Black Phosphorus Q-Switched Large-Mode-Area Tm-Doped Fiber Laser

We report on a passively Q-switched fiber laser with black phosphorus as saturable absorber. By employing the sol-gel fabricated large-mode-area Tm-doped fiber as gain medium, a high-energy Q-switched fiber laser has been demonstrated which delivers the maximum pulse energy of 11.72μJ with the pulse width of 660 ns at the wavelength of 1954 nm. Our experimental results indicate that BP Q-switched large-mode-area Tm-doped fiber laser is an effective and reliable approach to generate high-energy pulses at 2μm.

Black phosphorus (BP), as a newly emerged 2D material, has attracted wide attention recently for the common properties of 2D materials with wide bandwidth, ultrafast carrier dynamics, and planar characteristic [14].More importantly, BP has a thickness-dependent direct energy bandgap from 0.3 eV (bulk) to 2.0 eV (single layer) [15], filling up the interval between the zero gap of graphene and large gap of TMDCs, which is of great importance to the optical applications.Up to now, the broadband BP saturable absorbers have been widely applied to pulsed lasers in a wide spectral range from visible to mid-infrared [16][17][18][19][20][21][22][23][24][25].Specifically, in 2 m Q-switched fiber lasers, Wang et al. reported a passively Q-switched Tmdoped fiber laser using a BP deposited microfiber, delivering the maximum pulse energy of 154 nJ [26]; Jiang et al. deposited the BP powders onto a side-polished fiber and achieved Q-switched operation with the maximum pulse energy of 276 nJ [27].The output pulse energy was further improved up to 632 nJ with BP SA fabricated by optical deposition method [28].Although BP has been confirmed to be a reliable and excellent SA for 2.0 m Q-switched fiber lasers, the output pulse energy is limited below micro joule level due to the employment of single-mode fibers.Large-mode-area (LMA) fiber fabricated via sol-gel method combined with high temperature sintering will be a choice for generating high-energy laser pulses.The sol-gel method has a great merit of higher doping homogeneity for rareearth ions, preventing the cluster of the rare-earth ions and the fluorescence quench effect.With the advance of LMA fiber fabrication, dual-cladding Tm-doped fiber with core diameter as large as 38 m could be developed by sol-gel method combined with high temperature sintering [29], offering a good alternative for higher pulse energy generation at 2 m.
In this paper, we adopted the optimized LMA doublecladding Tm-doped fiber as gain medium with a core diameter as large as 30 m.Based on a mechanically exfoliated BP SA, the high-energy Q-switching operation was demonstrated.The Q-switched fiber laser emitted pulses with the maximum average output power of 615 mW, the maximum pulse energy of 11.72 J, and the shortest pulse width of 660 ns at 1954 nm.Compared with the previously reported pulse energies in BP Q-switched Tm-doped fiber lasers, the pulse energy was improved by an order of magnitude, indicating that the sol-gel fabricated LMA Tm-doped fiber together with BP SA is an effective way to generate Q-switched fiber laser with high pulse energy at the wavelength of 2 m.

Characteristics of BP Flake
Similar to the preparation approach of graphene, the layered BP flake could also be prepared via mechanical exfoliation method.By repeatedly exfoliating a bulk BP with scotch tape, the thin BP flake could be obtained.As displayed in Figure 1, the characteristics of as-prepared BP sample were analyzed.
Excited by a 532 nm laser, Raman spectrum (Figure 1(a)) of the sample reveals three apparent peaks at the wavenumbers of 362 cm −1 , 439 cm −1 , and 466 cm −1 , corresponding to mode vibrations A 1 g , B 2g , and A 2 g of phosphorus atoms in BP crystal lattice, respectively [30][31][32].Using the scanning electron microscopy (SEM), the morphology of the as-prepared BP flake was obtained with an amplification rate of 1000.As Figure 1(b) shows, the smooth surface indicated that the mechanically exfoliated BP flake is uniform with a relatively high quality.To further confirm the thickness of the BP flake, the thickness measurement was performed with the atomic force microscopy (AFM).The three-dimensional (3D) AFM image (Figure 1(c)) shows the existence of some bubble-like bulges on the surface.The inset of Figure 1(c) shows that the height difference between the mica sheet substrate (point A) and the BP flake (point B) is 7.0 m.Since the thickness of monolayer BP is 0.6 nm, the as-prepared BP flake could be regarded as bulk-like form with an energy gap of 0.3 eV, supporting the saturable absorption up to 4.1 m wavelength.The elemental components of BP flake were confirmed by energy-dispersive X-ray spectroscopy (EDS).The analysis of the spectroscopic data in Figure 1

Experimental Setup
The experimental setup schematic of BP passively Q-switched LMA Tm-doped fiber laser is depicted in Figure 2. The fibercoupled 793 nm laser diode (LD) with a core diameter of 105 m and a numerical aperture of 0.15 was adopted as the pump source.The pump laser from the LD was collimated and then focused onto the gain fiber by a pair of planoconvex lenses F1 and F2 ( 1 =  2 = 75 mm).The 4 m length LMA double-cladding Tm-doped fiber was adopted as the gain fiber.The double-cladding Tm-doped fiber has a glass composition of 0.1Tm 2 O 3 -1.5Al 2 O 3 -98.4SiO 2 in mol.% and a large core diameter of 30 m with a NA of 0.102.The first cladding diameter and NA of the gain fiber were 250 m and 0.366, respectively, guaranteeing the effective pump coupling.The normalized frequency of the fiber is about 4.9 for 1954 nm, indicating that the gain fiber supports multimode operation.In order to generate high-quality beam, the gain fiber was rolled up with a folding diameter of ∼15 cm to suppress high-order transverse mode oscillation.The input fiber end was cleaved perpendicularly and acted as cavity feedback with Fresnel reflectivity of 4% and output coupler with a transmissivity of 96%.The other fiber end was cut with an angle of 8 ∘ to avoid parasitic oscillation.In order to remove the heat while pumping, both ends of fiber were mounted in an aluminum heat sink with a V-groove.The intracavity laser beam was collimated and focused onto BP SAM by a pair of concave mirrors M1 and M2 with a radius of curvature (ROC) of 150 mm and 100 mm, respectively.The residual pump light was filtered by the two concave mirrors (M1 and M2) with antireflection coating for pump wavelength, avoiding the adverse effect such as additional heat load on BP SAM.In order to separate the output laser beam from the pump beam, the 45 ∘ -placed dichroic mirror (DM) was coated with high transmissivity for pump wavelength ( > 95%) and high reflectivity for laser wavelength ( > 99%).For the convenience of measurement, the output laser beam was collimated by a plano-convex CaF 2 lens.

Results and Discussion
Using a highly reflective mirror instead of BP SAM, the continuous-wave (CW) laser performances with differentlength Tm-doped fibers were tested and 4 m length LMA Tmdoped fiber demonstrated the best laser performance.CW operation with 4 m length gain fiber was achieved with a threshold of 10.7 W, as shown in Figure 3.With the increase of the launched pump power, the CW output power increased linearly with a slope efficiency of 16.42% and the maximum output power of 2.08 W was obtained at the launched pump power of 23 W. The output beam quality was measured by a commercial beam profiler (Thorlabs, M2MS, 400-2700 nm) with a  2 factor of 1.3 [33].Higher output power was only  limited by the available pump power in this experiment.In the whole pump power range from laser threshold to the maximum pump power, we did not observe the self-pulsed phenomenon.The inset of Figure 3 shows the cross section of the gain fiber.In order to achieve Q-switching operation, we replaced the highly reflective mirror with BP SAM.
By precisely adjusting the longitudinal and transverse position of BP SAM, self-started Q-switching operation was observed on a digital oscilloscope at the launched pump power of 16.6 W. Figure 4(a) shows three typical Q-switched pulse trains recorded at the output power of 363 mW, 505 mW, and 615 mW, respectively.Each output power corresponded to a repetition rate which increased with the launched pump power.The corresponding pulse profiles are shown in Figure 4(b), which were captured by a photoelectric detector (ALPHALAS, UPD-5N-IR2-P).The radio frequency (RF) spectrum (inset of Figure 4(b)), recorded at the average output power of 615 mW, shows a signal-to-noise ratio of 20 dB.
Figure 5(a) shows the average output power and pulse energy variation with the launched pump power in the passively Q-switched LMA Tm-doped fiber laser.Both average  1), there was a significant improvement of pulse energy in our experiment due to the employment of LMA fiber.With further increasing the pump power beyond 18.8 W, the pulse train of the passively Q-switched laser became unstable and then disappeared.This phenomenon might be attributed to performance degradation of BP flake due to excess heat.With the advance of fiber fabrication and optimization of BP SAM, we believe that higher pulse energy will be achieved.Figure 5(b) shows the evolution of repetition rate and pulse width with the launched pump power.As expected, with the increase of launched pump power from 16.6 W to 18.8 W, the repetition rate increased from 48.3 kHz to 52.5 kHz and the pulse width decreased from 1.15 s to 0.66 s, respectively.During the experiment, no Q-switching mode-locked pulses were obtained.
At the maximum average output power of 615 mW, the spectrum of Q-switched fiber laser was measured by a mid-infrared spectrum analyzer with a resolution of 0.2 nm (Ocean Optics, SIR5000).As depicted in Figure 6, the Qswitched spectrum centered at the wavelength of 1954 nm with a full width of half maximum (FWHM) of 3.2 nm.

Conclusions
In conclusion, we have experimentally demonstrated a BP passively Q-switched fiber laser with LMA double-cladding Tm-doped fiber as gain medium for the first time.The Qswitched fiber laser emitted pulses with a maximum pulse energy of 11.72 J and the shortest pulse width of 0.66 s at the wavelength of 1954 nm.The experimental results suggest that the sol-gel fabricated LMA dual-cladding Tm-doped fiber combined with BP SA is an effective way for generating highenergy laser pulses at the eye-safe wavelength of 2 m.

2 InternationalFigure 1 :
Figure 1: (a) Raman spectrum of the BP flake sample.(b) Morphology of BP flake measured by the scanning electron microscopy (SEM).(c) Three-dimension morphology of the BP flake scanned by the atomic force microscopy (AFM).Inset: height difference between mica sheet substrate (point A) and BP flake (point B).(d) Elemental components measured by the energy-dispersive X-ray spectroscopy (EDS).Inset: SEM image of BP flake.

Figure 4 :
Figure 4: (a) Typical pulse trains at the average output power of 363 mW, 505 mW, and 615 mW, respectively.(b) Corresponding pulse profiles.Inset: RF spectrum recorded at the average output power of 615 mW.

Figure 5 :
Figure 5: (a) Average output power and pulse energy versus the launched pump power.(b) Repetition rate and pulse width versus the launched pump power.

Table 1 :
Passively Q-switched Tm-doped and Tm/Ho-co doped fiber lasers based on low dimensional SAs.
output power and pulse energy increased linearly with the launched pump power in the Q-switched regime.When the pump power increased to 18.8 W, the maximum pulse energy of 11.72 J was achieved.Compared with the previous reports of 2D SAs Q-switched fiber lasers at 2 m (Table