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Abstract
We present high mode-locking performances from an erbium-doped fiber laser (EDFL) by using graphene oxide (GO) and reduced graphene oxide (r-GO) as saturable absorbers (SA) deposited onto the polished surface of a D-shaped optical fiber. The samples were prepared with different concentrations and its characterization was performed by using an optical microscope, a Raman spectrometer, nonlinear saturable absorption measurements, polarization setup, and laser mode-locking analysis. As a 1550-nm polarizer, the best GO (r-GO) samples exhibited higher polarization extinction ratio (PER) of 7.94 (7.65) dB, corresponding to 84 (83) %, both showing similar graphene TE absorption behavior. In a managed-intracavity dispersion laser, broadest bandwidths of 27.2 and 24.1 nm and the corresponding shortest pulse duration of 190 fs could be generated when incorporating the SA with high modulation depth (above 20%), being so far the best mode-locking results ever reported in the literature for GO and r-GO SA onto D-shaped optical fibers in EDFL.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Near infrared ultrashort pulse fiber lasers have become one of the most active fields in the laser research for ultrashort pulse generation by using mode-locking technique through nonlinear optical nanomaterials as physical saturable absorbers (SA), because of the wide and significant applications in basic science, industry, medicine and optical communications [1]. In past decades, semiconductor saturable absorber mirrors (SESAMs) [2] were mostly used as commercial ultrafast SA due their well-developed semiconductor technologies and stable mode-locking performance in fiber laser resonators, although their expensive and narrower wavelength tuning range fabrication processes. In recent years, single-walled carbon nanotubes (SWCNTs) [3, 4] have emerged to overcome the SESAM performance, exhibiting fast recovery time, large saturable absorption, and ease of fabrication, which became an effective and broadband SA because of the nanotube’s diameter/energy band gap tuning. However, as SESAM, it required a specific CNT diameter to tune its absorption band to generate mode-locking at near infrared laser wavelength, although the CNT exhibits much wider saturable absorption bandwidth compared to SESAM. In addition, unavoidable bundles, catalysts and attached amorphous carbons induce large non-saturable losses to CNT-based SA, becoming a great challenge for mode-locking applications.
Currently, graphene has emerged as promising atomically thin two-dimensional (2D) material in the fields of nano-electronics and photonics [5] because of its unique electrical and optical properties due to its gapless linear dispersion of Dirac electrons, ultrafast recover time, low saturable intensity and broadband saturable absorption, overcoming all the drawbacks of CNTs and SESAM. Since the demonstrations of mode-locked lasers using graphene as SA [3, 5–12], the emergence of other layered nanomaterials groups such as transition metal dichalcogenides [13–17], topological insulators [18–20] and black phosphorus [21–23] with remarkable nonlinear optical properties also have been used for the same purpose.
Also motivated by graphene development, low-cost precursors carbon-based-nanomaterial graphene oxide (GO) and reduced graphene oxide (r-GO) [24, 25] have attracted much attention as an alternative SA nanomaterials [26–32]. By graphite powder oxidation via Hummers method [33, 34], GO is synthesized as atomically thin sheet of carbon covalently bonded with functional oxygen groups of sp3 hybridized carbon atoms. Its reduced structure, r-GO can be commonly obtained using reducing agents such as hydrazine or sodium borohydride. Exhibiting strong hydrophilic, ease of functionalization and water solubility properties as well as its flexibility and processibility at large-scale production, these two carbon materials have been extensively explored for optoelectronics and laser applications [35]. Different from graphene, the gapless linear dispersion is modified by oxygen functional groups, but they possess broadband absorption, fast time relaxation of hot carriers and strong saturable absorption, almost comparable to those of graphene, making it potentials cost-effective broadband SAs for mode-locking applications.
Since the GO and r-GO broadband nonlinear optical properties [26–33] were demonstrated, firstly used by Bonaccorso [5] and Liu et al [36] as SA, several lasers in Q-switching and mode-locking configurations have been reported at 1.06 [37, 38], 1.55 [5, 36, 38–46] and 2 μm [46] infrared spectrum regions. In all works, the SA properties have been commonly explored using the direct interaction between the perpendicular optical beam intensity of the light and the nanomaterial deposited onto micrometric core area of optical fiber connector end-faces. Sobon et al made the only experimental comparative work involving the two carbon-based materials, showing for both similar EDFL mode-locking performances of 390 fs pulse duration and 9 nm bandwidth [40].
However, a fiber-ferrule based SA exhibit many problems such as limited nonlinear interaction superficial area, mechanical damage from back-to-back connector end-face contact and sometimes induced thermal damage, affecting directly the laser performance. Therefore, evanescent light field interaction with nanomaterials deposited onto photonic crystal fiber (PCF), tapered and D-shaped optical fibers surfaces can suppress all these issues and provide higher damage threshold, longer interaction millimeter-length and higher efficiency of the nonlinear effect, thereby facilitating laser mode-locking.
Current ultrafast fiber lasers based on GO and r-GO evanescent field based SA have been reported. Tapered fibers were covered with GO and r-GO solutions, resulting in long picosecond pulses generation in EDFL [47, 48]. In addition, hollow core PCF was filled with r-GO nanosheets solution, similar to Ref [36] and reported Q-switching and 616-fs mode-locking harmonic pulse generation [49]. By using D-shaped optical fiber, J. Lee et al used a high polarization sensitive D-shaped optical fiber based on sprayed GO particles as SA in an EDFL obtaining dual Q-switching and mode-locking regimes [50], the last one generating 3.13 nm bandwidth and pulse duration of 780 fs. Such GO-based sample was also incorporated as SA to generate 60-ps long long-pulse at 2 μm mode-locked TDFL [51]. Our group deposited a spin-coated GO film via wet transfer to the polished surface of D-shaped optical fiber, exhibiting high polarization performance comparable to Ref [8, 50], which generated 12 nm bandwidth and 300 fs pulse duration as SA in an EDFL cavity [52], and recently, by using the same GO/D-shaped fiber configuration sample, 227-fs ultrashort pulse generation has been also reported [53].
In this work, we present high mode-locking performances from an EDFL using both spin-coated GO and r-GO film SA deposited via wet transfer method onto polished surface of D-shaped optical fiber. Unlike the previous references, the spin-coating method can provide better control of concentration, homogeneity and thickness of nanomaterial film, resulting in a simple and powerful nanomaterial based solvent fabrication method. To date, high polarization extinction ratio (PER ~8 dB) and nonlinear modulation depth (above 20%) values were measured for both nanomaterials. When incorporated as SA in a managed-dispersion EDFL cavity, the broadest spectra of 27.2 (24.1) nm and the corresponding pulse durations of 190 fs could be generated, presenting so far, the best EDFL mode-locking performances and the unique experimental comparison of the literature for GO (r-GO) SA in D-shaped optical fibers.
2. GO/r-GO production and samples fabrication
The GO was obtained through the modified Hummers method [33, 34], dispersed in water and sonicated during 1 hour. For the film production, we used the methodology demonstrated by Domingues et al. [54]. As substrate, an electro-polished aluminum foil was used to remove the oxide layer and placed on a glass sheet (2.5 cm x 2.5 cm), which the GO dispersion was deposited on its surface. Homogeneous nanomaterial films were then formed by spin coating method. Then, the GO films were covered by 300 nm thick-layer of polymethyl-methacrylate (PMMA) again using spin coating. In the case of r-GO samples production, the GO films prepared by spin coating method were subsequently reduced by 200 µL hydrazine monohydrate vapor per sample at 100-120 °C for 12 h. To remove the aluminum substrate, we firstly took it out from the glass slide, cut it in the size of D-shaped optical fiber polishing area (0.8 cm x 1.0 cm) and placed into a 2M hydrochloric acid (HCl) solution until its complete corrosion. After this step, the film was washed several times with deionized water to remove the corrosion impurities and then deposited onto the polished surface of D-shaped optical fiber with 10 mm polishing length and no distance from core to polished surface via wet transfer method [8], as shown in Fig. 1. In order to improve the film adhesion onto the fiber surface, the samples were heated at 120 °C in a hotplate for 30 minutes. By using an atomic force microscope (AFM), we could measure the GO (r-GO) film thicknesses about 100 nm.
3. Optical and Raman characterization
The optical and Raman spectroscopy characterization of the GO (r-GO)/D-shaped optical fiber sample were realized by using a confocal Raman microscope/spectrometer Witec Alpha 300R. For optical characterization, microscope images of D-shaped optical fiber with the GO (r-GO) deposited onto the polished surface are illustrated Fig. 2(a) and 2(b) using 10 and 50X objective lens. As clearly seen, the nanomaterials clusters were scattered along the whole film, showing good homogeneity.
Fig. 2 Optical image of polished surface of the D-shaped optical fiber with r-GO film with (a) 10X and (b) 20X objective lens and (c) its Raman spectrum obtained from 532 nm laser excitation.
The Raman spectroscopy characterization was performed by using a 532 nm solid-state laser wavelength. The Raman spectrum reveals two characteristic D and G bands of multilayer GO (r-GO) at ~1350 cm−1 and ~1600 cm−1, respectively. In Fig. 2(c) are shown the GO (blue curve) and r-GO (green curve) Raman spectra, all normalized by G-band intensity. The presence of two bands Raman intensity ratio (ID/IG) close to 1, is characteristic of GO. In the case of r-GO, we observed a higher ID/IG value about 1.28, intrinsically due to the formation of new graphitic domains, which confirms the reduction process.
4. Polarization and nonlinear characterization
The GO (r-GO)/D-shaped optical fiber samples were optically characterized by measuring the polarization extinction ratio (PER) from the absorption difference between the parallel transverse electric (TE) and orthogonal transverse magnetic (TM) modes relative to graphene plane [55, 56] using the experimental setup as described by Zapata et al. [8]. The optically free-space beam from a 1550-nm EDFL was directed by mirrors to a Glan-Thompson (GT) polarizer (1000-1550 nm wavelength operation range), setting it at linear vertical polarization state (TM mode). A 1550-nm half wave plate was placed in front of the GT polarizer to rotate the input vertical polarization in steps of 10° from 0° to 360°. Then, the beam was coupled to the D-shaped optical fiber with the GO/r-GO film SA and its interaction was detected through the output power via an optical powermeter, as illustrated in Fig. 3(a).
Fig. 3 (a) Polarization experimental setup. (b) Transmitted power as function of the beam polarization angles through the D-shaped optical fiber without (black line) and with GO (blue curve)/r-GO (green curve) films.
According to the criterion demonstrated by Zapata [8] with monolayer CVD graphene, the concentrations were chosen to reach high values of PER in order to obtain the best EDFL mode-locking performances. By adjusting the input power to ~1 mW and rotating the half wave plate, output power as a function of the polarization angles curves were plotted. In Fig. 3(b) are shown the interaction curves of the light-evanescent field of D-shaped optical fiber light without (black curve), with GO (blue curve) of 2.5 mg/ml and with r-GO (green curve) of 5 mg/ml, relative to the best samples obtained from each nanomaterial.
As expected, the curves showed the same 90° periodicity in the half wave plate angle (corresponding to 180° in the polarization angle) and similar phases, resulting in a well-defined square cosine function. No polarization-induced loss was observed for D-shaped fiber without GO/r-GO. By observing the maximum (αTE) and minimum (αTM) polarization absorption values relative to the GO (r-GO) plane, 13.6 (13.4) and 5.66 (5.75) dB were obtained, which corresponded to PER values of 7.94 dB (84%) for GO and 7.65 dB (83%) for r-GO. Such PER values are comparable to CVD graphene/D-shaped optical fiber samples [8] and much higher than the minimum of 4 dB (60%) required to obtain good EDFL mode-locking performances.
In Fig. 3(b), we assumed that the peaks and valleys of the curves measured for the GO and r-GO refer to the TM and TE modes, respectively, considering its crystalline structure similar to graphene, but without the influence of oxygen functional groups bonded to them. To accurately determine the highest absorption mode of each sample, we analyzed the output beam of the previous polarization setup and directed it through a polarizer, measuring again the output power with the power meter. In Fig. 4, the output power measurements are shown as a function of the polarization angle for the GO (r-GO) samples with the polarizer adjusted at vertical (blue curve) and horizontal (red curve) polarization states, relative to TM and TE modes passage through the samples.
Fig. 4 Output power measurements from GO (r-GO) samples after the polarizer at vertical (blue curve) and horizontal (red curve) polarization configurations.
From Fig. 4, we clearly noted that the horizontal polarization power presented almost three times more absorption than the vertical polarization in both nanomaterials plane. Therefore, although they are composed of graphene sheets bonded by oxygen functional groups (less in r-GO) and defects in their crystalline structures due to their interaction, the most absorbed was the TE mode, indicating the strong interaction of light with the graphene sheets.
The GO and r-GO samples were fabricated by using different concentrations to achieve high PER values for obtaining good EDFL mode-locking performances [8, 52]. By calculating the measured PER using the following expressions,
we obtained high PER samples with 2.5 mg/ml for GO and 5 mg/ml optimal concentrations for r-GO. The best results of measured polarization parameters of each material are described in Table 1.
Table 1. Polarization parameters from GO/r-GO onto D-shaped optical fiber.
The nonlinear absorption characteristics of optimum concentrated GO/r-GO films were investigated using a laser with 150 fs pulse source, centered at 1550 nm with repetition rate of 89 MHz and 260 mW maximum average power. The GO/r-GO transmittances as a function of fluence were fitted using the fast saturable absorber equation [57–60], expressed by
where Fsat is the saturation fluence, T(F) is the fluence-dependent transmittance, αSAT is the modulation depth and αNS is the nonsaturable absorption, as depicted in Fig. 5(a) for GO and Fig. 5(b) for r-GO.Fig. 5 Transmission characteristics as a function of fluence of (a) GO and (b) r-GO films at 1550 nm.
The results showed high modulation depths of ~22% (~35%), linear transmittances of ~54% (~28%), non-saturable losses of ~20% (~29%) and low saturation fluences of ~7 μJ/cm2 (~9 μJ/cm2) for GO (r-GO) samples, which are comparable with some previously reports [40, 47]. Those values were very important for the high EDFL performance and the ultrashort pulse formation in the cavity.
5. EDFL setup and mode-locking results
For testing the mode-locking efficiency of the fabricated GO (r-GO)/D-shaped optical fiber samples, we used an EDFL setup as depicted in Fig. 6. As an active fiber, a 2-m long of Erbium doped fiber (absorption coefficient @ 1530 nm = 47.6 dB/m) with highly normal dispersion @ 1550 nm of D = −57 ps/nm/km (GVD = + 73.6 ps2/km) was used. The EDFL was co-directionally pumped via 980/1550 nm wavelength division multiplexer (WDM) by a high power 980 nm laser diode. A 50-dB optical fiber isolator ensures unidirectional signal propagation inside the cavity. Fiber-based in-line polarization controller (PC) allows adjusting the intra-cavity polarization to optimize the mode-locked operation.
The fabricated in-line GO(r-GO)/D-shaped optical fiber samples were inserted into the laser cavity, consisting of standard single mode fiber (SMF-28) based-optical components with anomalous dispersion @ 1550 nm of D = + 17 ps/nm/km (GVD = −22 ps2/km). The total cavity length of the EDFL was estimated to be 12.7 m, resulting in accumulated anomalous dispersion of DACCUMULATED = + 69 fs/nm and average anomalous dispersion (GVDACCUMULATED = −0.087 ps2) of DAVERAGE = + 5.4 ps/nm/km (GVDAVERAGE = −6.90 ps2/km), typically used to generate soliton-like pulses, being the optimal managed-intracavity dispersion condition to ensure good mode-locking performances when incorporating the best GO/r-GO samples. The laser performance was measured by a 0.06 nm resolution optical spectrum analyzer for the spectral analysis and a 1 GHz sampling oscilloscope / 7 GHz radio frequency (RF) spectrum analyzer for output pulse train analysis of mode-locking operation.
Incorporating the GO (r-GO)/D-shaped optical fibers SA samples into the laser cavity, the mode-locking was achieved at low pump power of ~40 mW. By adjusting the polarization controller, near-flat-top type [8, 61] spectra at ~1566 nm with broad bandwidths of 27.2 for GO on Fig. 7(a) and 24.1 nm for r-GO on Fig. 7(c). In addition, their corresponding pulse durations of ~300 fs could be measured with both samples at the cavity fundamental repetition rate (single pulse operation) of ~15.73 MHz (inset - Fig. 7(b) and 7(d)). The absence of conventional Kelly sidebands is due to non-dispersive wave soliton interference caused by shorter soliton period (LCAV > ZGO, Zr-GO) and the low managed-intracavity dispersion mode-locking operation [62–64].
Fig. 7 (a,c) Laser spectrum bandwidth (inset – log scale spectrum) and (b,d) pulse autocorrelation trace (inset – pulse train) obtained with the GO (blue curves) and r-GO (green curves).
The generated laser spectrum and chirped pulse profiles can be associated with stretched pulse-lasers, usually characterized by the alternate stretching and recompressing of the pulse into the fiber cavity by segments of large normal and anomalous dispersion [65–67]. The time bandwidth product (TBP) of the GO (r-GO) pulses was about 1.00 (0.890), which is ~3.20 (2.80) times larger than 0.315, expected in the case of transform-limited soliton-like pulses, with 0.2 mW output power, 0.277 kW peak power, 83 pJ energy pulse and 347 MW/cm2 peak intensity for both samples.
By placing additional 1.50 m SMF length at the laser output coupler, we compressed the stretched-soliton pulse duration from 300 to 190 fs for both GO and r-GO (Fig. 7(b) and 7(d)), assuming the sech2 pulse profile. These are, so far, the best mode-locking performances generated in EDFL using GO and the unique using r-GO SA onto D-shaped optical fibers, when compared to the current literature [48, 50, 52]. As the r-GO sample was prepared by further reduction of GO, we could, for the first time, made a direct comparison of both polarization and laser performances of the evanescent field-based samples at the same time. In the laser, the two nanomaterials presented similar behavior, as previously reported by Ref. 40. Furthermore, as the fabricated samples are acting as polarizers because of its high PER values, we can surely attributed the laser best mode-locking results from nonlinear polarization rotation (NPR) and GO/r-GO saturable absorption hybrid mechanism [20]. As the two NPR and saturable absorption are fast mode-locking mechanisms for ultrashort pulse generation, we can associate the NPR contribution to the ultrashort pulse formation and the saturable absorption to the high EDFL stability, therefore obtaining the best laser performance with hybrid mode-locking [50].
The stability of laser mode-locking regime and GO (r-GO)/D-shaped optical fiber samples quality are depicted in Fig. 8(a) and 8(b) respectively, showing the measured RF spectrum at both 1 GHz-broad frequency span with 100 Hz resolution bandwidth (RBW) and 2 kHz-narrow frequency span with 1 Hz RBW. As expected from the fundamental cavity repetition rate observed in the oscilloscope, the corresponding RF frequency was observed at 15.726 MHz for GO and 15.729 MHz for r-GO, with a high signal to noise ratio (SNR) around 74 dB for both samples. The origin of symmetrical side frequencies around the central spectrum could be possibly attributed to small fluctuations caused by the hybrid laser mode-locking mechanism composed by NPR and GO/r-GO saturable absorption [20]. Generally, pulses generated by NPR mode-locking are more unstable when compared to those generated by real saturable absorber, essentially because of its good spectral filter. Therefore, we can associate the good stability of our laser (high signal to the noise ratio of 74 dB) mainly with the nanomaterials saturable absorption and the small spectral noises with NPR influence [68].
Fig. 8 Output RF spectrum measured around the fundamental cavity repetition rate at 15.73 MHz with 1 Hz resolution (inset – wideband RF spectrum with 2 kHz resolution) obtained with (a) GO and (b) r-GO samples.
The long-term stability of the laser mode-locking was also measured by monitoring the optical spectrum and the repetition rate over time. By operating at single pulse condition (15.7 MHz), the laser remained stable for 15 minutes, but it shown very sensitive to environment fluctuations. So, in order to find the maximum stability, we check the optimal laser conditions adjusting the pump power and the polarization controller. At pump power of ~70 mW, the long term laser stability could be achieved at double pulse operation (~30 MHz), remaining stable for 300 minutes with mode-locking performances of ~26 nm using GO, (Fig. 9(a) and 9(b)) and ~22 nm average bandwidth using r-GO (Fig. 9(c) and 9(d)), proving its good stability as saturable absorbers.
Fig. 9 Monitoring of spectral bandwidth and repetition rate of the laser mode-locking regime over time obtained with (a, b) GO and (c, d) r-GO samples.
To compare our results, we tabulate all reported EDFL mode-locking performances based on evanescent field interaction obtained with GO (Table 2), with r-GO (Table 3) and with some other nanomaterials from literature (Table 4) by spectral bandwidth (Δλ), pulse duration (Δτ) and RF spectrum signal to noise ratio (SNR) parameters.
Table 2. Mode-locked EDFL performances with GO SA.
Table 3. Mode-locked EDFL performances with r-GO SA
Table 4. Comparison with other nanomaterials SA based on evanescent-field onto D-shaped optical fiber
As observed, our samples were able to provide good results on all laser parameters, comparing with all reported EDFL based on evanescent field saturable absorption samples. Considering only GO results, we could highlight the wide bandwidth of the laser and the shortest pulse generated by evanescent field, when compared with those reported by Ref. 53. For the first time, high performances of an EDFL were achieved with evanescent field-based r-GO sample onto D-shaped optical fiber, and so far the best in comparison to the entire literature. In addition, the mode-locking performances were very close or better than those obtained with other nanomaterials SA, as shown in Table 4, confirming our efficient liquid-phase GO and r-GO polarized samples fabrication method for mode-locking applications.
6. Conclusions
In summary, we presented high mode-locking performances from an EDFL using GO and r-GO SA film deposited onto the polished surface of a D-shaped optical fiber. By using 2.5 (GO) and 5 mg/ml (r-GO) optimal concentrations, 1550-nm polarizer based samples exhibited higher polarization extinction ratio (PER) values range with a graphene TE absorption behavior about ~8 dB (80%). Incorporating the high nonlinear GO (αS ~22%) and r-GO (αS ~35%) saturable absorbers, the broadest laser spectra of 27.2 and 24.1 nm with the shortest pulse durations of 190 fs, respectively, were obtained in a managed-intracavity dispersion EDFL, which we attributed these best laser results to hybrid mode-locking mechanism between nonlinear polarization rotation (NPR) and GO/r-GO saturable absorption. These are, so far, the best mode-locking performances obtained with GO and the first/unique with r-GO SA onto D-shaped optical fibers in EDFL.
Funding
Fapesp (2012/50259-8, 2015/11779-4 and 2016/25836-2); CAPES; CNPq; and Mackpesquisa.
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