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Abstract
We experimentally investigated the saturable absorption influence of graphene layers with natural stacking order in an erbium-doped fiber laser passive mode-locking. Mechanically exfoliated graphene saturable absorber (MEGSA) samples, ranging from 1 to 6 layers, were fabricated preserving their natural ABA stacking order and precisely characterized by 2D band profile from Raman spectroscopy. By incorporating the samples as saturable absorbers (SA) in the fiber laser, mode-locking performances with pulse duration from 670–780 fs and bandwidth from 3.8–4.6 nm could be generated. Also, we identified a transition in the mode-locking activation mechanism from non-self-starting, for monolayer and bilayer graphene, to self-starting, for trilayer and few-layer graphene, which is a strong indicative of fast-to-slow saturable absorption response dependence on the number of graphene layers.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers have been an important topic for scientific and industrial research, with important applications in areas such as biomedicine and optical communications [1–3]. Due to the advent of two-dimensional materials in the last two decades and its relative integration facility with optical fibers, passively femtosecond mode-locked fiber lasers have been widely studied using nanomaterials as saturable absorbers (SA) [4–6]. Among these materials, graphene stands out for being the pioneer with remarkable optical properties such as ultra-fast recovery time and broadband absorption due to the linear energy dispersion of Dirac electrons [7].
All these graphene properties are directly related to its electronic band structure [8], which also changes with the number of graphene atomic layers and their stacking order, owing to the interlayer interaction variation [9,10]. Since monolayer graphene is the main reference as the fundamental carbon-based nanomaterial unit, the study of few-layer graphene for a precise determination and manipulation of its optical and electronic properties appears as the next step [11–13]. In this direction, artificial production of bilayer and trilayer graphene with special stacking orders has been investigated [13–15]. From natural graphite, two stable stacking orders of graphene can be obtained: ABA (or Bernal) stacking and ABC (or rhombohedral) stacking. Via mechanical exfoliation of natural graphite, low defect graphene with stable stacking order, free impurities, and high structural quality can be obtained, in contrast to its structure as derived from other methods such as chemical vapor deposition (CVD) [16] or epitaxial growth on SiC [17] that usually presents a random stacking order with weak interlayer interaction.
Many relevant works have demonstrated passively ultrafast mode-locked fiber lasers using multilayer [18–20], few-layer [18,21–30] and monolayer [18,22,26,31–33] graphene as saturable absorbers (SA) obtained by different physical or chemical methods. However, few studies have reported graphene SA properties and mode-locked Erbium-doper fiber lasers (EDFL) dynamics as a function of the number of graphene layers [18,21–30], and to the best of our knowledge, none of them analyzed the influence of its ABA or ABC stacking. Rosa et al. [29] studied, for the first time, the EDFL mode-locking performance as a function of the precise number of graphene SA layers from 1 to 7 by accurately controlling the number of CVD-stacked graphene monolayers. Nevertheless, they concluded that each monolayer graphene was isolated from neighboring layers and the mode-locking performance was mainly determined by the SA linear absorption, which was directly proportional to the number of stacked layers.
For the first time, we present a study of a mechanically exfoliated monolayer, bilayer, trilayer, and few-layer graphene as SA for passive EDFL mode-locking (ML). In this work, mechanically exfoliated graphene saturable absorber (MEGSA) samples, ranging from 1 to 6 layers, were fabricated preserving their natural ABA stacking order and precisely characterized by 2D band profile from Raman spectroscopy [34–37]. By incorporating these samples as SA in EDFL, ML performances with pulse duration from 670–780 fs and bandwidth from 3.8–4.6 nm could be generated. We also identified a transition in the mode-locking activation mechanism from non-self-starting (NSS), for monolayer and bilayer graphene, to self-starting (SS), for trilayer and few-layer graphene, which is a strong indicative of fast-to-slow saturable absorption response dependence on the number of graphene layers.
2. Saturable absorber samples fabrication and characterization
2.1 Graphene samples preparation and Raman characterization
Graphene flakes of 1 to 6 layers were extracted from high-purity natural graphite from Nacional do Grafite Ltda. (Brazil), with a low level of defects and impurities. The graphite crystal was mechanically exfoliated using the scotch tape method [38] and deposited onto viscoelastic and transparent polydimethylsiloxane (PDMS) substrate via a fast and effective stamp method [39]. By microscopy optical contrast, we identified the positions and dimensions of the graphene flakes in the PDMS stamp.
The next step was the identification of the number of layers and the stacking order of each graphene flake. For this characterization, a Raman spectroscopy characterization in a confocal microscope-spectrometer configuration was performed (3 mW, 532 nm (2.33 eV) laser excitation). The graphene flakes were characterized by the ratio between the G (∼1585 cm−1) and 2D (∼2675 cm−1) Raman band intensities (IG/I2D), as shown in Fig. 1(a). Because this classic method is not sufficiently accurate to identify N > 2 graphene layers, we also performed the 2D band profile analysis to conclude the exact number of layers for each sample, as shown in Fig. 1(b). This 2D band spectral analysis also allowed us to characterize the ABA stacking order [35–37] of the samples. With the absence of the D band at 1345 cm−1 (Fig. 1(a)) we confirmed the low defect level of all graphene flakes [40].
Fig. 1. Raman spectra of mechanically exfoliated graphene samples (ranging from 1 to 6 layers) showing the (a) G and 2D bands intensities ratio (IG/I2D) and (b) 2D band profiles from the dashed mark in Fig. 1(a) obtained with a 532 nm (2.33 eV) excitation laser. In all samples, ABA stacking spectral signatures were observed, as confirmed by Refs. [35–37].
After Raman characterization, we transferred each selected graphene flake from the PDMS stamp onto the polished face of the optical fiber connector core via adapted XYZ positioning stages and an optical microscope, as reported by Rosa et al. [31]. We performed the Raman mapping characterization to verify both the graphene flakes structure and fiber core coverage after the transfer. The transfer procedure of graphene to fiber connector end face is the same procedure for stacking layers of different 2D materials for van der Waals heterostructures fabrication, therefore it is as reproducible as one’s ability for obtaining graphene flakes via micromechanical exfoliation. Different from other methods, mechanically exfoliated graphene flakes are well known for being single crystalline, relatively large areas, and uniform in the number of layers. Before the transfer procedure, the obtained graphene flake is characterized via Raman mapping, in which the intensity ratio of the G and 2D bands is characterized across the samples, so we can be sure about the number of layers and uniformity. After this characterization, the desired flake is deterministically transferred and aligned to the fiber connector end face to fully cover the fiber, guaranteeing full coverage of the fiber core with a single graphene crystal of a defined number of layers.
As shown by Rosa et al. [31], the Raman spectra of the as-transferred graphene flakes present bands from the germanium-doped silica fiber core and graphene flake. By mapping these bands, it is possible to gather information about the fiber core coverage by the graphene flake, as well as about the uniformity of the transferred graphene flake on top of the optical connector end face. As the germanium-doped silica fiber core and graphene flake Raman bands present intensities in different scales, we combine both information in a false-color Raman map, highlighting graphene’s uniformity and fiber core 100% coverage, as shown in Fig. 2.
Fig. 2. Optical microscope images and Raman maps of graphene flakes onto fiber core with 1 (I) to 6 (VI) layers. The Raman maps display false-colored images, with fiber core displayed in pink and graphene flakes 2D band intensity displayed in blue. The fiber core in each sample is completely covered by the graphene saturable absorber.
The optical images and the 2D band intensity Raman spectroscopy mapping of the transferred graphene samples onto the optical fiber connectors are shown in Fig. 2, demonstrating total coverage of the fiber core by graphene flakes. After this step, each sample was sandwiched with a complementary optical fiber connector thus completing the MEGSA samples fabrication.
2.2 Saturable absorption measurements
The characterization of the nonlinear transmittance of MEGSA samples was made using a balanced twin-photodetector transmittance system [21]. In this setup, a reference arm is used to measure the linear transmittance of an optical fiber sample without graphene, and all samples with graphene have their transmittance compared to this linear reference. As the excitation source, a 1560 nm pulsed laser with 430 fs pulse duration and 89 MHz repetition rate was used, providing a maximum achievable peak intensity of 300 MW/cm2. The measured saturable absorption curves (open squares) together with theoretical fitting (solid lines) are presented in Fig. 3(a). Based on the fast saturable absorbers model [41,42], the transmittance saturation profiles were fitted using the following equation:
where T(I), T0, Tns, and Isat are the intensity-dependent transmittance, linear transmittance, non-saturable transmittance, and saturation intensity, respectively.Fig. 3. (a) Experimental and theoretical saturation curves of all MEGSA samples. ΔT is the maximum change in transmittance obtained experimentally (saturated transmittance), while Tsat is the total saturable transmittance, obtained from the model fitting. (b) Saturated (ΔT, open squares) and saturable (Tsat, filled circles) transmittance obtained from the curves of Fig. 3(a).
The total linear transmittance (T0) is determined by graphene (T(N) = (1-πα)N, N = number of graphene layers, α = attenuation coefficient) [8–10] added to the patch-to-patch optical connectors transmittances, as they have different losses. The observed saturated transmittance change percentage (ΔT) was measured by the difference between non-saturable and linear transmittances [(Tns -T0)*100] from the experimental curve. Because of the setup power limitation, we could not observe the full saturation of the samples. Therefore, we extended the fitting to the saturation plateau to determine the saturable transmittance parameter (Tsat) for each curve, according to the model parameters. We tabulated all saturation parameters of all samples in Table 1.
Table 1. Saturation parameters of all MEGSA samples. T0: linear transmittance; Tns: non-saturable transmittance, Isat: saturation intensity; Tsat: saturable transmittance (model); ΔT: saturated transmittance.
From the relative transmittance graphs (T-T0), we observed that the saturated transmittance variation scales with the number of graphene layers, starting from ∼0.35%, for monolayer and bilayer, up to 0.85-1.03% for trilayer and a few-layer. From the model, we could estimate the Tsat parameters of 2.12 and 1.20%, for 1 and 2L, up to 1.82%, 1.91%, 2.04%, and 3.27% for 3, 4, 5, and 6L respectively, having the same tendency of the saturated transmittance ΔT (Fig. 3(b)). In the opposite direction, the saturation intensities of samples were inversely proportional to the number of graphene layers, going from 1315 MW/cm2 with monolayer to 267 MW/cm2 with 6 layers. We compared our saturation characterization results with the parameters for few-layers graphene reported in the literature in Table 2.
Table 2. Comparison of our MEGSA performances with few-layer graphene reported works.
3. EDFL setup and mode-locking results
An EDFL ring configuration (Fig. 4) was used to measure the ML performances and study the short-pulse generation at the standard telecommunication wavelength region. The cavity consisted of a 0.7 m Erbium-doped fiber pumped by a 980-nm diode laser via a minimum polarization dependent WDM/isolator coupler, a polarization controller, a 30% output coupler, and an additional 9 m single-mode fiber (SMF), totalizing 13.8 m cavity length with anomalous accumulated and average dispersions of +189 fs/nm and +13.62 ps/nm.km, respectively. The spectral and temporal ML performances were simultaneously measured by an optical spectrum analyzer, a 12.5 GHz InGaAs photodetector connected to a 1 GHz sampling oscilloscope, a power meter, an autocorrelator, and an RF spectrum analyzer.
To obtain the best ML results, the total losses - including the insertion loss of each sample - were minimized inside the cavity (spectrum emission at longer wavelengths) and its dispersion management was optimized by the implementation of additional 9 m of SMF inside the cavity and 3 m of dispersion compensating fiber (DCF) outside the cavity. After reaching the threshold pump power and activating the mode-locking with each sample inside the cavity, the best performance measurements were obtained at different pumping powers, all operating at cavity fundamental repetition rate (single pulse per cavity round-trip operation regime).
First, we incorporated an empty (without graphene) pair of optical fiber connectors of the same type and size as those used to fabricate the MEGSA samples. By increasing the pump power to ∼ 188 mW and tuning the polarization controller, we achieved a non-self-starting (NSS) ML at a multiple pulse regime, provided by the nonlinear polarization rotation (NPR) effect, as activated by the need of high intracavity power and/or localized external perturbation (polarization state change or external mechanical perturbation). By decreasing the pump power to 29 mW, soliton-like pulses at 1567 nm could be generated with 4.21 nm bandwidth and 711 fs pulse duration operating at a fundamental repetition rate of 14.42 MHz, resulting in an output power of 2.67 mW and calculated intracavity peak power of 605 W. The time-bandwidth product (TBP) of this pulse was ∼ 0.366, which is near (1.15 time) the pulse transformed limit of 0.315 for sech2 pulses. For all measurements, a continuous wave (CW) parasite peak was observed at the top of all spectra, caused by intracavity power excess and unlocked modes.
By inserting the monolayer and bilayer MEGSA inside the cavity, we also obtained the mode-locking regime with a high threshold pump power of 188 mW (in agreement with Rosa et al. [43] experimental reports) and 75 mW, respectively, both at multiple pulse regime, similar to NPR. In the case of monolayer graphene, 4.60 nm bandwidth and 685 fs pulse duration (TBP ∼ 0.385) could be generated at a pumping power of 27 mW (single pulse regime at 14.45 MHz) with 2.06 mW output power and 485 W intracavity power. For the bilayer graphene sample, we achieved the best ML performance among all samples. At pumping power of 35 mW, 4.74 nm bandwidth and 667 fs pulse duration (TBP ∼ 0.386) were measured at 14.55 MHz cavity fundamental repetition rate with 2.57 mW output power and 621 W peak power. To date, this is the shortest pulse in the literature using exfoliated bilayer graphene SA in EDFL.
By incorporating the trilayer MEGSA inside the cavity, a self-starting ML mechanism was observed at low pumping power of ∼ 10 mW, very close to the CW threshold (∼9 mW). As result, laser mode-locking performance of 3.82 nm bandwidth and 765 fs pulse duration (TBP ∼ 0.357) was obtained at low pumping power of 12 mW operating at 14.45 MHz. After trilayer graphene, the same self-starting mechanism was observed using 4L, 5L, and 6L MEGSA at the same low pumping power range (∼10-15 mW). The laser generated 3.75, 3.81, and 4.12 nm optimal bandwidths and corresponding 788, 761, and 723 fs pulse durations at pumping power of 18-35 mW, respectively, all operating at ∼ 14.50 MHz repetition rate, resulting in 1-3 mW output power and 200-620 W intracavity peak power, very similar ML performances to those obtained with trilayer graphene SA. Also, the lasers using all MEGSAs presented both similar RF spectra (14.41-14.55 MHz fundamental repetition rate (RR)) and high signal-to-noise (SNR) levels (55.2-59.6 dB).
In summary, the best mode-locking performances obtained with all MEGSA are depicted in Fig. 5, showing the soliton spectra (fitted by sech2 function in Fig. 5(a)) and pulse autocorrelation traces (Fig. 5(b)), followed by all EDFL measured parameters in Table 3. According to the results, all MEGSA with an even number of layers (2 L,4 L,6 L) showed the same pumping power for optimal operation (∼ 35 mW), suggesting a parity effect of ABA stacking graphene SA in the laser [44].
Fig. 5. Best mode-locking results with (or without) different MEGSA’s inside the EDFL cavity, showing the (a) soliton spectra and (b) pulse autocorrelation traces, all fitted by sech2 fitting (purple solid line).
Table 3. EDFL mode-locking optimal results without (NPR) and with MEGSA samples. Ppump (th) = threshold pump power; Ppump (op) = operation pump power, Pout = average output power; Ppeak = intracavity peak power.
We also analyzed the single and multiple pulse regimes for each laser by identifying the pump power ranges in the CW characterization curves (dotted line) for NPR, monolayer, and bilayer (top graph) and 3-6 layers (bottom graph), as shown in Fig. 6. By observing the single pulse regimes (blue region), the laser mode-locking using monolayer and bilayer provided about 50% wider pump power range (10-60 mW) and more stability against multiple pulse breakup than 3-6L (10-40 mW). At the multiple pulse regime (green/brown region), stable double or triple pulse operation regimes could be only generated by monolayer and bilayer graphene, in contrast to the unstable single-to-multiple pulse regime transition, observed for trilayer and few layers graphene.
Fig. 6. CW output power curves (dotted line) of EDFL cavity showing the single pulse (blue region) and multiple (green or brown region) pulse regimes at different pump power ranges achieved with each MEGSA sample.
4. Discussions
4.1 Mode-locking performance vs nonlinear saturable transmittance (Tsat)
We analyzed the average ML performance for each sample as a function of the number of layers, and results for output pulse duration and bandwidth according to the number of layers are shown in Fig. 7. By relating the Tsat parameter with the respective laser results, the best mode-locking performance should be obtained from graphene samples with high Tsat values (high modulation depth), this being one of the main responsible for pulse shortening process along with the dispersion and nonlinearity of the cavity. This behavior was clearly observed with 3–6-layers graphene samples. For 3-5 layers, the estimated Tsat values and their mode-locking performances were very similar to each other, while with the 6-layer sample, the same measurements were better, as expected. From this graph in Fig. 7, ML performances promoted by NPR, monolayer, and bilayer graphene, were almost 13% superior to those obtained by 3L, 4L, 5L, and 6L graphene. From the bilayer graphene sample, there is a behavior transition from fast-to-slow SA [45] with the trilayer graphene samples, which extends to up N = 6, due to the appearance of self-starting mode-locking at very low pumping powers.
Fig. 7. Average soliton-mode-locking performances (pulse duration and bandwidth) obtained without (NPR) and with each MEGSA (1-6 layers).
4.2 Saturation intensity vs mode-locking activation threshold
For mono- and bi-layer graphene SA, similarly to NPR, the laser needed high intracavity power operation and external perturbation (nonlinear change in the polarization state of light or mechanical perturbation) to stimulate the absorbing elements in the cavity, whereas, for 3 to 6 layers of graphene SA, the mode-locking operation of the laser is self-starting. To better understand this relation, we analyzed the single and multiple pulse regimes for each laser configuration and the power to achieve each regime. In Fig. 8 is shown the pump power level necessary to achieve a single-pulse mode-locking regime for 1-6 layers of graphene as a saturable absorber. This mode-locking threshold level presents the same trend as in the saturation intensity dependence on the number of layers in each graphene SA.
Fig. 8. Saturation intensity (black open squares) and mode-locking pump power threshold (blue-filled circles) tendencies obtained with all MEGSA (1-6 layers).
The intracavity power to achieve graphene saturation is much higher for 1 or 2 graphene layers if compared to 3 to 6 layers, and therefore saturable absorption is not the sole mechanism responsible for laser mode-locking for mono and bilayer graphene SA, but a hybrid mechanism with the combination of SA with NPR [46–52]. For a large number of graphene layers, saturable absorption is easily achieved, suppressing NPR effects, and making it the main mechanism for laser mode-locking.
5. Conclusion
In summary, we presented an experimental study of EDFL mode-locking as a function of mechanically exfoliated graphene saturable absorber (MEGSA) samples with 1 to 6 layers. By using Raman spectroscopy, the exfoliated flakes were accurately identified by their ABA stacking order via 2D Raman band profile and transferred to optical fiber connectors conserving their natural stacking order. By incorporating the samples inside the cavity, passively mode-locking performances were obtained with pulse duration (bandwidth) ranging from 667-788 fs (3.75–4.74 nm), in which the best result was achieved using bilayer graphene. Also, we observed a mode-locking transition from non-self-starting, as promoted by NPR, monolayer, and bilayer graphene, to a self-starting mechanism of the laser, as generated by 3L, 4L, 5L, and 6L MEGSA. Based on these results, we attribute this mode-locking behavior to the fast-to-slow saturable-absorption response dependence on the number of graphene layers with Bernal stacking and low defect level. Monolayer and bilayer graphene seems to have a saturable absorber recovery time faster than trilayer and few-layer graphene, as confirmed by all our experimental measurements and some mode-locked EDFL references from literature, although these materials need to be further investigated via pump-and-probe technique to determine their corresponding relaxation times at 1550 nm.
Funding
Brazilian Army; Fundo Mackenzie de Pesquisa e Inovação (MackPesquisa Project 221017); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (grant 1716664); Fundação de Amparo à Pesquisa do Estado de São Paulo (SPEC 2012/50259-8, SPEC 2015/11779-4, SPEC 2016/25836-2).
Acknowledgments
The authors gratefully acknowledge Prof. K. Novoselov for suggesting this work and FAPESP, CAPES, MackPesquisa, and the Brazilian Army for the financial support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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