A photonic crystal fiber (PCF) with high-quality graphene nano-particles uniformly dispersed in the hole cladding are demonstrated to passively mode-lock the erbium-doped fiber laser (EDFL) by evanescent-wave interaction. The few-layer graphene nano-particles are obtained by a stabilized electrochemical exfoliation at a threshold bias. These slowly and softly exfoliated graphene nano-particle exhibits an intense 2D band and an almost disappeared D band in the Raman scattering spectrum. The saturable phenomena of the extinction coefficient β in the cladding provides a loss modulation for the intracavity photon intensity by the evanescent-wave interaction. The evanescent-wave mode-locking scheme effectively enlarges the interaction length of saturable absorption with graphene nano-particle to provide an increasing transmittance ΔT of 5% and modulation depth of 13%. By comparing the core-wave and evanescent-wave mode-locking under the same linear transmittance, the transmittance of the graphene nano-particles on the end-face of SMF only enlarges from 0.54 to 0.578 with ΔT = 3.8% and the modulation depth of 10.8%. The evanescent wave interaction is found to be better than the traditional approach which confines the graphene nano-particles at the interface of two SMF patchcords. When enlarging the intra-cavity gain by simultaneously increasing the pumping current of 980-nm and 1480-nm pumping laser diodes (LDs) to 900 mA, the passively mode-locked EDFL shortens its pulsewidth to 650 fs and broadens its spectral linewidth to 3.92 nm. An extremely low carrier amplitude jitter (CAJ) of 1.2-1.6% is observed to confirm the stable EDFL pulse-train with the cladding graphene nano-particle based evanescent-wave mode-locking.
© 2013 Optical Society of America
In addition to the graphene based saturable absorbers [1–9] for the passively mode-locked fiber lasers, the graphene nano-particle has shown its potential to be the mode-locker for erbium-doped fiber lasers (EDFLs) [10–12]. Because graphite is easily cleaved due to the weak coupling of van der Waals forces between each graphene plane, a convenient polishing method to triturate the graphene nano-particle from highly oriented pyrolytic graphite (HOPG) foil was demonstrated previously [10, 11]. The graphene nano-particle exhibits similar optical properties with graphene, such as fast carrier relaxation time, wideband absorption and superior thermal conductivity etc. In addition to reduce the size of graphene nano-particle for detuning the coverage ratio at the interacting cross-section area, the effect of layer number of the graphite or graphene on the saturable absorption was investigated to optimize the mode-locking performance. Bao et al. have studied that reducing the graphene layer number can enhance the mode-locking force of graphene saturable absorber, as attributed to the enlarged modulation depth and decreased linear absorbance. A stabilized and shortened mode-locking pulse is obtained by using an atomic-layer graphene as compared to that by a multilayer graphene [1, 13].
At current stage, the size shrinkage, layer number reduction and uniformity of graphene nano-particle can only be roughly controlled by the polishing conditions of mechanical trituration. In the case of fabricating graphene nano-particle saturable absorber, the few-layer and multilayer graphene nano-particles are co-existed after the mechanical polish process. Therefore, the layer number of graphene nano-particle must be decreased when considering it as a saturable absorber. Besides, the uniformity of graphene nano-particles is mandatory to precisely control the coverage ratio as well as linear insertion loss on the fiber end-face. These drawbacks were left the unsolved issues up to now. Recently, the electrochemical exfoliation has emerged as a simple solution-processed fabrication for obtaining few-layer graphene nano-particles from the graphite foil [14–16]. Su et al. used the electrochemical exfoliation to fabricate a large-scale and few-layer graphene sheet by setting the graphite foil as an electrode under a bias voltage of + 10V . However, the nonuniform graphene sheet with numerous structural defects is caused by the fast exfoliation mechanism at such high bias voltage.
On the other hand, most of the carbon based saturable absorbers [2–8] were sandwiched between two fiber patchcords to provide the loss modulation [2–8, 17–33]. Sun et al. set a graphene-polymer composite obtained by a wet-chemical method between two fiber patchcords to passively mode-lock the EDFL with a 460-fs pulsewidth . Zhang et al. placed the atomic-layer graphene between two SMF patchcords to generate a mode-locked EDFL soliton with 30-nm wavelength tunability . Similar wavelength tunability at C- and L-bands with the insertion of a space between SMF patchcord connectors has also been reported in other kinds of fiber lasers [19, 20]. This patchcord/absorber/patchcord scheme is compact; however, the interaction is limited by the saturable absorber thickness. Increasing the thickness of saturable absorber in this scheme inevitably leads to a large insertion loss to increase the mode-locking threshold , and the thermal damage of graphene materials caused by the directly propagated laser beam also set a constrain for high-power operation. More recently, the EDFL passively mode-locked by evanescent-wave saturable absorption with graphene is developed to concurrently solve the reaction length and thermal damage problems [35–40]. Song et al. reported the 1.3-ps pulsewidth with the evanescent-field saturable absorption by graphene at cladding region, which endures an intracavity power of up to 21 dBm without damaging the graphene . Luo et al. demonstrated a graphene-deposited fiber taper to obtain the multi-wavelength mode-locked EDFL with 8.8-ps pulsewidth . Choi et al. utilized a graphene-injected hollow optical fiber (HOF) to achieve the evanescent-wave mode-locking of EDFL with 510-fs pulsewidth . Although the reacting length is lengthened, the interaction area is still limited on one-side of the taper fiber or on the single hole of HOF. This greatly lengthens the device and causes additional cavity loss.
In this work, the high-quality few-layer graphene nano-particle is obtained by using the stabilized electrochemical exfoliation at a threshold bias condition. The decreasing exfoliation rate significantly and precisely controls the layer number of the graphene nano-particles. Raman scattering spectroscopy is performed to determine the structural quality and layer number of graphene nano-particle. By syphoning the graphene nano-particles into a multi-core photonic crystal fiber (PCF), the evanescent-wave of the EDFL interacts with the graphene nano-particle uniformly in the hole cladding region. The multi-core PCF contains more graphene nano-particles in a shorter segment, which can strengthen the nonlinear saturable absorption as compared to that in a one-core HOF. By inserting the graphene nano-particle doped PCF with an interaction length of 200 μm, the evanescent-wave mode-locking of EDFL with low pumping threshold, sub-picosecond pulsewidth and ultra-low carrier amplitude jitter is demonstrated.
2. Experiment setup
Figure 1 illustrates the flow chart of electrochemical exfoliation, centrifugation, and syphoning of the graphene nano-particles into the PCF. In the electrochemical exfoliation, the HOPG foil and a Pt wire are respectively served as the anode and the cathode in the electrolyte of sulfuric acid aqueous solution, which is prepared by diluting 96% sulfuric acid and 100 mL of deionized water. The exfoliation process is operated by applying different DC bias voltages of + 3 and + 6V. When operating at a bias of + 6V, the HOPG foil is rapidly split and dissociated into small particles. The roughly exfoliated graphite stacks are observed right after turn-on the DC power supply. In contrast, the operation under a threshold bias of + 3V spends more than 20 sec to start the exfoliation after turn-on the DC power supply, and the exfoliated graphene nano-particles are more delicate. Subsequently, the graphene nano-particle aqueous solution is filtered to remove the large graphene sheets by a porous filter, and the percolated graphene nano-particles are preserved in acetone solution. After the ultrasonic agitation of graphene nano-particle contained in acetone solution for 10 min, the centrifugation of graphene nano-particle solution can separate small particles as the supernatants but deposit the large particles in the bottom.
The supernatants are syphoned into the PCF (NKT, LMA-10) to make the graphene nano-particles dispersed in the PCF. To avoid the modification of guiding mode in the PCF filled with the acetone solution in the hole cladding region, the passive mode-locking of EDFL is performed after evaporating the acetone solution in an oven at 70° for 1 hour. The SEM image confirms the sizes of graphene nano-particles are around 400 nm. The slowly exfoliated graphene nano-particle obtained with low bias voltage is the preferred candidate to be used in the EDFL system. In addition, the centrifugation is a necessary process to collect the uniform and delicate graphene nano-particles after the electrochemical exfoliation, because the few-layer graphene nano-particle with small size induces low insertion loss but high modulation depth. Afterwards, the PCF with dispersed graphene nano-particles is spliced with another PCF. The evanescent-wave interaction improves the nonlinear interaction length and increases the tolerance of high intra-cavity power. The advantages of the fabrication process are simple, convenient and high reproducibility.
Figure 2 demonstrates the experimental setup of the passively mode-locked EDFL. This system utilizes an erbium-doped fiber (EDF, nLIGHT Liekki Er80-8/125) as the gain medium, which is bi-directionally pumped by a 980-nm laser diode (LD, forward) and a 1480-nm LD (backward) through two wavelength-division multiplexers (WDMs). The circulated direction is determined by an isolator and the intra-cavity polarization is controlled by a polarization controller (PC). A 95/5 coupler is inserted to provide 5% output and feedback 95% of intra-cavity power. The EDFL consists of a 2-m long EDF with dispersion coefficient β2,EDF of −20 ps2/km, and a 6.2-m long SMF with β2,SMF of −20 ps2/km [12, 41]. In the inset of Fig. 2, the photographs show the optical microscopy images of PCF. The top-view of PCF indicates that the hole diameter is 3.1 μm with the spacing between each hold of 7.1 μm. The length of PCF is 200 μm as shown in the side-view image. The dispersion coefficient β2,PCF of the PCF is about −40 ps2/km.
3. Results and discussions
Figure 3(a) compares the Raman scattering spectra of the original HOPG foil and the electrochemically exfoliated graphene nano-particles obtained at bias voltages of + 3 and + 6V. The HOPG foil exhibits two prominent Raman signals at 1580 and 2730 cm−1, which represent G band and 2D band originated from the sp2 carbon network and the second-order double resonance [11,12,42–45]. As the graphene layer number increases to form graphite, the 2D band intensity significantly reduces with a broadening linewidth and an asymmetric spectral shape. The divided phonon branches contribute different phonon frequencies that could lead to the splitting of 2D band . After the electrochemical exfoliation at a bias of + 6V, the exfoliated graphene nano-particle shows a relatively broadened G band and an attenuated 2D band. To quantify, the intensity ratio of 2D band over G band (I2D/IG) is about 0.35. In addition, a distinct D band with an intensity ratio of D band over G band (ID/IG) of 1.57 is observed, which is mainly caused by the structural defects occurred outside the graphene nano-particle [11, 12]. The defects are inevitably generated outside the graphene nano-particle when the graphene nano-particles are rapidly exfoliated from the HOPG foil, such as cracks, vacancies, stretches and bending, as shown in Fig. 4.
Most of the defects are tensile strains after the electrochemical exfoliation, which lead to the red-shift of the Raman scattering peak wavenumber at 2D band [43, 45]. In contrast, the G band of the graphene nano-particle obtained under the exfoliated operation with bias voltage of + 3V becomes intense and sharp, and the 2D band increases it intensity with an enlarging I2D/IG ratio of 0.54, whereas the D band is approximately disappeared to show a dramatically decreased ID/IG ratio of 0.12. This observation indicates that the crystalline quality of graphene nano-particle can be improved and the size (or layer number) of graphene nano-particle is also reduced by decreasing the bias voltage, as shown in Fig. 4. Because the graphene nano-particles are slowly and softly exfoliated from the graphite foil under the stable exfoliation bias of + 3V, as compared to the operation of + 6V bias voltage. To be the saturable absorber, the large graphene nano-particle with plenty of defects obtained by the operation of + 6V bias voltage is not a good candidate due to the insufficient modulation depth and the enlarged absorption loss . Therefore, the electrochemical exfoliation operated under + 3V bias voltage is suitable to fabricate few-layer graphene nano-particle with better crystalline quality.
Subsequently, the linear transmittance of graphene nano-particles in PCF is measured and shown in Fig. 5(a). By illuminating with a tunable continuous-wave (CW) laser (Agilent 8164A) at 1570 nm, the linear transmittance of graphene nano-particles in PCF is determined as 0.56, which is attributed to the linear absorption of graphene nano-particle, the coupling loss and the attenuation from PCF (main contribution of up to 2 dB). To compare the evanescent-wave mode-locking with the core-region wave mode-locking, the acetone solution with dispersed graphene nano-particles is dropped on the end-face of SMF patchcord. After the evaporation of acetone solution, the free-standing graphene nano-particles on the end-face of SMF can be obtained. The linear transmittance of 0.54 is shown in Fig. 5(b), which originates from the large coverage ratio and the enlarged sizes caused by the self-aggregation of graphene nano-particles. The linear transmittance of graphene nano-particles in the PCF is kept approximately the same with the one by confining the graphene nano-particles on the end-face of SMF. However, embedding the graphene nano-particles in the PCF would benefit from a better nonlinear absorption with higher modulation depth, which will be discussed in the next section.
The saturable absorption of graphene nano-particles under high power excitation due to Pauling blocking effect occurs under feedback light circulation in the EDFL cavity [10–12]. Figure 6(a) and 6(b) demonstrate the nonlinear transmittance and the normalized absorbance of graphene nano-particles in PCF and graphene nano-particles on the end-face of SMF, respectively. The nonlinear transmittance is detected under a pulsed laser with wavelength of 1570 nm and pulsewidth of 700 fs. For the graphene nano-particles in PCF, the transmittance enlarges from 55% to 60% with an increasing ΔT of 5%, and the normalized absorbance shows a decreasing trajectory with a modulation depth of 13% by increasing the pumping power from 0.1 to 41 mW. In contrast, for the graphene nano-particles on the end-face of SMF, the transmittance enlarges from 0.54 to 0.578 with the corresponding increment of ΔT = 3.8% and the modulation depth of 10.8%. In comparison with the interaction of core-wave and graphene nano-particles, the evanescent-wave interaction contributes higher modulation depth due to the longer nonlinear interaction length. As a result, the characteristic parameters related to the saturable transmittance of graphene nano-particle in the PCF are numerically simulated by the following equation:
The schematic diagram of the evanescent-wave mode-locking for inducing the pulse formation with graphene nano-particles dispersed in hole-cladding region of the PCF is shown in Fig. 7. Assuming that the graphene nano-particle dispersed in the PCF has a nonlinear absorption coefficient given by
Within the hole-cladding region of the PCF, the evanescent-wave exponentially decays with the radial distance (x) away from the core/cladding interface, and the evanescent-wave intensity can be described as 47]. This consequently results in the saturable phenomena for both the extinction coefficient β and the field confinement factor Γ given by :
The saturable phenomena of the extinction coefficient β determines the decay length (1/2β) of the evanescent wave, which results in a Kerr-lens like refractive index change along the transverse direction of the PCF fiber, thus providing a loss modulation for the intracavity photon intensity. According the schematic diagram shown in Fig. 8, the decay length decreases to perform the low evanescent-wave intensity when β increases, whereas the decay length increases with reducing β value to broaden the evanescent-wave field. Such a phenomenon occurs back and forth by the saturable-absorption of graphene nano-particles distributed in the hole-cladding region of the PCF.
To perform the evanescent-wave interaction for starting the passively mode-locked EDFL, the graphene nano-particle injected hollow optical fiber (HOF)  with a length of 59 mm has been inserted into the EDFL ring cavity. In our case, because more graphene nano-particles can be siphoned into the multi-core PCF to enhance the nonlinear interaction, the PCF with a length of only 200-μm is required to produce the evanescent-wave interaction with ΔT of 5%. The attenuation loss caused by the PCF can certainly be decreased and a large GDD caused by the PCF can also be avoided by shortening the PCF, simultaneously. However, there is a trade-off between the degradation of saturable absorption and aforementioned effects. Figure 9(a) and 9(b) show the Pout vs. Pin transfer response and the power gain curves of the EDFA with increasing both the pumping currents of 980-nm LD and 1480 nm-LD from 700 to 900 mA. A continuous-wave laser with a wavelength of 1570 nm is passing through the EDFA to measure the power gain at different pumping currents. The pumping power of 980-nm LD enlarges from 235 to 290 mW, and the pumping power of 1480-nm LD enlarges from 153 to 200 mW by increasing the pumping current from 700 to 900 mA. The Gain curves of the EDFA are simulated by G = G0/(1 + Pin/Psat), where G0 is the small signal gain, Pin and Psat are the input power and the saturated power of EDFA, respectively. G0 rises from 8.42 to 8.62, and Psat enlarges from 0.55 to 0.65 with increasing the pumping currents of 980-nm LD and 1480-nm LD simultaneously.
Figure 10(a) and 10(b) depict the autocorrelation traces and the optical spectra of the passively mode-locked EDFLs under different pumping power. The autocorrelation traces and the optical spectra are measured by an autocorrelator (Femtochrome, FR-103XL) and an optical spectrum analyzer (Ando, AQ6317B), respectively. By simultaneously increasing the pumping currents of 980-nm and 1480-nm LDs from 700 to 900 mA, the enhanced net optical gain increases the synchronous oscillating modes . As a result, the passively mode-locked EDFL pulsewidth slightly shrinks from 668 to 650 fs with the corresponding spectral full-width at half maximum (FWHM) broadened from 3.75 to 3.92 nm. The wavelength of the EDFL in all cases remains unchanged at 1567.6 nm. The time-bandwidth products (TBPs) of the EDFLs are around 0.315, and the group delay dispersion (GDD) of the laser cavity is approximately −0.164 ps2/km. In the anomalous dispersion condition, the soliton pulse formation is periodically perturbed by the linear effect of GDD and the nonlinear effect of self-phase modulation (SPM) [12, 49].
The soliton mode-locking operation and the occurrence of Kelly sidebands on the shoulder of optical spectra are observed in the EDFL system. To confirm the optimized performances of passively mode-locked EDFL started by the graphene nano-particles doped in PCF, both the pulse shape and the optical spectrum are simulated by using Haus master equation with the experimentally obtained parameters. The master equation is given by :Figure 11(a) and 11(b) demonstrate the simulated autocorrelation traces and the optical spectra under different pumping currents. By simultaneously increasing the pumping current of 980-nm and 1480-nm LDs from 700 to 900 mA, the simulated pulse shapes show the pulsewidth shortening from 683 to 655 fs, with the spectral FWHM broadening from 3.71 to 3.86 nm, which are well correlated with the experimental results.
At last, the stabilization of the evanescent-wave mode-locking performance is monitored by characterizing the peak power fluctuation among the adjacent mode-locked pulses. The inset of Fig. 12 exhibits the oscilloscope traces of the passively mode-locked EDFLs under different pumping currents. The repetition time of the EDFL is 40 ns with the corresponding repetition rate of 25 MHz. By using the evanescent-wave mode-locking, the location of graphene nano-particles in the hole-cladding region of the PCF can ensure the high intracavity power in the core region without suffering from any thermal dissipation, which generates a highly stabilized mode-locking of EDFL at a larger output power. The quality of pulse-amplitude equalization can be determined by calculating the carrier amplitude jitter (CAJ) of the measured mode-locked pulse-train in time domain, which is defined as a ratio of the standard deviation (σ) on peak pulse intensity to the average pulse intensity (Iave), CAJ = (σ/Iave)x100% . The extremely low CAJ values of around 1.23%~1.66% are obtained for different pumping cases and shown in Fig. 12, indicating a very small peak power fluctuation for the evanescent-wave mode-locked EDFL started with the graphene nano-particles doped in the hole-cladding region of the PCF. The operating lifetime of such an evanescent-wave mode-locked EDFL can be stably operated up to 12 hours, which is at least 1.5 times longer than the same system using the core-region mode-locking with same graphene nano-particle based saturable absorber. This observation confirms the stabilization of the evanescent-wave mode-locked EDFL.
The high-quality graphene nano-particles obtained by the stabilized electrochemical exfoliation are uniformly dispersed in the hole-cladding region of a PCF to passively mode-lock the EDFL by evanescent-wave interaction. The electrochemical exfoliation operated at a threshold voltage of + 3V effectively reduces the structural defects and produces few-layer graphene nano-particle. Because the graphene nano-particles are slowly and softly exfoliated from the graphite foil under the operation of low bias voltage. In comparison with the operation under higher bias, the Raman scattering spectrum of the graphene nano-particle under + 3V exhibits an intense and sharp 2D band, whereas the defect related D band is approximately disappeared. The graphene nano-particles are siphoned into the hole cladding of PCF to induce the saturable absorption. The saturable phenomena of the extinction coefficient β in the cladding results in a Kerr-lens like refractive index change along the transverse direction of the PCF fiber, thus providing a loss modulation for the intracavity photon intensity. It contributes the increasing transmittance from 55% to 60% with ΔT of 5%, and the normalized absorbance shows a decreasing trajectory with the modulation depth of 13%. In contrast, the graphene nano-particles on the end-face of SMF under the same linear transmittance only provide a modulation depth of 10.8%. The evanescent-wave mode-locking with graphene nano-particles in hole-cladding of PCF provides higher modulation depth and lower linear loss than those imprinted on the SMF end-face, because the graphene nano-particles uniformly distributed in the PCF can enlarge interaction length and avoid self-aggregation. By simultaneously increasing the pumping current, the net optical gain enhances to shorten the passively mode-locked EDFL pulsewidth to 650 fs. The corresponding spectral linewidth broadens to 3.92 nm, leading to a transform limit operation with time-bandwidth of nearly 0.315. The temporal/spectral waveforms with Kelly sidebands are well simulated by using the Haus master equation. The extremely low carrier amplitude jitter of 1.23% indicates a very small peak power fluctuation for the evanescent-wave mode-locked EDFL started with the graphene nano-particle doped in the hole-cladding region of the PCF. A stabilized operation up to 12 hrs is confirmed to be 1.5 times longer than the core-wave mode-locking case, which ensures the handling of high intracavity power away from the thermal damage via the evanescent-wave interaction with the graphene nano-particles doped in the hole-cladding region of the PCF.
This work was supported by National Science Council and National Taiwan University under grants NSC101-2622-E-002-009-CC2, NSC101-2221-E-002-071-MY3 and NTU102R89083.
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