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All-Polarization-Maintaining Femtosecond Fiber Lasers Using Graphene Oxide Saturable Absorber Jia Xu, Sida Wu, Jiang Liu, Yanping Li, Jun Ren, Quan-Hong Yang, and Pu Wang Abstract— We demonstrate the mode-locking operation in an all-polarization-maintaining (all-PM) erbium-doped fiber laser by exploiting graphene oxide saturable absorber for the first time. The self-started, environmentally stable laser directly generates linearly polarized soliton pulses of 502 fs duration at a repetition rate of 48.2 MHz with an extinction ratio of 21.3 dB. The optical spectrum centered at 1555.92 nm has 3-dB bandwidth of 6 nm. This unique combination of the all-PM configuration and the novel graphene oxide saturable absorber offers an ideal approach to fabricate a compact single-polarization femtosecond source with high environmental stability. Index Terms— Erbium-doped fiber lasers, mode-locking, ultrafast lasers, graphene oxide.
I. I NTRODUCTION
C
OMPACT femtosecond erbium-doped fiber lasers, delivering high peak power, can be used as desirable light sources in many fields, such as ophthalmology, optical coherent tomography, terahertz generation and biomedical research. Passively mode-locking technology has been considered as the most common method to generate ultrashort pulses and several kinds of saturable absorbers, from semiconductor saturable absorber (SESAM) to carbon nanotube (CNT) and graphene, have been widely investigated to improve the mode-locking performance as well as to decrease the manufacturing cost. However, these passively mode-locked fiber lasers may suffer from the environmentally instability that means the lasers are sensitive to externally-induced changes, like significant temperature variations and mechanical perturbations which will influence the fiber birefringence property. One effective method to eliminate this instability is to build an allpolarization-maintaining (all-PM) fiber oscillator where the light polarizes only along the slow or fast axis in the PM fiber. And there is no polarization controller (PC) needed, which could simplify the cavity configuration. Furthermore, these
Manuscript received August 13, 2013; revised November 8, 2013; accepted November 27, 2013. Date of publication December 5, 2013; date of current version January 21, 2014. This work was supported in part by the National Natural Science Foundation of China under Grants 61235010 and 61177048, in part by the Beijing Municipal Education Commission under Grant KZ2011100050011, and in part by the National Science Foundation of Tianjin, China, under Grant 12JCZDJC27400. J. Xu, J. Liu, Y. Li, J. Ren, and P. Wang are with the Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, China (e-mail:
[email protected];
[email protected];
[email protected];
[email protected]; wangpuemail@bjut. edu.cn). S. Wu and Q.-H. Yang are with the School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2293624
all-PM fiber lasers delivering single-polarization pulses will have practical applications in some polarization-dependent process, such as nonlinear frequency conversion including frequency doubling and optical parametric generation. Up to now, all-PM passively mode-locked fiber lasers using SESAM [1], [2], CNT [3], [4] and graphene [5], [6] have been demonstrated. Recently, graphene oxide, the precursor of the pure graphene in chemical synthesis, has attracted a great deal of attention as a promising novel saturable absorber, due to its strong saturable absorption, ultrafast recovery time and broadband operation wavelength. Graphene oxide is an atomically thin sheet of carbon bonded with oxygen-containing functional groups and it can be obtained from natural graphite by simple oxidation and ultra-sonication process [7]. Moreover, graphene oxide has strong hydrophilic induced by the presence of oxygen-containing functional groups, which offers superior flexibility to form versatile saturable absorbers. Up to now, the graphene oxide based Q-switched and modelocked lasers at 1 µm [8], 1.5 µm [9]–[13] and 2 µm [14], [15] have been widely explored, indicating that the graphene oxide is comparable to graphene as a saturable absorber [16]–[22]. However, to the best of our knowledge, all the previous work related to graphene oxide mode-locked fiber laser adopted nonPM configurations. In this letter, we present single-polarization femtosecond pulse generation in a graphene oxide mode-locked erbiumdoped fiber laser which adopts a ring cavity configuration with all-PM fibers and components. In the all-anomalous dispersion regime, the laser directly generates 502 fs pulses at a repetition rate of 48.2 MHz and the polarization extinction ratio (PER) of the linearly-polarized pulse is 21.3 dB. It is considered to be a robust ultrafast source with high environmental stability for long-term operation. II. E XPERIMENTAL S ETUP The experimental setup of the all-PM graphene oxide modelocked fiber laser is illustrated in Fig. 1. The ring cavity consists of a 1 m Panda-type PM erbium-doped fiber, a 980/1550 PM wavelength division multiplexer (WDM), a 30% PM output coupler and a three-port PM circulator. All these PM fibers and PM components are spliced by a Vytran splicer (FFS-2000) with precisely alignment in slow axis. The oscillator is counter-directionally core-pumped by a 974 nm diode laser with the maximum pump power of 600 mW. The PM erbium-doped fiber (Nufern, PM-ESF-7/125) has ∼17.6 dB/m absorption at 974 nm and ∼15 ps/nm/km group velocity dispersion (GVD) at 1550 nm. A PM circulator with
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XU et al.: ALL-PM FEMTOSECOND FIBER LASERS
Fig. 1. Schematic setup of the all-PM graphene oxide mode-locked erbiumdoped fiber laser.
1.5 dB insertion loss is employed to assure the unidirectional propagation of the laser and incorporate the graphene oxide saturable absorber mirror (GOSAM) into the cavity. The fiber on the second-port of the circulator is perpendicularly cleaved and butted to the GOSAM, which can be packaged together, like what the commercial SESAM does. Compared with the transmissive method by using fiber connectors, this reflective method by butt-coupling offers more flexibility. The coupling loss between the fiber and the gold mirror without GO is measured to be 11%. The total length of this all-anomalous dispersion cavity is ∼4.15 m. If we ignore the dispersive terms above the second order, the total dispersion is estimated to be −0.093 ps2 . The GOSAM used in this experiment is the same sample in reference [9]. It is prepared by depositing the graphene oxide hydrosol with a concentration of 2 mg/ml on a broadband reflective mirror. As the widely used approach, the graphite oxide is obtained by modified Hummers method, and then the graphene oxide hydrosol is produced by ultrasonic peeling of graphite oxide in aqueous suspension. The GO samples used in this letter are mostly mono-layered. The thickness of the deposited GO membrane on the mirror is estimated to be several to tens µm by observing the cross-sectional surface of GO membrane prepared by using the similar method but free of the mirror. And the thickness can be easily controlled by changing the graphene oxide hydrosol concentration and the deposition time. According to the our previous results on the saturable absorption properties of GOSAM, the modulation depth, nonsaturable loss and saturation intensity are ∼2.6%,∼30.5% and ∼60 MW/cm2 respectively [9]. The spectrally and temporally mode-locking performance is simultaneously observed by an optical spectrum analyzer (Yokogawa, AQ6370), a 25 GHz real-time oscilloscope (Agilent DSO-X92504A), a 25 GHz photo-detector, an autocorrelator (Femtochrome, FR-103XL) and a 7.5GHz radiofrequency analyzer (Agilent N900A-507). III. E XPERIMENTAL R ESULTS AND D ISCUSSION The self-started single pulse mode-locking is initially achieved at the pump power of 96 mW. Fig. 2 shows a typical pulse train with ∼21 ns interval between the two adjacent pulses, thus giving a repetition rate of 48.2 MHz, which corresponds to the total cavity length of ∼4.15 m. The smooth optical spectrum is centered at 1555.92 nm with 3 dB spectrum bandwidth of 6.0 nm, measured by an optical spectral analyzer
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Fig. 2. Typical pulse train of the all-PM graphene oxide mode-locked erbiumdoped fiber laser.
Fig. 3. Optical spectrum of the all-PM graphene oxide mode-locked erbiumdoped fiber laser.
with a resolution of 0.02 nm, shown in Fig. 3. The clearly visible Kelly sidebands, induced by intra-cavity periodic perturbations, conforms the soliton-like pulse formation, which could be well described by nonlinear Schrödinger equation (NLSE). The laser pulse width is directly measured without any pre-amplification by a commercial autocorrelator with a maximum scale range of 90 ps and a resolution of 5 fs. Fig. 4 illustrates the pedestal-free clear autocorrelation trace which is well fitted by a sech2 temporal profile. The full width at half maximum (FWHM) of the pulse is measured to be 774 fs, resulting in pulse duration of ∼502 fs. The time-bandwidth product (TBP) is calculated to be ∼0.373, which is closed to the Fourier-transform limit of 0.315 for sech2 pulse. This laser operates in an all-anomalous-dispersion cavity without any dispersion compensation. Fig. 5 shows the clean radio-frequency (RF) spectrum measured at a span of 50 kHz with 100 Hz resolution bandwidth. The fundamental peak is located at a repetition rate of 48.187 MHz and the signal-to-noise ratio is 65 dB, indicating the stable single-pulse mode-locking operation. The maximum output power is 2.7 mW at the pump power of 121 mW, corresponding to single pulse energy of 56 pJ and peak power of 112 W. A further power increase results in multiple pulsing operation. Higher output power could be obtained by using a coupler with larger output ratio. Owing to the all-PM configuration, the linearly polarized output pulses are obtained with the extinction ratio of 21.3 dB, measured by an Extinction Ratio Meter (Thorlabs ERM100) with 0.1 dB resolution. This PM mode-locked laser is extremely robust against the thermal variations and mechanical stress and it can operate without interruptions for a long period of time. Repeatedly, when the pump diode
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 4, FEBRUARY 15, 2014
saturable absorber seems to provide an easy way to obtain linearly polarized femtosecond pulses. After further amplification, this kind of high power single-polarization femtosecond fiber laser can be used as a practical pump source in nonlinear frequency conversion process. R EFERENCES
Fig. 4. Autocorrelation traces of the all-PM graphene oxide mode-locked erbium-doped fiber laser. (Blue dots: Experimental data. Red line: Sech2 fit).
Fig. 5. RF spectrum of the PM graphene oxide mode-locked erbium-doped fiber laser.
Fig. 6. Optical spectrum of the long-cavity PM graphene oxide mode-locked erbium-doped fiber laser.
is turned off and on, the mode-locking always can quickly recover to its previous mode-locking state. In addition, this GOSAM shows good performance in a long cavity with 100 m PM single mode fiber, where we obtain the 2 ps pulses at 1.98 MHz repetition rate. As presented in Fig. 6, the central wavelength of the optical spectrum is 1555.96 nm, and the 3 dB bandwidth is 1.6 nm, yielding a TBP of ∼0.40. Since the fiber dispersion plays an important role for ultrafast pulse generation, further investigation in the dispersion-managed cavity will enable the laser to deliver shorter pulse duration. IV. C ONCLUSION We present an environmentally-stable, all-PM, graphene oxide mode-locked fiber laser for the first time. The laser in all-anomalous dispersion cavity directly generates 502 fs linearly-polarized pulses at a repetition rate of 48.2 MHz with an average output power of 2.7 mW and the polarization extinction ratio is 21.3 dB. This novel combination of the all-PM configuration and the attractive graphene oxide
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