe an approach to estimate the SBST power of a single-mode opti- cal fiber from its index profile. To study the acousto-optic interaction we use the approach of Refs. [1, 15]. We present a simple reasoning that allows us to deduce a single parameter, the acousto-optic effective area, that quantifies the SBST depending on the radial distribution of the core dopant concentration. We confirm our theoretical results by measurement of the SBST in several GeO2-doped silica fibers. The paper is organized as follows. In Section 2 we derive the expression for the acousto-optic effective area which unlike the commonly used optical effec- tive area determines the strength of the SBS process in optical fibers. We then calculate profiles of acoustic modes that are needed to evaluate the acousto-optic effective area (Section 3) and describe the calculation of SBST when input optical wave interacts with several acoustic modes (Section 4).

Andrey Kobyakov, Shiva Kumar∗, Dipak Q. Chowdhury, A. Boh Ruffin, Michael Sauer, and Scott R. Bickham

Corning Incorporated, Science and Technology Division, One Science Center Drive, SP-TD-01-1, Corning, NY 14831

KobyakovA@corning.com

Raj Mishra

Corning Incorporated, Corning Optical Fiber, 310 N. College Rd, MS-6 Wilmington, NC 28409

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