Graphene-clad microfibre saturable absorber for ultrafast fibre lasers

Graphene, whose absorbance is approximately independent of wavelength, allows broadband light–matter interactions with ultrafast responses. The interband optical absorption of graphene can be saturated readily under strong excitation, thereby enabling scientists to exploit the photonic properties of graphene to realize ultrafast lasers. The evanescent field interaction scheme of the propagating light with graphene covered on a D-shaped fibre or microfibre has been employed extensively because of the nonblocking configuration. Obviously, most of the fibre surface is unused in these techniques. Here, we exploit a graphene-clad microfibre (GCM) saturable absorber in a mode-locked fibre laser for the generation of ultrafast pulses. The proposed all-surface technique can guarantee a higher efficiency of light–graphene interactions than the aforementioned techniques. Our GCM-based saturable absorber can generate ultrafast optical pulses within 1.5 μm. This saturable absorber is compatible with current fibre lasers and has many merits such as low saturation intensities, ultrafast recovery times, and wide wavelength ranges. The proposed saturable absorber will pave the way for graphene-based wideband photonics.

Scientific RepoRts | 6:26024 | DOI: 10.1038/srep26024 was approximately 16 μ m. To increase the graphene-cladded length, we propose a new technique, in which the length of graphene is greater than 200 μ m (Fig. 3). Subsequently, we propose and demonstrate the use of a graphene-clad microfibre (GCM) as an SA in a mode-locked fibre laser for the generation of ultrafast soliton pulses. The improved saturable absorption properties originate from the enhanced light-graphene interaction due to the optical field confined to the wave-guiding microfibre 29 .

Results
GCM saturable absorber. The microfibre, with a minimum diameter of ~6 μ m, is fabricated via a flame-brushing technique and integrated onto a hollow glass substrate, as shown in Fig. 2(b). The monolayer graphene is grown on Cu foil using the chemical vapour deposition (CVD) method 30 . First, the polymethyl methacrylate (PMMA)/graphene sheet is wrapped around the microfibre, as shown in Fig. 3(b). The suspended PMMA/graphene is then cut to a width of ~10 μ m alongside the microfibre, as shown in Fig. 3(c). The detailed cutting procedure is illustrated in a video included in the online supplementary information. Finally, the PMMA is removed. As shown in Fig. 2(a), the GCM saturable absorber is realized by wrapping monolayer graphene around a microfibre. The detailed fabrication procedure of the GCM SA is provided in the Methods section. Figure 3(a) shows a top-view optical microscope image of the GCM before cutting. Only the microfibre is visible because the PMMA/graphene plane is parallel to the top-view direction. Figure 3(b,c) present lateral-view optical microscope images of the GCM before and after cutting, respectively. The photograph of the GCM together with the substrate before cutting is shown in supplementary information Fig. S1. We can see from Fig. 3(b) that the length of the GCM is approximately 210 μ m. The white rectangular area in Fig. 3(b) originates from the reflection of the PMMA/graphene plane. After cutting, the width of PMMA/graphene alongside the   microfibre is approximately 10 μ m, as shown in Fig. 3(c). The full lateral-view optical microscope image of the GCM after cutting is included in supplementary information Fig. S2. Figure 4(a) shows the diameter of microfibre along with the propagating position, where x 0 is the position of the microfibre at the minimum diameter. The circles are the experimental data, which are fitted with a polynomial function of degree 4 (red solid curve). The polynomial equation that produces the best fit is where D is the diameter of the microfibre and Δ x = x − x 0 is the relative position of the microfibre. The minimum diameter of the GCM is approximately 6 μ m. Figure 4(b) presents a scanning electron microscopy (SEM) image of the GCM. By comparing this image to Fig. 3, the SEM image reveals that the surface of the microfibre is smooth after the PMMA is removed. The Raman spectra of the graphene in the GCM are shown in Fig. 4(c). The symmetric Raman 2D band centred at ~2689 cm −1 exhibits a single Lorentzian feature with a narrow full width at half maximum (FWHM) of ~30.6 cm −1 , and the intensity of the 2D band is much larger than that of the G band, with a 2D-to-G intensity ratio of ~3, as shown in Fig. 4(c). According to the technique of identifying single-layer graphene [31][32][33] , the microfibre is covered by monolayer graphene. Figure 4(d) shows the normalized nonlinear absorption of the GCM SA, which is experimentally measured with a homemade ultrafast fibre laser at a central wavelength of ~1550 nm and a pulse duration of ~200 fs. The experimental data are fitted using the solid curve shown in Fig. 4(d) on the basis of a simplified two-level SA model [34][35][36] . Figure 4(d) illustrates that the linear limits of the saturable absorption (α 0 ), the nonsaturable absorption (α ns ), and the saturation intensity (I sat ) are approximately 4.37%, 95.27%, and 1.51 GW/cm 2 , respectively.  Figure 5(a,b) show the power density at the surface of the microfibre and the energy fraction in the air (i.e., P air /P) for the fundamental mode (i.e., HE 11 ) along the diameter of the microfibre (i.e., D), respectively. The central wavelength of the guided wave is 1531.3 nm. The inset illustrates the cross-sectional intensity distribution in a 10-μ m diameter microfibre. These figures were calculated using COMSOL. In the calculation, the peak power of the pulse wave is 200 W, which is consistent with the experimental results. One can see from Fig. 5(a) that the power density at the surface of the microfibre decreases exponentially with D. For instance, the power densities are approximately 20, 3, and 0.038 MW/cm 2 for D = 6, 10, and 30 μ m, respectively. Therefore, the light-graphene interaction is enhanced significantly in the GCM via a tightly confined evanescent field guided along the surface of the microfibre. The energy fraction in the air, P air /P, increases exponentially with decreasing D. For example, P air /P ≈ 22.7% and 77.6% for D = 1 and 0.5 μ m, respectively, as shown in Fig. 5(b).   The threshold pump power for CW lasing is approximately 5 mW. Self-starting mode-locking operation starts at the proper pump power when the polarization controller is adjusted appropriately. Once the stable laser is obtained, no further polarization controller adjustment is needed. Figure 7(a) demonstrates a typical laser spectrum at a pump power of 27 mW, with a central wavelength of ~1531.3 nm. The FWHM spectral bandwidth, Δ λ, is 2.08 nm. The sidebands at 1527.28, 1528.81, and 1533.82 nm are typical of soliton-like pulse formations, which originate from intracavity periodic perturbations 37 . Figure 7(b) illustrates the second harmonic generation (SHG) autocorrelation traces of the experimental data and a sech 2 -shaped fit. Assuming a sech 2 temporal profile, the deconvolution yields a pulse duration of 1.21 ps. The time bandwidth product (TBP) is 0.322, which is near the value of the transform-limited sech 2 -shaped pulses. Figure 7(c) shows that the repetition rate of the fundamental cavity frequency is 1.888847 MHz, corresponding to 529.4 ns of round-trip time (inset of Fig. 7(c)). The radio frequency (RF) spectrum in Fig. 7(c) gives a signal-to-noise ratio of > 60 dB (> 10 6 contrast), indicating low-amplitude fluctuations and good mode-locking stability 38 . No spectrum modulation is observed over 500 MHz (Fig. 7(d)), thereby indicating no Q-switching instabilities.

Discussion
In the experiments, the average power of pulses in the laser cavity is ~0.45 mW, corresponding to a peak power of ~200 W. The experimental observations demonstrate that the proposed fibre laser can generate ultrafast pulses when the diameter of GCM, D, is changed from ~1 to ~30 μ m. We can see from Fig. 5(a) that for D > 30 μ m, the light-graphene interaction is so weak (i.e., less than 0.038 MW/cm 2 ) that the mode locking fails. When D is less than 1 μ m, a considerable fraction of the energy of the pulse is from the microfibre, in which case slight roughness on the surface of the microfibre can cause large losses. As a result, the loss that occurs in the GCM prevents the mode-locking operation of a laser.
The experimental observations demonstrate that if the GCM-SA component is excluded from the proposed fibre laser, it fails to mode-lock; thus, the CW rather than ultrafast pulses is generated. Although the polarization effects can affect the characteristics of pulses, they hardly play such a role in emitting pulses. Both theoretical and experimental results indicate that the polarizer plays the key role in the nonlinear polarization rotation technique 39 . In this work, however, there is no polarizer that is artificially made from a polarization-sensitive isolator.
The loss that occurs in the GCM is dependent on the diameter of the taper and the length of the graphene. In this work, the loss is approximately 1 dB. Although the taper fibre is not protected, the proposed laser can work well over the long term and deliver pulses consistently in our ultraclean laboratory. Outside the ultraclean laboratory environment, the taper fibre must be protected appropriately. Because the diameter of the taper is approximately 6 μ m, the damage threshold is sufficiently high.

Methods
Preparation of the GCM SA. The microfibre is drawn from a standard telecom optical fibre using a flame-brushing technique. The microfibre has a minimum diameter of ~6 μ m, a length of ~30 mm, and a low insertion loss of 0.3 dB. The monolayer graphene is grown on Cu foil via the CVD method. After spin-coating the graphene with PMMA, the large-area PMMA/graphene/Cu film is cut into small pieces of width ~0.2 mm and length ~2 mm. The underlying Cu foil of the small piece is etched in FeCl 3 solution for ~10 h. After the Cu is etched completely, transparent PMMA/graphene sheet floats on the surface of the solution. The PMMA/graphene sheet is then transferred into distilled water five times and is immersed for approximately 2 h each time to remove the remaining FeCl 3 etchant. Subsequently, a microfibre, which is integrated onto a hollow glass substrate in advance, is placed under the centre of the PMMA/graphene sheet. When the microfibre is lifted together with the hollow substrate from the water, the PMMA/graphene spontaneously wraps around the microfibre (Fig. 3(b)). A femtosecond laser beam (at 790 nm with a pulse duration of 200 fs, repetition rate of 1 kHz, and power of 100 μ J) through a micro/nano platform is used to cut the PMMA/graphene to a ~10 μ m width alongside the microfibre (Fig. 3(c)). After cutting, the microfibre with the PMMA/graphene is placed into acetone solution for ~20 min to remove the PMMA. Finally, the graphene wrapped around the microfibre is transferred into alcohol for ~10 min to wash away the residue of the polymer. Measurement method. An optical spectrum analyser (Yokogawa AQ-6370), an autocorrelator, a 6-GHz oscilloscope, a radio-frequency (RF) analyser, and a 10-GHz photodetector are used to measure the laser output performance.