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With the intra-cavity nano-scale charcoal powder based saturable absorber, the 455-fs passive mode-locking of an L-band erbium-doped fiber laser (EDFL) is demonstrated. The size reduction of charcoal nano-particle is implemented with a simple imprinting–exfoliation–wiping method, which assists to increase the transmittance up to 0.91 with corresponding modulation depth of 26%. By detuning the power gain from 17 to 21 dB and cavity dispersion from −0.004 to −0.156 ps2 of the EDFL, the shortening of mode-locked pulsewidth from picosecond to sub-picosecond by the transformation of the pulse forming mechanism from self-amplitude modulation (SAM) to the combining effect of self-phase modulation (SPM) and group delay dispersion (GDD) is observed. A narrower spectrum with 3-dB linewidth of 1.83-nm is in the SAM case, whereas the spectral linewidth broadens to 5.86 nm with significant Kelly sideband pair can be observed if the EDFL enters into the SPM regime. The mode-locking mechanism transferred from SAM to SPM/GDD dominates the pulse shortening procedure in the EDFL, whereas the intrinsic defects in charcoal nano-particle only affect the pulse formation at initial stage. The minor role of the saturable absorber played in the EDFL cavity with strongest SPM is observed.
© 2013 Optical Society of America
1. Introduction
Since the first demonstration of graphene saturable absorber for passively mode-locked erbium-doped fiber lasers (EDFLs) by Bao et al. in 2009, versatile graphene samples in different forms have been developed, including single-layer graphene [1–6], few-layer graphene [7–13], multi-layer graphene [14–17], graphene polymer [18–23], graphene composite [24–26], graphene solution [27] and graphite nano-particle [28–30] etc., which progressively show the capabilities on initiating the ultrafast saturable absorption in the EDFL cavity. In particular, Zhang et al. investigated a dissipative soliton mode-locking laser with a wavelength tuning range of 1570-1600 nm by using few-layer graphene [8]. Sun et al. incorporated the exfoliated graphene flakes into the polyvinyl alcohol (PVA) film to obtain a 460-fs passively mode-locked EDFL [20]. Sobon et al. used graphene oxide to stabilize a mode-locked EDFL with soliton pulsewidth of sub-400 fs [25]. Liu et al. integrated the graphene-oxide with hollow-core photonic crystal fiber to demonstrate the passive mode-locking in nanosecond regime [27]. To simplify the production, Lin et al. directly brushed the polished graphite nano-particles on the fiber patchcord end-face in EDFL and achieve 400-fs mode-locking [28–30]. Up to now, almost all graphene, graphene oxide and graphite materials can initiate mode-locking of EDFL with ultrafast recovery time, broadband wavelength tunability, ultrahigh nonlinearity and large optical damage threshold.
Not long ago, Singh et al. preliminarily demonstrated a green and simple method to synthesis the graphene nano-sheets from a pencil by using the electrochemical exfoliation [31]. The bulk charcoal structure in a pencil is confirmed to be similar with graphite that contains multi-layer graphene. In the meantime, Lin's group also obtained the charcoal nano-particles by simply polishing the pencil [32], and presented that even the unprocessed charcoal nano-particles possess the ability of saturable absorption to passively mode-lock the EDFL. However, charcoal is a graphene contained raw material with plenty of structural defects, which can only achieve passive mode-locking of EDFL at picosecond regime due to its small modulation depth and large absorption loss [32]. Although the pulse compression into femtosecond regime is limited by the natural characteristic of charcoal nano-particles, which can essentially be improved via the parametric tuning of the EDFL cavity. In this work, the passively mode-locked EDFL at L-band is demonstrated with charcoal nano-particle based saturable absorber, and the mode-locking laser pulse can be shortened to <500 fs. The scanning electron microscope (SEM), X-ray photoelectron spectrum (XPS), Raman scattering spectroscopy and X-ray diffraction (XRD) diagnosis are utilized to investigate the structural properties of charcoal nano-particles. By detuning the power gain and cavity dispersion of the EDFL, the transformation of the pulse forming mechanism from self-amplitude modulation (SAM) to the effects of self-phase modulation (SPM) and group delay dispersion (GDD) is achieved in the EDFL. This eventually strengthens the mode-locking and pulse-shortening force. Although the structural defects degrade the nonlinear modulation behavior, the improved gain and dispersion of the EDFL cavity starts the high-order SPM effect to further compress the mode-locked pulsewidth afterwards.
2. Experiment setup
After mechanical polishing from pencil, the triturated charcoal nano-particles are directly brushed onto the end-face of single-mode fiber (SMF) patchcord connector, to be the saturable absorber for passively mode-locking the EDFL. However, the self-aggregation of charcoal nano-particles would enlarge its size to increase the absorption loss [32]. In previous work, the imprinting–exfoliation–wiping method was developed to reduce the layer number of graphite nano-particle by using the fiber patchcords [30]. Therefore, the size of charcoal nano-particle and its coverage ratio on the SMF end-face are expected to be decreased by utilizing the same method. Figure 1 shows the fabricating process for thinning and separating the charcoal nano-particles by employing the imprinting–exfoliation–wiping method. The photographs of the pencil, the triturated charcoal powder in the bottle and the SMF patchcord are shown in Fig. 1(a). The optical microscope (OM) image in Fig. 1(b) shows the adhesion of charcoal nano-particles on the SMF connector end-face after direct brushing. Subsequently, the SMF patchcord with charcoal nano-particles is tightly connected with another SMF patchcord, as shown in Fig. 1(c). By separating the connected SMF patchcords, the charcoal nano-particle can be exfoliated again to shrink its size. This reduces the layer number of the graphene inside the charcoal nano-particle, as illustrated in Fig. 1(d). After repeating the imprinting-exfoliation-wiping process by multiple times, the microscopic image shows the reduced coverage ratio of charcoal nano-particles on SMF connector end-face.
Fig. 1 The imprinting-exfoliation-wiping procedure for adhesion of charcoal nano-particles onto SMF patchcord connector end-face. (a) The photographs of pencil, charcoal powder and SMF. (b) The OM image of charcoal nano-particle directly brushed on SMF end-face. (c) The connected SMF patchcord. (d) Separation of SMF connectors. (e) The OM images of the split charcoal nano-particles distributed on both SMA connector end-faces.
Figure 2 illustrates the schematic diagram of a passively mode-locked EDFL system. In the beginning, a low-gain EDFA is employed as the gain medium, which uses 12-m long erbium-doped fiber (EDF) with a peak absorption of 1.5 dB/m (SDO, EFAS1B1438002A) to provide a power gain of 17 dB. The other SMF based components including the input/output couplers, the bi-directional pumping laser diodes (LDs), an isolator and a polarization controller (PC) are employed to form the EDFL system with a total length of 21 m. The cavity group delay dispersion (GDD) is minimized to a nearly dispersion free value of −0.004 ps2. Owing to the low-gain operation, such a low-gain EDFL system can only be operated in the self-amplitude-modulation (SAM) regime. For comparison, a homemade EDFA is established. To improve the intra-cavity gain and dispersion for operating the EDFL in the self-phase-modulation (SPM) regime with an enlarged negative GDD, a 2-m long high-gain erbium-doped fiber (HGEDF, nLIGHT Liekki Er80-8/125) with a peak absorption of 80 dB/m and a dispersion coefficient β2 of −20 ps2/km is used as the gain medium, which is bi-directionally pumped by two LDs (forward: 980-nm LD, backward: 1480-nm LD). A 980/1550 wavelength division multiplexer (WDM) and a 1480/1550 WDM are involved to deliver the pumping powers. An isolator set behind the EDF is used to decide the circulation direction and avoid feedback. A PC is inserted in front of the saturable absorber for controlling the intra-cavity polarization [33]. The charcoal nano-particles are confined between two SMF patchcord connectors. An output optical coupler provides 95% feedback ratio and 5% output coupling ratio. The pulse shape, optical spectrum and pulse train are measured by an autocorrelator (Femtochrome FR-103XL), an optical spectrum analyzer (Ando AQ6317B) and an oscilloscope (Tektronix TDS 2022), respectively.
Fig. 2 The schematic diagram of the passively mode-locked EDFL system. LD: laser diode, WDM: wavelength-division multiplexer, PC: polarization controller.
3. Results and discussions
3.1 Characterization of charcoal nano-particles and EDFAs
The size of the charcoal naon-particles after trituration is reduced down to 500 nm [32], which further shrinks to 300 nm after the exfoliation-imprinting-wiping process, as shown in Fig. 3(a). The XPS spectrum of charcoal nano-particle is demonstrated in Fig. 4(a), which indicates the photoelectron peaks of C1s and O1s located at 284.7 and 532 eV with the composition ratio of 67% and 33%, respectively [34]. The chemical content of the charcoal nano-particles is characterized by decomposing the C1s orbital electron related binding energy peak in the XPS spectrum, which consists of the main C-C (sp2) bond, and some other minor contributions from the C-OH, C-O, C = O and COOH bonds caused by chemicals, as shown in Fig. 4(b). The photoelectron peak of C-C bond at 284.6 eV originates from the sp2 carbon bond, which dominates the chemical structure of C1s peak. The other minor peaks of C-OH, C-O, C = O and COOH are appeared at 286.1, 286.5, 287.8, and 290.1 eV, respectively [35, 36]. The XPS results confirm that the C-C sp2 bond dominated main structure of charcoal is similar with graphite, however, the existence of oxygen related composites also causes the structural defects inside the charcoal nano-particles.
The Stokes Raman scattering spectrum of charcoal nano-particle shown in Fig. 5(a) demonstrates a prominent G band located at 1570 cm−1 corresponding to the existence of sp2 carbon bonds [32, 37–39], a distinct D peak at 1328 cm−1 caused by the breathing mode of k-point phonons from structural defects [32], and a weak 2D peak at 2771 cm−1 due to the second-order double resonant process. The structural defects also lead to a broadened G band with 3-dB linewidth of 81 cm−1 and an intensity ratio of 2D band over G band (I2D/IG) as high as 0.67. The 2D peak intensity attenuates whereas its bandwidth broadens by increasing the layer number of graphene, which correlate well with the highly disordered carbon structure in charcoal nano-particles. This results in the intensity ratio of 2D band over G band (I2D/IG) decreasing to 0.13. As evidence, the XRD analysis of charcoal nano-particle shown in Fig. 5(b) reveals a relatively broadened diffraction peak at 25.2° from the {002}-oriented lattice, indicating the bad crystallinity of the hexagonal graphite plane [29, 32]. In comparison with the {002} peak of nature graphite (26.54°), the small angle shift of −0.34° and the broadened linewidth of charcoal nano-particle elucidates the existence of structural defects including curvatures and distortions of graphene layers [40].
The saturable absorbance of charcoal nano-particle containing crystalline graphene structure is characterized by femtosecond laser illumination, in which the optical absorption of charcoal nano-particle is reduced under high optical power illumination, because the carrier transition from valence band to conduction band is forbidden by the Pauli blocking effect [32]. The optical absorbance α of charcoal nano-particle is correlated with the linear absorbance (qlin), the nonlinear (saturbale) absorbance (qnon), as described by [6,30]:
where Pin denotes the input power, Psat the saturation power of charcoal nano-particle, ω the optical angular frequency, Imχ(3) the imaginary part (the extinction constant) of third-order nonlinear refractive index, ε0 the dielectric permittivity, c the speed of light and n0 the refractive index. The modulation coefficient γ is defined as qnon/Psat, and the saturable absorbance can be simplified as qlin + qnon + γPin, which represents the SAM effect. After derivation, the saturable absorbance is expressed as a function of the Imχ(3).The triturated charcoal nano-particles with larger sizes exhibit lower transmittance changing from 0.66 to 0.72 with enlarged illuminating power, which provide a modulation depth (MD) of 22%, as shown in Fig. 6. Fitting the curve of saturable absorbance obtains the linear loss of qlin = 0.292, the nonlinear loss of qnon = 0.13, and the saturation power of Psat = 12 mW. After multiple imprinting-exfoliation-wiping process, the charcoal nano-particle with smaller sizes show the transmittance increasing from 0.87 to 0.91 with corresponding modulation depth enlarging up to MD = 26%. Not only the linear loss decreases to qlin = 0.1, but also the saturable absorbance attenuates to qnon = 0.036, and the saturation power reduces to Psat = 3.5 mW. The size shrinkage of charcoal nano-particle reduces the linear/nonlinear absorption loss, which concurrently enlarges the nonlinear modulation depth and scales down the saturation power for initiating the saturable absorption. Both the two factors are significant for improving the passively mode-locked EDFL performances.
The power and gain characteristics of the high-gain and low-gain EDFAs are demonstrated in Fig. 7(a). To shorten the EDFL pulse, a sufficiently large amount of the longitudinal modes must be phase-locked over a wide frequency range limited by gain bandwidth [41, 42]. The enhanced cavity gain helps the circulating pulse to overcome the saturated absorption threshold so as to reduce the passively mode-locked EDFL pulsewidth. The bi-directionally pumped high-gain EDFL operated at maximum current conditions (900 mA for both pumping LDs) provides forward and backward pumping powers of 290 mW at 980 nm and 200 mW at 1480 nm, respectively. The equation G = g0/(1 + Pin/Psat) is utilized to simulate the gain of two EDFLs in linear scale, as shown in Fig. 7(b), where g0 is the small signal gain and Psat is the saturation power of the EDFA. With an input power Pin as low as −10 dBm, the high-gain EDFL provide a power gain of up to 20 dB, which is nearly 4-dB larger than that provided by the low-gain EDFL (16 dB). The g0 = 8.62 for high-gain EDFA is larger than g0 = 7.3 for low-gain EDFA. Even at the output power saturated condition (at Pin>0 dBm), the high-gain EDFL still provides a 21-dB gain that is 3.1 dB higher than gain provided by the low-gain EDFL.
Fig. 7 (a) The Pout vs. Pin curves in logarithm scale of the high-gain and low-gain EDFAs. (b) The Gain vs. Pin curves in linear scale of the high-gain and low-gain EDFAs.
3.2 SAM and SPM dominated passive mode-locking of EDFLs with charcoal nano-particles
Figure 8(a) and 8(b) present the autocorrelation traces and optical spectra of the high-gain and low-gain EDFLs passively mode-locked by charcoal nano-particles. Without using high-gain EDF [43] or externally high-order soliton compression [44, 45] approach, the low-gain EDFL only delivers a pulsewidth of 1.36 ps at central wavelength of 1573 nm accompanied with spectral linewidth of 1.83 nm. The time-bandwidth product (TBP) of the low-gain EDFL is about 0.32, which is a nearly transform limited pulse but still operated at SAM condition. The pulse formation is thus determined by the saturable absorber when operating at SAM mode. The passively mode-locked EDFL enters into the SPM operation after enhancing the cavity gain and reducing the absorption loss of saturable absorber, which shortens its pulsewidth to 455 fs with corresponding linewidth of 5.86 nm. Moreover, the red-shift of central wavelength to 1577 nm is observed due to the enlarged intra-cavity gain [46]. The TBP of the improved passively mode-locked EDFL is about 0.322. The length and dispersion coefficient β2,EDF of EDF are 2 m and −20 ps2/km; For SMF used in the EDFL cavity, the length and β2,SMF are 5.8 m and −20 ps2/km. The resonant cavity is in the anomalous dispersion regime with group delay dispersion (GDD) of −0.156 ps2. Obviously, the high-gain EDFL is operated at strong SPM and negative GDD regime, which greatly improves the pulse shortening mechanism.
Fig. 8 The (a) auto-correlated pulses and (b) optical spectra of the high-gain(upper) and low-gain (lower) EDFLs mode-locked by charcoal nano-particles (solid: measured; dashed: fitting) [32].
When sharpening the intense pulse by high-gain operation, the intensity-dependent self-phase modulation (SPM) effect plays an important role to distort the pulse shape [47]. Only in the anomalous dispersion regime, the GDD and SPM can compensate each other to form a periodically breathing soliton pulse in the EDFL cavity [48]. The higher SPM dominates the pulse shortening force in negative GDD region at cost of a slight instability [41]. In our case, the picosecond high-power pulse initially generated in the high-gain EDFL cavity induces the SPM effect at very beginning. Afterwards, the gradually strong SPM induced by the circulating pulse with increasing power initiates the soliton mode-locking process, and the 1st-order Kelly sideband appears at both sides of the mode-locked spectrum. The frequency spacing Δυ of the Kelly sideband is ± 1.18 THz, which is related with the cavity GDD and the EDFL pulsewidth [30]. In contrast, the optical spectrum of the mode-locked spectrum from low-gain EDFL does not contain the Kelly sidebands due to the lack of intra-cavity SPM.
Figure 9(a) demonstrates the pulse-train of the high-gain EDFL passively mode-locked by charcoal nano-particles, which indicates the repetition time and frequency of 35 ns and 28.5 MHz, corresponding to the cavity length of 7.8 m (EDF: 2 m, SMF: 5.8 m). Even with such short cavity length, an extremely large GDD is caused by the high-gain EDF with a large negative dispersion. In contrast, Fig. 9(b) reveals pulse-train of the low-gain EDFL under passive mode-locking, the repetition time and frequency of 11 ns and 8.9 MHz corresponds to the cavity length of 21 m (EDF: 8.5 m, SMF: 12.5 m). The high-gain EDFL not only shortens the mode-locked pulsewidth but also shrinks the cavity length to increases the repetition rate by three times. In addition, the fluctuation of pulse peak power as well as the quality of pulse amplitude equalization is characterized by measuring the carrier amplitude jitter, which is defined as (σ/Iave) x 100%, where σ denotes the standard deviation of peak pulse intensity, Iave the average pulse intensity [49]. The CAJ value of the high-gain EDFL is 1.74%, which is better than the low-gain EDFL of 1.89%. Table 1 summarizes the parametric comparison on both EDFLs mode-locked by charcoal nano-particle based saturable absorber, in which the pulse shortening by three times can be attributed to the increased cavity gain (by EDF) and the reduced cavity loss (by small charcoal nano-particles).
Fig. 9 (a) The improved passively mode-locked EDFL pulse train. (b) The original passively mode-locked EDFL pulse train [32].
Table 1. Parametric comparisons of the high-gain and low-gain EDFL mode-locked bt charcoal nano-particles.
With the aforementioned parameters, the pulsating dynamics of passively mode-locked EDFL is simulated by Haus master equation. By considering a pulse with hyperbolic secant shape A(T,t) = A0sech(t/τ) propagating in the resonant cavity, the master equation is written as [42, 50]:
where g denotes the cavity gain, l0 the insertion loss, Dg,f the gain and filter dispersions, γ the modulation coefficient, D and δ are the intra-cavity GDD and SPM coefficient, respectively. The right-hand side is divided into two terms to distinguish the SAM from the SPM effect. The first square bracket in the right hand side of Eq. (2) leads to a self-amplitude modulation (SAM) mechanism, whereas the second square bracket is contributed by the self-phase modulation (SPM) mechanism. Without considering the effects of GDD and SPM, the pulse pulsewidth is only determined by the charcoal nano-particle based saturable absorber. In SAM case, the fast saturable absorber benefits from the short pulse generation with a pulsewidth given by [51]When considering the effects of GDD and SPM at high power condition, the phase perturbation induces an additional chirp to distort the pulse shape. However, the strong SPM mechanism can compensate the pulse distortion by the GDD in anomalous dispersion regime. The charcoal nano-particle behaves like rather a mode-locking starter than a pulse compressor. Under the effects of GDD and SPM, the pulsewidth τ' is modified as:
where β is the chirp correlated with the normalized dispersion of DN = D/Dg,f and the normalized nonlinearity δN = δ/γ . When the chirp is larger than 3DN under a strong soliton operation with δN >> 1, DN >> 1 and δN≈-DN, the soliton pulse can be further compressed by a factor of 2~3. The strong SPM and GDD dominate the pulse shaping, whereas the saturable absorber only plays the role to start and stabilize the pulse-train. That is, the gain enhancement not only enlarges the intra-cavity power to shorten the pulse, but also enhances the SPM effect to form the soliton mode-locking. However, a huge cavity gain could further induce giant SPM (ie. δN >> DN) to induce the extremely large phase instability and pulse amplitude jitter.Figure 10 shows the simulated passively mode-locked EDFL pulse shapes and optical spectra with (and without) the effects of GDD and SPM. These results are in good agreement with our experimental results. In our case, the soliton mode-locking is operated with the cavity GDD of −0.156 ps2 and the SPM coefficient of 1x10−2 W−1. With only the SAM effect inside the cavity, the charcoal nano-particles can mode-lock the EDFL to deliver a pulsewidth of 820 fs and a spectral linewidth of 3.21 nm. With a relatively strong SPM, the charcoal nano-particle only starts the pulse-train, and the SPM dominates the compression of the EDFL pulsewidth down to 460 fs with corresponding linewidth of 5.82 nm. Therefore, no matter the fast or slow saturable absorber, the mode-locked EDFL can stably deliver femtosecond pulse with the mechanism transferring from SAM to SPM.
Fig. 10 The (a) simulated autocorrelation traces and (b) optical spectra of the passively mode-locked EDFLs without (upper) and with (lower) the GDD and SPM effects.
4. Conclusion
With the triturated charcoal nano-particle based saturable absorber, the passively mode-locked EDFL at L-band is demonstrated with its pulsewidth shortened to fs regime via the aid of strong SPM and negative GDD. The processed charcoal nano-particle based saturable absorber exhibits average size of <500 nm, which consists of similar C-C sp2 bonds but more oxygen impurity related structural defects as compared to graphite. Raman scattering spectrum shows an intensity ratio I2D/IG of 0.13 and a defect related weak 2D band in charcoal nano-particle, leading to a broadened {002} diffraction peak with a azimuth angle shift of −0.34° in XRD spectrum. After shrinking its size by multiple imprinting-exfoliation-wiping process, the charcoal nano-particle increases its linear transmittance from 0.66 to 0.91 and enlarges the corresponding modulation depth up to MD = 26%. The pulsewidth shortening relies on the optimizations including intra-cavity gain enhancement and loss reduction, which transfer the pulse forming mechanism from SAM to the combining effects of SPM and GDD. With the high-gain EDFA and the size reduced charcoal nano-particles, the passively mode-locked EDFL pulsewidth is compressed from 1.36 to 455 fs, accompanied with its spectrum centered at 1577 nm and broadened to 5.86 nm. A nearly transform-limited pulse is obtained with a TBP of 0.322 under a negative cavity GDD of −0.156 ps2, which exhibits a repetition rate of 8.9 MHz and an amplitude jitter as low as 1.74%. The simulations support that the charcoal nano-particles can mode-lock the EDFL to deliver a pulsewidth of 820 fs with only the SAM effect, whereas a strong SPM and negative GDD compresses the charcoal nano-particle started EDFL pulse down to 460 fs. In the anomalous dispersion (negative GDD) regime, the high-gain operation induces strong SPM to sharpen the intense pulse into a periodically breathing soliton pulse in EDFL. The combining effect of strong SPM and negative GDD dominates the pulse shortening, whereas the nano-scale charcoal saturable absorber plays a minor role to start and stabilize the mode-locked EDFL pulse-train.
Acknowledgment
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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