1. Introduction

Femtosecond erbium-doped fiber lasers have many applications in various industrial and scientific research areas, such as optical communications [1], optical coherent tomography [2], optical atomic clock [3] and supercontinuum generation [4]. Passive mode-locking is a practical technique to generate ultrafast femtosecond pulses in highly compact fiber lasers. Recently, nano-material graphene-based saturable absorber has attracted considerable interest as an excellent wideband mode-locker due to its unique linear and nonlinear optical properties, such as saturable absorption characteristics for a broad wavelength range, ultrafast recover time, low saturable intensity, and pulses from sub 200 fs to a few picoseconds in the graphene mode-locked erbium-doped fiber lasers have been reported [5–20]. In 2004, Novoselov et al. first produced graphene by mechanical exfoliation [21], although this method suffers from the ultra-low success ratio. And then various methods for high quality, large scale fabrication of graphene are actively explored, such as chemical vapor deposition (CVD) [22], thermal decomposition from SiC [23], and chemical reduction method [24]. The chemical reduction method involves complex chemical processes and generates graphene with heavily functionalized organic groups.

The first step of chemical reduction method is to synthesize graphene oxide from natural graphite powder. Then graphene-based nanosheet can be obtained from graphene oxide by chemical methods using reductants such as hydrazine, dimethylhydrazine. Graphene oxide, served as the precursor for graphene, has also been widely investigated for its own physical and chemical characteristics. On one hand, the presence of oxygen-containing functional groups makes graphene oxide strongly hydrophilic and water soluble, which is different with graphene. The solubility offers superior flexibility and processibility for large-scale production of graphene oxide based optoelectronics. For example, we can fabricate graphene-oxide membrane on different kinds of substrates by a spin-coater, or inject graphene oxide solution into a hollow-core photonic crystal fiber [25]. On the other hand, although the oxygen functional groups destroy the gapless linear dispersion of Dirac electrons in graphene and make graphene oxide insulating, it has been demonstrated that graphene oxide also has a fast energy relaxation of hot carriers and strong saturable absorption, which is comparable with that of graphene [26,27]. These properties make graphene oxide as potential saturable absorber material in pulsed fiber lasers. Up to now, there are two reports of graphene oxide mode-locked fiber lasers. In 2010, F. Bonaccorso et al. reported the graphene oxide mode-locked fiber laser for the first time [14]. However, in this paper they only provided the autocorrelation trace and optical spectrum of ~743 fs pulses, and other important information were not given, such as the cavity configuration, pulse trains and nonlinear optical parameter of graphene oxide. In 2011, Liu et al. generated pulsed erbium-doped fiber laser based on a hollow-core photonic crystal fiber filled with graphene oxide solution [25], but the pulse width was 4.85 ns.

Here, we report femtosecond mode-locked erbium-doped fiber lasers, which adopted ring cavity configuration and self-assembled graphene oxide saturable absorber mirror. With dispersion management, the laser directly generated 200 fs pulses at a repetition rate of 22.9 MHz and the average output power was 5.8 mW. To the best of our knowledge, 200 fs is the shortest pulses obtained from graphene-oxide-based fiber laser. With the variation of net cavity dispersion, output pulses with pulse width of 0.2~3 ps were obtained at a repetition rate of 22.9~0.93 MHz. Considering the outstanding advantages of low price, easy fabrication and amphipathic properties, the graphene oxide is promising candidate as saturable absorber and can be used as practical and efficient photonic material for generation of ultrafast fiber lasers.

2. Preparation of the graphene oxide saturable absorber mirror (GOSAM)

The graphite oxide was synthesized from natural graphite powder by a modified Hummers method [28]. The graphene oxide hydrosol with concentration of 2 mg/ml was prepared by ultrasonic peeling of graphite oxide in aqueous suspension. Then a broadband reflective mirror was immersed into the graphene oxide hydrosol for 48 hr. Finally, a thin graphene oxide membrane was formed on the broadband reflective mirror. Figure 1(a) shows the Raman spectrum of the graphene oxide membrane, which was excited by a 514 nm Ar ion laser. The Raman spectrum reveals two prominent features of graphene oxide at 1347 cm⁻¹ and 1593 cm⁻¹, which are assigned to D and G bands, respectively. The D band is from the structural imperfections created by the attachment of hydroxyl and epoxide groups on the carbon basal plane. The G band corresponds to ordered sp² bonded carbon [29]. According to the reports in [6] and [10], there was an obvious band around 2700 cm⁻¹ named 2D band in the Raman spectrum of graphene, which is considered as an evident feature of graphene material. In Fig. 1(a), the 2D band was hardly observed, which indicates there was no graphene on GOSAM.

 

Fig. 1 (a) Raman spectrum of graphene oxide. (b) Saturable absorption of graphene oxide.

Download Full Size | PDF

Figure 1(b) shows the measured reflection of GOSAM at different incident power using a probe laser with ~600 fs pulse width at 38 MHz repetition rate. This source was achieved by a SESAM mode-locked erbium-doped fiber laser and 10% of the output beam was used to monitor the input power, while the 90% was used to pump the GOSAM. The modulation depth of GOSAM was ~2.6% at 1558 nm. The insert loss of GOSAM and three-port circulator was ~60.5% in total, shown in Fig. 1(b). The loss of the three-port circulator measured to be ~30%, so the non-saturable loss of the GOSAM was ~30.5%. The saturable incident power was ~0.7 mW, corresponding to saturation intensity of ~60 MW/cm². In [10], Sun et al. inserted graphene membrane between two FC/APC fiber connectors to generate mode-locked pulses. In this way, the modulation depth was measured to be 1.3%, the non-saturable loss was34.3% and the saturation intensity was 266 MW/cm².

3. Experimental results and discussions

3.1 Graphene oxide mode-locked fiber laser in anomalous dispersion cavity

The experimental configuration of femtosecond graphene oxide mode-locked erbium-doped fiber laser is shown schematically in Fig. 2 . The ring cavity included a piece of 1 m erbium-doped fiber and ~6.8 m single mode fiber. The cavity length was around 7.8 m, and the net dispersion was estimated to be −0.14 ps². The erbium-doped fiber with ~7 dB/m absorption was core pumped by a diode laser with a center wavelength of 974 nm and a maximum output power of 600 mW. An optical circulator was used to incorporate the graphene oxide saturable absorber mirror (GOSAM) into the cavity. The fiber of circulator 2nd-port was perpendicularly cleaved and butted to the GOSAM. A 30% fiber coupler was used to output the signal. An optical spectrum analyzer (Yokogawa, AQ6370), a 7.5GHz radio-frequency analyzer (Agilent N900A-507), and a 25 GHz real-time oscilloscope (Agilent DSO-X92504A) with a 25 GHz photo-detector were employed to monitor the laser output simultaneously.

 

Fig. 2 Schematic setup of the graphene oxide mode-locked erbium-doped fiber laser. WDM: wavelength division multiplexer; SMF: single mode fiber; GOSAM: graphene oxide saturable absorber mirror.

Download Full Size | PDF

When the diode pump power increased to 33 mW, the self-started mode-locking occurred. Figure 3(a) shows a typical pulse train at repetition rate of 25.6 MHz, which corresponds to the total cavity length of ~7.8 m. The spectral FWHM of 5.4 nm was centered at 1556.9 nm, measured by an optical spectral analyzer with resolution of 0.02 nm (Fig. 3(b)). Figure 3(c) shows a typical autocorrelation trace, which is well fitted by a sech² temporal profile, resulting in pulse duration of ~600 fs. The time-bandwidth product (TBP) was 0.405 at fundamental soliton-like operation, conformed by the clearly visible Kelly sidebands of optical spectrum [30]. The maximum output power was 3.3 mW at 98 mW pump power, corresponding to single pulse energy of 0.13 nJ and peak power of 220 W. Further increase of the pump power, the wave breaking occurred. Eventually, harmonic mode-locking with two times of the fundamental frequency was also observed. The radio-frequency spectrum (Fig. 3(d)) shows its fundamental peak located at the cavity repetition rate of 25.6 MHz, with a signal-to-noise ratio of 50 dB, indicating good mode-locking stability. To verify that the mode locking resulted from the graphene oxide, we purposely used a broadband mirror to replace the GOSAM from the cavity and then no mode locking was observed. In this work, the stability performance of the fiber laser was monitored for 8 hours.

 

Fig. 3 Characterization of graphene oxide mode-locked fiber laser in anomalous dispersion cavity: (a) Stable pulse train at 25.6 MHz repetition rate. (b) Optical spectrum. (c). 600 fs pulse width. (d) Frequency spectrum.

Download Full Size | PDF

3.2 Graphene oxide mode-locked fiber laser in near zero dispersion cavity

According to the dispersion management theory, shorter pulses can be achieved by adjusting the lengths of erbium-doped fiber and single mode fiber to give near zero round trip group velocity dispersion. In this experiment, we increased the length of erbium-doped fiber with GVD parameter of −11.7 (ps/nm/km) from 1 m to 3 m, in order to compensate the negative dispersion of single mode fiber. The single mode fiber length was optimized to be 5.7 m to get stable pulse trains, and the total dispersion was calculated to be −0.088 ps², which was closer to zero than −0.14 ps² in last work. Meanwhile, a 70% fiber coupler was used to increase the output power.

The stable mode-locking occurred at 27 mW pump power. Figure 4(a) shows a typical pulse train at repetition rate of 22.9 MHz, Fig. 4(c) shows the 200 fs pulse width under sech² assumption, and Fig. 4(b) shows the optical spectrum centered at 1560 nm. The central wavelength had a slightly red shift, which means the gain of the whole cavity was increased. The gain caused by longer erbium-doped fiber overweighed the loss of a coupler with higher output radio. There was an obvious “continuous wave component” (the narrow peak at the center of the spectrum) existing, which was caused by an insufficient modulation depth of GOSAM [31]. In [31], Gui et al. demonstrated the saturable absorber with larger modulation depth can suppress the continuous wave of mode-locked pulses. So we used a SESAM with 34% modulation depth in this cavity configuration and obtained ~200 fs pulse train without “continuous wave component”. That means if we can fabricate a larger modulation depth GOSAM, we can suppress the “continuous wave component” and the mode-locking stability will also greatly increased. Figure 4(d) shows the radio-frequency spectrum measured at a span of 4 kHz and a resolution bandwidth of 10 Hz. The fundamental peak located at the cavity repetition rate of 22.9 MHz has a signal-to-noise ratio of 60 dB. The maximum output power was 5.8 mW at 69 mW pump power. Further increase of the pump power, the pulse breaking occurred.

 

Fig. 4 Characterization of graphene oxide mode-locked fiber laser in near zero dispersion cavity: (a) Stable pulse train at 22.9 MHz repetition rate. (b) Optical spectrum. (c) 200 fs pulse width. (d) Frequency spectrum.

Download Full Size | PDF

In order to investigate the dependence of pulse width and net cavity dispersion, the length of single mode fiber was varied while the 3 m erbium-doped fiber maintained. Table 1 included six groups of data shows the relationship between cavity design and laser performance. When the net dispersion varied from −0.088 ps² to −4.9 ps², we generated pulses with pulse width of 200 fs ~3 ps at repetition rate of 23 MHz ~930 kHz.

Table 1. Optical Parameter of Graphene Oxide Mode-locked Fiber Laser

View Table

Compared with the reports [6,9], the mode-locking performance (such as stability, pulse width, spectrum shape) of the fiber lasers based on graphene oxide was as good as that of atomic-layer graphene. As we known, atomic-layer graphene has much better performance in mode-locking than multi-layer graphene because of larger modulation depth, but the current approaches cannot satisfy the large yields, layer controlled production of graphene. Monolayer graphene oxide can be easily peeled from graphite oxide by a simple ultra-sonication process, and the oxygen groups attached on the graphene oxide nanosheet provide good hydrophilic properties, making it easy for large-scale production. By further optimization of the cavity design and improvement on the GOSAM, we could generate mode-locked pulses with narrower pulse width and larger pulse energy.

4. Conclusion

In summary, we have demonstrated femtosecond graphene oxide mode-locked erbium-doped fiber laser. With dispersion management, the total dispersion can be decreased to −0.088 ps², where the pulse width was 200 fs at 22.9 MHz repetition rate and the average output power was 5.8 mW. With the variation of net cavity dispersion, output pulses with pulse width of 0.2~3 ps were obtained at a repetition rate of 22.9~0.93 MHz. The superiority of easy fabrication and strong solubility will facilitate potential applications of graphene oxide for ultrafast photonics.