Optical frequency comb generation based on chirping of Mach–Zehnder Modulators Jassim K. Hmood a,b , Siamak D. Emami a , Kamarul A. Noordin a , Harith Ahmad c , Sulaiman W. Harun a,c,n , Hossam M.H. Shalaby d a Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia b Laser and Optoelectronic Department, University of Technology, Baghdad 10066, Iraq c Photonics Research Center, University of Malaya, Kuala Lumpur 50603, Malaysia d Department of Electronics and Communications Engineering, Egypt-Japan University of Science and Technology, Alexandria 21934, Egypt article info Article history: Received 14 November 2014 Received in revised form 15 January 2015 Accepted 19 January 2015 Available online 21 January 2015 Keywords: Chirp factor Optical frequency comb Optical fiber communication Optical modulator abstract A new approach for the generation of an optical frequency comb, based on chirping of modulators, is proposed and numerically demonstrated. The setup includes two cascaded Mach–Zehnder Modulators (MZMs), a sinusoidal wave oscillator, and an electrical time delay. The first MZM is driven directly by a sinusoidal wave, while the second MZM is driven by a delayed replica of the sinusoidal wave. A math- ematical model of the proposed system is formulated and modeled using the Matlab software. It is shown that the number of the frequency lines is directly proportional to the chirp factor. In order to achieve the highest number of frequency comb lines with the best flatness, the time delay between the driving voltages of the two MZMs is optimized. Our results reveal that at least 51 frequency lines can be observed at the output spectrum. In addition, 27 of these lines have power fluctuations of less than 1 dB. The performance of the proposed system is also simulated using a split-step numerical analysis. An optical frequency comb, with tunable frequency spacing ranging from 5 to 40 GHz, is successfully gen- erated. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Nowadays, optical frequency comb generator (OFCG) is widely used as multi-wavelength source for optical communication sys- tems such as dense wavelength-division multiplexing (DWDM), optical time-division multiplexing, and optical orthogonal fre- quency-division multiplexing systems [1–6]. The generated comb is required to provide a stable frequency with a fixed phase and spacing for these applications. In addition, the flatness of spectral comb lines and low implementation costs are also vital require- ments. Till today, many techniques have been proposed and de- veloped to realize optical frequency combs (OFCs) such as the employment of a mode locked laser (MLL) [7,8]. The instability of carrier frequency due to the environment's dependency of laser cavity is the main disadvantage of MLL method [3]. Another technique is based on nonlinear effects in a highly nonlinear medium [9]. However, this technique requires a high power am- plifier and an optical filter to shape the spectrum. Optical modulation technique can also be used to realize OFC whereby multiple optical carriers with precise channel spacing can be ob- tained from one seed light source [10–14]. Low cost, low com- plexity, small size, less noise [14], and high stability are some of the main advantages of this technique. Many schemes have been used for generating OFCs such as by implementing optical modulation technique. This modulation is normally based on two approaches; amplitude-phase hybrid modulation and intensity modulation. In amplitude-phase hybrid modulation scheme, the number of generated comb lines is di- rectly proportion to phase modulation (i.e. to tune the number of comb lines, the amplitude of electrical-oscillator signal is con- trolled) [10–12]. The limitations of this technique are mainly due to the limited number of generated comb lines and the use of high power of external RF to drive the modulators. Wu et. al. success- fully demonstrated a 10 GHz comb with 38 lines within a spectral power variation of 1 dB by cascading intensity and phase mod- ulations [11]. The problem of high power of external RF can be solved by employing optical intensity modulators with a low driving voltage. Shang et. al. used two cascaded intensity mod- ulators to generate a 25 comb lines at low driving voltage [15]. In the optical intensity modulator, the change of phase of the output light with time causes a chirping in the optical signal. The Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications http://dx.doi.org/10.1016/j.optcom.2015.01.054 0030-4018/& 2015 Elsevier B.V. All rights reserved. n Corresponding author at: Department of Electrical Engineering, Faculty of En- gineering, University of Malaya, 50603 Kuala Lumpur, Malaysia. E-mail address: swharun@um.edu.my (S.W. Harun). Optics Communications 344 (2015) 139–146