0.16 nm spaced multi-wavelength Brillouin fiber laser in a figure-of-eight configuration R. Parvizi a , H. Arof b , N.M. Ali b , H. Ahmad a , S.W. Harun a,b,n a Photonics Laboratory, Department of Physics, University of Malaya, Kuala Lumpur 50603, Malaysia b Department of Electrical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia article info Article history: Received 31 August 2010 Received in revised form 15 October 2010 Accepted 19 October 2010 Available online 3 November 2010 Keywords: Figure-of-eight multi-wavelength Brillouin fiber laser Photonic crystal fiber Stimulated Brillouin scattering abstract A stable and compact multi-wavelength Brillouin fiber laser (BFL) operating at room temperature is experimentally demonstrated using a 100 m long photonic crystal fiber (PCF) in conjunction with a figure- of-eight configuration. At a Brillouin pump (BP) level of 15.3 dBm, 7 simultaneous lines with 20 GHz or 0.16 nm line spacing is achieved by removing the odd-order Stokes lines. The anti-Stokes lines are also generated via four wave mixing process in the laser cavity. Compared with the Erbium-based multi- wavelength laser, this BFL has advantages in term of channel spacing and flexibility in the choice of operating wavelength. The output spectrum of the proposed BFL can be tuned by 80 nm, dependent on the availability of an appropriate BP source. The multi-wavelength BFL shows a good stability with power fluctuations of less than 0.5 dB over more than 3 h. & 2010 Elsevier Ltd. All rights reserved. 1. Introduction Stimulated Brillouin scattering (SBS) is a nonlinear process in which a frequency down-shifted Stokes signal is generated by consuming the pump energy when the threshold is reached. The process involves the interaction between the pump wave and a nonlinear material, which generates an acoustic wave obeying the conservations of momentum and energy. The generated Stokes frequency is dependent on the velocity of the acoustic wave travelling in the medium. Therefore, the Brillouin shift varies with different types of fiber and is estimated around 10 GHz or 0.08 nm in the 1550 nm region for silica-based single-mode fibers (SMFs). SBS-based multi-wavelength lasers have been investigated intensively [1–3] due to their many advantages such as low threshold and narrow line-width, which are suitable for applications in microwave photonics [4], fiber sensors [5] and high capacity wavelength division multiplexing (WDM) communication networks [6]. Normally, hybrid Brillouin/Erbium [6–12] or Brillouin/ Raman fiber lasers [13–14] are used to achieve multiple Stokes lines with a constant spacing. For instance, the generation of 160 Brillouin Stokes lines has been experimentally demonstrated using a linear cavity self-seeded Brillouin/Erbium fiber laser (BEFL) [10]. A stable Brillouin/Raman fiber laser has also been demonstrated with 30 lasing lines [13]. However, the tuneability of these multi-wavelength lasers is limited by the Erbium or Raman gain bandwidths. The use of a Brillouin fiber laser (BFL) for multi-wavelength generation has not been considered seriously so far because of the limited number of lasing lines and the undesirable power flatness. Most BFL and the hybrid lasers generate a wavelength spacing of approximately 0.08 nm, which is not practical for the WDM applications. Recently the advent of small-core photonic crystal fibers (PCFs) with their unique guiding properties and increased optical nonlinea- rities led to the demonstration of a number of nonlinearity-based devices [8]. In PCFs with a solid silica core, the guidance mechanism is somewhat similar to the total internal reflection that occurs in conventional fibers, except that the effective cladding index is an appropriate average of the air and silica refractive indices. In this paper, we propose a new compact multi-wavelength BFL using a short piece of PCF as a nonlinear Brillouin gain medium in conjunction with a figure-of-eight configuration. The PCF is used to significantly reduce the length of Brillouin gain medium. In this work, only 100 m long PCF is used as a gain medium rather than the few km of single-mode fiber used in a conventional BFL. The proposed BFL can achieve stable multi-wavelength simultaneous lasing with 20 GHz or 0.16 nm line spacing, lower threshold power and is also able to mitigate the limitation of tuning range. Compared with the previous BFLs and hybrid Brillouin lasers, the line spacing of the proposed BFL is twice as wide due to the removal of odd-order Stokes waves with the proposed figure-of-eight configuration and therefore it has more potential to be implemented in future WDM systems. 2. Experimental setup The configuration of the proposed figure-of-eight PCF-based multi-wavelength BFL is shown in Fig. 1. It includes an optical Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology 0030-3992/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2010.10.008 n Corresponding author at: Department of Physics, Photonics Laboratory, University of Malaya, Kuala Lumpur 50603, Malaysia. E-mail addresses: parvizi1360@gmail.com (R. Parvizi), swharun@um.edu.my (S.W. Harun). Optics & Laser Technology 43 (2011) 866–869