Tunable microwave photonic notch filter using a dual-wavelength fiber laser with phase modulation D. Liu a, * , N.Q. Ngo a , G. Ning b , P. Shum b , S.C. Tjin a a Photonics Research Centre, School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 b Network Technology Research Centre, School of Electrical and Electronic Engineering, 50 Nanyang Drive, Research TechnoPlaza, 4th Storey, XFrontiers Block, Singapore 637553 Received 18 June 2005; received in revised form 16 March 2006; accepted 16 March 2006 Abstract A novel tunable microwave photonic notch filter using a phase-modulated dual-wavelength fiber laser is presented. A stable dual-wavelength erbium-doped fiber laser with a linear cavity is formed by a polarization-maintaining uniform fiber Bragg grating (PM-FBG) and a polarization maintaining linearly chirped fiber Bragg grating (PM-LCFBG), both of which were fabricated on a high-birefringence (Hi-Bi) fiber. It is found that a stable room-temperature dual-wavelength operation can be achieved due to the pres- ence of two reflection peaks arising from the orthogonal states of polarization (SOP) of the PM-FBG. Experimental results show stable dual-wavelength lasing operation with a wavelength separation of 0.36 nm and a large optical signal-to-noise ratio (OSNR) of over 40 dB under room temperature. The dual-wavelength fiber laser is combined with a phase modulator and a segment of single-mode fiber (SMF) as a dispersive device to form a tunable microwave photonic notch filter. By stretching the PM-FBG to tune the wavelength separation of the dual-wavelength fiber laser, a tunable microwave photonic notch filter with various free spectral ranges (FSRs) and a rejection ratio greater than 35 dB was developed. Ó 2006 Elsevier B.V. All rights reserved. PACS: 42.55.+W; 42.55.Wd Keywords: Dual-wavelength; Erbium-doped fiber laser; Fiber Bragg grating; High birefringence fiber; Notch filter; Microwave photonics 1. Introduction All-optical processing of microwave and millimeter-wave signals provides such advantages as large time-bandwidth products, insensitivity to electromagnetic interference, and light weight. Therefore, a number of microwave photonic filters have been reported in the literature [1–4]. By incorpo- rating a fiber Bragg grating (FBG) into the filter, e.g., [1,2], the filter response can be tuned. A laser array with an FBG was used in Ref. [1] to realize a tunable notch filter, and a fiber Mach–Zehnder section with a linearly chirped fiber Bragg grating (LCFBG) was reported in Ref. [2] to achieve a microwave filter with tunable notch frequency. However, most reported approaches are based on laser arrays to over- come the coherent operation of the filter [3,4], and are there- fore costly. We propose a novel continuously tunable microwave photonic notch filter by exploiting a dual-wavelength fiber laser with tunable wavelength spacing. The dual- wavelength fiber laser has a linear laser cavity, which consists of an 8-m long polarization-maintaining erbium- doped fiber (PM-EDF), a uniform FBG with a section for strain modulation and a LCFBG, both of which were fabricated on the Hi-Bi fiber. The dual-wavelength is 0030-4018/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2006.03.026 * Corresponding author. Tel.: +65 6790 5363; fax: +65 6792 6894. E-mail address: liuduan@pmail.ntu.edu.sg (D. Liu). www.elsevier.com/locate/optcom Optics Communications 266 (2006) 240–248