OPTICAL REVIEW Vol. 10, No. 3 (2003) 146-149 O 2003 The Optical Society of Japan All-Fiber Ultra-Fast SWitching USing Stinlulated Raman Scattering Feroz AHMED* and Naoto KISH11 Satellite Venture Business Laboratory, Faculty of Engineering, Gunlna University, Kiryu, Gunma 376-85J5, Japan IDepartment of Information and Commtinication Engineering, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan (Received March 8, 2002; Accepted March 18, 2003) We propose a method for all-fiber ultra-fast switching using stimulated Raman scattering. It is based on the transference of energy from the signal as pump to the control signal as first Stokes by stimulated Raman scattering. In the absence of a control signal, the transference of energy from the pump signal to the generated first Stokes is negligible, which results in high output power at the pump signal wavelength. To minimize the walk-off problem between two wavelengths, we chose pump signal and control signal to be equally spaced on opposite sides of the zero dispersion wavelength of the fiber. Based on this assumption, the all-fiber ultra-fast optical switch with low power consumption and high output extinction ratio can be realized. Key words: ultra-fast optical switch, all-fiber devices, stimulated Raman scattering, highly nonlinear fiber, fiber nonlinearities 1. Introduction All-optical ultra-fast switches are promising devices for high-speed all-optical telecommunication networks. Among these all-optical devices, much attention has been paid to fiber nonlinearity based switches.1-25) Optical Kerr effect in fiber has primarily been used for such switching function. One of the other important nonlinear effects in optical fiber is stimulated Raman scattering (SRS) by which many interesting applications have already been proposed and demonstrated.26~30) The most attractive feature of SRS based devices is the response time in optical fiber, which is on the order of a few femto-seconds. Thus the bit rate response is not a limiting factor for devices based on SRS. However, high pump power is required for minimizing the walk-off problem between pump and signal wavelengths. For example, a nonlinear-optical modulator based on SRS is applicable only for pump power up to I W and pulse widths larger than I ns.31) In the nonlinear optical switch based on SRS in a Sagnac loop, high pump power is required to minimize walk-off between pump and signal wavelengths.32) However, ultra-fast switching with lower pump power is in demand for communication applications. For example, ultra- fast wavelength conversion and logic operations at different wavelengths are essential in an all-optical packet switching circuit, and efficient switching with low power and simple configuration of the devices are also desired. In this paper, we propose a simple method for all-fiber ultra-fast optical switching based on SRS. In the absence of an input control signal, an incoming signal as pump generates a signal as first Stokes, which is initiated from amplified spontaneous emission (ASE). The fiber length is not sufficient for significant energy conversion into first Stokes, which results in high pump signal power at the output. In the presence of the input control signal, the pump signal transfers all of its energy to the control signal by efficient interaction with that signal, which results in null pump signal at the output. The asymmetric position of the pump and the control signal's wavelengths with the zero- dispersion wavelength of the fiber may cause walk-off between them. Such walk-off occurs due to the group delay difference between the pump signal and the control signal that could perturb the switching performance for an ultra- fast signal. To minimize the walk-off between two wavelengths, we minimize their group delay difference by choosing the pump signal and the control signal so that they are equally spaced on opposite sides of the zero dispersion wavelength of the fiber. Based on this assumption, an all- fiber optical switch with large on-off contrast ratio is realized. 2. Theoretical Modeling The proposed switching scheme is shown in Fig. I ; it consists of a WDM coupler, an optical isolator, an optical bandpass filter, and a certain length of highly nonlinear optical fiber (HNLF) as a Raman gain medium. The WDM coupler is used to couple the control signal power to the HNLF. An optical isolator is introduced to suppress backward Rayleigh scattering, back reflection from the optical filter and backward ASE at the control signal wavelength, which could perturb the switching performance. The optical bandpass filter suppresses all wavelengths other than the pump signal wavelength at the output. The steady-state equations governing the interaction of the signal (Ps) as a pump, and the control signal (Pcs) as first Stokes power are given by the following coupled equations:33) = -((L)s/cocs)(gR/Acs)PsPcs - otsPs dz = (gR/As) - PsPcs ~ cecsPcs dz Control signal Highly nonlinear optical ribers ( 1 665nm) ~ ~ Jl~L~, LLiiiJ_ pump si**nal WDM coupler ( ~ 555nm) ( 1 555 / 1 665) Optical f~Ilter HF ~ LJ~ ~:~ Optical isolator output ( 1 555nm) E mail address: feroz@vbl,gunma-u.ac.jp Fig. I . Proposed ultra-fast optical switching scheme.