Optik 123 (2012) 1149–1152 Contents lists available at ScienceDirect Optik jou rnal homepage: www.elsevier.de/ijleo Investigation on threshold power of stimulated Brillouin scattering in photonic crystal fiber R. Parvizi a , S.W. Harun a,b,∗ , N.M. Ali b , H. Arof b , H. Ahmad a a Photonics Laboratory, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia b Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia a r t i c l e i n f o Article history: Received 6 February 2011 Accepted 16 July 2011 Keywords: Stimulated Brillouin scattering Brillouin threshold gain Photonic crystal fiber a b s t r a c t We have developed a theoretical approach to simulate stimulated Brillouin scattering (SBS) generation in photonic crystal fiber (PCF). The threshold condition for the SBS occurrence has been derived as a function of fiber parameters and input pump power. A particular emphasis is given to the influence of the input pump power and fiber length on the Brillouin gain in the PCF. To assess threshold pump power accurately, the pump depletion effect has been included by employing the 1% criterion. This simulation can anticipate the Brillouin threshold gain value precisely. The threshold gain varies from 14 to 18 depending on the PCF length. © 2011 Elsevier GmbH. All rights reserved. 1. Introduction Stimulated Brillouin scattering (SBS), which originates from the  (3) nonlinearity of the medium especially related to acoustic phonon interaction, is one the most prominent nonlinear effects in optical fibers [1]. It has been implemented in a wide range of applications as a narrowband fiber laser source [2], narrow band fil- tering [3], and Brillouin fiber amplifier (BFA) [4]. Since the Brillouin frequency shift is depended on the material composition, the tem- perature and pressure of the medium, the BFA can also be applied in various optical fiber sensors [5]. In a development of distributed Brillouin sensors (DSB), the Brillouin shift is measured along the fiber length based on frequency (or time) domain modulation of the input pump wave [6]. One of the main characteristics of the nonlinear Brillouin scattering effect in optical fibers is its threshold power condition for the occurrence of the SBS. The threshold power is normally defined as the input pump power, P th , at which the out- put Stokes is some fraction (r), used as threshold criterion, of P th [7]. This threshold input pump power value is strongly depended on the providing Brillouin gain medium or fiber length which plays an important role in the distributed Brillouin sensors accuracy. In another words, a uniform resolution measurement over the entire sensing length cannot be achievable in case of strong pump depletion [4]. Consequently, a precise and comprehensive theo- retically measurement of the threshold input pump power with ∗ Corresponding author at: Photonics Laboratory, Department of Physics, Univer- sity of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail addresses: parvizi1360@gmail.com (R. Parvizi), swharun@um.edu.my (S.W. Harun). considering the pump depletion as well as sensing fiber length variation is required. Recently the advent of small-core photonic crystal fibers (PCFs) with their unique guiding properties and increased optical nonlin- earities led to the demonstration of a number of nonlinearity-based devices such as DSB [8]. Solid silica core comprising a Ge-doped center region has been chosen to avoid some drawbacks of PCFs by increment of the nonlinear refractive index of the core, creat- ing a smaller mode field diameter, and reducing the confinement loss [9]. In this paper, the Brillouin scattering process in a piece of PCF is theoretically studied for possible use in a development of a Brillouin fiber laser in DSB. In this study, the Stokes emission is con- sidered to be initiated by spontaneous Brillouin scattering, SpBS, and the 1% criterion is used for the threshold definition [7]. The threshold power is investigated based on PCF parameters, which determines the amount of transmitted pump and Stokes powers. This investigation also involves the experimental verification. 2. Theoretical principle If the input pump intensity is sufficient, SBS in optical fibers will convert a pump field with frequency ω p to a scattered counter- propagating probe field with frequency ω s (Stokes waves), through an electrostrictive process. Indeed, Brillouin scattering arises from the nonlinear interaction of launched light with propagating den- sity waves or acoustic phonons. Since momentum and energy can be conserved during these scattering events, the SBS effect is remarkable [1]. The interaction between the three fields of input pump signal, Brillouin scattered wave, and acoustic wave (phonon) 0030-4026/$ – see front matter © 2011 Elsevier GmbH. All rights reserved. doi:10.1016/j.ijleo.2011.07.043