Passively Mode-Locked Fiber Laser Based on Reduced ... - IEEE Xplore

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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 30, NO. 7, APRIL 1, 2012

Passively Mode-Locked Fiber Laser Based on Reduced Graphene Oxide on Microfiber for Ultra-Wide-Band Doublet Pulse Generation Xiaoying He, Zhi-Bo Liu, Dongning Wang, Member, IEEE, Minwei Yang, C. R. Liao, and Xin Zhao

Abstract—Graphene with zero-band gap can be easily saturated under strong excitation due to Pauli blocking. In this work, the graphene oxide membrane is reduced on the surface of microfiber by use of high temperature heating. Such reduced graphene oxide can interact with the strong evanescent field of the microfiber and enable saturable absorption in passively mode-locked fiber laser. The system can be effectively utilized to directly generate ultrawide-band doublet pulses for high-capacity wireless communication applications. Index Terms—Fiber laser, passive mode locking, ultra-wideband doublet pulses, reduced graphene oxide.

I. INTRODUCTION

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ASSIVELY mode-locked fiber lasers have attracted a great deal of attention from researchers because of their simple and compact design and widespread applications on optical communication, medicine, micro-processing and materials processing, etc. One of the efficient methods for high-quality passive mode-locking pulse generation is by use of saturable absorber (SA). The commonly used SA is semiconductor SA mirrors (SESAMs) which is rather complex and expensive in fabrication and exhibits only narrow-band saturable absorption [1], [2]. Recently, single wall carbon nanotubes (SWNTs) and graphene used as the SAs for passive mode-locking of femosecond or picosecond pulses have been reported [3]–[15]. Such SAs are featured with fast recovery time, large modulation depth, low cost and easy fabrication [3]–[7]. Moreover, graphene based SAs do not require band-gap design and diameter control to improve its performance, and the linear dispersion of Dirac electrons also provides an ideal solution for wideband and tunable operation for passive mode-locking [8]–[15].

Manuscript received July 10, 2011; revised November 01, 2011; accepted December 21, 2011. Date of publication January 02, 2012; date of current version March 02, 2012. This work was supported by the Hong Kong SAR government through a GRF (general research fund) grant PolyU 5298/10E and the Hong Kong Polytechnic University research grants G-YX2N and G-YX3R. X. He, Z.-B. Liu, D. N. Wang, M. Yang, and C. R. Liao are with the Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong (e-mail: [email protected]). Z.-B. Liu is with the Department of Electrical Engineering, The Hong Kong Polytechnic University, Hong Kong, and also with the Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School, Nakai University, Tainjin, China (e-mail: [email protected]). X. Zhao is with the Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School, Nakai University, Tainjin, China. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2011.2182499

To date, some graphene composite materials, such as graphene-polymer composites [8]–[10], [12], [13], chemical vapor deposition (CVD) grown films [11], [12], functionalized graphene (e.g., graphene oxide bonded with polymer) [13], [14] and reduced graphene oxide flakes [15] have been used as the SAs in different passively mode-locked fiber laser cavities. However, the full potentials of such graphene composites integrated with optical fiber have not been exploited yet. There are two approaches to integrate graphene SAs in fiber laser cavities for ultrafast pulse generation. One is to sandwich a graphene nano membrane between two fiber connectors within a fiber adaptor [8]–[14], and the other is to spray graphene suspension solution on the surface of a side-polished fiber and then evaporate its solvent [15]. However, in the first approach, a serious thermal damage on the graphene SA could be induced by the high optical pulse power, and the second approach exhibit large insertion loss, which could lead to unstable Q-switching operation in the fiber laser system. Furthermore, all these approaches are devoted to generate ultra-fast normal Gaussian and Sech pulses. Ultra-wide-band (UWB) radio technology based on UWB doublet pulses provides a promising solution to the future high-capacity wireless personal-area networks (PANs) due to its advantages such as low power consumption, high data capacity and multipath fading immunity [16]–[20]. A number of optical techniques have been proposed for the generation of UWB doublet pulses, such as using nonlinearly biased electro-optic modulator [16], Sagnac-interferometer-based intensity modulation [17], electro-optic phase modulation [18], and cross-absorption modulation effect in electro-absorption modulator [19] and semiconductor optical amplifier (SOA) [20]. However, the input Gaussian seed signals are needed in all the above mentioned schemes, which are then transformed into the doublet UWB pulses, thus the system is complex. Up to now, there is no report on direct generation of UWB doublet pulses by use of fiber laser system yet. In this paper, a direct generation of doublet UWB pulses by use of a passively mode-locked fiber laser is presented. The system is based on the reduced graphene oxide (RGO) deposited on the surface of the microfiber (MF) by use of high temperature heating. One of the unique and interesting properties of MF is its strong evanescent wave coupling [21]. Owing to the interaction of the RGO with the evanescent field of the MF in the fiber laser system, saturable absorption effect is created. Moreover, the UWB doublet pulses can be directly generated through the interaction between the dispersion and nonlinearity in the laser cavity.

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HE et al.: PASSIVELY MODE-LOCKED FIBER LASER

II. PREPARATION AND OPTICAL PROPERTIES REDUCED GRAPHENE OXIDE

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A. Preparation of Reduced Graphene Oxide on MF Graphene oxide (GO) is a water-soluble nano-material prepared through extensive chemical attack to oxide graphite. Here, we prepare the GO by using modified Hummers method [22] from flake graphite (Qingdao Tianhe Graphite Co. Ltd, 99.95% purity, 20 mm). Solution-processable GO can easily be used to fabricate paper-like films on other materials with massive production. Importantly, GO can be easily deposited on the surface of the quartz material, especially on MF. In this work, we firstly fabricate MF with the diameter of 6 m by use of “flamebrushing” technique [23], [24], and then deposit GO on the MF surface by use of spin-coating method. The nonlinear coefficient of the MF obtained is between 0.012 and 0.42 W /m, depends on its effective cross-section area. After spin-coated deposition, the insulating GO films are reduced on the MF through thermal annealed in inert conditions. The optical properties (e.g., broadband saturable absorption) of GO can be typically enhanced through removal of the oxidized moieties in the GO by thermal reduction. This will make the RGO on MF as a SA in fiber laser cavity for optical pulse generation. B. Optical Properties of Reduced Grapheme Oxide The RGO on quartz substrate shows a broadband absorption in the whole spectral region as demonstrated in Fig. 1(a). A typical Raman spectrum of the RGO obtained at the excitation wavelength of 633 nm is shown in Fig. 1(b). The first-order G and D peaks, arising from the vibrations of sp carbon, appear at around 1580 and 1350 cm , respectively. Degenerate pump-probe experiment was carried out by use of femtosecond laser pulses from a Ti: sapphire laser amplifier (1 K Hz, Spitfire, Spectra Physics) at 800 nm with the pulse duration of 130 fs. The pump pulses were modulated at 383 Hz with the help of an optical chopper, the probe pulses were delayed and the transmitted probe pulses were sent to the photodiode of a balanced detector, which was connected to a lock-in amplifier. Fig. 1(c) gives the transient differential transmission spectrum of the RGO. The pump-induced change of transmission is dominated by absorption saturation in the RGO, resulting in a positive differential transmission , where T and are the sample transmissions with and without excitation, respectively. We fit the data by using an exponentially decaying function, , convoluted with the cross correlation of the pump and the probe pulses. The fast decay time of the RGO is ps, which agrees well with that reported in [25] (0.14 ps) and [26] (0.09–0.18 ps), and can be considered to arise from carrier-phonon scattering. Fig. 2(a) is the illustration of the RGO on the surface of the MF. As shown in Fig. 2(a), the RGO on the MF can compress the pulse propagated along the MF due to the saturable absorption property. As shown in Fig. 2(b), the insertion loss of MF is varied between 0.5 and 2.7 dB in the wavelength region between 1480 and 1640 nm. A simple two-level SA model [27], [28] used for two-dimensional quantum wells of semiconductor can be adopted to analyze the nonlinear saturable absorption of graphene. The absorp-

Fig. 1. Optical properties of the RGO film. (a) UV-visa-NIR absorption. (b) Raman. (c) Transient differential transmission spectra.

tion as a function of increased light intensity can be expressed as (1) is the saturation intensity, and and are the where saturable and nonsaturable absorption. The measured transmission of the RGO on MF as a function of average pump power at the wavelength of 1550 nm (a probe laser with the pulse width of 2 ps and the repetition rate of 20 GHz) is shown in Fig. 2(b). The transmission change, as shown in Fig. 2(c), is similar to that of SESAMs [8] and polyvinyl alcohol (PVA) composite based on graphene [13]. At the relatively small input power level, the transmission is low and almost independent of the pump power. However, when the input average power is raised to 7.52 dBm (corresponding to the pump peak density of 0.506 MW/cm ) at 1550 nm, the transmission is in full saturation regime and increased by %, which leads that %, due to the fact that the RGO becomes more transparent after it is saturated. Thus, its modulation depth at wavelength of 1550 nm is %. The transmission loss (54.1%) under high input power level is caused by the nonsaturable absorption loss of RGO and the insertion loss of the MF. From Fig. 2(b), the insertion loss of MF is dB at the wavelength of 1550 nm. The nonsaturable absorption is % excluding the insertion loss of the MF. Compared with graphene thin films synthesized by CVD epitaxial growth [18] with of 33.5%, and PVA composite graphene [13] with of 34.3%, the nonsaturable absorption of % of the RGO is lower. However, the most important index to evaluate the pulse compression capability of SA is the modulation depth. Because the RGO on MF is reduced from

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Fig. 3. Experimental setup of the passively mode-locked fiber laser with the returned graphene on the MF; LD (laser diode), WDM coupler (wavelength division multiplexer), PC (polarization controller), EDF (Erbium-doped fiber), OSA (optical spectrum analyzer)

Fig. 2. Schematic and power-dependent saturable absorption properties of RGO on MF. (a) Schematic model. (b) Insertion loss of microfiber. (c) Measured typical transmission as a function of average pump power for wavelength of 1550 nm.

GO, its optical property on saturable absorption is not as good as pure graphene synthesized by CVD method and mechanical exfoliation of graphite. The RGO film has some residual oxygen and structural defects, which could lead to a reduced modulation depth. However, it can be easily produced by our method and utilized in a flexible manner. The modulation depth of our fabricated RGO is lower than that of graphene thin films reported in [18], but it is larger than that of PVA composite graphene reported in [13]. This indicates that the system based on RGO on MF has high potential to generate low-noise laser pulses in the optical communication band. III. PASSIVELY MODE-LOCKED FIBER LASER BASED REDUCED GRAPHENE OXIDE

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A. Experimental Setup The passively mode-locked fiber laser based on RGO on MF is shown in Fig. 3. A 1.5 m heavily erbium-doped fiber (OFS EDF-80) is used as the gain medium, pumped by a 1480 nm high power laser diode through a wavelength division multiplexer (WDM) coupler. To ensure slightly net positive cavity

dispersion, a 15 m long dispersion compensation fiber (DCF) is added in the cavity after the EDF. A section of MF with RGO plays the role of nonlinear SA. The polarization controller (PC) after the MF is for optimizing mode-locking pulses, and an isolator used to maintain the unidirectional pulse propagation. The mode locking signal can be directed out by using a 90:10 coupler. The group velocity dispersion (GVD) is one of the main factors to maintain the fiber laser operation stability. The GVD of the EDF used in the system is ps/nm/km, and that of the DCF is ps/nm/km, at the wavelength of 1560 nm. The MF with RGO is 4 cm in length and has a small anomalous dispersion of ps/nm/km, as RGO only acts on the evanescent field of the MF. The rest of the cavity consists of 11.26 m single mode fiber which has anomalous dispersion at 1560 nm, with GVD of ps/nm/km. Thus, the total intra-cavity dispersion is ps . The laser output spectrum is obtained by optical spectrum analyzer (ANDO AQ6319) with 0.01 nm resolution. The RF spectrum of the passively mode-locked fiber laser is measured by using a high speed photo-detector (Newfocus 1414, 25 GHz) connected to a real-time spectrum analyzer (Tektronix RSA 3303A, 3 GHz). The pulse is monitored by a second harmonic generation (SHG) autocorrelator (FEMTOCHROME FR-103XL, resolution fs) and a high speed photo-detector connected to an oscilloscope (Tektronix, TPS 2024). B. Results and Discussion In our fiber laser, the pump threshold for self-started modelocking is mW. When the pump power is increased to mW, a stable mode-locking operation can be established. To obtain the full characteristics of mode-locking, all the results are measured under the pump power of mW. Fig. 4 summarizes the mode-locked characteristics of the fiber laser based on the RGO on the MF. The optical spectrum is monitored from a 90:10 coupler output, by use of an optical spectrum analyzer. As shown in Fig. 4(a), it is a typical


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Fig. 4. Characteristics of the passively mode-locked fiber laser based on graphene on the MF. (a) Output spectrum centered at 1560.36 nm with the resolution of 0.1 nm. (b) Output pulse trains measured by oscilloscope. (c) Output power as a function of pump power. (d) Radio frequency spectrum with the span of 2 MHz. ps). (f) Autocorrelation traces of pulse peak and Gaussian fitting curve with a small scan range ( ps). (e) Autocorrelation traces with a large scan range (

output spectrum of mode-locking, with central wavelength of nm. The side bands are located at and nm respectively, resulting from the intra-cavity periodical perturbations [29], [30]. The pulse train is measured by use of a high speed photo-detector connected to an oscilloscope. Fig. 4(b) shows the output pulse train, with the period of 133.8 ns, as expected from the cavity length. The output power up to mW is recorded with a slope efficiency of 0.673%. To study the operation stability, the radio frequency (RF) spectrum

is measured by a high speed photo-detector connected to a real-time spectrum analyzer, as shown in Fig. 4(d). The measured RF span is up to 2 MHz, with the resolution bandwidth of 50 Hz. Its fundamental peak is located at the repetition rate of 7.47 MHz as shown in Fig. 4(d), with a high signal-to-noise ratio (SNR) of 70 dB ( contrast). This indicates the good mode-locking stability and high reliability of our system. Once the stable output is achieved in our fiber laser system, the high tolerance on the polarization state can be obtained. The stable

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doublet pulses obtained in our fiber laser well meets the FCC requirement. IV. CONCLUSION

Fig. 5. Measured RF spectrum of the generated doublet pulse (black line), and the FCC mark (red dotted line)

The RGO can be obtained by reducing the GO deposited on the surface of MF, with high temperature heating treatment. When the RGO interacts with the evanescent optical field of the MF, enhanced nonlinearity effects, such as nonlinear saturable absorption, photoluminescence, Raman effects, etc., can be induced. Here, we demonstrate that it can be effectively used as the SA in a passively mode-locked fiber laser with the modulation depth of %, for the generation of UWB doublet pulses ( ps) with repetition rate of 7.47 MHz and a high SNR of dB ( contrast). The UWB doublet pulses produced exhibit the FWHM value of ps, which can be used to generate UWB signals that has wide applications in high-capacity wireless communications. ACKNOWLEDGMENT

mode-locking can be kept unchanged for a long time such as more than half a day. Fig. 4(e) and (f) are the autocorrelation traces measured by use of an autocorrelator after a section of 120 cm long single mode fiber. As shown in Fig. 4(e), the AC trace of the mode-locked pulses measured at a long scan range of ps is the desired UWB doublet pulse, which is directly generated in the laser cavity and can be used to generate UWB microwave signal for high-capacity wireless communications [16]–[20]. The full-width at half-maximum (FWHM) value of such double pulses is ps, and the interval of two negative peaks at both sides is ps. The measured AC trace of the pulse peak with a small scan range ( ps) shown in Fig. 4(f) fits well with a Gaussian profile. According to the Gaussian temporal profile, its decorrelation factor is 0.707 for the pulses, leading to a pulse width of ps. In the fiber laser cavity, the dispersion and enhanced self-phase modulation (SPM) incurred by the MF and the whole fiber system result in the pulses with high chirp. The chirped mode-locking pulses are compressed by the RGO on MF in the cavity. As described in Section III part A, the total intra-cavity dispersion of our fiber laser is under large normal dispersion. This large normal dispersion in the cavity changes the shape of the chirped pulses. SPM has main effect on the narrow part of the chirped pulse waveform, which has relatively high intensity, thus leading to the generated doublet UWB pulses. The external single mode fiber at the output end can cause the red and blue edge of pulses to slightly split. A long range RF spectrum corresponding to the UWB doublet pulses of the fiber laser is measured by use of a high speed photo-detector connected to a RF spectrum analyzer (Agilent 8564EC, 40 GHz), as shown in Fig. 5. It is observed from this figure that the center frequency of such doublet pulses is estimated to be GHz, and its 10 dB bandwidth is GHz. The US Federal Communications Commission (FCC) Report and Order issued in 2002 defines the UWB signal as any signal that possesses a spectral bandwidth of more than 500 MHz in the 3.1–10.6 GHz band, with a spectral power density lower than dBm/MHz. Obviously, the RF spectrum of UWB

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Zhi-Bo Liu was born in Hebei, China, in 1978. He received the B.S. and Ph.D. degrees in physics from Nankai University in 2001 and 2006, respectively. He joined the faculty at Nankai University in 2004, where he is now an Associate Professor in Teda Applied Physics School. His research is focused on nanophotonics, nonlinear optics and micro-nanofabrication.

Dongning Wang (M’06) received the B.Sc. degree in telecommunications from Beijing University of Posts and Telecommunications, China, in 1982, the M.B.A. degree from the University of Ulster, U.K., in 1989, and the Ph.D. degree from City University, U.K., in 1995, respectively. Since 1998, he has been with the Department of Electrical Engineering, The Hong Kong Polytechnic University. His main research interests are ultrafast optics, femtosecond laser micromachining, fiber laser, optical fiber communications and optical fiber sensors. He has more than 130 international journal publications.

Minwei Yang received the B.Eng. degree in electronic information engineering in 2005, the M.Eng. degree in electromagnetic engineering from Shanghai Jiao Tong University, Shanghai, China, in 2008, and the Ph.D. degree from the Hong Kong Polytechnic University. His current research interests are optical fiber sensors and femtosecond laser micromachining.

C. R. Liao received the B.E. degree in optical information science and technology and the M.S. degree in physical electronics from Huazhong University of Science and Technology, Wuhan, China, in 2005 and 2007, respectively. He started the Ph.D. program in electrical engineering at The Hong Kong Polytechnic University in 2008. His current research is focused on optical fiber sensor and femtosecond laser micromachining.

Xin Zhao was born in Hebei, China, in 1985. She received the B.S. degree in physics from Hebei Normal University in 2008. She started her Master study in 2008 under the guidance of Prof. J. G. Tian and Prof Z. B. Liu in the Key Laboratory of Weak Light Nonlinear Photonics, Ministry of Education, Teda Applied Physics School, Nankai University. Her research is focused on the ultrafast dynamics of materials by femtosecond laser.