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Mode-locked fiber lasers are currently undergoing a significant evolution towards higher pulse energies and shorter pulse durations. A key enabler in this progress has been the discovery of novel saturable absorbers (SA) such as carbon nanotubes (CNT) and graphene. The exceptional properties of CNTs as SA have been extensively studied in recent years. Graphene, a one atom thick planar sheet of carbon atoms arranged into a hexagonal lattice, has been recently proposed as an alternative to CNTs in several photonics applications. Here, we propose a method for the integration of graphene into a fiber ferrule using an optical deposition technique, which has been also employed for the deposition of CNT directly on the core of a fiber edge and in tapered fibers. We investigate and compare the optical properties of CNT-SA and graphene-SA fabricated by this optical deposition technique. Soliton-like, mode-locked lasing is confirmed using an erbium doped optical fiber in an all-fiber ring cavity laser configuration.
©2010 Optical Society of America
1. Introduction
Passively mode-locked fiber lasers capable of producing ultrashort, high power pulses are rapidly gaining ground over conventional solid state lasers for many commercial applications. Such fiber laser devices are now routinely employed for material processing, supercontinuum generation, optical frequency metrology and biomedical applications and are often preferred to their solid state laser counterparts due to their high beam quality, reliability, efficient heat dissipation and compact size [1]. In addition, passively mode-locked laser technology is going through a fascinating evolution striving towards higher powers, shorter pulses and improved stability. This evolution is been fueled by the discovery of novel model-locking mechanisms such as stretched-pulse mode-locking [2, 3], self-similar pulsing [4], dissipative solitons [5, 6] and the recently proposed soliton-similariton mode-locking [7]. Pulse operation in a fiber laser is commonly obtained by introducing an intensity-dependent component into the optical system that discriminates in favor of pulse formation over continuous wave lasing. This approach is known as passive mode-locking and it is generally preferred to active mode-locking techniques due to its simplicity and the ability to produce transform-limited pulses without the need for any external active devices such as modulators. In practice, this is generally achieved by using a saturable absorber (SA) that absorbs the incoming light linearly until a given threshold intensity. Once this threshold is reached, the material saturates and becomes transparent to the incident light [8]. Semiconductor saturable absorber mirrors (SESAMs) have been generally employed as SA for most commercial applications, however SESAMs consist of multiple quantum-walls drawn on distributed Bragg reflectors [9]. Their fabrication is expensive, requiring clean room facilities and in order to reduce the saturation recovery time they require a complex post-fabrication process of ion implantation. A further drawback is that their integration into a fiber configuration is not straight forward, in particular for ring cavity configurations.
In recent years, the emergence of novel SA materials that could, in principle, replace and outperform SESAMs has gathered much attention within the research community. Those devices utilize the intensity-dependent, nonlinear optical absorption of materials such as carbon nanotubes (CNT) and graphene. Both CNTs and graphene are carbon allotropes, consisting of a carbon honeycomb sheet of sp2 bonded carbon atoms. CNTs are created when such carbon honeycomb sheet rolls in itself to form a cylinder which typically has a diameter between 1nm and 2nm. Depending on their chiral vector, single-wall CNTs may behave as semiconductors or metals, those that behave as semiconductors have a direct electronic bandgap which is directly proportional to the diameter of the nanotube. Optical absorption, as in standard direct bandgap semiconductors, is determined by their bandgap and broadband operation is a result of a large distribution of diameters form during the CNT fabrication [10]. Graphene, on the other hand, is a one-atom-thick planar sheet of sp2 bonded carbon atoms. Unlike standard semiconductors, graphene has a zero bandgap, hence it is a semi-metal. Saturable absorption results from its unique electric band structure and the Pauli blocking principle which means that the same graphene-based SA can be in principle used for all different operational wavelengths. In graphene, an incident photon may be absorbed by the promotion of an electron from the valence band to the conduction band. Equilibrium is reestablished through electron-hole recombination. As the density of photons increases, the concentration of carriers increases until the states near the edges of the valence and conduction bands are filled. This leads to the blocking of any further absorption, hence the saturation of its optical absorption [11].
CNT-based SA technology is the more mature of the two since the first devices were demonstrated as early as 2003 [12, 13]. CNTs exhibit exceptional nonlinear optical properties which include nonlinear saturable absorption, ultrafast recovery time, high third-order optical nonlinearity, and broad bandwidth operation. Since 2003, CNT-based devices have been demonstrated in multiple configurations [3, 6, 13–23] and for all the relevant operational wavelengths ranging from 1μm to 2μm [22]. In addition, CNTs have been a key enabler in the evolution of various mode-locking operating regimes including soliton, stretched-pulse mode-locking [3], and dissipative solitons [6, 23]. Despite the great promise of this material, there are also some drawbacks inherent to their physical properties. The optical absorption of CNTs is proportional to their diameter; only the CNTs that are in resonance with the operating wavelength of the laser contribute as saturable absorbers while the remaining CNTs are responsible for excess background losses and scattering. Thermal or optical damage of the CNT-SA is often observed during operation at relatively high intra-cavity optical powers. Recent reports have demonstrated that graphene exhibits ultrafast recovery and wavelength independent saturable absorption as well as lower scattering losses and potentially a higher damage threshold than CNTs [24–28]. In addition, graphene is known for its ultrahigh electron mobility and thermal conductivity. All of this makes graphene an intriguing material with enormous potential to be applied in photonics.
In this paper, we demonstrate the feasibility of using the optical deposition method to fabricate a saturable absorber of graphene into a fiber ferrule. This method has been previously employed to deposit CNT on fiber ferrules and on tapered fibers [29–31]. The method is a simple and effective approach to deposit the material directly onto the core of the optical fiber relying on a combination of optical trapping and heat convention effects. The optical properties of the mode-lockers fabricated by optical deposition of both graphene and CNT are investigated. The devices are then inserted into an all-fiber ring cavity laser with erbium doped fiber as gain media in order to confirm self-starting passively mode-locked laser operation.
2. Saturable absorber preparation
The CNTs used in this work were commercial available CNTs, made by the high-pressure CO conversion (HiPCO) method and the graphene was exfoliated from commercial graphite. Both the CNTs and the graphite were independently dispersed into Dimethylformamide (DMF) solvent by ultrasonification. In the case of the CNT, the aim is to separate individual CNTs and the breaking of bundles of CNT that are formed due to the van der Waal forces. Efficient dispersion was achieved after 30 minutes of ultrasonification. On the other hand, efficient exfoliation and dispersion of graphite into single-layer or few layer graphene required several hours of ultrasonification. Both solutions were then subjected to centrifugation in order to separate the remaining macroscopic flakes of graphite and the agglomerated CNTs. Only the visually homogeneous part of each solution was used for the optical deposition process. Photographs of the graphite and CNT prior to their dispersion in DMF are shown in Fig. 1(a) and Fig. 1(b) respectively. A photograph of the dispersion of graphene in DMF after ultrasonification and centrifugation is shown in Fig. 1(c). The optical absorption of both solutions was measured by a spectrometer and is shown in Fig. 1(d). The S11 and S22 absorption peaks characteristic of the CNT are clearly observed in Fig. 1(d), such peaks are not present in the graphene absorption. Fluctuations in the longer wavelengths of the graphene solution spectrometer trace are a result of fluctuations in the baseline measurement of the DMF solvent. Such fluctuations should have no influence on the performance of the device since there is no remaining DMF in the SA after the optical deposition process is completed.
Fig. 1 (a) graphite, (b) HiPCO-CNTs, (c) Solution of graphene in DMF after the ultrasonic and subsequent centrifugation treatment (d) Transmission of graphene (black) and CNT (gray) solutions on DMF.
Optical deposition was carried out as described by Kashiwagi et al. [30]. The experimental set-up used for the optical deposition of the samples is shown in Fig. 2(a) . Deposition is achieved by immersing a flat fiber connector into the solution of graphene or CNT on DMF and launching a high-power light into the fiber. The optical power required to achieve optical deposition is dependent on a number of parameters, including the concentration levels of the solution, and how well the CNTs (or graphene) have been dispersed on the solvent. The physical mechanisms responsible for the optical deposition have been described elsewhere, and consist of a combination of optical trapping and heat convention effects [30].
Fig. 2 (a) Set-up employed for the optical deposition, (b) and (c) scanning electron microscopy (SEM) of the optical fibers with graphene and CNT optical deposited in their core region, insertion losses were 8dB and 3dB respectively. The insets show the corresponding microscopic pictures of the fiber edge confirming the deposition into the core of the fiber.
Figure 2(b) and Fig. 2(c) show the scanning electron microscope (SEM) images of the deposited graphene and CNT into the core of the fiber respectively. The fiber in Fig. 2(b) exhibited insertion losses (IL) as high as 8dB, which were excessive for mode-locking. The insets of Fig. 2(b) and Fig. 2(c) correspond to the microscopic images taken from the same samples of optically deposited graphene and CNT respectively around the core area of the fiber. In Fig. 2(b) submicron graphitic flakes deposited on the fiber surface are observed, as well as the thinner, smaller single or bi-layer flakes of graphene. The deposition of multi-layer graphitic flakes leads to higher non-saturable losses in the device but does not contribute towards the saturation. In Fig. 2(c) the CNT appear finely disperse without excessive bundling on the core area of the fiber ferrule. Once the optical deposition process is completed, the fiber was removed from the DMF solution and was allowed to dry on air for several minutes prior to been butt-coupled to a second flat fiber connector. All the experiments in this work were carried out without applying any protection to either the CNT or the graphene saturable absorbers.
3. Results and discussions
The linear insertion losses and saturable absorption of the optically deposited CNT-SAs and graphene-SAs are shown in Fig. 3 . The insertion loses were approximately 2.5dB for the CNT sample and 2.0dB for the graphene sample (note that the samples used for the saturable absorption measurements and the modelocked laser are different to those shown in Fig. 2). Saturable absorption was measured using a mode-locked laser that operates at central wavelength of 1557nm with a repetition rate of 50MHz, a pulse duration of 500fs and an output power of 10dBm. The inset of Fig. 3 shows a microscope picture of the graphene-deposited fiber used for the saturable absorption measurements and the mode-locked laser described in the following section. The modulation depth was of the order of 5% for the CNT-SA and less than 4% for the graphene-SA. Reports in literature indicate that the modulation depth in graphene-SA is dependent on the number of graphene layers stacked together. Highest modulation depths are achieved when using single layer graphene SA (24, 25). Multiple-layered graphene and the presence of graphitic flakes leads to increased nonsaturable loses (24-26). Our measurements of saturable absorption indicate a modest modulation depth of 4% while the nonsaturable losses were of the order of 37%. Careful analysis of the graphene solution in DMF confirmed the presence of suspended graphitic flakes. We expect an improvement in the performance of the device by filtering out the larger flakes prior to the optical deposition procedure.
Fig. 3 Saturable absorption of a CNT-SA (black) and a graphene SA (grey). Inset, microscopic image of graphene-SA used for the saturable absorber measurements and for the mode-locking experiment.
Both SA were “sandwiched” between two flat fiber connectors and inserted in a ring laser cavity such as shown in Fig. 4 . The gain media consisted of 3m of Erbium doped fiber with a group velocity dispersion parameter of −13ps/nm/km, and >3dB/m absorption at 980nm. It was backward pumped by a 980nm laser diode. In addition, 20m of SMF fiber with anomalous dispersion at +15ps/nm/km were used ensuring net anomalous dispersion in the fiber cavity. Unidirectional operation was guaranteed by adding two isolators within the cavity. 50% of the intra-cavity power was coupled out. The ring cavity laser also consisted of a polarization controller (PC), nevertheless self-starting, mode-locked operation was observed by both SAs regardless of the state of polarization, and without any significant variation in their performance.
Fig. 4 All-fiber ring cavity laser configuration, WDM – wavelength division multiplexer, EDF – erbium doped fiber, ISO – isolator, SMF – single mode fiber, SA – saturable absorber “sandwiched” between two butt-coupled flat connectors PC – polarization controller.
The modelocking of the laser was self-starting at a pump power of approximately 20mW for the CNT-SA and at a significantly higher pump power for the graphene-SA (80mW). The laser operates in the anomalous-dispersion, soliton regime as evident from the optical spectrum of the laser output shown in Fig. 5(a) and Fig. 5(d), the central emission wavelength was approximately 1532nm. The pulse duration, inferred from the measured autocorrelator trace was approximately 820fs for the CNT-SA (Fig. 5(b)) and 850fs for the graphene-SA (Fig. 5(e)), assuming a sech2 pulse shape in both cases. The RF measurements indicate that the laser operate in its fundamental regime with a frequency of 5.27MHz in both cases, with peak-to-background ratios over 45dB as shown in Fig. 5(c) for the CNT-SA mode-locked laser and Fig. 5(f) for the graphene mode-locked laser. The measurement span was 10kHz and the resolution bandwidth 30Hz.
Fig. 5 Output spectra of the ring cavity laser when using the CNT-SA (a) and the graphene-SA (d), autocorrelation traces at the output of the laser using the CNT-SA (b) and the graphene-SA (e), RF spectrum in its fundamental repetition rate of the fiber laser when using the CNT-SA (c) and the graphene-SA (f)
In this paper, optical deposition of graphene into a fiber ferrule was confirmed and mode-locking with such graphene device was successful. However, the pump power required for self-starting modelocking was significant higher for devices employing graphene (80mW of pump power) than for devices employing optically deposited CNT (less than 20mW), even when the devices presented comparable insertion losses. This indicates that there is a higher ratio of nonsaturable to saturable absorption in the graphene sample compared to the CNT samples. This result is in direct contrast to previous reports indicating that the saturable to nonsaturable ratio of graphene is higher than that of CNT [24, 25]. The presence of microscopic flakes of graphite and multi-layer graphene in the DMF solution is the likely cause for the relative lower performance of the graphene-SA here presented. This drawback can be solved by further improving the pre-deposition steps of the exfoliation of graphene in DMF and the removal of the larger graphitic flakes.
It is also worth noting that the laser was capable of operating at intracavity powers as high as 24dBm using both the CNT-SA and the graphene-SA without any observation of damage on the device. At those higher pump powers soliton quantization was observed however this is relevant since previous reports indicate that such CNT-based SA under a direct interaction regime are damaged at intracavity powers lower than 15dBm. The source for such improved robustness to optical damaged is not fully understood and will be further investigated.
4. Conclusion
In this paper, we present a simple, convenient method for the exfoliation of graphene from commercial graphite and its optical deposition into a fiber ferrule. We confirm the validity of the method by using the optically deposited graphene to mode-lock an Erbium-doped fiber-based ring cavity laser. The performance of the device was compared to that of CNTs optically deposited on a fiber ferrule. Self-starting mode-locked lasing was observed with both devices operating at a central wavelength of 1532nm and with similar pulse durations (~0.8ps). However, the graphene-based saturable absorbers exhibited a significantly higher threshold for self-start mode-locking that their CNT counterparts. We expect that such threshold value may be significantly reduced by improving the ultrasonification process and by filtering out the larger (>1micron) graphitic particles responsible for the increased background un-saturable losses.
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