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Ittipiboon, Microstrip antenna design handbook, Artech House, Canton, MA, 2001. 26. M. Biswas and D. Guha, Input impedance and resonance characteris- tic of superstrate loaded triangular microstrip patch, IET Microwave Antennas Propag 3 (2009), 92–98. 27. V. Losada, R.R. Boix, and M. Horno, Resonant modes of circular microstrip patches in multilayered substrates, IEEE Trans Microwave Theory Tech 47 (1999), 488–497. 28. I.J. Bahl and S. Stuchly, Analysis of microstrip covered with a lossy dielectric, IEEE Trans Microwave Theory Tech 28 (1980), 104–109. 29. M. Deshpande and M. Bailey, Input impedance of microstrip anten- nas, IEEE Trans Antennas Propag 30 (1982), 645–650. 30. HFSS 13: Ansoft’s Corp., Pittsburgh, PA. VC 2015 Wiley Periodicals, Inc. A PASSIVELY HARMONICALLY MODE- LOCKED SOLITON ERBIUM-DOPED FIBER LASER WITH LOW PUMPING THRESHOLD USING A SINGLE-WALLED CARBON NANOTUBES Raja Zaimas Raja Rosdin, 1 Norfizah M. Ali, 1 H. Arof, 1 and Sulaiman Wadi Harun 1,2 1 Department of Electrical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia; Corresponding author: ahamzah@um.edu.my 2 Photonics Research Center, University of Malaya, Kuala Lumpur 50603, Malaysia Received 3 September 2014 ABSTRACT: We demonstrate a passively harmonically mode-locked soliton Erbium-doped fiber laser (EDFL) with low pumping threshold using single-walled carbon nanotubes (SWCNTs) embedded in poly eth- ylene oxide (PEO) thin film as a passive saturable absorber. The film with a thickness of 50 lm was fabricated using a prepared homogeneous SWCNT solution with 0.1% loading percentage, which was mixed with a diluted PEO solution and casted onto a glass petri dish to form a thin film by evaporation technique. The film is sandwiched between two fiber connectors to construct a saturable absorber, which is then integrated in an EDFL cavity to generate a stable passive mode-locking of a soliton laser operating at 1560 nm at the 4th, 7th, and 15th harmonic of funda- mental cavity frequency when the pump powers of 1480 nm laser diode are 14.9, 17.5, and 20.1 mW, respectively. The 4th harmonic pulses are characterized in detail with a repetition rate of 44.84 MHz, a transform- limited pulse width of 1.19 ps, side-mode suppression ratio of larger than 20 dB and pulse energy of 9.14 pJ. VC 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:799–803, 2015; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28964 Key words: single-walled carbon nanotubes; mode-locking; passive sat- urable absorber 1. INTRODUCTION Passively mode-locked soliton erbium-doped fiber lasers (EDFLs) have been used as sources in many applications ranging from basic research to telecommunications, medicine, and materi- als processing. They enjoy many advantages over their active counterparts such as simplicity, tenability, pico- and femtosecond operation [1, 2]. When the pump power extends beyond a certain value, multipulses will form in the cavity because of energy quantization effect [3]. It is well known that there are diverse dis- tributed multipulse modes in the cavity, such as multipulse bunching, disordered multipulse, harmonic mode-locking, and so on [4]. Among these operation modes, higher order harmonic mode-locking pulses with high repetition rates are important for many applications such as high bit-rate optical communication, biological imaging, and arbitrary wave form generation [5]. Cur- rently, most of the harmonic mode-locking (HML) approaches utilize a nonlinear polarization rotation (NPR) [6] or semiconduc- tor saturable absorber mirror (SESAM) [7]. The mode-locking method that utilize NPR are strongly dependent on the polariza- tion evolution and phase evolution of the optical pulse in the laser cavity such that with a long cavity they can be easily overdriven and affected by the environment induced fiber birefringence. The one using SESAM offers superior results especially in ultralong cavity mode-locking because the SESAM saturable absorption is independent of the cavity length. This is currently the dominant passive mode-locking technique in spite of the SESAM’s com- plex fabrication and narrow tuning range. Recently, a simple and cost-effective alternative using single-walled carbon nanotubes (SWCNTs) has also attracted considerable attention, owing to its advantages, such as ultrafast recovery time, large saturable absorption, ease of fabrication, and low cost [8]. Carbon nanotubes can be embedded in various composite materials such as metal [9], ceramic [10], and polymer [11]. In recent years, carbon nanotubes polymer composites have been extensively investigated due to the ease of handling as well as the carbon nanotubes property preservation during processing. They can be produced using various methods such as solution mixing [12], melt blending [13], and in-situ polymerization [14] and have been used for many applications such as pH sensor [15], actuators [16], and strain sensors [17]. Recently, carbon nano- tubes polymer composite has also been produced in the form of transparent thin films [18]. Some of the approaches reported to develop the film are, filtering [17], coagulation [19], and evapo- ration technique [20]. Among them, the evaporation technique is the most cost effective and simplest. It is implemented by mixing the nanotubes with water soluble polymer and the evaporation process is either using low temperature ovens or simply by letting it dry at room temperature [20]. Recently, Park et al. [21] demon- strated multiwalled carbon nanotubes—polyethylene oxide (PEO) composite film using evaporation technique that makes use of the low melting temperature of PEO. In this article, a passive harmonic mode-locked EDFL is demonstrated using a SWCNTs-PEO composite film-based satu- rable absorber (SA). The SWCNTs-PEO film is developed using similar technique of the earlier work [21]. The SA is constructed by sandwiching the film between two fiber connectors. It is incorporated in an EDFL system to demonstrate a passive mode-locking of a soliton EDFL at the 4th, 7th, and 15th har- monic of fundamental cavity frequency operating at 1560 nm. 2. FABRICATION AND CHARACTERIZATION OF SWCNTs-PEO FILM A homogeneous suspension was prepared by mixing 250 mg SWCNTs (99% pure, diameter of 1–2 nm and length of 3–30 DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 57, No. 4, April 2015 799